circular polarizers @ NSA '84?

[P3D Peter Abrahams <telscope@xxxxxxxxxx>]

I've been told that (as best as can be remembered) at the NSA convention in
1984 in New Hampshire (?), the glasses used for the stereo theater were
circularly polarized, with matching filters on the projectors.  Do any
readers remember this, and whether the filtration was any more effective
than with the linear polarizers used for projection?
This issue arose because my stereo x-rays are very high contrast, white on
black, and there was ghosting during the shows in Bellevue (worse than when
viewed at home with a TDC.)  This was not because the film depolarized, but
because the most effective extinction with crossed Polaroid filters is
about 99 per cent.  Even 1 per cent of a bright, white image will appear on
a jet black background.  
I do not know if circular polars would have any better extinction.
I believe it was slightly worse at Bellevue because the filters for the
Brackett projectors I've seen are not held flat (like a camera filter), but
bulge in the middle.  Extinction by crossed polars requires the filters to
be very flat and parallel to each other.  Do any readers know if any
Brackett projectors sandwich the filters between glass or use very flat
plastic filters?
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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