WO2025109089A1

WO2025109089A1 - Suppression of ghost reflections in a windscreen

Info

Publication number: WO2025109089A1
Application number: PCT/EP2024/083137
Authority: WO
Inventors: Jamieson Christmas
Original assignee: Envisics Ltd
Current assignee: Envisics Ltd
Priority date: 2023-11-22
Filing date: 2024-11-21
Publication date: 2025-05-30
Anticipated expiration: 2026-05-22
Also published as: GB2635699A

Abstract

A head-up display system comprises an optical combiner, and a display system The display system is arranged to output head-up display light to the optical combiner. The head-up display light is linearly polarised light that is S-polarised. The optical combiner comprises a primary surface and a secondary surface. The primary surface is arranged to direct a first portion of the display light towards a viewing plane to form a primary image. The secondary surface is arranged to direct a second portion of the display light towards the viewing plane to form a ghost reflection of the primary image. The optical combiner comprises a polarisation converter between the primary surface and the secondary surface arranged to reduce the intensity of the ghost reflection.

Description

SUPPRESSION OF GHOST REFLECTIONS IN A WINDSCREEN

FIELD

The present disclosure relates to a display system, and more particularly to a head-up display system, in which an image is formed using an optical combiner. Embodiments relate to a head-up display, such as a holographic head-up display, and include an automotive head-up display, in which a vehicle windshield is used as an optical combiner.

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 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 micromirrors, for example.

A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission.

A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, “HUD”.

SUMMARY

Aspects of the present disclosure are defined in the appended independent claims.

In an aspect, a head-up display is provided. The head-up display comprises a display system and an optical combiner. The display system is arranged to output head-up display light to the optical combiner. The display light comprises polarised light. The polarised light is S-polarised. It may be said that the display light is linearly polarised in a direction perpendicular to the plane of incidence. The optical combiner comprises a primary surface arranged to direct a first portion of the display light towards a viewing plane to form a primary image. The optical combiner further comprises a secondary surface arranged to direct a second portion of the display light towards the viewing plane to form a ghost reflection of the primary image. The optical combiner comprises a polarisation converter between the primary surface and the secondary surface arranged to reduce the intensity of the ghost reflection.

An optical combiner of a head-up display comprises an optically transparent substrate comprising first and second major surfaces. A real world scene is seen by a viewer at the viewing plane through the transparent substrate. In addition, display light incident on the first surface is reflected towards the viewing plane. Thus, a primary image is formed by the optical combiner, which is seen by the viewer at the viewing plane. As the skilled person will appreciate, the primary image is a virtual image. However, as well as reflection of display light by the first surface of the optical combiner, a proportion of the light may be transmitted through the transparent substrate and reflected by the second surface towards the viewing plane. This may lead to the appearance of an undesirable secondary or ghost reflection of the primary image at the viewing plane. Such a secondary or ghost reflection is typically dimmer and blurred in comparison to the primary image. In addition, due to refraction of the display light by the substrate, the secondary or ghost reflection is offset from the primary image. This represents a distraction for the viewer and detrimentally impacts on the overall image quality of the head-up display. Conventional approaches to reducing secondary or ghost reflections involve complex optical combiner designs, which may incorporate a wedge having an angle chosen to overlap the ghost reflection and the primary image at the viewing plane. However, such designs lead to increased cost and complexity and may increase the thickness of the optical combiner. In addition, such approaches are unable to overlap ghost reflections of images formed at different virtual image planes.

Accordingly, the inventor proposes improved measures to reduce the appearance of such secondary or ghost reflections of an image formed at a viewing plane by an optical combiner. In particular, the improved measures suppress, or even eliminate, reflection of display light towards the viewing plane by surfaces of the optical combiner other than the primary surface used to form the primary image, such as a secondary surface of the optical combiner as described herein.

In particular, the inventor realised that an optical combiner may preferentially reflect light that is S-polarised, i.e., light with an electric field that is polarised perpendicular to the plane of incidence. Accordingly, the display system may be configured to provide head-up display light that is S-polarised so as to maximise a first portion of the display light, which is reflected from a primary surface of the optical combiner towards the viewing plane to form the primary image. In addition, the optical combiner may include a polarisation converter between the primary surface and a secondary surface thereof. The polarisation converter may convert a remaining portion of the head-up display light, which is transmitted through the primary surface into the optical combiner, so that reflection is suppressed, or even eliminated, at the secondary surface. In this way, the intensity of a secondary or ghost reflection of the primary image formed at the viewing plane is reduced.

