Abstract
The contrast required to see Mach bands in a trapezoidal grating has been measured under different conditions. At photopic levels, increasing contrast is required as grating frequency increases. Mach bands are absent at all frequencies in scotopic viewing. Their absence at lower frequencies is not due simply to lack of suprathreshold contrast since at low frequencies contrast sensitivity is little affected by dark adaptation. Trapezoids of diminishing ramp width require increasing contrast until for a square edge a contrast of just greater than 1 would be required. This is difficult to achieve in most subjects. The Chevreul illusion behaves quite differently being unaffected by dark adaptation and grating frequency.
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