Spectral characteristics of bulk extrinsic photo-conductive detectors have been measured under conditions that only partially illuminated the photo-sensitive areas. The data presented were obtained at background photon flux levels of 10" to 10" photons/sec-cm'. The geometry of the detectors was such that the applied electric field was perpendicular to the incident radiation. Substantial changes in spectral responsivity have have been measured. These changes are dependent upon which portion of the detector's sensitive area is illuminated.