Re: Projector from Hell

[P3D Oliver Dean <3d-image@xxxxxxxxxxxxxx>]

Hi, Sam!

I caught sight of this thread while I was whipping through some old
digests that I couldn't check out in detail, and found that it has all
the symptoms of a little-known phenomenon -- you don't have a projector
from Hell, you have a slide with a film base from Hell!  I had some
experience exploring this many years ago, and discovered that a few
batches of film have a base material that unevenly rotates the axis of
polarization of polarized light passing through it. I'm not speaking of
those infamous copy films with an Estar base -- I'm talking about
ordinary Kodachome and, possibly, other commonly available slide films.
The nasty part of it is that the phenomenon is unpredictable -- most
film batches don't have it, but some do!  The film industry doesn't care
because purity of polarization through the film base is not a
requirement for the vast majority of uses for 35mm film. (Sort humbles
us Stereoscopic Projection fans, doesn't it?)

You can check this out quickly with your test slide as follows:
 
        1.  In the evening or in a dark room, set up a convenient diffuse light
source, like a light table or an illuminated slide sorter, or a light
shining away from you onto a white sheet of paper. Dim or extinguish the
room lights.

        2.  Over the light box, rotate two polarizers, such as those found in
ordinary polaroid glasses, until the light passing through them is
minimized, i.e., the polarizers look darkest.

        3.  Place your test slide between the two polarizers. If you have a
film base from Hell, the light passing through the transparent parts of
your side will be visibly lighter than the light passing through the
polarizers alone, and the image on the film will be more visible than it
should be.  If this is the case, it's your slide that's the culprit.

Reasons I think that this is your problem:

        a.  The image depolarized in your TDC, but not your Triad.  The TDC
puts the slide in the light path where the light passes through the
polarizers BEFORE it passes through the slide, thereby giving a slide
base from Hell the opportunity for unevenly spoiling the axis of
polarization.  The Triad, however, puts the polarizers in the light path
AFTER the light passes through the slide, which means that you can have
the most hideous of depolarizing films, even Estar based films, and the
Triad will project them with perfect polarization.  It is for this
reason that I modified my TDC projectors so that the polarizers are now
located like the ones in the Triad, namely, between the slide carrier
and the lenses.  (It increases the temperature of the light passing
through the slide, however, so if you do this, be careful that you don't
use any slides that have gelatin filters sandwiched with the film
between glass -- the gelatin may melt if the slide is left long enough
in the carrier!) 

        b.  The polarizing  worked OK when you put the polarizers in front of
the projector lenses.  Here you simply modified the TDC to work like a
Triad -- the film base from Hell had no effect with the polarizers
coming after it in the light path.

        C.  When you used the rear channel, there was no problem.  The rear
channel is designed for 2-inch high slides, such as twin 2X2's, so you
must have been using another test slide, one which doesn't have the same
film base batch as your slide from Hell. Check that one out between
polaroids as I described above, and I'll bet you find that it has much
less effect on the polarization than your slide from Hell did.

Hope this is the answer -- when I re-read your description, everything
seems to fit.  Please let us know!

-- 
Oliver Dean -- 3d-image@xxxxxxxxxxxxxx
Dominguez Hills (near Los Angeles), Calloushernia, USA


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