Optimisation of compact holographic projector

Abstract

A display system that comprises a hologram replicator 1210 arranged to output an array of replicas of a holographic wavefront, wherein angles of the holographic wavefront correspond to spatial coordinates of an image corresponding to the holographic wavefront; and an optical component 1200 arranged to receive the array of replicas, wherein the optical component has a primary surface 1202 arranged to direct a primary reflection of the array of replicas towards the viewing plane to form a primary array of composite holographic wavefronts at a viewing plane and a secondary surface 1204 arranged to direct a secondary reflection of the array of replicas towards the viewing plane to form a secondary array of ghost composite holographic wavefronts at the viewing plane. The hologram replicator and optical combiner are arranged such that primary and secondary arrays form a combined array in which the composite holographic wavefronts (1650, 1651, fig.16A) and the ghost holographic wavefronts (1650’, 1651’, fig.16A) are substantially interleaved. A corresponding method of holographic projection is also disclosed.

Description

FIELD The present disclosure relates to a display system and a method of holographic projection. More specifically, the present disclosure relates a display system comprising an optical combiner such as a windscreen or windshield of a vehicle. Even more specifically, the present disclosure relates to a display system comprising a wedge-shaped optical combiner. 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 ray tracing or a point cloud technique, for example. A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micro-mirrors, 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, aspects of the present disclosure relate to display systems comprising an optical component (such as an optical combiner such as a windscreen, or windshield, of a vehicle) comprising primary and secondary (opposing) surfaces. The display system may be arranged such that display light is relayed by the optical component to a viewing plane. A (projected) image may be viewable at the viewing plane. The display system according to aspects of the present disclosure is arranged such that ghost artefacts formed as a result of the dual-surface nature of the optical combiner are minimised, mitigated, or utilised, as will be explained in more detail herein. The inventors have found that, when an optical component comprising primary and secondary (opposing) surfaces is used to relay the display light to the viewing plane, primary and secondary reflections of the display light may be formed. The primary reflection of the display light may be directed or reflected by the primary surface to be relayed towards the viewing plane. The secondary reflection of the display light may be directed or reflected by the secondary surface to be relayed towards the viewing plane. Display light incident on the primary surface may be divided into reflected light (forming the primary reflection) and transmitted light (at least a portion of which may go on to form the secondary reflection). The transmitted light (transmitted by the primary surface) may propagate to the secondary surface of the optical combiner. At least a portion of the transmitted light may be reflected by the secondary surface, forming the secondary reflection. The light forming the secondary reflection may then be propagated back through the primary surface and on to the viewing plane. Because the display light may be divided by the primary surface, the display light may effectively be replicated at the primary surface. In other words, the light forming the secondary reflection may be a copy or replica of the light forming the primary reflection (because of the initial division at the primary surface). The display light forming the primary reflection may follow a first optical path (or first plurality of optical paths) to the viewing plane that is different to a second optical path (or second plurality of optical paths) followed by light of the secondary reflection to the viewing plane. For example, the first optical path or paths may be shorter than the corresponding second optical path or paths. If a pupil of a user or aperture of a viewing system in the viewing plane receives light from both the primary and second reflection, then ghost artefacts may be visible to the user / viewing system. This may be because of the differences in optical paths followed by light of the primary and secondary reflections. For example, the light of the primary reflection (received by the user / viewing system at the viewing plane) may be said to form a primary or main image and the light of the secondary reflection (received by the user I viewing system) may be said to form a secondary or ghost image. The main image may be offset from the ghost image (because the respective light footprints are offset from one another). The skilled reader will appreciate how the presence of the (offset) ghost image may cause the appearance of blurring of the primary image for a user in the viewing plane or otherwise adversely affect the viewing experience for the user. In embodiments, the display system further comprises a hologram replicator arranged to output an array of replicas of spatially modulated light (such as replicas of a holographic wavefront) and the optical combiner is arranged to direct a reflection of the array of replicas towards a viewing plane of the display system. As explained in more detail herein, the hologram replicator being arranged to form the array of replicas may have the effect of expanding a pupil of the display system. In embodiments of the display system comprising a hologram replicator, both of the primary and secondary reflections may correspond to or comprise the array of replicas. In other words, the optical component may be arranged to direct a primary reflection of the array of replicas (formed by the hologram replicator) towards the viewing plane and direct a secondary reflection of the array of replicas (formed by the hologram replicator) towards the viewing plane. The primary and second reflections of the array of replicas may effectively form first and second copies of the replicas. In other words, two arrays of (substantially identical) replicas may be received at the viewing plane. The two arrays of replicas may be offset from one another, for the reasons outlined above. The two arrays of replicas may, respectively, form a first array of composite holographic wavefronts and a second array of ghost composite holographic wavefronts. The inventors have found that, when wavefronts of the second array overlap with a primary composite holographic wavefront of the first array at an aperture (such as the entrance aperture of a viewing system or a pupil of an eye), a so-called ghost or ghost image may be formed. The so-called ghost image may be offset from a primary image (which may be the image that it was intended for the user to receive). The applicant has described in previous applications other mechanisms by which so-called ghosts or ghost artefacts may be formed - and mitigation of said ghosts. For example, the description of UK patent GB2603517 described the presence of ghost images being formed owing to the different possible light propagation paths through a waveguide (hologram replicator). Unless stated otherwise, the present disclosure relates to a different source of ghosts. In particular, as above, the present disclosure relates to so-called ghost images or ghost artefacts formed as a result of a dual-surfaced optical component / combiner forming both primary and secondary reflections (by primary and second surfaces of the optical component, respectively) of an array of replicas formed by the hologram replicator. 5 The display light is a holographic wavefront, that is, the holographic wavefront corresponds to light that has been spatially modulated in accordance with a hologram of an image. In some embodiments, the hologram may be a so-called channelling hologram. In other words, angular ranges of the holographic wavefront may correspond to (or encode) different respective parts of the image. Thus, it may be said that angles of the holographic wavefront correspond to spatial 10 coordinates of an image corresponding to or encoded in the holographic wavefront. There may be an angular dependence between angles of the holographic wavefront and spatial coordinates of the image correspond to the holographic wavefront. There is disclosed an unclaimed display system having a viewing plane from which an image is 15 visible. The display system comprises a hologram replicator or pupil expander and an optical component. The hologram replicator or pupil expander may be a waveguide. The hologram replicator is arranged to output an array of replicas of a holographic wavefront. The plurality of replicas extend in a first dimension. Angles of the holographic wavefront correspond to spatial coordinates (e.g. in the first dimension) of an image corresponding to (or encoded in) the 20 holographic wavefront. The angles may be light ray angles on a plane containing the first dimension and a second dimension corresponding to the propagation axis, wherein the first dimension and second dimension are perpendicular. The optical component may be an optical combiner such as a vehicle windscreen. The optical component may have curvature. The optical component is arranged to receive the array of replicas. The optical component has a 25 primary surface arranged to direct a primary reflection of the array of replicas towards the viewing plane to form a primary composite holographic wavefront at the viewing plane. The optical component has a secondary surface arranged to direct a secondary reflection of the array of replicas towards the viewing plane to form a ghost composite holographic wavefront at the viewing plane. An angle and / or distance between the primary surface and secondary surface are 30 arranged or optimised to: 1 / overlap the primary composite holographic wavefront and the ghost composite holographic wavefront at the viewing plane; and 2 / parallelise the corresponding angular components of the primary composite holographic wavefront and the ghost composite holographic wavefront arriving at the viewing plane. More specifically, parallelise (i.e. make parallel) the corresponding angular components of the primary and ghost passing through a pupil 35 of a viewer or viewing system at the viewing plane. Notably, corresponding angles of primary and ghost composite wavefronts are substantially parallel at the viewing plane. More specifically, angular components of the primary and ghost composite wavefront corresponding to the same respective spatial coordinates of the image make substantially the same angle at the viewing plane. The second condition of the first aspect is peculiar to the type of hologram disclosed herein and is a result of the image content of the picture in the spatial domain being encoded by angle in the hologram domain. Accordingly, the inventors found that if the angles in the hologram domain of corresponding picture content were not properly matched (i.e. substantially the same), a main image and a ghost image could not be positively reinforced by superposition. In some cases, it is not possible to satisfy the first condition of the first aspect without an angle between the first surface and second surface of the optical component. The technical problems addressed by the inventors in this disclosure are exasperated when the optical component has curvature such as curvature in two orthogonal directions. It will be understood from the detailed description below that a composite holographic wavefront is a holographic wavefront formed by the combination or superposition of a plurality of holographic wavefronts or portions thereof. A composite holographic wavefront appears as a light spot at the viewing plane. Each composite holographic wavefront in accordance with this disclosure comprises at least a first (holographic wavefront) component received from a first replica formed by the hologram replicator / pupil expander and a second (holographic wavefront) component received from a second replica form by the hologram replicator I pupil expander. In some embodiments, a plurality of different holographic wavefront replicas formed by the hologram replicator / pupil expander contribute to each composite holographic wavefront. Each replica may contribute different angular content and therefore different parts of the picture owing to the way the hologram divides the picture content in the spatial domain by angle in the hologram domain. That is, each replica contributing to a composite holographic wavefront may contribute a different / respective angular component of the picture. In accordance with the present disclosure, an array of composite holographic wavefronts is formed at the viewing plane. Not all of these composite holographic wavefronts pass through a pupil of the viewer but, in some embodiments, a plurality of composite holographic wavefronts pass through the pupil of the viewer and contribute to the perceived image. The presence of composite holographic wavefronts at the viewer can cause the appearance of ghost images of the picture which degrade the viewing experience. In some embodiments, the second composite holographic wavefront is a so-called “ghost” formed by a reflection from a secondary surface of an optical component of the system. The first / primary composite holographic wavefront may be formed via a reflection from a primary surface of the optical component. In accordance with this disclosure, however, positive image reinforcement between a first / primary composite holographic wavefront and a second / secondary composite holographic wavefront is achieved. In overview, positive reinforcement is achieved by matching both the spatial position and the angular composition of the two composite wavefronts at the pupil of the viewer by optimising an angle and spacing between the first and second surfaces of the optical component. In some embodiments, the ghost composite wavefront is a ghost of a secondary composite wavefront formed by the first surface not a ghost of the primary composite wavefront. This is analogous to combining the nth replica of a primary reflection with the (n+1)th replica of a ghost / secondary reflection. In other words, the first composite wavefront is the nth composite wavefront of a first array of replicas of the holographic wavefront and the second composite wavefront is the (n+1)th composite wavefront of the second array of replicas of the holographic wavefront. In some embodiments, the primary composite holographic wavefront is an nth composite holographic wavefront formed by the primary reflection; and the ghost composite holographic wavefront is an (n+1th) ghost composite holographic wavefront formed by the secondary reflection. In some embodiments, the display system (e.g. the hologram replicator and I or holographic wavefront) is arranged such that the primary reflection forms a first array of composite holographic wavefronts. The first array may comprise the primary composite holographic wavefront and a plurality of secondary composite holographic wavefronts. In some embodiments, the display system (e.g, the hologram replicator and / or holographic wavefront) is arranged such that the secondary reflection forms a second array of composite holographic wavefronts. The second array may comprise a ghost of the primary composite holographic wavefront and a ghost of each of the secondary composite holographic wavefronts of the first array. In some embodiments, angular ranges of the holographic wavefront correspond to different respective parts of the image. In