= Optical coherence
<Optical coherence> describes correlations between the <wave amplitudes> and <wave phases> of <light> sampled at different places or times. Its most direct classical test is <wave interference>: combine two samples of a field and see how much the detected <light intensity> changes when their relative <wave phase> is scanned. <First-order optical coherence> makes this statement quantitative; <temporal optical coherence> compares delayed samples, while <spatial optical coherence> compares different positions. These properties are relative to the samples and averaging procedure, rather than a single yes-or-no label for a source. https://www.rp-photonics.com/coherence.html[Rüdiger Paschotta, RP Photonics Encyclopedia, Coherence: field correlations and the two kinds of coherence]{ref}.
A useful surprise is that two copies of a fluctuating source can interfere strongly when they have matched delays: the copies share the fluctuations. Conversely, two individually steady-looking beams need not have correlated <wave phases>. The ideal <diffraction grating> calculation requires <spatial optical coherence> across its illuminated elements; unequal-arm measurements with a <Michelson interferometer> test <temporal optical coherence>.
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