20 Apr 2013
With the recent upsurge in interest in long range optical communications it is easy to forget that optical data communications over long distances predates radio communications by many years. In those far-off days the data was morse code and the light came from the sun in the form of a heliograph mirror that reflected the sun's rays over long distances in daylight. Keying was usually by tilting the mirror or by keying a grill placed in front of the mirror. See http://www.modulatedlight.org/Modulated_Light_DX/Heliograph.html . Best DX achieved in the 19th century was well over 200km using these methods!