GB2428101A

GB2428101A - Display device switchable between public and private viewing modes

Info

Publication number: GB2428101A
Application number: GB0513971A
Authority: GB
Inventors: Paul Antony Gass; Diana Ulrich Kean; Nathan Smith; Thomas Matthew Wynne-Powell
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: GB0513971D0

Abstract

A display device is provided that comprises a liquid crystal display panel (100) for displaying an image by spatial light modulation, and circuitry (101, 103) for switching liquid crystal (105) in the panel (100) between having a first configuration in a first mode to cause an image displayed using the panel (100) to be discernible from a wide range of viewing angles, and having a second configuration in a second mode to cause an image displayed using the panel (100) to be discernible substantially only from within a narrow range of viewing angles. Several types of display panel to achieve such in-panel switching between public and private viewing modes are disclosed.

Description

Display Device and Liquid Crystal Display Panel The present invention

relates to a display device and a liquid crystal display panel 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 which 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 (EP0S) 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 modify 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-240499J. 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.

WO 03/0 15424 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.

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.

According to a first aspect of the present invention, there is provided a display device comprising a liquid crystal display panel for displaying an image by spatial light modulation, and circuitry for switching liquid crystal in the panel between having a first configuration in a first mode to cause an image displayed using the panel to be discernible from a wide range of viewing angles, and having a second configuration in a second mode to cause an image displayed using the panel to be discernible substantially only from within a narrow range of viewing angles.

The second configuration of liquid crystal may cause an image-confusing pattern to be visible in the image discerned by a viewer outside the narrow range of angles.

The liquid crystal in the first configuration may comprise a single arrangement of liquid crystal across the display device.

The liquid crystal in the first configuration may comprise a plurality of lateral regions each having one of at least two different arrangements of liquid crystal The first configuration regions may be sized so as not to be resolvable by a viewer.

The liquid crystal in the second configuration may comprise a plurality of lateral regions each having one of at least two different arrangements of liquid crystal.

The second configuration regions may be sized so as to be resolvable by a viewer.

The second configuration regions may have a lateral dimension at least two times greater than a lateral dimension of a picture element of the panel.

The second configuration regions may have a lateral dimension at least five times greater than a lateral dimension of a picture element of the panel.

The second configuration regions may have a lateral dimension at least ten times greater than a lateral dimension of a picture element of the panel.

The second configuration regions having the same or similar liquid crystal arrangement may be arranged spatially in a predetermined manner.

The second configuration regions of the same or similar liquid crystal arrangement may be arranged spatially in a chequerboard pattern.

The circuitry may comprise electrodes disposed either side of each second configuration region for switching the liquid crystal to the second configuration.

The electrodes may be disposed on the same side of the liquid crystal display panel.

The electrodes may be disposed on the same side of the liquid crystal display panel as electrodes used for switching picture elements of the panel.

Neighbouring regions may be arranged to have different arrangements of liquid crystal.

The at least two arrangements may comprise liquid crystal having different respective substantially uniform orientations of liquid crystal.

The at least two arrangements may comprise one or more pairs of orientations, the orientations in the or each pair being disposed substantially symmetrically about a predetermined axis.

The predetermined axis may lie in the narrow range of viewing angles.

The at least two different arrangements of liquid crystal may have different respective angular transmission functions.

The respective angular transmission functions may be asymmetric about an axis lying within the narrow range of viewing angles.

The respective angular transmission functions for the second configuration may be substantially equal for viewing angles within the narrow range, and different for viewing angles outside the narrow range.

The respective angular transmission functions for the first configuration may be spatially averaged by a viewer in the first mode to provide a smoothly-varying average transmission function across at least part of the wide range of viewing angles.

The average transmission function may vary smoothly across the whole of the wide range of viewing angles The first and second configurations may be vertically aligned nematic configurations.

An image may be represented by a plurality of image elements, and the display device may comprise means for modi1'ing, in the second mode, the respective data values of at least some of the image elements such that when the modified image is displayed on a display panel having a first data value-to-luminance response, the image is perceived through spatial averaging to be substantially the same as the original image, and such that when the modified image is displayed on a display panel having a second data value-to-luminance response, different to the first data value-to-luminance response, the image perceived by the second viewer through spatial averaging is different to the original image, and the liquid crystal in the second configuration may arranged to provide the display panel with substantially the first data value-to-luminance response for viewing angles within the narrow range and the liquid crystal in the first configuration may be arranged to provide the display panel with substantially the second data value-to-luminance response for viewing angles outside the narrow range.

