Waveguide height reduction

Abstract

A head-up display having an eye-box, and sequentially: a first optical sub-system, a waveguide 900, and a second optical sub-system. The first optical sub-system receives an input wavefront having a f

Description

FIELD The present disclosure relates to pupil expansion or hologram replication. More specifically, the present disclosure relates to a display system comprising a waveguide and to a method of pupil expansion or hologram replication using a waveguide. The waveguide provides pupil expansion or hologram replication for a holographic wavefront or diffracted light field comprising diverging ray bundles. Some embodiments relate to two-dimensional pupil expansion or hologram replication. Some embodiments relate to a picture generating unit and a head-up display, for example an automotive head-up display (HUD). BACKGROUND AND INTRODUCTION Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or “hologram”, comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object. Computer-generated holography may numerically simulate the interference process. A computer-generated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. A Fourier hologram may be considered a Fourier domain / plane representation of the object or a frequency domain / plane representation of the object. A computer-generated hologram may also be calculated by coherent raytracing or a point cloud technique, for example. A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micromirrors, for example. A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, “HUD”. SUMMARY Aspects of the present disclosure are defined in the appended independent claims. There is disclosed a pupil expander or hologram replicator for a head-up display comprising an eye-box having a first dimension (e.g., horizontal or x dimension) and a second dimension (e.g. vertical or y dimension). A first waveguide is arranged to replicate an input wavefront, comprising a diverging or converging light ray bundle, in the first dimension of the eye-box. A second waveguide may be arranged to replicate the input wavefront, and any replicas thereof formed by the first waveguide, in the second dimension of the eye-box. In some embodiments, the wavefront is a holographic wavefront but the present disclosure is equally applicable to non-holographic wavefronts such as image wavefronts or wavefronts carrying image or picture content in the spatial domain rather than hologram domain. In implementations, each waveguide comprises a pair of parallel reflective surfaces between which input light is waveguided. These surfaces may be referred to as the waveguiding surfaces. Each of the waveguiding surfaces may have a length defined between a first end and a second end, and a width substantially perpendicular to the length. The pair of reflective surfaces may comprise a substantially reflective surface and a reflective-transmissive surface. The reflective-transmissive surface may form an output surface (or output port) of the waveguide. The pair of reflective surfaces may be arranged to provide waveguiding of input light received at the range of input angles and to form a plurality of replicas of the input light field. In some examples, the input port may be provided in the substantially reflective surface. The first waveguide may be a substantially elongate waveguide, such as a rod-shaped waveguide (i.e., a bulk optic waveguide). In particular, the first waveguide may be elongate in the direction of pupil expansion. The holographic wavefront or input light field may be diverging input light, such that the size of its wavefront increases with propagation distance. The first waveguide may configured so that the size of the wavefront does not exceed the width of the waveguiding surfaces along the length of the first waveguide. In some implementations, the holographic wavefront or light field (or a pupil corresponding thereto) comprises diverging light, for example a diffracted light field comprising diverging ray bundles. For example, the holographic wavefront may comprise spatially modulated light in accordance with a hologram. In some arrangements, a hologram is provided at an input plane of a first optical sub-system upstream of the waveguide. In some examples, an image of a hologram displayed on a display device is relayed to the input plane. In other examples, the plane of a display device arranged to display the hologram is disposed at the input plane of the first optical sub-system. An output plane of the first optical sub-system may be disposed in close proximity of an input port of the waveguide or inside the waveguide. A first example of the present disclosure is a hologram replicator for a head-up display. The head-up display has an eye-box defined by a first (e.g. horizontal) and second (e.g. vertical) dimension. The hologram replicator comprises a first optical sub-system and a first waveguide. The first optical sub-system has one-dimensional (positive) magnification in the second dimension. The first optical sub-system is arranged to receive an input holographic wavefront formed by a hologram. The input holographic wavefront may comprise a diverging light ray bundle characterised by a maximum divergence angle. The first optical sub-system is further arranged to output a stretched holographic wavefront. A size in the second dimension of the stretched holographic wavefront is greater than that of the input holographic wavefront in accordance with the one-dimensional magnification. The first waveguide is arranged to receive the stretched holographic wavefront from the first optical sub-system. The first waveguide is further arranged to output the stretched holographic wavefront and a plurality (e.g. linear array) of replicas thereof extending in the first dimension by waveguiding between a reflective surface and a reflective-transmissive surface thereof. A divergence angle of the stretched holographic wavefront in the second dimension is less than that of the input holographic wavefront. The rate of expansion of the (stretched) holographic wavefront in the second dimension is reduced during waveguiding. The divergence angle of the stretched holographic wavefront is non-zero or greaterthan zero. The divergence angle of the input holographic wavefront is non-zero or greater than zero. In head-up display, the size of the device is critical. For example, there is an overwhelming demand in the automotive industry to reduce the size of the device required to provide head- up display. A compact head-up display is disclosed herein using a small pupil, waveguiding and hologram-to-eye encoding. A first and second waveguide may be used to replicate the pupil or holographic wavefront in the horizontal (e.g. x) and vertical (e.g. y) direction, respectively. The first waveguide may be elongated and may provide horizontal replication. Some examples of the present disclosure address a problem resulting from the long optical path length within the first waveguide required to provide a large viewing window or so-called “eye-box”. In examples, the angular field of view of the head-up display corresponds to the divergence of the holographic wavefront that encodes the picture. The angular field of view may be defined by a first and second component. The first component may be a horizontal (e.g. x) angular field of view such as -15 to +15 degrees. The second component may be a vertical (e.g. y) angular field of view such as +5 to -5 degrees. Broadly, some examples address the problem that the vertical field of view and the long path length place a demand I requirement on the height of the first waveguide. In some examples, the height of the first waveguide has a significant impact on the overall size of the device. More specifically, some examples of the present disclosure address the problem of how to reduce the height (e.g. in the y-direction) of the first waveguide (which may be elongate in e.g. the x-direction). Typically, in a direct to eye holographic HUD display system, the horizontal pupil expansion / replication is done by passing the optical path into a first waveguide such that the light propagates along internally bouncing off the front and rear surfaces allowing some light to exit on each bounce off the exit surface. In a simple (non-astigmatic system) the optical footprint is smallest on entry into the first waveguide (where a pupil is formed) and diverges into an ever-increasing ray bundle size as the light propagates along the rod due to the angular connect to create the field of view. As a result, the height of the first waveguide needs to be tall enough to accommodate this increasing ray bundle size. This can be problematic when trying keep the height of the device small. The first example addresses this problem using a first optical sub-system having onedimensional magnification in the y-direction (e.g. the “height” direction of the device) prior to in-coupling into the waveguide. The size of the footprint of the holographic wavefront coupled into the first waveguide is therefore increased in one dimension (e.g. the y-direction). An aspect ratio of the footprint is therefore changed. In an optical system in which compactness is critical, increasing the size of the light field is a counterintuitive approach. It is further surprising to increase the size of the light field in only one dimension or change the aspect ratio of the light field prior to waveguiding. However, the inventors recognised that, in optical terms, this stretching or one-dimensional magnification of the footprint of the holographic wavefront is optically equivalent to stretching a pupil (i.e. a top or limiting aperture) of an optical system. The inventors confirmed in simulation and practice that this meant that the principle of conservation of etendue applied and that therefore an expansion or “stretch” of a footprint of the holographic wavefront in spatial terms caused a corresponding reduction or “compression” of a divergence of the holographic wavefront in angular terms. The inventors therefore identified that by stretching the footprint of the holographic wavefront prior to waveguiding they could reduce the divergence of the holographic wavefront and therefore reduce the required height of the waveguide - which is particularly significant given the relatively long optical path length required within the first waveguide. Importantly, the inventors further recognised, and confirmed, that this process was optically reversible even in the hologram domain / with a holographic wavefront. In accordance with this disclosure, the vertical field of view is reduced (with corresponding pupil size increase) prior to coupling into the first waveguide. By doing so, the rate at which the field of view increases the height of the ray bundle within the first waveguide is reduced and therefore reduces the height that the first waveguide needs to be. Consequently, on exiting the first waveguide, optical components need to be applied to increase the vertical field of view (with corresponding pupil size reduction). Advantageously, examples of the present disclosure provide a reduction in the height of the first waveguide and therefore package height and optic sizes. Further advantageously, examples of the present disclosure reduce the height of the first waveguide and also reduce the size of the coupling window of the second waveguide (when used) - further reducing package size and increasing optical efficiency increase. The inventors balanced the optical footprint at the exit face of the first waveguide with the optical footprint on entry. As the field of view angles are reduced - the pupil size is increased. If the field of view is excessively compressed, a tall waveguide results due to a large entry pupil tapering down to narrow footprint at the ends. In some examples, an optimum is achieved by reducing the field of view such that the height of the entry pupil is equal to the field of view height as propagated at the ends. The one-dimensional magnification may be 1.5 to 5.0 such as 2.0 to 4.0. A divergence angle of the (stretched) holographic wavefront in the second dimension in the waveguide may be less than 1.5 degrees such as less than 1 degree. The size and divergence angle of the stretched holographic wavefront in the second dimension and an optical path length of the waveguiding define an optical region or optical sub-region (e.g. a cross-sectional area or volume) of the waveguide. The word “optical” is used here to reflect that the waveguided light may not propagate through all parts of the waveguide. The term “physical” is used here to reflect the real-world or actual physical size of the waveguide. The optical area may be at least 50% such as at least 75% of the physical area of the waveguide. In some examples, the first waveguide is rectilinear in which case the optical area or volume and physical area or volume of the first waveguide may be different. In other examples, the height of the first waveguide is tapered in accordance with the (vertical) divergence angle of the holographic wavefront within the waveguide. In these examples, the difference between the optical area or volume and physical area or volume is reduced by the tapering. In these examples, the size of the first waveguide in the second dimension (e.g. the height) at the distal or exit end is no more than 20% greater than the size at the proximal or input end. In some examples, the size of the first waveguide in the second dimension (e.g. the height) at the distal or exit end is no more than 10% greater than the size at the proximal or input end. The words “proximal” and “distal” are used here with respect to the input port of the first waveguide - that is, the part of the first waveguide where the holographic wavefront enters or in-couples. In some examples, light ray angles of each holographic wavefront correspond to spatial coordinates of a picture encoded by the hologram. A divergence angle of each holographic wavefront may correspond to an angle of an (angular) field of view of the picture. The first optical system and second optical sub-system may each comprise at least one selected from the group comprising: an anamorphic prism pair; a cylindrical achromatic doublet lens; and a cylindrical mirror having optical power. The hologram replicator may further comprise a second optical sub-system having onedimensional de-magnification in the second dimension. The second optical sub-system may be arranged to receive the stretched holographic wavefront, and plurality of replicas thereof. The second optical sub-system may be arranged to output a compressed holographic wavefront, and plurality of replicas thereof. A size in the second dimension of the compressed holographic wavefront, and each replica thereof, may be less than that of the stretched holographic wavefront in accordance with the one-dimensional de-magnification. It was surprising that the effect disclosed herein could be perfectly reversed in the hologram domain without adverse consequences e.g. a loss of image content or the emergence of image artefacts. The one-dimensional de-magnification may be substantially the inverse of the onedimensional magnification. A divergence angle of the compressed holographic wavefront may be substantially equal to that of the input holographic wavefront. In some examples, a (non-zero) divergence angle of the compressed holographic wavefront in the second dimension is greater than that of the stretched holographic wavefront. The hologram replicator may further comprise a second waveguide arranged to replicate the input holographic wavefront in the second dimension. The second waveguide may comprise an input port arranged to receive the output of the first waveguide. A second example of the present disclosure is a method of head-up display or a method of reducing the volume of a head-up display device. The method comprises a first step of forming a holographic wavefront, comprising a diverging light ray bundle characterised by a maximum divergence angle, from a hologram of a picture for head-up display. The method comprises a second step of waveguiding the holographic wavefront between a reflective surface and a reflective-transmissive surface of a waveguide. The method comprises a third step of outputting, from the waveguide, the holographic wavefront and a plurality (e.g. linear array) of replicas thereof extending in the first dimension. The method reduces a rate of expansion of the holographic wavefront in the second dimension before waveguiding by stretching (i.e. increasing the size of) the holographic wavefront in the second dimension in accordance with a one-dimensional magnification of a first optical sub-system. Accordingly, it is found that a divergence angle of the holographic wavefront in the second dimension after one-dimensional stretching is less than that of the holographic wavefront received by the first optical sub-system. In some examples, the method further comprises compressing the holographic wavefront, and plurality of replicas thereof, after waveguiding in accordance with a one-dimensional demagnification of a second optical sub-system. Accordingly, it is found that a divergence angle of the holographic wavefront in the second dimension after one-dimensional compression is greaterthan that of the holographic wavefront during waveguiding. In some examples, the hologram is arranged to divide picture content of the picture by angle. Light ray angles of the holographic wavefront may correspond to spatial coordinates of the picture / angles of the angular field of view of the head-up display. In implementations comprising a second waveguide, the first and second waveguides are arranged in a substantially planar configuration. The planar configuration assists in providing a more compact arrangement. In some examples, the substantially planar configuration is in a plane of the propagations paths of the replicas formed by the first waveguide. There is provided a head-up display comprising a pupil expander as described above. The term “footprint” is used herein to refer to a cross-sectional shape or area of the light field or wavefront on the plane perpendicular to the general propagation axis or optical axis of the system (e.g. z-direction). The term “stretched” is used herein to reflect that the size of the holographic wavefront in only one direction (e.g. the vertical or y direction) is increased - in examples, prior to waveguiding. For the avoidance of doubt, the size of the holographic wavefront in the horizontal direction is not increased - e.g. prior to waveguiding. This step is therefore referred to herein as “stretching” the holographic wavefront but, importantly, the process is strictly one-dimensional only. The aspect ratio of the holographic wavefront is changed by the “stretching”. More specifically, the footprint of the holographic wavefront is stretched in one dimension. As explained further in the detailed description, the inventors identified that, in the optical system of the present disclosure, this process is optically equivalent or analogous to increasing the size of an optical pupil in one dimension. It may therefore be said that the input “pupil” of the waveguide or the “pupil” of the hologram or display device is stretched to reflect this analogy. The terms “stretching” and “compressing” a wavefront are used herein as shorthand for stretching or compressing a footprint of the wavefront. In some embodiments, only a sub-area of the replay field is used for displaying picture content. In other words, there is a reduced size available for the image (picture) on the replay field. The available image size may be reduced compared to the full replay field size due to the presence of a DC spot (a bright spot at the centre of the replay field). This DC spot is a bright spot of light that spoils the viewing experience. It may also be beneficial to allocate an area of the replay field for non-image content such as “noise”. The areas of the replay field containing the noise and DC spot are then