GB2415849A

GB2415849A - A switchable liquid crystal parallax barrier

Info

Publication number: GB2415849A
Application number: GB0414495A
Authority: GB
Inventors: Paul Bonnett; Diana Ulrich Kean; Heather Ann Stevenson
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2004-06-29
Filing date: 2004-06-29
Publication date: 2006-01-04
Also published as: GB0414495D0

Abstract

A liquid crystal parallax barrier comprises first and second substrates each carrying a uniformly continuous electrode 2 constituting addressing means. The electrodes 2 define a layer 4 of liquid crystal material between them, which they cooperate with to provide varying switching properties in the plane of the layer 4. The switching properties preferably comprise voltage thresholds, to allow selection between parallax barrier, and other modes of operation, which may include landscape, portrait, and non-barrier modes. Preferably the barrier operates in one of: ferroelectric; antiferroelectric; nematic or; zenithal, bistable modes. The layer 4 may have a first layer of permittivity composed of regions of different material thicknesses, preferably formed as a plurality of ramps or steps, which is planarised by a second layer of different permittivity. A barrier addressing method is disclosed; a first signal sets the first mode and at least one further signal sets a second mode, to vary the position of at least one portion of the barrier.

Description

Parallax Barrier The present invention relates to a parallax barrier for

use in direct view three- dimensional displays, and in particular those displays utilising liquid crystal materials. s

The use of parallax barriers in conjunction with a two-dimensional display in order to provide two different images for viewing at two different locations is well known. In particular, it is well known to use a display layer to provide a pair of images and a parallax optic to send the two images in different directions, one for viewing at the left eye of an observer, the other for viewing at the right eye of the observer, the perception of the two images by the observer giving rise to a threedimensional image. However, such systems require the observer to remain within a certain fixed viewing region to achieve the three-dimensional effect. If the observer's eyes move outside the regions directed for left eye and right eye viewing, the three-dimensional effect is lost. If the position of the viewer is tracked, the direction of the images may be adjusted in order to maintain the three-dimensional image. This may be achieved by shifting the parallax barrier relative to the image display layer in accordance with the movement of the viewer. Prior art methods of shifting the parallax barrier are known.

JP 03-119889 discloses a parallax barrier formed using an additional panel having vertically striped electrodes. The system disclosed in JP 09197344 achieves movement of the parallax barrier by forming the barrier in an additional panel from a large number of narrower electrodes and, switching sub-sets of these electrodes in order to move the barrier. EP 833183 extends this method by switching the inter-electrode gap in such an arrangement. Despite the ease of implementation, it is difficult to remove the effects of the gaps between the electrodes, which can result in fixed dark or light stripes. Further, the electrodes are narrow and thus sufficiently resistive to cause signal distortion in large panels. Several drivers are also required, and the formation of the indium tin oxide electrodes on the additional panel is expensive and time consuming.

Bistable materials have also been applied for use in direct view displays, with the resulting difficulty of achieving grey levels. Addressing of bistable ferroelectric liquid crystal materials to achieve grey levels is described in several patents by Canon. EP 453856, JP 2-94384 and US 4712877 disclose methods of achieving threshold patterning and a multi-pulse method of addressing the panel by driving pixels to achieve a certain grey level for information display. The multi- pulse method described uses pulses of alternating polarity to arrive at the correct grey level, although the purpose of the method is to achieve reproducible grey levels rather than a mobile parallax barrier.

According to the invention, there is provided a parallax barrier comprising first and second substrates carrying first and second addressing arrangements, respectively, each addressing arrangement comprising an electrode which is substantially uniformly continuous in the plane of the addressing arrangement, the electrodes defining therebetween a cell comprising a layer of liquid crystal material, the first and second addressing arrangements cooperating with the liquid crystal layer to provide switching properties which vary in the plane of the liquid crystal layer for selectively providing a first mode of operation providing a first parallax barrier, and a second mode of operation different from the first mode of operation.

The barrier may be arranged to operate in a bistable liquid crystal mode. The bistable liquid crystal mode may be any one of a ferroelectric liquid crystal mode, antiferroelectric liquid crystal mode, bistable nematic mode and zenithal bistable mode.

