GB2428100A

GB2428100A - Display device and optical device

Info

Publication number: GB2428100A
Application number: GB0513969A
Authority: GB
Inventors: Grant Bourhill; Diana Ulrich Kean; Nathan Smith
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2005-07-08
Filing date: 2005-07-08
Publication date: 2007-01-17
Also published as: GB0513969D0

Abstract

An optical device is provided that comprises a light refracting element (1) switchable between first and second modes in which the element (1) has first and second different light refracting functions respectively for incident light (9a, 9b) having a predetermined polarisation, the strength of the second light refracting function being dependent on the angle of incidence. There is also disclosed a display device including an array of said optical devices. When the optical devices are in a first mode the display operates in a wide viewing angle (public) mode. When the optical devices are switched into a second mode the display device operates in a narrow viewing angle (private) mode.

Description

Display Device and Optical Device The present invention relates to a

display device and an optical device for use in a display device.

Electronic display devices, such as monitors used with computers and screens built in to telephones and portable information devices, are usually designed to have a viewing angle as wide as possible, so that they can be read from as many viewing positions as possible.

However, there are some situations where it is useful to have a display that is visible from only a narrow range of angles. For example, where a person is reading a confidential or private document on the display of a mobile device in a crowded place, he would wish to minimise the risk of others around him also having sight of the document on the display.

It is therefore useful to have a display device that is switchable between two modes of operation. In a public' mode, the display device would have a wide viewing angle for general use. In a private' mode, the display device would have a narrow viewing angle, so that private information could be read in a public place.

For example, when certain secure web pages are accessed (e.g. bank site web pages), or when a certain PIN (personal identification number) is input to the keyboard (e.g. bank account PIN), the display could automatically go into the privacy mode. In the private mode, an indicator or icon could be shown on the screen to indicate that the private mode is active.

This concept can be applied to many other types of devices where a user may -wish to view confidential information, but cannot control who else may be watching. Examples are mobile phones, Personal Digital Assistants (PDAs), laptop PCs, desktop monitors, Automatic Teller Machines (ATMs) and Electronic Point of Sale (EPoS) equipment.

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

US 6,552,850 describes a method for the display of private information on a cash dispensing machine. Light emitted by the machine's display has a fixed polarisation state. The machine and its user are surrounded by a large screen of sheet polariser that absorbs light of that polarisation state but transmits the orthogonal state. Passers-by can see the user and the machine but cannot see information displayed on the screen.

One method for controlling the direction of light is the use of a "louvred" film. Such a film consists of alternating transparent and opaque layers in an arrangement similar to a Venetian blind. These layers may be perpendicular to the surface of the film or at some other angle. Like a Venetian blind, it allows light to pass through it when the light is travelling in a direction nearly parallel to the plane of the layers, but absorbs light travelling at large angles to the plane of the layers. Methods for the production of such films are described in USRE 27,617, US 4,766,023 and US 4,764,410.

Other methods exist for making films with similar properties to the louvred film. These are described, for example, in US 5,147,716 and US 5, 528,319.

The techniques described above may be used to restrict the range of angles from which a display can be viewed; in other words, they can be used to make a display private'.

However none of them gives a method by which the privacy function can easily be switched off to allow viewing from a wide range of angles.

Several methods are known for providing a display that can be switched between a public mode (with a wide viewing angle) and a private mode (with a narrow viewing angle).

US 2002/0158967 describes the use of a light control film mounted on a display so that the light control film can be moved over the front of the display to give a private mode, or mechanically retracted into a holder behind or beside the display to give a public mode. This method has the disadvantages that it requires moving parts that may fail or be damaged, and it adds significant bulk to the display.

One method for switching 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 that 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 the private mode. When the diffuser is switched on, it causes light travelling at a wide range of angles to pass through the panel and the display is in the 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 this type are described in US 5,831,698, US 6,211,930 and US 5,877,829. They share the disadvantage that the light control film absorbs a significant fraction of the light incident upon it, whether the display is in the public or the private mode. The display is therefore inefficient in its use of light. Since the diffuser spreads light through a wide range of angles in the public mode, these displays are also dimmer in the public mode than in the private mode, unless the backlight is made brighter to compensate.

