Contribution of evanescent incident waves to the scattered far field

It is a common perception that evanescent waves do not contribute to the far field. On scattering, however, the evanescent incident field may contribute to both the homogeneous and the evanescent scattered waves. With the method of angular spectrum representation of plane waves, we calculate the ratio between the contributions of the evanescent and homogeneous incident waves to the scattered far field for a δ-correlated scattering medium. The result may be applicable to the scattered near field of the scatterer as well as to the intermediate and far zones.