Display

A display is switchable between a first display mode with a first viewing angle range, a second display mode having a second viewing angle range smaller than the first viewing angle range and a multiple view directional display mode. The display includes an image display layer and a control element that is switchable between a first state in which it cooperates with the image display layer to provide the first display mode, a second state in which it cooperates with the image display layer to provide the second display mode, and a third state in which it cooperates with the image display layer to provide the multiple view directional display mode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display in which the angular output range of light is controllable so that the display can be switched between a wide angle viewing mode and a narrow angle viewing mode, and which can further be switched to a multiple view directional display mode.

2. Description of the Related Art

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

Another application of such a display may be as a display in a motor vehicle. The viewing angle of the display could be controlled such that the passengers are unable to see the display or such that the driver is unable to see the display. Alternatively, the viewing angle could be controlled in order to reduce the reflections of the display in the windscreen and the windows—so that, for example, the viewing angle could be reduced at night-time or in low light conditions. A brightness sensor could be provided to allow automatic switching between a wide viewing angle and a narrow viewing angle, and also to allow automatic control of the brightness of the display.

Displays are also known which display two or more images simultaneously, with each image being displayed in a different direction from the other image, and such displays are known as multiple view directional displays. The images may be still images or moving images (i.e., a sequence of images).

One type of a multiple view directional display is an autostereoscopic 3-D display, which displays two images that are the left eye image and right eye image of a stereoscopic image pair. The two images are displayed such that the left eye image is directed to the left eye of an observer and the right eye image is directed to the right eye of an observer, whereby the observer perceives a full 3-D image.

Another type of a multiple view directional display is a dual view display. A dual view displays two (or more) images, such that one image is directed to one observer and another image is directed to another observer. The images are independent from one another, and may be completely unrelated to one another such that the two observers see completely different images.

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

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

One known element for controlling the direction of light is a ‘louvered’ film that consists of alternating transparent layers and opaque layers provided in an arrangement similar to a Venetian blind. Such a film is shown schematically inFIG. 1. The film operates on the same principle as a Venetian blind, and it allows light to pass through it when the light is traveling in a direction parallel to, or nearly parallel to, the opaque layers, as shown by ray30inFIG. 1. However, light traveling at large angles relative to the plane of the opaque layers is incident on one of the opaque layers and is absorbed, as shown by ray31inFIG. 1. The layers may be perpendicular to the surface of the film, as shown inFIG. 1, or they may be arranged at some other angle relative to the surface of the film.

Louvered films of this type may be manufactured by stacking many alternating sheets of transparent material and opaque material and then cutting slices of the resulting block perpendicular to the layers. This method has been known for many years and is described in, for example, U.S. Pat. Nos. 2,053,173; 2,689,387 and 3,031,351.

Other manufacturing methods are known. For example, U.S. Pat. No. RE27,617 describes a process where a louvered film is cut continuously from a cylindrical billet of stacked layers. U.S. Pat. No. 4,766,023 describes how the optical quality and mechanical robustness of the resulting film can be improved by coating with a UV-curable monomer and then exposing the film to UV radiation. U.S. Pat. No. 4,764,410 describes a similar process where a UV-curable material is used to bond the louver sheet to a covering film.

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

Another example of a light-control film is described in U.S. Pat. No. 5,528,319. This film has a transparent body in which are embedded opaque regions that extend generally parallel to the plane of the film. The opaque regions are arranged in stacks, with each stack being spaced from a neighboring stack. The opaque regions block the transmission of light through the film in certain directions while allowing the transmission of light in other directions.

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

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

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

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

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

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

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

Japanese Patent Publication No. 2003-233074 describes a display having a switchable viewing angle. This uses an additional LC panel, which is segmented. Different segments of the additional LC panel modify the viewing characteristics of the associated areas of the display in different ways, with the result that the whole display panel is fully readable only from a central viewing position.

U.K. Patent Application No. 0320363.5 describes switchable privacy devices based on louvers, which operate only for one polarization of light. The louvers are switched on and off either by rotating dyed liquid crystal molecules in the louver itself or by rotating the plane of polarization of the incident light using a separate element.

U.K. Patent Application No. 0408742.5 describes a switchable privacy device constructed by adding one or more extra liquid crystal layers and polarizers to a display panel. The intrinsic viewing angle dependence of these extra elements can be changed by switching the liquid crystal electrically in the well-known way.

U.K. Patent Application No. 0401062.5 describes a display that is switched between a public mode and a private mode by using two different backlights which generate light with different angular ranges.

GB 2 410 116, which was published after the priority date of this application, relates to a display device that is switchable between a public display mode and a private display mode. This is achieved by providing two backlights with different angular output ranges, and selecting the narrow output backlight to obtain a private display mode or selecting the wide output backlight to obtain a public display mode. In one embodiment, the backlight is formed of two inter-digitated illumination systems, with each illumination system being illuminated either by a visible light source or by a UV light source. Each illumination system includes a phosphor sheet, and so emits visible light with a wide angular range when the respective UV light source is illuminated. If the visible light source is illuminated, however, the illumination system emits light with a narrow angular range. The backlight can therefore operate in various modes—(1) if both visible light sources are illuminated it emits visible light with a narrow angular range over its entire area; (2) if both UV sources are illuminated, it emits visible light with a wide angular range over its entire area; (3) if just one visible [or UV] source is illuminated, it emits visible light with a narrow [wide] angular distribution over the area corresponding to one of the inter-digitated illumination systems.

GB 2 405 544 is directed to a polarization-dependent light control structure in which the light control structure is arranged to act as a parallax barrier for light having a particular plane of polarization. Depending on the state of a switchable half-wave plate, the light control structure may be rendered ineffective, thus providing a public 2-D mode or it may act as a parallax barrier thereby providing a 3-D display mode. Furthermore, if the half-wave plate is switched so that the light control structure is enabled, a private 2-D display mode may be obtained by displaying a single image on the display panel.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodiments of the present invention provide a display switchable between a first display mode having a first viewing angle range, a second display mode having a second viewing angle range smaller than the first viewing angle range and a multiple view directional display mode, wherein the display includes an image display layer and a control element, the control element being switchable between a first state in which it cooperates with the image display layer to provide the first display mode, a second state in which it cooperates with the image display layer to provide the second display mode, and a third state in which it cooperates with the image display layer to provide the multiple view directional display mode. Such a display is more flexible in use than known displays. Not only does such a display have the advantages of conventional displays that are switchable between a public mode and a private mode, but it also may be switched to provide a multiple view directional display mode such as a 3-D mode or a dual view mode.

If a display according to a preferred embodiment of the present invention is installed in a motor vehicle, for example, it may be used in a wide display mode in which a displayed image is visible to the driver and to passengers, in a narrow view mode in which a displayed image is visible only to passengers (or only to the driver), and a dual view mode in which one image (such as a road map) is displayed to the driver and another image (such as an entertainment program) is displayed to passengers.

Preferred embodiments of the present invention may also be applicable to displays in mobile devices such as mobile telephones, to provide such devices with a private display mode and a 3-D display mode. Selection of a desired display mode is effected by switching the control element appropriately, and it is not necessary to, for example, switch from one backlight to another as in GB 2 410 116 nor to reconfigure an image displayed on the image display layer as in GB 2 405 544.

