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Thread: rgb laser, safety, 635 vs 660

  1. #1
    Join Date
    Dec 2007
    Posts
    93

    Default rgb laser, safety, 635 vs 660

    Hi there,

    I've been reading all kinds of laserforums lately, and I also want to built a modest RGB scanner for home use. I recently bought the ScanPRO20 from Dave and there you have it, the addiction started

    I would like to have very low powers because I want to do *some* audience scanning safely.
    I've been calculating some MPE numbers using the guide on pangolin.com. As long as the divergence is high enough (4mrad, dot of 30mm after 7m), a 100mw RGB seems to be safe enough. (this will project a static beam of 5mw into your eye in case of a galvo failure if my calculations are correct). 5mw seems to be the limit here in Belgium, so any moving beam should even be safer.

    I was thinking of 50mw green, 70mw blue, and 125mw red (635..660).
    Now, I assume that when you guys want to project green, you use the full 50mw. For blue, the full 70mw, and for red, 100mw max. (since that is my maximum for a safe show)
    If you want to project white, you use 15mw green, 30mw blue and 60mw red. In order to not go above 100mw. In other words, you "normalize" the show to 100mw, in stead of using the full (unsafe) 50+70+100=220mw. Correct ???

    Now, this brings me to the following point: 635 vs 660 vs beam quality.
    I've read that with 635, the ratio's are about 1:1.5:2.5 and for 660nm, the ratios are about 1:1.5:4.
    And I have seen some guys say 1 : 1.5 : 4 for 635 and : 8 for 660). anyway, perfect white doesn't exist, so i'll assume my first ratios are good enough.

    Safety wise, the 635nm blows the 660nm out of the sky. Since you only need half the power for the same visibility.
    Safety wise, the 635nm blows the 660nm out of the sky again, since the beam divergence against 660nm is higher. (
    placed against marconi's maxyz 225mw)

    Now, you all talk about "lousy" beam specs for the reds (I know, except marconi's modules). But, wouldn't it be better and safer for garage guys like me to have blue and greens with also +-4mrads ?

    I'm more interested in beamshows, but I wouldnt like a red circle around the green/blue in animations.
    But maybe if you take the 635nm 125mw red, and project a dot at 10meters. Then take the 50mw green and 70mw blue, and you put them slightly out of focus so that the dot is the same size as the red. Wouldn't this be a good system for home use ? No red glow around the beams/dot, and quite safe.

    I'm just wondering what to buy next.. marconi's 660nm-225mw module, laserwave's 635nm-100..150mw..??

    Thanks for any advice.
    Wim

  2. #2
    Join Date
    Jan 2006
    Location
    Charleston, SC
    Posts
    2,147,489,216

    Default

    I mentioned this in your sunlight to laser power comparison thread already, but in short, 635 nm red will be safer because it will appear brighter at a lower power level.

    The divergence isn't the issue. The lens in your eye can focus a divergent beam to a spot just like it can focus a parallel beam to a spot. It just has to change shape a little bit. (That's what your eyes do natually.)

    Expanding the beam makes it safer because the power is spread out over a larger cross-sectional area, so less power can make it through the pupil and be focused to a spot on the retina.

    Adam

  3. #3
    Join Date
    May 2008
    Location
    Wisconsin
    Posts
    449

    Default

    So is it more dangerous to view a class IIIa with focusing optics? Several Class listings I've seen say it is.

  4. #4
    Join Date
    Jan 2006
    Location
    Charleston, SC
    Posts
    2,147,489,216

    Cool

    "Focusing optics"? On the laser, or in front of your eye?

    Your eye will focus the beam even if it's divergent. (Well, within limits of course.) If you put a lens in the beam before your eye, that just means that the eye does less work. (Like wearing a pair of glasses.)

    Where you get into trouble is if the lens can gather more light than your eye normally would and concentrate all that energy before it enters your eye. (Where it will be further focused to a small point.)

    Most lasers have a beam diameter that is less than 7 mm, so the entire beam can enter the eye. In this case a lens has no effect on your exposure. The same amount of light enters your eye, and the only difference is that the lens in the eye doesn't have to work quite as hard to focus the beam to a spot.

    However, if you expand the beam so it is larger than 7 mm, then only a portion of the total beam power can enter the eye. This is the basis behind Bill's earlier statement that "divergence is the key to safety." This is where a lens can get you into trouble.

    I'm not exactly sure what the class listings you mentioned were referring to, but I suspect they're talking about viewing a laser with a pair of binoculars or something similar. Binoculars are dangerous because they have a large aperture when compared to the pupil of the eye. Thus, the binoculars can gather *much more* light. The problem is that they focus all that light down to a small beam. Thus all that extra light can pass through your pupil, which negates the benefit of expanding the laser beam in the first place.

    In this case, a beam that was greatly expanded in order to make it eye-safe would probably not be eye-safe if viewed with a pair of binoculars.

    Adam

  5. #5
    Join Date
    May 2008
    Location
    Wisconsin
    Posts
    449

    Default

    That was thorough and very well put, thank you.

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