Image projection

Abstract

A holographic projection system include provides spatially modulated light representative of a hologram to a viewing system which has an entrance pupil. The projection system comprises a spatial ligh

Description

FIELD The present disclosure relates to image projection. More specifically, the present disclosure relates to holographic projection and a method of determining a diffractive structure such as a hologram or kinoform. Some embodiments relate to virtual image projection. Other embodiments relate to projection of a real image. Some embodiments relate to viewing a projected image through a waveguide. Some embodiments relate to a light engine such as a picture generating unit. Some embodiments relate to a head-up display. BACKGROUND AND INTRODUCTION Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or "hologram", comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object. Computer-generated holography may numerically simulate the interference process. A computergenerated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. A Fourier hologram may be considered a Fourier domain / plane representation of the object or a frequency domain / plane representation of the object. A computer-generated hologram may also be calculated by coherent ray tracing or a point cloud technique, for example. A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micro-mirrors, for example. A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, "HUD". SUMMARY Aspects of the present disclosure are defined in the appended independent claims. 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. 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. a lens or lenses of the human eye) and a viewing plane (e.g. the retinas of the human eye or eyes). 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. The image is formed by illuminating a diffractive pattern (e.g. hologram) displayed on the display device. The display device comprises pixels. The pixels of the display device diffract light. 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. However, the principle of etendue governs any conventional magnification. In embodiments, the image is a real image. In other embodiments, the image is a virtual image that is perceived by a human eye (or eyes). The projection system, or light engine, may thus be configured so that the viewer looks directly at the display device. In some embodiments, light encoded with the hologram is propagated directly to the eye(s) and there is no intermediate holographic reconstruction formed, either in free space or on a screen or other light receiving surface, between the display device and the viewer. In such embodiments, the pupil of the eye may be regarded as being the entrance aperture of the viewing system and the retina of the eye may be regarded as the viewing plane of the viewing system. It is sometimes said that, in this configuration, the lens of the eye performs a hologram-to-image conversion. In other embodiments, light encoded with an image - or simply "an image" - is propagated to the eye. 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-motion 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 is possible to consider a plurality of different virtual image points of a virtual image. The distance from a virtual point to the viewer is referred to herein as a virtual image distance, for that virtual image point. Different virtual points may, of course, have different virtual image distances. Individual light rays, within ray bundles associated with each virtual point, may take different respective optical paths to the viewer, via the display device. However, only some parts of the display device, and therefore only some of the rays from one or more virtual points of a virtual image, may be within the user's field of view. In other words, only some of the light rays from some of the virtual points on the virtual image will propagate, via the display device, into the user's eye(s) and thus will be visible to the viewer. 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. There has previously been disclosure (for example in UK patent GB22603517B) of how to increase the field of view - i.e., how to increase the range of angles of light rays that are propagated from the display device, and which can successfully propagate through an eye's pupil to form an image - when the display device is (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 aperture of the display device (i.e., size of the array of pixels). More specifically, this prior disclosure addressed how to do this with so-called direct view holography in which a hologram of an image is propagated to the human eye rather than the image itself. In other words, the light received by the viewer is modulated according to a hologram of the image. A waveguide is used to enable a viewer to experience the full field of view from each viewing position or, in other words, increase the maximum propagation distance over which the full diffractive angle range of the display device may be used. Use of a waveguide increases the size of the eye-box, thus enabling some movement of the user to occur, whilst still enabling the user to see the entire image. The waveguide may be referred to as a waveguide pupil expander. It was previously found, however, that for a non-infinite virtual image distance - that is, near-field virtual images - so-called 'ghost images' appear owing to the different possible light propagation paths through the waveguide. A ghost image is a lower intensity replica of a main image. The main, highest intensity image may be referred to as the primary image. Each ghost image may be referred to as a secondary image. The presence of ghost images can significantly reduce the quality of a perceived virtual image. The ghost images may give the appearance of blurring of the primary image. Previous disclosures related to different approaches for addressing problems caused by the ghost images (i.e. by removing the appearance of ghost images). This included modifying / manipulating the ghost image in order to enhance or reinforce the primary / non-ghost image. However, the inventors discovered that these prior disclosures can result in "contributory areas" of the display device - that is, the active pixel area used to form the hologram - forming multiple so-called "lobes" in the point spread function resulting therefrom. That is, each individual pixel in the image reconstructed from the hologram will not be a single point of light, but instead will be a central point of light with multiple "lobes" of light (i.e. elongated shapes or "petals") extending therefrom in a radial pattern. As these "lobes" extend across a (relatively) large area, they may interact (e.g. interfere) with the "lobes" (and even the central points) of adjacent pixels. This so-called "pixel cross-talk" is stronger the more of these "lobes" are present and the larger the "lobes" are, and can reduce the quality of the image perceived by the viewer by, for example, reducing the pixel-per-degree and introducing speckles. According to a first aspect of the present disclosure, there is provided a light engine (in other words, a display system or holographic projector) arranged to provide a viewing system (e.g. an eye of a viewer / user) with spatially modulated light. The viewing system (eye) has an entrance pupil (e.g. a pupil of the eye). The light engine comprises a display device (which, as described above, may be an "LCOS") arranged to display a hologram and spatially modulate light in accordance with the hologram. The light engine further comprises a hologram engine arranged to receive information identifying at least one primary contributory area of the display device (in other words, at least one primary contribution area). This identification is based on the location of the entrance pupil. The at least one primary contributory area substantially propagates spatially modulated light passing through the entrance pupil at the location that contributes to (in other words, forms) a primary image. That is, the primary contributory area is the area of the display device from which light forms (at least part of) the image intended to be viewed, rather than any ghost images that the display device may also form (from different areas of the display device). Such formation may occur when the display device is illuminated by a light source (for example, a laser diode). The hologram engine is further arranged to receive information identifying at least one sub-area of the primary contributory area. The sub-area is also referred to as an "inscribed circle", as is discussed below. In other words, each sub-area is at least part of (i.e. is within the) at least one of the primary contributory areas. The hologram engine is arranged to receive information identifying the primary contributory area(s) and sub-area(s) of the display device meaning that the areas are called from memory or calculated when needed (e.g. on-the-fly), or a combination of both. That is, the shape, location, size, etc. of the areas may be taken from memory to reduce computation time and power, but may also / alternatively be calculated (for example, if the location of the entrance pupil changes). The hologram engine is further arranged to determine (e.g. calculate) the hologram in only the at least one sub-areas and to output the hologram to the display device for display. Each sub-area has a greater radial symmetry than (that of) the primary contributory area (that is, the corresponding primary contributory area that the sub-area is located within). In other words, each sub-area has a first radial symmetry and the primary contributory area has a second radial symmetry, the first radial symmetry being greater than the second radial symmetry. By radial symmetry (in other words rotational symmetry), it is meant a shape that looks the same when viewed after a partial turn or rotation about a central axis of the shape. Shapes having a larger number of different rotations through 360° for which this holds have a greater radial symmetry. In other words, each sub-area has more lines of symmetry that pass through a central point of the respective sub-area than the (corresponding) primary contributory area, or has a substantially constant radius. More generally, it may be said that the sub-area has greater symmetry (e.g. radial symmetry) than the primary contributory area. The inventors found that by increasing the radial symmetry of the area(s) of the display device in which the hologram is formed reduces the number and quantity of "lobes" produced in the resultant point spread functions. This in turn improves the quality of the image perceived by the viewer. In other words, the present disclosure significantly reduces the size and number of "lobes" of the point spread function of the system. This is critical to achieve high resolution, low speckle images-e.g. minimising (image) pixel crosstalk. To avoid the presence of ghost images, only pixels of the display device that are inside the primary contributory area were used to display the hologram. However, using the whole of the primary contributory area may degrade the point spread function. As such, the inventors discovered that selecting pixels from a radially symmetric sub-area inside the primary contributory area can improve the point spread function (leading to decreased image pixel crosstalk) and thereby the image quality. However, determining the hologram only in the sub-area(s) of the display device, which are smaller than the (corresponding) primary contributory area(s), reduces the area of the display device that is used for display of the target image. This reduces the intensity of the light and therefore the possible intensity of the image displayed. The inventors have surprisingly found that this reduction in intensity is worth the improvements made to the image quality as described above. In other words, although more pixels of the display device are effectively switched off, the optimised shape of the area of the display device in which the hologram is formed produces an improved point spread function. At least one of the sub-areas may be an equilateral (or regular) polygon in shape having at least 10 sides, such as at least 20 sides or at least 30 sides. Sides of the polygon may be equal in length. Angles of the polygon may be equal. The inventors have found that using sub-areas with these number of sides reduces the presence of sharp corners in the primary contributory area(s) sufficiently to remove or reduce the "lobes" in the point spread function that these corners would produce. At least one of the sub-areas may be substantially circular. By "substantially circular", it can be meant as close to a true circle as the hologram engine is capable of producing on the display device (depending on the pixel pitch of the display device). Otherwise, "substantially circular" refers to the sub-area(s) being sufficiently circular that the shape reduces the presence of the "lobes" in the point spread function, as described above. The use of a substantially circular shape produces an Airy disc, which is generally considered as the ideal point spread function for an optical imaging system. The substantially circular nature of the sub-area(s) within the primary contributory area(s) is reflected by the term "inscribed circle" to describe the sub-areas, as mentioned above. A least one of the primary contributory areas may have a cropped circular shape, such as a dumbbell shape. Such a shape is often formed during the identification of the primary contributory area(s) and maximising of said area(s) to maximise the intensity of light produced from the display device. These shapes however lead to the sharp corners as described above, which the inventors have recognised produces the "lobes" in the point spread function. The radius of at least of the sub-areas may be determined such that the size of the sub-area is substantially maximised within the corresponding primary contributory area. That is, the size of the sub area is determined such that it fills as much of the primary contributory area as possible, within the bounds set by e.g. the pixel pitch of the display device. In this way, the loss of light intensity as discussed above can be minimised as much as possible. At least one of the primary contributory areas may comprise a plurality of sub-areas, such as two subareas. Each sub-area within a corresponding primary contributory area may have substantially the same shape. Each sub-area may have a diameter greater than 1mm, such as greater than 1.5mm or greater than 2mm. The inventors have surprisingly found that using multiple sub-areas simultaneous can increase the intensity of the light produced from the display device whilst not negatively affecting the point spread function. That is, the inventors surprisingly found that, as long as each sub-area is substantially the same size and relatively large enough compared to the display device, using multiple sub-areas does not change the point spread function produced. However, if either of those principles are not followed, distortions may be introduced to the point spread function that may reduce the image quality perceived by the viewer. The light engine may further comprise a monitoring system arranged to determine the location of the entrance pupil of the viewing system. If the viewing system is an eye of viewer, the monitoring system may be an eye tracking system that tracks the position of the user's eyes (more specifically, their pupils). In this way, the location, shape, size, etc. of the primary contributory area can be adjusted based on the entrance pupil location. The viewing system may be arranged to form an image corresponding to the hologram. That is, as described above, the viewing system (e.g. the eye) performs a "hologram-to-eye" conversion. The hologram engine may be further arranged to receive information identifying at least one secondary contributory area of the display device propagating light to the viewing system that contributes to a secondary image. The hologram may be configured such that the primary image and at least one secondary image are virtual images each having a non-infinite virtual image distance. The contributory information (in other words, the contribution information) may further identify a non-contributory area of the display device based on the location of the entrance pupil, wherein the non-contributory areas of the display device substantially propagate spatially modulated light stopped by the entrance pupil at the location. The contributory information may identify respective contributory and non-contributory areas of the display device for each of a plurality of image points of the image. By identifying contributory and non-contributory areas of the display device, the light engine can determine which part or parts of the display device can usefully (i.e. should) be encoded by the hologram, in order to contribute positively to formation of the primary image, for a given location of the entrance aperture of the viewing system. For example, this may correspond to a location of a viewer's eye, at a given time. Moreover, the light engine can determine which parts of the display device cannot propagate light through the entrance aperture, and thus are not worth populating with hologram values (e.g. are not worth the compute time). In addition, the light engine can distinguish between parts of the display device that contribute positively to a 'main' target image and parts which contribute to a copy / replica or 'ghost' version of the primary image. Thus, hologram pixel values can be omitted in so-called secondary contributory areas, to eliminate the ghosts. The light engine may further