Oversized LCOS cover glass

Abstract

A method of holographic reconstruction / projection comprises a first step of displaying a hologram of a picture on a spatial light modulator (SLM). The spatial light modulator comprises a pixel array 510 and a cover glass 530 wherein the area of the cover glass is greater than the area of the pixel array. The method comprises a second step of illuminating the hologram with light. A footprint 520 of the light on the spatial light modulator overfills the pixel array, yet underfills the cover glass such that the footprint does not impinge an edge of the cover glass. A spread of a DC component of the holographic reconstruction is reduced.

Description

FIELD The present disclosure relates to a display device such as a spatial light modulator e.g. liquid crystal on silicon spatial light modulator. More specifically, the present disclosure relates to a display device having an oversized cover glass. The present disclosure also relates to a method of holographic projection and a method of reducing the size or spread of a DC component of a holographic reconstruction. BACKGROUND AND INTRODUCTION Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or “hologram”, comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object. Computer-generated holography may numerically simulate the interference process. A computer-generated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. A Fourier hologram may be considered a Fourier domain / plane representation of the object or a frequency domain / plane representation of the object. A computer-generated hologram may also be calculated by coherent raytracing or a point cloud technique, for example. A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micromirrors, for example. A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, “HUD”. SUMMARY Aspects of the present disclosure are defined in the appended independent claims. There is disclosed herein a method of holographic reconstruction of a picture. The method comprises a first step of displaying a hologram of the picture on a spatial light modulator. The spatial light modulator comprises a pixel array and a cover glass. An area of the cover glass is greater than an area of the pixel array. The method comprises a second step of illuminating the hologram with light. A footprint of the light on the spatial light modulator overfills the pixel array. A footprint of the light on the spatial light modulator underfills the cover glass. The footprint of the light does not impinge an edge of the cover glass. A spread of a DC component of the picture is reduced. The first step may be performed before the second step. The term “overfill” is used herein to indicate that an area of the footprint of light on the pixel array of the spatial light modulator is greaterthan an area of the pixel area. That is, the footprint extends beyond the pixel array in at least one direction. Some of the light illuminating the pixel array therefore illuminates outside of the pixel array. The term “overfill” therefore means that all the pixels of the pixel array are illuminated and some of the area outside or beyond the pixel array is also illuminated. The reader will be familiar with the idea that “overfilling” the pixel array with light ensures that all pixels are fully or entirely illuminated. By way of example only, the pixel array may be rectangular and the footprint of light may be circular. The pixel array and footprint may be concentric. In this example, the “overfill” is irregular. That is, a boundary or perimeter of the footprint may get closer to a boundary or perimeter of the pixel array at the corners of the pixel array or may even be coincident at the corners. However, it may still be said that the perimeter of the pixel array is generally or substantively overfilled. In other examples, the pixel array and footprint are concentric and have the same shape. The footprint is larger than the pixel array in order to achieve the “overfill”. In these cases, the reader will understand that the “overfill” is uniform or constant around the perimeter of the pixel array. The term “overfill” therefore includes regular and irregular overfill around the perimeter of the pixel array. In contrast, the term “underfill” is used herein to indicate that an area of the footprint of light illuminating the spatial light modulator is less than an area of the cover glass. More specifically, an area of the footprint of light illuminating the spatial light modulator is less than the surface area of the cover glass that receives the light. The term “underfill” therefore means that some but not all of the cover glass is illuminated. It may be said that a first subarea of the cover glass is illuminated and a second sub-area of the cover glass is not illuminated, wherein the first sub-area and second sub-area are non-overlapping. The reader will understand from the detailed description that follows that, notably, an edge of the cover glass is not substantively illuminated because the inventors have identified that such an overlap causes a feature referred to herein as “DC tails”. The term “underfill” is therefore used herein to reflect that the footprint substantively falls within the boundary of the cover glass. There is no substantive illumination of the edge of the cover glass. In this respect, it is said that the cover glass is “oversized” to indicate that it is larger than normal or absolutely necessary to receive the light. The person skilled in the art will understand that the intensity of the light illuminating the spatial light modulator may change (e.g. gradually decrease) with radial distance from the centre of the light footprint. That is, the spatial profile of the footprint may not necessarily have a “top-hat” shape. For example, the footprint may have a Gaussian spatial profile. In accordance with these embodiments, a perimeter or boundary of the footprint may be defined as bounding or