WO2007085658A1

WO2007085658A1 - Lens arrangement for a variable focus optical device

Info

Publication number: WO2007085658A1
Application number: PCT/EP2007/050855
Authority: WO
Inventors: Pierre Craen; Nicolas Tallaron
Original assignee: Varioptic Sa
Priority date: 2006-01-30
Filing date: 2007-01-29
Publication date: 2007-08-02

Abstract

A lens arrangement for a variable focus optical device comprising: a first lens group of positive optical power; a second lens group of a negative optical power; said first lens group or second lens group including at least two immiscible liquids whose interface can change shape in response to changes in an applied voltage; and a third lens group of a positive optical power.

Description

Lens arrangement for a variable focus optical device

Field of the invention

The present invention relates to a lens arrangement for a variable focus optical device in which a liquid lens is employed, particularly but not exclusively for an autofocus optical device of a minimized sized camera module having a high resolution.

Background art Camera modules for use in cameras, in telephones, bar code readers, web camera or any small optical systems are consistently being improved.

One recent improvement has been brought by the advent of variable focal length liquid lenses. Variable focal length liquid lenses are based on the change of a liquid-liquid interface under the application of an electric signal, said interface being moveable for example under the application of a voltage by principle of electro-wetting. An optical lens driven by electrowetting and of variable focal length is described for example in European Patent Application EP1662276, the content of which is incorporated herein by reference. Figure 1 of this application corresponds to Figure 12 of EP1 662 276. A variable focal length lens is formed by a cell defined by a fluid chamber covered by a lower plate 7 with a first transparent window 9, an upper plate 1 with a second transparent window, and a perpendicular, or near perpendicular, axis Δ. The lower plate, which is non-planar, comprises a conical or cylindrical depression or recess 3, which contains a non-conductive or insulating fluid 4. The remainder of the cell is filled with an electrically displaceable conductive fluid 5 along the axis Δ. The fluids are non- miscible, in contact over a meniscus (A, B), and have a different refractive index and substantially the same density. The cell comprises an electrical insulating substrate 2, arranged on at least an area of the lower plate, on which both fluids are in contact. A first electrode is separated from the conductive fluid and the insulating fluid by the insulating substrate. In this example, the lower plate comprises a conductive body 7 acting as the first electrode and the transparent window 9 for the passage of the beam of light. The windows are for example transparent plates, made of an optical transparent material, generally glass or a transparent plastic material but are preferably glass. Windows can be planar windows or fixed lenses centred on an optical axis of the variable focus lens. The conductive body in Figure 1 is used for the centering of the non conductive liquid. Another electrode 8 is in contact with the conductive fluid. The wettability of the insulating substrate by the conductive fluid varies under the application of a voltage V between the first and the second electrodes, such that through electrowetting phenomena it is possible to modify the shape of the meniscus, depending on the voltage V applied between the electrodes. Thus, a beam of light passing through the cell normal to the plates in the region of the drop will be focused to a greater or lesser extent according to the voltage applied. Voltage V may be increased from 0 volt to a maximum voltage, which depends on the materials used. For example, when the voltage increases, the non-conducting liquid drop 4 deforms to reach a limiting position (designated as B). While drop 4 deforms from its position A (rest position, without tension, concave interface with conductive fluid 5) to its position B (convex interface with conductive fluid 5), the focus of the liquid lens varies. The conductive fluid generally is a salt containing- aqueous fluid. The insulating fluid is typically an oil, an alkane or a mixture of alkanes that is possibly halogenated.

Liquid lenses have no moving parts and as such are rugged, have fast responses and can be made to be very small. In addition they provide a good optical quality and make an optimal low cost lens for an optical device such as a camera module. Auto-focus optical systems for camera module have been disclosed in US2006/0056042 A1 (Samsung Electromechanics Company Ltd). The system includes a first lens group having a liquid lens and a positive optical power; a second lens group also included having a positive optical power and third lens group having a negative optical power. Each of the lens groups 1 , 2 and 3 include aspherical lenses.

