Infrared Spectral Reflectance Independent of Emitted Flux at Controlled Temperatures
This research program consisted of the development of techniques and instrumentation for obtaining infrared spectral reflectance measurements independent of emitted flux at controlled temperatures for spectral range 4000 to 450 wavenumbers (2.5 to 22.2 microns) at temperatures ranging from −175°C (liquid Nitrogen temperatures) to approximately +1000°C. Supported by ARPA, a Cary-White infrared spectrophotometer was used equipped with a special designed (White, J. Opt. Soc. Am. 54, 1332, 196k) hemispherical reflectance attachment with temperature controls of heated substrates, water cooled sample holder, nichrome source input power control, heated sample holder, and specially designed liquid nitrogen cell holder. Measurements were made of frosts of H2O and CO2 (Keegan and Weidner, J. Opt. Soc. Am. 56, 523, 1966) N2O4, NH3, combinations of them, and dry ice; natural and man-made objects; highly absorbing materials used for black-body cavities; highly reflecting metals; the effect of temperature on painted surfaces, and time of exposure; and the effect of temperature on some rare-earth oxides. From spectral reflectance data independent of emitted flux at controlled temperatures, emissivity data can be obtained indirectly on opaque materials from the relationship: emissivity is equal to one minus the reflectance. This is particularly useful in obtaining emissivity data for samples at temperatures below 200°C where direct measurements of emissivity are not easily obtained on ablative materials such as polytetrafluoroethelyene.