For example, the source of the modulus data may be the Fraunhofer diffraction pattern formed when an object is illuminated with coherent light.
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It can be shown (see Fourier optics, Huygens-Fresnel principle, Fraunhofer diffraction) that the field radiated by a planar object (or, by reciprocity, the field converging onto a planar image) is related to its corresponding source (or image) plane distribution via a Fourier transform (FT) relation.
If all the terms in f(x', y) can be neglected except for the terms in x and y, we have the Fraunhofer diffraction equation.