The objective of this thesis was to develop a reliable multi-wavelength pyrometer for simultaneous measurement of the wafer temperature and its optical properties in the wavelength range of 1 to 20 microns and temperature range of 30 to 1500° C. The spectral emissometer has been utilized for measurement of the temperature dependent optical properties of InP, AlN and Sapphire. The experimental results presented in this thesis showed that the measurement of high temperature optical properties could be performed reliably with a novel approach using the spectral ernissometer. The temperature determination capability of the emissometer was tested and verified using a standard thermocouple embedded in a silicon wafer. The temperature measurement accuracy, with the emissometer, was found to be within +/- 10° C of the thermocouple temperature for a temperature range of 30 to 300° C. A particularly interesting results were the observed sharp peak in the emissivity of Fe doped InP at 14 microns, the deconvoluted values of the refractive indices from the measured optical properties of Fe doped InP are within +/- 10 % of the limited refractive index data available in the literature, sapphire exhibiting emissivity value of ~1 at 8 microns and the refractive indices of sapphire approach high values in the wavelength range of 12 to 16 microns resulting from its high reflectance. Spectral emissometry has been established as a reliable technique for simultaneously measurement of temperature and optical properties of semiconductors.