Some questions regarding special features

[joe307]

Hi,

Reading about how the darkfield feature works, at one point, I thought that in order to use this feature, one has only to make changes to the condenser. But then, I've stumbled upon http://www.microscopenet.com/100x125-16 ... -9423.html.
Do I have to use a darkfield objective in darkfield microscopy?(meaning: Is it necessary?)

I've read that Kohler illumination was obtained by putting a collector lens and a stop in the condenser(the light from the lamp being focused at the aperture of the condenser). Supposedly, http://www.microscopenet.com/microscope ... 10160.html would come attached on top of the condenser. With this device attached, does the condenser lens become the collector lens? How does this type of "Kohler illumination" fare compared to proper Kohler illumination?

Thank you in advance,

Joe

[The QCC]

Joe:
Dark Field lighting uses the same condenser as Bright Field. The microscope must have a swing out filter holder immediately below the condenser. A Dark Field stop is placed in the filter holdr.
Please read these two articles on the MicrobeHunter web site.
Differences in Dark and Bright field lighting
DarkField microscopy

Kohler Illumination requires a light source to have a field diaphragm or aperture. The aperture and lens are part of the light source, not the condenser.
The Microscopenet add-on sits on top of the light source and is not connected to the condenser.

Kohler illumination creates uniform lighting across the objective. It is a relatively simple operation to achieve Kohler Illumination for obj. greater than 5x.
The microscope is focused on the specimen.
Then the condenser is focused on the light sources aperture leaves.
The procedure must be repeated every time the objective is changed.

[Crater Eddie]

So, would it really be as simple as just adding the above referenced iris assembly to one's light source to convert to Kohler illumination? (Assuming of course that one has a centerable lamp and the iris assembly would fit) Too bad they don't provide a link to the installation instructions. I wonder if this would fit the LOMO Biolam? I'll have to do some measuring.

[The QCC]

Crater Eddie:

The mounting diameter is 49mm. If you look at one of the compatible microscopes on MicroscopeNet, you can see the light source collector is raised. The Kohler attachment fits over the raised portion and is held in place by an Allen screw.
I think you can assume your light source is already centered. The attachment has centering adjustments.

[Crater Eddie]

Thanks, yes I saw that measurement. I did look at the photos before I posted earlier, they look similar. What I meant was that I need to measure the corresponding area on my own scope to see if it is close enough for it to fit. It would be worth a little tool work if this could be made to fit and work. The lamp on the Biolam is adjustable / centerable, so no worries there.

[microgreen]

Eddie

You can't achieve true Kohler if you have a diffuser in the light path

Kohler requires you to see an image of the light source (the lamps filament) at the specimen plane,.If your lamp has a diffuser fitted that becomes a secondary light source that is focused on the specimen plane not the lamp flament.
My view is - if the back plane of the objective is filled with an even light be it Kohler, Nelson or Critical lighting - it.s OK

Tony
East Riding of Yorkshire(UK)

[The QCC]

It is not necessary to see the lamp filament to achieve Kohler Illumination.
LED microscopes do not have filaments and can achieve Kohler Illumination.
This five step procedure works for all microscopes with a field diaphragm.
This is the procedure I use on my Labomed LB-592. It works.

[gekko]

For what it may be worth, there was a rather long, earlier thread that also discussed the subject of Koehler illumination:
viewtopic.php?f=5&t=90

[Crater Eddie]

Thanks to all for the comments, gives me things to think about.
My apologies to Joe for hijacking the thread.

[microgreen]

The point I am making is that if your microscope has a DIFFUSED field lens you cannot possibly obtain TRUE Kohler and it is a waste of time trying
Most modern 'scopes seem to have these fitted and more often than not without a field iris.

I say again - If your lighting system provides for an even light at the objectives back focal plane is that not OK? - is there anything wrong with that? I don't think so
Like all things, time moves on and we are a long way from using candles and oil lamps for illumination
New budding microscopists must feel they have to have Kohler illumination be able to see anything if their 'scope is not equipped with Kohler. A small electric torch can provide suitable lighting to see the microscopical wonders of this world.


Rant over

Tony
East Riding of Yorkshire (UK)

[75RR]

I say again - If your lighting system provides for an even light at the objectives back focal plane is that not OK? - is there anything wrong with that? I don't think so
Like all things, time moves on and we are a long way from using candles and oil lamps for illumination

Curiously candles and oil lamps provided even illumination, just not very much of it.

The advent of electric bulbs provided more illumination, only it was not even. The lamp filament saw to that.
What Köhler was doing and now bears his name, was to try to achieve what was commonplace with candles and oil lamps: even illumination.

