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Axial illumination for the inside of a dark tube
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rjlittlefield
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PostPosted: Tue Jan 24, 2012 5:18 pm    Post subject: Axial illumination for the inside of a dark tube Reply with quote

In an earlier post, I promised to describe an axial lighting setup that I had to use to illuminate the inside of a dark tube.

To refresh your memories, here was the final image:



The challenge here was that the inside of the tube was surprisingly dark, and the stuff I wanted to photograph was pretty far down inside. The result was that I couldn't get even close to what I wanted, by the usual approach of shining light around the lens.

After thrashing around for a while, I ended up switching to an axial scheme, bouncing illumination off a thin piece of glass that I was also shooting through.

Here's a picture of the setup. The black card shields the subject from direct illumination from the side, and the white paper serves to diffuse a second halogen fiber that was providing backlight to shine through the melted hole in the subject, a broken CFL.



Here's an illustration of the result. The top picture is what the camera sees if no illumination is bounced off the glass, the bottom is including that light.





Hope this helps.

--Rik
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Peter De Smidt



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PostPosted: Tue Jan 24, 2012 6:06 pm    Post subject: Reply with quote

That's a neat idea, Rik.
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rjlittlefield
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PostPosted: Tue Jan 24, 2012 7:29 pm    Post subject: Reply with quote

Thanks, Peter. It's an old idea. I think I first ran into it in Kodak's old N-12 book on photomacrography. They were using it to illuminate a flat mirrored surface. The subject was some "microcircuit" from the late 1960's -- big enough to see with a magnifying glass -- and they were bouncing light off an angled coverslip. But what's here is the very same idea, just done bigger.

By carefully arranging some other lenses, you can make both the illumination and the imaging be perfectly telecentric -- chief rays lined up with the optical axis all across the field. See HERE, "Brightfield Illumination of Large Field Sizes" by Theodore M. Clarke. Fig 17 illustrates the basic scheme, down at the bottom where it says "Light cones normal to field plane". Carefully done, that sort of setup will let you take a picture of a mirror from straight on, and see nothing but uniform light. I've never needed to go nearly that far, though.

I should have mentioned also that the room lights were off when I was actually shooting the subject. It's important that the glass doesn't reflect anything that's illuminated back up into the camera. It's also important that the glass be thin enough to not mess up the image. This particular piece is only 1.15 mm thick, purchased as a 3" square microscope slide.

--Rik
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Craig Gerard



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PostPosted: Wed Jan 25, 2012 1:52 am    Post subject: Reply with quote

Most effective!

Thanks for the demonstration Rik.


Craig
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rovebeetle



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PostPosted: Wed Jan 25, 2012 5:07 am    Post subject: Reply with quote

Hm, self-made beam splitter. Lovely idea.

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ChrisR
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PostPosted: Wed Jan 25, 2012 5:04 pm    Post subject: Reply with quote

So, am I correct in thinking that :
    there's nothing telecentric about the setup used here

    thin glass would only serve to bring any images from reflections in the glass closer to the original

    a long working-distance set-up would do likewise

    maybe a coated piece of glass ( UV filter) would be better in that regard

    but you avoided them by careful positioning of the lights

    ?
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Peter De Smidt



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PostPosted: Wed Jan 25, 2012 5:14 pm    Post subject: Reply with quote

An anti-reflection coated glass would bounce less light on the subject.

The most ideal setup would be a thin piece of optical glass with no coating on the side facing the subject and multi-coating on the side facing the lens. This would give the maximum light on the subject with the minimum amount of contrast loss.
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rjlittlefield
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PostPosted: Wed Jan 25, 2012 9:28 pm    Post subject: Reply with quote

ChrisR wrote:
there's nothing telecentric about the setup used here

Correct.

Quote:
thin glass would only serve to bring any images from reflections in the glass closer to the original

No, thin is important mostly to avoid introducing aberrations. There would be no problem if the beam-splitter were in an infinity portion of the optical train, where the image-forming light is organized into pencils of parallel rays. But 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. (Think of cover glass thickness under a high-NA microscope objective.) Glass that's not perpendicular to the optical axis is even worse, since the path length through the glass not only increases but becomes different on opposite sides of the aperture.

To illustrate, here are three images that I shot just now. These are actual pixels from a Canon T1i and Canon 100 mm f/2.8 L IS USM macro lens at very close to 1:1, same as the original stacks were shot. However these were shot at f/2.8 to maximize the effect of the glass. As you can see, the thin glass has only a small effect on image formation, but the thick glass really messes things up.

Left = no glass; middle = 1.15 mm at 45 degrees; right = 3.1 mm at 45 degrees.



Quote:
but you avoided them by careful positioning of the lights

I didn't worry about reflections of the subject. Those are formed by a double-bounce from the two glass surfaces, so their intensity will be under 1%, too small to matter here.

However, anything on the side opposite the light will reflect in a single bounce, which basically means it's just as important as the subject. Of course that's exactly the direction light is shining toward, so it's easy for things over there to be bright. I could have stuck a big piece of Protostar over there, but it was simpler to just orient things so that what was being reflected was a far-away wall, and turn off the room lights. Then inverse square applies, the wall becomes relatively dark, and that's what the beam splitter reflects.

Quote:
The most ideal setup would be a thin piece of optical glass with no coating on the side facing the subject and multi-coating on the side facing the lens. This would give the maximum light on the subject with the minimum amount of contrast loss.

Sounds good at first thought, but I don't think that holds up under careful analysis. Check me out here...

