The Camera Phone is one of the hottest-selling items in all of consumer electronics, with anticipated sales this year of 170 million units. The little gadgets have become so ubiquitous that hardly anyone finds it odd anymore to see tourists squinting with one eye while pointing their cellphones at a Buddhist temple, a Greek statue, or a New York City skyscraper. It's easy to see why analysts expect that this year camera phones will outsell conventional digital cameras and traditional film cameras combined.
But as anyone who has ever seen them can attest, the images that come out of camera phones leave plenty to be desired. Part of the problem is their CMOS imaging chips, which typically have a sensor array of only about 300 kilopixels--a quarter or less of the number in a low-end digital camera. Of course, semiconductor industry fundamentals ensure that 1-megapixel camera phones will soon be the norm. When they are, however, the only thing we may see more clearly is the other weakness of these cameras: their tiny, fixed-focus lenses, which have poor light-gathering and resolving power.
We have a solution. It's modeled on the human eye, with its remarkable optical capabilities. We call it the FluidFocus lens. Like the lens of the eye, this lens, which we built at Philips Research Laboratories, in Eindhoven, the Netherlands, varies its focus by changing shape rather than by changing the relative positions of multiple lenses, as high-quality camera lenses do. Our tests of a prototype FluidFocus lens showed that it can be made nearly as small as a fixed-focus lens. Fixed-focus lenses use a small aperture and short focal length to keep most things in focus, but at the sacrifice of light-gathering power and therefore of picture quality.
At the same time, our prototype lens delivered sharpness that is easily on a par with that of variable-focus lenses. In fact, the optical quality of a liquid lens combined with a megapixel imaging chip could soon give cellphone snapshots quality that rivals images from conventional--and much bulkier--digital cameras.
The superior capabilities of FluidFocus lenses should make them ideal not only in camera phones but also in products whose design constraints demand a tiny but capable optical system. Just a few examples are pocket-size conventional digital cameras, PDA cameras, webcams, hidden security cameras, DVD recorders, and endoscopes. And with extensive bioengineering, it's even possible to imagine these lenses being a key component of a future implantable artificial eye--long a dream of ophthalmologists and science-fiction writers. The superhuman, zooming vision first popularized by the hero of the 1970s U.S. TV series "The Six Million Dollar Man" is still far off, but now, at least, we have an idea of how it might be achieved.
Conventional Autofocus Systems are not practical in today's camera phones and other portables, because they use motors and gears to shift the position of the lenses. Those assemblies are difficult to miniaturize and are vulnerable to wear. But our liquid-based lens has no moving parts or mechanical actuation, which makes it more efficient and potentially much longer-lived. Such features are a big plus in security cameras, for example, which are constantly refocusing.
The human eye focuses on objects at different distances by contracting and expanding muscles attached to the lens. The muscles change the shape of the lens and alter its focal length.
Our FluidFocus lens, on the other hand, uses electrostatic forces to alter the shape of a drop of slightly salty water inside a glass cylinder 3 millimeters in diameter and 2.2 mm long. One end of the cylinder points toward the image plane; the other is directed at the subject being imaged [see diagram, Shape Shifter].
The FluidFocus lens comprises a volume of water [blue] covered by a volume of oil [tan] inside a glass cylinder [light blue]. At the inner surface of the glass are cylindrical layers of an electrode, an insulator, and, on the very inside, a water-repellent material.
With no voltage on the electrode, the water surface is convex [top]. And because the refractive index of oil is greater than that of water, parallel light rays passing through the meniscus--the interface between the water and the oil spread out.
A voltage on the electrode attracts water molecules toward the cylinder's surface, making it act lessrepellent, and the water surface becomes concave [next diagram]. Here, parallel light rays passing throughthe meniscus converge at a focal point.