GB2428857A

GB2428857A - Directional display and dual mode illumination system

Info

Publication number: GB2428857A
Application number: GB0515807A
Authority: GB
Inventors: Jonathan Harrold; Graham John Woodgate
Original assignee: Ocuity Ltd
Current assignee: Ocuity Ltd
Priority date: 2005-08-01
Filing date: 2005-08-01
Publication date: 2007-02-07
Also published as: GB0515807D0

Abstract

A directional display apparatus comprising a transmissive spatial light modulator and a parallax element capable of directing light from respective pixels into one of a plurality of different viewing windows has an illumination system operable in two modes. In the first mode, the illumination system illuminates the spatial light modulator with light of a first 230 and a second 232 of the three primary colours red, blue and green but not the third 234 of the three primary colours. In the first mode, the illumination system illuminates the spatial light modulator with light of the third 234 of the three primary colours. The array of pixels includes interlaced arrays of first 220 and second 222 types of pixel having respective colour filters, wherein the colour filter of the first type of pixel 220 passes light of the first 230 primary colour and filters out the second 232 primary colour, the colour filter of the second type of pixel 222 filters out light of the first 230 primary colour and passes light of the second 232 primary colour, and the colour filter of at least one of the first and second types of pixel passes light of the third 234 primary colour. The display device is operable in a two phase time sequential manner to display colour data in a spatially and temporally multiplexed manner. The display is a dual-view display that is switchable between a 2D mode and a multi-view mode or an autostereoscopic 3D mode. Also disclosed is a directional display wherein pixels of a first and second type in adjacent rows are offset from one another (see figs 11-14).

Description

Directional Display Apparatus The present invention relates to directional

display apparatuses. Such an apparatus uses a parallax element to direct light from pixels of a spatial light modulator into a plurality of different viewing windows. The apparatus may be for example a three dimensional (3D) autostereoscopic display apparatus or a multiple view display apparatus. The display apparatus may be used in computer monitors, telecommunications handsets, digital cameras, laptop and desktop computers, games apparatuses, automotive and other mobile display applications.

Normal human vision is stereoscopic, that is each eye sees a slightly different image of the world. The brain fuses the two images (referred to as the stereo pair) to give the sensation of depth. Three dimensional stereoscopic displays replay a separate, generally planar, image to each of the eyes corresponding to that which would be seen if viewing a real world scene. The brain again fuses the stereo pair to give the appearance of depth in the image.

Fig. 1 a shows in plan view a display surface in a display plane 1. A right eye 2 views a right eye homologous image point 3 on the display plane and a left eye 4 views a left eye homologous point 5 on the display plane to produce an apparent image point 6 perceived by the user behind the screen plane.

Fig. lb shows in plan view a display surface in a display plane 1. A right eye 2 views a right eye homologous image point 7 on the display plane and a left eye 4 views a left eye homologous point 8 on the display plane to produce an apparent image point 9 in front of the screen plane.

Fig. ic shows the appearance of the left eye image 10 and right eye image 11.

The homologous point 5 in the left eye image 10 is positioned on a reference line 12.

The corresponding homologous point 3 in the right eye image 11 is at a different relative position 3 with respect to the reference line 12. The separation 13 of the point 3 from the reference line 12 is called the disparity and in this case is a positive disparity for points which will lie behind the screen plane.

For a generalised point in the scene there is a corresponding point in each image of the stereo pair as shown in Fig. la. These points are termed the homologous points.

The relative separation of the homologous points between the two images is termed the disparity; points with zero disparity correspond to points at the depth plane of the display. Fig. lb shows that points with uncrossed disparity appear behind the display and Fig. ic shows that points with crossed disparity appear in front of the display. The magnitude of the separation of the homologous points, the distance to the observer, and the observer's interocular separation gives the amount of depth perceived on the display.

Stereoscopic type displays are well known in the prior art and refer to displays in which some kind of viewing aid is worn by the user to substantially separate the views sent to the left and right eyes. For example, the viewing aid may be colour filters in which the images are colour coded (e.g. red and green); polarising glasses in which the images are encoded in orthogonal polarisation states; or shutter glasses in which the views are encoded as a temporal sequence of images in synchronisation with the opening of the shutters of the glasses.

Autostereoscopic displays operate without viewing aids worn by the observer. In autostereoscopic displays, each of the views can be seen from a limited region in space as illustrated in Fig. 2.

Fig. 2a shows a display device 16 with an attached parallax optical element 17.

The display device produces a right eye image 18 for the right eye channel. The parallax optical element 17 directs light in a direction shown by the arrow 19 to produce a right eye viewing window 20 in the region in front of the display. An observer places their right eye 22 at the position of the window 20. The position of the left eye viewing window 24 is shown for reference. The viewing window 20 may also be referred to as a vertically extended optical pupil.

Fig. 2b shows the left eye optical system. The display device 16 produces a left eye image 26 for the left eye channel. The parallax optical element 17 directs light in a direction shown by the arrow 28 to produce a left eye viewing window 30 in the region in front of the display. An observer places their left eye 32 at the position of the window 30. The position of the right eye viewing window 20 is shown for reference.

The system comprises a display and an optical steering mechanism. The light from the left image 26 is sent to a limited region in front of the display, referred to as the viewing window 30. If an eye 32 is placed at the position of the viewing window 30 then the observer sees the appropriate image 26 across the whole of the display 16. Similarly the optical system sends the light intended for the right image 18 to a separate window 20. If the observer places their right eye 22 in that window then the right eye image will be seen across the whole of the display. Generally, the light from either image may be considered to have been optically steered (i.e. directed) into a respective directional distribution.

The optical system serves to generate a directional distribution of the illumination at a window plane at a defined distance from the display. The variation in intensity across the window plane of a display constitutes one tangible form of a directional distribution of the light.

The respective images are displayed at the display plane, and observed by an observer at or near the window plane. The variation in intensity across the window plane is not defined by the variation in intensity across the image; however the image seen by an observer at the window plane may be referred to as the image at the viewing window for ease of explanation.

In this application the term "spatial light modulator" (or SLM) is used to include devices which modulate the transmitted or reflected intensity of an external light source, examples of which include liquid crystal displays, and also devices which generate light themselves, often termed emissive displays, examples of which include electroluminescent displays.

In this application the term "3D" is used to refer to a stereoscopic or autostereoscopic image in which different images are presented to each eye resulting in the sensation of depth being created in the brain. This should be understood to be distinct from "3D graphics" in which a 3D object is rendered on a 2D dimensional display and each eye sees the exact same image One type of prior art switchable 2D/3D display system uses a switchable backlight unit in order to achieve switching between different directional distributions as described in Proc. SPIE vol.1915 Stereoscopic Displays and Applications P1(1993) pp1 77-186, "Developments in Autostereoscopic Technology at Dimension Technologies Inc.", 1993. In a first mode, the light distribution from the backlight is substantially uniform and a 2D directional distribution from the display is generated. In a second display mode, light lines are produced by the backlight. These light lines are modulated by LCD pixels so that the windows of an autostereoscopic intensity distribution for viewing a 3D image are formed. The switching could, for example, be accomplished by means of a switchable diffuser element, controlled by a voltage applied across the diffuser. Such diffusers are well known in the prior art.

One type of spatial light modulator for use with autostereoscopic display is described in EP-A-0,625,861. The pixels are aligned in a manner so that the columns of the pixels are substantially contiguous so as to provide uniform viewing windows when combined with a parallax optic with power in a first direction only. Another type of spatial light modulator in which the viewing windows have substantially uniform intensity when combined with a parallax optic with power in a first direction only is described in EP-A-0,833,184.

