S/N, as defined in this work, plotted against the usual definition of S/N, based on Gaussian fits to the spectra. The spectra from the Heiles cloud 2 map are used. Each panel shows the scatter plot for a different value of the width of the velocity window, W. The parameter σ is the standard deviation of the spectrum averaged over the whole map. The value used to compute the SCF in this work is ﹩W=10\,\sigma ﹩.