GB2405043A

GB2405043A - Compensation for refraction effects in an autostereoscopic display

Info

Publication number: GB2405043A
Application number: GB0318940A
Authority: GB
Inventors: Jonathan Mather; David Montgomery; Grant Bourhill; Robert Winlow
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2003-08-13
Filing date: 2003-08-13
Publication date: 2005-02-16
Also published as: JP2005062867A; GB0318940D0; JP4367848B2

Abstract

A display enabling multiple views for the purpose of autostereoscopic viewing includes a spatial light modulator (SLM) and a parallax barrier. Additional means to compensate for refraction effects occurring at a refractive boundary of the screen are included. Such refraction at the periphery of a display leads to inaccurate alignment of the optimum viewing fields, known as crosstalk. Various compensation means are suggested to counter this effect to improve the viewer's perception of the display. Embodiments of the compensation means include variation (ie deliberate non-uniformity) in pitch and slit width of the parallax barrier, variation in the pixel positions of the SLM and provision of a transparent layer exhibiting a variation of refractive index, such that the refractive index at regions at the periphery of the display is lower than that at its centre.

Description

MULTIPLE VIEW DIRECTIONAL DISPLAY

The present invention relates to multiple view directional displays comprising a parallax optic such as a parallax barrier or a lenticular system, and particularly to such displays comprising large area parallax barriers.

One use of a multiple view directional display is as a dual view display. In a dual view display, one display panel is used typically to provide two-dimensional images to users in two separate two-dimensional viewing regions. The images may be displayed using spatial or temporal multiplexing. The supplied images may differ, so that viewers in the first region see a different image to the viewers in the second region. Such displays are generally used to provide two distinct two-dimensional images to the two viewing regions.

Another use of a multiple view directional display is as an autostereoscopic display to provide a three dimensional image. In normal vision, the two human eyes perceive views of the world from different perspectives due to their separate location within the head. These two two-dimensional 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: À Stereoscopic displays typically display both of the images over a wide viewing area. However, each of the views is encoded, for instance by colour, polarisation state or time of display, so that a filter system of glasses worn by the observer can separate the views and will let each eye see only the view that is intended for it.

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

Instead, the two views are each 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 region". If the observer is situated such that one of their eyes is in the correct viewing region for one image of a stereoscopic pair and the other eye is in the correct viewing region for the other 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 operates on the same general principle as a dual view display. However, the two images displayed are the right-eye and left-eye images of a stereoscopic image pair, and are displayed so that each image is visible to one observer, but to different eyes of the same observer.

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

This screen can be set in front of a spatial light modulator (SLM) with a two- dimensional array of picture element apertures as shown in Figure 1 of the accompanying drawings. The pitch of the slits in the parallax barrier is chosen to be close to an integer multiple of the picture element pitch of the SLM so that groups of columns of picture elements are associated with a specific slit of the parallax barrier.

Figure 1 shows an SLM in which two picture element columns are associated with each slit of the parallax barrier. The parallax optic may comprise an array of lenticular lenses.

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

The pixels of the LCD are arranged as rows and columns with the pixel pitch in the row or horizontal direction being p. The aperture array 8 comprises vertical transmissive slits 12 with a slit width of 2w and a horizontal pitch b separated by opaque regions 13.

The plane of the barrier aperture array 8 is spaced from the pixel plane 3 by a distance s.

In use, the display forms left and right viewing windows 10, 11 in a window plane at the desired viewing distance from the display. The window plane is spaced from the plane of the aperture array 8 by a distance rO. The windows 10 and 11 are contiguous in the window plane and have a width and pitch e corresponding to the average human eye separation. The half angle to the centre of each window 10, 11 from the display normal is illustrated as 0,.

Figure 2 of the accompanying drawings shows the angular zones of light created from an SLM 20 and parallax barrier 30 where the parallax barrier has a pitch of an exact integer multiple of the pixel column pitch. One or more columns of pixels of the SLM display a slice of an Image 1. These slices are interlaced with similar slices showing an Image 2. In this case, the angular zones coming from different locations across the display panel surface intermix and a pure zone of view for the Image 1 or the Iinage 2 does not exist. In order to address this, the pitch of the parallax barrier is reduced slightly so that the angular zones converge at a pre-defined plane (termed the "window plane") in front of the display. This change in the parallax barrier pitch is termed "viewpoint correction" and the effect is illustrated in Figure 3 of the accompanying drawings, with modified parallax barrier 31. The viewing regions, when created in this way, are roughly kite shaped in plan view. r

Parallax barriers known in the art have a constant pitch across the optic. This gives rise to disadvantages in the viewing of displays incorporating such barriers. The problems occur due to the change in refractive index as the light moves from the refractive medium of the panel into air. This change in refractive index causes refraction of the light travelling from the SLM to the observer (as given by Snell's law: nj sing = n, sintI,, where ni is the refractive index of the glass panel, Ri is the angle between the light ray outgoing from the SLM and the perpendicular to the plane of the screen, n, is the refractive index of air, and 0, is the angle between the refracted light ray outgoing from the parallax barrier and the perpendicular to the plane of the screen).