some embodiments, a first portion of the optical component comprises a (or the) portion of the primary surface that is arranged to direct the primary reflection and a (or the) portion of the secondary surface that is arranged to direct the secondary reflection. Said portions of the primary and second surface may substantially oppose one another. A cross-section of at least the first portion of the optical component may be substantially wedge-shaped. In some embodiments, the cross-section may be in a first plane that is perpendicular to a plane comprising two opposing edges of the optical component. Alternatively or additionally, the first plane may be perpendicular to a plane comprising or containing a tangent of the primary or second surface at the first portion. In some embodiments, the optical component comprises a distal or top edge or end and a proximal or bottom edge or end. A thickness (between the primary surface and the secondary surface) of at least the first portion of the optical component may increase from the distal edge to the proximal edge. Herein, such a wedge may be referred to as a “negative wedge”. It is very unconventional for optical components, in particular windscreens or windshields of vehicles, to comprise a so-called negative wedge. However, after thorough simulation and experimentation, the inventors have found that the wedge may preferably be negative to achieve the spatial overlap and parallisation described above as a general rule. In particular, all display systems in vehicles such as cars that were modelled by the inventors used a negative wedge (in combination with an increased thickness, as described below) to achieve the spatial overlap and parallisation. In embodiments, the proximal edge is closer to the hologram replicator (and / or a dashboard of a vehicle comprising the display system) than the distal edge. In some embodiments, the proximal edge of the optical component is larger than the distal edge of the optical component. This may be the case if the optical component is a windshield or windscreen for a vehicle such as a car. In some embodiments, the thickness (i.e. the distance between the primary surface and the secondary surface of the first portion pf the optical component) increases substantially linearly from the distal edge to the proximal edge. In some embodiments, an angle between the primary surface and the secondary surface of the first portion is substantially constant. In some embodiments, a thickness of the first portion of the optical component between the primary and second surfaces is between 7 millimetres and 10 millimetres. In the other words, the thickness of the first portion of the optical component may be greater than the thickness of a conventional windshield / windscreen of a vehicle such as a car. Herein, the thickness of the first portion of the optical combiner may refer to an average thickness of the first portion, a maximum thickness, or a minimum thickness. Generally, the thickness varies very slowly in the first portion so it does not matter which thickness is taken. The important point may be that the thickness is greater than a conventional windshield / windscreen. After thorough simulation and experimentation, the inventors have found that a thickness of between 7 millimetres and 10 millimetres is generally the thickness required to achieve the spatial alignment and parralisation described above in substantially any arrangement of a display system in a vehicle such as a car. The inventors have found that a thickness of between 7.5 millimetres and 9 millimetres may be preferred. The inventors have found that a thickness of between 8 and 8.5 millimetres may be even more preferred. In some embodiment herein the optical component is curved such that at least one of, optionally both of, the primary and secondary surfaces (of the optical component) are curved. The primary and secondary surfaces may be concave curve shaped. This may mean that the primary reflection and secondary reflection are directed by I reflected by concave shaped surfaces towards the viewing plane. In some embodiments, the optical component is arranged such that the primary surface and second surface oppose one another. In some embodiments, the optical component comprises a first layer comprising the primary surface, a second layer comprising the secondary surface, and a third layer between the first and second layer. The third layer may be sandwiched between the first and second layers. The third layer may be in contact with the first and second layers. The first, second and third layers may be laminated. In some embodiments, the third layer may be substantially wedge-shaped. In some embodiments, the first and second layers have a substantially constant thickness such that a distance between the first and second surfaces varies with distance in the second dimension as a result of the wedge-shaped third layer. In some embodiments, the first layer and the second layer comprise glass. In some embodiments, the third layer comprises a polymer such as polyvinyl butyral (PVB). In some embodiments, the optical component is arranged such that at least one of, optionally both, of the primary surface and secondary surface defines an interface between the optical component and air. In some embodiments, the primary surface is arranged to receive the array of replicas of the holographic wavefront from the hologram replicator and to partially-reflect / partially-transmit the received array of replicas. The partial reflection may form the primary reflection. The partial transmission may be received by the secondary surface of the optical component. The secondary surface may be arranged to at least partially reflect the received partial transmission from the primary surface to form the secondary reflection. In some embodiments, the optical component is an optical combiner such as windshield (or windscreen) of a vehicle. In some embodiments, the display system comprises a head-up display system comprising the 5 hologram replicator. In some embodiments, the hologram replicator comprises a waveguide comprising an input port arranged to receive a holographic wavefront. The waveguide maya comprise a pair of surfaces arranged to waveguide light therebetween. A first surface of the pair of surfaces may be partially transmissive-reflective such that the holographic wavefront is divided at each internal reflection to output an array of replicas. There is provided an unclaimed display system having a viewing plane from which an image is visible. The display system comprises a hologram replicator arranged to output an array of replicas of a holographic wavefront. Angles of the holographic wavefront correspond to spatial coordinates of an image corresponding to the holographic wavefront. The display system further 15 comprises an optical component arranged to receive the array of replicas. The optical component has a primary surface arranged to direct a primary reflection of the array of replicas towards the viewing plane to form a primary composite holographic wavefront at the viewing plane and a secondary surface arranged to direct a secondary reflection of the array of replicas towards the viewing plane to form a ghost composite holographic wavefront at the viewing plane. A thickness 20 of at least a portion of the optical component increases from a distal edge of the optical component to a proximal edge of the optical component, the proximal edge being closest to the hologram replicator. There is provided an unclaimed display system having a viewing plane from which an image is 25 visible. The display system comprises a picture generating unit arranged to output display light and an optical component (such as an optical combiner, such as the windscreen or windshield of a vehicle). The optical component has a primary surface arranged to direct a primary reflection of the display light towards the viewing plane. The optical component further comprises a secondary surface arranged to direct a secondary reflection of the display light towards the 30 viewing plane. A thickness of at least a portion of the optical component increases from a distal edge of the optical component to a proximal edge of the optical component, the proximal edge being closest to the picture generating unit. In embodiments, the proximal edge is a bottom edge and the distal edge is a top edge. 35 There is provided an unclaimed method of holographic projection. The method comprises receiving an array of replicas of a holographic wavefront at an optical component. Angles of the holographic wavefront correspond to spatial coordinates of an image corresponding to the holographic wavefront. The method further comprises directing, using a primary surface of the optical component, a primary reflection of the array of replicas towards a viewing plane. The method further comprises directing, using a secondary surface of the optical component, a secondary reflection of the array of replicas towards the viewing plane. An angle and a distance between the primary surface and secondary surface are arranged to: 1 / overlap the (n +1)th replica of the secondary reflection with the nth replica of the primary reflection at the viewing 5 plane; and 2 / parallelise the propagation axis of the (n+1)th replica of the secondary reflection with the propagation axis of the nth replica of the primary reflection. An aspect of the present disclosure is a display system having a viewing plane from which an image is visible. The display system comprises a hologram replicator and an optical component. 10 The hologram replicator is arranged to output an array of replicas of a holographic wavefront, wherein angles of the holographic wavefront correspond to spatial coordinates of an image corresponding to the holographic wavefront. The optical component is arranged to receive the array of replicas. The optical component has a primary surface arranged to direct a primary reflection of the array of replicas towards the viewing plane to form a primary array of composite 15 holographic wavefronts at the viewing plane and a secondary surface arranged to direct a secondary reflection of the array of replicas towards the viewing plane to form a secondary array of ghost composite holographic wavefronts at the viewing plane. The hologram replicator and optical combiner are arranged such that primary and secondary arrays form a combined array in which the composite holographic wavefronts and the ghost holographic wavefronts are 20 interleaved such that each ghost holographic wavefront is positioned in a space between adjacent composite holographic wavefronts. The hologram replicator and optical combiner may be arranged such that adjacent composite holographic wavefronts of the first array are non-overlapping at the viewing plane. 25 Each composite holographic wavefront of the first array may have the same shape and, optionally, size. The hologram replicator and optical combiner may be arranged such a pitch the composite 30 holographic wavefronts of the first array is equal to, or greater than, 1.5 times a width or a diameter of at least one of the composite holographic wavefronts. The hologram replicator and optical combiner may be arranged such adjacent composite holographic wavefronts and ghost composite holographic wavefronts are non-overlapping. 35 Adjacent replicas of the combined array of composite holographic wavefronts may be adjoined. The primary array of composite holographic wavefronts may comprise a primary composite holographic wavefront and a plurality of secondary composite holographic wavefronts. The second array of composite holographic wavefronts may comprise a ghost of the primary 5 composite holographic wavefront and a ghost of each secondary composite holographic wavefront of the first array. Athickness of at least a portion of the optical component may decrease from a distal edge ofthe optical component to a proximal edge ofthe optical component, the proximal edge being closest 10 to the hologram replicator. A first portion ofthe optical component may comprises: a portion ofthe primary surface that is arranged to direct the primary reflection; and a portion ofthe secondary surface that is arranged to direct the secondary reflection, wherein said portions ofthe primary and second surface 15 oppose one another. A cross-section of at least the first portion ofthe optical component may be wedge-shaped. An angle between the primary surface and the secondary surface ofthe first portion may be 20 constant for at least the first portion ofthe optical combiner. The primary surface may be arranged to receive the array of replicas ofthe holographic wavefront from the hologram replicator and to partially-reflect / partially-transmit the received array of replicas, the partial reflection forming the primary reflection, the partial transmission 25 being received by the secondary surface ofthe optical component. The secondary surface may be arranged to at least partially reflect the received partial transmission from the primary surface to form the secondary reflection. 30 The optical component is an optical combiner such as windshield of a vehicle. The display system may comprise a head-up display system comprising the hologram replicator. The hologram replicator may comprise a waveguide comprising an input port arranged to receive 35 a holographic wavefront, wherein the waveguide comprises a pair of surfaces arranged to waveguide light therebetween, a first surface ofthe pair of surfaces being partially transmissive-reflective such that the holographic wavefront is divided at each internal reflection to output an array of replicas. Another aspect is a method of holographic projection. The method comprising receiving an array of replicas of a holographic wavefront at an optical component, wherein angles of the holographic wavefront correspond to spatial coordinates of an image corresponding to the holographic wavefront. The method further comprises directing, using a primary surface of the optical 5 component, a primary reflection of the array of replicas towards a viewing plane to form a primary array of composite holographic wavefronts at the viewing plane. The method comprises directing, using a secondary surface of the optical component, a secondary reflection of the array of replicas towards the viewing plane to form a secondary array of ghost composite holographic wavefronts at the viewing plane. The hologram replicator and optical combiner are arranged 10 such that primary and secondary arrays form a combined array in which the composite holographic wavefronts and the ghost holographic wavefronts are interleaved such that each ghost holographic wavefront is positioned in a space between adjacent composite holographic wavefronts. 