An approach to high energy laser safety in open environments

High energy lasers present new challenges for safety evaluation. Not only is the direct beam hazardous, but the reflected beam may also cause injury. Two important issues with high energy laser safety have been addressed in simulations used to model reflected-beam hazards: the reflecting properties of materials and the time history of exposure. The reflecting properties are quantified by the Bi-directional Reflectance Distribution Function (BRDF), and a technique has been developed to extract parameters for BRDF models, primarily the Maxwell-Beard model, from measurement data and to calculate the reflected irradiance at an observer. In addition, the reflecting properties of materials often change upon heating, so a procedure using a camera and screen is being used to measure the time-varying BRDF of materials, which in turn is modeled empirically with spherical harmonics. Because the use of high energy lasers is often dynamic in nature, involving a moving laser, target, or observer, the time-history of exposure to the observer is calculated in the simulations. A sliding-window algorithm compares the time-history of exposure to maximum permissible exposure limits for a range of exposure times in order to determine if a hazardous condition exists for the observer.