[photo-3d] Ghosting / Cross-cancellation

["John Goodman" <jgood@xxxxxxxx>]

I gather that when using a non-depolarizing screen, perceived 
ghosting is entirely due to the less than ideal interaction of the 
projector and spectacle polarizing materials?

Looking at a bright light through two identical pairs of 
polarizing glasses and rotating them for maximum extinction, I 
find the light source is still plainly visible. At its darkest, the 
right-angle sandwich is like a strong neutral density filter, but 
(with the glasses I have) the light source appears deep blue.

I wonder what a graph of density vs angle of such polarizers 
would look like. Obviously it's a symmetrical cusp, but 
maximum extinction seems to "snap" in at the very last few 
degrees. When I see ghosting during projection, I naturally tilt 
my head slightly left and right to find the best possible angle.

Is there a theoretical limit to polarizing performance? Are there 
different (superior) types, beyond the typical plastic, that could 
be used for either the projector or glasses side? Are there any 
retail sources for small quantities of very high quality materials 
of different densities? Do polarizers degrade over time, say a 
few years, regardless of exposure to bright light?

John Rupkalvis recently posted lots of interesting polarization 
info, including:

> Although built-in polarizers are a convenience, if you find that
> your polarizer life is too short you may be better off mounting
> them externally.

I tried adding a second layer of polarizing material right on the 
front of a projector lens, when testing without a slide. This 
dramatically improved cross-cancellation, but didn't appear to 
cut the brightness unacceptably. Would simple plastic 
polarizing material placed at the front edge of the lenses be 
expected to degrade the sharpness of projected images?

To check polarizer performance, I turned on one projector 
lamp and, wearing polarizing glasses, alternately looked 
through one eye and then the other. I was shocked by how 
bright the "ghost" sides were, this with a new projector that 
hasn't been used more than a few hours. Even though the 
system works nicely, improvements would make it that much 
better.  

jeg

 

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