Theoretical integrated intensity emitted in Lα line versus emission measure (EM = n_e²D_eff) for the range of observed Lα intensities for selected temperatures, marked by colors. The solid lines are linear least-square fits to model data. For a given observed E_Lα at a temperature of 150 kK, the dashed lines indicate the uncertainty range in EM determination. The horizontal blue line shows the resulting uncertainty in the derived EM at the brightest point.