Fixed Köhler

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david_b
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Fixed Köhler

#1 Post by david_b » Wed Jul 01, 2020 7:16 am

I'm looking at the Olympus CX33 and CX43 microscopes and note they have "fixed Köhler" illumination.
I assume this means there is no adjustable field diaphragm?
How does fixed Köhler work, and does it have any disadvantages?

https://www.olympus-lifescience.com/en/ ... t/cx43-33/
Last edited by david_b on Wed Jul 01, 2020 12:41 pm, edited 2 times in total.

MichaelG.
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Re: Fixed Köeler

#2 Post by MichaelG. » Wed Jul 01, 2020 8:30 am

It appears to be a ‘convenient approximation’ for routine [non-critical] use, David

This, from Zeiss, goes a little further towards explaining:
http://focuspi.com/Site/AXIO_SCOPE_A-1_ ... 0004_e.pdf]
[see pp7-9]

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Hobbyst46
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Re: Fixed Köeler

#3 Post by Hobbyst46 » Wed Jul 01, 2020 12:11 pm

MichaelG. wrote:
Wed Jul 01, 2020 8:30 am
It appears to be a ‘convenient approximation’ for routine [non-critical] use, David

This, from Zeiss, goes a little further towards explaining:
http://focuspi.com/Site/AXIO_SCOPE_A-1_ ... 0004_e.pdf]
[see pp7-9]

MichaelG.
Probably, it has no field diaphragm...
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Re: Fixed Köeler

#4 Post by david_b » Wed Jul 01, 2020 12:41 pm

Hobbyst46 wrote:
Wed Jul 01, 2020 12:11 pm
MichaelG. wrote:
Wed Jul 01, 2020 8:30 am
It appears to be a ‘convenient approximation’ for routine [non-critical] use, David

This, from Zeiss, goes a little further towards explaining:
http://focuspi.com/Site/AXIO_SCOPE_A-1_ ... 0004_e.pdf]
[see pp7-9]

MichaelG.
Probably, it has no field diaphragm...
I'm still not clear what is the difference between "fixed Köhler" and a microscope without Köhler illumination.

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Re: Fixed Köeler

#5 Post by MichaelG. » Wed Jul 01, 2020 12:59 pm

david_b wrote:
Wed Jul 01, 2020 12:41 pm
I'm still not clear what is the difference between "fixed Köhler" and a microscope without Köhler illumination.
.
Do you have any detail pictures of the light source, David ??

My [hopefully ‘educated’] guess is that they are using large area LED sources ... which would reasonably approximate the illuminated area that normally exists in the plane of the true Köeler‘s Field Diaphragm.

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Re: Fixed Köhler

#6 Post by mrsonchus » Wed Jul 01, 2020 1:28 pm

I'd say Michael has the jist - I suspect that as Kohler is defined primarily as uniformity of field illumination, rather than the accepted means by which it's achieved (i.e. field diaphragm and condenser height)... The LED may be deemed to give a uniform field, regardless of it's diameter? The correct n.a. is of course handled by the condenser's iris - not the field iris.
Personally I'd like-to think that if Olympus says it's so it's so... maybe?
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Re: Fixed Köeler

#7 Post by david_b » Wed Jul 01, 2020 2:31 pm

MichaelG. wrote:
Wed Jul 01, 2020 12:59 pm
Do you have any detail pictures of the light source, David ??
I only have the brochure that I linked to and the (very interesting) one in your previous link.

I also searched Olympus lifescience website:
0 results for fixed kohler We are sorry but your search returned no results.

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Re: Fixed Köhler

#8 Post by apochronaut » Wed Jul 01, 2020 2:37 pm

Koehler is such a trope in amateur microscopy anymore. People who have never looked in a microscope launch themselves after Koehler illumination, as though it is going to make or break the quality of a microscope.
Every self illuminated microscope ever made was designed as closely as possible to achieve a Koehler type of illumination, within it's production cost and market target. A field diaphragm doesn't really matter in Koehler illumination. It just provides a location to focus on. If that location can't be defocused, then the field diaphragm as far as Koehler illumination is concerned is irrelevant.