The second data value-to-luminance response may be a non-linear data value-to- luminance response.

The first data value-to-luminance response may be a substantially linear data value-to- luminance response.

The first configuration regions may be adapted such that light travelling at an angle outside the narrow range of angles passes through at least two regions having different arrangements of liquid crystal so as to have the first data value-to-luminance response.

The second configuration regions may be adapted such that light travelling at an angle outside the narrow range of angles passes through a single region so as to have the second data value-to-luminance response.

The first and second configurations may be twisted nematic configurations.

The display device may comprise at least one patterned alignment layer for producing the first and second configurations.

The circuitry may be operable to apply an electric field to change the alignment properties of the at least one alignment layer to switch the liquid crystal between the first and second configurations.

The circuitry may be operable to apply an electric field across and/or in the plane of the liquid crystal to switch the liquid crystal between the first and second configurations.

The circuitry may be operable to apply fringe electric fields.

The circuitry may be adapted to operate the display panel to apply an electric field in first and second different ways in the first and second modes respectively to achieve the first and second liquid crystal configurations.

The circuitry may be adapted to operate the display panel using different respective ranges of applied electric field strengths in the first and second modes.

The circuitry may be adapted to operate the display panel using different respective directions of applied electric field in the first and second modes.

The circuitry may be adapted to operate the display panel using in-plane switching in the first mode and electrically controlled birefringence switching in the second mode.

The circuitry may comprise a first set of electrodes for performing the in-plane switching and a second set of electrodes for performing the electrically controlled birefringence.

The first set electrodes may be disposed on the same side of the display panel to apply a

field substantially in the plane of the panel.

The second set electrodes may be disposed on opposite sides of the display panel to

apply a field across the panel.

The circuitry may be adapted to operate in the second mode to use different respective driving voltage ranges to produce the at least two different arrangements of liquid crystal, the liquid crystal arrangements being such as to have substantially the same transmission to viewers within the narrow range of angles for different respective voltages selected from each of the driving voltage ranges and different respective transmissions for those selected voltages to viewers outside the narrow range of angles.

A first one of the driving voltage ranges may have a transmission-tovoltage function suitable for image display to viewers outside the narrow range of angles and a second one of the driving voltage ranges may have a transmission-to-voltage function unsuitable for image display to viewers outside the narrow range of angles.

The second one of the driving voltage ranges may have a substantially constant, low, transmission to viewers outside the narrow range of angles for voltages across at least most of the range.

The substantially constant, low, transmission may be a substantially zero transmission.

The second one of the driving voltage ranges may have a substantially constant, high, transmission to viewers outside the narrow range of angles for voltages across at least most of the range.

The circuitry may be adapted to operate in the first mode to use only the first one of the driving voltage ranges for each of the lateral regions to produce a substantially uniform arrangement of liquid crystal across the panel.

The narrow range of viewing angles may be disposed about the normal to the display panel.

According to a second aspect of the present invention, there is provided a liquid crystal display panel for use in a display device for displaying an image by spatial light modulation, the display panel being adapted to enable switching of liquid crystal in the panel between having a first configuration in a first mode to cause an image displayed using the panel to be discernible from a wide range of viewing angles, and having a second configuration in a second mode to cause an image displayed using the panel to be discernible substantially only from within a narrow range of viewing angles.

Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a side view of a display panel according to a first embodiment of the present invention showing operation in wide and narrow viewing modes; Figure 2 is a chart for use in explaining operation of the first embodiment; Figures 3(A) and 3(B) are charts for use in explaining operation of a second embodiment of the present invention; Figure 4 is a plan view of a display panel according to the second embodiment showing operation in wide and narrow viewing modes; Figure 5 shows plan and side views of a display panel according to a third embodiment of embodiment of the present invention showing operation in wide and narrow viewing modes respectively; and Figures 6 to 9 are diagrams for illustrating a fourth embodiment of the present invention, and a variant of the above-described third embodiment.

Figure 1 illustrates a display device incorporating a liquid crystal display panel 100 according to a first embodiment of the present invention. The liquid crystal display panel 100 displays an image by spatial light modulation, using opposed electrodes 101 and 103 disposed across a layer of liquid crystal material 105. The electrode 103 is segmented so as to enable switching of the liquid crystal into two or more different orientations within a single picture element. Within a single picture element the same voltage is applied to all areas of the segmented electrode 103. Preferably several regions of the two or more different orientations are formed within a single picture element. Switching into the different orientations is controlled by the fringing electric fields produced at the edges of the segmented electrode 103. Alternatively other methods of producing fringing electric fields, such as protrusions on the electrode surface, may be used.

