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.