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 : Chromatic adaptation uses this von Kries chromatic transform equation to find the corresponding color (uc,i, vc,i) for each sample. The mixed subscripts (t, i) refer to the inner product of the test illuminant spectrum and the spectral reflexivity of sample i: where subscripts r and t refer to reference and test light sources respectively. Test color samples As specified in , the original test color samples (TCS) are taken from an early edition of the Munsell Atlas. The first eight samples, a subset of the eighteen proposed in , are relatively low saturated colors and are evenly distributed over the complete range of hues. These eight samples are employed to calculate the general color rendering index . The last six samples provide supplementary information about the color rendering properties of the light source; the first four for high saturation, and the last two as representatives of well-known objects. The reflectance spectra of these samples may be found in , and their approximate Munsell notations are listed aside. R96a method In the CIE's 1991 Quadrennial Meeting, Technical Committee 1-33 (Color Rendering) was assembled to work on updating the color rendering method, as a result of which the R96a method was developed. The committee was dissolved in 1999, releasing , but no firm recommendations, partly due to disagreements between researchers and manufacturers. The R96a method has