Light control device

Abstract

A head-up display 700 for a vehicle has a replicator (520, figure 5b) and a light control device 704. The replicator is arranged to receive spatially modulated light and replicate the spatially modula

Description

FIELD The present disclosure relates to a head-up display suitable for use in a vehicle comprising a light control layer. In some embodiments, the light control layer acts as both a reflection suppression device and a glare mitigation device. More broadly, the present disclosure relates to the light control layer. Some embodiments relate to a holographic projector or picture generating unit. BACKGROUND AND INTRODUCTION 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. 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 raytracing or a point cloud technique, for example. 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. 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. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, “HUD”. SUMMARY Aspects of the present disclosure are defined in the appended independent claims. In general terms, there is provided a light control device or glare mitigation 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 nonlinear. The optical component has a complex curvature which introduces complex distortions when used in a display system particularly one based on holographic projection. 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 / eye-box of the display device thus forming glare. The light control device of the present disclosure is arranged to suppress such reflections. 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. The inventors have identified a number of problems associated with the lensing effect of the curved optical component. 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 / 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, the inventors have further 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. The inventors have previously proposed light control devices / glare mitigation devices for reflection / glare suppression. These can be seen, for example, in UK patent number GB2607672. The inventors have found that an airgap between the replicator (waveguide) and the light control device assists in the functioning of both. For example, the air gap ensures a consistent optical boundary between the waveguide and the light control device, such that light ray angles are well defined and uniform across the length of the components. For example, in some embodiments, the bottom surface of the light control device may comprise an array of (triangular) prisms that would form a non-uniform optical boundary with the waveguide in the absence of an air gap. An air gap allows the replicas output by the replicator to uniformly turn, as designed, before arriving at the light control device such that the replicas arrive at the light control device at a consistent angle relative to said device. However, it is also desirable to have the replicator and light control device be as thin as possible, in order that they can be installed within a vehicle dashboard, and e.g. weight can be kept to a minimum. As such, it is often preferable to use a thin light control device mounted / affixed parallel to the replicator with an airgap formed between the two components. The inventors have found that, due to the thin nature of the light control device when having supports around its perimeter (in order to provide an open optical aperture), the light control device can deflect (in other words, deform e.g. wobble, flex or vibrate) during use due to, for example, movements such as vibrations of the vehicle in which it is housed. Such deflections can cause distortions of the image displayed, as the replicas arrive at and leave the light control device at undesirable, or even changing, angles relative to the replicator, and cause further distortions if the light control device deflects sufficiently such that it contacts the replicator, as the absence of an airgap would change the light rays angles of the replicas as described above owing to a change in the refractive index difference at the contact point. Repeated deflections may also damage the light control device and even the replicator, if the light control device deflects such that it contacts the replicator. Such repeated contact between the light control device and the replicator can also create a squeak / rattle noise that is undesirable in all, especially high end, vehicles. Affixing the light control device directly to the replicator is not preferrable due to the desire for the airgap, as described above. Increasing the height of the gap between the light control device and the replicator could prevent the damage and rattle caused by contact between the deflecting light control device and the replicator, but it would not address the problem of damage caused to the light control device itself by the repeated deflections, nor the distortions to the image displayed caused by the deflections. Furthermore, increasing said height results in the system taking up more room in the vehicle dashboard, which is undesirable due to space therein being at a premium. In a first aspect, a head-up display for a vehicle is provided. In other words, a (head-up) display system or projection system is provided that is suitable for use in a vehicle (however other uses are also envisaged). The head-up display comprises a replicator and a light control device. The replicator may be a waveguide. The replicator is 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 spatially modulated light may be spatially modulated in accordance with a picture or a hologram of a picture, as will be described further below. The transmissive-reflective surface forms an output surface for the plurality of replicas of the spatially modulated light. The light control device is located in the optical path of the plurality of replicas of the spatially modulated light downstream from the output surface of the replicator. That is, the replicas output from the output surface of the replicator arrive at (i.e. are directed to) the light control device. Or, in other words, the light control device is located on an optical path between the replicator and a viewing plane (e.g. eye-box) or user / viewer. The light control device comprises an arch such that a non-uniform gap (in other words, a space, volume or cavity) is formed between the output surface of the replicator and light control device. In other words, the light control device comprises a curve or arc between two points of the light control device such that, when travelling along a cross-section of the replicator and light control device, the height (or distance) of the gap between the output surface of the replicator and the light control device varies at a plurality of points in at least one plane. The two points may be on the side or opposite sides of the light control device depending on the specifics of the curvature. The light control device “comprises” an arch in that not necessarily all of the light control device is curved, as will be discussed in greater detail below. The term “arch” is used herein not necessarily in the architectural sense - that