The anomalous diffraction approximation for extinction efficiency is valid for optically soft particles and large size parameter, x = 2πa/λ: where
The ratio of refractive indices has to be close to 1, and the size parameter should be large.
However, semi-empirical extensions to small size parameters and larger refractive indices are possible.
The main advantage of the ADT is that one can (a) calculate, in closed form, extinction, scattering, and absorption efficiencies for many typical size distributions; (b) find solution to the inverse problem of predicting size distribution from light scattering experiments (several wavelengths); (c) for parameterization purposes of single scattering (inherent) optical properties in radiative transfer codes.
Another limiting approximation for optically soft particles is Rayleigh scattering, which is valid for small size parameters.