Why isn't phase and darkfield done in the back focal plane of the objective?

[farnsy]

I was rewatching Peter Evennett's delightful lecture in which he does Abbe's experiments (https://www.youtube.com/watch?v=60_jgZtyR6U).

When he discusses darkfield at 51:41, instead of using a patch stop in the condenser, he uses little film with a tiny dot in the back focal plane of the objective that blocks out the 0th order light. Again, when he discusses phase contrast at 59:40, he does the same thing: he has a little film with a dot that retards the 0th order light. In both cases, he mentions that the way we do these things in practice is to modify the light in the condenser, and in the phase contrast case, to also use specialized objectives.

I wonder why the approach he uses at first not preferred in practical microscopy. In particular, with a phase contrast, it seems that a single film inserted at the back focal plane of the objective would work, rather than having a special phase condenser and objective for this purpose. If we did phase contrast in the back focal plane of the objective, we would be able to use phase contrast with super expensive apochromatic objectives or other really expensive objectives without having to swap them out for specialized phase contrast objectives, right?

I'm sure there is a reason it's not done that way, but I'm not sure what it is. Are there significant technical advantages to the approach current manufacturers use for phase and darkfield microscopy?

[apochronaut]

With phase, I'm not sure how you can get around the importance of the substage diaphragm and condenser. The substage diaphragm splits the wavefront into an undeviated wavefront and a deviated wavefront and the condenser focuses them at the object plane. Whatever is put at the rear focal plane is secondary to that but designed to recombine those two wavefronts slightly out of phase, the degree of which determines the effect. That wave front interference, need not take place at the rear focal plane.
Zernike's original design did not use a rotary carousel, or phase annuli in the objective. It was unlike what we have come to know as a phase contrast system, more as a proof of concept. The phase annuli seemed to be up near the eyepiece, probably at the intermediate image plane, although I don't know for sure. His original microscope had a rotating contraption up there.
Zeiss worked the concept through and shortly after the war, AO patented a rotary carrousel, containing alignable diaphragms.
Irregardless, there always seems to be an understage component to split the light into diffracted and undeviated wavefronts.
There have been a number of different phase contrast type systems. The AO Polanret used a combination of continuously variable frequency and amplitude shift. Reichert took the annuli out of the objective altogether and put them at the intermediate image plane in the body of the microscope, so any objective could be used. Hoffman Modulation used slits instead of circles, Baker used a cross.

In DF, you have to get the light to pass outside of the objective aperture and undergo tir in the slide itself. That is why DF condensers will work only with slides of a specified thickness.
I have difficulty seeing how that could exactly be accomplished above the object plane but manipulation of the image could be accomplished.

[TiggerScope]

Some microscopes do work in phase contrast with brightfield objectives by creating a plane inside the microscope which is conjugate with, or you might say a replica of, the aperture, BFP or pupil plane. One example is the Zeiss Axiomat where conjugate phase contrast was an option, allowing, as you say, brightfield planapos. Zeiss call it the intermediate pupil image but it's the same thing as the conjugate aperture plane.

I attach the Axiomat pdf on this. It's worth a look.

Other (40-50 year old) microscopes which do this are the Reichert Univar and Nikon's Interference Phase Contrast mounted on an S-type. I believe also the Zeiss Jena Amplival Interphaco. There may be others but nobody does it today as they are all too far too attached to the money they make selling the "unnecessary" phase contrast objectives. In any event the condenser annulus is always needed to preserve the diffrated rays in the formation of the image.

Most of the time, the back focal plane is located not at the back of the objective, but inside it at some inaccessible place, so conjugate is the only way to go if one needs to access the BFP. Providing a conjugate plane where the microscope has none, is not something I want to attempt.

The only specialised objectives for darkfield (usually but not always big-NA oil immersion) have a built-in iris for the case when the NA is bigger than the biggest possible illuminating cone. It then becomes necessary to reduce the objective's NA with the iris. In fact if the objective's NA is more tan about 1.2, DF is impossible, and that is why Leitz used to make 90x/1.15 oil objectives in the Heine system, for compatibility with DF. For all dry objectives there should be no problem in providing a big enough DF illuminating cone, e.g. a 0.7-0.85 DF cone for a 40x/0.65.

