  We have combined up-to-date stellar population synthesis models, a simple
radiative transfer approach, and a fully comprehensive dust model with the aim
of developing a simple but quantitative way of interpreting the mid-infrared
spectra of galaxies. We apply these models to the observed correlations of
mid-infrared luminosities (at 8 and 24 micron) with the star-formation rate of
normal galaxies and find that the observations are naturally reproduced by our
models. We further find that the observed 24 micron correlation places a weak
constraint on relative distribution of dust and stars.
