P3D Re De-polarising filter?

[Peter Homer <P.J.Homer@xxxxxxxxxxxxx>]

>I've finally gotten hold of the lenticular screen I'm planning to use for
>polarised PC 3D projection using my two LCD projectors.  Watching a single
>projector's output on the screen with polarised glasses however has revealed
>what should have been obvious (with hindsight) - ie. that the light from LCD
>projectors is already partially polarised!

I hoped to make use of this fact for stereo projection, my idea was to atattch
 another LCD cell like those used for field sequential glasses minus its
polaroids to the front of the projector . This would rotate the
polarisation of every other frame of a field sequentional stereo video
projected on a silver screen which should allow it to be watched with
conventional stereo glasses. I even thought the 45 degree convention might
be used because LCD cells such as those in the Tommytronic stereo games are
polarised this way. However LCD projectors use a combination of vertical
and horizontal polarisation.

>Well, from my (limited) understanding of LCD technology, polarising filters
>are used on both sides of the liquid crystal layer, so of course the emitted
>light is already polarised.  In practice, this is causing a marked colour
>difference between the left/right eye images of a single projector as seen
>through the glasses (I think the reason that the difference isn't more
>drastic is to do with the beamsplitting that's going on inside the
>projector).

I think beamsplitters or rather beamsplitters working in reverse,
beamcombiners are the cause of the problem. The three primary colours are
combined from the three LCD cells by reflecting two into the path and
transmitting one. This means that the reflected components must be
polarised in the plane of the reflecting surfaces but the transmitted must
be polarised perpendicular to them. This is also why horizontal\vertical
alignments are used. To produce a 45 degree output would require the whole
arrangment to be tilted at 45 degrees.

>The point is, although it looks like I could compensate for this colour
>difference, I'm obviously loosing light output once the projectors get their
>polarising filters attached, and they are dark enough already for this to
>become a problem... so, the question is, is there such a thing as a
>de-polarising filter?

Strictly speaking I dont think there is except for a diffuser which would
also ruin your projected image. However many materials effectively
"depolarise" light by converting to a form of polarisation other than
linear, idealy circular by the use of a quarter wave plate. This may still
reduce the light a little but will not be affected by the alignment of the
linear polars. At one time it was difficult to produce circular polarisers
for all colours because the quarter wave retardation depends on wavelength.
But they seem to have got around this problem and they are available for
cameras and computer screens, so you may be reading this through one. You
could try using a couple of these with the quarter plate side towards the
light source, which is the reverse of their normal use. They should convert
your projector outputs to circular polarisation both left and right handed
and the second layer will produce plane polarisation in the normal way. You
could also just use quarter wave plates and your existing polarisers but
quarter wave material alone may be more difficult to obtain. You could  try
experimenting with "depolarising" materials such as various plastics to
find one that gives no noticeable dimming of light between crossed
polaroids.

          P.J.Homer