Conclusion

We reviewed the basic concepts of the second-order spatial coherence functions described in the space-time domain and the space-frequency domain. While the spatial coherence function in the space-time domain such as the mutual coherence function describes the spatial correlation of the quasi-monochromatic wavefields, the coherence function in the space-frequency domain such as the cross-spectral density plays an important role in describing the spatial correlation of the wavefields with the wide spectral bandwidth. The introduction of the correlation-induced spectral changes was also reviewed. The condition for the spectral invariance was derived.