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 temperatures of a high-temperature black body from 2100 K to 3100 K. The measurements took place over a ten-day period in October 1998 at the Physikalisch-Technische Bundesanstalt (PTB, Braunschweig, Germany) using filter radiometers from the PTB and from the National institute of Standards and Technology (NIST, Gaithersburg, USA). The comparison revealed that the radiometric temperatures determined using the NIST and the PTB filter radiometers differed 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 measurements of absolute spectral responsivity were the main cause of the observed temperature differences.