Transmittance and Reflectance of Cesium Iodide in the Far Infrared Region
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668 LETTERS TO THE EDITOR Vol. 48 Transmittance and Reflectance of Cesium Iodide in the Far Infrared Region EARLE K. PLYLER AND NICOLO ACQUISTA National Bureau of Standards, Washington, D. C. (Received June 11, 1958) S INCE large crystals of cesium iodide became available in 1951 1 there has been a general use of the material for prisms FIG. 1. The percent transmittance of cesium iodide and of crystal quartz with optical axis parallel to the surface of window. Thickness of CsI 5 mm, and of quartz 0.95 mm.
Transcript of Transmittance and Reflectance of Cesium Iodide in the Far Infrared Region
668 L E T T E R S TO T H E E D I T O R Vol. 48
Transmittance and Reflectance of Cesium Iodide in the Far Infrared Region
EARLE K. PLYLER AND NICOLO ACQUISTA National Bureau of Standards, Washington, D. C.
(Received June 11, 1958)
SINCE large crystals of cesium iodide became available in 19511 there has been a general use of the material for prisms
FIG. 1. The percent transmittance of cesium iodide and of crystal quartz with optical axis parallel to the surface of window. Thickness of CsI 5 mm, and of quartz 0.95 mm.
September 1958 L E T T E R S T O T H E E D I T O R 669
and windows. Prisms of CsI can be used to 56 μ and the present measurements determine the long wavelength cutoff of a window 5 mm thick. The measurements were made on a small grating spectrometer which has been described previously.2 The stray-radiation of the instrument was less than 2% and the trans-mittance in the regions of absorption could be measured accurately. The results are shown in Fig. 1. No corrections have been applied for the reflection losses and in the region of 40 μ where the transmittance is 92% no absorption occurs in the material. The percent transmittance for any thickness can be calculated from the results of Fig. 1. For example, a thickness of 1.25 mm would transmit about 60% of the incident energy at 65 μ. For comparison crystal quartz has been measured at wave-
FIG. 2. The reflectance of CsI from 120 to 170 μ. The radiation was reflected from 3 surfaces in order to determine the rest-strahlen wavelength which occurs at 145 μ.
lengths greater than 45 μ, and its transmittance is also shown in Fig. 1. This sample was cut with the optic axis parallel to the surface and with a thickness of 0.96 mm.
The reflection of CsI .increases rapidly at wavelengths greater than 120 μ. A single reflection from a plate of the material is useful in a filter system from 120 to 170 μ. In Fig. 2 the radiant energy has been reflected from 3 plates in order to obtain a sharp maximum. The maximum or reststrahlen wavelength was observed at 145 μ.
1 E. K. Plyler and F. P. Phelps, J. Opt. Soc. Am. 42, 432 (1952). 2 E. K. Plyler and N. Acquista, J. Research Natl. Bur. Standards 56, 149 (1956).
