In this paper we present the V and I luminosity functions and color-magnitude diagrams derived from wide-field ( 23 \arcmin \times 23 \arcmin ) BVI photometry of the intermediate metallicity ( [ Fe/H ] \sim - 1.3 ) Galactic globular cluster M12 .
Using observed values ( and ranges of values ) for the cluster metallicity , reddening , distance modulus , and age we compare these data to recent \alpha -enhanced stellar evolution models for low mass metal-poor stars .
We describe several methods of making comparisons between theoretical and observed luminosity functions in order to isolate the evolutionary timescale information the luminosity functions contain .
We find no significant evidence of excesses of stars on the red giant branch , although the morphology of the subgiant branch in the observed luminosity function does not match theoretical predictions in a satisfactory way .
Current uncertainties in T _ { \mbox { eff } } -color transformations ( and possibly also in other physics inputs to the models ) make more detailed conclusions about the subgiant branch morphology impossible .
Given the recent constraints on cluster ages from the WMAP experiment ( Spergel et al .
2003 ) , we find that good fitting models that do not include He diffusion ( both color-magnitude diagrams and luminosity functions ) are too old ( by \sim 1 - 2 Gyr ) to adequately represent the cluster luminosity function .
The inclusion of helium diffusion in the models provides an age reduction ( compared to non-diffusive models ) that is consistent with the age of the universe being 13.7 \pm 0.2 Gyr ( Bennett et al .
2003 ) .