Spectra (nm) from three sources at different temperatures (K)

The excitation temperature of the emission source has a profound influence on the observed spectrum.

Spectra (nm) from three sources at different temperatures (K)

Low temperature sources have few atoms excited to the highest energy states. So the only strong lines observed are those de-exciting to or close to the ground state, ie resonance or near-resonance lines. Few ionic lines are seen. As the temperature of the source increases, more higher energy states are excited, more ions are created, and the spectrum becomes more and more complex. Resonance atomic lines may no longer be the dominant lines.