Successful home-made polarizer for Compco Triad (and Howto)

[P3D Michael Kersenbrock <michaelk@xxxxxxxxxxxxxxxxxx>]

Greetings,

	Last weekend after getting home from the Cascade Stereoscopic 
Club meeting, I found a "round tuite".

	I've been wanting to try and improve extinction on high
contrast slides by improving on my projector's filters.  I had 
not been getting total extinction in the "other" eye when showing 
a "null-slide".  This shows up on slides where one image has a
bright white "something" where the other eye's image is solid black.

	A new filter works a bit better , but there still is a little
leakage.  The residual leakage seems not to be from the projector's 
polarizer but from the screen itself (made by DaLite and I think 
called "Silver Pacer").  Test was to put a piece of polarizer in 
front of the projection lens "crossed" and see if I get 
extinction on the screen, and doing that way, I did.

	Making a replacement took less than an hour, even doing
it slow and mildly ugly.  However it remains to be seen if it'll 
last a long time.  :-)

	In case anybody wanted to try it, I thought I'd pass along
what I did -- it costed me practically nothing but the time, and
not even much of that.  Really shouldn't need a round tuite,
although I needed one.

	I took the old filter (two-sided metal piece with two round glass
polarizer openings) and traced it onto two pieces thin corregated cardboard
taken from the inner box of a CDROM software package.  I also traced the
holes and then cut the pieces out.  To have looked nicer, I should have painted
it black like the metal version was.  I then put pieces of polarizing material
between the two cardboard pieces and stapled the cardboard together in 
various places.  Bingo, a replacement -- and it fits snuggly into the
slot where the original was.  And works.  Difference on
most slides isn't visible, but on high-contrast white-on-black slides, it helps
noticeably.

	The polarizing material is some I got from NSA '97.  Three large
sheets for $5.  They were used originally for projecting movies in 
the 50's -- it even said that it was part of the "Natural-vision" system.  
One I used had some bad areas on it, but most of it was "good" (my 
projector uses a MUCH smaller area polarizer than the movie projector!).
Hated cutting up a semi-historical thing, ... but I needed the material
and it's what I had.  The movie polarizers cross-tested with total 
extinction by eyeball.  Obviously this material would work for projection
purposes.  :-)  

	The Triad has the polarizer between the slide and the lenses
and blows cool air up between the polarizer and the slide to cool them 
both.

	Don't know how stable the cardboard will be in use, but
"we'll see" -- in maybe 15~20 minutes of test projection, it's
worked fine so far w/o any adverse effects.

	Aligning was "trivial".  I compared the new material with
the existing polarizer to know how to align things -- then aligned
the first one "straight".  Then using polarizing glasses ("assumed" 
to be  properly 90-degree aligned) I aligned the second one to full
extinction with one eye while the other eye was simultaneously
at full-extinction (holding it up to the window and a cloudy day sky).

	If I ever get another round tuit, I'll trace the pattern onto
sheet aluminum instead or some such and then paint it black so it'll 
look kewl.  :-)

	That'd take a *BIG* round tuite though.  Especially if the
quickie hacked version works fine.

Mike K.


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