Normalized average intensity in the images for the 25 line levels and in the continuum as functions of 1 − μ in the northern hemisphere and of − 1 + μ in the southern hemisphere. The full curve shows the fifth-order polynomial fit of the 630 nm-continuum center-to-limb variation proposed in Neckel (2005). Upper panel: 2013 data in the selected INR images; lower panel: 2007 data in the selected INR images.