My project is experimental and computational study of optical properties of germanium. Germanium is an important material in the application of semiconductor and optical devices with some high-temperature optical properties. However, some optical properties are difficult for experiments due to the extreme environmental conditions. The first-principles simulations are used to calculate the electronic structure and optical properties of germanium. Room temperature simulations of refractive index and absorption coefficient can correlated to experimental results. Thus, high-temperature n and kappa spectra are calculated subsequently. Since the measured samples are in different thicknesses, the influence of thickness is also explored by the reflectance spectrum. Additionally, high-temperature emissivity spectra of silicon and germanium are measured to add to the optical properties study.