Anomalous diffraction approximation, van de Hulst approximation, eikonal approximation, high energy approximation, soft particle approximation.

Anomalous diffraction approximation (ADT) offers very approximate but computationally fast technique to calculate extinction, scattering, and absorption efficiencies. It agrees well with the "Mie" (modal) calculations for spheres, cylinders, spheroids, and coated spheres. Absolute value of refractive index has to be close to 1, and size parameter should be large. However, semi-empirical extensions to small size parameter, and larger refractive index are possible. It is NOT possible to calculate full range of phase function or asymmetry parameter - g in this approximation. The main advantage of the ADT is that one can (a) Calculate, in closed form, extinction, scattering, and absorption efficiency 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.

References:

One review is Mahood, Robert W. "The application of vector diffraction to the scalar anomalous diffraction approximation of van de Hulst", 1987, Thesis (M.S.)—Pennsylvania State University.

The ADT was first proposed by van de Hulst and is described in his book.

Anomalous diffraction approximation

Name	Author	Type	Language	Description
GGADT, see also https://ggadt.org	John A. Hoffman, Bruce T. Draine (2016)	ADT	Fortran	An open source Fortran suite, General Geometry Anomalous Diffraction Theory (GGADT). Journal reference ApJ 817 139 2016. See also preprint at https://arxiv.org/abs/1509.08987
adscat	P. J. Flatau (1992)	ADT	Fortran	ADSCAT is a FORTRAN program to calculate scattering and absorption of electromagnetic radiation by arbitary convex polyhedron target using the "Anomalous Diffraction Theory" (ADT). The target is represented by an array of vertices; the matrix of distances inside the target is generated as a first step (TRACE) which is followed by the ADT calculations (ADSCAT). Described in 1992 Ph. D. thesis Colorado State University