fluid focus lens

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ABSTRACT:- The camera phone is one of the hottest-selling items in all of consumer electronics. The little gadgets have become so ubiquitous that hardly anyone finds it odd anymore to see tourists squinting with one eye while pointing their cell phones at a Buddhist temple, a Greek statue, or a New York City skyscraper. It's easy to see why analysts expect that camera phones will outsell conventional digital cameras and traditional film cameras combined. But as anyone who has ever seen them can attest, the images that come out of camera phones leave plenty to be desired. Part of the problem is their CMOS imaging chips, which typically have a sensor array of only about one mega pixel a half or less of the number in a low-end digital camera. When they are, however, the only thing we may see more clearly is the other weakness of these cameras: their tiny, fixed-focus lenses, which have poor light-gathering and resolving power. Here is a solution. It's modeled on the human eye, with its remarkable optical capabilities. It is called the Fluid Focus lens. Like the lens of the eye, this lens, which we built at Philips Research Laboratories, in Eindhoven, the Netherlands, varies its focus by changing shape rather than by changing the relative positions of multiple lenses, as high-quality camera lenses do .The tests of a prototype Fluid Focus lens showed that it can be made nearly as small as a fixed-focus lens. Fixed-focus lenses use a small aperture and short focal length to keep most things in focus, but at the sacrifice of light-gathering power and therefore of picture quality. At the same time, the prototype lens delivered sharpness that is easily on a par with that of variable-focus lenses. In fact, the optical quality of a liquid lens combined 1

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Page 1: Fluid Focus Lens

ABSTRACT:- The camera phone is one of the hottest-selling items in all of consumer electronics. The little gadgets have become so ubiquitous that hardly anyone finds it odd anymore to see tourists squinting with one eye while pointing their cell phones at a Buddhist temple, a Greek statue, or a New York City skyscraper. It's easy to see why analysts expect that camera phones will outsell conventional digital cameras and traditional film cameras combined. But as anyone who has ever seen them can attest, the images that come out of camera phones leave plenty to be desired. Part of the problem is their CMOS imaging chips, which typically have a sensor array of only about one mega pixel a half or less of the number in a low-end digital camera. When they are, however, the only thing we may see more clearly is the other weakness of these cameras: their tiny, fixed-focus lenses, which have poor light-gathering and resolving power. Here is a solution. It's modeled on the human eye, with its remarkable optical capabilities. It is called the Fluid Focus lens. Like the lens of the eye, this lens, which we built at Philips Research Laboratories, in Eindhoven, the Netherlands, varies its focus by changing shape rather than by changing the relative positions of multiple lenses, as high-quality camera lenses do .The tests of a prototype Fluid Focus lens showed that it can be made nearly as small as a fixed-focus lens. Fixed-focus lenses use a small aperture and short focal length to keep most things in focus, but at the sacrifice of light-gathering power and therefore of picture quality. At the same time, the prototype lens delivered sharpness that is easily on a par with that of variable-focus lenses. In fact, the optical quality of a liquid lens combined with a good imaging chip could soon give cell phone snapshots quality that rivals images from conventional- and much bulkier- digital cameras.

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1. INTRODUCTION The camera phone is one of the hottest-selling items in all of consumer electronics. The little gadgets havebecome so ubiquitous that hardly anyone finds it odd anymore to see tourists squinting with one eye whilepointing their cell phones at a Buddhist temple, a Greek statue, or a New York City skyscraper. It's easy to see why analysts expect that camera phones will outsell conventional digital cameras and traditional film cameras combined. But as anyone who has ever seen them can attest, the images that come out of camera phones leave plenty to be desired. Part of the problem is their CMOS imaging chips, which typically have a sensor array of only about one mega pixel?a half or less of the number in a low-end digital camera. When they are, however, the only thing we may see more clearly is the other weakness of these cameras: their tiny, fixed-focus lenses, which have poor light-gathering and resolving power. Here is a solution. It's modeled on the human eye, with its remarkable optical capabilities. It is called the FluidFocus lens. Like the lens of the eye, this lens, which we built at Philips Research Laboratories, in Eindhoven, the Netherlands, varies its focus by changing shape rather than by changing the relative positions of multiple lenses, as high-quality camera lenses do.The tests of a prototype Fluid Focus lens showed that it can be made nearly as small as a fixed-focus lens. Fixed-focus lenses use a small aperture and short focal length to keep most things in focus, but at the sacrifice of light-gathering power and therefore of picture quality. At the same time, the prototype lens delivered sharpness that is easily on a par with that of variable-focus lenses. In fact, the optical quality of a liquid lens combined with a good imaging chip could soon give cell phone snapshots quality that rivals images from conventional- and much bulkier- digital cameras. Recently a number of new optical applications havearisen, which require a high degree of miniaturization. Photonics, optical communications, optical sensors,optical pick-up for CD/DVD reading or writing, aswell as miniature cameras for the security or theconsumer electronic markets are demanding opticalparts in the 1-10mm range. Whereas the realization oflens elements of this sizes is well mastered, using plastic or glass materials, the problems of actuation ofsuch lenses for bringing autofocus, or zooming functions is still a challenging problem.Optical engineers have naturally looked at adaptive optics coming from high-end scientific world to correc tthe wavefront curvature, in order to enable closed loop. Variable focus lenses offer these optical functions while having no moving part. There have been a number of principles of variable focus lenses whichhave been experienced since about 60 years, starting with the work of Graham1 who developed a principleof deformable

