  We present a detailed temperature and frequency dependence of the optical
conductivity measured on clean high quality single crystals of
URu$_{2}$Si$_{2}$ of $ac$- and $ab$-plane surfaces. Our data demonstrate the
itinerant character of the narrow 5f bands, becoming progressively coherent as
temperature is lowered below a cross-over temperature $T^*{\sim}75~K$. $T^*$ is
higher than in previous reports as a result of a different sample preparation,
which minimizes residual strain. We furthermore present the density-response
(energy-loss) function of this compound, and determine the energies of the
heavy fermion plasmons with $a$-and $c$-axis polarization. Our observation of a
suppression of optical conductivity below 50~meV both along $a$ and $c$-axis,
along with a heavy fermion plasmon at 18~meV, points toward the emergence of a
band of coherent charge carriers crossing the Fermi energy and the emergence of
a hybridization gap on part of the Fermi surface. The evolution towards
coherent itinerant states is accelerated below the hidden order temperature
$T_{HO}=17.5$~K. In the hidden order phase the low frequency optical
conductivity shows a single gap at $\sim 6.5$meV, which closes at $T_{HO}$.
