CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2018/058555, filed on Apr. 4, 2018, which claims the benefit of EP Patent Application No. EP 17165020.3, filed on Apr. 5, 2017. These applications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to a multi-view display device of the type that comprises a display panel having an array of display pixels for producing a display, and a lenticular lens arrangement arranged over the display panel through which the display pixels are viewed.

BACKGROUND OF THE INVENTION

There are two basic types of multi-view display device. One type is for simultaneously presenting different images to different viewers at different spatial locations. For example, one viewer can be in the driver's seat of a vehicle and another can be in the passenger's seat. The driver can be displayed driver-related content such as satellite navigation content, and the passenger can be displayed entertainment content. There may be more than two views, i.e. for multiple viewers.

Another type is autostereoscopic displays for displaying the views for different eyes to different spatial locations. There may be two views, e.g. for a single viewer at a single position, but there may be many more views (e.g. 9 or 15) so that multiple viewers can be in the field of view and/or so that a viewer can move relative to the display to experience a look around effect.

The principles behind the generation and spatial separation of the different views are the same in these devices. Essentially, different two-dimensional content is projected to different spatial locations simultaneously. The only difference is that the angular separation of views is typically less for the autostereoscopic display (approximately 2 degrees) than for multi-view displays, where both eyes of a viewer receive the same image. Typically, the different spatial locations extend along a horizontal line, since viewers' eyes are generally at the same vertical height, but at different horizontal positions with respect to the display. For the purposes of explanation, the following will be with reference to autostereoscopic display devices.

A known autostereoscopic display device comprises a two-dimensional liquid crystal display panel having a row and column array of display pixels acting as a spatial light modulator to produce the display. The display pixels are formed of sub-pixels, for example of different colours. Each sub-pixel is the smallest individually addressable display element. Thus in a monochromatic display above definition of sub-pixel coincides with the term pixel. Therefore in the following only the term sub-pixel is used but it is to be understood that in case of a chromatic display such as e.g. a black and white display or the well-known “green screen”, the commonly used term pixel falls under the term sub-pixel used in the present patent application. An array of elongate lenticular elements extending parallel to one another lies over the sub-pixel array, and the sub-pixels are observed through these lenticular elements. The lenticular elements are provided as a sheet of elements, each of which comprises an elongate semi-cylindrical lens element. The lenticular elements extend in the column direction of the display panel, with each lenticular element overlying a respective group of two or more adjacent columns of sub-pixels.

In an arrangement in which, for example, each lenticular element is associated with two columns of sub-pixels, the sub-pixels in each column provide a vertical slice of a respective two-dimensional sub-image. The lenticular sheet directs these two slices and corresponding slices from the sub-pixel columns associated with the other lenticular elements, to the left and right eyes of a user positioned in front of the sheet, so that the user observes a single stereoscopic image. The sheet of lenticular elements thus provides a light output directing function.

In other arrangements, each lenticular element is associated with a group of, say, four or more adjacent sub-pixels in the row direction. Corresponding columns of sub-pixels in each group are arranged appropriately to provide a vertical slice from a respective two-dimensional sub-image. As a user's head is moved from left to right a series of successive, different, stereoscopic views are perceived creating, for example, a look-around impression.

The above-described device provides an effective three-dimensional display. However, it will be appreciated that, in order to provide stereoscopic views, there is a necessary sacrifice in the horizontal resolution of the device (the same applies to the resolution of the different images in a multi-view two-dimensional system). This sacrifice in resolution is unacceptable for certain applications, such as the display of small text characters for viewing from short distances or graphics applications needing a high resolution. For this reason, it has been proposed to provide an autostereoscopic display device that is switchable between a two-dimensional mode and a three-dimensional (3D stereoscopic) mode. Such a device is described in U.S. Pat. No. 6,069,650, to which reference is invited. In this device, different groups of sub-pixels, forming one or more stereoscopic pairs, are seen by respective eyes of a viewer through the lenticular elements. The lenticular elements include electro-optic material having a refractive index that is switchable in order to enable removal of the refracting effect of the lenticular elements.

In the two-dimensional mode, the lenticular elements of the switchable device operate in a “pass through” mode, i.e. they act in much the same way as would a flat sheet of optically transparent material. The resulting display has a high resolution, equal to the native resolution of the display panel, which is suitable for the display of small text characters from short viewing distances. The two-dimensional display mode cannot, of course, provide a stereoscopic image.

