Extending the concept of Wehrl's entropy to s-parametrized quasiprobab
ility distributions, as they are observed in optical homodyne detectio
n with realistic detectors, we show that the marginal entropies with r
espect to phase and amplitude obey a rigourous inequality relation. Re
lating those entropies to phase and intensity uncertainties, respectiv
ely, we arrive at an uncertainty relation for operationally defined ph
ase and intensity that takes explicit account of the noise introduced
by non-ideal detectors.