  A Huygens bianisotropic medium is a linear homogeneous medium for which the
Huygens principle can be formulated. When a bounded 3D scattering object
composed of a linear bianisotropic medium, whether homogeneous or not, is
embedded in a Huygens bianisotropic medium, the excess field phasors inside
that object act as volume current densities, and the tangential components of
the internal field phasors on the surface of the same object act as surface
current densities, to radiate identical field phasors in the external region.
