Aperture mask

Abstract

A holographic projector is arranged to form a holographic reconstruction of a picture on a replay plane. It comprises a spatial light modulator arranged to spatially modulate light in accordance with

Description

FIELD The present disclosure relates to a mask for a display device. More specifically, the present disclosure relates to a mask defining an aperture of a display device, such as a spatial light modulator of a holographic display. Some embodiments relate to a holographic projector, picture generating unit or head-up display. BACKGROUND AND INTRODUCTION Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or “hologram”, comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object. Computer-generated holography may numerically simulate the interference process. A computer-generated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. A Fourier hologram may be considered a Fourier domain / plane representation of the object or a frequency domain / plane representation of the object. A computer-generated hologram may also be calculated by coherent raytracing or a point cloud technique, for example. A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micromirrors, for example. A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, “HUD”. SUMMARY Aspects of the present disclosure are defined in the appended independent claims. In an aspect, there is provided a holographic projector configured to form a holographic reconstruction of a picture on a replay plane. The holographic projector comprises a spatial light modulator. The spatial light modulator is arranged to display a hologram such that light incident on the displayed hologram is spatially modulated in accordance with the hologram. The spatial light modulator comprises a display area (e.g., active pixel area) arranged to display the hologram, and a non-display area (e.g., non-pixel area) adjacent the display area. For example, the non-display area may be an area adjoining, such as surrounding, the display area of the spatial light modulator. The holographic projector comprises a mask. The mask has a transmission area arranged in cooperation with the pixel area of the spatial light modulator, and a non-transmission area arranged in cooperation with the non-pixel area of the spatial light modulator. At least a portion of a boundary of the mask between the transmission area and the non-transmission area is apodised. There is disclosed herein a holographic projector comprising a display device. The display device comprises a spatial light modulator arranged to display a hologram of a picture for reconstruction at a replay plane. The spatial light modulator is arranged to spatially modulate light in accordance with a hologram and to output light comprising the spatially modulated light. A mask is arranged in cooperation with a major surface of the spatial light modulator having a display area (e.g., active pixel area) and a non-display area (e.g., non-pixel area). The mask comprises a transmission area arranged in cooperation with the display area of the spatial light modulator. The mask further comprises a non-transmission area arranged in cooperation with the non-display area of the spatial light modulator. Accordingly, it may be said that the mask comprises a transmission area that defines an aperture for light incident on, and optionally output from, the display area of the spatial light modulator. At least a portion of the boundary of the mask between the transmission area and the non-transmission area is apodised. The apodised boundary reduces a diffractive effect at the boundary (e.g., at the edge of the non-transmission area of the mask). In particular, the diffractive edge effect on an unmodulated or DC component of the output light, such as reflected light output from the non-display area (e.g. non-pixel area), is reduced to minimise artefacts in the holographic reconstruction formed on the replay plane. As described herein, a hologram may be displayed by a display device comprising a spatial light modulator, which is illuminated with incident light. The hologram may be a phasebased hologram and the spatial light modulator may be a phase modulator. A pixelated spatial light modulator, for example, may comprise regions, such as areas between pixels and around the active pixel area including unused “dummy” pixels, which do not modulate light. Furthermore, the spatial light modulator may comprise a cover glass having non zero reflectivity, which may specularly reflect a small proportion of incident light. This “unmodulated” light, sometimes called the “DC order” or “DC component”, may be output by such non-display areas and forms a light spot at the centre of the replay field, when focused by a lens to reconstruct an image. Although the amount of light in this DC order is a small proportion of the total output light from the spatial light modulator, since it is focused by a lens it can form a bright spot. This can appear to a viewer as a very bright (i.e., high intensity) “image pixel” at the centre of the replay field, in which the holographic reconstruction (or “replay image”) is formed at the replay plane. This bright image spot, or “DC spot”, constitutes noise. Accordingly there is desire to prevent the DC order light from reaching the replay plane in order to provide high image quality. One technique for addressing this need is to provide a spatial filter, such as a reflective component with a physical “hole”, to remove the central light spot of the DC order. In another technique, a spatial light modulator may be formed with a mask placed to overlay (e.g., positioned in an optical path directly or indirectly on top of / above) a major surface comprising the display area. This can reduce some of the factors that contribute to the DC spot. For example, a mask may be formed with an optically transparent quadrilateral window that overlays only the active pixel area of the major surface (e.g., a quadrilateral array of pixels), and a non-transparent light blocking area overlaying areas surrounding the active pixel area. Since the mask has two distinct areas with a single step or clear-cut boundary therebetween, it may be called a “binary mask”. It may be said that there is an abrupt or instantaneous change in transmissivity at the boundary between the transmission area and the non-transmission area of the mask. The mask may define an entrance aperture so that only the pixels of the active pixel area forming the display area are illuminated by incident light. The mask may also define an exit aperture through which light output from the spatial light modulator passes as it propagates towards the replay plane. This exit aperture allows the transmission of modulated light and the DC order light therethrough, and the above described spatial filter may be provided downstream of the mask for removal of the DC order. Accordingly, the above described mask is designed to at least prevent incident light from reaching at least some of the non-pixel areas of the major surface of the spatial light modulator such that these non-pixel areas cannot contribute to the 5 DC order that propagates to the replay plane. However, the inventors have found that, despite a combination of the above measures for preventing unmodulated DC order light from propagating to the replay plane, undesirable artefacts may nevertheless still appear in the holographic reconstruction. Such artefacts are 10 detrimental to image quality. The inventors investigated the reasons for the unwanted artefacts. The inventors surprisingly found that the unmodulated light of the DC order results in a focus, the central DC spot, which is spatially extended or spread to form long lines or “tails” of bright light. In 15 particular, by performing simulations using a spatial light modulator with a quadrilateral aperture mask, the inventors found that the DC spot is formed with four outwardly radiating “tails” extending horizontally and vertically (i.e., arranged at 90 degrees to each other) to form the shape of a cross. The inventors realised that these tails extend perpendicular to the aperture edges and so could be due to diffraction at the edge of the mask. Whilst a 20 “hole” or other spatial filter may substantially remove the DC spot at the centre of the output DC order light, these residual DC order “tails” may remain and so cause the unwanted artefacts that appear in the reconstructed image. According to the present disclosure, the inventors propose to “apodise” at least one portion 25 of the boundary defining the aperture of the mask, in order to reduce a diffractive effect. Specifically, apodisation of at least a portion of the boundary of the mask serves to “soften” the boundary / edge of the mask to reduce the diffractive effect thereof. For example, at least one side of a quadrilateral boundary may be apodised. Specifically, the apodised boundary may reduce the diffractive edge effect on the unmodulated light at the boundary. The 30 inventors have found that by apodising the boundary defining the aperture of the mask, the intensity and length of the above mentioned DC order “tails” is reduced. This, in turn, reduces the artefacts appearing in the reconstructed image. The inventors propose various techniques for apodising at least one portion of the boundary 35 of the mask, such as two opposite sides of the boundary of a quadrilateral mask, and optionally substantially the whole of the boundary of the mask, as described herein. Other possibilities will be apparent to the skilled person. In implementations, the transmission area of the mask has a maximum transmissivity value (e.g., 95% to 100% transmissivity) forthe output light, and the non-transmission area of the mask has a minimum transmissivity value (e.g., substantially 0% transmissivity) forthe output light. Light transmissivity may be expressed as a percentage. A transmissivity of 100% may mean fully transparent. Accordingly, the transmission area allows substantially all light to pass through unhindered. Thus, light propagating through the transmission area does not reduce in intensity. A transmissivity of substantially 0% may mean fully opaque. In other words, the portion having 0% transmissivity may prevent substantially all incident light from passing through that portion. Thus, light incident on the non-transmission area will be blocked by the mask. As the skilled person will appreciate, an area of a mask having a transmissivity that is between 100% and 0% may mean that a proportion of the incident light is allowed to pass and the remaining proportion of the light is blocked by that area of the mask. In implementations, at least one portion of the boundary of the mask has a characteristic that is non-uniform or variable. For example, the boundary may be quadrilateral, such as rectangular, and one side of the rectangle may be non-uniform. The non-uniform or variable characteristic may change with position along the boundary or with position across the width of an edge forming the boundary. In some implementations, the non-uniform or variable characteristic of at least one portion of the boundary of the mask is a shape of the boundary. It may be said that the geometry of the mask is changed to reduce the diffractive effect at the boundary between transmission and non-transmission areas thereof. In an example, the shape of the edge is non-linear (i.e., not straight) such as wavy. It may be said that the shape of the boundary is a periodic waveform, comprising a series of wave peaks, extending in the longitudinal direction ofthe boundary. For instance, an edge ofthe non-transmission area ofthe mask, which forms the boundary, may have a periodically varying shape such that it alternately extends into and out ofthe aperture it defines (i.e., the area that it bounds). In this way, small regions ofthe nontransmission area (e.g., corresponding to “wave peaks”) ofthe mask are arranged in cooperation with (i.e., overlay or overlap) corresponding small regions ofthe display area (e.g., active pixel area) ofthe spatial light modulator. In some implementations, the shape of the boundary is a periodic waveform, comprising a series of wave peaks and troughs, extending in the longitudinal direction ofthe boundary. The alignment ofthe wave peaks and troughs ofthe mask boundary with the straight edge perimeter ofthe display area may be so that small regions of the transmission area (e.g., corresponding to “wave troughs”) may additionally be arranged in cooperation with (i.e., to overlay or overlap) corresponding small regions of the non-display area (e.g., non-pixel area) surrounding the display areas of the spatial light modulator. 