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Disability Glare: a Study in Simulated Road Lighting Conditions

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Disability Glare: a Study in Simulated Road Lighting Conditions Lighting Res. Technol. 2014; Vol. 46: 695–705 Disability glare: A study in simulated road lighting conditions N Davoudian MArch PhD MSLL, P Raynham BSc MSc CEng FILP MCIBSE FSLL and E Barrett MSci PhD The Bartlett School of Graduate Studies, University College London, UK Received 31 May 2013; Revised 12 July 2013; Accepted 9 September 2013 Disability glare is associated with veiling luminance caused by light from bright sources being scattered within the eyes of observers, thereby reducing retinal luminance contrast. This study compares the reduction in observers’ performance in the presence of glare with veiling luminance in the eye, calculated using a non- subjective method. A total of 42 observers performed a target detection task in the presence of a glare source in conditions similar to street lighting at night. Luminance contrast thresholds were measured for each observer under different levels of glare. Results show that, while veiling luminance has a significant effect on the performance of observers, its effect is lower than expected from contrast loss. Furthermore, the performance of observers over the age of 50 is unaffected by increasing the glare level. Notation  theanglebetweentheobserver’sdirec- tion of view and the glare source (8) A the age of the observer (years) Ceff the effective luminance contrast of an object in the presence of glare C0 the luminance contrast of an object in the absence of glare 1. Introduction Eeye the illuminance caused by a particular glare source at the observer’s eye on a Improved street lighting, in general, is widely plane normal to the observer’s direc- thought to be an effective means of reducing tion of view (lx) the likelihood of road traffic accidents for k a disability glare model parameter, pedestrians, cyclists and vehicle users. In this constant for a given observer context, glare has long been recognised as one LB the luminance of the background of the main issues; this is due to the fact that (cdÁmÀ2) glare from street lamps and vehicle head- À2 LO the luminance of the object (cdÁm ) lamps can cause a reduction in the conspi- LV the veiling luminance created by a cuity of objects and discomfort for both particular visual environment motorists and pedestrians. The problem of (cdÁmÀ2) glare has been recognised for many years and n a disability glare model parameter, the current design guidance for road light- constant for a given observer ing1,2 includes provisions for limiting glare. This study focuses on the impact of glare on Address for correspondence: N Davoudian, The Bartlett road users’ performance in conditions similar School of Graduate Studies, University College London, to street lighting. Moreover, this study bene- Central House, 14 Upper Woburn Place, London WC1H 0NN, UK. fits from not using subjective measures to E-mail: [email protected] calculate veiling luminance in the eye. ß The Chartered Institution of Building Services Engineers 2013 10.1177/1477153513510168 696 N Davoudian et al. 1.1 Veiling luminance In common lighting practice the assessment of Disability glare has been studied for many veiling luminance is done with a formula that years, with perhaps the key initial work in the relates it to the illuminance at the eye of area being carried out by Holladay.3 The the observer due to the glare sources and the proposed mechanism for disability glare is angular separation of the glare source from that light is scattered by the ocular media, and the point of regard. The general form of most this results in extra light being added to areas formulae used to assess veiling luminance is of the retina surrounding the area directly given as equation 4 illuminated by the glare source. It is known that the amount of scattered light in the eye, X E L ¼ k eye ð4Þ and the angular size of the region affected, V n increases with age and this correlates with 4 increased susceptibility to glare. Older people For most lighting application standards the tend to have more problems in coping with value of k is taken to be 10 and n is taken to glare sources and this makes lighting difficult be 2. It is known that these values vary and since they also need more light to perform that older subjects tend to have more problems 5 common visual tasks. with disability glare. In the European standard Conventionally, disability glare has been for road lighting,6 equation 5 is provided for understood by treating the scattered light as if the calculation of k as a function of age. it added an extra veiling luminance across the "# visual field, as this would thereby reduce A 4 k ¼ 9:86 1 þ ð5Þ luminance contrasts. There are several pos- 66:4 sible equations that could be used for quan- tifying the luminance contrast, for simplicity, equation 1 is used. Consider the normal Recently, it has become possible to assess luminance contrast of an object against its scatter in the eye directly instead of depending background as on psychophysical measurements that are time consuming and generate large within- jjLO À LB observer variability. A new objective tech- C0 ¼ ð1Þ LB nique developed at City University, London, UK involves direct estimates of light scatter in In the presence of a veiling luminance due to a the eye using imaging techniques and is both glare source, the effective luminance contrast rapid (5-minutes test) and gives significantly is given by equation 2 more accurate results than conventional clin- 7 ical and visual psychophysical techniques. ðÞÀL þ L ðÞL þ L This technique is critical to the further inves- C ¼ O V B V ð2Þ eff ðÞL þ L tigation of disability glare as it gives the B V researcher the ability to collect together a group of observers for whom the veiling Thus, it can be seen that the effective lumi- luminance in any given scene can be calcu- nance contrast is less than the luminance lated precisely. contrast in the absence of glare according to Changes in the light scattering properties equation 3 of the eye were calculated using a scatter system applied on the P_SCAN 100 pupill- LB Ceff ¼ C0 ð3Þ ometer apparatus at City University LB þ LV (London, UK). This method permits Lighting Res. Technol. 2014; 46: 695–705 Disability glare in simulated road lighting 697 simultaneous measurement of pupil size and 2. Method eye movements.8 This programme uses extended scatter sources that can be produced The objective of the test was to assess the on a visual display and uses a flicker cancel- effect of glare in conditions that were similar lation technique similar to that used in the to those encountered in normal street lighting van den Berg et al. study.9 In order to conditions. A dual target detection task was maintain a constant illuminance in the plane employed to explore the effect of glare on of the pupil, the dimensions of the scattering peripheral target detection. source are adjusted for five eccentricities. The sinusoidally modulated scatter source causes 2.1 Observers short bursts of flicker. Light scattered from A total of 42 observers (21 males), between the source causes the dark centre disc of the the age of 20 and 75 participated in this annulus to flicker in phase with the scatter experiment. All observers were tested for source. In order to null out the modulation of visual acuity and ocular diseases. Each of the retinal illuminance caused of scattered light, observers had the scatter function of their eyes the luminance of the dark disc at the centre of measured using the method explained above. the annulus is modulated sinusoidally in Figure 1 illustrates the range of k and n values counterphase with the scatter source. The for subjects over and under 50 years of age. nulling display target luminance is required to balance the retinal illuminance by the scat- 2.2 Apparatus tered light. This was measured for each of the A street scene was presented on a white five annuli that formed the scatter source. screen at a distance of 4 m from the observer. During each run, for each scatter source The luminances of the scene were set to eccentricity, six readings were measured and closely resemble the luminances encountered averaged. The output of the programme on on a real street. To prevent problems asso- the P_SCAN 100 system produces the n and k ciated with colour contrast confounding the parameters used in equation 4. results of the experiment, an achromatic 70 Under 50s Over 50s 60 50 40 value k 30 20 10 0 1 1.5 2 2.5 3 n value Figure 1 k and n values for observers over and under the age of 50 Lighting Res. Technol. 2014; 46: 695–705 698 N Davoudian et al. Figure 2 Street image used in the experiment with embedded targets image was used. The scene was controlled by street luminaire seen at 40 m from the observer. custom software that permitted the scene to The illuminance the glare source created at the be overlaid by different targets for fixed eye of the observers was similar to that periods of time. The scene with targets is produced by a typical street luminaire shown in Figure 2. mounted on a 5 -m column, 10 m away from Two light-emitting diode (LED) light a pedestrian. A bent tube painted matt white sources were used as the glare sources and was used to create a uniform large glare source their spectra are shown in Figure 3. To make and an iris was used to create the small glare the scene look natural the glare source was source (Figure 4).
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