Two adjacent semicircular testfields of luminances l and L + delta L, were presented on an adapting field of luminance LU. In earlier experiments, it was found that the just noticeable difference of brightness (jnd) delta L, can be described by delta L = delta Ls + bLc (Lingelbach and haberich 1977). For foveal observations and LU values within the photopic range, delta Ls and b are power functions of LU. In this study, these investigations area continued for scotopic background luminances and extrafoveal conditions (6 degrees and 12 degrees temporal). The following results area obtained: 1. Under scotopic conditions of background luminance, LU, the jnd's for foveal observations are nearly independent of LU. delta L is dependent on the luminance L and can be expressed by the same curve for different levels of LU. 2. For extrafoveal conditions, the results can be described in the same way as for the fovea. The jnd is found to be delta L = delta Ls + bLc. In the total range of LU investigated (0.001 less than or equal to LU less than or equal to 100 cd/m2), the parameters delta Ls and b are power functions of LU. c increases with log L from about 1.1--1.5. 3. The sum of jnd-steps as separate "units of sensation" form a "dynamic characteristic" for each adaptational level. It is different for fovea and periphery. 4. The Fechner scaling of brightness, which corresponds to the sum of jnd-steps up to each adaptational luminance LU ("static characteristic"), shows an unexpected similarity to the Stevens power function.