The control element preferably may include a layer of electro-optical material.

The control element may further include at least one patterned electrode for addressing the layer of electro-optical material and a controller for addressing the at least one electrode thereby to put the control element into a desired one of the first state, the second state and the third state.

The control element may include a first set of patterned electrodes disposed on a first side of the layer of electro-optical material and a second set of patterned electrodes disposed on a second side of the layer of electro-optical material.

The first set of patterned electrodes may be addressable to define a first image in the electro-optical layer visible at viewing angles outside the second viewing angle range and not visible at viewing angles inside the second viewing angle range. Alternatively, the first and second sets of patterned electrodes may be addressable to co-operate to define a first image in the electro-optical layer visible at viewing angles outside the second viewing angle range and not visible at viewing angles inside the second viewing angle range. An observer viewing the display along a direction outside the second viewing angle range sees a superposition of an image displayed on the image display layer and the image defined in the electro-optical layer of the control element. The image defined in the electro-optical layer of the control element acts as an “obscuring image”, so that the observer has difficulty in making out the image displayed on the image display layer. An observer viewing the display along the axis normal to the display surface of the display, however, sees only an image displayed on the image display layer.

Typically, the second viewing angle range includes, and is often centered on, the normal to the display surface of the display.

The display may further include an optical retarder disposed in the path of light through the control element.

The control element may include at least one patterned alignment surface for aligning the electro-optical layer. It may include a set of patterned electrodes disposed on one side of the layer of electro-optical material.

The electro-optical layer may be addressable to define a first image in the electro-optical layer visible at viewing angles outside the second viewing angle range and not visible at viewing angles inside the second viewing angle range. In this preferred embodiment, the “obscuring image” arises as a result of the patterning of the patterned alignment surface, which defines regions of different optical properties in the layer of electro-optical material.

The second set of patterned electrodes, or the set of patterned electrodes, may be addressable to selectively define a parallax optic in the electro-optical layer. This provides a multiple view directional display mode.

The first image may be selected dependent on a second image to be displayed by the image display layer.

The layer of electro-optical material may include a first layer of liquid crystal material.

The liquid crystal material may be in a partially switched state when the control element is in the second state.

The liquid crystal material may be a nematic liquid crystal material in a Freedericksz alignment, or it may be a vertically aligned nematic liquid crystal material.

The image display layer may include a second layer of liquid crystal material.

The multiple view mode may be a 3-D display mode, or it may be a dual view display mode.

The second viewing angle range may be within the first viewing angle range.

The second viewing angle range may include the normal direction.

The second viewing angle range may have a bisector which is non-normal to the display.

The display may be arranged to display an indication when the display is in the second display mode.

The display may be arranged to adopt the second display mode in response to the content of data for display.

The display may include an ambient light sensor for causing the display to adopt the second display mode when the ambient light is below a threshold.

The display may comprise a vehicle display.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Like reference numerals denote like components throughout the drawings.

The present invention will be described with reference to a display switchable to an autostereoscopic 3-D display mode as the multiple view directional display mode, as an example. The present invention is not, however, limited to this mode, and the multiple view directional display mode may be, for example, a dual view display mode.

FIG. 2Ais a schematic sectional plan view of a display1according to a first preferred embodiment of the present invention. The display1is switchable so as to operate in any of a wide viewing mode, a narrow viewing mode, and a 3-D display mode.

The display1preferably includes an image display layer2which can be driven to display a desired image. The image display layer2may be of a conventional type, which does not require any change in order to provide a display that is operable in more than one mode. For example, the image display layer2may be a liquid crystal layer, and in particular may be the liquid crystal layer of a thin film transistor (TFT) liquid crystal panel that provides a pixelated full color or monochrome display in response to image data supplied to the panel.

FIG. 2Ashows the image display layer2disposed between first and second transparent substrates3,4. The image display layer2and the substrates3,4are themselves disposed between an entrance polarizer5and an exit polarizer6. The polarizers5,6, the substrates3,4and the image display layer2together constitute an image display device7.

In operation, the display is illuminated by a backlight (not shown) placed behind the image display device7. The backlight emits light with reasonable uniformity of intensity throughout a relatively wide angular distribution range. The backlight may be of a conventional type as used to illuminate known displays.

Other components of the image display device7such as, for example, electrodes for addressing the image display layer2, or alignment surfaces for aligning the image display layer (for example when the image display layer is a liquid crystal layer) have been omitted fromFIG. 2Afor clarity.

The display1further includes additional components9-14disposed in the path of light from the image display device7to an observer20,20a,20b. The additional components9-14together define a control element8. The control element8provides controllable angular light modulation and thereby enables the display mode of the display1to be varied. The control element8may, for example, change the output of the display1between a narrow angular light distribution16and a wide angular light distribution15. The control element8may further be switched to provide a parallax barrier so that, when two or more images are displayed in appropriate manner on the image display layer2, the control element8cooperates with the image display layer2to provide a multiple view directional display mode.

The control element8may alternatively be disposed between the backlight (not shown) and the image display device7. In such an alternative configuration, the image display layer7rather than the control element8is closer to an observer20,20a,20b.

The control element8includes a layer of electro-optical material11. In the foregoing description, it will be assumed that the layer of electro-optical material11constitutes a liquid crystal layer, but the invention is not in principle limited to this. The liquid crystal layer11is disposed between third and fourth transparent substrates9,13, formed, for example, of glass. A first set of patterned electrodes10and a second set of patterned electrodes12are provided between the third and fourth glass substrates9,13, to enable to liquid crystal layer11to be addressed, and these electrodes will be described further below.

Finally, the control element8includes an exit polarizer14.

Other components of the control element8, such as a controller for applying voltages to the first and second sets of patterned electrodes or alignment surfaces for aligning the molecules of the liquid crystal layer11of the control element, are conventional, and have been omitted fromFIG. 2Afor clarity.

As explained above, the control element8may be controlled so as to control the display mode of the display1. In one state of the control element, it has substantially no effect on the angular spread of light emitted from the display device7, so that an image displayed by the display device7is visible over a wide viewing range15thereby providing the display1with a wide viewing mode. In another state of the control element, an image displayed by the display device7is visible only over a narrow viewing range16thereby providing the display1with a narrow viewing mode.

In the preferred embodiment ofFIG. 2A, the narrow viewing range is obtained by the control element8creating an obscuring image that is visible at the viewing angles outside the narrow viewing range16but is not visible for viewing angles within the narrow viewing range16. An observer20located within the narrow viewing angle range16thus sees only the image displayed on the image display layer2, but an observer20a,20bviewing the display from outside the narrow viewing range16sees the superposition of the original image displayed on the image display layer2and the obscuring image generated by the control element8.

Finally, the control element may be switched to a third state in which it cooperates with the image display layer to provide a multiple view directional display mode. In the preferred embodiment ofFIG. 2A, the control element provides a parallax optic when switched to its third state so that, if two or more images are displayed on the image display layer2in an appropriate manner (for example, as interlaced images when the parallax optic has alternating transmissive and non-transmissive regions) a multiple view directional display mode is obtained.

