Light control device

Abstract

A compact head-up display having a layered stack of components The layered stack of components comprises a waveguide and a Pancharatnam-Berry, "PB", lens. The waveguide is arranged to receive a wavefront corresponding to a picture and output an array of replicas thereof by (partial) waveguiding between a pair of reflective surfaces. The PB lens is arranged to receive the array of replicas from the waveguide and apply (e.g. non-uniform) optical power thereto.

Description

[0001] LIGHT CONTROL DEVICE

[0002] FIELD

[0003] The present disclosure relates to a light control layer or corrective optic. The present disclosure also relates to a display system comprising the light control layer. The present disclosure further relates to methods of fabricating a light control layer or corrective optic. Some embodiments relate to a holographic projector, picture generating unit or head-up display.

[0004] BACKGROUND AND INTRODUCTION

[0005] Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or “hologram”, comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object.

[0006] Computer-generated holography may numerically simulate the interference process. A computer-generated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. A Fourier hologram may be considered a Fourier domain / plane representation of the object or a frequency domain / plane representation of the object. A computer-generated hologram may also be calculated by coherent ray tracing or a point cloud technique, for example.

[0007] A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micromirrors, for example.

[0008] A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission.

[0009] A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, “HUD”.

[0010] Diffractive optical elements composed of liquid crystal photopolymer (LCP) have a number of applications. A method for creating LCP having a spatially varying LCP tilt angle, to optical elements created with the same, and to applications of the LCP, such as beam-steering switch used in telecommunication networks have been described. In the case of non-flat diffractive optics, the etching and / or moulding processes used to form these optical elements are relatively complex and time consuming. In addition, the surface relief structure generally requires complex optical thin-film coating processes to protect the delicate structures. Because these elements are flat, there is an advantage in an optical projection applications where a coherent image bearing beam is transmitted by the element.

[0011] The present disclosure relates to reactive mesogens and Pancharatnam-Berry “PB” methods and optics. In particular, the inventors have recognised significant benefits that these can bring to compact head-up display based around a waveguide design. Reactive Mesogen optics can be designed to be much thinner and lighter than conventional corrective lenses, facilitating easier integration into compact optical systems. The polarization-independent nature of PB optics makes them highly versatile and effective in various lighting conditions, eliminating the performance variability associated with polarization-sensitive systems. The ability to achieve continuous tuning of the director in the PB system allows for precise control over the optical properties, enhancing the performance and adaptability of the optical system.

[0012] SUMMARY

[0013] Aspects of the present disclosure are defined in the appended independent claims.

[0014] A first aspect of the present disclosure is a (compact) head-up display having a layered stack of components. The layered stack of components comprises a waveguide and a Pancharatnam-Berry, “PB”, lens. The waveguide is arranged to receive a wavefront corresponding to a picture and output an array of replicas thereof by (partial) waveguiding between a pair of reflective surfaces. The PB lens is arranged to receive the array of replicas from the waveguide and apply (e.g. non-uniform) optical power thereto. The components of the layer stack of components may each substantially planar and parallel to each other such that the respective surface normals of components are substantially parallel.

[0015] The array of replicas output by the waveguide may comprise linearly-polarised light, the layered stack of components further comprises: a first quarter-wave plate disposed between the waveguide and PB lens; and a second quarter-wave plate disposed after the PB lens.

[0016] The (e.g. non-uniform) optical power of the PB lens may comprise negative optical power.

[0017] The compact head-up display may further comprise a turning film disposed between the waveguide and first quarter-wave plate.

[0018] The waveguide may be substantially planar such as slab-shaped.

[0019] The optical axis of the wavefront may be angled with respect to a surface normal of the waveguide or PB lens.

[0020] The optical axis of the wavefront may be angled with respect to a surface normal of the waveguide or PB lens on two orthogonal planes containing the surface normal.

[0021] A first angle of the wavefront with respect to the surface normal of the waveguide, or PB lens, on a first plane of the two orthogonal planes may be in the range 20-35 degrees.

[0022] A second angle of the wavefront with respect to the surface normal of the waveguide, or PB lens, on a second plane of the two orthogonal planes may be in the range 10-20 degrees.

[0023] A second aspect of the present disclosure is a display system comprising the compact head- up display of the first aspect and an optical combiner (e.g. vehicle windscreen). The optical combiner may be arranged to receive the output of the compact head-up display and direct (e.g. reflect) the array of replicas to a viewing plane of the display system, wherein the (e.g. non-uniform) optical power of the PB lens at least partially compensates for a non-uniform optical power of the optical combiner (arising from a spatially-variant curvature thereof). A third aspect of the present disclosure is a method of fabricating a Pancharatnam-Berry, “PB”, lens. The method comprises a first step of (evenly) splitting a (collimated) light beam into a first path comprising first (e.g. left circularly) polarised light and a second path comprising second (e.g. right circularly) polarised light forming a reference beam. The method comprises a second step of Illuminating a template lens on the first path with the first (e.g. left circularly) polarised light to form a sample beam. The method comprises a third step of combining the sample beam and reference beam to form a combined beam. The method comprises a fourth step of illuminating a photo-alignment layer with the combined beam and recording a polarisation distribution of the combined beam in the photo-alignment layer. A surface normal of the photo-alignment layer is angled with respect to the (optical axis of the) combined beam.

[0024] The surface normal of the photo-alignment layer may be angled with respect to the (optical axis of the) combined beam on two orthogonal planes containing the optical axis of the combined beam.

[0025] An angle of the surface normal with respect to the (optical axis of the) combined beam on a first plane of the two orthogonal planes may be in the range 20-35 degrees.

[0026] An angle of the surface normal with respect to the (optical axis of the) combined beam on a second plane of the two orthogonal planes may be in the range 10-20 degrees.

[0027] The first polarised light and second polarised light may be linearly polarised in opposing directions.

[0028] The method may further comprise imparting a quarter-wave phase delay on the combined beam before illuminating the substrate.

[0029] The first polarised light and second polarised light may be circularly polarised with opposing handedness.

[0030] The method may further comprise forming a liquid crystal layer on the photo-alignment layer. The liquid crystal may comprise reactive mesogen.

[0031] The light beam may comprise ultraviolet light.

[0032] Other examples and arrangements disclosed herein are described in the following. In general terms, there is provided a light control device for display light that is arranged to compensate for the curvature of a curved optical component on an optical path of the display light. In embodiments, the light control device or glare mitigation device is for display light of a display system. In embodiments, the curved optical component is on an optical path of the display system. In some embodiments, the curved optical component is an optical combiner, such as a vehicle windscreen, arranged to redirect display light from a display device to a viewing window or so-called eye-box. The optical component may have a first curvature in a first direction and a second curvature in a second direction perpendicular to the first direction. The first and / or second curvature may be non-linear. The optical component has a complex curvature which introduces complex distortions when used in a display system particularly one based on holographic projection. In overview, there is a pair of prismatic layers designed in cooperation to simultaneously mitigate glare and compensate for the complex curvature of the optical component particularly when the display light is replicated as described in the following. The pitch or periodicity of the replicas is critical to the viewing experience.

[0033] The display light may be spatially modulated light. The display system may be arranged to relay the spatially modulated light to a viewing plane or eye-box. In some embodiments, the display system is a holographic display system and the spatially modulated light is light that is spatially modulated in accordance with a hologram. The spatially modulated light may be referred to as a holographic wavefront. The light control device of the present disclosure provides a means for controlling reflections of ambient light to prevent or suppress glare from reaching the viewing plane while allowing the spatially modulated light to reach the viewing plane. For example, the display device may comprise an optical component comprising a reflective surface. In the absence of the light control device, ambient light may be reflected by the reflective surface towards the viewing plane I eye-box of the display device thus forming glare. The light control device of the present disclosure is arranged to suppress such reflections.

[0034] As above, the light control device of the present disclosure is further arranged to compensate for the curvature of a curved optical component on an optical path of the display system. The curved optical component being on the optical path of the display system may mean that the spatially modulated light, propagating through the display system, may be incident on, reflected by, transmitted through, or otherwise interact with the curved optical component. As the skilled person will appreciate, the curvature of the optical component may alter the divergence or convergence of the spatially modulated light and angles thereof. For example, if the spatially modulated light is substantially collimated upstream of the curved optical component (prior to interacting with the optical component), then the spatially modulated may be non-parallel (e.g. converging or diverging) downstream of the curved optical component (after interacting with the optical component). In other words, the curved optical component may have a lensing effect on the spatially modulated light incident thereon. If the curvature of the curved optical component is non-uniform, then the lensing effect may be non-uniform. For example, different portions of the curved optical component may have a different radius of curvature and so may have a different lensing effect on spatially modulated light incident thereon. In some embodiments, the curved optical component is a windscreen or windshield of a vehicle. A windscreen or windshield may have a complex curvature having a complex lensing effect on display light incident thereon.

[0035] A number of problems associated with the lensing effect of the curved optical component have been identified. One problem is that the lensing effect may distort the display light (of the display system). For example, the display light may be such that a picture is viewable at a viewing plane. For example, the display light may be spatially modulated in accordance with a hologram of a picture, or simply in accordance with a picture. The lensing effect of the curved optical component may distort the picture that is viewable at the viewing plane. This may adversely affect a viewing experience of the display system. Another problem identified by the inventors is specific to display systems comprising a replicator, upstream of the curved optical component. The replicator may be arranged to replicate the spatially modulated light to form a plurality of replicas of the spatially modulated light. For example, if the spatially modulated light is a holographic wavefront, the replicator may be arranged to form a plurality of replicas of the holographic wavefront. In embodiments, the replicator may be a waveguide, as described below. For example, the waveguide may comprise an input port arranged to receive the spatially modulated light. The waveguide may comprise a pair of surfaces arranged to waveguide the spatially modulated light received at the input therebetween. A first surface of the pair of surfaces may be partially-transmissive partially- reflective. The first surface may be arranged to form the plurality of replicas of the spatially modulated light. At least a portion of the first surface may be said to form an output port of the replicator / waveguide. The replicator may be arranged such that the plurality of replicas are relayed towards the curved optical component. The display system may be further arranged such that the plurality of replicas are relayed towards a viewing plane I eye-box of the display system. The inventors have found that the pitch of the replicas of the spatially modulated light (at the viewing plane) is important for ensuring a good viewing experience. Through simulation and experimentation, it was found that the pitch of the replicas may be affected by the lensing effect of the curved optical component. For example, the pitch of the replicas at the viewing plane may be increased or decreased. This may adversely affect the viewing experience. For example, if the pitch of the replicas is reduced, so-called ghosting effects, in which a copy of the intended picture or image content is displayed slightly offset from the intended picture or image content, may become more apparent. The pitch of the replicas may be reduced if the curved optical component has a concave shape, for example the inside surface of a windscreen or windshield. As used herein, the pitch of replicas refers to the separation or distance between the centres of adjacent replicas.

[0036] The inventors have previously proposed light control devices I glare mitigation devices for reflection I glare suppression.

[0037] For example, in UK patent number GB2607672, there is disclosed a light control device for glare mitigation. GB2607672B described a light control device having a serrated sunlightreceiving surface providing an array of angled surfaces that can be arranged to direct sunlight away from a particular area, for example away from the eye-box of a head-up display. In particular, each of the angled surfaces forms an interface between a material forming the serrated surface and air such that most of the sunlight incident thereon (e.g. 96% of incident sunlight) is reflected (i.e. not coupled into the light control device I component). The angled surfaces are orientated at an angle with respect to a plane of the light control device. The angled surfaces are arranged to direct the reflected sunlight away from a direction towards the eye-box, owing to the orientation angle thereof. It was also described in GB2607672B how, in examples, the angled surfaces of the sunlight-receiving surface change the angle of reflection of sunlight incident thereon. That is, the angle of rays of sunlight received by the angled surfaces is different from the angle of rays of the sunlight reflected by the angled surfaces, where the angles of the rays of sunlight are measured with respect to (the normal to) the plane of the light control film / optical component in the first and second dimensions. It may be said that each angled surface changes the course or path of specular reflection of incident sunlight in comparison to specular reflection by non-angled surfaces (parallel to a plane of the light control component).

[0038] There was proposed an improvement to the light control device in UK patent application number GB2303536.3 (publication number GB2627988A). GB2303536 discloses a reflection suppression device comprising a first layer, an intermediate layer and a second layer. The first layer comprises a first serrated surface arranged to receive a holographic wavefront from a waveguide and provide a first turn of the holographic wavefront having at least a component on a first plane containing a surface normal of the waveguide. The first layer is formed of a transparent material. The intermediate layer is arranged to receive the holographic wavefront from the first layer, wherein the intermediate layer comprises a plurality of louvres in an array. Each louvre comprises a light absorbing material. The second layer is arranged to receive the holographic wavefront from the intermediate layer. The second layer comprises a second serrated surface arranged to provide a second turn of the holographic wavefront having at least a component on the first plane. The component of the second turn on the first plane is equal and opposite to the component of the first turn on the first plane. In GB2303536, the second (top) serrated surface of the reflection suppression device may perform as the serrated sunlight-receiving surface of GB2607672B and so is arranged to deflect sunlight. The second serrated surface may be provided as an array of prisms, the prisms forming the serrated sunlight-receiving surface. The inventors recognised that such a prismatic layer introduced a (second) turn component on the first plane and so there is a need to correct for deviations of the holographic wavefront (in at least the first plane) to compensate for the changes introduced by the turn. Thus, the first (bottom) serrated surface of the reflection suppression device was introduced, having serrations (e.g. an array of prisms) arranged to cancel out the (second) turn component on the first plane with a (first) turn.

[0039] The turn of the second layer is determined by the refractive index of the material forming the second layer and an angle of the serrations of the serrated surface. As above, the angle of the serrations of the serrated surface are selected to mitigate I suppress glare. The inventors have found that there is a very narrow range of angles of the serrations that are optimized for glare mitigation. Thus, the turn of the second layer is constrained by the need for the serrations to fall within this very narrow range of angles. So, generally, all serrations I prisms of the second layer may have exactly the same angle (falling within the very narrow range).