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Re: Fixed Köhler

#9 Post by BramHuntingNematodes » Wed Jul 01, 2020 2:57 pm

apochronaut wrote:
Wed Jul 01, 2020 2:37 pm
Koehler is such a trope in amateur microscopy anymore. People who have never looked in a microscope launch themselves after Koehler illumination, as though it is going to make or break the quality of a microscope.
Every self illuminated microscope ever made was designed as closely as possible to achieve a Koehler type of illumination, within it's production cost and market target. A field diaphragm doesn't really matter in Koehler illumination. It just provides a location to focus on. If that location can't be defocused, then the field diaphragm as far as Koehler illumination is concerned is irrelevant.
It was very important in getting a good image in the Dynoptic, which had a mirror. I built two illuminators with optics and accessories. The field iris was important to reduce glare in some cases. Building the light from scratch helped me to understand what all the parts were for. Now I can look at high magnification phase images without straining to see them.

In the Dynazoom, which has a built in illuminator that ensures diffuse light that is pointed in the right direction, it's not something I have to think about.
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Re: Fixed Köeler

#10 Post by MichaelG. » Wed Jul 01, 2020 3:16 pm

david_b wrote:
Wed Jul 01, 2020 2:31 pm
MichaelG. wrote:
Wed Jul 01, 2020 12:59 pm
Do you have any detail pictures of the light source, David ??
I only have the brochure that I linked to and the (very interesting) one in your previous link.
.

Thanks, David ... it was just a hope

The Zeiss document does include an image that clearly shows an array of LEDs ... so I’m pretty sure that we are on the right track.

Like you, I can find no definition or explanation of the term ... but it seems to be in fairly widespread use:
Olympus, Zeiss, and Motic to name but three.

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Re: Fixed Köhler

#11 Post by apochronaut » Wed Jul 01, 2020 3:52 pm

BramHuntingNematodes wrote:
Wed Jul 01, 2020 2:57 pm
apochronaut wrote:
Wed Jul 01, 2020 2:37 pm
Koehler is such a trope in amateur microscopy anymore. People who have never looked in a microscope launch themselves after Koehler illumination, as though it is going to make or break the quality of a microscope.
Every self illuminated microscope ever made was designed as closely as possible to achieve a Koehler type of illumination, within it's production cost and market target. A field diaphragm doesn't really matter in Koehler illumination. It just provides a location to focus on. If that location can't be defocused, then the field diaphragm as far as Koehler illumination is concerned is irrelevant.
It was very important in getting a good image in the Dynoptic, which had a mirror. I built two illuminators with optics and accessories. The field iris was important to reduce glare in some cases. Building the light from scratch helped me to understand what all the parts were for. Now I can look at high magnification phase images without straining to see them.

In the Dynazoom, which has a built in illuminator that ensures diffuse light that is pointed in the right direction, it's not something I have to think about.
Closing the field diaphragm to reduce glare has nothing to do with Koehler illumination. It is a separate concept and is used as part of Koehler technique because it is useful in improving results. People were reducing glare by reducing the aperture with an iris diaphragm since before Koehler was born.
Closing the iris when adjusting Koehler is simply a way of centering and finding a focus point. Of course you have to open the iris again and adjust it for best contrast. You do the same thing on systems that are not Koehler, yet still contain a field diaphragm.

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Re: Fixed Köeler

#12 Post by 75RR » Wed Jul 01, 2020 4:07 pm

MichaelG. wrote:
Wed Jul 01, 2020 8:30 am

I'm still not clear what is the difference between "fixed Köhler" and a microscope without Köhler illumination.
Prefocused on the condenser front focal plane perhaps ...
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Re: Fixed Köhler

#13 Post by BramHuntingNematodes » Wed Jul 01, 2020 4:33 pm

I think it's somewhat relevant as Kohler himself describes the field diaphragm, the "blenden" close to his light source, as part of his light train whereas this system seems to lack one. Then again, Kohler, a man after my own heart, also talks about using a cardboard cutout for this and improvising a field lens using Opernglases. My German is not so great, but I think he also describes using an objective for a condenser and using funnel stops as his condenser iris. Truly a great system!
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Re: Fixed Köeler

