Comparison of the New NPL Primary Standard Ag Fixed-Point Blackbody Source with the Primary Standard Fixed Point of PTB

Above the freezing point of silver (961.78 deg C), the International Temperature Scale of 1990 is defined in terms of Planck's radiation law. The scale is maintained and disseminated using a validated and linear pyrometer in conjunction with a blackbody reference source at either the Ag, Au (1064.18 deg C) or Cu (1084.62 deg C) freezing point. In order to realize the scale with the highest precision high quality, well-characterised, reproducible fixed-point blackbody sources are required. Such sources have been maintained at NPL for a number of years, but it was felt that improvements to the design would be beneficial. A new Ag point blackbody source has therefore been constructed. The new design will improve the quality and reproducibility of the melting and freezing plateaux and reduce errors due to the `out-of-focus'size-of-source effect which is difficult to measure and to eliminate. Full details of the design of the new source, including results of the assessment of its performance, are described. Critical for the application of fixed-point blackbodies as primary temperature standards is the precise knowledge of the emissivity of the cavity, which causes a correction to the melting and freezing temperature of the ingot. As blackbody emissivities are difficult to assess experimentally, two different numerical approaches developed at NPL and PTB are used to calculate the blackbody emissivity. In order to further validate the performance of the new Ag fixed-point blackbody it has been compared with the Au primary fixed-point blackbody of PTB. For the comparison the ratios of the spectral radiances of the fixed-point blackbodies were measured at 650 nm and 950 nm using the PTB monochromator-based spectral radiance calibration facility, and at 654 nm and 953 nm using the PTB interference filter-based primary photoelectric pyrometer.