Light control device

Abstract

A head-up display for a vehicle is provided. The head-up display has a viewing window. The head-up display comprises a substantially planar waveguide 600. The waveguide has a reflective surface that

Description

FIELD The present disclosure relates to a display system comprising a light control layer. More broadly, the present disclosure relates to a light control layer, a reflection suppression device and a glare mitigation device. The present disclosure further relates to methods of mitigating glare when using a display system comprising a light control layer. Some embodiments relate to a holographic projector, picture generating unit or head-up display. BACKGROUND AND INTRODUCTION Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or “hologram”, comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object. Computer-generated holography may numerically simulate the interference process. A computer-generated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. A Fourier hologram may be considered a Fourier domain / plane representation of the object or a frequency domain / plane representation of the object. A computer-generated hologram may also be calculated by coherent raytracing or a point cloud technique, for example. A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micromirrors, for example. A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, “HUD”. SUMMARY Aspects of the present disclosure are defined in the appended independent claims. In general terms, there is provided a light control device or glare mitigation device for display light. In embodiments, the light control device or glare mitigation device is for display light of a display system. The display light may be spatially modulated light. The display system may be arranged to relay the spatially modulated light to a viewing window / plane or eye-box. In some embodiments, the display system is a holographic display system and the spatially modulated light is light that is spatially modulated in accordance with a hologram. The spatially modulated light may be referred to as a holographic wavefront. The light control device of the present disclosure provides a means for controlling reflections of ambient light to prevent or suppress glare from reaching the viewing window while allowing the spatially modulated light to reach the viewing window. For example, the display device may comprise an optical component comprising a reflective surface. In the absence of the light control device, ambient light may be reflected by the reflective surface towards the viewing window / eye-box of the display device thus forming glare. The light control device of the present disclosure is arranged to suppress such reflections. The inventors have previously proposed light control devices / glare mitigation devices for reflection I glare suppression. For example, in UK patent number GB2607672, the inventors proposed a light control device for glare mitigation. GB2607672B described a light control device having a serrated sunlight-receiving surface providing an array of angled surfaces that can be arranged to direct sunlight away from a particular area, for example away from the eye-box of a head-up display. In particular, each of the angled surfaces forms an interface between a material forming the serrated surface and air such that most of the sunlight incident thereon (e.g. 96% of incident sunlight) is reflected (i.e. not coupled into the light control device I component). The angled surfaces are orientated at an angle with respect to a plane of the light control device. The angled surfaces are arranged to direct the reflected sunlight away from a direction towards the eye-box, owing to the orientation angle thereof. It was also described in GB2607672B how, in examples, the angled surfaces of the sunlight-receiving surface change the angle of reflection of sunlight incident thereon. That is, the angle of rays of sunlight received by the angled surfaces is different from the angle of rays of the sunlight reflected by the angled surfaces, where the angles of the rays of sunlight are measured with respect to (the normal to) the plane of the light control film / optical component in the first and second dimensions. It may be said that each angled surface changes the course or path of specular reflection of incident sunlight in comparison to specular reflection by non-angled surfaces (parallel to a plane of the light control component). The inventors proposed an improvement to this light control device in UK patent application number GB2303536. GB2303536 discloses a reflection suppression device comprising a first layer, an intermediate layer and a second layer. The first layer comprises a first serrated surface arranged to receive a holographic wavefront from a waveguide and provide a first turn of the holographic wavefront having at least a component on a first plane containing a surface normal of the waveguide. The first layer is formed of a transparent material. The intermediate layer is arranged to receive the holographic wavefront from the first layer, wherein the intermediate layer comprises a plurality of louvres in an array. Each louvre comprises a light absorbing material. The second layer is arranged to receive the holographic wavefront from the intermediate layer. The second layer comprises a second serrated surface arranged to provide a second turn of the holographic wavefront having at least a component on the first plane. The component of the second turn on the first plane is equal and opposite to the component of the first turn on the first plane. In GB2303536, the second (top) serrated surface of the reflection suppression device may perform as the serrated sunlight-receiving surface of GB2607672B and so is arranged to deflect sunlight. The second serrated surface may be provided as an array of prisms, the prisms forming the serrated sunlight-receiving surface. The inventors recognised that such a prismatic layer introduced a (second) turn component on the first plane and so there is a need to correct for deviations of the holographic wavefront (in at least the first plane) to compensate for the changes introduced by the turn. Thus, the first (bottom) serrated surface of the reflection suppression device was introduced, having serrations (e.g. an array of prisms) arranged to cancel out the (second) turn component on the first plane with a (first) turn. However, the inventors have discovered that these light control devices can still transmit direct reflections to the viewing window / eye-box in some cases, identified by rigorous simulation and experimentation. That is, the inventors have found that some sunlight can be directly reflected from the light control device to the viewing window / eye-box, increasing the presence of glare observed by the user / viewer. As will be further described in relation to the Figures below, sunlight can reflect off the tips (in other words, the points) of the serrations / prism structures towards the viewing window / eye-box. Specifically, one tip of the serrated surface may reflect sunlight at certain angles to an optical path that passes the adjacent tips and causes direct glare. Sunlight in the context of this application refers to light that originates from the sun, as opposed to light of the same or similar wavelengths to that produced by the sun. The inventors have found that this is predominately (but not exclusively) due to the angles of the surfaces of the serrations / prism structures necessary to achieve suitable head-up display image quality and minimise the impact of dark banding leading to such reflections occurring at some positions of the sun in the sky. In other words, prevention of specular sun reflection from surfaces of the display device (such as the waveguide) is achieved through an array of serrations / prisms on top of said surfaces. The array of serrations / prisms are not necessarily directly on top of said reflective surfaces of the display device - i.e. there may be other components layered between said surfaces and the array of serrations / prisms. Individual serrations / prisms are characterised by a pair of angles relative to a plane of (or, in other words, a normal to) the reflective surface(s) of the display device (or relative to a plane perpendicular to said surface). These angles are chosen to achieve suitable head-up display image quality and to minimise the impact of dark banding on the displayed image. However, these angles can result in direct sun glare (reflection of sunlight from surfaces of the serrations / prisms directly to the viewing window / eye-box) for certain positions of the sun in the sky relative to the vehicle. The result can be a bright band across the eye-box, which can, in extreme cases, wash out the image / picture being displayed, significantly reduce the contrast of the displayed image and distract the user (i.e. the driver of the vehicle). In a first aspect, a head-up display fora vehicle is provided. The head-up display has a viewing window. In other words, the head-up display forms a viewing window. The viewing window may also be referred to as an eye-box. That is, it is the invisible window within 3D space in which the user / viewer can observe the displayed image / picture with a satisfactory quality (e.g. with an acceptable number and / or size of image artefacts visible). The head-up display comprises a substantially planar waveguide. Substantially planar in this case means that the waveguide is sufficiently flat that it can output a uniform 2D array of replicas from an inputted uniform 1D set of replicas. The waveguide has a reflective surface that is arranged, during head-up display operation, in a configuration that is conducive to sunlight glare. In a vehicle, this may be that a plane of the reflective surface (or indeed the waveguide itself) is parallel to the ground i.e. substantially horizontal. It may also be that the waveguide is located under the windscreen of the vehicle, with the windscreen acting as an optical combiner to reflect the array of replicas produced by the waveguide towards the user. Therefore, the waveguide cannot be shielded from the sunlight, as this would compromise (at least some of) the optical paths of the replicas to the user. The head-up display further comprises a light control layer arranged to receive sunlight on an optical path to the reflective surface. The light control layer acts to prevent or mitigate the reflection of sunlight from the reflective surface to the viewing window by altering the optical path of the sunlight before it reaches the reflective surface, as will be discussed in greater detail below. The light control layer comprises a plurality of (elongate) serrations. In other words, the light control layer has an array of prism structures, with each prism structure being described as a serration. Each serration has a tip. That is, each serration (or prism structure) narrows to a point at a free end. At least one of the serrations comprises a reflection suppression extension (also referred to herein as a reflection suppression structure) extending from the tip of the serration (or, in other words, the reflection compression structure protrudes from the tip of the serration, or more broadly is at least partially located at the tip of the serration). That is, the reflection suppression extension extends away from the tip of the respective serration. It can also be said that each serration extends away from the waveguide. The reflection suppression extension is arranged to receive sunlight on an optical path to the viewing window. In this way, the reflection suppression extension is in (or on) the optical path between the light control layer and the viewing window and so can prevent or mitigate sunlight from travelling this optical path, and can thus prevent or mitigate glare from reaching the viewing window (the eye-box). As will be further described below, the reflection suppression extension achieves this without a substantial change the angle of the surfaces of each serration (that is, without a change large enough to cause a level of degradation to the displayed image unacceptable to the user of the head-up display). Thus, the quality of the image displayed is preserved. To describe how the reflection suppression extension prevents or mitigates such glare, take a serration Sn in an array of serrations, wherein serration Sn has serrations Sn-i and Sn+i on either side. With previous light control layers, the inventors found that sunlight could, in some cases, reflect off the tip of serration Sn-i, pass over serration Sn and arrive at the viewing window / eye-box. Similarly, sunlight would reflect off the tip of serration Sn, pass over serration Sn+i and arrive at the viewing window / eye-box, and so on for the array of serrations. In other words, close examination of ray paths for direct glare by the inventors showed that rays reflected from the surfaces of each serration that do not absorb light pass close to the tip of an adjacent serration. As such, the inventors have found that utilising a reflection suppression extension that extends from the tip of the serrations can prevent / mitigate this glare. In the present disclosure, some of the sunlight that would reflect off the tip of serration Sn-i either is reflected away from the eye-box or attenuated (as will be discussed further below) by the reflection