Visible Light Communications

What is Visible Light Communication?

There is now a lot of talk about Visible Light Communication (VLC) and indeed this blog site is dedicated to the topic, but what is VLC?

On this site when we talk about VLC we tend to be referring to an illumination source (e.g. a light bulb) which in addition to illumination can send information using the same light signal. So in our terms:

VLC = Illumination + Communication

Imagine a flash light which you might use to send a morse code signal. When operated manually this is sending data using the light signal, but because it is flashing off and on it cannot be considered to be a useful illumination source, so it is not really VLC by our definition. Now imagine that the flash light is switched on and off extremely quickly via a computer, then we cannot see the data and the flash light appears to emitting a constant light, so now we have illumination and communication and this does fits our definition of VLC. Of course we would need a receiver capable of receiving the information but that is not too difficult to achieve.

Visible Light Communication

In literal terms any form of information that can be sent using a light signal that is visible to humans could be considered to be VLC, but by our definition we should be able to see the light, but cannot “see” the data. So although there seems to be no universally agreed definition of VLC is, we can at least agree what we mean by VLC.

The opportunity to send data usefully in this manner has largely arisen because of the widespread use of LED light bulbs. LEDs are semiconductor devices similar to silicon chips. Consequently we can switch these bulbs at very high speeds that were not possible with older light bulb technologies such as fluorescent and incandescent lamps. The rapid adoption of LED light bulbs has created a massive opportunity for VLC. The problem of congestion of the radio spectrum utilised by Wi-Fi and cellular radio systems is also helping to create the market for VLC.

There are other terms used in the VLC space which are quite widely used but have slightly different meaning to VLC. Three terms closely associated with VLC are:

Free space optical (FSO) communication is similar to VLC but is not constrained to visible light, so ultraviolet (UV) and infrared (IR) also fall into the FSO category. Additionally, there is no illumination requirement for FSO and so this tends to be used in narrow beams of focussed light for applications such as communication links between buildings. FSO often uses laser diodes rather than LEDs for the transmission.

Li-Fi is a term often used to describe high speed VLC in application scenarios where Wi-Fi might also be used. The term Li-Fi is similar to Wi-Fi with the exception that light rather than radio is used for transmission.  Li-Fi might be considered as complementary to Wi-Fi. If a user device is placed within a Li-Fi hot spot (i.e. under a Li-Fi light bulb), it might be handed over from the Wi-Fi system to the Li-Fi system and there could be a boost in performance.

Optical Wireless communication (OWC) is a general term which  refers to all types of optical communications where cables (optical fibres) are not used. VLC, FSO, Li-Fi and infra-red remote controls are all examples of OWC.