GB2405517A

GB2405517A - Multiple view display

Info

Publication number: GB2405517A
Application number: GB0320360A
Authority: GB
Inventors: Diana Ulrich Kean; Jonathan Mather; Heather Stevenson
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2003-08-30
Filing date: 2003-08-30
Publication date: 2005-03-02
Also published as: GB0320360D0

Abstract

A multiple view display comprises a first liquid crystal device (20-28) having a first asymmetric liquid crystal mode with first asymmetric viewing properties. The display comprises a second liquid crystal device (21'-28') having a second asymmetric liquid crystal mode with second asymmetric viewing properties oriented differently from and preferably opposite the first asymmetric viewing properties. A driving arrangement (29) drives the first device with a first drive scheme for displaying the first image in a first image direction and drives the second device with the second drive scheme which may be the same as or different from the first scheme, for displaying a second image in a second viewing direction different from the first viewing direction.

Description

24055 1 7 MUI,TIPLE VIEW DISPLAY The present invention relates to a

multiple view display. Such a display may be used to display two or more views containing images which are substantially different from each other. Such displays allow different viewers to see different images which may be unrelated to each other.

Figure I of the accompanying drawings illustrates the concept of a multiple view display which, in this example, comprises a dual view display. Viewers 1 and 2 located in viewing regions 1 and 2 view a dual view display 10. The display 10 has discrete viewing regions in which substantially different images may be displayed. For example, viewer 1 may be watching a film while viewer 2 may be reading a map. Such an example is applicable, for instance, to a display for automotive use so that a driver may see navigation information while a passenger simultaneously sees the film.

Figure 2 of the accompanying drawings illustrates another example of the use of a dual view display 10. In this case, the display is mounted in the plane of a desk or counter 11 so that viewers 1 and 2 on opposite sides of the desk or counter may view different images. Similarly, Figure 3 illustrates a horizontally oriented multi-view display which, in this case, displays four unrelated images to viewers 1 to 4 on the four sides of the display.

The viewing angle properties of the twisted nematic (TN) liquid crystal (LC) mode are well known and the viewing angle properties of such a liquid crystal display (LCD) are disclosed in P. Yeh and C. Gu, "Optics of Liquid Crystal Displays", John Wiley and Sons Inc., 1999, chapter 9. These properties are generally relatively uniform in the horizontal viewing angle direction but are asymmetric in the vertical viewing angle direction. Techniques for optimising the viewing angle properties of LCDs to allow the same image to be viewed from a wide range of angles are known.

A multiple view video display is disclosed in JP 06-236152. In this example, use is made of lenticular lenses to generate different viewing regions.

The directional viewing properties of TN and guest-host (GH) LCDs are disclosed in JP 2-146087, JP 60-211418, JP 60-211420, JP 60-211428 and in Okada et al, IEEE Transactions on Electron Devices, Vol. 45, No. 7, 1988, pp 1445-1452 "Possibility of Stereoscopic Displays by Using a Viewing Angle Dependence of Twisted Nematic Liquid Crystal Cells". These documents disclose spatially multiplexing two images on an LCD with the pixels for displaying one of the images having a different alignment from the pixels for displaying the other image. These documents refer to autostereoscopic or stereoscopic displays for displaying related images to provide a 3D display.

JP 08-101367 discloses a dual layer autostereoscopic display. One layer displays spatially multiplexed images behind another layer which acts as a direction control layer to direct the images into different viewing regions for autostereoscopic viewing.

Chen et al, Japanese Journal of Applied Physics, Vol. 36 (1997), pages L1685-L1688, "Simple Multimode Stereoscopic Liquid Crystal Display" discloses an arrangement in which a passive spatially patterned liquid crystal layer is overlaid on an image panel displaying spatially multiplexed left and right eye images. The passive layer comprises twisted nematic liquid crystal regions and Freedericksz mode liquid crystal regions for rotating the polarization of light for one view and for not rotating the polarization of light for the other view. The images may be viewed autostereoscopically without viewing aids or stereoscopically using polarising glasses.

JP 60-211428 discloses a stereoscopic display comprising a pair of stacked guest-host liquid crystal devices. Each of the devices displays a respective view and the device alignments are oriented perpendicularly to each other. The left and right views are thus polarised perpendicularly to each other and polarising glasses are used to view the display stereoscopically.

US 6 424 323 discloses an arrangement which uses a lenticular screen to provide two or more 2D or 3D images.

EP 1 250 013 discloses an arrangement in which a display screen displays spatially multiplexed left and right images and a plurality of further devices in front of the display screen control the visibility of the images by the left and right eyes of an observer.

JP 09-043540 discloses a stereoscopic display using electronic switching of two liquid crystal barriers to translate the effective position of a parallax barrier. This results in an autostereoscopic 3D display arrangement.

According to the invention, there is provided a multiple view display comprising; a first liquid crystal device having a first asymmetric liquid crystal mode with first asymmetric viewing properties; a second liquid crystal device having a second asymmetric liquid crystal mode with second asymmetric viewing properties oriented differently from the first asymmetric viewing properties; and a driving arrangement for driving the first device with a first drive scheme for displaying a first image in a first viewing direction and for driving the second device with a second drive scheme for displaying a second image in a second viewing direction different from the first viewing direction.

The first and second images may be unrelated to each other.

The first and second drive schemes may comprise first and second voltage ranges, respectively. The first and second voltage ranges may be substantially the same.

The first and second directions may be in a plane which is orthogonal to a display surface of the display and contains directions of maximum viewing angle asymmetry of the first and second devices. The first and second directions may be on opposite sides of a normal to the display surface. The first and second directions may be substantially symmetrical about the normal or may be asymmetric about the normal.

The angle between the first and second directions may be greater than or equal to substantially 10 .

The second device may be viewable through the first device. The second device may be disposed between the first device and a backlight.

The first and second devices may be substantially parallel to each other.

Each of the first and second devices may have uniform alignment.

Each of the first and second devices may be a transmissive mode device.

The first drive scheme may be arranged to provide a first contrast ratio in the first direction and a substantially zero contrast ratio in the second direction and the second driving scheme may be arranged to provide a second contrast ratio in the second direction and a substantially zero contrast ratio in the first direction.

The first and second liquid crystal modes may be of the same type.