There are a number of problem associated with the auto-focus optical system of this application. Particularly, the lens arrangement described includes one or more aspherical lenses in each group to reduce the aberration. To reduce the size of the overall lens arrangement, an aspherical lens in the first lens group corresponds to a cover surface of the liquid lens. The inclusion of an aspherical lens as a cover of the liquid lens reduces the aberration but makes it difficult to build the liquid lens. This arises because alignment of an aspherical lens with the cone of the liquid lens is not easy, as it requires highly accurate positioning of the cover respective to the other cover of the liquid lens (which can also be a lens) and respectively to all the other elements of the optical combination. Further, covers of the liquid lenses are preferably made of glass, which material is chemically inert regarding the liquids contained in the fluid chamber and making aspheric lenses in glass is very expensive with the current technologies. By contast, a number of commonly used plastic materials are not so inert and therefore not always suitable.,. The lens arrangement described in US 2006/0056042A1 leads to a complex design of liquid lens and packaging thereof and is in turn a less cost-effective and efficient solution.

The size of the lens in the prior art system is indeed small, but can be further reduced as will be identified by the present invention and will be described below. The issue of the size of lenses in this technical field is very critical. In particular a few micrometers here or there can make huge difference to the space constraints under which the design of cameras and the devices in which they are carried are used.

Summary of the present invention One object of the present invention is to provide a still smaller variable focus optical system with improved optical qualities when compared to that of the prior art.

As will be clear to the person skilled in the art, the design of optical systems is a complex science. To change the overall structure of an optical system requires a complete redesign of the optics and a complete redesign of all the different optical surfaces.

Accordingly a further object of the present invention is to design a new lens arrangement which reduces aberration and/or minimizes the same and reduces at the same time the telecentricity of the lens arrangement.

Another object of the present invention is to overcome at least some of the disadvantages of known auto-focus optical systems.

According to one object to the present invention is provided a lens arrangement for a variable focus optical device comprising: a first lens group of positive optical power; a second lens group of a negative optical power; said first lens group or second lens group including at least two immiscible liquids whose interface can change shape in response to changes in an applied voltage; and a third lens group of a positive optical power.

This inventive lens arrangement has a number of advantages. In particular, as the lens system is substantially symmetrical the aberration problems associated with this type of lens are substantially minimized and the number of asphehcal lenses can be substantially reduced. Preferably the positioning of the stop within the lens body is chosen to ensure the best compromise between aberration minimization and low telecentricity. This results in an overall optimal solution, namely a shorter length lens arrangement with optimal aberration and telecentricity. In addition, the inventive lens arrangement has a wide field of view and aperture which again improve the optical properties of the lens arrangement and optical system.

The design of the liquid lens also has a number of advantages, as higher accuracy is achieved by having a spherical lens closest to the non- conductive liquid. The design of the lens and the cone is in the present invention optimized to enable the lens to be well-centered with respect to the cone. The spherical lens being next to the cone, aids in the reduction of the overall length of the optical system. The position of the first interface of the liquid lens also assists in reducing the overall length of the lens arrangement or optical system. As the lenses or surfaces are spherical they are easier to center with respect to the cone, can be made of glass, making manufacture of the liquid lens included in the first lens group easier.

Preferably as the other lens groups include asphehcal surfaces, the overall aberration is reduced for the lens arrangement without requiring an asphehcal surface in the group containing the liquid lens.

Advantageously, the stop is in an optimal position for the advantages of telecentricity and reduction of optical aberrations as mentioned above. The stop may be a mask or a window on one of the lens surfaces in order to reduce the overall number of components located within the lens arrangement. The mask may be applied by any appropriate process and to any desired surface. Ideally, for reduction of optical aberrations, the stop will be located as close as possible to the center of the second lens group. Practically, a best compromise will be found in the positioning of the stop to reduce as well the telecentricity if necessary. With the design of the present invention it is possible to locate the stop in other positions and still maintain an operable and optimized lens arrangement.

