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Color Rendering Index

Color Rendering Index (CRI) is a specification of the color rendering property of a light source.

Description

The CRI is calculated by comparing the colorimetric values for a given set of test colors illuminated by the test light source against the values for the same test colors illuminated by a reference illuminant with a color temperature close to that of the test light source. The calculations produce a rendering index for each patch and the CRI for the light source is the mean of the rendering indices for the test color set. The resulting CRI has a value of 100 if the test light source gives identical rendering indices with the reference illuminant.

The choice of the reference illuminant changes depending on the test light source's color temperature. Below 4000 K a Planckian radiator is used for the reference illuminant. From 4000 K and above a daylight simulated reference illuminant is used.

The basic set of eight test color samples are moderate in saturation, approximately the same in lightness and span the hue circle. An additional six test color samples can be optionally added to the basic eight samples. These six samples vary widely in saturation and lightness, representing four strong primary colors, a flesh tone and a foliage color.

    CIE 1976 L*a*b*
D50, 2°
   
Sample ISCC-NBS Color Name L* a* b* Approximate
Color
Set
1 Light grayish red
62.01
18.98
12.89
Basic
2 Dark grayish yellow
61.07
2.74
29.27
Basic
3 Strong yellow green
62.16
-17.82
43.92
Basic
4 Moderate yellowish green
60.79
-31.68
15.26
Basic
5 Light bluish green
61.86
-19.06
-9.73
Basic
6 Light blue
60.92
-4.03
-29.22
Basic
7 Light violet
61.13
17.73
-23.87
Basic
8 Light reddish purple
63.22
27.32
-12.29
Basic
9 Strong red
41.87
62.01
31.81
Extended
10 Strong yellow
81.97
1.81
71.59
Extended
11 Strong green
51.62
-41.12
11.52
Extended
12 Strong blue
29.20
-5.22
-49.38
Extended
13 Light yellowish pink
80.79
13.67
21.83
Extended
14 Moderate olive green
40.81
-12.38
24.08
Extended

The CRI computed with the basic set is called the General Color Rendering Index.

Reference

CIE 13.2 1974 (Corrected 1988), Method of Measuring and Specifying Colour Rendering Properties of Light Sources.

Updated 15.5.2007

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