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chamber filled with liquids. More recently the work of many companies of industries hasbeen oriented to produce solutions at a very small scale.Liquid crystals actuators have been known since a longtime to produce wave front corrections, which are easy to configure in a fully addressable 2D pattern. Severalpractical realizations have come though, as in the group of Commander. Recently a simplified version has been produced by Naumov et al3, using aspecial design of electrodes which allows a gradient of electric field, thus producing a variable focal lens. The main drawback of the liquid crystal lens is that the amplitude is rather small, achieving a few dioptries of optical power variation. In the recent past, we could introduce a new principle of liquid lenses based on electrowetting. While the principle of manipulating a liquid droplet was known since a long time7-8, these principles were restricted to a single lens element systems, due to the lack of precise control of the optical axis. One can show that major advantages of these liquid lensesare coming from the small electrical dissipation, thereduced size, the long lifetime under voltage cycling (which is a side advantage of the no-moving part principle). In this paper, we will review first the operation of the liquidlens, on a given design example. The second paragraph will then examine quantitatively the most important feature of the liquid lens design: the centre-alignment of the liquid drop. Then we will present experimental results of our current lens, comparing with the theoretical model. This is a a unique variable-focus lens system that has no mechanical moving parts. This can come into application in areas such as digital cameras, camera phones, endoscopes, home security system, optical storage devices etc. It may consist of two immiscible fluids of different refractive indices: one a electrically conducting aqueous solution and the other an electrically non-conducting oil. They are housed in a short tube with end caps which are coated with a hydrophobic coating that makes this solution to form the shape of a hemispherical mass at the opposite end of the tube. This it acts as a spherically curved lens.

When an an electric field across the hydrophobic coating, electro wetting happens , i.e, it becomes less hydrophobic .Due to this surface-tension the aqueous solution begins to wet the sidewalls of the tube. this causes the radius of curvature of the meniscus between the two fluids to alter and hence altering the focal length of the lens. The lens can be made completely flat (no lens effect) or even concave. By changing the magnitude of the electric field applied. Thus the lens can be convex to flat to concave by just varying the electric field strength. This works on the principle of mimics the action of the human eye using a fluid lens that alters its focal length by changing its shape. This also overcomes the fixed-focus disadvantages of present lenses.

Engineering revision control developed from formalized processes based on tracking revisions of early blueprints or blue lines .Implicit in this control was the option to be able to

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return to any earlier state of the design, for cases in which an engineering dead-end was reached in iterating any particular engineering design.

Likewise ,in computer software engineering, revision control is any practice which tracks and provides controls over changes to source code. Software developers sometimes use revision control software to maintain documentation and configuration files as well as source code.

In theory, revision control can be applied to any type of information record. In practice, however, the more sophisticated techniques and tools for revision control have rarely been used outside software development circles (though they could actually be of benefit in many other areas).

However, they are beginning to be used for the electronic tracking of changes to CAD files, supplanting the "manual" electronic implementation of traditional revision control.

As software is developed and deployed, it is extremely common for multiple versions of the same software to be deployed in different sites, and for the software's developers to be working privately on updates. Bugs and other issues with software are often only present in certain versions (because of the fixing of some problems and the introduction of others as the program evolves).

Therefore ,for the purposes of locating and fixing bugs, it is vitally important for the debugger to be able to retrieve and run different versions of the software to determine in which version(s) the problem occurs.

It may also be necessary to develop two versions of the software concurrently (for instance, where one version has bugs fixed, but no new features, while the other version is where new features are worked on).

At the simplest level, developers can simply retain multiple copies of the different versions of the program ,and number them appropriately. This simple approach has been used on many large software projects.

While this method can work, it is inefficient (as many near-identical copies of the program will be kept around), requires a lot of self-discipline on the part of developers, and often leads to mistakes. Consequently, systems to automate some or all of the revision control process have been developed.

VARIABLE FOCUS LIQUID LENS:

Variable focus liquid lenses are attractive for cellular phone, camera, eyeglasses, and

othermachine vision applications. Similar to a glass lens, a liquid lens focuses light based on

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thesurface-relief profile and its focal length can be tuned by changing the surface profile.

According to the operation mechanism, liquid lenses can be classified into two types.

