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Paper Code and Title: H01RS Residential Space Designing Module Code and Name: H01RS19 Light – Measurement, Related Terms and Units Name of the Content Writer: Dr Paper Code and Title: H01RS Residential Space Designing Module Code and Name: H01RS19 Light – measurement, related terms and units Name of the Content Writer: Dr. S. Visalakshi Rajeswari LIGHT – MEASUREMENT, RELATED TERMS AND UNITS Introduction Light is that part of the electromagnetic spectrum which will stimulate a response in the receptors of the eye. Its frequency usually expressed as wavelength determines the colour of light and its amplitude determines its intensity. Accommodation from the individual’s part enables focusing of vision. Hence the need to study lighting in interiors. Especially when activities are carried out indoors it is necessary to provide some sort of artificial illumination. In such circumstances, the designer should be aware of what (lighting) is provided and the satisfaction the ‘user’ derives out of it. 1. The radiant energy spectrum Vs visible spectrum Light is visually evaluated radiant energy (electromagnetic), which moves at a constant speed in vacuum. The entire radiant energy spectrum consists of waves of radiant energy that vary in wavelength of a wide range; an array of all rays - cosmic, gamma, UV, infra red, radar, x rays, the visible spectrum, FM, TV- and radio broadcast waves and power transmission. The portion of the radiant energy which is seen as light, identified as the spectrum visible to the human eye ranges from about 380 (400) to 780 (700) mµ (referred to as nanometers or millimicrons). A nanometer (nm) is a unit of wavelength equal to 10 -9 m. Light can thus be thought of as the aspect of radiant energy that is visible. Colour perception is attributed to the varying wavelengths noticeable within the spectrum of visible light. A mixture of most of the wavelengths within this range will give rise to a ‘white light’, but if any wavelengths predominate the light will be coloured. Thus a coloured light can be defined in terms of the intensity and wavelength of its components. Paper Code and Title: H01RS Residential Space Designing Module Code and Name: H01RS19 Light – measurement, related terms and units Name of the Content Writer: Dr. S. Visalakshi Rajeswari 2. Sources of light There are two sources from which light is received. One is from incandescent bodies (sources which are hot like Sun, bright luminaires, natural or artificial flame etc) and luminescent bodies (cold sources, that is, all the objects in the environment which reflect light). Sunlight is a simple thermal source and approximately 44 per cent of sunlight energy reaching the ground is visible. The glowing solid particles in a flame are also common thermal light sources. Incandescent bulbs, another example emits only 10 percent of the energy as visible light. 3. Spectral sensitivity of the human eye Except when looking directly at a light source, sensations in the eye are caused by light reflected from the object in view. The human eye does not exhibit equal sensitivity to all wavelengths in the visible spectrum. The eye consists of cone and rod cells, is a known fact, in which the cone cells are of three types. They are found to respond differently across the visible spectrum. The response peaks is somewhere around 555 nm. Naturally, visible light of uniform intensity from twin sources of light are found not to appear equally bright. The eye responds to light as a function of the wavelength. This concept is represented as luminosity function. In this context, the eye responds differently in different situations. It is called photopic vision when it adapts to light, (luminance levels over three candela per sq.m), as scotopic vision when it adapts to dark conditions. Photopic response is the base for Photometry. Hence, the brightness of sources in dim lighting predicted from the data obtained from such a method may not be reliable. Here colours may not be that discernible. Photometry (measurement of light) is a method that assesses this. These units relate to luminous efficacy, a term used to explain raw power by a quantity. It is absolutely necessary to determine/ arrive at sufficient illumination for various tasks in lighting of buildings – both interiors and exteriors. 