Physical Optics

Physical Optics

Physical optics (PO) is one of the fundamental electromagnetic high-frequency theories for scattering and radiation problem. The total field of a source (such as an antenna) which radiates in the presence of a perfectly conducting surface may be expressed as a superposition of the incident field (Eⁱ, Hⁱ) and the field (E^s, H^s) which is scattered by the surface. The current fields are chosen in PO to denote the electric and magnetic fields of the source which exist everywhere, i.e., they exist as if the scatterer was “absent”; this is unlike the geometrical optics (GO) incident field, which exists in the presence of the surface of the scatterer. The scattered fields in this case can be expressed in terms of the radiation integrals over the actual currents induced on the surface of the scatterer. These currents also radiate the scattered fields in the absence of the scatterer, i.e., these currents are now viewed as equivalent sources, which are impressed over the mathematical boundary but with the perfecting conducting scatterer removed.

In the conventional PO method, the radiation integral for the scattered field is calculated by employing a GO approximation for the currents induced on the surface, which is assumed to be an electrically large plane; hence, the PO method is also a high-frequency method. In this chapter, the following are illustrated: the fundamental PO formulation and calculated results, and some topics which improve the conventional PO to the extended PO such as physical theory of diffraction (PTD) (Ufimtsev, 1971) and PO with transition current (PTD-TC) considering transition currents (Kobayashi, 1999). The PO method described here is the same concept as Kirchhoff’s integration derived from the Huygens’ principle.

Both the electromagnetic radiation and scattering which re-radiates electromagnetic waves around the scatterer from the induced currents are considered as a secondary source on the surface which is illuminated by incident waves. The PO is a typical high-frequency technology, and is a powerful calculation method with high accuracy of approximation if the size of scattering object is sufficiently large compared to the wavelength. There are two main features of PO from a practical point of view. First, there is a degree of freedom in how to express the electromagnetic current, and second, the scatterer surface which induces currents can be subdivided and calculated. As described later, the former can individually treat transition electromagnetic currents at discontinuities such as wedge, and the latter indicates that it can be divided into patches even if the target model has a complicated shape. It is well known that PO gives a finite value with a certain correctness near the focal point and focal line, and that the radiation integration includes a diffraction effect. In the general PO method, as mentioned above, the equivalent electromagnetic current of the radiation integral is obtained by the GO method. Since GO is a fundamental high-frequency theory that treats waves as ray, PO is also a high-frequency theory, so the electromagnetic current is zero in the shadow area of the scattering object. The electromagnetic current distribution in the transition region is also different from the actual one, and this is a reason that limits the application range of PO.