NIST-PTB measurements of the radiometric temperatures of a high-temperature black body using filter radiometers

Filter radiometers were used in a bilateral comparison of the radiometric t emperatures of a high-temperature black body from 2100 K to 3100 K. The mea surements took place over a ten-day period in October 1998 at the Physikali sch-Technische Bundesanstalt (PTB, Braunschweig, Germany) using filter radi ometers from the PTB and from the National institute of Standards and Techn ology (NIST, Gaithersburg, USA). The comparison revealed that the radiometr ic temperatures determined using the NIST and the PTB filter radiometers di ffered progressively from 2.4 K at 2200 K to 5.1 K at 3200 K, with the NIST temperature values always lower than the PTB values. Differences in the me asurements of absolute spectral responsivity were the main cause of the obs erved temperature differences.