The Color Rendition Index
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The color rendition index, is a measure of the ability of a light source to reproduce the colors of various objects being lit by the source. It is a method devised by the International Commission on
Illumination. The best possible rendition of colors is specified by a color rendition index of one hundred, while the very poorest rendition is specified by a color rendition index of zero. For a source like a low pressure sodium vapor lamp, which is monochromatic, the color rendition index is nearly zero but for a source like an incandescent light
bulb, which emits essentially black body radiation, it is nearly one hundred. The color rendition index is measured by comparing the color rendering of the test source to that of a perfect source which is generally a black body radiator, except for sources with color temperatures above 5000K, in which case a simulated daylight (e.g. D65) is used. For example, a standard cool white fluorescent lamp will have a color rendition index near 63. Newer fluorescent lamps often claim a color rendition index of 80 to 90.
Color rendition index is a quantitatively measurable index, not a subjective one. A reference source, such as black body radiation, is defined as having a color rendering index of 100 this is why incandescent lamps have that rating as they are, in effect, almost black body radiators, and the test source with the same color temperature is compared against this. Both sources are used to illuminate eight standard samples. The perceive colors under the reference and test illumination measured in the color rendition index 1931 color space are compared using a standard formula, and averaged over the number of samples taken usually eight to get the final color rendition index.
The standard formula consists of measuring the color uses eight sample colors on the 1964 W*U*V* uniform color space which is now obsolete. The uses of the samples are first measured while being illuminated by the reference source, yielding usage where the index i specifies the particular sample color. The usage of the samples are then measured under the test source yielding. The distances between the measured colors is the color rendering index is calculated which gives the color rendering index with respect to each sample. The factor 4.6 was so chosen that the Ra of a standard warm white transparent lamp would be about 50. It also appeared the Roi could be negative, and this was indeed calculated for some lamp test colors the general color rendering index Ra is the average of these eight separate usages.
In 1965, in order to be able to objectively compare the color rendering properties of light sources, the commission on illumination introduced a standard measuring method. This method calculation the color change of 14 test colors under the light source being tested relative to the colors measured under a reference code. The first eight test colors are relatively non-saturated colors and are evenly distributed over the complete range of hues. These 8 test colors are employed to calculate the general color are employed to supply extra information about the color rendering