The first type is the electro-wetting lens whose focal length can be tuned continuously by applying anexternal voltage . The pros of the electro-wetting lens are large focusing power and no mechanical moving part. However, the required driving voltage is relatively high and theshock wave stability and liquid evaporation are serious concerns. The second type is the chamber which, in turn, changes the curvature of the liquid profile . The operation mechanism of this lens is simple however it requires a fluid pumping system. As a result, the lens is sensitive to vibration and inconvenient for portable devices.In our previous , demonstrated a liquid lens whose focal length is variable based on pressure-induced liquid redistribution. In such a lens structure, the reservoir chamber is around the periphery which is wrapped using an elastic membrane. This approach isattractive for making a single large aperture lens. However the elastic membrane wrapped onthe lens border occupies a large area. As a result, the lens is bulky and has a relatively smalleffective aperture. Thus, it is difficult to extend this design to micro lens arrays where a largeaperture ratio is crucial.In this paper, we demonstrate a liquid lens based on pressure-induced liquid redistribution.The lens cell has two apertures and is flat in the initial non-focusing state. One aperture issealed with an elastic membrane on the outer side of the top substrate surface and anotheraperture is sealed with an elastic membrane on the inner surface of the bottom substrate.These two apertures do not overlap. Let us call the top aperture as reservoir hole and bottomone as lens hole. Because there is no periphery reservoir, the thickness of this new lens isreduced by ~50% as compared to that reported in if the same substrates are used. The thinlens will greatly suppress the lens aberration, reduce the gravity effect, and improve theresponse time. Such a lens design can be easily extended to micro lens arrays.

RAPID VARIABLE FLUID LENS WITH 2 MS RESPONSE Another potential advantage of the variable-focus lens is high-speed response. Such lenses can change their focal length in very short time, since the slight change of the surface shape results in significant focal position shift. And high-speed response is important especially for optical instruments that need high-speed axial scanning such as laser scanning on focal microscopes. It’s also important torealize high-speed focusing or zooming for visual inspections and security applications. The liquid variable-focus lenses based on electro wetting are ,however, not enough fast for such applications. This is because this type of driving mechanism changes only the boundary condition of the interface. The interface shape change occurs only because of inherent surface tension. There are no additional forces to help the interface to change their shape .The authors concentrated on speeding up focusing speed and proposed a variable-focus lens with1-kHz bandwidth . This lens transforms its lens surface rapidly using the liquid pressure generated

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by a piezo stack actuator. This mechanism also includes a built-in motion amplifier with high bandwidth to compensate for the short working range of the piezo stack actuator. The lens, however, included large aberrations because the refractive surface profile was not spheric.

2.FOCUS LENS

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Fixed-focus lens Fig:1

The same subject in the same conditions photographed with both a fixed-focus lens (on the left) and an autofocus lens (on the right). The difference in the picture quality shows the difficulty of properly focusing a camera with a fixed-focus lens.

A 4x magnified closeup of the above photograph shows the difference in picture quality even better.A photographic lens for which the focus is not adjustable is called a fixed-focus lens. The focus is set at the time of manufacture, and remains fixed. It is usually set to the hyperfocal distance, so that the depth of field ranges all the way down from half that distance to infinity, which is acceptable for most cameras used for capturing images of humans or objects larger than a meter.

Rather than having a method of determining the correct focusing distance and setting the lens to that focal point, a fixed-focus lens relies on sufficient depth of field to produce acceptably sharp images. Most cameras with focus free lenses also have a relatively small aperture which increases the depth of field. Fixed-focus cameras with extended depth of field (EDOF) sometimes are known as full focus cameras.

Concept

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In order to reach a short minimal focal distance the aperture and the focal length of the lens are reduced (a slow wide-angle lens), so that the hyperfocal distance is small. This allows for the depth of field to be extended from infinity to a short distance.

The disadvantage is the reduction of light that will reach the film through the small aperture. Therefore the lenses are usually not suitable for fast-moving objects which require short exposure times – see lens speed. The amount of collected light can be increased by opening the angle of view, which is achieved with an even shorter focal length resulting in a wide-angle lens. Telephoto lenses are not feasible at a reasonable lens speed.

The advantage of this design is that it can be produced very inexpensively, more so than autofocus or manual focus systems. The system is also effectively automatic; the photographer need not worry about focusing. It can also be more predictable than automatic systems.

Disadvantages include the inability to produce images as sharp as a lens that has been set to the best focal point for a given scene. Fixed-focus lenses are unable to produce sharp close-ups, or images of objects close to the camera, usually within 2.4 - 3.7 meters (8–12 feet). The latter limitation makes them unsuitable for portraits, as they cannot fill the frame of an image with a person's face and render it sharp at the same time. This limitation is likely to confuse inexperienced photographers.

Fixed focus can be a less expensive alternative to autofocus, which requires electronics, moving parts, and power. Since fixed-focus lenses require no input from the operator, they are suitable for use in cameras designed to be inexpensive, or to operate without electrical power as in disposable cameras, or in low-end 35 mm film point and shoot cameras, or in cameras featuring simple operation. These are usually wide-angle lenses with fixed aperture, and cameras with these lenses generally use a viewfinder for composition.

Especially suitable are fixed-focus lenses for low resolution CCD cameras as found in webcams, surveillance cameras and camera phones, because the low resolution of the detector allows a loose focusing on the CCD without noticeable loss of image quality. Therefore the circle of confusion gets bigger and hype rfocal distance smaller.

Special-purpose cameras such as the A giflite are used for situations like aerial photography from aircraft. Because the ground is far from the camera, focus adjustment is not necessary. For 35 mm cameras, some super wide fixed-focus lenses have been made.

