WO2011055280A1

WO2011055280A1 - Optical-beam manipulation device

Info

Publication number: WO2011055280A1
Application number: PCT/IB2010/054888
Authority: WO
Inventors: Dirk Kornelis Gerhardus De Boer; Marcellinus Petrus Carolus Michael Krijn; Steven Lawrence Nyabero
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2009-11-03
Filing date: 2010-10-28
Publication date: 2011-05-12
Anticipated expiration: 2012-05-03
Also published as: TW201207543A

Abstract

An optical-beam manipulation device comprises first and second optically transparent substrates (3,4), a liquid crystal layer (2) sandwiched therebetween, and in-plane switching electrodes (20) arranged at a side of said first substrate. The optical-beam manipulation device is controllable between optical-beam manipulation states, each permitting passage of light through said optical-beam manipulation device in a direction perpendicular thereto. The device comprises a dielectric layer (24) between the electrodes and the liquid crystal layer and having a thickness between IOnm and 10μm.

Description

Optical-beam manipulation device

FIELD OF THE INVENTION

The invention relates to an optical-beam manipulation device which is controllable between at least two optical-beam manipulation states, each permitting passage of an optical beam through the optical-beam manipulation device.

BACKGROUND OF THE INVENTION

Active manipulation of an optical beam (e.g. light) is useful for various applications ranging from general lighting to special lighting applications, such as a video flash in which the zoom function of the camera is coupled to the beam width control function of an active optical element. Liquid crystal optics would appear to be suitable for this purpose. The alignment orientation of liquid crystal molecules in a liquid crystal cell can be controlled by applying an electric field thereto. Using this reorientation of the liquid crystal molecules a refractive index gradient may be created within a layer of liquid crystal material, which leads to a light ray passing through the liquid crystal layer being redirected. Hereby, the direction and/or shape of a light beam can be controlled electrically.

One application of optical-beam manipulation devices of particular interest is in the field of autostereoscopic display devices, which include a display panel having for example a regular array of display pixels for producing a display and an imaging arrangement for directing different views to different spatial positions. It is well known to use an array of elongate lenticular elements which are provided extending parallel to one another and overlying the display pixel array as the imaging arrangement, and the display pixels are observed through these lenticular elements.

In an arrangement in which, for example, each lenticule is associated with two columns of display pixels, the display pixels in each column provide a vertical slice of a respective two dimensional sub-image. The lenticular sheet directs these two slices and corresponding slices from the display pixel columns associated with the other lenticules, to the left and right eyes of a user positioned in front of the sheet, so that the user observes a single stereoscopic image. The sheet of lenticular elements thus provides a light output directing function to create parallactic views. In other arrangements, each lenticule is associated with a group of four or more adjacent display pixels in the row direction. Corresponding columns of display pixels in each group are arranged appropriately to provide a vertical slice from a respective two dimensional sub-image. As a user's head is moved from left to right, a series of successive, different, stereoscopic views are perceived creating, for example, a look-around impression.

The above described device provides an effective three dimensional display. However, it will be appreciated that, in order to provide stereoscopic views, there is a necessary reduction in the horizontal resolution of the device. This reduction in resolution is disadvantageous for certain applications, such as the display of small text characters for viewing from short distances or generally small displays such as in handheld devices. For this reason, it has been proposed to provide a display device that is switchable between a two- dimensional mode and a three-dimensional (stereoscopic) mode.

One way to implement this is to provide an electrically switchable lenticular array. In the two-dimensional mode, the lenticular elements of the switchable device operate in a "pass through" mode, i.e. they act in the same way as would a planar sheet of optically transparent material. The resulting display has a high resolution, equal to the native resolution of the display panel, which is suitable for the display of small text characters from short viewing distances. The two-dimensional display mode cannot, of course, provide a stereoscopic image.

In the three-dimensional mode, the lenticular elements of the switchable device provide a light output directing function, as described above. The resulting display is capable of providing stereoscopic images, but has the inevitable resolution loss mentioned above.

In order to provide switchable display modes, the lenticular elements of the switchable device can be formed as an optical-beam manipulation arrangement of an electro- optic material, such as a liquid crystal material, having a refractive index that is switchable between at least two values. The device is then switched between the modes by applying an appropriate electrical potential to planar electrodes arranged above and below the lenticular elements. The electrical potential alters the refractive index of the lenticular elements in relation to that of an adjacent optically transparent layer. A more detailed description of the structure and operation of the switchable device can be found in US patent number

6,069,650.

