Re: Koehler illumination in conoscopic observation

Kevin Cunningham wrote:
On Nov 28, 3:34 pm, Andy Resnick <andy.resn...@xxxxxxxxxxx> wrote:

Kevin Cunningham wrote:

<snip>



A ground glass filter should be used, I'll bet there is a place for
one. Other wise I'd put it close to the lamp, if possible directly in
front of the lamp. Then, technically, your using critical
illumination, not Kohler but who cares?

I don't think that's correct- critical illumination is the Fourier
transform of Kohler illumination: the filament is focused onto the
sample plane rather than the entrance pupil of the condenser. And
actually- that's the better solution.

Using a ground glass plate is a kludge to destroy the image of the
filament in the back aperture of the objective lens; it may work, but it
isn't critical illumination.

--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University


Dr. Resnick, Good points! I've been having this discussion over a
few years with some buddies in Germany and Austria. I should have
known better. What NoSpam and I were discussing was whats commonly
called either critical or Kohler illumination.

In fact no one uses either exactly. The usual start point is a ground
glass assembly, take a look in the base of a modern microscope there
are between 1 and 3 ground glasses. This is because designers know
that if you use a specially designed lamp with a thick filament they
are a pain to get, expensive and in a few years very few will be
used. Its simpler to use ground glass and a lot cheaper.

For home use, I sort of agree with you- a diffuser allows a hobbyist to quickly and easily quasi-align the microscope for even illumination.

I think it's important to remember that Kohler/critical illumination is more than just simply focussing and centering the condenser field stop onto the sample- it means that all the aperture and field stops (condenser and objective) are at Fourier transform planes, and that the source is also at one of those planes. Which plane the source is at (field planes or pupil/aperture planes) determine if the microscope is "Kohler" or "critical".

In the lab, I remove the diffuser (Leica uses some wierd diffraction element as opposed to a ground glass plate). Our users don't use conoscopic viewing, so images of the filament are not really a problem. An alternative solution I use is to pipe the halogen light in with a liquid-core fiber.

--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
.