  We have established a model to systematically estimate the contribution of
the mid-infrared emission features between 3 and 11.6 micron to the IRAS
in-band fluxes, using the results of ISO PHT-S observation of 16 galaxies by Lu
et al. (1997). The model is used to estimate more properly the k-corrections
for calculating the restframe 12 and 25 micron fluxes and luminosities of IRAS
galaxies. We have studied the 12-25 micron color-luminosity relation for a
sample of galaxies selected at 25 microns. The color is found to correlate well
with the 25 micron luminosity, the far-infrared and the blue luminosities. The
relations with the mid-infrared luminosities are more sensitive to different
populations of galaxies, while a single relation of the 12-25 micron color vs.
the ratio of the far-infrared and the blue luminosities applies equally well to
these different populations. The luminous and ultraluminous infrared galaxies
have redder 12-25 micron colors than those of the quasars. These relations
provide powerful tools to differentiate different populations of galaxies. We
have also selected a sample of 668 galaxies from the IRAS Faint Source Survey
flux-density limited at 200 mJy at 12 microns. A 12 micron local luminosity
function is derived and, for the first time in the literature, effects of
density variation in the local universe are considered and corrected in the
calculation of the 12 micron luminosity function. It is also found that the 12
micron-selected sample are dominated by quasars and active galaxies, which
therefore strongly affect the 12 micron luminosity function at high
luminosities. The ultraluminous infrared galaxies are relatively rare at 12
micron comparing with a 25 micron sample.
