Perceptual Constancies- Brightness Constancy

Brightness constancy can be specifically defined as the tendency for an object to be visually perceived as having the same brightness under a range of different levels of illumination.

In simpler terms, and for example, if you look at a white piece of paper in very bright sunlight and then look at that same paper under less illumination, the paper still seems like it has the same brightness as it did when it was in very bright light. (Even though the levels of illumination were different.)

(The picture on the left was taken with the flash on, and the picture on the right was taken without the flash.)