S–B curves of the Mg I 12.32 μm (top) and Mg I 12.22 μm (bottom) lines using different calculation methods in the FALC (solid green line, data points are denoted by triangles) and MALTBY (dashed blue line, data points are denoted by diamonds) models for B_l. S1 (left) and S2 (right) represent different wavelength-integrated methods (see Eqs. (1) and (2)). The vertical lines are used to divide the position that deviates from linearity in the FALC model (solid red) and MALTBY model (dashed red).