Field diaphragm placement

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imnoexpert
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Field diaphragm placement

#1 Post by imnoexpert » Thu Apr 25, 2019 6:27 am

Hi guys. I've recently gotten into microscopy as a hobby and have bought my first microscope. I am hoping to use Kohler illumination but my scope doesn't have a field diaphragm (it does have a substage condenser with an iris diaphragm though). I've purchased a third party field diaphragm to install on the scope but something that I'm not certain about is how important the exact placement of the field diaphragm is. Can I simply install it above my collector lens in the base? Does the diaphragm itself create the image of the light source or do I need to figure out the correct height/position for its placement in order to match up with the image plane? Thanks in advance for your advice.

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75RR
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Re: Field diaphragm placement

#2 Post by 75RR » Thu Apr 25, 2019 7:58 am

...(it does have a substage condenser with an iris diaphragm though)
Note that to achieve Köhler having a Field Stop Diaphragm is not enough, you also need the lamp filament to be focused on the Front Focal Plane of the Condenser, i.e. the underside of the Condenser Iris Diaphragm. You can check this by closing the Condenser Diaphragm and holding a mirror under it, the lamp filament should be visible on the leaves of the diaphragm.

As can be seen in the image, the Field Stop Diaphragm(1) is part of the Conjugate Field Planes (left hand diagram), as such it should be visible and in focus in the Object Plane(2) when the specimen is in focus and the condenser is near the top of its range (1 or 2mm below the slide).
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imnoexpert
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Re: Field diaphragm placement

#3 Post by imnoexpert » Thu Apr 25, 2019 9:27 am

Thanks for the quick response and insight. I will definitely check these alignments/focal points out. I have one further question on checking these though. My light source is LED, not a filament bulb. Since there is no filament to view, what am I looking for or what could I do to check the focus of the light source at the condenser diaphragm?

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75RR
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Re: Field diaphragm placement

#4 Post by 75RR » Thu Apr 25, 2019 10:41 am

My light source is LED, not a filament bulb.
The LED itself is a filament and like the filament of an incandescent bulb needs a lens to be able to focus it.

See image of a focused LED - note sharp edges and the wires indicating focus. (image is from a focusable LED torch)

What microscope do you have? If it does not have a Field Diaphragm it is most probably rather basic.

It might be best to wait until you get a better one that incorporates Köhler illumination rather than mess about trying to adapt it.

One can get a used quality microscope with Köhler for a reasonable amount of money.
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Re: Field diaphragm placement

#5 Post by imnoexpert » Thu Apr 25, 2019 2:43 pm

The model I have is quite basic. It's an Amscope B450C-SP. As far as I can infer from what I've read so far, I should be able to setup Kohler with it. It's just the placement of the field diaphragm I'm uncertain of. I'll give it a bash when the part arrives and see what I can do. I really appreciate your input. I am really impressed by most of the people on this forum.

As regards buying another model, I can only spend so much on a hobby and although I am aware that there are great used models on the market, I live in South Africa and I don't know anyone else who works with a microscope or anyone who could assist me with maintenance or servicing of an old model. There are no reliable used sellers anywhere near me. I basically wasn't willing to take the risk right now. Once I am more comfortable with the system myself and if I find this hobby takes off nicely, I will almost definitely look at a more capable used model. Additionally, microscopes are really expensive here with the shipping and customs on top.

TLDR: I am aware of the argument of new vs old and I chose new due to where I live and my lack of microscopy buddies to help with a problematic used model.

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Re: Field diaphragm placement

#6 Post by MichaelG. » Thu Apr 25, 2019 3:45 pm

imnoexpert wrote:The model I have is quite basic. It's an Amscope B450C-SP. As far as I can infer from what I've read so far, I should be able to setup Kohler with it. ...
Please forgive me if I damp your enthusiasm; but how do you make that inference ?
I have just downloaded the 450 series manual:
https://www.amscope.com/download
... and can find nothing to suggest that Kohler can be set.

Obviously if you were to add a sub-base, with a collimated light source, then something could be done; but that's quite a significant modification.

Please do let me know what I have missed.

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Re: Field diaphragm placement

#7 Post by imnoexpert » Fri Apr 26, 2019 6:22 am

Hi Michael

Since you guys are the experts, I'm more inclined to believe that I am the one who has missed something rather than yourselves. I have read up on the theory behind microscopy and Kohler illumination from a lot of different online sources in an effort to understand how it all works and not one has indicated that there is anything more than a field diaphragm missing from my model in order to achieve Kohler. I was under the impression that the collector lens creates the collimated light beam (which I thought my model has - please correct me if I'm wrong here, is this not a collector lens or does it require another lens to enable collimation?).

If I am not able to achieve Kohler illumination, I would be a little disappointed but ultimately I'm very happy with this as my first model. It fits well into my cost/benefit analysis of cost vs quality, new vs old, etc. I'm having a lot of fun with it and can't wait until my son is old enough to enjoy it too so the only frustration for me is the wasted cash I spent on an extra diaphragm.

