  We have calculated optical conductivity [$\sigma(\omega)$] spectra of
ytterbium compounds (YbAl$_3$, YbAl$_2$, YbCu$_2$Si$_2$, YbNi$_2$Ge$_2$,
YbInCu$_4$, YbRh$_2$Si$_2$, YbIr$_2$Si$_2$, and YbB$_{12}$) based on the direct
interband transition derived from first-principle band calculation and compared
the results with the experimentally obtained $\sigma(\omega)$ spectra. The
spectral feature of a peak in the middle-infrared region (mid-IR peak) and a
shoulder structure in the far-infrared region (far-IR shoulder) in the
experimental $\sigma(\omega)$ spectra can be described by the band calculation
with a common renormalization factor. This result indicates that the infrared
spectra of Yb compounds originate from the interband transition from the Yb
$4f$ state but that the Yb $4f$ state shifts to the Fermi level with strong
electron correlation.
