Light control layer

Abstract

A head-up display (HUD) is housed in a support structure. The HUD includes an optical component 740 with a reflective surface which, when housed in the support structure is conducive to sunlight glare. A light control layer 706 receives light from the optical component and includes an array of louvres 710 to absorb a first portion of light incident upon them. At least one characteristic of at least a portion of each louvre ensures that a second portion of the light incident on the louvre array forms a first veiling glare 1502 to compensate for a second veiling glare 1104 caused by the support structure. The first veiling glare may compensate for the second veiling glare by reducing a gradient or contrast between the first and second veiling glare. The first veiling glare caused by the louvre array may match the second veiling glare caused by the support structure. The at least one characteristic may be that the louvre array diffusely reflects ≥ 5%, or ≤ 30% of light, incident upon it. Each louvre may include a core and an outer coating where the core includes a light absorbing material and the outer coating includes a diffusely reflective material.

Description

FIELD The present disclosure relates to a light control layer. More specifically, the present disclosure relates to a light control layer for providing reflection suppression to reduce sunlight glare. Even more specifically, the light control layer of the present disclosure is tuned / optimised such that, while a first portion of sunlight glare incident thereon is absorbed or suppressed, a second portion of the incident sunlight glare forms a veiling glare to compensate for (or match) a (second) veiling glare formed by a support structure for housing the light control layer. Yet more specifically, the present disclosure relates to a head-up display comprising the light control layer and a projection system comprising the head-up display. The light control layer is for suppressing reflections off a reflective optical component of the head-up display, such as a waveguide. The present disclosure further relates to a method of optimised glare suppression and a method of designing a glare suppression component. 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 ray tracing 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. Broadly, the disclosure relates to a head-up display comprising an optical component, such as a waveguide, having a reflective surface. The head-up display may be arranged such that light encoded with picture content is directly or indirectly relayed towards an eye-box or viewing window. The light encoded with picture content may be a holographic wavefront (in other words, light which is spatially modulated in accordance with a hologram of the picture). The head-up display further comprises a light control layer. The reflective surface may be susceptible or conducive to cause glare by reflecting sunlight incident thereon onto an optical path to a user. The reflective surface may be substantially planar and / or substantially aligned with general orientation of a housing or support structure such as a vehicle dashboard. This glare could be distracting and uncomfortable for a viewer such as the driver of a vehicle comprising the head-up display. This glare may also reduce the visibility or contrast of an image projected by the head-up display. Thus, the light control layer is provided to reduce said glare. The light control layer according to the disclosure comprises a plurality of louvres arranged in an array, each louvre being arranged to absorb light incident thereon (e.g. ambient light I sunlight glare). The louvres may be substantially elongate and may be substantially parallel. The head-up display may be arranged so that light encoded with picture content is relayed from the optical component (e.g. waveguide) over a limited range of angles. The louvres of the light control layer are arranged to allow for the transmission of the picture content light over the limited range of angles while substantially blocking I absorbing light incident on the light control layer outside of this range of angles (which may be ambient light / sunlight glare). In some embodiments, the light control layer further comprises a serrated surface. The serrated surface provides an array of angled surfaces that direct sunlight away from the eye-box or viewing window. In particular, most sunlight incident on the serrated top surface may be reflected and the angled surfaces changes the course or path of the specular reflection of the sunlight or glare. Herein, the light control layer may be referred to as a reflection suppression device. The terms light control layer and reflection suppression device may be used interchangeably. Given that, as above, glare is generally considered to be distracting and uncomfortable for a viewer and can reduce the visibility or contrast of an image projected by a head-up display, there is a strong prejudice in the technical field towards maximising glare suppression (i.e. preventing glare from the head-up display - which could be formed by the optical component or by the light control layer, for example - from reaching the viewing window I eye box). The inventors have designed, and previously disclosed, a light control layer comprising a louvre array and, optionally, a serrated top surface, that is very effective at reducing sunlight glare from reaching a viewing window or eye-box of the head-up display. In fact, the inventors have effectively eradicated said glare (i.e. such that substantially no glare reaches the viewing window or eye-box). Thus, given the prejudice in the field for maximising glare suppression, the inventors expected such an arrangement (in which glare is effectively eradicated) to provide an optimum viewing experience. However, after thorough experimentation and simulation, the inventors have surprisingly found that, in a real device, complete suppression of glare can result in visible artefacts being formed in a field of view of a viewer of the head-up display. These artefacts may be distracting and adversely affect the viewing experience. In more detail, typically, the head-up display is provided (e.g. received and I or housed) in a support structure. For example, if the head-up display is for a vehicle such as a car, the support structure may be the dashboard of the vehicle. The head-up display may be provided I received I housed in at least a portion of the dashboard. The support structure (e.g. dashboard) may comprise an opening in which the head-up display is receivable. The support structure may comprise a first surface, such as an external surface. The opening may be formed in the first surface. The support structure (e.g. the first surface of the support structure) may be susceptible I conducive to veiling glare. For example, ambient light I sunlight may be incident on the support structure / first surface. The ambient light / sunlight may be reflected by the support structure / first surface. The reflection may be a diffuse reflection. The (diffusely) reflected light may form the veiling glare. In this disclosure, this component of veiling glare is referred to as “second veiling glare”. At least some of the veiling glare may be relayed to the viewing window I eye-box such that veiling glare is visible at the viewing window I eye box. For example, the veiling glare may be reflected by an optical combiner such as a windscreen of the vehicle so as to be relayed to the viewing window / eye box. Thus, the veiling glare caused by the support structure may result in a region of elevated brightness being visible at the viewing window. In some examples, an image of the support structure may be visible in the region of elevated brightness. The inventors have found that, if a head-up display is provided comprising a light control layer arranged to effectively eradicate glare I substantially completely supress glare, then substantially no (veiling) glare may originate from the light control layer. The inventors have found that this may result in a conspicuously dark portion in the viewer’s field of view (e.g. a dark portion in the region of elevated brightness I in the image of the support structure formed by veiling glare). The inventors have found that this dark portion may be distracting for a viewer and may adversely affect the viewing experience. So, the inventors have surprisingly found that, contrary to the general prejudice in the field, an optimum viewing experience of a head-up display may not be achieved when glare suppression is maximised. The inventors have recognised that an improved I optimised viewing experience may be achieved be tuning the light control layer to intentionally cause a (modest amount of) veiling glare that at least partially compensates for or “matches” the veiling glare caused by the support structure. This intentionally-added glare is referred to herein as a “first veiling glare”. In this way, the appearance of the dark portion I region may be significantly reduced. In particular, the inventors have found that the louvres of the light control layer can be modified such that a portion of light incident thereon forms veiling glare (and the remaining portion of the light is absorbed such that the louvres continue to primarily function to suppress glare). For example, the louvres may be arranged to diffusely reflect a (relatively small) proportion of incident light. It was counter-intuitive for the inventors to provide louvres which intentionally introduce a source of (veiling) glare into the system. But the inventors have surprisingly found that doing so (to compensate for or match or reduce the visual impact of the veiling glare caused by the support structure) reduces