<div>Thin films with selective spectral properties have great potential for use in applications such as solar thermal absorbers, selective thermal emitters, thermal barrier coatings, and thermophotovoltaics. Understanding the dependence of their optical properties with temperature is crucial for practical applications. Relatively less-used films with high thermal stability, high refractive index, and infrared transparency, such as cerium oxide CeO2 and magnesium oxide (MgO) have not been studied extensively for high-temperature applications. In this paper, CeO2, MgO, and several multilayer samples containing these compounds will be studied their change of optical properties with increasing temperature across the visible and infrared spectrum. In this work, I introduce Spectroscopic Ellipsometry (SE) to analyze dielectric constants of films and measure its thickness at multiple incident angles in visible and near-Infrared (IR) wavelengths both at room temperature and elevated temperatures. Using ellipsometry, several single and multilayer thin films (CeO2 and other transparent materials-based) have been measured their dielectric constants and thickness at room temperature and high temperature up to 500°C using thermal control stages called INSTEC. The change of their optical properties with temperatures has been characterized using theoretical modelling. Fitting their optical properties have been observed by changing thickness of each layer and calculating the corresponding Cauchy parameters in transparent material. Finally, FTIR measurements will be used to further characterize direct emission of these films at higher temperatures. This will be performed at room temperature and high temperature up to 1400 °C.</div>