A blackbody radiation source (22) includes an insulated enclosure (40) having a viewing aperture (24) defining a line of sight (26) through the wall of the enclosure (40). A heat sink (50), preferably a pool of liquid nitrogen, is located within the insulated enclosure (40). There is a viewing surface (28) in thermal contact with the heat sink (50) but having an unobstructed view through the viewing aperture (24). The viewing surface (28) is inclined to the line of sight (26) through the viewing aperture (24). A sensor (20) is calibrated by placing the sensor (20) in facing relation to the aperture (24) and measuring an output black body signal of the sensor (20). The viewing surface (28) may be radiatively heated concurrently with the measuring to produce a higher equivalent radiometric temperature.
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