 Spectral line profile

A normalized spectral line profile distributes a transition's emission or absorption over frequency, with $\int \phi (\nu )d\nu =1$ and units $\mathrm{H}{\mathrm{z}}^{-1}$. Thermal motion, finite excited-state lifetime and collisions can give different contributions to the shape. Normalization lets an optically thin integrated line measure the relevant column density without requiring the detailed profile.