Luminance Requirements for Hue Perception in Small Targets

Abstract

This study investigated the luminances necessary to perceive red (642 nm), green (521 nm), and blue (468 nm) at nine visual angles ranging from over 1° to 21 sec of arc diam. Monocular, foveal measurements were made by the method of constant stimuli at exposures of 44 and 700 msec duration. Three color-normals served as observers.The results show that, with sufficient luminance, these hues can be seen even for stimuli as small as 0.35 min of arc diam. Red is generally perceived at lower luminances than blue or green. There is an inverse relationship between stimulus size and luminance necessary for hue perception which can be represented as two linear functions. At small angles, the area-luminance slopes bracket, but tend to be somewhat larger than the slope predicted by Ricco’s law; at larger angles, the function resembles that predicted by Piper’s law. The change of function from approximately A·I=C to approximately A1/2·I = C occurs at higher visual angles for shorter exposures.