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Optical communication, also known as optical telecommunication, is communication at a distance using light to carry information. It can be performed visually or by using electronic devices. The earliest basic forms of optical communication date back several millennia, while the earliest electrical device created to do so was the photophone, invented in 1880. An optical communication system uses a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal. When electronic equipment is not employed the 'receiver' is a person visually observing and interpreting a signal, which may be either simple (such as the presence of a beacon fire) or complex (such as lights using color codes or flashed in a Morse code sequence). Free-space optical communication has been deployed in space, while terrestrial forms are naturally limited by geography, weather and the availability of light. This article provides a basic introduction to different forms of optical communication. == Forms == Visual techniques such as smoke signals, beacon fires, hydraulic telegraphs, ship flags and semaphore lines were the earliest forms of optical communication.〔(Chapter 2: Semaphore Signalling ) ISBN 978-0-86341-327-8 Communications: an international history of the formative years R. W. Burns, 2004〕〔(Telegraph ) Vol 10, Encyclopaedia Britannica, 6th Edition, 1824 pp. 645-651〕 Hydraulic telegraph semaphores date back to the 4th century BCE Greece. Distress flares are still used by mariners in emergencies, while lighthouses and navigation lights are used to communicate navigation hazards. The heliograph uses a mirror to reflect sunlight to a distant observer.〔Harris, J.D. (Wire At War - Signals communication in the South African War 1899–1902 ). Retrieved on 1 June 2008. Note a discussion on the heliograph use during the Boer War.〕 When a signaler tilts the mirror to reflect sunlight, the distant observer sees flashes of light that can be used to transmit a prearranged signaling code. Naval ships often use signal lamps and Morse code in a similar way. Aircraft pilots often use visual approach slope indicator (VASI) projected light systems to land safely, especially at night. Military aircraft landing on an aircraft carrier use a similar system to land correctly on a carrier deck. The coloured light system communicates the aircraft's height relative to a standard landing glideslope. As well, airport control towers still use Aldis lamps to transmit instructions to aircraft whose radios have failed. In the present day a variety of electronic systems optically transmit and receive information carried by pulses of light. Fiber-optic communication cables are now employed to send the great majority of the electronic data and long distance telephone calls that are not conveyed by either radio, terrestrial microwave or satellite. Free-space optical communications are also used every day in various applications. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Optical communication」の詳細全文を読む スポンサード リンク
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