WO2007083082A1

WO2007083082A1 - Miniature camera lens arrangement

Info

Publication number: WO2007083082A1
Application number: PCT/GB2007/000061
Authority: WO
Inventors: James Allan
Original assignee: 1...Limited
Priority date: 2006-01-17
Filing date: 2007-01-10
Publication date: 2007-07-26
Also published as: GB0600912D0

Abstract

A camera lens arrangement comprises a front and a rear lens element, the front lens element comprising either a single lens or plural lenses fixed relative to each another, the rear lens element comprising plural lenses fixed relative to each other. All the lenses are arranged in series along an optical axis and focus light onto an image sensor. The rear lens element is fixed relative to the image sensor, and the front lens element is movable to change the focus. The lens(es) of the front lens element have a smaller width than the widest lens of the rear lens element and an actuator arranged to move the front lens element extends around the front lens element and is disposed at least partially inside the area of the widest lens of the rear lens element as viewed along the optical axis. This provides compactness.

Description

Miniature Camera Lens Arrangement

The present invention relates to miniature cameras such as may be incorporated into a portable electronic device such as a mobile telephone or personal digital assistant (PDA). In particular the present invention relates to a lens arrangement for a miniature camera which allows for focussing of the image formed by the lens arrangement.

Recent developments in mobile applications have increased the demand for miniature cameras that are sufficiently small to be incorporated into a portable device such as a mobile telephone or a PDA. There is an ever increasing drive to minimise the size of such portable devices and hence to minimise the size of its component devices such as a camera. Such reduction in size requires the use of different components from those conventionally used in camera products.

Early products incorporating a miniature camera in a mobile telephone used a lens arrangement having a fixed focus. However, this requires a lens arrangement with lenses of small aperture which limits the size and number of pixels of the image sensor. To incorporate larger image sensors with higher numbers of pixels, there have been developed miniature camera lens arrangements which allow for focussing of the image formed. Typically in such miniature camera lens arrangements, the focussing is achieved by movement of lenses relative to the image sensor. This movement is produced by an actuator. The presence of the actuator increases the size of the system which is undesirable in a miniature camera.

Examples of suitable lens arrangements for a miniature camera are disclosed in WO-02/103451 and WO-2004/077497. Both these documents disclose lens arrangements in which the motion of lenses for focussing is driven by an electro- active actuator of the type disclosed in WO-01/47041 which provides a suitable degree of movement with a relatively small size of actuator. In particular the electro- active actuator may be formed as a continuous member curving in a helix about a minor axis which is itself curved so that bending of the continuous member is concomitant with twisting of the actuator about the minor axis and relative movement of the ends of the actuator. Typically in such miniature camera lens arrangements which have been applied in commercial products to date, the focussing is achieved by movement of the entire lens arrangement relative to the image sensor (unit focusing).

Notwithstanding the above comments, it remains desirable to further reduce the size of the lens arrangements for use in a miniature camera.

According to the present invention, there is provided a camera lens arrangement comprising: a front lens element comprising either a single lens or plural lenses fixed relative to each another; and a rear lens element comprising either a single lens or plural lenses fixed relative to each other, the lenses of the first and second lens elements being arranged in series along an optical axis, the front lens element being movable relative to the rear lens element along the optical axis, and the single lens or all the plural lenses of the front lens element having a smaller width than the single lens or the widest lens of the rear lens element, the camera lens arrangement further comprising an actuator arranged, on activation, to move the front lens element relative to the rear lens element along the optical axis, the actuator extending around the front lens element and being disposed at least partially inside the area of the single lens or the widest lens of the rear lens element as viewed along the optical axis.

In the camera lens arrangement, the front lens element is driven to move by activation of the actuator. This alters the focussing of the lens arrangement. Therefore, focussing of the lens arrangement may be achieved moving only the front lens element, without the need to move the rear lens element. Thus, the rear lens element may remain in a fixed position relative to the image sensor. This lens arrangement provides a number of advantages, as follows.

The first advantage is to minimise the overall size of the camera lens arrangement. This is achieved for two reasons. The first reason is that the focussing may be achieved by movement solely of the front lens element, which has a relatively low mass and thereby reduces the force requirements of the actuator. The mass of the front lens element is for example less than the mass of an equivalent camera lens arrangement in which the entire lens arrangement is moved to achieve focussing. Furthermore, as the front lens element may be provided with a smaller width than the maximum width of the rear lens element, the front lens element has a mass which is low relative to widest lenses in the overall lens arrangement.

The second reason is that the focal length of the front lens element can be chosen to be short. This reduces the distance the lens has to be displaced when changing focus between two object distances. This in turn reduces the overall maximum length of the lens arrangement, and the displacement requirement on the actuator.

The relatively low force requirement and displacement requirement on the actuator allows the use of a relatively small actuator which in turn reduces the overall size of the camera lens arrangement. This allows reduction of the volume of the camera lens assembly and thus an entire camera module also including an image sensor.

Furthermore the reduced size of the actuator reduces the size of associated components, for example in the case of an electro-active actuator the size of any protective structure protecting the actuator against excessive damaging displacements caused by external impacts.

