  We report empirical comparisons of Cu K-edge indirect resonant inelastic
x-ray scattering (RIXS) spectra, taken at the Brillouin zone center, with
optical dielectric loss functions measured in a number of copper oxides. The
RIXS data are obtained for Bi$_2$CuO$_4$, CuGeO$_3$, Sr$_2$Cu$_3$O$_4$Cl$_2$,
La$_2$CuO$_4$, and Sr$_2$CuO$_2$Cl$_2$, and analyzed by considering both
incident and scattered photon resonances. An incident-energy-independent
response function is then extracted. The dielectric loss functions, measured
with spectroscopic ellipsometry, agree well with this RIXS response, especially
in Bi$_2$CuO$_4$ and CuGeO$_3$.
