GB2428128A

GB2428128A - A display having a holographic privacy device

Info

Publication number: GB2428128A
Application number: GB0513965A
Authority: GB
Inventors: Paul Antony Gass; Grant Bourhill
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2005-07-08
Filing date: 2005-07-08
Publication date: 2007-01-17
Also published as: GB0513965D0

Abstract

A display comprises an image display device (2) for displaying an image into at least a first viewing zone (zone B) and a second viewing zone (zone A, zone C) and a holographic optical element (1) which, when illuminated with light of a pre-determined wavelength, directs light into the second viewing zone (zone A, zone C) but directs substantially no light into the first viewing zone (zone B). Illuminating the holographic optical element (1) with light of the pre-determined wavelength thus provides a narrow display mode in which an observer in the second viewing zone cannot make out an image displayed on the display device (2). The transmission spectrum of the holographic optical element, for light propagating along at least one direction in the first viewing zone, is such that no minima occur in the spectrum at wavelengths at which the image displayed on the image display device has an intensity significantly different from zero. This can be arranged, for example, by suitable choice of the predetermined wavelength.

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. It particularly relates to a display having a holographic privacy device.

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.

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 polarisation state, and the machine and its user are surrounded by a large screen of sheet polariser which absorbs light of that polarisation state but transmits light of the orthogonal polarisation 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' film 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, 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 therefore 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'. However 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 film 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 from 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 travelling 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 diffuser in front of the light control film 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 film 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 switchable 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 of the 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. 5 825 436. The light control device in this patent is similar in structure to the louvred film described above. However, each opaque element in a conventional louvred film 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 cells with an appropriate shape. A second disadvantage is that, in 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 JP3607272 describes another display that is switchable between public and private display modes. This device uses an additional liquid crystal panel, which has a patterned liquid crystal alignment. Segments of the panel having different liquid crystal alignments modify the viewing characteristics of different areas of the display in different ways, with the result that the whole display panel is fully readable only from a central viewing position.

UK patent application No. 0320353.5 describes switchable privacy devices based on louvres, which operate only for one polarisation of light. The louvres are switched on and off either by rotating dyed liquid crystal molecules in the louvre itself, or by rotating the plane of polarisation of the incident light using a separate element.

UK patent application No. 0408742.5 describes a switchable privacy device constructed by adding one or more additional liquid crystal layers and polarisers to a display panel.

The intrinsic viewing angle dependence of these extra elements can be changed by switching the liquid crystal electrically in the well-known way.

UK patent application No. 0401062.5 describes a display having two different backlights which generate light with different angular ranges. The display can be switched between a public display mode and a private display mode by using the appropriate backlight.

UK patent application No. 0427303.3 discloses a display in which a polarisation modifying layer (PML) is placed behind the exit polariser of a liquid crystal display panel. Some parts of the PML are transparent. Other parts of the PML change the polarisation of light passing through them so that pixels viewed through these parts are inverted in colour (with bright pixels becoming dark and dark pixels becoming bright).

Data sent to pixels directly behind these parts is inverted so that when the display is viewed from a central position, the image appears normally. However, when the display is viewed from a non-central position, pixels that are supplied with non-inverted image data are viewed through the retarder elements of the PML, and the image is corrupted. Off-axis viewers see a confusing image which is a random dot pattern. The PML may be made from liquid crystal and switched off to give a public mode.

UK patent application No. 041227.0 describes a display which comprises a standard LC display and a guest host LC layer (containing a dye) with a patterned electrode. The guest host LC layer can be switched between an absorbing state and a non absorbing state. In the absorbing state, the absorption of the dye molecules is dependent upon the incident angle and polarisation of light - for a given polarisation and orientation, the absorption of the dye increases with larger viewing angles resulting in low brightness at high angles thereby giving a private display mode. Switching the guest host LC layer to the non-absorbing state gives a public display mode.

UK patent application No. 0318791.1 and European patent application No. 04270006.2 describe a further display switchable between a public viewing mode and a private viewing mode. Figure 1 is a schematic plan sectional view of such a display.

The display 5 comprises a holographic optical element I mounted in front of a liquid crystal display panel 2. The display panel 2 is illuminated by a backlight 3 that emits light with a wide angular range so that an image displayed on the display panel 2 may be seen from a central viewing zone (zone B) and also from one or more side viewing zones. Two side viewing zones (zone A, zone C) are shown in figure 1, located one of each side of the central zone B, but the display is not limited to this specific arrangement.. The full line arrows in figure 1 denote light emitted by the backlight 3.

