The characterization of non-paraxial radiation by moments is worked out. The Helmholtz equation is reformulated in terms of two equivalent first order equations of Schroedinger type. Left and right traveling parts of the solutions are separated and used to extract physically meaningful parts of generalized moments of the wave field. Based on the Schroedinger formulation the propagation behavior of nonparaxial moments is found.
Marcus Scholl, Marcus Scholl,
"Characterization of nonparaxial radiation", Proc. SPIE 2870, Third International Workshop on Laser Beam and Optics Characterization, (20 November 1996); doi: 10.1117/12.259890; https://doi.org/10.1117/12.259890