T3D Circular or Round

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

I have moved over from photo-3d for these questions as Brian Reynolds
suggested
he wrote.

Many modern cameras use off the film (OTF) metering.  The camera's
electronics measure the light reflected off the film during the
exposure in order to determine the exposure for the photo.  This light
becomes linearly polarized when it reflects off the film.  Using a
linear polarizing filter with such a metering system will give
inaccurate results because the combined polarizing results of the
filter and the reflection will lower the amount of light reaching the
metering system, but not the light reaching the film.  To correct for
this camera companies created a special polarizing filter.  The first
part of the filter is a linear polarizing filter, the second part
takes the polarized light (from which the glare has been reduced) and
randomizes it again so that it is no longer linearly polarized.  This
light can then reflect off the film and be read correctly by the
camera's metering system.  Unfortunately even thought the light from
this special polarizing filter is not circularly polarized, the camera
companies decided to call it a circular polarizing filter.

So it is just the reflection of the film that causes the polarisation? I
had thought that it was beamsplitters in the cameras optics diverting some
of the light. I am a little suprised that the light reaching the film from
the lens is polarised by reflection as it would seem to be largely
perpendicular to the film plane. Whereas polarisation by reflection
requires the light to strike at the polarisation or Brewster angle (Of
Stereo fame) which is about 55 degrees to the perpendicular normal ray.
Secondly as I wrote in photo-3d if the first part of a camera "Circular
polariser" is linear I would have thought that it could be orientated at 90
degrees to virtualy remove all of the linear polarised glare.
 I had taken the manafacturers word for it that their circular polarisers
were just that. In an early advert for them I noticed that for black and
white photography and special effects with colour they were offering
coloured circular polarisers in different colours cheaper than the
achromatic ones. One of the reasons circular polarisation could not be used
for stereo projection in the 50,s and even 60,s was that it was not
possible to produce an achromatic filter which did not introduce a
polarisation colour. But I believe this is now possible   and some people
are probably reading  this through a large circular polariser to remove
reflections I though that the cheap coloured camera filters at least were
old type circular polarisers and the achromatic ones the same as the
computer screen type.
 I canot see how the camera manafacturers filter can randomise the
polarisation again without using a diffuser of some sort to scatter the
light which would also affect the image .          P.J.Homer



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