OK what about Zeiss standard DIC?

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Pau
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Post by Pau »

viktor j nilsson wrote:Here's a few more question:

1) how is it possible for the first-generation Zeiss inko system to use a single objective prisms for all objectives? The more I think about it the weirder it seems. How would you design such a jack-of-all-trades prism?

2) is it really critical to place the objective prism exactly at the rear focal plane of the objective? It doesn't seem that way to me: some systems place the prisms immediately between the objective and the nose piece, and others (from the same maker) place them well above the objectives inside the nosepiece or inside an intermediate tube. Surely the location of the rear focal plane can't be that different among the objectives for those systems?
1- Using different condenser prisms for different magnification objectives. The goal in all cases is to match the interference band of both prisms at the rear focal plane of the objective. This is the original Nomarski design first implemented by Zeiss.
Zeiss moved to the individual objective prisms at the nosepiece design in their double arrow system but Olympus still uses the original design although with objective-dedicated changeable condenser prisms

2- In principle it is, although I think that you can place it a bit higher or lower if you still accomplish the condition of matching the interference band of both prisms at the same plane and close to the objective rear focal plane (I suppose that this is what I'm doing when I move a bit the condenser outside the Kölher position to obtain DIC in my hybrid setup)
It depends of the prisms design, the "section or beta angle" referred by abednego1995 is what determines the distance between the interference plane and the objective rear focal plane.
With a Wollaston prism this plane is placed inside the prism but the modified Wollaston prism by Nomarski places it outside the prism. I think that some Leitz DIC systems placed the prisms inside the objective for this reason (and so avoiding to pay Nomarski license fees)
Pau

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Post by viktor j nilsson »

Thanks, I think I understand a bit more now. I was confused because some places online claim to show Nomarski prisms but actually show Wollaston prisms located exactly at the rear focal plane. This one for example:

Image

In this case, the objective-side prism must be located exactly at the rear focal plane. I had to look around a bit for an illustration that actually showed Nomarski prisms and found this:

Image

So when we have a Nomarski prism, it's the front interference plane of the objective-side prism that must coincide with the objective rear focal plane - not the prism itself. Correct?

Image sources:
https://www.researchgate.net/figure/Exp ... 190278/amp

http://microscopy.berkeley.edu/courses/ ... asdic.html

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Post by viktor j nilsson »

Ooops, the seller accepted my low-ball offer, so soon I'll have four Nikon epi-DIC Nomarski prisms to play with.

abednego1995
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Post by abednego1995 »

So when we have a Nomarski prism, it's the front interference plane of the objective-side prism that must coincide with the objective rear focal plane - not the prism itself. Correct?
Exactly.

I've had the same problems in the past with DIC schematics showing Wollaston prisms in a Nomarski setup. Another thing... The upper wedge must be thinner than the lower wedge in a Nomarski prism, but most drawings just have a straight line connecting the corners. Small but important difference.[/code]

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Re: OK what about Zeiss standard DIC?

Post by blekenbleu »

Nikon has reflected DIC prisms with label: "10.40.100x";
M-DIC.jpg
... does that suggest some trade-off e.g. between magnification and distance from back focal plane?
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Re: OK what about Zeiss standard DIC?

Post by Pau »

blekenbleu wrote:
Sat Dec 10, 2022 9:31 am
Nikon has reflected DIC prisms with label: "10.40.100x";

... does that suggest some trade-off e.g. between magnification and distance from back focal plane?
Maybe, the Zeiss old InKo reflected (DIC) system uses the same prism for 4X and 8X objectives but not for higher magnifications.

Another approach is making the prism adjustable vertically like in some Reichert and Leica metallurgical models with only one prism for all objectives
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Re: OK what about Zeiss standard DIC?

Post by blekenbleu »

Elongated DIC sliders presumably have their interface between (quartz?) wedges change vertically when sliders move horizontally, so more universal but fussy to adjust among objectives.
I wonder whether Nikon objectives with DIC engraving were verified to both be strain-free as well as having precisely registered back focal planes to accommodate relatively fixed prisms.
Metaphot, Optiphot 1, 66; AO 10, 120, and EPIStar 2571
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Re: OK what about Zeiss standard DIC?

