Ga20Ge30Te50 thin films deposited by vacuum evaporation on various substrates have been studied for their structural and optical properties. The as-deposited amorphous films were crystallized by thermally annealing them. The optical constants of the amorphous films indicate semiconducting behaviour (n>k). The optical bandgap (E(g)) determined from Tauc’s plot is 0.7 ev. The change in reflectance on crystallization has been utilized to obtain maximum optical contrast by optimising the thickness of the film.