Identification of vector light fields with inhomogeneous linear polarization by an orbital angular momentum of polarization

We introduce and study a new propagation-invariant quantity for describing light fields, orbital angular momentum of polarization. For light fields with inhomogeneous linear polarization, it equals an angular velocity of changing polarization direction, averaged over a transverse plane with intensity as a weight function, similarly to the orbital angular momentum of scalar fields, equal to average angular velocity of phase change. We derive the orbital angular momentum of polarization for coaxial superpositions of cylindrical vector beams.