The varying switching properties may comprise varying voltage switching thresholds.

The varying switching properties may comprise different regions in the pulse duration- voltage plane. At least two of the regions may partially overlap.

The barrier may have a substantially clear non-barrier mode of operation.

The barrier may have a third mode of operation different from the first and second modes of operation, the third mode providing a second parallax barrier.

The liquid crystal layer may have adjacent regions of different thicknesses of liquid crystal material.

The liquid crystal layer may have adjacent regions comprising materials of different permittivities.

At least one of the substrates may comprise a stepped layer of a first permittivity planarised by a layer of a second permittivity different from the first perrnittivity.

At least one of the substrates may comprise a first layer of a first permittivity having a plurality of ramps planarised by a second layer of a second permittivity different from the first permittivity.

According to the invention, there is also provided a method of addressing a parallax barrier comprising applying to the parallax barrier a first signal for setting the parallax barrier into the first mode of operation; and applying at least one further signal, the at least one further signal for setting at least one portion of the parallax barrier into the 1 S second mode of operation.

The magnitude of the at least one further signal may be varied so as to vary the position of the at least one portion of the parallax barrier in the second mode of operation.

It is thus possible to provide a tracking parallax barrier in an additional panel without the need for complex electrode patterning in the additional panel, therefore eliminating the need for complex matrix addressing techniques. Threshold patterning within the panel is used to control the position of the parallax barrier stripes. The use of a bistable material in the parallax barrier enables switching between twodimensional and three dimensional modes and allows for low power operation since the additional panel requires no or relatively slow updating in either of the 2D or 3D modes. Appropriate selection of the scheme for driving the parallax barrier allows for minimization of the temperature dependence of the switching.

A parallax barrier according to the invention may be used in conjunction with a system for detecting the position of the viewer in order to correct the position of the images for three-dimensional viewing as the viewer moves with respect to the display. A parallax barrier according to the invention may also be used in a dual view display to provide two different images viewable from different directions, rather than one 3D view.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figures I a and lb are diagrammatic cross-sectional views of a parallax barrier constituting embodiments of the invention; Figure 2 is a graph showing the response of the embodiment of Figure 1 with voltage, for two different thickness regions.

Figure 3a to 3f illustrate diagrammatically a scheme for addressing the parallax barrier of the embodiments of the invention; Figure 4 illustrates possible switching curves for the three regions of a ferroelectric liquid crystal material utilised in the schemes of Figures 3a to 3f; Figure 5 is a graph illustrating the effect of temperature on the switching threshold; Figure 6 is a graph showing the effect of the driving pulse shape on the switching curve; Figures 7a to 7c illustrate rotatable parallax barrier regions in accordance with a further embodiment of the present invention; Figures 8a and 8b are graphs showing the threshold characteristics required for a rotatable parallax barrier in accordance with an embodiment of the present invention; Figures 9 to 11 are graphs showing the effective permittivities of the liquid crystal layers in parallax barriers according to the present invention; and Figure 12 is a diagrammatic cross-sectional view of a parallax barrier according to an embodiment of the invention.

Like reference numerals refer to like parts throughout the drawings.

Figures la and lb show parallax barrier elements in accordance with embodiments of the present invention. The parallax barrier element would be placed either above or below an image panel and suitably placed polarisers would be used to achieve optical contrast. The cell I is defined by substrates 2 and 3, the inner side of each substrate bearing an electrode (not shown). The parallax barrier element 4 is made from ferroelectric liquid crystal (FLC), a bistable material which may be used to achieve high contrast in-plane switching at relatively low voltages. The parallax barrier 4 is formed in stripes 5, and the switching threshold can be modified in units of these vertical stripes. In the embodiment shown in Figure la the thickness of the stripes is varied using a ramp, whereas in Figure lb the thickness of the stripes is stepped. To compensate for the corresponding optical change brought about by the varying thickness of the liquid crystal parallax barrier 4, it is preferable to planarise the structure using planarising material 6 to level the thickness of the cell to that of the dotted line 7.