15 There is also disclosed an unclaimed light engine arranged to provide a viewing system, having an entrance pupil, with spatially modulated light. The display system comprises a display device, an optical component and a hologram engine. The display device is arranged to display a hologram and spatially modulate light in accordance with the hologram. The optical component comprises a primary surface and a secondary surface. The hologram engine is arranged to 20 receive contribution information identifying contributory and non-contributory areas of the display device based on the location of the entrance pupil, wherein the contributory areas of the display device substantially propagate spatially modulated light passing through the entrance pupil at the location and non-contributory areas of the display device substantially propagate spatially modulated light stopped by the entrance pupil at the location. The contribution information 25 further identifies (i) at least one primary contributory area of the display device propagating light to the viewing system that contributes to a primary image using the primary surface of the optical component and (ii) at least one secondary contributory area of the display device propagating light to the viewing system that contributes to a secondary image. Notably, the secondary image is formed by a ghost reflection of the spatially modulated light from the secondary surface of the 30 optical component. The hologram engine is further arranged to determine the hologram based on only the at least one primary contributory area of the display device and to output the hologram to the display device for display. The primary contributory area is cropped to exclude an overlap area causing a ghost of the 35 composite holographic wavefront that does not satisfy the criteria for positive image reinforcement. That is, even though the primary and secondary composite holographic wavefronts do not overlap, at least one ghost composite holographic wavefront may overlap. If the overlapping ghost has not been optimised in accordance with the present disclosure using parameters of the optical combiner, such overlap may cause a ghost image rather than positive reinforcement. The primary contributory area is therefore cropped to remove the area causing the problematic ghost composite holographic wavefront and thereby improving the viewing experience. The primary surface and secondary surface may be non-parallel and, optionally, curved. The primary image may correspond to a primary composite holographic wavefront formed at the entrance pupil and the secondary image corresponds to a ghost composite holographic wavefront formed at the entrance pupil. The ghost composite holographic wavefront may correspond to a ghost image of a secondary composite holographic wavefront formed using the primary surface. At least one of an angle and spacing between the primary surface and secondary surface is arranged, such as optimized, to overlap the primary composite holographic wavefront and the ghost composite holographic wavefront at the entrance pupil. Notably, at least one of an angle and spacing between the primary surface and secondary surface may be arranged, such as optimized, to parallelise the corresponding angular components of the primary composite holographic wavefront and the ghost composite holographic wavefront arriving at the viewing plane. A first array of composite holographic wavefronts may be formed at the entrance pupil using the primary surface of the optical component and a second array of composite holographic wavefronts may be ghost composite holographic wavefronts formed at the entrance pupil using the secondary surface of the optical component. The first array of composite holographic wavefronts may comprise the primary composite holographic wavefront and a plurality of secondary composite holographic wavefronts. The light engine may further comprise a hologram replicator arranged to receive the spatially modulated light and form a plurality of spatially separated replicas thereof. Each composite holographic wavefront may comprise a contribution from a plurality of replicas of the holographic wavefront formed by the hologram replicator. Hologram pixel values of the hologram outside the at least one primary contributory area may not determined or are nullified. The at least one primary contributory area may have a cropped circular shape such as a dumbbell shape. The light engine may further comprise a monitoring system arranged to determine the location of the entrance pupil of the viewing system. The contribution information may identify respective contributory and non-contributory areas of the display device for each of a plurality of image points of the image. In the present disclosure, the term “replica” is merely used to reflect that spatially modulated light is divided such that a complex light field is directed along a plurality of different optical paths. The word “replica” is used to refer to each occurrence or instance of the complex light field after a replication event - such as a partial reflection-transmission by a pupil expander. Each replica travels along a different optical path. Some embodiments of the present disclosure relate to propagation of light that is encoded with a hologram, not an image - i.e., light that is spatially modulated with a hologram of an image, not the image itself. It may therefore be said that a plurality of replicas of the hologram are formed. The person skilled in the art of holography will appreciate that the complex light field associated with propagation of light encoded with a hologram will change with propagation distance. Use herein of the term “replica” is independent of propagation distance and so the two branches or paths of light associated with a replication event are still referred to as “replicas” of each other even if the branches are a different length, such that the complex light field has evolved differently along each path. That is, two complex light fields are still considered “replicas” in accordance with this disclosure even if they are associated with different propagation distances - providing they have arisen from the same replication event or series of replication events. A “diffracted light field” or “diffractive light field” in accordance with this disclosure is a light field formed by diffraction. A diffracted light field may be formed by illuminating a corresponding diffractive pattern. In accordance with this disclosure, an example of a diffractive pattern is a hologram and an example of a diffracted light field is a holographic light field or a light field forming a holographic reconstruction of an image. The holographic light field forms a (holographic) reconstruction of an image on a replay plane. The holographic light field that propagates from the hologram to the replay plane may be said to comprise light encoded with the hologram or light in the hologram domain. A diffracted light field is characterized by a diffraction angle determined by the smallest feature size of the diffractive structure and the wavelength of the light (of the diffracted light field). In accordance with this disclosure, it may also be said that a “diffracted light field” is a light field that forms a reconstruction on a plane spatially separated from the corresponding diffractive structure. An optical system is disclosed herein for propagating a diffracted light field from a diffractive structure to a viewer. The diffracted light field may form an image. The term “hologram” is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, regarding the object. The term “holographic reconstruction” is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The system disclosed herein is described as a “holographic projector” because the holographic reconstruction is a real image and spatially-separated from the hologram. The term “replay field” is used to refer to the 2D area within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of a plurality diffracted orders wherein each diffracted order is a replica of the zeroth-order replay field. The zeroth-order replay field generally corresponds to the preferred or primary replay field because it is the brightest replay field. Unless explicitly stated otherwise, the term “replay field” should be taken as referring to the zeroth-order replay field. The term “replay plane” is used to refer to the plane in space containing all the replay fields. The terms “image”, “replay image” and “image region” refer to areas of the replay field illuminated by light of the holographic reconstruction. In some embodiments, the “image” may comprise discrete spots which may be referred to as “image spots” or, for convenience only, “image pixels”. The terms “encoding”, “writing” or “addressing” are used to describe the process of providing the plurality of pixels of the SLM with a respective plurality of control values which respectively determine the modulation level of each pixel. It may be said that the pixels of the SLM are configured to “display” a light modulation distribution in response to receiving the plurality of control values. Thus, the SLM may be said to “display” a hologram and the hologram may be considered an array of light modulation values or levels. It has been found that a holographic reconstruction of acceptable quality can be formed from a “hologram” containing only phase information related to the Fourier transform of the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography. The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully-complex computer-generated hologram is calculated. Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for “phase-delay”. That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2tt) which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of tt / 2 will retard the phase of received light by tt / 2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term “grey level” may be used to refer to the plurality of available modulation levels. For example, the term “grey level” may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term “grey level” may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator. The hologram therefore comprises an array of grey levels - that is, an array of light modulation values such as an array of phase-delay values or complex modulation values. The hologram is also considered a diffractive pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light having a wavelength comparable to, generally less than, the pixel pitch of the spatial light modulator. Reference is made herein to combining the hologram with other diffractive patterns such as diffractive patterns functioning as a lens or grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the replay field on the replay plane or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a replay plane in the near field. Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged. BRIEF DESCRIPTION OF THE DRAWINGS Specific embodiments are described by way of example only with reference to the following figures: Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen; Figure 2 shows an image for projection comprising eight image areas / components, V1 to V8, and cross-sections of the corresponding hologram channels, H1-H8; Figure 3 shows a hologram displayed on an LCOS that directs light into a plurality of discrete areas; Figure 4 shows a system, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3; Figure 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 shows a prior art holographic display system comprising a conventional optical combiner; Figure 7 shows the light received at a viewing plane that forms a primary composite holographic wavefront and a ghost of the primary composite holographic wavefront; Figure 8 shows the system of Figure 4 but does not show the eye / pupil, instead Figure 8 schematically shows how a plurality of composite holographic wavefronts are formed by the system; Figure 9A shows the light received at a viewing plane that forms a primary composite holographic wavefront and first and second secondary composite holographic wavefronts; Figure 9B shows the light received by a pupil at the viewing plane of Figure 9A, the pupil being aligned with the primary composite holographic wavefront and at the viewing plane; Figure 10A shows the light received at a viewing plane that forms a primary composite holographic wavefront and first and second secondary composite holographic wavefronts as well as the light received at the viewing plane that forms respective ghosts of said composite wavefronts; Figure 10B shows the light received by a pupil being aligned with the primary composite holographic wavefront and at the viewing plane of Figure 10A; Figure 11 shows a representation of a ghost artefact; Figure 12 shows a schematic view of a first display system according to the present disclosure, the first display system comprising a negative wedge shaped optical combiner; Figure 13 shows a schematic view a pupil receiving non-parallel light from a primary composite holographic wavefront and a ghost composite holographic wavefront which may result in a ghost artefact; Figure 14 shows the light received at a viewing plane that forms a primary composite holographic wavefront and first and second secondary composite holographic wavefronts as well as the light received at the viewing plane that forms respective ghosts of said composite wavefronts in the context of the system of Figure 12; Figure 15A shows the light received at a viewing plane that forms a primary composite holographic wavefront and first and second secondary composite holographic wavefronts as well as the light received at the viewing plane that forms respective ghosts of said composite wavefronts in the context of a second display system according to the present disclosure; Figure 15B shows the light received by a pupil being aligned with the primary composite holographic wavefront and at the viewing plane of Figure 15A; Figure 16A shows the light received at a viewing plane that forms a primary composite holographic wavefront and first and second secondary composite holographic wavefronts as well as the light received at the viewing plane that forms respective ghosts of said composite wavefronts in the context of a third display system according to the present disclosure; Figure 16B shows the light received by a pupil being aligned with the primary composite holographic wavefront and at the viewing plane of Figure 16A; Figure 17 shows the individual optical paths for each of five ray bundles that contribute to five respective image points within a virtual image; Figure 18 shows an example of the pixel area of the display device in which a first area does not contribute to any image points that reach the viewer, a second area contributes to the main image points and a third area of the display device contributes only to ghost image points; and Figure 19 shows an embodiment in which the primary contributory area has been cropped to exclude an area of the display device that forms a secondary composite holographic wavefront that does not itself overlap the primary composite holographic wavefront at the viewing plane but a ghost thereof does. 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 co-dependent 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 frequency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, “LCOS”, device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser. 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 light-modulating 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 precalculated, 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 eyebox.) 