comprise a waveguide arranged to receive the spatially modulated light from the display device and provide a plurality of different light propagation paths for the spatially modulated light from the display device to the entrance pupil, wherein each contributory area corresponds to a different respective light propagation path provided by the waveguide. The primary image may be the image, from a plurality of images formed by the waveguide, having the greatest luminance. The hologram may comprise a plurality of sub-holograms, wherein each sub-hologram is determined by the hologram engine based on the contributory information of a respective image point of the image. That is, together the sub-holograms form the whole image displayed to the user. The sub-holograms may be tiled in order to form the full image. The hologram engine may be further arranged such that at least a portion of a boundary of at least one of the sub-areas is apodised to reduce a diffractive effect. In other words, at least a portion of a boundary of at least one of the sub-areas is modified to change the diffraction around a central intensity peak in the resultant point spread function. The apodisation may be achieved using a so-called window function (in other words, an apodization function or tapering function), such as the Blackman Harris function. In other words, the apodisation may be achieved by a function that produces a zero value outside of a chosen interval. The hologram engine may be arranged such that the boundary of at least one of the sub-areas (that is, the entire boundary) is apodised to reduce a diffractive effect. The inventors have found that edge apodisation of the boundaries produces a complementary effect to the improvement of the point spread function. In other words, the inventors have surprisingly found that taking the further step of using edge apodisation has a synergistic effect with the "inscribed circle". The edge apodisation reduces (i.e. softens) the steep change between the (sub-)areas of the display device used to display the hologram and those that are not (i.e. between the pixels that are on and those are off). The inventors have found that this step change contains high frequency components which will result in higher diffraction "lobes" in the point spread function. So, whilst the "inscribed circle" feature produces a smaller (i.e. improved) point spread function, a set of concentric rings still exists within the function. As such, the inventors have discovered that due to the smoothing process of the edge apodisation, the point spread function will consist mainly of the central "lobe" with most, if not all, the higher orders removed. As such, the resultant image produced has a higher quality. The inventors have surprisingly discovered that window functions (such as the Blackman Harris function) - from the field of signal processing - can also be used to improve holographic display. In some embodiments, the edge apodisation occurs without the "inscribed circle" feature described above in relation to the first aspect of the present disclosure. As such, according to a second aspect of the present disclosure there is provided a light engine arranged to provide a viewing system with spatially modulated light. The viewing system has an entrance pupil. The light engine comprises a display device arranged to display a hologram and spatially modulate light in accordance with the hologram. The light engine further comprises a hologram engine arranged to receive contributory information identifying at least one primary contributory area of the display device. The identification is based on the location of the entrance pupil. The at least one primary contributory area substantially propagates spatially modulated light passing through the entrance pupil at the location that contributes to a primary image. The hologram engine may be further arranged to receive information identifying at least one area of the primary contributory area, to determine the hologram in only the at least one primary contributory area (or sub-areas thereof) and to output the hologram to the display device for display. The hologram engine is further arranged such that at least a portion of a boundary of at least one of the primary contributory areas (or sub-areas thereof) is apodised to reduce a diffractive effect. That is, the edge apodisation may be applied to the primary contributory area, or the sub-areas thereof (if the "inscribed circle" feature of the first aspect is used). The edge apodisation may be applied to a portion or the whole of the boundary of the primary contributory area (or sub-area thereof). The inventors have surprisingly found that edge apodisation still produces the above-described positive effect on the point spread function without the "inscribed circle" feature. Features and advantages described in relation to the light engine of the first aspect may be applicable to the light engine of the second aspect, and vice versa. A display area of the display device may have a first dimension less than 5cm, such as less than 2cm or less than 1cm. Each sub-area may have a diameter greater than a quarter of the first dimension, such as greater than a third of the first dimension. A distance from the display device to the entrance pupil of the viewing system may be equal to or greater than 20cm or 50cm, such as greater than 75cm or greater than 100cm. According to a third aspect of the present disclosure, there is provided a method of determining a hologram for display on a display device. The method comprises a first step of determining the location of the entrance pupil of a viewing system arranged to view the hologram. The method comprises a second step of identifying at least one primary contributory area of the display device that substantially propagate light contributing to a primary image passing through the entrance pupil of the viewing system at the determined location. The method comprises a third step of receiving information identifying at least one sub-area of the primary contributory area. Each sub-area has a greater radial symmetry than the primary contributory area (in other words, has more lines or redial symmetry than the primary contributory area or has a substantially constant radius). The method comprises a fourth step of determining the hologram in only the at least one sub-areas of the display device. The third step may comprise determining a radius of at least one sub-area such that the size of the subarea is substantially maximised within the corresponding primary contributory area. The second step may further comprise identifying at least one secondary contributory area of the display device that provides light contributing to a secondary image. As with the second aspect in relation to the first aspect, the method of the third aspect may be performed without the "inscribed circle" feature, but with the edge apodisation feature. As such, according to a fourth aspect of the present disclosure, there is provided a method of determining a hologram for display on a display device. The method comprises a first step of determining the location of the entrance pupil of a viewing system arranged to view the hologram. The method comprises a second step of identifying at least one primary contributory area of the display device that substantially propagate light contributing to a primary image passing through the entrance pupil of the viewing system at the determined location. The method comprises a third step of apodising a portion of a boundary of at least one of the primary contributory areas to reduce a diffractive effect. The method comprises a fourth step of determining the hologram in only the at least one primary contributory areas of the display device. Features and advantages described in relation to the light engines of the first and second aspects may be applicable to the methods of the third and fourth aspects, and vice versa. In the present disclosure, the term "replica" is merely used to reflect that spatially modulated light is divided such that a complex light field is directed along a plurality of different optical paths. The word "replica" is used to refer to each occurrence or instance of the complex light field after a replication event - such as a partial reflection-transmission by a pupil expander. Each replica travels along a different optical path. Some embodiments of the present disclosure relate to propagation of light that is encoded with a hologram, not an image - i.e., light that is spatially modulated with a hologram of an image, not the image itself. It may therefore be said that a plurality of replicas of the hologram are formed. The person skilled in the art of holography will appreciate that the complex light field associated with propagation of light encoded with a hologram will change with propagation distance. Use herein of the term "replica" is independent of propagation distance and so the two branches or paths of light associated with a replication event are still referred to as "replicas" of each other even if the branches are a different length, such that the complex light field has evolved differently along each path. That is, two complex light fields are still considered "replicas" in accordance with this disclosure even if they are associated with different propagation distances - providing they have arisen from the same replication event or series of replication events. A "diffracted light field" or "diffractive light field" in accordance with this disclosure is a light field formed by diffraction. A diffracted light field may be formed by illuminating a corresponding diffractive pattern. In accordance with this disclosure, an example of a diffractive pattern is a hologram and an example of a diffracted light field is a holographic light field or a light field forming a holographic reconstruction of an image. The holographic light field forms a (holographic) reconstruction of an image on a replay plane. The holographic light field that propagates from the hologram to the replay plane may be said to comprise light encoded with the hologram or light in the hologram domain. A diffracted light field is characterized by a diffraction angle determined by the smallest feature size of the diffractive structure and the wavelength of the light (of the diffracted light field). In accordance with this disclosure, it may also be said that a "diffracted light field" is a light field that forms a reconstruction on a plane spatially separated from the corresponding diffractive structure. An optical system is disclosed herein for propagating a diffracted light field from a diffractive structure to a viewer. The diffracted light field may form an image. The term "hologram" is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, regarding the object. The term "holographic reconstruction" is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The system disclosed herein is described as a "holographic projector" because the holographic reconstruction is a real image and spatially-separated from the hologram. The 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 "phasedelay". That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2tc) 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 tc / 2 will retard the phase of received light by tc / 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 2A illustrates a first iteration of an example Gerchberg-Saxton type algorithm; Figure 2B illustrates the second and subsequent iterations of the example Gerchberg-Saxton type algorithm; Figure 2C illustrates alternative second and subsequent iterations of the example Gerchberg-Saxton type algorithm; Figure 3 is a schematic of a reflective LCOS SLM; Figure 4 shows an image for projection comprising eight image areas / components, VI to V8, and crosssections of the corresponding hologram channels, H1-H8; Figure 5 shows a hologram displayed on a display device that directs light into a plurality of discrete areas; Figure 6 shows a system, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3; Figure 7A shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each comprising pairs of stacked surfaces; Figure 7B shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each in the form of a solid waveguide; Figure 8A shows how with a finite virtual image and waveguide pupil expander, ghost images can be formed; Figure 8B shows a schematic view of the formation of ghost images; Figure 8C shows a virtual image that comprises a primary image and two ghost images; Figures 9A to 9C show an example in which the entire display device is used to form an primary image point and two corresponding ghost image points; Figures 10A to IOC show first, second and third propagation paths through a waveguide giving rise to a second ghost point, main image point and first ghost point, respectively; Figure 11A to 11C show three the propagation path and display device utilisation in relation to three different field / image points; Figure 12A shows a viewing system including a virtual image point and an image of that virtual image point formed by a viewing system and waveguide; Figure 12B shows the primary contributory area of the display device in relation to the example of Figure 12A; Figures 13A to 13C show formation of a primary contributory area on a display device; Figures 14A and 14B show the resultant point spread function from the primary contributory area of Figure 13C; Figures 14C to 14E show first examples of "inscribed circles" in accordance with the present disclosure and the resultant point spread function; Figures 15A and 15B show a further example of a primary contributory area and the resultant point spread function; Figures 15C and 15D show a second example of an "inscribed circle" in accordance with the present disclosure and the resultant point spread function; Figures 16A and 16B show a yet further example of a primary contributory area and the resultant point spread function; Figures 16C and 16D show a third example of an "inscribed circle" in accordance with the present disclosure and the resultant point spread function; Figures 17A and 17B show a further example of the resultant point spread function from the primary contributory area of Figure 13C; Figures 17C and 17D show an example of edge apodisation of the primary contributory area of Figure 17A in accordance with the present disclosure and the resultant point spread function; Figures 18A and 18B show an example of edge apodisation of an "inscribed circle" similar to that of Figure 13C in accordance with the present disclosure and the resultant point spread function; and Figures 19A to 19C show a further example of edge apodisation of an "inscribed circle" in accordance with 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 co-dependent relationship. Optical configuration of Holographic Picture Generating Unit Figure 1 shows an embodiment in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or frequency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, "LCOS", device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser. A light source 110, for example a laser or laser diode, is disposed to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally planar wavefront of light to be incident on the SLM. In Figure 1, the direction of the wavefront is off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer). However, in other embodiments, the generally planar wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in Figure 1, the arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a light-modulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to optics including a Fourier transform lens 120, having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives a beam of modulated light from the SLM 140 and performs a frequencyspace 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 lightmodulating 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. 