Very bad image quality when using polarizers on a microscope, why?

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Seta
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Very bad image quality when using polarizers on a microscope, why?

#1 Post by Seta » Fri Apr 02, 2021 5:36 am

Hi,
I
started a thread
at Photomacrography.net but I will do it also here, as it is more microscopy oriented
I realised I am getting very bad image quality when using polarizers, I have one on top of the source light and another one (Analyzer) between the microscope body and trinocular head (firstcut from 3D cinema glasses but I just receivednew filters ), the effect is not so evident at high magnification I guess due to higher effective f number but at low magnification with high NA lenses quality is pretty bad, fuzzy sometimes like double image. As you move focus the out of focus areas become very weird.

I was leaving the analyzer inside all the time as I thought without the polarizer on the source light was not doing anything; I was clearly wrong.

What is causing this? the filter being too far away from the camera? the prisms in the trinocular head? Is it the type of filter? Placing the filter close to the camera seems to solve the problem but I do not know if may cause some loss of contrast maybe from sensor reflections..

This is why some low magnification stacks were looking kind of weird, closing condenser iris fixed it somehow..

100% 400x400px central crops, with (right) and without the filter (left); no filter on the source light. Nikon CF Fluor 10/0.50, subject is a piece of printed paper with no cover glass, still shows the difference.

Image

Ok, it is clear to me that second polarizer (analyser?)placement has a big effect on the image quality; this time both polarizers in use.
Again 100% crops with Nikon CF Fluor 10/0.50; the first picture (left) is with analyzer 2cm away from camera sensor, second one (right) with it just bellow trinocular head. I feel so stupid... :oops:

Image

One more with (right) and without the analyser (left), apart from 1EV light loss it does not seem to affect resolution that much

Image


The only bad thing ( quite bad) is now I only see the effects on the camera, not on the eyepieces but the camera part is more important to me. I found a provisional solution though, did buy two 34mm filters (I am using one bellow the camera) and two 27mm; the 27mm filters fit perfect in the olympus eyepieces, I can use those when using polarizers, just have to rotate them to have same effect as in the camera

Still not sure if the issue is solve, maybe filter not being 100% flat is causing the problem, sometimes I see that weird effect sometimes I do not.

And a couple more tests, with and without filter (analyzer) close to the camera with fluor 10 and CFN 40 PlanAchro; condenser focused and iris wide open
There is some quality loss but this is a 10$ filter, I have been told a good analyser is 500$; I can live with it anyway. Weird enough with the CFN 40 seems to be a little bit less purple fringing with the filter on

Image

Image

Greg Howald
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Re: Very bad image quality when using polarizers on a microscope, why?

#2 Post by Greg Howald » Fri Apr 02, 2021 11:09 am

Hi Seta,
Good photos. There are two types of polarizing filters. They can be linear or circular. There are different qualities of polarizer and most sellers don't mention quality or provide much in the area of specification. There's a reason why an analyzer can cost $ 500.00.
But unless you are doing some really dramatic research you need not purchase one.
I have different kinds of polarizing materials and there is a difference in their performance. One pair doesn't seem to care how far apart they are and the other cares a Great deal in that I have to put the polarizer on the stage with the specimen on top of it and place the analyzer directly on top of the specimen.
I suggest you get the two filters as close to the specimen as possible and check results.
Also, some polarizing filters work in only a specific orientation and you have to play with them by holding them stacked on top of one another while holding them up to a light source to find extinction. In this, one of the filters may have to be turned over top to bottom for best results.

Suggest you try this things. Good luck.
Greg

Seta
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Re: Very bad image quality when using polarizers on a microscope, why?

#3 Post by Seta » Fri Apr 02, 2021 12:58 pm

Yes, these are linear polarizers..what I do not really understand is how their placement within the mechanical tube affects performance and why. I know a cheap filter is not going to work as well as a 500$ one but as long as it works wirh reasonable quality I am ok with it.

Greg Howald
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Re: Very bad image quality when using polarizers on a microscope, why?

#4 Post by Greg Howald » Fri Apr 02, 2021 2:36 pm

Don't worry. Some here on the forum will explain that to you with much more science than I can absorb. 😃😃
Greg

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Re: Very bad image quality when using polarizers on a microscope, why?

#5 Post by BramHuntingNematodes » Fri Apr 02, 2021 2:54 pm

Linear polarizers play havoc with some digital camera sensors in some situations. I would also not underestimate the effect of internal reflections. Microscope manufacturers have put an enormous amount of thought and effort into mitigating these over the last century to have some amateur just stick a flat bit of plate glass in the optical path wherever.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

Scarodactyl
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Re: Very bad image quality when using polarizers on a microscope, why?

#6 Post by Scarodactyl » Fri Apr 02, 2021 3:47 pm

I don't think they put much effort into mitigating that issue beyond putting a quarter wave plate above the analyzer to depolarize the light before it interacts with any flat surfaces. It might be worth trying with an appropriately oriented circular polarizer for your analyzer. I don't think this is how those polarization interactions usually manifest though(?), usually I think of that as affecting the polarization effects rather than blurring the image.

