It now well known in laser materials, that a refractive index change appears when the active ions are pumped from ground to excited state due to the polarizability difference between ground and excited states (metastable). In this paper this effect was investigated in Tb3+ doped glasses: calcium alumino phosphate (CAP), low-silica calcium aluminosilicate (LSCAS) and calcium aluminosilicate (CAS). The measurements were performed using the time resolved Z-scan technique, with an Ar+ laser at 488nm, close to the resonance of 7F6→5D4 absorption line, where 5D4 is a metastable state. We obtained for low-silica calcium aluminosilicate glass δαp ~10-24 cm3 which is the highest value ever reported for a RE doped material.