When a rectangular wave grating is binocularly viewed with a neutral density filter over one eye, an illusory rotation resembling that of a partially opened Venetian blind is perceived (Cibis and Haber, 1951). Using a binary classification task, in the first experiment, the probability of perceiving a rotation in a given direction was measured as a function of a factorial combination of inter-ocular contrast (see Note 1) and luminance ratios. The probability of a rotation in a given direction decreased monotonically with the luminance of the brighter bars when the grating contains a less than unity contrast. This result is inconsistent with (i) the model of the Venetian blind effect proposed by Cibis and Haber (1951), (ii) a mechanism based on irradiation with a compressive non-linearity (von Helmholtz, 1911/1924, pp. 186-193) and (iii) contemporary stereo-energy/cross-correlation models of stereopsis. In the second and third experiments, we tested the prediction that irradiation combined with an early compressive non-linearity in response implies a positive relationship between both the threshold contrast or average luminance disparity to perceive rotation and the magnitude of perceived rotation, and the blur width at the bar's edge. No support was found for the prediction. We propose an intensity difference model of the probability of perceiving a rotation in a given direction as a function of the interocular difference in luminance or contrast.