Editing Optical communication (section)

== Visual forms ==
Visual techniques such as [[smoke signals]], [[Beacon (signal fire)|beacon fires]], [[hydraulic telegraph]]s, [[ship flags]] and [[semaphore line]]s were the earliest forms of optical communication.<ref name="Burns2004">[https://books.google.com/books?id=7eUUy8-VvwoC&pg=PA29 Chapter 2: Semaphore Signalling] {{ISBN|978-0-86341-327-8}} Communications: an international history of the formative years R. W. Burns, 2004</ref><ref name="EncyBrit1824">[https://books.google.com/books?id=MsYnAAAAMAAJ&pg=PA645 Telegraph] Vol 10, Encyclopædia Britannica, 6th Edition, 1824 pp. 645-651</ref><ref>{{cite web |url=http://www.nps.gov/fire/utility/uti_tl_perspectivestext.cfm |title=Nation Park Service Fire History Timeline}}</ref><ref>{{cite web |url=http://lewisandclarkjournals.unl.edu/read/?_xmlsrc=1805-07-20.xml&_xslsrc=LCstyles.xsl |title=Lewis and Clark Journals, July 20, 1805}}</ref> Hydraulic telegraph semaphores date back to the 4th century BCE Greece. [[Distress flare]]s are still used by mariners in emergencies, while [[lighthouse]]s and [[navigation light]]s are used to communicate navigation hazards.

The [[heliograph]] uses a [[mirror]] to [[Reflection (physics)|reflect]] sunlight to a distant observer.<ref name="Boer">Harris, J.D. [http://rapidttp.com/milhist/vol111jh.html 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.</ref> 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. [[Navy|Naval]] [[ship]]s often use [[signal lamp]]s and Morse code in a similar way.

[[Aviator|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, [[Air traffic control#Airport control|airport control towers]] still use [[Aldis lamp]]s to transmit instructions to aircraft whose radios have failed.

=== Semaphore line ===
{{Main|Optical telegraph}}
[[File:OptischerTelegraf.jpg|thumb|left|upright|A replica of a [[Chappe telegraph]] tower (18th century)]]
A 'semaphore telegraph', also called a 'semaphore line', 'optical telegraph', 'shutter telegraph chain', '[[Chappe telegraph]]', or 'Napoleonic semaphore', is a system used for conveying information by means of visual signals, using towers with pivoting arms or shutters, also known as blades or paddles. Information is encoded by the position of the mechanical elements; it is read when the shutter is in a fixed position.<ref name="EncyBrit1824"/><ref name="EdEnc1832">[https://books.google.com/books?id=VhEbAQAAMAAJ&pg=PA657 Telegraph], Volume 17 of The Edinburgh encyclopaedia, pp. 664–667, 1832 David Brewster, ed.</ref>

Semaphore lines were a precursor of the [[electrical telegraph]]. They were far faster than [[post rider]]s for conveying a message over long distances, but far more expensive and less private than the electrical telegraph lines which would later replace them. The maximum distance that a pair of semaphore telegraph stations can bridge is limited by geography, weather and the availability of light; thus, in practical use, most optical telegraphs used lines of relay stations to bridge longer distances. Each relay station would also require its complement of skilled operator-observers to convey messages back and forth across the line.

The modern design of semaphores was first foreseen by the British [[polymath]] [[Robert Hooke]], who first gave a vivid and comprehensive outline of visual telegraphy in a 1684 submission to the [[Royal Society]]. His proposal (which was motivated by military concerns following the [[Battle of Vienna]] the preceding year) was not put into practice during his lifetime.<ref>Calvert, J.B. [https://web.archive.org/web/20120208081932/http://mysite.du.edu/~jcalvert/railway/semaphor/semhist.htm The Origin of the Railway Semaphore<!-- Bot generated title -->], [[Boston University]], 15 April 2000, Revised 4 May 2007.</ref><ref>McVeigh, Daniel P. [http://www.ilt.columbia.edu/projects/bluetelephone/html/part2.html An Early History of the Telephone: 1664-1865, Part 2<!-- Bot generated title -->] {{Webarchive|url=https://web.archive.org/web/20121128120005/http://www.ilt.columbia.edu/projects/bluetelephone/html/part2.html |date=2012-11-28 }}, [[Columbia University|Columbia University in The City of New York]], Institute For Learning Technologies, 2000.</ref>

The first operational optical semaphore line arrived in 1792, created by the French engineer [[Claude Chappe]] and his brothers, who succeeded in covering [[France]] with a network of 556 stations stretching a total distance of {{convert|4,800|km}}. It was used for military and national communications until the 1850s.

