Down the Line: Liquid Lens Sees Potential
Engineers at Royal Philips Electronics (Amsterdam) have developed a variable-focus lens system that has no mechanical parts. Potential optical imaging applications include digital cameras, camera phones, endoscopes, home security systems and optical storage drives.
The FluidFocus system mimics the action of the human eye, using a fluid lens that alters its focal length by changing its shape. The new lens, which lends itself to high-volume manufacturing, overcomes the fixed-focus disadvantages of many of today's low-cost imaging systems.
The lens consists of two non-mixing fluids of different optical properties. One is an electrically conducting aqueous solution; the other is an electrically nonconducting oil, contained in a short tube with transparent end caps. The internal surfaces of the tube wall and one of its end caps are coated with a water-repellent coating that causes the aqueous solution to form itself into a hemispherical mass at the opposite end of the tube, where it acts as a spherically curved lens.
The shape of the lens is adjusted by applying an electric field across the hydrophobic coating so it becomes less hydrophobic-a process called "electrowetting" that results from an electrically induced change in surface tension. As a result of this change in surface tension, the aqueous solution begins to wet the sidewalls of the tube, altering the radius of curvature of the meniscus between the two fluids and hence the focal length of the lens.
By increasing the applied electric field, the surface of the initially convex lens can be made completely flat (no lens effect) or even concave. As a result, it is possible to implement lenses that transition smoothly from being convergent to divergent and back again.
The fluid lens measures 3 millimeters in diameter by 2.2 millimeters in length, making it easy to incorporate into miniature optical pathways. Focal range extends from 5 centimeters to infinity. The lens is also extremely fast: Switching over the full focal range is obtained in less than 10 milliseconds. Controlled by a DC voltage and presenting a capacitive load, the lens consumes virtually zero power, which makes it attractive for portable battery-powered applications.