A blackbody radiation source includes an insulated enclosure having a viewing aperture defining a line of sight through the wall of the enclosure. A heat sink, preferably a pool of liquid nitrogen, is located within the insulated enclosure. There is a viewing surface in thermal contact with the heat sink but having an unobstructed view through the viewing aperture. The viewing surface is inclined to the line of sight through the viewing aperture. A sensor is calibrated by placing the sensor in facing relation to the aperture and measuring an output black body signal of the sensor. The viewing surface may be radiatively heated concurrently with the measuring to produce a higher equivalent radiometric temperature.
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