Abstract: Some embodiments described herein relate to a reader having a distributed source of radiation and a photodetector. The photodetector can be operable to sense radiation (e.g., light) emitted by an implanted sensor. The distributed source of radiation can at least partially surrounds the photodetector. The distributed source of radiation generates a photon cloud of excitation radiation within the skin, which can substantially envelopes a sensor that is implanted within the skin at a depth that is on the order of a centimeter or less.