The primary surface of the optical combiner may be arranged to receive the head-up display light before the secondary surface. It may be said that the head-up display light from the display is incident on the primary surface of the optical combiner.

The polarisation converter may be arranged to change the polarisation of the S-polarised head-up display light. In consequence, the head-up display light incident on the secondary surface of the optical combiner is no longer S-polarised.

In some embodiments, the polarisation converter comprises a half-wave component. The half wave component is positioned in the optical path between the primary surface and secondary surface of the optical combiner. Thus, S-polarised display light that is transmitted by the primary surface through the optical combiner is converted to P-polarised light, i.e., light with an electric field that is polarised parallel to the plane of incidence.

The inventor has found that, for typical angles of incidence, light polarised parallel to the plane of incidence experiences almost no reflection from the secondary surface of the optical combiner (i.e., it is substantially transmitted by the secondary surface). Thus, converting the display light, which is transmitted by the primary surface into the optical combiner, from S- polarised light to P-polarised light before it reaches the secondary surface significantly reduces the reflection of the display light therefrom. Thus the intensity of undesirable ghost reflections of the primary image at the viewing plane is reduced.

In other embodiments, the polarisation converter comprises a quarter-wave component. The quarter wave component is positioned in the optical path between the primary surface and secondary surface of the optical combiner. The S-polarised display light that is transmitted by the primary surface through the optical combiner is converted to circularly polarised light.

Display light that is circularly polarised may experience almost no reflection from the secondary surface of the optical combiner (i.e. , it is substantially transmitted by the secondary surface) at normal incidence. At other angles of incidence, the circularly polarised light will become elliptical due to the different reflectivity of the orthogonal S- and P-polarisation components. Thus, in some geometric configurations and applications, converting the display light, which is transmitted by the primary surface into the optical combiner, from S-polarised light to circularly polarised light, before the secondary surface may reduce the reflection of display light therefrom. Thus the intensity of undesirable secondary or ghost reflections of the primary image at the viewing plane is reduced.

In a further aspect of the invention, the polarised light is P-polarised. In this aspect, the polarisation converter may comprise a half-wave component positioned in the optical path between the primary surface and secondary surface of the optical combiner. Thus, P- polarised display light that is transmitted by the primary surface through the optical combiner is converted to S-polarised light.

In some embodiments, the polarisation converter and display system are arranged so that the polarisation converter reduces the intensity of the ghost reflection to substantially zero. Thus, negligible or even zero display light is directed towards the viewing plane by the secondary surface of the optical combiner, which would otherwise appear as a secondary or ghost reflection of the primary image. Thus, the appearance of secondary or ghost reflections of the primary image at the viewing plane is eliminated.

Embodiments described herein comprise a holographic head-up display system. The head- up display light may comprise spatially modulated light in accordance with a hologram of an image. In some arrangements, the head-up display light comprises a plurality of replicas of a spatially-modulated wavefront. In these arrangements, the plurality of replicas are spread over the primary and secondary surfaces of the optical combiner - i.e. are directed by areas of the surfaces, rather than a single point thereon. In other words, different replicas interact with different points of said surfaces. Each replica of the plurality of replicas may comprise a diverging light ray bundle. The spatially-modulated wavefront may comprise a holographic wavefront.

The display system may further comprise a substantially planar waveguide having a reflective surface arranged, during head-up display operation, in a configuration that is conducive to sunlight glare. That is, when the waveguide is positioned in used as described herein (for example, in the dashboard of a vehicle), the waveguide is in a position where it can be subject to sunlight. The substantially planar waveguide may be arranged to output the plurality of replicas of the spatially-modulated wavefront and further arranged such that it directs at least a portion of the sunlight towards the viewing plane via the first and second surfaces of the optical combiner. In other words, sunlight reflects off the waveguide, then off the surfaces of the optical combiner, before reaching the viewing plane. The polarisation converter may be further arranged to reduce the intensity of the sunlight directed towards the viewing plane by the substantially planar waveguide. The inventors have found that the polarisation converter can be used to reduce veiling glare (i.e. sunlight reflected back to the viewing plane via the optical combiner) in certain situations. If the veiling glare is primarily s- polarised on exit from the waveguide, the polarisation converter (for example, a half-wave plate) in the optical combiner can reduce (i.e. mitigate) reflection from the secondary surface of the optical combiner by same process as for the head-up display light. The veiling glare (i.e. the sunlight) may be s-polarised by reflection from the waveguide, or with the use of a further polarisation component located on an optical path between the waveguide and the optical combiner.

