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[photo-3d] Re: Ghosting / Cross-cancellation


  • From: "John Goodman" <jgood@xxxxxxxx>
  • Subject: [photo-3d] Re: Ghosting / Cross-cancellation
  • Date: Wed, 18 Apr 2001 06:59:51 +0900

 John A. Rupkalvis  wrote:

> As far as I know, there is no such thing as a perfect
> nondepolarizing surface.   It is a matter of how good the
> particular screen is. 

I should have intuited this truth.  :>)

> The amount of light reflected off of a screen is a very small
> percent of that emitted by the source. 

Another fact I overlooked. Is the reflected fraction typically a 
quarter or so? Even less?

> This is why StereoGraphics and some other companies opt for
> circular rather than linear polarizers.  They work no matter
> what your angle is. 

Though I've heard of circular polarizers, I can't visualize how 
they work. If they are clearly superior, is the reason they're not 
"standard issue" their higher cost or poorer extinction?

> You should test this under "typical" conditions for what
> trade-off you consider acceptable for your application. 

A rather daunting process, at this point. My current system 
works, but I know there's considerable room for improvement.

> If you matched the density of your uncrossed pair with a
> single denser polarizer (of density equal or close to that of
> your uncrossed pair together), your cancellation when crossed
> might be 5 to 10 times that of your duplex pair crossed. 

Amazing. Thank you for so many helpful details.

> But, so would polarizers placed internally (as supplied by
> the projector manufacturer) [degrade image sharpness]. 

In the projector I'm using, the polarizers are between the lamps 
and film, so I imagine they have no effect upon image clarity.

> [Ghosting] should not be apparent in normal contrast scenes. 
> If it is, you have a polarization problem that should be
> checked, including the screen itself. 

Aha. I imagine that the projector and glasses polarizers and 
screen properties can only be checked (at an amateur level) by 
substitution with various alternatives. Another daunting 
prospect for a beginner, but these variables are of common 
concern to stereo projection enthusiasts past and present. Is 
there any consensus, or pooled data, concerning which 
polarizing material specifications or catalog items are "best?" Or 
is this always a case by case fit of what's readily available?    

jeg

 

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