Audience scanning

  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.

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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). ^[1] 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

• 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 roughly 10 times below 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

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. [1] [2] 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.[3] 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.[4]

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.

• Do not scan with pulsed lasers (e.g., metal vapor, pulsed YAG, pulsed solid-state). They are inherently hazardous due to the power of each pulse. It requires exacting calculations to even consider scanning an audience with pulsed lasers. Because of the great potential danger, use continuous wave lasers (e.g., he-ne, argon, krypton, diodes, cw YAG, cw solid-state) only.
• Increase divergence. For ranges of less than about 30 meters (100 feet), using a lens to increase divergence can allow for visually effective power levels while maintaining controllable irradiance levels. A bright, fuzzy beam is far safer (and more visually effective) than a dim, tight beam with the same irradiance.
• Don't use a single beam. You should never aim a single beam into the audience. In general, if a single beam is safe, then any scanned effects such as cones and fans will spread the light out, and be too dim to be effective.
• Move the projected effects. When projecting a fan or tunnel, move the effect through the audience. This reduces the multiple pulse accumulation.
• Don't rely on faster scanning. In general, you will not increase safety by scanning faster. Although the beam spends less time in the eye, there are more crossings of the eye, and thus the total light energy delivered remains about the same.
• Attenuate power with size. The smaller the projected effect, the greater the concentration of energy. Any effect that grows from a point, or shrinks to a point should have a proportional fade in/ fade out.
• Limit anchor (dwell) points. Anchor points reduce beam velocity and increase exposure. Where possible, use blanking to emphasize beams, rather than anchor points.
• Use a scan fail interlock of some sort. Chances of a still beam from a laser entering someone's eye are small, but consider the consequences!
• Program "no-exposure" periods in the show. Allow time for the eyes to recover by parking effects outside of the audience area. A good "no-exposure" time is 10 seconds or longer.
• Measure the irradiance. Typical shows should not exceed 10 milliwatts per centimeter squared, or 100 watts per meter squared. You need to know what you are delivering to the audience.
• Quick test for aversion response. Note: The following tip is only for use when you believe your show is safe by using the above tips AND you are aware you could damage your eyes if your show is not safe. Use your computer or PCAOM controls to set the laser output to all green or all white. Run the show while standing at the closest audience access point. As the laser crosses your eyes, evaluate the brightness. If you have a desire to avert your eyes, you are probably approaching or exceeding the internationally agreed safety levels (MPE).
• Respect the audience. Not everyone enjoys bright lights in their eyes. Remember that they trust you to ensure their safety.

References

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. [5]. See especially section 5.5, Scanned Laser Beams, starting at PDF page 7 (printed page 53), available here.