Absolute measurements of black-body emitted radiance

Near-ambient-temperature black-body sources are routinely used for calibration in terms of radiance of a variety of infrared instruments such as those used in remote sensing and thermal imaging. The black-body radiance is usually determined by reference to a measured temperature and a calculated effective emissivity. The temperature is measured with one or more contact thermometers positioned close to the emitting black-body surface. In this case traceability to the International System of Units (SI) is to the kelvin through the ITS-90. This paper describes an alternative, more direct method based on the use of absolutely calibrated filter radiometers. These filter radiometers form part of a new facility called AMBER (Absolute Measurements of Black-body Emitted Radiance) which has been designed to determine the radiance of an ambient-temperature black body with an uncertainty of about 0.1% (which corresponds to a radiance temperature difference of 25 mK at 4 mum) and a resolution of 0.001% (0.3 mK). The facility obtains its traceability to the SI directly through radiometric standards in the form of a cryogenic radiometer rather than through the ITS-90.