High Accuracy Multi-color Pyrometry for High Temperature Surfaces

An evaluation of previously proposed least-squares multi-color pyrometry methods was carried out to determine their highest achievable accuracy. The study was limited to the visible spectrum and the temperature range of 1700 to 3000 K, typical of electrode surfaces in high-power electric propulsion thrusters. A Monte-Carlo simulation of the various methods showed the effects that the number of colors of the pyrometer, the errors associated with noise and calibration, and the number of measurements have on the uncertainty of the predicted temperature. Some of these methods were found to lead to inaccurate results, an underestimate of the uncertainty of the predicted temperature, or yield larger uncertainties than single-color pyrometry. The two methods that were found to yield the highest accuracy without underestimating the uncertainty are based on fitting the intensity versus wavelength data with three free parameters (temperature and the two coefficients of the emissivity versus wavelength model). Adding complementary measurements, at the price of spatial or temporal resolution, is shown to allow a reduction of the uncertainty in the predicted temperature well below that associated with single-color pyrometry.