Re: Question on Koehler Illumination
From: David Littlewood (david_at_nospam.demon.co.uk)
Date: 09/28/04
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Date: Tue, 28 Sep 2004 14:52:11 +0100
In article <3dc1da98.0409280145.4fe550fe@posting.google.com>, Rainer
<wilhelm.rainer@gmx.net> writes
>David Littlewood <david@nospam.demon.co.uk> wrote in message
>news:<Xb3$4LCA9$VBFwEr@dlittlewood.demon.co.uk>...
>> In article <3dc1da98.0409270048.524743bb@posting.google.com>, Rainer
>> <wilhelm.rainer@gmx.net> writes
>> >I have a question related to Koehler illumination:
>> >As far as I understood, Koehler illumination means imaging the light
>> >source to the back focal plane of the microscope's objective. In case
>> >of a point-like light source, this would that the illuminating light
>> >would leave the objective as a collimated beam (plane wavefront; since
>> >the Fourier transform of a "point" source is a plane wave). My problem
>> >begins here: When a manufacturer offers a microscope with a set of
>> >different objectives (with different magnifications), mounted on a
>> >rotating revolver, does he guarantee that for each individual
>> >objective, the light source will be imaged to the back focal plane of
>> >that objective? If yes, how is this achieved?
>> >The background of my question: I have the information on the locations
>> >of the back focal planes in objectives of different magnifications
>> >(e.g. 10x, 20x) from the *same* manufacturer (e.g. NIKON). The
>> >distance between the objective's flange (where it is screwed to the
>> >revolver) and the back focal plane changes with different objectives.
>> >So, how can they guarantee a correct Koehler illumination? Or is there
>> >a large tolerance in such an imaging setup?
>> >
>> >Any helpful comments are appreciated!!
>> >Rainer
>>
>> Surely the answer is that it doesn't matter.
>>
>> The essence of Kohler illumination is that the illuminating rays are
>> focussed by the lamp condenser on the aperture diaphragm at the front
>> focal plane of the condenser. The condenser then converts this into a
>> parallel beam of light which passes through the specimen.
>>
>> Since the light passing through the specimen is parallel, then it is
>> automatically brought to focus at the back focal plane of the objective.
>> If you change the objective for one with its back focal plane in a
>> different place, the light (i.e. the image of the lamp filament) will
>> still focus there.
>>
>> In practice, there will be some slight variations. This is taken care of
>> by the action (which you should always take on changing objective) of
>> adjusting the focus of the condenser so that you can see a sharp image
>> of the field (lamp) diaphragm. This will have the effect of ensuring any
>> minor differences due to the change of objective are taken into account.
>>
>> David
>
>Hi David,
>I understand your answer, but my question refers to a microscope where
>the specimen is illuminated "from above", i.e. the illumination source
>is located somewhere above the objective, and with an appropriate
>optics, we image the source into the back focal plane of the
>objective. Then, the light leaves the objective as a parallel beam and
>hits the specimen ("reflected mode"). In fact, this illumination also
>works for non-transparent specimen.
>If I understood you correctly, you are talking about an illumination
>where the light traverses the ((semi-)transparent) specimen from
>below, and then propagates to the objective. It is clear that in such
>a case, the position of the back focal plane of the objective does not
>matter.
>For me, the question remains, what to do in the "reflected mode" case?
>Thanks in advance for any comments
>Rainer
I think your reference to "a parallel beam of light which passes through
the specimen" made this rather hard to guess!
The only significant experience I have with epi-illumination (incident
illumination) is using Zeiss Epiplan HD lenses. These appear to be made
with a common back focal plane position; the instructions for the
Ultraphot include the paragraph "close the luminous field diaphragm
until its margin appears in the field of view. It is sharp without
taking any special measures." There is no provision for adjusting the
relative positions of the lamp and the objectives, and of course the
lens acts as its own condenser.
I cannot speak for other makes, but no doubt one of the several
contributors here with broader experience will be able to help.
David
-- David Littlewood
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