  Perfect (optical) vortices (PVs) have the mooted ability to encode orbital
angular momentum (OAM) onto the field within a well-defined annular ring.
Although this makes the near-field radial profile independent of OAM, the
far-field radial profile nevertheless scales with OAM, forming a Bessel
structure. A consequence of this is that quantitative measurement of PVs by
modal decomposition has been thought to be unviable. Here, we show that the OAM
content of a PV can be measured quantitatively, including superpositions of OAM
within one perfect vortex. We outline the theory and confirm it by experiment
with holograms written to spatial light modulators, highlighting the care
required for accurate decomposition of the OAM content. Our work will be of
interest to the large community who seek to use such structured light fields in
various applications, including optical trapping and tweezing, and optical
communication.
