In the gasdischarge light sources the radiator is the plasma - ionized quasineutral gas. The plasma appears if the gas temperature is high enough so between its particles there were a lot of such particles, whose energy of movement is approximately equal to the energy of ionization and chemical connection. In the similar case as a result of collision there are often processes of dissociation and ionization. As a result free electrical charges, equal quantity of positive and negative (ions and electrons) appear in gas among neutral atoms and molecules and it provides its electroconductivity.

Joul warmth educed by going electric current keeps the high gas temperature, which is necessary for excitation of plasma pluorescence in such sources as arc and spark. As the temperature is rising the gas ionization level is rising and besides single ionized atoms also twice and thrice (three times) ionized atoms appear. So, the real plasma is an ensemble which consists of heterogeneous particles - molecules, atoms, ions and electrons.

Besides processes requiring energy supply there are such processes in plasma as decontamination of excited states, recombination of electrons, ions, and atoms. These processes can occur as a result of treble strikings and strikings of the second type and also as a result of energy output as quanta of radiation. It explains the plasma radiation in spectral lines and in an uninterrupted spectrum. Relatively as a plasma limit one can take a condition of gas with such ionization level by which the collisions of charged particles (Kulon's interactions) have greater role than the collisions of neutral particles.

The requirement of quasineutrality puts the limitation on geometrical sizes of plasma: they must be much bigger than the average distance between the particles. From the macroscopic point of view the plasma is more or less inhomogeneous in its volume. The kind of its radiation is connected with excistence of bigger quantity of elementary processes, defining the occupying of energy conditions of molecules by atoms, ions. Radiation appearing in the single volume of plasma can be absorbed in the same volume or in the neighbour zones. Because of the variety of processes occurring in the spatial-inhomogeneous plasma, the analytic description of the real plasma is rather difficult in the common case. That's why simplified plasma models are usually considered and one mentions then as far the real plasma can be similar the accepted model.

The condition of the real plasma which is by the arbitrary pressure is defined by:

a) the concentration of the particles of all the kinds N (number of particles in the volume unit);

b) their functions of distribution on speeds N_i(n);

c). c) the occupying of excited levels N_k (number of particles in the volume unit, excited in the condition k);

d) spatial distribution of these values.

It's very difficult to get information of all the mentioned descriptions because the theoretical research of plasma condition requires working out and solution of equations system, connecting given values with external (outward) conditions.