Re: Polarization Filters

[T3D Peter Homer <P.J.Homer@xxxxxxxxxxxxxxxxx>]

>I agree, although I do not have figures (values, numbers) with me now,
>polarizers absorb *almost* 50 % of the intensity due to its basic principle.  
>The difference between 50% and the real value is negligible when calculating
>energy.

Actually I think it is more than 50% because as you say the basic principle
requires that it absorb about that amount to remove the unwanted components
but there is some unwanted light getting through. This would make it less
than 50% but then some of the wanted light is also absorbed.  The best
Polaroid I have heard of for light transmission was something like H42
where the number is the % of light transmitted.

>There is another absorption, natural and independent of polarization,
>much greater than this and that should be indicated also.

This is perhaps part of the reason that some of the wanted polarisation is
also lost.

>Usually, polarizers with high extinction rate have also a high absorption
>rate (appear darker).

I agree I think because as above when the intensity of the unwanted
component is reduced that reduces the total light as well but also more of
the wanted component is absorbed as well.

>Because we have in Brazil much Calcite (birefringent crystal) I thought
>many years ago of making an illuminator for projectors which divides the
>original beam in two with opposite polarizations, absorbing no (or
>almost no) energy.  It would be a double projector, using one lamp and a
>long crystal, about 30 cm long, 80 mm x 40 mm wide.
>I do not know if such a long crystal can be found, somebody talked to me
>about needing to assemble two or three.

I have had similar ideas myself but are you talking about using a calcite
crystal as it is, or assembled into a Nicol prism? This would probably not
need to be so large. Normally one of the polarised beams is absorbed at 
the sides of the prism which is painted black but this could probably be
removed although there may then be problems with total internal reflection.
There are variations on the Nicol prism which produce two cross polarised
beams such as the Wollaston and Rochon prisms.  One diverts one beam to the
side and splits it into the component colours, the other diverts and splits
both beams.  I cannot recall which does which. The one which diverts both
beams would probably be best to keep the path lengths the same with two
single separate prisms to bring the beams parallel again and recombine the
colours. One disadvantage, the polarisations would be vertical/horizontal
unless the whole arrangement was at 45 degrees which would mean that only
separate 2x2 slides could be used.

I knew that Brazil is rich in minerals but had not realised that calcite
was one of them.  Is there sufficient of sufficient quality?  The reason
Professor Land developed Polaroid in the first place was because supplies
of calcite had run out . Even the first person to do polarised stereo
projection, Anderton in 1893, before Polaroid was invented, had to use 
"pile of plates" polarisers rather than Nicol prisms. However he was using 
large lantern slides which would require large Nicols from large crystals.

>It could have been a task for some student of physics here, but the
>person finally did not come to work.
>
>There could be other ways of dividing polarization between the lamp and
>the photographs, did you hear of something similar?

Maybe the "pile of plates polariser" mentioned above could be used.  Anderton
used the transmitted light but the reflected light is also polarised at
right angles to the transmitted . There used to be a device in Anderton's
time called an "Elbow polariscope" which used light reflected from a pile of
glass plates for the polariser and a Nicol in the small objective lens as an
analyser for projecting images of microscopic birefringent specimens.
The deviation of the transmitted beam by refraction would probably not be
as much as that by reflection so there could be path length problems as
with one of the types of prism.
                        P.J.Homer


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