Suprathreshold contrast perception at different luminance levels.

Magnitude estimation experiments show that the perceived contrast of sine waves is a linear function of stimulus contrast at some mean luminance (Lm), spatial frequency and contrast levels, but not at others. For monocular vision at both low and high contrast levels, linearity is observed within a limited zone which, at high Lm levels, does not include intermediate frequencies. Binocular vision gives similar results at high contrast levels, while for low levels, the linearity zone comprises intermediate frequencies in a wide Lm range. It is demonstrated that if a threshold correction is introduced into the obtained psychophysical functions, the described linearity zones extend to wider Lm and contrast ranges.