What is Lightness Constancy?

White paper reflects 90 percent of the light falling on it; black paper, only 10 percent. In sunlight, a black paper may reflect 100 times more light than does a white paper viewed indoors, but it still looks black. This illustrates lightness constancy(brightness constancy);we perceive an object as having a constant lightness even while its illumination varies. Perceived lightness depends on relative luminance—the amount of light an object reflects relative to its surroundings. If you view sunlit black paper through a narrow tube so nothing else is visible, it may look gray, because in bright sunshine it reflects a fair amount of light. View it without the tube and it is again black, because it reflects much less light than the objects around it.(1)

Illusions

(1) Psychology, David G Myers

https://www.diliaranasirova.com/assets/PSYC579/pdfs/01.1-Ware.pdf