A colour spatial light modulator of the type described in EP-A-0,625,861 is disclosed in EP-A-0,752,610 which teaches compensation of colour balance in displays with more than three sub pixels by using a compensating reduction in efficiency of the device.

Colour filter patterns for two dimensional display devices in which more green pixels than red or blue are present are described in EP-A-0,322,106 and US-4,642,619.

In a directional display apparatus using a parallax element, there is a direct relationship between the nominal viewing distance and the separation between the spatial light modulator and the parallax element. Assuming the other parameters of the apparatus are the same, reduction of the viewing distance is achieved by reduction of the separation between the spatial light modulator and the parallax element. As a result, the provision of a display apparatus with a small viewing distance creates manufacturing difficulties. In terms of the manufacture, it is difficult to reduce the separation between the spatial light modulator and the parallax element, typically because it requires the use of thin substrates in the manufacture of the spatial light modulator. This increases manufacturing cost and reduces yield. This problem is well known and is a fundamental limitation on directional display apparatuses. It would be very desirable to produce a directional display apparatus in which this problem is eased.

According to the first aspect of the present invention, there is provided a directional display apparatus comprising: a transmissive spatial light modulator comprising an array of pixels; a parallax element capable of directing light from respective pixels into one of a plurality of different viewing windows; and an illumination system arranged to illuminate the spatial light modulator across the entire array of pixels, wherein the illumination system is operable in a first mode in which the illumination system illuminates the spatial light modulator with light of a first and a second of the three primary colours red, blue and green but not the third of the three primary colours and is operable in a second mode in which the illumination system illuminates the spatial light modulator with light of the third of the three primary colours, and the array of pixels includes interlaced arrays of first and second types of pixel having respective colour filters, wherein the colour filter of the first type of pixel passes light of the first primary colour and filters out the second primary colour, the colour filter of the second type of pixel filters out light of the first primary colour and passes light of the second primary colour, and the colour filter of at least one of the first and second types of pixel passes light of the third primary colour.

Such a directional display apparatus may be operated to display an image by operation in two sequential phases, typically under the control of a control circuit which forms part of the display apparatus.

In one phase, the illumination system is operated in the first mode so that the spatial light modulator is illuminated with both the first and second primary colours and the first and second types of pixel are addressed with the colour data of the first and second primary colours respectively. Light of both the first and second primary colours illuminates both types of pixel, but the first type of pixel only passes the first colour and the second type of pixel only passes the second primary colour. Furthermore, the light passed by the two types of pixel is modulated in accordance with the colour data of the same colour as the passed light. As a result the colour data of the first and second primary colours is displayed in this phase.

In the other phase, the illumination system is operated in the second mode so that the spatial light modulator is illuminated with the third primary colour arid the at least one of the first and second types of pixel is addressed with the colour data of the third primary colour. Light of the third primary colour illuminates both types of pixel and at least one of the types of pixel passes it. Furthermore the passed light of the third primary colour is modulated in accordance with the colour data of the third primary colour. As a result the colour data of the third primary colour is displayed in this phase.

The net result is that the colour data of all three primary colours is displayed over the two phases. This is achieved by time multiplexing of the colour data of the different colours. In addition, the display apparatus provides for spatial multiplexing of the colour data of the first and second primary colours. The degree of time multiplexing means that the spatial multiplexing is reduced as compared to a directional display apparatus of a conventional form employing colour pixels of all three primary colours. In particular, the number of pixels needed to display the colour data of all three primary colours is reduced from two to three. Similarly, given a particular area for a composite pixel needed to display the three-colour data, the area and pitch of the individual pixels is increased by a multiple of 1.5.

In a directional display apparatus, the increase in the pitch of the pixels has a beneficial effect that the pitch of the parallax element is increased by the same multiple of 1.5 and for a given viewing distance the separation of the parallax barrier and the spatial light modulator may be increased. This provides significant advantages in manufacture, For example in a typical spatial light modulator, the thickness of the substrates may be increased thereby allowing in a high resolution display the use of more standard thickness substrates, which are more compatible with standard manufacturing techniques. This increases display yield and reduces cost, as well as increasing the maximum number of panels that can be processed on a single substrate. This invention allows increased counter substrate thicknesses, which are easier to manufacture at high yields.

Conversely, the nominal viewing distance of the display can be reduced without reducing the separation of the pixel plane and optical element. Similarly, in a directional display apparatus providing multiple views, the viewing angle of each view can be increased without further degradation of image quality.

There are also further advantages as compared to a directional display apparatus of a conventional form employing colour pixels of all three primary colours, as follows.

These advantages basically follow from the increase in pitch and area of the individual pixels as set out above. Firstly, an increase in luminance frequency can be achieved without as significant a reduction in aperture ratio due to the finite area of electrodes and drive transistors. Thus the display brightness can be enhanced. There can be a reduction in the visibility of the zones between windows due to smaller relative size of the finite black mask width between pixels for the same increase in luminance frequency. The width of the gap between the windows can be reduced. This produces higher quality viewing windows with wider viewing freedom for a moving observer. An increased image brightness can be achieved.

Thus, as well as providing advantages relating to the separation between the parallax element and the spatial light modulator, the image quality of the display device can be improved in a number of ways.

As noted above, the advantages are achieved by the use of time multiplexing of three-colour data. Thus there are some similarities with time sequential display apparatuses which are known for the nondirectional display of two dimensional images and which employ threephase red, green and blue illumination of white transmissive pixels. An example of such a known display apparatus is described in WO-91/10223.

However, the present invention differs from such known display apparatuses by having a structure which employs both spatial and time multiplexing, and hence requires colour filters on the pixels. Furthermore, the present invention has particular advantages over the known time sequential display apparatuses if one were notionally to consider adapting them for use as a directional display apparatus by use of a parallax element. In particular, the absence of spatial multiplexing would cause the perceived size of the individual pixels in the viewing windows to double as compared to the present invention. This would increase the perception of blocks in the perceived image, sometimes called pixellation, as compared to the present invention. In addition, the increased amount of time-multiplexing would increase the degree of colour break-up artefacts perceived by the viewer, as compared to the present invention. In other words, the present invention has the advantages of reduced pixellation and reduced colour break-up artefacts over the notional application of a parallax barrier to a known time sequential 2D display apparatus.

Advantageously, the colour filters of both the first and second types of pixel pass light of the third primary colour. In this case, the illumination system in the second mode illuminates the spatial light modulator with light of the third of the three primary colours but not light of the first primary colour or the second primary colour, and in operation, both types of pixel are addressed with the colour data of the third colour. This choice of the colour filters has the advantage that in operation both types of pixel pass the third colour and are modulated in accordance with the colour signal of the third colour. This is a particular advantage if the third primary colour is green, in which case the colour filters of the first and second types of pixel are yellow and cyan filters. In this case the appearance of stripes on the display apparatus is reduced. This is for the following reason. The appearance of stripes arises because the perceived luminance of the green pixels is greater than the perceived luminance of the red and blue pixels. In general in a directional display apparatus with a parallax element this effect is more noticeable because the resolution of the pixels seen by a viewer is decreased, as compared to a display having a spatial light modulator with the same arrangement of pixels but no parallax element. This decreased resolution allows a viewer to resolve the luminance difference between the different colour pixels which is perceived as stripiness. However, by providing for both types of pixel to pass green light, this effect is greatly reduced or removed altogether in the case that the display has no further types of pixel.

