Abstract

The influence of size and symmetry has been studied on the contrast required for the recognition of rectangular targets against background brightnesses of 2950 and 17.5 footlamberts. Targets less than 2 minutes in diameter require the addition of a constant total light flux to the background. Larger targets require less contrast but more total flux as the area increases, until beyond 200 square minutes when the required contrast becomes independent of area. For areas below 100 square minutes, square targets are most efficient for their area; the greater the ratio of length to width the greater the contrast required. All the measurements can be unified on the supposition that the visually critical region of a target is a ribbon just inside its perimeter and about 1 minute wide. Evidently, contrast is not judged over the area of a target, but across its boundary.

© 1947 Optical Society of America

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