Display device and method

Abstract

A display device comprises a spatial light modulator (hologram) 404, a substrate 402, and an attachment material (Aluminium Nitride adhesive) 430 for mounting the spatial light modulator on the substr

Description

FIELD The present disclosure relates to a display device comprising a spatial light modulator and a method for assembling a display device comprising a spatial light modulator. More specifically, the present disclosure relates to a display device comprising a spatial light modulator mounted on a substrate and a method for mounting a spatial light modulator on a substrate. Some embodiments relate to a holographic display system comprising the display device for displaying a hologram of a picture Some embodiments relate to a head-up display. BACKGROUND AND INTRODUCTION Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or "hologram", comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object. Computer-generated holography may numerically simulate the interference process. A computergenerated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. A Fourier hologram may be considered a Fourier domain / plane representation of the object or a frequency domain / plane representation of the object. A computer-generated hologram may also be calculated by coherent ray tracing or a point cloud technique, for example. A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micro-mirrors, for example. A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, "HUD", and head-mounted displays, "HMD", including neareye devices, for example. SUMMARY Aspects of the present disclosure are defined in the appended independent claims. In general terms, a display device and a method of assembling a display device are provided. The display device comprises a spatial light modulator, such as a liquid crystal on silicon spatial light modulator, and a substrate. The spatial light modulator is mounted on a first major surface of the substrate by an attachment material. The properties of the attachment material are chosen to optimise optical performance of the display device over a large range of environmental conditions, such as temperatures associated with automotive applications. In particular, the attachment material may be chosen to have a selected combination of values of the thermal properties: (i) coefficient of thermal expansion and (ii) thermal conductivity. A display device according to the disclosure may be suitable for use in a picture generating unit or projector, such as a holographic projector. For example, the display device may be used in a system - such as a head-up display (HUD) - which is configured to display a picture to a viewer, without requiring the user to look away from their usual, or required, viewpoint. The display device may be particularly suitable for an HUD system comprised within a vehicle. In the context of a picture generating unit or projector, a spatial light modulator is encoded with an image or hologram of an image and is illuminated with light. Thus, the spatial light modulator modulates the amplitude and / or phase of the incident light according to the image or hologram and outputs spatially modulated light. In embodiments described herein, the spatial light modulator is a phase modulator. In some applications, a spatial light modulator is used in a relatively stable environment, for example in a static arrangement that is maintained at a stable temperature and humidity. However, in some applications, spatial light modulators may be used in less stable environments, for example in a portable arrangement, in which the spatial light modulator may experience substantial variations in temperature, humidity, movement and similar environmental changes. A particular example of such an environment is the inside of a vehicle or automobile, in which there may be extreme temperature variations. Components to be used in vehicles must be able to perform acceptably over a wide range of environmental conditions, such as ranges of temperature and humidity. For example, an electrical component located within the dashboard of a vehicle that is exposed to direct sunlight may be required to perform acceptably across a temperature ranging from as low as minus 40 degrees Celsius or lower to as high as 90 degrees Celsius or higher. An electrical component for use in a vehicle may be required to perform acceptably in a humidity ranging from 60 % or less to 90 % or more. The automotive industry typically apply strict testing methods to any electronic components to certify that the component is suitable before it can be used in a vehicle. These tests include thermal shock tests, in which the display device is quickly transferred between two extreme temperatures, such as a low temperature of -40 degrees Celsius and a high temperature of 95 degrees Celsius. The thermal shock test may be repeated for 1000 cycles or more, and this may be called thermal cycling. In addition, test may include thermal storage test, in which the display device is maintained at extreme temperatures, such as -40 degrees Celsius and 95 degrees Celsius, for a long time period such as 24 hours. These tests are far more rigorous than tests typically applied to electrical components not for use in the automotive industry. It is conventional to mount or attach electronic components on a surface of a substrate using an attachment material. For example, an adhesive material such as a thermal paste may be used between the underside of the electronic component (e.g., the lower major surface of a chip or die) and an upper major surface of a substrate in order to securely mount the component on the substrate. It may be said that the adhesive material bonds the electronic component to the substrate. In another example, an adhesive film or so-called "die attach tape" (e.g., a double sided thermoplastic film with an adhesive material formed as an even / uniform thickness layer on opposite sides thereof) may be arranged between the underside of the electronic component and an upper major surface of a substrate in order to securely mount the component on the substrate. In the case of a spatial light modulator, it is important to ensure that the attachment material between the spatial light modulator and the substrate is secure and has a uniform thickness throughout use. This ensures that the light modulating area (e.g., active pixel area) of the spatial light modulator, which is illuminated by light and outputs spatially modulated light as described herein, is held flat in a display plane, parallel to the substate, to achieve the required optical performance, such as image quality. Conventionally, a die attach tape is convenient to use to provide a uniform layer between the lower surface of the die or chip and the upper surface of the substrate so that the display device is held flat with its display plane / light modulation area parallel to the upper major surface of the substrate. The inventors have found that when a spatial light modulator is mounted on a substrate using conventional attachment techniques and subject to significant temperature variations, such as automotive testing, the optical performance is adversely affected. Specifically, the inventors found an adverse affect on optical performance of a display device comprising a spatial light modulator arranged for phase modulation after automotive testing in comparison to before automotive testing. In particular, the inventors observed abnormalities in (i) the output wavefront of light output by the spatial light modulator and (ii) the holographic reconstruction (or replay image) formed, including the presence of artefacts in the form of duplicate image spots (or image pixels) and, thus, reduced image resolution, at the replay plane. The inventors conducted investigations and realised that the abnormalities and artefacts may result from thermal changes to the attachment material during automotive testing. This was surprising because the attachment material used to attach a display device to a substrate is not normally considered relevant to the optical performance of a display device. In particular, the inventors noted that, during the attachment process, pressure is applied to the surface of an attachment material in the form of die attach tape on two occasions, firstly upon placement of the film evenly on top of the substrate, and secondly upon placement of the spatial light modulator die on top of the tape. In both cases a die bonding apparatus may be used to apply a predetermined pressure according to manufacturer recommendations. The application of an optimum pressure is particularly desirable in the case of an attachment material that is pressure sensitive, such as a pressure sensitive acrylic-based adhesive. However, the pressure applied is not perfectly uniform over the surface of attachment area (e.g., there are differences in the amount of pressure applied at the centre than at the edges). This is believed to lead to small non-uniformities in thickness and stress of the attachment material between the die and the substrate across the attachment area due to the changes such as deformation of the attachment material (i.e., thermal paste and / or film) under pressure. Furthermore, some attachment materials (e.g., that are not pressure sensitive) additionally require curing, which may affect this non-uniformity in thickness and stress of the attachment material across its area. Whilst the small non-uniformity in thickness of the attachment material between the spatial light modulator die and the substrate across the attachment area may provide acceptable optical performance under idea conditions, the inventors realised that the extreme temperature changes during automotive testing may cause thermal effects on the attachment material that exacerbate this problem, and / or cause other problems with the attachment material itself, leading to the observed adverse affects on optical performance. In particular, the inventors believe that, due to the thermal properties of the attachment material