# How do overhead projectors project black onto a screen?

• middlj
In summary: I thought that if one projects the colors red, green and blue on the same area of a white screen, the brain perceives that area as totally black. If you projected red green and blue, you would see white. If you projected light of 3 wavelengths: Red, green and blue. Of all the same intensity then it equals white. What about if you projected 4 different wavelengths? You just added another one in. Under what condition will it appear to be white? Generally the lights are dimmed while viewing a projected image or movie, and in theaters the dimmed lights are directed away from the screen so the screen is very dark if there is no projected light on it.
middlj
My housemates and I were having a discussion on how projectors work, and how they can project text on a white background. One idea was that white light is projected around the text with an absence of light where the text is, so it appeare relitively black. Then my friend posed the question of if you project a black frame around a white rectangle, it would still appear black. With an absence of projected light, surely it would appear the same colour as the background screen, which would be white, the same colour as the screen with projected white light on it.

So how does this work?

Welcome to PF.

Your eyes don't exactly measure light intensity, they more measure differences in light. So when one object is a lot darker than another, it appears black.

russ_watters said:
Welcome to PF.

Your eyes don't exactly measure light intensity, they more measure differences in light. So when one object is a lot darker than another, it appears black.

To add to what russ_watters is saying, check out the Chessboard (or checkerboard) illusion (at least, I think that it's a manifestation of the same sort of phenomenon):
http://www.popularscience.co.uk/features/feat16.htm

Thanks guys. I think i was on the right lines then.

That link is incredible by the way!

You can also take a light meter and measure it. You'll see that "black" isn't projected at all.

that would be cool if you could project black. is there some kind of particle you could launch in a beam that would absorb all photons?

tot said:
that would be cool if you could project black. is there some kind of particle you could launch in a beam that would absorb all photons?

With http://www.siliconhell.com/humour/darksucker.htm" ?

(for the humor-impaired: this is a joke)

Last edited by a moderator:
I could be wrong, but I thought that if one projects the colors red, green and blue on the same area of a white screen, the brain perceives that area as totally black.

If you projected red green and blue, you would see white

if you projected light of 3 wavelengths: Red, green and blue.
of all the same intensity then it equals white?
what about if you projected 4 different wavelengths? you just added another one in?

under what condition will it appear to be white?

Generally the lights are dimmed while viewing a projected image or movie, and in theaters the dimmed lights are directed away from the screen so the screen is very dark if there is no projected light on it.

tot said:
if you projected light of 3 wavelengths: Red, green and blue.
of all the same intensity then it equals white?
what about if you projected 4 different wavelengths? you just added another one in?

under what condition will it appear to be white?

I'm assuming it would appear white if red green and blue is projected in equal parts. If you added another, you would see that colour?

I may be wrong

## 1. How do overhead projectors project black onto a screen?

The overhead projector uses a special lens and a bright light source to project an image onto a screen. The lens focuses the light onto a small area, and this light passes through a transparency containing the black image. The black areas on the transparency block the light, creating a dark image on the screen.

## 2. Why does the black image appear on the screen instead of the transparency?

The transparency is made of a special material that blocks light from passing through it. This means that the black areas on the transparency prevent light from reaching the screen, while the clear areas allow light to pass through, creating the image on the screen.

## 3. Do all overhead projectors project black in the same way?

While most overhead projectors use the same basic principle of using a lens and light source to project an image, there may be slight differences in the materials and technology used. However, the end result of projecting black onto a screen remains the same.

## 4. Can other colors besides black be projected using an overhead projector?

Yes, an overhead projector can project any color as long as it is present on the transparency. The light passing through the transparency will mix with the color of the light source, resulting in the projected color on the screen.

## 5. How does the brightness of the projector affect the black image on the screen?

The brightness of the projector can affect the intensity of the black image on the screen. A brighter light source will create a more vibrant and darker black, while a dimmer light source may result in a lighter shade of black. Adjusting the brightness can help achieve the desired contrast and visibility of the black image on the screen.

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