The refraction is illustrated in Figure 4 of the accompanying drawings, showing the left hand edge of a multiple view directional display, where the parallax barrier 31 constitutes the refracting interface. For an observer 40 positioned as shown in Figure 4, the value of tori increases to the left of the Figure and the light is correspondingly refracted to a greater extent.

For low angles of Bi, 0, is substantially proportional to Bi. In this case the prior art display works effectively, i.e. with a parallax barrier of constant pitch, coherent viewing windows are formed in the viewing window plane, regardless of the point of origin on the parallax optic of the light rays forming the viewing window. This situation is illustrated in Figure 3.

When the constant pitch parallax barrier is sufficiently large, or the viewer is sufficiently close to the barrier (as is the case with immersive displays), tori is large at the edge of the barrier. For large values of Bi, the angle by which the light is refracted increases considerably; the relationship between to, and Ri is non-linear.

A result of the increased refraction at the edge of the barrier is that the component of the viewing window projected from near the edge of the barrier does not converge to the same point as the corresponding component of the viewing region projected from the centre of the barrier. This is illustrated in Figure 4. Left and right pixels provide different image components which correspond to the image to be viewed at the left and right eye respectively in order to perceive a three dimensional image. Left and right pixels 22 to 27 may alternate spatially with one another through the SLM 20. Each arrow shows the position of the centre point between the left-eye and right-eye viewing windows formed by one pair of a left-eye pixel and a right-eye pixel. These arrows should all converge to the centre point between the left and right eyes 41, 42 of the viewer 40 but, as clearly shown in Figure 4, they do not.

A further problem exists for viewing windows projected from large values of Bi. The problem is of less importance for autostereoscopic displays, however it may be significant for dual view displays where the angle between different views is large. The light that forms the left-eye viewing window exits the barrier at a different angle to light forming the right viewing window, since the images for each viewing window are provided by spatially alternating groups of picture elements. As illustrated in Figure 12a of the accompanying drawings, the light rays forming the view 1 and view 2 viewing windows are therefore refracted differently at the glass- air interface. The view 1 and view 2 windows become more widely separated with increasing distance from the panel than desired.

There are two adverse consequences of the effects detailed in the preceding paragraphs.

The viewing windows become narrower, decreasing the area in which each view may be viewed. Furthermore, crosstalk between the view 1 and view 2 images is increased.

Typically, the further away the user is from the centre of a viewing window, the higher the crosstalk becomes. As illustrated in Figure 4, the position of the boundary between the viewing regions depends on the area of the SLM from which the corresponding images originated. Accordingly no position exists where crosstalk is at a minimum from all points of the display.

The following table outlines the extent to which crosstalk causes a viewing problem.

Theoretical crosstalk values are provided for differently sized displays using a parallax barrier with a constant pitch and having a viewing distance of 600mm. Crosstalk rapidly becomes unacceptable for 18" (45.7cm) displays and above. High levels of crosstalk at the edges of the screen will give rise to significant visual discomfort. In the case of the 25" (63.5cm) screen, at the centre of the screen the user's left eye will see the left eye image, but at the edges of the screen the user's left eye will be able to see the right eye image (and vice versa). A 25" (63.5cm) display with a parallax optic of constant pitch will not show the correct images to the correct eyes over its entire area.

Display Size Average crosstalk (%) Crosstalk at the led and right edges of the display (%) 15" (38.1cm) 3.19% 4.02% 16' (406m) 3.24% 4.42% 18" (45.7cm) 3.39% 5.80% 21" (53.3cm) 3.86% 10.7% 25" (63.5cm) 5.13% >40% The paper Yamamoto, H.; Muguruma, S.; Sato, T.; Ono, K; Hayasaki, Y.; Nagai, Y.; Shimizu, Y.; Nishida, N., Optimum parameters and viewing areas of stereoscopic full- color LED display using parallax barrier, IEICE Transactions on Electronics, vol.E83- C, no.10 p. 1632-9, Oct. 2000 details the optimization of parallax barrier parameters such as pitch, slit width, and pixel aperture ratio. The paper identifies that users will look at the screen from an angle if they are not positioned in front of the centre of the screen. However, only a constant barrier pitch is considered and refraction of the light rays at the barrier-air interface is not mentioned.

