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Thread: macro photography: generating a plane of light?

  1. #11

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    Re: macro photography: generating a plane of light?

    If your samples are small enough there are some very sweet LED darkfield illuminators made for microscopes which project a plane of light into the centre of a ring. They are not usually specced for the thickness of the light sheet, but I'm sure their application engineers can help you out. One example:

    http://www.microscan.com/en-us/Produ...uminators.aspx

    There are also LED-based precision line illuminators which probably project a thin enough line (or can be focussed). Three or more of these would be easier to align than a bunch of projectors, and they come in lengths up to half a meter or more.

    Not cheap, but they're a) aligned, b) even and c) colour balance controlled. Homebrew solutions are possible, but aligning the teeny-weeny LED emitter to a collimator or line-projection optic isn't an easy job.

    If you can live with B+W, a laser pointer with a clip-on line projector will work, and again, will be easier to set up than a projector. Green pointers with pattern generators are fairly commonly available - but watch your eyes if you start peering into focussed laser light, some pointers on eBay and elsewhere are not eyesafe if focussed.

  2. #12
    jp's Avatar
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    Re: macro photography: generating a plane of light?

    How about a very thin plastic or glass disc that is completely mirror coated, then hole saw a piece out so it fits around the photographed object like a halo. Light it from a fiber(s) on the edge where some mirror coating is removed. The light should bounce around inside the interior of the disc and only exit from the center lighting the object. If you made it thin enough, you would have little or no moding and the light would be very well directed. A glass with different difraction could be substituted for the mirror coating like in SM fiber optic cable.

  3. #13
    David J. Heinrich
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    Re: macro photography: generating a plane of light?

    jp498,

    I'm not quite sure what you're referring to would work...it seems like the light would just bounce around the inner walls of the disk, if I'm understanding you. Is the diagram attached something like what you had in mind?

    Attachment 44572

  4. #14
    jp's Avatar
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    Re: macro photography: generating a plane of light?

    Almost: here's my 60 second sketch.



    It could just bounce around inside the edge if you put the light in at a narrow angle of incidence and if the light were a simple beam, but if aimed off center I imagine it could be fine. If it did bounce around the edges too much, some roughness on the edges before mirroring would certainly fix that.

  5. #15
    David J. Heinrich
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    Re: macro photography: generating a plane of light?

    Quote Originally Posted by jp498 View Post
    Almost: here's my 60 second sketch.



    It could just bounce around inside the edge if you put the light in at a narrow angle of incidence and if the light were a simple beam, but if aimed off center I imagine it could be fine. If it did bounce around the edges too much, some roughness on the edges before mirroring would certainly fix that.
    Ahhh, I see. That's similar to my initial idea, but an easier way to do it, although a more intense light-source would be needed.

  6. #16
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    Re: macro photography: generating a plane of light?

    well combined with your initial idea, you could leave the edges clear and put a circular flourescent light around the edge. You could make the disc of epoxy to custom fit the bulb exactly, and wrap the whole thing except the interior hole in reflective foil. A flourescent bult glows from the edges though, not the center, so a narrow interface with the bulb would be useful. You'd have to have the disc as thick as the circular fl bulb if you wanted to get as much as 1/2 the light from the bulb, and taper it inwards quickly toward the subject. So the disc would take on a concave shape.

  7. #17

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    Re: macro photography: generating a plane of light?

    dh003i, to answer your questions from above.

    1. I used a Newport XY stage single section with a connection to the dashpot piston directly below. The dashpot has an air bleed valve that adjusts the rate of descent of the free falling stage. Travel was about 4 inches for the stage and dashpot.

    2. Yes I was double masking the light source.

    3. I used camera movements to obtain focus over field of view. (Very tricky I might add).

    4. You show correctly one cylindrical slit. I used a second one inside the outer one. Since the gooseneck illuminators are extended light sources the outer slit acts as a longitudinal, circular, point slit source and when imaged on the inner slit produces a slit beam at the sample with little divergence of rays as a function of distance away from the inner slit. Of course intensity at the subject is greatly reduced. The whole idea is to get a sheet of light that is of uniform width along its path, (infinity corrected).

