GB2405544A

GB2405544A - Light control element for outputting polarised light over different angular ranges.

Info

Publication number: GB2405544A
Application number: GB0320363A
Authority: GB
Inventors: Allan Evans; Paul Bonnett; Grant Bourhill; Lyndon Hill; Jonathan Mather; Martin David Tillin; Emma Walton; Robert Winlow; Philip Stubbs
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2003-08-30
Filing date: 2003-08-30
Publication date: 2005-03-02
Also published as: CN1601331A; KR100818817B1; KR20050022958A; CN100351676C; JP4601360B2; GB0320363D0; JP2005078093A

Abstract

A light controlling element 15 comprises vertical linear polarisers 16 positioned between transparent substrates 17, and serves to output light having a first polarisation state in a first angular range and light having a second, different polarisation state in a second, greater angular range. The linear polarisers 16 may be horizontal (Figure 5), and the system may be incorporated in a display having a display panel such as an LCD, switchable half wave plate and backlight (Figure 6). The light controller may be used in a three dimensional, autostereoscopic or dual view display. Also disclosed are manufacturing methods adhering stacked polarising sheets together, removing selected polariser regions and providing polarising material or light transmitting substrates in recesses in the element.

Description

A light control element and a display incorporating the same.

The present invention relates to a light control element that can control the angular range of light output from the element. It also relates to a light control device that incorporates such a light control clement and that is controllable to vary the angular range of light output from the device, and also to a display incorporating such a light control device.

Electronic display devices such as, for example, monitors used with computers and- screens built in to mobile telephones and other portable information devices, are usually designed to have as wide a viewing angle as possible, so that an image displayed by the device can be seen from many different viewing positions. However, there are some situations where it is desirable for an image displayed by a device to be visible from only a narrow range of viewing angles. For example, a person using a portable computer in a crowded train might want the display screen of the computer to have a small viewing angle so that a document displayed on the computer screen cannot be read by other passengers on the train. For this reason, there has been considerable effort put in to developing display devices which are electrically switchable between two modes of operation - in a 'public' display mode they have a wide viewing angle for general use, but they can be switched to a 'private' display mode in which they have a narrow viewing angle so that private information can be displayed in public places without being visible to people other than the user of the device.

A number of devices are known which restrict the range of angles or positions from which a display can be viewed.

U.S. patent No. 6 552 850 describes a method for the display of private information on an automatic teller machine (ATM). Light emitted by the machine's display has a fixed polarisation state, and the machine and its user are surrounded by a large screen of sheet polariser which absorbs light of that polarisation state but transmits light of the orthogonal polarization state. Passers-by can see the user and the machine, but cannot see information displayed on the machine's screen.

One known element for controlling the direction of light is a 'louvred' film as illustrated in Figure 1. The film I consists of alternating transparent layers 2 and opaque layers 3 provided in an arrangement similar to a Venetian blind. The film 1 operates on the same principle as a Venetian blind, and it allows light to pass through it when the light is travelling in a direction parallel to, or nearly parallel to, the opaque layers 3, as indicated by ray path 4 in Figure 1. However, light travelling at large angles to the plane of the opaque layers 3 is incident on one of the opaque layers and is absorbed, as- indicated by ray path 5 in Figure 1. The layers 2,3 may be perpendicular to the surface of the film, as shown in Figure 1, or they may be at some other angle to the surface of the film.

Louvred films of the type shown in Figure 1 may be manufactured by stacking many alternating sheets of transparent material and opaque material and then cutting slices of the resulting block perpendicular to the layers. This method has been known for many years and is described in, for example, US patent Nos. 2 053 173, 2 689 387 and 3 031 351.

Other manufacturing methods are known. For example, US patent No. RE27617 describes a process where a louvred film is cut continuously from a cylindrical billet of stacked layers. US patent No. 4 766 023 describes how the optical quality and mechanical robustness of the resulting film can be improved by coating with a UV- curable monomer and then exposing the film to W radiation. US patent No. 4 764 410 describes a similar process where a UV-curable material is used to bond the louvre sheet to a covering film.

Other methods exist for making films with similar properties to the louvred film. For example, US patent No. 5 147 716 describes a lightcontrol film which contains many elongated particles which are aligned in the direction perpendicular to the plane of the film. Light rays which make large angles to this direction are therefore strongly absorbed, whereas light rays propagating in this direction are transmitted.

Figure 2 shows another example of a light-control film 6, as described in US patent No. 528 319. The film 6 has a transparent body 7 in which are embedded opaque regions 8 that extend generally parallel to the plane of the film 6. The opaque regions 8 are arranged in stacks 9, with each stack 9 being spaced from a neighbouring stack. The opaque regions 8 block the transmission of light through the film in certain directions (such as direction 1 in figure 2) while allowing the transmission of light in other directions (such as direction 2 in figure 2).

The prior art light control films may be placed either in front of a display panel or between a transmissive display panel and its backlight, to restrict the range of angles from which the display can be viewed. In other words, the prior art light control films make a display 'private'. However none of the prior art light control films enables the privacy function to be switched off to allow viewing from a wide range of angles.

There have been reports of a display which can be switched between a public mode (with a wide viewing angle) and a private mode (with a narrow viewing angle). For example, US patent application No. 2002/0158967 suggests that a light control film could be movably mounted on a display so that the light control film either may be positioned over the front of the display to give a private mode or may be mechanically retracted into a holder behind or beside the display to give a public mode. This method has the disadvantage that it contains moving parts which may fail or be damaged in use, and which add bulk to the display.

A method for switching a display panel from public to private mode with no moving parts is to mount a light control film behind the display panel, and to place a diffuser which can be electronically switched on and off between the light control film and the panel. When the diffuser is inactive, the light control film restricts the range of viewing angles and the display is in a private mode. When the diffuser is switched on, the light with a narrow angle range output from the light control film is incident on the diffuser, and the diffuser acts to increase the angular spread of the light - that is, the diffuser cancels out the effect of the light control film. Thus, the display is illuminated by light travelling at a wide range of angles and the display operates in a public mode. It is also possible to mount the light control film in front of the panel and place the switchable diffuser in front of the light control film to achieve the same effect.

Switchable privacy devices of the above type are described in US patent Nos. 5 831 698, 6 211 930 and 5 877 829. They have the disadvantage that the light control film always absorbs a significant fraction of the light incident upon it, whether the display is in public mode or private mode. The display is therefore inherently inefficient in its use- of light. Furthermore, since the diffuser spreads light through a wide range of angles in the public mode, these displays are also dimmer in public mode than in private mode (unless the backlight is made brighter when the device is operating in public mode to compensate).

Another disadvantage of these devices relates to their power consumption. Such devices often use a switchable polymer-dispersed liquid crystal diffuser which is not diffusive when no voltage is applied across the liquid crystal layer and which is switched on (into the diffusive state) by applying a voltage. Thus, to obtain the public mode of operation it is necessary to apply a voltage across the diffuser so that the diffuser is switched on. More electrical power is therefore consumed in the public mode than in the private mode. This is a disadvantage for mobile devices which are used for most of the time in the public mode and which have limited battery power.

Another method for making a switchable public/private display is given in US patent No. 5 825 436. The light control device in this patent is similar in structure to the louvred film of figure 1. However, each opaque element 3 in the louvred film of figure 1 is replaced by a liquid crystal cell which can be electronically switched from an opaque state to a transparent state. The light control device is placed in front of or behind a display panel. When the cells are opaque, the display operates in a private mode; when the cells are transparent, the display operates in a public mode.

One significant disadvantage of this device is the difficulty and expense of manufacturing liquid crystal cells with an appropriate shape. A second disadvantage is that, in the private mode, a ray of light may enter at an angle such that it passes first through the transparent material and then through part of a liquid crystal cell. Such a ray will not be completely absorbed by the liquid crystal cell and this may reduce the privacy of the device.

A stereoscopic display achieves a three-dimensional effect by allowing the left eye of the user to see a left-eye image of a stereoscopic image pair while the right eye sees the right-eye image. In an autostereoscopic display, this is done without the user wearing- specially designed glasses. Displays that can achieve this have been known for many decades, and are described, for example in the book 'Stereoscopy' by N.A. Valyus (Focal Press, 1966). One widely-used method for electronic displays is to use a parallax barrier or other type of parallax optic. Figures 3(a) and 3(b) are plan views of an autostereoscopic display of this type.

In the display 10 of Figures 3(a) and 3(b), a parallax barrier 11 is placed close to an image display panel 12. The parallax barrier may either be in front of the display panel 12 as shown in Figure 3(a) or behind the display panel as shown in Figure 3(b). The parallax barrier 11 comprises apertures or transmissive portions 13 separated by opaque portions 14. Two interlaced images, a left-eye image and a right-eye image, are displayed on the image display panel, as shown schematically in Figures 3(a) and 3(b) - the labels "L" and "R" denote pixels that display the left-eye image and right-eye image respectively. When the user is in the correct position, the parallax barrier 11 prevents light passing through pixels displaying the right-eye image of the display being seen by the observer's left eye, and prevents light passing through the pixels displaying the left- eye image being seen by the observer's right eye. Thus, the observer perceives a three- dimensional image The same method can be used to make a display which allows two or more different users located in different places to see different images. This is referred to as a 'dual- view' display. A dual view display is in principle similar to an autostereoscopic display, except that it displays two or more independent images rather than the left-eye and right-eye images of a stereoscopic image pair, and that the two images arc displayed so as to be visible to different observers rather than to the left and right eyes of one observer.

The basic design of parallax barriers is well known, and described, for example, by Valyus (above). Methods for the optimization of the barrier design are described by H. Yamamoto et al. in "IEICE Transactions on Electronics", vol. E83-C, no. 10, pp.l632- 1639 (2000).

One problem with using a parallax barrier is that it reduces the resolution of an image (number of pixels in the image) by a factor of two (or greater, if more than two views are shown). This is true even when the display is used to show a two-dimensional image: that is, when the same image is shown to both eyes. To avoid this problem, switchable displays have been developed, in which the parallax barrier can be disabled or "switched off" when the display is used to show two-dimensional images. Such switchable parallax- barrier displays are described in, for example, US patent No. 5 969 850 which describes how a liquid crystal device can be used to make a switchable parallax barrier and in US patent No. 6 055 103 which describes another switchable parallax barrier which uses a patterned retarder and a single switchable wave-plate.

A multiple view directional display of the type shown in figure 3(a) and 3(b) has the further disadvantage that the parallax barrier It must be carefully aligned with the display panel 12 in order to achieve the correct 3D effect. This adds to the manufacturing cost. The resolution of the display is also reduced by a factor of two when operating in the three-dimensional mode, as mentioned above.

