  We present ground-based mid-infrared imaging for 27 M-, S- and C-type
Asymptotic Giant Branch (AGB) stars. The data are compared with those of the
database available thanks to the IRAS, ISO, MSX and 2MASS catalogues. Our goal
is to establish relations between the IR colors, the effective temperature
$T_{eff}$, the luminosity $L$ and the mass loss rate $\dot M$, for improving
the effectiveness of AGB modelling. Bolometric (absolute) magnitudes are
obtained through distance compilations, and by applying previously-derived
bolometric corrections; the variability is also studied, using data accumulated
since the IRAS epoch. The main results are: i) Values of $L$ and $\dot M$ for C
stars fit relations previously established by us, with Miras being on average
more evolved and mass losing than Semiregulars. ii) Moderate IR excesses (as
compared to evolutionary tracks) are found for S and M stars in our sample:
they are confirmed to originate from the dusty circumstellar environment. iii)
A larger reddening characterizes C-rich Miras and post-AGBs. In this case, part
of the excess is due to AGB models overestimating $T_{eff}$ for C-stars, as a
consequence of the lack of suitable molecular opacities. This has a large
effect on the colors of C-rich sources and sometimes disentangling the
photospheric and circumstellar contributions is difficult; better model
atmospheres should be used in stellar evolutionary codes for C stars. iv) The
presence of a long-term variability at mid-IR wavelengths seems to be limited
to sources with maximum emission in the 8 -- 20 $\mu$m region, usually Mira
variables (1/3 of our sample). Most Semiregular and post-AGB stars studied here
remained remarkably constant in mid-IR over the last twenty years.
