Daylighting in Linear Atrium Buildings at High Latitudes
Norwegian University of Science and Technology Faculty of Architecture Department of Building Technology
DAYLIGHTING IN LINEAR ATRIUM BUILDINGS AT HIGH LATITUDES
av
Barbara Matusiak
Doktor ingeni0ravhandling
September 1998 DISCLAIMER
Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. ACKNOWLEDGEMENTS
I wish to express my gratitude to my advisor, professor 0yvind Achehoug, for an excellent guidance and support during the whole process of creating, developing and writing of the thesis. I am also grateful to professor B. Cold for giving me inspiration at the beginning, and professor A. G. Hestnes for discussing of the work at the end of the process. A special recognition to Paul Littlefair for guidance in use of the artificial sky at BRE in Watford, GB, and Raphael Compagnon for helping me with Radiance simulations. My appreciation also goes to Birgit Sudb0 for the help with all the expected and unexpected computer problems and my fellow doctoral students: Anne, Beate, Chandani and Heidi for all the discussions and the nice atmosphere we had. This project could not have succeeded without the financial support provided by the Hydro Aluminium dr.ing. programme at NTNU. My stipend was financed by the Faculty of Architecture, Planing and Fine Arts at NTNU.
Special thanks to my husband Miroslaw for the moral and spiritual support and to my children: Julia, Martin and Pawel for all the unforgettable moments we had together.
I SUMMARY
The content of the project can be divided into three parts that correspond to the three objectives of the thesis formulated in chapter 1. The first part is dedicated to visual comfort and is included in chapter 2. New criteria for visual comfort based on knowledge about visual perception, are proposed. The visual environment of an indoor space is divided into form, shape, and space, and visual comfort in vision of objects from each of these categories is discussed in detail. The modelling ability of light connected to the vision of small three-dimensional objects is discussed comprehensively. Radiance simulations are used for analysing the luminance distribution on small shapes. A method for estimating the modelling ability of light, using the inter-reflection calculations, is also proposed. At the end of chapter 2 visual comfort in atrium buildings is discussed. The second, theoretical part is included in chapters 3 and 4. Chapter 3 consists of simplified calculations of the daylight factor in linear building structures, using the projected solid angle principle. The calculations are done for uniform sky and for CIE overcast sky conditions. The results of calculations are compared to experimental results described in chapter 6. In chapter 4 simple diagrams are created on the base of calculation results of the mean daylight factor in rooms adjacent to a narrow street, using the Superlite program. The resulting formulas and tables from chapter 3 and diagrams from chapter 4 can be used as a simple design tool. In the third main part of the project, the daylighting strategies for linear atrium buildings at hi gh latitudes are developed and examined. This part is started in chapter 5 where daylighting strategies for linear atria are proposed and described. The strategies are divided into three groups: —> the atrium space and facades as light conductor/reflector, —> the glass roof as a light conductor, and —> light reflectors on the neighbouring roof. The strategies connected to the atrium space and facades are further divided into passive and active. The active strategies rely on the use of daylight systems in the atrium space, such as specular reflectors or laser cut panels. The passive strategies rely on the design of the facades. Two facade alternatives were examined. Facade A had glazing area that varied with floor level. The design of this facade was to a large degree based on the results of the Superlite calculations from chapter 4. Facade B had glazin g types that varied with floor level. The design of both facades resulted in balanced daylighting of the spaces adjacent to the atrium on all floor levels. The strategies connected to the glazed roof included different configurations of glazing: horizontal, single pitched, double pitched, and the use of laser cut panels and prismatic panels in the glazed roof. Three different shapes of reflectors on the neighbouring roof were examined: a flat reflector, a parabolic reflector and a parabolic concentrator. Strategies from all three groups were examined on a physical model in 1:20 scale in the artificial sky of mirror box type at Building Research Establishment in Watford near London; the results are presented and discussed in chapter 6. Simulations with artificial sun were done at the Department of Electrical Engineering in the Norwegian University of Science and Technology in Trondheim; the results are included in chapter 7.
n The strategies were also simulated using the Radiance rendering program. The program enabled parametric studies of the most pro mising alternatives, creating computer images, and calculation of the visual comfort probability, chapter 8. The results from model studies were compared to the results from Radiance simulations, and the final conclusions are formulated in chapter 9. All the active daylighting systems designed for use in the atrium space or on the atrium facades have a huge potential for use in atrium buildings. The glazed roof obstructs about 40- 50% of diffuse skylight The active daylight systems make it possible to utilise the remaining 50-60% effectively by redirecting diffuse skylight from the excessively daylit zones to the areas where it is most needed. In this way the negative effect of the glazed roof can be to a large degree reduced. From the strategies connected to the glazed roof the negatively sloped glass was found to be the best alternative for glazed roofs at high latitudes. All alternatives with the light deflecting panels on the roof performed very well. They multiplied the daylight levels in the atrium in the desired seasons. The alternative with laser cut panels sloped by 30° gave the best results of all alternatives, both in winter and in spring/autumn. Prismatic panels performed better as a sun shading. The study with roof reflectors showed that the flat one performs best, especially as an operable device. The thesis is supplemented with a list of the terms and definitions used in the thesis, appendix 1, and with calculations of the light transmission factor of different types of glass, appendix 2.
HI TABLE OF CONTENTS
CHAP. 1 INTRODUCTION 1
1.1 Motivation 1 1.2 Objectives 4 1.3 Methods 5
CHAP. 2 VISUAL COMFORT 6 2.1 Visual perception 8 2.1.1 The perception of brightness and darkness 8 2.1.2 Adaptation 8 2.1.3 Simultaneous contrast 8 2.1.4 Border contrast effect 8 2.1.5 Successive contrast 9 2.1.6 Illusory contours and forms 9 2.1.7 Colour perception 9 2.2 Glare, a sign of discomfort 10 2.3 Criteria of visual comfort 14 2.3.1 Criteria for visual comfort in vision of form 15 2.3.2 Criteria of visual comfort in vision of shape 20 2.3.3 Criteria of visual comfort in vision of space 28 2.3.4 Conclusions about visual comfort 30 2.4 Visual comfort in linear atrium buildings 30 2.4.1 Visual comfort in the atrium space 30 2.4.2 Visual comfort in adjacent rooms 34
CHAP. 3 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS 37 3.1 Calculations of sky factor for uniform sky 37 3.1.1 Sky factor on the floor 37 3.1.2 Vertical sky factor on facades 39 3.2 Calculation of the sky factor for the CIE overcast sky 40 3.2.1 Sky factor on the floor 40 3.2.2 Sky factor on the facades 44 3.3 Calculations of daylight factor 46
CHAP. 4 SUPERL1TE CALCULATIONS 49 4.1 Description of the calculation model 49 4.2 Calculations for the 5 floor linear atrium or street 50 4.3 Calculations for the 4 floor linear atrium or street 51 4.4 Conclusions 51
IV CHAP. 5 DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA 53 5.1 Materials used in the project 53 5.1.1 Traditional materials 53 5.1.2 Light redirective materials 55 5.2 Presentation of alternative strategies 57 5.2.1 Atrium space and facades as a light conductor/reflector 57 5.2.2 Glass roof as a light conductor/reflector 60 5.2.3 Light reflector on the neighbouring roof 62
Chao. 6 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY 64 6.1 Design of the base model 64 6.2 Sky details 66 6.3 Methods of measurement 66 6.4 Presentation of daylight systems 67 6.4.1 Atrium space and facades as a light conductor/reflector 67 6.4.2 Glass roof as a light conductor/reflector 69 6.4.3 Light reflector on the neighbouring roof 70 6.5 Comparison of facade alternatives 71 6.5.1 Facade alternatives A, B, C and Cx in the atrium space 72 6.5.2 Analysis of the luminance distribution on facade A 72 6.5.3 The impact of the choice of the facade alternative on the illuminance levels in adjacent rooms 75 6.5.4 Visual comfort in rooms adjacent to the atrium for alt A 79 6.6 Importance of the reflectance of atrium surfaces 82 6.6.1 Comparison of B-altematives on the street 82 6.6.2 Comparison of B-altematives in the room 84 6.7 Significance of the glass slope on the first floor 86 6.8 Comparing different active daylight strategies 87 6.9 Discussion of the significance of the roof glazing 98 6.10 Laser cut panels and prismatic panels on the roof 99 6.11 Reflectors on the neighbouring building 101
CHAP. 7 MODEL STUDIES WITH ARTIFICIAL SUN 104 7.1 Design of the artificial sun 105 7.2 Methods of measurement 106 7.3 Presentation of alternative daylighting ideas 107 7.4 Different glazing configurations on the roof 108 7.5 Laser cut panels and prismatic panels on the roof 112 7.6 Reflectors on the top part of south facing facade and on the neighbouring roof 119 7.7 Sun shading with laser cut panels on the facades 121
V CHAP. 8 RADIANCE SIMULATIONS 124 8.1 The data model 124 8.2 Automatization of the calculation process 125 8.3 Accuracy of calculations 126 8.4 Reflectors on the neighbouring roof 126 8.5 Glass roof as a light conductor 128 8.6 Atrium space and facades as a light reflector/conductor 140 8.6.1 M-fac 141 8.6.2 M-atr 141 8.6.3 M-shelf and M-curved 146
CHAP. 9 THE FINAL CONCLUSIONS 151 9.1 Visual comfort 151 9.2 Daylight design tools 152 9.2.1 Simplified daylight factor calculations for liner atrium 152 9.2.2 Estimation of the mean daylight factor in room adjacent to linear atrium 154 9.3 Daylighting ideas 155 9.3.1 Atrium space and facades as a light conductor/reflector 155 9.3.2 Glass roof as a light conductor/reflector 159 9.3.3 Light reflector on the neighbouring roof 161 9.4 Generality of the results and proposals for future studies 162
REFERENCES AND LITTERATURE 164
APPENDIX 1 Terms and definitions used in the project 170
APPENDIX 2 Calculation of the light transmission factor of glass 173
VI Chap. 11NTRODUCTION
1.1 Motivation
Why is a modern atrium building such an attractive building type? Climate shelter An atrium shelters buildings adjacent to it from rain, snow, wind, noise, pollution and at the same time enables daylight penetration. Sheltering of the space between buildings enables new possibilities for using it. Man in focus In large building complexes an atrium functions often as a public room during the whole year, giving space for social life and increasing contact between people. The green oasis If the temperature in an atrium is held over +5° during the year, an atrium is a suitable place for nearly all tropical plants. This is very much appreciated in a climate with a long and severe winter. If suitably landscaped, atria can provide an attractive view from the adjacent buildings. Potential for energy conservation Glazed atria offers opportunities for energy saving strategies in many categories of buildings. Even when unheated, the atrium will be semi-tempered by heat losses from the surrounding buildings, and solar gains. The buffering effect of such atria will reduce the overall heat losses [Hestnes, 86]. Economic status and potential for savings Atrium buildings are often considered as very expensive ones, because the atrium itself signals luxury. In reality it is possible to save construction costs by designing a complex of buildings connected by atria instead of separate buildings. The cost of glazed roofs can be more than balanced by savings due to simpler construction of facade walls and atrium floor, and reduced number of staircases and elevators. The operation costs of the building complex will also be smaller, because the facades sheltered from the influence of outside climate will not need frequent maintenance and the snow removal on the areas between buildings will be unnecessary [Tyholt, 88]. Daylight in atrium building An atrium in northern countries is most often physically closed to the adjacent buildings, due to energy conservation and fire regulations. For adjacent buildings, the atria function as a quasi-outdoor space and as an indirect daylight source. A successfully designed atrium should have the appearance of being open, bright and spacious, through high daylight levels and rich greenery. Daylight levels in an atrium space, compared to an open court of the same size, are considerably reduced due to the roof glazing and glazed roof structure. An analysis of Norwegian atria shows a reduction in daylight levels of about 40-50%. On the other side, glazing the space between buildings allows an increase in the size of the windows oriented towards that space without significant heat losses. It is important to distribute enough daylight from the atrium to the adjacent buildings to avoid unreasonably high levels of artificial lighting use. Unfortunately, the use of artificial lighting during the whole year can be observed in many atrium buildings in Norway. Since the same phenomenon can also be observed in richly daylit buildings, it is reasonable to believe that the low daylight level is not the only reason for excessive use of artificial lighting in atrium buildings. Other reasons can be reduction of the contrast between the window zone and the rear zone in the room or reduction of glare from windows. During overcast days it may be INTRODUCTION desired to add some direct warm light to the diffuse blue-white light from the sky. Other reasons not connected to the lighting technique can be the lack of motivation for energy conservation or desire to emphasise presence in the room by occupants. The main parameters influencing daylighting in atria and surrounding buildings are: —> structural loadbearing system for the glazed roof, -» glazing system, i.e. geometry of the glazing bars, number of glass sheets and their optical properties, (possible filling of cavity), and the maintenance of the glazing system. —» geometry of the atrium, —> reflectance of the atrium facades and floor, —> types and locations of plants within atria. -» size and type of glazing in the wall between atrium and adjacent buildings, —> dimensions of adjacent spaces, —> interior reflectances of the adjacent spaces, Photo1 1.1 Inside a linear atrium at the University Centre at Dragvoll, Trondheim. The shape of atrium In high latitudes, where the mean yearly noon solar altitude is about 30°, the sunlight penetration to a building structure from the top is very limited. A linear atrium enables comparatively the best sunlight penetration, because the sunlight will penetrate also through the gable walls. The linear atrium is not as compact as square one and enables distant views from rooms adjacent to it, so the visual contact with the outdoor environment is often maintained. Since the temperature in an atrium can be low in winter, it is best suitable for activities less sensitive to thermal comfort. The linear shape offers the possibility to use the atrium as a communication route in large building complexes, fig. 1.1. The linear atria have been build with success in northern countries, only in Trondheim three notable examples can be found: the University Centre at Dragvoll, photo 1.1, the Department of Electrical Engineering and Telecomunication, photo 2.3, and the Royal Garden Hotel. The atrium as a research subject The daylight levels in atria of different geometry’s have been investigated by several researchers. Maurice Aizlewood in the 1995 ASHRAE Conference publication “The daylighting
1 The following names are used for illustrations: photo - for pictures taken with camera picture - for computer made images figure - for the rest of illustrations
2 INTRODUCTION
of atria: a review” gave a very detailed and comprehensive review of recently published knowledge about daylighting in atrium buildings. In several books dedicated to atrium buildings, as “Atrium buildings: development and design ” by R. Saxon or “The new atrium ” by M. J. Bednar, daylighting was described and illustrated as one of many aspects of atrium design. In books dedicated to daylighting, as the European reference book “Daylighting in architecture ”, an atrium itself is dealt with as one of the possible daylighting strategies in large buildings and is compared to other daylighting strategies. Atrium buildings were studied comprehensively in TEA Task 11, solar heating and cooling progr amme “Passive solar commercial and institutional buildings" [Hastings, 94]. Daylighting in atrium was considered as one of factors influencing the energy conservation potential of atrium buildings. Although the literature dealing with atria is rather rich, very little have been published about daylight systems for atria. Also design tools for atria are rather complex, and most of the tools are developed for other shapes than linear.
DEL "2
DEL 2 :
Figure 1.1 Axonometric projection of the University Centre at Dragvoll, Trondheim. Adopted from [Knudsen, 94]
i
I
3
"'A \c/v V* -’7? ' #4 INTRODUCTION
1.2 Objectives
Atria have become a very popular architectural feature, but the literature dealing with daylighting in linear atrium buildings at high latitudes is not really comprehensive enough. The main objective of this thesis is increasing the knowledge about daylighting in linear atrium buildings at high latitudes. The main objective can be further divided into parts that correspond to the stages of the architectural design process. In the first stage architects develop several design proposals of the whole building complex, and after discussions with clients the decision about the main geometry of the atrium and the adjacent buildings is taken. In this stage only very simple design tools can be used, so that several alternative designs can be examined easily and quickly. On the other side, the tools should be precise enough in order to give correct answers, because decisions taken at this stage are often most important for the daylighting design of the building. The first objective of the thesis is therefore: 1. developing simplified design-related tools for evaluation of daylight levels in linear atrium buildings on the basis of the main geometry of the building. In the next stage of the architectural design process more specific design decisions are taken. The load-bearing structure of the glazed roof is designed, the glazing system of roof glazin g is chosen. The atrium facades are designed so that the size and type of windows and materials for opaque surfaces are chosen. Use of daylighting systems is under consideration. It is desired to give architects more detailed information about consequences of their choices and design decisions for daylighting. The second objective of the thesis is therefore: 2. developing and examining of daylighting ideas for linear atrium buildings in high latitudes. The ideas should include both passive and active strategies. Passive strategies rely on the design of the atrium surfaces alone. Active strategies rely on placing additional elements, daylight systems, in the atrium space that redirect light in the desired directions. In order to evaluate daylighting strategies a set of criteria should be used. Daylight design criteria: —> Daylight levels Daylight levels indicate the potential for energy savings; increasing the daylight levels means reduction of electrical light use —> Visual comfort Daylight design should not cause solar glare or visual fatigue. All objects in the space should be clearly visible and the visual impression should correspond to the impression of the same objects under ideal visual conditions. —> Impression of the room Daylight design influence the overall impression of the room, which can change from gloomy to bright, from narrow to spacious, from closed to open.
4 INTRODUCTION
The literature review showed that the best established criterion for visual comfort in daylit rooms is the lack of glare. In addition, some recommendations are established for reading writing tasks. Visual comfort and lighting/daylighting quality is high on the agenda for many scientists in the international lighting and daylighting community. In order to better define visual comfort and to take part in the discussion about visual comfort, developing visual comfort criteria for atrium buildings became important.
The third objective of the thesis is therefore: 3. developing of criteria for visual comfort for atrium buildings. The criteria for visual comfort should include all types of objects that can be found in a room. The objects can be divided into groups depending on size and shape. Visibility analysis of objects from each group should be performed.
1.3 Methods The development of simple design-related tools was achieved by using the following methods: 1. First, ‘the solid angle projection method ’ and an analytical integration were used in order to develop a simple calculation method, chap. 3. 2. Next, the Superlite data program was used for calculation of the mean daylight factor in rooms adjacent to the linear atrium in Norway, chap. 4. The development and evaluation of daylighting strategies, described in chap. 5, were done in three stages: 1. First, a series of model studies under an artificial overcast sky of the mirror box type were carried out at the Building Research Establishment in Watford near London in June/July 1996, described in chap. 6. 2. The next series of model studies with an artificial sun were carried out at Department of Electrical Engineering at Norwegian University of Science and Technology during 1997. The model was lit by a row of lamps, simulating the sun, described in chap. 7. 3. Finally, a series of simulations using the data program Radiance extended the spectrum of possible alternatives and enabled parameter studies of pro mising alternatives, chap. 8. The criteria for visual comfort were established on the basis of the knowledge about the visual perception, and with the help of the Radiance program.
5 Chap. 2 VISUAL COMFORT
Architecture is the masterly, correct and magnificent play of masses brought together in light.
Le Corbusier
This quotation gives a clue to how important light is for architectural quality. The ancient artists perceived architecture as the mother of design and the synthesis of arts. All the other arts that were placed in the building were integrated in architecture to make it more beautiful. Regarding architecture as a very complex art is relevant today, too. Demands on function are more detailed including a very important demand for flexibility. There are also higher demands to the loadbearing construction: it should be secure, efficient in use of materials, economical, and easy to build. The developments in building technology have caused, that wall and roof structure are more complex. The focus on environment in the last years has resulted in new demands for energy conservation and usage of environment friendly materials. And maybe the most important are the high demands for comfort. Professor B. Cold has presented the four main factors that create architectural quality: form, function, construction, and place in a graphic form, fig. 2.1 [Cold, 93]. The importance of the respective factors shown in the figure depends also on the person estimating architecture quality. I have a feeling, that the contemporary architects have the same conviction as the ancient architects had that the visual impression is most important in estimating quality of architecture. phUosophy, aesthetics Visual comfort Structure Form Room Architectu
technology Technique ral Function society Quality Resource Place
nature Figure 2.1. Factors influencing architectural quality. How is the visual impression built? We perceive the outside environment with the help of sensory organs. Each perceived stimulus generates an impulse to the brain, where it is interpreted. VISUAL COMFORT
William M. C. Lam in “Perception and lighting as formgivers for architecture ” [Lam, 77] emphasises the very important role that expectation and experience play in estimation of architectural quality. All temporary stimulus that comes to the brain from any sensory organ are first compared with expectations and interpreted afterwards. The experience is built as we perceive ‘new* objects or modified as we perceive ‘known ’ objects. In order to give the best condition for the functioning of our sensory organs, we demand high quality of our physical environment The concept thermal comfort is best established, and criteria for it are clearly defined: “Thermal neutrality for a person is defined as the condition in which the subject would prefer neither warmer nor cooler surroundings. .... In most cases, thermal neutrality will be the same as thermal comfort. ” [Fanger, 72].
Unlike thermal comfort, the concept visual comfort is used to describe the lack of visual discomfort Visual discomfort is then measured with the help of glare indexes, developed mostly for point sources. Most of the light falling on the retina is first reflected from objects we look at Therefore, unlike thermal comfort, visual comfort is directly connected with form and shape of the objects we look at The importance of visual quality is possibly a consequence of the key role that vision plays for the functioning of our organis m. The anatomic comparison of our sensory organs tells us that 60% of all nerve fibres from the sensory organs to the brain comes from the eye, 30 000 from the ear, 2 000 000 from the eye. In the cerebral cortex there are 800 000 nerve cells in the hearing centre and 500 000 000 nerve cells in the vision centre. [Valberg, 98]. The number of cells represents the amount of information that is possible to obtain, carry and interpret On this basis we can definitely say that the eye is our most important sense organ. Form, shape and colour are the most important attributes that we use to identify an object They are only possible to see if a minimum of light exists. Without light nothin g is visible. But the same object can make different impressions in different light conditions. Light level, spatial distribution and colour of light determine perception. One object can have many faces. Which impression is true? I think that the answer is: the impression, which leads to perception of all the properties and qualities of the object in the best way.
There exists a firm belief that to see the object properly one should see it in daylight on a sunny day. In order to give the best conditions for the functioning of our sensory organs, we demand high quality of our physical environment, we want comfort Visual comfort is present only if the light quality in a room is high. Analysis of criteria for visual comfort will give us cues about lighting quality.
7 VISUAL COMFORT
2.1 Visual perception
Because visual comfort is strongly connected with visual perception, it is reasonable to give a short resume of the most important terms connected with visual perception.
2.1.1 The perception of brightness and darkness.
The sensation of brightness is relative. It depends on the intensity of the light that the retina has been subjected to in the recent past and on the intensities of light fallin g on other regions of the retina.
2.1.2 Adaptation
The human retina can adjust to very different light levels and light qualities. This feature is called adaptation. In the adaptation process the eye changes its sensitivity to light The human eye can be light- or dark-adapted depending on the light level. In the dark-adapted condition the retina is extremely sensitive, detection of light can be made also if only few rods absorb a quantum energy each. The adaptation to darkness takes up to % hour, to brightness just some seconds. The eye has also ability to adjust and neutralise the large variations in the light colour. It means that the colour sensation of a lighted object changes much less, than it could be expected analysing spectral distribution of the reflected light. This ability is called the chromatic adaptation.
2.1.3 Simultaneous contrast
Another factor that affects sensation of brightness is the intensity of surrounding areas. Generally, the brightness sensation of given region is changed in the direction opposite to the surroundings, i.e. it looks brighter if its surroundings are dark and it looks darker if its surroundings are bright, fig. 2.2. The simultaneous contrast exists also for colour sensation. The chromatic colour looks more intense if it is surrounded by its complementary colour.
Figure 2.2 An example of simultaneous brightness contrast when looking at a grey square against a lighter or darker background.
2.1.4 Border contrast effect
Contrast enhancement is connected with the detection of borders. The existence of borders is signalled to the brain and the borders are simultaneously strengthened by the visual system, fig. 2.3 and 2.4. Very often regions of constant intensity, that do not need much information, lie between borders. The visual system extrapolates between borders, saving a lot of information handlin g by the peripheral parts of the system. It seems that the border strengthening does exist for borders between colours.
8 VISUAL COMFORT
.■ B
Distance
Figure 2.3 Upper: luminance border. Lower: perception of brightness. Adopted from [Spillman, 90].
Figure 2.4 Strengthening of luminance signals on the borders between areas of even luminance, the so-called Mach-bond effect.
2.1.5 Successive contrast
The colour sensation is influenced by the colour seen in the recent past in direction of the complementary colour i.e. the sensation of yellow colour will be changed in direction green- blue after looting at a large red region. The successive contrast was first discovered for colour vision, but it exists also for colours from a white-black scale.
2.1.6 Illusory contours and forms A contour that is not completed or that does not exist at all can be seen, too. It is due to the sensory systems ability toward organising the sensory data into objects. A few lines drawn by a cartoonist is enough to generate sensation of a face, or a whole scene. The human brain seeks and finds objects whenever possible,/?#. 2.5.
PI e + m ft v J s a
Figure 2.5 Some examples of subjective contours. Adopted from [Haber, 80].
2.1.7 Colour perception
The perception of colour is dependent on the spectral distribution of light falling on the retina and on the adaptation state of the eye.
9 VISUAL COMFORT
Detection of light and discrimination of colours The eye adjusts its sensitivity for light to the light level. If the light level is low enough, the sensitivity of radiation in the blue - green part of the visible spectrum is much higher than sensitivity for radiation in red - orange part, Purkinjes phenomenon. Colour appearance The spectral distribution of reflected light is dependent on the spectral distribution of incoming light and the spectral reflectance of the surface. Unlike our perception of sound that is selective, we are able to distinguish many simultaneous sounds that have different wavelengths, the perception of colours is additive. Our vision system adds light of different wavelengths. If the incoming light consists of more wavelengths than the surface is able to reflect, the rest will be absorbed in the surface. If the light lacks some of wavelengths that the surface is able to reflect, they will be missing in the light reflected from the surface, too. The colour may appear somewhat darker than it looks in lighting that does not lack those wavelengths. The spectral distribution of reflected light will be changed, it means that the hue can be somewhat different than in lighting that does not lack wavelengths that the surface is able to reflect In addition to the demand of presence of all wavelengths, reflectance of light of the respective wavelengths should correspond to the power of radiation of the same wavelength, e.g. the incandescent lamp gives a perfect colour rendering of red colours, but poor colour rendering of blue colours. The reason is probably that in the spectrum of light from incandescent lamp, the power of radiation in the blue spectrum is small in comparison with the power of radiation in the red spectrum. The power of radiation of the light has also influence on the colour perception. The colours change hue with the change of luminance. That phenomenon, called the Bezold-Brucke phenomenon, suggests a connection between light level and light quality, ./zg. 2.6.
Figure 2.6 Ilustration of Bezold-Brucke phenomenon. Figure shows results of experiment where three persons estimated hue of some stimuli changing luminance. Adopted from [Valberg, 98].
10 VISUAL COMFORT
Some light sources can give proper colour rendering for some colours, but a poor colour rendering of other colours. Only white light that consists of all wavelengths, such as sunlight, gives perfect colour rendering of all possible colours.
2.2 Glare, a sign of discomfort
It is much easier to describe the lack of quality, than quality as such [Cold, 95]. In the same way it is easier to describe conditions for lack of visual comfort than conditions for visual comfort.
The comfort aspect of daylight design is closely related to the problem of glare. [Hopkinson, 66]
Glare can be described as a subjective phenomenon occurring when visual perception is reduced or prevented by visual noise coming from an uncomfortably bright light source present in the visual field. There are two distinctly different forms of glare: disability glare and discomfort glare. scattered light from sclera scattered light from cornea scattered light from lens picture G ot an object scattered light from fluids scattered light from retina picture B'ofa light source Figure 2.7 Disability glare. Adopted from [Bj0rset, 92]. Disability glare occurs when light of high luminance is seen against a low lumin ance background close to the line of sight,/zg. 2.7. The light is scattered in the eye and generates a luminous veil across the retina. Holladay proved that the effect of that scattered light is equivalent with an extra, even luminance covering the visual field [Holladay, 26]. That equivalent luminance, called veiling luminance, can be calculated by Holladay’s formula: Ly = k Ebi /0” Ly veiling lumin ance Ebi illuminance on eye on a plan perpendicular to line of sight 0 the angle between the line of sight and the direction to the glare light source, k, n constants dependent on age. Discomfort glare occurs due to the variation of the luminance across the visual field influencing the central and the peripheral visual field. The literature produces a number of formulas for estimation of discomfort glare. Jens Christoffersen has described and compared all of known formulas in his dissertation [Christoffersen, 95]. The following summary of the most important formulas is based on his thesis. The parameters for sensation of glare, presented in fig. 2.8, are as follows:
Ls luminance of the glare source (cd/m2), Lb luminance of the background (cd/m2), % solid angle of the source seen from the point of observation (steradian),
11 VISUAL COMFORT
P Guth ’s position index, expressing the change in discomfort glare experienced relative to the azimuth and elevation of the position for the glare source and the observer ’s line of sight, n number of glare sources.
The British glare index fBREl: BGI
The British glare index was developed by Hopkinson and Petherbridge in 1950. The sensation of glare was estimated on the scale: just noticeable glare - just acceptable glare - just uncomfortable glare and just intolerable glare. The formula defines glare sensation from a single source: BGI = 10 log 10 0.478 Ii=1n (Ls16 to,0 8 / Lb P L6)
Luminance of the glare source
Solid angle subtended by the glare source
Backgroud luminance Fovea Line_of Sight
Displacement from line of sight 1 by elevation and azimuth angle Image of the glare source
Figure 2.8 A simplified illustration of the parameters influencing disability glare. Adopted from [ C1E, 83].
The Cornell glare index: DGI
The Cornell glare index is a modification of the BGI index, for a large source, e.g. window. The study was conducted at the BRE and Cornell University USA. The degree of glare was expressed by: DGI = 10 logjo 0.478 Ei=in [Ls16 £20'8 / ( Lb + 0.07 co,0'5 Ls)] where Q is a solid angle subtended by the source, modified for the position of the light source with respect to the field of view and position index P. Q — Jos (d Cps / P2)
12 VISUAL COMFORT
CIE Unified Glare Rating system: UGR
The intention of UGR is to compose the best parts of the recognised glare indexes in terms of the subjective glare response. UGR = 8 log 10 (0.25 /Lb )lLs2cos/P2
The American glare index fGuth): PGR
The discomfort glare equation for an individual glare source is: DGR = 0.5 Ls (20.4 cos + 1.52 ms0'2 - 0.075) / P F044
where F is luminance of the background including the luminance of the glare source. For multiple sources: DGR = (Ei=in DGR; )a, a = n (4)0914) The DGR system was used to define the percentage of people assessing an installation to be more comfortable than the borderline between comfort and discomfort, also called the visual comfort probability (VCP). The comparison of the corresponding magnitude of discomfort glare experienced for different glare indexes with the visual comfort probability (VCP), tab. 2.1, is adopted from [Christoffersen, 95], BGI Corresponding degree of glare CGI DGI DGR VCP UGR % No glare <20 Unnoticeable <10 <16 35 95 Just imperceptible 10 16 50 87 Acceptable but not imperceptible 13 18 65 75 Just acceptable 16 20 90 64 Border line comfort/discomfort 18.5 22 120 50 Just uncomfortable 22 24 220 20 Uncomfortable 25 26 300 11 Just intolerable 28 28 400 5 Intolerable 700
Table 2.1 Comparison of the corresponding magnitude of discomfort glare experienced for different glare indexes with the visual comfort probability.
The J index
A new method of estimating glare was elaborated at Laboratorie d’Ergonomie de la Vision at University in Geneva [Compagnon, 94]. The discomfort index is defined as: J = (Amax'A)/ Ajuaj physiological maximum acuity of one particular observer tested in optimal conditions A computed potential visual acuity in a given situation. Since the sensation of discomfort depends on the observer, both physical and physiological parameters are included in a computation procedure of the potential visual acuity A: -illumin ation of the eye measured at the pupil of the eye -contrast between target and background
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-contrast between background and surrounding -homogeneity of luminance inside the visual field -maximal visual acuity for an individual Vm, measured in laboratory -brightness of the target for this maximal acuity [cd/m2] The J index varies between 0 for intolerable and 1 for no glare. An interesting investigation about discomfort glare from the unobstructed sky was carried out by P. Chauvel, J. B. Collins R. Dogniaux and J. Longmore [Chauvel, 82]. After studying discomfort induced by seeing the sky direcdy through the windows they concluded that discomfort glare from a single window (except for a rather small one under 2 % floor area) is practically independent of size and distance from the observer but is strictly dependent on the sky luminance and on lesser extent, on the reflectance ’s of the room surfaces.
2.3 Criteria of visual comfort
B. Cold in Six perspectives for quality [Cold, 95] presented a method of describing quality by first describing the lack of quality and turning those statements to positive statements, e.g.: Lack of quality: when “things ” age badly. Positive statement: when “things ” age beautifully. Such a turning is very useful if the scale from lack of quality to quality is a one-dimensional scale. In case of visual comfort the problem is more complicated. “But is it really comfort just because you do not experience discomfort? ” asks Hans Allan Lofberg in a note written for a CIE task 21 meeting. He writes: “I would like to have a scale like: discomfort ...... neutral ...... comfort (if it is a one dimensional scale.) When evaluating glare you could say that the scale is high discomfort...... no discomfort which corresponds to discomfort...... neutral ” He writes on: “It is not certain that the scales discomfort.....neutral and neutral.....comfort are part of the same dimension. They could be orthogonal, i.e. reducing visual discomfort does not necessarily increase comfort. The remarks about glare (glitter, sparkle) indicates that it is not a one dimensional factor.” This opinion confirms that there exists a need for establishing criteria for visual comfort. I would like to propose a new definition of visual comfort based on visual perception: Visual comfort is experienced by an observer when she/he is subjected to the lighting condition that makes possible the best visual perception of objects and space in the visual field.
Such a definition of visual comfort can be understood more as readability or visual clarity, less as a feeling of pleasure. Readability can range from just readable to very easily readable. There can exist many different lighting designs that can create readability of the same object
14 VISUAL COMFORT and the same space. In that large group one can find the smaller group of lighting designs which is preferred by a person - that is visual comfort The sensation of visual comfort is very subjective; especially the age of a person has a colossal importance. In most beautiful buildings people experience visual comfort, especially in the functionally most important places in the building. But there exists also high quality architecture where visual comfort does not exist and poor architecture where visual comfort exists. In some types of room visual comfort is not the focus. Creating an atmosphere of intimacy, secrecy, or pleasure needs poor readability of space and shape. Enigmatic space gives a feeling of unsafely and the need for contact with other people. It is easier to stimulate the social life between people in such conditions. The criteria for visual comfort in the room can be divided according to the type of object we perceive. I will suggest the division into: 1. flat objects -form, 2. three-dimensional objects seen from outside - shape, 3. three-dimensional objects seen from inside - space. For most buildings it is reasonable to expect that visual comfort should be present for all those three categories. But there can exist examples where one category should have preference. In a room where reading a printed text on a flat paper is most important, high priority to visibility of form should be given, demands on visual comfort in visibility of shape and space can be reduced. 2.3.1 Criteria for visual comfort in vision of form. The necessary conditions for visual comfort in vision of form depends on the way the form is created: 1. luminance gradient between form and background a. difference in black - white reflectance (black letters on a white paper), b. difference in specularity (mirror versus diffuse surface), c. difference in roughness (smooth versus rough surface), 2. difference in colour between form and background, 3. form and background have different pattern, a. black - white pattern b. chromatic pattern
1. Luminance gradient between form and background l.a difference in black - white reflectance
For measurement of the luminance gradients for objects on a background the concept of local contrast was introduced: C = AL/L (3.1) L mean luminance in the visual field.
