Lightness constancy, ratio invariance and luminance profile.

The term simultaneous lightness constancy describes the capacity of the visual system to perceive equal reflecting surfaces as having the same lightness despite lying in different illumination fields. In some cases, however, a simultaneous lightness failure occurs, that is equal reflecting surfaces appear different in lightness when differently illuminated. An open question is if the luminance profile of the illumination edges affects simultaneous lightness constancy even when the ratio invariance property of the illumination edges is preserved. To explore this issue, we run two experiments by using bipartite illumination displays. Both the luminance profile of an illumination edge and the luminance ratio amplitude between the illumination fields were manipulated. Results revealed that the simultaneous lightness constancy increases when the luminance profile of the illumination edge is gradual (rather than sharp) and homogeneous (rather than inhomogeneous), whilst it decreases when the luminance ratio between the illumination fields is enlarged. Results are interpreted according to the scission hypothesis, stating that the visual system splits the luminance in the perceived lightness and in the apparent illumination components. We suggest that illumination edges having gradual and homogeneous luminance profiles facilitate the scission process, while wide luminance ratios impede it.