SPECKLE PATTERNS PRODUCED BY WEAKLY ROUGH RANDOM SURFACES - EXISTENCEOF A MEMORY EFFECT TWIN PEAK IN THE ANGULAR-CORRELATIONS AND ITS CONSEQUENCES

A new peak, as strong as the well known memory effect peak, is found i n the angular correlation of light and other waves scattered from slig htly rough random surfaces. The existence of this peak does not depend on whether there is single or multiple scattering, nor on whether the surface is dielectric or metallic, nor on the polarization. However, unlike the memory effect peak, it decreases as the rms surface height increases in wavelength units. Hence, it does nor appear in the scatte ring of waves from highly rough surfaces. Its existence also affects t he foundations of the theory of speckle patterns, as it implies that t he second order statistics of the scattered field, though being Gaussi an, is not circular, and hence the commonly used factorization approxi mation is never valid when the surface is slightly rough. Nevertheless , this peak cannot be inferred from speckle contrast observations, and thus it does not affect this quantity. (C) 1997 Elsevier Science B.V.