Atmospheric Boundaries in Lightness Perception

In judging the lightness of a surface, an observer must estimate and discount the optical viewing conditions: these include illumination level, haze, and interposed filters. The physical effects can be captured in terms of additive and multiplicative factors, which we call ‘atmosphere’. Local atmosphere can be estimated by gathering gray-level statistics over a local window. If the window is too small, the statistics will be impoverished. If the window is too large, it may encompass multiple atmospheres, making the statistics unreliable. We find evidence that the visual system imposes limits on the size and shape of the window, in accord with configural cues that signal atmospheric boundaries. Junctions with proper gray-level arrangements offer powerful cues to atmospheric boundaries; T-, X-, and psi-junctions are especially important. By combining statistics and configural cues, we have generated new lightness illusions that are much stronger than those seen in typical simultaneous-contrast displays. By changing the junctions we can also generate illusions that are quite weak, also in accord with the model.