The holographic optical element 1 is fabricated so that when it is illuminated with light of a particular wavelength from an additional light source 4 located in front of the holographic optical element I it diffracts this light strongly into the two side zones.

This wavelength will be referred to as the "operating wavelength" of the holographic optical element. The broken line arrows in figure 1 denote light emitted by the additional light source 4 and diffracted by the holographic optical element. The two side zones may for example extend, over the angular ranges (..5oo..3 o) or (+300,+500) horizontally and (25 ,+45 ) vertically. (The terms "horizontally" and "vertically" refer to the display as viewed by an observer in its normal orientation.) The diffracted light from the holographic element obscures the image from the display panel 2 when viewed from zone A or zone C, so that than an observer situate in either zone A or zone C will not be able to make out the image displayed on the panel 2, thereby providing a privacy function. No light is diffracted by the holographic element 1 into the angular range between the side zones. Therefore, a viewer located in zone B, on or near the normal axis of the display, sees an unaffected image as it is displayed on the display panel 2.

The "operating wavelength" of the holographic optical element used in UK patent application No. 0318791.1 is 530 nm.

If the additional light source 4 is switched off the holographic element has no effect, and an image displayed on the display panel 2 can be seen by observers in zone A, zone B or zone C, thereby providing a public display mode.

As illustrated in figure 1 the holographic element may be designed so that the additional illumination source 4 is located at an angle, horizontally and/or vertically, to the perpendicular axis of the hologram. This allows the additional illumination source 4 to be offset from the axis of the display 5, so that the additional illumination source does not obscure the main, on axis view seen by an observer located in zone B. Suitable illumination sources include light-emitting diodes (LED5) or laser diodes. The additional illumination source 4 may be a single LED or laser diode or may be a plurality of LEDs or laser diodes arranged together, for example in a strip.

A first aspect of the present invention provides a display comprising an image display device for displaying an image into at least first and second viewing zones; and a holographic optical element which, when illuminated with light of a pre-determined wavelength, directs light into the second viewing zone but directs substantially no light into the first viewing zone; wherein the transmission spectrum of the holographic optical element, for light propagating along at least one direction in at least one of the first arid second viewing zone, is such that no minima occur in the spectrum at wavelengths at which the image displayed on the image display device has an intensity significantly different from zero.

A transmission spectrum of a holographic optical element will in general contain local minima in the transmissivity. If one or more of these minima coincide with a peak wavelength in an image displayed on the image display device, the colour balance of an image is affected upon passing through the holographic optical element, so that an observer will not see a correctly coloured image. Arranging the holographic optical element such that any minima in the transmission of the holographic optical element occur at wavelengths at which the image displayed on the image display device has substantially no intensity ensures that an observer will see an image with correct colour balance.

In the case of a transmissive display that is illuminated by a backlight, the emission spectrum of the backlight defines the intensity distribution against wavelength in a displayed image. In the case of a transmissive display, therefore, the holographic optical element is such that any minima in the transmission of the holographic optical element occur at wavelengths at which the emission spectrum of the backlight has a zero or low intensity.

In the case of an emissive display, the emission spectrum of the display itself defines the intensity distribution against wavelength in a displayed image. In the case of an emissive display, therefore, the holographic optical element is such that any minima in the transmission of the holographic optical element occur at wavelengths at which the emission spectrum of the display has a zero or low intensity.

The pre-determined wavelength and/or the angle of incidence of light of the pre- determined wavelength on the holographic optical element may be such that minima in the transmission spectrum of the holographic optical element, for light propagating along the at least one direction in the first and second viewing zones, occur at wavelengths at which the image displayed on the image display device has substantially no intensity. The operating wavelength of the holographic optical element is one factor that determines the wavelengths at which minima occur in the holographic optical element transmission spectrum, and controlling the operating wavelength of the holographic optical element thus allows the wavelengths at which minima occur in the holographic optical element transmission spectrum to be controlled.

The pre-determined wavelength and/or the angle of incidence of light of the pre- determined wavelength on the holographic optical element may be such that minima in the transmission spectrum of the holographic optical element occur at wavelengths at which the image displayed on the image display device has substantially no intensity, for all viewing angles included in the first viewing zone.

