Optically thin atomic line intensity ($I_{ul}=\frac{h{\nu }_{ul}}{4\pi }\int n_u{A}_{ul}ds$)

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.