Re: Kohler illumination question...

Aaron wrote:

Hello Andy,

In one of the prior posts you said that "Kohler illumination
essentially allows each luminous point of the surce to uniformly
illuminate the sample, while critical illumination uses a coherent
image of the source to illuminate the sample."

That's true to the extent that the physical conditions are met. Kohler illumination essentially puts the source at infinity.


In my Zeiss illumination sources, there are lenses that focus the
image of the filament prior to the field stop and prior to the
condenser.  Since the filament is not a flat surface some portions of
the filament and its reflection will not be at the same plane and
therefore will not be in the same focus further on.  Would you
comment, please?

Right- that's one of the conditions that's not exactly met. One can fudge around by stating that when the depth of field of those lenses are sufficiently large, the entire filament is in focus. But, that's a requirement levied against the lens designer. The easiest way to see if the illumination optics are well-designed is to set the optics for critical illumination and to examine the image of the filament with the objective lens to determine how completely focused the filament is.


The frosted element in my illuminator is 1/4" in front of the halogen
bulb. The light passing through the frosted element is focused
thereafter. Would you explain why the light coming from the filament
is different from the light coming from the frosted element just prior
to the focusing?

The spatial information is different, obviously. Putting the ground glass element just after the filament allows one to use the ground glass surface as a source rather than the plane of the filament- so in this case, critical illumination should have an image of the ground glass on the sample, while for Kohler, the ground glass surface should be imaged to the entrance pupil of the condenser lens. In any case, the ground glass is not a requirement to establish Kohler or ciritical illumination, as the O.P. was wondering.


I think there is some confusion about the role of the diffusing
element since some designs place it at the plane of the field
diaphragm/field stop just prior to the light entering the condenser
while others place it immediately infront of the lamp filament prior
to the light being focused.   In this discussion we have not
differentiated between these two approaches.  Would you comment,
please?

Well, there are several planes of interest: the source plane, the field stop, the aperture stop, the entrance pupil of the condenser, and the sample plane. In a well-designed system, the various planes have very well-defined relationships between them, much like the orthoscopic/conoscopic relationship. For Kohler illumination, the filament is imaged onto the aperture stop and entrance pupil plane. For critical illumination, the filament is imaged onto the field stop and sample plane. Additionally, those other planes are at Fourier transform planes- the aperture stop plane and entrance pupil plane are at a Fourier transform plane of the field stop and sample plane.

So, placing a ground glass screen at the field stop is equivalent to placing a ground glass screen at the sample plane, and critical illumination is indicated. Placing the screen near the filament itself will provide the use the ability to set up either critical or kohler illumination by using the ground glass screen as a source plane.

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

Relevant Pages

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