Example of the spectral subtraction for the Na i D_1,2 and He i D₃ lines of Sz 97. Solid and dotted red lines have the same meaning as in Fig. 7. An offset of 0.5 was applied to both the observed and synthetic spectra for visualization purposes. The hatched green areas in the “difference” spectrum represent the Na i D_1,2 excess emissions, while the He i D₃ residual emission is filled with violet hatching.