During the recent development of a tactical Laser Engagement System (LES) it became necessary to accurately determine the effective beam geometry for a Gallium-Arsenide laser transmitter used in conjunction with a series of fixed threshold detectors. Of particular interest was the knowledge of the diameter of the effective detection zone as a function of range, laser power level, detector sensitivity and threshold level, laser beam divergence and atmospheric extinction. A theoretical model, based upon threshold detection at a critical irradiance level, results in a closed-form solution for the effective beam diameter as a function of all the stated parameters. The resulting equation successfully predicts the so-called "tube" effect, which has been discovered in experimental field tests, as well as the maximum effective range of the system. The equation has been programmed for the XDS Sigma 7, and computer-generated beam geometry plots are now available. The plots provide valuable system design data which has already helped to make the proposed EDM/LES more cost-effective.