Re: [Phys-l] Projecting Black

[John Denker <jsd@av8n.com>]

On 11/06/2007 03:35 PM, Ron Curtin wrote:
> A student asked me a question that I couldn't answer.  When I pick up the
> black pen on my Smart Board, I can write with a really black color --- much
> blacker than the appearance of the screen with no illumination at all.
> Where does the black come from? 

The problem of making black on a front-projection screen is a 
famous unsolved (and unsolvable) problem.

Now you know why the turn down the lights in a movie theater.

The root of the problem is in the screen, not the projector.
The screen doesn't know where the projector is, so all it
can do is offer diffuse scattering over a wide range of
angles.  Light comes in from the projector and goes out at
all angles into the room.  But that also means that every
light source in the room can hit the screen and be diffusely
scattered at a wide range of angles.

If you want a black, black background to your projected images
in this geometry, block the windows and turn out all the lights.
That's necessary and sufficient.

The alternative is to change the geometry.  That is, if you have
a ton of space to dedicate to the purpose, you can put the projector
*behind* the screen.  Light from the projector goes through the 
screen and is scattered in all directions into the room.  Light 
going the other direction is scattered all over the back-space, 
but that is not fatal because the back-space is painted black 
everywhere.

>  I can produce most all the other possible
> colors with a combination of the primary colors of light, but if I add all
> the colors of light together, it seems like I should get white, not black.

Of course.

If the screen isn't black with the projector off, it isn't going
to get any blacker with the projector on.

This is another nifty "practical physics" question.