A balloon borne instrument is described which measures the downward flux of thermal radiation from atmospheric water vapor in the region of 6.3 µ. Use is made of a liquid-air-cooled gold-doped germanium detector together with a 40 cps vibrating vane chopper which operates at the same temperature as the detector. The responsivity of the detector increases with height, enabling measurements to be made over about four orders of magnitude. Signals, after amplification and synchronous rectification, are transmitted to the ground via an FM/FM telemetry system. Regular energy calibration is performed by reference to a local blackbody at a known temperature. The complete assembly weighs 2.5 kg and is flown on a conventional rubber balloon as part of a regular meteorological sounding. Twelve ascents have been made and measurements obtained up to 32 km. Results have been used to calculate stratospheric humidity profiles.
E. J. Williamson and J. T. Houghton, "A Radiometer-Sonde for Observing Emission from Atmospheric Water Vapor," Appl. Opt. 5, 377-383 (1966)