Japanese Patent No. 6-82 934 discloses a stereoscopic display in which the parallax optic is a lenticular lens array. When a large stereoscopic display with a lenticular optic is made, the lenticular lenses at the periphery of the optic do not focus the image correctly. This document discloses overcoming this problem by changing the focal length of the lenses according to their position on the screen. The focal length is changed by altering the radius of curvature of the lenses, the refractive index of the lenses, or the thickness of the lenses. The positions of the lenses relative to the picture elements are not changed according to their position on the display.

United States Patent No. 5,900,972 discloses a method for making the pitch and/or slit widths of a parallax barrier variable. It does not consider that the pitch and slit widths should be varied according to the position of the slit on the screen. The parallax barrier of this document has variable pitch and slit width, but the pitch and slit widths are constant at all points on the screen.

According to a first aspect of the present invention there is provided a multiple view directional display comprising a display device and a parallax optic and having an interface between media of different refractive indices and compensation for refractive bending of the direction of light propagation at the interface. A display according to the invention increases the lateral and longitudinal viewing freedom of a user without detrimental effects on the crosstalk. The cost and ease of manufacture of such a display is substantially the same as that of known displays.

The parallax optic may comprise a plurality of parallax elements, each of which is aligned with a respective set of picture elements of the device. Each parallax element may be aligned with a respective group of columns of the picture elements. At least one of the lateral pitch of the parallax elements and the lateral pitch of the sets of pixels may vary across the display. The at least one lateral pitch may vary monotonically from the centre of the display to the lateral edges to all the edges of the display.

The parallax optic may be disposed between the device and a viewing position. The parallax element lateral pitch may increase from the centre. The picture element set lateral pitch may decrease from the centre.

Alternatively, the device may be disposed between the parallax optic and the viewing position. The parallax element lateral pitch may decrease from the centre. The picture element set lateral pitch may increase from the centre.

The lateral pitches of the picture elements within the sets may vary across the display.

The widths of the parallax elements may vary across the display.

Each of the parallax elements may comprise a plurality of sub-elements for passing light from the or each picture element of the set displaying a respective view.

The parallax optic may be controllable to vary the parallax element lateral pitch.

The parallax optic may be disableable for a single view mode of operation.

The display may comprise a layer whose refractive index varies across the display. The refractive index may be larger at the centre of the display than at lateral edges thereof.

The spacing between the parallax optic and an image generating plane of the device may vary across the display. The spacing may be larger at the lateral edges of the display than at the centre thereof.

Viewing windows generated at lateral edges of the display may be laterally offset from viewing windows generated at the centre thereof.

One of the media may be air.

The device may comprise a liquid crystal device.

The parallax optic may be a parallax barrier.

A second aspect of the present invention provides a parallax optic comprising a plurality of parallax elements having a lateral pitch which varies across the optic.

A third aspect of the present invention provides a display device comprising a plurality of sets of picture elements with the sets having a lateral pitch which varies across the device.

Existing multiple view displays suffer reduced lateral and longitudinal viewing freedom and higher crosstalk when they are made to cover a large part of the user's field of view.

Embodiments of the present invention enable the manufacture of multiple view displays that may cover a large part of a user's field of view without detrimental effects on crosstalk or lateral viewing freedom. The cost and ease of manufacture of a display implementing embodiments of the present invention will be substantially the same as

the prior art.

