Re: [Phys-l] Projecting Black

[Rick Tarara <rtarara@saintmarys.edu>]

For the projection, it is also the illusion effect of something fairly dark 
next to things that are bright.  This illusion is often shown with a 
checkerboard with light/dark grey squares but an object casting a shadow. 
The illusion is part of my Optics2006 set (see below).  One would swear that 
two marked squares are different--one darker than the other, but when 
connected, they are exactly the same.  So, as John says, the screen won't 
get blacker than it is without anything projected, but we see it as very 
dark when compared to anything brighter projected around an unilluminated 
section.

Rick

www.saintmarys.edu/~rtarara/GREYSQUARE1.jpg
www.saintmarys.edu/~rtarara/GREYSQUARE2.jpg


***************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN
rtarara@saintmarys.edu
******************************
Free Physics Software
PC & Mac
www.saintmarys.edu/~rtarara/software.html
*******************************
----- Original Message ----- 
From: "John Denker" <jsd@av8n.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Tuesday, November 06, 2007 4:09 PM
Subject: Re: [Phys-l] Projecting Black


> 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.
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