or eliminates the appearance of a distracting artefact (dark region) in the veiling glare of support structure. Thus, the inventors have surprisingly found that the viewing experience is improved if the louvres are turned to cause / form an intentional source of veiling glare. In a first aspect, there is provided a head-up display. The head-up display may be for an automotive vehicle such as a car. The head-up display is arranged for housing in a support structure (e.g. may be arranged to be receivable or received in a support structure). The support structure may be at least a portion of a dashboard of an automotive vehicle. The support structure may be conducive to forming a veiling glare when ambient I sunlight is incident thereon. For example, a surface, such as a top or exterior surface of the support structure, may be conducive to forming the veiling glare. The head-up display comprises an optical component (such as a waveguide). The optical component has a reflective surface (such as a reflective surface of the waveguide). The reflective surface is arranged such that, when the head-up display is housed in the support structure, the reflective surface is conducive to sunlight glare. The head-up display further comprises a light control layer arranged to receive light from the optical component. The light control layer comprises a plurality of louvres in an array. The louvre array is arranged to absorb a first portion of light (e.g. ambient or sunlight glare) incident thereon. At least one characteristic of at least a portion of each louvre is such that a second portion of the (ambient) light (sunlight glare) on the louvre array causes a first veiling glare. The at least one characteristic may be an optical characteristic. As described above, it is counterintuitive to provide a louvre array of a light control layer / reflection suppression device that is arranged to intentionally cause veiling glare. In some embodiments, the first portion of light (absorbed by the louvre array) comprises at least one of: direct sunlight and specularly-reflected sunlight. In some embodiments, the first veiling glare (caused I formed by the louvre array) is arranged to (at least partially) compensate fora second veiling glare caused by the support structure. As used herein, the first veiling glare being arranged to compensate for the second veiling glare may mean that the louvre array is arranged such that the first veiling glare substantially reduces or minimises the appearance of a dark area in the field of view of a viewer (in particular, in an otherwise bright region in the field of view caused by the second veiling glare). The first veiling glare at least partially compensating for the second veiling glare may mean that the first veiling glare balances, softens, substantially matches the second veiling glare. The louvre array may be arranged to form a first veiling glare so as to improve a match, even out, or reduce a difference between the first veiling glare and the second veiling glare. For example, the second veiling glare may illuminate a first area or region of the field of view of a viewer of the head-up display. There may be a hole or second area within the first area that is not illuminated by the second veiling glare. This hole or second area may be the result of an opening or aperture in (a surface of) the support structure in which the head-up display is received and may correspond to the position of the head-up display (light control layer) in the support structure). The inventors have surprisingly found that, if the light control layer substantially completely supressed glare from the head-up display and light control layer itself, then this hole or second area would be conspicuous and may distract a viewer and detract from the viewing experience of the head-up display. The inventors found that the viewing experience could be optimised by deliberately forming at least a portion of the louvres to introduce, add or enhance a veiling glare component. Specifically, the inventors have recognised that, by arranging the louvre array to form the first veiling glare, the hole or sub-area may be illuminated by the first veiling glare. This may compensate for the second veiling glare by reducing a contrast between the first area (illuminated by the second veiling glare) and the second area (illuminated by the first veiling glare). In some examples, the contrast of the first area and the second area may substantially match. Thus, the appearance or conspicuousness of the second area (relative to the first area) may advantageously be reduced. In some embodiments, the first veiling glare is arranged to compensate for the second veiling glare by reducing a gradient or contrast between the first veiling glare and the second veiling glare. This reduction may be relative to a light control layer which is arranged to absorb a first portion of ambient light but does not form a compensatory second veiling glare. In some embodiments, the second veiling glare caused by the louvre array substantially matches to a first veiling glare caused by the support structure. As used herein, the second veiling glare “matching” the first veiling glare may mean that the brightness or intensity of the first veiling glare is close enough to the respective brightness or intensity of the second veiling glare such that the first and second veiling glare appears similar (when viewing from a viewing window of the head-up display, for example). As above, this may be such that a contrast or gradient between areas of the field of view illuminated by the first veiling glare and the second veiling glare is reduced. In some examples, “matching” may mean that an parameter of the second veiling glare is at least 70% that of the first veiling glare, optionally at least 80%, optionally at least 90%. In some examples, “matching” may mean that the parameter of the second veiling glare is less than 130% that of first veiling glare, optionally less than 120%, optionally less than 110%. In some examples, the parameter is the intensity of the respective veiling glare, as measured from a viewing window of the head-up display. In some embodiments, the first and second veiling glare are substantially equal such as within 5% in intensity (for example, when measured from a viewing window or eye-box of the head-up display). In some embodiments, the at least one (optical) characteristic is such that the first veiling glare is formed by diffuse reflectance of the second portion (of light incident on the louvre array). For example, the at least one (optical) characteristic may be the diffuse reflectivity of the at least portion of each louvre. In some embodiments, the at least one (optical) characteristic is such that the louvre array is arranged to diffusely reflect 5% or more of light (sunlight glare) incident thereon, optionally 10% or more. In other words, the louvre array may be arranged such that the second portion of light (causing the first veiling glare) is at least 5% of the light incident on the array, optionally at least 10%. The inventors have found that such proportions of diffusely reflected light may result in the second veiling glare compensating for the first veiling glare. The inventors have recognised that, for the louvre array to sufficiently suppress glare, it may be advantageous for the louvre array to be arranged to absorb the majority of light incident thereon (e.g. absorb a larger proportion of light incident thereon than the proportion that is diffusely reflected). In some embodiments, that at least one (optical) characteristic is such that the louvre array is arranged to diffusely reflect no more than 30% of light (sunlight glare) incident thereon, optionally no more than 20%. In some embodiments, the louvre array is arranged such that the first veiling glare appears substantially in a region which is absent of second veiling glare (when viewed from a viewing window of the head-up display). This region may correspond to the second area described above. Thus, the region may be surrounded by a first region. In some embodiments, each louvre comprises a core layer and an outer coating. In such embodiments, the outer coating may be arranged to diffusely reflect light incident thereon. The outer coating may be arranged such that only a (minor) portion of the incident light may be diffusely reflected. This (minor) portion of the incident light may correspond to the second portion of incident light described above (forming the first veiling glare). The remaining portion of incident light may be transmitted by the outer coating to the core layer whereby the light may be absorbed. In this way, the intensity of the first veiling glare can be controlled by tuning the outer coating. The core layer may comprise or consist of a light absorbing material. The light absorbing material may be opaque. The light absorbing material may be black. The light absorbing material may comprise rubber, e.g. black rubber, or metal coated or painted with an opaque coating. The core layer may be arranged to absorb substantially all light incident thereon. The outer coating material may have properties selected to achieve a desired intensity of second veiling glare. The outer coating material may comprise or consist of a diffusely reflective material. In some embodiments, the light control layer comprises a serrated surface. The light control layer may comprise a light receiving surface. The light receiving surface may comprise the serrated surface. The serrated surface may be a first serrated surface. In some embodiments, a periodicity of the first serrated surface is substantially equal to a periodicity of the louvre array. In some embodiments, each serration of the serrated surface may comprise a first