What is a Liquid Lens?
By Alix Paultre
A liquid lens uses one or more fluids to create an infinitely-variable lens without any moving parts by controlling the meniscus (the surface of the liquid.) There are two primary types, transmissive and reflective. These are not to be confused with liquid-formed lenses that are created by placing a drop of plastic or epoxy on a surface, which is then allowed to harden into a lens shape.
Reflective liquid lenses are actually variable mirrors, and are used in reflector telescopes in place of traditional glass mirrors. When a container of fluid (in this case, mercury) is rotated, centripetal force creates a smooth reflective concavity that is ideally suited for telescope applications. Normally, such a smooth curved surface has to be meticulously ground and polished into glass in an extremely expensive and tricky process (remember the Hubble Space Telescope mirror fiasco?) A reflective liquid lens would never suffer from that problem, as a simple change in rotation speed would change the curve of the meniscus to the proper shape. Scientists at the University of British Columbia (UBC) have built a 236-inch (6-meter) Liquid Mirror Telescope (LMT). The world's 13th largest telescope, its reflective surface is made of a flat container of mercury spinning at about 5 RPM. The telescope costs only about $1 million, a significant reduction from the roughly $100 million cost of what a conventional telescope with a regular solid glass mirror of the same size would require.
Transmissive liquid lenses use two immiscible fluids, each with a different refractive index, to create variable-focus lenses of high optical quality as small as 10 µm (microns). The two fluids, one an electrically conducting aqueous solution and one a nonconducting oil, are contained in a short tube with transparent end caps. The interior of the tube and one of the caps is coated with a hydrophobic material, which causes the aqueous solution to form a hemispherical lens-shaped mass at the opposite end of the tube. The shape of the lens is adjusted by applying a dc voltage across the coating to decrease its water repellency in a process called electrowetting. Electrowetting adjusts the liquid's surface tension, changing the radius of curvature in the meniscus and thereby the focal length of the lens. Only 0.1 microjoules (µJ) are needed for each change of focus. Extremely shock and vibration resistant, such a lens is capable of seamless transition from convex (convergent) to concave (divergent) lens shapes with switching times measured in milliseconds. In addition, the boundary between the two fluids forms an extremely smooth and regular surface, making liquid lenses of a quality suitable for endoscopic medical imaging and other space-constrained high-resolution applications like microcameras and fiber-optic telecommmunications systems.
The aforementioned liquid-formed lenses are a cool technology as well, and used mostly on image sensors. Tiny drops of epoxy are placed on each pixel, which then form individual lenses to increase light-capturing ability. They are also used on novelty items to create a magnifying effect.
The Large Zenith Telescope at the UBC Malcolm Knapp Research Forest in Maple Ridge, British Columbia measures six meters in diameter. The adjustable mirror is created by rotating a pan of mercury.
The Philips FluidFocus lens consists of two immiscible (non-mixing) fluids of different refractive index (optical properties), one an electrically conducting aqueous solution and the other an electrically non-conducting oil, contained in a short tube with transparent end caps. Researchers show the miniature variable lens and the camera that contains the lens. Photo: Philips
Another kind of liquid lens technology is used for novelty items like these from X-trude FX Systems. liquid-formed lenses like these are created by placing a drop of plastic or epoxy on a surface, which is then allowed to harden into a lens shape, creating a magnifying effect.