Fresnel-Kirchhoff diffraction integral

A scalar aperture approximation that sums the secondary waves reaching an observation point. For a point source with incident spherical amplitude $A{e}^{iks}/s$, one common forward-normal convention isHere $s$ and $r$ are source-to-aperture and aperture-to-observer distances and $K$ is an obliquity factor. The Fresnel diffraction approximation retains quadratic path phase; Fraunhofer diffraction additionally neglects the quadratic phase across the aperture. Kirchhoff boundary assumptions do not give an exact electromagnetic solution at a sharp edge.