An active cavity radiometer of the electrical substitution type with a cone receiver that operates at 2–4 K has been developed for measuring radiant fluxes in the dynamic range of 20 nW to 100 μW within an uncertainty of ±1% (2σ level). It is a broadband absolute detector with a flat overall absorption efficiency that is > 99% for radiation from the visible to long-wavelength IR. The system is designed based on thermal modeling and experimental measurements of concepts. It has been installed in the cryogenic chamber for low-background infrared radiation calibrations at the National Institute of Standards and Technology (NIST) for testing cryogenic blackbody sources, detectors, and optical components. Its time constant, responsivity, and nonequivalence error have been measured. They are in agreement with design predictions. Radiant power measurements of an amplitude-stabilized He–Ne laser beam with the radiometer and an industry standard photodiode detector, QED-200, have been intercompared and found to be in agreement. The intercomparison ratio of the measurements with the absolute cryogenic radiometer and QED-200 was 1.004 in the 75–100-μW range with an uncertainty of 0.5% (the 3σ level).
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