The possibility of producing vacuum ultraviolet radiation by harmonic generation on an electron beam has stimulated the study of coherent emission of light from a modulated beam. Properties of coherent emission can be easily visualized on an w-θ diagram, and considering the electron beam as a non-uniform Poisson process, coherent and incoherent radiation are described in a unified way. The case of an undulator is analyzed in detail.
"Coherent And Incoherent Radiation From Charged Particle Beams", Proc. SPIE 0582, Insertion Devices for Synchrotron Sources, (5 May 1986); doi: 10.1117/12.950908; https://doi.org/10.1117/12.950908