P3D Re: "depolarising" problems

[Peter Homer <P.J.Homer@xxxxxxxxxxxxx>]

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.

This is actualy fairly normal for high contrast slides ,the light areas are
so light that some of it still gets through the crossed polars which never
give complete extinction.

Tom Hubbin wrote

>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.

I do not think the problem is birefringence in this case but the method Tom
mentioned above may cure the problem because as he says you lose light by
adding the second pair of polarisers. If this is not enough try rotating
the first set of polarisers relative to the second but keep them at 90
degrees to one another.
This will give a continuous neutral density dimming of the light untill the
problem disapears. I am about to go on vacation so I will not be able to
give    any further reply relating to this post untill 13 Sep.    P.J.Homer



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