At the turn of the eighteenth century, Stephen Gray, a British astronomer, carefully positioned droplets of water on tiny holes he had meticulously bored through thin sheets of brass. Gray realized that by exploiting the way in which the droplets could bend light, it was possible to use them to magnify the appearance of objects. This unusual set-up was to become the first microscope known to use lenses made out of liquid.
However, despite Gray's initial success in manipulating the optical properties of these lenses — by poking them with very small needles — it would take another three centuries and the unlikely catalyst of a boom in the cell-phone industry for the idea to finally catch on.
Over the next few years there is likely to be a massive shift as the traditional solid glass and plastic lenses used in camera phones, among other devices, are replaced with liquid ones whose shape and optical properties can be very precisely controlled using tiny electrical currents (Fig. 1).
Figure 1: Fun with fluids.
By controlling the curvature of the meniscus of a liquid droplet it is possible to create a variable focal length lens that mimics the function of the human eye. Here a 2-mm-diameter liquid lens is shown for four different pressures applied by a piezoelectric pump.
IMRE
Full size image (39 KB) Figures, schemes & tables indexIn effect, a spherical lens is formed by the meniscus of a liquid. By electrically manipulating the shape of a meniscus it is possible to create an autofocusing lens that can switch from a convex-shaped lens with a tight focus to a concave, divergent lens.
The shape-changing process typically requires a few tens of volts and takes just a few milliseconds. The result is a high-quality, compact autofocus lens that can tune its focal length from just a few centimetres to more than one metre.
Three organizations — the French start-up Varioptic, Philips of The Netherlands and the Institute of Materials Research and Engineering (IMRE) in Singapore — are now racing to turn the idea into a commercial reality and hopefully cash in on the booming market for camera-equipped mobile phones (Fig. 2).
Figure 2: Mobile magic.
A mobile phone equipped with a camera containing a prototype 'liquid lens' zoom system developed by IMRE in Singapore.
IMRE
Full size image (79 KB) Figures, schemes & tables indexAccording to John Barber, Vice President of Business Development for Varioptics in Lyon, France, the main advantage of the liquid-lens approach is that autofocus and zoom lenses can be made without the need for any mechanical moving parts. Current autofocus and zoom optics involve moving a rigid lens relative to the camera sensor. But such mechanisms are fragile and prone to failure or accidental damage.
With liquid lenses, these concerns disappear because there is nothing to break, says Saman Dharmatilleke, a liquid-lens researcher from IMRE. "In the liquid lens, we try to mimic the human eye by changing the shape of the liquid lens to focus an image onto a sensor," he says.
This also has a significant impact on how much space these lenses take up, making them ideal for a range of applications where size is an issue. Besides camera phones, liquid lenses will start to appear in anything from surgical endoscopes and barcode readers to security cameras and the next-generation Blu-ray DVD players (Fig. 3).
Figure 3: Compact answer.
A 'FluidFocus' lens developed by Philips. The Dutch company says that the lenses are as good as glass optics and has demonstrated their ability to read dual-layer Blu-ray optical discs — a successor to DVDs. Using this technology, the firm has also constructed a miniature autofocus camera that is just 5.5 mm high and has a lens diameter of 3 mm and focusing time of 10 ms.
PHILIPS
Full size image (27 KB) Figures, schemes & tables index"It's a field with enormous possibilities," says Stein Kuiper, a senior scientist with Philips. "We are discussing with several companies about licensing it." Not only are liquid lenses more compact, but they are also cheaper, more robust and will probably consume a fraction of the power of mechanically actuated lenses, meaning that batteries will last longer.
For designers of mobile phones this is a compelling proposition. After all, nobody wants to have a camera-equipped mobile phone that is bulky and fragile or runs out of power after a few hours just because a few pictures have been taken.
To date, there are two main approaches being developed. One, being pursued by both Varioptic and Philips, uses a phenomenon called 'electrowetting' to manipulate the surface tension between two neighbouring liquids, one being oil and the other water-based (Box 1).
The other approach, developed by IMRE scientists, relies on piezoelectric pumps to increase the pressure of a liquid within a channel, causing either a convex bulge or a concave-shaped hollow, depending on the pressure (see Box 2). Both approaches consume little power (of the order of milliwatts), and can withstand severe shaking without loss of function. Similarly, both approaches are at an advanced stage of research and are poised to hit the market.
"We are on the cusp of that right now," says Barber. Varioptic already has a production line up and running, and in January this year it announced general availability of its Arctic 320 autofocus liquid lens for camera phones (Box 3). A few months later the French firm secured third-round financing of 16.4 million euros and is now recruiting staff and expanding its business.
Webcams sporting Varioptic's lenses are likely to appear on the shelves by the end of the year with the first camera phones arriving in 2007. However, the other firms are likely to be hot on its heels. IMRE has also made significant strides forward with its commercialization plans, recently licensing its technology to a local firm in Singapore called PGS Precision, which specializes in injection moulding. Meanwhile, Philips confirms that it is exploring the opportunities for licensing its technology.
So what do the makers of cell phones think? "The rate of development in camera phone image quality is very fast," says Janne Haavisto, Director of Audio-Visual Systems at Nokia in Espoo, Finland. "We know of all these potential benefits of liquid lenses but we are constantly looking for extreme performance for the optics," he says.
As the number of pixels in camera phone image sensors steadily increase, the pixels get smaller and better lenses are required. "It's a trade off," says Haavisto. "The smaller you make the individual camera pixels the sharper the lens has to be," he explains. "And of course, smaller sensors capture less light".
With conventional lenses, one way to increase the amount of available light is to increase the size of the lens; with liquid lenses, however, this could become a problem. Provided the lenses are small enough it is possible to achieve a curvature and smoothness of the meniscus that rivals polished glass — with aberrations occurring almost at the molecular level. However, as Benno Hendriks from Philips points out, as liquid lenses become larger gravity starts to have an effect, deforming this perfect curvature.
Another limitation, particularly with electrowetting lenses, is that to ensure that the two liquids do not mix, they must be immiscible with matched densities and responses to thermal changes. "This is what makes them so robust," says Hendriks. "It's practically impossible to mix the liquids," he says. However, such matching restricts the number of liquids available and so limits the range of their optical properties.
According to Dharmatilleke this restriction is less of a problem for the pressure lens approach. Using a single liquid means there is a much broader range of liquids to choose from. As a result, IMRE has been able to create a lens with an optical transmittance as good as Carl Zeiss glass lenses. "Electrowetting approaches are not far behind," says Barber. "Their transmittance is still very good," he says, "as high as 95%", and Barber is confident that it won't take long before this can be increased to 98% to match the quality of glass.
