[en] A technique is presented to simplify calculations involving processes in which one or more leptons emit virtual photons, with the lepton spins summed over. It consists of the choice of a specific set of polarization vectors to respresent the virtual photon which diagonalizes the photon density matrix, thereby requiring only the square of the photon amplitude, summed over spins of other particles if desired. Although similar in spirit to the equivalent photon approximation, it is exact. An important application is the two-photon mechanism in colliding e⁺e^- beams. (Auth.)