Resonant excited state absorption (ESA) and relaxation processes in Tb³⁺-doped aluminosilicate glasses are quantitatively evaluated. A model describing the excitation steps and upconversion emission is developed and applied to interpret the results from laser-induced surface deformation using thermal mirror spectroscopy. The fluorescence quantum efficiency of level D5₄ was found to be close to unity and concentration independent while, for the level D5₃, it decreases with Tb³⁺ concentration. Emission spectroscopy measurements supported these results. ESA cross sections are found to be more than three orders of magnitude higher than the ground state absorption cross section.