Absorbing solar energy one moment and capturing images the next, a prototype video camera is said to be the first that is entirely self-powered. Developed by researchers at Columbia University, the camera can produce an image each second, indefinitely, of a well-lit indoor scene. It does not require a wall plug or battery. "Digital imaging is expected to enable many emerging fields including wearable devices, sensor networks, smart environments, personalized medicine and the Internet of Things," said Columbia University professor Shree K. Nayar. "A camera that can function as an untethered device forever – without any external power supply – would be incredibly useful." A video camera based on a self-powered image sensor can run indefinitely without an external power supply. Courtesy of the Computer Vision Laboratory, Columbia Engineering. Nayar, a computational imaging researcher, realized that although digital cameras and solar panels have different purposes – one measures light while the other converts light to power — both are constructed from essentially the same components. At the heart of any digital camera is an image sensor: a chip with millions of pixels. The key enabling device in a pixel is the photodiode, which produces an electric current when exposed to light. This mechanism enables each pixel to measure the intensity of light falling on it. Similar photodiodes are also used in solar panels to convert incident light to electric power. The photodiode in a camera pixel is used in the photoconductive mode, while in a solar cell it is used in the photovoltaic mode. Nayar, working with research engineer Daniel Sims and consultant Mikhail Fridberg of ADSP Consulting LLC, used off-the-shelf components to fabricate an image sensor with 30 × 40 pixels. In his prototype camera, which is housed in a 3-D-printed body, each pixel's photodiode is always operated in the photovoltaic mode. The pixel design is very simple and uses just two transistors. During each image-capture cycle, the pixels are used first to record and read out the image, and then to harvest energy and charge the sensor's power supply – that is, the image sensor continuously toggles between image-capture and power-harvesting modes. When the camera is not used to capture images, it can be used to generate power for other devices, such as a phone or a watch. The image sensor could use a rechargeable battery, Nayar said, "But we took an extreme approach to demonstrate that the sensor is indeed truly self-powered and used just as a capacitor to store the harvested energy." Funding came from the Office of Naval Research. The team is presenting its work at the International Conference on Computational Photography, April 24 to 26 at Rice University in Houston. For more information, visit www.cs.columbia.edu.