Patent ID: 12228835

DETAILED DESCRIPTION

Terms related to optical retarders for the purposes of the present disclosure will now be described.

In a layer comprising a uniaxial birefringent material there is a direction governing the optical anisotropy whereas all directions perpendicular to it (or at a given angle to it) have equivalent birefringence.

The optical axis of an optical retarder refers to the direction of propagation of a light ray in the uniaxial birefringent material in which no birefringence is experienced. This is different from the optical axis of an optical system which may for example be parallel to a line of symmetry or normal to a display surface along which a principal ray propagates.

For light propagating in a direction orthogonal to the optical axis, the optical axis is the slow axis when linearly polarized light with an electric vector direction parallel to the slow axis travels at the slowest speed. The slow axis direction is the direction with the highest refractive index at the design wavelength. Similarly the fast axis direction is the direction with the lowest refractive index at the design wavelength.

For positive dielectric anisotropy uniaxial birefringent materials the slow axis direction is the extraordinary axis of the birefringent material. For negative dielectric anisotropy uniaxial birefringent materials the fast axis direction is the extraordinary axis of the birefringent material.

The terms half a wavelength and quarter a wavelength refer to the operation of a retarder for a design wavelength λ0that may typically be between 500 nm and 570 nm. In the present illustrative embodiments exemplary retardance values are provided for a wavelength of 550 nm unless otherwise specified.

The retarder provides a phase shift between two perpendicular polarization components of the light wave incident thereon and is characterized by the amount of relative phase, Γ, that it imparts on the two polarization components; which is related to the birefringence Δn and the thickness d of the retarder by
Γ=2·π·Δn·d/λ0eqn.1

In eqn. 1, Δn is defined as the difference between the extraordinary and the ordinary index of refraction, i.e.
Δn=ne−noeqn. 2

For a half-wave retarder, the relationship between d, Δn, and λ0is chosen so that the phase shift between polarization components is Γ=π. For a quarter-wave retarder, the relationship between d, Δn, and λ0is chosen so that the phase shift between polarization components is Γ=π/2.

The term half-wave retarder herein typically refers to light propagating normal to the retarder and normal to the spatial light modulator.

Some aspects of the propagation of light rays through a transparent retarder between a pair of polarisers will now be described.

The state of polarisation (SOP) of a light ray is described by the relative amplitude and phase shift between any two orthogonal polarization components. Transparent retarders do not alter the relative amplitudes of these orthogonal polarisation components but act only on their relative phase. Providing a net phase shift between the orthogonal polarisation components alters the SOP whereas maintaining net relative phase preserves the SOP. In the current description, the SOP may be termed the polarisation state.

A linear SOP has a polarisation component with a non-zero amplitude and an orthogonal polarisation component which has zero amplitude.

A linear polariser transmits a unique linear SOP that has a linear polarisation component parallel to the electric vector transmission direction of the linear polariser and attenuates light with a different SOP.

Absorbing polarisers are polarisers that absorb one polarisation component of incident light and transmit a second orthogonal polarisation component. Examples of absorbing linear polarisers are dichroic polarisers.

Reflective polarisers are polarisers that reflect one polarisation component of incident light and transmit a second orthogonal polarisation component. Examples of reflective polarisers that are linear polarisers are multilayer polymeric film stacks such as DBEF™ or APF™ from 3M Corporation, or wire grid polarisers such as ProFlux™ from Moxtek. Reflective linear polarisers may further comprise cholesteric reflective materials and a quarter waveplate arranged in series.

A retarder arranged between a linear polariser and a parallel linear analysing polariser that introduces no relative net phase shift provides full transmission of the light other than residual absorption within the linear polariser.

A retarder that provides a relative net phase shift between orthogonal polarisation components changes the SOP and provides attenuation at the analysing polariser.

In the present disclosure an ‘A-plate’ refers to an optical retarder utilizing a layer of birefringent material with its optical axis parallel to the plane of the layer.

A ‘positive A-plate’ refers to positively birefringent A-plates, i.e. A-plates with a positive Δn.

In the present disclosure a ‘C-plate’ refers to an optical retarder utilizing a layer of birefringent material with its optical axis perpendicular to the plane of the layer. A ‘positive C-plate’ refers to positively birefringent C-plate, i.e. a C-plate with a positive Δn. A ‘negative C-plate’ refers to negatively birefringent C-plates, i.e. C-plates with a negative Δn.

‘O-plate’ refers to an optical retarder utilizing a layer of birefringent material with its optical axis having a component parallel to the plane of the layer and a component perpendicular to the plane of the layer. A ‘positive O-plate’ refers to positively birefringent O-plates, i.e. O-plates with a positive Δn.

Achromatic retarders may be provided wherein the material of the retarder is provided with a retardance Δn·d that varies with wavelength λ as
Δn·d/λ=κeqn.3

where κ is substantially a constant.

Examples of suitable materials include modified polycarbonates from Teijin Films. Achromatic retarders may be provided in the present embodiments to advantageously minimise color changes between polar angular viewing directions which have low luminance reduction and polar angular viewing directions which have increased luminance reductions as will be described below.

Various other terms used in the present disclosure related to retarders and to liquid crystals will now be described.

A liquid crystal cell has a retardance given by Δn·d where Δn is the birefringence of the liquid crystal material in the liquid crystal cell and d is the thickness of the liquid crystal cell, independent of the alignment of the liquid crystal material in the liquid crystal cell.

Homogeneous alignment refers to the alignment of liquid crystals in switchable liquid crystal displays where molecules align substantially parallel to a substrate. Homogeneous alignment is sometimes referred to as planar alignment. Homogeneous alignment may typically be provided with a small pre-tilt such as 2 degrees, so that the molecules at the surfaces of the alignment layers of the liquid crystal cell are slightly inclined as will be described below. Pretilt is arranged to minimise degeneracies in switching of cells.

In the present disclosure, homeotropic alignment is the state in which rod-like liquid crystalline molecules align substantially perpendicularly to the substrate. In discotic liquid crystals homeotropic alignment is defined as the state in which an axis of the column structure, which is formed by disc-like liquid crystalline molecules, aligns perpendicularly to a surface. In homeotropic alignment, pretilt is the tilt angle of the molecules that are close to the alignment layer and is typically close to 90 degrees and for example may be 88 degrees.

In a twisted liquid crystal layer a twisted configuration (also known as a helical structure or helix) of nematic liquid crystal molecules is provided. The twist may be achieved by means of a non-parallel alignment of alignment layers. Further, cholesteric dopants may be added to the liquid crystal material to break degeneracy of the twist direction (clockwise or anti-clockwise) and to further control the pitch of the twist in the relaxed (typically undriven) state. A supertwisted liquid crystal layer has a twist of greater than 180 degrees. A twisted nematic layer used in spatial light modulators typically has a twist of 90 degrees.

Liquid crystal molecules with positive dielectric anisotropy are switched from a homogeneous alignment (such as an A-plate retarder orientation) to a homeotropic alignment (such as a C-plate or O-plate retarder orientation) by means of an applied electric field.

Liquid crystal molecules with negative dielectric anisotropy are switched from a homeotropic alignment (such as a C-plate or O-plate retarder orientation) to a homogeneous alignment (such as an A-plate retarder orientation) by means of an applied electric field.

Rod-like molecules have a positive birefringence so that ne>noas described in eqn. 2. Discotic molecules have negative birefringence so that ne<no.

Positive retarders such as A-plates, positive O-plates and positive C-plates may typically be provided by stretched films or rod-like liquid crystal molecules. Negative retarders such as negative C-plates may be provided by stretched films or discotic like liquid crystal molecules.

Parallel liquid crystal cell alignment refers to the alignment direction of homogeneous alignment layers being parallel or more typically antiparallel. In the case of pre-tilted homeotropic alignment, the alignment layers may have components that are substantially parallel or antiparallel. Hybrid aligned liquid crystal cells may have one homogeneous alignment layer and one homeotropic alignment layer. Twisted liquid crystal cells may be provided by alignment layers that do not have parallel alignment, for example oriented at 90 degrees to each other.

Transmissive spatial light modulators may further comprise retarders between the input display polariser and the output display polariser for example as disclosed in U.S. Pat. No. 8,237,876, which is herein incorporated by reference in its entirety. Such retarders (not shown) are in a different place to the passive retarders of the present embodiments. Such retarders compensate for contrast degradations for off-axis viewing locations, which is a different effect to the luminance reduction for off-axis viewing positions of the present embodiments.

A private mode of operation of a display is one in which an observer sees a low contrast sensitivity such that an image is not clearly visible. Contrast sensitivity is a measure of the ability to discern between luminances of different levels in a static image. Inverse contrast sensitivity may be used as a measure of visual security, in that a high visual security level (VSL) corresponds to low image visibility.

For a privacy display providing an image to an observer, visual security may be given as:
V=(Y+R)/(Y−K)  eqn.4

where V is the visual security level (VSL), Y is the luminance of the white state of the display at a snooper viewing angle, K is the luminance of the black state of the display at the snooper viewing angle and R is the luminance of reflected light from the display.

Panel contrast ratio is given as:
C=Y/Keqn. 5

so the visual security level may be further given as:
V=(P·Ymax+I·ρ/π)/(P·(Ymax−Ymax/C))  eqn. 6

where: Ymaxis the maximum luminance of the display; P is the off-axis relative luminance typically defined as the ratio of luminance at the snooper angle to the maximum luminance Ymax; C is the image contrast ratio; ρ is the surface reflectivity; and l is the illuminance. The units of Ymaxare the units of I divided by solid angle in units of steradian.

The luminance of a display varies with angle and so the maximum luminance of the display Ymaxoccurs at a particular angle that depends on the configuration of the display.

In many displays, the maximum luminance Ymaxoccurs head-on, i.e. normal to the display. Any display device disclosed herein may be arranged to have a maximum luminance Ymaxthat occurs head-on, in which case references to the maximum luminance of the display device Ymaxmay be replaced by references to the luminance normal to the display device.

Alternatively, any display described herein may be arranged to have a maximum luminance Ymaxthat occurs at a polar angle to the normal to the display device that is greater than 0°. By way of example, the maximum luminance Ymaxmay occur may at a non-zero polar angle and at an azimuth angle that has for example zero lateral angle so that the maximum luminance is for an on-axis user that is looking down on to the display device. The polar angle may for example be 10 degrees and the azimuthal angle may be the northerly direction (90 degrees anti-clockwise from easterly direction). The viewer may therefore desirably see a high luminance at typical non-normal viewing angles.

The off-axis relative luminance, P is sometimes referred to as the privacy level. However, such privacy level P describes relative luminance of a display at a given polar angle compared to head-on luminance, and in fact is not a measure of privacy appearance.

