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An absolute cryogenic radiometer (ACR) has been developed at the National Institute of Metrology of China. The system was built based on a mechanical cooling system, with a cavity receiver working at 5.2 K and a noise-equivalent power of ~2 nanowatts. The ACR cavity receiver was made from pure copper by electroplating, weighing only <1 g. The inner surface of the ACR cavity receiver was coated with a polymer material mixed with carbon nanotubes which is absorptive in a wide spectral range from 250 nm to above 16 m. The relative standard uncertainty for the calibration of optical radiant power is 0.042% at near 100-microwatts level and 0.22% at near 1-microwatt level. The ACR system can be utilized for not only optical radiant power measurement but also irradiance measurement with a precision aperture of a calibrated area, and then can be used as laboratory irradiance standards, remote sensing detectors, and pyrheliometers. The ACR system can also be employed for total radiance measurements with precision measurement results on the effective areas of precision apertures and the distance between the precision apertures, in the cases that the measurement target overfills the field of view of the system. The distance between the precision apertures needs to be measured inside the cryogenic and vacuum chamber and hence in-situ measurement by time of flight method with an uncertainty of less than 0.1 mm was adopted. The ACR system can be then applied to calibrate the total radiance of a blackbody radiation source with a working temperature as low as 100 K with a relative standard uncertainty better than 0.5% and determine the effective radiant temperature with an uncertainty to <150 mK level when the emissivity of the blackbody is near unity. The ACR system can be further extended for spectral radiance measurements using band-limiting optical filters. The spectral transmission of the band-limiting optical filters can be used by laboratory spectrophotometers; but for higher accuracy, an integrating sphere source illuminated with a monochromatic light with tunable wavelength can be used for the spectral transmission measurement to eliminate the angle effect resulted from the spatially dispersive rays. Also, optical density outside the desired bandwidth of the band-limiting optical filters should be carefully analyzed. After the spectral radiance measurement at certain wavelength, the spectral radiance of the radiant source over a wider spectral range can be extrapolated by measuring the spectral diffuse reflectance over the interested wavelengths.
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