Liquid lensA 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.

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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 non conducting 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 electro wetting. Electro wetting 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 (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.

3.ELECTROWETTING:History : The electrowetting behavior of mercury and other liquids on variably charged surfaces was probably first explained by Gabriel Lippmann in 1875 and was certainly observed much earlier. Froumkin used surface charge to change the shape of water drops in 1936. The term

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electrowetting was first introduced in 1981 to describe an effect proposed for designing a new type of display device. The "fluid transistor" was first investigated by J. Brown in 1984-1988 under NSF Grants 8760730 & 8822197 employing insulating dielectric and hydrophobic layers, immiscible fluids, DC or RF power; and mass arrays of miniature interleaved electrodes with large or matching Indium tin oxide (ITO) electrodes to digitally relocate nano droplets and control fluid flow electronically or optically. Electrowetting using an insulating layer on top of the bare electrodes was studied by Bruno Berge in 1993. Electrowetting on this dielectric-coated surface is called electrowetting-on-dielectric (EWOD) to distinguish it from the conventional electro wetting on the bare electrode. Micro fluidic manipulation of liquids by electrowetting was demonstrated first with mercury droplets in water and later with water in air and water in oil. Manipulation of droplets on a two-dimensional path, rather than along a line path, was demonstrated later. If the liquid is discretized and programmably manipulated, the approach is called "Digital Micro fluidic Circuits" or “Digital Micro fluidics”. Discretization by electrowetting was first demonstrated by Cho, Moon and Kim, completing the four basic digital microfluidic functions of creating, transporting, dividing and merging droplets on chip by electro wetting.Since then, a large number of applications based on electro wetting have been demonstrated. Currently five companies are at the forefront in commercializing electro wetting-based applications based on Cytonix and Berge's later research: Clinical diagnostics by Advanced Liquid Logic which was spun out of Duke University, electronic paper by both Gamma Dynamics, which was spun out of the University of Cincinnati, and Liquavista which was spun out of Philips Research, liquid lenses by Varioptic,and Digital PCR by Life Technologies and Sequenom. In some of these applications, electro wetting allows large numbers of droplets to be independently manipulated under direct electrical control without the use of external pumps, valves or even fixed channels. In e-paper and liquid lenses, droplets are manipulated in-place whereas in clinical diagnostics applications, droplets are moved around on the platform.

Electro wetting theoryThe electro wetting effect has been defined as "the change in solid-electrolyte contact angle due to an applied potential difference between the solid and the electrolyte". The phenomenon of electro wetting can be understood in terms of the forces that result from the applied electric field. The fringing field at the corners of the electrolyte droplet tend to pull the droplet down onto the electrode, lowering the macroscopic contact angle and increasing the droplet contact area. Alternatively, electro wetting can be viewed from a thermodynamic perspective. Since the surface tension of an interface is defined as the Gibbs free energy required to create a certain area of that surface, it contains both chemical and electrical components, and charge becomes a significant term in that equation. The chemical component is just the natural surface tension of the solid/electrolyte interface with no electric field. The electrical component is the energy stored in the capacitor formed between the conductor and the electrolyte.The simplest derivation of electro wetting behavior is given by considering its thermodynamic model. While it is possible to obtain a detailed numerical model of electro wetting by

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considering the precise shape of the electrical fringing field and how it affects the local droplet curvature, such solutions are mathematically and computationally complex. The thermodynamic derivation proceeds as follows. Defining the relevant surface tensions as: - The total, electrical and chemical, surface tension between the electrolyte and the conductor - The surface tension between the electrolyte and the conductor at zero electric field - The surface tension between the conductor and the external ambient - The surface tension between the electrolyte and the external ambientθ - The macroscopic contact angle between the electrolyte and the dielectricC - The capacitance of the interface, єrє0/t, for a uniform dielectric of thickness t and permittivity єrV - The effective applied voltage, integral of the electric field from the electrolyte to the conductorRelating the total surface tension to its chemical and electrical components gives:

The contact angle is given by the Young-Dupre equation, with the only complication being that the total surface energy γws is used: Combining the two equations gives the dependence of θ on the effective applied voltage as:An additional complication is that liquids also exhibit a saturation phenomena: after certain voltage, the saturation voltage, the further increase of voltage will not change the contact angle, and with extreme voltages the interface will only show instabilities.However, surface charge is but one component of surface energy, and other components are certainly perturbed by induced charge. So, a complete explanation of electro wetting is un quantified, but it should not be surprising that these limits exist.It was recently shown that contact angle saturation can be explained if electro wetting is observed as a global phenomena affected by the detailed geometry of the system. Within this framework it is predicted that reversed electro wetting is also possible (contact angle grows with the voltage).