is, whilst the arch in the present disclosure is curved with two supports (as per the traditional architectural definition), it does not necessarily need to hold the weight of a load placed upon it. That is, when the display (i.e. the vehicle it is housed in) is at rest, the arch may be in a relaxed state and not under load. The arch may be formed when the light control device is moulded. In this way, the curvature of the arch increases the structural rigidity and stability of the light control device, thereby reducing the deflections or deformations that the light control device may exhibit when vibrations exert a load upon the light control device. Reducing the number of deflections the light control device will exhibit over a set period of time reduces the wear of the device itself, as well as reducing the number of impacts on the replicator. This can increase the lifespan of both components and increase their reliability. The reduction of contact between the components also reduces the occurrence of any rattling / squeaking noises. The arch also allows the gap between the replicator and the light control device to remain at a more constant height (that is, more consistent over consecutive time increments rather than constant across the output surface of the replicator). This helps stabilise the image displayed, as there are fewer changes in the system overtime that could cause varying distortions to the image. Finally, these benefits are achieved without having to increase the height of the gap between the replicator and the light control device, ensuring that the head-up display maintains a compact size. Adding the curvature of the arch to the light control device may create distortions to the image displayed, as the replicas emitted from the output surface of the replicator will arrive at, and be emitted from, the light control device at different angles depending on the position along the curvature of the arch that they interact with. However, the inventors have surprisingly found that the advantages described above are sufficient to justify the distortions in the image that will occur with the inclusion of the arch in the light control device and / or, in some embodiments, can be compensated for in the design of the light control device (as will be discussed in greater detail below). The arch may be formed between a first side and a second side of the light control device, the arch extending away from the output surface of the replicator. That is, the arch may extend from one side of the light control device to another. In other words, aside from comparatively small areas at each side of the light control device (as will be discussed in greater detail below), the entirety of the light control device may be arched. This further increases the structural benefits of the arch on the light control device. The arch may be symmetrical about a plane perpendicular to the output surface of the light control device and equidistant between the first and second sides of the light control device. In other words, in cross-section taken along the curvature of the arch, the arch may be symmetrical between the first and second sides of the light control device. Such symmetry adds further strength and stability to the light control device. The light control device may comprise a first attachment portion at the first side and a second attachment portion at the second side. Each attachment portion may be arranged substantially parallel to the output surface of the replicator. The replicator may be substantially flat, along with the attachment portions. In this case, substantially flat means that the replicator is sufficiently flat that no correction of the light emitted from the replicator is required because of curvature of the replicator and that the attachment portions are shaped to be parallel to the replicator. In other words, at each side of the light control device there is a flat portion that is not arched, the use of which will be described below. The light control device may have a lower surface and an upper surface. The terms upper and lower in this case refer to the orientation in which the surfaces would be arranged when in use, with the light emitted from the replicator travelling generally upwards (and away from the replicator). The lower surface may be arranged facing the output surface of the replicator. That is, the light emitted from the output surface of the replicator will arrive at the lower surface of the light control device first. In this way, the lower surface and upper surface may be called the upstream and downstream surfaces respectively. In other words, the lower surface is the inner surface of the curvature of the arch. A plane of the lower surface of the first and second attachment portions may be coincident with a plane of the output surface of the replicator. In other words, the arch may extend from a plane of the output surface of the replicator in a direction away from said output surface. The light control device may be affixed relative to the replicator via the first and second attachment portions, optionally using a pin or screw. That is, the light control device is secured such that movement of the light control device relative to the replicator (be that either away from or towards the replicator, or on a plane of the output surface of the replicator - i.e. lateral movement) is restricted or prohibited. The inventors found that the deflections of the light control device as described above may still occur if the light control device is unsecured. The load placed upon the arch by the vibrational forces may cause the sides of the light control device to separate as the arch deforms and / or lateral movement of the light control device to occur, potentially enough to cause the same issues as described above. Therefore, the inventors have found that, by restricting the horizontal / lateral movement of the sides of the light control device, this deformation can be prevented and the benefits described above preserved. The head-up display may further comprise a support structure. The replicator may be affixed relative to the support structure. The support structure may run around the perimeter of the replicator (in other words, the support structure may encircle or enclose the replicator in a plane of the output surface thereof). The light control device may be affixed relative to the support structure via the first and second attachment portions. That is, the support structure may act as a mounting point for both the light control device and the replicator and thus, by affixing them to the same solid object, movement between the light control device and the replicator can be reduced or prevented. The support structure may comprise a recess. At least the first and second attachment portions of the light control device may be located within the recess. That is, the light control device may be affixed relative to the replicator by the attachment portions being located within a recess. The arch may be a first arch or, in other words, may have a first arch component. The light control device may further comprise a second arch (or, in other words, a second arch component) formed between a third side and a fourth