Variation in the minimum continuum intensity in the umbra as a function of maximum magnetic field strength. The 1.1 μm data were multiplied by a factor 1.3. Red and black symbols mark the 1.1 and 1.6 μm data, respectively. P-incomplete and U-incomplete mark sunspots whose penumbra and umbra were not fully covered in the map. The solid line shows a least-squared fit, and the dashed lines indicate the 1σ interval. Numbers in parenthesis denote the 1σ uncertainty.