Coherence time
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For an electromagnetic wave, coherence time is the time over which a propagating wave may be considered coherent.
Note 1: In long-distance transmission systems, the coherence time may be reduced by propagation factors such as dispersion, scattering, and diffraction.
Note 2: In optical communications, coherence time, τ, is calculated by dividing the coherence length by the phase velocity of light in a medium; approximately given by τ = λ2/(cΔλ) where λ is the central wavelength of the source, Δλ is the spectral width of the source, and c is the velocity of light in vacuum.
Note 3: "Coherence time" is usually applied to the optical regime.
Source: from Federal Standard 1037C and from MIL-STD-188