The conversion from Luminosity Functions to Mass Functions

The aim of this talk is to review the methodology required to translate the observed luminosity function of a stellar sample into the mass function of the stars constituting the sample. Emphasis is given to the stellar models which necessarily underlie this operation, as the mass function is proportional to the derivative of the mass{luminosity relation, and this latter quantity can not be derived from observations with a suucient precision. On the other hand, when adopting a theoretical mass{luminosity relation, some useful rules must be kept in mind to obtain a physically meaningful result. The methodology is applied to the following speciic elds: 1. derivation of the initial mass function (IMF) of nearby low mass stars and of disk stars; 2. derivation of the IMF for the low masses in globular clusters; 3. derivation of the IMF in young stellar populations. In all cases, the stellar physics which enters into the determination of the mass{luminosity relation, and its uncertainties, is shortly discussed.