It is shown that circular nonmoving targets, detected at threshold, which appear as a “point source” for low values of the subtended visual angle (target diameter), pass smoothly into a subjective annular shape for larger diameters. The annulus is the locale of the luminance gradient and therefore provides the significant visual information. The annulus width is 0.9 min of arc when the adapting luminance B is 102 ft-L and rises to 1.3 min as B falls to 10-3 ft-L. Because of this continuous transition from a point source to an annulus, the “critical visual angle” is at best only an approximation.
Over a range of B from 10-3 to 102 ft-L and a duration of stimulus from 10-2 to 1 sec the threshold energy for a point source rises 140 fold. The corresponding rise in the threshold energy per sq min of annulus area is about 1600 fold.
The trends in threshold energy and in threshold contrast with increasing target size are believed to measure some of the effects of retinal (or neural) interaction and inhibition in the human fovea.
NELSON W. TAYLOR, "Foveal Vision: Dependence of Threshold Energy on the Visual Angle of a Circular Target," J. Opt. Soc. Am. 52, 820-825 (1962)