Laser reflections from relatively flat specular surfaces.

A major element in laser range control procedures has been the control of stray reflections from glass reflectors which may be near the laser target. These hazardous reflections have been thought to extend as far as the direct beam for near grazing angles of incidence. Modern military laser rangefinders and designators can be hazardous to the unaided eye to distances of 10 km or even greater. For this reason, many square kilometers of laser range area have been necessary to conduct laser tests when flat specular reflectors may be present on targets. Fortunately, sophisticated pointing systems have been developed with these laser systems to ensure that the direct beam is confined to the immediate target area. In most cases, flat specular reflectors also have been eliminated from the immediate target area. In some instances, however, specular reflectors still exist near or on laser targets. For these special cases, a more definitive mathematical treatment of hazardous laser reflections is desired. The divergence of a laser beam which has been reflected from a flat specular surface is dependent on the size of the reflector, the divergence of the laser creating the reflection, and the curvature of the reflecting surface. It can be shown mathematically that the curvature of even an optical flat, a reference surface used to compare the flatness of other surfaces, will produce a significant additional beam spread, thereby reducing the hazards of reflected beams. The natural curvature of plate glass or window glass is much greater, reducing the hazards even further. The extent of the hazards for reflections of various types of lasers and reflecting surfaces is discussed.