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The brightness of an atomic spectral line emitted by atoms in a gas (or plasma) can be proportional to the gas's temperature, pressure or a weighted sum of both.
Due to the completeness and accuracy of collisional radiative models for helium the temperature and density of plasmas which have helium present can be measured by taking ratios of the emission intensities of various atomic spectral lines.
Latimer, I; Mills, J.I.; Day, R.A. (1970), "Refinements in the helium line ratio technique for electron temperature measurement and its application to the precursor", Journal of Quantitative Spectroscopy and Radiative Transfer10 (6): 629, Bibcode:1970JQSRT..10..629L, doi:10.1016/0022-4073(70)90079-8