Since a computer monitor is a variable light source, brightness contrast can be observed dynamically. Turn down the brightness control of your monitor to a low, but not dim level of illumination. This phenomenon is called simultaneous brightness contrast or lateral brightness adaptation and is explained by the theory of lateral inhibition . See the commentary below. In our everyday visual experience we unconsciously follow an absolute theory of vision, namely, we assume that the perception of a scene in a small area of the visual field is determined entirely by the amount of light which emanates from that part of the scene and then becomes incident on the corresponding part of the retina.
Brightness contrast is also evident in colour.
For example, the gray squares with the blue backgrounds appear brighter than the gray square on the yellow background. This variation in apparent brightness among the gray squares occurs even if all background colours also emit equal levels of brightness or physical energy. The reason is that different colours have different levels of apparent brightness even if their physical light intensity is held constant. The observer may note that the gray squares reveal a slightly coloured tint which is complementary to the colour of their backgrounds. These special relations of colours to each other are part of a more complete discussion of simultaneous colour contrast and colour illusions .