In the three-dimensional mode, the lenticular elements of the switchable device provide a light output directing function, as described above. The resulting display is capable of providing stereoscopic images, but also suffers the inevitable resolution loss mentioned above.

In order to provide switchable display modes, the lenticular elements of the switchable device use an electro-optic material, such as a liquid crystal material, having a refractive index that is switchable between two different values for polarized light. The device is then switched between the modes by applying an appropriate electrical potential to electrode layers provided above and below the lenticular elements. The electrical potential alters the refractive index of the lenticular elements in relation to that of an adjacent optically transparent layer. Alternatively, the adjacent optically transparent layer may be formed of the electro-optic material, with the same result that the refractive index of the lenticular elements in relation to the optically transparent layer is altered.

Because of its static dielectric anisotropy, the orientation of the liquid crystal material can be controlled through an applied electric field. In the optical regime, there is also dielectric anisotropy, and the index of refraction of the liquid crystal material is related to the relative dielectric constant. The liquid crystal material has an ordinary and an extra-ordinary index of refraction, the former being applicable for light with electric field polarization perpendicular to the director and the latter being applicable for light with electric field polarization parallel to the director.

The applied electrical potential causes the lenticular elements in the selected portion of the display area to switch between maintaining and removing a light output directing function, which will now be explained with reference toFIGS. 1 and 2.

FIG. 1schematically depicts a cross-sectional view of a portion of the known lenticular element arrangement9when no electric potential is applied to the electrodes17,19. The lenticular element arrangement9is positioned over a display panel3comprising an array of sub-pixels5arranged in rows and columns. The electrode17is sandwiched between a glass plate13and a body, or replica,21. The other electrode19is sandwiched between a further glass plate15and an orientation layer26. Here, the rubbing directions of the orientation layers25and26and the polarization of the display light are in the z-direction (the direction of the lens axes); in this case extending into the plane of drawing ofFIG. 1. As a result, the effective lens, although being optically birefringent, can be approximated as an isotropic lens with an index of refraction corresponding to the extra-ordinary index of refraction of the liquid crystal material23. In this state, the refractive index (that is the extra-ordinary refractive index) of the liquid crystal material23for linearly polarized light provided by the display panel3is substantially higher than that of the body21, and the lenticular shapes therefore provide the light output directing function, as illustrated.

FIG. 2schematically depicts a cross-sectional view of a portion of the known lenticular element arrangement9when an alternating electric potential of approximately 50 volts is applied to the electrodes17,19. An electric field is created in the y-direction and the liquid crystal molecules align with the field lines. As a result, the director of the liquid crystal material23is also substantially in the y-direction. The polarization direction of light from the display panel3is still linearly polarized, that is, the E field of the light is in the z-direction. With the polarization of the light of the display in the z-direction, the effective lens will have the ordinary index of refraction, and light will not be refracted since there is an index match between the liquid crystal material23and the lenticular body21. In this state, therefore, the refractive index of the liquid crystal material23for light of the linear polarization provided by the display panel3is substantially the same as that of the inverse lenticular structure of the body21, so that the light output directing function of the lenticular shapes is cancelled, as illustrated. Thus, the array effectively acts in a “pass through” mode.

With the light output directing function maintained, as shown inFIG. 1, the lenticular lens elements11defined by the liquid crystal material23act as convex cylindrical lenses, and provide different images, or views, from the display panel3to the eyes of a user positioned in front of the lenticular element arrangement9. Thus, a three-dimensional image can be provided.

With the light output directing function removed, as shown inFIG. 2, the lenticular lens elements11defined by the liquid crystal material23act as if they were a flat sheet of transparent material that acts as a pass through layer with no view directing function. Thus, a high-resolution two-dimensional image can be provided employing the full native resolution of the display panel3.

The control of electrical potentials to switch between display modes is provided to the electrodes17,19of the lenticular element arrangement9by a controller12.

Further details of the structure of a known switchable autostereoscopic display device can be found in U.S. Pat. No. 6,069,650, to which reference is invited.

There are, however, disadvantages associated with such known switchable display devices. In particular, unwanted display artefacts in the two-dimensional mode may be observed when the display device is viewed at oblique angles.

