how do projectors make the color black on white screens?

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how do projectors make the color black on white screens?

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They don’t really, they just don’t shine any light at that area. That means that the “black” will be the same color as the screen, which is why projectors only really work in dark rooms.

The black on white screens is not the same as normal black. The black that they use usually isn’t as black as you would think. However, it’s usually black enough that viewers do not notice a difference.

They don’t – it is impossible for a projector to make a surface darker, only lighter.

The projectors get around this by primarily being used in dark rooms and projecting on special materials that help to isolate and direct light.

They also take advantage of a little quirk in our brains where the perceived lightness/darkness of a surface is impacted by the surrounding areas. We’ve all seen the [shadow illusion](https://en.wikipedia.org/wiki/Checker_shadow_illusion#/media/File:Checker_shadow_illusion.svg) where squares A and B are actually the same color, but our brain sees them as different because of the surrounding colors. Thanks to this, something can appear ‘black’ on a projector, despite the fact that it is actually just the color of the projector screen.

They don’t. Projectors work by projecting mixtures of light to make colors. White is a combination of all the light colors and black is the absence of all light. So in an image with black portions, no light gets projected to those parts.

A plain white screen is not a very good choice for projected images. What you really want is something that can reflect light very well when hit with light but otherwise will remain dark. This is why projectors work best in darkened rooms and we used to use silver screens to project to.

So basically the more light in the room and the lighter the screen you have, the more washed out looking and overall bright looking will be your projected image.

It does not. Black will be the absence of any light from the projector so it will have the same color as the surface you project on.

So you need a projector that emits a lot of light so the areas around it look a lot brighter and you see the unlit white areas as a lot darker.
Grey as a color is white with less light intensity compared to the surrounding.

Look at [this image.](https://en.wikipedia.org/wiki/Checker_shadow_illusion#/media/File:Checker_shadow_illusion.svg) square A and B have the same color but you think of A as gray and B as white because of the surrounding areas and the shading you see from the green object. If you open it in an image editor and pick the color they are the same, you can do the same by cutting holes in a paper so you only see those squares.

So human vision is based on the light relative to the surrounding not the absolute amount of light

The dark color in a projected image in a well-lit room will look washed because they are a lot brighter because of light that does not come from the projector

There is a reason that in a cinema you turn the light off in the theater so very little light hits the area that should be black.

A projector can not make the screen any darker then the ambient light in the room does. However your eyes are already used to this and will only perceive relative light levels. So when seeing the screen in both light and dark white we automatically interpret the darker areas as black even though it is actually fairly bright.

Take a white sock into a closet and close the door. If there’s no light, what color is the sock?

Black bits on a screen are not having any light pointed at them Same with if you walk in front of the projector and cast a shadow.

To add to the existing answers: they make some laser projector screens now that have special coatings/structures on them to only reflect light from one direction. Laser projectors sit underneath and directly in front of the screen, and the screen is designed so it only reflects light up from the bottom. Light coming from the side like from a window gets absorbed so it doesn’t brighten the screen.