In some embodiments, the refractive index of the optical combiner is greater than that of air, optionally, wherein the primary surface comprises an air-glass interface and the secondary surface comprises a glass-air interface. In some implementations, the optical combiner may be a windscreen of a vehicle. The primary surface may be an inner surface of the windscreen facing a viewer, such as a driver of the vehicle, disposed at the viewing plane. The secondary surface may be an outer surface of the windscreen facing away from the viewer.

In some arrangements, the optical combiner comprises a polymer layer disposed between the primary and secondary surfaces thereof. For example, an optical combiner comprising safety glass, such as the windscreen of a vehicle, may comprise a polymer layer of polyvinyl butyral or the like, positioned between its major surfaces forming the primary and secondary surfaces. In such arrangements, the polarisation converter may be arranged adjacent the polymer layer, such as disposed on the polymer layer. In examples, the polarisation converter may comprise a half-wave film or quarter-wave film.

The optical combiner may be arranged so that it is tilted towards the viewing plane. The head-up display light may be incident on an area of the optical combiner which has a tilt angle substantially in the range of 40 to 65 degrees, such as 45 to 60 degrees, relative to the vertical.

The optical combiner comprising the polarisation converter may be arranged such that the primary surface and secondary surface are substantially parallel. The primary and secondary surfaces of the optical combiner may be planar or curved. In embodiments in which the polarisation converter comprises a film, the overall thickness of the optical combiner is optimised.

In some embodiments, the head-up display may be arranged such that the angle of incidence of the head-up display light on the primary surface is substantially in the range of 45 to 70 degrees, such as 50 to 65 degrees. Typically, the refractive index of the optical combiner is about 1 .5. In consequence, in embodiments, the angle of incidence of the head- up display light on the secondary surface is substantially in the range of 25 to 45 degrees such as 30 to 37 degrees.

In another aspect, a method of of suppressing a ghost reflection of head-up display light from a secondary surface of an optical combiner of a head-up display is provided. The method comprises providing S-polarised head-up display light to the optical combiner. The method further comprises converting the polarisation of the head-up display light within the optical combiner after partial transmission through a primary surface thereof. In consequence, the intensity of the ghost reflection of the head-up display light from the secondary surface is reduced.

In embodiments, the method comprises providing the S-polarised head-up display light at an angle of incidence on the primary surface that is substantially in the range of 45 to 70 degrees, such as 50 to 65 degrees and an angle of incidence on the secondary surface secondary surface that is substantially in the range of 25 to 45 degrees such as 30 to 37 degrees.

In yet another aspect, there is provided a windscreen comprising a primary surface, a secondary surface and a polarisation converter therebetween. The polarisation converter is arranged to reduce the intensity of a ghost reflection from the secondary surface when the windscreen is arranged as an optical combiner for head-up display using S-polarised light.

In the present disclosure, the term “replica” is merely used to reflect that spatially modulated light is divided such that a complex light field is directed along a plurality of different optical paths. The word “replica” is used to refer to each occurrence or instance of the complex light field after a replication event - such as a partial reflection-transmission by a pupil expander. Each replica travels along a different optical path. Some embodiments of the present disclosure relate to propagation of light that is encoded with a hologram, not an image - i.e., light that is spatially modulated with a hologram of an image, not the image itself. It may therefore be said that a plurality of replicas of the hologram are formed. The person skilled in the art of holography will appreciate that the complex light field associated with propagation of light encoded with a hologram will change with propagation distance. Use herein of the term “replica” is independent of propagation distance and so the two branches or paths of light associated with a replication event are still referred to as “replicas” of each other even if the branches are a different length, such that the complex light field has evolved differently along each path. That is, two complex light fields are still considered “replicas” in accordance with this disclosure even if they are associated with different propagation distances - providing they have arisen from the same replication event or series of replication events.

A “diffracted light field” or “diffractive light field” in accordance with this disclosure is a light field formed by diffraction. A diffracted light field may be formed by illuminating a corresponding diffractive pattern. In accordance with this disclosure, an example of a diffractive pattern is a hologram and an example of a diffracted light field is a holographic light field or a light field forming a holographic reconstruction of an image. The holographic light field forms a (holographic) reconstruction of an image on a replay plane. The holographic light field that propagates from the hologram to the replay plane may be said to comprise light encoded with the hologram or light in the hologram domain. A diffracted light field is characterized by a diffraction angle determined by the smallest feature size of the diffractive structure and the wavelength of the light (of the diffracted light field). In accordance with this disclosure, it may also be said that a “diffracted light field” is a light field that forms a reconstruction on a plane spatially separated from the corresponding diffractive structure. An optical system is disclosed herein for propagating a diffracted light field from a diffractive structure to a viewer. The diffracted light field may form an image.

