🔬 What is Köhler Illumination? | Amateur Microscopy

Köhler Illumination increases contrast and evenness of illumination in a microscope. It reduces glare or stray light in the phototube.
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[@edward_grabczewski]

An excellent and much needed video on this topic. I've recently studied Köhler illumination, examining also the original German paper and reading the English translation. My children and I did an experiment on the properties of a bi-convex lens (e.g. a magnifying glass) in which we noticed that as you reach the focal point of the magnifying glass lens then the image becomes just an intense bright light, which gave Köhler the bright but even illumination he was trying to get from light sources at the time. This bright light is then focussed at the focal point of the condenser lens which then transmits it up through the specimen in a parallel (collimated) beam of even light. This light is so bright that I guess Köhler added the Köhler Field Diaphragm in order to reduce unnecessary light flooding the specimen when using high magnification objective lenses - as you clearly demonstrate in this video - which can reduce contrast. There are two ways to increase contrast on my cheaper microscopes: the first is to reduce the condenser diaphragm and the second is to reduce the intensity of the (halogen or LED) light source. I have created a Field Diaphragm by taking the diaphragm off a cheap condenser (bought on eBay) and placing this over the lamp. It seems to work but it's more trouble than it's worth and I've found I can get equally good results by altering the condenser diaphragm or changing the intensity of the light source. BTW Photoshop (or Gimp) have tools to gradually increase the contrast of images to give optimum results, but in photography it is normally better to underexpose your image than overexpose it as often more information is hidden in the darker, underexposed areas but information is lost in the lighter, overexposed ones. So, to be clear, you would slightly underexpose the image and then in Photoshop brighten it and increase the contrast of the image to see more detail.

[@enceendulue6388]

This is one of the most important things I have read here. I just stumbled upon your technique this very night while toying with my microscope. I like the way you described it. I will study it further

[@bonnitakhaliq9578]

So basically all Kohler is ...........Different Hole sizes that the Light shines through , simple as that , not complicated. So just make your own with whatever material you want , even a piece of Cardboard with a Hole in it , place on top of the Light as it is flat...............Job Done

[@edward_grabczewski]

@@bonnitakhaliq9578 If it was easy as that then everyone would have done it 🙂 As far as I can see, the important thing about Köhler illumination is the fact that the Field collector lens focusses the light at a particular focal point below the condenser lens so that it illuminates the specimen evenly. If you simply take a card with a hole in it then you'll see an upside down picture of the light source (blurred to a greater or lesser degree) on the specimen, thus creating an uneven source of illumination. The Köhler Field lens does the focussing of light evenly and the Köhler Field diaphragm controls the amount of light hitting the specimen. The technique I describe only mimics the Field Diaphragm but without a Köhler Field lens to provide the even illumination then, yes, it is equivalent to just a piece of cardboard with a hole in it. The important part of my post was to do with the lens. Without the ability to focus the light using a Field Collector lens then you're not going to have an evently illuminated specimen - which is the whole point of Köhler illumination.

[@paulfarrell5935]

I appreciate how you always explain all aspects of an issue. It really helps me to understand the and learn. As always, I thank you!

[@stargazer4574]

Very interesting type of illumination, I didn't know that.Thanks!

[@2omskwithlove]

Priceless! Thanks!

[@francantonvilgera7588]

Your videos make me very excited for my 1st microscope.😊

[@dtkbdksektionaugsburg1875]

Thanks for this interesting information, which helps me understand even more what to consider when purchasing a microscope for hobby purposes. It also is a good explanation of what Köhler illumination really does.

[@anniinthewoods8287]

Thank you! I really appreciate that you explain these details in a way that is easy to understand =) I try to learn fungi, moss and lichen ID for forest biodiversity protection. Did a course at university but we were 32 students and just 2 teachers so of course I couldn't get all my questions answered. Now I got a helpful book about fungi microscopy but it's in French... So it seems that every step on the way takes a lot of energy. And here you are just explaining it to me =)

[@Molko701]

Thank you!

[@littlejohn5652]

Thank you so much - maybe interesting to explain the difference to Heine Illumination?

[@alexandrevaliquette3883]

8:42 That is very nice demonstration indeed!!!!!! I like when you put a lot of visual in your explanations. So, for a 3D printed microscope, we should have a larger vertical tube and a darker paint to help reduce theses internals reflections. I'll keep this in mind. If anybody want to print his own microscope, you can find PUMA diy microscope open source project on KZbin and website. I'm on my way to design my own and your video helped me improve my design.

[@alexandrevaliquette3883]

Edit, I will use the Kohler illuminator anyway. It won't be very efficient to make the tube wider if the problem is inside the eye piece. I'll think about all that.

[@paralax8714]

Great content. Can i improve my microscope for building a Kohler device in it? My one has no Kohler illumination, and i would like to know if a can successfully adapt an iris in light lens in order to achieve the effect.

[@weirdsciencetv4999]

Can you do a video on epi-illumination using Kohler method?

[@ernestoabarca274]

My 100€ Traveler microscope (no Köhler of course) has metal gray (instead of black) shining tube causing similar 7:17 whiteish (but in an anular form when using barrow lens or just some archs depending on tube hole in prism chamber geometry). I solved it sticking some black 'no reflection' tape (it could also be painted black) to tube. Maybe you want to consider/review this in case it helps somebody :)

[@Bhodisatvas]

If you are shooting in RAW format (pretty much every photographer does these days) you should be able to pull out the contrast difference when editing but if you are simply studying a specimen and want the best image to observe then it is useful.

