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Proceedings Paper

A Helium-Cooled Absolute Cavity Radiometer For Solar And Laboratory Irradiance Measurement
Author(s): P. Foukal; P. Miller
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Paper Abstract

We describe the design and testing of a helium-cooled absolute radiometer (HCAR) devel-oped for highly reproducible measurements of total solar irradiance and ultraviolet flux, and for laboratory standards uses. The receiver of this cryogenic radiometer is a blackened cone of pure copper whose temperature is sensed by a germanium resistance thermometer. During a duty cycle, radiant power input is compared to electrical heating in an accurate resistor wound on the receiver, as in conventional self-calibrating radiometers of the PACRAD and ACR type. But operation at helium temperatures enables us to achieve excellent radia-tive shielding between the receiver and the radiometer thermal background. This enables us to attain a sensitivity level of 10-7 watts at 30 seconds integration time, at least 10 times better than achieved by room temperature cavities. The dramatic drop of copper specific heat at helium temperatures reduces the time constant for a given mass of receiver, by a factor of 103. Together with other cryogenic materials properties such as electrical superconductivity and the high thermal conductivity of copper, this can be used to greatly reduce non-equivalence errors between electrical and radiant heating, that presently limit the absolute accuracy of radiometers to approximately 0,2%. Absolute accuracy of better than 0.01% has been achieved with a similar cryogenic radiometer in laboratory measurements of the Stefan-Boltzmann constant at NPL in the U.K. Electrical and radiometric tests con-ducted so far on our prototype indicate that comparable accuracy and long-term reproducibility can be achieved in a versatile instrument of manageable size for Shuttle flight and laboratory standards uses. This work is supported at AER under NOAA contract NA8ORAC00204 and NSF grant DMR-8260273.

Paper Details

Date Published: 22 September 1983
PDF: 8 pages
Proc. SPIE 0416, Applications of Optical Metrology: Techniques and Measurements II, (22 September 1983); doi: 10.1117/12.935936
Show Author Affiliations
P. Foukal, Atmospheric and Environmental Research, Inc. (United States)
P. Miller, Atmospheric and Environmental Research, Inc. (United States)

Published in SPIE Proceedings Vol. 0416:
Applications of Optical Metrology: Techniques and Measurements II
Jar Jueh Lee, Editor(s)

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