For many years conventional display devices have been designed to be viewed by multiple users simultaneously. The display properties of the display device are made such that viewers can see the same good image quality from different angles with respect to the display. This is effective in applications where many users require the same information from the display—such as, for example, displays of departure information at airports and railway stations. However, there are many applications where it would be desirable for individual users to be able to see different information from the same display. For example, in a motor car the driver may wish to view satellite navigation data, for example GPS data, while a passenger may wish to view a film, as illustrated schematically in FIG. 1. These conflicting needs could be satisfied by providing two separate display devices, but this would take up extra space and would increase the cost. Furthermore, if two separate displays, i.e., a passenger's display 70 and a driver's display 80 were used in this example it would be possible for the driver to see the passenger's display 70 if the driver moved his or her head, which would be distracting for the driver. As a further example, each player in a computer game for two or more players may wish to view the game from his or her own perspective. This is currently done by each player viewing the game on a separate display screen so that each player sees their own unique perspective on individual screens. However, providing a separate display screen for each player takes up a lot of space and is costly, and is not practical for portable games.
To solve these problems, multiple-view directional displays have been developed. 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.
Examples of possible applications of multiple-view directional display devices have been given above, but there are many other applications. For example, they may be used in aeroplanes where each passenger is provided with their own individual in-flight entertainment programmes. Currently each passenger is provided with an individual display device, typically in the back of the seat in the row in front. Using a multiple view directional display could provide considerable savings in cost, space and weight since it would be possible for one display to serve two or more passengers while still allowing each passenger to select their own choice of film.
A further advantage of a multiple-view directional display is the ability to preclude the users from seeing each other's views. This is desirable in applications requiring security such as banking or sales transactions, for example using an automatic teller machine (ATM), as well as in the above example of computer games.
A further application of a multiple view directional display is in producing a three-dimensional display. 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 re-create 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. A stereoscopic display typically displays both images of a stereoscopic image pair over a wide viewing area. Each of the views is encoded, for instance by colour, polarisation state, or time of display. The user is required to wear a filter system of glasses that separate the views and let each eye see only the view that is intended for it.
An autostereoscopic display displays a right-eye view and a left-eye view in different directions, so that each view is visible only from respective 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 window”. If the observer is situated such that their left eye is in the viewing window for the left eye view of a stereoscopic pair and their right eye is in the viewing window for the right-eye image of the pair, then a correct view will be seen by each eye of the observer and a three-dimensional image will be perceived. An autostereoscopic display requires no viewing aids to be worn by the observer.
An autostereoscopic display is similar in principle to a dual-view display. However, the two images displayed on an autostereoscopic display are the left-eye and right-eye images of a stereoscopic image pair, and so are not independent from one another. Furthermore, the two images are displayed so as to be visible to a single observer, with one image being visible to each eye of the observer.
For a flat panel autostereoscopic display, the formation of the viewing windows is typically due to a combination of the picture element (or “pixel”) structure of the image display unit of the autostereoscopic display and an optical element, generically termed a parallax optic. An example of a parallax optic is a parallax barrier, which is a screen with transmissive regions, often in the form of slits, separated by opaque regions. This screen can be set in front of or behind a spatial light modulator (SLM) having a two-dimensional array of picture elements to produce an autostereoscopic display.
FIG. 2 is a plan view of a conventional multiple view directional device, in this case an autostereoscopic display. The directional display 1 consists of a spatial light modulator (SLM) 4 that constitutes an image display device, and a parallax barrier 5. The SLM of FIG. 2 is in the form of a liquid crystal display (LCD) device having an active matrix thin film transistor (TFT) substrate 6, a counter-substrate 7, and a liquid crystal layer 8 disposed between the substrate 6 and the counter substrate 7. The SLM is provided with addressing electrodes (not shown) which define a plurality of independently-addressable picture elements or “pixels”, and is also provided with alignment layers (not shown) for aligning the liquid crystal layer. Viewing angle enhancement films 9 and linear polarisers 10 are provided on the outer surface of each substrate 6, 7. Illumination 11 is supplied from a backlight (not shown).
The parallax barrier 5 comprises a substrate 12 with a parallax barrier aperture array 13 formed on its surface adjacent the SLM 4. The aperture array comprises transparent apertures 15 separated by opaque portions 14. The apertures 15 are vertically extending (that is, extending into the plane of the paper in FIG. 2), and have the form of slits. An anti-reflection (AR) coating 16 is formed on the opposite surface of the parallax barrier substrate 12 (which forms the output surface of the display 1).
The pixels of the SLM 4 are arranged in rows and columns with the columns extending into the plane of the paper in FIG. 2. The pixel pitch (the distance from the centre of one pixel to the centre of an adjacent pixel) in the row or horizontal direction is p. The width of the vertically-extending transmissive slits 15 of the aperture array 13 is 2w and the horizontal pitch of the transmissive slits 15 is b. The plane of the barrier aperture array 13 is spaced from the plane of the liquid crystal layer 8 by a distances.
