The ranges of refractive index and particle size over which the anomalous diffraction approximation is valid to within 15%, 50%O, and 100% are mapped. The approximation is found to have a broader range of applicability for the extinction efficiency than the Rayleigh-Gans approximation. The large range of validity explains the success of schemes for determining particle size distributions of spheres from extinction measurements based on analytic inversions of formulas containing the approximation.
William A. Farone and Matthew J. Robinson III, "The Range of Validity of the Anomalous Diffraction Approximation to Electromagnetic Scattering by a Sphere," Appl. Opt. 7, 643-645 (1968)