Radial profiles of the [O iii]/Hα ratio maps of the type B PNe NGC 3242 a), b), c) and NGC 6826 d). The radial profiles of NGC 3242 correspond to the inner shell a), the outer shell b), and the north-west blister of the outer shell c). The radial profile of NGC 6826 d) corresponds to the outer shell. The [O iii]/Hα peaks of the outer shell of NGC 3242 are not shown in the profile of the blister of this shell c).