Patent ID: 12259608

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·Δ⁢n·d/λ0eqn.1

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

Δ⁢n=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-plates, i.e. C-plates 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

Δ⁢n·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 colour 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 super-twisted 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 I 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 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:

Δ⁢ε=(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. Pat. Nos. 11,092,851, 10,948,648, and WIPO Publ. No. 2019-055755 (U.S. Pat. No. 11,099,433), 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. Pat. No. 10,976,578 and in U.S. Pat. No. 10,802,356, 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. Pat. No. 11,327,358, 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.

A switchable privacy display such as for a vehicle will now be described.

FIG.1Ais a schematic diagram illustrating a schematic top view of a privacy display device100for use by a passenger in a vehicle. Vehicles may include the automotive vehicle650ofFIG.1Aor trains, boats, and airplanes for example.

Display device100is arranged in a passenger information display (PID) location, (on the right-hand side of the vehicle for Left-Hand Drive), with light rays445,447output to the user45and user47respectively. In right-hand drive vehicles, the directions of light deflection referred to hereinbelow are typically reflected about a vertical axis, that is the lateral direction is reversed.

In a first mode of operation that is the privacy mode the display device100is arranged for viewing by the front passenger45near to an on-axis199location, and to inhibit viewing by the driver47. In operation in privacy mode, the light rays447may be directed towards a common off-axis point427in front of the display device100. In privacy mode, the light rays447may represent the direction for minimum luminance from each point on the display device100.

In the present disclosure, pupillation refers to the optical output of the display providing at least one common point such as off-axis point427from which rays from each at least part of the display device100are directed with substantially similar transmission, or luminance. An observer at a pupil such as point427may see a substantially uniform luminance or transmission from across the at least part of the display device100. As will be described hereinbelow, pupillation of various components in the display100may advantageously achieve increased luminance uniformity and increased uniformity of security factor.

It is desirable that the passenger45may view information such as entertainment without the image causing distraction to the driver47, that is the privacy mode refers to a low driver distraction mode. This mode is in comparison with a mode in which the passenger display turns off when the vehicle is in motion to prevent driver distraction. More specifically to minimise the visibility to the driver47of distracting images at both the nominal driver position and when the driver leans across towards the display while driving, it is desirable to maximise the security factor S at angles α from the optical axis199of greater than 30° and preferably greater than 25° in the direction from the optical axis199towards the driver47. Further it is desirable to achieve a high security factor. S for polar angles at least at angles β from the optical axis199.

Optional observer position location viewer tracking system200may comprise a camera with collection cone angle201and is provided to detect the location of at least the driver47during driver47movement540. The observer tracking system may optionally comprise more than one camera, for example one to detect the driver47and one to detect the passenger45. Viewer tracking system200may further comprise for example an image processing means to determine the location of an observer in a collected image. The image processing means may optionally detect the gaze direction of the driver47or passenger45as well as the head location. An additional function where the display brightness is reduced or turned off when the gaze is directed towards the display may be provided.

Further in a low stray light function of the privacy mode, it may be desirable to provide an image to the passenger45with desirable luminance while reducing the luminance to reflecting and scattering surfaces within the vehicle. Advantageously the brightness of internal surfaces of the vehicle650may be reduced during night-time operation, reducing driver distraction. Further, increased area displays may be provided while maintaining desirably low levels of stray illumination within the vehicle650cabin.

In a second mode that is the share mode, the display device100is arranged for viewing by driver47in an off-axis location. Such use may be for occasions when viewing the display content is safe such as when the vehicle is stationary, or the content is appropriate such as map or instrument data.

Further it would be desirable to achieve high image uniformity to the passenger45and high uniformity of security factor, S to the driver47.

FIG.1Bis a schematic diagram illustrating a schematic top view of a privacy display device100for use in off-axis use applications. Features of the embodiment ofFIG.1Bnot 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 present disclosure refers commonly to passenger45or driver47for an automotive vehicle650ofFIG.1A. The present embodiments hereinbelow are suited to applications with a preferred side on which privacy may be provided. The display devices100herein may be used in other applications, in which case user47or snooper47and user45may be equally substituted.

The display device100may be provided for applications including but not limited to point-of-sale, or medical monitors. Considering the alternative embodiment ofFIG.1B, such a display device100may be viewed on the right side by a user49who is a customer or medical practitioner, centrally by a user45who is a customer or patient and shielded from potential viewers47who may be other members of the public on the opposite side of the optical axis199to the user49.

In a share mode of operation the display100may be used to provide advertising information over a wide polar region, or to provide medical information to multiple display users for example. As described hereinbelow, the embodiments ofFIGS.23A-Cfor example may be used to provide switching of the side of the display on which the user47is located.

The embodiment ofFIG.1Bmay be provided with location viewer tracking system200and optionally or alternatively may comprise manual actuator202to provide user input indicating a direction of minimum light transmission of a view angle control arrangement310as will be described further hereinbelow.

An illustrative structure that can achieve the desirable characteristics of the display device100ofFIG.1Awill now be described.

FIG.2Ais a schematic side perspective view of a display device100providing uniformity in reduction of luminance in directions; andFIG.2Bis a schematic side view of an optical stacking for the display device ofFIG.2A. Features of the arrangement ofFIG.2Anot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals, including any potential variations in the features

FIG.2Aillustrates 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 SLM48. The input polariser210and the output polariser218are each linear polarisers.

The display device100also comprises an additional polariser318arranged on the input side of the input polariser210, that is the same side as the input polariser210. The additional polariser318is a linear polariser. Typical polarisers210,218,318may be polarisers such as dichroic polarisers.

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 SLM48.

The display device100also comprises at least one polar control retarder300which comprises a switchable liquid crystal retarder301comprising a layer314of liquid crystal material, and substrates312,316arranged between the input polariser210and the additional polariser318. The substrates312,316of the switchable liquid crystal retarder301respectively comprise electrodes320A,320B 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 electrodes320A,320B of the switchable liquid crystal retarder301.

Electrodes320A,320B are connected to control system500by means of at least one electrical bus bar contact321and voltage driver system352. Electrodes320A,320B are arranged to provide switching voltage across the layer314of liquid crystal material414. Voltage is applied with voltage profile322that varies across the lateral direction of the liquid crystal layer314as will be described further hereinbelow.

In typical use, for switching between a share mode and a privacy mode, the layer314of liquid crystal material414is switchable between two states. The first state being a share mode so that the display device100may 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 electrodes320A,320B.

In the embodiment ofFIG.2A, the display device100further comprises a backlight20arranged to illuminate the SLM48. The SLM48comprises a transmissive SLM48arranged to receive and spatially modulate the output light from the backlight20, the display device100further comprises a backlight20arranged to illuminate the SLM48, and said display polariser210is an input display polariser210arranged on the input side of the SLM48.

Backlight20may be arranged to illuminate the SLM48, thereby providing a transmissive SLM48and may comprise input light sources15A,15B, waveguides1A,1B, rear reflector3and optical stack5comprising diffusers, light turning films and other known optical backlight structures. Plural light sources15A,15B are shown by way of non-limitative example, but in general there may any number of one or more light sources15A,15B.

In operation for illumination of passenger45, most generally light is directed along ray bundle445output from the display device100. Light rays444A from the first light sources15A are directed into light cones460L,460C,460R after transmission at facets53A and reflection at facets53B of light turning film component50and after transmission through the polar control retarder300that may be arranged in a privacy or share mode of operation.

In operation for illumination of driver47, most generally light is directed along ray bundle447output from the display device100. Light rays444B from the second light sources15B are directed into light cones462L,462C,462R after transmission at facets53B and reflection at facets53A of light turning film component50and after transmission through the polar control retarder300that is typically arranged in a share mode of operation.

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. Advantageously image uniformity may be increased.

In the alternative embodiment ofFIG.2B, an optical stacking for the arrangement ofFIG.2Ais illustrated. Air gap57is provided between spatial light modulator48and substrate316of the polar control retarder300. Advantageously yield may be improved in manufacture. In an alternative embodiment the substrate316may be bonded to the polariser210so that reflection losses are advantageously reduced.

Light turning film50may be bonded to the additional polariser318by means of adhesive layer51. In operation, reflections from the top surface of the light turning film50are reduced in comparison to arrangements where the adhesive layer51is replaced by an air gap. Such an arrangement achieves reduced stray light directed into regions between the driver47and passenger45in the illustrative arrangement ofFIG.1Afor example. Advantageously distraction to driver47may be reduced.

Polar control retarder300may comprise liquid crystal material414arranged between alignment layer317A,317B arranged on transparent electrodes320A,320B and arranged to provide alignment of liquid crystal material414. Bus bar323may be provided on at least one edge of the electrode320B to achieve substantially uniform electrical connectivity to the transparent electrode320B. The electrodes320A and320B may have different sheet resistance (ohms per square) conductors (such as ITO or silver nanowire). The higher sheet resistance and edge bus bars321L,321R may be used to provide a larger range profile322. The opposing electrode320B for example may have a lower sheet resistance in order to provide a sufficiently uniform voltage across it without using full bus bars323. In the present embodiments, the electrodes321,323may be provided on different ones of either of the rearmost and forwardmost electrode320A,320B.

FIG.2Cis a front view of the stack of some of the layers of the display device100ofFIG.2A. Features of the arrangement ofFIG.2Cnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals, including any potential variations in the features. InFIG.2Cand 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.

The surface alignment layer419B closest to the display polariser210has a pretilt having a pretilt direction417Bp with a component in the plane of the layer314of liquid crystal material414that is parallel or perpendicular to an electric vector transmission direction211of the display polarisers210,218, and the surface alignment layer419A closest to the additional polariser318has a pretilt having a pretilt direction417Ap with a component in the plane of the layer314of liquid crystal material414that is parallel or perpendicular to an electric vector transmission direction319of the additional polariser318. In the illustrative embodiment ofFIG.2C, the pretilt direction417Ap is parallel to the electric vector transmission direction319, and the pretilt direction417Bp is parallel to the electric vector transmission direction211.

The electrodes320A,320B, which are optically transmissive comprising a material such as ITO, are arranged across the layer314to control the liquid crystal material414and thereby control the liquid crystal retarder301. The layer314of liquid crystal material414is switchable by means of adjusting the voltage being applied to the electrodes320A,320B. The electrodes320A,320B are 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 electrodes320A,320B and the liquid crystal material414of the layer314.

The polar control retarder301further 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.

A polar control retarder300wherein the SLM48is a twisted nematic (TN) SLM48may be arranged with the properties of TABLE 1 for example. When viewed from the front of the display device100, the optical axis direction412of the liquid crystal material414at the first alignment layer419A of the polar control retarder300has an in-plane component417Ap with an anti-clockwise rotation angle θAfrom easterly direction of 135°. Similarly, the optical axis direction417of the liquid crystal material414at the second alignment layer419B has an in-plane component417Bp with an anti-clockwise rotation angle θBfrom easterly direction of 45°.

TABLE 1In-planeIn-planeActive LC retarder 301rotationrotationAlignmentItemangleangleTwistlayersPretiltΔn.dVC318319, ϕA135°314417Ap, θA135°90°Homogeneous2°500 nm1.45 V417Bp, θB45°Homogeneous2°210211, ϕB45°218219, ϕB′135°

The liquid crystal material414has a positive dielectric anisotropy such that the optical axis of molecules of the liquid crystal material414lie at the pretilt angle to the plane of the layer314across the layer when no voltage is applied; and the molecules that are not near the alignment layers419A,419B are tilted towards the normal of the layer314when a voltage is applied. A wide polar angle of high transmission may be achieved advantageously with low power consumption for share mode operation.

