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How do overhead projectors project black onto a screen?

  1. Dec 13, 2009 #1
    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?
  2. jcsd
  3. Dec 13, 2009 #2


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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.
  4. Dec 13, 2009 #3


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    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):
  5. Dec 13, 2009 #4
    Thanks guys. I think i was on the right lines then.

    That link is incredible by the way!
  6. Dec 13, 2009 #5

    Vanadium 50

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    You can also take a light meter and measure it. You'll see that "black" isn't projected at all.
  7. Dec 13, 2009 #6


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    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?
  8. Dec 14, 2009 #7


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    With http://www.siliconhell.com/humour/darksucker.htm" [Broken] ? :smile:

    (for the humor-impaired: this is a joke)
    Last edited by a moderator: May 4, 2017
  9. Dec 14, 2009 #8
    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.
  10. Dec 14, 2009 #9
    If you projected red green and blue, you would see white
  11. Dec 14, 2009 #10


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    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?
  12. Dec 14, 2009 #11


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    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.
  13. Dec 14, 2009 #12
    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
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