The illuminance, I is the luminous flux per unit area that is incident on the display and reflected from the display towards the observer location. For Lambertian illuminance, and for displays with a Lambertian front diffuser illuminance I is invariant with polar and azimuthal angles. For arrangements with a display with non-Lambertian front diffusion arranged in an environment with directional (non-Lambertian) ambient light, illuminance I varies with polar and azimuthal angle of observation.

Thus in a perfectly dark environment, a high contrast display has VSL of approximately 1.0. As ambient illuminance increases, the perceived image contrast degrades, VSL increases and a private image is perceived.

For typical liquid crystal displays the panel contrast C is above 100:1 for almost all viewing angles, allowing the visual security level to be approximated to:
V=1+I·ρ/(π·P·Ymax)  eqn. 7

In the present embodiments, in addition to the exemplary definition of eqn. 4, other measurements of visual security level, V may be provided, for example to include the effect on image visibility to a snooper of snooper location, image contrast, image colour and white point and subtended image feature size. Thus the visual security level may be a measure of the degree of privacy of the display but may not be restricted to the parameter V.

The perceptual image security may be determined from the logarithmic response of the eye, such that
S=log10(V)  eqn. 8

Desirable limits for S were determined in the following manner. In a first step a privacy display device was provided. Measurements of the variation of privacy level, P(θ) of the display device with polar viewing angle and variation of reflectivity ρ(θ) of the display device with polar viewing angle were made using photopic measurement equipment. A light source such as a substantially uniform luminance light box was arranged to provide illumination from an illuminated region that was arranged to illuminate the privacy display device along an incident direction for reflection to a viewer position at a polar angle of greater than 0° to the normal to the display device. The variation I(θ) of illuminance of a substantially Lambertian emitting lightbox with polar viewing angle was determined by and measuring the variation of recorded reflective luminance with polar viewing angle taking into account the variation of reflectivity ρ(θ). The measurements of P(θ), r(θ) and I(θ) were used to determine the variation of Security Factor S(θ) with polar viewing angle along the zero elevation axis.

In a second step a series of high contrast images were provided on the privacy display including (i) small text images with maximum font height 3 mm, (ii) large text images with maximum font height 30 mm and (iii) moving images.

In a third step each observer (with eyesight correction for viewing at 1000 mm where appropriate) viewed each of the images from a distance of 1000 m, and adjusted their polar angle of viewing at zero elevation until image invisibility was achieved for one eye from a position near on the display at or close to the centre-line of the display. The polar location of the observer's eye was recorded. From the relationship S(θ), the security factor at said polar location was determined. The measurement was repeated for the different images, for various display luminance Ymax, different lightbox illuminance I(θ=0), for different background lighting conditions and for different observers.

From the above measurements S<1.0 provides low or no visual security, 1.0≤S<1.5 provides visual security that is dependent on the contrast, spatial frequency and temporal frequency of image content, 1.5≤S<1.8 provides acceptable image invisibility (that is no image contrast is observable) for most images and most observers and S≥1.8 provides full image invisibility, independent of image content for all observers.

In practical display devices, this means that it is desirable to provide a value of S for an off-axis viewer who is a snooper that meets the relationship S≥Smin, where: Sminhas a value of 1.0 or more to achieve the effect that the off-axis viewer cannot perceive the displayed image; Sminhas a value of 1.5 or more to achieve the effect that the displayed image is invisible, i.e. the viewer cannot perceive even that an image is being displayed, for most images and most observers; or Sminhas a value of 1.8 or more to achieve the effect that the displayed image is invisible independent of image content for all observers.

In comparison to privacy displays, desirably wide angle displays are easily observed in standard ambient illuminance conditions. One measure of image visibility is given by the contrast sensitivity such as the Michelson contrast which is given by:
M=(Imax−Imin)/(Imax+Imin)  eqn. 9
and so:
M=((Y+R)−(K+R))/((Y+R)+(K+R))=(Y−K)/(Y+K+2·R)  eqn. 10

Thus the visual security level (VSL), V is equivalent (but not identical to) 1/M. In the present discussion, for a given off-axis relative luminance, P the wide angle image visibility, W is approximated as
W=1/V=1/(1+I·ρ/(π·P·Ymax))  eqn. 11

The above discussion focusses on reducing visibility of the displayed image to an off-axis viewer who is a snooper, but similar considerations apply to visibility of the displayed image to the intended user of the display device who is typically on-axis. In this case, decrease of the level of the visual security level (VSL) V corresponds to an increase in the visibility of the image to the viewer. During observation S<0.1 may provide acceptable visibility of the displayed image. In practical display devices, this means that it is desirable to provide a value of S for an on-axis viewer who is the intended user of the display device that meets the relationship S≤Smax, where Smaxhas a value of 0.1.

In the present discussion the colour variation Δε of an output colour (uw′+Δu′, vw′+Δv′) from a desirable white point (uw′, vw′) may be determined by the CIELUV colour difference metric, assuming a typical display spectral illuminant and is given by:
Δε=(Δu′2+Δv′2)1/2eqn. 12

Switchable directional display apparatuses for use in privacy display for example and comprising plural retarders arranged between a display polariser and an additional polariser are described in U.S. Patent Publ. No. 2019-0086706, U.S. Patent Publ. No. 2019-0250458, U.S. Patent Publ. No. 2020-0225402, and WIPO Publ. No. 2019-055755, all of which are herein incorporated by reference in their entireties. Directional display apparatuses further comprising reflective polarisers arranged between the display polariser and retarders are described in U.S. Patent Publ. No. 2019-0250458 and in U.S. Patent Publ. No. 2019-0227366, both of which are herein incorporated by reference in their entireties. Directional display polarisers comprising passive retarders arranged between a display polariser and an additional polariser are described U.S. Patent Publ. No. 2018-0321553, which is herein incorporated by reference in its entirety.

Curvature is a property of a line that is curved and for the present disclosure is the inverse radius of curvature. A planar surface has a curvature of zero.

The structure and operation of various directional display devices will now be described. In this description, common elements have common reference numerals. It is noted that the disclosure relating to any element applies to each device in which the same or corresponding element is provided. Accordingly, for brevity such disclosure is not repeated.

FIG.1Ais a side perspective view of a display device100providing uniformity in reduction of luminance in directions; andFIG.1Bis a front view of the stack of layers of the display device ofFIG.1A. Features of the arrangement ofFIG.1Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals ofFIG.1A, including any potential variations in the features.

FIG.1Aillustrates the display device100, for use in ambient illumination604, which comprises a spatial light modulator (SLM)48arranged to output light400. The SLM48comprises an input polariser210arranged on the input side of the SLM48and an output polariser218, arranged on the output side of the SLM48, the input polariser210and the output polariser218being the two display polarisers of the SLM. The input polariser210and the output polariser218are each linear polarisers.

The display device100also comprises an additional polariser318arranged on the output side of the output polariser218, that is the same side as the output polariser218, and a reflective polariser302arranged between the output polariser218and the additional polariser318. The additional polariser318and the reflective polariser302are each a linear polariser. Typical polarisers210,218,318may be polarisers such as dichroic polarisers.

The display device100also comprises at least one polar control retarder300which is arranged between the additional polariser318and the output polariser218and wherein the polar control retarder300includes a liquid crystal retarder301. The polar control retarder300is also arranged between the reflective polariser302and the additional polariser318. Further, the reflective polariser302is arranged between the output polariser218and the polar control retarder300. The electric vector transmission direction303of the reflective polariser302is parallel to the electric vector transmission direction319of the additional polariser318, i.e. the reflective polariser302is a linear polariser arranged to pass the same linearly polarised component as the output polariser218. The electric vector transmission direction303of the reflective polariser302is parallel to the electric vector transmission direction219of the output polariser218.

The electric vector transmission direction211of the input polariser210is orthogonal to the electric vector transmission direction219of the output polariser218. Advantageously the transmission of partially polarised light from the waveguide1through the input polariser210may be increased.

In the present disclosure, the SLM48may comprise a liquid crystal display comprising substrates212,216, liquid crystal layer214and red, green and blue pixels220,222,224. The output polariser218may be arranged to provide high extinction ratio for light from the pixels220,222,224of the SLM48and to prevent back reflections from the reflective polariser302towards the pixels220,222,224.

Backlight20may be arranged to illuminate the SLM48, thereby providing a transmissive SLM48and may comprise input light sources15, waveguide1, rear reflector3and optical stack5comprising diffusers, light tuning films and other known optical backlight structures. Plural first light sources15are shown by way of non-limitative example, but in general there may any number of one or more light sources15. Asymmetric diffusers, that may comprise asymmetric surface relief features for example, may be provided in the optical stack5with increased diffusion in the elevation direction in comparison to the lateral direction may be provided. Advantageously image uniformity may be increased.

The display may further comprise a reflective recirculation polariser208arranged between the backlight20and the SLM48. The reflective recirculation polariser208is different to the reflective polariser302of the present embodiments. Reflective recirculation polariser208provides reflection of polarised light from the backlight20that has a polarisation that is orthogonal to the electric vector transmission direction of the dichroic input polariser210. Reflective recirculation polariser208does not reflect ambient light604to a snooper.

The SLM48may alternatively be provided by other display types that provide output light400by emission, such as organic LED displays (OLED), with output polariser218. Output polariser218may provide reduction of luminance for light reflected from the OLED pixel plane by means of one of more retarders518inserted between the output display polariser218and OLED pixel plane. The one or more retarders518may be a quarter waveplate and is different to the retarder330of the present disclosure.

In the embodiment ofFIG.1, the polar control retarder300comprises passive polar control retarder330, i.e. at least one passive compensation retarder, and a layer of liquid crystal material provided by a switchable liquid crystal retarder. In general, the polar control retarder300may comprise any configuration of at least one retarder, some examples of which are present in the devices described below.

The at least one polar control retarder300is capable of simultaneously introducing no net relative phase shift to orthogonal polarisation components of light passed by the reflective polariser302along an axis along a normal to the plane of the at least one polar control retarder300and introducing a relative phase shift to orthogonal polarisation components of light passed by the reflective polariser302along an axis inclined to a normal to the plane of the at least one polar control retarder300. The polar control retarder300does not affect the luminance of light passing through the reflective polariser302, the polar control retarder300and the additional polariser318along an axis along a normal to the plane of the polar control retarder300. The polar control retarder300does, however, reduce the luminance of light passing therethrough along an axis inclined to a normal to the plane of the polar control retarder300, at least in one of the switchable states of the switchable retarder301. The principles leading to this effect are described in greater detail below with reference toFIGS.23A-25Eand arises from the presence or absence of a phase shift introduced by the polar control retarder300to light along axes that are angled differently with respect to the liquid crystal material of the polar control retarder300. A similar effect is achieved in all the devices described below.

