The measurement of plasma quantities is a difficult task since the plasma cannot be treated like normal material. Any measurement of plasma quantities with solid electrostatic probes will yield interactions with the plasma and causes a perturbation of the measured quantity. Inside a hot plasma those methods are not applicable, since they lead to a disruption of the discharge.

An other way of diagnosing a hot plasma is the measurement of the emitted radiation in the infrared and microwave region as well as probing the plasma with infrared and micro waves. The measurement of the reflected wave yields also information on the plasma density from the refractive index. With microwave and far infrared diagnostics plasma properties can be measured quite accurate and reliable. Main plasma parameters as the electron density and the electron temperature can be measured. Even the measurement of fluctuations in density and temperature and the determination of the plasma current density are possible with sophisticated diagnostics.

In the following section the optical properties of the plasma will be developed. Sections III to VI are devoted to different diagnostic techniques.