A reduction in the overall dimensions perpendicular to the optical axis is also achieved, as a result of the use of a front lens element having a smaller width than the widest lens of the rear lens element. This allows the actuator to extend around the front lens element and to be disposed at least partially within the area of the widest lens of the rear lens element as viewed along the optical axis. As to the degree of overlap typically at least 20%, more preferably at least 50%, of the radial extent of the actuator is disposed inside the area of the single lens or the widest lens of the rear lens element as viewed along the optical axis. In some embodiments it is possible for the actuator to be disposed entirely inside the area of the single lens or the widest lens of the rear lens element.

This produces a relatively compact arrangement. The location of the actuator around the front lens element reduces the amount of unused space within the overall package, as well as reducing the size of the overall package of the camera lens arrangement in directions perpendicular to the optical axis. The size of the camera lens arrangement in this direction is particularly important because in practice the camera lens arrangement will be assembled with other components of a mobile device arranged adjacent the camera on one or other side of the optical axis. Lastly, this camera lens arrangement provides advantageous optical properties, as follows.

Image sensors with a large number of very small pixels are very demanding of lens resolution. This high resolution requirement when combined with the desire for a small number of lenses to be used (to minimise size and cost) results in designs which use aspheric surfaces or a mixture of spherical and aspheric surfaces on the individual lenses. However, the quality of the image produced by an aspheric lens is sensitive to errors in the orientation of the lens. Such errors occur when the lens tilts which can be a side effect of moving the lens with an actuator. hi the lens arrangement of the present invention, the relatively low displacement of the front lens element needed to achieve focussing correspondingly reduces the tilt of the single lens or plurality of lenses in that front lens element. This in turn reduces the reduction in the quality of the image produced by tilting of any aspheric lenses included in the front lens element. Conversely, movement of the rear lens element is unnecessary to achieve focussing and so the lens tilt and consequent reduction in image quality which would accompany such movement is avoided. These advantages may be maximised by using a design of lenses in which the aspherical lens surfaces included to reduce optical aberrations are included in the lenses of the rear lens element in preference to the front lens element. In this case, the single lens or all the plural lenses of the front lens element have a lower degree of asphericity than the single lens of the second lens element or the lens of the second lens element having the highest asphericity. Ideally, the single lens or all the plural lenses of the front lens element are spherical lenses, so that any lens tilt occurring on movement does not reduce image quality, but at least one lens of the rear lens element is aspherical, in order to reduce aberrations.

The advantages of the camera lens arrangement are particularly achieved when the actuator is an electro-active actuator extending between two ends along a minor axis which curves around the optical axis, the electro-active actuator being arranged, on activation, to twist around the minor axis concomitantly with relative displacement of the ends of the electro-active actuator in the same direction as the optical axis, the ends of the electro-active actuator being connected respectively to the first and second lens elements to move the front lens element relative to the rear lens element along the optical axis. The ends of the electro-active actuator maybe connected directly or indirectly to the lens elements. For example the "fixed" end of the actuator may be connected to a casing or other support structure which is in turn connected to the second lens element, so that the "fixed" end of the actuator is connected to the second lens element indirectly. In any event, the ends of the actuator are fixed relative to the first and second lens elements respectively. For example the electro-active actuator maybe of the type described in WO-01/47041 and WO- 02/103451 mentioned above, for example in which the electro-active actuator comprises a continuous electro-active member curving in a helix around the minor axis.

Beyond the intrinsic advantages of this type of electro-active actuator (for example providing a given displacement with a relatively small size of actuator), there is an advantage that the specific form of the actuator as a structure extending along a curved minor axis fits compactly around the front lens element, thereby maximising the degree of overlap with the area of the rear lens element as viewed along the optical axis. Thus, the use of this type of electro-active actuator provides a particularly compact package, hi many embodiments, the actuator need not extend outside the area of the rear lens element in any direction perpendicular to the optical axis, so that the actuator makes no marginal contribution to the dimensions of the camera lens arrangement in this direction. In the case of such an actuator or indeed any electro-active actuator, the present invention provides another advantage as compared to a camera lens arrangement where the entire lens system is moved for focussing, in particular an advantage of increased robustness. As the actuator is relatively small and the lens element moved by the actuator is also of relatively small mass, impacts occurring during use of the camera lens assembly, as are typical during normal use of a portable electronic device, cause a lower degree of relative movement of the ends of the actuator. This stresses the actuator less, thereby reducing the risk of damage. For example, the specific embodiment described below reduces the moment generated between the ends of the actuator by a factor of around 10, as compared to an equivalent camera lens assembly where the entire lens system is moved for focussing.

Although the use of an electro-active actuator has particular advantages, the present invention can equally be applied to other forms of actuator including a voice- coil motor or other electric motor. Advantageously, the single lens or the front lens of the rear lens element is no wider than the single lens or the widest of the plural lenses of the front lens element, the actuator extending around the front lens element and around the front lens of the rear lens element. By thus making the single lens or the front lens of the rear lens element of the same width or smaller than any lens of the front lens element, additional space is made available for accommodating the actuator without requiring any increase in the overall dimensions of the lens arrangement. This additional space may similarly be used to accommodate a suspension system which suspends the front lens and guides movement of the front lens element.

. Typically, the camera lens arrangement will be provided incorporating an image sensor, the rear lens element being fixed relative to the image sensor.