The first and second asymmetric viewing properties may be oriented in substantially opposite directions.

The first and second devices may have alignments which are oriented in substantially opposite directions.

At least one of the first and second liquid crystal modes may be one of twisted nematic, hybrid aligned nematic and twisted vertically aligned nematic.

At least one of the first and second devices may comprise sets of pixels with the pixels of each set being the same colour and being of a different colour from the pixels of the other sets. The at least one device may comprise a liquid crystal layer having different thicknesses at the pixels of different colours. The at least one device may comprise a patterned retarder with regions of different retardations being optically aligned with the pixels of different colours. The regions of different retardations may contain dyes of the different colours for acting as colour filters.

Each of the first and second devices may comprise sets of pixels of different colours.

One of the first and second devices may comprise sets of pixels of red, green and blue and the other of the first and second devices may comprise sets of pixels of cyan, magenta and yellow. Each of the first and second devices may comprise colour filter stripes extending parallel to a plane containing the first and second directions.

The display may comprise a multiple colour time-sequential backlight, the driving arrangement being arranged to drive the first and second devices colour time- sequentially.

The driving arrangement may be arranged to supply temporally multiplexed images to the first and second devices and to control synchronously a direction-switchable backlight.

Each of the first and second devices may comprise a spatial phase modulator.

It is thus possible to provide a multiple view display which makes use of asymmetrical viewing angle properties to allow unrelated images to be viewed in different viewing regions. It is unnecessary to use any parallax generating optic such as a parallax barrier or a lenticular screen, although it is possible to use a parallax optic to improve display contrast ratio for one liquid crystal device.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figures 1 to 3 are diagrams illustrating uses of a multiple view display; Figure 4 is a cross-sectional diagram of a display constituting an embodiment of the invention; Figure 5 is an exploded diagram illustrating the orientation of components of an example of the display of Figure 4; Figures 6 and 7 are graphs of transmittance against voltage for the LCDs of the display of Figure 4; Figure 8 is a graph of transmittance against voltage for a display of the type shown in Figure 4 but omitting a middle polariser; Figure 9 is a polar diagram illustrating display contrast ratio at different viewing angles; Figure to is a graph of intensity against viewing angle for different grey levels of the display of Figure 4; Figure 11 is a graph of intensity against grey level at two different viewing angles; Figure 12 is a graph of luminance against voltage illustrating two different voltage ranges; Figure 13 shows two graphs similar to that of Figure 12 illustrating driving schemes for the LCDs of the display of Figure 4; Figure 14 diagrammatically illustrates a display constituting an embodiment of the invention and having a single set of colour filters; Figure 15 diagrammatically illustrates a display constituting an embodiment of the invention and having two sets of colour filters; Figure 16 is a cross-sectional diagram of a display constituting another embodiment of the invention; Figure 17 illustrates an example of a pixel and colour filter arrangement; Figure 18is a cross-sectional diagram of a display constituting a further embodiment of the invention; Figure 19 illustrates diagrammatically the effect of display size and viewing distance on viewing angle at the edges of a display; Figure 20 illustrates an alternative mode of operation of the display of Figure 18; Figure 21 and 22 illustrate time-sequential modes of operation of the display of Figure 18; Figure 23 illustrates a higher resolution single view mode of operation of the display of Figure 18; Figure 24 illustrates a modified form of the mode illustrated in Figure 23; Figure 25 illustrates diagrammatically time sequential mode of operation; Figure 26 shows two graphs of transmittance against voltage for a display comprising two TVAN LCDs without a middle polariser; Figure 27 shows two graphs of transmittance against voltage for a display comprising two TVAN LCDs with a middle polariser; Figure 28 illustrates a display having a further time-sequential mode of operation; Figure 29 illustrates the use of angled or non-parallel LCDs for adjusting the angle between viewing directions; Figure 30 is a cross-sectional diagram of a display including a parallax barrier and constituting another embodiment of the invention; Figure 31 is a graph of intensity against grey level for different viewing angles and different pixel colours; Figure 32 is a graph of corrected grey level against image grey level illustrating the result of greyscale correction for different colours; Figure 33 is a cross- sectional diagram of a display constituting another embodiment of the invention; Figure 34 is a cross-sectional diagram of a display constituting a further embodiment of the invention; Figure 35 shows diagrammatic cross-sections illustrating a method of making a pixellated retarder; Figure 36 is a graph of transmission against voltage for two different viewing directions; and Figure 37 illustrates diagrammatically a time-sequential display constituting an embodiment of the invention.

Figure 4 illustrates a dual view display 10 comprising two thin film transistor (TFT) active matrix twisted vertically aligned nematic (TVAN) LCDs for directing images, which may be unrelated, in two views into viewing regions 1 and 2 for viewers 1 and 2, respectively, as illustrated in Figures I and 2. The first LCD comprises a front linear polariser 20 attached to or formed on the outer surface of a substrate 21. The substrate 21 may be made of glass or any suitable transparent non-birefringent material of sufficient stability. The substrate 21 carries on its inner surface a transparent electrode 22, for example made of indium tin oxide (ITO). The electrode 22 acts as a counter- electrode for the active matrix and uniformly covers the whole active area of the display 10. An aligning surface such as alignment layer 23, for example of rubbed polyimide, is formed on the electrode 22 and is uniformly rubbed so as to have the same uniform alignment direction throughout the active area of the display 10.

A second substrate 27 carries a middle linear polariser 28 and a TFT and electrode layer 26. The electrodes of the layer 26 are patterned so as to define pixels (picture elements). Such TFT and electrode arrangements are known and will not be described further. An aligning surface such as alignment layer 25, for example of rubbed polyimide, is formed on the layer 26. The alignment layer 25 also provides a uniform alignment direction throughout the active area of the display 10.

The substrates 21 and 27 are formed with the layers 22, 23, 25 and 26 and are brought together with the alignment layers 23 and 25 facing each other so as to define a liquid crystal cell with a liquid crystal layer 24 therebetween. The liquid crystal of the layer 24 is a nematic liquid crystal, for example comprising MJ97174 available from Merck UK. The polarisers 20 and 28 may be formed or provided before or after the liquid crystal cell is formed. The layers 22 and 26 include or are connected to a driving arrangement illustrated at 29 for providing the appropriate signals to address the individual pixels with voltages defining greyscales. The arrangement 29 may be formed entirely or partly of external components. Alternatively, the arrangement 29 may be integrated on the LCD, for example using continuous grain silicon.

