We have examined the influence of the mean luminance level on the detection thresholds for luminance and red-green chromatic gratings for three different spatial frequencies. The changes in detection thresholds according to the mean luminance level reflect the two different regions, the DeVries-Rose and Weber ranges, found in previous studies. The results for luminance gratings suggest that the transition luminance is proportional to the spatial frequency of the grating. Predictions based on the constant-flux hypothesis indicate, however, that the transition luminance is proportional to the square of the spatial frequency of the grating and so do not describe the distributions of luminance contrast thresholds adequately. For chromatic gratings, we obtained the same transition luminance for the two lowest spatial frequencies, showing that luminance and chromatic mechanisms behave differently as far as the dependence of the transition luminance on spatial frequency is concerned. Our results suggest that the transition luminance is related to the peak spatial frequency of visual mechanisms that respond to luminance and chromatic gratings. (C) 2004 Wiley Periodicals. Inc.