The polar control retarder300comprises a switchable liquid crystal retarder301comprising a layer314of liquid crystal material, and substrates312,316arranged between the reflective polariser302and the additional polariser318. Thus the at least one polar control retarder300comprises a switchable liquid crystal retarder301comprising a layer314of liquid crystal material414, wherein the at least one polar control retarder300is arranged, in a switchable state of the switchable liquid crystal retarder301, simultaneously to introduce no net relative phase shift to orthogonal polarisation components of light passed by the reflective polariser302along an axis along a normal to the plane of the at least one polar control retarder300and to introduce a net relative phase shift to orthogonal polarisation components of light passed by the reflective polariser302along an axis inclined to a normal to the plane of the at least one polar control retarder.

The substrates312,316of the switchable liquid crystal retarder301comprise electrodes413,415(illustrated inFIG.5A) arranged to provide a voltage across the layer314of liquid crystal material414for controlling the layer314. A control system352is arranged to control the voltage applied, by a voltage driver350, across the electrodes413,415of the switchable liquid crystal retarder301.

The polar control retarder further comprises two surface alignment layers419A,419B disposed adjacent to the layer314of liquid crystal material414and on opposite sides thereof. Each of the surface alignment layers419A,419B is arranged to provide alignment in the adjacent liquid crystal material414with an in-plane component417Ap,417Bp respectively that is in the plane of the layer314of liquid crystal material414.

In a region on the left side of the liquid crystal retarder301the in-plane component417ALp on the first alignment layer419A has an orientation angle617AL. In a region near the centre of the liquid crystal retarder301, the in-plane component417ACp on the first alignment layer419A has an orientation angle617AC. In a region on the right side of the liquid crystal retarder301the in-plane component417ARp on the first alignment layer419A has an orientation angle617AR.

As will be described further hereinbelow, in a region on the left side of the liquid crystal retarder301the in-plane component417BLp on the second alignment layer419B has an orientation angle617BL. In a region near the centre of the liquid crystal retarder301, the in-plane component417BCp on the second alignment layer419B has an orientation angle617BC. In a region on the right side of the liquid crystal retarder301the in-plane component417BRp on the second alignment layer419B has an orientation angle617BR.

In the present embodiments at least one of the angles617A,617B of said in-plane component of the alignment in the adjacent liquid crystal material414changes monotonically along the predetermined axis across at least part of the display device100. This will be further described hereinbelow, for example with reference toFIG.4below.

As illustrated inFIG.1Bin the case when the SLM48is a liquid crystal display, the input electric vector transmission direction211at the input polariser210provides an input polarisation component that may be transformed by the liquid crystal layer214to provide output polarisation component determined by the electric vector transmission direction219of the output polariser218. The electric vector transmission direction of the reflective polariser302is parallel to the electric vector transmission direction of the output polariser218. Further, the electric vector transmission direction of the output polariser218and the electric vector transmission direction303of the reflective polariser302are parallel to the electric vector transmission direction319of the additional polariser318in this example.

The at least one passive retarder330, of the at least one polar control retarder300, is arranged to introduce no net relative phase shift to orthogonal polarisation components of light passed by the reflective polariser302along an axis along a normal to the plane of the at least one passive retarder and to introduce a net relative phase shift to orthogonal polarisation components of light passed by the reflective polariser302along an axis inclined to a normal to the plane of the at least one passive retarder.

As will be described for example inFIG.5Ahereinbelow, the liquid crystal material414is aligned with an in-plane component that is in the plane of the layer314of liquid crystal material414.FIG.1Billustrates the alignment of liquid crystal material414at the alignment layer417A. The angle617AL,617AC,617AR of said in-plane components417ALp,417ACp,417ARp of the alignment in the adjacent liquid crystal material414at the alignment layer417A of the liquid crystal layer314changes monotonically along the predetermined axis500across at least part of the display device100, to provide liquid crystal material414molecules414ALp,414ACp and414ARp with varying alignment across alignment layer417A. When viewed from left to right, the angle617of the in-plane component of material414B increases from approximately 70 degrees for material414ALp, to 90 degrees at for material414ACp, to approximately 110 degrees for material414ARp. In this case, the angle has been determined from the axis500, rotating counter-clockwise, to the in-plane component of the liquid crystal material414. Determining the angle617AL,617AC,617AR in this manner results in a monotonically increasing angle. Alternatively, the angle may be determined from the axis500in a clockwise direction to the in-plane component of the liquid crystal material. In this case, the angle would be monotonically decreasing.

Passive polar control retarder330may comprise retardation layer with a solid birefringent material430, while switchable liquid crystal retarder301may comprise a layer314of liquid crystal material414, as will be described below.

An emissive display device will now be described.

FIG.2is a side perspective view of an emissive display device100providing uniformity in reduction of luminance in directions. Features of the arrangement ofFIG.2not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

FIG.2is an alternative embodiment wherein the spatial light modulator48is an emissive spatial light modulator and the display polariser218is an output polariser arranged on an output side of the transmissive spatial light modulator48. Pixels220,222,224may be provided by emissive elements such as OLED, micro-LED or other known emitting elements. Advantageously thickness may be reduced in comparison to the embodiment ofFIG.1.

By way of comparison withFIGS.1A-B.FIG.2also illustrates an alternative arrangement wherein the electric vector transmission directions219,303,319of the output polariser218, reflective polariser302and additional polariser318may be orthogonal to the alignment direction with in-plane component417ACp. The operation of the present embodiments is substantially the same for parallel or orthogonal alignments of the electric vector transmission directions219,303,319with the said in-plane components417ACp of the alignment in the adjacent liquid crystal material414AC. Advantageously the transmission of light through sunglasses with transmission direction parallel to the x-axis may be increased.

Arrangements of alignment layers419A,419B will now be further described.

FIG.3Ais a front view of a first surface alignment layer419A of the display device100ofFIG.1Awherein the angle of in-plane component of the alignment varies along first and second axes500,502. Features of the arrangement ofFIG.3Anot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

The surface alignment layer419A may have material that provides different alignment orientations across the surface alignment layer419A as will be described further hereinbelow.

The material on the upper left has alignment417ALU which has an angle617ALU to the axis500. The material in the upper centre has an alignment417ACU, which has an angle617ACU to the axis500. The material in the far right has an alignment417ARU, which has an angle617ARU to the axis500. As is apparent, the angle617AU of the material in the plane of the alignment layer419A increases monotonically from left to right along the axis500. This alignment layer419A will thus result in an angle of the in-plane component of the alignment in the adjacent liquid crystal material414, in the liquid crystal layer314, which changes monotonically along the predetermined axis across at least part of the display device100, which corresponds to the axis500of the alignment layer419A.

In the lower left portion of the alignment layer419A, the material has an alignment417ALD which has an angle617ALD to the axis500.

In the lower centre of the alignment layer419A, the material has the same alignment417ACD to the alignment417ACU which has an angle617ACD that is the same as the angle617ACU to the axis500. In alternative embodiments the angles617ACU,617ACD may be different to provide luminance control in the vertical axis502as will be described further hereinbelow.

Further, the material414on the lower right portion of the alignment layer419A has an alignment417ARD which has an angle617ARD to the axis500. As shown, the angle617ALU is larger than angle619ALD, and angle617ARU is larger than angle619ARD. Accordingly, in addition to the angle changing monotonically along axis500, the angle of the in-plane component of the alignment layer419A, and hence the alignment in the adjacent liquid material also changes monotonically along a further axis, shown by axis502which is perpendicular to axis500.

Advantageously as will be described below, increased uniformity of illumination to on-axis viewers may be increased.

FIG.3Bis a front view of a surface alignment layer419A of the display device ofFIG.1Awherein the angle of in-plane component of the alignment417A varies along the first axis500and not along the second axis502. Features of the arrangement ofFIG.3Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

Thus the alignment does not change along a further axes perpendicular to the predetermined axis500. Advantageously the alignment layer may be provided to achieve increased uniformity of luminance along the axis500. The alignment layers419A,419B may be more conveniently manufactured than the arrangement ofFIG.3A, as will be described further below with reference toFIGS.21-22.

The second surface alignment layer419B (not shown) may be the same as the first surface alignment layer419A, or alternatively it may be different. The alignment directions417B on the second surface alignment layer419B may typically be parallel or anti-parallel to the alignment directions417A on the first alignment layer. Advantageously increased uniformity may be provided in a lateral direction along axis500as will be described further below.

Some twist may be provided between the alignment directions417A,417B at the first and second alignment layers419A,419B. Advantageously increased rotational symmetry of output profile may be achieved. Further the number of substrates that have varying alignment directions may be reduced, reducing cost and complexity.

Variations of alignment angles617will now be described.

FIG.4is a graph of the in-plane component of angle617of the alignment provided by the surface alignment layer419A ofFIG.3Ain the predetermined direction500across the width of the active area of display device100. The alignment orientation shown by the profiles504,506,508change monotonically across at least part of the display100. Profiles504,506illustrates a linear variation of alignment angle617. In manufacture, the gradient of the profile may be varied to provide maximum uniformity for different nominal viewing distances of the primary viewer from the display device100. For example, a high gradient may be used for a short viewing distance while a lower gradient of profile506may be provided for displays arranged to be operated at longer viewing distances. Advantageously uniformity may be optimised.

Profile508illustrates a non-linear variation of angle617in the predetermined direction500. Such a profile may be provided to compensate for non-linear variations of luminance profile with viewing angle as will be further described with reference toFIGS.10A-Hbelow for example.

In these examples, the in-plane component of angle at least one of the angles617A,617B of said in-plane component of the alignment in the adjacent liquid crystal material414changes monotonically along the predetermined axis a mean direction that is parallel to the electric vector transmission directions of the output polariser218and the reflective polariser302and the additional polariser318, with a mean direction that is parallel to the electric vector transmission directions of the output polariser218, the reflective polariser302and the additional polariser318. More generally, in the case that the electric vector transmission directions of the output polariser218, the reflective polariser302and the additional polariser318are not parallel, the mean direction may be parallel to or orthogonal to the electric vector transmission directions of at least one of the output polariser218, the reflective polariser302and the additional polariser318. However, this is not essential. In other examples, the mean direction may be at a non-zero acute angle to one or more of the electric vector transmission directions of the output polariser218, the reflective polariser302and the additional polariser318.

In the examples described above, at least one of the angles617A,617B of said in-plane component of the alignment in the adjacent liquid crystal material414changes monotonically along the predetermined axis across the entirety of the display device100. In other examples described below, the at least one of the angles617A,617B of said in-plane component of the alignment in the adjacent liquid crystal material414changes monotonically along the predetermined axis across a part of the display device100, in which case the technical effects are achieved for that part.

An illustrative polar control retarder arrangement will now be described.

FIG.5Ais a perspective view of polar control retarders330,301that may be applied inFIG.1, comprising a homogeneously aligned switchable LC retarder301and passive crossed A-plate retarders308A,308B; andFIG.5Bis a perspective view of liquid crystal material414alignment orientations in the liquid crystal layer314of the liquid crystal polar control retarder301ofFIG.5A.