5 In other implementations, the non-uniform or variable characteristic of at least one portion of the boundary of the mask is an optical characteristic of the boundary. For instance, an edge of the non-transmission area of the mask, which forms the boundary, may have a non-uniform or variable optical characteristic. In an example, the optical characteristic is 10 transmissivity. For instance, the transmissivity at the edge of the non-transmission area of the mask, forming the boundary, changes as a function of distance perpendicular to the boundary. It may be said that edge of the non-transmission area that forms the boundary has a width, and the transmissivity of the edge varies across its width. Thus, there is a gradual (i.e., non-abrupt) change in transmissivity between the maximum value (of the 15 transmission area) and minimum value (of the non-transmission area) at the boundary. 20 25 In examples, the boundary is formed by an edge of the non-transmission area having varying transmissivity. The edge has a width in the range of about 1 to 250 microns, such as 5 to 100 pm or corresponding to the width of up to 4 to 10 pixels. The transmissivity profile of the edge changes from a maximum value to a minimum value across the width thereof (i.e., as a function of position perpendicular to the boundary). It may be said that the change in transmissivity between the maximum value and the minimum value is gradual (i.e., non-abrupt). In some examples, the transmissivity profile across the width of the edge may be continuous or smooth, such as a continuous linear (e.g., ramped) profile or a continuous curved profile. In other examples, the transmissivity profile across the width of the edge may be non-continuous, such as stepwise, for example comprising a sequence of discrete changes having a stepped or ramped profile. In the present disclosure, the term “replica” is merely used to reflect that spatially modulated 30 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. Embodiments of the present disclosure relate to propagation of light that is encoded with a hologram, not an image - i.e., 35 light that is spatially modulated with a hologram of an image, not the image itself. It may therefore be said that a plurality of replicas of the hologram are formed. The person skilled in the art of holography will appreciate that the complex light field associated with propagation of light encoded with a hologram will change with propagation distance. Use herein of the term “replica” is independent of propagation distance and so the two branches or paths of light associated with a replication event are still referred to as “replicas” of each other even if the branches are a different length, such that the complex light field has evolved differently along each path. That is, two complex light fields are still considered “replicas” in accordance with this disclosure even if they are associated with different propagation distances - providing they have arisen from the same replication event or series of replication events. A “diffracted light field” or “diffractive light field” in accordance with this disclosure is a light field formed by diffraction. A diffracted light field may be formed by illuminating a corresponding diffractive pattern. In accordance with this disclosure, an example of a diffractive pattern is a hologram and an example of a diffracted light field is a holographic light field or a light field forming a holographic reconstruction of an image. The holographic light field forms a (holographic) reconstruction of an image on a replay plane. The holographic light field that propagates from the hologram to the replay plane may be said to comprise light encoded with the hologram or light in the hologram domain. A diffracted light field is characterized by a diffraction angle determined by the smallest feature size of the diffractive structure and the wavelength of the light (of the diffracted light field). In accordance with this disclosure, it may also be said that a “diffracted light field” is a light field that forms a reconstruction on a plane spatially separated from the corresponding diffractive structure. An optical system is disclosed herein for propagating a diffracted light field from a diffractive structure to a viewer. The diffracted light field may form an image. The term “hologram" is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, regarding the object. The term “holographic reconstruction” is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The system disclosed herein is described as a “holographic projector” because the holographic reconstruction is a real image and spatially-separated from the hologram. The term “replay field” is used to refer to the 2D area within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of a plurality diffracted orders wherein each diffracted order is a replica of the zeroth-order replay field. The zeroth-order replay field generally corresponds to the preferred or primary replay field because it is the brightest replay field. Unless explicitly stated otherwise, the term “replay field” should be taken as referring to the zeroth-order replay field. The term “replay plane” is used to refer to the plane in space containing all the replay fields. The terms “image”, “replay image” and “image region” refer to areas of the replay field illuminated by light of the holographic reconstruction. In some embodiments, the “image” may comprise discrete spots which may be referred to as “image spots” or, for convenience only, “image pixels”. The terms “encoding”, “writing” or “addressing” are used to describe the process of providing the plurality of pixels of the SLM with a respective plurality of control values which respectively determine the modulation level of each pixel. It may be said that the pixels of the SLM are configured to “display” a light modulation distribution in response to receiving the plurality of control values. Thus, the SLM may be said to “display” a hologram and the hologram may be considered an array of light modulation values or levels. It has been found that a holographic reconstruction of acceptable quality can be formed from a “hologram” containing only phase information related to the Fourier transform of the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography. The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully-complex computer-generated hologram is calculated. Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for “phase-delay”. That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2tt) which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of 7t / 2 will retard the phase of received light by n / 2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term “grey level” may be used to refer to the plurality of available modulation levels. For example, the term “grey level” may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term “grey level” may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator. The hologram therefore comprises an array of grey levels - that is, an array of light modulation values such as an array of phase-delay values or complex modulation values. The hologram is also considered a diffractive pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light having a wavelength comparable to, generally less than, the pixel pitch of the spatial light modulator. Reference is made herein to combining the hologram with other diffractive patterns such as diffractive patterns functioning as a lens or grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the replay field on the replay plane or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a replay plane in the near field. Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged. BRIEF DESCRIPTION OF THE DRAWINGS Specific embodiments are described by way of example only with reference to the following figures: Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen; Figure 2 shows an image for projection comprising eight image areas / components, V1 to V8, and cross-sections of the corresponding hologram channels, H1-H8; Figure 3 shows a hologram displayed on an LCOS that directs light into a plurality of discrete areas; Figure 4 shows a system, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3; Figure 5A shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each comprising pairs of stacked surfaces; Figure 5B shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each in the form of a solid waveguide; Figure 6A shows an enlarged corner section of a conventional binary mask having a linear / straight line boundary between a non-transmission area and a transmission area defining a quadrilateral window arranged in cooperation with a corresponding quadrilateral display area of a spatial light modulator; Figure 6B shows an intensity plot of a reconstructed image of a blank hologram displayed on a spatial light modulator with the conventional mask of Figure 6A; Figure 7A shows an enlarged corner section of a mask, similar to Figure 6A, in which one side of the boundary is apodised in accordance with a first example; Figure 7B shows an intensity plot of a reconstructed image of a blank hologram displayed on a spatial light modulator with the mask of Figure 7A; Figure 8A shows an enlarged corner section of a mask, similar to Figure 6A, in which one side of the boundary is apodised in accordance with a second example; and Figure 8B shows an intensity plot of a reconstructed image of a blank hologram displayed on a spatial light modulator with the mask of Figure 8A. The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF EMBODIMENTS The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration. Terms of a singular form may include plural forms unless specified otherwise. A structure described as being formed at an upper portion / lower portion of another structure or on / under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between. In describing a time relationship - for example, when the temporal order of events is described as “after”, “subsequent”, “next”, “before” or suchlike - the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as “just”, “immediate” or “direct” is used. Although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims. Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in codependent relationship. In the present disclosure, the term “substantially” when applied to a structural units of an apparatus may be interpreted as the technical feature of the structural units being produced within the technical tolerance of the method used to manufacture it. Conventional optical configuration for holographic projection Figure 1 shows an embodiment in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or frequency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, “LCOS”, device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser. A light source 110, for example a laser or laser diode, is disposed to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally planar wavefront of light to be incident on the SLM. In Figure 1, the direction of the wavefront is off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer). However, in other embodiments, the generally planar wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in Figure 1, the arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a light modulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to optics including a Fourier transform lens 120, having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives a beam of modulated light from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at the screen 125. Notably, in this type of holography, each pixel of the hologram contributes to the whole reconstruction. There is not a one-to-one correlation between specific points (or image pixels) on the replay field and specific light-modulating elements (or hologram pixels). In other words, modulated light exiting the light-modulating layer is distributed across the replay field. In these embodiments, the position of the holographic reconstruction in space is determined by the dioptric (focusing) power of the Fourier transform lens. In the embodiment shown in Figure 1, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens but the performance of the lens will limit the accuracy of the Fourier transform it performs. The skilled person understands how to use a lens to perform an optical Fourier transform In some embodiments of the present disclosure, the lens of the viewer’s eye performs the hologram to image transformation. Hologram calculation In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or Fourier-based hologram, in which an image is reconstructed in the far field by utilising the Fourier transforming properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back to the lens plane. Computer-generated Fourier holograms may be calculated using Fourier transforms. Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms which may be calculated by a similar method. In some embodiments, the hologram is a phase or phase-only hologram. However, the present disclosure is also applicable to holograms calculated by other techniques such as those based on point cloud methods. In some embodiments, the hologram engine is arranged to exclude from the hologram calculation the contribution of light blocked by a limiting aperture of the display system. British patent application 2101666.2, filed 5 February 2021 and incorporated herein by reference, discloses a first hologram calculation method in which eye-tracking and ray tracing are used to identify a sub-area of the display device for calculation of a point cloud hologram which eliminates ghost images. The sub-area of the display device corresponds with the aperture, of the present disclosure, and is used exclude light paths from the hologram calculation. British patent application 2112213.0, filed 26 August 2021 and incorporated herein by reference, discloses a second method based on a modified Gerchberg-Saxton type algorithm which includes steps of light field cropping in accordance with pupils of the optical system during hologram calculation. The cropping of the light field corresponds with the determination of a limiting aperture of the present disclosure. British patent application 2118911.3, filed 23 December 2021 and also incorporated herein by reference, discloses a third method of calculating a hologram which includes a step of determining a region of a so-called extended modulator formed by a hologram replicator. The region of the extended modulator is also an aperture in accordance with this disclosure. In some embodiments, there is provided a real-time engine arranged to receive image data and calculate holograms in real-time using the algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are pre-calculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms. Large field of view using small display device Broadly, the present disclosure relates to image projection. It relates to a method of image projection and an image projector which comprises a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system, in which the image projector projects or relays light from the display device to the viewing system. The present disclosure is equally applicable to a monocular and binocular viewing system. The viewing system may comprise a viewer’s eye or eyes. The viewing system comprises an optical element having optical power (e.g., lens / es of the human eye) and a viewing plane (e.g., retina of the human eye / s). The projector may be referred to as a ‘light engine’. The display device and the image formed (or perceived) using the display device are spatially separated from one another. The image is formed, or perceived by a viewer, on a display plane. In some embodiments, the image is a virtual image and the display plane may be referred to as a virtual image plane. In other examples, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed either in free space or on a screen or other light receiving surface between the display device and the viewer, is propagated to the viewer. In both cases, an image is formed by illuminating a diffractive pattern (e.g., hologram or kinoform) displayed on the display device. The display device comprises pixels. The pixels of the display may display a diffractive pattern or structure that diffracts light. The diffracted light may form an image at a plane spatially separated from the display device. In accordance with well-understood optics, the magnitude of the maximum diffraction angle is determined by the size of the pixels and other factors such as the wavelength of the light. In embodiments, the display device is a spatial light modulator such as liquid crystal on silicon (“LCOS”) spatial light modulator (SLM). Light propagates over a range of diffraction angles (for example, from zero to the maximum diffractive angle) from the LCOS, towards a viewing entity / system such as a camera or an eye. In some embodiments, magnification techniques may be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of an LCOS. In some embodiments, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) - that may be informally said to be “encoded” with / by the hologram - is propagated directly to the viewer’s eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction I image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device. Reference is made herein to a “light field” which is a “complex light field”. The term “light field” merely indicates a pattern of light having a finite size in at least two orthogonal spatial directions, e.g. x and y. The word “complex” is used herein merely to indicate that the light at each point in the light field may be defined by an amplitude value and a phase value, and may therefore be represented by a complex number or a pair of values. For the purpose of hologram calculation, the complex light field may be a two-dimensional array of complex numbers, wherein the complex numbers define the light intensity and phase at a plurality of discrete locations within the light field. In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity / system, varies with the distance between the display device and the viewing entity. At a 1 metre viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye’s pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye’s pupil to form an image at the retina for a given eye position, determines the portion of the image that is ‘visible’ to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-box.) In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device - that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it may therefore be considered that the viewer is looking at a virtual image through an ‘display device-sized window’, which may be very small, for example 1cm in diameter, at a relatively large distance, e.g., 1 metre. And the user will be viewing the display device-sized window via the pupil(s) of their eye(s), which can also be very small. Accordingly, the field of view becomes small and the specific angular range that can be seen depends heavily on the eye position, at any given time. A pupil expander addresses the problem of how to increase the range of angles of light rays that are propagated from the display device that can successfully propagate through an eye’s pupil to form an image. The display device is generally (in relative terms) small and the projection distance is (in relative terms) large. In some embodiments, the projection distance is at least one - such as, at least two - orders of magnitude greater than the diameter, or width, of the entrance pupil and / or aperture of the display device (i.e., size of the array of pixels). Use of a pupil expander increases the viewing area (i.e., user’s eye-box) laterally, thus enabling some movement of the eye / s to occur, whilst still enabling the user to see the image. As the skilled person will appreciate, in an imaging system, the viewing area (user’s eye box) is the area in which a viewer’s eyes can perceive the image. The present disclosure encompasses non-infinite virtual image distances - that is, near-field virtual images. Conventionally, a two-dimensional pupil expander comprises one or more one-dimensional optical waveguides each formed using a pair of opposing reflective surfaces, in which the output light from a surface forms a viewing window or eye-box. Light received from the display device (e.g., spatially modulated light from a LCOS) is replicated by the or each waveguide so as to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide enlarges the viewing window due to the generation of extra rays or “replicas” by division of amplitude of the incident wavefront. The display device may have an active or display area having a first dimension that may be less than 10 cms such as less than 5 cms or less than 2 cms. The propagation distance between the display device and viewing system may be greater than 1 m such as greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide may be up to 2 m such as up to 1.5 m or up to 1 m. The method may be capable of receiving an image and determining a corresponding hologram of sufficient quality in less than 20 ms such as less than 15 ms or less than 10 ms. In some embodiments - described only by way of example of a diffracted or holographic light field in accordance with this disclosure - a hologram is configured to route light into a plurality of channels, each channel corresponding to a different part (i.e. sub-area) of an image. The channels formed by the diffractive structure are referred to herein as “hologram channels” merely to reflect that they are channels of light encoded by the hologram with image information. It may be said that the light of each channel is in the hologram domain rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram and the hologram domain is therefore the Fourier or frequency domain. The hologram may equally be a Fresnel or Fresnel transform hologram. The hologram may also be a point cloud hologram. The hologram is described herein as routing light into a plurality of hologram channels to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-areas, wherein each hologram channel would correspond to each image sub-area. Importantly, the hologram of this example is characterised by how it distributes the image content when illuminated. Specifically and uniquely, the hologram divides the image content by angle. That is, each point on the image is associated with a unique light