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. Color is an important part of the visual arts. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific setting, electromagnetic radiation of any wavelength. ... The International Commission on Illumination (usually known as the CIE for its French-language name Commission Internationale de lEclairage) is the international authority on light, illumination, colour, and colour spaces. ... A sodium vapor lamp is a gas discharge lamp which uses sodium in an excited state to produce light. ... An incandescent light bulb and its glowing filament. ... As the temperature decreases, the peak of the black body radiation curve moves to lower intensities and longer wavelengths. ... White light is commonly described by its color temperature. ... A compact fluorescent lamp with an integrated electronic ballast A fluorescent lamp is a type of lamp that uses electricity to excite mercury vapor in argon or neon gas, resulting in a plasma that produces short-wave ultraviolet light. ...

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 ΔE_i, 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 R_i is calculated for each of the eight samples:

which gives the colour rendering index with respect to each sample. The general colour rendering index R_a 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

• Formula to calculate CRI