However, the camera lens arrangement may be manufactured absent the image sensor for subsequent assembly with the image sensor. When assembled with an image sensor, the actuator is typically disposed at least partially, and preferably entirely, inside the area of the image sensor as viewed along the optical axis. This arises because, as a matter of the optical performance, the widest lens of the rear lens element is typically of comparable area to the image sensor.

Advantageously, the rear lens element comprises a housing mounting the single lens or the lenses of the rear lens element, hi this case, advantageously the housing is rigid and has one or more seats which abut respective lenses of the rear lens element around their periphery. Such an arrangement has the advantage of minimising assembly tolerances in the positions of the lenses of the rear lens element relative to each other and to the image sensor. This advantage is maximised if the housing of the rear lens element is a unitary member so there is no tolerance between any component parts of the housing. A similar advantage is achieved if the image sensor abuts the housing of the rear lens element.

Advantageously, the front lens element has a housing and the single lens or the plurality of lenses of the front lens element are fitted in a carrier mounted to the housing on a screw fitting which provides for adjustment of the position of the single lens or the plurality of lenses relative to the housing along the optical axis. This manner of assembly allows adjustment of the position of the front lens element to be made. This is typically necessary to compensate for the unavoidable tolerances both on the focal lengths of the individual lenses in both the front and rear lens elements, and on the relative positions of the lenses in the front and rear lens elements.

The terms "front" and "rear" as applied to the front and rear lens elements merely define front and rear relative to each other. The overall lens system may have further lenses in front of the front lens element. One example of this is where the entire camera lens arrangement is housed inside a casing which typically is sealed to prevent ingress of dirt and is opaque to prevent ingress of light. In this case, the casing has a window in series with the lenses of the front lens element and rear lens element along the optical axis. Whilst the window may be a planar sheet, it may alternatively be a lens forming part of the lens system with the lenses of the front lens element and rear lens element, in which case the front lens element is not in absolute terms the front of the lens system.

To allow better understanding, an embodiment of the present invention will now be described by way of non-limitative example with reference to the accompanying drawings, in which:

Fig. 1 is a cut-away perspective view of a camera lens assembly with some components omitted for clarity;

Fig. 2 is a cross-sectional view of the camera lens assembly of Fig. 1, taken along the same line as the cut-away in Fig. 1 but with all the components being shown;

Fig. 3 is a cross-sectional view of an alternative form of the front lens element of the camera lens assembly;

Fig. 4 is a perspective view of a suspension element of the camera lens assembly;

Fig. 5 is a perspective view of the electro-active actuator of the camera lens assembly;

Fig. 6 is a perspective view of a portion of the electro-active actuator of Fig. 5; and Fig. 7 is a schematic top view of the camera lens assembly.

A lens arrangement of a miniature camera is shown in Figs. 1 and 2, although for clarity Fig. 1 omits the suspension elements 32, flange 34 and casing 36 described below.

The lens arrangement mounts four lenses 1, 2, 3, 4 in series along an optical axis O. hi Figs. 1 and 2 the front of the lens arrangement is uppermost so that the lenses 1 to 4 are arranged from front to rear.

The front lens 1 is mounted in an annular holder 6, which acts as a housing, to constitute a front lens element 8. In particular, the front lens 1 is fixed, around its periphery, to an annular carrier 10. The carrier 10 is in turn mounted to the holder 6 by a screw fitting 12 formed by each of the carrier 8 and the holder 6 having mating screw threads.

The screw fitting 12 allows adjustment of the position of the front lens 1 along the optical axis O relative to the lens holder 6. The position of the front lens 1 along the optical axis O is adjusted during assembly in order to accommodate any variations in the focal lengths and relative positions of the four lenses 1 to 4 arising due to manufacturing tolerances. Thereafter, the lens 1 remains in the same position relative to the holder 6.

The rear three lenses 2 to 4 are mounted in a barrel 14, which acts as a housing, so that the rear three lenses 2 to 4 and the barrel 14 together constitute a rear lens element 16. The barrel 14 is formed as a unitary member and is therefore rigid. Along the optical axis O, the barrel 14 is formed with three seats 18, 20, 22 each taking the form of an annular ledge. The three rear lenses 2, 3, 4 are each fixed abutting a respective one of the seats 18, 20, 22 around the periphery of the given lens 2, 3, 4. By means of the three lenses 2, 3, 4 abutting the barrel 14, the position of the lenses 2, 3, 4 in the rear lens element 16 are fixed relative to one another to a high degree of tolerance.

An image sensor 24 is also mounted to the rear lens element 16. In particular, the image sensor 24 is formed in a die 26 which is fixed abutting a plurality of protrusions 28 formed on the rear side of the barrel 14. Thus, the image sensor 24 is positioned in series with the lenses 1 to 4 along the optical axis O. The lenses 1 to 4 are arranged to direct light onto the image sensor 24 to form an image thereon. As a result of the die 26 of the image sensor 24 abutting the lens barrel 16, the position of the image sensor 24 is fixed relative to the lenses 2, 3, 4 of the rear lens element 16 to a high degree of tolerance. The image sensor 24 is an electronic device which outputs an image signal representative of the image. The image sensor 24 may typically be a semiconductor device such as a CMOS device.

