Examples of spectra of underionized plasma (upper panel, region 3 in the western shell) and overionized plasma (lower panel, region 34 in the W28 center). In the upper panel, the contribution of the plasma and the stray light are marked with solid cyan lines and dashed red lines, respectively. The lower panel contains spectra of two observations, of which only the yellow and gray data points are significantly affected by the stray light.