  We put forward an operational degree of polarization that can be extended in
a natural way to fields whose wave fronts are not necessarily planar. This
measure appears as a distance from a state to the set of all its
polarization-transformed counterparts. By using the Hilbert-Schmidt metric, the
resulting degree is a sum of two terms: one is the purity of the state and the
other can be interpreted as a classical distinguishability, which can be
experimentally determined in an interferometric setup. For transverse fields,
this reduces to the standard approach, whereas it allows one to get a straight
expression for nonparaxial fields.
