Infrared Transmission of Contaminated Cryocooled Optical Windows

Normal incidence transmission of germanium at 20, 80, 200, and 300 K is reported for wave numbers between 500 and 6700 -1 (about 1.5 to 20 ^m). The absolute transmission in the 1000 to 4000 ~ ! region, in the absorption edge region near 6700 cm 1 , and in the lattice band region (500 to 800 cm 1 ) was found to be temperature sensitive. At the cryogenic temperatures 20 and 80 K, the germanium window was employed as a substrate for the transmission study of thin solid films of NH3 , CO2, CO, CH4, and HC1. The spectral interval covered in the thin film study was 630 to 3700 ~ J . Degradation in the transmission of a cryocooled window contaminated by these condensed gases was measured and the complex refractive index extracted from the transmission versus film thickness data for 80 K solid NH3 .