The double-heatpipe black body: A high-accuracy standard source of spectral irradiance for measurements of T-T-90

The Physikalisch-Technische Bundesanstalt (PTB) has developed a large-area black body (LABB) for accurate realization of the spectral irradiance in the red and near-infrared spectral range. Its temperature is measured by standard Pt-resistance thermometers according to the International Temperature Scale of 1990 (ITS-90). The black body consists of two concentric sodium heatpipes, the inner one with temperature stability of +/-2 mK forming the radiating cavity. The relative uncertainty of the spectral irradiance of the LABB in the red spectral range is 6 x 10(-4) at the freezing point of silver, the largest contribution resulting from the thermodynamic uncertainty of the temperature scale. This allows the ITS-90 to be checked by a comparison of the LABB with a cryogenic radiometer using calibrated filter radiometers as transfer standards. The thermodynamic temperatures obtained in the range 660 degrees C to 962 degrees C agree with T-90 within the combined 1 sigma uncertainties.