WO 2008/126049 discloses an optical-beam manipulation device which uses first and second in-plane electrodes, which generate an in-plane electric field within a liquid crystal layer. This is found to enable a larger refractive index gradient in the liquid crystal layer such that refraction of optical beams occurs. Thereby a more efficient beam

divergence/convergence can be achieved. In preferred arrangements, the optical-beam manipulation device has a set of electrodes, driven to different potentials, to define a smooth change in refractive index across the shape of the optical-beam manipulation device. This document also discloses the use of additional thick layers to increase the focal distance by influencing the electric field that is generated within the liquid crystal layer.

SUMMARY OF THE INVENTION

It has now been found that in cases where the size of the optical-beam manipulation device is to be reduced as far as possible, optical imperfections become noticeable. This invention is based on the recognition that these imperfections result because the electrodes function as gratings, giving rise to diffraction effects.

It is an object of the invention to provide an improved device with respect to the imperfections.

The object is achieved with the present invention which is defined by the independent claims. The dependent claims provide advantageous embodiments.

The arrangement of the optical-beam manipulation device according to the invention and having dielectric layers applied between the electrodes and the liquid crystal layer reduces the diffraction effects caused by the electrodes. The inventors have found that electric fields in the near vicinity of the electrodes, layer lead to diffractive layers being formed in the liquid crystal layer. The dielectric layers according to the invention reduce these electric fields and therewith reduce the formation of diffractive layers and their diffractive effect. They do so however without adversely affecting the overall optical-beam manipulation function of the device. The dielectric layer can for example comprise silicon nitride as that has appropriate dielectric properties.

Optical-beam manipulation in the context of the invention includes manipulation of an optical beam (light) with respect to its propagation direction and/or its crossectional shape and/or its diverging-converging characteristics. These manipulations may be applied independent to polarization state of the optical beam. For example either or both of the linear polarization directions of an optical beam may be manipulated.

Thus e.g. in a first manipulation state the optical beam may have a first direction and/or shape and/or polarization while in the second manipulation state the direction and/or the shape and/or the polarizations are different from the respective ones in the first manipulation state.

In one embodiment, the device, in its first manipulation state may be a lens (also called lenticular element or lenticule) with a first focal distance, while in the second state it may be a lens having a second focal distance different from the first focal distance. Alternatively or additionally, the lens in the second state may have dimensions such as width, diameter, circumference or pitch, when it is part of a lens array that differs from the corresponding parameter in the first state. This provides one way of implementing the optical beam manipulation as defined here above. In addition, when used in an autostereoscopic display according to the invention, depth modes and/or multi view modes of the display may be adjusted. In case the second state of the device represents the pass through mode, the autostereoscopic device is with this second state provided with the 2D mode of viewing in addition to its 3D mode of viewing given by the first (lens) state.

Each lens is preferably associated with a set of electrodes. This set of electrodes defines the refractive index pattern in the liquid crystal layer when the lens function of the device is activated, and enables a smooth refractive index gradient within the liquid crystal layer and a smooth transition of refractive index during switching (control) of the optical-beam manipulation device from its first optical beam manipulation state to its second optical-beam manipulation state.

A set of voltages is applied to the electrodes, and the same set of voltages can be applied to the set of electrodes of each lens when multiple lenses are defined by the device. In this case, the set of voltages can be provided on a bus of voltage lines, with the electrodes of the set connected to the voltage lines at taps. The set of voltages can be applied to the electrodes of one lenticular lens in opposite order to the electrodes of the adjacent lenticular lenses.

Through change of the voltages in a set of voltages the lens shape may be defined and adjusted to desire. In addition by altering the set of electrodes, i.e. adding or removing electrodes from a set also the size and/or circumference of the lens may be altered.

The autostereoscopic display device according to the invention having adjustable viewing mode and incorporating the optical-beam manipulation device benefits particularly from the improved performance of the optical-beam manipulation device as such display devices are directly observed by viewers often for prolonged periods of time, such that optical defects such as unwanted diffraction effects are easily noticed and may become particularly annoying. The imaging arrangement can comprise a multi- or one-dimensional array of lenticular lenses, particularly for the autostereoscopic display application, a one-dimensional array of parallel oriented semi-cylindrical lenses is preferably used. A preferred variant is a one dimensional array wherein multiple elongate semi-cylindrical lenticular elements are arranged side by side.

In one implementation of the lenticular lens array for an autostereoscopic display, the different spatial positions have an angular separation of ΔΘ, and wherein the number of electrodes associated with each lenticular lens satisfies: p

electrodes ' ^ wherein Punticuiar is the pitch of the lenticular lenses and λ is the wavelength of the optical beam.

This further reduces diffraction effects by maintaining a minimum electrode spacing, for example such that first and higher order diffraction effects are avoided.

