  Inspecting three-dimensional partially polarized light fields we show that
there is no unambiguous correspondence between the three-dimensional field and
coherence matrix (or light beam tensor). Therefore, it is needed to clarify the
definition of unpolarized light. We believe that unpolarized field should be
treated as light of equiprobable polarizations similar to the case of
two-dimensional light. Then degree of polarization bridges two definitions of
the three-dimensional degrees of polarization known in literature. We reveal
that only 6 Stokes parameters are sufficient to describe the coherence matrix.
All these parameters can be retrieved from the in-plane measurements of
two-dimensional coherence matrices.
