Brightness, discriminability and the "crispening effect".

Subjects adjusted the luminances, L, of 16 or 25 circles, all visible at the same time on a computer monitor, to make equal-interval brightness series. The background was black, white or grey. The luminance steps between adjacent circles behaved like the luminance discrimination thresholds of Whittle, P. [(1986) Vision Research, 26, 1677-1691)]. They showed a sharp minimum at the background luminance, Lb: the "Crispening Effect". They followed Weber's Law with respect to L when L was small, but with respect to delta L (= magnitude of L-Lb) near Lb. The Crispening Effect was abolished by a thin outline or a hue difference between circles and background.