As explained in more detail below, the liquid crystal display panel 100 also comprises circuitry 107 in the form of in-plane electrodes, for switching the liquid crystal 105 between having a first configuration Cl in a first (public or wide) mode and having a second configuration C2 in a second (private or narrow) mode. The first liquid crystal configuration Cl causes an image displayed using the panel 100 to be discernible by a viewer from a wide range of viewing angles, while the second liquid crystal configuration C2 causes an image displayed using the panel 100 to be discernible by a viewer substantially only from within a narrow range of viewing angles. As illustrated in Figure 1, the display device according to the first embodiment comprises only a single liquid crystal display panel 100. No additional optical components or layers are required in the first embodiment to achieve switching between the two modes of operation.

Figure 1 shows, in the left-hand portion, one suitable example of the first liquid crystal configuration CI in the first (wide) mode of operation. The first liquid crystal configuration Cl in this example has two or more regions or domains (referred to from herein onwards as regions) of different liquid crystal orientation within a single pixel.

Illustrated are four different regions RI to R4, with regions Ri and R3 having a first liquid crystal arrangement and regions R2 and R4 having a second liquid crystal arrangement, different to the first liquid crystalarrangement. In Figure 1 the combination of the four different regions RI to R4 forms a single picture element.

The first arrangement comprises liquid crystal having a substantially uniform first orientation of liquid crystal, while the second arrangement comprises liquid crystal having a substantially uniform second orientation of liquid crystal, different to the first orientation, with first and second orientations arranged in region pairs. The first and second orientations are disposed substantially symmetrically about the normal to the display panel 100. In the example illustrated in Figure 1, the first liquid configuration is a two-domain Vertically Aligned Nematic (VAN) configuration.

In the first (wide) mode, the size of these regions Ri to R4 is smaller than can be resolved by a viewer, for example of the order of 10 to 25 tm across. The properties of the unresolvable regions average to produce a wide viewing angle characteristic for the liquid crystal display panel 100. This will be explained further with reference to Figure 2 below. The multiple regions of different liquid crystal orientation within a pixel are formed by fringing fields from the patterned electrode 103 or from protrusions on the electrode surface. An image is displayed on the liquid crystal display panel 100 in a known way by applying a switching voltage VI across the liquid crystal layer 105.

In the second (narrow) mode, the in-plane electrodes 107 are used to provide electric fields substantially within the plane of the liquid crystal layer 105. To achieve this, a voltage of V2 is applied across adjacent in-plane electrodes 107, where V2 would typically be less than Vi. These in-plane fields overcome the effect of the fringing fields from the segmented electrode 103 and switch the liquid crystal layer into a second liquid crystal configuration C2 having larger regions R5 and R6, as illustrated in the right-hand portion of Figure 1. Regions R5 and R6 have different respective liquid crystal arrangements, the two liquid crystal arrangements having substantially uniform different respective orientation of liquid crystal. The two different orientations are disposed substantially symmetrically about the normal to the display panel 100.

The regions R5 and R6 in the second (narrow) mode are large enough to be resolved by a viewer, and would typically be much larger than a pixel, for example of the order of 1mm or more across. As a result, and as explained in more detail below, the effect of the regions R5 and R6 is clearly visible by an off-axis viewer (but the effect of the regions is hidden for an on-axis viewer). The regions R5 and R6 in the second (narrow) mode are arranged to provide a pattern that will obscure and confuse the underlying image for an off- axis viewer in the second (narrow) mode. One example of such a pattern is a chequerboard, although any suitable pattern can be used.

The function of the first embodiment is further illustrated in Figure 2. Plot 1 of Figure 2 shows the transmission as a function of angle of view for a single VAN region, measured relative to the perpendicular to the panel, i.e. for a single liquid crystal orientation. The angular transmission function is asymmetric relative to the on-axis direction due to the off-axis orientation of the liquid crystal molecules.

Suppose Plot 1 represents the angular transmission function for regions have the liquid crystal orientation as shown in region Ri, then the angular transmission function for regions having the liquid crystal orientation as shown in region R2 will be as for Plot 1, but reflected in the vertical axis.

