The determination of emissivity of the variable-temperature blackbody used in the dissemination of the us national scale of radiance temperature

The emissivity of the variable-temperature blackbody (VTBB) used to disseminate the US radiance temperature scale was determined using a pyrometer with narrow-band spectral filters centered at 650 nm, 900 nm, and 1000 nm. Such measurements are needed to compare the accuracies of the radiance temperature determinations of pyrometers which could have peak sensitivities at any arbitrary wavelength. At the three wavelengths, separate, independent determinations of the radiance temperature of the VTBB were performed from 1200 K to 2800 K via spectral radiance ratios to the Au fixed-point blackbody as described in the International Temperature Scale 1990 (ITS-90). In order to determine the radiance temperatures using the spectral radiance ratios, the relative spectral responsivity of the pyrometer was measured in the NIST Spectral Comparator Facility. The field stop of the optical system restricted the target area to 0.6 mm by 0.8 mm focused at the bottom of the 25.4 mm diameter cavity, and the temperature variations of the cavity bottom were found to be < 0.2 K over a diameter of 8 mm. Due to the temperature uniformity and the small viewing area of the pyrometer, the radiance temperature measurements were analyzed using a constant, or gray-body emissivity model. The measured emissivity of the VTBB was found to be 0.9985+/-0.0005 (k = 1).