Thermoreflectometry: a new system to determine the true temperature fields on surface with unknown emissivities
Résumé
In a context of quantitative thermography, the major problem in determining the true temperature of an object is the knowledge of its emissivity. This problem is very complicated, above all when its value changes during the measurement. This article deals with a new radiative method for measuring true temperature fields with an on-line determination of emissivity. This method, called thermoreflectometry, consists in the indirect emissivity measurement by a reflectometry method in addition to the radiance temperature measurement. It assumes that the shapes of bidirectional reflectivity distribution is homothetic for two wavelengths. This assumption is much less restrictive than the gray body one (emissivity equal for two wavelengths). Finally, those two measurements and the assumption are fused for determining the true temperature field and the diffusion factor field, a key parameter of the method. This parameter provides information on the surface properties (diffuse or specular) ans it is assumed to be independent of the wavelength. The theoretical basis of thermoreflectometry method are explained and a precise description of the apparatus is given. Measurements on instrumented samples, heated at a temperature of 350 degrees C and with non uniform emissivity, are in broad agreement with the theory and show a high accuracy of the method, in reference to thermocouples measurements. The main assumption of the method is also verified by additional measurements of the bidirectional reflectivity distribution function (BRDF). These results demonstrate the relevance of this method, based on a simple embedded sensor, for measuring the true temperature field on samples with non-uniform and unknown emissivity.