side of the light control device. The second arch (component) may extend away from the output surface of the replicator. In other words, the light control device may be curved in two perpendicular directions, said directions being taken along a plane of the output surface of the replicator, such that the light control device forms a square- or rectangular-based dome shape. The second arch (component) may have any (or all) of the features described above in relation to the first arch (component). In other words, the light control device comprises a curve or arc between two points of the light control device such that, when travelling along a cross-section of the replicator and light control device, the height (or distance) of the gap between the output surface of the replicator and the light control device varies at a plurality of points in at least two orthogonal planes. The introduction of a second arch (component) further increases the strength and stability of the light control device, but can introduce further distortions to the displayed image, as discussed above. The light control device may be configured to (optically) compensate for the curvature of each (or the) arch. In other words, the light control device may be designed in a way that reduces the distortion of the image (i.e. the image perceived through the light control layer is more faithful to the source e.g. more accurately resembles the source) and / or reduces the number or intensity of any distortions in the displayed image caused by the curvature of each (or the) arch. In this way, the potential downside to introducing the arch (as described above) can be mitigated. The gap may extend a maximum height away from the output surface of the light control device in the range of 1mm to 5mm, preferably in the range of 2mm to 3mm. By height it is meant the distance between a plane of the output surface of the replicator and the (lower surface of the) light control device (taken orthogonally from the plane) at the furthest point from the replicator (i.e. at the peak of the arch). In this way, a sufficiently large gap is provided to enable the correct (e.g. uniform) turning of the replicas, but not so large as to unnecessarily increase the size of the system within a vehicle dashboard. The replicator may be arranged to receive a plurality of replicas in a first direction and replicate the plurality of replicas in a second direction, the first direction being perpendicular to the second direction. That is, the replicator (e.g. the waveguide) replicates a onedimensional line of replicas across a two-dimensional plane (the plane of the output surface of the replicator). The spatially modulated light may be, or may comprise, a holographic wavefront. The replicator may be arranged to form a plurality of replicas of the holographic wavefront. The spatially modulated light may be, or may comprise, an image. The head-up display may have a viewing window (in other words, an eye-box). That is, the head-up display has (or forms) an area or volume in the space around the user / viewer in which the displayed image can be perceived with to a satisfactorily high quality (i.e. with a satisfactorily low amount of distortion and artefacts visible and / or without substantive cropping). The output surface of the replicator may be a (at least partially) reflective surface arranged, during head-up display operation, in a configuration that is conducive to sunlight glare. That is, the output surface may be arranged when the head-up display is being used such that could reflect sunlight. That is not to say that the output surface in this case is only reflective to sunlight in this location, merely that it is situated in a position that is exposed to sunlight. The light control device may be arranged to receive (or intercept) sunlight on an optical path to the viewing window. The light control device may be configured to attenuate sunlight, such as absorb sunlight. As such, the light control device acts to prevent sunlight glare from the replicator reaching the viewing window / eye-box. The light control device may be arranged to compensate for the curvature of a curved optical component downstream of the light control device. That is, the light control device may predistort the replicas - more specifically, the wavefront replicas - in order to counteract the distortions that are imparted upon them by the curved optical component, such that the replicas arrive at the user / viewer undistorted - e.g. as if the optical component had not been curved. It some embodiments, it may be said that the light control device has an opposite lensing effect to that of the curved optical element. The head-up display 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. As such, the light control device can be responsible for both glare mitigation and compensation for the curvature of the windscreen of the vehicle. 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 (in the hologram domain) correspond to spatial coordinates of the picture (in the spatial or image / picture domain). 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. 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. 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”. 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. 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. 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. 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 2ir) 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 ir / 2 will retard the phase of received light by k / 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. 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. 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. BRIEF DESCRIPTION OF THE DRAWINGS Specific embodiments are described by way of example only with reference to the following figures: Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen; 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; Figure 3 shows a hologram displayed on an LCOS that directs light into a plurality of discrete areas; Figure 4 shows a system, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3; Figure 5A shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each comprising pairs of stacked surfaces; 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 corrective glare mitigation device in accordance with the prior art; and Figure 7 is a schematic side view of a corrective glare mitigation device in accordance with the present disclosure. The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF EMBODIMENTS 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. 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. 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. 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. 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. 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. Conventional optical configuration for holographic projection 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 the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or freguency 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. 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. 