The second LCD comprises a substrate 21', an electrode 22', an alignment layer 23', a liquid crystal layer 24', an alignment layer 25', a TFT and electrode layer 26', a substrate 27' and a polariser 28' of the same types as the corresponding components 21 to 28. The layers 22' and 26' are connected to the driving arrangement 29. The display is provided with a backlight 30, which may also be controlled by the driving arrangement 29.

The driving arrangement 29 provides first and second driving schemes for the first and second images which are to be displayed on the first and second LCDs. These driving schemes are described in more detail hereinafter.

Although the first and second LCDs have been illustrated as substantially independent devices which are joined together, the LCDs may be constituted by a single integral device. For example, the substrates 27 and 21' may be replaced by a single common middle substrate. Also, in some embodiments, the middle polariser 28 may be omitted.

Figure 5 illustrates diagrammatically in an exploded view the polarisers 20, 28 and 28' and the layers 23 to 25 and 23' to 25'. Figure 5 also illustrates the vertical direction for normal orientation of the display 10 as illustrated in Figure 1 and the horizontal direction. The vertical upward reference direction is referred to as 0 and the horizontal right direction as 90 . The various directions illustrated for the components shown in Figure 5 are referred to the upward vertical 0 direction.

The front polariser 20 has a transmission axis 31 oriented at an angle + 90 relative to the upward vertical. The alignment layer 23 has a uniform alignment direction 32 oriented at an angle +90 relative to the upward vertical direction. The alignment layer has a uniform alignment direction 33 oriented at 0 to the upward vertical direction.

The polariser 28 has a transmission axis 34 oriented at +180 to the upward vertical direction. The alignment layer 23' has a uniform alignment direction 35 oriented at +270 to the upward vertical direction. The alignment layer 25' has a uniform alignment direction 36 oriented at +180 to the upward vertical direction. The polariser 28' has a transmission axis 37 oriented at +90 to the upward vertical direction.

The arrangement illustrated in Figures 4 and 5 comprises two TVAN (twisted vertically aligned nematic) LCDs operating in a normally black mode of operation. In the absence of an applied voltage across either of the liquid crystal layers 24 and 24', the liquid crystal is aligned substantially homeotropically so that no light is transmitted. If both of the layers 24 and 24' have a sufficiently large voltage applied across them, light is transmitted. However, as the voltage across the liquid crystal layer 24 is reduced, the amount of light entering the other liquid crystal layer 24is progressively attenuated so that the range of grey levels which may be displayed is progressively reduced. A similar effect occurs when the voltage across the liquid crystal layer 24is reduced.

The effects of this are illustrated in Figures 6 and 7. Figure 6 illustrates light transmission through the liquid crystal layer 24 against applied voltage with the liquid crystal layer 24 being switched at a selection of voltages for a viewing angle direction of -30 to the normal to the display. Conversely, Figure 7 illustrates transmission through the liquid crystal layer 24' with the liquid crystal layer 24 being switched at a selection of voltages at a viewing angle direction of +30 .

Figure 8 illustrates the effect of removing or omitting the middle polariser 28. If a voltage is applied across one of the layers, such as the layer 24, with the other layer 24 having no applied voltage, the display behaves as a normally white TVAN LCD. When a voltage above the threshold for switching is applied to the layer 24, the display increasingly resembles a normally black TVAN LCD. Figure 8 illustrates this in terms of transmission through the liquid crystal layer 24 against voltage with the layer 24' being switched at a selection of voltages for a viewing angle of -300.

Techniques for overcoming or reducing the problem of the switching of one liquid crystal layer affecting the light available through the other liquid crystal layer are described hereinafter.

Figure 9 illustrates the variation in contrast ratio at different viewing angles of a single LCD similar to those illustrated in Figures 4 and 5 but operating in the twisted nematic (TN) mode. The display 10 is effectively rotated through 90 compared with the typical orientation of a TN LCD so that the asymmetric viewing angle direction is substantially horizontal. In this example, the display is arranged for viewing at -30 and +30 on either side in the horizontal plane of the display normal.

The viewing angle characteristics of the LCD are illustrated in Figure 10 with the characteristics at viewing angles of -30 and +30 being emphasised. The LCD is of the type in which discrete grey levels O to 255 are addressable and Figure lO illustrates the intensities of a selection of the grey levels at various viewing angles and for a conventional drive scheme intended to provide substantially evenly spaced grey levels when the LCD is viewed on-axis.

Figure 11 illustrates the transmission as intensity against grey level for the -30 and +30 viewing directions. For example, when the grey level 96 is being displayed, those pixels which are displaying this level will appear substantially black from the +30 viewing region but will appear just below half maximum brightness when viewed from the -30 viewing region. By suitably selecting the voltage levels used to select the grey levels in accordance with a drive scheme, it is possible to display an image which is substantially only visible in a defined viewing region. The drive scheme is chosen such that it creates a contrast ratio adequate for displaying the image in the intended viewing direction and a very low or substantially zero contrast ratio in another viewing region.

Suitable drive schemes are illustrated diagrammatically in Figure 12, which illustrates luminance (in transmission mode) against the voltage applied to the pixels for selecting the grey levels to be displayed when viewed from -30 and +30 . By using the image l voltage range illustrated at 40, the image is visible in the -30 viewing direction but the pixels appear black in the +30 viewing region. Alternatively, by using the image 2 voltage range 41, the image is visible in the +30 viewing region whereas the pixels appear white in the -30 viewing direction.

Figure 13 illustrates drive schemes which may be used for the two LCDs forming the display as shown in Figure 4. The drive scheme for the upper LCD including the LC layer 24 is shown at 42 whereas that for the lower LCD including the LC layer 24' is shown at 43. The LCDs are of the same type and are substantially identical to each other but the lower LCD is effectively rotated through 180 with respect to the upper LCD. Thus, the alignment direction of the alignment layers 23' and 25' are rotated by i 13 180 with respect to (or opposite) the alignment directions of the alignment layers 23 and 25, respectively.

