Emissivity (j_line) of the fine-structure lines in the optical range (solid blue line: [O III] λ5007; solid green: [N II] λ6584) is a strong function of the electron temperature T_e in the 2000–30 000 K range, in contrast to the IR lines (solid pink: [Ne II]12.8 μm; dot-dashed orange: [Ne III]15.6 μm; dashed blue: [O III]88 μm; dashed green: [N II]122 μm). The emissivities are computed using PYNEB (Luridiana et al. 2015) for a fixed n_e = 100 cm⁻³. For comparison, the emissivity of the Hα recombination line is also shown (dashed black line).