Abstract: A control program for controlling a source of electromagnetic radiation, with which a cross-linking in eye tissue of a patient is implemented by means of a photosensitiser introduced into the tissue in order to bring about a change in the eye tissue with reference to its form and/or one of its mechanical properties, the control program having been produced with the following steps: a) measurement data with reference to the patient and the eye are acquired, b) with the measurement data and treatment parameters, which involve a predetermined auxiliary control program for controlling the source of electromagnetic radiation, an outcome with reference to the change in the eye tissue obtained with these treatment parameters is simulated in a computer, c) the change simulated in this way is compared with a change in the eye tissue to be achieved, d) when step c) yields a sufficient agreement between the simulated change and the change in the eye tissue to be achieved, the auxiliary control program applied in step