Stereopsis is dependent on the average level of illumination, especially if it differs between the two eyes. We manipulate the mean luminance seen by both eyes or the interocular difference in mean luminance by using neutral density (ND) filters placed in front of both eyes or just one eye respectively. Stereo acuity was measured using a one temporal interval forced choice task for detecting the sign of a Gaussian depth perturbation in a noise field with a comparable spectrum to that found in natural images. We show that the effect of changing mean luminance is spatial scale independent within the range of 0.5 to 4 cpd, certainly not larger at higher spatial scales. To investigate its origin we manipulate two factors, the temporal synchrony between the two eyes and the interocular contrast. Both factors are implicated in the loss of stereo performance when the mean luminance is different between the eyes, suggesting an underlying explanation in terms of temporal low-pass filtering resulting in the combination of a luminance-dependent temporal delay and a luminance-dependent change in contrast gain.