The use of Probabilistic Risk Assessment (PRA) techniques in the establishment of safe operating regimes for high-output lasers in outdoor environments is becoming a more widely accepted alternative to the traditional deterministic approach based on Maximum Permissible Exposure (MPE) limits. The United Kingdom (UK) Ministry of Defence (MOD) has been sponsoring the development of a PRA laser safety approach for nearly four decades, and several PRA models for air-to-ground laser safety applications have been developed during that time, one of which is currently being encoded in the joint USAJK MATILDA laser safety tool.
Behind every PRA model there is a considerable amount of preparatory study that has to be undertaken to characterise the laser system, particularly the laser directional control subsystem, extending from the human operator through the host platform to the laser sightline steering mechanism. Of particular interest is the laser sightline behaviour in the event of a fault or a failure in the laser directional control system. Drawing on the UK MOD experience, this paper describes a typical laser system safety investigation, shows its effect on the PRA model definition, and demonstrates the resulting MATILDA PRA analysis, when the airborne laser is used to illuminate a target on a test range.