  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.