defining the area containing 90% of the optical energy of the light. Accordingly, the footprint “overfills” the pixel array if the boundary or perimeter of the pixel array is inscribed inside the boundary of the footprint containing 90% of the optical energy. Likewise, the footprint “underfills” the cover glass if this boundary of the footprint is inscribed inside the shape defined by the perimeter or edge of the cover glass. The pixel area may be overfilled by more than necessary to ensure illumination of all pixels of the pixel array. In theory, a boundary of the footprint may coincide with a boundary of the cover glass. However, in a real-world scenario, the shapes of the pixel area and light footprint are not perfect complementary and it is necessary to overfill to ensure adequate illumination of all pixels. That is, in these embodiments, there is some light outside the boundary of the pixel array. The term “more than necessary” is therefore used herein to indicate that all pixels are adequately illuminated (i.e. receive a light level above a threshold for proper operation) but the illumination area (i.e. footprint) is slightly increased beyond this point to provide a separation between the boundary of the pixel area and that of the footprint. In some embodiments, a boundary of the light footprint may be spatially separated from a boundary of the pixel area. The light footprint is, of course, larger than the pixel area in accordance with the concept of “overfilling”. The pixel area may be overfilled such that more than 20%, such as more than 30% or more than 40%, of the light energy of the footprint falls outside the pixel area. The pixel area may be overfilled around its entire perimeter. That is, a boundary of the footprint is spatially separated from the boundary of the pixel area at all points around the pixel area. In alternative embodiments, the boundary of the footprint coincides with the boundary of the pixel area at the four corners of the pixel array - for example, when the pixel area is square or rectangular and the footprint is circular or elliptical - but is spatially separated at all other points. In these alternative embodiments, it may therefore be said that the pixel area is substantially overfilled around its entire perimeter-such as around at least 95% of its perimeter. In some embodiments, it may be said that the boundary of the pixel area is inscribed inside the boundary of the footprint. The pixel area may be overfilled around its entire perimeter by at least 2 mm such as at least 3 or 4 mm. A shape of the footprint may be different to that of the pixel array. For example, the pixel array may be rectangular and / or the footprint may be circular or elliptical. Alternatively or additionally, the cover glass may be rectangular or circular. The method may further comprise the step of collimating the light prior to illuminating the spatial light modulator. The pixel array may be surrounded by a mask that absorbs or blocks light that falls outside the pixel array. The method may further comprise focusing the output light of the spatial light modulator using a lens to form the picture. There is also disclosed herein a spatial light modulator comprising a pixel area and a cover glass. An area of the cover glass is greater than that of the pixel area. An area of the cover glass is at least 1.2 such as at least 1.5 or 2.5 times that of the pixel area. Accordingly, the pixel area can be overfilled when illuminated without impinging an edge of the cover glass. There is further disclosed herein a holographic projector comprising the spatial light modulator arranged to display a hologram of a picture and form a holographic reconstruction of the picture on a replay plane spatially separated from the spatial light modulator. A spread of the DC component of the picture is reduced owing to the size of the area of the cover glass. In some embodiments, at least one wavefront replicator is used. In these embodiments, the term “replica” may be used to reflect that spatially modulated light is divided such that a complex light field is directed along a plurality of different optical paths. The word “replica” is used to refer to each occurrence or instance of the complex light field after a replication event - such as a partial reflection-transmission by a pupil expander. Each replica travels along a different optical path. Some embodiments of the present disclosure relate to propagation of light that is encoded with a hologram, not an image - i.e., light that is spatially modulated with a hologram of an image, not the image itself. It may therefore be said that a plurality of replicas of the hologram are formed. The person skilled in the art of holography will appreciate that the complex light field associated with propagation of light encoded with a hologram will change with propagation distance. Use herein of the term “replica” is independent of propagation distance and so the two branches or paths of light associated with a replication event are still referred to as “replicas” of each other even if the branches are a different length, such that the complex light field has evolved differently along each path. That is, two complex light fields are still considered “replicas” in accordance with this disclosure even if they are associated with different propagation distances - providing they have arisen from the same replication event or series of replication events. A “diffracted light field” or “diffractive light field” in accordance with this disclosure is a light field formed by diffraction. A diffracted light field may be formed by illuminating a corresponding diffractive pattern. In accordance with this disclosure, an example of a diffractive pattern is a hologram and an example of a diffracted light field is a holographic light field or a light field forming a holographic reconstruction of an image. The holographic light field forms a (holographic) reconstruction of an image