Another disadvantage relates to the power consumption of such devices. In the public mode of operation, the diffuser is switched off. This would typically mean that a voltage is applied to a switchable polymerdispersed liquid crystal diffuser. More power is therefore consumed in the public mode than in the private mode. This is a disadvantage for displays that are used for most of the time in the public mode.

Another known method for providing a switchable public/private display is described in US 5,825,436. The light control device disclosed is similar in structure to the louvred film described above. However, each opaque element in the louvred film is replaced by a liquid crystal cell that 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 is in a private mode; when the cells are transparent, the display is in a public mode.

One disadvantage of this method relates to the difficulty and expense of manufacturing liquid crystal cells with an appropriate shape. Another 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.

Another method for producing a switchable public/private display device is disclosed in JP 3607272. The disclosed device uses an additional liquid crystal panel, which has patterned liquid crystal alignment. Different aligned segments of the panel modif' the viewing characteristics of different areas of the display in different ways, with the result that the whole display panel is fully readable only from a central position.

GB-A-2405544 and JP 2005-078093 describe switchable privacy devices based on louvres, which operate only for one polarisation of light. The louvres are switched on and off either by rotating dyed liquid crystal molecules in the louvre itself or by rotating the plane of polarisation of the incident light using a separate element.

Co-pending British Patent Application No. 0401062.5 and PCT Patent Application No. PCT/1B2005/050 170 disclose various backlight arrangements for use in a display device having the ability to switch the viewing angle between public and private modes, for example. Further known systems and techniques in this area are also described therein.

Co-pending British Patent Application No. 0408742.5 discloses a switchable privacy device that is constructed by adding one or more extra liquid crystal layers and polarisers to a display panel. The intrinsic viewing angle dependence of these extra elements can be changed by electrically switching the liquid crystal.

Co-pending British Patent Application No. 0427303.3 discloses a polarisation modifying layer (PML) that is placed behind the exit polariser of a liquid crystal display panel. Some parts of the PML are transparent. Other parts change the polarisation of light passing through them so that pixels viewed through these parts are inverted in colour (bright pixels becoming dark and dark pixels becoming bright). Data sent to pixels directly behind these parts are inverted so that when the display is viewed from a central position, the image appears normally. However, when the display is viewed from a different angle, different pixels are viewed through the retarder elements and the image is corrupted. Off-axis viewers see a confusing image, for example a random dot pattern. The PML may be made from liquid crystal and switched off to give a public mode.

Co-pending British Patent Application No. 0421227.0 discloses a device in which a guest host (dyed) LC layer with a patterned electrode is added to a standard TFT LC display. The dyed LC layer can be switched between an absorbing (private) and non- absorbing state (public). The dye molecules absorption is dependent upon the incident angle and polarisation of light. For a given polarisation and orientation the absorption of the dye increases with larger viewing angles resulting in low brightness at high angles (narrow mode).

Co-pending British Patent Application No. 0510422.9 discloses the combination of a privacy function and a 3D function provided by a single additional switch cell. The display has three operating states: a wide mode; a private mode; and a 3D mode. Both patterned and unpatterned LC alignment examples are described.

The concept of using a hologram to provide a privacy function which was first described in GB-A-240499 I. However, due to unwanted diffraction of light from the display by the hologram, the colour of the image seen by viewers may be affected.

Furthermore, for applications using a touch screen mounted on the front of the display, the user's hand can block the illumination of the hologram and so reduce the effectiveness of the privacy mode.

Co-pending British Patent Application No. 0511536.5 discloses the use of an extra liquid crystal layer located between the existing polarisers of an LCD panel. In this location the extra switch cell can modify the greyscale curves for off-axis light. This provides a higher level of privacy for images than the techniques disclosed in, for example, copending British Patent Application No. 0408742.5.

US 5,109,219 describes a method for controlling the viewing angle of a LC display by converting a digital view angle parameter to an analogue bias voltage which is applied to the LC. However, this technique will only serve to modify the view angle characteristics of the display, and will not tend to hide the image at wide angles.

US 5,936,596 and JP 2002-263235 describe changing the voltage range applied to the pixels in an LC display to change the viewing angle. Lookup tables are used to change the display between narrow and wide viewangle modes. However, this method does not conceal displayed information as such when in the narrow mode, it only modifies the grey-scale mapping to distort the image.