To prevent ingress of light, the lens barrel 16 in which the lenses 2 to 4 are mounted is opaque and the barrel covers the image sensor 24 entirely around the optical axis O by means of being provided with a skirt 30 which extends outwardly from the optical axis O and overlaps the edges of the die %6 in which the image sensor 24 is formed.

A casing 36 fits on the skirt 30 and encloses the other components of the miniature camera. The casing 30 protects the miniature camera and is opaque to prevent the ingress of light into the optics. The casing 30 has a window 38 in series with the lenses 1 to 4 along the optical axis O. The window 30 is a planar sheet of transparent material but could alternatively be shaped as a lens, as shown by the dotted line 39, forming part of the lens system with the lenses 1 to 4.

The lenses 1 to 4 together form a system of lenses which focus light onto the image sensor 24. The individual lenses may be designed using conventional lens design techniques. Ih particular, the lenses are designed so that movement of the front lens element 6 (including a single lens 1) relative to the rear lens element 16 including the lenses 2 to 4 which are fixed) changes the focus of the image formed on the image sensor 24. As only the front lens element 6 is being moved, the degree of movement required to achieve focussing is relatively small, for example of the order of ± 0.075mm from the centre position to achieve a range of object positions from infinity to 100mm.

In this regard, it is noted that the number of lenses in the lens arrangement shown in Fig. 1 is merely exemplary and other numbers of lenses could be used, hi general, the rear lens element 16 could incorporate any number of lenses. Although at the expense of aberration, it is even possible for the rear lens element 16 to include a single lens, in which case it has the construction shown in Figs. 1 and 2 with lenses 2 and 4 omitted. Similarly, the front lens element 8 could house, instead of a single lens 1, plural lenses which are fixed relative to the holder 6 and hence fixed relative to each other. As an example, Fig. 3 shows an alternative form for the front lens element 8 in which the single lens 1 is replaced by two lenses 11 and 13.

The number of lenses employed is a design choice which depends on the required optical performance of the overall system of lenses in the same manner as common in the design of lens systems, hi general terms, better optical performance may be achieved by increasing the number of lenses, but this increases both the overall size of the lens arrangement and also the manufacturing costs due to the increased complexity and number of components. hi the particular system of lenses 1 to 4 of the lens arrangement shown in Figs. 1 and 2, the rearmost lens 4 is of a similar width to the width of the image sensor 24. This is common in lens systems of miniature cameras to minimize the angle of incidence of light on the image sensor 24. The lens 3 in front of the rear lens 4 is also of a similar width. However, the lens 1 of the front lens element 8 and also the front lens 2 of the rear lens element 16 have a smaller width than the other two lenses 3 and 4 of the rear lens element 16. Thus, focussing of the image formed on the image sensor 24 may be achieved by movement of the lens 1 which is relatively small, as compared to other lenses within the overall system, in particular the rear lenses 3 and 4 of the rear lens element 16. This provides particular advantages as will be described below.

To suspend the front lens element 8 on the rear lens element 16 and to guide movement of the front lens element 8 along the optical axis O, there is provided a suspension system comprising two suspension elements 32 as illustrated in Fig. 2. The two suspension elements 32 are identical and are shown in perspective view in Fig. 4.

Each suspension element 32 comprises an inner frame 70 and an outer frame 72. Each of the inner frame 70 and the outer frame 72 are annular and rigid. Each suspension element 32 further comprises three flexure members 74 which extend between the inner frame 70 and the outer frame 72 and are rigidly fixed thereto along the entire extent of flexure members 74 around the optical axis O.

One possible construction of a suspension element 32 is that the flexure members 74 are all formed from a single sheet of elastic material in which the flexure members 74 are integrally formed together with an inner ring of elastic material of the same shape as the inner frame 70, as viewed along the optical axis O, and also having an outer ring having the same shape as the outer frame 72, as viewed along the optical axis O. In that case the inner frame 70 may be formed from two inner frame portions which are fixed to the inner ring on opposite sides of the sheet, and the outer frame 72 may be formed from two outer frame portions which are fixed to the outer ring on opposite sides of the sheet, the inner frame portions and outer frame portions being made of a rigid material, for example a plastics material or metal. An annular flange 34 protrudes from the rear lens element 16 extending around the front lens element 8. As shown in Fig. 2, the suspension elements 32 are each rigidly coupled between the holder 6 and the annular flange 34. In particular, the inner frame 70 fits around the holder 6 and is rigidly coupled to the holder 6, for example by clips formed on the holder 6 which provide a snap fitting for ease of assembly. Similarly, the outer ring 72 fits inside the annular flange 34 and is rigidly coupled to the annular flange 34, for example by being clipped into detents formed in the annular flange 34 which provide a snap fitting for ease of assembly.

The flexure members 74 are formed as pieces of elastic material which extend radially from the inner ring frame 70 the outer frame 72 so that they are coupled to the inner frame 70 and the outer frame 72 at the same angular positions around the optical axis O. The flexure members 74 are formed as flat sheets which are co-planar so that they extend parallel to one another perpendicularly to the optical axis O and at the same position along the optical axis O. The individual flexure members 74 have an identical form, in particular with an identical width, that is the extent around the optical axis O. In the present arrangement shown in Fig. 4, there are three flexure members 74, but in general any number of flexure members 74 could be provided. The flexure members 74 have a thickness parallel to the optical axis O which is less than their extent around the optical axis O, that is along the width of their rectangular shape.

