The design of solar concentrating systems and receivers requires the spatial distribution of the solar flux on the receiver. This article presents an integral formulation of the optical model for the multiple reflections involved in solar concentrating facilities, which is solved by a Monte Carlo ray-tracing (MCRT) algorithm that handles complex geometries. An experimental validation of this model is obtained with published results for a dish configuration. The convergence of the proposed algorithm is studied and found faster than collision-based algorithms. In addition, an example of the use of the sensitivity of the power on a target to the mirror rms-slope is given by treating an inverse-problem consisting in finding the equivalent rms-slope of mirrors that best match the flux map measurements.