A measurable counterpart to the optical concept of an object

The traditional optical concept for the object does not provide an experimental feasibility to speak for itself, due to the fact that no measuring instrument catches up with the fluctuation of light fields. Using the theory of coherence, we give it a measurable counterpart, and thus to give a satisfactory explanation of what the concept has said. To our knowledge it is the first time to valuate an object in optics in the terms of observable quantities. As a useful example, the applicability to obtain the full knowledge of an object under the partial coherent illuminations is suggested. This work hit the thought which had been advanced by E. Wolf that one measures the correlation function rather than the optical fields themselves. The suggested applicability is a supplementary to the recently found solution to the determination of phases of the diffracted beam.