The voltages applied to the set of electrodes can vary non-linearly with the position of the respective electrodes across the lenticular lens. This enables the optical function to be controlled to give improved optical performance. For example, the voltages applied to the set of electrodes can vary quadratically with the position of the respective electrodes across the lenticular lens.

The set of voltages can be provided by a ladder of impedances with the voltages provided at tapping points between adjacent impedances. This provides a simple way of generating multiple voltage values.

The device and/or the autostereoscopic display may have a controller for providing the sets of voltages to the electrodes. Such controller preferably is an electrical device comprising a computer chip or integrated circuit.

The invention also provides a method of controlling an optical-beam manipulation device, the method benefitting from the advantages of the optical-beam manipulation device.

The invention provides a method for controlling an autostereoscopic display between at least two viewing modes. BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention, wherein:

Fig. la is a perspective view of an exemplary optical-beam manipulation device which can be modified in according with the present invention;

Fig. lb is a cross-section view of the optical-beam manipulation device in Fig. la along the line A- A' when no voltage is applied across the electrodes;

Fig. lc is a cross-section view of the optical-beam manipulation device in Fig. la along the line A- A' when a voltage V is applied across the electrodes;

Figs. 2a and 2b show the characteristics of a known lens design;

Figs. 3a and 3b show how a known lens design is modified in accordance with the invention;

Fig. 4 shows the improved lens characteristics obtained by the invention; Fig. 5 shows a known autostereoscopic display device;

Figs. 6 and 7 are used to illustrate how a known switchable autostereoscopic display device can function;

Fig. 8 shows the required lens function for an autostereoscopic display device;

Fig. 9 shows a way to provide voltages to the lenses of the lenticular array of the autostereoscopic display device;

Fig. 10 shows the arrangement of Fig. 9 in perspective view;

Fig. 11 shows a first way to implement the generation of different voltages using an impedance ladder;

Fig. 12 is used to explain the benefit of a non-linear voltage profile on the electrodes of the lenses of the lenticular array; and

Fig. 13 shows a second way to implement the generation of different voltages.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, the present invention is described with reference to an optical beam manipulation device having a homeotropically aligned liquid crystal layer - the liquid crystal (LC) molecules comprised in the LC layer are oriented perpendicular to the substrates when no voltage is applied to the electrodes. It should be noted that this by no means limits the scope of the present invention, which is equally applicable to optical-beam manipulation devices in which the liquid crystal layer is aligned in any other way, such as a planar orientation in which the LC-molecules are oriented in a plane parallel with the substrates. In this orientation, the LC-molecules may be aligned in parallel with or perpendicular to the electrodes, or have a hybrid orientation where the LC molecules have a first orientation adjacent to the first substrate and a second orientation, orthogonal to the first orientation, adjacent to the second substrate.

Furthermore, in order not to obscure the present invention by details not directly related thereto, further layers well known to a person skilled in the art, such as alignment layers for aligning the LC-molecules etc have neither been depicted in the accompanying drawings, nor described in detail herein.

It should be noted that the drawings are not to scale. To, however, give an idea of suitable dimensions, it can be said that the width of a conductor line in the electrodes would typically range from 1 micrometer to 20 micrometer. Furthermore, the conductor lines are typically spaced apart by 10 micrometer to 100 micrometer, and the thickness of the LC layer is generally between 5 micrometer and 50 micrometer.

In one aspect, the invention relates generally to optical-beam manipulation devices suitable for many different applications, and in another aspect, the invention relates more specifically to additional features which make the use of the optical-beam manipulation device of particular interest for a 3D display device or a 2D/3D switchable display device. The general concepts and design of the optical-beam manipulation device will first be described, followed by an explanation of the additional features particularly relevant to the 3D display field (although these additional features also have more general application).

The present invention builds on the approach described in WO 2008/126049. All examples of use of the optical-beam manipulation device described in WO 2008/126049, and the different electrode designs presented, can be employed in the device of the present invention. A description of all of these variations will not be presented in this application, and the reader is referred to WO 2008/126049 for further details.

Figs, la-c schematically illustrates an exemplary optical-beam manipulation device as described in WO 2008/126049 and to which this invention can be applied.