and the pre-existing small non-uniformities in thickness and stress of the die attach tape across the attachment area, thermal expansion / contraction as a result of temperature changes during automotive testing (e.g., thermal cycling) may exacerbate (i.e., increase) the non-uniformities. This may cause deformation, wrinkling, lifting and / or warping of the film or tape. Furthermore, the inventors believe that thermal expansion / contraction may impart mechanical stress and strain on the spatial light modulator itself, with the possibility of deformation and warping of one or more layers of the die, such as glass or aluminium layers. In consequence, the spatial light modulator, in particular the display plane / light modulation area thereof, may not be completely flat, and may not be parallel to the upper major surface of the substrate. The correct orientation or tilt of a display plane of the spatial light modulator in a display system, as well as a flat surface thereof, is of vital importance in ensuring optimum image quality, especially in holographic display systems, such as highly sensitive phasebased display systems as described herein. The inventors conducted investigations to identify properties of attachment materials that would mitigate at least some of these problems. There is provided a display device comprising a spatial light modulator, a substrate and an attachment material for mounting the spatial light modulator on the substrate. The spatial light modulator comprises opposed first and second major surfaces. The first major surface comprises a display area. The attachment material is positioned between the second major surface of the spatial light modulator and a first major surface of the substrate. The attachment material is selected to have: (i) a coefficient of thermal expansion less than about 200 ppm / °C, optionally less than about 150 ppm / °C, and (ii) a thermal conductivity greater than about 1 W / m-°C, optionally greater than or equal to about 1.4 W / m-’C. 20 08 24 The inventors found that the selection of an attachment material (e.g., adhesive or thermal paste) with a relatively low coefficient of thermal expansion, which would be expected to mitigate problems arising from thermal expansion / contraction due to extreme temperature variations (e.g., thermal cycling), did not alone address the above problems with optical performance following automotive testing. However, the inventors found that the selection of an attachment material with the combination of parameters (i) a (relatively low) coefficient of thermal expansion less than about 200 ppm / °C, and (ii) a (relatively high) thermal conductivity greater than about 1 W / m-°C did not lead to abnormalities in the output wavefront of light output by the spatial light modulator following automotive testing. In embodiments, the spatial light modulator is a phase modulator or an amplitude and phase modulator. In some examples, the spatial light modulator comprises a liquid crystal on silicon (LCOS) spatial light modulator comprising a silicon backplane. Optionally, the thermal properties (e.g., thermal conductivity) of the attachment material is selected to as match the thermal properties of the silicon backplane as closely as possible. In some embodiments, the substrate comprises a ceramic substrate such as an alumina substrate. An alumina substrate has a high thermal conductivity allowing the transfer of heat to and from the display device so as to maintain an optimal operating temperature. The attachment material has been chosen to additionally have a selected value of the property: (iii) hardness. The attachment material is selected to have: (iii) a hardness value less than 65 Shore A, such as in the range 30 to 50 Shore A. The inventors surprisingly found that the selection of an attachment material with the additional property of (ii) a medium soft to soft material, with a Shore A hardness of less than 65, led to reduced the presence of artefacts in the holographic reconstruction (or replay image), such as duplicate image spots (or image pixels) at the replay plane formed by the substrate-mounted spatial light modulator following automotive testing. The inventors expected that a relatively soft attachment material would lead to increased non-uniformity in thickness due to greater differences in the amount of deformation from pressure differences applied across the attachment area during assembly. However, the inventors unexpectedly found that the selection of an attachment material that is medium soft to soft material, but is sufficiently hard for secure attachment, improved the resolution of the image following automotive testing. The inventors believe that this may be due to the fact that the mechanical stress and strain imparted on the spatial light modulator by a softer attachment material as a result of thermal changes during automotive testing is reduced, so that the layers of the spatial light modulator such as glass and aluminium do not deform or warp. However, an attachment material with a hardness less than 65 Shore A and optionally in the range 30 to 50 Shore A is sufficient to provide secure attachment to prevent movement of the spatial light modulator on the substrate after assembly. In addition, some examples may further improve the quality of the holographic reconstruction by an attachment material chosen to additionally have a selected value of the property: (iv) glass transition temperature. In some examples, the attachment material is selected to have: (iii) a glass transition temperature less than -40°C, optionally in the range -40 to -80°C. This ensures that the attachment material does not become hard and brittle within the temperature ranges associated with automotive testing. In some embodiments, the attachment material is an adhesive paste such as a thermal paste. In some examples, the thermal paste comprises a single uniform layer between the upper surface of the substrate and the lower surface of the spatial light modulator. In other examples, the attachment material comprises a double-sided adhesive film, such as a thermoplastic film comprising an adhesive or thermal paste applied uniformly to opposite sides of a film or tape, and the thermoplastic film is arranged between the upper surface of the substrate and the lower surface of the spatial light modulator. The use of a thermoplastic film is convenient for providing a uniform thickness of attachment material across the attachment area. The attachment material may be a pressure sensitive adhesive. There is provided a method of attaching a spatial light modulator to a substrate to form a display device. The method comprises applying a layer of an attachment material to one of a lower surface the spatial light modulator or an upper surface of the substrate. The method further comprise applying a pressure to the upper surface of the spatial light modulator to securely mount the spatial light modulator on the substrate. The attachment material has the following properties: coefficient of thermal expansion less than about 200 ppm / °C, optionally less than about 150 ppm / °C, and (ii) thermal conductivity greater than about 1 W / m-°C, optionally greater than or equal to about 1.4 W / m-’C). 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 2n) 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 n / 2 will retard the phase of received light by ti / 2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term "grey level" may be used to refer to the plurality of available modulation levels. For example, the term "grey level" may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term "grey level" may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator. The hologram therefore comprises an array of grey levels - that is, an array of light modulation values such as an array of phase-delay values or complex modulation values. The hologram is also considered a diffractive pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light having a wavelength comparable to, generally less than, the pixel pitch of the spatial light modulator. Reference is made herein to combining the hologram with other diffractive patterns such as diffractive patterns functioning as a lens or grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the replay field on the replay plane or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a replay plane in the near field. Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged. BRIEF DESCRIPTION OF THE DRAWINGS Specific embodiments are described by way of example only with reference to the following figures: Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen; Figure 2A illustrates a first iteration of an example Gerchberg-Saxton type algorithm; Figure 2B illustrates the second and subsequent iterations of the example Gerchberg-Saxton type algorithm; Figure 2C illustrates alternative second and subsequent iterations of the example Gerchberg-Saxton type algorithm; Figure 3 is a schematic of a reflective LCDS SLM; Figure 4 is a schematic plan view of a display device comprising a spatial light modulator mounted on a substrate; Figure 5 shows a schematic cross section along line X-X of Figure 4; Figure 6A show wavefront measurements of spatially modulated light output from a spatial light modulator mounted on a substrate that has not been exposed to thermal tests, and Figure 6B shows wavefront measurements of spatially modulated light output from a spatial light modulator mounted on a substrate that has been exposed to thermal tests, and Figure 7A-C show measurements of resolution of an image formed by a spatial light modulator mounted on a substrate before exposure to thermal tests, after exposure to low temperature thermal tests and after exposure to high temperature thermal tests, respectively. The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF EMBODIMENTS The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration. Terms of a singular form may include plural forms unless specified otherwise. A structure described as being formed at an upper portion / lower portion of another structure or on / under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between. In describing a time relationship - for example, when the temporal order of events is described as "after", "subsequent", "next", "before" or suchlike - the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as "just", "immediate" or "direct" is used. Although the terms "first", "second", etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims. Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in co-dependent relationship. Optical configuration 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. 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. A Fourier transform hologram may be calculated using an algorithm such as the Gerchberg-Saxton algorithm. Furthermore, the Gerchberg-Saxton algorithm may be used to calculate a hologram in the Fourier domain (i.e. a Fourier transform hologram) from amplitude-only information in the spatial domain (such as a photograph). The phase information related to the object is effectively "retrieved" from the amplitude-only information in the spatial domain. In some embodiments, a computer-generated hologram is calculated from amplitude-only information using the Gerchberg-Saxton algorithm or a variation thereof. The Gerchberg Saxton algorithm considers the situation when intensity cross-sections of a light beam, lA(x, y) and lB(x, y), in the planes A and B respectively, are known and lA(x, y) and lB(x, y) are related by a single Fourier transform. With the given intensity cross-sections, an approximation to the phase distribution in the planes A and B, MJA(x, y) and MJB(x, y) respectively, is found. The Gerchberg-Saxton algorithm finds solutions to this problem by following an iterative process. More specifically, the Gerchberg-Saxton algorithm iteratively applies spatial and spectral constraints while repeatedly transferring a data set (amplitude and phase), representative of lA(x, y) and lB(x, y), between the spatial domain and the Fourier (spectral or frequency) domain. The corresponding computer-generated hologram in the spectral domain is obtained through at least one iteration of the algorithm. The algorithm is convergent and arranged to produce a hologram representing an input image. The hologram may be an amplitude-only hologram, a phase-only hologram or a fully complex hologram. In some embodiments, a phase-only hologram is calculated using an algorithm based on the Gerchberg-Saxton algorithm such as described in British patent 2,498,170 or 2,501,112 which are hereby incorporated in their entirety by reference. However, embodiments disclosed herein describe calculating a phase-only hologram by way of example only. In these embodiments, the Gerchberg-Saxton algorithm retrieves the phase information MJ [u, v] of the Fourier transform of the data set which gives rise to a known amplitude information T[x, y], wherein the amplitude information T[x, y] is representative of a target image (e.g. a photograph). Since the magnitude and phase are intrinsically combined in the Fourier transform, the transformed magnitude and phase contain useful information about the accuracy of the calculated data set. Thus, the algorithm may be used iteratively with feedback on both the amplitude and the phase information. However, in these embodiments, only the phase information MJ[u, v] is used as the hologram to form a holographic representative of the target image at an image plane. The hologram is a data set (e.g. 2D array) of phase values. In other embodiments, an algorithm based on the Gerchberg-Saxton algorithm is used to calculate a fully-complex hologram. A fully-complex hologram is a hologram having a magnitude component and a phase component. The hologram is a data set (e.g. 2D array) comprising an array of complex data values wherein each complex data value comprises a magnitude component and a phase component. In some embodiments, the algorithm processes complex data and the Fourier transforms are complex Fourier transforms. Complex data may be considered as comprising (i) a real component and an imaginary component or (ii) a magnitude component and a phase component. In some embodiments, the two components of the complex data are processed differently at various stages of the algorithm. Figure 2A illustrates the first iteration of an algorithm in accordance with some embodiments for calculating a phase-only hologram. The input to the algorithm is an input image 210 comprising a 2D array of pixels or data values, wherein each pixel or data value is a magnitude, or amplitude, value. That is, each pixel or data value of the input image 210 does not have a phase component. The input image 210 may therefore be considered a magnitude-only or amplitude-only or intensity-only distribution. An example of such an input image 210 is a photograph or one frame of video comprising a temporal sequence of frames. The first iteration of the algorithm starts with a data forming step 202A comprising assigning a random phase value to each pixel of the input image, using a random phase distribution (or random phase seed) 230, to form a starting complex data set wherein each data element of the set comprising magnitude and phase. It may be said that the starting complex data set is representative of the input image in the spatial domain. First processing block 250 receives the starting complex data set and performs a complex Fourier transform to form a Fourier transformed complex data set. Second processing block 253 receives the Fourier transformed complex data set and outputs a hologram 280A. In some embodiments, the hologram 280A is a phase-only hologram. In these embodiments, second processing block 253 quantises each phase value and sets each amplitude value to unity in order to form hologram 280A. Each phase value is quantised in accordance with the phase-levels which may be represented on the pixels of the spatial light modulator which will be used to "display" the phase-only hologram. For example, if each pixel of the spatial light modulator provides 256 different phase levels, each phase value of the hologram is quantised into one phase level of the 256 possible phase levels. Hologram 280A is a phase-only Fourier hologram which is representative of an input image. In other embodiments, the hologram 280A is a fully complex hologram comprising an array of complex data values (each including an amplitude component and a phase component) derived from the received Fourier transformed complex data set. In some embodiments, second processing block 253 constrains each complex data value to one of a plurality of allowable complex modulation levels to form hologram 280A. The step of constraining may include setting each complex data value to the nearest allowable complex modulation level in the complex plane. It may be said that hologram 280A is representative of the input image in the spectral or Fourier or frequency domain. In some embodiments, the algorithm stops at this point. However, in other embodiments, the algorithm continues as represented by the dotted arrow in Figure 2A. In other words, the steps which follow the dotted arrow in Figure 2A are optional (i.e. not essential to all embodiments). Third processing block 256 receives the modified complex data set from the second processing block 253 and performs an inverse Fourier transform to form an inverse Fourier transformed complex data set. It may be said that the inverse Fourier transformed complex data set is representative of the input image in the spatial domain. Fourth processing block 259 receives the inverse Fourier transformed complex data set and extracts the distribution of magnitude values 211A and the distribution of phase values 213A. Optionally, the fourth processing block 259 assesses the distribution of magnitude values 211A. Specifically, the fourth processing block 259 may compare the distribution of magnitude values 211A of the inverse Fourier transformed complex data set with the input image 510 which is itself, of course, a distribution of magnitude values. If the difference between the distribution of magnitude values 211A and the input image 210 is sufficiently small, the fourth processing block 259 may determine that the hologram 280A is acceptable. That is, if the difference between the distribution of magnitude values 211A and the input image 210 is sufficiently small, the fourth processing block 259 may determine that the hologram 280A is a sufficiently-accurate representative of the input image 210. In some embodiments, the distribution of phase values 213A of the inverse Fourier transformed complex data set is ignored for the purpose of the comparison. It will be appreciated that any number of different methods for comparing the distribution of magnitude values 211A and the input image 210 may be employed and the present disclosure is not limited to any particular method. In some embodiments, a mean square difference is calculated and if the mean square difference is less than a threshold value, the hologram 280A is deemed acceptable. If the fourth processing block 259 determines that the hologram 280A is not acceptable, a further iteration of the algorithm may be performed. However, this comparison step is not essential and in other embodiments, the number of iterations of the algorithm performed is predetermined or preset or user-defined. Figure 2B represents a second iteration of the algorithm and any further iterations of the algorithm. The distribution of phase values 213A of the preceding iteration is fed-back through the processing blocks of the algorithm. The distribution of magnitude values 211A is rejected in favour of the distribution of magnitude values of the input image 210. In the first iteration, the data forming step 202A formed the first complex data set by combining distribution of magnitude values of the input image 210 with a random phase distribution 230. However, in the second and subsequent iterations, the data forming step 202B comprises forming a complex data set by combining (i) the distribution of phase