GB2418518A

GB2418518A - A Display

Info

Publication number: GB2418518A
Application number: GB0421227A
Authority: GB
Inventors: Paul Bonnett; Grant Bourhill; Allan Evans; Diana Ulrich Kean; Martin David Tillin; Emma J Walton; Robert Winlow; Koji Yabuta
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2004-09-24
Filing date: 2004-09-24
Publication date: 2006-03-29
Also published as: JP2006091871A; GB0421227D0

Abstract

A display (7) comprises an image display panel (1) for displaying a first image and a switchable privacy device (14) disposed in an optical path through the image display panel (1). The privacy device comprises a guest-host liquid crystal material (8) and is switchable between a public mode and a private mode. In its private mode, the privacy device displays a second image in a direction different from a pre-determined viewing direction whereby the image displayed by the display (7) along the direction different from the pre-determined direction is a superposition of the first image and the second image. The second image degrades the observed image, so that an observer located at the side, for example, of the display cannot clearly make out the first image. The second image is not displayed in the intended viewing direction, so that the first image is clearly seen by an observer viewing the display along the intended viewing direction.

Description

A Display The present invention relates to a display in which the angular

output range of light is controllable, so that the display can be switched between a wide angle viewing mode and a narrow angle viewing mode.

Electronic display devices such as, for example, monitors used with computers and screens built in to mobile telephones and other portable information devices, are usually designed to have as wide a viewing angle as possible, so that an image displayed by the device can be seen from many different viewing positions. However, there are some situations where it is desirable for an image displayed by a device to be visible from only a narrow range of viewing angles. For example, a person using a portable computer in a crowded train might want the display screen of the computer to have a small viewing angle so that a document displayed on the computer screen cannot be read by other passengers on the train. For this reason, there has been considerable effort put in to developing display devices which are electrically switchable between two modes of operation - in a 'public' display mode they have a wide viewing angle for general use, but they can be switched to a 'private' display mode in which they have a narrow viewing angle so that private information can be displayed in public places without being visible to people other than the user of the device.

Another application of such a display may be as a display in a motor vehicle. The viewing angle of the display could be controlled such that the passengers are unable to see the display or such that the driver is unable to see the display. Alternatively the viewing angle could be controlled in order to reduce the reflections of the display in the windscreen and the windows - so that, for example, the viewing angle could be reduced at nght-time or h1 low light conditions. A brightness sensor could be provided to allow automatic switching between a wide viewing angle and a narrow viewing angle, and also to allow automatic control of the brightness of the display.

A number of devices are known which restrict the range of angles or positions from which a display can be viewed.

U.S. patent No. 6 552 850 describes a method for the display of private information on an automatic teller machine (ATM). Light emitted by the machine's display has a fixed polarization state, and the machine and its user are surrounded by a large screen of sheet polariser which absorbs light of that polarization state but transmits light of the orthogonal polarization state. Passers-by can see the user and the machine, but cannot see information displayed on the machine's screen.

One known element for controlling the direction of light is a 'louvred' fihn that consists of alternating transparent layers and opaque layers provided in an arrangement similar to a Venetian blind. The film operates on the same principle as a Venetian blind, and it allows light to pass through it when the light is travelling in a direction parallel to, or nearly parallel to, the opaque layers. However, light travelling at large angles to the plane of the opaque layers is incident on one of the opaque layers and is absorbed. The layers may be perpendicular to the surface of the film, as shown in Figure 1, or they may be at some other angle to the surface of the film.

Louvred films of this type may be manufactured by stacking many alternating sheets of transparent material and opaque material and then cutting slices of the resulting block perpendicular to the layers. This method has been known for many years and is described in, for example, US patent Nos. 2 053 173, 2 689 387 and 3 031 351.

Other manufacturing methods are known. For example, US patent No. RE27617 describes a process where a louvred film is cut continuously from a cylindrical billet of stacked layers. US patent No. 4 766 023 describes how the optical quality and mechanical robustness of the resulting film can be improved by coating with a UV- curable monomer and then exposing the film to UV radiation. US patent No. 4 764 410 describes a similar process where a UV-curable material is used to bond the louvre sheet to a covering film.

Other methods exist for making films with similar properties to the louvred film. For example, US patent No. 5 147 716 describes a lightcontrol film which contains many elongated particles which are aligned in the direction perpendicular to the plane of the film. Light rays which make large angles to this direction are theret'ore strongly absorbed, whereas light rays propagating in this direction are transmitted.

Another example of a light-control film is described in US patent No. 5 528 319. This film has a transparent body in which are embedded opaque regions that extend generally parallel to the plane of the film. The opaque regions are arranged in stacks, with each stack being spaced from a neighbouring stack. The opaque regions block the transmission of light through the film in certain directions while allowing the transmission of light in other directions.

The prior art light control films may be placed either in front of a display panel or between a transmissive display panel and its backlight, to restrict the range of angles from which the display can be viewed. In other words, the prior art light control films make a display 'private'. I lowever none of the prior art light control films enables the privacy function to be switched off to allow viewing from a wide range of angles.

There have been reports of a display which can be switched between a public mode (with a wide viewing angle) and a private mode (with a narrow viewing angle). For example, US patent application No. 2002/0158967 suggests that a light control fihn could be movably mounted on a display so that the light control film either may be positioned over the front of the display to give a private mode or may be mechanically retracted into a holder behind or beside the display to give a public mode. This method has the disadvantage that it contains moving parts which may fail or be damaged in use, and which add bulk to the display.

A method for switching a display panel prom public to private mode with no moving parts is to mount a light control film behind the display panel, and to place a diffuser which can be electronically switched on and off between the light control film and the panel. When the diffuser is inactive, the light control film restricts the range of viewing angles and the display is in a private mode. When the diffuser is switched on, the light with a narrow angle range output from the light control film is incident on the diffuser, and the diffuser acts to increase the angular spread of the light - that is, the diffuser cancels out the effect of the light control film. Thus, the display is illuminated by light travailing at a wide range of angles and the display operates in a public mode. It is also possible to mount the light control film in front of the panel and place the switchable dil'f'user in front of' the light control fihn to achieve the same effect.

Switchable privacy devices of the above type are described in US patent Nos. 5 831 698,6 211 930 and 5 877 829. They have the disadvantage that the light control fihn always absorbs a significant fraction of the light incident upon it, whether the display is in public mode or private mode. The display is therefore inherently inefficient in its use of light. Furthermore, since the diffuser spreads light through a wide range of' angles in the public mode, these displays are also dimmer in public mode than in private mode (unless the backlight is made brighter when the device is operating in public mode to compensate).

