  Light beams carrying orbital angular momentum are key resources in modern
photonics. In many applications, the ability of measuring the complex spectrum
of structured light beams in terms of these fundamental modes is crucial. Here
we propose and experimentally validate a simple method that achieves this goal
by digital analysis of the interference pattern formed by the light beam and a
reference field. Our approach allows one to characterize the beam radial
distribution also, hence retrieving the entire information contained in the
optical field. Setup simplicity and reduced number of measurements could make
this approach practical and convenient for the characterization of structured
light fields.
