| Revision as of 00:31, 22 August 2014 edit88.104.20.79 (talk) →ILDA Audience Scanning Guidelines: remove WP:QUOTEFARM← Previous edit | Revision as of 00:32, 22 August 2014 edit undo88.104.20.79 (talk) →Possible reasons for the lack of reported injuries: remove, pending references; I don't think Misplaced Pages should state "Possible reasons" for lack of injuries without good refsTag: section blankingNext edit → | ||
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| 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).<ref>"Is Deliberate Audience Scanning Unsafe?", Patrick Murphy, Pangolin Laser Systems. As reported in ''Proceedings of the 1997 International Laser Safety Conference'', Vol. 3, pp. 493-502, Laser Institute of America, Orlando, Florida 32826.</ref> Accounting for an estimated 90% under-reporting 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. | 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).<ref>"Is Deliberate Audience Scanning Unsafe?", Patrick Murphy, Pangolin Laser Systems. As reported in ''Proceedings of the 1997 International Laser Safety Conference'', Vol. 3, pp. 493-502, Laser Institute of America, Orlando, Florida 32826.</ref> Accounting for an estimated 90% under-reporting 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. | ||
| ==Possible reasons for the lack of reported injuries== | |||
| {{Unreferenced section|date=April 2014}} | |||
| ===Injuries are not being noticed=== | |||
| *'''Affected viewers simply don't notice any vision problem.''' The human vision system has ways of compensating for damaged areas, especially when this happens on the periphery. Often sophisticated tests are needed to map out the retina and determine areas with a loss of vision. | |||
| ===Injuries are not attributed to laser exposure=== | |||
| *'''If vision loss is noticed, this may be put down to other sources instead.''' Persons affected may not realize that a laser exposure caused their visual problems. | |||
| ===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: | |||
| *'''MPEs have a built-in safety factor.''' MPE levels are set at roughly 1/10 the level at which injury could be seen in 50% of typical human eyes. Thus, if audience scanning is done at a level of 10 times the MPE, there is still only roughly a 50% chance that a detectable change to the retina can be seen. | |||
| *'''Viewers are not always looking at the laser source.''' In a disco or concert situation, the focus is not usually exclusively at the laser source. Therefore, laser light will either miss those looking away from the source, or will enter the eye at an oblique angle and be focused on the periphery of vision. | |||
| *'''Viewers take aversion actions if the light is too bright.''' This has been routinely observed at shows with bright lasers. Beams do not usually pop on; it is often possible to tell that a particular beam or effect is about to cross one's face. Aversion actions include moving one's head, looking away, glancing down, squinting, and blinking. | |||
| *'''The pupil is smaller than the dark-adapted size of 7 mm.''' Most displays are not performed in total darkness; thus a pupil size of 4–5 mm is more likely. At 5 mm, the pupil is only about 50% of the area of a 7 mm pupil. | |||
| *'''The pupil is relatively far from the laser source.''' In laboratory and industrial accidents, the pupil is often within a meter or two of the final reflected surface. In most displays the audience is much further. This gives the beam more room to diverge. | |||
| *'''There is a small chance of hitting a pupil,''' in cases of accidents or poor design where a powerful static beam may stay fixed for many seconds. For example, the total pupil area of 100 persons in a nightclub (scanfield of 10 x 10 meters) is roughly 1/25000 of the total area scanned by the laser. Thus, any randomly positioned beam would have only a 1/25000 chance of directly hitting a pupil. | |||
| *'''The audience is spread out.''' The MPE must be calculated for the closest audience members, meaning that those farther back will receive less light, for two reasons: 1) the beam can diverge more and 2) the linear velocity of a scanned beam increases with distance. Depending on the situation, the beam power may be much-reduced for farther-back audience members | |||
| ==Audience scanning calculations== | ==Audience scanning calculations== | ||
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Audience scanning (sometimes called "crowd scanning") occurs when a laser beam is directed toward the people observing a laser show or display. Although this is preferred for many laser effects such as tunnels, it is potentially dangerous due to the high levels of laser radiation exposed to the eyes of audience members.
Laser effects involving audience scanning are created by scanning the beam very rapidly across the audience, ensuring that the net exposure to any one eye is very small. Should the scanning mechanism fail, however, it is possible to expose people to laser light at levels that may produce eye injury.
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 actually 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 accepted in the United Kingdom, despite having been stopped for a period, and in 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% under-reporting 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.
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.
ILDA Audience Scanning Guidelines
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 guidelines and a cautionary statement.
"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."
References
- "Is Deliberate Audience Scanning Unsafe?", Patrick Murphy, Pangolin Laser Systems. As reported in Proceedings of the 1997 International Laser Safety Conference, Vol. 3, pp. 493-502, Laser Institute of America, Orlando, Florida 32826.
- Scanguard
- Scanguard Lite
- The Laserist, Spring 99
External links
"July 5, 2008 Russian accident", International Laser Display Association, statement on audience scanning accident.
"Making shows safe and enjoyable", William Benner, chair, International Laser Display Association Technical Committee, fall 1997. and also at
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.
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