Suppose the two surfaces of the beam splitter reflect fractions f1 and f2 of the light that hits them, and for simplicity let's assume that f1 and f2 are both much less than 1 so that we can leave second-order effects out of the algebra. Let the wasted illumination that gets through the beam splitter hit a surface with net reflectivity W, while the reflectivity of the subject is S. Let the brightness of the illumination be I.

Then the illumination reflected onto subject is I*(f1+f2), and (1-(f1+f2)) of whatever reflects from the subject is what gets through the beam splitter and back to the camera to form the desired image. Overall product for the subject is I*(f1+f2)*S*(1-(f1+f2)).

On the other side, wasted illumination getting through the beam splitter is I*(1-(f1+f2)), and of whatever reflects back toward the beam splitter, the fraction reflecting into the camera is (f1+f2). Overall product for the waste light is I*(1-(f1+f2))*W*(f1+f2).

Take the ratio between subject image and waste light, and everything cancels except S/W. S is fixed, so all you really get to play with is W.

In other words, what you really need is to avoid reflecting waste light back toward the beam splitter. The reflectivity of the beam splitter, and how that's divided across the two surfaces, does not matter for minimizing the effect of waste light that becomes flare. It's kind of a surprising result, but on the other hand it implies you don't need to hunt up clever glass to make a beam splitter. Just figure out how to deal with the waste light.

--Rik

Edit: typo


Last edited by rjlittlefield on Fri Jan 27, 2012 1:14 am; edited 1 time in total
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Peter De Smidt



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PostPosted: Wed Jan 25, 2012 9:45 pm    Post subject: Reply with quote

I'm glad to be wrong! Fancy glass gets expensive.
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Harold Gough



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PostPosted: Thu Jan 26, 2012 12:44 am    Post subject: Reply with quote

There are much thinner glass cover slips available, down to about 0.08mm thick and up to 24 x24mm or 24 x 60mm.

There is also this rarity:

http://www.alanwood.net/photography/olympus/mirror-housings.html

Harold
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rjlittlefield
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PostPosted: Thu Jan 26, 2012 12:54 am    Post subject: Reply with quote

Harold Gough wrote:
There are much thinner glass cover slips available, down to about 0.08mm thick and up to 24 x24mm or 24 x 60mm.

Yes, and these would be very handy if one were to use this technique in front of a high NA objective.

--Rik
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Chris S.
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PostPosted: Thu Jan 26, 2012 1:12 am    Post subject: Reply with quote

Rik, thanks for this excellent analysis on beam splitting mirrors!

One of the things on my project list is the addition of axial illumination capability to the Bratcam. So I've been reading about the wide variety of beam splitting materials available. Missing from the technical descriptions is whether the various differences actually matter for my application. From what you've described, they mostly don't--I can choose the component based mostly on mechanical convenience. Since I'll be putting it between objective and tube lens (in "an infinity portion of the optical train"), even the thickness isn't going to be critical. I do want 50/50 reflection/transmittance, so will either go with a purchased product with stated performance, or scavenge a beam splitter from one of the derelict instruments accumulating in my basement.

It's notable that controlling the wasted illumination may be more important than the mirror material. I've been intending to do this with Protostar flocking, but might try one of your magic 90 degree cones.

Incidentally, for those interested in trying something specifically marketed as a beam-splitting mirror, with stated characteristics, they are not necessarily expensive. Check out Anchor Optics' offerings in either commercial or experimental grade. A quick search will turn up other sources.

--Chris
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rjlittlefield
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PostPosted: Thu Jan 26, 2012 11:02 am    Post subject: Reply with quote

Chris S. wrote:
might try one of your magic 90 degree cones.

The 90 degree analysis was for a cone used as lens hood. Different requirements there, and I'm still not sure I got that analysis exactly right.

For use as a light trap, a narrower cone would be better. 90 degrees seems likely to act as a corner reflector, so the specular reflections might come right back out.

Sorry for the confusion.

--Rik
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g4lab



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PostPosted: Thu Jan 26, 2012 11:07 am    Post subject: Reply with quote

http://www.microscan.com/en-us/products/nerlite-machine-vision-lighting/smart-series-doal-illuminators.aspx

http://www.microscan.com/en-us/products/nerlite-machine-vision-lighting/scdi-illuminators.aspx


http://www.microscan.com/en-us/products/nerlite-machine-vision-lighting/cdi-illuminators.aspx




This company makes these things. Their target market it industrial machine vision so they have gone completely to LEDs They used to have them with fiber optics.
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ChrisR
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PostPosted: Thu Jan 26, 2012 3:56 pm    Post subject: Reply with quote

Quote:
I do want 50/50 reflection/transmittance

Yes, I believe you do!
This is not the same application as camera/viewing ports, where other ratios are often quoted.
It never previously occurred to me that they might be different.
Doodling with Rik's analysis showed, (if I am right having tried simple differentiation for the first time this century), that 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.
It looked as though it had to be 50% but it was nice to derive it.
Too little reflection and all your light goes to "waste", too much and no light makes it into the lens from the subject.


When interrupting the beam of a finite objective it makes sense to use a thin mirror, as Rik explained, though the generality of beamsplitters uses cubes - a summary happens to be with a predictable source:
http://www.thorlabs.hk/NewGroupPage9_PF.cfm?Guide=10&Category_ID=21&ObjectGroup_ID=4129
The panoply of applications with birefringent materials pops up, familiar to microscopists.
(Came across an interesting variant here:
http://www.meadowlark.com/store/applicationNotes/Beam%20Splitting%20Polarization%20Cube.pdf)

Doing things on the cheap though, I thinkthe 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.
I don't recall seeing such things though, so I'm probably wrong... Sad
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