  We investigate the optical conductivity of Sr$_{1-x}$Ca$_x$RuO$_3$ across the
ferromagnetic to paramagnetic transition that occurs at $x=0.8$. The thin films
were grown by metalorganic aerosol deposition with $0 \leq x \leq 1$ onto
NdGaO$_3$ substrates. We performed THz frequency domain spectroscopy in a
frequency range from 3~cm$^{-1}$ to 40~cm$^{-1}$ (100~GHz to 1.4~THz) and at
temperatures ranging from 5~K to 300~K, measuring transmittivity and phase
shift through the films. From this we obtained real and imaginary parts of the
optical conductivity. The end-members, ferromagnetic SrRuO$_3$ and paramagnetic
CaRuO$_3$, show a strongly frequency-dependent metallic response at
temperatures below 20~K. Due to the high quality of these samples we can access
pronounced intrinsic electronic contributions to the optical scattering rate,
which at 1.4~THz exceeds the residual scattering rate by more than a factor of
three. Deviations from a Drude response start at about 0.7~THz for both
end-members in a remarkably similar way. For the intermediate members a higher
residual scattering originating in the compositional disorder leads to a
featureless optical response, instead. The relevance of low-lying interband
transitions is addressed by a calculation of the optical conductivity within
density functional theory in the local density approximation (LDA).
