Folded Lens Promise

Physorg.com: Researchers at UC San Diego have built an ultrathin digital camera by folding up the telephoto lens. This technology may yield lightweight, ultrathin, high resolution miniature cameras for unmanned surveillance aircraft, cell phones and infrared night vision applications.

"This type of miniature camera is very promising for applications where you want high resolution images and a short exposure time. This describes what cell phone cameras want to be when they grow up," said Joseph Ford, a professor of electrical and computer engineering at the Jacobs School who leads the camera project within UCSD's Photonic Systems Integration Lab.

Instead of bending and focusing light as it passes through a series of separate mirrors and lenses, the new folded system bends and focuses light while it is reflected back and forth inside a single 5 millimeter thick optical crystal. The light is focused as if it were moving through a traditional lens system that is at least seven times thicker.

On a disk of calcium fluoride – a transparent optical crystal – the engineers cut a series of concentric, reflective surfaces that bend and focus the light as it is bounced to a facing flat reflector. The two round surfaces with 60 millimeter diameters are separated by 5 millimeters of transparent calcium fluoride.

This kind of lens folding has not been widely implemented as a means to slim cameras down, but recent advances in the mechanical machining process of "diamond turning" are changing that. The engineers used a diamond tip to cut all the reflective surfaces onto a single calcium fluoride disk.

"You mount the optic once and the diamond machining tool cuts all the optical surfaces without having to adjust the setup," explained Eric Tremblay, the first author on an Applied Optics paper published February 1, 2007, and an electrical and computer engineering Ph.D. candidate at UCSD's Jacobs School of Engineering. Not having to realign the optic during the machining of the reflective surfaces reduced an important source of error and helped make folding a viable approach for camera slimming.