= Comoving spectral emissivity
{title2=$\epsilon_\nu=dL_\nu/dV_c$}
<Comoving spectral emissivity> is emitted <spectral luminosity> per <comoving volume>, measured using source-rest-frame time and <frequency>. For identical sources with <comoving number density> $n_c$, it is $\epsilon_\nu=n_cL_\nu$. For a population with a <luminosity function>, integrate its rest-frame <spectral luminosity> over that distribution. Multiplying a proper-volume emissivity by $a^3$ gives this comoving convention for dimensionless <scale factor> $a$.
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