Example spectrum at 630 nm from POLIS. Blue-dotted line: average spectrum without correction for intensity gradients over the wavelength range. Black line: corrected spectrum. Red line: FTS atlas spectrum. The dashed vertical lines denote the “continuum” range, the solid vertical lines the range that was considered for determining the line parameters of Fe i 630.15 nm and Fe i 630.25 nm, respectively. The coloured horizontal lines inside the absorption profiles illustrate the method used to determine line properties at different line depression levels.