The spectral linewidth characterises the width of a peak in the spectrum of a system, e.g. of an optical spectrum or of the frequency spectrum in an acoustic or electronic system. For example, the emission of an atom usually has a very thin spectral linewidth, as only transitions between discrete energy levels are allowed, leading to emission of photons with a certain energy.
Several definitions are used to quantify the spectral linewidth, e.g. the Full width at half maximum (FWHM).
Whereas the spectral width of a resonator in electronics depends on the parameters of the used components and therefore can be adjusted according to the wishes of the engineer, things are different in physics. Even a resting atom which does not interact with its environment has a non-zero linewidth, called the natural linewidth, which is a consequence of the transform limit (classical description) and the Heisenberg uncertainty principle (quantum mechanical description). This linewidth is a fundamental property of atomic emission and cannot be adjusted arbitrarily (see Homogeneity (physics)).
