Uniform Chromaticity Scales — New Experimental Data

Sets of coloured paint and textile samples have been prepared, each set consisting of pairs of samples giving small chromaticity differences with a good coverage of the possible directions. For most sets the lightness differences were negligible, but for a few sets extra samples showing significant lightness differences were included. For each set one difference was taken to be standard and the other differences were assessed as a ratio of the standard difference by panels of 19 to 24 observers. Chromaticity discrimination ellipses were calculated for each set. Experiments in which pairs from different sets were inter‐compared enabled the sizes of the ellipses to be correctly adjusted relative to each other. In total 42 sets (536 sample pairs) were investigated under Illuminant D65 and 39 sets (531 sample pairs) were investigated under Illuminant A. When plotted on a chromaticity diagram the ellipses formed a fairly regular pattern, but marked differences compared to other sets of ellipses were noted. The ellipses increased in size as Y decreased, particularly at very low values of Y. The agreement between ΔE values from various colour‐ difference equations and the experimental results was poor, although newer equations (JPC and FCM) appeared to be somewhat better than the 1976 CIELAB equation. Various checks on the repeatability and consistency of the experimental results gave confidence in the latter and suggest that the equations are seriously in error.