Tests with polarizing filters

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fdupre
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Tests with polarizing filters

#1 Post by fdupre » Sun Feb 13, 2022 7:51 pm

Hello everyone,

I'm a complete beginner and new to the subject, but I've just done some tests with polarizing filters, and well, I find this technique highly interesting!
So I did some tests and here are the results:
Filters configuration:
config_filtres_polarisants_ang.jpg
config_filtres_polarisants_ang.jpg (136.73 KiB) Viewed 6547 times
I used half of a variable polarizing filter intended for astronomy directly placed on the slide and a photo polarizing filter directly placed on the light source.
With this config, I manage to have a total extinction without any problem and also to vary the brightness with surprising effects.

I therefore first looked for total extinction by rotating the photo filter placed at the bottom on the light source (it was more practical than turning the one placed on the blade) and the results obtained surprised me. I will do other tests with other subjects because given the results and the ease of obtaining a black background, I find that this technique is very very interesting and deserves to be well understood and deepened with other subjects . . .

Sand:
without polarization:
001Sable_normal10x_StackeBH.jpg
001Sable_normal10x_StackeBH.jpg (329.15 KiB) Viewed 6547 times
with polarization (complete extinction):
002Sable_fermerMax10x_StackeBH.jpg
002Sable_fermerMax10x_StackeBH.jpg (286.13 KiB) Viewed 6547 times
A hair :
without polarization:
003Cheveux_normal10x.jpg
003Cheveux_normal10x.jpg (73.45 KiB) Viewed 6547 times
with polarization (complete extinction):
004Cheveux_fermerMax10x.jpg
004Cheveux_fermerMax10x.jpg (82.54 KiB) Viewed 6547 times
It is with this test that I admit to having been the most surprised by the results because without polarization, it is impossible to have the hair correctly. I turned up the luminosity, breaking the curves during development but impossible to go through the hair, whereas with the polarization, I had the hair not too badly had the first time without forcing anything!

I have the impression that this technique illuminates the objects as if by "magic" and it tickles my curiosity :)
Dear friends, if you have any comments or advice, please don't hesitate.

Sincerely,
Franck
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EYE C U
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Re: Tests with polarizing filters

#2 Post by EYE C U » Sun Feb 13, 2022 7:54 pm

SEE IF YOU CAN PLACE ONE UNDER THE BINOCULAR HEAD IN BETWEEN

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josmann
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Re: Tests with polarizing filters

#3 Post by josmann » Sun Feb 13, 2022 8:24 pm

Definitely pick up some linear polarizing film (originally known simply as Polaroid). You can get surprisingly great results with it and you can experiment putting it in all sorts of different places in the optical system. If you want to do high mag polarization, you can stick a little piece to the back of an objective for best results.

Some other things to try:

Crossed polarizing film directly below and above the slide. (I think this may actually yield the best results for low mags!)

Put cellophane or other plastics between condenser and field lens polarizer and turn them to observe the effects

Combine crossed polarizers with darkfield

Try using a piece of polaroid as a coverslip for high mag lenses (never tried it, but I’m curious)

ShinyaVision! (https://youtu.be/z6TUnvAD-Pg)

There’s a seller on eBay who sells all sorts of polarizers and waveplates at good prices and often throws in a free crystalline sample slide which is always fun. Worth stocking up on some of his stuff!
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Re: Tests with polarizing filters

#4 Post by Greg Howald » Sun Feb 13, 2022 9:18 pm

I took the illuminator lens assembly apart and placed the polarizer on top of the diffuser. I adjust polarization by rotating the illuminator lens with the analyzer in the head. This seems to work well in most situations.
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Re: Tests with polarizing filters

#5 Post by richbart » Sun Feb 13, 2022 11:47 pm

On some samples I use just one layer of polarizing film over the field diaphragm. It seems to help with some lighting. It can reduce aberration.

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Re: Tests with polarizing filters

#6 Post by fdupre » Mon Feb 14, 2022 10:22 am

Hello,
Thank you very much for your very interesting advice.
Sincerely,
Frank
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Re: Tests with polarizing filters

#7 Post by fdupre » Mon Feb 14, 2022 2:52 pm

Hello again friends,

Here is some tests with subjects (onion cells and red rose petal) which I did not see any improvement with the polarized lighting system :

Onion cells without polarization - 10x:
oignon_sans polarisation10xMH.jpg
oignon_sans polarisation10xMH.jpg (281.19 KiB) Viewed 6450 times
Onion cells with polarization - 10x:
oignon_polarisation10x_extinctionCompletMH.jpg
oignon_polarisation10x_extinctionCompletMH.jpg (241.53 KiB) Viewed 6450 times
Red rose petal without polarization -10x:
roserougex10_sansMH.jpg
roserougex10_sansMH.jpg (237.89 KiB) Viewed 6450 times
Red rose petal with polarization -10x:
roserougex10_polacompletMH.jpg
roserougex10_polacompletMH.jpg (209.35 KiB) Viewed 6450 times
I have the impression, but tell me if I'm wrong, that if the subject does not take grazing light and if it does not have a certain finesse such as dust or hair or sand and well the polarization has no effect on the subject. I have the impression that the subjects react like with the technique of the black background with a patch of dark field.