For a better understanding of the present invention and in order to show how the same may be carried into effect, preferred embodiments of the invention will now be described, by way of example, to the accompanying drawings in which: Figure 1 illustrates a conventional multiple view display having a spatial light modulator with a two-dimensional array of picture element apertures; Figure 2 illustrates the angular zones of light created from a spatial light modulator and parallax barrier where the parallax barrier has a pitch of an exact integer multiple of the pixel column pitch; Figure 3 illustrates formation of viewing windows via viewpoint correction of the parallax barrier pitch; Figure 4 illustrates the refractive distortion present in a constant pitch parallax barrier with a known spatial light modulator; Figure 5 illustrates a parallax barrier with variable pitch according to a first embodiment of the present invention; Figure 6a illustrates the parallax barrier of Figure 5 made up of discrete pixels; Figure 6b illustrates a parallax barrier with variable pitch only in the horizontal direction according to an embodiment of the invention; Figure 6c illustrates a parallax barrier according to an embodiment of the invention in which both the pitch and the slit width vary; Figure 7 is a schematic illustration of a multiple view display according to an embodiment of the invention; Figures 8a to 8d illustrate user eye positions when observing different parts of a display; Figures 9a and 9b illustrate user eye positions when viewing a spatial light modulator with variable pitch pixels and a constant pitch parallax barrier; Figure 10 illustrates a multiple-view display in accordance with a further embodiment of the present invention; Figures lla to lid illustrate the variation of pitch and distance between picture elements in a multiple view display in accordance with a further embodiment of the present invention; Figures 12a and 12b illustrate a multiple view display in accordance with a further embodiment of the present invention; Figure 13a and 13b illustrate an autostereoscopic display in accordance with a further embodiment of the present invention; Figure 14 illustrates a display comprising a parallax barrier in accordance with a further embodiment of the present invention; Figures 15a to 15c illustrate a switchable parallax barrier in accordance with a further embodiment of the present invention; and Figure 16 illustrates a multiple view display in accordance with a further embodiment of the invention.

Figure 5 illustrates in front view a parallax optic according to a first embodiment of the present invention. Unlike known parallax optics, the pitch of the optic varies across the optic. The pitch may increase or decrease away from the centre of the optic, and the variation may be monotonic from the centre to the edges of the display. This embodiment is described with reference to a parallax barrier, but the invention is not limited to this type of parallax optic.

Parallax barrier 32 comprises a series of transparent slits 12 and opaque regions 13, the width of the slits being distance 2w and the pitch of the apertures being distance b. The slit width 2w is constant across the barrier, whilst pitch b increases with distance from the centre of the barrier.

There is a small lateral shift in the position, relative to the associated columns of pixels, of the slits along the parallax barrier as the distance from the centre of the barrier changes. The change in barrier pitch at each point along the barrier is substantially sufficient to alter the path of light refracted at a media interface disposed in relation to the parallax barrier.

The variation in the barrier pitch at a point on the barrier may depend on the vertical position on the barrier in addition to the horizontal position. For example, the adjustment required at the top of the barrier may be different to that required at the centre. This results in the curved nature of the slits 12 shown in Figure 5. The pitch varies horizontally across the display, with the variation in pitch horizontally across the display depending on the vertical position on the display.

For ease of manufacture, it may be convenient to approximate the curved slits of Figure to a finite number of discrete steps. A parallax optic 32 according to a further embodiment of the present invention is illustrated in Figure 6a. The parallax optic 32 of Figure 6a is again a parallax barrier, and corresponds to the parallax barrier 32 of Figure except that the boundaries between transparent slits 12 and opaque portions 13 are approximated as a series of discrete steps.

In certain circumstances it may not be desirable to produce a parallax optic with a curved shape as shown in Figure 5. According to a further embodiment of the present invention there is provided a parallax optic in the form of a parallax barrier 37 illustrated in Figure 6b and corresponding generally to the first embodiment, but differing in that the slits of the barrier are vertical and parallel. The parallax barrier 37 has a pitch that varies along a horizontal line on the barrier. The pitch of the barrier 37 is independent of vertical position on the screen, and the variation in the pitch may be optimised subject to the constraint that the parallax optics must be vertical. The slit width may be constant across the barrier 37 of this embodiment.

A further embodiment of the present invention illustrated in Figure 6c of the accompanying drawings comprises a parallax barrier 38 having different slit widths at different positions on the barrier as well as a varying pitch. The pitch of the barrier 38 varies along a horizontal line on the barrier. The pitch also varies vertically across the barrier. The slit width varies across the barrier 38, for example to reduce crosstalk at the edges of the display, or to compensate for brightness variations over the panel.

The parallax optics described herein are parallax barriers. However, they may alternatively be any other conventional parallax optic such as a lenticular lens array, prism barrier, etc..

The parallax optic of the present invention may be disposed in front of or behind the SLM. The parallax optic of the present invention may be applied to parallax optics that provide two views, or a plurality of views. The parallax optic of the present invention may further be applied to parallax optics such as those disclosed in pending British Patent Applications 0315170.1, 0306516.6 and 0228644.1, or any other type of configuration.