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. Computergenerated Fourier holograms may be calculated using Fourier transforms. This is further discussed, for example, in UK patent number GB2610203. A Fourier transform hologram may be calculated using an algorithm such as the Gerchberg-Saxton algorithm. Furthermore, the Gerchberg-Saxton algorithm may be used to calculate a hologram in the Fourier domain (i.e. a Fourier transform hologram) from amplitude-only information in the spatial domain (such as a photograph). The phase information related to the object is effectively "retrieved" from the amplitude-only information in the spatial domain. In some embodiments, a computer generated hologram is calculated from amplitude-only information using the Gerchberg-Saxton algorithm or a variation thereof. The Gerchberg Saxton algorithm considers the situation when intensity cross-sections of a light beam, Ia(x, y) and Ib(x, y), in the planes A and B respectively, are known and Ia(x, y) and Ib(x, y) are related by a single Fourier transform. With the given intensity cross-sections, an approximation to the phase distribution in the planes A and B, TMx, y) and UMx, y) respectively, is found. The Gerchberg-Saxton algorithm finds solutions to this problem by following an iterative process. More specifically, the Gerchberg-Saxton algorithm iteratively applies spatial and spectral constraints while repeatedly transferring a data set (amplitude and phase), representative of Ia(x, y) and Ib(x, y), between the spatial domain and the Fourier (spectral or frequency) domain. The corresponding computer-generated hologram in the spectral domain is obtained through at least one iteration of the algorithm. The algorithm is convergent and arranged to produce a hologram representing an input image. The hologram may be an amplitude-only hologram, a phase-only hologram or a fully complex hologram. In some embodiments, a phase-only hologram is calculated using an algorithm based on the Gerchberg-Saxton algorithm such as described in British patent 2,498,170 or 2,501,112 which are hereby incorporated in their entirety by reference. However, embodiments disclosed herein describe calculating a phase-only hologram by way of example only. In these embodiments, the Gerchberg-Saxton algorithm retrieves the phase information tp [u, v] of the Fourier transform of the data set which gives rise to a known amplitude information T[x, y], wherein the amplitude information T[x, y] is representative of a target image (e.g. a photograph). Since the magnitude and phase are intrinsically combined in the Fourier transform, the transformed magnitude and phase contain useful information about the accuracy of the calculated data set. Thus, the algorithm may be used iteratively with feedback on both the amplitude and the phase information. However, in these embodiments, only the phase information ^[u, v] is used as the hologram to form a holographic representative of the target image at an image plane. The hologram is a data set (e.g. 2D array) of phase values. In other embodiments, an algorithm based on the Gerchberg-Saxton algorithm is used to calculate a fully-complex hologram. A fully-complex hologram is a hologram having a magnitude component and a phase component. The hologram is a data set (e.g. 2D array) comprising an array of complex data values wherein each complex data value comprises a magnitude component and a phase component. In some embodiments, the algorithm processes complex data and the Fourier transforms are complex Fourier transforms. Complex data may be considered as comprising (i) a real component and an imaginary component or (ii) a magnitude component and a phase component. In some embodiments, the two components of the complex data are processed differently at various stages of the algorithm. Figure 2A illustrates the first iteration of an algorithm in accordance with some embodiments for calculating a phase-only hologram. The input to the algorithm is an input image 210 comprising a 2D array of pixels or data values, wherein each pixel or data value is a magnitude, or amplitude, value. That is, each pixel or data value of the input image 210 does not have a phase component. The input image 210 may therefore be considered a magnitude-only or amplitude-only or intensity-only distribution. An example of such an input image 210 is a photograph or one frame of video comprising a temporal sequence of frames. The first iteration of the algorithm starts with a data forming step 202A comprising assigning a random phase value to each pixel of the input image, using a random phase distribution (or random phase seed) 230, to form a starting complex data set wherein each data element of the set comprising magnitude and phase. It may be said that the starting complex data set is representative of the input image in the spatial domain. First processing block 250 receives the starting complex data set and performs a complex Fourier transform to form a Fourier transformed complex data set. Second processing block 253 receives the Fourier transformed complex data set and outputs a hologram 280A. In some embodiments, the hologram 280A is a phase-only hologram. In these embodiments, second processing block 253 quantises each phase value and sets each amplitude value to unity in order to form hologram 280A. Each phase value is quantised in accordance with the phase-levels which may be represented on the pixels of the spatial light modulator which will be used to "display" the phase-only hologram. For example, if each pixel of the spatial light modulator provides 256 different phase levels, each phase value of the hologram is quantised into one phase level of the 256 possible phase levels. Hologram 280A is a phase-only Fourier hologram which is representative of an input image. In other embodiments, the hologram 280A is a fully complex hologram comprising an array of complex data values (each including an amplitude component and a phase component) derived from the received Fourier transformed complex data set. In some embodiments, second processing block 253 constrains each complex data value to one of a plurality of allowable complex modulation levels to form hologram 280A. The step of constraining may include setting each complex data value to the nearest allowable complex modulation level in the complex plane. It may be said that hologram 280A is representative of the input image in the spectral or Fourier or frequency domain. In some embodiments, the algorithm stops at this point. However, in other embodiments, the algorithm continues as represented by the dotted arrow in Figure 2A. In other words, the steps which follow the dotted arrow in Figure 2A are optional (i.e. not essential to all embodiments). Third processing block 256 receives the modified complex data set from the second processing block 253 and performs an inverse Fourier transform to form an inverse Fourier transformed complex data set. It may be said that the inverse Fourier transformed complex data set is representative of the input image in the spatial domain. Fourth processing block 259 receives the inverse Fourier transformed complex data set and extracts the distribution of magnitude values 211A and the distribution of phase values 213A. Optionally, the fourth processing block 259 assesses the distribution of magnitude values 211A. Specifically, the fourth processing block 259 may compare the distribution of magnitude values 211A of the inverse Fourier transformed complex data set with the input image 510 which is itself, of course, a distribution of magnitude values. If the difference between the distribution of magnitude values 211A and the input image 210 is sufficiently small, the fourth processing block 259 may determine that the hologram 280A is acceptable. That is, if the difference between the distribution of magnitude values 211A and the input image 210 is sufficiently small, the fourth processing block 259 may determine that the hologram 280A is a sufficiently-accurate representative of the input image 210. In some embodiments, the distribution of phase values 213A of the inverse Fourier transformed complex data set is ignored for the purpose of the comparison. It will be appreciated that any number of different methods for comparing the distribution of magnitude values 211A and the input image 210 may be employed and the present disclosure is not limited to any particular method. In some embodiments, a mean square difference is calculated and if the mean square difference is less than a threshold value, the hologram 280A is deemed acceptable. If the fourth processing block 259 determines that the hologram 280A is not acceptable, a further iteration of the algorithm may be performed. However, this comparison step is not essential and in other embodiments, the number of iterations of the algorithm performed is predetermined or preset or user-defined. Figure 2B represents a second iteration of the algorithm and any further iterations of the algorithm. The distribution of phase values 213A of the preceding iteration is fed-back through the processing blocks of the algorithm. The distribution of magnitude values 211A is rejected in favour of the distribution of magnitude values of the input image 210. In the first iteration, the data forming step 202A formed the first complex data set by combining distribution of magnitude values of the input image 210 with a random phase distribution 230. However, in the second and subsequent iterations, the data forming step 202B comprises forming a complex data set by combining (i) the distribution of phase values 213A from the previous iteration of the algorithm with (ii) the distribution of magnitude values of the input image 210. The complex data set formed by the data forming step 202B of Figure 2B is then processed in the same way described with reference to Figure 2A to form second iteration hologram 280B. The explanation of the process is not therefore repeated here. The algorithm may stop when the second iteration hologram 280B has been calculated. However, any number of further iterations of the algorithm may be performed. It will be understood that the third processing block 256 is only required if the fourth processing block 259 is required or a further iteration is required. The output hologram 280B generally gets better with each iteration. However, in practice, a point is usually reached at which no measurable improvement is observed or the positive benefit of performing a further iteration is out-weighted by the negative effect of additional processing time. Hence, the algorithm is described as iterative and convergent. Figure 2C represents an alternative embodiment of the second and subsequent iterations. The distribution of phase values 213A of the preceding iteration is fed-back through the processing blocks of the algorithm. The distribution of magnitude values 211A is rejected in favour of an alternative distribution of magnitude values. In this alternative embodiment, the alternative distribution of magnitude values is derived from the distribution of magnitude values 211 of the previous iteration. Specifically, processing block 258 subtracts the distribution of magnitude values of the input image 210 from the distribution of magnitude values 211 of the previous iteration, scales that difference by a gain factor a and subtracts the scaled difference from the input image 210. This is expressed mathematically by the following equations, wherein the subscript text and numbers indicate the iteration number: = ^'{exp(zy„[M,v])} = ZF{t? •exp(zZFB[x,j])} ri = T[x, j] - a(\R„ [x, j]| - T[x, j]) where: F1 is the inverse Fourier transform; F is the forward Fourier transform; R[x, y] is the complex data set output by the third processing block 256; T[x, y] is the input or target image; Z is the phase component; tp is the phase-only hologram 280B; r] is the new distribution of magnitude values 211B; and a is the gain factor. The gain factor a may be fixed or variable. In some embodiments, the gain factor a is determined based on the size and rate of the incoming target image data. In some embodiments, the gain factor a is dependent on the iteration number. In some embodiments, the gain factor a is solely function of the iteration number. The embodiment of Figure 2C is the same as that of Figure 2A and Figure 2B in all other respects. It may be said that the phase-only hologram QJ(u, v) comprises a phase distribution in the frequency or Fourier domain. In some embodiments, the Fourier transform is performed using the spatial light modulator. Specifically, the hologram data is combined with second data providing optical power. That is, the data written to the spatial light modulation comprises hologram data representing the object and lens data representative of a lens. When displayed on a spatial light modulator and illuminated with light, the lens data emulates a physical lens - that is, it brings light to a focus in the same way as the corresponding physical optic. The lens data therefore provides optical, or focusing, power. In these embodiments, the physical Fourier transform lens 120 of Figure 1 may be omitted. It is known how to calculate data representative of a lens. The data representative of a lens may be referred to as a software lens. For example, a phase-only lens may be formed by calculating the phase delay caused by each point of the lens owing to its refractive index and spatially-variant optical path length. For example, the optical path length at the centre of a convex lens is greater than the optical path length at the edges of the lens. An amplitude-only lens may be formed by a Fresnel zone plate. It is also known in the art of computer-generated holography how to combine data representative of a lens with a hologram so that a Fourier transform of the hologram can be performed without the need for a physical Fourier lens. In some embodiments, lensing data is combined with the hologram by simple addition such as simple vector addition. In some embodiments, a physical lens is used in conjunction with a software lens to perform the Fourier transform. Alternatively, in other embodiments, the Fourier transform lens is omitted altogether such that the holographic reconstruction takes place in the far-field. In further embodiments, the hologram may be combined in the same way with grating data - that is, data arranged to perform the function of a grating such as image steering. Again, it is known in the field how to calculate such data. For example, a phase-only grating may be formed by modelling the phase delay caused by each point on the surface of a blazed grating. An amplitude-only grating may be simply superimposed with an amplitude-only hologram to provide angular steering of the holographic reconstruction. The second data providing lensing and / or steering may be referred to as a light processing function or light processing pattern to distinguish from the hologram data which may be referred to as an image forming function or image forming pattern. In some embodiments, the Fourier transform is performed jointly by a physical Fourier transform lens and a software lens. That is, some optical power which contributes to the Fourier transform is provided by a software lens and the rest of the optical power which contributes to the Fourier transform is provided by a physical optic or optics. 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. 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. The present disclosure is also applicable to holograms calculated by other techniques such as those based on point cloud methods. Light modulation A spatial light modulator may be used to display the diffractive pattern including the computer-generated hologram. If the hologram is a phase-only hologram, a spatial light modulator which modulates phase is required. If the hologram is a fully-complex hologram, a spatial light modulator which modulates phase and amplitude may be used or a first spatial light modulator which modulates phase and a second spatial light modulator which modulates amplitude may be used. In some embodiments, the light-modulating elements (i.e. the pixels) of the spatial light modulator are cells containing liquid crystal. That is, in some embodiments, the spatial light modulator is a liquid crystal device in which the optically-active component is the liquid crystal. Each liquid crystal cell is configured to selectively-provide a plurality of light modulation levels. That is, each liquid crystal cell is configured at any one time to operate at one light modulation level selected from a plurality of possible light modulation levels. Each liquid crystal cell is dynamically-reconfigurable to a different light modulation level from the plurality of light modulation levels. In some embodiments, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) spatial light modulator but the present disclosure is not restricted to this type of spatial light modulator. A LCOS device provides a dense array of light modulating elements, or pixels, within a small aperture (e.g. a few centimetres in width). The pixels are typically approximately 10 microns or less which results in a diffraction angle of a few degrees meaning that the optical system can be compact. It is easier to adequately illuminate the small aperture of a LCOS SLM than it is the larger aperture of other liquid crystal devices. An LCOS device is typically reflective which means that the circuitry which drives the pixels of a LCOS SLM can be buried under the reflective surface. The results in a higher aperture ratio. In other words, the pixels are closely packed meaning there is very little dead space between the pixels. This is advantageous because it reduces the optical noise in the replay field. A LCOS SLM uses a silicon backplane which has the advantage that the pixels are optically flat. This is particularly important for a phase modulating device. A suitable LCOS SLM is described below, by way of example only, with reference to Figure 3. An LCOS device is formed using a single crystal silicon substrate 302. It has a 2D array of square planar aluminium electrodes 301, spaced apart by a gap 301a, arranged on the upper surface of the substrate. Each of the electrodes 301 can be addressed via circuitry 302a buried in the substrate 302. Each of the electrodes forms a respective planar mirror. An alignment layer 303 is disposed on the array of electrodes, and a liquid crystal layer 304 is disposed on the alignment layer 303. A second alignment layer 305 is disposed on the planar transparent layer 306, e.g. of glass. A single transparent electrode 307 e.g. of ITO is disposed between the transparent layer 306 and the second alignment layer 305. Each of the square electrodes 301 defines, together with the overlying region of the transparent electrode 307 and the intervening liquid crystal material, a controllable phase-modulating element 308, often referred to as a pixel. The effective pixel area, or fill factor, is the percentage of the total pixel which is optically active, taking into account the space between pixels 301a. By control of the voltage applied to each electrode 301 with respect to the transparent electrode 307, the properties of the liquid crystal material of the respective phase modulating element may be varied, thereby to provide a variable delay to light incident thereon. The effect is to provide phase-only modulation to the wavefront, i.e. no amplitude effect occurs. The described LCOS SLM outputs spatially modulated light in reflection. Reflective LCOS SLMs have the advantage that the signal lines, gate lines and