discarded, to leave a smaller available image area (referred to herein as a “sub-area” of the replay field), but one that can display a high-quality holographic reconstruction. In some embodiments, over half of the replay field has been discarded in this way, and so it is therefore important to ensure that the available image area is not decreased any further. In this respect, it has been identified by the inventor that the magnification and demagnification (in other words, the stretching and compression) in above-described system can be used (i.e. tuned) to optimise onward optical coupling. That is, in real-world embodiments, the aspect ratio of the sub-area of the replay field has a different aspect ratio to that of the, or required by the, optics downstream thereof. Any difference, or mismatch, in this aspect ratio can result in areas of the replay field that effectively cannot be used to display image content because they won’t be receivable by the optics downstream. In other words a holographic system using an SLM with square pixels will natively form a square replay field with a DC order in the middle of that replay field. Previous DC removal techniques (e.g. blocking with a mask) result in a ‘dead’ region of the image formed on the diffuser. This limits the system to use less than half of the replay field or requires a more complex optical arrangement. The number available pixels that form the final virtual image are distributed over the full replay field. The PPD (pixel per degree) in the final virtual image perceived by the viewer is determined by the proportion of the replay field that is used to form that image (in other words, the number of pixels of the sub-area of the replay field that is used to display picture content). As described above, the amount of sub-area of the replay field area that can be used for picture content is a function of the aspect ratio that the downstream optics (e.g. windscreen corrective optics) require. This typically means some PPD is wasted, as the aspect ratio required by the downstream optics (e.g. windscreen corrective optics such as freeform optics) does not match the aspect ratio available at the plane of the holographic reconstruction (e.g. the plane of a diffuser). In a first aspect of the present disclosure, a head-up display (or wavefront replicator therefor) is provided. The head-up display has an eye-box defined by a first and second dimension. The head-up display comprises a first optical sub-system having one-dimensional magnification in the second dimension. The first optical sub-system is arranged to receive an input wavefront having a first aspect ratio and output a stretched wavefront. A size in the second dimension of the stretched wavefront is greater than that of the input wavefront in accordance with the one-dimensional magnification. The head-up display further comprises a waveguide arranged to receive the stretched wavefront and output the stretched wavefront and a plurality of replicas thereof extending in the first dimension by waveguiding between a reflective surface and a reflective-transmissive surface. The head-up display also comprises a second optical sub-system having one-dimensional de-magnification in the second dimension. The second optical sub-system is arranged to receive the stretched wavefront, and plurality of replicas thereof, and output a compressed wavefront, and plurality of replicas thereof. A size in the second dimension of the compressed wavefront, and each replica thereof, is less than that of the stretched wavefront in accordance with the one-dimensional de-magnification. The one-dimensional de-magnification is mismatched with respect to the one-dimensional magnification. The mismatch corresponds to the difference between the first aspect ratio and a second aspect ratio, the second aspect ratio being of an optical component downstream of the second optical sub-system. For the avoidance of doubt, a “magnification" has a magnitude greater than one (or unity) and a “de-magnification” has a magnitude less than one (or unity). In accordance with this disclosure, a magnification and a de-magnification are “matched” when the (magnitude of the) magnification is equal to the inverse of the (magnitude of the) de-magnification, or vice versa. In other words, the product of a magnification and its “matched” de-magnification is one (or unity). Thus, in accordance with this disclosure, a magnification is said to be mismatched with a de-magnification (or vice versa) when the product is not quite unity, such as 0.75 to 1.25 but not exactly 1.00. In short, the inventor has recognised an advantage of introducing a small “mismatch” between the magnification and de-magnification of the optics used either side of the (first) waveguide. This advantage comes at no cost. Conventionally, these optics would be “matched” to ensure there is no net optical effect. However, the inventor has recognised that the “mismatch” may be used to optimise coupling with any optics that follow downstream in the system - such as windscreen corrective optic / s. More specifically, the inventor recognised that the deliberate mismatch may be used to optimise coupling with any optics that follow. This approach is flexible and tuneable to the optics that follow downstream by changing the magnitude of the mismatch. In other words, the mismatch means that the magnification is substantially the inverse of the de-magnification, but with a small over- or under-compensation. That is, if (for example) the magnification is 3.0, the de-magnification may be (3.1)1. In this way, by purposefully choosing a mismatch between the magnification and demagnification, one can adjust (e.g. optimise) the aspect ratio, such that the aspect ratio of the image content better matches the aspect ratio of the available image area. As such, there is a reduction in area of the replay field that would have been unusable for picture content. This means that the image content can be displayed over the largest possible area of the replay field and PPD can be maximised in the final virtual image. In other words, by using optical elements to optimise the aspect ratio after the (first) waveguide, the proportion of replay field area used can be increased - and therefore the number of pixels used in the final virtual image will be maximised. This is in contrast to the previous design assumption of matching the prism pair ratios. Thus, in accordance with the present disclosure, the PPD in the final virtual image can be maximised. The mismatch in accordance with this disclosure may comprise the product of the onedimensional magnification and the one-dimensional de-magnification being (a) less than 1 but greater than 0.75 or (b) greater than 1 but less than 1.25. In other words, the mismatch in accordance with this disclosure may be a mismatch between the first and second aspect ratios of up to + / - 25% such as +1-10%. That is, the magnification can be anywhere from 25% stronger than the de-magnification to 25% weaker than the de-magnification, so long as the magnification and de-magnification are not the same. The head-up display may further comprise the optical component downstream of the second optical subsystem. The optical component may be an optic arranged to compensate for the curvature of an optical combiner downstream thereof. The optic may be a freeform mirror or corrective optic. The optical combiner may be a windscreen of a vehicle housing the head-up display. As in the example described above, the one-dimensional magnification may be 1.5 to 5.0, such as 2.0 to 4.0. Also as in the example described above, if the wavefront is a holographic wavefront, light ray angles of each holographic wavefront may correspond to spatial coordinates of a picture encoded by a hologram such that a divergence angle of each holographic wavefront corresponds to an angle of a field of view of the picture. However, the present disclosure is equally applicable to the holographic wavefront as discussed above as it is to non-holographic (e.g. picture) wavefront. As in the examples described above, a divergence angle of the stretched wavefront in the second dimension may be less than that of the input wavefront in order to reduce the rate of expansion of the wavefront in the second dimension during waveguiding. The divergence angle of the wavefront in the second dimension in the waveguide may be less than 1.5 degrees such as less than 1 degree. The size and divergence angle of the stretched wavefront in the second dimension and an optical path length of the waveguiding may define an optical footprint of the waveguide, wherein the optical footprint may be at least 50% such as at least 75% of the physical footprint of the waveguide. As in the examples described above, the first optical system may comprise an anamorphic prism pair. The first optical system may comprise a cylindrical lens such as a cylindrical achromatic doublet lens. The first optical system may comprise a cylindrical mirror having optical power. As in the examples described above, a (non-zero) divergence angle of the compressed wavefront in the second dimension may be greater than that of the stretched wavefront. As in the examples described above, the second optical sub-system may comprise at least one selected from the group comprising: an anamorphic prism pair; a cylindrical achromatic doublet lens; and / or a cylindrical mirror having optical power. The first optical sub-system may comprise a first anamorphic prism pair and the second optical sub-system may comprise a second anamorphic prism pair. The first anamorphic prism pair may be shaped relative to the second anamorphic prism pair to produce the mismatch. By changing the shape of the respective prism pairs relative to one another, the aspect ratio of the replay field can be better filed by the available image area, as described above. As in the examples described above, the head-up display may further comprise a second waveguide arranged to replicate the input wavefront in the second dimension, wherein the second waveguide comprises an input port arranged to receive the output of the (first) waveguide. In a second aspect of the present disclosure a method of head-up display is provided. The method comprises a first step of reducing a rate of expansion of an input wavefront by stretching the wavefront in the second dimension in accordance with a one-dimensional magnification of a first optical sub-system to form a stretched wavefront. The method comprises a second step of waveguiding the stretched wavefront between a reflective surface and a reflective-transmissive surface of a waveguide. The method comprises a third step of outputting, from the waveguide, the stretched wavefront and a plurality of replicas thereof extending in the first dimension. The method comprises a fourth step of compressing the stretched wavefront, and plurality of replicas thereof, in accordance with a one-dimensional de-magnification of a second optical sub-system. An optical component is located downstream of the second optical sub-system, the optical component having a second aspect ratio. The one-dimensional de-magnification is mismatched with respect to the one-dimensional magnification, the mismatch corresponding to the difference between the first and second aspect ratios. As in the examples described above, the reduction in the rate of expansion of the input wavefront may be such that a divergence angle of the stretched wavefront in the second dimension after the one-dimensional stretching is less than that of the input wavefront received by the first optical sub-system. The compression of the stretched wavefront, and plurality of replicas thereof, may be such that a divergence angle of the output wavefront in the second dimension after the one-dimensional compression is greater than that of the stretched wavefront during waveguiding. As in the examples described above, if the wavefront is a holographic wavefront, the compression of the holographic wavefront, and plurality of replicas thereof may such that a divergence angle of the holographic wavefront in the second dimension after the compression is greaterthan that of the holographic wavefront during waveguiding. As discussed above, the mismatch may comprise the product of the one-dimensional magnification and the one-dimensional de-magnification being (a) less than 1 but greater than 0.75 or (b) greater than 1 but less than 1.25. In other words, as also discussed above, the mismatch in accordance with this disclosure may be an inverse between the first and second aspect ratios of up to 25%. In a third aspect of the present disclosure a method of designing a head-up display is provided. The head-up display comprises a first optical sub-system having one-dimensional magnification in the second dimension. The first optical sub-system is arranged to receive an input wavefront having a first aspect ratio and output a stretched wavefront. A size in the second dimension of the stretched wavefront is greaterthan that of the input wavefront in accordance with the one-dimensional magnification. The head-up display further comprises a waveguide arranged to receive the stretched wavefront and output the stretched wavefront and a plurality of replicas thereof extending in the first dimension by waveguiding between a reflective surface and a reflective-transmissive surface. The head-up display also comprises a second optical sub-system having one-dimensional de-magnification in the second dimension. The second optical sub-system is arranged to receive the stretched wavefront, and plurality of replicas thereof, and output a compressed wavefront, and plurality of replicas thereof. A size in the second dimension of the compressed wavefront, and each replica thereof, is less than that of the stretched wavefront in accordance with the one-dimensional de-magnification. The method comprises a step of mismatching the one-dimensional demagnification with respect to the one-dimensional magnification. The mismatch corresponds to the difference between the first aspect ratio and a second aspect ratio, the second aspect ratio being of an optical component downstream of the second optical sub-system. As discussed above, the mismatch is purposefully introduced into the head-up display, which goes against engineering principles, in order to better fit the available image area to the replay field. Thus, the head-up display can be designed in a way that it maximises the area of the replay field that can display the image to be projected. 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. The person skilled in the art of holography will appreciate that the complex light field associated with propagation of light encoded with a hologram will change with propagation distance. Use herein of the term “replica” is independent of propagation distance and so the two branches or paths of light associated with a replication event are still referred to as “replicas” of each other even if the branches are a different length, such that the complex light field has evolved differently along each path. That is, two complex light fields are still considered “replicas” in accordance with this disclosure even if they are associated with different propagation distances - providing they have arisen from the same replication event or series of replication events. A “diffracted light field” or “diffractive light field” in accordance with this disclosure is a light field formed by diffraction. A diffracted light field may be formed by illuminating a corresponding diffractive pattern. In accordance with this disclosure, an example of a diffractive pattern is a hologram and an example of a diffracted light field is a holographic light field or a light field forming a holographic reconstruction of an image. The holographic light field forms a (holographic) reconstruction of an image on a replay plane. The holographic light field that propagates from the hologram to the replay plane may be said to comprise light encoded with the hologram or light in the hologram domain. A diffracted light field is characterized by a diffraction angle determined by the smallest feature size of the diffractive structure and the wavelength of the light (of the diffracted light field). In accordance with this disclosure, it may also be said that a “diffracted light field” is a light field that forms a reconstruction on a plane spatially separated from the corresponding diffractive structure. An optical system is disclosed herein for propagating a diffracted light field from a diffractive structure to a viewer. The diffracted light field may form an image. The term “hologram” is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, regarding the object. The term “holographic reconstruction” is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The system disclosed herein is described as a “holographic projector” because the holographic reconstruction is a real image and spatially-separated from the hologram. The term “replay field” is used to refer to the 2D area within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of a plurality diffracted orders wherein each diffracted order is a replica of the zeroth-order replay field. The zeroth-order replay field generally corresponds to the preferred or primary replay field because it is the brightest replay field. Unless explicitly stated otherwise, the term “replay field” should be taken as referring to the zeroth-order replay field. The term “replay plane” is used to refer to the plane in space containing all the replay fields. The terms “image”, “replay image” and “image region” refer to areas of the replay field illuminated by light of the holographic reconstruction. In some embodiments, the “image” may comprise discrete spots which may be referred to as “image spots” or, for convenience only, “image pixels”. The terms “encoding”, “writing” or “addressing” are used to describe the process of providing the plurality of pixels of the SLM with a respective plurality of control values which respectively determine the modulation level of each pixel. It may be said that the pixels of the SLM are configured to “display” a light modulation distribution in response to receiving the plurality of control values. Thus, the SLM may be said to “display” a hologram and the hologram may be considered an array of light modulation values or levels. It has been found that a holographic reconstruction of acceptable quality can be formed from a “hologram” containing only phase information related to the Fourier transform of the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography. The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully-complex computer-generated hologram is calculated. Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for “phase-delay”. That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2nj which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of ir / 2 will retard the phase of received light by tt / 2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term “grey level” may be used to refer to the plurality of available modulation levels. For example, the term “grey level” may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term “grey level” may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator. The hologram therefore comprises an array of grey levels - that is, an array of light modulation values such as an array of phase-delay values or complex modulation values. The hologram is also considered a diffractive pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light having a wavelength comparable to, generally less than, the pixel pitch of the spatial light modulator. Reference is made herein to combining the hologram with other diffractive patterns such as diffractive patterns functioning as a lens or grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the replay field on the replay plane or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a replay plane in the near field. Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged. BRIEF DESCRIPTION OF THE DRAWINGS Specific embodiments are described by way of example only with reference to the following figures: Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen; Figure 2 shows an image for projection comprising eight image areas / components, V1 to V8, and cross-sections of the corresponding hologram channels, H1-H8; Figure 3 shows a hologram displayed on an LCOS that directs light into a plurality of discrete areas; Figure 4 shows a system, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3; Figure 5A shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each comprising pairs of stacked surfaces; Figure 5B shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each in the form of a solid waveguide; Figure 6 is a side view of an elongate waveguide replicator for pupil expansion of a diverging input light field in a first dimension showing the optical footprint at the output surface in accordance with an example; Figure 7 is a side view of an elongate waveguide replicator for pupil expansion of a diverging input light field in a first dimension showing the optical footprint at the output surface in accordance with the present disclosure; Figure 8 is a top view of an elongate waveguide replicator for pupil expansion of first and second input light fields in opposite directions of a first dimension in accordance with an example; Figure 9 is a top view of an elongate waveguide replicator for pupil expansion of an input light field in opposite directions of a first dimension in accordance with the present disclosure; Figure 10 shows a method of reducing the height of the light footprint, and therefore optionally the height of the waveguide, in accordance with examples; Figure 11 shows a first example using anamorphic prism pairs to stretch and compress a holographic wavefront before and after waveguiding, respectively; Figure 12 shows a second example wherein the second optical sub-system comprises a pair of cylindrical achromatic doublet lenses; Figure 13 shows a third example wherein the second optical sub-system comprises a pair of a mirrors having optical power; Figure 14 shows a replay field of a head-up display according to the prior art; and Figure 15 shows a replay field of a head-up display according to the present disclosure. The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF EMBODIMENTS The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration. Terms of a singular form may include plural forms unless specified otherwise. A structure described as being formed at an upper portion / lower portion of another structure or on / under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between. In describing a time relationship - for example, when the temporal order of events is described as “after”, “subsequent”, “next", “before” or suchlike - the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as “just”, “immediate” or “direct” is used. Although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims. Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in codependent relationship. In the present disclosure, the term “substantially” when applied to a structural units of an apparatus may be interpreted as the technical feature of the structural units being produced within the technical tolerance of the method used to manufacture it. Conventional optical configuration for holographic projection Figure 1 shows an embodiment in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or freguency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, “LCOS”, device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser. A light source 110, for example a laser or laser diode, is disposed to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally planar wavefront of light to be incident on the SLM. In Figure 1, the direction of the wavefront is off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer). However, in other embodiments, the generally planar wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in Figure 1, the arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a lightmodulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to optics including a Fourier transform lens 120, having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives a beam of modulated light from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at the screen 125. Notably, in this type of holography, each pixel of the hologram contributes to the whole reconstruction. There is not a one-to-one correlation between specific points (or image pixels) on the replay field and specific light-modulating elements (or hologram pixels). In other words, modulated light exiting the light-modulating layer is distributed across the replay field. In these embodiments, the position of the holographic reconstruction in space is determined by the dioptric (focusing) power of the Fourier transform lens. In the embodiment shown in Figure 1, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens but the performance of the lens will limit the accuracy of the Fourier transform it performs. The skilled person understands how to use a lens to perform an optical Fourier transform In some embodiments of the present disclosure, the lens of the viewer’s eye performs the hologram to image transformation. Hologram calculation In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or Fourier-based hologram, in which an image is reconstructed in the far field by utilising the Fourier transforming properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back to the lens plane. Computer-generated Fourier holograms may be calculated using Fourier transforms. Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms which may be calculated by a similar method. In some embodiments, the hologram is a phase or phase-only hologram. However, the present disclosure is also applicable to holograms calculated by other techniques such as those based on point cloud methods. In some embodiments, the hologram engine is arranged to exclude from the hologram calculation the contribution of light blocked by a limiting aperture of the display system. British patent application 2101666.2, filed 5 February 2021 and incorporated herein by reference, discloses a first hologram calculation method in which eye-tracking and ray tracing are used to identify a sub-area of the display device for calculation of a point cloud hologram which eliminates ghost images. The sub-area of the display device corresponds with the aperture, of the present disclosure, and is used exclude light paths from the hologram calculation. British patent application 2112213.0, filed 26 August 2021 and incorporated herein by reference, discloses a second method based on a modified Gerchberg-Saxton type algorithm which includes steps of light field cropping in accordance with pupils of the optical system during hologram calculation. The cropping of the light field corresponds with the determination of a limiting aperture of the present disclosure. British patent application 2118911.3, filed 23 December 2021 and also incorporated herein by reference, discloses a third method of calculating a hologram which includes a step of determining a region of a so-called extended modulator formed by a hologram replicator. The region of the extended modulator is also an aperture in accordance with this disclosure. In some embodiments, there is provided a real-time engine arranged to receive image data and calculate holograms in real-time using the algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are pre-calculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms. Large field of view using small display device Broadly, the present disclosure relates to image projection. It relates to a method of image projection and an image projector which comprises a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system, in which the image projector projects or relays light from the display device to the viewing system. The present disclosure is equally applicable to a monocular and binocular viewing system. The viewing system may comprise a viewer’s eye or eyes. The viewing system comprises an optical element having optical power (e.g., lens / es of the human eye) and a viewing plane (e.g., retina of the human eye / s). The projector may be referred to as a ‘light engine’. The display device and the image formed (or perceived) using the display device are spatially separated from one another. The image is formed, or perceived by a viewer, on a display plane. In some embodiments, the image is a virtual image and the display plane may be referred to as a virtual image plane. In other examples, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed either in free space or on a screen or other light receiving surface between the display device and the viewer, is propagated to the viewer. In both cases, an image is formed by illuminating a diffractive pattern (e.g., hologram or kinoform) displayed on the display device. The display device comprises pixels. The pixels of the display may display a diffractive pattern or structure that diffracts light. The diffracted light may form an image at a plane spatially separated from the display device. In accordance with well-understood optics, the magnitude of the maximum diffraction angle is determined by the size of the pixels and other factors such as the wavelength of the light. In embodiments, the display device is a spatial light modulator such as liquid crystal on silicon (“LCOS”) spatial light modulator (SLM). Light propagates over a range of diffraction angles (for example, from zero to the maximum diffractive angle) from the LCOS, towards a viewing entity / system such as a camera or an eye. In some embodiments, magnification techniques may be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of an LCOS. In some embodiments, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) - that may be informally said to be “encoded” with / by the hologram - is propagated directly to the viewer’s eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction / image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device. Reference is made herein to a “light field” which is a “complex light field”. The term “light field” merely indicates a pattern of light having a finite size in at least two orthogonal spatial directions, e.g. x and y. The word “complex” is used herein merely to indicate that the light at each point in the light field may be defined by an amplitude value and a phase value, and may therefore be represented by a complex number or a pair of values. For the purpose of hologram calculation, the complex light field may be a two-dimensional array of complex numbers, wherein the complex numbers define the light intensity and phase at a plurality of discrete locations within the light field. In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity / system, varies with the distance between the display device and the viewing entity. At a 1 metre viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye’s pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye’s pupil to form an image at the retina for a given eye position, determines the portion of the image that is ‘visible’ to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-box.) In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device - that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it may therefore be considered that the viewer is looking at a virtual image through an ‘display device-sized window’, which may be very small, for example 1cm in diameter, at a relatively large distance, e.g., 1 metre. And the user will be viewing the display device-sized window via the pupil(s) of their eye(s), which can also be very small. Accordingly, the field of view becomes small and the specific angular range that can be seen depends heavily on the eye position, at any given time. A pupil expander addresses the problem of how to increase the range of angles of light rays that are propagated from the display device that can successfully propagate through an eye’s pupil to form an image. The display device is generally (in relative terms) small and the projection distance is (in relative terms) large. In some embodiments, the projection distance is at least one - such as, at least two - orders of magnitude greater than the diameter, or width, of the entrance pupil and / or aperture of the display device (i.e., size of the array of pixels). Use of a pupil expander increases the viewing area (i.e., user’s eye-box) laterally, thus enabling some movement of the eye / s to occur, whilst still enabling the user to see the image. As the skilled person will appreciate, in an imaging system, the viewing area (user’s eye box) is the area in which a viewer’s eyes can perceive the image. The present disclosure encompasses non-infinite virtual image distances - that is, near-field virtual images. Conventionally, a two-dimensional pupil expander comprises one or more one-dimensional optical waveguides each formed using a pair of opposing reflective surfaces, in which the output light from a surface forms a viewing window or eye-box. Light received from the display device (e.g., spatially modulated light from a LCOS) is replicated by the or each waveguide so as to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide enlarges the viewing window due to the generation of extra rays or “replicas” by division of amplitude of the incident wavefront. The display device may have an active or display area having a first dimension that may be less than 10 cms such as less than 5 cms or less than 2 cms. The propagation distance between the display device and viewing system may be greater than 1 m such as greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide may be up to 2 m such as up to 1.5 m or up to 1 m. The method may be capable of receiving an image and determining a corresponding hologram of sufficient quality in less than 20 ms such as less than 15 ms or less than 10 ms. In some embodiments - described only by way of example of a diffracted or holographic light field in accordance with this disclosure - a hologram is configured to route light into a plurality of channels, each channel corresponding to a different part (i.e. sub-area) of an image. The channels formed by the diffractive structure are referred to herein as “hologram channels” merely to reflect that they are channels of light encoded by the hologram with image information. It may be said that the light of each channel is in the hologram domain rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram and the hologram domain is therefore the Fourier or frequency domain. The hologram may equally be a Fresnel or Fresnel transform hologram. The hologram may also be a point cloud hologram. The hologram is described herein as routing light into a plurality of hologram channels to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-areas, wherein each hologram channel would correspond to each image sub-area. Importantly, the hologram of this example is characterised by how it distributes the image content when illuminated. Specifically, and unusually, 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. subrange of light ray angles) that may be considered in the spatially modulated light will be associated with a respective part or sub-area of the image. That is, all the information needed to reconstruct that part or sub-area of the image is contained within a sub-range of angles of the spatially modulated light formed from the hologram of the image. When the spatially modulated light is observed as a whole, there is not necessarily any evidence of a plurality of discrete light channels. Nevertheless, the hologram may still be identified. For example, if only a continuous part or sub-area of the spatially modulated light formed by the hologram is reconstructed, only a sub-area of the image should be visible. If a different, continuous part or sub-area of the spatially modulated light is reconstructed, a different sub-area of the image should be visible. A further identifying feature of this type of hologram is that the shape of the cross-sectional area of any hologram channel substantially corresponds to (i.e. is substantially the same as) the shape of the entrance pupil although the size may be different - at least, at the correct plane for which the hologram was calculated. Each light / hologram channel propagates from the hologram at a different angle or range of angles. Whilst these are example ways of characterising or identifying this type of hologram, other ways may be used. In summary, the hologram disclosed herein is characterised and identifiable by how the image content is distributed within light encoded by the hologram. Again, for the avoidance of any doubt, reference herein to a hologram configured to direct light or angularly-divide an image into a plurality of hologram channels is made by way of example only and the present disclosure is equally applicable to pupil expansion of any type of holographic light field or even any type of diffractive or diffracted light field. The system can be provided in a compact and streamlined physical form. This enables the system to be suitable for a broad range of real-world applications, including those for which space is limited and real-estate value is high. For example, it may be implemented in a head-up display (HUD) such as a vehicle or automotive HUD. In accordance with the present disclosure, pupil expansion is provided for diffracted or diffractive light, which may comprise diverging ray bundles. The diffracted light field may be defined by a “light cone”. Thus, the size of the diffracted light field (as defined on a two-dimensional plane) increases with propagation distance from the corresponding diffractive structure (i.e. display device). It can be said that the pupil expander / s replicate the hologram or form at least one replica of the hologram, to convey that the light delivered to the viewer is spatially modulated in accordance with a hologram. In some arrangements, 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 modulation The display system comprises a display device which in embodiments defines the exit pupil of the display system. The display device is a spatial light modulator. The spatial light modulation may be a phase modulator. The display device may be a liquid crystal on silicon, “LCOS”, spatial light modulator. Light channelling The hologram formed in accordance with some embodiments, angularly-divides the image content to provide a plurality of hologram channels which may have a cross-sectional shape defined by an aperture of the optical system. The hologram is calculated to provide this channelling of the diffracted light field. In some embodiments, this is achieved during hologram calculation by considering an aperture (virtual or real) of the optical system, as described above. Figures 2 and 3 show an example of this type of hologram that may be used in conjunction with a pupil expander as disclosed herein. However, this example should not be regarded as limiting with respect to the present disclosure. Figure 2 shows an image 252 for projection comprising eight image areas / components, V1 to V8. Figure 2 shows eight image components by way of example only and the image 252 may be divided into any number of components. Figure 2 also shows an encoded light pattern 254 (i.e., hologram) that can reconstruct the image 252 - e.g., when transformed by the lens of a suitable viewing system. The encoded light pattern 254 comprises first to eighth sub-holograms or components, H1 to H8, corresponding to the first to eighth image components / areas, V1 to V8. Figure 2 further shows how a hologram may decompose the image content by angle. The hologram may therefore be characterised by the channelling of light that it performs. This is illustrated in Figure 3. Specifically, the hologram in this example directs light into a plurality of discrete areas. The discrete areas are discs in the example shown but other shapes are envisaged. The size and