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AL difference between the mean luminance in the visual field and the luminance of the object. The relation between illuminance E falling on a perfectly diffuse surface and luminance L of that surface is: L = pE/jt (3.2) where p is the reflectance factor of the surface. The luminance contrast for a perfectly diffusing surface is defined as: C = (L0-Lb )/Lb where: L0 luminance of an object, Lb luminance of background,
If the illuminance E is the same, E/jc is constant, than:
C = (p 0 - p b )/ pb (3.3) where: p 0 reflectance factor of an object, p b reflectance factor of the background If the illuminance E fallin g on the perfectly diffuse surface is the same over the whole area, the expected luminance gradients must be caused by differences in reflectance alone, and the contrast is constant. It also means that for perfectly diffusing surfaces the azimuth direction of light has no importance as long as the angle of incidence is the same. For not perfectly diffuse surfaces, the azimuth direction of light has importance in addition to the mean luminance. If the light source is placed in the mirror angle in relation to the eye, veiling reflections result If the luminance is very high, the reflected light is perceived not as the object but as a light, which means that the object is not visible. A form is distinguishable only if the contrast between it and the background is higher than the threshold contrast The threshold contrast varies with the luminance of the background and the size of the object, fig. 2.9.
The results of many psychological experiments show, that the visual acuity is also dependent on background luminance, fig. 2.10. The visual acuity increases with increasing background luminance up to above ca. 100 cd/m2. Further increasing of the background luminance gives only a minimal contribution to visual acuity. On that basis the recommendations for illumination levels were established in form of illumination tables. The value of illuminance is only indicative in relation to the luminance level and it can be used only as a preliminary evaluation of lighting.
l.b difference in specularity
Light falling on a perfectly specular surface is reflected in a mirror angle. Light falling on perfectly diffuse surface is reflected in all possible directions. Most objects have partly diffuse, partly specular surfaces. The change of specularity degree causes a change of the appearance of a surface. The border between more and less specular surface is perceived as the contour of a form.
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■4L/L
! o.
•10 10- cd/mi Background luminance Figure 2.9 The threshold contrast as a function of background luminance for different object sine. Adopted from [ Bj^rset, 92].
Visual acuity • Visual angle in minutes
10* W5 Kf* w;J to2 icT if 10’ io' m' tf . —:----- »- Background luminance (cd/m2)
Figure 2.10 Visual acuity as a function of background luminance. Adoptedfrom [ Bjfirset, 92].
1. c difference in roughness
The concept roughness is connected with the concept texture. Texture can be defined as a perturbation of the surface normal [TEA, 94b]. The unevenness of the surface is supposed to be very small in comparison to object size. Roughness is a degree of perturbation. The change of roughness on a surface can cause a change of appearance. The effect is dependent on the lighting. Texture is best visible in direct lighting falling on a surface under a large incidence angle, preferably 45°-90°. In such lighting the smallest change in roughness causes a considerable change in appearance. In perfectly diffuse lighting differences in roughness are poorly visible. The border between more and less rough surface is perceived as the contour of a form.
2. Difference in colour between form and background
A form can have the same luminance as the background, but a different hue, e.g. red on green. This means that the light reflected from the object and the background can have the same luminous intensity but different spectral power distribution. In this situation the spectral distribution of incomin g light is of crucial importance.
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If the light produces perfect colour rendering of only one of the colours, the other colour will appear darker, in an extreme situation it will look black. The colour difference will be changed into luminance difference. The form will be visible, but the visual impression will be different If the light does not produce perfect colour rendering for any of the colours, both colours can appear as darker than theoretically possible. The chrominance contrast will be lower and may be changed to luminance contrast. The form may be poorly visible. Because for high luminance levels colours change hue with the change of luminance, the level of the incoming light also has influence on the perception of colour contrast. Strong light will have little influence on a strong colour contrast, as red-green, but the contrast between colours with neighbouring wavelengths, e.g. yellow-red or blue-green can disappear in strong light. Readability of form will be greatly reduced or disappear. These changes are somewhat subjective.
3. Form and background have different pattern
The vision of pattern depends on the contrast between the elements that produces pattern, and the background.
3.a black - white pattern
In the case of black-white pattern, the inco min g light on the surface will have the same influence on the perception of pattern as on perception of form made by difference in white - black reflectance. The vision of a pattern will be strongly influenced by the spatial frequency of its surface elements. For a spatial frequency between 5-2 periods/degree, where the eye’s contrast sensitivity is highest, the light level has lower influence on the readability of pattern. For very small elements high lighting level has a positive influence on pattern rendering because it increases visual acuity.
Figure 2.11 Examples of form made by changes in pattern.
3.b chromatic pattern
If the pattern is made up of colour contrast alone, e.g. red points on green background, the visibility of differences in pattern will depend on colour rendering in the same way as visibility of form. The sensitivity to colour contrast is lower than sensitivity to luminance contrast for small objects, fig. 2.12. Simultaneous contrast causes a pattern made of little dots of complementary colours to look brilliant, when seen from short distant The colours will be
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strengthened mutually. The same pattern seen from a long distant will look grey, the colours will be mixed. This phenomenon was experienced by the pointillist painters.
Luminance
Chrominance
Spatial frequency (period per deg)
Figure 2.12 Contrast sensitivity as a function of spatial frequency. Adopted from [Valberg, 97].
Discussion On the basis of the knowledge about visual perception some conclusions about daylightingZlightmg can be formulated, depending on the way the form is created: A form can have many of the attributes simultaneously, this causes sometimes contradictory demands to the lighting. A form can also be created be the light itself. The clearly visible form created by shadow is just an illuminance gradient Another example is a light spot that occurs when direct light comes through a hole. The shape of a hole gives form to a light spot Such forms are very interesting because they are not materialistic and they change size and intensity with changes of direction or strength of light But they can cause confusion and reduce visual comfort if they overlap forms created in different ways.
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Method: Comments for lighting reflectance Readability of form made by reflectance gradient depends gradient on the size of the form and on the background luminance. Generally, illuminance should increase with reduction of the size of the form and with reduction of the contrast. Luminance higher than 100cd/m2 will not help.
specularity A perfectly diffuse light will not make a difference in specularity readable. A small component of direct light is Luminance necessary. The incidence angle of direct light should not be mirrored to the viewing angle in order to avoid veiling gradient reflections that reduces visibility. If the direct light is too strong, the veiling reflections can cause glare. roughness A perfectly diffuse light will not make texture readable. Some of incoming light should be direct. A large incidence angle is preferable. Colour chrominance The luminance level should be high enough to generate contrast photopic vision. The light source should be composed of all wavelengths that the form is able to reflect and in proper proportions. Too high luminance can change the colour perception in an unexpected way. Pattern luminance The same as for reflectance gradient If the pattern is built contrast of very small elements, a high illuminance level is necessary. chrominance The same as for colour. A proper distance to the object is contrast important, especially if complementary colours are used.
Table 2.2 Comments about lighting/daylighting of form.
2.3.2 Criteria of visual comfort in vision of shape.
Two different problems should be considered in vision of three-dimensional objects: the three- dimensional geometry of a shape should be clearly readable, and a shape should be readable as somethin g different than the background. The readability of shape as a three-dimensional object is mostly dependent on the ability of the lighting to describe shape, i.e. modelling. Additionally, surface features such as reflectance, specularity, roughness and especially pattern can have a positive impact. The criteria for readability of shape as distinctive element from the background are the same as criteria for readability of two-dimensional form. Because the lumin ance and colour of a shape varies along its surface, the difference between the background and contour of the shape has importance.
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The necessary conditions for visual comfort in vision of a shape are: 1. The three-dimensional geometry of the shape should be clearly readable: a. modelling b. surface features, especially pattern. 2. The shape should be readable as something different than the background: a. luminance gradient between background and the contour of the shape difference in black-white reflectance
difference in specularity, difference in roughness b. difference in colour between background and the contour of the shape c. shape and background can have different pattern, black - white pattern chromatic pattern
Modelling
Shading is an important cue for the perception of depth and shape. Under most natural light sources, shading consists mainly of differences in brightness rather than hue. Therefore, depth information conveyed by shading is primarily luminance-contrast information, so the part of the visual system coding “shape from shading" need not carry any colour information. [Spillman, 90] The light direction has a fundamental importance for perception of shape, because the incidence angle of light is changing on a three-dimensional surface. For perfectly diffuse surfaces: L= pBfn but the illuminance is dependent on the incidence angle. In accordance with the first cosine law, the illuminance that a surface receives is proportional with the cosine of the incidence angle D:
E — EjnaA. COS (t)) Emax. the maximal illuminance value that a surface can get, incidence angle t> is zero. Then: L = (pEmax. COS (U))/7t
A contrast is possible to obtain also without differences in reflection factor or hue, variation of the incidence angle is enough. The luminance contrast between two places on a three- dimensional surface will be: C = (Lj-L^) / Ljnaan 1
1 The trigonometric functions are written cos (u) instead of cos u. This form of writing is dictated by the use of MathcadPLUS program.
21
# /< 1 VISUAL COMFORT
If the background has the same reflectance factor as an object, the contrast will be: C = (cos (uO - cos (o 2))/cos Cob ) where D] incidence angle of light in point 1 o 2 incidence angle of light in point 2 vb incidence angle of light on the background The contrast is dependent on incidence angles. Because of the importance of angles for contrast sensation, it is important to know the spatial distribution of light. The contrast is dependent on the point of view, the same object seen from one point can look pretty, seen from another may look too harsh or too soft. What happens in the shadows? A shadow appears when the light falls predominantly from one direction, so there is a large difference in illuminance between the parts of an object, that get light from the main direction and the parts that do not That part of an object that lies in shadow is usually also three- dimensional. If the contrast between the brightest and the darkest point on the object is too high, it is impossible to perceive the luminance differences of surface areas lying in shadow. I will propose a new criteria for modelling, based on contrast rendering. Because the perception of shape is dependent on shading and contour reading, good modelling should be conditioned by: 1. The minimum contrast, contrast between two points on the object that have a very low luminance and that are desired to be seen as distinguishable (e.g. Lmin and Ltow in pic. 2.1) should be higher than the threshold contrast 2. The maximum contrast (contrast between the brightest and the darkest point of the object, Lmin and LmoT in pic. 2.1) should be very high, preferably higher than 0.5. It is also dependent on the spatial size of the object, see pic. 2.2. For very small objects the demand of contrast should be higher than for large ones.
3. The local object-background contrast (contrast between the object’s contour and the background, e.g. 1 and L in pic. 2.1) should be higher than the threshold contrast.
Picture 2.1 Illustration of the luminances used in calculation of contrasts. The two first criteria are contradictory. Increasing the lumin ous intensity of direct light causes an increase of the maximum contrast and a reduction of the minimum contrast. Only a balance between them can produce good modelling. If the second criterion is fulfilled, the third criterion is usually fulfilled, too. Anyway, it is important to check that criterion, because it will guarantee that the object is distinguishable from the background. Additionally, the shadow thrown by a shape can give a clue for the readability of this shape, and especially for the placing of the shape in space. The fragment of surface lying in the
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shadow can have a very low luminance. In some cases, i.e. if the shadow is large and lies close to the object, it can considerably reduce the mean luminance level in the visual field and create a strong contrast The shadow is strongly dependent on the placing of the surface it falls on, so it can be poorly visible or not visible from some viewpoints. Including it in the criteria of modelling would cause demands on the surface it falls on.
Radiance
The data program Radiance is a wonderful tool for estimating the modelling ability of light A picture is created dependent on the viewpoint and the view direction data, light/daylight data and room geometry. Room and object surfaces can be simulated precisely using many accessible material types. A value of the calculated luminance in a desired point on the object surface can be exposed directly on the picture. A very wide range of luminance values can be experienced in the real world. The human visual system is capable of perceiving scenes sp annin g 5 orders of magnitude, but the media used to display these results, VD screen or print on paper, cannot reproduce the computed luminances. The new program pcond included in RADIANCE was created to filter the calculated picture to make it appear as a human eye would see it So the visibility of objects in the picture can be judged directly be looking at the picture. The following example was tested using Radiance: three balls of reflectances 0.8,0.3 and 0.15 were daylit by the CIE clear sky with sun, the solar azimuth was 30° and the solar altitude was 45°, and by the CIE overcast sky. They were placed free in the space, the black ball nearest, the white one farthest away from the observer. A vertical background oriented in the east - west direction was placed behind them The distance between the white ball and the background was large enough to avoid casting of shadows. Here the same three balls are presented on three different backgrounds: white (p=0.8), grey (p=0.3) and black (p=0.15).
Picture 2.2. White, grey and black balls on white, grey and black background. To the left lighted by the CIE clear sky with sun and to the right by the CIE overcast sky. The luminance values for calculation of contrasts were chosen in the same points on all the respective balls. The maximum and minimum luminances, and were'read on the
23 VISUAL COMFORT most and less lighted points lying by the contour. The point for neighbouring luminance Llow was chosen on the axis between and Lmi„ in the distance of V a diameter from the point where L,j„ was read. Additionahy, the local contrast was calculated for the maximum and the minimum luminance:
(Lmin"La)/La< Cl <(Lmax-La)/La and the mean local contrast CT.|m,m was calculated based on the luminance readings in eight points uniformly placed along the contour:
Ci.__ F=(IOi I + IC2I + IC2I + IC3I + IC4I + IC5I + IC6I + IC7I + ICgl)/8 Contrast sensitivity is directly dependent on the size of the object The reference threshold contrast Cref> a contrast between a 4-minutes disc and the background, seen by a reference observer in the reference lighting,[CIE, 81a], was calculated:
'l.639\ 0'4 2.5 C ref = 0.05936 .“J " where L is the adaptation luminance, in this example L = La white background, grey background black background La=15170 La=5672 Lj=2836 Cref=0.063 Cref=0.065 Crd=0.065 white ball: Cmax =1.22 Cnm=3.27 Cnmt=6.53 1^=21674 Cnan =0.16 Cmm= 0.45 Cmn= 0.89 Liow=5683 -0.79 Lmm=105 Ct TTw-an —0. S Cl _m«m=0.47 CL.mcan =0.47 Table 2.4. White, grey and black balls on white, grey and black background, lighted by the CIE overcast sky. 24 VISUAL COMFORT It is interesting to observe that for both skies the maximum and the minimum contrasts were lowest for the black ball exposed on the white background. It was also the only case where the minimum contrast was lower than the reference contrast Both increasing the ball reflectance and reducing the background reflectance, increases maximum and minimum contrasts. The mean local contrast between the ball and the background is nearly alike and low for all cases where the ball and the background have the same reflectance. The lowest one was found for the white ball on the white background for both skies. The lowest mean local contrasts were larger for clear sky than for overcast sky. It influences the possibility of distinguishing the balls from the background. The local contrast varies along the object contour. If it changes from negative to positive value, it must be near or equal to zero somewhere along the contour length, i.e. lower than the threshold contrast The visibility of the object depends on the length of contour with extremely low local contrast Anyway, because the human eye has a mechanism that strengths the contour, the local contrast can be very low. Another mechanism in the human eye helps to read a not complete form. In the case of a sphere the contour has a shape of a ring, a very strong and easy readable form. Inter-reflection calculations The three rules of good modelling can also be examined using principles for inter-reflection calculation. Maximum contrast CL,- Let us imagine a room with a ball placed in a point P. Only half of the ball surface is visible from a given view point V. Imagine division of the hemisphere visible from V into elements. If the elements are very small, the surface elements are nearly flat It is possible to calculate the lumin ance on each element of the hemisphere. From all surface elements an element that has the highest luminance L^, can be compared with one that has the lowest luminance L^;„. Qnmc — (Lnrnx-Lminj / La where: La the middle luminance in the visual field Minimum contrast The minimum contrast can be calculated as the contrast between the theoretically lowest luminance L,i„ and the lowest of the neighbouring elements. Cjnjn Lmin “ LQmin / T*a where: Ln m,„ luminance of that neighbouring element Q that has the lowest luminance of all neighbouring elements. Local obiect-background contrast Cu. Each contour element on the ball ’s surface will make a local contrast with the background element: Qoc — (Leo - Ly)/ La where: L^, luminance of contour element of the object Ly luminance of background element lying behind object element 25 VISUAL COMFORT To simplify the investigation, let us divide the room walls, floor and ceiling into small elements, each with an even luminance. Because of the small dimensions of the room elements compared to the distance to point P, it is possible to regard the surface elements as point sources, fig. 2.13. AP Figure 2.13 Illustration of the inter-reflection calculation. The illuminance on the small surface element AP placed in point P due to element AA is: AE = Iaa cos (a)/ r2 From the definition of luminance the luminance LA of element AA: La = Iaa / AA cos (e) then: AE = LA AA cos (e) cos (a) / r2 For perfectly diffusing surfaces the luminance of surface AP due to element AA is: AL = p La AA cos (e) cos (a) / tt r2 In this way it is possible to calculate the contribution to the luminance of a surface AP from room surface element AA. If the geometry of the room is known, the division of room surfaces on elements and calculation of r, cos. e, and cos.a is easy. The sum of contributions from all elements in the room will give the luminance of the area AP: L = p / 71 Xn=i n Ln An cos (ej cos (oj /rn 2 (3.1) where: n number of room surface elements in the room P reflectance of element AP L. luminance of element n An area of element n On angle of incidence on element AP from element An £n angle of incidence from element A„ rn distance between elements AP and An The formula 3.1 can be further used in calculations of L^*, Li„ LQmin and Lro . A mean local contrast will be the average value of local contrasts along the contour. Cl = 1/n Xn=i Cjocj, 26 VISUAL COMFORT The proposed method of calculation of luminances gives the possibility to investigate contributions to object elements ’ luminance from the respective room surface elements. It gives information about the shadows, too. The object elements, that are in some way screened from the room element/elements that give the highest contribution to illuminance of the object, are laying in shadow. Surface features Pattern Pattern is a very important clue for readability of shape. It changes appearance with changing of the object ’s geometry. A strong pattern is often a better clue for vision of shape than shading is. A way a pattern changes when seen from oblique angles, i.e. pattern shortening, give an immediate clue to the geometry of an object, because our eyes compare pattern forms seen in different incidence angles with the same pattern forms seen in more or less perpendicular incidence angle, which is a reference view. Proper colour rendering is important if the pattern is based on colour contrast alone. Texture The rendering of texture is strongly dependent on the lighting geometry. Because the incidence angle of direct light falling on a three-dimensional surface varies with the geometry of that surface, the changing appearance of texture gives additional information about the shape of an object Direct light is recommended for texture rendering. Colour In accordance with the law of constant colour perception, we perceive a three-dimensional object as having the same colour, although the hue of the object varies with the lighting. We judge the colour of an object on the basis of the highest lighted part of it, except the places where a veiling reflections occur. Because of that, the light falling on those high lighted places on the object has a crucial importance on colour rendering. A weak light reflected from room surfaces has usually little significance for colour rendering. The remainin g rules of colour rendering of three-dimensional objects are the same as for fiat objects. Conclusions about modelling Modelling is dependent on the light level, quality and distribution, geometry of the room, the placing of the point where the modelling is to be estimated and the placing of the point of view. From the definition of luminance contrast follows that: 1. The minimum contrast Cmin and the maximum contrast CL,? will be lowest on a black object exposed on a white background. In this case the mean local object-background contrast will be very high because of the high difference in the reflectances of object and background. In accordance with simultaneous luminance contrast, the background will influence the object in direction of the complementary colour to white, i.e. black. The object will look still darker. 2. The local object-background contrast Cl will be lowest in the case when the object and the background have nearly alike reflectances. To estimate the modelling in a desired point P in a room seen from a desired point of view V, small three-dimensional objects, e.g. balls, can be placed in point P, exposed on the white background. Because the extreme cases should be examined, minimum two colours should be 27 VISUAL COMFORT examined: white and nearly black (the black colour should not be perfectly black, otherwise it would absorb all incoming light). Additionally, some grey colours can be used in order to find out how large is the distance to the good modelling. The three contrasts described before can be measured or calculated. The Cmin on the nearly black object and CL on the white object can be compared to threshold contrast that corresponds with the background luminance and the size of the ball. If light has modelling capability for a nearly black object on a white background, and a white object is clearly distinguishable from the background, it will have modelling capability for any other object of the same or larger dimensions and background, too. In the model studies, chapters 6 and 7, the modelling sensor was made of two balls, one white and one dark grey, exposed on the white background. A similar modelling sensor was also used in Radiance simulations, chap. 8. Direct contra diffuse light for vision of human faces We prefer to look at side lighted faces: • we are familiar with side light • top light carries less information because the face appears symmetrical. 2.3.3 Criteria of visual comfort in vision of space It is difficult to find lighting designs which create visual comfort in each place in a room, but it is desirable to create visual comfort in the functionally most important places in the room and especially in the most important view directions from those places. The comparison of contrast sensitivity for colour contrast and luminance contrast, fig. 2.12, shows that for a low spatial frequency, that means large objects, the sensitivity to colour contrast does not diminish, and is higher than the sensitivity to luminance contrasts. The conclusion from this is that the use of different chromatic colours on room surfaces makes it more readable than the use of colours from the white-black scale. Local contrast between room surfaces on the border Because the eye is especially sensitive to border contrast, and estimates surface brightness on the basis of that contrast, it is desirable that the contrast is present on the borders between surfaces in order to give the eye a reference. In the case when neighbour surfaces have the same physical properties, the contrast must be created by lighting alone. If not, the border will not be visible and the room ’s geometry will be poorly readable. Since the eye has a mechanism that reinforces contour, the contrast needs not to be strong. Luminance or chromatic contrast between room surfaces. The lighting often varies along a surface, especially for large surfaces. In accordance with the constancy law, the eye manages to compensate for that We perceive the surface as having constant colour although luminance and/or chrominance vary. The eye perceives the light reflected from a surface, not the light falling on it The sensation of brightness is dependent both on the illuminance of light falling on the surface, and the reflectance of the surface. The lighting of a room should be designed in such a way that the eye should be able to clearly distinguish the impact of those two elements. The eye should be capable to distinguish a highly illumin ated dark surface from a low illuminated light surface, although the measured luminance of those two could be equal. To obtain this, the lighting 28 VISUAL COMFORT should not strike precisely only one surface, but be spread on neighbouring surfaces too, in order to give the possibility for comparing surfaces in nearly the same light If the mean reflectance or chrominance of the neighbouring room surfaces are different, the readability of the room will be high. This case occurs very often, e.g. the floor has usually lower reflectance than the walls, and the ceiling higher reflectance than the walls. Even with the same lighting all around the room, we will experience a clear contrast between room surfaces. Emphasis of texture on rough surfaces The visual impression of a surface with texture changes considerably with the incidence angle of light. The same texture used on neighbouring surfaces can look very different in direct light if the incidence angle on those surfaces is different and can give a clue to room geometry. If room surfaces differ in texture, the visual impression will differ even more. Recommended interior luminance ratios The eye cannot adapt to very different luminance levels simultaneously. Rapid change of focus between surfaces with highly contrasting luminances causes visual stress and fatigue, which means that the contrasts in a room should not be too high. Uniform brightness, on the other hand, produces emotional fatigue and gives no positive focus. The task of interest should therefore be slightly brighter than the immediate surroundings, to ensure attention and avoid distraction. The brightness ratios given in tab. 2.5 and the values of illuminance and reflection factors given in fig. 2.14 are an attempt to give balanced proportions between room surface luminances PES, 77]. Ceiling cavity reflectance 0.6 minimum Effective reflectance of Relative wall walls between 03 and 0.8 illuminance 05 to 0.8 Task illuminance 1.0 Reflectance of task immaterial Relative floor illuminance Reflectance of floor cavity between 0.2 and 0 3 1.0 Figure 2.14 Recommended ranges of surface reflectance and illuminance ratios relative to task illuminance. Adopted from PES, 77]. Areas Brightness Ratio Task and immidiate surroundings 3:1 Task and general surroundings 5:1 Task and remote surroundings 10:1 Light sources and to large adjacent area 20:1 Maximum contrast (except if decorative) 40:1 Table 2.5 Recommended brightness ratios for indoor lighting. Adopted from PES, 77]. 29 VISUAL COMFORT Discussion In a daylit room the room surfaces are the secondary light sources, they take part in the inter reflection of light. High reflective walls increase the diffuse part of light in the room. It is especially important in the rear zones of the side lighted room, where indirect light is the only light source. High reflection factors for the walls will essentially increase the daylight level in such zones. On the other side, the room surfaces are backgrounds. A wall lying far away is a background for pillars, walls are the backgrounds for people and furniture. Since modelling depends on background luminance, a dark wall will function better as a background than a light one. A very important factor, which influences the visual comfort in a space, is integration of light sources in the room. To make the space clearly readable, the lighting should be integrated with architecture. It is easier to obtain that goal with dayligh ting than with electrical light Daylight openings are usually well integrated with architecture. A window, contrary to electrical light, can not be placed by chance in relation to the building construction. 2.3.4 Conclusions about visual comfort The criteria for visual comfort should be chosen in accordance with the function of a room, i.e. the activities that take place in it. The functionally most important places in a room, and most important view directions from those places should be chosen. For all those places the estimation of visual comfort should be made in two steps: 1. Examination of glare and maximum contrasts in the room. 2. Dependent on function, use of one/two/three of the three following criteria described above: a. form reading b. modelling-shape reading c. space reading. 2.4 Visual comfort in linear atrium buildings It is reasonable to divide our investigations between the atrium space and the adjacent rooms. These two types of room have different size, geometry, function and daylighting. 2.4.1 Visual comfort in the atrium space The visual comfort in the atrium space is important first of all in the lower part of the atrium space, where people activities take place. Unheated atrium, that has low temperature in winter, functions often as a communication route or a temporary meeting place. High demands for form reading will seldom occur, but the modelling and space reading are important Since the atrium space functions as a light source for adjacent rooms and contains often tropical plants and trees, the mean light level needs often to be very high. Most of the tropical plants and trees need about 750 - 2000 lux 12 hours/day, some of them even more [Baker, 93]. The shape and lumin ance distribution of the visible part of sky has a crucial importance. In a linear atrium the sky visible from the middle of the floor and projected on the floor has a 30 VISUAL COMFORT shape of an ellipse, i.e. it is widest in the zenith and narrower towards the horizon. In the case of the densely overcast sky, most daylight can be expected from the top, gradually less from the lower parts of the sky and least from the sky band lying directly above the horizon. An obstruction of the daylight caused by roof construction is often larger for the light falling obliquely through the roof than from the top. It is due to the construction elements, i.e. mullions that are often oriented perpendicularly to the main atrium axis and have rectangular, elongated upwards sections. The visual comfort in the atrium space varies with the sky type. Let us consider two extremely different sky types, overcast sky and clear sky with sun. Overcast skv Since changes in overcast sky luminance are smooth and modest, neither solar glare nor extremely high luminance ratios can be expected. The perfectly overcast sky has highest luminance in the zenith direction. The sky luminance decreases gradually to 1/3 of zenith luminance at the horizon. It causes, together with the shape of the visible sky, that the luminous intensity of skylight falling from the top is many times stronger than the skylight falling obliquely from the lower parts of the sky. Light reflected from the atrium facades and floor is only a small addition to the light from the sky. In a linear atrium the natural direction of people movement is along the atrium axis. People walking through the atrium are daylit mostly from the top and less from the front. As it was discussed earlier, top light is not the most appropriate one for lighting of the human face. Due to the high daylight level, the human face and other objects placed in the lower part of the atrium space are clearly visible, but there is no light-shadow play, that could increase the visual interest or emphasise the distinctive features of the object. The modelling ability of daylight is moderate. Photo 2.1 Window in the atrium building at Faculty of Electrical Engineering and Telecommunication in Trondheim, overcast sky 31 VISUAL COMFORT The readability of the atrium space is high, all surfaces are well visible. The white colour of daylight from the clouds ensures good colour rendering. The overall impression while visiting the atrium buildings when the sky is overcast is often rather sad. They look gloomy and calm, colours look subdued. Since there exist no highlights or contrasts that has a high visual interest, the eye seeks people, interesting building details, furniture, plants etc. All building defects are clearly visible, photo 2.1. If some patterns are rendered in high contrast, illusion effects can be experienced. The Herman grid effect, see fig. 2.15 and photo 2.3, can occur if the roof or gable-facade construction is seen on the background of sky of high luminance; the effect is especially strong if the roof construction elements have low reflectance. The same effect can be observed when white glazing construction pattern is seen against the background of the dark sky at night. Figure 2.15 The Herman grid. Photo 2.3 Atrium at Department of Electrical Engineering and Telecomunication in Trondheim. A view toward the gable wall. Clear skv with sun Two very different elements make up this type of sky: the sun that is a source of direct parallel light and the sky, a source of diffuse light. The luminous intensity of sunlight is many times larger than the luminous intensity of skylight. The sky is a source of blue light, while the colour of sunlight varies dependent on the elevation angle, from red-orange in a position just over the horizon through yellow to white when it is high in the sky. Sunlight in atria may cause glare by direct sunlight or sunlight reflected from walls, especially from specular surfaces such as window glass. Since an atrium functions as a semi-outdoor space for adjacent rooms, and as a place for temporary occupancy, it is reasonable to expect that solar glare is more tolerable here than in adjacent rooms. The modelling ability of daylight under clear sky with sun depends strongly on the position of the sun. Photos 2.4 and 2.5 show dramatic effects of the sun radiation on a sunny winter day, when the sun’s elevation angle is about 5° and the sun penetrates to the atrium through the glazed gable wall. Both the placing of the object in the space and the view point and direction have a crucial importance for the sensation of the modelling ability of light. People standing in the shadow are much less visible than those that are directly sunlit. If an observer looks at 32 VISUAL COMFORT those in the direction of sun rays, the people are very good lighted, photo 2.4, the modelling ability of light is high. If the observer looks from the opposite direction, photo 2.5, only the contours are clearly visible, the full shape of a face is not visible at all; the modelling ability of light is very low. Photo 2.4 The linear atrium at the University Centre Photo 2.5 The linear atrium at the University Centre at Dragvoll in Trondheim. The view direction is at Dragvoll in Trondheim. The view direction is consistent with the direction ofsunrays. opposite to the direction ofsunrays. The depth of sun penetration into the atrium differs during the year. A sunlit fragment of facade becomes a secondary light source of considerably high luminance. If sunlight falls only on the upper part of one of the facades, and the objects placed on the floor level lie in shadow, the modelling ability of daylight is similarly moderate as under overcast sky. The modelling ability of daylight is very high if sunlight penetrates deeply into the atrium space, such as most part of one of the facades is sunlit. Objects placed on the ground floor level are lighted mostly from one side, i.e. lighting which is ideal for human faces. The readability of the atrium space depends on the sunlight penetration, too. The contrast between sunlit and not sunlit parts of atrium surfaces is very strong, photo. 2.7. The sunlit parts become an interesting visual focus due to the high luminance and the light-shadow play. If the sunlight falls only on the roof and a smaller upper part of the atrium facade, as it occurs in winter, the readability of atrium space is high, but the eye has a tendency to look up at the highly lighted areas. The atrium seems to be more closed and narrow. Anyway, even a small sunlit area make it nicer and more interesting, comparing to the overcast sky conditions. Sunlight has also a positive influence on the perception of warm colours, as red, yellow or brown. 33 VISUAL COMFORT Photo2.6 The linear atrium at the University Centre at Dragvoll in Trondheim. The sunlight creates a pattern of shadows. A deep penetration of sunlight enables full perception of the atrium with all its qualities. If the light falls from the side in relation to the atrium axis, one facade is basking in the sun. The other facade and the floor are also sufficiently lighted by inter-reflections. The sunlight emphasises the presence of the glazed roof and glazed gable walls. The design of the glazed roof construction becomes visible on the sunlit facade or floor as a pattern of shadows. The pattern can add visual interest by animating the scene, photo 2.6. Shadows can be made also by other elements, such as plants, sculptures or technical installations. Sometimes a multitude of shadow patches can make the perception of the space ambiguous, see photo 2.1 in comparison with photo 2.2. The plentiful use of glass causes a myriad of reflections. The glass roof is reflected in windows that are further reflected in the glass roof again. That play, together with the shadows of roof construction, makes the atrium look chaotic and restless, or brilliant. It is dependent on the atrium design, especially on the harmony of design between roof, floor and facades, and richness of forms, shapes, colours and details in the design of each of those surfaces. 