[Dubious]

...
I attach the Axiomat pdf on this. It's worth a look.

Other (40-50 year old) microscopes which do this are the Reichert Univar and Nikon's Interference Phase Contrast mounted on an S-type. I believe also the Zeiss Jena Amplival Interphaco. There may be others but nobody does it today as they are all too far too attached to the money they make selling the "unnecessary" phase contrast objectives. In any event the condenser annulus is always needed to preserve the diffrated rays in the formation of the image.
...

This is interesting--I had no idea there were microscopes that provided phase contrast without a phase objective! But, has anyone compared the results, to see if for some reason having the phase plate at the objective's rear focal plane produces a superior image? While I'm sure there is more profit to be made in selling dedicated phase objectives, still, it's hard to see them taking over the entire industry so rapidly unless they also provided some optical advantage.

[apochronaut]

When Reichert developed their infinity corrected 45mm parfocal system in the late 60's, which resulted in the Univar, they opted for using universal objectives. All objectives over 10X could be used for all contrast systems, of which they claimed there were 17. All objectives over .75 N.A. had an iris diaphragm. Phase annuli were in the intermediate image plane.
http://www.science-info.net/docs/reichert/A4/ 34th manual on the list, pg. 15/16. The concept was continued in some of the Poly microscopes such as the Polyvar.

Later, they did produce phase objectives for the Diavar 2, probably the Polylite and maybe others.

At the same time AO( Reichert's parent company) was pursueing a different path of liberating the phase annulus from the objective. They produced the Polanret, which allowed the use of standard objectives. Their system did not sell well, likely due to the emergence of DIC and the fact that adding a Polanret to an existing microscope was difficult.
https://opg.optica.org/josa/abstract.cf ... a-37-9-726

https://www.semanticscholar.org/paper/S ... 5a41ccb112
Max Pluta (PZO) fiddled with it too but I don't know if PZO did any work on that. They were winding down by the time that paper was written.

Note that in all such systems you still need to have a compatible substage diaphragm and condenser.

[Dubious]

...
https://www.semanticscholar.org/paper/S ... 5a41ccb112
Max Pluta (PZO) fiddled with it too but I don't know if PZO did any work on that. They were winding down by the time that paper was written.
...

Really too bad no one continued with this idea. The AO Polanret was apparently quite expensive, but per the abstract Apochronaut linked above, the PZO variant on the idea was potentially a lot cheaper (and better):

"An extremely simple, modified polanret phase‐contrast system has been developed. It uses only a single polarizing ring, instead of three zonal polarizers as required in Osterberg's original polanret system (manufactured by the American Optical Corporation). The modified system is much more easily manufactured and its economy of light is twice as good. However, it does require an easily accessible microscope objective exit pupil, where the polarizing ring must be located."

The earlier Nikon variable phase contrast system also appears to have been more affordable than the Polanret:

"Way back in the late 1960's Nippon Kogaku (Nikon) introduced a "polanret" type system of their own, intended for use on their Model "S" series of stands. However, unlike the later AO system the Nikon system was simple, compact and rather affordable. It also distinguished itself by creating user-controllable interference colors in the phase images!"
https://bettermicroscopy.blogspot.com/2 ... trast.html
(Series of articles on Polanret and other variable phase contrast systems begins in Jan. 2017 issue)

BTW, if anyone here is interested in experimenting with the Nikon Interference Phase Contrast System, there is presently one listed on Ebay (probably overpriced, but maybe the seller would accept less). The seller also provides photos said to have been taken with Olympus S Plan objectives.

https://www.ebay.com/itm/324753478739

[zzffnn]

farnsy,

Please note that Peter Evennett used ONE low power objective for demonstration purpose only. In reality, you have multiple high power objective where back focal plane (BFP) is not easily accessible.