Figure 7 is a partial plan view of a multiple view directional display according to an embodiment of the present invention and having compensation for refraction of light at an interface between media of different refractive index. Figure 7 shows only the pixel structure of the SLM and the parallax barrier aperture array 32 of the display 20. Other components are not relevant to the description below and so have been omitted.

The display device of Figure 7 comprises an SLM 20, substantially similar to the SLM described with reference to Figure 1. Left and right pixels 22 to 27 provide images for left and right eye viewing respectively. The parallax barrier 32 of the device is a parallax barrier according to any of the preceding embodiments, i.e. featuring at least a variable pitch in a lateral direction.

Pixels 22, 23 are disposed at one edge of the SLM and the pitch of the parallax barrier opposite those pixels differs from the pitch of the barrier opposite pixels 26, 27 which are near the centre of the SLM. The change in barrier pitch at each point along the SLM is substantially sufficient to alter the path of refracted light so that the light rays from each slit of the barrier converge at the viewing position 40. The pitch b of the parallax barrier increases from the centre of the SLM towards the edge of the display, resulting in Bi falling in value. According to Snell's Law, also decreases, resulting in the image from pixels 22, 23 converging at the same location as the image from pixels 26, 27.

Figure 8a shows a user 40 looking at the centre of a large display represented schematically by 33. The arrows indicate that the images displayed near the edge of the display are correctly directed to the left and right eyes. If the user now looks at a different part of the display, towards the edge of the display, it is natural for them to turn their eyes, as shown in Figure 8b, or to turn their head, as shown in Figure 8c.

These movements result in the user's eyes moving away from the optimal viewing position.

In a further embodiment of a parallax optic of the present invention the variable pitch of the parallax optic 33 is adjusted, for example, so that viewing regions originating from the sides of the screen are projected to the appropriate side of the user, illustrated in Figure 8d. The compensation to the refraction provided by the earlier embodiments is modified to shift the viewing region corresponding to the edge of the display to a laterally offset location. In order to achieve this, the pitch of the slits at the edges of the barrier must be increased to a greater extent than in the earlier embodiments in order to reduce 0, by a larger amount.

In this case, when the user looks at the centre of the display, their eyes will be in the optimal position for the viewing regions originating from the centre of the display. In this position the eyes of the user will be looking away from the viewing regions originating from the edges of the display. The user will therefore see higher crosstalk at the edges of the display, but this will be of little consequence as it lies in the user's peripheral vision, where the detail of the image is not so greatly appreciated.

When the user desires to look at the periphery of the screen, their eyes and/or head naturally move into the best viewing position for looking at the edge of the screen, where the viewing windows originating from the edges of the screen are already being projected. The edges of the screen are then in the centre of the user's field of view, and the user will see low crosstalk from the edges of the screen.

In certain circumstances it may not be desirable to produce a parallax optic with variable pitch. The pitch of the parallax optic may still be optimised taking into account the considerable refraction of light at the edges of the panel. Known parallax optics take into account the refraction of light at the glass-air interface for low angles of Ri (i.e. for small screens) only.

The right-eye viewing region ray paths for a user 40 viewing a known display are shown in Figure 9a, illustrating that near the centre of a display 31, the viewing windows are perfectly positioned relative to the user, whereas when the viewing windows originate further away from the centre of the display 31, they miss the user to a significant extent.

Figure 9b illustrates a user 40 viewing a display 31 according to a further embodiment of the present invention. The display 31 comprises an SLM 20 corresponding to that provided in the embodiment illustrated in Figure 7. The pitch of the parallax barrier of the display is constant in this embodiment, and is made slightly larger than that which would provide accurate on-axis performance relative to the centre of the display. Thistrades off performance of the centre of the screen for performance of the edges of the screen. In this way crosstalk may be made more uniform across the screen. In this embodiment, viewing windows originating from the centre of the display 33 appear correct to a user, with minimal crosstalk. Viewing windows originating from just right of centre of the display 33 go just to the right of the user. Viewing windows originating from further right, when refraction of the light increases, appear correctly again.

Viewing windows originating from the edge of the screen, when refraction has increased further, go slightly to the left of the user. The result is that the viewing regions converge within a smaller area than they would have if the barrier was optimised only for the centre of the screen.

The preceding embodiments describe how adjusting the pitch of the parallax barrier in a display may compensate for refraction of light at large angles of tit. It is also possible to compensate for refraction by adjusting the positions of the pixels in the panel.