However, it is not essential that both types of pixel transmit light of the third primary colour. As an alternative it is possible for one of the first and second types of pixel to pass light of the third primary colour and for the other one of the first and second types of pixel to filter out light of the third primary colour. In this case the illumination system may be arranged to emit in the second mode either (1) no light of the first and second primary colours, or else (2) light of one of the first and second primary colours, being the same one of the first and second primary colours which is passed by the type of colour pixel which filters out the third primary colour. Taking as an example the first to third primary colours being red, blue and green respectively, an example of case (1) is that the colour filter of the first type of pixel is red, the colour of the second type of pixel is cyan (so it passes green and blue light) and the illumination system emits only green light in the second mode. Taking as an example the first to third primary colours being red, blue and green respectively, an example of case (2) is that the colour filter of the first type of pixel is red, the colour of the second type of pixel is cyan (so it passes green and blue light) and the illumination system emits both green light and red light in the second mode. Other combinations of the primary colours are similarly possible. In general, the intensity of the light needs to be controlled depending on whether it is output in one or both phases and whether it passes through both or one type of pixel.

In one type of embodiment only the first and second types of pixel are present, but this is not essential. For example, in another type of embodiment, the array of pixels further includes a third type of pixel which passes light of all three primary colours interlaced with the arrays of first and second types of pixel. The third type of pixel may be thought of as a white pixel and has the advantage of decreasing the brightness of the display apparatus, albeit at the expense of decreasing the pixel pitch. Nonetheless, the display apparatus may still be operated with two sequential phases by addressing the third type of pixel with data derived as a combination of the colour data of all three colours by colour-gamut mapping.

As to configurations of pixels in a directional display apparatus, one type of prior art pixel configuration for autostereoscopic displays uses the well known stripe configuration as shown in Fig. 7a as used for standard 2D displays. This is described in more detail below and has pixels arranged in columns of red, green and blue pixels as shown in Fig. 8. To generate an autostereoscopic display, a parallax element such as a lenticular array is aligned with pairs of colour sub-pixels as shown in Fig. 7b which shows the appearance of the right eye image to an observer viewing the stereoscopic image through the parallax element.

In this kind of two view spatially multiplexed autostereoscopic image, the horizontal pixel resolution of the stereoscopic image is half of the 2D horizontal pixel resolution. Suitable parallax elements include lenticular screens, parallax barriers, holographic optical elements and polarisation selective elements in combination with a suitably polarised illumination source. The reduction in horizontal resolution may lead to the image appearing stripy for an observer positioned at a nominal viewing distance. It would be desirable to reduce this appearance of stripiness.

According to a second aspect of the present invention, there is provided a directional display apparatus comprising: a spatial light modulator comprising an array of pixels arranged in rows and comprising at least two types of pixel interlaced along each row, each type of pixel being arranged to output light of a different colour; and a parallax element capable of directing light from respective pixels into one of a plurality of different viewing windows, the parallax element having a structure which is uniform in a direction perpendicular to the rows, wherein the pixels of each type of pixel in adjacent rows are offset from one another.

The offset of each type of pixel in adjacent rows reduces the appearance of stripiness in the image seen by a viewer as compared to the case that the pixels of each type are aligned. This results from a physiological phenomenum. The appearance of stripes arises because the perceived luminance of the green pixels is greater than the perceived luminance of the red and blue pixels. In a directional display apparatus with a parallax element this effect is more noticeable because the resolution of the pixels seen by a viewer is decreased, as compared to a display having a spatial light modulator with the same arrangement of pixels but no parallax element. This decreased resolution allows a viewer to resolve the luminance difference between the different colour pixels which is perceived as stripiness. In a display in which the pixels of the same type are aligned the stripes extend parallel to the windows which is vertical in typical applications of the directional display apparatus for example as a multiviewer display or an autostereoscopic display. The offsetting of each type of pixel in adjacent rows reduces the perception of the stripes as a result of the fact that the pixels of the same color in successive rows are arranged at an angle, known as the screen angle, which is not vertical. It is a physiological effect that the presence of such a screen angle reduces the appearance of stripes. This effect is used to particular advantage in a directional display apparatus employing a parallax element.

In one advantage form, the spatial light modulator is a transmissive spatial light modulator and the display apparatus further comprises an illumination system arranged to illuminate the spatial light modulator across the entire of array of composite pixels.

Both aspects of the present invention may be applied to any type of parallax element, including both parallax barriers and lenticular arrays, and including parallax elements which are switchable between a first mode in which the parallax element is effective to direct light from respective pixels into said different viewing windows and a second mode in which the parallax element has no effect.

With both aspects of the present invention, the spatial light modulator may take any form. For example it may be a transmissive liquid crystal spatial light modulator.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. I a shows the generation of apparent depth in a 3D display for an object behind the screen plane; Fig. lb shows the generation of apparent depth in a 3D display for an object in front of the screen plane; Fig. 1 c shows the position of the corresponding homologous points on each image of a stereo pair of images; Fig. 2a shows schematically the formation of the right eye viewing window in front of an autostereoscopic 3D display; Fig. 2b shows schematically the formation of the left eye viewing window in front of an autostereoscopic 3D display; Fig. 3 shows a switchable 2D/3D system; Fig. 4 shows a 3D autostereoscopic display in which the directional distribution is switched by means of an electronically controlled polarisation switching element; Fig. 5 shows a further 3D autostereoscopic display in which the directional distribution is switched by means of an electronically controlled polarisation switching element between a lens array and an output polariser; Fig. 6 shows a further 3D autostereoscopic display in which the directional distribution is switched by means of an electronically controlled polarisation switching element between an output polariser and a lens array;

Fig. 7a shows a prior art colour filter pattern;

Fig. 7b shows the appearance of Fig. 7a when used in conjunction with a two view parallax optic in the right eye of an observer; Fig. 8 shows the arrangement of data on a two view lenticular display of the type shown in Fig. 7a; Fig. 9 shows a graph of a known computational model for perceived contrast sensitivity against spatial frequency; Fig. 10 shows the effect of higher green pixel luminance on the spatial luminance distribution of colour pixellated displays; Fig. 11 shows a pixel configuration of a first embodiment of the invention and the respective position of a lenticular screen parallax optic; Fig. 12 shows a pixel configuration of a further embodiment of the invention and the respective position of a lenticular screen parallax optic; Fig. 13 shows a pixel configuration of a further embodiment of the invention and the respective position of a lenticular screen parallax optic; Fig. 14 shows a pixel configuration of a further embodiment of the invention and the respective position of a lenticular screen parallax optic; Fig. 15 shows a time sequential embodiment of the present invention and the respective position of a lenticular screen parallax optic; Fig. 16 shows a further time sequential embodiment of the present invention and the respective position of a lenticular screen parallax optic; Fig. 17 shows a further embodiment of the present invention comprising a tilted lenticular screen; Fig. 18 shows a further embodiment of the present invention comprising a white pixel aperture; Fig. 19 shows a further embodiment of the present invention showing control apparatus for the backlight and display of the present invention;and Fig. 20 shows a spatial light modulator of the invention.

The present embodiments are 2D/3D switchable directional display devices of the general type disclosed in WO-03/0l 5424, but with some modifications as described in detail below. First the overall construction of particular display devices will be described. WO- 03/015424 discloses further details of the constructions and operation of the display devices, which may be applied to the present invention. WO- 03/015424 which is incorporated herein by reference.