facet surface. The first facet surface may comprise the angled portion and may have a first angle with respect to a plane of the light control layer. Each serration of the serrated surface may comprise a second facet surface having a second angle with respect to the plane of the light control layer. Thus, the serrated surface may comprises the first and second facet surfaces in an alternating configuration. Each serration may be arranged such that holographic wavefront of the head-up display passes through the first facet surface. Each serration may be arranged such that the second facet is substantially aligned with the holographic wavefront received by the first serrated surface. Each of the serrations of the serrated surfaces may be formed by a portion of the serrated surface that is angled with respect to a general plane of the light control layer. The or each portion of the first serrated surface forming a serration may have a constant angle with respect to the plane of the reflection suppression device. In other words, each serration I angled surface may have a linear profile. The first serrated surface may be described as having a sawtooth configuration or shape. In such cases, the angled surface portion associated with each serration of the first serrated surface may be substantially planar. This may result in the thickness of the first layer in the first plane linearly varying along a length of each individual serration in the first plane. Alternatively, the or each portion of the first serrated surface forming a serration may have a changing angle with respect to the plane of the reflection suppression device. The louvres may extend longitudinally in a first direction, and may be spaced apart in a second direction that is orthogonal to the first direction. The separation between adjacent louvre slats in the second direction may be defined as the pitch. In some arrangements the pitch may be the same for all louvre slats of the array. It may be said that the spacing or periodicity of the louvres is uniform for the array. In other arrangements, the pitch may vary between slats of the array, such as from a first end to a second end of the array. It may be said that the spacing or periodicity of the slats of the array is non-uniform for the array. As above, the louvres of the louvre array may comprise a light absorbing material. Said light absorbing material may have one or more of: high absorption; high attenuation; low specular reflectivity, and high diffusivity of light. The person skilled in the art of optics appreciates what constitutes “high” and “low” in relation to the optical properties of a material. In some embodiments, the term “high” means greater than 80% such as greater than 90% or 95% and the term “low” means less than 20% such as less than 10% or 5%. For example, the term “high attenuation” may mean that the intensity of incident light (e.g. sunlight) is attenuated (i.e. reduced) by at least 95%. The louvre structure comprises a one-dimensional array of parallel longitudinal slats, each slat having a length, width (height) and a thickness. In embodiments, the louvres / louvre slats have fixed positions in the array and remain static in use. In some embodiments, the slats have a uniform thickness. In other embodiments, the slats may vary in thickness, for example the thickness may be tapered along their width from the proximal end / edge to the distal end / edge. In some arrangements, the angle of the (sidewalls of the) louvres / louvre slats may be uniform (i.e. constant) across the array. In other arrangements, the angle of the (sidewalls of the) louvres / louvre slats may vary across the array, such as from a first end to a second end of the array. The louvres I slats (or slat sidewalls) may be orientated so as to be aligned with I parallel to a central or “gut” ray of the holographic wavefront received by the louvre array. In this way, the louvre array may be arranged to substantially transmissive to the holographic wavefront (even though the louvres themselves may comprise or consists of a light absorbing material). Typically, the holographic wavefront received at the intermediate layer from the first layer will be non-parallel to the normal of the waveguide. So, the louvre / slats (or slat sidewalls) may be orientated (inclined) at a (non-zero) angle to the normal of the waveguiding surface. However, in some examples, the louvres may be parallel to the normal of the waveguiding surface. In some embodiments, the louvre array comprises louvres / louvre slats that are spatially separated by air. In other embodiments, the louvre array includes a transparent structure between adjacent louvres / louvre slats of the array. The transparent structure may improve the structural and functional integrity of the louvre structure, so that it is more robust (less easily damaged) and may be more easily cleaned. In some such embodiments, the louvre array may be embedded in the transparent structure. For example, the intermediate layer may comprise a film or microfilm comprising the array of (micro) louvres embedded in a transparent material. In some embodiments, the optical component is a waveguide. In some embodiments, the waveguide is arranged to receive light encoded with picture content, such as a holographic wavefront, and waveguide the light I holographic wavefront between a pair of reflective opposing surfaces thereof. The waveguide may be arranged to waveguide the light I holographic wavefront in a waveguiding direction. In a second aspect, there is provided a projection system. The projection system comprises a head-up display. The head-up display is a head-up display as described in the first aspect. The head-up display is received in or housed in a support structure. The support structure may be at least a portion of a dashboard for an automotive vehicle. The support structure is arranged such that light (such as ambient light I sunlight glare) is receivable thereon. The support structure is arranged to to cause the second veiling glare (in particular, the support structure is arranged such the light receivable thereon forms the second veiling glare). In some embodiments, the support structure comprises an surface (e.g. an external surface). The (external) surface of the structure may be arranged to receive the light thereon. In some embodiments, the (external) surface comprises an opening I aperture in which the light control layer is received. In some embodiments, the opening I aperture does not contribute to the second veiling glare such that the first veiling glare comprises a dark region corresponding to the opening I aperture. In some embodiments, the louvre array is arranged such that the second veiling glare substantially fills the dark region of the first veiling glare (to reduce the appearance of the dark region). In a third aspect, there is provided a method of glare mitigation for a head-up display housed in a support structure. The head-up display comprises a light control layer arranged to receive light from an optical component having a reflective surface arranged in a configuration that is conducive to sunlight glare. The light control layer comprises a plurality of louvres in an array. The method comprising the step of receiving a sunlight glare incident on the light control layer. The method further comprises the step of absorbing, by the louvre array, a first portion of the sunlight glare incident on the light control layer, and causing, by the louvre array, a second portion of the sunlight glare to form a first veiling glare. The method further comprises receiving sunlight glare incident on a (surface of) the support structure to form a second veiling glare. The louvre array are arranged such that the first veiling glare is arranged to compensate for the second veiling glare. Features and advantages described in relation to one aspect may be applicable to other aspects. 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 2tt) which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of tt / 2 will retard the phase of received light by tt / 2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term “grey level” may be used to refer to the plurality of available modulation levels. For example, the term “grey level” may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term “grey level” may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator. 