Philips’ fluid lenses bring things into focus

At the recent CeBIT exhibition in Hannover, Germany, Philips demonstrated a unique, variable-focus lens system that does away with mechanical moving parts, making it suitable for a wide range of optical imaging applications like digital cameras, camera phones, endoscopes, home security systems and optical storage drivers.

The Philips 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 Philips FluidFocus lens consists of two immiscible (non-mixing) fluids of different refractive index (optical properties).

One is an electrically conducting aqueous solution and the other, an electrically non-conducting 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 hydrophobic (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 such that it becomes less hydrophobic – a process called "electrowetting" that results from an electrically induced change in surface tension.

Because of this, the aqueous solution begins to wet the sidewalls of the tube, changing 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 a mere three millimeters in diameter by 2.2 millimeters in length, making it easy to incorporate into miniature optical pathways.

The focal range provided extends from five centimeters to infinity and is extremely fast; switching over the full focal range is obtained in less than 10 ms.

Controlled by a DC voltage and presenting a capacitive load, the lens consumes virtually zero power, giving it a real advantage over battery-powered portable applications.

Lens durability is also very high. Philips has tested the lens with over one million focusing operations without loss of optical performance.

It also has the potential to be both shock-resistant and capable of operating over a wide temperature range, making it suitable for mobile applications. Moreover, its construction is compatible with high-volume manufacturing techniques.