Another disadvantage of these devices relates to their power consumption. Such devices often use a switchabie polymer-dispersed liquid crystal diffuser which is not diffusive when no voltage is applied across the liquid crystal layer and which is switched on (into the diffusive state) by applying a voltage. Thus, to obtain the public mode of operation it is necessary to apply a voltage across the diffuser so that the diffuser is switched on. More electrical power is therefore consumed in the public mode than in the private mode. This is a disadvantage for mobile devices which are used for most ofthe time in the public mode and which have limited battery power.

Another method for making a switchable public/private display is given in US patent No. 5 Y25 436. 'the light control device in this patent is similar in structure to the louvred film described above. I lowever, each opaque element in a conventional louvred fihn is replaced by a liquid crystal cell which can be electronically switched from an opaque state to a transparent state. The light control device is placed in front of or behind a display panel. When the cells are opaque, the display operates in a private mode; when the cells are transparent, the display operates in a public mode.

One significant disadvantage of this device is the difficulty and expense of manufacturing liquid crystal ceils with an appropriate shape. A second disadvantage is that, ha the private mode, a ray of light may enter at an angle such that it passes first through the transparent material and then through part of a liquid crystal cell. Such a ray will not be completely absorbed by the liquid crystal cell and this may reduce the privacy of the device.

Japanese patent application No. 2003-28263 describes a switchable viewing angle control mechanism for a liquid crystal (LC) panel. This uses an additional twisted nematic (TN) I,C panel, which is patterned in a checkerboard pattern. In the narrow viewing mode, the limited viewing angle characteristics of a standard TN LC panel are used to make a checkerboard pattern appear when the LC panel is viewed from an angle well away from the normal direction. 'This checkerboard pattern is confusing for the viewer and degrades the quality of the image seen from an angle well away from the normal direction. This does have the disadvantage that an additional LC panel and an additional polariser are required.

The present invention provides a display comprising an image display panel for displaying a first image and a switchable privacy device disposed in an optical path through the image display panel, the privacy device comprising a guest-host liquid crystal material and being switchable between a public mode and a private mode; wherein the privacy device, in its private mode, displays a second image in a direction different from a pre-determined direction whereby the image displayed by the display along the direction different from the pre-determined direction is a superposition of the first hnage and the second image.

The first image, which is displayed by the image display device, is an hnage that it is desired to display to an observer who is viewing the display along a pre-determined direction (which is the intended viewing direction of the display). Where the invention is applied to an ATM, as an example, the first image would consist of the instructions, key-pad etc that are displayed on the display screen of a conventional ATM.

I'he second image, which is displayed by the privacy device when the privacy device is in its private mode, is displayed along directions diff'ercnt from the intended viewing angle of the display. 'I'hc second image is not displayed along the intended viewing direction, so that an observer viewing the display along the intended viewing direction will see only the first image (i.e., will see only the image that it is desired to display to the observer).

An observer viewing the display from a direction that is well away from the intended viewing direction will however see an overall image that is a superposition of the first hnage and the second image. The second image is chosen to be an image (for example a pattern of light and dark areas) that obscures the first image, so that it is very difficult for an observer viewing the display from a direction that is not the intended viewing direction to see clearly and to understand the first image. In the example of the invention applied to an ATM, for example, it would be very difficult for a criminal viewing an ATM from the side to make out the images displayed on the ATM and obtain the personal identification number (PIN) of or personal information about the person using the ATM.

The privacy device may comprise a plurality of first regions, each first region being switchable between a light-transmissive state in which the region is transmissive for a pre-determined polarization component and a light-absorbing state in which the region is absorbing for the predetermined polarization component propagating in a direction different from the pre-determined direction. The pre-determined direction may be, but it not limited to, the normal direction to the display.

The public mode is obtained by switching each first region to its lighttransmissive state. The private mode is obtained by switching some first regions to their light- absorbing state while leaving other first regions in their light- transmissivc state.

Someone viewing the display from a wide viewing angle in the private mode will see a pattern of light areas and dark areas superposed over a displayed image. The light areas of'the pattern correspond to those first regions that are in their light-transmissive state and the dark areas correspond to those first regions that are in their light-absorbing state.

The pattern of light and dark areas will degrade the image seen from a large viewing angle, thereby improving the privacy of the display. 'I'he pattern of light and dark areas will not however be visible to an observer viewing the display along the pre-determined viewing direction, so that the displayed image will be clear to such a viewer.

The term "wide viewing angle" as used herein denotes a viewing angle that is away prom the predetermined viewing direction of'the display. In general, a display will have a range of intended viewing angles that are centred on an intended viewing direction.

The range of intended viewing angles will be determined by the intended viewing distance of the display and by the desired width of the viewing window of the display at the intended viewing distance. A typical range of viewing angles is up to 30 on each side of the intended viewing direction (which is often, but not always, the normal direction to the display face of the display), but the range of viewing angles for a particular device may be greater or smaller than this. The term "wide viewing angle", in connection with a display, is intended to cover any viewing angle that is outside the range of intended viewing angles of'that display.

The pattern of light and dark areas perceived by an observer viewing the display prom a wide viewing angle, when the display is in its private mode, will be referred to as a "confusing pattern" for convenience, since the light and dark areas serve to confuse and degrade the image seen at wide viewing angles. I lowever, the term "pattern" does not imply that the light and dark areas are necessarily arranged in a repeating manner.

The pattern of light and dark areas that provides the best privacy will depend on the displayed image, for example on whether the image is text or pictures, on the size of characters in the case of a text image etc. Accordingly, each first region may be independently switchable between the light-transmissive state and the light-absorbing state. This allows the pattern of light areas and dark areas to be re-configured if desired, thereby enabling a pattern to be displayed that provides good privacy for a particular image.

Alternatively, where a display is intended for one particular application the pattern of light and dark areas may be chosen in advance to suit this application and, in such a case, it is not necessary for each first area to be individually switchable.

The privacy device may comprise an electrode patterned to define the first regions.

This is a simple way of defining the first regions in the liquid crystal layer.

