  We review and re-examine the description and separation of the spin and
orbital angular momenta (AM) of an electromagnetic field in free space. While
the spin and orbital AM of light are not separately-meaningful physical
quantities in orthodox quantum mechanics or classical field theory, these
quantities are routinely measured and used for applications in optics. A
meaningful quantum description of the spin and orbital AM of light was recently
provided by several authors, which describes separately conserved and
measurable integral values of these quantities. However, the electromagnetic
field theory still lacks corresponding locally-conserved spin and orbital AM
currents. In this paper, we construct these missing spin and orbital AM
densities and fluxes that satisfy the proper continuity equations. We show that
these are physically measurable and conserved quantities. These are, however,
not Lorentz-covariant, so only make sense in the single laboratory reference
frame of the measurement probe. The fluxes we derive improve the canonical
(non-conserved) spin and orbital AM fluxes, and include a `spin-orbit' term
that describes the spin-orbit interaction effects observed in nonparaxial
optical fields. We also consider both standard and dual-symmetric versions of
the electromagnetic field theory. Applying the general theory to nonparaxial
optical vortex beams validates our results and allows us to discriminate
between earlier approaches to the problem. Our treatment yields the complete
and consistent description of the spin and orbital AM of free Maxwell fields in
both quantum-mechanical and field-theory approaches.
