V - band luminosity functions ( LF ) have been obtained for the upper main-sequence ( MS ) , sub-giant branch ( SGB ) and red giant branch ( RGB ) of 18 galactic globular clusters ( GGC ) from HST data .
A comparison with four sets of theoretical models has been performed .
In contrast with what was found in several previous works , a good general agreement has been found between the observed and theoretical LF at any metallicity [ M/H ] < -0.7 .
Possible discrepancies at higher metallicity , in the upper part of the RGB , need to be confirmed with further observational data and by extending all the models to the most metal rich regime .
The SGB shape has been used to set an upper limit to the cluster age , and consequently a lower limit on the cluster distance .
A discussion on the still open problem of the mismatch between the observed and theoretical RGB bump location is also presented .