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Audience scanning (sometimes called "crowd scanning") refers to when a laser effect is allowed to come into direct contact with the people observing a laser show or display. Although this is necessary for many laser effects such as tunnels, it is often considered potentially dangerous due to the possibility of eye exposure to damaging levels of light.
Despite lasers having an extremely intense and focused output of light, in practice laser effects are created by scanning the beam very rapidly, and thus even if it is directly viewed by the eye, the net exposure of light is actually very small. Although should any part of the scanning mechanism fail then it is possible to expose people for far longer.
Additional recommended knowledge
Legality and regulation
International legislation concerning the legality of audience scanning varies greatly. The biggest point of contention is the method of calculating the level of exposure actual received. The MPE (Maximum Permissible Exposure) is essentially the same throughout the world, but some countries are far more conservative in their estimations of the amount of light received by the eye. Audience scanning is not widely practiced in the United States; it is far more acceptable in the United Kingdom, despite being stopped for a period, and the rest of Europe.
Relatively few laser injury reports
The lasers used for many laser displays have energy levels that can exceed the MPE within a few tens of microseconds. Measurement and calculation techniques both show that the beam durations that audience members are routinely subjected would indicate that the MPE is being exceeded, often by a significant factor. Yet reported injuries from medical reports, and even anecdotal reports are rare.
For example, a 1996 study commissioned by a lasershow-related company tried to find worldwide reports of audience scanning injuries at any time during the then-20 years of laser light shows. As reported in a 1997 paper presented at the International Laser Safety Conference, the study found only five accidents (claimed injuries) and two incidents (potential injury).  Accounting for an estimated 90% underreporting factor, the ILSC paper estimated that there were roughly 70 injuries per decade -- a relatively small number considering the total number of concert-goers and disco patrons exposed to laser light, some night after night.
(Since 1997, there have been one or two reports of serious accidents involving pulsed lasers being used at discos. Pulsed lasers can be much more dangerous than the continuous-wave lasers used for audience-scanning displays. Because of the danger, they should never be used for audience scanning, or where there is a danger of the beam going into the audience.)
Since reports of laser shows substantially exceeding the MPE are so common, yet reported injuries are so infrequent, this leads to the question of "where are all the injuries?".
Many safety experts are certain that the injuries are occurring, and are going unreported for several reasons. It is also a fact that the eye can often receive several thousand "damage spots" or lesions to the retina without it causing a significant problem to the subject's visual acuity.
The consensus among many respected safety professionals is that the although the study conducted in the mid nineties had good intentions in trying to establish its goals, the methods in which the study was conducted were somewhat flawed, and could not give a proper indication as to how many injuries are occurring.
Possible reasons for the lack of reported injuries
Injuries are not being noticed
Injuries are not attributed to laser exposure
Other mechanisms are reducing or preventing noticeable injuries
The 1997 ILSC paper presented some additional reasons why injuries are either not really happening, or are not being reported. Among the suggestions:
Audience scanning calculations
In making safety calculations for audience scanning, there are two steps. The first is to determine the static laser beam parameters, such as laser power, divergence, audience distance etc. The result will give the irradiance, MPE, Nominal Ocular Hazard Distance (NOHD), and other safety-related characteristics for a fixed beam at the closest point of audience access. This is the "worst case".
The second step is to calculate the effect of scanning this fixed beam. As the beam passes one or more times over the eye, it creates one or more pulses. Thus, single- and multiple-pulse MPEs come into play. For simple, repeated scan patterns, it is possible to calculate the maximum possible exposure, by looking at the location (such as an edge) where the beam is moving slowest (longest dwell time).
But for a laser show, where there are many different scan patterns, it is nearly impossible to calculate the "worst case" location for viewing the show. This is where making multiple measurements of the show can help.
Doing measurements requires caution in using the proper type of instruments, and in correctly setting up and interpreting the measurements. There is no single right answer, since a detector could "pass" the show in one location but "fail" it in another. Taking measurements at a number of different locations -- say 10 or 20 -- can give a general indication as to the show's intensity. However this also requires running the show 10 or 20 times, which can be difficult in many applications (e.g., a one-time, one-night show).
There are a few commercial systems available which help with audience scanning calculations and measurements.   One software program is available in a free "Lite" version which can be used to determine basic beam parameters, and worst-case static beam and single pulse calculations. Knowing these parameters is a good first step in making audience scanning safe.
How to make audience scanning safer
In November 1998, a panel of safety experts and laser operators, convened by the International Laser Display Association, issued a joint statement regarding audience scanning. It included these guidelines and tips, plus a cautionary statement.
Caution: No system or test can absolutely guarantee eye safety when deliberately scanning the audience. You should use accepted instruments and practices to check the questionable parts of your show. The following tips are general ways to make your show safer through good design practices, and if accepted instruments are not available at your show site.
Audience scanning is discussed as a laser show risk in "A Risk Assessment Methodology for the Use of Lasers in the Entertainment Industry", PhD thesis of John O'Hagan of the U.K. Health Protection Agency, Radiation Protection Division. . See especially section 5.5, Scanned Laser Beams, starting at PDF page 7 (printed page 53), available here.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Audience_scanning". A list of authors is available in Wikipedia.|