So when you manage to achieve even illumination, no matter how, you have arrived. No need to chase will o' the wisps.

[gekko]

I say again - If your lighting system provides for an even light at the objectives back focal plane is that not OK? - is there anything wrong with that? I don't think so
Like all things, time moves on and we are a long way from using candles and oil lamps for illumination

Curiously candles and oil lamps provided even illumination, just not very much of it.

The advent of electric bulbs provided more illumination, only it was not even. The lamp filament saw to that.
What Köhler was doing and now bears his name, was to try to achieve what was commonplace with candles and oil lamps: even illumination.

So when you manage to achieve even illumination, no matter how, you have arrived. No need to chase will o' the wisps.

I agree with both Tony and 75RR, and would emphasize Tony's point that you quoted above, that the back focal plane of the objective must be filled with light, in order to realize the resolution that the objective is capable of. For what it is worth, if I wanted to improve the image quality of my microscope (given any kind of built-in illumination), I think I would start by upgrading one or more objectives.

[Crater Eddie]

Agreed.

[Oliver]

One aspect that has not been mentioned in this thread is that Koehler illumination also allows you to limit the illuminated area on the specimen to the area which is actually observed. this is done by closing the field diaphragm. There are two advantages: a. this reduces the heting of the specimen (halogen lamps) and b. it reduces internal reflections of light and therefore increases contrast, which is more noticable for higher magnifications.
Oliver.

[75RR]

One aspect that has not been mentioned in this thread is that Koehler illumination also allows you to limit the illuminated area on the specimen to the area which is actually observed. this is done by closing the field diaphragm.

Was a field diaphragm incorporated in microscopes in order to be able to achieve Köhler, or is it an independent yet useful addition that allows one to improve Köhler?

[joe307]

Yeah, it would probably have been a good idea to reserve philosophical discussions to another thread.

I admire all of your imagination but for the sake of not getting stucked in arguments, I was hoping for someone to answer my questions in relation to the knowledge aquired by experts(e. g. http://micro.magnet.fsu.edu/primer/anatomy/kohler.html).
Proper use of terminology would be a good bonus, too.

@The QCC
Check http://www.microscopenet.com/brighter-d ... -9426.html
Chatting with microscopenet.com, I found out that I would need to change(on most microscopes) the condenser when making the change from brightfield to darkfield(there is no microscope on microscopenet.com that has a condenser which accomadates all of the phase contrast, brightfield and kohler illumination features). This is not the case for Meiji or Zeiss, for instance.

I'll post my questions again.

Reading about how the darkfield feature works, at one point, I thought that in order to use this feature, one has only to make changes to the condenser. But then, I've stumbled upon http://www.microscopenet.com/100x125-16 ... -9423.html.
Do I have to use a darkfield objective in darkfield microscopy?(meaning: Is it necessary?)

I've read that Kohler illumination was obtained by putting a collector lens and a stop in the condenser(the light from the lamp being focused at the aperture of the condenser). Supposedly, http://www.microscopenet.com/microscope ... 10160.html would come attached on top of the condenser. With this device attached, does the condenser lens become the collector lens? How does this type of "Kohler illumination" fare compared to proper Kohler illumination?

[The QCC]

Joe:

As stated earlier, Darkfield illumination does NOT require a special condenser, just a filter holder below the condenser.

Kohler illumination is only achieved by having a field aperture in the light source.
The Microscopenet attachment fits over the light source and has an aperture in it. Thus giving the light source a field aperture.

the light from the lamp being focused at the aperture of the condenser

To achieve Kohler illumination, the CONDENSER is focused on the leaves of the FIELD aperture.

With this device attached, does the condenser lens become the collector lens?

The Microscopenet attachment has no connection or relation to the microscope condenser. The lens in the attachment is a collector lens for the light source.

How does the attachment compare to a built in field aperture? Not having the attachment I can only guess it would be similar.

Please note, Kohler illumination is intended to give uniform lighting across the objective,
If your microscope has LED lighting, the light is already quite uniform and Kohler illumination may well be redundant.

I have a Labomed microscope with a condenser slot that accepts a phase contrast and darkfield slider.
This is the link to Labomed CxL microscope
If you want options, the CxL has lots and lots of options.

The Labomed CxL Monocular LED sells for approx. $700.00CDN

[75RR]

Yeah, it would probably have been a good idea to reserve philosophical discussions to another thread.

Sorry joe307,
We had all taken your question as answered. Did not mean to hijack your thread.