Brief description of the drawings

Reference will now be made, by way of example to the accompanying drawings, in which:

- Figure 1 is a simplified cross-section view of a variable-focus liquid lens according to the prior art; - Figure 2 is a first embodiment of a lens arrangement in accordance with the present invention;

- Figure 3 is a diagram to explain the definition of the optical power of a lens group;

- Figure 4a is a graph of distortion versus field for the first embodiment lens arrangement;

- Figure 4b is the graph of relative illumination versus field for the first embodiment lens arrangement;

- Figure 5a is a graph of the sagittal modulation transfer function performances in the lens in accordance with the first embodiment in the case of an object at an infinite distance, for three values of the field of view (0°, 24°, 31 °);

- Figure 5b is a graph of the tangential modulation transfer function performances in the lens in accordance with the first embodiment in the case of an object at an infinite distance, for three values of the field of view (0°, 24°, 31 °);

- Figure 5c is a graph of the sagittal modulation transfer function performances in the lens in accordance with the first embodiment in the case of an object at 10 cm from the front lens for three values of the field of view (0°, 24°, 31 °); - Figure 5d is a graph of the tangential modulation transfer function performances in the lens in accordance with the first embodiment in the case of an object at 10.0 cm from the front lens for three values of the field of view (0°, 24°, 31 °); - Figure 6 is a table of an example of the surface data for a lens according to the first embodiment of the invention;

- Figure 7 is a lens arrangement according to a second embodiment lens arrangement of the present invention;

- Figure 8a is a graph of distortion versus field for the second embodiment;

- Figure 8b is a graph of relative illumination versus field for the second embodiment lens arrangement;

- Figure 9a is a graph of the sagittal modulation transfer function performances in the lens in accordance with the second embodiment in the case of an object at an infinite distance, for three values of the field of view (0°, 24°, 31 °);

- Figure 9b is a graph of the tangential modulation transfer function performances in the lens in accordance with the second embodiment in the case of an object at an infinite distance, for three values of the field of view (0°, 24°, 31 °);

- Figure 9c is a graph of the sagittal modulation transfer function performances in the lens in accordance with the second embodiment in the case of an object at 10 cm from the front lens for three values of the field of view (0°, 24°, 31 °); - Figure 9d is a graph of the tangential modulation transfer function performances in the lens in accordance with the second embodiment in the case of an object at 10.0 cm from the front lens for three values of the field of view (0°, 24°, 31 °);

- Figure 10 is a table of an example of the surface data for a lens according to the second embodiment of the invention; - Figure 11 is a third embodiment of a lens arrangement in accordance with the present invention;

- Figure 12a is a graph of distortion versus field for the third embodiment lens arrangement; - Figure 12b is a graph of relative illumination versus field for the third embodiment lens arrangement;

- Figure 13a is a graph of the sagittal modulation transfer function performances in the lens in accordance with the third embodiment in the case of an object at an infinite distance, for three values of the field of view (0°, 24°, 31 °);

- Figure 13b is a graph of the tangential modulation transfer function performances in the lens in accordance with the third embodiment in the case of an object at an infinite distance, for three values of the field of view (0°, 24°, 31 °); - Figure 13c is a graph of the sagittal modulation transfer function performances in the lens in accordance with the third embodiment in the case of an object at 10 cm from the front lens, for three values of the field of view (0°, 24°, 31 °);

- Figure 13d is a graph of the tangential modulation transfer function performances in the lens in accordance with the third embodiment in the case of an object at 10 cm from the front lens, for three values of the field of view (0°, 24°, 31 °);

- Figure of 14 is a table of an example of the surface data for a lens is according to the third embodiment of the invention; - Figure 15 is a schematic view of an autofocus optical device, which may be found for example in a digital camera, mobile telephone, etc. Detailed description of the present invention Description of the First embodiment:

Figure 2 shows a lens arrangement shown in generally at 100. The lens arrangement includes a first group of positive optical power 102, a second group of negative optical power 104 and third group of positive optical power 106. A sensor position 108 is shown and will be used as the position for a sensor in the final device. The sensor could be of any appropriate type depending on the overall device and the lens arrangement.

