Illustration of the transmission of stellar radiation (left-hand side) through an exoplanet atmosphere (transit) towards an observer (right-hand side). R₀ is the reference radius at which the atmosphere becomes fully opaque. A light ray at altitude z propagates along the line-of-sight x. The atmosphere is separated in N_L layers for size Δ_z, which are labelled by the index l = j + k, where j refers to the z-component and k to the x-component. The discretized altitude z_l corresponds to the altitude at the lower boundary of the layer l.