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

[jamesabc]

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?

 

[Oerg]

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.

 

[pcrespoy]

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?

 

[russ_watters]

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.

 

[xxChrisxx]

It would look black, or more precisely some very dark colour that isn't quite black.

 

[ImAnEngineer]

Does your teacher have a projector that can emit anti-photons? :D

 

[pcrespoy]

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?

 

[cragar]

white will reflect what ever color hits it

 

[pallidin]

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"