[Fe II]5.34 μm/[Fe II]4.89 μm ratio as a function of the electron density at 5000 K (solid black line), 10000 K (dotted red line), and 20000 K (dashed green line). The [Fe II]17.94 μm /[Fe II]4.89 μm ratio is fixed and equal to 10.9 since both transitions have the same upper level (3d⁷ a₄ F_7/2). Therefore, both transitions can be used interchangeably to determine the electron density together with the [Fe II]5.34 μm line. The line emissivities are calculated using PyNeb (Luridiana et al. 2015) and the Fe⁺ atomic parameters from Smyth et al. (2019).