Many national services adopted signaling systems different from the Chappe system. For example, [[UK|Britain]] and [[Sweden]] adopted systems of shuttered panels (in contradiction to the Chappe brothers' contention that angled rods are more visible). In [[Spain]], the engineer [[Agustín de Betancourt]] developed his own system which was adopted by that state. This system was considered by many experts in Europe better than Chappe's, even in France.{{cn|date=August 2023}}

These systems were popular in the late 18th to early 19th century but could not compete with the electrical telegraph, and went completely out of service by 1880.<ref name="Burns2004"/>

=== Semaphore signal flags ===
{{Main|Flag semaphore}}
[[File:020118-N-6520M-011 Semaphore Flags.jpg|thumb|A naval signaler transmitting a message by flag semaphore (2002).]]
Semaphore flags are the system for conveying information at a distance by means of visual signals with hand-held flags, rods, disks, paddles, or occasionally bare or gloved hands. Information is encoded by the position of the flags, objects or arms; it is read when they are in a fixed position.

Semaphores were adopted and widely used (with hand-held flags replacing the mechanical arms of [[semaphore line|shutter semaphores]]) in the maritime world in the 19th century. They are still used during [[Underway replenishment|underway replenishment at sea]] and are acceptable for emergency communication in daylight or, using lighted wands instead of flags, at night.

The newer flag semaphore system uses two short poles with square flags, which a signaler holds in different positions to convey letters of the alphabet and numbers. The transmitter holds one pole in each hand, and extends each arm in one of eight possible directions. Except for in the rest position, the flags cannot overlap. The flags are colored differently based on whether the signals are sent by sea or by land. At sea, the flags are colored red and yellow (the [[International maritime signal flags#Letters|Oscar flags]]), while on land, they are white and blue (the [[International maritime signal flags#Letters|Papa flags]]). Flags are not required, they just make the characters more obvious.

=== Signal lamps ===
{{Main|Signal lamp|Aviation light signals}}
[[File:TC with light gun.JPG|thumb|left|An [[air traffic controller]] holding a signal light gun that can be used to direct aircraft experiencing a radio failure (2007).]]
Signal lamps (such as Aldis lamps), are visual signaling devices for optical communication (typically using Morse code). Modern signal lamps are a focused lamp which can produce a pulse of light. In large versions this pulse is achieved by opening and closing shutters mounted in front of the lamp, either via a manually operated pressure switch or, in later versions, automatically.

With hand held lamps, a [[Curved mirror#Concave mirrors|concave mirror]] is tilted by a trigger to focus the light into pulses. The lamps are usually equipped with some form of optical sight, and are most commonly deployed on naval vessels and also used in airport control towers with coded [[aviation light signals]].

[[Aviation light signals]] are used in the case of a [[NORDO|radio failure]], an [[aircraft]] not equipped with a radio, or in the case of a hearing-impaired pilot. [[Air traffic controller]]s have long used signal light guns to direct such aircraft. The light gun's lamp has a focused bright beam capable of emitting three different colors: red, white and green. These colors may be flashing or steady, and provide different instructions to aircraft in flight or on the ground (for example, "cleared to land" or "cleared for takeoff"). Pilots can acknowledge the instructions by wiggling their plane's wings, moving their [[aileron]]s if they are on the ground, or by flashing their [[Aircraft landing lights|landing]] or [[navigation light]]s during night time. Only 12 simple standardized instructions are directed at aircraft using signal light guns as the system is not utilized with Morse code.

=== Heliograph ===
[[Image:Australian Heliograph in Egyptian Desert 1940.png|thumb|right|upright|Heliograph: Australians using a heliograph in North Africa (1940).]]
{{Main|Heliograph}}
A heliograph ({{Langx|el|Ἥλιος}} ''[[helios]]'', meaning "sun", and {{lang|el|γραφειν}} ''[[wikt:-graphy|graphein]]'', meaning "write") is a wireless solar [[telegraph]] that signals by flashes of [[sunlight]] (generally using Morse code) reflected by a [[mirror]]. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam with a shutter.

The heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th century. Its main uses were in military, surveys and forest protection work. They were standard issue in the British and Australian armies until the 1960s, and were used by the Pakistani army as late as 1975.<ref name="Boer"/>