The Spatial Properties of Contrast

Contrast is the well-known observation in which a gray in a white surround appears darker than in a black one. An example is a 32 by 32 pixel gray area subtending 0.75 degrees visual angle on a black background - 256 pixels on a side. The gray appears darker when surrounded by a band of white -12 pixels wide. The white band is made up of 2112 individual white pixels. If these white pixels are redistributed uniformly in the black background the gray appears much lighter. This paper measures the gray appearance as influenced by 2112 white pixels in 27 different spatial configurations. The set of different spatial patterns of white pixels that generate the same matching lightness for gray are defined as equivalent backgrounds. The paper then analyzes the spatial properties of equivalent backgrounds. Gray appears darkest when the solid white surrounds the gray and is contiguous with it. In the case of the distributed white pixels, the gray appears lighter. This paper presents an analysis of the spatial properties of intermediate surrounds that give the gray center equal visual appearances.