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

  1. #1
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    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
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    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 19:01.
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    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.

    https://youtu.be/p6mZSwKPx-o?t=23

    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.

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    .. 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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    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.

    https://youtu.be/Z4b6BPaO944?t=110

    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?

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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.



    Steve
    Last edited by mixedgas; 09-20-2023 at 09:59.
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    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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    Back in most, but not all, of the 80's decade we did a handful of varianced audience or sky scanning. The agency of the time that interrogated and analyzed the empirical evidence we had to gather that proved the worst-case energy density per unit time was below the established standards was the Bureau of Radiological Health (BRH), later the Center for Devices of Radiological Health (CDRH).

    I'm fairly sure I still have some of the variance applications, data and measurement collection methods, approved measurement equipment lists for audience and sky scanning. Sky scanning at night also required additional close coordination and approval from the FAA based on location, airport and air traffic patterns and airways, area of sky scanned, beam angles above horizon, beam divergence angles, traversing beam speeds etc., once the BRH/CDRH agencies were satisfied. Worst-case failure modes also had to be identified, addressed and prevented. My sky scanning was done for a multi-state group of GM car dealerships from Dallas, TX up through Ohio using one 20W Spectra Physics 171 Argon ion laser and G120PD scanners.

    One thing I remember without looking is that the required worst-case measurement collection aperture (at the time) was 50mm, the diameter of a common set of binoculars that could be focused through the human eye. The power density entering a 50mm aperture had to be less than so many microwatts per unit time (I think it was measured in milliseconds, but I won't swear to it.

    No doubt, we were lucky back then. Safety criteria now must be hugely different.

    I can't recall the name of the high-speed diode detector and accompanying power meter we used and was approved for measurement but it had the same meter case as ye' old Simpson Model 262 volt-ohm meter.
    Last edited by lasermaster1977; 09-29-2023 at 10:10.
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    Quote Originally Posted by lasermaster1977 View Post
    One thing I remember without looking is that the required worst-case measurement collection aperture (at the time) was 50mm, the diameter of a common set of binoculars that could be focused through the human eye. The power density entering a 50mm aperture had to be less than so many microwatts per unit time (I think it was measured in milliseconds, but I won't swear to it.)

    Safety criteria now must be hugely different.
    The rules really haven't changed all that much. The collection aperture consideration is still applicable at venues where it would be reasonable for audience members to bring binoculars, such as a large outdoor football stadium, for example. Not so much in a nightclub, however.

    Regarding the original post, I agree with people above - the show in the video is most likely NOT being done in accordance with US regulations for audience-scanning laser shows. I do admit that it is very difficult (if not impossible) to judge power and exposure levels from a video, but given how difficult it is to perform an audience-scanning show legally, and how dim and, frankly, un-impressive the effect is when it's done correctly, I feel comfortable saying that this show is almost certainly illegal. Companies that have the ability to do audience-scanning shows safely and legally would not waste the effect by throwing random shapes into the crowd.

    Here's a general rule of thumb regarding an audience-scanning laser show: If the show is run by a mobile DJ in a nightclub, chances are good that it's NOT a legal show. By this, I mean that it's very likely that the laser projector is NOT certified for audience-scanning, the laser operator probably does NOT have an audience-scanning variance, and you can almost guarantee that no one measured the irradiance levels of the show in the venue before the public was allowed to enter. Illegal crowd-scanning shows like this are more common that one might expect, sadly.

    Now, if you are in a club and you see a show as described above, should you be concerned? There are 2 answers to this question, depending on your perspective:

    1) From a regulatory and liability standpoint, this is a major problem. If you are the venue owner, or worse, if you're the laserist, this is something you want to avoid at all costs.
    2) From a personal injury standpoint, this may actually not pose much of a threat.

    The disconnect here is due to the very restrictive irradiance limits imposed by the CDRH with regard to audience-scanning shows. And indeed, if you look at the physics, the low exposure limits would seem to be more than justified. Consider the power density of a 1 mw beam from a HeNe laser with an initial diameter of 2 mm when that beam is focused down to a diffraction-limited spot at the surface of your retina. Do the math (see pages 16-17), and you'll discover that the focused spot from that 1 mw HeNe laser will have a power density of around 1300 WATTS per square centimeter. This is more than 200 times greater than the power density of the focused spot from looking directly into the sun!

    So the optics math tells us that even a 1 mw laser is super scary. But if that's true, then why are laser eye injuries so uncommon? Laser shows are happening all over the world, many times with audience scanning elements (either intentional or accidental), and most of these shows are using very powerful lasers that are well above the permissible exposure level, yet we're not all blind. What gives? The answer is complex.

    First, the lens on the eye isn't perfect, so you may never get down to the diffraction limit. Second, the angle of incidence (how the beam enters the eye during a show) is usually not head-on, and off-axis beams create all sorts of odd refractions that interfere with the focus of the lens. Third, the retina is surrounded by fluid (the vitreous humor inside the eye) and is also well-perfused with blood; all that fluid helps to mitigate any local heating. Fourth, the human body is always repairing itself, so small lesions caused by excessive irradiance will often heal on their own in a few weeks. And finally, even if the damage is permanent, it's usually limited to a very small area, and the brain is exceptionally adept at mapping around any blind spots in our visual field. (Everyone has a blind spot measuring ~ 10% in size in each eye where the optical nerve connects to the retina, yet we never notice this blind spot unless we're specifically trying to make it appear using a special visual test.)

    When you consider all these mitigating factors, the rarity of laser eye injuries becomes easier to understand. That's not to say that this is a license to be reckless with regard to how you use your lasers, but it does explain the disconnect between the regulations and the reality of personnel exposure over the past several decades.

    Personally, if I was in a club and saw audience-scanning going on, I'd probably just leave. I don't enjoy being flashed by a laser, even if the exposure level is below the MPE. I might mention something to the laserist / DJ, or even to the venue manager, but I also realize that I'm not the laser police. Plus, there are many, many ways that any laser advice I might offer could be interpreted as a threat, and I don't have the patience to deal with that sort of thing anymore.

    The dirty secret regarding illegal laser shows is that enforcement is nearly non-existent. True, if the CDRH comes down on you, they can absolutely ruin your day (see what happened to LaserWord's US operations), but the likelihood of that happening is very small. To be honest, the local Fire Marshal is probably the one who is most likely to shut down an illegal show, but even there, they've almost always got bigger fish to fry.

    Adam

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    Thanks Adam, great info and write-up.

    The meter we used was made by United Detector Technology or UTD, Model 40x Opto-Meter. Here are a few photos from my Ebay posting when I sold it about 5 years ago.

    Click image for larger version. 

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    UDT is now consolidated into this group https://www.osioptoelectronics.com/company.

    Edit/Addendum: I had forgotten about this. Prior to the BRH, radiological health CFR 21, fell under the Department of Health and Human Services (DHHS).
    Last edited by lasermaster1977; 09-29-2023 at 13:56.
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