P3D Re Why are polarising glasses orientated at 45 deg?

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

George Themelis quoted Lenny Lipton "This makes it possible to fold
cardboard temples in either direction while maintaning the proper
orientatio  of the axis. If the axis were at right angles (pseudoscopic
imaging would result) when these commonly used viewers are improperly
folded."
  This used to puzzle me to untill I got a copy of Leny liptons book "The
Foundations of the Stereoscopic Cinema" but I will have to check again what
he actualy wrote as the axis are actualy at right angles to one another
just at 45 degrees  to the vertical or horizontal IE + 45 and - 45 or 45
and 315 like so /\ . Rotation about the vertical is still /\ whereas |-
becomes -| making the image pseudo. Until Lipton the only explanation I had
come across was in Fritz Wraack who implies that it is to help eliminate
extraneous polarised light which he suggests is likely to be
verticaly/horizontaly polarised . Presumably he is refering to light
reflected of polished surfaces glass etc and the natural polarisation of
blue sky light. But these sources of light should be eliminated anyway by
an efficient black out so his explanation is not as convincing.
  When I first heard of stereo projection using polarisers I assumed that
they would use a vertical/horizontal arrangement as indeed the very first
system that of John Anderton 1891 did but then he used bulky goggles
containing piles of glass plates which could not have been put on back to
front and as Michael Watters pointed out the Fed in its original state
still does use vertical\horizontal.
 There is no need to use diagonal except for cardboard glasses which are
probably not used much now especialy by 3d enthusiasts and there is a
disadvantage to the use of the diagonal.
 Most other clear plastic materials if they have a molecular alignment have
it with its edges in the direction it is machined or stretched, as does
polaroid which is then cut diagonaly for our purposes . This means that it
is at 45 degrees to the direction of any aligned material in the system  so
is forced to split into two planes at right angles to each other to pass
through. They then constitute a pair of vectors at right angles to one
another so revert to a single plane at 45 degrees to them so back to the
original configuration . However with anything other than a cubic cystal
type structure one component will in the meantime have been retarded
relative the other ,if this is by half a wavelenth peak will now coincide
with trough and the direction of one vector will have reversed in respect
to the other and the diagonal alignment will have twisted through 90
degrees. So it will now pass through the polar it should be blocked by and
vice versa. With a retardation of 1/4 wavelenth cicular polarisation will
be produced with varying degrees of elliptical polarisation in between.
These will more or less pass equaly through both polars so the effect is
much like depolarisation and is frequently refered to as such although its
actualy repolarisation to a different configuration.  P.J.Homer



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