When signal images appear on the phosphorescent screen of a cathode-ray tube, they persist for some time before disappearing from view. This study investigated the visibility of signal images on one type of screen at different intervals from excitation to final disappearance.
Data were furnished by five observers viewing signal images on a P-7 screen of a PPI cathode-ray tube. It was found that the tube bias, which sets the brightness intensity of the rotating sweep, affects both the level of visibility and the duration of visibility. Increasing the bias, and hence dimming the scope, makes the signal traces visible over a longer period of time. Greatest visibility, at intervals longer than seven seconds after excitation, was obtained at the highest bias used. For more recent signals, optimum visibility results from using a lower bias but which yet presents a dim sweep. At high biases, a signal is at maximum visibility at the instant when it is excited by the sweep. At low biases, a signal is most visible immediately after its excitation. These patterns of visibility can be explained in terms of the contrast of the brightness level of the signal image with that of the scope background and by the lessened differential sensitivity of the eye at low intensities.
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