More generally, liquid crystal cells tend to perform better when they are viewed at normal incidence. For instance, in liquid crystal displays compensation foils are used between the second polarizer and the liquid crystal cell in order to compensate for deviations from perpendicular incidence and the associated light leakage/bleeding effects. Similar problems are associated with the use of a switchable lenticular lens, albeit that the optical function is different. In particular, different refractive indices are perceived at oblique angles compared to when the display device is viewed at normal incidence. This may result in different strength or focusing power of the lenticular lens when viewed at oblique angles, leading to ‘residue’ being observed at oblique angles in two-dimensional mode. This effect may also be detrimental to the performance of the display in three-dimensional mode.

A further disadvantage of such known switchable display device is that it relies on polarized light being used. With non-polarized light, two-dimensional and three-dimensional modes cannot be separated. Therefore certain display types which, for instance, do not emit polarized light, such as OLED displays, can only be used in combination with a conventional switchable lenticular if an additional polarizer is used. The necessity to employ an additional polarizer results in significant light losses.

US 2016/202493 discloses a liquid crystal lens for a display panel which makes use of a cholesteric helical liquid crystal material. US 2014/0118646 discloses a liquid crystal lens panel which also makes use of a cholesteric liquid crystal material.

SUMMARY OF THE INVENTION

There is a need for a multi-view display device that addresses the aforementioned problems.

The invention is defined by the claims.

In accordance with an aspect, there is provided a multi-view display device which is switchable between a single view and a multiple view mode, the display device comprising a display panel having rows and columns of sub-pixels configured to produce a display output, and a lenticular lens arrangement arranged over the display panel on the display output side of the display panel, the lenticular lens arrangement comprising an array of lenticular lens elements which comprise an electro-optic material having a lenticular shape adjacent a non-switchable optically transparent layer having an inverse lenticular shape,

wherein the electro-optic material comprises a cholesteric liquid crystal component having an effective refractive index which is controllable by application of an electrical stimulus, the cholesteric liquid crystal component having an ordinary refractive index and an extra-ordinary refractive index which differs from the ordinary refractive index, and comprising a helical structure defined by a pitch, wherein the pitch is selected such that a product of the pitch and said difference is equal to or less than a visible light wavelength of the display output.

The present invention is based on the realization that the polarization-related disadvantages of conventional switchable display devices can be addressed by using an electro-optic material comprising a cholesteric liquid crystal component having a helical pitch which is selected such that a product of the pitch and the difference between the values for the ordinary and extra-ordinary refractive indices is equal to or less than a visible light wavelength of the display output. Such a cholesteric liquid crystal component exhibits an effective refractive index for such a visible wavelength which is independent of the polarization direction. Accordingly, such a material has an isotropic effective refractive index which reflects the fact that the pitch is sufficiently short that the chiral structure of the cholesteric liquid crystal component is not recognized by incident light having such a visible wavelength.

The effective refractive index is controllable by application of an electrical stimulus, e.g. an applied electric field. In this way the lenticular lens element may be switched from a first state in which the effective refractive index of the electro-optic material is controlled to be substantially equal to a further refractive index of the non-switchable optically transparent layer, to a second state in which the effective refractive index differs from the refractive index of the non-switchable optically transparent layer, and vice versa. In the second state, the lenticular lens element performs a light output directing function. In the first state, the light output directing function is removed. The first and second states respectively correspond to the single view and a multiple view modes of the display device.

The pitch may be in the range from 0.1 to 10 μm. A pitch in this range may assist the cholesteric liquid crystal component to satisfy the condition that a product of the pitch and the difference between the extra-ordinary and ordinary refractive indices is equal to or less than a visible light wavelength of the display output.

The cholesteric liquid crystal component may comprise a cross-linked polymer. The cross-linked polymer may assist to stabilize a helical structure of the cholesteric liquid crystal component. This stabilization may, for instance, be realized by tuning the cross-linked polymer content in the cholesteric liquid crystal component. The stiffness or mobility of the cross-linked polymer may, for instance, be controlled by the degree of cross-linking of the polymer used.

