Diffraction spectrum of a finite grating section from the sole diffraction S-matrix of the infinite grating

—The possibility is demonstrated to obtain the diffraction pattern at infinity in the Fraunhofer zone of a finite section of a grating from the diffraction S-matrix of a plane wave by the infinite grating without limitation on the surface corrugation depth. A truncated plane wave impinges on the grating section which may consist of an integer number of periods of an actually periodic grating, or a finite section of an aperiodic surface corrugation repeated periodically, considered as the period of a virtual grating. Concrete examples illustrate the usefulness of the method and its highlighting physical meaning