The principles and application of optical thermometry based on the fluorescence intensity ratioFluorescence intensity ratio technique in rare-earth-ionRare-earth ions-doped glasses are detailed. The theoretical foundation is established, deriving the relationship between the intensity ratio of two thermally coupled energy levels and temperature via the Maxwell–Boltzmann distribution. Key performance metrics for evaluating a thermal sensor, including absolute sensitivity, relative sensitivity, temperature resolution, and repeatability, are defined and formulated. A practical case study using an Er3+-Yb3+-doped tellurite glass is presented, meticulously comparing two data analysis methodologies: direct integration and spectral deconvolution for calculating the emission band areas. The investigation validates the sensor’s performance through heating and cooling cycles, assessing its precision and accuracy. The analysis is also extended to non-thermally coupled levels, which are modeled using a polynomial fit. This chapter provides a comprehensive guide to the experimental implementation, calculation, and performance assessment of luminescent thermometers for advanced sensing applications.