[0040] It might be assumed that the first layer of the reflection suppression device of GB2607672B would be constrained to exactly cancel out the turn applied by the second layer such that all serrations I prisms of the first layer also have exactly the same angle. However, it was found that there is some freedom to deviate slightly from this constraint such that the net turn differs slightly from zero for at least some of the prism pairs of the first and second layers. It was recognised that providing a varying or changing net-turn for different prism pairs (that is still close to zero) enables the reflection suppression device I light control device to be arranged such that the curvature of a curved optical component on an optical path of a display system can be compensated for - i.e. is nullified. There was prejudiced against arranging the reflection suppression device such that the net turn provided by different prism pairs varies. Generally, the reflection suppression device is optimised so that all the prisms are the same. This may mean that all louvres or passive faces of the prisms can be orientated in the same way, to be precisely aligned with HUD light. It is generally recognised as important to orientate the louvres I passive faces in this way to avoid the introduction of dark bands in the HUD light (where the HUD light is absorbed by the louvres I passive faces). Dark bands can be minimised when all HUD passes through the reflection suppression device at substantially the same propagation angle. This is no longer the case when different prism pairs provide different net turns. So, there was prejudiced based on the thinking it would introduce dark bands into the HUD-light, unacceptably deteriorating the viewing experience. However, after throughout simulation and experimentation, it was recognised that only small angular differences were needed to achieve the desired compensation. These small angular differences result in minimal banding of the HUD-light.

[0041] Furthermore, arranging the reflection suppression device such that the net turn provided by different prism pairs varies increases the complexity of the reflection suppression device, making it more complicated and expensive to manufacture. It was found that this increase in complexity is worth it for the benefits achieved. Furthermore, providing a varying or changing net-turn for different prism pairs has the effect of distorting the spatially modulated light propagating through the device. Normally, it would be expected that this distortion would adversely affect the viewing experience. It is counterintuitive to intentionally distort the spatially modulated light. But it was recognised that the distortion can be selected to at least partially if not exactly compensate (or pre-compensate) for another source of distortion caused by the curved optical element (e.g. the windscreen I windshield). In other words, it was recognised that a layer of prisms can be selected which effectively combine to act as a composite lens having an opposite lensing effect of the curved optical element causing unwanted distortion (e.g. the windscreen I windshield).

[0042] In a first example, there is provided a light control device for display light. The light control device may be described as processing display light. The processing may be to pre-distort I compensate for distortion caused by a curved optical component. The light control device comprises a first surface. The first surface comprises an array of first prisms. Each first prism is arranged to provide a first turn of a respective portion of display light received thereon. The light control device further comprises a second surface. The second surface comprises an array of second prisms. Each second prism is arranged to receive a respective portion of the display light from a corresponding first prism. Thus, it may be said that there is a plurality of optically coupled prism pairs. Alternatively, it may be said that there is a plurality of prismatic substructures in which each substructure comprises one or more first prism optically coupled to one or more second prisms. In other words, a single sub-structure may be defined by optically coupled prisms of the first and second surfaces. As used herein, a sub-structure may refer to an area, region or volume of the light control device containing the optically coupled prisms.

[0043] Each second prism is arranged to provide a second turn of the respective portion of the display light.

[0044] In examples, a net turn provided by a first prism pair is different to the net provided by a second prism pair. In other words, different pairs of optically coupled prism pairs may provide a varying (i.e. non constant) net turn. In other words, the first and second pairs of prisms are arranged slightly differently to one another. The prisms of the second surface are not simply arranged to exactly cancel out the turn of the prisms of the first surface. Counterintuitively, the inventors have provided a more complex arrangement in which the net turn differs from prism pair to prism pair. As described above, this may enable the light control device to compensate or pre-compensate for distortions caused by other optical components in a display system. At least one, optionally both, of the first and second pairs of prisms provides a non-zero net turn.

[0045] This effect can alternatively be described in terms of the net turn provided by different substructures of the light control layer. Thus, it may alternatively or additionally be said that a net turn provided by a first prismatic sub-structure of the light control device is different to the net turn provided a second prismatic sub-structure. As above, each prismatic substructure comprises one or more first prisms optically coupled to one or more second prisms. Thus, the first prismatic sub-structure may comprise the first prism pair and the second prismatic sub-structure may comprise the second prism pair.

[0046] Each of the turns (provided by the prisms of the first and second layers) may be described as refractive turns. As the skilled reader will appreciate, prisms typically turn or redirect light as a result of refraction. The refraction of light that occurs at an optical interface of a prism happens because the spatially modulated light changes speed as it passes from a first medium having a first refractive index to a second medium having a second refractive index (such as air to glass or glass to air). As light enters the prism, it may bend towards the normal if it's entering a denser medium. When the light exits the prism it may bend away from the normal if it’s entering a less denser medium. The prisms of the first and second layers may be triangular in shape, with two triangular ends and three rectangular sides or faces. The angles and shape of the prism determine how much the light is bent.

[0047] Herein, a prism pair refers to a pair of optically coupled prisms. In some embodiments, the number of first prisms in the array of the first surface is substantially equal to (or exactly equal to) the number of second prisms in the array of the second surface. In such embodiments, each first prism may be optically coupled to one (and only one) second prism. In other words, there may be a one to one ratio between the optical coupling of first prisms to second prisms. Thus, in such embodiments, there may be as many prism pairs as there are first or second prisms in the respective arrays. In other embodiments, however, each first prism may be optically coupled to more than one second prism or vice versa. In other words, there may be a many to one ratio between the optical coupling of first prisms to second prisms or vice versa. This may be the case if the number of first prisms in the array of the first surface is not equal to the number of second prisms in the array of the second surface. For example, the first prisms may extend further in a first dimension than the second prisms, optionally twice as far. In other words, the first prisms may be larger than the second prisms. It should be clear to the skilled reader that, even in many-to-one embodiments, there exists a prism pair comprising a first prism optically coupled to a second prism. The second prism in the pair is arranged to provide a second turn to a portion of display light received from the respective first portion. In arrangements in which there is a many-to-one or one-to-many relationship between prisms of one layer being optically coupled to prisms of the other layer, it may be convenient to refer to sub-structures defined by the optically coupled prisms.

[0048] The “net” turn may refer to the vector sum of the first and second turns (provided, respectively, to spatially modulated light is as it passes through a first prism of the first layer and a second prism of the second layer that is optically coupled to the first prism). The net turn may refer to turns or components of turns on a particularly plane, e.g. a first plane. The first, second and net turns may be turns on a plane containing the propagation axis or so- called k-vector of the display light. This may be the first plane.

[0049] Herein, a first and second pair of the plurality prism pairs have been described as providing different net turns. It should be appreciated that there may be more (or many more) prism pairs each providing different net turns. For example, the plurality of prism pairs may comprise 50 prism pairs or more, 100 prism pairs or more, or 200 prism pairs or more. At least 25%, optionally at least 50%, optionally at least 75%, optionally all of the prism pairs may provide a unique (i.e. different) net turn. As described, the prism pairs may be arranged such that the varying differences in net-turn provided by the prism pairs combines to form a composite lens - that is, an optical structure providing the optical effect of a composite lens or equivalent to a composite lens. A Fresnel lens may be an example of a composite lens. The composite lens may be arranged to compensate for the curvature of the curved optical component.

[0050] In some arrangements, the net turn provided by the plurality of prism pairs or prismatic substructures varies from pair to pair according to a continuous function such as a linear or nonlinear function. While the net turn provided by individual pairs is continuous discrete, the This may mean that the net turn varies as a function of the position of the first I second prisms within the respective first and second arrays of prisms. The continuous function may correspond to a shape of the optical component that the light control device is arranged to compensate for. For example, if the optical component has a curved shaped, the continuous function may be a curved function having an inverse of the shape of the curved shape of the optical component. In some embodiments, the curved function is a polynomial function such as a quadratic function. In some embodiments, a net turn provided by at least one of the prism pairs or at least one of the prismatic sub-structures is zero. The net turn provided by other prism pairs or prismatic sub-structures may increase with distance from a prism pair providing a net-zero turn.

[0051] In some arrangements, the first turn provided by at least some, optionally each, first prism is arranged to (only) partially compensate for a second turn provided by the or each corresponding second prism on the respective spatially modulated light. Complete compensation of the second turn by the respective first prism may mean that the first prism or prism pair or prismatic sub-structure provides a net-zero turn. Partial compensation of the second turn by the respective first prism may mean that the first prism or prism pair or prismatic substructure provides a non-zero net turn. As described above, the second surface and second prisms may be arranged to mitigate glare I deflect ambient light away from a viewing plane of a display system. The introduction of a second surface comprising an array of second prisms results in a turn I deflection of the display light. Thus, it is advantageous to provide the first surface also comprising an array of (first) prisms to compensate for the turn of the second prisms. Also as described above, the inventors assumed that it would be necessary for the first prisms to completely compensate for the turn of the second prisms. However, the inventors have recognised that it is possible to only partially compensate for the second turn (for at least some of the prism pairs or prismatic sub-structures). The gap, delta or difference between complete and partial compensation can be used to compensate for optical components in the display system such as to compensate for the curvature of a curved optical component of the display system, as described above. In some arrangements, a difference between the maximum net turn provided by a prism pair of the plurality of prism pairs and the minimum net turn provided by a prism pair of the plurality of prism pairs is 5 degrees or less, optionally 2 degrees or less, optionally 1 degree or less. In other words, the net turn may vary by 5 degrees or less, optionally 2 degrees or less, optionally 1 degree or less.

[0052] Unless otherwise specified, features described in relation to prism pairs may also apply to prismatic sub-substructures. Thus, a difference between the maximum net turn provided by a prismatic sub-substructure of the plurality of prismatic sub-substructures and the minimum net turn provided by a prismatic sub-substructure of the plurality of prismatic subsubstructures is 5 degrees or less, optionally 2 degrees or less, optionally less than 1 degree or less.

[0053] Because the difference between the maximum and the minimum net turn provided by prism pairs may be relatively low (e.g. less than 5 or 2 degrees), and because the net turn may change is a continuous function, and because there may be relatively many prism pairs (e.g. more than 50, more than 100, or more than 200 prism pairs), the difference between the net turn provided by adjacent prism pairs may be relatively very low. In some embodiments, a difference between the net turn provided by adjacent prism pairs (optionally between each adjacent prism pair) is less than 1 / 10thof a degree, optionally less than 1 / 25thof a degree, optionally less than 1 / 50thof a degree.

[0054] The difference between the net turn provided by the first and second prism pairs may be such that the divergence of spatially modulated light transmitted by the second surface is increased relative to the divergence of spatially modulated light received by the first structure (or the convergence of spatially modulated light transmitted by the second surface is decreased relative to the convergence of spatially modulated light received by the first structure). This may compensate for decreased divergence I increased convergence caused by a curved optical component upstream or downstream of the light control layer

[0055] In some arrangements, the second prism pair is adjacent to the first prism pair. In some embodiments, the net turn provided by all adjacent prism pairs is different. In some embodiments, all prism pairs are different. In some arrangements, the first turn and second turn of each pair is different owing to a different angle of attack / incidence or different refractive index for different pairs. In some embodiments, the first turn and second turn of each pair are opposite in direction.

[0056] In some arrangements, each first prism comprises a receiving surface for receiving respective spatially modulated light. A normal of each receiving surface of the first prisms of the first surface may make an angle with a normal of the light control device. The first structure is arranged such that said angle varies between first prisms for at least some of the first prisms of the array, optionally for all of the first prisms of the array. In some embodiments, the receiving surface of each first prism is substantially planar.

[0057] In some arrangements, each second prism comprises a transmitting surface for transmitting respective spatially modulated light. A normal of each transmitting surface of the second prisms of the second structure may make an angle with a normal of the light control device. The second structure may be arranged such that said angle is substantially equal for all second prisms.

[0058] In some arrangements, a first angle is definable between the receiving surface of the first prism and the transmitting surface of the second prism of the first pair. A second angle may be definable between the receiving surface of the first prism and the transmitting surface of the second prism of the second pair. The first angle may be different to the second angle.

[0059] In some arrangements, the first surface (and I or first prisms) is I are formed of a material having a refractive index, n1 , greater than 1. The second surface (and I or second prisms) is I are formed of a material having a refractive index, n2, greater than 1. In some embodiments, n1 is substantially equal to n2. In other words, the refractive index of the materials forming the first and second surfaces is substantially equal. In some embodiments, the first serrated structure and the second serrated structure are formed of the same material.

[0060] Each first prism of the first surface may comprise an active face. Each first prism of the first surface also comprises a passive face. The active face and the passive face of each prism may meet at a corner of the prism. The active face and the passive face of each prism may form a triangle shape. Because each first prism may comprise an active face and a passive face, and the first prisms are arranged in an array, the first surface may comprise a plurality of active faces and a plurality of passive faces in an alternating configuration. Each active face may be described as forming a first facet surface of an individual serration. Each passive face may be described as forming a second facet face of an individual serration. Each active face may make a first angle with respect to a plane of the component. Each passive face may make a second angle with respect to a plane of the component. The first angle may be different to the second angle.

[0061] Each first prism may extend longitudinally in a second dimension. The second dimension may be perpendicular to the first dimension, described above. The first and second dimensions may define or lie in a second plane. The light control device may be substantially planar. A plane of the planar light control device may be parallel to the second plane.

[0062] Normals of the first plane and the second plane may be perpendicular to one another.

[0063] Each active face and each passive face of each first prism may extend longitudinally in the second dimension. The array of first prisms may extend in the first dimension. Herein, unless otherwise specified, the angles and shape of the active and passive face may be defined in terms of a cross-section of the component in the first plane. The first plane may contain a third dimension that is perpendicular to the first and second dimensions.

[0064] Each of the second prisms of the array of prisms of the second surface may also comprise an active face and a passive face. The active and passive faces of the second prisms may have corresponding features to those described with respect to the first prism.

[0065] As used herein, an active face of a prism may refer to a face through which it is intended for display light to be transmitted. In other words, these may be the faces of the first prisms that receive the display light and I or the faces of the second prisms that transmit the display light. Thus, the receiving faces of the first prisms, described above, may be active faces, as may the transmitting faces of the second prisms, also described above. A prism pair, being optically coupled, may refer to the fact that at least a portion of the display light received by an active face of a first prism may be relayed to and received by an active face of a second prism. The respective first and second prisms may be referred to as forming a prism pair.

[0066] If the device of the first example is used in a head-up display, the active faces may be the faces of the prism array through which the spatially modulated light of the head-up display passes. The head-up display light is referred to as HUD-light herein. The HUD-light may be spatially modulated light and / or a holographic wavefront and / or a plurality of replicas thereof having a (complex) replica pitch. As the skilled reader will appreciate, it may be important for the active faces to have a minimal impact on the HUD-light. For example, the active faces may be polished so as to allow for transmission of the HUD-light through the active face without surface roughness adversely affecting the HUD-light, for example by reducing image quality.

