Spectral line strength ($S=\int {\kappa }_{\nu }d\nu$)

The integrated absorption coefficient of an isolated spectral line is its line strength. For a mass coefficient write ${\kappa }_{\nu }=S{\varphi }_{\nu }$, where $\int {\varphi }_{\nu }d\nu =1$. 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.