Poor man’s "Kohler" illumination

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ZodiacPhoto
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Poor man’s "Kohler" illumination

#1 Post by ZodiacPhoto » Sat Mar 25, 2023 10:01 pm

Some basic microscopes, like Olympus CX23 I am playing with now, are not equipped with Kohler illumination, thus not being able to achieve better contrast and clarity Kohler illumination provides.

One of my previous microscopes, Olympus CH2, also didn’t have Kohler illumination, and I made the same attachment to add simplified version of Kohler to CH2 then. I decided to share this idea, maybe somebody will use it to upgrade their microscopes too.

Pre-requirement: your microscope needs to have a focusable condenser – able to move vertically with a knob, like both my Olympus microscopes had.

To implement the simplified Kohler illumination, we need to accomplish three steps:
- Limit the projected diameter of the light source using the field diaphragm, so it is just a bit larger than the field of view for your selected objective.
- Focus the condenser so the edge of the field diaphragm opening looks sharp.
- Center the light spot to the field of view.

Normally, this is accomplished by using an iris field diaphragm and centering screws in the condenser holder. Our simple microscopes do not have this option (actually, Olympus makes the kit to add Kohler to the scopes that don’t come with it from the factory, but it is hard to find and pricey).

My solution consists of three 3d-printed parts – Base, Arm, and Sector. By rotating the Arm and the Sector, you can easily center the light spot in your view. To find the diaphragm’s diameters for my objectives, I placed a brightly lit ruler in the same plane where the diaphragm would be, focused the condenser, and noted how many ruler’s millimeters the field of view diameter includes. For the 4X objective, I just move the Sector away to open the light source fully.
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The Sector part is shown transparent to see the parts under it, but, of course, everything is printed from black PLA:





Hope this helps somebody!
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Last edited by ZodiacPhoto on Fri Mar 31, 2023 11:05 am, edited 1 time in total.

Phill Brown
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Re: Poor man’s Kohler illumination

#2 Post by Phill Brown » Sun Mar 26, 2023 9:10 pm

It must be said that's a good effort.
Poor and 3D printer is all relative.
I don't know enough to say if it's effectively a filter patch in that position but I'm sure it offers lighting effects that could be interesting.
Maybe some with and without images would be good if possible?

ZodiacPhoto
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Re: Poor man’s Kohler illumination

#3 Post by ZodiacPhoto » Sun Mar 26, 2023 11:04 pm

Phill Brown wrote:
Sun Mar 26, 2023 9:10 pm
It must be said that's a good effort.
Poor and 3D printer is all relative.
I don't know enough to say if it's effectively a filter patch in that position but I'm sure it offers lighting effects that could be interesting.
Maybe some with and without images would be good if possible?
A very good 3d printer, Creality Ender 3, is $190: https://www.amazon.com/Official-Crealit ... AA3MFMEI5L. I was using this printer for a few years. Considering how many different things you can design and make, it would pay for itself very quickly. And print quality is virtually the same as from a 5x more expensive Prusa i3 mk3 that I use currently.

I determined that the diaphragm must be at least 10mm above the frosted glass surface - if it is closer, you will see the frosted glass pattern overlaying your view when you try to focus the condenser on the diaphragm opening. I found this contraption just as effective in improving contrast as proper Kohler alignment in my Olympus BX40 scope.

PeteM
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Re: Poor man’s Kohler illumination

#4 Post by PeteM » Sun Mar 26, 2023 11:40 pm

Another approach is to use an iris; either scavenged from an old camera or a new one (under $20) like this:

https://www.ebay.com/itm/Microscope-Iri ... 5929871591

Sure Squintsalot
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Re: Poor man’s Kohler illumination

#5 Post by Sure Squintsalot » Mon Mar 27, 2023 2:54 am

For a complete, integrated, and finished solution, this is pretty damn sexy! Well done!

Did you calculate the various apertures or come to them through trial and error?

