Understanding Microscope Patents

[apochronaut]

Yes but a certain amount has been made of the AO measurements not agreeing with a theoretical program but the collimating lens in a Microstar can be measured even with ones eyeballs at no more than 70mm from the front condenser plane. That is the practical part . And since the AO drawing does show the collimating lens forward of the mirror, why would anyone consider that the patent is anything but a generalized formula for an illuminator design that is likely patentable only because it provides a good laterally corrected beam courtesy of an aspheric lens . The dispersion filter does help because an aspheric lens is not perfect and has no capacity to rectify the intrinsic chroma spit out by the bulb/collector lens duo.
The essential design was already in the series 4 , which I think did not include an aspheric lens but it has been a long time since I cleaned one of those.
It is the aspheric optics that have been a key to some of AO's remarkably efficient illuminating system optics. When I first disassembled a series 4 phase condenser, in the back of my mind was: what the hell? It's a two lens abbe with a 3/4" back lens ! It looks about as trick as a 10X Huygens eyepiece. Given what that system is, or rather how sophisticated it seems,
the phase image quality.....when properly set up , is remarkable. I will disassemble a series 4 illumination pathway. It might have been a prototype.
The patent application may well represent a patentable generalized description of a system that is patentable based on only one or two features. The measurements and specs. represent a system that includes those features . Designs manufactured to include any of those features could include a statement such as " manufactured under the following patents". It doesn't mean the entire mechanism is. I don't recall Microscope IV having a notice of patent related to the illuminator.

[hans]

And since the AO drawing does show the collimating lens forward of the mirror, why would anyone consider that the patent is anything but a generalized formula for an illuminator design...

Yeah the 410 clearly does not follow that patent very closely other than in the general sense of being a "modified" Koehler system, so I think then maybe the confusion is because there are two fairly separate issues being discussed:

1. Whether or not a simulation of the exact design disclosed in that patent is expected to show the filament conjugate with the condenser diaphragm, ignoring questions about which, if any, products actually follow the patent, and how closely.
2. How does the 410 illumination system work?

Regarding actual implementations, I am curious what you know about the swing-in auxiliary lenses under the condensers and which illuminator/condenser/objective combinations required them? I see in the 410 brochure it says a swing-in lens was available for use with the 2.5X objective, neither of which I have ever seen. The swing-in lenses appear to be more common on the older stands.

[hans]

Another quick test -- did the usual setup of the field diaphragm and condenser with the 10X objective, then moved the objective and calibration slide out of the way and taped a blank slide in place with the frosted end (frosted side up) hanging over the condenser. Dual-gooseneck light guide from wstenberg's recent moving out sale is pointed at the frosted end of the slide from a low angle so there should be reasonably uniform and diffuse illumination in the normal working plane of the condenser. Small square of lens paper is laying on top of the aspheric surface showing where the light would be passing through the system. The field diaphragm is not in very sharp focus but almost all of the light is hitting within the inner region radius 10 mm. The radius of the field diaphragm opening is just under 6 mm, so the magnification going from the field diaphragm to the object plane is around 0.17X. I think this is pretty good evidence that the outer region of collector lens with reversed curvature will not be having much effect when using the 10X and higher power objectives.

[apochronaut]

And since the AO drawing does show the collimating lens forward of the mirror, why would anyone consider that the patent is anything but a generalized formula for an illuminator design...

Regarding actual implementations, I am curious what you know about the swing-in auxiliary lenses under the condensers and which illuminator/condenser/objective combinations required them? I see in the 410 brochure it says a swing-in lens was available for use with the 2.5X objective, neither of which I have ever seen. The swing-in lenses appear to be more common on the older stands.

