Illustration of how the activity number method transforms the intensity values of the spectral lines of other atlases. Top panels: the transformed spectral lines after being convolved and interpolated. The two vertical lines indicate the wavelength region used to calculate the activity number. Bottom panels: the remainder of the ratio differences, as described in Section 3.2. For HRS23, IAG16 and KPW11 atlases, the wavelengths were converted from vacuum to air using the formula from Birch & Downs (1994). (a) Hα, (b) Na D1.