Re: looking for a polarizing filter

From: David Littlewood (david_at_nospam.demon.co.uk)
Date: 09/30/04 

Date: Thu, 30 Sep 2004 11:48:04 +0100

In article <300920042038005115%nospam@somewhere.invalid>, Denis de
Castro <nospam@somewhere.invalid> writes
>Hi, looking for a circular polarising filter, about 43mm diameter, screw
>mount, but diameter and mounting not critical as we can machine a
>mount. What's important is high light transmission.
>
>The lens came from a Leitz Diaplan 20 (a condenser stage held two of
>these lenses which could be moved relative to each other), but is now
>used in a laboratory 'composite' microscope used to study single live
>neurons with calcium ratio imaging. Apparently a layer of polarising
>material is sandwiched between layers of glass, and the film comes away
>from the glass causing an oil-slick interference pattern (we originally
>thought the lens was broken until I learnt this). We soon found a
>polarising filter at a camera shop, but apparently photographers'
>polarising filters all block a considerable amount of light ('2
>stops'), and we need a lens that allows plenty of transmitted light.
>
>Local agents of microscopy companies haven't been able to help
>satisfactorily. If anyone could provide a pointer to where a
>replacement could be found we'd be most grateful.
>
>
>cheers and thanks in advance,
>
Dennis,

The very nature of a polariser is that at least half the light is
removed - a perfect specimen passes all the component of light whose
electric vector is parallel to the plane of least extinction, and
absorbs all the light component whose electric vector is perpendicular
to this direction. This 50% reduction corresponds to a 1-stop reduction
in intensity.

In practice, the polarising material always falls some way short of
perfection. In my experience, a typical photographic polariser will
reduce light intensity by approximately 1.5 stops.

Your best bet would be to look at specialist filter catalogues and find
a Kasemann linear polariser. These use the very best polarising
material, sandwiched between two sheets of optically flat glass and
hermetically sealed to prevent moisture ingress. They cost 2-3 times the
price of commonplace photographic polarisers, but if you want the best
properties these would be the ones. B&W are among the prestigious
manufacturers who make them.

I saw someone say that all Kasemann polarisers are linear, but this is
not so; they are available in linear or circular form. You would need, I
imagine, a linear one - which would probably be a little cheaper.

David 

-- 
David Littlewood

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