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A heliograph is a device which is used to send optical messages with the assistance of sunlight. The use of heliography of some form or another dates back to the Ancient World, where numerous cultures used reflected sunlight to send messages or signals across considerable distances. As late as the 1970s, heliographs were issued by some militaries so that people could maintain lines of communication if other communication methods broke down.
Sunlight reflected from a shiny object can carry a considerable distance, especially if someone is looking for it. Numerous human populations throughout history noted this and utilized it to their advantage. This principle was harnessed in an organized way in the 1800s, when several inventors developed heliographs such as the Mance heliograph, used in the Middle East in the 1870s and 1880s.
The device consists of a shiny object mounted on a stand. To use it, the object may be pivoted or shuttered to create a series of bursts of light which can be read by an observer. These transmissions were commonly done in Morse code, which uses a series of long and short transmissions to convey information. Heliographs could also be set up in a relay, with messages being passed across long distances via a series of heliographs.
There are a number of disadvantages to the heliograph as a communications device. The first is that it is limited to daytime use in lines of sight only, and if there is cloud cover, it can interfere with the operation of the device. The transmission can also be intercepted by anyone who happens to be viewing, which may require people to use code. This, in turn, requires maintenance and updating of codes so that enemies cannot crack the code and read intercepted messages.
However, the heliograph also has some advantages. It cannot be jammed, allowing people to communicate when jamming systems are operational. It is also not limited by power supply and functionality of equipment; even if a heliograph is damaged, as long as the reflective part is intact, it will be possible to send messages with it.
Students learning Morse code sometimes enjoy playing with heliography as a means of communication. It's important to be careful when practicing or demonstrating heliography, because the reflected light can damage the eyes if it is directed at someone. It can also be momentarily dazzling, which is a major issue for drivers, pilots, and other people operating motor vehicles. When using a heliograph, people should take care to make sure that they know where the light is being directed at all times, and the device should be covered or shuttered when not in use.