Tech question on illumination

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GaryB
Posts: 521
Joined: Fri Jul 29, 2016 12:18 pm

Tech question on illumination

Post by GaryB »

I'm building a lamp for Köhler illumination for my 'scope and have some questions about lens setups.

I have 2 options:

1 a single movable lens to focus the filament on the condenser iris.
2 a fixed colimator and movable field lens.

Is there any reason to choose one over the other as both seem to accomplish the same task in a slightly different way. My scope is antique with a mirror so I don't have any restrictions on placement and don't have any preconceived notions on which is better. I've seen numerous examples of both types and my testing is really inconclusive as both work fine for my application. Simplicity leans towards the single lens, is colimation only necessary for illuminated bases where longer or more complex light paths are involved?

From my testing, I can colimate to a virtual infinity, but focusing is a separate matter, which is why I assume a field lens is used. The single lens setup just acts like a magnifying glass, making the filament bigger and focusing where you need it. Is that correct?

Light gathering isn't an issues so I don't need to worry about that, I'm just curious. I know what each does but cannot find an explanation of why to use one or the other, only explanation of how to use them. For DIY I like to understand why, not just how.

Pau
Site Admin
Posts: 6051
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Post by Pau »

The few models I've used move the collimator lens to focus. In microscopes with a field diaphragm at the base of course it is at a fixed position but I also have a Nikon external illuminator with a big focusing range of the multi element lens placed behind its front diaphragm.
I think this is the correct way, for Köhler you not only need to focus the filament at the condenser aperture but also that it was placed at conjugate planes with the objective rear aperture and eye pupil.

I any case I don't understand well what you mean with "2 a fixed colimator and movable field lens"
Pau

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