Scientists think that more than 95% of Earth’s oceans have never been seen. This means that we know less about Earth’s oceans than we do about the far side of the moon or the surface of Mars.

Underwater investigation is hindered by the difficulty and expense of maintaining long-term camera power, either by tying it to a research vessel or sending a ship to recharge its batteries.

By creating a battery-free, wireless underwater camera that is around 100,000 times more energy-efficient than other underwater cameras, MIT researchers have made a significant progress toward solving this issue. Even in low-light conditions underwater, the camera captures color images and wirelessly sends them.

Sound delivers the camera’s power. To power its image and communications technology, it transforms mechanical energy from sound waves moving through water into electrical energy. The camera employs sound waves to convey data to a receiver that reconstructs the image after the image has been captured and encoded.

Since the camera doesn’t need a power source, it could run for weeks without being picked up. This would let scientists look for new species in remote parts of the ocean. It could also be used to take pictures of pollution in the ocean or to check on the health and growth of fish in aquaculture farms.

“One of the most exciting applications of this camera for me personally is in the context of climate monitoring,” says senior author Fadel Adib, adding, “we are building climate models, but we are missing data from over 95 percent of the ocean. 

“This technology,” according to the author, “could help us build more accurate climate models and better understand how climate change impacts the underwater world.”

Co-lead authors and research assistants from the Signal Kinetics group Sayed Saad Afzal, Waleed Akbar, and Osvy Rodriguez, research scientist Unsoo Ha, and former members of the group Mario Doumet and Reza Ghaffarivardavagh also collaborated on the work with Adib. Nature Communications has published the paper.

Getting rid of batteries

The researchers wanted a tool that could autonomously harvest energy underwater while using very little power in order to construct a camera that could run independently for extended periods.

Placing transducers constructed of piezoelectric materials on the camera’s outside allows it to acquire energy. When a mechanical force is applied to piezoelectric materials, an electric signal is generated. The transducers vibrate and change the mechanical energy of a sound wave passing through the water into electrical energy when it strikes them.

The source of such sound waves could be anything, such as a passing ship or aquatic life. Until it has accumulated enough to operate the circuits that take photographs and transmit data, the camera retains the harvested energy.

To minimize power usage, the researchers utilized commercially available ultra-low-power image sensors. These sensors, however, can only record grayscale images. They also needed to design a low-power flash because most underwater situations are dark.

“We were trying to minimize the hardware as much as possible, and that creates new constraints on how to build the system, send information, and perform image reconstruction. It took a fair amount of creativity to figure out how to do this,” Adib adds.

They used red, green, and blue LEDs to concurrently solve both issues. When the camera takes a picture, it shines a red LED and then employs image sensors. With green and blue LEDs, the same procedure is repeated.

Even though the image appears black and white, Akbar says that red, green, and blue light are reflected in the white portions of each photograph. The color image can be recreated by combining the image data during post-processing.

“When we were kids in art class, we were taught that we could make all colors using three basic colors. The same rules follow for color images we see on our computers. We just need red, green, and blue — these three channels — to construct color images,” he adds.

Using sound to send data

Using a technique known as underwater backscatter, image data are encoded as bits (1s and 0s) and delivered to a receiver one bit at a time after being acquired. The camera serves as a mirror to reflect the sound waves that are transmitted by the receiver through the water. The camera either sends a wave back to the receiver by reflecting it or transforms its mirror into an absorber to prevent reflection.

If a signal is reflected back from the camera, it is detected by a hydrophone close to the transmitter. It receives a bit-1 if there is a signal, and a bit-0 if there is no signal. This binary data is used by the system to reconstruct and post-process the image.

“This whole process, since it just requires a single switch to convert the device from a nonreflective state to a reflective state, consumes five orders of magnitude less power than typical underwater communications systems,” Afzal adds.

The camera was tested by the researchers in a variety of underwater settings. In one, they managed to photograph in color plastic bottles drifting in a pond in New Hampshire. Additionally, they were able to capture an African starfish in such exquisite detail that the small tubercles along its arms could be seen. The device was also able to take multiple pictures of the underwater plant Aponogeton ulvaceus in the dark over the course of a week to track its growth.

The researchers intend to improve the device so it may be used in real-world circumstances now that they have shown a functioning prototype. They plan to enhance the camera’s memory so that it can take underwater videos, broadcast images, and take photos in real-time.

They also aim to increase the camera’s field of view. Although they were able to transmit data 40 meters away from the receiver, extending the range would allow the camera to be employed in more underwater environments.

The Office of Naval Research, the Sloan Research Fellowship, the National Science Foundation, the MIT Media Lab, and the Doherty Chair in Ocean Utilization are all helping to fund this research.

Source: 10.1038/s41467-022-33223-x

Image Credit: Getty

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