InFIGS.5A-Band other schematic diagrams below, some layers of the optical stack are omitted for clarity. For example the switchable liquid crystal retarder301is shown omitting the substrates312,316. Features of the arrangement ofFIG.5Anot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

The switchable liquid crystal retarder301comprises a layer314of liquid crystal material414with a positive dielectric anisotropy.

The switchable liquid crystal retarder301comprises two surface alignment layers419A,419B disposed adjacent to the layer of liquid crystal material421and on opposite sides thereof and each arranged to provide homogeneous alignment in the adjacent liquid crystal material414. The layer314of liquid crystal material414of the switchable liquid crystal retarder301comprises a liquid crystal material414with a positive dielectric anisotropy. The layer314of liquid crystal material414has a retardance for light of a wavelength of 550 nm in a range from 500 nm to 900 nm, preferably in a range from 600 nm to 850 nm and most preferably in a range from 700 nm to 800 nm. The passive polar control retarder330further comprises a pair of passive retarders308A,308B which have optical axes in the plane of the retarders, that is they are A-plates, that are crossed. Each passive retarder308A,308B of the pair of passive retarders308A,308B has a retardance for light of a wavelength of 550 nm in a range from 300 nm to 800 nm, preferably in a range from 350 nm to 650 nm and most preferably in a range from 450 nm to 550 nm.

In the present embodiments, ‘crossed’ refers to an angle of substantially 90° between the optical axes of the two retarders308A,308B in the plane of the retarders308A,308B. To reduce cost of retarder materials, it is desirable to provide materials with some variation of retarder orientation due to stretching errors during film manufacture for example. Variations in retarder orientation away from preferable directions can reduce the head-on luminance and increase the minimum transmission. Preferably the angle310A is at least 35° and at most 55°, more preferably at least 40° and at most 50° and most preferably at least 42.5° and at most 47.5°. Preferably the angle310B is at least 125° and at most 145°, more preferably at least 130° and at most 135° and most preferably at least 132.5° and at most 137.5°.

The passive retarders308A,308B may be provided using stretched films to advantageously achieve low cost and high uniformity. Further field of view for liquid crystal retarders with homogeneous alignment is increased while providing resilience to the visibility of flow of liquid crystal material during applied pressure.

It may be desirable to provide the additional polariser318with a different electric vector transmission direction to the electric vector transmission direction of the output polariser218and reflective polariser302.

The liquid crystal retarder301further comprises electrodes413,415, which are transmissive, arranged across the layer314to control the liquid crystal material414and thereby control the liquid crystal retarder. The layer314of liquid crystal material414is switchable by means of adjusting the voltage being applied to the electrodes413,415. The electrodes413,415are on opposite sides of the layer314of liquid crystal material414and may for example be indium-tin-oxide (ITO) electrodes.

The alignment layers419A,419B may be formed between electrodes413,415and the liquid crystal material414of the layer314.

The orientation of the liquid crystal molecules414will now be further described.

Considering the molecule414A that is adjacent the first alignment layer417A, the optical axis direction417A is determined by the pretilt angle619A of the alignment layer417A. For non-zero pretilt angles619A, a component417Az of liquid crystal molecule414A optical axis alignment direction is provided that is out of the plane in which the liquid crystal retarder301extends. The optical axis direction417A is also determined by the orientation direction617A (that may be also referred to as the rubbing direction) of the alignment layer417A in the region in which the molecule414A is arranged. A component417Ap of liquid crystal molecule414A optical axis alignment direction is provided that is in the plane in which the liquid crystal retarder301extends.

Considering the molecule414B that is adjacent the second alignment layer417B, the optical axis direction417B is determined by the pretilt angle619B of the alignment layer417B. For non-zero pretilt angles619B, a component417Bz of liquid crystal molecule414B optical axis alignment direction is provided that is out of the plane in which the liquid crystal retarder301extends. The optical axis direction417B is also determined by the orientation direction617B (that may be also referred to as the rubbing direction) of the alignment layer417B in the region in which the molecule414B is arranged. A component417Bp of liquid crystal molecule414B optical axis alignment direction is provided that is in the plane in which the liquid crystal retarder301extends.

In other words, the orientation angle617A,617B is determined by the pretilt directions619A,619B of the alignment layers419A.419B so that each alignment layer419A,419B has a pretilt. In the embodiment of FIGURE SA, the pretilt of each alignment layer419A,419B has a pretilt direction with a component417Ap,417Bp in the plane of the layer314that is parallel or anti-parallel or orthogonal to each other.

In the embodiment ofFIG.5A, each of the surface alignment layers419A,419B is arranged to provide homogenous alignment in the adjacent liquid crystal material414. In such homogeneous alignment layers419A,419B may be provided with a pretilt angle619A,619B that is 2° for example.

Driver350provides a voltage V to electrodes413,415across the layer314of switchable liquid crystal material414such that liquid crystal molecules are inclined at a tilt angle to the vertical, forming an O-plate. The plane of the tilt is determined by the pretilt direction of alignment layers419A,419B formed on the electrodes413,415that are formed on the inner surfaces of substrates312,316(seen inFIG.1).

In typical use, for switching between a public mode and a privacy mode, the layer314of liquid crystal material414is switchable between two states. The first state being a public mode so that the display100may be used by multiple users, the second state being a privacy mode for use by a primary user with minimal visibility by snoopers. The switching may be by means of a voltage being applied across the electrodes413,415.

In general such a display100may be considered having a first wide angle state and a second reduced off-axis luminance state. Such a display100may provide a privacy display. In another use, or to provide controlled luminance to off-axis observers, for example in an automotive environment when a passenger or driver may wish some visibility of the displayed image, without full obscuration, by means of intermediate voltage levels. Stray light may be reduced for night-time operation. The display100may also provide more uniform luminance reduction across at least part of the display100for off-axis viewers whilst also providing more uniform luminance across at least part of the display for on-axis viewers.

The embodiment ofFIG.5Bfurther illustrates the variation of alignment orientations with location along the lateral axis500. Alignment layers419A.419B are provided with pretilts619A,619B respectively that are of the same magnitude and arranged to provide homogeneous alignment in the adjacent liquid crystal material414.

On the left side of the display on the first alignment layer419A, liquid crystal molecule414AL has an alignment direction417AL with an out-of-plane component417ALz and an in-plane component417ALp that has an acute angle617AL from the axis500direction. For the second alignment layer419B, liquid crystal molecule414BL has an alignment direction417BL with an out-of-plane component417BLz and an in-plane component417BLp that has an acute angle617BL from the axis500direction. The directions417ALp,417BLp are anti-parallel.

On the right side of the display on the first alignment layer419A, liquid crystal molecule414AR has an alignment direction417AR with an out-of-plane component417ARz and an in-plane component417ARp that has an acute angle617AR from the axis500direction. For the second alignment layer419B, liquid crystal molecule414BR has an alignment direction417BR with an out-of-plane component417BRz and an in-plane component417BRp that has an obtuse angle617BR from the axis500direction. The directions417ARp,417BRp are anti-parallel.

In the centre of the display, on the first alignment layer419A, liquid crystal molecule414AC has an alignment direction417AC with an out-of-plane component417ACz and an in-plane component417ACp that has an acute angle617AC from the axis500direction. For the second alignment layer419B, liquid crystal molecule414BC has an alignment direction417BC with an out-of-plane component417BCz and an in-plane component417BCp that has a right angle617BC from the axis500direction. The directions417ACp,417BCp are anti-parallel.

Variations of liquid crystal molecule414orientations through the liquid crystal layer314will now be described.

FIG.6is a graph of the LC director angle of the homogeneously aligned switchable LC inFIG.5Athrough its thickness, showing liquid crystal director angle407against fractional location440through the switchable liquid crystal retarder301for various different applied voltages. Profile441illustrates liquid crystal material414tilt angle for no applied voltage, tilt profile443illustrates director orientations for a first applied voltage to be use in privacy mode and tilt profile445illustrates director orientations for a higher applied voltage to be used in wide angle mode of operation. Thus the liquid crystal layers are typically splayed in desirable switched states, and compensated by the compensation retarders330. Increasing the voltage progressively reduces the thickness of the retarder301in which splay is present, and above the level for privacy operation advantageously increases the polar field of view over which the transmission is maximised.

The splay illustrated inFIG.6by profile443,445is through the thickness of the liquid crystal layer314and is different to the variation in angle617of the alignment direction at the alignment layer419.

The propagation of polarised light from the output polariser218will now be considered for on-axis and off-axis directions.

FIG.7is a side view of propagation of output light from the SLM48through the optical stack ofFIG.1Ain a privacy mode. When the layer314, seen inFIG.5A, of the switchable liquid crystal retarder301, is in a second state of said two states, the polar control retarder300provides no overall transformation of polarisation component360to output light rays400passing therethrough along an axis perpendicular to the plane of the switchable retarder. The polar control retarder does, however, provide an overall transformation of polarisation component361to light rays402passing therethrough for some polar angles which are at an acute angle to the perpendicular to the plane of the retarders. Features of the arrangement ofFIG.7not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

Polarisation component360from the output polariser218is transmitted by reflective polariser302and incident the polar control retarder300, which comprises the switchable liquid crystal retarder301and the passive polar control retarder330. On-axis light has a polarisation component362, upon exiting the polar control retarder300, that is unmodified from component360. Whereas off-axis light has a polarisation component364, upon exiting the polar control retarder300, that is transformed by the polar control retarder300. The polarisation component361may be transformed to a linear polarisation component364which is absorbed by additional polariser318. More generally, the polarisation component361may be transformed to an elliptical polarisation component, that is partially absorbed by additional polariser318.

Thus, in a polar representation of transmission by the polar control retarder300and additional polariser318in a privacy mode at different polar angles, regions of high transmission and regions of low transmission are provided.

The polar distribution of light transmission modifies the polar distribution of luminance output of the underlying SLM48. In the case that the SLM48comprises a directional backlight20then off-axis luminance may be further be reduced as described above.

Advantageously, a privacy display is provided that has low luminance to an off-axis snooper while maintaining high luminance for an on-axis observer.

FIG.8is a side view of propagation of ambient illumination light through the optical stack ofFIG.1Ain a privacy mode. Features of the arrangement ofFIG.8not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

Ambient light source604illuminates the display100with unpolarised light. The additional polariser318transmits light ray410normal to the display surface with a first polarisation component372that is a linear polarisation component parallel to the electric vector transmission direction319of the additional polariser318.

As depicted, the light ray410passes through the polar control retarder300before being incident on the reflective polariser302. In both states of operation, the polarisation component372remains unmodified by the polar control retarder300and so transmitted polarisation component382is parallel to the transmission axis of the reflective polariser302and the output polariser218. Ambient light is therefore directed through the SLM48and lost.