4. PRINCIPLE OF THE LIQUID LENS DESIGN: Figure 1 shows examples of a liquid lens structure :between two glass windows, two non miscible liquids are trapped in a closed cell. One of the liquid is based on a water solution and thus it is conducting electricity. The other liquid is a polar, and should be non conducting (the oil

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phase). The natural interface between the liquids thus forms a natural diopter, due to the index difference of the two liquids. The actuation of the liquid-liquid interface is using electro wetting, which enables to change the relative wettability of the two liquids by a simple voltage application. In these conditions, the liquid-liquid interface has a spherical shape, with a variable radius of curvature .In order to work properly, the liquid lens needs several keyfeatures: Density requirement: the two liquids should have exactly the same density. This allows the lens towork in every possible orientation: optical axis ,horizontal, vertical or in any orientation to gravity. This density equality is tuned by allowing a small adjustment of the liquid composition(mixture of dense and less-dense fluids). A centering mean for controlling the stability of the optical axis when the voltage is applied.The two configurations above are representing two different ways of achieving this stability(see the next paragraph). An integral liquid-liquid interface: it is important that the design allows to haveconducting phases to be connected to the outside world without having to touchphysically the liquid-liquid interface.

Insulator

Figure2: two examples of the liquid lens practical realization. Upon voltage application the liquid-liquidinterface is dislaced. The continuous line shows the zero voltage situation. (a) gradient configuration. (b)geometrical centering.

In the following, we will consider in details the centering of the liquid drop, discussing the relative advantages of the configurations shown above. Centering by a dielectric thickness gradient

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We will analyse the configuration of fig. 1(a) first :here the centering of the drop is obtained through agradient of the electric field. The Figure 2(a) shows the principle of electro wetting: the contact angle of the oildrop on a planar surface which is made of an electrode covered with an insulator film of thickness “e” and ofdielectric constant “ε” is well described with the following equation:cos(θ)=cos(θ0)- 1/2 εε0 / (eγ) V2. (1)It can be shown from eq (1) that for a 60V actuation, using a low-k dielectric (e.g. polyethylene ε=2,4) using liquids with a liquid-liquid interfacial tension of about 30mN/m,leads to a thickness of the dielectric film of about e=1,3μm.The electric field in the dielectric is then of the order of E=0,4 MV/cm. This E-field is quite high, significantly belowthe dielectric breakdown of good polymer insulators .Nevertheless, one can consider that the nature of the dielectric material should be carefully chosen, for sustaining the E field. This situation corresponds to figure

Figure 3: Electro wetting situation (a) “normal” planar geometry. (b) radial gradient of dielectric thicknessallowing the centering of the liquid drop.

On figure 2b one uses the same principle for the drop actuation, but one builds a radial gradient of the dielectric thickness: e(r) , where “e” the thickness now depends on radius “r”. In this situation there are two main differences compared to the “normal planar geometry .The dependency of the contact angle versus voltage is modified. The figure 3 for instance shows the expected evolution of the contact angle versus voltage, with and without the thickness gradient. The figure 3 has been calculated using the modified formula:cos(θ)=cos(θ0)-1/2 εε0/ (e(r)γ) V2. (2)where r is the radius of the drop base in contact with the solid surface. For Figure 3 calculations, one considers a gradient where r1=3mm, e(r1)=1,3μm, r2=2,5mm,e(r2)=4μm.The second consequence of installing the gradient is the centering of the drop, which is the expected effect.If we suppose that the drop is off-centred by a distance “s”, then the excess electrostatic energy of the off-centerd drop is approximated by (assuming s<<r0 the drop radius):ΔE= 1/2 εε0 V2 r/e2 |de/dr| s2 (3) “r” is the drop radius and de/dr is the gradient of thickness. This calculation gives only an order of magnitude and should be taken as a rough estimate. Nevertheless, if one plugs reasonable

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orders of magnitude, one can obtain the equivalent “spring” constant for the mechanical centering, which would be defined asΔE= 1/2 Κ s2 (4)K Would be estimated as K ~ 0,1 N/m. There is no friction force to compare to K at this stage ,rather this value will be useful for relative comparisons.

Figure 4: electro wetting curves with gradient(squares) or without gradient (circles). The electro wetting response is lower with the gradient, as the relevant dielectric thickness, at the wetting contact line, increases with voltage.

Centering by the geometryWe now consider that the liquid drop is deposited into a cavity with arbitrary shape with full rotational symmetryaround optical axis. In this configuration (see Figure 4) , using a 2Dsimplified model, the excess energy of the drop can be shown to be given by:ΔE= (c0-c) γ r sinθ s2 (5)Where c is the local curvature of the surface at the point of contact of the drop, c0 is the curvature of thetangent sphere on the contact points (shown as a dashed line on Fig 4), γ is the liquid-liquid interfacialtension, θ is the contact angle of the drop on the surface(material property) and r is the drop radius. The curvatures are counted positively if directed towards the interior of the cavity, for instance in the particular case of the fig. 4ageometry, c0>0. Eq. 5 is an exact result in the simplified2D model. The Eq. 5 shows that a solid surface which curvature is outside of the tangent sphere will produce stable centering mean of the liquid drop, whereas the surfaces which are inside the tangent sphere will give unstable drops.

