The effect on foveal dark adaptation of pre-exposure of the eye for brief durations to light of relatively low brightnesses was investigated after the eye was adapted to different levels of brightness. Monocular measurements of absolute brightness sensitivity were made using a square, one degree on a side, centrally fixated, test patch. The course of dark adaptation as measured by the RL was investigated following ten minutes exposure to each of three adaptation levels, 0.10, 1.0, and 10. ft-L. In addition, the effect of superimposing certain combinations of pre-exposure brightnesses (100., 10., and 1.0 ft-L) and durations (100, 10, and 1 sec) upon each of the adaptation levels was investigated. The effect of increasing adaptation level, in the absence of pre-exposure, was to increase the loss of sensitivity, and hence the extent of subsequent dark adaptation. The effects of superimposing various pre-exposures upon the different adaptation levels were found to be complex. With two exceptions, the only instances in which pre-exposure superimposed upon adaptation level produces more subsequent adaptation than adaptation level alone occurred when the product of pre-exposure brightness and duration exceeded 100 ft-L-seconds. Occasional combinations of pre-exposure brightness and duration superimposed upon an adaptation level appeared to produce no subsequent adaptation which could be measured with the apparatus and technique used.
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