Abstract
Apparent brightness was measured by the method of magnitude estimation as a function of three parameters: luminance of the target, locus of stimulation, and level of light-adaptation. For any given level of light adaptation, brightness grows as a power function of luminance and thereby conforms to the general psychophysical law proposed by S.S. Stevens. The exponent of the power function does not appear to vary appreciably from one retinal locus to another. From the brightness functions it can be shown that a 1° white target of constant luminance appears brightest in the fovea; its brightness decreases steadily as the locus of stimulation is made more and more eccentric. Relative sensitivity (reciprocal of luminance at threshold) also decreases with distance from the fovea, except at low levels of light-adaptation, where sensitivity is greatest at about 5°.
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