ToThe distance estimate the distance to a galaxy using the PNLF onerequires mustdiscovery firstof locatesuch pointan sourcesobject withinin the target galaxy that areis visible at λ5007 but not when the entire spectrum is considered. These points are candidate PNe, however, there are three other types of objects that would also exhibit such an emission line that must be filtered out: [[HII region|HII regions]], [[supernova remnant]]s, and [[Lyα galaxy|Lyα galaxies]]. After the PNe are determined, to estimate a distance one must measure their monochromatic <nowiki>[O III]</nowiki> λ5007 lumonisity. What remains is a statistical sample of
PNe. The observed luminosity function is then fitted to some standard law.<ref name="Ciardullo2004">{{harvnb|Ciardullo|2004}}</ref>