As explained above, in the first (wide) mode, neighbouring regions, for example regions Ri and R2 in Figure 1, are not resolvable by a viewer. Since these two regions RI and R2 are not resolvable, the viewer sees an average transmission for the two regions RI and R2 for all viewing angles. The resulting angular transmission function is shown in Plot 2 of Figure 2, which varies across viewing angle within acceptable bounds.

As explained above, in the second (narrow) mode the size of the VAN regions R5 and R6 is such that, instead of an average of the two domains, a viewer sees the domains as a distinct pattern because of the difference in transmission between the two regions for a particular viewing angle. This difference will increase with viewing angle.

Due to the symmetry of liquid crystal orientation about the perpendicular to the display panel 100, when viewed on-axis both regions R5 and R6 exhibit the same transmission, and so the effect of the different regions is not apparent to the on-axis viewer. An image is displayed in the standard way by applying a switching voltage VI across the liquid crystal layer 105.

Away from the perpendicular direction, the two regions R5 and R6 give different transmission for the same applied voltage. Therefore an offaxis viewer will see the pattern of the VAN domains as a pattern of differing brightness. This pattern will obscure the underlying image. The contrast between the two regions in the second (private) mode is illustrated by Plot 3 of Figure 2, which is the difference in transmission between adjacent regions R5 and R6 for each viewing angle.

In this way a good privacy function can be achieved with the first embodiment of the present invention by switching the liquid crystal layer 105 itself, without the need for additional layers.

Instead of or as well as using electric field switching of the liquid crystal layer 105 as described above, the alignment layer can also be switched to produce the wide and narrow viewing modes. One method of switching the alignment is the use of very fine patterning of an alignment layer as described by Kim et al., Surface alignment bistability of nematic liquid crystals by orientationally frustrated surface patterns', Applied Physics Letters, Vol 78, Is 20 (2001) 3055. Another method, disclosed in US 6,549,255, uses a polymer alignment layer whose alignment properties can be switched

with an applied field.

A second embodiment of the present invention, also making use of an inpanel liquid crystal switching technique to achieve switching between a public and private mode, will now be described with reference to Figures 3(A), 3(B) and 4.

A liquid crystal display device including a liquid crystal display panel 200 according to the second embodiment is illustrated in Figure 4, and is intended for use in conjunction with the method described in co-pending British Patent Application No. [Agent's ref P53506GB], which is incorporated herein by reference. The co-pending application describes a method of providing a privacy function by modifying image data before display. The reader is referred to the co-pending application for further details, but a brief overview of the technique therein disclosed will now be provided.

When a viewer is located more than a predetermined distance away from the display panel 200 of the display device, the viewer is unable to resolve each individual pixel or image element being displayed. Instead, the human eye will spatially average a localised group of displayed image elements to perceive a single overall luminance.

The technique takes advantage of this, together with the inherent data value-to- luminance response of the display device, as follows.

Consider a localised group comprising two image elements having the same grey level.

Before display, an image processor splits the original grey level data value equally into two new grey levels, such that the grey level of one of the image elements is equal to the original grey level minus the splitting amount, and the grey level of the other image element is equal to the original grey level plus the splitting amount.

When displayed on a display device having a linear data value-toluminance response, a viewer will perceive the two modified grey levels of the localised group to have the same overall luminance as those image elements would have done without such modification. This is because, due to the linear response of the display device, the single original grey level maps to the same luminance as the average luminance of the two modified image elements. This is illustrated in Figure 3(A).

On the other hand, when the modified image elements in the localised group are displayed on a display device having a non-linear data value-toluminance response, the luminance of these image elements is no longer spatially averaged by the eye of the viewer to have the same overall luminance as those image elements would have done without such modification. Instead, the viewer perceives a luminance which differs from a straight average by an amount which depends on the non-linearity of the display.

This is illustrated in Figure 3(B).

The switching voltage applied to a pixel in a liquid crystal display device is usually compensated such that, when viewed on-axis, a change in the data sent to the pixel causes a proportional change in the observed luminance. However, light passing through the panel 32 at an angle other than the normal to the panel 32 will travel a different optical path length through the liquid crystal and will experience a different optical retardation and will therefore be affected differently. This change in optical path length and retardation introduces a non-linear relationship between the pixel data and the observed luminance off-axis.

Because of this, the data value-to-luminance response to an on-axis viewer will be substantially linear, and the image perceived by such a viewer through spatial averaging will be substantially the same as the original image.

