Color Rendering Index (CRI) - is a measure of the quality of color light, expressed as a rating from 0 to 100. It was devised by the International Commission on Illumination (CIE). It describes how a light source makes the color of an object appear to human eyes and how well subtle variations in color shades are revealed. The higher the CRI rating is, the better its color rendering ability.
Imagine two objects, one red, one blue, which are lighted by a cool light source with a low CRI. The red object appears muted while the blue object appears a rich blue. Now take out the lamp and put in a cool light source with a high CRI. The blue object still appears a rich blue, but the red object appears more like its true color.
Technically speaking, CRI is a quantitatively measurable index, not a subjective one. A reference source, such as blackbody radiation, is defined as having a CRI of 100 (this is why incandescent lamps have that rating, as they are, in effect, blackbody radiators), and the test source with the same color temperature is compared against this. Both sources are used to illuminate several standard samples. The perceived colors under the reference and test illumination are compared using a standard formula, and averaged over the number of samples taken (usually eight) to get the final CRI.