Reconstruction of a representative training spectrum for different truncation levels. Each panel corresponds to a wavelength range and displays the original spectrum (black) together with reconstructions obtained using K = 4, 11, 30, and all principal components (red), vertically offset for clarity. The blue curves show the corresponding residuals (reconstruction original) plotted above each case. The examples illustrate the progressive improvement in the reconstructed line profiles with increasing K. The −selected windows contain strong and weak diagnostic features (H, He, and metals), including the wind–sensitive Hα region.