The additional polariser318and display polariser210are illustrated as having respective electric vector transmission directions319,211that are crossed, that is they are at 90° to each other. In other embodiments including those described hereinbelow, the polarisers may be parallel or may be inclined to angles that are not parallel or crossed. Polar profiles of transmission may advantageously be modified to achieve desirable viewing characteristics for driver47or passenger45.

It would be desirable to provide high luminance uniformity to passenger45and high uniformity of security factor to driver47across at least the lateral direction of the display device100. At locations that are between the alignment layers419A,419B, and may vary across the lateral direction (x-axis) of the display, the optical axis direction412of the liquid crystal material414varies as will now be described.

FIG.3Ais a perspective view of the polar control retarder300ofFIG.2C, comprising a twisted nematic switchable LC retarder301and voltage profile322in the lateral direction (x-axis) across at least one region of the polar control retarder300in a privacy mode of operation. Features of the arrangement ofFIG.3Anot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals, including any potential variations in the features.

View angle control arrangement310comprises: additional polariser318arranged on the same side of the SLM48as the display polariser210, the additional polariser318being a linear polariser; and at least one polar control retarder300arranged between the display polariser210and the additional polariser318. The at least one polar control retarder300comprises a switchable liquid crystal retarder301comprising a layer314of liquid crystal material414; two surface alignment layers419A,419B disposed adjacent to the layer314of liquid crystal material414and on opposite sides thereof. The surface alignment layers419A,419B are each arranged to provide homogenous alignment in the adjacent liquid crystal material414, the liquid crystal material414being provided with a twist.

In other words, the switchable liquid crystal retarder301comprises two surface alignment layers419A,419B disposed adjacent to the layer314of liquid crystal material414and on opposite sides thereof and each arranged to provide homogeneous alignment in the adjacent liquid crystal material414.

FIG.3Aillustrates that a cholesteric dopant411is provided in the layer of liquid crystal material414to minimise degeneracy in twist direction of the liquid crystal material414across the liquid crystal layer314. Twist may further be achieved by the different alignment directions417Ap,417Bp. Rotation angles different to 90 degrees, for example 80 degrees may achieve non-degenerate modes without the use of dopant411. Advantageously complexity and cost is reduced.

In the embodiment ofFIG.3A, the electric vector transmission direction319of the additional polariser318is oriented at a non-zero angle with respect to the electric vector transmission direction211of the display polariser210and the non-zero angle is the same as the twist. Further arrangements of polarisers, electric vector transmission directions319,211and alignment layer orientations417Ap,417Bp will be described hereinbelow with respect to for example TABLE 5 wherein the non-zero angle may be different to the twist.

First and second electrode arrangements320A.320B are disposed on opposite sides of the layer314of liquid crystal material414, wherein first and second electrode arrangements320A,320B are arranged to provide an electric field370perpendicular to the layer314of liquid crystal material414. For explanatory purposes, the magnitude of the electric field370is illustrated with field strength indicated by field lines with differing densities across a predetermined axis.

In the embodiment ofFIG.2AandFIG.3A, the predetermined axis is illustrated as the x-axis, that is the axis that runs from left to right across the display. The predetermined axis may be the nominal axis along which the driver47and passenger45are located, and may be horizontal along the X-axis ofFIG.1Afor example.

The coordinate system of x-y-z for the display device100may be offset from the coordinate system X-Y-Z for the vehicle650. The predetermined axis may differ from the illustrative example. In an alternative embodiment, the predetermined axis may be along a direction tilted with respect to the sides of the display device100active area, for example if the driver47ofFIG.1Ahas a nominal height offset along the Z-axis compared to the passenger45.

Returning to the description ofFIG.3A, the magnitude of the electric field370changes monotonically along a predetermined axis across at least part of the display device100. The change may be with monotonic profile322of electric field370as will be described further hereinbelow. On the left side of the display device100voltage VLis provided across the liquid crystal layer314to provide corresponding electric field370of ELwhich is higher than the magnitude of the electric field370of ECachieved by voltage VCthat is applied at the centre of the display device100. On the right side of the display device100, electric field370of ERis provided by voltage VRacross the liquid crystal layer314which is lower than the magnitude of the electric field370of ECprovided by voltage VC that is applied at the centre of the display device100.

Applied voltage results in an out-of-plane rotation of liquid crystal material414in locations through the liquid crystal layer314that are separated from the alignment layers419A,419B.

In the present embodiment, the in-plane components417Ap,417Bp of liquid crystal alignment at the alignment layers419A,419B are constant across the area of the display device100, that is the alignment layers419A,419B are uniformly processed during manufacture, for example by uniform rubbing or uniform photoalignment methods.

A full description of operation of the layer of liquid crystal layer at each point on the display surface is determined by evaluating the propagation of phase fronts through twisted and tilted liquid crystal layers. However, for purposes of illustration, the operation of the layer314may be determined by considering the optical alignment of liquid crystal material414in a plane410that is halfway between the alignment layers419A,419B. Such an alignment may represent an average alignment of the liquid crystal material414in the direction perpendicular to the layer314. Thus considering the liquid crystal material414alignment within the layer314, components417pof the optical axis417of the liquid crystal material414in the plane of the layer314of liquid crystal material414have an average direction that is along the predetermined direction. Thus components417Rp,417Cp and417Lp are illustrated as having an average direction that is along the x-axis.

On the left-hand side of the polar control retarder300, the optical axis417L has an in-plane component417Lp that is pointed towards the left-hand direction and is inclined at an angle ρLto the plane410; in the centre of the polar control retarder300, the optical axis417C has an in-plane component417Cp that is pointed towards the left-hand direction and is inclined at an angle ρCto the plane410; and on the right side of the polar control retarder300, the optical axis417R has an in-plane component417Rp that is pointed towards the left-hand direction and is inclined at an angle ρRto the plane410. The angles ρL, ρC, ρRare different due to the different applied voltages VL, VC, VRthat provide electric fields EL, EC, ERrespectively wherein EL>EC>ER, in this illustrative embodiment.

In other words, the magnitude of the electric field370changes monotonically along a predetermined axis across at least part of the display device100so that directions448of minimum light transmission of the view angle control arrangement310from points of said at least part of the display device100are directed towards a common off-axis point427in front of the display device100.

The transmission of various rays transmitted through the switchable liquid crystal retarder301to the passenger45varies as446R,446C,446L and the transmission of the switchable liquid crystal retarder to the driver47varies as448R,448C,448L.

The luminance profile for rays445,447directed towards passenger45and driver47will now be described further by illustrating schematically the propagation of polarised light from the output polariser218for on-axis and off-axis directions.

FIG.3Bis a side view of propagation of output light from the SLM48through the optical stack ofFIG.2Ain a privacy mode. Features of the arrangement ofFIG.3Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals, including any potential variations in the features.

For illustrative purposes only, in comparison to the arrangement ofFIG.3Aand TABLE 1, the polarisation orientations are illustrated as being in the plane of the paper or out of the plane of the paper. However, in practice the 45° and 135° orientations are used in the present embodiment.

Light rays from backlight20are output unpolarised or partially polarised. Additional polariser318transmits polarisation state with electric vector transmission direction319.

Considering light rays to the driver47, light ray447C that passes from point470C to the driver47undergoes substantially no phase shift so that the output polarisation state364C remains substantially the same as the polarisation state319. Such light is absorbed at the input display polariser210, and thus the driver sees low luminance in the direction447C. Light rays447R.447L that pass from points470R,470L respectively to the driver47also undergo substantially no phase shift so that the output polarisation state364R.364L remain substantially the same as the polarisation state319. Such light is absorbed at the input display polariser210, and thus the driver sees low luminance in the direction447R,447L. Note that the angles457R,457C,457L from the normal199to the plane of the display device100of output of the rays447R,447C,447L vary across the width of the polar control retarder301, in correspondence with the field of the view of the display device100as seen by the driver47. As will be described hereinbelow, the uniformity of reduced luminance to the driver47in privacy mode is advantageously increased, achieving increased uniformity of security factor, S.

Considering light rays to the passenger45, light ray445C that passes from point470C to the passenger45undergoes a phase shift so that most generally an elliptical output polarisation state362C is incident on the display input polariser, at least some of which is transmitted by the display polariser210to the passenger45. The passenger sees high luminance modulated with the image data of the SLM48in the direction445C. Light rays445R.445L that pass from points470R,470L respectively to the passenger45also undergo a similar phase shift so that the output polarisation state362R,362L has a similar elliptical polarisation state. Such light is transmitted at the input display polariser210, and thus the passenger sees a high luminance in the direction445R,445L with similar luminance to that in direction445C. Note that the angles455R,455L,455C from the normal199to the plane of the display device100of output of the rays445R,445L,445C vary across the width of the polar control retarder301, in correspondence with the field of the view of the display device100as seen by the passenger45. As will be described hereinbelow, the uniformity of high luminance to the passenger45in privacy mode is advantageously increased, achieving increased observed image uniformity.

FIG.3Cis a perspective view of a polar control retarder that may be applied inFIG.2A, comprising a twisted nematic switchable LC retarder and uniform voltage in the lateral direction across at least one region of the polar control retarder in a privacy mode of operation. Features of the arrangement ofFIG.3Cnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals, including any potential variations in the features.

In comparison to the arrangement ofFIG.3BA, a common electric field EL, EC, ERmay be applied perpendicular to the layer314of liquid crystal molecules414and the magnitude of the electric field is uniform across at least part of the display device100.

The in-plane components417Lp,417Cp,417Rp are the same and directed towards the left-hand direction and is inclined at common angles ρL, ρC, ρRto the plane410. The angles ρL, ρC, ρRare different due to the common applied voltages VL, VC, VRrespectively. Light rays448L,448C,448R propagating along parallel directions are directed towards the driver47and light rays446L.446C,446R propagating along parallel directions are directed towards the passenger45.

The embodiment ofFIG.3Cmay be advantageously achieve lower cost and complexity in comparison to the embodiment ofFIG.3A.

It may be desirable to provide further control of the common off-axis point427.

FIG.3Dis a schematic perspective side view of a switchable liquid crystal retarder301that may be applied inFIG.2A, comprising a twisted nematic switchable LC retarder with alignment layer orientations417A,417B that vary across the lateral direction, and further comprising a profile322of voltage in the lateral direction across at least one region of the switchable liquid crystal retarder301; andFIG.3Eis a graph of in-plane angle617against position in the predetermined (lateral) direction and illustrates exemplary profiles604,606,608of the in-plane components417Ap,417Bp provided by a surface alignment layers419A,419B ofFIG.3D. Features of the arrangements ofFIGS.3D-Enot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals, including any potential variations in the features.

In the alternative embodiment ofFIG.3D, the surface alignment layer419A is arranged to provide homogenous alignment in the adjacent liquid crystal material414the surface alignment layer419A having an in-plane component417ALp,417ACp,417ARp in the plane of the layer314of liquid crystal material414having an angle617ALp,617ACp,617ARp that changes monotonically along a predetermined axis across at least part of the display device100; and the surface alignment layers419B is arranged to provide homogenous alignment in the adjacent liquid crystal material414the surface alignment layer419B having an in-plane component417BLp,417BCp,417BRp in the plane of the layer314of liquid crystal material414having an angle617BLp,617BCp,617BRp that changes monotonically along a predetermined axis (such as the x-direction inFIG.3D) across at least part of the display device100.

In other words, the two surface alignment layers419A,419B are disposed adjacent to the layer314of liquid crystal material414and on opposite sides thereof wherein the surface alignment layers419A.419B are arranged to provide alignment in the adjacent liquid crystal material414with an in-plane component417Ap,417Bp that is in the plane of the layer of liquid crystal material, wherein the angle of said in-plane component417Ap,417Bp of the alignment in the adjacent liquid crystal material414changes monotonically along a predetermined axis across at least part of the display device100. Further, the components417ALp,417ACp,417ARp vary monotonically across the lateral direction on the alignment layer419A, and the components417BLp,417BCp,417BRp vary monotonically across the lateral direction on the alignment layer419B, wherein the liquid crystal material414twists between the alignment layers419A,419B.

