An instrumentation strategy for laser radiation safety assessments

The Maximum Permissible Exposure (MPE) to laser radiation defines a level below which an acute injury will not be sustained. Values for the MPE are defined by international standards, based on research into biological laser damage mechanisms and thresholds. To verify that a given laser installation does not present a hazard it is necessary to compare accessible levels of laser radiation to the MPE. In most cases this can only be done by a combination of measurement and calculation. The standard presents MPE values as tables of formulae. Users find the standards difficult to interpret and use for practical assessment tasks. It is shown by theoretical analysis and practical investigation that the measurement process is non-trivial. This research has identified that general purpose laser radiation measurement equipment is not capable of undertaking the critical pulsed measurements needed for MPE assessments. These instruments are usually limited to measurements of average power, or pulse energy over a limited range of pulse parameters. Analysis of the standards shows that energy measurement is a critical aspect of the radiation hazard assessment process. During the practical investigation a radiation hazard was demonstrated to exist in many laser displays used for entertainment purposes. To meet the measurement criteria laid down in the standard a novel detector strategy is developed to provide accurate pulse energy measurement. No single detector can meet these current criteria over the complete laser radiation spectrum. Three types of detector are used to measure the output of all common lasers. This leads to the concept of a modular instrument design, incorporating common detector interfaces with signal conditioning to provide a standard output signal format to an interface unit which extracts the relevant parameters from the data for processing by a palmtop computer. The software guides the user through the measurement process, controls the hardware, determines the measured radiation level from the data, calculates the appropriate MPE and displays the results. Techniques were developed to minimise the occurrence of user errors. This required consideration of human-computer interfacing techniques in the software design and unique coding of each instrument element. The measurement precision of the instrument was determined using stable laser and light emitting diode sources. A scanned laser display system was then used to determine the measurement precision of the combination of a typical source and the meter. It was found that the instrument precision exceeded that of the source, essential if the instrument measurement results were to be reliable. For safety critical instrumentation, calibration is identified as an important issue. Electrical and optical techniques are discussed. Alternative applications for the instrument were considered. A technique for high power laser measurement using a beam sampling technique was demonstrated. This had advantages when compared to traditional methods of high power laser measurement. The sampling technique was extended to the construction of a laser beam delivery monitor capable of monitoring beam power and position and shutting off the laser in the event of a fault developing. Since the completion of the research project the instrument has been developed commercially in collaboration with a UK company. The commercial instrument uses the same strategy, hardware and software designs as developed during the research project.