The propagation distance from a coherent source to a point where an electromagnetic wave maintains a specified degree of coherence. Note 1: In long-distance transmission systems, the coherence length may be reduced by propagation factors such as dispersion, scattering, and diffraction. Note 2: In optical communications, the coherence length, L, is given approximately by L = 2/ (n,) where is the central wavelength of the source, n is the refractive index of the medium, and is the spectral width of the source. Note 3: Coherence length is usually applied to the optical regime.