FIG. 2Bis a perspective view of the control element8, showing the first and second sets of patterned electrodes10,12.FIG. 2Bshows the control element from the direction in which it would be viewed by an observer, so that the first set of patterned electrodes12are closer to the observer than the second set10of patterned electrodes. The liquid crystal layer11, the substrates9,13and the polarizer14have been omitted fromFIG. 2B.

As is shown inFIG. 2B, the first set of patterned electrodes12includes an array of generally rectangular electrodes17(a)-17(f). The electrodes are arranged in an array of rows and columns. The electrodes17(a)-17(f) are addressed via addressing lines18a-18cthat allow the controller33to apply a desired voltage to the electrodes. However, the electrodes are connected to the addressing lines such that all electrodes in one row of electrodes are not connected to the same addressing line. Rather, each row of electrodes is arranged between two addressing lines, and the electrodes are connected alternately to the addressing line above the row of electrodes and the addressing line below the row of electrodes. Thus, in the first row of electrodes17a-17cshown inFIG. 2B, the first and third electrodes17a,17c(and the fifth, seventh electrodes, etc.) are connected to the addressing line18bbelow the first row of electrodes, and the second electrode17b(and the fourth, sixth, etc. electrodes) are connected to the addressing line18aabove the row of electrodes. Similarly, in the lower row of electrodes17e,17fshown inFIG. 2B, the first and third electrodes17d,17f(and the fifth, seventh, etc. electrodes) are connected to the addressing line18cbelow the row of electrodes whereas the second electrode17e(and the fourth, sixth etc. electrodes) are connected to the addressing line18babove that row of electrodes.

The second set of patterned electrodes10includes an array of stripe electrodes19. These are arranged so that they extend substantially vertically when the display1is in its normal orientation. The strip electrodes19are again addressable via addressing lines that allow the controller32to apply a desired voltage.

FIG. 3Aillustrates operation of the display in its wide viewing mode.FIG. 3Ashows only the first and second patterned electrodes of the control element8, and other components have been omitted for clarity. In the wide viewing mode, the optical control element8has substantially no effect on the angular range of light emitted by the display element7. This can most conveniently be achieved by arranging the liquid crystal layer11of the control element to rotate the plane of polarization of light passing through it in the wide viewing mode, and for the exit polarizer of the control element to be arranged with its transmission axis at an angle to the transmission axis of the exit polarizer6of the display element7. Preferably, in the wide viewing mode, the liquid crystal layer11of the control element rotates the plane of polarization of light passing through it by substantially 90°, and the exit polarizer of the control element has its transmission axis at about 90° relative to the transmission axis of the exit polarizer6of the display element7. An image generated by the display device7is thus transmitted through the control element with little or no change. In an alternative configuration, the wide display mode may be obtained by arranging the exit polarizer of the control element with its transmission axis parallel to the transmission axis of the exit polarizer6of the display element7, and by arranging the liquid crystal layer11of the control element to have substantially no effect on the plane of polarization of light passing through it. However, this alternative configuration may produce a less effective dark state.

In the wide viewing mode, the first and second sets of patterned electrodes10,12are addressed such that the liquid crystal layer11of the control element has a uniform liquid crystal state over its entire area. This requires that a uniform electric field is applied over the entire area of the liquid crystal layer11of the control element8, and this may be done by applying a first reference voltage to all electrodes17a-17fof the first set of patterned electrodes and by applying a second reference voltage to all stripe electrodes19of the second set of patterned electrodes. One (or possibly both) of the first and second reference voltages may be zero.

The particular electric field that must be applied across the liquid crystal layer11of the control element to obtain the wide display mode will depend on the operating mode of the liquid crystal layer11of the control element8. In some cases, zero electric field would be required to obtain the wide viewing mode, and in this case the same voltage would be applied to all of the electrodes17a-17fand the stripe electrodes19inFIG. 3A(i.e., the first and second reference voltages would be equal to one another, and could both be zero). In practice it is not usual to apply a zero voltage to an electrode of a liquid crystal device, because of the nature of the drive electronic that are commonly used. Thus, a zero electric field is in practice normally obtained by the applying the same non-zero voltage to opposing electrodes.

The wide display mode is obtained when zero electric field is applied as described in the above paragraph if the liquid crystal layer11of the control element8includes, for example, a nematic liquid crystal in a Freedericksz alignment. In the Freedericksz alignment, the liquid crystal molecules lie substantially in the plane of the substrate when zero electric field is applied across the liquid crystal layer. The liquid crystal layer is preferably disposed between crossed linear polarizers, with the liquid crystal molecules orientated at an angle of 45° relative to the transmission axis of each polarizer so that the liquid crystal layer acts effectively as a half-wave plate that rotates the polarization of light by 90°. The result is that display is transmissive at zero applied electric field (a “normally white mode”). When a sufficiently large electric field is applied (typically requiring an applied voltage of around 5V rms across a typical thickness of liquid crystal), the liquid crystal molecules are re-oriented to a perpendicular alignment in which they are substantially orthogonal to the plane of the substrate and so have no effect on the polarization of light passing through the layer. This produces a black state. At some intermediate applied electric field (typically requiring an applied voltage of around 2.5V rms across a typical thickness of liquid crystal), the liquid crystal molecules adopt an intermediate state in which they are partially switched up to the perpendicular alignment. This is a very asymmetric configuration.

Alternatively, a uniform electric field may be required across the liquid crystal layer11to obtain the wide viewing mode of the display1. This would be the case if the liquid crystal layer11of the control element8includes a nematic liquid crystal material in a vertically aligned nematic (VAN) configuration, for example. A VAN mode liquid crystal layer is switchable between the same states as described above for a Freedericksz mode nematic liquid crystal, but the liquid crystal molecules adopt the alignment perpendicular to the plane of the substrate in zero applied field (thus giving a “normally black” mode if the liquid crystal is disposed between crossed linear polarizers). When a sufficiently large electric field is applied (typically requiring an applied voltage of around 5V rms across a typical thickness of liquid crystal), the liquid crystal molecules are re-oriented to be parallel to the plane of the substrates and, provided the molecules are substantially at 45° relative to the transmission axes of the two polarizers, the liquid crystal layers acts as a half-wave plate and a bright state is obtained. As in the case of the Freedericksz mode, at some intermediate applied electric field, the liquid crystal molecules adopt an intermediate state in which they are partially switched to the perpendicular alignment.

FIG. 3Bshows a plot for the transmissivity of the control element in its wide viewing mode. The center of the plot (which is marked by a white circle) corresponds to an observer viewing the display along its normal axis, and moving left or right from the center corresponds to a change in azimuth angle. Similarly, moving up or down from the center corresponds to a change in the polar angle. The transmissivity for a particular viewing angle is denoted both by contour lines and by shading.FIG. 3Bcovers an azimuth angular range of 360° and an angular range of 90° in the vertical direction (from an in-plane direction to the normal direction).

FIG. 3Bis the plot for the transmissivity of the control element for the region of the control corresponding to the electrode17dof the first set of patterned electrodes.FIG. 3Cshows a similar transmissivity plot, but for the region of the control element corresponding to the adjacent electrode17eof the first set of patterned electrodes. As can be seen, both regions of the control element have a high transmissivity, even at viewing angles well away from the normal axis of the display. This shows that the display1has a wide viewing mode, since the control element will have little or no effect on an image displayed by the display device7and the image is visible through the wide angular range15ofFIG. 2A. All observers20,20a,20bwill see an image displayed on the image display layer2, and the display has a wide display mode.

