of sunlight for communication purposes is attributed to ancient Greeks and Romans who used polished shields to send signals by reflecting sunlight during battles. In 1810, Carl Friedrich Gauss invented the heliograph which uses a pair of mirrors to direct a controlled beam of sunlight to a distant station. Although the original heliograph was designed for the geodetic survey, it was used extensively for military purposes during the late 19th and early 20th century. In 1880, Alexander Graham Bell invented the photophone, the world’s first wireless telephone system. Military interest in photophones continued after Bell's time. For example, in 1935, the German Army developed a photophone where a tungsten filament lamp with an IR transmitting filter was used as a light source. Also, American and German military laboratories continued the development of high-pressure arc lamps for optical communication until the 1950s. Modern OWC uses either lasers or light-emitting diodes (LEDs) as transmitters. In 1962, MIT Lincoln Labs built an experimental OWC link using a light-emitting GaAs diode and was able to transmit TV signals over a distance of 30 miles. After the invention of the laser, OWC was envisioned to be the main deployment area for lasers and many trials were conducted using different types of lasers and modulation schemes. However, the results were in general disappointing due to the large divergence of laser beams and the inability to cope with atmospheric effects. With the development of low-loss fiber optics in the 1970s, they became the obvious choice for long distance optical transmission and shifted the focus away from OWC systems. Current status Over the decades, interest in OWC was mainly limited to covert military applications, and space applications including inter-satellite and deep-space links. OWC’s mass market penetration has been so far limited with the exception of IrDA which is a highly successful wireless short-range transmission solution. Applications Variations of OWC can be potentially employed in a diverse range of communication applications ranging from optical interconnects within integrated circuits through outdoor inter-building links to satellite communications. OWC can be divided into five categories based on the transmission range: Ultra-short range: chip-to-chip communications in stacked and closely packed multi-chip packages. Short range: wireless body area network (WBAN) and wireless personal area network (WPAN) applications under standard IEEE 802.15.7, underwater communications. Medium range: indoor IR and visible light communications (VLC) for wireless local area networks (WLANs) and inter-vehicular and vehicle-to-infrastructure communications. Long