2.4.2 Visual comfort in adjacent rooms Solar glare is not acceptable in the adjacent rooms. In rooms where direct sunlight can be expected during the year, sun shading devices should be used. 34 VISUAL COMFORT Overcast sky The visual environment in rooms adjacent to the atrium under overcast sky is not very different from similar rooms adjacent to a traditional street The daylight level depends strongly on the size of sky patch visible from the room. Due to the light transmission of the glass in the roof and the obstruction of daylight caused by the roof construction, the mean daylight level both in the atrium and in adjacent rooms is reduced, often as much as 50%. The adaptation ability of the human eye causes, that we are not able to sense this. A glazed roof will usually not change the light distribution in the atrium considerably. Since the sensation of glare is mostly dependent on the luminance of the light source, the danger of glare in rooms adjacent to the atrium is lower than for similar rooms adjacent to a traditional, uncovered street, and it can be expected only on top floors in periods when the sky luminance is very high. The mean daylight level in a room adjacent to an atrium depends strongly on the placing of the room in the atrium height A room on the top floor, that ‘sees’ a considerable large patch of sky will be well daylit, while a room on the ground floor, that hardly ‘sees’ the sky, will be daylit poorest for equal window area. Additionally, other parameters such as atrium geometry, reflectance of the atrium surfaces, windows area, design of the glazed roof construction and the type of glass used on the roof and in the windows, size and placing of plants, geometry of the adjacent room and reflectance of room surfaces, have considerably influence on the daylighting in adjacent room. The form reading, that is considerably dependent on the light level, is usually much higher in the window zone than in the rear zone and as a consequence of differences in daylighting with the atrium height, increases with the height of the room floor level. Modelling and space reading depend strongly on the size and placing of the windows, and will be discussed further. Clear skv with sun Unlike for the overcast sky conditions, not the size of visible sky patch, but the presence of direct sunlight has strongest influence on the daylight level in the adjacent room. Since the luminance of the blue sky is many times lower than the luminance of the sun, view to the blue sky has lower significance. The placing of the room in the atrium height has significance for the sun oriented part of building, because higher placing of the room increases the probability of sunshine during the year. As the sunlit part of the facade becomes a light source for the opposite facade, the rooms placed high on the opposite side will receive more inter-reflected light than rooms placed lower. Similarly for rooms adjacent to a traditional street, solar glare will occur in rooms where the sunrays penetrate. The reduction of light level caused by glass placed on the roof or in the windows is usually not enough protection, sun shading devices should be used. The widespread use of VD screens causes another problem. Since the VD screen luminance is rather low, about 80 cd/m2 for a light grey screen background, and the VD screens are often placed by the windows, the window luminance should be rather low in order to avoid an extremely large luminance ratio. If the 1:10 ratio between remote surrounding and the visual task is to be respected, the window should not have higher luminance than 800cd/m 2. To achieve this an effective sun shading device should be used. But most of the commonly used shading devices reduce the window lumin ance not only in the lower part where the neighbourhood to the VD screen is an important factor, but also for the whole window, making the room gloomy or even dark. The mean daylight level in the room is so low that 35 VISUAL COMFORT electrical light is necessary. There are many possible solutions to this problem. If moving of the VD screen from the window zone to the rear zone is difficult, the design of the window should be changed such that it should be possible to use two different sun shading devices. In the lower part of window sun shading that effectively reduces the luminance level of the window area, in the upper part a daylight system that redirect sunlight to the ceiling. 36 {Chap. 3 SlMPLlFfED DAYLIGHT FACTOR CALCULATIONS ~~ 1 The first objective of this thesis, formulated in chap. 1, is the development of simplified and precise design-related tools for linear atria. An analysis of daylighting in linear building structures in the preliminary phase of the work led to the observation, that the solid angle projection method [Hopkinson, 66] can be easily used in simplified calculations of daylighting in linear buildings structures. The main simplification made in calculations presented in this chapter rely on assumption that all atrium surfaces are perfectly diffuse, i.e. the Lambert law can be used in calculations. The calculation method presented in this chapter can be used in the first stage of the daylight design process, when the main geometry of the building is discussed. 3.1 Calculations of sky factor for uniform sky The concept of uniform sky is often used in daylighting research because it gives very simple formulas that can be used as a rule of thumb. For this reason, it is interesting to compare calculations using uniform sky with calculations using the CIE overcast sky in order to evaluate the difference in precision. The illumination due to the whole uniform sky on a horizontal plane is E:= ti-L where L is the uniform lumin ance of the sky. The illumin ation due to the whole CIE overcast sky is 7 E := 9 z where Lz is the luminance at the zenith. Let us assume, that the total illumin ation from those two sky models is equal, that is 0.393 0.785 1.178 1571 Q (radians) 0.0 45° 90° (degrees) Figure 3.1 The comparison of the luminance of the CIE overcast shy with the luminance of the uniform sky that is represented by the L(6)=l line. 3.1.1 Sky factor on the floor For a point Ps lying in the middle of the street floor, see fig. 3.1, the projection of the sly visible from P5 has a shape of an ellipse. The proportions between the length and the width of this ellipse depends on the proportions of the street: SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS sin(0) - - a 0 - half of the opening angle of the street. Figure 3.2. Section through an infinite long street and the vertical projection of the sky on the street floor in points P, and P. The sky factor at Ps is equal to the ratio of the area of the ellipse to the area of the circle. Jt-a-b SF 7t-a For an unit sphere with the radius equal 1: SF(ps):=sh For a point P randomly selected on the floor, the projection of the sky that is seen from P have a shape of two half ellipses. The sky factor at P is: SF(P) := ^sin^0 ^ + sin^02^ (2) For a point Pj selected on the comer line where the facade and the floor meets: SF(Pj) =1,^6,) (3) Fig. 3.3 shows changes in sky factor values on the floor of the unit width street 0.5 0.4 0.3 SF(w) ----- 0.2 0.1 0 0 0.2 0.4 0.6 0.8 1 W street width Figure 3.3. The sky factor SF on the floor of the unit width street. The ratio of the height to the width is equal 1. 38 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS It is interesting to observe that the sky factor at point Pj will always be smaller than at point Ps- Figure 3.4 Comparison of two streets of different proportions. w SF 2-e The ratio r between the sky factor at Pj and Ps is shown on fig. 3.5: Jw2i-h2 (4) 0.8 ~ ------0.4 “ k=h/w Figure 3.5. Variation of the ratio between the shy factor at the comer line and the centre line with the streets proportions. For a wide street where 1 is much larger than h, the sky factor at the centre line is nearly twice the sly factor at the comer line. For a narrow street, where the height h is much larger than the width 1, the difference between those two is very small. 3.1.2 Vertical sky factor on facades The vertical daylight factor at a point on a given vertical plane is defined as the relation between the illumin ation on the vertical plane at that point and the simultaneous exterior illumin ation on a horizontal plane from the whole of an unobstructed sky. The projected solid angle principle can be used to estimate the vertical sky factor. 39 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS For a selected point Q lying on the facade the projection of the sky visible from that point has a shape of a crescent with the area: 1 2 A := --it-a •( 1 - cos(y)) where y is the opening angle of the street. Figure 3.6 Section through an infinite long street and the horizontal projection of the sky on the street facade in point Q. The vertical sky factor in Q is equal to the ratio of the crescent area to the area of the circle that represents the illumination on a horizontal plane. SF(Q) := - (i - cos(y)) 2 (5) The vertical sky factor at a point Q approaching the top edge of the facade is approaching 0.5. At a point Q lying on the comer line where the facade and the floor meets, the value of the vertical sky factor depends on the streets proportions, s&ofig. 3.7. SF(h) height Figure 3.7 Variation of the vertical sky factor with the height over the floor for two streets with different proportions. It is interesting to observe that the vertical sky factor at the point lying on the comer line is lower than the horizontal sky factor at the same point because ^•sin(Y)>^(l- cos(y)) 3.2 Calculation of the sky factor for the CIE overcast sky 3.2.1 Sky factor on the floor The sky projected on a horizontal plane can be divided into elemental rings. The luminance of these rings changes with the radius. 40 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS L r--L z- (l + 2-Jl - T2) (6 ) where r is the radius of the ring, figure 3.8, that changes with the elevation angle 0 in the following way: r := cos(0) The area of the elemental ring is: Ar:=2-7t-rdr The sky factor in the given point P due to the elemental ring of sky is: LfAr SF(ring) := 'sky (8) Figure 3.8 The sky projected vertically on the horizontal plane. The illuminance from the whole sky [Hopkinson, 66] is: 'sky := --Tt-L, Integrating to obtain the sky factor from the top part of the sky, that projected on a horizontal plane has a shape of a circle with the radius n rr - L z- (l + 2-Jl - f)-2-m-r dr SF(r) := 7-Tt-L, The solution to this is: SF(r) :=- + --r2 - -• (jl - r2) (9) or expressed in elevation angle: SF(6) :=^ + “*(cos(0)) 2 - —(sin(0)) 3 (10) 41 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS The sky factor due to the elemental sector of a sky of horizontal radius r and azimuth angle a is: dSF(r,oc) = —-SF(r) 2-Tt In order to calculate the sky factor due to the part of the sky that projected on the horizontal plane has a shape of an ellipse sector, the integration of the elemental circle sectors is necessary. •«2 1 4 32 4(i 2V 2-Jt [77 7 1 \ “1 where the radius r can be calculated from the intersection between the circle and the ellipse, both lying on the centre of the co-ordinate system as shown in fig. 3.5. Figure 3.9 Calculation of the sky factor due to the sector of the CIE overcast sky. The co-ordinates x and y of a point N lying on the intersection of en ellipse and a ring must satisfy the two following equations: x 2 y2 = 1 x 2 + y2 := r2 The angle a between a line ON and the X axis can be expressed by the tangent function y tan(a) := - x After some transformations we obtain the following expression for r2 2 , 2 1 + (tan(a)) 2 ri =h~2------; b +■ (tan(a)) where a=l. 42 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS The last integration changes form to: f“2 4 3 2 l + (tan(a)) 2 4 . ,.2 i + (tan(a)) 2]3 SF(a i,a2) := —• 1 - D ------da v ' 2-n 7 7 b 2 + (tan(a)) 2 7 b 2 + (tan(a)) 2 a l (11) Calculations of the sky factor on the middle of the floor, for the unif orm sky using the equation 1, and for the CEE overcast sky using the equation 11, are presented in tab. 3.1. 0 - half of the b = sin(0) Sky factor in %, Sky factor in %, opening angle in deg uniform sky CIE overcast sky SFo/SF„ SF„ SF„ 05 0.0872 08.7 10.1 1.16 10 0.1736 17.4 20.1 1.15 15 0.2588 25.9 29.7 1.15 20 0.342 34.2 39.0 1.14 25 0.4226 42.3 47.9 1.13 30 0.5 50.0 56.2 1.12 35 0.5736 57.4 63.9 1.11 40 0.6428 64.3 70.9 1.10 45 0.7071 70.7 77.0 1.09 50 0.7660 76.6 82.5 1.08 55 0.8191 81.9 87.1 1.06 60 0.8660 86.6 91.0 1.05 65 0.9063 90.6 94.0 1.04 70 0.9397 94.0 96.4 1.03 75 0.9659 96.6 98.1 1.02 80 0.9848 98.5 98.5 1.01 85 0.9962 99.6 99.8 1.00 90 1.0 100.0 100.0 1.00 Table 3.1 The sky factor in the middle of the floor of an infinite long street with opening angle varied from 10 to 180 degrees. SF(8) 03 - 0.167 - 0.314 0.628 0.942 1.2571.571 g (radians) 0.314 0.628 0.942 12571.571 ©(radians) 45° 90° (degrees) 45° 90° (degrees) Figure 3.10 Variation of sky factor in the middle of the floor of an infinite long street with the opening angle 29. Uniform sky, to the left, CIE overcast sky to the right. For any point on the street’s floor the sky factor can be calculated using tab. 3.1. If the opening angle of the street from a randomly selected point P is a sum of 8% and 02, the sky factor is: 43 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS SF=l(SF(91)tSF(62)) (12) In the case of an atrium the final result should be multiplied by the transmission factor of the glazing system, including structural obstructions. 3.2.2 Sky factor on the facades For a given point Q on the atrium facade, the projection of the CDS overcast sky on a vertical plane has the shape of a half circle. Let us place the circle on the co-ordinate system and divide it into elemental horizontal stripes. Figure 3.11 Calculation of the sky factor due to part of the CIE overcast sky on a vertical plane. The luminance of such a strip changes with the distance h from the middle of the projection ring. L. L^TZ.(!frf-y 'h" h = cos(y) Lh = Lz 3 (13) The distance between two points: N lying on the intersection of the line y = h and the ellipse (b,a) and M lying on the intersection of the same line y = h and a circle (r = a), fig. 3.11, is the difference between the x co-ordinates of those two points. The co-ordinates of point N can be calculated from the equations: a2 b 2 for y = h and a = 1 x .= 1----- b 2 (14) In the same way the co-ordinates of point M can be calculated from the equations: x*+y*:=f for y = h and r = 1 '= J1 - (15) 44 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS The length of the section NM is the difference between those two x co-ordinates. The area of an elemental strip lying at the distance y from the origo is: The sky factor due to the strip is: SF(h) := ——• Ah -Lh sky (17) Angle y b=cos(Y) Sky factor in %, Sky factor in %, in degrees uniform sky CIE overcast sky SFoZSF„ SF„ SF„ 90 0.0 50.00 39.62 0.79 85 0.087 45.64 37.61 0.82 80 ■ 0.174 41.32 35.35 0.86 75 0.259 37.06 32,85 0.89 70 0.342 32.90 30.16 0.92 65 0.423 28.87 27.31 0.95 60 0.5 25.00 24.36 0.97 55 0.574 21.32 21.34 1.00 50 0.643 17.86 18.33 1.03 45 0.707 14.64 15.37 1.05 40 0.766 11.70 12.53 1.07 35 0.819 9.04 - 9.86 1.09 30 0.866 6.69 7.42 1.11 25 0.906 4.69 5.26 1.12 20 0.940 3.02 3.42 1.13 15 0.966 1.70 1.95 1.14 10 0.985 0.76 0.87 1.15 05 0.996 0.19 0.22 1.16 00 1.0 0.0 0.0 Table 3.2 The vertical sky factors on the facade of an infinite long street with opening angle varied from 0 to 90 degrees. 0.6 0.5 05 “ 0.4 SF(Y)oj SF(y) 05 “ 0.2 0.1 0.1 “ 0 0 0.314 0.628 0.942 1.2571571 0 0514 0.628 0.942 15571571 7 7 sky angle in radians sky angle in radians Figure 3.12 Variation of the vertical sky factor for uniform sky, left, and CIE overcast sky, right. Integration between the borders 0 and 1 gives the total sky factor for a point Q on the atrium facade. 45 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS h 2 SF(Q) = -•Lz'(l +■ 2* h) *2* [ a/ l - h 2 - l-----dh 7- ti-L. (18) The solution of this integral is: SF(Q) = —(l -b)+ —•(! - b 2) 14 7-7T (19) or expressed in angles: SF(Q) = —-(l- cos(y)) +■ —-(sin(Y)) 2 14 7-jt (20) The results of sky factor calculations using equation 20 for y changing from 90 to 0 degrees are presented in tab. 3.2 together with the results of sky factor calculations for uniform sky using equation 5. The same results are presented graphically in fig. 3.12. 3.3 Calculations of daylight factor The projected solid angle principle can also be used in calculation of the daylight factors in an infini te long street The street floor and the facades were supposed to have a constant luminance, respectively: Lf and and to have perfectly diffuse surfaces of reflection factor, respectively p f and p w. Two points, P lying on the middle of the floor and Q lying at the middle height of the facade, were selected. The illuminance in P and Q is: E(P) := SF(P).Esky + A'w-Lw (21) E(Q) =SF(Q)-Esky + A"w-Lw + AfLf (22) where: A'w the area of the facades projected on a horizontal plane A"w the area of the opposite facade projected on the vertical plane Af the area of the floor Esky the illuminance from the unobstructed sky on a horizontal plane. From the relation between the luminance and the illumin ance for a perfectly diffuse surface: E(P) = —-L; Pf E(Q) -Lw p w The set of two equations with two unknown quantities: L* and Lf, is therefore: A,w-Lw---Lf+SF(P)-Esky:=0 Pf (23) 46 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS n A" w'Lw " “—Lw + AfLf+ SF(Q)-Esky0 P w (24) Lw and Lf calculated from those equations are: L f ' ^SF(P)-E sky + A w‘^ w) (25) SF(Q)-Esky + ^AfSF(P).Esky Lw:=- n P f (26) Aw Af Figure 3.13 Illustration of daylight factor calculations. The total daylight factors calculated from the equations: DF(P) := —-Lf^— P f ^sky DF(Q):=JL.L^ w w sky are: Pf A' W-SF(Q) + —-A' w-A fSF(P) DF(P) := SF(P) + n Pf A"w—AfA' w P w (27) 47 SIMPLIFIED DAYLIGHT FACTOR CALCULATIONS Pf SF(Q) +■—SF(P) n DF(Q) .=------P w n Pf A"w- "'AfA'w w (28) The calculated daylight factors were compared with measured daylight factors for alternative A, chap. 6, and presented in tab. 3.3. Af Sky DF DF Pf Pw 9 Y A ' A " factor calcu measu DFc/DFm lated red p 0.33 0.583 15.93 2.294 31.4 44.3 45.8 0.97 Q 0.33 0.568 45.65 0.473 2.196 15.8 32.8 32.5 1.01 Table 3.3 Comparison of calculated and measured values of daylight factor for two selected points P and Q in the infinite long street. The facades in the model experiments were not perfectly diffuse, the window glazing area constituted about 34.5% of the facades. This could possibly be one of the reasons for the discrepancy between calculated and measured values. Anyway, the close consistence between calculated and measured values gives encouragement to further work with the projected solid angle principle. The calculations can be carried out more precisely by dividing of the facades and the floor into horizontal stripes. The use of the projected solid angle method in other types of buildings is also possible. All surfaces seen from the selected point have the shape of ellipse sectors or can be divided into ellipse sectors or the subtraction/addition of ellipse sectors. If we divide the surfaces in a room into parts with constant luminance, the calculation of the daylight factor should be the same as shown before. A large amount of surfaces results in a larger number of equations and unknown quantities. The area of the sector of the ellipse (l,b) limited by the angles cq and oq can be calculated from the following equation: «2 l + (tan(a)) , ------da b2 +• (tan(a))2 The advantage of this method is also that the internally reflected component is not calculated separately. In rooms that has a shape very different from a sphere, this method should give more correct results than the method based on the split-flux principle. Equations 9 and 10 can be used in calculations of sky factors in square atrium buildings. The radius r can be calculated as the middle value of rx and r2. In the case of a glazed roof covering the atrium, a roof shading factor Tj should be estimated. The calculated daylight factors should be multiplied by (l-q). 48 [CAap. 4 SUPERL/TE CAlCUUmONS______j In a conventional, uncovered street the open space between buildings can be regarded as a light source. All building surfaces, and the sky itself, take part in the infinite number of inter reflections. Because the sky has much higher luminance than the street surfaces, it is regarded as a daylight source. During each inter-reflection, some of the daylight fallin g on the street surfaces is absorbed by the atrium surface materials, some is transmitted to the adjacent rooms, and the rest is reflected. Except for the part absorbed by the materials, the rest takes part in the inter-reflection between building surfaces and the sky. The reflection coefficients, as well as reflection patterns of materials used on the facades are of crucial importance for the light distribution in the street 4.1 Description of the calculation model As the illuminance on the facades decreases with the distance from the roof level, a reasonable strategy is to increase the glass area as one move down to the lower floors. This strategy was intuitively used in old towns. The same strategy was used in the linear atrium buildings of the University Centre at Dragvoll in Trondheim. The size of window area is varied from 40% of the facade area on the third floor, 70% on the second floor and 90% on the first floor [Aschehoug, 89b]. The smallest width of a street that is accepted by the fire regulation in Norway is 8m. It is reasonable to expect that this width will be used in highly urban areas. Since a mean daylight factor of 2% is regarded as a minimum value for daylit rooms, the interesting questions is how high can buildings facing such a narrow street be in order to obtain a mean daylight factor of minimum 2% in each room adjacent to the street, and how large should the windows be on the respective floors. In order to answer these questions, the following investigation was carried out, using the SUPERUTE data simulation program. The simulation model consisted of narrow, 8m wide and over 100m long street, simulated by two facade walls and a simple room adjacent to the street, placed in the middle of the street length. The calculation of the mean daylight factor in the adjacent room was done with two changeable parameters: • the height of the room floor over the street floor, this is connected with the shading angle, • the window glass area in the adjacent room calculated as a percent of the room floor area. Since the SUPERUTE program has some limits, the model of the street had to be simplified. All surfaces had to be perfectly diffuse. The street could been simulated only by two opaque diffuse surfaces. As the investigation should include any building design, it was reasonable to simulate a poorly daylit room. The room height of 2.4m was equal the minimum height for the living rooms in accordance to the Norwegian building regulations. The gross height of one storey was 2.7m. The room was rather deep with the sidewall twice as long as the facade wall (3.5m x 7.0m x 2.4m). The reflectances of room surfaces were: ceiling 0.70, walls 0.50, and floor 0.30. The room had one window. The window glazing was simulated as 6mm + 12mm + 6mm Kappa Energy with 73% normal transmittance and 16% normal reflectance. The window had the form of a horizontal rectangle, and was placed in the middle of the facade wall; the windows SUPERUTE CALCULATIONS centre point was at the height of 1.5m above the floor level. The following sizes of window area were investigated: 10%, 15% 20% and 25% of the room floor area. Two series of numerical computation were carried out, in the first one a 5 floor building was simulated, in the second one a lower, 4 floor building. Both series were done for street ground reflectances equal 0.20 and 0.50. The analysis is based on calculation of the mean daylight factor on a horizontal surface situated 0.8m over the floor level in the room. In the calculations, the CEE overcast sky model was used for 21 March, 12:00 and the atmospheric and geographical data for Trondheim. 4.2 Calculations for the 5 floor linear atrium or street In the first series the buildings were 5 floor high, i.e. 13.5m in total, the reflection factor of the facade walls had a constant value of 0.4. 5.00 — 3.00 1.00 — 5th floor |4th floor 3rd floor 2nd floor {1st floor !I I i III I I I I I I I I I 10.00 20.00 30.00 40.00 50.00 60.00 Shading angle Figure 4.1 Daylight factor in a room adjacent to the narrow street, 5 floor building. The thin line refer to street ground reflectance of 0.50, the thick one to reflectance of 0.20. The results from the first series of calculations,/zg. 4.1, showed that to obtain a mean daylight factor of minimu m 2% in the room on the first floor, the window should have an area of minimum 25% of the floor area, the reflectance of the opposite wall in the street should be minimum 0.4 and the reflectance of the street floor should be as high as 0.5. Since such a combination of the parameter values is difficult to fulfil in a real street, it was decided that the height of the building should be reduced to 4 floors. Because the windows on the 1st and 2nd floors should be very large, the reflectance of the facades along the street should vary with the height 50 SUPERLITE CALCULATIONS 4.3 Calculations for the 4 floor linear atrium or street In the second series the street was 4 floor high, i.e. 10.8 m in total. The reflection factors of the facades were: 0.30 on the 1st floor, it corresponds approximately to a glazing area equal 25% of the floor area, and a reflectance of the opaque part of the facade of 0.50, 0.35 on the 2nd floor, it corresponds approximately to a glazing area equal 20% of the floor area, and a reflectance of the opaque part of the facade of 0.50, 0.40 on the third and forth floors, it corresponds approximately to a glazing area equal 10 to 15% of the floor area, and a reflectance of the opaque part of the facade of 0.50 «Bo O) >. «B Q 20.00 30.00 40.00 50.00 60.00 Shading angle Figure 4.2 Daylight factor in a room adjacent to a narrow street, 4 floors buildings on both sides. The thin line refer to street ground reflectance of 0.50, the thick one to reflectance of 0.20. 4.4 Conclusions The results show, fig. 4.2, that for the low reflectance of the street floor, i.e. p=0.20, the minimum glazing area for the respective floors should be as shown in tab. 4.1. The glazing area can be reduced if the reflectance of the street floor is increased, especially on the 1st floor. The results show also that the effect of the street floor reflectance decreases as one move to upper floors. The effect of the street floor reflectance increases also with the glazing area. 51 SUPERUTE CALCULATIONS Floor: % of the floor area: 4th floor 8 3rd floor 11 2nd floor 15 1“ floor 25 Table 4.1 The minimum glazing area for deep rooms adjacent to the four floor high narrow street, to reach a mean daylight factor of 2%. The main geometry of the atrium building used as a base case in the model studies was designed using the results of the SUPERLITE calculations described above and the experience from existing atrium buildings at high latitudes [Gunnarshaug, 85], [Aschehoug, 89a]. 52 Ictmp. 5 DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA 5.1 Materials used in the project Since the investigations in this project are limited to the northern climate, where the probability for overcast sky during the year is over 60%, daylighting components designed for collection or transmission of sunlight alone, such as movable heliostats or light guides were not considered. In heavily obstructed buildings such as those lining a street or a linear atrium, the access to daylight is very low, especially on the bottom level. Materials that reject diffuse daylight or have low transmittance or reflectance were not used in the project 5.1.1 Traditional materials The materials used on buildings envelope can be divided into two main categories: 1. Transparent/translucent materials used in windows and other daylighting openings. 2. Opaque, reflective materials used typically on the rest of facades. Baker, Fanchiotti and Steemers in “Daylighting in architecture ” proposed a simple classification that cover the theoretically unlimited number of possible distribution patterns [Baker, 93]. They found that all traditional materials can be classified as reflective or transmissive, fig. 5.1. Both transmission and reflection patterns can be divided into the same six groups: 1. Specular reflection and transmission 2. Diffuse reflection and transmission 3. Scatter wide reflection and transmission 4. Scatter narrow reflection and transmission 5. Diffuse and specular reflection and transmission 6. Complex prismatic reflection and transmission The dispersion angle, 8, is defined as the angle between the direction of maximum intensity (Imax) of reflected or transmitted light and the direction of intensity with a value of 1^ /2, when the intensity distribution curve can be supposed to be symmetrical about the direction of Ittirt , fj%. 5.2. Fig. 5.3 shows the classification of light distribution patterns from specular, where 8 is 0°, to perfectly diffuse, where 8 is 60°. Dispersion angle in between these two represent different levels of scattering. For narrow scatter reflection/transmission 0°<8<15 o, for wide scatter reflection/transmission 15°<8<45 0, if 45°<8<60 o reflection/transmission is classified as diffuse. I a. "V-' DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA SPECULAR SCA W SCA N SCATTER NARROW ' o»<*h» DIF+SPE TJBTD5E73FECUEAJT CMP TxmcExmsnmr Figure 5.1 Suggested classification of both transmission and reflection patterns, where delta (8) is the dispersion angle for half the intensity. Adopted from [Baker, 93]. LIGHT SOURCE Intensity Distribution Curve Figure 5.2 Definition of the dispersion angle (8) for transmitted light of normal incidence. Adopted from [Baker, 93]. 5 = 0° 8=15° 8 =45° 8 = 60° Specular Narrow Scatter Wide Scatter Diffuse Figure 5.3 Suggested classification of light distribution patterns as a function of the dispersion angle (8). Adopted from [Baker, 93] 54 DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA The classification suggested by Baker, Franchiotti and Steemers is very simple and understandable, because the proposed figures can be directly associated with the visual observation. On the other side this classification does not emphasise the importance of the incidence angle of light on the distribution pattern, the absorption of light by the material and the colour change. Glass Most materials usually recognised as transparent, changes distribution pattern with the incidence angle. Glass can be characterised as a specular transmittive material for incidence angles between 0° and 50°, for larger incidence angles the transmittance decreases with the increasing of the incidence angle. For incidence angles between 70° and 90° the reflectance is many times higher than the transmittance. The values of normal transmittance and reflectance of the glazing varies with the glass type, number of glass sheets and the fillin g of the space between the glass sheets. Data for some glazing examples are shown in fig. 5.4. 6 mm floot (bronze) 6 mm float (bronze) + 6 mm float 6 mm float fsdver] + low E Reflectance 0.8 Reflectance N. 02 Total transmittance 0.4 § Total transmittance 8 0.6 \ S £ J 04 0.6 | V 06# Direct transmittance Total transmittance \ £ Dired transmittance 0l2 ------r-xl0.8 Direct transmittance ^ 0 20 40 60 80 0 20 40 60 80 C) 20 40 60 80 Ang'a of incidence (degrees) Angle of incidence (degrees) Angle of incidence (degrees) Figure 5.4 Transmission factors of light as a function of incidence angle for different types of glass. Adopted from [Button, 93]. Many different metal coatings can be used on the glass surface in order to change the normal transmittance and reflectance of light In this project glass from the group Kappa Optima was used, because it is characterised by low absorbtance of light The specification of the optical properties of this glazing is included in chap. 5.2.1. Opaque wall materials Most opaque materials used on building surfaces, as concrete and plaster work come under the “wide scatter” heading. Rough cardboard was used to simulate opaque wall in the model studies. The specification of the reflection factors of the materials used on the respective walls is included in chap. 5.2.1. Specular reflective material for reflectors The specular reflective material is a material that has a very high reflection factor, about 0.9, for all incidence angles. In a real building shiny polished aluminium can be used. In the model studies this material was modelled by minor-paper, a thin cardboard with a very thin layer of specular aluminium covering. This material was used in the fiat and curved reflectors in the model studies, chap 6 and 7. 5.1.2 Light redirective materials There also exists some new materials, such as prismatic devices, holographic films and laser cut light deflecting panels, that were developed to redirect sunlight 55 DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA Prismatic glazing The principle behind prismatic systems is to use refraction in order to redirect sunlight. Specially designed Luxfer-prisms were used already at the beginning of this century in Berlin in heavily obstructed rooms to redirect diffuse light from the zenith sky towards the back of the room [Baker, 93]. Prismatic glass blocks were used also in the 1940s and 1950s in American schools. Prismatic glazing was mentioned even in classical “Daylighting ” [Hopkinson, 66]. In this case the prism system is employed solely to redirect diffuse light from the sky near zenith towards the back of the room. The prismatic system was proposed to be used in heavily obstructed rooms, which would otherwise receive no direct skylight. Two types of prismatic glazing are used in current buildings: sunlight redirecting prisms, and sunlight excluding prisms. Because this investigation refers to heavily obstructed buildings, it was reasonable to focus attention on sunlight redirecting prisms alone. The prismatic film used in the project is shown in fig. 5.5 in the vertical position. For optimally adjusted prism angles to the sun elevation angle only a small part of the daylight is directed downward, the larger part is redirected up to the ceiling. Prismatic glazing of that type functions only for a narrow range of incidence angles. The film was pasted on a 5mm thick acrylic sheet Figure 5.5 Sunlight ray paths through a prismatic panel. Adopted from [Aizlewood, 93], Prismatic glazing obscures the view out and causes rainbow effect on the borders of the light path created by the prismatic panels. Laser cut light deflective panel The panel, often called Ed-panel, is under development at Queensland University of Technology in Australia. The name originate from the name of inventor dr. Ian Edmonds. The panel is made of acrylic sheet that is cut perpendicularly by laser beams. As the laser cuts by melting, the internal surfaces are very smooth and provides a precisely formed light deflecting slats. This ensures efficient light reflection by total internal reflection. Edmonds panel was originally invented for sunlight If placed in the vertical position in a typical window, fig. 5.7, the sunlight falling at slanting incidence is deflected by the panel mostly to the ceiling of the room. Some light can pass through the material in the same way as through glass and make an image of parallel beams of rays. The ratio between the width of laser cuts W and distance between laser cuts D will determin e the ratio between the deflected and the pass-through parts of daylight,/#. 5.6. The panels used in this study had: W = 4mm, D = 24mm. 56 DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA The fabrication will influence the optical properties of the panels. Edmonds writes: ..in practice it is found that laser cutting yields surfaces which are not optically flat. The result is that the deflection of direct sunlight is not specular but exhibits some spreading [Edmonds, 92]. The spreading can be observed on fig. 5.7. Figure 5.6 Deflection of light in a transparent rectangular parallelepiped. Adopted from [Edmonds, 92]. Figure 5.7 Laser cut light deflecting panel placed in the window. Adopted from [Edmonds, 92]. At near normal incidence the material transparency can be compared to glass equipped with very thin Venetian blinds. The visual contact with the outside is maintained. From a few meters distance the laser cuts are not visible at all. 5.2 Presentation of alternative strategies During studies of the daylighting in a linear atrium, the following strategies were examined: the atrium space and facades as a light conductor/reflector, the glass roof as a light conductor, and light reflectors on a neighbouring roof. 5.2.1 Atrium space and facades as a light conductor/reflector The strategies belonging to that group were divided into passive and active. The passive strategies rely on the design of the facades alone. The active strategies rely on placing additional elements, daylight systems, in the atrium space that redirect light in the desired directions. 57 DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA Passive strategies The facades facing a street are usually composed of minimum two components that have different functions: 1. transparent/translucent materials, usually glass, used in windows that transmit light to the adjacent rooms, 2. opaque materials used on the remaining parts of the facades. Both the specific optical proprieties of each component and the ratio of the areas of those two components have an influence on the distribution of daylight in a building. Alternative A As the illumination on the street facades decreases with the decreasing height on a facade, a reasonable strategy is to increase the windows glass area as one moves down to the lower floor. This strategy was intuitively used in old towns. In this study it was desirable, that the rooms adjacent to the street/atrium were lit significantly by daylight; the criterion of minim um 2% mean daylight factor was used. To obtain this, the results of Superlite calculations described in chap. 4, see tab. 4.1, were used as an indicator. Additionally, aesthetic criteria were used in designing windows openings. As a result, the glazing areas as percentage of the facade areas in alt. A were: 4th fl. —> 16.2% 3rd fl. —> 23.5% 2nd fl. -» 36.5% 1st fl. —> 68.8% Figure 5.8 One unit of facade A to the left and facade B to the right. 58 DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA The glazing area was calculated excluding any glazing bars, the wall area was calculated using the gross floor height of 3.3 m. As I was interested in obtaining the highest possible illuminance values, a hi ghly reflective material was chosen on the facades as well as on the street’s floor. The cardboard sheets simulating the opaque parts of the facades were nearly perfectly diffusing and had a reflection factor of about p = 0.76. Double clear glass was used in windows. The specification of the optical properties of materials used in alt. A is included in tab. 5.2. Alternative B Modem glass technology makes it possible to use another strategy, based on reflective glass. It is theoretically possible to design buildings with equal glass area and equal mean daylighting level in adjacent rooms on all floors. The amount of light penetrating to the adjacent rooms to the street can be controlled by the degree of transmission /reflection of glazing and not by its size, using highly reflective glass on the top floor, glass with gradually lower reflectance as one move down the street’s height and a clear glass on the ground floor. This strategy assumes a large assortment of reflective glass having common hue. In this study the following types of glass, easily accessible on the market, were used: normal normal Floor Specification of the glazing reflection transmission factor factor 1st 6-12-4 double clear glass p = 0.14 X = 0.78 8 CL 253 6-12-4 Kappa Optima clear & clear glass II x = 0.66 3rd & 4th 6-12-4 Kappa Optima silver & clear glass p = 0.41 x = 0.44 Table 5.1. The specification of the glazing used in the alternatives B, Bb, Ba and Baa. The code 6-12-4 refers to the thickness in nun of the glazings and the cavity. The opaque part of facade B was composed of white flumes around the windows, p = 0.88, and grey facade elements, p = 0.32, fixed between the window bands. The mean reflection factor for facade B, including glazing, was calculated to: p = 0.42. In addition to alternative B, the following alternatives were examined: Ba alternative B with lighter walls and floor. Bb alternative B with lighter walls only. The grey facade elements lying between the windows bands were removed so that the entire opaque wall was white and had a reflection factor of p = 0.88. Baa alternative B with lighter walls and floor and the cardboard facades laying behind the glazing as seen from the street Stripes of the white cardboard, 2cm wide, were fixed to cover the joint lines between different types of glazing. The specific values of reflection factors of the floor and the facades for all B-altematives are presented in tab. 5.2. Alternative C To estimate the significance of variation of glass area and variation of glass reflectance, alternative C was composed of constant window glass area; the same cardboard facades as for alt B were used, and constant reflectance of the glazing , i.e. clear glass on all floors. The mean reflection factor of facade C, including glazing, was calculated to p = 0.35 59 i DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA In addition to alt C, alt Ci was examined. It consisted of only one pane of clear glass on all floors on the opposite side of the street Altema reflection factor mean reflection normal reflection normal transmission tives of the floor factor of the factor of the glass, factor of the glass, facade P X A 0.33 0.59 0.14 0.78 B 0.33 0.42 0.14-0.41* 0.44-0.78* Bb 0.33 0.57 0.14-0.41* 0.44-0.78* Ba 0.61 0.57 0.14-0.41* 0.44-0.78* Baa 0.61 0.34 0.14-0.41* 0.44-0.78* C 0.33 0.35 0.14 0.78 c, 0.33 0.31 0.07 0.87 * see table 5.1. Table 5.2. The specification of the optical properties of the materials used in major alternatives. Active strategies - daylight systems The active strategies were intended to be examined only with alternative A. The systems were supposed to reflect or deflect diffuse daylight fallin g into different zones of the atrium to the adjacent room on the first floor. Two types of materials were used in order to construct the daylight systems: mirrors and laser cut panels (Lcp). Fig. 5.9 shows as an example the way of functioning of mirror daylight systems. The laser cut panels were used in the same places in the atrium but were sloped differently. A more specific description of the systems is included in chap. 6. Since the systems from this group are supposed to reflect diffuse skylight only, they were examined in an artificial overcast sky. The CIE overcast sky model was used when they were simulated with the Radiance data program. Figure 5.9 Functioning of the mirror daylight systems: M-fac, M-atr and M-shelf. 