Interstellar extinction is the attenuation of light by absorption and scattering from dust between a source and observer. Its wavelength dependence commonly reddens a source as well as dimming it.
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.
The observed colour minus the inferred intrinsic colour in specified photometric passbands; for B and V it is .
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