Classical Radiation Processes in the Weizsacker-Williams Approximation
  The main features of radiation by relativistic electrons are well approximated in the Weizsacker-Williams method of virtual quanta. This method is best known for its application to radiation during elementary particle collisions, but is equally useful in describing "classical" radiation emitted during the interaction of a single relativistic electron with an extended system, such as synchrotron radiation, undulator radiation, transition radiation and Cerenkov radiation. 
Max S. Zolotorev, Kirk T. McDonald