Increments and decrements: luminance discrimination

Photopic luminance discrimination thresholds were measured between two simultaneous spatially separate 1 degree squares presented for durations between 6 and 800 msec on an 11 degree background. For increments and weak decrements, threshold was proportional to the luminance difference, delta L, between square and background. Stronger decrements, except at the shortest durations, were more discriminable than increments with the same delta L, threshold being proportional to absolute luminance down to a 'black limit'. All the data for durations over 100 msec obey to a first approximation the rule delta W/W = constant, where W = delta L/Lmin, and Lmin is the smaller of the square and background luminances.