Five types of Schott glass (GG435, GG455, GG475, BG18, and BG38) have been irradiated up to 1 Mrad using 1.2-MeV γ rays from a Co60 source. The effect of the irradiation on the transmittance of the glasses was studied as a function of the dose for wavelengths up to 800 nm. Three parameters are used to describe the increase of the optical thickness: the saturation coefficient in inverse millimeters, the slope in inverse kilorads times inverse millimeters, and the saturation speed in kilorads; they are given as a function of wavelength. For each glass, a single color center could explain the increase in the optical thickness. This single color center creates three Gaussian-shaped absorption bands that apply to both the saturation coefficient and the slope; the energy, the full width at half maximum, and the amplitude of these bands are given. The data can be used either by an engineer for predicting radiation damage and for choosing the best glass for a given application or by a scientist interested in solid state physics. Guidelines for minimizing the effect of radiation on glass are given and applied to a space experiment to be flown in 1995 on the European Space Agency/National Aeronautics and Space Agency (ESA/NASA) Solar and Heliospheric Observatory (SOHO) spacecraft.