A second type of autostereoscopic display avoids these disadvantages by displaying the images for the left and right eye at different times. The full resolution of the display is used for each image. The images are directed to different locations using a directional backlighting system. In the first time frame, the backlight is switched so that it sends light through the display panel towards the left eye of an observer, and the left-eye image is displayed. In the next time-frame, the backlight is switched so that it sends light through the display panel to the right eye of the observer, and the right-eye image is displayed. This sequence is repeated rapidly so that the images are not perceived to flicker. An example of this type of display is given in US patent No. 5 132 839. Such an arrangement may also be used in a dual-view display. A disadvantage of this type of display is that a switchable backlighting system is generally complex and expensive, which adds to the cost of manufacture.

A first aspect of the present invention provides a light control element which outputs light having a first polarisation state with a first angular range and outputs light having a- second polarisation state different from the first polarisation state with a second angular range greater than the first angular range.

The light control element may be opaque or substantially opaque to light having the first polarisation state and travelling in a first range of directions, may be substantially not opaque to light having the first polarisation state and travelling in a second range of directions different from the first range of directions, and may be not opaque to the second polarisation state.

When this polarisation-dependent light-control element is placed behind or in front of a transmissive displayed image that is illuminated with light of the first polarisation state, the light control element restricts the angular range of light output, and the image is displayed over a narrow angular range so that a private display mode is obtained.

However, when the display is illuminated with light of the second polarisation state an image is displayed with a greater angular extent, thereby giving a public display mode.

Switching between the public display mode and the private display mode is effected simply by changing the polarisation state of light incident on or output from the displayed image; thus, switching between the public mode and the private mode can be effected with no moving parts. Further advantages are that little or no light is absorbed by the light-control element in the public mode, so that the light efficiency is high, and that a light control element of the invention may be manufactured easily and cheaply.

The light control element of the invention may also be used with an image that is emissive or reflective. In this case, the light control element must be placed between the image and the observer.

The second polarisation state may be orthogonal to the first polarisation state. The two polarisation states may be linear polarisation states.

The element may comprise a plurality of regions opaque or substantially opaque to light of the first polarisation state and transmissive to light of the second polarisation, each pair of neighbouring regions being spaced from one another by a material transmissive- to light of the first polarisation and to light of the second polarisation.

Each region may extend substantially parallel to a direction crossed with the plane of the device. Alternatively, each region may extend substantially parallel to a direction perpendicular to the plane of the device.

The element may comprise a substrate transmissive to light of the first polarisation state and to light of the second polarisation state; a plurality of recesses in the substrate; and a material opaque or substantially opaque to light of the first polarisation state and transmissive to light of the second polarisation state disposed in each recess.

The element may alternatively comprise a substrate opaque or substantially opaque to light of the first polarisation state and transmissive to light of the second polarisation state; a plurality of recesses in the substrate; and a material transmissive to light of the first polarisation state and to light of the second polarisation state disposed in each recess.

Each region may be switchable between a first state in which it is opaque to light of the first polarisation state and a second state in which it is substantially transmissive to light of the first polarisation state. In the second state, the element is transmissive to light of both polarisation states.

Each region may extend substantially parallel to the plane of the element; the regions may be arranged in a plurality of stacks of two or more elements; and the stacks may be spaced laterally from one another with each pair of adjacent stacks being separated by a material transmissive to light of the first polarisation and to light of the second polarisation.

Each region may comprise a linear polariser material.

Each region may comprise a liquid crystal material. i A second aspect of the invention provides a light control device comprising a light control element as defined above and a polarisation switch disposed in an optical path through the light control element. The angular range of output light can be changed by controlling the polarisation state of light incident on or output from the display using the polarisation switch. The angular range of output light can be changed without requiring any moving parts.

The polarisation switch may comprise a switchable wave-plate. It may comprise a switchable half wave-plate disposed in series with a linear polariser.

A third aspect of the invention provides a display comprising a light control element as defined above.

The display may further comprise a polarisation switch disposed in an optical path through the light control element. The angular range of light output from the display can be changed by controlling the polarisation state of light incident on or output from the display using the polarisation switch. The angular range of light output from the display can be changed without requiring any moving parts.

The light control element may form, in use, a parallax optic for light of the first polarisation whereby the display is operable as a multiple view directional display. This enables the display to operate in a private autostereoscopic display mode.

A fourth aspect of the invention provides a display comprising: a first light control element as defined above; a second light control element as defined above; and a first polarisation switch; wherein the angular output range of the first optical control element for light of the first polarisation is different to the angular output range of the second optical control element for light of the first polarisation. Thus may for example allow' the angular range of light output from the display to be restricted in both the horizontak and vertical directions.

The display may further comprise a second polarisation switch. This allows each of the light control elements to be independently enabled, and so provides four display modes.

A fifth aspect of the invention provides a display comprising a first light control element as defined above; a second light control element opaque or substantially opaque to light having the second polarisation state and travelling in a third range of directions while substantially not opaque to light having the second polarisation state and travelling in a fourth range of directions different from the third range of directions and while substantially not opaque to light having the first polarisation state; and a polarisation switch disposed in the path of light through the first and second light control elements. In this aspect, one or other of the optical control elements may be enabled at any one time.

A sixth aspect of the invention provides a method of manufacture of a light control element comprising the steps of: stacking a plurality of polariser sheets one above the other, each polariser sheet comprising a polariser layer and at least one light- transmissive substrate; and adhering each polariser sheet to its neighbouring polariser sheet(s). If the polariser sheets have an appropriate size, the stack may constitute a light control element of the invention.

The method may comprise: disposing light-curable adhesive over a first polariser sheet, stacking a second polariser sheet over the first polariser sheet; and irradiating the adhesive to cure the adhesive and thereby adhere the first polariser sheet to the second polariser sheet. This is a convenient method of adhering layers of the stack to one another, although other methods may be used if desired.

The method may further comprise providing a light transmissive layer between each pair of neighbouring polariser sheets. This allows the separation between the polarising regions of the light control element to be made equal to a desired spacing.

The method may comprise the further step of removing selected regions of the polarising layers of all polariser sheets of the stack. This method may be used to manufacture a light control element in which the polarising regions extend generally parallel to the plane of the element.

The method may comprise the further step of cutting the stack into slices, wherein the cutting direction may be perpendicular to the plane of the polariser sheets or alternatively in another direction not parallel to the plane of the polariser sheets. Each cut portion of the stack may constitute a light control element of the invention.

A seventh aspect of the invention provides a method of manufacture of a light control element comprising the steps of: removing selected regions of the polariser layer of a first polariser sheet, the polariser sheet comprising a light-transmissive substrate and the polariser layer; stacking a second polariser sheet over the first polariser; and removing selected regions of the polariser layer of the second polariser sheet. This method may also be used to manufacture a light control element in which the polarising regions extend generally parallel to the plane of the element An eighth aspect of the invention provides a method of manufacture of a light control element comprising the steps of: forming a plurality of recesses in a light-transmissive substrate; and providing a polarising material in the recesses.

A ninth aspect of the invention provides a method of manufacture of a light control element comprising the steps of: forming a plurality of recesses in a substrate formed of a polarising material; and providing a light-transmissive material in the recesses.

The step of forming the recesses in the substrate may comprise the step of selectively irradiating the substrate.

Preferred embodiments of the present invention will now be described by way of illustrative example, with reference to the accompanying Figures in which: Figure 1 shows a known light control film with a louvre structure; Figure 2 shows a known light control film with a multiple layer structure; Figure 3 shows the principle of a parallax-barrier autostereoscopic display; Figure 4 shows one possible structure for a polarisation-dependent light control element of the invention; Figure 5 shows an alternative structure for a polarisation-dependent light control element of the invention; Figure 6 shows a display of the invention that is switchable between private and public viewing modes; Figure 7 illustrates the operation of the display of Figure 6; Figures 8(a) to 8(c) show a method of manufacture of the polarisation- dependent light- control element of the display of Figure 6; Figures 9(a) to 9(c) show an alternative method of manufacture of a polarisation- dependent light-control element; Figures lO(a) to lO(d) show two methods of cutting grooves in a transparent material for the construction of a polarisation-dependent light-control element; Figures 1 l(a) and (b) show two alternatives for the application of an alignment layer for liquid crystal material in a polarisation-dependent light-control element; Figure 12(a) and 12(b) illustrate how a polarisation-dependent light- control element may be constructed by removing part of a film of absorbing material; i Figure 13(a) to (f) show two further methods for the construction of a polarisation- dependent light-control element; Figure 14 shows another display switchable between private and public viewing modes; Figures 15(a) and 15(b) illustrate the operation of the display of Figure 14; Figure 16 shows a display of the invention switchable between four viewing modes; Figures 17(a) to (f) illustrate three modes of operation of the display of Figure 16; Figure 18 shows a display of the invention operable in an autostereoscopic private display mode or a public display mode; Figures 19(a) shows an alternative structure for the light-control element in the display of Figure 18; Figure 19(b) illustrates manufacture of the light- control element of figure l9(a); Figure 20 shows a display of the invention capable of showing a different private image to each of two viewers; Figure 21 shows a display of the invention including a polarisation- dependent light- control element that is switchable by control of liquid crystal internal to the device; Figures 22(a) and 22(b) show the principle of operation of one possible construction of the display of Figure 21; Figures 23(a) and 23(b) shows the principle of operation of a second possible construction the display of Figure 21; and Figures 24(a) and (b) shows the principle of operation of a third possible construction of- the display of Figure 21.

Like reference numerals denote like components throughout the drawings.

Figure 4 is a schematic perspective view of a light control element 15 of the invention.

The light control element comprises a plurality of regions 16 that are opaque or substantially opaque to light having a particular polarization state but that are transparent to an orthogonal polarization state. The light control element of figure 4 is conveniently embodied by making each region 16 of a linear polariser material, so that each region 16 is opaque to plane-polarised light having its direction of polarization crossed with the region's transmission axis. The regions 16 will therefore be referred to as "polarising regions", and the direction of the arrow on each region 16 in figure 4 indicates the direction of the transmission axis of the region. As can be seen, the polarising regions 16 are arranged such that their transmission axes are substantially parallel to one another.