The predetermined wavelength and\or the angle of incidence of light of the pre- determined wavelength on the holographic optical element may be such that the minima in the transmission spectrum of the holographic optical element occur at wavelengths at which the image displayed on the image display device has a substantially low intensity, for at least part of a viewing angle range included in the second viewing zone.

The pre-determined wavelength and/or the angle of incidence of light of the pre- determined wavelength on the holographic optical element may be such that at least one minimum in the transmission spectrum of the holographic optical element occurs at a wavelength outside the visible wavelength range. A minimum in the transmission spectrum of the holographic optical element that is outside the visible wavelength range will not affect the image seen by a human observer.

The holographic optical element may be disposed on the front side of the image display device.

The image display device may be a transmissive image display device. It may comprise a backlight for illuminating the image display layer.

Alternatively, the image display device may be an emissive image display device.

The display may comprise a first light source for illuminating the holographic optical element with light of the predetermined wavelength. The display may comprise a second light source for illuminating the holographic optical element with light of the predetermined wavelength, the holographic optical element having first and second gratings for diffracting light from the first and second light sources, respectively, the first and second light sources being disposed so that the behaviour of the transmission minima for the first and second gratings is similar as the direction from which the display is viewed changes. The display may comprise a second light source for illuminating the holographic optical element with light of the predetermined wavelength, the first and second light source is being disposed on opposite sides of a normal axis of the display. The first and second sources may be disposed in a third viewing zone and a second viewing zone, respectively. The holographic optical element may be arranged to detect light from the first and second light sources into the second and third viewing zones, respectively.

As an alternative, the light source may be laterally offset from the normal axis of the display, and the display may further comprise means for receiving light from the source of light and directing it onto the holographic optical element.

A second aspect of the present invention provides a display comprising an image display device for displaying an image into at least first and second viewing zones; a holographic optical element which, when illuminated with light of a pre-determined wavelength, directs light into the second viewing zone but directs substantially no light into the second viewing zone; and a means for receiving light from a source of light and directing it onto the holographic optical element.

This aspect of the invention addresses a second problem with the prior art display of figure 1, which is that a user's hand can partially or even completely block the path of light from the additional light source to the holographic optical element. If this happens, the intensity of light from the holographic optical element diffracted into the side zones is reduced, and it becomes easier for an observer in the side zone to make out an image. This is a particular disadvantage for applications that use a touch screen mounted on the front of a display, for instance Automatic Teller Machines (ATMs) and Electronic Point of Sale (EPOS) equipment, where it is necessary for a user's hand to touch the face of the display.

It might appear that this problem could be solved simply by placing the additional light source close to the plane of the holographic optical element (that is, reducing the axial separation dA between the holographic optical element and the light source), thus making it harder for a user's hand to block the light path from the additional light source to the holographic optical element. However, if this is done light from the additional light source is incident on the holographic optical element at a glancing angle, and a large amount of light is lost by reflection from the front face of the holographic optical element.

Providing the means for receiving light from the additional light source of light and directing it onto the holographic optical element makes it possible for the axial separation between the light source and the holographic optical element to be made small, while preventing a high reflection loss. This allows the additional light source to be positioned such that a user's hand cannot block the path of light from the additional light source to the holographic optical element.

The display may comprise a light source for emitting light of the predetermined wavelength, the light source being laterally offset from the axis of the display.

The means may comprise a lightguide.

The light source may be positioned adjacent to a side edge face of the lightguide.

The holographic optical element may be positioned adjacent to a front face or a rear face of the lightguide.

The refractive index of the lightguide may be substantially equal to the refractive index of the holographic optical element.

The means may comprise a plurality of light-directing surfaces for directing light onto the holographic optical element.

The light source may be placed outside the first viewing angle range. It may be placed outside both the first viewing angle range and the second viewing angle range.

The means may direct light onto substantially the entire area of the holographic optical element.

Preferred embodiments of the present invention will now be described by way of illustrative example with reference to the accompany figures in which: Figure 1 illustrates a prior art display having a holographic privacy device; Figure 2 illustrates the performance of a holographic element designed according to the

prior art;

Figure 3 illustrates the performance of a display according to an embodiment of the present invention; Figure 4 illustrates a display having a holographic privacy device according to another embodiment of the present invention; Figure 5 illustrates a display having a holographic privacy device according to a further embodiment of the present invention; Figure 6 illustrates the performance of a display according to a further embodiment of the present invention; and.

