Difference between the intrinsic polarization angle of the filament, ψ_fil, and the observed polarization angle, ψ, as a function of Δψ = | ψ_fil − ψ_bg |. The polarized intensity contrast of the filament is indicated close to the corresponding curves. It increases in steps of 0.5 from the top to the bottom curve. We note that for Δψ = 90° and f_d = 1, ψ is not defined and the resulting observed polarization fraction is null.