An Illuminated Surface May be Perceived as a Shining Surface

The relationship between the whiteness of a surface (W) and its perceived illumination (E) has been described by the equation: W + kE = Y, where Y is brightness and k is a constant. When brightness remains constant and perceived illumination decreases, whiteness increases from black to white. If, starting from a pair of values W = W₀, E = E₀ (where E₀ is a low value of perceived illumination) at which the surface is seen as white (state A), E is further decreased while brightness remains constant, the surface may be perceived as shining (state B).

We investigated the conditions under which the perceptual transition from state A to state B occurred. The subject looked at an illuminated white paper screen on which objects cast shadows. Two test patches were presented: one on a well-illuminated area, the other on a shaded area. The scene and the patches were illuminated by two separate light sources. The brightness of the patches was held constant. The subject was instructed to set the ‘scene’ illumination so that a test patch was perceived as shining, separately for each patch. The results showed that the ‘scene’ illumination where the transition occurred was different for the shaded and the well-illuminated patches. We conclude that the perception of a surface as shining depends on local illumination.