Effect of centre-to-limb variation in the radiance histograms of the Mg x 625 Å line. The original radiance map from the USUN mosaic taken on December 15, 2008 is shown in the upper-left panel. The fit to the average centre-to-limb variation at the minimum of solar activity is shown in the upper-right panel as a solid line (i.e.: a = 0.23, b = 0.44); for comparison, the measured variation for that date (dots) is also shown, along with the value of limb brightening at ρ = 0.85 (dashed lines). The map of radiances corrected for that limb brightnening function is shown in the lower-left panel. The lower-right panel shows the radiance histograms formed from all on-disk pixels (grey-shaded histogram), as compared with the histogram from pixels with ρ < 0.85 (green-shaded histogram); the histogram from the radiances corrected for the centre-to-limb variation is shown in red. The widths of the Gaussian fit to the core of the histograms are, respectively: 0.21, 0.19, and 0.17.