values 213A from the previous iteration of the algorithm with (ii) the distribution of magnitude values of the input image 210. The complex data set formed by the data forming step 202B of Figure 2B is then processed in the same way described with reference to Figure 2Ato form second iteration hologram 280B. The explanation of the process is not therefore repeated here. The algorithm may stop when the second iteration hologram 280B has been calculated. However, any number of further iterations of the algorithm may be performed. It will be understood that the third processing block 256 is only required if the fourth processing block 259 is required or a further iteration is required. The output hologram 280B generally gets better with each iteration. However, in practice, a point is usually reached at which no measurable improvement is observed or the positive benefit of performing a further iteration is out-weighted by the negative effect of additional processing time. Hence, the algorithm is described as iterative and convergent. Figure 2C represents an alternative embodiment of the second and subsequent iterations. The distribution of phase values 213A of the preceding iteration is fed-back through the processing blocks of the algorithm. The distribution of magnitude values 211A is rejected in favour of an alternative distribution of magnitude values. In this alternative embodiment, the alternative distribution of magnitude values is derived from the distribution of magnitude values 211 of the previous iteration. Specifically, processing block 258 subtracts the distribution of magnitude values of the input image 210 from the distribution of magnitude values 211 of the previous iteration, scales that difference by a gain factor a and subtracts the scaled difference from the input image 210. This is expressed mathematically by the following equations, wherein the subscript text and numbers indicate the iteration number: Rn+i [x,^] = F'{exp(z>„[M,v])} = ZF{Tj-exp(iZRn[x,y])} mr i / 1 r% r t nrrr t\ rj = T[x,y|-a(|£„[x, v ||^7 | x.y|) where: F' is the inverse Fourier transform; F is the forward Fourier transform; R[x, y] is the complex data set output by the third processing block 256; T[x, y] is the input or target image; Z is the phase component; tp is the phase-only hologram 280B; q is the new distribution of magnitude values 211B; and a is the gain factor. The gain factor a may be fixed or variable. In some embodiments, the gain factor a is determined based on the size and rate of the incoming target image data. In some embodiments, the gain factor a is dependent on the iteration number. In some embodiments, the gain factor a is solely function of the iteration number. The embodiment of Figure 2C is the same as that of Figure 2A and Figure 2B in all other respects. It may be said that the phase-only hologram ip(u, v) comprises a phase distribution in the frequency or Fourier domain. In some embodiments, the Fourier transform is performed using the spatial light modulator. Specifically, the hologram data is combined with second data providing optical power. That is, the data written to the spatial light modulation comprises hologram data representing the object and lens data representative of a lens. When displayed on a spatial light modulator and illuminated with light, the lens data emulates a physical lens - that is, it brings light to a focus in the same way as the corresponding physical optic. The lens data therefore provides optical, or focusing, power. In these embodiments, the physical Fourier transform lens 120 of Figure 1 may be omitted. It is known how to calculate data representative of a lens. The data representative of a lens may be referred to as a software lens. For example, a phase-only lens may be formed by calculating the phase delay caused by each point of the lens owing to its refractive index and spatially-variant optical path length. For example, the optical path length at the centre of a convex lens is greater than the optical path length at the edges of the lens. An amplitude-only lens may be formed by a Fresnel zone plate. It is also known in the art of computer-generated holography how to combine data representative of a lens with a hologram so that a Fourier transform of the hologram can be performed without the need for a physical Fourier lens. In some embodiments, lensing data is combined with the hologram by simple addition such as simple vector addition. In some embodiments, a physical lens is used in conjunction with a software lens to perform the Fourier transform. Alternatively, in other embodiments, the Fourier transform lens is omitted altogether such that the holographic reconstruction takes place in the far-field. In further embodiments, the hologram may be combined in the same way with grating data - that is, data arranged to perform the function of a grating such as image steering. Again, it is known in the field how to calculate such data. For example, a phase-only grating may be formed by modelling the phase delay caused by each point on the surface of a blazed grating. An amplitude-only grating may be simply superimposed with an amplitude-only hologram to provide angular steering of the holographic reconstruction. The second data providing lensing and / or steering may be referred to as a light processing function or light processing pattern to distinguish from the hologram data which may be referred to as an image forming function or image forming pattern. In some embodiments, the Fourier transform is performed jointly by a physical Fourier transform lens and a software lens. That is, some optical power which contributes to the Fourier transform is provided by a software lens and the rest of the optical power which contributes to the Fourier transform is provided by a physical optic or optics. In some embodiments, there is provided a real-time engine arranged to receive image data and calculate holograms in real-time using the algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are precalculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms. Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms which may be calculated by a similar method. The present disclosure is also applicable to holograms calculated by other techniques such as those based on point cloud methods. Light modulation A spatial light modulator may be used to display the diffractive pattern including the computergenerated hologram. If the hologram is a phase-only hologram, a spatial light modulator which modulates phase is required. If the hologram is a fully-complex hologram, a spatial light modulator which modulates phase and amplitude may be used or a first spatial light modulator which modulates phase and a second spatial light modulator which modulates amplitude may be used. In some embodiments, the light-modulating elements (i.e. the pixels) of the spatial light modulator are cells containing liquid crystal. That is, in some embodiments, the spatial light modulator is a liquid crystal device in which the optically-active component is the liquid crystal. Each liquid crystal cell is configured to selectively-provide a plurality of light modulation levels. That is, each liquid crystal cell is configured at any one time to operate at one light modulation level selected from a plurality of possible light modulation levels. Each liquid crystal cell is dynamically-reconfigurable to a different light modulation level from the plurality of light modulation levels. In some embodiments, the spatial light modulator is a reflective liquid crystal on silicon (LCDS) spatial light modulator but the present disclosure is not restricted to this type of spatial light modulator. A LCOS device provides a dense array of light modulating elements, or pixels, within a small aperture (e.g. a few centimetres in width). The pixels are typically approximately 10 microns or less which results in a diffraction angle of a few degrees meaning that the optical system can be compact. It is easier to adequately illuminate the small aperture of a LCOS SLM than it is the larger aperture of other liquid crystal devices. An LCOS device is typically reflective which means that the circuitry which drives the pixels of a LCOS SLM can be buried under the reflective surface. The results in a higher aperture ratio. In other words, the pixels are closely packed meaning there is very little dead space between the pixels. This is advantageous because it reduces the optical noise in the replay field. A LCOS SLM uses a silicon backplane which has the advantage that the pixels are optically flat. This is particularly important for a phase modulating device. A suitable LCOS SLM is described below, by way of example only, with reference to Figure 3. An LCOS device is formed using a single crystal silicon substrate 302. It has a 2D array of square planar aluminium electrodes 301, spaced apart by a gap 301a, arranged on the upper surface of the substrate. Each of the electrodes 301 can be addressed via circuitry 302a buried in the substrate 302. Each of the electrodes forms a respective planar mirror. An alignment layer 303 is disposed on the array of electrodes, and a liquid crystal layer 304 is disposed on the alignment layer 303. A second alignment layer 305 is disposed on the planar transparent layer 306, e.g. of glass. A single transparent electrode 307 e.g. of ITO is disposed between the transparent layer 306 and the second alignment layer 305. Each of the square electrodes 301 defines, together with the overlying region of the transparent electrode 307 and the intervening liquid crystal material, a controllable phase-modulating element 308, often referred to as a pixel. The effective pixel area, or fill factor, is the percentage of the total pixel which is optically active, taking into account the space between pixels 301a. By control of the voltage applied to each electrode 301 with respect to the transparent electrode 307, the properties of the liquid crystal material of the respective phase modulating element may be varied, thereby to provide a variable delay to light