By Paul Gabrielsen
— An insect's compound eye is an engineering marvel: high resolution, wide field of view, and incredible sensitivity to motion, all in a compact package. Now, a new digital camera provides the best-ever imitation of a bug's vision, using new optical materials and techniques. This technology could someday give patrolling surveillance drones the same exquisite vision as a dragonfly on the hunt.
Human eyes and conventional cameras work about the same way. Light enters a single curved lens and resolves into an image on a retina or photosensitive chip. But a bug's eyes are covered with many individual lenses, each connected to light-detecting cells and an optic nerve. These units, called ommatidia, are essentially self-contained minieyes. Ants have a few hundred. Praying mantises have tens of thousands. The semicircular eyes sometimes take up most of an insect's head.
While biologists continue to study compound eyes, materials scientists such as John Rogers try to mimic elements of their design. Many previous attempts to make compound eyes focused light from multiple lenses onto a flat chip, such as the charge-coupled device chips in digital cameras. While flat silicon chips have worked well for digital photography, in biology, "you never see that design," Rogers says. He thinks that a curved system of detectors better imitates biological eyes. In 2008, his lab created a camera designed like a mammal eye, with a concave electronic "retina" at the back. The curved surface enabled a wider field of view without the distortion typical of a wide-angle camera lens. Rogers then turned his attention to the compound eye.
Over the last 3 years, Rogers and students in his lab at the University of Illinois, Urbana-Champaign, have developed an array of 180 ommatidia (about the same number as in the eye of a fire ant), each of which contains a lens, tiny silicon photodetectors, and circuitry to read the image. But nanoscale manufacturing on a flexible, curved surface is tricky, says electrical engineer R. Fabian Pease of Stanford University in Palo Alto, California, who was not involved in the research. "It's not even easy on a nice, flat, rigid silicon wafer."
So Rogers and his team embedded the artificial ommatidia in a flexible rubber sheet and wired them together with stretchable silicon circuits. The researchers then inflated the sheet like a balloon until it reached the optimum curvature. The lenses are also made of rubber. "We moved completely away from glass optics," Rogers says, making even smaller ommatidia a possibility. The camera is about a centimeter in diameter, smaller than a penny.
Rogers and his students tested the camera with high-contrast images of soccer balls and, appropriately, the Chinese character for "eye." Images from the device are found in the team's paper online Wednesday in Nature. The camera boasts a 160° field of view, although 180° would be better. The resolution isn't as high as on smart phones, but the blurry images are recognizable. Shapes placed near the edges of the camera's field of view appear as if right next to each other, with no stretching or other distortion.
A wide-angle, compact camera would be ideal for a high-flying, motion-sensing surveillance drone or a miniature, snakelike endoscopic medical device, Rogers says. Next, the team will tinker with the radius and curvature of the flexible ommatidia array to see what other optical feats the camera is capable of.
Pease calls Rogers's work a "terrific tour de force of nonconventional microfabrication." Still, Rogers admits limitations of size and technical sophistication because his current manufacturing facilities are limited. "We're in an academic environment," Rogers says, "not in a digital camera manufacturing world."
This is adapted from ScienceNOW, the online daily news service of the journal Science. http://news.sciencemag.org