  A theoretical model of the transition radiation (TR) emission of an N
electron bunch must comply with the covariance and the temporal-causality
principles. A charge-density-like covariance must indeed imprint the formal
expression of the TR energy spectrum. A causality relation must constrain the
emission phases of the radiation pulse to the temporal sequence of the N
electron collisions onto the metallic screen. Covariance and causality are the
two faces of the same coin: failing in implementing one of the two constraints
into the model necessarily implies betraying the other one. The main formal
aspects of a covariance and causality consistent formulation of the TR energy
spectrum of an N electron beam will be here described with reference to the
case of a radiator surface with an arbitrary size.
