The contrast threshold for perception of square-wave gratings as depending on spatial frequency is measured for varying viewing distances, adaptation and exposure time. The luminance varied between 1.4 and 110 cd/m2; target distances were 1, 3.1, and 7 m. Exposure times ranged from 1.5 msec to 1 sec and unlimited. A distinct minimum threshold contrast is observed for a definite spatial frequency, which depends on the viewing distance and luminance. A decrease in exposure time causes a less significant minimum. Exposure times from 40 to 1.5 msec do not alter the curve decisively. With exposure times less than 2 msec and spatial frequencies somewhat above 0.02 lines/min of arc the optical transfer function of the eye can be measured by determination of thresholds.
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