The theorem of equivalence between the Kirchhoff aperture integration (AI) and the physical optics (PO) approach applied to the interior walls of a open-ended waveguide is generalized to the case of any open-ended structure (OES).
Rius, J. M.; Michalski, K. A. Generalization of the equivalence between physical optics and aperture integration for radiation from open-ended structures. IEEE Transactions on Antennas and Propagation, 2002, vol. 50, núm. 6, p. 891-893.