friendly,
Franck
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Re: Tests with polarizing filters

#8 Post by josmann » Mon Feb 14, 2022 5:50 pm

richbart wrote:
Sun Feb 13, 2022 11:47 pm
On some samples I use just one layer of polarizing film over the field diaphragm. It seems to help with some lighting. It can reduce aberration.
Very interesting you should say this! My diatom test slide has some features which are basically at the diffraction limit (a little under, actually!) and I noticed the other day that by rotating a single polarizer as you've described I've been able to make them appear and disappear. I still haven't fully figured out why this is but one possibility is that it makes the light a little more coherent and thus more prone to interference. Since a microscopic image can be largely understood as an interference pattern, it would make some intuitive sense. Very much worth experimenting more with!
fdupre wrote:
Mon Feb 14, 2022 2:52 pm
I have the impression, but tell me if I'm wrong, that if the subject does not take grazing light and if it does not have a certain finesse such as dust or hair or sand and well the polarization has no effect on the subject. I have the impression that the subjects react like with the technique of the black background with a patch of dark field.
The most notable way you'll see an effect of polarized light is when your subject is "birefringent." Birefringence means that a material has a different optical interaction with one polarization versus another. In many cases, birefringence arises when there is a regular, repeating molecular or atomic pattern in the sample. The easiest to understand are probably crystals - such as the sand grains where you observed strong birefringence. At about 7:40 in to this video https://youtu.be/auHbRi7bHfk I show a calcite crystal and how it can turn from light to dark depending on the angle of the incident polarized light. Crystals are formed by regular repeating atomic structures and are almost always birefringent.

In nature, certain organisms will grow crystals formed from their diets which you can observe easily in polarized light. One classic example are sulfur-fixing bacteria like beggiatoa which often contain sulfur granules which are birefringent. And certain living structures can also be birefringent. If you have any muscle fiber or root cross-sections you might try them out.

Whenever you see a distinct pattern of birefringence (like in those sand grains), that's telling you something about the molecular structure of what you're looking at. It's saying "hey, there is a highly organized structure in this thing." And if you're an expert polarized light microscopist, you can make measurements of that polarization response to understand the molecular structure without actually seeing the molecules :)

Another effect of using polarized light in microscopy is that structures which are highly scattering or refractive can scramble the polarization of incident light. The light leaving them is no longer highly polarized and will pass through the second filter. This is why dust and I believe the hair as well show up under crossed polarizers - they are scattering the light and so you get something like a dark field effect.
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Re: Tests with polarizing filters

#9 Post by richbart » Mon Feb 14, 2022 8:55 pm

I have a video of Difflugia that shows bifringence in the test. I’ll have to try and post it.

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Re: Tests with polarizing filters

#10 Post by fdupre » Tue Feb 15, 2022 10:18 am

Hello everyone,
@ Josmann: Thank you very much for your very very interesting and instructive feedback.
@ Richard: Thanks for the video
Sincerely,
Frank
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Re: Tests with polarizing filters

#11 Post by Sansub2 » Wed Feb 16, 2022 1:44 am

Frank,

Remove the condenser and see whether it makes the difference. For some reason, the condenser seems to reduce or change the polarization light for me. The way I achieved the maximum polarization is a) by removing the condenser for lower objectives b) by placing the bottom CPL on top of the condenser. The top one(2nd one) goes inside the head.

If you have 3d movie glasses, you can remove the film's from there easily and use it. It works very well. The side facing your eye is the liner Polarizer side, the side facing the screen is QWP side. I used the right eye film at the bottom and left eye at the top and got good results.

Halogen light makes a difference as well. Do you have 3d printer?


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Re: Tests with polarizing filters

#12 Post by Sansub2 » Wed Feb 16, 2022 2:48 am

Few more samples.

Above condenser:



Below Condenser:

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Re: Tests with polarizing filters

#13 Post by fdupre » Wed Feb 16, 2022 9:34 am

Hello Sansub2,

Thank you very much for your feedback and your very interesting tips. I'm going to try, I just have to find how I'm going to position the CPL above the condensor that I would have removed if I understood you correctly. Maybe you could post a pic of the setup?
Your videos are really great!
No, I don't have a 3D printer
FYI, my light source is a 3W led.

Thanks again for your tips!

Sincerely,
Frank
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Re: Tests with polarizing filters

#14 Post by Sansub2 » Wed Feb 16, 2022 1:07 pm

Frank, you are welcome.

