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

Abstract

This article addresses the problem of measuring an accurate temperature field on a multi-material part which exhibits spatial, temporal, spectral and thermal emissivity variations. The article analyses the contribution of trichromatic thermoreflectometry method compared to bichromatic thermoreflectometry method. Thermoreflectometry, an active thermography method, measures in-situ the emissivity, together with the temperature. The emissivity is measured indirectly by measuring the bidirectional reflectivity of the sample and by estimating its diffusion function. The bichromatic thermoreflectometry assumes an independent diffusion function with wavelength. For trichromatic thermoreflectometry, the diffusion function varies linearly with the wavelength. This article demonstrates the benefit of trichromatic thermoreflectometry on both simulated and experimental data. The simulated data come from measurements of emissivity and diffusion function of six different materials (metallic and dielectric) performed with a FTIR (Fourier Transform InfraRed) spectrometer. The addition of noise on these estimated values enables the propagation of uncertainties, which shows that the bias on temperature estimation is lower with trichromatic thermoreflectometry. Finally, an experimental demonstration on three of the six materials confirms a lower temperature measurement error (difference between the measured temperature and a reference temperature) with trichromatic thermoreflectometry.