On the other hand, the data value-to-luminance response to an off-axis viewer will be non-linear, and the image perceived by such a viewer through spatial averaging will be different to the original image. A component of the degree of splitting used is introduced. If the degree of splitting is varied across the image by an amount proportional to a masking image, then when viewed off-axis the original and the second image will both be visible. If the masking has a confusing patterning such as a chequerboard, then the original image will be substantially hidden to an off-axis viewer.

This provides a private mode of operation, in which only the on-axis viewer has an undisturbed view of the original image.

Although it is generally assumed above that the on-axis luminance response of the display device is linear, such that equal splitting of data values will result in the viewer seeing substantially the original image through spatial averaging, if in fact the on-axis luminance response is non-linear, then the splitting can easily be compensated to account for this so that the on-axis viewer still sees substantially the original image through spatial averaging.

The in-panel switching technique of the second embodiment of the present invention is intended to ensure a non-linear data value-to-luminance response off-axis for use with the method disclosed in the abovementioned co-pending application.

Figure 4 illustrates a single-layer liquid crystal display panel 200 according to the second embodiment. The liquid crystal in the display panel 200 is operated to switch between a first configuration Cl in the first (wide) mode of operation as illustrated in the top portion of Figure 4, and a second configuration C2 in the second (narrow) mode of operation as illustrated in the bottom portion of Figure 4.

In the illustration of Figure 4, the first liquid crystal configuration Cl is a four-domain (four-region) twisted nematic (TN) configuration, with four regions RI 1 to R14 shown.

Each of the four regions Ri 1 to RI 4 has different TN arrangements, each arrangement oriented at 90 degrees with respect to another arrangement.

The second liquid crystal configuration C2 is a two-domain (two-region) twisted nematic (TN) configuration, with two regions R15 and R16 shown. Region R15 occupies the same space as regions RI 1 and R12, while region R16 occupies the same space as regions R13 and R14. Region Ri5 has a TN arrangement that is oriented 180 degrees with respect to the TN arrangement of region R16.

The four-region liquid crystal configuration Cl can be fabricated by patterning of the liquid crystal alignment, for example by multi-rubbing or photoalignment. In the first (wide) mode the average transmission of the four regions provides a linear data value- to-luminance response for light transmitted at oblique angles. In this mode, light passes through all four regions having different arrangements of liquid crystal. For each individual region there will be certain directions for which the transmission of light travelling at oblique angles will have a linear data value-to-luminance response and other directions for which the transmission of light travelling at oblique angles will have a non-linear data value-to-luminance response. However due to the averaging of the transmitted light over the four regions the overall data value-to-luminance response is substantially linear in all directions.

In the second (narrow) mode, the alignment is switched to a two-region TN configuration C2, with the regions being orientated so that in the horizontal viewing plane (a plane normal to the page and parallel with the top and bottom edges of the page) the data value-to-luminance response is non-linear for light transmitted at oblique angles. In this mode, both regions R15 and R 16 are arranged so that the directions for which the transmission of light travelling at oblique angles will have a non-linear data value-to-luminance response are in the horizontal viewing plane. Therefore the overall data value-to-luminance response, which is an average of the response of the two regions, is non-linear. When such a mode is used in combination with an image processing method disclosed in the above-mentioned co-pending application, a good privacy function can be achieved.

One method of switching the alignment is the use of very fine patterning of an alignment layer as described by Kim et al., Surface alignment bistability of nematic liquid crystals by orientationally frustrated surface patterns', Applied Physics Letters, Vol 78, Is 20 (2001) 3055. Another method, disclosed in US 6,549,255, uses a polymer alignment layer whose alignment properties can be switched with an applied field.

Instead of or as well as switching the alignment layer as described in the embodiment above, the liquid crystal layer can also be switched using an electric field to produce the wide and narrow viewing modes. Examples are applying an electric field across the liquid crystal layer, applying an electric field in the plane of the liquid crystal layer, and

applying fringe fields from a patterned electrode.

Figure 5 illustrates a display device incorporating a liquid crystal display panel 300 according to a third embodiment of the present invention. In the third embodiment the first (wide) mode of the liquid crystal display is provided by switching a liquid crystal layer 305 with an electric field substantially parallel to the layer a using a first set of(in- plane) electrodes 307, 308. This in-plane switching (IPS) is known to give a wide viewing angle. The liquid crystal configuration Cl in the first (wide) mode of operation is illustrated in the top portion of Figure 5, which is a plan view of one liquid crystal in- plane switching cell.