An SLM according to a further embodiment of the present invention is illustrated in Figure 10. The novelty of the SLM resides in the pixel pitch, and accordingly only the pixels are shown in Figure 10. The SLM 20a has pixels arranged at varying pitch p, where p is the distance between the centre points of consecutive left-eye and right-eye pixel pairs. The pitch of the pairs of left-eye and right-eye pixels 22 to 25 at the edge of the SLM is Pe, and the pitch at the centre of the SLM ispc' where Pc > Pe Figure 10 further shows the novel SLM 20a incorporated in a display of the invention that also includes a parallax optic (in Figure 10 a parallax barrier 31) that has constant pitch b across the parallax barrier. Although the barrier pitch b is constant, the change in the pixel pair pitch p compensates for the increased refraction of light at the edge of the panel and makes the viewing regions converge to the same point.

In a display according to a further embodiment the pitch of the parallax barrier may vary in addition to the pixel pitch p. For example, the barrier pitch b may vary in accordance with the embodiment in which the image from the edge of the display is projected to one side of the viewing position.

The dimensions Pe and Pc may need to be varied as a function also of vertical position on the SLM, for the reasons explained above with reference to Figure 5.

Correction for the refraction of light due to the glass-air interface may also be effected by means of a combination of variations in pixel positions and variations in the parallax optic pitch, for example, a combination of the parallax barrier of Figure 5 or 6a with the SLM of Figure 10.

Figures lla and Fib illustrate a problem with known parallax optics. They are schematic plan views of a conventional multiple view display. Only the pixels 22 to 25 and the parallax optic 31 of the display are shown, as in Figure 4. Dimension pi is the pitch of adjacent pairs of left and right eye pixels 22 to 25. Dimension dr is the distance between the centres of the left and right eye pixels 24 and 25 of a pixel pair. Dimension s' is the distance between the SLM 20 and a parallax optic of constant pitch 31. LR iS the angle between the centres of the left and right eye viewing windows at viewing position 40. The distance from the parallax barrier to the viewing plane is rn. In the optimum viewing position rays from pixels 24, 25 would pass through the centre of the slit 12a in the parallax barrier and reach the user's eyes.

LR becomes large due to the increased refraction at the glass-air interface at the edge of the display. In Figure lla the user's eyes are too narrowly separated to fit in the optimum positions for both left and right viewing windows, the paths for which are shown by the solid and dashed lines respectively. Figure llb shows the reverse situation that may occur if the user looks at the display from an angle. The user's eyes become too widely separated to fit in the optimum positions for the left and right eye viewing windows.

According to a further embodiment of the present invention, LR iS adjusted by adjusting the distance do between the centres of the left and right eye pixels 24, 25. Figure l l c is a schematic plan view of a directional display according to a further embodiment of the invention. Only the pixels 22 to 25 and the parallax optic 31 of the display are shown, for convenience of description. Figure l to illustrates the separation between pixels of a pair having been reduced to distance d2, where d2 < d. This decreases OLR. Adjusting distance do continuously for each pair of pixels across the SLM whilst maintaining a constant value of the pitch p' can compensate for the increase in LR due to refraction at the glass-air interface.

According to another embodiment of the present invention (not illustrated) , it may be advantageous to decrease both the pitch pi of pairs of left and right eye pixels 22 to 25 and the pitch of the barrier, so that there is no unused area between the left and right pixel pairs.

Figure 1 Id is a schematic plan view of a multiple view display according to another embodiment of the invention. Only the pixels 22 to 25 and the parallax optic 31 of the display are shown, for convenience of description. In this display, the separation between pixels in a pair has been increased to distance d3, where d 3 > d. This increases all. Increasing the separation between pixels of a pair across the SLM whilst maintaining a constant value of the pitch pi can provide the increase in OLR that is needed if the user were to look at these pixels from an angle.

Increasing the separation do between pixels in pair and keeping the pair pitch pi constant would result in unused areas between the pixels. It may therefore be advantageous to increase pixel separation d,, and also increase pi and the barrier pitch. This would allow for larger pixel apertures to be used so that there are no, or fewer, unused areas between the pixels.

