Re: Kohler illumination question...

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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