The voltage range 40 may be used as the drive scheme for each of the LCDs. Thus, as shown at 42, the upper LCD can be used to display an image by suitably mapping the voltages in the range 40 to the appropriate grey levels and the resulting image is visible in the -30' viewing direction. However, the pixels of the upper LCD appear substantially black with substantially zero contrast ratio when viewed in the +30 direction.

The same drive scheme is shown at 43 but with the viewing directions corresponding to the viewing directions of the lower LCD. Thus, in the + 30 viewing direction, the lower LCD displays an image which is visible to a viewer. However, the lower LCD appears substantially black with substantially zero contrast ratio in the -30 viewing direction.

The display 10 thus creates two viewing regions for viewers to view unrelated images or sequences of images in their respective viewing regions without requiring any parallax optic and without the use of multidomain liquid crystal techniques with different alignments for pixels displaying different images. Such a display is therefore easier and cheaper to manufacture. Techniques for overcoming or reducing any problem of the switching of one liquid crystal layer affecting the light available through the other liquid crystal layer are described hereinafter.

As an alternative to the drive scheme described above, the voltage range 41 may be used for each of the LCDs. In that case, the upper LCD displays an image which is visible in the +30 viewing direction but appears bright or "white" in the -30 viewing direction.

Conversely, the lower LCD displays an image which is visible in the -30 viewing direction but appears white in the +30 viewing direction.

Because of the use of two "stacked" or "sequential" LCDs in the light path through the display 10, consideration must be given to the use of colour filters in order to provide a colour display. If only one of the LCDsis provided with colour filters, then light from the other LCD must pass through the colour filters in the correct way in order to avoid incorrectly colouring the image which it displays. For example, Figure 14 illustrates diagrammatically the liquid crystal layers 24 and 24' with the finite liquid crystal layer separation 44. The LCDs are pixellated with the same pixel pitch, for example as shown at 45. The display is of the type in which the first and second viewing angles are substantially the same and are defined by a minimum angle and a corresponding maximum angle, a and b respectively. In order to ensure that light passes along the correct light paths 46 and 47 through the display, absorbing regions illustrated as parts of black masks 48 and 49 are provided so as to block at least some incorrect light paths.

Figure 15 illustrates an embodiment in which both of the LCDs have colour filters. The filters for each colour are illustrated by the same cross hatching at the liquid crystal layers 24 and 24'. Light passing through the pixel 50 is encoded with the corresponding colour and is incident on the colour filters of the upper LCD. Depending on the colour of the colour filter on which it is incident, the light will either be transmitted or absorbed. Figure 15 illustrates the relative positions of the colour filters for the two LCDs with the pixels 51 and 52 having filters of the same colour as the filter of the pixel 50.

As in Figure 14, Figure 15 illustrates a primary angular range through the pixel 51 for light from the pixel 50. This may be approximately defined by the relationship between the pitch of the colour pixels of the layers 24 and 24' and the separation 44 between the liquid crystal layers of the LCDs. The separation 44 is important because light must be able to reach the layer 24' in order to be encoded with data to form both images to be displayed. This therefore provides a constraint on the separation 44 between the layers for a given colour pixel pitch. In general, for practical purposes, the separation 44 is required to be relatively small and, may be smaller than would normally be possible for conventional LCD components as illustrated in Figure 4.

In order to reduce the thickness, the two substrates 27 and 21' may be replaced by a common substrate as illustrated at 55 in Figure 16 for an embodiment in which there is no middle polariser. The common substrate 55 is processed on both sides to provide the 15 alignment and electrode layers 25, 26, 22', 23'. The colour filters for the two LCDs are illustrated at 56 and 57.

Alternatively or additionally, the middle substrate or substrates may be formed from very thin glass. A suitable glass is known as "microsheet" and is available from Schott, Germany.

There are various different pixel configurations which allow flexibility in the angles with which light of a certain colour may pass through both LCDs. For example, the configuration illustrated in Figure 15 may be used. For a pixel pitch of 200 micrometres and a liquid crystal layer separation 44 of 700 micrometres, light within the angular range in air from about 12 to about 61 passes through colour filters of the same colour in the layers 24 and 24'. There is a secondary angular range where light from the pixel 50 would be passed by the pixel 52 of the same colour but this light is totally internally reflected at the glass: air interface. By taking into account the effect of interfaces between media of different refractive indices, it is possible to produce an arrangement in which light which is not in the desired primary angular range is totally internally reflected so as to avoid undesirable visual effects.

In the arrangement illustrated in Figure 15, each of the liquid crystal layers is provided with the same type of filtering, for example so that each layer has red, green and blue filters. However, the LCDs may be provided with different coloured filtering so that, for example, the filters for one of the LCDs may comprise red, green and blue filterswhereas the filters for the other LCD may comprise cyan, magenta and yellow filters.

Figure 17 illustrates another form of colour filtering in which the individual colour filters comprise horizontal stripes extending throughout the width of the display. For example, as shown in Figure 17, the colour filters comprise repeating groups of red 60, green 61, and blue 62 filters. The individual colour filters for the two LCDs are aligned with each other such that the red filter for the upper LCD is directly above the red filter for the lower LCD. Such an arrangement therefore permits a wider range of viewing angles in the horizontal plane relative to the display for transmission of light but does not itself prevent propagation of light in inappropriate angles.

In order to avoid having to consider colour filter geometries, the display may be embodied using time sequential colour techniques. In this case, no colour filtering is necessary because the LCDs display different colour component images in consecutive time frames or slots and a switched multi-colour backlight is used. For example, in order to achieve a sufficiently high frame rate such that the colour component images are fused by the human vision into a full colour image, coloured light emitting diodes may be used to form the backlight with red, green and blue diodes being actuated cyclically.

Alternatively, a colour wheel arrangement and white light source may be used for this purpose. Such an arrangement is illustrated in Figure 37 and comprises display components 20 to 28, 21'to 28' with a backlight in the form of a white light source 30a and a colour wheel Job. The wheel 30b comprises three colour filters in the shape of equal-sized sectors and the wheel is arranged to rotate so that each colour filter in turn passes between the light source 30a and the remainder of the display.

As a further alternative, in order to avoid colour breakup issues which may be associated with time-sequential colour displays, scrolling bands of colour may be used.

