By James Stewart MSc, LVR Limited, United Kingdom
The assessment of laser beam exposure used for entertainment applications is a challenging undertaking; both the emission and the environment pose particular obstacles to persons with the responsibility of ensuring emissions are below the permissible exposure limits. This article discusses how use of Ophir’s BC20 detector is able to offer significant improvements in measurements quality over traditional laser power detectors intended for CW beams. In addition, the BC20 simplifies the measurement process and allows measurement of live effects, opening the way for assessments to be undertaken with a much greater degree of accuracy. This provides benefits to assessors, whether they are operators, venue safety staff, or regulators, who can now additionally monitor emissions and ensure they are not exceeded during performances.
Rock and pop concerts, along with other entertainment venues such as nightclubs and theme parks, use laser sources to create colourful and vivid lighting effects. Even with advances in non-coherent lighting, technology has not been able to replicate the unique appearance of laser lighting effects.
The types of lasers used for light show applications typically emit radiant powers in excess of 20W over a range of common wavelengths based on the technology employed in the projection device. Devices emitting such powers place them firmly into Class 4 laser product territory, posing a risk to eyes and skin, as well as being a potential fire hazard.
In display applications where personnel or members of the public are directly irradiated with laser radiation, (a practice more commonly referred to as audience scanning), a combination of separation distances, limiting exposure time, and attenuating the beam at source are necessary to ensure that exposure levels are not exceeded.
EU Optical Directives
Since April 2010, across European member states, through the implementation of the EU Physical Agents Directive (Artificial Optical Radiation) 2006/25/EC, the laser exposure limit values (ELV’s), derived from the internationally accepted guidance values published by the International Commission on Non-Ionizing Radiation Protection (ICNIRP), have become mandatory occupational exposure levels, not to be exceeded. They carry potential penalties if found. This legislation therefore places specific duties on the laser show provider and venue management to ensure employees in the venue are not exposed to excessive laser emissions. And with employees likely to be supervising in areas where the public is viewing the laser effects, the audience is afforded a similar level of protection.
Irradiating people safely with laser radiation inevitably means that exposure to visible beams has to be very brief, given that the amount of laser energy permissible in the exposure limits decreases with increasing exposure duration; brighter effects are therefore possible through limiting the exposure duration.
Limitations of Standard Sensors
Using a standard laser power meter and silicon photodiode sensor poses a significant problem when trying to assess a typical lightshow emission. This is because standard meters are intended to measure stationary CW laser beams. These are normally what would be considered quite sterile environments in comparison to an entertainment venue. And more so, standard sensors are unable to perform adequately in the brief time the beam directly irradiates the detector for laser effects evaluation. This problem is due to the fact that the time the laser light spends on the sensor is insufficient to provide a reading that can sensibly be used to evaluate the exposure. Essentially, although silicon photodiode based detectors react to incident light much faster than their thermal counterparts, most designs are still not sensitive enough to detect and register the peak power of the emission, and at best, will only provide an average power level indication, which is not suitable to undertake an exposure limit assessment.
Ophir BC20 Detector
The Ophir BC20 detector overcomes this limitation by providing a peak-hold detector/amplifier in its connector. This makes it capable of accurately registering short exposure emissions and reporting the peak value to the user via any of Ophir’s standard meter displays, such as the Nova II and Vega.
In practice, the difference between using this type of detector vs a standard silicon photodiode is quite startling. For example, the measurement of a stationary scanning cone effect with each type of device indicates a difference by a factor of 10. If an assessor were using the standard silicon detector for an evaluation and set the emission level, the actual exposure could be in excess of the safe exposure limit by a similar factor. The difference between the two types of detectors becomes even greater when measuring effects with increased and variable dynamics present.
|In lightshow applications, a single laser beam is usually modified in appearance by employing two galvanometermounted small mirrors placed in an orthogonal arrangement that rapidly scan the beam in an imaginary x-y field, (Figure 1). These movements are controlled from computer programs, which are used to create a large range of mid-air patterns that constitute a typical laser display. Other methods of modifying the propagation of the beam may include directing the beams to mirror balls, polygon mirrors, and other remote effects, as well as using diffractive optic elements (either internal or external to the laser projection device) to split the beam into multiple sub-beams. In each case, the resulting emission, because of the movement present in the effect, results in brief single or repetitive exposure potential — the effective pulse width.||
The results from surveying the exposure duration of a wide range of lightshow effects in differing venue sizes indicates that the time a beam takes to traverse a 7mm aperture ranges from several milliseconds for slow bright looking finger beam type effects, to tens of microseconds for the fastest moving effects. A large number of effects typically create effective pulse widths of several hundred microseconds.
The BC20 is able to overcome the deficiencies in using a standard power meter detector. The peak hold detector of the BC20 is able to register pulses as short as 10us, thus making the device suitable for directly measuring the peak power present in the majority of scanned laser show emissions. Once the peak power present in the emission at the point of accessible contact has been determined, and following a straightforward power-to-irradiance conversion, the assessor is able to evaluate the exposure against the applicable irradiance based exposure limit. With only a single pulse needed to register a reading, the BC20 can be used to immediately determine the peak power present in an operational laser effect. This is something that has been impossible to achieve using a standard power meter.