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 noninfinite 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 subarea of the spatially modulated light formed by the hologram is reconstructed, only a sub-area of the image should be visible. If a different, continuous part or sub-area of the spatially modulated light is reconstructed, a different sub-area of the image should be visible. A further identifying feature of this type of hologram is that the shape of the cross-sectional area of any hologram channel substantially corresponds to (i.e. is substantially the same as) the shape of the entrance pupil although the size may be different - at least, at the correct plane for which the hologram was calculated. Each light / hologram channel propagates from the hologram at a different angle or range of angles. Whilst these are example ways of characterising or identifying this type of hologram, other ways may be used. In summary, the hologram disclosed herein is characterised and identifiable by how the image content is distributed within light encoded by the hologram. Again, for the avoidance of any doubt, reference herein to a hologram configured to direct light or angularly-divide an image into a plurality of hologram channels is made by way of example only and the present disclosure is equally applicable to pupil expansion of any type of holographic light field or even any type of diffractive or diffracted light field. The system can be provided in a compact and streamlined physical form. This enables the system to be suitable for a broad range of real-world applications, including those for which space is limited and real-estate value is high. For example, it may be implemented in a head-up display (HUD) such as a vehicle or automotive HUD. In accordance with the present disclosure, pupil expansion is provided for diffracted or diffractive light, which may comprise diverging ray bundles. The diffracted light field may be defined by a “light cone”. Thus, the size of the diffracted light field (as defined on a two-dimensional plane) increases with propagation distance from the corresponding diffractive structure (i.e. display device). It can be said that the pupil expander / s replicate the hologram or form at least one replica of the hologram, to convey that the light delivered to the viewer is spatially modulated in accordance with a hologram. In some embodiments, two one-dimensional waveguide pupil expanders are provided, each onedimensional 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 (l.e., hologram) that can reconstruct the image 252 - e.g., when transformed by the lens of a suitable viewing system. The encoded light pattern 254 comprises first to eighth sub-holograms or components, H1 to H8, corresponding to the first to eighth image components / areas, V1 to V8. Figure 2 further shows how a hologram may decompose the image content by angle. The hologram may therefore be characterised by the channelling of light that it performs. This is illustrated in Figure 3. Specifically, the hologram in this example directs light into a plurality of discrete areas. The discrete areas are discs in the example shown but other shapes are envisaged. The size and shape of the optimum disc may, after propagation through the waveguide, be related to the size and shape of an aperture of the optical system such as the entrance pupil of the viewing system. Figure 4 shows a system 400, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3. The system 400 comprises a display device, which in this arrangement comprises an LCOS 402. The LCOS 402 is arranged to display a modulation pattern (or ‘diffractive pattern') comprising the hologram and to project light that has been holographically encoded towards an eye 405 that comprises a pupil that acts as an aperture 404, a lens 409, and a retina (not shown) that acts as a viewing plane. There is a light source (not shown) arranged to illuminate the LCOS 402. The lens 409 of the eye 405 performs a hologram-to-image transformation. The light source may be of any suitable type. For example, it may comprise a laser light source. The viewing system 400 further comprises a waveguide 408 positioned between the LCOS 402 and the eye 405. The presence of the waveguide 408 enables all angular content from the LCOS 402 to be received by the eye, even at the relatively large projection distance shown. This is because the waveguide 508 acts as a pupil expander, in a manner that is well known and so is described only briefly herein. In brief, the waveguide 408 shown in Figure 4 comprises a substantially elongate formation. In this example, the waveguide 408 comprises an optical slab of refractive material, but other types of waveguide are also well known and may be used. The waveguide 408 is located so as to intersect the light cone (i.e., the diffracted light field) that is projected from the LCOS 402, for example at an oblique angle. In this example, the size, location, and position of the waveguide 408 are configured to ensure that light from each of the eight ray bundles, within the light cone, enters the waveguide 408. Light from the light cone enters the waveguide 408 via its first planar surface (located nearest the LCOS 402) and is guided at least partially along the length of the waveguide 408, before being emitted via its second planar surface, substantially opposite the first surface (located nearest the eye). As will be well understood, the second planar surface is partially reflective, partially transmissive. In other words, when each ray of light travels within the waveguide 408 from the first planar surface and hits the second planar surface, some of the light will be transmitted out of the waveguide 408 and some will be reflected by the second planar surface, back towards the first planar surface. The first planar surface is reflective, such that all light that hits it, from within the waveguide 408, will be reflected back towards the second planar surface. Therefore, some of the light may simply be refracted between the two planar surfaces of the waveguide 408 before being transmitted, whilst other light may be reflected, and thus may undergo one or more reflections, (or ‘bounces’) between the planar surfaces of the waveguide 408, before being transmitted. Figure 4 shows a total of nine “bounce” points, B0 to B8, along the length of the waveguide 408. Although light relating to all points of the image (V1-V8) as shown in Figure 2 is transmitted out of the waveguide at each “bounce” from the second planar surface of the waveguide 408, only the light from one angular part of the image (e.g. light of one of V1 to V8) has a trajectory that enables it to reach the eye 405, from each respective “bounce” point, B0 to B8. Moreover, light from a different angular part of the image, V1 to V8, reaches the eye 405 from each respective “bounce” point. Therefore, each angular channel of encoded light reaches the eye only once, from the waveguide 408, in the example of Figure 4. The waveguide 408 forms a plurality of replicas of the hologram, at the respective “bounce” points B1 to B8 along its length, corresponding to the direction of pupil expansion. As shown in Figure 4, the plurality of replicas may be extrapolated back, in a straight line, to a corresponding plurality of replica or virtual display devices 402’. This process corresponds to the step of 5 “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’. 10 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. 15 20 25 30 35 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 I diverging, the light field size increases with propagation distance. In some embodiments, the diffracted light field is spatially-modulated in accordance with a hologram. In other words, in such aspects, the diffractive light field comprises a “holographic light field”. The hologram may be displayed on a pixelated display device. The hologram may be a computer-generated hologram (CGH). It may be a Fourier hologram or a Fresnel hologram or a point-cloud hologram or any other suitable type of hologram. The hologram may, optionally, be calculated so as to form channels of hologram light, with each channel corresponding to a different respective portion of an image that is intended to be viewed (or perceived, if it is a virtual image) by the viewer. The pixelated display device may be configured to display a plurality of different holograms, in succession or in sequence. Each of the aspects and embodiments disclosed herein may be applied to the display of multiple holograms. The output port of the first waveguide pupil expander may be coupled to an input port of a second waveguide pupil expander. The second waveguide pupil expander may be arranged to guide the diffracted light field - including some of, preferably most of, preferably all of, the replicas of the light field that are output by the first waveguide pupil expander - from its input port to a respective output port by internal reflection between a third pair of parallel surfaces of the second waveguide pupil expander. The first waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a first direction and the second waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a second, different direction. The second direction may be substantially orthogonal to the first direction. The second waveguide pupil expander may be arranged to preserve the pupil expansion that the first waveguide pupil expander has provided in the first direction and to expand (or, replicate) some of, preferably most of, preferably all of, the replicas that it receives from the first waveguide pupil expander in the second, different direction. The second waveguide pupil expander may be arranged to receive the light field directly or indirectly from the first waveguide pupil expander. One or more other elements may be provided along the propagation path of the light field between the first and second waveguide pupil expanders. The first waveguide pupil expander may be substantially elongated and the second waveguide pupil expander may be substantially planar. The elongated shape of the first waveguide pupil expander may be defined by a length along a first dimension. The planar, or rectangular, shape of the second waveguide pupil expander may be defined by a length along a first dimension and a width, or breadth, along a second dimension substantially orthogonal to the first dimension. A size, or length, of the first waveguide pupil expander along its first dimension make correspond to the length or width of the second waveguide pupil expander along its first or second dimension, respectively. A first surface of the pair of parallel surfaces of the second waveguide pupil expander, which comprises its input port, may be shaped, sized, and / or located so as to correspond to an area defined by the output port on the first surface of the pair of parallel surfaces on the first waveguide pupil expander, such that the second waveguide pupil expander is arranged to receive each of the replicas output by the first waveguide pupil expander. The first and second waveguide pupil expander may collectively provide pupil expansion in a first direction and in a second direction perpendicular to the first direction, optionally, wherein a plane containing the first and second directions is substantially parallel to a plane of the second waveguide pupil expander. In other words, the first and second dimensions that respectively define the length and breadth of the second waveguide pupil expander may be parallel to the first and second directions, respectively, (or to the second and first directions, respectively) in which the waveguide pupil expanders provide pupil expansion. The combination of the first waveguide pupil expander and the second waveguide pupil expander may be generally referred to as being a “pupil expander”. It may be said that the expansion / replication provided by the first and second waveguide expanders has the effect of expanding an exit pupil of the display system in each of two directions. An area defined by the expanded exit pupil may, in turn define an expanded 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 deliver 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. Dual-surface optical combiner Figure 6 shows a cross-sectional schematic view of a display system comprising an optical combiner 600 and a head-up display (HUD) 610. In this example, the display system is a display system for a vehicle such as a car. The HUD 610 is positioned within a dashboard of the vehicle (not shown in Figure 6). A driver of the vehicle 620 is shown in Figure 6. The driver is positioned such that one or more eyes of the driver 620 are within an eye-box or viewing plane of the HUD 610. In this example, the optical combiner 600 is a windscreen or windshield. In this example, the HUD 610 comprises the first and second replicators I waveguides (as shown in Figure 5A and 5B). The optical combiner 600 comprises a primary surface 602 and a secondary surface 604. The primary surface 602 opposes the secondary surface 604. The optical combiner 600 may be referred to herein as a dual-surface optical combiner given that it comprises a primary surface 602 and a secondary surface 604. In this example, the primary surface 602 is an internal surface. In other words, the primary surface 602 faces the interior of the vehicle and faces the driver 620. In this example, the secondary surface 604 is an external surface. In other words, the secondary surface 604 faces the real-world, outside of the vehicle and way from the driver. In this example, the optical combiner 60 is curved. In particular, both the primary surface 602 and the secondary surface 604 are curved. In this example, the HUD 610 comprises first and second replicators that are arranged to receive, as an input, a holographic wavefront. The holographic wavefront corresponds to (or is encoded with) a hologram of an image. In particular, as described above, the holographic wavefront is formed by illuminating a spatial light modulator (such as a liquid crystal on silicon spatial light modulated) displaying a hologram of the image. Thus, light incident on the spatial light modulator is spatially modulated in accordance with the hologram to form a holographic wavefront. The holographic wavefront is received by an input of the waveguide I replicator. A first waveguide is arranged to replicate the holographic wavefront to form a 1-D array of replicas of the holographic wavefront. A second waveguide is arranged to receive and replicate the 1-D array of replicas to form a 2-D array of replicas of the holographic wavefront. The HUD 610 is arranged to relay the 2-D array of replicas (in this example) of the holographic wavefront towards the optical combiner 600 such that at least a portion of the light of the 2-D array of replicas ofthe holographic wavefront is reflected / directed towards a viewing plane ofthe HUD 610 (where the driver 620 is positioned in Figure 6). Figure 6 represents the propagation of a single ray of light from the HUD 610 to the viewing plane / driver 620. The skilled reader will appreciate that, in reality, a great many rays of light will propagate from the HUD 610 to the viewing plane / driver 620 given that a 2-D array of replicas are transmitted from the HUD 610 and that each array of replicas comprises a diverging bundle of light rays (see Figure 4). For simplicity, however, a single light ray 612 of a single replica is shown in Figure 6 as propagating from the HUD 610. The single light ray 612 is propagating along an optical axis or propagation axis of one ofthe replicas emitted by the HUD 610. The single light ray 612 represents how the dual-surface nature ofthe optical combiner 600 can result in the formation of so-called ghosts. As shown in Figure 6, the light ray 612 propagates from the HUD 610 to the optical combiner 600. The light ray 612 is incident on the primary surface 602 of the optical combiner 600. A first portion of the light of the light ray 612 is reflected by the primary surface 602 of the optical combiner 600. This may be referred to as the primary reflection 614 of the light ray 612. The primary reflection 614 of the light ray is directed by the primary surface 602 of the optical combiner 600 to the viewing plane I driver 620. A second portion of the light of the light ray 612 is transmitted by the secondary surface 604 of the optical combiner 600 forming transmitted light ray 612’. The transmitted light ray 612’ propagates through the optical combiner 600 to the secondary surface 604 where at least a portion of the transmitted light ray 612’ is reflected by the secondary surface 604. This forms secondary reflection 614’ of the light ray 612. The secondary reflection 614’ propagates back through the optical combiner 600 towards the primary surface 602. The secondary reflection 614’ is then transmitted by the primary surface 602 towards a