Such an arrangement is disclosed, for example, in Katoh et al, "A Novel High- Definition Projection System using Single CG-Silicon TFT-LCD and an Optical Image Shift Device", LN-S, Eurodisplay 2002.

In such time sequential colour embodiments, because only one colour is being displayed in each time slot, the gray-scale correction which is used in each time slot may be different for each colour so as to compensate for colour dispersion in the LCDs.

With the embodiments described hereinbefore, consideration has to be given to the effect of the two LCDs variably attenuating light passing through the display with the two images. In order to reduce or avoid this difficulty, the two LCDs may be arranged B 17 to act as two spatial phase modulators between polarisers as illustrated in Figure 18. In this arrangement, the LCDs comprise spatial phase modulators SPM 70 and 71 disposed between polarisers 20 and 28' with the modulators 70 and 71 being pixellated lo provide pixels illustrated at a to l. The display can then be controlled so as to provide spatial phase modulation and hence intensity modulation of light passing through the display in first directions, such as R1 and R2, and in second directions, such as L1 and L2. In particular, by appropriately controlling the modulators 70 and 71, light propagating in the first and second directions can be modulated independently of each other. Both images are formed with the same spatial resolution as the spatial resolution of each of the modulators 70 and 71.

Light propagating in the direction L1 for the "left" image and passing through the pixel b also passes through the pixel c. The resulting intensity is a combination of the phase changes induced by the pixels b and c. Similarly, light propagating in the direction L2 has an intensity which results from the combination of the phase changes induced by the pixels d and e. For the "right" image, light propagating in the directions R1 and R2 is affected by the combination of phase changes in the pixels b and a and the pixels d and c, respectively. Each pixel is capable of providing a range of phase changes spanning substantially 180 .

It is necessary to select a boundary condition for one direction for either the first or last pixels of each modulator 70 and 71. This allows the individual image pixel intensities to be controlled independently of each other by solving a sufficiently large set of simultaneous equations. For example, for the propagation paths L1, L2, R1 and R2 illustrated in Figure 18, the light intensities (I) produced may be expressed as the following simultaneous equations; l(R1)=C(A(a)+A(b)) I(R2)=C(A(c)+A(d)) J(L1)=C(A(b)+A(c)) I(L2)=C (A (d) +A (e) ) where A is the phase change produced by a respective pixel and C is a constant. Values of the phase changes can be found such that the intensities of the two images being displayed can be selected independently of each other.

The above set of simultaneous equations comprises four equations with five variables.

However, in the case where pixel a is an end pixel of the modulator 70 and is required to change the phase of light passing in only one direction, unlike for example the pixel c which controls the phase of light passing in two directions, the pixel a may be set to a (boundary) value for the left image, for example providing a notional black pixel for the left image. In this case, the phase change A(a) of the pixel a may be set to no where n is an integer. This provides a fifth equation and so enables the set of simultaneous equations to be solved for the five variables.

The phase change is preferably optimised for the angle at which light passes through the liquid crystal layers in order to take into account the viewing angle dependence of these layers. The phase change is preferably also optimised for the wavelength of light passing through the liquid crystal layers. This may be the same or may be different for each LCD which is used. The performance is preferably optimised for a particular angular viewing position and will be substantially correct for a range of viewing angles.

When a dual view display is being viewed, the size of the display and the distance to the viewer affect the angles which the extreme edges of the display make with the viewer's eyes as illustrated in Figure 19. In the case of a display 72 which is 8 centimetres wide and for a viewing distance of 30 centimetres, assuming a fixed observer, the angular range over which the gray-scale correction is required to operate is from 23 to 35 .

However, for a display 73 which is 30 centimetres wide and for a viewing distance of centimetres, the angular viewing range increases to 16 to 41 . As the angular range increases, software image correction techniques may be used to prevent changes from being observed at the extremes of the image.

The separation between viewing regions may also be changed by changing the distance between the LCDs.

The pitch of the pixels may be adjusted to arrange for the viewing windows to be correctly formed.

There may be opaque regions of "black mask" between pixels of the LCDs. Increasing the width of the black mask may be used to increase the size of the viewing windows such that that angular range over which incorrect image data is displayed may be I substantially blocked.

Use may be made of different combinations of pixels to alter the angle between the viewing directions for the two views being displayed, for example as illustrated in Figure 20. In order to increase the angular separation of the viewing directions, each pixel of each modulator does not cooperate with the nearest pixels of the other modulator but, instead, with the next nearest pixels. Thus, the intensity of one pixel of: the right image is determined by the effects of the pixels c and f, as opposed to c and d as illustrated in Figure 18. In fact, switching between different angles of separation of the views may be performed electronically by changing the angular range for which the modulators 70 and 71 are optimised.

Figure 21 illustrates how a wider angular range may be achieved by the use of time multiplexing and a directional backlight. The directional backlight (not shown) is such that it provides illumination in only certain different angular ranges at different times.

For example, during time frame 1 as illustrated in the upper part of Figure 21, the backlight is set so that it only illuminates views in the directions of the right image 1 and the left image 2. During this frame, the modulators 70 and 71 operate in accordance with the operation illustrated in Figure 20. During the next time frame 2, the backlight is set to illuminate views only in the directions of the right image 2 to and the left image 1. During this time frame, the modulators 70 and 71 operate as illustrated in Figure 18. ; Such an arrangement allows four views to be shown in different viewing directions.

This may be used to provide a multiple view display or may be used to form a dual autostereoscopic 3D display. As a further alternative, such an arrangement may be used to increase the viewing range of a dual view display where the right images 1 and 2 are the same and the left images 1 and 2 are the same.

As an alternative, the views may be displayed in a different order, for example as illustrated in Figure 22.

Such an arrangement may be used in an alternative high resolution single view mode as illustrated in Figure 23. Light for each image pixel passes through a pixel in each of the modulators 70 and 71. Apart from boundary pixels of the modulators 70 and 71, each pixel modulates light for two image pixels. Simultaneous equations as described hereinbefore can be set up and solved so that all of the image pixels can be controlled independently of each other. A viewer viewing the display from within an angular range of normal incidence sees an image which has twice the spatial resolution of each of the modulators 70 and 71.