#14 Post by Hobbyst46 » Wed Jul 01, 2020 4:40 pm

75RR wrote:
Wed Jul 01, 2020 4:07 pm
MichaelG. wrote:
Wed Jul 01, 2020 8:30 am

I'm still not clear what is the difference between "fixed Köhler" and a microscope without Köhler illumination.
Prefocused on the condenser front focal plane perhaps ...
I think that the "fixed" in this context refers to the lamp only. The lamp has been optimized (within the cost limitations as set by the manufacturer) in size, centration and distance from the next optical element, and these are fixed. There are modern expensive scopes where Kohler setting includes centration of the lamp, adjustment of its distance etc. For brightfield as well as epi-fluorescence.
Specifically in the Zeiss manual, it appears that the least expensive model of the three does lack a field aperture - hence lacks the functions provided by the aperture, as detailed by apochronaut above. This is my personal interpretation of the specifications as described.
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Re: Fixed Köeler

#15 Post by MichaelG. » Wed Jul 01, 2020 5:03 pm

75RR wrote:
Wed Jul 01, 2020 4:07 pm
MichaelG. wrote:
Wed Jul 01, 2020 8:30 am

I'm still not clear what is the difference between "fixed Köhler" and a microscope without Köhler illumination.
Prefocused on the condenser front focal plane perhaps ...
Forgive me mentioning it, but ... david_b wrote that, not me

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Re: Fixed Köeler

#16 Post by 75RR » Wed Jul 01, 2020 5:04 pm

.
Sorry, my bad!
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Re: Fixed Köeler

#17 Post by david_b » Wed Jul 01, 2020 8:40 pm

MichaelG. wrote:
Wed Jul 01, 2020 3:16 pm
Thanks, David ... it was just a hope

The Zeiss document does include an image that clearly shows an array of LEDs ... so I’m pretty sure that we are on the right track.

Like you, I can find no definition or explanation of the term ... but it seems to be in fairly widespread use:
Olympus, Zeiss, and Motic to name but three.

MichaelG.
Motic seem confused too.
In the product description for the Panthera they state "full Kohler", but on the same page in the features list they say "fixed Kohler".
I don't see a field diaphragm or anything unusual about the lamp.
https://www.motic.com/Am_LifeSciences_U ... t_780.html

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Re: Fixed Köeler

#18 Post by MichaelG. » Wed Jul 01, 2020 9:02 pm

david_b wrote:
Wed Jul 01, 2020 8:40 pm
Motic seem confused too.
In the product description for the Panthera they state "full Kohler", but on the same page in the features list they say "fixed Kohler".
I don't see a field diaphragm.
.
Curiouser and Curiouser : Have a look at p12 of this document [p13 of the PDF]
https://genexpress.cl/wp-content/upload ... ra_Eng.pdf

... but I can still find no actual description of the illumination optics.

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Re: Fixed Köeler

#19 Post by Hobbyst46 » Wed Jul 01, 2020 9:45 pm

MichaelG. wrote:
Wed Jul 01, 2020 9:02 pm
david_b wrote:
Wed Jul 01, 2020 8:40 pm
Motic seem confused too.
In the product description for the Panthera they state "full Kohler", but on the same page in the features list they say "fixed Kohler".
I don't see a field diaphragm.
.
Curiouser and Curiouser : Have a look at p12 of this document [p13 of the PDF]
https://genexpress.cl/wp-content/upload ... ra_Eng.pdf

... but I can still find no actual description of the illumination optics.

MichaelG.
Why, the Motic Panthera looks the same as the Zeiss: the basic model (student, S) has no field diaphragm and a "fixed Kohler". The University model has "full Kohler". But the instructions for Kohler in the Panthera manual do not mention the lamp. Hence my interpretation: The lamps are fixed, their distance is fixed, they are precentered, need not be touched. The "fixed kohler" means no field aperture, the lamp and its position have been optimized and thats it.
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Re: Fixed Köeler

#20 Post by MichaelG. » Wed Jul 01, 2020 9:58 pm

MichaelG. wrote:
Wed Jul 01, 2020 9:02 pm
... but I can still find no actual description of the illumination optics.
.
Surely one of these companies could have the courtesy to describe what’s in their microscope.

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Re: Fixed Köhler

#21 Post by 75RR » Thu Jul 09, 2020 9:53 am

I'm still not clear what is the difference between "fixed Köhler" and a microscope without Köhler illumination.
Decided to contact Motic, this is their reply
.
Attachments
Motic.png
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Re: Fixed Köhler

#22 Post by 75RR » Thu Jul 09, 2020 9:55 am

To save interested parties having to type out the sentence in a translator, here it is:

Fixed Köhler significa que esta pre-centrado de fabrica y que no debe ser necesario el reajuste.

