Perceived Brightness as a Function of Duration of Dark-Adaptation

luminance in the course of dark-adaptation was measured by means of a direct psychophysical scaling method. The same trend wu found for all 6 luminance levels of the experiment. It could be characterized as composed of two functions, both growing at a decelerated rate and intersecting at about 8 rnin. A further analysis revealed that the empirical trend could be represented by the sum of two logarithmic functions of time. Much excellent work has been devoted to the study of dark-adaptation in human Ss. Most investigations have been concerned with detection thresholds. Results of many studies using this approach regularly show the well-known trend: the threshold decreases at a decelerating rate during the first 7 to 10 min. of dark-adaptation, and this trend is followed by a second similar trend; the two curve branches represent adaptation on the photopic and the scotopic levels, respectively. The process of dark-adaptation of the human eye has been very thoroughly studied on the liminal level (see, e.g., Hecht, Haig, & Wald, 1935; Hecht, Haig, & Chase, 1937; Hecht & Shlaer, 1938; Mote, 1955). This kind of work is of great interest in its own right, but it does not provide much information on perceived brightness, since we are not in a position to predict supraliminal sensation from threshold measures. The problem of dark-adaptation has also been approached by determining intensity levels matched for eqzdal brightness as perceived by one adapting eye and one eye at a constant level of adaptation (e.g., Pitt, 1939; Craik, 1940). Results of such investigations show, not unexpectedly, that equal brightness is produced by lower intensity as the time of dark-adaptation increases, and it is thus demonstrated that brightness at constant stimi~lus intensity grows with time of dark-adapation. That equal brightness funccions are similar in shape to threshold funccions has been empirically demonstrated by van den Brink ( 1962), who rightly points out that the threshold function may be considered a special case (brightness just barely above zero) of the equal brightness function. Measures of stimulus intensity under equal-brightness conditions are subject to the same limitations as threshold measures. However, van den Brink (1962) attempted to take one step further toward describing the relation between adaptation time and perceived brightness. He converted the luminance values obtained in an equal-brightness matching ex