In order to understand the photochromic effect in terbium gallium garnet doubly doped with calcium and cerium, the spectral characteristics of absorption changes induced by heating and irradiation are studied. Based on the findings we conclude that the photochromic band at 420 nm originates from defects involving Tb4+. Previously the band was attributed to Ce3+. Two possible models are suggested and discussed, both of which are in agreement with the obtained experimental results.