The embodiments ofFIGS.3D-Eprovide for further control of common off-axis point427. Such further control is provided by modifying the angle617of the respective alignment layer419A,419A in-plane components417Ap,417Bp during manufacture. In the embodiments ofFIG.3E, the profile of angle617may vary across the predetermined direction as shown by illustrative profiles604,606,608to provide desirable control of the common off-axis point427.

Said further control controls the transmission by the polarisation control retarder300with respect to the polar viewing angle across the predetermined direction. For example, consideringFIG.4Ahercinbelow, the direction of maximum transmission of the polar control retarder300may be different across the predetermined direction, for example the lateral direction.

In comparison to the embodiment ofFIG.2A, said further control is fixed at manufacture of the alignment layers419A,419B and control is also provided by modification of the voltage profile322provided across the electrodes320A,320B and across the layer314of liquid crystal material414.

The alternative embodiments ofFIGS.3C-Dmay be provided with each of the embodiments described herein comprising voltage profile322. By way of comparison with the embodiment ofFIG.2A, the control provided by the profile322may be provided by a smaller voltage gradient across the lateral direction. Advantageously the cost and complexity of the transparent electrodes320A,320B and the control system352may be reduced.

Control of the common off-axis point427by means of alignment layer419A,419B arrangement is described further in U.S. Pat. No. 11,079,646, which is herein incorporated by reference in its entirety.

FIGS.4A-Care graphs illustrating the variation of transmission with polar direction for the polar control retarder300ofFIG.2Ausing the twisted nematic polar control retarder300ofFIG.3Aand TABLE 1 for different points on the display device100.

FIGS.4A-Crepresent the polar variation in relative transmission of the polar control retarder301of TABLE 1 for three illustrative drive voltages VR, VC, VLrespectively. Region435represents the angular field of the outline of a 12.3″ passenger infotainment display device100viewed from 750 mm and from a head-on position. Region437represents the angular field of the outline of the same display device100viewed from the same 750 mm viewing plane and from an off-axis position centred at 30 degrees off-axis in the lateral direction.

The angular locations455R,457R,455C,457C,455L,457L ofFIG.3Bfor zero elevation are marked on respective plots. It can be seen that the transmission of approximately 80%˜90% is achieved for all three points455R,455C,455L, and the transmission of approximately 15%˜25% is achieved for each of the locations457R,457C,457L. If the driver47were to move to higher off-axis angles, then the corresponding transmission would fall; however, the uniformity of low transmission across the display area will be substantially maintained. Advantageously security factor uniformity is increased in comparisons to polar control retarders301with no voltage profile322.

In the embodiment ofFIG.3C, a single voltage may be applied, and one of the polar profiles of transmission ofFIGS.4A-Cmay be provided across at least part of the display device100.

The operation of the polar control retarder in a share mode of operation will now be described.

FIG.5Ais a perspective view of the polar control retarder300ofFIG.2C, comprising a twisted nematic switchable LC retarder301and no voltage applied across the liquid crystal layer for a share mode of operation. Features of the arrangement ofFIG.5Anot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals, including any potential variations in the features.

FIG.5Ais similar in structure toFIG.3AandFIG.3Cother than no voltage is applied across any of at least one region of the liquid crystal layer314for at least part of the display device100.

Thus at the intermediate plane417L, the directors417L,417C,417R of the liquid crystal molecules lie uniformly within the plane410and a substantially uniform transmission profile is achieved across the polar control retarder.

FIG.5Bis a side view of propagation of output light from the SLM48through the optical stack ofFIG.2Ain the share mode. Features of the arrangement ofFIG.5Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals, including any potential variations in the features.

As forFIG.3B, for illustrative purposes only, in comparison to the arrangement ofFIG.5Aand TABLE 1, the polarisation orientations are illustrated as being in the plane of the paper or out of the plane of the paper. However, in practice the 45° and 135° orientations are used in the present embodiment.

In comparison to the explanation ofFIG.3B, the polarisation state319is rotated to output polarisation state362across at least a region of the polar control retarder301. The light rays445,447are transmitted by the input display polariser210and transmitted to the passenger45and driver47, modulated with the image data of the SLM48.

FIG.5Cis a schematic graph illustrating the variation of transmission with polar direction for the polar control retarder300ofFIG.16Awith no voltage V profile322across the polar control retarder300.

For each of the subtended display areas, the uniformity of transmission is advantageously high, and high image uniformity of image appearance is achieved.

An alternative arrangement of additional polariser318and display polariser210will now be described.

FIG.5Dis a schematic graph illustrating the variation of transmission with polar direction for a polar control retarder300using the twisted nematic polar control retarder301with additional polariser318and display polariser210that have electric vector transmission directions319,211that are parallel and operating in privacy mode and for the embodiment of TABLE 2; andFIG.5Eis a schematic graph illustrating the variation of transmission with polar direction for a polar control retarder300using the twisted nematic polar control retarder301with additional polariser318and display polariser210that have electric vector transmission directions319,211that are parallel operating in share mode and for the embodiment of TABLE 2 but with 10V uniform drive voltage of the liquid crystal layer314compared to the zero volts forFIG.5C.

TABLE 2In-planeIn-planeActive LC retarder 301rotationrotationAlignmentItemangleangleTwistlayersPretiltΔn.dVC318319, ϕA45°314417Ap, θA45°90°Homogeneous2°500 nm1.55 V417Bp, θB135°Homogeneous2°210211, ϕB45°

In privacy mode, the luminance profile for the passenger45is extended, advantageously achieving increased image uniformity in the vertical direction for the passenger45.

It would be desirable to reduce the cost of the optical stack to achieve increased viewing freedom of driver47for high security factor, S.

FIG.6Ais a schematic top view illustrating schematically the operation for a driver47of a privacy display device100comprising the polar control retarder300with a voltage profile322aacross the polar control retarder300and viewer tracking system200arranged to adjust the voltage profile322ato voltage profile322bin response to the measured location of the driver47; andFIG.6Bis a schematic graph illustrating the variation in voltage V profile322applied to the polar control with respect to lateral position across the polar control retarder300for two different driver47lateral locations and for a display of width L in the predetermined direction (x-axis in this embodiment). Features of the arrangement ofFIGS.6A-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.

In the embodiment ofFIGS.6A-B, the viewer tracking system200is arranged to track the location of a viewer47movement540in the predetermined direction (laterally), wherein the control system500is arranged to vary the voltages to the first and second electrode arrangements320A,320B in response to the location of the viewer47.

Thus the control system500is arranged to control the spatial light modulator48and to supply voltages to the first and second electrode arrangements320A,320B for providing the electric field perpendicular to the layer314of liquid crystal material414, the control system being arranged to vary the voltages to the first and second electrode arrangements320A,320B for controlling a direction of minimum light transmission of the view angle control arrangement310.

In operation, the voltage profile322awith left and right side voltages VLa, VRa is provided to the polar control retarder300for a first measured viewing position, such that a uniformly low security factor is provided across the display device100. After movement540of the driver47away from the optical axis199, the voltage profile322ais adjusted by voltage change348to maintain the location of the minimum transmission to the driver47with voltage profile322bwith left and right side voltages VLb, VRb.

For the first voltage profile322a, a first point427ais provided for points across at least part of the display device100which is towards the driver47. Similarly for the second voltage profile322b, a second displaced point427bis provided for points across at least part of the display device100. The transmission directed towards the driver47may advantageously be minimised in response to the location of the driver across at least part of the display device100.

FIG.6Cis a schematic top view illustrating schematically the operation for a driver of a privacy display comprising the polar control retarder with a voltage profile across the polar control retarder and a head tracking system arranged to adjust the voltage across the polar control retarder in response to the measured location of the driver moving towards the display; andFIG.6Dis a schematic graph illustrating the variation in voltage profile applied to the polar control with respect to lateral position across the polar control retarder for two different driver lateral locations for the driver moving towards the display. Features of the arrangement ofFIGS.6C-Dnot 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 embodiment ofFIGS.6C-D, the viewer tracking system200is arranged to track the location of a viewer47movement540in towards the centre of the display, wherein the control system500is arranged to vary the voltages to the first and second electrode arrangements320A,320B in response to the location of the viewer47. In operation, the voltage profile322awith left and right side voltages Via. VRa is provided to the polar control retarder300for a first measured viewing position with a first monotonic profile322a, such that a uniformly low security factor is provided across the display device100for the driver near point427a. After movement540of the driver47in this illustrative example towards the display device100centre, then light rays447C to point427bare in substantially in the same direction for both viewing positions. In the centre of the display device100, the voltage is thus unmodified. However light rays447R,447L have different angles so that the profile322bis modified accordingly with respect to profile322aas illustrated inFIG.6D.

The transmission directed towards the driver47by view angle control element310of display device100may advantageously be minimised in response to the location of the driver across at least part of the display device100. Uniformity of security factor may be increased over a wider driver47headbox. It may be desirable to reduce the complexity of the polar control retarder300.

FIG.6Eis a schematic top view illustrating schematically the operation for a driver47of an alternative privacy display device100comprising the polar control retarder300with a common voltage V across the polar control retarder300and a head tracking system arranged to adjust the common voltage V across the polar control retarder300in response to the measured location of the driver47; andFIG.6Fis a schematic graph illustrating the variation in common voltage V applied to the polar control for two different driver47lateral locations for the arrangement ofFIG.6E.

In comparison to the arrangement ofFIGS.6A-B, the magnitude of the electric field perpendicular to the layer314of liquid crystal material is uniform across the entirety of the display device100.

Luminance reduction to a moving driver47may be provided by tracking the driver47polar location448and adjusting the voltage across the liquid crystal layer of the polar control retarder in correspondence. High image security may be provided at the user. At a first measured polar location448aof the driver47, then profile322awith constant voltage Va is applied and at a second measured polar location448bof the driver47then profile322bwith constant voltage Vb is applied.

The luminance supplied to the driver may be minimised for at least one location on the display device100. Reduced luminance is achieved over an increased polar region. Advantageously the cost and complexity of the transmissive electrodes320A,320B and drive circuit352may be reduced.

An illustrative example of tracking of the location of the driver47using controllable transmission profile of polar control retarder300will now be described.

FIG.7Ais a schematic top view of operation of a privacy display device100for a moving driver47and a head tracking system arranged to minimise luminance to the driver47; andFIG.7B-Dare graphs illustrating the variation of transmission with polar direction for the polar control retarder300ofFIG.2Ausing the twisted nematic polar control retarder300ofFIG.3operating with different voltage V profiles322across the polar control retarder300and the embodiment of TABLE 2.

As illustrated forFIGS.7A-D, luminance reduction to a moving observer may be provided by tracking the driver47polar location448and adjusting the voltage profile of the polar control retarder in correspondence.

For the location47B, the average voltage across electrodes320A,320B may be adjusted to provide transmission profile ofFIG.7Bof the view angle control element; for the location47C, the average voltage across electrodes320A.320B may be adjusted to provide transmission profile ofFIG.7Cof the view angle control element; and for the location47C, the average voltage across electrodes320A,320B may be adjusted to provide transmission profile ofFIG.7Cof the view angle control element.

Advantageously high image security for multiple driver locations47is achieved as will now be described.

