Light-weight waveguide

Abstract

A transmissive-reflective surface (303, figure 3) is formed on a first glass substrate 620 and has gradient thickness layers 401-404 arranged to provide a transmissivity that increases with distance i

Description

FIELD The present disclosure relates to a method of replicating a wavefront and a device for the same. More specifically, the present disclosure relates to a method of fabricating a waveguide and a waveguide. Yet more specifically, the present disclosure relates to a hybrid glass-plastic waveguide and a method of fabricating the same. The present disclosure also relates to a head-up display and a method of fabricating wavefront replicator or expander for 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 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. Summary of holographic projection and wavefront replication 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 embodiments, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. The image is formed by illuminating a diffractive pattern (e.g., hologram) displayed on the display device. The display device comprises pixels. The pixels of the display device diffract light. In accordance with well-understood optics, the magnitude of the maximum diffraction angle is determined by the size of the pixels (and other factors such as the wavelength of 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 examples, an image (formed from the displayed hologram) is propagated to the eyes. For example, spatially modulated light of an intermediate holographic reconstruction / image formed either in free space or on a screen or other light receiving surface between the display device and the viewer, may be propagated to the viewer. In some other examples, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) - that may be informally said to be “encoded” with / by the hologram - is propagated directly to the viewer’s eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction / image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device. In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity / system, varies with the distance between the display device and the viewing entity. At a 1 metre viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye’s pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye’s pupil to form an image at the retina for a given eye position, determines the portion of the image that is ‘visible’ to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-motion box.) In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device - that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it 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 particularly relates to non-infinite virtual image distances-that is, near-field virtual images - but is equally applicable to virtual images formed at infinity or even real images formed downstream of the display device I hologram. 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. Broadly, a system is disclosed herein that provides pupil expansion for an input light field or wavefront. The input light field may be a diffracted or holographic light field comprising diverging ray bundles and the display system may be described as hologram-to-eye. In this case, the input light field is spatially modulated in accordance with a hologram of an image. Alternatively, the input light field may be an image and the display system may comprise a screen, such as a diffuser, arranged to form the image thereon. In this case, the input light field is spatially modulated in accordance with an image. In both cases, the input light field is replicated to form a 1D or 2D array of replicas. For the avoidance of doubt, the present disclosure is equally applicable to both techniques. In particular, the coating or thin film or method of forming the same in accordance with the present disclosure is equally applicable to waveguiding and replicating a hologram or an image. In the present disclosure, the term “replica” is merely used to reflect that spatially modulated light is divided such that a complex light field or wavefront 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 or wavefront after a replication event - such as a partial reflectiontransmission by a pupil expander. Each replica travels along a different optical path. Some embodiments of the present disclosure relate to propagation of light that is encoded with a hologram, not an image - i.e., light that is spatially modulated with a hologram of an image, not the image itself. The person skilled in the art of holography will appreciate that the complex light field associated with propagation of light encoded with a hologram will change with propagation distance. Use herein of the term “replica” is independent of propagation distance and so the two branches or paths of light associated with a replication event are still referred to as “replicas” of each other even if the branches are a different length, such that the complex light field has evolved differently along each path. That is, two complex light fields are still considered “replicas” in accordance with this disclosure even if they are associated with different propagation distances - providing they have arisen from the same replication event or series of replication events. As above, an optical waveguide for use as a pupil expander may lightguide or waveguide a light field or wavefront between a pair of parallel surfaces. This may be achieved by internal reflection between the parallel surfaces. A first surface of the pair of surfaces may be partially transmissive-reflective. A second surface of the pair of surfaces may be reflective. The light field will therefore be divided at each internal reflection at the first surface such that a plurality of replicas of the light field is transmitted through a region of the first surface that forms an output port of the waveguide. Thus, a viewing window (and eye-box) is enlarged by the waveguide. The intensity and spectrum of each successive replica emitted by the waveguide is required to be substantially constant from replica to replica. For example, an integral of the intensity of the light of each replica may be substantially constant, a mean average of the intensity of the light of each replica may be substantially constant and I or a distribution of the intensity of the light of each replica may be substantially constant. Replicas having substantially constant intensity and spectra are referred to herein as being spatially homogenous and a waveguide emitting such replicas is referred to herein as providing spatially homogenous emission. Spatially homogenous replicas advantageously reduce or minimize the variability in the brightness of the image perceived (or different areas thereof) by a viewer moving around the (expanded) viewing window. Furthermore, the overall quality of the hologram received by the viewer (and / or the resulting image perceived by the viewer) may be improved, particularly if the hologram comprises a plurality of different wavelengths of light. The intensity of the light being waveguided between the first and second surfaces of the waveguide will decrease after each division of the light field at the first surface. A sub-optimal waveguide may typically comprise a first surface with constant reflectivity. This results in the intensity of each successive replica decreasing. In other words, such conventional waveguides do not provide spatially homogenous emission. A graded coating or gradient thickness coating can be applied to the waveguide to provide a first surface having a varying reflectivity and therefore transmissivity. In particular, the coating may be arranged so that the transmissivity of the first surface increases in the direction of