For an optically thin plasma composed of a single, neutral atomic species in local thermal equilibrium (LTE), the density of photons emitted by a transition from level i to level j is
where :
*  is the emission rate density of photons (in m sr s)
*  is the number of neutral atoms in the plasma (in m)
*  is the transition probability between level i and level j (in s)
*  is the degeneracy of the upper level i (2J+1)
*  is the partition function (unitless)
*  is the energy level of the upper level i (in eV)
*  is the Boltzmann constant (in eV/K)
*  is the temperature (in K)
*  is the line profile such that 
*  is the wavelength (in nm)
The partition function  is the statistical occupation fraction of every level  of the atomic species :