In Fig. la, an optical-beam manipulation device 1 is shown, comprising a homeotropically aligned liquid crystal (LC) layer 2 sandwiched between first 3 and second 4 transparent substrates. On the first substrate 3, facing the LC layer 2, first 5 and second 6 comb-shaped transparent electrodes are provided. By applying a voltage V over these electrodes 5, 6, a collimated light beam 7 incident on the optical-beam manipulation device can be deflected as is schematically illustrated in Fig. la. Fig. lb, which is a cross-section view along the line A- A' in Fig. la, schematically shows the situation where no voltage is applied across the electrodes 5, 6. Since no voltage is applied, no electric field is formed, and, consequently, the LC-molecules have the orientation imposed on them by the alignment layers (not shown). In the case illustrated in Fig. lb, the LC-molecules are homeotropically aligned, and the shape of the incident light beam 7, here represented by three parallel rays 1 la-c of light is unchanged by the passage through the optical-beam manipulation device 1.

With reference to Fig. lc which schematically shows the situation where the voltage V is applied across the electrodes 5, 6, the optical-beam manipulation mechanism utilized by the optical-beam manipulation device in Fig. la will now be described in more detail.

As is schematically shown in Fig. lc, the liquid crystal (LC) molecules lOa-c comprised in the LC layer 2 are aligned to the electric field lines between the electrodes 5, 6. Due to this reorientation, regions of the LC layer 2 having different refractive indices are formed. In the exemplary case illustrated in Fig. lc, the refractive index experienced by a light beam 7 hitting the optical-beam manipulation device 1 in a direction which is (locally) perpendicular thereto varies between the ordinary refractive index n₀ resulting from LC molecules 10a oriented perpendicular to the LC layer 2 and the extraordinary refractive index n_e resulting from LC molecules 10c oriented in parallel with the LC layer 2. Light hitting the optical-beam manipulation device 1 between a portion thereof with "perpendicular" LC- molecules 10a and a portion thereof with "parallel" LC-molecules 10c will experience an intermediate refractive index, hitting LC-molecules 10b. The molecule alignment follows an in-plane electric field. By this is meant that the field lines pass between electrodes which are substantially in the same plane. The field lines are curved and extend into the LC, but the field lines are parallel to the plane of the LC layer over at least a part of their length in order to define a continuous path from one electrode to another. The overall effect is to define a graded refractive index (GRIN) lens within the LC layer.

In Fig. lc, the three rays 12a, b, c representing the linear polarization component of unpolarized light having a direction of polarization which is perpendicular to the long axis of the LC molecules (ordinary rays) pass through the optical-beam manipulation device 1 practically without experiencing a refractive index gradient. Thus neither of these rays 12a-c has its direction altered significantly during passage through the LC-layer 2.

The other polarization component, rays 13a, b, c, representing light polarized in the plane of the long axis of the molecules (extraordinary rays) on the other hand experience a refractive index gradient and are therefore refracted as is schematically indicated in Fig. lc.

Consequently, a maximum of 50% of the light in an unpolarized light beam 7 is controllable by the optical-beam manipulation device 1 in Figs. la-c.

As described in WO 2008/126049, by stacking of optical-beam manipulation elements, control of substantially all of the light in an unpolarized light beam can be achieved.

As mentioned above, this invention is based on the recognition that the optical properties of the optical-beam manipulation device deteriorate when the dimensions of the device are reduced. Simulations of GRIN lenses suggest that diffraction is caused by the LC grating structures formed by the electrodes. The refractive index and angular profiles obtained have undulations, with the undulations observed in the angular profiles being more prominent.

It is advantageous to have more than two electrodes per lens in order to be able to vary the voltages non-linearly across the lens. Fig. 2 shows a theoretical analysis of a GRIN lens with 23 electrodes. The refractive index profile is shown in Fig. 2(a) for two different drive voltages and the angular profile is shown in Fig. 2(b). These plots have prominent undulations that are a result of diffraction effects in GRIN lenses.

The number of undulations corresponds to the number of electrodes under a single lens. This implies that the electric field lines between two neighboring electrodes form an unwanted grating within the LC layer which causes diffraction.

This realization has also been confirmed by illuminating a GRIN lens sample (when it is on) with a laser spot, and finding that a diffraction pattern is observed. The resulting diffraction pattern agrees with the electrode periods of the respective GRIN lens sample.

The invention provides a dielectric (dielectric) layer between the electrodes and the LC layer. The dielectric layer serves the purpose of suppressing relatively weak electric fields between adjacent electrodes, while preserving the desired parabolic electric fields lines that orient LC directors to form a lens.

Fig. 3 shows schematically how the dielectric layer is used to modify the optical-beam manipulation device when implemented as a GRIN lens.

Fig. 3(a) shows the basic GRIN lens design, with the in-plane electrodes 20 at the surface of an insulator layer 21 , in contact with the LC layer 22 in which the refractive index pattern is defined. Fig. 3(b) shows the dielectricdielectric layer 24 between the electrodes and the LC layer which is used to reduce the effects of the grating in the LC layer.

