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Thread: Beam expander after scanners.. possible?

  1. #21
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    Quote Originally Posted by jors View Post
    Unfortunately, there would be no angular scanning action this way.
    Thanks for your input. What would be the primary constraint? Is the off-axis nature going to change the focal length somehow or is it just a lens size thing?

  2. #22
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    To answer a few questions... The reason I specified diopters vs focal length is because the optics is as simple as eyeglasses.
    Just like if you put on a set of frames, it doesn't mean that somehow your field of view gets compressed down to a point spot. (No bets if this also involves Ketamine)... As long as one side of the lens is sufficiently concave (the other side will be convex which determines the final diopter) on the meniscus, the scan angle will only distort negligibly, while markedly improving the point size at a given distance (in our case usually 200ft or so)

    So to correct the raw output of a diode is silly, but in real life, anything that can fit on a scanhead mirror can conceivably see some improvement. We're not using lambda/20 mirrors on our rigs, so most projectors will gain some distortion after the number of bounces before the final output. And in the field, the last thing you want to do is crack open a laser head unless things are horrifically wrong, so you if can slap a lens on it can call it a day then its good to go. Now we were using these on gas lasers which don't normally have a fast and slow axis so YMMV in this age of laser flashlights.

    As for source, these days, everything is made out of china so I'd look there for a set... e.g. https://www.alibaba.com/showroom/blank-glasses.html

    Hope it helps?

    [edit]
    P.S. An easy and cheap way to test if it'll improve your long range is have a couple buddies, on who is slightly near-sighted and one who is slightly far-sighted throw their glasses in front of the scanhead and see if one of them helps.
    Last edited by yaddatrance; 06-27-2018 at 23:10.

  3. #23
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    I browsed and found a couple pictures, they're from the 90s so forgive the quality

    A lens slapped on a old scanner


    you can see a lens on the CD case, the aluminum mount let us rubber band it to the front of the fiber-fed projector.

  4. #24
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    Yaddatrance - thanks for all the info. Great that you have tested it yourself and you know it works. I will look into this further over the weeks and report back.

    Is it essentially changing the beam focus from infinity, to a closer focal point like spectacles would do for the retina? Or if I am wrong, would you perhaps mind explaining why this works in a short sentence of two? From what I understand, one would need to switch lenses based on the projection distance.

  5. #25
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    I think you can also just offset the lens from the scanner mirror for the focus distance. But it sounds like this will suffer from noticeable chromatic aberration, field curvature and spherical distortion at least.

  6. #26
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    In practical terms, the lens is "ruining" the divergence numbers, by focusing the beam at a given distance. For graphics, you'd focus on the target... for beam shows you'd focus somewhere halfway to the terminating wall. Somewhere between infinity and rayleigh length, the beam diameter would be pretty bad...

    For eyeglass blanks, you'll really only see chromatic aberration if you really cheap out. Most eyeglasses (that you'd actually wear to correct vision) are aspheric achromats of some type and are normally made of deep magic^H^H^H^H^H^H multiple lenses bonded together with different indices of refraction.... If you're worried about aberration, google 'Abbe number' and ask the supplier what theirs is as that directly corresponds to the amount of aberration.

    If you want to be super nit-picky, it is true that even achromats still suffer from aberration, especially visible at higher diopter values... (The lenses are normally calculated with red and blue and everything is else is considered linear, which is close, but not quite exactly true) But those who can whip out deep pocketbooks can send for some custom flourite lenses, ground to be superacromats...

  7. #27
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    Humphry, better is the enemy of good. Placing a simple beam expander after the galvo will definitely work, especially since the scanning angle will be 5 degrees or less. My advice is to test this with a moderate sized lens system and then scale up once you have the basic dimensions established. Keplerian systems are a little easier to align, but a Galilian system will be more compact and the second lens can be a little smaller for a given magnification ratio. Get the first lens as close to the galvos as possible.

  8. #28
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    Through trial and error I discovered a very long focal length lens (approx 1m) placed after the scanners could reduce the spot size by half on a 300-500' throw with SP171's in a stadium. I carried several focal lengths and used whatever worked best.

  9. #29
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    Quote Originally Posted by planters View Post
    Humphry, better is the enemy of good. Placing a simple beam expander after the galvo will definitely work, especially since the scanning angle will be 5 degrees or less. My advice is to test this with a moderate sized lens system and then scale up once you have the basic dimensions established. Keplerian systems are a little easier to align, but a Galilian system will be more compact and the second lens can be a little smaller for a given magnification ratio. Get the first lens as close to the galvos as possible.
    Mr. Planters, thanks for weighing in.
    I have gone ahead and purchased a -15mm plano-concave and 70mm plano-convex lens (75mm diameter) and will see what happens. Will use an optical adjustment rail to get the distance between optics right. Would I be correct in assuming that if I finely manipulate the the distance between the two optics, I may even be able to "focus" the spot and optimise for a distance of 300m (instead of infinity) and end up with a smaller beam spot at 300m? Or is this not how beam expanders work?

    Quote Originally Posted by Photonbeam View Post
    Through trial and error I discovered a very long focal length lens (approx 1m) placed after the scanners could reduce the spot size by half on a 300-500' throw with SP171's in a stadium. I carried several focal lengths and used whatever worked best.
    Interesting. Thanks for sharing.

  10. #30
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    Did you make any progress?
    Can you share your results?

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