Kirchhoff's integral theorem (sometimes referred to as the Fresnel–Kirchhoff integral theorem)[1] is a surface integral to obtain the value of the solution of the homogeneous scalar wave equation at an arbitrary point P in terms of the values of the solution and the solution's first-order derivative at all points on an arbitrary closed surface (on which the integration is performed) that encloses P.[2] It is derived by using Green's second identity and the homogeneous scalar wave equation that makes the volume integration in Green's second identity zero.[2][3]