viewing plane of the display system. As shown in Figure 6, the primary reflection 614 is offset from the secondary reflection 614’ at the viewing plane. The implications of this will be described in more detail below. The skilled reader will appreciate that the proportions of the light ray 612 that are reflected and transmitted will depend on the angle of incidence of the light ray 612 on the respective surface, the refractive index of the material at the interface between the primary surface 602 and air, and the reflectivity of the material. However, the important point here is that the arrangement of the display system is such that light ray 612 is divided at the primary surface 602 such that, eventually, primary and secondary reflections of the light ray 612 are received at the viewing plane. Furthermore, the skilled reader will appreciate that a proportion of the transmitted light ray 612’ may be transmitted at the secondary surface 604 (rather than reflected) and a portion of the secondary reflection 614’ of the light ray 612 may be reflected by the primary surface 602 (rather than transmitted as shown in Figure 6). These transmitted and reflected portions, respectively, are not shown in Figure 6 and are not important in the context of the present disclosure because said portions do not reach the viewing plane of the display system. In this example, the holographic wavefront has been formed by spatially modulating light in accordance with a channelling hologram. In other words, there is a relationship between the angles of the holographic wavefront (with respect to a propagation axis of the holographic wavefront), as described above. In particular, angles of the holographic wavefront correspond to spatial coordinates of an image corresponding to the holographic wavefront. The single light ray 612 represents a single angular component of a single replica of the holographic wavefront. The effect of the single light ray forming primary and second reflections 614, 614’ is that the angular content encoded by the single light ray is duplicated (but offset at the viewing plane). It should be understood that, while a single light ray 612 is shown in Figure 6, every light ray emitted by the HUD 600 may be duplicated by the dual-surface optical combiner 600. The offset (at the viewing plane) between the primary reflection of every light ray emitted by the HUD 600 and the respective secondary reflection may be the same. Thus, an offset copy of the light received at the eye 405 viewing plane may effectively be formed. Composite holographic wavefronts at the viewing plane and ghost formation As described above, in relation to Figure 4, light from different angular parts of an image, V1 to V8, reach the eye 405 from different respective “bounce” points. Each “bounce” point represents a replica of a holographic wavefront, in this example. Different angular parts of an image are received from different replicas. For example, V1 may reach the eye from a different replica to V2. The result of this is that each angular channel of encoded light reaches the eye, from a waveguide, such that the eye can receive all angular content, despite the pupil of the eye being relatively small, the display device 402 being relatively small, and the distance between the eye and display device being relatively very large. Although the description of Figure 4 is in terms of the angular content delivered to an eye, it should be understood that this angular content is in actual fact delivered to a region 450 of the viewing plane of the display system. The hologram I holographic wavefront has been calculated / arranged such that said region 450 corresponds to a position of the pupil of one of the eyes of the driver 620. For example, the position of the respective eye of the driver may be known from an eye-tracking system of the display system (which is not illustrated) and the hologram calculated accordingly such that all angular content is delivered to a region 450 of the viewing plane that is substantially aligned with the respective eye of the driver 620. In the present disclosure, the display system is described as forming a composite wavefront at the viewing plane. The composite wavefront corresponds to the combined I mixed / blended angular content received from multiple replicas at region 450 viewing plane. If the composite wavefront comprises all angular content of the image (because V1 to V8 are received from a plurality of replicas / bounces), then a viewing system / eye at the viewing plane receiving the composite wavefront may receive all angular content. Thus, by arranging the display system such that a composite wavefront containing all angular content is delivered to region 450 of the viewing plane (which is aligned with a pupil of the driver 620), the driver receives all angular content of the image. Figure 7 schematically shows some of the light received at the viewing plane from the display system of Figure 6. The view in Figure 7 is in a plane that is perpendicular to a plane of Figure 6. In particular, whereas Figure 6 shows the y-z plane, Figure 7 shows the x-y plane. Figure 7 shows region 450 of the viewing plane and how a primary composite wavefront 750 is delivered to region 450. In this example, the primary composite wavefront 750 is represented as black circle. It should be understood that the shape and size of the primary composite wavefront is determined in the hologram calculation. Specifically, the hologram calculation may comprise a masking step which masks areas of the hologram I display device that do not contribute to a primary composite wavefront 750 received by a pupil of an eye at the viewing plane. In this example, the shape and size (and position) of the primary composite wavefront has been selected to substantially correspond to the pupil. Thus, the primary composite wavefront 750 is shaped, sized and positioned to fill and completely overlap a pupil positioned at region 450 (without substantially extending beyond the pupil). More specifically, it could be said that the hologram has been calculated / arranged such that a plurality of replicas of the holographic wavefront deliver respective (different) angular content to region 450. Each of the replicas is arranged to deliver the respective angular content to fill the region 450 such that the primary composite wavefront 750 is effectively formed as a superposition of a plurality of discs of light, each disc of light corresponding to different angular content delivered by a different replica. A cross-section of the primary composite wavefront 750 on the viewing plane could be described as a light field. When the primary composite wavefront 750 has a circular shape, as in this example, a cross-section of the primary composite wavefront 750 on the viewing plane could be described as a light disc. Figure 7 shows that the display system of Figure 6 does not only form a primary composite wavefront 750, butthat an offset copy or replica of the primary composite wavefront 750 is also formed. This will be referred to as a ghost 750’ of the primary composite wavefront 750 (formed by the external surface of the optical combiner 600) herein. As described above, light from the HUD 610 will be divided at the optical combiner 600 such that primary and secondary reflections of each light ray are delivered to the viewing window. In this example, the primary composite wavefront 750 comprises content received from the primary reflections of light rays from the primary surface 602 of the optical combiner 600. In fact, the hologram / holographic wavefront has been arranged and calculated based on this optical path from the primary surface 602. Light from the primary reflection is what is intended to be received by the pupil of the eye. In other words, the display system 600 has been arranged such that the light from the primary reflection is received by the pupil of the eye. However, as described above, light from the secondary reflection 614’ is also received at the viewing plane. Because of the differences between the primary and secondary surfaces 602, 604 (e.g. because of the differences in position of the primary and secondary surfaces), the light from secondary reflection 614’ is offset from the light of the primary reflection 614. The result of this is that light of the secondary reflection 614’ forms a ghost composite wavefront 750’ at the viewing plane. The ghost composite wavefront 750’ may effectively be a copy or duplication of the primary composite wavefront 750. Because the primary composite wavefront 750 and its respective ghost 750’ are offset from one another, if light of both the primary composite wavefront 750 and its respective ghost 750’ were received by a pupil at the same time, then there is a risk that a ghost image may be visible to the driver 620. However, the inventors have found that, for display systems comprising conventional optical combiners, the overlap of the ghost composite wavefront 750’ with the pupil of the driver 620 is negligible. As above, the primary composite wavefront 750 is arranged to be positioned, sized and shaped to substantially correspond to the pupil. Thus, the fact the overlap of the primary composite wavefront 750 and the ghost composite wavefront 750’ is negligible also shows that the overlap of the ghost composite wavefront 750’ with the pupil is substantially negligible and a negligible amount of light is received by the pupil from the ghost composite wavefront 750’. Thus, in this example, the display system happens to be arranged such that the second composite wavefront 750 is not a significant contributor to so-called ghosts. However, as will be described, other composite wavefronts, and their respective ghosts, are formed by the display system and these composite wavefronts (and ghosts) may contribute to the formation of so-called ghosts. Figure 8 again shows the schematic waveguide 408 of Figure 4. Like Figure 4, Figure 8 shows how replicas of a holographic wavefront / light cone is replicated by waveguide 4 at nine “bounce” points, B0 to B8, along the length of the waveguide 408. Unlike Figure 4, the eye 405 is not shown in Figure 8. It should be understood that the eye 405 may actually still be present at the viewing plane in Figure 8. But the eye is not shown in Figure 8. Figure 8 shows how only light from one angular part of the image (e.g. light of one of V1 to V8) has a trajectory that enables it to form the primary composite wavefront 750 from each respective “bounce” point, B0 to B8. This was described above. Figure 8 shows how further composite wavefronts 750 are also inevitably formed by the replicas / bounce points. For example, B1 may contribute light of V1 to the primary composite wavefront 750. But the full angular range of content is contained in the replica of the holographic wavefront formed at B1. So, while B1 may not contribute light of VO and V2 to V8 to the first primary composite wavefront 750 (for example), that light still reaches the viewing plane. Said light contributes to further composite holographic 5 wavefronts. Herein, these holographic wavefronts are referred to as secondary composite holographic wavefronts. For example, as shown in Figure 8, B1 may contribute light of VO to a first secondary composite wavefront 851 and a may contribute light of V2 to a second secondary composite wavefront 849. As shown in Figure 8, the first secondary composite wavefront 851 is to the left of the primary composite wavefront 750 and the second secondary composite 10 wavefront 849 is to the right of the primary composite wavefront 750. It should be noted that the first and second secondary composite wavefronts 851,849 are formed by light that has been reflected by the primary surface 602 of the optical combiner 600 in this example. Figure 8 only shows first and second secondary composite wavefronts 851,849. 15 However, many more secondary composite wavefronts will typically be formed (which are not shown in Figure 8). The number of secondary composite wavefronts will depend on the number of bounces and the proportion of the total angular content that is provided by each bounce to each composite. It may be said that a first array of composite wavefronts is formed at the viewing plane by the light from the bounces / replicas that has been reflected by the primary surface 602 20 of the optical combiner. The first array comprises the primary composite wavefront 750 and the first and second secondary composite wavefronts 851,849. Figure 9A is a similar view to Figure 7 in that Figure 9A shows the viewing plane of the display device. Unlike Figure 7, Figure 9A additionally shows the first and second secondary composite 25 wavefronts 851,849. Figure 9A shows how the first and second secondary composite wavefronts 851,849 each overlap different portions of the primary composite wavefront 750. In particular, the first secondary composite wavefront 851 overlaps a top portion of the primary composite wavefront 750 and the second secondary composite wavefront 849 overlaps a bottom portion of the primary composite wavefront 750. As described above, the display system is arranged such 30 that primary composite wavefront 750 substantially exactly corresponds to the pupil of an eye of the driver 820. Thus, the overlap of the first and second secondary composite wavefronts 851,849 with the primary composite wavefront 750 means that said secondary composite wavefronts also overlap with the pupil. In other words, light of the first and second secondary composite wavefronts 851,849 may be receivable by the pupil. 