Planetary Nebulae (PNe) in external galaxies are mostly regarded
either as tracers of the gravitational potential as indicators for the
distance of their galactic hosts, with the latter advantage owing to
the almost universal -- though not fully understood -- shape of the
PNe luminosity function (PNLF, generally in the [OIII]5007 emission).

Yet extra-galactic PNe can also be used as probes of their parent
stellar population and understanding in particular the origin of the
PNLF is a puzzle that, once solved, promises to reveal new clues on
the late stages of stellar evolution and on the formation of PNe
themselves.

I will show how integral-field spectroscopy allows to detect PNe in
the optical regions of galaxies to flux levels otherwise inaccessible
to standard narrow-band photometry. This allows to trace the PNLF up
to a larger completeness limit and in the same galactic regions where
the properties of stellar populations can be well characterised, thus
making it possible to understand how the shape of PNLF relates to the
parent stellar population of PNe.

I will present results based on the detailed analysis of SAURON data
for M32 and M31, showing for the first time that the PNLF is in fact
not universal, and the future prospect of this field with next
generation integral-field units such as MUSE on VLT.