The cholesteric liquid crystal component may comprise a helical structure defined by a helical axis. The helical axis may extend along an axis of a plane lying co-planar with respect to the display panel. Alternatively, the helical axis may extend normal to the display panel such that the cholesteric liquid crystal component reflects a portion of the display output with the same handedness as the helical structure back towards the display panel, wherein the pitch of the helical structure is selected such that the portion has a wavelength in the UV region of the spectrum.

It is a known phenomenon of cholesteric liquid crystals that they separate light traveling along the helical axis into right- and left-handed circularly polarized components. The component with the same handedness as the helical structure is reflected, while the other is transmitted. In the case that the helical axis extends normal to the display panel, the pitch of the helical structure may be selected such that the reflected portion of light has a wavelength in the UV region of the spectrum. This may ensure that no visible light is reflected by the electro-optic material, thereby improving the optical efficiency of the display device for visible wavelengths.

The non-switchable optically transparent layer may comprise a further liquid crystal material. The further liquid crystal material may comprise a further cholesteric liquid crystal component comprising a further helical structure defined by a further pitch, and having a further ordinary refractive index and a further extra-ordinary refractive index which differs from the further ordinary refractive index by a further difference; wherein the further pitch is selected such that a further product of the further pitch and the further difference is equal to or shorter than the visible light wavelength of the display output.

By the non-switchable optically transparent layer comprising a further cholesteric liquid crystal component, close matching of the respective refractive indices of the electro-optic material and the non-switchable optically transparent layer may be achieved. This may enhance the quality of the image displayed in the single view mode, particularly at oblique viewing angles.

The further helical axis may extend along an axis of a further plane lying co-planar with respect to the display panel. Alternatively, the further helical axis may extend normal to the display panel such that the further cholesteric liquid crystal component reflects a further portion of light with the same handedness as the further helical structure back towards the display panel, wherein the further pitch of the further helical structure is selected such that the further portion has a further wavelength in the UV region of the spectrum.

As explained in relation to the cholesteric liquid crystal component, selecting the further pitch so as to minimize reflection of visible wavelengths back towards the display panel may improve the optical efficiency of the display device for visible wavelengths.

The display panel may comprise a liquid crystal display panel or an OLED display panel. Owing to the polarization-independent refractive index tuning provided by the electro-optic material, a display panel, such as an OLED display, which delivers non-polarized light may be used, without any requirement for additional polarizing means.

The display device may comprise an autostereoscopic display device, the single view mode comprising a two-dimensional mode, and the multi-view mode comprising a three-dimensional mode.

The display device may comprise a dual view display device, wherein the single view mode comprises a two-dimensional mode in the full field of view of the display, and the multi-view mode comprises two at least partly separated two-dimensional modes at different parts of the full field of view of the display.

In accordance with another aspect, there is provided a method of controlling a display output comprising: providing a display panel having rows and columns of sub-pixels configured to produce a display output; arranging a lenticular lens arrangement over the display panel on the display output side of the display panel, the lenticular lens arrangement including an array of lenticular lens elements which comprise an electro-optic material having a lenticular shape adjacent a non-switchable optically transparent layer having an inverse lenticular shape, the electro-optic material comprising a cholesteric liquid crystal component comprising a helical structure defined by a pitch, and having an ordinary refractive index and an extra-ordinary refractive index which differs from the ordinary refractive index, wherein the pitch is selected such that a product of the pitch and said difference is equal to or less than a visible light wavelength of the display output; and applying an electrical stimulus to the cholesteric liquid crystal component material thereby to control an effective refractive index of the electro-optic material.

In the absence of said electrical stimulus the lenticular lens arrangement may be in a first mode in which the effective refractive index of the electro-optic material is substantially equal to a further refractive index of the non-switchable optically transparent layer. Applying the electrical stimulus may comprise switching the lenticular lens arrangement from the first mode to a second mode in which the effective refractive index is different from the further refractive index.

Alternatively, in the absence of the electrical stimulus the lenticular lens arrangement may be in a second mode in which the effective refractive index of the electro-optic material is different from a further refractive index of the non-switchable optically transparent layer. Applying the electrical stimulus may comprise switching the lenticular lens arrangement from the second mode to the first mode in which the effective refractive index is substantially equal to the further refractive index.

The first mode and the second mode may respectively correspond to the single view mode and the multiple view mode of the display device as defined above.

