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. Insect Eyes Can Improve Optical Systems

by Charles Choi
New York (UPI) Apr 06, 2004
The next generation of smart weapons may sport artificial versions of insect eyes, say scientists funded by the U.S. Defense Department. The new technology uses diffractive optics -- lenses that actually scatter light waves instead of focusing or reflecting them.

"We think we can use this concept to make smart weapons smarter," said lead researcher Paul Holloway at the University of Florida in Gainesville.

"We're working with scientists and engineers at Raytheon Missile Systems to try and demonstrate the feasibility of this in a real system," Holloway told United Press International. "If these lenses could take even a small percentage of the optics market, they have significant economic impact."

Today's smart weapons see targets using refractive optics, or lenses that bend light that pass through them, just like the ones found in cameras and eyeglasses. Refractive optical systems are relatively bulky, because they must use mechanical systems to move the lenses when focusing. This bulk drives up the weight, which in missiles translates into more propellant, greater size and higher costs.

Also, the view refractive optical systems provide is like the view through a telescope -- the target is in sharp focus, but the surroundings are lost. This limits a weapon's ability to track moving targets.

Diffractive optics, in contrast, scatter light waves, providing a wider view.

"If I take a plate and make a series of holes in it, when the light comes through, it can actually be treated as a spherical wave coming out of those holes," Holloway explained. "The waves can interfere with each other, and provide an image just like the one through that of an eyeglass lens."

One common example of diffractive optics can be found in a device called a Fresnel zone plate, he continued. "In a Fresnel zone plate, you have a series of concentric rings that allow light to come through and form this image. If you've ever used a transparency projector and seen concentric rings on the lens or on where the light is reflected, that's actually a Fresnel zone plate," Holloway said.

"We're breaking up those concentric rings into an array of concentric dots. This gives us better control of the interference between light waves coming through the various holes, to give us a sharper image," he said.

Such devices are called photon sieves -- a photon is a wave or pulse of light. The wavelengths of light used depend on the size and spacing of the holes. The researchers are working on systems that can see from the infrared to the visible spectrum and ultraviolet. Working in the infrared allows for night vision.

Holloway said his group has made and tested small prototype lenses. Once perfected, the goal is to put many lenses together on a surface, with each lens working at different resolutions or focal distances, in a pattern that would resemble the compound eyes of insects.

Non-military applications of diffractive lenses could improve vision in robots used to fight oil well fires, transport nuclear materials or other tasks too dangerous for people. The lenses -- potentially lighter and cheaper than conventional lenses -- also could lead to better night vision goggles.

"The military's interested in night vision goggles, but so are more and more sportsmen. And it could help provide safety vision at night for automobiles and trucking," Holloway said.

"You could also have policemen using night vision goggles, or firefighters wanting to look through smoke or other things that obscure vision," said Gary McGuire, president of the International Technology Center in Raleigh, N.C., a nonprofit research corporation established to foster economic development through innovation in advanced micro-fabrication technologies.

"There's a lot of interest at the Department of Defense for lightweight imaging system for unmanned aerial vehicles," McGuire told UPI.

"Most of the lenses developed for the visible spectrum are 3 millimeters in diameter," Holloway said. "A typical camera lens for a video camera system might be on the order of 1 to 2 inches. We can easily fill that entire space up with photon sieves."

The result would be an optical system with panoramic and precise vision and no moving parts. "The advantage of diffractive optics over refractive optics is it can be made small and thus lightweight and extremely compact," he explained.

Moreover, not all photon sieves in such a system need work with visible light. Some can work in the infrared range as well.

"The ability to have both visible and infrared imaging in one system has not been done before on a simple basis and in a cheap application," Olga Shenderova, a computational material scientists with the International Technology Center in Research Triangle Park, N.C., told UPI.

Diffractive lenses do not have to be made of fragile materials such as glass. Holloway and his team are currently working on thin metal films patterned on glass surfaces about 1 millimeter thick, although they hope to switch from glass to more durable plastic. Also, diffractive optics can be patterned on a curved or even spherical surface, allowing for up to a 360-degree field of view, McGuire said.

The scientists have applied for patents on their findings. "We're scheduled to meet requirements for a prototype system in the course of a year and a half," Holloway said. The work is funded by the U.S. Defense Advanced Research Projects Agency.

Holloway does not expect diffractive optics to replace refractive optics in common applications such as cameras, because refractive optics have had far more time to become refined in such niches. Also, because light essentially must be filtered through a grate for the effect, the disadvantage of diffractive optics are images dimmer than those through refractive optics.

Although refractive lenses might see 99 percent to 50 percent transmission of light, diffractive optics commonly transmit only 30 percent to 10 percent.

The researchers are working to improve the amount of light their optics can transmit. Depending on how the holes are spaced, a charged field develops around their surfaces that can act like an antenna, "and the amount of light you would expect to go through an open hole is greater," Holloway said, adding the team hopes to double the amount of light their systems can transmit.

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