The Development and Characterization of an Absolute Pyrometer Calibrated for Radiance Responsivity

Abstract

For temperatures above the freezing temperature of silver, the International Temperature Scale of 1990 (ITS‐90) is defined in terms of spectral radiance ratios to one of the silver, gold or copper freezing‐temperature blackbodies using the Planck radiance law. However, due to the use of spectral radiance ratios, the uncertainties in the realization of thermodynamic temperatures using ITS‐90 increase as the square of the temperature ratios. Such increases in the temperature uncertainties can be reduced by using absolute radiometry with pyrometers traceable to cryogenic radiometers, and the resulting temperature uncertainties can be smaller than those measured using the ratio pyrometry as prescribed in ITS‐90. We describe the development and the characterization of an absolute pyrometer (AP1) constructed at NIST and calibrated for absolute radiance responsivity. The calibrations are performed with the pyrometer as a single unit; thus separate measurements of the lens transmittance and the spectral responsivities of the filters and detector are not required. The temperature measurement capabilities of the AP1 are shown using the melt and freeze cycle of the gold‐point blackbody.