INTRODUCTION

INTRODUCTIONThe theory behind liquid lens is based on the properties of one or more fluids to create magnifications within a small amount of space. Liquid lens can be considered as "infinitely variables" lens with variable focus, and the focus is controlled without using any moving parts. The focus of a liquid lens is controlled by the surface of the liquid. Water forms naturally a bubble shape when adhered to materials such as glass or plastics. This desirable property makes water a very suitable candidate for the production of a liquid lens. To generate a liquid lens, a mixture of two liquids is sandwiched between two pieces of clear plastics or glass. The second liquid needs to encapsulate the water drop and to fill any free space or void. It is well known that water and oil do not mix, and oil is also inexpensive and safe to use. Therefore, oil is chosen to be used as the other liquid mixture for the liquid lens system. The surface profiles of the liquids determine the focal length of the liquid lens system, and ultimately, how the liquid lens focuses light. In other words, by altering the surface profile of the liquids, the focal length can be adjusted. This is done by changing the shape and size of the drop of water within the liquid lens.

IMRE has made a breakthrough in lens technology. The lens is cheaper to make has optical zooming abilities and uses only a fraction of the space of most conventional lenses are called as fluidlens or liquidlens. In the past 2-3 decades, the need for miniaturization of optical systems has increased dramatically, especially incoherent light handling, for various applications including communications, data storage, security or personal identification. More recently this trend has extended to imaging systems. Nowadays camera modules, integrating a digital sensor and an optical system altogether, have entered into mobile phones and slim digital cameras, bringing the need for develop in miniature optical systems.

The camera module were developed first with low count pixels and ultra small format sensors (CIF resolution, single element lens), but the need for better image quality leads now to the development of mega pixels sensors, 1/4 or less. These sensors are now commercially available, but the need for auto focus and zoom compound lenses remains open: no commercial solution exists up to now at reasonable prices for this very large scale market.

The liquid lens technology that we present here could be the solution to this demanding application.A new principle of variable lenses with tunable focal length will be demonstrated : two iso-density non-miscible liquids are trapped inside a transparent cell. The liquid-liquid interface forms a drop shape. The natural interfacial tension between liquids produces a smooth optical interface, which curvature is actuated by electrowetting. In addition, in order to have a usable lens, it is necessary to incorporate a centering mechanism, such that optical axis remains stable. Intrinsic physical limitations will be presented as well as actual performances of the technology. Several applications will be discussed in the autofocus/macro/zoom optics for CMOS and CCD miniature imagers. But, because the technique relies on the surface tension of the liquids inside the lens, it cannot be used to make lenses larger than a centimetre in diameter. This would place a limit on the resolution of images.Nonetheless, Kuiper believes that FluidFocus lenses could be especially useful for reading from Blu-Ray DVD disks, which store information more densely than ordinary DVDs. Blu-Ray players require highly accurate optical systems capable of adjusting for distortions that naturally occur during dual layer disc reading and writing.

The FluidFocus lens will be demonstrated at the technology fair CeBit, in Hannover, Germany, next week. Kuiper says the first devices that incorporate fluid lenses be available by 2005.What is a liquid _ ^lens?

To generate a liquid lens, a mixture of two liquids is sandwiched between two pieces of clear plastics or glass. The second liquid needs to encapsulate the water drop and to fill any free space or void. It is well known that water and oil do not mix, and oil is alsoinexpensive and safe to use. Therefore, oil is chosen to be used as the other liquid mixture for the liquid lens system.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 pm (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 micro joules (pJ) 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 micro cameras and fiber-optic telecommunications 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.

WORKING PRINCIPLE:The magnifying principle of a liquid lens is similar to that of our eye. When we try to see an object, the light which comes from the object falls on our eye ball. Our eye ball(pupil) has the ability to contract or expand itself depending upon the position of the object. Which then leaves the perfect light ray to fall on the retina which results visibility of the object.If the pupil cant adjust itself then we are not able to see the object.The liquid lens acts on the same principle.

