A luminous bar of 4 min of arc was superimposed on an evenly illuminated 1°-of-arc circular field of 3, 30, or 300 mL, and the whole target was stabilized with respect to the retina. The bar luminance was varied sinusoidally at frequencies between 2.5 and 40 Hz, and the amplitude of variation yielded time-average contrast values between 0.06 and 6. Subjects indicated the length of time the edges of the bar stayed visible within a 20-sec viewing period. Judgments were also made at 0 Hz for each of the average contrast values. When the target is steady, visibility of the bar is a monotonically increasing function of contrast. Visibility of a target of any contrast can be enhanced to a value above the level obtained at 0 Hz, when its luminance is varied around its average contrast with frequencies between 2.5 and 15 Hz. Within this range, visibility is a decreasing function of frequency, and is found to be independent of contrast. Frequencies higher than 15 Hz are ineffective in lengthening the time during which the stabilized target stays clearly visible, and target visibility at frequencies above 15 Hz becomes once more a function of time-average contrast. At frequencies between 5 and 30 Hz, perception of flicker persists after the edges of the bar target have disappeared.
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