A transparent dielectric or insulating layer can be used to suppress the electric fields that form grating structures in the LC layer. Silicon nitride (S1₃N₄; ε_Γ=6) is preferred. The thickness of the dielectric layer ranges from 10 nanometer to 10 micrometer. Generally, the thickness of the dielectric layer is less than 10 micrometer and it may be less than 5 micrometer. It is preferably thicker than 100 nanometer. The use of a thin dielectricdielectric layer avoids the lens function being affected, so that the function of the dielectricdielectric layer is limited to the suppression of the diffraction effects.

Fig. 4 shows the simulated GRIN lens refractive index profile obtained when a 5 micrometer thick dielectricdielectric layer of S1₃N₄ is used, for the same electrode design as represented in Fig. 2(a). This shows that the undulations have been removed.

Experimentation with samples with the dielectric layer also shows significant improvement compared to samples without the dielectric layer. The first order peaks of the diffraction pattern obtained from a GRIN sample with a dielectric layer are found to be significantly smaller.

An additional advantage of the dielectric layer 24 is that it prevents shorts between neighboring electrodes. Such shorts may be especially occurring as a consequence of minimization of size of the device, which causes that electrodes become situated nearer to each other.

Various techniques can be used to deposit the desired dielectric material layers. Methods such as chemical vapor deposition/low pressure chemical deposition are preferred to deposit silicon nitride.

As mentioned above, optical-beam manipulation devices designed in accordance with the principles of the invention can have particular application in the field of 3D displays such as those switchable between two or more viewing modes one prominent example being displays that offer a 2D and a 3D viewing mode.

Fig. 5 is a schematic perspective view of a known direct view autostereoscopic display device 100. The known device 100 comprises a liquid crystal display panel 103 of the active matrix type that acts as a spatial light modulator to produce the display.

The display panel 103 has an orthogonal array of display pixels each one being subdivided into a number of sub-pixels 105 arranged in rows and columns according to regular standard buildup. For example in the embodiment described the display pixels may be composed of triplets of sub-pixels, where the sub-pixels are Red, Green and Blue. For the sake of clarity, only a small number of display sub-pixels 105 are shown in the Fig.. In practice, the display panel 103 might comprise about one thousand rows and several thousand columns of display sub-pixels 105.

The structure of the liquid crystal display panel 103 is entirely conventional. In particular, the panel 103 comprises a pair of spaced transparent glass substrates, between which an aligned twisted nematic or other liquid crystal material is provided. The substrates carry patterns of transparent indium tin oxide (ITO) electrodes on their facing surfaces.

Polarizing layers are also provided on the outer surfaces of the substrates.

Each display sub-pixel 105 can comprise opposing electrodes on the substrates, with the intervening liquid crystal material there between. The shape and layout of the display sub-pixels 105 are determined by the shape and layout of the electrodes. The display sub-pixels 105 are regularly spaced from one another by gaps.

Each display sub-pixel 105 is associated with a switching element, such as a thin film transistor (TFT) or thin film diode (TFD). The display sub-pixels are operated to produce the display by providing addressing signals to the switching elements, and suitable addressing schemes will be known to those skilled in the art.

The display panel 103 is illuminated by a light source 107 comprising, in this case, a planar backlight extending over the area of the display pixel array. Light from the light source 107 is directed through the display panel 103, with the individual display sub- pixels 105 being driven to modulate the light and produce the display.

In case of a black and white display instead of a color display, the display panel has black and white pixels such that in the description given above for the color display the display sub-pixels are the same as the black and white display pixels.

The display device 100 also comprises a lenticular sheet 109, arranged over the display side of the display panel 103, which performs a view forming function. The lenticular sheet 109 comprises a row of lenticular elements 111 extending parallel to one another, of which only one is shown with exaggerated dimensions for the sake of clarity.

The lenticular elements 111 are in the form of convex cylindrical lenses, and they act as a light output directing means to provide different images, or views, from the display panel 103 to the eyes of a user positioned in front of the display device 100.

The autostereoscopic display device 100 shown in Fig. 5 is capable of providing several different perspective views in different directions. In particular, each lenticular element 111 overlies a small group of display sub-pixels 105 in each row. The lenticular element 111 projects each display sub-pixel 105 of a group in a different direction, so as to form the several different views. As the user's head moves from left to right, his/her eyes will receive different ones of the several views, in turn.

It has been proposed to provide electrically switchable lens elements, as mentioned above. This enables the display to be switched between 2D and 3D modes.