The article "A Method for Concealment of Displayed Data", M. Dogruel, Displays, vol. 24, no. 3, October 2003, describes a method for concealing data shown on a display by time-sequentially rendering the image and its inverse at a rate faster than the human eye can perceive. The eye of a casual observer thus averages the images and therefore sees a uniform grey display screen. To see the private image, the user must wear shuttered glasses synchronised with the display, such that the inverse image is blocked. This method has a number of drawbacks: firstly, the user must wear shuttered glasses in order to observe the correct image; secondly, image privacy can also be compromised by rapidly moving a toothed object across the view of the display and thus obscuring some parts of the cancelling image; and thirdly a ghost image can be observed as it is very difficult to design the two images to cancel perfectly. This article also describes adding a third image to act as a confusing image, but this requires the display to run at three times the normal video rate.

Rocket Software, Inc. (http://www.rocketsoftware.com) have developed a software package that provides some level of privacy using the inherent properties of an LC display. The software modifies the image sent to the display by applying an extra patterning across the whole image that reduces the grey levels or contrast of the image.

Due to the non-linear response of the display, the level of reduction is such that, when viewed on-axis, the image is only slightly disturbed but, when viewed off- axis, the non- linear response of the display leads to an enhanced contrast patterning. However, this solution does inevitably affect the on-axis performance of the display in some degree, and the pattern visibility will disturb even the authorised user when using the display in the private mode. Further, in practice, the patterning is not sufficient to provide an adequate level of privacy off-axis.

JP 09230377 and US 5,844,640 describe a method of changing the viewing angle properties of a single layer LCD panel. This is achieved for a Vertically Aligned Nematic (VAN) LC mode. Electric fields in the plane of the display panel are used to control how the LC material tilts in a pixel area. The number and orientation of different tilt domains within a pixel can be controlled by the in-plane fields. A pixel with several tilt domains will have a wide viewing angle, while a pixel with one tilt domain will have a narrower viewing angle. The use of such a method to vary the viewing angle of a display is described. However, the viewing angle of a single tilt domain of the VAN mode described is generally not sufficiently narrow to provide a good privacy mode.

JP 3405972 describes a single LC panel which uses patterned LC alignment to provide a narrow viewing angle mode LCD. However, this narrow mode is fixed, and there is no wide viewing mode.

WO 03/015424 discloses a light switching apparatus that comprises a passive birefringent lens and a switchable polariser. By switching the polarisation, different directional distributions of output light are provided. However, when activated, the lenses do not discriminate in angle which light is imaged.

US 6,369,949 discloses an optically anisotropic micro-lens window. The imaging element described is not switchable, and consequently a device making use of this technology could not be switchable between public and private modes of operation.

Co-pending British Patent Application No. 0401190.4 discloses the use of multiple arrays of polarisation sensitive lenses in a polarisation optical conversion system.

According to a first aspect of the present invention, there is provided an optical device comprising a light refracting element switchable between first and second modes in which the element has first and second different light refracting functions respectively for incident light having a predetermined polarisation, the strength of the second light refracting function being dependent on the angle of incidence.

The strength of the first light refracting function may be substantially independent of the angle of incidence.

The first and second light refracting functions may be substantially the same for a predetermined angle of incidence.

The strength of the second light refracting function may be substantially zero at the predetermined angle of incidence.

The predetermined angle of incidence may be substantially normal to the light refracting element.

The first light refracting function may comprise passing the incident light substantially without convergence, whether positive or negative.

The first light refracting function may comprise passing the incident light substantially without deviation.

The strength of the second light refracting function may vary substantially continuously with the angle of incidence.

The predetermined polarisation may be a linear polarisation.

The second light refracting function may comprise a light converging function.

The second light refracting function may comprise a lens function.

The lens function may comprise a positive lens function.

The element may comprise a polarisation sensitive light refracting arrangement and a switchable polarisation modifying layer for providing light to the arrangement having different respective polarisations in the first and second modes for incident light of the predetermined polarisation, at least for incident angles away from a predetermined angle of incidence.

The polarisation modifying layer may be adapted to pass light of the predetermined polarisation in the first mode substantially without affecting its polarisation.

