Schematic of the geometry of a light ray crossing the atmosphere. The inner circle is the arbitrary reference surface of the planet of radius R_p. The lighter-grey region is the atmosphere. The distance from the centre of the planet is r = R_p + z. A light ray is defined by its distance of closest approach to the centre of the planet, ρ, and the corresponding tangential altitude z_t = ρ − R_p. The direction of the light ray defines the x direction. As we further discuss the extent of the limb along the ray, we introduce x_limb so that the absorption outside the [−x_limb, x_limb] segment is negligible in determining the transit radius, and the corresponding limb opening angle ψ.