The second (narrow) mode of operation is provided by switching the liquid crystal layer 305 with an electric field applied across the liquid crystal layer (electrically controlled birefringent or ECB switching) using a second set of electrodes 301, 303. The switching of the liquid crystal layer 305 out of the plane of the layer gives a narrow viewing angle. The liquid crystal configuration C2 in the second (narrow) mode of operation is illustrated in the bottom portion of Figure 5, which is a side view of one liquid crystal ECB switching cell.

Alternatively, the first (wide) and second (narrow) viewing modes can be provided in an embodiment of the present invention by driving a liquid crystal display panel in two different respective voltage ranges. An example of a suitable such device is shown in Figure 7, with associated viewing angle characteristics shown in Figure 9.

A fourth embodiment will be described with reference to Figures 6 to 9. In this embodiment, switchable privacy is achieved using an LC mode which has two voltage ranges, denoted in Figure 6 as ranges A and B, with similar greyscale variation on axis but a single voltage range, denoted as A in Figure 6, with normal greyscale variation off axis. In the public mode, the voltage range A having normal greyscale variation both on axis and off axis is used and good image quality is seen at all viewing angles. In the private mode, some of the pixels achieve the desired greyscale on axis using the first voltage range A and other pixels achieve the same greyscale on axis using the second voltage range B. Pixels that use the first voltage range A will appear normal both on and off axis. However, pixels that use the second voltage range B will not appear normal off axis. By patterning the pixels that use the first and second voltage ranges A and B, a confusing image will appear off axis.

An example of such a greyscale response is shown in Figure 6. For each greylevel on axis, there are two voltages that can be used. However, these two voltages do not give the same greylevels off axis. By patterning the pixels which use the first and second voltage ranges A and B a confusing image will be seen off axis. The pattern is not seen on axis because the two voltage ranges A and B are matched to the same greylevels for this viewing angle.

An example of a liquid crysal mode that can achieve this is shown in Figure 7, and would be readily understood by the skilled person. Figure 8 shows that, for the first voltage range of-O.9V to l.6V, the greyscale curve on axis is similar to the greyscale curve off axis (+1- 45deg). However, for the second voltage range of-4.6V to 2.4V, the greyscale curve is inverted but similar in range on axis but is virtually unchanged off axis. Therefore two voltages can be selected to give identical greylevels on axis but these voltages will give very different greylevels off axis. Figure 9 shows the viewing angle dependency of the greylevels for the two voltage ranges.

Therefore, in the fourth embodiment, two voltage ranges are used to achieve similar greylevels on axis, one of these voltage ranges giving normal greyscale variation off axis and the other giving abnormal greyscale off axis. Driving the panel only in voltage the first voltage range gives good viewing at all angles. Driving the panel in the second voltage range gives abnormal viewing off axis. Privacy can be enhanced by patterning the pixels which use first and second voltage ranges such that the abnormal viewing off axis results in a pattern which confuses the image seen by the off axis viewer.

It will be appreciated that each of the above-described embodiments is not limited to operation using the particular liquid crystal configurations described. Each embodiment of the present invention can be used with any of the following types of liquid crystal display: two or four domain Twisted Nematic (TN) or Super Twisted Nematic (STN); two or four domain Vertically Aligned Nematic (VAN) and Twisted Vertically Aligned Nematic (TVAN); two or four domain Hybrid Aligned Nematic (HAN); Multidomain Vertical Aligned (MVA); and Continuous Pinwheel Aligned (CPA). In addition liquid crystal modes that have intrinsic bistability, such as Bistable Twisted Nematic (BTN), Bistable Hybrid Aligned Nematic (BHAN), Zenthally Bistable Nematic (ZBN) and azimuthally bistable modes produced by gratings or other surface structures can be employed. Other suitable liquid crystal modes would be readily available to those skilled in the art.

