GB2406731A

GB2406731A - Multiple view display having directional backlight

Info

Publication number: GB2406731A
Application number: GB0401126A
Authority: GB
Inventors: Jonathan Mather; Emma J Walton; Grant Bourhill; Diana Ulrich Kean; Neil Barratt
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2003-08-30
Filing date: 2004-01-20
Publication date: 2005-04-06
Also published as: GB0401126D0

Abstract

A display for displaying three dimensional (3D) stereoscopic or multiple directional images comprises a dual view display 19 which may be an LCD and a directional backlight or illumination arrangement 20. Back lighting system 20 comprises a partially collimated backlight source 21 and a prism structure 22 directing outputted light into different directions such that an image may be viewed in, for example, two positions at various angles. As shown in more detail in Figure 6b, the illumination source 21 may comprise a rear reflector (26), light source and waveguide arrangement (23a, 25), and collimating prism structures (27, 28). The multiple direction display may be arranged to direct no light into a region between imaging viewing directions, i.e. provide a "black window" normal to the plane of the display between dual image regions. A parallax barrier may further prevent the projected images being observed in certain regions, and pixel colour filters may be provided.

Description

A multiple view display and a multi-direction display The present

invention relates to a multiple-view directional display, which displays two or more images such that each image is visible from a different direction. Thus, two observers who view the display from different directions will see different images to one another. Such a display may be used in, for example, an autostereoscopic 3-D display device or a dual view display device. The invention also relates to a multi- direction display for displaying a single image at a time so as to be visible throughout a range of directions including first and second ranges for first and second viewers.

One application of a multiple-view directional display is as a 'dual-view display', which can simultaneously display two or more different images, with each image being visible only in a specific direction - so an observer viewing the display device from one direction will see one image whereas an observer viewing the display device from another, different direction will see a different image. A display that can show different images to two or more users provides a considerable saving in space and cost compared with use of two or more separate displays.

A further application of a multiple view directional display is in producing a three- dimensional image. In normal vision, the two eyes of a human perceive views of the world from different perspectives, owing to their different location within the head.

These two perspectives are then used by the brain to assess the distance to the various objects in a scene. In order to build a display which will effectively display a three dimensional image, it is necessary to recreate this situation and supply a so-called "stereoscopic pair" of images, one image to each eye of the observer.

Three dimensional displays are classified into two types depending on the method used to supply the different views to the eyes; Stereoscopic displays typically display both of the images over a wide viewing area. However, each of the views is encoded, for instance by colour, polarization state or time of display, so that a filter system of glasses worn by the observer can separate the views and will only let each eye see the view that is intended for it.

Autostereoscopic displays require no viewing aids to be worn by the observer.

Instead, the two views are only visible from defined regions of space. The region of space in which an image is visible across the whole of the display active area is termed a "viewing region". If the observer is situated such that the left eye is in the left image viewing region and the right eye is in the right image viewing region, then a correct set of views will be seen and a three-dimensional image will be perceived.

For flat panel autostereoscopic displays, the formation of the viewing regions is typically due to a combination of the pixel structure of the display unit and an optical element, generically termed a parallax optic. An example of such an optic is a parallax barrier, which is a screen with vertical transmissive slits separated by opaque regions.

This screen can be set in front of a spatial light modulator (SLM) with a two dimcnsional array of pixel apertures as shown in figure 1. The pitch of the slits in the parallax barrier is chosen to be close to an integer multiple of the pixel pitch of the SLM so that groups of columns of pixels are associated with a specific slit of the parallax barrier. Figure 1 shows an SLM in which two pixel columns are associated with each slit of the parallax barrier.

The display shown in figure 1 comprises an SLM in the form of a liquid crystal device (LCD) having an active matrix thin film transistor (TFT) substrate 1 and a counter substrate 2, between which are disposed a liquid crystal layer forming a picture element (pixel) plane 3 with associated electrodes and alignment layers (not shown) as appropriate. Viewing angle enhancement films 4 and polarisers 5 are provided on the outer surfaces of the substrates 1 and 2 and illumination 6 is supplied from a backlight (not shown). A parallax barrier comprises a substrate 7 with a barrier aperture array 8 formed on its surface adjacent the LCD and an antireflection (AR) coating 9 formed on the other surface thereof.

The pixels of the LCD are arranged as rows and columns with the pixel pitch in the row or horizontal direction being p. The aperture array 8 comprises vertical transmissive slits with a slit width of 2w and a horizontal pitch b. The plane of the barrier aperture array 8 is spaced from the pixel plane 3 by a distance s.

In use, two interlaced images - a left eye image and a right eye image in the case of an autostereoscopic 3-D display - are displayed on the pixel plane 3 of the SLM. The display forms a left viewing window 10 in which the left eye image is visible and a right viewing region 11 in which the right eye image is visible in a window plane at the desired viewing distance of the display. The window plane is spaced from the plane of the aperture array 8 by a distance rO. The windows 10 and 11 are contiguous in the window plane and have a width and pitch e corresponding to the average human eye separation. The half angle to the centre of each window 10, 11 from the display normal is illustrated at a: Figure 2 of the accompanying drawings shows the angular zones of light created from an SLM 12 and parallax barrier 13 where the parallax barrier has a pitch of an exact integer multiple of the pixel column pitch. In this case, the angular zones coming from different locations across the display panel surface intermix and a pure zone of view for image I or image 2 does not exist. In order to address this, for example for a front parallax optic, the pitch of the parallax optic is reduced slightly so that the angular zones converge at a pre-defined plane (termed the "window plane") in front of the display.

This change in the parallax optic pitch is termed "viewpoint correction" and the effect is illustrated in figure 3 of the accompanying drawings. The viewing regions, when created in this way, are roughly kite shaped in plan view.

Figure 4 of the accompanying drawings illustrates another known type of directional display in the form of a rear parallax barrier display. This is generally similar to the display of figure 1 except that in the front parallax barrier display shown in figure 1 the parallax barrier is disposed between the SLM and the viewing windows 10 and 11 whereas, in the rear parallax barrier display shown in figure 4, the SLM is disposed between the parallax barrier and the viewing windows 10 and 11.

Figures I and 4 describe an autostereoscopic display. A dual view display operates in an identical manner, but the angle of separation between the different images is larger.

Instead of the two images being sent to left and right eyes (approximately 6.2 cm apart), images are sent to left and right people (separated by, for example, one metro). The two images displayed on the pixel plane of the SLM are not the left eye image and the right eye image of a stereoscopic image pair, but are two independent images. Where a dual view display is installed in a motor vehicle, for example, one image may be sent to the driver and another image may be sent to a passenger in the front passenger seat. The driver may see a road map, whereas the front seat passenger may see a film.

A multiple view directional display may display more than two images. To use the above example of a display installed in a motor vehicle, a display may be arranged to send a further view to a passenger in the back seat. The term "dual view display" as used herein is not limited to a display that displays two independent views in two different directions, but also includes a directional display that display three (or more) independent images in three (or more) different directions.

Figure 5a is a schematic plan view showing a dual view display installed in a motor vehicle. The display is displaying one image to the driver l 5 of the vehicle and is displaying a second image to a front seat passenger 16. The regions l5a,16a outlined in broken lines indicate the viewing regions for the driver's image and the passenger's image respectively. A dual view display in a motor vehicle is generally installed in the vehicle's dashboard, so that the driver 15 and front seat passenger 16 both view the display at a direction of approximately 40 to the normal of the display. Reference 17 denotes an "image mixing region" in which both the driver's image and the passenger's image are visible. The image mixing region is centred about the normal to the display 14, and an observer located in the image mixing region 17 will perceive cross-talk.

Trace (a) in figure 5b shows how the intensity of a typical liquid crystal display panel varies as a function of the lateral position of the observer relative to the display. It can be seen that the LCD panel is optimised for viewing in the direction normal to the display face of the panel (referred to as "on axis"). At viewing angles greater than approximately +20 to the normal of the display the intensity decreases significantly.

When the LCD panel is viewed from an angle of +40 the intensity is reduced by almost 50% compared to the on-axis intensity. When a conventional LCD panel is used in the s dual view display 14 of figure 5a, therefore, the driver 15 and passenger 16 will see an image that has a low intensity.

Furthermore, at a viewing angle of +40 the intensity of the conventional LCD panel varies steeply with changes in the viewing angle. Thus, if the driver l 5 or passenger 16 in figure 5a should move their head sideways the intensity of the image they see will vary, and this is irritating and could possibly cause discomfort.

Trace (b) in figure 5b shows an ideal intensity profile for the dual view display 14 of figure 5a. The intensity would ideally have maxima at viewing angles of +40 to the normal to the display face and, moreover, the variation of intensity with viewing angle would ideally be small at viewing angles of +40 . The intensity of the display would ideally be low for viewing angles near 0 (i.e. for angles near the normal to the display face), since light emitting at angles near 0 is wasted. Furthermore viewing directions l 5 that are along, or close to, the normal to the display face are in the image mixing region 17, and making the intensity of the display low for viewing angles near 0 means that an observer positioned in the image mixing region 17 will not experience cross-talk.

