THE VISIBILITY OF NON-UNIFORM TARGET-BACKGROUND COMPLEXES. 2. FURTHER EXPERIMENTS

Abstract
Studies have been made of targets of uniform luminance presented against backgrounds of nonuniform luminance and consisting of an array of ball-bearings painted gray, which produced a regular pattern of luminance non-uniformity, each element of which subtended 8 minutes of arc. Targets were circles measuring 4, 8, and 24 minutes in diameter which were superimposed over the background, thus obscuring it over their area. Experiments were conducted in which the overall contrast of the target-background complex was systematically varied until the target was at the visibility threshold. When the visibility of the targets was considered described by their luminance with respect to the average luminance of the background, the visibility thresholds of targets presented against nonuniform backgrounds differed from those of targets presented against uniform backgrounds in a complex manner which differed as a function of target size. However, when the visibility of the targets was considered described by their luminance with respect to the luminance of the immediately adjacent elements of the background, visibility thresholds were the same for targets presented against uniform and non-uniform backgrounds. This implies that visibility thresholds are determined by target contrast at target borders rather than by some kind of average contrast. Similar results were obtained at exposure durations of 1.9 and 0.1 seconds.

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