Full spectrum fitting method: a new approach for instantaneous phosphor thermometry in harsh environments

This paper proposes a new approach for the post-treatment of Laser Induced Phosphorescence (LIP) signals for instantaneous surface temperature measurements. LIP is a semi-invasive optical diagnostic based on coated thermographic phosphor surfaces, emitting a temperature-dependent signal under laser irradiation. The so-called “Full Spectrum Fitting method” (FSF method) uses the dependence of the entire phosphorescence spectrum with temperature. \(\text {Mg}_{3.5}\text {FGeO}_5\text {:Mn}\) phosphorescence spectra are acquired to build a database at each kelvin in a temperature range from 100 to 900 K with a unique set of experimental acquisition parameters. The constituted database is used to retrieve a single-shot temperature measurement using a least mean square algorithm. Statistical studies show excellent performance of the FSF method for single-shot temperature determination with a 12 K precision and no significant influence of temperature. In the same conditions, the error on temperature raises with the temperature with the more common Intensity Ratio method (IR method). Since it is difficult or even impossible to know the exact energy exciting the phosphor in harsh high-absorbing environments, an investigation on the influence of the laser fluence is carried out. It shows dependencies on the phosphorescence spectrum leading to errors during the temperature determination when the calibration and the measurement are not performed at the same laser fluence. Acquiring multiple databases at different laser fluences and systematically checking them with the least mean square algorithm allows being nearly independent of the laser fluence with the FSF method.