I explain your question above in the thread below . It is true that the aux. swing in condenser is uncommon but it was only necessary with the 2.5X as you point out. There were theoretically as many aux. condenser lenses sold as 2.5X objectives but probably many more ended up in dumpsters. Scopes fitted with # 1201 .90 achromat condensers were obviously not fitted with a 2.5X objective because they are not designed to accomodate one. I have seen maybe four 2.5X objectives , two fitted to Diastars which were capable of carrying the full complement of 6 planachros. One was sold on ebay . I think one recently on a Microstar.
So many of the microscopes in the second hand market have been handled by surplus dealers that parts can go missing, or get swapped around. Surplus dealers can't really be expected to understand the technicals of objective/condenser matching and I'm sure there have been more than one employee who liked playing musical objectives.

The 2.5X objective is one that AO really went to town on when they built up the 34mm objective lineup into the 45mm lineup. The #1028 2.5X that they designed for the series 100 was not great and they don't seem to have relied too much on it's intricacies to develop the 1730. It is a really great objective. In fact, the 2.5X, 4X and 10X planachros were all improved greatly. The 20X, 40X and 100X were less in need. They also left a space in the numbering scheme for a 60X but the program never got that far. I'll bet there are some prototypes in existence.
viewtopic.php?f=24&t=10730&sid=e88f139d ... 3d79573cf2 .

[MichaelG.]

... why would anyone consider that the patent is anything but a generalized formula for an illuminator design that is likely patentable only because it provides a good laterally corrected beam courtesy of an aspheric lens ...

I am not familiar with any of the AO instruments, so this needs input from the cognoscenti:

The reason might be that, in the closing sentence of the patent [i.e Claim 2], a specific focal length of condenser is referenced. ... This might help identify the instrument for which this illuminator was intended.

Just a thought
MichaelG.

[apochronaut]

... why would anyone consider that the patent is anything but a generalized formula for an illuminator design that is likely patentable only because it provides a good laterally corrected beam courtesy of an aspheric lens ...

I am not familiar with any of the AO instruments, so this needs input from the cognoscenti:

The reason might be that, in the closing sentence of the patent [i.e Claim 2], a specific focal length of condenser is referenced. ... This might help identify the instrument for which this illuminator was intended.

Just a thought
MichaelG.

Infinity focal length, which covers quite a few condensers.

If the patent is then considered to be an accurate description and depiction of an illuminating system that went into a microscope, the collimating lens would then need be placed forward of the mirror. So , looking at all the microscopes that were produced in the era that the patent could cover and could have been designed by Arthur Shoemaker, the AO 20 seems to be the only one with a centrally located collimating lens. That one makes a bit of sense but the actual measurement between the collimating lens and the focal plane of the condenser would be somewhat more.

The other thing is the condenser. The 20 used a 1088, a 1242 and a 2110 but not a 1084. All standard models used a 1088. The 10 used a 1088 and a 1084, not a 2110 and a 1242 with an optical modification required in the bottom. Not a lot of that has ever made much sense to me.

However, what does make sense is that they could rely on the 1088 as a condenser. A 2 lens abbe, when the 10 had the option of the abbe aspheric 1084. The illuminatiing system of the 10, needed an aspheric condenser in order to get the off axis light coherent. With the 20, they moved the aspheric element to the illuminating system, maybe even engineering compensation into it, in order to correct for the deficiencies of the 1088. I never understood why the creme de la creme 20 only had an abbe condenser available, when the 10 had an abbe aspheric. Now something might be clarifying.

[MichaelG.]

Infinity focal length, which covers quite a few condensers.

An infinity corrected condenser of focal length 10.5mm, by my reading

... does that still cover quite a few ?

MichaelG.

[apochronaut]

Infinity focal length, which covers quite a few condensers.

An infinity corrected condenser of focal length 10.5mm, by my reading

... does that still cover quite a few ?

MichaelG.

I never meaured a 1088 but if 2 other condensers are catalogued for use with the same stand, then it follows that they also must be 10.5mm focal length and infinity corrected. That is if the patent pertains to the 20. There is no other stand that meets the criterion. All the other stands have multiple possible condensers too and all of them have a collimating lens located downstream of the mirror, not upstream, with a fairly short lens to condenser focal plane distance.