In order to define the optical power of an optical group reference is made to figure 3. The optical group is shown as 30 and ray 32 enters the group in parallel with the optical axis of the optical group, the optical axis being generally defined as the axis of symmetry of the optical group. The ray is a distance h from the optical axis. This ray exits the group with an angle α from the optical axis. The optical power P of the group is defined to be the opposite of the derivative of α with respect to h at h = 0. This is shown in the equation below:

Where h is the distance from a ray entering the lens group parallel to the optical axis to said optical axis and α is the angle at which said ray exits from the lens group with respect to the optical axis. The optical group is said to be positive if P is positive and negative if P is negative.

According to this example, the first group 102 of positive optical power includes a liquid lens shown generally at 1 10. The liquid lens includes a first fluid 112, in this case for example an oily phase liquid, and the second fluid 114, in this case for example an aqueous phase liquid. The liquids are retained by means of a cone 116 and surfaces 118 and 120. The surface 118 includes a stop 122 shown as a dotted line in the figure. This stop can be applied by means of a mask or paint or other means of blocking a part of the surface 118 to control entry of light into the lens arrangement. At the same position as the stop 122, there is a means 124, which can be used to attach the overall lens arrangement to the device.

Returning now to the liquid lens, the two immiscible fluids 1 12 and 1 14 form a liquid-liquid interface 126 there between. The shape of the liquid-liquid interface 126 or meniscus between the two fluids may be adjusted by means of a voltage applied thereto. The details of the connection of the voltage are not shown but will be apparent to those skilled in the art.

A change in the voltage across the liquid-liquid interface gives rise to a change in the shape of the meniscus there between and thus the optical characteristics of the first group and thus the overall lens arrangement. This process provides the variable focus function of the lens. Accordingly, the variable focus function includes no moving parts and is enabled by means of a change in the meniscus of the interface between the two liquids that changes the focal length of the lens group and thus the lens arrangement. The first group of the lens arrangement has a positive optical power and the first face of the lens arrangement is a spherical surface 128. In this embodiment, one spherical surface and three plane surfaces are used, making the system easy to manufacture. The spherical surface is more easily aligned with the cone 116 than would be the case for an asphehcal surface. Further, a spherical surface can be made of glass easily, which material is more compatible with the liquids used in the liquid lens than plastic materials. The spherical surface can form a cover of the fluid chamber of the liquid lens as it appears in the first embodiment. Thus, the length and construction of the first lens group is improved and optimized. The second lens group 104 has a negative optical power. The negative optical power in this case is provided by lens 130. Other formations of lens or lens combinations may be used to provide an equivalent negative power to this lens group 104. The third lens group 106 includes a lens 132 and a field flattener

134. The overall optical effect of the third lens group 106 is that of a positive lens group. The field flattener is an advantageous feature especially in large field of view systems to reduce the effects of curvature of field and obtain a better focus on the sensor over the whole field. Lens groups one, two and three are respectively positive, negative and positive in terms of their optical power. This provides a degree of symmetry to the lens arrangement. This degree of symmetry at least partially mitigates the problems associated with aberration.

In the present example, the stop is shown on a surface of the liquid lens for the first lens group, which enables a reduction in the diametric size of the liquid lens. This is an optimal position for the stop as it is located close to the center of the second group, which would be an ideal location for the stop if the design permitted in term of reduction of aberration. It is possible to design the lens arrangement to include the stop at other locations than that shown. For the optimal degree of telecentricity of the lens arrangement this would ideally be at the front of the first lens group. Accordingly, the stop will be positioned on any appropriate surface or position in order to get the best compromise between reduction of the telecentricity and symmetry of the overall lens arrangement, while keeping it as close as possible to the liquid lens.