Figs. 6 and 7 schematically show an array of electrically switchable lenticular elements 115 which can be employed in the autostereoscopic display. The array comprises a pair of transparent glass substrates 119, 121, with transparent electrodes 123, 125 formed of indium tin oxide (ITO) provided on their facing surfaces. An inverse lens structure 127, formed using a replication technique, is provided between the substrates 119, 121, adjacent to an upper one of the substrates 119. Liquid crystal material 129 is also provided between the substrates 119, 121, adjacent to the lower one of the substrates 121.

The inverse lens structure 127 causes the liquid crystal material 129 to assume parallel, elongate lenticular shapes, between the inverse lens structure 127 and the lower substrate 121, as shown in cross-section in Figs 2 and 3. Surfaces of the inverse lens structure 127 and the lower substrate 121 that are in contact with the liquid crystal material are also provided with an orientation layer (not shown) for orientating the liquid crystal material.

Fig. 6 shows the array when no electric potential is applied to the electrodes 123, 125. In this state, the refractive index of the liquid crystal material 129 for light of a particular polarization is substantially higher than that of the inverse lens array 127, and the lenticular shapes therefore provide a light output directing function, i.e. a lens action, as illustrated.

Fig. 7 shows the array when an alternating electric potential of approximately 50 to 100 volts is applied to the electrodes 123, 125. In this state, the refractive index of the liquid crystal material 129 for light of the particular polarization is substantially the same as that of the inverse lens array 127, so that the light output directing function of the lenticular shapes is cancelled, as illustrated. Thus, in this state, the array effectively acts in a "pass through" mode.

The skilled person will appreciate that a light polarizing means must be used in conjunction with the above described array, since the liquid crystal material is birefringent, with the refractive index switching only applying to light of a particular polarization. The light polarizing means may be provided as part of the display panel or the imaging arrangement of the device. Further details of the structure and operation of arrays of switchable lenticular elements suitable for use in the display device shown in Fig. 5 can be found in US patent number 6,069,650.

Fig. 8 shows the principle of operation of a lenticular type imaging arrangement as described above and shows the backlight 130, display device 134 such as an LCD and the lenticular array 138.

The manufacture of the device, shown in Figs. 6 and 7, uses replica lenticulars, which requires equipment that is not standard in production facilities. The use of an optical- beam manipulation device as described above, having laterally controlled graded index lens function, thus simplifies the manufacturing process.

When the optical-beam manipulation device is used to implement lenticular lenses, the electrodes run parallel to the elongate lens axis (so that the lens shape is defined across the lens width).

In the case of more than two electrodes per lens, a set of voltages is applied to the electrodes, and the same set of voltages is applied to the set of electrodes of each lenticular lens. In order to simplify the supply of voltages to the electrodes, the set of electrodes can be provided on a bus of voltage lines 140 as shown in Fig. 9.

Fig. 9 shows part of a layout in which each cylindrical lens of 0.38 mm wide is driven by 23 different voltages. The Fig. shows part of four cylindrical lenses, each of which is covered by 23 ITO electrodes running in the up-down direction.

The set of voltages are applied to the electrodes of one lenticular lens in opposite order to the electrodes of the adjacent lenticular lenses. This gives the triangular connection pattern of tapping points shown between the bus 140 and the electrode lines 142.

An increasing ramp of voltages and a decreasing ramp of voltages result in the same refractive-index profile. The electrodes are as long as the height of the display (tens of centimeters), i.e. they extend over the entire height of the display in the column direction, but only the top part of the electrodes 142 is shown in the vicinity of the bus 140. The bus lines 140 are on a different depth level to the ITO electrodes 142, separated by an isolation layer. The tapping points are implemented by connection vias between the two electrode layers.

Fig. 10 shows a three-dimensional sketch of the structure and shows the set of voltages 144 coupling to the voltage bus 140, and the silicon nitride insulator 146 through which the vias are formed.

A typical LC-based GRIN lenticular has a cell gap of tens of micrometers and requires DC voltages of several tens of volts for driving. The simplest way of driving is by applying the desired voltages directly to the electrodes or bus lines, from respective voltage sources. However, this is an expensive solution.

By knowing the desired ratios of voltages that have to be applied to the different electrodes, a resistive or capacitive divider can instead be used to form the different desired voltages. Fig. 11 shows a ladder of impedances using to form a set of voltages.

The simplest implementation is to generate a set of voltages which vary linearly with distance across the lenticular lens. However, improved lens performance can be obtained by providing a more complicated voltage profile. In one example, a voltage profile which varies quadratically with distance across the lens can be used.

Fig. 12 shows calculations for GRIN structures with multiple line electrodes at one side of the LC.

