Searchlight

The searchlight signal – which uses a single lamp with a system of movable lenses to project three colors – first appeared in 1920. Lens improvements in 1930 made signals with a 1-mile range and low power consumption possible, and the searchlight found widespread use. A relay-type mechanism moves colored lenses in front of a white light, not unlike a semaphore. Searchlight signals' use became widespread mostly due to their relatively low maintenance, high visibility, low power-consumption, and after 1932 using a compound lens with a 4-watt 3-volt bulb that worked quite well in territory with battery-powered signaling.