The arrangement may comprise optically isotropic material in contact with optically anisotropic material having a shaped interface therebetween.

The isotropic material or the anisotropic material may be shaped as a convex lens.

The isotropic material or the anisotropic material may be shaped as a cylindrical lens.

The anisotropic material may be distinct from the polarisation modifying layer.

The anisotropic material may be formed of reactive mesogen material.

The anisotropic material may be indistinct from the polarisation modifying layer, such that the anisotropic material performs the function of the polarisation modifying layer.

The isotropic material may have a refractive index substantially equal to one of the refractive indices of the anisotropic material, at least in the first mode.

The isotropic material may have a refractive index substantially equal to one of the refractive indices of the anisotropic material in the first and second modes.

The anisotropic material may have a refractive index substantially equal to one of the refractive indices of the polarisation modifying layer, at least in the first mode.

The one of the refractive indices may be that experienced by incident light of the predetermined polarisation.

The polarisation modifying layer may be adapted to modify the polarisation in the second mode by an amount dependent upon the angle of incidence.

The polarisation modifying layer may be formed of ECB material.

The polarisation modifying layer may comprise a retarder.

The retarder may be a half wave plate.

The anisotropic material may comprise VAN liquid crystal material.

The light refracting element may be electronically switchable between the first and second modes.

The optical device may comprise an array of such light refracting elements.

Each element of the array may have one of a plurality of different types of second light refracting function.

The optical device may comprise a plurality of different strengths of lens function for the same incident angle.

The different strengths may be arranged in a random fashion across the array.

The elements of the array may cooperate to provide a light scattering function in the second mode.

According to a second aspect of the present invention, there is provided a display device comprising an optical device according to the first aspect of the present invention.

The display device may comprise a spatial light modulator disposed between a backlight and the optical device.

Each element of the array may be optically aligned with a respective pixel of the modulator.

The modulator may be a liquid crystal device.

In the first mode the elements of the array may have substantially no effect on an image formed by the modulator, and in the second mode the elements of the array may cooperate to disturb the image for viewers receiving light incident on the elements away from a predetermined angle of incidence.

The first mode may be a public mode of the display device and the second mode may be a private mode of the display device.

Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 illustrates an optical device according to a first embodiment of the present invention; Figure 2 illustrates a display device incorporating an array of light refracting elements of the first embodiment; Figure 3 illustrates a display device according to a second embodiment of the present invention; Figure 4 illustrates a display device according to a third embodiment of the present invention; Figure 5 illustrates a display device according to a fourth embodiment of the present invention; and Figure 6 illustrates a display device according to a fifth embodiment of the present invention.

As mentioned above, the concept of electronically switchable liquid crystal lenses is generally known. However, known switchable lens designs have in common that, when the lens is switched on, an imaging function is performed for light incident at all angles.

An embodiment of the present invention, on the other hand, provides a lens that can perform an imaging function for light incident at oblique angles while light incident at or near normal incidence is not imaged by the lens. The lens can also be switched off so that substantially no imaging is performed for any incident angle.

Such a lens design is illustrated in Figure 1, which shows an optical device according to a first embodiment of the present invention. The optical device according to the first embodiment comprises a light refracting element 1 that is switchable between first and second modes. The left half of Figure 1 shows the light refracting element 1 operated in the first mode, while the right half of Figure 1 shows the light refracting I operated in the second mode.

The light refracting element I comprises optically isotropic material 3 in contact with optically anisotropic material 5, having a shaped interface 7 therebetween. In the first embodiment, the interface 7 is shaped such that the anisotropic material 3 forms a convex lens.

The anisotropic material 5 in the first embodiment comprises Electrically Controlled Birefi-ingent (ECB) liquid crystal material. In the first mode, the liquid crystal in the ECB material 5 is oriented in a predetermined direction substantially parallel to the planar outer surface of the light refracting element 1. In operation of the light refracting element 1, incident light 9a, 9b is arranged to have a linear polarisation in the same predetermined direction as the liquid crystal orientation of the ECB material 5 in the first mode. Further, the refractive index experienced by such polarised light passing through the ECB material 5 in the first mode is arranged to be substantially the same as the refractive index of the isotropic material 3.