on a replay plane. The holographic light field that propagates from the hologram to the replay plane may be said to comprise light encoded with the hologram or light in the hologram domain. A diffracted light field is characterized by a diffraction angle determined by the smallest feature size of the diffractive structure and the wavelength of the light (of the diffracted light field). In accordance with this disclosure, it may also be said that a “diffracted light field” is a light field that forms a reconstruction on a plane spatially separated from the corresponding diffractive structure. An optical system is disclosed herein for propagating a diffracted light field from a diffractive structure to a viewer. The diffracted light field may form an image. The term “hologram” is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, regarding the object. The term “holographic reconstruction” is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The system disclosed herein is described as a “holographic projector” because the holographic reconstruction is a real image and spatially-separated from the hologram. The term “replay field” is used to refer to the 2D area within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of a plurality diffracted orders wherein each diffracted order is a replica of the zeroth-order replay field. The zeroth-order replay field generally corresponds to the preferred or primary replay field because it is the brightest replay field. Unless explicitly stated otherwise, the term “replay field” should be taken as referring to the zeroth-order replay field. The term “replay plane” is used to refer to the plane in space containing all the replay fields. The terms “image”, “replay image” and “image region” refer to areas of the replay field illuminated by light of the holographic reconstruction. In some embodiments, the “image” may comprise discrete spots which may be referred to as “image spots” or, for convenience only, “image pixels”. The terms “encoding”, “writing” or “addressing” are used to describe the process of providing the plurality of pixels of the SLM with a respective plurality of control values which respectively determine the modulation level of each pixel. It may be said that the pixels of the SLM are configured to “display” a light modulation distribution in response to receiving the plurality of control values. Thus, the SLM may be said to “display” a hologram and the hologram may be considered an array of light modulation values or levels. It has been found that a holographic reconstruction of acceptable quality can be formed from a “hologram” containing only phase information related to the Fourier transform of the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography. The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully-complex computer-generated hologram is calculated. Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for “phase-delay”. That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2ti) 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 2A 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 2B shows an intensity plot of a reconstructed image of a blank hologram displayed on a spatial light modulator with the conventional mask of Figure 2A; Figure 3 shows the DC component of an example holographic reconstruction when insufficient DC filtering has been used; Figure 4 shows an overfilled pixel array; Figures 5A and 5B represent embodiments of the present disclosure using an oversized cover glass and an overfilled pixel array; Figure 6 shows the DC component formed by a rectangular beam; Figure 7 shows the DC component formed by a circular beam; Figure 8 shows an example replay field in accordance with the prior art; and Figure 9 shows the corresponding replay field in accordance with the present disclosure. The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF EMBODIMENTS The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration. Terms of a singular form may include plural forms unless specified otherwise. A structure described as being formed at an upper portion / lower portion of another structure or on / under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between. In describing a time relationship - for example, when the temporal order of events is described as “after”, “subsequent”, “next”, “before” or suchlike - the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as “just”, “immediate” or “direct” is used. Although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims. Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in 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 lightmodulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to optics including a Fourier transform lens 120, having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives a beam of modulated light from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at the screen 125. Notably, in this type of holography, each pixel of the hologram contributes to the whole reconstruction. There is not a one-to-one correlation between specific points (or image pixels) on the replay field and specific light-modulating elements (or hologram pixels). In other words, modulated light exiting the light-modulating layer is distributed across the replay field. In these embodiments, the position of the holographic reconstruction in space is determined by the dioptric (focusing) power of the Fourier transform lens. In the embodiment shown in Figure 1, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens but the performance of the lens will limit the accuracy of the Fourier transform it performs. The skilled person understands how to use a lens to perform an optical Fourier transform In some embodiments of the present disclosure, the lens of the viewer’s eye performs the hologram to image transformation. Hologram calculation In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or Fourier-based hologram, in which an image is reconstructed in the far field by utilising the Fourier transforming properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back, to the lens plane. Computer-generated Fourier holograms may be calculated using Fourier transforms. Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms which may be calculated by a similar method. In some embodiments, the hologram is a phase or phase-only hologram. However, the present disclosure is also applicable to holograms calculated by other techniques such as those based on point cloud methods. In some embodiments, the hologram engine is arranged to exclude from the hologram calculation the contribution of light blocked by a limiting aperture of the display system. British patent application 2101666.2, filed 5 February 2021 and incorporated herein by reference, discloses a first hologram calculation method in which eye-tracking and ray tracing are used to identify a sub-area of the display device for calculation of a point cloud hologram which eliminates ghost images. The sub-area of the display device corresponds with the aperture, of the present disclosure, and is used exclude light paths from the hologram calculation. British patent application 2112213.0, filed 26 August 2021 and incorporated herein by reference, discloses a second method based on a modified Gerchberg-Saxton type algorithm which includes steps of light field cropping in accordance with pupils of the optical system during hologram calculation. The cropping of the light field corresponds with the determination of a limiting aperture of the present disclosure. British patent application 2118911.3, filed 23 December 2021 and also incorporated herein by reference, discloses a third method of calculating a hologram which includes a step of determining a region of a so-called extended modulator formed by a hologram replicator. The region of the extended modulator is also an aperture in accordance with this disclosure. In some embodiments, there is provided a real-time engine arranged to receive image data and calculate holograms in real-time using the algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are pre-calculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms. Large field of view using small display device Broadly, the present disclosure relates to image projection. It relates to a method of image projection and an image projector which comprises a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system, in which the image projector projects or relays light from the display device to the viewing system. The present disclosure is equally applicable to a monocular and binocular viewing system. The viewing system may comprise a viewer’s eye or eyes. The viewing system comprises an optical element having optical power (e.g., lens / es of the human eye) and a viewing plane (e.g., retina of the human eye / s). The projector may be referred to as a ‘light engine’. The display device and the image formed (or perceived) using the display device are spatially separated from one another. The image is formed, or perceived by a viewer, on a display plane. In some embodiments, the image is a virtual image and the display plane may be referred to as a virtual image plane. In other examples, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed either in free space or on a screen or other light receiving surface between the display device and the viewer, is propagated to the viewer. In both cases, an image is formed by illuminating a diffractive pattern (e.g., hologram or kinoform) displayed on the display device. The display device comprises pixels. The pixels of the display may display a diffractive pattern or structure that diffracts light. The diffracted light may form an image at a plane spatially separated from the display device. In accordance with well-understood optics, the magnitude of the maximum diffraction angle is determined by the size of the pixels and other factors such as the wavelength of the light. In embodiments, the display device is a spatial light modulator such as liquid crystal on silicon (“LCOS") spatial light modulator (SLM). Light propagates over a range of diffraction angles (for example, from zero to the maximum diffractive angle) from the LCOS, towards a viewing entity / system such as a camera or an eye. In some embodiments, magnification techniques may be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of an LCOS. In some embodiments, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) - that may be informally said to be “encoded” with / by the hologram - is propagated directly to the viewer’s eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction I image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device. Reference is made herein to a “light field” which is a “complex light field”. The term “light field” merely indicates a pattern of light having a finite size in at least two orthogonal spatial directions, e.g. x and y. The word “complex” is used herein merely to indicate that the light at each point in the light field may be defined by an amplitude value and a phase value, and may therefore be represented by a complex number or a pair of values. For the purpose of hologram calculation, the complex light field may be a two-dimensional array of complex numbers, wherein the complex numbers define the light intensity and phase at a plurality of discrete locations within the light field. In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity / system, varies with the distance between the display device and the viewing entity. At a 1 metre viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye’s pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye’s pupil to form an image at the retina for a given eye position, determines the portion of the image that is ‘visible’ to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-box.) In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device - that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it may therefore be considered that the viewer is looking at a virtual image through an ‘display device-sized window’, which may be very small, for example 1cm in diameter, at a relatively large distance, e.g., 1 metre. And the user will be viewing the display device-sized window via the pupil(s) of their eye(s), which can also be very small. Accordingly, the field of view becomes small and the specific angular range that can be seen depends heavily on the eye position, at any given time. A pupil expander addresses the problem of how to increase the range of angles of light rays that are propagated from the display device that can successfully propagate through an eye’s pupil to form an image. The display device is generally (in relative terms) small and the projection distance is (in relative terms) large. In some embodiments, the projection distance is at least one - such as, at least two - orders of magnitude greater than the diameter, or width, of the entrance pupil and / or aperture of the display device (i.e., size of the array of pixels). Use of a pupil expander increases the viewing area (i.e., user’s eye-box) laterally, thus enabling some movement of the eye / s to occur, whilst still enabling the user to see the image. As the skilled person will appreciate, in an imaging system, the viewing area (user’s eye box) is the area in which a viewer’s eyes can perceive the image. The present disclosure encompasses non-infinite virtual image distances - that is, near-field virtual images. Conventionally, a two-dimensional pupil expander comprises one or more one-dimensional optical waveguides each formed using a pair of opposing reflective surfaces, in which the output light from a surface forms a viewing window or eye-box. Light received from the display device (e.g., spatially modulated light from a LCOS) is replicated by the or each waveguide so as to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide enlarges the viewing window due to the generation of extra rays or “replicas” by division of amplitude of the incident wavefront. The display device may have an active or display area having a first dimension that may be less than 10 cms such as less than 5 cms or less than 2 cms. The propagation distance between the display device and viewing system may be greater than 1 m such as greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide may be up to 2 m such as up to 1.5 m or up to 1 m. The method may be capable of receiving an image and determining a corresponding hologram of sufficient quality in less than 20 ms such as less than 15 ms or less than 10 ms. In some embodiments - described only by way of example of a diffracted or holographic light field in accordance with this disclosure - a hologram is configured to route light into a plurality of channels, each channel corresponding to a different part (i.e. sub-area) of an image. The channels formed by the diffractive structure are referred to herein as “hologram channels” merely to reflect that they are channels of light encoded by the hologram with image information. It may be said that the light of each channel is in the hologram domain rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram and the hologram domain is therefore the Fourier or frequency domain. The hologram may equally be a Fresnel or Fresnel transform hologram. The hologram may also be a point cloud hologram. The hologram is described herein as routing light into a plurality of hologram channels to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-areas, wherein each hologram channel would correspond to each image sub-area. Importantly, the hologram of this example is characterised by how it distributes the image content when illuminated. Specifically and uniquely, the hologram divides the image content by angle. That is, each point on the image is associated with a unique light ray angle in the spatially modulated light formed by the hologram when illuminated - at least, a unique pair of angles because the hologram is two-dimensional. For the avoidance of doubt, this hologram behaviour is not conventional. The spatially modulated light formed by this special type of hologram, when illuminated, may be divided into a plurality of hologram channels, wherein each hologram channel is defined by a range of light ray angles (in two-dimensions). It will be understood from the foregoing that any hologram channel (i.e. sub-range of light ray angles) that may be considered in the spatially modulated light will be associated with a respective part or sub-area of the image. That is, all the information needed to reconstruct that part or sub-area of the image is contained within a sub-range of angles of the spatially modulated light formed from the hologram of the image. When the spatially modulated light is observed as a whole, there is not necessarily any evidence of a plurality of discrete light channels. Nevertheless, the hologram may still be identified. For example, if only a continuous part or sub-area of the spatially modulated light formed by the hologram is reconstructed, only a sub-area of the image should be visible. If a different, continuous part or sub-area of the spatially modulated light is reconstructed, a different sub-area of the image should be visible. A further identifying feature of this type of hologram is that the shape of the cross-sectional area of any hologram channel substantially corresponds to (i.e. is substantially the same as) the shape of the entrance pupil although the size may be different - at least, at the correct plane for which the hologram was calculated. Each light I hologram channel propagates from the hologram at a different angle or range of angles. Whilst these are example ways of characterising or identifying this type of hologram, other ways may be used. In summary, the hologram disclosed herein is characterised and identifiable by how the image content is distributed within