Double Modulation Pyrometry Applied to Radiatively Heated Surfaces With Dynamic Optical Properties

The accuracy of radiometric temperature measurement in radiatively heated environments is severely limited by the combined effects of intense reflected radiation and unknown, dynamically changing emissivity, which induces two correlated and variable error terms. While the recently demonstrated double modulation pyrometry (DMP) eliminates the contribution of reflected radiation, it still suffers from the shortcomings of single-waveband pyrometry: it requires knowledge of the emissivity to retrieve the true temperature from the thermal signal. Here, we demonstrate an improvement of DMP incorporating the in situ measurement of reflectance. The method is implemented at Paul Scherrer Institute (PSI) in its 50 kW high-flux solar simulator and used to measure the temperature of ceramic foams (SiSiC, ZrO₂, and Al₂O₃) during fast heat-up. The enhancement allows DMP to determine the true temperature despite a dynamically changing emissivity and to identify well-documented signature changes in ZrO₂ and Al₂O₃. The method also allows us to study the two dominant error sources by separately tracking the evolution of two error components during heat-up. Furthermore, we obtain measurements from a solar receiver, where the cavity reflection error limits measurement accuracy. DMP can be used as an accurate radiometric thermometer in the adverse conditions of concentrated radiation, and as a diagnostic tool to characterize materials with dynamic optical properties. Its simple design and ability to correct for both errors makes it a useful tool not only in solar simulators but also in concentrated solar facilities.