P3D Re: Ghosting

[Tom Hubin <thubin@xxxxxxxxx>]

>Help. I'm giving two slide shows tomorrow for a community event on historic
>preservation. I have about a hundred slides, mostly houses & details of
>houses that I shot with a Realist and mounted each to the window using the
>little mounting allignment guide from Reel 3D. About half a dozen slides
>have horizontal ghosting. Any ideas on why that is and suggestions on how to
>fix?

Three causes have been suggested. First you need to determine if the
cause is the slide, the projector or the glasses. Since you have lots of
slides that work well it is not likely that it is your glasses or your
projector. So I will assume that it is the slides. 

If the projectors polarizers are located between the slide and the
screen then the quality of the slide does not matter. But, most
projectors have the polarizers between the light source and the slide.
If the slide alters the polarization of the light as the light passes
through the slide then the screen image will not be purely polarized
enough and you will see ghosting. This does happen with some slide film.

A quick fix is to add polarizers between the slide and the screen.
Usually attached just outside the projection lenses. This can be a
problem if you need to rotate the lenses to focus. You rotate the
polarizers for appropriate viewing. Once the polarizers are properly
oriented you do not want to rotate them again until the show is over.
You will loose about 24% of your nearly polarized light doing this but
it will clear up the ghosting if the cause is the slide film itself.

A way to do this without losing light is to also remove the polarizers
that are inside the projector. But if it is like the TDC716 then this is
not a good idea. The polarizer before the slide only passes 38% of the
unpolarized light and IR energy hitting it. If you remove the polarizer
then the slide is exposed to almost 3 times as much light and heat
causing IR. This can ruin the slide due to excessive heat. 

There are ways to do this. If you ever plan to make a mod to your
projector condenser optics, IR absorbing filter, or polarizers, first
read my report on projector improvements that damage film.
http://www.clark.net/pub/thubin/aosd/photo3d/ir_study.txt

A long term fix is to copy the slides onto a more acceptable slide
film.   

There are tests that you can do to determine the cause of the problem.
Too many to write about at this late hour. If you want to pursue this
then I need to know more about your projector. 

Tom Hubin
thubin@xxxxxxxxx


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