Re: Kohler illumination question...

Aaron schrieb/wrote:

> The frosted element in my illuminator is 1/4" in front of the halogen
> bulb. The light passing through the frosted element is focused
> thereafter. Would you explain why the light coming from the filament
> is different from the light coming from the frosted element just prior
> to the focusing?
>
> I think there is some confusion about the role of the diffusing
> element since some designs place it at the plane of the field
> diaphragm/field stop just prior to the light entering the condenser
> while others place it immediately infront of the lamp filament prior
> to the light being focused. In this discussion we have not
> differentiated between these two approaches. Would you comment,
> please?

Hello Aaron!

reading your qestions to Andy Resnick and earlier contributions in
this thread, it seems to me, the term "frosted" does not take into
account properly, that the first lens-surface of some Zeiss
lamp-collectors ist not ground. Its mat surface is achieved by
hydrofluoric acid, which leaves thousands of little bowl-like concave
hollows in the lens-surface. They are of different diameters, but all
of the same focus-width - tiny concave lenses on the surface of a
concave lens.

The result is, that light-rays are not being scattered in all
directions, as would be the result of a simple ground glass, but the
light beams entering the first collector lens, are just "widened" and
defocussed, changing the image of the lamp filament into a homogeneous
image, mixed together out of thousands of little lenses. This design
does not fully destroy the definition of the field diaphragm.

Frosted in that special way the lighting force is reduced about
20-40%; that is not much, compared with reduction of 60-80% through a
normal ground glass.

There were, to my understanding, some rather different reasons, which
lead the design team at the plant of Carl Zeiss (Winkel) in Goettingen
1949 to this interesting and well working solution for a
Koehler-illumination.

Transatlantic greetings from Bavaria!
Klaus Henkel
.

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