The idea is to make the most of VLC even in daylight when indoor lighting would typically be turned off. What's more, mobile users relying on VLC see how shining light to a receiver can drain their batteries. Using off-the-shelf, low-cost LEDs and photodiodes, the researchers came up with novel data encoding and LED driving schemes they presented in a paper, "The DarkLight Rises: Visible Light Communication in the Dark"
They had to tackle various challenges, such as designing an effective driving circuit that would react sufficiently fast (in nanoseconds) for minimal delay and to boost the gain of low cost photodiodes (for improved communication distance). They also optimized a lightweight modulation scheme to maximize the number of bits encoded within the ultra-low LED duty cycle and developed a robust demodulation scheme to reliably extract the bits from the light pulses.
"DarkLight", as they call this invisible VLC implementation, achieves 1.6kbps data rate and supports up to 1.8m of communication distance while drastically reducing the power consumption of the LED front end from 19.8W to 104mW. They achieved that with 500ns light pulses and a LED duty cycle of only 0.007%, making the DarkLight LED indistinguishable from a LED in the "off" state.
They say DarkLight broadens the applicable scenario of VLC, as it could just be a special mode that a VLC link could seamlessly switch to, whenever perceptible light beams are not desired (in a sunny day or even at night when one would want to connect discreetly). This mode would allow light-based communication to be always-on, regardless of the actual light luminance.