suppression extension extending from the tip of serration Sn-i. At least some of the remaining sunlight that is reflected off the tip of serration Sn-i cannot pass over Sn due to the reflection suppression extension extending from the tip of serration Sn. Similarly, some of the sunlight that would reflect off the tip of serration Sn either is reflected away from the eyebox or attenuated by the reflection suppression extension extending from the tip of serration Sn+i, whilst at least some of the remaining sunlight that is reflected off the tip of serration Sn cannot pass over Sn+i due to the reflection suppression extension extending from the tip of serration Sn+i. This is repeated for the array of serrations, thereby removing or mitigating the glare that is directly transmitted from the light control layer to the viewing window / eye-box. In other words, and in summary, the increase in height provided by the reflection suppression extension blocks light rays reflected from the specular surface of adjacent prisms, but without an increase in blocking of head-up display light (for angles of the surfaces of the serration that are feasible), thus causing no detrimental impact on the head-up display image. In one embodiment of the first aspect, each reflection suppression extension comprises two substantially parallel surfaces extending from the tip of the respective serration (either side of the tip) substantially perpendicularly to (a plane of) the reflective surface of the waveguide (or, indeed, the waveguide itself). As such, a so-called “baffle” is formed at the tip of the respective serration. The surfaces being substantially parallel and them extending substantially perpendicularly to the reflective surface is a recognition that in real-world manufacturing true parallel and a 90° angle relative to the reflective surface is unlikely to be possible. The skilled person would understand that the word “substantially” in this case means a 1-2° tolerance from true parallel / perpendicularto account for manufacturing capabilities, but that such an inaccuracy would still produce the desired results described herein. In other words, and in summary, an absorbing element or baffle of a suitable height and thickness is placed at the tip of each serration to block the direct glare rays from reaching the eye-box. Each reflection suppression extension may have a central axis along the length of the structure’s extension from the tip of the respective serration. That is, the axis is perpendicular / orthogonal to (a plane of) the reflective surface (i.e. is a normal to a plane of the reflective surface). This central axis may be described as the axis of symmetry or the longitudinal axis. The central axis may intersect (or is coincident with) said tip of the respective serration. In other words, the reflection suppression extension (the “baffle”) is symmetrical about the tip of the respective serration. The cross-sectional shape of each (or the) reflection suppression extension is substantially rectangular. In other words, each (or the) reflection suppression extension may be substantially rectangular in shape in cross section. That is, when viewing the reflection suppression extension in cross section, the structure approximates a rectangular shape, with two sets of substantially parallel sides of unequal lengths (see the comment above regarding the term substantially parallel). The two surfaces of the reflection suppression extension may have a minimum separation in the range of 25pm to 250pm, preferably 50pm to 150pm. The two surfaces of the reflection suppression extension may have an extension from the tip of the respective serration in the range of 30pm to 250pm, preferably 50pm to 200pm. The inventors have found that this size of “baffle” provides a good level of glare mitigation whilst not blocking so much light that dark bands form in the image displayed. In other words, the maximum height (the maximum extension from the tip of the respective serration) is limited by the light ray paths of the head-up display to avoid blocking such rays and creating visible dark bands in the head-up display image. The head-up display light rays (i.e. the display light or, in other words, the array of replicas) is output from the waveguide at a range of angles relative to a plane perpendicular to the reflective surface (or the waveguide itself) to achieve the required field of view across the full viewing window / eye-box. The inventors have found that light rays with the largest angle are the limiting factor for the height of the reflection suppression extension from the tip of the respective serration. The surfaces (i.e. each surface) of the reflection suppression extension may be configured to (in other words, arranged to) attenuate sunlight, such as absorb sunlight. This light attenuation may be achieved by painting the required surfaces with a light absorbing paint, such as a (matte) black paint. As such, following on from the example given above, the sunlight reflected off the tip of serration Sn-i is absorbed by the reflection suppression extension extending from the tip of serration Sn, the sunlight reflected off the tip of serration Sn is absorbed by the reflection suppression extension extending from the tip of serration Sn+i, and so on for each serration with a reflection suppression extension. A first surface of each serration may be configured to (in other words, arranged to) attenuate sunlight, such as absorb sunlight. The phrase “attenuate sunlight” means (in other words, comprises) attenuating the intensity of the specular reflection of the sunlight. This light attenuation may be achieved by painting the first surface with a light absorbing paint, such as a (matte) black, paint. A second surface of each serration may be configured to (in other words, arranged to) transmit (display) light. The phrase “configured to transmit light” means (in other words, comprises) a surface configured such that any light reflection is substantially specular (rather than diffuse) and light absorption is low (e.g. less than 1%). This can be achieved by using a transparent material to form the serration and leaving the second surface unpainted. In this way, the light control layer prevents sunlight from reaching the reflective surface of the waveguide (and hence being reflected towards the viewing window / eye-box), whilst still allowing the light forming the image to be displayed to be transmitted. The first surface may be arranged at a first angle with respect to a normal of (in other words, a plane perpendicular to) the reflective surface in the range of 20° to 25°, preferably 29° to 38°. The second surface may be arranged at a second angle with respect to a normal of (in other words, a plane perpendicular to) the reflective surface in the range of 290° to 340°, preferably 320° to 330°. As discussed above, it is at these angles that the head-up display image quality is maximised whilst the presence of dark bands in the image is minimised. In other words, the inventors have found that for these particular ranges of angles, the problem of direct glare is particularly significant. The present invention allows these angles to be used whilst preventing / minimising the direct glare from the light control layer to the viewing window / eye-box. In a further embodiment of the first aspect, each reflection suppression extension comprises a primary (in other words, a downstream) surface and a secondary (in other words, an upstream) surface (in other words, a primary / downstream face and an secondary / upstream face). The secondary surface is the functional equivalent to the first surface of the previously described embodiment, as will be described in greater detail below. The primary surface extends from a proximal end to a distal end substantially perpendicularly to (a plane of) the reflective surface. As with regards to the previously described embodiment, the use of the word “substantially” in this case means a 1-2° tolerance from true perpendicular to account for manufacturing capabilities, butthat such an inaccuracy would still produce the desired results described herein. The proximal end is located at the tip of the respective serration. The secondary surface extends from a base of the respective serration to the distal end of the primary surface. That is, the serration extends and narrows from a base until it culminates at a tip. In other words, in effect the vertex of the light absorbing surface of the serration is increased in height, whilst the other, light transmitting, surface of the serration is lowered. This results in a near-vertical surface between the light absorbing and light reflecting surfaces of the serration. The reflection suppression extension of this further embodiment has a number of benefits. The overall structure of the serration is largely maintained, meaning that the strength and stability of the serration is maintained. This also allows the light control layer to be manufactured using techniques such as injection moulding, due to a reduced risk of breakage of the serrations when being removed from the mould or tooling afforded by the maintained strength and stability. The secondary surface of the reflection suppression extension may be configured to (in other words, arranged to) attenuate sunlight, such as absorb sunlight. This light attenuation may be achieved by painting the secondary surface with a light absorbing paint, such as a (matte) black paint. The primary surface of the reflection suppression extension may be configured to (in other words, arranged to) diffuse sunlight. In other words, the primary surface is configured to make any light reflection substantially diffuse, rather than substantially specular whilst, optionally, not modifying the surface for increased light absorption. This may be achieved by texturing the primary surface such that it is optically diffuse. As such, following on from the example given above, the sunlight reflected off the tip of serration Sn-i is absorbed by the secondary, upstream, surface of serration Sr, the sunlight reflected off the tip of serration Sn is absorbed by the upstream surface of serration Sn+i, and so on for each serration with a reflection suppression extension. Meanwhile, some of the sunlight is diffused by the primary, downstream, surface of the reflection structure of each serration. In other words, reflection suppression extension comprises a first component (namely, a block coating) arranged to increase light absorption and a second component (namely, a diffuseness or roughness) arranged to make the / any light reflection substantially diffuse rather than substantially specular. The primary surface of the reflection suppression extension may have a length from the proximal end to the distal end in the range of 30pm to 250pm, preferably 50pm to 200pm. As with regards to the previously described embodiment, the inventors have found that this size of primary surface provides a good level of glare mitigation whilst not blocking so much light that dark bands form in the image displayed. In other words, the maximum height (the maximum extension from the tip of the respective serration) is limited by the light ray paths of the head-up display to avoid blocking such rays and creating visible dark bands in the head-up display image. The head-up display light rays (i.e. the display light or, in other words, the array of replicas) is output from the waveguide at a range of angles relative to a plane perpendicular to the reflective surface (or the waveguide itself) to achieve the required field of view across the full viewing window / eye-box. The inventors have found that light rays with the largest angle are the limiting factor for the height of the reflection suppression extension from the tip of the respective serration. The secondary surface may be arranged at a first angle with respect to a plane perpendicular to the reflective surface in the range of 20° to 45°, preferably 29° to 38°. As discussed above, it is at this angle that the head-up display image quality is maximised whilst the presence of dark bands in the image is minimised. The present disclosure allows this angle to be used whilst preventing / minimising the direct glare from the light control layer to the viewing window / eye-box. Each of the serrations may further comprise a second surface extending from a base of the respective serration to the tip of said serration (that is, the tip of the “original” serration and not a tip of the reflection suppression extension). The second surface of each serration may be configured to (in other words, arranged to) transmit (display) light. This surface is the equivalent of the second surface in the previously described embodiment and allows the light forming the image to be displayed to be transmitted through the light control layer. The second surface may be arranged at a second angle with respect to a normal of (in other words, a plane perpendicular to) the reflective surface in the range of 290° to 340°, preferably 320° to 330°. As discussed above, it is at this angle that the head-up display image quality is maximised whilst the presence of dark bands in the image is minimised. The present invention allows this angle to be used whilst preventing / minimising the direct glare from the light control layer to the viewing window / eye-box. In either of the above described embodiments of the first aspect, each