Abstract
The increase of general index R a for some illuminants having chromaticities on the purple side of the Planckian locus compared with those on the green side is only partly due to the von Kries correction in the revised CIE method, and the location of the Judd PDT primaries used has little effect. Uniformity of the u, v diagram in the area involved is questionable, and its geometry is a likely cause of some of the results. Gamut areas of the eight test colours suggest a correlation between colour discrimination and colour rendering but only for certain illuminants. Variable sensitivity of R a to the spectral distribution of the illuminant is demonstrated; for wavelengths near 580 nm there is evidence of a marked effect on the index.
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