= Phase covariance in the first Rytov approximation
{title2=$\operatorname{Cov}(\varphi_1,\varphi_2)=\iint b_1b_2C_W$}
For a centered real <scattering potential> fluctuation $W$ and the imaginary part $b$ of the incident-field-weighted <Green function>, $\varphi(\mathbf r)=-\int b(\mathbf r,\mathbf r')W(\mathbf r')d\mathbf r'$. Its mean vanishes, and its <covariance function> is the double integral of the two deterministic kernels against the <covariance function> of $W$. The correlation of the <refractive index> itself is insufficient when the <scattering potential> depends quadratically on that index.
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