By comparison, for ray412which is off-axis, the additional polariser318transmits a portion of the light ray412with a first polarisation component372that is a linear polarisation component parallel to the electric vector transmission direction319of the additional polariser. The off-axis light ray412is directed through the polar control retarder300and its first polarisation component372is modified to become a modified polarisation component374. The ray412having the modified polarisation component374is incident on the reflective polariser302. Because the modified polarisation component374is different to the transmission axis of the reflective polariser302, the ray412may be reflected by the reflective polariser302. The modified polarisation component374is re-converted into polarisation component376after passing through retarders300and is transmitted through the additional polariser318.

Thus, when the layer314of liquid crystal material is in this state, the polar control retarder300provides no overall transformation of polarisation component372to ambient light rays410passing through the additional polariser318, and then the polar control retarder300, along an axis perpendicular to the plane of the switchable retarder. The reflective polariser302thus provides no reflected light for ambient light rays410passing through the additional polariser318and then the polar control retarder300along an axis perpendicular to the plane of the polar control retarder300. The polar control retarder300does, however provide an overall transformation of polarisation component372to ambient light rays412passing through the absorptive polariser318and then the polar control retarder300at some polar angles which are at an acute angle to the perpendicular to the plane of the polar control retarder300. This results in reflected light rays412for ambient light passing through the additional polariser318, and then the polar control retarder300, at some polar angles which are at an acute angle to the perpendicular to the plane of the polar control retarder300, wherein the ray412, reflected by the reflective polariser302, passes back through the polar control retarder300and is then transmitted by the additional polariser318.

Advantageously, the polar distribution of light reflection provides high reflectivity at typical snooper locations by means of the privacy state of the polar control retarder300, while maintaining low reflectivity for an on-axis observer. Thus, in the privacy mode of operation, the reflectivity for off-axis viewing positions is increased, and the luminance for off-axis light from the SLM is reduced. As is described above, such increased reflectivity provides increased visual security level for the display in an ambiently illuminated environment.

Operation of a privacy display to a viewer and snooper will now be further described.

FIG.9Ais a schematic top view of the display device100illustrating variation in the viewing angle of a display device100across its width,FIG.9Bis a graph illustrating the variation of luminance with polar direction for the light output from a centre point of the display device100including the polar control retarder300ofFIG.5A; andFIG.9Cis a perspective view of a user45and the surface alignment layer of a prior art display device1.

The display100comprises a liquid crystal polar control retarder301, wherein the angle617of said in-plane component417of the alignment in the adjacent liquid crystal material414changes monotonically along the predetermined axis500across at least part of the display device as illustrated elsewhere herein.

In the alternative embodiment ofFIG.9Athe angle of said in-plane component417of the alignment in the adjacent liquid crystal material414changes monotonically along a predetermined axis across the entire display device100.

Directions as illustrated by light rays450R,450C,450L of maximum light transmission of the display polarizer218, the additional polariser318and said at least one polar control retarder300from points470R,470C470L of said at least part of the display device100are directed towards a common optical window26in front of the display device100. In operation, the optical window26in window plane197may be provided at a window distance Zw that is typically equal to or greater than the nominal viewing distance480. The optical window26may alternatively be referred to as an optical pupil, and the output of the display100may be described as being pupillated. Advantageously desirable uniformity variation across the display100may be provided for the user45.

Referring toFIG.9A, an on-axis observer45, positioned centrally with respect to the display100, is aligned with optical axis199that is normal to the display. Thus ray450C that is with a polar angle of zero degrees from a central location point470C is directed towards the primary user45in the normal direction.

A first ray450R propagating from point470R on the right side of the display100, propagates at a first angle453relative to a normal199to the display100towards the user45. For a snooper47, positioned at a distance away from the display100to the right side, for a second ray456propagating from point470L at the left edge of the display100, towards the snooper47, propagates at a second angle457relative to a normal199to the display100.

As depicted, the first angle453and second angle457may be comparatively similar in size.

Referring toFIG.9B, in a display device100with a large field of view angle460to the primary user45in which the angle of the in-plane component of the liquid crystal in the plane of the retarder is uniform across the display as illustrated inFIG.9C, the luminance463of the point470R on the display from which the ray450R emanates at angle453, for the user45, may be undesirably the same or similar to the luminance467for the snooper47for ray456at angle457from point470L.

As illustrated inFIG.9C, this further results in a luminance roll-off across the display100wherein the luminance is at its brightest in the centre of the display and reduces gradually towards a minimum at the edges of the display100. The change in luminance across the display100may be noticeable by an observer45which may not be desirable.

In comparison to the arrangement ofFIG.9C, it is generally desirable for the ray450R to be observable by the user45and the ray456to have low luminance and high reflectivity for the snooper47. Further it is desirable that the luminance across at least part of the display device100is provided with high uniformity to the user45at a nominal viewing distance480.

In the present embodiments the display100provides alignment of the adjacent liquid crystal material414with an in-plane component that is in the plane of the layer of liquid crystal material314and wherein the angle of said in-plane component of the alignment in the adjacent liquid crystal material changes monotonically along the predetermined axis across at least part of the display device100. A different angular dependent luminance is achieved for different points470C,470R,470L across at least part of the display. Accordingly, with an appropriate in-plane angle of the liquid crystal material314it may be possible for the first ray450R to have a luminance that is observable by the user45, and ray456, which travels at a similar angle457to the angle453of first ray450R, to have a reduced luminance which is not observable by the snooper47. This may be achieved across the entire width of the display100by the liquid crystal material314adjacent the two surface alignment layers419A,419B, having an in-plane component which has an angle which changes monotonically across an axis across at least part of the display device100.

As can be seen, the luminance of the of the centre point on the display decreases from a maximum at 0° lateral angle to a minimum at around a 45° lateral angle. Accordingly, an on-axis user will see a maximum luminance from the centre of the display device100, whereas a snooper, i.e. an off-axis observer, positioned at a lateral angle of around 45° will see a minimum luminance from the centre of the display device100. It may therefore be possible for the minimum luminance to be sufficiently small that the snooper cannot observe the display device100.

The polar output of an illustrative embodiment will now be described.

FIGS.10A-Dare graphs illustrating the variation of luminance with polar direction for the light output from a display device100ofFIG.1Ausing the plural retarders ofFIG.5Afor different points on the display device for the left-side of the centre of the display with the angle617L of in-plane component of the alignment of 90°, 85°, 80°, 70° respectively; andFIGS.10E-Hare graphs illustrating the variation of luminance with polar direction for the light output from a display device ofFIG.1Ausing the plural retarders ofFIG.5Afor different points on the display device for the right-side of the display with the angle617R of in-plane component of the alignment of 90°, 95°, 100°, 110° respectively.

The illustrative embodiments are provided for properties of TABLE 1.

TABLE 1Active LC retarderPassive retarder(s)AlignmentPretilt/Δn.d/Voltage/TypeΔn.d/nmlayersdegnmΔϵVCrossed+565 @ 45°Homogeneous275013.22.5A+565 @ 135°Homogeneous2

It would be further desirable to provide low reflectivity to the primary user45while providing high reflectivity to the snooper47.

FIGS.11A-Dare graphs illustrating the variation of reflectivity with polar direction for the light reflected from a display device ofFIG.1Ausing the plural retarders ofFIG.5Afor different points on the display device for the left-side of the centre of the display with the angle617L of in-plane component of the alignment of 90°, 85°, 80°, 70° respectively; andFIGS.11E-Hare graphs illustrating the variation of reflectivity with polar direction for the light output from a display device ofFIG.1Ausing the plural retarders ofFIG.5Afor different points on the display device for the right-side of the display with the angle617R of in-plane component of the alignment of 90°, 95°, 100°, 110° respectively.

Each graph shows the variation in luminance with polar direction from the axis199, for a given point on the display100. Each graph is labelled with the angle617of the alignment direction. Further the angle at which the luminance drops below a certain level, i.e. the angle at which a point is not visible for a snooper, varies across the width of the display. This therefore means that for a given position of a snooper, different parts of the display will have different luminance.

Each of the profiles further illustrates the subtended angular size of the display for a primary user45as shown by polar outline490. The profiles further illustrate the polar location492of the maximum luminance for zero elevation and the location494of the maximum luminance for the elevation of light at the top of the display device100as seen by the user45.

As the angle617varies from 90 degrees, the polar locations492,496of maximum luminance shift in polar direction. As shown in the embodiment ofFIG.3A, the angles617vary with spatial location, and thus the direction of peak luminance also varies with spatial position. Advantageously display luminance to the primary user45is increased. Further the reduction to the snooper is increased for points across at least part of the display. Further the uniformity of luminance reduction to the snooper47is increased. Visual security level of the display is increased.

FIGS.11A-Hillustrate the polar locations of the points490,492ofFIG.10A-H, and thus provide the reflectivity as seen by the primary user45. Advantageously reflectivity to the primary user may be reduced and reflectivity to the snooper may be increased.

In combination, the minimum visual security level, VSL as seen by the snooper47is increased for points across at least part of the display device100, and the uniformity of VSL may be increased.

Arrangements of alignment directions for planar and curve displays will now be described.

FIG.12Ais a perspective view of a user45in front of a planar display device100comprising a polar control retarder300comprising a liquid crystal retarder301comprising alignment layer417A. Features of the arrangement ofFIG.12Anot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

In the embodiments herein wherein only one of the two alignment layers419A,419B is illustrated then the other of the alignment layers that is not shown may be arranged with corresponding alignment directions, for example as illustrated inFIG.5A,FIG.5B,FIG.16,FIG.18A,FIG.19A.FIG.19CorFIG.20Bas illustrated elsewhere herein.

InFIG.12A, the display device100has a planar structure such that it is flat. For every point on the display100, an on-axis user45views each point at a different lateral and elevation angle as illustrated inFIG.9A. As seen above inFIGS.10A-HandFIGS.11A-H, these angles impact the luminance and reflectivity for an on-axis observer for each of the points.

Thus the different alignment directions417ALp,417ACp,417Arp with orientation angles617AL,617AC,617AR respectively are arranged to provide variations in the direction of maximum luminance across the angular size of the display as described elsewhere herein. Advantageously the varying alignment directions achieve increased uniformity across the display device100of both luminance and reflectivity as observed by the user45at viewing distance480. Visual security to off-axis snooper47is also increased.

In other words, in comparison to the arrangement ofFIG.9C,FIG.12Aillustrates that the uniformity is increased by means of varying the polar angle of the maximum luminance at different lateral locations across the display device100.

An alternative arrangement for a curved display100will now be described.

FIG.12Bis a perspective view of a user45in front of a curved display device100comprising a polar control retarder300comprising a liquid crystal retarder301comprising alignment layer417A. The display device100is curved about the y-axis and has a centre of curvature on the same side of the display device100as the user45. Features of the arrangement ofFIG.12Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

In the alternative ofFIG.12B, wherein the display100comprises an arrangement such as illustrated inFIG.1Athe additional polariser318A and the at least one polar control retarder300are curved with a concave curvature. The spatial light modulator48and backlight20is also curved with a concave curvature.