At least one first region of the privacy device may comprise, in its light-absorbing state, a first sub-region having a first liquid crystal alignment and a second sub-region having a second liquid crystal alignment different from the first liquid crystal alignment. This allows the pattern of light and dark areas seen in the private mode to appear more strongly at viewing angles near the normal. '[his reduces the angular range over which a displayed image is not obscured by the confusing pattern of light and dark areas, and so provides increased privacy in the private mode.

The first liquid crystal alignment may be tilted from an homeotropic alignment, and the second liquid crystal alignment may be tilted from an homeotropic alignment. The first and second liquid crystal alignments may be tilted in different directions from the homeotropic alignment.

The directors of liquid crystal molecules in the first liquid crystal alignment and the directors of liquid crystal molecules in the second liquid crystal alignment may lie in a common plane perpendicular to the directors of liquid crystal molecules in the first region when the first region is in its light-transmissive state.

The privacy device may further comprise a plurality of second regions, each second region being in a light-transmissive state when the privacy device is in its public mode.

The transmissivity of each second region may be substantially constant between the public mode and the private mode ofthe privacy device.

Each second region may be formed by selective polymerization of the guesthost liquid crystal material.

The director of the liquid crystal material in each first region may be tilted with respect to the normal direction of the display when the privacy device is in its private mode, the direction and angle of tilt of the directors being substantially the same in each first region. This provides a display which, in the private mode, can be viewed from a predetermined viewing direction that is not coincident with the normal direction to the display.

A second aspect of the invention provides a use of a guest host liquid crystal panel as a controllable privacy device for a display.

The controllable privacy device may be switchable between a public mode and a private mode.

The privacy device may comprise a plurality of first regions, each first region being switchable between a light-transmissive state in which the region is transmissive for a pre-determined polarization component and a light-absorbing state in which the region is absorbing for the predetermined polarization component propagating in a direction different from a pre-determined direction.

Preferred embodiments of the present invention will be described with reference to the accompanying figures in which: Figure I (a) is a schematic plan view of a display according to the present invention; F igure I (b) is a schematic plan view of a display according to the present invention; Figures 2(a) and 2(b) are partial enlarged views of figures 1 (a) and 1 (b) respectively; Figure 3(a) shows a privacy device ofthe invention in its public mode; Figure 3(b) shows the privacy device of figure 3(a) in its private mode; F igure 4 shows the variation of the contrast against viewing angle tor the privacy device of figure 3(a); Figure 5 is a sectional view of a display according to an embodiment of the present invention; Figure 6 is an illustration of an off-axis image as seen in the private mode for a display of the present invention; Figures 7(a) to 7(d) show further examples of "confusing patterns" of light and dark areas for a display ofthe present invention; Figure 8 is a schematic sectional view of a display according to a further embodiment of the present invention; F igure 9 is a schematic sectional view of a display according to a further embodiment of the present invention; Figure 10 is a schematic sectional view of a display according to a further embodiment of the present invention; I igure l I(a) Is a schematic sectional view of a display according to a further embodiment ofthe present invention; and Figure I I (b) shows the variation of the transmission against viewing angle for a display ol figure I I(a).

Like reference numerals denote like components.

Figure l(a) is a schematic plan view of a display 7 illustrating the principle of the present invention. 'I'he display 7 comprises a transmissive image display panel I that is illuminated by a backlight 3. The backlight 3 is disposed on the opposite side of the transmissive image display panel I to an observer 6.

The backlight 3 emits light having a wide angular range, as shown schematically in figure l(a). If the image display panel I were simply illuminated by the backlight 3, the wide angular spread of light emitted by the backlight 3 would result in light output from the display 7 having a wide angular spread so leading to a wide viewing angle. In order to allow the angular distribution of light from the display 7 to be controlled, a privacy device 2 is placed between the backlight 3 and the image display panel 1. The privacy device 2 is switehable between a public mode and a private mode. In the private mode, the privacy device 2 has a relatively narrow angular distribution 4 of output light, whereas in the public mode the privacy device 2 has a wide angular output distribution of output light. Thus, by switching the privacy device 2 between its public mode and its private mode, the light output from the display 7 can be chosen to have either a wide angular distribution or a narrow angular distribution.

The construction and operation of the image display panel I is unaffected by the provision of the privacy device 2. The privacy device 2 simply modulates the angular extent of light that has passed through the image display panel 1.

Figure I (b) shows a further example of a display 7h This corresponds generally to the display 7 of figure I (a), except that the privacy device 2 is placed in front of the image display layer 1 (that is, the privacy device 2 is placed between the observer 6 and the image display panel 1). The image display panel I again functions normally, and emits light with a wide angular distribution. Light from the image display panel I is incident on the privacy device 2 which, as in the display 7 of figure I (a), is switchable between a public mode and a private mode. In the public mode the privacy device 2 transmits light from the image display panel 1 with no significant effect on its angular distribution - so that light is transmitted by the privacy device 2 with a wide angular distribution 5.

When the privacy device 2 is switched to its private mode, howcvcr, it transmits light only in a narrow angular range 4, so that an image displayed on the image display panel I can be viewed only from angles close to the normal of the display 7 and cannot be viewed from angles that are a long way from the normal ofthe display.

In the display 7' of figure l(b), the image display panel I may be a transmissive display panel, a renective display panel or an emissive display panel. If the image display panel I is a transmissive display panel a separate backlight (not shown) is required. The construction and operation of the display panel are conventional, and the control of the angular output range ofthe display 7' is effected by the privacy device 2.

According to the present invention, the privacy device 2 comprises a guest-host liquid crystal layer. 'I'he privacy device may be switched between a public mode and a private mode by applying a suitable voltage across the liquid crystal layer. There is therefore no need to mechanically remove the privacy element in order to obtain the public mode, in contrast to some of the prior art displays described above.

Figure 2(a) is a block schematic sectional view of a display of the present invention.

The display corresponds generally to the display 7 of figure l(a), and comprises an image display panel I illuminated by a backlight 3. In figure 2(a) the image display panel I is shown as a liquid crystal image display panel having a layer 9 of liquid crystal material disposed between linear polarisers 10, 11. The liquid crystal layer is addressable in any suitable method to allow an image to be displayed, and the liquid crystal panel may be, for example, a thin fihn transistor (TFT) liquid crystal panel.

However, the precise nature of the image display panel I is not relevant to the present invention and, in principle, any suitable transmissive display panel may be used.

A layer 8 of guest host liquid crystal material is provided between the backlight 3 and the image display panel 1. The guest host liquid crystal layer 8 acts as the privacy element 2 of figure I (a), as will be described in more detail below.

