equivalent width

Equivalent width. The shaded rectangle has the same area as the shaded area within the line profile. A measure of the strength of a spectral line. When a spectrum is displayed as a graph of light intensity against wavelength, absorption lines show up as bell-shaped dips in an otherwise smooth continuum level (see illustration). The equivalent width is calculated by measuring the area of the dip and dividing by the height of the continuum. The result is the wavelength range that the line would occupy if the amount of absorption were redistributed into a rectangle having the height of the continuum. This equivalent width is measured in units of wavelength, often milliångströms.