FIG. 4Acorresponds toFIG. 2Band shows the display in its narrow view mode. Again,FIG. 4Ashows only the first and second patterned electrodes of the control element8, and other components have been omitted for clarity.

To obtain the narrow view mode, an electric field of non-uniform strength is applied across the liquid crystal layer of the control element8, so that some regions of the liquid crystal layer of the control element8experience a first electric field strength whereas other regions of the liquid crystal layer of the control element8experience a second, different electric field strength. For ease of implementation, one of the first or second electric fields is preferably zero. As a result, the liquid crystal layer11does not have a uniform liquid crystal state over its entire area, and so the optical properties of the liquid crystal layer vary over its area.

In the preferred embodiment ofFIG. 4A, a non-zero voltage is applied to alternate ones of the addressing lines18a-18cfor the first patterned electrodes17a-17f. InFIG. 4A, a first, non-zero voltage has been applied to the second addressing line18b(and to the fourth, sixth, etc. addressing lines), whereas a second voltage (different from the first voltage and preferably non-zero) is applied to the first and third addressing lines18a,18c(and the fifth, seventh, etc. addressing lines) and to all of the stripe electrodes19of the second set of patterned electrodes. The first voltage is therefore applied to the first and third electrodes17a,17cof the first row, and to the second electrode17eof the second row, so that an electric field is applied across the corresponding regions of the liquid crystal layer11of the optical control element. A zero electric field is applied across other regions of the liquid crystal layer11of the optical control element. The first and second voltages are set such that one liquid crystal state exists in the regions of the liquid crystal layer11corresponding to the first and third electrodes17a,17cof the first row and to the second electrode17eof the second row, and a different liquid crystal state, having different optical properties, exists in the regions of the liquid crystal layer11corresponding to the other electrodes17b,17d,17f. The first patterned electrodes17a-17fthus act as a patterned counter electrode that can obtain a narrow display mode.

FIG. 4Ais appropriate for the case where the liquid crystal layer11of the control element8is a nematic liquid crystal material having a Freedericksz alignment. In this case, regions of the liquid crystal layer11corresponding to those electrodes to which the second voltage is applied—the second electrode17bof the first row of electrodes, and the first and third electrodes17d,17fof the second row of electrodes—have the same optical properties as inFIG. 3A, as shown by the transmissivity plot ofFIG. 4B. However, in regions of the liquid crystal layer corresponding to those electrodes to which the first voltage is applied, the liquid crystal state is changed to the partially switched alignment as a result of the applied electric field set up across those regions. As a result, these regions of liquid crystal produce a variation of intensity upon viewing angle. This is shown by the transmissivity plot ofFIG. 4C. As can be seen in this Figure, the intensity is high only for viewing angles near the normal direction. As the observer moves away from normal viewing direction, in a horizontal plane, the intensity quickly decreases.

Thus, an observer who views the display along its normal axis, or at a viewing angle close to the normal axis, will see an image displayed on the image display device7. All regions of the control element8have a high transmissivity when viewed along the normal axis, and all regions of the control element8have substantially the same transmissivity as one another when viewed along the normal axis, so the control element8has no effect on the image seen by an observer viewing the display along its normal axis or along a direction close to the normal axis.

However, an observer who views the display at a wide viewing angle will see a “confusing image” generated by the control element, as a consequence of the substantial difference in transmissivity, at high viewing angles, between the regions of the control element corresponding to the electrodes17a,17cand17eand the regions of the control element corresponding to the electrodes17b,17dand17f. An observer20a,20boutside the narrow angular range16sees the “confusing image” superposed over the image displayed on the image display layer2, and is thus unable to easily make out the image displayed on the image display layer2.

If the first set of patterned electrodes is arranged and operated as shown inFIG. 4A, the “confusing image” visible at high viewing angles is a checkerboard pattern as shown inFIG. 4D. The checkerboard pattern makes it difficult for the observer to make out the image displayed on the image display panel.

The liquid crystal state in the regions of the liquid crystal layer11of the control element corresponding the electrodes17a,17c,17eshould provides a high transmissivity for light propagating along or close to the normal axis but a low transmissivity for light propagating a large angles to the normal axis. The partially switched alignment of a Freedericksz mode nematic liquid crystal or a VAN liquid crystal is suitable for this. Such effects are described in more detail in co-pending UK Patent Application No. 0408742.5 and the corresponding European Patent Application No. 05103193.8.

It should be noted that the details of the addressing of the first set of patterned electrodes will depend on the nature of the liquid crystal layer11of the control element8. While the addressing scheme described above will be appropriate for a nematic liquid crystal having the Freedericksz configuration, liquid crystal layers having other liquid crystal alignments may require addressing in a different way. For example, in the case of a nematic liquid crystal material having a vertically aligned nematic alignment, the wide view mode is obtained by applying a uniform non-zero electric field across the liquid crystal layer to orient the liquid crystal molecules parallel to the substrates and at 45° relative to the transmission axes of the substrates. In this case, the narrow view mode would again be obtained by addressing the liquid crystal layer using alternate ones of the addressing lines18a,18b,18c. An electric field high enough to obtain the parallel alignment of the liquid crystal molecules would be applied to some liquid crystal regions, and these regions would have a high transmissivity over a wide range of viewing angles. Other liquid crystal regions would have an intermediate electric field applied across them, so that the molecules would adopt the partially switched state and so would have a low transmissivity at high viewing angles.

FIGS. 5A to 5Cillustrate the narrow display mode of the display1in more detail.FIG. 5Ashows plots of the transmissivity of the control element8for an observer viewing the display along its normal axis, with the position of the observer being indicated by a “X” in the transmissivity plots. The left hand transmissivity plot is for a region of the control element8corresponding to the electrode17dinFIG. 4A, and the right hand transmissivity plot is for a region of the control element8corresponding to the electrode17einFIG. 4A. As can be seen, for an observer viewing the display along its normal axis, all regions of the liquid crystal layer11of the control element8have high transmissivity, and have substantially the same transmissivity as one another. The “obscuring image” generated by the control element8is not visible to the observer viewing the display along its normal axis. As a result, the observer sees only the image displayed on the image display layer2, as represented in the left hand portion ofFIG. 5B, which is a representation of the view seen by an observer, and the image displayed on the image display layer2is thus seen clearly.

FIG. 5Cshows plots of the transmissivity of the control element8for an observer viewing the display from a position that is laterally displaced from the normal axis. The position of the observer is again indicated in the intensity plots ofFIG. 5Cby an “X”. The left hand transmissivity plot is for a region of the control element8corresponding to the electrode17dinFIG. 4A, and the right hand transmissivity plot is for a region of the control element8corresponding to the electrode17einFIG. 4A. It can now be seen the regions of the control element do not have the same transmissivity as one another, so that the observer will perceive a superposition of the “obscuring pattern” created by the control element8and the original image displayed on the image display layer2. This is illustrated in the right hand portion ofFIG. 5B, and it can be seen that the obscuring pattern (the checkerboard pattern in this preferred embodiment) makes it very hard to make out the image displayed on the image display layer2(the text in this preferred embodiment).

