Spectral luminous efficiency functions in the mesopic range.

Spectral luminous efficiency functions for mesopic vision were measured extensively to establish a basic data set for the mesopic photometry system. These functions were obtained by the direct brightness matching for a 10 degrees field at nine retinal illuminance levels from 100 to 0.01 Td in 0.5-log-unit steps. Data for 12 subjects were collected. In the mesopic range, the spectral luminous efficiency function, averaged over 12 subjects, gradually changes form with retinal illuminance. This suggests that the mesopic function should be established for all the levels in that range. An attempt was made to find an empirical formula that expresses the mesopic function as a simple combination of the photopic and the scotopic brightness functions.