Example of the correction made for each individual spectrum. Here one of the most affected spectra for the galaxy NGC 3923 (not sky-subtracted) is shown. The top black spectrum represent the original spectrum at one spatial position. The middle blue spectrum shows the correction spectrum derived with our method. The bottom red spectrum shows the corrected spectrum. The black dashed vertical lines indicate the central bandpasses of the line-strength indices Hγ_A, Hβ, Mg b, and Fe5270.