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It is important to remember that illuminance is not the same as luminance. Illuminance is the light falling on the surface and is independent of reflectivity. Luminance is the amount of light reflected from the surface and has nothing to do with the original light source.
Colour is a psychological phenomenon. It is an interpretation by the brain of sen- sations that are stimulated by radiation. Objects themselves are not coloured, they only appear coloured because they reflect light from a particular wavelength of the visible spectrum.
The illuminating light in our environment is white, but we see most objects as coloured, not white. This is because objects absorb certain wavelengths contained in the white light and reflect the rest. This reflected light creates a particular sensation of colour. Colours therefore are a result of the light they reflect, and not the light they absorb.
The phenomenon of reflectivity, when expressed as the amount of light reflected by a particular colour, is called its Light Reflectance Value, commonly known by the acronym, LRV.
British Standard (BS 8300:2001/A1:2005) defines Light Reflectance Value (LRV) as the total quantity of visible and usable light reflected by a surface in all directions and at all wavelengths when illuminated by a light source.
LRV is a numerical rating assigned to each colour and is a scientifically determined assessment of the amount of light that colour will reflect on a scale of 0 to 100. The reflected radiations considered are those in the visible region only. The term albedo is broadly used when the total reflectance is considered, and is not restricted to the visible region only.
Pure black which has an LRV of zero, absorbs all light. On the other hand, pure white has an LRV of nearly 100, and can reflect all the incident light. All colours fit in between these two extremes. A colour with an LRV of 50 will reflect 50% of the light that falls on it.
In standard LRV notation, absolute black is assigned a Light Reflectance Value of zero, and a perfectly reflective white is assumed to have an LRV of 100. However, absolute black and perfectly reflective white do not occur in everyday scenarios. In practical application, the blackest black has an LRV of 5, and the whitest white has an LRV of 85. Yellows are also highly reflective, some yellows have LRVs close to those of the brightest whites.
There are several terms that are commonly used when describing or measuring light reflectance. LRV, luminance, and luminosity function are terms describing the same concept—the amount of incident light that a surface reflects. Tools such as a spectro- photometer measure LRV using the CIE Standard, established by the International Commission on Illumination (Commission Internationale de l’Éclairage).
As an industry rule of thumb, an LRV of 70 units of contrast between sign text and background colours is an ideal guideline to follow when evaluating colour and finish options. 70 is not a fixed number and contrasts of a few units below the recommended figure, depending on the colours chosen, can also yield highly legible results. Yellow is one of the most reflective colours in the spectrum and hence has a high LRV. Some yellows have an LRV of more than 80. However, at times this can be misleading. Choosing a yellow using just its LRV could result in an intense yellow as opposed to a calmer, softer yellow.
From a sustainability point of view, a wall colour with higher LRV supports lighting plans by helping propagate daylight deep into the room space. LRVs of fittings such as tiles, carpets, and furnishings also help in reflecting existing light within the room. Colours with LRVs higher than 50 will be lighter and will reflect more light back into the room. This reduces the requirement of the standard number of lighting fixtures required to enable efficient and safe performance of tasks. The reflectivity of exterior paint colours can even keep inner environments cool which helps consume less electricity for cooling purposes.
Temperature Control: The heating effect of the sun is due to the Near Infrared Radiations (NIR) in sunlight. As LRV is restricted to the visible region of radiations, it theoretically does not govern the heating effect. However, it has been observed that whites tend to reflect radiation not only in the visible region but also in the NIR region and hence are able to reduce the heating effect of sunlight. Most blacks tend to have low LRVs in the visible, as well as the NIR region, which results in an increase in overall temperature.
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