Researchers learn optics lessons from biology
WASHINGTON - When nanotech researcher Luke Lee is looking for inspiration for the next generation of optical gadgets, he ponders the lobster. And the house fly. And the octopus.
Lee and other “bioengineers” are borrowing ideas from all corners of the animal kingdom to design artificial vision systems that could be used for high-tech cameras, motion detectors, navigation devices in unmanned vehicles, or perhaps even synthetic retinal implants.
Lee and his colleague at the University of Berkeley, Robert Szema, describe the latest progress on this front in an article in the November 18 issue of the journal Science, published by AAAS, the nonprofit science society. advertisement
Researchers have been fascinated with the phenomenon of vision since the days of the ancient Greeks, who thought sight involved rays emanating from the eye to touch surrounding objects. It wasn’t until fifteen hundred years later that the Arab scholar al-Haytham described how lenses can focus and magnify images.
Today, we know that natural selection has produced at least ten animal vision systems, each tailored to fit the specific needs of its owner. Eyes for different species are adapted for seeing in the day or night, short or long distances, with wide or narrow fields of view, etc.
But, all of these systems capture light and use it produce some sort of picture in the brain representing the surrounding environment. These systems are superior to the imaging technologies that humans have produced in many ways. They can be more efficient and powerful, and often simpler and more elegant than their synthetic counterparts.
How animals see
Animals have two main types of vision systems: camera-type eyes, which use a single lens to focus images onto a retina, and compound eyes which have multiple lenses — sometimes thousands of them.
Animals with camera-type eyes use a variety of ways to focus the lenses so they can see objects at various distances. Humans and birds have specialized muscles that change the lens’ curvature. The single lens in the octopus eye has layers like an onion, each with slightly different optical properties, to help sharply focus the light, even with a wide field of view.
In whales, a chamber behind the lens fills and empties with fluid to move the lens closer or farther from the retina, while in some amphibians a muscle moves the lens back and forth.
Lee and other researchers have made lenses that are similar to those in camera-type eyes, whose focus can be tuned by changing the pressure of fluid in special chambers. These so-called “microdoublet” lenses can assume two different shapes — with both sides either bulging away from each other or curving in the same direction — to help adjust the focal length and field of view. Lenses like these may be useful for applications such as cellular cameras, medical imaging inside the body or optical data storage.
Researchers are also working on building synthetic retinas. This is a challenge because most retinas are curved in nature, but conventional arrays of light-sensors, including those inside cameras, are typically flat and rigid.
Scientists also haven’t yet figured out how to put all the parts together in order to produce a full-fledged artificial eye of this type, according to Lee.
Research is progressing much faster with compound eyes, which are made up of many individual lenses (as many as ten thousand in some dragonflies) and found in insects and other arthropods.
The lenses are part of separate imaging units called “ommatidia,” which each provide fragments of an image. In many cases, the ommatidia send their signals simultaneously, allowing for the fast motion detection and image recognition that allows flies, for example, to evade fly-swatters time and again.
Lee and his colleagues have made artificial ommatidia, each with a tiny lens connected to a tube-like “waveguide” that directs the light down to an optoelectronic imaging device. The ommatidia can be arranged around a dome, projecting outwards in all directions. Putting two of these structures back-to-back could hypothetically allow for a device with 360-degree vision.
“The whole thing could potentially be smaller than a vitamin. What if you could swallow it to get a look inside the body? That’s just a concept right now, though,” Lee said.