In this paper, recent advances in the research of coherence properties of electromagnetic radiation are discussed. Using the conception of photon degeneracy and degree of coherence the author is able to explain many problems such as light mixing, intensity fluctuations and the properties of coherent light source. Since the occurrence of radiation process is at random, the intensity of the signal generated by the mixing of different sources can be determined by the correlation of radiation processes as derived in the present paper. Regarding photons as bosons, the phenomena "intensity interferense" is explained as quantum fluctuations. The coherence properties of radiation may thus be interpreted well on the basic conception of quantum machanics.