FIG. 6Aillustrates the display1in a 3-D display mode.FIG. 6Ashows only the first and second patterned electrodes of the control element8, and other components have been omitted for clarity. In this preferred embodiment, the stripe electrodes19of the second set of patterned electrodes are driven so as to provide a parallax optic, in this example, a parallax barrier, in the optical control element8. This can be done by applying a first voltage to every alternate one of the stripe electrodes19, and by applying a second, different voltage (which may be zero) to every other one of the stripe electrodes19. At the same time, the addressing lines18a-18cof the first set of patterned electrodes are connected to a uniform voltage, so that every one of the electrodes17a-17fof the first set of patterned electrodes is at a uniform voltage. This creates a parallax barrier in the control element8, which includes dark stripes21alternating with light stripes22, as shown inFIG. 6D. The stripe electrodes thus enable a parallax barrier to be defined for a 3-D display mode.

FIG. 6Bis a plot of the transmissivity of a region of the control element8corresponding to a stripe electrode19ato which the first voltage is applied. It can be seen that this region of the control element has a very low transmissivity when viewed along, or close to, the normal axis.FIG. 6Cis a plot of the transmissivity of a region of the control element8corresponding to a stripe electrode19bto which the second voltage is applied, and this shows that this region of the control element has a high transmissivity when viewed along the normal axis. The control element thus creates a parallax barrier including dark stripes21alternating with light stripes22, as shown inFIG. 6D, in directions along or close to the normal axis. AlthoughFIG. 6Bhas a slightly different form fromFIG. 6C, both figures are plots of the transmissivity of the control element.

In most applications of the display, the opaque regions of the parallax barrier in the 3-D display mode are required to be visible in directions near the normal axis. This means that the liquid crystal state required to create the opaque regions of the parallax barrier is not the same as the liquid crystal state required to create the opaque regions of the obscuring pattern in the narrow display mode (which, as explained above, are not visible along the normal axis). This in turn means that the electric field required across the liquid crystal layer to create the opaque regions of the parallax barrier is not the same as the electric field required to create the opaque regions of the obscuring pattern in the narrow display mode. Depending on the liquid crystal mode, the electric field required to create the opaque regions of the parallax barrier may be either greater than or lower than the electric field required to create the opaque regions of the obscuring pattern in the narrow display mode.

In a display where the transmission axis of the exit polarizer14of the control element is perpendicular to the transmission axis of the exit polarizer6of the display device7, the liquid crystal state in the regions of the liquid crystal layer11of the control element that form the transmissive regions of the parallax barrier preferably rotates the plane of polarization of light propagating along the normal axis by substantially 90°. The liquid crystal state in the regions of the liquid crystal layer11of the control element that form the opaque regions of the parallax barrier preferably have substantially no effect on the plane of polarization of light propagating along the normal axis, and this creates opaque regions visible at angles near normal incidence.

In a case where the liquid crystal layer of the control element8includes a nematic liquid crystal in a Freedericksz alignment, the wide display mode occurs in the absence of any electric field applied across the liquid crystal layer, as stated above. Application of a first voltage to alternate electrodes17a,17c,17eof the first set of patterned electrodes (while applying another voltage (different from the first voltage) to the other electrodes of the first set of patterned electrodes or to the stripe electrodes19) will produce the narrow display mode. Application of a second voltage, different from the first voltage, to alternate stripe electrodes19(while applying the said another voltage to the other stripe electrodes19or to the first set of patterned electrodes) will produce the 3-D display mode.

In the example of a display in which the liquid crystal layer of the control element8includes a nematic liquid crystal in a Freedericksz alignment or a vertical alignment disposed between cross linear polarizers, the narrow display mode is obtained by causing the liquid crystal molecules in some regions of the liquid crystal layer of the control element to adopt the partially switched state, whereas liquid crystal molecules in other regions of the liquid crystal layer of the control element are oriented parallel to the substrates and at 45° relative to the transmission axes of the polarizers. This generates the “confusing image” at high viewing angles. To obtain a directional display mode, the liquid crystal molecules in some regions of the liquid crystal layer of the control element are caused to adopt the fully switched state in which they are perpendicular to the plane of the substrates, to define the opaque regions of the parallax barrier. Liquid crystal molecules in other regions of the liquid crystal layer of the control element are oriented parallel to the substrates and at 45° relative to the transmission axes of the polarizers, to define the transmissive regions of the parallax barrier.

In order to obtain a good 3-D display mode, it is necessary for the dark regions21of the parallax barrier23to have as low a transmissivity as possible. It may therefore be advantageous to include one or more retarders or one or more optical compensator layers in the control element8, in order to obtain an achromatic, low transmissivity dark state for the dark regions21of the parallax barrier23.

FIG. 7illustrates a display according to a modification of the preferred embodiment ofFIG. 2A.FIG. 7shows only the first and second sets of patterned electrodes10,12, and other components of the display have been omitted for clarity (and correspond to the components described above with reference toFIG. 2A). In this preferred embodiment, the second set10of patterned electrodes includes stripe-shaped electrodes19that are arranged generally parallel to one another. In this preferred embodiment, the first set of patterned electrodes12also includes an array of stripe-shaped electrodes24arranged generally parallel to one another and having a general uniform thickness over their length. The stripe electrodes24of the first set of patterned electrodes extend generally perpendicularly to the stripe electrodes19of the second set of patterned electrodes. In the display shown inFIG. 7, the stripe electrode24of the first set of patterned electrodes extend generally horizontally when the display is in its normal orientation, and the stripe electrode19of the second set of patterned electrodes extend generally in the vertical direction when the display is in its normal orientation.

In this preferred embodiment, the wide display mode may be obtained by applying a first voltage to each of the electrodes24of the first set of patterned electrodes12, and by applying a second voltage to each of the stripe electrodes19of the second set of patterned electrodes10so as to obtain a uniform electric field (which, depending the liquid crystal mode, may be zero or non-zero) across the liquid crystal layer. A wide display mode is then obtained as described with reference toFIG. 3Aabove. Similarly, a 3-D display mode may be obtained by applying a first voltage to each of the stripe electrodes24of the first set of patterned electrodes, by applying a second voltage to every alternate stripe electrode19of the second set of patterned electrodes, and by applying a third voltage different from the second voltage to every other of the stripe electrodes19of the second set of patterned electrodes. A parallax barrier is then set up as described above with reference toFIG. 6A. The stripe electrodes19of the second set of patterned electrodes thus enable a parallax barrier to be defined for a 3-D display mode. As also described above, the magnitudes of the voltages required to obtain the wide display mode and the 3-D display mode will depend on the nature of the liquid crystal layer11of the control element8.

In order to obtain a narrow display mode, the first and second sets of patterned electrodes are driven in order to define an “obscuring image” in the liquid crystal layer11. Essentially, a pixel is defined in the liquid crystal layer11of the control element at every location where one of the stripe electrodes24of the first set of electrodes12overlaps one of the strip electrodes19of the second set of electrodes, and these pixels may be driven to be either transmissive or in an intermediate state that appears opaque to an off-axis viewer. The first and second sets of electrodes10,12may be driven using any standard “passive matrix addressing” technique to drive the liquid crystal layer11; such addressing techniques are well-known, and will not be described here. The stripe electrodes24of the first set of patterned electrodes12thus act as a patterned counter electrode that can obtain a narrow display mode.

