  Detector requirements for far infrared astronomy generally result in devices
which exhibit a few-moded response to incident radiation. The sensitivity and
spatial form of the individual modes to which such a detector is sensitive can
be determined with knowledge of the complex valued cross-spectral density of
the system, which we label the detector response function (DRF). A matrix
representing the discretized cross-spectral density can be measured from the
complex amplitudes of interference fringes generated by two identical sources
as they are independently scanned through the field of view. We provide
experimental verification of this technique using monochromatic THz beams
generated by photomixers in which the relative phase is varied with fiber
stretchers. We use this system to characterize the modal response of a single
pixel from an array of microwave kinetic inductance detectors (MKIDs).