transistors are below the mirrored surface, which results in high fill factors (typically greater than 90%) and high resolutions. Another advantage of using a reflective LCOS spatial light modulator is that the liquid crystal layer can be half the thickness than would be necessary if a transmissive device were used. This greatly improves the switching speed of the liquid crystal (a key advantage for the projection of moving video images). However, the teachings of the present disclosure may equally be implemented using a transmissive LCOS SLM. As described above, the principles of the present disclosure are applicable to non-holographic picture generating units as well as holographic picture generating units as described above. Compact Head-up Display The picture generating unit described in relation to Figure 1 is typically provided as part of a head-up display system (HUD-system). The HUD system further comprises an optical relay system arranged to relay light of a picture from a display area of a picture generating unit to an eye-box such that a virtual image of the picture is visible therefrom. As described herein, the eye-box comprises an area, optionally a volume, from which the virtual image can be fully perceived by a viewer. As the skilled person will appreciate, the virtual image becomes increasingly less completely visible from viewing positions further away from the eye-box. Ray tracing techniques may be used to measure parameters, such as distortion and horizontal / vertical disparity, in order to objectively identify viewing positions where the virtual image is clear. Based on such measurements, the inventors have recognised that the optical relay system may be configured in order to define the eye-box area to satisfy design requirements, such as packing requirements. Large field of view using small display device Broadly, the present disclosure relates to image projection. It relates to a method of image projection and an image projector which comprises a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system, in which the image projector projects or relays light from the display device to the viewing system. The present disclosure is equally applicable to a monocular and binocular viewing system. The viewing system may comprise a viewer's eye or eyes. The viewing system comprises an optical element having optical power (e.g., lens / es of the human eye) and a viewing plane (e.g., retina of the human eye / s). The projector may be referred to as a 'light engine'. The display device and the image formed (or perceived) using the display device are spatially separated from one another. The image is formed, or perceived by a viewer, on a display plane. In some embodiments, the image is a virtual image and the display plane may be referred to as a virtual image plane. In other examples, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed either in free space or on a screen or other light receiving surface between the display device and the viewer, is propagated to the viewer. In both cases, an image is formed by illuminating a diffractive pattern (e.g., hologram or kinoform) displayed on the display device. The display device comprises pixels. The pixels of the display may display a diffractive pattern or structure that diffracts light. The diffracted light may form an image at a plane spatially separated from the display device. In accordance with well-understood optics, the magnitude of the maximum diffraction angle is determined by the size of the pixels and other factors such as the wavelength of the light. In embodiments, the display device is a spatial light modulator such as liquid crystal on silicon ("LCOS") spatial light modulator (SLM). Light propagates over a range of diffraction angles (for example, from zero to the maximum diffractive angle) from the LCOS, towards a viewing entity / system such as a camera or an eye. In some embodiments, magnification techniques may be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of an LCOS. In some embodiments, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) - that may be informally said to be "encoded" with / by the hologram - is propagated directly to the viewer's eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction / image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device. Reference is made herein to a "light field" which is a "complex light field". The term "light field" merely indicates a pattern of light having a finite size in at least two orthogonal spatial directions, e.g. x and y. The word "complex" is used herein merely to indicate that the light at each point in the light field may be defined by an amplitude value and a phase value, and may therefore be represented by a complex number or a pair of values. For the purpose of hologram calculation, the complex light field may be a two-dimensional array of complex numbers, wherein the complex numbers define the light intensity and phase at a plurality of discrete locations within the light field. In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity / system, varies with the distance between the display device and the viewing entity. At a 1 metre viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye's pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye's pupil to form an image at the retina for a given eye position, determines the portion of the image that is 'visible' to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-box.) In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device - that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it may therefore be considered that the viewer is looking at a virtual image through an 'display device-sized window', which may be very small, for example 1cm in diameter, at a relatively large distance, e.g., 1 metre. And the user will be viewing the display device-sized window via the pupil(s) of their eye(s), which can also be very small. Accordingly, the field of view becomes small and the specific angular range that can be seen depends heavily on the eye position, at any given time. A pupil expander addresses the problem of how to increase the range of angles of light rays that are propagated from the display device that can successfully propagate through an eye's pupil to form an image. The display device is generally (in relative terms) small and the projection distance is (in relative terms) large. In some embodiments, the projection distance is at least one - such as, at least two - orders of magnitude greater than the diameter, or width, of the entrance pupil and / or aperture of the display device (i.e., size of the array of pixels). Use of a pupil expander increases the viewing area (i.e., user's eye-box) laterally, thus enabling some movement of the eye / s to occur, whilst still enabling the user to see the image. As the skilled person will appreciate, in an imaging system, the viewing area (user's eye box) is the area in which a viewer's eyes can perceive the image. The present disclosure encompasses non-infinite virtual image distances - that is, near-field virtual images. Conventionally, a two-dimensional pupil expander comprises one or more one-dimensional optical waveguides each formed using a pair of opposing reflective surfaces, in which the output light from a surface forms a viewing window or eye-box. Light received from the display device (e.g., spatially modulated light from a LCOS) is replicated by the or each waveguide so as to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide enlarges the viewing window due to the generation of extra rays or "replicas" by division of amplitude of the incident wavefront. The display device may have an active or display area having a first dimension that may be less than 10 cms such as less than 5 cms or less than 2 cms. The propagation distance between the display device and viewing system may be greater than 1 m such as greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide may be up to 2 m such as up to 1.5 m or up to 1 m. The method may be capable of receiving an image and determining a corresponding hologram of sufficient quality in less than 20 ms such as less than 15 ms or less than 10 ms. In some embodiments - described only by way of example of a diffracted or holographic light field in accordance with this disclosure - a hologram is configured to route light into a plurality of channels, each channel corresponding to a different part (i.e. sub-area) of an image. The channels formed by the diffractive structure are referred to herein as "hologram channels" merely to reflect that they are channels of light encoded by the hologram with image information. It may be said that the light of each channel is in the hologram domain rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram and the hologram domain is therefore the Fourier or frequency domain. The hologram may equally be a Fresnel or Fresnel transform hologram. The hologram may also be a point cloud hologram. The hologram is described herein as routing light into a plurality of hologram channels to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-areas, wherein each hologram channel would correspond to each image sub-area. Importantly, the hologram of this example is characterised by how it distributes the image content when illuminated. Specifically and uniquely, the hologram divides the image content by angle. That is, each point on the image is associated with a unique light ray angle in the spatially modulated light formed by the hologram when illuminated - at least, a unique pair of angles because the hologram is two-dimensional. For the avoidance of doubt, this hologram behaviour is not conventional. The spatially modulated light formed by this special type of hologram, when illuminated, may be divided into a plurality of hologram channels, wherein each hologram channel is defined by a range of light ray angles (in two-dimensions). It will be understood from the foregoing that any hologram channel (i.e. sub-range of light ray angles) that may be considered in the spatially modulated light will be associated with a respective part or sub-area of the image. That is, all the information needed to reconstruct that part or sub-area of the image is contained within a subrange 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 realestate value is high. For example, it may be implemented in a head-up display (HUD) such as a vehicle or automotive HUD. In accordance with the present disclosure, pupil expansion is provided for diffracted or diffractive light, which may comprise diverging ray bundles. The diffracted light field may be defined by a "light cone". Thus, the size of the diffracted light field (as defined on a two-dimensional plane) increases with propagation distance from the corresponding diffractive structure (i.e. display device). It can be said that the pupil expander / s replicate the hologram or form at least one replica of the hologram, to convey that the light delivered to the viewer is spatially modulated in accordance with a hologram. In some embodiments, two one-dimensional waveguide pupil expanders are provided, each onedimensional waveguide pupil expander being arranged to effectively increase the size of the exit pupil of the system by forming a plurality of replicas or copies of the exit pupil (or light of the exit pupil) of the spatial light modulator. The exit pupil may be understood to be the physical area from which light is output by the system. It may also be said that each waveguide pupil expander is arranged to expand the size of the exit pupil of the system. It may also be said that each waveguide pupil expander is arranged to expand / increase the size of the eye box within which a viewer's eye can be located, in order to see / receive light that is output by the system. Light channelling The hologram formed in accordance with some embodiments, angularly-divides the image content to provide a plurality of hologram channels which may have a cross-sectional shape defined by an aperture of the optical system. The hologram is calculated to provide this channelling of the diffracted light field. In some embodiments, this is achieved during hologram calculation by considering an aperture (virtual or real) of the optical system, as described above. Figures 4 and 5 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 4 shows an image 452 for projection comprising eight image areas / components, VI to V8. Figure 4 shows eight image components by way of example only and the image 452 may be divided into any number of components. Figure 4 also shows an encoded light pattern 454 (i.e., hologram) that can reconstruct the image 452 - e.g., when transformed by the lens of a suitable viewing system. The encoded light pattern 454 comprises first to eighth sub-holograms or components, Hl to H8, corresponding to the first to eighth image components / areas, VI to V8. Figure 4 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 5. 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 6 shows a system 600, including a display device that displays a hologram that has been calculated as illustrated in Figures 4 and 5. The system 600 comprises a display device, which in this arrangement comprises an LCOS 602. The LCOS 602 is arranged to display a modulation pattern (or 'diffractive pattern1) comprising the hologram and to project light that has been holographically encoded towards an eye 605 that comprises a pupil that acts as an aperture 604, a lens 609, and a retina (not shown) that acts as a viewing plane. There is a light source (not shown) arranged to illuminate the LCOS 602. The lens 609 of the eye 605 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 600 further comprises a waveguide 608 positioned between the LCOS 602 and the eye 605. The presence of the waveguide 408 enables all angular content from the LCOS 602 to be received by the eye, even at the relatively large projection distance shown. This is because the waveguide 608 acts as a pupil expander, in a manner that is well known and so is described only briefly herein. In brief, the waveguide 608 shown in Figure 6 comprises a substantially elongate formation. In this example, the waveguide 608 comprises an optical slab of refractive material, but other types of waveguide are also well known and may be used. The waveguide 608 is located so as to intersect the light cone (i.e., the diffracted light field) that is projected from the LCOS 602, for example at an oblique angle. In this example, the size, location, and position of the waveguide 608 are configured to ensure that light from each of the eight ray bundles, within the light cone, enters the waveguide 608. Light from the light cone enters the waveguide 608 via its first planar surface (located nearest the LCOS 602) and is guided at least partially along the length of the waveguide 608, 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 608 from the first planar surface and hits the second planar surface, some of the light will be transmitted out of the waveguide 608 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 608, 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 608 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 608, before being transmitted. Figure 6 shows a total of nine "bounce" points, BO to B8, along the length of the waveguide 608. Although light relating to all points of the image (V1-V8) as shown in Figure 4 is transmitted out of the waveguide at each "bounce" from the second planar surface of the waveguide 608, only the light from one angular part of the image (e.g. light of one of VI to V8) has a trajectory that enables it to reach the eye 605, from each respective "bounce" point, BO to B8. Moreover, light from a different angular part of the image, VI to V8, reaches the eye 605 from each respective "bounce" point. Therefore, each angular channel of encoded light reaches the eye only once, from the waveguide 608, in the example of Figure 6. The waveguide 608 forms a plurality of replicas of the hologram, at the respective "bounce" points Bl to B8 along its length, corresponding to the direction of pupil expansion. As shown in Figure 6, the plurality of replicas may be extrapolated back, in a straight line, to a corresponding plurality of replica or virtual display devices 602'. 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 602 and the replica display devices 602'. 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 6 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 6 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 7A shows a perspective view of a system 700 comprising two replicators, 704, 706 arranged for expanding a light beam 702 in two dimensions. In the system 700 of Figure 7A, the first replicator 704 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 608 of Figure 6. 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 702 is directed towards an input on the first replicator 704. 