In use, the display device 1 forms a left-eye image and a right-eye image, and an observer who positions their head such that their left and right eyes are coincident with the left-eye viewing window 2 and the right-eye viewing window 3 respectively will see a three-dimensional image. The left and right viewing windows 2,3 are formed in a window plane 17 at the desired viewing distance from the display. The window plane is spaced from the plane of the aperture array 13 by a distance ro. The windows 2,3 are contiguous in the window plane and have a pitch e corresponding to the average separation between the two eyes of a human. The half angle to the centre of each window 2, 3 from the normal axis to the display normal is αs.
The pitch of the slits 15 in the parallax barrier 5 is chosen to be close to an integer multiple of the pixel pitch of the SLM 4 so that groups of columns of pixels are associated with a specific slit of the parallax barrier. FIG. 2 shows a display device in which two pixel columns of the SLM 4 are associated with each transmissive slit 15 of the parallax barrier.
In operation, the pixels are driven to display two images that are the left image and right image of a stereoscopic image pair. The images are interlaced on the pixels with, in the display of FIG. 2, alternate columns of pixels being assigned to each image.
A dual view display is similar in principle to the autostereoscopic 3-D display of FIG. 2. However, the pixels are drives to display two independent images intended for display to different observers. Moreover, since the images are intended for display to different observers the pitch e of the two viewing windows is greater in a dual view display than in an autostereoscopic 3-D display—the pitch e is typically of the order of a metre in a dual view display, and of the order of ten cm in an autostereoscopic 3-D display.
A high quality dual view display requires that each user is able to see a high quality, bright image of the desired data content without any interference from the other user's data content. Additionally, each user will require some freedom to move their viewing position again without degradation in image quality and without any interference from the other user's data content. If a user can see interference from the other user's data content then this is typically termed crosstalk or image mixing.
In order to construct a dual view display using a parallax barrier, a set of lines can be drawn from the centre of every odd-numbered pixel column P1,P3 etc to the right view position 3 and another set of lines can be drawn from the centre of every even-numbered pixel column P2,P4 etc to the left view position 2. (This is for a dual view display in which, in use, the left view will be displayed on the even-numbered pixel columns P2,P4 etc. and the right view will be displayed on the odd-numbered pixel columns P1,P3 etc.) At some distance from the plane 18 of the pixels, there are positions 19 where these sets of lines first intersect with one another, and, in a display where the pixels lie in a common plane, the positions of intersection 19 define a line 20, referred to as the “line of intersection”, that is spaced from but parallel to the plane 18 of the pixels. In fact, the positions of intersection 19 define a plane that extends into the plane of the paper, parallel to the plane of the pixels. This is shown in FIG. 3(a).
By placing a parallax barrier in the plane 20, such that the opaque regions 22 of the barrier 21 block light everywhere except at the points of intersection 19, as shown in FIG. 3 (b), a dual view display can be created such that the a user in the left view window cannot see the odd-numbered pixel columns and a user in the right view window cannot see the even-numbered pixel columns. In fact if one now draws a set of lines from the edges of each of the pixels, with each line passing though one of the apertures 23 of the parallax barrier 21, it can be shown that such a system naturally has a wide range of positions in which each user can see only the intended pixels—as shown in FIG. 3(b), the left and right view windows 2,3 have a large angular extent. This freedom of movement is one of the requirements for a high quality dual view display. However, in practice having apertures in the form of slits of only infinitesimal width through which light can pass will not provide a useable brightness for the display. In addition, light which passes through narrow apertures will be diffracted, and this light could cause unwanted crosstalk even in positions where geometrically one should not see any crosstalk.
It is possible to increase the brightness of the display by increasing the width of the apertures of the parallax barrier 21, however the increase in brightness is at a cost of reduced viewing freedom. As shown in FIG. 4, increasing the width of the apertures of the parallax barrier leads to a large window 24 between the left view window 2 and the right view window 3 where it is possible to view all of the pixels of the display. An observer positioned in the central region 24 will see both the left image and the right image, and will there fore experience image mixing.
A display having wider apertures in the parallax barrier 21 may again suffer from diffraction effects, which may give crosstalk in positions where geometrically there should not be any crosstalk. Additionally, if the opaque regions of the parallax barrier are made of a material which is not completely absorbing then light from the pixels can leak into the wrong viewing window and add to the unwanted crosstalk.
One known approach to solving these problems is to use a lenticular array (an array of semi-cylindrical lenses) as the parallax optic to provide image separation rather than a parallax barrier. A lenticular array can in principle give good brightness and large observer head freedom without a central image-mixing region. However in practice it may be difficult to achieve this performance for several reasons. The lenses may not be perfectly formed in manufacture, the mechanical stability of the display with thin lenses may suffer, and thermal expansion of the lenses might cause a mismatch in alignment with the pixels.
UK patent application No. 0320358.5 describes a multiple view display in which a prism array is disposed behind a parallax barrier aperture array such that a prism of the array is disposed behind every aperture of the parallax barrier. The prism array alters the angular separation between the left view window and the right view window. However, the angular extent of the central image mixing region is scaled up or down by the same factor as the angular extent of the viewing windows.
UK patent application No. 0501469.1 discloses a dual view display in which the content of data displayed on the pixels of the image display layer is adjusted to compensate for cross-talk between a left image and right image. In one embodiment a faint predetermined masking image is added to the content of the data.