Illustration of method used in this work to relate the line cord level to the corresponding line residual intensity (in arbitrary units). At each pixel along the spectrograph slit, the widths of the red and blue wings at a given residual intensity (red and blue curves respectively), are added to get the variation of the line cord width as a function of the residual line intensity (green curve). We then consider 25 line cord levels evenly distributed from the line center (level 1) to Δλ_c (see the text for the definition of Δλ_c).