In this paper a determinate equation is derived for the propagation of a finite beam of radiation in a random medium. The radiation is described by a mutual coherence function. The analysis is restricted to beam diameters that are large compared to the characteristic correlation lengths in the random medium. A scalar theory is used and the characteristic wavelength is assumed to be very small compared to the smallest correlation length. The method used is an iteration procedure similar to that considered in the propagation of a plane wave. The resulting equation is suitable for numerical integration.
MARK BERAN, "Propagation of a Finite Beam in a Random Medium," J. Opt. Soc. Am. 60, 518-521 (1970)