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.
R. Coisson, R. Coisson,
"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