  Based on very deep photometry, Surface Brightness Fluctuations (SBF) have
been traditionally used to determine galaxy distances. We have recently
computed SBF spectra of stellar populations at moderately high resolution,
which are fully based on empirical stars. We show that the SBF spectra provide
an unprecedented potential for stellar population studies that, so far, have
been tackled on the basis of the mean fluxes. We find that the SBFs are able to
unveil metal-poor stellar components at the one percent level, which are not
possible to disentangle with the standard analysis. As these metal-poor
components correspond to the first stages of the chemical enrichment, the SBF
analysis provides stringent constrains on the quenching epoch.
