An optical radiation source has been developed by coupling a dye laser to a small integrating sphere with an optical fiber. The radiant power from this source, which is monochromatic and spectrally tunable, has been stabilized to +/-0.02%. Nonuniformities in the emitted optical radiation field caused by speckle have been overcome by vibrating the fiber at ultrasonic frequencies. The source has been successfully used in a spectrophotometer to measure the transmittance of a large lens with an uncertainty of +/-0.01%, and the spectral responsivity of a filter radiometer with an uncertainty of +/-0.04%.

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.

Two spectral responsivity scales have been established at the National Physical Laboratory, UK. High quantum efficiency silicon photodiodes have been used to establish a continuous spectral responsivity scale for the spectral region 400 nm to 920 nm. The scale can be disseminated by the NPL with an uncertainty of f 0 , l YO. Pyroelectric detectors fitted with a reflecting hemispherical cavity have been used to establish a spectral responsivity scale for the spectral region 1 pm to 20 pm. This scale has an uncertainty of less than f l,6 YO.

This paper describes a radiometric determination of the melting/freezing point of gold by measurement of the spectral radiance of a black body at that temperature. A simple optical system was used to image the black-body aperture onto a filter radiometer. The advantages and disadvantages of this approach are discussed. The techniques to characterize the optical system are described together with the assessment of uncertainties. The freezing point of gold was found to be 1 337,330 K ± 28 mK.

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/21/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx 7 MARTIN J E, FOX N P and KEY P J, 1985, Metrologia 21, 147 -155.A cryogenic radiometer for absolute radiometric measurements.A cryogenic radiometer for absolute radiometric measurements.