[hans]

This inspired me to bring in one of the surplus 110s I have been accumulating. I did not realize before, but apparently the 110 illumination is modular like the 10, since all the illumination stuff is removable as a unit just taking four screws out of the bottom. Mine has a model 1130 illuminator which looks a lot closer to that patent than the 410 does. The collimating lens is before the mirror and the window below the condenser (sitting on the desk behind the illuminator) is just a flat glass plate. There is a frosted surface but it is inside the collector lens assembly which I have not tried to take apart yet, and if the internal frosted surface is curved as in the patent then immersion oil and cover slip will not work.

[hans]

...a specific focal length of condenser is referenced. ... This might help identify the instrument for which this illuminator was intended.

I never meaured a 1088...

Have either of you tried measuring condenser focal length? Seems not so straightforward since the condenser is not at all "thin" relative to the focal length. Two possibilities I can think of:

1. Measure the focal length of the collimator, which should be easier to do accurately, then calculate condenser focal length from that and field-diaphragm-to-object-plane magnification which I already just measured on the 410, for example.
2. Put a mirror below the condenser view some distant reference with known subtended angle (via condenser -> stage micrometer -> objective -> eyepieces, not sure how well that would actually work?) then calculate focal length directly.

[hans]

The collector appears to match the the patent pretty closely, first lens is plano convex, second is positive meniscus with the concave surface R3 frosted and convex surface R4 clearly aspheric but much more mildly so than in the 410, more parabolic-looking with no change in the sign of the curvature.

[apochronaut]

...a specific focal length of condenser is referenced. ... This might help identify the instrument for which this illuminator was intended.

I never meaured a 1088...

Have either of you tried measuring condenser focal length? Seems not so straightforward since the condenser is not at all "thin" relative to the focal length. Two possibilities I can think of:

1. Measure the focal length of the collimator, which should be easier to do accurately, then calculate condenser focal length from that and field-diaphragm-to-object-plane magnification which I already just measured on the 410, for example.
2. Put a mirror below the condenser view some distant reference with known subtended angle (via condenser -> stage micrometer -> objective -> eyepieces, not sure how well that would actually work?) then calculate focal length directly.

I usually just focus them directly onto a transparent screen.

[apochronaut]

The collector appears to match the the patent pretty closely, first lens is plano convex, second is positive meniscus with the concave surface R3 frosted and convex surface R4 clearly aspheric but much more mildly so than in the 410, more parabolic-looking with no change in the sign of the curvature.

I considered the 110 but I thought it might have too short an S5 distance but I never measured it.

[hans]

I usually just focus them directly onto a transparent screen.

How do you decide where to measure from on the condenser?

I considered the 110 but I thought it might have too short an S5 distance but I never measured it.

Yeah I have not measured anything yet, just observed that the general arrangement of surfaces matches.

[hans]

...when I take their suggested spacings/lens radii (from Table II) and plug them into spectral simulation software, I get an image like this:

The rendered lens shapes look very similar to the lenses in the 1130 except for the outer diameter of the collimator, which is larger in the 1130. The spacings S1, S3, S4 are all very close, within 10% of the patent. I can't measure S5 since I don't have an assembled 110.

...but the focal point of the second lens falls short of the suggested condenser aperture F2 (marked by the white line on the right in the image).

I smeared the frosted surface with a thin layer of immersion oil (no cover glass because it is not flat) before putting the colllector back together. The view through is a bit streaky/ripply but there is still a reasonably sharp image of the filament 50-60 mm beyond R6, definitely would fall short of the condenser aperture.

[hans]

Also, unlike in the 410, the 1130 collector does not image the filament on it's own, so the simulation showing the rays diverging slightly between the collector and collimator appears to be correct.

[MichaelG.]

Have either of you tried measuring condenser focal length? Seems not so straightforward since the condenser is not at all "thin" relative to the focal length. Two possibilities I can think of:
[…]

I have not ... and at present do not have access to my optical bench, so cannot offer to do so
But it should, I believe, be a ‘textbook exercise’

Ref. https://www.cis.rit.edu/class/simg232/l ... points.pdf

MichaelG.

