Relation of Increment Thresholds to Brightness and Luminance*

Under ordinary conditions, both the brightness and the increment threshold of an illuminated disk vary directly with its luminance. However, when the disk is surrounded by an annulus more intense than the disk, the brightness of the disk decreases while its luminance remains unchanged.This set of experiments was performed to determine whether, when brightness is varied independently of luminance, the increment threshold depends upon the luminance or upon the brightness of the disk, or upon both factors. We measured the increment threshold for a flash added to the center of a large illuminated disk when the disk was surrounded by a contiguous annulus whose luminance could be varied. Measurements were also taken of the increment threshold as a function of time after the onset of the annulus. Correcting for light scattered in the eye, we found the increment threshold under all conditions to be independent of the luminance of the annulus (and thus independent of the brightness of the region), and dependent only upon the retinal illuminance of the region to which the test flash was added. It is concluded that brightness and the increment threshold cannot depend upon the same properties of the visual system.