5.2.2 Glass roof as a light conductor/reflector In a glass covered street some light will be obstructed by the glass itself, and some by the roof construction. Additionally, the glazing itself, or innovative glazing materials, if used on the roof, can change the direction and pattern of incoming light In the study different shapes of glazed roof were examin ed: horizontal, single pitched and double pitched. In the case of single pitched roof the orientation of the roof is important. Two orientations were examined: 60 DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA —> positive: with the sloping glass oriented to the south, i.e. the lower edge is on the south side of the street —> negative: with the sloping glass oriented to the north. The negative single pitched roof was supposed to control penetration of sunlight into the atrium, by reflecting some sunlight into the atrium in the winter and reflecting some sunlight out of the atrium in the summer, jig. 5.10. Figure 5.10 An atrium building with the negative glazed roof. Winter to the left, summer to the right. The light deflecting panels placed on the roof were supposed to redirect sunlight down to the lower parts of the atrium space. The single pitched shape of the roof enabled also catching the sunlight that otherwise would fall on the neighbouring mof, fig. 5.11. IMmmMmmm Figure 5.11 The change of sunlight direction caused by glazed roof with the light deflective panels. The dashed lines show the penetration of sunlight to the atrium without glass roof. 61 DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA 5.2.3 Light reflector on the neighbouring roof The strategies from this group are supposed to increase the total amount of sky radiation falling into the atrium by using the radiation that otherwise would fall on the roof of the neighbouring building. Since a specular reflector placed on the roof can cause solar glare, the reflectors from this group can be used only on the north-facing facade. In the study, they were placed symmetrically on both facades, to examine their effectiveness. Measurements and calculations could be repeated for both sides and the mean value was presented. Another possibility is to design the reflector as an operable device, such as the sloping can be adjusted depending on the sun position on the sky. Figure 5.12 Functioning of the roof reflectors: overcast sky to the left, sun to the right. The main daylight systems examin ed in the study are presented in tab. 5.3. 62 DESIGNING DAYLIGHT SYSTEMS FOR LINEAR ATRIA i Light reflector on the atrium Glass roof as a light conductor Light reflector on the facades and in the atrium neighbouring roof space Graphic Description Graphic Description Graphic Description symbol symbol symbol M-atr \ 4 flat reflector in Horizontal glass Flat roof A the middle of roof reflector atrium space 1 I 1 M-fac V J flat reflector on Double pitched Parabolic t V the facades glass roof reflector —I— + M-shelf & J flat light shelf Single pitched Parabolic glass roof concentrator —i— M-curved Lcp Flat reflector on the top part of curved light shelf laser cut panels the facade j v on the roof '-+-J Lcp-atr PP A A Flat reflector on the roof and lcp in the atrium prismatic panels facades \z space on the 1“ on the roof a Lcp*2-atr z lcp in the atrium sz space on the 1“ and 2nd a Lcp-fac lcp on the % / facades on the 1st a Lcp*2-£ac lcp on the V V / facades on the 1st and 2nd a Table 5.3 The main daylight systems examined in the study. 63 -to. v.V Chap. 6 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY 6.1 Design of the base model The base model represents a linear atrium in 1: 20 scale, see fig. 6.1. It consists of two buildings standing about 8 m apart. Each building has a height of four floors and a depth of 6 m. The street length is divided into 8.20 m long modules that are clearly seen on the facades. Each of those modules can be further divided into ‘office units ’ of the inside width of 2.5 m. The model has a length of five modules. Two mirrors were used on both ends of the street in order to simulate a desired infinitely long street Figure 6.1. Vertical section through the base model with facade B and the small box. Figure 6.2. Horizontal section through the model with facade B and the small box. MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY The facades were made of double glazing units and sheets of cardboard. Two sets of glazing together with two sets of cardboard sheets made it possible to compose quite a few facade alternatives. The two sets of glazing were as follows: —> The first one was just a double clear glazing of the size of the entire facade, —> The second one consisted of three different double glazing units. Each of these had different reflectance/transmittance properties and was placed on different floors. The optical properties of the respective types of glazing are described in chap. 5.2. Each double glazing unit had the length of the entire facade, i.e. 206.5cm. The units used on 1st and 2nd floor had a height of 16.5cm that corresponds with the gross height of one floor. The third double glazing unit, used on 3rd and 4th floor, had a height of 35cm that corresponds with the gross height of two floors and a parapet The two sets of cardboard facades were as follows: —> The first set, representing alternative A, was made of a beige cardboard and had rectangular openings simulating windows of varied glass area from the smallest windows on 4th floor, gradually to windows filling the hole facade area on 1st floor, see fig. 5.8. —> The second set representing alternative B, had equal openings simulating constant glass area on all floors beside 1st floor, where, alike in alt A, nearly the whole facade area was filled inn by windows, see fig. 5.8. A white cardboard sheet was used here. Additional pieces of grey cardboard were fixed to the main sheet between the window bands. In order to have the possibility of fixing daylighting devices in the upper parts of the windows, the windows in both alternatives were divided into an upper and a lower part Only the central piece of the lower part was designed as being operable. Two boxes were used in order to measure illuminance levels inside the building, fig. 6.3. The first one simulated a big room, e.g. a classroom, of following dimensions: 5.1m width and 6.0 m depth. The second one simulated a smaller one, e.g. a room of dimensions: 5.1 m width and 4.2m depth, that can be divided into two ‘office units ’ of dimensions 2.5 m x 4.2 m. The boxes had the same inside height corresponding to 2.9 m in the real building. The space between main floor levels was adjusted such as it was possible to push each box on to each floor. The gross floor height was 3.3 m. The two boxes had the same type of surfaces, the reflection factors for the box surfaces were: floor: p = 0.45 walls: p = 0.66 ceiling: p = 0.88 The values of reflection factors were chosen in order to correspond with a typical office or classroom in Norway. 65 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY I i ttl e box section through the boxes bi g box Figure 6.3 Two boxes with sensors. 6.2 Sky details The Building Research Establishments artificial sky of the mirror box type (4.5m x 4.5m x 1.9m) was used for the overcast sky measurements. The sky was observed to vary within a few percent of value during measurements. This meant that “outdoor ” measurement had to be taken together with each indoor measurement The model was placed on a 76cm high table. For most alternatives the outdoor sensor was placed directly on the model, i.e. at the height of 114cm over the floor. For alternatives with a reflector on the roof, the outdoor sensor was placed on a small platform 162.5cm over the floor. The distance between the outdoor sensor and the ceiling was 47cm or 27.5cm, depending on the roof configuration. As a result, the outdoor sensor showed a few percent higher results if placed on the model with reflectors on the roof, than if placed on the model without any reflector. All results were corrected for this discrepancy, depending on how high the outdoor sensor was placed. 6.3 Methods of measurement All daylight factor measurements were made with two colour and cosine corrected illuminance meters, each employing a single photocell. The illuminance meters was of ‘Pocket lux ’ type and originate from LMT Lichtmesstechnik, Berlin. The outdoor illuminance was measured by the first sensor placed on the top of the model; the inside illuminance in the respective points was measured by the second one. Calibration of sensors was made shortly before the measurements took place. The difference between values measured by those two sensors was 66 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY below 0.5%. The illuminance measurements, both on the street and on the facades were taken in the middle of the street’s length. Illumin ances on the facades were measured on the middle of the operable part of a window on each floor. Illumin ances on the street’s floor were measured on the centerline of the street and at a distance of 1 m from each facade. Illuminances in the rooms were measured both on a horizontal plane 0.75 m over the floor and on a vertical plane on the walls at a height of 1.15 m. The placing of the measurement points in the street and in the rooms is shown in figures 6.1, 6.2 and 6.3. The central back part of the model was covered by fabric. Both the artificial sky and the principles of measurement are described in [Litdefair, 93]. 6.4 Presentation of daylight systems During our investigations of daylighting in a linear atrium, the following strategies were examined: the atrium space and facades as a light conductor/reflector, the glazed roof as a light conductor, and light reflectors on a neighbouring roof. 6.4.1 Atrium space and facades as a light conductor/reflector The strategies belonging to that group was divided into passive and active strategies. The passive strategies are described in chap. 5. The active strategies, based on the use of daylighting systems, were examined only with alternative A. Two types of materials were used to construct the daylighting systems: mirrors and laser cut panels (Lcp). __M-fac. A strip of mirror placed on the facades between window bands on the 1st and the 2nd floor, fig. 6.4. This mirror strip was inclined in relation to the facade in ^ t order to reflect the skylight washing the facade to the windows on the 1st floor on . 1 the opposite side of the street The mirror strip was 5cm wide i.e. 1.0 m in the real building and the inclination angle to tire wall was 32°. The strip of mirror was simulated by a mirror-covered paper. — — M-atr. Two strips of mirror placed in the middle of the street over pedestrian height Such mirrors were supposed to reflect the sky light falling on the middle of z\ the street to the rooms on the 1st floor on both sides of the street fig- 6.4. The —=— mirrors were 5 cm wide, i.e. 1.0 m in the real building, and were carried by a specially designed structure. The inclination angle was 45°. The strips of mirror were simulated by mirror-covered paper. Figure 6.4 Section through the model with daylighting systems: M-fac to the left and M-atr to the right. __ M-curved A curved reflector fixed between the lower and the upper part of the windows on the 1st floor. It was supposed to concentrate and reflect a part of skylight washing the facade to the rooms on the 1st floor, fig. 6.5. The curved __ !_J reflector was simulated by aluminium foil, which had a little lower specular reflectance than the mirror-covered paper. 67 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Figure 6.5 Section through the model with daylighting system M-curved. _ Lcp-fac. The laser cut panels used on the facades on the 1st floor were 8 cm wide i.e. 1.6 min the real building. The panels were fixed in front of the upper parts of the windows and were inclined at an angle of 40° to the facade, fig. 6.6. They were supposed to deflect skylight washing the facade deeply into the rooms on the 1st floor. The panels were designed such that 1 cm on each side was without cuts, the reflecting width of the panel was thus reduced to 6 cm i.e. 1.2m in the real building. _ Lcp-atr. The same panels were used on the middle of the street, fig. 6.6. They were hung up at the height of the upper part of the 1st fl. windows and were supposed to deflect the skylight falling in the middle of the street to the rooms on the 1st floor on both sides of the street In a real building, the structure for the panels can also be used for hanging lighting fixtures, signs etc. Figure 6.6 Section through the model with daylighting systems: Lcp-fac to the left and Lcp-atr to the right _ Lcp x 2 - fac. This alternative was developed from alt Lcp-fac. In addition to laser cut panels on the 1st floor, similar laser cut panels were fixed in front of the ' upper parts of the windows on the 2nd floor, fig. 6.7. The panels used on the 2nd __ ;__ floor was 6.5 cm wide, with 4 cm reflecting width i.e. 0.8 m in a real building. __ Lcp x 2 - atr. This alternative was also developed from alt Lcp-atr. In addition to laser cut panels on the 1st floor, similar 6 cm wide panels were hung up at the ,, / height of the upper parts of the 2nd floor windows. In order to avoid shadowing of the 1st floor panels, the 2nd floor panels were hung somewhat to the side, fig. 6.7. 68 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Figure 6.7 Section through the model with daylighting systems: Lcp*2-fac to the left andLcp*2-atr to the right 6.4.2 Glass roof as a light conductor/reflector Three glass roof configurations were examined; each of them composed exclusively of single, 6 mm thick, clear glass panes. Horizontal glass roof. One single glass pane was placed on the model’s roof in order to cover the street Double pitched glass roof . Two single glass panes inclined at an angle of 45° made a double-pitched glass roof. The glass panes were taped with transparent duct tape on the top. Two strips of wood, fixed to the roof of the model, prevented , the glass from sliding sideways, fig. 6.8. Single pitched glass roof . This alternative was composed of two single glass panes. The first one was placed vertically between a strip of wood and a couple of aluminium brackets. The second one was laid down on the first one on one side of the street and directly on the roof on the other side of the street, forming a desired slope angle, fig. 6.8. Figure 6.8 Section through the model with the double-pitched glass roof to the left and the single pitched glass roof to the right. Additionally two innovative glazing materials were examined in the glass roof. 69 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Laser cut panels . The construction of the roof with the laser cut panels, described in chap. 5, was the same as the construction of the single pitched glass roof. Three laser cut panels of dimensions: 57 x 69 cm and the thickness of 6 mm, were put together in the sloping position instead of the glass pane. In order to examine different sloping angles, three different glass panes were used as a vertical support The following angles of slope were examined in the experiments: 0°, 15°, 30°, and 35°. Prismatic panel . Prismatic, light deflective film, fixed to an acrylic sheet of 6mm thickness is described in chap. 5. The panel was set at 10° slope, because this angle was supposed to maximise the sunlight redirection into the street The panels were placed in two positions: with the prisms turned up and down. 6.4.3 Light reflector on the neighbouring roof The following reflectors were examined: A L Flat and sloping mirror reflector. The slope of the mirror was chosen in accordance with the proportion of the street A slope lower than 61°, that is equal the main section angle of the street, does not secure reflection of skylight to the entire opposite facade. On the other hand, the reflector will obstruct the sky. As long as the reflector has higher luminance than the sky it obstructs, usage of reflectors should be beneficial, but too steep reflectors can obstruct more than they reflect. The sloping of 61° was chosen for alt B,fig. 6.9, a steeper sloping of 65° was chosen for alt A. V J Parabolic shaped mirror reflector. The reflection factor of facade A varies with the height of the street because of the variation of glazin g area. This alternative was based on the idea of concentrated reflection of the vertical light from the brightest . part of the CIE sky, i.e. from zenith, to the upper part of the opposite facade. The parabola was designed such that its focal point was lying in the space between the facades as shown in fig. 6.9. Figure 6.9 Section through the model with the flea, sloping mirror reflector to the left and the parabolic shaped minor reflector to the right. Parabolic concentrator made of mirrors . Its geometry is shown in fig. 6.10. Both sides of the small curved reflector were specular. This shape of reflector was intended to give more light down on the facade of the 1st floor and on the floor of the street 70 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Figure 6.10 Section through the model with the parabolic concentrator made of mirror. __ Strip of mirror. 5 cm width, i.e. 1.0 m in the real building, on the top of the facades was examined as an addition to the flat and parabolic reflectors. Mirror-covered paper was used to simulate the flat, sloping and the parabolic shaped reflectors. In the parabolic concentrator mirror-covered paper was used on the parabolic and flat surfaces, aluminium foil was used on the small reflector. 6.5 Comparison of facade alternatives Figure 6.11 Perspective of the street with facades A to the left and BorCto the right. 71 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY 6.5.1 Facade alternatives A, B, C and in the atrium space vert. D.F 0.20 0.30 0.40 0.50 13.20 Alternatives: -A— alternative A ■fi— alternative B -€— alternative C alternative C1, one gl. pane on the opposite side 0.50 o.40 her. D.F. 0.00 0.30 0.00 3.76 7.52 width of the street Figure 6.12 Comparison of alternatives A, B, C and Cj on the street section. For a selected point on the facade there is an equal amount of light that comes from the sky in all alternatives. Because the floor reflectance in all alternatives shown in fig. 6.12 was the same, p = 0.33, all the variations of the daylight factor (DF) values must be caused by differences in wall reflectances only. The mean reflection factor of the walls is highest for alt A. Therefore alt. A gives the highest DF values, both on the facades and on the floor. The changes of the ratio DF A / DF c on the facade for the different floors: 4th fl. —» 1.16 3rd fl. —» 1.27 2nd fl. —» 1.35 1st fl. —» 1.26 shows that the significance of the light from the opposite facade increases with the depth of the street more for alt. A than for alt. C and decreases on the 1st fl. where the light reflected from the floor has more importance. The shapes of curves for alt B, C and Q are very similar; the DF values are quite similar too. Compared to alt C, alt B causes the following increase of DF on the respective facades: 4th fl. -> 4%, 3rd fl. 6%, 2nd fl. -> 8%, 1st fl. -> 7%. Alt Ci causes an additional reduction of the DF values of ca. 2-3% in relation to alt C. 6.5.2. Analysis of the luminan ce distribution on facade A On the basis of the DF measurements on the facades, it is possible to estimate the luminance distribution on the facades as seen from the middle point on the floor. 72 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Facade A was divided into horizontal strips, such that the border between the strips was at the half of the distance between the sensors. In order to simplify the analysis it was assumed that i each strip is a homogeneous perfectly diffuse surface, where the relation between lumin ance and illuminance is as follows: L: (1) where: p reflection factor of the facade strip E illuminance in the measure point The mean reflection factors of those four strips were calculated, tab. 6.1. (2) Eout : (3) From equation (1), (2) and (3) [Hopkinson, 66] the luminance values were calculated. L := “p-DF-L„ » z (4 The results of experimental measurements of DF on the facades were used in the calculation of luminances. The luminances calculated for L% equal 1.0 are shown on fig. 6.13 together with the sky luminances for the visible part of the sky. part of C1E Overcast Sky visible from point P ------part of CIE Overcast Sky invisible from point P Luminance on facades 1.00 -] 0.90 - 0.80 - 0.70 - 0.60 - 0.50 - 0.40 - 0.30 - 0.20 - 0.10 - Angle in degrees Figure 6.13 Luminance distribution of the facades and the sky seen from a point P lying on the middle of the floor of an infinite long street. In order to examine the luminance changes on the facades, the lumin ance relative to the luminance on the 4th floor was calculated. 73 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Sensor height over the mean reflection DF luminance luminance relative and floor the floor, factor p in % calculated for to the 4th no. in m of the strip Lz=1.0 floor in % 4 11.5 0.70 45.99 0.25 100 3 8.20 0.64 34.93 0.17 69.6 2 4.86 0.56 26.59 0.12 45.7 1 1.50 0.35 21.69 0.06 23.3 Table 6.1 The results of calculation of the luminances on the facades. ------CIE Overcast Sky relative to the 4th fl. -----1— luminance relative to the 4th fl. ■ fit function nr 1 fit function nr 2 Angle in degrees Figure 6.14 Luminance distribution of the facades of the infinite street seen from a point P lying on the middle of the floor. The shape of the diagram is quite different from the diagram of lumin ance distribution of the CIE overcast sky. Its shape looks more like the asin 1 function. The fit function no 1. was found as: L(0) .= —-asm/ 8- 180 82.3-71 The fit function no 2. is: L(6) = —-—-asin/ 0— 1 -i------Jt 20 \ 82.371 20 The fit function for the Lz equal 1.0 based on the fit function no 1. will be: 2 . L 180 L(0) —•0.25 asin 0- \ 82.371 1 asin=arcsin 74 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY We expect, that the function will have minimu m two parameters: A that depends mainly on the reflection factor of the facades and B that depends on the opening angle of the street The formula will therefore have this general form: L(0) := — A asin(8 B) + C 7t More experiments are needed in order to find the relation between A and the reflection factor of the facades. 6.5.3. The impact of the choice of the facade alternative on the illuminance levels in adjacent rooms. Alternative A As shown in fig. 6.15, the DF levels for alt A on the 2nd , 3rd and 4th floor are nearly equal, both on the horizontal plane and on the walls. The largest difference of about 1.5% is between 4th and 3rd floor in the window zone. Alternatives: vert. D.F. 0.0 2.0 4.0 6.0 t * l ■ l ■ l 10.0 8.0 6.0 hor. D.F. vert. D.F. 4.0 2.0 0.0 6.00 4.00 2.00 0.00 Section Plan Figure 6.15 Alt. A, comparison ofDF measured on different floors, section through the centre of the window. Left: section through the window of the big room with diagrams showing DF measured on an horizontal plan 0.75 m over the floor. Right: plan of the same room with diagrams showing DF measured on the walls 1.15 m over the floor. Clearly, higher values are on the 1st floor than on all others. That phenomena is caused by the light reflected from the floor of the street that penetrates mostly to the rooms on the 1st floor and is reflected by the white ceiling and the comparatively light walls. In a real building the rooms on the 1st floor adjacent to the atrium are usually more obstructed by advertising, plants, curtains and peoples. Because of that it is always advantageous to design such rooms for high daylight penetration. Alternative B For alt. B, fig. 6.16, the DF is highest on the 1st and 4th floors. Because the same type of glazing was used on the 3rd and 4th floor and the glazing area was the same, it is obvious that DF on the 4th ft. is higher than on the 3rd fl., both on the horizontal plane and on the walls. On the 1st floor the same phenomenon as in alt. A is possible to observe. As a result DF is highest on the 1st and 4th fl., both on the horizontal plane and on the walls. The DF on the 2nd and 3rd fl. is about 30-40% lower than on the 1st and 4th fl. 75 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Alternatives: vert. D.F. 4— Bl.ll. 0.0 2.0 4.0 6.0 I i I i 1 i I -2— BII.IL -3— B 111. fl. 10.0 8.0 6.0 hor. D.F. vert. D.F. 4.0 2.0 0.0 Figure 6.16 Alt. B, comparison ofDF measured on different floors. Left: section through the window of the bigger room with diagrams showing DF measured on an horizontal plan 0.75 m over the floor. Right: plan of the same room with diagrams showing DF measured on the walls 1.15 m over the floor. Alternative C The DF diagrams on the section for alL C, fig. 6.17, have a typical shape representing dramatic changes of DF on a horizontal plane, especially on the 4th fl., from 14.5% in the window zone to 2.5% in the rear zone. The curves of DF on the 4th , 3rd and the 2nd floor have similar shapes. Only the curve for the 1st floor is different In the window zone, where the sky component is most important, there is less light on the 1st floor, than on the second floor. In a distance of 2.25 m from the window the DF value for the 1st floor is higher than for the 2nd floor and in the rear zone it is on the level of the 3rd floor. The reason for that must be that the internally reflected component (IRC) on the 1st floor has much larger significance than on all other floors and causes a flatting of the curve. Alternatives: vert. D.F. —1— cut. 0.0 2.0 4.0 6.0 l ■ l ' I ■ I ■2— C IL fl. CIII.lt. C IV. II. 10.0 hor. D.F. vert. D.F. 4.0 2.0 0.0 Section Figure 6.17 Alternative C, comparison of DF measured on different floors. Left: section through the window of the bigger room with diagrams showing DF measured on an horizontal plan 0.75 m over the floor. Right: plan of the same room with diagrams showing DF measured on the walls 1.15 m over the floor. The changes of light level on the walls are less dramatic. On the 4th fl. DF changes from the value of ca. 8% in the window zone to 3% in the rear zone. The diagrams representing DF on the rear wall are almost flat; the IRC is more significant there. Alternatives A. B and C on the respective floors The design of the windows of the 1st floor was equal for all three alternatives, the floor of the street and surfaces in the room was the same, too. The shape of the DF diagrams on the 1st fl. 76 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY is nearly alike, fig. 6.18. Alternative A gives the highest values of DF as a consequence of the highest mean reflection factor of the street facades. The consequence of differences between the mean facade reflectances of the street: Pa = 0.59, p B = 0.42 and pc = 0.35 appear clearest on the 1st fl. wall diagrams. The significance of reduced transmission of the glazing, alt. B compared to alt. C, is best visible on the 3rd and 4th floors, especially in the window zone and to a certain degree in the rear zone. The reduction of the normal glass transmission from 0.78 for alt. C to 0.44 for alt B results in ca. 50% reduction of DF in the window zone and ca. 40% reduction in the rear zone. The significance of reduced glazing area from 42.3% of the facade area on the 4th floor for alt. C to 16.2% of facade area for alt A is also clearly seen in the diagrams. The reduction of light levels in the window zone is nearly equal to the reduction of the glass area: On the 4th floor: Glazing area A/ Glazing area c = 0.383 DF A / DF c = 0.399 77 ’7? MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Alternatives: vert. D.F. 0 0 2.0 4.0 6.0 -A— AtV.lt. I » l t I > l -B— B IV. II. C IV. II. 10.0 10.0 8.0 8.0 6.0 6.0 hor. D.F. vert. D.F. 4.0 4.0 2.0 2.0 0.0 0.0 Section Plan Alternatives: Alternatives: 0 0 2.0 4.0 6.0 -A- AIL fL I i I i I i I —fi— BILfl. —€— C II. fL 10.0 8.0 vert. D.F. 2.0 o.o Alternatives: 0.0 2.0 4.0 6.0 —A— A I. fl. —B— Bl.fl. C I. fl. 10.0 6.0 vert. D.F. 4.0 2.0 0.0 Plan Figure 6.18 Comparison of alt. A, B and C in the big room. 78 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY 6.5.4 Visual comfort in rooms adjacent to the atrium for alt A Alternative A gives the possibility to compare the visual environment in four adjacent rooms that have nearly equal daylight levels, but different design of windows. As can be observed by comparison of photos 6.1, 6.2, 6.3 and 6.4, the danger for glare is strongest in the room on the 4th floor. It is caused by two reasons. The luminance of the windows is highest, because the opposite facade seen through the window is brightest on the top floor level. The distance between the windows and the sidewalls is largest, such that the sidewall is richly daylight only in a distance from the front wall. The front wall together with the parts of the side walls and the ceiling lying in the direct neighbourhood make a dark background for the excessively bright windows. The contrast between windows and their background is highest. As one move gradually down to the lower floors, the area of the opaque, dark front wall decreases, the side walls and the ceiling become more daylight in the neighbourhood of the front wall, at the same time the luminance of the windows area decreases. Since the visual acuity is dependent on the adaptation luminance, the readability of form will depend mostly on the outside illuminance and the reflectance of the visual task and the immediate surroundings. To estimate the modelling ability of light, two balls: one white and one dark grey were exposed on the white background in different places in the room, photos 6.5, 6.6, 6.7 and 6.8. It is possible to see all balls on all floors, but the visibility of balls as a three-dimensional objects and not as a disks decreases from the 4th floor gradually down to the 1st floor, although DF in the 1st floor room is highest, both on the horizontal and the vertical plane. Uniform, diffuse light gives the poorest modelling, irrespective of the mean daylight level. Modelling depends more on the light distribution, than on the light level. Due to the high daylight level, the readability of the space is rather high on all floors. Anyway, the three vertical walls placed in the rear zone of the room are better visible in the room on the 4th floor, because the contrast between luminances of differently oriented surfaces of those walls is strongest. It is even possible to observe their shadows, something that helps to estimate their depth. Also the spatial placing of the right, triangular joining element beneath the table is best visible in the 4th floor room. The smallest and brightest windows give the largest danger for glare, but much better modelling and space reading. The overall impression of the room is also different. The 4th floor room looks most closed, clear, lively and bright, when it is seen toward the sidewall. The 1st floor room looks most open, spacious and most gloomy in the rear zone. It looks as if it does not make a separate unit, but belongs more to the atrium space. 79 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Photo 6.1 4th floor Photo 6.23 rd floor 80 ; MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY \ i I Photo 6.5 4th floor Photo 6.6 3rd floor Photo 6.72 nd floor Photo 6.81st floor 81 ** ' MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY 6.6 Importance of the reflectance of atrium surfaces 6.6.1. Comparison of B-alternatives on the street, vert. D.F 0.20 0.30 0.40 0.50 13.20 - Alternatives: B base case 9.90 - Ba, alt. B with lighter facades & floor Bb, all. B with lighter facades only Baa, as Ba, facades behind glass 6.60 - 0.50 3.30 - 0.40 hor. D.F. 0.30 width of the street Figure 6.19 Comparison of alternatives B, Ba, Bb, and Baa. Daylight factors for different variants of alternative B are shown in fig. 6.19. The reflectances of the facades and the floor were differentiated in order to find the significance of such changes for the illumination on facades and on the floor. The alternative Baa is a variant of alt. Ba with the facade cardboard lying behind the double glazing. In this alternative nearly the whole facade has specular reflection. Increasing the mean reflection factor of the facades from 0.42 for alt. B to 0.57 for alt. Bb caused an increase of the DF on the floor of about 14%. At the facades the significance of facade reflectance varies with the height. On the height of the 1st and the 4th floor it caused an increase of the DF of about 11%, at the height of the 2nd and the 3rd fl. an increase of about 30%. It shows that the impact of the light reflected from the opposite facade is highest at the height of the 2nd and the 3rd floor, i.e. at the middle of the street height. An increase of the reflection factor of the floor from 0.33 in alt. Bb to 0.61 in alt. Ba caused a significant increase of the DF on the facades. The DF increased with 43% at the height of the 1st floor, 14% at the height of the 2nd fl., 8% and 4% respectively at the height of the 3rd and 4th fl. Thus the significance of the reflectance of the floor decreased with increasing height. 82 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY vert. D.F 0.00 0.20 0.40 13.20 - Alternatives: -0— Ba, as alt. B lighter facades & floor 9.90 — -X— Ba, DF measured on inside glass pane O— Baa, as Ba, facades behind glass ^— Baa, DF measured on Inside glass pane 6.60 — 3.30 - width of the street Figure 6.20 Alt. Ba and Baa. DF values measured on outside and inside of the glazing. A comparison of alt Ba and Baa shows the importance of the choice between specular and diffuse facade elements. There is no difference between the results for alt Ba and Baa at the height of the 1st floor where the light reflected from the floor dominates and at the height of the 4th floor, where the impact of skylight is most signifi cant The DF at the height of the 2nd and 3rd floor for alt Baa compared to Ba is reduced only by 5%, although the mean reflection factors of the walls are respectively: p = 0.57 for alt Ba, and only p = 0.34 for alt Baa A comparison of alt Ba and Baa based on the DF measurements taken on the inside of the glass shows even better consistence. The facade with specular facade elements of mean reflectance of 0.34 works like a diffuse facade of mean reflectance of 0.57. Why? The normal reflection factor of glass was used in calculations of the mean reflection factor of the facades, also of facade Baa that is composed nearly exclusively of glass. The results show that the reflection factor measured in the model is higher, than the normal one. In order to compare different reflection/transmission factors and to estimate which of those are most close to the measured ones, calculations of the light transmission factors using the Rivero equation were done, appendix 2. The measured transmission factor of the glazing in alt B decreases with the atrium height The mean incidence angle of skylight in the atrium varies in the same manner. Since the transmission factor of glass decreases with incidence angle, the positive impact of high transmission factor is cancelled by the negative impact of the high incidence angle of the skylight The result is that the vertical DF measured on inside glass pane on all floors is nearly equal. 83 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY 6.6.2. Comparison of B-altematives in the room. An increase of the mean reflection factor of the street’s facades from 0.42 for alt. B to 0.57 for alt. Bb, fig. 6.21, caused an increase of the DF in the room as shown in tab. 6.2. floor: window zone of the room rear zone of the room 4th fl. 8%, 12% 3rd fl. 15% 30% 2nd fl. 20% 45-50% 1st fl. 18% 30% Table 6.2 Increase of the DF in adjacent room caused by the increase of the mean reflection factor of the street facades. The significance of the reflectance of the opposite facade increased with the depth in the room, both on horizontal and vertical surfaces. The light from the opposite facade penetrates deep into the room, because it has nearly horizontal direction. The largest increase of the DF was on the wall opposite to the windows and on the rear parts of the sidewalls. Inter-reflections between room surfaces caused increased DF on the horizontal plane, too. An increase of the reflectance of the street floor from 0.33 in alt Bb to 0.61 in alt Ba caused an increase of the DF in the rooms as shown in tab. 6.3 and are somewhat similar to the changes on the facades.______ floor: increase of DF inside the adjacent room increase of DF on the atrium facades 4th fl. 2,5% 4% 3rd fl. 7% 8% 2nd fl. 18% 14% 1st fl. 30% 43% Table 6.3 Increase of the DF in adjacent room and on the atrium facades caused by the increase of the mean reflection factor of the street floor. It is interesting to observe that on the horizontal plane the significance of the street floor reflectance increases with the depth in the room, e.g. on the 1st fl. from 15% near the window to 35% near the rear wall. On the walls the significance of the floor reflectance was not dependent on the depth of the room to the same degree, e.g. on the 1st fl. the increase on the side wall was 33% by the window, 31% by the rear wall. The light reflected from the street floor penetrates the room from down up, and is first reflected by the white ceiling in the room, then take part in the inter-reflections between room surfaces. It increases only the internally reflected component If the significance of the street floor reflectance for the room walls is independent of the depth of the room, it must be caused by the fact, that the sky component on the room walls is so small that it can be overlooked. On the horizontal plane, the sky component has more importance in the window zone than in the rear zone, therefore the significance of street reflectance increases with the depth of the room. 84 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY IV. floor vert. D.F. 0.0 2.0 4.0 6.0 —B, bite ease i —Bb, il B wto Cghtii tictdis ■■ 0~ Bi,itBwithCgMirfictdei 1 Boor ■~0~ Bn,eiBt,fieidisbehhdghu 10.0 8.0 6.0 hor. D.F 4.0 vert. D.F. 2.0 0.0 4.20 2.10 0.00 Section Plan Figure 6.21 Comparison of different variants of alt. B in a room adjacent to the street/linear atrium. 85 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY The relation between daylight factors for alt. Ba and Baa is quite different inside the rooms, than on the street facades. The DF values for Baa are higher than the DF values for Ba, especially on the horizontal plane. Because the DF values for Ba and Baa measured on the inside of the glazing are nearly exactly the same for those two alternatives and the area of glazing is the same, it must be caused by the different design of the junction glass-wall, fig. 6.22. The thickness of the glazing, that is in 1:1 scale, is rather large in relation to the room dimensions that are in 1:20 scale. Two reasons cause the phenomena described before. The first one is the difference in the viewing angle. For a selected point P on the horizontal plane in the room, the cardboard facade lies further away in alt. Ba than in alt. Baa, so the viewing angle from point P in Baa is larger than in alt Ba. The second one is the increased transmission factor of the glass along window edges, caused by the absence of frame. The daylight falling on the window at high angles of incidence that in other facade design would be reflected or absorbed by the frame is partly transmitted through the outside pane, partly reflected from the inside glass pane or facade element and partly reflected from the outside glass pane to the room. In a real building the window glass is usually fixed in a frame, so the increased transmission of the window edges will not occur. Figure 6.22 Comparison of alt. Ba, to the left, and alt. Baa to the right. 