page_title: Color rendering index
section_title: Test method
hierarchical_section_title: Color rendering index / Test method
caption_attribution_description: CIE 1960 UCS. MacAdam (u,v) chromaticity diagram
context_page_description: A color rendering index is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as neonatal care and art restoration. It is defined by the International Commission on Illumination as follows:
  Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.
  The CRI of a light source does not indicate the apparent color of the light source; that information is given by the correlated color temperature. The CRI is determined by the light source's spectrum. An incandescent lamp has a continuous spectrum, a fluorescent lamp has a discrete line spectrum; the incandescent lamp has the higher CRI.
  The value often quoted as "CRI" on commercially available lighting products is properly called the CIE Rₐ value, "CRI" being a general term and CIE Rₐ being the international standard color rendering index.
context_section_description: 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.
   
  Using the 2° standard observer, find the chromaticity co-ordinates of the test source in the CIE 1960 color space.
  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.
  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.
  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. 
  Illuminate the first eight standard samples, from the fifteen listed below, alternately using both sources.
  Using the 2° standard observer, find the co-ordinates of the light reflected by each sample in the CIE 1964 color space.
  Chromatically adapt each sample by a von Kries transform.
  For each sample, calculate the Euclidean distance  between the pair of co-ordinates.
  Calculate the special (i.e., particular) CRI using the formula 
  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 :
caption: CIE 1960 UCS. Planckian locus and co-ordinates of several illuminants shown in illustration below.
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