Two‐frequency intensity cross‐spectrum

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Abstract
The intensity cross‐spectrum (spatial Fourier transform of the two‐frequency intensity correlation) for scintillations caused by a plane wave passing through a random phase screen is considered. Two series solutions (one valid for low and the other for high spatial frequencies) are obtained which are the generalizations of previous results for the monochromatic intensity spectrum. We show that the Gaussian‐field approximation (modeling the cross‐spectrum as the transform of the square of the second moment) breaks down when the outer scale is large compared with the diameter of the scattering disk.