Recovery of foveal acuity following exposure to various intensities and durations of light

Abstract

The discrimination of visual detail at night or under conditions of dim illumination is required for the execution of many naval duties, such as piloting a ship, operating a periscope, or plotting navigation data. These tasks may depend more on good dark-adapted central visual acuity, which requires about five minutes of dark-adaptation, than on the more time-consuming peripheral night vision. It was the purpose of the study to determine the decrement in this ability caused by ordinary brief exposure to bright lights. Results showed that the recovery times varied systematically with the intensity and duration of the flash from essentially zero (or no effect) for the dim, brief flashes, to a maximum of five minutes (or complete loss of dark-adapted foveal acuity) for the brighter, longer flashes. Furthermore, the amount of the recovery time needed can be predicted by a knowledge of the product of the intensity X the duration of the flash. Thus the decrement produced by 30 seconds of 100 ft-L is the same as that caused by a 3-second flash of 1000 ft-L. The results are applicable to any operator needing good visual discrimination at night, since they assess the effect of any glare source, from the flare of a match to a burst of fire, on his ability to see his task.