FIG.8Ais a schematic graph illustrating a simulated polar profile of the security level, S of the arrangement ofFIG.2Afor an ambient illuminance measured in lux that is equal to the head-on display luminance measured in nits and for the polar profile of backlight luminance ofFIG.16Ahereinbelow, and polar profile of polar control retarder transmission ofFIG.7C, with Fresnel front surface reflectivity of the display device100; andFIG.8Bis a schematic graph illustrating a simulated polar profile of the security level. S of the arrangement ofFIG.2Afor an ambient illuminance measured in lux that is equal to the head-on display luminance measured in nits and for the polar profile of backlight luminance ofFIG.16Aand polar profile of polar control retarder transmission ofFIG.7D, with Fresnel front surface reflectivity of the display device100.

Region750represents the polar region for the passenger45to achieve high image visibility with S<0.2. The angular locus of display outline435is thus provided in the region of high image visibility and the passenger can easily see the image data from the SLM48. Region752comprises the polar region over which the image is neither clearly visible or clearly private with 0.2≤S<1. Region754comprises the polar directions over which most image content appears to be private with 1≤S<1.8 while observers located in region754see no images, independent of content.

In the present embodiments using the view angle control arrangement310ofFIG.3Afor example, the polar control retarder300is pupillated so that the security factor at the centre of the display is similar to that at the edges of the display, and thus the driver47may see a uniformly private image (S=1) for a viewing angle of 25°.

In systems without pupillation then parts of the display region437will have S<1 and so parts of the display will appear to be non-private.

In regions758for which S<1 at higher viewing angles for the driver, the image may appear to drop below a threshold of acceptable privacy.

The variation in location of polar region754with acceptable security factor may be provided by control of voltage profile322. Thus the arrangement ofFIG.8Ausing the transmission profile ofFIG.4B, may be adjusted by changing the voltage profile322by change348to achieve the security factor profile ofFIG.8Bas the driver47moves closer to the axis, that is closer to the angle α inFIG.1A.

It would be desirable to provide further improvements in image security in the polar region near to the driver47. In embodiments hereinbelow such as illustrated inFIGS.23A-C.FIG.24AandFIG.25Aincreased image security may be provided by using a further additional polariser and a further polar control retarder. Advantageously the embodiment illustrated by the results ofFIGS.8A-Bmay be achieved with fewer polar control retarders300, achieving reduced cost, thickness, weight and complexity.

Methods to control a polar control retarder300will now be described.

FIG.9Ais a flow chart illustrating a method to control an observer-tracked polar control retarder300. Features of the arrangement ofFIG.9Anot 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 location of the observer47may be determined by means of viewer tracking system200and associated image processing. The ambient light level may be measured by means of ambient light sensor204. The ambient light level may be used as an input to set the display brightness so that the viewability of the display device100may be optimised to a minimum or a maximum as desired. The ambient light system204may detect levels impinging on it as a function of input angle. The control system500may be set to track the observer47by processing sensor data910.

The control system500may be operated with both pupillation and tracking control of the voltage profile322across electrodes320A,320B.

Alternatively the control system500may be operated without pupillation needing to be adjusted in response to driver47location. The voltage profile may be adjusted for movement of the driver47in the predetermined direction by modifying the average voltage of the profile, for example by means of voltage change348as illustrated inFIG.6BorFIG.6D.

All or optionally part of the display device100may be set or determined912to be pupillated by options in the control system500. The pupillation may be controlled by an adjustment914of voltages on the polar control retarder300of the view angle control arrangement310as set by system500control options.

In the case where the pupillation is not tracked, for example as illustrated inFIGS.6E-Fa similar method with appropriately adjusted voltages applied to the polar control retarder300of the view angle control arrangement310to optimise the display security factor for the driver47location is performed as illustrated on the right-hand side ofFIG.9A.

The observer47location signal from viewer tracking system200may be mathematically differentiated with respect to time in order to determine velocity and acceleration of the observer47so that any of this data can be used in a predictive tracking control loop tracking the observer to further improve dynamic tracking performance as the observer47moves.

FIG.9Bis a flow chart illustrating a method for manual control of an adjustable polar control retarder300, for example for use with the arrangement ofFIG.1B. Features of the arrangement ofFIG.9Bnot 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 display device100, is not tracked by means of a viewer tracking system200, the display device100may nevertheless be adjusted and controlled by a manual actuator202such as dial or buttons so that these signals may be processed911and the viewing conditions set or adjusted by the observer45or user. The control of the system may be achieved by means of adjustment of the voltages in the polar control retarder300, included in the view angle control arrangement310. Advantageously cost and complexity may be reduced.

The operation of a pupillated display device100in privacy mode will now be further described.

FIG.10Ais a schematic top view illustrating schematically the operation for a passenger45of a privacy display device100comprising the polar control retarder300with voltage V profile322ofFIG.2A; andFIG.10Bis a schematic top view illustrating schematically the operation for a driver47of a privacy display device100comprising the polar control retarder300with voltage V profile322ofFIG.2A. Features of the arrangement ofFIGS.10A-Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals, including any potential variations in the features.

In comparison to the description ofFIG.3B, the output from the polar control retarder300is illustrated as cones460R,460C,460L, where the width of the respective cone460R,460C,460L represents the angular width comprising a limited range of transmission values, for example the cone angles for which transmission is within 20% of a nominal value.

As illustrated inFIG.10A, the light cones460R,460C,460L towards the passenger45may be arranged to overlap in an illumination window325at a plane197at distance482from the display device100that may be different from the nominal viewing distance480of the passenger45from the display device100along the display normal199. The distance482may be greater than the distance480. Advantageously the variation in uniformity as the passenger45moves in a lateral direction may have a natural sensation of roll-off rather than a rapid switching behaviour.

Similarly, as illustrated inFIG.10Bthe cones462R,462C,462L of low light transmission towards the passenger45may be arranged to overlap in an illumination window327, and at a greater distance from the display device100than the nominal driver47locations.

The illumination windows325,327are sometimes referred to as pupils, and the output transmission profile from the polar control retarder provided by the voltage variation322may be referred to as pupillated, that is the illumination windows325,327are at a finite distance from the display device100.

By way of comparison with the embodiment ofFIGS.3A-Bthe output of a display without the voltage profile322will be described.

FIG.11Ais a schematic top view illustrating schematically the operation for a passenger45and a driver47of a privacy display device100comprising the polar control retarder300with a common voltage V in the lateral direction across at least one region of the polar control retarder300; andFIG.11Bis a schematic graph illustrating the variation of transmission across the display device100surface with polar direction for a polar control retarder300with a common voltage V across the polar control retarder300of TABLE 1. Features of the arrangement ofFIGS.11A-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.

In comparison with the arrangement ofFIG.3B, the output is not pupillated and each point470R,470C,470L on the display device100directs the maximum transmission in a common direction458and the minimum transmission in a common direction459.

FIG.11Billustrates that the angles455L,455C,455R ofFIG.3Bhave different transmissions and thus the uniformity of the image is reduced for the passenger45. Similarly, the transmission to the driver varies with the angles457L,457C,457R ofFIG.3Bhave different transmissions and thus the uniformity of security factor is reduced for the driver47.

To achieve the pupillation effects described above it would be desirable to achieve a one-dimensional variation in the electro-optic response of the liquid crystal material414of the switchable liquid crystal retarder301. An illustrative structure that can achieve the desirable characteristics of the display device100ofFIG.2Awill now be described.

FIG.12Ais a schematic diagram illustrating a front perspective view a transparent electrode320A comprising part of a switchable liquid crystal retarder301of a switchable display device100comprising a transmissive SLM48. 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.

The first electrode arrangement320A comprises a continuous electrode extending over the entirety of the layer314of liquid crystal material414, the continuous electrode having two contacts321L,321R disposed at opposite ends of the continuous electrode along the predetermined axis and arranged to supply respective voltages that create a voltage profile322on the continuous electrode320A along the predetermined axis for providing the electric field370perpendicular to the layer314of liquid crystal material414. In alternative embodiments, the electrode320B rather than320A may comprise the structure ofFIG.12A. The voltage profile322may be a voltage gradient that is monotonic. The voltage profile322provides the variation of electric field370across the predetermined direction, for example as illustrated inFIG.3A.

In this embodiment, the second electrode arrangement320B is arranged to supply a common voltage across the area of the electrode arrangement320B. In other words, the second electrode arrangement320B comprises a continuous uniform resistance per square electrode extending over the entirety of the layer314of liquid crystal material414.

The control system352ofFIG.2Ais arranged to supply voltages to the first and second electrode arrangements320A,320B for providing the electric field370perpendicular to the layer314of liquid crystal material414.

In the illustrative example ofFIG.3A, the electric field370perpendicular to the layer314of liquid crystal is produced by means of x-axis variation in the electrode voltage in the predetermined direction which is the lateral or x-axis direction leading to a variation in the electro optic response of, for example, a liquid crystal material414in the layer314.

In the embodiment ofFIG.12Adifferent voltages VL, VR are applied at the left and right ends of the transparent electrode320A. This differs from a conventional liquid crystal display where the voltage applied to the transparent electrode320A is either applied at a single point or the same voltage VL is applied at multiple points. This is to establish a uniform potential on the transparent electrode320A. Note that in the conventional case there is no current flow in the predetermined direction so the potential across the resistance of the transparent electrode320remains the same. By way of comparison in the present embodiments current does flow between electrodes321L,321R.

InFIG.12Adifferent voltage contacts321are applied at different points specifically VL and VR where VL>VR is illustrated and a voltage profile322across (along the x-axis direction of) the transparent electrode320A is produced because of its sheet resistance. Typical sheet resistances for transparent conductive coatings are in the range 10 to 1000 ohms per square. The point contact of the voltage VL and VR may provide a non-uniformity of the magnitude of the electric field around the contact321L,321R and this can cause the voltage profile322to deviate from uniformity in the y-axis near to the contacts. This may be particularly undesirable for low aspect ratio (more square) displays.

It would be desirable to reduce the deviation from uniformity of the voltage profile322near to the contacts321L,321R of VL and VR.

FIG.12Bis a schematic diagram illustrating in a front perspective view a transparent electrode320A with additional bus bars comprising the contacts321L and321R. 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.

The two contacts321L,321R comprise conductive bars extending perpendicular to the predetermined axis.

The bus bars contacts321L,231R comprise material with higher conductivity than that of the transparent electrode320A and may for example comprise metal or a stack of thin film metals such that the voltage VL is effectively applied with a low impedance line contact to the transparent electrode320A rather than a point contact. The voltage profile322in the y-axis direction is therefore more uniform compared to that ofFIG.12Aand the display device100can advantageously provide reduced visibility of edge effects from non-uniform electric fields370in the y-axis direction.

The voltage profile322could be reversed by making VR>VL. The voltage profile322could alternatively be produced in the y direction, if so desired, by altering the location of voltage contacts321L,321R (not illustrated).

FIG.12Cis a schematic diagram illustrating in front perspective an exemplary method of implementing voltages VL and VR, VL and VR are applied to transparent electrode320A by means of driver330,331which may be an integrated circuit comprising an op-amp. Features of the arrangement ofFIG.12Cnot 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.

VL may be an alternating potential such as a square or sine wave. The output of drive circuit330is scaled down, illustrated schematically by variable resistor334and that potential is then buffered by driver331to create alternating voltage VR which is synchronised with VL. The voltage scaling may be achieved by other known circuits such that the scaling may be under electronic control by a control system in response to for example the location of an observer. The lateral voltage profile322sprovided by this circuit creates electric fields in the predetermined direction with magnitudes that are typically much lower than the magnitude of the electric field370perpendicular to the switchable liquid crystal retarder301i.e. in the z-axis direction.

FIG.12Afurther illustrates Vref that is a reference potential, which may be the potential applied to transparent electrode320B (not shown) for example.

FIG.12Dis a schematic diagram illustrating in front perspective an alternative embodiment of a control system352for providing voltages VL and VR. Features of the arrangement ofFIG.12Dnot 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.

