  We study both systematic and statistical errors in radiation density matrix
measurements. First we estimate the minimum number of scanning phases needed to
reduce systematic errors below a fixed threshold. Then, we calculate the
statistical errors, intrinsic in the procedure that gives the density matrix.
We present a detailed study of such errors versus the detectors quantum
efficiency $\eta$ and the matrix indexes in the number representation, for
different radiation states. For unit quantum efficiency, and for both coherent
and squeezed states, the statistical errors of the diagonal matrix elements
saturate for large n. On the contrary, off-diagonal errors increase with the
distance from the diagonal. For non unit quantum efficiency the statistical
errors along the diagonal do not saturate, and increase dramatically versus
both $1-\eta$ and the matrix indexes.