[0067] As used herein, a passive face of a prism may refer to a face through which it is not intended for light to be transmitted. For example, each passive face may comprise a layer, such as a paint layer, arranged to absorb light such that light may not be transmitted through the passive face. In other words, each passive face may be arranged to be substantially non- transmissive or opaque. The passive face may be angled so as to be substantially parallel to display light or HUD-light passing through the component when the component is in use. This may minimise the impact of the passive face on the HUD-light. In particular, this may minimise the amount of HUD-light that is absorbed by the passive face. Thus, each passive face may advantageously be arranged to not block the HUD-light (which may be intended to be received at an eye-box) while blocking light at other angles which may include sunlight I glare or otherwise scattered light.

[0068] In some arrangements, each prism pair is optically aligned with a sub-area of the curved optical component arranged to receive the display light from the light control device. Each prism pair may provide a net turn which compensates for the local curvature of the corresponding sub-area.

[0069] In some arrangements, the difference between the net turn provided by the first and second prism pairs is or corresponds to the changing difference compensating for a curvature of the curved optical component.

[0070] In some arrangements, the light control device further comprises a plurality of louvres in an array, each louvre comprising a light absorbing material.

[0071] In some arrangements, the light control device comprises an intermediate layer comprising the array of louvres. The intermediate layer may be positioned between the first and second layers. In such arrangements, the intermediate layer may comprise a transparent core material in which the array of louvres may be embedded.

[0072] In some arrangements, the array of louvres is provided on the first or second prisms of the first or second layer. In some embodiments, individual louvres are provided on the passive face of each prism of the respective layer. In some arrangements, a first array of louvres is provided on the first prisms of the first layer and a second array of louvres is provided on the second prism of the second layer. The louvres may extend longitudinally in the second dimension and may be spaced apart in the first dimension. The separation between adjacent louvre slats in the first dimension may be defined as the pitch. In some arrangements the pitch may be the same for all louvre slats of the array. It may be said that the spacing or periodicity of the louvres is uniform for the array. In other arrangements, the pitch may vary between slats of the array, such as from a first end to a second end of the array. It may be said that the spacing or periodicity of the slats of the array is non-uniform for the array.

[0073] The louvres of the array of louvres may comprise a material having one or more of: high absorption; high attenuation; low specular reflectivity, and high diffusivity of light. The person skilled in the art of optics appreciates what constitutes “high” and “low” in relation to the optical properties of a material. In some embodiments, the term “high” means greater than 80% such as greater than 90% or 95% and the term “low” means less than 20% such as less than 10% or 5%. For example, the term “high attenuation” may mean that the intensity of incident light (e.g. sunlight) is attenuated (i.e. reduced) by at least 95%.

[0074] The louvre structure may comprise a one-dimensional array of parallel longitudinal slats, each slat having a length, width (height) and a thickness. In embodiments, the louvres / louvre slats have fixed positions in the array and remain static in use. In some embodiments, the slats have a uniform thickness. In other embodiments, the slats may vary in thickness, for example the thickness may be tapered along their width from the proximal end / edge to the distal end / edge.

[0075] In some arrangements, the angle of the (sidewalls of the) louvres / louvre slats may be uniform (i.e. constant) across the array. In other arrangements, the angle of the (sidewalls of the) louvres / louvre slats may vary across the array, such as from a first end to a second end of the array.

[0076] The louvres I slats (or slat sidewalls) may be orientated so as to be aligned with I parallel to a central or “gut” ray of the HUD-light received at the intermediate layer. In this way, the louvres may be arranged to substantially transmissive to the HUD-light. Typically, the HUD- light received at the intermediate layer will be non-parallel to the normal of the component I light control device. So, the louvres I slats (or slat sidewalls) may be orientated (inclined) at a (non-zero) angle to said normal. In a second example, there is provided a light control device. The light control device may be for a display system. The light control device may be arranged to relay spatially modulated light to a viewing plane. The light control device may be arranged to relay the spatially modulated light on an optical path comprising a curved optical component (such as a windshield I windscreen). The light control layer of the second example comprises a first structure I first layer I first surface comprising an array of first prisms. Each first prism comprises a receiving surface for receiving spatially modulated light. Each first prism is arranged to provide a first turn on the received spatially modulated light. The light control layer further comprise a second structure I second layer I second surface comprising an array of second prisms. Each second prism is optically coupled with (e.g. paired with or associated with) a respective first prism. This may mean that spatially modulated light received by the respective first prism is received by one or more respective second prism to be transmitted by a transmitting surface of the one or more second prisms. In embodiments, the first turn provided by each first prism is arranged to (only) partially compensate for a second turn provided by the or each corresponding second prism on the respective spatially modulated light. Because of the partial compensation (rather than complete compensation), there may be a difference between the first turn and the second turn provided by each prism pair. A difference between the first turn and the second turn provided by the first and second prisms changes between at least some of the optically coupled prisms. The changing difference in net turn compensates for a curvature of the curved optical component.

[0077] In a third example, there is provided a display system comprising the light control device of the first or second example. The display system further comprises a curved optical component. The display system may be arranged to relay display light (such as spatially modulated light) to a viewing window or eye-box of the display system along an optical path. The curved optical component may be on the optical path. The light control device may also be on the optical path. Thus, the display light may interact with the curved optical component and the light control device when relayed to the viewing plane. The curved optical component may be an optical combiner such as a windshield or a windscreen, such as a windshield or a windshield of a vehicle. The curved optical component may have a complex curved shape. As described above, the light control device may be arranged to compensate for the distortion of display light incident on

[0078] In a fourth example, there is provided a method of processing display light. The method comprises the step of receiving display light at a first surface of a light control device. The first surface comprises an array of first prisms. The method further comprises the step of providing a first turn to each portion of the display light received by a first prism. The method further comprises the step of receiving the turned display light at a second surface of the light control device. The second surface comprises an array of second prisms such that there are a plurality of optically coupled prism pairs. The method further comprises providing a second turn to each portion of the display light received by a second prism. The net turn provided by a first prism pair is different to the net turn provided by a second prism pair.

[0079] Herein, processing the display light means that the different prism pairs apply different net turns to different portions of the display light. This effect compensates for a distortion caused by a curved optical component (e.g. a curved optical combiner such as a windscreen or windshield).

[0080] In some arrangements, the method of the processing display light comprises use of the light control device of the first and second examples. In other words, the first and second surfaces referred to above in the fourth example may correspond to the first and second surfaces of the light control device of the first example (or the second example).

[0081] In a first arrangement, a display system is provided. The display system comprises a replicator arranged to receive spatially modulated light and replicate the spatially modulated light to form a plurality of replicas of the spatially modulated light by waveguiding between a reflective surface and a transmissive-reflective surface. The transmissive-reflective surface forms an output surface for the plurality of replicas of the spatially modulated light. The display system further comprises a light control device located in the optical path of the plurality of replicas of the spatially modulated light downstream from the output surface of the replicator. The light control device is arranged to compensate for the curvature of an curved optical component downstream from the light control device. The compensation is a function of the position on the output surface.

[0082] The spatially modulated light may be a holographic wavefront and the replicator may be arranged to form a plurality of replicas of the holographic wavefront.

[0083] The light control device may have an opposite lensing effect of the curved optical element. The lensing effect of the curved optical component may distort the picture that is perceived from the viewing plane. In some embodiments, the light control device provides a “counterdistortion” or compensation so that the picture is undistorted after the spatially modulated light has interacted with the curved optical component. The light control device may provide the optical effect of a composite lens or equivalent to a composite lens. The light control device may be a composite lens.

[0084] The light control device may be a Fresnel structure. The light control device may be a Fresnel lens or part of a Fresnel lens. In this way, the size of the component is reduced.

[0085] The light control device may comprise a first surface and a second surface, the first and second surface being complementary. The complementary nature of the first and second surfaces means that the only distortion applied to the spatially modulated light by the light control device is that desired to produce the compensation described herein. No further distortion is added by the light control device itself.

[0086] The light control device may comprise a first surface comprising an array of first prisms, each first prism arranged to provide a first turn of a portion of display light received thereon. The light control device may also comprise a second surface comprising an array of second prisms, each second prism being arranged to receive a portion of the display light from a corresponding first prism thereby forming a plurality of optically coupled prism pairs. Each second prism may be arranged to provide a second turn of the portion of the display light. The net turn provided by a first prism pair may be different to the net turn provided by a second prism pair.

[0087] The light control device may be a diffractive optic element. Alternatively, the light control device may be a holographic optic element.

[0088] The compensation may have a negative optical power. This power is in contrast to the positive optical power provided by the curved optical element, thus aiding the compensation.

[0089] The compensation may be a phase function, optionally a phase-delay function. The inventors have found that the optical power of the optical element can be cancelled using a phase function. That is, a phase value that varies over the area of the waveguide. It may be said that the phase function is a global function across the surface of a waveguide to reflect that the phase value is a function of location on the surface of the waveguide. This means that the spatially modulated light arrives at the user correctly and the perceived picture is undistorted by the optical element.

[0090] The light may be spatially modulated in accordance with a picture. Alternatively, the light may be spatially modulated in accordance with a hologram of a picture. The hologram may be arranged to divide the spatial content of the picture by angle such that angles of the spatially modulated light correspond to spatial coordinates of the picture.

[0091] The display system may further comprise a numerical aperture expander arranged to increase a numerical aperture of the spatially modulated light received by the replicator. The numerical aperture expander may be a diffuser.

[0092] The display system may further comprise a view tracking system arranged to determine an eye-box position (of a viewer). The finite size of the eye-box forms a correlation between areas of the output surface (of the waveguide) and areas of the curved optical component. The tracking of the position of the viewer allows the display system to adjust to the movement of the user such that the spatially modulated light arrives at the user’s pupils as intended. The correlation between the output surface and the areas of the curved optical component allow for a global phase function to compensate for the shape of the optical element as described herein.

[0093] The display system may further comprise the curved optical component located downstream from the light control device. The curved optical component may be an optical combiner. The curved optical component may be a windscreen of a vehicle.

[0094] There is disclosed herein a light control device or corrective optic. The light control device comprises a plurality of elongate (or elongated) elements. The plurality of elongate elements is arranged on a plane. The elongate elements are substantially aligned (e.g. parallel) and adjoined in the elongate direction. The elongate elements are each ramped in cross section to form a plurality of first (elongate) surfaces that are inclined with respect to the plane (e.g. for glare suppression). At least one first surface comprises curvature.

[0095] The curvature may comprise first curvature of the ramp in cross section. The ramp is defined (or definable) by a first mathematical function. The first mathematical function may comprise a quadratic component or term that provides optical power (when illuminated). The first mathematical function may additionally comprise a linear component or term that provides an optical turn (when illuminated). The first mathematical function may additionally or alternatively comprise at least one component or term to the power of three or more that corresponds to an aberration (e.g. provides aberration correction). The first curvature of at least one elongate element may vary in the elongate direction. The curvature may comprise second curvature (of the first surface) in the elongate direction that defines an astigmatism (e.g. of the optical power of / provided by the first curvature). The second curvature may comprise second curvature of a proximal edge (with respect to the plane) and / or second curvature of a distal edge (with respect to the plane) of the first surface in the elongate direction. The second curvature of the proximal edge may be different to second curvature of the distal edge. The second curvature may be circular or an arc of a circle - when viewed in plan. The second curvature may be elliptical or an arc of an ellipse - when viewed in plan. The second curvature may further comprise at least one component or term to the power of three or more.

[0096] A periodicity or pitch of the ramp in cross section may be non-uniform (e.g. varies).

[0097] Each elongate element may further comprise a return surface that connects the distal edge of the ramp to the plane. Each return surface may be substantially perpendicular to the plane.

[0098] The at least one first surface may comprise a plurality of adjacent first surfaces forming a continuous section of the light control device.

[0099] The curvature may provide non-uniform optical power across the plane of the light control device.

[0100] The light control device may comprise a pair of (generally) orthogonal arrays of elongate elements as described above. The two orthogonal arrays may be different - e.g. comprise different curvature.

[0101] In the present disclosure, the term “replica” is merely used to reflect that spatially modulated light is divided such that a complex light field is directed along a plurality of different optical paths. The word “replica” is used to refer to each occurrence or instance of the complex light field after a replication event - such as a partial reflection-transmission by a pupil expander. Each replica travels along a different optical path. Some embodiments of the present disclosure relate to propagation of light that is encoded with a hologram, not an image - i.e. , light that is spatially modulated with a hologram of an image, not the image itself. It may therefore be said that a plurality of replicas of the hologram are formed. The person skilled in the art of holography will appreciate that the complex light field associated with propagation of light encoded with a hologram will change with propagation distance. Use herein of the term “replica” is independent of propagation distance and so the two branches or paths of light associated with a replication event are still referred to as “replicas” of each other even if the branches are a different length, such that the complex light field has evolved differently along each path. That is, two complex light fields are still considered “replicas” in accordance with this disclosure even if they are associated with different propagation distances - providing they have arisen from the same replication event or series of replication events.

[0102] A “diffracted light field” or “diffractive light field” in accordance with this disclosure is a light field formed by diffraction. A diffracted light field may be formed by illuminating a corresponding diffractive pattern. In accordance with this disclosure, an example of a diffractive pattern is a hologram and an example of a diffracted light field is a holographic light field or a light field forming a holographic reconstruction of an image. The holographic light field forms a (holographic) reconstruction of an image on a replay plane. The holographic light field that propagates from the hologram to the replay plane may be said to comprise light encoded with the hologram or light in the hologram domain. A diffracted light field is characterized by a diffraction angle determined by the smallest feature size of the diffractive structure and the wavelength of the light (of the diffracted light field). In accordance with this disclosure, it may also be said that a “diffracted light field” is a light field that forms a reconstruction on a plane spatially separated from the corresponding diffractive structure. An optical system is disclosed herein for propagating a diffracted light field from a diffractive structure to a viewer. The diffracted light field may form an image.

[0103] The term “hologram” is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, regarding the object. The term “holographic reconstruction” is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The system disclosed herein is described as a “holographic projector” because the holographic reconstruction is a real image and spatially- separated from the hologram. The term “replay field” is used to refer to the 2D area within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of a plurality diffracted orders wherein each diffracted order is a replica of the zeroth-order replay field. The zeroth-order replay field generally corresponds to the preferred or primary replay field because it is the brightest replay field. Unless explicitly stated otherwise, the term “replay field” should be taken as referring to the zeroth-order replay field. The term “replay plane” is used to refer to the plane in space containing all the replay fields. The terms “image”, “replay image” and “image region” refer to areas of the replay field illuminated by light of the holographic reconstruction. In some embodiments, the “image” may comprise discrete spots which may be referred to as “image spots” or, for convenience only, “image pixels”.