The suspension elements 32 take the same form as described in copending British application No. 0600911.2 which is incorporated herein by reference. AU of the teachings of that application may equally be applied to the suspension elements 32.

The pair of suspension elements 32 allow movement of the lens barrel 8 along the optical axis O. On such movement, the orientations of the flexure members 74 change, from the orientation perpendicular to the optical axis O in the rest position shown in Fig. 2 to an orientation at an angle to a line perpendicular to the optical axis O. The length of the flexure members 74 thus increases by a small amount, this being accommodated by stretching of the flexure members 74.

For such movement of the lens barrel 8 relative to the housing 4 along the optical axis O, the stiffness of the suspension elements 32 is relatively low, being dependent on the configuration and material of the flexure members 74. As the flexure members 74 extend in the rest state radially of the optical axis O, the increase in length of the flexure members 74 is smaller than the degree of movement along the optical axis O, so the tension generated elastically in the flexure members 74 is relatively low and only a component of this tension is directed along the optical axis O.

Relative to such stiffness along the optical axis O, the stiffness generated by each of the suspension elements 32 against lateral displacement perpendicular to the optical axis O is relatively high, as compared to the stiffness along the optical axis O. This is because such lateral movement tends to bend at least one of the flexure members in a direction radially of the optical axis. As the flexure members 74 each have a large extent around the optical axis O, as compared to their thickness parallel to the optical axis O₅ the flexure members 74 resist such bending with a relatively high force, thereby providing a high stiffness against such lateral motion. As the flexure members 74 are spaced around the optical axis O, this high lateral stiffness is present in all directions, the symmetrical nature of the flexure members 74 providing similar stiffness in all lateral directions.

Furthermore, the use of two suspension elements 32 spaced apart along the optical axis O provide a high stiffness against tilting of the lens barrel 8, because such tilting involves lateral displacement of the lens barrel 8 at the position of one or both of the suspension elements 32. To improve the stiffness against such tilting movement of the front lens element 8, the holder 6 has an extended length which is greater than that necessary merely to support the lens 1.

The suspension elements 32 are advantageous because they are compact and provide good performance, but they could be replaced by any other suspension system which suspends the lens for movement along the optical axis, for example a suspension system of the type described in WO2005/003834 including parallel link elements of the type described in WO-03/048831 including a flexure having two regions of opposite curvature, or indeed any other type of suspension, including a bearing. Although the suspension elements 32 reduces the tilt of the front lens element 8 during movement, it is nonetheless inevitable that there will remain a small degree of tilt of the lens 1 when the front lens element 8 moves. For this reason, the overall lens system formed by the lenses 1 to 4 is designed so that any aspherical lenses included to reduce optical aberrations are constituted by one or more of the lenses 2, 3, 4 of the rear lens element 16: As these lenses 2, 3, 4 of the rear lens element 16 are fixed, there is no tilting which can occur when a lens moves and thus a reduction in the optical performance which can occur when an aspherical lens tilts is avoided. hi contrast, the lens 1 of the front lens element 8 is arranged to have a lower degree of a asphericity than any of the lenses 2, 3, 4 of the rear lens element 16 which are aspherical, the lens 1 of the front lens element 8 preferably being a spherical lens, hi this manner, to the extent that tilt of the lens 1 of the front lens element 8 occurs, there will be a minimal amount of reduction in the quality of the image formed on the image sensor 24 and indeed no reduction of the quality to the extent that the front lens 8 is precisely spherical. To drive movement of the front lens element 8 along the optical axis O, there is provided an electro-active actuator 40. The actuator 40 is shown in isolation in Fig. 5 and will now be described, hi the following description, the electro-active actuator 40 is described with reference to minor and major axes which are imaginary, but are nonetheless useful for visualising and defining the actuators. The actuator 40 has a structure in the form of a continuous electro-active member 42 curving in a helix around a minor axis 43 so that the actuator 40 extends along the minor axis 43. The minor axis 43 is curved, extending in a curve which is an arc of a circle around a geometrical major axis 44 perpendicular to the plane of the minor axis 43, i.e out of the plane of the paper in Fig. 5. As the minor curve 43 is planar, the thickness of the actuator 40 parallel to the major axis 44 is merely the thickness of the helical structure of the electro-active member 42.

Fig. 6 illustrates a portion 60 of the continuous member 42 of the actuator 40. The electro-active portion 60 is a finite portion of the continuous member 42 and hence the electro-active member 42 may be considered as a plurality of adjacent portions 60 as illustrated in Fig. 6 disposed successively along the minor axis 43. Hence, the portion 60 extends along part of a helical curve around the minor axis 43 as shown in Fig. 6.

Fig. 6 illustrates the construction of the electro-active portion 60. This construction is preferably uniform along the entire length of the minor axis 43 in order to provide uniform properties on activation. Alternatively, the actuator 40 may be designed with some variation along the length of the minor axis 43, either in the construction of the continuous member 42 in the shape of the curve of the continuous member 42 around the minor axis 43.

