Geometry for the plane-parallel atmosphere. In the reference system Σ we consider a fixed point P located at height z corresponding to line optical depth t_L. The point P′ lies on the plane Γ parallel to the x–yplane; its height is z′ corresponding to line optical depth . The cylindrical coordinates (r,α) define the position of P′ in the plane Γ. The angles θ and χ specify the direction Ω.