Why Does a Projected Black Background Not Make a White Screen Look Black?

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    Light Physics
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Discussion Overview

The discussion revolves around the optical perception of colors when a black background is projected onto a white screen. Participants explore how a white board reacts to projected images, particularly focusing on whether it appears black or remains white under certain conditions. The scope includes conceptual reasoning and technical explanations related to light emission and perception.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the white board would stay white because a projector displays black by not emitting light for those areas.
  • Others argue that the white board could appear black or a very dark color when parts of the light are blocked, similar to how sunspots appear black against the bright sun.
  • A participant questions how black characters can be seen on a white board if black is achieved by not emitting light.
  • One participant mentions that the perception of black in projected images is not the same as the absence of light, suggesting that RGB/CMYK values contribute to the perceived color.
  • Another participant shares personal experience with projection systems, noting that screens can appear mostly black even when the original screen is white.

Areas of Agreement / Disagreement

Participants express differing views on whether the white board would appear black or remain white when a black background is projected. The discussion remains unresolved, with multiple competing perspectives on the optical effects involved.

Contextual Notes

Participants reference concepts related to light emission, perception of brightness, and the mechanics of projection without reaching a consensus on the implications of these factors.

jamesabc
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in class once the teacher was showing some things to us from his computer by a projector. here's what happened. the background (wallpaper) on his computer was black and he was projecting onto a white board.

question: did you think the white board went black or stayed white?
 
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wouldn't it stay white? A projector displays black by just not emitting light for the required parts. So if the lights in the room were on, I think you would still see a white board.
 
o?
I think it is similar with black character projected onto the white board and looks black,so do the background (wallpaper),right?
Hi Oerg,u said "A projector displays black by just not emitting light for the required parts",but how we can see the black character on white board?
 
It's the same as sunspots. Sunspots are so bright that if the entire sun were covered with them, you'd still damage your eyes by looking at them. But since the rest of the sun is much brighter, they look black by comparison. Your eyes' ability to distinguish differences in brightness is somewhat limited.

[edit] I should have said that last sentence a little differently I think. Our eye don't record absolute brightness, just relative brightness. They adjust to keep the absolute brightness as constant as they can (by dilating and contracting the pupils) and try keep a consistent relative scale.
 
It would look black, or more precisely some very dark colour that isn't quite black.
 
Does your teacher have a projector that can emit anti-photons? :D
 
Oh,russ_watters,thanks,I understand!
Just as a white wallpaper projected onto a white board,and someone pass through front of the projector,shut out the part of the light,so part of the white board looks black,though it's a white board.
right?
 
white will reflect what ever color hits it
 
xxChrisxx said:
It would look black, or more precisely some very dark colour that isn't quite black.

Exactly right!

I've set-up a number of laptop/projection systems. When the laptop goes into, say, screen-saver mode, the image from the external screen appears mostly black, even though the screen is white.

Projected "black' is not the same thing as "absence of light"
Rather, this projection of black requires the emission of, for example, RGB/CMYK values that result in a perceived "black"
 

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