incident thereon. The effect is to provide phase-only modulation to the wavefront, i.e. no amplitude effect occurs. The described LCOS SLM outputs spatially modulated light in reflection. Reflective LCOS SLMs have the advantage that the signal lines, gate lines and transistors are below the mirrored surface, which results in high fill factors (typically greater than 90%) and high resolutions. Another advantage of using a reflective LCOS spatial light modulator is that the liquid crystal layer can be half the thickness than would be necessary if a transmissive device were used. This greatly improves the switching speed of the liquid crystal (a key advantage for the projection of moving video images). However, the teachings of the present disclosure may equally be implemented using a transmissive LCOS SLM. Display Device A spatial light modulator, such as the LCOS SLM described in relation to Figure 3, comprises a chip or die, which may be packaged as a display device. In some embodiments, the display device is provided as part of a HUD system for use in a vehicle. The display device may be manufactured separately, as a standalone component, and then assembled into a HUD system for a particular application. Figure 4 shows a schematic top view of such a display device 400 and Figure 5 shows a cross section along line X-X of Figure 4. The display device 400 comprises a substrate 402 in the form of ceramic or alumina carrier and a spatial light modulator 404 mounted on the substrate 402. In this example, the spatial light modulator 404 is an SLM LCOS having a silicon backplane as described in relation to Figure 3. However, it will be clear to the skilled person that other spatial light modulators may take the place of the LCOS SLM of Figure 3 in the display device. In addition, other types of carrier substate or board may be used according to application requirements. A connector 406 is provided for connection to the spatial light modulator 404. In this example, the connector 406 is a flexible printed cable capable but may alternatively be a flat flex connector or a flexible printed circuit board. In particular, electrical connections 418, which are illustrated as wire bonds, extend between contact pads 414 in an electrical connection region 412 of the spatial light modulator and corresponding contact pads 416 in an electrical connection region 416 of the connector 406. The connector 406 is provided on the substrate 402 to connect the spatial light modulator 404 to at least one other part of a HUD system, such as a controller or display driver (not shown). The spatial light modulator 404 comprises a first major surface 420 including a light modulating region 410 (also called a display area or display panel) and the electrical connection region 412, and a second major surface 422 opposite the first major surface 420. The second major surface 422 of the spatial light modulator 404 is attached to a first major surface of the substrate 402 so that the display area 410 is exposed, in order to be illuminated with light. In one example, during use of the display device 404, the spatial light modulator 404 receives a computer-generated hologram from a controller via the connector 406 so as to encode the array of pixels of the spatial light modulator 400 in the display area 410. The array of pixels of the spatial light modulator 400 is then illuminated with light from a light source, such as coherent light from a laser or laser diode, such that light is incident on the spatial light modulator. The light is then modulated in accordance with the hologram and output as spatially modulated light. Although not shown in Figures 4 and 5, the substrate 402 may comprise a heat exchange portion or surface (not shown), for example extending from a second major surface of the substrate 402. The heat exchange portion of the substrate 402 may comprise an arrangement of metal heat dissipating members (e.g., aluminium fins) arranged to conduct heat to and from the spatial light modulator 404 so as to maintain it at an optimum operating temperature. As shown in Figure 5, an attachment material 430 is provided between the second major surface 422 of the spatial light modulator 404 and the first major surface of the substrate 402 in order to securely mount the spatial light modulator 404 onto the substrate 402. In particular, the attachment material 430 may comprise an adhesive paste (e.g., thermal paste) or an adhesive film (e.g., thermoplastic film) such as a so-called "die attach tape". Many attachment materials are known for use in mounting electronic components, such as chips and dies, to the surface of a carrier substrate, including pressure sensitive adhesives and other types of bonding material. The inventors performed measurement relating to optical performance of a display device comprising a spatial light modulator mounted using a conventional die attach tape to a ceramic substate, both before and after automotive testing. The adhesive material was an acrylic based attachment material. Wavefront Measurements Wavefront measurement, which may be used to determine how flat the spatial light modulator is, involve illuminating the light modulating area without encoding the pixels. In this configuration, the reflective surface of the spatial light modulator reflects the incident light to output an unmodulated wavefront. Measurements of the output wavefront are performed to determine the shape of the wavefront, as well known in the art. Figure 6A shows 2D and 3D representations of wavefront measurements of light output from a display device comprising a reflective spatial light modulator mounted on a substrate using a conventional die attach tape before exposure to thermal tests. In particular, the die attach tape was an acrylic-based adhesive material conventionally used in display applications. The wavefront has a substantially uniform beam profile in the x dimension, with only minor non-uniformities at the beam edges in the y dimension. Furthermore, as shown in the 3D representation, the beam profile is generally planar at its centre. These results are generally consistent with a beam profile for light reflected from a substantially planar mirror of an ideal reflective display device. Figure 6B shows corresponding 2D and 3D representations of wavefront measurements of light output from the same display device after it has been exposed high temperature thermal tests. In contrast to the wavefront of Figure 6A, the beam profile is non-planar. Instead, as shown in the 3D representation, the wavefront has a divergent beam profile resembling a beam profile of light reflected from a convex mirror. Thus, the beam profile includes non-uniformities from the centre to the beam edges in both the x and y dimensions. Such a beam profile is undesirable, especially in the context of a phase-modulator or amplitude and phase-modulator. Accordingly, a detrimental impact on the shape of the output wavefront from the spatial light modulator was observed, as a result of automotive testing, in particular exposure to high temperatures. Image Resolution Measurements Image resolution measurements, which may be used to determine the resolution of an image formed by the spatial light modulator, comprise illuminating the light modulating area with encoded pixels so as to form an image comprising image spots (or image pixels) at an image / replay plane. In this configuration, the image formed at the image / replay plane can be captured, and the size and shape of the image spots analysed, to assess image resolution, as well known in the art. Figures 7A-C shows the resolution of an image formed by the same display device as Figures 6A and 6B, comprising a reflective spatial light modulator mounted on a substrate using the conventional die attach tape when exposed to different environmental conditions, specifically thermal storage tests. In particular, each of the drawings shows an array of image spots of an image formed by the display device at an image plane. Accordingly, in each case, the array of image spots represents the resolution of the image seen by a viewer. Figure 7A shows the resolution of the image before thermal storage testing. In particular, the observed number and spacing of image spots formed corresponds to the intended image resolution. In addition, the image spots are substantially uniform in size, shape and intensity throughout the array and no artefacts are observed. The inventors also found that this resolution of the array of image spots forming the image remained substantially stable and consistent with the display of the image following thermal storage testing in normal operating conditions, in particular at a typical (medium-high) display device operating temperature and humidity for a number of days. Figure 7B shows the resolution of the image after exposure to very low temperature thermal storage testing, in particular at an extremely low temperature for a number of days. The image resolution is similar to the results shown in Figure 7A. In particular, the observed number and spacing of image spots corresponds to the intended image resolution. In addition, the image spots are substantially uniform in size and shape throughout the array and no significant artefacts are observed. Figure 7C shows the resolution of the image after exposure to very high temperature thermal storage testing, in particular at an extremely high temperature for a number of days. In this case, the image resolution differs from the results shown in Figure 7A due to "doubling" or "duplication" of image spots. In particular, the observed number of image spots is increased and the spacing between image spots is reduced in comparison to Figure 7A. In addition, artefacts are observed in the form of "duplicate" image spots of lower intensity between columns of the array. The inventors observed this change in the resolution of the array of image spots even after exposure to high temperatures for only a few hours. Accordingly, a detrimental impact on the resolution of the image formed by the spatial light modulator