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

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seta666
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Very bad image quality when using polarizers on a microscope, why?

Post by seta666 »

Hi,

I just realized 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 (first from 3D cinema glasses now proper 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 (Edit: solves) 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% crops, with and without the filter; no filter on the source light. Nikon CF Fluor 10/0.50, subject is a piece of paper

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 is with analyzer 2cm away from camera sensor, second one 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

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, compared to those 500$ a good analyser costs; 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
Last edited by seta666 on Fri Apr 02, 2021 1:14 am, edited 1 time in total.

Ichthyophthirius
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Joined: Thu Mar 07, 2013 5:24 am

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

Post by Ichthyophthirius »

Hi,

Ideally, use a non-anisotropic (i.e. isotropic) object for a resolution and contrast test, like a stage micrometer. If you use crystals, there are additional pol-optical effects that make the images difficult to analyse.

The analyser is critical for pol microscopy. A poor quality or damaged filter or an incorrect placement can introduce ghosting, light scattering, loss of resolution etc.

On biological microscopes, the analyser is usually placed a few cm above the objective, ideally in an infinity space (not always), the filter is also usually very thin and sandwiched between AR-coated plan glass windows. The analyser can be angled by 5 degrees or so if it is placed in an infinity space; it should be horizontal if it's not in an infinity space. The list price for these analysers start at around $500 so they are expensive bits of equipment.

Using other filters for this purpose is always going to be a compromise. Can you say more about your filter and the placement? Is there a slot in your intermediate tube for an analyser? Are you sure the filter isn't damaged?

Regards, Ichty

seta666
Posts: 1071
Joined: Fri Mar 19, 2010 8:50 am
Location: Castellon, Spain

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

Post by seta666 »

First I was using the polarizer from 3d cinema glasses (just under the trinocular dovetail, which maybe too close to the objectives)and yesterday received some filters I bought (10$ each)
I am happy with the quality I am getting with the new placement, if you see the first two pictures you can see the difference before and after and in the last one with and without. Those are 100% 400px crops with no sharpenning

The only bad thing ( quite bad) is I only see the effects on the camera, not on the eyepieces but the camera part is more important to me.

I did buy two 34mm filters (I am using one bellow the camera) and two 27mm, those 27mm filters almost fit inside the outer part of the eyepieces, could be a solution. The polarizer intermediate piece for BH2 is not cheap

EDIT: I found a provisional solution, the 27mm filter fit perfect in the olympus eyepieces, I will 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 it sometimes I do not.

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

Post by zzffnn »

Analyzer not being flat or not being big enough will definitely degrade image quality.

So with 27mm filters in/near eyepiece, you don't have significant image quality degradation anymore?

seta666
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Joined: Fri Mar 19, 2010 8:50 am
Location: Castellon, Spain

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

Post by seta666 »

zzffnn wrote:
Mon Apr 05, 2021 8:34 pm
Analyzer not being flat or not being big enough will definitely degrade image quality.
So with 27mm filters in/near eyepiece, you don't have significant image quality degradation anymore?
For photography I do not use an eyepiece (normally), works via direct projection. The analyzer close to the camera performs way better than under the trinocular head.

100% crops from first post but just to clarify, on the left analyzer is 2cm away from camera sensor, on the right under the trinocular head.
Performance close to the sensor is way better
Image

And this one on the left no analyzer on the right analyzer in place close to sensor (no polarizer on source light), image without anything on the mechanical tube is better but they are pretty close

Image

The only con is placing the analy¡zer close to the camera makes me use analysers also on the eyepieces if I want to see the effects of polarization when viewing; I suse the 27mm filters on the Olympus WHK 10/20 eyepieces. This is a provisional solution but it does work.

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