P3D Re: Projection problem

[Paul Talbot <ptww@xxxxxxxxx>]

Tom Hubin wrote:
> 
> Polarized glasses upside down will blank the screen. Right side up will
> pass the images unchanged in polarization but about 75% as bright
> (dimmer by about one stop).

That's the opposite of what I've observed.  Glasses right
side up in front of the projector blank the screen; upside
down they just dim the images.

> Your polarizers seem to be ok from your test. 

Thanks, that was what I thought after doing the tests.

> The slides may be a problem. Are you testing with the slide loaded or
> empty? You should be able to project the offending slide pair and test
> the projected images for polarization just as you did above. The screen
> should be blank if the viewing lenses are upside down over the
> projection lenses.

I couldn't remember for sure whether I'd tried it with the
slide in or not, so I went back and did it again.  Same result
with slide or without.  Right side up glasses in front of the
lenses blank the screen.

> If you then place the viewing glasses rightside up over the projection
> lenses you will be projecting properly polarized images. It does not
> matter if your internal polarizers are bad or if the slides are
> repolarizing the light. External polarizers correct both of these
> problems. If, under these circumstances, you still have ghosting while
> viewing the screen with good leses, then the screen is the problem.

Except for having to put the glasses upside down, that is
exactly what is happening.  I'd say you've confirmed my
suspicion that the screen is the problem.

> Most common screens repolarize light and cannot be used for 3d
> projection because of excessive ghosting.

Does that mean that just specifying a "silver" screen is not
sufficient?  What is the correct specification for a 3D-
compatible screen?

> You can test your screen or
> any screen before buying it.
> 
> Shine a flashlight through a polarizer so that you now have a polarized
> light source. Or you can use a laser pointer if you happen to have one
> (assuming it is polarized). Shine the polarized light onto the screen
> and look at the light through a polarizer. Rotate the either polarizer
> until you cannot see the light on the screen. If you can do this then
> the light leaving the screen is still linearly polarized. If you cannot
> reduce the light coming from the screen to your eye to a very low level
> then it is not linearly polarized and ghosting will result.

Great tip!

Thanks, Tom!

Paul Talbot