Phenomenological theory of photodetection

The probability of photon absorption by atoms is found when the atoms are in the thermal equilibrium with thermal radiation inside a closed cavity. We explain the experimental observation of photon bunching in the absence of spatial coherence. The existing theory of photodetection by Glauber and Mandel is shown to be an approximation, which contains internal contradictions. We discuss the discrepancy between theory and experiment in the method of absolute calibration of photon multipliers with the help of entangled pairs of photons, which can be eliminated by the use of obtained expressions for photon counting probabilities.