BramHuntingNematodes
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Re: Poor man’s Kohler illumination

#6 Post by BramHuntingNematodes » Mon Mar 27, 2023 3:43 am

Kohler himself used cardboard with holes cut in them. It seems also like you could focus on the frosted glass and then simply move the condenser up about 10mm or so.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

ZodiacPhoto
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Re: Poor man’s Kohler illumination

#7 Post by ZodiacPhoto » Mon Mar 27, 2023 11:20 am

Sure Squintsalot wrote:
Mon Mar 27, 2023 2:54 am
For a complete, integrated, and finished solution, this is pretty damn sexy! Well done!

Did you calculate the various apertures or come to them through trial and error?
Thanks! As I stated above, to find the diaphragm’s diameters for my objectives, I placed a brightly lit ruler in the same plane where the diaphragm would be, focused the condenser on the ruler, and noted how many ruler’s millimeters are visible through each objective. For the 4X objective, I just move the Sector away to open the light source fully.

apochronaut
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Re: Poor man’s Kohler illumination

#8 Post by apochronaut » Mon Mar 27, 2023 10:13 pm

I'm not sure that I am following you directly but Köhler illumination is simply a method of defocusing the illumination source at the field diaphragm plane. It has nothing to do with centering anything or providing limiting apertures anywhere in the system. Those are entirely separate controls one can exert on the illumination beam. You don't actually require a field diaphragm to adjust for köhler illumination, just any physical device to provide a reference that you have focused the condenser at the appropriate plane.
Can you explain further?

ZodiacPhoto
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Re: Poor man’s Kohler illumination

#9 Post by ZodiacPhoto » Tue Mar 28, 2023 1:03 am

apochronaut wrote:
Mon Mar 27, 2023 10:13 pm
I'm not sure that I am following you directly but Köhler illumination is simply a method of defocusing the illumination source at the field diaphragm plane. It has nothing to do with centering anything or providing limiting apertures anywhere in the system. Those are entirely separate controls one can exert on the illumination beam. You don't actually require a field diaphragm to adjust for köhler illumination, just any physical device to provide a reference that you have focused the condenser at the appropriate plane.
Can you explain further?
I was talking about this procedure recommended by Olympus: https://www.olympus-lifescience.com/en/ ... ple-steps/
This fixture allows similar results without buying additional parts (assuming you have access to a 3d printer).

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Re: Poor man’s Kohler illumination

#10 Post by apochronaut » Tue Mar 28, 2023 11:32 am

Well, they are just plain wrong. It seems that Evident is on a campaign to justify their existence as well as appeal to the mass market. It is hard to understand why they would title that little 6 step blurb " How to align Köhler illumination in 6 simple steps" because they are misguiding those that do not know what Köhler illumination is into believing that it is the act of centering the field diaphragm. It should be called " How to centre and adjust the Field Diaphragm for the best intensity and Contrast".
They are certainly easy steps but there is only one sentence on the entire page that relates to Köhler illumination ; " If it is unfocused, adjust the condenser’s height until the blades of the field stop are as sharp as possible, matching the focus of the sample."
It may be that they feel obliged to cling to such a persistent concept as Kòhler to make their led microscopes more capable or something because technically there is no need for Köhler illumination on most led illuminated microscopes at all, since an led as built into most microscopes are broad light sources and therefore can be a focused illumination source, or just slightly defocused.

Köhler illumination was invented to make the best out of a bad situation. Prior to it's existence, illuminators were designed based on existing lighting technology to produce a wide flat source, so they could be adjusted critically. The illumination source was focused at the front focal plane of the condenser. The illuminators were purpose built and expensive. When the coiled filament came along, it revolutionized the microscope illuminator because it was so inexpensive and compact but it was too small and intense to be focused critically so it was figured out that by spreading or defocusing the light at the front focal plane of the condenser rather than critically focusing it, the illumination would be bright and even. The condenser then focuses that "field" at it's back focal plane which is adjusted to coincide with the focused image. There wasn't even a field diaphragm in the concept originally. The field iris diaphragm is a device that controls illumination intensity and contrast and works just as well with critical illumination as it does with Köhler illumination. It is often but not always placed at the front focal plane of the condenser, so the blades can be used to focus on for Köhler adjustment. It is used as a tool in adjusting köhler but is itself not part of köhler illumination.

