Cryogenic absolute radiometers as laboratory irradiance standards, remote sensing detectors, and pyroheliometers.

The dramatic improvement in heat diffusivity of pure copper at liquid helium temperatures makes possible very important advances in the absolute accuracy, reproducibility, sensitivity, and time constant of cryogenic electrical substitution radiometers (ESRs), relative to conventional ESRs. The design and characterization of a table top cryogenic ESR now available for detector calibration work to the 0.01% level of absolute accuracy under laser illumination is discussed. A sensitive cryogenic ESR recently delivered to the NIST for radiometric calibrations of black bodies is also described, along with the design and testing of a very fast cryogenic ESR developed for NASA's remote sensing studies of the earth's radiation budget. Finally, the improvements that could be achieved in total and UV solar irradiance measurement using cryogenic ESRs are mentioned.