Another important measure of color quality used by the lighting industry is the color
rendering index (CRI). CRI indicates how well a light source renders colors, on a scale of 0 to 100, compared to a reference light source of similar color temperature.

The test procedure established by the International Commission on Illumination (CIE) involves measuring the extent to which a series of eight standardized color samples differ in appearance when illuminated under a given light source, relative to the reference source.

The average “shift” in those eight color samples is reported as Ra or CRI. In addition to the eight color samples used by convention, some lighting manufacturers report an “R9” score, which indicates how well the light source renders a saturated deep red color.