Figure 7 illustrates a display having a holographic privacy device according to a further embodiment of the invention.

A disadvantage of the holographic privacy technique described by the prior art is illustrated in figure 2 which shows the intensity spectrum for a cold cathode fluorescent (CCFL) backlight as typically used to illuminate liquid crystal displays. Superposed on figure 2 is the transmission spectrum of a holographic optical element fabricated to give strong diffraction when illuminated from the front by light of wavelength 530nm as used in the display of UK patent application No. 0318791.1. This transmission spectrum is measured in the direction perpendicular the hologram, illuminated from the side of the hologram on which the display panel 2 is disposed, and so represents the transmissivity of the hologram (along the normal axis of the display) for an image displayed on the display panel 2.

Firstly, it can be seen that the transmission spectrum of the hologram shows a generally continuous reduction in transmitted intensity moving from long wavelengths to short wavelengths. This is due to Raleigh scattering from the silver particles in the hologram, and this effect can be minimised using known holographic techniques.

Secondly, it can be seen that the transmission spectrum of the hologram shows two minima in transmission within the wavelength range of 400-700nm. One minimum in transmission is at a wavelength of around 470nm, and the other is at a wavelength of around 6IOnm (these minima will be referred to as the "lower minimum" and "upper minimum" respectively, for convenience). Unfortunately, these minima are co-incident with wavelengths at which the light from the backlight has non-zero intensity. In particular, the upper minimum in transmission of the holographic optical element (at a wavelength of around 61 Onm) is almost exactly coincident with the red wavelength peak of the spectrum of the CCFL backlight. The CCFL backlight also has a broad peak at blue wavelengths of 450-SOOnm, and the lower minimum in transmission of the holographic optical element (at a wavelength of around 450nm) is coincident with this.

The transmission spectrum of the hologram does not, however, have a minimum at or near a wavelength of 550nm, where the spectrum of the CCFL backlight has a peak (corresponding to green light). As a consequence, the colour balance of an image displayed on the display panel 2 of the display of figure 1 will alter as a result of transmission through the hologram the intensity of the image at red or blue wavelengths will be reduced by a greater factor than will the intensity of the image at green wavelengths. Thus, an observer viewing the display along the normal axis will see an image having a predominantly green appearance.

The wavelengths at which the minima in the transmission spectrum of the holographic optical element occur are to some extent dependent on viewingangle, but minima will exist in the transmission spectrum of the holographic optical element for all viewing angles near the normal direction. Thus, an observer located anywhere in zone B will see an image with incorrect colour balance in the narrow display mode. (Indeed, the colour balance of an image seen by an observer in zone B might change as the observer moved their head from side to side within zone B.) According to the present invention, therefore, the holographic optical element 1 of the display 5 is chosen such that the minima in the transmission spectrum of the hologram, for light propagating along at least one direction in viewing zone B, occur at wavelengths at which a displayed image has little or no intensity. This may be obtained by appropriate choice of the operating wavelength of the holographic optical element.

This is illustrated with respect to figure 3.

In many cases it is preferable for the at least one direction to be the normal axis of the display, particularly if viewing zone B is symmetric about the axis of the display.

Moreover, it is preferable if the holographic optical element is such that minima in the transmission spectrum occur at wavelengths at which a displayed image has little or no intensity, for all viewing directions in zone B and preferably for all or most viewing directions in zones A and C. Figure 3 again shows the intensity spectrum for a cold cathode fluorescent (CCFL) backlight as typically used in liquid crystal displays. Superposed on figure 3 is the transmission spectrum of a holographic optical element fabricated to give strong diffraction when illuminated from the front by light of wavelength 645nm. The transmission spectrum of the holographic optical element is again measured in the direction perpendicular the hologram, illuminated from the side of the hologram on which the display panel 2 is disposed, and so represents the transmissivity of the hologram for an image displayed on the display panel 2.

