Speckle Patterns Generated by Rough Surfaces with a Periodic Component

A real diffraction grating is replaced by a coordinate-dependent surface impedance that is constructed as the sum of two contributions: a perfectly periodic contribution and a randomly varying contributon. Numerical results obtained with this model show the presence of diffuse light bands on the speckle pattern that constitutes the diffuse background of the spectrum. We emphasize the role played by the electromagnetic surface waves supported by this structure on the generation of the bands.