Uncertainty of non-linearity in radiation thermometers calibrated by interpolation among multiple fixed points

We applied a new method for estimating the uncertainty due to non-linearity on the n = 3 scheme to the interpolating scheme below the copper point. The formula of uncertainty is mathematically derived extending the random walk method. The expression is simple and practical because it requires only the temperatures of the fixed points and a relative uncertainty value for each flux-doubling derived from the non-linearity measurement. The method is applied to actual data of non-linearity measurements with a 0.9 μm radiation thermometer.