Re: Kohler illumination question...

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."

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?

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?

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?

Aaron

On Wed, 27 Apr 2005 09:01:50 -0400, Andy Resnick
<andy.resnick@xxxxxxxxxxx> wrote:

>Aaron wrote:
>> I believe the problem originates with the design of the source lamp
>> itself. The ideal light source should be a flat surface emmitting
>> uniform amounts of light from every point.
>
>That's not really true. What matters is the coherence of the light
>coming off the source, not the shape of the source. But, I can see why
>you would say what you did- I suspect you tend to use critical
>illumination rather than Kohler (see below)
>
><snip>
>
>Without the
>> frosted diffuser, it becomes necessary to fuss with lighting for each
>> objective to eliminate the image of the coils in the field background.
>
>That's true if you are using critical illumination rather than Kohler.
>In any case, it's possible to perform image subtraction to eliminate the
>image of the filament.
>
>What is potentially interesting to investigate is that for Kohler
>illumination, the source is imaged at the pupil plane of the condenser
>lens, where phase contrast filters (and DIC prisms, etc) exist. I
>wonder what the effect is of non-uniform illumination of these filters.
> Probably not much. But maybe not....

.

Relevant Pages

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