reflection suppression extension may be integral with the respective serration. That is, each reflection suppression extension is formed of the same material as the serrations as a single component. This further increases the strength and stability of each reflection suppression extension. In either of the above described embodiments of the first aspect, each serration may comprise the reflection suppression extension. The height of the extension of each reflection suppression extension may be uniform. That is, the height of the extension of each reflection suppression extension may be constant for all the serrations. Alternatively, the height of the extension of each reflection suppression extension may vary between a first serration and a last serration. In this context, the “height” of the reflection suppression extension refers to the length of the extension from the tip of the respective serration taken in a direction perpendicular to (a plane of) the reflective surface. The serrations may be formed of a first group of serrations and a second group of consecutive serrations. Each group of serrations may have a constant height of the extension of each reflection suppression extension. That is, the serrations are split into two defined groups (i.e. no alternating or overlapping serrations of different groups). The serrations may be split into the two groups in half by number of serrations, but as will be described below the location of the split may depend on the height of the extension of the reflection compressions structure required. The height of the extension of each reflection suppression extension in the first group of serrations may be different to that of the height of the extension of each reflection suppression extension in the second group of serrations. In other words, there may be a step change in height of the reflection suppression extensions across the array of serrations. The height of the extension of each reflection suppression extension may vary linearly between the first serration and the last serration. The height of the extension of each reflection suppression extension may vary exponentially between the first serration and the last serration. The height of the extension of the reflection suppression extension of the first serration may be greater than the height of the extension of the reflection suppression extension of the last serration, the first serration being closer to the viewing window than the last serration. In other words, the height of the reflection suppression extension of each serration may increase (for example by a step change, linearly or exponential as described above) as the serrations get closer to the viewing window / eye-box (or as the serrations get closer to the user). As discussed above, the inventors have found that light rays with the largest angle (relative to a normal of, ora plane perpendicular to, the reflective surface) are the limiting factor for the height of the reflection suppression extension from the tip of the respective serration. The inventors have further found that, at regions of the waveguide closer to user (i.e. the driver of the vehicle), the maximum head-up display ray angle is smaller (in other words, the ray paths are steeper). As such, the maximum height of the reflection suppression extensions in these regions can be larger, allowing them to block more direct rays from the sun whilst still not blocking display light rays (and causing dark bands in the display image). The inventors have further found that the majority of direct light rays from the sun reflect from the rearward half of the light control layer (when positioned in the vehicle, i.e. the closest half to the user / driver), which coincides with the regions where the height of the extension of the reflection suppression extensions can be increased. Therefore, this height can be varied across the light control layer (for example by a step change, linearly or exponential as described above) and, in particular, an increase in such a height can be implemented over the main region of direct glare reflection (the “rearward” section of the light control layer as described above) without negatively affecting the head-up display image. This has the benefit of increasing the blocking of direct light rays, without any increase in dark banding of the head-up display image. At least some of the sunlight on the optical path to the viewing window may have originated from an adjacent serration. That is, as described above, sunlight can, in some cases, reflect off the tip of serration Sn-i, pass over serration Sn and arrive at the viewing window / eye-box. Similarly, sunlight can reflect off the tip of serration Sn, pass over serration Sn+i and arrive at the viewing window / eye-box. The light control layer may further comprise a wavefront-receiving surface arranged between the sunlight-receiving surface and the reflective surface of the waveguide. The sunlightreceiving surface is the surface of the light control layer that the sunlight reaches first. The wavefront-receiving surface is the first surface of the light control layer that the array of replicas output by the waveguide reach. This surface does not require the reflection suppression extensions, as there is no optical path of sunlight directly from this surface to the viewing window / eye-box. The cross-sectional shape of each serration may be substantially triangular. In other words, each serration may be substantially triangular in shape in cross section. That is, when viewing the reflection suppression extension in cross section, the structure approximates a triangular shape. In a second aspect, a method of mitigating (in other words, reducing or even preventing) glare during operation of a head-up display for a vehicle is provided. The head-up display comprises a substantially planar waveguide. The head-up display has a viewing window. The method comprises receiving sunlight at a light control layer on an optical path to a reflective surface of the waveguide. The light control layer comprises a sunlight-receiving surface having a plurality of serrations. Each serration has a tip. The method further comprises preventing reflection of the sunlight from at least one of the serrations to the viewing window by means of a reflection suppression extension extending from the tip of the serration. In a third aspect, a light control layer for a head-up display is provided. The light control layer comprises a plurality of serrations. Each serration has a tip. At least one of the serrations comprises a reflection suppression extension extending from the tip of the serration and arranged to receive sunlight on an optical path to a viewing window of the head-up display. Features and advantages described in relation to the first aspect may be applicable to the second aspect and / or the third aspect. 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 2nj which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of ir / 2 will retard the phase of received light by k / 2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term “grey level” may be used to refer to the plurality of available modulation levels. For example, the term “grey level” may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term “grey level” may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator. The hologram therefore comprises an array of grey levels - that is, an array of light modulation values such as an array of phase-delay values or complex modulation values. The hologram is also considered a diffractive pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light having a wavelength comparable to, generally less than, the pixel pitch of the spatial light modulator. Reference is made herein to combining the hologram with other diffractive patterns such as diffractive patterns functioning as a lens or grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the replay field on the replay plane or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a replay plane in the near field. Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged. BRIEF DESCRIPTION OF THE DRAWINGS Specific embodiments are described by way of example only with reference to the following figures: Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen; Figure 2 shows an image for projection comprising eight image areas / components, V1 to V8, and cross-sections of the corresponding hologram channels, H1-H8; Figure 3 shows a hologram displayed on an LCOS that directs light into a plurality of discrete areas; Figure 4 shows a system, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3; Figure 5A shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each comprising pairs of stacked surfaces; Figure 5B shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each in the form of a solid waveguide; Figure 6 is a first schematic side view of a reflection suppression device in combination with a turning layer; Figure 7 is a schematic side view of a display system comprising the reflection suppression device and a curved optical combiner; Figure 8 is a schematic side view of a single prism structure of the reflection suppression device of Figure 6; Figure 9 is a second schematic side view of the reflection suppression device of Figure 6; Figure 10 is a schematic side view of a reflection suppression device according to one embodiment of the present disclosure; Figure 11 is a first schematic side view of a reflection suppression device according to a further embodiment of the present disclosure; Figure 12 is a first schematic side view of a reflection suppression device according to a variation of the embodiment of Figure 11; Figure 13 is a section view of Figure 10; Figure 14 is a section view of Figure 11; Figure 15 is a second schematic view of the reflection suppression device of Figures 11 or 12; Figure 16 is a third schematic view of the reflection suppression device of Figures 11 or 12; Figure 17 is a schematic view of a series of reflection suppression devices with a step change in height; and Figure 18 is a schematic view of a series of reflection suppression devices with a linear change in height. The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF EMBODIMENTS The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration. Terms of a singular form may include plural forms unless specified otherwise. A structure described as being formed at an upper portion / lower portion of another structure or on / under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between. In describing a time relationship - for example, when the temporal order of events is described as “after”, “subsequent”, “next”, “before” or suchlike - the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as “just”, “immediate” or “direct” is used. Although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims. Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in codependent relationship. In the present disclosure, the term “substantially” when applied to a structural units of an apparatus may be interpreted as the technical feature of the structural units being produced within the technical tolerance of the method used to manufacture it. Conventional optical configuration for holographic projection Figure 1 shows an embodiment in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or freguency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, “LCOS”, device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser. A light source 110, for example a laser or laser diode, is disposed to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally planar wavefront of light to be incident on the SLM. In Figure 1, the direction of the wavefront is off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer). However, in other embodiments, the generally planar wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in Figure 1, the arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a lightmodulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to optics including a Fourier transform lens 120, having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives a beam of modulated light from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at the screen 125. Notably, in this type of holography, each pixel of the hologram contributes to the whole reconstruction. There is not a one-to-one correlation between specific points (or image pixels) on the replay field and specific light-modulating elements (or hologram pixels). In other words, modulated light exiting the light-modulating layer is distributed across the replay field. In these embodiments, the position of the holographic reconstruction in space is determined by the dioptric (focusing) power of the Fourier transform lens. In the embodiment shown in Figure 1, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens but the performance of the lens will limit the accuracy of the Fourier transform it performs. The skilled person understands how to use a lens to perform an optical Fourier transform In some embodiments of the present disclosure, the lens of the viewer’s eye performs the hologram to image transformation. Hologram calculation In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or Fourier-based hologram, in which an image is reconstructed in the far field by utilising the Fourier transforming properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back to the lens plane. Computer-generated Fourier holograms may be calculated using Fourier transforms. Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms which may be calculated by a similar method. In some embodiments, the hologram is a phase or phase-only hologram. However, the present disclosure is also applicable to holograms calculated by other techniques such as those based on point cloud methods. In some embodiments, the hologram engine is arranged to exclude from the hologram calculation the contribution of light blocked by a limiting aperture of the display system. British patent application 2101666.2, filed 5 February 2021 and incorporated herein by reference, discloses a first hologram calculation method in which eye-tracking and ray tracing are used to identify a sub-area of the display device for calculation of a point cloud hologram which eliminates ghost images. The sub-area of the display device corresponds with the aperture, of the present disclosure, and is used exclude light paths from the hologram calculation. British patent application 2112213.0, filed 26 August 2021 and incorporated herein by reference, discloses a second method based on a modified Gerchberg-Saxton type algorithm which includes steps of light field cropping in accordance with pupils of the optical system during hologram calculation. The cropping of the light field corresponds with the determination of a limiting aperture of the present disclosure. British patent application 2118911.3, filed 23 December 2021 and also incorporated herein by reference, discloses a third method of calculating a hologram which includes a step of determining a region of a so-called extended modulator formed by a hologram replicator. The region of the extended modulator is also an aperture in accordance with this disclosure. In some embodiments, there is provided a real-time engine arranged to receive image data and calculate holograms in real-time using the algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are pre-calculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms. Large field of view using small display device Broadly, the present disclosure relates to image projection. It relates to a method of image projection and an image projector which comprises a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system, in which the image projector projects or relays light from the display device to the viewing system. The present disclosure is equally applicable to a monocular and binocular viewing system. The viewing system may comprise a viewer’s eye or eyes. The viewing system comprises an optical element having optical power (e.g., lens / es of the human eye) and a viewing plane (e.g., retina of the human eye / s). The projector may be referred to as a ‘light engine’. The display device and the image formed (or perceived) using the display device are spatially separated from one another. The image is formed, or perceived by a viewer, on a display plane. In some embodiments, the image is a virtual image and the display plane may be referred to as a virtual image plane. In other examples, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed either in free space or on a screen or other light receiving surface between the display device and the viewer, is propagated to the viewer. In both cases, an image is formed by illuminating a diffractive pattern (e.g., hologram or kinoform) displayed on the display device. The display device comprises pixels. The pixels of the display may display a diffractive pattern or structure that diffracts light. The diffracted light may form an image at a plane spatially separated from the display device. In accordance with well-understood optics, the magnitude of the maximum diffraction angle is determined by the size of the pixels and other factors such as the wavelength of the light. In embodiments, the display device is a spatial light modulator such as liquid crystal on silicon (“LCOS”) spatial light modulator (SLM). Light propagates over a range of diffraction angles (for example, from zero to the maximum diffractive angle) from the LCOS, towards a viewing entity / system such as a camera or an eye. In some embodiments, magnification techniques may be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of an LCOS. In some embodiments, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) - that may be informally said to be “encoded” with / by the hologram - is propagated directly to the viewer’s eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction / image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device. Reference is made herein to a “light field” which is a “complex light field”. The term “light field” merely indicates a pattern of light having a finite size in at least two orthogonal spatial directions, e.g. x and y. The word “complex” is used herein merely to indicate that the light at each point in the light field may be defined by an amplitude value and a phase value, and may therefore be represented by a complex number or a pair of values. For the purpose of hologram calculation, the complex light field may be a two-dimensional array of complex numbers, wherein the complex numbers define the light intensity and phase at a plurality of discrete locations within the light field. In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity / system, varies with the distance between the display device and the viewing entity. At a 1 metre viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye’s pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye’s pupil to form an image at the retina for a given eye position, determines the portion of the image that is ‘visible’ to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-box.) In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device - that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it may therefore be considered that the viewer is looking at a virtual image through an ‘display device-sized window’, which may be very small, for example 1cm in diameter, at a relatively large distance, e.g., 1 metre. And the user will be viewing the display device-sized window via the pupil(s) of their eye(s), which can also be very small. Accordingly, the field of view becomes small and the specific angular range that can be seen depends heavily on the eye position, at any given time. A pupil expander addresses the problem of how to increase the range of angles of light rays that are propagated from the display device that can successfully propagate through an eye’s pupil to form an image. The display device is generally (in relative terms) small and the projection distance is (in relative terms) large. In some embodiments, the projection distance is at least one - such as, at least two - orders of magnitude greater than the diameter, or width, of the entrance pupil and / or aperture of the display device (i.e., size of the array of pixels). Use of a pupil expander increases the viewing area (i.e., user’s eye-box) laterally, thus enabling some movement of the eye / s to occur, whilst still enabling the user to see the image. As the skilled person will appreciate, in an imaging system, the viewing area (user’s eye box) is the area in which a viewer’s eyes can perceive the image. The present disclosure encompasses non-infinite virtual image distances - that is, near-field virtual images. Conventionally, a two-dimensional pupil expander comprises one or more one-dimensional optical waveguides each formed using a pair of opposing reflective surfaces, in which the output light from a surface forms a viewing window or eye-box. Light received from the display device (e.g., spatially modulated light from a LCOS) is replicated by the or each waveguide so as to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide enlarges the viewing window due to the generation of extra rays or “replicas” by division of amplitude of the incident wavefront. The display device may have an active or display area having a first dimension that may be less than 10 cms such as less than 5 cms or less than 2 cms. The propagation distance between the display device and viewing system may be greater than 1 m such as greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide may be up to 2 m such as up to 1.5 m or up to 1 m. The method may be capable of receiving an image and determining a corresponding hologram of sufficient quality in less than 20 ms such as less than 15 ms or less than 10 ms. In some embodiments - described only by way of example of a diffracted or holographic light field in accordance with this disclosure - a hologram is configured to route light into a plurality of channels, each channel corresponding to a different part (i.e. sub-area) of an image. The channels formed by the diffractive structure are referred to herein as “hologram channels” merely to reflect that they are channels of light encoded by the hologram with image information. It may be said that the light of each channel is in the hologram domain rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram and the hologram domain is therefore the Fourier or frequency domain. The hologram may equally be a Fresnel or Fresnel transform hologram. The hologram may also be a point cloud hologram. The hologram is described herein as routing light into a plurality of hologram channels to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-areas, wherein each hologram channel would correspond to each image sub-area. Importantly, the hologram of this example is characterised by how it distributes the image content when illuminated. Specifically and uniquely, the hologram divides the image content by angle. That is, each point on the image is associated with a unique light ray angle in the spatially modulated light formed by the hologram when illuminated - at least, a unique pair of angles because the hologram is two-dimensional. For the avoidance of doubt, this hologram behaviour is not conventional. The spatially modulated light formed by this special type of hologram, when illuminated, may be divided into a plurality of hologram channels, wherein each hologram channel is defined by a range of light ray angles (in two-dimensions). It will be understood from the foregoing that any hologram channel (i.e. sub-range of light ray angles) that may be considered in the spatially modulated light will be associated with a respective part or sub-area of the image. That is, all the information needed to reconstruct that part or sub-area of the image is contained within a sub-range of angles of the spatially modulated light formed from the hologram of the image. When the spatially modulated light is observed as a whole, there is not necessarily any evidence of a plurality of discrete light channels. Nevertheless, the hologram may still be identified. For example, if only a continuous part or sub-area of the spatially modulated light formed by the hologram is reconstructed, only a sub-area of the image should be visible. If a different, continuous part or sub-area of the spatially modulated light is reconstructed, a different sub-area of the image should be visible. A further identifying feature of this type of hologram is that the shape of the cross-sectional area of any hologram channel substantially corresponds to (i.e. is substantially the same as) the shape of the entrance pupil although the size may be different - at least, at the correct plane for which the hologram was calculated. Each light I hologram channel propagates from the hologram at a different angle or range of angles. Whilst these are example ways of characterising or identifying this type of hologram, other ways may be used. In summary, the hologram disclosed herein is characterised and identifiable by how the image content is distributed within light encoded by the hologram. Again, for the avoidance of any doubt, reference herein to a hologram configured to direct light or angularly-divide an image into a plurality of hologram channels is made by way of example only and the present disclosure is equally applicable to pupil expansion of any type of holographic light field or even any type of diffractive or diffracted light field. 