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 5 shows a perspective view of a first example two-dimensional pupil expander comprising two replicators; Figures 6A and 6B are schematic views of an automotive head-up display system showing potential areas of sunlight glare; Figure 7 is a schematic view of a reflection suppression device formed on a transmission surface of a pupil expander of an automotive head-up display system; Figure 8 is a schematic view of a portion of the reflection suppression device of Figure 7, showing the refraction of light rays passing through the suppression device; Figure 9 is a schematic view of a second reflection suppression device in combination with a waveguide and a turning layer; Figure 10 is a schematic cross-sectional view of a vehicle showing veiling glare formed by a dashboard of the vehicle; Figure 11 is a schematic perspective view of the interior of the vehicle; Figure 12 is a schematic cross-sectional view of a reflection suppression device according to the present disclosure, comprising an array of louvres that are arranged to be diffusely reflective; Figure 13 is a schematic cross-sectional view of an individual louvre of the array of louvres of Figure 12, showing a portion of a light ray incident thereon being diffusely reflected; and Figure 14 is a schematic perspective view of the interior of a vehicle comprising a reflection suppression device according to 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 and eye-box 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 egually 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, ata 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, BO 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 5, 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 / 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 / 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 I 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. Reflection Suppression to Mitigate Glare In operation, the transmission / exit surface (i.e. expanded exit pupil) of the second replicator 506 of the two-dimensional pupil expander of Figure 5B forms an external surface or “output port” from which image light is transmitted through air to an eye-box area for viewing. Accordingly, the transmission surface may be exposed to sunlight from the environment in which the head-up display is used. Received sunlight may cause glare to the viewer, in particular due to reflections of sunlight associated with the pupil expander 506 and / or a turning film, if used in conjunction with the pupil expander 506. For example, glare may arise if rays of sunlight are directly reflected from the external transmission surface, or other surfaces of the pupil expander 506, at angles such that rays of sunlight follow an optical path directly to the viewing area / eye-box. This is described herein as “direct glare”. In another example, glare may arise if sunlight is coupled into the pupil expander 506 at angles such that rays of sunlight follow the same optical path within the pupil expander as rays of image light, or are otherwise reflected by surfaces thereof, in order to reach the viewing area / eye-box indirectly (e.g. via an optical combiner, such as a vehicle windscreen). This is described herein as “veiling glare”. Figure 6A shows the optical path of rays of sunlight S incident on the transmission / exit surface 642 of a (bulk optic) waveguide pupil expander 640 of a head-up display (HUD) in an automotive application. In particular, sunlight S at a relatively high elevation angle to the horizon is incident through a vehicle windscreen 630 onto the external transmission / exit surface 642 of the pupil expander 640. In the example, the transmission / exit surface 642 is located in a substantially horizontal plane in an aperture in the vehicle dashboard (not shown). Some sunlight rays D may be directly reflected from the pupil expander 640 (e.g. by one or more reflective layers thereof) towards the viewing area / eye-box and cause “direct glare”. Some other light rays V may be indirectly reflected from the pupil expander 640 (e.g. by one or more reflective layers thereof) towards the viewing area / eye box, via the windscreen 730 and cause “veiling glare”. Thus, light rays V may follow the same optical path(s) as image light output from the pupil expander 640. In either case, the glare arising from reflected sunlight may impair a viewer / drivers view of the real world and may reduce the contrast of projected images formed by light emitted by the head-up display. Figure 6B shows the view at the viewing area / eye-box indicating the areas of the windscreen and dashboard, from which the viewer / driver may receive sunlight glare. An area of direct glare D is seen at the exit / transmission surface in the vehicle dashboard (not shown) and an area of viewing glare V is seen at the vehicle windscreen. The skilled person will appreciate that the presence of glare from different positions within the illustrated areas D and V at a particular point in time may depend on the elevation angle of the sun and the configuration of the display system (both internally and in situ). Reflection Suppression Device Accordingly, there is a reflection suppression device to be position over the transmission surface of the second replicator / pupil expander, or more generally the output port of the HUD or any other optical component comprising a reflective surface, to reduce the risk of glare to the viewer. The reflection suppression device comprises a louvre structure comprising a plurality of parallel louvres comprising a light absorbing material. The orientation (e.g. the side-wall angle(s), pitch and geometry (e.g. length, width and thickness)) of the louvres is chosen to allow image light to be transmitted from the HUD at the desired range of angles necessary to reach the viewing area / eye-box. The inventors have recognized that an louvre array, typically in the form of longitudinal rectangular-shaped louvre slats, may be used to control the direction and / or supress reflections of sunlight that may be incident on the transmission surface / output port of the HUD due to its upwardly facing / horizontal orientation in a vehicle dashboard adjacent the vehicle windscreen. The reflection suppression device further comprises a transparent support structure configured to provide mechanical strength and rigidity to the reflection suppression device. In particular, the support structure may be configured to substantially prevent deformation of the array louvres which would otherwise cause the louvres to deviate from the desired angle. The inventors have identified that one problem with the inclusion of the transparent structure is that sunlight (or other unwanted light) may be reflected by a surface of the transparent support structure. So, the transparent structure may introduce another source of glare into the optical system. The transparent support structure can be shaped so as to form a serrated top (or external) surface, the serrations of the serrated surface being co-ordinated with the louvres. Each of the serrations forms an angled surface. The angle of the angled surface is chosen so that light that is reflected by the angled top surface in such a way that the reflected light is not relayed to the eye-box. Thus, the serrated surface of the transparent structure acts to suppress glare as well as to improve mechanical stability. Figure 7 shows an example of a reflection suppression device. The reflection suppression device comprises a light control layer in the form of louvre structure 706 for reducing glare due to reflected sunlight in accordance with reflection suppression devices of the present invention. The louvre structure 706 comprises a plurality of parallel louvres / louvre slats 710. Each louvres 710 has a length extending in the x direction. Thus, the length (long dimension) of the louvres / louvre slats 710 is parallel to a first dimension (illustrated as the x dimension). Each louvre is angled with respect to the z axis (in other words, is angled with respect to the normal of a plane defined by the x and y axis). The louvre slats 710 comprise a light absorbing or light attenuating (e.g. light diffusing) material in order to block light rays incident thereon. In the illustrated arrangement, the louvre structure 706 is disposed on a substantially planar transmission surface 742 forming the output port of a second replicator / waveguide pupil expander 740, which is arranged to internally reflect and replicate image light to provide pupil expansion in the second dimension. However, in other embodiments, the transmission surface 742 could be replaced with a reflective surface of any other optical component. Light rays of the image light are formed and output by the waveguide 740 at the transmission surface 742. A single ray of image light 712 is shown in Figure 7. However, it should be clear that rays of image light will be output along the waveguide pupil expander 740 in the y direction. Each ray of image light will be parallel and so emitted at the same angle as 712. The louvre slats are inclined at angle 0 (as shown in Figure 7). Angle 0 is defined as the angle with respect to the normal of the transmission surface (i.e. the angle with respect to the z axis in Figure 7). Angle 0 is chosen to be equal to the angle of the image light 712. The angled orientation of the louvre slats 710 enables the light rays of the replicas of the image light 712 to pass between the louvre slats 710 without deviation from the required optical path / s via the windscreen to the viewing area / eye-box. The louvres are configured with an orientation, pitch and geometry arranged to limit the range of transmission angles 708 from the second (top / external) surface, as shown by dashed lines. So, while image light 712 is shown being parallel to the louvres 710, in actual fact all light having an angle falling with range of angles 708 may pass through the reflection suppression structure without deviation and be received at an eye-box. In this example, the width of the louvres 710 and the pitch p between adjacent louvres 710 and I or the orientation angle 0 of the louvres 710 are selected such that there is an overlap (or overhang) between adjacent louvres (in the y dimension). In particular, the distal end / edge 711 - the end that is remote / distal with respect to the transmission surface 742 - of one louvre overlaps (or overhangs) the proximal end / edge 712 of the adjacent / neighbouring louvre in the second dimension, as shown by overlap o. It may be said that the proximal end / edge 712 of one or a pair of neighbouring louvres / louvre slats 710 is in the shadow of the overlap (or overhang) o of the distal end / edge 711 of the other of the pair of neighbouring louvres / louvre slats 710. The reflection suppression device further comprises a transparent support structure. In this example, the transparent support structure comprises a plurality of transparent elements 760 made of a transparent material such as glass or a transparent polymer material. In this