Correct viewing of the image will occur within a certain angular range, for example around normal incidence viewing of the display. In order to increase the angular range, a scatterer or defuser 75 may be disposed in front of the display as illustrated in Figure 24. The scatterer 75 may be switchable into a non-scattering mode to return to a multiple view mode of operation.

Figure 25 illustrates an alternative time sequential switching mode of operation which does not require simultaneous equations to be solved. In each time frame, one of the LCDs displays an image whereas the other LCD is switched to a uniform configuration, such as a uniform retarder or a homeotropic layer. Each LCD displays its image in alternate time frames. Thus, as illustrated in Figure 25, in odd-numbered time frames, one of the LCDs displays an image which is visible in a first viewing direction but appears black in the second viewing direction. In the even-numbered time frames, the other LCD displays an image which is visible in the second viewing direction but appears black in the first viewing direction. By choosing a sufficiently high frame rate, the images are fused by a viewer in each viewing direction.

The LCDs may or may not have a polariser therebetween. The LCDs may operate in liquid crystal modes such as normally black or normally white TN or normally white or normally black TVAN (twisted vertically aligned nematic). For example, Figure 26 illustrates greyscales as transmission against applied voltage for different viewing angles for TVAN LCDs with no polariser and with 90 twist. Figure 27 illustrates the effect of a polariser between the 90 twisted TVANs.

Consideration should be given so as to avoid situations in which there is a polariser between the LCDs and one of the LCDs is switched such that it is not transmitting sufficient light for the image produced by the other LCD to be visible.

In another single view mode of operation, for substantially normal incidence viewing, one of the LCDs may remain switched so as to act as a uniform layer and the image may be displayed by the other LCD. The uniform layer may have a substantially planar alignment, in which case the LCD acts as a uniform retarder, may be substantially homeotropic, or may be switched to a state between these two. Figures 26 and 27 illustrate the performance where the LCD not displaying the image is substantially homeotropic for the curve marked 0 .

When using the display in a single view time sequential mode, the plane in which the image is produced is different for the two LCDs forming the display. This results in parallax between the images as a viewer changes position. This problem may be overcome or reduced by using a switchable scatterer as described hereinbefore or by using switchable lenses as illustrated in Figure 28. The display shown in Figure 28 comprises a backlight in the form of a light source 80 and a collimator 81. A switchable lens arrangement 82 is disposed between the collimator 81 and the LCDs 70 and 71 and is switchable between an inactive mode in a first time frame and an active mode in a second time frame as illustrated in the left and right parts, respectively, of Figure 28. In the inactive mode, the lens arrangement 82 is index-matched to an adjacent medium so as to have substantially no effect on the collimated light from the collimator 81. In the active mode, the index-matching is disabled so that the lenses act as converging lenses.

In the first time frame, the LCD 70 displays the image for normal incidence viewing whereas the LCD 71 acts as a uniform retarder. In the second time frame, the lenses 82 are activated and focus the pixels of the LCD 71 to the same plane as that to which the pixels of the LCD 70 were focused in the previous timeframe. The LCD 70 now acts as a uniform retarder. The images are therefore formed in the same plane for both types of time frame so as to avoid parallax problems.

Figure 29 illustrates at 83 the LCDs 70 and 71 being disposed substantially parallel to each other as in the previously described embodiments. For individual LCDs which have a relatively large optimum viewing angle, such as 30 , from the normal to the device, this provides a relatively large angular separation between the viewing angles.

However, where it is desired to use LCDs of this type to provide a smaller difference between the optimum viewing angles, the LCDs may be oriented non-parallel to each other as illustrated at 84 in Figure 29. In this example, the LCDs 70 and 71 are rotated by 5 in opposite directions in a horizontal plane towards the respective normal. Thus, the angle between optimum viewing directions is reduced from 60 to 50 .

Figure 30 illustrates a dual view display which may be used to form two pairs of autostereoscopic images in the first and second viewing directions so that the display functions as a dual autostereoscopic display. The display differs from the embodiments described hereinbefore mainly or only in that a parallel barrier 85 is disposed between the LCDs. The parallax barrier 85 acts as a rear parallax barrier for the LCD comprising the components 21, 24 and 27 and as a front parallax barrier for the LCD comprising the components 21', 24' and 27'. Appropriate values are chosen for the parallax barrier pitch and for the spacing of the parallax barrier 85 from each of the pixel planes at the liquid crystal layers 24 and 24'. Different pixel pitches may be needed for the LCDs. Operation of front and rear parallax barrier autostereoscopic displays is well-known and will not he described further.

In order to be able to provide a full colour LCD, the effect of colour on the LC mode has to be taken into consideration when selecting the drive schemes for the first and second images. In a typical colour display, colour filters filter the light from respective sets of pixels. The colours may be red, green and blue or cyan, magenta and yellow.

Because of the dispersion of the liquid crystal layer 24, the optical properties of the liquid crystal mode used in the LCD vary with the wavelength of light. For example, Figure 31 illustrates the variation for the -30 and +30 viewing directions for red, green and blue pixels as intensity against grey level for the individual colours.

In order to produce a good colour dual view display, the effects of the dispersion of the liquid crystal mode on the grey scale curves may be overcome by performing careful mapping of grey levels individually for each of the colour components. Figure 32 illustrates the results of such a mapping procedure to allow the same grey level to be selected and displayed for each colour. Thus, the intensity of the image is the same for a given grey level irrespective of which colour is displaying the image.

The colour mapping may be chosen in accordance with the application of the display.

For example, for some applications, it may be desirable to have a higher relative intensity of green light in either or both images relative to red and/or blue. The mapping may be selected in order to take account of such requirements.

Figure 33 illustrates an alternative LCD arrangement for displaying colour images. The LCD of Figure 33 differs from each LCD of Figure 4 in that colour filters 86 are provided on the inner surface of the substrate 21, 21' and the liquid crystal layer thickness 87 is different for different colour pixels.

The optical properties of the pixels are determined by the retardation of the liquid crystal layer 24, 24' at the pixel. The retardation is the product of the birefringence and the thickness 87 of the liquid crystal layer 24, 24'. Thus, by varying the thickness of the layer 24 for the different colour pixels, each pixel can have its properties optimised or, at least, improved for the colour or range of colours which it is to display.

Alternatively, liquid crystal materials with different birefringence values may be used, separated by polymer walls. The retardation is matched to the wavelength of the respective colour pixel.