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 7A), which will be familiar to the skilled reader, light of the light beam 702 is replicated in a first direction, along the length of the first replicator 704. Thus, a first plurality of replica light beams 708 is emitted from the first replicator 704, towards the second replicator 706. The second replicator 706 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 708 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 708, 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 7A), 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 710 is emitted from the second replicator 706, wherein the second plurality of light beams 710 comprises replicas of the input light beam 702 along each of the first direction and the second direction. Thus, the second plurality of light beams 710 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 704, 705 of Figure 7A combine to provide a two-dimensional replicator (or, "two-dimensional pupil expander"). Thus, the replica light beams 710 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 7A, the first replicator 704 is a waveguide comprising a pair of elongate rectilinear reflective surfaces, stacked parallel to one another, and, similarly, the second replicator 704 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 7B shows a perspective view of a system 700 comprising two replicators, 720, 740 arranged for replicating a light beam 722 in two dimensions, in which the first replicator is a solid elongated waveguide 720 and the second replicator is a solid planar waveguide 740. In the system of Figure 7B, the first replicator / waveguide 720 is arranged so that its pair of elongate parallel reflective surfaces 724a, 724b are perpendicular to the plane of the second replicator / waveguide 740. Accordingly, the system comprises an optical coupler arranged to couple light from an output port of first replicator 720 into an input port of the second replicator 740. In the illustrated arrangement, the optical coupler is a planar / fold mirror 730 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 7B, the mirror 730 is arranged to receive light - comprising a one-dimensional array of replicas extending in the first dimension - from the output port / reflective-transmissive surface 724a of the first replicator / waveguide 720. The mirror 730 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 740 at an angle to provide waveguiding and replica formation, along its length in the second dimension. It will be appreciated that the mirror 730 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 724a of the first replicator 720 is adjacent the input port of the first replicator / waveguide 720 that receives input beam 722 at an angle to provide waveguiding and replica formation, along its length in the first dimension. Thus, the input port of first replicator / waveguide 720 is positioned at an input end thereof at the same surface as the reflective-transmissive surface 724a. The skilled reader will understand that the input port of the first replicator / waveguide 720 may be at any other suitable position. Accordingly, the arrangement of Figure 7B enables the first replicator 720 and the mirror 730 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 720 is located - in the second dimension (illustrated as the y dimension) is reduced. The mirror 730 is configured to direct the light away from a first layer / plane, in which the first replicator 720 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 740 is located (i.e. a "second planar layer"). Thus, the overall size or "height" of the system - comprising the first and second replicators 720, 740 and the mirror 730 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 7B for implementing the present disclosure are possible and contemplated. The image projector may be arranged to project a diverging or diffracted light field. In some embodiments, the light field is encoded with a hologram. In some embodiments, the diffracted light field comprises diverging ray bundles. In some embodiments, the image formed by the diffracted light field is a virtual image. In some embodiments, the first pair of parallel / complementary surfaces are elongate or elongated surfaces, being relatively long along a first dimension and relatively short along a second dimension, for example being relatively short along each of two other dimensions, with each dimension being substantially orthogonal to each of the respective others. The process of reflection / transmission of the light between / from the first pair of parallel surfaces is arranged to cause the light to propagate within the first waveguide pupil expander, with the general direction of light propagation being in the direction along which the first waveguide pupil expander is relatively long (i.e., in its "elongate" direction). There is disclosed herein a system that forms an image using diffracted light and provides an eye-box size and field of view suitable for real-world application - e.g. in the automotive industry by way of a head-up display. The diffracted light is light forming a holographic reconstruction of the image from a diffractive structure - e.g. hologram such as a Fourier or Fresnel hologram. The use diffraction and a diffractive structure necessitates a display device with a high density of very small pixels (e.g. 1 micrometer) - which, in practice, means a small display device (e.g. 1 cm). The inventors have addressed a problem of how to provide 2D pupil expansion with a diffracted light field e.g. diffracted light comprising diverging (not collimated) ray bundles. In some embodiments, the display system comprises a display device - such as a pixelated display device, for example a spatial light modulator (SLM) or Liquid Crystal on Silicon (LCoS) SLM - which is arranged to provide or form the diffracted or diverging light. In such aspects, the aperture of the spatial light modulator (SLM) is a limiting aperture of the system. That is, the aperture of the spatial light modulator - more specifically, the size of the area delimiting the array of light modulating pixels comprised within the SLM - determines the size (e.g. spatial extent) of the light ray bundle that can exit the system. In accordance with this disclosure, it is stated that the exit pupil of the system is expanded to reflect that the exit pupil of the system (that is limited by the small display device having a pixel size for light diffraction) is made larger or bigger or greater in spatial extend by the use of at least one pupil expander. The diffracted or diverging light field may be said to have "a light field size", defined in a direction substantially orthogonal to a propagation direction of the light field. Because the light is diffracted / diverging, the light field size increases with propagation distance. In some embodiments, the diffracted light field is spatially-modulated in accordance with a hologram. In other words, in such aspects, the diffractive light field comprises a "holographic light field". The hologram may be displayed on a pixelated display device. The hologram may be a computer-generated hologram (CGH). It may be a Fourier hologram or a Fresnel hologram or a point-cloud hologram or any other suitable type of hologram. The hologram may, optionally, be calculated so as to form channels of hologram light, with each channel corresponding to a different respective portion of an image that is intended to be viewed (or perceived, if it is a virtual image) by the viewer. The pixelated display device may be configured to display a plurality of different holograms, in succession or in sequence. Each of the aspects and embodiments disclosed herein may be applied to the display of multiple holograms. The output port of the first waveguide pupil expander may be coupled to an input port of a second waveguide pupil expander. The second waveguide pupil expander may be arranged to guide the diffracted light field - including some of, preferably most of, preferably all of, the replicas of the light field that are output by the first waveguide pupil expander - from its input port to a respective output port by internal reflection between a third pair of parallel surfaces of the second waveguide pupil expander. The first waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a first direction and the second waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a second, different direction. The second direction may be substantially orthogonal to the first direction. The second waveguide pupil expander may be arranged to preserve the pupil expansion that the first waveguide pupil expander has provided in the first direction and to expand (or, replicate) some of, preferably most of, preferably all of, the replicas that it receives from the first waveguide pupil expander in the second, different direction. The second waveguide pupil expander may be arranged to receive the light field directly or indirectly from the first waveguide pupil expander. One or more other elements may be provided along the propagation path of the light field between the first and second waveguide pupil expanders. The first waveguide pupil expander may be substantially elongated and the second waveguide pupil expander may be substantially planar. The elongated shape of the first waveguide pupil expander may be defined by a length along a first dimension. The planar, or rectangular, shape of the second waveguide pupil expander may be defined by a length along a first dimension and a width, or breadth, along a second dimension substantially orthogonal to the first dimension. A size, or length, of the first waveguide pupil expander along its first dimension make correspond to the length or width of the second waveguide pupil expander along its first or second dimension, respectively. A first surface of the pair of parallel surfaces of the second waveguide pupil expander, which comprises its input port, may be shaped, sized, and / or located so as to correspond to an area defined by the output port on the first surface of the pair of parallel surfaces on the first waveguide pupil expander, such that the second waveguide pupil expander is arranged to receive each of the replicas output by the first waveguide pupil expander. The first and second waveguide pupil expander may collectively provide pupil expansion in a first direction and in a second direction perpendicular to the first direction, optionally, wherein a plane containing the first and second directions is substantially parallel to a plane of the second waveguide pupil expander. In other words, the first and second dimensions that respectively define the length and breadth of the second waveguide pupil expander may be parallel to the first and second directions, respectively, (or to the second and first directions, respectively) in which the waveguide pupil expanders provide pupil expansion. The combination of the first waveguide pupil expander and the second waveguide pupil expander may be generally referred to as being a "pupil expander". It may be said that the expansion / replication provided by the first and second waveguide expanders has the effect of expanding an exit pupil of the display system in each of two directions. An area defined by the expanded exit pupil may, in turn define an expanded eye-box area, from which the viewer can receive light of the input diffracted or diverging light field. The eye-box area may be said to be located on, or to define, a viewing plane. The two directions in which the exit pupil is expanded may be coplanar with, or parallel to, the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. Alternatively, in arrangements that comprise other elements such as an optical combiner, for example the windscreen (or, windshield) of a vehicle, the exit pupil may be regarded as being an exit pupil from that other element, such as from the windscreen. In such arrangements, the exit pupil may be non-coplanar and non-parallel with the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. For example, the exit pupil may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. The viewing plane, and / or the eye-box area, may be non-coplanar or non-parallel to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. For example, a viewing plane may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. In order to provide suitable launch conditions to achieve internal reflection within the first and second waveguide pupil expanders, an elongate dimension of the first waveguide pupil expander may be tilted relative to the first and second dimensions of the second waveguide pupil expander. Combiner shape compensation An advantage of projecting a hologram to the eye-box is that optical compensation can be encoded in the hologram (see, for example, European patent 2936252 incorporated herein by reference). The present disclosure is compatible with holograms that compensate for the complex curvature of an optical combiner used as part of the projection system. In some embodiments, the optical combiner is the windscreen of a vehicle. Full details of this approach are provided in European patent 2936252 and are not repeated here because the detailed features of those systems and methods are not essential to the new teaching of this disclosure herein and are merely exemplary of configurations that benefit from the teachings of the present disclosure. 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 ID array of apertures or windows, wherein each aperture or window independently switchable between a light transmissive and a light non-transmissive state in order to control the delivery of hologram light channels, and their replicas, to the eye-box. Each aperture or window may comprise a plurality of liquid crystal cells or pixels. Ghost Image Formation The top part of Figure 8A shows a system comprising a display device 802 propagating light 803, that has been encoded with (i.e., modulated in accordance with) a hologram displayed on the display device 802, towards a viewing system (i.e. an eye) that comprises an entrance aperture 804 (i.e. a pupil) and a viewing plane 806 (i.e. a retina). There is a light source (not shown) arranged to illuminate the display device 802. The system further comprises a waveguide 808 positioned between the display device 802 and the entrance aperture 804, to act as a pupil expander as described in detail in relation to Figures 6 to 7B, above. The middle part of Figure 8 shows a magnified view of the entrance aperture 804 and the viewing plane 806, and the lowest part of Figure 8 shows a further magnified view of the viewing plane 806. This Figure is schematic and therefore physiological detail of the eye is not shown. In this arrangement, the eye perceives the virtual image 801 as being located at a finite distance, upstream of the display device 802. The rays between the virtual image 801 and display device are divergent because the virtual image distance is finite. The presence of the waveguide 808 in Figure 8 effectively enables the full diffractive angle of the display device 802 to be accessed at a relatively large projection distance, such that the full image content is visible to the user at the viewing position shown. This is because all angular content output by the display device 802 may be present, at a greater number of positions on the display plane (and at a greater number of positions on the aperture plane) than would have been the case, in the absence of the waveguide 808. This means that light from each ray bundle may enter the entrance aperture 804 and contribute to an image formed by the viewing plane 806, despite the relatively large projection distance. In other words, all angular content from the display device 802 can be received by the eye. Therefore, the full diffraction angle of the display device 802 is utilised and the viewing window is maximised for the user. In turn, this means that all the light rays contribute to a perceived virtual image 801. However, a further technical problem is introduced. The different optical paths of light from different parts of the display device 802, for certain of the ray bundles, can lead to those ray bundles each forming multiple image points on the retina 806 when the virtual image is formed at a finite virtual image distance. This is shown in relation to the ray bundles labelled R3' and R5' in Figure 8a. The additional image points that are formed, which are subsidiary to a main image point for a given point within the virtual image, can be referred to as 'ghost image points' and collectively they form 'ghost images' or simply 'ghosts'. As the person skilled in the art of image formation will appreciate, the formation of ghosts can cause blurring and a general decrease in the perceived quality of a virtual image, from the viewer's perspective. This is particularly true if the "ghost" partially overlaps the "main" image. This technical problem is further shown in Figure 8B, wherein a first replica 803a and a second replica 8303b are emitted from a waveguide (as discussed in relation to Figures 6 to 7B above, not shown) towards a viewing system (i.e. an eye) that comprises an entrance aperture 804 (i.e. a pupil), a lens 809 and a viewing plane 806 (i.e. a retina). As also discussed above, both replicas 803a, 803b comprise light rays carrying the same angular content. Due to the difference in propagation path length taken by each replica inside the waveguide (as shown in Figure 6), there is a lateral and axial displacement between the adjacent replicas, including the first and second replicas 803a, 803b. This is a natural result of using an extended modulator (i.e. a waveguide) to form a 3D array of replicas. When the virtual image distance is not equal to infinity, light rays coming from each virtual image point are not parallel to one another (i.e. they diverge from the bounce point B0 to B9 as described above). As a result, a ghost image may be seen whenever light rays carrying the same angular content pass through the entrance aperture 804 and arrive at the viewing plane 806 simultaneously from different replicas. As shown in Figure 8B, both the first and second replicas 803a, 803b are from the same virtual image point and will be detected by the viewing system. Due to the displacement between the two replicas 803a, 803b, two image points (a primary point and a ghosting point) will be detected (i.e. seen). Figure 8C shows an example of a virtual image of the numbers '5' and '9', created using a viewing system similar to that shown in Figure 8A, that includes ghost images in addition to a main