FIG.12Billustrates an alternative structure wherein the display device100is curved. In such a case, the additional polariser318, the at least one polar control retarder300and optionally the spatial light modulator48are curved with a concave curvature. The other components of the device100, as described above, may also be curved with a concave curvature.

The curved display device100may further improve the luminance for an on-axis observer of the display device as the lateral angle for individual points across the display may be reduced.

In this instance, the in-plane component of the liquid crystal material414adjacent the alignment layers419A,419B, may not need to be as large to create the desired effect. The alignment directions417ALp′,417ARp′ are different to the alignment directions417ALp,417ARp, the deviations of angles617AL′,617AR′ from 90 degrees (that is the direction orthogonal to the axis500) being reduced. Advantageously the uniformity of the display may be increased and visibility of the display to off-axis snoopers may be reduced.

FIG.13Ais a top view of a centre stack display for an automotive vehicle650. Display100comprises a first part101P that is provided with modified alignment directions best suited to passenger that is user45whereas second part101D is provided with modified alignment directions best suited to driver that is user47. In comparison to the embodiments illustrated elsewhere herein, the alternative ofFIG.13Aillustrates that the display100comprises first and second parts101D,101P.

In comparison to the embodiment ofFIG.9A, the left part101D of the display100provides an optical window26D in the region of the driver47and the right part101P of the display100provides an optical window26P in the region of the passenger45. Advantageously uniformity of luminance is increased for viewing of the part101D by the driver47and uniformity is increased for viewing of the part101P by the passenger45. Further uniformity of security factor is increased for viewing of the part101P by the driver47and uniformity of security factor is increased for viewing of the part101D by the passenger45.

In other embodiments (not shown), the first and second parts101P,101D may be interleaved, for example to provide a camouflage effect, for example with features that are a few millimetres or less in width. Advantageously increased obscuration of image data may be provided.

FIG.13Bis a top view of an alignment layer419A for the display100ofFIG.13Acomprising the arrangement ofFIG.1Aand further comprising first and second parts101P,101D. Features of the arrangement ofFIG.13Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

Considering one alignment layer417A of the pair of alignment layers419A,419B, the angle617DAL,617DAC,617DAR of said in-plane component417DALp,417DACp,417DARp of the alignment in the adjacent liquid crystal material414changes monotonically along the predetermined axis500across a first part10D of the display device100. The angle617PAL.617PAC,617PAR of said in-plane component417PALp,417PACp,417PARp of the alignment in the adjacent liquid crystal material404also changes monotonically along a predetermined axis across a second part of the display device.

Referring further toFIG.13A, the angle617D of said in-plane component of the alignment417Dp in the adjacent liquid crystal material414changes monotonically along the predetermined axis500across the first part101D of the display device100so that directions of maximum light transmission of the display polarizer218, the additional polariser318and said at least one polar control retarder300from points470DL,470DC,470DR of the first part of the display device100in are directed towards a first common optical window26D in front of the display device100, and the angle617P of said in-plane component417Pp of the alignment in the adjacent liquid crystal material414changes monotonically along the predetermined axis500across the second part101P of the display device100so that directions of maximum light transmission of the display polarizer218, the additional polariser318and said at least one polar control retarder300from points of the second part101P of the display device100are directed towards a second common optical window26P in front of the display device100different from the first common optical window26D.

FIG.13Billustrates alignment layer418A alignment directions417PALp,417PACp,417PARp with angles617PAL,617PAC,617PAR for the first part101P. Such an alignment advantageously achieves increased uniformity from the right side of the display100to the passenger user45.FIG.13Bfurther illustrates alignment layer418A alignment directions417DALp,417DACp,417DARp with angles617DAL,617DAC,617DAR for the second part101D. Such an alignment advantageously achieves increased uniformity from the right side of the display100to the driver user47.

Thus for the two surface alignment layers419A.419B disposed adjacent to the layer of liquid crystal material414and on opposite sides thereof, at least one of the surface alignment layers is arranged to provide alignment in the adjacent liquid crystal material with an in-plane component417that is in the plane of the layer314of liquid crystal material414, wherein the angle617of said in-plane component of the alignment in the adjacent liquid crystal material414changes monotonically along the predetermined axis across a first part101D of the display device100and changes monotonically along the predetermined axis across a second part101P of the display device100.

As shown inFIG.13B, in this example in each of the first part101D and the second part101P, the respective mean directions of the in-plane component of the alignment in the adjacent liquid crystal material are at non-zero acute angles to the electric vector transmission directions of the output polariser218, the reflective polariser302and the additional polariser318.

An alternative switchable privacy display for off-axis display will now be described.

FIG.13Cis a top view of an alternative centre stack display100for an automotive vehicle650comprising the display100ofFIG.1Awith the reflective polariser302omitted and the polar control retarder ofFIG.19A, hereinbelow. Features of the arrangement ofFIG.13Cnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

In comparison to the arrangement ofFIG.13A, in the alternative display ofFIG.13C, a central privacy display is provided for use by the passenger45and to provide image invisibility to the driver47. A single optical window26is provided for the passenger45. Advantageously image invisibility may be provided to the driver47while the passenger may view otherwise distracting content such as entertainment information.

FIG.13Dis a top view of an alignment layer419A for the display ofFIG.13C. Features of the arrangement ofFIG.13Dnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features. In comparison to the embodiment ofFIG.13B, the average angle617DAL,617DAC,617DAR across the display100is non-zero.

FIG.13Eis a schematic top view of the display device illustrating observation viewing angles450L,450C,450R of the display from points470L,470C,470R ofFIG.13C. Features of the arrangement ofFIG.13Enot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

The angles63L,63c,63R of maximum transmission are all inclined to axis199and are directed to the common optical window26that is off-axis.

FIGS.13F-Hare graphs illustrating the variation of luminance with polar direction for the light output from a display device ofFIG.13Cfrom different positions across the display100. Advantageously uniform luminance may be provided to the passenger45at the window26and uniform security factor may be provided to the driver47, reducing visibility of distracting image data.

Arrangements of alignment directions for tiled displays will now be described.

FIG.14AandFIG.14Bare top and perspective views, respectively, of a pair of tiled display devices100that are angled with respect to each other such that the user45is aligned to the optical axis199A,199B for the centre of each respective display device100A,100B. Features of the arrangements ofFIGS.14A-Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

The direction of ray450LA from display100A is parallel to ray450RB from display100B. The arrangement of alignment directions417RA,417CA,417LA may be the same as the arrangement417RB,417CB,417LB. Advantageously the same alignment may be used for both displays and cost is reduced.

FIG.15AandFIG.15Bare top and perspective views, respectively, of a pair of tiled display devices100that are in-plane with respect to each other.

In comparison to the arrangement ofFIGS.15A-Bdifferent arrangements of alignment layers419are used for display devices100A,100B.

As is apparent fromFIG.15A, a user45may no longer be arranged on-axis with the centre of each display. Instead, the user may, approximately, be positioned on-axis with an edge of each display. Accordingly, as shown inFIG.15B, at least the alignment layer419B may be arranged to provide alignment in the adjacent liquid crystal material414which differs to that for a display100which is viewed with a user on-axis with the centre of the display100, for example as shown inFIG.3A. The alignment in this arrangement of the display100may be arranged such that the luminance appears uniform for points across at least part of the display, despite the observer45not being arranged on-axis with the centre of the display100.

A further illustrative embodiment will now be described.

FIG.16is a perspective view of a polar control retarder300that may be applied in the display device100seen inFIG.1A. The polar control retarder300comprises a homogeneously aligned switchable LC retarder301. This is shown by the aligned directions417A,417B on the electrodes413,415. The polar control retarder300further comprises a passive polar control retarder provided by a negative C-plate retarder330. The negative C-plate has an optical axis perpendicular to the plane of the retarder330, illustrated schematically by the orientation of the discotic material430.

Features of the arrangement ofFIG.16not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

FIGS.17A-Care graphs illustrating the variation of luminance with polar direction for the light output from a display device ofFIG.1Ausing the plural retarders ofFIG.16for different points on the display device with the angle of in-plane component of the alignment of 80°, 90°, 100° respectively; andFIGS.17D-Fare graphs illustrating the variation of reflectivity with polar direction for the light output from a display device ofFIG.1Ausing the plural retarders ofFIG.16for different points on the display device with the angle of in-plane component of the alignment of 80°, 90°, 100° respectively.

The profiles ofFIGS.17A-Fare provided for the properties of TABLE 2.

TABLE 2Active LC retarderPassive retarder(s)AlignmentPretilt/Δn.d/Voltage/TypeΔn.d/nmlayersdegnmΔϵVC-plate−700Homogeneous275013.22.3Homogeneous2

Advantageously thickness and cost may be reduced in comparison to the embodiment ofFIG.5A.

FIG.18Ais perspective views of polar control retarders300that may be applied inFIG.1, comprising a homeotropically aligned switchable LC retarder and negative C-plate retarder in privacy mode of operation; andFIG.18Bis a graph of the LC director angle407of the homeotropically aligned switchable LC414inFIGS.17D and17Ethrough its thickness, shown as the fraction440through LC retarder301. The polar control retarder300comprises a homeotropically aligned switchable LC retarder301and negative C-plate retarder330. Features of the arrangement ofFIG.18Anot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

In comparison to the homogeneous alignment ofFIGS.5A-B,FIG.18Aillustrates that the homeotropically aligned switchable LC retarder301has two surface alignment layers419A,419B, each having alignment directions417AL,417AC,417AR,417BL,417BC,417BR which provide homeotropic alignment in the adjacent liquid crystal material414with a pre-tilt providing the in-plane component, albeit that these in-plane components417ALp,417ACp,417ARp and417BLp,417BCp,417BRp are relatively small in magnitude, and the pretilt components417ALz,417ACz,417ARz and417BLz,417BCz,417BRz that are out of the plane are relatively large in magnitude.

FIG.18Billustrates the LC director angle407shown by profile444when a drive voltage is applied, and profile442illustrates the LC director angle407when no voltage is applied. This graph illustrates how when a drive voltage is applied the liquid crystal material414has a varying director angle407through the thickness of the layer314and when no voltage is applied, the liquid crystal material414has a constant LC director angle of close to but not exactly 90°.

In comparison to the homogeneous liquid crystal embodiments ofFIG.5AandFIG.16, the liquid crystal material414may provide wide viewing angle with no applied voltage. The homeotropic alignment at the alignment layers419may be arranged with a small pretilt as illustrated by profile442, such that there remains a preferred alignment direction617arrangement within the layer314, and thus the molecules414are provided with a preferred orientation within the layer that varies in the predetermined direction500.