[0104] The terms “encoding”, “writing” or “addressing” are used to describe the process of providing the plurality of pixels of the SLM with a respective plurality of control values which respectively determine the modulation level of each pixel. It may be said that the pixels of the SLM are configured to “display” a light modulation distribution in response to receiving the plurality of control values. Thus, the SLM may be said to “display” a hologram and the hologram may be considered an array of light modulation values or levels.

[0105] It has been found that a holographic reconstruction of acceptable quality can be formed from a “hologram” containing only phase information related to the Fourier transform of the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography.

[0106] The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully- complex computer-generated hologram is calculated.

[0107] Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for “phase-delay”. That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2TT) which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of TT / 2 will retard the phase of received light by TT / 2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term “grey level” may be used to refer to the plurality of available modulation levels. For example, the term “grey level” may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term “grey level” may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator.

[0108] The hologram therefore comprises an array of grey levels - that is, an array of light modulation values such as an array of phase-delay values or complex modulation values. The hologram is also considered a diffractive pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light having a wavelength comparable to, generally less than, the pixel pitch of the spatial light modulator. Reference is made herein to combining the hologram with other diffractive patterns such as diffractive patterns functioning as a lens or grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the replay field on the replay plane or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a replay plane in the near field.

[0109] Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged.

[0110] BRIEF DESCRIPTION OF THE DRAWINGS

[0111] Specific embodiments are described by way of example only with reference to the following figures:

[0112] Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen;

[0113] Figure 2 shows an image for projection comprising eight image areas / components, V1 to V8, and cross-sections of the corresponding hologram channels, H1-H8;

[0114] Figure 3 shows a hologram displayed on an LCOS that directs light into a plurality of discrete areas;

[0115] Figure 4 shows a system, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3;

[0116] Figure 5A shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each comprising pairs of stacked surfaces;

[0117] Figure 5B shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each in the form of a solid waveguide; Figure 6 is a schematic side view of a reflection suppression device in combination with a turning layer;

[0118] Figure 7 is a schematic side view of a portion of the reflection suppression device of Figure 6 showing a holographic wavefront propagating therethrough;

[0119] Figure 8 is a schematic side view of a display system comprising the reflection suppression device and a curved optical combiner;

[0120] Figure 9 is schematic side view of a display system comprising a reflection suppression device and a curved optical combiner, wherein the reflection suppression device is arranged to compensate for a curvature of the curved optical combiner;

[0121] Figure 10 is an example of a global phase function;

[0122] Figure 11 A shows a top view of a first elongate element;

[0123] Figure 11 B shows a side view of the first elongate element of Figure 11 A;

[0124] Figure 12A shows a top view of a second elongate element;

[0125] Figure 12B shows a side view of the second elongate element of Figure 12A;

[0126] Figure 13A shows a top view of a third elongate element;

[0127] Figure 13B shows an end side view of the third elongate element of Figure 13A;

[0128] Figure 13C shows an cross-section side view of the third elongate element of Figure 13A, taken along line C-C;

[0129] Figure 14A shows a top view of a section of a light control device;

[0130] Figure 14B shows a side view of the section of the light control device of Figure 14A, showing how a group of elongate elements form a continuous curved surface;

[0131] Figure 15A shows an isometric view of a section of a light control device;

[0132] Figure 15B shows a side view of a section of the light control device of Figure 15A;

[0133] Figure 16A shows a method of manufacturing a Pancharatnam-Berry lens;

[0134] Figure 16B shows the liquid crystal configuration;

[0135] Figure 17 shows a method of manufacturing a Pancharatnam-Berry lens in accordance with this disclosure;

[0136] Figure 18 shows a schematic representation of a waveguide and an optical combiner;

[0137] Figure 19 shows a schematic representation of a head-up display in accordance with this disclosure, viewed on the x-z plane;

[0138] Figure 20 shows the schematic representation of Figure 19, viewed on the y-z plane;

[0139] Figure 21 shows a simplified version of Figure 19;

[0140] Figure 22 shows a simplified version of Figure 20; and

[0141] Figure 23 shows a method of manufacturing a freeform optic in accordance with this disclosure.

[0142] The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF EMBODIMENTS

[0143] The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration. Terms of a singular form may include plural forms unless specified otherwise.

[0144] A structure described as being formed at an upper portion / lower portion of another structure or on / under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between.

[0145] In describing a time relationship - for example, when the temporal order of events is described as “after”, “subsequent”, “next”, “before” or suchlike - the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as “just”, “immediate” or “direct” is used.

[0146] Although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims.

[0147] Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in codependent relationship.

[0148] In the present disclosure, the term “substantially” when applied to a structural units of an apparatus may be interpreted as the technical feature of the structural units being produced within the technical tolerance of the method used to manufacture it.

[0149] Conventional optical configuration for holographic projection

[0150] Figure 1 shows an embodiment in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of 1 the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or frequency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, “LCOS”, device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser.

[0151] A light source 110, for example a laser or laser diode, is disposed to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally planar wavefront of light to be incident on the SLM. In Figure 1, the direction of the wavefront is off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer). However, in other embodiments, the generally planar wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in Figure 1, the arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a lightmodulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to optics including a Fourier transform lens 120, having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives a beam of modulated light from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at the screen 125.

[0152] Notably, in this type of holography, each pixel of the hologram contributes to the whole reconstruction. There is not a one-to-one correlation between specific points (or image pixels) on the replay field and specific light-modulating elements (or hologram pixels). In other words, modulated light exiting the light-modulating layer is distributed across the replay field.

[0153] In these embodiments, the position of the holographic reconstruction in space is determined by the dioptric (focusing) power of the Fourier transform lens. In the embodiment shown in Figure 1, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens but the performance of the lens will limit the accuracy of the Fourier transform it performs. The skilled person understands how to use a lens to perform an optical Fourier transform In some embodiments of the present disclosure, the lens of the viewer’s eye performs the hologram to image transformation. Hologram calculation

[0154] In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or Fourier-based hologram, in which an image is reconstructed in the far field by utilising the Fourier transforming properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back to the lens plane. Computer-generated Fourier holograms may be calculated using Fourier transforms. Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms which may be calculated by a similar method. In some embodiments, the hologram is a phase or phase-only hologram. However, the present disclosure is also applicable to holograms calculated by other techniques such as those based on point cloud methods.

[0155] In some embodiments, the hologram engine is arranged to exclude from the hologram calculation the contribution of light blocked by a limiting aperture of the display system. British patent application 2101666.2, filed 5 February 2021 and incorporated herein by reference, discloses a first hologram calculation method in which eye-tracking and ray tracing are used to identify a sub-area of the display device for calculation of a point cloud hologram which eliminates ghost images. The sub-area of the display device corresponds with the aperture, of the present disclosure, and is used exclude light paths from the hologram calculation. British patent application 2112213.0, filed 26 August 2021 and incorporated herein by reference, discloses a second method based on a modified Gerchberg-Saxton type algorithm which includes steps of light field cropping in accordance with pupils of the optical system during hologram calculation. The cropping of the light field corresponds with the determination of a limiting aperture of the present disclosure. British patent application 2118911.3, filed 23 December 2021 and also incorporated herein by reference, discloses a third method of calculating a hologram which includes a step of determining a region of a so-called extended modulator formed by a hologram replicator. The region of the extended modulator is also an aperture in accordance with this disclosure.

[0156] In some embodiments, there is provided a real-time engine arranged to receive image data and calculate holograms in real-time using the algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are pre-calculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms. Large field of view using small display device

[0157] Broadly, the present disclosure relates to image projection. It relates to a method of image projection and an image projector which comprises a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system, in which the image projector projects or relays light from the display device to the viewing system. The present disclosure is equally applicable to a monocular and binocular viewing system. The viewing system may comprise a viewer’s eye or eyes. The viewing system comprises an optical element having optical power (e.g., lens / es of the human eye) and a viewing plane (e.g., retina of the human eye / s). The projector may be referred to as a ‘light engine’. The display device and the image formed (or perceived) using the display device are spatially separated from one another. The image is formed, or perceived by a viewer, on a display plane. In some embodiments, the image is a virtual image and the display plane may be referred to as a virtual image plane. In other examples, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed either in free space or on a screen or other light receiving surface between the display device and the viewer, is propagated to the viewer. In both cases, an image is formed by illuminating a diffractive pattern (e.g., hologram or kinoform) displayed on the display device.

[0158] The display device comprises pixels. The pixels of the display may display a diffractive pattern or structure that diffracts light. The diffracted light may form an image at a plane spatially separated from the display device. In accordance with well-understood optics, the magnitude of the maximum diffraction angle is determined by the size of the pixels and other factors such as the wavelength of the light.

[0159] In embodiments, the display device is a spatial light modulator such as liquid crystal on silicon (“LCOS”) spatial light modulator (SLM). Light propagates over a range of diffraction angles (for example, from zero to the maximum diffractive angle) from the LCOS, towards a viewing entity / system such as a camera or an eye. In some embodiments, magnification techniques may be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of an LCOS.

[0160] In some embodiments, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) - that may be informally said to be “encoded” with / by the hologram - is propagated directly to the viewer’s eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction I image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device.

[0161] Reference is made herein to a “light field” which is a “complex light field”. The term “light field” merely indicates a pattern of light having a finite size in at least two orthogonal spatial directions, e.g. x and y. The word “complex” is used herein merely to indicate that the light at each point in the light field may be defined by an amplitude value and a phase value, and may therefore be represented by a complex number or a pair of values. For the purpose of hologram calculation, the complex light field may be a two-dimensional array of complex numbers, wherein the complex numbers define the light intensity and phase at a plurality of discrete locations within the light field.

[0162] In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity / system, varies with the distance between the display device and the viewing entity. At a 1 metre viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye’s pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye’s pupil to form an image at the retina for a given eye position, determines the portion of the image that is ‘visible’ to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-box.)

[0163] In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device - that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it may therefore be considered that the viewer is looking at a virtual image through an ‘display device-sized window’, which may be very small, for example 1cm in diameter, at a relatively large distance, e.g., 1 metre. And the user will be viewing the display device-sized window via the pupil(s) of their eye(s), which can also be very small. Accordingly, the field of view becomes small and the specific angular range that can be seen depends heavily on the eye position, at any given time. A pupil expander addresses the problem of how to increase the range of angles of light rays that are propagated from the display device that can successfully propagate through an eye’s pupil to form an image. The display device is generally (in relative terms) small and the projection distance is (in relative terms) large. In some embodiments, the projection distance is at least one - such as, at least two - orders of magnitude greater than the diameter, or width, of the entrance pupil and / or aperture of the display device (i.e., size of the array of pixels).

[0164] Use of a pupil expander increases the viewing area (i.e., user’s eye-box) laterally, thus enabling some movement of the eye / s to occur, whilst still enabling the user to see the image. As the skilled person will appreciate, in an imaging system, the viewing area (user’s eye box) is the area in which a viewer’s eyes can perceive the image. The present disclosure encompasses non-infinite virtual image distances - that is, near-field virtual images.

[0165] Conventionally, a two-dimensional pupil expander comprises one or more one-dimensional optical waveguides each formed using a pair of opposing reflective surfaces, in which the output light from a surface forms a viewing window or eye-box. Light received from the display device (e.g., spatially modulated light from a LCOS) is replicated by the or each waveguide so as to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide enlarges the viewing window due to the generation of extra rays or “replicas” by division of amplitude of the incident wavefront.

[0166] The display device may have an active or display area having a first dimension that may be less than 10 cms such as less than 5 cms or less than 2 cms. The propagation distance between the display device and viewing system may be greater than 1 m such as greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide may be up to 2 m such as up to 1.5 m or up to 1 m. The method may be capable of receiving an image and determining a corresponding hologram of sufficient quality in less than 20 ms such as less than 15 ms or less than 10 ms.

[0167] In some embodiments - described only by way of example of a diffracted or holographic light field in accordance with this disclosure - a hologram is configured to route light into a plurality of channels, each channel corresponding to a different part (i.e. sub-area) of an image. The channels formed by the diffractive structure are referred to herein as “hologram channels” merely to reflect that they are channels of light encoded by the hologram with image information. It may be said that the light of each channel is in the hologram domain rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram and the hologram domain is therefore the Fourier or frequency domain. The hologram may equally be a Fresnel or Fresnel transform hologram. The hologram may also be a point cloud hologram. The hologram is described herein as routing light into a plurality of hologram channels to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-areas, wherein each hologram channel would correspond to each image sub-area. Importantly, the hologram of this example is characterised by how it distributes the image content when illuminated. Specifically and uniquely, the hologram divides the image content by angle. That is, each point on the image is associated with a unique light ray angle in the spatially modulated light formed by the hologram when illuminated - at least, a unique pair of angles because the hologram is two-dimensional. For the avoidance of doubt, this hologram behaviour is not conventional. The spatially modulated light formed by this special type of hologram, when illuminated, may be divided into a plurality of hologram channels, wherein each hologram channel is defined by a range of light ray angles (in two-dimensions). It will be understood from the foregoing that any hologram channel (i.e. sub-range of light ray angles) that may be considered in the spatially modulated light will be associated with a respective part or sub-area of the image. That is, all the information needed to reconstruct that part or sub-area of the image is contained within a sub-range of angles of the spatially modulated light formed from the hologram of the image. When the spatially modulated light is observed as a whole, there is not necessarily any evidence of a plurality of discrete light channels.

[0168] Nevertheless, the hologram may still be identified. For example, if only a continuous part or sub-area of the spatially modulated light formed by the hologram is reconstructed, only a sub-area of the image should be visible. If a different, continuous part or sub-area of the spatially modulated light is reconstructed, a different sub-area of the image should be visible. A further identifying feature of this type of hologram is that the shape of the cross-sectional area of any hologram channel substantially corresponds to (i.e. is substantially the same as) the shape of the entrance pupil although the size may be different - at least, at the correct plane for which the hologram was calculated. Each light I hologram channel propagates from the hologram at a different angle or range of angles. Whilst these are example ways of characterising or identifying this type of hologram, other ways may be used. In summary, the hologram disclosed herein is characterised and identifiable by how the image content is distributed within light encoded by the hologram. Again, for the avoidance of any doubt, reference herein to a hologram configured to direct light or angularly-divide an image into a plurality of hologram channels is made by way of example only and the present disclosure is equally applicable to pupil expansion of any type of holographic light field or even any type of diffractive or diffracted light field.

[0169] The system can be provided in a compact and streamlined physical form. This enables the system to be suitable for a broad range of real-world applications, including those for which space is limited and real-estate value is high. For example, it may be implemented in a head-up display (HUD) such as a vehicle or automotive HUD.

