Parallel development of night-vision systems occurred in the US. The M1 and M3 infrared night sighting devices, also known as the "sniperscope" or "snooperscope", saw limited service with the US Army in World War II, and were also used in the Korean War, to assist snipers. These were active devices, using a large infrared light source to illuminate targets. Their image intensifier tubes used an anode and an S-1 photocathode, made primarily of silver, cesium, and oxygen, and electrostatic inversion with electron acceleration was used to achieve gain.