  The organic salt $\alpha$-(BEDT-TTF)$_2$I$_3$ is considered a model system
for metal-insulator transition due to electronic charge ordering at $T_{\rm
CO}=135$~K. The optical properties obtained from polarized reflection
measurements above and below $T_{\rm CO}$ can be well described by calculations
based on first-principle density-functional theory (DFT). We discuss the effect
of electronic correlations on the metal-insulator transition.