The desired switching threshold of the FLC may then be set by using materials of different permittivity for the ramps/steps and the planarising material.

The operation of the parallax barrier will now be described with reference to Figure 2.

Application of a voltage pulse 8 across the electrodes enables switching of certain regions of the FLC layer of the parallax barrier in accordance with the parameters of the applied pulse, those parameters including the pulse voltage and the pulse width. Figure 2 is a graph of pulse duration against pulse voltage, defining a plane which is known as the pulse duration-voltage plane. The graph of Figure 2 shows two pairs of curves, 9, and 9', 10'. Considering the first of these pairs of curves, the region I 1 between the curves 9 and 10 indicates the pulse width and pulse voltage conditions for which partial switching occurs. Below curve 9 the liquid crystal material is not switched. Above curve 10 the material is fully switched. The arrow indicates the effect of increasing thickness, so that the curves 9', 10' show the response to the pulse of a region of the parallax barrier having a greater thickness of liquid crystal material than the first curves 9, 10. The increased thickness causes a change in the switching threshold, the effect of which is to shift the response curves to a higher pulse voltage. At low voltages, the width of the partial switching region 11, 11' is only two to three volts, and the pairs of curves shift by approximately twenty volts per micrometer change in thickness. For a given pulse width, the pulse voltage may be changed in order to switch different thicknesses of liquid crystal, and therefore vary the position of the parallax barrier.

Considering the point marked A on Figure 2, at a given pulse width and pulse voltage, for the curves 9, 10 indicating the switching behaviour for the first thickness the material is fully switched, since the point A lies above the curve 10, but for the greater thickness represented by curves 9', 10' the material is not switched.

The addressing scheme for operating the parallax barrier as described above is- illustrated in Figures 3a to 3f and Figure 4. Referring to Figure 3, an initial blanking pulse 12 is a large pulse which switches all of the thicknesses to one state, either white or black. This pulse could exist in the region A of Figure 4. Typically a fixed pulse width would be used, and only the pulse voltage varied, as shown, although it is also possible to vary pulse width. Two subsequent pulses 13, 14 of opposing voltage directions are then supplied, their magnitudes arranged such that the horizontal position of a black stripe forming an opaque region of the barrier can be controlled. The magnitudes of pulses 13 and 14 are selected to alter the position of the black stripe, and suitable selection ranges are shown schematically in Figure 4. The partial overlap between the different switching properties in the pulse duration-voltage plane for the different pulses should be noted. The voltages Vl, V2 and V3 at which the second and third pulses operate may be discrete or part of a continuous range. The values of V1, V2 and V3 can be selected depending on feedback received from a sensor which determines the whereabouts of the viewer's head, these values determining the position of the opaque region. The sensor may involve face andlor eye recognition or be a simple infrared sensor.

The temperature sensitivity of the switching may result in the edges of the opaque regions of the parallax barrier not being straight due to temperature variations over the panel. It is therefore preferable for the stepped threshold variation of Figure lb to be implemented over the ramp implementation of Figure 1 a.

Figure 5 corresponds generally to Figure 2 but indicates the change in the threshold response curve with temperature. As temperature increases, the voltage response curves typically shift downwards and to the right. The change in pulse voltage is of the order to 0.2V per degree and the change in pulse width is approximately three microseconds per degree around room temperature. At low voltages, the shift of the response curve with temperature may be almost parallel to the curve itself. The precise shape of the response curve depends on the shape of the drive pulse, as illustrated in Figure 6. The description above considers only a rectangular pulse. It is therefore possible to choose a pulse shape which minimises the change in switching threshold with temperature by choosing a pulse shape which causes the same movement in the switching curve as al change in temperature. This also allows the temperature to be sensed and the pulse shape modified to maintain the switching curve in the same position on the T-V plot.

Since there is no patterning of the electrodes, there is no requirement for complex matrix addressing techniques, giving the freedom to choose any shape for the addressing pulse to minimise the temperature sensitivity. The freedom to choose the shape of the switching pulse is not an option for the matrix-addressed display of the

above-described threshold patterning prior art.

