Heliograph

A heliograph is a solar telegraph system that uses flashes of sunlight as morse code. The sunlight is reflected by a mirror. The name comes from the Greek ‘hḗlios’ meaning ‘sun’ and 'gráphein' meaning ‘to write’.

The heliograph was a simple and effective form of communication over long distances in sunny environments and was used by the British Army, until the 1960s. It is however quite heavy and difficult to carry.