  We have performed evolutionary calculations of very-low-mass stars from 0.08
to 0.8 $\msol$ for different metallicites from [M/H]= -2.0 to -1.0 and we have
tabulated the mechanical, thermal and photometric characteristics of these
models. The calculations include the most recent interior physics and improved
non-grey atmosphere models. The models reproduce the entire main sequences of
the globular clusters observed with the Hubble Space Telescope over the
afore-mentioned range of metallicity. Comparisons are made in the WFPC2 Flight
system including the F555, F606 and F814 filters, and in the standard
Johnson-Cousins system. We examine the effects of different physical
parameters, mixing-length, $\alpha$-enriched elements, helium fraction, as well
as the accuracy of the photometric transformations of the HST data into
standard systems. We derive mass-effective temperature and mass-magnitude
relationships and we compare the results with the ones obtained with different
grey-like approximations. These latter are shown to yield inaccurate relations,
in particular near the hydrogen-burning limit. We derive new hydrogen-burning
minimum masses, and the corresponding absolute magnitudes, for the different
metallicities. We predict color-magnitude diagrams in the infrared NICMOS
filters, to be used for the next generation of the HST observations, providing
mass-magnitudes relationships in these colors down to the brown-dwarf limit. We
show that the expected signature of the stellar to substellar transition in
color-magnitude diagrams is a severe blueshift in the infrared colors, due to
the increasing collision-induced absorption of molecular hydrogen with
increasing density and decreasing temperature.