Figure 2(b) is a schematic sectional view of a display of the invention corresponding generally to the display 7' of figure l(b). This corresponds generally to the display of figure 2(a), except that the guest host liquid crystal layer 8 is provided in front of the image display panel 1.

In figure 2(b) the image display panel I is a transmissive display panel illuminated by a backlight 3. However, as explained with reference to figure l(b) above, the image display panel could alternatively be a reflective display panel or an emissive display panel.

The operation of the guest host liquid crystal layer 8 of the present invention is illustrated in figures 3(a) and 3(b). As is known, a guesthost liquid crystal material contains guest dye molecules in a liquid crystal material host. The liquid crystal material may be any standard liquid crystal material and may have positive or negative dielectric anisotropy. 'I'he liquid crystal material is doped with dye molecules to provide optical absorption. The liquid crystal material may contain a wide-band dye that absorbs over the entire visible region of the spectrum or it may contain a mixture of a dye that absorbs in the red spectral region, a dye that absorbs in the green spectral region and a dye that absorbs in the blue spectral region (or any combination of dyes that gives good absorption over the entire visible spectrum). The dichroic dye molecules are highly anisotropic, and absorb light that is planepolarised in a direction parallel to the long axis of the dye molecules. Light that is plane-polarised in a direction perpendicular to the long axis of the dye molecules is, however, not significantly absorbed by the dye molecules.

Possible liquid crystal materials include, for example E7 or ZLI-4619-000 available from Merck. A suitable dye is Mitsubishi black dye 4, at a concentration of between 1% and 3% by weight.

The dye molecules tend to align themselves along the same direction as the liquid crystal molecules. 1; igure 3(a) shows the liquid crystal layer 8 having a planar alignment, in which the liquid crystal molecules extend generally parallel to the plane of the substrates 12,13. It is assumed that the liquid crystal layer 8 is illuminated by light that is plane-polarised in a direction generally perpendicular to the long axis of the dye molecules. This polarization component is therefore transmitted through the liquid crystal layer 8 without significant absorption, since the polarization direction is perpendicular to the long axis of the dye molecules.

In figure 3(a) the substrates 12,13 are shown oriented parallel in the xy plane, and the liquid crystal molecules extend generally along the xaxis (which is into the plane of the paper in figure 3(a)). It is assumed that the liquid crystal layer 8 is illuminated by light that is planepolarised in the y-direction (which is parallel to the plane of the paper in figure 3(a), as indicated by the symbol). The y-polarisation component is therefore transmitted through the liquid crystal layer 8 through the wide angular range, as shown in figure 3(a). The liquid crystal layer acts as a linear polariser with its absorption axis arranged along the x- axis' as shown by the symbol in figure 3(a). Figure 3(b) shows the liquid crystal molecules having a homeotropic

alignment, in which the liquid crystal molecules extend generally perpendicular to the substrates.

(that is, along the z-direction in figure 3(a)). Light that is planepolarised along the y- direction and that propagates along the direction in which the liquid crystal molecules extend will be transmitted without significant absorption, since light propagating along this direction "sees" only the end of the dye molecules and so is not significantly absorbed. However, as the angle between, on the one hand, the direction of propagation of the light and, on the other hand, the direction in which the liquid crystal molecules extend increases from zero, the incident light "sees" more and more of the long axis of the dye molecules. The y-plane polarised component is therefore absorbed by the dye molecules, with the absorption becoming increasingly strong as the angle between, on the one hand, the direction of propagation of the light and, on the other hand, the direction in which the liquid crystal molecules extend increases.

The guest host liquid crystal layer 8 thus acts as a privacy device for yplane polarised light. When the liquid crystal material is switched to the planar alignment of figure 3(a), the y-polarisation component is transmitted with lime absorption over a wide angular range. When the liquid crystal material is switched to the homeotropic alignment of figure 3(b), however, the y-polarisation component is transmitted without significant absorption in the normal direction, but is increasingly strongly absorbed as the angle of incidence moves away from the normal direction.

Figure 4 shows the variation of the transmissivity of the guest host liquid crystal layer against the viewing angle. Trace (a) shows the transmissivity of the liquid crystal layer for the y-component of plane polarised light, when the liquid crystal layer has the planar alignment of figure 3(a). Trace (b) of figure 4 shows the transmissivity of the liquid crystal layer 8, for the y-polarisation component, when the liquid crystal material has the homeotropic alignment of figure 3(b). It will be seen that the transmissivity decreases significantly as the viewing angle increases away from the normal direction when the liquid crystal layer has the homeotropic alignment of figure 3(b) . The guest- host liquid crystal layer is therefore switchable between a private mode (corresponding to the homeotropic alignment of figure 3(b)) and a public mode (corresponding to the planar alignment of figure 3(a)).

In the embodiment of figure 2(b), if the privacy device 14 is oriented as shown in figure 3(a), the exit polariser 11 of the image display panel I is preferably arranged with its absorption axis along the x-direction so that output light from the image display panel I contains only the y-polarisation component. The angular range of light output from the display can therefore be controlled by switching the alignment of the guest-host liquid crystal layer 8 between the planar alignment and the homeoiropic alignment, so that the guest host liquid crystal layer 8 transmits the y- polarisation component with either a wide angular range (planar alignment) or a narrow angular range (homeotropic al ignment).

Conversely, in the display of figure 2(a), if the privacy device 14 is again oriented as shown in figure 3(a), the input polariser 10 of the image display panel I would preferably be arranged with its absorption axis along the x-direction so that any x polarised light that passes through the guest-host liquid crystal layer 8 would be absorbed in the input polariser 10 and would not contribute to the output of the display.

The liquid crystal layer 8 may contain a liquid crystal material having positive dielectric anisotropy. In this case, the liquid crystal material will adopt the planar alignment of figure 3(a) when no electric field is applied across the liquid crystal layer, and can be switched to a substantially homeotropic alignment by applying a suitable electric field across the liquid crystal layer. Alternatively, the liquid crystal layer may have a negative dielectric anisotropy, and the substrates may have a substantially homeotropic alignment. In this case the liquid crystal material would adopt a homeotropic alignment under no applied field, and the application of a suitable electric filed would switch the liquid crystal material into the planar alignment. (In practice, the liquid crystal alignment may be required to be a slightly oft homeotropic alignment when a negative dielectric anisotropy liquid crystal material is used, in order to influence the direction in which the liquid crystal molecules switch when an electric field is applied.) If a display is to operate predominantly in the public mode it would be preferable (from the power consumption of the display) to use a positive dielectric liquid crystal material in the privacy device, so that the public mode is obtained when no voltage is applied across the liquid crystal layer. Conversely, if a display is to operate predominantly in the private mode it would be preferable (from the power consumption of the display) to use a negative dielectric liquid crystal material in the privacy device.