35 Figure 9B shows the light of the primary and first and second secondary composite wavefronts 750,851,849 that is received by the pupil of the driver 920. The pupil 920 is at the viewing plane. As shown in Figure 9B, much of the first and second secondary composite wavefronts 851,849 is not received by the pupil 920. Only the overlapping portions of said composite wavefronts are received by the pupil 920. As above, the dual-surfaces nature of the optical combiner means that each light ray is replicated 5 because of primary and secondary reflections of the light by the primary and secondary surface 602, 604 of the optical combiner 600. This results in ghosts of each of the composite wavefronts shown in Figure 9A and 9B being formed. The formation of the (primary) ghost 750’ of the primary composite wavefront was described above in relation to Figure 7. First and second secondary ghosts 1049’, 1051’ of the first and second secondary composite wavefronts 851,849, 10 respectively, are formed in a similar way. This is shown in Figure 10A. Figure 10A is a similar view to Figure 7 and 9A in that Figure 10A shows the viewing plane of the display device. Unlike Figure 9A, Figure 10A shows the primary and first and secondary ghosts 750’,1049’,1051’ in addition to the primary and first and second secondary composite wavefronts 15 750, 851,849. Each of the ghosts is offset from its respective composite wavefront by substantially the same amount. Figure 10A only shows secondary ghosts of the first and second secondary composite wavefronts 851,849. However, as above, there may actually be a first array comprising many 20 more secondary composite wavefronts. For every secondary composite wavefront of the first 1 array, there may be a respective ghost formed. It may be said that a second array of ghost composite wavefronts is formed at the viewing plane by the light from the bounces I replicas that । has been reflected by the primary surface 602 of the optical combiner. The second array comprises the ghost of the primary composite wavefront and the ghosts of the first and second 25 secondary composite wavefronts 851,849. Figure 10B shows the light of the primary and first and second secondary composite wavefronts 750,851,849, and their respective ghosts 75O’,1O49’,1O5T, that is received by the pupil of the driver 920. As in Figure 9B, the pupil 920 receives all of the light of the primary composite 30 wavefront 750 and additionally receives light from the portions of the first and second secondary composite wavefronts 851,849 that overlaps that primary composite wavefront 750. Additionally, the pupil 902 receives a significant amount of light from the second secondary ghost 1051 ’. This is because the majority (more than half) of the primary composite wavefront 750 / pupil 920 is overlapped by the second secondary ghost 1051 ’. The pupil 902 does not receive light from the 35 first secondary ghost 1049’ or the primary ghost 750’ because these ghosts do not overlap the pupil 902 at the viewing plane. The inventors have found that the light received at the pupil coming from composite wavefront(s) other than the primary composite wavefront 750, or from ghost wavefronts, may contribute to the formation of secondary or so-called ghost images which may adversely affect the viewing experience of a user at the viewing window. A ghost image is a lower intensity replica of a main image. The main, highest intensity image may be referred to as the primary image. Each ghost image may be referred to as a secondary image. The presence of ghost images can significantly reduce the quality of a perceived virtual image. The ghost images may give the appearance of blurring of the primary image. Figure 11 shows an example of a virtual image of the numbers ‘5’ and ‘9’, created using a viewing system similar to that shown in Figure 6, that includes ghost images in addition to a main image. The main image can be seen as the brightest, central image for each number, with ghosts to the left and the right. In the example of Figure 8, the ‘9’ is formed when the viewing distance is larger than it is for the ‘5’, therefore the blurring is more pronounced for it. The inventors have recognised that it would be desirable to improve the quality of images visible at the viewing plane by the driver 620. The inventors have recognised that one way to address the ghosts issue is to apply a mask to the hologram displayed on the spatial light modulator / LCoS. This approach involves selectively only calculating hologram pixel values for hologram pixels that contribute only to the primary composite wavefront 750. Other hologram pixel values are not calculated. The composite holographic wavefronts (and ghosts thereof) may have a shape substantially corresponding to the shape of the hologram display on the spatial light modulator. Thus, masking the hologram has the effect of masking the composite holographic wavefronts. A shape can be chosen which results in there being substantially no overlap of secondary composite wavefronts or ghosts with the primary composite wavefront. This prevents the formation of the copy of the holographic reconstruction and improves the viewing experience. The LCOS masking approach is described in more detail below. Masking of the LCOS masking has the same effect on each of the composite wavefronts and ghosts. In the example of Figure 10B, the majority of the primary composite wavefront 750 is overlapped by the second secondary ghost 1051’. Thus, masking the LCOS so as to remove the second secondary ghost 1051’ would result in the majority of the primary composite wavefront 750 being masked. This may be undesirable because it may cause the brightness and I or the quality and / or resolution of the projected image may be reduced. The inventors have developed improved optical combiners and / or display systems comprising an improved optical combiner that are optimised for positive image reinforcement (to reduce the appearance of ghost images and I or to form a ghost image that reinforces the primary image). The (improved) LCOS masking approach described below may be used in combination with the 5 improved optical combiner(s) and / improved display system(s). Wedge-shaped optical combiner After thorough simulation and experimentation, the inventors have found that the separation and 10 the angle between the primary and secondary surfaces of an optical combiner can be arranged, adjusted, optimised, or otherwise configured to reduce the appearance of ghost images visible by a pupil at the viewing plane. In a first set of embodiments, the inventors have devised an arrangement in which the angle and the distance between the primary and the secondary surface of the optical combiner cause the primary composite wavefront to substantially completely 15 spatially overlap with a ghost composite and to parallelise light of the primary composite wavefront with said ghost composite. In such embodiments, the ghost composite forms a ghost image that reinforces the primary image formed by the primary composite wavefront. In a second set of embodiments, the inventors have devised an arrangement in which a pitch of 20 the composite holographic wavefronts at the viewing plane is increased and the optical combiner is arranged such that the first array of composite holographic wavefronts is interleaved with the second array of ghosts. In other words, a combined array of the first and second array may be formed comprising composite holographic wavefronts and ghosts in an alternating configuration. 25 First set of unclaimed embodiments - overlapping ghost Figure 12 shows a schematic cross-sectional view of an unclaimed first embodiment of a display device according to the present disclosure. The display device comprises an optical combiner 1200 according to the disclosure. In this example, the optical combiner 1200 is a windscreen or 30 windshield of a vehicle such as a car. The vehicle comprises a dashboard, a top surface of which is represented by broken line 1250 in Figure 12. Embedded in the dashboard is a HUD 1210 which is similar to the HUD 610 described in relation to Figure 6. The optical combiner 1200 of Figure 12 comprises a primary surface 1202 and an opposing 35 secondary surface 1204. The optical combiner 1200 according to present disclosure is arranged such that the distance between the primary surface 1202 and the secondary surface 1204 is such that a ghost image reinforces the primary image formed by a primary composite wavefront. In this example, this achieved by providing an optical combiner 1200 at least a portion of which is substantially wedge-shaped and having a thickness that is greater than is conventional. The (at least portion of the) optical combiner being substantially wedge-shaped means that a cross-section of the optical combiner 1200 is wedge-shaped. The (at least portion of the) optical combiner being substantially wedge-shaped may mean that a distance between the primary and secondary surfaces 1202, 1204 varies with distance from a proximal end or edge to a distal end or edge. In other words, a thickness of the wedge-shaped portion of the optical combiner varies with distance from the proximal end or edge to the distal end or edge. In this example, the proximal end or edge is the end or edge that is closest to the top 1250 of the dashboard (and also closest to the HUD 1210). In other words, the proximal end or edge of the optical combiner is the end or edge forming the bottom of the optical combiner. In this example, the distal end or edge is the end or edge that is furthest from the top 1250 of the dashboard (and also furthest from the HUD 1210). In other words, the distal end or edge of the optical combiner is the end or edge forming the top of the optical combiner. The optical combiner 1200 has a length, I, extending from the proximal end or edge to the distal end or edge. In this example, the thickness (i.e. the distance between the primary and second surfaces 1202,1204) ofthe wedge-shaped portion of the optical combiner 1200 increases from the distal end or edge to the proximal end or edge. Figure 12 shows an increase in the thickness ofthe optical combiner from t1 at the proximal end or edge to t2 at the distal end of edge. t1 >t2. Such a wedge-shape may be referred to as a negative wedge. Figure 12 schematically shows an example in which a cross-section ofthe entire optical combiner is wedge-shaped. However, in other examples, a cross-section of only a portion ofthe optical combiner is wedge-shaped. For example, the wedge-shaped portion ofthe optical combiner may correspond to a portion ofthe optical combiner that receives light from the HUD 1210. Outside ofthe wedge-shaped portion, the thickness ofthe optical combiner may be substantially constant. The schematic of Figure 12 significantly exaggerates the wedge. In particular, the difference between t1 and t2 is exaggerated in Figure 12. In reality, the wedge angle (i.e. corresponding to an angle between normals of corresponding portions ofthe optical combiner 1200) may be relatively very small. For example, the wedge angle may be less than 1 degree, optionally less than 1 / 10th of a degree, optionally less than 1 / 50th of a degrees. Furthermore, the schematic of Figure 12 shows the first and second surfaces 1202, 1204 as being straight (in at least the y-z plane). In reality, these surfaces may be curved (similar to what is shown in Figure 6). The inventors have recognised that, by adjusting the wedge angle and thickness of the optical combiner 1200, the primary and secondary composites can be manipulated separately from (and with respect to) their respective ghost composites. This is because the primary and secondary composites are formed by a primary reflection of light from the primary surface 1202 whereas the ghost composites are formed by a secondary reflection of light from the secondary surface 1204. By adjusting the angle and the position of the second surface 1204 (by providing a wedge), the propagation paths of the ghost composites can be altered without changing the propagation paths of the primary and secondary holographic composites. In the example of Figure 12, the thickness and the angle of the wedge have been selected such that the ghost composites are translated with respect to the primary and secondary composites. In particular, the thickness and the wedge angle have been selected such that the second secondary ghost 1051’ substantially spatially overlaps the primary composite wavefront 750 at the viewing plane. After thorough simulation and experimentation, the inventors have found that it is not enough to simply completely spatially overlap the second secondary ghost and the primary composite wavefront. The inventors have found that it is also necessary for corresponding angular components of the primary composite wavefront and the second secondary ghost to be substantially parallel at the viewing plane. If this is not done, then a ghost image will be visible (formed using light of the second secondary ghost) that is offset from a primary image (formed using light of the primary composite wavefront). As described above, each composite wavefront comprises angular content of an image received from a plurality of replicas. The inventors have found that the corresponding angular content forming both the primary image and the ghost image should be incident on the viewing plane at substantially the same angle. For example, if the V0 content the primary image is formed by light rays incident on the viewing plane at 3 degrees to a normal of the viewing plane, then the V0 content of the ghost image must also be formed by light rays that are also incident on the viewing plane at 3 degrees to a normal of the viewing plane. If this is not achieved, then the V0 content of the ghost image (visible at the viewing plane) may appear offset from the V0 content of the primary image. The inventors have found that angular parallelism should be maintained for all angular content. Figure 13 demonstrates a case in which there is no angular parallelism of corresponding angular content. Pupil 1300 of Figure 13 positioned at a viewing plane of a display system which comprises an improperly designed optical combiner. A first ray of light 1310 is received by the pupil 1300. The first ray of light 1310 corresponds to V0 image content, for example, and 5 contributes to a primary holographic composite. A second ray of light 1310’ is received by the pupil 1300. The second ray of light 1310’ also corresponds to V0 image content (i.e. the same angular content as the first ray of light 1310). The first ray of light 1310 is not parallel to the second ray of light 1310’. Thus, the V0 image content in the primary image will be offset to the V0 image content in the ghost image. The offset will depend on the angle between 1310 and 10 1310’. The inventors have found that it is not generally possible to constrain one of: a) the optical wedge thickness; and b) the optical wedge angle and design the improved optical combiner. For example, one of thickness or wedge angle could be altered to translate a ghost composite 15 wavefront to overlap the primary composite wavefront. But this would break the angular parallelism of corresponding angular components of the two composites. Both thickness and angle must be selected to achieve spatial overlap and angular parallelism. Figure 14A shows light received at the viewing plane of the display system 1200. 20 In this example, the primary surface 1202 of the optical combiner 1200 forms a primary reflection which is responsible for the formation of a primary composite wavefront 1450 and first and second secondary composite wavefronts 1449, 1451 above and below the primary composite wavefront 1450, respectively. The primary surface 1202 of the optical combiner 1200 is 25 substantially the same as the primary surface 602 of the optical combiner 602. Thus, the primary composite wavefront 1450 and first and second secondary composite wavefronts 1449, 1451 of Figure 14A are substantially similar to the corresponding composite wavefronts750, 851,849 of Figure 10A. 30 In this example, the secondary surface 1204 of the optical combiner 1200 forms a secondary reflection which is responsible for the formation of ghost composite wavefronts of the primary composite wavefront 1450 and first and second secondary composite wavefronts 1449, 1451. In other words, a primary ghost composite wavefront 1450’ of the primary composite wavefront 1450 is formed and first and second secondary ghost composite wavefronts 1449’ and 1451’ of 35 the first and second secondary composite wavefronts are formed above and below the primary ghost composite wavefront 1450, respectively. The optical combiner 1200 has a wedge shape arranged such that the second secondary ghost composite wavefront 1451’ substantially completely overlaps the primary composite wavefront 1450; and such that corresponding angular content of said overlapping wavefronts is parallel. The primary composite wavefront 1450 is substantially aligned with pupil 1220. Thus, the pupil 1220 receives light of both the primary composite wavefront 1450 and the second secondary ghost composite wavefront 1450. Because of the parallelism and spatial overlapping nature of the two composite wavefronts, substantially coincident and identical primary and ghost images are visible to the viewing system I user comprising pupil 1220. In other words, the ghost image is no longer offset from the primary image. Thus, the ghost image does not substantially degrade the viewing experience for the viewing system / user. In fact, the ghost image may advantageously boost a brightness of the image viewable from the viewing window. In other words, it may be said that the ghost image reinforces the content of the primary image. In the described example, it is the (n+1)th ghost of the second array (of ghost composite wavefronts) that is overlapped with the nth (or primary) composite wavefront of the first array. It should be understood that any ghost of the second array could be spatially and angularly aligned to the nth (or primary) composite to achieve a similar affect. But it is convenient for it to be the (n+1 )th ghost in this example because, in the described example, the (n+1 )th ghost the second array was already closest to the nth (or primary) composite wavefront in the display system 1200. So, After thorough experimentation and simulation with many different display system arrangements, the inventors have found that, as a general rule, the (n+1)th ghost is closest to the nth (primary) composite wavefront in most systems and so it is generally convenient to overlap the (n+1 )th ghost and the nth (primary) composite wavefront. The exact wedge angle and thickness needed to achieve the spatial overlap and angular parallelism described above will vary from display system to display system. For example, these parameters may vary with rake angle of a windscreen and radii of curvature of the windscreen. However, after thorough experimentation and simulation, the inventors have found that, as a general rule, the optical wedge should have a "negative” wedge (as described above) and a thickness of between 7 and 10 millimetres, optionally between 7.5 and 9 millimetres, optionally between 8 and 8.5 millimetres. Conventionally, windscreens or windshields have a thickness of about 5 or 6 millimetres. It is generally preferable not to increase the thickness (and so weight) of a windscreen. However, the inventors have found that the increased thickness (above convention) is worth the benefit of improved viewing experience, described above. The skilled reader will appreciate that there is partial overlap of the primary ghost 1450’ and the first and second secondary composites 1449 and 1451 with the primary composite 1450. This overlap can be removed with LCOS masking, as described below. 