Absolute Radiometry for the MeP-K: The Irradiance Measurement Method

Abstract

The “Mise en pratique for the definition of the kelvin” (MeP-K) as a guide for the realization of the kelvin is presently revised to incorporate the temperature scales in current use as the best formal approximations of the thermodynamic temperature and, additionally, direct methods of thermodynamic temperature measurement. Within the MeP-K, the CCT WG 5 “Radiation Thermometry” develops a document to facilitate the implementation of thermodynamic temperature measurements at temperatures above the silver fixed point (961.78 °C). Three thermodynamic temperature measurement methods, all relying on absolute spectral-band radiometry, are proposed. To make these techniques more accessible for the broader thermometry community, each of these methods is described in detail in an accompanying paper to the MeP-K. These descriptions shall not be understood as a restriction to other possible radiometric methods of thermodynamic temperature measurement. One of the methods of radiometric temperature measurement above the silver point is the absolute measurement of irradiance in a well-defined spectral band. This is a straightforward method, which relies on filter radiometers with known spectral irradiance responsivity and a well-defined observation geometry, without any imaging technique. It has been developed for thermodynamic temperature measurements of precision blackbodies as primary standards for radiometry and thermometry and for the investigation of the uncertainty of the ITS-90 as an approximation of the thermodynamic temperature over a period of nearly 20 years at PTB. The article describes the design of the applied filter radiometers, their calibration chain, and their long-term stability. Additionally, the details and requirements of the experimental procedure for obtaining thermodynamic temperatures with this method are presented. Finally, relevant results obtained with this method, which is also applicable for temperatures significantly below the silver fixed point, are reviewed.