There are three plots on the left for a linear change of voltage with distance. The corresponding three plots on the right are for a quadratic change of voltage with distance. The top plot in each case shows the deflection angle 150, the linear fit to this 152 and the refractive index versus distance perpendicular to the electrodes 154. The centre plot shows the cross section of the LC cell showing calculated director profiles (small dashes) and the equi-potential lines. The electrodes are in layer 156. The bottom plots show the applied voltage versus distance perpendicular to the electrodes (i.e. across the lenticular lens).

The resulting refractive-index profile and deflection-angle profile for the linear voltage profile are not ideal, since for a good lens action, the deflection-angle profile should be as close to straight line sections as possible.

By changing the voltage as a function of distance, a desired refractive- index profile and deflection-angle profile can be obtained. The quadratic (parabolic) function shown in the bottom right plot of Fig. 12 gives a greatly improved resulting refractive-index profile and deflection-angle profile. The deflection-angle profile essentially consists of straight line sections, implying a good lens action.

The desired function can be, but need not be implemented with the impedance ladder shown in Fig. 11. The impedance values are chosen in such a way that the desired voltage profile is obtained. For example, for resistors, it can easily be shown that, if the ratio of the resistance values between the n^th and the 1^st resistor is R_n/Ri = 2n-l, a quadratic voltage profile V_n °= n² is obtained.

More generally, the ratio of the impedance values between the n^th and the 1^st impedance is

= 2n-l . This can be achieved, for instance, by using a resistor strip 160 with a changing width, from which the connections with the electrodes are branched off at positions that are chosen such that the desired voltage profile is obtained. This is shown in Fig. 13.

Resistors are only one example of a possible implementation of the

impedances. Instead, capacitors may be used. In this case, the capacitance of the electrode lines themselves are taken into account to ensure the capacitances used for the impedance ladder are larger than the electrode capacitances.

The examples above form the desired voltage profile in discrete steps.

Alternatively, however, a continuous voltage profile could be obtained. One embodiment could be the use of a high-ohmic continuous electrode plate with changing thickness, where the thickness profile is chosen such that the desired resistance profile is obtained. A quasi- continuous solution is also possible with a high-ohmic electrode plate that contains holes. Then the ratio between open area and electrode area determines the local voltage. To avoid discontinuities in the voltage profile, the holes should be small.

Thus, by changing the voltage set one can induce different lens shapes within the lc layer. Alternatively, or additionally, by adjusting the amount or ensemble of electrodes one can change the width or pitch of the lenticulars. The latter is advantageous for altering the number of views to be displayed in the 3D mode, as using that the number of sub-pixels within a group of sub-pixels that is directed into different views may be altered. As explained above, the additional dielectric layer reduces the diffraction effects. There may still be some residual electric fields left that form grating structures. Their effect on the optical quality can be minimized by choosing the number of electrodes beneath a GRIN lens not to exceed a certain number.

For the example of an autostereoscopic display using a lenticular lens arrangement, let Δφ be the angular separation between the 0^th order and 1^st order peak of the residual diffraction pattern (when a laser spot is used to illuminate the lens). Let ΔΘ be the angular separation between two neighboring views - determined by the pixel pitch of the display and the lenticular design. In order not to increase the overlap between neighboring views too much, it is preferred that Δφ < αΔΘ, with c being a constant. This implies an upper limit for the number of electrodes, N_eiectrodes, beneath a GRIN lens.

Since

and, in approximation the grating law gives

Δφ=λ/ά:

Here, d is the electrode pitch and P lenticular is the pitch of the lenticular lenses; λ is the wavelength of the light.

For example, in case Pi_enticuiar ⁼ 0.377 mm and the distance between neighboring views ΔΘ = 2.6°, this yields N_eiectrodes≤ 16 (based on c=0.5 and

nm).

Preferably, c < 0.5. Thus, preferably, N ^~ electrodes obeys the equation above with c=0.5.

There is of course also a lower limit for N ^~ electrodes^'- too low a value will result in a deviation from the ideal lens profile and will in turn result in a deterioration of the 3D quality.

Silicon nitride is only one example of a dielectric material suitable for the dielectric layer. Other examples are silicon oxide, silicon oxynitride and polymers.

Generally, the layer needs to be transparent when deposited with the desired thickness, and should have a value of ε_Γ ίη the range 1 to 20.

As mentioned above, various additional modifications are disclosed in WO 2008/126049 which can be employed. One modification of particular interest is to provide a counter electrode on the opposite side of the LC layer to the in-plane switching electrodes. This can be in the form of an additional layer of transparent material in contact with a transparent conductor, such as Indium-Tin-Oxide (ITO). This can be used to reduce the lens thickness (and therefore increase its focal length) by effectively compressing the electric field. The layer can be grounded, and the influence is that it imposes conditions on the electric field that are beneficial for the field distribution needed in the layer of LC.

