Direct comparison of copper and gold freezing-point black-body primary standards from the NRLM and the NIST

Abstract

Freezing-point black bodies used as primary standards of spectral radiance at two national standards laboratories are directly intercompared for the first time. The spectral radiance ratio is directly measured for a copper freezing-point black body from the National Research Laboratory of Metrology (NRLM), Japan, and a gold freezing-point black body from the National Institute of Standards and Technology (NIST), USA. Narrowband spectral radiance measurements at five wavelengths between 546 nm and 878 nm show the NRLM copper to NIST gold freezing-point temperature interval to be 20,39 K ± 0,12 K (2σ), in good agreement with the International Temperature Scale of 1990 (ITS-90) value of 20,44 K, and comfortably within the combined uncertainties assigned to the realization of the ITS-90 from each source.