example, the transparent material has a refractive index of greater than 1. Each of the transparent elements 760 are provided between adjacent louvres 710. Each transparent element 760 is in contact with a first louvre 710 at a first end and in contact with a second louvre 710 at a second end. In some examples, the transparent element 760 is fixed such as glued to each of the first and second louvres. A top surface 720 of each of the transparent elements is angled with respect to the transmission surface 742 (i.e. is angled with respect to the normal of a plane defined by the x and y axis). The top surface 720 of each of the transparent elements 760 forms a serrated surface, with each top surface 720 forming one of the serrations such that the serrated surface comprises an array of serrations. In this embodiment, the serrations are uniformly spaced and are aligned with the louvres. Thus, it may be said that the periodicity of the serrations is substantially equal to the periodicity of the louvre array. The reflection suppression device of Figure 7 is configured to prevent sunlight glare to a viewer at the viewing area / eye-box from various angles of elevation and as a result of passing through various routes within the reflection suppression device. Figure 7 shows a ray of sunlight Si from the sun at a first elevation angle (with respect to the horizon) and incident on the louvre structure 706. Some of these rays (not shown) of sunlight Si may be incident directly on the louvre slats 710 where the light is absorbed or attenuated without reaching the transmission surface 742. In addition, some of these rays (shown) of sunlight Si may be incident on the transparent elements 760 between the louvre slats 710 as shown. Owing to the serrated configuration and orientation of the transparent structures 760, these rays of sunlight Si may be specularly reflected by one or more surfaces of the transparent structure 760. Because of the angled top surface of the serrated transparent surface, the reflected rays of sunlight Sir are directed on an optical path away from the viewing area / eye-box as shown by the arrow. In particular, Sir is reflected in such a way as to fall outside of the range of angles 706. Figure 7 further shows a ray of sunlight S2 from the sun at a second elevation angle, which is higher / greaterthan the first elevation angle and incident on the louvre structure 706. Owing to the configuration and orientation of the transparent structures 760, the rays of sunlight S2 may be specularly reflected by one or more surfaces of the transparent structure 760 in a direction onto one of the louvres 710, and thus absorbed or attenuated. This blocks / reduces the intensity of reflected rays of sunlight S2R which may be directed towards the viewing area / eye-box as shown by the arrow. However, rays of sunlight S2 that are incident on, and transmitted through, the transparent structures 760 may reach the transmission surface 742. Figure 7 further shows ray of sunlight S3 from the sun at a third elevation angle which is different to the first and second angles of elevation. In this scenario, sunlight S3 is received at an angle that enables the light rays to pass between the louvres / louvre slats 710 (e.g. parallel to the sidewalls of the louvre slats 710) and to be (directly) incident on the transmission surface 742 of the waveguide pupil expander 740 (after being transmitted through a transparent element 760. In this scenario, a proportion of the incident sunlight S3 may be specularly reflected from the transmission surface 742 as reflected rays S3R. However, due to the parameters of the louvre structure 706, as described herein, the reflected rays S3R are specularly reflected along an optical path that is blocked by a louvre / louvre slat 710, and thus absorbed or attenuated as shown by cross X. Furthermore, the remaining proportion of incident sunlight S3 may be transmitted through the transmission surface 742 into the waveguide pupil expander 740. In this case, as shown in Figure 7, the light rays may be internally reflected between the opposed reflective surfaces of the waveguide 740 as shown by arrows (from right to left - the opposite direction to the direction of pupil expansion). At each internal reflection / bounce at the partially reflective-partially transmissive surface 742, some light rays are emitted from the transmission surface 742 as transmitted rays S3T. However, once again, the transmitted rays S3T are emitted from the transmission surface 742 along an optical path that is blocked by a louvre / louvre slat 710, and thus also absorbed or attenuated as shown by cross X. The overlap 0 of the louvres described above may ensure that the sunlight cannot escape being absorbed by the louvres 710 (whether that be after transmission or reflection). Accordingly, the optical path of rays of sunlight incident on the reflection suppression device comprising the louvre structure and the transparent support structure, at substantially all angles of elevation of the sun follow an optical path that is blocked, and thus absorbed or attenuated, by at least one of the louvres / louvre slats 710. Thus, the risk of sunlight being reflected towards the viewing area / eye-box and causing glare is reduced / mitigated. This is achieved while the transparent support structure also provides mechanical strength and stability to the reflection suppression device. For simplicity, refraction effects (particularly, refraction caused when light is transmitted through the transparent support structure) have not shown in Figure 7. However, because, in this example, the transparent support structure comprises transparent elements 760 formed of a transparent material having a refractive index of greater than 1, light will be refracted as it passes from air into the transparent element 760. This is shown in Figure 8 which shows a close-up view of three of the louvres 710 of Figure 7 and the associated transparent elements 760. A single light ray 812 of the image light formed and output by the waveguide 740 at the transmission surface 742 is shown in Figure 8. Figure 8 shows how each transparent element has a parallelogram structure. In particular, the top surface 720 of each transparent element 760 is parallel to a bottom surface 806. In other words, the top and bottom surface 720, 806 correspond to one another and the thickness of the transparent element 760 (i.e. the distance between the top surface 720 and the bottom surface 806) is constant. The single light ray 812 is initially output from the waveguide 740 at an angle such that the single light ray 812 is parallel to the adjacent louvre. When the single light ray 812 is input into the transparent element 760, the change in refractive index at the air I transparent element boundary causes the direction I angle of the single light ray 812 to deviate slightly. When the single light ray 812 is output from the transparent element 760, the change in refractive index at the transparent element I air boundary causes the direction I angle of the single light ray 812 to deviate slightly in an opposite direction to the air / transparent element boundary. Advantageously, because the top and bottom surfaces of the transparent element 760 are parallel, the overall shift in light rays passing through transparent element 760 will be constant for all light rays at all positions. Thus, while the transparent element 760 may cause a slight positional shift in the light rays emitted, this shift does not cause distortion of an image viewed at the eye-box. Sunlight that is input into the transparent element 760 will also be refracted by the transparent element. Sunlight is represented by single ray 802 in Figure 8. Figure 8 shows how 802 is deviated I shifted slightly as a result of refraction, but this does prevent the sunlight from eventually being absorbed by the louvres 710. Above, it has been described how a (spatial) overlap of louvres 710 ensures that sunlight cannot escape being absorbed by a louvre. This (spatial) overlap comprises the proximal end of one of the of louvres 710 being in shadow of the distal end of another of the louvres 710. However, in some embodiments, a spatial overlap may not be strictly necessary. This is because the transparent element can be arranged so that the refraction effect caused by the transparent element steers the sunlight towards a nearby louvre, as shown in Figure 8. The upshot of this is that the reflection suppression device may optically appear to have overlapping louvres, even if the louvres do not (quite) spatially overlap. This may be referred to as an optical overlap. So, in some embodiments, the louvres 710 may not spatially overlap but do at least optically overlap. In such cases, the louvres 710 may still be referred to as overlapping. Head-up display comprising a second reflection suppression device Figure 9 shows a schematic side-view of a features of a head-up display comprising another example reflection suppression device 900. The head-up display of Figure 9 comprises a waveguide 902, a turning layer 903, and the reflection suppression device 900. The head-up display further comprises a coherent light source (in this example, a laser) and a display device (in this example, a liquid crystal on silicon spatial light modulator) arranged to display a hologram of a picture. The light source and display device are not shown in Figure 9. In use, the light source of the head-up display illuminates the display device such that light is spatially modulated in accordance with the hologram displayed on the display device, thus forming a holographic wavefront. The holographic wavefront is coupled into a first waveguide (not shown in Figure 9) where the holographic wavefront is