Sketch of the infinite plane-parallel slab setup. Radiation enters the slab of transverse optical depth τ = τ_max through the top layer. The observer detects radiation that is transmitted through the slab and is located below the bottom layer of the slab. The initial direction of a photon package is described by the penetration angle θ₀. If the photon package scatters, its direction is chosen randomly according to its corresponding PDF and the new direction is described by the angle θ.