Convergence history of the I Stokes element for a conservative Rayleigh atmosphere over a black surface with cos(SPA) = cos(OPA) = 1 and two different optical thicknesses. Thin and thick curves represent the solutions obtained with the pre-conditioned and classical sampling schemes, respectively. Abrupt changes in the solution with the classical sampling scheme for optical thickness of 4 occur, but they are not discernible on the scale of the graph.