Primary Radiometry for the Mise-en-Pratique for the Definition of the Kelvin: The Hybrid Method

A task group of CCT-WG5 (radiation thermometry) was established in May 2008 to write text for the mise-en-pratique for the definition of the kelvin ( MeP- K) for high temperatures. This task group reviewed and gave summaries for the existing techniques for filter radiometry as a means of determining the absolute radiance, and hence the thermodynamic temperature of a blackbody source. Three approaches were described—the radiance method, which calibrates the radiation thermometer for radiance responsivity, the irradiance method, which calibrates a filter radiometer for irradiance responsivity and then measures the source through two apertures, and the hybrid method that introduces a lens to the irradiance method. In the “hybrid method” the radiation thermometer consists of a filter radiometer, a double aperture system, and a lens. The lens allows the instrument to view a small area blackbody source. The system is calibrated “in parts”—i.e., the filter radiometer is calibrated for irradiance responsivity, and the transmittance of the lens and the geometric factor are determined separately. The main drawbacks of this single lens instrument are its high size-of-source effect (~0.2 %), and that this effect has to be determined in an “absolute” sense—relative to a theoretical infinite source. However, although the correction is large, with careful evaluation, the associated uncertainty can be made sufficiently small to measure the temperature of fixed-point cell transitions with low uncertainties. This article reviews the hybrid method and gives a comprehensive discussion of the associated uncertainty components.