Note 1: In long-distance transmission systems, the coherencetime may be reduced by propagation factors such as dispersion, scattering, and diffraction.
Note 2: In optical communications, coherencetime, τ, is calculated by dividing the coherence length by the phase velocity of light in a medium; approximately given by τ = λ
The temporal coherence of a wave is related to the spectral bandwidth of the source.
Spatial coherence is high for spherical waves and plane waves as it is related to the size and coherence of the light source.
A non-monochromatic point source of zero diameter emits spatially coherentlight, while the light from a collection of incoherent point-sources which are resolved by the imaging system would have lower coherence (as the point sources are not coherent with each other, the resulting light has low spatial coherence).
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