Observed (symbols) and fitted (curves) Stokes I, Q, U, and V profiles for a sample MBP marked in Fig. 1. Red (squares and solid line): non-reconstructed, spatially smoothed IMaX data (see main text); fitted with the SPINOR inversion code. Blue (triangles and dot-dashed line): non-reconstructed data; fitted with the SPINOR code. Green (asterisks and dashed line): phase-diversity reconstructed data; fitted with the SPINOR code. Black (triple-dot-dashed line): fit to the phase-diversity reconstructed data returned by the SIR inversion code. The vertical dotted lines represent the IMaX filter wavelength positions. All profiles are normalised to the IMaX Stokes I continuum.