ray angle in the spatially modulated light formed by the hologram when illuminated - at least, a unique pair of angles because the hologram is two-dimensional. For the avoidance of doubt, this hologram behaviour is not conventional. The spatially modulated light formed by this special type of hologram, when illuminated, may be divided into a plurality of hologram channels, wherein each hologram channel is defined by a range of light ray angles (in two-dimensions). It will be understood from the foregoing that any hologram channel (i.e. sub-range of light ray angles) that may be considered in the spatially modulated light will be associated with a respective part or sub-area of the image. That is, all the information needed to reconstruct that part or sub-area of the image is contained within a sub-range of angles of the spatially modulated light formed from the hologram of the image. When the spatially modulated light is observed as a whole, there is not necessarily any evidence of a plurality of discrete light channels. Nevertheless, the hologram may still be identified. For example, if only a continuous part or sub-area of the spatially modulated light formed by the hologram is reconstructed, only a sub-area of the image should be visible. If a different, continuous part or sub-area of the spatially modulated light is reconstructed, a different sub-area of the image should be visible. A further identifying feature of this type of hologram is that the shape of the cross-sectional area of any hologram channel substantially corresponds to (i.e. is substantially the same as) the shape of the entrance pupil although the size may be different - at least, at the correct plane for which the hologram was calculated. Each light / hologram channel propagates from the hologram at a different angle or range of angles. Whilst these are example ways of characterising or identifying this type of hologram, other ways may be used. In summary, the hologram disclosed herein is characterised and identifiable by how the image content is distributed within light encoded by the hologram. Again, for the avoidance of any doubt, reference herein to a hologram configured to direct light or angularly-divide an image into a plurality of hologram channels is made by way of example only and the present disclosure is equally applicable to pupil expansion of any type of holographic light field or even any type of diffractive or diffracted light field. The system can be provided in a compact and streamlined physical form. This enables the system to be suitable for a broad range of real-world applications, including those for which space is limited and real-estate value is high. For example, it may be implemented in a head-up display (HUD) such as a vehicle or automotive HUD. In accordance with the present disclosure, pupil expansion is provided for diffracted or diffractive light, which may comprise diverging ray bundles. The diffracted light field may be defined by a “light cone”. Thus, the size of the diffracted light field (as defined on a two-dimensional plane) increases with propagation distance from the corresponding diffractive structure (i.e. display device). It can be said that the pupil expander / s replicate the hologram or form at least one replica of the hologram, to convey that the light delivered to the viewer is spatially modulated in accordance with a hologram. In some embodiments, two one-dimensional waveguide pupil expanders are provided, each one-dimensional waveguide pupil expander being arranged to effectively increase the size of the exit pupil of the system by forming a plurality of replicas or copies of the exit pupil (or light of the exit pupil) of the spatial light modulator. The exit pupil may be understood to be the physical area from which light is output by the system. It may also be said that each waveguide pupil expander is arranged to expand the size of the exit pupil of the system. It may also be said that each waveguide pupil expander is arranged to expand / increase the size of the eye box within which a viewer’s eye can be located, in order to see / receive light that is output by the system. Light channelling The hologram formed in accordance with some embodiments, angularly-divides the image content to provide a plurality of hologram channels which may have a cross-sectional shape defined by an aperture of the optical system. The hologram is calculated to provide this channelling of the diffracted light field. In some embodiments, this is achieved during hologram calculation by considering an aperture (virtual or real) of the optical system, as described above. Figures 2 and 3 show an example of this type of hologram that may be used in conjunction with a pupil expander as disclosed herein. However, this example should not be regarded as limiting with respect to the present disclosure. Figure 2 shows an image 252 for projection comprising eight image areas / components, V1 to V8. Figure 2 shows eight image components by way of example only and the image 252 may be divided into any number of components. Figure 2 also shows an encoded light pattern 254 (i.e., hologram) that can reconstruct the image 252 - e.g., when transformed by the lens of a suitable viewing system. The encoded light pattern 254 comprises first to eighth sub-holograms or components, H1 to H8, corresponding to the first to eighth image components / areas, V1 to V8. Figure 2 further shows how a hologram may decompose the image content by angle. The hologram may therefore be characterised by the channelling of light that it performs. This is illustrated in Figure 3. Specifically, the hologram in this example directs light into a plurality of discrete areas. The discrete areas are discs in the example shown but other shapes are envisaged. The size and shape of the optimum disc may, after propagation through the waveguide, be related to the size and shape of an aperture of the optical system such as the entrance pupil of the viewing system. Figure 4 shows a system 400, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3. The system 400 comprises a display device, which in this arrangement comprises an LCOS 402. The LCOS 402 is arranged to display a modulation pattern (or ‘diffractive pattern') comprising the hologram and to project light that has been holographically encoded towards an eye 405 that comprises a pupil that acts as an aperture 404, a lens 409, and a retina (not shown) that acts as a viewing plane. There is a light source (not shown) arranged to illuminate the LCOS 402. The lens 409 of the eye 405 performs a hologram-to-image transformation. The light source may be of any suitable type. For example, it may comprise a laser light source. The viewing system 400 further comprises a waveguide 408 positioned between the LCOS 402 and the eye 405. The presence of the waveguide 408 enables all angular content from the LCOS 402 to be received by the eye, even at the relatively large projection distance shown. This is because the waveguide 508 acts as a pupil expander, in a manner that is well known and so is described only briefly herein. In brief, the waveguide 408 shown in Figure 4 comprises a substantially elongate formation. In this example, the waveguide 408 comprises an optical slab of refractive material, but other types of waveguide are also well known and may be used. The waveguide 408 is located so as to intersect the light cone (i.e., the diffracted light field) that is projected from the LCOS 402, for example at an oblique angle. In this example, the size, location, and position of the waveguide 408 are configured to ensure that light from each of the eight ray bundles, within the light cone, enters the waveguide 408. Light from the light cone enters the waveguide 408 via its first planar surface (located nearest the LCOS 402) and is guided at least partially along the length of the waveguide 408, before being emitted via its second planar surface, substantially opposite the first surface (located nearest the eye). As will be well understood, the second planar surface is partially reflective, partially transmissive. In other words, when each ray of light travels within the waveguide 408 from the first planar surface and hits the second planar surface, some of the light will be transmitted out of the waveguide 408 and some will be reflected by the second planar surface, back towards the first planar surface. The first planar surface is reflective, such that all light that hits it, from within the waveguide 408, will be reflected back towards the second planar surface. Therefore, some of the light may simply be refracted between the two planar surfaces of the waveguide 408 before being transmitted, whilst other light may be reflected, and thus may undergo one or more reflections, (or ‘bounces’) between the planar surfaces of the waveguide 408, before being transmitted. Figure 4 shows a total of nine “bounce” points, BO to B8, along the length of the waveguide 408. Although light relating to all points of the image (V1-V8) as shown in Figure 2 is transmitted out of the waveguide at each “bounce” from the second planar surface of the waveguide 408, only the light from one angular part of the image (e.g. light of one of V1 to V8) has a trajectory that enables it to reach the eye 405, from each respective “bounce” point, BO to B8. Moreover, light from a different angular part of the image, V1 to V8, reaches the eye 405 from each respective “bounce” point. Therefore, each angular channel of encoded light reaches the eye only once, from the waveguide 408, in the example of Figure 4. The waveguide 408 forms a plurality of replicas of the hologram, at the respective “bounce” points B1 to B8 along its length, corresponding to the direction of pupil expansion. As shown in Figure 5, the plurality of replicas may be extrapolated back, in a straight line, to a corresponding plurality of replica or virtual display devices 402’. This process corresponds to the step of “unfolding” an optical path within the waveguide, so that a light ray of a replica is extrapolated back to a “virtual surface” without internal reflection within the waveguide. Thus, the light of the expanded exit pupil may be considered to originate from a virtual surface (also called an “extended modulator” herein) comprising the display device 402 and the replica display devices 402’. Although virtual images, which require the eye to transform received modulated light in order to form a perceived image, have generally been discussed herein, the methods and arrangements described herein can be applied to real images. Two-Dimensional Pupil Expansion Whilst the arrangement shown in Figure 4 includes a single waveguide that provides pupil expansion in one dimension, pupil expansion can be provided in more than one dimension, for example in two dimensions. Moreover, whilst the example in Figure 4 uses a hologram that has been calculated to create channels of light, each corresponding to a different portion of an image, the present disclosure and the systems that are described herebelow are not limited to such a hologram type. Figure 5A shows a perspective view of a system 500 comprising two replicators, 504, 506 arranged for expanding a light beam 502 in two dimensions. In the system 500 of Figure 5A, the first replicator 504 comprises a first pair of surfaces, stacked parallel to one another, and arranged to provide replication - or, pupil expansion -in a similar manner to the waveguide 408 of Figure 4. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially elongate in one direction. The collimated light beam 502 is directed towards an input on the first replicator 504. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), which will be familiar to the skilled reader, light of the light beam 502 is replicated in a first direction, along the length of the first replicator 504. Thus, a first plurality of replica light beams 508 is emitted from the first replicator 504, towards the second replicator 506. The second replicator 506 comprises a second pair of surfaces stacked parallel to one another, arranged to receive each of the collimated light beams of the first plurality of light beams 508 and further arranged to provide replication - or, pupil expansion - by expanding each of those light beams in a second direction, substantially orthogonal to the first direction. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially rectangular. The rectangular shape is implemented for the second replicator in order for it to have length along the first direction, in order to receive the first plurality of light beams 508, and to have length along the second, orthogonal direction, in order to provide replication in that second direction. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), light of each light beam within the first plurality of light beams 508 is replicated in the second direction. Thus, a second plurality of light beams 510 is emitted from the second replicator 506, wherein the second plurality of light beams 510 comprises replicas of the input light beam 502 along each of the first direction and the second direction. Thus, the second plurality of light beams 510 may be regarded as comprising a two-dimensional grid, or array, of replica light beams. Thus, it can be said that the first and second replicators 504, 505 of Figure 5A combine to provide a two-dimensional replicator (or, “two-dimensional pupil expander”). Thus, the replica light beams 510 may be emitted along an optical path to an expanded eye-box of a display system, such as a head-up display. In the system of Figure 5A, the first replicator 504 is a waveguide comprising a pair of elongate rectilinear reflective surfaces, stacked parallel to one another, and, similarly, the second replicator 504 is a waveguide comprising a pair of rectangular reflective surfaces, stacked parallel to one another. In other systems, the first replicator may be a solid elongate rectilinear waveguide and the second replicator may be a solid planar rectangular shaped waveguide, wherein each waveguide comprises an optically transparent solid material such as glass. In this case, the pair of parallel reflective surfaces are formed by a pair of opposed major sidewalls optionally comprising respective reflective and reflective-transmissive surface coatings, familiar to the skilled reader. Figure 5B shows a perspective view of a system 500 comprising two replicators, 520, 540 arranged for replicating a light beam 522 in two dimensions, in which the first replicator is a solid elongated waveguide 520 and the second replicator is a solid planar waveguide 540. In the system of Figure 5B, the first replicator / waveguide 520 is arranged so that its pair of elongate parallel reflective surfaces 524a, 524b are perpendicular to the plane of the second replicator / waveguide 540. Accordingly, the system comprises an optical coupler arranged to couple light from an output port of first replicator 520 into an input port of the second replicator 540. In the illustrated arrangement, the optical coupler is a planar / fold mirror 530 arranged to fold or turn the optical path of light to achieve the required optical coupling from the first replicator to the second replicator. As shown in Figure 5B, the mirror 530 is arranged to receive light - comprising a one-dimensional array of replicas extending in the first dimension - from the output port / reflective-transmissive surface 524a of the first replicator / waveguide 520. The mirror 530 is tilted so as to redirect the received light onto an optical path to an input port in the (fully) reflective surface of second replicator 540 at an angle to provide waveguiding and replica formation, along its length in the second dimension. It will be appreciated that the mirror 530 is one example of an optical element that can redirect the light in the manner shown, and that one or more other elements may be used instead, to perform this task. In the illustrated arrangement, the (partially) reflective-transmissive surface 524a of the first replicator 520 is adjacent the input port of the first replicator / waveguide 520 that receives input beam 522 at an angle to provide waveguiding and replica formation, along its length in the first dimension. Thus, the input port of first replicator / waveguide 520 is positioned at an input end thereof at the same surface as the reflective-transmissive surface 524a. The skilled reader will understand that the input port of the first replicator / waveguide 520 may be at any other suitable position. 10 Accordingly, the arrangement of Figure 5B enables the first replicator 520 and the mirror 530 to be provided as part of a first relatively thin layer in a plane in the first and third dimensions (illustrated as an x-z plane). In particular, the size or “height” of a first planar layer - in which the first replicator 520 is located - in the second dimension (illustrated as the y dimension) is reduced. The mirror 530 is configured to direct the light away from a first layer / plane, in which the first replicator 520 is located (i.e. the “first planar layer"), and direct it towards a second layer / plane, located above and substantially parallel to the first layer / plane, in which the second replicator 540 is located (i.e. a “second planar layer”). Thus, the overall size or “height” of the system - comprising the first and second replicators 520, 540 and the mirror 530 located in the stacked first and second planar layers in the first and third dimensions (illustrated as an x-z plane) - in the second dimension (illustrated as the y dimension) is compact. The skilled reader will understand that many variations of the arrangement of Figure 5B for implementing the present disclosure are possible and contemplated. 15 The image projector may be arranged to project a diverging or diffracted light field. 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. 20 25 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). 30 35 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 extent by the use of at least one pupil expander. The diffracted or diverging light field may be said to have “a light field size”, defined in a direction substantially orthogonal to a propagation direction of the light field. Because the light is diffracted / diverging, the light field size increases with propagation distance. In some embodiments, the diffracted light field is spatially-modulated in accordance with a hologram. In other words, in such aspects, the diffractive light field comprises a ‘‘holographic light field”. The hologram may be displayed on a pixelated display device. The hologram may be a computer-generated hologram (CGH). It may be a Fourier hologram 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 eyebox area, from which the viewer can receive light of the input diffracted or diverging light field. The eye-box area may be said to be located on, or to define, a viewing plane. The two directions in which the exit pupil is expanded may be coplanar with, or parallel to, the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. Alternatively, in arrangements that comprise other elements such as an optical combiner, for example the windscreen (or, windshield) of a vehicle, the exit pupil may be regarded as being an exit pupil from that other element, such as from the windscreen. In such arrangements, the exit pupil may be non-coplanar and non-parallel with the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. For example, the exit pupil may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. The viewing plane, and / or the eye-box area, may be non-coplanar or non-parallel to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. For example, a viewing plane may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. In order to provide suitable launch conditions to achieve internal reflection within the first and second waveguide pupil expanders, an elongate dimension of the first waveguide pupil expander may be tilted relative to the first and second dimensions of the second waveguide pupil expander. Combiner shape compensation An advantage of projecting a hologram to the eye-box is that optical compensation can be encoded in the hologram (see, for example, European patent 2936252 incorporated herein by herein). The present disclosure is compatible with holograms that compensate for the complex curvature of an optical combiner used as part of the projection system. In some embodiments, the optical combiner is the windscreen of a vehicle. Full details of this approach are provided in European patent 2936252 and are not repeated here because the detailed features of those systems and methods are not essential to the new teaching of this disclosure herein and are merely exemplary of configurations that benefit from the teachings of the present disclosure. Control device The present disclosure is also compatible with optical configurations that include a control device (e.g. light shuttering device) to control the delivery of light from a light channelling hologram to the viewer. The holographic projector may further comprise a control device arranged to control the delivery of angular channels to the eye-box position. British patent application 2108456.1, filed 14 June 2021 and incorporated herein by reference, discloses the at least one waveguide pupil expander and control device. The reader will understand from at least this prior disclosure that the optical configuration of the control device is fundamentally based upon the eye-box position of the user and is compatible with any hologram calculation method that achieves the light channeling described herein. It may be said that the control device is a light shuttering or aperturing device. The light shuttering device may comprise a 1D array of apertures or windows, wherein each aperture or window independently switchable between a light transmissive and a light non-transmissive state in order to control the delivery of hologram light channels, and their replicas, to the eye-box. Each aperture or window may comprise a plurality of liquid crystal cells or pixels. DC Order / aperture mask The present disclosure relates to an aperture mask arranged in cooperation with the display area, such as an active pixel area, of the spatial light modulator. For example, the mask may be formed on a major surface of a transparent substrate overlaying, or forming part of the pixels of, the active pixel area. The mask may define a transparent window that forms an entrance aperture for light incident on the spatial light modulator. Thus, the mask / entrance aperture may substantially block light incident on areas outside (e.g. surrounding) the display area of the spatial light modulator, so that the such areas are not illuminated and so do not output unmodulated light that may contribute to the DC order. At the same time, the transparent window of the mask / entrance aperture may allow all the active pixels to be substantially uniformly illuminated for efficient modulation and output of spatially modulated light. However, despite the use of both a spatial light filter and an aperture mask as described above, the inventors have found that artefacts may still appear in the holographic reconstruction and thus the image visible to the viewer. The inventors investigated the cause of the unwanted artefacts described herein by simulating the profile of a light field (i.e. holographic wavefront) output