  Hong-Ou-Mandel interference is a cornerstone of optical quantum technologies.
We explore both theoretically and experimentally how the nature of unwanted
multi-photon components of single photon sources affect the interference
visibility. We apply our approach to quantum dot single photon sources in order
to access the mean wavepacket overlap of the single-photon component - an
important metric to understand the limitations of current sources. We find that
the impact of multi-photon events has thus far been underestimated, and that
the effect of pure dephasing is even milder than previously expected.
