Test of the direct computation of the perturbed intensity (I_λ − I_λ⁰)/I_λ⁰ (dots) for μ = 0.5 and comparison with first-order computations δI_λ/I_λ⁰ (solid line). The thermodynamic, geometrical, and line contributions are represented separately in blue, green, and red, while the full intensity is in black. The wavelength offset is with respect to the center of the HMI line λ_HMI = 6173.33 Å.