The ORG measures rainfall by detecting the optical irregularities induced within the sample volume by precipitating particles falling through a beam of partially coherent infrared light. These irregularities are known as scintillation. By detecting the intensity of the scintillation, the actual rainfall rate can be measured.
This particular rain gauge is an optical independent analog unit consisting of a frame that contains 1) the transmit head, 2) a receive head and electronics, and 3) a 15-meter-long power/signal cable. The transmit head contains an infrared emitting diode and lens with heater. The other head contains a photodiode, lens and aperture, heater, electronics, and cable connectors. All wiring between the heads runs within the frame.