As is shown by figure 4, the use of the guest host liquid crystal layer 8 of the invention provides a simple viewing angle control mechanism. However, the decrease in transmissivity in the private mode as the viewing angle increases is relatively slow, and it may be difficult to obtain a private mode that has a sufficiently narrow viewing angle - as trace (b) in figure 4 suggests, it is possible that an image would still be readable even at viewing angles that are a long way from the normal direction. While the transmissivity at off-axis viewing angles can be reduced further, compared to trace (b) of figure 4, by using very high levels of dye doping in the liquid crystal material, this would cause high attenuation of the image in the normal direction and may therefore be undesirable.

According to the invention, therefore, the privacy device 14 of the invention is patterned. One possible embodiment of a patterned privacy device is shown in figure 5.

Figure 5 shows a privacy device 14 that comprises, in addition to the guest host liquid crystal layer 8 and the upper and lower substrates 12, 13, an electrode 15A,15B for applying an electric field across the liquid crystal layer 8. The electrode is transparent, for example an ITO (indium tin oxide) electrode, and is disposed on the lower substrate 13 (in practice a counter-electrode will be disposed on the upper substrate 12, but this has been omitted from figure 5 for clarity). The electrode is patterned, and figure 5 shows two regions 15A and 15B. The electrode regions 15A and 15B are controllable independently from one another, and define two corresponding regions (denoted by A, B) in the liquid crystal layer 8.

In the private mode of the privacy device, the liquid crystal material and the dye molecules of region B of the liquid crystal layer 8 are caused to adopt a planar alignment (in the case of a liquid crystal material having a positive dielectric anisotropy this requires that no electric field is applied across the region B of the liquid crystal layer 8, whereas in the case of a liquid crystal material having a negative dielectric anisotropy this requires that a voltage is applied across region B of the liquid crystal layer 8.) The y-polarisation component is therefore transmitted through region B of the guest-host liquid crystal layer 8 with a wide angular range.

In the private mode of the privacy device, the liquid crystal material and the dye molecules of region A of the liquid crystal layer 8 are caused to adopt the homeotropic alignment. (In the case of a positive dielectric liquid crystal material this requires that an electric filled is applied across region A of the liquid crystal layer by the electrode 1 5A, whereas in the case of a liquid crystal material having negative dielectric anisotropy this requires that no voltage is applied across region A of the liquid crystal layer.) The y-polarisation component is therefore transmitted through region A of the guest-host liquid crystal layer 8 without significant absorption only for directions around the intended viewing direction (which is the normal direction in figure 5) - as the angular separation between the viewing angle and the intended viewing direction increases, the y-component is increasingly strongly absorbed as explained above.

When a display incorporating the privacy device 14 of figure 5 is viewed from a wide viewing angle, an observer Will see a pattern of light areas (corresponding to region B) and dark areas (corresponding to region A). The pattern of light and dark areas will be superposed on any image displayed on the image display panel 1. The pattern of light and dark areas will provide a "confusing pattern" which will degrade the image seen by an observer viewing the display from a wide viewing angle. The abrupt variations in brightness of the "confusing pattern" seen by an observer viewing the display from a wide viewing angle in the private mode obscure a displayed image far more effectively than does a privacy device that produces a low (but non-zero), slowly-varying transmissivity.

Figure 6 gives an example of how a display incorporating the privacy device of figure 5 would be seen by an observer at a wide viewing angle. The regions A and regions B are arranged in a "chequerboard" arrangement, so as to produce a pattern of alternating light and dark squares. As can be seen in figure 6, the pattern of light and dark squares severely degrades the quality of the image, and makes it very hard to decipher the text in figure 6 (which corresponds to the image displayed on the image display panel 1).

When the display is viewed from the normal direction, the pattern of light and dark areas does not exist. This is because region A and region B in figure 5 both have a high transmissivity for light travelling along the normal direction. An observer viewing the display along the normal direction would therefore see the image displayed on the image display panel I against a uniform white background.

lithe 'chequerboard" pattern of light and dark areas of figure 6 is shown without any obscuring text in figure 7(a).

The invention is not limited to the particular chequerboard pattern of light and dark areas of figure 7(a). Any pattern that will obscure an image may be used. Examples of possible other patterns are shown in figure 7(b) to 7(d). In figure 7(b) each dark area has the general form of a sine wave, with the areas between adjacent sine waves being light In figure 7(c), the dark areas have the form of letters of the alphabet. The letters of the alphabet arc arranged randomly to produce a "confusing pattern"; some letters are superposed over other letters to make the pattern more confusing.

In figure 7(d), the dark areas are arranged in the forms of horizontal and vertical lines.

The light areas are rectangles defined by two adjacent horizontal lines and by two adjacent vertical lines.

The pattern of light and dark areas can be chosen depending on the image that is to be displayed on the display device. Some patterns are better for obscuring text, and some patterns arc better for obscuring pictures. Furthermore, the extent to which the pattern obscures the image depends on the size of the light and dark areas relative to the size of the image. To demonstrate this, figure 6 contains text having different fonts and dif'fcrcnt sizes, and it can be seen that a chequerboard pattern having light and dark areas of a particular size is more effective at obscuring certain fonts and certain sizes of text than others. The sizes of the light and dark areas are therefore preferably chosen so that they are effective at obscuring the image that is intended to be displayed on the image display panel.

