In addition to the spin angular momentum carried by its photons, a beam of light can also have orbital angular momentum if the electric field varies inside the beam. Roughly speaking, a beam with non-zero orbital angular momentum has a sort of twist in the electric field within a wavefront.
While significant effort has gone into thinking about what kind of torques such beams might exert, there has been little attention paid to what diffraction patterns they produce. This has changed with a simple and beautiful result from Jandir Hickmann of the Universidade Federal de Alagoas in Brazil and colleagues.
They have shown that the diffraction pattern from a triangular aperture shows a set of bright spots that can be counted to give the orbital angular momentum in the beam. This is also probably the simplest way to measure this quantity, and an extremely simple one at that.