shape of the optimum disc may, after propagation through the waveguide, be related to the size and shape of an aperture of the optical system such as the entrance pupil of the viewing system. Figure 4 shows a system 400, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3. The system 400 comprises a display device, which in this arrangement comprises an LCOS 402. The LCOS 402 is arranged to display a modulation pattern (or ‘diffractive pattern') comprising the hologram and to project light that has been holographically encoded towards an eye 405 that comprises a pupil that acts as an aperture 404, a lens 409, and a retina (not shown) that acts as a viewing plane. There is a light source (not shown) arranged to illuminate the LCOS 402. The lens 409 of the eye 405 performs a hologram-to-image transformation. The light source may be of any suitable type. For example, it may comprise a laser light source. The viewing system 400 further comprises a waveguide 408 positioned between the LCOS 402 and the eye 405. The presence of the waveguide 408 enables all angular content from the LCOS 402 to be received by the eye, even at the relatively large projection distance shown. This is because the waveguide 508 acts as a pupil expander, in a manner that is well known and so is described only briefly herein. In brief, the waveguide 408 shown in Figure 4 comprises a substantially elongate formation. In this example, the waveguide 408 comprises an optical slab of refractive material, but other types of waveguide are also well known and may be used. The waveguide 408 is located so as to intersect the light cone (i.e., the diffracted light field) that is projected from the LCOS 402, for example at an oblique angle. In this example, the size, location, and position of the waveguide 408 are configured to ensure that light from each of the eight ray bundles, within the light cone, enters the waveguide 408. Light from the light cone enters the waveguide 408 via its first planar surface (located nearest the LCOS 402) and is guided at least partially along the length of the waveguide 408, before being emitted via its second planar surface, substantially opposite the first surface (located nearest the eye). As will be well understood, the second planar surface is partially reflective, partially transmissive. In other words, when each ray of light travels within the waveguide 408 from the first planar surface and hits the second planar surface, some of the light will be transmitted out of the waveguide 408 and some will be reflected by the second planar surface, back towards the first planar surface. The first planar surface is reflective, such that all light that hits it, from within the waveguide 408, will be reflected back towards the second planar surface. Therefore, some of the light may simply be refracted between the two planar surfaces of the waveguide 408 before being transmitted, whilst other light may be reflected, and thus may undergo one or more reflections, (or ‘bounces’) between the planar surfaces of the waveguide 408, before being transmitted. Figure 4 shows a total of nine “bounce” points, BO to B8, along the length of the waveguide 408. Although light relating to all points of the image (V1-V8) as shown in Figure 2 is transmitted out of the waveguide at each “bounce” from the second planar surface of the waveguide 408, only the light from one angular part of the image (e.g. light of one of V1 to V8) has a trajectory that enables it to reach the eye 405, from each respective “bounce” point, BO to B8. Moreover, light from a different angular part of the image, V1 to V8, reaches the eye 405 from each respective “bounce” point. Therefore, each angular channel of encoded light reaches the eye only once, from the waveguide 408, in the example of Figure 4. The waveguide 408 forms a plurality of replicas of the hologram, at the respective “bounce” points B1 to B8 along its length, corresponding to the direction of pupil expansion. As shown in Figure 5, the plurality of replicas may be extrapolated back, in a straight line, to a corresponding plurality of replica or virtual display devices 402’. This process corresponds to the step of “unfolding” an optical path within the waveguide, so that a light ray of a replica is extrapolated back to a “virtual surface” without internal reflection within the waveguide. Thus, the light of the expanded exit pupil may be considered to originate from a virtual surface (also called an “extended modulator” herein) comprising the display device 402 and the replica display devices 402’. Although virtual images, which require the eye to transform received modulated light in order to form a perceived image, have generally been discussed herein, the methods and arrangements described herein can be applied to real images. Two-Dimensional Pupil Expansion Whilst the arrangement shown in Figure 4 includes a single waveguide that provides pupil expansion in one dimension, pupil expansion can be provided in more than one dimension, for example in two dimensions. Moreover, whilst the example in Figure 4 uses a hologram that has been calculated to create channels of light, each corresponding to a different portion of an image, the present disclosure and the systems that are described herebelow are not limited to such a hologram type. Figure 5A shows a perspective view of a system 500 comprising two replicators, 504, 506 arranged for expanding a light beam 502 in two dimensions. In the system 500 of Figure 5A, the first replicator 504 comprises a first pair of surfaces, stacked parallel to one another, and arranged to provide replication - or, pupil expansion -in a similar manner to the waveguide 408 of Figure 4. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially elongate in one direction. The collimated light beam 502 is directed towards an input on the first replicator 504. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), which will be familiar to the skilled reader, light of the light beam 502 is replicated in a first direction, along the length of the first replicator 504. Thus, a first plurality of replica light beams 508 is emitted from the first replicator 504, towards the second replicator 506. The second replicator 506 comprises a second pair of surfaces stacked parallel to one another, arranged to receive each of the collimated light beams of the first plurality of light beams 508 and further arranged to provide replication - or, pupil expansion - by expanding each of those light beams in a second direction, substantially orthogonal to the first direction. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially rectangular. The rectangular shape is implemented for the second replicator in order for it to have length along the first direction, in order to receive the first plurality of light beams 508, and to have length along the second, orthogonal direction, in order to provide replication in that second direction. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), light of each light beam within the first plurality of light beams 508 is replicated in the second direction. Thus, a second plurality of light beams 510 is emitted from the second replicator 506, wherein the second plurality of light beams 510 comprises replicas of the input light beam 502 along each of the first direction and the second direction. Thus, the second plurality of light beams 510 may be regarded as comprising a two-dimensional grid, or array, of replica light beams. Thus, it can be said that the first and second replicators 504, 505 of Figure 5A combine to provide a two-dimensional replicator (or, “two-dimensional pupil expander”). Thus, the replica light beams 510 may be emitted along an optical path to an expanded eye-box of a display system, such as a head-up display. In the system of Figure 5A, the first replicator 504 is a waveguide comprising a pair of elongate rectilinear reflective surfaces, stacked parallel to one another, and, similarly, the second replicator 504 is a waveguide comprising a pair of rectangular reflective surfaces, stacked parallel to one another. In other systems, the first replicator may be a solid elongate rectilinear waveguide and the second replicator may be a solid planar rectangular shaped waveguide, wherein each waveguide comprises an optically transparent solid material such as glass. In this case, the pair of parallel reflective surfaces are formed by a pair of opposed major sidewalls optionally comprising respective reflective and reflective-transmissive surface coatings, familiar to the skilled reader. Figure 5B shows a perspective view of a system 500 comprising two replicators, 520, 540 arranged for replicating a light beam 522 in two dimensions, in which the first replicator is a solid elongated waveguide 520 and the second replicator is a solid planar waveguide 540. In the system of Figure 5B, the first replicator / waveguide 520 is arranged so that its pair of elongate parallel reflective surfaces 524a, 524b are perpendicular to the plane of the second replicator / waveguide 540. Accordingly, the system comprises an optical coupler arranged to couple light from an output port of first replicator 520 into an input port of the second replicator 540. In the illustrated arrangement, the optical coupler is a planar / fold mirror 530 arranged to fold or turn the optical path of light to achieve the required optical coupling from the first replicator to the second replicator. As shown in Figure 5B, the mirror 530 is arranged to receive light - comprising a one-dimensional array of replicas extending in the first dimension - from the output port I reflective-transmissive surface 524a of the first replicator / waveguide 520. The mirror 530 is tilted so as to redirect the received light onto an optical path to an input port in the (fully) reflective surface of second replicator 540 at an angle to provide waveguiding and replica formation, along its length in the second dimension. It will be appreciated that the mirror 530 is one example of an optical element that can redirect the light in the manner shown, and that one or more other elements may be used instead, to perform this task. In the illustrated arrangement, the (partially) reflective-transmissive surface 524a of the first replicator 520 is adjacent the input port of the first replicator / waveguide 520 that receives input beam 522 at an angle to provide waveguiding and replica formation, along its length in the first dimension. Thus, the input port of first replicator / waveguide 520 is positioned at an input end thereof at the same surface as the reflective-transmissive surface 524a. The skilled reader will understand that the input port of the first replicator / waveguide 520 may be at any other suitable position. Accordingly, the arrangement of Figure 5B enables the first replicator 520 and the mirror 530 to be provided as part of a first relatively thin layer in a plane in the first and third dimensions (illustrated as an x-z plane). In particular, the size or “height” of a first planar layer - in which the first replicator 520 is located - in the second dimension (illustrated as the y dimension) is reduced. The mirror 530 is configured to direct the light away from a first layer / plane, in which the first replicator 520 is located (i.e. the “first planar layer"), and direct it towards a second layer / plane, located above and substantially parallel to the first layer / plane, in which the second replicator 540 is located (i.e. a “second planar layer”). Thus, the overall size or “height” of the system - comprising the first and second replicators 520, 540 and the mirror 530 located in the stacked first and second planar layers in the first and third dimensions (illustrated as an x-z plane) - in the second dimension (illustrated as the y dimension) is compact. The skilled reader will understand that many variations of the arrangement of Figure 5B for implementing the present disclosure are possible and contemplated. The image projector may be arranged to project a diverging or diffracted light field. In some embodiments, the light field is encoded with a hologram. In some embodiments, the diffracted light field comprises diverging ray bundles. In some embodiments, the image formed by the diffracted light field is a virtual image. In some embodiments, the first pair of parallel / complementary surfaces are elongate or elongated surfaces, being relatively long along a first dimension and relatively short along a second dimension, for example being relatively short along each of two other dimensions, with each dimension being substantially orthogonal to each of the respective others. The process of reflection / transmission of the light between / from the first pair of parallel surfaces is arranged to cause the light to propagate within the first waveguide pupil expander, with the general direction of light propagation being in the direction along which the first waveguide pupil expander is relatively long (i.e., in its “elongate” direction). There is disclosed herein a system that forms an image using diffracted light and provides an eye-box size and field of view suitable for real-world application - e.g. in the automotive industry by way of a head-up display. The diffracted light is light forming a holographic reconstruction of the image from a diffractive structure - e.g. hologram such as a Fourier or Fresnel hologram. The use diffraction and a diffractive structure necessitates a display device with a high density of very small pixels (e.g. 1 micrometer) - which, in practice, means a small display device (e.g. 1 cm). The inventors addressed a problem of how to provide 2D pupil expansion with a diffracted light field e.g. diffracted light comprising diverging (not collimated) ray bundles. In some embodiments, the display system comprises a display device - such as a pixelated display device, for example a spatial light modulator (SLM) or Liquid Crystal on Silicon (LCoS) SLM - which is arranged to provide or form the diffracted or diverging light. In such aspects, the aperture of the spatial light modulator (SLM) is a limiting aperture of the system. That is, the aperture of the spatial light modulator - more specifically, the size of the area delimiting the array of light modulating pixels comprised within the SLM - determines the size (e.g. spatial extent) of the light ray bundle that can exit the system. In accordance with this disclosure, it is stated that the exit pupil of the system is expanded to reflect that the exit pupil of the system (that is limited by the small display device having a pixel size for light diffraction) is made larger or bigger or greater in spatial extent by the use of at least one pupil expander. The diffracted or diverging light field may be said to have “a light field size”, defined in a direction substantially orthogonal to a propagation direction of the light field. Because the light is diffracted / diverging, the light field size increases with propagation distance. In some embodiments, the diffracted light field is spatially-modulated in accordance with a hologram. In other words, in such aspects, the diffractive light field comprises a “holographic light field”. The hologram may be displayed on a pixelated display device. The hologram may be a computer-generated hologram (CGH). It may be a Fourier hologram ora Fresnel hologram or a point-cloud hologram or any other suitable type of hologram. The hologram may, optionally, be calculated so as to form channels of hologram light, with each channel corresponding to a different respective portion of an image that is intended to be viewed (or perceived, if it is a virtual image) by the viewer. The pixelated display device may be configured to display a plurality of different holograms, in succession or in sequence. Each of the aspects and embodiments disclosed herein may be applied to the display of multiple holograms. The output port of the first waveguide pupil expander may be coupled to an input port of a second waveguide pupil expander. The second waveguide pupil expander may be arranged to guide the diffracted light field - including some of, preferably most of, preferably all of, the replicas of the light field that are output by the first waveguide pupil expander - from its input port to a respective output port by internal reflection between a third pair of parallel surfaces of the second waveguide pupil expander. The first waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a first direction and the second waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a second, different direction. The second direction may be substantially orthogonal to the first direction. The second waveguide pupil expander may be arranged to preserve the pupil expansion that the first waveguide pupil expander has provided in the first direction and to expand (or, replicate) some of, preferably most of, preferably all of, the replicas that it receives from the first waveguide pupil expander in the second, different direction. The second waveguide pupil expander may be arranged to receive the light field directly or indirectly from the first waveguide pupil expander. One or more other elements may be provided along the propagation path of the light field between the first and second waveguide pupil expanders. The first waveguide pupil expander may be substantially elongated and the second waveguide pupil expander may be substantially planar. The elongated shape of the first waveguide pupil expander may be defined by a length along a first dimension. The planar, or rectangular, shape of the second waveguide pupil expander may be defined by a length along a first dimension and a width, or breadth, along a second dimension substantially orthogonal to the first dimension. A size, or length, of the first waveguide pupil expander along its first dimension make correspond to the length or width of the second waveguide pupil expander along its first or second dimension, respectively. A first surface of the pair of parallel surfaces of the second waveguide pupil expander, which comprises its input port, may be shaped, sized, and / or located so as to correspond to an area defined by the output port on the first surface of the pair of parallel surfaces on the first waveguide pupil expander, such that the second waveguide pupil expander is arranged to receive each of the replicas output by the first waveguide pupil expander. The first and second waveguide pupil expander may collectively provide pupil expansion in a first direction and in a second direction perpendicular to the first direction, optionally, wherein a plane containing the first and second directions is substantially parallel to a plane of the second waveguide pupil expander. In other words, the first and second dimensions that respectively define the length and breadth of the second waveguide pupil expander may be parallel to the first and second directions, respectively, (or to the second and first directions, respectively) in which the waveguide pupil expanders provide pupil expansion. The combination of the first waveguide pupil expander and the second waveguide pupil expander may be generally referred to as being a “pupil expander”. It may be said that the expansion / replication provided by the first and second waveguide expanders has the effect of expanding an exit pupil of the display system in each of two directions. An area defined by the expanded exit pupil may, in turn define an expanded eye box area, from which the viewer can receive light of the input diffracted or diverging light field. The eye-box area may be said to be located on, or to define, a viewing plane. The two directions in which the exit pupil is expanded may be coplanar with, or parallel to, the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. Alternatively, in arrangements that comprise other elements such as an optical combiner, for example the windscreen (or, windshield) of a vehicle, the exit pupil may be regarded as being an exit pupil from that other element, such as from the windscreen. In