5 Second set of embodiments - increased spacing In a second set of embodiments, the impact of ghost images on the viewing experience of a user / viewing system of display system is improved by introducing a space between adjacent composite holographic wavefronts formed by the primary reflection. In examples, this is achieved 10 by modifying the pitch and I or the size of the composite holographic wavefronts. As the skilled reader will appreciate, this effect could be achieved by increasing the pitch of replicas of the holographic wavefront (e.g. by modifying angle and / or size of the waveguides) and / or by modifying the hologram calculation. This may ensure that the composite holographic wavefronts are spatially separated. Thus, unlike in Figure 9A, there may be a gap between the primary 15 composite holographic wavefront and the first and second primary composite holographic wavefronts, for example. Said composites may no longer overlap one another. The corresponding ghost composite holographic wavefronts will have the same gap or spatial separation as the primary and secondary composite holographic wavefronts. 20 The inventors have recognised that a wedge-shaped optical combiner (similar to that described above) can then be used to translate the ghost composite holographic wavefronts (formed by the reflection by the secondary surface of the optical combiner) with respect to the primary and secondary holographic wavefronts (formed by the primary surface of the optical combiner) such that each ghost composite holographic wavefront is positioned in the space between adjacent 25 primary and / or secondary composite holographic wavefronts. In other words, the ghost composite holographic wavefronts and the primary and second holographic wavefronts together form a combined array in which composite wavefronts of the first and second arrays are substantially interleaved. This is what is shown in Figure 15A. 30 Figure 15A shows light received at a viewing plane. Figure 15A shows a primary composite holographic wavefront 1550 and first and second secondary composite holographic wavefronts 1549, 1551. Comparing Figure 15A with the primary and first and second secondary holographic wavefront composites 750, 851,849 of Figure 10A, it should be clear that the holographic wavefront composites of Figure 15A have the larger spacing and are not overlapping with one 35 another. Figure 15A also shows ghost composite holographic wavefronts 1550 of the primary composite holographic wavefront 1550’ and first and second secondary composite holographic wavefronts 1549’, 1551’. The ghosts composite wavefronts similarly do not overlap one another. The wedge of the optical combiner of the display system is arranged such that the array of ghosts is positioned such that each ghost composite wavefront separates a non-ghost composite wavefront. The benefit of the arrangement of this example is that the overlap of the ghosts with the primary composite holographic wavefront 1550’ can be significantly reduced (compared to Figure 10B, for example). For example, Figure 15B shows how a relatively small portion of the primary composite holographic wavefront 1550 is overlapped by the primary and second secondary ghosts 1550’, 1551’ at the pupil 1520. This overlap can be removed with LCOS masking, as described below, if necessary. Alternatively, in some examples, the spacing of the composite holographic wavefronts could be arranged such that there is no overlap at all between ghost composite wavefronts and the nonghost composite wavefronts. This is shown in Figure 16A. Figure 16A shows light received at a viewing plane. Figure 16A shows the following nonoverlapping, exactly interleaved, composite holographic wavefronts and ghost composite wavefronts, in order: a ghost first secondary composite wavefront 1649’; the corresponding first secondary composite wavefront 1649; a ghost primary composite wavefront 1650’; the corresponding primary composite wavefront 1650; a ghost second secondary composite wavefront 1651’; and the corresponding second secondary composite wavefront 1651. Figure 16B shows the light received by a pupil 1620 at the viewing plane 1620. In this example, the pupil 1620 is exactly aligned with the primary composite wavefront 1650. As such, there is no need for LCOS masking in this example. It should be noted that in the second set of embodiments, the optical combiner may comprise be a positive wedge rather than a negative wedge as in the first set of embodiments. This means that the thickness of the wedge-shaped portion of the optical combiner increases in thickness from the proximal end or edge to the distal end or edge, rather than the other way around. LCOS masking British patent 2,603,517, which is incorporated herein by reference in its entirety, discloses a device and method in which a primary contributory area and a secondary contributory area of the display device are identified. The inventors found that a primary contributory area of the display device gave rise to a main holographic reconstruction (image) and a secondary contributory area of the display device gave rise to a copy of the holographic reconstruction (referred to in the earlier British patent as a “secondary image”). GB 2,603,517 discloses a hologram engine arranged to calculate a hologram based on the primary contributory area or areas. Hologram pixel values outside the primary contributory area are not calculated. This prevents the formation of the copy of the holographic reconstruction and improves the viewing experience. Figure 17 shows the individual optical paths for each of five ray bundles that contribute to five respective image points within a virtual image 1701 - labelled from top to bottom as R1 to R5, respectively. For more details, the reader is referred to GB 2,603,517. As can be seen therein, the light of each of R1 and R2 is simply refracted and then transmitted by the waveguide 1708. The light of R4, on the other hand, encounters a single bounce before being transmitted. The light of R3 comprises some light from a corresponding first part of the display device 1702 that is simply refracted by the waveguide 1708 before being transmitted, and some light from a second, different corresponding part of the display device 1702 that encounters a single bounce before being transmitted. Similarly, the light of R5 comprises some light from a corresponding first part of the display device 1702 that encounters a single bounce before being transmitted and some light from a second, different corresponding part of the display device 1702 that encounters two bounces before being transmitted. For each of R3 and R5, two different parts of the display device propagate light corresponding to that part of the virtual image. Accordingly, different parts of the display device contribute to different virtual image points. Figure 17 shows how some image points (e.g. R3 and R5) are repeated (i.e. form a main image point and a ghost image point) because a plurality of different optical paths through the waveguide are possible forthat image point. GB 2,603,517 discloses how the hologram determination process may be used to eliminate these ghosts by excluding hologram pixel values in parts of the display device that contribute to the ghost image (i.e. form ghost image points). These excluded areas are referred to as “secondary contributory areas” in the earlier British patent. Figure 18 shows an example of the pixel area of the display device 1800 in which a first area 1810 does not contribute to any image points that reach the viewer (“non-contributory area”), a second area 1820 contributes to the main image points (“primary contributory area”) and a third area 1830 of the display device 1800 contributes only to ghost image points (“secondary contributory area”). In some examples, hologram pixel values are only determined in the primary contributory area. In some embodiments, the primary contributory area is cropped to avoid overlap between the primary composite holographic wavefront arriving at the viewing plane and at least one secondary composite holographic wavefront arriving at the viewing plane. These embodiments can be advantageous when used with a non-uniform or curved optical combiner (e.g. windscreen). An example of overlapping composite holographic wavefronts is shown in Figure 9A. Figure 9B shows how a cropped shape can be defined excluding the partially overlapping composite holographic wavefronts. This cropping is mirrored I matched at the display device. That is, the corresponding contributory area of the display device has a corresponding shape. In a yet further advancement disclosed herein, it was recognised that this approach may be extended to exclude other areas of the display device that contribute to a windscreen ghost, as described above. As described above, a plurality of composite holographic wavefronts are formed and, furthermore, a plurality of ghosts or repeats thereof may also be formed by a second surface of the optical combiner (e.g. windscreen). In the embodiments described above, parameters of an optical combiner (e.g. windscreen) are optimised to so that one specific ghost of the composite holographic wavefront can positively reinforce the primary composite holographic wavefront. The ghost may be the ghost of the primary composite holographic reconstruction or a repeat / replica thereof. In some embodiments, the hologram determination process is modified so that hologram pixel values are not calculated in areas of the display device that will produce a ghost (of a composite holographic wavefront that overlaps the primary composite holographic wavefront) at the viewing plane but has not been optimised for positive image reinforcement. As described in more detail above, optimising comprises satisfy the two criteria of spatial overlap and parallelism between equivalent angular components. In some embodiments, the primary composite holographic wavefront and at least one secondary composite holographic wavefront are at least partially overlapping at the viewing plane. This can cause the appearance of a ghost image. In these embodiments, the primary contributory area many therefore be cropped (i.e. reduced in size) in order to eliminate the overlap. More specifically, hologram pixel values are not determined in the area of the display device corresponding to the overlap area. In other embodiments, the primary composite holographic wavefront and secondary composite holographic wavefronts are sufficiently spaced such that the primary contributory area does not need to be cropped to avoid an overlap with any of the secondary composite holographic wavefronts. However, in further embodiments, the primary contributory area is cropped to exclude an overlap area causing a ghost of the composite holographic wavefront that does not satisfy the criteria for positive image reinforcement. That is, even though the primary and secondary composite holographic wavefronts do not overlap, at least one ghost composite holographic wavefront does overlaps. If the overlapping ghost has not been optimised in accordance with the present disclosure using the optical combiner, such overlap may cause a ghost image rather than positive reinforcement. In these embodiments, the primary contributory area is therefore cropped to remove the area causing the problematic ghost composite holographic wavefront. Figure 19 shows an embodiment in which the primary contributory area has been cropped to exclude an area of the display device that forms a secondary composite holographic wavefront that does not itself overlap the primary composite holographic wavefront at the viewing plane but a ghost thereof does. The secondary composite holographic wavefront may be composite holographic wavefront that is immediately adjacent the primary or it may be a higher order repeat of the primary. Figure 19 shows the pixel area of a display device 1900 comprising a cropped primary contributory area 1920 and a non-contributory area 1910. The cropped primary contributory area 1920 may be said to have a dumbbell shape. The primary contributory area is generally circular -that is, a circular sub-area of the pixel array of the display device. The cropped primary contributory area may comprise two cropped areas each corresponding to part of a circle. The two cropped areas may be opposing. The resulting shape resembles a dumbbell. Accordingly, in these embodiments, a secondary contributory area of the display device is identified wherein that secondary contributory area corresponds to a secondary image formed by a ghost reflection from a secondary surface of the optical combiner. 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 5 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 display system having a viewing plane from which an image is visible, the display system comprising:5 a hologram replicator arranged to output an array of replicas of a holographic wavefront,wherein angles of the holographic wavefront correspond to spatial coordinates of an image corresponding to the holographic wavefront; andan optical combiner arranged to receive the array of replicas, wherein the optical combiner has a primary surface arranged to direct a primary reflection of the array of replicas10 towards the viewing plane to form a primary array of composite holographic wavefronts at the viewing plane and a secondary surface arranged to direct a secondary reflection of the array of replicas towards the viewing plane to form a secondary array of ghost composite holographic wavefronts at the viewing plane,wherein the hologram replicator and optical combiner are arranged such that primary and15 secondary arrays form a combined array in which the composite holographic wavefronts and the ghost holographic wavefronts are interleaved such that each ghost holographic wavefront is positioned in a space between adjacent composite holographic wavefronts.

