The complex refractive index is one of the basic characterization of thermo-optical properties occuring in thermal exchanges. We present the methods and the apparatus which help us determine this main parameter for either solids or aerosols. For solids, the theoretical study follows DRUDE's theory for conducting materials. For aerosols, some industrial applications are "in situ" measurements of particles size (like coal dust) and scattering spectral phenomena leading to a better knowledge of combustion in flames.