Differential surface brightness for (from top to bottom) the (V, J), (V, H) and (V, Ks), photometric pairs versus spectral type. The (V, J) and (V, Ks) curves have been shifted by ± 0.2 for clarity. The entries were binned, with an interval of 2.5 subspectral type. The dotted lines represent the best fit with polynomials of degree 6. Note that the spectral metric is not smooth (see text).