[@sachinrathod9725]

I have question that could one have the bright field and dark field microscopy in one system, as there simple difference in diaphragm!!

[@scarlingdarIing]

Hi! I have a question. I first heard about Kohler Illumination on a website I found while researching the best microscope suggestions for looking mostly at fungal spores and this was a feature the writer thought was an important one.. I know with a lot of fungi, reagants/stains may need to be used to be able to really see much - is it true that by having Kohler Illumination, one could then forgo having to use stains? It seems a little difficult for the average person to get many of these regeants that would be used in a lab, so I was hoping that kohler Illumination was a way around that?

[@Faquarhl]

Hey, did you learn anything new on this? And could you forward me the website or the microscope suggstions you are talking about?

[@williammauzey8342]

Excellent description! Had the diaphragms and did not know what they were for. Now I do, Thank you very much!

[@Crispy_Bee]

Hmmm to me this looks more like lens flare and/or internal reflections - so a weakness in the lens optics and/or treatment of the microscope tube. On an old Reichert microscope you could remove/insert small rings (with variable inner diameters) into the rear of the microscope objective in order to prevent accidental reflections from stray light. This would only allow a rather narrow beam of light to enter the microscope which wouldn't cause those reflections and they have a very clear image, even in very high contrast scenes.

[@ti62bu54]

so for viewing thru the eyepiece, with eye or camera, kohler not that much of improvement? only viewing thru camera(3rd) tube will see a good amount of difference? Show this!!

[@fredashay]

So to turn a "normal" microscope to a DIC microscope is to put an iris (like a camera shutter) above the light source below the stage to limit the light source to a small spot of light?

[@legoworks-cg5hk]

Where did you get DIC from?

[@meilabnitrourkela5314]

@Microbehunter Microscopy: May I use some of the screenshots from your videos for my classroom teaching? Needless to say, the proper source of credit will be given. I will upload the videos in my classroom teaching KZbin channel. Please advise.

[@MicrobehunterMicroscopy]

There's a much better way. Embed a link of the video in your website (if you have one). No need to upload it if it is already on KZbin. If you want to start the video at a certain time, then add for example #t=1m10s to the link (to start the video after 1 minute and 10 sec). I also recommend that you create playlists in your own youtube channel, where you can add videos from all of youtube. This also allows you to sort and collect videos for different topics. You then send the students a link of the playlist, which contains all of the videos for a lesson that you want them to watch. This has the additional advantage, that youtube will not penalize your own youtube account for duplicate content (the algorithm automatically finds duplicate videos that were taken from somewhere else, and channels can be shut down because of this.)

[@meilabnitrourkela5314]

@@MicrobehunterMicroscopy no issues. I would have taken some still pictures (mainly some micrographs as an example) only and not any portion of your videos. 😊 Anyways, thanks for your advise. Stay safe and keep well.

[@karl3549]

I wonder how difficult it would be to make a Köhler Illumination kit for pretty much any microscope, using the aperture diaphragm from an old camera lens and a 3D printed holder (or maybe even a piece of PVC pipe) of some kind.

[@MicrobehunterMicroscopy]

The diaphragm might not be the problem, but you do need a lens that is placed over the light. But by limiting the diameter of the lamp (diaphragm only), one might also already get an improvement. worth to try out....

[@karl3549]

@@MicrobehunterMicroscopy Good point. I may be able to fit one of the achromatic lenses between the light and the condenser though. I'm not able to experiment yet since I don't have a proper lens spanner and I'm still waiting on the microscope as well, haha. It looks like a very tight fit, so I may not be able to place a lens under the stage at all. It'll be a fun project though, at least.

[@lotharmayring6063]

wieso muss denn der Kondensor zentriert werden. Seine optisch Achse sollte doch in jedem Fall mit der optischen Achse des Objektives uebereinstimmen und nicht mehr veraendert werden. Wohl aber sollte die Lichtquelle bzw die Leuchtfeldblende zentrierbar sein. Im Uebrigen sollte man immer die Lichtquelle optimieren das bedeutet auch eine zentrierbare Punktlichtquelle, Steuerung der Lichtintensitaet etc

[@user-zu4hm1pp1b]

In my experience, Koehler illumination brings almost the same result as using the phase contrast method. What do you think about it?

[@MicrobehunterMicroscopy]

The purposes of these two are different. Koehler increases image quality by making sure that there is no stray light. Phase Contrast converts refractive index differences to brightness. You can combing Koehler and Phase but they do different things.

[@user-zu4hm1pp1b]

@@MicrobehunterMicroscopy what can you observe with phase contrast , that you can’t see in bright field? In your experience?

[@anniinthewoods8287]

With phase contrast you limit the light in the centre and only allow light from the sides to shine on your specimen. I use it for fungi spores because you see the cell walls better

[@lotharmayring6063]

Koehler is nonsense, you have to center the the lightfield-iris and the lightsource not the condenor-riris. The condensor-iris has to be centered to the objectiv-okular -axis previous. Then you close the condensor-ris a much as it is possibel. Koehler is a historical technic on times where the sun was the only lightsource. Using modern microscops Koehlern is nonsense and of course the illumination depend on the specimen and not on condensors

[@miguelelento5637]

you're just pronouncing it "kulah"