was observed, in particular, an increase in the number and non-uniformity in intensity of image spots in the array of image spots, as a result of automotive testing, in particular exposure to high temperatures. Analysis On the basis of the wavefront and image resolution measurements, the inventors realised that the spatial light modulator, in particular the reflective surface / layer thereof, has been deformed or warped across the display area as a result of the thermal tests. In particular, the spatial light modulator appears to have been warped and distorted from a substantially flat to a generally convex shape to produce the observed change in wavefront, with some degree of tilt leading to the observed duplicate image spot artefacts. The inventors considered whether the warping, distortion and / or tilt of the spatial light modulator could be due to differences in mechanical stress imparted on the spatial light modulator by the attachment material during thermal cycling. Inspection of the attachment material after testing revealed that the thermoplastic film was deformed and wrinkled, suggestive of movement and lifting of the die attach tape during thermal testing. In addition, the adhesive or thermal paste was non-uniform in thickness and had an uneven tone as a result of the thermal testing. Given that the attachment material is likely to have a preexisting non-uniform thickness and stress across the attachment area, as discussed above, the inventors realised that the deformation and changes to the attachment material during thermal tests may cause deformation, warping and / or tilt of the spatial light modulator across the display area. Selection of Die Attachment Material The inventors investigated a large range of adhesive materials, across a range of chemical compositions and having different thermal properties, for use as the attachment material between a spatial light modulator (e.g., LCOS SLM) and a substate (e.g., ceramic or alumina substrate). In each case, the material was used to attach the die to the substrate in accordance with manufacturer recommended conditions (e.g., applied pressure), and the device was subject to automotive thermal testing. The device was then evaluated by wavefront and image resolution measurements. The results for a small sample of five of the tested materials, for the purposes of illustration and explanation of the present disclosure only, are set out in Table 1. Wavefront Anomalies Present? (Indicative of Stress on Die) Image Spot Artefacts Present? (Indicative of Tilt) Material 1 Yes No Material 2 Yes Yes Material 3 Yes Yes Material 4 No Yes Material 5 No No Table 1 It should be understood that the Table 1 does not represent all the materials tested, or the chronological order of the tests, which were carried out over a long period of time. Rather, Table 1 is provided to demonstrate the often surprising findings, analysis and progression of the investigations carried out by the inventors, in order to arrive at solutions to the problems of wavefront anomalies and image resolution artefacts as described above. Coefficient of Thermal Expansion The inventors considered the coefficient of thermal expansion (also referred to as linear thermal expansion) of all the sample materials (unit ppm / °C). The inventors noted that wavefront anomalies were (always) present in materials having a coefficient of thermal expansion greater than 200 ppm / °C. For example, example materials 1 and 2 in Table 1 have a coefficient of thermal expansion of 500 and 400 ppm / °C respectively. However, example materials 3-5 in Table 1 have a coefficient of thermal expansion less than 200 ppm / X. Accordingly, a suitable adhesive material for die attachment has (i) a coefficient of thermal expansion less than about 200 ppm / °C, optionally less than about 150 ppm / °C. The inventors believe that the use of adhesive materials having a high coefficient of thermal expansion (i.e., greater than 200 ppm / °C) may lead to movement of the spatial light modulator die relative to the substate due to thermal testing, causing the die to lift, the film to unwrap, and / or causing stress on the die, leading to non-uniformity across the panel and undesirable wavefront anomalies. However, the inventors found it surprising that the choice of an adhesive material for die attachment with a low coefficient of thermal expansion alone did not resolve this issue, as illustrated by the presence of wavefront anomalies with material 3. Thermal Conductivity The inventors considered the thermal conductivity of all the sample materials (unit W / m-°C). The inventors noted that both wavefront anomalies and image spot artefacts were (always) present in materials with a thermal conductivity less than 1 W / m-°C. For instance, example materials 2 and 3 in Table 1 have a thermal conductivity of less than 1 W / m-’C . Example materials 1,4 and 5 in Table 1 have a thermal conductivity greater than 1 W / m-°C. As noted above, material 3 has a coefficient of thermal expansion of less than 200 ppm / °C but nevertheless wavefront anomalies were observed. Material 4 had a thermal conductivity of 1.4 W / m-°C and material 5 had the highest thermal conductivity of the materials 1-5. Thus, the inventors realised that an adhesive material for die attachment should be chosen to have the correct combination of coefficient of thermal expansion and thermal conductivity. Accordingly, there is provided a suitable adhesive material for die attachment that has (i) a coefficient of thermal expansion less than about 200 ppm / °C (optionally less than about 150 ppm / °C) and (ii) a thermal conductivity greater than about 1 W / m-°C (optionally greater than or equal to about 1.4 W / m-°C). The thermal conductivity of an attachment material between a spatial light modulator and a ceramic substate (having a high thermal conductivity) should allow adequate heat flow between the die and the substate during thermal testing. It is also desirable to match the thermal conductivity of the adhesive material (as closely as possible) with both the material of the lower part of the die (e.g. silicon backplane) and the material of the substrate (e.g. ceramic or alumina) is desirable for good heat flow. Since the materials of the lower part of the die and the substrate are different, it is desirable to use an adhesive material that has a thermal conductivity between the two, whilst being sufficiently high for good heat transfer. The inventors believe that it is possible that the ineffective dissipation of heat thought the attachment material during thermal testing may lead to changes to the adhesive material, and thus may cause the deformation and warping of the die and consequent non-uniformity across the panel, resulting in undesirable wavefront anomalies. The inventors surprisingly found that the combination of two different thermal properties of a material improved the shape of the wavefront output by the display device. These improvements were observed with attachment materials having (i) coefficient of thermal expansion less than about 200 ppm / °C and (ii) a thermal conductivity greater than about 1 W / m-°C. Other Properties Whilst using adhesive materials with the above thermal properties (i) and (ii) were found to improve the shape of the wavefront of light output by the display device, the results for image resolution were variable, suggestive of unacceptable tilt. The inventors therefore considered other (nonthermal) properties and their effects. Hardness The inventors tested sample materials with similar thermal properties but different hardness values (unit Shore A). As the skilled person will appreciate, the Shore A hardness of an adhesive material is typically specified by the manufacturer and is measured according to ASTM D 2240. In particular, the inventors compared the hardness of sample materials, having (i) coefficient of thermal expansion less than 200 ppm / °C and (ii) a thermal conductivity greater than about 1 W / m-°C. Notably, example materials 4 and 5 in Table have the same a low coefficient of thermal expansion below 200 ppm / °C and good thermal conductivity above 1 W / m-°C. The inventors noted that double image spot artefacts were present in materials having a hardness greater than 65 Shore A. For example, example materials 3 and 4 in Table 1 have a hardness greater than or equal to 65 Shore A, whilst example material 5 in Table 1 has a hardness less than 65 Shore A (specifically between 30 and 50 Shore A). It was surprising to the inventors that the relative hardness of the material influenced whether image spot artefacts were present. Accordingly, there is provided a suitable adhesive material for die attachment that has (i) a coefficient of thermal expansion less than about 200 ppm / °C, optionally less than about 150 ppm / °C (ii) a thermal conductivity greater than about 1 W / m-°C, optionally greater than or equal to about 1.4 W / m-°C and (iii) a hardness of less than about 65 Shore A, optionally between about 30 and 50 Shore A. The inventors expected that the observed wavefront anomalies and image spot artefacts shown in Figures 6B and 7C may arise when the attachment material is too soft, because the spatial light modulator die may move (e.g., tilt) relative to the substate during, before and / or after thermal testing. Thus, the inventors originally considered that a relatively "hard" material was desirable. However, the findings surprisingly revealed that, when comparing the "medium soft" sample material 5 with the "medium hard / hard" sample material 4 (both with the above properties (i) and (ii))), the sample material 5 did not lead to image artefacts whereas the sample material 4 did. Accordingly, based on their findings, the inventors realised that if the attachment material is too hard, the attachment material may deform in response to temperature changes during thermal testing, which may lead to an undesirable tilt and result in the observed adverse effects. Thus, a hardness value less than 65 Shore A, and optionally in the range 30 to 50 Shore A, strikes a