Spectral Radiance Comparisons of Two Blackbodies with Temperatures Determined Using Absolute Detectors and ITS-90 Techniques

Abstract

For calibrations of spectral irradiance standards, a high‐temperature blackbody (HTBB) is used as a source of spectral radiance or irradiance as derived from Planck’s radiance law. The temperature of such a blackbody can be determined using radiance ratios to the gold freezing‐temperature blackbody based on the technique described in the International Temperature Scale of 1990 (ITS‐90), or by using a primary thermometer, which is an absolute detector referenced to a cryogenic radiometer. One of the primary motivations of using the detector‐based method is that the uncertainties in the temperature determination of a HTBB can be lower than those assigned using ITS‐90. Previous comparisons of temperatures measured using the two techniques have been at a few, selected wavelengths due to the limited measurement wavelengths of the comparison pyrometer. In this study, the spectral radiances of a HTBB from 250 nm to 2400 nm are assigned using a spectroradiometer from the known spectral radiances of a variable‐temperature blackbody (VTBB) with a 0.999 emissivity, and its temperature is found using ITS‐90 techniques. The spectral radiances of the HTBB are also assigned using a primary thermometer. These measurements are performed on four separate occasions over a period of one month. The uncertainties in the detector‐based radiance temperature measurements are propagated directly from the spectral responsivity determinations, and propagating the correlations in the calculation of the uncertainties results in the detector‐based temperature uncertainty of 0.21 K (k = 2) at 3000 K, which is more than a factor of 6 lower than the uncertainties in the temperatures determined using ITS‐90. The agreement in radiance temperatures between the two methods is within the total combined expanded uncertainty of the temperature determination of the HTBB at 3000 K of 1.2 K (k = 2).