If you want to get technical, the algorithm for computing CRI is:
- take a specific set of arbitrary paint chips, and record their colors when illuminated by the target lamp (using the CIE 1931 2° standard observer, and the 1960s CIEUVW color space)
- perform an outdated and not very effective type of chromatic adaptation transformation so that the white point matches either a black body radiator (like an incandescent bulb) for lamps of correlated color temperature of <5000K, or a point on the “Illuminant D” series of approximations to daylight for lamps of CCT >5000K
- measure the distances in UVW space between those target colors and the colors of the paint chips as illuminated by the reference black body radiator or D series illuminant.
- sum up all the color distances, multiply by an arbitrary factor, and subtract from 100.
Everything about this process was totally ad hoc and arbitrary 50 years ago when it was first invented, and is absurdly outdated today.
There was an attempt at the CIE to replace the CRI with something better in IIRC the late 1990s, but the lighting industry didn’t want to follow the recommendations of the color scientists, so it fell apart. (I could be misrepresenting what happened; I’m not an expert in CIE politics or the history here, and haven’t ever researched it in detail.)
- take a specific set of arbitrary paint chips, and record their colors when illuminated by the target lamp (using the CIE 1931 2° standard observer, and the 1960s CIEUVW color space)
- perform an outdated and not very effective type of chromatic adaptation transformation so that the white point matches either a black body radiator (like an incandescent bulb) for lamps of correlated color temperature of <5000K, or a point on the “Illuminant D” series of approximations to daylight for lamps of CCT >5000K
- measure the distances in UVW space between those target colors and the colors of the paint chips as illuminated by the reference black body radiator or D series illuminant.
- sum up all the color distances, multiply by an arbitrary factor, and subtract from 100.
Everything about this process was totally ad hoc and arbitrary 50 years ago when it was first invented, and is absurdly outdated today.
There was an attempt at the CIE to replace the CRI with something better in IIRC the late 1990s, but the lighting industry didn’t want to follow the recommendations of the color scientists, so it fell apart. (I could be misrepresenting what happened; I’m not an expert in CIE politics or the history here, and haven’t ever researched it in detail.)