Threshold color differences compared to supra-threshold color differences

Threshold color difference ellipses, as represented by the MacAdam ellipses have been compared to unit contours implicit in the CIE94 color difference formula, found to represent small supra-threshold color differences well, both in the CIE chromaticity diagram as well as the a*, b* diagram. The res ults indicate that the two types of contours are significantly different. W hile threshold differences are well described by incremented changes in con e sensitivity supra-threshold differences are well-described by increments in the opponent color parameters a*, and b*. A comparison of the hue angles in the a*, b* diagram of the MacAdam ellipses and the CIE94 contours for t he same 25 colors indicates no correlation (correlation coefficient -0.1). The conclusion is that threshold and supra-threshold color difference perce ption operates at different levels in the color vision system.