Thresholds for Mach bands were obtained by filtering discontinuous luminance gradients until the bands disappeared. Visibility was related to two properties of the stimulus discontinuity at which the bands were seen: the change in the rate of change of luminance with respect to distance and the luminance at the discontinuity. The former clearly affects visibility. The latter also appears to affect visibility, but its influence was not always demonstrable. Light and dark bands are independently perceived. Light-band thresholds vary with exposure duration according to Bloch’s law.
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