Figure 1.

A field E(r), with wavefront shown by the brown-yellow surface, propagates into the half-space z>0 along arbitrary directions (light-brown arrows). In the 0XY Z framework, the propagation vector along s of each plane wave component of E(r) has polar and azimuthal angles θ and ϕ. The polarization of each of these plane waves is characterized by the orthonormal system {ϵ_⊥,ϵ_∥,s}. The unit vector ϵ_∥ is in the polar plane containing both s and its projection(green broken line) on OXY , and points in the rotation sense of θ. On the other hand, ϵ_⊥ is normal to this plane and points against the sense of rotation of ϕ. (Online version in colour.)