With such an arrangement, in the first mode of operation, incident light 9a, 9b is passed through the ECB material 5 and into the isotropic material 3 substantially without refraction at the interface 7 between the ECB material 5 and the isotropic material 3.

The incident light 9a, 9b passes through the light refracting element 1 substantially undeviated.

In the second mode of operation, as illustrated in the right half of Figure 1, a suitable voltage is applied across the light refracting element I to change the orientation of liquid crystal in the bulk of the ECB material 5 towards having a substantially non- horizontal director orientation. In this switched configuration, off-axis (oblique) incident light 9b experiences some degree of polarisation modification, such that oblique light 9b arriving at the interface 7 has a polarisation-converted component that is orthogonal to the predetermined linear polarisation of the incident light 9b.

Due to the birefringence of the anisotropic 5, this polarisationconverted component experiences a different refractive index compared to light incident at the same angle in the first mode of operation. In other words, the refractive index experienced by the polarisation-converted component in the second mode for oblique light is different to the refractive index of the isotropic material 3 and such light is therefore refracted at the interface 7 between the anisotropic material 5 and the isotropic material 3. Therefore, light 9b that is incident at an oblique angle experiences some degree of refraction at the lens interface 7, and a lensing or imaging function is present.

The size of the polarisation-converted component increases as a function of the angle of incidence, and accordingly, the lensing function in the second mode has a strength that is dependent on the angle of incidence. For light incident normal to the light refracting element I, no polarisation conversion is experienced, and no imaging function is performed by the light refracting element I. Equivalently, the strength of the lensing function for on-axis (normal incidence) light is substantially zero.

It will be appreciated that, although the light refracting element I in the first embodiment is adapted to have no imaging function in the first mode, in other applications it would be possible to alter the refractive indices of the anisotropic material 5 and/or the isotropic material 3 such that an imaging function is also performed in the first mode. With such a device, the light refracting element would be switchable between first and second modes in which the element has first and second different light refracting functions respectively for incident light having a predetermined polarisation, with the strength of the second light refracting function being dependent on the angle of incidence.

Figure 2 shows the light refracting element 1 of Figure 1 incorporated into a display device 11. The display device II comprises a backlight 13, an input polariser 15, a liquid crystal spatial light modulator 17 disposed between two substrates 1 9a and 1 9b, and an output polariser 21. These parts 12 together are known from existing display devices.

In a display device according to a first embodiment, in addition to the known components mentioned above, the display device 11 comprises an array 31 of light refracting elements 1, as described above with reference to Figure 1, disposed between two substrates 23a and 23b in a layer structure. The ECB material 5 described above with reference to Figure 1 for an individual light refracting element 1 forms a continuous layer when incorporated into an array 31 of image refracting elements 1.

The orientation of liquid crystals in the ECB layer 5 in the first mode of operation is arranged to be substantially parallel with the output polariser 21 of the standard display components 12.

In the first embodiment the array 31 comprises light refracting elements I having a variety of different f-number lenses (i.e. lenses with different focal lengths and/or of a different physical size) arranged in a random spatial distribution so as to provide spatially varying imaging functions in the second mode of operation that distort the off- axis image of the underlying spatial light modulator 17 in a random fashion. For light incident obliquely on the spatial light modulator 17, and subsequently the array 31, a spatially varying imaging function is performed by the array 31, causing confusion and disturbance to the image destined to a an off-axis viewer. The second mode of operation is therefore suitable as the private mode in a public/private display device. In the private mode, as described above, on-axis light destined to an on- axis viewer is substantially undeviated by the array 31 of light refracting elements 1, and therefore the on-axis viewer receives an undistorted image.

When the display device 11 is switched back to the first mode of operation, substantially no imaging function is performed by the array 31 of light refracting elements I for any viewing angle, and accordingly the first mode of operation is the public mode for a viewing angle restriction (VAR) display device. Figure 3 illustrates a display device 41 according to a second embodiment

of the present invention comprising an array 231 of light refracting elements 241. The display device 41 of the second embodiment is similar to the display device 11 of the first embodiment, and only the differences will be described herein.

