This work presents an experimental procedure for measuring high temperature spectral optical functions of materials. The complex refractive index is determined over a spectral range of 1.13-4.96 eV at high temperatures by ellipsometry in a reduced pressure, inert gas environment. On the basis of the measured complex refractive index, relevant optical functions such as the complex dielectric function, normal incidence reflectance, and absorption coefficient are also obtained. Silicon is selected due to the fact that, even though it has numerous applications in microelectronics fabrication and processing, the available spectral optical property data are incomplete at high temperatures. The experimental results are compared with the existing published data.