Advantages of ‘circular polarimetry’ (determination of the Stokes parameter V) in active regions as compared to ‘elliptical polarimetry’ (determination of all Stokes parameters) are discussed:

1. (1) Complicated and doubtful reduction procedures by means of line formation theory generally are needed for ‘elliptical polarimetry’. This is not the case for ‘circular polarimetry’.

2. (2) Even if the difficulties (1) could be removed, ‘elliptical polarimetry’ would not help to determine the total magnetic vector outside the disc center. This is due to the fact that the azimuth φ of the linear polarization is only defined for 0° ⩽ φ < 180°. When converting the measured inclination Ψ to the line-of-sight into the true inclination γ to the solar surface normal for active regions outside the disc center, the ambiguity of φ always yields an ambiguity of γ. The normal field component and thus the flux can therefore not be determined without making an important assumption on the unknown field structure (see Wiehr, 1970a).

3. (3) For active regions near the disc center, however, where the difficulty (2) disappears, ‘circular polarimetry’ is sufficient for the determination of the normal field component and flux. It furthermore avoids the difficulty (1).