= Validity of the first Rytov approximation
The <Rytov approximation> omits the quadratic gradient source in the <logarithmic wave perturbation> equation. Its next correction solves $\mathcal L_i\chi_2=-\nabla\chi_1\cdot\nabla\chi_1$. Control of the propagated correction, weak <amplitude> fluctuations and avoidance of field zeros are needed. An appreciable smooth accumulated phase can be retained by exponentiation; small total phase is not the same condition as small local phase gradients.
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