This chapter explains a preliminary consideration of the statistics of a thermal electromagnetic field in a one-photon approximation. It outlines of the macroscopic method of calculating thermal radiation by means of the fluctuation-dissipation theorem. The chapter examines the connection between field moments in the far zone of a radiator and moments of the creation and annihilation operators. It is usually postulated in quantum optics that the observable is the probability of excitation or ionization of one of the detector's atoms. With the help of perturbation theory, this probability can, in turn, be expressed in terms of the normally ordered second moments of the free field incident on the detector and averaged in accord with its geometry and frequency characteristics. It should be noted that if spatial filters are placed before the photomultiplier tube, then the counting probability will also depend on moments nondiagonal by direction.