Measurement of temporal summation of visual acuity with use of modified tachistoscope

Visual acuity was measured in 18 normal eyes of 10 subjects ranging in age from 20 to 30 years, using a modified tachistoscope. We changed the exposure duration of the target (1-1000 msec) or background luminance (0.1-200 cd/m2). Visual acuity improved with increasing duration of exposure. At background luminance over 10 cd/m2, critical duration time was approximately 500 msec. At low luminances below 10 cd/m2, however, critical duration was prolonged. This tendency was also seen when pupil diameter was fixed at 3 mm using an artificial pupil. Visual acuity also could be determined as the product of background luminance and exposure duration. We concluded that these findings follow the Bloch-Bunsen-Roscoe law.