  In this paper, we discuss how the integrated properties of intermediate-age
single burst population, especially in the near-infrared, behave as a function
of age and metallicity. Our models take into account all stellar evolutionary
phases that affect the evolution of the integrated optical and near-infrared
spectrum of such a population. Particular care was dedicated to the Asymptotic
Giant Brach stars, which can be dominant at near-infrared wavelengths. First we
present new synthetic model that takes into account the relevant physical
processes that control the evolution through the thermally pulsing AGB. In the
isochrone presented in this paper the lifetime and the nature of the AGB stars
are established as consequences of the interplay between the physical processes
that control the AGB star evolution. The contribution of these stars to the
integrated light of the population is thus obtained in a consistent way. We
optimize our models by using a new stellar library that explicitly takes into
account the spectral features thar characterize only AGB stars in comparison to
other cool and luminous stars (abriged)
