I was under the impression that anything over 5mW can be a danger (i.e. not compensated by the blink reflex) so how are clubs allowed to shine 100mW beams onto dancefloor revellers?!
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Answer, in most places, it is not legal, without doing it correctly and with notification to the authorities. However, enforcement can be lax.
Simple question, what legal jurisdiction are you in? If the US, IIIB does not legally touch the audience in any way, unless you have one of the five to ten audience scanning variances rumored to still exist in the US. A variance is a de facto permit to use a entertainment laser in public, in the US.
In Europe and elsewhere, except Sweden, if you can get the light fairly randomly distributed and kept below a certain well defined cumulative exposure per unit time, your fine. In most nations, if a incident happens, the operator gets charged with Assault, sued, etc. Some nations do require inspection or certification by a third party.
The math for the distribution is not easy to explain in one post, and suffice it to say it does depend on a large number of parameters, you can't just aim and shoot without calculations and measurements, even in jurisdictions where it is legal. Many factors are involved, diameter, divergence, exposure time, pulse width, wavelength, distance to the audience, etc.
If you can keep incident energy below the threshold, there is a low chance of eye damage. However incidents have happened in Bulgaria, Russia and other places where damage to the vision of many people in a matter of seconds has occurred. Usually involving a industrial or medical laser being misused, and in one well documented case, a pointer at a rave that had safe lasers in the background.
Static, slow moving, or stopped beams are highly dangerous, however scan speed is not the main driving force behind safety. Its very much NOT a case of faster = safer.
The fact that it is not always legal, does not stop many sellers of low cost products from just drop shipping units from China. That does bend/break the rules in many places, such as the US,UK, Germany, Australia, and Sweden. Its hard to intercept a horde of small international shipments, that are often mislabeled, so people get away with things they should not. In fact most vendors these days may not even know they are selling a potential hazard.
I should mention that people who do this professionally often have sophisticated software and hardware devices known as "scanfails" as well as good measuring instrumentation, to make sure their effects are safe.
The debate over why some governments fail in enforcing the rules is a matter of another post, and has been debated to death here and elsewhere. As lasers are starting to follow Moore's law, I suspect you will see incidents and rule changes in the future. This is due to lasers getting cheaper, easier to make, and more powerful at the same time, while being sold to people who assume if its in a catalog, its safe.
If you have further technical questions, pop into PhotonLexicon and I'll show you some math and documentation. Again, its based on cumulative energy per unit exposure time. The fact of the matter is, it is never 100% safe to audience scan. Audience scanning, and all other laser safety, is based on probability and statistics, and physics. What actually happens is a scenario is set where the odds of damage occurring is low, say 1 case in 100,000 or 1 case in 1,000,000 exposures.
Steve
