Re: Kohler illumination question...

Aaron:

If the geometry of the coiled filament were the problem, I do not understand
why Nikon's new scopes still use a frosted glass. The new Eclipse series
scopes use a diode array instead of a filament.

I think that one of the imperfections of all implementations of Koehler
illumination has something to do with the fact that the coherence length of
the incandescent light source is limited to around 30cm (l = c*t, assuming
the optical transition lasts for roughly 10^-9 sec). Since all forms of
interference in light microscopy require rays that are partially coherent,
the longer the coherence length the better Koehler will function. This makes
me think that in order to create a truly Koehler illumination, I would have
to use light generated by metastable optical transitions (see laser).

Why the situation might improve with a frosted glass, is unknown to me. I
think a frosted glass does not improve the situation at all. It actually
makes things worse regarding light interference. But since the illumination
looks more even (or pleasing to the user) it is generally accepted. I
believe that you are right in saying that the frosted glass is a quick and
dirty solution for providing even lighting. But it comes at a price of
further degrading resolution. But since the implementation of Koehler
illumination is already suffering under other, more severe limitations, the
loss in resolution may no longer be measurable.

Gregor

"Aaron" <nghy@xxxxxxxxxxx> wrote in message
news:ohrt61l8ko2nbg676u25ondboaqd6uecs1@xxxxxxxxxx
>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.
>
> A lamp with a coiled filament does not provide a uniform a flat
> emmitter of light. If it did, there would be no need for the frosted
> glass. Further, to avoid wasting light output, most systems employ a
> mirror to reflect the light coming from the rear of the filament
> forward adjacent to the direct image of the filament. Spacing between
> the coils and spacing between the direct and reflected images prevent
> truly flat even illumination. Under some circumstances the uneven
> light output shows up in the field background. The frosted glass is a
> quick and dirty solution to providing even lighting. 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.
>
> My Zeiss 100 watt halogen light source has a frosted plate that can be
> introduced easily in front of the lamp. I have tried adjusting the
> lighting with and without the froted plate and found no difference
> discernable to my eyes except the absence of the filament image. This
> is not to say that there may not be some theoretical advantage to be
> had without the frosted plate, but as a practical matter it does not
> seem to be important.
>
> Aaron
>
>
>
>
>
> On 26 Apr 2005 09:55:46 -0700, "justbeats" <steve_beats@xxxxxxxxxxx>
> 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?
>>
>> Thanks
>> Beats
>


.

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