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, BO 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 I reflective-transmissive surface 524a of the first replicator / waveguide 520. The mirror 530 is tilted so as to redirect the received light onto an optical path to an input port in the (fully) reflective surface of second replicator 540 at an angle to provide waveguiding and replica formation, along its length in the second dimension. It will be appreciated that the mirror 530 is one example of an optical element that can redirect the light in the manner shown, and that one or more other elements may be used instead, to perform this task. In the illustrated arrangement, the (partially) reflective-transmissive surface 524a of the first replicator 520 is adjacent the input port of the first replicator / waveguide 520 that receives input beam 522 at an angle to provide waveguiding and replica formation, along its length in the first dimension. Thus, the input port of first replicator / waveguide 520 is positioned at an input end thereof at the same surface as the reflective-transmissive surface 524a. The skilled reader will understand that the input port of the first replicator / waveguide 520 may be at any other suitable position. Accordingly, the arrangement of Figure 5B enables the first replicator 520 and the mirror 530 to be provided as part of a first relatively thin layer in a plane in the first and third dimensions (illustrated as an x-z plane). In particular, the size or “height” of a first planar layer - in which the first replicator 520 is located - in the second dimension (illustrated as the y dimension) is reduced. The mirror 530 is configured to direct the light away from a first layer / plane, in which the first replicator 520 is located (i.e. the “first planar layer"), and direct it towards a second layer / plane, located above and substantially parallel to the first layer / plane, in which the second replicator 540 is located (i.e. a “second planar layer”). Thus, the overall size or “height” of the system - comprising the first and second replicators 520, 540 and the mirror 530 located in the stacked first and second planar layers in the first and third dimensions (illustrated as an x-z plane) - in the second dimension (illustrated as the y dimension) is compact. The skilled reader will understand that many variations of the arrangement of Figure 5B for implementing the present disclosure are possible and contemplated. The image projector may be arranged to project a diverging or diffracted light field. In some embodiments, the light field is encoded with a hologram. In some embodiments, the diffracted light field comprises diverging ray bundles. In some embodiments, the image formed by the diffracted light field is a virtual image. In some embodiments, the first pair of parallel / complementary surfaces are elongate or elongated surfaces, being relatively long along a first dimension and relatively short along a second dimension, for example being relatively short along each of two other dimensions, with each dimension being substantially orthogonal to each of the respective others. The process of reflection / transmission of the light between / from the first pair of parallel surfaces is arranged to cause the light to propagate within the first waveguide pupil expander, with the general direction of light propagation being in the direction along which the first waveguide pupil expander is relatively long (i.e., in its “elongate” direction). There is disclosed herein a system that forms an image using diffracted light and provides an eye-box size and field of view suitable for real-world application - e.g. in the automotive industry by way of a head-up display. The diffracted light is light forming a holographic reconstruction of the image from a diffractive structure - e.g. hologram such as a Fourier or Fresnel hologram. The use diffraction and a diffractive structure necessitates a display device with a high density of very small pixels (e.g. 1 micrometer) - which, in practice, means a small display device (e.g. 1 cm). The inventors have addressed a problem of how to provide 2D pupil expansion with a diffracted light field e.g. diffracted light comprising diverging (not collimated) ray bundles. In some embodiments, the display system comprises a display device - such as a pixelated display device, for example a spatial light modulator (SLM) or Liquid Crystal on Silicon (LCoS) SLM - which is arranged to provide or form the diffracted or diverging light. In such aspects, the aperture of the spatial light modulator (SLM) is a limiting aperture of the system. That is, the aperture of the spatial light modulator - more specifically, the size of the area delimiting the array of light modulating pixels comprised within the SLM - determines the size (e.g. spatial extent) of the light ray bundle that can exit the system. In accordance with this disclosure, it is stated that the exit pupil of the system is expanded to reflect that the exit pupil of the system (that is limited by the small display device having a pixel size for light diffraction) is made larger or bigger or greater in spatial extend by the use of at least one pupil expander. The diffracted or diverging light field may be said to have “a light field size”, defined in a direction substantially orthogonal to a propagation direction of the light field. Because the light is diffracted / diverging, the light field size increases with propagation distance. In some embodiments, the diffracted light field is spatially-modulated in accordance with a hologram. In other words, in such aspects, the diffractive light field comprises a “holographic light field”. The hologram may be displayed on a pixelated display device. The hologram may be a computer-generated hologram (CGH). It may be a Fourier hologram ora 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 eye box 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 Device Figure 6 shows a schematic side-view of features of a head-up display comprising a first reflection suppression device 600 (in other words, a light control layer). The head-up display of Figure 6 comprises a waveguide 602, a turning layer 603, and the first reflection suppression device 600. The head-up display further comprises a coherent light source (in this example, a laser) and a display device (in this example, a liquid crystal on silicon spatial light modulator) arranged to display a hologram of a picture. The light source and display device are not shown in Figure 6. In use, the light source of the head-up display illuminates the display device such that light is spatially modulated in accordance with the hologram displayed on the display device, thus forming a holographic wavefront. The holographic wavefront is coupled into a first waveguide (not shown in Figure 6) where the holographic wavefront is replicated in a first direction a plurality of times (as described previously) to form a one dimensional array of replicas which are then coupled into the waveguide 602 (which is shown in Figure 19). The waveguide 602 comprises a pair of opposing surfaces. A first surface 604 of the waveguide 602 is partially transmissive-reflective. A second surface 606 of the waveguide 602 is reflective. The waveguide 602 is arranged to waveguide the replicas of the holographic wavefront coupled in to the waveguide 602 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 604 is represented by the dotted arrows 608 in Figure 6. In the y-z plane, the replicas 608 are angled with respect to a normal of the waveguide 602 (i.e. the replicas are angled with respect to the z-direction in the y-z plane). In this example, the replicas 608 of the holographic wavefront (at the first surface of the waveguide 604) do not have a component on the x-z plane. So, as the skilled reader will appreciate, in a side view of the waveguide 602 in the x-z plane, the replicas 608 would appear to be emitted vertically up the page. It should be understand that, although a holographic head-up display is described in relation to this example, that the glare suppression device 600 could in fact be used with a conventional head-up display. In such cases, the light that propagates through the glare suppression device 600 may be more conventional light, modulated in accordance with an image rather than a hologram. The replicas 608 of the holographic wavefront emitted by the waveguide 602 are received by the turning layer 603 whereby the (replicas of) the holographic wavefront are turned by the turning layer 603. The turned replicas 608 are then received by the reflection suppression device 600. The reflection suppression device 600 comprises a first surface 620 and a second surface 624. The first layer 620 is closest to the waveguide 608 I turning layer 603. As such, the (replicas of) holographic wavefront (propagating through the reflection suppression device 600) are received by the first surface 624. The first (bottom) surface 624 has a serrated or sawtooth shape. The second surface 624 comprises a prismatic structure comprising a plurality I an array of first prism elements 621 (in other words, a series of first serrations 621). The first surface 624 is integrally formed such that the array of first prism elements 621 form a single component (forming the first surface 624). The first serrated surface 624 is defined by first and second surfaces or faces of each of the prism elements 621, thus forming a sawtooth-type structure when viewed in the y-z plane (as in Figure 6). The first surface 6241 first prism elements 621 are formed of a transparent material which, in this example, is a hard plastic (transparent) material. The first layer 624 may have been manufactured by, for example, injection moulding, hot embossing, extruding or cutting a source of the transparent material. The second (top) surface 620 also forms a (second) serrated surface. The second surface 620 comprises a prismatic structure comprising a plurality I an array of second prism elements 623 (in other words, a series of second serrations 623). Like the first surface 624, the second surface 620 is integrally formed such that the array of second prism elements 623 form a single component (forming the first layer 620). The second serrated surface 620 is defined by first and second surfaces or faces of each of the prism elements 623, thus forming a sawtooth-type structure when viewed in the y-z plane (as in Figure 6). The second surface 620 is formed of a transparent material which, in this example, is a hard plastic (transparent) material. In this example, the second surface 620 is formed of the same transparent material as the first surface 624 (or, at least, a material having the same refractive index as the transparent material of the first surface 624). The first layer 624 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 second and first serrated surfaces 624,620 is equal. Each of the first and second prism elements 621,623 comprises an active face 625 and a passive face 626. The active face 625 of the first prism elements 621 may be referred to as a receiving face because it is this face that receives the holographic wavefront 608. The active face 625 of the second prism elements 621 may be referred to as a transmitting face because it is this face that transmits the holographic wavefront 608. The holographic wavefront 608 is received by the first serrated surface 624 (in particular, by an active face 625 of a first prism element 621). A first turn is provided to the holographic wavefront 608 by the first prism element 621 as a result of the angle of the active face 625 and the refractive index of the first prism element 621. The turned holographic wavefront 608 then propagates through the reflection suppression layer to be received by a second prism element 623 of the second serrated surface 620 (in particular, to be received by an active face 625 of a second prism element 623). When the holographic wavefront 608 is emitted at the second serrated surface 620 a second turn is provided to the holographic wavefront 608 by the second prism element 623 as a result of the angle of the active face 625 and the refractive index of the second prism element 623. In this example, the reflection suppression 600 device further comprises two arrays of louvres 632. A first array 630 of louvres 632 is provided on the first surface 624. In particular, each louvre 632 of the first array 630 is provided on a passive face 626 of a first prism 621. A second array 631 of louvres 632 is provided on the second surface 620. In particular, each louvre 632 of the second array 631 is provided on a passive face 626 of a second prism 623. Because the first and second arrays 630,631 of louvres 632 is provided on the passive face 626 of the respective first and second prism elements 621,623, the periodicity of the louvres 632 of the first and second arrays is equal to the periodicity of the prism elements 621,623. In some examples, the louvres 632 are provided as coatings on the respective passive faces 626. In some examples, the louvres 632 are provided as black paint on the respective passive faces 626. Each of the second and first surfaces 620,624 are formed of a transparent material which, in this example, has a refractive index greater than 1. In this example, each of the second first surfaces 620 and 624 are formed of polymethyl methacrylate (PMMA). Each of the second and first serrated surfaces form a transparent material I air interface. Light (in particular, the holographic wavefront 608) propagating through the reflection suppression device will be turned, twice. A first turn will be provided by the first layer 620 and a second turn will be provided by the second layer 624. In this example, the shape of the serrations, and the refractive index of the transparent material, are selected such that the component of the first turn on the first plane (the y-z plane) is equal but opposite to the component of the second turn on the first plane (the y-z plane). The first serrated surface 624 of the reflection suppression device 600 may be referred to herein as an input side of the reflection suppression device 600 (because the first serrated surface receives the holographic wavefront). The second serrated surface 620 of the reflection suppression device 600 may be referred to herein as an output side of the reflection suppression device 600 (because the second serrated surface 620 emits the holographic wavefront once the holographic wavefront has propagated though the reflection suppression device 600). The turning layer 603, in this example, also has a prismatic structure. However, the prism elements 1935 of the turning layer 603 extend longitudinally in a direction