Quasi-intrinsic angular momentum and the measurement of its spectrum.

We introduce the concept of quasi-intrinsic angular momentum to denote fields for which the mean value of the angular momentum is unaltered by a lateral shift of the rotation axis but the spectrum changes. This property is exemplified by the orbital angular momentum of a beam of light about its propagation direction. We propose an interferometric experiment to measure efficiently the exact angular momentum spectrum and variance for light beams with any arbitrary spatial distribution.