Color rendering index: Difference between revisions

Revision as of 15:18, 18 December 2005

Revision as of 15:19, 18 December 2005

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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 colour temperature is compared against this. For colour-temperatures of 5000K and above, the reference source is a simulated daylight (e.g. [[D65]]) rather than a true blackbody. Both sources are used to illuminate several standard samples. The perceived colours under the reference and test illumination (measured in the [[CIE 1931 colour 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.

The standard formula consists of taking the colour differences <math>\Delta E_i</math>, between the test colour and the eight samples, on the 1964 W*U*V* uniform colorcolour space (which is now obsolete). The colour rendering index <math>R_i</math> is calculated for each of the eight samples:

:<math>R_i=100-4.6\Delta E_i\,</math>