Normal incidence transmission of germanium at 20, 80, 200, and 300 K is reported for wave numbers between 500 and 6700 cm -1 (about 1.5 to 20 ^m). The absolute transmission in the 1000 to 4000 cm ~! 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 NH 3 , CO 2 , CO, CH 4 , and HC1. The spectral interval covered in the thin film study was 630 to 3700 cm ~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 NH 3 .

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The development and testing of an ir hemi-ellipsoidal mirror reflectometer (HEMR), operational over a wavelength interval of 2-34 microm, are described. This optical system measures the hemispherical-directional reflectance of room temperature samples relative to a specular gold-coated surface. For a source and sample area commensurate with detectable energy requirements, it is shown experimentally that the HEMR is functional with very tolerable errors. Finally, the hemispherical-directional reflectance of test samples, e.g., black paints, gold diffuser, sulfur, cesium iodide, and others, is presented for wavelengths from 2 microm to 34 microm.

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