Intensity profile of the K I resonance line at disk center. Top: comparison of the K I 7699 line profile at disk center between the SST/CRISP data (black dots) and the IAG Atlas convolved with the instrumental profile (red dashed line). The original profile from the IAG Atlas is also plotted with a black solid line. The grey-shaded region highlights the selected wavelength range for Wλ computation. A telluric spectrum (Hinkle et al. 2000) is overplotted as a grey line in the top panel, to show that there is negligible contamination in the W_λ window. Bottom: difference between the convolved IAG Atlas and the SST data in percentage.