[0170] In accordance with the present disclosure, pupil expansion is provided for diffracted or diffractive light, which may comprise diverging ray bundles. The diffracted light field may be defined by a “light cone”. Thus, the size of the diffracted light field (as defined on a two- dimensional plane) increases with propagation distance from the corresponding diffractive structure (i.e. display device). It can be said that the pupil expander / s replicate the hologram or form at least one replica of the hologram, to convey that the light delivered to the viewer is spatially modulated in accordance with a hologram.

[0171] In some embodiments, two one-dimensional waveguide pupil expanders are provided, each one-dimensional waveguide pupil expander being arranged to effectively increase the size of the exit pupil of the system by forming a plurality of replicas or copies of the exit pupil (or light of the exit pupil) of the spatial light modulator. The exit pupil may be understood to be the physical area from which light is output by the system. It may also be said that each waveguide pupil expander is arranged to expand the size of the exit pupil of the system. It may also be said that each waveguide pupil expander is arranged to expand / increase the size of the eye box within which a viewer’s eye can be located, in order to see / receive light that is output by the system.

[0172] Light channelling

[0173] The hologram formed in accordance with some embodiments, angularly-divides the image content to provide a plurality of hologram channels which may have a cross-sectional shape defined by an aperture of the optical system. The hologram is calculated to provide this channelling of the diffracted light field. In some embodiments, this is achieved during hologram calculation by considering an aperture (virtual or real) of the optical system, as described above. Figures 2 and 3 show an example of this type of hologram that may be used in conjunction with a pupil expander as disclosed herein. However, this example should not be regarded as limiting with respect to the present disclosure.

[0174] Figure 2 shows an image 252 for projection comprising eight image areas / components, V1 to V8. Figure 2 shows eight image components by way of example only and the image 252 may be divided into any number of components. Figure 2 also shows an encoded light pattern 254 (i.e., hologram) that can reconstruct the image 252 - e.g., when transformed by the lens of a suitable viewing system. The encoded light pattern 254 comprises first to eighth sub-holograms or components, H1 to H8, corresponding to the first to eighth image components / areas, V1 to V8. Figure 2 further shows how a hologram may decompose the image content by angle. The hologram may therefore be characterised by the channelling of light that it performs. This is illustrated in Figure 3. Specifically, the hologram in this example directs light into a plurality of discrete areas. The discrete areas are discs in the example shown but other shapes are envisaged. The size and shape of the optimum disc may, after propagation through the waveguide, be related to the size and shape of an aperture of the optical system such as the entrance pupil of the viewing system.

[0175] Figure 4 shows a system 400, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3.

[0176] The system 400 comprises a display device, which in this arrangement comprises an LCOS 402. The LCOS 402 is arranged to display a modulation pattern (or ‘diffractive pattern') comprising the hologram and to project light that has been holographically encoded towards an eye 405 that comprises a pupil that acts as an aperture 404, a lens 409, and a retina (not shown) that acts as a viewing plane. There is a light source (not shown) arranged to illuminate the LCOS 402. The lens 409 of the eye 405 performs a hologram-to-image transformation. The light source may be of any suitable type. For example, it may comprise a laser light source.

[0177] The viewing system 400 further comprises a waveguide 408 positioned between the LCOS 402 and the eye 405. The presence of the waveguide 408 enables all angular content from the LCOS 402 to be received by the eye, even at the relatively large projection distance shown. This is because the waveguide 508 acts as a pupil expander, in a manner that is well known and so is described only briefly herein. In brief, the waveguide 408 shown in Figure 4 comprises a substantially elongate formation. In this example, the waveguide 408 comprises an optical slab of refractive material, but other types of waveguide are also well known and may be used. The waveguide 408 is located so as to intersect the light cone (i.e. , the diffracted light field) that is projected from the LCOS 402, for example at an oblique angle. In this example, the size, location, and position of the waveguide 408 are configured to ensure that light from each of the eight ray bundles, within the light cone, enters the waveguide 408. Light from the light cone enters the waveguide 408 via its first planar surface (located nearest the LCOS 402) and is guided at least partially along the length of the waveguide 408, before being emitted via its second planar surface, substantially opposite the first surface (located nearest the eye). As will be well understood, the second planar surface is partially reflective, partially transmissive. In other words, when each ray of light travels within the waveguide 408 from the first planar surface and hits the second planar surface, some of the light will be transmitted out of the waveguide 408 and some will be reflected by the second planar surface, back towards the first planar surface. The first planar surface is reflective, such that all light that hits it, from within the waveguide 408, will be reflected back towards the second planar surface. Therefore, some of the light may simply be refracted between the two planar surfaces of the waveguide 408 before being transmitted, whilst other light may be reflected, and thus may undergo one or more reflections, (or ‘bounces’) between the planar surfaces of the waveguide 408, before being transmitted.

[0178] Figure 4 shows a total of nine “bounce” points, B0 to B8, along the length of the waveguide 408. Although light relating to all points of the image (V1-V8) as shown in Figure 2 is transmitted out of the waveguide at each “bounce” from the second planar surface of the waveguide 408, only the light from one angular part of the image (e.g. light of one of V1 to V8) has a trajectory that enables it to reach the eye 405, from each respective “bounce” point, B0 to B8. Moreover, light from a different angular part of the image, V1 to V8, reaches the eye 405 from each respective “bounce” point. Therefore, each angular channel of encoded light reaches the eye only once, from the waveguide 408, in the example of Figure 4.

[0179] The waveguide 408 forms a plurality of replicas of the hologram, at the respective “bounce” points B1 to B8 along its length, corresponding to the direction of pupil expansion. As shown in Figure 4, the plurality of replicas may be extrapolated back, in a straight line, to a corresponding plurality of replica or virtual display devices 402’. This process corresponds to the step of “unfolding” an optical path within the waveguide, so that a light ray of a replica is extrapolated back to a “virtual surface” without internal reflection within the waveguide. Thus, the light of the expanded exit pupil may be considered to originate from a virtual surface (also called an “extended modulator” herein) comprising the display device 402 and the replica display devices 402’.

[0180] Although virtual images, which require the eye to transform received modulated light in order to form a perceived image, have generally been discussed herein, the methods and arrangements described herein can be applied to real images.

[0181] Two-Dimensional Pupil Expansion

[0182] Whilst the arrangement shown in Figure 4 includes a single waveguide that provides pupil expansion in one dimension, pupil expansion can be provided in more than one dimension, for example in two dimensions. Moreover, whilst the example in Figure 4 uses a hologram that has been calculated to create channels of light, each corresponding to a different portion of an image, the present disclosure and the systems that are described herebelow are not limited to such a hologram type.

[0183] Figure 5A shows a perspective view of a system 500 comprising two replicators, 504, 506 arranged for expanding a light beam 502 in two dimensions.

[0184] In the system 500 of Figure 5A, the first replicator 504 comprises a first pair of surfaces, stacked parallel to one another, and arranged to provide replication - or, pupil expansion - in a similar manner to the waveguide 408 of Figure 4. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially elongate in one direction. The collimated light beam 502 is directed towards an input on the first replicator 504. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), which will be familiar to the skilled reader, light of the light beam 502 is replicated in a first direction, along the length of the first replicator 504. Thus, a first plurality of replica light beams 508 is emitted from the first replicator 504, towards the second replicator 506.

[0185] The second replicator 506 comprises a second pair of surfaces stacked parallel to one another, arranged to receive each of the collimated light beams of the first plurality of light beams 508 and further arranged to provide replication - or, pupil expansion - by expanding each of those light beams in a second direction, substantially orthogonal to the first direction. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially rectangular. The rectangular shape is implemented for the second replicator in order for it to have length along the first direction, in order to receive the first plurality of light beams 508, and to have length along the second, orthogonal direction, in order to provide replication in that second direction. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), light of each light beam within the first plurality of light beams 508 is replicated in the second direction. Thus, a second plurality of light beams 510 is emitted from the second replicator 506, wherein the second plurality of light beams 510 comprises replicas of the input light beam 502 along each of the first direction and the second direction. Thus, the second plurality of light beams 510 may be regarded as comprising a two-dimensional grid, or array, of replica light beams.

[0186] Thus, it can be said that the first and second replicators 504, 505 of Figure 5A combine to provide a two-dimensional replicator (or, “two-dimensional pupil expander”). Thus, the replica light beams 510 may be emitted along an optical path to an expanded eye-box of a display system, such as a head-up display.

[0187] In the system of Figure 5A, the first replicator 504 is a waveguide comprising a pair of elongate rectilinear reflective surfaces, stacked parallel to one another, and, similarly, the second replicator 504 is a waveguide comprising a pair of rectangular reflective surfaces, stacked parallel to one another. In other systems, the first replicator may be a solid elongate rectilinear waveguide and the second replicator may be a solid planar rectangular shaped waveguide, wherein each waveguide comprises an optically transparent solid material such as glass. In this case, the pair of parallel reflective surfaces are formed by a pair of opposed major sidewalls optionally comprising respective reflective and reflective- transmissive surface coatings, familiar to the skilled reader.

[0188] Figure 5B shows a perspective view of a system 500 comprising two replicators, 520, 540 arranged for replicating a light beam 522 in two dimensions, in which the first replicator is a solid elongated waveguide 520 and the second replicator is a solid planar waveguide 540.

[0189] In the system of Figure 5B, the first replicator / waveguide 520 is arranged so that its pair of elongate parallel reflective surfaces 524a, 524b are perpendicular to the plane of the second replicator / waveguide 540. Accordingly, the system comprises an optical coupler arranged to couple light from an output port of first replicator 520 into an input port of the second replicator 540. In the illustrated arrangement, the optical coupler is a planar / fold mirror 530 arranged to fold or turn the optical path of light to achieve the required optical coupling from the first replicator to the second replicator. As shown in Figure 5B, the mirror 530 is arranged to receive light - comprising a one-dimensional array of replicas extending in the first dimension - from the output port I reflective-transmissive surface 524a of the first replicator / waveguide 520. The mirror 530 is tilted so as to redirect the received light onto an optical path to an input port in the (fully) reflective surface of second replicator 540 at an angle to provide waveguiding and replica formation, along its length in the second dimension. It will be appreciated that the mirror 530 is one example of an optical element that can redirect the light in the manner shown, and that one or more other elements may be used instead, to perform this task.

[0190] In the illustrated arrangement, the (partially) reflective-transmissive surface 524a of the first replicator 520 is adjacent the input port of the first replicator / waveguide 520 that receives input beam 522 at an angle to provide waveguiding and replica formation, along its length in the first dimension. Thus, the input port of first replicator / waveguide 520 is positioned at an input end thereof at the same surface as the reflective-transmissive surface 524a. The skilled reader will understand that the input port of the first replicator / waveguide 520 may be at any other suitable position.

[0191] Accordingly, the arrangement of Figure 5B enables the first replicator 520 and the mirror 530 to be provided as part of a first relatively thin layer in a plane in the first and third dimensions (illustrated as an x-z plane). In particular, the size or “height” of a first planar layer - in which the first replicator 520 is located - in the second dimension (illustrated as the y dimension) is reduced. The mirror 530 is configured to direct the light away from a first layer / plane, in which the first replicator 520 is located (i.e. the “first planar layer"), and direct it towards a second layer / plane, located above and substantially parallel to the first layer / plane, in which the second replicator 540 is located (i.e. a “second planar layer”). Thus, the overall size or “height” of the system - comprising the first and second replicators 520, 540 and the mirror 530 located in the stacked first and second planar layers in the first and third dimensions (illustrated as an x-z plane) - in the second dimension (illustrated as the y dimension) is compact. The skilled reader will understand that many variations of the arrangement of Figure 5B for implementing the present disclosure are possible and contemplated.

[0192] The image projector may be arranged to project a diverging or diffracted light field. In some embodiments, the light field is encoded with a hologram. In some embodiments, the diffracted light field comprises diverging ray bundles. In some embodiments, the image formed by the diffracted light field is a virtual image. In some embodiments, the first pair of parallel I complementary surfaces are elongate or elongated surfaces, being relatively long along a first dimension and relatively short along a second dimension, for example being relatively short along each of two other dimensions, with each dimension being substantially orthogonal to each of the respective others. The process of reflection / transmission of the light between / from the first pair of parallel surfaces is arranged to cause the light to propagate within the first waveguide pupil expander, with the general direction of light propagation being in the direction along which the first waveguide pupil expander is relatively long (i.e. , in its “elongate” direction).

[0193] There is disclosed herein a system that forms an image using diffracted light and provides an eye-box size and field of view suitable for real-world application - e.g. in the automotive industry by way of a head-up display. The diffracted light is light forming a holographic reconstruction of the image from a diffractive structure - e.g. hologram such as a Fourier or Fresnel hologram. The use diffraction and a diffractive structure necessitates a display device with a high density of very small pixels (e.g. 1 micrometer) - which, in practice, means a small display device (e.g. 1 cm). The inventors have addressed a problem of how to provide 2D pupil expansion with a diffracted light field e.g. diffracted light comprising diverging (not collimated) ray bundles.

[0194] In some embodiments, the display system comprises a display device - such as a pixelated display device, for example a spatial light modulator (SLM) or Liquid Crystal on Silicon (LCoS) SLM - which is arranged to provide or form the diffracted or diverging light. In such aspects, the aperture of the spatial light modulator (SLM) is a limiting aperture of the system. That is, the aperture of the spatial light modulator - more specifically, the size of the area delimiting the array of light modulating pixels comprised within the SLM - determines the size (e.g. spatial extent) of the light ray bundle that can exit the system. In accordance with this disclosure, it is stated that the exit pupil of the system is expanded to reflect that the exit pupil of the system (that is limited by the small display device having a pixel size for light diffraction) is made larger or bigger or greater in spatial extend by the use of at least one pupil expander.

[0195] The diffracted or diverging light field may be said to have “a light field size”, defined in a direction substantially orthogonal to a propagation direction of the light field. Because the light is diffracted I diverging, the light field size increases with propagation distance.

[0196] In some embodiments, the diffracted light field is spatially-modulated in accordance with a hologram. In other words, in such aspects, the diffractive light field comprises a “holographic light field”. The hologram may be displayed on a pixelated display device. The hologram may be a computer-generated hologram (CGH). It may be a Fourier hologram or a Fresnel hologram or a point-cloud hologram or any other suitable type of hologram. The hologram may, optionally, be calculated so as to form channels of hologram light, with each channel corresponding to a different respective portion of an image that is intended to be viewed (or perceived, if it is a virtual image) by the viewer. The pixelated display device may be configured to display a plurality of different holograms, in succession or in sequence. Each of the aspects and embodiments disclosed herein may be applied to the display of multiple holograms.