In this preferred embodiment, the electric field applied across a region of the liquid crystal layer11of the control element to define an opaque region of the parallax barrier will again be different from (in general, greater than) the electric field applied across a region of the liquid crystal layer11of the control element to define an opaque region of the obscuring image in the narrow display mode.

The preferred embodiment ofFIG. 7has the advantage that any type of pattern or image may be used as the obscuring image, and the display1is not restricted to the use of one particular obscuring image. One advantage of this is that, as can be seen inFIG. 5D, the effectiveness of a checkerboard pattern at obscuring text depends on the relative size of the squares of the checkerboard pattern to the characters of the text. The preferred embodiment ofFIG. 7would allow the size of the squares of the checkerboard pattern to be chosen based on knowledge of the character size of a text image to be displayed on the image display layer2. Yet another different pattern may be selected if the display layer2is primarily displaying graphical images or photographs. Further, the obscuring pattern may be selected based on the nature of an image being displayed on the image display layer2, in order to give the most effective privacy effect.

In principle, a display having first and sets of patterned electrodes as shown inFIG. 2Bcould provide an obscuring pattern that could be varied depending on the nature of an image being displayed on display layer2. For example, if the electrodes17a-17fof the first set of patterned electrodes are individually addressable, the size of the opaque and transmissive regions of the checkerboard pattern may be varied, for example by driving the electrodes in 2×2 groups, 3×3 groups etc. This would allow the size of the squares of the checkerboard pattern to be chosen based on knowledge of the character size of a text image to be displayed on the image display layer2.

Alternatively, the preferred embodiment ofFIG. 7allows use of a “moving” obscuring image—that is, an obscuring image that varies over time. This can increase the privacy of a displayed image, by making it harder for an observer outside the narrow viewing range16to decipher an image displayed on the image display layer2.

The preferred embodiment ofFIG. 7may also use, for example, an advertising message or other message conveying information as the “obscuring image”.

The preferred embodiment ofFIG. 7may be used to provide an alternative form of a parallax barrier, namely the slanted or staggered type as is known in the art. Such a barrier has transmissive apertures with boundaries that are inclined, rather than parallel, with respect to the boundaries of the underlying pixel of the image display layer2. The advantage is that resolution in each image may be more evenly distributed between horizontal and vertical directions. Also the stripes of the barrier may appear less visible to a user.

FIG. 8Ais a schematic plan sectional view of a display25according to a further preferred embodiment of the present invention. The display25includes a control element8disposed in front of a transmissive image display device7. The image display device7corresponds to the image display device7of the display1ofFIG. 2A, and its description will not be repeated here.

The control element8includes a layer of electro-optic material, in this example, a liquid crystal layer11′, disposed between first and second transparent substrates9,13, (for example, glass substrates). Electrodes for addressing the liquid crystal layer11′ are provided between the first substrate9and the liquid crystal layer11′, and between the liquid crystal layer11′ and the second substrate13. Finally, the control element8includes an exit polarizer14. Other components, such as a controller for applying voltages to the electrodes and alignment films, have been omitted for clarity.

In the preferred embodiment ofFIG. 2A, the liquid crystal layer11of the control element8had a uniform liquid crystal alignment over its entire area. In the preferred embodiment ofFIG. 8A, however, the liquid crystal layer11′ does not have a uniform liquid crystal alignment over its entire area, but has a patterned alignment. As a result, this preferred embodiment requires only one set of patterned electrodes, to define the parallax barrier in the liquid crystal layer, and one of the sets of patterned electrodes of the display1ofFIG. 2Amay be replaced by a uniform electrode26. In the display25ofFIG. 8A, the uniform electrode26takes the place of the first set12of patterned electrodes17a-17fof the display ofFIG. 2A.

FIG. 8Bis a plan view showing the liquid crystal layer and the patterned electrodes19, looking from the position of an observer. Other components of the control element have been omitted. In this preferred embodiment, the liquid crystal layer11′ is patterned into a plurality of liquid crystal layer regions27a-27f. The liquid crystal layer regions are arranged in a matrix of rows and columns so that a checkerboard obscuring image can be obtained. The liquid crystal layer is patterned in that the stable liquid crystal alignment, under zero applied electric field, varies between one region and an adjacent region in a column or row.

In this preferred embodiment, the liquid crystal layer11′ includes a twisted nematic liquid crystal material, in which the projection of liquid crystal molecules adjacent the first substrate9onto the first substrate makes a non-zero twist angle α (with the projection of liquid crystal molecules adjacent the second substrate13onto the second substrate. In this preferred embodiment, the twist angle α is approximately 90°. InFIG. 8B, the full arrow represents the alignment direction of liquid crystal molecules adjacent to the first substrate9, and the broken arrow represents the direction of liquid crystal molecules adjacent to the second substrate13. It can be seen that the alignment direction for one region17ais at 180° relative to the alignment direction of a neighboring region17bin a row or a neighboring region17din a column, which applies to both the alignment direction adjacent to the first substrate9and to the alignment direction adjacent to the second substrate13.

At least 1-2 degrees pre-tilt of the liquid crystal molecules is needed to break the degeneracy of possible liquid crystal configurations which would otherwise reduce contrast, as is known in the art. Following convention, the arrowheads inFIG. 8Bindicate a pre-tilt direction, and so it can be seen that different regions of the liquid crystal layer will have pre-tilt of an opposite sign (i.e., the alignment direction is different by 180 degrees from one region to another).

FIG. 8Balso shows the patterned electrodes10. These correspond to the second patterned electrodes10ofFIG. 2A, and include an array of stripe electrodes19that are arranged to extend generally vertically when the display is in its normal orientation.

The liquid crystal layer alignment shown inFIG. 8Bmay be obtained by use of one or more patterned alignment surfaces (not shown inFIG. 8A). Each patterned aligning surface induces an alignment in adjacent liquid crystal molecules that is not uniform over the area of the aligning surface but that spatially varies across the surface so that regions27a,27bhaving different liquid crystal configurations, and hence different optical properties, may be obtained. For example, the patterned alignment surfaces may be arranged so as to provide regions of twisted nematic liquid crystal alignment, with different orientations of twist as shown inFIG. 8B.

A patterned alignment surface may be achieved by any known or suitable technique, for example, multiple rubbing techniques (where an alignment layer is rubbed, and is subsequently selectively masked and re-rubbed) by multiple evaporation of SiOx or by use of a photo-alignment technique. The production of a patterned alignment surface does not form part of the present invention, and will therefore not be described in detail.

FIGS. 9A to 9Cillustrate the general principle of this preferred embodiment of the present invention.FIG. 9Acorresponds generally toFIG. 8B, but shows only four of the liquid crystal regions, including27a,27b,27dand27e. Of these, regions27aand27ehave one liquid crystal alignment, and regions27dand27bhave a different liquid crystal alignment.

FIG. 9Billustrates generation of a parallax barrier by applying suitable voltages to the stripe electrodes19of the set of patterned electrodes. As before, a parallax barrier is obtained by applying a first voltage to every alternate stripe electrode19of the set of patterned electrodes, by applying a second, different voltage to every other stripe electrode19, and by applying a third voltage, different from at least one of the first and second voltages, to the uniform electrode26. In the simplest implementation, the second and third voltages are equal to one another, and may both be zero.