balance between the conflicting effects of soft and hard materials. Glass Transition Temperature The inventors also identified another (non-thermal) property in the sample materials may have a role in reducing wavefront anomalies and image spot artefacts as described herein, namely glass transition temperature Tg (unit °C). This is the temperature at which the material transitions between a hard, glasslike state and a soft, deformable state. This property had not been previously considered to be relevant in relation to the selection of die attachment materials. The inventors noted that there was a difference in the Tg of all the sample materials (unit °C). For instance, example materials 1 and 2 in Table 1 have positive glass transition temperatures in the range 20 to 100°C. Thus, glass transition of materials 1 and 2 takes place within the range of the temperatures experienced by the spatial light modulator during thermal cycling and normal operating temperatures. However, example materials 3, 4 and 5 in Table 1 have negative glass transition temperatures in the range -40 to -80°C. Thus, glass transition of example materials 3,4 and 5 takes place outside the range of the temperatures experienced by the spatial light modulator during thermal cycling and normal operating temperatures. The inventors believe that the glass transition of the attachment material during thermal testing contributed to the observed anomalies, in particular the image spot artefacts, because glass transition leads to a hard, glasslike state. Thus, glass transition temperature is a property that is also relevant to the above conflicting effects associated with hardness, which may undesirably affect tilt. Accordingly, there is provided a suitable adhesive material for die attachment that has (i) a coefficient of thermal expansion less than about 200 ppm / °C, optionally less than about 150 ppm / °C (ii) a thermal conductivity greater than about 1 W / m-°C, optionally greater than or equal to about 1.4 W / m-°C and (iv) a glass transition temperature less than about -40°C, optionally in the range of about -40 to -80°C. Optionally, the material additionally has (iii) a hardness of less than about 65 Shore A, optionally between about 30 and 50 Shore A, Sample material 5 in Table 1 is an example of a material having the characteristics (i) to (iv), and comprises an aluminium nitride filled thermoplastic adhesive. However, a variety of attachment materials, having different chemical compositions, may possess the characteristics (i) and (ii), and optionally (iii) and / or (iv), and are suitable for die attachment, as described herein. Additional features Embodiments refer to an electrically-activated LCOS spatial light modulator by way of example only. The teachings of the present disclosure may equally be implemented on any spatial light modulator capable of displaying a computer-generated hologram in accordance with the present disclosure such as any electrically-activated SLMs, optically-activated SLM, digital micromirror device or microelectromechanical device, for example. In some embodiments, the light source is a laser such as a laser diode. In some embodiments, the detector is a photodetector such as a photodiode. In some embodiments, the light receiving surface is a diffuser surface or screen such as a diffuser. The holographic projection system of the present disclosure may be used to provide an improved head-up display (HUD) or head-mounted display. In some embodiments, there is provided a vehicle comprising the holographic projection system installed in the vehicle to provide a HUD. The vehicle may be an automotive vehicle such as a car, truck, van, lorry, motorcycle, train, airplane, boat, or ship. The quality of the holographic reconstruction may be affect by the so-called zero order problem which is a consequence of the diffractive nature of using a pixelated spatial light modulator. Such zero-order light can be regarded as "noise" and includes for example specularly reflected light, and other unwanted light from the SLM. In the example of Fourier holography, this "noise" is focussed at the focal point of the Fourier lens leading to a bright spot at the centre of the holographic reconstruction. The zero order light may be simply blocked out however this would mean replacing the bright spot with a dark spot. Some embodiments include an angularly selective filter to remove only the collimated rays of the zero order. Embodiments also include the method of managing the zero-order described in European patent 2,030,072, which is hereby incorporated in its entirety by reference. In some embodiments, the size (number of pixels in each direction) of the hologram is equal to the size of the spatial light modulator so that the hologram fills the spatial light modulator. That is, the hologram uses all the pixels of the spatial light modulator. In other embodiments, the hologram is smaller than the spatial light modulator. More specifically, the number of hologram pixels is less than the number of light-modulating pixels available on the spatial light modulator. In some of these other embodiments, part of the hologram (that is, a continuous subset of the pixels of the hologram) is repeated in the unused pixels. This technique may be referred to as "tiling" wherein the surface area of the spatial light modulator is divided up into a number of "tiles", each of which represents at least a subset of the hologram. Each tile is therefore of a smaller size than the spatial light modulator. In some embodiments, the technique of "tiling" is implemented to increase image quality. Specifically, some embodiments implement the technique of tiling to minimise the size of the image pixels whilst maximising the amount of signal content going into the holographic reconstruction. In some embodiments, the holographic pattern written to the spatial light modulator comprises at least one whole tile (that is, the complete hologram) and at least one fraction of a tile (that is, a continuous subset of pixels of the hologram). In embodiments, only the primary replay field is utilised and system comprises physical blocks, such as baffles, arranged to restrict the propagation of the higher order replay fields through the system. In embodiments, the holographic reconstruction is colour. In some embodiments, an approach known as spatially-separated colours, "SSC", is used to provide colour holographic reconstruction. In other embodiments, an approach known as frame sequential colour, "FSC", is used. The method of SSC uses three spatially-separated arrays of light-modulating pixels for the three single-colour holograms. An advantage of the SSC method is that the image can be very bright because all three holographic reconstructions may be formed at the same time. However, if due to space limitations, the three spatially-separated arrays of light-modulating pixels are provided on a common SLM, the quality of each single-colour image is sub-optimal because only a subset of the available light-modulating pixels is used for each colour. Accordingly, a relatively low-resolution colour image is provided. The method of FSC can use all pixels of a common spatial light modulator to display the three singlecolour holograms in sequence. The single-colour reconstructions are cycled (e.g. red, green, blue, red, green, blue, etc.) fast enough such that a human viewer perceives a polychromatic image from integration of the three single-colour images. An advantage of FSC is that the whole SLM is used for each colour. This means that the quality of the three colour images produced is optimal because all pixels of the SLM are used for each of the colour images. However, a disadvantage of the FSC method is that the brightness of the composite colour image is lower than with the SSC method - by a factor of about 3 - because each single-colour illumination event can only occur for one third of the frame time. This drawback could potentially be addressed by overdriving the lasers, or by using more powerful lasers, but this requires more power resulting in higher costs and an increase in the size of the system. Examples describe illuminating the SLM with visible light but the skilled person will understand that the light sources and SLM may equally be used to direct infrared or ultraviolet light, for example, as disclosed herein. For example, the skilled person will be aware of techniques for converting infrared and ultraviolet light into visible light for the purpose of providing the information to a user. For example, the present disclosure extends to using phosphors and / or quantum dot technology for this purpose. Some embodiments describe 2D holographic reconstructions by way of example only. In other embodiments, the holographic reconstruction is a 3D holographic reconstruction. That is, in some embodiments, each computer-generated hologram forms a 3D holographic reconstruction. The methods and processes described herein may be embodied on a computer-readable medium. The term "computer-readable medium" includes a medium arranged to store data temporarily or permanently such as random-access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" shall also be taken to include any medium, or combination of multiple media, that is capable of storing instructions for execution by a machine such that the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methodologies described herein, in whole or in part. The term "computer-readable medium" also encompasses cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example form of a solid-state memory chip, an optical disc, a magnetic disc, or any suitable combination thereof. In some example embodiments, the instructions for execution may be communicated by a carrier medium. Examples of such a carrier medium include a transient medium (e.g., a propagating signal that communicates instructions). It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. The present disclosure covers all modifications and variations within the scope of the appended claims and their equivalents. 20 08 24

