Maps of departure coefficients at heights where τ_λ0 = 1, with λ₀ being the rest wavelength of the Fe I 6173 Å line. Top panel: departure coefficients of the lower level (β_low). Middle panel: departure coefficient of the upper level (β_up). Bottom panel: ratio of β_up to β_low. Heights corresponding to τ_λ0 = 1 surface computed from LTE (white line) and NLTE (blue line) are plotted in panel d. In the background is the temperature map of the slice marked by the horizontal yellow line in Fig. 1. The positions of the four sample pixels chosen for detailed analysis are marked by vertical lines in panel d.