The IRIS radiometer is a new transfer standard instrument for the measurement of atmospheric longwave irradiance. The requirement for irradiance measurements required the development of a novel integrating sphere concept with integrated shutters and a pyroelectric detector. The radiometer is traceable to SI via the calibration by a well characterised cavity. Simultaneous outdoor measurements with four independently calibrated IRIS radiometers gave consistent measurements to within ±1 Wm~(-2).
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