T3D Re: Circular Polarizers (fwd)

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

>Some info a friend sent me re circular polarizers...
>
>--Elliott
>
>>
>> Polarizing screens (aka linear polarizers) block all waves except
>> those in a certain plane.
>>
>> Some TTL metering systems use beam-splitters and/or half-silvered
>> mirrors to redirect some of the incoming light to the meter.


This would apear to confirm what I originaly thought although I dont think
silvering will have any effect.


>> If the light is plane-polarized, it disrupts the function of the beam
>> splitter.

Yes because partialy reflecting/transmitting  surfaces themselves tend to
polarise light  the light perpendicular to the surface passing into and
that at right angles reflecting. If the light is already polarised in
either of these two directions it tends to be just reflected or transmitted.
 Also beamsplitters such as Rochon and Wollaston prisms are variations of
the Nicol polarising prism and definetly split the beam into two beams
polarised at right at angles to one-another. Although if these were used I
think they will still screw-up circular polarised light as they are
birefringent devices and would act as retarders as well . Retardation by a
further 1/4 wavelength would give a total of 1/2 and it would return to
plane polarization although at right angles to the original direction .By
3/4 to a give a total of a whole wavelength
would restore the original plane polarisation either way the problem would
return.

>> Using a linear polarizer on such cameras will send your metering to
>> shit.
>>
>> A "Circular Polarizer" consists of an ordinary polarizing screen,
>> followed by a "quarter wave plate" which "depolarizes" the light.
>> The effectively unpolarized light will then behave OK in the
>> metering system.

I dont agree with that a "quarter wave plate" converts linear polarised
light to circular which will behave normaly with partial reflecting/
transmitting surfaces the only change being that the reflected component
will have its rotation reversed. The effect is much the same as if the
light was depolarised but strictly it is only de-planepolarised

>> So if you use a PL (linear) filter, the light is polarized all
>> the way to film, but with a PC (circular) filter the polarized
>> light gets "scrambled" again after passing the screen.

It could only be truly "scrambled" by a diffuser of the sort of material
that is useless for stereo back projection (Trying Desperately to Get back
on topic)
which would probably not allow any sort of recognisable image to get
through to the film.     P.J.Homer




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