MicrobeHunter.com Microscopy Forum

Hello,

Maybe this is a stupid question, but is there any difference between epi Kölher in a finite and infinity corrected system ?

Isn't the light beam supposed to present to the infinity-corrected objective as parallel rays?
But in this case the light source image can't form at the objective back focal plane right?

Or is it supposed to work in exactly the same way as with finite objectives?

Thanks.

The condenser has to focus the diffuse Köhler beam on the object, and at a working distance dependent distance, irregardless of whether the objective is infinity or finity. Interestingly, there are infinity condensers , which were used mostly in older microscopes. When the sun or a remote illuminator is used as a light source, the front condenser focus is at a much greater distance than a condenser focusing on an led source right under the stage for instance. Older condensers tend to be infinity corrected and modern ones most likely finite unless they were designed for an unusually large microscope with an illuminator in the back 40.

Thank you.

But in the case of epi illumination, the objective is the condenser right?
It is the same than an infinity condenser?
That's why I was wondering if parallel beams were needed in this specific case.

onilink_ wrote: ↑

Sat Mar 09, 2024 6:53 pm

Thank you.

But in the case of epi illumination, the objective is the condenser right?
It is the same than an infinity condenser?
That's why I was wondering if parallel beams were needed in this specific case.

Wild guess: Kohler is needed in the epi- and infinity optics as well, since the illumination path is not necessarily an infinity space. In contrast to the observation path (objective-telan- ocular). The illuminating filament (or other light source) is not placed at infinity, is not very far away from the objective (which serves as condenser, as you mention).

Thanks for the precision.

I'm not really sure why I have more troubles to understand Köhler in epi illumination, but I'll have to play with a real world application to understand I guess

This kind of modular epi illumination seems pretty cool: https://www.thorlabs.com/newgrouppage9. ... up_id=8565

I'll try to reproduce something like this with generic lenses.
I found a bunch of good quality FL=50mm and FL=270mm lenses. I assume I can build a system like this by combining them with various distances to get different focal lengths than just 50mm and 270mm.

Maybe someone here played with this kind of setup?

Also, I took a look at how my Olympus BH2-MA was made and it seems they added some diffuser inside of the illumination tube. Is this common practice? But why it is needed if they use some kind of Köhler illumination?

onilink_ wrote: ↑

Sat Mar 09, 2024 11:20 pm

Also, I took a look at how my Olympus BH2-MA was made and it seems they added some diffuser inside of the illumination tube. Is this common practice? But why it is needed if they use some kind of Köhler illumination?

In the old Zeiss Standard illumination collector, next to the light bulb, a mild diffuser lens is fitted as well.
Maybe because of non-ideal optic and light components, some diffusion was needed in addition to the alignment of conjugate planes.

I think also due to the not perfect nature of filaments. Some might wonder why certain scientific bulbs are so expensive. Some of that expense is due to the effort put into matching the filament coil to the illuminator optic. The illuminator optic is trying to convert that filament image which is of varying convolutions but essentially linear into a flat circular image. Having diffusion as well as lensing can only help get that blank flat background that is required.

Oh I see, thanks!

Illuminations techniques are really an interesting subject.