Post by viktor j nilsson »

blekenbleu wrote:
Sat Dec 10, 2022 10:21 am
I wonder whether Nikon objectives with DIC engraving were verified to both be strain-free as well as having precisely registered back focal planes to accommodate relatively fixed prisms.
A couple of years ago, when I bought the epi-DIC prisms discussed above, I made some experiments to learn more about this exact issue. I wanted to know the location of the interference fringe of each prism, so that I could know at least roughly where to place the prisms in my DIY DIC setup. I didn't know how to observe the location of the fringe directly, so I instead measures the location of the back focal plane of the objectives the prisms where designed for. I didn't have all the M plan DIC objectives, but I did have a regular M plan 10x, a Mplan 20x DIC, and a M plan 40x DIC.

To measure the location of the BFP, I used Paolo Pozzi's method described here: https://www.researchgate.net/post/Is_th ... ctive_lens

That is, shine a laser pointer through the front lens, and measure the diameter of the projected circle at different distances from the back of the objective, fit a regression and calculate where the diameter is expected to reach zero.

I'll post some screenshots of the data in another post, but the results were quite interesting. Here's some highlights:

M plan 10x: -1.36 mm
M plan 20x DIC: -4 mm
M plan 40x DIC: -1.13mm
CF Fluor 10x: +0.5mm
CF E plan 4x: roughly +15mm

Measurements are given as a distance from the back edge of the objective, not at the shoulder. So -1.36 means that the BFP is located inside the barrel, and +0.5 just outside of it.

This shows that the 10x and 40x M plans, that use the same DIC prisms, also have remarkably similar BFP locations. And the 20x M plan, that has a dedicated prism, has a BFP in a pretty different location. Furthermore, the 10x in particular appears to have been designed so that the BFP falls unusually far into the objective for a relatively low-magnification objective, while other 10x presumably have their BFPs at least some distance outside the objective barrel.

Personally, I don't think that the BFP locations differs between M plan objectives marked DIC and those lacking the DIC designation. But I do think that the 10x M plan was designed to have a BFP location that matched that of the 40x and 100x M plans.

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Re: OK what about Zeiss standard DIC?

Post by viktor j nilsson »

And here's the spreadsheet with raw data and regressions:
Plots show A~B, where A is diameter of illuminated circle and B is distance from rear of objective.

Image

Image

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Re: OK what about Zeiss standard DIC?

Post by viktor j nilsson »

Note that data for the 4x objective is nonsense, the laser pointer method don't work well with low magnification objectives with front lenses much larger than the beam diameter.

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Re: OK what about Zeiss standard DIC?

Post by rjlittlefield »

This process and data are very interesting to me. Thanks for the full description.

Data for the 4x objective does not strike me as nonsense. It also makes sense in terms of an optical design that places the rear principal plane far forward. You could check this possibility by pointing the objective at a distant scene and looking to see where it forms its image. If I understand correctly the measurement you made, then the image of the distant scene should form at the same place indicated by the laser measurement.

--Rik

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Re: OK what about Zeiss standard DIC?

Post by viktor j nilsson »

Glad you found it interesting.

What data on the 4x do you refer to? The data in the graph, or the 15mm estimate? As I can recall, the 15mm estimate was based on pointing the objective at the sun and observing where the image was formed on a piece of paper. But the data in the graph suggests that the BFP is located 2.48mm behind the objective, which I interpreted as a spurious result caused by the fact that the laser pointer only filled a small central portion of the entrance pupil (is that the correct term?).

I just double-checked the E plan 4x, and when I aim it at two semi distant street lamps it forms a sharp image on a matte screen located ~15mm behind the objective.

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Re: OK what about Zeiss standard DIC?

Post by rjlittlefield »

I was looking at the good linearity of data in the graph.

