  We provide a combined theoretical and experimental study of the electronic
structure and the optical absorption edge of the orthorhombic perovskite
LaInO$_{3}$. Employing density-functional theory and many-body perturbation
theory, we predict a direct electronic quasiparticle band gap of about 5 eV and
an effective electron (hole) mass of 0.31 (0.48) m$_{0}$. We find the
lowest-energy excitation at 0.2 eV below the fundamental gap, reflecting a
sizeable electron-hole attraction. Since the transition from the valence band
maximum (VBM, $\Gamma$ point) is, however, dipole forbidden the onset is
characterized by weak excitations from transitions around it. The first intense
excitation appears about 0.32 eV above. Interestingly, this value coincides
with an experimental value obtained by ellipsometry (4.80 eV) which is higher
than the onset from optical absorption spectroscopy (4.35 eV). The latter
discrepancy is attributed to the fact that the weak transitions that define the
optical gap are not resolved by the ellipsometry measurement. The absorption
edge shows a strong dependency on the light polarization, reflecting the
character of the involved valence states. Temperature-dependent measurements
show a redshift of the optical gap by about 120 meV by increasing the
temperature from 5 to 300 K. Renormalization due to zero-point vibrations is
extrapolated from the latter measurement to amount to 150 meV. By adding the
excitonic binding energy of 0.2 eV obtained theoretically to the experimental
optical absorption onset, we determine the fundamental band gap at room
temperature to be 4.55 eV.