waveguiding. The graded coating may be arranged such that the increased transmissivity of the first surface in the direction of waveguiding accounts (e.g. at least partially compensates) for the decrease in the intensity of light being light guided. However, current graded coatings often result in high absorption losses of the waveguided light. Furthermore, these coatings typically allow for very limited control of spectral performance, particularly disadvantageous when the light to be waveguided comprises multiple wavelengths. While some improved graded coatings are available, these are expensive, time consuming and complex to manufacture and generally cannot be manufactured reliably. For example, such coatings may comprise a plurality (often 20 or more) layers of dielectric material, each layer having a unique percentage change of the thickness from a first end of the layer to a second end. Such a complex layered structure may be necessary to provide substantially spatially homogenous emission for the full visible spectrum but is slow to manufacture and difficult to manufacture reliably (for example, a moving grader may be required to be present in the coating chamber). Summary of waveguide coating Broadly, the present disclosure addresses a technical problem of providing a spatially homogenous emission from a waveguide pupil expander such that the intensity and spectrum of each replica is substantially similar. The present disclosure proposes an improved waveguide pupil expander which provides substantially homogeneous emission at least at specific wavelengths such as a red, green and blue wavelength. A first surface of the waveguide comprises a plurality of alternating layers of a first and a second dielectric that differ in refractive index. In some examples, each layer has a unique percentage change of thickness. In some examples, each layer of dielectric has a percentage change of thickness. The multilayer structure resulting from conventional design techniques is very complex and has low suitability for industrial scale-up. The complexity is partly caused by the need to achieve uniformity at three different wavelengths. This design process has been rethought and focused on restricting it in ways that simplify the manufacturing method that will be needed and relaxing it in other ways that do not impact the image-forming (e.g. holographic) process. In particular, focused has been placed on how to increase speed and reduce the number of hardware changes required during coating because these may require time inefficient processes related to opening the coating chamber such as re-evacuating the chamber. According to examples, a waveguide is provided. The waveguide comprises a pair of parallel I complementary surfaces arranged to provide waveguiding therebetween. A first surface of the pair of parallel surfaces comprises a plurality of layers of a first dielectric and a plurality of layers of second dielectric arranged in an alternating configuration. Each layer of the first and second dielectric has a first end and a second end. The first end may be a (light) input end and the second end may be a (light) output end for the final replica or the end of an output port / window. A percentage change in the thickness of each layer from the first end to the second end of that layer has one of a plurality of discrete (e.g. allowable) values. The total number of layers of the first and second dielectric is greater than the total number of discrete values. A difference in refractive index between the first dielectric and second dielectric is greater than 0.4, optionally greater than 0.5. References to “percentage change in the thickness” of a layer herein refer to the percentage change in thickness of that layer from the first end of that layer to the second end of that layer, unless specified otherwise. The percentage change in the thickness of a layer may be: 100 x (tf -1) / tj, wherein ti is the thickness of the respective layer at the first end and tf us the thickness of the respective layer at the second end. Generally, each layer of the plurality of layers may be parallel to the other layers. The first end of each of layer may be aligned with the first end of the other layers and the second end of each layer may be aligned with the other layers. In other words, each layer may have a substantially similar length and / or width as the other layers but may differ in thickness or depth. The plurality of layers of the first and second dielectric may be referred to as a stack of layers or, simply, dielectric stack. As used herein, the “alternating configuration” of the layers of the first and second dielectric means that each layer of the first dielectric is separated from the next closest layer(s) of first dielectric by a layer of the second dielectric, and vice versa. In some examples, more than two, optionally four or more, optionally 6 or more, optionally 8 or more, optionally 10 or more layers of the plurality of layer may have the same percentage change of thickness from the respective first end to the respective second end (and so these layers may be associated with the same discrete value). In some examples, the layers that have the same percentage change of thickness as each other are layers of the same dielectric (i.e. all layers having a particular percentage change of thickness value are layers of the first dielectric or layers of the second dielectric). Each of the layers of the first or second dielectric may increase or decrease in thickness in the direction of waveguiding. A first direction may be defined from the first end to the second end of each of the plurality of layers. The pair of parallel surfaces may be arranged to provide waveguiding in the first direction. In other words, light waveguided by the pair of surfaces may interact with the first end before interacting with the second end. Furthermore, each of the layers of the first or second dielectric may increase or decrease in thickness in the first direction. The minimum thickness of each of the plurality of layers of the first and second dielectric, as well as the discrete values for the percentage change in thickness, may be selected to provide a waveguide with desired optical properties, in particular with a desired transmissive behaviour such that a spatially homogenous emission is generated. The parameters may depend on (among other things) the material properties (in particular, the refractive index) of the first and second dielectric, the angle of incidence of light entering the waveguide, the wavelength(s) of the incident light and the distance between the pair of parallel surfaces of the waveguide. As will appreciated by the skilled person, there will be a plurality (usually a relatively large number) of arrangements of the plurality of layers that provide a waveguide with desired optical properties. However, common to all of these arrangements are the advantages that the layered structure of the first surface provides desired optical properties and can be manufactured quickly, inexpensively and reliably. One example of a fast, inexpensive and reliable method of manufacturing the layers of the first and second dielectric may comprise forming the layers by depositing the respective dielectric material on a waveguide substrate. A shadow mask may be used to control the flow of the dielectric material onto the substrate. Preferably, a shadow mask with a trapezoidal shape may be used. The percentage change in thickness of each layer may be determined by the shape of the mask and the overall thickness of each layer may be determined by the length of time that the dielectric material is allowed to flow. When the mask is trapezoidal, the percentage change in thickness of each layer may correspond to