Histograms showing the relative errors in percentage of the radiation field tensors, , at the height where the optical depth at the line centre is unity along the disc centre line of sight. The different panels show the histograms for different truncation orders of the spherical harmonics expansion for the S₀, S₁, and S₂ quadratures with the following number of rays in total, respectively: (a) 62, 64 and 72 rays, (b) 100, 112 and 120 rays, and (c) 124, 144 and 152 rays in total.