[apochronaut]

I have used a stand as a crude bench , setting an object on the illuminator window and focusing on a screen for a slide.

In the past, i tested all of the condensers except the l.w.d. phase condensers that I had , that would thread into what became AO's default standard condenser thread. 1242 3 element phase : 2110 1.3 oil Achromat/aplanat : 1087 1.25 oil abbe aspheric : 1201 .90 achromat/aplanat : 1973 1.4 oil achromat/aplanat and the two self built condensers I made based on 1973 blanks containing a 1.4 N.A. front lens. The shortest , one of the d.i.y.'s came out at 1.9mm, the longest ; the other d.i.y. came out at 4.5mm. All the factory built AO condensers were between 2 and 3mm. The cat.# 1087 abbe aspheric which was the standard condenser for the series 100 microscopes is pretty short. I just tested it using my cride system and got 2.3mm.
I also just tested the 1970 abbe aspheric, the w.f. condenser often used on a Microstar IV and it came out at 10.
I have not tested the 1096A. the flip top condenser used with the #1028 2.5X objective. The only reason for it's existence, requiring rotating the top element out of the way for the 2.5X.

There is mention in the patent of a flip in lens, which would match the system used for the 1970, in order to accomodate a 2.5X objective.

[hans]

But it should, I believe, be a ‘textbook exercise’

Ref. https://www.cis.rit.edu/class/simg232/l ... points.pdf

If I understand correctly, the two methods I was proposing are equivalent to "foco-collimator" in that PDF except I would just observe two different collimated points with known angle between them instead moving a single collimated source?

1. Measure the focal length of the collimator, which should be easier to do accurately, then calculate condenser focal length from that and field-diaphragm-to-object-plane magnification which I already just measured on the 410, for example.
2. Put a mirror below the condenser view some distant reference with known subtended angle (via condenser -> stage micrometer -> objective -> eyepieces, not sure how well that would actually work?) then calculate focal length directly.

In #1 the angle would be determined by the measured thin-lens focal length of the collimator in the illumination system and the diameter of the field diaphragm. (This assumes the field diaphragm is accurately collimated by the collimator in the microscope, which I did verify roughly above, but could check more carefully.) In #2 the angle between two distant reference points would be determined by some other means: direct measurement, geometric calculation, etc.

I have used a stand as a crude bench , setting an object on the illuminator window and focusing on a screen for a slide.

It is not obvious to me what you are actually measuring/calculating to get the numbers in the 2-3 mm range? I have only measured the focal length of the 410 collimator roughly, but it is in the 60-70 mm range. Earlier I measured the magnification between the field diaphragm and object plane:

...did the usual setup of the field diaphragm and condenser with the 10X objective... The radius of the field diaphragm opening is just under 6 mm, so the magnification going from the field diaphragm to the object plane is around 0.17X.

By proposed method #1 this implies a focal length of the condenser around 65 mm * 0.17 = 11 mm, in close agreement with the 10.5 mm "preferred embodiment" number from the 1130 patent.

[Edit: Should clarify, 11 mm was with the 1970 condenser, which just noticed you did measure at 10 mm, so good agreement. Not sure about the 10.5 mm "preferred embodiment" vs. your much shorter 2-3 mm numbers for condensers that would be used with the 1130, I have not tried any other condensers yet.]

[hans]

The cat.# 1087 abbe aspheric which was the standard condenser for the series 100 microscopes is pretty short. I just tested it using my cride system and got 2.3mm. I also just tested the 1970 abbe aspheric, the w.f. condenser often used on a Microstar IV and it came out at 10.

I cleaned up a 1087 to compare side-by-side with a 1970 in a 410 and do not see such a large difference. I only see a difference of 10-20% in magnification, 12 mm diameter field diaphragm opening to fill the 10X field with the 1970 vs. 14 mm with the 1087. I have not yet checked more carefully how well the collimator is actually collimating the field diaphragm, but assuming reasonable collimation and using 65 mm focal length for the collimator gives 65 mm * 2 mm / 12 mm = 10.8 mm for the 1970 and 65 mm * 2 mm / 14 mm = 9.3 mm for the 1087.

