  The complementarity relation between the visibility and the spectral
distinguishability of frequency-displaced photonic wave-packets in a
Hong-Ou-Mandel interferometer is studied. An experimental definition of $K$,
the distinguishability parameter, is proposed and tested for the
$K^2+\mathcal{V}^2\leq 1$ complementarity inequality when a consistent
visibility parameter is defined. The results show that the spectral
distinguishability is, indeed, complementary to the visibility and that the
quantum aspect of the two-photon interference phenomenon can be examined by
employing weak-coherent states.