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. 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 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. 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. 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 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. 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. 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. 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 / 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. 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. 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.) 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). 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. 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. 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. 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. 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 / 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. 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. 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. 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. Light channelling 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. 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. 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. 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. 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. 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. 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’. 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. Two-Dimensional Pupil Expansion 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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). 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. 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. 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 / diverging, the light field size increases with propagation distance. 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. 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. 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. 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. 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”. 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. 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. 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. 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. Combiner shape compensation 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. Control device 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 1D 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. Deformations in Corrective Glare Mitigation Devices Figure 6 shows a (head-up) display system 600 according to the prior art with a waveguide 602 (or, more broadly, a replicator 602) as per the second waveguide 540 described above in relation to Figure 5B. Waveguide 602 has an output surface 602a from which the replicas are emitted. Positioned above the waveguide 602 (i.e. downstream of the waveguide 602 in the z-direction) is a corrective glare mitigation device 604 (or, more broadly, a light control device 604). The corrective glare mitigation device 604 serves two main roles to the system 600. Firstly, the corrective glare mitigation device 604 prevents or mitigates sunlight glare that would otherwise reflect off the output surface 602a and towards the user / viewer. For example, the corrective glare mitigation device 604 may comprise carefully designed and angled louvres that allow display light pass through the corrective glare mitigation device 604 whilst dealing with the different sunlight angles for different times of the day in different ways. This is further described, for example, in UK patent number GB2607672. In other words, the corrective glare mitigation device 604 does this by absorbing the sunlight that would be directed towards the user whilst allowing the replicas emitted from the output surface 602a to pass therethrough. Secondly, the corrective glare mitigation device 604 compensates for the curvature of a curved optical component (not shown) such as a windscreen of a vehicle or other optical combiner. For example, the corrective glare mitigation device 604 may comprise a layer (or a pair of layers) of prisms that effectively combine to act as a composite lens having an opposite lensing effect of the curved optical element causing unwanted distortions (e.g. the windscreen / windshield). In this way, the distortion applied by the corrective glare mitigation device 604 can be selected to at least partially if not exactly compensate (or precompensate) for another source of distortion caused by the curved optical element (e.g. the windscreen / windshield). In other words, the corrective glare mitigation device 604 does this by purposefully applying a distortion to the replicas that has an opposite optical power to that of the curved optical component, thereby removing or mitigating any distortion that would be visible to the user. The corrective glare mitigation device 604 is located above the waveguide 602 (in the z-direction) such that a gap 603 is formed therebetween. The gap 603 is an air gap that ensures that the replicas undergo the correct turn as they are emitted from the output surface 602a of the waveguide 602 and before they arrive at the corrective glare mitigation device 604. This gap 603 is therefore required (i.e. the corrective glare mitigation device 604 cannot be mounted directly to the waveguide 602). However, this gap 603 may cause problems as, when the system 600 is subjected to vibrations (e.g. as the vehicle the system 600 is housed in is used), the load L placed on the corrective glare mitigation device 604 by said vibrations may cause it to deform to the deflected shape 604’. The load L as concerns the present disclosure acts generally in the z-direction, although the skilled person would appreciate that the vibrations would cause forces in other directions. That is, the corrective glare mitigation device 604 may change shape under load L, resulting in a curved shaped 604’ that extends from the intended shape towards the waveguide 602 (i.e. the corrective glare mitigation device 604 deforms by extending in the negative z-direction). As can be seen in Figure 6, the deflected / deformed shape 604’ can cause the corrective glare mitigation device 604 to come into contact with the output surface 602a of the waveguide 602. This repeated deflection / deformation and contact therebetween can cause damage to both the waveguide 602 and the corrective glare mitigation device 604, as well as creating a rattle or squeak noise. The movement / contact may disrupt and / or damage the above-described carefully designed and angled louvres, which may result in a cropping of the display light or allow glare to reach the user that would otherwise have been prevented. Furthermore, the movement / contact may disrupt and / or damage the above-described layer(s) of prisms, which may result in an incorrect pre-distortion being applied, resulting in distortion from the curved optical combiner. This contact could be prevented