VL is applied to transparent electrode320A by means of driver330which may be an integrated circuit such as an op-amp.

Bus bar contact321R is illustrated schematically connected to tuncable variable resistor336and to voltage VG. A current path is created from VL to VG and the relative voltage drop to VR across the sheet resistance of transparent electrode320A may be adjusted by means of tuneable resistor336. The tuneable resistor336may be replaced by an electronically controlled variable voltage that enable the degree of pupillation to be controlled, for example in response to the location of the observer. Voltage VL may be an alternating e.g. square wave voltage in order to meet the DC balance conditions across the liquid crystal layer314. Voltage VG may be a fixed potential such as ground or for example an alternating voltage to contribute to the DC balance design of the switchable liquid crystal retarder301. VG may also be the potential applied to transparent electrode320B.

FIG.13Ais a schematic diagram illustrating in front perspective an exemplary assembly of switchable liquid crystal retarder301comprising opposing transparent electrodes320A and320B. Features of the arrangement ofFIG.13Anot 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 withFIG.12A, voltages may be applied to multiple contacts321L,321R on transparent electrode320B to create voltage profile322and transparent electrode320A may have a uniform voltage.

FIG.13Afurther shows that contacts321L,321R may be provided at more than one point on at least one side of the electrode arrangement320A. Advantageously non-uniformities of electric field profile370may be reduced.

FIG.13Bis a schematic diagram illustrating in front perspective a further exemplary assembly of switchable liquid crystal retarder301comprising opposing transparent electrodes320A and320B. 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.

In comparison with reference toFIG.13A, bus bars contacts321L and321R are used, as described with reference toFIG.12B, to create a lateral voltage profile322on transparent electrode320B. The voltage profile322may be present on the front or the back of the switchable liquid crystal retarder301. The lateral voltage profile322creates a difference in the magnitude of the electric field370in the perpendicular (z-axis direction) over the layer314of liquid crystal material414.

FIG.13Cis a schematic diagram illustrating in front perspective a further exemplary assembly of switchable liquid crystal retarder301comprising opposing transparent electrodes320A and320B. 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 this case a voltage profile322is produced on both transparent electrode320A and320B by means of bus bar contacts321LA,321RA and321LB,321RB respectively. The voltages VLA, VLB, VRA, VRB applied may be adjusted to give in the required predetermined direction voltage profile322to produce the desirable electric field370profile in layer314of liquid crystal414and difference in electro optic response of the liquid crystal in the z-axis of the switchable liquid crystal retarder301.

In the embodiments ofFIG.12AtoFIG.13Cthe x-axis voltage profile322created depends on the applied voltages VL and VR and the resistance uniformity of the respective transparent electrode320A or320B. It is common for deposited transparent electrodes to have a uniform resistance per area and therefore the voltage profile322across the transparent electrode320A from bus bar contacts321L to321R is essentially linear. However, the electro optic response of the liquid crystal contained in the switchable liquid crystal retarder301may require a different response function to achieve the desired optical effect. It would be desirable to be able to produce a prescribed variation in the profile across one or more of the transparent electrodes.

FIG.13Dis a schematic diagram illustrating different profiles of voltage with lateral position across the display device100. 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.

Voltage profile322aillustrates a linear voltage profile322, that is a uniform gradient, between the left and right ends of the electrode layer320A. However, it may be desirable to produce a non-linear voltage profile322bbetween the left and right ends, for example a curve. In this way the voltage VC2at the centre of the electrode may differ from the linear gradient case voltage VC1. This is so that the electro optic response of the liquid crystal layer314varies spatially as desired to provide the desired pupillation of the display. The gradient322acan be implemented by varying the resistance per square of the electrode material for example.

The curve322bmay be provided to achieve increased accuracy of directing minimum transmission of light rays to point427across the at least part of the display device100. Advantageously increased uniformity may be provided.

It would be desirable to be able to implement curve322bwithout control during manufacture of the variation of the sheet resistance per square of the area of the electrode material.

FIG.13Eis a schematic diagram illustrating in front perspective a further exemplary assembly of switchable liquid crystal retarder301comprising opposing transparent electrodes320A and320B; andFIG.13Fis a schematic graph illustrating an approximation to desired voltage profile322curve322aproduced by stepwise approximation using wires321a-n. Features of the arrangement ofFIGS.13E-Fnot 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 first electrode arrangement320A comprises plural electrodes323a-nprovided along the predetermined axis, the plural electrodes323a-nbeing arranged to supply different voltages that change monotonically along the predetermined axis, for example as illustrated by profile323a-nofFIG.13F.

In this alternative embodiment, the transparent electrode320A is segmented into a number n of y-axis fingers, each of which is connected by respective wire321a-nto drive circuit326, which may be an integrated circuit addressed from a control system (not shown). Drive circuit326is arranged to apply a different voltage to each of the respective wires321a-nso that in principle any desired voltage variation with lateral (x-axis) direction may be approximated, including non-monotonic profiles. Advantageously complexity of the electrode resistance per square variation of320A is reduced.

Referring toFIG.13F. “n” may be any reasonable integer number, for example 256.

By controlling the voltages on wires321a-n, areas of the switchable liquid crystal retarder301may achieve pupillation and other areas may achieve a different pupillation or no pupillation. This effect can be used to control the viewing of the parts of the display device100from different directions. Segmenting plural electrodes323a-nalso provides different parts of the display device100to operate to produce different pupillation. In this case the change in voltage within each section is monotonic, but the monotonicity may be broken across the boundary between respective parts.

It may be desirable to provide further control of profile322.

FIG.13Gis a schematic diagram illustrating in a perspective front view the opposing electrodes320A,320B of a switchable liquid crystal retarder301comprising corner bus bar contacts321TL,321BL,321TR,321BR. Features of the arrangement ofFIG.13Gnot 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 embodiment ofFIG.13G, the bus bar contacts321TL,321BL,321TR,321BR are provided with shapes and/or locations to provide improved control of profile322. Separate drive circuits330TL,330BL,330TR,330BR respectively are provided to achieve variations such as shown by illustrative profiles322TBL,322TBR that are in addition to the lateral profile322LR.

The sheet resistance of the resistive conductive coating (for example ITO) of the electrode320A may provide a lower resistance at each of the corners than in the centre of electrode320A, so that when viewed from the front, the corners and centre of electrode320A may therefore experience a slightly different voltage.

In operation, the profiles322TBL,322TBR and322LR may be arranged to provide modification of the luminance directed into a common pupil325from across the display area such as illustrated inFIG.6A. Advantageously uniformity may be increased.

In alternative embodiments (not illustrated) a similar arrangement may be implemented on electrode320B. When the display has a high rectangular aspect ratio, the length of the bus bars321may be longer in the x direction, i.e., Λx>Λy to retain a higher voltage along the edge near the corners of the long side of the rectangle. Advantageously visual uniformity may be improved.

It would be desirable to further increase the uniformity of the display device100.

FIG.14Ais a schematic top view illustrating schematically the operation for a passenger45of a privacy display device100comprising a backlight20ofFIG.2Acomprising a first illuminated waveguide1A, pupillated turning component50and polar control retarder300. Features of the arrangement ofFIG.14Anot 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.

During operation in privacy mode, light rays447from light source15A are guided through waveguide1A, and are directed onto light turning film component50, being directed towards the passenger45.

As will be described further hereinbelow with respect toFIGS.26A-C, light turning film component50may be pupillated, that is arranged to provide light cones466R,466C,466L that vary in nominal pointing direction across the display device100. The transmission profiles460R,460C,460L may be arranged in alignment with the profiles466R,466C,466L from both the backlight20illumination and for the transmission profiles and pupil325may be provided. Advantageously uniformity of luminance provided to the passenger45may be increased.

FIG.14Bis a schematic top view illustrating schematically the operation for a driver47of a privacy display device100comprising a backlight20ofFIG.2Acomprising a second waveguide1B comprising a second luminance profile. Features of the arrangement ofFIG.14Bnot 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.

During operation in privacy mode, stray light rays451from light source15A are guided through waveguide1A, and are directed onto the waveguide1B. Such light rays451are then output onto the light turning film and directed towards the driver47into light cones468R.468C,468L that are pupillated towards driver47illumination window (optical pupil)327such that the uniformity of stray light is increased.

Light cones464R,464C,464L similarly provide reduction of the luminance of the light cones468R.468C.468L that is uniform across the display device100. The reduction of luminance can be arranged to minimise the visibility of stray light451, advantageously achieving increased uniformity of security factor.

An alternative arrangement for achieving improved uniformity using a curved display device100will now be described.

FIG.15Ais a perspective view of a user45in front of a curved display device100comprising a curved polar control retarder300comprising a liquid crystal retarder301as described elsewhere herein. Features of the arrangement ofFIG.15Anot 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.15A, the display device100is curved about the y-axis and has a centre of curvature on the same side of the display device100as the user45. At least one of the backlight20and polar control retarder300may be curved. In the illustrative example ofFIG.15Aboth the backlight20and polar control retarder300are curved.

By way of comparison with the arrangement ofFIG.15A, if the polar control retarder300and backlight were flat then the respective pupillation properties would be arranged to provide a viewing window325F at a window plane distance197F. In the present embodiment, the curvature of the display device100provides a window plane325C at a shorter window plane distance197C.

The voltage profile322across the polar control retarder301may be modified in correspondence to the curvature of the display.

The variation of voltage profile322across the polar control retarder301may be reduced to achieve a desirable window325C. The optical transmission profiles of the polar control retarder301provided across the display device100may have a smaller variation than for the corresponding flat polar control retarder and advantageously increased uniformity may be achieved.

In alternative embodiments, the curvature may be non-uniform to provide a shaped display to match the profile of the surface of a vehicle, with the voltage profile322adjusted accordingly to provide common optical windows325,327for points across the display device100. Advantageously high image uniformity and security factor uniformity may be provided for displays with non-uniform surface profiles.

It would be desirable to provide multiple displays that are adjacent to each other. Arrangements of alignment directions for tiled displays will now be described.

FIG.15Bis a schematic diagram illustrating a schematic top view of a pair of co-planar tiled display devices100A.100B; andFIG.15Cis a schematic diagram illustrating a schematic top view of a pair of tilted planar tiled display devices100A,100B. Features of the arrangements ofFIGS.15B-Cnot 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 embodiments ofFIGS.15B-C, display devices100A,100B have respective voltage profiles322A,322B, wherein the profiles322A,322B are different to desirably achieve common points427A,427B at or near the driver47wherein the transmission of the polar control retarder300is minimised. Advantageously uniformity of security factor S to the driver and uniformity of image to the passenger45may be achieved.

Common points427A.427B may be aligned as in the illustrative embodiment ofFIG.15B. Advantageously the lowest luminance is achieved for a driver47at said common location427A,427B.

In alternative embodiments the common points427A,427B may be offset as illustrated inFIG.15C. For the first display device100A, the visibility of reflected light rays606A from ambient light source604to the driver47may be lower than the visibility of reflected light rays606B for the second display device100B. For a given luminance reduction at the common points427A,427B, the security factor of display device100B would be higher than for display device100A. It would be desirable to provide a uniform security factor for various locations of the driver47while the reflectivity varies.

Driver47at point427A sees a low luminance from display device100A but lower frontal reflections from rays606A. By comparison the driver sees higher front reflections from rays606B from display device100B but higher luminance due to the offset from point427B. The offset of common points427A,427B advantageously achieves a wider viewing region of driver47for which the security factor is above a desirable threshold.

As is apparent fromFIGS.15B-C, a passenger45may no longer be arranged on-axis near to the centre of each display device100A,100B. Instead, the user may, approximately, be positioned on-axis with an edge of each display device100A,100B.