The holographic optical element having the transmission spectrum shown in figure 3 was fabricated from bleached silver halide so that it strongly diffracted light of wavelength 645nm into two side views, extending over the angle ranges (5O0,300) and (+300,+500) horizontally and (-25 ,+45 ) vertically. This hologram was mounted in front of an image display panel, to produce a display similar to the display 5 shown in figure 1. The additional light source 4 comprised one or more LEDs emitting in the red region of the spectrum. When the LEDs were illuminated, the holographic optical element diffracted red light from the LEDs into zone A and zone C, and the high- intensity red diffracted light obscured an image displayed on the image display device 2 from viewers located in zone A or zone C. A viewer in zone B was however able to perceive the displayed image, since the holographic optical element diffracted little or no red light form the LEDs into zone B. Thus, a narrow view mode was obtained.

When the LEDs were OFF, the displayed image could be seen in any of zone A, zone B or zone C, giving a wide display mode.

The wavelengths at which the minima in the transmission of the holographic optical element, at normal or near-normal incidence, occur are related to the operating wavelength of the holographic optical element. In figure 2, the operating wavelength of the holographic optical element is 530nm, and the minima in the transmissivity of the holographic optical element are at wavelengths of approximately 470nm and 6lOnm - that is, one minimum is at a wavelength below the operating wavelength, and the other is at a wavelength greater than the operating wavelength. (The wavelengths at which the minima occur are also dependent on factors such as the angle of the incident light from the additional source, orientation of the diffraction grating with respect to the layer of the holographic optical element and refractive index of the holographic optical element.) A change in the operating wavelength of the holographic optical element therefore causes a change in the wavelength in which the minima occur in the transmission spectrum of the holographic optical element for normal or near-normal incidence. This can clearly be seen in figure 3 - as a result of the change in operating wavelength of the holographic optical element from 530nm to 645nm, the wavelength at which the "upper" minimum occurs has been increased to significantly above 645nm, so that the upper minimum is not visible in figure 3. The upper minimum does not now occur at a wavelength in the visible spectrum but instead occurs in the infra-red part of the spectrum (i.e., it occurs at a wavelength that is greater than 700nm). As a result, the upper minimum has no effect on the image seen by a person observing an image in the visible spectrum.

The wavelength at which the "lower minimum" occurs has also been increased as a result of increasing the operating wavelength of the holographic optical element. The lower minimum in the transmission spectrum of figure 3 is now at a wavelength of approximately 570nm. While this wavelength is still in the visible spectrum, it can be seen that a wavelength of 570nm does not coincide with either the 550nm peak or the 610 nm peak in the spectrum of the CCFL backlight. The lower minimum in the transmission spectrum of the holographic optical element does not therefore affect the colour balance of the image seen by an observer.

A similar result could have been obtained by reducing the operating wavelength of the holographic optical element compared with the holographic optical element of figure 2.

In an holographic optical element with an operating wavelength in the blue region of the spectrum, for example in the 400-450nm wavelength range, the lower minimum in the normal incidence transmission spectrum of the holographic optical element would occur at a wavelength in the ultraviolet region of the spectrum (i.e., at a wavelength below 400nm). Provided that the operating wavelength was selected such that the upper minimum occurred at a wavelength at which the spectrum of the CCFL backlight had a low intensity (for example, at a wavelength of 570nm or 520nm), the holographic optical element would not affect the colour balance of an image seen by an observer.

Figure 6 illustrates a further embodiment of the invention. Figure 6 shows the intensity spectrum for a backlight comprising a combination of coloured light-emitting diodes (LEDs) that provide a white backlight. The spectrum of this backlight has three peaks, corresponding to LEDs that emit in the red, green and blue regions of the spectrum respectively. The intensity of the backlight is substantially zero for wavelengths between adjacent peaks.

Figure 6 also shows the transmission spectrum at normal incidence of two holographic optical elements. One holographic optical element corresponds to the holographic optical element of figure 2, and exhibits minima in transmission at wavelengths of approximately 470nm and 6lOnm - this is shown by trace A in figure 6. The upper minimum at 61 Onm coincides with one of the peaks of the spectrum of the LED backlight (corresponding to red light), so that this holographic optical element will alter the colour balance of a displayed image.

Trace B (which has been vertically separated from Trace A, for clarity) shows the transmission spectrum of an holographic optical element with an operating wavelength selected according to the invention, such that the minima in the transmission spectrum occur at wavelengths between the narrow peaks in the spectrum of the LED backlight (or occur at wavelengths outside the visible region of the spectrum). Therefore this holographic optical element does not alter the colour balance of an image, and the appearance of a displayed image is unaffected.