a percentage change in the short base to the long base of the trapezoid shape of the shadow mask. A different mask may be associated with each of the discrete values. For example, if there are first to fourth percentage change values then four different masks may be used to manufacture the layers of the coating. The waveguide substrate may advantageously simply be moved between different material sources having different masks. Preferably, the rate of change of the thickness of each layer may be constant. In other words, each of the layers may have a linear profile. It may be simpler to manufacture dielectric layers having a linear profile. Furthermore, the skilled person will appreciate that it may be more straightforward to calculate / determine discrete allowable values for a stack of layers having a linear profile. In some embodiments, the first surface may provide a plurality, n, of light emission zones for light waveguided between the first surface and second surface. The plurality of light emission zones may be distributed along a length of the first surface, in the direction of waveguiding. A replica of the input wavefront may be generated at each emission zone. The first end of each layer of the first and second dielectric layer may be at or adjacent to the first light emission zone. The second end of each layer of the first and second dielectric later may be at or adjacent to the nth light emission zone. An angle of internal incidence at each emission zones may be in the range 0 to 70 degrees, preferably in the range of 10 to 50 degrees. At other wavelengths (i.e. not the first, second and third visible wavelength), the transmissivity of the first surface may or may not increase in the direction of waveguiding. As previously described, the waveguide of the present disclosure may be particularly advantageous in the context of waveguiding light at discrete wavelengths, rather than waveguiding light having a continuous spectrum. The inventors have appreciated that the parameters such as thickness and the values for the percentage change of thickness of the plurality of layers of the first and second dielectrics may be selected to provide a desired changing transmissivity behaviour at the first, second and third wavelengths only and that there may be no need for the layers to provide that same transmissivity behaviour at other wavelengths. The inventors have found that by confining the problem of providing a changing transmissivity behaviour to only certain wavelengths, a coating comprising multiple layers having the same rate of change of thickness can provide the desired transmissivity behaviour which, as above, can be manufactured inexpensively, quickly and reliably while still providing acceptable transmissive properties. The first wavelength, referred to above, may preferably be in the range 630-670 nm. The second wavelength may preferably be in the range 500-540 nm. The third wavelength may preferably be in the range 430-470 nm. In other words, the first wavelength may correspond to red visible light. The second wavelength may correspond to green visible light. The third wavelength may correspond to blue visible light. Preferably, the transmissivity of the first surface at each emission point, T(n), may satisfy the following equation: T(n)=(T(n-1)) / ([1-T(n-1)]x[1-L]) wherein L is an optical loss factor of the waveguide material. The first dielectric may be a first oxide, fluoride, sulfide or nitrate of a first transition metal or semiconductor. The second dielectric may be a second oxide, fluoride, sulfide or nitrate of a second transition metal or semiconductor. In some embodiments, the first dielectric comprises silicon, titanium, tantalum or hafnium. In some embodiments, the second dielectric comprises another of silicon, titanium, tantalum or hafnium. Each layer may have a thickness in the range 2 to 300 nm. The thickness of each layer may not fall outside of this range at any point between the first end and second end. In some embodiments, each layer may have a thickness in the range 20 to 300 nm. A minimum thickness of each layer may be between 2 and 300 nm, optionally between 20 and 300 nm. A maximum thickness of each layer may be between 2 and 300 nm, optionally between 20 and 300 nm. The minimum thickness of each layer is less than the maximum thickness of that respective layer. The minimum or maximum thickness of each layer may be at the first end of that layer. The other of the minimum or maximum thickness of the respective layer may be at the second end of that layer. At least one percentage change in thickness may be positive. At least one percentage change in thickness may be negative. Each percentage change in thickness may be in the range -150% to +150%. The number of layers of the plurality of layers may be at least 10, optionally at least 15, optionally at least 20. The number of layers of the plurality of layers may be in the range 10 to 30, optionally, 15-25. Summary of an improved waveguide coating In overview, the present disclosure relates to a method of forming a transmission coating comprising a plurality of layers. There is disclosed herein a method for providing an unlimited number of gradient thicknesses with the same hardware. The present disclosure is based on a dual target and a shadow mask that provides a different coating contribution from each target such that small adjustments achievable without opening the coating chamber can caused significant changes to the coating gradient in order to provide enhanced design freedom. The present disclosure is synergistic with the method of United States Patent US 11,852,832 and British patent application GB 2402387.1 which are each therefore incorporated herein by reference in their entirety. United States Patent US 11,852,832 discloses an improved efficiency method of fabricating graded coatings on a waveguide. The patent discloses that a percentage change in the thickness of each layer of the coating, from the first end to the second end, has one of a plurality of discrete allowed values, wherein the total number of layers is greater than the total number of discrete allowable values. British patent application GB 2402387.1 discloses a method of optimising coating by measuring a deviated coating function and modifying the coating design based on the deviated coating function in order that an improved coating may be provided without making hardware changes within the coating chamber such as changing the coating target. The method of the present disclosure may comprise a step a of determining a first coating parameter and a coating function for each layer to optimise the transmissivity at a plurality of locations along the waveguide for a plurality of different wavelengths. The coating function is chosen from a plurality of allowable coating functions. Next, in these examples, there is a step b of forming the plurality of layers using the determined coating parameters and coating functions. Then, there is a step c of measuring a thickness of at least one layer at each of the plurality of locations. The measurements indicate that the coating function deviates from that selected during the optimisation of step a. Finally, there is a step d of determining a second coating parameter for at least one layer by repeating the optimisation of step a using the coating function derived from the measurements of step c. Notably, the coating function is not adjusted. That is, the coating function determined in step a is retained by step d. However, the present disclosure is not limited to these steps. The coatings disclosed herein may be formed by any coating technique or by any coating geometry such as a planar coating geometry or a so-called rotating drum geometry, for example. The coatings