Location of the Break in the Dark Adaptation Curve in Relation to Pre-Exposure Brightness and Pre-Exposure Time

Dark adaptation curves are obtained by presenting a 2° square test field, subdivided by vertical bars, 6° in the lateral parafovea. When pre-exposure time is constant (10 minutes), and pre-exposure brightness is progressively decreased, the cone plateau gradually drops to lower brightness levels, while the final rod level remains constant. However, when pre-exposure brightness is constant (1510 millilamberts), and pre-exposure time is varied, the cone plateau and the final rod level remain unchanged. The break in the duplex curve shifts to earlier times with decreasing pre-exposure brightness as well as with decreasing preexposure time. Linear relationships exist between time of appearance of the break (minutes) and log preexposure brightness, or log pre-exposure time. The slope constants are different, however, indicating that pre-exposure brightness and pre-exposure time are not reciprocally interchangeable as far as the location of the break is concerned.