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The ``tail'' of the distribution (a straight line in semi-log scale) corresponds  to exponentially-distributed time-delays, indicating that those counts are generated by a  Poisson process. At short  timescales, the distribution departs from the an  exponential due to the contribution of the higher rate process of single molecules traversing the excitation volume.  To estimate the background rate (i.e. the inverse of the exponential time constant),  it is necessary to define a time-delay threshold above which the distribution