# Re: Entrance Pupil Microscope Objective, Now what about the condenser?

**From:** B. Gambier (*Parataxy_at_Nowhere.Com*)

**Date:** 06/11/04

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Date: Fri, 11 Jun 2004 10:20:53 -0400

"Andrew Resnick" wrote

B. Gambier wrote:

* > >> Where is the entrance pupil of a microscope objective
*

(metallographic)?

*> >>
*

*> >> More or less same question:
*

*> >>
*

*> >> Can such an objective be considered telecentric in object space?
*

*> >
*

*> > Objective telecentric in object space indeed, thanks for your answers.
*

*> >
*

*> >
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*> > Now what about the condenser?
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*> >
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*> > What are the consequences of this fact for the illumination?
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*> >
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*> > I mean for Koehler illumination.
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*> >
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*> > Must the condenser be somehow telecentric also?
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*> >
*

*> > I would think so, but how?
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*> >
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*> > And where could I find the schematic of such a condenser?
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*> >
*

*>
*

*> Hang on there a second: a condenser for a metallographic microscope?
*

*> Those use reflected light, correct?
*

Right, so the objective is the last component of the illumination system.

Perhaps not a true condenser in the conventional sense therefore.

However there is a beam splitter above the objective.

Between the objective and the so called tube lens.

Before the beam splitter there are additional optics and finally the light

source.

So perhaps the illumination system, again while not a true a condenser, has

to

fulfill the expected role of a condenser.

*> Here's my take on microscope optics- consider just the condenser, the (
*

*> 2D) sample, and the objective (transillumination).
*

*>
*

*> For either critical or Koehler illumination, both lenses are focused on
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*> the sample. In critical illumination, the source is imaged onto the
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*> sample by the condenser lens, while for Kohler, the Fourier transform of
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*> the source is imaged onto the sample by the condenser. That is, for
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*> Koehler illumination, the source is at minus infinity as far as the
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*> condenser lens is concerned. This is immaterial to the lens being
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*> telecentric or not.
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*>
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*> Infinity-corrected means something different than telecentricity (
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*> although I don't know specifically about the Mitutoyo lenses).
*

OK, telecentricity in object space appears to be a property of all

microscope objectives where as infinity correction refers specifically to

metallographic objectives.

*> Infinity
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*> corrected objectives put the image at plus infinity, but the exit pupil
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*> is located at or near the back surface of the lens (at least they are
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*> for Leica objectives). The entrance pupil should be located at the
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*> sample, as can be inferred by maximizing the light-gathering efficiency
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*> of the system. The front focal length of infinity corrected lenses
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*> seems, AFAIK, to be defined by a tautology: the magnification is defined
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*> in terms of a ficticious tube length, and so the front focal length is
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*> likewise defined in terms of this fiction.
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*>
*

*> Or am I missing something
*

I am a little lost here...

OK but the tube length is not really fictitious, as a tube lens becomes

necessary.

This tube length brings the image (originally at infinity) provided by the

objective to a focus where it can be observed.

So the tube length is replaced conceptually by a tube lens whose focal

length is generally more or less equal to the tube length.

At least that's my understanding.

*>
*

*> Andrew Resnick, Ph. D.
*

*> National Center for Microgravity Research
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*> NASA Glenn Research Center
*

Thanks

BG

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