Figure 5 has been simplified for the purpose of explanation and shows only two regions of the guest-host liquid crystal layer 8. As can be seen from figures 7(a) to 7(e), the liquid crystal layer will in general contain many more than two regions. If desired, every region of the guest- host liquid crystal layer 8 may be controllable independently from every other region. This would enable the pattern of light and dark areas to be reconfigured, and this may be of use when the image display panel I is intended to display many types of images. As an example, if the chequerboard pattern of figuec 7(a) is assumed to have light and dark areas each having a size of I unit x I unit, making each region of the guest-host liquid crystal layer 8 independently controllable would allow the chequerboard pattern to be re- conOgured to give light and dark areas each having a size of, for example 2 units x 2 units. 20

In principle, however, it is not necessary for every region of the liquid crystal layer 8 to be independently addressable. For example, all regions A (that produce dark regions in the pattern) may be controlled together, and all regions B of the liquid crystal layer 8 (that produce the light regions in the pattern) may also be controlled together. this would reduce the number of switching components needed, so that a privacy device in which all regions A are controlled by one switching element would have the lowest manufacturing cost. In practice, if all the regions are controlled by one switching element, it would be necessary for there to be a conductive path that connects all the regions that will be switched - so, for example, in this embodiment all the black areas in figures 7(a)-(d) would have to be electrically connected to each other (and these connections are most conveniently provided in the layer in which the electrodes MA, 15B are defined).

Where the guest-host liquid crystal material privacy device of the present invention is used in conjunction with an hnage display panel that contains a polariser, it is not necessary for the privacy device to contain a separate polariser. The privacy device of the present invention therefore requires the minimum of further components to be added to the display. Furthermore, there is no need for the privacy device to be precisely aligned with the pixels of the image display panel. The privacy device of the invention is therefore stipple to make, and is simple to incorporate into a display.

Furthermore, when the privacy device is in its public mode, the guesthost liquid crystal layer does not significantly absorb light of the polarization component passed by the privacy device (the y-polarisation for a privacy device oriented as shown in figure 3(a)).

The privacy device therefore does not significantly reduce the brightness of the display in its public mode.

Figure 8 is a schematic illustration of another privacy device of the present invention.

This privacy device again consists of a layer 8 of guest-host liquid crystal material disposed between upper and lower substrates 12, 13. The substrates are provided with electrodes to allow an electric field to be applied across selected regions of the liquid crystal layer 8, but these electrodes are omitted from figure 8 for clarity. Figure 8 also shows a polariser 10, and this may be the entrance polariser of the image display panel (although it could possibly be a separate polariser). The polariser 10 has its absorption axis arranged parallel to the direction of the liquid crystal molecules in the planar aligmnent (region B); with the components oriented as in figure 8 the absorption axis of the poiariser 10 is along the x-axis, as shown by the symbol in figure 8.

In this embodiment the alignment of the liquid crystal layer is also patterned, so that the alignment of the liquid crystal layer in its private mode is not an exact homeotropic alignment.

Regions A and B shown in figure 8 correspond to regions A and B of figure 5. Thus, region A is in the private mode and has a homeotropic liquid crystal alignment, whereas region B is in its public mode and has a planar alignment. (It is again assumed that light passing through the privacy device is plane-polarised along the y-direction.) I'he effect of patterning the alignment of the liquid crystal materials is shown in regions C and D. In both region C and region D the liquid crystal alignment is tilted from an homeotropic alignment when the region is in its private mode, with the direction of tilt in region C being opposite to the direction of tilt in region D. As a result, the dark areas of the "confusing pattern" produced by the liquid crystal layer 8 appear more strongly at a given angle from the normal direction, which is the intended viewing direction in this embodiment. This is because region C is more strongly absorbing than region A for light directed at an angle tic from the normal direction, whereas region D is more strongly absorbing than region A for light directed at an angle 0 (which is on the opposite side of the normal direction to Oc) to the normal direction. By providing regions C and D with approximately equal areas, it is thus possible to reduce the transmissivity of the liquid crystal layer 8, in its private mode, for ot't:axis viewing directions. (It should be noted that tilting the liquid crystal molecules as shown in regions C and D will cause a small reduction in the transmissivity in the normal direction in the private mode, but the disadvantage of this is normally compensated by the reduction in transmissivity at off-axis directions.) 22 In the public mode, region C and region D would both have a planar liquid crystal alignmcut, as shown in figure 3(a).

If the patterned alignment embodiment of figure 8 is applied to the privacy device of figure 5, each region A of figure 5 could be replaced partly by region C and partly by region D. Thus, in the private mode the privacy device would consist of regions B and regions that were part C and part 1). A possible disadvantage of this, however, is that a viewer looking at normal incidence might see some patterning in the private mode owing to the difference in transmission between region B and regions C and D. It might be possible to overcome the above problem by matching the transmission of region B to the transmission of regions C and D. In a more preferred embodiment, however, regions C and D are the only regions present in the privacy device. When figure 8 is applied to the embodiment of figuec 7(a) in this way, each region shown in figure 7(a) would, in the private mode, be a region in part of which the liquid crystal orientation is as shown for region C of figure 8 and in part of which the liquid crystal orientation is as shown for region D of figure 8. These have the same transmissivity at normal incidence, so that an observer viewing the display along the normal direction would not see any patterning, but have transmissivities that vary in opposite directions as the viewer moves *om one side of the display to the other thereby creating the patterning ef'f'ect when the privacy device is viewed from a wide angle. When the privacy device is switched into the public mode, the entire region would adopt the liquid crystal orientation of region B of figure 8, so that no patterning is seen at any viewing angle. (This embodiment may require the additional use of a wave plate, because the switching down is in a dit'ferent direction to the pre-tilt on these two regions, as mentioned below.) The regions C and D may be defined by providing a patterned alignment film on one or both of the substrates 12, 13. The pre-tilt would be patterned to provide alignment in region C and in region D that was tilted by a few degrees from an homeotropic alignment, with the alignment in region C being tilted in the opposite direction from the normal than the alignment in region D. This embodiment could be ef'f'ectcd using a liquid crystal material having a negative dielectric anisotropy, so that the tilted homeotropic states were the stable state under zero applied field. Application of a suitable electric field would switch region C and region D to the planar alignment to produce the public mode.

Alternatively, this embodiment could be af'f'ected with a liquid crystal material having a positive dielectric anisotropy. Fringe field effects could be used to produce the off- homeotropic alignment when an electric field is applied. (Under zero applied field, the planar state would be the stable state in both regions C and region D).