Here are the picture of my DIY Polarizer. 3 seperate parts were printer and glued together with polarized film in the middle.

ImageImage

Here is the video of how it is placed and used. ( Sorry for the shaking 😃. ).

The last 3 samples above were taken using my DIY cheap LED conversation. With your superior camera and lighting, it will come even better.


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Re: Tests with polarizing filters

#15 Post by Greg Howald » Wed Feb 16, 2022 1:37 pm

It can be very helpful in determining molecular or crystal structure to be able to determine the degree of polarization. Some specimens will give a better analysis at degrees of polarization other than 90 degrees . Also, you may have a better result when using three polarizer rather than two. One way to do that is by placing a polarizer in the head and one in the filter holder at extinction, then adjusting the degree of polarization by rotating a third polarizer placed on top of the illuminator.
Greg

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Re: Tests with polarizing filters

#16 Post by Sansub2 » Wed Feb 16, 2022 1:43 pm

Greg, Thanks for the suggestion. I can give it a try on the 3 polarizers and post results. Does it have to be Linear or CPL or can it be mixed? Reversing the CPL will make it linear I assume.

BTW, as for sample, what would be the best household item to test?

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Re: Tests with polarizing filters

#17 Post by Sansub2 » Wed Feb 16, 2022 2:35 pm

Pink Himalayan Salt - supposed to have multiple trace elements in it.



BTW, adding 3rd polarizer didn't do much in my setup.

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Re: Tests with polarizing filters

#18 Post by fdupre » Wed Feb 16, 2022 4:23 pm

well, here are some very very interesting feedback.
Thank you Sansub2 for your very explicit video !
I will study this with the greatest interest.
Greg, I will try your trick = thank you :)

Sincerely,
Frank
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Re: Tests with polarizing filters

#19 Post by marcel_ » Wed Feb 16, 2022 6:09 pm

The way I achieved the maximum polarization is a) by removing the condenser for lower objectives b) by placing the bottom CPL on top of the condenser.
Why not just underneath the slide? That would be practically the same place. Or am I missing something here?

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Re: Tests with polarizing filters

#20 Post by Sansub2 » Wed Feb 16, 2022 8:23 pm

Marcel,

Good thinking. That was my very first try as well. See below picture for one of the various, on the stage options I tried ( shown upside down).

Issues were:


1. When I rotate the polarizer handle, it has high chance of disturbing the slide above.
2. When I move the stage, it disturbing the polarizer handle.

I was spending more time in adjusting. Moving it under the stage removed these problems.

Image

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Re: Tests with polarizing filters

#21 Post by dtsh » Thu Feb 17, 2022 12:16 am

When I designed a polarizer setup for my scopes, I put one polarizer under the head, laid one on the field lens of the illuminator, and the 1/4 wave plate clips onto the bottom of the condenser. Not that my way is the best, but might give some ideas.

https://www.thingiverse.com/thing:5167470

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Re: Tests with polarizing filters

#22 Post by Sansub2 » Thu Feb 17, 2022 1:30 pm

Dtah, that looks like an interesting bdeaign. Even though I don't have the microscope listed in that link, I am going to try to see whether I can adopt it to mine. Thanks for sharing :)

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Re: Tests with polarizing filters

#23 Post by fdupre » Sat Feb 19, 2022 11:01 am

Good morning all,

Here are tests with a grain of rice under different lighting (bright background - polarized complete extinction - dark background with patch):

light background:
0001_nopolarisation_stackMH.jpg
0001_nopolarisation_stackMH.jpg (69.08 KiB) Viewed 6123 times
polarized complete extinction:
0002_polarisation_stackMH.jpg
0002_polarisation_stackMH.jpg (74.7 KiB) Viewed 6123 times
black background with central circle black background patch 15 mm in diameter:
0003_patchrond15mm_stackMH.jpg
0003_patchrond15mm_stackMH.jpg (70.5 KiB) Viewed 6123 times
Sincerely,
Franck
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Re: Tests with polarizing filters

#24 Post by Chas » Sat Feb 19, 2022 1:59 pm

Placing some clear sticky-tape ( selloptape) on a glass slide, between the two polarisers, for example, over the bottom polariser can give a bit of extra colour /range of colours.
[The 'diffuse' surfaced sticky tape doesn't work].
viewtopic.php?p=116833#p116833

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Re: Tests with polarizing filters

#25 Post by fdupre » Sun Feb 20, 2022 4:55 pm

Hello Chas,
Very interesting as a tip.
Sincerely,
Frank
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Re: Tests with polarizing filters

#26 Post by Sansub2 » Mon Feb 21, 2022 4:21 pm

Looks nice Frank. I like the deep black background in the dark field.

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Re: Tests with polarizing filters

#27 Post by fdupre » Mon Feb 21, 2022 4:28 pm

Hello Sansub2,
Thank you :)
Franck
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