In figure 4 the polarising regions 16 have the form of plates of approximately constant thickness. The plates 16 are also arranged such that they are substantially parallel to one another, and such that neighbouring plates are separated by a layer of transmissive material 17. In Figure 4 the polarization plates are arranged such that their transmission axes are substantially parallel to the upper surface of the element 15, but the invention is not limited to this. Indeed, theinvention is not limited to polarising regions shaped as plates. The polarising regions may have any shape provided that they are effective to output light of one polarisation state with a wide angular spread and output light of the orthogonal polarisation state with a narrow angular spread Consider light that is propagating in the z-direction through the light-control element IS of figure 4, where the x-, y- and z-directions are denoted by the axes in the figure. Light that is plane-polarised in the x-direction will not be absorbed by the polarising regions 16, since the plane of polarisation of the light is parallel to the transmission axis of the polarising regions. Light polarised in the x- direction will therefore pass through the optical control element 15 both when propagating along the z-direction and when- propagating at an angle to the z-direction, as shown by the ray paths 18 and 19 in figure 4.

Light that is plane-polarised in the y-direction and that is travelling along, or close to, the z-direction will be able to pass through the layers 17 of transparent material, as again indicated by the ray path 18. Light that is plane-polarised in the y-direction and that is incident on the optical control element IS at an angle to the z-direction will, however, be incident on one of the polarising regions 16 and, since the transmission direction of the polarising region is orthogonal to the polarisation direction of the light, the polarising region will be opaque to the light. This is indicated by ray path 20 - the ray path 20 represents light that is incident on the optical control element IS, from below as seen in Figure 4, and is absorbed by one of the polarising regions 16.

Accordingly, the optical control element IS of figure 4 has the property that it is opaque to light of one polarisation state travelling in a first range of directions but is not opaque to light of that polarisation that is travelling in another range of directions (the specific embodiment of figure 4 is opaque to light plane-polarised in the Z- direction that is travelling at an angle to the z-direction, but transmits light planepolarised in the y- direction travelling along or close to the z-direction). Furthermore, the optical control element IS is not absorbing for light of an orthogonal polarisation state (the element of figure 4 is not absorbing for light that is plane-polarised parallel to the x-direction). The optical control element IS of figure 4 may therefore be considered to be a polarisation dependent control element, since light of one polarisation state is output with a wide angular spread whereas light of the orthogonal polarisation is output with a narrow angular spread (in figure 4, light plane-polarised along the y-axis is transmitted only in directions parallel to the xz-plane). The angular spread of light output from the optical control element 15 may be controlled by selecting the polarisation of light passing through the element using a suitable polarisation switch.

(It should be noted that the above description applies to the case of a perfect polariser.

Any practical polariser will inevitably transmit a small amount of the polarisation component that it is intended to block and will absorb a small amount of the polarisation- component that it is intended to transmit, and terms such as "opaque", "not absorbing", "transmissive", "transparent" etc. are therefore not to be interpreted as requiring perfect absorption or transmission.) In an alternative embodiment (not illustrated) the polarising regions are arranged in a grid structure. For example, further polarising regions that extend in the z- and ydirections but have limited extent in the x-direction may be provided, in addition to the polarising regions shown in Figure 4. The light control element of this embodiment can confine light of one polarisation state, propagating in the z-direction, in both the x- and y-directions. Polarising regions having a grid structure may be formed by, for example, the methods of Figures 9(a) to 9(c) or lO(a) to lO(d) to be described below, by forming a grid arrangement of grooves.

Figure 5 shows an alternative light control element 15' of the present invention. The light control element 15' of figure S again comprises a plurality of regions 16 that arc opaque or substantially opaque to light having a particular polarisation state but that are transmissive to an orthogonal polarisation state. In the embodiment of figure S the regions 16 are again in the form of plates and are arranged such that they are substantially parallel to the upper surface of the optical control element 15.

Furthermore, the regions 16 are arranged in stacks 21 in which a plurality of regions 16 are arranged one above the other. The stacks 21 are laterally separated from one another. Regions between the stacks, and between plates within a stack, are filled with a light-transmissivc material 17.

The regions 16 are arranged such that the polarisation properties of the regions do not vary within a stack or between different stacks. Preferably, the regions 16 are constituted by a material that is a linear polariser and, in this case, the polarising regions 16 in each stack are arranged such that their transmission axes are parallel to one another and such that the transmission axes of the polarising plates in one stack is substantially parallel to the transmission axes of the polarising plates of other stacks.

The principle of operation of the optical control element IS' of figure 5 is generally similar to that of the optical control element 15 of figure 4. That is to say, light of one polarisation is transmitted through the optical control element 15' with no absorption regardless of its direction of travel (in the embodiment of Figure 5 this is light that is plane-polarised in the direction parallel to the transmission axes of the polarising regions 16). Light of an orthogonal polarisation state (that is, plane-polarised in the direction perpendicular to the transmission axes of the polarising regions 16) is, however, transmitted only if it can pass through the control element 15' without being incident on one of the polarising regions 16 - that is, if it is propagating along the xz- plane or very close thereto. Light of this polarisation component that is travelling at an angle to the xz-plane will be incident on one of the polarising regions 16, and will be absorbed as shown by the ray path 20 in figure S. The optical control element 15' of figure 5 will output light of one polarisation state with a wide angular spread but will output light of an orthogonal polarisation state with a narrow angular spread. The angular spread of light output from the optical control element 15 may again be controlled by selecting the polarisation of light passing through the element using a suitable polarisation switch.

Figure 6 illustrates a display 21 according to an embodiment of the invention. The display comprises an image display panel 22 that, in use, is driven by drive means (not shown) to display a desired image. The display panel may be any conventional image transmissive display panel such as a liquid crystal display panel. The display panel 22 is preferably a pixellated display panel.

The image display panel 22 is illuminated by a backlight 25. The backlight 25 preferably provides the display panel 22 with even illumination over its entire area. The backlight 25 is shown as a wide area backlight in figure 6, but any suitable backlight may be used. The backlight may be integral to the display or it may be a separate component.

The display 21 further comprises an optical control element 15 as shown in figure 4,- and a switchable half-wave plate 24 and a linear polariser 23 provided between the backlight 25 and the display panel 22. The polariser 23 and the half-wave plate 24 are arranged substantially parallel to the plane of the display panel. Although the polariser 23 is shown as a separate component in Figure 6 the polariser 23 may be an input polariser of the display panel 22, if the display panel 22 has an input polariser (for example if the panel 22 is a liquid crystal display panel).

The transmission axis of the polariser 23 is crossed with, and is preferably at 90 to, the transmission axes of the polarization regions 16 of the light control element. The optic axis of the half-wave plate is preferably at 45 to the transmission axis of the polariser.

The backlight 25 emits, in use, light that is unpolarized. The light is assumed to be travelling generally along the z-axis, as shown by the axes defined in figure 6. The component of the light that is plane-polarised in the x-direction is, as explained above, transmitted without significant absorption by the optical control element 15. Light plane- polarised in the x-direction will therefore be output by the optical control element with a wide angular range. Light that is plane-polarised in the y-direction however, is only transmitted by the optical control element 15 if it is travelling parallel to the z- direction or close thereto, and so will be output from the optical control element 15 with a narrow angular range centred on the z-axis.

The switchable half-wave plate 24 acts, in conjunction with the polariser 23, as a polarisation switch. When the half-wave plate is switched ON, it will have \/2 retardation and will therefore rotate the plane of polarisation of planc-polarised light incident on it by twice the angle between the optic axis of the half-wave plate and the plane of polarisation of the incident light. The half-wave plate 24 is arranged with its optical axis at approximately 45" to the x- and y- axes. Thus, when the half-wave plate is switched ON, it will rotate the plane of polarisation of light plane-polarised in both the x-direction and the ydircction by 90 .

The polarisation switch (de, the switchable half-wave plate 24 and the polariser 23) is- disposed between the optical control element 15 and the display panel in Figure 6, but it could alternatively be disposed between the backlight and the optical control element so as to control the polarisation of light incident on the optical control element. It could also be incorporated within the backlight 25.

Figure 7(a) and 7(b) illustrate the operation of the display 21 of figure 6. Figure 7(a) illustrates operation of the display when the half-wave plate is ON and so rotates the plane of polarisation of x-polarised or ypolarised plane polarised light by 90". The backlight 25 is omitted from Figures 7(a) and 7(b). Light plane-polarised in the y- direction is denoted by a short arrow in the y-direction, and light plane- polarised in the x-direction is denoted by two concentric circles.

As explained above, light plane-polarised in the x-direction that is incident on the optical control element 15 is transmitted with substantially no absorption. After passing through the optical control element, the plane of polarisation of the light is rotated by 90 by the half-wave plate 24, so that the plane of polarisation is made parallel to the y- direction; this component of the light is therefore transmitted through the polariser 23, since this has its transmission axis arranged parallel to the y-axis.

Light that is initially plane-polarised along the y-direction is output from the optical control element with a narrow angular range. This plane of polarisation is then rotated by the half-wave plate so as to be along the x-direction, and this component is therefore blocked by the polariser 23. Thus, the display panel is illuminated by the component of light from the backlight that was originally emitted with a plane of polarization parallel to the x-direction. This is transmitted by the optical control element 15 with a wide angular range, so that the display panel 22 is illuminated with light having a wide angular range as shown in figure 7(a) . This therefore provides a public mode of operation of the display.

Figure 7(b) illustrates operation of the display 21 when the switchable half-wave plate 24 is OFF. When the half-wave plate 24 is OFF, it has no effect on the plane of polarization of light that is incident on it. The x-polarised component of light from the- backlight is transmitted by the half-wave plate 24 with no change in its polarization, and is therefore incident on the polariser 23 still polarised in the x-direction. This component of light is therefore blocked by the polariser 23. As a result, the display panel 22 is illuminated with the y-polarised component of light from the backlight, since this is transmitted through the polariser 23. As mentioned above, this is transmitted through the optical control element 15 with a narrow angular range about the xz plane but with a wide angular extent in the vertical direction, and the display panel is therefore illuminated with light having only a narrow angular range in the horizontal direction. This provides a private mode of operation of the display, since an image displayed on the display 22 is transmitted along a narrow range of directions.

This is shown in figure 7(b).

The display 21 of figure 6 may therefore be switched between a public display mode and a private display mode simply by switching the half-wave plate 24 between a OFF state and a ON state in which it has half-wave retardation. This may be done electrically, and no moving parts are therefore required.

The switchable half-wave plate 24 may be a liquid crystal cell. There are many known ways of making a switchable half-wave plate using a liquid crystal cell, as described by, for example, E Lueder in "Liquid Crystal Displays: addressing schemes and electro-optic effects" Wiley-SID series in display technology (2001).