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Re: Field diaphragm placement

#8 Post by apochronaut » Fri Apr 26, 2019 11:39 am

Koehler illumination is an over emphasized qualitative aspect of microscopy. I guarantee you that you could take 2 photographs through 2 different properly adjusted microscopes using identical objectives; one instrument designed to use Koehler, the other not and you would not be able to tell the difference.
Koehler is a method that assists to make the best of a bad situation; the inadequacy of a lamp filament in completely and evenly filling the illumination field of a microscope. It's a method that partially rectifies the faults of that inadequate illumination system by defocusing the illuminator and blurring those faults. If the microscope wasn't designed with Koehler in mind then it is unlikely that modifying it to use Koehler is going to improve it's performance because the optics in the illumination path are designed otherwise.
The field iris in the illumination system is irrelevant to Koehler anyway, it is simply a marker by which one can determine if they have adjusted things correctly and it's real purpose is to adjust the illumination intensity, a function distinct from Koehler illumination.

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Re: Field diaphragm placement

#9 Post by MichaelG. » Fri Apr 26, 2019 11:53 am

imnoexpert wrote:I have read up on the theory behind microscopy and Kohler illumination from a lot of different online sources in an effort to understand how it all works and not one has indicated that there is anything more than a field diaphragm missing from my model in order to achieve Kohler. I was under the impression that the collector lens creates the collimated light beam (which I thought my model has - please correct me if I'm wrong here, is this not a collector lens or does it require another lens to enable collimation?).
Sorry, this will have to be brief: Have another look at the diagram which 77RR posted.
It's rather simplified, but it shows a collector lens between the lamp and the field diaphragm ... this is not part of the condenser; it usually resides in the base of the instrument.
Do you have anything equivalent to that lens in your microscope ?

Duty calls ... but I will try to find a more detailed diagram later.

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apochronaut
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Re: Field diaphragm placement

#10 Post by apochronaut » Fri Apr 26, 2019 12:43 pm

Just because there is a collector lens in the illumination path, that doesn't mean the system is capable of Koehler illumination, nor should be. That depends on the focus of that lens and the source of illumination. I can think of several microscope brands that had no need for Koehler illumination because they were designed to defocus and spread the beam at the condenser when it was correctly positioned and oiled. No field diaphragm. The literature on the development of Koehler illumination is a historical explanation going back to when broad sources of light( sky, gas flame) were perfectly adequate when adjusted for critical illumination. With the advent of tiny filaments, a change was needed to create a false broad source of illumination but there are many ways of accomplishing that. We are now back to the possibility of broad sources of illumination . Microscopes can have disc sources of light, so no manipulation of the beam is necessary.

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Re: Field diaphragm placement

#11 Post by wporter » Fri Apr 26, 2019 1:41 pm

I agree with apochronaut. Kohler is a compensatory arrangement for use with 'point' sources of illumination. These days, with COB LEDs, properly placed diffusers, and sufficient LED brightness, Kohler is not necessary for many microscopes; it is still useful in high-powered lab scopes that may use arc lamps or other very bright point sources.

The only main thing to adjust remains the distance of the condenser top to the specimen: usually the closer the better, but the adjustment for Kohler made this easy, by raising the condenser until the inner edges of the field iris were in focus (with the specimen also in focus), then opening up the field iris. If you are curious how the condenser adjustment procedure in your manual matches that for Kohler, simply make a 'fake' field iris by punching a hole in a piece of cardboard and laying it (centered) on top of the light well. Focus on your specimen, and raise the condenser until the edges of the punched hole are in focus. You have raised the condenser to the correct position; remove the cardboard and you're good to go.
Last edited by wporter on Fri Apr 26, 2019 2:59 pm, edited 1 time in total.

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Re: Field diaphragm placement

#12 Post by MichaelG. » Fri Apr 26, 2019 2:54 pm

MichaelG. wrote:Duty calls ... but I will try to find a more detailed diagram later.
The diagrams on this page are more representative of practical reality:
https://www.olympus-lifescience.com/en/ ... my/kohler/

That said ... I do broadly agree with the comments made by apochronaut and wporter
[as, presumably, does AmScope]

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Re: Field diaphragm placement

#13 Post by 75RR » Fri Apr 26, 2019 2:55 pm

To make Köhler just about even illumination is to miss half the point.

See this extract from Fundamentals of Light Microscopy and Electronic Imaging by Douglas B. Murphy
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Re: Field diaphragm placement

#14 Post by MichaelG. » Fri Apr 26, 2019 3:43 pm

Whilst we're at it ...
This is an excellent page:
http://www.microscopist.co.uk/essential ... umination/
and Peter Evenett's explanation is thorough [see link in the sidebar]

MichaelG.
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imnoexpert
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Re: Field diaphragm placement

#15 Post by imnoexpert » Fri Apr 26, 2019 6:03 pm

Wow guys, the slew of information and references you've thrown at me are fantastic thanks. I've just gotten home and need to deal with life but I'll check these out tomorrow.

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