Brightness Constancy

Brightness constancy is the tendency to perceive an object as having a constant lightness even while the illumination in its surroundings vary.  For example, the bottom of this picture I took in Baltimore shows a beam of light on the sidewalk ahead of me; if you look beyond that, the very same sidewalk continues but looks as though it's colored differently because of the relative luminance or the amount of reflection relative to surroundings.