FIG. 9Cillustrates an “obscuring pattern” that can be provided by the liquid crystal layer11′. This includes light regions and dark regions, with the light regions corresponding to the regions27a,27eof one liquid crystal alignment and the dark regions corresponding to regions27d,27bof the other liquid crystal alignment. In this preferred embodiment, the confusing pattern arises from the patterned alignment of the liquid crystal layer, rather than from use of a patterned electrode.

FIGS. 10A to 10Cillustrate the wide display mode of the display25of this preferred embodiment. In this mode, the stripe electrodes19of the set of patterned electrodes10are all maintained at a uniform voltage. In the particular case of a twisted nematic crystal material, the wide display mode may be obtained by applying a voltage to every of the stripe electrodes19, and also by applying the same voltage to the uniform counter electrode26. In this case, a region27ahaving one liquid crystal alignment and a region27bhaving the other liquid crystal alignment produce substantially the same optical properties as one another, and preferably both provide high transmissivity. In a preferred embodiment, both liquid crystal alignments rotate the plane of polarization of light by substantially 90°, and the exit polarizer14of the control element8has its transmission axis perpendicular to the transmission axis of the exit polarizer6of the display device7.

FIG. 10Bis a plot of transmissivity, in the wide display mode, of a region27b,27dof the control element8in which the liquid crystal layer11′ has one liquid crystal alignment, andFIG. 10Cshows a transmissivity plot, in the wide display mode, for a region27a,27eof the control element8having the other liquid crystal alignment. As can be seen, both intensity plots show a high intensity for an observer viewing the display along its normal axis, and also show that the intensity remains high as the observer moves laterally away from the normal axis, on either side of the normal axis. Thus, an image displayed on the image display layer2is visible over a wide viewing angle range, such as the wide viewing angle range15indicated inFIG. 2A.

FIG. 11Aillustrates a narrow view mode of the display25. In this mode, the stripe electrodes19of the set10of patterned electrodes are again all set at a uniform voltage. However, the voltage applied to the stripe electrodes19in this mode is sufficiently large to re-orient the liquid crystal molecules and cause them to adopt a different liquid crystal state.

FIG. 11Bis a plot of transmissivity, in the narrow display mode, of a region27b,27dof the control element8having the first liquid crystal alignment, andFIG. 11Cis a plot of transmissivity, in the narrow display mode, of a region27a,27eof the control element8having the second liquid crystal alignment. As can be seen fromFIG. 11B, a region27b,27dhaving a first liquid crystal alignment appears light to a viewer who is looking at the display along its normal axis or who is positioned to the left of the normal axis, but appears dark to a viewer who is positioned to the right of the normal axis of the display. Conversely,FIG. 11Cshows that a region27a,27eof the liquid crystal layer11′ having the second liquid crystal alignment appears light to a viewer viewing the display along the normal axis or who is positioned to the right of the normal axis, but appears dark to a viewer positioned to the left of the normal axis. A viewer who is viewing the display along the normal axis, or within a narrow viewing range around the normal axis such as the viewing range16inFIG. 2Awill therefore be able to make out an image displayed on the image display layer2, since all regions of the control element appear bright to such a viewer, they will not perceive any “obscuring image”.

A viewer who is viewing the display from a viewing angle outside the narrow viewing angle range16will, however, perceive the checkerboard “obscuring pattern”. If the viewer is to the left of the normal axis, the regions of the control element corresponding to the regions27b,27dof the first liquid crystal alignment will appear light and the regions of the control element corresponding to regions27a,27ehaving the second liquid crystal alignment will appear dark, whereas a viewer viewing the display from a viewing angle that is to the right of the normal axis will perceive the regions of the first liquid crystal alignment as dark and regions of the second liquid crystal alignment as light. In either case, the viewer will see the checkerboard pattern overlaid on the image displayed on the image display layer, and the checkerboard pattern will obscure the image as explained above. An image displayed on the image display layer2will therefore be readable only by an observer disposed within the narrow range16of viewing angles.

FIGS. 12A to 12Cfurther illustrate operation of the display25in its narrow view mode.FIG. 12Ashows two plots of transmissivity of the control element in the narrow display mode, the left plot being for a region of the control element in which the liquid crystal layer11′ has the first liquid crystal alignment and the right hand intensity plot being for a region of the control element8in which the liquid crystal layer11′ has the second liquid crystal alignment. The position of an observer viewing the display along its normal axis is marked on the intensity plots by an “X” and it can be seen that the two regions of the control element have substantially the same transmissivity as one another for this observer. An observer viewing the display along its normal axis will therefore not see the obscuring image, and will see only the original image displayed on the image display layer2, as shown in the left hand part ofFIG. 12B, which represents the view seen by an observer. The image displayed on the image display layer2is thus seen clearly.

FIG. 12Cshows two transmissivity plots corresponding to those ofFIG. 12A, but the position of an off-axis observer is marked using an “X”. It can be seen that a region of the control element8in which the liquid crystal layer11′ has the first liquid crystal alignment appears dark to this observer, whereas a region of the control element in which the liquid crystal layer11′ has the second liquid crystal alignment appears bright to this observer. As a result, the observer will perceive the checkerboard “obscuring image” superposed over the image from the image display layer2. This is shown in the right hand portion ofFIG. 12B. The right hand portion ofFIG. 12Bcorresponds generally to the right hand portion ofFIG. 5B, and further description will not be repeated.

FIG. 13AtoFIG. 13Cillustrate the display25in its 3-D mode. This can be obtained by applying a first voltage to every alternate stripe electrode19of the set of patterned electrodes, and by applying a second, different voltage (which may be zero) to every other stripe electrode19. This causes alternating light and dark stripe/shaped regions to be defined in the control element8, thereby producing a parallax barrier.

Details of how liquid crystal states that provide low intensity off-axis (to provide the narrow display mode as inFIGS. 11B and 11C) or that provide low intensity on-axis (as inFIG. 13C, to provide the opaque regions of the parallax barrier) may be obtained using a TN liquid crystal material are contained in the above-mentioned UK Patent Application No. 0408742.5 and corresponding European Patent Application No. 05103193.8, to which attention is directed. In brief, however, at an intermediate applied voltage the liquid crystal molecules in the middle of the layer are tilted with respect to the liquid crystal plane resulting in an asymmetric angular distribution of transmitted light, due to the effective birefringence of the liquid crystal being tilted, as is well known in the art. The provision of a dark state for a fully switched TN is also well known in the art.

FIG. 13Bis a plot of transmissivity for a region of the control element8corresponding to one of the light stripes of the parallax barrier. As can be seen, a high intensity is seen not only by an observer viewing the display along its normal axis but also by observers in a wide range of viewing angles, such as the wide viewing angle range15ofFIG. 2A.FIG. 13Cis a plot of transmissivity for a region of the control element8corresponding to one of the dark regions of the control element, and it can be seen that a very low intensity is obtained over a wide range of viewing angles, including the normal direction. The control element thus acts as a parallax barrier, and a 3-D display mode is obtained.