FLC devices commonly exist in either a Cl or a C2 state, named after the two chevron configurations of the director of LC molecules throughout the thickness of the device.

In general, the C2 state is easier to achieve since it has a low pre-tilt angle, but the resulting contrast in the absence of an applied electric field is generally inferior to that of the Cl state. If suitable cone angle materials are chosen, good contrast levels may be achieved with either of the C 1 or the C2 state, although there may be some temperature dependence of the contrast.

Other bistable liquid crystal modes which respond to changes in the switching threshold in a similar way to that of FLC material may also be used. These include the bistable and nematic mode (BTN), which has several different types (180 twist, 360 twist and binem 180 nematic mode), and the zenithal bistable mode (ZBD). The nematic materials have the advantage that the cell thickness can be approximately twice that required for the FLC mode described above which require a layer thickness of 2 micrometers or less in order to achieve the required optical effect.

The BTN switching threshold is thickness sensitive, so responds well to threshold patterning with thickness changes. ZBD liquid crystal mode uses a ridged or grating type alignment on one surface to achieve bistability. The pitch of this alignment may be varied over the LC layer to induce variations in the switching threshold on the LC layer.

It may be desirable to rotate a three-dimensional viewing panel through 90 such that both portrait and landscape formats can be viewed in threedimensions. This is difficult to achieve on panels based upon parallax barriers since the parallax barrier generally runs in a single direction. Rotating the barrier by 90 about the nommal to the display would mean that the barrier would provide vertical separation of two images rather than horizontal separation. The use of threshold patterning enables rotation of the three dimensional panel with the addition of a single optical element.

Figures 7a to 7c illustrate a parallax barrier according to a second embodiment of the present invention which divides the barrier up into nine regions of four different types, indicated by the numbers I to 4. The barrier has the same general construction as the barrier of the first embodiment. Regions of type 1, 2 and 3 are constructed from a liquid crystal material which switches once a suitable threshold voltage has been exceeded such that a parallax barrier for either landscape or portrait orientation may be produced by selecting type 1, 3 or type 2, 3, respectively. Region type 1 and type 2 may be composed of a material which only transmits light under certain conditions. Region type 3 is composed of a material which is required to transmit light in a 2D mode.

Region type 4 is composed of a clear polymer or resin, as it is required to always transmit light. The resin used for a region of type 4 may be a birefringent photopolymer such as a reactive mesogen. The same polariser configuration may be used throughout the liquid crystal layer or, alternatively, a series of patterned polarisers may be used.

Although this adds complexity the polariser may be patterned on a pixel level and be placed as a layer inside the panel, which can have benefits in temns of parallax and addressing if a complex pattern is required.

Figure 7a illustrates the orientation of the viewer when the system is operating in the portrait mode. Figure 7b shows the parallax barrier in the landscape mode. The dark strip 21 indicates an opaque region of the parallax barrier. It will be noted that the barrier is always vertical when seen by the viewer so as to provide the appropriate horizontal separation of 2 images. Figure 7c shows all four different regions types of the display transmitting light since the system is operating in a twodimensional mode in which no parallax barriers are required.

Different heights of indium tin oxide can be used to simultaneously provide the step effect and the electrode. This can be applied to any embodiment, but may also benefit from planarisation of the resulting structure.

A different dielectric material may be used in each of regions of types I to 4 to control the voltage dropped across the liquid crystal layer.

Figures 8a and 8b illustrate examples of the required threshold characteristics of regions of type 1 to 3 of the barrier shown in Figure 7. Figure 8a shows the characteristics of the regions when it is required that the 2D display mode is an active mode of the device, i.e. a voltage must be supplied in order for the 2D mode to be enabled. Regions of type 2 and 4 transmit light at low voltages whilst regions 1 and 3 do not. Regions of type 1 and 3 therefore form a portrait parallax barrier at low voltages. As the voltage is increased above voltage threshold Tl, region type 2 no longer transmits light and region type 1 begins to transmit light. Region types 3 and 4 remain non-transmissive and transmissive, respectively. The parallax barrier therefore changes from portrait configuration to landscape configuration at the threshold T1. As the voltage is further increased beyond the voltage threshold indicated approximately by T2, all regions become transmissive, including region type 3, providing an active two-dimensional display mode.

