The effect of adaptation and contrast on apparent brightness

Abstract

Measurements have been made of the variation of the brightness-sensitivity of the eye over a range of adaptation brightnesses extending from 0.001 to 400 equivalent foot-candles. The method adopted involves the use of binocular matching, the left eye, at a constant state of adaptation, being used as a reference standard, and the right eye, adapted to various brightnesses, viewing a test patch. From the experimental results, curves each of which represents physical brightnesses differing from those for subjective black by a constant number of just noticeable brightness-differences, and curves of equal apparent brightness have been constructed. It is shown that the brightness of the surround field has a very great effect on the apparent brightness of the test patch, the change caused by changing the surround brightness being almost instantaneous. The effect of this surround field, viewed with the right eye, on the comparison patch, as viewed with the left eye, both fields merging binocularly, is discussed and is shown to be negligible at least in comparison with the first-mentioned effect. The results obtained show that it may be impossible to construct sensation curves of the simple type hitherto postulated for considering the reproduction of photographic tone.