A farther technique for varying LR iS provided by a display according to a further embodiment of the present invention, illustrated in Figure 12a. Only the pixels 22 to 25 and the parallax optic 31 of the display are shown, for convenience of description. The distance s2 between the SLM 20 and the parallax optic 31 is varied across the display, in contrast to keeping this distance constant in the display of Figure 1 1 a. In regions of the display of Figure 12a where the SLM-to-parallax optic separation is greater than the distance sit of Figure 1 1 a, LR iS lower for viewing windows originating from those areas of the display than in Figure 1 1 a. Similarly, LR may be increased by making the SLM- to-parallax optic separation less than the distance sit of Figure lla. By varying the distance between the parallax optic and the SLM, OLR can be made different for different regions on the display, thereby compensating for the change in OLR at the refractive interface.

Figure 12b is a farther schematic plan view of a display according to this embodiment of the present invention and illustrates the varying separation between the parallax optic 31 and the SLM 20 of a display. At the edges of the SLM 20 the distance between the SLM and the parallax optic is greater than at the centre of the SLM in order to reduce the magnitude of OUR for viewing windows originating at the edges of the SLM. The separation may vary in a horizontal direction and/or in a vertical direction.

In general use, a three-dimensional display 20 is viewed by observer 40 from a central position, as schematically shown in Figure 13a. The display 20 of Figure 13a may comprise a parallax barrier 32 such as that illustrated in Figure 5. It may, however, be desirable to view such a display from an off-centre position, for example from the right hand edge 36 of the display as illustrated in Figure lab. In this case, the light forming the viewing windows originating from the left hand edge 35 of the display is refracted more than the light forming viewing windows originating from the right hand edge 36.

According to a further embodiment of the present invention illustrated in Figure 13b there is provided a different arrangement of the slit positions in the parallax barrier to that of the first embodiment to compensate for this. The parallax barrier 32a of Figure 13b has a pitch that changes monotonically away from the position that is directly in front of the observer 40. The slit width of the barrier 32a is constant at all points on the barrier. In order to compensate for the observer 40 being nearer to the right hand edge 36, the pitch of the barrier 32a increases from the point on the barrier closest to the observer towards the left hand edge 35.

The preceding embodiments describe how adjusting the pitch of the parallax barrier in a display or adjusting the positions of the pixels in the SLM may compensate for refraction of light at large angles of Bi. It is also possible to compensate for refraction by varying the refractive index term ni of Snell's Law as opposed to varying the angle of incidence. According to a further embodiment illustrated in Figure 16, a refractive medium 60 between the SLM 20 and the constant pitch parallax optic 30 that has a high refractive index in the centre 60b, changing to a lower refractive index at the edges 60a may be used. As illustrated in Figure 4, light rays from pixels 22, 23 nearer the edges of the SLM 20 are refracted to such an extent that they do not converge with the rays from pixels 26, 27 near the centre of the SLM. According to the present embodiment, the refractive index of the medium in the region of pixels 22, 23 near the edge of the SLM is lower than the refractive index of the medium in the regions of pixels 26, 27 near the centre of the SLM. The change in the refractive index of the medium between the two regions may be continuous or in discrete steps. The lower refractive index at the edge of the display results in the light ray originating from that region being refracted to a lesser extent, i.e. t), for the ray is smaller in magnitude than the corresponding value of 0, for the ray shown in Figure 4, and consequently the rays from each region on the display all converge at the correct point at the observer 40.

A multiple view display according to a further embodiment of the present invention is illustrated in plan view in Figure 14. Only the pixels 22 to 25 and the parallax optic 34 of the display are shown in Figure 14. The novel parallax barrier 34 has two types of aperture. The first type of aperture 50 (denoted by a square dotted line) allows only light from the left-eye pixels 23, 25 to pass and the second type of aperture 51 (denoted by a circular dotted line) allows only light from the right-eye pixels 22, 24 to pass. The first type of apertures and the second type of apertures are disposed at different positions on the barrier to one another so that the left-eye and right-eye image directions can be controlled separately. This allows the left and right eye images to be directed precisely to the left 41 and right 42 eyes respectively, irrespective of the different amounts of refraction at the glass-air interface.

At points of overlap between the two types of aperture, a third type of aperture may be provided that allows light from both the left-eye and right-eye pixels to pass.

The structure of the parallax barrier 34 of the above embodiment may be achieved in various manners. A first technique involves rendering the images from left and right eye pixels as being orthogonally polarised. The two aperture types 50 and 51 are made from orthogonal polarisers. There is no polariser provided for the third aperture type, so that light from left-eye and right-eye pixels can pass. A second technique involves rendering the images from left-eye and right-eye pixels in different colours. The first aperture type 50 is a colour filter allowing only the colour of the left-eye pixels 23, 25 to pass. The second aperture type 51 is a colour filter allowing only the colour of the right-eye pixels 22, 24 to pass. The third aperture type is a region where no colour filter exists or a colour filter exists that allows light from left-eye and right-eye pixels to pass.

