Engineers at UC San Diego have built a powerful yet ultrathin digital camera by folding up the telephoto lens, which could lead to high-resolution miniature cameras for a variety of applications.
This technology could in the future be used in unmanned surveillance aircraft, cell phones and infrared night vision applications.
To reduce camera thickness but retain good light collection and high-resolution capabilities, the researchers replaced the traditional lens with a ‘folded’ optical system that is based on an extension of conventional astronomical telescopes that employed mirrors.
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 5mm thick optical crystal. The light is focused as if it were moving through a traditional lens system that is at least seven times thicker.
Folding the optic addresses performance issues facing cell phone cameras by increasing the effective focal length without increasing the distance from the front of the optic to the image sensor.
The engineers cut a series of concentric, reflective surfaces on a disk of calcium fluoride that bend and focus the light as it is bounced to a facing flat reflector. The two round surfaces with 60mm diameters are separated by 5mm of transparent calcium fluoride.
This design strategy 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.
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.
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.
In mass production, diamond turning would be used to create a master for a moulded-glass element, bringing the cost in line with the inexpensive moulded glass aspheres found in current compact cameras.
In the laboratory, the engineers compared a 5mm thick, 8-fold imager optimised to focus on objects 2.5m away with a conventional high-resolution, compact camera lens with a 38mm focal length.
At best focus, the resolution, colour and image quality are very similar between the two cameras, the authors report.
One initial drawback with the new folded camera was its limited depth of focus. Digital post-processing techniques and design changes were successfully implemented in the latest generation of the camera.
The team is now designing variable-focus folded optical systems that have air between the reflective surfaces of the imager. Such imagers may be especially useful for lightweight, inexpensive infrared vision applications. The all-reflective systems also enable ultra-broad-spectrum imaging and may be useful for ultraviolet lithography.