The Heliograph.
Updated: 1 May 2003
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The Heliograph was a simple but highly effective instrument for instantaneous optical communication over 50 miles or more in the 19th century. Its major uses were for military and survey work. It was still in serious use at least up to 1935, for example by Glubb Pasha's Arab Legion in Palestine.

The heliograph sent its signals by reflecting sunlight towards the intended recipient with a mirror or mirrors, the beam being keyed on and off with a shutter or a tilting mirror, allowing Morse code to be sent. Heliographs were used by the armies of several countries during the late 1800's; they were highly popular with British forces in India because of the dependable sunlight.

The classic heliograph is the Mance pattern, devised by Sir Henry Mance at Bombay in 1869. See illustration below.

Left: A British Mark V Mance pattern 5-inch heliograph. The operating key is behind the mirror.

Heliograph equipment varied somewhat from country to country. The U.S. Army heliograph had a mirror with a sighting rod--or else two mirrors--mounted on a tripod. A screen or shutter for interrupting the flashes was mounted on another tripod. If the sun was in front of the sender, its rays were reflected directly from a mirror to the receiving station. The sender used the sighting rod to line the flash up with the receiver. If the sun was behind the sender, its rays were reflected from one mirror to another, to send the beam on to the target receiver.

The distance that heliograph signals could be seen depended on the clearness of the sky and the size of the mirrors used. Obviously a clear line of sight was required, and the earth's surface is curved, so the highest convenient points were used. Under ordinary conditions, a flash could be seen 30 miles (48 km) with the naked eye and much farther with a telescope. You might think that reflecting a beam to hit a target 30 miles away would require some precision optical alignment- fortunately this is not so because of the finite size of the sun's disc. The maximum range was considered to be 10 miles for each inch of mirror diameter; mirrors varied from 1.5 inches to 12 inches or more.

Heliographs could be used with moonlight, but at much reduced range.

Speed was 5 to 12 words per minute, depending on the Morse skills of the operators.

Left: The working parts of a Mance pattern heliograph.

The heliograph had some powerful advantages. It allowed long-distance communication with no fixed infrastructure, though it could also be linked to make a fixed network extending over hundreds of miles, as in the fort-to-fort network used in the Geronimo campaign. It was highly portable, required no power source, and was relatively secure in that it was invisible if you were not close to the axis of operation.

On the minus side, the heliograph was obviously vulnerable to bad weather, and the mirrors were fragile.

The directional transmission feature became a problem instead of an advantage when a heliograph was used from a ship under way or swinging at anchor. It was solved in British practice by adding a dispersing lens which broadened the beam out to about 15 degrees in two dimensions, at some loss of range.

USERS OF THE HELIOGRAPH
included:

  • Xenophon's Hellenica contains the phrase "to signal with a shield" which is thought to refer to reflecting sunlight with a polished shield, in 450 BC.
  • Some evidence for use by the Chacoan Indians (circa AD 850) in North-Western New Mexico, using mica mirrors.
  • British forces in India from 1870 onwards. Heliograph training in the British Army was still going on in 1907.
  • Both sides in the Boer War
  • USA Cavalry in the campaign against Geronimo in 1886
  • Glubb Pasha's Arab Legion before WW2 in Palestine
  • Portugese forces. No details known.
  • Afghan forces during the Soviet invasion of 199*.

HELIOGRAPH LINKS.

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