Laser Safety in Stage Marking
laserfabrik relies on high-performance measurement technology
What would live concerts, festivals and TV shows be without breathtaking laser shows and stunning special effects? laserfabrik GmbH is active in this field worldwide and has more than 20 years of experience. One area that has become increasingly important in recent years is stage marking. The aim here is to indicate the positions of actors and technical crew on stage by using laser projections. But the safe usage of the lasers remains the highest priority for the laserfabrik team. The company's proprietary series of measurements, taken with Ophir sensors, yielded instructive insights on the laser power for this purpose.
Fast-changing colors and shapes, unique projections, and everything choreographed precisely to the music: at festivals, the laserfabrik team employs several dozen show lasers. The event experts plan and execute dazzling laser displays, special effects and — together with art2O — water fountain shows all around the world. And ever more frequently, the laserfabrik team also receives targeted requests to undertake stage marking with lasers. The advantages are obvious: The positions can be changed dynamically and activated just at the right moment. Plus, the different laser colors provide a clear distinction between the positions of different actors. However, because laser marking takes place in the 'audience area', the safety requirements governing their use for this purpose are very stringent. According to current directives, the protected zone extends from the ground up to a height of 2.7m, since even at festivals it must be ensured that no persons are injured.
Irradiance in the audience area is regulated throughout Europe and must not exceed a maximum permissible exposure limit (MPE). The laser systems used in stage marking are outfitted with scanning technology; within the framework of safety regulations, they are considered pulsed lasers. This is why special formulas are prescribed for calculating the MPE limit values. Drawing on a series of proprietary investigations, the laserfabrik team collected information on the safest practices when using lasers for stage marking, including which aspects must be taken into account when dimming the lasers. The test series, using a TARM® laser (max. power 300mW), was carried out by Lucas Hamacher as part of a bachelor thesis.
"For laserfabrik, the safety of employees, artists and spectators is paramount in all laser applications. Our measurements allowed us to determine the actual levels of the exposure limits."
Jan Eiserloh, Managing Director of laserfabrik
High-quality measurement technology
One thing was clear from the start: The experiments certainly demanded a lot from the measurement technology! The RGB diode laser that was used employs one diode for each wavelength in play. To determine the pulse duration and frequency of the scanned laser beam, a fast photodiode was required. At the same time, despite the beam divergence when projected to a distance of 10 m, it was necessary to capture the entire incident beam. A product from MKS Instruments was able to fulfill both basic requirements: The Ophir IS1.5-VIS-FPD-800 integrating sphere measures divergent light with angles up to ±60 degrees.
Figure 2: By using the laser, marks can be flexibly projected onto the stage.
Figure 3: Techniques for stage marking are complex.
While the large 20mm aperture permits greater working distances, the inner diameter of the sphere is only 38mm, which enables a reaction time of just 2.8ns. Furthermore, a precision photodiode delivers calibrated power measurements, and a fast photodiode reproduces the pulse shape for display on the oscilloscope. In addition, the laser power is recorded with the Ophir 3A-FS thermal sensor in combination with the Ophir Juno external USB interface. For the laserfabrik team, a significant advantage of the two measurement systems was that the resultant data could be evaluated directly on the PC using the Ophir Starlab software.
True-to-life measurement setup
The measurements had to represent the on-stage situation as realistically as possible and, at the same time, be easy to carry out. In a hall, the diode laser was set up at a distance of 10 meters from the wall. The measurements were taken at 7.3m, or right at the leading edge of the audience area. As a basis for the experiments, Lucas Hamacher developed four practical projections of varying complexity, including, for example, denoting the amperage of electrical power on the ground or adding markings at the corners of the stage area.
The laser beam used for the stage marking traces out the selected projection. For this purpose, two mirrors are used to quickly change the position of the beam in the x- and y-directions so that — due to the slowness of the human eye — the complete symbol is perceived. Every pulse of the laser beam that hits the viewer's retina must be taken into account. In order to simulate the human eye, a diaphragm of 7mm is prescribed for the measurements for determining the MPE. However, since at 7.3m the laser beam has already expanded somewhat, in the experiments Lucas Hamacher used the integrating sphere with the Ophir 1.5-VIS-FPD-800 photodiode; the device contains the appropriate optics and provides a sufficiently large aperture. Laser power was measured using both the photodiode and an Ophir 3A-FS thermal sensor.
Safe stage marking
The measurements showed that the shape of the projection exerts significant influence on the power. In the projection of a triangle, the pulse duration measured on its edges was only 1/17 of what was measured at its vertices. Due to the inertia of the mirrors, changing the direction of the beam leads to a longer pulse duration and thus to a higher irradiation at these measuring points. In order to reliably adhere to the limit values, the dimming of the laser must be adapted individually to the projection. A general reduction in laser power makes no sense here, as this would come at the expense of the marking's visibility. These findings were operationalized for the application by comparing the limit value with the maximum measured power. The maximum power at a distance of 7.3m from the laser source was measured with both the Ophir photodiode and the Ophir 3A-FS sensor. Taking into account the respective wavelengths, in the next step, Lucas Hamacher determined the required attenuation (dimming) of the laser.
Since most commonly-used show lasers operate at higher powers, the attenuation required for stage marking is extreme. But in the case of very low powers, the adjustment must be exceedingly precise. In this case, the investigations revealed that 24-bit analog/digital converters should be used.
"Thanks to MKS' high-quality Ophir laser measurement devices we were able to accurately measure both the temporal characteristics per wavelength and the actual power of our projections."
Lucas Hamacher, Master Student Event Technology
Figure 4: Ophir measurement technology enabled the safety assessment by precisly termining the laser power.
Depending on the respective wavelength, a certain offset of the output power may occur in diode lasers. This is where the strength of the Ophir measurement technology was clearly demonstrated: The measured wavelength is entered into the Starlab software and the system automatically uses the calibration curve of the sensor with the corresponding correction factors. Lucas Hamacher explains: "Being able to accurately measure both the temporal characteristics per wavelength and the actual power emitted for the projections equipped us with essential knowledge for meeting the safety standards. These can only be correctly determined by top-notch measurement technology."
Different color mixtures were also taken into account during the tests. Each of the four test projections was measured in 10 different colors. With the appropriate calibrations and the individual dimming of the laser, more than 98% of the 260 measured values met the limits straight away. For the remaining 2%, the deviation was 1.9% or less, in a range that already falls within the measurement tolerance. However, it also became apparent that mixing colors – when each one has to comply individually with the limit values per wavelength made no sense, as this requires a division of the permissible power, which in turn exacts a toll on visibility. Overall, even with a 300mW laser, the power range cannot be fully exploited in stage marking. It would still exceed the limit values. The findings of this test series offer important insights, as Jan Eiserloh, Managing Director of laserfabrik, explains: "For laserfabrik, the safety of employees, artists and spectators is paramount in all laser applications. Our measurements allowed us to determine the actual levels of the exposure limits for various complex projections and verify the conditions for compliance in our applications."