Epi / coax illumination, beamsplitter, polarizer, etc

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bokemon
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Epi / coax illumination, beamsplitter, polarizer, etc

Post by bokemon »

Hello folks,

Now I'm setting up an epi illumination system that goes: sample -> window -> Mitutoyo 10x -> plate beamsplitter -> polarizer -> tube lens ->camera. The samples I am looking at are flakes of material (single or few atomic layers) on wafers, so it's reflective and the contrast shows up as different colors due to thickness (interference). So far, all I tested out was the coax module from a Qioptiq unit, which is just a plate beamsplitter and linear polarizer (analyzer) for the light that goes back to the camera. The guy who originally got it forgot to also buy the polarizer for the light source, but it worked fine for my purpose, so [shrug]. However, I need to replace this with my own unit due to vignette reasons, and so I have a few questions on the setup:

1) In the unit I'm replacing, normally you'd use two linear polarizers to block out stray light from non-specular reflections in the beamsplitter assembly. Is there any other reason? What if I use 1 or zero polarizers?

2) Is there any reason to use circular polarizers or 1/4 wave plate? Normally this would be used to block out glare from the window, but since the samples are also reflective, maybe it won't help? Or if it does help, where does the 1/4 wave plate go? (Can't put it in front of the objective since it will cause aberrations)

3) Are photography circular polarizers good enough optical quality? Here I'm referring to about 10 um resolution at the camera sensor. Something from B&W might cost around $70 and it already has mounting threads and the ability to rotate, vs $200 for a good quality bare one from Edmunds.

4) For the light source beam dump, is it good enough to just have a black matte or fuzzy surface, or do I need to resort to some kind of baffled assembly?

5) I might not have the option for AR coating, so what can I do to minimize reflections from the window ruining the contrast?

For now, I'm going to piece together something from Thorlabs and Edmunds parts, but perhaps somebody with more direct experience can warn me of any gotchas?

Chris S.
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Re: Epi / coax illumination, beamsplitter, polarizer, etc

Post by Chris S. »

bokemon wrote:
Wed Oct 05, 2022 10:27 am
3) Are photography circular polarizers good enough optical quality? Here I'm referring to about 10 um resolution at the camera sensor. Something from B&W might cost around $70 and it already has mounting threads and the ability to rotate, vs $200 for a good quality bare one from Edmunds.
When I first set up cross-polarization, I tried using the polarizing filters I already had for general photography (Tiffen and B&W). I don't think these filters hurt resolution (though I didn't pay much attention to this, given other deal breakers for me). What I dislike was these filters' level of extinction, and their poor color neutrality. So I went with a unmounted polarizing filter from Edmund Optics behind the lens, and Edmund Optics polarizing sheets for the lights. These have worked very well for my purposes (very high extinction and almost zero effect on color). I also have an Olympus analyzer whose output is nearly indistinguishable from the Edmund Optics. The EO analyzer is mounted to fit behind Mitutoyo objectives; the Olympus analyzer is mounted to fit behind finite Nikon objectives.

But your use case is different--for example, you may not need very high extinction. Neutral color, on the other hand, may be important to you. Also, general photography polarizers vary a lot, even between low, medium, and high-quality product lines from a given maker; so the experiments I did on my polarizing filters may have limited extensibility. Edmund optics does provide extinction and color neutrality specifications, so I tend to think of their filters as a known--or at least a knowable--quantity. (Though Edmund has changed the product names since I bought mine, which adds confusion.)

For the setups we build, I think of the unmounted aspect of these filters as a feature, not a liability. Both Edmund Optics and Thorlabs have components for integrating unmounted glass filters. Some of these components permit rotation of the filter. Many of these components are are designed for easy integration within the company's lines of optical assembly products. The better of these components seem much more robust than the mounting rings of photographic polarizers.

--Chris S.

blekenbleu
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Re: Epi / coax illumination, beamsplitter, polarizer, etc

Post by blekenbleu »

bokemon wrote:
Wed Oct 05, 2022 10:27 am
normally you'd use two linear polarizers to block out stray light from non-specular reflections in the beamsplitter assembly. Is there any other reason?
Depending on how EPI illumination is accomplished, a polarizer that can be rotated between lamp and subject can reduce specular reflections from subjects,
then a rotating analyzer between beamsplitter and camera manages beamsplitter stray light.

Unmounted laminated polarizers (one plopped in an illuminator filter slot) and lambda plates from techno2020 on eBay have been satisfactory for me.
Metaphot, Optiphot 1, 66; AO 10, 120, and EPIStar 2571
https://blekenbleu.github.io/microscope

bokemon
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Re: Epi / coax illumination, beamsplitter, polarizer, etc

Post by bokemon »

Can I put the polarizer between the tube lens and camera sensor? Yes it's a converging beam, but I think the F number should be something like the mitutoyo exit pupil (11mm) divided by the tube lens focal length (200 or 125mm). If so, then spherical aberration shouldn't be a problem. Anyway, I'm just asking in case the Raynox performs much better closer to the Mitu than far away, even though vignetting won't be a problem in either case.

Also, what is a lambda plate supposed to do for you?

blekenbleu
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Re: Epi / coax illumination, beamsplitter, polarizer, etc

Post by blekenbleu »

bokemon wrote:
Fri Oct 07, 2022 7:25 am
Can I put the polarizer between the tube lens and camera sensor?
Yes, that is equivalent to what is done with finite microscopes.
In theory, minor spherical aberrations might be mitigated by fiddling tube length and refocusing...
Also, what is a lambda plate supposed to do for you?
Once polarizers are involved, introducing retardation can increase perceived color contrasts
and/or evaluate birefringence.
Metaphot, Optiphot 1, 66; AO 10, 120, and EPIStar 2571
https://blekenbleu.github.io/microscope

bokemon
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Re: Epi / coax illumination, beamsplitter, polarizer, etc

Post by bokemon »

I think I will go for the 2" polarizer and M42-sized rotation mount from Edmund. However, they have both the "High Performance" and "High Contrast XP42" polarizers, and I'm not sure what is the real difference between them. The XP42 seems to have a more uniform wavelength response, but what I really care about is a spec that they don't list - Total Wavelength Distortion, which I assume is a spec for not degrading the resolution.