2. A display system as claimed in claim 1 wherein the hologram replicator and optical20 combiner are arranged such that adjacent composite holographic wavefronts of the first array are non-overlapping at the viewing plane.

3. A display system as claimed in any preceding claim wherein each composite holographic wavefront of the first array has the same shape and, optionally, size.

254. A display system as claimed in any preceding claim wherein the hologram replicator and optical combiner are arranged such a pitch the composite holographic wavefronts of the first array is equal to, or greater than, 1.5 times a width or a diameter of at least one of the composite holographic wavefronts.

305. A display system as claimed in any preceding claim wherein the hologram replicator and optical combiner are arranged such that adjacent composite holographic wavefronts and ghost composite holographic wavefronts are non-overlapping.35 6. A display system as claimed in any preceding claim wherein adjacent replicas of thecombined array of composite holographic wavefronts are adjoined.

7. A display system as claimed in any preceding claim wherein the primary array ofcomposite holographic wavefronts comprises a primary composite holographic wavefront and a plurality of secondary composite holographic wavefronts.5 8. A display system as claimed in claim 7, wherein the second array of compositeholographic wavefronts comprises a ghost of the primary composite holographic wavefront and a ghost of each secondary composite holographic wavefront of the first array.

9. A display system as claimed in any one of the preceding claims, wherein a thickness of 10 at least a portion of the optical combiner decreases from a distal edge of the optical combiner to a proximal edge of the optical combiner, the proximal edge being closest to the hologram replicator.

10. A display system as claimed in any one of the preceding claims, wherein a first portion of 15 the optical combiner comprises: a portion of the primary surface that is arranged to direct theprimary reflection; and a portion of the secondary surface that is arranged to direct the secondary reflection, wherein said portions of the primary and second surface oppose one another.

11. A display system as claimed in claim 10, wherein a cross-section of at least the first20 portion of the optical combiner is wedge-shaped.

12. A display system as claimed in claim 10 or 11, wherein an angle between the primary surface and the secondary surface of the first portion is constant for at least the first portion of the optical combiner.2513. A display system as claimed in any preceding claim, wherein the primary surface is arranged to receive the array of replicas of the holographic wavefront from the hologram replicator and to partially-reflect I partially-transmit the received array of replicas, the partial reflection forming the primary reflection, the partial transmission being received by the secondary 30 surface of the optical combiner.

14. A display system as claimed in claim 13, wherein the secondary surface is arranged to at least partially reflect the received partial transmission from the primary surface to form the secondary reflection.

15. A display system as claimed in any one of the preceding claims, wherein the optical combiner is a windshield of a vehicle.16 102416. A display system as claimed in any one of the preceding claims, wherein the display system comprises a head-up display system comprising the hologram replicator.

17. A display system as claimed in any one of the preceding claims, wherein the hologram5 replicator comprises a waveguide comprising an input port arranged to receive a holographic wavefront, wherein the waveguide comprises a pair of surfaces arranged to waveguide light therebetween, a first surface of the pair of surfaces being partially transmissive-reflective such that the holographic wavefront is divided at each internal reflection to output an array of replicas.10 18. A method of holographic projection, the method comprising:receiving an array of replicas of a holographic wavefront from a hologram replicator at an optical combiner, wherein angles of the holographic wavefront correspond to spatial coordinates of an image corresponding to the holographic wavefront;directing, using a primary surface of the optical combiner, a primary reflection of the array15 of replicas towards a viewing plane to form a primary array of composite holographic wavefronts at the viewing plane; anddirecting, using a secondary surface of the optical combiner, a secondary reflection of the array of replicas towards the viewing plane to form a secondary array of ghost composite holographic wavefronts at the viewing plane;20 wherein the hologram replicator and optical combiner are arranged such that primary andsecondary arrays form a combined array in which the composite holographic wavefronts and the ghost holographic wavefronts are interleaved such that each ghost holographic wavefront is positioned in a space between adjacent composite holographic wavefronts.25