A score, with 100 as the maximum, for how accurately a light source shows colours compared with a reference light of the same colour temperature.
CRI (Ra) is a CIE-defined metric comparing a light source's rendering of a standard set of test colours against a reference illuminant of the same colour temperature.
CRI and colour temperature measure different things — a light can be a flattering warm colour temperature but still have poor CRI, rendering colours inaccurately.
Many cheap LED bulbs have lower CRI than daylight or high-quality bulbs, which is part of why paint samples can look 'off' under them.
A low-CRI bulb is a common, under-recognised reason a paint colour looks different at home than in a shop or outdoors.
The lamp's spectrum is measured, and the colours of a set of standard test samples are calculated under it and under a reference light of the same correlated colour temperature: a blackbody below 5,000 K, a phase of daylight at or above it. The colour shift of each sample becomes a special index, and the general index Ra is the average for the first eight samples, all of moderate saturation. The reference scores 100 by definition, and poor sources can score below zero. Six further samples, including the strong red reported as R9, are calculated but are not part of Ra.
LED lamps are bought on Ra alone for a kitchen, a clothes shop or a make-up mirror.
Ra averages eight moderately saturated samples and leaves out strong red, so a lamp can reach 80 while rendering skin, meat, wood and red fabric as dull. Ask for R9 as well, or for the TM-30 figures.
A colour match is judged under a high-CRI lamp and assumed safe everywhere.
A high index means the lamp behaves like its reference, not that a metameric pair will stay matched under some other light. Critical matches are checked under at least two different sources.