Claims

CLAIMS: 1. A display device comprising a liquid crystal display panel for

displaying an image by spatial light modulation, and circuitry for switching liquid crystal in the panel between having a first configuration in a first mode to cause an image displayed using the panel to be discernible from a wide range of viewing angles, and having a second configuration in a second mode to cause an image displayed using the panel to be discernible substantially only from within a narrow range of viewing angles.
2. A display device as claimed in claim I, wherein the second configuration of liquid crystal causes an image-confusing pattern to be visible in the image discerned by a viewer outside the narrow range of angles.
3. A display device as claimed in claim I or 2, wherein the liquid crystal in the first configuration comprises a single arrangement of liquid crystal across the display device.
4. A display device as claimed in claim 1 or 2, wherein the liquid crystal in the first configuration comprises a plurality of lateral regions each having one of at least two different arrangements of liquid crystal
5. A display device as claimed in claim 4, wherein the first configuration regions are sized so as not to be resolvable by a viewer.
6. A display device as claimed in any preceding claim, wherein the liquid crystal in the second configuration comprises a plurality of lateral regions each having one of at least two different arrangements of liquid crystal.
7. A display device as claimed in claim 6, wherein the second configuration regions are sized so as to be resolvable by a viewer.
8. A display device as claimed in claim 6 or 7, wherein the second configuration regions have a lateral dimension at least two times greater than a lateral dimension of a picture element of the panel.
9. A display device as claimed in claim 8, wherein the second configuration regions have a lateral dimension at least five times greater than a lateral dimension of a picture element of the panel.
10. A display device as claimed in claim 9, wherein the second configuration regions have a lateral dimension at least ten times greater than a lateral dimension of a picture element of the panel.
11. A display device as claimed in any one of claims 6 to 10, wherein the second configuration regions having the same or similar liquid crystal arrangement are arranged spatially in a predetermined manner.
12. A display device as claimed in claim 11, wherein the second configuration regions of the same or similar liquid crystal arrangement are arranged spatially in a chequerboard pattern.
13. A display device as claimed in any one of claims 6 to 12, wherein the circuitry comprises electrodes disposed either side of each second configuration region for switching the liquid crystal to the second configuration.
14. A display device as claimed in claim 13, wherein the electrodes are disposed on the same side of the liquid crystal display panel.
15. A display device as claimed in claim 13 or 14, wherein the electrodes are disposed on the same side of the liquid crystal display panel as electrodes used for switching picture elements of the panel.
16. A display device as claimed in any one of claims 4 to 15, wherein neighbouring regions are arranged to have different arrangements of liquid crystal.
17. A display device as claimed in any one of claims 4 to 16, wherein the at least two arrangements comprise liquid crystal having different respective substantially uniform orientations of liquid crystal.
18. A display device as claimed in claim 17, wherein the at least two arrangements comprise one or more pairs of orientations, the orientations in the or each pair being disposed substantially symmetrically about a predetermined axis.
19. A display device as claimed in claim 18, wherein the predetermined axis lies in the narrow range of viewing angles.
20. A display device as claimed in any one of claims 4 to 19, wherein the at least two different arrangements of liquid crystal have different respective angular transmission functions.
21. A display device as claimed in claim 20, wherein the respective angular transmission functions are asymmetric about an axis lying within the narrow range of viewing angles.
22. A display device as claimed in claim 20 or 21, when dependent on claim 6, wherein the respective angular transmission functions for the second configuration are substantially equal for viewing angles within the narrow range, and different for viewing angles outside the narrow range.
23. A display device as claimed in claim 20, 21 or 22, when dependent on claim 5, wherein the respective angular transmission functions for the first configuration are spatially averaged by a viewer in the first mode to provide a smoothly-varying average transmission function across at least part of the wide range of viewing angles.
24. A display device as claimed in claim 23, wherein the average transmission function varies smoothly across the whole of the wide range of viewing angles
25. A display device as claimed in any preceding claim, wherein the first and second configurations are vertically aligned nematic configurations.
26. A display device as claimed in any one of claims I to 6, wherein an image is represented by a plurality of image elements, and comprising means for modifying, in the second mode, the respective data values of at least some of the image elements such that when the modified image is displayed on a display panel having a first data value- to-luminance response, the image is perceived through spatial averaging to be substantially the same as the original image, and such that when the modified image is displayed on a display panel having a second data value-to-luminance response, different to the first data value-toluminance response, the image perceived by the second viewer through spatial averaging is different to the original image, and wherein the liquid crystal in the second configuration is arranged to provide the display panel with substantially the first data value-to-luminance response for viewing angles within the narrow range and wherein the liquid crystal in the first configuration is arranged to provide the display panel with substantially the second data value-to-luminance response for viewing angles outside the narrow range.
27. A display device as claimed in claim 26, wherein the second data value-to- luminance response is a non-linear data value-to-luminance response.
28. A display device as claimed in claim 26 or 27, wherein the first data value-to- luminance response is a substantially linear data value-to-luminance response.
29. A display device as claimed in claim 26, 27 or 28, when dependent on claim 4, wherein the first configuration regions are adapted such that light travelling at an angle outside the narrow range of angles passes through at least two regions having different arrangements of liquid crystal so as to have the first data value-to-luminance response.
30. A display device as claimed in any one of claims 26 to 29, when dependent on claim 6, wherein the second configuration regions are adapted such that light travelling at an angle outside the narrow range of angles passes through a single region so as to have the second data value-to-luminance response.
31. A display device as claimed in any one of claims 26 to 30, wherein the first and second configurations are twisted nematic configurations.
32. A display device as claimed in any one of claims 26 to 31, comprising at least one patterned alignment layer for producing the first and second configurations.
33. A display device as claimed in claim 32, wherein the circuitry is operable to apply an electric field to change the alignment properties of the at least one alignment layer to switch the liquid crystal between the first and second configurations.
34. A display device as claimed in any one of claims 26 to 33, wherein the circuitry is operable to apply an electric field across and/or in the plane of the liquid crystal to switch the liquid crystal between the first and second configurations.
35. A display device as claimed in claim 34, wherein the circuitry is operable to

