  We have extended our evolutionary synthesis code, GALEV, to include Lick/IDS
absorption-line indices for both simple and composite stellar population models
(star clusters and galaxies), using the polynomial fitting functions of Worthey
et al. (1994) and Worthey & Ottaviani (1997). We present a mathematically
advanced Lick Index Analysis Tool (LINO) for the determination of ages and
metallicities of globular clusters (CGs): An extensive grid of GALEV models for
the evolution of star clusters at various metallicities over a Hubble time is
compared to observed sets of Lick indices of varying completeness and
precision. A dedicated chi^2 - minimisation procedure selects the best model
including 1-sigma uncertainties on age and metallicity. We discuss the age and
metallicity sensitivities of individual indices and show that these
sensitivities themselves depend on age and metallicity; thus, we extend
Worthey's (1994) concept of a "metallicity sensitivity parameter" for an old
stellar population at solar metallicity to younger clusters of different
metallicities. We find that indices at low metallicity are generally more age
sensitive than at high metallicity. Our aim is to provide a robust and reliable
tool for the interpretation of star cluster spectra becoming available from 10m
class telescopes in a large variety of galaxies: metal-rich & metal-poor,
starburst, post-burst, and dynamically young. We test our analysis tool using
observations from various authors for Galactic and M31 GCs, for which reliable
age and metallicity determinations are available in the literature, and discuss
in how far the observational availability of various subsets of Lick indices
affects the results. For M31 GCs, we discuss the influence of non-solar
abundance ratios on our results.
