Axial illumination for the inside of a dark tube

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rjlittlefield
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Post by rjlittlefield »

ChrisR wrote:for axial illumination of a subject you DO want 50% transmission / reflection to maximise the amount of light getting from the subject into your lens.
I agree. It's only for flare that the reflectance doesn't matter. If maximizing the illumination is important, then 50% is right.

That said, it's probably good to keep the magnitudes in mind. If uncoated glass is 10% reflective, then the net is 9% (0.10*0.9). Swap in a 50/50 beamsplitter, and it rises to 25% (0.5*0.5). In the grand scheme of things you have to balance the 2.8X gain in efficiency against the cost & time of acquiring a suitable 50/50 material. As I mentioned, Kodak's book used a coverslip.
Doing things on the cheap though, I think the better place to put a semi silvered mirror, in a simple finite system such as when using a reversed enlarger lens, is between the lens and the camera. The "NA" that side is a lot smaller, so I'm guessing the aberrations introduced would also be much less.
That sounds right with respect to aberrations. The only wrinkle is that then all the light that illuminates the subject also has to go through the aperture of the lens. So instead of getting the dreaded "black hole" effect, I think we get a "white hole" instead. But I haven't tried it either.

--Rik

Chris S.
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Post by Chris S. »

Rik, I hadn’t given it that much thought—noted your 90-degree cone in passing, thought it nifty, and marked it as a potentially useful tool for later. But it makes sense that it would be counterproductive in my use, which is sort of backwards compared with your implementation. I’m not terribly worried about the light-trap issue, which is why I hadn’t given it overmuch thought; trapping waste light in my particular circumstances should be much easier than in the case of a lens hood.

Rik and Chris, regarding the desirability of 50/50 split, it seemed self evident. I didn’t work it out algebraically, but probably should, if only to prove to myself that I still can. It would have been simple in high school—wonder if it still is?

From the diagrams I’ve seen, Mitutoyo seems to be using half-silver mirrors between objective and tube lens. I’ve wondered if the cube approach would offer any advantages, but in this scenario, haven’t seen any. So unless I find a convincing argument otherwise, I’ll probably go with a mirror, rather than a cube.

I do, definitely, not want a polarizing beam splitter.

This is only for the infinite optics configuration of the Bratcam. If I were going to do it for the finite configuration of the rig, I think I’d go with thin glass in front of the lens, in order to sidestep the dreaded black hole—but have no plans to do so at present.

For the record, a purchased beam splitter need not be expensive. Anchor Optics experimental grade beamsplitters are $10-15 USD, available in a wide variety of sizes, thicknesses, and reflectivities. One model is only 0.5mm thick. Not so cheap as a cover slip, but with the advantage of being able to choose particular characteristics. I haven’t tried these, so can’t vouch for them.

--Chris

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Post by DQE »

100 lifetimes ago, I had a couple of expensive, high-precision scanning microdensitometers in my lab, at the Kodak Research Labs. They were the Perkin-Elmer PDS 1010 and 2020GM models, I believe. The latter was a beast, being composed of granite, weighing about 5000 pounds (2000 kg). It could scan a 20x20 inch film sample, and is/was used to digitize film based astronomy plates as might be produced for large-field sky survey projects. Below is a link to photos of these instruments:

http://www.astro.virginia.edu/~rjp0i/mu ... Micro.html

Assuming that I am recalling some design details correctly, I believe they were equipped with a *very* thin membrane type of beamsplitter for viewing while scanning photographic films. This design benefited from its physical thinness as discussed above, I believe, especially for high-magnification work.

This wikipedia entry seems releated to my rusty memories of this component:

https://en.wikipedia.org/wiki/Pellicle_mirror

Would a pellicle beamsplitter have any significant advantages for Rik's project or for similar projects at other magnifications?
-Phil

"Diffraction never sleeps"

rjlittlefield
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Post by rjlittlefield »

Would a pellicle beamsplitter have any significant advantages for Rik's project or for similar projects at other magnifications?
Sure. Except for infinity sections, the higher the magnification the more important it is for the beam splitter to be thin. I'm pretty sure that if I were to stick even an ordinary cover glass in front of my 50X NA 0.55 ELWD objective, the image quality would become atrocious, something like that 3 mm glass in front of the macro lens. In a case like that, the 0.02mm thickness pellicle mentioned in the Wikipedia article would be just what the doctor ordered -- assuming it's thin enough!

--Rik

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Post by Chris S. »

The Pellicle beamsplitters I've seen offered by the likes of Thorlabs and Newport are thinner than those described by Wikipedia. They are 2 microns thick, made of nitrocellulose, and can not be touched without destroying them. Also, they are hygroscopic, and need to be kept in low-humidity. I can see how they could be used within the protected confines of a solid instrument, but hanging one out in front of a lens sounds problematic.

--Chris

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Post by rjlittlefield »

Problematic? I love the understatement!

