1 Introduction

Grids of theoretical stellar spectra providing models at low and high metallicities are indispensable for modelling the chemical evolution of the integrated light of stellar systems from theoretical isochrones used to describe the time-dependent stellar population. However, even the combined uses of available modern stellar libraries in evolutionary synthesis studies (e.g. Buzzoni 1989; Worthey 1994; Bressan et al. 1994) have been handicapped by intrinsic inhomogeneities and incompleteness. Furthermore, one of the most serious difficulties arises from the fact that the synthetic spectra and colors provided by most theoretical libraries still show large systematic discrepancies with calibrations based on spectroscopic and photometric observations. This is particularly true at low effective temperatures for which an accurate modelling of the stellar spectra requires important molecular opacity data which are not yet completely available. Ultimately, these limitations lead unavoidably to serious uncertainties in the interpretation of the population model.

In order to overcome these major shortcomings, we have undertaken the construction of a comprehensive combined library of realistic stellar flux distributions intended for population and evolutionary synthesis studies. A preliminary version of such a standard grid was presented in Lejeune et al. (1997, Paper I, referred to hereafter as LCB97), along with an algorithm developed for correction and calibration of the (theoretical) spectra. In this previous grid, M dwarf models, which are important for the determination of mass-to-light ratios in stellar populations, were missing. We present here a more comprehensive library which incorporates these dwarf spectra. The construction of this basic combined library is presented in the following section. The empirical $T_{\mathrm{eff}}$-color relations in UBVRIJHKL photometry, required to calibrate the spectra, are presented in Sect. 3. Section 4 is dedicated to the correction of the theoretical spectra; in particular, we also examine the properties and the limitations of the method used when applied to the M dwarf models. Finally, we summarize the main properties of this new standard library in view of its synthetic photometry applications.