[apochronaut]

The # 1970 condenser will fill the 5000 micron field required to illuminate a 4X objective X 10/20 eyepiece fully under a Koehler condition. A # 1087 will only fill about a 4750 micron field under the same conditions, or the equivalent of using 19m oculars. I would put the magnification factor of the 1970 a little higher, probably 30%.
I redid my test of the 1087 and I still got only got a 3.4mm focal length....the longest of any except my d.i.y. apochromat and the 1070.

[hans]

I redid my test of the 1087 and I still got only got a 3.4mm focal length...

To explain in more detail how I got 9.3 mm, I found some course notes similar to the ones Michael posted but a bit clearer, I think:
Sophie Morel - Methods for measuring a lens focal length

I assume the condenser focal length being referred to in the 1130 patent is the effective focal length since that is what is most relevant to the overall behavior of the system. The introduction section in the PDF defines EFL vs. BFL vs. FFL and notes:

Measuring the back focal distance is usually easy. Determining the distance between P’ and V’ is a bigger issue.

This is what I was referring to with my initial comment about the condenser being not at all "thin" relative to the focal length, and why I asked how you were deciding where on the condenser to measure from.

"Figure 3: y’/tan(θ) setup" matches almost exactly what I am doing: "reticle" is the field diaphragm, "collimator" is the existing collimator in the illumination system, "test lens" is the condenser, and "reticle image" is formed in the object plane and observed via the rest of the microscope as usual. The diameter of the field diaphragm opening and the focal length of the collimator (easier to measure because it is relatively thin) are both being measured directly so is not necessary to explicitly calculate θ -- the term tan(θ) just the ratio of those two measurements. Using abbreviations IFD (illuminated field diameter), FDD (field diaphragm diameter), and CFL (collimator focal length) the given formula is IFD/(FDD/CFL) or CFL*(IFD/FDD) as I arranged the calculations in my previous message, where the term IFD/FDD is the magnification I have been referring to.

I believe there is a close analogy with (basic geometry is exactly the same as) giving the magnification of an infinity-corrected telelens/objective combination as the effective ("reference" in microscope context) focal length of the telelens system divided by the effective focal length of the objective. Except instead of a telelens and objective we have a collimator and condenser.

Note that effective focal length and magnification are directly related -- if you conclude that the magnification of the 1970 going from the field diaphragm to the object plane is 30% greater than the 1087, then I think you must also conclude that the effective focal length is 30% greater, unless you are using different definitions of focal length and magnification than I am.

[apochronaut]

Yes the effective focal length and magnification are related. A shorter focal length should indicate a higher magnification.
Condensers being basically large low power objectives, I assumed the focal plane is somewhere inside the aspheric lens, so I based my measurements on an approximation there , to arrive at the effective focal length. It isn't so thick when that becomes a target for the focal plane but yes, thick as a lens system relative to the focal length, if you consider the entire condenser lens pack. The location I used for the focal plane might have varied some between the two but not by much.

Not sure where the mistake is but it does seem that the 1087 should be closer to the 1970

I used the 1/o + 1/i = 1/f formula.

[MichaelG.]

To explain in more detail how I got 9.3 mm, I found some course notes similar to the ones Michael posted but a bit clearer, I think:
Sophie Morel - Methods for measuring a lens focal length

Great that you found that link, Hans
... I had wondered whether to add it to my earlier post

MichaelG.

[hans]

A shorter focal length should indicate a higher magnification.

Depends on which direction we are defining magnification in. For objective/telelens the convention goes in the direction of propagation so shorter EFL of the objective gives a larger intermediate image for given object size. I was choosing to also go in the direction of propagation for collimator/condenser in which case longer condenser EFL gives a larger image of the field diaphragm in the object plane.