FIG.19Ais perspective views of polar control retarders300that may be applied inFIG.1, comprising a homeotropically and homogeneously aligned switchable LC retarder301and a negative C-plate retarder and wherein the alignment direction of the homeotropic alignment layer419A is common across the lateral direction and wherein the alignment direction of the homogeneous alignment layer419B varies across the lateral direction. Features of the arrangements ofFIG.19Anot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features. An exemplary embodiment is illustrated in TABLE 6.

TABLE 6Passive polarActive LC retardercontrol retarder(s)AlignmentModeTypeΔn.d/nmlayersPretilt/degΔn.d/nmΔϵVoltage/VPublicNegative−1100Homogeneous21300+4.315.0PrivacyCHomeotropic882.8

In the alternative embodiment ofFIG.19A, one of the surface alignment layers419B is arranged to provide homogeneous alignment in the adjacent liquid crystal material414, wherein the angle617BL,617BC,617BR of said in-plane component of the alignment in the liquid crystal material414adjacent to said one of the surface alignment layers419B changes monotonically along the predetermined axis500across at least part of the display device100, and the other of the surface alignment layers419A is arranged to provide homeotropic alignment in the adjacent liquid crystal material414, wherein the angle617A of said in-plane component of the alignment in the liquid crystal material adjacent to said other of the surface alignment layers619A does not change along the predetermined axis500across at least part of the display device100.

When the surface alignment layer419aarranged to provide homeotropic alignment is between the layer314of liquid crystal material414and the polar control retarder330, the liquid crystal retarder301has a retardance for light of a wavelength of 550 nm in a range from 500 nm to 1800 nm, preferably in a range from 700 nm to 1500 nm and most preferably in a range from 900 nm to 1350 nm. The polar control retarder300may further comprise a passive polar control retarder330having its optical axis perpendicular to the plane of the retarder330, the passive polar control retarder330having a retardance for light of a wavelength of 550 nm in a range from −300 nm to −1600 nm, preferably in a range from −500 nm to −1300 nm and most preferably in a range from −700 nm to −1150 nm; or the retarder330may further comprise a pair of passive retarders which have optical axes in the plane of the retarders that are crossed, each retarder of the pair of retarders having a retardance for light of a wavelength of 550 nm in a range from 400 nm to 1600 nm, preferably in a range from 600 nm to 1400 nm and most preferably in a range from 800 nm to 1300 nm.

FIG.19Aillustrates that the aligned switchable LC retarder301has a surface alignment layer419A, having alignment direction417A which provides homogeneous alignment in the adjacent liquid crystal material414and having alignment direction417B which provides homeotropic alignment in the adjacent liquid crystal material414with a pre-tilt providing the in-plane component.FIG.19Afurther illustrates that the homeotropic alignment layer419A is arranged between the passive retarder330and the liquid crystal retarder301.

In comparison to the embodiments ofFIGS.5A-BandFIG.18A, the homeotropic alignment layer has an alignment direction that is common in the direction parallel to the axis500. Further, as the components417ALp,417ACp and417ARp are small, then the effect of the respective alignment layers417ALp,417BLp and417Arp,417BRp not being parallel or anti-parallel is small. The pupillation of the output, that is redirection of the direction of maximum transmission is thus mostly attributed to the homogeneous alignment layer and desirable pupillation of output is achieved. Advantageously the alignment layer417A has reduced cost and complexity.

FIG.19Bis a graph of the LC director angle of the homeotropically and homogeneously aligned switchable LC inFIG.19Athrough its thickness440.

FIG.19Billustrates the LC director angle407shown by profile444when a drive voltage is applied, and profile442illustrates the LC director angle407when no voltage is applied. This graph illustrates how when a drive voltage is applied the liquid crystal material414has a non-linear variation of director angle407through the thickness of the layer314and when no voltage is applied, the liquid crystal material414has a constantly varying LC director angle.

In comparison to the arrangement ofFIG.18A, the arrangement ofFIG.19Amay provide increased polar area for desirable visual security level. Advantageously privacy performance and uniformity may be improved for off-axis snoopers.

FIG.19Cis perspective views of polar control retarders300that may be applied inFIG.1, comprising a homeotropically and homogeneously aligned switchable LC retarder301arranged between negative C-plate retarders330A,330B and wherein the alignment directions of the homogeneous alignment layer and homeotropic alignment layer each varies across the lateral direction. Features of the arrangement ofFIG.19Cnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

FIG.19Cillustrates an alternative toFIG.19Awherein the alignment layers417A,417B may be formed on passive retarders330A,330B, advantageously achieving reduced cost and thickness.

FIG.19Cillustrates another alternative toFIG.19Awherein both alignment layers417A,417B have in-plane orientations that vary monotonically along the direction of the axis500. Advantageously increased transmission efficiency may be achieved for off-centre locations of the display.

Stacked plural retarders and additional polarisers comprising alignment layers with alignment that changes monotonically will now be described.

FIG.20Ais a side perspective view of a display device100that is modified compared to the display device100ofFIG.1A; andFIG.20Bis s a perspective view of two sets of polar control retarders300,300A that may be applied inFIG.1, each comprising a homogeneously aligned switchable LC retarder301,301A and negative C-plate retarder330,330A. Features of the arrangements ofFIGS.20A-Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, particularly with reference toFIG.1, including any potential variations in the features.

In comparison to the arrangement ofFIG.1A, the display device100comprises a further additional polariser318B arranged on the input side of the input polariser210which is arranged on the input side of the SLM48. As mentioned previously, the input polariser210is a linear polariser.

The polar control retarders300ofFIG.1may be provided by respective polar control retarders300A ofFIG.20A.

The display device100thus comprises polar control retarder300A which is arranged between the output polariser218(and reflective polariser302) and the additional polariser318A.

The at least one polar control retarder300A includes a liquid crystal retarder301A which comprises a layer of liquid crystal material314A and two surface alignment layers419AA,419AB disposed adjacent to the layer of liquid crystal material314A and on opposite sides thereof. At least one of the surface alignment layers419AA,419AB is arranged to provide alignment in the adjacent liquid crystal material314A with an in-plane component that is in the plane of the layer of liquid crystal material314A. The angle617AA of said in-plane component of the alignment in the adjacent liquid crystal material314A changes monotonically along the predetermined axis500across at least part of the display device100. This is illustrated inFIG.20Aby the arrows417ALp,417ACp,417ARp which show the in-plane component of the alignment layer419AA, which determines the in-plane component of the alignment in the adjacent liquid crystal material314A.

The display device100also comprises at least one further polar control retarder300B which is arranged between the input polariser210and the further additional polariser318B. The at least one further polar control retarder300B includes a liquid crystal retarder301B which comprises a layer of liquid crystal material314B and two surface alignment layers419BA,419BB disposed adjacent to the layer of liquid crystal material314B and on opposite sides thereof. At least one of the surface alignment layers419BA,419BB is arranged to provide alignment in the adjacent liquid crystal material314B with an in-plane component that is in the plane of the layer of liquid crystal material314B. The angle617BA of said in-plane component417BA of the alignment in the adjacent liquid crystal material314B changes monotonically along the predetermined axis across at least part of the display device100. This is illustrated inFIG.20Aby the arrows417BLp,417BCp,417BRp which show the in-plane component of the alignment layer419BA, which determines the in-plane component of the alignment in the adjacent liquid crystal material314B. As mentioned above, the use of two polar control retarders300A,300B may make it possible to further reduce the luminance for off-axis snoopers through the multiplicative effect of the reduction in luminance achieved by each of the polar control retarders300,300A.

The polar control retarder300A,300B are identical to the polar control retarder300seen inFIG.19A. As depicted, it may be possible to apply a voltage V to the polar control retarder300A and a voltage VAto the further polar control retarder300A. The polar control retarder300A,300B may have the same or different prescription for properties of retarders301A,330A and301B,330B. Different prescriptions may provide control of different polar viewing angles. Advantageously uniformity of privacy image to the primary user and uniformity of visual security to off-axis snoopers may be increased.

In alternative embodiments ofFIGS.20A-B, both polar control retarders300A,300B may be arranged on the input side of a transmissive spatial light modulator48or both polar control retarders300A,300B may be arranged on the output side of a transmissive spatial light modulator48or on the output side of an emissive spatial light modulator48.

FIG.21is a side view of an apparatus600for manufacturing a surface alignment layer419, for example the alignment layer419A,419B described above, having a varying angle of alignment across its area. The apparatus600comprises a rotating roller602configured to translate a substrate604with an alignment layer419thereon. The apparatus600further comprises a light source606such as an ultra-violet light source arranged to provide a beam of light607towards the alignment layer419. Arranged in the optical path of the light source606is a polariser608and a mask610. The mask610serves to only allow the light beam607to be incident on a desired portion605of the alignment layer419. The apparatus further comprises a controller612operatively connected to a means610to rotate the polarisation603from the polariser608and the roller602. The polariser608may be a linear polariser that is physically rotated or may comprise a fixed linear polariser and a controllable retarder.

Operation of the apparatus600will now be described. In use, the roller602may translate the substrate604, and hence the alignment layer419, relative to the light source606and mask610such that the light beam607can be incident on different portions of the alignment layer419that may be a photoalignment layer. As the roller602moves the substrate604, the polariser608may simultaneously be controlled such that the beam of light607incident on the alignment layer419has a desired polarisation direction603. The portion of the alignment layer419illuminated by the beam of light may align based on the polarisation of the incident beam of light607. The beam of light607may also cure the alignment layer419. Through control of the polariser608orientation to change the polarisation direction603of the beam of light607as the roller602moves the substrate604, it may be possible to illuminate the alignment layer419with different polarisation directions603at different portions of the alignment layer419, and thus change the angle of alignment along the alignment layer419. Through appropriate control, it may be possible to achieve a monotonically varying in-plane angle of alignment of material in the alignment layer419. Whilst a roller602is described above, any other suitable means may be provided for causing relative movement between the beam of light607and the alignment layer419. For example, the light source606may be movably mounted to scan the beam of light607over the surface layer419.

FIG.22is front view illustrating the mask610, of the apparatus600, seen inFIG.21over different portions605,605R of the alignment layer419, and the polarisation direction603,603R of the beam of light607at those corresponding portions. Double ended arrows419,419R illustrate the direction of alignment in the alignment layer419with the light607, at the different portions of the alignment layer419. As shown inFIG.22by the arrows417,417R, the material of the alignment layer aligns at 90°, i.e. perpendicular, to the polarisation direction603,603R of the incident light. The beam of light607may also be directed at an angle to the plane of the alignment layer419in order to achieve a pre-tilt of the material in the alignment layer419.

FIG.23Ais a schematic diagram illustrating in perspective view illumination of a polar control retarder630layer by off-axis light. Polar control retarder630may comprise birefringent material, represented by refractive index ellipsoid632with optical axis direction634at 0 degrees to the x-axis, and have a thickness631. The polar control retarder630is representative of any of the passive or switchable retarders described above having a homogeneous alignment. Features of the arrangements ofFIGS.23A-25Ebelow that are not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

Normal light rays636propagate so that the path length in the material is the same as the thickness631. Light rays637are in the y-z plane have an increased path length; however the retardance of the material is substantially the same as the rays636. By way of comparison light rays638that are in the x-z plane have an increased path length in the birefringent material and further the retardance is different to the normal ray636.