that is orthogonal to the direction of extension of the prism elements of the first and second layers. Specifically, the prism elements 1935 extend longitudinally in the y-direction rather than the x-direction. As such, the prism elements 1935 are arranged to turn the holographic wavefront exclusively on the second plane (in the y-z plane) rather than on the first plane (the x-z plane). Glare mitigation The reflection suppression device 600 is arranged to mitigate or suppress glare from being received at a viewing window or eye-box. In the absence of the reflection suppression device 600, there is a risk that ambient light incident on the waveguide may be reflected and be received at the viewing window or eye-box. This ambient light may then be distracting. Said glare is suppressed by the reflection suppression device 600 via a number of different mechanisms. The arrays 630,631 of louvres 632 are formed of / consist of a light absorbing material. The array of louvres 632 are angled such that the replicas 608 of the holographic wavefront can substantially pass between the louvres 632 without being absorbed. However, ambient light passing through the reflection suppression device 600 following a different propagation path (that is not parallel to the louvres) will tend to be incident on one of the louvres 632. Said ambient light will be absorbed by said louvre 632 and therefore will not be able to propagate on to the waveguide 602 and so will not be reflected by the waveguide 602 back to the viewing window / eye-box. As should be clear to the skilled reader, there will be a range of angles at which ambient light may be able to pass between adjacent louvres 632 in both the first and second arrays 630,631. This range of angles will be defined by the pitch of the louvres 632 and the shape I dimensions / orientation of the louvres 632. After a first pass through the reflection suppression device, said ambient light may be reflected by the waveguide 602 and returned back towards the reflection suppression device 600 to pass through for a second time. On the second pass, the reflected ambient light will generally be absorbed by one of the louvres 632. Thus, even if the ambient light is not absorbed by the louvres 632 on a first pass through the reflection suppression device, it will generally be absorbed by the louvres 632 on the second pass. The second serrated surface 620 provides another mechanism for glare mitigation. In particular, the array of angled surfaces of the second serrated surface 620 are arranged such that a portion of the light incident on second serrated surface 620 may be specularly reflect ambient light incident thereon. The second serrated surface 620 is arranged such that this specularly reflected light is reflected in a direction that is generally away from viewing window / eye-box. The reflection suppression device 600 described above comprises first and second serrated surface 624, 620 comprising, respectively, arrays of first or second prisms 621,623. The second serrated surface 620 is arranged to direct specular reflections away from a viewing window I eye-box of a display system. In particular, the angle of the passive faces with respect to a normal of the light control layer is arranged to direct specular reflections on the passive faces away from the viewing window I eye-box. Through experimentation and simulation, the inventors have found that there may be only one angle, ora very narrow range of angles, of the passive face that may be optimised for directing specular reflections away from the viewing window / eye-box. Thus, to optimize the suppression of glare, the angle of the passive faces 626 the second surface 620 is constrained to optimised angle or very narrow range of angles. In this example, each passive face 626 of the second is arranged to be at the same (optimised) angle. In other words, each passive face 626 of the second surface 620 is parallel. As described above, the provision of the second prism elements 623 of the second surface 620 will each provide a (second) turn to the holographic wavefront 608. The reason for the provision of the first prism elements 621 of the first surface 620 in the reflection suppression device 600 is to cancel out this second turn. Thus, in the reflection suppression device of this example, the first turn provided by the first prism elements 621 is arranged to completely compensate for the second turn provided by the second prism elements 623. In other words, the net turn (of the sum of the first and the second turns) is zero in this example. The reason for this is so that the introduction of the reflection suppression device 600 into a display system does not result in a net turn of the holographic wavefront 608 which might result in unwanted translation of the viewing plane / eye-box. Curved optical component Figure 7 is a schematic cross-sectional view of a display system comprising the reflection suppression device 600 and a windscreen 650 (or windshield). The display system also comprises a spatial light modulator and light source for illuminating the spatial light modulator to form the holographic wavefront 608. The display system further comprises one or more waveguides arranged to replicate the holographic wavefront 608, and a turning layer, as described above. However, Figure 7 only shows a portion of the reflection suppression device 600 and a portion of the windscreen 650. Figure 7 schematically represents a distortion caused by the display system comprising a curved optical component. In this example, the curved optical component is the windscreen 650. The problem is that the curved optical component I windscreen 650 introduces distortion to the holographic wavefront 608 / to the light received at the eye-box / viewing window. This is represented by the propagation of three different rays of the holographic wavefront 608 through the reflection suppression device in Figure 7. The holographic wavefront 608 is shown as being incident on the active faces 625 of three first prism elements 621. Here, the three rays of the holographic wavefront 608 are substantially parallel. At the interface between air and the first prism elements 621, a first turn is provided to the three rays of the holographic wavefront 608. As described above, each of the first prism elements 621 is arranged provide a first turn which exactly cancels out or compensates for the respective second prism element 623 to which the first prism element 621 is optically coupled. Because each second prism element is constrained by the need for reflection suppression (and so each second prism element is identical), each first prism element 621 is also identical. Thus, the identical first prism elements 621 all provide exactly the same first turn to each of the rays of the holographic wavefront 608. Thus, the three rays of the holographic wavefront 608 remain substantially parallel while propagating through the reflection suppression device, between the first and second surfaces 624,620. At the interface between the second prism elements 623 and air, a second turn is provided to the holographic wavefront 608. Again, each of the second prism elements 623 of the reflection suppression device 600 is substantially identical. Thus, the identical second prism elements 623 all provide exactly the same second turn to each of the rays of the holographic wavefront. Thus, the three rays of the holographic wavefront 608 remain substantially parallel immediately downstream of the reflection suppression device 600. In other words, the angles of the three rays of the holographic wavefront 608 are maintained upstream and downstream of the reflection suppression device 600. As such, the reflection suppression device 600 is arranged to not distort the rays of the holographic wavefront 608. The three rays of the holographic wavefront 608 continue to propagate towards the windscreen 650 (or optical combiner) and are incident on the windscreen 650, after which the three rays of the holographic wavefront 608 are reflected by at least an interior surface of the windscreen 650. The reflected rays of the holographic wavefront 608 are directed towards an eye-box or viewing window of the display system. The windscreen 650 has a curved surface. The three different rays of the holographic wavefront 608 are incident on different portions of the windscreen 650. Because the windscreen 650 has a curved surface, the rays make different angles with different portions of the windscreen 650 and so are reflected at different angles to one another. In this example, the windscreen 650 has a substantially concave curve shape. This results in the reflected rays of the holographic wavefront 608 converging. Thus, downstream of the windscreen 650, the rays of the holographic wavefront 608 are no longer substantially parallel. The angular relationship between the different rays of the holographic wavefront 608 that existed upstream of the reflection suppression device 600 has been broken by the windscreen 650. In other words, the holographic wavefront 608 has been distorted by the windscreen 650. This has the effect of distorting a picture viewable from the eye-box / viewing window. In this example, the three rays of the holographic wavefront 608 are shown as being substantially parallel in Figure 7. However, it should be clear that the rays of the holographic wavefront 608 are typically slightly diverging. This may be because a holographic wavefront 608 typically comprises diffracted light. The important point is that the curved windscreen 650 changes the convergence (or divergence, in some examples) the holographic wavefront 608 such that the angular relationship between rays of the holographic wavefront 608 is not maintained between the reflection suppression device 600 and the windscreen 650. In this example, the curved optical element is a windscreen 650. However, it should be understood that any curved optical element on the optical path of the holographic wavefront 608 could introduce a distortion effect. Furthermore, in this example, the curved optical element (windscreen 650) is provided downstream of the reflection suppression device 600. In other examples, a curved optical element could be provided upstream of the reflection suppression device 600. In such examples, the curved optical element introduces the distortion to the holographic wavefront 608 prior to the holographic wavefront being received by the reflection suppression device 600. The reflection suppression device 600 then maintains the distortion because the distortion is maintained by the combination of the first and second prismatic structures. Thus, a distorted image may be delivered to a eye-box or viewing window of the system. It should be clear that Figure 7 is a schematic drawing. Figure 7 is not drawn to scale and the shapes and relative positions of components / features shown in Figure 7 are not drawn accurately or to scale. For example, the curvature of windscreen 650 is exaggerated in Figure 7, and the location of the windscreen with respect to the reflection suppression device 600 is merely representative. Furthermore, the reflection suppression device 600 is not drawn to scale. The individual prism elements 621,623 of the reflection suppression device 600 are relatively very small (for example, less than 1 millimetre) in reality. The reflection suppression device 600 may comprise upwards of 200 first and second prism elements 621,623. Herein, the light that propagates through the display system I reflection suppression device 600 has been described as a holographic wavefront. In other examples, the light that propagates therethrough may be otherwise spatially modulated, for example spatially modulated in accordance with a picture (i.e. conventional head-up display light). Glare Mitigation Control Figure 8 shows an individual second prism element 623 or, in other words (as the component will henceforth be referred to as), an individual second serration 623. That is, Figure 8 shows an individual serration 623 from the second surface 620 of reflection suppression device or, in other words (as the component will henceforth be referred to as), a light control layer 600. As described above, the serration 623 has an active face 625 and a passive face 626, the passive face 626 being painted with a light absorbing material to form a (light) louvre 632. The serration 623 extends in the z-direction and from a base 802 to a tip 804. The base 802 is the wide end of the serration 623 (i.e. the end of the serration 623 closest to e.g. the waveguide 602). The base 802 extends from a first side 802a to a second side 802b along the y-direction (i.e. the direction parallel to the length of the waveguide 602). The first side 602a joins with a second side of a further serration in the negative y-direction, whilst the second side 602b joins with a first side of a further serration in the positive y-direction. The z-axis / direction is perpendicular to the first surface 604 of the waveguide 602 (i.e. the surface of the waveguide 602 that is partially reflective such that, without the light control layer 600, sunlight would reflect off towards the eye-box). The passive face 626 is at a first angle a with respect to the z-axis, whilst the active face 625 is at a second angle p with respect to the z-axis. The first angle a is in the range of 29° to 38°, however angles in the range of 20° to 45° are also