The color rendering index (CRI) (sometimes called color rendition index), is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as photography and cinematography.[1] It is defined by the International Commission on Illumination as follows:
Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant[2]
Note that the CRI by itself does not indicate what the color temperature of the reference light source is; therefore, it is customary to also cite the correlated color temperature (CCT).
According to (Schanda & Sándor 2005), CRI is being deprecated in favor of measures based on color appearance models, such as CIECAM02 and, for daylight simulators, the CIE Metamerism Index. (Guo & Houser 2004) and (CIE 1995) note that CRI is not a good indicator for use in visual assessment, especially for sources below 5000 kelvin (K).
A newer version of the CRI has been developed (R96a), but it has not replaced the better-known Ra (general color rendering index).
Typical values Light source |
CCT (K) |
CRI |
Low Pressure Sodium (LPS/SOX) |
1800 |
~5 |
Clear Mercury-vapor |
6410 |
17 |
High Pressure Sodium (HPS/SON) |
2100 |
24 |
Coated Mercury-vapor |
3600 |
49 |
Halophosphate Warm White Fluorescent |
2940 |
51 |
Halophosphate Cool White fluorescent |
4230 |
64 |
Tri-phosphor Warm White Fluorescent |
2940 |
73 |
Halophosphate Cool Daylight Fluorescent |
6430 |
76 |
"White" SON |
2700 |
82 |
Quartz Metal Halide |
4200 |
85 |
Tri-phosphor Cool White fluorescent |
4080 |
89 |
Ceramic Metal Halide |
5400 |
96 |
Incandescent/Halogen Light Bulb |
3200 |
100 |
A reference source, such as black body radiation, is defined as having a CRI of 100. This is why incandescent lamps have that rating, as they are, in effect, almost black body radiators. The best possible faithfulness to a reference is specified by a CRI of one hundred, while the very poorest is specified by a CRI of zero. A high CRI by itself does not imply a good rendition of color, because the reference itself may have an imbalanced SPD if it has an extreme color temperature (see next section).
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