Illustration of the Fresnel-Kirchhoff diffraction formulation (Eq. (3)) for a reflecting surface. A point source at P₀ illuminates the reflector surface S, producing an incident scalar field. The field reflected from each surface point q on the reflector surface S is treated as a secondary source, and the scattered field at point p on the target surface Τ is obtained by integrating over S. Here r₀ and r are the distance from P₀ to the point q and from q to p, respectively. The angles θ_in and θ_οut denote the local incident and scattering angles at q.