How do overhead projectors project black onto a screen?

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middlj
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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?
 
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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!
 
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" ? :smile:

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