The temperature describing the state of optically thin plasma in the conditions of the local thermodynamic equilibrium can be defined if one measures the intensity of some line of radiated by plasma spectral lines l in absolute units on the formula:

Where the values N₀, g_k, g₀, g_i, A_ki, h, n_ki, E_k are taken respectively for transitions in an atom, ion or molecule. But the exact measuring of absolute intensities of spectral lines is connected with great experimental difficulties. And in many cases only relative values of probabilities of transitions are known. The spectrascopic finding out the temperature on measurings of relative intensities of spectral lines is assumed as a basis of the method of measuring of plasma temperature (Method was worked out by L.S. Ornshtein.). It gives the opportunity to avoid the measuring of absolute values and doesn't require the knowledge the absolute values of concentrations of atoms and ions. The main point of the method is the following. 2 special lines are considered which appear by transition between excited levels ki and l m of one sort of particles (atoms, ions of the given division etc.). If the occupyings of all these levels answer the distribution of Bolzman with the same temperature of excitation Т_е, then having put down the expression (11) for both lines and also combining them we'll get the ratio in the form

Where l_ki and l_lm - waves lengths of chosen spectral lines. Here it is taken into consideration that the main state for the given particles of the same sort is the same. Having measured the relative intensities of 2 lines by using the formula (12) the electronic temperature can be calculated.