As TigherScope also pointed out:

Quote

Most of the time, the back focal plane is located not at the back of the objective, but inside it at some inaccessible place, so conjugate is the only way to go if one needs to access the BFP.

Unquote

Also one objective’s BFP location may or may not match the other when you have different brands and models. That is why Nikon IPC does not work well with many long objectives.

Yes, you can try to access BFP, but it is really a gamble if you can get to it, which is not always easy with high power objectives. I did try to get to BFP of my objectives on my scopes, but consider it difficult to implement conveniently or comfortably. With low power objectives, there are so many contrast methods or imaging tricks (for example a black cardboard far away from underneath and light from above or periphery can provide decent darkfield for 4x-10x objectives), such that access BFP may not be worth the inconvenience.

Edit: I am also under the impression that Peter Evennett modified that objective or scope or combination thereof specifically for demonstration purpose. With standard objectives and scopes, it is not that easy.

[apochronaut]

Access to the BFP is what makes these systems work, so in order to use objectives with complicated lens formulas, the BFP must be moved. With non-Plan objectives the BFP can be accessed, that's why the Nikon Interference Phase System was cheaper and easier to use ; because it only worked with standard achromats. There were no optics in it to move the BFP.

In the Reichert system, all objectives were plan, so the back focal plane was moved by an apochromatic optical system to another location where it could be accessed easily. That is part of the cost. In the Reichert system, all objectives over 10X are useable for all contrast systems.
AO were selling a lot of infinity planapos from 1967 on, so they seemed to focus on those for the Polanret; thus it too had an apochromatic optical system built in to move the BFP to take advantage of plan optics. Probably by the time they brought out their infinity planapos, the Polanret was being abandoned but those probably would do the system justice.
The Polanret also required the use of a 1.3 N.A. achromat aplanat condenser. Apparently DJ Jackson did not have one of those, so was using an inadequate condenser for the system.

[TiggerScope]

Various points:

Access to the BFP is what makes these systems work, so in order to use objectives with complicated lens formulas, the BFP must be moved. With non-Plan objectives the BFP can be accessed, that's why the Nikon Interference Phase System was cheaper and easier to use ; because it only worked with standard achromats. There were no optics in it to move the BFP.

This is not really so, I'm sorry to say. Nikon IPC did move the BFP by creating a new conjugate aperture plane inside the optical relay module which fits between the trino head and the body. See below for the optical setup of this, noting that the conjugate aperture plane is located at PA, which is the phase ring (actually a clear annulus in polarising film, which they call the Polarising Aperture). Any objective can be used if its image of the phase annulus overlays the corresponding phase ring at PA in the slider. I use Nikon CFN planapos; with some tweaking they work very well and it's no surprise that the images are much better than the 1960s achros.

Also one objective’s BFP location may or may not match the other when you have different brands and models. That is why Nikon IPC does not work well with many long objectives.

A follow-on from above. The phase rings in the IPC slider actually have fore and aft adjustments to get the annulus and ring parfocal, and racking the condenser slightly can change the size of the annulus image. Nikon CFN plans and planapos all seem to give good results, except the 60x/1.4.

BTW, if anyone here is interested in experimenting with the Nikon Interference Phase Contrast System, there is presently one listed on Ebay (probably overpriced, but maybe the seller would accept less). The seller also provides photos said to have been taken with Olympus S Plan objectives.

Good of you to give a heads-up, but it certainly is overpriced, and also faulty as the seller says. When these come up at auction from private sellers, they "usually" sell at £300- £400 if in good condition. The one you are pointing to was bought as a BIN for $200 from a private seller in Canada, so this seller is hoping to make a packet, not that it's my business, and good luck to him but I'd steer clear. Be aware that the IPC condenser is exactly the same as the normal phase condenser, with slightly wider ("thicker") annuli to let more light through, which compensates for the 3 layers of pol film in the relay unit. So if anyone were to find just the relay unit with no condenser, a normal phase condenser will do.