The retardance of the polar control retarder630is thus dependent on the angle of incidence of the respective ray, and also the plane of incidence, that is rays638in the x-z will have a retardance different from the normal rays636and the rays637in the y-z plane.

The interaction of polarized light with the polar control retarder630will now be described. To distinguish from the first and second polarization components during operation in a directional backlight20, the following explanation will refer to third and fourth polarization components.

FIG.23Bis a schematic diagram illustrating in perspective view illumination of a polar control retarder layer by off-axis light of a third linear polarization state at 90 degrees to the x-axis andFIG.23Cis a schematic diagram illustrating in perspective view illumination of a polar control retarder layer by off-axis light of a fourth linear polarization state at 0 degrees to the x-axis. In such arrangements, the incident linear polarization states are aligned to the optical axes of the birefringent material, represented by ellipse632. Consequently, no phase difference between the third and fourth orthogonal polarization components is provided, and there is no resultant change of the polarization state of the linearly polarized input for each ray636,637,638. Thus, the polar control retarder630introduces no phase shift to polarisation components of light passed by the polariser on the input side of the polar control retarder630along an axis along a normal to the plane of the polar control retarder630. Accordingly, the polar control retarder630, and the polarisers (not shown) on each side of the polar control retarder630, do not affect the luminance of light passing therethrough. AlthoughFIGS.23A-Crelate specifically to a polar control retarder630that which is passive, a similar effect is achieved by the polar control retarders in the devices described above.

FIG.23Dis a schematic diagram illustrating in perspective view illumination of a polar control retarder630layer by off-axis light of a linear polarization state at 45 degrees. The linear polarization state may be resolved into third and fourth polarization components that are respectively orthogonal and parallel to optical axis634direction. The polar control retarder thickness631and material retardance represented by refractive index ellipsoid632may provide a net effect of relatively shifting the phase of the third and fourth polarization components incident thereon in a normal direction represented by ray636by half a wavelength, for a design wavelength. The design wavelength may for example be in the range of 500 to 550 nm.

At the design wavelength and for light propagating normally along ray636then the output polarization may be rotated by 90 degrees to a linear polarization state640at −45 degrees. Light propagating along ray637may see a phase difference that is similar but not identical to the phase difference along ray636due to the change in thickness, and thus an elliptical polarization state639may be output which may have a major axis similar to the linear polarization axis of the output light for ray636.

By way of contrast, the phase difference for the incident linear polarization state along ray638may be significantly different, in particular a lower phase difference may be provided. Such phase difference may provide an output polarization state644that is substantially circular at a given inclination angle642. Thus, the polar control retarder630introduces a phase shift to polarisation components of light passed by the polariser on the input side of the polar control retarder630along an axis corresponding to ray638that is inclined to a normal to the plane of the polar control retarder630. AlthoughFIG.23Drelates to the polar control retarder630that is passive, a similar effect is achieved by the polar control retarders described above, in a switchable state of the switchable liquid crystal polar control retarder corresponding to the privacy mode.

To illustrate the off-axis behaviour of polar control retarder stacks, the angular luminance control of C-plates for example C-plate330, between the additional polariser318and the output display polariser218, or C-plate330A, will now be described for various off-axis illumination arrangements with reference to the operation of a C-plate between the parallel polarisers500,210will now be described.

FIG.24Ais a schematic diagram illustrating in perspective view illumination of a C-plate layer by off-axis polarised light638with a positive elevation. Incident linear polarisation component704, achieved by polariser500with electric vector transmission direction501, is incident onto the birefringent material561of the polar control retarder560that is a C-plate with optical axis direction507that is perpendicular to the plane of the polar control retarder560. Polarisation component704sees no net phase difference on transmission through the liquid crystal molecule and so the output polarisation component is the same as component704. Thus a maximum transmission is seen through the polariser210as the polarisation direction is parallel to the electric vector transmission direction211. The same is also true for the incident light636which travels at a zero lateral and zero elevation angle. Thus the polar control retarder560has an optical axis507perpendicular to the plane of the polar control retarder560, that is the x-y plane. The polar control retarder560having an optical axis507perpendicular to the plane of the polar control retarder comprises a C-plate.

FIG.24Bis a schematic diagram illustrating in perspective view illumination of a C-plate layer by off-axis polarised light with a negative lateral angle. As with the arrangement ofFIG.24A, polarisation state704sees no net phase difference and is transmitted with maximum luminance. Thus, the polar control retarder560introduces no phase shift to polarisation components of light passed by the polariser on the input side of the polar control retarder560along an axis along a normal to the plane of the polar control retarder560. Accordingly, the polar control retarder560does not affect the luminance of light passing through the polar control retarder560and polarisers500,210on each side of the polar control retarder560.

FIG.24Cis a schematic diagram illustrating in perspective view illumination of a C-plate layer by off-axis polarised light with a positive elevation and negative lateral angle. In comparison to the arrangement ofFIGS.24A-B, the polarisation state704resolves onto eigenstates703,705with respect to the birefringent material561providing a net phase difference on transmission through the polar control retarder560. The resultant elliptical polarisation component656is transmitted through polariser210with reduced luminance in comparison to the rays illustrated inFIGS.24A-B. AlthoughFIGS.24A-Crelate specifically to the polar control retarder560that is passive, a similar effect is achieved by the polar control retarders in the devices described above.

FIG.24Dis a schematic diagram illustrating in perspective view illumination of a C-plate layer by off-axis polarised light with a positive elevation and positive lateral angle. In a similar manner toFIG.24C, the polarisation component704is resolved into eigenstates703,705that undergo a net phase difference, and elliptical polarisation component660is provided, which after transmission through the polariser reduces the luminance of the respective off-axis ray. Thus, the polar control retarder560introduces a phase shift to polarisation components of light passed by the polariser on the input side of the polar control retarder560along an axis that is inclined to a normal to the plane of the polar control retarder560. AlthoughFIG.24Drelates to the polar control retarder560that is passive, a similar effect is achieved by the polar control retarders described above, in a switchable state of the switchable liquid crystal polar control retarder corresponding to the privacy mode.

FIG.24Eis a schematic graph illustrating the variation of output transmission with polar direction for transmitted light rays inFIGS.24A-D. Thus, the C-plate may provide luminance reduction in polar quadrants. In combination with switchable liquid crystal retarder301described elsewhere herein, this may achieve: (i) removal of luminance reduction of the C-plate in a first wide angle state of operation and (ii) extended polar region for luminance reduction may be achieved in a second privacy state of operation.

To illustrate the off-axis behaviour of polar control retarder stacks, the angular luminance control of crossed A-plates308A,308B between an additional polariser318and output display polariser218will now be described for various off-axis illumination arrangements.

FIG.25Ais a schematic diagram illustrating in perspective view illumination of crossed A-plate retarder layers by off-axis polarised light with a positive elevation. Linear polariser218with electric vector transmission direction219is used to provide a linear polarisation state704that is parallel to the lateral direction onto first A-plate308A of the crossed A-plates308A,308B. The first A-plate308A may comprise birefringent material, represented by refractive index ellipsoid408with optical axis direction309A is inclined at +45 degrees to the lateral direction. The first A-plate308A has a first thickness531. The retardance of the polar control retarder308A for the off-axis angle θ1in the positive elevation direction provides a resultant polarisation component650that is generally elliptical on output. Polarisation component650is incident onto the second A-plate308B of the crossed A-plates308A,308B. The second A-plate308A may comprise birefringent material, represented by refractive index ellipsoid410with has an optical axis direction309B that is orthogonal to the optical axis direction309A of the first A-plate330A. The second A-plate308B has a second thickness533. The first thickness531and second thickness533may be the same or different. In the plane of incidence ofFIG.25A, the retardance of the second A-plate308B for the off-axis angle θ1is equal and opposite to the retardance of the first A-plate308A. Thus a net zero retardation is provided for the incident polarisation component704and the output polarisation component is the same as the input polarisation component704. The same is also true for ray636which has zero lateral and zero elevation angle.

The output polarisation component is aligned to the electric vector transmission direction of the additional polariser318, and thus is transmitted efficiently. Advantageously substantially no losses are provided for light rays that have zero lateral angle angular component so that full transmission efficiency is achieved.

FIG.25Bis a schematic diagram illustrating in perspective view illumination of crossed A-plate retarder layers308A,308B, seen inFIG.25A, by off-axis polarised light with a negative lateral angle. Thus the linear input polarisation component704is converted by the first A-plate308A to an intermediate polarisation component652that is generally an elliptical polarisation state. The second A-plate308B again provides an equal and opposite retardation to the first A-plate308A so that the output polarisation component704is the same as the input polarisation component704and light is efficiently transmitted through the polariser318.

Thus the polar control retarder comprises a pair of retarders308A,308B which have optical axes309A,309B in the plane of the retarders330A,330B that are crossed, that is the x-y plane in the present embodiments. The pair of retarders308A,308B have optical axes309A,309B that each extend at 45° with respect to an electric vector transmission direction that is parallel to the electric vector transmission of the polariser318.

Advantageously substantially no losses are provided for light rays that have zero elevation angular component so that full transmission efficiency is achieved.

FIG.25Cis a schematic diagram illustrating in perspective view illumination of crossed A-plate retarder layers by off-axis polarised light with a positive elevation and negative lateral angle. Polarisation component704is converted to an elliptical polarisation component654by first A-plate308A. A resultant elliptical component656is output from the second A-plate308B. Elliptical component656is analysed by input polariser318with reduced luminance in comparison to the input luminance of the first polarisation component704.

FIG.25Dis a schematic diagram illustrating in perspective view illumination of crossed A-plate retarder layers308A,308B by off-axis polarised light with a positive elevation and positive lateral angle. Polarisation components658and660are provided by first and second A-plates308A,308B as net retardance of the first and second A-plates308A,308B does not provide compensation.

Thus luminance is reduced for light rays that have non-zero lateral angle and non-zero elevation components. Advantageously display privacy can be increased for snoopers that are arranged in viewing quadrants while luminous efficiency for primary display users is not substantially reduced.

FIG.25Eis a schematic graph illustrating the variation of output transmission with polar direction for transmitted light rays inFIGS.25A-D. In comparison to the arrangement ofFIG.24E, the area of luminance reduction is increased for off-axis viewing. However, the switchable liquid crystal layer314may provide reduced uniformity in comparison to the C-plate arrangements for off-axis viewing in the first public mode state of operation.

While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of this disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages.

Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the embodiment(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any embodiment(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the embodiment(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple embodiments may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the embodiment(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.