Chromatic discrimination in relation to luminance level

We have measured the chromatic-discrimination ellipses with different lumin ances for 66 stimuli distributed throughout the CIE-1931 chromatic diagram. The distribution of these stimuli enabled us to analyze the influence of l uminance on discrimination from clearly photopic levels to levels that coul d enter the mesopic range. The results show a clear influence of the lumina nce level on the areas of the chromatic-discrimination ellipses. These area s remain almost constant when the luminance level is clearly photopic, and clearly increase when the luminance diminishes. However, it is not necessar y that the luminance level diminish far below 2-3 cd/m(2) to detect a subst antial increase in the area of the ellipses. According to our data, within the photopic range appears a transition interval of luminance above which t he most pronounced increase in the area of the ellipse would be appreciable . Other parameters characteristic of discrimination ellipses, such as orien tation and relation to semi-axes, vary with the luminance level, although w ithout clearly following any rule. (C) 2001 John Wiley & Sons, Inc.