Phase annuli were in the intermediate image plane... The concept was continued in some of the Poly microscopes such as the Polyvar.

Do you mean aperture plane? I have a Polyvar, and I'm not aware of any conjugate aperture plane. There is a graticule slider at a conjugate image plane above the tube lens but obviously that's no use for phase as anything inserted there will be in focus with the specimen. Perhaps some models have conjugate aperture plane; have you any details?

But, has anyone compared the results, to see if for some reason having the phase plate at the objective's rear focal plane produces a superior image?

Displaced-aperture phase images seem pretty good. However, I do need to make that comparison on the Nikon using BF and phase planapos in 20x and 40x.

[TiggerScope]

This might be of interest. Compare with Nikon IPC. This is the Zeiss Jena Amplival Interphako, a pretty amazing system that will do lots of cool stuff, like classical interference (Jamin-Lebedeff) as well as interference phase, DIC etc etc.

The next two are Professor Pluta's easy-reading description of exactly how the Nikon IPC system works.

[apochronaut]

Yes, you are using newer objectives that were not part of the original concept. They just happen to work "with a little tweaking" but in it's original embodiment the system used only achromats. It could not use apochromats nor apparently even plan objectives. The schematic shows achromat correction. It is good that you have found a way of working the system with more modern objectives.
The reason it wasn't designed with better corrected objectives in mind is likely this.

Around 1960 due to the development of newer glass types, manufacturers could design apochromats and achromats with identical correction requirements. The same eyepieces could be used with both. AO had this since 1961, and Reichert since about 1970, so their systems work with all objectives.
Designing such a system for a less advanced optical system that required separate corrections for achromat and apochromat systems would require a different remote optical transfer system for each.
Did Nikon make a different system for their apochromats?

[TiggerScope]

The tweaking is just moving the condenser up and down while looking through a phase telescope. Not every 45mm objective can be used. I need to get back here with a list of what can and what can't be used. Of course nobody says they have to be Nikon. Oly, Zeiss, Leitz could I'm sure give good results (obviously, with the associated eyepieces and photoprojective).

Nikon offered high-eyepoint compensating eyepieces which would work with everything, and they also offered cheap Huyghenian eyepieces including a 10x/12 which is probably like looking through a drinking straw. I'm not sure if you're asking if there is a different optical relay module designed for apos, well, no there wasn't, there is only one IPC, and since all residual LCA compensation is done in the eyepiece, the intermediate optics must just be neutral. There's no need for them to be apochromatic either as simple achro doublets don't create visible CA. E'g' the Zeiss Optovar is composed of simple doublets and isn't "apo" though it might as well be called so.

I have no reason to think that Nikon specified non-plan and non-apo achros for IPC. AFAIK any Nikon short mount was ok. Where does this idea originate, I wonder. The IPC instruction manual makes no mention of objective type, just the 4 basic magnifications.

[apochronaut]

Maybe check in a manual whether the compensating eyepieces worked with everything. They would not be compensating otherwise. Compensating eyepieces are called compensating because they are designed to compensate , not correct : for over correction of ca applied in the objective once sa has been corrected for. Huygens or earlierl W.F. eyepieces are correcting eyepieces. The opposite of compensating. That has been always the case until it became possible for achromat and apochromat objectives to be corrected uniformly, a feature that the shorty Nikon objective families did not possess . Maybe later ones but I haven't come across any. Once the condition of correction uniformity has been met, the term compensating no longer needs to be used because the eyepieces can be anywhere on a continuity from heavily correcting to heavily compensating, whatever condition serves the objective family best. AO and B & L for instance had no need for compensating eyepieces after 1961 and 1965 or so respectively because all objectives were equally tuned.
It is often said that compensating eyepieces can and in some cases should be used with achromat objectives and they are better than non-compensating eyepieces. They are only in center field and they work better only for photography where there is a defined frame that excludes the overcorrected peripheral part of an achromat image created through compensating eyepieces. This is also mitigated somewhat by the field stop but not enough : a reason older systems had smaller fields.