Uniform theory of diffraction
In numerical analysis, the uniform geometrical theory of diffraction (UTD) is a high-frequency method for solving electromagnetic scattering problems from electrically small discontinuities or discontinuities in more than one dimension at the same point.  UTD is an extension of Joseph Keller's geometrical theory of diffraction (GTD). 
The uniform theory of diffraction approximates near field electromagnetic fields as quasi optical and uses ray diffraction to determine diffraction coefficients for each diffracting object-source combination. These coefficients are then used to calculate the field strength and phase for each direction away from the diffracting point.
These fields are then added to the incident fields and reflected fields to obtain a total solution.
- R. G. Kouyoumjian and P. H. Pathak, "A uniform geometrical theory of diffraction for an edge in a perfectly conducting surface," Proc. IEEE, vol. 62, pp. 1448–1461, November 1974.
- J. B. Keller, "Geometrical theory of diffraction", J. Opt. Soc. Am., vol. 52, no. 2, pp. 116–130, 1962.