are generally formed by a high temperature and / or high pressure technique such as physical vapour deposition e.g. sputtering. Summary of aspects and embodiments of the present disclosure A first aspect of the present disclosure is a method of fabricating a waveguide. The waveguide comprises a plastic core substrate, a first glass substrate and a second glass substrate. Each substrate may be substantially planar and / or may have a rectangular or square cross-section such as slab shaped. The method comprises forming a transmissive-reflective surface on a first glass substrate. The transmissive-reflective surface may be formed on a first major (e.g. largest surface area) side or face of the first glass substrate and the reflective surface may be formed on a second major (e.g. largest surface area) side or face of the second glass substrate. The first and second glass substrate may be the same size and shape. The transmissive-reflective surface comprises a plurality of gradient thickness layers. The plurality of gradient thickness layers is arranged (collectively) to provide a transmissivity that increases with distance in a first dimension of the first glass substrate and / or a reflectivity that decreases in the same direction. The first dimension may be the general direction of waveguiding of the waveguide. In some embodiments, the transmissive-reflective surface forms an output port for a plurality of replicas of a wavefront received at an input port of the waveguide. The first dimension may correspond to a direction of replication of the received wavefront. The method further comprises sandwiching a plastic core substrate between the transmissive-reflective surface of the first glass substrate and a reflective surface of a second glass substrate. The transmissive-reflective surface and reflective surface form a pair of complementary surfaces for (partial) waveguiding in the plastic core substrate. The transmissive-reflective surface and reflective surface are said to be enclosed because they encapsulate the plastic core substrate. The transmissive-reflective surface and reflective surface are therefore protected from the environment and other external factors. Typically, a glass substrate is used for waveguiding. The glass substrate would typically comprise a top surface comprising a transmissive-reflective coating and a second surface comprising a reflective coating. Each coating may comprise a complex stack of different dielectrics designed to form a plurality of uniform intensity replicas of a plurality of different colours. The design of such a dielectric stack is not trivial but can be achieved, and implemented on glass. Glass is expensive but can be polished to within 1 micrometre which makes it a preferred choice for imaging applications. A plastic waveguide might be desirable from a weight and cost perspective, but it is difficult -if not, impossible - to obtain high quality plastic optics that are large enough for high quality head-up display. It is particularly challenging to extend the quality of a large plastic optic to more than one surface because, as the plastic cools, it shrinks and bows. This makes it difficult to maintain the thickness of a planar e.g. slab shaped plastic substrate over a few hundreds of millimetres. Even if a high quality plastic waveguide could be fabricated, plastic cannot be polished to the same level as glass and techniques for depositing dielectric stacks are not suitable for a plastic substrate. It is therefore very difficult to find a path from a glass waveguide to a plastic equivalent for head-up display based on wavefront replication. Techniques for coating glass with dielectric stacks are well established. With this in mind, the inventor has devised the concept of a hybrid glass-plastic waveguide in which the core material for waveguiding is plastic but the optical coatings are formed on glass that at least partially surrounds the plastic. In some embodiments, a first and second coated glass layer are laminated onto a plastic core for waveguiding. This configuration is counter-intuitive because it might be expected that the core would need to be glass for high quality waveguiding and plastic could be clad around the glass to e.g. provide protection but the inventor has realised that the advantages of using glass on the outer layers to facilitate the complex optical coatings out-weight the loss of performance from waveguiding in plastic rather than glass. Imperfections in the plastic can be at least partially addressed by techniques such as rolling or pressing and index matching using a e.g. polymer gel or glue to fill voids between the plastic and glass owing to the imperfect plastic structure. Notably, the use of glass as an outer surface, or surfaces, of the waveguide provides a robust surface that can be used to provide additional functionality. For example, a liquid crystal cell could be integrated with the outer glass surface / s or the outer glass surface / s could be e.g. etched to provide a surface relief structure. These further advancements are synergistic with the concept of providing the complex optical coatings with wavefront replication on an outer glass laminate layer of a plastic core for waveguiding. The processes to form a liquid crystal cell using glass or form a surface relief structure are well established for glass but may be impossible with plastic. The top and bottom glass substrate of the hybrid waveguide disclosed herein may be put to different uses. In another example, a patterned electrochromic layer may be formed on one or both glass layers e.g. to provide switchability between a first mode e.g. transmissive and a second mode e.g. non-transmissive. In yet another example, an electrode - e.g. ITO electrode or array of electrodes - may be formed on one or both glass layers of the hybrid waveguide. This may not be possible on plastic. A second aspect of the present disclosure is a waveguide. The waveguide comprises a plastic core substrate sandwiched by a first glass substrate and a second glass substrate. A first surface of the first glass substrate is transmissive-reflective and a second surface of the second glass substrate is reflective. The waveguide is arranged to receive spatially modulated light and replicate the spatially modulated light by waveguiding in the plastic core between the transmissive-reflective surface and the reflective surface. The transmissive-reflective surface forms an output surface for the plurality of replicas of the spatially modulated light. Summary of terms The terms “transmissive” and “reflective” are used herein with reference to a plurality of discrete wavelengths such as a red, green and blue wavelength. It may be said that the wavefront comprises a plurality of single colours or it may equally be said that the wavefront comprises a plurality of single colour wavefront-lets or simply wavefronts. The term “transmissive-reflective” means that some light of each colour is reflected and some light of each colour is transmitted by the surface at each internal reflection. The term “gradient thickness” is used herein to refer to the thickness of the layer increasing or decreasing from an input side to an output side of the surface or waveguide. An increase in thickness is considered a positive thickness gradient and a decrease in thickness is considered a negative thickness gradient. The transmissive-reflective surface comprises at least two layers that have a gradient thickness. The material of each layer may vary. In some embodiments, the plurality of layers comprises an arrangement of alternating first and second dielectrics. The thickness of a gradient thickness layer may increase or decrease gradually or in uniform or non-uniform steps aligned with reflection - or so-called “bounce” -point or zone of the waveguide. Some layers may have a positive thickness gradient and others may have a negative thickness gradient. The term “partial waveguiding” is used herein to indicate that the wavefront (or different colour wavefronts) is not perfectly waveguided. That is, 100% of the light is not guided by internal reflection at each reflection or “bounce”. One surface is transmissive-reflective in order that a portion of the light escapes at each internal reflection or “bounce”. As explained further herein, the portion or percentage of light that escapes at each internal reflection or “bounce” increases with distance in order to improve uniformity of the wavefront replicas output by the waveguide. The reflective surface may be substantially 100% reflective. In accordance with this disclosure, the term “active layer” is used to refer to component that has a function or effects the functioning of the device. That is, the layer is not passive but instead performs a role in the behaviour of the device. The “active layer” may be an “optically active layer” meaning that the layer affects light such as refracts or diffracts light. The “active layer” may be an “electrically active layer” meaning that the layer performs an electrical function such as forms an electrode or other electrically switchable function. The term “hologram” is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, regarding the object. The term “holographic reconstruction” is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The system disclosed herein is described as a “holographic projector” because the holographic reconstruction is a real image and spatially-separated from the hologram. The term “replay field” is used to refer to the 2D area within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of a plurality diffracted orders wherein each diffracted order is a replica of the zeroth-order replay field. The zeroth-order replay field generally corresponds to the preferred or primary replay field because it is the brightest replay field. Unless explicitly stated otherwise, the term “replay field” should be taken as referring to the zeroth-order replay field. The term “replay plane” is used to refer to the plane in space containing all the replay fields. The terms “image”, “replay image” and “image region” refer to areas of the replay field illuminated by light of the holographic reconstruction. In some embodiments, the “image” may comprise discrete spots which may be referred to as “image spots” or, for convenience only, “image pixels”. The terms “encoding”, “writing” or “addressing” are used to describe the process of providing the plurality of pixels of the SLM with a respective plurality of control values which respectively determine the modulation level of each pixel. It may be said that the pixels of the SLM are configured to “display” a light modulation distribution in response to receiving the plurality of control values. Thus, the SLM may be said to “display” a hologram and the hologram may be considered an array of light modulation values or levels. It has been found that a holographic reconstruction of acceptable quality can be formed from a “hologram” containing only phase information related to the Fourier transform of the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography. The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully-complex computer-generated hologram is calculated. Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for “phase-delay”. That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2nj which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of ir / 2 will retard the phase of received light by k / 2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term “grey level” may be used to refer to the plurality of available modulation levels. For example, the term “grey level” may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term “grey level” may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator. The hologram therefore comprises an array of grey levels - that is, an array of light modulation values such as an array of phase-delay values or complex modulation values. The hologram is also considered a diffractive pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light having a wavelength comparable to, generally less than, the pixel pitch of the spatial light modulator. Reference is made herein to combining the hologram with other diffractive patterns such as diffractive patterns functioning as a lens or grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the replay field on the replay plane or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a replay plane in the near field. Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged. BRIEF DESCRIPTION OF THE DRAWINGS Specific embodiments are described by way of example only with reference to the following figures: Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen; Figure 2 shows a perspective view of a pair of wavefront replicators arranged for replication in two dimensions; Figure 3 shows a cross-sectional schematic view of a first waveguide according to the present disclosure; Figure 4 shows a close-up cross-sectional schematic view of a portion of the first waveguide of Figure 3; Figure 5 shows a graph of the ideal increasing transmissivity of a waveguide in the direction of waveguiding; and Figure 6 shows a process of forming a hybrid waveguide 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. 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 the example of Figure 1, an image is formed on a screen 125 by holographic reconstruction or transformation. The image may be replicated by a waveguide of the present disclosure. In this example, the waveguide receives, and replicates, a wavefront comprising spatially modulated light in accordance an image. In other examples of the present disclosure, an image is not formed on a screen and instead the hologram is propagated directly to the viewer. This may be described as hologram-to-eye and, at least conceptually, it may be said that the lens of the viewer’s eye performs the hologram to image transformation. In these examples, it may be said that the waveguide receives, and replicates, a wavefront comprising spatially modulated light in accordance with a hologram of an image. 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. Two-Dimensional Pupil Expansion or Wavefront Replication Figure 2 shows a perspective view of a system 200 comprising two pupil expanders or replicators, 204, 206 arranged for expanding a pupil or replicating a wavefront 202 in two dimensions. In the system 200 of Figure 2, the first replicator 204 comprises a first pair of surfaces, stacked parallel to one another, and arranged to provide replication - or, pupil expansion The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially elongate in one direction. The wavefront 202 is directed towards an input on the first replicator 204. The wavefront comprises spatially modulated light in an accordance with an image or a hologram of the image. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 2), which will be familiar to the skilled reader, light of the wavefront 202 is replicated in a first direction, along the length of the first replicator 204. Thus, a first plurality of replica wavefronts 208 is emitted from the first replicator 204, towards the second replicator 206. The second replicator 206 comprises a second pair of surfaces stacked parallel to one another, arranged to receive each of the collimated light beams of the first plurality of wavefronts 208 and further arranged to provide replication, or pupil expansion, by expanding each of those light beams in a second direction, substantially orthogonal to the first direction. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially rectangular. The rectangular shape is implemented for the second replicator in order for it to have length along the first direction, in order to receive the first plurality of wavefronts 208, and to have length along the second, orthogonal direction, in order to provide replication in that second direction. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 6), light of each light beam within the first plurality of wavefronts 208 is replicated in the second direction. Thus, a second plurality of wavefronts 210 is emitted from the second replicator 206, wherein the second plurality of wavefronts 210 comprises replicas of the wavefront 202 along each of the first direction and the second direction. Thus, the second plurality of wavefronts 210 may be regarded as comprising a two-dimensional grid, or array, of replica wavefronts. Thus, it can be said that the first and second replicators 204, 205 of Figure 2 combine to provide a two-dimensional replicator (or, “two-dimensional pupil expander”). Improved Waveguide As described in relation to Figure 2, light in the waveguide is reflected between a partially reflective, partially transmissive surface and a reflective surface of a waveguide. Light may undergo one or more reflections or bounces between the two reflective I reflective-transmissive planar surfaces and, at each bounce point on the partially transmissive surface, the light is divided such that a portion of the light is emitted out of the waveguide and the remaining (typically larger) portion of the light is reflected to continue to propagate between the two surfaces of the waveguide. This effectively results in the partially transmissive surface of the waveguide providing a plurality, n, of light emission zones for light waveguided between the first surface and second surface. After each bounce point I emission zone, the intensity of the light propagating in the waveguide will decrease. In other words, the intensity of the light propagating in the waveguide decreases in the direction of waveguiding. It is desirable for the intensity of the light emitted out of the waveguide at each of the n light emission zones to be substantially the same. This can be achieved this by providing an improved waveguide in which a layered coating is provided on the partially transmissive surface of the waveguide to cause the transmissivity of the partially transmissive surface to decrease in the direction of waveguiding. This accounts for the decrease in the intensity of the propagating light in the direction of waveguiding. Figure 3 is schematic cross-sectional view of a waveguide 308 according to the disclosure. The waveguide 308 comprises a first surface 302 and a second surface 304. A light field or wavefront 306 (represented by one light ray in Figure 3) is shown propagating through the waveguide 308. The second surface 304 comprises an input port arranged to receive the light field. The first surface 302 is partially transmissive, partially reflective and comprises a coating 303. The term “coating” is merely used herein for convenience and the person skilled in the art will appreciate that components described as a “coating” may be formed by any method including, but not limited to, a coating process. The second surface 304 is substantially fully reflective (other than at the input). Figure 3 shows the path of the light field or wavefront through the waveguide, bouncing between the first and second surfaces. On each reflection at the first surface, the light field divides such that a portion of the light field is emitted through the first surface and a remaining portion is reflected and continues to propagate between the first and second surfaces by reflection. So, an emission zone is effectively formed at each reflection point. Figure 3 shows six emission zones, however the skilled person will understand that there could, of course, be a larger or smaller number of reflections and emission zones. Figure 3 is merely illustrative. In some embodiments, the coating 303 comprises a plurality of layers of a first dielectric and a plurality of layers of a second dielectric in an alternating configuration. This is illustrated in Figure 4. The layers of the coating will be referred to herein by number, with the layer in contact with the first surface 302 being the first layer (layer 401). Layer 402 is on top of layer 401 and layer 403 is on top of layer 402. The layer furthest from the first surface 302, which is in top of layer 403, is the fourth layer 404. In this example, layers 401 and 403 are formed of silicon dioxide (SiO2) and layers 402 and 404 are formed of titanium dioxide (TiO2) such that the layers are in an alternating configuration in which subsequent layers of SiO2 (the first dielectric) are separated by layers of TiO2 (the second dielectric). Each of layers 401 to 404 has a varying thickness in the direction of waveguiding (from left to right in Figure 4) and, in this embodiment by way of example, has a linear profile. In other words, the rate of change of thickness of each layer is constant. The profile of each layer can be characterized using a percentage change in thickness. Each layer has a first end 406 and a second end 408. The percentage change in thickness is defined as the change in thickness from the first end 906 to the second end 408 divided by the thickness at the first end 406 multiplied by 100. For the case of the first layer 401, the percentage change in thickness is 100 x (final thickness 412 - initial thickness 410) I initial thickness 410. Layer 403 has the same percentage change in thickness as layer 401. Furthermore, the percentage change value is positive for both layers 401 and 403 (i.e. the thickness of the layer increases from the first end 406 to the second end 408). Layer 402 and 404 both have different percentage changes to one another and to layers 401 and 403. Furthermore, both the percentage change of layers 402 and 404 is negative (i.e. the thickness of the layers decreases from the first end 406 to the second end 408). It has been found that by, selecting an appropriate number of alternating layers of the first and second dielectric, with the layers having an appropriate thickness and percentage change in thickness from the first end to the second end, a first surface of the waveguide can be provided having a transmissivity that increases in the direction of waveguiding. In this way, the intensity of the light field emitted at each emission zone (i.e. the intensity of each replica emitted at each emission zone) is substantially constant. This may advantageously achieve a substantially spatially homogeneous emission of light from the waveguide. An ideal exponential increase of the transmissivity of the first surface 302 is shown in Figure 5 which is a graph showing transmissivity on the Y axis and position along the first surface 302 on the X axis. The numbers on the X axis represent the n emission zone. Specifically, the transmissivity increases according to the following equation: T(n) = where L is the optical loss factor of the waveguide material. The coating method used to fabricate such layers typically subjects the substrate to harsh environmental conditions such as high temperature e.g. from a plasma or high pressure. These conditions are not suitable for a plastic substrate. Hybrid glass-plastic waveguide Figure 6 shows a method of fabricating a waveguide in accordance with the present disclosure. The waveguide 600 comprises a plastic core substrate 610. The method comprises attaching or abutting (e.g. laminating) a first glass substrate 620 onto one side of the plastic core substrate 610 and a second glass substrate 630 onto the opposing side of the