[0197] The output port of the first waveguide pupil expander may be coupled to an input port of a second waveguide pupil expander. The second waveguide pupil expander may be arranged to guide the diffracted light field - including some of, preferably most of, preferably all of, the replicas of the light field that are output by the first waveguide pupil expander - from its input port to a respective output port by internal reflection between a third pair of parallel surfaces of the second waveguide pupil expander.

[0198] The first waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a first direction and the second waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a second, different direction. The second direction may be substantially orthogonal to the first direction. The second waveguide pupil expander may be arranged to preserve the pupil expansion that the first waveguide pupil expander has provided in the first direction and to expand (or, replicate) some of, preferably most of, preferably all of, the replicas that it receives from the first waveguide pupil expander in the second, different direction. The second waveguide pupil expander may be arranged to receive the light field directly or indirectly from the first waveguide pupil expander. One or more other elements may be provided along the propagation path of the light field between the first and second waveguide pupil expanders.

[0199] The first waveguide pupil expander may be substantially elongated and the second waveguide pupil expander may be substantially planar. The elongated shape of the first waveguide pupil expander may be defined by a length along a first dimension. The planar, or rectangular, shape of the second waveguide pupil expander may be defined by a length along a first dimension and a width, or breadth, along a second dimension substantially orthogonal to the first dimension. A size, or length, of the first waveguide pupil expander along its first dimension make correspond to the length or width of the second waveguide pupil expander along its first or second dimension, respectively. A first surface of the pair of parallel surfaces of the second waveguide pupil expander, which comprises its input port, may be shaped, sized, and / or located so as to correspond to an area defined by the output port on the first surface of the pair of parallel surfaces on the first waveguide pupil expander, such that the second waveguide pupil expander is arranged to receive each of the replicas output by the first waveguide pupil expander.

[0200] The first and second waveguide pupil expander may collectively provide pupil expansion in a first direction and in a second direction perpendicular to the first direction, optionally, wherein a plane containing the first and second directions is substantially parallel to a plane of the second waveguide pupil expander. In other words, the first and second dimensions that respectively define the length and breadth of the second waveguide pupil expander may be parallel to the first and second directions, respectively, (or to the second and first directions, respectively) in which the waveguide pupil expanders provide pupil expansion. The combination of the first waveguide pupil expander and the second waveguide pupil expander may be generally referred to as being a “pupil expander”.

[0201] It may be said that the expansion / replication provided by the first and second waveguide expanders has the effect of expanding an exit pupil of the display system in each of two directions. An area defined by the expanded exit pupil may, in turn define an expanded eyebox area, from which the viewer can receive light of the input diffracted or diverging light field. The eye-box area may be said to be located on, or to define, a viewing plane.

[0202] The two directions in which the exit pupil is expanded may be coplanar with, or parallel to, the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. Alternatively, in arrangements that comprise other elements such as an optical combiner, for example the windscreen (or, windshield) of a vehicle, the exit pupil may be regarded as being an exit pupil from that other element, such as from the windscreen. In such arrangements, the exit pupil may be non-coplanar and non-parallel with the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. For example, the exit pupil may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion.

[0203] The viewing plane, and / or the eye-box area, may be non-coplanar or non-parallel to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. For example, a viewing plane may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion.

[0204] In order to provide suitable launch conditions to achieve internal reflection within the first and second waveguide pupil expanders, an elongate dimension of the first waveguide pupil expander may be tilted relative to the first and second dimensions of the second waveguide pupil expander.

[0205] Combiner shape compensation

[0206] An advantage of projecting a hologram to the eye-box is that optical compensation can be encoded in the hologram (see, for example, European patent 2936252 incorporated herein by herein). The present disclosure is compatible with holograms that compensate for the complex curvature of an optical combiner used as part of the projection system. In some embodiments, the optical combiner is the windscreen of a vehicle. Full details of this approach are provided in European patent 2936252 and are not repeated here because the detailed features of those systems and methods are not essential to the new teaching of this disclosure herein and are merely exemplary of configurations that benefit from the teachings of the present disclosure.

[0207] Control device

[0208] The present disclosure is also compatible with optical configurations that include a control device (e.g. light shuttering device) to control the delivery of light from a light channelling hologram to the viewer. The holographic projector may further comprise a control device arranged to control the delivery of angular channels to the eye-box position. British patent application 2108456.1, filed 14 June 2021 and incorporated herein by reference, discloses the at least one waveguide pupil expander and control device. The reader will understand from at least this prior disclosure that the optical configuration of the control device is fundamentally based upon the eye-box position of the user and is compatible with any hologram calculation method that achieves the light channeling described herein. It may be said that the control device is a light shuttering or aperturing device. The light shuttering device may comprise a 1 D array of apertures or windows, wherein each aperture or window independently switchable between a light transmissive and a light non-transmissive state in order to control the delivery of hologram light channels, and their replicas, to the eye-box. Each aperture or window may comprise a plurality of liquid crystal cells or pixels. Reflection Suppression Device

[0209] Figure 6 shows a schematic side-view of features of a head-up display comprising a first reflection suppression device 1900. The head-up display of Figure 6 comprises a waveguide 1902, a turning layer 1903, and the first reflection suppression device 1900. The head-up display further comprises a coherent light source (in this example, a laser) and a display device (in this example, a liquid crystal on silicon spatial light modulator) arranged to display a hologram of a picture. The light source and display device are not shown in Figure 6. In use, the light source of the head-up display illuminates the display device such that light is spatially modulated in accordance with the hologram displayed on the display device, thus forming a holographic wavefront. The holographic wavefront is coupled into a first waveguide (not shown in Figure 6) where the holographic wavefront is replicated in a first direction a plurality of times (as described previously) to form a one dimensional array of replicas which are then coupled into the waveguide 1902 (which is shown in Figure 19). The waveguide 1902 comprises a pair of opposing surfaces. A first surface 1904 of the waveguide 1902 is partially transmissive-reflective. A second surface 1906 of the waveguide 1902 is reflective. The waveguide 1902 is arranged to waveguide the replicas of the holographic wavefront coupled in to the waveguide 1902 from the first waveguide between the pair of opposing surfaces. In this way, the holographic wavefront is replicated in a second direction (that is orthogonal to the first direction) a plurality of times (again, as described previously) to form a two dimensional array of replicas. The emission of replicas from the first surface 1904 is represented by the dotted arrows 1908 in Figure 6. In the y-z plane, the replicas 1908 are angled with respect to a normal of the waveguide 1902 (i.e. the replicas are angled with respect to the z-direction in the y-z plane). In this example, the replicas 1908 of the holographic wavefront (at the first surface of the waveguide 1904) do not have a component on the x-z plane. So, as the skilled reader will appreciate, in a side view of the waveguide 1902 in the x-z plane, the replicas 1908 would appear to be emitted vertically up the page.

[0210] It should be understand that, although a holographic head-up display is described in relation to this example, that the glare suppression device 1900 could in fact be used with a conventional head-up display. In such cases, the light that propagates through the glare suppression device 1900 may be more conventional light, modulated in accordance with an image rather than a hologram.

[0211] The replicas 1908 of the holographic wavefront emitted by the waveguide 1902 are received by the turning layer 1903 whereby the (replicas of) the holographic wavefront are turned by the turning layer 1903. The turned replicas 1908 are then received by the reflection suppression device 1900. The reflection suppression device 1900 comprises a first surface 1920 and a second surface 1924. The first layer 1920 is closest to the waveguide 19081 turning layer 1903. As such, the (replicas of) holographic wavefront (propagating through the reflection suppression device 1900) are received by the first surface 1924.

[0212] The first (bottom) surface 1924 has a serrated or sawtooth shape. The second surface 1924 comprises a prismatic structure comprising a plurality I an array of first prism elements 1921. The first surface 1924 is integrally formed such that the array of first prism elements 1921 form a single component (forming the first surface 1924). The first serrated surface 1924 is defined by first and second surfaces or faces of each of the prism elements 1921, thus forming a sawtooth-type structure when viewed in the y-z plane (as in Figure 6). The first surface 19241 first prism elements 1921 are formed of a transparent material which, in this example, is a hard plastic (transparent) material. The first layer 1924 may have been manufactured by, for example, injection moulding, hot embossing, extruding or cutting a source of the transparent material.

[0213] The second (top) surface 1920 also forms a (second) serrated surface. The second surface 1920 comprises a prismatic structure comprising a plurality I an array of second prism elements 1923. Like the first surface 1924, the second surface 1920 is integrally formed such that the array of second prism elements 1923 form a single component (forming the first layer 1920). The second serrated surface 1920 is defined by first and second surfaces or faces of each of the prism elements 1923, thus forming a sawtooth-type structure when viewed in the y-z plane (as in Figure 6). The second surface 1920 is formed of a transparent material which, in this example, is a hard plastic (transparent) material. In this example, the second surface 1920 is formed of the same transparent material as the first surface 1924 (or, at least, a material having the same refractive index as the transparent material of the first surface 1924). The first layer 1924 may have been manufactured by, for example, injection moulding, hot embossing, extruding or cutting a source of the transparent material.

[0214] A periodicity of the serrations of the second and first serrated surfaces 1924,1920 is equal.

[0215] Each of the first and second prism elements 1921, 1923 comprises an active face 602 and a passive face 604. The active face 802 of the first prism elements 1921 may be referred to as a receiving face because it is this face that receives the holographic wavefront 1908. The active face 802 of the second prism elements 1921 may be referred to as a transmitting face because it is this face that transmits the holographic wavefront 1908. This is shown more clearly in Figure 7 which is a cross-sectional schematic view of just one first prism element 1921 and one second prism element 1923 of the reflection suppression device 1900. Figure 7 further shows the path of a gut ray of a holographic wavefront 1908 through the reflection suppression device 1900.

[0216] The holographic wavefront 1908 is received by the first serrated surface 1924 (in particular, by an active face 802 of a first prism element 1921). A first turn is provided to the holographic wavefront 1908 by the first prism element 1921 as a result of the angle of the active face 802 and the refractive index of the first prism element 1921. The turned holographic wavefront 1908 then propagates through the reflection suppression layer to be received by a second prism element 1923 of the second serrated surface 1920 (in particular, to be received by an active face 802 of a second prism element 1923). When the holographic wavefront 1908 is emitted at the second serrated surface 1920 a second turn is provided to the holographic wavefront 1908 by the second prism element 1923 as a result of the angle of the active face 802 and the refractive index of the second prism element 1923.

[0217] In this example, the reflection suppression 1900 device further comprises two arrays of louvres 1932. A first array 806 of louvres 1932 is provided on the first surface 1924. In particular, each louvre 1932 of the first array 806 is provided on a passive face 804 of a first prism 1921. A second array 808 of louvres 1932 is provided on the second surface 1920. In particular, each louvre 1932 of the second array 808 is provided on a passive face 804 of a second prism 1923. Because the first and second arrays 806,808 of louvres 1932 is provided on the passive face 604 of the respective first and second prism elements 1921 ,1923, the periodicity of the louvres 1932 of the first and second arrays is equal to the periodicity of the prism elements 1921 ,1923.

[0218] In some examples, the louvres 1932 are provided as coatings on the respective passive faces 604. In some examples, the louvres 1932 are provided as black paint on the respective passive faces 804.

[0219] Each of the second and first surfaces 1920,1924 are formed of a transparent material which, in this example, has a refractive index greater than 1. In this example, each of the second first surfaces 1920 and 1924 are formed of polymethyl methacrylate (PMMA). Each of the second and first serrated surfaces form a transparent material I air interface. Light (in particular, the holographic wavefront 1908) propagating through the reflection suppression device will be turned, twice. A first turn will be provided by the first layer 1920 and a second turn will be provided by the second layer 1924. In this example, the shape of the serrations, and the refractive index of the transparent material, are selected such that the component of the first turn on the first plane (the y-z plane) is equal but opposite to the component of the second turn on the first plane (the y-z plane).

[0220] The first serrated surface 1924 of the reflection suppression device 1900 may be referred to herein as an input side of the reflection suppression device 1900 (because the first serrated surface receives the holographic wavefront). The second serrated surface 1920 of the reflection suppression device 1900 may be referred to herein as an output side of the reflection suppression device 1900 (because the second serrated surface 1920 emits the holographic wavefront once the holographic wavefront has propagated though the reflection suppression device 1900).

[0221] The turning layer 1903, in this example, also has a prismatic structure. However, the prism elements 1935 of the turning layer 1903 extend longitudinally in a direction that is orthogonal to the direction of extension of the prism elements of the first and second layers. Specifically, the prism elements 1935 extend longitudinally in the y-direction rather than the x-direction. As such, the prism elements 1935 are arranged to turn the holographic wavefront exclusively on the second plane (in the y-z plane) rather than on the first plane (the x-z plane).

[0222] Glare mitigation

[0223] The reflection suppression device 1900 is arranged to mitigate or suppress glare from being received at a viewing window or eye-box. In the absence of the reflection suppression device 1900, there is a risk that ambient light incident on the waveguide may be reflected and be received at the viewing window or eye-box. This ambient light may then be distracting. Said glare is suppressed by the reflection suppression device 1900 via a number of different mechanisms.

[0224] The arrays 806,808 of louvres 1932 are formed of / consist of a light absorbing material. The array of louvres 1932 are angled such that the replicas 1908 of the holographic wavefront can substantially pass between the louvres 1932 without being absorbed. However, ambient light passing through the reflection suppression device 1900 following a different propagation path (that is not parallel to the louvres) will tend to be incident on one of the louvres 1932. Said ambient light will be absorbed by said louvre 1932 and therefore will not be able to propagate on to the waveguide 1902 and so will not be reflected by the waveguide 1902 back to the viewing window I eye-box. As should be clear to the skilled reader, there will be a range of angles at which ambient light may be able to pass between adjacent louvres 1932 in both the first and second arrays 806,808. This range of angles will be defined by the pitch of the louvres 1932 and the shape I dimensions I orientation of the louvres 1932. After a first pass through the reflection suppression device, said ambient light may be reflected by the waveguide 1902 and returned back towards the reflection suppression device 1900 to pass through for a second time. On the second pass, the reflected ambient light will generally be absorbed by one of the louvres 1932. Thus, even if the ambient light is not absorbed by the louvres 1932 on a first pass through the reflection suppression device, it will generally be absorbed by the louvres 1932 on the second pass.

[0225] The second serrated surface 1920 provides another mechanism for glare mitigation. In particular, the array of angled surfaces of the second serrated surface 1920 are arranged such that a portion of the light incident on second serrated surface 1920 may be specularly reflect ambient light incident thereon. The second serrated surface 1920 is arranged such that this specularly reflected light is reflected in a direction that is generally away from viewing window I eye-box.