replicated in a first direction a plurality of times (as described previously) to form a one dimensional array of replicas which are then coupled into the waveguide 902 (which is shown in Figure 9). The waveguide 902 comprises a pair of opposing surfaces. A first surface 904 of the waveguide 902 is partially transmissive-reflective. A second surface 906 of the waveguide 902 is reflective. The waveguide 902 is arranged to waveguide the replicas of the holographic wavefront coupled in to the waveguide 902 from the first waveguide between the pair of opposing surfaces. In this way, the holographic wavefront is replicated in a second direction (that is orthogonal to the first direction) a plurality of times (again, as described previously) to form a two dimensional array of replicas. The emission of replicas from the first surface 904 is represented by the dotted arrows 908 in Figure 19. The replicas 908 of the holographic wavefront emitted by the waveguide 902 are received by the turning layer 903 whereby the (replicas of) the holographic wavefront are turned by the turning layer 903. The turned replicas 908 are then received by the reflection suppression device 900. The reflection suppression device 900 comprises a layered structure which comprises a first layer 920, an intermediate layer 922, and a second layer 924. The first layer 920 is closest to the waveguide 902 I turning layer 903. As such, the (replicas of) holographic wavefront (propagating through the reflection suppression device 900) are received in turn by the first layer 920, the intermediate layer 922 and the second layer 924. The first (bottom) layer 920 comprises a first serrated surface 928. An opposing face of the first layer 920 is a non-serrated (planar) surface. The non-serrated surface of the first layer 920 is in contact with, and adhered to, a surface of the intermediate layer 922. The first layer 920 is a prismatic structure comprising a plurality I an array of prism elements 921. The first layer 920 is integrally formed such that the array of prism elements 921 form a single component (forming the first layer 920). The first serrated surface 928 is defined by first angled surface of each of the prism elements 921, thus forming a sawtooth-type structure when viewed in the y-z plane (as in Figure 19 and 21 A, for example). The first layer 920 is formed of a transparent material which, in this example, is a hard plastic (transparent) material. The first layer 920 may have been manufactured by, for example, injection moulding, hot embossing, extruding or cutting a source of the transparent material. The intermediate layer 922 comprises a transparent material 930 separating an array of individual louvres 932. Each louvre 932 is in the form of a slat, the length of which extends substantially in the x-direction. The louvres 932 are embedded in the transparent material 930. In this example, the transparent material 930 is a material having the same refractive index as the material forming the first layer 920. The second (top) layer 924 comprises a second serrated surface 929. An opposing face of the second layer 924 is a non-serrated (planar) surface. The non-serrated surface of the second layer 924 is in contact with, and adhered to, an opposite surface of the intermediate layer 922 to the non-serrated surface of the first layer 920. The second layer 924 is a prismatic structure comprising a plurality I an array of prism elements 923. Like the first layer 920, the second layer 924 is integrally formed such that the array of prism elements 923 form a single component (forming the first layer 922). The second serrated surface 929 is defined by first angled surfaces of each of the prism elements 923, thus forming a sawtooth-type structure. The second layer 924 is formed of a transparent material which, in this example, is a hard plastic (transparent) material. In this example, the second layer 924 is formed of the transparent material as the first layer 920 (or, at least, a material having the same refractive index as the transparent material of the first layer 920). The second layer 924 may have been manufactured by, for example, injection moulding, hot embossing, extruding or cutting a source of the transparent material. A periodicity of the serrations of the first and second serrated surfaces 928,929 is equal. Furthermore, the periodicity of the louvres 932 in the louvre array is equal to the periodicity of the serrations of the first and second serrated surfaces. Thus, for each serration of the first serrated surface 928 of the first layer there is a corresponding serration of the second serrated surface 929 of the second layer and a corresponding louvre 932. Each of the first and second layers 920,924 are formed of a transparent material which, in this example, has a refractive index greater than 1. Each of the first and second serrated surfaces 928, 929 form a transparent material / air interface. Light (in particular, the holographic wavefront 908) propagating through reflections suppression device will be turned, twice. A first turn will be provided by the first layer 920 and a second turn will be provided by the second layer 1924. The turning layer 1903, in this example, also has a prismatic structure. However, the prism elements 935 of the turning layer 903 extend longitudinally in a direction that is orthogonal to the direction of extension of the prism elements of the first and second layers. As in the example of Figures 7 and 8, the louvres of the second reflection suppression device 900 suppress sunlight glare, by absorption. The second surface 928 is arranged to reflect a proportion of light incident thereon and direct that light away from an eye-box of the HUD. Veiling glare The inventors have found that the reflection suppression devices of Figures 7 and 9 (comprising an louvre array and a serrated top surface) can be arranged to be very effective - almost perfect - at reducing sunlight glare from reaching a viewing window or eye-box of the head-up display. For example, the inventors have found that it is possible to effectively eradicate said glare. Given the prejudice in the field that glare should be suppressed (to improve the viewing experience and reduce distractions), it would be expected that such a reflection suppression device (which effectively eradicates glare) would provide an optimised viewing experience. However, the inventors have surprisingly found that this is not the case because, in use, such a reflection suppression device can result in a conspicuous dark region in the field of view of a viewer using a head up display comprising the reflection suppression device. This is explained in more detail in Figures 10 and 11. Figure 10 shows a schematic cross-sectional view of an automotive vehicle 1000 (in this example, a car). Only a portion of the car 1000 is shown. In particular, Figure 10 shows a portion of the bonnet (or hood) 1002, the windscreen 1004 (or windshield) and a portion of a roof 1006 of the car 1000. Figure 11 further shows a portions of a dashboard 1008 of the car 1000. A head-up display comprising a reflection suppression device as described above (such as reflection suppression device 900) is received in the dashboard 1008 but is not shown in Figure 10. Figure 10 shows how sunlight glare 1010 may be incident on the dashboard 1008. In this example (as is typical), the dashboard 1008 of the car 1000 is formed of an opaque material having a surface roughness due to a non-uniform pattern being formed in the dashboard. A portion of the sunlight glare 1010 is absorbed by the dashboard 1008. Another (in this example, smaller) portion of the sunlight glare 1010 is (diffusely) reflected by the dashboard 1008 and relayed to the windscreen 1004. A ray of said diffusely reflected light is represented by dotted or broken arrow 1012 in Figure 10. At least some of the ray 1012 of light (incident on the windscreen 1004) is reflected by the windscreen 1004 and relayed to a viewer 1014 (e.g. driver) in the car 1000. Said light, incident on the dashboard 1008 and being relayed to the viewer 1014 via the windscreen 1004 is referred to as veiling glare. In Figure 11, a single ray (forming veiling glare) is shown. However, it should be understand that sunlight glare may be incident on any and all of an exposed upper surface of the dashboard 1008. Furthermore, when light is diffusely reflected by a surface, it is generally emitted over a range of angles. For example, light may be emitted in all directions within a half (hemi) sphere. In some examples, the dashboard may act to cause hemispherical diffusion of light incident thereon. In such cases, light may be emitted equally (i.e. of equal intensity) in all directions or at all angles within a hemisphere. The result of this is that veiling glare may be emitted by the dashboard over a wide range of angles and over a wide area. Thus, a relatively large area of a field of view of the viewer 1014 may be illuminated with veiling glare. This is shown in Figure 11. Figure 11 shows a schematic cross-sectional view of a portion of the inside the automotive vehicle 1000. The car 1000 in this examples is left-hand drive (with the steering wheel 1102 on the left). Figure 11 further show a portion of the dashboard 1008 and the windscreen 1004. Figure 11 is shown from the perspective of the viewer 1014 (of Figure 10). Figure 11 represents the veiling glare 1104 received by the viewer 1014 by the angled shading in the windscreen 1004. As above, the veiling glare 1104 may be received by the viewer 1014 over a large range of angles and may fill a relatively large area of a field of view. In this example, veiling glare 1104 reaches the viewer 1014 from substantially all of the windscreen 1004. In other words, veiling