= Emission coefficient
{title2=$j_\nu$}
The <emission coefficient> $j_\nu$ measures emitted <energy> per unit volume, time, <solid angle> and <frequency>. Its SI units are $\mathrm{W\,m^{-3}\,sr^{-1}\,Hz^{-1}}$. Along a ray it contributes $j_\nu ds$ to <specific intensity>. For an <isotropic> <spontaneous emission> transition with a normalized <spectral line profile>, $j_\nu=h\nu_{21}n_2A_{21}\varphi(\nu)/(4\pi)$; integration over <solid angle> removes the $4\pi$ denominator.
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