Upper panel: center-to-limb variation of the Strömgren (b + y)/2 intensity calculated for the quiet Sun, faculae, penumbra, and umbra models (black, red, magenta, and blue curves). Plotted are the ratios to the quiet-Sun intensity at the disk center. Lower panel: quadratic polynomial parameterization of the intensity center-to-limb variation at the Ca II K line and the nearby continuum (solid and dashed orange curves) according to Skumanich et al. (1984). Both intensities are normalized to unity at the disk center.