The LCD of Figure 33 has discrete stepped thicknesses for the pixels of different colours. In this particular example, this is achieved by forming polymer steps such as 88 on the TFT substrate 27, 27' with the alignment layer 25, 25' being formed on top of the steps. Such steps may be formed on the other substrate beneath the alignment layer or on both substrates. The steps 88 may be formed by photolithographic processing of suitable resist materials. Alternatively, the steps 88 may be formed by screen printing i of suitable polymer materials directly onto the or each substrate. In a further alternative, the colour filters 86 may have stepped thicknesses. In another example, the variation in liquid crystal layer thickness may be achieved using wedge-shaped structures of suitable gradient or similar structures without sharp edges so as to reduce the effects of any misalignment of the LC caused by the sharp edges of the steps.

Figure 34 illustrates another technique for compensating for the liquid crystal dispersion. In this display, a pixellated retarder 90 is provided. Each "pixel region" of the retarder provides an amount of retardation which substantially compensates for the l dispersion effect of the liquid crystal of the associated pixel. In order to reduce parallax, the pixellated retarder 90 is disposed between the substrates 21, 21' and 27, 27'. In Figure 34, the retarder 90 is shown as being located on the TFT substrate 27, 27' but the retarder could alternatively be located on the colour filter substrate 21, 21' Various techniques are available for making the pixellated retarder 90. Examples of such techniques are disclosed in van der Zande et al, Sid 03 Digest, "Technologies towards Patterned Optical Foils", pp 194-197. A specific example of a suitable technique is illustrated in Figure 35 and makes use of polymerisable liquid crystals such as reactive mesogens, an example of which is RMM 34 available from Merck UK.

A substrate 93 is prepared with an aligning surface such as an alignment layer 92 for aligning the optic axis of the reactive mesogen. The reactive mesogen is then coated on the alignment layer 92 by any suitable technique, such as spin coating. The reactive mesogen is of the type whose birefringence when unpolymerized varies with temperature and which is polymerised so as to fix the orientation of the optic axis and the birefringence upon exposure to light, such as ultraviolet light.

In order to form the regions of the retarder for a first colour, the layer 91 is exposed through a photomask 92 to ultraviolet radiation with the layer 91 being maintained at a suitable temperature for controlling its birefringence. The first regions are those which require the largest retardation and hence birefringence.

Following the first ultraviolet polymerization, the layer 91 is heated to a second temperature in order to provide the desired birefringence for second regions of the finished retarder. This is illustrated at 95 in Figure 35. The birefringence of the unpolymerized reactive mesogen is thus reduced to a desired value, after which the second regions for the next colour are exposed to ultraviolet radiation through a second photomask 96. The second regions are thus polymerized and their properties are fixed.

The temperature is then increased again as illustrated at 97 so as to reduce the birefringence of the remaining unpolymerized regions, which are polymerized by exposure to ultraviolet radiation through a third mask 98. The retarder is then effectively ready for use and may be removed from the substrate 93 and the alignment layer 92 for inclusion in a display device of the type shown in Figure 34. Alternatively, the retarder may be made directly on the TFT substrate as illustrated at 99 in Figure 35 with the alignment layer being formed on its upper surface, before being used with the upper substrate and its associated layers to form the liquid crystal cell.

It may also be possible to add suitable dyes to the polymerisable liquid crystal in order to form the colour filters for the display. In such cases, the retarder and colour filters can be made in a single layer so as to reduce the number of layers required in the display, which simplifies manufacture and reduces the number of alignment steps which are necessary.

The viewing angle characteristics of any of the polarisers 20, 28, 28' may be chosen or optimised for non-normal incidence of light so as to improve performance and, in particular, image quality in the viewing regions of the display.

The voltages used for addressing the grey levels for the different views may be selected or optimised by techniques similar to gamma correction which is well known in the technical field and which is described, for example, in "Frequently Asked Questions about Gamma" by Charles Poynton available at www.inforamp.net/poynton/. This may be done by first remapping the grey levels in the original image of each view to the grcy levels which are visible from the viewing region for that view. The remapping may be to a single linear range or may be to two or more ranges of intensity level.

Adjustments are then made in accordance with the appearance of the image from its viewing region so as to improve the appearance of the image. This may be performed by suitable gamma correction of the image data. This type of correction may not take the greyscales outside the existing greyscale range for that view. Any suitable gamma value may be used according to the effect which it is desired to achieve. For example a value of 2.2 may be used or a lower value of gamma correction may be used, such as 1.7. Applying gamma correction to images after they have been colour- corrected is unlikely to produce a good result. Gamma correction may therefore be applied to the colour correction curve or to the original image before colour correction.

Other image intensity adjustment techniques may be used, such as "histogram equalization".

The grey level ranges of the pixels for the first and second viewing regions may be suitably chosen or optimised. For example, the grey level range may be chosen so as to give good image quality in the viewing region from which the image is to be viewed, for example with maximal contrast ratio throughout the greyscale range, and to give the best grey level state when the pixel is viewed from the or each other viewing range, which level may be substantially black or substantially white.

The voltage range selected for operating each LCD of the dual view display is dependent on the properties of the liquid crystal mode used. For example, each LCD may be driven with a different voltage range. Alternatively, both LCDs may be driven with substantially the same voltage range. The appropriate voltage ranges for dual-view operation and, where appropriate, for single view or dual autostereoscopic view modes are provided by the driving arrangement 29.

Although the embodiments described hereinbefore are based on TN and TVAN modes, any liquid crystal mode which produces the appropriate asymmetric viewing angle may be used. For example, suitable smectic or ferroelectric liquid crystal modes may be used. Also, other twisted nematic modes may be used, such as the hybrid aligned nematic (HAN) mode. The twisted vertically aligned nematic (TVAN) mode, for example as disclosed in EP 1103840, has a substantially untwisted vertical structure with respect to the substrates below the threshold voltage for switching. Above this threshold voltage, this mode progressively switches to a more planar twisted structure, which is similar to that of the twisted nematic mode below its threshold voltage. An example of the transmission voltage characteristic of a TVAN mode liquid crystal device for -30 and +30 viewing angles is illustrated in Figure 36 and such a mode may be used in either or both of the LCDs of the dual view display.

