Left panel: linear polarization calculated at the core of the spectral line . Middle panel: transversal magnetic field inferred during the inversion. Right panel: magnetic field bias generated with a noise of σ = 10⁻⁴ I_c. Some solid contours have been drawn in the left panel indicating where the inferred value is above the bias. The rest of values are overestimated. The white dashed contour draws the shape of the filament seen in Hα by CHROTEL.