Realistic Retinas Make Better Bionic Eyes

Following nature’s example more closely could lead to better visual sensors

4 min read
shape of eye with different colored dots

New visual sensors inspired by the human eye could help the blind see again and provide powerful new ways for machines to sense the world around them. Recent research shows that more faithfully copying nature’s hardware could be the key to replicating its powerful capabilities.

Efforts to build bionic eyes have been underway for several decades, with much of the early research focused on creating visual prostheses that could replace damaged retinas in humans. But in recent years, there’s been growing recognition that the efficient and adaptable way in which the eye processes information could prove useful in applications where speed, flexibility, and power constraints are major concerns. Among these are robotics and the Internet of things.

Now, a pair of new research papers describe significant strides toward replicating some of the eye's capabilities by more closely imitating the function of the retina—the collection of photoreceptors and neurons at the back of the eye responsible for converting light into visual signals for the brain. “It’s a very exciting extension from where we were before,” says Hongrui Jiang, a professor of engineering at the University of Wisconsin–Madison. “These two papers [explain research aimed at] trying to mimic the natural visual system’s performance, but on the retina level right at the signal-filtering and signal-processing stage.”

One of the most compelling reasons to do this is the retina’s efficiency. Most image sensors rely on components called photodiodes, which convert light into electricity, says Khaled Salama, professor of electrical and computer engineering at King Abdullah University of Science and Technology (KAUST), but photodiodes constantly consume electricity, even when they’re on standby, which leads to high energy use. In contrast, the photoreceptors in the retina are passive devices that convert incoming light into electrical signals that are then sent to the brain. In an effort to recreate this kind of passive light-sensing capability, Salama’s KAUST team turned to an electrical component that doesn’t need a constant source of power—the capacitor.

“The problem is that capacitors are not sensitive to light,” says Salama. “So we decided to embed a light-sensitive material inside the capacitor.” The team sandwiched a layer of perovskite—a material prized for its electrical and optical properties—between two electrodes to create a capacitor whose ability to store energy, or capacitance, changed in proportion to the intensity of the light to which it is exposed. The researchers found that the resulting device mimicked the characteristics of the rod-cell photoreceptors found in the retina.

To see if the devices they created could be used to make a practical image sensor, the team fabricated a 100-by-100 array of them, then wired them up to simple circuits that converted the sensors’ change in capacitance into a string of electrical pulses, similar to the spikes of neural activity that rod cells use to transmit visual information to the brain. In a paper published in the February issue of Light: Science & Applications, they showed that a special kind of artificial neural network could learn how to process these spikes and recognize handwritten numbers with an accuracy of roughly 70 percent.

An array of the bio-inspired sensors produces strings of electrical pulses in response to light, which are then processed by a spiking neural network.An array of the bio-inspired sensors produces strings of electrical pulses in response to light, which are then processed by a spiking neural network.Dr. Mani Teja Vijjapu/King Abdullah University of Science and Technology

Thanks to its incredibly low energy requirements, Salama says future versions of the KAUST team’s bionic eye could be a promising solution for power-constrained applications like drones or remote camera systems. “The best application for something like this is security, because often nothing is happening,” he says. “You are wasting a lot of power to take images and take videos and process them to figure out that there is nothing happening.”

Another powerful capability of our eyes is the ability to rapidly adapt to changing light conditions. Image sensors can typically operate only within a limited range of illuminations, says Yang Chai, an associate professor of materials science at the Hong Kong Polytechnic University. Because of this, they require complex workarounds like optical apertures, adjustable exposure times, or complex postprocessing to deal with varying real-world light conditions. By contrast (pun intended), when you transition from a dark cinema hall to a brightly lit lobby, it takes only a short while for your eyes to adjust automatically. That’s thanks to a mechanism known as visual adaptation, in which the sensitivity of photoreceptors changes automatically depending on the level of illumination.

In an effort to mimic that adaptability, Chai and his colleagues designed a new kind of image sensor whose light sensitivity can be modulated by applying different voltages to it. In a paper in Nature Electronics, his team showed that an array of these sensors could operate over an even broader range of illuminations than the human eye. They also paired the array with a neural network and showed that the system’s ability to recognize handwritten numbers improved drastically as the sensors adapted, going from 9.5 percent to 96.1 percent accuracy as it adjusted to bright light and 38.6 percent to 96.9 percent as it adjusted to darkness. These capabilities could be very useful for machines that have to operate in a wide range of lighting conditions. One application for which it will be quite helpful, says Yang, is in a self-driving car, which has to keep track of its position with respect to other objects on the road as it enters and exits a dark tunnel.

While there’s still a long way before bionic eyes approach the capabilities of their biological cousins, Jiang says the kinds of in-sensor adaptation and signal processing achieved in these papers show why researchers should be paying more attention to the finer detail of how the retina achieves its impressive capabilities. “The retina is an amazing organ,” he says. “We’re only just scratching the surface.”

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The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

This article appears in the February 2023 print issue.