Called Li-Fi, which is short for light fidelity, the technology is what is known as Visible Light Communication (VLC). Unlike infrared-based systems, VLC involves transmitting data using light visible to the human eye. In this case it is transmitted as intermittent, imperceptible flickers of light emitted by LEDs.
The team started out with audio, cabling up a protoboard table to a smartphone via its 3.5 mm audio jack. The protoboard table converted the audio signal into an optical signal that is transmitted by a special emitter across the spectrum of light generated by an LED lamp. At the receiving end, a receptor located in a speaker captured the signal and converted it back into an audio signal that was played by the speaker.
For wireless internet transmission, the principle is the same but makes use of a receptor device designed to be placed above a router. The router incorporates an LED lamp to transmit data so anyone falling within the halo of light emitted by the LED will be in range. However, only those with a receptor/transmitter device will be able to send and receive the signal.