Complex rays in transient scattering from smooth targets with inflection points

It is reported that transient scattering by targets with smooth concave-convex boundary shape reveals certain features that cannot be interpreted as conventional specular reflection and creeping wave contributions. This observation is based on numerical reference data generated for the prototype configuration of a perfectly conducting cylinder with sinusoidal azimuthal modulation of its surface contour; excitation is by a third-derivative Gaussian pulsed plane wave which has a bandlimited spectrum, deemphasizing low frequencies. The nonconventional response characteristics in the data are found to be related to travel times from nonspecular points near concave-to-convex surface inflections on the illuminated and shadowed portions. These characteristics are explained in terms of complex incident as well as creeping rays, which satisfy on the complex extension of the obstacle contour the complex specular reflection law that manifests itself nonspecularly in real coordinate space. By comparison with the reference results, it is shown that conventional reflected and creeping ray theory augmented by the complex ray concept furnishes an excellent quantitative tool for constructing the scattered fields from nonconvex smooth objects subjected to pulsed excitation with weak low-frequency spectrum. Asymptotic diffraction theory for general target shapes should therefore be based on complex- as well as real-ray phenomena