Telecentricity is defined for a given field of view as the angle of incidence on to the image plane of the chief ray for said value of the field; the chief ray is the ray of light traveling from the object point at said value of the field through the center of the entrance pupil and on to the image surface. An example of a possible arrangement of optical components to form the above described lens is shown in figure 6. Figure 6 shows a surface data summary which tabulates surface, type, comment (where appropriate), radius, thickness, type of material, diameter and conic measurement headings and details. The surfaces are numbered 1 through to 14 with surface 3 being identified as STO (standing for the stop). The surfaces 1 to 14 correspond with the surfaces of the various optical elements from left to right in figure 2, i.e. from the object end of the lens arrangement to the image end. For those optical elements that are aspherical (EVENASPH) under the type heading, the table continues to include details of the coefficients of radius of curvature of the surface. This is applicable to surfaces 9 through to 14 and is also shown in figure 6. The rotational symmetric polynomial surface which are aspherical are described by a polynomial expansion of the deviation from a spherical (or aspherical described by a conic) surface. The EVENASPHERE surface model uses only the even powers of the radial coordinate to describe the asphericity. The model uses the appropriate base radius of curvature and the conic constant. This can be expressed in the following formula:

_{z =} ^{C ' γ2} + Cx₁ T² + α₂ T⁴ + α₃ T⁶ + α₄ T⁸ + 0C₅ T¹⁰ + 0C₆ T¹² + 0C₇ T¹⁴ + 0C₈ T¹⁶ l + Vl - (l + k) - c² - r²

The optical length or track of the lens arrangement according to this first embodiment is of the order of 5.5 mm and the sensor (not shown) is of the order of 0.5 mm thick; and in this example of sensor 3 mega pixels and format % inches (measured diagonally across the surface thereof).

Thus, the camera module has an overall thickness of the order of 6.1 mm. Further details of the mechanical and optical characteristics are shown in the Table A below.

In the above example, the detailed characteristics of the elements forming the lens arrangement have been designed with the oil having a refractive index of 1.49892 and an Abbe number of 35.9 and the aqueous phase liquid having a refractive index of 1.41211 and an Abbe number of 56.6. As will be appreciated these are only example of the relevant attributes of the liquids and others are equally relevant. Table A

The application of this type and size of lens arrangement is wide. The lens arrangement may be found in any appropriate device including for example a camera module. The camera module may be used in, for example, bar code readers, web cameras, phone cameras, camera phones, cameras used in cars for driving assistance, medical equipment and industrial equipment (such as medical endoscope and industrial endoscope) etc. It will be appreciated that this camera module can also be used in many other potential devices.

Referring now to figure 4a and 4b, the distortion and relative illumination curves versus field for the figure 2 lens arrangement are shown respectively. The distortion is given as a percentage of the deviation of magnification relative to the nominal magnification as a function of the field value in the image plane (Y axis on the figure 2). The illumination curve gives the value of illumination normalized relatively to the center of the field value. As can be seen from figure 4a the maximum distortion of this lens arrangement is of the order of less than 2.5 %. This can also be seen in table A. At a field of view of 80 %, the relative illumination is greater that 60 % as is also shown in table A and the graph in figure 4b. Referring now to figures 5a to 5d, the modulation transfer function performances are shown for the figure 2 embodiment of the lens arrangement. The evaluation frequency was made at 110 line pairs per mm. Each figure shows the performance at three field points, each field point corresponding to a given line type, 0 degrees (solid line), 24 degrees (dashed line) and 31 degrees (dotted line).

The modulation transfer function is a measure of the visibility of the object at the sensor position. The higher the value of the modulation transfer function the better the quality of the picture. Ideally, a lens should transmit every bit of light it receives. However, as no lens is ideal and the light passing through the lens is affected by the materials thereof. The lens typically modulates the light to some extent or another. The modulation transfer function is a measure of this modulation. The quality of a lens is generally characterized by its Modulation Transfer Function (MTF) at a given spatial frequency, measured using a target with black and white line spaced at the spatial frequency. Tangential and Sagittal MTFs are the values measured respectively for the lines of the target being in two perpendicular directions.

From the graphs for this embodiment it can be seen that the lens arrangement according to the first embodiment gives a level of performance which is excellent for an object at an infinite distance (see figure 5a and 5b) and sufficient for all the potential devices identified herein for an object at a close distance (10 centimeters in figures 5c and 5d).