Static Laser Beams in Challenging Environments
The BC20 also helps with measuring static laser beams in what can easily be considered as a challenging measurement environment. A common difficulty experienced when a laser system is installed in an entertainment setting is that the distance the beam propagates until it reaches the audience may often be several tens of meters. Over such distances, any instability in the beam, such as slight beam movement caused through vibration of the structure, the laser projector, or any of its external optical components are secured to, are amplified and cause difficulties for a standard silicon detector.
A rapidly vibrating beam due to high sound pressure levels present in the venue results in a beam profile that cannot be effectively measured. In these circumstances, the peak-hold detector of the BC20 makes it much easier to identify moving hotspots that are often present in such beam profiles. Furthermore, the BC20 overcomes another common problem evident in the output of most common laser effects control systems, which is that even when they are commanded to output a single beam, the output is not true CW. Instead, the beam has a strange duty cycle, due to control software constraints, which again leads to sources of measurement error in traditional silicon detectors. In these circumstances, the BC20 effectively ignores the off time of the laser, and is able to still present a peak power reading to the user.
Removing Background Light
The BC20 was originally designed in mind for measuring the red laser emission of barcode scanning products, and like the Ophir PD300 sensor, the detector has Ophir’s patented automatic background subtraction system incorporated in the front face of the sensor. This feature is intended to remove background light from the measured emission, a useful aid for measuring small diameter beams that are fairly close to the source. However, for light show applications, the beams to be measured are much larger, perhaps a 100mm or more in diameter to possess an irradiance less than the exposure limit value. For this type of beam measurement it is better to disable the background subtraction feature during installation.
Ophir’s PD300-CDRH adapter over the BC20, which has the effect of covering up the secondary background detector, introduces a 7mm aperture over the front of the primary detector. The installation of the PD300-CDRH adapter prevents the large diameter beams from interfering with the automatic background light subtraction system. But this means a manual background light subtraction should be carried out prior to measurement by pressing the zeroing key on the meter and staying aware of other illumination sources.
Determining Exposure Limit
The appropriate Maximum Permissible Exposure (also known as exposure limit) value should be determined by the type of emission that the lighting effect is likely to expose people to. In practice, this may consist of one or more short doses of laser light causing direct exposure. If an assumption is made that the total duration for all the doses in a performance could be 10s or greater, then the small source ocular limit of 10W.m-2 (1mW.cm-2) may be appropriate. This limit, when considered as a power reading through the 7mm aperture of the PD300-CDRH adapter, equates to 400uW. Similarly, if the total exposure duration of all doses could be determined to be less than 0.25s, then the small source ocular limit of 25.4W.m-2 (2.5mW.cm-2) would be appropriate — the equivalent of a 1mW power reading through the 7mm aperture.
For a light show operator, the values being suggested may seem to be restrictive. However, one has to remember that the reading on the meter is only that accessible through the PD300-CDRH aperture, while the entire expanded beam will have more power present, and hence look much brighter than a 1mW tight beam.
As is common with most silicon photodiode based detectors, the response to different wavelengths of laser radiation varies across the visible spectrum as a result of the related conversion efficiency of photodiode technology. This requires the meter to either be set to measure the correct wavelength, and, for those wavelengths that the meter does not have a pre-programmed builtin calibration factor, the meter reading must be adjusted manually to determine the true exposure level. Figure 2 shows the relative sensitivity of the BC20, while Table 1 lists a range of correction factors to be used for wavelengths across the visible spectrum.
Fig 2 Normalised Sensitivity for the BC20
Note With the meter set to 635nm, the reading obtained must be multiplied by the correction factor nearest to laser emission wavelength to obtain the true power.
460nm Blue Laser, reading 5.0mW on the 635nm setting True Power = 5.0mW x 0.34 = 1.7mW The peak-hold detector in the BC20 is implemented through analogue design. This removes problems such as Nyquest error that can be evident in systems using digitising methods that have been noted as introducing when attempting to measure these types of exposure on other instrumentation.
The combination of the BC20 and PD300-CDRH with an Ophir meter provides instrumentation that, with a little care, is straightforward to use and allows for a better assessment of a range of difficult exposure conditions commonly experienced in entertainment applications. Some of these measurements would be virtually impossible to undertake with standard silicon photodiode detector systems. Furthermore, the system provides a means of measuring the actual emissions from live laser effects. This allows operators and venue safety management to monitor emissions in real-time, rather than having to rely solely on pre-show checks, which may, for a range of foreseeable reasons, alter at performance time. This is a laser measurement system that helps to ensure the safety of workers and patrons at venues and equips employers with the tools to evaluate compliance with recently introduced workplace laser exposure regulations in Europe.
About James Stewart
James Stewart is a qualified health and safety practitioner based in the UK, having a strong specialism in optical radiation hazard assessment. His skills in laser safety management are regularly employed at many of the UK’s leading entertainment venues where James advises venue operators and production alike on laser safety management issues unique to this sector. Away from entertainment applications, James works across a wide range of industries, advising, evaluating, and training on both laser safety and broadband optical radiation safety management, as well as assisting with safe product design and classification.