such arrangements, the exit pupil may be non-coplanar and non-parallel with the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. For example, the exit pupil may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. The viewing plane, and / or the eye-box area, may be non-coplanar or non-parallel to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. For example, a viewing plane may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. In order to provide suitable launch conditions to achieve internal reflection within the first and second waveguide pupil expanders, an elongate dimension of the first waveguide pupil expander may be tilted relative to the first and second dimensions of the second waveguide pupil expander. Combiner shape compensation An advantage of projecting a hologram to the eye-box is that optical compensation can be encoded in the hologram (see, for example, European patent 2936252 incorporated herein by herein). The present disclosure is compatible with holograms that compensate for the complex curvature of an optical combiner used as part of the projection system. In some embodiments, the optical combiner is the windscreen of a vehicle. Full details of this approach are provided in European patent 2936252 and are not repeated here because the detailed features of those systems and methods are not essential to the new teaching of this disclosure herein and are merely exemplary of configurations that benefit from the teachings of the present disclosure. Control device The present disclosure is also compatible with optical configurations that include a control device (e.g. light shuttering device) to control the delivery of light from a light channelling hologram to the viewer. The holographic projector may further comprise a control device arranged to control the delivery of angular channels to the eye-box position. British patent application 2108456.1, filed 14 June 2021 and incorporated herein by reference, discloses the at least one waveguide pupil expander and control device. The reader will understand from at least this prior disclosure that the optical configuration of the control device is fundamentally based upon the eye-box position of the user and is compatible with any hologram calculation method that achieves the light channeling described herein. It may be said that the control device is a light shuttering or aperturing device. The light shuttering device may comprise a 1D array of apertures or windows, wherein each aperture or window independently switchable between a light transmissive and a light non-transmissive state in order to control the deliver of hologram light channels, and their replicas, to the eye-box. Each aperture or window may comprise a plurality of liquid crystal cells or pixels. One-dimensional Pupil Expansion of Diverging Light In some implementations, light coupled into the first replicator is diverging. For example, as described herein, the input light may comprise a diffracted light field comprising diverging light ray bundles. In particular, a holographic light field comprising spatially modulated light in accordance with a hologram may be coupled into the first replicator (e.g., in a so-called “direct view” system) such that the holographic light field expands with propagation distance. It may be said that the size of the holographic wavefront increases with propagation distance (from and input port) along the first replicator. Figure 6 shows a side view of an example waveguide 600 forming a first replicator for providing pupil expansion of an input light field in a first dimension (illustrated as the x dimension), equivalent to the first replicator 520 of Figure 5B. In particular, Figure 6 shows an elongate, quadrilateral output surface of a rod-shaped waveguide 600, equivalent to the reflective-transmissive surface 524a of the first replicator 520 of Figure 5B, for the output of a one-dimensional array of replicas extending in the first dimension. As the skilled person will appreciate, waveguide 600 also has a corresponding reflective surface, equivalent to the reflective surface 524b of the first replicator 520 of Figure 5B, having the same elongate, quadrilateral shape. The two reflective surfaces of the waveguide 600 may be arranged in a plane generally extending in the first and second dimensions (illustrated as the x and y dimensions), as in the arrangement of Figure 5B. In the illustrated arrangement, an input holographic light field is coupled into the waveguide 600 at an input port (not shown) at a first end 601, and is waveguided between its two reflective surfaces in a first direction of the first dimension towards a second end 602, as shown by arrow 610. A one-dimensional array of replicas of the input light field, extending in the first dimension, are output from the output surface, as described herein. Since the light field is a diffracted light field comprising diverging ray bundles, the size of the wavefront increases with distance from the first end 601 to the second end 602. In consequence, the wavefront of the output replicas also increase in size. Figure 6 illustrates a boundary of an optical footprint 615 of the replicas output from the output surface. As shown, due to the diverging light ray bundles, the size of the optical footprint 615 increases with distance in the first direction of the first dimension from a smallest size in the second dimension at the first end 601 to a largest size in the second dimension at the second end 602 of waveguide 600. It may be said that the optical footprint 615 tapers outwardly from the first end 601 to the second end 602 of the waveguide 600. Accordingly, as shown in Figure 6, a height 620 of the waveguide 600, corresponding to the width of the illustrated output surface in the second dimension (illustrated as the y dimension), must be greater than the maximum size of the optical footprint 615 at the second end 602 of the waveguide 600. In some applications, the amount of pupil expansion / replication required from the first replicator necessitates a relatively long waveguide in the elongate, dimension of pupil expansion (i.e., the first dimension). Furthermore, depending upon the preferred configuration, the first replicator may need to be tilted in the plane of the first planar layer (i.e., the plane in the first and second dimensions or x-y plane). For example, the longitudinal axis of the first replicator may be at an angle with respect to the first dimension (illustrated as the x dimension), to allow the effective propagation and coupling of replicas output from the first replicator into the second replicator as described in UK patent application publication, GB2610875A. In this tilted configuration, a minimum separation or gap is required between the first planar layer / replicator and second planar layer / replicator. Thus, the required height 620 of the waveguide 600 for the desired amount of pupil expansion, as well as the need for the above described tilt, is a constraint on the overall size of the replicator system, such as the arrangement shown in Figure 5B. This may mean that the replicator system is too bulky for use in applications where available space is at a premium, such as under the dashboard of a vehicle in an automotive head-up display. Central Launch Waveguide for Pupil Expansion of Diverging Light Field Figure 7 shows a side view of a waveguide 700 in accordance with the present disclosure. Similar to the above-described waveguide 600 of Figure 6, waveguide 700 forms a first replicator for providing pupil expansion of an input light field in a first dimension (illustrated as the x dimension), equivalent to the first replicator 520 of Figure 5B. In particular, Figure 7 shows an elongate, quadrilateral output surface of a rod-shaped waveguide 700, equivalent to the reflective-transmissive surface 524a of the first replicator 520 of Figure 5B, for the output of a one-dimensional array of replicas extending in the first dimension. As the skilled person will appreciate, waveguide 700 also has a corresponding reflective surface, equivalent to the reflective surface 524b of the first replicator 520 of Figure 5B, having the same elongate, quadrilateral shape. The two reflective surfaces of the waveguide 700 may be arranged in a plane generally extending in the first and second dimensions (illustrated as the x and y dimensions), as in the arrangement of Figure 5B. As shown in Figure 7, a height 720 of the waveguide 700, corresponding to the width of the illustrated output surface in the second dimension (illustrated as the y dimension), is reduced compared to the height 620 of the waveguide 600 of Figure 6. This may be achieved by changing the way in which an input light field is coupled into, and waveguided between the two reflective surfaces of, waveguide 700. In the illustrated embodiment, an input holographic light field is coupled into the waveguide 700 at an input port (not shown) at its centre 701, and is waveguided between its two reflective surfaces in opposite first and second directions of the first dimension towards respective first and second ends 702, 702', as shown by arrows 710, 710'. A onedimensional array of replicas of the input light field, extending in the first dimension, are output from the output surface, as described herein. Since the light field is a diffracted light field comprising diverging ray bundles, the size of the wavefront increases with distance from the centre 701 to the first end 702 and from the centre 701' to the second end 702'. In consequence, the wavefront of the output replicas also increases in size. Figure 7 illustrates a boundary of first and second optical footprints 715, 715' of the replicas output from the output surface. As shown, due to the diverging light ray bundles, the size of the first optical footprint 715 increases with distance in the first direction of the first dimension, as illustrated by arrows 710, from a smallest size in the second dimension at the centre 701 to a largest size in the second dimension at the first end 702 of waveguide 700. Similarly, the size of the second optical footprint 715' increases with distance in the second direction of the first dimension, as illustrated by arrows 710', from a smaller size in the second dimension at the centre 701' to a largest size in the second dimension at the second end 702' of waveguide 700. It may be said that each of the first and second optical footprints 715, 715' tapers outwardly from the centre 701,701' to the respective end 702, 702' of waveguide 700. In the illustrated arrangement, the first and second optical footprints 715, 715' are substantially the same, thus providing symmetry in the first and second directions of the first dimension about the centre 701 of waveguide 700. Since the input light field is coupled into the centre 701 of the waveguide 700 and provides pupil expansion in two opposite, first and second directions of the first dimension, corresponding to the dimension of pupil expansion, the maximum size of the first and second optical footprints 715, 715' is reduced, in comparison to the arrangement of the waveguide 600 of Figure 6 when configured for the same amount of pupil expansion. Accordingly, the height 720 of the waveguide 700 of Figure 7, corresponding to the width of the illustrated output surface in the second dimension (illustrated as the y dimension), is reduced, in comparison to the height 620 of the waveguide 600 of Figure 6. This may lead to a reduction in the overall size (e.g., height) of a two replicator system, such as that shown in Figure 5B, and a more compact arrangement for applications in which space is at a premium. In addition, the inventors found that it is possible to reduce the tilt of the waveguide 700 in the plane of the first planar layer, such as a tilt or angle of the longitudinal axis of waveguide 700 with respect to the first dimension (illustrated as the x dimension). This, in turn, enables a reduction in the gap between the first and second replicators of a two replicator system, further reducing height and contributing to a more compact arrangement. In the arrangement shown in Figure 7, a single holographic light field (or pupil) is input into the centre of the waveguide 700, and is waveguided between the two reflective surfaces thereof to provide pupil expansion in opposite, first and second directions of the first dimension. The following description discloses to one a suitable technique for coupling a single light field, for replication across a desired field of view, into the centre of the waveguide 700. As the skilled person will appreciate, other techniques for coupling a single light field (or pupil) into the centre of the waveguide (in the first dimension), described herein as a “central launch” waveguide are possible and contemplated. As the skilled person will appreciate, in other implementations, first and second holographic light fields may be coupled into the waveguide 700 at its centre, such that the first holographic light field is waveguided in the first direction of the first dimension, and the second holographic light field is waveguided in the second direction of the first dimension as described below with reference to Figure 8. In such arrangements, each of the first and second holographic light field corresponds to a respective half of a desired field of view in the first dimension, and is received as an exit pupil from a separate optical system. In-coupling of a Diverging Light Field into “Central Launch” Waveguide Figure 8 is a top view of a conventional waveguide 800 for providing pupil expansion in a first dimension, comprising an input port at its centre (in the first dimension). In particular, waveguide 800 comprises an elongate, quadrilateral sided rod-shaped waveguide, which may be used as a first replicator for providing pupil expansion of an input light field in a first dimension (illustrated as the x dimension). Waveguide 800 comprises a reflective surface 810, illustrated as the bottom surface in Figure 8, and a transmissive-reflective surface 820, illustrated as the top surface in Figure 8. An input port 830 is provided at the centre in the bottom surface, such as an aperture or gap in the reflective surface 810, and the transmissive-reflective top surface 820 forms an output surface (or output port). A first light field 850 is incident on the input port 830 of the waveguide 800 at the centre of the waveguide at a first input angle (or angle of incidence) 0 and is waveguided between the reflective bottom surface 810 and transmissive-reflective top surface 820 thereof in a first direction of the first dimension. A first one-dimensional array of replicas R1, R2, ... Rn of the first light field 850, extending in the first direction towards a first end 802 of waveguide 800, are formed and output at the transmissive-reflective output surface 820. The first onedimensional array of replicas R1, R2,... Rn are a first subset, corresponding to about half of the total number of replicas formed by waveguide 800. Similarly, a second light field 850', substantially identical to first light field 850, is incident on the input port 830 of the waveguide 800 at a second input angle (or angle of incidence) 6' and is waveguided between the reflective bottom surface 810 and transmissive-reflective top surface 820 thereof in a second direction, opposite to the first dimension, of the first dimension. A second one-dimensional array of replicas R1', R2', ... Rn' of the second light field 850 ', extending in the second direction towards a second end 802' of waveguide 800, are formed and output at the transmissive-reflective output surface 820. The second onedimensional array of replicas R1', R2', ... Rn' are a second subset, corresponding to about half of the total number of replicas formed by waveguide 800. As the skilled person will appreciate, the first input angle (or angle of incidence) 0 is equal in magnitude, and opposite in direction, to the second input angle (or angle of incidence) 9'. Thus, the first and second one-dimensional arrays of replicas formed at the transmissive-reflective output surface 820 of waveguide 800 together form the total number of replicas corresponding to the desired field of view. Figure 8 shows just one example of the first and second input angles 9, 9' of the respective first and second light fields 850, 850'. As the skilled person will appreciate, a conventional waveguide pupil expander 800 comprising an input port at its centre in a first dimension, may be configured to waveguide first light, received at a first range of input angles, in a first direction of the first dimension, and to waveguide second light, received at a second range of input angles, in a second direction, opposite to the first direction, of the first dimension. The first range of input angles is (substantially) equal in magnitude, and opposite in direction, to the second range of input angles. Accordingly, in a conventional waveguide arranged for in-coupling of light at its centre (in the first dimension of pupil expansion) and providing pupil expansion in opposite directions, it is necessary to provide two separate optical systems (e.g., lens systems), having respective exit pupils for coupling into the waveguide 800 at different angles. As noted above, each of the two exit pupils represents half the field of view, but, in practice, correspond to the same light field 850 for replication. Thus, the requirement to provide two separate optical systems (e.g., lens systems) for coupling two separate light fields (i.e., exit pupils) into the waveguide at different input angles increases cost and complexity of the system for pupil expansion. Furthermore, the exit pupils from the two optical systems should be substantially identical, and provided at the same angle of incidence, but in opposite directions. This is challenging due to different tolerances of two physically separate optical systems, which may lead to differences in the two light fields (exit pupils) coupled into the waveguide and differences in the magnitude of their respective angles of incidence. The inventors found that slight differences in the two light fields may lead to poor pupil expansion and image quality, such as poor image uniformity across the complete field of view. Figure 9 is a top view of a waveguide 900 for providing pupil expansion in a first dimension, comprising an input port at its centre (in the first dimension). Similar to the waveguide 800 of Figure 8, waveguide 900 comprises an elongate, quadrilateral sided rod-shaped waveguide, which may be used as a first replicator for providing pupil expansion of an input light field in a first dimension (illustrated as the x dimension). Waveguide 900 comprises a reflective surface 910, illustrated as the bottom surface in Figure 9, and a transmissive-reflective surface 920, illustrated as the top surface in Figure 9. Similar to the waveguide 800 of Figure 8, the transmissive-reflective top surface 920 forms an output surface (or output port). However, the input port 930 at the centre of the bottom surface 910 of the waveguide 900 is configured differently from the waveguide 800 of Figure 8, to allow the input, and in-coupling of, of a single light field 955 rather than two separate light fields, such as an exit pupil from a single optical system. In particular, the input port 930 comprises a diffraction grating 960 at, in particular in front of (or upstream of), an aperture at the centre of the bottom, reflective surface 910. Diffraction grating 960 is arranged to turn the propagation direction of the optical path of an input light field into two optical paths (e.g., having equal and opposite directions) for waveguiding in first and second directions of the first dimension. In accordance with implementations, the input light field 955 is a holographic light field. For example, the input light field 955 is an exit pupil of a display device comprising spatially modulated light in accordance with a hologram displayed on the display device, as described