image. The main image can be seen as the brightest, central image for each number, with ghosts to the left and the right. In the example of Figure 8C, the '9' is formed when the viewing distance is larger than it is for the '5', therefore the blurring is more pronounced for it. Ghost Image Mitigation In overview, ghost images are mitigated by providing a light engine for generating a hologram that, in effect, identifies one or more areas of a display device that would contribute to one or more ghost images, wherein the hologram is derived in order to control the contributions from those one or more areas of the display device, and thus to avoid or reduce the formation of ghost image points when the hologram is displayed on the display device and illuminated. A hologram engine can provide such a hologram, and can provide an improved viewing system for the display and illumination of an improved hologram, for the formation of improved images, even when the projection distances in the viewing system are relatively large and the display device and / or the viewing aperture is relatively small. Because of the angular restrictions imposed by having a viewing system that includes a waveguide - such as those shown in Figures 6 to 8A herein - comprising a relatively small viewing aperture, and optionally also a relatively small display device, it is possible to consider the different possible propagation paths within the waveguide separately. As a result of such consideration, it is possible to identify each of: areas of the display device that are the source of light contributing to a desired 'main' image; areas of the display device that are the source of light contributing to an undesirable 'ghost' image; and areas of the display device that are the source of light that is blocked by the aperture and thus do not contribute to either the main image or a ghost image. It is possible to limit the hologram calculation to only areas of the display device that contribute to the main image. An improved hologram may be provided, which in effect may cause one or more of the ghost images to be translated, so as to be superimposed onto a main image. Figure 9A shows a display device 902, which in this example is an LCOS (liquid crystal on silicon) spatial light modulator, however the teachings of the present disclosure are not limited to an LCOS display device. Figure 9B traces the light rays in relation to one virtual image point from the display device 902, via a waveguide 908, towards a viewing entity / system 905, which in this example comprises a viewer's eye. Figure 9C further comprises a magnified view of the eye 905, showing the rays at the pupil 904 (i.e., the entrance aperture) and the retina 906 (i.e., the sensor or viewing plane). In this example, the entire display device area contributes to formation of the image point on the retina 906. In other words, the entirety of the display device 902 is 'visible' to the viewer. This contribution of the entire display device 902, to the image, is illustrated by the entirety of the display device being shaded, denoting its whole surface area as a 'contributory area'. As can be seen, the light traced from the display device 902 in Figure 9C leads to the formation of three image points - labelled as Gl, M and G2, respectively - on the retina 906 for this particular virtual image point. The middle image point 'M' comprises a main image point, which contributes to the primary / main virtual image perceived by the viewer. The top image point Gl comprises a first ghost image point and the bottom image point G2 comprises a second, different ghost image point of the same virtual image point. It is possible to identify the region(s) of the display device 902 that contribute to the main image point M and / or to the ghost image points Gl, G2. Figures 10A to 10C show the display device 902 and ray diagram of Figures 9A to 9C divided up into three respective propagation paths - the first of which comprises the light that contributes to the bottom ghost image point G2, the second of which comprises the light that contributes to the main image point M, and the third of which comprises the light that contributes to the top ghost image point Gl. As can be seen in Figure 10A, the light that contributes to G2 bounces three times before being transmitted by the waveguide 908. As can be seen in Figure 10b, the light that contributes to M bounces twice before being transmitted by the waveguide 908. As can be seen in Figure 10C, the light that contributes to Gl bounces once before being transmitted by the waveguide 908. Each figure (10A, 10B, IOC) also shows, illustrated by shading, the portion(s) of the display device 902 that contribute to the respective image point. Thus, it can be seen that the bottom ghost image point G2 is contributed to by a region towards the lower part of the display device 902, the top ghost image point Gl is contributed to by a region towards the upper part of the display device 902, and the main image point is contributed to by the entire display device 902. The aperture 904 (i.e., the viewer's pupil) is relatively wide in the example of Figures 9A to 10c, which explains why the entire display device 902 contributes to the main image point. In other words, the f-number of the viewing system is relatively low in this example. Figures 10A to 10C show that, although parts of the display device 902 also contribute to one or other ghost image Gl, G2, there is a region of the display device 902 that contributes to neither ghost image Gl, G2, but only contributes to the main image point M. This region may be identified as being a contributory area, for the display device 902 in this example - more specifically, it may be identified as being a 'primary contributory area', as will be understood further from the description of the subsequent Figures. It can therefore be seen, in this case, that the primary contributory region is not limited to being a circle or ellipse and may take other more complex shapes. Figures 11A to 11C show the corresponding ray diagrams for different points of the virtual image when the entrance aperture is relatively small (i.e. the f-number is relatively high). Figure 11A relates to a first field point of the virtual image (i.e. a first virtual image point), Figure 11B relates to a second field point of the virtual image and Figure 11C relates to a third field point of the virtual image. Figures 11A to 11C show that not all of the display device 902 contributes to the main image point. In fact, Figures 11A to 11C show that a first region of the display device correspond to the main image points (herein refer to as a "primary contributory area") and a second region of the LCOS corresponding to the ghost image points (herein referred to as a "secondary contributory area"). Under certain conditions, different respective regions of the display device 902 will contribute either to a main image or to a ghost image or will not contribute to any visible part of an image. The hologram determination process can be optimised using this information. For example, light from certain parts of the display device may be omitted, or, in some cases, the manner in which they are encoded, by the hologram, may be changed so as to contribute positively to the main image, instead of contributing to a ghost image. Furthermore, additional areas of the display device may be identified, which may be configured to contribute to positively to the main image. This will be described below in relation to point cloud holograms by way of one example. However, they may be applied to other types of hologram such as a Fourier or Fresnel hologram. That is, other hologram calculation methods can be optimised using the display device information that can be determined in accordance with this disclosure. As will be well understood, usually for calculation of a point cloud hologram of an image (such as a virtual image), the image is broken down into (i.e., represented by) a plurality of individual points - referred to herein as 'virtual points', since we describe the formation of virtual images. A spherical wave (or 'wavelet') is then propagated computationally - i.e., using a model or other theoretical tool - from each virtual point, at its intended or desired location, within the virtual image, to the plane of the display device - such as the plane of the display device, in the examples described hereabove. The way in which such wavelets would interfere with one another is considered and the resulting amplitude and / or phase of the wavelet that would be received at each pixel of the display device is calculated. The display device can then be tuned, in a manner that is well known and so will not be described herein, to exhibit the amplitude and / or phase modulation that is required at each pixel location, in order to mimic the calculated wavelets, and thus to create a hologram of the image. For a viewing system with a waveguide and large viewing distance as described herein, if the entire display device is populated with the net amplitude and phase of the corresponding wavelets of all the virtual points, the hologram that will be created may, when displayed and illuminated, generate one or more ghost images as well as a main image. For example, this may occur when the viewing system is configured so that a virtual image is perceived at a finite distance from the viewer. Moreover, in many cases, light rays emitted from the pixels in some parts of the device will be wasted (i.e., they will not contribute to the image that the viewer sees or perceives) because the physical constraints of the viewing system (such as a small aperture and / or a small display device and / or a large projection distance) will dictate that light from those parts of the device will not enter the viewer's eye. Therefore, an intelligent selection may be applied, in respect of which portions of the display device are tuned to provide a hologram. Specifically, if only those portions (or, parts, or, regions) of the display device that contribute to a main image are selected - and if wavelets are computationally propagated only from the virtual points of the intended virtual image, to those portions of the display device - and not to other portions of the display device, which do not contribute to the main image - the resulting amplitude and / or phase of the wavelet that would be received at each pixel within the selected region of the display device can be calculated. No calculation is needed for any respective other parts of the display device. The display device can then be tuned, in accordance with the improved calculation, to exhibit the amplitude and phase modulation that is required at each pixel location, within the selected portion(s), in order to mimic the calculated wavelets, and thus to create a hologram of the main image. When this is done, there will be no tuning of any other portions of the display device, and therefore no image information will propagate from those other portions, to the viewer's eye (or other viewing entity), when the calculated hologram is displayed on the display device and illuminated. Therefore, there will be no information available to the viewer, which could lead to it forming an undesirable "ghost" image point. As a result, the ghost(s) is / are eliminated or 'quenched'. Moreover, no computation or image information is wasted, since only those pixels of the display device that are known to provide light that will be admitted through the viewer's pupil (or, through the aperture of a corresponding other viewing entity), for a given set of conditions (such as for a particular aperture width and location of the eye), will be tuned. Figures 12A and 12B show a system 1200 forming a virtual image comprising an example virtual point 1201. The viewing system 1200 comprises a display device 1202, which in this example is an LCOS SLM, which comprises a contributory area 1203 and a non-contributory area 1207, identified in accordance with the present disclosure. The display device 1202 is arranged to display a hologram of the virtual image and to project light that has been encoded in accordance with the hologram, towards an eye 1205 that comprises a pupil (not shown) that acts as an aperture, a lens 1209, and a retina 1206 that acts as a viewing plane. The lens 1209 and retina are separated by a separation distance 'A'. There is a light source (not shown) arranged to illuminate the display device 1202. The viewing system 1200 further comprises a waveguide 1208 positioned between the display device 1202 and the eye 1205. This image is schematic and therefore physiological detail of the eye is not shown. The virtual point 1201 is located upstream of the display device 1202, which in Figure 12A is depicted by the virtual point 1201 being to the left of the display device 1202. The virtual point 1201 has a location defined by spatial coordinates, which in this example comprise Cartesian (x, y, z) coordinates but other coordinate systems or other means of identifying the virtual point's location may be used. A distance 'z' is defined between the virtual point 1201 and the display device 1202, in a direction substantially parallel to the optical axis of the display device 1202. There is also a display-to-lens distance T defined between the display device 1201 and the eye lens 1209 is, in a direction substantially parallel to the optical axis of the display device 1201. The numerical values of both 'z' and T will vary dependent on the particular arrangements of the viewing system 1200, at a given time, including viewer position. For example, the display-to-lens distance T may be of the order of approximately 1 metre and the display-to-image distance 'z' may be larger, for example of the order of a few metres. But these numerical examples are purely illustrative and should not be regarded as being limiting. If a virtual image comprising the virtual image point 1201 is to be perceived, by a viewer, at the location depicted in Figure 12A, a corresponding image point 1211 must be formed on the retina 1206. Light rays can be tracked from the virtual point 1201 of the virtual image to a corresponding point 1211 on the retina 1211, via the display device 1202. It will be appreciated that more than one possible optical path may be taken, between the virtual point 1201 and its corresponding point 1211 on the retina, via the display device 1202 owing to the possible paths created / generated by the waveguide 1208. According to embodiments, a chief light ray may be determined, which comprises a light ray path amongst a plurality of light ray paths, between the virtual image point 1201 and the corresponding point 1211, on the viewing plane (i.e., the retina 1206). When this chief ray path is identified, the number of bounces that the light undergoes within the waveguide is determined. That number of bounces (B) can be set as being the number of bounces for which rays should be traced, between the virtual image and the viewing plane. According to embodiments, the chief ray - and the associated number (B) of bounces - may be identified, as an initial step. In the present example, ray tracing can determine that the portion of the display device 1202 through which the 'chief ray' light travels, between each virtual image point 1201 to the corresponding point 1211 on the retina, in order to identify the 'contributory area' 1203 for that virtual image point 1201. There is therefore a light ray 'r' depicted as propagating between the virtual image point 1201 and the contributory area 1203 of the display device 1202 in Figure 12B. In accordance with the recognitions made by the inventors, only wavelets contributing to the contributory area of the display device need to be modelled (or otherwise computationally considered), from the virtual image point 1201 and the display device 1202. In other words, only the identified contributory area 1203 of the display device 1202 needs to be encoded (or, 'tuned') - in order to generate an appropriate hologram. Such a hologram, when encoded on the display device and suitably illuminated, would enable the virtual image point 1201 to be perceived by the viewer without any ghost images of that virtual point 1201 also being present. The contributory area 1203 in Figure 12B may be sized and shaped based on the size and shape of the entrance aperture of the corresponding viewing entity and of the associated optics (e.g., waveguide geometry, any reflections within a larger optical system, and so on). Therefore, when the viewing entity is a human eye, the contributory area on the display device may, in some cases, comprise a substantially circular, or elliptical, shape, or any other suitable shape such as a complex shape, of a similar size to the receiving pupil. However, the present disclosure encompasses more complex shapes for the contributory area. Eye pupil diameter may be measured or estimated in any suitable way. For example, measurement of the eye pupil diameter may be carried out by an eye tracking system. Alternatively, it may be estimated based on known ranges of pupil diameter of the eye (e.g. 2-6 mm) or based on another estimate given the ambient light conditions at a given time. The contributory area may be set so as to deliberately contribute to an area (on the aperture plane) that is a little larger than the pupil, and / or to contribute to an area (on the aperture plane) that is a slightly different shape to the pupil (or other aperture). In such a case, not all light from a "contributory area" may pass through the pupil at all times, but the eye would be able to move around a little while still collecting sufficient light to form a good image on the retina. It is desirable to provide a viewing system in which a virtual image