Spectral width

In telecommunication, spectral width is the wavelength interval over which the magnitude of all spectral components is equal to or greater than a specified fraction of the magnitude of the component having the maximum value.

Note 1: In optical communications applications, the usual method of specifying spectral width is the full width at half maximum. This method may be difficult to apply when the spectrum has a complex shape. Another method of specifying spectral width is a special case of root-mean-square deviation[?] where the independent variable is wavelength, λ, and f (λ) is a suitable radiometric quantity.

Note 2: The relative spectral width , Δλ/λ, is frequently used where Δλ is obtained according to note 1, and λ is the center wavelength.

Source: from Federal Standard 1037C and from MIL-STD-188