The optical properties of an organic polycrystalline layer of NTCDA embedded in a microcavity show exciton-photon coupling between the cavity mode and both the 0 - 0 and 0 - 1 transitions. In this work we present a theoretical microscopic model that reproduces the experimental data and explains the creation of an additional third polaritonic dispersion relation. Furthermore, a theoretical insight into the problem of polariton relaxation and photoluminescence in presence of vibronic replicas is given using a general model based on a master equation approach.