  In this article, we review the charge excitations of heavy fermion compounds
probed by infrared spectroscopy. The article is not meant to be a comprehensive
survey of experimental investigations. Rather it focuses on the dependence of
charge excitations on the hybridization strength. In this context, the infrared
properties of the Ce$_m$M$_n$In$_{3m+2n}$ family are discussed in detail since
the hybridization strengths differ dramatically in different members despite
their similar lattice structures. Investigations on some mixed valent compounds
are also presented aiming to elucidate the generic trend on the evolution. In
particular, we address the scaling between hybridization energy gap
$\Delta_{dir}$ and the hybridization strength $\tilde{V}$($\propto\sqrt{WT_K}$)
in a wide range of heavy fermions compounds which demonstrates that the
periodic Anderson model can generally and quantitatively describe the
low-energy charge excitations.
