Single-wavelength calibration curves for different wavelengths. Panel a: calibration relation of the B_l and Stokes V/I for the Mg I 12.32 μm line in the FALC model with a magnetic field strength 0–400 G. Panel b: same as panel a, but the calibration relationship between B_t and Stokes (Q/I, U/I). Different colors are for different wavelength positions toward the line center (λ₀ = 12319.246 nm). The selected wavelength positions are the same for B_l and B_t.