Note on the Optical Equivalence Theorem

Abstract

The optical equivalence theorem is examined for the particular class of the electromagnetic fields. It is shown that the distribution function can be taken as a non-negative function, if the density operator describing the states of the electromagnetic field is diagonal in the occupation number representation. Hence the equivalence is not only formal but also essential for this class of the electromagnetic fields, so long as the description of the optical coherence theory is concerned. As a typical example the “diagonal” representation of the density operator for blackbody radiation is studied.