Relation of brightness to duration and luminance under light- and dark-adaptation.

Abstract These experiments determined how the brightness of a brief flash grows with duration and intensity in dark-adapted and light-adapted eyes. Observers matched the brightness of a 1-sec flash to that of shorter flashes over a wide range of luminances. They also matched the briefer flashes to the 1-sec flash. The matching functions exhibit the brightness enhancement known as the Broca-Sulzer effect. The maximum enhancement shifts to a lower duration as the intensity is increased. Below about 10 msec in the dark-adapted eye the product of duration and intensity is constant for a given brightness (Bloch's law). In an eye adapted to 95 dB (300 mL) this reciprocity was not found at the durations used, 0.15 msec and greater. The results accord with other evidence to the effect that brightness grows as a power function of luminance, but that two basically different exponents are involved. When the duration is above the region of the Broca-Sulzer enhancement, the exponent has the value13. When the duration is short enough for Bloch's law to apply, the exponent is larger. Present evidence places the flash exponent between 0.4 and 0.5 for the dark-adapted eye.