Formulae for the oscillator strength of optical transitions, at which excitons trapped at a lattice defect are formed, were deduced by means of the effective mass method. The magnitude of the necessary interband elements characterizing the band structure can be deduced e.g. from the measured oscillator strength of “free” excitons. The resulting formulae were used for investigating the assumptions of an exciton mechanism of infrared luminescence in Cu2O.