Field Stop

[microb]

For diagrams like this, what is the positioning of the field stop in relation to the focal length?

[MichaelG.]

I think, for diagrams like that, the field stop is dimensioned and positioned to not overfill the collimating lens.

It would probably be easier if you work backwards through the system.

MichaelG.

[BramHuntingNematodes]

what's the lens system doing for this setup? Just widening the collimated beam of gradient diffused light? I can't think that the erecting going on would be at all necessary since you should be able to rotate the quarter annulus. Does the field stop play a role in further collimating the beam after the diffusion?

[Greg Howald]

Michael makes a good point. Generally the field stop is placed at 1/2 the focal length so as not to overwhelm the culminating lens. The beam of light reaching the culminating lens should never be wider than the lens diameter.
Greg

[hans]

what is the positioning of the field stop in relation to the focal length?

Which focal length? If you are looking at the slight offset shown between the ray intersection and field stop, I was wondering about that too. Not sure how much to read into that diagram, though, since it looks like a fairly crude schematic.

[MichaelG.]

Looking at the Thorlabs text ... this statement is obviously relevant:

The rotating knob on the front controls an integrated field stop diaphragm, which can be used to adjust the illumination intensity and match the back aperture of a condenser.

... and supports my comment that: It would probably be easier if you work backwards through the system.

MichaelG.

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Reference, for the sake of courtesy :
https://www.thorlabs.com/newgrouppage9. ... p_id=10095

[hans]

Looking at the Thorlabs text ... this statement is obviously relevant:

I don't understand this part of the statement:

...which can be used to adjust the illumination intensity and match the back aperture of a condenser.

Since everything else is consistent with it being a field stop:

1. It is labeled "Field Stop."
2. It is shown near the focus of the collimating lens.
3. Diagram near the top of the page of condensers they sell indicates they are infinity-corrected.
4. The condensers they sell appear to be standard Nikon microscope condensers which already have the usual integrated aperture stop, so an additional one in the Dodt module would be redundant?

There is also some discussion of the Thorlabs Dodt module in this thread: viewtopic.php?f=28&t=10964

[microb]

what is the positioning of the field stop in relation to the focal length?

Which focal length? If you are looking at the slight offset shown between the ray intersection and field stop, I was wondering about that too. Not sure how much to read into that diagram, though, since it looks like a fairly crude schematic.

So the diagram shows the field stop near but not on the crossing of the rays (the depiction of a focal lengths fl1 and fl2 added together). But why is it not on that crossing of rays? I'm trying to use this as an example to understand epi design for aperture stop and field stop, but some designs elude me on figuring out conjugate planes.

[hans]

So the diagram shows the field stop near but not on the crossing of the rays (the depiction of a focal lengths fl1 and fl2 added together). But why is it not on that crossing of rays?

Yeah strange it is shown like that, especially since the usual Köhler setup procedure involves adjusting condenser height to focus the field stop in the specimen plane. If the drawing is accurate then I think it is necessary to conclude that rays between conjugate points in the specimen and field stop planes are actually not quite collimated going between the collimator and condenser in normal operation?

[MichaelG.]

There is also some discussion of the Thorlabs Dodt module in this thread: viewtopic.php?f=28&t=10964

Yes, I have been following that thread with interest ... which is why I knew where to find appropriate credit for the image.

On the technical point : I think the explanation probably lies with those double diffusers at the light source ...
If I understand correctly, this is not a conventional Dodt [as per its inventor] system, but Thorlabs’ own variant.

MichaelG.
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Edit: quoting from post #15 on the referenced thread:
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Thorlabs seems to use a different system, interestingly! At least based on what they show on the website, their system is no Dodt contrast. They replaced the two lenses with their engineered diffuser, if I understand it correctly. The engineered diffuser is not the Dodt diffuser but a second one before the annulus. After the annulus it seems they have a traditional diffuser.

[hans]

On the technical point : I think the explanation probably lies with those double diffusers at the light source ...
If I understand correctly, this is not a conventional Dodt [as per its inventor] system, but Thorlabs’ own variant.

Edit: quoting from post #15 on the referenced thread:
.

Thorlabs seems to use a different system, interestingly! At least based on what they show on the website, their system is no Dodt contrast. They replaced the two lenses with their engineered diffuser, if I understand it correctly. The engineered diffuser is not the Dodt diffuser but a second one before the annulus. After the annulus it seems they have a traditional diffuser.

I replied in #19, perhaps not very clearly. What I meant was, there are differences getting the light to the mask (including the engineered diffuser), but from the mask onward (including the second, standard diffuser) it looks equivalent? The Thorlabs diagram does not explicitly show whether the mask/diffuser are conjugate with the objective read focal plane, but since they are in a region where the field stop is shown collimated, I don't think there is any contradiction?

[MichaelG.]

The Thorlabs diagram does not explicitly show whether the mask/diffuser are conjugate with the objective read focal plane, but since they are in a region where the field stop is shown collimated, I don't think there is any contradiction?

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Perhaps we are at cross-purposes, Hans ... in which case, I apologise: But it seems to me that Field Stop in a diffuse ‘beam’ is almost a contradiction in terms.

MichaelG.

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Edit: This, from the Scientifica text may be significant:

Additionally, if your microscope has an additional diffuser between the illumination port and the condenser this must be removed, as it will dilute and disperse the contrast output provided by the spatial filter.

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... or I may be barking up completely the wrong tree !

[hans]

The Thorlabs diagram does not explicitly show whether the mask/diffuser are conjugate with the objective read focal plane, but since they are in a region where the field stop is shown collimated, I don't think there is any contradiction?

Perhaps we are at cross-purposes, Hans ... in which case, I apologise: But it seems to me that Field Stop in a diffuse ‘beam’ is almost a contradiction in terms.

There is no diffuser after the field stop so it can still be imaged sharply through the collimator and condenser, conjugate with the specimen, as usual. I believe that is what is shown in the Thorlabs diagram (ignoring the confusing blurb in the description you pointed out) and was speculating in the other thread that could be the reason to locate the Dodt mask and diffuser earlier, between an extra pair of relay lenses, rather than putting them in the condenser rear focal plane where other types of illumination masks are commonly placed:

But I don't understand the significance of the mask being earlier in the optical path if it is still conjugate with the condenser/objective apertures...

Thinking about it a bit more, if the mask/diffuser were placed just below the condenser as is more common the diffuser would also be blurring the image of the field stop. So perhaps the motivation to add the additional pair of relay lenses and place it earlier is to avoid interfering with the imaging of the field stop?

[apochronaut]

Simplified schematic? Probably understated.
It is the ground glass diffuser that is responsible for widening the beam sufficiently enough to fill the optical pathway to the maximum aperture of the focusing lens used. The beam shows considerable shading off from one side of the beam to the other due to the effect of the diffuser. No doubt the design, like any design, was part engineering and part intuition and had to be tested, since the singlet focusing lens used would issue a confused bundle of rays, despite attempts to diffuse everything at source. The effect of the aperture on the gradient would be difficult to determine without a fair amount of experimentation.
They would have had to choose the best case location for the field aperture. It is hard to tell from the schematic, just where they chose to depict it in the diagram but it looks like they chose the theoretical focal point, probably that of green light, for the aperture and the crossing point of the rays might depict the circle of least confusion?
Presumably, the collimating lens does a fair bit of correcting for the final assault on the object plane but at the field aperture, I would guess there is a fair amount of chaos.

[hans]

More on the interpretation of the Thorlabs diagram back in the other thread: #33