Imagine your smart wristband, wireless sensors, and even the smart light bulbs in your home never needing to be plugged in for charging or have their batteries replaced again. An invisible beam of infrared light can be emitted from a transmitter on the ceiling, precisely tracking and continuously powering these devices, whether it is day or night. Does this sound like science fiction? A team from the Tokyo Institute of Technology in Japan has made this technology a reality.
The Dilemma of Wireless Charging: Why Do We Need ‘Light Charging’?
As the number of IoT devices increases, traditional charging methods are becoming cumbersome. Batteries need to be replaced frequently, which is neither environmentally friendly nor convenient; wires limit the placement of devices. Although wireless radio wave charging already exists, it has a fatal weakness: it requires large antenna arrays, is inefficient, and is easily affected by electromagnetic interference.
Thus, scientists thought of using light to transmit energy. Just like we used to focus sunlight with a magnifying glass to ignite paper, Optical Wireless Power Transmission (OWPT) can convert electrical energy into light energy, which is then emitted and received by solar panels on devices, converting it back into electrical energy. This method is highly directional, unaffected by electromagnetic interference, and the transmitter can be made very compact.
However, the problem is that while lasers are concentrated in energy, their power is too high and can be harmful to human eyes when used indoors. On the other hand, ordinary LEDs are safe but face three major challenges: energy disperses over longer distances, they cannot charge multiple devices simultaneously, and they easily lose track of targets when light conditions change.
The Innovation of This Research: A ‘Thinking’ Charging System
The research team designed an LED charging system that can automatically track multiple targets, with the standout feature of seamlessly switching between day and night. The core of the system consists of three clever designs:
1. “Zoom Flashlight”: Adaptive Optical System
Imagine you have a smart flashlight that can automatically adjust its aperture size based on the distance to the target. The system uses a liquid lens that can quickly focus like the human eye’s lens by changing the voltage. Combined with a large convex lens, it can keep the light spot at an appropriate size from 0.5 meters to 5 meters away.
Traditional systems lose 90% of energy beyond 1 meter, while this system can dynamically adjust to keep the light spot focused within the target size range, just like a sniper always keeping the scope aimed at the bullseye.
2. “Smart Eyes”: Dual-Camera Positioning
The system is equipped with a depth camera that has “two eyes”: a regular color camera and an infrared camera.
– Day Mode: The color camera directly identifies the outline of the solar panel.
– Night Mode: When the environment darkens, the infrared camera locates the panel using reflective stickers. Researchers have placed special reflective materials around the solar panel (similar to bicycle tail lights), and the weak infrared light emitted by the camera creates bright spots, allowing for precise identification even in complete darkness.
3. “Flexible Mirror”: Dual-Axis Reflector
Once positioning is complete, how do we ensure the light beam accurately hits the target? The team placed a dual-axis reflector behind the lens that can rotate up, down, left, and right. Two stepper motors control the angle of the mirror with an accuracy of 0.018 degrees—this means that at a distance of 5 meters, the light spot’s error can be controlled within 1.6 millimeters, thinner than a toothpick.
Actual Performance: Operating in an Organized Manner Like a Lawnmower
In experiments, the system successfully achieved a “cruise charging” mode:
– Multi-Target Management: It can simultaneously identify five solar panels of different sizes, the largest being about 10 cm square, medium about 7.5×10 cm, and the smallest only 4.5×6.5 cm.
– Intelligent Sequencing: Like a lawnmower, it scans from left to right, charging each device for 5 seconds before moving to the next.
– Adaptive Adjustment: It adjusts the light spot size in real-time based on the panel size. At a distance of 1 meter, the light spot can shrink to a minimum of 2 cm; at 5 meters, it can still be controlled within 8.5 cm.
– Seamless Switching: It can switch between a bright office environment of 500 lux and a completely dark environment of 0 lux with a delay of less than 50 milliseconds—before the human eye can react, the system has already adjusted.
Test results show that at a distance of 5 meters, regardless of day or night, the system can stably maintain the output voltage of the solar panels. Although the current overall energy transmission efficiency is about 56%, the main losses come from the aperture limitations of the liquid lens and material absorption. Researchers indicate that by improving the LED divergence angle and optimizing lens temperature control, efficiency is expected to exceed 80%.
Future Applications: Making Smart Environments Truly ‘Wireless’
This technology opens up possibilities for future smart homes, factories, and IoT deployments:
– Smart Homes: Install transmitters on room ceilings, keeping all small devices permanently online.
– Industrial Sensors: Deploy wireless sensors in hazardous or hard-to-reach locations without needing battery replacements.
– Medical Implants: Provide safe energy for miniature medical devices inside the body (infrared light is harmless to humans).
– Emergency Equipment: Power critical devices in dark environments during power outages.
The current main limitation is that the liquid lens generates heat during continuous operation, causing focus drift. The team is researching temperature compensation algorithms to allow the system to automatically “correct” the effects of thermal expansion and contraction.
Conclusion
This research has brought the concept of “airborne electricity” into practicality. By combining intelligent optics, machine vision, and precision control, a wireless charging system that can automatically track, adjust, and serve multiple devices under any lighting conditions has been developed. Perhaps in the near future, we can truly say goodbye to charging cables, allowing energy to fill our living spaces invisibly, just like Wi-Fi signals.
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This article is based on the research by Zhao Mingzhi and Miyazaki Tomoyuki from the Tokyo Institute of Technology published in the “Optics Express” in 2025 titled “Automatic and adaptive optical wireless power transmission for IoT with dual mode of day and night charging.”