A further advantage of this display is that, in many known switchable half-wave plates, the plate is ON when no voltage is applied across the plate, and the plate is switched OFF to give zero retardation by applying a suitable voltage. This has the advantage that when the display is operating in its public display mode of figure 7(a), the wave plate is ON and no power is therefore consumed by the half-wave plate. It is necessary to apply a voltage only when the display is desired to operate in the private display mode.

Methods of fabricating a polarisation-dependent light control element of the present invention will now be described. i Linear polariser sheet is commercially available, and the structure of a typical linear polariser sheet is shown in figure 8(a). As can be seen, a linear polariser sheet 26 typically consists of an active layer 27 which absorbs light having a particular linear polarization. The active layer 27 may be, for example, a layer of a stretched polymer that contains an absorbing dye such as iodine. The active layer 27 is supported on both sides by films or layers of a transparent material such as, for example, cellulose acetate butyrate (CAB). The total thickness of the polariser sheet is typically of the order of 2501lm, while the thickness of the active layer 27 is much smaller and is typically 10- 2Olm although it may be as little as slim.

In some commercially available linear polariser sheets, the active layer is supported by only one transparent layer. In such a polariser sheet, one of the transparent layers 28, 28' of figure 8(a) would not be present.

One method of manufacturing a light control element 15 of the type shown generally in figure 4 is illustrated in figures 8(b) and 8(c). In essence, the light control element 15 is made by stacking many layers of a conventional linear polariser sheet such as the sheet 26 of figure 8(a). Depending on the desired separation s between adjacent polarising regions 16 in the light control element, it may be necessary to provide additional transparent spacer layers 29 between adjacent polariser sheets 26. If spacer layers 29 are provided, their refractive index preferably matches the refractive index of the transparent substrates 28,28' of the polariser. It is therefore preferable to make the spacer layers of the same material as used for the transparent substrates 28,28' of the polariser sheet, as this ensure that refractive indices of the spacer layer 29 and the substrates 28,28' are equal to one another. This also has the advantage that the mechanical properties (and in particular the elastic modulus) of the spacer layers 29 are matched to those of the transparent substrates 28, 28' of the polariser sheet so that, where a mechanical cutting process is used to cut the stack into slices as described below, the cutting process will deform the polariser layers and the spacer layers in the same way and the stack will undergo the least distortion possible.

The stacked layers of the polariser sheet 26, and the transparent spacer layers 29 if- provided, are attached to one another by any suitable method. This may be done, for example, using an adhesive such as a light-curable (for example UV-curable) transparent adhesive. If such adhesive is used, the stack of polariser sheets 26, and the transparent spacer layers 29 if present, would be assembled as shown in figure 8(b) with a layer of transparent adhesive between each pair of adjacent layers. Once the stack had been assembled, the entire stack would be subject to irradiation with ultra- violet light to cure the adhesive.

It would alternatively be possible to irradiate each layer of adhesive separately, before the next adhesive layer is deposited. While this might increase the overall time required to manufacture the stack 30, it has the advantage that it overcomes the problem that, when all adhesive layers are irradiated in a single step, absorption of UV light in the upper layers reduces the intensity of irradiation incident on the lower adhesive layers of the stack and may prevent them from curing properly.

The adhesive used preferably has a refractive index, when cured, which is as close as possible to the refractive index of the transparent spacer layers 29 and to the refractive index of the transparent layers 28, 28' of the polariser sheet. This provides good optical performance of the finished element 15. If the refractive indices of the adhesive layers and the transparent layers 28, 28', 29 are not matched, refraction and reflection will occur at boundaries within the element, thereby degrading the optical performance of the element.

Once the adhesive has cured, the block 30 may be divided into slices 31 as indicated in figure 8(b). The block 30 may be sliced by, for example, mechanical cutting. In order to ensure that the cutting step is successful, it is preferable that the mechanical properties (and in particular the elastic modulus) of the cured adhesive are matched to those of the transparent layers 28, 28', 29 of the stack 30.

It is not necessary to attach the polariser sheets 26, and spacer layers 29 if provided, to one another using a transparent adhesive. In principle, any suitable method can be used' such as, for example, welding.

In principle, a slice 31 cut from the block 30 could be used as the optical control element 15 of figure 4. In practice, however, it is preferable to provide transparent layers 32, 33 over the front and back surfaces of the slice 31, as shown in figure 8(c).

The layers 32, 33 provide physical protection for the optical control element, and also improve the optical quality of the element by planarising the output and input faces.

The layers 32, 33 may, for example, be layers of cured adhesive. Alternatively, the layers 32, 33 may be thin polymer films that are attached to the slice 31 using a TV curable adhesive.

In a modification of figures 8(b) - 8(c), the sheet of linear polariser 26 may be initially cut into strips having a width w that is equal to the desired thickness t of the light- control element (excluding the transparent protective layers 32, 33). This is shown in broken lines in figure 8(a). These strips may then be stacked together to form a structure similar to the slice 31 shown in figure 8(b). If desired, strips of a transparent spacer material may be interposed between adjacent strips of the polariser sheet. The strips of polariser sheet and, if present, spacer material may be adhered together using any suitable technique such as, for example, bonding using a transparent adhesive or welding. The protective layers 32, 33 may then be provided on the front and back surfaces of the stack of desired.

An alternative method of fabricating a light control element 15 of the present invention is shown in figures 9(a) to 9(c). In this embodiment, the light control element is manufactured from a transparent sheet 34 such as, for example, a film of transparent polymer. This may he, for example, a CAB film.

As is shown in figure 9(b), a plurality of recesses 35 are defined in the transparent film 34. The recesses 35 correspond in size, shape and location to the desired positions of the polaising regions 16 of the optical control element 15. The recesses may for example be in the form of parallel-sided grooves if the polarizing regions are to have the plate-shape shown in figure 4. In figure 9(b) the recesses 35 extend over the full depth- of the transparent film 34, and the transparent film 34 is therefore attached to a substrate or backing sheet 36. The substrate 36 ensures that the discreet portions of the transparent film created when the grooves 35 are formed are retained in correct relation to one another. The substrate 36 is preferably transparent so that it can be incorporated into the completed optical control element without adversely affecting its performance.

This eliminates the need to remove the substrate 36. The substrate 36 may form one of the two surface layers 32,33 of Figure 8(c).

In principle it would it be possible for the grooves not to extend through the entire depth of the transparent film 34, in which case the substrate 36 could be omitted. However, it is generally difficult to control the depth of the grooves precisely, and disposing the transparent film 34 on a substrate and forming grooves through the entire depth of the film is likely to provide a more reliable manufacturing process.

The recesses 35 are then filled with a material 37 that absorbs light of a particular polarization state but that does not absorb light of an orthogonal polarization state. This material may be, for example, a liquid crystal material incorporating a dye that absorbs light of a particular linear polaisation.

The grooves 35 in the transparent film 34 may be formed using standard lithographic techniques. One suitable technique is shown in figures 10(a) and 10(b). In this embodiment, the transparent film 34 is a film of a photosensitive material. This material is applied over the backing film 36 to provide a layer of uniform thickness by any suitable technique such as spinning, roll coating, printing, etc. The thickness of the layer 34 of photosensitive material is chosen to be the desired thickness t of the light control element (excluding the substrates 32,33 if provided).

The layer 34 of photosensitive material is then exposed to irradiation, for example to ultra-violet (W) light, through a mask 38 as shown in figure 10(a). The transparent apertures 39 of the mask correspond to the locations of the recesses 35 that are required.

Once the layer 34 of photosensitive material has been exposed, the mask is removed and- the layer is washed. The washing step will remove the regions of the photosensitive layer that were irradiated in the irradiation step. Accordingly, recesses 35 are formed in the layer 34, as shown in figure 10(b). The portions of the layer 34 that remain after the irradiation and washing steps form "walls" 55 between ncighbouring grooves and correspond to the transmissive regions 17 in Figure 4.

The embodiment of figures 10(a) and 10(b) uses a layer 34 of a positive photosensitive material, in which the regions of the layer that are not irradiated are retained in the washing step. It would alternatively be possible to use a negative photosensitive layer in which the nonirradiated regions would be removed in the washing process. To do this, it would be necessary to use a mask in which the transparent apertures corresponded to the regions of the layer that were to form the walls 55.

An alternative method of forming the grooves 35 is shown in figures 10(c) and 10(b).

Initially, a layer of transparent material is deposited over the substrate 36 to a uniform thickness that corresponds to the desired thickness t of the light control element (excluding the substrates 32,33 if provided). The layer 34 may again be deposited by any suitable technique such as, for example, spinning, roll coating, printing, etc. In the embodiment of figures 10(c) and 10(d) the grooves are formed by removing the unwanted parts of the transparent layer 34 using a high energy beam source. This embodiment may use, for example, a laser ablation technique, a reactive ion etching (ROE) technique, etc. As shown in figure 10(c), the layer 34 of transparent material is exposed through a mask 38 to the high energy beam source, and the regions of the layer 34 that correspond to the apertures 39 in the mask are removed, to form the recesses 35 as shown in figure 10(d).

Once the recesses have been formed, whether by the method of Figures 10(a) and (b), by the method of Figures 10(c) and (d), or by another method, a material that is opaque to light of one polarisation state but that transmits light of an orthogonal polarisation state is deposited in the recesses. For example, a liquid crystal material incorporating a dye that absorbs light of a particular linear polarization may be disposed in the grooves in the transparent layer 34. The liquid crystal material and the dye must be aligned so that all the molecules of the liquid crystal layer are oriented in the same direction.

This can be achieved by providing a polyimide alignment layer. Figures ll(a) and 11(b) illustrate two possible ways of providing the alignment layer. In the embodiment of figure ll(a) an alignment layer 40 is disposed over the entire upper surface of the substrate 36. The alignment layer 40 is then provided it with a desired alignment direction, for example using a rubbing technique or by photoalignment (that is, aligned by exposure to polarised ultra-violet radiation). The layer 34 of transparent material is then deposited over the rubbed alignment layer 40, and the grooves 35 are formed as described above. When liquid crystal material incorporating dye is disposed in the grooves 35, it will be aligned by the alignment layer 40.

In an alternative embodiment, shown in figure 11(b), the alignment layer 40 is applied after the grooves 35 have been defined - that is, the alignment layer 40 is provided after formation of the grooves in figure 9(b) but before deposition of the liquid crystal material in figure 9(c). In this embodiment, the alignment layer is deposited only in each groove 35, as shown in figure 11(b). The alignment layer may then be provided with a desired alignment direction, for example using a rubbing technique or by photoalignment.

