Transmittance of Optical Materials at High Temperatures in the 1-μ to 12-μ Range
Transmittance measurements of optical materials were made at 25°C, 100°C, 200°C, 300°C, and 400°C in the 1-μ to 12-μ range with a Perkin-Elmer Model 21 spectrophotometer. A continuous proportioning temperature control system, using a modified Loyola LC-2 4KVA Power Manipulator in conjunction with a Wheelco Capacitrol, provided a given temperature level in high-temperature cells. The following materials, which are dielectrics and semiconductors of potential use as windows and IRDOMES in optical systems at high temperatures, were selected: Corning glasses, Nos. 0160, 8363, and 7905; fused, water-free quartz, type 106, General Electric; Barr and Stroud calcium aluminate, 37A and 39A; sapphire, Linde Company; Irtran-1 and Irtran-2; silver chloride; sodium chloride; silicon; and germanium. A final transmittance trace, corrected for cell characteristics, of each optical material at a given temperature was obtained by drawing a smooth curve through the point-by-point adjustments of the specimen spectrum. The transmittance of dielectrics remains relatively unaffected up to 400°C. Optical materials of this class are restricted in use at the higher temperatures only in the shift of the long-wavelength transmittance limit. With the semiconductors silicon and germanium, the absorption edge is shifted to longer wavelengths, and the over-all transmittance is greatly reduced with increase of temperature.