Brightness of a Field as a Function of its Area

The brightness of a test field as a function of its area was investigated in the fovea at different levels of luminance. A circular test field, which varied in area, was presented to the subject’s right eye. A circular match field, held at a constant intermediate area, was presented to the subject’s left eye. The subject’s task was to match the two fields in brightness employing the psychophysical method of limits. The dependent variable was the test luminance required either to match a constant match luminance in brightness or to produce a constant threshold effect. The test-field radius was varied in six steps from 2.69′ to 26.86′, for any one of six levels of match-field luminances from threshold to 2.56 logmL. The results showed that only at threshold were there systematic differences in test luminance as a function of test area. Threshold luminance decreased as area increased. At suprathreshold test luminances, differences that did occur were not systematic nor greater than might occur from day to day. A theoretical account is presented based upon possible inhibitory interaction in the retina of “on” by “off” nerve fibers. Curves generated by this theory are fitted to the data.