Blue profiles: observations of the Fe I 15648.52 Å (air wavelength) line in the quiet sun (courtesy of Dr. J.M. Borrero; see Kiess et al. 2018). Red profiles: NLTE synthesis obtained with our model assuming a two-level atom for the transition , with A_uℓ = 1.94 × 10⁻⁶, effective Landé factor , negligible damping constant a = 0, T = 8000 K, and . The magnetic field and the LOS are vertical. For obtaining profile 1 (left panel): B_LOS = 500 G, T_ν0 = 1.0, v^LOS = −6.3 km s⁻¹, , , r_c = 12 (ratio continuum to line opacity). For the profile 2 (right panel): B_LOS = 1100 G, T_ν0 = 0.35, v^LOS = −3.2 km s⁻¹, S_cont/S_line = 1.37, , r_c = 18. The I₀ was obtained by using the shape of the observed I as seed profile and scaling it with and to reproduce simultaneously the observed intensity and Stokes V profiles within a tolerance of 5% of the continuum intensity.