Comparison of δI computed from the first-order perturbation theory to the difference of direct computation of emergent intensity in the initial and perturbed model atmospheres I₀(μ₀)−I(μ). Intensities are calculated from the centre to the limb for the radial mode l = 0, m = 0, n = 20. The red line on the panel in the left corner shows the positions on the solar disk in the observer frame where intensity perturbations are computed and which corresponds to i = 90°, ϕ = 0 and 0 ≤ θ < 90°. The intensity perturbation is measured in [erg cm⁻² s⁻¹ cm⁻¹ sr⁻¹].