The electro-active portion 60 has a bimorph bender construction comprising two layers 61, 62 of electro-active material extending along the length of the portion 60. The layers 61, 62 of electro-active material both face the minor axis 43. The electro-active layers 61, 62 preferably extend, across the width of the portion 60, parallel to the minor axis 43, although there maybe some distortion of the electro- active portion 60 of the continuous member 42 due to the nature of the curve around the minor axis 43. Alternatively, the layers 61, 62 may extend, across the width of the portion 60, at an angle to the minor axis 43 so that one edge along the electro-active portion 60 is closer to the minor axis 43 than the opposite edge.

The material of the electro-active layers 61, 62 is preferably piezoelectric material. The piezoelectric material may be any suitable material, for example a piezoelectric ceramic such as lead zirconate titanate (PZT) or a piezoelectric polymer such as polyvinylidenefluoride (PVDF). However, the material of the electro-active layers 61, 62 maybe any other type of electro-active material, for example electrostrictive material, which constricts on application of an electric field.

The electro-active portion 60 further comprises electrodes 63 to 65 extending parallel to the layers 61, 62 of piezoelectric material. Outer electrodes 63, 64 are provided outside the electro-active layers 61, 62 on opposite sides of the electric- active portion 60. A centre electrode 65 is provided between the electro-active layers 61 and 62. The electrodes 63 to 65 are used to apply poling voltages and to operate electro-active portion 60 in a bending mode. On electrical activation, activation voltages are applied to the electrodes 63 to 65. On activation, the electro-active layers 61 and 62 undergo a differential change in length concomitant with bending of the portion 60 due to the constraint of the layers being coupled together at their interface formed by the centre electrode 65. For maximum displacement, on activation one of the electro-active layers 61, 62 expands and the other one of the electro-active layers 61 and 62 contracts. The relative direction and magnitude of the activation and poling voltages may be selected in the same manner as for known linear electro- active actuators having a bender construction. For example, poling voltages of sufficient magnitude to pole the electro-active layers 61 and 62 may be applied in opposite directions across the electro-active layers 61 and 62 by grounding the centre electrode 65 and applying poling voltages of the same polarity to both the outer electrodes 63, 64. hi this case, the electro-active portion 60 is electrically activated by applying activation voltages in the same direction across the electro-active layers 61 and 62 by applying voltages of opposite polarity to the two outer electrodes 63 and 64. On activation the electro-active portion 60 bends around the minor axis 43, either towards or away from the minor axis 43, depending on the polarity of the activation voltages: On electrical activation the activation voltages are applied from a circuit 66 through external terminals 67 electrically connected to the electrodes 63 to 65 in the manner known for known straight piezoelectric actuators having a bender construction.

Electrical connection to the electrodes 63 to 65 may be made in the same way as is known for known straight actuators having a bender construction, in principle at any point along the length of the actuator 40 of which the portion 60 forms part but preferably at the end. The preferred technique is to provide the electrodes with fingers (not shown) extending at the end of the actuator 40 at different lateral positions across the width of the actuator 40 as known for straight actuators having a bender construction.

It will be appreciated that other bender constructions could equally be applied to the portion 60, for example a unimorph bender construction comprising a layer of electro-active material and an inactive layer or a multimorph bender construction comprising a plurality of layers of electro-active material.

Whilst the bender construction illustrated in Fig. 6 is preferred for simplicity and ease of manufacture, it will be appreciated that the continuous member 42 could in fact have any construction which bends around the minor axis 43 on activation. For example, the continuous member 42 could be an electro-active actuator of the type described in WO-02/103818 in which the elements have two pairs of electrodes extending along the length of the member for bending across the width on activation.

On activation, the electro-active portions 60 of the continuous member 42 bends around the minor axis 43. As a result of the continuous electro-active member 42 curving around the minor axis 43, in particular in a helix, such bending is concomitant with twisting of the continuous member 42 around the minor axis 43. This may be visualised as the turns of the continuous member 42 as the bending tightening or loosening causing a twist of the member 42 along the minor axis 43. The twist of the continuous member 42 occurs along the entire length of the minor axis 43. It may be thought of as each section of the actuator 40 along the minor axis 43 twisting around the minor axis 43. This causes a relative rotation of the ends 45, 46 of the actuator 40.

It will be appreciated that the continuous member 42 could curve around the minor axis 43 in curves other than a helix to produce such twisting, for example by having the shape as though formed by twisting a flat member round the minor axis. It will also be appreciated that structures other than the continuous member 42 could be applied to produce twisting around the minor axis. For example the structure of the electro-active actuator 40 could consist of a plurality of electro-active portion disposed successively along the minor axis and coupled together so that the bending of each individual portion twists the adjacent portion around the minor axis causing twisting of the actuator 40 as a whole. Alternatively the electro-active actuator 40 could be an actuator of the type described in WO-02/103817 which comprises a plurality of electro-active torsional actuators which may comprise electro-active elements activated in shear mode. The twisting of the continuous member 42 around the minor axis 43 is concomitant with relative displacement of the ends 45 and 46 of the actuator 40 perpendicular to the curve of the minor axis 43, that is parallel to the major axis 44. The relative displacement of the ends 45 and 46 derives from the twisting of the continuous member 42 around the minor axis 43 in combination with the curve of the minor axis 43. It is an inevitable result that twisting of a curved object causes relative displacement of the ends of that object perpendicular to the local curve of the object. The relative displacement caused by any given small section of the actuator 40 along the minor axis 43 causes relative displacement of the ends of that section perpendicular to the local curve of the minor axis 43. The overall displacement of the ends 45, 46 of the actuator 40 is the sum of the displacements of all the sections which results in an overall relative displacement parallel to the major axis 44.

