P3D Re: Ghosting

[Tom Hubin <thubin@xxxxxxxxx>]

Paul Talbot wrote:
> 
> Dr. George A. Themelis wrote:
> 
> > - I assume that you have checked your projector's polarizing filters
> >   and they are still strong and also that your screen is OK.
> 
> What steps would one take to check the screen and the
> projector's polarizers?
> 
> Paul Talbot

Ghosting occurs when you cannot block the image that is viewed through
the opposite lens. The right eye must not see the left image and the
left eye must not see the right image. Ghosts are caused by improper
polarization.

Sometimes it is just a rotation problem. Project one of your ghosty
slides. Rotate your head slightly to see if the ghost goes away. If you
can make either the right ghost or the left ghost go away but not both
at the same time then it is a rotation problem. Either your viewing
lenses are not polarized 90 degrees apart or your projection polarizers
are not 90 degreees apart or both.

You will need some dependable polarizers as test tools. Use good viewing
lenses or remove some polaroid sunglass lenses or buy some polarizer
sheet from Edmunds. Stack 2 of these polarizers and look through both at
some light. A lighted wall or screen is fine. Just use room lighting.
Rotate one of the polarizers while looking through them. There should be
some orientation where very little light is visible. If not, one or both
of the polarizers are not good enough. 

You only need one polarizer for testing but it must be a good one. If
you are not sure of the quality then finding two that pass the test
above assures that you now have two good test polarizers. 

Once you have polarizers that you trust you can test other things that
are supposed to be polarized. Test the polarizers in your viewing
glasses. Stack a viewing lens with your test polarizer and rotate your
test polarizer until very little light passes. If you cannot block the
light enough then the viewing lens is not good enough.

Project the right image only on the screen. Look through your test
polarizer. Rotate the test polarizer until the image is very dark. You
are now looking at the left ghost. If it is very dark then it is not a
problem. If it is too visible then something is wrong in the projector
or on the screen. Try the left image too. You might only have ghosting
on one side.

Place your test polarizer between the projector and the screen. A few
inches from the screen is good. There will be a shadow on the screen
from your test polarizer. Rotate the polarizer until the shadow is very
dark as viewed with the naked eye. If the shadow is very dark then the
projector is projecting a well polarized image. If the shadow is not
very dark then the projector is not polarizing the image very well. This
can be caused by poor polarizers in the projector or by any optical
surface between the polarizers and the screen. Do this test with only
one side projecting at a time.

The first suspect is the slide itself. Remove the slide and see if the
problem goes away. Try other slides. If the projector passes these tests
with some slides and not with others then the slide is the culprit.

It can also be dirty or scratched optics. Any contaminated optical
surface between the polarizer and the screen can alter the polarization
of the light. Dirty optics between the light source and the polarizer
may make the image dimmer but it will not corrupt the polarization of
the image. A ghost is a polarization problem.  

Start with these tests. I'll answer your questions as they arise.

Tom Hubin
thubin@xxxxxxxxx


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