The characteristics of the regions in Figure 8b are similar to those of Figure 8a, except that the two-dimensional mode is present at a zero or low voltage range so that the display operates in two-dimensional mode when no power is applied to the parallax barrier. Above the voltage threshold T3 region type 3 stops transmitting light and between voltage thresholds T4 and T5, region type 2 stops transmitting light. Between T4 and T5 only region types 1 and 4 transmit light, therefore giving a landscape parallax barrier operation mode. Above threshold T5, region type I stops transmitting light and region type 2 becomes transmissive, therefore giving a portrait operation mode.

The threshold effect may be obtained in a similar manner to that described above for the first embodiment. A series of different thicknesses of liquid crystal may be provided for regions of type I, 2 and 3. The steps or ramps may be made using a polymer, such as a photoresist. The always-transmitting region 4 is formed entirely from polymer. Such structures are well known and may be fabricated, for example, by photolithography.

The step structure is fabricated on a substrate which is provided with a uniform electrode. The opposing substrate is also provided with a uniform electrode. The gap between the polymer steps and the opposing substrate is filled with a suitable liquid crystal. An aligning surface may be formed on each substrate so as to orient the liquid crystal and to provide the ability to vary the director configuration between the regions in the case of a nematic liquid crystal.

This type of structure may be implemented using a ferroelectric liquid crystal as the switchable media, in a similar way to that described above for the previous embodiments. As shown in Figure 4, there are two points on the pair of curves, indicated by B and C, where it is possible to drive two thicknesses simultaneously, thereby producing either the landscape or portrait barrier.

Instead of a bistable ferroelectric liquid crystal material, a monostable nematic liquid crystal material may also be used, which has some advantages in terms of ease of use and alignment.

If a field variation is achieved using different dielectric layers, it should be noted that the effective dielectric constant of the liquid crystal layer will vary as a voltage is applied and also upon switching. Figure 9 shows the effective permittivity for a 3.7 micrometer layer of negative nematic liquid crystal.

Figure 10 shows a modelled response of the transmission with voltage for a 7 micrometer layer of liquid crystal which could be used for region of type 2 of Figure 7.

The voltage indicated on the scale is the voltage dropped across the liquid crystal layer.

There is a bright state at approximately 3.2V which could be used for the portrait mode of Figure 8a, a dark state at 4.1V for use in the landscape mode of Figure 8a, and a further bright state at 6.5V which could be used for the active two-dimensional display mode of Figure 8a.

Figure 11 is a further modelled transmission-voltage response for a 3.7 micrometer layer of liquid crystal which may be used for either region type 1 or 3 in Figure 7.

There is a dark state below approximately 2.6V and a bright state at about 4.0V. If the thickness step between regions of type 1 and 3 is created using material having a different dielectric constant, then the barrier of Figure 8a may be obtained by using the 3.7 micrometer layer of liquid crystal for regions 1 and 3 and the 7 micrometer layer of liquid crystal for region type 2. Region type 4 is always transmissive. This arrangement is illustrated in Figure 12. Disposed between the two substrates 1, 2 are the liquid crystal 4 and the resin, arranged in steps to form the region types 1 to 4 as described above. Region type 4 contains purely resin since it is purely transmissive.

Region types 1 and 3 have the 3.7 micrometers thick layer of liquid crystal and equal depths of resin of differing dielectric constant. Region type 2 contains a 7 micrometer thick layer of liquid crystal and a correspondingly smaller thickness of resin. The electric permittivity of region types 2 and 4 may be the same. The electric permittivities of regions 1 and 3 are different to one another and to that of regions 2 and 4. The substrate upon which the liquid crystal is disposed may be rubbed so as to provide different liquid crystal director configurations in the different regions, as required, during manufacture of the liquid crystal cell.

