P3D Re: depolarizing problems

[Tom Hubin <thubin@xxxxxxxxx>]

Klaus Kemper wrote:
> 
> Hello, I have problems with depolarizing when I project my stereo-macro slides.
> Especially if the subject is a light butterfly for example or a blossom with
> light and dark contrasts. Is that normal? I already changed the polfilters in the
> RBT-projector but there is no difference. May it be that the lens separation of
> my stereo-macro-camera is to wide? Is there someone who has experience with
> projecting macro-slides? When I look at the slides in a handviewer it's all o.k.
> 
> Klaus

Hello Klaus,

The slides are probably birefringent. That just means that they alter
the polarization as the light passes through. 

To test this, start with two polarizers crossed at 90 degrees so that
little or no light passes through them. Look through them at some light
source as you orient them so that as little light as possible passes
through.

Then insert the slide between the polarizers. If light now passes then
the slide is changing the polarization. Try this with good slides too so
that you can verify that good slides do not change the light
polarization.

If the slides are the problem you have two choices. One is to make
copies of the slides using a film type that is not birefringent. The
other is to mount the polarizers after the slide rather than before the
slide. You can do this by attaching polarizers to the screenside of your
projection lenses. You loose light by adding the polarizers but it will
cause no damage. 

In theory you could remove the original polarizers and get that lost
light back. But, the slides will then be exposed to about 3 times as
much infrared and visible light. Tripling the visible light is not a
problem but tripling the infrared may cause your slides to get very hot.
If you have a very good infrared filter (heat absorbing glass) in your
projector you might not have a problem when you remove the internal
polarizers.

If you decide to remove the internal polarizers be sure to use slides
that are not valuable to test this. Dark slides will get the hottest.
Very transparent slides will not absorb as much energy so will be
cooler. Project slides for a couple of minutes. If 2 or 3 minutes does
not warp them you may be ok.

Read http://www.clark.net/pub/thubin/aosd/photo3d/ir_study.txt for some
detailed info about overheating slides.

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design


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