6.7 Significance of the glass slope on the first floor The significance of the direct sky light for day lighting in rooms adjacent to a linear atrium is greatest on the top floor and decreases gradually as we move down to the lower floors. In a street that has dark facades, the significance of skylight will be very important On the bottom of the street the light from the sky have nearly vertical direction, the window glass is placed in vertical position, too. The large incidence angle of skylight fallin g on the glazing will cause reflection for the most part of the incoming skylight A small experiment was carried out to examine the significance of the sloping of glass in relation to the facade in order to reduce the incidence angle of skylight falling on the glazing. On one side of the street facade C was constructed, the other side was without facade, only a single glass pane was placed before the rooms on the 1st floor. The inclination angle of the glass pane was varied from 0° to 45°. As expected, the largest influence of sloping the glass was noticed for the points lying in the window zone. The influence increases with the slope. Tab. 6.4 shows the increase of the DF on the horizontal plane in comparison to vertical glass. 86 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Inclination angle of the glass pane to the facade Point Distance from the CO o 15° 45° no glass window 1 0.5m 13% 17% 21% 37% 2 1.5m 2% 5% 8% 20% 3 2.5m 1% 4% 7% 16% 4 3.5m 2% 5% 8% 12% Table 6.4 Increasing of the horizontal DF in the adjacent room on the 1” floor caused by the sloped glass in comparison to vertical glass. Alternatives: vert. D.F. no glass 2.0 4.0 6.0 O glass 0-deg her. D.F. vert. D.F. i- 12.0 - 6.0 — 4.0 u 2.0 2.10 Section Figure 6.23 The significance of the sloping of the window glass for illuminance level in the room. 6.8 Comparing different active daylight strategies In order to increase the illumination level in the rear zone of the rooms adjacent to the street/linear atrium, the active daylighting systems described in chap. 5 and section 6.4.1 were examined. The active daylight systems were examined with the facade A only. For alternatives that were supposed to change illumin ation values differently in the lower and the upper part of the window, one additional sensor was placed in the middle of the upper part. One additional sensor was also used on the upper part of the 2nd floor window for alternatives: Lcp x 2-atr and Lcp x 2-fac, tab. 5.3. 87 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY vert. D.F 0.20 0.30 0.40 0.50 13.20 Active daylight systems, mirrors: —^— A. M-fac. —^— A. M-aV. —— A, M-curved width of the street Base case V < M-fac A M-atr ) V M-cutved Figure 6.24 The influence of three different active daylighting systems based on the usage of mirrors on illumination distribution in a street. Active daylight systems, mirrors: vert. D.F. A, base cue 0.0 2.0 4.0 6.0 —X' A.U-eunea her. D.F. vert. D.F. - 6.0 — 4.0 — 4.0 — 2.0 - 2.0 •— 0.0 L- 0.0 4.00 2. Section Base case t 4 M-fac A M-atr J v. M-curved Figure 6.25 The influence of three different active daylighting systems based on the usage of mirrors on illumination distribution in a room adjacent to a street. 88 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY M-fac Mirrors on the facades reduce slightly the illuminance level on the street floor, fig. 6.24. They increase the illuminance levels on the 1st fl. windows of about 12% and on the 2nd fl. windows of about 9%. The mirrors were inclined to the facades at an angle of 32°, the angle was calculated as the optimal angle for reflection of skylight to the 1st floor room on the opposite side of the atrium. The mirrors shade the 1st floor windows from skylight Changing the design of the street section in order to reduce that shading could increase the significance of that alternative. Inside the room ,fig. 6.25, the mirrors on the facades increase the DF of about 14% in the rear zone, both on the horizontal and vertical surfaces. On the horizontal plane in the window zone the DF was reduced by 5% because of skylight shading. M-atr The changes in the light distribution caused by the mirrors placed in the middle of the street were much more dramatic. Beneath the mirrors in the middle of the street the illumin ation level was reduced by 65%, simultaneously the illumin ance level on the upper part of the 1st floor window increased by about 30%, fig. 6.24. Inside the room, fig. 6.25, alt. M-atr was most efficient of all active daylight systems based on the usage of mirrors examined in the study. It caused an increasing of the DF of about 30% in the rear zone on all surfaces. Illuminance in the window zone, both on horizontal and vertical surfaces was increased by about 10%. M-curved The curved mirror reflector fixed on the front of the upper part of the 1st fl. window caused an increasing of illuminance on the upper part of the 1st fl. window of about 20%. The reflector, protruding 4.2 cm from the facade, i.e. 84 cm in a real building, shades the lower part of the window and the street floor lying by the facades. Indeed, the DF on the lower part of the window was reduced by 30% and the DF on the street floor in the distance of 1.0 m from facades by 16%. In the room the illuminance in the window zone was slightly reduced. In the rear zone of the room the increase in illuminance was minim al (5-8%). The efficiency of alt M-curved was smallest of all active alternatives examined in this experiment The reason for that can be that the aluminium foil used for simulating a curved mirror had not good enough specular reflection. An increase of the DF on the 3rd and 4th fl. windows is probably caused by the fact, that the light supposed to be reflected from the curved mirror to the room was instead partly reflected up to the upper parts of facades. Anyway, the light reflected from the street floor have a crucial importance for illumination in the room on the 1st fl. adjacent to the street as was investigated in section 6.6. Any obstruction of the light falling on the street floor, specially the floor area close to the facades, will reduce the illumina tion values in the room significantly. Discussion Alt M-atr was the most effective of these three alternatives. The mirrors were placed in the middle of the street width, where the sky factor is largest; the facade windows were not shadowed. The negative consequence of M-atr alternative is the deep shadow caused by the mirrors on the middle of the street floor. 89 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY vert. D.F 0.20 0.30 0.40 0.50 13.20 Active daylight systems, Icp.: A, base case -0— A.Lcp-fac. A. Lcp-atr. A. Lcp x 2 • fac. - - © - A,Lcpx2-atr. width of the street V y Base case x y Lcp-fac V Lcp-atr X y Lcp'2-fac V Figure 6.26 The influence of active daylighting systems based on the usage of laser cut panels on illumination distribution on a street. Active daylight systems, Icp: vert. D.F. A, bste ctse 0.0 2.0 4.0 6.0 her. D.F. vert. D.F. — 6.0 — 4.0 — 4.0 - 2.0 - 2.0 ■— 0.0 •— 0.0 4.00 2. Section Base case x y Lcp-fac V Figure 6.27 The influence of active daylighting systems based on the usage of laser cut panels on illumination distributionin the f fl. roomadjacent to the street. 90 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Lcp-fac The laser cut panels fixed in front of the upper part of the 1st fL window,/?#. 6.26, caused an increase of the DF on the upper part of the window by 35% and simultaneously a reduction of the DF on the lower part of the window by 18%. The illumination level on the street floor was increased by 3% in the middle and reduced by 12% by the facades. In the adjacent room,/?#. 6.27, the illuminance level in the rear zone was increased by 50% on the horizontal plane and on the rear wall, and by 40% on the sidewalls. Lcp-atr The idea of placing of laser cut panels in the middle of the street was based on the principle of reducing of light level in most bright parts of the street space by redirecting it to the adjacent rooms. Indeed, the illuminance level was reduced by 43% on the middle of the street floor and increased by 29% on the upper part of the window and 14% on the lower part of the window, fig. 6.26. Unlike alt Lcp-fac, the DF levels in the room was increased both in the window zone and in the rear zone,/?#. 6.27. In the rear zone the illuminance increased by ca. 35% both on the horizontal plane and on the sidewalls, and by 44% on the rear wall. The increase in illuminance levels in the rear zone of the room was smaller than for alt Lcp-fac due to the dispersion of light from the panels. Laser cut panels on the I & lift: vert. D.F. A,Lcpx2-itr.,llfl. hot D.F. vert. D.F. — 4.0 — 4.0 — 2.0 — 2.0 t— 0.0 Section —l i— -] t y ✓ Base case v y Lcp-2-fac V Lcp'2-atr Figure 6.28 The influence of active daylighting systems based on the usage of laser cut panels on illumination distribution in the 2?*fl. room adjacent to the street. Leo x 2 - fac Laser cut panels fixed in front of the upper part of the 2nd fl. window increase the DF on the upper part of the window by 40% and reduce the DF on the lower part of the window by only 10%. It seems that the efficiency of Lcp on the 2nd fL is larger than Lcp used on the 1st fl. in spite of the fact, that the width of Lcp on the 2nd fl. was smaller. It must be caused by the much higher sky factor at the 2nd fl. In the room on the 2nd floor,/?#. 6.28, illuminance level in the rear zone increased by 55% on the horizontal plane, by 40% on the side walls and by 51% on the rear wall, i.e. slightly more than in the room on the 1st floor. 91 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Although laser cut panels on the 2nd floor shadow the windows on the 1st fl., it was very interesting to observe how great changes in illumin ation of the street and the room can occur if Lcp was used both at the 1st and the 2nd floor. Comparison of alt Lcp-fac and alt Lcp x 2 - fac shows the negative influence of laser cut panels on the 2nd fl. on the illuminance on the 1st fl., fig. 6.28. The DF on the upper part of the window was increased by about 35% for alt Lcp-fac compared to the base case, and only by 17% for alt Lcp x 2 - fac. In the rear zone of the room on the 1st floor, fig. 6.30, alt Lcp x 2 - fac increased the DF by 40% on the horizontal plane, by 34% on the side walls and by 46% on the rear wall, compared to the base case. It is still a considerable improvement of lighting in the rear zone of the room, but smaller than the respective numbers for alt Lcp-fac. Lcp x 2 - atr Laser cut panels placed in the street space at the height of the upper part of the 1st and the 2nd fl. window, alt Lcp x 2 - atr, increased the DF on the upper part of the 1st fl. window by 24% and on the lower part of the window by 8%, i.e. slightly lower values than for alt Lcp-atr. It seems that the efficiency of laser cut panels on the 2nd fl. is nearly alike the one used on the 1st fl. in spite of the fact, that the width of laser cut panels on the 2nd fl. was smaller. In the room on the 2nd floor, fig. 6.28, the illuminance levels in the rear zone increased by 45%, both on the horizontal plane and on the side walls, and by 50% on the rear wall, i.e. more than in the room on the 1st floor. Alternatives Lcp x 2 - fac and Lcp x 2 - atr gives nearly the same results in the rear part of the room on the 2nd floor. Placing of another Lcp in the street space at the height of the upper part of the 2nd floor window in addition to the Lcp on the 1st floor caused a 40% reduction of illuminance levels on the 1st floor window. In addition the illuminance level on the street floor near facades was reduced by 14%. In the 1st fl. room, fig. 6.32, alt Lcp x 2 -atr caused an increase of the illuminance levels in the rear zone of about 25% on the horizontal plane and the side walls, 30% on the rear wall, compared with the base case, i.e. a slight reduction compared with alt Lcp-atr. Discussion A comparison of daylight systems placed on the facades, fig. 6.29, shows that the best performance on the facade at the height of the 1st floor can be expected from alt Lcp-fac. All the facade alternatives shadow the facades on the height of the 1st floor, and the street floor near the facades. The difference between the bright upper part of facades and the gloomy lower part of the facades is stronger than in the base case. The middle part of the street floor was very little influenced by the systems. In the room, fig. 6.30, alt Lcp-fac gives the best results for all measured points, alt M-fac the poorest All the daylight systems placed in the street space, fig. 6.31, reduced the illuminance levels in the middle of the street The changes in illumin ance levels are most dramatic for mirrors, alt M-atr, and lowest for alt Lcp x 2 - atr. Diagrams of the DF for alternatives M-atr and Lcp-atr on the street facade have nearly the same shape. Only on the lower part of the 1st fl. window will Lcp-atr give a little better results than M-atr. In the room, fig. 6.32, alternatives M-atr and Lcp-atr give nearly the same results, only on the rear wall does alt Lcp-atr give a little better results. Of all alternatives that use the street space will alt Lcp x 2 - atr give the lowest illuminance values in the room on the 1st fl. 92 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY vert. D.F 0.20 0.30 0.40 0.50 V > Base case L J M-fac lxp-tac v y Figure 6.29 Comparing active daylighting systems placed on facades, measurements in the street. Daylight systems on facades: vert. D.F. A, base case 0-0 2.0 4.0 6.0 —X— U.fac. —©— A, Lep-fac. —0— A, Lcpx2-fac. her. D.F vert. D.F, - 6.0 2.55 ------ -4.0 — 4.0 — 2.0 — 2.0 t— 0.0 t— 0.0 Section Plan i < > Base case k 4 V / Lcp-fac < / Lcp*2-fac Figure 6.30 Comparing active daylighting systems placed on facades, measurements in the 1st fl. room. ! 93 A MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY vert. D.F 0.20 0.30 0.40 0.50 13.20 — width of the room Base case M-atr X/ Lcp-atr V Figure 6.31 Comparing active daylighting systems placed in the street space , measurements in the street. Active daylight systems, mirrors: vert. D.F. A, base case 0.0 2.0 4.0 6.0 A, M-str. A, Lcp • ttr. A, Lcp z 2 - atr. vert. D.F. — 6.0 - 6.0 — 4.0 — 4.0 — 2.0 — 2.0 * — 0.0 L- 0.0 4.00 2. Section / Base case A M-atr \/ Lcp-atr V Figure 6.32 Comparing active daylighting systems placed in the street space, measurements in the 1st fl. room. 94 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY Generally the daylight systems placed on facades are a little more effective in the rear part of the room than daylight systems placed in the street space. They reduce the illuminance values in the window zone and increases illuminance values in the rear zone. The daylight systems placed in the street space increase the illuminance values equally in the whole room. Conclusions In narrow streets the illuminances on the street floor are usually much higher than illuminances on the lower part of the facades. In the study the values for facade A were respectively DFno0r = 45%, DFfac. i fl. = 23%. Such ratios of illuminance levels cause the street to look gloomy, but at the same time objects placed in the lower part of the street are strongly illuminated from the top, photo 6.9. Daylight systems placed on the facades increase the contrast between the excessively bright middle part of the street and relatively low illuminated lower part of facades. The street looks gloomier and the modelling in the space near the facades is considerably reduced, photos 6.13, 6.14, 6.15 and 6.16. Daylight systems placed in the street space function in the opposite manner. They reduce the illumination on the middle of the street by redirecting the light to the facades. In that way they improve the appearance of the street, make it more bright and spacious, photos 6.10, 6.11 and 6.12. The strong contrasts in alt. M-atr reduce the modelling properties of the light in the shadow below the mirrors. The study shows also, that the use of laser cut panels to redirect the diffuse skylight is a very interesting solution for deep atria. They perform as well as mirrors and are transparent for perpendicular light, such that the visual contact between different parts of the atrium space can be maintained. 95 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY 96 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY 97 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY 6.9 Discussion of the significance of the roof glazing The choice of roof configurations was dictated by the climate. Only totally transparent roofs: horizontal, double pitched and single pitched, were examined. The same type of glass i.e. clear 6 mm thick glass was used in all roof configurations. vert. D.F vert. D.F 0.20 0 30 0.40 0.50 0.10 0.20 0.30 0.40 Alternatives: — B double pitched gl — B single pitched gl hor. D.F. Base case Double Single Horiz. gl. no glass pitched gl.45° pitched gl.35° Figure 6.33 Comparing different glazing configurations on the roof for alt. A and B. All forms of glazing on the roof reduce the illuminance levels, both on the facades and on the street floor, fig. 6.33. The amount of reduction varies with the configuration of the glazing. The reason for that is the variation of light transmission factor of glass with incidence angle. Because the differences between the illumination levels for these three glazing configurations were veiy little, the same experiment was carried out for two facade alternatives: A and B. The DF, both on facades and on the floor, was reduced by a minimum of 10%. The smallest discrepancy between alt. A and B was found on the floor. For both alternatives the horizontal glass obstruct the light less than each of two inclined glass roofs, i.e. about 10%. Double pitched glass obstruct the light the most, 16.5% in A and 15-20% in B. This is especially well seen for the measure point placed on the floor 1.0m from the facade. Because the facades in alternatives A and B were exactly alike for all glass configurations, the sort of glass used on the roof was the same and facades were illuminated in the same manner, the differences between measurements must be caused by the glass roof configuration alone. The results of measurements on facades do not give such a clear picture. It is possible to observe that the lowest light reduction occurs for horizontal glass on the lower part of facade on alt. A and B, i.e. ca. 10%, the largest light reduction occurs for double pitched glass, especially in alt. B, i.e. ca. 20%. Besides the measure point on the 4th fl. in alt. B, double- pitched glass obstructs the diffuse skylight less than single pitched glass does. Again the configuration of glass on the roof is the most probable reason for this. In addition to comparison between different glass roofs, a comparison of two sides of the same street was carried out for alt. A with single pitched glass, fig. 6.34. 98 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY vert. D.F 0.20 0.30 0.40 0.50 Alternatives: Abase case A single pitched gL right side A single pitched gL left side 0.60- — 0.40 hor. D.F. width of the street Single Single Base case pitched gl. pitched gl. no glass left side right side Figure 6.34 Comparison of illumination levels on the left and the right side of the street under the single pitched glass for alt. A. The difference between the right and the left side of the street covered by the single pitched glass is clearly shown, both on the facades and on the floor. In the points that 'see' the sloped glass the DF is increased by about 5% comparing to points on the other side of the street. 6.10 Laser cut panels and prismatic panels on the roof vert. D.F vert. D.F 0.10 0.20 0.30 0.40 0.10 0.20 0.30 0.40 Alternatives: 13.20 ...— base case —.— base case o 9.90 —0— Lcp-0 —Lcp-35 6.60 — —Q— Lcp-35 + gL 3.30- 3.30- 0.40 hor. D.F. hor. D.F. 0.30 I0 3.76 7. width of the street width of the street Lcp-15* Lcp-35* Lcp-35* Figure 6.35 To the left: laser cut panels on the roof inclined to different angles. Measurements taken on the facade below the high end of the sloped roof. To the right: Laser cut panels with and without glass . 99 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY All the measurements shown in fig. 6.35 were taken on the right side of the street, from which the sloped surface was seen. It shows the changes in illumination on facades and on the floor as the inclination angle of the laser cut panels increases. The differences are best visible on the floor. Laser cut panels in a horizontal position reduce the illumination level on the floor by 13-15%. Increasing incidence angle causes further reduction of the illuminance levels in the street. For 35° slope the reduction is about 30%. On the walls the results for 30° and 35° are nearly the same. The reduction of the DF increases as one move down the street height, tab. 6.5. At the 4th floor the Lcp obstructs the light more if placed horizontally, than if inclined to 15°. Floor: Lcp sloped by 0° and 15° Lcp sloped by 30° and 35° 4th fl. 13-14% 13% 3rd fl. 13-14% 18.5% 2nd fl. 15% 21% 1st fl. 13% 22% Table 6.5 The reduction of DF on facades caused by Lcp placed on the top of the atrium space sloped by angles: 0°, 15°. 30°and 35°. In fig. 6.35 an alternative with an additional pane of glass was added. The results shows that the illumin ation on the floor is additionally reduced by ca. 10% as a single glass pane is used in addition to the laser cut panels on the roof. vert. D.F 0.20 0.30 0.40 0 SO Alternatives: base case 9.90- PP prisms up PP prisms down Lcp*15 1.60- 0.50 0.40 her. D.F. 0.30 width of the street PP PP Lcp-15* prisms up prisms down Figure 6.36 Comparison of laser cut panels and prismatic panels on the roof. Two configurations of prismatic panel were examined, one with the prisms oriented up, another with the prisms oriented down. The results show very little difference between those two alternatives, i.e. about 2-3%. The prismatic panels (PP) were inclined at 10°. In order to compare the Lcp and the PP, the Lcp inclined at 15° was chosen because the incidence angle was closest to the incidence angle 100 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY of the PP. The results show very little difference between those two systems. The PP obstruct the diffuse skylight slightly more than the Lcp, both on the floor and on the facades except for the 4th floor. The difference in light reduction between the panels in comparison with the base case is only about 3-4%. 6.11 Reflectors on the neighbouring building In order to examine the efficiency of reflectors placed on the roof of the neighbour building, facade A as well as facade B was used in our experiments with reflectors. vert D.F vert. D.F 0.20 0.30 0.40 0.50 0.20 0.30 0.40 0.50 width of the street ------width of the streetg- 3—Z: T—T Flat, sloping Parabolic Base case Parabolic mirror minor concentrator Figure 6.37 The flat and the parabolic reflector composed with the facades B & A. Fig. 6.37 shows that both the flat and the parabolic reflector performs better together with facade A than with facade B. This is probably caused by the higher reflectance of facade A, that has a positive influence for daylighting, specially in the lower part of the street The parabolic reflector has nearly the same efficiency on the top parts of facades for both facade alternatives. It increases the DF at the height of the 4th floor by 8-10% compared to the base case. The parabolic concentrator, as it was constructed in the study, does not give better results than parabolic reflector. A change of material used on the small reflector can probably increase the efficiency of this system. The flat reflector performs better if sloped at the angle of 65° than 61°. The flat reflector inclined at the angle of 65° was the most efficient one of all reflectors examined in this experiment The results in fig. 6.38 show that the mirror strip placed on the top part of the facade is more effective if placed on facade A, than if placed on facade B. The reason for that is probably that the top part of facade B is lighter than the corresponding part of facade A. The difference between the optical effect of the white cardboard and the mirror is smaller than between the beige cardboard and the mirror. MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY vert. D.F vert. D.F 0.20 0.30 0.40 0.50 0.20 0.30 0.40 0.50 13.20- Altarnstlves: B, base case B, mirror >.90- B. parab. rofl B. parab. rofl. + mirror 6.60 — 3.30- — 0.40 hor. D.F. width of the street r Z % Z Parabolic mirror Strip of Parabolic Base case and mirror mirror strip of mirror Figure 6.38 The parabolic reflector composed with the facades A andB. The fig. 6.39 results show that the flat reflector increases illuminance levels more than the strip of mirror, especially on the floor and on the top part of the facade. It looks like the mirror strip and the flat reflector work independently of each other, and that the increase of the DF in alternative A with the flat reflector and the mirror is the sum of the gains due to the mirror on the roof and the strip of mirror on the facadeA. The increase of the DF caused by the flat reflector on the facades varies from 9% at the height of the 1st floor to 13% at the height of the 4th floor. On the street floor the increase is about 7%. The flat roof reflector with the strip of mirror increases the DF on the facades by 10% at the height of the 1st floor and up to 17% at the height of the 4th floor. On the street floor the improvement is about 7%. From the fig. 6.40 results that horizontal glass placed between the reflectors and the facades reduces considerably the efficiency of the reflectors. In the case of the flat reflector, the reduction of the DF on the facades due to the glass is between 13-15%. In the case of the parabolic reflector the reduction of the DF due to the glass varies with the height The largest reduction of about 23% occurs for parabolic reflector and facade B on the top parts of the facades. This is caused by the fact that the light reflected from the parabolic reflector falls on the glass with a large incidence angle, where the reflectance of glass is high. A considerable part of the light falling on the glass is reflected outward. The study with the horizontal glass placed between the roof reflectors and the facades shows also that the negative impact on the illumination in the atrium caused by the glass on the roof can not be fully cancelled by reflectors on the neighbouring roof. 102 MODEL STUDIES UNDER AN ARTIFICIAL OVERCAST SKY vert. D.F 0.20 0.30 0.40 0.50 width of the street 4 4—4 Flat, sloping Strip of Flat, sloping and strip of minor Figure 6.39 The flat reflectors with facade A. vert. D.F vert D.F 0.20 030 0.40 050 0.10 030 0.30 0.40 width of the street width of the street — —: o —t T Flat, mirror Rarab. mirror Flat, sloping Parabolic Base case and and mirror hor. class hor. class Figure 6.40 The flat and the parabolic reflector with and without a horizonted glass roof. 103 # CAap. 7M dbEi S7%WfE& #^ SUM ^ The model used for overcast sty simulations was also used for sun simulations, fig. 6.1 and 6.2. All alternatives were examined with facade A, fig. 5.8. The model was oriented with the main axis in the east-west direction. In order to examine the daylighting ideas most thoroughly, seven sun positions were chosen on the sun diagram for Oslo, fig. 7.1. The noon altitudes were chosen in order to best represent the mean altitudes in respective seasons. The sun altitudes for 30° and 60° azimuth were read from the sun diagram on the curves corresponding with the respective noon altitudes, tab. 7.1. N Elevation Junc4 Tt 9 p.m.v 7 p.m. Apr. 5 y " Sep. 19 Mar. 6 Oct. 19 Feb. 4 Nov. 18 Dec. 21 Year's 1st half Year's 2nd half : Indicated local mean time 5 GMT+1.0 h Figure 7.1 The sun diagram for Oslo, longitude 10.75° E, latitude 59.93° N, traced with the SunOrbl.O program [Unger], Summer Spring/Autumn Winter Time, 12:00 10:00/14:00 8:00/16:00 12:00 10:00/14:00 8:00/16:00 12:00 about Azimuth 0° U> o 60° 0° 30° 60° 0° o 9 Altitude 40° W =3 27° 00 10° Table 7.1. Azimuth and altitude angles used in sun simulations. MODEL STUDIES WITH ARTIFICIAL SUN 7.1 Design of the artificial sun Because of the great distance to the sun as a light source, the average distance during the year is 149,600,000 km, -»the rays of sunlight falling on the earth are virtually parallel and -> the illumination intensity on a building will not vary with the distance to the sun. The large dimensions of the model (206 x 102 x 70.75cm) made it impossible to simulate the sun with a single lamp. Because the model had a linear shape, a row of lamps of the type: SYLVANIA Halogen HI- SPOT 120, 100W, spot 10°, lOOOOcd, 240V, fixed to a horizontal beam was used. The lamps had parabolic reflectors; the reflected part of radiation from the lamps was parallel. In order to limit the diffuse part of radiation, a single tube with internal black velvet surface was fixed directly in front of each lamp. The tubes were 51cm long and had an inner radius equal to the radius of the lamp, i.e. 12 cm. Even so, the diameter of the light beam from a single lamp increased with the distance from the lamp. Also the illumination intensity of the beam decreased with the distance from the middle axis of the beam. The difference between the illuminances measured respectively in the middle and near the border of the light beam was about 20-30%. A \ \ \ \ - Figure 7.2 The artificial sun and the model. To light the atrium space of horizontal dimensions 206x 40cm, five lamps spaced by 41cm were used. To obtain a light beam of diameter larger than 60cm from each lamp, the op timal distance between the row of lamps and atrium axis at roof level of the model was found to be between 3 and 4m. In order to obtain this desired distance, the horizontal beam with lamps was moved vertically. Additionally, the model was placed on a platform with wheels. An accurate simulation of the desired azimuth and altitude was possible by moving and turning the model and by adjusting the lighting direction from the lamps. 105 MODEL STUDIES WITH ARTIFICIAL SUN The experiments with artificial sun were carried out at the Lighting Laboratory of the Electrical Engineering Department at Norwegian University of Science and Technology, NTNU. 7.2 Methods of measurement All measurements were taken with an IDAS illuminance data acquisition system, fig. 8.3. The IDAS is a portable, computer-based system suitable for measuring illuminances at several locations simultaneously in models and full-scale buildings. The system consists of an IBM compatible portable computer, a SAM data acquisition module with auto ranging, 8 Li-Cor remote photometric sensors and an operating program in Basic. RS- 232 data cablfe BATERY | charger! LIGHT SENSORS Figure 7.3. The IDAS illuminance acquisition system. The lamps were not observed to vary considerably during measurements. This meant that the outdoor measurement could been taken once for a series of about five indoor measurements. The indoor measurements were taken on the atrium facades and floor in the middle of the atrium length. The accurate placement of the eight base sensors is shown in fig. 7.4. In some cases, as for the winter measurements, an additional measure was taken in a point shown on the section and named ‘winter ’. 3 Winter Figure 7.4. Section of the model with placement of the sensors. Because the illumin ation from the lamps on the south facing facade was not perfectly even as it is under the real sun, the illumin ance measurements were taken in 15 points evenly 106 MODEL STUDIES WITH ARTIFICIAL SUN distributed on the sunlit part of the middle unit of facade. An eveness correction factor X was calculated for each sun position: X = E1/Ev>fac were: Ej the illuminance measured in the base case at point no. 1 placed at mid-height of the 4th floor window. Eyjac the mean value of illuminance on the sunlit part of the facade. All measurements taken at point no.l in all alternatives for a given sun position were multiplied by the factor X. Unlike the real sun radiation, the intensity of the illumination from the lamps changed with the distance. To reduce this problem, the vertical and the horizontal illuminances were measured in 15 evenly distributed points at roof level in the middle section of the model. The mean values Ej^oof and E^of were calculated respectively for horizontal and vertical illuminances. Because the distance to the lamps measured from the middle of the sunlit facade was larger than the distance measured from the middle of the top plane, E^ was smaller than Ey^f. To compensate for this, a distance correction factor v was calculated, V — Ey^fae / Ev>roof and the outdoor horizontal illuminance Eout was calculated as: Eout= v * Ejjjoof The sunlight factor (SF) at a measurement point N: SF = En / Eout were Eq is the illuminance at point N. In addition to the measurements, photos of the atrium space were taken for each alternative system. 7.3 Presentation of alternative daylighting ideas The following daylighting ideas were examined with the artificial sun: 1. Different configurations of glass cover over the atrium: • horizontal, • single pitched glass roof, sloped by 18°, • single pitched glass roof, sloped by 30°, • single pitched glass roof, negatively sloped by 35°, chap. 5. 2. 3M prismatic panels covering the atrium: • sloped by 10°, • sloped by 18°. 3. Laser cut light deflecting panels covering the atrium: • sloped by 18°, • sloped by 30°. 4. Reflector on the top part of the south facing facade. 5. Reflector on the neighbouring roof on the top of the south facing facade. 6. Laser cut panels as sun shading. 107 MODEL STUDIES WITH ARTIFICIAL SUN Strategies 1,2 and 3 belong to the group: glass roof as a light conductor/reflector, chap. 5. The first four groups of systems are described in chap. 6. Unlike for the overcast sky situation, the orientation of the sloped glass is of key importance for direct sunlight The term ‘negative slope ’, chap. 5, is used for an alternative where the low side of the sloped glass sheet is located on the north side of the atrium. Strategies 5 and 6 were added with the goal of examining different moveable systems, that can be used only for clear sky with sun and removed when the sky is overcast or when the increased sun shine is not desired. J _ The reflector on the neighbouring roof . A strip of mirror placed at the top of the atrium was supposed to reflect the sunlight falling on the neighbouring roof into the atrium itself, increasing the total level of illumination in the atrium. It was possible to turn the mirror around a horizontal axis placed on Reflector on the roof the top of the south facing facade. The mirror was 10 cm wide i.e. 2 m in a real building. The mirror was simulated by a strip of mirror-covered paper. Laser cut panels as sun shading . The panels were fixed over the windows. The panels used on the 2nd floor were 8 cm wide, while those used on 3rd and 4th floor were 6 cm wide. The slope of the panels could be adjusted. They were supposed to deflect sunlight falling on the windows to the atrium Lcp-2,3,4 fl. floor. Both strategies were examined for the most effective slope angle for a given sun position. Their position is supposed to be changed under overcast sky, in order to not obstruct the diffuse light The roof reflector is supposed to be hinged in such a way that it also can increase the atrium illumin ance under overcast sky, this was investigated in chap. 6. The laser cut panels are also supposed to be hinged, which enables panels to be parked in a vertical position. Alternatively they can be turned downwards to a sloping position in front of the upper part of the windows, increasing the illumination level in the rooms adjacent to the atrium, chap. 6. 7.4 Different glazing configurations on the roof The diagrams in the following figures show the sunli ght factors on the facades and on the floor of the linear atrium, measured at points shown in fig. 7.4. To make the diagrams more readable, the symbols representing different alternatives are spread a little bit from each other. As can be seen in the diagrams, the illuminance distribution in the atrium space can be changed considerably by the glass configuration alone. This phenomenon is due to the fact that the light transmittance of glass varies with the angle of incidence. Winter The horizontal glass sheet obstructed incoming sunlight most of all examined glass configurations, fig. 7.5. Because of the large incidence angle of the sunligh t in winter, most of the sunlight is reflected out, only about 39% is transmitted. Therefore, the use of horizontal glass on the roof caused min. 50% reduction of the sunligh t factor, both on the facades and on the atrium floor. There was nearly no difference between glass sheets sloped by 18° and 30°. Both reduced the direct sunlight penetration by about 8%. These cases caused negligible reduction of the sunlight factor on the facades and on the floor in comparison with the base case, i.e. open atrium. 108 MODEL STUDIES WITH ARTIFICIAL SUN Very interesting results were obtained for the alternative with the negative sloped glass roof. The direct sunlight was reduced only by 5%, which is less than what can be expected because of the normal light transmittance of glass (89%). Sunlight factor Sunlight factor 0.0 1.0 2.0 3.0 4.0 5.0 6.0 1.0 Oj 0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 1.0 0.5 0.0 J I J 1 ! J I I I 1 J 111 J ] III ij ill I | I III | [TTTTJTTTTJ -B „ 1M 1 # -9.90 Positive sloping glass: — best case + gbit, 0 deg + gla»i,16deg o gUsi,30d#g •j®----- # # : i------i------r00 0.00 3.76 7.52 width of the street width of the street Positive Positive Negative U. Horiz. gl. slope gl 18® slope gL 30® u slope gl 35® Figure 7.5 Winter. Sunlight factors in atrium with positive slope glass on the roof to the left and negative slope glass to the right. 0.08 060 1.00 160 260 066 0.00 060 060 160 160 260 0.50 0.00 ("TTT-r-pTTTT'J'TTTT"j"TTTT'J i i | i ri t | | i i i > j ‘ i t i | i i • i j i i i ■ j ‘T‘ 1 1 1 Oq mo. ++ -mo ++ ++N LOc Positive sloping glass: Negative sloping glass: — base case at 12:00 — + glass, 0 deg base case at 12:00 + glass, 16 deg + glass, 0 deg o glass, 30 deg o glass, 35 deg 4§b=—#---- # : I------1------0.00 - i i i' 0.00 3.76 7.52 0.00 3.76 7.52 width of the street width of the street "j ^ Base case I Positive Positive | Negative Honz-gl - I I slope gl 18° u slope gl 30® | slope gL 35® Figure 7.6 Spring/autumn. Sunlight factors in atrium with positive slope glass on the roof to the left and negative slope glass to the right. The size of symbols is connected with the time of the day: 12:00 large symbols, 10:00/14:00 middle size symbols, 08:00/16:00 small ones. Three line types are used for base case: normal one indicates 12:00, dashed 10:00 /14:00, dotted 08:00/16:00. 109 MODEL STUDIES WITH ARTIFICIAL SUN The sunlight factor on the floor and on the lower part of facades was increased by over 50% in comparison with the base case. Such good results are caused by the sunlight that after penetrating the vertical glass sheet was to a considerable degree reflected from the lower surface of the negatively sloped glass, down on the south facing facade. Spring/autumn The changes in the level of atrium illuminance in spring/autumn, fig. 7.6, are similar as in winter, but not so dramatic. The horizontal glass sheet reduced the direct sunlight penetration most of all the alternatives, but the reduction was dependent on the time of day. At 8:00/16:00, when the incidence angle of sunlight was largest, the reduction was about 30%, at noon only 15%. It is due to the low light transmittance of glass for large incidence angles. Consequently, the sunlight factor on all atrium surfaces is mostly reduced at 8:00/16:00, i.e. about 20%. The differences between the results for glass sheet sloped by 18° and 30° are negligible and are caused more by the inaccuracy of the measurements than by the real differences between those alternatives. The best results, the highest illuminances, were obtained for the negative sloped glass sheet, both on the floor and on the lower part of the facades, but the mean increasing of the sunlight factor was not larger than 10%. 0 00 0 50 1 00 0 50 0 00 o oo oio i.oo 0-50 0 00 I ‘ “ 1 I 1 “ * I “ PH* 1 * | j i 1 i r jj i i i | i ! P"‘111 , 13.20 - Positive pitched glees roof: -13.20 Negative pitched glass roof: — base cate at 12:00 — base case at 12:00 © + glass, 0 deg + glass. 0 dog o glass. 30 deg -9.90 o glass. 