In this embodiment, the dye molecules in the private mode are ideally in a plane perpendicular to the axis of the dye molecules when in the planar configuration. If the dye molecules extend along the x-direction in the planar configuration, as in region B. the y-polarisation component is transmitted through the liquid crystal layer without substantial absorption. To obtain the greatest absorption of the y-polarisation component when the liquid crystal layer is in the private mode, the dye molecules would preferably extend in the y-z plane, so that the ypolarisation component "sees" the great length of the dye molecules and so that the greatest possible absorption of the y-component occurs. This is, however, dit'ficult to achieve in practice. In a more realistic embodiment, theret'ore, a switchable halt:waveplate would be placed between the guest-host liquid crystal layer 8 and the polariser 10. In the private mode, the liquid crystal molecules would be aligned along the x-direction, and the switchable half- waveplate would be switched to give a zero retardation. The y- polarisation component would thus be passed by the liquid crystal layer 8 without substantial absorption, and would then be passed by the polariser 10 (which is assumed to have its absorption axis arranged along the x- axis).

When the liquid crystal layer was switched to its private mode, the molecules in region C and region D would be tilted out of the plane of the paper, so that the x-polarisation component would undergo the greatest absorption. 'I'he switchable half-waveplate would be switched to give half-wave retardation, and so the x-polarisation component transmitted by the liquid crystal layer 8 would be converted to y- polarisation by the half-waveplate and would then be passed through the polariser 10. The y-polarisation component passing through the liquid crystal layer would be switched to the x- polarisation component to the half-waveplate, and would then be blocked by the polariser 10. A typical pre-tilt of the liquid crystal molecules in this embodiment would be between 80 and 90 from the plane of the substrate.

As an alternative, fringe fields from pattern or inter-digitated electrodes may be used to produce the required liquid crystal alignment in regions C and D in the private mode.

These fringe fields may be directed either substantially across the liquid crystal cell so as to produce the patterned tilt direction shown in figure 8 in the private mode, and in this case the use of patterned alignment films is unnecessary. Alternatively, the fringe fields may be substantially in the plane of the liquid crystal layer, and produce the planar alignment for the public mode (in which case patterned aligmnent films are required to produce the patterned tilt directions in the private mode).

In the above embodiments the liquid crystal layer 8 is continuous over the area of the privacy device, and means are provided for changing the alignment of liquid crystal molecules in one or more regions of the layer independently of the alignment of molecules in other regions of the liquid crystal layer. It is alternatively possible for the liquid crystal layer to be non-continuous over the plane of the privacy device, and a further embodiment of the invention having this feature is shown in figure 9. Figure 9 is again a schematic sectional view through a privacy device of the present invention.

In the privacy device of figure 9, liquid crystal material is disposed in a plurality of discrete regions 8A, 8B between the upper and lower substrates 12, 13. The liquid crystal regions 8A, 8B again contain a dichroic dye, and are switchable between a planar alignment (as in region B ol figure 9) and a homeotropic alignment (as in region A of figure 9). Figure 9 also shows a polariser 10, and this may be the entrance polariser of the image display panel (although it could possibly be a separate polariser).

The polariser 10 has its absorption axis arranged parallel to the direction of the liquid crystal molecules in the planar alignment (region B); with the components oriented as in figure 9 the absorption axis of the polariser 10 is along the x-axis, as shown by the symbol in figure 9.

The regions 8E,8F adjacent to the liquid crystal regions 8A, 8B contain a non- switching, light-transmissivc material. The regions 8E, 8F may contain, for example, a light-transmissive polymer material such as the polymer SW-8. The regions of light- transmissive material remain light-transmissive when the liquid crystal regions 8A, 8B are switched into their private mode, and so correspond to the light regions in the "interfering pattern" generated by the privacy device. In the pattern of figure 7(a), for example, the white regions may correspond to a region of polymer material, whereas the black regions correspond to a region 8A, 8B of switchable liquid crystal material.

It is desirable that the transmissivity of the non-switching regions 8E, 8F is matched with the transmissivity of the liquid crystal regions in their public mode. This is to ensure that the privacy device has a substantially uniform transmissivity over its entire area in its public mode. Where the fixed regions 8E, 8F are formed of a light- transmissive polymer material, it may be necessary to add dye to the polymer material in order to match transmissivity of the polymer to the transmissivity of the liquid crystal regions in their public mode.

The fixed light-transmissive regions 8E, 8F and the switchable liquid crystal regions 8A, 8B may again be arranged to provide any desired interfering pattern when the privacy device is in its private mode.

It should be noted that the embodiment of figure 9 does not operate as a "louvre". The fixed regions 8E,8F and the switchable liquid crystal regions 8A,8B are again intended to provide a "confusing image" that degrades the image seen by a viewer at a wide angle. The size of the regions will be determined by the nature and font size of the image of the image that is to be obscured, as explained with reference to figure 6 above.

In practice the regions are likely to have dimensions of the order of a milihnetre square, although in principle they could be as small as a pixel of the display layer (typically 300U11 square) or as large as tens of millimetres.

The liquid crystal molecules in the liquid crystal regions 8A,8B could be tilted from the homeotropic alignment in their private mode, as explained with reference to figure 8 above, in order further to reduce the transmissivity at off-axis directions.

The fixed transparent regions 8E, 8F of figure 9 can be formed of any suitable light- transmissive, non-switching material. As one alternative to the use of' a light- transmissive polymer material, the fixed transmissive regions BE, 8F could be formed by in-situ polymerization of the guest host liquid- crystal layer 8. This can be done by, for example, selective irradiation of the liquid crystal layer using a mask having apertures corresponding to the desired positions of the fixed light-transmissive regions 8E, 8F.

Figure 10 illustrates a further embodiment of a privacy device of the present invention, in which the polariser 10 of figures 8 and 9 is not present. In the public mode, the guest-host liquid crystal layer 8 has a planar alignment, and operates as a polariser.

This is shown by region B of figure 10. In this region, the liquid crystal molecules are aligned along the y-direction and so absorb the ypolarisation component while transmitting the x-polarisation component. Region B in figure 10 therefore acts as a linear polariser having its absorption axis along the y-direction.