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The lens has an actuator which is driven by the dielectric power. Which results in adjustment of the lens, hence we are able to take the picture.The figure below depicts the configuration of a liquid lens actuated by the dielectric force. The liquid lens consists of a 15pL (liquid) droplet with a low dielectric constant and a sealing liquid with a high dielectric constant. The bottom diameter of the droplet was 7mm when no voltage was applied. The two liquids were injected inside a 3mm thick PMMA (polymethyl methacrylate) chamber that was sealedbetween two ITO glass substrates. The concentric ITO electrodes on the bottom glass substrate were coated with 1pm thick Teflon to reduce friction between the droplet and the glass substrate. The width and spacing of the ITO electrodes was 50pm. The mass density of the sealing liquid was adjusted to match that of the droplet to minimize the gravitational effect, since the gravitational effect may induce no uniform deformation of the droplet profile, causing optical aberrations. As the voltage was applied, a dielectric force arose on the droplet due to the difference in the dielectric constant between the two liquids. The dielectric force shrunk the droplet, increasing the droplet's contact angle and shortening the focal length of the liquid lens. The dielectric force induced is described by equation given below

where z0 is the permittivity of free space, z1 and z2 are dielectric constants of the sealing liquid and the droplet, respectively. E denotes the electric field intensity across the interface of the two liquids

( fig. A typical liquid lens)

,...oooo..,Electrowetting Principle:

This principle states that whenever no voltage is applied to the drop of liquid then it is phobic of the surface. As the voltage increases the liquid wets the surface more.This principle helps in adjusting the lens which can make the lens to behave like concave & convex as per the requirement.

Two non miscible liquids of same density instead of having a water drop in air, one works with water and oil. This condition is necessary for suppressing any optical distortion of the gravity on the liquid-liquid interface, which enables to use the lens in every orientation.

Inversion of the conducting and non conducting fluid. In current electrowetting experiments, the water is used as a drop immersed inthe non conducting fluid (air). For application, it will be preferable to work with a drop of the insulating fluid (oil) immersed in the conducting fluid (water). This is to avoid any optical perturbation of the liquid-liquid interface due to the liquid meniscus at the electrode touching the conducting fluid. It is preferable to use an oil drop immersed into a conducting fluid (water based solution) which can be connected to the outside without perturbing the liquidliquid interface. This inversion is not strictly necessary, as in former publications it is mentioned that contact could be made through the insulating . Nevertheless, in practical realization the inversion of oil and water is preferable. Centering mean : some publications have mentioned in the past how to use small liquid droplets as optical lenses [6], but if this lens has to be inserted in a more complex system, it needs precise alignments of optical lenses. The fig 1 shows that if no centering mean is applied, the drop can freely move in the transverse directions while the focal length is changed.We have the experience of such random displacements which prevent to use the lens. The liquid-liquid interface thus needs then to be precisely controlled and any physical realization of lenses have to incorporate such a centering mean This centering of the liquid-liquid interface can be obtained by several ways. The following are given as example, and many others can be found: applying electric field gradient using variable thickness of the insulator film. the natural gradient present at the edge of an electrode can be used. In the case of lenses developed by Lucent, a decentering force can be applied through angular sector electrodes. Such decent ring force can only be used if a centering force (restoring force) exists, such that the balance between the decentering forces and the centering forces can bring a stable equilibrium. Although this was not explicitly discussed in the publications of Lucent, we believe that in their case the centering effect comes either from the edge effect of the ring electrode, or from the gap between sector electrodes, which could play this role too, if well designed.it can also be obtained as a result of the geometry of the supporting surface for the two fluids. It has been shown that inwards cones, cylinder and some toroidal shapes are centering surfaces for the liquid-liquid interface. Cylinder the contrary, some surfaces having an inward high insides and cylinder edges have also been proposed. On curvature are not suited for centering the liquid-liquid interface.

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13WORKING DIFFERENCE BETWEEN CONVENTIONAL & LIQUID LENS Liquid lens working like human eye. So here we compare conventional lens with human eye.Conventional CameraLength between lens and focal plane adjusted to focus image.Human EyeShape and curvature o the lens changed to focus image

Leus stietchiug to incleast focal le

The optical characteristic of the liquid lens that were measured experimentally included the droplet's contact angle and its hysteresis, the conic constant of the droplet, focal length tuning, and focal spot size. Further, focal length tuning, focal spot size and spherical aberration were verified using simulation tools and theory. The contact angle of the droplet in the packaged liquid lens was measured at various voltages as shown in Figure 2(a). The intrinsic contact angle of the droplet was measured to be 25. The contactangle began to significantly increase at voltages over 50 volts and reached 58 at 200 volts. Hysteresis of the droplet's contact angle was observed and its maximum was found to be 12.5 at 120 volts. Figure 2(b) shows that the conic constants of the droplet were close to zero at various voltages, implying that the droplet maintained a spherical profile at all focal lengths. Hence, the surface profile of the droplet could be assumed to be spherical during actuation. The actuation of the droplet in the liquid lens was captured by a highspeed CCD camera. The rise time was measured to be about 650ms when the liquid lens was actuated from the rest state to 200 volts. When the applied voltage was switched off, the measured fall time was 300ms.Fig. The measured receding contact angles and advancing contact angles of the droplet in a liquid lens versus the applied voltages. The insets show the droplets actuated at various voltages. Left: the droplet was at the rest state. Right: the droplet was actuated at 200V.

