In this paper we establish an expansion of complex degree of coherence by using the properties that we can obtain from its definition. In the third section we utilize this expansion for studying the problem of object reconstruction; we conclude that, if the optical system fulfils some conditions, it is possible to construct, in an unique way, the modulus of object transmittance from intensity data at the image and to characterize, in an unique way, the behaviour of object transmittance with respect to every new source or optical system.