A further embodiment of the present invention is shown in figure 4. In this embodiment, the additional light source 4 for operating the holographic optical element is not mounted in front of the holographic optical element as in figure 1, but is mounted at the side of the holographic optical element. Means are provided for receiving light from the offset additional light source 4 and directing the light onto the holographic optical element. Providing the means for receiving light from the additional light source 4 and directing it onto the holographic optical element allows the additional light source to have a low axial separation dA from the holographic optical element, and to be offset from the axis of the display and even allows the additional light source to be placed outside the active display area of the display in its wide mode (i. e., outside any of zone A, zone B and zone C of figure 1). Alternatively, the additional light source could be offset so that it is outside zone B but is within zone A or zone C. This overcomes the problem with the display of figure 1 of a user's hand coming between the additional light source and part of the holographic optical element, blocking the illumination of the hologram and reducing the effectiveness of the privacy in the narrow display mode.

In the embodiment of figure 4, the means for receiving light from the additional light source 4 and directing it onto the holographic optical element is a lightguide 6. The additional light source 4 is located adjacent to one side edge face of the lightguide, and the holographic optical element is placed adjacent to the upper or lower face of the lightguide 6. Figure 4 shows the lightguide 6 located above the holographic optical element, with the holographic optical element being directly adjacent to the lightguide 6. The lightguide 6 and the holographic optical element preferably have the same refractive index as one another, or have refractive indices that are very close to one another. Thus, light propagating in the lightguide 6 can pass into the holographic optical element, without there being significant reflection at the interface between the holographic optical element and the lightguide 6. The light from the additional light source that enters the holographic optical element is then diffracted, according to the normal operation of the holographic optical element.

The holographic optical element may alternatively be placed in front of the lightguide 6.

In this case, light propagating within the lightguide would undergo internal reflection at the lower face of the lightguide, which would be in contact with air or another material with a lower refractive index than the lightguide. Internal reflection would not however occur at the interface between the upper surface of the lightguide 6 and the holographic optical element, so that light passes into the holographic optical element and is then diffracted according to the normal operation of the holographic optical element.

The lightguide 6 may be formed by a lightguide film mounted on a face of the holographic optical element. Examples of material which are suitable for such films are PMMA and polycarbonate.

Where a display of the invention is used in an ATM or EPOS equipment, the lightguide 6 can be mounted behind the touch screen of the ATM or EPOS terminal. If the lightguide is mounted next to the hologram, as shown in figure 4, it is impossible for a user's hands to block the path of light from the additional light source 4 to the holographic optical element 1. The privacy effect of the light diffracted by the holographic optical element is therefore preserved, by avoiding shadowing of the holographic optical element by a user.

The use of the lightguide 6 also enables the additional light source 4 to be positioned at a very low axial separation from the holographic optical element without there being excessive reflection loss.

The additional light source 4 may be located outside the active area of the image display device 2, as shown in figure 4. The additional light source can therefore be arranged not to block part of an image displayed on the image display device 2, even when the display is in the wide display mode Because the lightguide is mounted next to the hologram it is impossible for a user's hands to block the hologram. Therefore the privacy effect of the obscuring image is preserved by avoiding shadowing.

According to a further embodiment of the present invention, the holographic optical element, the lightguide 6 and a touch screen may be integrated into a single component.

This component has combined touch screen and privacy functions and the lightguide 6 may acct as a support for, for example a conventional touch screen film, thus reducing the thickness of the arrangement..

In another embodiment of the invention the lightguide film is provided with optical elements, for example a prismatic array, to control the uniformity and angular range of the light illuminating the holographic optical element. An example of such an embodiment of the invention is shown in figure 5. In this embodiment, the means for receiving light from the additional light source 4 and directing it onto the holographic optical element comprise a plurality of light-directing surfaces 8a,8b for directing light onto the holographic optical element. The light directing surfaces are inclined with respect to the path of light from the additional light source, so that light incident on the light- directing surfaces is reflected or refracted towards the holographic optical element.

In the embodiment of figure 5, the light directing surfaces are the surfaces of prisms 9 formed of a material having a refractive index greater than 1, such as PMMA having a refractive index of approximately 1. 5, so that some light-directing surfaces 8a are included in one direction and other light-directing surfaces 8b are inclined in another direction. Light enters a prism 9 through a surface 8a, and then undergoes total internal reflection at the interface between the prism and air at a surface 8b so that the light propagation direction is turned by approximately 90 ., The light-directing surfaces 8a,8b are again used to direct the light from the additional light source 4 onto the holographic optical element 1 with reduced reflection loss. This allows the additional light source 4 (which may again be one or more LEDs) to be mounted with a low axial separation from the holographic optical element.

