An approximation for the rapid computation of nonspherical particle optical properties is presented. The approximation is based on complex angular momentum theory applied to the equal volume-to-area particle. The optical properties calculated are the extinction coefficient, single-scattering albedo, and the asymmetry parameter. The phase function is calculated from a set of simple trigonometric functions, this analytic phase function is intended to represent the general scattering properties of cirrus. The validity of the analytic phase function is tested against aircraft radiometric observations of cirrus. At infrared wavelengths the approximation is combined with electromagnetic theory so that 3D particle optical properties can be rapidly calculated at all size parameters.