Color rendering index
Color rendering index, or CRI, is a measure of the quality of color light, generally ranging from zero for a source like a low-pressure sodium vapor lamp, which is monochromatic, to one hundred, for a source like an incandescent light bulb, which emits essentially blackbody radiation. It is related to color temperature, in that the CRI measures for a pair of light sources can only be compared if they have the same color temperature. A standard "cool white" fluorescent lamp will have a CRI near 62.
CRI is a quantitatively measurable index, not a subjective one. A reference source, such as blackbody radiation, is used to illuminate several standard samples. The apparent color under the reference and test illumination is compared in a particular formula, and averaged over the number of samples taken (usually eight) to get the final CRI.