[0226] The reflection suppression device 1900 described above comprises first and second serrated surface 1924, 1920 comprising, respectively, arrays of first or second prisms 1921 ,1923. The second serrated surface 1920 is arranged to direct specular reflections away from a viewing window I eye-box of a display system. In particular, the angle of the passive faces with respect to a normal of the light control layer is arranged to direct specular reflections on the passive faces away from the viewing window I eye-box. Through experimentation and simulation, the inventors have found that there may be only one angle, or a very narrow range of angles, of the passive face that may be optimised for directing specular reflections away from the viewing window I eye-box. Thus, to optimize the suppression of glare, the angle of the passive faces 604 the second surface 1920 is constrained to optimised angle or very narrow range of angles. In this example, each passive face 604 of the second is arranged to be at the same (optimised) angle. In other words, each passive face 604 of the second surface 1920 is parallel.

[0227] As described above, the provision of the second prism elements 1923 of the second surface 1920 will each provide a (second) turn to the holographic wavefront 1908. The reason for the provision of the first prism elements 1921 of the first surface 1920 in the reflection suppression device 1900 is to cancel out this second turn. Thus, in the reflection suppression device of this example, the first turn provided by the first prism elements 1921 is arranged to completely compensate for the second turn provided by the second prism elements 1923. In other words, the net turn (of the sum of the first and the second turns) is zero in this example. The reason for this is so that the introduction of the reflection suppression device 1900 into a display system does not result in a net turn of the holographic wavefront 1908 which might result in unwanted translation of the viewing plane I eye-box.

[0228] Curved optical component

[0229] Figure 8 is a schematic cross-sectional view of a display system comprising the reflection suppression device 1900 and a windscreen 902 (or windshield). The display system also comprises a spatial light modulator and light source for illuminating the spatial light modulator to form the holographic wavefront 1908. The display system further comprises one or more waveguides arranged to replicate the holographic wavefront 1908, and a turning layer, as described above. However, Figure 8 only shows a portion of the reflection suppression device 1900 and a portion of the windscreen 902.

[0230] Figure 8 schematically represents a distortion caused by the display system comprising a curved optical component. In this example, the curved optical component is the windscreen 902. The problem is that the curved optical component I windscreen 902 introduces distortion to the holographic wavefront 19081 to the light received at the eye-box I viewing window. This is represented by the propagation of three different rays of the holographic wavefront 1908 through the reflection suppression device in Figure 8.

[0231] The holographic wavefront 1908 is shown as being incident on the active faces 602 of three first prism elements 1921. Here, the three rays of the holographic wavefront 1908 are substantially parallel. At the interface between air and the first prism elements 1921 , a first turn is provided to the three rays of the holographic wavefront 1908. As described above, each of the first prism elements 1921 is arranged provide a first turn which exactly cancels out or compensates for the respective second prism element 1923 to which the first prism element 1921 is optically coupled. Because each second prism element is constrained by the need for reflection suppression (and so each second prism element is identical), each first prism element 1921 is also identical. Thus, the identical first prism elements 1921 all provide exactly the same first turn to each of the rays of the holographic wavefront 1908. Thus, the three rays of the holographic wavefront 1908 remain substantially parallel while propagating through the reflection suppression device, between the first and second surfaces 1924,1920. At the interface between the second prism elements 1923 and air, a second turn is provided to the holographic wavefront 1908. Again, each of the second prism elements 1923 of the reflection suppression device 1900 is substantially identical. Thus, the identical second prism elements 1923 all provide exactly the same second turn to each of the rays of the holographic wavefront. Thus, the three rays of the holographic wavefront 1908 remain substantially parallel immediately downstream of the reflection suppression device 1900. In other words, the angles of the three rays of the holographic wavefront 1908 are maintained upstream and downstream of the reflection suppression device 1900. As such, the reflection suppression device 1900 is arranged to not distort the rays of the holographic wavefront 1908.

[0232] The three rays of the holographic wavefront 1908 continue to propagate towards the windscreen 902 (or optical combiner) and are incident on the windscreen 902, after which the three rays of the holographic wavefront 1908 are reflected by at least an interior surface of the windscreen 902. The reflected rays of the holographic wavefront 1908 are directed towards an eye-box or viewing window of the display system. The windscreen 902 has a curved surface. The three different rays of the holographic wavefront 1908 are incident on different portions of the windscreen 902. Because the windscreen 902 has a curved surface, the rays make different angles with different portions of the windscreen 902 and so are reflected at different angles to one another. In this example, the windscreen 902 has a substantially concave curve shape. This results in the reflected rays of the holographic wavefront 1908 converging. Thus, downstream of the windscreen 902, the rays of the holographic wavefront 1908 are no longer substantially parallel. The angular relationship between the different rays of the holographic wavefront 1908 that existed upstream of the reflection suppression device 1900 has been broken by the windscreen 902. In other words, the holographic wavefront 1908 has been distorted by the windscreen 902. This has the effect of distorting a picture viewable from the eye-box I viewing window.

[0233] In this example, the three rays of the holographic wavefront 1908 are shown as being substantially parallel in Figure 8. However, it should be clear that the rays of the holographic wavefront 1908 are typically slightly diverging. This may be because a holographic wavefront 1908 typically comprises diffracted light. The important point is that the curved windscreen 902 changes the convergence (or divergence, in some examples) the holographic wavefront 1908 such that the angular relationship between rays of the holographic wavefront 1908 is not maintained between the reflection suppression device 1900 and the windscreen 902.

[0234] In this example, the curved optical element is a windscreen 902. However, it should be understood that any curved optical element on the optical path of the holographic wavefront 1908 could introduce a distortion effect. Furthermore, in this example, the curved optical element (windscreen 902) is provided downstream of the reflection suppression device 1900. In other examples, a curved optical element could be provided upstream of the reflection suppression device 1900. In such examples, the curved optical element introduces the distortion to the holographic wavefront 1908 prior to the holographic wavefront being received by the reflection suppression device 1900. The reflection suppression device 1900 then maintains the distortion because the distortion is maintained by the combination of the first and second prismatic structures. Thus, a distorted image may be delivered to a eye-box or viewing window of the system.

[0235] It should be clear that Figure 8 is a schematic drawing. Figure 8 is not drawn to scale and the shapes and relative positions of components I features shown in Figure 8 are not drawn accurately or to scale. For example, the curvature of windscreen 902 is exaggerated in Figure 8, and the location of the windscreen with respect to the reflection suppression device 1900 is merely representative. Furthermore, the reflection suppression device 1900 is not drawn to scale. The individual prism elements 1921 ,1923 of the reflection suppression device 1900 are relatively very small (for example, less than 1 millimetre) in reality. The reflection suppression device 1900 may comprise upwards of 200 first and second prism elements 1921 ,1923.

[0236] Herein, the light that propagates through the display system I reflection suppression device 1900 has been described as a holographic wavefront. In other examples, the light that propagates therethrough may be otherwise spatially modulated, for example spatially modulated in accordance with a picture (i.e. conventional head-up display light).

[0237] Improved reflection suppression device

[0238] Figure 9 shows a portion of a first example reflection suppression device 1000 according to the present disclosure. The reflection suppression device 1000 has similarities to the reflection suppression device 1900 described above. In particular, the reflection suppression device 1000 comprises first and second serrated surfaces 1024,1020 corresponding to the first and second serrated surface 1924,1920 of the reflection suppression device 1900. The first and second serrated surfaces 1024, 1020 comprise prismatic structures comprises first and second prism elements 1021 ,1023 respectively. Each of the first and second prism elements 1021 ,1023 comprises an active face 1002 and a passive face 1004. The second (top) serrated surface 1020 is arranged similarly to the second (top) serrated surface 1920 of the reflection suppression device 1900. In particular, the passive faces 1004 of the second prism elements 1023 of the second serrated surface 1020 are angled to suppress ambient light incident thereon and suppress glare from reaching the eye-box I viewing window. Thus, each of the second prism elements 1023 is identical. The reflection suppression device 1000 differs from the reflection suppression device 1900 in that each first prism element 1021 is not identical to every other first prism element 1021. In particular, the inventors have gone against their assumption that each first prism element 1021 should exactly compensate for the turn introduced by the second prism element 1023 that is optically coupled to the first prism element 1021.

[0239] It may be said that each first prism element 1021 is optically coupled to a second prism element 1023. As used herein, the first and second prism elements 1021,1023 being optically coupled means that a particular second prism element 1023 receives spatially modulated light that was relayed from a particular first prism element 1021. In Figure 9, three rays of spatially modulated light 1908 are shown. Each ray passes through an active face 1002 of a first prism element 1021, and an active face 1002 of a second prism element 1023. The first and second prism elements 1021, 1023 through which an individual ray passes may be referred to as being optically coupled to one another.

[0240] In the reflection suppression device 1900, the net turn provided by each prism pair is the same and equal to zero. This arrangement is most straightforward from a manufacturing perspective and means that the reflection suppression device 1900 does not provide a net turn to light propagating therethrough. Furthermore, the louvres 1932 can be arranged to align with the holographic wavefront 1908 in a way which minimises the amount of holographic wavefront 1908 light that is absorbed by the louvres 1932 (e.g. as a result of the thickness of the louvres). This is advantageous as any light that is absorbed by the louvres may result in dark bands. The inventors have gone against this by introducing (relatively small) differences between the prism pairs. In particular, while each second prism element 1023 is identical to every other second prism element 1023, slight differences exist between the first prism elements 1021. In this example, the angle of the active face 1002 of each first prism element 1021 is adjusted slightly. The effect of this is to (slightly) change the turn provided by each first prism element 1021 with respect to every other first prism element 1021. The turn provided by each second prism element 1023 is still identical, thus the net turn provided by different prism pairs is different (e.g. the net turn provided by a first prism pair is different to the net turn provided by a second prism pair).

[0241] The differences in net turn provided by different prism pairs is shown schematically in Figure 9. The three active faces 1002 of the three first prism elements 1021 shown in Figure 9 are all angled differently to one another. In particular, the angle of the leftmost active face 1002 (of a first prism element 1021) is turned slightly anti-clockwise relative to the centre active face 1002 and the angle of the right most active face 1002 is turned slightly clockwise relative to the centre active face 1002. This means that the turns provided by each of first prism element 1021 is different for the holographic wavefront 1908 incident thereon. In particular, the divergence of the rays of the holographic wavefront 1908 is increased as the rays pass into the first prism elements 1021. This increased divergence is maintained as the rays pass out of the second prism elements 1023 into air because each turn provided by the second prism elements 1023 is the same.

[0242] The differences in net turn provided by the different prism pairs is arranged to increase the divergence of the holographic wavefront 1908 to pre-compensate for the convergence provided by the windscreen 902. Thus, the angular relationship of the rays of holographic wavefront 1908 incident on the first layer 1024 is the same as the angular relationship of the rays of the holographic wavefront 1908 after reflection on the windscreen 902. In other words, the first prism elements 1021 of the first layer 1024 are arranged to compensate for the distortion of the windscreen 902. In effect, the turns by the first prism elements 1021 are arranged to combine to have a lensing effect which compensates for a lensing effect of the windscreen 902.

[0243] Figure 9 only shows three prism pairs to exemplify the concept of the present disclosure. As above, in reality, each layer may comprise a relatively large number of prism elements (e.g. 200 or more prism elements). Furthermore, the differences in net turn between adjacent prism pairs is exaggerated in Figure 9. In reality, the difference in net turn between adjacent prism pairs may be relatively small, e.g. less than a fraction of a degree. There may be only a maximum of couple of degrees between the prism pairs providing a the smallest and the largest difference in net turn.

[0244] Figure 10 shows an example of a global phase function (of a light control layer) in accordance with the present disclosure. The y-axis of Figure 10 represents a phase value or a phase-delay value applied to light transmitted therethrough. In some embodiments, the waveguide is substantially planar and the x-axis and z-axis are the primary dimensions of the waveguide. Figure 10 shows how a phase (or phase-delay) value of the light control device is a function of spatial position e.g. spatial position on the surface of a planar waveguide. In other embodiments, the waveguide is substantially elongate. The phase function may be formed in a number of different ways. The phase function may be implemented in bulk optics. However, in other embodiments, the phase function is a Fresnel structure or a section thereof. For example, the phase may be wrapped or may be modulo 2K. In other embodiments, the phase function is formed as a diffractive optic element or a holographic optic element. The phase function may be complementary to a shape (or function) of the optical element (e.g. windscreen) or, at least, the shape of the optical element in the sub-area thereof that corresponds to the output port or output area of the waveguide.

[0245] Curvature of a dimension of the elongate elements

[0246] The present disclosure refers to prismatic structures, prisms and prism elements by way of examples of elongate, or elongated, elements. The present disclosure is not limited to elongate structures that are straight or linear in all dimensions. For example, the present disclosure is not limited to elongate elements that that are perfectly triangular in crosssection - i.e. have three straight sides in cross section. In accordance with this disclosure, at least one dimension of each elongate element, or a subset thereof, may be curved - that is, may comprise curvature. The curvature provides an optical effect when illuminated with the display light (e.g. picture content or wavefront) such as optical power or a “turn”. The term “turn” is used herein to describe a change of direction of a received wavefront.

[0247] At least one elongate element of the plurality of elongate elements forming the light control device may be curved in one or more dimensions such as the elongate or longitudinal direction. Alternatively, or additionally, at least one side of the elongate element / s in crosssection may be non-linear e.g. curved.

[0248] Figures 11A to 13C show three example elongate elements in accordance with this disclosure. Figures 11 A, 12A and 13A comprise a top (or plan) view, whilst Figures 11 B, 12B, 13B and 13C comprise a side view of the corresponding elongate element. Each top view shows the elongate, long or longitudinal dimension. The side views of Figures 11 B and 12B show the corresponding cross section of the element, whilst Figure 13B shows the corresponding end face of the element. Figure 13C shows the corresponding cross section of the element taken along line C-C. The inclined side (or face) shown in each side view may provide glare suppression and / or an optical turn owing to the angle made with the base.

[0249] Each cross section shows a generally inclined face (with respect to the base) which is referred to herein as the “active face” because it receives light. Each cross section also shows a return face which connects a distal edge (with respect to the base) of the active face with the base. The return face is also referred to herein as the “passive face”.

[0250] The base may engage with the generally planar surface of an optical component - such as a waveguide or cover glass - susceptible to cause sunlight glare when installed in a vehicle. A group of these elements are adjoined and aligned as described in earlier embodiments. The plurality of active or inclined faces forms a louver type structure.