by increasing the height of the gap 603 (i.e. moving the corrective glare mitigation device 604 away from the waveguide 602 in the positive z-direction). However, this would increase the size of the system 600 and would not prevent the strain put on the corrective glare mitigation device 604 by the repeated deformations, nor the above described rattle or squeak noise. Improved Corrective Glare Mitigation Device Figure 7 shows a (head-up) display system 700 according to the present disclosure. The system 700 has the same waveguide 602 with an output surface 602a as described above. The system 700 comprises a corrective glare mitigation device 704 (or, more broadly, a light control device 704). As described above, the corrective glare mitigation device 704 provides glare mitigation and compensates for the curvature of an curved optical component (such as a vehicle windscreen or other optical combiner, not shown). The corrective glare mitigation device 704 comprises an arch 705 extending between a first attachment portion 710 and a second attachment portion 712 on either side of the corrective glare mitigation device 704. The first and second attachment portions 710, 712 of the corrective glare mitigation device 704 are flat (that is, parallel to the output surface 602a, or a plane of the output surface 602a, of the waveguide 602). The arch 705 formed therebetween has a curvature that is symmetrical about a centre point of the corrective glare mitigation device 704 in the y-direction. That is, the arch 705 is symmetrical between the first and second attachment portions 710, 712. The corrective glare mitigation device 704 has a lower or upstream surface 706 and an upper or downstream surface 708. The lower surface 706 is the surface of the corrective glare mitigation device 704 that faces the output surface 602a of the waveguide 602. In other words, after being emitted from the output surface 602a, the replicas arrive at the lower surface 706 of the corrective glare mitigation device 704 first. The lower surface 706 of the first and second attachment portions 710, 712 are coincident with a plane of the output surface 602a of the waveguide 602. As such, it can be said that the arch 705 extends in the z-direction from a plane of the output surface 602a of the waveguide 602. The curvature of the arch 705 forms a gap 703 that varies in the z-direction across the y-direction. At the peak of the curve of the arch 705, the gap 703 has a height h of between 2mm and 3mm, however values of h in the range of 1 mm to 5mm are also envisaged. By the peak of the curve, it is meant the point at which the distance in the z-direction from the output surface 602a (or a plane of the output surface 602a) of the waveguide 602 to the lower surface 706 is the largest. This is how the term “height” of the arch 702 is defined herein. The arch 705 in the embodiment shown in Figure 7 is curved in the y-z plane, however in another embodiment (not shown), the arch 705 is also curved in the x-z plane. The curvature of the arch 705 in the x-z plane is the same as the curvature in the y-z plane. That is, the arch 705 forms the corrective glare mitigation device 704 into a square- or rectangular-based dome shape. The corrective glare mitigation device 704 is secured to a support structure 714 that is (although not shown) also secured to the waveguide 602. In other words, relative movement between the waveguide 602, corrective glare mitigation device 704 and support structure 714 in the x-, y- and z-directions is prevented or restricted. The support structure 714 may be a singular component or multiple parts secured together. The support structure 714 extends the full perimeter of the replicator 602, thereby encircling or enclosing the replicator 602 in the x-y plane. The embodiment shown in Figure 7 has the corrective glare mitigation device 704 being secured to the support structure 714 via attachment means 716 passing through the first and second attachment portions 710, 712 of the corrective glare mitigation device 704. The attachment means 716 may be pins or screws, however other suitable attachment methods are also envisaged. In a further embodiment (not shown), the support structure 714 has a recess into which the first and second attachment portions 710, 712 are slotted to facilitate the securement of the corrective glare mitigation device 704 to the support structure 714. The curvature of the arch 704 adds strength and stability to the corrective glare mitigation device 704 that mitigates or prevents the deflections / deformations described above in relation to the system 600 of Figure 6. As the load L from the vibrations is applied to the corrective glare mitigation device 704 in the negative z-direction (as described above in relation to the system, 600 of Figure 6), the load L is distributed across the curvature of the arch 704 via compression of the arch 704, with any deflection happening in the y-direction as the first and second attachment points 710, 712 move apart from one another. Securing the first attachment points 710, 712 relative to one another via the support structure 714 prevents this deflection or any lateral movement (i.e. movement in the x-y plane) that may occur. As such, the corrective glare mitigation device 704 maintains its shape and position under the load L. This mitigates or prevents the strain exerted on the corrective glare mitigation device 704 due to repeated deflections / deformations. Furthermore, it mitigates or prevents contact between the corrective glare mitigation device 704 and the waveguide 602, increasing the lifespan of the components and reducing or preventing any squeak or rattle noises that would otherwise have been produced. Finally, this ensures that the gap 703 can be maintained without it needing to be so large that the system 700 has an increased size. The corrective glare mitigation device 704 is formulated such that it accounts for the curvature of the arch 704 and the distortions this will impart on the replicas being emitted from the waveguide 602. That is, the corrective glare mitigation device 704 is arranged such that it compensates for the effect the curvature of the arch 705 has on the replicas. For example, the layer(s) of prisms as described above may be shaped or formed in such a way that they distort the replicas in such a way that the distortion caused by the curvature of the arch 705 is compensated for. The inventors have surprisingly found that the benefits to the strength and stability of the corrective glare mitigation device 704 as described above outweigh the need to perform this compensation. Additional features 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. 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). 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. A head-up display for a vehicle comprising: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 forming an output surface for the plurality of replicas of the spatially modulated light; anda 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, wherein the light control device comprises an arch such that a non-uniform gap is formed between the output surface of the replicator and light control device.

