ABSOLUTE SPECTRAL RADIOMETRIC DETERMINATION OF THE THERMODYNAMIC TEMPERATURES OF THE MELTING FREEZING POINTS OF GOLD, SILVER AND ALUMINUM

Narrow band filter radiometers have been used to measure the spectral radiance of black bodies held at the melting/freezing points of aluminium, silver and gold with an accuracy of 0,04%. Descriptions are given of the optical components and the techniques used for their characterization. As the measurements are absolute, the thermodynamic temperatures of these points can be calculated directly from the Planck radiation law. The values obtained are: T (aluminium) = 933,509 +/- 0,027 K, T (silver) = 1 235,009 +/- 0,044 K and T (gold) = 1 337,330 +/- 0,049 K. These values are analysed with reference to the International Temperature Scale of 1990 (ITS-90) and to the data that were used to derive the scale. The conclusion drawn is that, while the ITS-90 has a sounder thermodynamic basis than its predecessor (IPTS-68), the temperature intervals between the high temperature fixed points, which were thought to be well established, may need further investigation.