As long as we're talking about polarizers and birefringence, I also wanted to ask about windows. My window choices are c-cut sapphire or fused silica. Sapphire is harder and therefore can be made thinner for less spherical ab. But will the birefringence cause any issues? For C-cut orientation, it is not birefringent, but I think that is only true for light rays that go straight thru? But when imaging an object at close distance, light rays from one point of the sample will go thru the window at all angles, only to reconverge back at the camera sensor. Do these angled rays experience bi-refringence? And will that mess up the image resolution?
Will polarizing the incoming or outgoing light automatically "fix" the off-axis birefringence issue?

blekenbleu
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Re: window

Post by blekenbleu »

Not knowing your application or why any window, much less sapphire or silica, is wanted,
consider that plenty of objectives are designed to minimize aberrations with glass coverslips.
Metaphot, Optiphot 1, 66; AO 10, 120, and EPIStar 2571
https://blekenbleu.github.io/microscope

J_Rogers
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Re: Epi / coax illumination, beamsplitter, polarizer, etc

Post by J_Rogers »

bokemon wrote:
Sat Oct 08, 2022 3:32 pm
but what I really care about is a spec that they don't list - Total Wavelength Distortion, which I assume is a spec for not degrading the resolution.

Wavefront error (sometimes called distortion) is the deviation of a specific wavelength of light - using one mode, transmitted or reflected - over a particular area, using either PV or RMS values of flatness - given in units of waves or fringes.

For example, transmitted wavefront P-V: lambda/10 @ 633nm is the wavefront error measured with a 633nm interferometer using peak-to-valley values.

If you meant wavelength distortion, i.e., a thin-film polarizer, will pass any un-polarized light at a slightly longer wavelength than the initial wavelength, then this definition will not have any associated error value since you either remedy this, or you don't. Adding a bandpass filter after the polarizer and selecting a polarizer with a higher extinction ratio will solve this. Higher extinction ratios will give less contrast, so you must implement what blekenbleu mentioned for better results.

bokemon
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Re: Epi / coax illumination, beamsplitter, polarizer, etc

Post by bokemon »

It's a typo, I meant waveFRONT

bokemon
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Re: window

Post by bokemon »

blekenbleu wrote:
Sun Oct 09, 2022 1:15 pm
Not knowing your application or why any window, much less sapphire or silica, is wanted,
samples under vacuum

blekenbleu
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Re: polarized coaxial illumination

Post by blekenbleu »

bokemon wrote:
Sun Oct 09, 2022 9:50 pm
vacuum
If EPI illumination is coaxial, is sapphire or silica requirement is for strength or transparency at illumination wavelength?
I ask mostly out of curiosity; whatever coaxial illumination you want polarized is almost certainly beyond my experience.
Metaphot, Optiphot 1, 66; AO 10, 120, and EPIStar 2571
https://blekenbleu.github.io/microscope

wowjl
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Re: Epi / coax illumination, beamsplitter, polarizer, etc

Post by wowjl »

Chris S. wrote:
Wed Oct 05, 2022 9:38 pm

These have worked very well for my purposes (very high extinction and almost zero effect on color). I also have an Olympus analyzer whose output is nearly indistinguishable from the Edmund Optics. The EO analyzer is mounted to fit behind Mitutoyo objectives; the Olympus analyzer is mounted to fit behind finite Nikon objectives.
Can I easily compare "extinction" without using an instrument?

With my presbyopia, I see with the naked eye.
With two polarizers rotated, there is almost no difference in the light blocking effect between a camera polarizer and a professional polarizer (claiming an extinction ratio of 1:1000).

Best regards
wowjl

Chris S.
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Re: Epi / coax illumination, beamsplitter, polarizer, etc

Post by Chris S. »

Wowjl,

To make sure we are talking about the same thing, here are two scenarios:
1) You hold two polarizers side by side, such that you can see through each of them separately but simultaneously. You rotate these polarizers in parallel, and don’t notice any difference. If this is what you’re doing, I’d be skeptical that it is a good test for your purposes.

2) You hold two polarizers, one in front of the other, such that you are looking through a stack of two. You rotate these polarizers against one another until no light passes through. You take a different pair of polarizers and repeat. Then you hold both pairs of polarizers up to a bright light, and both pairs appear equally opaque. If this is what you’re doing, I’d be inclined to trust it as an acceptable test of extinction for most purposes.
If yours is scenario number two--and you are testing a pair of camera polarizers and a pair of “professional” polarizers--I’d say your camera polarizers have very good extinction. (This is possible--there is a wide range of extinction among camera polarizers. I recall seeing certain camera polarizing filters advertised as having less light loss; if this claim was true, extinction couldn’t have been very complete.)

For color neutrality, on the other hand, I would not trust my eye, as it’s connected to a human brain, which is evolved to negate differences in color temperature of light. For color neutrality, I’d test with a camera.

Best,

--Chris S.

bokemon
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Re: polarized coaxial illumination

Post by bokemon »

blekenbleu wrote:
Mon Oct 10, 2022 5:42 am

If EPI illumination is coaxial, is sapphire or silica requirement is for strength or transparency at illumination wavelength?
It's just that those are the two most common materials that you can get in terms of vacuum chamber viewports with a metal-glass seal.

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