plastic core substrate 610. The plastic core substrate 610, first glass substrate 620 and second glass substrate 630 are substantially planar into this example. The planes of the plastic core substrate 610, first glass substrate 620 and second glass substrate 630 are substantially parallel. A first major - e.g. largest area - face 621 (e.g. bottom face) of the first glass substrate 620 is adjoined to a second major surface (e.g. top face) of the plastic core substrate 610. A second major face 632 (e.g. top face) of the second glass substrate 630 is adjoined to a first major surface (e.g. bottom face) of the plastic core substrate 610. The first glass substrate 620 and second glass substrate 630 may be laminated to the plastic core substrate 610. The first major surface 621 of the first glass substrate 620 is coated with a first dielectric stack to provide a transmissive-reflective surface or interface, as disclosed herein, such that substantially uniform intensity replicas at a plurality of discrete wavelengths are output therefrom. A second major surface 632 of the second glass substrate 630 - is coating with a second coating - which may or may not be a dielectric stack but could be a simple reflective coating - to provide reflectivity at the plurality of discrete wavelengths. Accordingly, light or a wavefront received at non-normal incidence may be at least partially waveguided between the transmissive-reflective coating (of the first glass substrate 620) and the reflective coating (of the second glass substrate 630). The laminated structure may be rolled or pressed to improve optical and / or mechanical performance. Index matching gel or glue may be disposed on the interface between each glass substrate and the plastic core substrate to improve the optical interface and / or compensate for any imperfections of the plastic e.g. resulting from an injecting moulding process. The transmissive-reflective coating of the first glass substrate 620 or the reflective coating of the second glass substrate 630 may comprise an in-coupling window which forms an input port for light or a wavefront for replication. The wavefront may correspond to an image or a hologram of the image for head-up display as previously described. In accordance with this disclosure, the optical coatings for waveguiding are thus protected from the outside environment. These surfaces may be referred to as inner surfaces. The outer surfaces may be utilised to provide additional optical or electrical functionality because glass is used as the outer most element. The additional functionality described in the following could not be achieved with an all-plastic waveguide. In further improvements, the opposing surface of one or both glass substrates may form an active surface of the waveguide - e.g. an optically active surface or an electrically active surface. In an example, the second major surface 622 of the first glass substrate 620 (and / or the first major surface 631 of the second glass substrate 630) is processed (e.g. etched) to form a surface relief structure. The surface relief structure may comprise an array of triangular prisms arranged to provide an optical turn (to the plurality of wavefront replicas) by refraction. The surface relief structure may additionally or alternatively at least partially compensate for a non-linear curvature of an optical combiner - e.g. vehicle windscreen -used with the waveguide to form a head-up display. This may be achieved by non-uniform shaping or curving of the prisms, or a face thereof, as described in British patent application GB 2405029.6 of 9 April 2024 incorporated herein by reference in its entirety. The surface relief structure may comprise a Fresnel-type structure. The surface relief structure may be optically active meaning it has a refractive or diffractive effect on the wavefront - or plurality of replicas thereof. That is, the surface relief structure may be arranged to refractive light and / or diffractive light. The surface relief structure may contribution to reflection suppression such as glare mitigation. In another embodiment, the second major surface 622 of the first glass substrate 620 (and / or the first major surface 631 of the second glass substrate 630) form a layer of a liquid crystal cell. The liquid crystal cell may comprise liquid crystal arranged (e.g. aligned or orientated) to provide the equivalent optical effect as the surface relief structure described above. Alternatively, or additionally, the liquid crystal cell may be used for light shuttering as described in British patent 2612153B incorporated herein by reference in its entirety. The light shuttering device may comprise a plurality of apertures, wherein each aperture (of the plurality of apertures) is individually switchable between a transmissive and a non-transmissive state. Alternatively, the liquid crystal cell may be used to form a Pancharatnam-Berry structure as disclosed in British patent application GB 2409773.5 of 5 July 2024 incorporated herein by reference in its entirety. The liquid crystal cell may comprise Reactive Mesogen and / or polymer dispersed liquid crystal. Alternatively, or additionally, the second major surface 622 of the first glass substrate 620 (and / or the first major surface 631 of the second glass substrate 630) may formed an electrically active layer of the waveguide - e.g. an electrode layer such as an array of indium tin oxide electrodes. Alternatively, or additionally, the second major surface 622 of the first glass substrate 620 (and / or the first major surface 631 of the second glass substrate 630) may be patterned with an electrochromic material. The functionally described above may be shared or divided between the second major surface 622 of the first glass substrate 620 and the first major surface 631 of the second glass substrate 630 in combination. For example, one surface may provide one of the functions described above and the other may provide another, different function described above. Any one or more of the functions described above may be shared between the two outer surfaces of the glass substrates. By way of example only, the glass may be so-called Gorilla Glass and / or the plastic may be PMMA. A largest linear dimension of each substrate may be 200 to 600 millimetres which is challenging for plastic. Additional features The methods and processes described herein may be embodied on a computer-readable medium. The term “computer-readable medium” includes a medium arranged to store data temporarily or permanently such as random-access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" shall also be taken to include any medium, or combination of multiple media, that is capable of storing instructions for execution by a machine such that the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methodologies described herein, in whole or in part. The term "computer-readable medium" also encompasses cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example form of a solid-state 5 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). 10 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 fabricating a waveguide, the method comprising:forming a transmissive-reflective surface on a first glass substrate, wherein the transmissive-reflective surface comprises a plurality of gradient thickness layers arranged to provide a transmissivity that increases with distance in a first dimension of the first glass substrate; andsandwiching a plastic core substrate between the transmissive-reflective surface of the first glass substrate and a reflective surface of a second glass substrate such that the transmissive-reflective surface and reflective surface form enclosed surfaces for partial waveguiding in the plastic core substrate.

2. A method as claimed in claim 1 wherein the step of forming the transmissive-reflective surface comprises coating the first glass substrate in environmental conditions (e.g. temperature and / or pressure) that would cause deformation or failure of the plastic core substrate.

3. A method as claimed in claim 1 or 2 wherein at least one layer of the plurality of gradient thickness layers comprises dielectric.

4. A method as claimed in any preceding claim wherein the plurality of gradient thickness layers comprises alternating layers of a first dielectric and a second dielectric.

5. A method as claimed in any preceding claim further comprising forming an active layer on the first glass substrate or the second glass substrate.

6. A method as claimed in claim 5 wherein the active layer is formed on the surface of the first glass substrate opposite the transmissive-reflective surface or on the surface of the second glass substrate opposite the reflective surface.

7. A method as claimed in claim 5 or 6 wherein the active layer is a surface relief structure.

8. A method as claimed in claim 7 wherein the surface relief structure comprises a Fresnel-type structure.

9. A method as claimed in claim 7 or 8 wherein the surface relief structure comprises a substantially linear array of substantially triangular prisms.

10. A method as claimed in any of claims 7 to 9 wherein the surface relief structure is diffractive or refractive.

11. A method as claimed in any of claims 7 to 10 wherein the surface relief structure is arranged to provide at least one selected from the group comprising: an optical turn of light; non-uniform optical power; reflection or glare suppression; and selective light shuttering comprising areas of transmission and areas of non-transmissive.

12. A method as claimed in claim 5 or 6 wherein the active layer is a component of a liquid crystal cell integrated with the waveguide.

13. A method as claimed in claim 12 wherein the liquid crystal cell comprises liquid crystal arranged to be optical equivalent to a Fresnel-type structure.

14. A method as claimed in claim 12 or 13 wherein the liquid crystal cell comprises liquid crystal arranged to be optically equivalent to a substantially linear array of substantially triangular prisms.

15. A method as claimed in any of claims 12 to 14 wherein the liquid crystal cell comprises liquid crystal arranged in a diffractive or refractive configuration.

16. A method as claimed in any of claims 12 to 15 wherein the liquid crystal cell comprises liquid crystal arranged to provide at least one selected from the group comprising: an optical turn of light; non-uniform optical power; reflection or glare suppression; and selective light shuttering comprising areas of transmission and areas of non-transmissive.

17. A method as claimed in any of claims 12 to 16 wherein the component of the liquid crystal cell comprises at least one electrode thereof.

18. A waveguide comprising a plastic core substrate sandwiched by a first glass substrate and a second glass substrate, wherein first surface of the first glass substrate is transmissive-reflective and a second surface of the second glass substrate is reflective, wherein the waveguide is arranged to receive spatially modulated light and replicate the spatially modulated light by waveguiding in the plastic core between the transmissive-reflective surface and the reflective surface, wherein the transmissive-reflective surface forms an output surface for the plurality of replicas of the spatially modulated light.30

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