In the first embodiment described above with reference to Figures 1 and 2, there may be some residual lensing effect on-axis in the second mode due to the refractive index change from the tilt of the ECB molecules of the anisotropic material 5. In order to ensure that there is no refractive index change at the lens interface 7 for normal (on- axis) incidence in both modes of operation, a reactive mesogen (RM) planarisation layer 43 is added in the second embodiment between the ECB layer 5 and the isotropic elements 3. This effectively decouples the polarisation conversion switching at oblique angles from the requirement for an unswitchable matching of refractive index for one polarisation state. The refractive index is matched between the ECB layer 5, the RM planarisation layer 43 and the lenses 3 for incident light of the predetermined polarisation so that no lensing action is performed in the first mode. To achieve this, the director of the RM material is substantially parallel to the ECB director.

In the second embodiment, therefore, when a voltage is applied across the light refracting elements 241 in the array 231, there is no change in LC orientation in the RM planarisation layer 43, and therefore no change in refractive index at the interface 7 experienced by on-axis incident light, and accordingly no refraction at the interface 7.

In effect, for each light refracting element 241 in the second embodiment, the RM planarisation layer 43 and the isotropic material 3 form a polarisation-sensitive light refracting arrangement, having a refracting function that is dependent upon the polarisation of light incident on the arrangement, which in turn is dependent upon the angle of incidence through the ECB layer 5 beforehand. The ECB layer 5 acts as a polarisation modifying layer for providing light to the arrangement 3, 43 having different respective polarisations in the first and second modes for oblique light having the predetermined polarisation. In the first embodiment, the ECB layer 5 also acts as a polarisation modifying layer, with the polarisation-sensitive light refracting arrangement being formed by the isotropic lenses 3 and the portion of ECB material 5 immediately surrounding the lenses 3 (since it is the LC configuration of that part which determines the refraction at the interface 7).

Figure 4 illustrates a display device according to a third embodiment of the present invention. The standard display elements 12 are as described above in connection with the first embodiment. In the third embodiment, each light refracting element 341 of an array 331 comprises a switchable half wave plate 305 disposed between substrates 323a and 323b. En the light path following the half wave plate 305, a vertically aligned nematic (VAN) layer 343 is arranged in contact with an isotropic lens 3 of the type described above with reference to the first embodiment. The array 331 of isotropic lenses 3 is disposed on a further substrate 325.

The VAN material 343 is adapted to have an ordinary refracted index n0 equal to the refractive index of the isotropic lens 3. In the first (public) mode of operation, light from the output polariser 21 is vertically polarised (into the plane of the page). The switchable half wave plate 305 has no effect on the polarisation in the first mode of operation, so that the polarisation of light entering the VAN layer 343 is vertical. In this mode, therefore, every angle of light in the plane of the page experiences a refractive index n0 passing through the VAN material 343, and since this matches the refractive index of the isotropic lenses 3, there is no lensing effect for any incident angle. Some incident angles out of the plane of the page experience a refractive index n (the extraordinary index of the VAN material 343) and no. However, in the third embodiment the lenses 3 are cylindrical lenses arranged vertically (relative to the horizontal page), so that there is no lensing action. This is the first, or public, mode of operation.

In the second, or private, mode of operation, the half wave plate 305 is switched so that it rotates the polarisation from the output polariser 21 to be horizontal (in the plane of the page and parallel to the top and bottom edges of the page). At normal incidence, this polarisation experiences a refractive index n0, and therefore is no lensing function.

At oblique angles of incidence the light experiences both fle and n0 of the VAN material 343, such that the lens "develops" and causes confusion at oblique incidence to an off- axis viewer. This is the private mode of operation.

In the third embodiment, the switchable polarisation modifying layer mentioned above in connection with the first and second embodiments is provided by the half wave plate 305, while the polarisation-sensitjve light refracting arrangement is provided by the VAN material 343 in combination with the isotropic material 3.

Figure 5 illustrates a display device 411 according to a fourth embodiment of the present invention, comprising the standard display components 12 as mentioned above in addition to an array 441 of light refracting elements 441. Each light refracting element 411 comprises liquid crystal material 443 in a twisted nematic (TN) configuration in contact with an isotropic lens 3, disposed between two substrates 423a and 423b.