Further in an alternative embodiment the control system500may be provided to control the location of the points427A,427B in response to ambient illumination604measured by ambient light sensor204and driver47location measured by observer position location viewer tracking system200to achieve optimised security factor for a measured driver47location and illumination conditions.

In other embodiments the number of parts101may be increased to advantageously achieve improved visual performance for privacy and share mode operation.

In alternative embodiments, at least one of the display devices100may be curved as illustrated inFIG.15A.

FIG.15Dis a schematic diagram illustrating a schematic top view of a segmented curved display device comprising offset common points427A,427B. Features of the arrangement ofFIG.15Dnot 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.15C, a single display device100may be provided, with part101A and part101B. Profiles332A.332B may be provided that are different on the parts101A.101B respectively to provide different common points427A,427B. In a similar manner toFIG.15C, the security factor across the display may be provided with increased uniformity.

FIG.15Eis a schematic diagram illustrating a schematic top view of a segmented curved display device100comprising different polar control retarder300drive voltages. Features of the arrangement ofFIG.15Enot 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 with the arrangement ofFIG.15D, the parts101A,101B are provided with different drive voltages, but no profile of voltage is provided across the respective parts101A,110B. The output light rays with minimum luminance447A,447B may be pupillated at least to an extent by the curvature of the display device100as described inFIG.15Aabove.

The direction of light rays447A,447B from respective sides of the display parts101A,101B may be different. The display device100A may have an electrode arrangement320that is advantageously less complicated than forFIG.15D. The directions of light rays447A,447B may be modified by control of the voltages VA. VB across the liquid crystal layer314of the respective parts101A,101B that are different. The voltages VA. VB may be arranged to achieve improved security factor for moving driver47, and may be in response to measured driver47location and/or in response to ambient illumination from ambient light source604in a similar manner to that described inFIG.15D. Advantageously the security factor uniformity may be improved and cost may be reduced.

It may be desirable to provide displays with alternative arrangements of curvature.

FIG.15Fis a schematic diagram illustrating a schematic top view of a segmented curved display device100comprising a planar part101A and a curved part101B. Features of the arrangement ofFIG.15Fnot 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 embodiment ofFIG.15F, the display device100may be arranged to provide an aesthetically desirable profile across the dashboard of a vehicle600for example.

The profiles322A.322B may be modified to provide the desirable common points427A,427B as described hereinabove. Planar part101A may be arranged with a profile322A that is different to the profile322B to achieve points427A,427B that in the illustrative example ofFIG.15Fare coincident. Advantageously a uniform display uniformity may be achieved for the passenger47, and the driver45may see a display100with no driver distraction.

In alternative embodiments, not shown, the parts101A.101B may be provided by separate displays100A,100B. Advantageously yield of manufacture may be increased and replacement cost reduced.

In alternative embodiments, the part101A may be arranged to provide images that are arranged to be seen by the driver, and the profile322A may provide a uniform voltage profile across the liquid crystal retarder301. Advantageously cost and complexity may be reduced.

The operation of the backlight20ofFIG.2Awill now be further described.

FIG.16Ais a schematic graph illustrating the simulated polar variation of luminance output for an illustrative backlight20ofFIG.2AandFIG.26Aprimarily operating to direct light to the passenger45, that is light source15A is illuminated and light source15B is not illuminated.

Advantageously most light is directed towards the passenger45in direction445and high suppression of luminance is achieved in the driver47in direction447. High power efficiency for passenger45illumination is achieved. However, such an illumination profile is not sufficient to achieve desirable security factor, S as will be described further hereinbelow.

FIG.16Bis a schematic graph illustrating the polar variation of luminance output for an illustrative alternative backlight20ofFIG.2AandFIG.26Aprimarily operating to direct light to the driver47, that is light source15B is illuminated and light source15A is not illuminated. Advantageously most light is directed towards the driver47in direction447and low luminance is achieved in the passenger45direction. High power efficiency for driver47illumination is achieved in a mode of operation in which the light source15B is illuminated.

FIG.17Ais a schematic graph illustrating the polar variation of luminance output for an illustrative backlight20ofFIG.2AandFIG.26Aoperating to direct light to the passenger45and to the driver47, that is both light sources15A,15B are illuminated; andFIG.17Bis a schematic graph illustrating the variation of relative luminance output at zero elevation for the alternative backlight profile ofFIG.17Aoperating to direct light to the passenger45and driver47in a share mode of operation. Such a profile may be provided by a backlight20as will be described with reference toFIG.26Ahereinbelow for example.

The backlight20in a second mode of operation has a second luminance distribution having an output luminance profile having first and second maxima at polar locations466,468in luminance at first and second different polar locations445,447with a minimum524in luminance therebetween. The luminance of the minimum524is desirably at most 25% of the luminance of the first and second maxima.

Advantageously a display device100that may be observed by both driver47and passenger45may be provided.

Other arrangements of display will now be described.

FIG.18is a schematic top view of a centre stack display for an automotive vehicle650. Features of the arrangement ofFIG.18not 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 embodiment ofFIG.18, display device100comprises a right-hand part101P that is a switchable privacy display arranged to provide privacy and share modes; and a left-hand part101D that is a fixed share mode display. Other arrangements of mixed function display may be provided, for example to provide switchable privacy functions to each of driver47and passenger45on respective sides of the display.

In the alternative embodiment ofFIG.18, the left part101D of the display is provided such that the magnitude of the electric field across the layer314of liquid crystal material is uniform across the area of the polar control retarder300. Such a uniform region could for example be provided with no transparent electrode, or with a segmented electrode that is separately driven for the part101D and101P. Both driver47and passenger45may see a high luminance image provided by high transmission from the polar control retarder300over a wide polar range, such as illustrated inFIG.5Cfor example.

By comparison the region of the polar control retarder300arranged for the right-hand part101P of the display may be provided with a voltage profile322such that a pupillated low transmission profile is directed to the driver to achieve high uniformity of security factor for the right-hand part101P of the display device100while achieving high image uniformity for the passenger45in the manner described hereinabove.

Various alternative embodiments of display device100structures will now be described.

It may be desirable to use an SLM48with polariser210,218electric vector transmission directions211,219that are not at 45 degrees to the edge of the active area of the display device100.

FIG.19Ais a schematic side perspective view of a display device100providing uniformity for a display user wherein the polariser transmission directions of the display polarisers are different to those ofFIG.2A; andFIG.19Bis a front view of the stack of layers of the display device100ofFIG.19A. Features of the arrangement ofFIGS.19A-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 at least one polar control retarder300further comprises passive retarders332A,332B that are half wave plates that may be an A-plate with a retardance for light of a wavelength of 550 nm in a range of 250 nm to 300 nm.

Backlight20typically provides partially polarised light420incident onto the additional polariser318. In the present embodiments, desirably the electric vector transmission direction319of the additional polariser318is aligned to the linear polarisation transmission direction of the backlight20. It would be desirable to provide electric vector transmission direction319of additional polariser318to be at 0 degrees.

In the alternative embodiment ofFIGS.19A-B, half wave plate retarder332A has an optical axis direction333A that is inclined at 67.5 degrees to the easterly direction, so that the polarisation state of light incident onto the liquid crystal retarder301is 135 degrees. Advantageously transmission efficiency of light from the backlight20is improved.

In the embodiment ofFIGS.19A-B, an SLM48input polariser210with horizontal transmission direction211and vertical output polariser218transmission direction219is provided. Such polariser arrangements are typically provided for fringe field switching LCDs and vertically aligned nematic LCDs. Advantageously wide angle contrast of images may be improved compared to the TN LCD ofFIG.2A. Further the output polarisation state219may be aligned to the transmission of polarised sunglasses so that advantageously display brightness is improved in displays where the display user is wearing sunglasses.

In the embodiment ofFIG.19A, a further passive retarder332B has an optical axis direction333B that is inclined at 22.5 degrees so that for light rays directed to the point427, the output polarisation state from the polar control retarder300is aligned to be absorbed by the input polariser210as described elsewhere herein.

In alternative embodiments one of the retarders332A,332B may be omitted and the additional polariser318and display polariser210,218aligned accordingly with respect to the directions417Ap,417Bp of the polar control retarder300. Advantageously performance may be improved and cost reduced.

In alternative embodiments, retarders332A,332B may comprise multi-layer retarders such as Pancharatnum retarders. Advantageously undesirable colouration of transmitted light may be reduced.

Alternatives to the arrangement of TABLE 1 will now be described.

FIGS.20A-Fare graphs illustrating the variation of transmission with polar direction for various arrangements of liquid crystal polar control retarder, display polarisers and additional polarisers.

More generally, the switchable liquid crystal retarder301may have a retardance for light of a wavelength of 550 nm in a range from 300 nm to 1500 nm, preferably in a range from 400 nm to 1200 nm. The twist may be in a range from 60° to 120°, and preferably in a range from 70° to 90°.

TABLE 3 andFIGS.20A-Billustrate an embodiment with 80° twist. In comparison to the embodiment ofFIG.4B, the amount of light directed to the side passenger window may be reduced. In comparison to the embodiment ofFIG.5C, share mode performance may be enhanced, with reduced colouration.

TABLE 3Active LC retarder 301In-planeIn-planeVCrotationrotationAlignment(representativeItemangleangleTwistlayersPretiltΔn.dprofile figure)318319, ϕA130°314417Ap, θA130°80°Homogeneous2°650 nm0 V (FIG. 20A)417Bp, θB50°Homogeneous2°1.6 V (FIG. 20B)210211, ϕB50°

TABLE 3 andFIGS.20C-Dillustrate an embodiment with 70° twist. In comparison to the embodiment of TABLE 3, the size of the high transmission viewing zone for the passenger45is increased, and on-axis luminance is increased for passenger45near to the optical axis199of the display. Further if retarders332A,332B are provided as described with respect toFIGS.19A-Bthen the angle of adjustment of polarisation to match desirable backlight20and SLM48alignment directions is advantageously reduced, reducing chromaticity and off-axis degradations from said retarders332A,332B.

TABLE 4Active LC retarder 301In-planeIn-planeVCrotationrotationAlignment(representativeItemangleangleTwistlayersPretiltΔn.dprofile figure)318319, ϕA125°314417Ap, θA125°70°Homogeneous2°1000 nm0 V (FIG. 20C)417Bp, θB55°Homogeneous2°1.55 V (FIG. 20D)210211, ϕB55°

TABLE 5 andFIGS.20E-Fillustrate an embodiment with 80° twist and additional polariser318that is crossed to display polariser210. In comparison to the embodiment of TABLE 3, the alignment of polarisers may be conveniently achieved without the cost and complexity of retarders332A,332B such as illustrated inFIGS.19A-B.

TABLE 5Active LC retarder 301In-planeIn-planeVCrotationrotationAlignment(representativeItemangleangleTwistlayersPretiltΔn.dprofile figure)318319, ϕA135°314417Ap, θA130°80°Homogeneous2°650 nm0 V (FIG. 20E)417Bp, θB50°Homogeneous2°1.55 V (FIG. 20F)210211, ϕB50°

It may be desirable to provide privacy operation for an emissive display.

FIG.21is a schematic side perspective view of a display device100comprising an emissive SLM48and a polar control retarder300and additional polariser arranged to receive light from the SLM48. Features of the arrangement ofFIG.21not 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 embodiment ofFIG.21, the SLM48comprises an emissive SLM48arranged to emit the spatially modulated light. 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.1A. Said display polariser210is an output display polariser218arranged on the output side of the SLM48.

Advantageously thickness is reduced in comparison to the embodiment ofFIG.2A. Emissive displays also typically advantageously exhibit high image contrast for the passenger45in comparison to conventional LCDs.