"Dual directional backlight for Stereoscopic LCD", Mitsubishi Electric Corporation, SID 03 Digest, p399 describes a directional backlight having two light sources that can be used with a conventional LCD panel to generate a tirne-rr.ultiplexed stereoscopic image pair. A prism and lens structure is used to direct the light from one source to the user's left eye and to direct light from the other source to the user's right eye. In a first time frame a left eye image is displayed and only the first light source is illuminated so that the left eye image is sent to the user's left eye. In a second time frame a right eye image is displayed and only the second light source is illuminated so that the right eye image is sent to the user's right eye. This backlight has the disadvantage that light would not go to the left user and the right user simultaneously so that the average brightness of the display would be low. Furthermore, in order to display moving images the LCD panel is required to operate at approximately 120Hz (twice the normal picture refresh rate), and this is difficult to achieve.

A first aspect of the invention provides a multiple view display comprising: a directional display device for displaying a first image or sequence of images so as to be mainly visible from a first range of directions relative to the device and for simultaneously displaying a second image or sequence of images so as to be mainly visible from a second range of directions relative to the device different from the first range; and a directional backlight for directing light through the device at least mainly in the first and second ranges.

Since the directional backlight directs light through the device at least mainly in the first and second ranges, the display provides users located in the first and second ranges with images of greater intensity than the conventional display of figure 5a. Where this aspect of the invention is applied to a dual view display, for example, the first and second ranges would correspond to the angular positions of two users viewing the display.

The first and second ranges may be on opposite sides of the normal to a display surface of the device.

The first and second ranges may be substantially symmetrical with respect to the normal.

The first image may be substantially not visible from the second range.

The second image may be substantially not visible from the first range.

The first and second ranges may be separated by a third range of directions.

The backlight may be arranged to direct substantially no light in at least part of the third range. This provides a "black window" between the first and second ranges. The intensity of the display in the black window is lower than in other ranges, and is possibly zero or close to zero so that an observer located in the black window will not perceive an image.

The backlight may comprise a light source and an optical arrangement for directing light in the first and second ranges.

The light source may be arranged to supply at least partially collimated light.

The light source may comprise a plurality of apertures aligned with imaging elements.

The apertures may be separated by reflectors for reflecting light.

The arrangement may comprise a plurality of prisms.

The prisms may have a prism angle of less than l 10 . They may have a prism angle of greater than 70 . They may have a prism angle of approximately 83 .

The prisms may be arranged as an array of variable pitch.

The arrangement may comprise: an array of substantially non-lighttransmissive blocking elements; and an array of imaging elements; and the array of imaging elements may be substantially aligned with the blocking elements thereby reducing or substantially preventing light from being directed between the first and second ranges.

The blocking elements may be reflective for recycling light.

The second image or sequence of images may be spatially multiplexed with the first image or sequence of images.

A second aspect of the present invention provides a multi-direction 2-D display comprising: a display device for displaying a single image at a time, which image is visible throughout a range of directions including first and second ranges for first and second viewers, the first and second ranges being disposed on opposite sides of and spaced from the normal to a display surface of the device: and a directional backlight for directing light through the device with higher intensity in the first and second ranges than in a third range including the normal. A display according to this aspect of the invention may be used to provide a time- multiplex directional display, provided that the backlight can be switched between a mode in which it directs light primarily in the first range of directions and a mode in which it directs light primarily in the second range of directions. in a time-multiplex directional display the image displayed in each time frame can be displayed over the entire display area of the display device, so that the images are displayed with the full resolution of the display device. Since the directional backlight directs light through the device at least mainly in the first and second ranges, the display provides users located in the first and second ranges with images of greater intensity than the conventional display of figure 5a. The first and second ranges again correspond to, for example, the angular positions of two users viewing the display.

The intensity in the third range may be non-zero.

The backlight may be switchable between a first mode, in which light is mainly directed in the first range, and a second mode, in which light is mainly directed in the second range.

The display may comprise a controller for repeatedly switching the backlight between the first and second modes and for controlling the device to display a first image or sequence of images when the backlight is in the first mode and a second image or sequence of images when the backlight is in the second mode. As noted above, this enables the display to operate as a time-multiplex directional display.

The backlight may comprise first and second light sources disposed at opposite edges of a substantially rectangular generally planar light guide.

A third aspect of the present invention provides a multiple view display comprising: a display device for displaying first and second images or sequences of images; a backlight for directing light through the device; and an observing arrangement for permitting the first and second images or sequences of images to be visible from first and second ranges of directions, respectively, and for preventing the first and second images or sequences of images from being visible from a third range of directions including the normal to a display surface of the device, the first and second ranges being non-overlapping with the third range.

Where the invention is applied to a dual view display intended for use in a motor vehicle, for example, the third range of directions, in which the images cannot be seen, may cover the position of a central rear seat passenger. With a conventional dual view display a central rear seat passenger would be positioned in the image mixing region and would see both images, and this would be irritating and could cause discomfort. If the third range of directions of a display of the invention is made large enough to accommodate the head of a central rear seat passenger, the passenger will not see either image. (It is preferable if the lateral extent at the third range of directions, at the rear seat of the vehicle is comparable in extent to width of the head of a human, so that a left or right rear seat passenger would not be located in the third range of directions.) The first and second ranges may be on opposite sides of the normal.

The backlight may be a Lambertian backlight.

The device may comprise a plurality of pixels.

The observing arrangement may substantially block light in the third range.

The observing arrangement may comprises a first parallax barrier.

The first parallax barrier may comprise a plurality of slits, each of which is disposed above a gap between an adjacent pair of pixels.

The parallax barrier may comprise a plurality of slits, each of which is arranged to be substantially non-transmissive to light from at least one adjacent pixel and to be substantially transmissive to light from at least one non-adjacent pixel.

The pixels may be arranged as groups of different colours and the parallax barrier may comprise a plurality of colour filter slits, each of which substantially blocks the colour of light from at least one adjacent pixel and transmits the colour of light from at least one non-adjacent pixel.

The pixels may be arranged as groups of different output polarizations and the parallax barrier may comprise a plurality of polarising slits, each of which substantially blocks the polarization of light from at least one adjacent pixel and transmits the polarization of light from at least one non-adjacent pixel.

The first parallax barrier may comprise a plurality of slits and the observing arrangement may comprise a second parallax barrier having a plurality of slits, each of which is disposed above a gap between adjacent slits of the first parallax barrier.

The pixels may be arranged as groups of different colours and the observing arrangement may comprise a plurality of colour-selective reflectors, each of which is arranged to reflect the colour of light from at least one adjacent pixel and to transmit the colour of light from at least one non-adjacent pixel.

The observing arrangement may comprise a directional scatterer for scattering light in the third range.

The first image or sequence of images may be spatially multiplexed with the second image or sequence of images.

In a device according to any aspect of the centres of the first and second ranges may subtend an angle at the device of between 20 and 140 . The centres of the first and second ranges may subtend an angle at the device of less than 130 , of less than 120 , of less than 110 , of less than 100 , or of less than 90 . The centres of the first and second ranges may subtend an angle at the device of greater than 30 , of greater than 40 , of greater than 500, of greater than 60', or of greater than 700. The centres of the first and second ranges may subtend an angle at the device of approximately 800.

A display of any aspect of the invention may comprise a liquid crystal display.

Preferred embodiments of the present invention will now be described by way of illustrative example with reference to the accompanying figures in which: Figure 1 is a schematic plan view of a conventional front-barrier multiple-view directions display; Figure 2 is a schematic plan view of viewing windows formed by a conventional multiple-view directional display; Figure 3 is a schematic plan view showing viewing windows formed by a multiple-view directional display that incorporates viewpoint correction; Figure 4 is a schematic plan view of a rear-barrier multiple view directional display; Figure Sa is a schematic plan view showing a dual-view display installed in a motor vehicle; Figure 5b shows the angular variation of intensity of a conventional liquid crystal display panel; Figure 6a is a schematic plan view of a multiple view display according to a first embodiment of the present invention; Figure 6b is a schematic view of a partially collimated light source; Figure 7a shows the angular distribution of intensity produced by the backlight of Figure 6b; Figure 7b shows the prism array of the display of Figure 6a; Figure 7c shows an alternative prism structure; Figure 7d shows the angular distribution of intensity according to another embodiment; Figure 8 is a schematic plan view of another partially collimated light source; Figure 9 shows a multiple view display according to a second embodiment of the present invention; Figure 10 is a schematic perspective view of a backlight for use in a multi-direction display according to a third embodiment of the present invention; Figure 11 illustrates the angular distribution of intensity provided by the backlight of Figure 10; Figure 12a illustrates a multiple view display according to a fourth embodiment of the present invention; Figure 12b and 12c illustrate modifications of the fourth embodiment of the present invention; Figure 13 is a schematic plan view of a multiple view display according to a fifth embodiment of the present invention; Figure 14 is a schematic plan view of a display according to a sixth embodiment of the present invention; Figure 15 is a schematic view of a multiple view display according to a seventh embodiment of the present invention; Figure 16a is a schematic plan view of a multiple view display according to an eighth embodiment of the present invention; Figure 16b shows the angular variation of intensity provided by the display of Figure 16a for a uniform backlight; Figure 17a shows the angular distribution of intensity of a backlight suitable for use with the display of figure 16a; Figure 17b shows the overall angular distribution of intensity obtained by providing the backlight of figure 17a in the display of figure 16a; Figure 18a is a schematic plan view of a backlight having the intensity distribution of figure 17a; and Figure 18b shows the transmissivity of the parallax barrier of the backlight of figure 18a.