by a spatial light modulator arranged with a conventional aperture mask, when displaying a blank or “zero” hologram. As the skilled person will appreciate, this means that the active pixels are not encoded with a hologram, and so act to transmit / reflect incident light without modulation. In particular, the inventors investigated the intensity profile of the light field (replay image) at the replay plane in the absence of a spatial light filter or “hole” to remove the central DC order spot. Figure 6A shows an enlarged corner section of a conventional binary mask comprising a masked area forming a non-transmission area (shown in black) and an unmasked area forming a transmission area (shown in white). The mask defines a quadrilateral transmission window or aperture that is arranged in cooperation with (e.g., to overlay) a corresponding quadrilateral display area (e.g. active pixel area) of a spatial light modulator. It may be said that the transmission area or window is complementary to the display area of the spatial light modulator, in particular the window is aligned with, and is matched in size and shape to, the display area. The mask may be formed directly on the display area of the spatial light modulator, or may be formed on a transparent substrate overlying the display area such that the mask is spatially separated from the display area in a direction perpendicular thereto. Figure 6A shows an intersection of two portions forming the sides of a quadrilateral boundary between the masked / non-transmission area and the unmasked / transmission area. In accordance with the conventional arrangement, all four sides of the quadrilateral boundary defining the transmission area are linear (i.e. straight lines) and form a step or abrupt / clear-cut transition between the non-transmission area and the transmission area of the mask. Figure 6B shows a simulated intensity plot of the light field corresponding a reconstructed image (i.e. replay field / image at the replay plane) of a blank or zero hologram displayed on a spatial light modulator with the conventional mask of Figure 6A. In the simulation, the SLM comprises a phase modulator, in particular a Liquid Crystal on Silicon (LCOS) spatial light modulator. The plot represents a greyscale light intensity, in which the black areas correspond to a minimum intensity of received light and the white areas correspond to a maximum intensity of received light. Since the displayed hologram is a blank or zero hologram, the output light does not include a component of spatially modulated light. Thus, the light field may be considered to correspond to the DC order. As shown in Figure 6B, the inventors surprisingly found that, instead of a single central DC spot at the focus of the optical relay, the DC order light is spread (or extends) outwardly from a spot at the centre of the replay field. In particular, the DC order light includes four long lines or “tails”, two extending horizontally and two extending vertically, which radiate outwardly from the central DC spot to form a cross shape. These “DC order tails” have decreasing intensity (or brightness) with distance from the centre. The inventors realised that these bright tails may be a cause of artefacts appearing in images seen by a viewer. Apodised Aperture Mask As a result of their findings, the inventors postulated that the observed “DC order tails” may be a consequence of the diffractive effect caused at the boundary between the transmission and non-transmission areas of the aperture mask. Accordingly, the inventors investigated the effect on the DC order light of “apodising” a portion of the quadrilateral boundary of the aperture mask to verify this theory. Example 1 Figures 7 A and 7B illustrate a first example for apodising the boundary of the aperture mask investigated by the inventors. Figure 7A shows an enlarged corner section of a mask, similar to Figure 6A, in which one portion of the boundary between the masked and unmasked areas thereof is apodised in accordance with the first example. In this example, the bottom side of the quadrilateral boundary between the masked / non-transmission area and the transparent / transmission area is apodised so that the shape of the boundary is non-uniform, in particular continuously varying. Thus, the bottom side of the boundary is not a straight line corresponding to (or matching) the straight line perimeter of the display area of the spatial light modulator over which the mask is formed. Rather, the bottom side of the boundary is wavy, such that at least some portions, corresponding to “wave peaks” of the boundary, are perpendicularly displaced from a straight line corresponding to (i.e., aligned with) the straight line perimeter of the display area. In particular, the perpendicular displacement of points on the boundary varies continuously with distance along the boundary. In the illustrated example, the variable displacement is periodic such that boundary has a shape that varies periodically along its length. It may be said that the shape of the bottom side of the boundary corresponds to a periodic waveform. In the example, the wave peaks and troughs of the waveform are curved. In particular, the shape of the boundary generally corresponds to a sine wave (i.e., sinusoidal shape), having an amplitude and a wavelength, whereby the wave peaks extend into the aperture defined by the boundary and the wave troughs extend out of the aperture defined by the boundary. The skilled person will appreciate that the maximum amount that the wave peaks and troughs respectively extend into and out of the aperture (in the perpendicular direction) is dependent upon the amplitude of the peaks (and troughs) and the alignment of the waveform axis with the straight line perimeter of the display area. Accordingly, it may be said that an edge of the masked / non-transmission area of the aperture mask formed at the boundary with the unmasked / transmission area has a width corresponding to perpendicular distance between the peaks (crests) and troughs of the waveform measured from the waveform axis. Accordingly, in this example, the edge of the masked / non-transmission area is defined to have a periodically varying shape such that it periodically extends into the aperture / transmission area it defines (i.e., the area or window that it bounds). In this way, the masked / non-transmission area has “waves peaks” extending into the aperture / transmission area arranged periodically along the length of the boundary. These wave peaks at the boundary of the masked / non-transmission area of the mask are thus arranged in cooperation with (i.e., to overlay or overlap) correspondingly shaped small regions of the display area (e.g., pixel area) adjacent the perimeter thereof, thereby blocking light from illuminating these small regions of the spatial light modulator. In particular, the edge of the masked / non-transmission area may block light incident on active pixels of the display area (e.g., active pixel area) corresponding to small regions at the edge of the display area. Each wave peak may block incident light on between a fraction of a pixel up to a small number of pixels at the edge of the display area. It may be said that the straight line edge of the bottom side of the boundary of the mask is “softened” such that the edge of the illumination light field incident on the display area is (gradually) “attenuated”. In this first example, the waves of the shape of the edge of the masked / non-transmission area are spaced at periodic intervals (i.e., with a wavelength) of the order of microns (e.g., 1 to 250 pm such as 5 to 100 pm) along the length of the bottom side of the boundary and extend into the unmasked / transmission area by a maximum perpendicular distance or displacement (i.e., an amplitude) of the order of microns (e.g., 1 to 250 pm or up to the width of 4 to 10 pixels). The skilled person will appreciate that the frequency and amplitude of the waveform may be chosen according to design requirements for optimising the apodisation of the boundary of the mask. It should be noted that, in the first example, the top side (not shown) of the quadrilateral boundary of the mask is apodised in the same way as the bottom side, but the left side and the right side (not shown) of the quadrilateral boundary of the mask are not apodised but formed as a straight line as in Figure 6A. This allows observation of the effect of apodising the boundary of the mask on the DC order light propagating to the reconstructed image in the replay field at the replay plane. Figure 7B shows an intensity plot of the reconstructed image of a blank or “zero” hologram displayed on a spatial light modulator using a mask in accordance with the first example shown in Figure 7A. As shown in Figure 7B, the horizontally extending DC order tails radiating from the centre of the replay field are substantially the same as those shown in Figure 6B. However, the vertically extending DC order tails of light radiating from the centre of the replay field have a reduced length in comparison the horizontally extending DC order tails. In particular, the decrease in brightness (intensity) of the vertically extending tails with distance from the centre is greater, so that they appear shorter. Furthermore, the intensity of the vertical DC order tails is reduced overall. Accordingly, this reduction in intensity of the unwanted light can be attributed to the apodisation of the top and bottom sides of the edge of the mask as shown in Figure 7A. This difference from the vertically extending tails in Figure 6B supports the inventor’s theory that the DC order tails are, at least in part, due to the diffractive effect of the straight line edges at the boundary between the masked / non-transmission and unmasked / transmission areas of the aperture mask. As shown in Figure 7B, as a consequence of the apodisation of the top and bottom sides of the edge at the boundary of the mask in accordance with example 1, some “ghosting” appears in the DC order intensity profile. In particular, the vertically extending DC order tails are repeated in the reconstructed image at periodic intervals, and with decreasing intensity, from the centre radiating outwardly (to the left and right). The inventors believe this “ghosting” effect is partly due to the shape of the geometry of the apodised edge, which may be optimised to reduce the repeating ghost DC order tails. For example, the non-uniform illumination of the display area (e.g. the active pixels) due to the apodised edge of the mask blocking light from illuminating small regions near the edge of the display area may contribute to the ghosting effect. Specifically, it is likely that some of the active pixels in the small regions corresponding to the “wave peaks” extending into the aperture or transmission area defined by the boundary of the mask receive no illumination at all. In addition, the illumination of small regions of the surface of the display device, outside (surrounding) the active display area, corresponding to the “wave troughs”, may also to contribute to the ghosting effect. Finally, the geometry of the shape of the apodised boundary itself may cause diffractive effects that contribute to the ghosting effect. Accordingly, the inventors propose the use of an optimisation algorithm to tailor the shape of the apodisation of the edge of the mask to balance the conflicting requirements to: (i) “soften” the abrupt change in transmissivity at the boundary between the masked / non-transmission and unmasked / transmission areas defining the aperture of the mask, thus “attenuating” the edge to the illumination light field in order to reduce the “DC order tails”, and (ii) uniformly