Color rendering index: Difference between revisions

Revision as of 09:50, 11 December 2005

Revision as of 13:52, 16 December 2005

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'''Colour rendering index''', or '''CRI''', is a measure of the quality of [[colour]] [[light]], devised by the [[International Commission on Illumination]] (CIE). It generally ranges from zero for a source like a low-pressure [[sodium vapor lamp]], which is monochromatic, to one hundred, for a source like an [[incandescent light bulb]], which emits essentially [[blackbody radiation]]. It is related to [[colour temperature]], in that the CRI measures for a pair of light sources can only be compared if they have the same colour temperature. A standard "cool white" [[fluorescent lamp]] will have a CRI near 62. Newer "triphosphor" fluorescents often claim a CRI of 80 to 85 (although this author subjectively is not so impressed!)

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

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which gives the colour rendering index with respect to each sample. The general colour rendering index <math>R_a</math> is then the average of these eight separate indices.

Although an objective measure, the CRI has come under a fair bit of criticism in recent years as it does not always correlate well with the subjective colour-rendering quality for real scenes, particularly for modern (eg fluoresent) lightsources with spikey emission specta, or white LEDs. It is understood that the CIE is looking at developing newer colour-rendering performance metrics.

==External link==