s300>3OA 1=P1 *Applications:Applications of the liquid lenses based on electro wetting can be found in many areas. Typical possible sizes for the lens pupil range from less than a millimeter to one centimeter, using the current technology. This makes this technology ideal for millimetrtic lenses needed now in the mobile phone applications. The very small power consumption (less than one mW dissipated in the lens) is also a great advantage compared to conventional motorized systems. All electronic sets integrating optics could benefit from the simplicity of this technology. Optical pickups, displays, cameras, computers etc... Again the size under consideration is well fitted between macro- and microscopic systems. Of course photonic professional applications could also present good opportunities for our technology. Many other applications could be envisaged. The liquid ends

(a)

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19is one adaptive optical components, with a huge amplitude, but rather limited flexibility on the pattern of phase shifts, limited to what can be done with a liquid liquid interface. Directly every application where Z scanning is required could be of interest: the dynamic behavior shown in this paper demonstrates s possible to apply to the lens a triangular ramp (eventually damped in order to avoid shocks generated at the reversing of the ramp) in order to use the full range of dioptric correction upon very fast scans. Telemetry could use focus information in order to produce 2D and 3D images at quite good resolutions. Medical applications could also be very promising, as endoscopes develop on many complex optical functions including confocal microscopy or Optical Coherent Tomography .

Telecommunications: Optical switches, fibre optic coupling, mobile phone cameras, webcamsData storage: CD, DVD, barcode readersAnalytical equipment: Portable ^5": microscopes, Sensors"ft/1 om ifa/'ti irinn1 I aeair to/*hnAlAnw

As a final remark, lasers could be monitored or controlled by the liquid lens.Liquids can sometimes produce interesting properties as materials, which could be of use, even in high power pulsed laser systems.It hasDyamic field of view.Easy to mass produce: manufactured using lithography. Difficulties:E.3E13E13iLens for cameras:Far from being a fully developed product.

Varioptic and Sunny Optics announced last week that they were making the Varioptic Arctic 416 auto focus oil and water lens available in high-end camera phones. The oil and water lens, which has no moving parts, replaces traditional mechanical lens focusing systems.

(Oiland water lensuseselectrowetting)The Varioptic oil and water lens uses a phenomenon called "electrowetting" to focus the system. A water droplet is deposited on a metal substrate covered by an insulating layer. The voltage applied to the substrate modifies the contact angle of the droplet. A liquid lens uses two liquids with the same density; one is an insulator while the other is a conductor. The variation of voltage leads to a change of curvature of the liquid-liquid interface, which in turn leads to a change of the focal length of the lens.Liquid lenses have many advantages over their mechanical counterparts, including ruggedness (no moving parts), faster response, excellent optical quality, wide operating temperaturerange and very low energy consumption (ideal for small mobile devices). Science fiction fans have been waiting for this since Frank Herbert wrote about oil lenses in his 1964 classic Dune:Paul lay ... in a slit of rock high on the shield wall rim, eye fixed to the collector of a Fremen telescope. The oil lens was focused on a starship lighter exposed by dawn in the basin below them. (Read more about oil lens from Dune)Philips is also working on this technology (there may be some patent fights involved); see Philips FluidFocus: Variable Focus Fluid Lens. Varioptic is ramping up production in (where else) Shanghai and expects to produce 100,000 lenses per month in addition to the production in its plant in Lyon, France. Read more in the 2MP Autofocus Camera Module with Varioptic Liquid Lens press release and Varioptic comes into focus with liquid phone camera lenses. Scroll down for more stories in the same category. (Story submitted 2/18/2007) .Schreiber and colleagues worked with Varioptic, French pioneers of liquid lenses, to come up with a design that switches from a normal view to 2.5-times magnification. The design consists of four liquid lenses and three fixed plastic lenses and offers a magnification of 2.5 times, while when all four lenses are at their flattestthereisnomagnification.The complete length of the system from outer lens to image sensor is 29mm, but it should be possible to reduce that, says Schreiber. Varioptic is now considering how to take the design on tothenprototypestage.The lenses are arranged to prevent image distortion while minimising colour distortion. Red, green and blue images must be recorded in sequence and then combined digitally, a process that would increase exposure times, says Schreiber, finding less distorting liquids to build the lenses out of is the answer to that problem.So although it potentially sounds like great news, this is probably another new technology which wont find its way into DSLR cameras for a few years yet. For smaller lenses such as camera phones it could find a market, but well have to see how this one pans out.Conclusion: