Windscreen correction with glare mitigation and diffractive global turn

Abstract

The replicator is arranged to receive spatially modulated light. The replicator is further arranged to replicate the spatially modulated light to form a plurality of replicas 608 of the spatially modu

Description

FIELD The present disclosure relates to a light control device and a method of providing a global turn of a replicated wavefront for head-up display. More specifically, the present disclosure relates to a display system, structure for the output surface of a waveguide and a method of display such as 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 micromirrors, for example. A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, “HUD”. SUMMARY Aspects of the present disclosure are defined in the appended independent claims. There is provided a light control device (or “corrective optic”) that cooperates with an optical component (such as a waveguide or cover glass) of a display device (such as a head-up display) to provide at least one optical function selected from the group comprising: correction for the complex curvature of an optical combiner (e.g. windscreen) used to the relay the image; and reflection suppression (or glare mitigation) of the optical component. The optical combiner may have first curvature in a first direction and a second curvature in a second direction perpendicular to the first direction. The first and / or second curvature may be non-linear. The optical combiner has a complex curvature which introduces complex distortions when used in a display system particularly one based on holographic projection. The display light may be spatially modulated light. The display system may be arranged to relay the spatially modulated light to a viewing plane or eye-box. In some embodiments, the display system is a holographic display system and the spatially modulated light is light that is spatially modulated in accordance with a hologram (of a picture or image). The spatially modulated light may be referred to as a holographic wavefront. In another embodiment, the display system is a picture generating unit and the spatially modulated light is light that is spatially modulated in accordance with a picture (in other words, an image). The spatially modulated light, in these embodiments, may be referred to as a picture wavefront (or an image wavefront). The light control device of the present disclosure may optionally provide a means for controlling reflections of ambient light to prevent or suppress glare from reaching the viewing plane while allowing the spatially modulated light to reach the viewing plane. For example, the display device may comprise an optical component comprising a reflective surface such as a substantially planar (e.g. glass) waveguide or cover glass. In the absence of the light control device, ambient light may be reflected by the reflective surface towards the viewing plane / eye-box of the display device thus forming glare. The light control device of the present disclosure may be arranged to suppress such reflections. More broadly, the light control device of the present disclosure is arranged to compensate for the curvature of a curved optical combiner on an optical path of the display system. Another purpose may be reflection suppression, such as glare mitigation. The curved optical combiner being on the optical path of the display system may mean that the spatially modulated light, propagating through the display system, may be incident on, reflected by, transmitted through, or otherwise interact with a curved optical combiner, such as a windscreen. As the skilled person will appreciate, the curvature of the optical combiner may alter the divergence or convergence of the spatially modulated light and angles thereof. For example, if the spatially modulated light is substantially collimated upstream of the curved optical combiner (prior to interacting therewith), then the spatially modulated may be nonparallel (e.g. converging or diverging) downstream of the curved optical combiner (after interacting therewith). In other words, the curved optical combiner may have a lensing effect on the spatially modulated light incident thereon. If the curvature of the curved optical combiner is non-uniform, then the lensing effect may be non-uniform. For example, different portions of the curved optical combiner may have a different (local) radius of curvature and so may have a different lensing effect on spatially modulated light incident thereon. In some embodiments, the curved optical combiner is a windscreen or windshield of a vehicle. A windscreen or windshield may have a complex curvature having a complex lensing effect on display light incident thereon. The lensing effect of the curved optical combiner may distort the display light (of the display system). For example, the display light may be such that a picture is viewable from a viewing plane. For example, the display light may be spatially modulated in accordance with a hologram of a picture, or simply in accordance with a picture. The lensing effect of the curved optical combiner may distort the picture that is viewable at the viewing plane. This may adversely affect a viewing experience of the display system. The replicator may be arranged to replicate the spatially modulated light to form a plurality of replicas of the spatially modulated light. In embodiments, the replicator may be a waveguide, as described below. For example, the waveguide may comprise an input port arranged to receive the spatially modulated light. The waveguide may comprise a pair of surfaces arranged to waveguide the spatially modulated light received at the input therebetween. A first surface of the pair of surfaces may be partially-transmissive partially-reflective. The first surface may be arranged to form the plurality of replicas of the spatially modulated light. At least a portion of the first surface may be said to form an output port of the replicator / waveguide. The replicator may be arranged such that the plurality of replicas are relayed towards the curved optical component. The display system may be further arranged such that the plurality of replicas is relayed towards a viewing plane / eye-box of the display system. It is found that the pitch of the replicas of the spatially modulated light (at the viewing plane) is important for ensuring a good viewing experience. Through simulation and experimentation, it was further found that the pitch of the replicas may be affected by the lensing effect of the curved optical combiner. For example, the pitch of the replicas at the viewing plane may be increased or decreased. This may adversely affect the viewing experience. For example, if the pitch of the replicas is reduced, so-called ghosting effects, in which a copy of the intended picture or image content is displayed slightly offset from the intended picture or image content, may become more apparent. The pitch of the replicas may be reduced if the curved optical combiner has a concave shape, for example the inside surface of a windscreen or windshield. As used herein, the pitch of replicas refers to the separation or distance between the centres of adjacent replicas. In other words, in display systems (such as head-up display systems), the effective pupil of the display system (more specifically the image projecting system used to project to image to the user) is expanded. This expansion may be by means of a plurality (e.g. a pair) of waveguides that each expand the pupil in a different dimension. These waveguides multiply reflect light bundles, providing a “replication” of the pupil in two dimensions in order to fill an extended viewing window (in other words, an eye-box) many times greater in size than the pupil of the display system. The light received and subsequently transmitted by the waveguides may be collimated such that, for any single point in the light field, there is no variation in angle (and resultant projected image position) as the light is reflected within the waveguides. This light is then reflected off an optical combiner (such as a windscreen) to fill a virtual viewing window (an “eye-box”) at the user’s / viewer’s head position. There has previously been proposed a light control device. Such a structure has the advantage of being able to provide the required optical functions in a relatively flat, lightweight format. This has been disclosed, for example, in British patent application GB2401627.1. However, the present disclosure relates to a non-Fresnel, light control device or, alternatively, a combination of Fresnel and non-Fresnel elements performing the equivalent optical functions. Attention has focused on providing all these functions using refraction. A “Fresnel structure” is reduced size version of an optical component that retains optical functionality of the optical component but in a more compact form factor. The size of the optical component is effectively reduced in at least one dimension (e.g. height) by dividing or slicing the structure in a second dimension (e.g. radial distance form a geometric centre or minima) into a plurality of zones (e.g. concentric rings) and “wrapping” the values in the first dimension (e.g. wrapping or capping the height in accordance with a modulus). The zones may have a regular interval in one dimension such as height or radial distance from a geometric centre and the “wrapping” may occur in the other dimension. The step of wrapping the values in the first dimension may comprise determining the modulo of the value in the first dimension. Accordingly, the values in the first dimension of the Fresnel structure may be the modulo of the values in the first dimension of the optical component. For example, the Fresnel structure corresponding to a wedge (i.e. linear ramp function) may be a sawtooth function, wherein the vertical returns between the inclines or ramps represent the wrapping of the values in the first