IESNA Definition: measure of the degree of color shift objects undergo when illuminated by the light source as compared with the color of those same objects when illuminated by a reference source, of comparable color temperature.

Light sources differ in their ability to render the color of objects "correctly." The color rendering capability of a lamp is expressed by the Color Rendering Index (CRI). To determine the CRI rating of a specific lamp, testing eight standard color samples are illuminated by a reference light source defined as having a CRI of 100, which very closely matches the test lamp in color temperature. The chromaticity of the samples under the reference source is calculated. The test lamp is then used to illuminate the same samples. All "shifts" in chromaticity between the two tests are tabulated, and the results are averaged to arrive at a single CRI number for the test lamp. Since the CRI of a lamp is related to the reference source of similar color temperature, comparing the CRIs of lamps with different correlated color temperatures (CCT) provides no useful information.

CRI=90	CRI=70	CRI=50
CRI is a general indicator of how "natural" object colors will appear when illuminated by a particular light source. Generally, a CRI of 70 and above will be required for most lighting applications.

	CRI=70		CRI=70
Since the CRI rating for any given lamp is an average of eight test results, it can give no particular insight into the effect of the appearance of any one color. In this case, the two light sources illuminating the object have a CRI of 70, however the light source on the right renders blue more naturally than the one on the left.

The following are typical CRI values for commonly used light sources.