Abstract: The invention concerns a process for producing a security feature and a print medium which is equipped in that respect, which affords the possibility of not having to implement any change and in particular mechanical change in the surface of the card body. In that respect the substrate includes at least one change-over substance which by irradiation with light of a given wavelength experiences an irreversible change in color from a starting color to a final color, wherein the substrate in the initial condition is so irradiated with a controlled light beam of that wavelength, in particular a laser light beam that, due to the change in color caused thereby in the change-over substance, an image which can be recognized with the naked eye is produced within the volume of the substrate.