Aims. We provide an empirical calibration relation to convert Lick indices into abundances for the integrated light of old, simple stellar populations for a large range in the observed [Fe/H] and [α/Fe]. This calibration supersedes the previously adopted ones because it is based on the real abundance pattern of the stars instead of the commonly adopted metallicity scale derived from the colours. Methods. We carried out a long-slit spectroscopic study of 23 Galactic globular clusters (GCs) for which detailed chemical abundances in stars have previously been measured. The line-strength indices, as defined by the Lick system and Serven and collaborators, were measured in low-resolution integrated spectra of the GC light. The results were compared to average abundances and abundance ratios in stars taken from the compilation by Pritzl and collaborators, as well as synthetic models. Results. The Fe-related indices increase linearly as a function of [Fe/H] for ?[Fe/H]??>????2. The Mg-related indices respond in a similar way to [Mg/H] variations, although Mgb turns out to be a less reliable metallicity indicator for ?[Z/H]??
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