Having heard the rest of the story, I agree that the data in the graph looks nonsense.

That shifts my curiosity to what caused the nonsense. An incoming parallel beam that fills the aperture will produce a wide-angle cone that comes to a small focus in back, which is how the image of your street lamps gets formed. An incoming parallel beam that does not fill the aperture will produce a narrower cone in the back, but barring insane aberrations it will come to a focus at the same distance. I just checked some of my own objectives and that's the way it worked with them. The narrowness and high intensity of the beam may make measuring its diameter difficult, but if there were a measurement problem then I would expect noisy data, which I did not see.

Having looked again at your data, I'm wondering if the laser beam was actually scattering off something inside the objective, like the edge of a lens element, and what you were measuring was a cone of light coming off the scattering point.

Do you have any ideas along those lines?

--Rik

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Re:

Post by Macro_Cosmos »

Pau wrote:
Tue Jan 29, 2019 6:31 am
Mixing and matching DIC components is risky and full of uncertainty although in some cases it works. *
waterbear wrote:...
I'm guessing parameters would be relating to numerical aperture and focal ratio (steepness of the light cone emanating from the objective).

I imagine that is thickness and angle, and extinction amount seems to be constant in their specs.
...
AFAIK the amount of shear and the focal distance where the interference band is placed (Nomarski prisms have it located outside the prism) to match the objective rear focal plane are main parameters. Unfortunately makers do not disclose them and I lack the knowledge and tools to measure it.
Also the objective rear focal plane is not standardized and many objectives can't deliver DIC. With my system I can't get DIC with any phase objectives I've tested, for example, but I get decent DIC with some non phase Leitz NPL fluotar and Pl Fluotar (160), Zeiss Planapo and and Nikon CF Plan Apo, even with a highly mismatched Nikon CFI Plan Apo 20/0.75.

I agree with Johny about the PZO system: because it is highly regulable it can work well with many objectives not designed to work with it like Nikon CF and Leitz NPL fluotars

* see my hybrid setup at viewtopic.php?t=15606
The shear angle of the normal type Olympus DIC slider prism (U-DICT, U-DICTS, WI-DICT) is 74μrad. Nikon's fitted nosepiece objective-specific prism is 76μrad and Zeiss is 71μrad.
A smaller shear angle and therefore shorter shear distance correspond to higher resolution, and vice versa for higher contrast. This is why Zeiss and Nikon's DIC "seems to have better resolution", it is not their "superior optics" but rather their vision of what an optimised DIC system looks like. There is nothing stopping Olympus and Leica from doing the same with matching prisms.
Assuming the normal DIC is being used.
Leica: higher contrast, common nosepiece slider with condenser prisms for specific objetives.
Nikon and Zeiss: contrast and resolution balanced, most people prefer the look of these two. Each objective has its own slider while condenser prisms are tailored for different NA.
Olympus: Between Leica and Nikon/Zeiss. Common nosepiece slider with condenser prisms for specific objectives. High contrast at lower magnifications while contrast and resolution is balanced at higher magnifications.
Image
My post is here: https://www.microbehunter.com/microscop ... 27&t=15169

Measuring this number is rather tricky, hopefully, I settle down enough and build something to do that next year. If I get all the numbers accurately measured, mixing and matching prisms from different brands even could become a possibility.

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Re: OK what about Zeiss standard DIC?

Post by viktor j nilsson »

rjlittlefield wrote:
Sat Dec 10, 2022 6:17 pm

Having looked again at your data, I'm wondering if the laser beam was actually scattering off something inside the objective, like the edge of a lens element, and what you were measuring was a cone of light coming off the scattering point.

Do you have any ideas along those lines?

--Rik
It was a couple of years ago, I don't remember exactly what the symptoms were I'm afraid. But I'd be happy to dig out my laser pointer and set up my optical bench again to try to see if I can figure it out. And by optical bench I mean a ruler, a caliper, and a couple of dabs of Blu-tack to hold things in place.

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