What you have designed is similar to a disc diaphragm, such as has been installed on various student microscopes. It's a nice design but duplicates the function of the centered field diaphragm and has nothing to do with Köhler illumination. Köhler is a method of focusing light not a method of aperture control.

ZodiacPhoto
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Re: Poor man’s Kohler illumination

#11 Post by ZodiacPhoto » Wed Mar 29, 2023 2:30 am

apochronaut wrote:
Tue Mar 28, 2023 11:32 am
Well, they are just plain wrong. It seems that Evident is on a campaign to justify their existence as well as appeal to the mass market. It is hard to understand why they would title that little 6 step blurb " How to align Köhler illumination in 6 simple steps" because they are misguiding those that do not know what Köhler illumination is into believing that it is the act of centering the field diaphragm. It should be called " How to centre and adjust the Field Diaphragm for the best intensity and Contrast".
It's a much bigger conspiracy than Olympus / Evident alone. Look what Zeiss, Leica and Nikon are saying:

https://www.zeiss.com/microscopy/en/res ... ation.html
https://www.leica-microsystems.com/scie ... asy-steps/
https://www.microscopyu.com/tutorials/kohler

They all repeat the same procedure step-by-step. Yes, it is very different from the original Kohler's idea. And with today's microscopes, many of them using a frosted glass between the light source / collector and the field diaphragm, there is no way to focus on the bulb filament. Instead, they focus on the opening of the field diaphragm. All the major players just "agreed" to call this approach "Kohler" illumination. At least, the name of August Kohler is not forgotten...

apochronaut
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Re: Poor man’s Kohler illumination

#12 Post by apochronaut » Wed Mar 29, 2023 12:09 pm

Those links don't really alter the picture. Referring to the adjustment of the field diaphragm as a function of Köhler illumination is just a way of dumbing down the procedure because there is a general recognition that the procedure is intimidating to many and some well read procedural instructions are over complicated. That has to be included without isolation, due to the broad range of general understanding that the potential readership would hold.
Clearly, adjusting both the centering and the aperture of the field diaphragm is a tool you use to achieve the correct Z axis location for Köhler illumination, not an aspect of Köhler illumination itself. If you put a funnel in a rain barrel in order to catch a stream of water off the corner of the roof, is the funnel part of the barrel of water? No. You have de-focused the stream of water in the barrel but there are many ways of getting that done without the funnel. You could precisely locate the barrel under the stream, you could remove the lid, you could get a downspout that goes directly to the barrel. All are just adjustment tools.
In the case of the field diaphragm or facsimile, in just the same way you could remove it entirely and you would still be able to de-focus and adjust for Köhler illumination. The field diaphragm is put at the front focal plane of the condenser as a practical location in order to see it for the adjustment of it's function, not the adjustment of Köhler illumination. It so happens that in being there, it serves as a convenient reference device in order to focus the condenser.


If someone were to print your device and install it on a microscope that had a fairly plain illumination system with a diffuser and so no precise way to focus the condenser but your device location was conjugate with the object plane and thus could be seen in focus through the eyepiece, would that give you Köhler illumination? No. Your device would be simply a tool with which to alter the illumination aperture at it's location, which is the only thing it does. Köhler illumination is integral to the design of the illumination system against which you focus the condenser. Not an aperture that you can add on to a microscope.
It's not that I don't like your device, I just think your title should be amended to reflect what it actually does.

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Re: Poor man’s Kohler illumination

#13 Post by Phill Brown » Thu Mar 30, 2023 9:55 am

I'd still be interested in with and without images.
Diffused LED and defocusing the concept to a component that fits in the hand is more cost effective than producing a Kohler light source.
There's only so many things some people are comfortable with twiddling,cars seem to be going the opposite way,or offer screens that have to be looked at to demist the windscreen while driving, usually when it's raining and not caring where else the water goes.
Not that poor people can afford cars with touch screens either.