herein. The hologram displayed on the display device may be relayed to the plane of the input port 930, or, more specifically, the plane of the diffraction grating 960. It may be said that the hologram forms the input wavefront coupled into the waveguide 900. Alternatively, it may be said that the input diffraction grating 960 is disposed at the plane of a hologram. Figure 9 shows a first order of the diffraction grating 960, comprising a positive primary diffraction order (e.g., positive first diffraction order) of the input light field 955 formed by the diffraction grating 960. The first order light is incident on the input port 930 (e.g., an aperture in bottom reflective surface 910) of waveguide 900 at a first input angle (or angle of incidence) 0, and is waveguided between the reflective bottom surface 910 and transmissive-reflective top surface 820 thereof in a first direction of the first dimension. A first one-dimensional array of replicas R1, R2, ... Rn of the light field 955, extending in the first direction towards a first end 902 of waveguide 900, are formed and output at the transmissive-reflective output surface 920. Figure 9 additionally shows a second order, comprising a negative primary diffraction order (e.g., negative first diffraction order) of the input light field 955 formed by the diffraction grating 960. The second order light is incident on the input port 930 of the waveguide 900 at a second input angle (or angle of incidence) 0', and is waveguided between the reflective bottom surface 910 and transmissive-reflective top surface 820 thereof in a second direction of the first dimension. A second one-dimensional array of replicas RT, R2', ... Rn'of the first light field 950, extending in the second direction towards a second end 902' of waveguide 900, are formed and output at the transmissive-reflective output surface 920. As the skilled person will appreciate, the first input angle 0 is equal in magnitude, and opposite in direction, to the second input angle 9'. Thus, the first and second one-dimensional arrays of replicas formed at the transmissive-reflective output surface 920 of waveguide 900 form a substantially continuous field of view. Furthermore, since the first and second onedimensional arrays of replicas forming the complete, desired field of view replicate the same / identical unput light field 955 (e.g., the exit pupil of a single optical system) coupled into the waveguide 900, pupil expansion and image quality is optimised. As the skilled person will appreciate, the output light, comprising the first and second onedimensional arrays of replicas formed by a waveguide in accordance with the present disclosure, may be coupled into a second replicator to provide pupil expansion in a second dimension, corresponding to a second dimension of the eye box of a head-up display. For example, second replicator may comprise a slab-shaped waveguide as illustrated in Figure 5B and described above. The output surface of the waveguide forming the first replicator may be arranged perpendicular to the input port of the second replicator, and an optical component, such as a fold mirror, may be provided to optically couple the first and second replicators, as shown in Figure 5B. In addition, the inventors have found that it is possible to arrange the first replicator so that its longitudinal axis is parallel to the first dimension, and thus parallel to two opposite sides (and perpendicular to the other two opposite sides) of the second replicator. First waveguide height reduction Figures 11 to 13 illustrate embodiments that provide the technical advancement represented by Figure 10. In overview, the holographic wavefront is magnified or “stretched” in the vertical direction (only) prior to waveguiding (by the first waveguide). Again, for the avoidance of doubt, the holographic wavefront is not stretched in any other direction. More strictly, the footprint of the holographic wavefront is stretched in the vertical direction such that the size of the footprint in the vertical direction is increased. The inventors hypothesised that stretching the footprint of the holographic wavefront might be optically equivalent to stretching an optical pupil and might therefore be expected to obey the principle of conservation of etendue. It was confirmed in simulation and experiment that etendue was conserved. It was therefore found that the process of stretching the holographic wavefront caused a decrease of the divergence / field of view in the direction of stretching. Accordingly, in some embodiments, the field of view of the holographic wavefront is reduced prior to coupling into the waveguide (or first waveguide in a system using two orthogonal waveguides). In some embodiments, the vertical field of view (of the holographic wavefront) determines the required height (size in the vertical direction) of the waveguide. Therefore, in accordance with this advancement, the rate at which the field of view increases the height of the ray bundle within the waveguide is reduced. Figure 10 represents the change in light footprint that is achieved by stretching the (footprint of the) holographic wavefront before waveguiding. In detail, Figure 10 is a side view of a waveguide 1000 arranged to hologram replication in the x-direction. Figure 10 shows a central launch geometry byway of example only. The present disclosure is equally applicable to an end or one-side launch geometry or any other launch geometry that may be conceived. In this embodiment, the holographic wavefront is received at the centre of the waveguide and waveguided in the +x and -x directions towards a first end 1020 and second end 1020’ of the waveguide, respectively. The holographic wavefront comprises a diverging light ray bundle as described above. The holographic wavefront is therefore characterised by a divergence angle or, more specifically, a maximum divergence angle. In some embodiments, the maximum divergence angle is determined by a pixel pitch of a display device used to display the hologram used to form the holographic wavefront. To illustrate a technical advancement achieved by the inventors, Figure 10 shows the volume of space corresponding to two difference holographic wavefronts. A first holographic wavefront 1050 having a first divergence is represented by the dashed lines within the waveguide 1000. The first holographic wavefront 1050 occupies or uses a first volume of the waveguide 1000. A second holographic wavefront 1060 having a second divergence is represented by the solid lines within the waveguide 1000. The second holographic wavefront 1060 occupies or uses a second volume of the waveguide 1000. The first divergence is greater than the second divergence. At the input I centre of the waveguide 1000, a height of the (footprint of the) first holographic wavefront 1050 is less than that of the second holographic wavefront 1060. At the respective ends 1020,1020’ of the waveguide, a height of the (the footprint of the) first holographic wavefront 1050 is greater than that of the second holographic wavefront 1060. The first holographic wavefront 1050 represents the expansion (in the vertical direction) of a holographic wavefront without stretching in accordance with this disclosure. The second holographic wavefront 1060 represents the reduced expansion (in the vertical direction) achieved with stretching, prior to waveguide launch, in accordance with this disclosure. Notably, the vertical size of the holographic wavefront is increased by stretching but the divergence is decreased in accordance with principle of etendue conservation. Accordingly, the holographic wavefront is actually smaller (in the vertical direction) by the time it reaches the end of the waveguide. A first height 1030 of (a footprint of the) first holographic wavefront 1050 is greater than a second height 1040 of (the footprint of) the second holographic wavefront 1060. Accordingly, the height of the waveguide 1000 (i.e. the size in the y-direction) may be reduced. Furthermore, more efficient use of the volume of the waveguide 1000 is made. It may be said that an optical fillfactor of the waveguide 1000 has been increased. In some embodiments, the (maximum) vertical field of view of the holographic wavefront is 1 to 6 degrees before stretching and 0.3 to 2 degrees after stretching but the present disclosure is not limited to these embodiments. The one-dimensional magnification of the first optical sub-system may be 1.5 to 5.0 such as 2 to 4. The optical path length (of the holographic wavefront) in the waveguide may be 500 to 1500 mm. The refractive index of the waveguide may be 1.1 to 1.8. The size of the (footprint of the) holographic wavefront received by the first optical sub-system may be 10 to 100 mm. The height (y-direction size) of the waveguide required may be reduced from 100 to 25 mm in accordance with the present disclosure. The inventors recognised that the optical stretching performed by the first optical sub-system may be fully reversible in order that a field of view of the head-up display, or image quality, is not compromised. The stretching may comprise an optical transform such as a onedimensional magnification. The stretching may be reversed by using the inverse optical transform at the output of the waveguide such as a one-dimensional de-magnification. The one-dimensional magnification (e.g. 2) may be the numerical inverse of the one-dimensional magnification (e.g. 0.5). The process of reversing the stretching performed by the first optical sub-system may be referred to as “compressing”. The stretched holographic wavefront, and the plurality of replicas thereof formed by the waveguide, may each be compressed by a second optical sub-system. In notable embodiments, the second optical sub-system comprises the same type of optical component / s as the first optical sub-system. For example, the first optical sub-system may be a first anamorphic prism pair with a positive magnification (e.g. 2) and the second optical sub-system may be a second anamorphic prism pair with an inverse magnification (e.g. 0.5). The prism parameters, such as prism angles, of the first anamorphic prism pair may be the same to those of the second anamorphic prism pair - at least in terms of their magnitude. The optical components of the first optical sub-system may be said to each have a “corresponding” optical component of the same type in the second optical sub-system. For example, corresponding optical components may be said to have the same (magnitude) surface curvatures and / or same (magnitude) angles. In this respect, it may be said that the first optical sub-system and second optical sub-system form an identical, matching pair to reflect that the same optical components and the same characterising optical parameters thereof are used in the first and second optical sub-systems - at least in terms of magnitude but not necessarily direction. It may also be said that the first and second optical subsystems are perfectly complementary in their physical form to reflect that any unwanted optical effects, such as aberrations, caused by the first optical sub-system are exactly cancelled out by the second optical sub-system. This may only be achieved if the first and second optical sub-systems are formed using “matching” components. The inventors found that any optical imbalance between the first optical sub-system and second optical subsystem were magnified by the hologram replicator of the present disclosure. In some embodiments, the angular field of view of the holographic wavefront after the second optical sub-system is the same as the angular field of view of the holographic wavefront formed by the hologram - i.e. before “stretching” by the first optical sub-system. The holographic wavefront waveguided by the waveguide has a reduced divergence. That divergence is not zero. Therefore, the physical size of the holographic wavefront still increases with propagation distance. The size of the footprint of the holographic wavefront at the exit of the first waveguide is therefore greater than that at the entry of the first waveguide. In relative terms, the size reduction (compression) associated with the second optical sub-system is equal (but opposite) to the size increase (stretch) associated with the first optical sub-system. But, in absolute terms (e.g. number of millimetres), the size reduction associated with the second optical sub-system is greater than the size increase associated with the first optical sub-system. This reflects an additional gain, or further benefit, associated with the present approach. In some embodiments, the size of the holographic wavefront, in the second dimension, after the second optical sub-system is substantially the same (e.g. within 25%) of that received by the first optical sub-system. In this respect, it may be said that the present disclosure substantially nullifies the effect of the long optical path length within the waveguide - which may be 500 to 1500 mm, for example. The hologram replicator of the present disclosure may further comprise a second waveguide downstream of the second optical sub-system. The second waveguide may be slab-shaped and may provide hologram replication in the second dimension. The second waveguide may comprise an input port arranged to receive the compressed holographic wavefront from the second optical sub-system. In some embodiments, the second waveguide can use a shorter input port owing to the present disclosure. This can improve image quality for reasons explained in British patent publication, GB 2,614,066A which is incorporated herein in its entirely by reference. In this respect, it may be said that the present disclosure provides a yet further additional technical benefit when a second waveguide having an input port arranged to receive the (optical) output of the first waveguide is used. Figures 11-14 show four different examples of a first and second optical sub-system. In each example, the first and second optical sub-system form an identical matching pair as described above in order to optimise image quality. Figure 11 shows a first anamorphic prism pair 1110 arranged to receive an input holographic wavefront 1160 (e.g. from a hologram) and output a stretched holographic wavefront. The stretched holographic wavefront is waveguided and replicated by a first waveguide 1100. The output of the first waveguide 1100 is compressed by a second anamorphic prism pair 1120 and directed (e.g. turned) by an optional mirror 1130 to an input port (not shown) of a second waveguide 1150. In Figure 11, the first optical sub-system is therefore a first anamorphic prism pair 1110 and the second optical sub-system is a second anamorphic prism pair 1120. The first and second anamorphic prism pair are complementary in their physical form, as described above. Figure 12 shows an embodiment in which the second optical sub-system comprises a pair of cylindrical lenses 1220. In this embodiment, the pair of components of the second optical sub-system are separated by an optical turn mirror 1230 arranged to direct the compressed holographic wavefront to an input port of a second waveguide 1250. Figure 13 shows an embodiment in which the second optical sub-system comprises a pair of cylindrical mirrors comprising a first cylindrical mirror 1310 and a second cylindrical mirror 1312 each having optical power. In this embodiment, the holographic wavefront is bounced between the two cylindrical mirrors 1310,1312 having optical power by a planar mirror 1320. The holographic wavefront is directed to an input port of a second waveguide 1350 by the second cylindrical mirror 1312 having optical power. Replay Field Fitting The available image area within the replay field of the above head-up display is shown in Figure 14. Whilst it the replay field 1400 is relatively large, the area of the available image area 1410 (that is, the area that is projected to the eye-box / viewing window) is comparatively much smaller. It is known that the centre of the replay field 1400 will always form a bright spot (known as the DC spot), which can produce a visual artefact at the eye-box and / or distort the image(s) intended for display at the centre of the replay field 1400. This DC spot is contained within the central area 1420. This issue can be solved by masking the DC spot (i.e. blocking the bright light of the DC spot with an opaque mask). However, masking in this way causes a black spot in the centre the replay field 1400 where no image could be displayed. As such, the hologram could be generated such the image content is solely with in the (image) subarea 1410 ofthe replay field 1400, with any noise content of the hologram being “dumped” in the noise area 1430. The light in the central area 1420 and the noise area 1430 are then discarded, leaving only (image) sub-area 1410 to be projected further downstream in the optical system. In this way, the only light content that reaches the user / viewer is light ofthe image(s) intended for display (from the image sub-area 1410). Moreover, the inventor recognised that, in practice, there will inevitably be a mismatch between the aspect ratio ofthe sub-area 1410 ofthe replay field 1400 used to display picture content and the aspect ratio of optics further downstream. In practice, this results in cropped areas 1440a,b that are wasted (i.e., the pixels ofthe replay field are effectively not utilised). Given that the available image area 1410 is already reduced (due to discard ofthe central area 1420 and the noise area 1430), it is desired that the remaining area is maximised. The inventor has found that these wasted cropped areas 1440a,b can be reduced or eliminated by purposefully introducing a “mismatch” between the stretching and compression (magnification and de-magnification) ofthe wavefront described above. This produces the replay field 1400’ shown in Figure 15. That is, by stretching the wavefront more than it is compressed (orvice versa), the aspect ratio ofthe image area 1410’ is improved to better match the aspect ratio ofthe downstream optics, thus reducing or removing the wasted cropped areas 1440a,b. As such, whilst the central area 1420 and noise area 1430 are still discarded, the available image area 1410’ is maximised. This ensures that the most possible image pixels are used in the final virtual image, which in turn increases the quality of the image perceived and the PPD. This “mismatch” can be achieved by adjusting the relative shapes of the prism pairs before and after the waveguide (as discussed above). Whilst having such prism pairs being matched in their effect on the wavefront was previously considered the best option, the inventor has found that purposefully introducing such a mismatch (contrary to engineering practices) can provide the benefits described above. Additional features The methods and processes described herein may be embodied on a computer-readable medium. The term “computer-readable medium” includes a medium arranged to store data temporarily or permanently such as random-access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term “computer-readable medium” shall also be taken to include any medium, or combination of multiple media, that is capable of storing instructions for execution by a machine such that the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methodologies described herein, in whole or in part. The term "computer-readable medium” also encompasses cloud-based storage systems. The term “computer-readable medium” includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example form of a solid-state memory chip, an optical disc, a magnetic disc, or any suitable combination thereof. In some example embodiments, the instructions for execution may be communicated by a carrier medium. Examples of such a carrier medium include a transient medium (e.g., a propagating signal that communicates instructions). It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. The present disclosure covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

