In physics, **coherence theory** is the study of optical effects arising from partially coherent light and radio sources. Partially coherent sources are sources where the coherence time or coherence length are limited by bandwidth, by thermal noise, or by other effect. Many aspects of modern coherence theory are studied in quantum optics.

The theory of partial coherence was awoken in the 1930s due to work by Pieter Hendrik van Cittert and Frits Zernike.

- Visibility
- Mutual coherence function
- Degree of coherence
- Self coherence function
- Coherence function
- Low frequency fluctuations
- General interference law
- Van Cittert–Zernike theorem
- Michelson stellar interferometer
- Correlation interferometry
- Hanbury–Brown and Twiss effect
- Phase-contrast microscope
- Pseudothermal light
- Englert–Greenberger duality relation
- Coherence Collapse

- Eugene Hecht and Alfred Zajac,
*Optics*, (1974) Addison-Wesley Publishing, Reading, Massachusetts