  Linear polarimetric transformations of light polarization states by the
action of material media are fully characterized by the corresponding Mueller
matrices, which contain in an implicit and intricate manner all measurable
information on such transformations. The general characterization of Mueller
matrices relies on the nonnegativity of the associated coherency matrix, which
can be mathematically formulated through the nonnegativity of its eigenvalues.
The enormously involved explicit algebraic form of such formulation prevents
its interpretation in terms of simple physical conditions. In this work, a
general and simple characterization of Mueller matrices is presented based on
their statistical structure. The concepts associated with the retardance,
enpolarization and depolarization properties as well as the essential coupling
between the two latter are directly described in the light of the new approach.
