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Robert Hanbury Brown and Richard Q. Twiss sought angular sizes through correlations of detected light intensities. Their method multiplies detector outputs after detection, whereas a Michelson interferometer combines fields before detection.^[ref]

A monochromatic discharge source illuminated a half-silvered beam splitter and two photodetectors. The averaged current product showed an excess near coincidence. Moving one detector introduced a delay that removed it, supplying a control. Slow response weakened the peak. This photon bunching is compatible with fluctuating classical light intensity.^[ref]

Photon antibunching means suppression of close pairs compared with pairs at some later delay: $g^{(2)}(0)<g^{(2)}(\tau )$ for some $\tau$. If correlations decay to the independent-count reference, $g^{(2)}(0)<1$ is a sufficient signature. It violates the classical intensity bounds in photon bunching. For the specified single-mode number measurement, second-order optical coherence also shows that $g^{(2)}(0)<1$ is equivalent to $\mathrm{Var}N<\mu$. This variance criterion concerns that mode or gate; it must not be applied unchanged to arbitrarily long integrated counting intervals.

Photon bunching is enhanced near-coincident detection relative to an appropriate independent-count reference. It need not require nonclassical light. In a classical intensity model with nonnegative instantaneous light intensity $I$, ideal detection with a conditional Poisson distribution givesA fluctuating intensity therefore bunches counts. For a stationary classical intensity with finite second moment, the Cauchy-Schwarz inequality further gives $⟨I(t)I(t+\tau )⟩\le ⟨I^2⟩$. A zero-delay correlation smaller than one at a later delay cannot be explained by this model.