Relation between visual surface brightness S_V as a function of the color index (V − K)₀. The black, light blue, green, brown, and red measurements are from Di Benedetto (2005), Boyajian et al. (2012), Hanbury Brown et al. (1974a), Maestro et al. (2013), and VEGA (this work), respectively. The red line corresponds to our fit when considering all stars. The rms of the difference between the surface brightness computed from our fit and measured surface brightness is presented in the lower panels (see the text for more detail).