A model for the infrared radiation emitted by a film/substrate system has been developed which includes both the effects of interference in the growing film and of scattering from its rough growth surface. Predictions of the model for the time-dependence of the apparent temperature Tapp of the film/substrate system measured in-situ by both one-color and two-color infrared pyrometers are presented for the case of diamond growth on Si. Using this model, the following information can be obtained from in-situ pyrometric results in real time: the true temperature of the film/substrate system, the instantaneous film growth rate, and the rms surface roughness σ of the film.