PREDICTION OF APPARENT EXTINCTION FOR OPTICAL-TRANSMISSION THROUGH RAIN

At optical wavelengths, geometrical optics holds that the extinction e fficiency of raindrops is equal to two. This approximation yields a wa velength-independent extinction coefficient that, however, can hardly be used to predict accurately rain extinction measured in optical tran smissions. Actually, in addition to the extinct direct incoming light, a significant part of the power scattered by the rain particles reach es the receiver. This leads to a reduced apparent extinction that depe nds on both rain characteristics and link parameters. A simple method is proposed to evaluate this apparent extinction. It accounts for the additional scattered power that enters the receiver when one considers the forward-scattering pattern of the raindrops as well as the multip le-scattering effects using, respectively, the Fraunhofer diffraction and Twersky theory. It results in a direct analytical formula that ena bles a quick and accurate estimation of the rain apparent extinction a nd highlights the influence of the link parameters. Predictions of app arent extinction through rain are found in excellent agreement with me asurements in the visible and IR regions. (C) 1996 Optical Society of America.