Mailinglist Archives:
Infrared
Panorama
Photo-3D
Tech-3D
Sell-3D
MF3D

Notice
This mailinglist archive is frozen since May 2001, i.e. it will stay online but will not be updated.
<-- Date Index --> <-- Thread Index --> [Author Index]

P3D Re: Projection problem


  • From: Tom Hubin <thubin@xxxxxxxxx>
  • Subject: P3D Re: Projection problem
  • Date: Wed, 25 Aug 1999 00:35:39 -0600

Hello Paul,

> However, I've not yet been successful at projecting.  :-(
> I'm getting such extreme ghosting that the images are
> effectively unviewable.  The problem seems to be lack of
> (or insufficient) extinction in the right-eye view.  My
> initial reaction was that the right polarizer must be at
> fault.  But when I place a pair of polarized glasses in
> front of the projector lenses, the screen goes completely
> dark.  This suggests the polarizers in the projector must
> be OK, right?

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). Your polarizers seem to be ok from your
test. If they are upside down you will get pseudo 3d, not ghosting. So I
would say that the polarizers are ok.

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.

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.

> I'm left with suspecting a problem with the screen, which
> was given to me about 1-1/2 years ago.  It is a silver screen,
> brand name "Radiant Glo-Master." 

> Given the common advice to pick up cheap silver screens
> at thrift stores, if problems with silver screens causing
> depolarization are common, how does one avoid buying, say,
> 1/2 dozen "bad" silver screens?

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

If you want to be picky, the polarizer on the flashlight and the
polarizer that you view with should be 90 degrees apart when the light
viewed is minimized. But it is unlikely that the screen will alter the
light by simply rotating the polarization but otherwise leave it
linearly polarized. The ususal problem is that the screen alters the
polarization so that it is only partly polarized.

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design