Laser action inside matter is possible only for those wavelengths for which this material have fluorescent emission.

Fluorescence line is described by plotting spontaneous emission radiation intensity as a function of frequency (or wavelength), for the specific lasing transition.

Figure 5.1 describes a simplified energy level diagram for Helium Neon laser (other examples of He-Ne laser were shown in examples: 3.1, 4.2, 4.3, 4.4, and will be explained more in section 6.1.1).

Figure 5.1: Energy Level Diagram for Helium Neon laser.

The main transition in the visible spectrum is from level E₅ to level E₂, and the emission is at red wavelength of 632.8 [nm].