anticipated. Meanwhile, the second angle p is in the range of 30° to 40°, however angles in the range of 20° to 70° are also anticipated. The first angle a and the second angle p are opposed to one another, and so these angle values are taken as they are shown in Figure 8 (i.e. in cross section viewed along the x-axis, with the base 802 of the serration 623 extending from the first side 802a to the second side 802b in the positive y-direction). As such, another way of describing the second angle (taken in a clockwise direction as is the first angle a is in Figure 8) is in the range of 320° to 330°. The first angle a and second angle p are chosen to achieve the desired head-up display image quality and to minimise the impact of dark banding the serrations 623 may have on the image. Direct Glare However, at some positions of the sun in the sky relative to the vehicle (and more specifically, relative to the head-up display and the light control layer 600), the inventors have found that these values for the first and second angles a, p can still cause glare directly from the serration 623 to the eye-box, as shown in Figure 9. Figure 9 shows a series of second serrations 623a-d, taken as a section view of part of the second surface 620 of the light control layer 600. Each serration 623a-d has an active face 625a-d and a passive face 626a-d, and culminates at a tip 804a-d. Due to the first and second angles a, p, sunlight rays 908a-d may reflect off the active face 625a-d of the respective serration 623a-d at a point close to the respective tip 804a-d towards the eye-box. That is, most sunlight rays that reflect off the active face 625a-d of the respective serration 623a-d (i.e. at a point further towards the base of each serration 623a-d) will be reflected into the passive face 626a-d of the subsequent serration. These light rays will therefore be absorbed by the passive face 626a-d and will not reach the eye-box. However, for example, sunlight ray 908a reflects off the active face 625a near to the tip 804a of the serration 623a. The reflected sunlight ray 908a passes over the next serration 623b and then the rest of the serrations 623c, d. The reflected sunlight ray 908a may then reach the eye-box. Likewise, sunlight ray 908ba reflects off the active face 625b near to the tip 804b of the serration 623b and then passes over the next serration 623c. This is then repeated for each serration 623a-d. As such, some sunlight rays 908a-d can be reflected off the serrations 623a-d towards the eye-box. This can wash out the image being displayed, create light bands in the image, or otherwise distract the user (i.e. the driver of the vehicle). Direct Glare Mitigation Figure 10 shows one embodiment of the present disclosure of a side section view of a light control layer configured to prevent or mitigate the above described direct glare. The serrations 623a-d are as shown in Figure 9, apart from an addition of a reflection suppression structure (also referred to as a reflection suppression extension) 1002a-d (referred to in this embodiment as a “baffle”) on each serration 623a-d. The structure of the baffles 1002a-d is discussed in greater detail below with respect to Figure 11. Each baffle 1002a-d is positioned on the tip 804a-d of each serration 623a-d and is painted with a light absorbing paint as per the passive face 626 of each serration 623. Each baffle 1002a-d has two methods for direct glare mitigation. For example, a first sunlight ray 1008a is absorbed by the baffle 1002a (due to the aforementioned light absorbing paint). Meanwhile, a second sunlight ray 1009a reflects off the active face 625a near to the tip 804a of the serration 623a. That is, the second sunlight ray 1009a reflects off a region of the active face 625a that is not low enough (in the z-axis) that it is then absorbed by the passive face 626b of the next serration 623b, nor is it high enough that it is absorbed by the baffle 1002a. The second sunlight ray 1009a, having been reflected off the active face 625a of the serration 623a, is then absorbed by the baffle 1002b of the next serration 623b. This is repeated across the array of serrations 623a-b. For example, a first sunlight ray 1008b is absorbed by the baffle 1002b, whilst a second sunlight ray 1009b reflects off the active face 625b near to the tip 804b of the serration 623b and is absorbed by the baffle 1002c of the next serration 623c. In this way, sunlight rays 1008a-d, 1009a-d are prevented from reaching the eye-box via reflection from the serrations 623a-d. Figure 11 shows a further embodiment of a light control layer having a reflection suppression structure 1102 (referred to in this embodiment as an “offset” 1102). The light control layer has a first surface 620 and a second surface 624 with serrations 623, 621. The second surface 624 is as described above in relations to Figures 6 and 7. Meanwhile, the serrations of 623 of the first surface 620 have an offset 1102, the structure of which will be described below in relation to Figure 14. Figure 12 shows a variation of the embodiment of Figure 11. In this variation, the second surface 1224 does not have serrations and is instead a flat surface. Although not shown in the Figures, this is also possible with the baffle 1002 of the previously described embodiment. That is, the invention of present disclosure still functions as intended even with light control layers that do not require serrations on the surface closest to the waveguide 602 (not shown in Figure 12). This is not least because the reflection suppression structure 1002,1102 of the present disclosure is not required on the second surface 624,1224 as this surface is not subject to sunlight rays. Figure 13 shows section A of Figure 10, showing the baffle 1002 in more detail. As can be seen, the baffle 1002 is located, and extends from, the tip 804 of the serration 623. The baffle have a substantially rectangular shape - although the skilled person would understand that this rectangular shape is schematic and would recognise that in a physical embodiment slight discrepancies from a true rectangular shape would exist due to manufacturing constraints. The baffle 1002 is located centrally with respect to the intersection of the active and passive faces 625, 626 of the serration 623. The extension of the baffle 1002 in the z-direction from the tip 804 of the serration 623 (i.e. the height of the baffle 1002) is in the range of 50pm to 200pm, however heights of 30pm to 250pm are also anticipated. The thickness of the baffle 1002 in the y-direction (i.e. the width of the baffle 1002) is in the range of 50pm to 150pm, however widths of 25pm to 250pm are also anticipated. Figure 14 shows section B of Figure 11, showing the offset 1102 in more detail. As in Figure 8, the serration 623 extends to a tip 804. However, the active face 1425 and passive face 1426 have effectively be lowered and raised. This allows the downstream face 1425 and the upstream face 1426 to be at the same angles a, p as the active and passive faces 625, 626 of Figure 8, ensuring that the glare mitigation of the serration of Figure 8 is maintained, as will be discussed further below. As per the active and passive faces 625, 626, the downstream face 1425 is transparent and the upstream face 1426 is painted with a light absorbing (e.g. a matte black) paint. This effective raising and lowering of the active and passive faces 1425,1426 creates an offset face 1427 that extends from the tip 804 of the serration 623. This offset face 1427 extends parallel to the z-axis, although the skilled person would understand that a 1° to 2° deviation from true parallel is to be expected due to manufacturing constraints. The extension of the offset face 1427 in the z-direction from the tip 804 of the serration 623 (i.e. the height of the offset 1102) is in the range of 50pm to 200pm, however heights of 30pm to 250pm are also anticipated. The offset face 1427 is textured so that it is optically diffuse. This provides a distance between the painted passive face 1426 and the transparent active face 1425, reducing the chance of paint meant for the passive face 1426 obscuring the active face 1425. Figure 15 shows the path of the display light rays 1508 through the light control later of Figure 11. As can be seen, the display light rays 1508 are received and transmitted by the active faces 1425 of the serrations 623. The display light rays 1508 are turned as described above in relation to Figure 7, but the offsets 1102 do not affect the optical path of these rays 1508. As such, the offsets 1102 prevent / mitigate direct glare (as will be further described below), whilst still allowing the display light to be transmitted as required without an increase in dark banding in the head-up display image. Figure 16 shows the functioning of the offset 1102 across a selection of serrations 623a-d, as in Figure 10. Each offset 1102a-d has two methods of direct glare mitigation. For example, a first sunlight ray 1608a is diffused by the offset face of the offset 1102a (due to the aforementioned textured surface). Meanwhile, a second sunlight ray 1609a reflects off the active face 1425a near to the tip 804a of the serration 623a and is absorbed by the passive face 1426b of the next serration 623b. This is repeated across the array of serrations 623a-b. For example, a first sunlight ray 1608b is diffused by the offset face of the serration 623b, whilst a second sunlight ray 1609b reflects off the active face 1425b near to the tip 804b of the serration 623b and is absorbed by the passive face 1426c of the next serration 623c. In this way, sunlight rays 1608a-d, 1609a-d are prevented from reaching the eye-box via reflection from the serrations 623a-d. Although the reflection suppression structures (i.e. the baffle 1002 and offset 1102) as described above have been shown with a constant height across the serrations 623, the height can vary across the first surface 620. Figure 17 shows a first surface 620 with a series of serrations 623 that have baffles 1702a, b that show a step change in height along the y-direction (that is, between each serration 623). That is, a first set of baffles 1702a have a first height, whilst a second set of baffles 1702b have a second height, which is greater than the height of the baffles 1702a of the second set. As can be seen in Figure 17, the two sets of baffles 1702 a, b are separated (i.e. there is no mixing of baffles 1702 of different heights). Similarly, Figure 18 shows a first surface 620 with a series of serrations 623 that have baffles 1802a-b that show a linear increase in height along the y-direction (that is, between each serration 623). That is, the height of the baffles 1802a-b linearly increase from a first baffle 1802a to a last baffle 1802b. Although not shown, other types of height increase are also envisaged in a similar way, such as an exponential increase. For both Figures 17 and 18 the reflection suppression structure is a baffle. However, this inconstant height could also occur when the reflection suppression structure is an offset, with the height of the offset face (as described above) changing along the y-direction (for example as a step change, or a linear or exponential increase, as described above). In both the examples of Figures 17 and 18 an increase in the y-direction is to move closer to the user (i.e. the driver when the head-up display is positioned as it would be used in a vehicle). The angle of the display light rays 608 leaving the first surface 604 of the waveguide 602 (as per Figure 6) and arriving at the eye-box increases relative to the z-direction as the y-direction increases. That is, the angle of the ray 608 paths are steeper as they get closer to the user. As such, the inventors have found that the reflection suppression structures can have a larger height closer to the user without increasing the dark banding in the image. The inventors have also found that this coincides with where the majority of the sunlight rays are reflected off the serration 623 (that is, as serrations 623 closer to the user reflect more sunlight rays). As such, increasing the height of the reflection suppression structures on serrations 623 closer to the user does not further impact the image displayed whilst providing an increased direct glare mitigation effect. Additional features The methods and processes described herein may be embodied on a computer-readable medium. The term “computer-readable medium” includes a medium arranged to store data temporarily or permanently such as random-access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" shall also be taken to include any medium, or combination of multiple media, that is capable of storing instructions for execution by a machine such that the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methodologies described herein, in whole or in part. The term "computer-readable medium" also encompasses cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example form of a solid-state memory chip, an optical disc, a magnetic disc, or any suitable combination thereof. In some example embodiments, the instructions for execution may be communicated by a carrier medium. Examples of such a carrier medium include a transient medium (e.g., a propagating signal that communicates instructions). It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. The present disclosure covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

