IRAS data opened completely new possibilities to address the question of
bolometric luminosity functions in star forming regions. The IRAS
sensitivity limits the studies to objects with L_bol > 0.1 L_sun. With
ISO it is possible to reach much lower flux levels. Therefore ISO gives
an access to stellar objects of the lowest masses and to brown dwarves.
While it is most difficult to resolve the true nature of an individual
very low mass object, statistical means can be used to examine the
physical nature of a group of low mass stars. We want to use bolometric
luminosity function as such a diagnostic tool. We have specifically
selected regions claimed to form the lowest mass stars in the solar
neighbourhood: Lupus 3 and globules in the Taurus filaments. ISO mapping
of these regions at 7, 16 and 60 microns is essential in recovering the
bolometric luminosities of the low mass objects. In addition, our three
wavelength survey will discover more luminous members which are too
embedded to be seen with ground-based facilities. The deduced bolometric
luminosity functions can be directly compared with existing theoretical
models. This way we can address some of the most basic questions of
astrophysics: time scales involved in star formation and initial mass
function. It is most important to dedicate survey type time to the
closest star forming regions. There is only a limited number of star
forming clouds within 150 pc of the Sun. If we want to understand
formation of the very lowest mass objects, these are the regions to be
observed with ISO.