The light-directing surfaces may be provided by a commercially available optical turning film 7. The optical turning film 7 may be, for example, TRAF film as supplied by 3M.

The embodiment shown in Figure 1 uses an additional light source 4 located at a single position to illuminate the hologram. Therefore two substantially different gratings must be formed in the holographic optical element 1, a first grating diffracting light from the additional source into zone A and the second grating diffracting light into zone C. If, as shown in Figure 1, the additional light source 4 is located within zone A, then the first grating acts to diffract light approximately back in the direction of the additional light source. Therefore the planes for this grating will lie substantially perpendicular to the direction of incidence for the light from the additional source, taking into account refraction at the air-hologram interface. The second grating acts to diffract light to an approximately equal angle in zone C. Therefore the planes for this grating will lie substantially parallel to the layer of the holographic optical element.

The two diffraction gratings described above have different transmission spectra for light propagating from the display panel into zone B. For example light propagating perpendicular to the holographic optical element will have two different transmission minima as shown bellow. The transmission minimum for the first grating will occur at wavelength XmInl = Xo cos OG where Xo is the "operating wavelength" of the holographic optical element and e0 is the angle between the grating planes and the layer of the holographic optical element. The transmission minimum for the second grating will occur at wavelength Xmin2 = Xo/cos lnt where nt is the angle of incidence, after refraction at the air/hologram interface, of the light from the additional source. For the first grating, the wavelength of the transmission minimum for light propagating perpendicular to the holographic optical element will be less than the "operating wavelength". For the second grating, the wavelength of the transmission minimum will be greater than the "operating wavelength".

Due to the different behaviour of the transmission minima for the two gratings required to diffract light from a single additional light source, it may be difficult to select an "operating wavelength" wherein the transmission spectrum of the holographic optical element is such that no minima occur in the spectrum at wavelengths at which the image displayed on the image display device has an intensity significantly different from zero.

However, according to a further embodiment of this invention, the HOE is illuminated by two or more additional light sources 4, 10. The additional light sources are located in both zones A and C, so that only one type of grating is required. For example Figure 7 shows how a grating lieing substantially parallel to the layer of the holographic optical element can be used to diffract light from an additional source 4 in zone A into zone C and from an additional source 10 in zone C into zone A. For a single type of grating, all the transmission minima of the HOE can be arranged to be either greater than or less than the "operating wavelength". For example, the transmission minima can all be outside the visible wavelength range and so will not affect the image seen by a human observer.

Preferably the holograms described in this invention are of a volume type. Suitable materials for making the hologram include bleached silver halide, dichromated gelatine or photopolymer. Alternatively, the holographic optical element may be a surface reflection hologram arranged to work with only a fixed range of operating wavelengths.

Such a surface relief hologram is disclosed in "Aztec surface-relief volume diffraction structure", Journal of the Optical Society of America A, vol. 7, No. 8, 1990.

The invention has been described with reference to a display in which the image display panel 2 is a liquid crystal display panel. The invention is not, however, limited to displays having a liquid crystal display panel but can be applied to displays having other types of image display panels. Indeed, the invention is not limited to a display having a transmissive image display panel, but may be applied to a display having an emissive image display pane. As examples, the invention can be applied to display in which the image display panel is an organic light-emitting device (OLED) image display panel, a cathode ray tube (CRT) image display panel, a plasma display panel (PDP) image display panel or a field emission display (FED) image display panel.

In the embodiments of figures 4 and 5, the holographic optical element is preferably chosen such that minima in the transmission spectrum of the holographic optical element, for light propagating along at least one direction in the first viewing zone, occur at wavelengths at which the image displayed on the image display device has substantially no intensity. However, the embodiments of figures 4 and 5 may in principle be applied with any holographic optical element.

