Reflected light Kohler Illumination

[urbanpiper3]

I am unable to find any definite guide or reference for how to setup a kohler illumination design for a reflected light microscope. I am trying to build a reflected light microscope with off-the-shelf optics from Edmund Optics but have no idea where to look for a reference design to build a Kohler reflected/epi-illumination.

Any guidance will be appreciated.

Thanks!

[BramHuntingNematodes]

I don't remember Kohler mentioning epi illumination in his article. You're using the objective as a condenser, so I assume the most important part is getting a field lens after your light source with some adjustability.

[blekenbleu]

Elements of Kohler illumination:

• a field iris in focus that blocks light outside the image field of view

• a collector iris in focus at the objective back focal plane to block light beyond its field

• an undiffused illumination source also focused at the objective back plane

Reflected light (episcopic) microscopy can be bright or dark field.
An episcopic (AKA vertical) illuminator can have a field iris and a second iris corresponding to a condenser iris.

from https://www.zeiss.com/microscopy/us/sol ... train.html
Bright field episcopic illumination can use the objective as a condenser.
Literally reflected light would be specular, e.g. from mirrored surfaces.
At least in theory, by focusing on a first surface mirror,
one could e.g. focus the filament of an incandescent illumination source on an objective's back plane.
Practically, much episcopic microscopy images scattered light,
avoiding specular reflections and employing diffuse illumination:

reflected.jpg (37.76 KiB) Viewed 2048 times

from https://www.microscopyu.com/techniques/ ... lumination
...at most adjusting only those 2 irises.

More/better reference:
http://www.microscopist.co.uk/wp-conten ... ond-MA.pdf

[BramHuntingNematodes]

The field lens is the element that distinguishes Kohler from critical illumination in practice. The irises are useful but secondary.

[urbanpiper3]

Thanks to all for your comments. Having trouble finding a existing model of epi-illumination that can be built on workbench with off the shelf optics.

Question(s):
1. The aperture stop should be conjugate to the objective BFP ? In most cases, especially with Chinese manufacturers this information is absent. In this case how would one experimentally find out the approximate BFP of a given objective?
2. The field stop is technically conjugate to object and image plane. Therefore, in a infinite conjugate microscope, the tube lens relays the image formed at intermediate image plane by the objective lens to the camera. So distance(field stop → object) == distance(field stop → intermediate image plane)?

I found this: WFA2001 Ready-to-Use Epi-Illuminator Module https://www.thorlabs.com/newgrouppage9. ... pn=WFA2001 as a starting design model

[blekenbleu]

1. The aperture stop should be conjugate to the objective BFP ?
In most cases, especially with Chinese manufacturers this information is absent.
In this case how would one experimentally find out the approximate BFP of a given objective?

I use a centering telescope or Bertrand lens to bring the objective's stop into focus.

2. The field stop is technically conjugate to object and image plane. Therefore, in a infinite conjugate microscope,
the tube lens relays the image formed at intermediate image plane by the objective lens to the camera.
So distance(field stop → object) == distance(field stop → intermediate image plane)?

In my (likely typical) Reichert EpiStar infinity microscope,
episcopic illumination is introduced below the (image) tube lens.
Its illumination optics may have a focal length different from that image tube lens,
but its field stop should be conjugate to the object plane.
I focus an eyepiece and objective on something bright and flat (e.g. a mark on paper),
then close down the field stop enough to see it in the image field.
If not sharp, then the stop and/or illumination optics would need moving
to bring that field stop also into sharp focus on the paper.
Similarly for the condenser aperture stop and objective stop.

[urbanpiper3]

The field lens is the element that distinguishes Kohler from critical illumination in practice. The irises are useful but secondary.

Thanks for your comment. I agree that the field lens is the element that distinguishes Kohler from a critical illumination (nelsonian). But what exactly does the field lens provide ? Because it is conjugate to the object and image plane, I am unable to understand the rationale behind having a field lens and field stop.

[BramHuntingNematodes]

Your field lens will collect light coming off the filament and direct it in more or less a straight line to your condenser. The field iris or stop can prevent some glare but also provides a centered surface to focus on when adjusting the condenser to make sure the filament is perfectly unfocused. In practice you can use your forceps or a wire if you don't have an iris.