  We examine the emergent fluxes from helium-core white dwarfs following their
evolution from the end of pre-white dwarf stages down to advanced cooling
stages. For this purpose, we include a detailed treatment of the physics of the
atmosphere, particularly an improved representation of the state of the gas by
taking into account non-ideal effects according to the so-called occupation
probability formalism. The present calculations also incorporate hydrogen line
opacity from Lyman, Balmer and Paschen series, pseudo-continuum absorptions and
new updated induced-dipole absorption from H$_2$-H$_2$, H$_2$-He and H-He
pairs. We find that the non-ideal effects and line absorption alter the
appearance of the stellar spectrum and have a significant influence upon the
photometric colours in the UBVRI-JHKL system. This occurs specially for hot
models $T_{\rm eff}\ga 8000$ due to line and pseudo-continuum opacities, and
for cool models $T_{\rm eff}\la 4000$ where the perturbation of atoms and
molecules by neighbour particles affects the chemical equilibrium of the gas.
In the present study, we also include new cooling sequences for helium-core
white dwarfs of very low mass (0.160 and 0.148 M$_\odot$) with metallicity
$Z=0.02$. These computations provide theoretical support to search for and
identify white dwarfs of very low mass, specially useful for recent and future
observational studies of globular cluster, where these objects have began to be
detected.