The advantage of the present invention are obtained primarily in a dual view display, since the angular separation between the two viewing windows of a dual view display is relative large (typically around 80 ). In an autostereoscopic display the lateral separation between the centres of the two viewing windows at the intended viewing distance of the display is equal to the separation between the two eyes of a human, and the angular separation between the two viewing windows is therefore generally much lower than in the case of a dual view display. The invention will therefore be described primarily with reference to a dual view display, but the invention can in principle be applied to other directional displays such as, for example, an autostercoscopic display.

In the case of a dual view display intended for installation in a motor car, for example, if a conventional display is used a central rear seat passenger would be located in the image mixing region 17, as shown in figure 5a, and so will perceive a mixture of the driver's image and the front passenger's image. This will be irritating, and could cause discomfort. If a display of the invention is used, however, a central rear seat passenger will be located in a "black window" (a region where the display provides low or zero intensity) between the viewing window of the driver's image and the viewing window of the front passenger's image and so will not perceive cross-talk.

Embodiments of the invention may also be applied to a multi-direction display. A multi-direction display displays a single image at a time, such that the image is visible throughout a first range of directions for a first viewer and a second range of directions for a second viewer. The first and second ranges of directions are generally disposed on opposite sides of the normal to a display surface of the device. The first and second ranges of directions do not overlap with one another, and are generally separated by a third range of directions (which includes the normal direction) in which it is intended that the image is not visible. Such a display may be operated in a time-multiplex manner to provide a directional display. Since the display is not intended to display an image in the third range of directions any output light emitted in the third range of directions is wasted, and it is preferable that the output light from the display is concentrated as much as possible in the first and second ranges.

Figure 6a is a schematic plan view of a multiple-view display 18 according to a first embodiment of the present invention. The display 18 comprises a directional display device 19 that is able to display first and second images such that the first image is primarily visible in a first viewing window that covers a first range of directions relative to the device and for displaying a second image that is visible in a viewing window defined by a second range of directions, different from the first range, relative to the device. The first and second images displayed on the directional display device 19 may be still images, or they may be moving images (that is, the first image and second image each consist of a sequence of images). The first and second images may be displayed simultaneously and be spatially multiplexed with each other. The directional display device l9 may be a conventional multiple view directional display that comprises a spatial light modulator for displaying the first and second images and a parallax optic for directing the first and second images into the first and second ranges of direction respectively Thus, the first image is not visible from the second range of directions and the second image is not visible from the first range of directions. The spatial light modulator may comprise a liquid crystal display layer. The directional display device l9 may have the general form of, for example, the multiple viewdirectional displays shown in figure I or figure 4 of the present application.

As explained above, it will be assumed that the directional display device 19 is a dual view display and that the first and second displayed images are intended to be viewed by first and second observers.

The multiple view display 18 of figure 6a further comprises a directional backlight 20.

The directional backlight directs light through the directional display device 19 at least mainly in the first and second ranges of directions. When the display 18 is viewed by an observer from the first range of directions, or from the second range of directions, the observer will see a bright image. This is because light from the backlight 20 is directed primarily along the directions in which the display 18 is viewed. The directional backlight 20 is preferably arranged so that the display 18 emits little or no light along a direction normal to the display face of the display. This ensures that as much as possible of the light generated by the directional backlight 20 is emitted into the first and second ranges so that the first and second images are displayed with as great an intensity as possible. It also provides a "black window" between the first and second ranges of directions.

In the embodiment of figure 6a, the first range of directions subtends an angle, at the display 18, of 2a, and the second range of directions subtends an angle at the display of 23. The first and second ranges are on opposite sides of the normal to the display face 19a of the device and may be, as in this embodiment, substantially symmetrical with respect to the normal to the display face of the display. The first range and the second range are separated by a third range, which includes the normal to the display.

The directional backlight 20 comprises a light source 21 and an optical arrangement 22 for directing light into the first and second ranges. In this embodiment the light source 21 is a source of partially collimated light, and generates an output beam that has its greatest intensity at directions along, or close to, a the normal to the display face of the display 18.

Figure 6b is a schematic perspective view ofthe light source 21 showing its components separated for clarity. As can be seen, the light source 21 comprises a waveguide 23, with first and second light sources 24, 25 arranged along opposite side edges of the waveguide. Light emitted by the light sources 24, 25 passes into the waveguide 23, undergoes internal reflection within the waveguide 23 since the waveguide 23 has a greater refractive index than its surroundings, and finally passes out of the front face 23a of the waveguide. A reflector 26 is preferably positioned behind the waveguide 23, so that any light that is emitted from the rear face of the waveguide 23 is reflected back to the waveguide 23 and is not lost.

Light emitted from the front face 23a of the waveguide 23 is collimated by two collimating structures 27, 28. Each structure comprises a flat plate, provided on their rear surface with an array of prisms. The prisms of one structure 27 extend generally perpendicular to the prisms of the second structure 28.

The partially collimated light source 21 shown in figure 6b is described in, for example, "Highly efficient backlight for liquid crystal display having no optical films", Applied Physics Letters, Vol. 83, No. 13, p215 (2003), to which attention is directed. Figure 7a shows the angular distribution of intensity provided by the light source 21 of figure 6b.

It will be seen that the intensity is greatest along the direction of the light beam, and falls to a minimum in the range of from approximately + 40 to + 60 from the direction of the light beam. The intensity then rises to a subsidiary peak at an angle of approximately + 75 from the direction of the light beam.

The optical arrangement 22 of the display 18 of figure 6a directs light from the light source 21 such that the hght is directed in primarily the first and second ranges of directions. One suitable structure for the optical arrangement is a prism structure 22, and this is shown in more detail In figure 7b. The prism structure 22 comprises a base S plate 23, and an array of prisms 24 provided on a surface of the base plate 23. In use, the prism structure 22 is oriented such that light from the light source 21 Is incident on the prism array 24. Light propagating along the axis of the display 18 that is incident on the prism array 24 is split by the prism array, and is directed primarily along the first and second ranges of directions. This is shown by ray paths 29a and 29b in figure 7b.

The prism array 24 and base plate 23 are made of a material that has a refractive index higher than the refractive index of the surrounding materials. The prism angle y of the prisms is chosen depending on the desired angular direction of the light leaving the prism structure 22. Where the invention is applied to a dual view display, it is likely that the first and second ranges will be centred on angles in the approximate ranges of from 30 to 40 and from -30 to 40 from the axis of the display. The prism angle is preferably in the range of from 70 to 110 , and is particularly preferably approximately 90 . The refractive index of the prism array 24 and the base plate 23 is preferably in the range of from 1.3 to 1.8 and is particularly preferably approximately 1.5. In one particularly preferred embodiment, the prism array had a refractive index of approximately 1.5, and it was found that a prism angle of 83 produced the greatest angular extent of the black window. The prism angle that produces the greatest angular extent of black window will depend on the angular distribution of intensity from the backlight, but for most backlights a prism angle of around 90 will produce a well defined black window.

Although the prism array 24 and the base plate 23 are shown as separate components in figure 7b they are in practice preferably integral with one another. The prism structure 22 may be formed by, for example, a moulding process.

In the embodiment of figure 6a, the light source 21 provides partially collimated light having the intensity profile shown in figure 7b. This embodiment is particularly advantageous, because light propagating at an angle of approximately + 75 to +80 to the normal of the display, corresponding to the subsidiary peak in the intensity spectrum of figure 7a, is also directed into the first and second ranges by the prism array. This is shown by the light paths 30a and 30b in figure 7b. Light travclling along these paths is incident on one angled face of a prism of the array and is transmitted into the prism with refraction, undergoes internal reflection at the adjacent angled face of the prism, and finally leaves the prism structure 22 via the rear face of the base plate 23 where it undergoes further refraction. The result of the refraction, reflection and refraction steps is that light incident on the prism structure 22 at an angle of approximately + 80 to the axis is directed into the first or second ranges. Thus, light from both the principal l O intensity peak in the spectrum of figure 7a centred on an angle of 0 , and light from the subsidiary intensity peaks at approximately + 75 is all directed into the first or second ranges - that is, towards the intended viewing positions of the display. The use of a source of partially collimated light is therefore preferable, since this provides bright images. In principle, however, this embodiment of the invention is not limited to a ] 5 source of partially collimated light.

The backlight 20 of figure 6a has an output intensity profile similar to the ideal profile shown in figure 5b - the light is directed primarily into first and second ranges of directions, and the first and second ranges of directions are separated by a third range of directions in which little or no light is directed. The centres of the first and second ranges preferably subtend an angle at the device of between 20 and 140 . They may subtend an angle at the device of less than 130 , of less than 120, of less than 110, of less than 100 or of less than 90 . They may subtend an angle at the device of greater than 30 , of greater than 40 , of greater than 50 , of greater than 60 or of greater than 70 . They may subtend an angle at the device of approximately 80 .

