looking for a polarizing filter

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Denis de Castro - 30 Sep 2004 09:38 GMT

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,

Denis de Castro

neurophysiology lab, Auckland NZ

(www.health.auckland.ac.nz/physiology/neurophysiology)

(my email address is dfkx2001 and the domain is sneakemail.com)

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David Littlewood - 30 Sep 2004 11:48 GMT

>Hi, looking for a circular polarising filter, about 43mm diameter, screw
>mount, but diameter and mounting not critical as we can machine a
[quoted text clipped - 16 lines]
>
>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

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Kevin Cunningham - 30 Sep 2004 13:23 GMT

> >Hi, looking for a circular polarising filter, about 43mm diameter, screw
> >mount, but diameter and mounting not critical as we can machine a
[quoted text clipped - 43 lines]
>
> David

David, good points. Zeiss used to use a prism polarizer but they no longer
make them for the very good reason that they don't polarize as much light.
The good thing about conventional Polaroid film polarizers is the amount of
light polarized is very high. I would be tempted to just buy a good high
quality replacement part from Leica, it fits and works.

Kevin

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Natural Science Forum / Physics / Optics / September 2004

looking for a polarizing filter