Claims

1. A display device comprising:a spatial light modulator;a substrate, andan attachment material for mounting the spatial light modulator on the substrate, wherein the attachment material has the following properties:(i) coefficient of thermal expansion less than about 200 ppm / °C,(ii) thermal conductivity greater than about 1 W / m-°C, and(iii) hardness value less than 65 Shore A.

2. A display device as claimed in claim 1 wherein the coefficient of thermal expansion is less than about 150 ppm / °C.

3. A display device as claimed in claim 1 or claim 2 wherein the thermal conductivity is greater than or equal to about 1.4 W / m-°C.

4. A display device as claimed in any preceding claim wherein the hardness value is in the range 30 to 50 Shore A.

5. A display device as claimed in any preceding claim wherein the attachment material comprises an adhesive paste such as a thermal paste.

6. A display device as claimed in any preceding claim wherein the attachment material comprises an adhesive coated film such as a thermoplastic film or die attach tape.

7. A display device as claimed in any preceding claim wherein the spatial light modulatorcomprises opposed first and second major surfaces, wherein the first major surface comprises a display area and the attachment material is positioned between the second major surface of the spatial light modulator and a first major surface of the substrate.

8. A display device as claimed in any preceding claim wherein the spatial light modulator comprises a die or chip.20 08 249. A display device as claimed in any preceding claim wherein the spatial light modulator comprises a liquid crystal on silicon (LCOS) spatial light modulator comprising a silicon backplane.

10. A display device as claimed in claim 9 wherein:a difference between a magnitude of at least one thermal property of the attachment material and the magnitude of the thermal property of the silicon backplane is minimised.

11. A display device as claimed in claim 9 or claim 10 wherein:a difference between a magnitude of at least one thermal property of the attachment material and the magnitude of the thermal property of the substrate is minimised.

12. A display device as claimed in claim 10 or claim 11 wherein a difference between the thermal conductivity of the attachment material and the thermal conductivity of the silicon backplane and the substrate is minimised.

13. A display device as claimed in any preceding claim wherein the substrate comprises a ceramic substrate such as an alumina substrate.

14. A display device as claimed in claim 13 wherein the substate includes a heat exchanging portion or surface.

15. A display device as claimed in any preceding claim wherein the attachment material further comprises the property:(iv) glass transition temperature less than about -40°C.

16. A display device as claimed in claim 15 wherein the glass transition temperature is in the range of about -40 to -80°C.

17. A display device as claimed in any preceding claim wherein the spatial light modulator is arranged as a phase modulator or an amplitude and phase modulator.

18. A display device as claimed in claim 17 wherein the display device is arranged to display a hologram and to be illuminated with light so as to form a holographic wavefront at a replay plane.20 08 2419. A display device as claimed in claim 18 wherein the illumination light is coherent light.

20. A head-up display or head-mounted display comprising the display device as claimed in any preceding claim.

21. A method of attaching a spatial light modulator to a substrate to form a display device, the method comprising:applying a layer of an attachment material to at least one of a lower surface of the spatial light modulator or an upper surface of the substrate, andapplying a predetermined pressure to the upper surface of the spatial light modulator, to mount the spatial light modulator on the substrate;wherein the attachment material has the following properties:(i) coefficient of thermal expansion less than about 200 ppm / °C,(ii) thermal conductivity greater than about 1 W / m-°C, and(iii) hardness value less than 65 Shore A.

22. A method as claimed in claim 21 wherein the coefficient of thermal expansion is less than about 150 ppm / °C.

23. A method as claimed in claim 21 or claim 22 wherein the thermal conductivity is greater than or equal to about 1.4 W / m-°C.

24. A method as claimed in any of claims 21 to 23 wherein the hardness value is in the range 30 to 50 Shore A.

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