Coherence

Summary

Coherence measures how well a wave field at one point and time is correlated with the same field at another point or a later time. It is described by the mutual coherence function, the correlation of the field values. Its normalised form, the complex degree of coherence, has modulus at most $1$. Coherence decides whether two parts of a wave can show stable interference fringes: the fringe visibility is proportional to the degree of coherence. Temporal coherence concerns delays at one point and is linked to the spectral width. Spatial coherence concerns two points at the same time and is linked to the size of the source.