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Figure 5: (a) general stability analysis of the drop center ,on a given surface. The surface is assumed to have a full rotational symmetry around the vertical axis (optical axis).The tangent sphere curvature c0 is counted positively on the figure, whereas the curvature of the real supporting surface “c” is counted negatively. (b) examples of surfaces which are self-centering the drop, according to Eq. (5):conical recess, cylindrical hole, toroidal cavity etc… . (c)Examples of surfaces on which the drop will be unstable(non self centering).

In the case of a conical recess, the situation for the drop is to be always stable at the centre whatever the contact angleθ. The energy of an off-centred drop is then given byΔE= 2 γ sinξ sinθ s2 (6)where ξ is the cone angle (see fig. 4 for definition), and the equivalent spring constant can also been estimated as K ~0,2N/m for a 45° cone angle.Other centering meansThe calculations above show that dielectric gradient and geometry centering can have roughly the same strength bringing similar lens quality .There are a number of other conceptual ways of achieving the centering of the liquid drop. One could think of any gradient in the system, which would affect the liquid- liquid interface, assuming this gradient has the right direction .Also electrode edge effects can produce such gradients. Similar effect has been used in liquid crystal lenses recently.

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5. VARIABLE FLUID FOCUS LENS EXPERIMENT Device structureFigure 1 depicts the fabrication process of the lens. Two clear glass or plastic slabs are used aslens frames. Each slab is drilled with a hole and each hole is sealed with an elastic membrane,as Figs. 1(a) and 1(b) show. The two slabs are sandwiched together to form a flat cell. Theperiphery of the cell is sealed with epoxy glue except for a hole which connects to thechamber. A liquid was injected into the chamber through the hole and afterwards the hole wassealed with glue. Figure 1(c) shows the cross-sectional view of the lens cell in a flat state.Because the volume of the liquid is not constringent, when an external pressure is applied todeform the outer elastic rubber inward the liquid in the lens chamber is redistributed causingthe inner elastic membrane to swell outward. As shown in Fig. 1(d), the resultant lens is aplano-convex lens and the incident light is focused. Unlike most of liquid crystal lenses such a liquid lens exhibits a large dynamic range and its focusing behavior ispolarization independent.

Fig:6

To activate the liquid lens, an electrically controlled actuator or a mechanical lever can beemployed . To prove concept, here we just use a mechanical lever which has a sphericalhead for pressing the elastic membrane. The moving distance of the mechanical lever wascontrolled by a precision low-profile ball bearing linear stage (from Newport). By pressing theouter elastic membrane using the mechanical lever, the liquid inside the cell chamber isredistributed which causes the inner elastic membrane to swell outward and form a convexprofile.

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ExperimentTo fabricate a liquid lens according to Fig. , we drilled a 5-mm hole on each of the disc likeslabs. One hole was sealed using a polydimethylsiloxane (PDMS) membrane from insideand the other was sealed using an elastic rubber from outside. The thickness of the PDMSmembrane is ~50 μm and its Young’s modulus is ~3 MPa. The outer rubber that we chose hasa similar Young’s modulus as that of the PDMS but with a thickness of ~100 μm. The gapbetween the two slabs is controlled at ~1 mm. The thickness of each slab is ~4.5 mm. The cellperiphery was sealed with epoxy glue. After a pure water (refractive index n=1.333) wasinjected into the chamber through a hole and the hole was then sealed with the same glue.To characterize the light focusing properties of the lens, we measured the twodimensional focused spot pattern of the outgoing beam. The lens cell was intentionally set invertical direction so that the gravity effect of the liquid on the membrane curvature is takeninto consideration. An expanded He–Ne laser beam was used as the probing light source. ACCD camera was set at ~16 cm behind the lens cell to record the images.

Figures (a) and (b) show the three dimensional intensity profiles of the liquid lens at nulland activated state, respectively. In the non-focusing state, as Fig. (a) shows, the recordedbeam spot is larger than 5 mm. The output intensity profile is not symmetric because theincident He-Ne laser beam is not very uniform. The beam size is somewhat larger than thediameter of the PDMS membrane. In the activated state, as Fig. (b) shows, the laser beam istightly focused. To avoid saturating the CCD camera, we used a neutral density filter toreduce the light intensity. The focused beam diameter is about 200 μm.

Fig:7

results with the following Fraunhofer diffraction equation

ρ = 2.44λf / D , (1)

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where λ is the incident light wavelength, D is the diameter of the lens hole, and f is the focallength of the lens. In our experiment, D=5 mm, λ=0.633 μm, and f=160 mm. Usiparameters, we find ρ~50 μm from Eq. (1). Our measured result is ~4X larger than that ofdiffraction limited. This difference is believed to originate from the spherical lens aberration.For a long focal length, the PDMS film is only slightly deformed so that the sphericalaberration is small. As the focal length gets shorter, the PDMS membrane is deformed moretightly and the curvature may no longer be spherical. Our theoretical calculation shows that aseverely deformed PDMS membrane is basically a paraboloid. Under such a circumstance, thelens aberration is worsened and the image quality degraded. In our plano-convex liquid lens,the observed lens aberration is attributed to the non uniformity of the PDMS film, a relativelythick top slab, and rough edge of the drilled hole.To evaluate the lens performance during focus change, we recorded the image of anobject through the liquid lens under white light environment. A resolution target was set at ~7cm in front of the lens and a digital CCD camera (C-3040 zoom with 3.3M pixels) was behindthe lens cell. Figure 3 shows the photos taken at 5 different focusing stages of the liquid lens.Initially, the lens is in the non-focusing state so that the observed image has the same size asthe object. When we apply a pressure to the reservoir hole, the lens starts to focus. Since thedistance between the object and the lens cell is shorter than the focal length, the observedimage is upright but virtual. When the focal length becomes shorter, the observed imaged ismagnified further. In stages 2 and 3, the observed images at the border of the field-of-view areas clear as that observed in the center, although the lens is in vertical direction. In stages 4 and5, there is a little blur at the aperture border due to lens aberration. Before the aberrationbecomes severe, our liquid lens can resolve better than 25 lp/mm clearly.

