PNe. The observed luminosity function is then fitted to some standard law.<ref name="Ciardullo2004">{{harvnb|Ciardullo|2004}}</ref>
Finally, one must estimate the foreground [[interstellar extinction]]. There areThe two sources of thisextinction, are from within the [[Milky Way]] and the internal extinction of the target galaxy. The first is well known and can be taken from sources such as reddening maps computed from [[H I region|H I]] measurements and galaxy counts or from [[IRAS]] and [[Diffuse InfraRed Background Experiment|DIRBE]] satellite experiments. The later,onlytype of extinction, occurs only in target galaxies which are either late type [[spiral galaxy|spiral]] or [[irregular galaxy|irregular]]. However, this extinction is difficult to measure. In the Milky Way, the scale height of PNe is much bigger than that of the dust. Observational data and models support that this holds true for other galaxies, that the bright edge of the PNLF is primarily due to PNe in front of the dust layer. The data and models support a less than 0.05 [[apparent magnitude|magnitude]] internal extinction of a galaxy's PNe.<ref name="Ciardullo2004" />
