Audience scanning is a unique and very beautiful art form. Audience scanning shows are very popular around the world and have been performed for many years. Indeed, audience scanning is the one thing that sets laser shows apart from any other medium-the ability to reach out and touch the audience. While there is no question as to the effectiveness of these shows, there is a continuing debate as to their eye-safety.
Traditionally, it has been difficult for both show producers and safety regulaiot's to evaluate the safety of audience scanning shows. This has led to scenarios ran,,ing from show producers unknowingly creating unsafe shows, to ultraconservative regulators not allowing audience scanning at all. Recently, two new instruments have emerged to aid in the safety evaluation of audience scanning and other laser effects. They are the Laser MPE meter from Preci sion- Optical Engineering in the United Kingdom and the Laser Safety Measurement System (LMS-2) from LOBO electronic in Germany. Each of these instruments uses a different approach to safety measurement, so each will be covered by its own article. This article looks at the Laser MPE meter by Precision-Optical Engineering, which I recently had the opportunity to evaluate. A follow-up article in the next Laserist will review the LMS-2 system from LOBO
When evaluating laser exposure from AOW-L a safety standpoint, show producers and regulators should refer to the laser safety standard for their country. These standards recognize the concept of a damage threshold and prescribe certain Maximum Permissible Exposure (MPE) values. Essentially, the MPE is the maximum amount of laser light that you can be exposed to without experiencing "adverse biological effects." The MPE depends on wavelength, laser power, exposure duration, pulse characteristics and the target organ (eye or skin). Although there are multiple safety standards issued by various regulatory bodies, the main difference is the unit of measurement (watts per square centimeter vs. watts per square meter, for example). The various standards are in surprising agreement as to actual MPE levels. Safety evaluation of audience scanning shows is a complex task. From a pure calculation standpoint, it generally involves six steps: 1) calculate the beam velocity; 2) calculate the pulse width as it crosses the eye; 3) calculate the stationary beam power at the audience; 4) calculate the energy per pulse; 5) calculate the average power; 6) using the applicable MPE, calculate the permissible time for the effect to be viewed. Each of these "steps" generally involves more than one actual calculation. When these steps are done by hand, each step is tedious, error prone and takes a surprisingly long amount of time. While calculations of this type can be performed to evaluate simple effects such as cones, tunnels and fans, it is much more difficult or even impossible to evaluate more typical effects like multiple beams, moving waves and overlapping sheets all being projected simultaneously. Having done these calculations by hand myself, I have often wished for a meter that I could hold up into the scanned image, or place in the audience, which would tell me whether a particular effect was safe or hazardous. Both the Laser MPE meter and the LMS-2 system are answers to my silent wishes. The Laser MPE meter is made by Precision-Optical Engineering which is a division of British Aerospace Defense Ltd. The Laser MPE meter is a portable, battery operated, high quality instrument that can evaluate the safety of continuous-wave or pulsed lasers. The unit is calibrated to traceable standards.
The Laser MPE meter was not designed specifically for audience- scanning laser light shows. Instead, it was designed to be a general laboratory and field instrument. It can, for example, measure indirect scatter from a dirty mirror or light leakage from a projector. The Laser MPE meter consists of two parts: a measurement head and a digital processing unit. The measurement head is the part that you hold in your hand and aim at the laser radiation emission. It contains a detector, selectable aperture and attenuator, and signal processing circuitry. The measurement head can also be placed on a tripod for fixed measurements. The digital processing unit is a molded plastic case which houses a digital signal processing board and a Psion palmtop computer. Use of the Laser MPE is extremely simple. The user interface on the computer and the overall operation of the unit are well thought out. After turning on the system and selecting MPE mode, you are prompted for your name, password and location. This provides an audit trail for reporting and record keeping. Once you have logged on, you are prompted for the laser wavelength. In many cases you know this exactly. If multiple wavelengths are present, Precision-Optical Engineering recommends using the lowest wavelength present, as this produces
the most conservative measurement. After you have entered the wavelength, the system prompts you for the exposure duration. For example, if you are wondering whether scattered light on a wall is safe for audience members to view for one hour, you would enter one hour. If you want to know if someone will be able to blink fast enough to protect their eye from injury, you can enter 0.25 seconds (the natural blink response time). For audience scanning measurements, I usually measure the MPE over 10 seconds (because laser exposures are cumulative, determining the length of time to measure an effect for MPE purposes is not as simple as it seems; look for a future article on this subject) Once you have entered the wavelength and exposure duration, the Laser MPE system checks to make sure that you have the appropriate aperture and attenuator selected. There are several apertures selectable on the measurement head depending on whether you are evaluating skin or eye hazards. Once you have the correct aperture and attenuator selected, the Laser MPE meter will measure the laser light. Regardless of the amount of time you enter for the exposure duration, the meter takes a one second sample and extrapolates the measurement. At that time, the display will show the measured irradiance and applicable MPE. In larger bold print you will see either "MPE EXCEEDED" or "MPE NOT EXCEEDED." A visual aid is also provided in the form of a handy analog-style indicator that shows how close you are to the MPE.
On the positive side, I found the Laser MPE meter extremely useful for laser hazard analysis. Not only can it be used to effectively measure audience scanning, but also other hazards such as light scatter, stray beams, etc. Precision-Optical Engineering also stated that they would add "Power Meter" functionality to the Laser MPE meter so that it could be used as a general power meter.
On the down side, I saw two potential problems. The first was that the computer interface asks you the same questions over and over for each measurement. When you are making repeated measurements of the same wavelength and duration, this becomes annoying. I evaluated a pre-production unit, however, and would not be surprised to see this minor software problem corrected in subsequent models. The second problem is more specific to audience scanning. Since the meter actually measures for only one second, it could not be placed in a "stare" mocie. In other words, you could not place the meter on a tripod in the audience, run a show or show segment for an extended period of time, and then see the measured results. For computer users who can pause a show to measure specific effects, this is not a major problem. However, this cannot be done for shows recorded on ADAT tape because if you pause the tape, the beam stops scanning. When we mentioned that it would be desirable to have this "stare" feature, Precision -Optical Engineering seemed receptive. This feature may be implemented by the time of this writing. Overall, I feel the Laser MPE meter is a very valuable tool for serious laserists as it allows for quick evaluation of many laser safety hazards. Be sure to read the next issue of The Laserist for a report on the Laser Safety Measurement System (LMS-2) from LOBO electronic. For more information on the Laser MPE Meter, contact: Precision- Optical Engineering: phone: (+44) 1462 440328;,fax: (+44) 1462 440329; www.p-oe.co.uk