The local principal curvatures (shape) of a wavefront propagating through an optical system can be calculated from geometrical optics. Formulas are derived for the change in shape during transfer, refraction, reflection, and diffraction by a grating in a system of homogeneous isotropic media. The results not only have immediate application to design, but also provide necessary data for treating points where the geometric approximation fails.
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John A. Kneisly (1964) studied this question.
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