An intensity modulated cathode-ray screen of the PPI type used in radar depends on differential brightness discrimination for detection of weak signals. The brightness quantities have been largely unknown, because of photometric difficulties, but it has commonly been assumed that dark-adaptation is required for efficient visual detection. Therefore, an experimenal test was undertaken of the effects of light- and dark-adaptation on the contrast threshold and on time required for detection of supra-threshold signals appearing on a typical screen (P7 phosphor). Adapting brightnesses ranged from zero to 2000 millilamberts (ml.). By special photometry, the four screen intensities used were estimated as ranging from 0.0001 ml. to 0.22 ml. Length of exposure to the brightest adapting light and the degree of contrast between signal and background were also varied. The lowest contrast thresholds and the shortest detection times were found to occur when the test and adapting illuminations were equal or nearly so. When the two illuminations were unequal, the loss in sensitivity was roughly in proportion to their difference. The optimal field brightness of the phosphor screen used was on the order of 0.1 ml. With this field the level of adaptation of the eye can be a relatively insignificant factor in radar detection, even of weak signals. Increasing values of signal contrast allow progressive reduction in detection delay after exposure to the highest brightness.
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