In the embodiment ofFIG.21, there is not a reflective polariser arranged between the output display polariser218and the at least one polar control retarder300. In liquid crystal modes that are asymmetric in the predetermined direction (such as shown inFIG.4B), reflectivity of light from a reflective polariser (as will be described hereinbelow with respect toFIG.23CandFIG.25A) is reduced because of high input transmission of light from an ambient light source (such as illustrated by source604inFIG.2A) but low reflected luminance. Such reflective polarisers as will be described thus have reduced efficacy for asymmetric modes.

FIG.22is a schematic side perspective view of an optical component comprising a polar control retarder300comprising electrodes arranged to provide a variable voltage V profile322in the lateral direction across at least one region of the polar control retarder300. Features of the arrangement ofFIG.22not 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.

It may be desirable to provide reduction of luminance to the driver over an increased polar range and to a lower light level.

FIG.23Ais a schematic side perspective view of a display device100providing uniformity for a display user47comprising first and second polar control retarders300A,300B with further additional polariser318B arranged on the input side of the additional polariser318. Features of the arrangement ofFIG.23Anot 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 ofFIGS.2A-C, in the alternative embodiment ofFIG.23A, the display device100further comprises: a further additional polariser318B arranged on the same side of the SLM48as the additional polariser318A outside the additional polariser318A, the further additional polariser318B being a linear polariser; and at least one further polar control retarder300B arranged between the additional polariser318A and the further additional polariser318B.

The at least one further polar control retarder300B comprises a switchable liquid crystal retarder301B comprising a layer314B of liquid crystal material414B, and the display device100further comprises first and second electrode arrangements320BA,320BB disposed on opposite sides of the layer314B of liquid crystal material414, wherein first and second electrode arrangements320BA,320BB are arranged to provide an electric field370perpendicular to the layer314B of liquid crystal material414, wherein the magnitude of the electric field370changes monotonically along a predetermined axis across at least part of display device100.

The retardance, twists, and or pre-tilts of the liquid crystal layer314A.314B may be different for the polar control retarder300A,300B respectively. The profiles322A,322B may be different and may provide common points427or different points427at which transmission uniformity is greatest as described hereinbefore. Additional passive retarders332AA,332AB,332BA,332BB (not illustrated) may be provided to achieve desirable polarisation rotations as described elsewhere herein, for example with reference toFIGS.19A-B.

Advantageously different polar regions may have reduced transmission, increasing polar region of driver47location for high image security.

The embodiment ofFIG.23Ahas the SLM48arranged between the view angle control arrangement310and the output side of the display device100. Advantageously reduced visibility of frontal reflections from additional layers may be achieved.

FIG.23Bis a schematic side perspective view of a display device100providing uniformity for a display user comprising first and second polar control retarders300A,300B with further additional polariser318B and second polar control retarder300B arranged on the output side of the display output polariser218. Features of the arrangement ofFIG.23Bnot 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.23A, in the alternative embodiment ofFIG.23B, the display device100comprises: an output display polariser218arranged on the output side of the SLM48; a further additional polariser318B arranged on the output side of the SLM48; and at least one further polar control retarder300B arranged between the further additional polariser318B and the output display polariser218.

The at least one further polar control retarder300B comprises a switchable liquid crystal retarder301B comprising a layer314B of liquid crystal material414B, and the display device100further comprises first and second electrode arrangements320BA,320BB disposed on opposite sides of the layer314B of liquid crystal material414, wherein first and second electrode arrangements320BA,320BB are arranged to provide an electric field370perpendicular to the layer314B of liquid crystal material414, wherein the magnitude of the electric field370changes monotonically along a predetermined axis across at least part of display device100.

The retardance, twists, and or pre-tilts of the liquid crystal layer314A.314B may be different for the polar control retarder300A,300B respectively. The profiles322A.322B may be different and may provide common points427or different points427at which transmission uniformity is greatest as described hereinbefore. Additional passive retarders332AA,332AB,332BA,332BB (not illustrated) may be provided to achieve desirable polarisation rotations as described elsewhere herein, for example with reference toFIGS.19A-B.

Advantageously different polar regions may have reduced transmission, increasing polar region of driver47location for high image security.

The display device100may further comprise a reflective polariser302arranged between the output polariser218and the at least one polar control retarder300, the reflective polariser being a linear polariser arranged to pass the same linearly polarised polarisation component as the output polariser218. The polarisers218,318B may have electric vector transmission directions that are parallel and the polar control retarder300B may be arranged to provide a more rotationally symmetric polar output than that provided by the polar control retarder300A. In alternative embodiments, the profile322B may be omitted and a uniform voltage applied to the further polar control retarder300B. Advantageously the reflectivity of the display to off-axis light may be improved when the layer314is arranged for privacy mode operation. Security factor, S may advantageously be improved.

FIG.23Cis a schematic side perspective view of a display device100providing uniformity for a display user comprising first and second polar control retarders300A,300B arranged on the output side of the output display polariser318. Features of the arrangement ofFIG.23Cnot 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 ofFIGS.23A-B, in the alternative embodiment ofFIG.23A, such an arrangement may be provided for an emissive display. Further, the view angle control element310may be provided as a single component, reducing cost and complexity.

It may be desirable to further improve the security factor of a display.

FIG.24Ais a schematic side perspective view of a display device100providing uniformity for a display user comprising a first polar control retarder300with a voltage V profile322and a second polar control retarder300with a profile of alignment layer orientations.

In the alternative embodiment ofFIG.24A, the polar control retarder300A may comprise a twisted liquid crystal layer314A as described elsewhere hereinbefore.

By way of comparison, the further polar control retarder300B comprises passive polar control retarder330, i.e. at least one passive compensation retarder, and a layer314B of liquid crystal material414B provided by a switchable liquid crystal retarder301B. In general, the polar control retarder300B may comprise any configuration of at least one retarder, some examples of which are present in the devices described below.

An illustrative arrangement of alignment layers419BA,419BB for the alternative embodiment ofFIG.24Awill now be further described.

FIG.24Bis perspective views of polar control retarder300B that may be applied inFIG.24A, comprising a homeotropically and homogeneously aligned switchable LC retarder301and a negative C-plate passive retarder330and wherein the alignment direction of the homeotropic alignment layer419BA is common across the lateral direction and wherein the alignment direction of the homogeneous alignment layer419BB varies across the lateral direction. Features of the arrangements ofFIG.24Bnot 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 polar controlretarder 330Active LC retarder 301Δn.d/AlignmentPretilt/Δn.d/Voltage/ModeTypenmlayersdegnmVPublicNegative−900Homogeneous2100010.0PrivacyCHomeotropic881.4

FIG.24Bis a front view of a first surface alignment layer419BA of the display device100ofFIG.24Awherein the angle of in-plane component of the alignment varies along the predetermined direction, for example the x-axis. Features of the arrangement ofFIG.24Bnot 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 layer419BA may have material that provides different alignment orientations417BLp,417BCp,417BRp across the surface alignment layer419BB. The operation of the different alignment layer orientations is described further in U.S. Pat. No. 11,079,646 and in U.S. Pat. No. 11,099,448, both of which are herein incorporated by reference in their entireties.

The operation and function of the arrangement ofFIG.24Bis different to that achieved by the arrangement ofFIG.3A, other than appropriate phase shifts are provided across the predetermined direction. Advantageously increased uniformity of illumination to on-axis viewers may be increased.

The alignment orientation417BLp,417BCp,417BRp change monotonically across at least part of the display device100. 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 profile may be provided for displays arranged to be operated at longer viewing distances. Advantageously uniformity of luminance may be optimised for the passenger45and uniformity of security factor may be optimised for the driver47.

FIG.24Cis a schematic graph illustrating the variation of transmission with polar direction for the polar control retarder ofFIG.24Bin a privacy mode of operation; andFIG.24Dis a schematic graph illustrating the variation of transmission with polar direction for the polar control retarder ofFIG.24Bin a share mode of operation.

In comparison withFIG.4B, the profile ofFIG.24Chas higher symmetry. Symmetry may be reduced by increasing the average rotation of the alignment layer417BB. In comparison to the profile ofFIG.5C, there is substantially no change with viewing angle and thus colour may be improved.

Returning to the description ofFIG.24A, a display may be provided with higher off-axis reduction of luminance levels, increasing polar region of high security factor.

FIG.25Ais a schematic side perspective view of a display device100providing uniformity for a display user comprising a first polar control retarder300with a voltage V profile322arranged between the backlight20and transmissive SLM48, and a reflective polariser, further polar control retarder300and further additional polariser arranged to receive light from the SLM48. Features of the arrangement ofFIG.25Anot 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 ofFIGS.23B-C, increased symmetry of the second polar control retarder300B may be achieved, and a larger region with high reflectivity achieved. Advantageously the polar region for high security factor may be increased. The second polar control retarder300B may have variable alignment across the predetermined direction as illustrated inFIG.24Bor may have a common alignment across the predetermined direction. Advantageously the size of the polar region of increased reflectivity may be increased and security factor may be increased.

The embodiments ofFIGS.23A-C.FIG.24AandFIG.25Aillustrate that the polar control retarders300A,300B are provided with a common predetermined direction. In other embodiments, not shown, the predetermined directions may be different.

For example, the predetermined directions may be opposite. Returning to the description ofFIG.1B, one of the polar control retarders300A.300B may be arranged to provide privacy to the left side of the display device100for user47, and the other of the polar control retarders300A,300B may be arranged to provide privacy to the right side of the display device100for user49.

In other embodiments, the predetermined directions may be crossed and one of the polar control retarders300A,300B may be arranged to provide privacy to the left side of the display device100for user47, and the other of the polar control retarders300A.300B may be arranged to provide privacy for the vertical direction, for example to minimise windscreen reflections in an automotive vehicle650.

FIG.25Bis a schematic graph illustrating the variation of reflectivity with polar direction for the polar control retarder300B and reflective polariser302ofFIG.25Aand TABLE 6 in a share mode of operation; andFIG.25Cis a graph illustrating a simulated polar profile of the security level, S of the arrangement ofFIG.25for an ambient illuminance measured in lux that is the same as the head-on display luminance measured in nits and for the polar profile of backlight luminance ofFIG.16A, polar profile of polar control retarder300B transmission ofFIG.24Cand reflectivity of polar control retarder300B and reflective polariser302ofFIG.25B.

In comparison to the arrangement ofFIG.25C, the polar region for desirable security factor is substantially increased. Further the pupillation of the polar control retarders300A,300B and backlight20may achieve advantageously high image security uniformity around the direction448across the area of the display device100.

The structure and operation of various alternative backlights20that provide desirable illumination characteristics for the switchable privacy display device100of the present embodiments will now be described further.

FIG.26Ais a schematic diagram illustrating a side view of the switchable backlight20ofFIG.2comprising waveguides1A,1B, a rear reflector3and an optical turning film component50and outputting light beams445,447with the angular distributions as illustrated inFIGS.4A-C;FIG.26Bis a schematic diagram illustrating a front perspective view of an optical turning film component50for the backlight20ofFIG.26A; andFIG.26Cis a schematic diagram illustrating a side view of an optical turning film component50. Features of the embodiments ofFIGS.26A-Cnot 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 backlight20comprises: at least one first light source15A arranged to provide input light; at least one second light source15B arranged to provide input light in an opposite direction from the at least one first light source15A; a waveguide arrangement11comprising at least one waveguide1, the waveguide arrangement11being arranged to receive the input light from the at least one first light source and the at least one second light source and to cause light from the at least one first light source and the at least one second light source to exit from the waveguide arrangement11by breaking total internal reflection; and an optical turning film component50comprising: an input surface56arranged to receive the light exiting from a waveguide1through a light guiding surface8of the waveguide1by breaking total internal reflection, the input surface56extending across the plane; and an output surface58facing the input surface56, wherein the input surface56comprises an array of prismatic elements51. The prismatic elements51may be elongate.