[0251] Figures 11 A and 11 B show an example of a first elongate element 1100 that does not comprise curvature. The first elongate element 1100 is linear in the elongate direction and all three sides in cross section are straight. The cross-sectional shape is therefore a triangle. The inclined face forms a first surface which is also referred to herein as the “active face”.

[0252] Figures 12A and 12B show an example of a second elongate element 1200 that is curved in the elongate or longitudinal direction. In this example, as shown in the top view of Figure 12A, the inner and outer curves are the same but they may be different. The inner curve may be described as a proximal edge with respect to the base and the outer curve may be described as being the distal edge with respect to the base. This is an example of one type of curvature in accordance with this disclosure. This is referred to herein as “second curvature”.

[0253] Figures 13A, 13B and 13C show an example of a third elongate element 1300 that, in cross section, has additional curvature. Specifically, the third elongate element 1300 has a curved side in cross section. This is referred to herein as “first curvature”. In this example, the curved side is the active or inclined side. In this example, the curvature of the active or inclined side varies along the length of the element as shown by the side view taken along line C-C (shown in Figure 13C). That is, the profile of the curved face (i.e. the active face) may be different at different positions along the element.

[0254] The inventors found that quadratic terms describing the first curvature provide, or correspond to, optical power when illuminated. The changing profile of the curved face may provide different optical power at different locations on the light control device. Higher order terms (e.g. terms to the power three or above) describing the curvature may provide aberration correction. Linear terms that describe the general incline of the active face may provide a turn (i.e. change of direction of the received wavefront).

[0255] Notably, the inventors found that curvature in the elongate direction provides astigmatism. That is, second curvature can determine the ratio of two orthogonal components of the optical power provided by the first curvature (e.g. the ratio of optical power in x to the optical power in y). The inventors found that using these principles, they could provide a single component that is substantially planar (optimal for head-up display) and provides at least one selected from the group comprising: glare mitigation, turn and windscreen correction. The inventors have successfully designed and tested a light control device that serves all three purposes for a waveguide head-up display.

[0256] The exact distribution of shapes and curvature required to achieve this multifunctionality is determined by the shape of the windscreen and precise location of the display device relative to the windscreen and eye-box. The person skilled in the art of head-up display will be familiar with the variation in these parameters across vehicle lines and manufacturers. The present disclosure is not therefore limited to any specific design. Instead, the present disclosure is focused on the concept of introducing curvature in one or more ways (e.g. physical dimensions) to the elongate elements of a louver-type structure (e.g. for glare mitigation) in order to introduce optical effects (e.g. power) that address the complexities of the wider system - namely, the complex curvature of the windscreen. Disclosure of these principles is sufficient for a person skilled in the art of optical design and in possession of optical data characterizing the optical power of a windscreen (as a function of x, y and x) to design the structure.

[0257] Figures 14A to 15B show some further drawing which aid this disclosure.

[0258] Figure 14A shows the top view of a section of a light control device in accordance with this disclosure, whilst Figure 14B shows a side view of such a light control device. The pitch, p, may be constant but does not have to be. The maximum height, h, of each zone may be constant but, again, does not have to be. Figure 14B shows how if the face of each zone is placed end-to-end, the resulting curve is continuous.

[0259] Figure 15A shows an embodiment in which the top and bottom face of the light control device comprise an array of elongate elements in accordance with examples described above. That is, the top and bottom surfaces may both have elongate features. The structures of the top and bottom surfaces may be generally orthogonal in the elongate direction as shown in the drawing. Figure 15B shows one surface in cross section. Light may only interact with the active faces so, in some embodiments, the passive faces can be e.g. painted black. That is, the optics can be designed such that light only interacts with one set of faces, such that the other surface could be e.g. made absorbing and / or glare mitigating. Pancharatnam-Berry corrective optic

[0260] In embodiments of the present disclosure, the light control layer, or corrective optic, comprises a Pancharatnam-Berry, “PB”, component or lens. The term “lens” is used herein as shorthand to refer to an optical component or optic having optical power. Notably, in accordance with embodiments of this disclosure, the optical power provided by the Pancharatnam-Berry component is non-uniform. That is, the Pancharatnam-Berry component may be more complex than a simple lens.

[0261] In summary, Figure 16A represents a conventional arrangement for forming a conventional PB lens. Figure 16A is a Mach Zehnder interferometer for creating a PB lens on a substrate. Notably, there is normal incidence upon the substrate. The light source comprises ultraviolet, “UV”, light. The essential point is that the half-wave plate at the output of the interferometer creates right circularly polarized, “RCP”, and left circularly-polarised, “LCP”, light which interfere to give a polarisation hologram.

[0262] PB lenses and the method of fabrication shown in Figure 16A are known in the art. Figure 16A shows an incident light beam 1601. The incident light beam 1601 is split into a first path comprising first linear polarization 1603 which in this example is vertically polarized and a second path comprising second linear polarization 1605 which is polarized into the page in this example by a first polarizing beam splitter, “PBS”, 1610. At this point, the first and second paths are coplanar and perpendicular. The first path comprises an object beam and the second path comprises a reference beam. The first path comprises a mirror 1630 that turns the first polarized light 1603 by a right angle and onto the target or object which in this example is a simple lens 1650 e.g. a converging or positive lens. The light of the first path is modified (e.g. focussed) by the simple lens 1650 to form a modified (e.g. converging or diverging) beam 1605 which is received by a first input of a second PBS 1640. The second path comprises a third PBS 1620 that directs the second linear polarization 1603 to a second input of the second PBS 1640. The second PBS combines the first and second inputs. A quarter waveplate, “QWP”, 1670 receives the combined beams and illuminates a substrate 1680 comprising a photo-alignment layer that is responsive or sensitive to the wavelength of the light. An alignment structure corresponding to the simple lens 1650 is recorded or encoded in the photo-alignment layer. Photoalignment is well-known - see, for example, Muravsky Alexander , Murauski Anatoli , “Q&A of liquid crystal alignment: theory and practice” Frontiers in Soft Matter vol. 4, 2024. The photo-alignment layer formed using the method of Figure 16A may be used to encode a simple lens, which will subsequently be used as a template or a liquid crystal layer such as a polymerizable reactive mesogen in a manner known in the art. Reactive mesogens are available commercially and have been actively researched for applications in AR / VR - see, for example, Owain Parri, Graham Smith, Richard Harding, Hyun-Jin Yoon, Iain Gardiner, Joe Sargent, and Karl Skjonnemand "Patterned retarder films using reactive mesogen technology", Proc. SPIE 7956, Advances in Display Technologies; and E-papers and Flexible Displays, 79560W (2 February 2011).

[0263] The simple lens can be used either, as formed, by illuminating the polymeric reactive mesogen with collimated light of the same circular polarization as the reference beam, or by illuminating it with collimated light of the orthogonal circular polarization, in which case it behaves as a diverging lens.

[0264] In order to give an equal efficiency to the three colours used in a HUD, the PB lens is composed of three layers, known in the literature as QHQ (Quarterwave, Halfwave, and Quarterwave retarders). Pancharatnam showed that a combination of three birefringent plates, the first and last having identical retardations and their fast axes mutually parallel, will behave as a single birefringent plate with a retardation 2A whose principal plane is inclined at an angle a to the fast axes of the first and last plates, where 2A and a are given by the relations: cos A = cos 2pi cos P2 - sin 2pi sin P2 cos 2p, and cot 2a = cosec 2p (sin 2Pi cot P2 - cos 2Pi cos 2p).

[0265] In these relations, 2 Pi is the retardation of each of the two outer plates, 2 P2 is that of the middle plate, and p is the angle between the fast axes of the middle plate and the two outer plates. An example of the in-plane nematic liquid crystal director profile created by the photoalignment layer is shown in Figure 16B. The first line shows one period of the profile, and the second line shows one period of a profile shifted by 45 degrees.

[0266] In summary, an embodiment of the present disclosure is shown in Figure 17 which creates a PB lens on a tilted substrate. Figure 17 differs from Figure 16A in that the substrate is tilted. The substrate may be tipped and tilted on two orthogonal planes as described in more detail below in relation to head-up display. Specifically, some embodiments require that the optical axis remains tilted, wherein the divergence of the beam is with respect to this axis. For the avoidance of doubt, the concept of encoding a tilted PB lens is distinguishing over Figure 16A. Figure 17 shows the tilted substrate 1780 comprises a photo-alignment layer. The photo-alignment layer formed using the method of Figure 17 may be used to encode a tilted lens in a liquid crystal layer such as a polymerizable reactive mesogen in a manner known in the art.

[0267] In summary, Figure 18 establishes the coordinate system with respect to a waveguide and the optical combiner (e.g. windshield) with the orientation of the E vector of a HUD projector

[0268] In summary, Figure 18 shows a plan view of a slab-shaped waveguide 1806 as part of an automotive head-up display using a windscreen 1802 as an optical combiner. The HUD light 1804 - that is, the light emitted by the head-up display for the viewer - is polarized in the x- direction. The z-direction is the vertical direction in normal operation. The windscreen 1802 spans over the top of the waveguide 1806. Figure 18 establishes the geometry used by the following figures.

[0269] In summary, Figure 19 shows the full layer structure of the layered stacked including the PB lens. Because it requires RCP / LCP illumination and HUD light is typically linearly polarised, QWPs are included on both the input and output surfaces of the PB lens.

[0270] In more detail, Figure 19 shows an embodiment of the present disclosure in side view - that is, the x-z plane. Figure 19 shows a layered stack of components comprising a waveguide 1906, a light turning film 1925, a first quarter waveplate, “QWP” 1970, a PB lens 1980 and a second QWP 1971. For the avoidance of doubt, the light turning film 1925 is optional and, in other embodiments, the light turn of the light turning film 1925 is also encoded in the PB lens, or omitted altogether. The HUD light 1904 is polarized in the x-direction. On the x-z plane shown, the HUD light makes an angle a with the surface normal of the components of the layered stack.

[0271] In summary, Figure 20 shows the same as Figure 19 for the orthogonal view. On the y-z plane shown, the HUD light makes an angle p with the surface normal of the components of the layered stack.

[0272] Figures 21 and 22 are simplified versions of Figures 19 and 20, respectively.

[0273] In summary, Figure 23 shows copying a freeform optic into a Reactive monomer. Figure 23 largely corresponds to Figure 17 except that the target (or object or template optic) is a freeform optic 2350 which means an optic having spatially variant optical power. In accordance with this disclosure, the spatially variant optical power of the PB lens formable using the recording apparatus of Figure 23 may at least partially compensate for the non- uniform curvature of the windscreen of a vehicle.

[0274] Additional features

[0275] The methods and processes described herein may be embodied on a computer-readable medium. The term “computer-readable medium” includes a medium arranged to store data temporarily or permanently such as random-access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" shall also be taken to include any medium, or combination of multiple media, that is capable of storing instructions for execution by a machine such that the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methodologies described herein, in whole or in part.

[0276] The term "computer-readable medium" also encompasses cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example form of a solid-state memory chip, an optical disc, a magnetic disc, or any suitable combination thereof. In some example embodiments, the instructions for execution may be communicated by a carrier medium. Examples of such a carrier medium include a transient medium (e.g., a propagating signal that communicates instructions).

[0277] It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. The present disclosure covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

1. CLAIMS1. A head-up display having a layered stack of components comprising: a waveguide arranged to receive a wavefront corresponding to a picture and output an array of replicas thereof by waveguiding between a pair of reflective surfaces; and a Pancharatnam-Berry, “PB”, lens arranged to receive the array of replicas from the waveguide and apply optical power thereto.

2. A head-up display as claimed in claim 1 wherein the components of the layer stack of components are each substantially planar and parallel to each other such that the respective surface normals of components are substantially parallel.

3. A head-up display as claimed in any preceding claim wherein the array of replicas output by the waveguide comprises linearly-polarised light.

4. A head-up display as claimed in claim 3 wherein the layered stack of components further comprises: a first quarter-wave plate disposed between the waveguide and PB lens; and a second quarter-wave plate disposed after the PB lens.

5. The head-up display of any preceding claim wherein the optical power of the PB lens comprises negative optical power.

6. The head-up display of claim 4 or claim 5 when dependent upon claim 4 further comprising a turning film disposed between the waveguide and first quarter-wave plate.

7. The head-up display of any preceding claim wherein the waveguide is substantially planar such as slab-shaped.

8. The head-up display of any preceding claim wherein the wavefront is angled with respect to a surface normal of the waveguide.

9. The head-up display of claim 8 wherein the wavefront is angled with respect to a surface normal of the waveguide on two orthogonal planes containing the surface normal.

10. The head-up display of claim 9 wherein a first angle of the wavefront with respect to the surface normal of the waveguide on a first plane of the two orthogonal planes is in the range 20-35 degrees.

11. The head-up display of claim 9 or 10 wherein a second angle of the wavefront with respect to the surface normal of the waveguide on a second plane of the two orthogonal planes is in the range 10-20 degrees.

12. A display system comprising: a compact head-up display as claimed in any preceding claim; and an optical combiner arranged to receive the output of the compact head-up display and direct the array of replicas to a viewing plane of the display system, wherein the optical power of the PB lens at least partially compensates for a non-uniform optical power of the optical combiner.

13. A method of fabricating a Pancharatnam- Berry, “PB”, lens, the method comprising: splitting a light beam into a first path comprising first polarised light and a second path comprising second polarised light forming a reference beam;Illuminating a template lens on the first path with the first polarised light to form a sample beam; combining the sample beam and reference beam to form a combined beam; and illuminating a photo-alignment layer with the combined beam and recording a polarisation distribution of the combined beam in the photo-alignment layer, wherein a surface normal of the photo-alignment layer is angled with respect to the combined beam.

14. The method of claim 13 wherein the surface normal of the photo-alignment layer is angled with respect to the combined beam on two orthogonal planes containing the optical axis of the combined beam.

15. The method of claim 14 wherein an angle of the surface normal with respect to the combined beam on a first plane of the two orthogonal planes is in the range 20-35 degrees.

16. The method of claim 13 or 14 wherein an angle of the surface normal with respect to the combined beam on a second plane of the two orthogonal planes is in the range 10-20 degrees.

17. The method of any of claims 13 to 16 wherein the first polarised light and second polarised light are linearly polarised in opposing directions.

18. The method of any of claims 13 to 17 further comprising: imparting a quarter-wave phase delay on the combined beam before illuminating the substrate.

19. The method of any of claims 13 to 18 wherein the first polarised light and second polarised light are circularly polarised with opposing handedness.

20. The method of any of claims 13 to 19 comprising forming a liquid crystal layer on the photo-alignment layer, optionally, wherein the liquid crystal comprises reactive mesogens.

21. The method of any of claims 13 to 20 wherein the light beam comprises ultraviolet light.

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