In figure 7b the prisms are shown as having a constant pitch over the prism structure 22.

It is, however, possible to change the pitch of the prisms randomly over the prism structure, and this would have the advantage of reducing the generation of Moire effects that could occur if the prisms have a uniform pitch over the prism structure 22 as a result of alignment between the prisms and other components of the display that have a regular pitch.

Additionally or alternatively, the prism angle may change across the display as shown schematically in figure 7c which shows an alternative prism structure 22'. A prism 24a in the centre of the structure has a symmetric section (the section is generally an isosce]cs triangle). However, the section of the prisms, and the prism angle, change away from the centre of the prism structure, as shown by prisms 24b,24c. The widths of the base of the prisms 24a, 24b, 24c are substantially equal to one another, so that the change in the prism section leads to a reduction in the height of the prism. The change of the prism angle across the display provides "viewpoint correction".

The prism structure 22 may also be arranged to provide only partial redirection of light way from directions close to the normal to the display face. This embodiment provides an output intensity profile that provides increased intensity at the intended viewing directions but that has a nonzero intensity at the normal to the display face, as shown in figure 7d. This embodiment may be used in a display that is intended to be viewed along the normal direction such as, for example, a display that is switchable between a multiple view directional display mode and a standard two-dimensional display mode, and it may also be used in a conventional two-dimensional display.

Figure 8 is a schematic plan view of another partially collimated light source 21 that is suitable for use in the embodiment of figure 6a. In this embodiment the light source 21 comprises a backlight 31 that emits light substantially uniformly over its area. The backlight 31 may for example comprise a waveguide and light sources disposed along opposite side edges of the waveguide, as shown in figure 6b.

The light source comprises a plurality of imaging elements 33. In the embodiment of figure 8 the imaging elements 33 constitute a lenticular sheet 33a, but the imaging elements are not limited to a lenticular sheet. The light source 21 further comprises a barrier 34 having a plurality of transmissive regions or apertures 32 that are aligned with the imaging elements. The apertures 32 are separated by regions 34a that do not transmit light. In general, the lens will have a generally semicylindrical shape and extend into the plane of the paper, and the transmissive regions 32 and opaque regions 34 will be in the form of strips that extend into the plane of the paper. Light that passes through the apertures 32 is at least partially collimated by the imaging elements 33 so that the light source outputs a beam of at least partially collimated light. The degree of collimation of the output light will depend on the imaging power of the imaging elements, the distance between the imaging elements and the barrier 34, the width of the transmissive regions 32, the pitch of the transmissive regions 32, and the pitch of the imaging elements.

The opaque regions 34a of the barrier are preferably reflective, so that light incident on an opaque region 34a is reflected back into the backlight 3 l. The light may then be re- reflected out of the backlight 31, and pass through a transmissive aperture 32.

The pitch of the barrier is preferably not an exact integral multiple of the pitch of the imaging elements, to provide the "viewpoint correction" effect mentioned earlier.

The opaque regions 34a of the barrier may be incorporated in the backlight 31. Where the backlight is a backlight with a waveguide as shown in figure 6b, for example, the opaque regions 34a of the barrier may be provided on the surface of the waveguide by any suitable deposition, printing or stamping technique.

The barrier may alternatively constitute a liquid crystal layer that is suitably addressed to define the opaque regions 34a of the barrier. This would provide a barrier that can be switched OFF by addressing the liquid crystal layer so that it is uniformly transparent over its area. This provides a backlight that can be switched between a directional backlight mode (when the barrier is defined in the liquid crystal layer) and a conventional backlight mode (when the barrier is OFF).

If the barrier is defined in a liquid crystal layer, it is further possible to "move" the opaque regions 34a of the barrier laterally with respect to the imaging elements by re- addressing the liquid crystal layer. This alters the angular positions at which the backlight provides maximum intensity, and this may be of advantage in a display in which the angular positions of the viewing wmdows can be altered to track the movement of an observer using any of the known so-called "observer tracking" techniques.

Figure 8 shows only the principal components of the light source 21. In a practical embodiment extra components such as, for example, a diffuser placed in front of the backlight 31 may be present.

Figure 9 is a schematic plan view of a multiple view display 18 according to a second embodiment of the present invention. The display 18 again comprises a directional display device l9, in this embodiment a dual view display, that can display a first image so as to be mainly visible from a first range of directions relative to the device and to display a second image so as to be mainly visible from a second range of directions (different from the first range) relative to the device. The directional display device 19 is illuminated by a directional backlight 20 that directs light through the directional display device 19 at least mainly in the first and second ranges.

As in the embodiment of figure 6a, the directional display device 19 may be any conventional directional display, and will not be described further.

The directional backlight 20 of this embodiment comprises a light source 35 that emits light generally uniformly over its entire area. An optical arrangement for directing light from the light source 35 primarily in the first and second directions is provided in front of the light source 35. The arrangement 36 comprises an array of imaging elements 37.

In this ernkodiment the imaging elements 37 constitute a lenticular lens array, but the invention is not limited to this particular form for the imaging elements 37.

A barrier 38 is disposed between the light source 35 and the imaging elements 37. The barrier 38 contains regions 38a that are opaque to light and regions 38b that are transmissive to light from the light source. The imaging elements 37 extend into the plane of figure 9, and the transmissive region 38b and opaque regions 38a of the barrier 38 extend into the plane of the paper in the form of transmissive strips or opaque strips.

The array of imaging elements is substantially aligned with the opaque regions 38a of the barrier so as to prevent, or substantially prevent, light from being directed between along, or close to, the normal direction to the display surface of the display. One opaque region 38a of the barrier is aligned with the axis of each imaging element 37.

Thus, all light that is incident on the imaging element 37 is incident on the off-axis portions of the imaging elements, and so is directed by the imaging elements 37 in an off-axis direction. Thus the output from the backlight 20 contains little or no light directed along the axis of the display, and substantially all light emitted by the backlight 20 is directed m off-axis directions. The imaging power of the imaging elements 37, the separation between the imaging elements 37 and the barrier 38, and the width and pitch of the apertures 38b of the barrier are chosen so that the directions in which the backlight emits light are coincident with the first and second ranges in which the directional display device 19 displays the first and second images. As in the embodiment of figure 6a, the first and second ranges, in which the display 18 displays the first and second images, are separated by a third range of directions which includes the normal direction. The backlight is arranged to direct substantially no light into at least part of the third range of directions.

The opaque regions 38a of the barrier 38 may be reflective, so that light incident on the opaque regions 38a is reflected back into the light source 35. The light may then be re- emitted, and can pass through a transmissive region 38b of the barrier.

In some applications of a multiple-view display it may be desirable if some light is emitted in the normal direction. For example, where a dual view display is intended for use in a motor vehicle, it may be desirable to provide a third display for a rear-seat passenger, and this view may be directed in substantially the normal direction. If it is desired that some light is emitted in the normal direction, the directional backlight 20 of figure 9 may be modified by replacing the opaque regions 38a of the barrier by semi opaque regions. Some light will then pass through the semi-opaque regions, and the backlight 20 will emit some light along the axis of the display 18. Alternatively, the opaque barrier strips 38a may be replaced by regions that are semi- reflective and semi- transmissive.

Figure 10 shows another directional backlight 20 suitable for use in a multiple view display of the present invention. The backlight comprises a waveguide 39 and first and second light sources 40, 41. Each light source is arranged along one side edge of the waveguide. Light emitted by the light sources 40, 41 is transmitted through the respective side edge of the waveguide, and propagates through the waveguide. The waveguide 39 has a greater refractive index than its surroundings and light initially undergoes internal reflection at the front and rear surfaces of the waveguide 39 until it is eventually incident on the front surface of the waveguide at an angle less than the critical angle and is emitted from the front face of the waveguide.

The light sources 4O, 41 are arranged along the left and right side edges of the waveguide 39, with the terms "left" and "right" referring to how the components are perceived when viewed by an observer looking at the display. Figure 11 shows the intensity of the backlight 21 of figure 10 as a function of the lateral angular displacement from the normal, and it will be seen that the intensity contains two maxima, one either side of the normal to the display. The angles at which the intensity maxima occur are determined by the width and thickness of the waveguide, and by the difference between the refractive index of the waveguide and the refractive index of the surrounding material. The directional backlight 21 of figure 10 is therefore suitable for use in a multiple view display of the present invention - by arranging the backlight to illuminate a directional display such that the angular position of one maximum in the output intensity of the backlight lies within the first range of directions (in which the directional display device displays the first image) and such that the angular position of the second maximum in the output intensity of the backlight lies within the second range of directions (in which the directional display displays a second image), it is possible to increase the brightness of the displayed first and second images.

