The spectral emissivity of a semi-transparent thin film on a heated substrate varies with film thickness due to optical interference within the layer. This leads to oscillations in the emitted blackbody radiation intensity when the film thickness changes with time. We show how measurements of these oscillations have been used to determine the crystallization kinetics and optical constants of ion-implanted amorphous Si and Ge layers undergoing solid phase epitaxial crystallization. A comparision of emission and reflectivity data illustrates that the emission behavior can be understood in terms of a straightforward optical model.