Example of measured Stokes profiles, normalized to the maximum intensity, of the He I D₃ line observed on the prominence. The zero of the wavelength scale is at 5875.97 Å. A significant positive Stokes Q signal can be clearly observed. Weak U and V signals are also detected. However, the significance of the latter strongly depends on the accuracy of the cross-talk correction. The reference direction for positive Stokes Q was chosen to be parallel to the solar limb. Error bars represent the statistical photon noise uncertainty.