A probabilistic risk analysis model for receiving laser eye injury from space based lasers

The European Space Agency ESA initiated a study, which dealt with the development of a risk model for laser induced eye injury. This study contributes to the minimisation of risks to the public caused by space flight projects and is summarised in this paper. The probability of an eye injury occurring in a given potential laser exposure scenario is a combination of the probability of being exposed to the laser beam and the probability of the level of the incident radiation energy producing eye injury. A probabilistic risk model with uncertainties has been developed to quantitatively model the risk for ocular injuries due to laser beams being emitted from satellite based lidars (atmospheric laser measurement systems). The risk model, to the knowledge of the authors, for the first time accounts for uncertainties associated with the variability that an ocular lesion is formed for a given laser exposure, as described by a dose-response curve. The needs of the user of the model will be summarised, the physical model discussed and the realisation, as generally applicable software, presented. INTRODUCTION AND USER’S REQUIREMENTS The use of lidars is considered in a number of ESA’s future missions, as well in missions conducted by NASA and NASDA. Spacecraft based lidars are used to measure a range of atmospheric or earth surface properties by analysis of the part of the laser radiation which is directed back to the lidar (lidar can be considered as a laser radar and is an acronym for light detection and ranging). As only part of the laser radiation is scattered or absorbed by the atmosphere, the remaining laser radiation as emitted from the spacecraft is incident on the earth surface, where it might lead to injuries, especially to the eye, if thresholds are exceeded. Exposure of the eye, either naked or through small optical instruments, is usually harmless but exposure of the eye via large telescopes can result in ocular damage. In order to minimise the risk for ocular injury due to exposure to the laser beam, ESA has initiated a study “Human Risk Analysis Simulator for Space Lidars”, carried out by the Austrian Research Centers Seibersdorf with participation of a number of international experts in the field of laser bioeffects and risk analysis (Schulmeister, 2001). The requirements of ESA were to follow a probabilistic approach to determine the occurrence rate for exposure to the laser beam for relevant population groups, to calculate the ocular radiation exposure level with atmospheric scintillation effects included, and then predict the frequency distribution of an ocular injury from a space borne lidar. The model was to be implemented in a flexible and user friendly software to facilitate the risk evaluation of future missions involving lidars. Following ESA's requirements, the probabilistic risk model was developed for the full range of possible orbit and laser beam parameters. Parameters such as orbit inclination, pointing direction of the laser beam, laser wavelength, laser energy, and footprint diameter are specified as input parameters by the user. The uncertainty and variability of model parameters are described by distributions which are propagated through the model with Monte Carlo simulation to produce a distribution of the expected number of ocular injuries per mission (collective risk) and the frequency for ocular injury per hour of using a given type of optical instrument (individual risk). SCENARIO DESCRIPTION, OCULAR ENERGY The scenario is schematically depicted in figure 1. Laser radiation with a given wavelength between the ultraviolet and the far infrared (180 nm – 20 μm) is emitted from the lidar as short pulses (pulse durations less then 1 μs) with a given repetition rate (typically in the order of 10 100 Hz) and energy per pulse (in the order of 100 mJ). The lidar output can also consist of two different wavelengths with different energies, which is included as option in the model. The direction of the line of sight (LOS) of the laser beam is specified as azimuth angle clockwise from the flight direction and by the angle off nadir. The laser energy is decreased by wavelength dependent atmospheric scattering and absorption. This is included in the model by transmittance curves calculated for nadir pointing with atmospheric model software MODTRAN and FASCODE for three atmospheric conditions which form a triangular distribution with “US-Standard” as most likely value. The decrease of transmittance with increasing path-length for off-nadir pointing is accounted for by the cosine law.