An Etymological Dictionary of Astronomy and Astrophysics

Of an → electromagnetic wave, the area of a surface perpendicular to the direction of → propagation, over which the wave maintains a specified → degree of coherence. According to the van Cittert-Zernike theorem, the coherence area is given by: A_c = D²λ²/(πd²),

where d is the diameter of the light source and D is the distance away.

The coherence area is an important parameter in photon correlation experiments. In the → Young’s experiment the → interference pattern is only seen if slits are inside one coherence area.

See also: → coherence; → area.