dimension (e.g. the modulo value) and the zones in the second dimension represent the regular or irregular periodicity of the Fresnel structure. The Fresnel structure comprises a plurality of grooves (such as concentric rings or zones) having a groove width and a groove height. At least one of the groove width and groove height may be constant. In some embodiments, the groove width is variable (e.g. increases or decreases with distance from the centre of the structure) and the groove height is constant. But other embodiments are the other way round. In contrast, the present disclosure principally relates to a bulk optic which would not ordinarily be described as a Fresnel structure. In accordance with this disclosure, the “bulk optic” comprises a pair of light-receiving surfaces that are both continuous in a mathematical sense - that is, can be described by a respective mathematical function that is continuous (such as one defining a plane or curved surface) rather than a repeating function or function comprising discontinuities (e.g. as per a Fresnel structure). The term “bulk optic” reflects that the component is not a film or layer but rather a physical piece of e.g. plastic or glass. In some embodiments, a bulk optic is formed by injection moulding. In accordance with this disclosure, a bulk optic is an optic having thickness up to 15 mm such as up to 10 mm at the thickest point, wherein the thickness is the dimension parallel to the (planar) surface normal. An example of a conventional, bulk component in accordance with this disclosure would be a common lens (e.g. biconvex lens) having a pair of opposing spherical surfaces - that is, surfaces both definable by a spherical function based on x-squared and y-squared terms. In some embodiments of the present disclosure, the bulk optic is a plano-convex component -i.e. the first (light-receiving) surface is piano or flat and the second (light-receiving) surface is concave and bends light by refraction owing to a refractive index change. In some embodiments, the bulk optic is a plastic optic. In some embodiments of the present disclosure, at least one (light-receiving) surface of the bulk optic (e.g. the second or concave surface) is definable by a mathematical function comprising x-squared and y-squared terms and higher order terms. In comparative examples, the light control device is fundamental based refraction using an arrangement of triangular prisms. The elongate dimension of the triangular prims may be non-linear. The triangular prisms of the array are configured (e.g. shaped such as curved in the elongate direction or arranged in a concentric ring formation) to encode regular or irregular optical power which may compensation forthat of an optical combiner (e.g. windscreen). The triangular prism structure can also suppress reflection such as sunlight glare. The previously proposed light control devices for reflection / glare suppression is described in GE32607672E3 and GB2627988A for example. Due to the relative angles between the display device, the surface of the optical component, the user’s position and the user’s line of sight, there is also a need to optically adjust (e.g. “globally turn”) the display light emerging from the optical component. That is, there is a need to provide a so-called “global turn” of the display light. A turning function defining the “global turn” may ensure that the display light travels in a direction coincident with the user’s line of sight (referred to as the optical axis). The turning function is in the azimuthal direction. A “global turn”, in accordance with the present disclosure, is a rotation of a light propagation axis of a wavefront that maintains the integrity of the wavefront because all parts of the wavefront (or all light rays thereof) are turned by the same amount (and in the same direction). Accordingly, a single value may define the global turn of the wavefront. A global turn may be provided by refraction using a linear ramp or uniform blazed grating. A “global turn function” or “global turning function” is a function (e.g. mathematic equation or function) that defines a “global turn”. In contrast, the present disclosure also refers to components having optical power which requires a non-uniform turn (e.g. refraction) of light rays of the wavefront - that is, different light rays or parts of the received wavefront may be turned by different amounts. In some embodiments, there is a need for a global turn on two orthogonal planes of the optical component. That is, on both planes orthogonal to the optical component and orthogonal to each other. In these embodiments, it may therefore be said that there is a need for a global turn comprising a first component and a second component. A first aspect of the present disclosure is a display system. The display system comprising a replicator, a light control device and a diffractive optical element. The replicator is arranged to receive spatially modulated light. The replicator is further arranged to replicate the spatially modulated light to form a plurality of replicas of the spatially modulated light by waveguiding between a reflective surface and a transmissive-reflective surface. The transmissive-reflective surface forms an output surface for the plurality of replicas of the spatially modulated light. The light control device is located in the optical path of the plurality of replicas of the spatially modulated light. The light control device is downstream from the output surface of the replicator. The light control device is a bulk optic. For the avoidance of doubt, the bulk optic does not comprise a Fresnel structure. The bulk optic comprises at least one light-receiving surface definable by a continuous function. The at least one lightreceiving surface has non-uniform power that compensates for the non-uniform power of an optical combiner downstream thereof. The non-uniform power of the bulk optic is refractive (i.e. provided by refraction). The diffractive optical element is arranged to provide a global turn of the spatially modulated light and replicas thereof. The global turn is diffractive (i.e. provided by diffraction). A display system comprising a replicator, in accordance with the present disclosure, typically requires a light control device to e.g. manage sunlight glare and address the optical distortion caused when used with an optical combiner having complex curvature. In some applications, such as head-up display fora vehicle, it is also advantageous for the component to provide a global turn of the display light to be performed. The system and structure of the present disclosure perform a global turn of an image-forming wavefront and plurality of replicas thereof formed by a replicator. This global turn is typically provided by the light control device which is a Fresnel structure arranged to refract light. Broadly, the present disclosure addresses the technical problem of how to compensate for the complex curvature of the optical combiner using a refractive structure and provide a global turn without introducing image artefacts The light control device may comprise a glare mitigation structure partially embedded therein. The glare mitigation structure may comprise a louvre array (or prismatic array) in which louvres (triangular prism) of the louvre array (or prismatic array) protrude from a surface of the light control device. More generally, it may be said that glare mitigation elements of the glare mitigation structure protrude the light control layer or are partially embedded therein. The louvre array (or prismatic array) may be arranged to receive direct sunlight - that is, face the sun when in-use. The louvres (or triangular prims) may be partially embedded on a piano-side of the light control device. The louvres (or triangular prisms) may be at least partially surrounded by air. For example, a sunlight-receiving portion of the louvres (or prisms) may be surrounded by air. It may also be said that a portion of the louvres (or prisms) is surrounded by (or embedded within) material e.g. plastic of the light control device. The light control device and partially embedded glare mitigation elements may be formed by injection moulding using plastic. For the avoidance of doubt, in accordance with embodiments, the refractive structure of the present disclosure does not perform a global turn. In other words, a global turn associated with the refractive light control device is zero. Accordingly, the requirements (e.g. optical demands) placed on the structure of the light control device are relaxed to an extent that image quality improvements are realised. The diffractive optical element may be arranged to principally redirect the spatially modulated light and replicas thereof into a non-zero diffraction order. The term “principally redirect” is used herein to mean that, in these embodiments, the spatial modulated light is redirected into a non-zero (such as the first) diffractive order with near maximum efficiency -e.g. at least 95% such as at least 98% of the optical energy is redirected into the non-zero diffractive order. An angle between the zero-order and non-zero diffraction order may be non-zero on a second plane. The first plane may be substantially perpendicular to the second plane. The diffractive optical element may comprise liquid crystal such as a Reactive Mesogen. The global turn of the diffractive optical element may be achromatic. The diffractive optical element may be located downstream, or upstream, of the light control device. The diffractive optical element may be optically equivalent to a lineargrating such as a linear blazed grating. The light control device may have negative optical power. The light control device may comprise a phase function such as a phase-delay function. Additionally, or alternatively, the diffractive optical element may comprise a phase function such as a phase-delay function. The spatially modulated light may comprise a holographic wavefront or the spatially modulated light comprises a picture wavefront. A second aspect of the present disclosure is a structure (e.g. bulk optic such as plastic optic) for the output surface of an optical component of a head-up display or a waveguide. The structure comprises a light control device and a diffractive optical element. The light control device comprises a bulk optic having a light-receiving surface comprising non-uniform power that compensates for the non-uniform power of an optical combiner downstream thereof. The non-uniform power of the structure is refractive. The diffractive optical element is arranged to provide a global turn of the spatially modulated light and replicas thereof. The global turn is diffractive. A third aspect of the present disclosure is a method of display. The method comprises a first step of receiving and replicating spatially modulated light to form a plurality of replicas thereof by waveguiding between a reflective surface and a transmissive-reflective surface. The method comprises a second step of compensating for the non-uniform optical power of an optical combiner using a refractive component upstream thereof. The refractive component is a conventional, bulk optic such as a plastic optic. The method comprises a third step of globally turning the spatially modulated light and replicas thereof using a diffractive optical element. The term “non-uniform power” is used herein to refer to a non-uniform turning or optical power of a component that is non-constant or varying across the component - i.e. is a function of position or location on the component. More specifically, the term reflects that the turning or optical power of the component varies across a light-receiving or input surface thereof - i.e. is a function of a least one dimension of the component such as x or y. That is, a first sub-area (or point) of the (surface of the) component has a first optical power and a second sub-area thereof has a second optical power, wherein the first optical power and second optical power are unequal. In some embodiments, the non-uniform power is nonlinear power or irregular optical power. For example, the term “non-uniform power” (or “complex power”) may be used to describe a vehicle windscreen in accordance with the present disclosure, wherein the optical power of the windscreen is a function of position (e.g. in x and / or y) on the inner surface thereof. In the case of a windscreen in a head-up display, an optical turn is provided by reflection and so the turn may be described as a reflective turn. The curvature of the windscreen is complex (e.g. may be defined by a polynomial containing spherical and higher order terms) and so the reflective turn imparted on a light ray varies across the inner surface of the windscreen. In the case of a bulk optic of the present disclosure, an optical turn is provided by refraction using a curved surface. In some embodiments, the surface or profile of the structure may be described by a polynomial comprising squared terms and higher powers such that the turning power and optical power are also functions (i.e. non constant). In accordance with the present disclosure, the term “compensates” is used to refer to an optical effect of a first component (e.g. the optical power of a light control device) being the opposite or inverse of a second component (e.g. the optical power of an optical combiner such as windscreen) such that the net effect is substantially zero. In other words, the second component nullifies an optical effect of the first component. For example, if a first component compensates for the optical power of a second optical combiner, the net or resultant optical power associated with the combination of the first and second component is zero. In some embodiments, it may be said that the magnitude of a compensation provided by the light control device is a function of position thereon. In some embodiments, this is because an optical power of an optical combiner (e.g. windscreen) of the display system is non-uniform - i.e. varies across a surface (e.g. inner surface) thereof. The term “refractive” is used herein to refer to an optical effect provided by or resulting from refraction - that is, obeying the rules of refraction or characterised by the rules of refraction. The reader will be familiar with the idea that refraction is a rotation or bending of light caused by a refractive index difference or change. The term “diffractive” is used herein to refer to an optical effect provided by or resulting from diffraction - that is, obeying the rules of diffraction or characterised by the rules of diffraction. The reader will be familiar with the idea that diffraction is caused by light spreading as it passes through an aperture or around an object. 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 5E3 shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each in the form of a solid waveguide; Figure 6 is a schematic side view of a display system according to a first embodiment of the present disclosure; Figure 7 is a schematic side view of a corrective optic in combination with a diffractive optical element; Figure 8 is a schematic side view of a display system according to a second embodiment of the present disclosure; Figure 9 is a schematic side view of a display system according to a third embodiment of the present disclosure; Figure 10 is a schematic side view of a diffractive optical element according to a fourth embodiment of the present disclosure; Figure 11 represents a linear ramp function for global turn; Figure 12 represents a light control device or corrective optic for windscreen correction; and Figures 13A and 13B represent embodiments in which the light control device is a plastic optic with a partially embedded glare mitigation structure. The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF EMBODIMENTS The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration. Terms of a singular form may include plural forms unless specified otherwise. A structure described as being formed at an upper portion / lower portion of another structure or on / under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between. In describing a time relationship - for example, when the temporal order of events is described as “after”, “subsequent”, “next”, “before” or suchlike - the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as “just”, “immediate” or “direct” is used. Although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims. Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in codependent relationship. In the present disclosure, the term “substantially” when applied to a structural units of an apparatus may be interpreted as the technical feature of the structural units being produced within the technical tolerance of the method used to manufacture it. Conventional optical configuration for holographic projection Figure 1 shows an embodiment in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or freguency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, “LCOS”, device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser. A light source 110, for example a laser or laser diode, is disposed to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally planar wavefront of light to be incident on the SLM. In Figure 1, the direction of the wavefront is off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer). However, in other embodiments, the generally planar wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in Figure 1, the arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a 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 pre-calculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms. Large field of view using small display device Broadly, the present disclosure relates to image projection. It relates to a method of image projection and an image projector which comprises a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system, in which the image projector projects or relays light from the display device to the viewing system. The present disclosure is equally applicable to a monocular and binocular viewing system. The viewing system may comprise a viewer’s eye or eyes. The viewing system comprises an optical element having optical power (e.g., lens / es of the human eye) and a viewing plane (e.g., retina of the human eye / s). The projector may be referred to as a ‘light engine’. The display device and the image formed (or perceived) using the display device are spatially separated from one another. The image is formed, or perceived by a viewer, on a display plane. In some embodiments, the image is a virtual image and the display plane may be referred to as a virtual image plane. In other examples, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed either in free space or on a screen or other light receiving surface between the display device and the viewer, is propagated to the viewer. In both cases, an image is formed by illuminating a diffractive pattern (e.g., hologram or kinoform) displayed on the display device. The display device comprises pixels. The pixels of the display may display a diffractive pattern or structure that diffracts light. The diffracted light may form an image at a plane spatially separated from the display device. In accordance with well-understood optics, the magnitude of the maximum diffraction angle is determined by the size of the pixels and other factors such as the wavelength of the light. In embodiments, the display device is a spatial light modulator such as liquid crystal on silicon (“LCOS”) spatial light modulator (SLM). Light propagates over a range of diffraction angles (for example, from zero to the maximum diffractive angle) from the LCOS, towards a viewing entity / system such as a camera or an eye. In some embodiments, magnification techniques may be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of an LCOS. In some embodiments, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) - that may be informally said to be “encoded” with / by the hologram - is propagated directly to the viewer’s eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction I image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device. Reference is made herein to a “light field” which is a “complex light field”. The term “light field” merely indicates a pattern of light having a finite size in at least two orthogonal spatial directions, e.g. x and y. The word “complex” is used herein merely to indicate that the light at each point in the light field may be defined by an amplitude value and a phase value, and may therefore be represented by a complex number or a pair of values. For the purpose of hologram calculation, the complex light field may be a two-dimensional array of complex numbers, wherein the complex numbers define the light intensity and phase at a plurality of discrete locations within the light field. In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity / system, varies with the distance between the display device and the viewing entity. At a 1 metre viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye’s pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye’s pupil to form an image at the retina for a given eye position, determines the portion of the image that is ‘visible’ to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-box.) In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device - that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it may therefore be considered that the viewer is looking at a virtual image through an ‘display device-sized window’, which may be very small, for example 1cm in diameter, at a relatively large distance, e.g., 1 metre. And the user will be viewing the display device-sized window via the pupil(s) of their eye(s), which can also be very small. Accordingly, the field of view becomes small and the specific angular range that can be seen depends heavily on the eye position, at any given time. A pupil expander addresses the problem of how to increase the range of angles of light rays that are propagated from the display device that can successfully propagate through an eye’s pupil to form an image. The display device is generally (in relative terms) small and the projection distance is (in relative terms) large. In some embodiments, the projection distance is at least one - such as, at least two - orders of magnitude greater than the diameter, or width, of the entrance pupil and / or aperture of the display device (i.e., size of the array of pixels). Use of a pupil expander increases the viewing area (i.e., user’s eye-box) laterally, thus enabling some movement of the eye / s to occur, whilst still enabling the user to see the image. As the skilled person will appreciate, in an imaging system, the viewing area (user’s eye box) is the area in which a viewer’s eyes can perceive the image. The present disclosure encompasses non-infinite virtual image distances - that is, near-field virtual images. Conventionally, a two-dimensional pupil expander comprises one or more one-dimensional optical waveguides each formed using a pair of opposing reflective surfaces, in which the output light from a surface forms a viewing window or eye-box. Light received from the display device (e.g., spatially modulated light from a LCOS) is replicated by the or each waveguide so as to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide enlarges the viewing window due to the generation of extra rays or “replicas” by division of amplitude of the incident wavefront. The display device may have an active or display area having a first dimension that may be less than 10 cms such as less than 5 cms or less than 2 cms. The propagation distance between the display device and viewing system may be greater than 1 m such as greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide may be up to 2 m such as up to 1.5 m or up to 1 m. The method may be capable of receiving an image and determining a corresponding hologram of sufficient quality in less than 20 ms such as less than 15 ms or less than 10 ms. In some embodiments - described only by way of example of a diffracted or holographic light field in accordance with this disclosure - a hologram is configured to route light into a plurality of channels, each channel corresponding to a different part (i.e. sub-area) of an image. The channels formed by the diffractive structure are referred to herein as “hologram channels” merely to reflect that they are channels of light encoded by the hologram with image information. It may be said that the light of each channel is in the hologram domain rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram and the hologram domain is therefore the Fourier or frequency domain. The hologram may equally be a Fresnel or Fresnel transform hologram. The hologram may also be a point cloud hologram. The hologram is described herein as routing light into a plurality of hologram channels to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-areas, wherein each hologram channel would correspond to each image sub-area. Importantly, the hologram of this example is characterised by how it distributes the image content when illuminated. Specifically and uniquely, the hologram divides the image content by angle. That is, each point on the image is associated with a unique light ray angle in the spatially modulated light formed by the hologram when illuminated - at least, a unique pair of angles because the hologram is two-dimensional. For the avoidance of doubt, this hologram behaviour is not conventional. The spatially modulated light formed by this special type of hologram, when illuminated, may be divided into a plurality of hologram channels, wherein each hologram channel is defined by a range of light ray angles (in two-dimensions). It will be understood from the foregoing that any hologram channel (i.e. sub-range of light ray angles) that may be considered in the spatially modulated light will be associated with a respective part or sub-area of the image. That is, all the information needed to reconstruct that part or sub-area of the image is contained within a sub-range of angles of the spatially modulated light formed from the hologram of the image. When the spatially modulated light is observed as a whole, there is not necessarily any evidence of a plurality of discrete light channels. Nevertheless, the hologram may still be identified. For example, if only a continuous part or sub-area of the spatially modulated light formed by the hologram is reconstructed, only a sub-area of the image should be visible. If a different, continuous part or sub-area of the spatially modulated light is reconstructed, a different sub-area of the image should be visible. A further identifying feature of this type of hologram is that the shape of the cross-sectional area of any hologram channel substantially corresponds to (i.e. is substantially the same as) the shape of the entrance pupil although the size may be different - at least, at the correct plane for which the hologram was calculated. Each light I hologram channel propagates from the hologram at a different angle or range of angles. Whilst these are example ways of characterising or identifying this type of hologram, other ways may be used. In summary, the hologram disclosed herein is characterised and identifiable by how the image content is distributed within light encoded by the hologram. Again, for the avoidance of any doubt, reference herein to a hologram configured to direct light or angularly-divide an image into a plurality of hologram channels is made by way of example only and the present disclosure is equally applicable to pupil expansion of any type of holographic light field or even any type of diffractive or diffracted light field. The system can be provided in a compact and streamlined physical form. This enables the system to be suitable for a broad range of real-world applications, including those for which space is limited and real-estate value is high. For example, it may be implemented in a head-up display (HUD) such as a vehicle or automotive HUD. In accordance with the present disclosure, pupil expansion is provided for diffracted or diffractive light, which may comprise diverging ray bundles. The diffracted light field may be defined by a “light cone”. Thus, the size of the diffracted light field (as defined on a two-dimensional plane) increases with propagation distance from the corresponding diffractive structure (i.e. display device). It can be said that the pupil expander / s replicate the hologram or form at least one replica of the hologram, to convey that the light delivered to the viewer is spatially modulated in accordance with a hologram. In some embodiments, two one-dimensional waveguide pupil expanders are provided, each one-dimensional waveguide pupil expander being arranged to effectively increase the size of the exit pupil of the system by forming a plurality of replicas or copies of the exit pupil (or light of the exit pupil) of the spatial light modulator. The exit pupil may be understood to be the physical area from which light is output by the system. It may also be said that each waveguide pupil expander is arranged to expand the size of the exit pupil of the system. It may also be said that each waveguide pupil expander is arranged to expand / increase the size of the eye box within which a viewer’s eye can be located, in order to see / receive light that is output by the system. Light channelling The hologram formed in accordance with some embodiments, angularly-divides the image content to provide a plurality of hologram channels which may have a cross-sectional shape defined by an aperture of the optical system. The hologram is calculated to provide this channelling of the diffracted light field. In some embodiments, this is achieved during hologram calculation by considering an aperture (virtual or real) of the optical system, as described above. Figures 2 and 3 show an example of this type of hologram that may be used in conjunction with a pupil expander as disclosed herein. However, this example should not be regarded as limiting with respect to the present disclosure. Figure 2 shows an image 252 for projection comprising eight image areas / components, V1 to V8. Figure 2 shows eight image components by way of example only and the image 252 may be divided into any number of components. Figure 2 also shows an encoded light pattern 254 (i.e., hologram) that can reconstruct the image 252 - e.g., when transformed by the lens of a suitable viewing system. The encoded light pattern 254 comprises first to eighth sub-holograms or components, H1 to H8, corresponding to the first to eighth image components / areas, V1 to V8. Figure 2 further shows how a hologram may decompose the image content by angle. The hologram may therefore be characterised by the channelling of light that it performs. This is illustrated in Figure 3. Specifically, the hologram in this example directs light into a plurality of discrete areas. The discrete areas are discs in the example shown but other shapes are envisaged. The size and shape of the optimum disc may, after propagation through the waveguide, be related to the size and shape of an aperture of the optical system such as the entrance pupil of the viewing system. Figure 4 shows a system 400, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3. The system 400 comprises a display device, which in this arrangement comprises an LCOS 402. The LCOS 402 is arranged to display a modulation pattern (or ‘diffractive pattern') comprising the hologram and to project light that has been holographically encoded towards an eye 405 that comprises a pupil that acts as an aperture 404, a lens 409, and a retina (not shown) that acts as a viewing plane. There is a light source (not shown) arranged to illuminate the LCOS 402. The lens 409 of the eye 405 performs a hologram-to-image transformation. The light source may be of any suitable type. For example, it may comprise a laser light source. The viewing system 400 further comprises a waveguide 408 positioned between the LCOS 402 and the eye 405. The presence of the waveguide 408 enables all angular content from the LCOS 402 to be received by the eye, even at the relatively large projection distance shown. This is because the waveguide 508 acts as a pupil expander, in a manner that is well known and so is described only briefly herein. In brief, the waveguide 408 shown in Figure 4 comprises a substantially elongate formation. In this example, the waveguide 408 comprises an optical slab of refractive material, but other types of waveguide are also well known and may be used. The waveguide 408 is located so as to intersect the light cone (i.e., the diffracted light field) that is projected from the LCOS 402, for example at an oblique angle. In this example, the size, location, and position of the waveguide 408 are configured to ensure that light from each of the eight ray bundles, within the light cone, enters the waveguide 408. Light from the light cone enters the waveguide 408 via its first planar surface (located nearest the LCOS 402) and is guided at least partially along the length of the waveguide 408, before being emitted via its second planar surface, substantially opposite the first surface (located nearest the eye). As will be well understood, the second planar surface is partially reflective, partially transmissive. In other words, when each ray of light travels within the waveguide 408 from the first planar surface and hits the second planar surface, some of the light will be transmitted out of the waveguide 408 and some will be reflected by the second planar surface, back towards the first planar surface. The first planar surface is reflective, such that all light that hits it, from within the waveguide 408, will be reflected back towards the second planar surface. Therefore, some of the light may simply be refracted between the two planar surfaces of the waveguide 408 before being transmitted, whilst other light may be reflected, and thus may undergo one or more reflections, (or ‘bounces’) between the planar surfaces of the waveguide 408, before being transmitted. Figure 4 shows a total of nine “bounce” points, BO to B8, along the length of the waveguide 408. Although light relating to all points of the image (V1-V8) as shown in Figure 2 is transmitted out of the waveguide at each “bounce” from the second planar surface of the waveguide 408, only the light from one angular part of the image (e.g. light of one of V1 to V8) has a trajectory that enables it to reach the eye 405, from each respective “bounce” point, BO to B8. Moreover, light from a different angular part of the image, V1 to V8, reaches the eye 405 from each respective “bounce” point. Therefore, each angular channel of encoded light reaches the eye only once, from the waveguide 408, in the example of Figure 4. The waveguide 408 forms a plurality of replicas of the hologram, at the respective “bounce” points B1 to B8 along its length, corresponding to the direction of pupil expansion. As shown in Figure 4, the plurality of replicas may be extrapolated back, in a straight line, to a corresponding plurality of replica or virtual display devices 402’. This process corresponds to the step of “unfolding” an optical path within the waveguide, so that a light ray of a replica is extrapolated back to a “virtual surface” without internal reflection within the waveguide. Thus, the light of the expanded exit pupil may be considered to originate from a virtual surface (also called an “extended modulator” herein) comprising the display device 402 and the replica display devices 402’. Although virtual images, which require the eye to transform received modulated light in order to form a perceived image, have generally been discussed herein, the methods and arrangements described herein can be applied to real images. Two-Dimensional Pupil Expansion Whilst the arrangement shown in Figure 4 includes a single waveguide that provides pupil expansion in one dimension, pupil expansion can be provided in more than one dimension, for example in two dimensions. Moreover, whilst the example in Figure 4 uses a hologram that has been calculated to create channels of light, each corresponding to a different portion of an image, the present disclosure and the systems that are described herebelow are not limited to such a hologram type. Figure 5A shows a perspective view of a system 500 comprising two replicators, 504, 506 arranged for expanding a light beam 502 in two dimensions. In the system 500 of Figure 5A, the first replicator 504 comprises a first pair of surfaces, stacked parallel to one another, and arranged to provide replication - or, pupil expansion -in a similar manner to the waveguide 408 of Figure 4. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially elongate in one direction. The collimated light beam 502 is directed towards an input on the first replicator 504. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), which will be familiar to the skilled reader, light of the light beam 502 is replicated in a first direction, along the length of the first replicator 504. Thus, a first plurality of replica light beams 508 is emitted from the first replicator 504, towards the second replicator 506. The second replicator 506 comprises a second pair of surfaces stacked parallel to one another, arranged to receive each of the collimated light beams of the first plurality of light beams 508 and further arranged to provide replication - or, pupil expansion - by expanding each of those light beams in a second direction, substantially orthogonal to the first direction. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially rectangular. The rectangular shape is implemented for the second replicator in order for it to have length along the first direction, in order to receive the first plurality of light beams 508, and to have length along the second, orthogonal direction, in order to provide replication in that second direction. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), light of each light beam within the first plurality of light beams 508 is replicated in the second direction. Thus, a second plurality of light beams 510 is emitted from the second replicator 506, wherein the second plurality of light beams 510 comprises replicas of the input light beam 502 along each of the first direction and the second direction. Thus, the second plurality of light beams 510 may be regarded as comprising a two-dimensional grid, or array, of replica light beams. Thus, it can be said that the first and second replicators 504, 505 of Figure 5A combine to provide a two-dimensional replicator (or, “two-dimensional pupil expander”). Thus, the replica light beams 510 may be emitted along an optical path to an expanded eye-box of a display system, such as a head-up display. In the system of Figure 5A, the first replicator 504 is a waveguide comprising a pair of elongate rectilinear reflective surfaces, stacked parallel to one another, and, similarly, the second replicator 504 is a waveguide comprising a pair of rectangular reflective surfaces, stacked parallel to one another. In other systems, the first replicator may be a solid elongate rectilinear waveguide and the second replicator may be a solid planar rectangular shaped waveguide, wherein each waveguide comprises an optically transparent solid material such as glass. In this case, the pair of parallel reflective surfaces are formed by a pair of opposed major sidewalls optionally comprising respective reflective and reflective-transmissive surface coatings, familiar to the skilled reader. Figure 5E3 shows a perspective view of a system 500 comprising two replicators, 520, 540 arranged for replicating a light beam 522 in two dimensions, in which the first replicator is a solid elongated waveguide 520 and the second replicator is a solid planar waveguide 540. In the system of Figure 5B, the first replicator / waveguide 520 is arranged so that its pair of elongate parallel reflective surfaces 524a, 524b are perpendicular to the plane of the second replicator / waveguide 540. Accordingly, the system comprises an optical coupler arranged to couple light from an output port of first replicator 520 into an input port of the second replicator 540. In the illustrated arrangement, the optical coupler is a planar / fold mirror 530 arranged to fold or turn the optical path of light to achieve the required optical coupling from the first replicator to the second replicator. As shown in Figure 5B, the mirror 530 is arranged to receive light - comprising a one-dimensional array of replicas extending in the first dimension - from the output port I reflective-transmissive surface 524a of the first replicator / waveguide 520. The mirror 530 is tilted so as to redirect the received light onto an optical path to an input port in the (fully) reflective surface of second replicator 540 at an angle to provide waveguiding and replica formation, along its length in the second dimension. It will be appreciated that the mirror 530 is one example of an optical element that can redirect the light in the manner shown, and that one or more other elements may be used instead, to perform this task. In the illustrated arrangement, the (partially) reflective-transmissive surface 524a of the first replicator 520 is adjacent the input port of the first replicator / waveguide 520 that receives input beam 522 at an angle to provide waveguiding and replica formation, along its length in the first dimension. Thus, the input port of first replicator / waveguide 520 is positioned at an input end thereof at the same surface as the reflective-transmissive surface 524a. The skilled reader will understand that the input port of the first replicator / waveguide 520 may be at any other suitable position. Accordingly, the arrangement of Figure 5B enables the first replicator 520 and the mirror 530 to be provided as part of a first relatively thin layer in a plane in the first and third dimensions (illustrated as an x-z plane). In particular, the size or “height” of a first planar layer - in which the first replicator 520 is located - in the second dimension (illustrated as the y dimension) is reduced. The mirror 530 is configured to direct the light away from a first layer / plane, in which the first replicator 520 is located (i.e. the “first planar layer"), and direct it towards a second layer / plane, located above and substantially parallel to the first layer / plane, in which the second replicator 540 is located (i.e. a “second planar layer”). Thus, the overall size or “height” of the system - comprising the first and second replicators 520, 540 and the mirror 530 located in the stacked first and second planar layers in the first and third dimensions (illustrated as an x-z plane) - in the second dimension (illustrated as the y dimension) is compact. The skilled reader will understand that many variations of the arrangement of Figure 5B for implementing the present disclosure are possible and contemplated. The image projector may be arranged to project a diverging or diffracted light field. In some embodiments, the light field is encoded with a hologram. In some embodiments, the diffracted light field comprises diverging ray bundles. In some embodiments, the image formed by the diffracted light field is a virtual image. In some embodiments, the first pair of parallel I complementary surfaces are elongate or elongated surfaces, being relatively long along a first dimension and relatively short along a second dimension, for example being relatively short along each of two other dimensions, with each dimension being substantially orthogonal to each of the respective others. The process of reflection / transmission of the light between / from the first pair of parallel surfaces is arranged to cause the light to propagate within the first waveguide pupil expander, with the general direction of light propagation being in the direction along which the first waveguide pupil expander is relatively long (i.e., in its “elongate” direction). There is disclosed herein a system that forms an image using diffracted light and provides an eye-box size and field of view suitable for real-world application - e.g. in the automotive industry by way of a head-up display. The diffracted light is light forming a holographic reconstruction of the image from a diffractive structure - e.g. hologram such as a Fourier or Fresnel hologram. The use diffraction and a diffractive structure necessitates a display device with a high density of very small pixels (e.g. 1 micrometer) - which, in practice, means a small display device (e.g. 1 cm). 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 reference). 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. Light shuttering device The present disclosure is also compatible with optical configurations that include a light shuttering device to control the delivery of light from a light channelling hologram to the viewer. The holographic projector may further comprise a control device arranged to control the delivery of angular channels to the eye-box position. British patent application 2108456.1, filed 14 June 2021 and incorporated herein by reference, discloses the at least one waveguide pupil expander and control device. The reader will understand from at least this prior disclosure that the optical configuration of the control device is fundamentally based upon the eye-box position of the user and is compatible with any hologram calculation method that achieves the light channeling described herein. It may be said that the control device is a light shuttering or aperturing device. The light shuttering device may comprise a 1D array of apertures or windows, wherein each aperture or window independently switchable between a light transmissive and a light non-transmissive state in order to control the delivery of hologram light channels, and their replicas, to the eye-box. Each aperture or window may comprise a plurality of liquid crystal cells or pixels. Diffractive global turn Figure 6 shows a side schematic view of a display system 600 of a first embodiment of the present disclosure. A waveguide 602 replicates inputted light as per the second waveguide 540 described above. That is, a one-dimensional array (i.e. a line) of replicas in the x-direction are input into the waveguide 602. An output surface of the waveguide 602 emits a two-dimensional array of replicas on the x-z plane that is received by a corrective optic 604 (more broadly termed a light control device or, more specifically, termed a plastic optic). The corrective optic 604 may optionally reduce the glare that would otherwise reflect off the waveguide 602 towards the user. The corrective optic 604 principally provides compensation for the complex curvature of a windscreen 610 (more broadly termed an optical combiner). The function of a synergistic glare mitigation device is further described in GB2607672B, GB2627988A and GB2401627.1, which are incorporated herein by reference. The replicas output from the corrective optic 604 are then received by a diffractive optical element 606, the function of which will be described below. Replicas 608 are emitted from the diffractive optical element 606 towards the windscreen 610. The diffractive optical element 606 applies a global turn to the replicas output by the waveguide 602, as will be described in more detail below. The replicas 608 reflect off the windscreen 610 to arrive at an eye-box 612 (in other words, a viewing window). The eye-box 612 is determined to be the area in which the user can view the replicas 608 to form the desired image to a satisfactory quality and / or with an acceptably low number of optical anomalies or distortions (as decided upon by the designer of the system 600). In other words, in this example, the diffractive optical element 606 is arranged on top of the corrective optic 604 (in the y-direction), which is in turn arranged on top of the waveguide 602 (in the y-direction). That is, the optical path the display light through the display system 600 is: from the waveguide 602 to the corrective optic 604, then to the diffractive optical element 606 before being reflected off the windscreen 610 towards the eye-box 612. Figure 7 is a section view of Figure 6, showing a schematic representation of the path the replicas 608 take through the corrective optic 604 and the diffractive optical element 606. The diffractive optical element 606 applies a global turn to each replica 608 (by diffraction) which may, for example, ensure the light ultimately arrives at the eye-box 612 in the correct position for usability and comfort of the viewer. The diffractive optical element 606 is designed to substantially diffract the light into one diffractive order. For example, the diffractive optical element 606 may receive the light at normal incidence (defining a “zero” order direction) and substantially redirect all the light into e.g. the +1- nth diffractive order, wherein n is an integer greater than zero. The person skilled in the art of diffractive optical elements can design such a component in accordance with established design methodologies. The diffractive optical element may comprise liquid crystal such as Reactive Mesogen. As discussed above, the diffractive optical element applies a global turn to each replica 608. The diffractive optical element 606 may be optically equivalent to a linear, blazed grating. The global turn provided by the diffractive optical element 606 may be a function of wavelength e.g. non-linear with wavelength of light. However, in other embodiments e.g. based on Reactive Mesogen, the global turn is achromatic. Figure 8 shows a side schematic view of a display system 800 of a second embodiment of the present disclosure. This embodiment has the same components as discussed in relation to the first embodiment and Figures 6 and 7, but in this embodiment the diffractive optical element 606 is located between the waveguide 602 and the corrective optic 604. In other words, the optical path of the display light through the display system 600 is: from the waveguide 602 to the diffractive optical element 606, then to the corrective optic 604 before being reflected off the windscreen 610 towards the eye-box 612. Figure 9 shows a side schematic view of a display system 900 of a third embodiment of the present disclosure. In this embodiment, a first diffractive optical element 906i is located before the waveguide 602 on the optical path through the system 900. As such, the first diffractive optical element 906i receives a one-dimensional array of replicas 908 (for example from the first waveguide 520 as described above in relation to Figure 5). This onedimensional array of replicas 908 is then globally turned by the first diffractive optical element 906i (as described above in relation to the diffractive optical element 606) before entering the corrective optic 604 as previously described. The output of the corrective optic 604 is then directed to a second diffractive optical element 906ii which provides a second global turn. The first diffraction optical element 906i may be stronger (i.e. greater global turn) than the second diffractive optical element 906ii or vice versa. Figures 6 to 9 are schematic and by example only. As such, the skilled person would understand that the relative sizes, proportions and positions of the components may be different in physical implementations of the embodiments shown therein. Reactive Mesogen for diffractive global turn In a fourth embodiment (a variation of the first, second and third embodiments), the diffractive optical element 1006 is formed of a reactive mesogen material that contains polymerizable liquid crystal materials patterned to impart the above-described turn (more specifically, a spatially varying geometric phase retardation) by means of the Pancharatnam-Berry effect. The design of such a reactive mesogen diffractive optical element 1006 is described below in relation to Figure 10. This turn is independent of wavelength and so can be achieved egually for all wavelengths of replica 608, 908. The periodicity d of the diffractive optical elements 606, 906 of the first, second and third embodiments produces a turn angle 0 for a given single wavelength A following the diffraction eguation: d ■ sin 0 = A As described above, the turn (i.e. the diffraction) angle 0 is dependent on the wavelength A. Meanwhile, the direction of light through a reactive mesogen diffractive optical element 1006 is normal to the wavefront emerging therefrom and can be determined such that: (2 ■ it / X) ■ Ax ■ sin 0 = A0 where Ax is the length across the reactive mesogen diffractive optical element 1006 (i.e. the length in the x-direction of Figure 10) and △<£ is the phase retardation caused by the reactive mesogen diffractive optical element 1006 across the length Ax. As such, it has been found that the same global turn can be produced by the reactive mesogen diffractive optical element 1006 as the diffractive optical elements 606, 906 of the first, second and third embodiments so long as the linear varying retardance of the reactive mesogen material (that is, the rate at which A4> changes over Ax) is chosen according to the equation: A0 / Ax = 2 ■ n / d As this does not have a wavelength dependence, the global turn provided by the reactive mesogen diffractive optical element 1006 occurs equally across all wavelengths and so efficiency can be maximised in all wavelengths without the need to prioritise one colour of light. In some embodiments, the diffractive optical element is therefore described as being achromatic. In some embodiments, the Reactive Mesogen layer is sandwiched between a pair of quarter-wave plates. Examples In some embodiments, the diffractive optical element is optically equivalent to a linear ramp - at least in the way it provides a global turn. Figure 11 shows an example linear ramp that could be used to form the diffractive optical element of the present disclosure. By way of example only, the linear ramp of Figure 11 is divided into twenty discrete zones 1104a, 1104b, 1104c ... 1104s, 1104t which progressively increase in height to form a wedge shape. In this example, the first zones on the lefthand side 1104a corresponds to a minimum value of height (i.e. the thin end of the wedge) and the twenty zone 1104t shown on the righthand side of the figure has the maximum height (i.e. the thick end of the wedge). The zones are substantially parallel and arranged to provide a 1D or 2D global turn. In the example of Figure 11, the global turn has an x and y component. For the avoidance of doubt, Figure 11 is merely representative of the optical functionality of the component which is fundamentally diffractive. In accordance with the present disclosure, Figure 12 shows an example of a non-Fresnel light control device that compensates for the complex curvature of a windscreen and, optionally, suppress glare. This component is a refractive optical element. By way of example only, the refractive structure of Figure 12 is divided into discrete zones 1204a, 1204b, 1204c ... 1204i, 1204j which progressive increase in height. The surface defined by the changing height provides optical power. The complex shape of the component may be determined by any number of different experimental or computational approaches familiar to the person skilled in the art of optical design such as ray tracing. Figures 13A and 13B show two examples of the light control device of the present disclosure in the form of a bulk optic e.g. plastic optic. In the embodiments represented by Figures 13A and 13B, the plastic optic has a plano-concave shape. Figure 13A shows a waveguide 1304 which may be the second of a pair of waveguides as described herein. Figure 13A further shows the plastic optic 1320 and a glare mitigation structure 1360 thereof. In this embodiment, the glare mitigation structure is a louvre array 1360 comprising a plurality of louvres. The louvres are at least partially embedded in the plastic optic 1320. The louvres are an array of inclined (relative to the surface that causes glare e.g. waveguide 1304 surface) surfaces or slats that suppress reflection by e.g. absorption as described in the above referenced documents. The angle and spacing of the slats are determined to absorb direct sunlight and reflections whilst not adversely affecting the display light. In Figure 13A, the concave side of the plastic optic faces the waveguide 1804. Figure 13A shows the general direction of the display light 1380 and the general direction of sunlight 1370 which may cause glare owing to e.g. a reflective surface of the waveguide 1304. The plastic optic 1320 may be formed by injection moulding. Accordingly, the louvres may be at least partially embedded in the plastic. In this embodiment, the louvres of the louvre array at least partially protrude the plastic optic e.g. piano surface thereof. The louvres are at least partially surrounded by air. The louvre array is arranged in-use to receive direct sunlight. These embodiments may be advantageous because the medium surrounding a portion (e.g. optically-active portion) of the louvres is air not plastic. This may simplify the louvre design process. In other words, this geometry is advantageous from a glare mitigation perspective. Figure 13B shows a variation in which the piano side of the plastic optic faces the waveguide 1304 rather than the concave side. In this variation, the louvres at least partially protrude the curved surface providing the refractive power for windscreen compensation. This variation may be advantageous from a packaging and fixing perspective because the output surface of the waveguide is also piano. The plastic optic may have a refractive index of 1.4 to 1.6 such as 1.55 +1- 0.05 and any plastic having suitable transmission characteristics may be used. The curved surface of the plastic optic may be described by a polynomial in x and y comprising mixed terms e.g. up to and including the sixth powers. The plastic optic may be described as having a (irregular) bowl-shape. The diffractive component for global turn may be disposed before or after the plastic optic. Shared turning In another embodiment, the global turn is shared between the (refractive) plastic optic and the diffractive component. In some embodiments, the majority of the global turn (e.g. at least 75% thereof in degrees) is provided by the diffractive component and the rest is provided by the refractive component. In these embodiments, the advantages of the diffractive global turn may be substantially maintained. In another variation, the corrective function and turning function are both performed by a liquid crystal component - e.g. Reactive Mesogen sandwich between two quarter-wave plates. These variations can be advantageous for image resolution (owing to the larger Fresnel zone width), contrast (fewer dark band artefacts), optical disparity and astigmatism. Nevertheless, comparative examples in which the refractive component performs both the corrective function and global turn do have some advantages such as it can be easier to manage chromatic performance and simultaneously incorporate glare mitigation. Examples in which the global turn is shared between the plastic optics and the diffractive component can strike a good balance. 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. 5 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 10 medium. Examples of such a carrier medium include a transient medium (e.g., a propagating signal that communicates instructions). It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. The present disclosure 15 covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

