Thresholds of visual detectability were determined for signals on a modified 7BP7 cathode-ray tube. Thresholds were determined in units of signal voltage for three pip sizes, all relatively small, and for four screen brightnesses at each of several positions from the line of binocular regard to 20 degrees out from it. The purpose was to determine the limits within which chance eccentricity of view could be a factor in radar detection. The range of impairment from zero to twenty degrees of eccentricity was about 17 db of signal voltage. The course of the impairment is probably not quite linear, with the optimal scope brightness (about 0.16 footlambert for this phosphor), but the slope is a function of scope brightness. For very small pips the data show no region of equal visibility, even within the fovea; direct fixation is required for maximal detection. There being no data based on optically precise stimuli available for comparison with the radar thresholds, an estimate was made of the luminances of the pips and their backgrounds. As estimated, the visual threshold probably varies up to one-log unit or greater over a range of 20 degrees of viewing angle, for the optimal scope background.
© 1949 Optical Society of AmericaFull Article | PDF Article
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