Empirical BRDF modeling of a window to improve accuracy of laser reflection hazard analyses

A reflection hazard analysis is required for laser test and training scenarios where the target is inside the direct beam Nominal Hazard Zone (NHZ) of a laser. For example, consider a hypothetical scenario in which a laser range finder illuminates a glass window on the side of a parked aircraft or the window of a stationary vehicle during training exercises on a military range. If we assume nominal laser range finder parameter values (1.06 micron wavelength; 100 mJ output energy; 10 Hz PRF, 10 nsec pulse duration, a 1 mrad 1/e full-angle divergence and an 8 millimeter 1/e beam waist diameter) the Nominal Ocular Hazard Distance (NOHD) is 2480 meters. If the range target is within 1000 meters of the laser source, then a reflection hazard analysis must be performed. The traditional and simplest approach for analyzing the reflections off an illuminated window is to assume that the window surface acts as an ideal specular reflector. We will demonstrate that for a glass window target, the ideal specular reflector assumption can lead to an estimated Reflected Nominal Ocular Hazard Distance (RNOHD) that is much larger than the true RNOHD for systems with relatively small beam divergences. Overly conservative RNOHD estimates based on the ideal specular target assumption can unnecessarily constrain test planning and hinder the achievement of test objectives.