Top left and center left: radiative energy and radiative flux as a function of x for the vertical diffusion test. Empty circles show the cell-centered values. Crosses show the values of radiative flux from the Riemann solver and radiative energy reconstructed from the Riemann fluxes of energy and flux. The black line shows the analytical solution. Top right and center right: difference between our simulation and the analytical solution. Bottom: L1 relative norm for three different reduced speed of light ĉ/c = 10⁻², 10⁻³, and 10⁻⁴ as blue, red, and yellow lines, respectively.