By: Michael Higlett and John O’Hagan
Health Protection Agency, UK
Figure 1. Laser show circular pattern passing over the two “eyes” of the detector
Lasers have been used in live entertainment since the 1970s to make the overall experience more spectacular for the audience. To form visual effects, a moving high power laser beam is required, obtained by moving the laser beam by computer-controlled galvanometers. Many laser display companies wish to direct laser beams towards the audience – so-called audience scanning. However, some scan patterns require the same location to be repeatedly scanned, which may lead to the potential for injury, particularly to the eye, where a person receives a burst of laser pulses as the pattern passes across the face. IEC/TR 60825-3, “Safety of laser products – Part 3: Guidance for laser displays and shows” [1], provides guidance with recommendations for safety on the planning and design, set-up and conduct of laser shows that use high power lasers. These guidelines contain safety criteria for the protection of the public and persons in the vicinity of laser shows. If the guidelines are not followed, the likelihood of injury is increased. Although safety of laser shows is increasingly better appreciated, every year a few suspected eye injuries are reported.
The simplified approach for assessing the safety of a laser display is to compare the highest power static laser beam and assume that all of the power enters the eye. Assuming an exposure duration of 0.25 s, the maximum power that can be permitted without exceeding the maximum permissible exposure (MPE) for a visible laser beam is 1 mW. This approach would show that audience scanning would not be permitted for almost all laser displays. A moving laser beam is not much safer if the same location (e.g. the eye) is repeatedly exposed [2]. Ideally, on-site measurements to complement the initial numerical assessments would be advantageous, allowing experimental measurements to confirm whether the laser show is eye-safe.
Figure 2. Captured a simple circle pattern at different scan rates
To make this possible, a prototype instrument has been developed to carry out a comprehensive real-time safety assessment of a laser show. The Laser Scanner System (LSS) can be used in a laser show to quantify the hazard from the laser beam(s) providing feedback in real-time and stating any risks so that the show can be modified or, if necessary, stopped. The limiting exposure criterion can be the MPE for the actual exposure conditions, or can be higher if accepted by enforcing authorities, as proposed by Murphy and Makhov [3].
The LSS has been designed to capture every possible exposure to laser radiation, to be as compact and as portable as possible, while resembling a human head. To capture the exposure, photodiodes represent the human eyes with pupil diameters of 7 mm and are independently connected to a fast data acquisition card. The location of the LSS during the laser show will be determined from an initial assessment considering the overall layout and defining where the worst case scenario would appear most likely to occur. It can also be used to assess occupational exposure. The software has been designed so that all parts of the laser show can be measured, from a single pass of the beam across the detector to multiple passes at various stages throughout the performance. This is analyzed close to real time to provide feedback to the interested parties.
Figure 3. Laser Passes across left eye photodiode over a 10 second period for complex pattern
Initial testing of the LSS was undertaken using a He-Ne laser beam passing through a pair of motor-mounted mirrors to create simple patterns, from a simple circle to more complex structures. Figures 2 and 3 show some results acquired by the laser scanner system. None of the measured test patterns resulted in exposure above the MPE.
An interesting outcome from the two sets of measurements was the number of laser passes across the photodiode in 0.25 second. For the simple circle pattern, multiple passes were observed in 0.25 second, while the more complex pattern only single passes were observed in 0.25 seconds which were shorter than those measured for the simple circle. This clearly shows the assessment of the laser show depends on the scanning pattern. The next step is to carry out further testing of the laser scanner system hardware and to carry out tests with more complex laser shows allowing us to push the hardware and software to its limits.
References
[1] IEC/TR 60825-1. “Safety of laser products – Part 3: Guidance for laser displays and shows”. International Electrotechnical Commission, Geneva, 2008.
[2] Corder, D A, O’Hagan, J B and Tyrer, J R. (1997) Safety assessment of visible scanned laser beams. J. Radiol. Prot. 17, 4, 231-238.
[3] Murphy, P and Makhov, G. (2009) Scanning audiences at laser shows: theory and practice. In Proceedings of the International Laser Safety Conference, March 23-26, 2009, 334-343, ISBN 978-0-912035-24-6, Laser Institute of America, 2009.