The resulting mechanical and optical characteristics of the lens arrangement according to the first embodiment are summarized in the Table A above.

Description of the second embodiment:

Referring now to figure 7, a second embodiment lens arrangement 400 is shown. The lens arrangement is similar to the figure 2 embodiment in that this includes a first lens group 402, a second lens group 404 and a third lens group 406 located in front of a sensor position 408. The sensor again is not shown but would be appreciated to exist in any appropriate format. The lens groups one, two and three are respectively positive, negative and positive optical power as in the first embodiment. In this example of lens arrangement the stop 410 is located in front of the first lens in the first lens group of the lens arrangement. The first lens group 402 includes a liquid-liquid interface 412 separating two liquids to form part of the liquid lens. The cone 414 supports the outer edge of the liquid- liquid interface and a voltage is applied across the liquids to change the meniscus and therefore the focus length of the lens. All other lens elements in the first group 402 are formed by spherical optical elements having surfaces 416, 418, 420 and 422. As can be seen in the figure, the spherical optical elements having surfaces 416, 418 on the one hand, 420, 422 on the other hand form the two covers of the fluid chamber containing the two immiscible liquids, thus enabling a reduction in the overall length of the lens arrangement.

The second lens group 404 comprises a lens 424 and the third lens group comprises a lens 426 and a field flattener 428. A filter 430 may also be included in front of the sensor.

The fact that all the surfaces of the first lens group are spherical means that the manufacture of this particular group of lenses is simplified. Similarly, it is easier to align all this spherical surfaces with the cone 414. The precise characteristics of each optical component of the lens arrangement of figure 4 are shown in the tables of figure 10 and table B below.

Table B

Once again an important feature is the fact that the first, second and third lens groups are positive, negative, positive optical powers respectively. These provide similar advantages in respect of reduction of aberration as the first embodiment. In addition, the asphehcal surfaces of the second and third lens group assists in minimizing the aberration problems. In this second embodiment, four spherical surfaces are used in the first lens group, leading to a reduction of the overall track length of the lens arrangement. The positioning of the stop in front of the first lens in the first lens group of the lens arrangement enables to reduce the telecenthcity of the system.

It can be seen from graph 8a that the distortion level of this embodiment is further improved with respect to the first embodiment and are generally less than plus or minus 1 %. At the same time, the relative illumination of this embodiment is not significantly impacted as shown in figure 8b. .

Figures 9a, 9b, 9c and 9d show the modulation transfer function performance of the second embodiment lens arrangement. It can be seen, as in figure 5, that this second embodiment gives a very good level of performance in terms of modulation transfer function.

Table B above summarizes further details the mechanical and optical characteristics obtained with the lens arrangement according to this second embodiment. Description of the third embodiment

Referring now to figure 11 , a third embodiment lens arrangement 500 is shown. The lens arrangement is similar to the figure 2 embodiment in that this includes a first lens group 502, a second lens group 504 and a third lens group 506 located in front of a sensor position 508. The sensor again is not shown but would be appreciated to exist in any appropriate format. The lens groups one, two and three are respectively positive, negative and positive optical power as in the first and second embodiments. In this example of lens arrangement, the second lens group of negative optical power includes a liquid-liquid interface 510 separating two liquids to form part of the liquid lens and a stop 512 located on the rear surface of the lens in the first lens group of the lens arrangement.

The first group 502 is formed by a spherical optical lens having surfaces 514 and 516. The second lens group 504 comprises the liquid lens 518 and a lens 520 and the third lens group comprises a lens 522 and a field flattener 524. A filter 526 may also be included in front of the sensor.

The manufacture of the lens arrangement of this third embodiment is simplified regarding first and second embodiments since the covers 528 and 530 of the liquid lens are both planar. On the other hand, the track length of the lens arrangement is longer than that of the first and second embodiments. Figure 14 sets out the surface data summary of this third embodiment, including of coefficients of radius for any aspherical surfaces. Once again an important feature is the fact that the first, second and third lens groups are positive, negative, positive optical powers respectively. In addition, the aspherical surfaces of the second and third lens group assists in minimizing the aberration problems. In this third embodiment, the positioning of the stop in front between the first and the second lens groups of the lens arrangement enables to reduce the telecentricity of the system still maintaining a good symmetry of the lens arrangement.