The exact construction and dimensions of the member 42 and the form of the electro-active actuator 40 may be freely varied to produce the desired response. A suitable member 42 has a 0.24mm thickness tape wound as a 1.05mm diameter minor helix around the minor axis 43. When this forms the actuator 40 in which the minor curve extends around about three quarters of a circle of 7mm diameter the observed displacement is about ±0.325mm.

The actuator 40 is an example of the type of actuator disclosed in WO- 01/47041 which is incorporated herein by reference. The actuator 40 could have any of the alternative features disclosed in WO-01/47041.

In accordance with the present invention the actuator 40 is electrically activated to create mechanical displacement between the ends 45 and 46 , although the actuator 40 is capable of being mechanically activated in which case relative displacement of the ends 45 and 46 causes an electrical voltage to be developed across the electrodes 63 to 65.

The actuator 40 may be manufactured using the techniques described in WO- 01/47041 and WO-02/103451.

The actuator 40 is attached at one end 45 to the annular flange 34 which protrudes from the rear lens element 16. The actuator 40 is attached at the other end 46 to the holder 6 of the front lens element 8, as best seen in Fig. 7, in which the suspension elements 32 are omitted for clarity. Within the camera lens arrangement, the actuator 40 is disposed around the front lens element 8 and also around the front lens 2 of the rear lens element 16. Thus, the actuator 40 is arranged with the minor axis 43 extending around the optical axis O and the major axis 44 extending parallel to the optical axis O. Thus, on activation of the actuator 40, the relative displacement of the ends 45 and 46 of the actuator 40 is in the same direction as the optical axis O and so the actuator 40 drives movement of the front lens element 8 relative to the rear lens element 16 along the optical axis O. This movement is guided by the suspension system comprising the two suspension elements 32, as discussed above. The actuator 40 has a diameter which is slightly larger than the outer diameter of the holder 6 of the front lens element 8 so that the actuator 40 sits closely therearound. Consequently, as the lens 1 of the front lens element 6 has a smaller width than the widest lens 4 of the rear lens element 16, as viewed along the optical axis O the actuator 40 is disposed partially inside the area of the widest lens 4 of the rear lens element 16 and similarly partially within the area of the image sensor 24. Ih this case more than 50% of the radial extent of the actuator 40 is inside the area of the widest lens 4 of the rear lens element 16 and inside the area of the image sensor 24. hi other embodiments, the actuator 40 is entirely inside the area of the widest lens 4 of the rear lens element 16 and entirely inside the area of the image sensor 24. The overlap of the actuator 40 with the widest lens 4 of the rear lens element

16 and the image sensor 24 minimises the dimensions of the lens arrangement in directions perpendicular to the optical axis O as compared to an equivalent arrangement in which the actuator 40 were to be arranged extending around the widest lens of a system of lenses, as would be the case for an equivalent lens arrangement in which all the lenses are moved relative to the image sensor 24 to achieve focussing.

Furthermore, as compared to a lens arrangement in which all the lenses are moved to achieve focussing, in the present lens arrangement the force and displacement requirements of the actuator 40 are greatly reduced. It is only needed to move the front lens element 8 which has a low mass compared to the overall mass of both lens elements 8 and 16. For example in the miniature camera described above the front lens element 8 has a mass of 35mg, whereas in a camera module previously produced in which the entire lens system is moved for focussing and providing similar optical performance the movable lens holder including all the lenses has a mass of 300mg. Also, the front lens element 8 only needs to move a relatively short distance to achieve focussing due to the relatively small size of the lens 1. This reduces the size that the actuator 40 needs to be and hence the overall size of the lens arrangement.

For example in the miniature camera described above, the actuator 40 has an outer diameter of 7mm, an inner diameter of 4.8 mm and a length around the minor axis 43 of 200° to 240°. In contrast a camera module previously produced in which the entire lens system is moved for focussing and providing similar optical performance used an actuator of the same form having an outer diameter of 1 lmm, an inner diameter of 8.8 mm and a length around the minor axis of 280°. One can also compare the volume of camera modules including a camera lens assembly together with an image sensor. The miniature camera module described above has external dimensions of 8.55mm by 8.55mm by 8.15mm giving a volume of about 0.6cm³. hi contrast considering currently commercially available products providing similar optical performance, a camera module having a fixed focus typically has a volume of the order of 0.7cm³ and a camera module having a variable focus provided by movement of the entire lens system typically has a volume of the order of 1.0cm³ or more.

The reduced size of the actuator 43 and the fact it is coupled to the front lens element 8 being of relatively low mass also improves the robustness, hi particular, impacts on the camera module as commonly occur in portable electronic products cause a lower degree of displacement of the actuator 40 providing a lower risk of stressing the actuator 40 to cause damage. For example one can compare the miniature camera described above in which the front lens element 8 has a mass of 35mg and the actuator 40 has an outer diameter of 7mm with the equivalent camera module in which the entire lens system is moved for focussing and the moving lens holder has a mass of 300mg and the actuator has an outer diameter of 1 lmm. On these figures, it can be seen that the moment acting on the actuator 40 in the event of an impact is of the order of 10 times for the miniature camera described above.