A number of different embodiments are described which use many elements in common. The common elements bear the same reference numerals and for brevity the description thereof is not repeated. For clarity the drawings illustrate only a part of the area of the display devices. Practical display devices will have much larger numbers of pixels formed by repeating the structure shown in the drawings. All the components are arranged in series Fig. 3 which shows one type of switchable directional display as described in WO-03/015424. A backlight 1034 acts as an illumination system. The backlight 1034 produces an optical output 1036 which is incident on a liquid crystal spatial light modulator comprising an input linear polariser 1038, a LCD TFT substrate 1040, a pixel plane 1042 comprising an array of LCD pixels 1044-1058 and an LCD counter substrate 1064. Each pixel 1044-1058 comprises a separate region of addressable liquid crystal material and a colour filter and is surrounded by a black mask 1060 which defines pixel apertures 1062. Thus the spatial light modulator is transmissive and the backlight illuminates the entire array of pixels 1044-1058. The pixels 1044-1058 may be addressed with data and they modulate the light passing therethrough in accordance with that data.

Arranged in series with the spatial light modulator to receive light passing therethrough, the display device has a lenticular array comprising a birefringent microlens array 1072 comprising a layer of birefringent material 1068 and an isotropic lens microstructure 1070. The birefringent microlens array 1072 is mounted between a carrier substrate 1066 fixed to the LCD counter substrate 1064 and a lens substrate 1074.

The lenticular array further comprises a polarisation modifying device 1076 arranged in series with the birefringent microlens array 1072 to receive light therefrom.

The polarisation modifying device 1076 is switchable between a two modes in which the light output from the device is of two respective, different polarisation components. One of the polarisation components is directed into a directional distribution by the birefringent microlens array 1072 in which light from respective pixels 1044-1058 is directed into one of a plurality of viewing windows, for example to provide multiple viewing windows or an autostereoscopic effect. Thus in the mode in which the first polarisation component is output, the birefringent microlens array 1072 acts as a parallax element. The other polarisation component is not affected by the birefringent microlens array 1072 and thus light from the pixels 1044-1058 is not directed into the viewing windows.

Fig. 4 shows a further type of switchable directional display, as described in WO- 03/015424, in which the directional distribution is switched by means of a switchable polariser element. The display device has a backlight 1034 and a spatial light modulator having the same construction as in the device of Fig. 3 except for the addition of an LCD output polariser 1414 between the LCD counter substrate 1064 and the carrier substrate 1066. The display device also has a birefringent microlens array 1072 having the same construction as in the device of Fig. 3 except that the polarisation modifying device 1076 is replaced by a polarisation modifying device 1416.

The polarisation modifying device 1416 may be embodied as for example a twisted nematic liquid crystal layer sandwiched between surfaces treated with transparent electrodes and liquid crystal alignment layers 1418 as well known in the art.

A sensing device 1424 may be used to monitor the electrical driving of the polarisation switching layer 1416. A second substrate 1420 is formed on the output side of the polarisation modifying device 1416 and has a polariser 1422 attached to its outer side.

The polariser 1422 may be a linear polariser with a transmission direction aligned at 45 degrees to the birefringent optical axis of the microlens array 1072. The birefringent axis of the microlens array 1072 is the direction of the extraordinary axis of the birefringent material used in the birefringent microlens array 1072. The polarisation state incident on to the birefringent microlens array 1072 will resolve on to the two axes of the birefringent material. In a first axis, the refractive index of the birefringent material is substantially index matched to the isotropic index of the birefringent microlens array 1072 and so the lens has substantially no imaging function. In a second axis, which maybe orthogonal to the first axis, the refractive index of the birefringent material has a different refractive index to the isotropic material and thus the lens has an imaging function.

In a non-directional mode of operation, no voltage is applied across the polarisation modifying device 1416, and an incident polarisation state is rotated. In a directional mode of operation, a voltage is applied across the polarisation modifying device 1416, and the incident polarisation state is substantially unrotated.

If the polarisation modifying device 1416 is set so that the polarisation state transmitted through the polariser 1422 is parallel to the first axis, then the display will have a non-directional distribution. If the switch 1416 is set so that the polarisation state transmitted through the polariser 1422 is parallel to the second axis, then the display will have an directional distribution, for example to provide an autostereoscopic effect. The sensing device 1424 thus determines the display mode of the optical switching apparatus by determining the electrical driving of the polarising element.

Fig. 5 shows a further type of switchable directional display, as described in WO- 03/0 15424, in which the directional distribution is switched by means of a switchable polariser element. This device is similar in structure to the architecture of Fig. 4 except that the polariser 1414 is omitted and the orientation of polarisation angles is different.

Such a device operates is a similar way to the device of Fig. 3 except that the mechanically reconfigurable polariser is replaced by an electrically switched polariser 1416 which may be for example a twisted nematic liquid crystal layer sandwiched between surfaces 1418 comprising transparent electrodes and alignment layers and an absorbing linear polariser 1422.

As described for Fig. 4, the device may be switched between 2D and 3D directional distributions by selecting the polarisation state that is transmitted by the final polariser 1422.

Fig. 6 shows a further type of switchable directional display, as described in WO- 03/01 5424, in which the directional distribution is switched by means of a switchable polariser element. The device is the same as that of Fig. 4 except that the polarisation modifying device 1416 is positioned between the an LCD output polariser 1414 and the birefringent microlens array 1072. The output linear polarisation of the display transmitted by an LCD output polariser 1414 is transmitted though a switch substrate 1432, to the polarisation modifying device 1416 which comprises transparent electrodes and alignment layers 1418 sandwiching a twisted nematic layer 1430.

In the 2D mode, the polarisation modifying device 1416 rotates the incident polarisation so that it is incident on to the ordinary axis of the material in the birefringent microlens. The ordinary index is matched to the index of the isotropic material and thus the lens has no effect. In the 3D mode, an electric field is applied to the liquid crystal layer 1430 so that the polarisation state is not rotated by the polarisation modifying device 1416 and the light is incident on the extraordinary axis of the birefringent microlens. The lens then has an optical effect which produces the autostereoscopic directional distribution.

The sensing device 1424 thus determines the display mode of the optical switching apparatus by determining the electrical driving of the polarising element.

The embodiments of this invention described below may be applied to any of the types of directional display device described above, although they could equally be applied to other lenticular screen devices including active lens devices and parallax barrier devices, in which the parallax elements are fixed 3D or switchable 2D/3D.

The configuration of the pixels in the spatial light modulator will now be considered.

Fig. 7a shows the well known stripe' pixel configuration used in many display types, comprising columns of red 1228, green 1234 and blue 1238 pixels. A lens array is shown in cross section while the pixels are shown in plan view for ease of explanation in the figures of this document. If a cylindrical lens array 100 is placed over the surface of this pixel configuration then each eye of the observer will see half of the horizontal pixels. This is illustrated in Fig. 7b for the right eye image comprising columns of red 102, blue 104 and green 106 image pixels. In this case, the horizontal gap 108 between the pixels is substantially zero because the lenticular screen serves to distribute the light from the respective pixel across the whole of the aperture of the lens.

The use of colour pixels in a two view autostereoscopic display is shown in more detail in Fig. 8. The lens 1214 of the lens array 1208 serves to cover pixel columns 1228 and 1234. The column 1228 contains red right eye data and the column 1234 contains green left eye data. The pixels 1222 are imaged to the right eye by the lens 1214 and appear to fill the aperture of lens 1214. In the adjacent lens 1216, the blue pixel column 1238 is imaged to the right eye and the red pixel column 1230 is imaged to the left eye.

