The planetary nebula luminosity function (PNLF) is a model consisting of a (non-normalized) probability density function of the luminosity of planetary nebulae. With it, planetary nebulae are used as standard candles, using the fact that there is a (rough) maximum brightness of planetary nebulae, which can be used in the manner of distance-determination of the brightest star of a galaxy, the globular cluster luminosity function, or a brightest cluster galaxy in a galaxy cluster.
Typically, observation is carried out for an [O III] forbidden line (wavelength 500.7 nm, which is produced by planetary nebulae and can be observed from the ground). If it can be established as from a planetary nebula (i.e., other possible sources eliminated), by observing a number of them in a galaxy, the distance to the galaxy can be estimated based upon the brightest of the group. The function is typically stated in a way that relates a distance modulus to the apparent magnitude of this line from the galaxy's brightest planetary nebulae.
Planetary nebulae occur throughout galaxies, including easy-to-view portions, which makes them better for this than some other such standard candle candidates within galaxies.