'Fold up' zoom lens could fit slim gadgets


By Will Knight A “fold up” telephoto lens that sends light on a zigzagging path to its centre has been demonstrated by researchers in the US. The lens could fit inside ultra-thin camera-phones and other devices, providing higher quality images within a more compact package. The lens performs the same job as one seven times thicker by reflecting light along a path that “folds” back upon itself, from the edge of the lens to its core. This increases the focal length without requiring the distance between the lens and the imaging device to also be increased, which normally constrains the thickness of a camera. “The elements are ‘folded’ on top of one another to reduce the thickness of the optic,” says Eric Tremblay, who developed the lens with colleagues at the University of California in San Diego, US. The lens was created by etching a 5-millimetre-thick circular piece of calcium fluoride to create the grooves that guide light through it. Two flat mirrors were then placed on top and beneath the etched crystal, leaving just the edge exposed to incoming light. Grooves within the material guide light in a zigzag path between mirrors from the outer aperture of the lens to its centre, where it is then imaged using a light-sensitive chip. “It is not technically a lens, since it is reflective,” Tremblay notes: “‘Imager’, or ‘folded optic’ are more accurate.” Similar, simpler designs have previously been used to shorten camera and telescope lenses. But it has never before been possible to bounce light through such a lens so many times, its designers claim. The diamond-etching fabrication employed was only recently perfected. It eliminates errors that would normally prevent such a complex lens from working. Tremblay’s team used the lens to focus on an image 2.5 metres away. They found the resolution, colour and image quality to be equal to that of a compact camera lens with a 38 mm focal length. “This type of miniature camera is very promising for applications where you want high resolution images and a short exposure time,” says Joseph Ford, Tremblay’s supervisor. “Today’s cellphone cameras are pretty good for wide angle shots, but when you zoom in, they’re terrible. They’re blurry, dark, and low contrast.” Andy Harvey at Herriot Watt University in Edinburgh, Scotland, says the lens could provoke significantly interest from cellphone makers. “The big commercial requirement with mobile phone cameras is to make them as small as possible,” he says. However, for it to be a success, Harvey says cellphone manufactures will need to find cheaper material to make the lens from and ensure the process is both cost effective and reliable. He also told New Scientist that the complexity of the folded lens would inevitably reduce the quality of the final image. The researchers claim that digital post production techniques could be used to adjust for these imperfections, but Harvey says this will only go so far. “You’ll never get back to the quality of a normal lens,” he believes. Journal reference: Applied Optics (vol 46,
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