Coaxial Kohler Conundrum

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bokemon
Posts: 20
Joined: Sun Jan 16, 2022 6:06 pm
Location: Silicon Valley, CA

Coaxial Kohler Conundrum

Post by bokemon »

Hello folks,

Now I'm working on the illumination part of my imaging setup. It consists of Mitutoyo 10x NA 0.28 objective, an extension tube after it (since the objective is inserted into a tube with a window at the end), beamsplitter, tube lens, etc. The initial illumination tests with a cheapo LED light source did not work so well and gave excessive vignetting, even though I checked that there was no mechanical vignetting. Thus I decided to try out Kohler illumination. Schematically my setup is similar to the diagram on this web page:

https://www.olympus-lifescience.com/en/ ... ectkohler/
Image

...except that I will use a LED light instead of filament. Short summary is that the light source plus 3 lenses and two irises eventually create a light source image at the back focal plane of the objective. I'm still figuring out all the details and requirements, but have a few questions:

BTW, I'm using this paper as a starting point since they're rather explicit about having a parts list and setup guide:
https://arxiv.org/abs/1902.10521

1) Regarding the NA of the condenser lens limiting the effective NA of the objective, I came upon this problem: Because of the extension tube and beamsplitter box, I physically can't place the last / condenser lens closer than about 60mm from the objective's back focal plane. And the beamsplitter box has Thorlabs SM1 holes in the sides (about 1" diameter). I think that means the best NA I can get is sin(arctan(lens radius/focal length)) where radius approx 12mm, focal length approx 60-75, so NA = 0.2. But my objective is NA 0.28. Any way for me to get a larger NA?

2) Unclear to me whether the light source should be diffused first? Most Kohler articles don't mention it, yet the arxiv paper does. If the LED is almost a point source, and un-diffused, then I'm not sure what an aperture diaphragm is supposed to do since the "image" it tries to close off is really small.

3) Is there a shortcut to "cheap out" and only have 1 or 2 lenses, as long as I still try to make an light source image at the objective's back focal plane? And maybe only 1 or 2 irises can also be inserted at clever locations which can save me from having to purposely make collimated and then convergent light?

I should mention that the samples are mostly specular reflective.

patta
Posts: 86
Joined: Mon Apr 20, 2020 9:51 am
Location: Stavanger Norge
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Re: Coaxial Kohler Conundrum

Post by patta »

Try to draft some answer

1) the NA 0,28 refers to the object-side; instead the beamsplitter is on the image-side; on image-side the NA is smaller, divided by magnification:
NAimage = NAobject/mag =0,28/10=0,028 so you shouldn't have issues. Just make sure that the beamsplitter diameter is same or larger than the Mitutoyo back lens.

2) Diffusing the light helps but not mandatory; if you're using a single-die LED, then it is already a very nice uniform surface that don't need diffusion - unlike old bulbs with a weird incandescent filament. The main purpose is to avoid seeing the filament projected on the specimen; either you diffuse it, or you defocus it. Khoelher goes for defocusing. There are some further issues that would need several books to discuss.

3) The setup with 3 optical groups (one collector and two "illumination objectives") is the most complete and flexible. It allows to place conveniently one "field iris" and one "aperture iris".
Note that the "back focal plane", where the aperture stop iris should be, is usually not well defined in microscope objectives, so you can put it ballparked. Microscope objectives usually do not have an aperture stop (!!!!).
It would be nice to be able to put filters and masks at the "aperture stop" position, so you can play with lateral illumination, colors etc.
With the single-die LED you can skip one optical group and put an iris/stop straight in face of the LED; this is the setup of many industrial inspection illuminators.
I've done collimated illumination using three old 50mm f/2 objectives with M42 screw as "groups", it works great and not too expensive.
The simplest setup may be a single 50mm f/2 doing the job of collector and its iris working as aperture stop.

4) one useful trial you can do is to "illuminate from the specimen": put a LED light under the mitutoyo, where the specimen should be; and look where the light beam goes and how large it is.

5) one expensive alternative is to put your hands on a "metallographic microscope" that have the coaxial illuminator ready, or give a couple salaries to ThorLabs.
Last edited by patta on Wed Nov 02, 2022 9:00 am, edited 1 time in total.

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