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Lightness constancy: Ratio invariance and luminance profile

This study investigated the lightness constancy phenomena in relation to shadow recognition - a perceptual phenomenon whereby equal reflecting surfaces are perceived as having the same achromatic colour (i.e., lightness) despite lying in different illumination fields (for example one surface is highly illuminated and another equal reflecting surface is in shadow). This phenomenon constitutes a problem for scientists since visual objects are perceived by means of light rays reaching the eyes. The amount of light reflected by the surfaces reaching the eyes is the product of the light intensity and the reflectance of the surfaces. Therefore, a highly illuminated surface projects a greater amount of light to the eyes than an equal reflecting surface placed in shadow. Nevertheless, under many conditions, two surfaces sharing the same reflectance are perceived as having the same lightness, even when one is highly illuminated and the other is in shadow. Hence, in order for the lightness constancy to be achieved, we must recognise the illumination level of each visual field. In particular, we must recognise the shadows in the visual scene.

Soranzo, in collaboration with Agostini (University of Trieste) prior to the census period (2006, 'Perception and Psychophysics', 68 (1), 102-113), suggested that the number of surfaces crossed by the shadows' edges is an important factor in shadow recognition.

Research occurring within the census period includes the study of the penumbra along the shadow edges. Results showed that a shadow is better recognised, and consequently, the lightness constancy phenomenon is favored, when the edge along the shadow is gradual (i.e. there is a penumbra) rather than sharp. However, this happens only whether the penumbra is consistent along all the length of the shadow edge.