Colour rendering index (CRI) is a quantitative measure of how accurately a light source renders the colors of objects. It is not a measure of colour temperature - indeed it is independent of colour temperature. Absolute faithfulness to a reference source is represented by a CRI of 100, and the poorest faithfulness is represented by a CRI of zero. Note that a high CRI does not necessarily imply good rendition of color, because a reference source having an extremely high or low colour temperature may have been characterized by an imbalanced spectral power distribution (SPD).

Calculation of CRI involves comparing the appearance of eight standard colour samples such as those shown opposite under both the source in question and a known reference source. The average measured differences are subtracted from 100 to obtain the CRI for the source in question. Small average differences therefore produce high CRI values and larger average differences give lower CRI values. The eight-colour swatch used features pastel rather than saturated colours.

Incandescent bulbs render all colours reasonably well, but fluorescent sources render some colours better than others. The best fluorescent sources have a CRI of about 95.