The term “hologram” is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, regarding the object. The term “holographic reconstruction” is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The system disclosed herein is described as a “holographic projector” because the holographic reconstruction is a real image and spatially- separated from the hologram. The term “replay field” is used to refer to the 2D area within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of a plurality diffracted orders wherein each diffracted order is a replica of the zeroth-order replay field. The zeroth-order replay field generally corresponds to the preferred or primary replay field because it is the brightest replay field. Unless explicitly stated otherwise, the term “replay field” should be taken as referring to the zeroth-order replay field. The term “replay plane” is used to refer to the plane in space containing all the replay fields. The terms “image”, “replay image” and “image region” refer to areas of the replay field illuminated by light of the holographic reconstruction. In some embodiments, the “image” may comprise discrete spots which may be referred to as “image spots” or, for convenience only, “image pixels”.

The terms “encoding”, “writing” or “addressing” are used to describe the process of providing the plurality of pixels of the SLM with a respective plurality of control values which respectively determine the modulation level of each pixel. It may be said that the pixels of the SLM are configured to “display” a light modulation distribution in response to receiving the plurality of control values. Thus, the SLM may be said to “display” a hologram and the hologram may be considered an array of light modulation values or levels.

It has been found that a holographic reconstruction of acceptable quality can be formed from a “hologram” containing only phase information related to the Fourier transform of the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography.

The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully- complex computer-generated hologram is calculated.

Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for “phase-delay”. That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2TT) which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of TT/2 will retard the phase of received light by TT/2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term “grey level” may be used to refer to the plurality of available modulation levels. For example, the term “grey level” may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term “grey level” may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator.

The hologram therefore comprises an array of grey levels - that is, an array of light modulation values such as an array of phase-delay values or complex modulation values. The hologram is also considered a diffractive pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light having a wavelength comparable to, generally less than, the pixel pitch of the spatial light modulator. Reference is made herein to combining the hologram with other diffractive patterns such as diffractive patterns functioning as a lens or grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the replay field on the replay plane or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a replay plane in the near field.

Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments are described by way of example only with reference to the following figures:

Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen;

Figure 2 shows an image for projection comprising eight image areas/components, V1 to V8, and cross-sections of the corresponding hologram channels, H1-H8;

Figure 3 shows a hologram displayed on an LCOS that directs light into a plurality of discrete areas;

Figure 4 shows a system, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3;

Figure 5A shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each comprising pairs of stacked surfaces;

Figure 5B shows a perspective view of a first example two-dimensional pupil expander;

Figure 6 shows an example of a head-up display package comprising a waveguide and an eye-tracker;

Figure 7 shows the reflection of head-up display light towards a viewing plane by a conventional optical combiner, and

Figure 8 shows the reflection head-up display a towards a viewing plane by an optical combiner according to the present disclosure.

The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration.

Terms of a singular form may include plural forms unless specified otherwise. A structure described as being formed at an upper portion/lower portion of another structure or on/under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between.

In describing a time relationship - for example, when the temporal order of events is described as “after”, “subsequent”, “next”, “before” or suchlike - the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as “just”, “immediate” or “direct” is used. Although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims.

Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in codependent relationship. In the present disclosure, the term “substantially” when applied to a structural units of an apparatus may be interpreted as the technical feature of the structural units being produced within the technical tolerance of the method used to manufacture it.

Conventional optical configuration for holographic projection

Figure 1 shows an embodiment in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or frequency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, “LCOS”, device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser.

A light source 110, for example a laser or laser diode, is disposed to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally planar wavefront of light to be incident on the SLM. In Figure 1 , the direction of the wavefront is off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer). However, in other embodiments, the generally planar wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in Figure 1 , the arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a lightmodulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to optics including a Fourier transform lens 120, having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives a beam of modulated light from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at the screen 125.

Notably, in this type of holography, each pixel of the hologram contributes to the whole reconstruction. There is not a one-to-one correlation between specific points (or image pixels) on the replay field and specific light-modulating elements (or hologram pixels). In other words, modulated light exiting the light-modulating layer is distributed across the replay field.

In these embodiments, the position of the holographic reconstruction in space is determined by the dioptric (focusing) power of the Fourier transform lens. In the embodiment shown in Figure 1 , the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens but the performance of the lens will limit the accuracy of the Fourier transform it performs. The skilled person understands how to use a lens to perform an optical Fourier transform In some embodiments of the present disclosure, the lens of the viewer’s eye performs the hologram to image transformation.