Viewing angle restriction in the fourth embodiment is achieved with multirubbed TN material. At certain voltages, the transmission verses angular response of a TN device is very asymmetric. It is therefore possible to have a configuration where the TN material rotates the polarisation by 900 on-axis and for a wide range of positive angles off-axis. For a range of negative angles off-axis the polarisation will not be rotated. By patterning the orientation of the TN material, it is possible to cause the lens to develop for light that has no rotation of the polarisation, while the light that is rotated by 900 does not "see" the lens. This patterning could be formed in any orientation so that privacy caused by the lensing off-axis can be realised in any direction. Figure 5 shows an example where the light refracting elements 441 in region A develop lenses at 35 , while the light refracting elements 441 in region B develop lenses at +35 .

Figure 6 illustrates a display device 511 according to a fifth embodiment of the present invention. The display device 511 comprises the same basic display components 12 as described above, in addition to an array 531 of light refracting elements 541. Each light refracting element 541 comprises an optically anisotropic lens 513 receiving light from an ECB layer 5, these being disposed between substrates 523a and 523b.

Each anisotropic lens 513 is polarisation dependent such that one linear polarisation of light is imaged while the orthogonal linear polarisation is not. Such a lens can be realised using a liquid crystal material that has a spatially varying tilt angle so as to realise a lensing structure. Such LC lensing structures can be realised using patterned electrodes (T. Nose and S. Sato, Liq. Cryst. 5, 1425 (1989)), micropatterned alignment (described in co-pending United Kingdom patent application number 0405165. 2), or curved electrodes (S. Sato, Jpn. J. Appi. Phys. 18, 1679 (1979)).

In one-dimensional (cylindrical) LC lensing structures, a spatially varying LC tilt angle is designed to occur parallel to a single plane and so the lensing effect only occurs for light linear polarised parallel to the plane of the LC deformation. In this embodiment, lens deformation is a splay-bend type and occurs in the plane of the page. For light polarised in the plane of the page, an effective refractive index nefi is experienced that is a function of the lateral lens position and the angle of incidence. For a positive LC material, n0 = eJf A pure twist deformation lens can also be realised, although the light from the output polariser 21 should be polarised in the plane of the page for such a configuration.

Operation of the fifth embodiment in the first (public) mode is as follows. Light from the output polariser 21 is vertically polarised (into the plane of the page). Such light will experience a refractive e in the ECB material 5 and n0 in the lens element 513 for all angles of incidence. No polarisation conversion occurs in the ECB material 5, and therefore no imaging occurs.

Operation of the fifth embodiment in the second (private) mode is as follows. Again, light from the output polariser 21 is vertically polarised (into the plane of the page).

Such light will experience polarisation conversion in the switched ECB layer 5, and the polarisation converted component will be imaged by the optically anisotropic lenses 513 to achieve the private mode of operation.

