Flash of brilliance?

posted by Ian Johnson, CBC News Online

It's usually easy to tell when a photo was snapped with a cellphone camera – just look for images that are grainy, blurry and poorly lit. A team of engineers at the University of California, San Diego, has been focusing on the problem, and after a flash of brilliance, says it has developed — OK, OK, no more weak photography references — a high-powered but compact imager that can fit into everything from cellphones to small aerial surveillance drones.

The researchers at the school's Photonic Systems Integration Lab say they've been able to "fold up" a telephoto lens. The technique, they admit, is not entirely new – it's based on conventional astronomical telescopes that use mirrors, such as the Cassegrain telescope developed in 1672 - but the UCSD team was able to build its imager using a single component.

According to a UCSD statement, "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."

The researchers took a disc of transparent calcium fluoride, and "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 millimetre diameters are separated by 5 millimetres of transparent calcium fluoride. This forces light entering the ring-shaped aperture to bounce back-and-forth between the two reflective surfaces. The light follows a predetermined zigzag path as it moves from the largest of the four concentric optic surfaces to the smallest and then to the CMOS light sensor."

The imager has a limited depth of focus, but this can be compensated for through software. The UCSD says the cost of mass-producing the imagers would be in the same range as producing the glass lenses used now in small cameras.