Heliographs use sunlight to send optical messages over long distances, with origins dating back to ancient times. The device consists of a shiny object that can be rotated or closed to create flashes of light that convey information in Morse code. While limited to daytime use and susceptible to interception, heliographs are advantageous in their ability to communicate when other methods fail and not being limited by equipment functionality. However, caution must be exercised when using heliography to avoid eye damage and temporary blindness.
A heliograph is a device which is used to send optical messages with the help of sunlight. The use of heliography in one form or another dates back to the ancient world, where numerous cultures used reflected sunlight to send messages or signals over considerable distances. As late as the 1970s, some militaries issued heliographs so people could maintain lines of communication if other methods of communication failed.
Sunlight reflected from a shiny object can carry a considerable distance, especially if someone is looking for it. Numerous human populations throughout history noticed it and used it to their advantage. This principle was exploited 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 can be rotated or closed to create a series of flashes of light that 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 installed in a relay, with messages transmitted over long distances via a series of heliographs.
There are a number of disadvantages to the heliograph as a communication device. The first is that it is limited to daytime use only in lines of sight and, in the event of cloud cover, can interfere with the operation of the device. The broadcast can also be intercepted by anyone watching, which may prompt people to use the code. This, in turn, requires the codes to be maintained and updated so that enemies cannot crack the code and read intercepted messages.
However, the heliograph also has some advantages. It cannot be blocked, allowing people to communicate when jamming systems are operational. It is also not limited by the power supply and functionality of the equipment; even if a heliograph is damaged, as long as the reflective part is intact, you will be able to send messages with it.
Students learning Morse code sometimes enjoy playing with heliography as a means of communication. It is important to be careful when practicing or demonstrating heliography, because reflected light can damage the eyes if directed at someone. It can also be dazzling momentarily, which is a big deal for drivers, pilots, and other people who operate motor vehicles. When using a heliograph, people should ensure that they know at all times where the light is directed and that the device should be covered or closed when not in use.
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