--Rik

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Post by oxkarthemighty »

So by "thrashing" do you actually mean losing your temper and swearing :)? I would not have thought of that setup. I probably would have come up with a less successful snoot lighting idea.
If your photo lacks interest, you aren't close enough.

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Post by rjlittlefield »

oxkarthemighty wrote:So by "thrashing" do you actually mean losing your temper and swearing :)?
No, I mean just "try this, try that, scratch head, try something else, let mind wander through dim memories of other people's clever ideas", and so on.

I actually enjoy wrestling with technical problems. I get particular amusement from seeing how many ways there are for things to actually go wrong, in addition to the many possibilities that occurred to me beforehand. :?

--Rik

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Post by oxkarthemighty »

Now that sounds like developer talk. You don't develop software do you? ha ha
It used to drive me nuts when I would have to script (probably why I got out of it) I wish I could have that type of patience. I have to put things down for a while due to the frustration involved in tricky lighting situations. Take a step back and think about it for a few days before I go another round. I'm curious to know what was your time frame in "thrashing" before your solution arose?
If your photo lacks interest, you aren't close enough.

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Post by rjlittlefield »

oxkarthemighty wrote:I'm curious to know what was your time frame in "thrashing" before your solution arose?
I don't remember clearly. Probably a couple of hours before I had things assembled well enough to actually shoot the stack.

The problem was easy to spot by eye as I tried flying normal lights around, and I already knew the basics of axial illumination so the concept came to mind pretty quickly.

But as always the devil is in the details. It took a while to make that split-dowel holder for the glass, and to get the glass clean enough to work well, and get the lamp and glass aligned properly, all this in addition to the usual issues of picking the right lens, deciding how to focus-step the stack, etc etc.

Checking timestamps on what images remain, I see that the outside views were shot on Jan 12, the inside of the broken end on Jan 15, the filament on the other end on Jan 19, and the filament from the other bulb on Jan 20, same as the posting date for the original thread. Hard to say how many wall clock hours actually got spent over those 8 days, but it's safe to say there was a lot of think time about what to shoot and how to shoot it.

--Rik

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Post by dunksargent »

I read a photomacrography article years ago which advocated making a large beam splitter (for coaxial illumination) using a wooden embroidery hoop with cling film or shrink wrap stretched over it. I actually tried to make one but the film 'burst' every time it was heated - the membrane needs heating to shrink it 'flat' and remove the ripples. This might be easier to make on a smaller scale using a wire frame or similar because a smaller membrane area is probably easier to heat evenly.

Best wishes

dunk
And now for something completely different.

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Re:

Post by mjkzz »

rjlittlefield wrote:
Sat Jan 28, 2012 4:14 pm
oxkarthemighty wrote:So by "thrashing" do you actually mean losing your temper and swearing :)?
No, I mean just "try this, try that, scratch head, try something else, let mind wander through dim memories of other people's clever ideas", and so on.

I actually enjoy wrestling with technical problems. I get particular amusement from seeing how many ways there are for things to actually go wrong, in addition to the many possibilities that occurred to me beforehand. :?

--Rik
:D

Lou Jost
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Re: Axial illumination for the inside of a dark tube

Post by Lou Jost »

As long as this thread has been revived, I'll just mention that this solution to axial illumination causes aberrations as Rik mentioned ("...located in front of the lens like this where the light is focusing in a cone, even glass that's perpendicular to the optical axis will introduce spherical aberration"). As I've mentioned elsewhere on this forum, these aberrations can be completely eliminated by using a glass surface (ideally half-silvered) to reflect the image rather than the illumination. The reflected image is photographed. The light leaving the subject never enters any glass. The illumination light goes through the glass rather than the light that makes the image. A prism could do this.

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Re: Axial illumination for the inside of a dark tube

Post by Pau »

Lou Jost wrote:
Sun Jan 23, 2022 4:41 pm
...these aberrations can be completely eliminated by using a glass surface (ideally half-silvered) to reflect the image rather than the illumination. The reflected image is photographed. The light leaving the subject never enters any glass. The illumination light goes through the glass rather than the light that makes the image.
The first part will be fully realized if you use a 100% reflecting first surface mirror, although in this case you can't have axial illumination
If you use a plain glass or a partially silvered plate beamsplitter there still is chance of ghost reflection on the second surface of the glass. The intensity of that reflection can be small and so you can have a workable solution, I ignore if really better than the classic configuration.
A prism could do this
Please let me ask again about the prism setup, did you test it? I think that a single glass triangular prism will decompose the light and only could be adequate with monochromatic illumination.
Pau

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Re: Axial illumination for the inside of a dark tube

Post by Lou Jost »

Pau, I don't have a prism, so I have not tested this.

"If you use a plain glass or a partially silvered plate beamsplitter there still is chance of ghost reflection on the second surface of the glass. The intensity of that reflection can be small and so you can have a workable solution, I ignore if really better than the classic configuration. "

A prism could be arranged so that the first-order internal reflections did not go towards the camera lens.

One of these days I'll find a prism here. I really want to find a prism with a half-silvered surface.

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