Conventionally, right below the head should be OK.

The perceived difference in fringing looks to be just a slightly different focal point. Nikon fluors sometimes have significant green/purple axial aberrations which will change dramatically with a slight focus adjustment.

Seta
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Re: Very bad image quality when using polarizers on a microscope, why?

#7 Post by Seta » Fri Apr 02, 2021 3:47 pm

BramHuntingNematodes wrote:
Fri Apr 02, 2021 2:54 pm
Linear polarizers play havoc with some digital camera sensors in some situations. I would also not underestimate the effect of internal reflections. Microscope manufacturers have put an enormous amount of thought and effort into mitigating these over the last century to have some amateur just stick a flat bit of plate glass in the optical path wherever.
I do not think the sensor is responsible because the filter performs better close to the sensor than under the trinocular head.
My photo tube has a tube of black flocking material to avoid reflections, same I use in all extension tubes and on the back of the lenses.

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Re: Very bad image quality when using polarizers on a microscope, why?

#8 Post by BramHuntingNematodes » Fri Apr 02, 2021 3:54 pm

Black flocking will not affect rays normal to the filter surface.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

Seta
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Re: Very bad image quality when using polarizers on a microscope, why?

#9 Post by Seta » Fri Apr 02, 2021 3:56 pm

Scarodactyl wrote:
Fri Apr 02, 2021 3:47 pm
I don't think they put much effort into mitigating that issue beyond putting a quarter wave plate above the analyzer to depolarize the light before it interacts with any flat surfaces.......
A CPL is just a linea PL with a quater wave plate, right. I could try a CPL and see.
The focus point is the same but performance under some circumstances is so poor it looks out of focus
BramHuntingNematodes wrote:
Fri Apr 02, 2021 3:54 pm
Black flocking will not affect rays normal to the filter surface.
You mean reflections on the filter itself? then it should work worst close to the sensor I guess


Any way, the place it is now performs OK and is easy to remove when not needed.

This is aceta minophen (came with the filters) with Nikon CFN 40/0.70 direct projection on a BHT, stack of 5 images or so

Image

Full size
Last edited by Seta on Fri Apr 02, 2021 4:45 pm, edited 1 time in total.

BramHuntingNematodes
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Re: Very bad image quality when using polarizers on a microscope, why?

#10 Post by BramHuntingNematodes » Fri Apr 02, 2021 4:42 pm

Seta wrote:
Fri Apr 02, 2021 3:56 pm
You mean reflections on the filter itself? then it should works worst close to the sensor I guess

I am not sure that this necessarily follows or is trivial to demonstrate. If a disturbance creates reflections at an off angle, for instance, the apparent distortion will be a positive function of the distance between the disturbance and sensor. I am glad you have found a solution. Trial and error is often useful!
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

EYE C U
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Re: Very bad image quality when using polarizers on a microscope, why?

#11 Post by EYE C U » Sun Apr 04, 2021 8:31 am

look at the filter with your microscope and see why.

Seta
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Re: Very bad image quality when using polarizers on a microscope, why?

#12 Post by Seta » Sun Apr 04, 2021 1:08 pm

Bad performance on my case depends on placement more than the filter used, I can not see anything unusual on them under the scope. What am I supposed to see?

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woyjwjl
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Re: Very bad image quality when using polarizers on a microscope, why?

#13 Post by woyjwjl » Wed Mar 30, 2022 8:23 am

BramHuntingNematodes wrote:
Fri Apr 02, 2021 3:54 pm
Black flocking will not affect rays normal to the filter surface.
DIC polarizer is placed obliquely....
Micrographers from China, thanks to the forum for providing a platform for exchange

Chas
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Re: Very bad image quality when using polarizers on a microscope, why?

#14 Post by Chas » Wed Mar 30, 2022 6:38 pm

"Ok, it is clear to me that second polarizer (analyser?)placement has a big effect on the image quality"..."Placing the filter close to the camera seems to solve the problem".
-Thanks for pointing out this possibilty; I have been puzzled sometimes by poor photographs and it might have been simply where I put the analyser filter :-)

mineral rob
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Re: Very bad image quality when using polarizers on a microscope, why?

#15 Post by mineral rob » Sat Apr 23, 2022 6:46 pm

I would also note to make sure polarizers are as flat in their mount as possible if using a sheet film and if they are not already in a glass filter mount. Slightly tilting the whole filter can help with reflections.

I check polarizers (linear or CPL) outside the scope to make sure the pair I use gives great extinction by shining a flashlight through them and rotating the pair to extinction (and flipping a CPL over if I forget which way to place it.) Since I like to look at rocks in thin section I also note the orientation on each polarizer to place them N-S, or E-W in the microscope along with top/bottom orientation.

If using a CPL as an analyzer place the 1/4 wave side towards the camera sensor so you can get extinction. The 1/4 wave (depolarizer) was added to linear polarizers to a make them circular (CPL) for camera metering systems to work properly even before digital cameras. They tend to be cheap on the used market and come in many sizes. I often add polarization by putting a good flat linear analyzer just under the head and a cheap camera CPL over the field lens below the condenser.

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