35 deg # L. (Do -#+*! ------,------1------1-0.00 J T“ 3.76 0.00 3.76 7.52 width of the street width of the street f\_ Positive Positive Negative Base case Hortz. gl. slope gl 18® slope gl 30® slope gL 35* Figure 7.7 Summer. Sunlight factors in atrium with positive slope glass on the roof to the left and negative slope glass to the right. Summer In summer at 8:00/16:00, the only case where the whole south facing facade was sunlit could be observed, fig. 7.7. A large patch of sunlight reflected from the 1st floor window caused excessively high results also on the sensor placed on the floor next to the south facing facade. The horizontal glass sheet and the positively sloped glass sheets gave nearly exactly equal results, i.e. they all caused about 10% reduction, both of the direct sunlight and of the sunlight 110 i i i MODEL STUDIES WITH ARTIFICIAL SUN i factor on atrium surfaces. Because the differences between the results for positively pitched i glass roof sloped by 18° and 30° were negligible, only the results for 30° slope are presented. Totally different changes in the level of atrium illuminance can be observed for the negative pitched glass roof. The reduction of the direct sunlight was over 40%. Nearly equal reduction of the sunlight factor was observed on all atrium surfaces. Discussion The experimental results show the potential of sunlight control in linear atrium using different glass configurations alone. Horizontal glass roof is most problematic. It obstructs most of the direct sunlight coming from low altitudes, i.e. just when it is most desired. For a climate, were there is a large request for sunlight during winter, a horizontal glass roof should be avoided. Sloping the glass by 18° in the positive direction is enough to avoid the sunlight obstruction from low sun altitudes. Positive pitch glass roof with the glass sloping by 18° - 30° is the most attractive roof configuration if the goal is to give free passage for sunlight, without changing the sunlight distribution. The positively sloped glass sheet will reduce the sunlight factor by about 10% on all facades. Photo 7.1. Winter. Changes in atrium illuminance distribution. Base case to the left, negative slope glass roof to the right. The negative pitch glass roof controls the sunlight best. The level of sunlight illumination coming from low altitudes, i.e. winter and 8:00/16:00 in spring/autumn, is increased by about 50%! In summer, the excessive sunlight illumination coming from high altitudes is reduced by min 40%. The negative pitch glass roof is the most suitable one for high latitudes. Most of the different glass configurations cause changes only in daylighting level. The adaptation mechanism in the eye causes that they will not be perceived. Only the negative slope glass causes noticeable changes in daylight distribution, photo 7.1. In winter, the stripes of light reflected from the windows appear on the floor, giving more life to the room. The Ill ! Ill ' .'4: -vi-V MODEL STUDIES WITH ARTIFICIAL SUN modelling property of the light is also increased; it can be observed by comparing the black balls in the modelling sensors on the left side of the atrium. The theoretical background for the modelling sensors can be found in chap. 2. 7.5 Laser cut panels and prismatic panels on the roof Both the laser cut light deflecting panels and the panels with the 3M prismatic film, were supposed to change the direction of the sunlight. Examining of many sloping angles is troublesome and time consuming. It was concluded that two angles should be examined for each product. In the case of laser cut panels, the two most extreme angles were chosen. The Lcp panels sloped by 30° was supposed to deflect sunlight in spring/autumn vertically down to the floor and deflect sunlight to the north facade in winter. Further increasing of the slope will cause the sunlight deflected from the panels in spring/autumn to fall on the south facing facade. The model studies under overcast sky, chap. 6, showed that the laser cut panels ability to transmit diffuse light differs with the slope. Laser cut panels sloped by 30° obstruct diffuse light more than the same panels sloped by 15° The laser cut panels sloped by 18° was chosen as the lowest reasonable sloping. In the case of prismatic panels it was advised by one of scientists from Building Research Establishment to examine 10° sloped panels with the prisms oriented up. The prismatic side had its 78° triangular part on the top and the 62° triangular part on the bottom, if observed from the side, chap. 5. Preliminary experiments with the prismatic panels in the laboratory showed, that the slope angle could be a little larger, but 30° slope seemed to be too large. Finally, 18° slope was chosen as the most reasonable. 10 20 30 50 60 20 10 00 00 10 20 30 40 50 60 1 1 1 ' 1 ,,,,,, 1 "8 " 1 " 1 TjTTTTj-rrrq 13.20 - > 1 -13.20 1 ° \ P Laser cut panels: o — base case -9.90 - o Lcp-18 4 lcp-30 - 30 -6.60 - Prismatic panels: o 1 25 ------base case 20 A PP-10 c 1 -3.30 _ O PP-18 1 ___ o 2 -I 0 O O - 05 jf) ZQ ; O £> 0.00 3.76 7.52 0.00 3.76 7.52 width of the street width of the street Lcp-18° Lcp-30° Figure 7.8 Winter. Sunlight factors in atrium with laser cut panels on the roof to the left and prismatic panels on the roof to the right. 112 MODEL STUDIES WITH ARTIFICIAL SUN 113 MODEL STUDIES WITH ARTIFICIAL SUN Winter The effectiveness of the laser cut light deflecting panels depends strongly on the slope angle of the panels, fig. 7.8. Panels sloped by 18° nearly totally obstruct the direct sunlight. The large increase of the sunlight factor on atrium surfaces, the sunlight factor was more than doubled, was caused by the light deflected from the sloping panel. The absolutely best results were obtained for panels sloped by 30°. The sunlight factor on the atrium floor and on the lower part of the facades was increased about seven times! The prismatic panels sloped 18° worked slightly better than the panels sloped 10°. The sunlight factor on the atrium floor under the 18° sloped panel was doubled in comparison with the base case. Anyway, both slopes gave poorer results than the laser cut panel sloped by 18° on all atrium surfaces. 00 05 10 1 5 20 25 100500 00 05 10 15 20JTTTT, 25 20iMM 15iM 101 05 00 . SP i - a? Laser cut panels: Prismatic panels: 6.60 - & — 6.60 — base case, 12 00 base case, 12.00 o Lcp-18 A PP-10 Lcp-30 ) 0 0 PP-18 $ •■rg I s 001------r~------1 0.00 3.76 7.52 0.00 3.76 7.52 width of the street width of the street IT Base case Lcp-18* Lcp-30 C PP-10” PP-18® Figure 7.9 Spring/autumn. Sunlight factors in atrium with laser cut panels on the roof to the left and 3M panels to the right. Spring/autumn Also in spring/autumn the effectiveness of the laser cut light deflecting panels depended considerably on the sloping angle. The laser cut panels sloped by 18° (Lcp-18°) deflected more light to the north facade and a narrow strip of floor close to the north facade; the Lcp- 30° deflected sunlight nearly exclusively down to the floor. The results vary also with the time during the day. Generally, the laser cut panels gave the best results at noon. Only about 20% of the direct sunlight was transmitted by Lcp-18°, and about 35% by Lcp- 30°, the rest of sunlight falling on the panels was redirected. The sunlight factor on the north facing facade was increased on the average by 25% by Lcp- 18°. Lcp-30° caused a slight reduction of the sunlight factor on the upper part of the north facing facade, up to 20%; and a slightly increasing of the sunlight factor on the lower part, up to 15%. 114 MODEL STUDIES WITH ARTIFICIAL SUN t i j ! ! 115 < i MODEL STUDIES WITH ARTIFICIAL SUN The PP-18° panels distributed the sunlight more evenly on the whole north facade. It more than doubled the sunlight factor on the north facade and on the floor. The results varied considerably with the time of the day. Generally, the prismatic panels gave the best results at noon, a little worse at 10:00/14:00. At 8:00/16:00 the prismatic panels gave sometimes results contrary to the expected, e.g. PP-18° at the middle height of the north facade. It seems that a part of the direct sunlight was scattered outside the building. 0 00 0 60 100 050 000 060 000 aO, Co Laser cat panels: Prismatic panels: — base case. 12 00 — base case. 12 00 0 lcp-18 A PP-10 0 0 kp-30 PP-18 M . o =?rc 4 0S0 1------1------rc I------1----- 0.00 3.76 7.52 0.00 3.76 7.52 width of the street width of the street Figure 7.10 Summer. Sunlight factors in atrium with laser cut panels on the roof to the left and prismatic panels to the right. Summer The laser cut light deflecting panels gave quite different results depending on the sloping angle. Lcp-18° transmitted only about half of the direct sunlight, the rest was deflected down to the floor, causing an increase of the sunlight factor by about 50%. Lcp-30° transmitted much more sunlight than Lcp-18°, about 80%, so the deflected part of the sunlight was negligible. The prismatic panels transmitted less than 50% of the sunlight. The direction of the refracted sunlight depended on the sloping angle. PP-10° increased the sunlight factor mostly on a strip of the floor by the north facade, four times, and on the lower part of both facades, mainly by inter-reflection. PP-I8 0 increased the sunlight factor on most parts of the floor, multiplying it more than three times, only a narrow strip of floor next to the south facing facade was unchanged. The sunlight factor on the upper part of the north facing facade was slightly reduced, mainly because the opposite facade had lower luminance than in the base case. On the floor the sunlight factor was increased on the average five times by Lcp-30°. The results for Lcp-18° varied with the position of the sensor on the atrium section. Near the north facing facade the sunlight factor was increased five times, when on the rest part of the floor the sunlight factor was doubled. 116 I MODEL STUDIES WITH ARTIFICIAL SUN I Upper-left: Laser cut panels sloped by 18° Upper-right: Laser cut panels sloped by 30° Right: Base case Lower-left: Prismatic panels sloped by 10° Lower-right: Prismatic panels sloped by 18° Photo 7.4. Summer. Changes in daylight distribution in the atrium caused by the light deflecting panels. 117 MODEL STUDIES WITH ARTIFICIAL SUN The sunlight factor on the lower part of the south facing facade was more than doubled for both slope alternatives. This was caused mainly by inter reflected light, especially from the floor. The results varied somewhat with the time of the day. At 8:00/16:00 the Lcp panels transmitted more of the direct light and deflected less. The prismatic panels sloped at both angles transmitted only about 20% of the direct sunlight. Like in the case of Lcp panels, the results varied strongly with the sloping angle. Prismatic panels sloped by 10° (PP-10°) increased the sunlight factor mostly on the upper part of the north facade, three times. On the floor the increase was between 20-50%. Discussion Compared to the base case, all alternatives with the light deflecting panels on the roof increased the daylight level in atrium when this was desired. Lcp-30° gave the best results of all alternatives, both in winter and in spring/autumn. In addition to their positive impact on the daylight level in the atrium, PP panels sloped by both angles and Lcp sloped by 18° function as sun shading in all seasons. This is very desirable, especially in summer. The light was often deflected vertically down to the floor, as with prismatic panels sloped by 18°, without causing problems with solar glare. Only Lcp sloped by 30° did not obstruct the direct sunlight considerably in summer. An additional sunshade will be necessary on the south facing facade for that alternative. Overheating can be overcome by opening some parts of the glass roof; this method is with success used in atrium buildings situated at high latitudes. In an atrium without any glass roof, the base case, only the upper part of the south facing facade is directly illuminated, the rest is in shadow. The size of the sunlit part of the south facing facade, that has a crucial significance on the atrium illuminance, differs during the year. The light reflected from it illuminates mostly the upper part of the opposite facade. The floor and lower parts of the facades have the poorest illumination in all seasons. The lower part of the atrium looks gloomy and sad, especially in winter. The usage of light deflecting panels on the roof has often a very positive impact on the lighting quality in the street. The sunlight is redirected down to the floor or to the lower part of the north facade, causing positive changes in the mood of the atrium space. If the whole north facing facade is sunlit, as in PP-180 in spring/autumn, the atrium seems to be more spacious and lively. The modelling property of the light on the floor is also improved. The balls on the left side are a little better visible than in the base case, because the light falling on them is more horizontal. The balls on the right side are nearly perfectly visible as three-dimensional shapes. If only the floor is daylit, as in Lcp-30° in spring/autumn, the atrium seems to be shallower and more inviting. The vertical light falling on the balls light them excessively from the top, giving a clear but not the best modelling. The atrium looks probably best when both the lower part of the north facade and the floor are sun lighted, as in Lcp-30° in winter. The room looks spacious, inviting and lively. The modelling property of light is very good, especially on the south side of the street. By reducing the daylight level under overcast sky and increasing the daylight level under clear sky with sun, both Lcp and prismatic panels cause an increase of the contrast between those 118 MODEL STUDIES WITH ARTIFICIAL SUN two different sky types, something that can be very interesting to experience in the real building. 7.6 Reflectors on the top part of south facing facade and on the neighbouring roof Two types of reflectors were examined in the artificial sun studies: facade reflector and roof reflector. The mirror in the alternative ‘facade reflector ’ was permanently fixed. The alternative ‘roof reflector ’ is based on the assumption that the slope of reflector will be changed with the changes of seasons. Both alternatives were examined in winter and spring/autumn, when they can improve daylighting in the atrium by reflecting sunlight down to the desired zones in the atrium. In summer there is no need for more light. The facade reflector as a fixed building element will reflect sunlight in summer, too. The proportions of the atrium section cause the strip of reflected light to fall on the atrium floor. So the problem with solar glare will not be serious. 0.0 1.0 2.0 4.0 5.0 6 ( 3.0 2.0 1.0 0 0 00 0.5 1.0 14 2.0 24 2.5 2.0 14 1.0 05 0.0 11 m I m i 11 l~i | I i l IJ 11 11 | TTJTTTTJTTTTJTTTTJ 9 □ -13.20 -mo □ Q QJa -9.90 -9.90 Reflectors: o 6.60 Reflectors: tf 6.60- base case ------base case. 12:00 □ facade reflector □ facade reflector O flat roof reflector O O flat tool reflector O 9 04 r- 0.00 -J l-0.00 ooL 1------1------L-0.00 0.00 3.76 7.52 0.00 3.76 width of the street width of the street i— — / r- Reflector Reflector Base case on facade on the roof Figure 7.11 Sunlight factors in atrium with mirrors on the top part of the sunlit facade and on the neighbouring roof. Winter to the left, Spring/autumn to the right. Winter The slope angle of the mirror for the roof reflector alternative was chosen such that the reflected light beam was directed to the north facing facade between the 1st and the 2nd floor window bands,/!#. 7.11. The reflector on the roof multiplied the sunlight factors for base case on the lower part of the north facing facade and a large part of floor next to this facade. Also the sunlight factor on the rest of the floor and on the lower part of the south facing facade was considerably increased by inter-reflection. The results are quite different for the facade reflector. The vertical position of the reflector and nearly horizontal inclination of sunrays caused the reflected light beam to appear on the north facade at the height of the 4th floor windows. The sunlight factor measured on this strip was 119 MODEL STUDIES WITH ARTIFICIAL SUN multiplied by more than 5 times, but the daylight level on the rest of the atrium surfaces was unchanged. Spring/autumn The spring/autumn results for the facade reflector are very similar to the results from winter, fig 7.11. The reflected light beam on the north facing facade moved down to the space between 3rd and 4th floor window bands. Like in winter, the roof reflector sent the light beam to the north facade, between the 1st and 2nd window bands. The sunlight factor measured in that beam area was many times multiplied. The sunlight factor on the lower part of both facades and on the floor was increased by about 50% because of inter-reflections. The rest of the atrium surfaces were unchanged. The facade reflector did not change the light level on atrium surfaces besides the reflected light beam area, were the sunlight factor was multiplied three times. Photo 7.5 Spring/autumn. Facade reflector to the left, roof reflector to the right. Discussion As expected, the reflector placed on the top of the south facing facade gave much better results than the reflector placed on the top part of the facade. The main reason for this is that the first one utilises the sunlight that otherwise would fall on the roof of the neighbouring building, increasing the total light flux coming into atrium. A reflector placed on the top part of the south facing facade gives even lower results than expected. A vertical position is probably not the most advisable. The sunlight reflected from the reflector in winter falls on the windows on the opposite facade, causing solar glare. The use of a reflector placed above the south facing facade caused very positive changes in the light distribution in the atrium. It looks as if a new, linear light source was placed on the north facade between the 1st and the 2nd floor window bands. The dominant vertical direction of daylight was changed to be more horizontal. The atrium looks more spacious and lively, the modelling properties of the light are improved, photo 7.5. If solar glare has to be avoided, the 120 MODEL STUDIES WITH ARTIFICIAL SUN sloping angle has to be accurately adjusted, something that increases the maintenance costs of the building. The light can be also reflected to the floor, were the danger of solar glare is lower. 7.7 Sun shading with laser cut panels on the facades The idea of using laser cut light deflecting panels as sun shading was based on utilisation of the excellent deflection capability of the panels [Edmonds, 92]. In the atrium building, the sunlight deflected from the panels was used to increase the illumination on the atrium floor. The winter tests were not carried out because no sun shading is necessary then, as the elevation angle of the sun is too low. 0X0 0 50 t.00 1.50 2 CO 2.50 1X0 0X0 0 00 0 09 0X0 100 0 50 000 | i 11 i 1111111 i 11 j 111111 i i 11 11111111111 j i I I 1 |l I M | I I rrrrn 13.20- H -13.20 Lcp on facades: — base case. 12.00 o lcp 2.3,4 fl. o lcp-4 fL 1 Lcp on facades: ------base case.12 00 O Lcp.4 H. 1-0.00 —r 0.00 3.76 7.52 7.52 width of the street width of the street Base case Lcp-4 fl. Lcp-2,3,4 fl. Figure 7.12 Sunlight factors in atrium with laser cut panelsfixedover the 4th floor window, alternatively over the 2™*, 3rd and 4th floor windows. Spring/autumn to the left. Summer to the right. Spring/autumn In spring/autumn only the laser cut panels fixed over the 4th floor windows was examined. The windows on the 3rd and 4th floor were not sunlit. As can be seen in fig. 7.12, the laser cut light deflecting panels function very well as the sun shading. If properly inclined, they transmit only about 25% of the direct sunlight. The rest is deflected to the atrium floor, causing three to four times greater sunlight factors in the middle of the deflected light beam. Because of the small size of the panels compared to the atrium dimensions, the changes made by the Lcp panels were small; the sunlight factor on the rest of the atrium surfaces was not changed. Summer To avoid solar glare, the sloping angle of the panels was chosen such that the deflected light was directed to the floor. The panels transmitted ca 50% of the direct sunlight. If stronger shading is desired, the sunlight has to be reflected to the north facing facade. It is also possible to use Lcp with slanting cuts. 121 MODEL STUDIES WITH ARTIFICIAL SUN The results for Lcp on the 4th floor were nearly exactly equal to spring/autumn results. For the Lcp on the 2nd , 3rd and 4th floor, the total level of reflected sunlight was higher, especially on the floor. The sunlight factor was increased not only in the deflected light beam, but on the whole floor. Photo 7.6. Spring/autumn. Base case to the left, laser cut panels over the 4,h fl. window to the right, 8:00/16:00 Photo 7.7. Summer. Base case to the left, laser cut panels over the 2nd , 3rd and 4th fl. window to the right, 8:00/16:00 122 MODEL STUDIES WITH ARTIFICIAL SUN Discussion The effectiveness of the panel as sun shading elements depends on the sloping angle. In summer, the panels could function better if they were hinged upwards, but the reflected light would fall on the north facade. To avoid solar glare in the rooms on the opposite side, the inclination angle had to be exactly adjusted something that can be troublesome in a real building. A better solution is to change the design of panels by reducing the distance between cuts or by changing the cutting angle. More light could be deflected, less transmitted. The use of the laser cut light deflecting panels as sun shading has a very positive influence on the daylighting quality in the atrium. In spring/autumn, the placing of Lcp over the 4th floor window was enough to change the overall mood of the room, photo 76. The atrium looks nicer and more inviting than in the base case. In summer, the Lcp produced bright and soft light stripes on the floor; the room was nicer. The modelling property of light was also better than in the base case, see the modelling sensor on the left side of the atrium, photo 7.7. 123 RADIANCE SIMULATIONS J ...... ' The Radiance data simulation program is a rendering system that was developed at Lawrence Berkeley Laboratory in California and Ecole Polytechnique Federale de Lausanne in Switzerland. It is described by the author Gregory J. Ward as “a physically based rendering system...which blends deterministic and stochastic ray-tracing techniques ” [Ward, 94], The Radiance program was used in this study for the following reasons: —> it is able to handle complicated geometry and a huge amount of surfaces -> it supports a variety of reflection models, —> it has the ability to model unconventional materials, such as prismatic panels, light deflective panels, and materials with combined transmission and specular reflection, —> it has the ability to model any sky luminance distributions using mathematical models. The program was used in order to expand the parameter studies of daylighting systems examined earlier in the physical scale model. This enabled increasing the precision in finding the best design for each daylighting system. Some new systems, e.g. specular-reflective light shelves, were examin ed in addition to the earlier ones. It was also possible to examine the solar glare and to estimate the luminance distribution. In order to evaluate the modelling ability of light, three small balls: white, grey and near black, exposed on a white background, were used as modelling sensors. 8.1 The data model The Radiance data simulation program gave the opportunity to make the atrium building model more like a real building. The length of the model for data simulations was extended to 11 units; this corresponds to 90.4m for a real building. The thickness of the wall between the atrium and the adjacent rooms was reduced to 10 mm, i.e. 200 mm in a real building, the windows were simulated as a double glazing fixed in 80 mm thick and 50 mm high frames, fig. 8.1. The size of the glazing areas was not changed. Figure 8.1 A fragment of section through the physical model to the left and the data model to the right. The glass in windows situated in the wall between the atrium and the adjacent rooms was modelled as the Radiance ‘glass’ material of 0.78 transmittance, i.e. equal to double glazing. The glass on the roof was modelled as two layers of glass, each with a transmittance of 0.87, placed on the roof with a 20 mm cavity. A layer of prismatic panels or a layer of light deflective panels was placed between those. 140 RADIANCE SIMULATIONS Each opaque surface was modelled using the Radiance ‘plastic ’ description. The Radiance definition specifies five material properties i.e. red, green and blue (RGB) colour reflection values, specularity and roughness. Because the reflectance of each surface was of main importance, and many of the surfaces in the physical model had a colour from the white-black scale, grey scale rendering was acceptable. The reflectances for each surface used in model studies were measured and used in the Radiance simulations. The specular material used in reflectors was simulated using ‘metal’ description. The RGB reflectances were set equal 0.9, specularity to 0.9 and roughness to 0. In order to define the reflective/transmissive material used in the canopy in the middle of the atrium, the function ‘BRDFfunc ’ was used. This function gives the maximum flexibility over surface reflectance and transmittance, providing spectrally dependent specular rays and reflectance and transmittance distribution functions PEA, 94b]. The computer simulations were carried out on a 166 MHZ personal computer with 64 Mb RAM and the LINUX operative program. In order to reduce rendering time most of the pictures were parallel rendered using the CRAY T3E machine. The images were divided into many small pieces; more pieces than there were processors. If one piece takes longer than the others, the processors that had easy pieces are not all waiting for the processor with the difficult piece to finish, but start the calculation of the next piece. The pieces were put together at the end of the process. The parallel rendering reduced the calculation time from 2-3 weeks on a PC to maximum 24 hours on CRAY T3E. 8.2 Automatization of the calculation process The calculation process was totally automated. The method elaborated by Raphael Compagnon was used [Compagnon, 94]. The organising of the calculation process is shown in fig. 8.2. Project directory ------Lpgm variant 1 variant 2 Figure 8.2 The scheme for the project organising structure. A clear structure for the main project directory enabled easy use of programs generating automatization. The files defining sky types were placed in the .skies directory, programs generating automatisation processes in the .pgm directory. Files defining atrium surfaces that were common for all alternatives, were placed in the common directory. The files defining surfaces specific for a given variant, e.g. variant 1, were placed in directory of the same name as that variant, i.e. variant 1 directory. All results for a given variant were automatically placed in the same directory. 125 RADIANCE SIMULATIONS It was possible to produce many types of calculations for each alternative: illuminance could be calculated in desired points, several types of pictures could be created, solar glare could be calculated etc. The automatization method made it possible to work systematically, accurately and to avoid mistakes, which were easy to make due to the large amount of variables and options accessible for most of the programs included in the Radiance package. 8.3 Accuracy of calculations Since both, the calculation time and the simulated results are dependent on rendering parameters [Jarvis, 97], a test was made to compare the experimental and the calculated results. The main goal was to estimate a reasonable number of indirect bounces, i.e. the -ab parameter that has the strongest impact on the calculation time. Daylight factors were calculated on the atrium surfaces under CIE overcast sky for different numbers of indirect bounces and compared with the results from the model studies. The atrium building was without glazed roof. For two indirect bounces the daylight factor values on the lower parts of facades and on the floor were about 13% and 5% lower than experimental ones, respectively. For three indirect bounces the differences were reduced to 6% and 4%. For five indirect bounces the daylight factor values on the lower parts of the facades were 2% higher than the experimental ones, on the floor 1% lower. Since the accuracy of measurements in the model studies was about 5%, and it was necessary to finish the calculations in a reasonable period of time, three indirect bounces were chosen for all calculations. 8.4 Reflectors on the neighbouring roof Since the flat specular reflector gave the best results in the model studies, it was reasonable to carry out the parameter study for that system. In order to obtain accurate results, the reflector was placed symmetrically on both sides of the atrium on top of the neighbouring buildings. Calculations of the daylight factor on atrium surfaces were repeated for both sides of the atrium, i.e. north and south. The mean value for each height was calculated and is presented in the tables 8.1, 8.2 and 8.3. Two parameters were examined: • the width of the reflector varied from 2 to 4m, • the sloping angle a varied from 50° to 70°. Base case o=50° 0=54° 0=58° 0=62° o=66° 0=70° DF% +/-% +/-% +/-% +/-% +/-% +/-% floor: in the middle: 43.15 +0.21 +1.14 +1.12 -0.43 -0.42 facade: lower part 21.34 +0.23 +0.32 +0.57 +0.58 +0.51 +6*92 upper part 47.67 +6.11 +6.17 +6.98 +6.58 +5.59 Table 8.1 Changes of daylight factor on atrium surfaces caused by a linear 2m wide roof reflector. 126 RADIANCE SIMULATIONS Base case 0=50° 0=58° 0=62° 0=66° 0=70° DF% +/-% II +/-% +/-% +/-% +/-% floor: in the middle: 43.15 +0.32 +1.12 40.91 +1.08 +0.70 facade: *0 lower part: 21.34 40.73 rwi +0.81 40.65 +0.82 °° upper part 47.67 +7.64 +8.37 +8.94 +9.30 +8.90 Table 8.2 Changes of daylight factor on atrium surfaces caused by a linear 3m wide roof reflector. Base case 0=50° 0=54° 0=58° 0=62° 0=66° o=70° DF% +/-% +/-% +/-% +/-% +/-% +/-% atrium floor: in the middle: 43.15 +0.84 +1.19 +1.33 ; -tam : +0.98 +1.08 facade: lower part 21.34 40.84 +1.15 !...+LZ7 : +1.13 +1.10 +0.56 upper part 47.67 +9.09 +10.51 +10.33 +10.93 +10.10 +8.96 Table 8.3 Changes of daylight factor on atrium surfaces caused by a linear 4m wide roof reflector. The results for the lower part of the facades, presented in tab. 8.1-8.3, correspond to the middle of the upper part of the 1st floor window; the results for the upper part of the facades correspond to the middle of the upper part of the 4th floor window. The results for the atrium floor and the lower part of the facades are rather low and lie within the boundaries of accuracy; they are all smaller than 5%. The best results for the floor and for the lower part of the facades are found for the slope angles 54-62° for all reflector widths. vert. D.F 20.00 30.00 40.00 50.00 60.00 13.20 — Daylight systems: ------base case —^— roof reflector —Q— facade reflector 1.60- roof and facade reflector 50.00 - 40.00 D.F. 0.00 -J 30.00 3.76 width of the street "5 —ZT X—7 Roof Base case Facade Roof and reflector reflector facade reflector Figure 8.3 Changes in the atrium caused bythelm wide facade reflector and the 4m wide roof reflector. 127 RADIANCE SIMULATIONS The alternatives can be evaluated more accurately on the basis of the results for the upper part of facade. A sloping angle of 62° is optimal for all reflector widths. It corresponds to the main section angle of the atrium a, defined as tan(a) = (facade height)/(floor width), that is equal to 61.7° for the data model. The daylight factor on the atrium surfaces increases with the width of the reflector from about 7% for 2m wide reflector to 11% for 4m wide reflector. The results depend strongly on the specular reflection property of the reflectors. An additional small series of simulations were done to examine the impact of the reflection propriety of the reflectors. Increasing the reflection factor of the 62° slope roof reflectors from 0.90 to 0.95 caused an additional increase of the daylight factor by: —> 2% for 2m wide reflector, —> 3% for 3m wide reflector, —» 5% for 4m wide reflector. A specular reflection factor larger than 0.95 is not realistic in a real building. The results for the 4m wide roof reflector and the lm wide facade reflector are presented in fig. 8.3. They were calculated for a reflection factor of 0.9 for both reflectors. The results show the difference in the way the top reflector and the facade reflector function. The roof reflector increased the daylight level mostly on the upper part of the facades. It is due to the fact that the roof reflector changes the luminance distribution primarily for a view from the top floor. An observer on the 4th floor, instead of seeing the lower part of the sky of relatively low luminance, sees the roof reflector with the brightest part of the sky reflected in it. Increasing the width of the reflector causes an increase of the area of the reflected sky in the visual field. Since the roof reflector increases the daylight factor mostly on the upper part of the facades, the window glazing area on the top floors can be reduced so that more daylight can be reflected from the opaque part of facades downwards. The change of the reflection pattern of the opaque part of the facades from diffuse to specular, i.e. by using of the facade reflector, is a good method for increasing the daylight factor in the lower part of the atrium. From fig. 8.3 results that if both reflectors are used, the total increase of the daylight factor is a sum of the gains due to each reflector. 8.5 Glass roof as a light conductor Since both the prismatic panels and the light deflecting laser cut panels used on the roof gave promising results in the artificial sun simulations, it was reasonable to simulate the same panels also with the Radiance program. Figure 8.4 Design of the glazed roof. 128 RADIANCE SIMULATIONS Unlike the model studies, where the panels were placed directly over the atrium building model, the panels in the data model were placed between two layers of glass in an accurately simulated glazed roof. The glazed roof had a main load bearing structure made of cylindrical tubes, fig. 8.4, and a secondary structure made of rectangular profiles. Both the main and the secondary structure were simulated with white diffuse surfaces of 0.8 reflection factor. Different shapes of glazed roof with and without light deflecting panels were examined using both the CIE overcast sky model and seven models of a clear sky with sun. The clear sky models were created using the same sun positions that were used in the model studies, tab. 7.1. The values of the total and the direct illuminance for the respective sun positions are shown in tab. 8.4. The solar daylight factor in a given point was calculated as the ratio between the illuminance at this point and the total illuminance from the sky on an unobstructed horizontal plane outdoors. Season Time Azimuth Altitude total illuminance direct illuminance lux lux Winter 12:00 0° o 8 622.6 4032.4 9 Spring/ 12:00 0° w 41 445.2 32 917.0 Autumn 10:00/14:00 30° 27° 36 942.8 28 870.4 00 1-4 8:00/16:00 60° 22 382.4 16 006.8 Summer 12:00 0° 50° 66 845.6 56 504.1 10:00/14:00 30° 64744.6 54501.3 8:00/16:00 60° 40° 55 207.6 45 549.3 Table 8.4 The values of the total and the direct illuminance for the respective sun positions. Different shapes of the glazed roof The results of daylight factor calculations for overcast sky, fig. 8.5, for an atrium covered with different shapes of glazed roof show that the reduction of the DF due to the roof depends on the roof shape. The double pitched glazed roof obstructs daylight most, the DF was reduced on all surfaces to about 50% - 60% of values for the atrium without roof. The single pitched roof obstructs daylight least, the daylight factor was reduced to about 60-65% of the value for an uncovered atrium. It is probably due to the fact that the large number of structural elements in the single pitched roof is situated to the side of the atrium opening. It is possible to observe an asymmetry in results for the single pitched roof. The DF values on the facade lying below the high end of the sloped glass are slightly higher than tire DF values on the opposite facade. 129 RADIANCE SIMULATIONS vert. DF ).0 20.0 40.0 40.0 20.0 0.C | i i i i j i i i i [ i i i | 'itti | ( i ( i | Glazed roofs, -13.20 overcast sky: double pAched 30 deg —O-* single pitched 30 deg —Q— single pitched 35 deg -9.90 ------no roof •oO 9.90« OC8 a o O 6.60 • 3.30- 0.00 1-0.00 floor single pitched single pitched double pitched no roof positively sloped negatively sloped 30° 30° 35° Figure 8.5 Daylight factors in atrium covered by different shapes of glazed roof. Overcast sky. Laser cut panels: A comparison of daylight factors under overcast sky, fig. 8.6, for glazed roof sloped by 24° with and without laser cut panels shows, that the panels reduce the daylight factor on the average by about 20%. The obstruction increases with increasing of the sloping angle and varies from about 17% for 18° sloped roof to 26% for 30° sloped roof. Without any special daylight system on the overhead glazing, a double pitched roof would be preferred in most cases, for practical reasons. The change of the roof shape from double pitched to single pitched causes an increase of daylight factors by minimum 15%, especially on the facades. This positive change is cancelled out by the negative change caused by the panels. The simulations show that the daylight factor values for the single sloped glazed roof with LCP are nearly equal to the daylight factor values for the double pitched roof without panels, especially on the facades. The way the laser cut panels function on a sunny day in winter can be observed in fig. 8.7. The direct sunlight falling on the top part of the south facing facade is strongly reduced because the sunlight is redirected to the opposite facade. The level of daylighting on the north facing facade depends on the sloping angle of the roof. The 30° slope is most efficient in winter. The solar daylight factor on the floor and on the lower part of the south facing facade is doubled. On the lower part of the north facing facade the solar daylight factor is multiplied seven times. 130 RADIANCE SIMULATIONS vert. OF 0.0 10.0 20.0 30.0 30.0 20.0 10.0 0.0 11111 11111111 pTTTJTTTTJTT n 13.20- -13 JO 01 9.90 ■ Roofs with lessr cut panels -9 JO 1 overcast sky: s □ LCP, 18 dig o> A LCP.24 dig o 6.60 • o LCP, 30 dig -6.60 cs — no LCP.24 dig — bo LCP, dbLpttcb, 30dtg 3 JO 1 8 -3.30 0.00 J —I— l-0.00 0.00 3.76 7.52 floor no LCP no LCP single pitched Lcp-18 0 Lcp-24 6 Lcp430° double pitched 24° Figure 8.6 Daylight factor in atrium covered by a glazed roof sloped by different angles. Overcast sky. Solar DF 0.0 50.0 100.0 150.0 200.0 100.0 50.0 0.0 p''l I I I I I I I'I'i I I I I I I I I I | i i i i | i i i i | -13.20 -9.90 •6.60 -3.30 ^0.00 Figure 8.7 Solar daylight factor in atrium covered by the glazed roof with laser cut panels. Winter. RADIANCE SIMULATIONS Solar DF 0.0 50.0 50.0 0.0 |—i—i—i—r- i—i—|—i—i—«—j—| (p |-13.20 () ■\ o j-9.90 Roots with Icp •prlng/autu mn: o -B- kp,18 dig '() I-6.60 kp.24 dag -©- kp. 30 dig | no kp 24d.g 0 3=6 1 00 I -a 0.00 J T l-o.oo 7.52 Figure 8.8 Solar daylight factor in atrium covered by the glazed roof with laser cut panels. Spring, 12:00. LCP-18” LCP-24° LCP-300 Figure 8.9 Solar daylight factors in atrium covered by glazed roof with laser cut panels. Spring/autumn. The size of symbols correspond with the time of the day, 12:00 - large symbols, 10:00/14:00 - middle symbols and 8:00/16:00 - small symbols. On a sunny day in spring/autumn at noon, fig. 8.8, the sunlight is effectively redirected. It seems that both LCP-30 and LCP-24 give very good results. The variation of effectiveness of the panels during the day can be observed in fig. 8.9. Each of the three small diagrams is a diminished version of the diagram shown in fig. 8.8. The diagrams show very little difference between the daylight factor values at 12:00 and 10:00/14:00, and a considerable reduction in effectiveness of the panels at 8:00/16:00. 132 RADIANCE SIMULATIONS Solar DF 0.0 50.0 50.0 0.0 I ‘ 1 ‘ ' I ■ ■ 1 ' I 1 13.20 -13.20 <0 9.90 -9.90 Roofs with Icp o 6.60 - 6.60 Icp. 30 dag no Icp. 24 dag - 50.0 -3.30 0.00 Figure 8.10 Solar daylight factors in atrium covered by glazed roof with laser cut panels. Summer, 12:00. LCP-18° LCP-24° LCP-30° Figure 8.11 Solar daylight factors in atrium covered by glazed roof with laser cut panels. Summer. The size of symbols correspond to the time of the day, 12:00 - large symbols, 10:00/14:00 - middle symbols and 8:00/16:00 - small symbols. On a sunny day in summer the elevation angle of the sun is about 50° at noon. For panels sloped by 30° the incidence angle of sunlight is only 10°, i.e. the direction of sunrays in the material is nearly parallel to the cuttings. The result is that most of the sunlight goes through the panels without being deflected. Since the main strategy in summer is shading of sunlight, laser cut panels sloped by 18° are the best choice in summer, sloped by 30° the poorest. The variation of effectiveness of the panels during the day can be observed in fig. 8.11. The ■ diagrams show very little difference between the daylight factor values at 12:00 and 10:00/14:00. At 8:00/16:00 sunlight penetrates to the whole south facing facade. 