Stokes I continuum intensity of the Hinode/SP map of a sunspot umbra of NOAA AR 10933. The original data are plotted in the top panel. The Stokes I continuum resulting from the 2D inversion is shown in the bottom panel. The intensity is normalized to the mean quiet-Sun intensity I_QS. The outer contour line in the bottom panel indicates the edge of the umbra as retrieved from the magnetic field inclination map (see main text), the inner contour line separates central from peripheral umbral dots (UDs). Four typical UDs are marked by circles and letters.