Calibration of standard radiation thermometers using two fixed points

Radiation thermometry based on the measurement of two fixed points, the so-called n = 2 scheme, is studied in this work. Verification of the scheme was undertaken for various measurement conditions using seven radiation thermometers with nominal centre wavelengths of 650 nm, 890 nm, 900 nm, and 1600 nm. Comparison between the signals calculated using the n = 2 scheme and the signals measured directly with fixed-point cells was studied. For temperatures between the two reference fixed points, the equivalent temperature difference was less than 0.14 K in the temperature range from the In to Cu fixed points, and less than 0.3 K in the temperature range from the Cu to Re-C fixed points. The maximum normalised error was 0.35, confirming the validity of the n = 2 scheme; the consistency was observed not only in the interpolation temperature range, but also in the extrapolation range. The results also indicate that temperature realisation in the range from 150 degrees C to 2500 degrees C can be achieved by maintaining only the Zn, Cu, and Re-C fixed-point facilities. The n = 2 scheme would be of great benefit to developing national metrology institutes in terms of time consumed for scale realisation and cost for maintenance of the measurement system.