Bi-color near infrared thermoreflectometry: A method for true temperature field measurement

Abstract

In a context of radiative temperature field measurement, this paper deals with an innovative method, called bicolor near infrared thermoreflectometry, for the measurement of true temperature fields without prior knowledge of the emissivity field of an opaque material. This method is achieved by a simultaneous measurement, in the near infrared spectral band, of the radiance temperature fields and of the emissivity fields measured indirectly by reflectometry. The theoretical framework of the method is introduced and the principle of the measurements at two wavelengths is detailed. The crucial features of the indirect measurement of emissivity are the measurement of bidirectional reflectivities in a single direction and the introduction of an unknown variable, called the "diffusion factor." Radiance temperature and bidirectional reflectivities are then merged into a bichromatic system based on Kirchhoff's laws. The assumption of the system, based on the invariance of the diffusion factor for two near wavelengths, and the value of the chosen wavelengths, are then discussed in relation to a database of several material properties. A thermoreflectometer prototype was developed, dimensioned, and evaluated. Experiments were carried out to outline its trueness in challenging cases. First, experiments were performed on a metallic sample with a high emissivity value. The bidirectional reflectivity was then measured from low signals. The results on erbium oxide demonstrate the power of the method with materials with high emissivity variations in near infrared spectral band.

Temperature monitoring on a plasmatron experiment by pyroreflectometry

“…Finally, it would be interesting to visualize this scene with a camera, to take into account surface phenomenon. Pyroreflectometry knows a matrix extension called thermoreflectometry [10], which will be soon tested on the Plasmatron facility.…”

Trichromatic thermoreflectometry for an improved accuracy of true temperature field measurement on a multi-material part

“…The indirect active method consists in measuring the reflectance of the material illuminated by a source of lower power [18]. Rather than measuring reflectance, another solution only measures the bidirectional reflectivity [19], which is easier to grasp with an active thermography system, called thermoreflectometry. This method performs simultaneous measurements of the sample thermal signal through the radiance temperature (thermography) and of its emissivity with a bidirectional reflectivity measurement (reflectometry) instead of a reflectance measurement.…”

“…The resolution of radiometric equations provides the true temperature and the n model parameters of the diffusion function. Bichromatic thermoreflectometry [19] assumes that the diffusion function is only a constant over a given spectral range. This assumption means that the reflectivities are homothetic at two wavelengths.…”

Automated thermal 3D reconstruction based on a robot equipped with uncalibrated infrared stereovision cameras

“…The ideal architecture is then only based on infrared cameras for a direct D 3 thermal reconstruction. Assuming that thermal methods already described in [14][15][16] are not in the scope of this paper, the challenge is then to provide a dense image-based D 3 reconstruction [17,18] with infrared cameras.…”