  The electronic properties of Cerium (Ce) and ytterbium (Yb) intermetallic
compounds may display a more local or more itinerant character depending on the
interplay of the exchange interactions among the $4f$ electrons and the Kondo
coupling between $4f$ and conduction electrons. For the more itinerant case,
the materials form heavy-fermions once the Kondo effect is developed at low
temperatures. Hence, a temperature variation occurs in the electronic structure
that can be traced by investigating the optical conductivity ($\sigma(\omega)$)
spectra. Remarkably, the temperature variation in the $\sigma(\omega)$ spectrum
is still present in the more localized case, even though the Kondo effect is
strongly suppressed. Here, we clarify the local and itinerant character in the
electronic structure by investigating the temperature dependence in the
$\sigma(\omega)$ spectra of various Ce and Yb compounds with a tetragonal
ThCr$_2$Si$_2$-type crystal structure. We explain the temperature change in a
unified manner. Above temperatures of about 100 K, the temperature dependence
of the $\sigma(\omega)$ spectra is mainly due to the electron-phonon
interaction, while the temperature dependence below is due to the Kondo effect.
