The perceived brightnesses of the maxima and minima in spatial sinusoidal light variations have been determined by suprathreshold psychophysical techniques in the photopic region. For example, the response/ stimulus contrast ratio at 20 cd/m<sup>2</sup> average luminance and spatial frequencies between 1.5 to 7.5 lines/deg is 2.4 to 3.4 for 25% and 1.8 to 2.0 for 50% object contrast. Contrast transfer has a pronounced peak at about 5.5 lines/deg for all object contrasts. The visual system is nonlinear and a contrast transfer function does not exist in the photopic region. The measurements have been performed under normal viewing conditions where both successive and simultaneous contrast phenomena are operative.
OLOF BRYNGDAHL, "Characteristics of the Visual System: Psychophysical Measurements of the Response to Spatial Sine-Wave Stimuli in the Photopic Region," J. Opt. Soc. Am. 56, 811-812 (1966)