  Although stellar SED models are constantly improving and seem to show similar
differential tendencies as measured spectra, their absolute properties (eg.
absolute flux in different wavelength regions) can still differ significantly
from the measured ones. Therefore, it is necessary to "color-calibrate" the
theoretical spectra to obtain realistic model-based stellar spectra, which can
then be used for many purposes, such as population synthesis models. In order
to calibrate a hybrid library (consisting of the Kurucz -, the Allard and
Hauschild -, and the Scholz-models) of theoretical SEDs, UBVRIJHKLM photometric
data of solar neighborhood stars were collected from the literature, and the
solar metallicity models were calibrated using an algorithm developed by
Cuisinier et. al. The corrections were then propagated through the whole grid
of models, resulting in the "semi-empirical" models (available on ftp from the
university of Basel). In order to extend the calibration to non-solar
abundances, we have collected UBVRIJHKL photometric data from well-known
Galactic globular clusters, covering a wide range of metallicities. We shall
present first results, using the metallicity-calibrated library for calculating
isochrones of metal-poor clusters. In particular, the observed slope and/or
curvature of the RGB appears to be reproducible in a systematically correct
manner for the first time.
