When making single-molecule experiments on intrinsically-disordered proteins that have been labelled with donor and acceptor fluorophores, the distribution of energy transfer efficiencies can only be transformed into a distribution of donor-acceptor distances if the observation time is at least an order of magnitude smaller than the relaxation time of the donor-acceptor distance. In other words, the relaxation time of the donor-acceptor distance must be small enough for this distance to remain unchanged during the observation time. Otherwise the molecules would sample all of the possible donor-acceptor distances during the observation time, which would result in a narrow energy transfer distribution which would not contain much information on the donor-acceptor distance distributions. Since in the case of this sort of experiments the count rates are in the order of 10⁵ s^-1, the intermolecular relaxation times must be greater than 1 ms.