  The determination of the quantum properties of a single mode radiation field
by heterodyne or double homodyne detection is studied. The realistic case of
not fully efficient photodetectors is considered. It is shown that a large
amount of quite {\em precise} information is avalaible whereas the completeness
of such information is also discussed. Some examples are given and the special
case of states expressed as a finite superposition of number states is
considered in some detail.
