A concept to determine the true temperature of opaque materials using a tricolor pyroreflectometer

Abstract

Despite the existence of several possible optical pyrometric methodologies, the determination of the true temperature has still not received a universal and perfect solution. We present here an analysis of our theoretical simulations and experimental measurements that have been carried out in order to validate a three-color method. Our goal is to test a pyroreflectometer (working wavelengths 0.84, 1.3, and 1.55μm) equipped with an optical fiber probe. This apparatus has been developed for in situ measurements in severe conditions (high temperature, difficult accessibility, controlled atmosphere, vacuum,…) to determine the true temperature of opaque materials. The method is based on three monochromatic measurements proportional to the emitted radiance; three monochromatic measurements proportional to normal reflectivities; the hypothesis of the same bidirectional reflectivity distribution function (BRDF) for the instrumented sample at the three working wavelengths. Using the six simultaneous measurements, the diffuse factor η and the apparent diffuse factor η∗ are introduced to determine the convergence temperature T∗ that is assumed to be equal to the true temperature T. After theoretical simulation, the method has been experimentally verified on several materials and an analysis of the causes of errors and their impact on the method accuracy and the operator modus, is discussed. The quality of the results obtained demonstrates the utility of the tricolor pyroreflectometer in a situation where fundamental parameters, such as the temperature and radiative properties, are difficult to measure.