Similarly for the lens 1218 the green pixel column 1236 is imaged to the right eye and the blue pixel column 1240 is imaged to the left eye.

In the 2D mode, a colour pixel 1200 is made from adjacent colour subpixels 1202, 1204 and 1206. However, the 3D image colour pixel is formed from pixels that have twice the spacing for example 1224, 1242 and 1207.

As shown in Fig. 7b, the horizontal resolution of a conventional stripe image produced by attaching a parallax optic such as a lenticular screen or parallax barrier to a conventional stripe panel is half the full panel resolution. The disadvantage of this approach is that the stereo image may appear to contain artifacts, for example appearing to contain vertical stripes.

One origin of the appearance of these stripes may be due to the human contrast sensitivity function as shown for example in Fig. 9, taken from the known relationship described in J. L. Mannos, D. J. Sakrison, "The Effects of a Visual Fidelity Criterion on the Encoding of Images", IEEE Transactions on Information Theory, pp. 525-535, Vol. 20, No 4, (1974). This relationship describes the variation of visual contrast sensitivity against spatial frequency 112 of a luminance function. The luminance spatial frequency for a stripe panel may be defined as the spatial frequency of the triplet of colour pixels.

For a typical display using a stripe panel of pixel pitch 8Oum, viewed from 400mm, the spatial frequency of the green channel for example is 29 cycles per degree and is shown by the arrow 114. When a lenticular screen is added and the device is viewed in the 3D mode then the spatial frequency is halved to 14.5 cycles per degree and the arrow 116 has been marked. In this case, the contrast sensitivity function has increased from 0.2 to 0.8, close to the peak of the human contrast sensitivity function.

Clearly this value can be reduced by increasing the distance of the observer from the display, but the image will be less easily viewed and so this approach is not desirable.

Fig. 10 shows schematically the horizontal luminance function in images with RGB stripe pixel patterns. An array of pixels containing red 118, green 120 and blue 122 data columns is shown. The overall colour balance of the combined image is set as a standard white. The equivalent photopic luminance 124 of the three channels against position 126 is shown below. The graph shows that the luminance perceived from the green pixels is greater than the luminance of the red and blue pixels due to the human photopic efficacy function. At high resolutions, the human visual system cannot resolve the separate luminance levels of the RGB pixels and so this luminance difference is not perceived and the image appears uniform. However, if the pixel resolution falls as is the case with the half horizontal resolution stereo image, then the difference between the luminance of the red and blue pixels and the luminance of the green pixel may become apparent. The brighter green pixel columns may thus be seen as interspersed by dimmer red and blue columns, causing stripes to appear in the stereo image. Such a stripe appearance would be visible irrespective of the pixel shape on the display. Thus, at these spatial frequencies, removing the gap between the pixels would not be expected to have any significant impact on image stripes in the stereoscopic mode of the display.

Multi-view displays advantageously increase the viewing angle of 3D displays compared to two view displays. Thus, conventional pixel patterns, particularly when used in multi-view displays suffer from image stripiness caused by visibility of horizontal luminance functions in the 3D mode.

Embodiments of the second aspect of the present invention will now be described. The following description explains the nature of the pixels 1044-1058 in the spatial light modulators of the display devices of Figs. 3 to 6. In the following, the birefringent microlens array 1072 is shown schematically as a lenticular screen 200.

A first embodiment of the invention is shown in Fig. 11. For illustrative purposes, the pixel pattern is shown with the relative orientation of a four-view lenticular screen 200, that is the light is directed into four viewing windows. The geometric axis of the lenticular screen is parallel to the columns of pixels. The pixel pattern comprises groups of red, green and blue pixels arranged in a mosaic pattern in rows 208,210,212 and columns 202,204,206 arranged perpendicular to each other. The rows 208,210,212 extend perpendicular to the lenticular screen 200 which thus has a structure which is uniform parallel to the columns 202,204,206. The types of pixel in each row 208,210,212 are red, green and blue pixels interlaced with each other. The pixels of each type (e.g. the green pixels) are offset from each other by an amount equal to the pitch of the pixels along each row 208, 210,212. Thus, within each row 208,210,212, each group of three pixel columns 202,204,206 comprises a red, green and blue pixel, and that within each column 202,204,206 each group of three pixel rows 208,210,212 comprises a red, green and blue pixel.

The views imaged at the window plane are associated with each column 202,204,206 of pixels. Thus, the viewer's eye in the first window sees a red pixel from row 208, column202, a blue pixel from row 210, column 202 and a green pixel from row 212, column 202. Thus, within each view, green pixels are seen under each lens, in particular under adjacent lenses in adjacent rows, rather than from every third lens. In a display in which the horizontal pixel pitch of the SLM is 1/3 the vertical pixel pitch, the angle of the green pixel grid to the vertical, termed screen angle, 201 of the green pixels is thus changed from 0 degrees (parallel to the vertical) to 53 degrees in a 4 window system. Such a screen angle substantially reduces the visibility of the luminance frequency in the 3D multi-view mode.

It is well known, for example in the printing industry, that optimum screen angles at 45 degrees to the vertical minimise visibility of luminance patterns in images. In a further embodiment of the present invention, the screen angle 201 of the 3D mode may be advantageously set to be 45 degrees, by adjusting the vertical to horizontal pixel pitches of the pixels. In the case of a 4 window system, the lateral pitch of the pixels may for example be /4 of the vertical pitch of the pixels, to enable a 45 degree screen angle in the 3D mode.

Screen angle in the 3D mode can be more accurately be determined using the lens pitch rather than N x lateral pixel pitch, where N is an integer number of views produced in a single row by a lens. The lens pitch is generally slightly smaller (in front parallax optical systems) or slightly larger (in rear parallax optical systems) than an integer number of pixel pitches.

In such a system, the data input may need to be adjusted so as to correct for distortions in the size of the image in both 2D and 3D modes. Distortion correction can be achieved as by sampling the data and interpolating the missing data points appropriately. In the general case of a display with N viewing windows, the horizontal to vertical pitch of the pixels would optimally be set to be substantially I/N, so as to achieve an optimised screen angle.

Although the pixel configuration shown in Fig. 11 relates to a lenticular screen 200 providing four viewing windows, in general the same principle applies to any directional display with any number of viewing windows. In general, if the pixels are arranged in columns 202,204,206, the pixels of each type are offset in adjacent rows 208,210,212 by an amount equal to the pitch of the pixels in each row 208, 210,212 multiplied by an integer. To provide a reduction in the visibility of the luminance patterns, the screen angle is preferably in the range from 250 to 650, more preferably 300 to 60 , more preferably 35 to 55 .

Alternatively, the pixels may be arranged as shown in Fig. 12 in a delta pattern such that alternate rows are offset by an amount equal to the pitch of the pixels in a row multiplied by a half. In this case, the screen angle, 203 is 49 degrees between green pixels in adjacent lenses. Such a system advantageously does not require distortion of input images, while reducing the visibility of screens in the 3D mode. More generally, the alternate rows may be offset by an amount equal to the pitch of the pixels in a row multiplied by any fraction.

Thus advantageously, the resolution loss of the 3D mode is optimally compensated by setting the screen angle of the green channel at or near to 45 degrees, and by ensuring that each pixel column has green data within it. Such a display is achieved without the requirement to incorporate pixels or different size, for example as described in WO- 2005/006775. Thus, the present invention reduces the number of additional column and row drivers. Further, the aperture ratio of the display is optimised by reducing the total number of pixels on the display.