can be formed at a finite virtual image distance, which comprises all the angular image content that is output by a display device, and which reduces or removes the formation of ghost images. As the size of a viewing aperture increases in a conventional viewing system, the risk of forming ghost image points increases, because the aperture can admit additional light rays, which may form additional image points on the display plane. Therefore, it is desirable to provide an improved viewing system that can accommodate apertures of different sizes, whilst still reducing or removing the formation of ghost images. The solutions provided by the inventors, detailed below, are applicable to a range of different sizes of - and arrangements of - aperture, waveguide, and display device, and may be applied for different propagation distances, for which one or more ghost images may conventionally be formed. The Inscribed Circle Figures 13A to 13C show the formation of a further contributory area on a display device 1302 (e.g. an LCOS). A region 1312 of the display device 1302, as shown in Figure 13A, that produces the light that passes through the aperture (i.e. pupil) of the viewing system (i.e. eye) can be found by, for example, ray tracing the light from the viewing system to the display device 1302, as described above. In other words, the region 1312 may have a size based on a diameter of the entrance pupil. A primary replica (i.e. the "chief ray") carries the information illuminated on a first area 1310a of the display device 1302. Meanwhile, secondary replicas to either side of the primary replicas carry information illuminated on second and third areas 1310b, 1310c of the display device 1302. The first replica has a region 1312a of light that passes through the aperture of the viewing system that matches the region 1312 described above. The second and third replicas have respective regions 1312b, 1312c that are intended to be used if the viewing system (i.e. the viewer) moves within the viewing window (i.e. eye-box), as described above. As can be seen Figure 13B, the overlap between the first area 1310a and the neighbouring second and third areas 1310b, 1310c results in parts of the regions 1312b, 1312c of the second and third replicas overlapping with the region 1312a of the primary replica. As discussed above, this may result in the same angular content arriving at the viewing system from multiple replicas, thus producing ghost images. Therefore, as seen in Figure 13C, the hologram is determined only in a "contributory area" 1303 and not in the "non-contributory area" 1307. The "contributory area" 1303 has a dumbbell-like shape (i.e. a cropped circular shape) due to the aforementioned overlap between parts of the regions 1312b, 1312c of the second and third replicas with the region 1312a of the primary replica. In other words, for each individual image point, pixels of the display device 1302 that are visible to the viewing system in more than one replica are switched off. To suppress ghosting, only pixels that are in the primary replica but not in the adjacent replicas are switched on to produce the required wavefront. As the dumbbell shape of the "contributory area" 1303 is not radially symmetric, the point spread function 1420a (i.e. each individual pixel in the reconstructed image) will exhibit multiple "lobes" spanning across a large area and emanating from a central point. The resultant point spread function 1420a of the "contributory area" 1303 of Figure 14A (as per Figure 13C) is shown in Figure 14B. Such point spread functions, PSFs, produce strong pixel cross-talk that may affect the overall image quality visible to the viewing system, such as a reduction in pixel-per-degree and the introduction of (or a greater number of) speckles being visible. The inventors have found that determining the hologram only in areas of an "inscribed circle" can improve the PSFs produced. The hologram is determined in the largest circular area possible within the "contributory area" 1403 as shown in Figure 14C, with this area being referred to as the "inscribed circle" 1413. Whilst the term "circle" is used, any polygon with a sufficiently constant radius to achieve the technical effects described herein would also be suitable. Only determining the hologram in the area of the "inscribed circle" 1413 (i.e. turning off the pixels in the rest of the "contributory area" 1403 and the "non-contributory area" 1307) produces the improved PSF 1420b shown in Figure 14E. As can be seen, there are no longer multiple "lobes" extending from the central point of the PSF 1420b, which in turn reduces the pixel cross-talk and thereby increases the quality of image viewable by the viewing system. The "inscribed circle" 1413 is positioned such that it has the largest radius possible, in order to maximise the area of the display device 1302 that can be used for display of the hologram. However, even then the area of the display device 1302 that can be used is less than that of the "contributory area" 1403 and even less than that of the original region 1312 of the display device 1302 that produces the light that passes through the aperture of the viewing system. As such, the intensity of the light produced when illuminating the "inscribed circle" 1413 is lower than if the aforementioned larger areas were to be illuminated instead. However, the inventors have surprisingly found that the improvements in perceived image quality is worth the reduction in intensity. The intensity of the light can be improved by using multiple "inscribed circles", as shown in Figure 14D. In this example, a second "inscribed circle" 1413' is formed in the righthand part of the dumbbell shape of the "contributory area" 1403'. As with the first "inscribed circle" 1413, the second "inscribed circle" 1413' is positioned such that it can have the maximum radius possible. Only determining the hologram in the area of both "inscribed circles" 1413,1413' (i.e. turning off the pixels in the rest of the "contributory area" 1403' and the "non-contributory area" 1307) produces the same improved PSF 1420b as described above and shown in Figure 14E, but with an increased intensity as a greater area of the display device 1302 can be illuminated. Although the example of Figure 14D shows two "inscribed circles" 1413,1413', the inventors have also envisaged using any number of "inscribed circles", so long as each circle is of substantially the same size and large enough relative to the overall size of the display device 1302. If either of these criteria are broken, the inventors have found that further distortions of the PSF can be introduced. By introducing more "inscribed circles", the illuminated area can be further increased whilst still producing the improved PSF. Figures 15A to 16D show further examples of the "inscribed circle" and the corresponding PSF improvements resulting therefrom. Figure 15A shows an area 1510 carried by a primary replica with closer neighbouring replicas than in the example of Figures 13A to 14E. As such, the dumbbell-like shape of the "contributory area" 1503 is more compressed, producing further "lobes" in the PSF 1520a shown in Figure 15B. The "inscribed circle" 1513 shown in Figure 15C removes these "lobes" from the resultant PSF 1520b, as seen in Figure 15D, thereby improving the perceived image quality. Meanwhile, Figure 16A shows an example in which the user has moved such that the "contributory area" 1603 is to one side of the area 1610, producing "lobes" in the PSF 1620a of Figure 16B in the horizontal and vertical directions. However, the "inscribed circle" 1613 of Figure 16C once again removes these "lobes" from the resultant PSF 1620b of Figure 16D. Although only one "inscribed circle" is shown in these examples, multiple may be used simultaneously, as described above. The examples shown in Figures 13A to 16D are schematic only and as such the exact size, shape, proportions, number etc. of the "contributory area", "non-contributory area" and "inscribed circle" may be different. The skilled person would understand that, as long as the principles described above are adhered to, then the improvements to the resultant PSFs and thereby the resultant perceived images will be achieved. Edge Apodisation The inventors have further discovered that the presence of "lobes" in PSFs can be reduced (and therefore the quality of the resultant image improved) by using edge apodisation on the boundary of the relevant "contributory area" or "inscribed circle". Figure 17A shows a further example of the first area 1310, dumbbell-like shaped "contributory area" 1303 and "non-contributory area" 1307 as shown in Figures 13A to 13C and described above, along with the resultant PSF 1720a shown in Figure 17B. The PSF 1720a has a number of "lobes" extending therefrom that may reduce the quality of the image perceived, as discussed above. Meanwhile, Figure 17C shows an example of a first area 1710 (as per the first area 1310 of Figure 17A) with a "contributory area" 1703 that has been edge apodised. In other words, the boundary between the "contributory area" 1703 and the "non-contributory area" 1707 has been processed with an apodisation function such that the binary change between the two areas is smoothed. As can be seen in Figure 17D, the resulting PSF 1720b has a reduced number of "lobes" extending from the central point, as the edge apodisation has removed the higher orders of diffraction. This reduces the pixel cross-talk and thereby increases the quality of image viewable by the viewing system. The inventors have also found that the edge apodisation has a complementary affect to that of the "inscribed circle" as described above. Figure 18A shows a first area 1810, dumbbell-like shaped "contributory area" 1803 and "non-contributory area" 1807 similar to that shown in Figure 14C and described above, but wherein the "inscribed circle" 1813 has undergone edge apodisation. The resultant PSF 1820 of Figure 18B has fewer concentric rings around the central point as compared to the PSF 1420b of Figure 14E, further improving the quality of the perceived image. The inventors have further found that the edge apodisation does not have to occur on every boundary of the "contributory area" for it to have a beneficial effect, as shown by example in Figures 19A to 19C. Figure 19A shows a first area 1910, copped-circle shaped "contributory area" 1903 and "non-contributory area" 1907 similar to that shown in Figures 16A to 16D and described above, but with the "contributory area" 1903 being instead moved to the top edge of the first area 1910. Without edge apodisation, the "inscribed circle" produces the PSF 1920i of Figure 19B, which is an improvement over illumination of the full "contributory area" 1903, but that produces tails in the vertical direction (due to the dropped nature of the "contributory area" 1903). However, with edge apodisation, the "inscribed circle" 1913 produces the PSF 1920ii of Figure 19C, which substantially removes these tails. Whilst the width of the central point of the PSF 1920ii is reduced (due to the increase in width of the edge apodised "inscribed circle" 1913), the size of the PSF 1920ii does not substantially change and thus the inventors have found that the image quality can be improved without a significant loss in light intensity over the "inscribed circle". The inventors have found that the Blackman-Harris function performs the best (i.e. provides the best image quality improvement), the Blackman-Harris function being defined as: / 2nn\ / 4nn\ / 6nn\ win] = an — a, cos --- + a7 cos --- — a,cos --- \ N J \ N J \ N J Where a0 = 0.35875,¾ = 0.48829,¾ = 0.14128,¾ = 0.01168 However, other edge apodisation (i.e. window) functions would also be suitable. Improved Method of Hologram Determination As such, the inventors have developed an improved method of determining a hologram for display on a display device. As described above, the display device is part of a display system that comprises a waveguide arranged to receive spatially modulated light from the display device and provide a plurality of different light propagation paths for the spatially modulated light from the display device to an entrance pupil of a viewing system (i.e. a pupil of an eye of a viewer). In a first step, the location of the entrance pupil of the viewing system arranged to view the hologram is determined. This may be achieved, for example, by tracking the position of the viewing system (i.e. by tracking the viewer's pupils). In a second step, the areas of the display device corresponding to each replica as described above are identified for each point of the image to be displayed. As also described above in relation to Figures 13A to 13C, this allows for the identification of the "contributory area" and "non-contributory area" by using a replica forming the primary image, said replica being neighboured by replicas that may form ghost images. This step also includes determining, for each point of the image to be displayed, a number of internal reflections, B, within the waveguide pupil expander corresponding to the primary image. This is based on an angle associated with the corresponding image point, the angle being the angle to the optical axis of a line formed by extrapolating to the image aline connecting the centre of the display device and the determined entrance pupil location. This is further based on identifying a location at which a light ray, travelling from said image point to the entrance pupil, intersects the display device. In other words, this identification is achieved for each image point by ray tracing from the image point to a viewing plane of the viewing system (i.e. a retina of the eye) for B light reflections within the waveguide to identify a position on the viewing plane. From this, coordinates of the chief light ray at the display device for light propagation with B reflections from the image point to the position on the viewing plane can be determined. The "contributory areas" for the neighbouring replicas for each image point are identified in the same process but for B+AB bounces. In this way, active pixels of the display device within the "contributory area" or areas can be identified defined by these coordinates. In an third step, information identifying the "inscribed circle" or circles is received, as also discussed above. The properties and principles guiding the formation of the circle or circles as described above are followed. Alternatively / additionally, the third step involves performing edge apodisation upon the boundary of the "contributory area" or areas, or (in the case where there is at least one present) the "inscribed circle" or circles. As described above, these features have a complementary affect but can be used individually. In a fourth step, the hologram is determined in only the "inscribed circle" (or circles) or, when only the edge apodisation feature is being used, in only the "contributory area" (or areas) of the display device for each image point. This involves determining one or more values for the hologram only in the "inscribed circle" (or circles) or "contributory area" (or areas) of the display device, whilst excluding values for the hologram in an area of the display device that is not comprised within the "inscribed circle" (or circles) or "contributory area" (or areas). This further involves limiting the hologram determination solely to the "inscribed circle" (or circles) or "contributory area" (or areas) of the display device. This involves determining a sub-hologram within the respective "inscribed circle" (or circles) or "contributory area" (or areas) for each image point and combining the sub-holograms in order to form the hologram. In this case, each sub-hologram comprises an amplitude and / or phase hologram component determined by propagating a light wave from each image point to the corresponding "inscribed circle" (or circles) or "contributory area" (or areas). Additional features Examples describe illuminating the SLM with visible light but the skilled person will understand that the light sources and SLM may equally be used to direct infrared or ultraviolet light, for example, as disclosed herein. For example, the skilled person will be aware of techniques for converting infrared and ultraviolet light into visible light for the purpose of providing the information to a user. For example, the present disclosure extends to using phosphors and / or quantum dot technology for this purpose. Some arrangements describe 2D holographic reconstructions by way of example only. In other arrangements, the holographic reconstruction is a 3D holographic reconstruction. That is, in some arrangements, each computer-generated hologram forms a 3D holographic reconstruction. 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 holographic projector arranged to provide a viewing system with spatially modulated light, the viewing system having an entrance pupil and wherein the holographic projector comprises:5 a display device arranged to display a hologram and spatially modulate light in accordancewith the hologram; anda hologram engine arranged to receive information identifying at least one primary contributory area of the display device, based on the location of the entrance pupil, that substantially propagates spatially modulated light passing through the entrance pupil at the location 10 that contributes to a primary image; andwherein the hologram engine is further arranged to receive information identifying at least one sub-area of the primary contributory area, to determine the hologram in only the at least one sub-area and to output the hologram to the display device for display, each sub-area having a greater radial symmetry than the primary contributory area.

