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The '''ColourColor rendering index''', or '''CRI''', is a measure of the qualityability of [[colour]]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). ItThe generallybest rangespossible fromrendition of colors is specified by a CRI of one hundred, while the very poorest rendition is specified by a CRI of zero. forFor a source like a low-pressure [[sodium vapor lamp]], which is monochromatic, tothe oneCRI hundredis nearly zero, but for a source like an [[incandescent light bulb]], which emits essentially [[blackbody radiation]], it is nearly a hundred. ItThe CRI is relatedmeasured toby [[colourcomparing temperature]],the incolor thatrendering of the CRItest measuressource forto that of a pair"perfect" ofsource lightwhich sourcesis cangenerally onlya beblack comparedbody ifradiator, theyexcept havefor thesources samewith colour[[color temperature]]s above 5000K, in which case a simulated daylight (e.g. A[[D65]]) is used. For example, a standard "cool white" [[fluorescent lamp]] will have a CRI near 62. Newer "triphosphor" fluorescents often claim a CRI of 80 to 85.
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 colourcolor 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 severaleight standard samples. The perceived colourscolors under the reference and test illumination (measured in the [[CIE 1931 colourcolor 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 measuring the color indices of eight sample colors on the 1964 W*U*V* uniform colourcolor space (which is now obsolete). The indices of the samples are first measured while being illuminated by the reference source, yielding indices <math>[W_{i0},U_{i0},V_{i0}]</math> where the index ''i'' specifies the particular sample color. The indices of the samples are then measured under the test source yielding indices <math>[W_i,U_i,V_i]</math>. The distances <math>\Delta E_i</math> between the measured colors is then calculated:
:<math>\Delta E_i=\sqrt{(U_i-U_{i0})^2+(V_i-V_{i0})^2+(W_i-W_{i0})^2}\,</math>
The colourcolor 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 colourcolor rendering index with respect to each sample. The general colourcolor rendering index <math>R_a</math> is then the average of these eight separate indices.
:<math>R_a=\frac{1}{8}\sum_{i=1}^8 R_i</math>
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 colourcolor-rendering quality for real scenes, particularly for modern (e.g. fluorescent) lightsources with spikey emission specta, or white LEDs. It is understood that the CIE is looking at developing newer colourcolor-rendering performance metrics.
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