1. A head-up display having an eye-box defined by a first and second dimension, wherein the head-up display comprises:a first optical sub-system having one-dimensional magnification in the second dimension, wherein the first optical sub-system is arranged to receive an input wavefront having a first aspect ratio and output a stretched wavefront, wherein a size in the second dimension of the stretched wavefront is greater than that of the input wavefront in accordance with the one-dimensional magnification;a waveguide arranged to receive the stretched wavefront and output the stretched wavefront and a plurality of replicas thereof extending in the first dimension by waveguiding between a reflective surface and a reflective-transmissive surface; anda second optical sub-system having one-dimensional de-magnification in the second dimension, wherein the second optical sub-system is arranged to receive the stretched wavefront, and plurality of replicas thereof, and output a compressed wavefront, and plurality of replicas thereof, wherein a size in the second dimension of the compressed wavefront, and each replica thereof, is less than that of the stretched wavefront in accordance with the one-dimensional de-magnification;wherein the one-dimensional de-magnification is mismatched with the onedimensional magnification, wherein the mismatch corresponds to the difference between the aspect ratio of the input wavefront and that of an optical component downstream of the second optical sub-system.

2. A head-up display as claimed in claim 1, wherein the mismatch comprises the product of the one-dimensional magnification and the one-dimensional de-magnification being (a) less than 1 but greater than 0.75 or (b) greater than 1 but less than 1.25.

