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Color Rendering Index
Author: Jack Chow    Date: 2015-7-15

 CRI - Color Rendering Index

The Color Rendering Index (CRI) is the relative ability of a light source to replicate colors generated by a reference light source of the same color temperature. It is measured as a percentage. 
What this means is that a CRI of 100%, taken at a color temperature of noon day sunlight (6000k) will exactly reproduce the colors found on a sunny day at noon. 
A CRI of 50% taken at noon day sun will partially reproduce the colors when compared to that of a normal noon day. 
Some marketers use differing color temperatures as a reference to attain 100% CRI, when the actual CMI would be much less than 100% if noon day sun light was used as a reference. 
If you notice how a landscape changes color as the sun sets. This is a change in the color temperature. 100% CRI at sunset would result in a yellow or orange appearance. That same measure against noon day sun might only produce a CRI of 60% or less. 
If this has not confused you try this wikipedia definition: 
"The Color Rendering Index (CRI) (sometimes called 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 (CIE). The best possible rendition of colors is specified by a CRI of one hundred, while the very poorest rendition is specified by a CRI of zero. For a source like a low-pressure sodium vapor lamp, which is monochromatic, the CRI is nearly zero, but for a source like an incandescent light bulb, which emits essentially black body radiation, it is nearly one hundred. The CRI 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 CRI near 63. Newer "triphosphor" fluorescent lamps often claim a CRI of 80 to 90. 
CRI is a quantitatively measurable index, not a subjective one. A reference source, such as black body radiation, is defined as having a CRI of 100 (this is why incandescent lamps have that rating, as they are, in effect, almost blackbody radiators), and the test source with the same color temperature is compared against this. Both sources are used to illuminate eight standard samples. The perceived colors under the reference and test illumination (measured in the CIE 1931 color space) are compared using a standard formula, and averaged over the number of samples taken (usually eight) to get the final CRI. Because eight samples are usually used, manufacturers use the prefix "octo-" on their high-CRI lamps." 
Examples of color temperature and CRI
Candle: 1700k 100 CRI High Pressure Sodium: 2100k 25 CRI Incandescent: 2700k 100 CRI Tungsten Halogen: 3200k 95 CRI Cool White: 4200k 62 CRI Clear Metal Halide: 5500k 60 CRI Natural Sunlight: 5000-6000k 100 CRI Daylight Bulb: 6400k 80 CRI 
Many products marketed as 'natural sunlight' light sources are far from noon day sun.
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