4. Methods used for measurement of light Light is measured adopting two methods. Radiometry which measures light power individually at all wavelengths and photometry which on the other hand measures light based on wavelength weighted on par with a standardized model of perception of brightness by the human eye are the two sets of units in practice to measure light. For quantifying lighting intended for human use, latter method which measures only the visible light is used. It considers human eye as a detector and gives data in multiple photometric units and measures. There is a factor representing the sensitivity of the eye to particular wavelengths. Photometry allots a weightage to the measured power to individual wavelengths with that factor. Paper Code and Title: H01RS Residential Space Designing Module Code and Name: H01RS19 Light – measurement, related terms and units Name of the Content Writer: Dr. S. Visalakshi Rajeswari Light propagates through three dimensional spaces in many ways. It can spread out, concentrate and/ or reflect off shiny surfaces. Similarly, light consists of different wavelengths. This attribute enables scientists to fundamentally measure different kinds of light in a broad range and also represent them in varied quantities and units. Photometry considers three aspects in measuring light: amount of meaningful light incident on a surface, quantum of light emitted by the source and the wavelengths with their specific colours associated with the light. Therefore measurement of useful light should always consider luminosity function. The unit used is the candela (cd) from which all other units are derived. Light is considered as a form of energy and when it is measured as energy units, it becomes a physical unit. In applications where light is measured based on its brightness, they are explained in terms of subjective units such as candela, lux and the like. It is because of this subjective difference that light is described in multiple units. The word ‘bright’ (usually used to refer to source of light) can be used as an adjective to refer to light in many ways. A bright light is a source: from which emerges a high density light output ( given in units of lumens) from which channelizes and focuses the flux into a very narrow beam ( expressed as candela) focused against a dark background Naturally, when considering the amount of light required in relation to the task being undertaken it is the brightness of the object which is of importance. This brightness depends on the illumination and on the proportion of the incident light which the objects reflect. Brightness, scientifically speaking is described in two terms: luminosity and luminance. 4.1. Luminosity: defines the subjective characteristics of the visual sensation. They are influenced by various factors not directly related to the light reflected into the eye. 4.2. Luminance (brightness): refers to the quantity of reflected light measured photo metrically. It is the amount of light per unit area reflected from or emitted by a surface, per se, that gets channelized within a given solid angle. This factor called luminance is measured in terms of a basic unit known as the Lambert in non – SI unit. The Lambert is distinctly stated adopting the symbols - L, la or Lb, proving that it is a non – SI unit of luminance. Contrarily, the SI unit of luminance is candela / sq. m (cd/m2) and the CGS unit is stilb. 5. Measurement of light (Photometry) There are many concepts and terms that relate to the intensity of light. Each one has their own units of measurement as explained here individually Paper Code and Title: H01RS Residential Space Designing Module Code and Name: H01RS19 Light – measurement, related terms and units Name of the Content Writer: Dr. S. Visalakshi Rajeswari 5.1. Candela (cd): Formerly called the ‘candle’ is the international unit used to express an estimate of luminous intensity, specifically, its denseness emitted from a source. Luminous intensity is the power of the light source. 5.2. Candle power (cp): It is the unit used to express an estimation of the luminous intensity of a light source as expressed in candelas. It basically is considered a measure of the light produced by a bulb. It is the light measured at level of the bulb, not what falls on a medium. Measurement of the intensity of light is based on the concept of density of Luminous Flux (F), flowing from a standard source of radiant energy, which is arbitrary and is called the “international or standard candle”. 5.3. Luminous Flux: It is defined as the amount of useful light emitted from the source, its light output. More specifically it is the light energy in toto that a source emits across visible wavelengths of light. It relates to the time rate of flow of light measured in Lumens (lm). This refers to the flow of light related to a unit of solid angle. Evidently it is the quantum of light that leaves the lamp. 5.4. Lumen (lm): is considered as the photometric unit of light flow or light output. It is given as the term referring to the unit used for measuring luminous flux. The light emerging from a known point source of one candela (12.57 lumens) falling on an area of one feet square (1 ft2) at a distance of one feet is one lumen. It is also given as equal to one foot candle falling on a surface of one foot square. It is the quantity of continuously generated light. Packs of an incandescent bulb emitting 60 watt and a compact fluorescent (CFL) labeled 15 watt both indicate that they provide 900 lumens each.
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