Claims

CLAIMS: 1. An optical device comprising a light refracting element

switchable between first and second modes in which the element has first and second different light refracting functions respectively for incident light having a predetermined polarisation, the strength of the second light refracting function being dependent on the angle of incidence.
2. An optical device as claimed in claim I, wherein the strength of the first light refracting function is substantially independent of the angle of incidence.
3. An optical device as claimed in claim 1 or 2, wherein the first and second light refracting functions are substantially the same for a predetermined angle of incidence.
4. An optical device as claimed in claim 3, wherein the strength of the second light refracting function is substantially zero at the predetermined angle of incidence.
5. An optical device as claimed in claim 3 or 4, wherein the predetermined angle of incidence is substantially normal to the light refracting element.
6. An optical device as claimed in any preceding claim, wherein the first light refracting function comprises passing the incident light substantially without convergence.
7. An optical device as claimed in any preceding claim, wherein the first light refracting function comprises passing the incident light substantially without deviation.
8. An optical device as claimed in any preceding claim, wherein the strength of the second light refracting function varies substantially continuously with the angle of incidence.
9. An optical device as claimed in any preceding claim, wherein the predetermined polarisation is a linear polarisation.
10. An optical device as claimed in any preceding claim, wherein the second light refracting function comprises a light converging function.
11. An optical device as claimed in claim 10, wherein the second light refracting function comprises a lens function.
12. An optical device as claimed in claim 11, wherein the lens function comprises a positive lens function.
13. An optical device as claimed in any preceding claim, wherein the element comprises a polarisation sensitive light refracting arrangement and a switchable polarisation modifying layer for providing light to the arrangement having different respective polarisations in the first and second modes for incident light of the predetermined polarisation, at least for incident angles away from a predetermined angle of incidence.
14. An optical device as claimed in claim 13, wherein the polarisation modifying layer is adapted to pass light of the predetermined polarisation in the first mode substantially without affecting its polarisation.
15. An optical device as claimed in claim 13 or 14, wherein the arrangement comprises optically isotropic material in contact with optically anisotropic material having a shaped interface therebetween.
16. An optical device as claimed in claim 15, wherein the isotropic material or the anisotropic material is shaped as a convex lens.
17. An optical device as claimed in claim 15 or 16, wherein the isotropic material or the anisotropic material is shaped as a cylindrical lens.
18. An optical device as claimed in claim 15, 16 or 17, wherein the anisotropic material is distinct from the polarisation modifying layer.
19. An optical device as claimed in claim 18, wherein the anisotropic material is formed of reactive mesogen material.
20. An optical device as claimed in claim 15, 16 or 17, wherein the anisotropic material is indistinct from the polarisation modifying layer, such that the anisotropic material performs the function of the polarisation modifying layer.
21. An optical device as claimed in any one of claims 15 to 20, wherein the isotropic material has a refractive index substantially equal to one of the refractive indices of the anisotropic material, at least in the first mode.
22. An optical device as claimed in claim 21, when dependent on claim 18, wherein the isotropic material has a refractive index substantially equal to one of the refractive indices of the anisotropic material in the first and second modes.
23. An optical device as claimed in claim 22, or claim 21 when dependent on claim 18, wherein the anisotropic material has a refractive index substantially equal to one of the refractive indices of the polarisation modifying layer, at least in the first mode.
24. An optical device as claimed in claim 21, 22 or 23, wherein the one of the refractive indices is that experienced by incident light of the predetermined polarisation.
25. An optical device as claimed in any one of claims 13 to 24, wherein the polarisation modifying layer is adapted to modify the polarisation in the second mode by an amount dependent upon the angle of incidence.
26. An optical device as claimed in any one of claims 13 to 25, wherein the polarisation modifying layer is formed of ECB material.
27. An optical device as claimed in any one of claims 13 to 22, wherein the polarisation modifying layer comprises a retarder.
28. An optical device as claimed in claim 27, wherein the retarder is a half wave plate.
29. An optical device as claimed in claim 27 or 28, when dependent on claim 15, wherein the anisotropic material comprises VAN liquid crystal material.
30. An optical device as claimed in any preceding claim, wherein the light refracting element is electronically switchable between the first and second modes.
31. An optical device as claimed in any preceding claim, comprising an array of such light refracting elements.
32. An optical device as claimed in claim 31, wherein each element of the array has one of a plurality of different types of second light refracting function.
33. An optical device as claimed in claim 32, when dependent on claim 11, comprising a plurality of different strengths of lens function for the same incident angle.
34. An optical device as claimed in claim 33, wherein the different strengths are arranged in a random fashion across the array.
35. An optical device as claimed in any one of claims 31 to 34, wherein the elements of the array cooperate to provide a light scattering function in the second mode.
36. A display device comprising an optical device as claimed in any preceding claim.
37. A display device as claimed in claim 36, when dependent on claim 31, comprising a spatial light modulator disposed between a backlight and the optical device.
38. A display device as claimed in claim 37, wherein each element of the array is optically aligned with a respective pixel of the modulator.
39. A display device as claimed in claim 37 or 38, wherein the modulator is a liquid crystal device.
40. A display device as claimed in claim 37, 38 or 39, wherein in the first mode the elements of the array have substantially no effect on an image formed by the modulator, and in the second mode the elements of the array cooperate to disturb the image for viewers receiving light incident on the elements away from a predetermined angle of incidence.
41. A display device as claimed in claim 40, wherein the first mode is a public mode of the display device and the second mode is a private mode of the display device.
42. An optical device substantially as hereinbefore described with reference to the accompanying drawings.
43. A display device substantially as hereinbefore described with reference to the accompanying drawings.