illuminate the display area (i.e., active pixel area) to prevent “banding”. In particular, it is envisaged that the width of the edge of the boundary, defined by the perpendicular distance from the waveform axis between wave peaks (crests) and troughs or equivalent, may be optimised to be sufficiently small to minimise non-uniformities in illumination whilst being sufficiently large to soften the boundary edge. The shape of the apodised edge including the frequency and profile shape, across the width of the boundary edge may be optimised, accordingly. As the skilled person will appreciate, the alignment of the apodised edge of the mask with the perimeter of the display area (e.g. active pixel area) may also be optimised for application requirements, considering factors such as spatial separation between the respective planes of the mask and display area and orientation, including the angle of illumination of light on the display area and any tilt of the plane of the spatial light modulator, which affect the attenuation of the edge of the illumination light field / wavefront. Example 2 Figures 8A and 8B illustrate a second example for apodising the boundary of the aperture mask investigated by the inventors. In particular, Figure 8A shows an enlarged corner section of a mask, similar to Figure 6A, in which one portion of the boundary between the masked and unmasked areas thereof is apodised in accordance with the second example. In this example, the bottom side of the quadrilateral boundary between the masked / non-transmission area and the transparent / transmission area is apodised so that the transmissivity of the mask is non-uniform at the boundary between the masked / non-transmission area and the transparent / transmission area. In particular, instead of an abrupt or clear-cut change in transmissivity in a single step at the boundary, the transmissivity changes as a function of distance in the direction perpendicular to the boundary. Accordingly, the edge of the masked / non-transmission area has a transmission profile that changes (e.g., increases) over its width (i.e., a distance in a direction perpendicular to the boundary) from a minimum transmissivity value at the masked / non-transmission area to a maximum transmissivity value adjacent the unmasked / transmission area. In the illustrated example, the change in transmissivity between a minimum and a maximum value may extend over a distance of microns. It may be said that the edge of the masked / non-transmission area has a varying transmissivity profile that extends over a distance of the order of microns (e.g. about 1 to 250 pm, such as 5 to 100 pm) in a direction perpendicular to the boundary thereof. Since the masked / non-transmission area, with the minimum transmissivity value, is arranged in cooperation with (i.e., aligned with) the straight line perimeter of the display area of the spatial light modulator on which the mask is formed, so that the width of the edge extends into the aperture / transmission area it defines (i.e., the area or window that it bounds). Thus, the edge of the masked / non transmission area having variable transmissivity is arranged in cooperation with (i.e., to overlay or overlap) a narrow strip of the display area (e.g., active pixel area) adjacent the perimeter thereof, thereby blocking a proportion of the light incident thereon in accordance with the transmissivity profile thereof. In the second example, the transmissivity profile across the width of the edge of bottom side of the boundary of the mask has a substantially linear stepwise-change from minimum to maximum transmissivity. This stepwise function may result from forming the mask using a graded light absorption coating, which, in practice, is formed by applying a series of light absorption coatings onto the surface of the modulator, as well known in the art. However, a more gradual change in transmissivity profile is generally preferred for the purposes of apodisation. As the skilled person will appreciate, any suitable transmissivity profile across the width of the edge of the boundary between the masked and unmasked areas of the mask may be chosen according to design requirements for optimising the apodisation thereof. Importantly, the change in transmissivity across the width of the edge of the boundary is not abrupt or instantanous, but otherwise may include continuous, stepwise, linear and non-linear change in transmissivity as a function of distance, between maximum and minimum transmissivity. Accordingly, in the second example, the apodised edge of the masked / non-transmission area may variably partially block light incident on pixels around the perimeter of the display area (e.g., active pixel area). Thus, depending upon the width of the edge of the boundary of the mask, a fraction of a pixel up to a small number of pixels in a strip at the edge of the display area may be only partially illuminated. It may be said that the straight line edge of the bottom side of the boundary of the mask, is “softened”, such that the edge of the illumination light field incident on the display area is (gradually) “attenuated”. As with the first example, in the second example, the top side (not shown) of the quadrilateral boundary of the mask is apodised in the same way as the bottom side, but the left side and the right side (not shown) of the quadrilateral boundary of the mask are not apodised but formed as a straight line as in Figure 6A. This allows observation of the effect of apodising the boundary of the mask on the DC order light propagating to the reconstructed image in the replay field at the replay plane. Figure 8B shows an intensity plot of the reconstructed image of a blank hologram displayed on a spatial light modulator using the mask of the first example shown in Figure 8A. As shown in Figure 8B, the horizontally extending DC order tails radiating from the centre of the replay field are substantially the same as those shown in Figure 6B. However, the vertically extending DC order tails of light radiating from the centre of the replay field have a reduced length in comparison the horizontally extending DC order tails. In particular, the decrease in brightness (intensity) of the vertically extending tails with distance from the centre is greater, so that they appear shorter. Furthermore, the intensity of the vertical DC order tails is reduced overall. Accordingly, this reduction in intensity of the unwanted light can be attributed to the apodisation of the top and bottom sides of the mask as shown in Figure 8A. However, unlike the intensity profile in example 1 shown in Figure 7B, the apodisation of the top and bottom sides of the edge at the boundary of the mask in example 2 does not lead to the appearance of significant visible “ghosting” in the DC order intensity profile, as shown in Figure 8B. In particular, compared to Figure 7B, there are no repeating vertically extending DC order tails in the reconstructed image. The inventors believe this may due to the fact that the apodised edge of the mask only partially blocking light from illuminating small regions near the edge of the display area, thereby improving the uniformity of illumination across the display area compared to example 1. In addition, since the “softened” boundary edge in example 2 is a straight line, the apodisation itself may not cause additional diffractive effects that may be associated with the geometry of the apodised edge in example 1 . As with example 1, the inventors propose the use of an optimisation algorithm to tailor the apodisation / change in transmittivity across the width of the edge of the mask to balance the conflicting requirements to: (i) “soften” the abrupt change in transmissivity at the boundary between the masked / non-transmission and unmasked / transmission areas defining the aperture of the mask., thus “attenuating” the edge to the illumination light field in order to reduce the “DC order tails”, and (ii) uniformly illuminate the display area (i.e., active pixel area) to prevent “banding”. In particular, it is envisaged that the width of the edge of the boundary, having variable transmissivity, may be optimised to be sufficiently small to minimise non-uniformities in illumination whilst being sufficiently large to soften the boundary edge. The transmissivity profile, including the rate of change of transmissivity, across the width of the edge may be optimised accordingly. As the skilled person will appreciate, the alignment of the apodised edge of the mask with the perimeter of the display area (e.g. active pixel area) may also be optimised for application requirements, considering factors such as spatial separation between the respective planes of the mask and display area and orientation, including the angle of illumination of light on the display device and any tilt of the plane of the display device, which affect the attenuation of the edge of the illumination light field / wavefront. As the skilled person will appreciate, other methods for apodising the edge of the mask are possible and contemplated. In addition, different apodising techniques may be used in combination to achieve the optimisations in image quality as described herein. Accordingly, the inventors have found that apodising or “softening” the mask edge / boundary has an unexpected beneficial effect. In examples 1 and 2, the apodised mask may (i) allow a small amount of illumination light to be incident on, and reflected by, the non-pixel area to pass through the aperture to the replay plane, and / or (ii) reduce the intensity of the holographic / modulated content passing through the aperture to the replay plane, as result of attenuation of the illumination light at the edge of the active pixel area. Both (i) and (ii) would be expected to reduce the signal to noise ratio at the replay field, by either increasing the DC order light or reducing the spatial modulated light. In other words, the proposed approach of apodising the boundary would be expected to make the DC order problem worse. Instead, the inventors surprisingly found that apodising the mask reduces the artefacts in the image, by reducing the spatial reach or extension of the DC spot due to the “DC order tails”. It is also surprising that the centre of the image (where the DC spot occurs) is affected so much by the boundary at the edge of the hologram. As such, a spatial light filter having a Gaussian hole advantageously improves the holographic pictures as seen by a viewer. This is because the image artifacts with conventional spatial light filters are substantially removed. Additional features The methods and processes described herein may be embodied on a computer-readable medium. The term “computer-readable medium” includes a medium arranged to store data temporarily or permanently such as random-access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" shall also be taken to include any medium, or combination of multiple media, that is capable of storing instructions for execution by a machine such that the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methodologies described herein, in whole or in part. The term "computer-readable medium" also encompasses cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example form of a solid-state memory chip, an optical disc, a magnetic disc, or any suitable combination thereof. In some example embodiments, the instructions for execution may be communicated by a carrier medium. Examples of such a carrier medium include a transient medium (e.g., a propagating signal that communicates instructions). It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. The present disclosure covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

