Upper panel: angular distribution of ordinary photons for 0 ≤ μ ≤ 1 and optical depth τ₀ = 20. Solid line: angular distribution for the sum of the first ten eigenfunctions and relative coefficients c_k obtained with the algorithm described by AS08. Dashed line: same function calculated with the first eigenfunction and coefficient from the algorithm of AS08. Dot-dashed line: same function, but defined as in Eq. (29) of L88. Lower panel: angular distribution of the ordinary photons emerging at the top of the slab (0 < μ < 1) for eigenfunctions and relative weights up to k = 10 for the cases of optical depth τ₀ = 5 (solid line), τ₀ = 20 (dashed line), and τ₀ = 40 (dot-dashed line).