  We apply dynamical mean field theory to study a prototypical model that
describes charge ordering in the presence of both electron-lattice interactions
and intersite electrostatic repulsion between electrons. We calculate the
optical and d.c. conductivity, and derive approximate formulas valid in the
limiting electron-lattice coupling regimes. In the weak coupling regime, we
recover the usual behavior of charge density waves, characterized by a transfer
of spectral weight due to the opening of a gap in the excitation spectrum. In
the opposite limit of very strong electron-lattice coupling, instead, the
charge ordering transition is signaled by a global enhancement of the optical
absorption, with no appreciable spectral weight transfer. Such behavior is
related to the progressive suppression of thermally activated charge defects
taking place below the critical temperature. At intermediate values of the
coupling within the polaronic regime, a complex behavior is obtained where both
mechanisms of transfer and enhancement of spectral weight coexist.

Strongly Correlated Electrons