152. A holographic projector as claimed in claim 1, wherein at least one of the sub-areas is an equilateral polygon in shape having at least 10 sides, such as at least 20 sides or at least 30 sides.

3. A holographic projector as claimed in claim 1 or claim 2, wherein at least one of the sub-20 areas is substantially circular.

4. A holographic projector as claimed in any preceding claim, wherein at least one of the primary contributory areas has a cropped circular shape, such as a dumbbell shape.25 5. A holographic projector as claimed in any preceding claim, wherein the radius of at least ofthe sub-areas is determined such that the size of the sub-area is substantially maximised within the corresponding primary contributory area.

6. A holographic projector as claimed in any preceding claim, wherein at least one of the30 primary contributory areas comprises a plurality of sub-areas, such as two sub-areas.

7. A holographic projector as claimed in claim 6, wherein each sub-area within a corresponding primary contributory area has substantially the same shape.12 11 258. A holographic projector as claimed in any preceding claim, wherein each sub-area has a diameter greater than 1mm, such as greater than 1.5mm or greater than 2mm.

9. A holographic projector as claimed in any preceding claim, further comprising a monitoring5 system arranged to determine the location of the entrance pupil of the viewing system.

10. A holographic projector as claimed in any preceding claim, wherein the viewing system isarranged to form an image corresponding to the hologram.10 11. A holographic projector as claimed in any preceding claim, wherein the hologram engine isfurther arranged to receive information identifying at least one secondary contributory area of the display device propagating light to the viewing system that contributes to a secondary image.

12. A holographic projector as claimed in claim 11, wherein the hologram is configured such that 15 the primary image and at least one secondary image are virtual images each having a non-infinite virtual image distance.

13. A holographic projector as claimed in any preceding claim, wherein the contributory information further identifies a non-contributory area of the display device based on the location of 20 the entrance pupil, wherein the non-contributory areas of the display device substantially propagate spatially modulated light stopped by the entrance pupil at the location.

14. A holographic projector as claimed in claim 13, wherein the contributory information identifies respective contributory and non-contributory areas of the display device for each of a 25 plurality of image points of the image.

15. A holographic projector as claimed in any preceding claim, wherein the holographic projector further comprises a waveguide arranged to receive the spatially modulated light from the display device and provide a plurality of different light propagation paths for the spatially modulated 30 light from the display device to the entrance pupil, wherein each contributory area corresponds to a different respective light propagation path provided by the waveguide.12 11 2516. A holographic projector as claimed in any preceding claim, wherein the hologram comprises a plurality of sub-holograms, wherein each sub-hologram is determined by the hologram engine based on the contributory information of a respective image point of the image.5 17. A holographic projector as claimed in any preceding claim, wherein the hologram engine isfurther arranged such that at least a portion of a boundary of at least one of the sub-areas is apodised to reduce a diffractive effect.

18. A holographic projector as claimed in claim 17, wherein the apodisation is achieved using a 10 window function, such as the Blackman Harris function.

19. A holographic projector as claimed in claim 17 or claim 18, wherein the boundary of at least one of the sub-areas is apodised to reduce a diffractive effect.15 20. A holographic projector as claimed in any preceding claim, wherein a display area of thedisplay device has a first dimension less than 5cm, such as less than 2cm or less than 1cm.

21. A holographic projector as claimed in claim 20, wherein each sub-area has a diameter greater than a quarter of the first dimension, such as greater than a third of the first dimension2022. A holographic projector as claimed in any preceding claim, wherein a distance from the display device to the entrance pupil of the viewing system is equal to or greater than 20cm or 50cm, such as greater than 75cm or greater than 100cm.25 23. A method of determining a hologram for display on a display device arranged to spatiallymodulate light in accordance with the hologram; the method comprising:(i) determining the location of the entrance pupil of a viewing system arranged to view the hologram;(ii) identifying at least one primary contributory area of the display device that substantially30 propagates light contributing to a primary image passing through the entrance pupil of the viewing system at the determined location;(iii) receiving information identifying at least one sub-area of the primary contributory area, each sub-area having a greater radial symmetry than the primary contributory area; and(iv) determining the hologram in only the at least one sub-area of the display device.

24. A method as claimed in claim 23, wherein step (iii) comprises determining a radius of at least one sub-area such that the size of the sub-area is substantially maximised within the correspondingprimary contributory area.

525. A method as claimed in claim 23 or claim 24, wherein step (ii) further comprises identifying at least one secondary contributory area of the display device that provides light contributing to a secondary image.12 11 25s

Images