= Spectral line strength
{title2=$S=\int\kappa_\nu\,d\nu$}
The integrated <absorption coefficient> of an isolated <spectral line> is its line strength. For a mass coefficient write $\kappa_\nu=S\phi_\nu$, where $\int\phi_\nu d\nu=1$. The units of $S$ include the chosen spectral-coordinate unit; a strength integrated over <wavenumber> is not numerically interchangeable with one integrated over <frequency>. Changing normalized width at fixed strength redistributes opacity without changing its spectral integral.
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