The infrared emissivity of glassy carbon, stainless steel, and stainless steel with a selective coating of NiNiO has been determined using a thermal-wave resonant cavity heated with a modulated laser beam. This is achieved performing a length scan of the cavity at a fixed frequency of 5 Hz. It is observed experimentally that (1) the mechanisms of heat conduction and radiation co-exist inside the cavity, through the coupling of the dc and ac components of the temperature, and (2) the radiation effect shows up in both the amplitude and phase signals for cavity thicknesses greater than the diffusion length of the intra-cavity air. Using a suitable theoretical model, the experimental data for the amplitude and phase have allowed the determination of the infrared emissivity of the studied materials.