P3D Re Circular polarizing filters,etc

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

Ted Morris asked "Does anyone know where we can purchase circular polarized
filters and /or glasses?."

"The filters must be multichromatic (not just limited to R,G,B spectrum)."

 I am not shure what he means by R,G,B  spectrum if he means that they are
individuly Red Green or Blue  then it is certainly possible to get  filters
that are more multichromatic than that . You can get "white " light
circular polarisers for SLR,s and in fact they are tending to replace
linear polarisers because they can cause problems with the beamsplitters in
some exposure systems. Many people are also probably reading this through a
computer circular polarising screen which is effectively achromatic. But if
he means that a filter that is aproximately multichromatic by circularily
polarising a red green and blue wavelength I think that is all they can do
just as colour monitors and colour photography just work with the three
primary colours.
  Because as John Vala has pointed out a circular polariser consists of a
linear  polariser followed by a 1/4 wave plate orientated at 45 degrees to
it to convert it to circular polarisation. If it is only 1/4 wavelength
retarding then it can only convert the wavelength that it is a 1/4
wavelength of to circular polarisation. With increasing thickness of
waveplate the higher frequencies reach this condition first because they
are shorter wavelength anyway and are also retarded more as with a prism
which is why it causes them to refract more .As thickness increases the
lower frequency longer wavelengths will reach this condition but then the
higher frequency shorter will have been retarded more than 1/4 wavelenth
and will not have 1/4 wave difference again and so be circularily polarised
again untill they reach 3/4 wavelenth etc.
 Wiliam Shurcliff and Stanley S.Ballard in their book on polarised light
1964 put it mathematicaly J must be directly proportional to wavelength so
that Jt/Lamda is independant of wavelength. Where J is the birefringence
(difference between the two refractive indexes of the 1/4 wave plate
material)  t = magnitude of electric field strength at any given time and
Lamda is the wavelengh.
 Perhaps it would be possible to get a 3 primary colour filter with blue
retarded 11/4 Lambda green 3/4 and red 1 which is perhaps what Ted Morris
means by R,G,B but that would be in whats called the 3rd order polarisation
colours which are still not white but Violet- red, Indigo, Green, Carmine,
Redish-Violet and Grey-Blue. You dont get white untill the high order
whites in the 6th order.   Many materials that are said to "depolarise
light" actualy repolarise it to a different configuration perhaps even
circular and pass white light. If you rotate the second polariser while
viewing such a piece of material and you cannot regain extinction it is
probably circularily polarising and could perhaps be used as a 1/4 Lamda
plate .  P.J.Homer



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