Liquid Lenses Take Various Forms
Research groups have produced dielectric and wireless devices.
Michael J. Lander
Having the potential to allow compact adaptive optical features in many settings, liquid lenses have captured the interest of numerous researchers. In two published studies, scientists describe an adaptive lens that could be used for mobile phones and another system that might one day be implanted in the eye.
An electrically activated liquid lens synthesized by Taiwanese researchers appears small compared with the one-yuan coin beside it. Courtesy of J. Andrew Yeh, National Tsing Hua University.
At National Tsing Hua University in Hsinchu, Taiwan, researchers led by J. Andrew Yeh designed a liquid lens that requires little energy and occupies a small volume. In space-limited contexts, the lens may prove more practical than those based on fluid redistribution, which require bulky accessory fluid reservoirs. Unlike in lenses that use electrowetting, no evaporation or microbubble generation occurs in the system. Although previously synthesized devices using the dielectric effect overcame these problems, their liquid crystal filling yielded blurred images at lower ambient temperatures.
The assembly housing the lens comprised a plastic ring covered on either side with glass plates. On the bottom plate, the investigators placed a 15-μl drop of fluid with a low dielectric constant. They filled the rest of the chamber with a liquid of equal density but a higher dielectric constant to encourage subsequent interaction between the chemicals. The fluids had different refractive indices as well, a key factor in producing a focusing effect.
Researchers in the Netherlands analyzed the optical properties of a wireless adaptive lens submerged in a clear aqueous bath. Courtesy of Aleksey N. Simonov, Technische Universiteit Delft.
Equally important for the device’s operation was a series of concentric ring electrodes beneath the liquid of low dielectric constant. Under no current, the droplet spread itself out over the electrode region with a contact angle of 25°. Applying a 200-V, 1-kHz current, however, induced a dielectric force that decreased the droplet’s diameter, increased its height and changed its contact angle to 58° within 650 ms, as established using a high-speed CCD camera.
As these changes occurred, the focal length of the lens decreased from 34 to 12 mm — a fact verified with a 532-nm laser and a beam profiler. In practical terms, these values suggest that, in a camera, the lens could automatically focus from a few centimeters to infinity. Focal spot size remained approximately 80 μm at all tunable focal lengths, meaning that such devices would produce images of uniform quality. Finally, power consumption was around 1 mW, low enough for compact applications. Some spherical aberration of the lens occurred, but it remained constant for focal lengths greater than 20 mm, which would make designing an aberration correction system for the lens much easier.
According to Yeh, the most difficult task in creating the lens was finding suitable liquids. Not only did the substances have to possess different dielectric constants but they also had to fulfill about 10 other criteria, including immiscibility, an appropriate difference in refractive index and the ability to stay in the liquid state at operating temperatures.
Despite the technology’s advantages, other challenges may slow its adoption. The researchers have tested the device’s response to shock and thermal cycling but must analyze the lens for electromagnetic compatibility and interference. The device is inexpensive, however, requiring less than $1 worth of material — three times less than current systems. If the prototype performs well in future testing, the scientists may adapt it for use in camera phones, endoscopes and security cameras.
In related research, scientists led by Gleb Vdovin at Technische Universiteit Delft in the Netherlands crafted a device based on an established technology — that of the adaptive modal liquid crystal lens. In contrast to older designs, however, their lens is controlled externally, without wires.
Planoconvex glass lenses served as the outer sections of the device. To the flat inner surface of one lens, the investigators applied a highly conductive transparent film. A film with low conductivity was deposited on the flat side of the other glass. After placing a rubbed polyimide layer over both surfaces, they filled a narrow gap between them with nematic liquid crystals and sealed the halves together. Exposed to an alternating current, changes in electrical impedance between the high- and low-conductivity surfaces would change the crystal layer’s refractive index, permitting adjustment of the lens.
A radio-frequency control system for the device consisted of a rectifying diode and an antenna encircling the completed lens. The diode took radio waves collected by the antenna and turned them into an AC voltage. Supplying the signal was an external transmitter with adjustable amplitude and frequency. To allow sufficient power generation, the researchers mounted the transmitter within 8 cm of the lens.
To measure the optical properties of the device, they used a Melles Griot HeNe laser emitting at 543.5 nm and a Shack-Hartmann wavefront sensor from Flexible Optical BV of Delft. Exposed to no radio signal, the 5-mm lens possessed a focusing power of +12.5 diopters because of the convex profile of the glass. When an amplitude-modulated radio-frequency control signal of ∼6 MHz was applied — translating to a voltage of 2.83 V at the lens — focusing power increased by 2.51 diopters within 4 s. Total power drawn by the system remained in the milliwatt range.
With a pupil-size aperture and low voltage and power requirements, the lens design may lend itself to implantation into the capsular bag of the eye. Because reading from a typical distance requires a focus of 2.5 diopters, for example, the technology could help individuals who have lost the ability to focus. Additionally, devices that rely on techniques such as electrowetting require high voltages that make them unsuitable for implantation.
Use of the lens in the human eye, however, remains far in the future. For one thing, the flexibility offered by the wireless design could become problematic if not regulated by the brain signals or other bodily responses. The researchers are working on biofeedback algorithms. Those wishing to further apply the technology also will have to address biocompatibility. Most likely, a silicone-based sac would house the entire lens assembly to allow use in the eye.
The scientists also seek to address slow response time. Optimally, they could obtain a settling time of 0.5 s. Although this may prove difficult because focusing power and response time are inversely proportional, they could eliminate the device’s requirement for polarized light through the use of a second liquid crystal layer. With a unique potential for remote control, the lens could initially see service as an adjustable lens for machine vision applications in industrial settings.
Optics Express, June 11, 2007, pp. 7140-7145 and pp. 7468-7478.
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