  We provide the whole set of Lick indices from CN1 to TiO2 of Simple Stellar
Population models with, for the first time, variable element abundance ratios,
[alpha/Fe]=0.0, 0.3, 0.5, [alpha/Ca]=-0.1, 0.0, 0.2, 0.5, and [alpha/N]=-0.5,
0.0. The models cover ages between 1 and 15 Gyr, metallicities between 1/200
and 3.5 solar. Our models are free from the intrinsic alpha/Fe bias that was
imposed by the Milky Way template stars up to now, hence they reflect
well-defined alpha/Fe ratios at all metallicities. The models are calibrated
with Milky Way globular clusters for which metallicities and alpha/Fe ratios
are known from independent spectroscopy of individual stars. The metallicities
that we derive from the Lick indices Mgb and Fe5270 are in excellent agreement
with the metallicity scale by Zinn & West (1984), and we show that the latter
provides total metallicity rather than iron abundance. We can reproduce the
relatively strong CN-absorption features CN1 and CN2 of galactic globular
clusters with models in which nitrogen is enhanced by a factor three. An
enhancement of carbon, instead, would lead to serious inconsistencies with the
indices Mg1 and C24668. The calcium sensitive index Ca4227 of globular clusters
is well matched by our models with [Ca/Fe]= 0.3, including the metal-rich Bulge
clusters NGC 6528 and NGC 6553. From our alpha/Fe enhanced models we infer that
the index [MgFe] defined by Gonzalez (1993) is quite independent of alpha/Fe,
but still slightly decreases with increasing alpha/Fe. We define a slight
modification of this index that is completely independent of alpha/Fe and
serves best as a tracer of total metallicity. Searching for blue indices that
give similar information as Mgb and Fe, we find that CN1 and Fe4383 may be best
suited to estimate alpha/Fe ratios of objects at redshifts z~1. (Abridged)