1. A head-up display for a vehicle, the head-up display having a viewing window and comprising:a substantially planar waveguide having a reflective surface arranged, during head-up display operation, in a configuration that is conducive to sunlight glare; anda light control layer arranged to receive sunlight on an optical path to the reflective surface, the light control layer comprising a plurality of serrations, each serration having a tip;wherein at least one of the serrations comprises a reflection suppression extension extending from the tip of the serration and arranged to receive sunlight on an optical path to the viewing window,2. The head-up display as claimed in claim 1, wherein each reflection suppression extension comprises:a primary surface extending from a proximal end to a distal end substantially perpendicularly to the reflective surface, wherein the proximal end is located at the tip of the respective serration; anda secondary surface extending from a base of the respective serration to the distal end of the primary surface.

3. The head-up display as claimed in claim 1 or claim 2, wherein the primary surface of the reflection suppression extension is configured to diffuse sunlight.

4. The head-up display as claimed in any of claims 1 to 3, wherein the primary surface of the reflection suppression extension has a length from the proximal end to the distal end in the range of 30pm to 250pm, preferably 50pm to 200pm.

5. The head-up display as claimed in any of claims 1 to 4, wherein the secondary surface is arranged at a first angle with respect to a normal of the reflective surface in the range of 20° to 45°, preferably 29° to 38°.

6. The head-up display as claimed in any of claims 1 to 5, wherein each of the serrations further comprises a second surface extending from a base of the respective serration to the tip of said serration, the second surface of each serration configured to transmit light.

7. The head-up display as claimed in claim 6, wherein the second surface isarranged at a second angle with respect to a plane perpendicular to the reflective surface in the range of 290° to 340°, preferably 320° to 330°.

8. The head-up display as claimed in claim 1, wherein each reflection suppression extension comprises two substantially parallel surfaces extending from the tip of the respective serration substantially perpendicularly to the reflective surface of the waveguide.

9. The head-up display as claimed in claim 8, wherein each reflection suppression extension has a central axis along the length of the structure’s extension from the tip of the respective serration, the central axis intersecting said tip of the respective serration.

10. The head-up display as claimed in claim 8 or claim 9, wherein the cross-sectional shape of each reflection suppression extension is substantially rectangular.

11. The head-up display as claimed in any of claims 8 to 10, wherein the two surfaces of the reflection suppression extension have a minimum separation in the range of 25pm to 250pm, preferably 50pm to 150pm, and / or wherein the two surfaces of the reflection suppression extension have an extension from the tip of the respective serration in the range of 30pm to 250pm, preferably 50pm to 200pm.

12. The head-up display as claimed in any of claims 8 to 11, wherein a first surface of each serration is configured to attenuate sunlight, such as absorb sunlight, and a second surface of each serration is configured to transmit light.

13. The head-up display as claimed in claim 12, wherein the first surface is arranged at a first angle with respect to a normal of the reflective surface in the range of 20° to 45°, preferably 29° to 38°, and / or wherein the second surface is arranged at a second angle with respect to a normal of the reflective surface in the range of 290° to 340°, preferably 320° to 330°.

14. The head-up display as claimed in any proceeding claim, wherein each reflection suppression extension is integral with the respective serration.

15. The head-up display as claimed in any proceeding claim, wherein each serration comprises the reflection suppression extension.

16. The head-up display as claimed in claim 15, wherein the height of the extension of each reflection suppression extension is uniform.

17. The head-up display as claimed in claim 15, wherein the height of the extension of each reflection suppression extension varies between a first serration and a last serration.18 The head-up display as claimed in claim 17, wherein the serrations are formed of a first group of serrations and a second group of consecutive serrations, each group of serrations having a constant height of the extension of each reflection suppression extension, the height of the extension of each reflection suppression extension in the first group of serrations being different to that of the height of the extension of each reflection suppression extension in the second group of serrations.

19. The head-up display as claimed in claim 17, wherein the height of the extension of each reflection suppression extension varies linearly between the first serration and the last serration.

20. The head-up display as claimed in claim 17, wherein the height of the extension of each reflection suppression extension varies exponentially between the first serration and the last serration.

21. The head-up display as claimed in any of claims 17 to 20, wherein the height of the extension of the reflection suppression extension of the first serration is greaterthan the height of the extension of the reflection suppression extension of the last serration, the first serration being closer to the viewing window than the last serration.

22. The head-up display as claimed in any proceeding claim, wherein at least some of the sunlight on the optical path to the viewing window has originated from an adjacent serration.

23. The head-up display as claimed in any proceeding claim, wherein the serrations form at least part of a sunlight-receiving surface and light control layer furthercomprises a wavefront-receiving surface arranged between the sunlight-receiving surface and the reflective surface of the waveguide.

24. The head-up display as claimed in any proceeding claim, wherein the cross-5 sectional shape of each serration is substantially triangular.

25. A method of mitigating glare during operation of a head-up display for a vehicle, the head-up display comprising a substantially planar waveguide and having a viewing window, the method comprising:10 receiving sunlight at a light control layer on an optical path to a reflective surface ofthe waveguide, the light control layer comprising a plurality of serrations, each serration having a tip; andpreventing reflection of the sunlight from at least one of the serrations to the viewing window by means of a reflection suppression extension extending from the tip of the15 serration.52

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