Probability density function f(θ,φ)=f_θ(θ)f_{φ | θ}(φ | θ) in the pre-condioned sampling scheme of photon propagation directions, Eqs. (23)–(24), for a Rayleigh-scattering medium. Note the changes in f(θ,φ) with q, especially near the maximum of | b₁(θ) /a₁(θ) | for θ = 90°. By ignoring polarisation, the classical sampling scheme determines the θ and φ values of the incident propagation direction from f(θ,φ;q ≡ 0). It is apparent that the classical sampling scheme is more likely to fail in strongly polarising media that involve high q values during the backtracing of photons.