It will be noted, however, that the output intensity of the backlight 21 of figure 10 is not close to zero for directions near the normal direction, but is approximately 70% of the maximum intensity. This means that the backlight 21 of figure 10 is not suitable for use in a multiple view display that is desired to provide a dark display in the normal direction. The backlight 21 of figure 10 is, however, suitable for use in a multiple view display that provides three or more images, or in a multiple view display that can be switched to give a conventional twodimensional display mode.

The backlight 21 of figuec 10 may also be used in a time-multiplexed multiple view directional display. This may be done by incorporating the backlight of figure 10 in a display that also comprises a display device for displaying a single image at a time, which image is visible throughout a range of directions including first and second ranges for first and second viewers, the first and second ranges being disposed on opposite sides of and spaced from the normal to a display surface of the device. The display device may be any suitable display device, and may be a liquid crystal display device. Again, the angular position of the maxima in the output intensity of the backlight are arranged such that one maximum in the output intensity of the backlight lies within the first range of directions and such that the angular position of the second maximum in the output intensity of the backlight lies within the second range of directions. The intensity maxima in the output intensity of the backlight preferably subtend an angle at the device of between 20 and 140 . They may subtend an angle at the device of less than 130 , of less than 120, of less than 110, of less than 100 or of less than 90 . They may subtend an angle at the device of greater than 30 , of greater than 40 , of greater than 50 , of greater than 60 or of greater than 70 . They may subtend an angle at the device of approximately 80 . The position of the minimum in the output intensity of the backlight is arranged to be substantially coincident with the normal to a display face of the display.

To operate this display as a time multiplex directional display the backlight is switched between one mode in which light is primarily directed in the first range of directions and another mode in which light is primarily directed in the second range of directions.

This is done by switching the light sources 40,41 so that they are illuminated in sequence.

The display device would be controlled, by a suitable controller (not shown), to display a first image in a first time frame. If the first image is intended to be displayed in the first range of directions, for example to a first viewer, the backlight is accordingly controlled to emit light primarily in the first range of directions in this time frame. For example, the controller may control the backlight so that the first light source 40 is ON and the second light source 41 is OFF in this time frame. Thus, in the first time frame the first viewer would see the displayed image but a second viewer located in the second range of directions would not see the image (since the backlight was emitting no light in the second range of directions.) In the second time frame, the display device would be controlled, by the controller, to display a second image that is intended to be displayed in the second range of directions, for example to a second viewer. The backlight is controlled to emit light primarily in the second range of directions in this time frame. For example, the controller may control the backlight so that the second light source 41 is ON and the first light source 40 is OFF in this time frame. Thus, in the second time frame the second viewer would see the displayed image but the first viewer located in the first range of directions would not see the image (since the backlight was emitting no light in the first range of directions).

In the third time frame the controller would control the backlight so that the first light source 40 is again ON and the second light source 41 is again OFF. The display device would display the next image intended for the first viewer (and this might be the same as the image displayed in the first time frame, in the case of a still image, or it might be the next of a sequence of images in the case of a moving image.) Thus, in the third time frame the first viewer would see the displayed image but the second viewer would not see the image.

Similarly, in the fourth time frame the controller would control the backlight so that the second light source 41 is again ON and the first light source 40 is again OFF. The display device would display the next image intended for the second viewer (and this might be the same as the image displayed in the second time frame, in the case of a still image, or it might be the next of a sequence of images in the case of a moving image.) Thus, in the fourth time frame the second viewer would see the displayed image but the first viewer would not see the image.

In a conventional backlight in which light sources are arranged alongside edges of a waveguide, it is normal to dispose the light sources along the top and bottom side edges of the display. The width of a display is typically greater than its height, so that placing the hght sources along the top and bottom side edges of the waveguide allows longer, and hence brighter, light sources to be used. In order to offset any reduction In intensity of the backlight that may be caused by the need to provide the light sources 40, 41 along the side edges of the waveguide, it would be possible to provide two light sources along each side edge of the waveguide 39. The light sources 40, 41 may be, for example, fluorescent light tubes or arrays of light-emitting diodes.

Figure 12a is a schematic plan view of a multiple view display according to a further embodiment of the present invention. The display 47 comprises an image display device for displaying first and second images. The image display device has an image display layer 43 that includes a plurality of pixels. Other components of the image display device have been omitted from figure 12a for clarity. The image display device may be any suitable image display device, and may be, for example, a liquid crystal display device. The display device is illuminated by a backlight 42 disposed behind the display device. The images may be displayed in a spatially multiplexed manner, and figure 12a indicates that columns of pixels are displaying alternately an image for a left observer (denoted by L) and an image for a right observer (denoted by R). The pixel columns extend into the plane of the paper in figure 12a.

The backlight 42 of the display 47 is not a directional backlight, and emits light in a range of directions. The display 47 is provided with an observing arrangement that permits the first and second images to be visible from first and second ranges of directions respectively, and that prevents the first and second images from being visible from a third range of directions that includes the normal to a display surface of the device. The first and second ranges are not overlapping with the third range, and are preferably on opposite sides of the normal to a display surface of the display to one another. The first and second ranges may, as in previous embodiments, be substantially symmetrical about the normal to the display surface. The centres of the first and second ranges preferably subtend an angle at the device of between 20 and 140 . They may subtend an angle at the device of less than 130 , of less than 120, of less than 110, of less than 100 or of less than 90 . They may subtend an angle at the device of greater than 30 , of greater than 40 , of greater than 50 , of greater than 60 or of greater than 70 . They may subtend an angle at the device of approximately 80 . The observing arrangement preferably prevents the first image from being visible from the second range of directions, and also preferably prevents the second image from being visible from the first range of directions. In figure 12a the observing arrangement comprises a colour filter barrier

44. The colour filter barrier 44 comprises a plurality of transmissive regions separated by opaque portions 46. The transmissive regions are each transmissive to a selected wavelength range. A region 45G is transmissive to light in the green wavelength of the spectrum and is not transmissive to red or blue light, a region 45R is transmissive to hght in the red portion of the spectrum and is not transmissive to green or blue light, and a regions 45B is transmisssive to light in the blue portion of the spectrum and is not transmissive to red or green light. The transmissive regions are therefore referred to as "green regions", "red regions", or "blue regions" respectively.

The image display layer 43 comprises colour pixels. Pixels 43R transmit light in the red portion of the spectrum, but block blue and green light from the backlight 42. The pixels 43R produce an image in the red portion of the spectrum, and will therefore be referred to as "red pixels". Similarly, pixels 43G are transmissive only to light in the green portion of the spectrum and so produce a green image and are known as "green pixels", and pixels 43B transmit only light in the blue portion of the spectrum and are known as "blue pixels".

The green, red and blue regions in the colour filter barrier are arranged, relative to the pixels, so that a region that transmits light of a particular colour is not placed immediately in front of a pixel of that colour. The green regions 45G of the colour filter barrier, for example, are not placed in front of green pixels 43G; the green apertures 45G of the colour filter barrier 44 are laterally displaced with respect to the green pixels 45G. Light from the green pixels 43G is therefore transmitted in first and second ranges of directions, at an angle to the normal to the display face of the display 47. Similarly, red regions of the colour filter barrier are not placed in front of red pixels, and blue regions 45B of the colour filter barrier are not placed in front of red pixels 43R. Light from red and blue pixels is therefore not transmitted through the colour filter barrier in directions parallel to or close to the normal direction to the display face of the display.

Red and blue light is again emitted in first and second ranges of directions that are spaced from the normal direction.

As an example, the "blue" region 45B in the colour filter barrier 44 blocks light from the red and green pixels 43R, 43G that are placed substantially directly behind it, as shown by the arrows in full lines in figures 12a.

It will be noted that this embodiment does not re-direct light from the normal direction into the viewing directions - light emitted by the backlight in the normal direction will be blocked by the colour filter barrier, as shown by the full arrows. This embodiment therefore does not increase the intensity of light emitted along the intended viewing directions, although it does create a "black window" between the viewing windows.

The angular extent of the "black window" is determined by the width of the transmissive regions 45B, 45G, 45R, and the smaller is the width of the transmissive regions 45B, 45G, 45R the greater will be the angular extent of the black central window. The width of the transmissive regions 45B, 45G, 45R in a particular display can therefore be selected to give a black window having the angular extent desired for that display.

The colour pixels in this embodiment may, as is well-known, be obtained by providing colour filters over a conventional image display layer.

Figure 12b shows a further multiple view display 48 of the present invention. This again comprises a directional display device for displaying first and second images; only the image display layer 43 of the display device is shown, and other components of the display device are omitted for clarity. The image display layer is again a pixellated display layer that comprises a plurality of colour pixels including red pixels 43R, green pixels 43G and blue pixels 43B. Figure 12b illustrates the image display device displaying two interlaced images, with images for left and right observers being displayed on alternate columns of pixels as denoted by L (for pixel columns displaying the left image) and R (for pixels displaying the right image).

