[He] Line Ratio

In optical plasma diagnostics, line intensity ratio method is one of method to determine electron temperature and density of plasma. The intensity of radiation, I_ki, is expressed by

E_ki : Difference between energy levels E_k and E_i (E_k>E_i)
A_ki : Transition probability
N_k : Population of upper energy state of k.

Line intensity ratio of plasma is expressed by

: Wavelength corresponding to the transition from energy state k to energy state i.

In experiment, the optical system cannot collect whole radiation from plasma volume, the transmittance of optical system has a dependency on wavelength, and quantum efficiency of detecting system is also dependent on a wavelength. Thus, the measured line intensity ratio needs to be relatively calibrated for whole optical system and detector.