ZodiacPhoto
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Re: Poor man’s Kohler illumination

#14 Post by ZodiacPhoto » Fri Mar 31, 2023 12:37 am

Phill Brown wrote:
Thu Mar 30, 2023 9:55 am
I'd still be interested in with and without images.
Here you are, with the 40x objective:
"Open" diaphragm:
open.jpg
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"Closed":
closed.jpg
closed.jpg (94.62 KiB) Viewed 3967 times
This is exactly what I was trying to achieve with this contraption: reduce the size of the illuminated circle on the specimen to reduce the stray light and improve contrast.

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Re: Poor man’s Kohler illumination

#15 Post by MichaelG. » Fri Mar 31, 2023 1:48 am

.

Q.E.D. … as the Romans would put it !

Not classic Koehler, but very effective.

MichaelG.
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apochronaut
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Re: Poor man’s Kohler illumination

#16 Post by apochronaut » Fri Mar 31, 2023 2:38 am

It has nothing to do with Köhler. You are simply opening and closing an aperture, making a lateral adjustment in order to alter the illumination intensity and contrast. Köhler illumination is a longitudinal adjustment of the illumination focus in order to create an even background illumination. It is important , since there are a lot of new people taking up microscopy and will read the title of this thread , to be accurate. What you have made is great but you should change the title of thread to reflect what you have actually done and not confuse it with Köhler illumination and confuse new microscopists.

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Re: Poor man’s Kohler illumination

#17 Post by BramHuntingNematodes » Fri Mar 31, 2023 5:04 am

You can find and read Kohler's paper--its actually quite entertaining and not really overly technical. Although he did make use of a field aperture, really the essential components are a field lens and an adjustable condenser. Without these you can't really get the even illumination effect. Without the field diaphragm, you can, meaning the diaphragm is neither necessary nor sufficient.

Reducing glare by itself is commendable, but we also don't want to call the black paint on the inside of the tubus Kohler illumination either. I would call this device a field diaphragm turret, or some other such.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

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Re: Poor man’s Kohler illumination

#18 Post by BramHuntingNematodes » Fri Mar 31, 2023 5:07 am

Also, if you get a chance to read the paper I think you will find that a "poor man's Kohler" is exactly Kohler, as he had limited equipment but a good understanding of the principles and a knack for ingenious improvisation.
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Re: Poor man’s Kohler illumination

#19 Post by MichaelG. » Fri Mar 31, 2023 5:55 am

apochronaut wrote:
Fri Mar 31, 2023 2:38 am
[…] It is important , since there are a lot of new people taking up microscopy and will read the title of this thread , to be accurate. What you have made is great but you should change the title of thread to reflect what you have actually done and not confuse it with Köhler illumination and confuse new microscopists.
.
… and for their benefit: This is a good introductory page
https://micro.magnet.fsu.edu/optics/tim ... ohler.html

MichaelG.
.

PostScript: __ digressing slightly, this paper is interesting:
https://www.researchgate.net/publicatio ... advantages
Too many 'projects'

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Re: Poor man’s Kohler illumination

#20 Post by shutterbug » Fri Mar 31, 2023 8:29 am

Whether it's an actual Köhler Illumination or not - the results are great and as long as Zodiac Photo is happy we shouldn't get too hung up on nomenclature.

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Re: Poor man’s "Kohler" illumination

#21 Post by apochronaut » Fri Mar 31, 2023 11:45 am

Ure rite wot duz it matur iyl jus go bak tu mi lensoscope tern du liteon an luk at all mi ameebas did yall no sum uv em av got hares?

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Re: Poor man’s "Kohler" illumination

#22 Post by MichaelG. » Fri Mar 31, 2023 12:25 pm

8-)
Too many 'projects'

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Re: Poor man’s "Kohler" illumination

#23 Post by Phill Brown » Fri Mar 31, 2023 12:40 pm

I'm just happy that poor people now have 3D printers and PC's.

