Optically thin atomic line intensity
= Optically thin atomic line intensity
{title2=$I_{ul}=\frac{h\nu_{ul}}{4\pi}\int n_uA_{ul}ds$}
For isotropic emission without significant absorption, integrating the spontaneous photon-production rate along the sightline gives the displayed energy intensity per solid angle. Photon intensity omits the factor $h\nu$. Summing unresolved transitions is necessary before comparing with an observed blended spectral feature.