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  National Geographic 5 Dec 05
Animal Eyes Provide High-Tech Optical Inspiration
Brian Handwerk for National Geographic News

Today's cutting-edge optical technologies could progress by leaps and bounds if scientists can better imitate animal-eye evolution spanning billions of years, two bioengineers report.

Using biology as inspiration, scientists hope to make advances in optics that would enhance camera and video technology, surveillance systems, missile defense, remote navigation, and even human vision aids.

"It's amazing to see the beauty of nature and see if we can apply this to everyday life," said Luke Lee, a bioengineer at the University of California, Berkeley.

Lee and Berkeley colleague Robert Szema wrote on the state of animal-eye optics research in a recent issue of the journal Science.

In his lab, Lee is refining three-dimensional polymer structures that can mimic the components of an eye, from lenses to light receptors. He believes soft, flexible polymers may be the key to replicating natural sight systems that outperform their mechanized competition.

"Many, many biologists have studied animals' eyes," Lee said. "Some of those studies are decades old. But they didn't have the tools to make the artificial structures that are now possible. "[Now is] really a good time to figure out how to make complex three-dimensional structures, like compound eyes."

Flexible Camera Lenses?

Sight works when eyes admit light and focus the rays on some type of detector so that the brain can recognize images. Nature has many ways of accomplishing this goal depending on a critter's needs and habitat.

Humans have camera-type eyes, as do many fish, birds, and reptiles. These eyes use a single lens to focus images onto a light detector called a retina. The human eye focuses at different distances by using tiny muscles to change the curvature of the eye's flexible crystalline lens. Changing the lens's curvature changes the focal length, so that images at different distances appear in focus.

Other animals use more unusual techniques adapted to their lifestyles.

Whales, for example, have an internal hydraulic system adapted to their air-breathing ocean existence. A chamber behind the eye's lens alternately fills with fluid to move the lens closer to the retina and empties to move it farther away. This changes focal length in a way that allows the whales to enjoy clear sight above and below the water's surface. The system may also help whales deal with the water pressure they experience at depth.

Lee has created polymer versions of whale-eye lenses in his lab for use in cameras and other equipment.

"If you can make a dynamic lens, you can change its curvature by the amount of pressure that you control," he said.

"If a company could make an inexpensive, plastic, membrane-based lens, you wouldn't have to rotate [between different glass] lenses," he explained. "You might just have one lens and a system that you can use to change the magnification." Such a system could someday replace "fisheye" type lenses, which capture a wider field of view. "That's a huge monster, because there are actually many lenses in a fisheye lens," Lee said. "But if you can control [curvature] with a liquid, you can reduce all those layers of lens. By playing with this polymer you can do a lot."

360-Degree Sight

Lee and other researchers have created only components of camera-type eyes. But scientists are much closer to reproducing entire vision systems like the lenslets found in compound eyes.

Compound eyes, common in insects such as dragonflies, may utilize up to 29,000 lenslets per eye. These individual lens systems, or ommatidia, function separately from each other. Each captures its own tiny piece of the overall picture. All these tiny images are processed simultaneously in the eye, which enables insects to have outstanding fast-motion detection.

"People buy those [kaleidoscopelike] things where you see a whole bunch of little images that look pretty much like the same image. That's not at all what insects see," said dragonfly-vision expert Robert Olberg.

Olberg explains that compound eyes afford a panoramic view, allowing insects to see in many directions at once. "The dragonfly's field of vision is actually 360 degrees," said Olberg, a biology professor at Union College in Schenectady, New York. "They can look back and see their wings going up and down, though their vision isn't very acute in that direction."

Dragonflies enjoy a wide field of vision, but clarity is much higher for them within a limited arc 60 degrees above the horizon. "You can really think of it that if you have a thousand ommatidia, you have a picture that's made of a thousand pixels, and the pixel size varies over the image."

To make artificial compound eyes similar to insect eyes, researchers combine a polymer lens with a tubelike "waveguide," which connects to an optoelectronic detector that recognizes images.

Lee has created 180-degree hemispheres with ommatidia of this type, though they may display only half of the possible picture. "In our case we can bond two [half hemispheres] together for a 360-degree view," Lee said. "We can create an omnidirectional sensor that can also detect fast movement?the insect eye is very good at fast movement."

Surveillance is an obvious candidate for such an application. Sensors could constantly monitor locales in 360 degrees and real time, from parking lots to the skies above major cities.

Lee also notes that imaging sensors could theoretically take Fantastic Voyage-esque journeys inside the human body. "They can be made small enough to be swallowed," he notes, "which would allow doctors to see 360-degree images of the entire digestive system."

Lee believes that such breakthroughs will be greatly aided by flexible, three-dimensional polymers, which were unavailable to researchers in previous decades of animal-vision study.

"It's not that we're smarter today--past researchers were very smart to identify so many details of the eye," he said. "But they didn't have the tools that we have."
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