Summary: The time-dependent intensity of a UV-photon source, located inside an interstellar cloud, is determined by formulating and solving an inverse problem for the integro-differential transport equation of photons in a one-dimensional slab. Starting from a discretization of the direct problem, an iterative procedure is used to compute the values of the source intensity at increasing values of time, and it is applied in some numerical simulations, whose results are presented and discussed.