Panel a: taken from Fig. 1, the HMI longitudinal magnetogram from 12 June 2014. The yellow contours encompass the areas we identified as active regions. For the entire data set (i.e. all data days), panel b: ⟨B⟩_loc and panel c: the Fe I 6173 Å continuum intensity contrast of the quiet-Sun network (purple) and active region faculae (green) at disc centre (0.99 <  μ ≤ 1.00). The filled circles represent the mean within successive intervals of 40 G. In panel b the error bars denote the corresponding standard deviation, and in panel c they denote the standard error. The curves correspond to the contrast versus B profiles calculated with the empirical model described in Sect. 4.4. Panel d: zoomed inset of the boxed region.