Re: Kohler illumination question...

justbeats wrote: 
Given that the final step is to hide it behind frosted glass, why is it
necessary to get a focussed image of the lamp filament when setting up
for Kohler illumination?

I tried to answer this myself by taking images at different powers with
the filament defocussed to varying degrees. I couldn't detect any
difference in final image quality for any of the settings (as long as
the filament remains nicely centered of course).

After unsuccessfully Googling for the answer, I've another (maybe
related) question. Is critical illumination considered an old and
inferior technique, or a modern and superior one?

Well... the semi-obvious thing to do is remove the frosted glass. Truth is, the manufacturers put it (or a similar diffracting element) in the beam to pacify people who don't know how to properly align a scope. Honestly, or at least that's what my sales rep told me. So I make sure to remove it as a matter of practice.

Critical illumination, for all practical purposes, is functionally identical to Kohler illumination. Both maximize throughput, both maximize the "performance" of the illumination system, etc. etc. Some people like critical, most prefer Kohler. Critical illumination generally means one ends up examining a magnified image of the bulb filament. (Guess which I prefer?)

Hope this helps....

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

Relevant Pages

  • Re: Kohler illumination question...
    ... In one of the prior posts you said that "Kohler illumination ... 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. ... 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. ...
    (sci.techniques.microscopy)
  • Re: Kohler illumination question...
    ... necessary to get a focussed image of the lamp filament when setting up for Kohler illumination? ... the filament defocussed to varying degrees. ... required - a frosted glass helps a lot in hiding imperfections. ... shape and radiation uneveness of the light source itself. ...
    (sci.techniques.microscopy)
  • Re: Koehler illumination in conoscopic observation
    ... front of the lamp. ... transform of Kohler illumination: the filament is focused onto the sample plane rather than the entrance pupil of the condenser. ... 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, ...
    (sci.techniques.microscopy)
  • Re: =?windows-1252?Q?K=F6hler_illumination_question=2E=2E=2E?=
    ... If you look at the illumination design of larger research microscopes, especially those which have a relay for illumination then you find these devices implanted there. ... Rene wrote: ... also not in theory (when applied with field iris). ... iris, instead of the filament. ...
    (sci.techniques.microscopy)
  • Re: Kohler illumination question...
    ... If the geometry of the coiled filament were the problem, ... me think that in order to create a truly Koehler illumination, ... Why the situation might improve with a frosted glass, ... >I believe the problem originates with the design of the source lamp ...
    (sci.techniques.microscopy)