glare 1104 is received by the viewer 1014 over a relatively large area. In some examples, the veiling glare 1104 forms an image. For example, the shaded area in Figure 11 may appear as an image of dashboard 1008. The skilled reader should understand that the size and shape of the area of the field of view that contains veiling glare 1104 from the dashboard 1008 will depend on the (range of) angles of the light incident on the dashboard 1008 as well as the size and shape of the dashboard 1008 and the position of the viewer 1014. Herein, it is assumed that the viewer has their eyes positioned in an eyebox of the HUD. In this example, a top portion 1106 of the windscreen does not contain veiling glare 1104 when viewed by the viewer 1014 from their respective viewing position. This is merely exemplary. It should be understood that the veiling glare 1104 shown in Figure 111 has been described as being formed by light incident on the dashboard 1008. However, it should be clear that other features or components of the car may additionally or alternatively contribute to veiling glare 1104 such as the steering wheel 1102). The presence of the veiling glare 1104 (caused by the dashboard 1008) in itself is generally not a problem. For example, the dashboard 1008 can be arranged in such a way that the veiling glare 1104 is maintained below tolerable levels and does not impinge on the viewer 1014 / driver’s ability to view the real world through the windscreen 1004. For example, a surface roughness, material and I or colour of the dashboard 1008 may have been selected to ensure that veiling glare 1104 remains at acceptable levels. The head-up display is received in the dashboard 1008. A top surface 1106 of the head-up display (e.g. a top surface of the reflection suppression device 900) is shown in Figure 11. It may be said that a portion of the dashboard 1008 is formed by the top surface 1106 of the head-up display. The reflection suppression device 900 is arranged to substantially eliminate glare. Thus, the inventors have found that sunlight incident on the top surface 1106 of the head-up display does not contribute to the veiling glare 1104. This results in a dark region 1110 in the field of view. The inventors have surprisingly found that the contrast between the dark region 1110 and the veiling glare 1104 when reflection suppression device 900 is used (arranged to substantially eliminate glare) results in the dark region 1110 being rather conspicuous. The inventors have recognised that said conspicuous dark region 1110 could draw the driver’s attention to the area of their field of view containing the dark region 1110. In other words, dark region 1110 could be distracting. Thus, even though it would be expected that the reflection suppression device 900 (which substantially eliminates glare) would provide an optimal viewing experience (in which contrast of the projected HUD image is maximised), the inventors have surprisingly found that this may not actually be the case. Reflection suppression device comprising diffusely reflecting louvre The inventors have recognised reflection suppression device can be tuned such that, while a first portion of sunlight glare incident thereon is absorbed, a second (smaller) portion of the sunlight glare forms a first veiling glare (viewable from the eye box of the HUD). It is very counterintuitive to arrange the reflection suppression device so as to intentionally introduce a veiling glare. As above, the general prejudice in the field (and the expectation of the inventors) was that an optimised viewing experience would be achieved by eliminating as much glare as possible. However, the inventors have recognised that the first veiling glare can be selected or tuned to compensate for or match the (second) veiling glare 1204 (originating from the dashboard 1008). In particular, the inventors have recognised that the louvres of the louvre array (which would normally be expected to be arranged to maximise light absorbance) can instead be arranged to be diffusely reflective. This is explained in more detail in Figures 12 and 13. Figure 12 shows a schematic cross-sectional view of an example of a reflection suppression device 1200 according to the present disclosure. The reflection suppression device 1200 comprises a first prismatic layer 1220, an intermediate layer 1222 and a third prismatic layer 1224. Each layer is substantially the same as the respective layer of the second reflection suppression device 900 (described above), except that the louvres of the intermediate layer 1222 have been modified. In particular, the intermediate layer 1222 comprises an louvre array 1230, each of which is arranged to be diffusely reflective. The diffuse reflectively of an individual louvre 1230 is shown in Figure 13 (which is a also a schematic cross-sectional view). Figure 13 shows a ray of sunlight glare 1302 incident on a surface 1230 of the louvre 1230. A first portion of the ray of sunlight glare 1302 is absorbed by the louvre 1230. In this example, the first portion corresponds to a majority of the intensity of the light (e.g. at least 50% of the incident light is absorbed by the louvre 1230, optionally at least 80% or at least 90%). The louvre 1230 is further arranged such a second portion of the ray of sunlight glare 1302 is diffusively reflected by the louvre 1230. This results in the emitted light being scattered and emitted over a range of angles. In this example, the louvre 1230 surface is arranged to be hemispherically diffusive and so an equal intensity of light is emitted by the louvres 1230 over all angles. The louvres 1230 are arranged such that the louvre array forms a first veiling glare arranged to match the second veiling glare caused by the dashboard 1008. This is shown in Figure 14. Figure 14 shows a schematic perspective view of the inside of a car 1200 that corresponds to the view shown in Figure 11. However, in Figure 14, the car 1200 comprises the reflection suppression device 1200 described above, the top surface 1106 of which is visible in Figure 14 and which is arranged to form the first veiling glare. Thus, the dark region 1110 of Figure 11 has been illuminated by the first veiling glare 1502 (represented by diagonal shading in Figure 15, the diagonal shading being in an opposite direction to the second veiling glare 1204). In this example, the louvre array is arranged such that the first veiling glare 1502 has an intensity (when viewed from the eye-box) that is substantially equal to the intensity of the second veiling glare 1204. However, it should be understood that this is not necessary. What is generally important is that the dark region 1110 is illuminated first veiling glare 1502 which is chosen to reduce the contrast between the dark region 1110 and the second veiling glare 1204 and so reduce the appearance of the dark region 1110. As the skilled reader will appreciate, the louvres 1230 can be tuned to provide a desired amount of first veiling glare by an appropriate selection of material, surface roughness and colour, for example. In some examples, each louvre may be provided as a metal strip with a black paint coating. The coating may have a surface roughness arranged to provide a desired amount of diffuse reflectivity. In other examples, each louvre may comprise a light absorbing core (formed of, for example, a strip of black material such as black rubber). The light absorbing core may be coated (on either side, for example) with a diffusely reflective material. Thus, the coating may achieve a desired diffuse reflectively while any remaining portion of the light may be absorbed by the core material. In some examples, the louvre material is designed to match the diffuse reflectivity of the dashboard surface, so that diffuse reflection from louvers will look similar to the diffuse reflection from the dashboard when the (diffusely) reflected sunlight reach driver’s eyes. Specular reflection will continue to be blocked from louver surfaces. The improved louvre reflectivity may always be diffuse. In some examples, this may be achieved by altering surface morphology, changing material packing / index, adding additives or based on other material selections. In some examples, the diffuse reflectivity from louvres may be tuned to match the colour of the dashboard. In some examples, the louvres are configured to show a gradient in colour, a pattern (e.g. to match the perforated structure of an audio speaker) or even carry a subtle contrast logo that would be visible on direct view but barely visible in veiling glare. Thus, the design aesthetic improvement if the HUD (or light control layer) (top) surface matches the surrounding dashboard. This may be when the dashboard / HUD surface is viewed directly, not in relation to veiling glare. 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 arranged for housing in a support structure, wherein the head-up5 display comprises:an optical component having a reflective surface arranged, when housed in the support structure, in a configuration that is conducive to sunlight glare;a light control layer arranged to receive light from the optical component, the light control layer comprising a plurality of louvres in an array arranged to absorb a first portion of10 light incident thereon; anda viewing window;wherein at least one characteristic of at least a portion of each louvre is such that a second portion of the light incident on the louvre array forms a first veiling glare arranged to at least partially compensate for a second veiling glare caused by the support structure when15 viewed from the viewing window; andwherein the first veiling glare caused by the louvre array substantially matches the second veiling glare caused by the support structure.