In a further embodiment, not illustrated, the material of the walls 55 are used to align the liquid crystal material disposed in the recesses. The side faces of the walls can be rubbed, along the direction of the recesses, to induce alignment in the side faces of the walls that will be effective to align the liquid crystal material deposited in the recesses.

This is an effective technique of the substrate is formed from a soft material such as, for example a photoresist.

In order to introduce the liquid crystal material 37 into the grooves 35, a second substrate or other sealing layer may be placed over the walls. The liquid crystal material with dye may then be introduced into the grooves 35 using a capillary or vacuum technique.

Figures 12(a) and 12(b) illustrate a further method of manufacturing a light control element 15 of the present invention. This method is similar to the method of figures 9(a) - 9(c), except that the method starts with a layer 41 of material that is opaque to light of a given polarisation state but that transmits light of an orthogonal polarisation state. The materialmay be, for example, a polymerised liquid crystal doped with an absorbing dye, or a stretched polymer such as PVA doped with an absorbing dye.

Grooves or recesses 35 are formed in this layer, and are filled with a transparent material. The grooves filled with transparent material correspond to the transparent portions 17 of the light control element 15, and the remaining portions of the layer 41 of material that is opaque to light of a given polarisation form the regions 16 of the light control element lS of figure 4.

In one embodiment of this method, an alignment layer 40 is initially deposited over a substrate 36 and is then aligned using, for example, a rubbing or photoalignment technique.

A layer 41 of photosensitive material containing an absorbing dye is then deposited over the alignment layer 40. The alignment layer 40 serves to align the dye molecules of the layer 41, so that the layer 41 absorbs light of a particular linear polarisation. The short lines in the alignment layer 41 in figures 12(a) and 12(b) indicate the alignment of the dye molecules of the layer 41.

The photosensitive layer is then subject to UV or other irradiation through a mask 38.

In the embodiment of figure 12(a) the layer 41 is a layer of positive photosensitive material, so the apertures 39 of the mask correspond to the regions where it is desired to remove the layer 41. The size, shape and position of the apertures 39 of the mask therefore correspond to the desired shape, size and positions of the transmissive regions 17 of the light control element 15.

After the irradiation step, the mask 38 is removed, and the layer 41 is washed to remove the photosensitive material from the irradiated regions. The remaining portions of the layer 41 constitute the polarising plate 16 of the light control element 15. The grooves may be filled with a transparent material such as, for example, a transparent polymer or a transparent resin, to form the transmissive regions 17.

In a modification of this embodiment, the optical control element is formed by removing unwanted parts of a layer of a material that absorbs light of one linear polarization by means of an ablation technique using a high energy beam source. This may be a laser ablation technique, a RIE technique, etc. This embodiment may, in principle, be used with any material that absorbs light of a given linear polarization, and is not limited to use with a photosensitive material. In this embodiment, the alignment layer 40 can be omitted if the material that absorbs light of a given linear polarization does not require the alignment layer. The regions of the layer removed by ablation may then be filled with a transparent material as explained above.

Figures 13(a) to 13(d) illustrate a further method of manufacturing a light control element of the present invention. This embodiment uses, as shown in Figure 13(a) a commercially available linear polariser sheet, of the type comprising an active layer 27 disposed over a single transparent substrate 28. The second transparent substrate 28' of the polariser sheet of figuec 8(a) is not present.

Initially, selected portions of the active layer 27 of a first polariser sheet 26a are removed using, for example, a patterned photo-etching process or a laser ablation process. The result of this step is, as shown in figure 13(b) a series of discrete regions 27' of linear polarising material disposed over the transparent substrate 28. Each of the discrete regions 27' of polarising material in figure 13(b) corresponds to one of the polarising regions 16 in the light control element 15' of figure 5.

Next, a second polariser sheet 26b is adhered to the upper surface of the first polariser sheet 26a. This step may consist of depositing a layer of adhesive, for example UV- curable adhesive, over the upper surface of the first polarising sheet, and adhering the second polarising sheet over the first polarising sheet. The W-curable adhesive acts, firstly, to adhere the two polarising sheets to one another and, secondly, to fill the gaps left by removal of parts of the active layer of the first polarising sheets. The adhesive is preferably chosen such that its refractive index, when cured, is close to the refractive index of the transparent substrate 28 of the polarising sheets. The stacked polarisers are the irradiated to cure the adhesive.

Selected regions of the active layer of the second polarising sheet may then be removed, again using a patterned photo-etching technique or a laser ablation technique. The regions of the active layer of the second polarising sheet that are removed prclerably lie directly over, and have the same size and shape as, the regions where the active layer of the first polarising sheet was removed. Conversely, the regions of the active layer of the second polarising sheet that are retained lie directly over, and have the same size and shape as, the retained regions 27' of the active layer of the first polarising sheet 26a.

The steps of adhering a further layer of polarising sheet 26c to the stack, and removing selected regions of the active layer may be repeated as many times as necessary.

Finally, a protective transparent layer 32 may be deposited over the uppermost polariser sheet to serve as a protective layer.

In this embodiment, the regions 27' of the active layer that are retained form the stacks 21 of polarising regions 16 in the light control film 15' of figure 5. Figure 13(d) shows a light control element having three polarising sheets 26a-26c, so that each stack of polarising sheets contains three polarising sheets; however, the invention is not limited to this number of polarising sheets.

In this embodiment it is in principle possible to assemble the complete stack 32 and cure all the adhesive layers in one step. However, it is important to ensure that each polariser sheet is correctly aligned with the preceding polariser sheet, and waiting until the complete stack has been fabricated before curing the adhesive layers has the potential disadvantage that a polariser sheet may be inadvertently moved when a subsequent sheet is stacked. In practice, therefore, it may well be preferable to cure each adhesive layer before the next adhesive layer and polariser sheet are deposited, to prevent one polariser sheet from moving relative to other sheets.

Figures 13(e) to 13(f) show a modification of this embodiment. In this embodiment, the polarising sheets 26a-26c and the protective substrate 32 are initially stacked together, with a layer of curable adhesive disposed between each component. The stack 42 is then irradiated to cure the adhesive layers.

A plurality of recesses or grooves 35 are then formed in the stack 42, as shown in figure 13(f). Each groove is made deep enough to remove the active layer of each polarising sheet 26a-26c of the stack, and to remove the substrate of all polariser sheets but the lowest, leaving only the transparent substrate 28a of the lowest polariser 26a.

The portions 42' of the stack that are retained correspond to the stacks 21 of polarising regions in the light control element lS' of figure 5. The position, size and shape of the retained portions 42' of the stack are therefore chosen appropriately. The recesses 35 between the retained portions 42' may then be filled with a transparent material and, if desired, a protective substrate (not shown) may be deposited over the upper surface of the light control element.

Figure 14 shows a display according to a further embodiment of the present invention.

As with the display 21 of figure 6, the display 21' of figure 14 contains a light control element 15 of the present invention. In the display 21' of figure 14, however, the light control element 15 is placed in front of the image display panel 22 (that is, between the display panel and the observer) rather than behind as in figure 6. This embodiment may be applied to a display panel 22 that is an emissive display panel such as an organic light-emitting device (OLED) display, a cathode ray tube display, or a field-emission display. It may also be applied to a transmissive image display panel, such as a liquid crystal display panel.

A polarization switch is provided between the display panel and the observer to control the polarization of light reaching the observer. In figure 14 the polarization switch is formed by a linear polariser23 and a switchable half-wave plate 24 placed between the display panel 22 and the optical control element Is. If the display panel incorporates an output polariser, for example if the display panel 22 is a liquid crystal display panel,- then the output polariser of the display panel may be used as the polariser 23. The switchable half-wave plate 24 is disposed in front of the polariser 23, with the optic axis of the polariser arranged at 45 to the transmission axis of the linear polariser 23.

The polarization switch may alternatively be provided between the optical control element and the observer or, in the case of a transmissive display, behind the display panel. In principle the display may also be embodied using an emissive display that emits light having a controllable output polarization, in which case an external polarization switch would not be required.

If the display panel 22 is a transmissive display panel, the display 21' would require to be illuminated by light from a suitable backlight (not shown).

In the embodiment of figure 14, the polarising regions 16 of the optical control element Is have their transmission axes parallel to the transmission axis of the polariser 23. In this embodiment the transmission axis of the horizontal polariser 23 is parallel to the x- axis, and the polarising plate 16 of the optical control element 15 accordingly have transmission axes that are parallel to the x-axis.

The operation of the display 21 is illustrated in figure lS(a) and lS(b). Figure lS(a) illustrates the operation of the display 21 when the switchable half-wave plate is OFF and thus provides no retardation. In this embodiment, the horizontal polariser 23 transmits light that is plane-polarised parallel to the x-axis. When the plate 24 is OFF it has no effect on the plane of polarisation of light passing through it, so that the light incident on the optical control element 15 is still planepolarised parallel to the x-axis.

Since the polarising plates 16 ot the optical control element have transmission axes arranged in the x-direction, the polarising plates 16 do not absorb the light incident on the optical control element 15. Thus, light is output from the optical control element with a wide viewing angle range, as shown in figure 15(a), thereby providing a public display mode.

Figure 15(b) shows operation of the display when the switchable plate 24 is ON ton provide 712 retardation. In this case, the polariser 23 again passes light that is plane- polarised in the x-direction. Since the optical axis of the plate is at 45" to the plane of polarization of the light incident on it, the plane of polarization of the light is rotated by 90" as it passes through the plate 24. Accordingly, the light incident on the optical control element is plane-polarised in the y-direction - and so the polarising regions 16 of the optical control element will absorb any light that is incident upon them.

Accordingly, the optical control clement 15 outputs light having only a narrow angular range as shown in figure 1 5(b), thereby providing a private display mode.

Figure 16 shows a further display 43 according to the present invention. The display 43 comprises two light-control elements similar to the element 15 of figure 4. The two light control elements 15a, 15b both transmit light of a particular linear polarization regardless of the direction of travel of the light. The two light control elements 15a, 15b both transmit light of the orthogonal linear polarization state only over a certain range of directions, but the two light control elements have different geometries to one another so that the range of ray angles over which they transmit the orthogonal linear polarization is different between the two elements. The display 43 can therefore operate in more than two display modes.

The display 43 comprises an image display panel 22. This may be a transmissive display panel or an emissive display panel, as in the embodiment of figure 14. If the display panel is a transmissive display panel, the display requires illumination by a suitable backlight (not shown).

