  The optical properties of LaNiO$_3$ thin films are investigated over a wide
energy and temperature range. Thin films of varying thickness were epitaxially
grown by pulsed laser deposition on LaAlO$_3$ and SrTiO$_3$ substrates. The
optical conductivity data of the films reveal a number of interband transitions
above 1 eV which are in good agreement with band structure calculations. No
well defined Drude peak is observed however, in stark contrast with LDA theory
predicting a finite density of states at the Fermi energy. This experimental
finding of a vanishing Drude spectral weight, compared to a finite electron
kinetic energy obtained from band structure calculations, highlights the
importance of strong electronic correlations in LaNiO$_3$.
