re Transmax

[P3D Peter Homer <P.J.Homer@xxxxxxxxxxxxxxxxx>]

I have just got back on line having been on holiday over christmas and
noticed Larry Berlins quotes of and questions about Paul Ivester's post
about the Retina "Beamsplitter" and new polarising material . There are
variations on an older device for producing polarised light the Nicol prism
called Wollaston and Rochon prisms which retain two beams of light
polarised at right angles to each other rather than sacrificing one of them
. One allows one beam to carry on as normal and displaces the other to one
side the other displaces them both equaly in opposite directions ( I cant
remember which does which). The latter would probably be best for our
purposes as the path lenths are the same ,both are split into the spectral
colours but this could be corrected with two more standard prisms which
would be needed anyway to bring the beams parrallel. Another possibility is
an even earlier device the "pile of plates" polariser this uses the fact
that the light reflected of a transparent surface such as glass is highly
polarised provided  the rays  are parrallel and strike it at the
polarisation or Brewster angle (yes the Brewster of stereo fame). The
intensity is low while the transmited light is much more intense but only
slightly polarised at right angles to the reflected light by subtraction of
the reflected polarised light. If more layers are used the reflected
polarised light becomes more intense the transmitted less but better
polarised at right angles to the reflected untill 50% is reflected and 50%
transmitted although this is the theoretical ideal . The optimum number of
glass plates is about 24 but a device like this would probaly suffer the
same disadvantage as one of the above prisms in that athough the the
transmitted portion would be refracted it would not be displaced as much as
the reflected so there may a difference in intensity.
 Appart from that any device like this would produce the polarisations
vertical and horizontal to each other so they would not be compatible with
the standard glasses they would have to have their own like the Fed
projectors. But this would also be true of the system using circular
polarisers. If the prisms/plates were held at 45 degrees to produce the
standard polarisation the beams would be seperated diagonaly so could only
be used for 2X2 projection withe seperate projectors converging vertical
and horizontaly which could be awkward. One advantage to a vertical
/horizontal system is that it would not be "depolarised" by certain film
bases to the same extent if at all. P.J.Homer



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