Re: outdated glasses? (longish)

[P3D Gregory J. Wageman <gjw@xxxxxxxxxx>]

Steven Lederman wrote:

>I recently won a box of old polarized glasses from the '50s [...]
>I set up my FED projector, and put in a slide. LUNCHBAG LETDOWN!!!
>No stereo effect! [...]  I then put on a pair of the FED glasses, and
>WOOOOAAAAHHHHHHHH!!! The person's foot in the picture leaped out and
>almost kicked me in the face!!  My question to the group; do the
>polarized lenses in the projector and the glasses have to match?

This is a more complicated question than it may at first seem.  Yes,
the polarizers in the projector and the glasses must "match".  There
are two ways in which they must match: the type of polarization (linear
vs. circular), and, if linear, in the angle of orientation.

The most commonly-available polarizing glasses use linear polarizers,
with the axes of polarization being 90 degrees to each other (obviously),
and at a 45 degree angle from vertical, like this: \ /.

>Is that why I'm not seeing the stereo effect with the older glasses?
>Or is this because the older glasses have all lost their polarization?

That's easy enough to test.  Take two pairs of glasses and put them
lens-to-lens.  The paired polarizers which are now crossed, should be
nearly opaque.  If they're not, then they're no longer polarizing.  This
is true whether they're circular or linear.

To see if it's a mismatch between projector polarizer axis and glasses
axis, look through one lens at the projected images and rotate the
glasses around the center of one lens.  If one of the images disappears
at some point, then that's the problem, and you need to re-orient the
projector's polarizers to match the glasses.

If the glasses cancel each other, but not the projected image regardless
of orientation, the it's a mismatch of polarization type.

>Does one have to buy glasses that are manufactured by the same
>folks as the projector?

No, as long as you have the correct type.

	-Greg W.


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