Equivalent width

A diagram indicating the equivalent width corresponding to the absorption line, which is shown in red.

The equivalent width of a spectral absorption line is the width of the rectangle which has the same area as that between the intensity profile of the absorption line as a function of wavelength, and the continuum level, with the height of the rectangle being the magnitude of the continuum emission. It is a measure of the strength of spectral features that is primarily used in astronomy. The intensity profile of the absorption line depends upon column density, . Column density is defined as the number of atoms per unit area along the line of sight. The curve of growth describes the dependence of the equivalent width, which is an effective measure of the strength of a feature in an emission or absorption spectrum, on the column density. The shape of this profile is initially Gaussian but moves towards a Lorentzian profile as the column density increases.

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