Space-time diagram of the radiation energy front evolving in an optically thin slab of gas. In the optically thin limit, the FLD flux limiter λ prohibits radiation from moving faster than the speed of light. In this plot, the speed of light has been indicated by the black dotted lines. In addition to the colormap, the position of the radiation front has been evaluated at three arbitrarily values with the red contour lines. The total optical thickness of the slab is 0.002.