ABSTRACT
The perception of the lightness of a subject or image area corresponds roughly to its luminance but, unlike luminance, lightness is not directly measurable. Because the perception of lightness involves physiological as well as psychological factors, an area of constant luminance can appear to change in lightness for various reasons, including whether the viewer had previously been adapted to a high or a low light level. Psychological scaling procedures have been devised to determine the effect of viewing conditions on the perception of lightness and contrast. It has been established, for example, that a photograph appears lighter when viewed against a dark background than when viewed against a light background; and that the contrast of a photograph appears lower when viewed with a dark surround than when viewed with a light surround.