The preferred embodiments of the present invention have been described with reference to nematic liquid crystals, in particular to a Freedericksz aligned nematic liquid crystal, a vertically aligned nematic liquid crystal, or a twisted nematic liquid crystal material. The present invention is not, however, limited to these liquid crystal materials, and the control element8may include other liquid crystal modes. For example, the control element8may include a layer of, for example, a super twisted nematic liquid crystal material, a hybrid aligned nematic liquid crystal material, a twisted vertically aligned nematic liquid crystal material, a pi-cell alignment, a ferro-electric liquid crystal material, etc.

The preferred embodiment ofFIG. 8Ahas been described with reference to a patterned liquid crystal layer that produces a checkerboard “obscuring image”. The preferred embodiments are not, however, limited to this, and the liquid crystal layer may be patterned to provide any suitable “obscuring image” that, when overlaid on an image displayed on the image display layer2, makes it difficult or impossible for a viewer to decipher the original image. As examples, the liquid crystal layer11′ of the display25may be patterned to produce an obscuring image in the form of text or a manufacturer's name.

FIGS. 14A and 14Billustrate the wavelength dependence of transmissivity of the wide viewing mode for the embodiments ofFIGS. 8A and 2A, respectively. It can be seen that although the preferred embodiment ofFIG. 8A(in which the liquid crystal layer11′ of the control element8has a patterned alignment) has a transmissivity with a relatively low dependence on wavelength over the visible spectrum in its wide viewing mode, the preferred embodiment ofFIG. 2A(in which the counter electrode12of the control element8is patterned) has a transmissivity in its wide viewing mode that is strongly wavelength-dependent. This strong wavelength-dependence of the wide viewing mode in the preferred embodiment ofFIG. 2Aarises because a relatively thick liquid crystal layer is required for the control element8in the preferred embodiment ofFIG. 2A, and it leads to undesirable coloration of a displayed image in the public display mode.

The wavelength-dependence of the wide viewing mode in the preferred embodiment ofFIG. 2Amay be reduced by providing an optical retarder in the path of light through the liquid crystal layer11of the control element ofFIG. 2A.

FIG. 15is a schematic plan sectional view of a display28according to a further preferred embodiment of the present invention. The display28includes a control element8disposed in front of a transmissive image display device7. The image display device7corresponds to the image display device7of the display1ofFIG. 2A, and its description will not be repeated here.

The control element8includes a layer of electro-optical material11, for example, a liquid crystal layer, disposed between third and fourth transparent substrates9,13, formed, for example, of glass. A first set of patterned electrodes10and a second set of patterned electrodes12are provided between the third and fourth glass substrates9,13, to enable to liquid crystal layer11to be addressed. These sets of electrodes correspond to the sets of patterned electrodes of the control element8of the display ofFIG. 2Aand so will not be described further. The control element8also includes an exit polarizer14.

The display28ofFIG. 15further includes an optical retarder29provided to reduce the coloration of the display in its wide viewing mode. InFIG. 15, the retarder29is disposed within the control element8, between the fourth substrate13and the exit polarizer14. However, the retarder is not limited to this location, and the retarder may alternatively be disposed, for example, between the image display device7and the control element8.

The retarder is preferably arranged with its optical axis substantially parallel to, or substantially perpendicular to, the alignment direction of the liquid crystal layer11of the control element8.

FIGS. 16A and 16Bshow the improvement provided by the embodiment ofFIG. 15.FIG. 16Ashows the wavelength dependence of transmissivity of the wide viewing mode for a display according to the preferred embodiment ofFIG. 2A, for a case where the liquid crystal layer11of the control element8is an ECB liquid crystal layer. As noted above, the transmissivity in the wide viewing mode is strongly wavelength-dependent.

FIG. 16Bshows the wavelength dependence of transmissivity of the wide viewing mode for a display according to the preferred embodiment ofFIG. 15, again for a case where the liquid crystal layer11of the control element8is an ECB liquid crystal layer. As can be seen, providing the retarder29has significantly reduced the wavelength dependence of the transmissivity in the wide viewing mode.

AlthoughFIGS. 16A and 16Bshows the transmissivity for a display in which the control element has an ECB liquid crystal layer, similar results are obtained for displays in which the control element has, for example, a VAN liquid crystal layer.

The preferred embodiments of the present invention have been described with reference to a display that is operable in a 3-D display mode. The present invention is not, however, limited to the use of a 3-D display mode as the multiple view directional display mode, and the display of the present invention may alternatively be operable in a dual view mode.

The preferred embodiments of the present invention have been described with reference to a display incorporating a transmissive image display device7. The present invention is not limited to a transmissive image display device7, and a display of the present invention may have an emissive image display device, a reflective image display device, or a transflective image display device.

In the case of a display having a transmissive image display device, the control element8may be disposed in front of the image display device (i.e., between the image display device and an observer), or it may be disposed behind the image display device (i.e., between a backlight and the image display device). In the case of a display having an emissive display device or a reflective display device, the control element8is necessarily disposed in the path of light from the image display device to an observer.

Any of the preferred embodiments described herein may be arranged to provide an indication to a user of when the display is in the private or narrow viewing angle mode. For example, this may be provided within software which causes an image or icon to be displayed to show that the display is in the private mode. Such an icon may be overlaid, for example, on a displayed image at the bottom of the screen of the display, and may include the word ‘private’. Alternatively, this function may be provided in the image display or in the additional components so that, when the display is switched to the private mode, a portion of the image display of the additional components is activated in order to display an appropriate icon.

The displays described herein may be combined with or provided in association with a device or arrangement which automatically switches the display to the private mode when the content of the image to be displayed is of the appropriate type. For example, if the display is used for viewing internet pages, any of the software flags associated with internet pages may be used to trigger the display so that it operates in the private mode. An example of such an application is when a browser is working in a secure encrypted mode, for example, when personal bank details are being viewed or when secure transactions are being conducted.

It is also possible to arrange for the display to switch to the private mode when the display is part of or is associated with a display for data entry and the type of data being entered or about to be entered is such that the private display mode is required. For example, the entering of a personal identification number (“PIN”) may automatically cause the display to switch to the private mode. Such an arrangement may, for example, be used with “chip and pin” technology in retail trading outlets.

In many of the preferred embodiments described above, the narrow viewing angle range has been such that its bisector is parallel, or substantially parallel, to the normal axis of the display. In some applications, however, it may be desirable for the narrow viewing angle range to be such that its bisector is not parallel to the normal axis of the display. The feature may be desirable when the display is used in an automotive application, for example, in the dashboard of a vehicle. Such an arrangement could be used so that, in the narrow viewing angle mode, the passenger or driver is unable to view the displayed image. This may be achieved, for example, by using a display as shown inFIGS. 2A and 2B, in which the bisector of the narrow viewing angle range shown inFIG. 2Ais not parallel to the normal axis of the display.

The displays described above may include an ambient light sensor, and may arrange for the display to switch to the private mode when the output from the ambient light sensor indicates that the level of ambient light has fallen below a pre-set threshold. This may be of use in, for example, a display in a vehicle, since the viewing angle of the display could be controlled in order to reduce the reflections of the display in the windscreen and the windows, so that, for example, the viewing angle could be reduced at nighttime or in low light conditions.