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09 Feb 2007
US researchers explain to optics.org how they have produced an "origami" lens that could form the basis of thinner cameras for mobile phones.
Engineers from the University of California at San Diego (UCSD) have produced a camera lens that's more than five times thinner than its conventional counterparts by embedding tiny mirrors inside a single optical crystal (Applied Optics 46 463). The new design derives its operation from Cassegrain telescopes, and could lead to a new generation of slim lenses for cell phone cameras.
While today's cameras focus light by using a set of compound refractive lenses placed next to each other, the new lens reflects light beams back and forth inside a thin optical crystal made of calcium fluoride. "All the power in this crystal is reflective, and there is only a small refractive component that gets introduced when light enters the plane annular aperture," UCSD's Eric Tremblay explained to optics.org.
The prototype plano-aspheric optic is about 6 cm wide and 5 mm thick. Its front side is flat and has a reflective central core surrounded by a thin ring-shaped aperture, while the rear face is aspheric and has a series of black and silver rings surrounding an aperture for connecting the CMOS sensor.
Light entering from the annular aperture is forced to bounce along the two reflective surfaces while making its way to the sensor. "We call it the eightfold annular design, because two incoming light rays would encounter exactly eight mirrors on their way to the sensor," says Tremblay.
Four concentric mirrored rings are located at precise positions on the disk to create a pathway for the light rays. Light entering the annular aperture is forced to strike the largest of the concentric reflectors, and it then follows a zigzag path to the smallest reflector before arriving at the CMOS sensor. Baffles made from a black light-absorbing material were fabricated between the mirrors to prevent stray light from interfering with the images.
The mirrors were cut into the calcium fluoride slab using diamond turning, which is widely used to make high-quality aspheric optical elements. "As only one of the surfaces is aspheric, all of the reflective surfaces (in the rear portion of the lens) can be shaped in a single mounting," said Tremblay.
To demonstrate the crystal, the team has built a basic digital camera with a resolution of 1024x768 pixels. Images taken from conventional camera lenses and the origami lens were compared and found to be very similar. However, the new design suffers from a smaller field-of-view (FOV) and depth than conventional cameras. While the FOV cannot be enhanced – which might preclude its use in high-end digital cameras – wavefront coding can be used to increase its depth.
The current prototype has a fixed focal length, but the team is now designing variable-focus crystals that would have air between the two reflective surfaces. Being lightweight and inexpensive, they could be attractive for infrared surveillance cameras that require high resolution. And because they are completely based on reflection, they could also find uses in broad-spectrum imaging and ultraviolet lithography.
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