eli5 How does projectors projects black in a bright room?

Question

I am in psychology class, the light’s are on and I clearly see this black squares on white background. How is this possible? https://www.instructables.com/Optical-Illusion-Black-Squares-and-Gray-Dots/

In: 1

Answer 1

The projector obviously can’t remove light from the wall. It can only add more. When we observe an image, we look for relative differences and accept the darkest portion to be black. Pure black almost never exists. Materials always reflect some light. Common video monitors with a backlight usually emit light when fully dark.