National Physical Laboratory

Supply and calibration of radiation thermometers using high temperature eutectic fixed points

Metal carbon eutectic alloy fixed points have been extensively researched as high temperature reference standards. They provide a wide range of potential fixed points above the copper point at 1084.62 °C and up to 2750 °C. Their proven stability and reproducibility allows NPL to assign transition temperatures to them with very low uncertainty. This in turn makes it possible to calibrate suitable radiation thermometers with lower uncertainty than is usually possible. This is done by measuring the instrument's output at a series of fixed points (at least three) and fitting an interpolation function based on Planck's radiation law. While the exact uncertainty depends on the pyrometer being calibrated, it has been found realistic to provide a calibration certificate with a UKAS uncertainty of 0.05%, for even a relatively low cost instrument. This makes it possible to maintain a temperature scale at the level of uncertainty of a primary ITS-90 realisation, but at reduced cost and complexity.

The requirements for a suitable radiation thermometer are:

  • Small target size: A field of view (fov) of 1.5 mm or less at a working distance of 0.5 m or above is recommended.
  • Good size-of-source (sse) response: The optical design should be such that the signal is minimally affected by radiance from outside the nominal field of view. Less than 0.2% change from 3 mm to 25 mm source size is recommended.
  • Band pass spectral response: If the instrument spectral response does not behave as a band pass with adequately narrow bandwidth an additional fixed point measurement is required.

A suitable radiation thermometer can be supplied with a 1 mm fov at 750 mm distance, a bandpass filter at a centre wavelength of 715 nm and sse less than 0.2%. This instrument can be calibrated to an uncertainty of 0.05% to 2000+ °C (k = 2). In a practical situation this has been shown to potentially halve the uncertainties achieved in a calibration laboratory simply by changing to a standard calibrated by this method1.

1 Reducing uncertainty in practical radiation thermometer calibration based on proposed changes to the mise en pratique of the definition of the kelvin, David Lowe, Mick Broughton, Jon R Willmott, International Congress of Metrology 15010 (2013).


Calibration certificate uncertainty of NPL design ATK radiation thermometer

Calibration certificate uncertainty of NPL design ATK radiation thermometer


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Last Updated: 12 Dec 2013
Created: 12 Dec 2013


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