The colour rendering index (CRI), sometimes called as colour rendition index, is a quantitative measure of the ability of a light source to reveal the colours of various objects faithfully in comparison with an ideal or natural light source.
Light sources with a high CRI are desirable in colour critical applications such as photography and cinematography.
It is defined by the International Commission on Illumination (CIE, in French) as the effect of an illuminant on the colour appearance of objects by conscious or subconscious comparison with their colour appearance under a reference illuminant.
The CRI of a light source does not indicate the apparent colour of the light source. That information is under the rubric of the Correlated Colour Temperature (CCT).
CRI’s ability to predict colour appearance has been criticized in favour of measures based on colour appearance models, such as CIECAM02 and for daylight simulation, the CIE metamerism index.
CRI is not a good indicator for use in visual assessment, especially for sources below 5000 kelvin (K)
Numerically, the highest possible CRI is 100, for a Black body (incandescent lamps are effectively black bodies), dropping to negative values for some light sources.
Low pressure sodium lighting has negative CRI, fluorescent lights range from about 50 for the basic types, up to about 90 for the best tri phosphor type.
Typical LEDs have about 80+ CRI, while some manufactures claim that LEDs have achieved up to 98 CRI.
A newer version of the CRI, R96a, has been developed but it has not replaced the better known Ra general colour rendering index.
