Spectral line profile
= Spectral line profile
{title2=$\varphi(\nu)$}
A normalized <spectral line profile> distributes a transition's emission or <absorption> over <frequency>, with $\int\varphi(\nu)d\nu=1$ and units $\mathrm{Hz^{-1}}$. 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.