= Equivalent width
{title2=$W$}
{wiki}
An absorption <equivalent width> is $W_\lambda=\int[1-I_\lambda/I_\lambda^0]d\lambda$, the <wavelength> width of a completely black interval that removes the same continuum-normalized area. For a narrow <spectral line profile>, small <optical depth> and negligible <stimulated emission>, the energy-density convention for <Einstein coefficients> gives $W_\lambda=hB_{12}N_1/\nu_{12}$. This follows by replacing $1-e^{-\tau_\nu}$ by $\tau_\nu$ and using $|d\lambda/d\nu|=c/\nu^2$. At <redshift> $z$, the observed and rest-frame values satisfy $W_{\rm obs}=(1+z)W_{\rm rest}$.
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