Calculations have been carried out for first- and second-order backscattering from water clouds illuminated by a continuous 0.9-µ beam with a finite divergence angle. In the single-scattering calculations several cloud types were used, while only an approximation to fair weather cumulus clouds was used for double scattering. It was found that the intensity and hence the reflectivity varied with the transceiver-cloud distance for both orders of scattering. Second-order backscattering also varied with field of view. From these results a criterion is suggested for determining when the plane parallel atmosphere theories can be used with finite beams.
R. C. Anderson and E. V. Browell, "First- and Second-Order Backscattering from Clouds Illuminated by Finite Beams," Appl. Opt. 11, 1345-1351 (1972)