= Emission measure
{title2=$\mathrm{EM}=\int n_e^2dl$}
For a hydrogen-dominated fully ionized <plasma>, the line-of-sight <emission measure> is $\mathrm{EM}=\int n_e^2\,dl$. Its units distinguish it from a volume-integrated <emission measure> $\int n_e^2dV$. Thermal free-free <optical depth> is proportional to $\mathrm{EM}\,T^{-3/2}\nu^{-2}\bar g_{\rm ff}$ in the radio limit. Inferring <density> also requires depth, ionization and clumping assumptions.<Essential Radio Astronomy by James J. Condon and Scott M. Ransom>[chapter 4, equations (4.57)–(4.58)]{ref}
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