P3D Re: LCD lenses in T-Rex

[roberts@xxxxxxxxxxxxxxxxx (John W Roberts)]

>Date: Tue, 29 Dec 1998 16:51:33 -0700
>From: "William Carter, Ph.D." <wc@xxxxxxxxxxxx>
>Subject: P3D Re: LCD lenses in T-Rex

>It might be helpful to note that the "liquid crystal", in and of it's self,
>only alters polarization. A polarizing filter must be the first filter
>element. The "liquid crystal" then alters that polarization, 

More specifically, it rotates the plane of polarization of incoming linearly
polarized light, usually a multiple of 90 degrees (for example, 90 degrees
or 270 degrees). (Some LCD displays used to have a second LC layer, partly to
compensate for color shift - I don't know whether any of the new ones do.)

>depending upon an applied potential.. 

The applied potential actually *disrupts* the helical structure that rotates 
the plane of polarization. So, for example, if an LCD or LCS has
polarizers in front and in back with parallel planes of polarization, and
if the LC material rotates the plane of polarization 270 degrees, then the
cell will be opaque with no voltage applied (LC material rotates the
polarized light from the first polarizer, so it's cross-polarized to the
second polarizer), and transmissive with a voltage applied (LC material
does not rotate the plane of polarization, so the light just goes through
both polarizers). But don't forget to change the polarity of the driving
signal regularly, to avoid applying a DC bias to the LC material...

>usually +/- 5v.., 

Of the LCD panels I've used that required the user to provide the driving
voltage, none of them were 5V. I think most were in the 12-35 volt range.
I believe most display panels these days use voltage multipliers to
generate their driving voltages internally.

>and after that there's the analyzer at the aft end. 

"analyzer" = "polarizer"?

>In this scheme, polarization becomes either crossed, or
>un-crossed, depending upon the applied voltage; either "opaque" or "clear".

I think the main items of interest for 3-D projection are:
1) The LCS glasses alternately block the left and right eyes, so when a
   frame-sequential stereo movie (or field-sequential interlaced video)
   is shown, the left eye sees just the left frames, and the right eye
   sees just the right frames. In 3D IMAX, the movie is shown at 48 frames
   per second, so each eye sees 24 frames per second.
2) Since the LCS glasses use linear polarizers, it makes a difference
   whether the incoming light from the screen is polarized, and in which
   direction the polarization is.
3) It's possible to build LCS glasses so the two eyes are "cross-polarized",
   and project the left and right views accordingly, so that:
    - separation (extinction) of the left and right views is enhanced
    - it might be possible to view the same movie with passive polarized 
      glasses (perhaps with reduced extinction)
    - The LCS glasses might provide some stereo effect even if the
      battery runs out.

John R


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