light encoded by the hologram. Again, for the avoidance of any doubt, reference herein to a hologram configured to direct light or angularly-divide an image into a plurality of hologram channels is made by way of example only and the present disclosure is equally applicable to pupil expansion of any type of holographic light field or even any type of diffractive or diffracted light field. The system can be provided in a compact and streamlined physical form. This enables the system to be suitable for a broad range of real-world applications, including those for which space is limited and real-estate value is high. For example, it may be implemented in a head-up display (HUD) such as a vehicle or automotive HUD. In accordance with the present disclosure, pupil expansion is provided for diffracted or diffractive light, which may comprise diverging ray bundles. The diffracted light field may be defined by a “light cone”. Thus, the size of the diffracted light field (as defined on a two-dimensional plane) increases with propagation distance from the corresponding diffractive structure (i.e. display device). It can be said that the pupil expander / s replicate the hologram or form at least one replica of the hologram, to convey that the light delivered to the viewer is spatially modulated in accordance with a hologram. In some embodiments, two one-dimensional waveguide pupil expanders are provided, each one-dimensional waveguide pupil expander being arranged to effectively increase the size of the exit pupil of the system by forming a plurality of replicas or copies of the exit pupil (or light of the exit pupil) of the spatial light modulator. The exit pupil may be understood to be the physical area from which light is output by the system. It may also be said that each waveguide pupil expander is arranged to expand the size of the exit pupil of the system. It may also be said that each waveguide pupil expander is arranged to expand / increase the size of the eye box within which a viewer’s eye can be located, in order to see / receive light that is output by the system. Light channelling The hologram formed in accordance with some embodiments, angularly-divides the image content to provide a plurality of hologram channels which may have a cross-sectional shape defined by an aperture of the optical system - as described in British patent 2,603,518 for example which is incorporated herein in its entirety by reference. Two-Dimensional Pupil Expansion Embodiments of the present disclosure may be used with an optical system providing two-dimensional pupil expansion using a pair of orthogonal pupil expanders (or wavefront replicators) - as described in British patent 2,614,286 for example which is incorporated herein in its entirety by reference. 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 2,936,252 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 2,936,252 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 2,607,899, incorporated herein in its entirety 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 When a collimated beam hits the spatial light modulator (e.g. LCOS), a portion of light is not modulated and will form a very bright DC spot in or near the image plane. This DC spot must be removed as otherwise it will appear in the middle of the image and affect user experience. Removing DC spot usually comes with a cost of losing some contents in the image as the spot has a finite size. Generally, the larger the size of the DC spot, the more content is lost. Therefore, minimizing the size of the spot is desirable. The terms “DC order”, “DC component”, “DC spot”, “DC beam” and “DC cross” are used herein interchangeably to refer to the same phenomenon. When the active area of the LCOS is properly masked, most of the energy in the DC component comes from the direct reflection off the front cover glass of the LCOS. The rectangular aperture of the LCOS will shape the DC beam to a rectangular cross profile and after focused by a lens a clear cross-shape DC spot is formed at the centre of the image. Depending on the illumination energy and the amount of image content in the reconstruction, the DC cross can sometimes be very pronounced. Figures 2A and 2B are reproduced from British patent application 2311246.9 of 21 July 2023 which relates to masking the pixel array of a spatial light modulator in accordance with this disclosure. Figure 2A 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 2B 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 2B 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. Apodised Aperture Mask The central, bright spot of the DC component can be removed by spatial filtering or blocking a small region at the centre of the holographic replay field. However, insufficient filtering will leave residual tails in the image. Figure 3 shows residual tails resulting from inadequate DC filtering. These long horizontal and vertical tails of the DC component are difficult to remove without sacrificing image content. The observed “DC order tails” are a consequence of the diffractive effect caused at the boundary between the transmission and non-transmission areas of the aperture mask. Accordingly, an approach of “apodising” a portion of the quadrilateral boundary of the aperture mask was disclosed in the aforementioned British patent application. British patent application 2311246.9 of 21 July 2023 discloses an apodised mask arranged in cooperation with the display area, such as an active pixel area (or “pixel array”), of the spatial light modulator in order to suppress the DC order / component of a holographic reconstruction. Oversized cover glass There is disclosed herein an alternative solution to the image quality problem caused by the DC component - in particular, the “DC order tails” - of the holographic reconstruction. The tails of the DC cross come from the rectangular aperture of the LCOS or mask thereof. One way to mitigate the tails is to change