apply fringe electric fields.
36. A display device as claimed in claim 1, wherein the circuitry is adapted to operate the display panel to apply an electric field in first and second different ways in the first and second modes respectively to achieve the first and second liquid crystal configurations.
37. A display device as claimed in claim 36, wherein the circuitry is adapted to operate the display panel using different respective ranges of applied electric field strengths in the first and second modes.
38. A display device as claimed in claim 36, wherein the circuitry is adapted to operate the display panel using different respective directions of applied electric field in the first and second modes.
39. A display device as claimed in claim 38, wherein the circuitry is adapted to operate the display panel using in-plane switching in the first mode and electrically controlled birefringence switching in the second mode.
40. A display device as claimed in claim 39, wherein the circuitry comprises a first set of electrodes for performing the in-plane switching and a second set of electrodes for performing the electrically controlled birefringence.
41. A display device as claimed in claim 40, wherein the first set electrodes are disposed on the same side of the display panel to apply a field substantially in the plane of the panel.
42. A display device as claimed in claim 40 or 41, wherein the second set electrodes are disposed on opposite sides of the display panel to apply a field across the panel.
43. A display device as claimed in any one of claims 6 to 15, or any one of claims 16 to 22 when dependent on claim 6, wherein the circuitry is adapted to operate in the second mode to use different respective driving voltage ranges to produce the at least two different arrangements of liquid crystal, the liquid crystal arrangements being such as to have substantially the same transmission to viewers within the narrow range of angles for different respective voltages selected from each of the driving voltage ranges and different respective transmissions for those selected voltages to viewers outside the narrow range of angles.
44. A display device as claimed in claim 43, wherein a first one of the driving voltage ranges has a transmission-to-voltage function suitable for image display to viewers outside the narrow range of angles and a second one of the driving voltage ranges has a transmission-to-voltage function unsuitable for image display to viewers outside the narrow range of angles.
45. A display device as claimed in claim 44, wherein the second one of the driving voltage ranges has a substantially constant, low, transmission to viewers outside the narrow range of angles for voltages across at least most of the range.
46. A display device as claimed in claim 45, wherein the substantially constant, low, transmission is a substantially zero transmission.
47. A display device as claimed in claim 44, wherein the second one of the driving voltage ranges has a substantially constant, high, transmission to viewers outside the narrow range of angles for voltages across at least most of the range.
48. A display device as claimed in claim 44, 45, 46 or 47, wherein the circuitry is adapted to operate in the first mode to use only the first one of the driving voltage ranges for each of the lateral regions to produce a substantially uniform arrangement of liquid crystal across the panel.
49. A display device as claimed in any preceding claim, wherein the narrow range of viewing angles is disposed about the normal to the display panel.
50. A liquid crystal display panel for use in a display device for displaying an image by spatial light modulation, the display panel being adapted to enable switching of liquid crystal in the panel between having a first configuration in a first mode to cause an image displayed using the panel to be discernible from a wide range of viewing angles, and having a second configuration in a second mode to cause an image displayed using the panel to be discernible substantially only from within a narrow range of viewing angles.
51. A display device substantially as hereinbefore described with reference to the accompanying drawings.
52. A liquid crystal display panel substantially as hereinbefore described with reference to the accompanying drawings.