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Fig. 8. Five-stage image magnification of the liquid lens. (1.6 MB)

To correlate how the volume change affects the focal length of the liquid lens, we assume the swelled state of the PDMS membrane has an ideal spherical profile if the volume change is not too largewhere Rv is the radius of the variable curvature of the PDMS membrane and nliquid is therefractive index of the employed liquid. We calculate how the volume change in reservoirhole affects the radius of the PDMS curvature and the effective focal length of the lens hole. The liquid we used for calculation is pure water. For simplicity, the influences of the PDMS thickness variation during deformation and its refractive index on the lens are neglected.Figure 4 shows the calculated results. As the displaced volume is increased, the PDMSfilm is deformed further so that its radius of curvature is reduced. As a result, the focal lengthof the lens decreases, according to Eq. (2). Three experimental data (open triangles) areincluded in Fig. 4 for comparison. The simulated and experimental results agree well. Becausethe lens performance is affected by the f-number and aberration, the resolution could varyduring focus change. At resolution of 25 lp/mm, as shown in Fig. 6, the f-number is estimatedto be ~f/20. Further decreasing the f-number will decrease the lens resolution correspondingly.

Fig. 9. Calculated PDMS radius of curvature and focal length of the liquid lens as a function of volume change. The open triangles are experimental data.

Response time is another important parameter for active imaging devices as it determinesthe data acquisition rate. We tried to estimate the response time of the liquid lens by probingthe lens activation using an expanded He-Ne laser beam. A diaphragm with ~2.5 mm diameterwas placed behind the liquid lens to control the transmitted laser beam. When the lens is in thenon-focusing state, the beam is large and some light is blocked by the diaphragm. Thus, thereceived beam intensity is weak. When the PDMS membrane is pressured to form a convex

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lens, the beam is converged so that more light can pass through the diaphragm. In experiment,we deformed the PDMS membrane by gently pressing the outer elastic membrane of the lensby a pen. The transient laser transmission was recorded by a digital oscilloscope.Figure 5 shows the measured results. The rise time is ~35 ms and the recovery time is~40 ms. The rise time is inversely proportional to the pressure acting on the rubber membrane.For a fixed volume change, as the applied force increases the rise time decreases. To achievefast rise time, we could use a piezoelectric actuator. As soon as the external force is removed,the lens recovers quickly to its original state. The measured relaxation time is not sensitive tothe force we applied.It would be desirable to quantitatively correlate the recovery time with volume changeand material parameters. However, this is not an easy task as it involves the dynamics ofliquid redistribution which is governed by the elastic and viscous torques. Qualitativelyspeaking, to shorten the lens recovery time we could take the following steps: 1) to choose theouter elastic rubber with a high elastic modulus, 2) to decrease the cell gap of the liquid lensand the distance between the reservoir hole and the lens aperture so that less liquid is involvedin the flow, and 3) to select a liquid with low density. Some liquids, such as silicon oil andliquid monomer, are suitable for the lens because they have a high refractive index and low

density. By optimizing the lens parameters, it is possible to improve the response time of theliquid lens and achieve video rate for real time active imaging applications.

Fig. 10. Measured response time of the liquid lens. Volume change is induced by an impulse pressure onthe outside elastic membrane of the lens.

In our experiment, water was chosen for feasibility demonstration. Water has two majorshortcomings: low refractive index (n=1.33) and high frozen temperature (4oC). For practicalapplications, some high refractive index and low frozen temperature liquids, such asmicroscope immersion oil (n=1.51), dimethyl silicon oil (n=1.60) or liquid polymer (Norlandoptical adhesive, n=1.56) can be considered. For a higher index liquid, a smaller volumedisplacement will achieve the same focal length without deforming the PDMS membrane too

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severely. Thus, the lens aberration will be decreased.In addition to a single large aperture lens, we can also fabricate lens array. To do so, wesimply drill arrays of holes on the glass or plastic substrate and seal the holes with an elasticmembrane. The device structure is similar to that sketched in Fig. 1, except the single lensaperture is now replaced by an array of holes.We have demonstrated a variable-focus liquid lens based on pressure-induced liquidredistribution. The aperture size of the liquid lens is determined by the uniformity andsturdiness of the PDMS film which could range from millimeters to centimeters. The focallength of the lens can be controlled by a mechanical or piezoelectric actuator. The resolutionof the lens is better than 25 lp/mm and response time is ~40 ms. This compact liquid lens canbe used for cellular phone zoom lens, machine vision, and real time satellite imaging. EXPERIMENTAL RESULTS On Figure 8, one can see an experimental measurement on an actual lens made with an parylene layer 5 μm thick. The experimental points are shown, recorded during a cycle.

