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Grendelkhan (talk | contribs) brought in some data from color temperature---left out the bit about the measure being inaccurate, as it smelled pov and i can't verify it |
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CRI is a quantitatively measurable index, not a subjective one. A reference source, such as blackbody radiation, is defined as having a CRI of 100 (this is why incandescent lamps have that rating, as they are, in effect, blackbody radiators), and the test source with the same color temperature is compared against this. Both sources are used to illuminate several standard samples. The perceived colors under the reference and test illumination (measured in CIE 1931 form) 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.
The standard formula consists of taking the color differences <math>\Delta E_i</math>, between the test color and the eight samples, on the 1964 W*U*V* uniform color space (which is now obsolete). The color rendering index <math>R_i</math> is calculated for each of the eight samples:
:<math>R_i=100-4.6\Delta E_i\,</math>
which gives the color rendering index with respect to each sample. The general color rendering index <math>R_a</math> is then the average of these eight separate indices.
==External link==
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