It can be seen from graph 12a that the distortion level of this embodiment is less than plus or minus 3%. At the same time, the relative illumination of this embodiment is not significantly impacted as is shown in figure 12b.

Figures 13a, 13b, 13c and 13d show the modulation transfer function performance of the third embodiment lens arrangement. It can be seen as for the first and the second embodiments that this third embodiment gives a very good level of performance in term of modulation transfer function. Table C below summarizes the mechanical and optical characteristics obtained with the lens arrangement according to this third embodiment: Table C

Table C : mechanical and optical characteristics of the third embodiment

Figure 15 is a schematic view of an optical device 1500, which is for example an autofocus optical device for a compact digital camera, mobile phone, etc. Optical device 1500 comprises a camera module 1502 comprising a lens arrangement according to one of the embodiments described herein which includes a variable liquid lens 1504 and a number of fixed lenses 1503. Lens 1504 has a lower electrode 1506 and one or a number of upper electrodes, two of which are shown labelled 1508 and 1510. Driving circuitry 1516 is provided connected to the electrodes of the liquid lens 1504. The camera module further comprises an image sensor 1512 which captures images formed from light rays received via the lens arrangement.

A processing unit 1514 is provided which is for example a image signal processor or the baseband processor of a mobile telephone. Processing unit 1514 implements algorithms for controlling the driving circuitry 1516 to provide the correct voltages to the electrodes of the liquid lens 1504, as well as controlling image sensor 1512 to capture images. Processing unit receives captured images from image sensor 1512 and stores them in a memory 1518 and displays them on a display 1520. It will be appreciated that the present invention could embrace several different, alternative embodiments, relating to features such as, the application, the actual optical elements, the materials, the surface curvatures etc... However, the feature of the lens arrangement that provides many of the advantages of the present invention is to have a positive-negative-positive optical power combination within a small module wherein the first or the second lens group includes a liquid-liquid lens.

It should be noted that the surface of the liquid-liquid interface is moveable by electro-wetting in the examples described but it will be appreciated that other techniques may be equally valid.

Another important feature is the ability to place the stop at different points within the module depending on the specific design thereof in order to minimize the impact of aberration and telecentricity on the lens arrangement as a whole.

Claims

1. A lens arrangement for a variable focus optical device comprising: a first lens group of positive optical power; a second lens group of a negative optical power; the first lens group or the second lens group including at least two immiscible liquids whose interface can change shape in response to changes in an applied electrical signal; and a third lens group of a positive optical power.

2. The lens arrangement of claim 1 , wherein the interface can change in response to changes in an applied voltage by an electro-wetting effect.

3. The lens arrangement of claim 1 or claim 2, wherein the first lens group includes said interface between the immiscible liquids and at least one spherical surface.

4. The lens arrangement of claim 3, wherein at least one of said spherical surface forms a cover of a fluid chamber including the immiscible liquids.

5. The lens arrangement of any of claims 1 to 4, further comprising a stop which is located in front of the centre of the second lens group.

6. The lens arrangement of claim 5, wherein the stop is located on a surface of the first lens group.

7. The lens arrangement of claim 6, wherein the stop is formed as a mask on the surface.

8. The lens arrangement of claim 5, wherein the stop is located in front of the first lens group.

9. The lens arrangement of any preceding claim, wherein the third lens group includes a field flattener.

10. A variable focus optical device including the lens arrangement of any of claims 1 to 9.

11. An auto-focusing optical device of claim 10 including a sensor and a processing means for processing the signal generated at the sensor.

12. A camera including a variable focus optical device according to claim 10 or claim 1 1.

13. A system including a camera according to claim 12.

14. The system of claim 13 which is a mobile phone, a webcam, a barcode reader.