  We analyze the scattering of elliptically polarized plane waves normally
incident at the planar interface between two different materials; we consider
two cases: dielectric-dielectric and dielectric-conductor interfaces. The
scattering matrix S in both cases is obtained using the boundary conditions and
Poynting's theorem. In the dielectric-dielectric case we write S using two
different basis, the usual xy and a rotated one. For the dielectric-conductor
interface, the use of the rotated basis together with an energy balance
argument leads us, in a natural way, to construct a unitary S matrix after
recognizing the need to introduce two equivalent parasitic channels due to
dissipation in the conductor, and the transmission coefficient into these
parasitic channels measures the absorption strength.
