In physics, coherence length is the propagation distance from a coherent source to a point where an electromagnetic wave maintains a specified degree of coherence . This concept is commonly used in telecommunication engineering.

In long-distance transmission systems, the coherence length may be reduced by propagation factors such as dispersion, scattering, and diffraction.

In optical communications, the coherence length L is given approximately by L = λ²/(n Δλ), where λ is the central wavelength of the source, n is the refractive index of the medium, and Δλ is the spectral width of the source.

Coherence length is usually applied to the optical regime.

Source: from Federal Standard 1037C in support of MIL-STD-188