Sept. 27, 2007 – At the epicenter of a photographer’s camera is the lens. While most photographers are accustomed to traditional mechanical lenses made of glass or plastic, they could soon see their electronics instead use liquid crystal or liquid lenses. University of Central Florida (UCF) researchers have developed two types of adaptive lens technology, liquid crystal (LC) and liquid lenses. With electric voltage or applied compression, respectively, the UCF scientists can control the focal length and aperture of these adaptive lenses. Because of their compact size, greater zoom abilities, and inexpensive production costs, the adaptive lenses could be used for camera phones and digital still cameras, according to UCF researchers.
A Different Kind of Lens: Adaptive Lens
Adaptive lenses have already been implemented in simple cameras in mobile phones. Unlike traditional lenses, adaptive lenses require no moving parts and therefore consume less power.
“We have several approaches, but each one has its own complexity,” said UCF professor of Optics and Photonics Dr. Shin-Tson Wu in an interview with DigitalCameraInfo.com. “Every approach has its own pros and cons."
Making Liquid Crystal Lenses with Electricity
The first adaptive lens technology is liquid crystal lenses. Most gadget lovers are already familiar with liquid crystal material, which is common in the LCDs (liquid crystal display) of laptops, computers, and television sets. The same liquid crystal material can be applied to zoom lenses, according to Wu, only with better light sensitivity.
Many lenses, such as those in sunglasses, use polarizers, or thin plates, to filter incoming light. The LC lenses, on the other hand, are “polarization independent," which is advantageous, said Wu. Since LC lenses do not require polarizers, the liquid crystal lens allows more ambient light to pass though and, therefore, increase light sensitivity.
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Fabrication process of parabolic electrode: (a) put a drop of prepolymer on the top substrate surface, (b) turn the substrate up-side down and cure the polymer by UV light, (c) coat a conductive polymer layer as electrode, and (d) flatten the lens surface by the same polymer.
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What makes the UCF model unique is the construction of the liquid crystal lens. The liquid crystal is sandwiched between two flat substrates that confine the liquid. The top substrate has a deposited curved indium-tin-oxide (ITO) electrode, a well-known transparent conductive. The bottom substrate has a flat ITO. Together, the planar and parabolic electrodes generate a gradient refractive index profile.
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| Device structure of a LC lens cell. The top electrode is a conducting polymer and bottom electrode is ITO. |
“This shape is new, and that's why we can get such a short focal length," said Wu. "This design is really superb compared to prior ones."
With standard zoom lenses, users have to mechanically change focal length. With LC lenses, users can adjust focal length electrically.
The researchers then apply AC (alternating current) voltage to activate the liquid crystal and create an inhomogeneous electric field, which in turn creates the curvature of the liquid lens. The more voltage applied, the shorter the focal length becomes.
Researchers were able to achieve a short focal length of 15 centimeters with the liquid lens. That short focal allows users to capture subjects very close to the lens.
These LC lenses may be attractive to manufacturers because they are easy to fabricate and exhibit benefits of a wide focal length, low power consumption, stability, and a fast response time.
“Our new design has a 25 times faster response time than [other LC structures],” Wu said.
Unlike mechanical lenses, though, liquid crystal lenses are susceptible to temperature changes. As temperatures increase, focal length will become shorter. Liquid crystal can operate in -10 to 50 degrees Celsius.
“We are overcoming these problems one-by-one,” said Wu. “With continuing research, we can make liquid crystal lenses more practical."
The Pressure is on for Liquid Lenses
While liquid crystal lenses depend on electric voltage, the second technology of variable focus liquid lenses depends on applied pressure. This liquid lens is constructed of an elastic membrane, rubber membrane, glass plate, and liquid (in this case, water). In its dormant state, there is no pressure applied. When force is placed on the elastic rubber membrane using a string or by hand, the liquid causes the elastic membrane to swell into a convex lens. By changing the curvature of the elastic membrane, the researchers were able to adjust focal length. With the shortest possible focal length of just 2 centimeters, liquid lenses produce a greater range of focusing power, Wu said.
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Structure of a liquid lens cell: (a) top slab, (b) bottom slab, and side view of the lens cell in (c) non-focusing and (d) focusing states.
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Like the liquid crystal lenses, the liquid lens is independent of polarization, allowing for more light sensitivity. The liquid lens also only requires low voltage, like the liquid crystal.
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| Focusing behavior of the liquid lens in (a) planar state and (b) focusing state. The aperture of the lens is 15 mm and the lens thickness is 6 mm. |
The liquid lens, called a “broadband device,” according to Wu, can focus light for any width in the visible region.
As with any emerging technology, there are a few drawbacks. The liquid lens is mechanically driven and relies on mechanical adjustments, that is, applied pressure to the membrane. Also, as Wu points out, “Any liquid could leak.”
While liquid lenses are cost effective, the major costs are attributed to the lens system that prevents the liquid from leaking.
With greater zoom capabilities, the liquid lens is ideal for cameras in mobile phones. UCF is currently investigating infrared possibilities to use an IR transparent substrate with liquid lenses, Wu said.
Currently, the UCF technology is under a licensing agreement with specialized lens supplier Holochip, Corp., which holds exclusive worldwide rights to five of the lens patents. Holochip provides adaptive optics for camera manufacturers and the medical, military, and automotive fields. According to the abstracts, adaptive lenses can have far-reaching applications for optical signal processing, eyeglasses, and zoom lenses for camera phones and digital still cameras.
Images courtesy of University of Central Florida, College of Optics and Photonics.