the shape and profile of the unmodulated beam. In most scenarios, illumination is obtained by collimating a laser with a positive lens, and the beam often has a circular cross section. To minimize dark areas in the image, the diameter of the beam leaving the lens must be at least as large as the diagonal of the LCOS. The beam hitting the region outside the rectangular area is usually blocked or absorbed. If the size of the cover glass is extended to be larger than the illumination spot, then the reflected beam will maintain its circular cross section rather than being cropped by the LCOS aperture. In this case, the DC beam will have a circular shape as shown in Figure 4. In more detail, Figure 4 shows the rectangular aperture 410 of a spatial light modulator (e.g. LCOS). The rectangular aperture 410 defines the area of the pixel array. In some embodiments, the rectangular aperture corresponds to the boundary of a mask around the pixel array. Figure 4 also shows a circular spot of light 420 illuminating the rectangular aperture 410. The circular spot of light 420 is larger than the rectangular aperture 410. It may therefore be said that the light overfills the rectangular aperture 410 or pixel array of the display device. All pixels of the pixel array are illuminated. A significant area outside the pixel array is also illuminated. The circular spot of light 420 and rectangular aperture are concentric in this example. Figures 5A and 5B show the cover glass 530 of the display device. The presence of a cover glass in a display device, such as a liquid crystal display device, will be familiar to the person skilled in the art of display. Figures 5A and 5B show the pixel array 510 of the display device, the illumination light spot (or “footprint”) 520 and cover glass 530. As described in relation to Figure 4, the illumination light spot 520 is larger than the pixel array 510. In this example, the pixel array 510 is rectangular and the illumination light spot 520 is circular. The pixel array is “overfilled” with light meaning that all pixels are illuminated and an area outside the pixel array 510 is also illuminated. In this example, the cover glass 530 is also circular. Notably, the cover glass is larger (in area) than the pixel array 510 and larger (in area or diameter) than the illumination light spot 520. The illumination light spot does not therefore impinge or illuminate a boundary or perimeter or edge of the cover glass 530. In this example, the pixel array 510, illumination light spot 520 and cover glass 530 are concentric but that is not an essential feature of this disclosure. It may be said that the cover glass of the display device is “oversized” or “extended” to reflect that it is larger than the pixel array or, more specifically, larger than necessary to cover the pixel array. This is unconventional because extending (i.e. increasing the size of) the cover glass causes more optical energy to be directed into the DC component. Conventional approaches to the problem caused by the DC component are generally directed to reducing the amount of optical energy in the DC component. However, the inventor has provided an approach which increases the amount of energy of the DC component in order to enhance a destructive interference process which effectively erodes the DC tails. In accordance with the arrangement of the present disclosure, the inventors found that the circular DC beam forms a round spot with a Bessel function profile rather than a cross, with no horizontal or vertical tails. The removal of the tails in the DC spot is advantageous as this reduces the loss content during DC filtering. Another advantage is that extending the DC beam of any shape will further compress (shrink) the DC spot, regardless of what shape the DC beam has. The extended cover glass does not have to be circular, as long as it is larger than the spot size of the illumination beam. In addition, if the incoming illumination beam has a soft edge, such as a Gaussian beam with gradually declining intensity (i.e. apodizing edge), the size of the DC spot will be even smaller as higher order of the Bessel profile will be further suppressed. The DC spots from rectangular and circular beams are shown in Figures 6 and 7, respectively. Further experimental results are shown in Figures 8 and 9. Figure 8 shows a conventional arrangement, including a conventional size cover glass, in which DC component comprises DC tails. Conventionally, the size of the cover glass may be substantially the same as the pixel array in order to reduce stray light. Figure 9 corresponds to embodiments using an oversized cover glass (i.e. a cover glass larger than necessary to span the pixel array) and an overfilled pixel array (i.e. a significant area outside the pixel array is illuminated - e.g. at least 20% of the optical energy falls outside the pixel area) in which the DC tails of the DC component are reduced (e.g. in size / spread and / or intensity) owing to the enhanced destructive interference process described above. It should be understood that any magnitude of overfill and any magnitude of oversizing of the cover glass will lead to reduction in the visual impact of the DC tails. Notably, all pixels of the pixel array are illuminated but an edge or boundary of the cover glass is not illuminated. This is achieved by the arrangement of features defined in the appended claims. 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 method of holographic reconstruction of a picture, the method comprising: displaying a hologram of the picture on a spatial light modulator comprising a pixel array and a cover glass, wherein an area of the cover glass is greater than an area of the pixel array; andilluminating the hologram with light, wherein a footprint of the light on the spatial light modulator overfills the pixel array but underfills the cover glass such that the footprint of the light does not impinge an edge of the cover glass.

