For many decades, theory has predicted that Bose-Einstein statistics are a fundamental feature of thermal emission into one or a few optical modes; however, the resulting Bose-Einstein-like photon noise has never been experimentally observed. There are at least two reasons for this: 1) relationships to describe the thermal radiation noise for an arbitrary mode structure have yet to be set forth, and 2) the mode and detector constraints necessary for the detection of such light is extremely hard

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... to fulfill. Herein, photon statistics and radiation noise relationships are developed for systems with any number of modes and couplings to an observing space. The results are shown to reproduce existing special cases of thermal emission and are then applied to resonator systems to discuss physically realizable conditions under which Bose-Einstein-like thermal statistics might be observed. Examples include a single isolated cavity and an emitter cavity coupled to a small detector space. Low mode-number noise theory shows major deviations from solely Bose-Einstein or Poisson treatments and has particular significance because of recent advances in perfect absorption and subwavelength structures both in the long-wave infrared and terahertz regimes. These microresonator devices tend to utilize a small volume with few modes, a regime where the current theory of thermal emission fluctuations and background noise, which was developed decades ago for free space or single-mode cavities, has no derived solutions.