- Liquid Used as Lens for Miniature Cameras
Anne L. Fischer
As cell phone cameras and other optical devices become smaller and smaller, the lenses become more complex. When the surface-to-volume ratio increases, so does friction, making it difficult to fabricate tiny lenses with moving parts. As a possible solution, a group at Philips Research Eindhoven in the Netherlands has developed a variable-focus liquid lens with no moving parts.
The meniscus between two immiscible liquids forms the lens. One of the liquids is an electrically conducting aqueous solution such as a salt solution, and the other is an insulating one such as a nonpolar oil. The liquid is placed in a short tube with end caps. The tube and one end cap are treated with a hydrophobic coating that causes the solution to form into a hemispherical mass at the other end of the tube. This mass acts as a spherically curved lens.
A schematic cross section shows the FluidFocus lens principle (A). When a voltage is applied, charges accumulate in the glass wall electrode, and opposite charges collect near the solid/liquid interface in the conducting liquid (B). The resulting electrostatic force lowers the solid/liquid interfacial tension, and with it the contact angle θ and, hence, the focal distance of the lens. A 6-mm-diameter lens subjected to different applied voltages forms different shapes (C-E). Courtesy of Philips Research Eindhoven.
The shape of the lens can be altered through the process of electrowetting, which changes the focus. Applying an electric field across the coating affects the surface tension to alter the shape of the lens. Applying a high voltage flattens the lens, and decreasing the voltage returns it to a curved shape.
In their experiments with the device, the researchers used a direct voltage of approximately 50 V. They used a 1000-fps high-speed Philips VGA CMOS camera to study the meniscus from the side. By applying a voltage step and varying the insulating liquid and the cylinder diameter, they observed menisci that were underdamped, critically damped and overdamped. Critically damped lenses are of interest for use in cameras because they react quickly to a voltage step without any oscillations.
In previous attempts by other groups investigating electrowetting, a drop of liquid on a flat plate was employed. A major obstacle was the difficulty in centering the laterally unstable drop on the optical axis. The Philips researchers were able to demonstrate that the meniscus can act as a lens with adjustable focal length and without the centering problems encountered in previous attempts because the walls of the tube prevent lateral displacement of the liquid.
The optical quality of the lens is related to the radial symmetry of the meniscus. For example, nonuniformities may be caused by surface roughness or varying thickness in the coating. When measuring the meniscus using an interferometer, the researchers found that wavefront aberration was well below the diffraction limit. They measured over a diameter of 4 mm and obtained an rms optical path length difference of 0.027 l.
Another potential problem with camera lenses is chromatic aberration, caused by the dependence of the refractive index on the various colors of light. Conventional lenses use grating structures that are susceptive to haze, or they use costly color-correction methods. This problem is avoided with liquid lenses because by dissolving specific substances in the liquid we can tune their optical properties.
The variable-focus fluid lens measures 3 mm in diameter by 2.2 mm in length, making it suitable for use in digital cameras, camera phones, endoscopes, home security systems and optical storage devices. One potential problem is the harsh environments to which these lenses may be subjected. To prevent freezing, a highly concentrated salt solution with a low atomic mass, such as lithium chloride, may be used.
In tests of the performance of the lens with a VGA CMOS sensor, the group observed that the lens focused faster than the refresh rate of the sensor and that the resolution was better than that of a fixed-focus camera. After switching the lens more than a million times, it noticed no decrease in performance.
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