color rendition index (CRI)
Eight standard color samples used in the test-color method for measuring and specifying the color rendering properties of light sources. Adapted from IESNA Handbook.
The color rendition index (CRI), also called color rendering index, is a measure of the quality of light. The maximum CRI value of 100 is given to natural daylight and incandescent lighting. The closer a lamp's CRI rating is to 100, the better its ability to show true colors to the human eye.
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.