1. A holographic projector arranged to form a holographic reconstruction of a picture on a replay plane, wherein the holographic projector comprises:a spatial light modulator arranged to display a hologram of the picture and spatiallymodulate light in accordance with the hologram;a mask comprising a transmission area arranged in cooperation with a display area of the spatial light modulator and a non-transmission area arranged in cooperation with a non-display area of the spatial light modulator adjacent the display area,wherein at least a portion of a boundary of the mask between the transmission areaand non-transmission area is apodised to reduce a diffractive effect, wherein the at least one portion of the boundary of the mask has a shape that variesnon-linearly along a length or across a width thereof.15 2. A holographic projector as claimed in claim 1 wherein the shape of the boundary isperiodically varying with distance along its length, such as in the shape of a waveform.Cd3. A holographic projector as claimed in claim 1 or 2 wherein the boundary extends in and out of the transmission area that it bounds.

4. A holographic projector as claimed in claim 1 wherein the characteristic is an optical characteristic of the boundary.

5. A holographic projector as claimed in claim 4 wherein the optical characteristic is 25 transmissivity.

6. A holographic projector as claimed in claim 4 or 5 wherein the transmissivity of the mask gradually (non-abruptly) changes across the boundary, optionally wherein the transmissivity changes across the boundary from a minimum value, corresponding to the 30 transmissivity of the non-transmission area, to a maximum value corresponding to the transmissivity of the transmission area.

7. A holographic projector as claimed in claim 6 wherein an edge of the nontransmission area of the mask at the boundary has a width, wherein the transmissivity 35 changes with distance across the width of the edge.

8. A holographic projector as clamed in claim 6 or 7 wherein the transmission profile across the width of the boundary is continuous or discontinuous, optionally a continuous linear or non-linear profile such as ramped or continuously curved or a discontinuous profile such as a stepped profile.

9. A holographic projector as claimed in any preceding claim wherein the non-pixel area of the spatial light modulator at least partially surrounds the pixel area, such that the nontransmission area defines an aperture for light, optionally wherein the aperture is a quadrilateral shape.

10. A holographic projector as claimed in any preceding claim wherein the boundary is apodised along substantially the whole of its length.

11. A holographic projector as claimed in any preceding claim wherein boundary of the 15 mask comprises an apodised edge of the non-transmission area, optionally wherein the edge has a width in the range of 1 to 250 microns or up to the width of 4 to 10 pixels of theCMspatial light modulator.

12. A holographic projector as claimed in any preceding claim wherein the spatial light20 modulator is a reflective spatial light modulator such that the mask defines an entrance aperture and an exit aperture for light.CM13. A holographic projector as claimed in any preceding claim wherein the spatial light modulator is a phase modulator.

14. A holographic projector as claimed in any preceding claim wherein the spatial light modulator is a Liquid Crystal on Silicon, “LCOS”, spatial light modulator.

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