According to a further embodiment of the present invention, the parallax optic may be made switchable. This is illustrated in Figures 15a to lSc. Figure 15a illustrates a constant pitch parallax barrier in which the transparent stripes 12 are substantially straight. This is appropriate for a user a large distance from the screen, where the effects of refraction at the edges of the display are minimal. As the user moves closer to the display, the pitch of the transparent stripes further away from the centre of the barrier would progressively need to increase to adjust for the new user position. The barrier stripes would become more curved, illustrated in Figure l5b, as provided by earlier embodiments of the present invention, and particularly with regard to Figure 5

and the corresponding description.

This embodiment may be combined with a known system of observer tracking to determine the user's position. The display may be controlled on the basis of the output from the observer tracking system. Alternatively, the barrier may be switched off entirely, leaving the barrier transparent to the SLM, illustrated in Figure l5c. In this case the LCD would function as a conventional LCD panel with no barrier present.

Such a switchable barrier may be constructed as a separate conventional monochrome LCD.

The present invention provides a multiple view directional display comprising a display device and a parallax optic and having an interface between media of different refractive indices and compensation for refractive bending of the direction of light propagation at the interface. It will be appreciated by the person skilled in the art that various modifications may be made to the above embodiments without departing from the scope of the present invention.

Claims

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

and a parallax optic and having an interface between media of different refractive indices and compensation for refractive bending of the direction of light propagation at the interface.
2. A display as claimed in claim 1, in which the parallax optic comprises a plurality of parallax elements, each of which is aligned with a respective set of picture elements of the device.
3. A display as claimed in claim 2, in which each parallax element is aligned with a respective group of columns of the picture elements.
4. A display as claimed in claim 2 or 3, in which at least one of the lateral pitch of the parallax elements and the lateral pitch of the sets of picture elements varies across the display.
5. A display as claimed in claim 4, in which the at least one lateral pitch varies monotonically from the centre of the display to the lateral edges of the display.
6. A display as claimed in claim 5, in which the at least one lateral pitch varies monotonically from the centre of the display to all the edges of the display.
7. A display as claimed in claim 5 or 6, in which the parallax optic is disposed between the device and a viewing position.
8. A display as claimed in claim 7, in which the parallax element lateral pitch increases from the centre.
9. A display as claimed in claim 7 or 8, in which the picture element set lateral pitch decreases from the centre.
10. A display as claimed in claim 5 or 6, in which the device is disposed between the parallax optic and a viewing position.
11. A display as claimed in claim 8, in which the parallax element lateral pitch decreases from the centre.
12. A display as claimed in claims 8 or 9, in which the picture element set lateral pitch increases from the centre.
13. A display as claimed in any one of claims 2 to 10, in which the lateral pitches of the picture elements within the sets varies across the display.
14. A display as claimed in any one of claims 2 to 11, in which the widths of the parallax elements varies across the display.
15. A display as claimed in any one of claims 2 to 12, in which each of the parallax elements comprises a plurality of sub-elements for passing light from the or each picture element of the set displaying a respective view.
16. A display as claimed in any one of claims 2 to 13, in which the parallax optic is controllable to vary the parallax element lateral pitch.
17. A display as claimed in any one of the preceding claims, in which the parallax optic is disableable for a single view mode of operation.
18. A display as claimed in any one of the preceding claims, comprising a layer whose refractive index varies across the display.
19. A display as claimed in claim 16, in which the refractive index is larger at the centre of the display than at lateral edges thereof.
20. A display as claimed in any one of the preceding claims, in which the spacing between the parallax optic and an image generating plane of the device varies across the display.
21. A display as claimed in claim 18, in which the spacing is larger at lateral edges of the display than at the centre thereof.
22. A display as claimed in any one of the preceding claims, in which viewing windows generated at lateral edges of the display are laterally offset from viewing windows generated at the centre thereof.
23. A display as claimed in any one of the preceding claims, in which one of the media is air.
24. A display as claimed in any one of the preceding claims, in which the device comprises a liquid crystal device.
25. A display as claimed in any one of the preceding claims, in which the parallax optic is a parallax barrier.
26. A parallax optic comprising a plurality of parallax elements having a lateral pitch which varies across the optic.
27. A display device comprising a plurality of sets of picture elements with the sets having a lateral pitch which varies across the device.