    Your silver shim stock at .0005 in. thick would be too fragile. I used brass at IIRC about .005 in thick, which stays fairly stabile when wrapped in a 6 inch diameter circle. I used a cardboard cylinder of appropriate diameter to jig up the slit shim stock with shims poking thru the cardboard to set the slit gap precisely.

    5. Not missing something. A 250 um COC is poor resolution but OK for my purpose. I suppose one could shoot for even 1 mil (25 um), but the smaller, the greater the challenge in fabrication. As you note it all depends on the image quality you want.

    6. Your Beacher frame structure looks promising but there is a lot of other support rigging required.

    7. I used trial and error by Polaroid exposure only. Need to be careful of extraneous light. I worked using a lidded box.

    Nate Potter, Austin TX.

  8. #18

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    Re: macro photography: generating a plane of light?

    The dashpot provides a resisting force proportional to velocity so that your sample can quickly reach its terminal velocity.

    You need a dashpot with a stroke larger than your sample travel or otherwise connected via a lever so that its resistance will be uniform through the sample's entire travel.

    Another ancient approach to providing resistance proportional to velocity is to connect the moving stage to some fan or rotor spinning in a fluid (possibly air.)

  9. #19
    David J. Heinrich
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    Re: macro photography: generating a plane of light?

    Nathan,

    Thank you very much for your response.

    Your silver shim stock at .0005 in. thick would be too fragile. I used brass at IIRC about .005 in thick, which stays fairly stabile when wrapped in a 6 inch diameter circle. I used a cardboard cylinder of appropriate diameter to jig up the slit shim stock with shims poking thru the cardboard to set the slit gap precisely.
    I was actually thinking more along the lines as using the 0.0005 in thick shim to set he slit gap to 0.0005 in, not as the outer and inner hollow cylinders. I'd use brass shim stock folded into a cylinder to make the top and bottom halves of the cylinder, with the slit between them (and some kind of support bars or something to maintain the slit distance, lest gravity take over). e.g., something like this:

    Attachment 44663

    I think the difficult thing will be getting the slits aligned between the inner and outer cylinders. Maybe it will be made easier by four pieces of brass shim stock presumably of precisely the same dimensions, for alignment purposes vis-a-vis a perfectly flat ground.

    Quote Originally Posted by Nathan Potter View Post
    4. You show correctly one cylindrical slit. I used a second one inside the outer one. Since the gooseneck illuminators are extended light sources the outer slit acts as a longitudinal, circular, point slit source and when imaged on the inner slit produces a slit beam at the sample with little divergence of rays as a function of distance away from the inner slit. Of course intensity at the subject is greatly reduced. The whole idea is to get a sheet of light that is of uniform width along its path, (infinity corrected).
    I wonder how one could calculate the theoretical reduction in light-intensity. It seems like the reduction in intensity comes not from blocking off light above and below the slit (as that just reduces total light), but from blocking off light that isn't parallel to the desired plane of light to be created by the slit. (i.e., light that would be hitting the same spot from numerous non-parallel sources is eliminated, hence intensity is reduced).

  10. #20
    David J. Heinrich
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    Re: macro photography: generating a plane of light?

    Quote Originally Posted by aduncanson View Post
    The dashpot provides a resisting force proportional to velocity so that your sample can quickly reach its terminal velocity.

    You need a dashpot with a stroke larger than your sample travel or otherwise connected via a lever so that its resistance will be uniform through the sample's entire travel.

    Another ancient approach to providing resistance proportional to velocity is to connect the moving stage to some fan or rotor spinning in a fluid (possibly air.)
    Thanks for the links to he dashpots! I think I understand why the dashpot stroke needs to be larger than the sample travel -- so it has time to slow the sample's descent to terminal velocity, right?

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