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The Colour Rendering Index is a quantification of the faithfulness of colour appearance under the test light source compared to the colour appearance under the standard illuminant.
The test compare the appearance of eight color samples (see below) under the light in question and a reference light source. The average differences measured are subtracted from 100 to get the CRI. Small average differences will result in a higher score, while larger differences give a lower number. Of all the colors possible, only these eight are measured. Samples used are pastels, not saturated colors.
CRI is calculated by measuring the difference between the lamp in question and a reference lamp in terms of how they render the eight color samples. If the lamp to be tested has a correlated color temperature (CCT) of less than 5000 Kelvin (K), the reference source is a black body radiator (approximately like an incandescent lamp). For higher CCT sources, the reference is a specifically defined spectrum of daylight. Light sources that mimic incandescent light or daylight for the eight color samples are, by definition, the ones that will score highest on the CRI.
Main 8 patches are pastel colors and 8 more patches provide diversity adding more saturated colors and few other useful ones.
Unfortunately, high quality, high CRI leds have less efficiency compared to low quality ones, especially if they are not warm.
Light Measurement Handbook by Alexander D. Ryer
Useful, on point and short.
Language of light