2. A method as claimed in claim 1 wherein the pixel area is overfilled by more than necessary to ensure illumination of all pixels of the pixel array.

3. A method as claimed in any preceding claim wherein the pixel area is overfilled such that more than 20%, such as more than 30% or more than 40%, of the light energy of the footprint falls outside the pixel area.

4. A method as claimed in any preceding claim wherein the pixel area is overfilled around its entire perimeter.

5. A method as claimed in claim 3 wherein the pixel area is overfilled around its entire perimeter by at least 2 mm such as at least 3 or 4 mm.

6. A method as claimed in any preceding claim wherein a shape of the footprint is different to that of the pixel array.

7. A method as claimed in any preceding claim wherein the pixel array is rectangular.

8. A method as claimed in any preceding claim wherein the cover glass is rectangularor circular.

9. A method as claimed in any preceding claim wherein the footprint is circular or elliptical.

10. A method as claimed any preceding claim further comprising collimating the light prior to illuminating the spatial light modulator.

11. A method as claimed in any preceding claim wherein the pixel array is surrounded by a mask that absorbs or blocks light that falls outside the pixel array.

12. A method as claimed in any preceding claim further comprising focusing the output light of the spatial light modulator using a lens to form the picture.

13. A spatial light modulator comprising a pixel area and a cover glass, wherein an area of the cover glass is at least 1.2 such as at least 1.5 or 2.5 times that of the pixel area such that the pixel area is overfillable with light without illuminating an edge of the cover glass.

14. A holographic projector comprising the spatial light modulator of claim 13 arranged to display a hologram of a picture and form a holographic reconstruction of the picture on a replay plane spatially separated from the spatial light modulator, wherein a spread of the DC component of the picture is reduced owing to the size of the cover glass.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:-04 03 25CLAIMS1. A method of holographic reconstruction of a picture, the method comprising: displaying a hologram of the picture on a spatial light modulator comprising a pixel 5 array and a cover glass, wherein an area of the cover glass is greater than an area of the pixel array; andilluminating the hologram with light, wherein a footprint of the light on the spatial light modulator overfills the pixel array but underfills the cover glass such that the footprint of the light does not impinge an edge of the cover glass.

102. A method as claimed in claim 1 wherein the pixel area is overfilled by more than necessary to ensure illumination of all pixels of the pixel array.

3. A method as claimed in any preceding claim wherein the pixel area is overfilled such 15 that more than 20%, such as more than 30% or more than 40%, of the light energy of thefootprint falls outside the pixel area.

4. A method as claimed in any preceding claim wherein the pixel area is overfilled around its entire perimeter.

205. A method as claimed in claim 3 wherein the pixel area is overfilled around its entire perimeter by at least 2 mm such as at least 3 or 4 mm.

6. A method as claimed in any preceding claim wherein a shape of the footprint is 25 different to that of the pixel array.

7. A method as claimed in any preceding claim wherein the pixel array is rectangular.

8. A method as claimed in any preceding claim wherein the cover glass is rectangular30 or circular.

9. A method as claimed in any preceding claim wherein the footprint is circular or elliptical.35 10. A method as claimed any preceding claim further comprising collimating the lightprior to illuminating the spatial light modulator.04 03 2511. A method as claimed in any preceding claim wherein the pixel array is surrounded by a mask that absorbs or blocks light that falls outside the pixel array.

12. A method as claimed in any preceding claim further comprising focusing the output 5 light of the spatial light modulator using a lens to form the picture.

13. A spatial light modulator, for holographic projection of a picture, comprising a pixel area and a cover glass, wherein an area of the cover glass is at least 1.2 that of the pixel area such that the pixel area is overfillable with light without illuminating an edge of the cover 10 glass.

14. A spatial light modulator as claimed in claim 13, wherein the area of the cover glassis at least 1.5 or 2.5 times that of the pixel area.15 15. A holographic projector comprising the spatial light modulator of claim 13 or 14arranged to display a hologram of a picture and form a holographic reconstruction of the picture on a replay plane spatially separated from the spatial light modulator, wherein a spread of the DC component of the picture is reduced owing to the size of the cover glass.

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