Positive Afterimage as a Background Luminance*

Abstract

In one experimental session, each of six subjects received six flashes of 1.4-msec duration from a centrally fixated 10° field of 4 × 105 L. The brightness of the afterimage was tracked, following the first two flashes, by means of a monocular bipartite photometric field. The bipartite field was formed by blocking one-half of the flash field to provide a semicircular afterimage and arranging a semicircular comparison field in juxtaposition. The comparison field could be varied over an 8-log-unit range by the subject. The density of the wedges was continuously recorded as the match was maintained. Recovery times for recognition of 28.7′ and 16.3′ Sloan Snellen letters at various luminance levels were measured following the other flashes. The letters were transilluminated and viewed against a dark surround. They were presented at one-second intervals and were at 140 mL immediately following the flashes. As soon as the subject correctly identified two successive letters, the luminance was reduced by introducing a neutral density filter. In this manner, ten luminance levels were measured following each flash, with the lowest 0.007 mL. The letters were superimposed on a 10° variable-luminance field and each subject determined the field luminance necessary for threshold recognition of each letter condition. The recovery times for the various letter conditions were then predicted for each subject from his afterimage brightness measurements and cross correlated with the measured recovery times. The correlation coefficient for the measured and predicted values was 0.82.