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 irradi ance in the red and near-infrared spectral range. Its temperature is m easured by standard Pt-resistance thermometers according to the Intern ational Temperature Scale of 1990 (ITS-90). The black body consists of two concentric sodium heatpipes, the inner one with temperature stabi lity 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 re sulting from the thermodynamic uncertainty of the temperature scale. T his 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 de grees C to 962 degrees C agree with T-90 within the combined 1 sigma u ncertainties.