TIA's Glossary of Telecommunication Terms
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degree of coherence
A dimensionless unit, expressed as a ratio, used to indicate the extent of coherence of an electromagnetic wave such as a lightwave. (188) [After 2196] Note 1: For lightwaves, the magnitude of the degree of coherence is equal to the visibility, V , of the fringes of a two-beam interference test, as given by 
where I max is the intensity at a maximum of the interference pattern, and I min is the intensity at a minimum. Note 2: Light is considered to be highly coherent when the degree of coherence exceeds 0.88, partially coherent for values less than 0.88 but more than nearly zero values, and incoherent for nearly zero and zero values.