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shutterbug
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Re: Poor man’s "Kohler" illumination

#24 Post by shutterbug » Fri Mar 31, 2023 12:48 pm

@Apochronaut - I get your point but if it's an approximation of the principle by the use of low-cost alternatives and the result is as intended... well..

I mean you yourself prove your point by using "poor man's English" to communicate :-D It's not English per se, but close enough to get the concept, idea and result.

But maybe we're getting too deep into philosophy hahah

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Re: Poor man’s "Kohler" illumination

#25 Post by viktor j nilsson » Fri Mar 31, 2023 1:10 pm

But... It's not an approximation of the Kohler principle. The principle of Köhler is about focal points and image planes. An image of the light source should be in sharp focus at the objective back focal plane. Just look down an eyepiece tube with an eyepiece removed (or use a phase telescope) and have a look. Do you see a led chip or lamp filament? Then you have Kohler. If not, you don't. Simple as that.

Adding a field aperture has no effect on whether your light source is focused at your objective back focal plane. So adding a field aperture has the exact same effect on whether you achieve Kohler illumination as painting your microscope pink.

But the importance of Köhler is often overstated. My Vanox AH has one of the most refined true (diffuser-less) Köhler illumination systems around, and it definitely takes a lot more fiddling around to get the illumination perfect at all magnifications, than modern, user-friendly 'modified Köhler' systems with a diffuser. I can see why the big four has more or less abandoned the true Kohler concept, especially since so many microscope users in today's labs are completely oblivious and disinterested in fine tuning things like this.

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Re: Poor man’s "Kohler" illumination

#26 Post by apochronaut » Fri Mar 31, 2023 2:09 pm

thunk ya viktir
i kudent uv sed it bettur miself
wot aboyt dem hary ameebas anyhow is dey still ameebas?
stlll
stiii
still
y i new der wuz sumthen iz fergettin i left de pressur skrew clozd
iyl be rite bak.............

Phill Brown
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Re: Poor man’s "Kohler" illumination

#27 Post by Phill Brown » Fri Mar 31, 2023 2:36 pm

I can't think of many optically worse things than using a phase telescope to look at an LED chip, thankfully it's just a diffused bright field with an adjustable aperture, says Kohler on the box, but then it says baked beans on the side of the bus.
I have plenty Kohler also so all is well here.
Does anyone still make them on compound?
I have an inverted with Kohler which is still current but it's a bit spendy.

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Re: Poor man’s "Kohler" illumination

#28 Post by Phill Brown » Sat Apr 01, 2023 9:47 am

IMG-20230401-WA0003.jpg
IMG-20230401-WA0003.jpg (104.58 KiB) Viewed 3749 times
Part of the problem with Kohler is it's not competitive to produce.
I was wondering about getting a 3D printer.

ZodiacPhoto
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Re: Poor man’s "Kohler" illumination

#29 Post by ZodiacPhoto » Sat Apr 01, 2023 11:02 am

Phill Brown wrote:
Sat Apr 01, 2023 9:47 am
Part of the problem with Kohler is it's not competitive to produce.
I was wondering about getting a 3D printer.
Prusa just announced the new MK4 printer - expect plenty of Prusa i3 Mk1 - Mk3's on the used market.
Last edited by ZodiacPhoto on Sat Apr 01, 2023 11:42 am, edited 1 time in total.

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Re: Poor man’s "Kohler" illumination

#30 Post by Phill Brown » Sat Apr 01, 2023 11:13 am

ZodiacPhoto wrote:
Sat Apr 01, 2023 11:02 am
Phill Brown wrote:
Sat Apr 01, 2023 9:47 am
Part of the problem with Kohler is it's not competitive to produce.
I was wondering about getting a 3D printer.
Prusa just announced the new MK4 printer - expect plenty of Prusa i3 Mk1 - M3's on the used market.
I could get one and make things from old Kohler bottles.
I have a lathe and milling machine that don't need a computer, it's better that way for my patience levels.

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