Lenticular display systems may suffer from chromatic aberration. The eye spot, being the image of a nominal observer's pupil (which may have a diameter 2-6mm, preferably 4mm) at the pixel plane when imaged by the lens may have a different size for red, green and blue wavelengths. In multi-view displays, the intensity difference as the observer moves across the window plane should be minimised. Typically multi-view displays may use an eye spot which is nominally the same width as the pixel pitch. In the case of a chromatically aberrated spot due to the chromatic dispersion of the materials from which the lens is formed, the eye spot may have a reduced or increased spot width in red or blue wavelengths. Thus, the intensity uniformity at the window plane can be optimised for the green channel, but may provide residual intensity differences in the red and blue channels. The width of the red, green and blue pixel apertures can be adjusted to compensate for this effect, as shown in Fig. 13. Thus the green pixel 214 may have a nominal width, the blue pixel 216 may have a smaller width and the red pixel 218 may have a greater width. Alternatively or additionally, the pixels 214,216,218 may have different heights as shown in Fig. 14, to compensate for the lateral pixel aperture change, thus maintaining the aperture ratio of the panel. Thus the area of the pixel is adjusted to compensate for the difference in the eye spot size between red, green and blue pixels.

Advantageously, such a system can substantially improve the uniformity of the display in the window plane.

In the embodiments of the invention above, green data is placed in each row and each column of the display, with pixels of equal size. Such a configuration may not require the fabrication of pixels of different sizes to standard pixel sizes, and conveniently requires an adjustment of colour filter layout and addressing circuitry.

Particularly for multi-view displays in which the lateral resolution is reduced, the loss of luminance frequency is compensated by ensuring each view column comprises green pixel data. In the case of a purely spatially multiplexed display, an improvement in image appearance is obtained by adjusting the screen angle in the 3D mode to be close to degrees.

Embodiments of the first aspect of the present invention will now be described.

The following description explains, in the display devices of Figs. 3 to 6, the nature of the pixels 1044-1058 in the spatial light modulator and the backlight 1034 which acts as an illumination system. In the following, the birefringent microlens array 1072 is shown schematically as a lenticular screen 200.

The following embodiments involve a combination of spatial multiplexing and time multiplexing of the colour data of the three primary colours red, blue and green.

There are known in the art time sequential 2D displays in which a composite pixel comprising red, green and blue pixels is replaced with a single white pixel and phased red, green and blue illumination in synchronization with red, green and blue data. In principle a directional display such as a four view autostereoscopic display could be configured in which a lenticular screen or other parallax barrier is placed over four columns of pixels of this display. However, such an element would have a lens width of three times that of an RGB stripe display. Such a display will show image blockiness, compared to the image stripiness of the RGB stripe configuration produced by a display similar to that shown in Fig. 8 for example. Image blockiness, sometimes referred to as pixellation, is due to a wide lateral distribution of the image data from a pixel and can degrade high frequency features in the image. Further, such elements will have a high thickness, which may become unacceptable. Continuing the above example, the lens pitch and thus the width of each of the red, green and blue pixels for a four view system would be 600microns. The separation of pixel to lens would be 2.55mm. Such a thickness is considered to great for many applications, and requires substrates of thickness greater than typically used for LCD manufacture, also disadvantageously increasing device weight. Accordingly the present invention uses an alternative approach which reduces this problem.

An embodiment of the first aspect of the present invention is shown in Fig. 15. In this embodiment, a pixel configuration comprises two types of pixels, namely cyan pixels 220 and yellow pixels 222 of substantially equal size. In this case, the cyan pixels 220 and yellow pixels 222 may be considered as a composite colour pixel 224. The cyan pixels 220 have a colour filter which filters out red light but passes blue and green light, while the yellow pixels 222 have a filter which filters out blue light, but passes green and red light.

The backlight 1034 comprises red, green and blue light sources 230, 232, 234, respectively, which may for example be LEDs are arranged to illuminate the entire array of pixels in the spatial light modulator through an optical system 236, which may comprise optical waveguides and other elements known in themselves in the art. The three light sources 230, 232, 234 are independently operable. As is conventional, the references to light of each of the three primary colours refers to light which is predominantly of the colour concerned but which typically has a range of wavelengths which may include small amounts of other colours, depending on the precise nature of the sources 230, 232, 234. Similarly the references to the colour filters filtering and passing colour of light refer to this being the predominant action of the colour filters.

A control circuit 235 controls the operation of the light sources 230, 232, 234 and also addresses the pixels in the spatial light modulator with three colour data of red, blue and green colours. In particular, the control circuit 235 operates in two phases which are alternated time sequentially. The operation under the control of the control circuit 235 is as follows.

In the first illumination phase, the red light source 230 and blue light source 232 are simultaneously operated without the green source 234 being operated. As a result red and blue light illuminates the pixels. At the same time the cyan pixels 220 are addressed with blue data and the yellow pixels 222 are addressed with the red data. The cyan pixels 220 pass the blue light and modulate it in accordance with the blue data and the yellow pixels 222 pass the red light and modulate it in accordance with the red data. Thus in the first phase the pixels 220, 222 spatially multiplex the red and blue data and display the red and blue components of the image.

In the second illumination phase, the green source 234 is operated without the red light source 230 and blue light source 232 being operated. As a result green light illuminates the pixels. At the same time the cyan pixels 220 and the green pixels 222 are addressed with green data. The cyan pixels 220 and the yellow pixels 222 both pass the green light and modulate it in accordance with the green data. Thus in the second phase the cyan and yellow pixels 220, 222 display the green component of the image.

The use of the two phases temporally multiplexes the red and blue data with the green data so that the entire image is displayed in each cycle of the two phases.

The composite colour pixel 224 may be square, so that the individual pixels 220,222 may have an increased width compared to the equivalent RGB pixels of a purely spatially multiplexed display. This is particularly advantageous for autostereoscopic displays. The nominal viewing distance of such a display is determined by the pixel pitch, the refractive index of the optical system and the separation of the lens from the pixel plane. If the pixel pitch is increased, the nominal viewing distance is reduced.

However, the pitch is not increased so much that image blockiness artefacts become dominant, and the glass thickness becomes excessive.

The panel can be compared to an RGB stripe panel with a colour sub-pixel pitch of 50microns (colour pixel pitch of 150 microns). Such a 2D panel does not show image stripiness for a viewing distance of 400mm. In a four view autostereoscopic 3D mode, the RGB panel has a lens pitch of 200 microns and a 3D luminance pitch of 600 microns. For a nominal window size of 65mm at a nominal viewing distance of 750mm, and a refractive index of glass substrate of 1.5, this requires a separation of the lens from the pixel plane of less than 860 microns. Such a display would have a minimum viewing distance of typically 250mm.

Given typical thickness of waveplates and parallax optic substrates is greater than 600 microns, then a counter substrate of thickness less than 36Omicrons is required.

Such a display will show distinct image stripes at the apertures of the lenses or other parallax optic. In the display device of Fig. 15, the pixel column pitch can be increased

which has unexpected advantages for the manufacturability and cost of autostereoscopic displays. In the embodiment of Fig. 15, for the same colour pixel 224 pitch as for an RGB stripe panel on a single row, the pixel may be 50% larger in width. Therefore for the same nominal viewing distance of 750mm, the pixel pitch of pixels 220,222 may be increased to 75um, and the total separation of pixel to lens increased to 1.29mm. Such an element can be produced using standard polariser and device substrate thicknesses.