A linear polariser 23 is disposed in front of the display panel 22. If the display panel 22 has an output linear polariser (for example if the display panel 22 is a liquid crystal display panel), the output polariser of the display panel 22 could be utilised as the polariser 23.

A first optical control element 15a is disposed in front of the polariser 23. A' polarisation switch is provided to select the polarisation of light that is output from the- first optical control element. In Figure 16, the first polarisation switch is constituted by the polariser 23 and a first switchable half:wave plate 24a disposed between the polariser and the first optical control element 15a. The half-wave plate is arranged with its optic axis at 45 to the transmission axis of the polariser 23.

A second optical control element 15b is disposed in front of the first optical control element 15a. A second polarisation switch is provided to select the polarisation of light that is output from the first optical control element. In Figure 16, the first polarisation switch is constituted by the polariser 23 and a second switehable half-wave plate 24b disposed between the first optical control element 15a and the second optical control element 15b. The optic axis of the second switehable halfwave plate 24a is again arranged with its optic axis at 45 to the transmission axis of the polariser 23.

The order of the components is not limited to the order shown in Figure 16. For example, the components might be placed in the order of: backlight, light-control element, switehable wave plate, light-control element, switehable wave plate, polariser, and the display panel.

The optical control elements 15a, 15b are arranged such that they each transmit light of the linear polarisation passed by the polariser 23 in all directions. However, the light control elements are chosen such that their output angular ranges for light plane- polarised at 90 to the transmission axis of the polariser 23 are different to one another.

In the display shown in figure 16, the polariscr 23 is arranged with its transmission axis parallel to the z- (vertical) direction. The optic axes of the two switchable half-wave plates 24a, 24b are parallel to one another and are at 45 to the transmission axis of the polariser 23. The light control elements 15a, 15b both have no absorption effect for vertically-polarised light, and so transmit vertically-polarised light regardless of its incident on the light control element. The first polarisation-dependent light control element lSa allows light that is polarised in the horizontal direction to pass through only in directions that are close to the horizontal (x-y) plane, and blocks horizontally- j polarised light that is propagating at large angles to the horizontal plane. The second- light-control element 15b allows horizontally-polarised light that is propagating in a direction close to the vertical (y-z) plane to pass, but blocks light that is propagating at a large angle to the vertical plane.

Figures 17(a) to 17(c) illustrate three of the possible four modes of operation of the display 43 of figure 16. Figure 17(a) illustrates the mode of operation when both switchable wave-plates 24(a), 24(b) are OFF and so both provide zero retardation. In this mode of operation, light output from the display panel 22 is plane-polarised by the polariser 23, with its plane of polarisation in the vertical direction. The plane of polarisation is unaffected by either of the switchable half-wave readers 24a, 24b, and light propagating through the display 43 remains planepolarised in the vertical direction. (The polarisation direction of light is illustrated schematically in figure 17(a)- 17(c) - a vertical arrow denotes plane-polarised light, whereas two concentric circles denotes horizontally polarised light (that is, light polarised out of the plane of the paper.) Since the light control elements 15a, 15b are transmissive for vertically- polarised light, regardless of its direction of propagation, light is output from the display over a wide range of viewing angles in both horizontal and vertical directions. This is illustrated schematically in figure 17(d), which is a schematic front view of the display.

Figure 17(b) illustrates the mode of operation of the display 43 when the first wave- plate 24a is ON and the second wave-plate 24b is OFF. In this mode of operation, the polariser 23 again outputs light plane-polarised in the vertical direction, but the plane of polarisation is rotated by 90 by the first wave-plate 24a. Since the second wave-plate 24b is OFF, it has no effect on the plane of polarization of light passing through it accordingly, the plane of polarization of light passing through the display is converted to horizontal polarization by the first wave-plate 24a, and the light remains horizontally polarised thereafter as it passed through the display.

In the mode of operation of figure 17(b), light is polarised in the horizontal direction when it passed through both light control elements lSa, 15b. Since the first light control element 24a allows horizontally polarised light to pass through only in directions close; to the horizontal plane whereas the second light control element 15b allows horizontally polarised light to pass through only in directions close to the vertical plane, the overall result is that only light propagating in a direction close to the normal direction to the surface of the display is transmitted through the entire system. In this mode, the display has a narrow range of viewing angles in both horizontal and vertical directions, as illustrated schematically in figure 17(e) which is a schematic front view of the display.

In figure 17(c) both the half-wave readers 24a and 24b are ON. The polariser 23 again passes light that is vertically plane-polarised, the plane of polarization is rotated by 90" by the first half-wave plate so that light incident on the first polarisation-dependent control element 15a is horizontally plane-polarised. However, when the light passes through the second half-wave plate, the plane of polarization is again rotated by 90", so that light incident on the second optical control element 24b is plane-polarised in the vertical direction. In this mode, therefore, the first light control element 24a is opaque to light travelling at large angles to the horizontal plane and transmits only light that is travelling close to the horizontal plane. However, since light reaching the second light control element 15b has been returned to vertical plane-polarisation, the second light control element 15b does not absorb the light. Accordingly, as shown in figure 17(f), which is a schematic front view of the display, the output from the display has a wide viewing angle in the horizontal direction, but has a narrow viewing angle in the vertical direction.

In a fourth mode, not shown in figuec 17, the first half-wave plate 24a is OFF while the second half-wave plate 24b is ON. In this mode, the first light control element 15a has no absorbing effect (since it receives vertically polarised light), whereas the second light control element 15b is opaque to light travelling at large angles to the vertical plane (since it receives horizontally-polarised light). In this mode, the display has a narrow viewing angle in the horizontal direction, but has a wide viewing angle in the vertical direction.

Thus, the display 43 has a public display mode with a wide viewing range, and three modes of restricted viewing range.

In a modification of this embodiment, the second polarization switch could be omitted (by omitting the second wave plate 24b). In this modification, only the display modes of figures 17(a) and 17(b) would be available however such a display would be able to switch between a public display mode and a private display mode by suitably switching the first wave-plate 24a.

Other combinations of two light control elements and wave-plates are also possible.

For example, the embodiment of figure 16 could be modified such that both polarisation-dependent light control elements allow light that is plane polarised in one direction, either substantially parallel to or substantially perpendicular to the transmission axis of the polariser, and that is travelling along a direction close to the same plane to pass, but with different viewing angle ranges. For example, the first device might absorb horizontally polarised light which makes an angle of greater than 20 to the vertical plane whereas the optical control element might absorb horizontally polarised light which makes an angle of more than 10 to the vertical plane. This would provide a display that could be switched between a display mode with a wide viewing angle, a display mode with a horizontal viewing angle range of 20 , and a display mode with a horizontal viewing angle range of 40 .

Figure 18 shows a display 44 according to a further embodiment of the present invention. This display can provide an autostereoscopic private display mode.

The structure of the display 44 of figure 18 is generally similar to the structure of the display of display 21' of figure 14. It again comprises a display panel 22, and an optical control element 15 of the present invention disposed in front of the display panel. A polarisation switch is provided between the display panel and an observer. The image display panel 22 may be a transmissive display panel illuminated by a backlight (not shown) or an emissive display panel. In figure 18, the polaisation switch is formed by a linear polariser 23 and a switchable half-wave plate 24 disposed between the display panel and the optical control element. Again, the order of the components is not limited to the order shown in Figure 18 and the components could for example be placed in the following order: backlight, light-control element, switchable wave plate, polariser, and the display panel; this order provides a rear-barrier display.

In this embodiment, the polarising regions 16 of the light control element 15 are arranged so that they transmit light plane-polarised in a direction that is perpendicular to the transmission axis of the polariser 23. The optic axis of the switchable half-wave plate is arranged at 45 to the transmission axis of the polariser 23. In figure 18 the polariser 23 has its transmission axis arranged parallel to the y-axis and the polarising plates 16 of the light control element 15 have their transmission axes parallel to the x- axis, but the display is not limited to this specific arrangement.

When the half-wave plate is ON, the light plane-polarised in the ydirection by the polariser 23 has its plane of polarisation rotated by 90 by the switchable half-wave plate 24. The light leaving the switchable half-wave plate is therefore plane-polarised in the x- direction, and is accordingly unaffected by the light control element 15. The device therefore operates in a public display mode.

When the switchable half-wave plate is OFF, it has no effect on the plane of polarisation of the light transmitted by the polariser 23, so that light plane-polarised in the y-direction is incident on the light-control element 15. Since the polarising regions 16 of the light control element l S absorb light that is plane-polarised in the y-direction, the light control element will transmit only light that is travelling close to the xz-plane and so can pass through the transparent regions 17 of the element 15. The device therefore operates in a private display mode, as explained previously. In the display of figure 18, the display panel 22 is driven by any suitable drive means (not shown) to display two interlaced images. The two images are the left- eye image and the right-eye image of a stereoscopic image pair. The interlacing is shown schematically in figure 18 by the letters "R" and "L" which denote columns of pixels that display the right-eye image or the left-eye image respectively.

The thickness of the optical control element 15, and the width of the transparent j portions 17, are chosen so that the optical control element 15 also acts as a parallax- barrier for light that is plane-polarised parallel to the y-direction. The parallax barrier separates the two images displayed on the display panel 22, so that the left eye of an observer sees the Icft-eye image and the right eye of an observer sees the right-eye image. In general, the width of the transparent portions 17 of the optical control element 15 must be chosen such that the pitch p of the optical control element/parallax barrier ensures that the parallax barrier correctly separates the two images.

Accordingly, the private display mode of the device 44 may be an autostereoscopic display mode. The display 44 is therefore switchable between an autostereoscopic, private display mode and a public, twodimensional display mode. The display of figure 18 may also be used in a private, 2-dimensional display mode. To achieve this, a single image is displayed on the display panel 22 rather than two interlaced images.

The embodiment of Figure 18 may also be applied to a rear barrier display by arranging the components in a suitable order as explained above, or used in a dual-view display.

The image display panel 22 of the display 44 will typically be several tens of pixels deep and several tens of pixels wide. The polarising regions 16 of the optical control element 15 form the opaque portions of the parallax barrier when the device is operating in its autostereoscopic private mode, and accordingly must extend over the full depth of the image display panel (into the plane of the paper in Figure 18). Furthermore, the opaque portions of a parallax barrier of an autostereoscopic display have a width of typically at least one pixel of the image display panel of the display. Thus, the polarising regions 16 of the image control element 15 must be both wide (having a width equal to at least one pixel ol the display panel 22) and must be tall (many pixels deep). If the segments of polarising material that form the polarising region 16 are made of polarising material throughout, as suggested in figure 18, they are likely to absorb light of both polarisation states as it passes through the entire thickness of the segment, and this will detract from the performance of the device in its public mode.

