A team of researchers at the University of California, Berkeley has figured out how to inexpensively assemble more than 8,000 tiny plastic lenses into a dome-shaped array, mimicking the compound eye of a bee or dragonfly. Their innovative structure can capture light and images across a 180-degree radius – twice the range of wide-angle “fish-eye” lenses.
The new fabrication process could be used in small, inexpensive cameras for medical procedures such as endoscopies or video-guided surgery. Or, say the researchers, the array could be integrated into mobile-phone cameras to increase their picture-viewing area. The dome-shaped collection of lenses, described in the current issue of Science, consists of 8,370 individual lenses, each with a diameter of 25 micrometers, packed in a honeycombed pattern.
[For images of these arrays of lenses, click here.]
Luke Lee, professor of bioengineering at Berkeley and lead researcher on the study, says the array design was inspired by insects’ compound eyes, in which thousands of lenses work together to collect light from different directions and focus it onto photoreceptor cells inside the eye. It allows insects with compound eyes to use “thousands of lenses looking at different angles,” says Lee. His array of lenses mimics, almost exactly, the structure and function of these natural eyes.
It has been difficult to build working arrays of tiny lenses, partially due to the lens-making process itself. To create his array, Lee has developed a clever three-dimensional lens fabrication technique, says Rashid Bashir, professor of electrical, computer, and biomedical engineering at Purdue. “Arrays of ‘microlenses’ have been fabricated in the past…but usually on flat surfaces,” and it has been difficult to make them work at all, he says.
Traditionally, the lenses would be formed, and then, in a separate step, another component, called a waveguide, would be added to them. The waveguide, usually made of a polymer fiber, directs light collected by the lenses onto a detector chip. The challenge in this two-step process, says Bashir, who’s familiar with Lee’s work, is to effectively align each tiny waveguide with each tiny lens. Alignment is hard enough with a flat array of microlenses, he says – and it’s far more difficult with lenses in a dome.
Lee’s fabrication process sidesteps the need to align waveguides and lenses. Instead, his team builds the waveguides into the lens-making process. First, the researchers shape the lenses by pouring a clear liquid polymer into a curved, dimpled mould, which is heated so that the material hardens somewhat. Next, they shine ultraviolet light onto the lenses-filled dome. Each lens collects that light, which induces a chemical reaction within the rest of the polymer material. This reaction causes the polymer to solidify further around the focal point of the lens, creating a cone shape. The cone further guides light through the polymer, and the beam forges a solid fiber that acts as a waveguide for the individual lens. Lastly, the researchers heat the entire dome structure to 150 degrees Celsius to solidify the remaining soft polymer.
By using this “self-aligned process,” Bashir says, each lens has its own perfectly aligned waveguide. Lee says his team has shown that it is “possible to micro-fabricate these structures without alignment with a machine.” This lens-making method lends itself well to mass production, he adds, because it uses only common materials and fabrication equipment. Additionally, Lee says, the process is inexpensive because the waveguides and lenses do not need to be bonded together in further, tedious steps.
Although Lee’s work is just the first demonstration of this three-dimensional lens-making technique, Bashir says it is detailed and robust: “All the key ingredients are there, and currently the process is amenable to manufacturing.”
Bashir suspects that the ideal application for the dome-shaped array is to attach it to a charge-coupled device (CCD) – a chip commonly found in digital cameras that converts light into electrical signals. The diameter of the dome matches well the millimeter length of the chip. This combining of technologies would enable cameras to take wider-angle images. This application, he says, could be just two to five years away.
Although these arrays of lenses can extend a camera’s field of view, they don’t offer a high resolution yet, says Russell Chipman, professor of optical sciences at the University of Arizona. “The number of pixels is equal to the number of lenses,” he says. Yet this resolution may still be “good enough to be applicable in some instances,” such as endoscopies. And, Chipman adds, it should not be too difficult to provide more resolution by adjusting the design of the lenses.
Overall, Chipman believes the technology is promising – the optics industry is always looking for better ways to capture images. And the lenses created by Lee and his team can collect more light than other types of lenses. “If something like this gets into the marketplace it could be competitive,” Chipman says.
Home page image courtesy of Science magazine.