Best Polarizing Filters

Here you can discuss different microscopic techniques and illumination methods, such as Brightfield, Darkfield, Phase Contrast, DIC, Oblique illumination, etc.
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microbob3
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Best Polarizing Filters

#1 Post by microbob3 » Wed Jun 10, 2020 7:49 pm

I was going to fool around with polarizing filters. I read a article here but cant find it now. You need 2 I know and turn them to where it looks black. But is their a certain type thats best. Thanks

BramHuntingNematodes
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Re: Best Polarizing Filters

#2 Post by BramHuntingNematodes » Wed Jun 10, 2020 7:58 pm

I like surplus photographic filters. They can be had for almost nothing when in unpopular sizes.

There are also so-called circular polarizers, which are polarizers with a quarter wave plate attached to one side. Quarter wave plate film can be obtained relatively cheaply from Edmunds Optics or American Polarizers to make your own. These films can also, I believe, be used to make a compensator.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

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75RR
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Re: Best Polarizing Filters

#3 Post by 75RR » Wed Jun 10, 2020 8:03 pm

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Zuul
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Re: Best Polarizing Filters

#4 Post by Zuul » Wed Jun 10, 2020 8:04 pm

Most photographic filters are circular polarizers. They did make some linear polarizers, too, but they are rare. Linear is typical for this application. If you use circular, I believe the “front” of the filter needs to face the specimen for best results.

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Re: Best Polarizing Filters

#5 Post by BramHuntingNematodes » Wed Jun 10, 2020 8:11 pm

Heh, maybe these days, but I have bought a bucket of surplus filters a few decades old for a pittance and they are all linear. They also have the added benefit of having a rotating bezel.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

Zuul
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Re: Best Polarizing Filters

#6 Post by Zuul » Wed Jun 10, 2020 8:41 pm

BramHuntingNematodes wrote:
Wed Jun 10, 2020 8:11 pm
Heh, maybe these days, but I have bought a bucket of surplus filters a few decades old for a pittance and they are all linear.
Could be an age thing. I don't remember the exact reason, but I thought "pure" polarized light messed with phase detect focus or something like that. That means circular would have become the norm when auto-focus cameras got popular.

PeteM
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Re: Best Polarizing Filters

#7 Post by PeteM » Thu Jun 11, 2020 1:11 am

Better polarizer sets will go to a deep dark extinction - and make a difference in something like DIC.

The affordable camera type polarizers vary from fairly cheap (and only a sort of dark gray when crossed) to approaching the deep extinction of the best polarizer/analyzer sets.

For casual use, it's hard to beat the prices of cheap import camera polarizers, which come in a wide variety of sizes and can usually get pulled out of their mounts with a small spanner if desired. Since most are now circular polarizers, be sure to get the orientation right.

microbob3
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Re: Best Polarizing Filters

#8 Post by microbob3 » Thu Jun 11, 2020 12:56 pm

Thanks guys I found linear polarizing filter set on ebay for $10. Many sizes . I thinking about 33mm.

microb
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Re: Best Polarizing Filters

#9 Post by microb » Thu Jun 11, 2020 5:46 pm

If you just want to experiment, just buy this set of 10 square films for $8: https://www.amazon.com/Polarizing-Film- ... 183&sr=8-4

If you want what the microscopes use then these from Edmund Optics go for a much higher price but they go far darker when cross polarizing (extinction ratio 10000:1): https://www.edmundoptics.com/f/high-con ... ers/12725/

If you want cheap plastic filter discs to put into a slider you have, probably diameters like 25.4mm or 20mm, ebay has these available in different sizes (one month mailing time and maybe slower nowadays): https://www.ebay.com/itm/2-Pcs-25-4-mm- ... SwaU5eFJaR

The ebay disc filters will match the level of the square films mentioned above.

sleekdigital
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Re: Best Polarizing Filters

#10 Post by sleekdigital » Sat Jul 25, 2020 2:08 pm

I am currently using 2 old filters I had used on my camera. I put one over the lamp and one over the slide. That works well, but I can only use up to a 10x objective because of the thickness of the filters. I just ordered some of the 8$ sheets from amazon. I'm hoping that I can use one of the camera filters over my light source and cut small pieces of the sheets to use as cover slips :lol:

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Re: Best Polarizing Filters

#11 Post by PeteM » Sat Jul 25, 2020 3:49 pm

sleekdigital wrote:
Sat Jul 25, 2020 2:08 pm
I am currently using 2 old filters I had used on my camera. I put one over the lamp and one over the slide. That works well, but I can only use up to a 10x objective because of the thickness of the filters. I just ordered some of the 8$ sheets from amazon. I'm hoping that I can use one of the camera filters over my light source and cut small pieces of the sheets to use as cover slips :lol:
If your microscope has a detachable head (as most binocular and trinocular ones do), try to see if you can insert a small polarizing filter in the space above the objectives and below the head. There's room to adapt or drop in something, such as a small camera or iPhone polarizer, in a majority of microscopes I've seen.

sleekdigital
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Re: Best Polarizing Filters

#12 Post by sleekdigital » Sun Jul 26, 2020 12:41 pm

Thanks for the idea, I will give that try. The film works pretty well, but only up to 10X. However, when I move up to 40X, the field no longer looks very dark and I can't focus. I also tried both the film and my camera polarizer with an eyepiece on the camera port (where it seems the prism doesn't affect the polarization). There I could focus but the field was still not very dark. So, I'm still not sure what I need to get above 10x. Maybe higher quality polarizers are required as you increase magnification?

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