It should be noted that the above-mentioned modifications and embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that the combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. An optical-beam manipulation device comprising first and second optically transparent substrates (3,4), a liquid crystal material (22) sandwiched therebetween, and electrodes (20) arranged at a side of said first transparent substrate for orienting at least part of the liquid crystal material, wherein said optical-beam manipulation device is controllable between at least first and second optical-beam manipulation states that differ from each other, each state permitting passage of an optical-beam through said optical-beam manipulation device from one of the optically transparent substrates to the other, wherein the optical-beam manipulation device comprises a dielectric layer (24) between at least part of the electrodes and the liquid crystal layer, the dielectric layer having a thickness between 10 nanometer and 10 micrometer.

2. An optical-beam manipulation device as claimed in claim 1, comprising an array of lenticular lenses (111).

3. An optical-beam manipulation device as claimed in claim 2, wherein each lenticular lens (111) is associated with a set of the electrodes (20; 142).

4. An optical-beam manipulation device as claimed in claim 3, wherein a set of voltages is applied to the electrodes (20; 142), and wherein the same set of voltage is applied to the set of electrodes of each lenticular lens, and wherein the set of electrodes are provided on a bus (140) of voltage lines, with the electrodes of the set connected to the voltage lines at taps.

5. An optical-beam manipulation device as claimed in claim 4, wherein the set of voltages are applied to the set of electrodes (142) of one lenticular lens in opposite order to the set of electrodes of the adjacent lenticular lenses.

6. An optical-beam manipulation device as claimed in claim 4, wherein the voltages applied to the set of electrodes (142) vary non-linearly with the position of the respective electrodes across the lenticular lens.

7. An optical-beam manipulation device as claimed in claim 6, wherein the voltages applied to the set of electrodes (142) vary quadratically with the position of the respective electrodes across the lenticular lens.

8. An optical-beam manipulation device as claimed in claim 3, wherein a set of voltages is applied to the electrodes (20; 142), and wherein the set of voltages is provided by a ladder of impedances (Z_{l s} Z₈) with the voltages provided at tapping points between adjacent impedances.

9. An autostereoscopic display device (100) switchable between a 3D viewing mode wherein a stereoscopic image may be viewed and a second viewing mode different from the first viewing mode, the autostereoscopic display device comprising:

a display panel (107) having an array of display pixels (105) for producing a display, the display pixels being arranged in rows and columns; and

an imaging arrangement (109) which in a first imaging mode directs the output from different pixels to different spatial positions to enable the 3D viewing mode, and which in a second imaging mode operates to provide the second viewing mode

wherein the imaging arrangement (109) comprises an optical-beam manipulation device as claimed in claim 1 , the first optical-beam shaping state for providing the first imaging mode and the second optical-beam shaping state for providing the second imaging mode..

10 An autostereoscopic display device as claimed in claim 9 wherein the second viewing mode is a 2D viewing mode, and wherein the second optical beam shaping state is a pass through state wherein the device acts as a transparent plate for providing a 2D imaging mode of the imaging arrangement.

11. An autostereoscopic display device as claimed in claim 9 or 10, wherein the optical-beam manipulation device in its first state comprises an array of parallel lenticular lenses (111), wherein each lenticular lens is associated with a set of electrodes, and wherein the different spatial positions have an angular separation of ΔΘ, and wherein the number of electrodes (20; 142) associated with each lenticular lens satisfies:

p

electrodes " λ

wherein P lenticular is the pitch of the lenticular lenses and λ is the wavelength of the light.

12. A method of manipulating an optical beam using a controlling an optical-beam manipulation device according to any one of the previous claims,

wherein the method comprises:

controlling the optical-beam manipulation device between optical-beam manipulation states, each permitting passage of light through said optical-beam manipulation device in a direction perpendicular thereto by applying a voltage across said electrodes (20; 142) thereby to generate an in-plane electric field, and applying the in-plane electric field to the LC layer through a dielectric layer (24) having a thickness between lOnm and ΙΟμιη.

13. A method as claimed in claim 12 for controlling the lens function of a lens of an autostereoscopic display device (100).

14. A method as claimed in claim 13, wherein the imaging arrangement comprises an array of parallel lenticular lenses (111), wherein each lenticular lens is associated with a set of electrodes (20; 142), wherein the voltages applied to the set of electrodes vary non- linearly with the position of the respective electrodes across the lenticular lens.

15. A method as claimed in claim 14, wherein the voltages applied to the set of electrodes vary quadratically with the position of the respective electrodes across the lenticular lens (111).