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Thread: Beam divergence is too low

  1. #11
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    Quote Originally Posted by mixedgas View Post
    [cut]
    Doing this, i will increase the beam diameter, not the divergence.. right? I prefer to increase the divergence to avoid loosing power on the scanners (the mirrors are 5mm wide)

    I think that both green and blue have IR filter. If i watch in the output hole i can see a light cyan filter, that is very similar to an ir filters that i found in an old webcam.

    If i don't find the appropriate lenses soon, i will not do audience scanning.

  2. #12
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    Steve,

    Can you suggest a method in which one could increase both the beam size AND divergence of a laser beam simultaneously ?

    Cheers,

    Adrian
    Now proudly stocking and offering the best deals on laser-wave

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  3. #13
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    Quote Originally Posted by aijii View Post
    Can you suggest a method in which one could increase both the beam size AND divergence of a laser beam simultaneously?
    Steve's 1:1 collimator will do exactly that if you adjust the lenses so that they are farther apart.

    Adam

  4. #14
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    denny-
    i apologize for impugning a lack of knowledge on your part regarding doing shows.
    i had not considered you were talking about audience scanning and i knew that the power levels you mentioned would not burn stuff unless left stationary on a target.
    also,i failed to retain the fact you mentioned about damage to your camera.
    i know people that have had cameras and digital projectors damaged by getting coherent laser light directly into the lenses.
    here in the u.s., most of us are conditioned against audience scanning due to the laws we have.
    i will admit to letting friends stand inside rotating tunnels after the audience has left the building
    NEVER with the cubr as it is just way too dangerous.
    i hope you have a great show man.
    peace,
    wes

  5. #15
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    Quote Originally Posted by wes View Post
    ...
    It was my fault. I haven't explained well what i was talking about. (when i write in english i concentrate more on how to say things, and less on what i should say )

  6. #16
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    mixedgas is online now Creaky Old Award Winning Bastard Technologist
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    the laws of physics say, for laser show people there are main three classes of collimator.

    case I if you use a collimator to go up in size, the divergence WILL go down,

    case II
    If you use a collimator to go down in size, the divergence WILL go up.

    case III (newtonian focal point)
    If you use the collimator at unity, nothing happens, but in between the collimating lenses if you use convex lenses with teh same or similar focal lengths, you get a tiny focal point where you put the blanking scanner and teh beam comes out the same size, but the blanking scanner only needs to move very little and is thus faster.

    the lower the ratio of the focal lengths, the less change.
    ie a 2:1 collimator will only reduce divergence by 1/2
    a 10:1 will make the same beam 10 times tighter,
    so by using like a 1.6:1 or 2:1, you can get it on the mirrors but not loose
    much divergence for beam effects, also you can put another one after the scanners and boost things up, ven with just a single long focal length lens.

    You can use one convex and one concave, but that gets tricky math wise.
    buy two convex lenses, ie PCX, not PCV like I misspoke below.

    Steve Roberts
    Last edited by mixedgas; 09-07-2007 at 15:38.

  7. #17
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    Here is a good intro to audience scanning...

    http://www.pangolin.com/resguide09a.htm

    Divergence is an important variable to control. Any unsafe irradiance level can be made safe by adding more divergence.

    But, until you are confident in your math, I wouldn't mess with it.

  8. #18
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    Quote Originally Posted by mixedgas View Post
    the laws of physics say, for laser show people there are main three classes of collimator.<snip>
    Steve, there is another case you're forgetting. It is possible to expand both the beam diameter *and* increase the divergence at the same time. In fact, any single lens will do this, but a colimator will give you very precise control. When configured as a beam expander, it can be adjusted to give you a larger beam and still have increased divergence. I suppose I should say "mis-adjusted", since that's essentially what you have to do. You don't want it to be aligned for minimum divergence, you want it to be just off that point. That way your beam is fatter, but it still diverges. It's not common to want this result, and indeed it goes against the very purpose of a colimator, but there's nothing in the laws of physics preventing you from doing it.

    Adam

  9. #19
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    With audience scanning and large shows, would adding divergence be the best way?

    First, there is dwell time, which reduces further out on the beam path. Second, in a beam far enough over head it's not easy to see actual width variation, only intensity variation due to width change.

    That might mean that gradual convergence is better, so long as the beam never stops while scanning an audience, which is an extremely unlikely event, the one thing most deliberately avoided.

    No beam will come to a dangerous focus if the convergence is gradual enough, so this could allow people at the back to see a sharp beam just as those at the front will, and you can use dwell time to reduce the energy of light cast to any point in a crowd. to offset the increase due to convergence. The problem with this is you need big scan mirrors, but for big shows, people already do.

    To a limited extent, this trick is used even in small beams to overcome the divergence of cheap DPSS lasers, if you look closely at many you'll see they converge for the first few yards.

  10. #20
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    It's true, you don't need to use divergence to create a wider, safer beam. A wide, converging beam would look great and still be safe as long as the long-distance focal point is far enough away.

    Divergence is just more convenient - easy to do and you get a beam that still fits on the galvos.

    Dwell time is also effective, but it's more complicated to estimate, and is riskier - any problem that can cause the galvos to stop moving creates a dangerous situation.

    If you you use divergence to create a beam that's safe at a 1+ second dwell-time (given some minimal distance between the audience and projector), the safety level is built into the setup - you can't blind people with a hardware or software glitch, or a pattern that has too much dwell-time in some area that someone happens to walk through.

    I guess it's less than optimal, but if you must do audience scanning it's an easy way to ensure safety. Especially if you're not an expert.

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