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Thread: Picture of crowd-scanning effect -- why is this safe?

  1. #1
    Join Date
    Sep 2020
    Bellevue, WA

    Default Picture of crowd-scanning effect -- why is this safe?

    I've posted a number of dumb questions here and gotten great answers which have basically scared me away from ever having a laser projector, haha. But I remain curious because I do have a bit of the bug... You know how it is. I'd like to take courses but have not yet had the opportunity.

    I have another rookie big picture question here and if anyone can explain what I am seeing I would sure appreciate it.

    The other day I had a dance club stream on and I noticed that they were doing crowd scanning laser effects. The picture is crummy because it was a dark room, but I lightened it so you can better see the layout. You are looking at a dance floor full of people. The green arrows show segments of a color laser display--slowly-moving geometric shapes and simple animations. This projector is approximately at the camera's location and the beams are all over the floor and the patrons.

    The red arrow show a beam from another color laser effect off to the right side of the dance floor, shining more towards the camera. This beam also seems to scan the crowd. I can't really tell what kind of animations it does.

    Click image for larger version. 

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    I know these are lasers and they use Quickshow, I asked. I could not get the names of the fixtures, though.

    So, why is this kind of crowd scanning OK? My impression was that these kinds of displays with slow-moving effects that could have long dwell time on the eye were inherently unsafe. But obviously, I must be mistaken.

    Thanks a bunch if you can help me understand this scene!

  2. #2
    Join Date
    Sep 2007

    Default Crowd Scanning

    Others will comment as well, I’d imagine…

    It was hard for me to see much in your photo, but…
    In the US, crowd scanning is only legal under certain conditions:

    1) Proper training and safety understanding, LSO, etc. IMO
    2) Proper hardware to safely perform crowd scanning
    3) Varianced show that includes crowd scanning

    I’d expect that most shows in the US where crowd scanning is being performed are likely not being done under legal conditions. There are many that would argue against what I have listed as too restrictive, not necessary, etc., but it’s the law and an approved variance is the minimum with expectations of safety measures taken to allow for this. As a result, I would expect that many folks doing legal laser shows would avoid crowd scanning in the US, both as a result of the constraints of the variance, as well as the liabilities that could result from doing so. Personally, I would not do crowd scanning if I was doing a show - I don’t have the right equipment and being insured to do this is likely a whole other consideration.

    Just my .02…
    Last edited by Displaser; 09-18-2023 at 18:01.
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  3. #3
    Join Date
    Sep 2020
    Bellevue, WA


    That sounds like you are thinking "this show is probably breaking the rules" but don't want to say it. I am not trying to get someone to formally evaluate this show as naughty as a gotcha. I'm really just tying to understand what kind of things are possible to do safely. Despite poking around this subject for a while I have yet to develop that understanding. I've just become more wary of lasers, haha.

    If I can get a better handle on the big picture I can decide to pursue training and paperwork. If my own desired uses are just impractical and unsafe, I am hoping to figure that out before I spend that time and money.

    This promo video from the club shows a better view of some crowd scanning, including scanning the band on stage. It also shows a projector being used to write on a screen behind a band. You can watch from about 25 to 60 sec and see a good sample, but there are other laser effects through the end at 3:25. These examples are relevant to my own interests.

    For the sake of illustration... Assume this was done 100% by the book and this was a safe crowd scanning show. Is it possible to say things like "clearly the projector could not be greater than XX mW" and other generalities that illustrate the effort?

    I know this is kind of like asking, "what size scalpel do you need to remove an appendix?" It's not that simple a question. But any way it can be simplified would be extremely helpful.

    And if the answer really is "that looks crazy, I wouldn't set foot in there" that's great too.

  4. #4
    Join Date
    Oct 2012


    .. if crowd-scanning is "safe" (legal) or not, depends mostly on energy density and timing of the beam - if it's higer than a specific threshold, then eye (or even skin) damages couldn't be avoided, so an absolute nogo!

    To get the laser below this "threshold" you can reduce the power and/or enhance the scanning speed (damage level ist mainly depending on energy density per time for the specific wavelength - and so thermal interaction with the surface, hit by the beam) ... and the electronics have to secure, that no "stranding beam" will occure, if the scanner will block for failure or mechanical issues ...

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  5. #5
    Join Date
    May 2014


    While we're on the topic of crowd scanning, this looks totally not legal artist-scanning.
    Check out the bass player around the 2:00 minute mark.
    He's getting a constant face-full of laser -- it goes on that way for most of the video.

    The divergence does not look that wide, and the white beams seem to be relatively static lines across his face.

    Seems odd for a professional show like this. I wonder if any of this is legal/varianced?

  6. #6
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    May 2007
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    Audience scanning requires a variance, approved control system such as Pangolin's "Pass", each effect must be tested and documented as safe using a variety of precise, expensive, and calibrated equipment. A pre-show, instrumented, quality control session is required, and in some cases cumulative exposure must be monitored and timed. There is a minimum distance to the audience that has to be observed. Lots of documentation, and generally a safety evaluation by an outside, qualified, non ionizing radiation specialist.

    At any given time there are maybe 5-10 such variances in the US max. If done properly, choreographed properly, it is enjoyable, but somewhat weak effect.

    My friend who has three such projectors spent about 25,000$ over the top of the projector cost, hired an expert, bought the logging radiometer, LPM, oscilloscope, fast photodiode, etc... He spent beaucoup money on training and insurance, and is reluctant to use the gear for anything but the most special of gigs. Yearly traceable calibration of the measuring gear is expensive. Such gigs are filmed, because one of the bigger hazards of such gigs is an audience person with a high power pointer mimicking the effects, which has happened with bad results in Europe. Having video saved the laserist in that case from a huge financial settlement.

    If I ever do gigs again on my own again, the show rider says "Pointers will be banned from the facility, a notice of such will be posted, and event security agrees to confiscate ANY active pointers, any questions, see 21 CFR 1040.1.J"

    Guess who is legally responsible during a show if an exposure comes from ANY coherent source. Hint, it may not be the pointer owner...

    That said, it can be done, and really you just need good insurance, avove average hardware, math skills*, and a tight Quality Control (QC) proceedure.

    *College Level General Math / low end Algebra.. class, not even Calculus.

    Last edited by mixedgas; 09-20-2023 at 08:59.
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  7. #7
    Join Date
    Sep 2020
    Bellevue, WA


    Thank you Steve, that is super interesting and exactly the kind of overview I was hoping to get!

    Since I do see crowd scanning happening in casual shows like the video I linked, I was really confused about what was kosher. I thought surely, so many people aren't breaking the rules by blasting animations onto the dance floor? I now have a healthier level of skepticism. And I finally understand that anything like the video I linked is absolutely impractical for personal use.

    (I don't actually want to do crowd scanning myself -- I want to use a laser projector to display art on a stage behind a musician. But I cannot lock down access to the beam path, so my concern is making accidental exposure safe. Same answer as deliberate exposure, though... Making it safe is complicated and expensive.)

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