3. A head-up display as claimed in claim 1 or claim 2, wherein the optical component is an optic arranged to compensate for the curvature of an optical combiner downstream thereof.

4. A head-up display as claimed in claim 3, wherein the optic is a freeform mirror or corrective optic and / or the optical combiner is the windscreen of a vehicle housing the head-up display.

5. A head-up display as claimed in any preceding claim, wherein a divergence angle of the stretched wavefront in the second dimension is less than that of the input wavefront in order to reduce the rate of expansion of the wavefront in the second dimension during waveguiding.

6. A head-up display as claimed in claim 5, wherein the divergence angle of the wavefront in the second dimension in the waveguide is less than 1.5 degrees such as less than 1 degree.

7. A head-up display as claimed in claim 5 or claim 6, wherein the size and divergence angle of the stretched wavefront in the second dimension and an optical path length of the waveguiding define an optical footprint of the waveguide, wherein the optical footprint is at least 50% such as at least 75% of the physical footprint of the waveguide.

8. A head-up display as claimed in any preceding claim, wherein the first optical system comprises an anamorphic prism pair.

9. A head-up display as claimed in any preceding claim, wherein the first optical system comprises a cylindrical lens such as a cylindrical achromatic doublet lens.

10. A head-up display as claimed in any preceding claim, wherein the first optical system comprises a cylindrical mirror having optical power.

11. A head-up display as claimed in any of claims 5 to 10, wherein a (non-zero) divergence angle of the compressed wavefront in the second dimension is greater than that of the stretched wavefront.

12. A head-up display as claimed in any of preceding claim, wherein the second optical sub-system comprises at least one selected from the group comprising: an anamorphic prism pair; a cylindrical achromatic doublet lens; and / or a cylindrical mirror having optical power.

13. A head-up display as claimed in any of preceding claim, wherein the first optical sub-system comprises a first anamorphic prism pair and the second optical subsystem comprises a second anamorphic prism pair, and wherein the first anamorphic prism pair is shaped relative to the second anamorphic prism pair to produce the mismatch.

14. A head-up display as claimed in any preceding claim further comprising a second waveguide arranged to replicate the input wavefront in the second dimension, wherein the second waveguide comprises an input port arranged to receive the output of the (first) waveguide.

15. A method of head-up display, the method comprising:reducing a rate of expansion of an input wavefront by stretching the wavefront in the second dimension in accordance with a one-dimensional magnification of a first optical subsystem to form a stretched wavefront;waveguiding the stretched wavefront between a reflective surface and a reflective-transmissive surface of a waveguide;outputting, from the waveguide, the stretched wavefront and a plurality of replicas thereof extending in the first dimension; andcompressing the stretched wavefront, and plurality of replicas thereof, in accordance with a one-dimensional de-magnification of a second optical sub-system;wherein an optical component is located downstream of the second optical subsystem, the optical component having a second aspect ratio, and wherein the onedimensional de-magnification is mismatched with respect to the one-dimensional magnification, the mismatch corresponding to the difference between the first and second aspect ratios.

16. A method of head-up display as claimed in claim 15, wherein the reduction in the rate of expansion of the input wavefront is such that a divergence angle of the stretched wavefront in the second dimension after the one-dimensional stretching is less than that of the input wavefront received by the first optical sub-system, and wherein the compression of the stretched wavefront, and plurality of replicas thereof, is such that a divergence angle of the output wavefront in the second dimension after the one-dimensional compression is greater than that of the stretched wavefront during waveguiding.

17. A method of head-up display as claimed in claim 15 or 16, wherein the mismatch comprises the product of the one-dimensional magnification and the onedimensional de-magnification being (a) less than 1 but greater than 0.75 or (b) greater than 1 but less than 1.25.

18. A method of designing a head-up display, the head-up display comprising:a first optical sub-system having one-dimensional magnification in the second dimension, wherein the first optical sub-system is arranged to receive an input wavefront having a first aspect ratio and output a stretched wavefront, wherein a size in the second dimension of the stretched wavefront is greater than that of the input wavefront in5 accordance with the one-dimensional magnification;a waveguide arranged to receive the stretched wavefront and output the stretched wavefront and a plurality of replicas thereof extending in the first dimension by waveguiding between a reflective surface and a reflective-transmissive surface; anda second optical sub-system having one-dimensional de-magnification in the second 10 dimension, wherein the second optical sub-system is arranged to receive the stretched wavefront, and plurality of replicas thereof, and output a compressed wavefront, and plurality of replicas thereof, wherein a size in the second dimension of the compressed wavefront, and each replica thereof, is less than that of the stretched wavefront in accordance with the one-dimensional de-magnification;15 the method comprising:mismatching the one-dimensional de-magnification with respect to the onedimensional magnification, the mismatch corresponding to the difference between the first aspect ratio and a second aspect ratio, the second aspect ratio being of an optical component downstream of the second optical sub-system.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:-07 01 25CLAIMS1. A head-up display having an eye-box defined by a first and second dimension, wherein the head-up display comprises:5 a first optical sub-system having one-dimensional magnification in the seconddimension, wherein the first optical sub-system is arranged to receive an input wavefront having a first aspect ratio and output a stretched wavefront, wherein a size in the second dimension of the stretched wavefront is greater than that of the input wavefront in accordance with the one-dimensional magnification;10 a waveguide arranged to receive the stretched wavefront and output the stretchedwavefront and a plurality of replicas thereof extending in the first dimension by waveguiding between a reflective surface and a reflective-transmissive surface; anda second optical sub-system having one-dimensional de-magnification in the second dimension, wherein the second optical sub-system is arranged to receive the stretched15 wavefront, and plurality of replicas thereof, and output a compressed wavefront, and plurality of replicas thereof, wherein a size in the second dimension of the compressed wavefront, and each replica thereof, is less than that of the stretched wavefront in accordance with the one-dimensional de-magnification;wherein the one-dimensional de-magnification is mismatched with the one-20 dimensional magnification, wherein the mismatch corresponds to the difference between the aspect ratio of the input wavefront and that of an optical component downstream of the second optical sub-system.

2. A head-up display as claimed in claim 1, wherein the mismatch comprises the 25 product of the one-dimensional magnification and the one-dimensional de-magnification being (a) less than 1 but greater than 0.75 or (b) greater than 1 but less than 1.25.

3. A head-up display as claimed in claim 1 or claim 2, wherein the optical component is an optic arranged to compensate for the curvature of an optical combiner30 downstream thereof.

4. A head-up display as claimed in claim 3, wherein the optic is a freeform mirror or corrective optic and / or the optical combiner is the windscreen of a vehicle housing the head-up display.07 01 255. A head-up display as claimed in any preceding claim, wherein a divergence angle of the stretched wavefront in the second dimension is less than that of the input wavefront in order to reduce the rate of expansion of the wavefront in the second dimension 5 during waveguiding.

6. A head-up display as claimed in claim 5, wherein the divergence angle of the wavefront in the second dimension in the waveguide is less than 1.5 degrees such as less than 1 degree.

107. A head-up display as claimed in claim 5 or claim 6, wherein the size and divergence angle of the stretched wavefront in the second dimension and an optical path length of the waveguiding define an optical footprint of the waveguide, wherein the optical footprint is at least 50% such as at least 75% of the physical footprint of the waveguide.

158. A head-up display as claimed in any preceding claim, wherein the first optical system comprises an anamorphic prism pair.

9. A head-up display as claimed in any preceding claim, wherein the first optical 20 system comprises a cylindrical lens such as a cylindrical achromatic doublet lens.

10. A head-up display as claimed in any preceding claim, wherein the first optical system comprises a cylindrical mirror having optical power.25 11. A head-up display as claimed in any of claims 5 to 10, wherein a (non-zero)divergence angle of the compressed wavefront in the second dimension is greater than that of the stretched wavefront.

12. A head-up display as claimed in any of preceding claim, wherein the second 30 optical sub-system comprises at least one selected from the group comprising: an anamorphic prism pair; a cylindrical achromatic doublet lens; and / or a cylindrical mirror having optical power.

13. A head-up display as claimed in any of preceding claim, wherein the first 35 optical sub-system comprises a first anamorphic prism pair and the second optical subsystem comprises a second anamorphic prism pair, and wherein the first anamorphic prism pair is shaped relative to the second anamorphic prism pair to produce the mismatch.07 01 2514. A head-up display as claimed in any preceding claim further comprising a second waveguide arranged to replicate the input wavefront in the second dimension, wherein the second waveguide comprises an input port arranged to receive the output of the 5 (first) waveguide.

15. A method of head-up display, the method comprising:reducing a rate of expansion of an input wavefront having a first aspect ratio by stretching the wavefront in the second dimension in accordance with a one-dimensional10 magnification of a first optical sub-system to form a stretched wavefront;waveguiding the stretched wavefront between a reflective surface and a reflective-transmissive surface of a waveguide;outputting, from the waveguide, the stretched wavefront and a plurality of replicas thereof extending in the first dimension; and15 compressing the stretched wavefront, and plurality of replicas thereof, in accordancewith a one-dimensional de-magnification of a second optical sub-system;wherein an optical component is located downstream of the second optical subsystem, the optical component having a second aspect ratio, and wherein the onedimensional de-magnification is mismatched with respect to the one-dimensional20 magnification, the mismatch corresponding to the difference between the first and second aspect ratios.

16. A method of head-up display as claimed in claim 15, wherein the reduction in the rate of expansion of the input wavefront is such that a divergence angle of the stretched 25 wavefront in the second dimension after the one-dimensional stretching is less than that of the input wavefront received by the first optical sub-system, and wherein the compression of the stretched wavefront, and plurality of replicas thereof, is such that a divergence angle of the output wavefront in the second dimension after the one-dimensional compression is greater than that of the stretched wavefront during waveguiding.3017. A method of head-up display as claimed in claim 15 or 16, wherein the mismatch comprises the product of the one-dimensional magnification and the onedimensional de-magnification being (a) less than 1 but greater than 0.75 or (b) greater than 1 but less than 1.25.3518. A method of designing a head-up display, the head-up display comprising:07 01 25a first optical sub-system having one-dimensional magnification in the second dimension, wherein the first optical sub-system is arranged to receive an input wavefront having a first aspect ratio and output a stretched wavefront, wherein a size in the second dimension of the stretched wavefront is greater than that of the input wavefront in5 accordance with the one-dimensional magnification;a waveguide arranged to receive the stretched wavefront and output the stretched wavefront and a plurality of replicas thereof extending in the first dimension by waveguiding between a reflective surface and a reflective-transmissive surface; anda second optical sub-system having one-dimensional de-magnification in the second 10 dimension, wherein the second optical sub-system is arranged to receive the stretched wavefront, and plurality of replicas thereof, and output a compressed wavefront, and plurality of replicas thereof, wherein a size in the second dimension of the compressed wavefront, and each replica thereof, is less than that of the stretched wavefront in accordance with the one-dimensional de-magnification;15 the method comprising:mismatching the one-dimensional de-magnification with respect to the onedimensional magnification, the mismatch corresponding to the difference between the first aspect ratio and a second aspect ratio, the second aspect ratio being of an optical component downstream of the second optical sub-system.

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