  We recall and generalize the analysis of the output of the so-called balanced
homodyne detectors. The most important feature of these detectors is their
ability to quantify the vacuum fluctuations of the electric field, that is
expectation values of products of (quantum-) electric-field operators. More
precisely, the output of BHDs provides information on the one- and two-point
functions of arbitrary states of quantum fields. We generalize the analysis of
the response of BHDs to the case of quantum fields under influence of static
external conditions such as cavities or polarizable media. By recalling the
expressions for two-point functions of quantum fields in Casimir geometries we
show, that a rich, position- and frequency-dependent pattern of BHD responses
is predicted for ground states. This points to a potentially new
characterization of quantum fields in Casimir setups which would not only
complement the current global methods (Casimir forces), but also improve
understanding of sub-vacuum energy densities present in some regions in these
geometries.
