We extend the colour calibration of the widely used BaSeL standard stellar library ( Lejeune , Cuisinier , & Buser 1997 , 1998 ) to non-solar metallicities , down to { [ Fe / H ] } \sim - 2.0 dex .
Surprisingly , we find that at the present epoch it is virtually impossible to establish a unique calibration of UBVRIJHKL colours in terms of stellar metallicity { [ Fe / H ] } which is consistent simultaneously with both colour-temperature relations and colour-absolute magnitude diagrams ( CMDs ) based on observed globular cluster photometry data and on published , currently popular standard stellar evolutionary tracks and isochrones . The problem appears to be related to the long-standing incompleteness in our understanding of convection in late-type stellar evolution , but is also due to a serious lack of relevant observational calibration data that would help resolve , or at least further significant progress towards resolving this issue . In view of the most important applications of the BaSeL library , we here propose two different metallicity calibration versions : ( 1 ) the ” WLBC 99 ” library , which consistently matches empirical colour-temperature relations and which , therefore , should make an ideal tool for the study of individual stars ; and ( 2 ) , the ” PADOVA 2000 ” library , which provides isochrones from the Padova 2000 grid ( 41 ) that successfully reproduce Galactic globular-cluster colour-absolute magnitude diagrams and which thus should prove particularly useful for studies of collective phenomena in stellar populations in clusters and galaxies .