The waveguide arrangement11comprises: a first waveguide1A extending across a plane and comprising first and second opposed light guiding surfaces arranged to guide light along the waveguide, the second light guiding surface being arranged to guide light by total internal reflection; and a first input end2A arranged between the first and second light guiding surfaces6A.8A and extending in a lateral direction between the first and second light guiding surfaces6A,8A; wherein the at least one first light source15A is arranged to input light445into the first waveguide1A through the first input end, and the first waveguide1A is arranged to cause light from the at least one first light source15A to exit from the first waveguide1A through one of the first and second light guiding surfaces6A.8A by breaking total internal reflection; a second waveguide1B extending across the plane arranged in series with the first waveguide1A and comprising first and second opposed light guiding surfaces6B,8B arranged to guide light along the waveguide1B, the second light guiding surface8B being arranged to guide light by total internal reflection, and a second input end2B arranged between the first and second light guiding surfaces6B.8B and extending in a lateral direction between the first and second light guiding surfaces6B.8B; wherein the at least one second light source15B is arranged to input light447into the second waveguide1B through the second input end2B, and the second waveguide1B is arranged to cause light from the at least one second light source15B to exit from the second waveguide1B through one of the first and second light guiding surfaces6B.8B by breaking total internal reflection, and wherein the first and second waveguides1A,1B are oriented so that at least one first light source15A and at least one second light source15B input light445,447into the first and second waveguides1A,1B in opposite directions.

The optical turning film component50comprises: an input surface56arranged to receive the light444A,444B exiting from the waveguide arrangement11through a light guiding surface of the at least one waveguide1A,1B of the waveguide arrangement by breaking total internal reflection, the input surface56extending across the plane; and an output surface58facing the input surface, wherein the input surface56comprises an array of prismatic elements52. The prismatic elements each comprise a pair of elongate facets52defining a ridge54therebetween. Angles ϕA, ϕBof prism surfaces53A.53B are provided to direct the nominal light output from waveguides1A,1B to directions445,447by refraction and reflection at surfaces53A,53B. Advantageously desirable illumination directions such as illustrated inFIGS.4A-Fmay be achieved by selection of angles ϕA, ϕB.

The backlight20ofFIG.26Amay provide the exemplary luminance profiles ofFIGS.16A-Bhereinabove. In operation, the light444A from the first light source15A exits the backlight20with a first angular distribution445as illustrated inFIG.16Aand the light from the second light source15B exits the backlight20with a second angular distribution457as illustrated inFIG.16Bdifferent from the first angular distribution455. The first angular distribution455may be symmetrical about an axis199of symmetry of the backlight20and the second angular distribution457is asymmetrical about the same axis199of symmetry of the backlight20. In a left-hand drive vehicle, the asymmetrical distribution457may be to the left of the axis199of symmetry of the backlight20and in a right-hand drive vehicle the asymmetrical distribution457may be to right of the axis199of symmetry of the backlight20.

Waveguides1A,1B comprise surface relief features that are arranged to leak some of the guiding light either towards the rear reflector3or towards the light turning component50. Each waveguide1A.1B comprise a surface relief30arranged on the first side6A,6B that may comprise prism surfaces32,33. Further the second sides8A,8B may further comprise surface relief31that may comprise elongate features or prism features as illustrated inFIG.14Dhereinbelow. In operation the surface reliefs30,31provide leakage of light445,447from the waveguide1A,1B for light guiding along the waveguide1A,1B.

FIG.27is a schematic diagram illustrating a side view of a switchable backlight20comprising a waveguide1, first and second light sources15at respective opposite input sides of the waveguide1, a rear reflector and an optical turning film and outputting light beams for passenger45and driver47nominal directions. Features of the embodiment ofFIG.27not 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 waveguide arrangement11comprises: a waveguide1extending across a plane, wherein the waveguide1is an optical waveguide, and comprising: first and second opposed light guiding surfaces6,8arranged to guide light along the waveguide1, the second light guiding surface being arranged to guide light by total internal reflection8, and first and second input ends2A,2B arranged between the first and second light guiding surfaces6,8and extending in a lateral direction between the first and second light guiding surfaces6,8; wherein the at least one first light source15A is arranged to input light445into the waveguide1through the first input end2A and the at least one second light source15B is arranged to input light447into the waveguide1through the second input end2B, and the waveguide1is arranged to cause light from the at least one first light source15A and the at least one second light source15B to exit from the waveguide1through one of the first and second light guiding surfaces6,8by breaking total internal reflection.

It may be desirable to pupillate the output of the backlight20in a manner similar to the pupillation of the transmission of the polar control retarder300.

FIG.28Ais a schematic diagram illustrating in side-view operation of a backlight20comprising a pupillating light turning film component50; andFIG.28Bis a schematic diagram illustrating in front perspective view, operation of a pupillating light turning film component50. Features of the embodiments ofFIG.28AandFIG.28Bnot 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 embodiment ofFIG.28A, the tilt of the facets53A,53B are adjusted so that the highest luminance output from the backlight20from each part of the light turning component50is directed towards a common point825.

In operation,FIG.28Aillustrates that light rays415G are output from the waveguide1and directed onto the facets53B,53A of the light turning component50. The inclination angles of the facets53A,53B are adjusted so that light rays415U,415C,415D are directed towards a common point825, wherein the light rays415have the maximum luminous intensity of the light cone425.

FIG.28Billustrates that the light turning film component may further have curved elongate facets with facet tangents γL, γC, γRthat vary along the predetermined direction. In this manner, pupillation may be provided in both x and y directions from the convergence of light rays415DR.415CR,415UR,415DC,415CC,415UC,415DL,415CL,415UL. Advantageously increased uniformity may be provided in both lateral and vertical directions to the display user45.

The point825may be arranged to be at or near the nominal viewing location of the passenger45. Advantageously uniformity of the backlight20is improved for the user45. Such improved uniformity may cooperate with the improved uniformity from the polar control retarder300described elsewhere herein.

Alternative arrangements of backlights20suitable for use in the display ofFIG.2Awill now be described.

FIG.29is a schematic diagram illustrating a side view of a switchable backlight20comprising a first waveguide1A, light turning component50, a micro-louvre film66and second waveguide1B. Features of the embodiment ofFIG.29not 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.26A, the backlight20comprises waveguide1A arranged between rear reflector3and light turning component50, the operation of which is described hereinabove.

In a privacy mode of operation, the light sources15A are illuminated and the waveguide1A and optical turning film component50and provides light rays445with cone angle461. Such cone angle461may be larger than desirable for viewing locations of driver47at angles near to angle α ofFIG.1Asuch that undesirable security factor may be achieved.

Reflective recirculation polariser64may optionally be provided to achieve light recirculation in the backlight20. Advantageously efficiency may be increased. The reflective recirculation polariser64is different to the reflective polariser302ofFIG.25A. 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 polariser210or additional polariser318A. Reflective recirculation polariser64does not reflect ambient light604to a snooper.

Micro-louvre film66(such as ALCF™ from 3M Corporation) may be provided that for input light cone461outputs light cone467that has a smaller angular size for a given proportional luminance roll-off. Such light cone467is substantially transmitted by the waveguide1B. Features30B,32B arranged on the light guiding surfaces6B,8B of the waveguide1B may be arranged to minimise scatter in the predetermined direction.

In driver47illumination and in share mode the light sources15B are operated and light cones457around light rays447are output from the waveguide1B. As the illumination is predominately off-axis then a second light turning film component to redirect light rays447is not provided.

Advantageously a switchable backlight20with a narrow cone angle to user45may be provided. Stray light to driver47may be reduced in comparison to the arrangement ofFIG.26A, advantageously achieving improved security factor.

FIG.30is a schematic diagram illustrating a side view of a switchable backlight20comprising a first waveguide1A, prismatic recirculation films60,62, a micro-louvre sheet66and second waveguide1B. Features of the embodiment ofFIG.30not 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 embodiment ofFIG.30, the light turning film component50is omitted and at least one brightness enhancement film60,62provides recirculation of on-axis scattered light from the rear reflector3. In comparison to the alternative embodiment ofFIG.29, the output cone461may have a larger size and so losses may increase and off-axis luminance may also increase in the region around the angle α ofFIG.1A. Advantageously cost and complexity may be reduced and uniformity may be increased.

It may be desirable to provide increased dynamic range and reduced power consumption.

FIG.31is a schematic diagram illustrating a side view of a switchable backlight20comprising a mini-LED array15Aa-n, prismatic recirculation films60,62, a micro-louvre sheet66, waveguide1B and light source15B. Features of the embodiment ofFIG.31not 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 toFIG.30, the embodiment ofFIG.31comprises array15Aa-n of mini-LEDs that are distributed across the area of the backlight20. The array15Aa-n may provide blue output light and additional diffusers70and wavelength conversion sheet72may be provided to achieve white light output.

In operation for illumination of passenger45, the array15Aa-n may be modulated with image data and aligned to pixels220,222,224of the SLM48. Advantageously image contrast may be increased.

FIG.32Ais a schematic diagram illustrating a perspective side view of a steerable backlight20comprising a waveguide arrangement11comprising a stepped waveguide1, and addressable light source array15a-n; andFIG.32Bis a schematic diagram illustrating a side view of a display steerable backlight20comprising a stepped waveguide, and addressable light source array and rear reflector. Features of the embodiment ofFIGS.32A-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.

Such a stepped waveguide is further described in U.S. Pat. No. 9,519,153 and in U.S. Pat. No. 10,054,732, both of which are herein incorporated by reference in their entireties.

Waveguide1comprises an input end2, a reflective end4and first and second light guiding surfaces6,8arranged between the input end2and reflective end4. The second light guiding surface8may be a planar surface and the first light guiding surface6may comprise a stepped structure comprising steps12and intermediate regions10that may be planar.

In operation, light from at least some of light sources15a-nis input at the input end2and guided substantially without loss to the reflective end4. Reflected light rays are guided back towards the steps12by means of the surface8,10at which point they are extracted from the waveguide through total internal reflection or by refraction. ConsideringFIG.32A, light sources15in region31are directed towards driver47and light sources in region33are directed towards the passenger45.

ConsideringFIG.32B, refracted light rays445bare incident on rear reflector3comprising reflective facets34,36and directed towards passenger45, similarly refracted light rays447b(not shown) are directed towards driver47. Advantageously output brightness is increased.

The light extraction features12may be curved and optical pupils325,327may be provided towards passenger45, and driver47. Advantageously image uniformity may be increased.

FIG.32Cis a schematic diagram illustrating a perspective rear view of a steerable backlight20comprising a stepped waveguide1, addressable light source array15spatial light modulator48and polar control retarder300to illuminate a driver47and passenger45. Features of the embodiment ofFIG.32Cnot 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 operation for a moving passenger47with movement of measured distance545from a reference location, the light sources15a-nare controlled such that region33is adjusted to move location by distance542in correspondence to the distance of movement540. The light sources15are controlled accordingly such that the input illuminated region31is moved by distance542. Optical window325is then maintained at a location near to the passenger, and light leakage to window327is minimised. As described elsewhere herein, the polar control retarder300may similarly be controlled to achieve desirable luminance reduction at the driver47measured distance of movement540from a reference location. Advantageously luminance to passenger45is increased and security factor to driver47is increased.

In share mode operation, all of the light sources15a-nmay be operated to achieve wide angle operation, together with share mode operation of the polar control retarder300, described elsewhere herein.

A thin and steerable backlight for a switchable privacy display with high image security may advantageously be achieved.

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.