  In radiation processes such as a transition radiation, diffraction radiation,
etc. based on relativistic electrons passing through or near an opaque screen,
the electron self-field is partly shadowed after the screen over a distance of
the order of the formation length $\gamma^2\lambda$. This effect has been
investigated on coherent diffraction radiation (DR) by electron bunches.
Absorbing and conductive half-plane screens were placed at various distances L
before a standard DR source (inclined half-plane mirror). The radiation
intensity was reduced when the screen was at small L and on the same side as
the mirror. No reduction was observed when the screen was on the opposite side.
It is worth noting that absorbing and conductive half-plane screens produce the
same shadowing effect. The shadowing effect is responsible for a bound on the
intensity of Smith-Purcell radiation.