The multiple view display 48 of figure 12b further comprises an observing arrangement for permitting the first and second images to be visible from first and second range of directions respectively, while preventing the first and second images from being visible from a third range of directions that includes the normal to the display surface of the device. In this embodiment the observing arrangement comprises two parallax barriers 49, 50. The first parallax barrier 49 is disposed in front of the image display layer 43, and the second parallax barrier 50 is disposed in front of the first parallax barrier. Each parallax barrier comprises transmissive region 49a, 50a, separated by opaque regions 49b, 50b. The transmissive region 49a, 50a and the opaque regions 49b, 50b extend into the plane of the paper and so have the form of transmissive strips or opaque strips respectively. The two parallax barriers 49, 50 are arranged such that a transmissive region 50a of the second barrier 50 is not disposed directly in front of an transmissive region 49a of the first parallax barrier 49. The two parallax barriers are arranged such that an transmissive region 50a in the second parallax barrier 50 is disposed in front of an opaque region 49b of the first parallax barrier, and so that an opaque region 50b of the second parallax barrier 50 is disposed in front of a transmissive region 49a of the first parallax barrier. As a result, light emitted by the backlight 42 in a direction parallel to, or close to, the normal of the display face of the display is blocked by one or other of the parallax barriers 49, 50. This is indicated by the full black arrow 51 in figure 12b.

Because the two parallax barriers are arranged such that transmissive region 50a in the first parallax barrier 50 are laterally offset with respect to transmissive region 49a in the first parallax barrier, light that leaves the second parallax barrier 50 is travelling in first and second ranges of directions, as shown in figure 12b. This embodiment again provides a multiple view display in which the first and second images are not visible from a range of directions that includes the normal to the display surface of the device.

One or both of the two barriers 49, 50 could be disposed between the backlight 42 and the pixellated display device. If one or both of the two barriers 49, 50 is disposed between the backlight 42 and the pixellated display device, it is preferable that the opaque regions of the barrier nearest to the backlight 42 are reflective, so that any light that is blocked by the opaque portions is returned to the backlight and can subsequently be re-emitted.

The angular extent of the "black window" between the first and second ranges is determined by the width of the transmissive regions 49a, 50a of the two parallax barriers 49,50, and the smaller is the width of the transmissive regions of the parallax barriers the greater will be the angular extent of the black central window. The width of the transmissive regions 49a,50a of the parallax barriers in a particular display can therefore be selected to give a black central window having the angular extent desired for that display. s

figure 12c shows a multiple view display 52 according to a further embodiment of the present invention. This multiple view display 52 again comprises a display device for displaying first and second images. Only the image display layer 43 of the display device is shown, and other components of the display device are omitted for clarity.

The image display layer 43 is again a pixellated image display layer that comprises colour pixels - pixels 43R are red pixels, pixels 43G are green pixels, and pixels 43B are blue pixels. The image display layer 43 is shown as displaying two interlaced images, with a left image and a right image for left and right observers being displayed on alternate columns of pixels as denoted by L and R in figure 12c.

The image display layer 43 in this embodiment is provided with a black mask between the pixels. That is, regions 53 of the image display layer between adjacent pixels are made non-transmissive to light from the backlight 42.

The display 52 further comprises a parallax optic 49 disposed in front of the image display layer 43. In the embodiment of figure 12c the parallax optic is a conventional parallax barrier having transmissive portions 49a separated by opaque portions 49b.

The opaque portions 49b and transmissive portions 49a extend into the plane of the paper.

The parallax optic 49 services to spatially separate the two images displayed on the image display layer, so that one image is displayed along a first range of directions and the second image is displayed along a second range of directions different from the first range of directions. The manner in which the parallax optics does this is conventional and will not be described further. In this embodiment, however, the presence of the opaque portions 53 of the black mask between adjacent pixels of the image display layer 43 reduces the intensity of light that is transmitted in directions along or close to the normal to the display face of the display 52. The parallax optic and the image display layer are arranged so that, as far as possible, an opaque portion 53 of the black mask of the image display layer is arranged behind an aperture 49a in the parallax barrier 49.

The opaque portions 53 of the black mask therefore prevent light being transmitted through the apertures 49a of the parallax barrier in a direction parallel to, or close to, the normal to the display face of the display. The display 52 of figure 12c will therefore emit a much lower intensity of light in directions parallel or close to the normal direction of the display face of the device. The intensity of the emitted light in, for example, the image mixing region shown in figure 5a will therefore be much lower in the display 52 than in a conventional display.

The angular extent of the black central window is determined by the width of the non transmissive black mask regions 53, and the greater is the width of the non-transmissive black mask regions the greater will be the angular extent of the black central window.

The width of the non-transmissive black mask regions in a particular display can therefore be selected to give a black central window having the angular extent desired for that display.

Figure 13 shows a multiple view display 70 according to a further embodiment of the present invention. The display 70 again comprises an image display device disposed in front of a uniformly-emitting backlight 42. The image display device has an image display layer having pixels 43. The image display layer is able to display two images as described in previous embodiments and, as in previous embodiments, other components of the image display device have been omitted for clarity.

The multiple view display 70 further comprises an observing arrangement that permits first and second images displayed on the image display device to be viewed in first and second ranges of direction respectively, while preventing the first and second images from being visible from a third range of directions that includes the normal to a display surface of the device. In this embodiment the observing arrangement comprises a parallax barrier 54 having transmissive portions 54a and opaque portions 54b. In this embodiment the transmissive portions 54a of the parallax barrier 54 are polarising apertures and transmit light of one polansation while substantially blocking light of an orthogonal polarization. The pixels 43 emit light of either the first polarization state or the second polarisation state. In figure 13 the two polarization states are taken to be the P- and S- linear po]arisation states. Pixels 43 are labelled with an "S" or a "P" to denote whether they denote light having the Spolarisation or light having the P-polarisation respectively. The transmissive portions 54a of the parallax barrier 54 are also labelled with a P or a S to denote whether they transmit light having the Ppolarisation or the S- polarisation respectively.

The parallax barrier 54 is arranged such that an aperture 54a that transmits light of a particular polarization is not in front of a pixel that emits light of that polarization.

Thus, the apertures 54a that transmit the P-polarisation state are not arranged in front of pixels 43 that emit the P-polarisation state, and apertures 54a that transmit the S- polarisation state are not arranged in front of pixels that emit the S- polarisation state.

As a result, the light that is emitted by a pixel of one polarization state can only pass through the parallax barrier 54 in first and second ranges of directions that are different from, and lie on opposite sides of, the normal to the display face of the display. Light that is emitted by, for example, a S-pixel in a direction parallel or close to the normal direction will be incident on an aperture 54a that transmits only the Ppolarisation or on an opaque portion 54b of the parallax barrier, and so will be blocked. The intensity of light emitted by the display of this embodiment in the normal direction, or in directions close to the normal direction, is therefore low. The device thus provides a black window between the viewing windows of the two images.

A black mask (not shown) is provided between adjacent pixels 43. The angular extent of the black central window can be varied by altering the black mask: pixel ratio (while keeping the pixel pitch constant). The greater is the width of the black mask between adjacent pixels, the greater is the angular extent of the black central window.

The angular extent of the black central window is also determined by the width of the polarising apertures 54a of the parallax barrier 54. The angular extent of the black central window may also be varied by changing the width of the polarising apertures (while keeping the aperture pitch constant). The smaller is the width of the polarising apertures of the parallax barrier the greater will be the angular extent of the black central window.

Figure 14 shows a display 54 according to a further embodiment of the present invention. The display again comprises a display device disposed in front of a backlight 42. The display device has a pixellated display layer that can display first and second images in the manner described above for previous embodiments. In this embodiment the display device is a full-colour display device and has red pixels 43R, green pixels 43G and blue pixels 43B. Other components of the display device, such as a parallax optic for ensuring that each displayed image is visible from only a particular respective range of viewing angles, are omitted from figure 14 for clarity.

The display 55 is further provided with an observing arrangement for permitting first and second images displayed on the image display layer 43 to be visible from first and second ranges of directions respectively, while preventing the first and second images from being visible from a third range of directions that includes the normal 2a display surface of the display. In this embodiment the observing arrangement comprises a plurality of Bragg stacks 56R, 56G, 56B, with one Bragg stack being arranged in front of each pixel of the image display layer 43. As is wellknown, a Bragg stack comprises a plurality of layers that are partially reflective and partially transmissive. The layers are arranged such that, when light passes perpendicularly through the Bragg stack, interference occurs between light reflected from the various layers of the Bragg stack.

In this way, it is possible to provide a Bragg stack that will block light from passing through it in the normal direction.

The parameters of each Bragg stack, such as the refractive indices of the layers, the thickness of the layers, and the number of layers, are optimised for each colour pixel.

Thus, a Bragg stack 56R in front of a red pixel 43R will be optimised so as to block the transmission of red light propagating in the normal direction through the Bragg stack.

Similarly, a Bragg stack 56G in front of a green pixel will be optimised to block the transmission of green light in the normal direction, and a Bragg stack 56B disposed in front of a blue pixel will be optimised to block the transmission of blue light in the normal direction. Thus, the display 55 of figure 14 will emit little or no light in the direction normal to the display face of the device, since this light is blocked by the Bragg stacks 56R, 56G, 56B.2 A Bragg stack will not, however, block light that is passing through the stack at an angle that is significantly different from normal direction. Thus, the display 55 will emit light in directions that are significantly different from the normal direction, as shown by the arrows 57, 57' in figure 14. The display 55 will therefore display the first and second images along the first and second directions, respectively, while providing a substantially black window between the first and second images.