Figure 11: optical power of the lens (1/f) versus the ac applied voltage V. The continuous line corresponds tothe modelization .It can be seen on figure 5 that there is very little hysteresis.

On the Figure 6, one shows measurements of the stability of the optical axis: a laser beam is focused through the liquid lens, and the XYZ position of the focus point is shown: X and Y as a function of Z.It can be seen that the centering effect (in this case due to the conical geometry) is working very efficiently: The slope of curves onfigure 6 is due to misalignment of the translation stages compared to the optical axis :the important information in Figure 6 is the error compared to the dashed line. This error is of the order to 50μm maximum across the whole 15mm variation of the Z position.

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Figure 12: XYZ position of the focus of a laser beampassing through the lens, during a voltage cycle: at V=0,Z=34mm, Z decreasing when voltage increases.

.

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6.FLUID FOCUS LENS FEATURES AND APPLICATIONS

FEATURES:•Low cost optical focusing system•No moving parts•Compact in size•Excellent transparency•Concave and convex lens formed from single lens•Zoom lens created by adding to fixed lens•Not prone to scratches•Low power usage for automatic focus (1mW ) andno power for manual focus•Fast response time (ms)

APPLICATIONS:

Telecommunications: Optical switches,fibre optic coupling, mobile phonecameras, webcamsData storage: CD, DVD, barcode readersAnalytical equipment: Portablemicroscopes, SensorsManufacturing: Laser technologyMedicine: Endoscopes

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7.CONCLUSION:

We might have seen in digital cameras that when it becomes on or captures or zooms the lens make two and fro. This is done by the internal motor inside the camera. This lens uses most of the power of the camera. But now-a-days wewant to save power as much as possible. So, here liquid lens shows its ability. It needs very less power as compared to the typical motor driven lens. So, it may be the best alternativeof typical motor driven lens. But the problem is that as liquids are used sothere may be problem in the extreme conditions .Major camera production companies like Canon, Nikon & cell phone company Sony Ericson have already tested and might be in the final process sto apply .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 liquid lens. 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 bethe solution to this demanding application. By using molecular simulations The structure of interface between liquid layers and molecular arranges have been understood. The chemical reaction and by-products were predicted. By modifying the liquid ensemble composition of the interface depth decreased. The transmissivity after high-temperature aging improved. Therefore mentioned liquid-formed lenses are a cool technology a swell, 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. We have demonstrated a variable-focus liquid lens based on pressure-induced liquid redistribution. The aperture size of the liquid lens is determined by the uniformity and sturdiness of the PDMS film which could range from millimeters to centimeters. The focallength of the lens can be controlled by a mechanical or piezoelectric actuator. The resolutionof the lens is better than 25 lp/mm and response time is ~40 ms. This compact liquid lens canbe used for cellular phone zoom lens, machine vision, and real time satellite imaging.

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8. REFERENCES:1 - R. Graham, A variable focus lens an dits use, J.O.S.A. vol 30, pp560-563, 1940.2 - L.G. Commander, S.E. Day, C.H. Chia, D.R. Selviah, Microlenses immersed in nematic liquid crystal withelectrically controllable focal length, Third EuropeanOptical Society “Microlens array”, topical meeting, NPL,May 11th -12th 1995.3 - A.F. Naumov, M.Yu. Loktev, I.R. Guralnik, G. Vodkin,Liquid crystal adaptative lenses with modal control.4 - B. Berge, J. Peseux, Variable focal lens controlled by anexternal voltage: an application of electrowetting, Eur.Phys. J. E. 3, pp159-163, 2000.5 - C. Gabay, B. Berge, G. Dovillaire and S. Bucourt,“Dynamic study of a Varioptic variable focal lens” SPIEproceedings vol 4767 , 159-165, 2002.6- L. Saurei, G. Mathieu, B. Berge, “ Design of anautofocus lens for VGA 1/4" CCD and CMOS sensors”Proceedings of SPIE vol 5249, 288-296, 2004.7- N.K. Sheridan, United States Patent. “ElectrocapillaryColor Display Sheet” Patent number 5,659,330. (1996).8- C.B. Gorman, H.A. Biebuyck, G. M. Whitesides,“Control of the shape of Liquid lenses on a modifiedGold surface Using an Applied Electrial Potential acrossa Self-Assembled Monolayer”. Langmuir, 11, 2242-2246 (1995).9- Ye, Y. Yokoyama, S. Sato, paper # 5639-23 of the^proceedings of SPIE, Photonics Asia. “Liquid crystallens with voltage and azimuth dependent focus”, 2004.10 -B. Berge and J. Peseux; "Variable focal lens controlled byan external voltage: an application of electrowetting”.

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