Elementary particles. Smallest unit of the electromagnetic radiation. Each time an electron passes from a higher to a lower excitation status, one photon is generated. The higher the energy of a photon, the higher the frequency of the electromagnetic radiation. When the electrons pass from the first excited state into the basic state of the atoms or molecules of an excited process gas, photons with frequencies in the range of the visible light are radiated (frequency approx. 800 THz to 400 THz, corresponding to a wave length of approx 400 - 800 nm). This light gives the plasma the colour which is characteristic for the respective process gas.