Since then there have been numerous research activities focussed on VLC. More recent work began in 2003 at Nakagawa Laboratory, in Keio University, Japan, using LEDs to transmit data by visible light.
This pre-dates the transmission of speech by radio. when the Scottish-born scientist Alexander Graham Bell invented the photophone, which transmitted speech on modulated sunlight over several hundred meters. The history of visible light communications (VLC) dates back to the 1880s in Washington, D.C. 2.3.2 Variable Pulse Position Modulation.2.2 Multi-Carrier Modulated Transmission.2.1 Single-Carrier Modulated Transmission.VLC can be used as a communications medium for ubiquitous computing, because light-producing devices (such as indoor/outdoor lamps, TVs, traffic signs, commercial displays and car headlights/taillights ) are used everywhere. Such a sensor may provide either multi-channel (down to 1 pixel = 1 channel) or a spatial awareness of multiple light sources. The image sensor used in these devices is in fact an array of photodiodes (pixels) and in some applications its use may be preferred over a single photodiode. Specially designed electronic devices generally containing a photodiode receive signals from light sources, although in some cases a cell phone camera or a digital camera will be sufficient. Systems such as RONJA can transmit at full Ethernet speed (10 Mbit/s) over distances of 1–2 kilometres (0.6–1.2 mi). The technology uses fluorescent lamps (ordinary lamps, not special communications devices) to transmit signals at 10 kbit/s, or LEDs for up to 500 Mbit/s over short distances. VLC is a subset of optical wireless communications technologies. Visible light communication ( VLC) is a data communications variant which uses visible light between 400 and 800 THz (780–375 nm). Visible light is only a small portion of the electromagnetic spectrum.
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