2. A head-up display as claimed in claim 1, wherein the first veiling glare is arranged to20 compensate for the second veiling glare by reducing a gradient or contrast between the first veiling glare and the second veiling glare.

3. A head-up display as claimed in any one of the preceding claims, wherein the at least one characteristic is such that the first veiling glare is formed by diffuse reflectance of the25 second portion of the light incident on the louvre array by the louvre array.

4. A head-up display as claimed in any one of the preceding claims, wherein the at least one characteristic is diffuse reflectivity.30   5. A head-up display as claimed in any one of the preceding claims, wherein the at leastone characteristic is such that the louvre array is arranged to diffusely reflect 5% or more of light incident thereon.

6. A head-up display as claimed in any one of the preceding claims, wherein the at least 35 one characteristic is such that the louvre array is arranged to diffusely reflect no more than30% of light incident thereon.27 06 247.     A head-up display as claimed in any one of the preceding claims, wherein the louvrearray is arranged such that the first veiling glare is formed substantially in a region that is substantially absent of second veiling glare when viewed from a viewing window of the head-up display.

58. A head-up display as claimed in any one of the preceding claims, wherein each louvre comprises a core and an outer coating.

9. A head-up display as claimed in claim 8, wherein the core comprises a light10 absorbing material.

10. A head-up display as claimed in claim 8 or 9, wherein the core is arranged to absorb substantially all light incident thereon.15   11. A head-up display as claimed in any one of claims 8 to 10, wherein the outer coatingmaterial comprises a diffusely reflective material.

12. A head-up display as claimed in any one of the preceding claims, wherein the optical component is a waveguide.2013. A head-up display as claimed in any one of the preceding claims, wherein the support structure is at least a portion of a dashboard of a vehicle.

4. A head-up display as claimed in any one of the preceding claims, wherein the first 25 portion of light is a greater proportion of the incident light than the second portion of light.

15. A projection system comprising a head-up display as claimed in any one of the preceding claims received in a support structure, wherein the support structure is arranged to cause the second veiling glare when light is incident thereon.3016. A projection system as claimed in claim 15, wherein the support structure comprises a first surface arranged to receive light, wherein the light control layer is received on a first portion of the first surface or received in an opening in the first surface.35    17. A projection system as claimed in claim 16, wherein the first portion or opening offirst surface does not contribute to the second veiling glare in a first region of a field of view when viewed from a viewing window of the head-up display.27 06 2418. A projection system as claimed in any one of the claims 16 or 17, wherein the louvre array is arranged such that the second veiling glare illuminates the first region of the first veiling glare.

519. A method of glare mitigation in a head-up display housed in a support structure, the head-up display comprising a viewing window and a light control layer arranged to receive light from an optical component having a reflective surface arranged in a configuration that is conducive to sunlight glare, the light control layer comprising a plurality of louvres in an10 array, the method comprising the steps of:receiving light incident on the light control layer;absorbing, by the louvre array, a first portion of the light incident on the light control layer, and causing, by the louvre array, a second portion of the light incident on the light control layer to form a first veiling glare;15           receiving light incident on the support structure to form a second veiling glare;wherein the louvre array is arranged such that the first veiling glare is arranged to compensate for the second veiling glare when viewed from the viewing window; andwherein the louvre array is arranged such that the first veiling substantially matches the second veiling glare.

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