2. The head-up display as claimed in claim 1, wherein the arch is formed between a first side and a second side of the light control device, the arch extending away from the output surface of the replicator.

3. The head-up display as claimed in claim 2, wherein the arch is symmetrical about a plane perpendicular to the output surface of the light control device and equidistant between the first and second sides of the light control device.

4. The head-up display as claimed in claim 2 or claim 3, wherein the light control device comprises a first attachment portion at the first side and a second attachment portion at the second side, each attachment portion arranged substantially parallel to the output surface of the replicator.

5. The head-up display as claimed in claim 4, wherein the light control device has a lower surface and an upper surface, the lower surface arranged facing the output surface of the replicator, a plane of the lower surface of the first and second attachment portions being coincident with a plane of the output surface of the replicator.

6. The head-up display as claimed in claim 4 or claim 5, wherein the light control device is affixed relative to the replicator via the first and second attachment portions, optionally using a pin or screw.

7. The head-up display as claimed in any of claims 4 to 6 further comprising a support structure, wherein the replicator is affixed relative to the support structure and the light control device is affixed relative to the support structure via the first and second attachment portions.

8. The head-up display as claimed in claim 7, wherein the support structure comprises a recess, and wherein at least the first and second attachment portions of the light control device are located within the recess.

9. The head-up display as claimed in any preceding claim, wherein the arch is a first arch and the light control device further comprises a second arch formed between a third side and a fourth side of the light control device, the second arch extending away from the output surface of the replicator.

10. The head-up display as claimed in any preceding claim, wherein the light control device is configured to compensate for the curvature of each arch.

11. The head-up display as claimed in any preceding claim, wherein the gap extends a maximum height away from the output surface of the light control device in the range of 1 mm to 5mm, preferably in the range of 2mm to 3mm.

12. The head-up display as claimed in any preceding claim, wherein the replicator is arranged to receive a plurality of replicas in a first direction and replicate the plurality of replicas in a second direction, the first direction being perpendicular to the second direction.

13. The head-up display as claimed in any preceding claim, wherein the spatially modulated light is a holographic wavefront and the replicator is arranged to form a plurality of replicas of the holographic wavefront.

14. The head-up display as claimed in any preceding claim, wherein the head-up display has a viewing window and the output surface of the replicator is a reflective surface arranged, during head-up display operation, in a configuration that is conducive to sunlight glare, wherein the light control device is arranged to receive sunlight on an optical path to the viewing window.

15. The head-up display as claimed in claim 14, wherein the light control device is configured to attenuate sunlight, such as absorb sunlight.

16. The head-up display as claimed in any preceding claim, wherein the light control device is arranged to compensate for the curvature of a curved optical component downstream of the light control device.

17. The head-up display as claimed in claim 16, wherein the light control device has an opposite lensing effect to that of the curved optical element.

18. The head-up display as claimed in claim 16 or claim 17 further comprising the curved optical component located downstream from the light control device, wherein the curved optical component is an optical combiner, optionally wherein the curved optical component is a windscreen of a vehicle.

19. The head-up display as claimed in any preceding claim, wherein the light is spatially modulated in accordance with a picture.

20. The head-up display as claimed in any of claims 1 to 18, wherein the light is spatially modulated in accordance with a hologram of a picture.

21. The head-up display as claimed in claim 20, wherein the hologram is 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.

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