Polarizing filter for microscope objective

[Chris S.]

This question may be elementary for the experienced microscopists at this forum, as polarizing microscopes are so widely used--but coming from a non-microscopic background, I so far haven't found an answer that is clear to me.

On the Bratcam (http://www.photomacrography.net/forum/v ... php?t=8247), I'm using microscope objectives on a bellows. I need to photograph a subject that requires cross polarization. Adding a polarizer to the lights is pretty easy--what isn't obvious to me is how to put a polarizer on the objective lens.

Does anyone know where to find a small, high-quality polarizing filter for the front of a microscope objective that won't take up too much of my working distance or hang out far to the sides? I can use any polarizing film for the lights, but for the lens, I should have something of high optical quality.

Also, am I correct in my understanding that I cannot put a polarizer behind the objective, somewhere between it and the camera sensor? That might be easiest, but I have it in my head that a polarizer needs to be in front of the lens.

Thanks, folks. Cheers,

--Chris

[AndrewC]

You can put them between the objective and "viewer":

http://www.microscopyu.com/articles/pol ... intro.html

I'm sure an optical expert will correct/modify my answer but typically good quality strain-free optics don't depolarise light.

In fact I've been searching for a T-mount polarizer for a long time with no success as that would be the easiest for me to integrate in my setup. Otherwise I need a T to filter_thread, then the filter, then a filter_thread back to T. Or maybe M42 instead of the T-threads ...

[Chris S.]

Thanks, Andrew. Makes sense. I'd like to see some other folks weigh in on the accuracy of this source, since it has been caught in substantial error a few times. However, it also appears that Nikon polarized optiphots were built this way.

If so, we really need a drop in-filter option for the bellows. Andrew, you have me thinking about just doing it with it with adapters. Was comptemplating something in PVC with the appropriate mounts epoxied to front and back, fitted for a Nikon 39-mm drop in filter. Black paint on outside, flocking on inside. You approach sounds better.

Cheers, (Still would like to hear confirmation/amplification.)

--Chris

[Craig Gerard]

Chris,

The first method that comes to mind; is to attach a Nikon BR2A to the bellows, then attach a polarising filter to the BR2A via the 52mm thread, next, screw a Nikon BR3 onto the exposed filter thread, then finally the T-Mount of the Beljan cone attaches to the Nikon bayonet on the BR3.

Are you familiar with these pieces; I can upload a photo? I just hobbled it together.

Regarding the polarising filter and impact on resolution.....hmmm...don't know; but use the best quality filter you have or can buy, something like a B+W MRC CPL; probably doesn't need to be a CPL.

As Andrew suggested, it would be better to have the analyser behind the objective. Hope others will chime in with their thoughts.


Craig

[Chris S.]

Craig, I am familiar with those pieces--but please add a photo to this thread--I'd love to see how you did it.

Like you, I'm thinking to go with the best polarizer money can buy. Mine are all B&W, but they live in the field bag and there they should probably stay. Will likely purchase another B&W for the Bratcam. As you say, probably not much need for CPL.

Thanks! Looking forward to see you photo(s) of this setup.

Cheers,

--Chris

[Pau]

Cris,
The polarizer (in microscopes this one is called analyzer) must be placed between the objective and the sensor, like in any polarizing microscope, usualy near the objective. If you place it betweeen the objective and the subjet, apart of reducing the useful working distance you are likely inducing severe spherical aberration and flare. High quality microscope polarizers are usualy expensive, but a good photographic one may be adequate. First test if when you cross the polarizers they do not induce any color cast (someones do).

If your are using the normal camera photometer, you need a circular pol in the camera side, because photometeres (and AF systems) are sensitive to pol light as they collect ligth by means of a semitransparent mirror.

If you browse my posted images, most of them are done with this approach.

[Chris S.]

Thanks, Pau. Ok, I'll put the polarizer behind the objective--nice that is happens to be the easiest way to do it--especially if I go with Craig's approach. There are sources out there that say (apparently wrongly) that the polarizer must be in front of the lens. Coming from a standard photography background, this is of course the only way we generally do it. But if putting the polarizer behind the lens works, then this is actually a great thing--much easier to place.

My B&H polarizers have imparted (in normal photography) no color cast; my cheap Tiffen ones (from long ago) do impart a green-yellow cast.

With the Bratcam, I focus and set exposure totally by hand, so otherwise legitimate concerns about circular polarizers probably don't apply here. Right?

Thanks again. I appreciate the good advice I'm getting here tonight.

Cheers,

--Chris

[Craig Gerard]

Here's a quick image.

The filter in the photo is a UV (but you get the idea). I don't have a 52mm CPL.



Craig

[Chris S.]

Thanks a bunch, Craig. Looks beautiful. I like it, and will likely cobble together something very much like what you came up with.

Best,

--Chris

[Pau]

Glad if it can help

There are sources out there that say (apparently wrongly) that the polarizer must be in front of the lens.

I never saw this refered to microscopes. The only reason I can think for doing it whith a microscope objective is if it has highly stressed glass, but I only had problems whith a very damaged one. (there are special pol objectives strain free, but in my experience it is't necessary for normal X pol light)

My B&H polarizers have imparted (in normal photography) no color cast; my cheap Tiffen ones (from long ago) do impart a green-yellow cast.

OK, but test the crossed filters in front of a bright light before. The color cast of Xpol may be different and much more intense.

With the Bratcam, I focus and set exposure totally by hand, so otherwise legitimate concerns about circular polarizers probably don't apply here. Right? .

exposure totally by hand?. you don't use the camera photometer?
If you are buying a new one go whith a circular one, if you already have one, try it, it problabily will work OK.

[AndrewC]

... You approach sounds better.

...

--Chris

Possibly, possibly not - rotating filters aren't designed to hold much weight. A better solution might well be to use a drop in filter housing if you can get all the right size pipes ... or just buy a trinoc microscope !!

[RogelioMoreno]

With the Bratcam, I focus and set exposure totally by hand, so otherwise legitimate concerns about circular polarizers probably don't apply here. Right?
--Chris

In this case, if were you I would buy a linear polarizer. Linear polarizer has better performance than circular polarizer. The microscope polarizers are linear polarizer.

Rogelio

[RogelioMoreno]

Possibly, possibly not - rotating filters aren't designed to hold much weight. A better solution might well be to use a drop in filter housing if you can get all the right size pipes ... or just buy a trinoc microscope !!

Thank you for the tip!

Rogelio

[Pau]

Linear polarizer has better performance than circular polarizer. The microscope polarizers are linear polarizer.

Not all of them!. I have an high end nikon microscope polarizer set and the analizer is a circular one. The extintion is very good (nearly total) whith no visible color cast.

[RogelioMoreno]

Pau,

I would like to see a picture of the Nikon analyzer.

Rogelio