This embodiment is not limited to Bragg stacks. Any suitable colour sensitive reflector that reflects on-axis light of a particular wavelength while transmitting off-axis light of that wavelength such as, for example, a cholesteric reflector, may be used.

Figure 15 shows a multiple view display 58 according to a further embodiment of the present invention. The display 58 comprises a backlight 43, a directional display device 59 such as, for example, a dual view display, and a directional scatterer 60 disposed in front of the multiple view display 59. The directional display device 59, as is known in the art, displays two images such that a first image is visible from a first range of directions and a second image is visible from a second range of directions different from the first range.

The directional scatterer 60 is effective at scattering light that is propagating along the normal axis of the display, and so will scatter the image that is seen when the display is viewed in the normal direction. Thus, an observer viewing the display along a direction parallel or close to the normal direction of the display face will not see an image, since the image is blurred by the directional scatterer. The directional scatterer does not, however, scatter light that is passing through it at an oblique angle. Thus, the two images displayed on the directional display device 59 will be visible in the normal viewing directions - as explained above, a typical viewing angle for a dual view display is + 40 from the normal direction, and light incident on the directional scatterer at an angle of + 40 to the normal axis will not be scattered. Thus, display of the two images in their intended viewing directions will not be affected by the directional scatterer 60.

Thus, when the display 58 is viewed from the left or right, it will appear as a normal multiple view directional display, since the directional scatterer 60 has no effect on light passing through it in these directions. However, when the display is viewed along the normal direction, the observed intensity will be low because light emitted in the normal direction by the dual view display 59 will be scattered by the directional scatterer 60. It should be noted that, when the display 58 is viewed in the normal direction, it will appear blurred owing to the scattering caused by the directional scatterer. This may be preferable to seeing a mixture of the two images, as can happen with a conventional dual view display.

A suitable directional scatterer for this embodiment is the "Lumisty" (trade mark) directional scattering film form Sumitomo Chemical Co. Limited.

The angular extent of the region in which a blurred image is seen will depend on the range of incident angles that are scattered by the directional scattering film. Thus, by using different directional scattering films it is possible to provide displays in which the region in which a blurred image is seen has different angular extents.

In the embodiments of figures 12a - 15, the backlight 42 is not a collimated backlight, but may be any conventional backlight such as, for example, a Lambertian backlight.

Figure 16a is a schematic plan view of a further multiple view display 61 according to the present invention. The display comprises a display device for displaying first and second images, a backlight 42, and a parallax barrier 49. Only the pixellated image display layer 43 of the display device is shown in figure 16a. The image display layer 43 and the parallax barrier 49 correspond generally to the parallax barrier and image display layer of the display 52 of figure 12c, and description of these components will not be repeated.

Figure 16b shows the light transmission through the image display layer 43 and the parallax barrier 49 of the display 61 of figure 16a, for the case where the width of the transmissive apertures 49a in the parallax barrier 49 is similar to the width of the pixels of the image display layer 43. As can be seen in figure 16b, the transmission of the image display layer and parallax barrier has maxima at angles of approximately + 40 from the normal to a display face of the display. Substantially no light is transmitted in the normal direction, and the output light is directed primarily into first and second ranges of direction that are centred on + 40 . The display will therefore provide a central black window between the first and second images.

However, the transmission shown in figure 16b varies steeply with changes in viewing angle around the viewing angles at which the transmission is a maximum. This means that if an observer moves their head from side to side, the brightness of the image seen lO by the observer will vary noticeably, and this can be distracting and uncomfortable for the observer.

According to this embodiment of the invention, therefore, the backlight 42 is not a conventional backlight that emits with substantially uniform intensity in all directions.

In this embodiment, the intensity of light emitted by the backlight depends on the angle of emission, and the variation in intensity with angle is selected so as to at least partially compensate for the angular changes in transmission of the image display layer and the parallax barrier 49.

Figure 17a shows the ideal angular distribution of intensity of light emitted by the backlight 42. In this ideal distribution, the backlight would emit no light in a direction parallel to, or close to, the normal to the display face of the display. The output light from the backlight would be concentrated around the intended viewing directions, in this case + 40 . The intensity is not a maximum at the intended viewing angles, however, but in fact displays a local minimum at each intended viewing angle.

The overall angular distribution of intensity of light emitted by the display 61 is shown in figure 1 7b. The overall intensity is obtained by multiplying the intensity distribution of the backlight shown in figure 1 7a by the transmission of the image display layer and barrier shown in figure 16b. In the overall intensity distribution of figure 17b, the intensity does not show maxima at the intended viewing angles, but rather exhibits plateau 62a, 62b in the intensity, with each plateau being centred around one of the intended viewing angles. The overall intensity does not vary significantly with the changes in viewing angle in the plateau regions 62a, 62b (and in the ideal case would exhibit no change with angle in the plateau regions 62a, 62b). The plateau regions 62a, 62b extend over the angular ranges from - b4 to -3 and from H1 to 82 , and the overall intcusity of the display does not vary significantly until the viewing angle is outside the plateau regions. If an observer viewing the display 61 of this embodiment moves their head from side to side, they will experience significantly no change in intensity, provided that they remain within a viewing angle range of from I to b2 or of from- 83 to-84.

In a preferred embodiment the display 61 has viewing properties that arc symmetric about the normal to the display face. In this case 83 = 81 and 82 = 04.

Figure 18a is a schematic plan view of a backlight 42 having an angular distribution of output intensity similar to that shown in figure 17a. The backlight 42 comprises an extended light source 63 that emits light over its area with a substantially isotropic intensity distribution.

A plurality of imaging elements 65 are arranged in front of the light source 63. The imaging elements may constitute, for example, a lenticular lens array. A parallax barrier 64 is disposed between the imaging elements 65 and the light source 63. The parallax barrier comprises a plurality of transmissive regions 64a that are separated by non- transmissive regions 64b. The light-transmissive regions 64a and non- transmissive regions 64b of the parallax barrier extend generally into the plane of the paper in figure 18.

The transmissive regions 64a of the parallax barrier do not have a uniform transmissivity over their width. The transmissivity of each transmissive region 64 a has a local minimum approximately mid-way across its width. One possible profile of the transmissivity of the parallax barrier 64 is shown in figure 18b.

The parallax barrier and the imaging elements are arranged so that the transmissive regions 64a of the parallax barrier are not disposed on the axis of any of the imaging elements. The imaging elements 65 form images of the transmissivc apertures of the parallax barrier, and since the transmissive apertures are not disposed on the axis of the imaging elements the images of the apertures are directed in off-axis directions that is, they are directed in directions that are not along the normal to the display surface of the display. The directions in which the images of the transmissive apertures 64a are formed can be varied by changing, for example, the imaging power of the imaging elements or the distance between the imaging elements and the barrier.

Furthermore, since the transmissivity of the transmissive apertures 64a has a local minimum approximately mid-way along its width, the intensity of an image of one of the transmissive apertures 64a will also display a local minimum approximately mid- way along the width of the image. Thus, a profile of intensity-against- angle similar to that shown in figure 17b is obtained.

The transmissive apertures 64a may be obtained by depositing an opaque material over a substrate in such a way that the thickness of the deposited material is not constant.

The transmissivity of the aperture will vary in an inverse dependence on the thickness of the deposited material, and a transmissivity profile having a local minimum may be obtained by depositing the material appropriately. Alternatively, a reflective material may be deposited with varying thickness to obtain a transmissivity profile having a local minimum, and this would have the advantage that light blocked by the barrier would be reflected into the light source 63 and could subsequently be re-emitted.

The backlights shown in figures 8, 9, 12a, 112b, 12c, 13, 14 and IS and the light source of figure 18a may each have the general form of a waveguide illuminated by one or more light sources disposed along side edges of the waveguide. If the backlight comprises one (or more) light sources arranged along one side edge of the waveguide and one (or more) light sources arranged along an opposite side edge of the waveguide as shown in figure 10, these embodiments may be used to provide a timemultiplexed display by controlling the light sources of the backlight to be illuminated in sequence as described with reference to figure 10. In the embodiments described above, the angular extent of the window

between the two viewing windows (the window in which the image is blurred in the embodiment of figure 15, or the "black window" in other embodiments) may be chosen to suit the intended application of the display. For example, the angular extent may be arranged so that the width of the window, at the intended viewing distance of the display, corresponds generally to the width of a human head. This allows an observer to position themselves in the window. Alternatively, the window may extend from an angle of approximately 5 on one side of the normal to the display surface of the device to an angle of approximately 5 on the other side of the normal to the display surface of the device, or from an angle of approximately 10 on one side of the normal to the display surface of the device to an angle of approximately 10 on the other side of the normal to the display surface of the device, or from an angle of approximately 15 on one side of the normal to the display surface of the device to an angle of approximately 15 on the other side of the nommal to the display surface of the device, or from an angle of approximately 20 on one side of the normal to the display surface of the device to an angle of approximately 20 on the other side of the normal to the display surface of the 1 5 device.