The areas of region types 1 to 3 which transmit light in the portrait, landscape or 2D modes must transmit light which matches the colour and luminance of region type 4 (which is always transmissive) so that an image of sufficient 2D or 3D quality and consistency is displayed.

An alternative method to pattern the switching threshold is to pattern the alignment at a microscopic or sub-pixel level. This patterning may be achieved by using a suitable photopolymer and using UV irradiation to impart an alignment condition to the material. Alternatively, multirubbing methods or other direct contact methods are known in the prior art for improving the viewing angle. Using these methods, the pretilt or alignment direction may be patterned so that, for example, the ferroelectric material is patterned in Cl and C2 stripes and nematic materials are patterned in stripes with different pretilts or stripes of different arrangements (e.g. TN, parallel, HAN, etc.) which may be expected to have different switching thresholds.

According to a further embodiment of the present invention, the parallax barrier assembly described above is used in conjunction with a highresolution static image display rather than a changing image. The parallax barrier is changed in accordance with one of the above-described addressing systems to provide different images so that, for example, a moving image is seen by an observer. The images are not necessarily threedimensional images, and may not necessarily require tracking of the observer.

Such a system requires a fast switching parallax barrier using, for example, FLC material. Using an ultra high-resolution still image behind the parallax barrier assembly could allow a short video clip of lower resolution than that of the still image to be viewed very cheaply by virtue of the parallax barrier. The expense and equipment needed to operate and drive a separate video display unit is substantially eliminated.

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 as set out in the accompanying claims.

Claims

CLAIMS: 1. A parallax barrier comprising first and second substrates

carrying first and second addressing arrangements, respectively, each addressing arrangement comprising an electrode which is substantially uniformly continuous in the plane of the addressing arrangement, the electrodes defining therebetween a cell comprising a layer of liquid crystal material, the first and second addressing arrangements cooperating with the liquid crystal layer to provide switching properties which vary in the plane of the liquid crystal layer for selectively providing a first mode of operation providing a first parallax barrier, and a second mode of operation different from the first mode of operation.
2. A barrier as claimed in claim 1, arranged to operate in a bistable liquid crystal mode.
3. A barrier as claimed in claim 2, in which the bistable mode is any one of a ferroelectric liquid crystal mode, antiferroelectric liquid crystal mode, bistable nematic mode and zenithal bistable mode.
4. A barrier as claimed in any one of the preceding claims, in which the varying switching properties comprise varying voltage switching thresholds.
5. A barrier as claimed in any one of the preceding claims, in which the varying switching properties comprise different regions in the pulse duration-voltage plane.
6. A barrier as claimed in claim 5, in which at least two of the regions partially overlap.
7. A barrier as claimed in any one of the preceding claims, having a substantially clear non-barrier mode of operation.
8. A barrier as claimed in any one of the preceding claims, in which there is provided a third mode of operation different from the first and second modes of operation, the third mode providing a second parallax barrier.
9. A barrier as claimed in any one of the preceding claims, in which the liquid crystal layer has adjacent regions of different thicknesses of liquid crystal material.
10. A barrier as claimed in any one of the preceding claims, in which the liquid crystal layer has adjacent regions comprising materials of different permittivities.
11. A barrier as claimed in any one of claims I to 8, in which at least one of the substrates comprises a stepped layer of a first permittivity planarised by a layer of a second permittivity different from the first permittivity.
12. A barrier as claimed in any one of claims 1 to 8, in which at least one of the substrates comprises a first layer of a first permittivity having a plurality of ramps planarised by a second layer of a second permittivity different from the first 1 S permittivity.
13. A method of addressing a parallax barrier as defined in any one of the preceding claims, comprising applying to the parallax barrier a first signal for setting the parallax barrier into the first mode of operation; and applying at least one further signal, the at least one further signal for setting at least one portion of the parallax barrier into the second mode of operation.
14. A method as claimed in claim 13, wherein the magnitude of the at least one further signal is varied so as to vary the position of the at least one portion of the parallax barrier in the second mode of operation.