Emission coefficient ($j_{\nu }$)

The emission coefficient $j_{\nu }$ measures emitted energy per unit volume, time, solid angle and frequency. Its SI units are $\mathrm{W}{\mathrm{m}}^{-3}\mathrm{s}{\mathrm{r}}^{-1}\mathrm{H}{\mathrm{z}}^{-1}$. Along a ray it contributes $j_{\nu }ds$ to specific intensity. For an isotropic spontaneous emission transition with a normalized spectral line profile, $j_{\nu }=h{\nu }_{21}{n}_2{A}_{21}\phi (\nu )/(4\pi )$; integration over solid angle removes the $4\pi$ denominator.