Line radiation from the plasma due to bound state transitions of the electrons can yield information about the power losses and impurity concentrations, and also about the velocities of impurities. An excellent secondary reference summarizing the physics is Hutchinson's Principles of Plasma Diagnostics 2nd Edition
Soft X-Ray References
Bolometer References
VUV Spectrometer References
Visible Spectrometer References
Ion Doppler Spectrometer References