In the private mode, the liquid crystal layer 8 has a patterned talignment, as in the embodiment of figure 8. This is illustrated by the liquid crystal regions C and D of figure 10 (which correspond to the liquid crystal regions C and D of figure 8). Regions C and D act as a polariser (albeit a not particularly efficient polariser) when the display is viewed along the normal axis. (As explained above, the tilt of'the liquid crystal alignment should not be too great, since increasing the tilt ofthe alignment will increase the absorption in the normal direction.) Region C is absorbing for the y-polarisation component that is propagating in direction 01 to the normal direction, but is not significantly absorbing for the y- component propagating along a direction 02 to the normal direction. Conversely, region D is transmissive to the y-component propagating along a direction 01 to the normal direction, but is absorbing for the y- component propagating along the direction 02 to the normal direction. Thus, in the private mode, the liquid crystal layer acts as a patterned first polariser when seen from a wide viewing angle. Titus, when the privacy device is viewed from a wide viewing angle one of region C and region D will give a better hnage than when viewed from the normal direction, whereas the other of region C and region D will give a worse image (or possibly no image) since effectively only one polariser is present. In the private mode, the image seen by an observer viewing from an of'f:axis direction will be patterned into areas of good contrast and areas of poor contrast, owing to the tilt of the liquid crystal alignment. An observer viewing the device along angic 01 in the private mode will perceive a pattern of light areas (corresponding to region D) and dark areas (corresponding to region C); an observer observing the privacy device along direction 02 in the private mode will similarly see a pattern of light and dark areas. 'I'his is because, from an off-axis direction, the image seen is equivalent to having a patterned first polariser.

Figure ll(a) is a schematic illustration of a privacy device according to a further embodiment of the present invention. This corresponds generally to the embodiment of figures 3(a) and 3(b) except that the liquid crystal alignment is such that, in its private mode, the liquid crystal molecules are uniformly tilted with respect to the normal direction as shown in figure I l(a). 'I'he tilt direction of the liquid crystal molecules is substantially uniform over the area of the privacy device, in contrast to the embodiments of figures 8 or 10. This embodiment may be obtained by providing each substrate 12, 13 with a uniform alignment layer, with the two alignment layers arranged to produce a pre-tilt offset from the normal direction in opposite directions to one another.

The embodiment of figure ll(a) operates in generally the manner described with reference to figure 3(a) and 3(b) above. 1-lowevcr, because of the tilted liquid crystal alignment in the private mode, the direction of maximum visibility of a displayed image in the private mode is not the normal direction. The direction of maximum visibility is indicated by the direction OM in figure 1 1 (a).

Figure I I(b) shows the transmissivity of a privacy device of the type shown in figure I I(a) in which the liquid crystal molecules are aligned at 20 to the normal direction. It can be seen that in the private mode (trace b), the direction of maximum transmissivity is of iset from the normal direction by approximately 30 . The embodiment of figure I I(a) is therefore suitable for use in an application where a single viewer is intended to view the display from a direction other than normal incidence.

The embodhnent of figure I I(a), in which the intended viewing direction of a privacy device of the invention is not the normal direction, may be applied to the embodiments described above such as, for example the embodiments of figure 8, 9 or 10.

In embodiments in which a patterned liquid crystal alignment is required, this may be achieved in any suitable manner. For example, an alignment layer may be patterned using standard masking and rubbing techniques, the alignment layer may be micro- patterned using, for example, using standard UV sensitive photo alignment materials, or the electrode structure may be patterned to produce the required tilt within the bulk of the guest-host liquid crystal layer.

Claims

CLAIMS: 1. A display comprising an image display panel for displaying a

first image and a switchable privacy device disposed in an optical path through the image display panel, the privacy device comprising a guesl-host liquid crystal material and being switchable between a public mode and a private mode; wherein the privacy device, in its private mode, displays a second image in a direction different from a pre-determined direction whereby the hnage displayed by the display along the direction dif't'erent from the pre- determined direction is a superposition of the first hnage and the second image.
2. A display as claimed in claim I wherein the privacy device comprises a plurality of first regions, each first region being switchable between a light-transmissive state in which the region is transmissive for a predetermined polarization component and a light-absorbing state in which the region is absorbing Nor the pre-determined polarization component propagating in a direction different from the pre-determined direction.
3. A display as claimed in claim 2 wherein each first region is independently switehable between the light-transmissive state and the light-absorbing state.
4. A display as claimed in elahn 2 or 3 wherein the privacy device comprises an electrode patterned to define the first regions.
5. A display as claimed in claim 2, 3 or 4 wherein at least one first region of the privacy device comprises, in its light-absorbing state, a first sub-region having a first liquid crystal alignment and a second subregion having a second liquid crystal alignment dit't'erent Irom the first liquid crystal alignment.
6. A display as claimed in claim 5 wherein the first liquid crystal aligmnent is tilted t'rom an homeotropie alignment, and wherein the second liquid crystal alignment is tilted from an homeotropie alignment.
7. A display as claimed in claim 6 wherein the first and second liquid crystal alignments are tilted in different directions from the homeotropic alignment.
8. A display as claimed in claim 5, 6 or 7 wherein the directors of liquid crystal molecules in the first liquid crystal alignment and the directors of liquid crystal molecules in the second liquid crystal alignment lie in a common plane perpendicular to the directors of liquid crystal molecules in the first region when the first region is in its light-transmissive state
9. A display as claimed in any preceding claim wherein the privacy device further comprises a plurality of second regions, each second region being in a light- transmissive state when the privacy device is in its public mode.
10. A display as claimed in claim 9 wherein the transmissivity ol each second region is substantially constant between the public mode and the private mode of the privacy device.
11. A display as claimed in claim 10 wherein each second region is formed by selective polymerization of the guest-host liquid crystal material.
12. A display as claimed in any of claims I to 4 wherein the director of the liquid crystal material in each first region is tilted with respect to the normal direction of the display when the privacy device is in its private mode, the direction and angle of tilt of the directors being substantially the same in each first region.
13. A method of controlling the angular extent of an image displayed on an image display panel, the method comprising disposing a switchable privacy device comprising a guest-host liquid crystal material in an optical path through the image display panel, and switching the privacy device between a public mode and a private mode.
14. A method as claimed in claim 13 wherein the privacy device comprises a plurality of first regions, each first region being switchable between a light-transmissive state in which the region is transmissive for a predetermined polarization component and a light-absorbing state in which the region is absorbing for the pre-determined polarization component propagating in a direction different from a pre-determined direction, and wherein the method comprises switching one or more of the first regions thereby to generate, in the private mode, a pattern visible in a direction other than the intended viewing direction.
15. Use of a guest host liquid crystal panel as a controllable privacy device for a display.
16. A use as claimed in claim 15 wherein the controllable privacy device is switchable between a public mode and a private mode.
17. A use as claimed in claim 15 or 16 wherein the privacy device comprises a plurality of first regions, each first region being switchable between a light-transmissive state in which the region is transmissive for a pre-detennined polarization component and a light- absorbing state in which the region is absorbing for the pre-determined polarization component propagating in a direction different from a pre- determined direction.