Kohler Illumination

[LouiseScot]

It's interesting that Omax and Euromex sell add-on field diaphragms--apparently someone thinks Kohler illumination is worthwhile even on lower end scopes. A pity, though, they didn't just incorporate a field diaphragm into the original design. Probably would have added all of $5 to the manufacturing cost...

A field diaphragm improves image contrast by blocking stray light,
which may be more problematic in lower end scopes.
If, as it seems, these diaphragms are added between field lens and condenser,
then they cannot be at an illumination focal plane unless extra lenses are also involved.

I think the Omax Kohler is a field lens assembly:

https://omaxmicroscope.com/a195k-kohler ... copes.html
and https://omaxmicroscope.com/UserManuals/M93SBDK.pdf

although just the field iris part is available as a separate spare.

The Euromex iScope comes with the Kohler fitted but the diaphragm part is available as an accessory (spare?)
See data sheet:
https://www.euromex.com/en/products/pro ... es/iscope/

Re accessories: I only know what I've seen advertised. Maybe someone else on here has bought one? However, the 'Kohler' field iris on my Amscope T720Q appears to be the same field iris as on the Euromex iScope above. Without undoing it, I don't know exactly what lies beneath. I'm thinking maybe that scopes like the T720 that come with diffused LED are cheating a bit with their 'Kohler' designation. But, inasmuch as they allow focusing on the diaphragm via the condenser height adjustment on the field iris (as in my short youtube video above), they perhaps achieve the same outcome as a more complicated lens assembly and which may use a halogen bulb.
I'm no expert on the optics of Kohler but it's just my sense of things. I'm open to correction and others wisdom on this!

Louise

[GerryR]

....... If, as it seems, these diaphragms are added between field lens and condenser,
then they cannot be at an illumination focal plane unless extra lenses are also involved.

Your statement would indicate that on Kohler systems the diaphragm is between the light source and light source lens (field lens??). Am I interpreting this correctly? Or, are you saying that the source lens focal plane is designed to be at the location of the field diaphragm on a Kohler designed light system? Forgive my ignorance here, but just adding an iris diaphragm over any light source will not necessarily give Kohler lighting, if that is the case. Is this what you are referring to?

[blekenbleu]

Your statement would indicate that on Kohler systems the diaphragm is between the light source and light source lens (field lens??). Am I interpreting this correctly?
Or, are you saying that the source lens focal plane is designed to be at the location of the field diaphragm on a Kohler designed light system?

Much of what I think I understand about Köhler illumination comes from:
https://zeiss-campus.magnet.fsu.edu/art ... uniformity



.. where their Images Planes are field diaphragm, specimen, intermediate image, and retina.
Meanwhile, so-called Aperture Planes are lamp filament, condenser diaphragm, objective rear focal, and pupil.

Sadly, that diagram omits a field lens, which all my compound microscopes have,
even those (such as AO 10) which lack adjustable diaphragms.
Those with adjustable field diaphragms have them between collector and field lens.
It may be possible to construct a compound microscope with a combined collector and field lens,
which presumably requires a different condenser lens prescription,
for which case I have yet to sort where one could locate a field diaphragm that would be in the specimen plane,
while keeping the lamp filament in the objective rear focal plane.
I am fairly certain that, for best illumination uniformity, typical objectives want condenser diaphragm
and light source filament both in focus on their rear focal plane (which in my experience is difficult to consistently establish).

FWIW, diagrams at https://www.photometrics.com/learn/micr ... mination-2
are probably more correct, but somehow to me more confusing.

[Greg Howald]

I had an extra diaphragm/ filter holder from a condenser. It works well on two of my scopes. The illuminator lenses were such that centering was automatic. The diaphragm just pretty much fell in place. It works well.
Greg

[blekenbleu]

It works well.

That is what matters, and LED illumination (which need not provide for replacement)
allows things to be usefully simplified, much as AO did with frosted glass in some Series 10 illuminators.

[Chas]

Is this helpful? :
https://www.youtube.com/watch?v=60_jgZtyR6U

[LouiseScot]

As I comprehend it there's more than one schema for achieving essentially the same thing. The elimination of the bulb filament and substitution by LED + diffuser simplifies the components and light paths. I don't think a field lens is an absolute necessity but is maybe traditionally part of 'Rolls Royce' configurations. An Amscope isn't an Olympus, Zeiss, Nikon or whatever... This article is more comprehensive to me than graphic pics of all the different light planes but ymmv:

https://www.microscopemaster.com/kohler ... ation.html

Louise

[blekenbleu]

Is this helpful? :
https://www.youtube.com/watch?v=60_jgZtyR6U

It is good or at least consistent with my "understanding".
Years ago, I used a ground glass to find conjugate image planes
(3:00, 4:30 or 8:30 / 1:18:26) to set up camera extension tubes and helicoid in place of a microscope head.
That works well for monochrome light or highly corrected plan objectives.

Practically, viewing and focusing filament planes (10:45, 13:00, 14:45 / 1:18:26) is more problematic:

• Many condensers lack diaphragms.

• My Nikon condenser with a diaphragm has it too deeply recessed to view.

• I lack provisions for inserting ground glass at objectives' back focal plane.

• Filament image focus changes appreciably among nominally parfocal objectives.

What is wanted:

• video showing that filament moving in and out of focus, while the condenser diaphragm is in focus, without ground glass

In theory, one can hold ground glass over an eyepiece, with condenser diaphragm in focus,
then move the lamp or collector lens to focus the filament on that same ground glass.
Practically, I have as yet been unable to accomplish that.
Seemingly, some folks (but not I) can simply peer down an eye tube with ocular removed
and adjust filament focus without ground glass. Alternatively, one could try:

• mounting a camera without lens over the ocular,

• spacing it to bring the condenser diaphragm in focus on its sensor,

• then adjusting lamp filament to also be in focus.

Having to do that for each objective change,
then reconfiguring the camera for capturing wanted images is a big ask.

[viktor j nilsson]

Don't you have a phase telescope?

[blekenbleu]

Don't you have a phase telescope?

Not yet; my AO Bertrand insert helps not at all...
Oops, your question provoked me to disassemble and clean it;
that now works nicely for the AO 120; thanks!

July 5 update: a generic centering telescope works nearly as well in Optiphots.

[viktor j nilsson]

Don't you have a phase telescope?

Not yet; my AO Bertrand insert helps not at all.

That's odd, your Bertrand lens should let you see the objective back focal plane.

[blekenbleu]

That's odd, your Bertrand lens should let you see the objective back focal plane.

That's correct; I've edited the previous reply. Thanks!

[viktor j nilsson]

Side note. I find it kind of funny that many companies promote these add-on field diaphragms as a "Kohler device". The only thing that's relevant for achieving Kohler is whether the light source is in focus at the objective back focal plane. You really just need to remove an eyepiece and look down the tube to see if your microscope can achieve Kohler illumination or not. Even a LED die tends to have enough visible detail to be observed at the BFP. Adding a field diaphragm has zero effect on whether this is achieved or not.

And I seriously doubt that the location of these add-on field diaphragms is really conjugate with the specimen plane while the light source is focused at the objective back focal plane anyway, so it probably won't help you setup Kohler anyway. Could help a little with stray light, I guess.