Uncertainty of radiation thermometers calibrated by interpolation between fixed points

An interpolation scheme for calibration of radiation thermometers in high-temperature is discussed, which utilizes one metal fixed point and one high-temperature fixed point of metal-carbon systems. The two-fixed-point interpolation scheme can realize smaller uncertainty than conventional schemes which make reference to a single fixed point, or to three or more fixed points. The two-fixed-point interpolation scheme has the advantage that no precise evaluation of the radiation thermometer spectral responsivity function is required. This reduces the need of a regular re-evaluation of the relative spectral responsivity in the presence of filter drift, or even evaluation of the spectral responsivity at all if the approximate filter bandwidth is known.