...I assumed the focal plane is somewhere inside the aspheric lens, so I based my measurements on an approximation there , to arrive at the effective focal length. It isn't so thick when that becomes a target for the focal plane but yes, thick as a lens system relative to the focal length, if you consider the entire condenser lens pack. The location I used for the focal plane might have varied some between the two but not by much.

Not obvious to me how accurate this heuristic should be, but it is surprising that it gives such different results for 1087 and 1970, which appear to be very similar designs other than the somewhat larger diameter of the 1970 optics.

...at present do not have access to my optical bench...

Curious what your optical bench looks like, have you set up something like the nodal slide in those class notes? Would be interesting and educational to play with something like that.

[MichaelG.]

...at present do not have access to my optical bench...

Curious what your optical bench looks like, have you set up something like the nodal slide in those class notes? Would be interesting and educational to play with something like that.

It’s an old, and very basic, Zeiss pattern [prismatic rail] which, like most of my stuff, needs some work.
... a nodal slide [whether bought or built] is high on that infamous list of ‘projects’

MichaelG.

.
Edit: Just spotted this, which might be of interest:
https://www.thingiverse.com/thing:2940905

[MichaelG.]

It’s time to recommend one of my favourite reference books:
I have a hard copy, but the ‘PDF with text’ is on my iPad

https://archive.org/details/EngineeringOptics

ENGINEERING OPTICS by Habell and Cox

THE PRINCIPLES OF OPTICAL METHODS IN ENGINEERING MEASUREMENT

Measurement of Focal Length by Nodal Slide is discussed on p27 et seq

MichaelG.

[apochronaut]

Not obvious to me how accurate this heuristic should be, but it is surprising that it gives such different results for 1087 and 1970, which appear to be very similar designs other than the somewhat larger diameter of the 1970 optics.

It can't be that far off, especially if the relative same location is chosen for each lens system. The curvatures are different between the two .
One thing that should be obvious is that in use, the 1970 will focus an object at it's image plane at a considerably greater distance from it's front lens than the 1087 will.
One thing is that I don't know which of them, if either, is infinity corrected. The patent says that the required condenser is but without defining which condenser it is. There is no specific reason why either of them should or need be, since unlike older systems where the illumination source could be at variable distances, the illumination sources for both the 100 series and the 400 series are fixed. Since they both were used in the 100 watt and standard wattage systems, it would make sense that they be infinity only if the collector lens focal planes are at different distances between the 100 watt version and the 20/24 watt version. Intuitively, I think there would be a difference with the Diastar and IV. Maybe not with the series 100 version.
One might be infinity corrected and the other not. The lamp house on the 120 is a transferee from Reichert Austria but that doesn't mean the lens system employed is.

[hans]

One thing is that I don't know which of them, if either, is infinity corrected.

I forgot to mention, but before writing the more detailed explanation of the method I was using I did verify that both the 1130 and 410 have the field diaphragm accurately collimated by checking that the collimators focus a distant scene onto a lens paper within a couple mm of the field diaphragm. Easy to do with the 1130 removed from the 110, more awkward with the 410, had to use a mirror so the view wasn't obstructed by the nosepiece.

One thing that should be obvious is that in use, the 1970 will focus an object at it's image plane at a considerably greater distance from it's front lens than the 1087 will.

You are talking about the image plane in your test where you focus an object that is resting on the window/collimator below the condenser, correct? I did notice that in normal use the 1087 seems to have slightly shorter working distance than the 1970, but the difference was a small fraction of the EFL. If I understand correctly what arrangement you are talking about I think it is a fairly direct indication that the positions of the principal points relative to the top lens surface are fairly different between the two condensers, which of course could explain the large difference in measurement results.

[hans]

It’s time to recommend one of my favourite reference books: ... ENGINEERING OPTICS by Habell and Cox

Thanks for the link, looks very readable compared to some others I have found.

... a nodal slide [whether bought or built] is high on that infamous list of ‘projects’

Of course, I never knew I needed one, now I must have one...