Brightness constancy is our visual ability to perceive objects as having the same level of brightness even though the level of lighting changes.
For example, something white will appear to be the same shade of white no matter how much light it is being exposed to - noontime sunlight or a soft lamplight at night. Although the lighting has changed the shade of white seems to be the same. An object will retain brightness constancy as long as the object and its surroundings are in light of the same intensity. Brightness constancy is not maintained when the brightness of the background is different from the object. An object will appear darker if the background is lighter than the object.