P3D Re: Re De-polarising filter?

["gl@xxxxxxxx" <gl@xxxxxxxxxxxxxx>]

Thanks for the insights.  Just to clarify; what exactly does a quarter-wave
retarder do to the polarised light?  And if I can get hold of one, that
sandwiched between the lens and a standard linear polariser should do the
trick?

I guess I should really read up on exactly what polarisation actually is - I
understand the effect, but not the underlying mechanics.
--
gl

----- Original Message -----
From: "Peter Homer" <P.J.Homer@xxxxxxxxxxxxx>
To: "Multiple recipients of list PHOTO-3D" <photo-3d@xxxxxxxxxxxxxxxxx>
Sent: Friday, January 21, 2000 3:32 AM
Subject: P3D Re: Re De-polarising filter?


> >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
>