  The integrated spectra of stellar systems contain a wealth of information,
and its analysis can reveal fundamental parameters such as metallicity, age and
star formation history. Widely used methods to analyze these spectra are based
on comparing the galaxy spectra to stellar population (SP) models. Despite
being a powerful tool, SP models contain many ingredients, each with their
assumptions and uncertainties. Among the several possible sources of
uncertainties, it is not straightforward to identify which ingredient dominates
the errors in the models. In this work we propose a study of one of the SP
model ingredients -- the spectral stellar libraries -- independently of the
other ingredients. To that aim, we will use the integrated spectra of globular
clusters which have color-magnitude diagrams (CMDs) available. From these CMDs
it is possible to model the integrated spectra of these objects without having
to adopt -- or make assumptions -- on the other two main ingredients of SP
models, evolutionary tracks or an IMF. Here we tested four widely used stellar
libraries. We found that the libraries are able to reproduce the integrated
spectra of 18 of the 30 cluster spectra inside a mean flux uncertainty of 5%.
For the larger wavelength range tested, a theoretical library outperforms the
empirical ones in the comparison. Without the blue part of the spectra,
empirical libraries fare better than the theoretical, in particular when
individual features are concerned. However, the results are promising for
theoretical libraries, which are equally efficient to reproduce the whole
spectrum.
