The paper uses a general treatment for obtaining the expressions of the scattered field and intensity in the Fraunhofer space, within the range of validity of the quasi-monochromatic approximation. As a result, the relation between the object wave and the object is obtained. It is suggested that the coherence characteristics of the object wave are best described by a volume average of the two-point coherence factor normally used, and an expression of the deterioration of coherence in a random medium is obtained. The paper suggests that the definition of mutual intensity ought to be analysed.