Re: Köhler illumination question...

Rene,
I did not write that this is the right way to do, but it is done and it helps - that is the reason why I wrote "improving" and not improving....
The problem with the frosted glass in the aperture plane is that it needs to be optically pretty thin to avoid significant losses of brightness. If you look at the illumination design of larger research microscopes, especially those which have a relay for illumination (for Phasecontrast etc) then you find these devices implanted there.
rgds George


Rene wrote:
Hello to all, 

The Koehler vs critical discussion crops up some now and then and is
always good for some excitement. I'm surprised however by the high
level of microscopists involved in this discussion though!
In my own experience I have not seen any differences between critical
vs Koehler, also not in theory (when applied with field iris). I think
‘large source Koehler' would be the best system for microscopy, it is
based on the projection of the diffusing screen onto the condenser
iris, instead of the filament. Needless to say this is far more
homogenous, and makes far less critical demands on the whole of the
illumination train. My own setup is slightly different, with an
opaline glass that can be inserted just behind the lamp. I believe
this to be in the focal distance of the collector lens, as with slight
condenser adjustment I can see the structure in the image.
Interestingly, it is not entirely the same spot as in BF, but when the
system is used with circular oblique light above NA 1.0, the field
iris is sharp together with the structure of the diffusing screen.
Besides of this detail, the field iris behaves perfectly normal in
this setup, with closure of the field iris resulting in cut-off of
straylight, as is clearly visible when looking in the tube without
eyepiece.

George's remark however is a bit off:

A last but not least reason is that the frosted glass increases the 

nA

of the illumination which helps "improving" the imaging performance 

of

the microscope. 


Out of practicality, most illuminator systems I've seen do project the
filament too small on the condenser iris, thereby not able to fill the
highest apertures (1.3-1.4) fully. Surely a diffuser will help, but it
is not really in this respect the right tool for improvement.

Now the Nelson way of illumination is slightly different as well, it
assumes illumination by a lamp wick or similar. Even though the
illumination train is the same, several people have described this as
a superior illumination when details are checked against the edge of
the flame, probably due to interference mechanisms, see eg Michael
Bingley, Quekett J Micr, 1984, 480-482. He mentions the visibility of
the lamp filament with Koehler in the back focal plane of the
objective and regards this as a serious optical compromise, while the
visibility of the filament in the image as with Nelsonian is
optically, hardly more then an aesthetic problem.

Rene. 
.

Relevant Pages

  • =?ISO-8859-1?Q?K=F6hler_illumination_question...?=
    ... The Koehler vs critical discussion crops up some now and then and is ... level of microscopists involved in this discussion though! ... also not in theory (when applied with field iris). ... >of the illumination which helps "improving" the imaging performance ...
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  • Re: Kohler illumination question...
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