Dark Adaptation in the Presence of Waning Background Luminances*

The course of dark adaptation following low-level pre-exposure was determined against background luminances whose logarithms decreased linearly with time. Luminance was reduced by 7 log units over periods of 3.5, 7, 14, and 21 min. Transient adapting fields of this kind yield curves that remain increasingly behind the time course of normal dark thresholds; the delay varies with the rate of background change. Increment thresholds derived from such dark-adaptation curves exceed corresponding thresholds for stationary adapting fields by as much as 1.25 log unit, particularly during the scotopic portion of steep luminance descents. The deficit in contrast sensitivity may be attributed to the additive action of the real background plus the equivalent background resulting from the pre-exposure bleach. Dark thresholds measured during temporary removal of a gradually decreasing field luminance are persistently elevated. This suggests that prolonged exposure to waning illumination may slow dark adaptation below its maximum rate.