133 RADIANCE SIMULATIONS Winter, 12:00 Spring/autumn, 12:00 Summer, 12:00 Picture 8.1 Radiance images of the atrium covered by 24° slope glazed roof with laser cut panels for different seasons. The south facing facade is on the left side. The visual impression of daylight in a linear atrium building covered by a glazed roof with laser cut panels sloped by 24° can be observed in pic. 8.1. The border between sunlight and shadow is very sharp. Real panels create some spreading of deflected light, so the borders between shadow and light are softer. Since the exact measurements of light reflection/transmission through the material are not accessible yet, the data program simulating laser cut panels simulates the ideal material without spreading of light. Anyway, the pictures give very realistic images of a real building and clearly show how the daylight changes during a year. Pic. 8.2 show the same building with three different slopes of the glazed roof on a sunny day in spring/autumn. The direct sunlight deflected down to the lower part of the atrium should be very useful, both for people and plants situated in the atrium space, but the direct penetration of sunlight to the adjacent rooms can cause glare. Sun shading devices should be used to control it. Is shading necessary in adjacent rooms where occupants may look at the sunlit facade on the opposite side? In order to find the answer, a Radiance fish-eye picture was created, pic. 8.3. It was chosen a check in winter season at 12:00 and a room on the 4th floor lying behind the south facing facade, pic. 8.2. The visual comfort probability (VCP) based on the Guth formula, chap. 2, calculated for this room was only 33%. Since the sunlight illumination during winter is low, it is reasonable to expect, that the glare caused by excessively bright opposite facade will also occur in other seasons. Sun shading will be necessary not only in rooms where sunlight penetrates directly, but also in rooms on the opposite side. 134 RADIANCE SIMULATIONS LCP-18, 12:00 LCP-24,12:00 LCP-30,12:00 Picture 8.2 Radiance images of the atrium building in spring/autumn covered by sloped glazed roof with laser cut panels sloped by 18°, 24° and 30°. South facing facade is on the left side. Picture 8.3 Radiance image ofthe4th floor room adjacent to atrium building covered by 24° sloped glazed roof with laser cut panels Prismatic panels Since the daylight factors from the model studies of an atrium covered with prismatic panels were considerably lower than for laser cut panels, Radiance simulations were based on a different strategy, sunlight was directed only to the south facing facade or to the floor. Similarly to Radiance simulations with the laser cut panels, the panels were placed between to layers of glass. The prisms were oriented up, chap. 5. The prismatic side has its 62° triangular part on the top and the 78° triangular part on the bottom, if observed from the side. A preliminary simple calculation of sun rays travelling through the panel, based on Snell ’s law and a total reflectance principle, led to the conclusion that prismatic panels oriented this way will send sunlight to the south facing part of building for sun angles between 10° and 50°. 135 RADIANCE SIMULA TIONS Solar DF 0 0 50.0 100.0 100.0 50.0 13.20 - m j Q [] A - Roofs with prism, panels o> 9.90 - I];A -9.90 I n/i -a- PP. t8 dag -A- 45 PP. 24 dag o> ; I -8- PP. 30 dag c no PP. 24 dag o 6.60 - :i A - 6.60 :? j t' 3.30 - i Id -3.30 I1! Id 0.00 J I —I— 1-0.00 0.00 3.76 floor prism, panels prism, panels prism, panels No prism 24° 18° 24* 30° Figure 8.12 Solar daylight factors in atrium covered by differently sloped glazed roofs with prismatic panels placed between two layers of glass. Winter, 12:00. The daylight distribution in winter is shown in fig. 8.12. The sunlight, instead of falling on the narrow top part of the facade, is evenly distributed to the whole south facing facade and partly to the floor for 24° and 30° sloping. The roof sloped by 30° was most effective. The vertical solar daylight factor on the lower part of the south facing facade was multiplied six times. The effects of prismatic panels in spring/autumn are similar to the effect in winter, fig. 8.13. The 24° sloping is most effective, the vertical solar daylight factor on the lower part of the south facing facade is multiplied five times. From fig. 8.14 results, that the effectiveness of prismatic panels changes with the change of sun position during a day. The performance of panels at 08:00/16:00 can be reduced up to 50% of the performance at noon. 136 RADIANCE SIMULATIONS Solar DF 0.0 50.0 50.0 1.0 j i i—i i j i—i c—t—|—i—r -13.20 [] A© ffi 9.90 - -9.90 Roof with prism, panels sprfng/autumn: • ■ Q— PP,18dig o 6.60 • -6.60 —A— PP, 24 dag —©— PP. 30 deg ------no PP, 24 dsg -3.30 0.00 J Lo.00 Figure 8.13 Solar daylight factors in atrium covered by differently sloped glazed roofs with prismatic panels. Spring/Autumn. I 1 1 1 1 I PP-24 PP-30 Figure 8.14 Solar daylight factors in atrium covered by differently sloped glazed roofs with prismatic panels. Spring/Autumn. In summer, fig. 8.15, the prismatic panels function more as sun shading devices. At noon, the glazed roof with prismatic panels sloped by 24° reduce the solar daylight factor on the upper part of the south facing facade to about 40% of the value for the same roof without panels. The Radiance simulations for the summer day, fig. 8.16, show that the way the prismatic panels work is strongly dependent on the incidence angle. The glazed roof with prismatic panels sloped by 30° reduce the solar daylight factor to 10% of the reference value at noon, but already at 10:00/14:00 the sun shading potential of the roof is strongly reduced. Since overheating in atrium buildings due to sunshine increases with the elevation angle of the sun, a thermal analysis should be done to predict if the sun shading due to the prismatic panels is sufficient 137 RADIANCE SIMULATIONS Solar DF 50.0 l"‘ 1 11 I 13.20- G) 5] ;□© -13.20 o i] o 9.90 ■ O 43 ;ii. -9.90 T3 Roof with prism, ptnols m summer: () i: -e- PP. 16 dig PP. 24 dig o 6.60 - ^ ) [ ] PP.30 dag IB* -6.60 noPP, 24 dig H % 3.30 111 0.00 -> ^0.00 Figure 8.15 Solar daylight factors in atrium covered by differently sloped glazed roofs with prismatic panels. Summer at noon. Figure 8.16 Solar daylight factors in atrium covered by differently sloped glazed roofs with prismatic panels. Summer day. If the main goal in the design process is daylight enhancement, the 24° sloping should be chosen. This alternative gives a huge increase of daylight on the south facing facade, pic. 8.4, and a reduction of daylight on the north facing facade. The calculations of visual comfort probability, pic. 8.4, show that solar glare can occur in all seasons. With additional sun shading on the lower part of the windows and light deflecting device on the upper part, sunlight can be reflected to the ceiling, creating very good daylighting in adjacent rooms. The main advantage of this strategy is that confusion about the sun position on the sky or orientation of the building is avoided, sun is shining always on the south facing facade. At the same time, the penetration of sunlight is deeper and more even. A sloping angle between 24° and 30° should be chosen, depending on the design objectives. The disadvantage of this strategy is poor daylighting of the north facing facade. 138 RADIANCE SIMULATIONS Winter, 12:00 Spring/autumn, 12:00 Summer, 12:00 VCP=74.6% VCP=38.8% VCP=55% Picture 8.4 Upper: Rradiance images of the linear atrium covered by 24° sloped glazed roof with prismatic panels, south facing facade is on the left side. Lower: 4,h floor room adjacent to the atrium from the north. 139 RADIANCE SIMULATIONS 8.6 Atrium space and facades as a light reflector/conductor Since the Radiance program gave the possibility to simulate a ‘real’ glazed roof, daylight systems from this group were simulated in the roof covered atrium. A double pitched roof, sloped by 30° was chosen as a very common one, fig. 8.17. As with the scale model simulations under artificial sun, the main axis of the building was oriented in the east-west direction. 1-shelf Figure 8.17 Section through the atrium building with all the daylight systems placed in the atrium space and on the atrium facades. The daylighting systems described in this chapter are placed mostly at the height of the first floor. The proportions of the atrium section cause that in the theoretically indefinitely long linear atrium, the direct solar radiation will nearly never fall on those systems. In a real building, a linear atrium ends with gable walls where sun shading systems should be fixed if solar glare is to be avoided. Since all systems described in this chapter, i.e. 8.6, reflect diffuse skylight, only the CIE overcast sky model was used in simulations. All systems were placed symmetrically in the atrium. Daylight factors were calculated on the atrium floor, atrium facades and inside the adjacent room in nearly the same points as in physical scale model. The calculations were made for both sides, north and south. The mean values, calculated using the results from both sides for each height point and width point, are presented in tab. 8.5 - 8.8. 140 RADIANCE SIMULATIONS 8.6.1 M-fac As in the scale model studies, chap. 6, a strip of specular reflector was fixed to the facades between window bands on the 1st and the 2nd floor. The width of the reflector was lm, i.e. the same as in the physical model, the sloping angle varied from 0° to 30°. Sloping angle in relation to vertical base case o=0° ct=10° o=15° o=20° 0=25° o=30° facade a DF% +/-% +/-% +/-% +/-% +/-% +/-% atrium floor: lm from facade 23.21 +22.8 +41.8 +41.7 +35.2 +13.9 +11.7 middle of the atrium floor 26.13 +11.3 +26.7 +22.5 +20.7 +14.2 +9.9 facade: lower part of Is1 fl. window 10.43 +26.6 +29.2 +28.1 +36.0 +35.4 +22.1 upper part of 1st fl. window 11.32 +23.1 +25.6 +24.5 +31.4 +31.1 +16.3 adjacent room: 1.00 m from the window 4.01 -0.7 -0.2 -2.0 +16.2 +1.0 2.50 m from the window 1.75 -0.6 +1.1 +0.5 +43.4 +25.1 4.00 m from the window 1.05 -2.9 -3.8 -5.7 +20.9 +11.4 5.50 m from the window 0.72 -2.7 -1.4 -1.4 +22.2 +22.2 Table 8.5 Changes of daylight factor in atrium building caused by M-fac. The specular reflector M-fac reflects more sky light to the atrium floor than the diffuse facade does. The results of Radiance calculations show that the M-fac reflector increases the daylight factor on the atrium floor. The amount of light reflected to the lower part of the opposite facade depends on the sloping angle. On the other side, the sloped reflector shades the window lying beneath it. The best results for rooms adjacent to the atrium were found for 20° sloping. The daylight system M-fac could function more effectively if the shading of the 1st floor window could be avoided by moving the reflector in a horizontal direction into the facade construction, this would change the section of the building. The M-fac system can cause strong luminance contrasts in the atrium space, pic. 8.6, therefore it is preferred to use it on walls that has rather high reflectance. As can be seen by comparing the balls in pic. 8.5 and pic. 8.6, the M-fac system does not considerably change the modelling ability of the light. 8.6.2 M-atr Two parameters were varied for the system M-atr, the sloping angle: 45° and 60° and the height over the atrium floor: 2.3-3.2m, see chap. 5. The M-atr system was simulated as two specular-reflective, sloped, lm wide, surfaces placed as a canopy in the middle of the atrium space at the height of 2.30m, i.e. the same height as in the physical model. For 45° slope, the canopy was also simulated at the height of 2.50m and 2.70m. Increasing the height did not increase the illuminance in the adjacent rooms. This confirmed the correctness of the M-atr system design used in the model studies. The modelling ability of daylight in the atrium space with the M-atr system can be observed in pic. 8.9. In the area beneath the system it is impossible to see the white ball, both grey balls appear as a flat disks. The modelling ability of the light is thus rather poor. In zones near the facades the modelling ability of the light is as good as in the base case. 141 RADIANCE SIMULA TIONS Picture 8.5 Base case: atrium without systems. Picture 8.6 M-fac. 142 I I 143 | RADIANCE SIMULATIONS In order to improve the modelling ability of the light in the atrium beneath the canopy, the material of the reflector was changed to specular-reflective/transsmissive; i.e. the transmission factor was increased to 10% and 20%. The specular-reflection factor was reduced respectively to 80% and 70%. As expected, the modelling ability of the light beneath the canopy was improved, pic. 8.10. At the same time the daylight factors in the adjacent rooms were reduced, compared to the best M-atr system. Increasing the reflector area could help to maintain equally high daylight levels in the adjacent rooms. In a real building, a semi-specular surface can be made of perforated metal plate or a glass sheet covered by a highly reflective film. Height over the atrium floor H, base case, H=2.3m H=2.5m H=2.7m H=2.3m H=2.3m transmission 1 and specular DF% 1=0 t=0 1=0 1=10% 1=20% reflection p. p=90% p=90% p=90% p=80% p=70% +/-% +/-% +/-% +/-% +/-% atrium floor: lm from facade 23.21 +2.9 +14.2 +12.4 +11.8 middle of the atrium floor 26.13 -81.9 -72.6 -70.7 -62.4 facade: lower part of 1st fl. window 10.43 +19.6 +24.7 +20.8 +36.6 +29.2 upper part of 1SI fl. window 11.32 +22.2 +33.8 +20.0 +38.1 +31.4 adjacent room: 1.00 m from the window 4.01 +6.9 +6.9 +2.9 +1.5 2.50 m from the window 1.75 +14.3 +13.7 +7.4 +6.3 4.00 m from the window 1.05 +20.9 +16.2 +15.2 +13.3 5.50 m from the window 0.72 +41.6 +34.7 +34.7 +29.2 Table 8.6 Changes in illuminance caused by specular reflectors placed in the middle of the atrium space as a canopy, sloping angle 45°. Since the width of the canopy was the most critical factor for the modelling, the horizontal width of the canopy sloped by 60° was decided to be the same as for 45° slope, i.e. 1.41m. In this way the reflector area was increased by 41% compared to the 45° slope. The best results were found for a height of 3.1m above the floor. The modelling ability of daylight beneath the canopy was not improved by increasing of the canopy height, see balls in pic. 8.8 and 8.9. Height over the atrium floor H base case H=2.3m H=2.5m H=2.7m H=2.9m H=3.1m H=3.2m DF% +/-% +/-% +/-% +/-% +/-% +/-% atrium floor: lm from facade 23.21 +21.3 +20.8 +17.7 +16.4 +ii.i +9.9 middle of the atrium floor 26.13 -76.9 -74.1 -72.7 -70.8 -68.9 -67.4 facade: lower part of 1st fl. window 10.43 +78.9 +69.4 +61.3 +58.8 +54.4 +44.8 upper part of 1st fl. window 11.32 +35.9 +44.5 +56.7 +58.9 +66.2 +58.4 adjacent room: 1.00 m from the window 4.01 +33.2 +30.4 +31.7 +27.2 2.50 m from the window 1.75 +25.7 +14.3 +14.8 +8.6 4.00 m from the window 1.05 +4.8 +26.7 +45.7 +57.1 5.50 m from the window 0.72 +29.2 +65.3 +69.4 +54.2 Table 8.7 Changes in illuminance caused by the specular reflector placed in the middle of the atrium space as a canopy, sloping angle 60°. 144 RADIANCE SIMULATIONS The results of the daylight factor calculations for the most promising systems from the M-atr group are presented in fig. 8.18 and 8.19. All systems reduce the daylight factor on the atrium floor dramatically, i.e. to 1/3 of the value for base case. The M-atr-45 reduces daylight the most The results on the atrium facades are best for M-atr-60. An increase of daylight factors on the 1st floor of over 50% can be noticed. vert. D.F 0.10 0.20 0.30 13.20 — Daylight systems: base case ■X— M-atr, 45 9.90 — -0— M-atr, 45 deg, t=20% 6.60 — 3.30 — 0.20 D.F. 0.00 —1 3.76 width of the street Figure 8.18. Changes in the atrium caused by the daylight systems placed in the middle of the atrium. Daylight systems: M-atr, 45 deg, 1=20% — 4.0 — 4.0 her. D.F. 2.55 Z—j— - 2.0 - 2.0 vert. D.F. L- 0.0 4.00 2.01 Section 60 deg Figure 8.19 Changes in the adjacent room caused by the daylight systems placed in the middle of the atrium. 145 RADIANCE SIMULATIONS The system M-atr-60 gave the best results also inside the adjacent rooms, causing a 70% increase of the daylight factor on the horizontal plane in the rear zone of the rooms. The systems M-atr-45 and M-atr-45-T, with the partly transmissive material, increase the daylight factor on the horizontal plane by 40% and 30%, respectively. The results for the room walls are somewhat lower. The good results for system M-atr-60 are probably caused by the fact that for this type of reflector the area of specular-reflective material is the largest All M-atr systems gave very promising results and can be recommended for use in real buildings. The system with the partly transmissive reflector is very interesting, because it maintains the visual contact with the sky and the upper part of the facades. 8.6.3 M-shelf and M-curved The daylight system M-shelf was not simulated in the model studies. It is just a strip of a flat specular reflector fixed to the facades between the lower and the upper part of the 1st floor window. The width of the reflector is 1m, i.e. the same as the width of M-fac, the slope relative to the horizontal varied from 0° to 40°. The calculation results show an increase of the daylight factor with increasing sloping angle. A flat specular reflector sloped to an angle larger than 30.6° will reflect skylight partly to the room space lying beneath 2.20m. The danger of glare was examined, using the ‘findglare ’ program. Sloping angle a in relation to the base case o=10° o=20° o=30° o=40° curved horizon DF% +/-% +/-% +/-% +/-% +/-% atrium floor: lm from facade 2321 -3.3 -2.9 +5.5 +4.4 +4.3 middle of the atrium floor 2613 +12.9 +5,9 +53 +8.6 +6.8 facade: lower part oElafl. window 10.43 -aw -33.4 -28.8 -23,4 4272 upper part of 1® fi. window 1132 +1192 +148.3 +1692 +161.6 +1882 adjacent room: 1.00 m from the window 4.01 +8.0 +17.2 +17.7 ' +&S +142 2 JO infram the window 1.75 +6.8 +183 +31,4 +19,4 +32.6 4.00 m from the window 145 -1.6 +7.6 - +28.6 +45.7 +37.1 5.50 m from the window 0.72 +1.4 +9.7 +36.1 +91.7 +52.7 Table 8.8 Changes in illuminance caused by the specular flat and curved light shelfs placed between the lower and the upper part of windows on the first floor. In order to avoid problems with glare, the M-curved reflector was designed such that all of skylight was reflected to the ceiling of the loom, fig. 8.20. The reflector was composed of five, 200mm wide, flat specular pieces. The sloping of each piece was calculated such that the skylight from the middle of the sky ‘visible from the calculation point ’ was reflected to the rear zone of the ceiling, about 5m from the window wall. The sloping angle f of the first reflector element was calculated as follows: f = 180 - a - 0.5(90 - a + 0.5b) -90 The point B, the end of the first element and the start of the second element, was calculated on the basis of the sloping angle and the distance from the point A, fig. 8.20. 146 RADIANCE SIMULATIONS Figure 8.20 Design of the M-curved reflector to the left and the first fragment, scaled three times, to the right. The visual comfort probability was also calculated, using the Guth formula. The results were: VCP = 37% for M-shelf-40, VCP = 100% for M-curved. The visual impression of these two reflectors can be observed in pic. 8.11. The room looks very dark, because the exposure of the computer images was adjusted to the luminance level of the reflectors. Picture 8.11 Daylight systems M-shelf-40 to the left and M-curved to the right. The room looks very dark, because the exposure of the pictures was adjusted to the luminance of the reflectors. The results of daylight factor calculations for M-curved and the most effective M-fac and M- shelf are presented in fig. 8.21 and fig. 8.22. The daylight factor on the atrium floor was increased considerably by the M-fac system only, by about 30%. All systems increased the daylight factor on the facades, for 2. - 4. floors by 15 -20%. On the 1st floor M-fac increased the daylight factor on the whole window area by 20%, M-shelf and M-curved reduced slightly the daylight factor in the lower part of the window and multiplied the daylight factor nearly three times on the upper part of the window. The highest results in the adjacent rooms were found for the M-shelf system. It caused a 90% increase of the daylight factor in the rear zone of the room, both on the horizontal and the vertical planes. It did not change the daylighting in the window zone. 147 RADIANCE SIMULATIONS System M-curved increased the daylight factor in the rear part of the room by about 50% on all surfaces. System M-fac caused a 20-25% increase of the daylight factor, both in the rear and the window zone. vert. D.F 0.10 0.20 0.30 13.20 Daylight systems: ...... base case Q— M-fac, 20 deg —|— M-shelf, 40 deg -0— M-courved 0.20 D.F. width of the street M-fac Base case M-shelf 20 deg 40 deg M-curved Figure 8.21 Changes in the atrium daylight factors caused by the daylight systems placed on the atrium facades. Daylight systems: ----- Base case 0.0 2.0 4.0 •Q— M-lac, 20 deg i— M-shelf, 40 deg M-fac M-shelf M-curved 20 deg 40 deg Figure 8.22 Changes in the adjacent room caused by the daylight systems placed on the atrium facades. 148 RADIANCE SIMULATIONS The pictures of the room, pictures 8.12,8.13 and 8.14 were rendered to study the modelling in the adjacent room. The results of the model studies discussed in chap. 6, showed that the modelling ability of daylight in the 1st floor room where the light source, i.e. the window wall, is large, is worse than the modelling at the upper floors, where the light source is smaller and brighter. Indeed, the dark grey ball is visible as a three-dimensional shape only in the zone close to the window. The systems placed in the upper part of the windows do not change the modelling, except for M-shelf that increased the modelling ability of daylight only close to the upper part of the wall. All the simulated systems meant to be used on the atrium facades, performed as expected and can be recommended for real buildings. The M-fac system that gave the poorest results can probably be improved by increasing its area. Taking into consideration the large depth of the adjacent rooms, i.e. 6m, and the diffuse sky as a light source, the results seem very promising. RADIANCE SIMULATIONS Picture 8.12 A room adjacent to the atrium on the i" floor without any daylight system. Fish-eye view from the back of the room, left, and a parallel view of the sidewall, right. Picture 8.13 A room adjacent to the atrium on the Is' floor with the reflector M-shelf-40. Fish-eye view from the back of the room, left, and a parallel view of the sidewall, right. Picture 8.14 A room adjacent to the atrium on the 1“ floor with the reflector M-curved. Fish-eye view from the back of the room, left, and a parallel view of the sidewall, right. 150 Chap. 9 THE FINAL CONCLUSIONS 9.1 Visual comfort The dissertation starts with the discussion about visual comfort, chap. 2. As an answer to the third objective of the thesis, chap. 1, a new set of criteria for visual comfort based on knowledge about visual perception, is proposed: 1. Examination of glare and maximum contrasts in the room. 2. Dependent on function, use of one/two/three of the three following criteria: a. form reading b. modelling - shape reading c. space reading. The visual environment of an indoor space is divided into form, shape and space. Visual comfort in vision of objects from each of these categories is discussed in detail taking visual perception as a point of issue. Conclusions about lighting/daylighting of form, resulting from this discussion, are presented in tab. 2.2. Method: Comments for lighting reflectance gradient Readability of form made by reflectance gradient depends on the size of the form and on the background luminance. Generally, illuminance should increase with reduction of the size of the form and with reduction of the contrast Luminance higher than 100cd/m2 will not help. specularity A perfectly diffuse light will not make a difference in Luminance specularity readable. A small component of direct light is necessary. The incidence angle of direct light should not gradient be mirrored to the viewing angle in order to avoid veiling reflections that reduces visibility. If the direct light is too strong, the veiling reflections can cause glare. roughness A perfectly diffuse light will not make texture readable. Some of incoming light should be direct A large incidence angle is preferable. Colour chrominance The luminance level should be high enough to generate contrast photopic vision. The light source should be composed of all wave lengths that the form is able to reflect and in proper proportions. Too high luminance can change the colour perception in an unexpected way. Pattern luminance contrast The same as for reflectance gradient If the pattern is built of a very small elements, a high illuminance level is necessary. chrominance The same as for colour. A proper distance to the object is contrast important especially if complementary colours are used. Table 2.2 from chap. 2. Comments about lighting/daylighting of form. THE FINAL CONCLUSIONS The modelling ability of light connected to the vision of small three-dimensional objects was discussed comprehensively and the new criteria for modelling ability of light based on lumin ance contrast was proposed, chap. 2.3.2: 1. The minimu m contrast, contrast between two points on the object that have a very low lumin ance and that are desired to be distinguishable (e.g. Lm and LIow in pic. 2.6) should be higher than the threshold contrast 2. The maximum contrast (contrast between the brightest and the darkest point of the object, Ljuiu and in pic. 2.6) should be very high, preferably higher than 0.5. 3. The local object-background contrast (contrast between the object ’s contour and the background, e.g. 1 and L in pic. 2.6) should be higher than the threshold contrast Picture 2.6from chap. 2 Illustration of the luminances used in calculation of contrasts. It was also found, that if light has modelling capability for a nearly black object on a white background, and a white object is clearly distinguishable from the background, it will have modelling capability for any other object of the same or larger dimensions and background, too. On this base a modelling sensor was created of two balls, one white and one dark grey, exposed on the white background. The modelling sensor was used in further studies of daylighting ideas. At the end of chap. 2 many aspects of visual comfort in atrium buildings were discussed in detail. 9.2 Daylight design tools 9.2.1 Simplified daylight factor calculations for liner atrium As an answer to the first objective of the thesis, chap. 1, the first design-related tool was developed using projected solid angle principle, chap. 3. The tool consists of algorithms and tables for calculation of: —» sky factor (SF) for any surface point in an atrium —> the horizontal daylight factor (DF) in the middle of the atrium floor, and the vertical DF at the middle height of the atrium facades, for any geometry and any surface reflectances of an atrium that has a linear shape and is open, i.e. is not covered with a glazed roof. If high accuracy is not necessary, the uniform sky model can be used. The algorithm for calculation of SF at a point P randomly selected on the floor is: equation 2 from chap. 3 where angles fly or fly can be measured on the atrium section between the normal at a point P and a lines connecting point P with the top points on the facades, see fig. 3.1. 152 THE FINAL CONCLUSIONS The sky factor at a point Q randomly selected on the atrium facade is: SF(Q) := "(l- cos(y)) 2 equation 5 from chap. 3 where yis an opening angle of atrium measured at point Q, see fig. 3.3. For more precise calculation of the SF, the CIE overcast sly model should be used. The algorithm for calculation of SF at a randomly selected point on the atrium floor is rather complicated, equation 11, chap. 3. Therefore SF for CIE overcast sky was calculated for a middle point on the atrium floor and is presented in tab. 3.1. 8 - half of the Sky factor in %, Sky factor in %, opening angle in deg b = sin(8) uniform sky CIE overcast sky SFVSF. SF. SF. 05 0.0872 08.7 10.1 1.16 10 0.1736 17.4 20.1 1.15 15 0.2588 25.9 29.7 1.15 20 0.342 34.2 39.0 1.14 25 0.4226 42.3 47.9 1.13 30 0.5 50.0 56.2 1.12 35 0.5736 57.4 63.9 1.11 40 0.6428 64.3 70.9 1.10 45 0.7071 70.7 77.0 1.09 50 0.7660 76.6 82.5 1.08 55 0.8191 81.9 87.1 1.06 60 0.8660 86.6 91.0 1.05 65 0.9063 90.6 94.0 1.04 70 0.9397 94.0 96.4 1.03 75 0.9659 96.6 98.1 1.02 80 0.9848 98.5 98.5 1.01 85 0.9962 99.6 99.8 1.00 90 1.0 100.0 100.0 1.00 Table 3.1 from chap. 3 The sky factor in the middle of the floor of an infinite long street with opening angle varied from 10 to 180 degrees. The algorithm for calculation of the vertical SF, for the CIE overcast sky, at a randomly selected point Q on the atrium facades is: SF(Q) := -~(l - cos(y)) +—-(sin(Y)) 2 7,71 equation 20 from chap. 3 The results for vertical sly factor calculations are also presented in tab. 3.2. The DF can be calculated using the results of SF calculations for points P and Q and the projected solid angle principle: A'W-SF(Q)+—-A'w-AfSF(P) DF(P) := SF(P) +------71 Pf A", -AfA' w Pw equation 27 from chap. 3 153 THE FINAL CONCLUSIONS Pf SF(Q) + —SF(P) n it DF(Q) := Pw^"A"""rAfA'»' equation 28 from chap. 3 where A'*, A"*, Af are projected areas of atrium facades and floor, shown in fig. 3.7. Angle y b=cos(y) Sky factor in %, Sky factor in %, in degrees uniform sky CIE overcast sky SFo/SFn SF„ SF„ 90 0.0 50.00 39.62 0.79 85 0.087 45.64 37.61 0.82 80 0.174 41.32 35.35 0.86 75 0.259 37.06 32,85 0.89 70 0.342 32.90 30.16 0.92 65 0.423 28.87 27.31 0.95 60 0.5 25.00 24.36 0.97 55 0.574 21.32 21.34 1.00 50 0.643 17.86 18.33 1.03 45 0.707 14.64 15.37 1.05 40 0.766 11.70 12.53 1.07 35 0.819 9.04 9.86 1.09 30 0.866 6.69 7.42 1.11 25 0.906 4.69 *5.26 1.12 20 0.940 3.02 3.42 1.13 15 0.966 1.70 1.95 1.14 10 0.985 0.76 0.87 1.15 05 0.996 0.19 0.22 1.16 00 1.0 0.0 0.0 Table 3.2 from chap. 3 The vertical sty factors on the facade of an infinite long street with opening angle varied from 0 to 90 degrees. This method is a simple and precise tool for sky factor and daylight factor calculations in a linear, open atrium. If the obstruction of daylight due to the glazed roof is to be considered, the final result should be multiplied by (1-T|), where T| is a shading factor of the glazed roof. 9.2.2 Estimation of the mean daylight factor in room adjacent to linear atrium The second design-related tool was developed for linear open atrium buildings, i.e. without glazed roof, situated in Norway, see the diagram in fig. 4.2. The diagram was developed on 154 THE FINAL CONCLUSIONS the base of the results for mean daylight factor calculations in a room adjacent to an atrium using the Superlite program. The Norwegian buildings regulations are decisive for the width and the floor height of the simulated atrium. The size of windows in a room adjacent to the linear atrium expressed as a per cent of room floor area can be read on the diagram for a given floor level and for a desired daylight factor level. The diagram can be also used for any horizontal outside shading lying between 25° and 57° and having similar reflectance, see chap. 4 for details. 5.00 —1 4.00 — 3.00 — 1.00 — 4 th floor ■3rd floor ■ 2nd floor 1st floor 20.00 30.00 40.00 50.00 60.00 Shading angle Figure 4.2 from chap. 4 Daylight factor in a room adjacent to a narrow street, 4 floors building on both sides. The thin line refer to street ground reflectance of 0.50, the thick one to reflectance of 0.20. 9.3 Daylighting ideas The daylighting ideas were evaluated using the daylight design criteria presented in chap. 1. Depending on the sky and sun model, the daylight level was estimated using one of three following factors: daylight factor (DP) for CIE overcast sky, sunlight factor (SF) for sunlight alone, and solar daylight factor (SDF) for clear sky with sun. The visual comfort probability (VCP), based on the estimation of glare, was calculated for some strategies. The modelling ability of light was estimated using the modellin g sensors developed in chap. 2, and the overall impression of the room was described on the base of the photographs taken on the physical scale model or computer images created using the Radiance program. 9.3.1 Atrium space and facades as a light conductor/reflector Passive strategies In a typical atrium building the daylighting of spaces adjacent to the atrium varies considerably with floor level. Two different strategies to overcome this problem were examined: varying the glazing area or varying the glazing type with floor level. Both strategies 155 THE FINAL CONCLUSIONS achieve the goal of improving the balance of daylighting within the adjoining spaces. For both strategies there is a significant increase of daylight reaching the bottom of the atrium. Varying the glazing type is less flexible as long as the number of glasses on the market, that differ in reflectance and match in hue, is limited. Also there could be problems with sunlight reflected from the highly reflective glazin g placed on the upper part of the south facing facade. Varying glazin g area encourages varied facade designs. It is also more advantageous in cold climates where the heat loss through windows is large. The shape and placing of the windows in the facade wall were not discussed in detail in this study, but it is clear that the results could be worse if the windows on the upper floors were e.g. horizontal and placed low in the facade wall. The study shows also, that the mean reflectance of the atrium surfaces has a considerable influence on the daylight distribution in the atrium. An increase of the atrium floor reflectance is a very efficient method for increasing daylighting in the 1st floor rooms, increasing the reflectance of the facades influence the daylighting on 1st floor only marginally, but is very effective for daylighting at the upper floors. The comparison of the results for two facade alternatives that were made with the cardboard lying behind glass, and with the cardboard lying in front of glass, led to one important observation. The daylight factors measured for those alternatives were nearly equal, but the mean reflection factors of those facades calculated using the normal reflection factor of glass were considerable different This observation encouraged calculations of the reflection- transmission factors of glass and comparison with the transmission factor measured in the physical model, appendix 2. It indicated a considerably discrepancy between the normal reflection/transmission factor on one side and measured or calculated values on the other side. A hemispherical reflection/transmission factor should be used instead of normal one if a high accuracy in calculations is necessary. The study of daylight in adjacent rooms showed, that if a nearly equal mean daylight factor level on the horizontal plane in the adjacent rooms is to be obtained by varying the size of windows, the visual impression of the adjacent rooms on different floors will vary considerably, too. The rooms situated on the 1st floor, that have large windows will look open and spacious, but the modelling ability of light will be low. In rooms situated on the top floor, that have smaller and brighter windows, the modelling ability of light will be much better, but those rooms will be perceived as more closed and narrow. Active strategies The model studies of the daylight distribution in the atrium showed, that the vertical daylight factor measured on the lower part of the facades is about 50% of the horizontal daylight factor measured on the floor. This indicates, that daylight in the lower part of the atrium has a vertical direction. All active strategies were designed to increase the daylight level in rooms adjacent to the atrium on the 1st floor by redirecting the vertical light in the atrium space to those rooms. i / M-fac (sloped mirror reflectors placed above la ft. windows) i The system M-fac was designed to reflect the vertical daylight washing the facades to the adjacent rooms situated on the opposite side of the atrium. The results of model studies and Radiance simulations showed, that the system causes only moderate increasing of the daylight 156 THE FINAL CONCLUSIONS in the adjacent rooms, i.e. between 10 and 20%. The 20% increase was noticed for the optimal sloping angle of the system, 20°, that was found as a result of parameter studies done with the Radiance program. This optimal angle is lower than the angle used in the model studies, i.e. 32°, that was calculated manually as an optimal angle for reflection of daylight from the zenith to the rear part of the adjacent room. The main reason why the optimal angle found with Radiance simulations is lower than the optimal angle calculated manually, is that the effect of the shading of the 1st floor window lying beneath the system was not taken into account in the manual calculation. In a real building the M-fac system can be designed as an integrated part of the facade. The effectiveness of the system could be improved by moving the reflector horizontally into the facade construction in order to avoid shading of the 1st floor window and increasing the reflector area. It seems that specular surfaces placed on the facades, especially on the upper parts, reflect diffuse daylight down to the atrium or to the desired directions very effectively. A further study of daylighting in an atrium building should be made with the whole north facing facade covered with the specular material. In a real building the specular surface can be made of polished aluminium or steel. M-atr (sloped mirror canopy placed along the streets centerline) ■ The daylight system M-atr gave very good results, both in scale model studies and in Radiance simulations. In model studies it gave an increase of the daylight factor in the rear zone of the adjacent rooms by about 30%, in Radiance simulations by about 40%. The parameter studies done with the help of the Radiance program show that the sloping angle of this system and the height of the canopy over the floor level should be designed in conjunction. For a low height of tire canopy, the sloping angle should be low, too. Increasing the height makes it necessary to increase the sloping angle of the reflectors. The height of the system over the floor should be also low enough to avoid solar glare. The best results were obtained for a 60° sloping canopy situated with the bottom line 3.1m over the floor, it increased the daylight factor in the adjacent room by about 70%. The M-atr system changes considerably the visual impression of the atrium space. The atrium looks wider, brighter and nicer compared to an open street The most problematic consequence of using the M-atr system is the reduction of light modelling ability beneath the canopy. This problem can be solved by using a specular- reflective and partly transmissive material. The effectiveness of the system will be reduced, dependent on the degree of transmittance, but the modelling ability of light will be improved and, at the same time, the visual contact with the upper parts of the atrium and the sky will be maintained. In a real building the M-atr system can be made of a highly reflective glass or a perforated metal sheet The effectiveness of this system depends strongly on the specular reflection ability of the material and the area of the reflector. 