  We experimentally verify uncertainty relations for mixed states in the
tomographic representation by measuring the radiation field tomograms, i.e.
homodyne distributions. Thermal states of single-mode radiation field are
discussed in details as paradigm of mixed quantum state. By considering the
connection between generalised uncertainty relations and optical tomograms is
seen that the purity of the states can be retrieved by statistical analysis of
the homodyne data. The purity parameter assumes a relevant role in quantum
information where the effective fidelities of protocols depend critically on
the purity of the information carrier states. In this contest the homodyne
detector becomes an easy to handle purity-meter for the state on-line with a
running quantum information protocol.
