In fig. 2 we observe a plane wave which is diffracted in one plane. for this wave in the x y plane directly behind the plane z=0 with the following transmission distribution (x,y):
where Ee (x,y) : electric field distribution of the incident wave. the further expansion can be described by the assumption that a spherical wave emanates from each point (x,y,,0) behind the diffracting structure ( Huygens' principle). this leads to Kirchhoff's diffraction integral: