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Thread: Info on scan fail systems

  1. #71
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    Quote Originally Posted by mliptack View Post
    I perplexed my high school physics teacher when he was talking about the inverse square law - I asked him if lasers follow the law.
    The inverse square law refers specifically to isotropic radiators like black bodies, and lasers are not isotropic. The square term comes from the formula for the surface area of a sphere, a = 4.pi.r^2

    Dig it yo, maths in da house.

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    Interesting.

    Can anyone who can calculate MPE throw this in the mix:

    1. 200-250mw - what's the min safe distance?

    2. What effect does having a lens of minus 12 diopters on the front of the above make to min safe distance. ie what's the new safe distance?

    I'm trying to work out where to go next with my laser and I'm wanting a minimum safe distance of around 2-3 metres.

    Also, does the min safe distance work out for scanned patterns or is it for pencilled beams?

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    Quote Originally Posted by White-Light View Post
    Interesting.

    Can anyone who can calculate MPE throw this in the mix:

    1. 200-250mw - what's the min safe distance?

    2. What effect does having a lens of minus 12 diopters on the front of the above make to min safe distance. ie what's the new safe distance?

    I'm trying to work out where to go next with my laser and I'm wanting a minimum safe distance of around 2-3 metres.

    Also, does the min safe distance work out for scanned patterns or is it for pencilled beams?

    what is your beam diameter and divergence?

    Steve

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    2 to 3 meters... That seems a bit close. My projector is about that from where I usually sit to watch and can't imagine making my beams safe for scanning myself without an attenuation map or some serious optics that would probably kill the "beam" effect. The beams are about 7 to 10 mm at the wall a meter... yard, behind my chair. Now 20 to 30 meters, I would consider doable.

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    Ok as I haven't bought any other lasers at the moment I'm going to give you Laserwave figures for their green as I reckon that's about the tightest I'm going to get from them.

    Divergence Full Angle >2mrad

    Beam Diameter at Aperture ~ 3mm

    Yes I know the viewing distance is close but for home parties unless you live in a mansion people are going to be potentially close up so I need to plan for the worst case scenario. TBH I think in many small clubs, the nearest person probably isn't much if any further away although I know that doesn't mean they're doing it safely!

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    your nominal ocular distance works out to roughly 113 meters.
    for a 1 hour viewing the software says a minimum of 70 Cm for a diffuse viewing. I'd triple that.

    Steve

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    Hi Steve,

    Just so I understand this (sorry for being a bit newb at this), 113 metres is for a static pencil beam and 70cm is for a fast scan, right?

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    113 meters is your nominal ocular hazard distance, in other words you want to be at least that far away before taking a direct hit the eye.

    The 70 cm distance is scattered spot on the wall distance, that is a static beam assumed to be uniformly scattered, and I'd prefer you use 210 cm for that at least.

    The scanning calculation has to be done for each individual effect in the show, with respect to power, time of exposure, scan angle, and total overall exposure.

    I can't give you one number fits all, it just doesnt work that way except for static, or nonmoving, beams.

    Otherwise the only remotely safe assumption is to use a class II laser at 0.95 mW or less, and don't do that one in the states.

    for the basic math, see this thread, and takeit with a grain of salt, because you have to measure each effect with a photodiode and a oscilloscope, or a commercial MPE meter to get it right.

    start here, for the math, and read my disclaimers. I posted scans of the CDRH's (then the BRH) own guidelines for audience scanning. You must calculate each effect and the total exposure for the show. You also cannot make assumptions like scanning faster = better, because the effect repetition rate can come back to haunt you as at some point it actually gets you more exposures. You also have to check your power at each show, you cannot assume the laser is safe just because it was at 200 mW last week.

    You are also trusting that your software makes no changes to the frame from show to show,and that its timing doesn't change from show to show or even frame to frame.

    SEE : http://www.photonlexicon.com/forums/...+scanning+cdrh


    Steve

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    Ok Cheers Steve.

    I guess I'd need to measure the effect the lenses have on diameter to work out a correct figure.

    I'd like to get down to a point where I can have an RGB that can scan safely from small distances. I do realise effects are cumulative but I guess if you can get it down to a figure where there's an hours safe exposure and then mix in regular overhead effects with the scans to giving occular cooling off periods and to lengthen that hour out, you can be pretty sure, you're ok for the duration of most home BBq's / party's although I realise any assumption is a risk in itself.

    If that means going down to 20mw blue, 10mw red, 10mw green, then so be it. As long as I can get beams bright and highly visible I'm not too concerned. I guess the obvious choices here would be 635, 473 and 532 although I don't know of anywhere I can get 635 at 10mw. I'd even go to 5mw (30mw white) if thats what it took.

    However, if higher power levels are possible through lens use then that would be an advantage. Like I said earlier I have - 12 diopters worth of convex lenses so thats a pretty powerful diverging effect. Given most people can't see their hand in front of their face with - 5 diopter glasses (or should that be without!), thats quite some power.

    I suppose the ohter way is to put more powerful lasers in then modulate the power down but then you're dependant on the software / absence of programming errors for your safety. On this subject, I wonder if anyone has ever put a switch in a projector to reduce the voltage through the use of a resistor to manually give 2 power levels eg. full ananlogue controlled and at the flick of the switch, mechanical reduction in power to a pre-set level. Maybe thats an answer.

    I was very impressed from the results of Anthony's scanner as it seems to be punching well above its station although I noted that it tended to be quite tightly focused beam effects rather than diffuse tunnels. However, at 200mw +, it might still be over power for safety for my purposes.
    Last edited by White-Light; 02-01-2009 at 15:02.

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    Red face

    I strongly urge you to use the lenses. 1 it will catch the smoke better, and 2, its mondo safer!

    um, measuring the diameter is easy. Put a razor blade on a micrometer stage. Move the slide in until the power decreases by 13%, note the reading.
    Move the slide in until power decreases to 87%, note the reading. Assuming a TEM00 beam, ie not donut mode, that is your 1/e^2 diameter, which is used for the divergence calculations. Do this at two points along the beam, but make sure you are past the beam waist if it is outside the cavty.

    then Theta = 2 arctan ((Df -Di)/2l)

    where L is the distance between the two measurements.


    or see:

    http://www.uslasercorp.com/envoy/diverge.html

    http://www.answers.com/topic/beam-divergence

    http://cord.org/cm/leot/Module6/module6.htm

    I have nothing against audience scanning if you do the measurements and that math. I do have something to say if people dont!

    Steve
    Last edited by mixedgas; 02-01-2009 at 15:34.

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