Multi-Angle method to retrieve infrared spectral properties of a high-transparency material at high temperatures

A Multi-Angle (MA) method was proposed to determine the high-temperature spectral properties of high- transparency and low-absorption materials. This method overcomes the experimental difficulties of temperature gradient and low recognition. Through just rotating the sample to tune the incident/transmitted angle, this method can provide multiple distinctly different and independent transmittances, and there is barely any influence on the temperature field of the sample. Measurements are performed on an ultraviolet fused silica between 0.8 and 5 µm for several high temperatures, using a modified FTIR system coupling with a heating cell. Based on the transmittance data, a genetic-algorithm-based least-square method was used to extract the refractive index and the absorption index. In this identifying process, a Monte-Carlo ray-tracing (MCRT) method was adopted to predict the experimental value. The genetic algorithm was used to search and optimize the association of the indexes of refraction and absorption. Using this experimental identifying method, temperature and frequency dependence of transmission properties of the UV fused silica up to 1800 K were obtained. The spectral refractive index gradually decreases with wavelength and slightly rise with increasing temperature. The spectral absorption index is also dependent on wavelength as well as temperature. As the absorption bands broadening with the increasing temperature, three absorption peaks all shift toward longer wavelengths and vary differently from 1200 to 1800 K. The indexes of refraction and absorption can be used for predicting the transmitting rates of fused silica windows based on different entrance angles at high temperatures.