Electron temperature

The ratio of the flux of the auroral line [O iii]λ4363 to that of nebular line [O iii]λλ4959,5007 is sensitive to the electron temperature of the ionized gas. Based on estimates of the electron temperature and density, the oxygen abundance can be inferred from ratios of strong oxygen lines (e.g., [O iii]λλ4959,5007 and [O ii]λ3727) to Balmer lines.

However, the direct electron-temperature based method cannot be applied to the majority of local galaxies with spectroscopic observations because [O iii]λ4363 is typically ∼ 100 times weaker than [O iii]λ5007 at low metallicity, and much weaker still at solar and higher abundances.