Claims

CLAIMS: 1. A multiple view display comprising: a directional display

device for displaying a first image or sequence of images so as to be mainly visible from a first range of directions relative to the device and for simultaneously displaying a second image or sequence of images so as to be mainly visible from a second range of directions relative to the device different from the first range; and a directional backlight for directing light through the device at least mainly in the first and second ranges.

2. A display as claimed in claim 1, in which the first and second ranges are on opposite sides of the normal to a display surface of the device.

3. A display as claimed in claim 2, in which the first and second ranges are substantially symmetrical with respect to the normal.

4. A display as claimed in any one of the preceding claims, in which the first image is substantially not visible from the second range.

5. A display as claimed in any one of the preceding claims, in which the second image is substantially not visible from the first range.

6. A display as claimed in any one of the preceding claims, in which the first and second ranges are separated by a third range of directions.

7. A display as claimed in claim 6, in which the backlight is arranged to direct substantially no light in at least part of the third range.

8. A display as claimed in any one of the preceding claims, in which the backlight comprises a light source and an optical arrangement for directing light in the first and second ranges.

9. A display as claimed in claim 8, in which the light source is arranged to supply at least partially collimated light.

10. A display as claimed in claim 9, in which the light source comprises a plurality of apertures aligned with imaging elements.

11. A display as claimed in claim 10, in which the apertures are separated by reflectors for reflecting light.

12. A display as claimed in any one of claims 8 to 11, in which the arrangement comprises a plurality of prisms.

13. A display as claimed in claim 12 wherein the prisms have a prism angle of less than 1 10 .

14. A display as claimed in claim 12 or 13 wherein the prisms have a prism angle of greater than 70 .

15. A display as claimed in claim 12, 13 or 14 wherein the prisms have a prism angle of approximately 83 .

16. A display as claimed in claim 12, in which the prisms are arranged as an array of variable pitch.

17. A display as claimed in claim 8, in which the arrangement comprises: an array of substantially non-light-transmissive blocking elements; and an array of imaging elements; wherein the array of imaging elements is substantially aligned with the blocking elements thereby reducing or substantially preventing light from being directed between the first and second ranges.

18. A display as claimed in claim 17, in which the blocking elements are reflective for recycling ligl1t.

19. A display as claimed in any preceding claim in which the second image or sequence of images is spatially multiplexed with the first image or sequence of images.

20. A display as claimed in any one of the preceding claims, in which the centres of the first and second ranges subtend an angle at the device of between 20 and 140 .

21. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of less than 130 .

22. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of less than 120 .

24. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of less than 110 .

25. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of less than 100 .

26. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of less than 90 .

27. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of greater than 30 .

28. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of greater than 40 .

29. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of greater than 50 .

30. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of greater than 60 .

31. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of greater than 70 .

32. A display as claimed in claim 20, in which the centres of the first and second ranges subtend an angle at the device of approximately 80 .

33. A display as claimed in any preceding claim and comprising a liquid crystal display.

34. A multi-directional two-dimensional display comprising: a display device for displaying a single image at a time, which image is visible throughout a range of directions including first and second ranges for first and second viewers, the first and second ranges being disposed on opposite sides of and spaced from the normal to a display surface of the device: and a directional backlight for directing light through the device with higher intensity in the first and second ranges than in a third range including the normal.

35. A display as claimed in claim 34, in which the first and second ranges are substantially symmetrical with respect to the normal.

36. A display as claimed in claim 34 or 35, in which the intensity in the third range is non-zero.

37. A display as claimed in any one of claims 34 to 36, in which the backlight is switchable between a first mode, in which light is mainly directed in the first range, and a second mode, in which light is mainly directed in the second range.

38. A display as claimed in claim 37, comprising a controller for repeatedly switching the backlight between the first and second modes and for controlling the device to display a first image or sequence of images when the backlight is in the first mode and a second image or sequence of images when the backlight is in the second mode.

39. A display as claimed in any one of claims 34 to 38, in which the backlight comprises first and second light sources disposed at opposite edges of a substantially rectangular generally planar light guide.

40. A display as claimed in any one of claims 34 to 39, in which the centres of the first and second ranges subtend an angle at the device of between 20 and 140 .

41. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of less than 130 .

42. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of less than 120 .

44. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of less than 110 .

45. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of less than 100 .

46. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of less than 90 .

47. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of greater than 30 .

48. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of greater than 40 .

49. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of greater than 50 .

50. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of greater than 60 .

51. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of greater than 70 .

52. A display as claimed in claim 40, in which the centres of the first and second ranges subtend an angle at the device of approximately 80=.

53. A display as claimed in any of claims 34 to 52 and comprising a liquid crystal display.

54. A multiple view display comprising: a display device for displaying first and second images or sequences of images; a backlight for directing light through the device; and an observing arrangement for permitting the first and second images or sequences of images to be visible from first and second ranges of directions, respectively, and for preventing the first and second images or sequences of images from being visible from a third range of directions including the normal to a display surface of the device, the first and second ranges being non-overlapping with the third range.

55. A display as claimed in claim 54, in which the first and second ranges are on opposite sides of the normal.

56. A display as claimed in claim 55, in which the first and second ranges are substantially symmetrical with respect to the normal.

57. A display as claimed in any one of claims 54-56, in which the backlight is a Lambertian backlight.

58. A display as claimed in any one of claims 54-57, in which the device comprises a plurality of pixels.

59. A display as claimed in any one of claims 54-58, in which the observing arrangement substantially blocks light in the third range.

60. A display as claimed in claim 59 when dependent on claim 58, in which the observing arrangement comprises a first parallax barrier.

61. A display as claimed in claim 60, in which the first parallax barrier comprises a plurality of slits, each of which is disposed above a gap between an adjacent pair of pixels.

62. A display as claimed in any of claims 54 to 60, in which the observing arrangement comprises a plurality of transmissive regions, each of which is arranged to be substantially non-transmissive to light from at least one adjacent pixel and to be substantially transmissive to light from at least one non-adjacent pixel.

63. A display as claimed in claim 62, in which the pixels are arranged as groups of different colours and the observing arrangement comprises a plurality of colour filter slits, each of which substantially blocks the colour of light from at least one adjacent pixel and transmits the colour of light from at least one non-adjacent pixel.

64. A display as claimed in claim 62, in which the pixels are arranged as groups of different output polarizations and the observing arrangement comprises a plurality of polarising slits, each of which substantially blocks the polarization of light from at least one adjacent pixel and transmits the polarization of light from at least one non-adjacent pixel.

65. A display as claimed in claim 60, in which the first parallax barrier comprises a plurality of slits and the observing arrangement comprises a second parallax barrier having a plurality of slits, each of which is disposed above a gap between adjacent slits of the first parallax barrier.

66. A display as claimed in claim 59 when dependent on claim 58, in which the pixels arc arranged as groups of different colours and the observing arrangement comprises a plurality of colour-selective reflectors, each of which is arranged to reflect the colour-selective reflectors, each of which is arranged to reflect the colour of light from at least one adjacent pixel and to transmit the colour of light from at least one nonadjacent pixel.

67. A display as claimed in any one of claims 54 to 58, in which the observing arrangement comprises a directional scatterer for scattering light in the third range.

68. A display as claimed in any of claims 54 to 67 wherein the first image or sequence of images is spatially multiplexed with the second image or sequence of images.

69. A display as claimed in any one of claims 54 to 68, in which the centres of the first and second ranges subtend an angle at the device of between 20 and 140 .

70. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of less than 130 .

71. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of less than 120 .

72. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of less than 110 .

73. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of less than 100 .

74. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of less than 90 .

75. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of greater than 30 .

76. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of greater than 40 .

77. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of greater than 50 .

78. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of greater than 60 .

79. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of greater than 70 .

80. A display as claimed in claim 69, in which the centres of the first and second ranges subtend an angle at the device of approximately 80 .

81. A display as claimed in any of claims 54 to 80 and comprising a liquid crystal display.

82. A display as claimed in any of claims 6, 7 to 33 when dependent directly or indirectly from claim 6, and 34 to 81 in which the third range extends from an angle of approximately 5 on one side of the normal to the display surface of the device to an angle of approximately 5 on the other side of the normal to the display surface of the device.

83. A display as claimed in any of claims 6, 7 to 33 when dependent directly or indirectly from claim 6, and 34 to 81 in which the third range extends from an angle of approximately 10 on one side of the normal to the display surface of the device to an angle of approximately 10 on the other side of the normal to the display surface of the device.

84. A display as claimed in any of claims 6, 7 to 33 when dependent directly or indirectly from claim 6, and 34 to 81 in which the third range extends from an angle of approximately 15 on one side of the normal to the display surface of the device to an angle of approximately 15 on the other side of the normal to the display surface of the device.

85. A display as claimed in any of claims 6, 7 to 33 when dependent directly or indirectly from claim 6, and 34 to 81 in which the third range extends from an angle of approximately 20 on one side of the normal to the display surface of the device to an angle of approximately 20 on the other side of the normal to the display surface of the device.