Color Rendering Index - Test Method

Test Method

The CRI is calculated by comparing the color rendering of the test source to that of a "perfect" source which is a black body radiator for sources with correlated color temperatures under 5000 K, and a phase of daylight otherwise (e.g. D65). Chromatic adaptation should be performed so that like quantities are compared. The Test Method (also called Test Sample Method or Test Color Method) needs only colorimetric, rather than spectrophotometric, information.

  1. Using the 2° standard observer, find the chromaticity co-ordinates of the test source in the CIE 1960 color space.
  2. Determine the correlated color temperature (CCT) of the test source by finding the closest point to the Planckian locus on the (u,v) chromaticity diagram.
  3. If the test source has a CCT<5000 K, use a black body for reference, otherwise use CIE standard illuminant D. Both sources should have the same CCT.
  4. Ensure that the chromaticity distance (DC) of the test source to the Planckian locus is under 5.4×10−3 in the CIE 1960 UCS. This ensures the meaningfulness of the result, as the CRI is only defined for light sources that are approximately white.
  5. Illuminate the first eight standard samples, from the fifteen listed below, alternately using both sources.
  6. Using the 2° standard observer, find the co-ordinates of the light reflected by each sample in the CIE 1964 color space.
  7. Chromatically adapt each sample by a von Kries transform.
  8. For each sample, calculate the Euclidean distance between the pair of co-ordinates.
  9. Calculate the special (i.e., particular) CRI using the formula
  10. Find the general CRI (Ra) by calculating the arithmetic mean of the special CRIs.

Note that the last three steps are equivalent to finding the mean color difference, and using that to calculate :

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