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The spectral linewidth characterizes the width of a spectral line, such as in the electromagnetic emission spectrum of an atom, or the frequency spectrum of 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).
While the spectral width of a resonator in electronics depends on the parameters of the components, and therefore can be easily adjusted over a wide range, linewidths are typically more difficult to adjust in physics. For example, even a resting atom which does not interact with its environment has a non-zero linewidth, called the natural linewidth (also called the decay width), which is a consequence of the Fourier transform limit (classical description) and the Heisenberg uncertainty principle (quantum mechanical description). According to the uncertainty principle the uncertainty in energy, ΔE, of a transition is inversely proportional to the lifetime, Δt of the excited state:
In practice lines are further broadened by effects such as Doppler broadening.
See also 
- Bandwidth (signal processing), a more generic term for a range of electromagnetic frequencies
- Q factor and linewidths in electronics, acoustics and optics
- Spectral lines in optics. (This article also includes a list of sources of linewidth broadening.)
- Spectral width in telecommunications
- Oscillator linewidth
- Laser linewidth
- MOSFET scaling, discussing transistor length (which is rapidly shrinking)