Relation between degrees of coherence for electromagnetic fields.

In terms of the electric spectral density tensors associated with a random electromagnetic field, it is shown that, under certain conditions, a link can be established between two definitions of the degree of coherence for electromagnetic beams introduced not long ago by Wolf [Phys. Lett. A312, 263 (2003)] and by Setälä [Opt. Lett.29, 328 (2004)]. This connection is formulated as a certain equivalence relationship concerning the behavior of the above degrees of coherence and the degree of polarization of the field at two points.