157 THE FINAL CONCLUSIONS M-shelf and M-Curved (flat/curved specular exterior lightshelfat la fl). • I •. J The M-shelf system was examined only in Radiance simulations. The system is very effective if sloped by a large angle, i.e. 40°. It increases the daylight factor in the rear part of the room by 90%! In order to avoid glare, the 30-35° sloping angle, that gives about 35% increased daylight factor, should be used. The system is easy to construct In a real building it can be made of polished aluminium or steel. The results obtained for the M-curved system in model studies and Radiance simulations were totally different This discrepancy was probably caused by the aluminium foil used in the model studies, that had lower specular reflectance than the mirror covered paper used for other systems in the model studies. Additionally, the system was simulated more precisely in the data model. The Radiance simulations showed, that the M-curved system can reflect diffuse skylight effectively to the adjacent rooms without causing glare. The daylight factor in the rear part of the adjacent room can be increased by about 50%,. Model studies showed, that the visual impression of the atrium with the daylight systems placed on the facades is not improved compared to the open street By reducing the daylight factor on the facades, the systems increase the contrast between excessively daylit floor and poorly daylit facades. In order to improve the design of the atrium, the possibility of using hi ghl y reflective glass should be examined for both the M-shelf and M-curved systems. Lcp-fac and Lcp*2-fac (sloped, laser cut panels above la and 2*^ fl. windows) The laser cut panels placed on the facades in a sloping position are very effective. The optimal sloping angle of the system Lcp-fac, i.e. 40°, was found during model studies by changing slightly the sloping angle of the panels and taking a single measurement of the daylight factor in the middle of the adjacent room. The panels sloped at the optimal angle then caused an increase of the daylight factor in the rear part of the adjacent room by about 50%. Laser cut panels can be used on all floors. But the panels situated higher on the facade shade the ones situated lower. Comparison of Lcp-fac and Lcp x 2 - fac in the adjacent rooms on the 1st floor show that the 50% increase of daylighting factor caused by laser cut panels on the 1st floor was reduced to 40% if an additional panel was fixed on the 2nd floor. The effectiveness of the laser cut panels depends also on the placing of the panels along the height of the building. Panels used on the 2nd floor were more effective than if used on the 1st floor, because the sky factor on the 2nd floor was considerably larger. The practical conclusion is that the area of panels used on the 2nd floor can be considerably smaller than the area of panels used on the 1st floor. The panels sloped down out from the facades looks very much in place on the atrium facades. An immediate association with different shading devices is made. Really, the laser cut panels can be effectively used as sun shading. If appropriately sloped, they can reflect the sunlight falling on the window down to the floor. On south facing windows the same panels, if mounted above windows as operable devices with adjustable slope, can be used as sun shading on sunny days as well as a reflector of light for the adjacent room on overcast days. 158 THE FINAL CONCLUSIONS Since the laser cut panels are transparent for perpendicular light, the visual contact between adjacent room with the 40° sloped panels and the upper parts of the atrium is maintain ed. Lcp-atr and Lcp*2-atr (sloped laser cut panel canopy in the middle of the street) The same panels placed in the middle of the atrium gave slightly lower results, than if fixed to the facades. The daylight factor was increased by a maximum of 40% in the rear part of the adjacent rooms. It is due to the dispersion of light caused by the panels. The skylight is deflected not only to the upper part of the window, but also to the lower part of the window and to the part of facade lying over the 1st floor windows. The laser cut panels placed in the middle of the atrium space improve the impression of the atrium. It looks nicer than the open street, because the panels improve the balance between the brightness of facades and the atrium floor. Also the modelling ability of light in the zone beneath the panels is quite good. Since the panels are transparent for perpendicular light, visual contact with the upper part of facades will be maintained for observers standing on the atrium floor, to the side of the main atrium axis. The panels mounted this way cause a dividing of the atrium space of a right and a left side, and can be used in atrium buildings were such a division is acceptable or desireable. An additional laser cut panel placed in the atrium space at the 2nd floor level, Lcp*2-atr, caused a reduction of the effectiveness of the laser cut panels on the height of the 1st floor. Main conclusions about daylight systems in the atrium All the active daylighting systems designed for use in the atrium space or on the atrium facades have a huge potential for use in atrium buildings. The glazed roof obstructs about 40- 50% of diffuse skylight The active daylight systems make it possible to utilise the remaining 50-60% effectively by redirecting diffuse skylight from the excessively daylit zones to the areas where it is most needed. In this way the negative effect of the glazed roof can be to a large degree reduced. The increase of the daylight factor in adjacent rooms was calculated relative to the richly daylit base case, that had windows filling the whole facade wall and surfaces of high reflectance. Even better effectiveness should be obtained for systems placed in a building that, from the start, are poorly daylit Use of daylighting systems in an atrium should be more economical, than increasing the atrium width, reducing the atrium height or using electrical lighting. Also the maintenance of daylighting systems placed in the atrium space beneath the glazed roof should be easy. 9.3.2 Glass roof as a light conductor/reflector Horizontal, double pitched and single pitched glass roof The model studies under the overcast sky conditions showed, that all configurations of a single glass sheet on the roof reduced the illuminance levels, both on the facades and on the street floor by a minimum of 8%. The amount of reduction varied, horizontal glazing obstructed daylight the least, double pitched glass the most 159 THE FINAL CONCLUSIONS The Radiance simulations of an atrium building with different shapes of realistically simulated glazed roof showed, that the single sloped roof obstruct diffuse skylight considerably less, i.e. about 20%, than the double pitched one. The model studies showed, that the illuminance distribution in the atrium space during ‘clear sky with sun ’ conditions can be changed considerably by the glass configuration alone. This phenomenon is due to the fact that the light transmittance of glass varies with the angle of incidence. A horizontal glass roof gives most problems. It obstructs the direct sunlight coming from low altitudes, i.e. just when it is most desired. For a climate where there is a large demand for sunli gh t during winter, a horizontal glass roof should be avoided. Sloping the glass by 18° in the positive direction, i.e. with the lower edge of roof located on the south side, is enough to avoid obstruction of sunlight from low altitudes. The positive pitched glass roof with the glass sloping by 18° - 30° is the most attractive roof configuration if the goal is to give free passage for sunlight, without changing the sunlight distribution. The negative pitch glass roof controls the sunlight best. The level of sunlight illumination coming from low altitudes, i.e. winter and at morning/aftemo on in spring/autumn, is increased by ca. 50%. In summer, the excessive sunlight illumination coming from high altitudes is reduced by min 40%. The negative pitch glass roof is the most suitable one for high latitudes. Lcp (laser cut panels on the roof) The model studies in the overcast sky conditions showed, that the obstruction of diffuse skylight by laser cut panels placed over the atrium depends on the sloping angle of the panels. The reduction of the daylight factor on the atrium floor, compared to the open street, varies from 14% for panels placed horizontally to 30% for panels sloped by 30°. Both model studies and Radiance simulations showed a huge potential in sunlight enhancement for laser cut panels placed in a sloped position over the atrium. The single pitched shape of the roof makes it possible to utilise the sunlight that otherwise would fall on the neighbouring roof. The effectiveness of the panels varies with the sloping angle. Since the glazed roof construction is usually a fixed one, the choice of the roof sloping angle has a crucial importance for the sunlight enhancement potential during the year. If daylight enhancement is the main goal, the sloping of the roof in latitudes similar to Oslo should be chosen between 24° and 30°. The effectiveness of the panels varies with the variation to the sun position on the sky during the day. The best results were obtained for noon, poorest for 8:00/16:00 o ’clock. PP (prismatic panels on the roof) . Two different strategies were examined using prismatic panels. In the model studies the prismatic panels were supposed to redirect sunlight, as for the laser cut panels, to the north facing facade or to the floor. The daylight factors were considerably higher than the values for the open street, especially in spring/autumn, but lower than for laser cut panels. In summer the panels worked as sun shading, sending sunlight vertically down to the floor. Such solution will solve the problem with solar glare in summer, but will not reduce overheating. The sloping angle of 18° seems to be optimal for latitudes similar to Oslo. 160 THE FINAL CONCLUSIONS Another strategy was examined in the Radiance simulations. This time the sunli gh t was redirected to the south facing facade, but the distribution of sunli ght was deeper and more even, than in an atrium without glazed roof. The results were really high on the south facing facade, but the daylight on the opposite facade was reduced. It is not possible to compare those two alternatives directly. Both have a potential for utilisation in a real building. The choice of one of the strategies should be dictated by the function and the form of the building. The obstruction of the diffuse skylight by the prismatic panels under overcast sky conditions, examined in the model studies, showed that the prismatic panels perform nearly exactly as laser cut panels. Main discussion about light deflecting panels Compared to the base case, all alternatives with the light deflecting panels on the roof multiplied the daylight levels in the atrium in the desired seasons. The alternative with laser cut panels sloped by 30° gave the best results of all alternatives, both in winter and in spring/autumn. Prismatic panels performed better as a sun shading in summer. From the study one can conclude, that the laser cut panels have a larger potential for daylight enhancement, than the prismatic panels. Since the effectiveness of both panels varies with the variation of the sun position in the sky during the day and the year, a time based calculation is needed to estimate the daylight enhancement potential of the glazed roof with the panels during a whole year. Another advantage of laser cut panels is that they are transparent for perpendicular light, so the visual contact with the sky is maintained from the south facing facade and atrium floor. The use of light deflecting panels on the roof has a positive impact on the lighting quality in the street Sunlight is redirected down to the floor or to the one of facades, causing positive changes in the mood of the atrium space. 9.3.3 Light reflector on the neighbouring roof A 4 Flat roof reflector The model studies showed, that under overcast sky conditions the flat roof reflector was the most efficient one of all reflectors examined in this study. It increased the daylight factor on the facades by 5-10%. The parameter studies using the Radiance program showed that the optimal sloping angle of the reflector is equal to the main section angle of the atrium. The effectiveness of this reflector increases with its area and with the specular reflection factor of the material. The simulations showed also, that the flat reflector is most effective for the upper part of the facades. The results from this study can be used in the design of real buildings, especially in projects, where the height of the building is discussed. The study showed that under overcast sky an additional top floor will not reduce the daylight level in the atrium or street if the walls are sloped to the optimal angle and a highly specular material is used on the sloping facades. The sloping of the top facade will not cause a solar glare problem, this could occur if the specular reflectors are placed on the atrium walls. Another concept of using specular reflectors is to construct them as operable devices, such that the sloping angle can be chosen depending on the sky and sun conditions. In model 161 THE FINAL CONCLUSIONS experiments with the artificial sun, the sloping angle of the south facing roof reflector was chosen such that the reflected light beam was directed to the north facing facade between the 1st and the 2nd floor window bands during all seasons except summer. The roof reflector multiplied the sunlight factor values on the atrium surfaces. The main reason for this is that the reflector utilise the sunlight that otherwise would fall on the roof of the neighbouring building. The use of a roof reflector caused very positive changes in the light distribution in the atrium. The dominant vertical direction of daylight was changed to be more horizontal. The atrium looked more spacious and lively, the modelling properties of the light were also improved. If solar glare is to be avoided, the sloping angle has to be accurately adjusted, something that increases the operating costs of the building. The sunligh t can be also reflected to the floor, where the danger of solar glare is lower. 9.4 Generality of the results and proposals for future studies Many of conclusions from this study are general, and can be used in other contexts, also. Although the set of criteria for visual comfort was developed for linear atrium buildings, it can be used for any building. The comments about lighting of form are very universal and refer to any form for electrical lighting as well as for daylighting. Also the proposed criteria for the modelling ability of light and modelling sensors developed in this thesis are universal. Modelling sensors can be used in physical scale model studies, in studies of lighting or daylighting in real buildings or in computer simulations. The size and the placement of the modelling sensor should be chosen carefully. In order to be more certain about the use of modelling sensors in computer simulation, further studies should be carried out It would be interesting to use modelling sensors of different dimensions and different reflectances of the ‘black ball ’ in real rooms in order to evaluate modelling and simulate the whole experiment using the Radiance program. The first daylight design tool can be used in the form it is presented in this thesis. However, in order to create a really used friendly tool for architects a data program should be written. The program could calculate the sky factor and the daylight factor on linear atrium surfaces on the base of the main geometry and reflectances of atrium surfaces. The atrium facades can be divided into horizontal stripes of even reflectance corresponding to the height of one floor, such as the vertical DF could be calculated on the height of each floor. Further, the mean daylight factor in adjacent rooms could be calculated using algorithms developed by Lerum [Lerum, 96]. The projected solid angle principle can be also easily used for square atria. Further work is needed to develop similar algorithms for square and rectangular atriums as for linear ones and include these in the program. The second daylight design tool is even simpler and easier to use for architects than the first one. However, for other atrium shapes a new similar diagram or diagrams should be created. The conclusions about daylight strategies developed for overcast sky conditions are valid for any atrium shape. Both passive strategies, i.e. varying the glazing area and varying the glazing type with floor level should be valid for any shape of atrium. However, the ratio between window areas on different floors should differ for different atrium shapes. In the case of the strategy based on varying the glazin g type, the strongest obstacle is a very limited number of reflective glazing types on the market Also the conclusions about daylight systems developed to operate under overcast sky are valid for any atrium shape. Daylight systems placed in an atrium space and on atrium facades, as M- atr, Lcp-atr, M-fac, or Lcp-fac can be used in atria of any shape, but the effectiveness of the 162 THE FINAL CONCLUSIONS systems will vary, depending on the size of sky visible from the place where the system is to be placed, the size and sloping angle of the system and the optical proprieties of the system. Therefor the design of the systems should be adjusted to the design of the atrium. The placement of systems in relation to the adjacent room should be similar as in this thesis. A care should be taken to avoid solar glare. In the case of systems developed basically for sunlight, such as prismatic panels or laser cut panels on the glazed roof, small changes of the atrium orientation or shape will not have a great impact on the results. Anyway, it is not possible to take a general conclusions about such systems for atria that differ considerably from the one presented in this thesis, because the incidence angle of sunlight on the system and the placement of the system in relation to the atrium space have a crucial impact on the effectiveness of these systems. REFERENCES AND LITTERATURE Aizlewood 93 M. E. Aizlewood Innovative daylighting systems: an experimental evaluation. Lighting Research and Technology, vol. 25,1993. Alzlewood 95 M. E. Aizlewood The daylighting of atria: a review. ASHARE Conference, San Diego, 1995. Aschehong 86 0. Aschehong Daylight design for glazed spaces. Second International Daylighting Conference, Long Beach CA.USA, 1986. Aschehong 87 0. Aschehong Daylight in glazed streets Nordic Building Physics Symposium, Lund, 1987. Aschehong 87 0. Aschehong, Glazed spaces : research and development in Norway Atrium Conference of the UK Section of the International Solar Energy Society, London, 1987. 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A guide to modem architectural glass performance Pilkington Glass Limited, Spain, 1993. Cartwright 85 V. Cartwright The use oflightwells as a daylight strategy Proceedings Solar’85, American SES, Raleigh NC, 1985. Cartwright 86 V. Cartwright Sizing atria for daylighting presented at International Daylighting Conference, Long Beach -CA,1986. CIESla International Commission on illumination An analytic model for describing the influence of lighting parameters upon visual performance volumel: Technical foundations publication QE no 19/2.1, Paris, 1981. CIE 81b International Commission on illumination An analytic model for describing the influence of lighting parameters upon visual performance volumel: Summary and application guidelines publication CIE no 19/2.2, Paris, 1981. cm 83 International Commission on illumination Discomfort glare in the interior working environment Technical Committee TC - 3.4, No. 55,1983 cm 89 International Commission on illumination International lighting vocabulary publication CIE no 17.4, 4th edition, 1989 Chauvel 82 P. Chauvel, LB. Collins, R. Dogniaux, J. Longmore Glare from windows: current views of the problem Lighting Research and Technology, vol. 14 No. 1,1982. 165 REFERENCES ChristofFersen 95 J. Chris toffersen Daylight utilisation in office buildings Danish Building Research Institute, H0rsholm, 1995. Cold 85 B. Cold, H. Fahti, S. Asmervik Evaluering av den overdekte gatenpa Universitetssenteret pa Dragvold SINTEF rapport ; SFT62 A84007, Trondheim, 1985. Cold 93 B. Cold Arlatekturevaluering i teori og praksis Fakultet for arldtektur, plan og billedkunst, NTNU, Trondheim, 1993. Cold 95 B. Cold Six perspectives on quality Oslo School of Architecture, Research magazine, no. 2,1995. Compagnon 93 R. Compagnon, J.L. Scartezzini, B. Paule Application of nonimaging optics to development of new daylighting systems ISES Solar World Congress, Budapest, 1993. Compagnon 94 R. Compagnon Simulations numeriques de systemes diclairage naturel a penetration laterale EPFL, Lausanne, 1994. Cuttle 97 C. Cuttle Cubic illumination Lighting Research and Technology, vol. 29,1997. Edmonds 92 Ian R. Edmonds Performance of laser cut light deflecting panels in daylighting applications Solar Energy Materials and Solar Cells 29 (1993) 1-26, North Holland. Fanger 72 P.O. Fanger Thermal comfort: analysis and applications in environmental engineering Kbh. : Danish technical press, 1972. Fontoynont 86 M. R. Fontoynont Day lighting of large enclosures. 1986 International Daylighting Conference, Proceedings II, Long Beach - CA.1986. Gregory 66 R. L. Gregory Eye and brain, the psychology of seeing. Officine Grafiche Amoldo Mondadori, Verona, 1966. Gunnarshaug 85 J. Gunnarshaug Erfaringerfra glassoverdelaiinger i Trondheim Sintef rapport STF62 A 85013, Trondheim, 1985. Haber 80 R.N. Haber, M. Hershenson The psychology of visual perception.2nd ed. Holt, Rinehart and Winston, New York, 1980. Hastings 94 International Energy Agency, Paris, France ; principal editor: S.R. Hastings Passive solar commercial and institutional buildings : a sourcebook of examples and design insights Chichester : Wiley, cl994. Hendee 97 W. R. Hendee, P.N.T. Wells, editors The perception of visual information Springer, New York, 1997. 166 REFERENCES Hestiles 86 A. G. Hestnes IEA task XI: Passivfe] and hybrid solar commercial buildings. SINTER report ; STF62 A86012, 1986. Hestnes 88 A. G. Hestnes Norwegian Atria Experiences, Proc of solar 88, 13th National Passive Solar Conference, Cambridge M.A.1988. Hestnes 89 A. G. Hestnes Atrium buildings for cold climates. International Solar Energy Society's Conference: "Clean and safe energy forever", Kobe, Japan, 1989. Holladay 26 L.L. Holladay The fundamentals of glare and visibility Journal of the Optical Society of America and Review of Scientific Instruments, Vol. 12, No. 4.1926. Hopkinson 66 R.G. Hopkinson, P. Peterbridge, J. Longmore Daylighting William Heinemann Ltd, London, 1966 IES77 IBS IES Code for interior lighting Illuminating Engineering Society, London, 1977 IEA94a IEA Adeline, Superlite User Manual, SuperlitelEA 1.0 Lawrence Berkeley Laboratory, USA and Fraunhofer Institute of Buildings Physics, Stuttgart, 1994. DBA 94b IEA Adeline, Radiance Reference Manual. Lawrence Berkeley Laboratory, USA and Fraunhofer Institute of Buildings Physics, Stuttgart, 1994. Jarvis 97 D. Jarvis, M. Donn Comparison of computer and model simulations of a daylit interior with reality 5th International Conference of Building Performance and Simulation Association, Prague, 97 Jones 91 J. R. Jones Illumination predictions for malls and atria Journal of the Illuminating Engineering Society, Vol. 20, Iss. 2,1991. Kim 86 K. Kim, L. L. Boyer Development of daylight prediction methods for atrium design. 1986 International Daylighting Conference, Proceedings H, Long Beach - CA.1986. Kim 87 K. Kim Development of daylight prediction methods for atrium design. Texas A&M University PhD Thesis, 1987. Knudsen 94 P. Knudsen Arkitektkontor AS Universitetssentret pa Dragvoll del II, Trondheim. Byggekunst, nr. 3,1994. 167 REFERENCES Lam 77 W. M.C. Lam Perception and lighting as formgivers for architecture McGraw-Hill, USA, 1977. Larsen 94 B. Larsen 15 ar etter, en samtale med Per Knudsen Byggekunst, nr. 3,1994. Lam 86 W. M.C. Lam Sunlight as formgiver for architecture Van Nostrand Reinhold, New York, 1986. Lerum96 V. Lerum Energy efficient office building at high latitudes NTH, Trohdheim, 1996. Littiefair 90 P.J. Littiefair Innovative daylighting - a review of systems and evaluation methods. Lighting Research & Technology, Vol 22, No 1,1990 Littiefair 93 P.J. Littiefair Measuring daylight - the effective use of scale models Building Research Establishment, Garston, Watford, 1993. Lynes79 J. A. Lynes A sequence for day lighting design. Lighting Research & Technology, Vol 11, No 2,1979. Matnsiak 98 B. Matusiak, 0. Aschehoug Daylighting systems for linear atria at high latitudes An International Conference on Daylighting Technologies for Energy Efficiency in Buildings, Ottawa, Canada, 1998. Matusiak98 B. Matusiak, 0. Aschehoug Criteria for Estimation of the Modelling Ability of Daylight/Electrical Light CIE International Symposium on Lighting Quality, Ottawa, Canada, 1998. Michel 96 L. Michel LIGHT: The Shape of Space, Designing with Space and Light Van Nostrand Reinhold, USA, 1996. Nawab 84 M. Nawab, S. Selkowitz Daylighting data for atrium design. Proceedings 9th National Passive Solar Conference, Columbus, Ohio, 1984 Nawab 86 M. Nawab, V. Bezjanac Daylight design for Atria. 1986 International Daylighting Conference, Proceedings 1,1986. Neal 92 T. J. Neal, S. Sharpies The infuence of well geometry on daylight levels in atria. Proceedings of C3BSE national lighting conference, 1992. Saxon 83 R.Saxon Atrium buildings, Development and Design London: Architectural Press, 1983. 168 REFERENCES Spillmann 90 L. Spillmann, J. S. Werner editors Visual perception: the neurophysiological foundation Academic Press, San Diego, 1990 Tyholt 88 M. Tyholt, 0. Aschehoug Kostnadsanalyse over de glassoverdekkete gatene IELA-byggetpa NTH.. Sintef rapport STF 62 A88015, Trondheim, 1988 Unger H. Unger SunOrb 1.0 Department for Nuclear and New Energy Systems, Bochum, Germany. Valberg 98 A. Valberg Lys, syn, farge. Tapir forlag, Trondheim, 1998 WDG 85 Windows and Daylighting Group Superlite 1.0. Evaluation manual LBL, Berkely, 1985 Ward 88a G. J. Ward, F. M. Rubinstein, R. D. Clear A Ray Tracing Solution for Diffuse Interreflection Computer Graphics, vol. 22, no. 4,1988 Ward 88b G. J. Ward, F. M. Rubinstein, A New Technique for Computer Simulation of Illuminated Spaces Journal of the Illuminating Engineering Society, Winter 1988. Ward 90 G. J. Ward Visualization Lighting Design + Application (LD?A), vol. 20, no 6,1990 Ward 91a G. J. Ward RADIANCE Visual Comfort Calculation Rapport interne, LESO, EPFL1991. Ward 91b G. J. Ward, F. M. Rubinstein, R. D. Clear A Ray Tracing Solution for Diffuse Interreflection Computer Graphics, vol. 22, no. 4,1988 Ward 92 G. J. Ward Measuring and Modelling Anisotropic Reflection Computer Graphics, vol. 26, no 2,1992 Ward 94 G. J. Ward The RADIANCE Lighting Simulation and Rendering System SIGGRAPH 94, Computer Graphics Proceedings, Annual Conference Series, Orlando, Florida, 1994. Wright 91 J. A. Wright A brighter future for atria? Building Services, October 1991. 169 APPENDIX 1 Terms and definitions used in the project Absorbtance: The ratio of absorbed flux to the incident flux. Adaptation: The process including contraction or dilation of the pupil by which the eye’s retina adjusts sensitivity to the intensity or quality of light stimulation. Altitude: The vertical, angular distance of a point in the sky above the horizon. Angle of Incidence: Angle at which light rays strike a surface, measured between the ray and a line perpendicular to the surface. Atrium: An open area placed usually in the centre of a building or building complex that can be covered by a glazed roof. Historically the term was used for open courts. In last years the term atrium has been used also for spaces covered by a daylight-transparent roofs. Azimuth: The horizontal angular distance between the vertical plane containing a point in the sky and the true south. Brightness: The subjective perception of luminance. CIE: Commission Internationale tie PEclairage, International Lighting Commission, head quartered in Paris, France. CIE Overcast Sky: Complete overcast sky, in which the sun position cannot be detected. The luminance changes slightly with elevation angle such as the zenith luminance is approximately three times that at the horizon. CIE Clear Sky: A reference cloudless sky having the greatest luminance near the sun and least luminance 90 degrees from the sun on the opposite side of zenith. The relative luminance of sections of the sky hemisphere are defined according to their position in relation to the sun. The area around the sun is brightest, the area opposite the sun on the sky hemisphere is darkest, and the horizon generally has low luminance. Clear Sky: The sly that has less than 30% cloud cover. Colour: The aspect of light or of any object that may be described in terms of hue, lightness, and saturation. Colour is associated specifically with electromagnetic radiation of a certain range of wavelengths visible to the human eye. Radiation of such wavelengths comprises that portion of the electromagnetic spectrum known as the visible spectrum —i.e., light Hue is an attribute associated with each of the dominant wavelengths of the spectrum. Saturation pertains to relative purity, or the amount of white light mixed with a hue. High-saturation colours contain little or no white light Brightness refers to intensity, distinguished by the degree of shading. Colour Rendering: The effect of a light source on the colour appearance of objects in conscious or unconscious comparison with their colour appearance under a reference source. Contrast: The relationship between the luminance of an object and its immediate background. Mathematically, the difference between the luminances divided by the luminance of the background. APPENDIX 1 Daylight: Light from the sky: direct sunlight or/and diffuse light from the sky. Daylight Factor (DF): A relative measure of daylight illuminance at an interior point or plane expressed as the ratio of the illuminance on the given plane to the simultaneous exterior illuminance on a horizontal plane from the whole unobstructed CIE overcast sky. Disability Glare: Glare resulting in reduced visual performance and visibility; often accompanied by discomfort glare. Discomfort Glare: Glare producing discomfort, interfering with the perception of visual information but not necessarily interfering with visual performance or visibility. Flux: see Luminous Flux Glare: The sensation produced by luminance within the visual field that is sufficiently greater than the luminance to which the eyes are adapted; causes annoyance, discomfort, or loss of visual performance and ability. Glare Index: The degree of glare caused by all apertures in the field of view based on the relationship between source and background luminance. Hue: an attribute of colour associated with each of the dominant wavelengths of the spectrum. Internally Reflected Component (IRC): Component of the Daylight Factor which consists of light reflected from internal reflecting surfaces. Illuminance: The density of the luminous flux incident on a surface, expressed in lux. Illumination: The act of illumination or state of being illuminated. Latitude: The angular distance north or south of the equator Light: Radiant energy that is capable of exciting the retina and producing visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 780 nm. Luminance: The physical measure of brightness: lumin ous intensity per unit projected area of any surface, as measured from a specific direction. Luminous Flux: The time rate of flow of light In physics: radiant flux in the visible- wavelength range expressed in lumens instead of watts. Luminous Intensity: the magnitude of luminous flux per solid angle. Orientation: The relationship of a building surface with respect to compass orientation. Overcast Sky: A sky that has more than 70% cloud cover. Overcast Sky (CIE): see CIE Overcast Sky. Reflection: The process by which incident flux leaves a surface or medium on the incident side of that surface or medium. Reflectance or Reflection Factor: The ratio of reflected flux to incident flux. Sensor: A photocell device used for measuring of illuminance. Sky Component (SC): The ratio of daylight illumination at a point on a given plane that is received from a sky of assumed or known lumin ance distribution, to the illuminance on a horizontal plane due to an unobstructed hemisphere of the sky. 171 APPENDIX 1 Solar Daylight Factor (SDF): A relative measure of daylight illuminance at an interior point or plane expressed as the ratio of the illuminance on the given plane to the simultaneous exterior illuminance on a horizontal plane from the whole unobstructed C1E clear sky with sun. Sunlight: Light directly from the sun; excludes light from other portions of the sky. Sunlight Factor (SF): A relative measure of sunlight illuminance at an interior point or plane expressed as the ratio of the illuminance on the given plane to the simultaneous exterior illuminance on a horizontal plane from the sun alone. Transmission: The process by which incident flux leaves a surface or a medium on a side other than the incident side, without change in frequency. Transmittance or Transmission Factor: The ratio of the transmitted flux to the incidence flux. Uniform Sly: An isotropic sky in which the lumin ance in all directions is equal. Visibility: The quality or state of being perceivable by the eye. Visual Acuity: A measure of the ability to distinguish fine details. Visual Comfort Probability (VCP): The rating of a lighting system expressed as percent of people who, when viewing from a specified location and a specified direction, will be expected to find it acceptable in terms of discomfort glare. Visual Field: The locus of objects or points in space that can be perceived when the head and eyes are kept fixed; the field may be monocular of binocular. Visual Perception: The interpretation of impressions transmitted from the retina to the brain in terms of information about a physical world displayed before the eye. Visual Task: Conventionally designates those details and objects that must be seen for the performance of a given activity; includes the immediate background. Zenith: The point at the top of the hemispherical sky dome. Source: Adeline, Superlite User Manual [ IEA 94a ] and Encyclopaedia Britannica [Britannica 98]. The description of terms and definitions is made as simple as possible in order to make the thesis understandable for readers not familiar with daylighting terms. The really scientific description of the terms can be found in International lighting vocabulary [CEE, 89]. 172 APPENDIX 2 Calculation of the light transmission factor of glass The direct transmission factor defined in Daylighting [HopMnson, 66] is: the ratio of the luminous flux transmitted in accordance with the laws of direct transmission to the total incident flux. For a given fragment of glass the ratio of the transmitted and incident flux will be equal to the ratio of transmitted and incident illuminance measured on both sides of glass surface. The transmission factor rof a piece of glazing is: T := (1) where E* and E^, am respectively the transmitted and the incident illuminance. The measured values of DF on inside and outside of glazing in alt Ba and Baa was used in the calculation of the transmission factor te, the results are presented in the tab. A2.1. Alt Ba Alt Ba Transmis Alt Baa Alt Baa Transmis The mean Floor DF on DF on sion factor DF on DF on sion factor transmis outside of inside of altBa outside of inside of alt Baa sion factor the glazing the glazing the glazing the glazing % 4th 0.4681 0.1762 0.376 0.4631 0.1670 0.361 0.37 3rd 0.3778 0.1432 0.379 0.3583 0.1391 0.388 0.38 2nd 0.3103 0.1595 0.514 0.2963 0.1608 0.542 0.53 1st 0.2884 0.2040 0.707 0.2888 0.2043 0.707 0.707 Table A2.1 Transmission factors of glazing used in alt. Ba and Baa based on the experimental results. The transmission factors calculated on the basis of experiments were compared with the calculated ones. The calculations were carried out twice. In the first one the glazing was placed horizontally under the uniform sky of constant luminance L. In the second one the glazing was placed vertically on the facade of the infinite long street The CIE overcast sky type was used. The Rivero equation was used in both calculations: Te := 1.018-T Q-[cos(e) + (sin(e)) 3-cos(e)] where Te is the transmission of a single pane of glass at a given angle of incidence e, and T0 is the transmission at normal incidence, [HopMnson 66]. Transmission factor t, of the glazing placed horizontally under a sky of uniform luminance. After HopMnson, section 3.2.1, the illumiantion from the whole hemisphere of sky is: E = tc-L (3) In order to calculate the transmission factor of glass two values of illuminance are needed: incident illuminance that can be calculated using equation 3, and transmitted illuminance fit, that was calculated using the Rivero formula. APPENDIX 2 Consider a hemispherical sky of uniform luminance L with a reference point situated beneath a fragment of the horizontal glass at its centre, fig. A2.1. Figure A2.1 Illumination from a sky of uniform luminance, transmitted by the fragment of glazing. On the basis of fig. 3.1. from "Daylighting ” [Hopkinson, 66]. An elemental ring of the hemisphere of angular width dy at an angle of elevation y above the horizon will have an area Rdy2jtRcos(y). The intensity I of the ring will be LRdy2xtRcos(y). The illuminance dE* at the centre of hemisphere under the glass due to the elemental ring will be given by the product of this intensity, the inverse square of the distance and the transmission of glass TE, i.e. I Te/R2, multiplied also by the cosine of the angle of incidence e. The relation e = 90 - y gives: dEjj := 1.018-To-2-Jt-(sin(y))2-cos(y)-[l)-[l + (cos(y))3]-J-dy (4) Integrating to obtain the illumin ation from the whole sky gives: Ett:= 1.018To-2-Jt-L- (sin(y))2cos(y)-(i + (cos(y))3) dy (5) Because = xc-L, the transmission factor -q, will be: ftt (sin(y))2-cos(y)-[l + (cos(y))3] dy Tu:= 1.018-To -2- 0 (6 ) The results of calculations for the respective glasses are presented in tab. A2.2. The results can be used also for inclined glazin g in calculations where the assumption of the uniform luminance distribution in the outside environment does not depart too much from the real luminance distribution, Transmission factor t, of the glazing placed vertically on the facade of the infinitely lone street Consider the luminance distribution of the outside environment for a given piece of glazin g placed vertically on the facade of the infini te long street There are three elements that make the outside environment the sky, the opposite facade and the floor. The mean luminance of the street surfaces was calculated using formula 4 in chap. 6 and the DF measurements on the street in alt Ba. The mean luminances were respectively: Lf%c = 0.16-1^, Lfl00I = 0.23 Both values are much lower than the sky luminance, that varies from L* on zenith to (1/3)- Lz on the horizon. 174 APPENDIX2 It is reasonable to divide the outside environment into two parts. The CIE overcast sky makes up the first part, the street surfaces the second. The luminance of the sky varies with the elevation angle 0: Le,L.1^5 (6 ) 3 (7) The luminance of the street surfaces was assumed to be unif orm and equal 0.2 L^. The transmission factor -rs will be: ® sky tr + ® surfjr ? g'------® sky_inc + ® surfhu rg\ where: Esiyjr the illumination due to sky light transmitted by the glazing, Esurf q. the illumination due to light reflected from the street surfaces and transmitted by the glazing Eskyjnc the illumination due to incident sky light Esurfmc the illumination due to incident light reflected from the street surfaces Consider a piece of glazing with a reference point O placed vertically on the facade of the infinite long street Consider also a half of a hemispherical sky of CIE overcast type with a reference point O lying at its centre, fig. A2.2 Let point O be the origin of the co-ordinate system and axis X be the intersection line of the horizontal and the vertical plane. Let the radius of hemisphere be taken as R. Because of obstruction from the opposite facade, the sky visible from the reference point will have a shape of a spherical wedge. Let the angle of the wedge be taken as EL,.,. Figure A2.2 Illumination from the spherical wedge of the sky. Consider an elemental spherical wedge of angular width dp at an angle between wedge and vertical glass surface p. For a point P lying on the wedge consider an element dP of the wedge 175 APPEND1X2 of angular width da and at an angular distance a from axis X. The area of element dP is da-db, where da = R-doc, and db = PPx -dp. Px is the projection of point P on axis X, but PPX = R-sin(a), so the area of dP is R2-sin(a)-da-d|J. Let point Pn be the projection of point P on XZ plane and point P*y be the projection of point P on XY plane. Let the angle between PO and P^O be taken as y and the angle between PO and PxyO as 0. The luminance of the sky element dP depends on sin9. PPxy sin(8) := where R PP xy := PP x -cos(p) PP x := R-sin(a) sin(8) := sin(a)-cos(P) The equation 7 will be changed to: 1 + 2 sin(a) cos(P) Le;=Lz 3 (10) The intensity of the spherical element dP will be given by the product of its luminance L@ and its area. a, L7-L^2^R2-siK K max E skyjnc (l +■ 2-sin(a)-cos(P))-(sin(a)) 2-sin(p) dP da 0 (13) The rest of the environment represented by the spherical wedge of the angular width (90-PmaJ is supposed to have the uniform luminance equal 0.2-Lz. The same procedure as before leads to equation 14: n "71 p surf inc ^^z" (sin(a)) 2-sin(P) dp da P max (14) A calculation of the illuminance of the transmitted light must include the light transmission properties of glass. Let replace the incidence angle £ in the Rivero equation by (90-y). The Rivero equation will be changed to: 176 APPENDIX2 Ty := l.0l8T o-(sin(Y) + (cos(y))3-sin(Y)) (15) The sine and cosine of angle y will be calculated: PP xz sin(Y) := ■ R where: PPxz:= PPPP' x “x:= Rsh sin(Y) := sin(a)-sin(|3) (16) The cosine function will be given by: cos(Y) := Jl - (sin(a))2-(sin(P))2 (17) The light transmission of glazing dependent on angles a and p will be: 3" T^ := 1.018To-sin( (18) The illumination dE* due to the light from the sky element dP transmitted by the glazing will be given by the product of dE% calculated for incident light and the light transmission of glass. Integrating to obtain the illumination from the whole sky visible from the point O: ^max 3 E sky_tr:= ~7 "1,018T c (1 + 2-sir(a)-cos(P))- ,l + (l-(sm(a))2.(sic(P))2)2 (sin(oO) 3-(sin(P)) 2dpdoc Jo • 0 (19) The illumin ation due to the rest of the environment will be: "it 'll 3' Esurf_trr 0-2-Lz-1'018 To i .i + (i-(sin(cO) 2-(sm(P))2)2 (sin(a)) 3.(sir 177 APPENDIX 2 The results show considerable differences between the normal transmission factor T0 and all the other transmission factors presented in tab. A2.2. Compared to T0 the transmission factor Tc based on the measurements has 12-18% lower values. In the same way the real reflection factors will be 12-18% higher than the normal reflection factors. The transmission factor ts calculated for C1E overcast sky and the infinite long street correspond best with tc. During calculation of ts the sky light angles measured on sensors were used. The illuminances measured by sensor placed on the inside of the glazing was obstructed by the thickness of the facade. That fact was ignored in calculations of transmitted illuminances: E^^ and E^fg, sky angles on sensors were used. The glazing on the 1st floor is larger than the glazing on all other floors, so are the real angles of incidence. It is probably the reason why xs is smaller than tc only on the 1st floor. In a real building both the thickness of the sensor and the thickness of glazing are usually minimal comparing to streets dimensions and can be overlooked. The calculation of the transmission factor for uniform sky Tu gives quite good results, specially on the 1st floor where the outside luminous environment is most similar to the environment of the uniform luminance distribution. The sky angle is only about 30°. It seems that Tu can be used instead T, if an accuracy of about 7% is acceptable. Anyway, the difference between Tu and Ts can be considerably larger if calculated for the street that have facades of low reflection factor, because the illumination due to sky light will be much more important than it is in our example. 178