color rendering index

A figure of merit, on a scale of 0 to 100, used by manufacturers of fluorescent, metal halide and other nonincandescent lighting equipment to describe the visual effect of the light on colored surfaces. Natural daylight and any light source approximating a blackbody source (see color temperature) is assigned a color rendering index (CRI) of 100.

To determine a CRI value, observers view 8 standard pastel colors under the light source being rated and under light from a blackbody source (such as an incandescent lamp) having the same color temperature. The CRI is calculated, roughly speaking, by averaging the observers' estimation of the extent of the differences in the appearance of the colors under the two lights. Although the CRI is intended to apply only to the visual quality of light, photographers have also used it in choosing light sources.

The CRI can only be used to compare two light sources that have the same color temperature. A 5000 K, 80 CRI light source is not necessarily superior to a 4000 K, 70 CRI light source.

some typical
color rendering index
(CRI)
values for light sources
Light source	CRI
clear mercury	17
white deluxe mercury	45
warm white fluorescent tube	55
cool white fluorescent tube	65
deluxe warm white fluorescent	73
daylight fluorescent	79
metal halide 4200K	85
deluxe cool white fluorescent	86
metal halide 5400K	93
low pressure sodium	0-18
high pressure sodium	25
100-watt incandescent	100

Illuminating Engineering Society.
Interim Method of Measuring and Specifying Color Rendering of Light Sources, LM-19, NY, 1962.

For an exposition of the mathematics of calculating the CRI, visit www.kruschwitz.com/cri.htm