The alignment tolerances of the lens elements with respect to the display are similarly relaxed, and due to the similar size of electrodes, transistors and storage capacitors, the aperture ratio of the panel is increased. Thus the panel is of higher brightness. The gaps between the pixels may be reduced in proportion to the pixel width and thus the gaps between the windows in the window plane may be equivalently reduced.

The vertical luminance frequency is the same as the RGB stripe panel. The total number of addressable sub-pixels in a colour pixel has reduced from three to two, with no increase in the number of row drivers. Such a display advantageously is lower cost, easier to manufacture, thinner and lighter while providing higher image quality by minimizing image stripiness, particularly in the green channel.

Thus this invention allows the production of a display which has higher spatial luminance frequency in both 2D and 3D modes of operation when combined with a spatial multiplexing parallax optical element such as a lenticular screen or parallax barrier. This is advantageously achieved without changing the colour balance of the panel or substantial degradation in brightness.

As shown in Fig. 15, the appearance of the screen can also be improved for red and blue channels by offsetting the cyan and yellow pixels 220, 222 on adjacent pixel rows 226,228. For a four view system, the red/blue screen angle 227 is 45 degrees if the lateral pixel pitch is half of the pixel height. Such a four view system thus has optimal image appearance for both green and red/blue image phases. Such a system should minimise the appearance of colour fringes in the 3D mode of operation of the display.

As shown in Fig. 16, the width (and height, not shown) of the cyan and yellow pixels can be modified in a similar manner to that shown in Fig. 14 so as to optimise the window uniformity for different colours. The difference in pixel aperture can be compensated by calibrating the image data accordingly.

The pixel apertures of the present invention are nominally the same size, and thus the number of row and column drivers is optimised, and the aperture ratio of the panel maximized.

The present invention can use two coloured illumination phases. Thus, colour break up effects are minimised on the panel, and a switching mode, such as a liquid crystal switching mode may more optimally be employed. The resolution of the 2D mode is also improved by the structures of this invention, thus the number of row and column drivers may be reduced to provide the same resolution performance as for RGB stripe panels. This invention allows the fabrication of two view and multi-view time sequential autostereoscopic displays in which the glass thickness is optimised for typical pixel sizes and glass thicknesses. Thus the embodiments of this invention are particularly advantageous for autostereoscopic 3D displays, as well as enhancing the 2D mode performance.

Fig. 17 shows a further embodiment of the present invention in which columns of cyan pixels 220 and columns of yellow pixels 224 are used together with a tilted lenticular screen 241 in which the geometric lens axis 240 is tilted with respect to the column direction of the display. Such an apparatus takes advantages of the increased pixel size and glass separation of the present invention.

Fig. 18 shows a further embodiment of the invention in which a further white pixel 242 is used, to provide a white pixel 244. Such a pixel can be used to increase display luminance, and reduce colour break up artefacts, as it can be illuminated in both the first and second phases for example. In this embodiment, a green pixel is produced in C, Y and W pixels during the green illumination phase, thus optimising the appearance of image stripiness and colour break-up. Appropriate colour gamut mapping is used to convert RGB images into RGBW time sequential images.

In this way, the appearance of a 3D image in the embodiments of this invention may be improved by removal of stripe structure in the final image. Such an improvement may be obtained without the need for increases in the panel resolution as is required in the prior art, and uses pixels of nominally the same size, thus increasing device efficiency. The configurations given further do not change the panel colour balance and do not significantly reduce the aperture ratio of the individual pixels. Further, in the 3D mode of operation, a short viewing distance may be maintained while using standard thickness glass substrates. Such configurations have particular advantages for multi-view displays in which resolution loss may be significant, for example four view displays.

Such pixel arrangements are particularly suitable for switchable 2D-3D displays where the improvement of appearance will be obtained in both 2D and 3D modes compared to the standard configurations.

Many compression schemes use different resolutions for chrominance and luminance functions. The different resolutions of the red and blue channels of the invention can be matched with the chrominance output of compression algorithms.

In the time sequential system described so far, a display cycle comprises two phases of illumination on to cyan and yellow pixels, or red, green and blue illumination onto cyan yellow and white pixels. Other combinations of illumination are permitted in this methodology. A complete image cycle comprises a series of individual colour fields.

The phases may be further interlaced so as to reduce colour break-up effects. For example, it is also possible to use multiple phases to form a frame cycle. In such cases, the multiple phases may be of the same or different lengths in order to provide some greyscale in the temporal domain and/or to reduce the appearance of image breakup.

Such techniques are known in the art of field sequential displays.

The pixel configurations of the present invention may be used for autostereoscopic 3D displays, switchable 2D/3D displays, multi-view displays or switchable multi-view displays. Multi-view displays provide different images in different directions, for use for example in automobiles in which the driver wishes to see a different image to a passenger.

Further detail of the display devices will now be given to explain the nature of the control. Fig. 19 shows the control of the display apparatus of the present invention.

The backlight 1034 comprises light sources 254, 256 and 258 and a waveguide element 250 arranged to direct light from the light sources 254, 256 and 258 so as to produce an emitting area. Various films 252 are placed on one side of the waveguide 250 so as to efficiently extract light from the waveguide. Such films are well known in the art. Each of the light sources 254, 256, 258 may for example comprise LED sources, or maybe other sources such as cold cathode fluorescent tube (CCFT) with appropriate emitting phosphors. The sources 260,262,264 may comprise for example green, red and blue emitters, as primary colours. The backlight 1034 comprising elements 250,252,254,256,258 is placed behind the pixel plane., for example as shown by 1042 in Fig. 4. The pixel plane comprises pixels and control elements as shown in Fig. 19.

The control circuit 252 comprise the following elements.

The emitters 260 from each of the sources 254, 256 and 258 are connected in parallel by means of control line 265 whije the red and blue emitters 262,264 are connected in parallel for each of the emitters 254,256,258 be means of control line 267.

Control signals are transmitted along control lines 265,267 by means of a light source controller 266. Tn this way, the sources 260 may be controlled in anti-phase to sources 262, 264. Alternatively, the sources 262,264 may have different switching response functions, so that there may be a requirement that the switching of sources 262,264 may be independently controlled, but that the sources are essentially controlled in phase with each other.

A system controller 268 may be used to control the backlight controller 266 and a panel controller 270. Panel controller receives image data 271 which is directed to the pixels of the panel 220,222 by means of column drivers 270 and row drivers 276 along column electrodes 274 and row electrodes 278. In an active matrix panel, active elements such as transistors can be positioned at the intersection of each column and row electrode so as to control the data that each pixel receives. A lens 200 is positioned over the panel.

Thus the panel controller may take a red, green, and blue image frame, and convert into green and blue/red fields. The green data is applied to each pixel of the panel in the green illumination phase, while the red data is applied to the yellow pixels and the blue data to the cyan pixels during the magenta illumination phase.

The present invention allows the use of a switching mode which is slower than that required by a three phase illumination scheme (such as that which requires separate red, green and blue illumination phases). The driver electronics is thus not required to have high response speed, and thus has reduced cost compared to the three phase system.

Further, the response speed of the switching electro-optic effect can be reduced, while maintaining the same optical efficiency of the mode. Alternatively, the frame rate can be set higher than that for a three phase illumination system. Thus the colour break-up artefacts will be reduced. Further, the colour break-up will be between green and magenta, rather than red, blue and green, and so may be less visible. The panel can use voltage addressed grey levels and so may not suffer from false dynamic contouring effects.