1. A display system comprising:a replicator arranged to receive spatially modulated light and replicate the spatially5 modulated light to form a plurality of replicas thereof by waveguiding between a reflective surface and a transmissive-reflective surface, wherein the transmissive-reflective surface forms an output surface for the plurality of replicas of the spatially modulated light;a light control device located in the optical path of the plurality of replicas of the spatially modulated light, wherein the light control device comprises a bulk optic having non-10 uniform power that compensates for the non-uniform power of an optical combiner downstream thereof, wherein the non-uniform power of the bulk optic is refractive, wherein the light control device is plano-concave; anda diffractive optical element arranged to provide a global turn of the spatially modulated light and replicas thereof, wherein the global turn is diffractive.

152. A display system as claimed in claim 1 wherein the concave surface of the light LO control device faces the reflective or transmissive-reflective surface of the replicator.CXI"1” 3. A display system as claimed in any preceding claim wherein the concave surface of1 20 the light control device has the non-uniform power that compensates for the non-uniformpower of the optical combiner downstream thereof.

4. A display system as claimed in any preceding claim wherein the bulk optic has a dimension (e.g. height) that is no more than 15 mm such as no more than 10 mm.

5. The display system as claimed in any preceding claim wherein the light control device has a focal length of 1 to 4 metres such as 2 to 3 metres.

6. The display system as claimed in any preceding claim wherein the light control30 device comprises a glare mitigation structure partially embedded therein, optionally, wherein the glare mitigation structure comprises a louvre array in which louvres of the louvre array protrude from a surface of the light control device.

7. The display system as claimed in claim 6 wherein the louvres are at least partially35 embedded on a piano-side of the light control device, optionally, wherein the piano side is arranged to receive direct sunlight when in-use.

8. A display system as claimed in any preceding claim wherein the diffractive optical element is arranged to principally redirect the spatially modulated light and replicas thereof into a non-zero diffraction order.

59. A display system as claimed in claim 8 wherein an angle between the zero-order and non-zero diffraction order is non-zero on a second plane, wherein the first plane is substantially perpendicular to the second plane10 10. A display system as claimed in any preceding claim wherein the diffractive opticalelement comprises liquid crystal such as a Reactive Mesogen.

11. A display system as claimed in any preceding claim wherein the global turn of the diffractive optical element is achromatic.1512. A display system as claimed in any preceding claim wherein the diffractive optical LO element is located downstream of the light control device.CXI13. A display system as claimed in any of claims 1 to 11 wherein the diffractive optical20 element is located upstream of the light control device."1” 14. The display system as claimed in any preceding claim wherein the light controldevice comprises negative optical power.25 15. The display system as claimed in any preceding claim wherein the spatiallymodulated light comprises a holographic wavefront or the spatially modulated light comprises a picture wavefront.

16. A structure for the output surface of waveguide, the structure comprising:30 a light control device comprising a bulk optic having non-uniform power thatcompensates for the non-uniform power of an optical combiner downstream thereof, wherein the non-uniform power of the conventional, bulk optic is refractive, wherein the light control device is plano-concave; anda diffractive optical element arranged to provide a global turn of the spatially35 modulated light and replicas thereof, wherein the global turn is diffractive.

17. A method of display comprisingreceiving and replicating spatially modulated light to form a plurality of replicas thereof by waveguiding between a reflective surface and a transmissive-reflective surface;compensating for the non-uniform optical power of an optical combiner using a refractive component upstream thereof, wherein the refractive component is a bulk optic,5 wherein the refractive component is plano-concave; andglobally turning the spatially modulated light and replicas thereof using a diffractive optical element using a diffractive component.14 11 25s

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