The shape of an absorption line depends on the number of photons that are absorbed at a particular wavelength. In order to compare the strengths of different absorption lines from a source, or the same absorption line from several different sources, we can use the equivalent width.

To obtain the equivalent width, first we measure the area, A, of the spectral line below the continuum intensity level, as shown in the diagram below:

The area, A, of a spectral line measured below the continuum level is related to a rectangular line profile with the same area, and equivalent width, b.

We then replace the spectral line profile by a rectangle with the same area such that

where I is the intensity level of the continuum and b is the equivalent width of the absorption line.