Figures 19(a) and (b) illustrate one way in which this problem can be overcome. Figure 19(a) shows a modified light-control element 15"of the invention, which is generally- similar to the light control element 15 of figure 4 but in which the polarising regions 16 are not formed of polarising material throughout their entire width. Instead, as shown in figure 19(a) each polarising region 16 is formed of polarising material arranged in a channel having a U-shaped or inverted U-shaped cross-section. The interior of the channel is filled with light-transmissive material 46. The polarising regions 16 are again separated by regions 17 of light-transmissive material. Arranging the polarising regions 16 as shown in figure 19(a) would have no effect on light of the polarisation which is intended to be absorbed - the absorption will be as good in the control element 15" of figure 19 as in the control element 15 of figure 4. However, because the thickness of polarising material is much less in the embodiment of figure 19, the polarising region 16 of the light control element 15" of figure 19(a) will not significantly absorb the polarization that is intended not to be absorbed.

Figure 19(b) illustrates one method of manufacturing the light-control element 15" of figure l9(a). In this method, the control element 15" ismade from a layer of transparent material which is placed over a substrate 33. Selected regions of the transparent film are removed, for example using the irradiation of photoresist material or selective ablation techniques described above. Material is removed to define U- shaped channels 47 in the transparent material. The resultant channels 47 are then filled with a liquid crystal containing a dye, which acts as the polarising material to absorb light of a defined linear polarization. The liquid crystal material may be aligned by any of the methods described above with reference to figures 1 l(a) and 1 l(b).

The light control element 15" of figure 19(a) may also he fabricated from a layer of material that is opaque to one polarisation state but that transmits light of the orthogonal polarisation state. Regions of this layer are removed, for example as described above with reference to figure 12(b) to leave U-shaped regions, and a transmissive material is then deposited.

Figure 20 shows a further display 48 of the present invention. This is able to display private images to two separate observers 45A, 45B.

The display 48 comprises a transmissive image display panel 22 that is illuminated by a backlight 25. Between the backlight 25 and the display panel 22 are disposed a first optical control element 15a, a second optical control element 15b and a polarisation switch. In figure 20 the polarisation switch is formed by a switchable half-wave plate 24 and a linear polariser 23 provided between the second optical control element lSb and the display panel. The switchable half-wave plate is arranged with its optic axis at 45 to the transmission axis of the polariser 23. The polariser 23 may be the input polariser of the display panel 22.

The first optical control element is opaque to light of a first linear polarisation travelling in some, but not all, directions, and has no effect on light of a second linear polarisation orthogonal to the first polarisation. The second optical control element is opaque to light of the second linear polarisation travelling in some, but not all, directions, and has no effect on the light of the first linear polarisation. The polariser 23 is arranged to block one of the first or second linear polarizations.

In the embodiment of figure 20, the first optical control element 15a is effective to control the angular spread of horizontally-polarised light but has no absorbing effect on vertically plane-polarised light, the second optical control element l5b is effective to control the angular spread of vertically-polarised light but has no absorbing effect on horizontally plane-polarised light, and the polariser 23 has its transmission axis arranged vertically. The invention is not, however, limited to this specific orientation.

In the optical control element 15 of figure 4, the polarising regions 16 are arranged with their plane parallel to the input and output surfaces of the optical control element. The invention is not limited to this orientation however. In the light control elements of the figure 20, the polarising regions 16 of the first and second optical control elements 15a, lSb are not arranged to be perpendicular to the input and output faces of the control elements. The orientation of the polarising regions 16 of the two optical control elements 15a, 15b is arranged so that the direction in which the first optical control element 15a directs horizontally polarised light is different from the direction in which the second optical control element directs vertically polarised light. In the specific- embodiment of figure 20, the first optical control element passes horizontally-polarised light in a narrow range of angles centred on approximately +30 to the axis of the display and the second optical control element 15b passes vertically-polarised light in a narrow range of angles centred on approximately -30 to the axis of the display.

The optical control elements 15a, 15b of figure 20 may be manufactured in a similar way to the optical control element of figure 4. For example, layers of polarising plate may be stacked as shown in figure 8(b), and the stack may be cut at an angle other than 90 to the upper surface of the stack.

In one mode of operation of the display 48, the switchable half-wave plate 22 is OFF. It therefore has no effect on the polarisation of light passing through it. In this mode, the first polarisation-dependent light control element 15a is effectively disabled, since it has no effect on the vertically-polarised light that is transmitted by the polariser 23. The first light control element 15a controls the angular spread of horizontally polarised light, but this is blocked by the polariser 23. Vertically polarised light is output from the second optical control element 15b directed towards the second observer 45B and is passed by the polariser 23. In this display mode, therefore, the second user 45B will see the image displayed on the display panel 22, whereas the first user will see a dark display panel.

When the half-wave plate 22 is ON, it is effective to rotate the plane of polarisation of light by 90 . Thus, the horizontally plane-polarised component of light from the backlight is converted to vertically-plane polarised light by the half-wave plate 22 and is passed by the polariser. This light is directed, by the first optical control element 15a, towards the first observer 45A. The vertically-plane polarised component of light from the backlight 25 is converted to horizontally-polarised light by the half-wave plate 22, and is therefore blocked by the polariser 23. When the half-wave plate 22 is ON, therefore, the first user sees an image displayed on the display panel 22, whereas the second user sees a dark display panel.

If it is desired to display a private image to each user, the wave-plate is rapidly switched between its ON and OFF states, and the image displayed on the panel is switched in synchronization with the switching of the wave-plates so that a first image is always displayed while the wave-plate is OFF and a second image is always displayed while the wave- plate is ON. If the switching is performed quickly enough, each user will see a different image without flicker.

In a different display mode, the display 48 may display a single image to both users.

This can be done by rapidly switching the wave plate between its ON and OFF states without changing the image displayed on the display panel 22. This display mode may also be obtained by applying an intermediate control voltage to the wave-plate so that it has a retardation of greater than zero but less than Jo. In this display mode, the horizontally- polarised component and the vertically-polarised component of light from the backlight will each be converted to a circular or elliptically polarised component by the switchable half-wave plate 24, and each component will therefore be partially transmitted by the polariser 23.

Figure 20 shows the display 48 directing different views to different observers, to provide a dual-view display. The display 48 may alternatively be arranged to direct two different images to one observer, so that one eye of the observer receives one image and the other eye of the observer receives another image. In this case, the display may act as an autostercoscopic display by displaying the left-eye image and the right-eye image of a stereoscopic image pair time-sequentially on the display.

In a modification of the display 48 (not illustrated), the backlight is a switchable directional backlight. When the first light control clement lSa is active, the backlight emits light primarily in the direction in which the first light control element 15a transmits light, but when the second light control element lSb is active the backlight emits light mainly in the direction in which the second optical control element lSb transmits light. The advantage of this embodiment is that the light emitted by the' backlight is used more efficiently, leading to a brighter display for the same backlight.- lt also has the advantage that the cross-talk between the two views displayed by the display is reduced. (In this context, the term "cross- talk" is intended to mean visibility of an image intended for the first user (or the first eye of a user) to the second user (or second eye), or vice versa).

In the embodiment of Figure 20 the polarization switch may alternatively be provided within the backlight, between the backlight and the first optical control element lSa, between the first optical control element lSa and the second optical control element I Sb, or in front of the display panel 22.

The embodiment of figure 20 may also be effected by providing the light control elements between the display panel and the observer, for example by providing the components in the following order: image display panel; polariser; switchable half- wave plate; first light control element; and second light control element. If the components are arranged in this order, it is possible to use an emissive display or a transmissive display (which would require a backlight). When the components are arranged in this order, the polariser may be constituted by an exit polariser of the image display panel, if the image display panel has an exitpolariser.

The embodiment of Figure 20 has the further advantage that it does not require the display panel 22 to be precisely aligned with the light control elements. The need for precise registration of the components in prior art displays is therefore avoided in this embodiment.

Figure 21 shows a display 49 according to a further embodiment of the present invention. The display 49 comprises an active optical control element 50 that, in one state, is opaque to light of a first linear polarization travelling in a certain direction and transmits light of the first linear polarization travelling in another direction while also transmitting light of a second, orthogonal polarization. Thus, in this state, the optical control element 50 functions in the same manner as the optical control element 15 of j figure 4. However, the optical control element 50 in this embodiment is switchable, and - can be switched to a second state in which it is opaque to light of neither linear polarization.

In the display 49 of figure 21, the optical control element 50 is disposed between the backlight 25 and the transmissive display panel 22 of the display 49. A linear polariser 23 is provided between the backlight and the display panel, and this may be placed either between the backlight and the optical control element or between the optical control element and the display panel as shown in figure 21. Where the polariser 23 is placed betwocn the optical control element 50 and the display panel 22 as shown in figure 49, if the display panel 22 has an input polariser this may be used as the polariser 23.

The transmission axis of the linear polariser 23 is arranged to be parallel to the transmission axes of the polarising regions of the optical control element 50. In figure 21, the transmission axis of the polariser 23 and the transmission axes of the polarising regions 16 of the optical control element 50 are parallel to the x-axis.

The polarising regions 16 of the optical control element 50 are formed of a liquid crystal material containing dye that is switchable between an absorbing state and a non- absorbing state.

Figures 22(a) and (b) illustrate two modes of operation of the display 49 of figure 21.

These figures show the linear polariser 23 disposed between the backlight 25 and the optical control element SO, but this does not affect the operation of the device. The linear polariser 23 is arranged with its transmission axis parallel to the x-axis. In the mode shown in figure 22(b), the dye molecules of the liquid crystal molecule are aligned along the x-direction so that the regions 16 containing dye are opaque to xpolarised light. The polariscr 23 transmits light that is plane-polarised along the x- direction, and this is absorbed in the polarising regions 16. Thus, light that is travelling along, or close to, the xz-plane is passed by the optical control element as shown by path A in figure 22(b), but light that is travelling at a large angle to the -z plane is absorbed by the polarising regions 16 so that a private display mode is obtained. The angular spread in this mode depends on the ratio between the width of the transmissive regions 17 and the thickness of the optical control element SO.