Claims

CLAIMS: I. A display comprising an image display device for displaying an

image into at least first and second viewing zones; and a holographic optical element which, when illuminated with light of a pre-determined wavelength, directs light into the second viewing zone but directs substantially no light into the first viewing zone; wherein the transmission spectrum of the holographic optical element, for light propagating along at least one direction in at least one of the first and second viewing zones, is such that no minima occur in the spectrum at wavelengths at which the image displayed on the image display device has an intensity significantly different from zero.
2. A display as claimed in claim I wherein the pre-determined wavelength and//or the angle of incidence of light of the pre-determined wavelength on the holographic optical element is such that minima in the transmission spectrum of the holographic optical element, for light propagating along the at least one direction, occur at wavelengths at which the image displayed on the image display device has substantially no intensity.
3. A display as claimed in claim 2 wherein the pre-determined wavelength and/or the angle of incidence of light of the pre-determined wavelength on the holographic optical element is such that minima in the transmission spectrum of the holographic optical element occur at wavelengths at which the image displayed on the image display device has substantially no intensity, for all viewing angles included in the first viewing zone.
4. A display as claimed in claim 2 or 3 wherein the predetermined wavelength and/or the angle of incidence of light of the pre-determined wavelength on the holographic optical element is such that the minima in the transmission spectrum of the holographic optical element occur at wavelengths at which the image displayed on the image display device has substantially no intensity, for at least part of a viewing angle range included in the second viewing zone.
5. A display as claimed in any of claims 2 to 4 wherein the predetermined wavelength and/or the angle of incidence of light of the predetermined wavelength on the holographic optical element is such that at least one minimum in the transmission spectrum of the holographic optical element occurs at a wavelength outside the visible wavelength range.
6. A display as claimed in any preceding claim wherein the holographic optical element is disposed on the front side of the image display device.
7. A display as claimed in any preceding claim wherein the image display device is a transmissive image display device.
8. A display as claimed in claim 7 and further comprising a backlight for illuminating the image display layer.
9. A display as claimed in any one of claims 1 to 6 wherein the image display device is an emissive image display device.
10. A display as claimed in any preceding claim and comprising a first light source for illuminating the holographic optical element with light of the predetermined wavelength.
11. A display as claimed in claim 10 and comprising a second light source for illuminating the holographic optical element with light of the predetermined wavelength, the holographic optical element having first and second gratings for diffracting light from the first and second light sources, respectively, the first and second light sources being disposed so that the behaviour of the transmission minima for the first and second gratings is similar as the direction from which the display is viewed changes.
12. A display as claimed in claim 10 and comprising a second light source for illuminating the holographic optical element with light of the predetermined wavelength, the first and second light sources being disposed on opposite sides of a normal axis of the display.
13. A display as claimed in claim 12, wherein the first and second light sources are disposed in a third viewing zone and a second viewing zone, respectively.
14. A display as claimed in claim 13 wherein the holographic optical element is arranged to direct light from the first and second light sources into the second and third viewing zones, respectively.
15. A display as claimed in claim 10 wherein the light source is laterally offset from the normal axis of the display, and wherein the display further comprises means for receiving light from the source of light and directing it onto the holographic optical element.
16. A display comprising an image display device for displaying an image into at least first and second viewing zones; a holographic optical element which, when illuminated with light of a pre-determined wavelength, directs light into the second viewing zone but directs substantially no light into the second viewing zone; and a means for receiving light from a source of light and directing it onto the holographic optical element.
17. A display as claimed in claim 16 and further comprising a light source for emitting light of the predetermined wavelength, the light source being laterally offset from the axis of the display.
18. A display as claimed in claim 15, 16 or 17 wherein the means comprises a lightguide.
19. A display as claimed in claim 18 wherein the light source is positioned adjacent to a side edge face of the lightguide.
20. A display as claimed in claim 18 or 19 wherein the holographic optical element is positioned adjacent to a front face or a rear face of the lightguide.
21. A display as claimed in claim 20 wherein the refractive index of the Iightguide is substantially equal to the refractive index of the holographic optical element.
22. A display as claimed in claim 15, 16 or 17 wherein the means comprise a plurality of light-directing surfaces for directing light onto the holographic optical element.
23. A display as claimed in claim 15, claim 17, or in any of claims 18 to 22 when dependent directly or indirectly from claim 15 or 17, wherein the light source is placed outside the first viewing angle range.
24. A display as claimed in claim 23 wherein the light source is placed outside both the first viewing angle range and the second viewing angle range.
25. A display as claimed in any of claims 15 to 22 wherein the means directs light onto substantially the entire area of the holographic optical element.