With the use of a suitable liquid crystal mode, each LCD of the display may have a different viewing angle. This may be tuned for each specific application of the display.

This provides increased flexibility in achieving and tuning the viewing angle characteristics. For example, each view may be optimised for a different angle and this may be preferable in some applications such as automotive applications. With a suitable liquid crystal mode, by adjusting the gray-scale range which is used, the viewing angle which is optimised may be adjusted to be optimum for a particular viewer.

Viewing angle compensation films are well known for optimising the characteristics of the liquid crystal mode used, in particular the viewing angle characteristics of the black/white and grey level states. In the prior art, such films are used to produce a viewing angle characteristic which is as uniform as possible with respect to vertical and horizontal directions, as well as grey levels.

For dual view displays, the objective is not to obtain uniform viewing angle. The requirements for a dual view display are to have separation of the images between the first viewing direction, for example the +30 direction, and the second viewing direction, for example the -30 direction. A combination of liquid crystal mode in each LCD with or without viewing angle compensation may be chosen so that, for both viewing directions, it is possible to produce grey scale characteristics which produce an image grey level for one view while producing black for the other view for all grey levels.

The very different requirements for the grey scale viewing angle characteristics of the LC mode require very different optimization from a standard 2D panel and so very different design for the viewing angle compensation films. The viewing angle compensation film will work such that, from the first viewing direction, the pixels which contain the image for the other view are made as black as possible while, for the pixels which contain the image for this view, the image is optimised. The opposite will be required from the second viewing position.

Additionally, as described hereinbefore, retarder layers may be used toimprove the chromatic properties of the image. The retarder layer may be uniform or patterned. The retarder may be made from a liquid crystal polymer or polymerisable liquid crystal, such as a reactive mesogen. The patterned retarder may be used inside the switching liquid crystal cell cavity. This substantially eliminates parallax effects between any patterned retarder and the LC pixels. Alternatively, a thin substrate may be used between the patterned retarder and the liquid crystal. Alternatively, the retarder may be an external layer to the switching liquid crystal. Alternatively, a uniform retarder layer may be laminated or fixed onto the outside of the switching liquid crystal cavity.

Claims

CLAIMS: 1. A multiple view display comprising: a first liquid crystal

device having a first asymmetric liquid crystal mode with first asymmetric viewing properties; a second liquid crystal device having a second asymmetric liquid crystal mode with second asymmetric viewing properties oriented differently from the first asymmetric viewing properties; and a driving arrangement for driving the first device with a first drive scheme for displaying a first image in a first viewing direction and for driving the second device with a second drive scheme for displaying a second image in a second viewing direction different from the first viewing direction.
2. A display as claimed in claim 1, in which the first and second images are unrelated to each other.
3. A display as claimed in claim 1 or 2, in which the first and second drive schemes comprise first and second voltage ranges, respectively.
4. A display as claimed in claim 3, in which the first and second voltage ranges are substantially the same.
5. A display as claimed in any one of the preceding claims, in which the first and second directions are in a plane which is orthogonal to a display surface of the display and contains directions of maximum viewing angle asymmetry of the first and second devices.
6. A display as claimed in claim 5, in which the first and second directions are on opposite sides of a normal to the display surface.
7. A display as claimed in claim 6, in which the first and second directions are substantially symmetrical about the normal.
8. A display as claimed in claim 6, in which the first and second directories are asymmetric about the normal.
9. A display as claimed in any one of the preceding claims, in which the angle between the first and second directions is greater than or equal to substantially 10 .
10. A display as claimed in any one of the preceding claims, in which the second device is viewable through the first device.
11. A display as claimed in claim 10, in which the second device is disposed between the first device and a backlight.
12. A display as claimed in any one of the preceding claims, in which the first and second devices are substantially parallel to each other.
13. A display as claimed in any one of the preceding claims, in which each of the first and second devices has uniform alignment.
14. A display as claimed in any one of the preceding claims, in which each of the first and second devices is a transmissive mode device.
15. A display as claimed in any one of the preceding claims, in which the first drive scheme is arranged to provide a first contrast ratio in the first direction and a substantially zero contrast ratio in the second direction and the second driving scheme is arranged to provide a second contrast ratio in the second direction and a substantially zero contrast ratio in the first direction.
16. A display as claimed in any one of the preceding claims, in which the first and second liquid crystal modes are of the same type.
17. A display as claimed in any one of the preceding claims, in which the first and second asymmetric viewing properties are oriented in substantially opposite directions.
18. A display as claimed in any one of the preceding claims, in which the first and second devices have alignments which are oriented in substantially opposite directions.
19. A display as claimed in any one of the preceding claims, in which at least one of the first and second liquid crystal modes is one of twisted nematic, hybrid aligned nematic and twisted vertically aligned nematic.
20. A display as claimed in any one of the preceding claims, in which at least one of the first and second devices comprises sets of pixels with the pixels of each set being the same colour and being of a different colour from the pixels of the other sets.
21. A display as claimed in claim 20, in which the at least one device comprises a liquid crystal layer having different thicknesses at the pixels of different colours.
22. A display as claimed in claim 20 or 2l, in which the at least one device comprises a patterned retarder with regions of different retardations being optically aligned with the pixels of different colours.
23. A display as claimed in claim 22, in which the regions of different retardations contain dyes of the different colours for acting as colour filters.
24. A display as claimed in any one of claims 20 to 23, in which each of the first and second devices comprises sets of pixels of different colours.
25. A display as claimed in claim 24, in which one of the first and second devices comprises sets of pixels of red, green and blue and the other of the first and second devices comprises sets of pixels of cyan, magenta and yellow.
26. A display as claimed in claim 24 or 25, in which each of the first and second devices comprises colour filter stripes extending substantially parallel to a plane containing the first and second directions.
27. A display as claimed in any one of claims 1 to 19, comprising a multiple colour time-sequential backlight, the driving arrangement being arranged to drive the first and second devices colour time-sequentially.
28. A display as claimed in any one of the preceding claims, in which the driving arrangement is arranged to supply temporally multiplexed images to the first and second devices and to control synchronously a directionswitchable backlight.
29. A display as claimed in any one of the preceding claims, in which each of the first and second devices comprises a spatial phase modulator.