Claims

1. A camera lens arrangement comprising: a front lens element comprising either a single lens or plural lenses fixed relative to each another; and a rear lens element comprising either a single lens or plural lenses fixed relative to each other, the lenses of the first and second lens elements being arranged in series along an optical axis, the front lens element being movable relative to the rear lens element along the optical axis, and the single lens or all the plural lenses of the front lens element having a smaller width than the single lens or the widest lens of the rear lens element, the camera lens arrangement further comprising an actuator arranged, on activation, to move the front lens element relative to the rear lens element along the optical axis, the actuator extending around the front lens element and being disposed at least partially inside the area of the single lens or the widest lens of the rear lens element as viewed along the optical axis.

2. A camera lens arrangement according to claim 1 , wherein the actuator is an electro-active actuator extending between two ends along a minor axis which curves around the optical axis, the electro-active actuator being arranged, on activation, to twist around the minor axis concomitantly with relative displacement of the ends of the electro-active actuator in the same direction as the optical axis, the ends of the electro-active actuator being connected between the first and second lens elements to move the front lens element relative to the rear lens element along the optical axis.

3. A camera lens arrangement according to claim 2, wherein the electro-active actuator comprises electro-active portions disposed successively along the minor axis and arranged to bend, on activation, around the minor axis.

4. A camera lens arrangement according to claim 3, wherein the electro-active actuator comprises a continuous electro-active member curving around the minor axis, said electro-active portions being adjacent finite portions of the continuous member.

5. A camera lens arrangement according to claim 4, wherein the continuous electro-active member curves in a helix around the minor axis.

6. A camera lens arrangement according to any one of claims 3 to 5, wherein the successive electro-active portions have a bender construction including a plurality of layers including at least one layer of electro-active material and electrodes for application of an electric field to activate the at least one layer of electro-active material.

7. A camera lens arrangement according to any one of claims 2 to 6, wherein the minor axis extends in a curve which is planar.

8. A camera lens arrangement according to any one of claims 2 to 7, wherein the electro-active actuator is a piezoelectric actuator.

9. A camera lens arrangement according to claim 1, wherein the actuator is a voice-coil motor.

10. A camera lens arrangement according to any one of the preceding claims, wherein the single lens or the front lens of the rear lens element is no wider than the single lens or the widest of the plural lenses of the front lens element, the actuator extending around the front lens element and around the single lens or the front lens of the rear lens element.

11. A camera lens arrangement according to any one of the preceding claims, wherein at least 20% of the radial extent of the actuator is disposed inside the area of the single lens or the widest lens of the rear lens element as viewed along the optical axis.

12. A camera lens arrangement according to any one of the preceding claims, wherein at least 50% of the radial extent of the actuator is disposed inside the area of the single lens or the widest lens of the rear lens element as viewed along the optical axis.

13. A camera lens arrangement according to any one of the preceding claims, wherein the entirety of the actuator is disposed inside the area of the single lens or the widest lens of the rear lens element as viewed along the optical axis.

14. A camera lens arrangement according to any one of the preceding claims, further comprising an image sensor, the rear lens element being fixed relative to the image sensor.

15. A camera lens arrangement according to claim 14, wherein the actuator is disposed at least partially inside the area of the image sensor as viewed along the optical axis.

16. A camera lens arrangement according to claim 14, wherein the actuator is disposed entirely inside the area of the image sensor as viewed along the optical axis.

17. A camera lens arrangement according to any one of claims 14 to 16, wherein all the single lens or all the plural lenses of the front lens element have a lower degree of asphericity than the single lens of the second lens element or the lens of the second lens element having the highest asphericity.

18. A camera lens arrangement according to any one of the preceding claims, wherein the rear lens element comprises a housing mounting the single lens or all the plural lenses of the rear lens element.

19. A camera lens arrangement according to claim 18, wherein the housing is rigid and has a seat which abuts the single lens of the second lens element around its periphery or seats which abut each lens of the rear lens element around its periphery.

20. A camera lens arrangement according to claim 18 or 19, wherein the housing is a unitary member.

21. A camera lens arrangement according to any one of claims 18 to 20 when appendant to claim 14, wherein the image sensor abuts the housing of the rear lens element.

22. A camera lens arrangement according to claim 21, wherein the housing is an opaque barrel inside which the single lens or all the lenses of the rear lens element are mounted, the barrel covering the image sensor entirely around the optical axis.

23. A camera lens arrangement according to any one of the preceding claims, wherein the front lens element has a housing mounting the single lens or the plurality of lenses.

24. A camera lens arrangement according to claim 23, wherein the single lens or the plurality of lenses of the front lens element are fitted in a carrier mounted to the housing on a screw fitting providing adjustment of the position of the single lens or the plurality of lenses relative to the housing along the optical axis.

25. A camera lens arrangement according to any one of the preceding claims, further comprising a suspension system which suspends the front lens element on the rear lens element and guides movement of the front lens element relative to the rear lens element.