As is shown in figures 22(a) and 22(b) the optical control element SO is provided with electrodes S1, 52 on its upper and lower surfaces. These are used to apply an elcctic field across the liquid crystal layer to cause the liquid crystal molecules to orient themselves generally in the z-direction. This causes the dye molecules to re-orient themselves with the liquid crystal molecules, so that they are also aligned along the zdirection. This is indicated in figure 22a by the short lines shown within the regions 16 of liquid crystal material. Accordingly, the light plane-polarised in the x-direction that is transmitted by the polariser 23 is not absorbed by the liquid crystal regions 16, and a wide display mode is obtained as shown in Figure 22(a).

The optical control element SO of this embodiment may be manufactured in a number of ways. For example, it may be manufactured substantially as described in figures 9(a) to 9(c) using a lower substrate 36 that has a uniform transparent electrode over its entire area. A second substrate, having a uniform transparent electrode, is placed over the structure, and the recess are filled with liquid crystal material containing dye. The liquid crystal material with dye may be oriented in any of the methods described above with reference to figure l l(a) and l l(b). In this construction method, the liquid crystal material can have an positive dielectric anisotropy and be aligned homogenously, or it may have a negative dielectric anisotropy and be aligned homeotropically.

A second constructional method of the switchable optical control element is to use patterned alignment layers. This is shown in figures 23(a) and 23(b). In this embodiment, each substrate 36, 36' of the element is provided with a transparent electrode on one surface and an alignment film (not shown) over the electrode. The alignment layers are patterned, and the substrates are then aligned parallel to one another with a separation equal to the desired thickness of the liquid crystal layer. The gap between the substrates is then filled with a liquid crystal material including a dye Because of the patterned alignment layers, when no voltage is applied across the liquid crystal layer it adopts regions of two different alignments. In regions 53 the liquid crystal molecules, and consequently the dye molecules, are aligned substantially in the zdirection. In regions 54, however, the liquid crystal molecules are aligned substantially in the x-direction (planar alignment) and the dyemolecules are accordingly also aligned in this direction. The regions 54 therefore act as absorbing regions for light that is plane polariscd in the x-direction.

When an electric field is applied across the liquid crystal layer, via the electrodes 51, 52, liquid crystal molecules in the regions 54 of planar alignment re-orient themselves in generally the z-direction, and the dye-molecules are accordingly also re-aligned to lie in the z- direction. The orientation of the dye molecules in the regions 53 where alignment in the z-direction already exists is unaltered. The entire liquid crystal layer is therefore non-absorbing for light plane-polarised in the x-direction, as shown in figure 23(a).

Figures 24(a) and 24(b) show a modification of the embodiment of figures 23(a) and 23(b). In this embodiment, the electrodes 51, 52 are patterned rather than the alignment films. (The alignment films and substrates have been omitted from figure 24(a) and 24(b) for clarity). The alignment films are arranged to produce a liquid crystal alignment in the z-direction (homeotropic alignment) as shown in figure 24(a). When an electric field is applied between the electrodes, regions of the liquid crystal layer between electrodes are re-oriented to adopt a planar alignment (parallel to the x direction), whereas liquid crystal material in regions where electrodes are not present retains its original homeotropic alignment. This is shown in figure 24(b). In the regions 54 of the liquid crystal layer between the electrodes, the liquid crystal molecules adopt a planar alignment when a voltage is applied across the liquid crystal layer, causing the dye-molecules also to re-orient themselves parallel to the x-direction. The regions 54 therefore become absorbing for light that is plane-polarised in the x-direction when a voltage is applied across the liquid crystal layer.

In describing the operation of the embodiments of figures 22(a) to 24(b) it was assumed i that the liquid crystal material contained a positive dichroic dye. The liquid crystal - material may also contain a negative dichroic dye, in which case the mode of operation of the optical control element is reversed.

As a further alternative, it will be possible to use an electrode structure that generates in- plane electric fields and rotate the liquid crystal and dye molecules in a plane parallel to the substrates by suitably applying a voltage. In this case, the liquid crystal molecules and dye molecules can be switched to be either along the x-direction or along the y- direction.

The liquid crystal materials used in the embodiments described above may be either a nematic liquid crystal or a smectic liquid crystal material. Alternatively, a bistable liquid crystal material may be used, in which the orientation of the liquid crystal material is stable in both operating states, so that application of a voltage is required only to switch the material from one operating state to another.

The light control element of the invention has been described in connection with a display. However, the light control elements of the invention are not limited to use in a display. The combination of a light control element of the invention and a polarization switch disposed in the path of light through the light control element forms a light control device that has many applications. For example, such a light control device might be sold separately so that it could be attached to a preexisting display to make it into a display switchable between public and private modes. Such a light control device also has other applications as one example, it might be used to make a window that can be switched between a narrow range of transmission angles and a wide range of transmission angles. The window would be effective to cut out glare when switched to have a narrow range of transmission angles. Such a window would have applications as, for example a switchable sun visor for a motor vehicle.

Claims

CLAIMS: 1. A light control element which outputs light having a first

polarisation state with a first angular range and outputs light having a second polarisation state different from the first polarisation state with a second angular range greater than the first angular range.
2. A light control element as claimed in claim 1 which is opaque or substantially opaque to light having the first polarisation state and travelling in a first range of j directions, which is substantially not opaque to light having the first polarisation state - and travelling in a second range of directions different from the first range of directions, and which is substantially not opaque to the second polarisation state.
3. An element as claimed in claim 1 or 2 wherein the second polarisation state is orthogonal to the first polarisation state.
4. An element as claimed in claim 1, 2 or 3 and comprising a plurality of regions opaque or substantially opaque to light of the first polarisation state and transmissive to light of the second polarisation, each pair of neighbouring regions being spaced from one another by a material transmissive to light of the first polarisation and to light of the second polarisation.
5. An element as claimed in claim 4 wherein each region extends substantially parallel to a direction crossed with the plane of the device.
6. An element as claimed in claim 4 wherein each region extends substantially parallel to a direction perpendicular to the plane of the device.
7. An element as claimed in claim 4, 5 or 6 and comprising a substrate transmissive to light of the first polarisation state and to light of the second polarisation state; a plurality of recesses in the substrate; and a material opaque or substantially opaque to light of the first polarisation state and transmissive to light of the second polarisation state disposed in each recess.
8. An element as claimed in claim 4, 5 or 6 and comprising a substrate opaque or substantially opaque to light of the first polarisation state and transmissive to light of the second polarisation state; a plurality of recesses in the substrate; and a material transmissive to light of the first polarisation state and to light of the second polarisation state disposed in each recess.
9. An element as claimed in any of claims 4 to 8 wherein each region is switchable-- between a first state in which it is opaque to light of the first polarisation state and a second state in which it is substantially transmissive to light of the first polarisation state.
10. An element as claimed in claim 4 wherein: each region extends substantially parallel to the plane of the element; the regions are arranged in a plurality of stacks of two or more elements; and the stacks are spaced laterally from one another with each pair of adjacent stacks being separated by a material transmissive to light of the first polarisation and to light of the second polarisation.
11. An element as claimed in any of claims 4 to 10 wherein each region comprises a linear polariser material.
12. An element as claimed in any of claims 4 to 11 wherein each region comprises a liquid crystal material.
13. A light control device comprising a light control element as defined in any of claims 1 to 12 and a polarisation switch disposed in an optical path through the light control element.
14. A device as claimed in claim 13 wherein the polarisation switch comprises a switchable wave-plate.
15. A device as claimed in claim 13 wherein the polarisation switch comprises a switchable half wave-plate disposed in series with a linear polariser.
16. A display comprising a light control element as defined in any of claims I to 12.
17. A display as claimed in claim 16 and further comprising a polarisation switch disposed in an optical path through the light control element.
18. A display as claimed in claim 17 wherein the polarisation switch comprises a- switchable wave-plate.
19. A display as claimed in claim 18 wherein the polarisation switch comprises a switchable half wave-plate disposed in series with a linear polariser.
20. A display as claimed in claim 16, 17 or 18 wherein the light control clement forms, in use, a parallax optic for light of the first polarisation whereby the display is operable as a multiple view directional display.
21. A display comprising: a first light control element as defined in any of claims 1 to 12; a second light control element as defined in any of claims 1 to 12; a first polarisation switch; wherein the angular output range of the first optical control element for light of the first polarisation is different to the angular output range of the second optical control element for light of the first polarisation.
22. A display as claimed in claim 21 and further comprising a second polarisation switch.
23. A display comprising a first light control element as defined in any of claims 1 to 12; a second light control element opaque or substantially opaque to light having the second polarisation state and travelling in a third range of directions while substantially not opaque to light having the second polarisation state and travelling in a fourth range of directions different from the third range of directions and while substantially not opaque to light having the first polarisalion state; and a polarisalion switch disposed in the path of light through the first and second light control elements.
24. A display as claimed in claim 23 wherein the first light control element outputs, in use, light of the first polarisation in an angular range centred about a first direction and the second light control clement outputs, in use, light of the second polarisation in an angular range centred about a second direction different from the first direction.
25. A method of manufacture of a light control element comprising the steps of:- slacking a plurality of polariser sheets one above the other, each polariser sheet comprising a polariser layer and at least one lighttransmissive substrate; and adhering each polariser sheet to its neighbouring polariser sheet(s).
26. A method as claimed in claim 25 and comprising: disposing lightcurable adhesive over a first polariser sheet, stacking a second polariser sheet over the first polariser sheet; and irradiating the adhesive to cure the adhesive and thereby adhere the first polariser sheet to the second polariser sheet.
27. A method as claimed in claim 25 or claim 26 and further comprising providing a light transmissive layer between each pair of neighbouring polariser sheets.
28. A method as claimed in any of claims 25 to 27 and comprising the further step of removing selected regions of the polarising layers of all polariser sheets of the stack.
29. A method as claimed in any of claims 25 to 28 and comprising the further step of cutting the stack into slices, wherein the cutting direction may be perpendicular to the plane of the polariser sheets or alternatively in another direction not parallel to the plane of the polariser sheets.
30. A method of manufacture of a light control element comprising the steps of: removing selected regions of the polariser layer of a first polariser sheet, the polariser sheet comprising a light-transmissive substrate and the polariscr layer; stacking a second polariser sheet over the first polariser; and removing selected regions of the polariser layer of the second polariser sheet.
31. A method of manufacture of a light control element comprising the steps of: forming a plurality of recesses in a light-transmissive substrate; and providing a polarising material in the recesses.
32. A method of manufacture of a light control element comprising the steps off forming a plurality of recesses in a substrate formed of a polarising material; and- providing a lighttransmissive material in the recesses.
33. A method as claimed in claim 31 or 32 wherein the step of forming the recesses in the substrate comprises the step of selectively irradiating the substrate.