Measurement principle of SO/PHI. Panel a: solar spectrum around 617 nm (red; FTS atlas; see Neckel & Labs 1984), tunable filter profile (blue) and prefilter bandpass (yellow); FSR and Δλ_FWHM denote the free spectral range and the full width at half maximum of the Filtergraph, Δλ_OSPF is the full width of the order-sorting prefilter. Panels b–e: Fe I 6173 Å Stokes profiles obtained from one spatial point of an MHD simulation (red) and ideally simulated SO/PHI primary observables (light blue). The blue asterisks denote the expected SO/PHI measurements when tuning the filter pass-band to the dedicated wavelength positions.