= Same-upper-level extinction diagnostic
{title2=$j_{ji}/j_{jk}=A_{ji}\nu_{ji}/(A_{jk}\nu_{jk})$}
Two optically thin lines from the same upper level have an intrinsic energy-emissivity ratio fixed by their radiative branching rates and <photon> energies. The upper population cancels, so a common electron-temperature or abundance change cannot alter that ratio. Comparing an optical branch with a weakly attenuated infrared branch can determine differential dust extinction, subject to matched emitting <volumes> and calibrated emissivities.
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