+ google translate:

Fixed Köhler means that it is pre-centered at the factory and no re-adjustment is necessary.

+ DeepL translate:

Fixed Köhler means that it is pre-centered at the factory and that no readjustment should be necessary.
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Re: Fixed Köhler

#23 Post by apochronaut » Thu Jul 09, 2020 10:54 am

They must have a lot of faith in their ability to churn out absolutely parfocal objectives: one after another after another after another after another.

seems to me they know it doesn't really matter anyway and all the customer needs is the buzzword to make them happy.

they can unpack it and point it out in a video.

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Re: Fixed Köhler

#24 Post by MichaelG. » Thu Jul 09, 2020 12:52 pm

75RR wrote:
Thu Jul 09, 2020 9:55 am
To save interested parties having to type out the sentence in a translator, here it is:
[…]
.
Thanks for that

... So give or take one hyphen, and the small but significant difference between ‘is’ and ‘should be’: it’s not adjustable.

... Seems like a reasonable description of ‘fixed’

MichaelG.
.

Thought for the Day :

There was a wise old saying amongst Scientific Instrument Makers :
If you can’t make it right, make it adjustable.
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Re: Fixed Köhler

#25 Post by Hobbyst46 » Thu Jul 09, 2020 1:16 pm

MichaelG. wrote:
Thu Jul 09, 2020 12:52 pm
75RR wrote:
Thu Jul 09, 2020 9:55 am
To save interested parties having to type out the sentence in a translator, here it is:
[…]
...There was a wise old saying amongst Scientific Instrument Makers :
If you can’t make it right, make it adjustable.
:lol: :lol:
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Re: Fixed Köhler

#26 Post by david_b » Thu Jul 09, 2020 3:38 pm

I understand that the lamp is pre-centered, but my understanding of Kohler is that a field diaphragm is essential to match the spread of illumination to the objective in use...
https://www.olympus-lifescience.com/en/ ... my/kohler/
If a field diaphragm is an essential part of Kohler, then is "fixed Kohler" a misnomer?

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Re: Fixed Köhler

#27 Post by Zuul » Thu Jul 09, 2020 3:47 pm

david_b wrote:
Thu Jul 09, 2020 3:38 pm
I understand that the lamp is pre-centered, but my understanding of Kohler is that a field diaphragm is essential to match the spread of illumination to the objective in use...
https://www.olympus-lifescience.com/en/ ... my/kohler/
If a field diaphragm is an essential part of Kohler, then is "fixed Kohler" a misnomer?
I believe it’s the condenser diaphragm that performs that function. The field diaphragm is used to center and focus the condenser, then opened fully. Am I mistaken?

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Re: Fixed Köhler

#28 Post by 75RR » Thu Jul 09, 2020 3:52 pm

david_b wrote:
Thu Jul 09, 2020 3:38 pm
I understand that the lamp is pre-centered, but my understanding of Kohler is that a field diaphragm is essential to match the spread of illumination to the objective in use...
https://www.olympus-lifescience.com/en/ ... my/kohler/
If a field diaphragm is an essential part of Kohler, then is "fixed Kohler" a misnomer?
A field diaphragm is one of the essential parts of what they are now calling Full Köhler, I think Fixed Köhler is perilously close to being a kissing cousin to 2000x magnification.
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Re: Fixed Köhler

#29 Post by 75RR » Thu Jul 09, 2020 4:03 pm

Zuul wrote:
Thu Jul 09, 2020 3:47 pm

I believe it’s the condenser diaphragm that performs that function. The field diaphragm is used to center and focus the condenser, then opened fully. Am I mistaken?
Yes to center and focus, partially closed depending on the objective to reduce glare.
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Re: Fixed Köhler

#30 Post by david_b » Thu Jul 09, 2020 4:41 pm

As above, the field diaphragm is set to be just outside the field of view for whatever objective is in use, not normally wide open.
The principle is the same as the lens hoods, 'barn doors', bits of polystyrene etc. that pre-smartphone photographers used to reduce flare and maintain contrast.
Last edited by david_b on Thu Jul 09, 2020 6:11 pm, edited 1 time in total.

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