Further the size of the pixels is reduced compared to the equivalent three phase system. In the three phase system, the pixel is square. Thus, the pitch of the lens for a four view system will be substantially four times the pixel aperture. In the two phase system of the present invention, the pitch of the lenses will be twice the colour pixel pitch. Thus, the image will be substantially less blocky in appearance than the three phase system.

Further the size of the pixels is increased compared to the equivalent one phase (spatially multiplexed system). This allows the viewing distance of the directional mode to be reduced, or the thickness of the device substrates to be increased.

Fig. 20 shows an example structure of the spatial light modulator used in the embodiments described above. A substrate 280 has an ITO layer formed on it. A black mask layer 284 is formed on the ITO layer, producing pixel apertures 286 and light blocking regions 287. A colour filter layer 288 comprising colour filters 289, 290 is formed, for example by means of screen printing or laser induced thermal transfer. The colour filters may comprise dyes or pigments for example with absorption characteristics required for the colour filters of the invention, so as to optimise display colour gamut and brightness. Further planarisation layers may be present, but are not shown.

Alignment layer 292 is formed over the colour filters. A second substrate 300 has electrodes, transistors, capacitors and other required addressing components 298 formed on its surface, and has an alignment layer 296 formed on its surface.

Alignment layers 292,296 are arranged to orient the liquid crystal material 294 in a cell spaced by means of spacer balls 302. Alternatively, other spacing mechanisms may be used such as polymer walls.

Claims

Claims 1. A directional display apparatus comprising: a transmissive

spatial light modulator comprising an array of pixels; a parallax element capable of directing light from respective pixels into one of a plurality of different viewing windows; and an illumination system arranged to illuminate the spatial light modulator across the entire array of pixels, wherein the illumination system is operable in a first mode in which the illumination system illuminates the spatial light modulator with light of a first and a second of the three primary colours red, blue and green but not the third of the three primary colours and is operable in a second mode in which the illumination system illuminates the spatial light modulator with light of the third of the three primary colours, and the array of pixels includes interlaced arrays of first and second types of pixel having respective colour filters, wherein the colour filter of the first type of pixel passes light of the first primary colour and filters out the second primary colour, the colour filter of the second type of pixel filters out light of the first primary colour and passes light of the second primary colour, and the colour filter of at least one of the first and second types of pixel passes light of the third primary colour.
2. A directional display apparatus according to claim 1, wherein the colour filters of both the first and second types of pixel pass light of the third primary colour and the illumination system in the second mode illuminates the spatial light modulator with light of the third of the three primary colours but not light of the first primary colour or the second primary colour.
3. A directional display apparatus according to claim 2, wherein the third primary colour is green.
4. A directional display apparatus according to any one of the preceding claims, wherein the display apparatus further comprises a control circuit arranged to control the operation of the spatial light modulator and the illumination system to display an image in accordance with image data comprising colour data of the three primary colours, the control circuit being operative in two sequential phases, wherein in one phase the control circuit controls the illumination system to operate in the first mode and addresses the first and second types of pixel with the colour data of the first and second primary colours respectively, and in the other phase the control circuit controls the illumination system to operate in the second mode and addresses the at least one of the first and second types of pixel with the colour data of the third primary colour.
5. A directional display apparatus according to any one of the preceding claims, wherein the pixels are arranged in rows with the different types of pixel interlaced along each row, the parallax element having a structure which is uniform in a direction perpendicular to the rows, and the pixels of each type of pixel in adjacent rows are offset from one another.
6. A directional display apparatus according to any one of the preceding claims, wherein the first and second types of pixel have different areas which compensate for chromatic dispersion in the parallax element of the light passed by the first and second types of pixel.
7. A directional display apparatus according to any one of the preceding claims, wherein the array of pixels further includes a third type of pixel which passes light of all three primary colours interlaced with the arrays of first and second types of pixel.
8. A directional display apparatus according to any one of the preceding claims, wherein the illumination includes three sources which, on operation, emit light of a respective one of the three primary colours, the illumination system being operable in the first mode by simultaneous operation of the two sources which emit light of the first colour and the second colour respectively, and being operable in the second mode by operation of the source which emits light of the third colour.
9. A directional display apparatus according to any one of claims 1 to 7, wherein the illumination includes two sources, the first source, on operation, emitting light of both the first and second primary colours and the second source, on operation, emitting light of the third primary colour, the illumination system being operable in the first mode by operation of the first source and being operable in the second mode by operation of the second source.
10. A directional display apparatus according to any one of the preceding claims, wherein the illumination system comprises a waveguide extending across the entire of array of composite pixels and a plurality of sources arranged to emit light into the waveguide.
11. A directional display apparatus according to any one of the preceding claims, wherein the display apparatus is switchable between a first mode in which the parallax element is effective to direct light from respective pixels into said different viewing windows and a second mode in which the parallax element has no effect.
12. A directional display apparatus according to any one of the preceding claims, wherein the parallax element has a structure which is uniform in a first direction and which repeats in a second direction perpendicular to the first direction.
13. A directional display apparatus according to any one of the preceding claims, wherein the parallax element is a lenticular array.
14. A directional display apparatus comprising: a spatial light modulator comprising an array of pixels arranged in rows and comprising at least two types of pixel interlaced along each row, each type of pixel being arranged to output light of a different colour; and a parallax element capable of directing light from respective pixels into one of a plurality of different viewing windows, the parallax element having a structure which is uniform in a direction perpendicular to the rows, wherein the pixels of each type of pixel in adjacent rows are offset from one another.
15. A directional display apparatus according to claim 14, wherein the pixels of each type of pixel in adjacent rows are offset by an amount which is equal to the pitch of the pixels along the rows multiplied by an integer.
16. A directional display apparatus according to claim 14, wherein the pixels of each type of pixel in adjacent rows are offset by an amount which is equal to a fraction of the pitch of the pixels along the rows
17. A directional display apparatus according to claim 16, wherein said fraction is a half.
18. A directional display apparatus according to any one of claims 14 to 17, wherein the array of pixels comprises at least three types of pixel.
19. A directional display apparatus according to any one of claims 14 to 17, wherein the array of pixels consists of three types of pixel.
20. A directional display apparatus according to claim 18 or 19, wherein the three types of pixel being arranged to output light of red, blue and green colours, respectively.
21. A directional display apparatus according to any one of claims 17 to 20, wherein the three types of pixel have different areas which compensate for chromatic dispersion in the parallax element of the light output by the different types of pixel
22. A directional display apparatus according to any one of claims 14 to 21, wherein the screen angle between pixels of the same type in adjacent rows and from which light is directed by the parallax element into the same window is between 250 and 650.
23. A directional display apparatus according to claim 22, wherein said screen angle is45 .
24. A directional display apparatus according to any one of claims 14 to 23, wherein adjacent pixels in adjacent rows are offset from each other.
25. A directional display apparatus according to any one of claims 14 to 24, wherein the spatial light modulator is a transmissive spatial light modulator and the display apparatus further comprises an illumination system arranged to illuminate the spatial light modulator across the entire of array of composite pixels.
26. A directional display apparatus according to any one of claims 14 to 25, wherein the display apparatus is switchable between a first mode in which the parallax element is effective to direct light from respective pixels into said different viewing windows and a second mode in which the parallax element has no effect.
27. A directional display apparatus according to any one of claims 14 to 26, wherein the parallax element has a structure which repeats in a direction along the rows and is uniform in a direction across the rows.
28. A directional display apparatus according to any one of claims 14 to 27, wherein the parallax element is a lenticular array.