Hologram calculation

In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or Fourier-based hologram, in which an image is reconstructed in the far field by utilising the Fourier transforming properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back to the lens plane. Computer-generated Fourier holograms may be calculated using Fourier transforms. Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms which may be calculated by a similar method. In some embodiments, the hologram is a phase or phase-only hologram. However, the present disclosure is also applicable to holograms calculated by other techniques such as those based on point cloud methods.

In some embodiments, the hologram engine is arranged to exclude from the hologram calculation the contribution of light blocked by a limiting aperture of the display system. British patent application 2101666.2, filed 5 February 2021 and incorporated herein by reference, discloses a first hologram calculation method in which eye-tracking and ray tracing are used to identify a sub-area of the display device for calculation of a point cloud hologram which eliminates so called “ghost images”. As the skilled person will appreciate, such “ghost images” are formed by the head-up display system (e.g., as a result of pupil expansion) that outputs display light to the optical combiner. Thus, these “ghost images” are different from secondary or ghost reflections of a primary image formed by the optical combiner, as described herein. The sub-area of the display device corresponds with the aperture, of the present disclosure, and is used exclude light paths from the hologram calculation. British patent application 2112213.0, filed 26 August 2021 and incorporated herein by reference, discloses a second method based on a modified Gerchberg-Saxton type algorithm which includes steps of light field cropping in accordance with pupils of the optical system during hologram calculation. The cropping of the light field corresponds with the determination of a limiting aperture of the present disclosure. British patent application 2118911.3, filed 23 December 2021 and also incorporated herein by reference, discloses a third method of calculating a hologram which includes a step of determining a region of a so- called extended modulator formed by a hologram replicator. The region of the extended modulator is also an aperture in accordance with this disclosure.

In some embodiments, there is provided a real-time engine arranged to receive image data and calculate holograms in real-time using the algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are pre-calculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms. Large field of view and eye-box using small display device

Broadly, the present disclosure relates to image projection. It relates to a method of image projection and an image projector which comprises a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system, in which the image projector projects or relays light from the display device to the viewing system. The present disclosure is equally applicable to a monocular and binocular viewing system. The viewing system may comprise a viewer’s eye or eyes. The viewing system comprises an optical element having optical power (e.g., lens/es of the human eye) and a viewing plane (e.g., retina of the human eye/s). The projector may be referred to as a ‘light engine’. The display device and the image formed (or perceived) using the display device are spatially separated from one another. The image is formed, or perceived by a viewer, on a display plane. In some embodiments, the image is a virtual image and the display plane may be referred to as a virtual image plane. In other examples, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed either in free space or on a screen or other light receiving surface between the display device and the viewer, is propagated to the viewer. In both cases, an image is formed by illuminating a diffractive pattern (e.g., hologram or kinoform) displayed on the display device.

The display device comprises pixels. The pixels of the display may display a diffractive pattern or structure that diffracts light. The diffracted light may form an image at a plane spatially separated from the display device. In accordance with well-understood optics, the magnitude of the maximum diffraction angle is determined by the size of the pixels and other factors such as the wavelength of the light.

In embodiments, the display device is a spatial light modulator such as liquid crystal on silicon (“LCOS”) spatial light modulator (SLM). Light propagates over a range of diffraction angles (for example, from zero to the maximum diffractive angle) from the LCOS, towards a viewing entity/system such as a camera or an eye. In some embodiments, magnification techniques may be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of an LCOS.

In some embodiments, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) - that may be informally said to be “encoded” with/by the hologram - is propagated directly to the viewer’s eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction I image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device.

Reference is made herein to a “light field” which is a “complex light field”. The term “light field” merely indicates a pattern of light having a finite size in at least two orthogonal spatial directions, e.g. x and y. The word “complex” is used herein merely to indicate that the light at each point in the light field may be defined by an amplitude value and a phase value, and may therefore be represented by a complex number or a pair of values. For the purpose of hologram calculation, the complex light field may be a two-dimensional array of complex numbers, wherein the complex numbers define the light intensity and phase at a plurality of discrete locations within the light field.

In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity/system, varies with the distance between the display device and the viewing entity. At a 1 metre viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye’s pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye’s pupil to form an image at the retina for a given eye position, determines the portion of the image that is ‘visible’ to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-box.)

In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device - that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it may therefore be considered that the viewer is looking at a virtual image through an ‘display device-sized window’, which may be very small, for example 1cm in diameter, at a relatively large distance, e.g., 1 metre. And the user will be viewing the display device-sized window via the pupil(s) of their eye(s), which can also be very small. Accordingly, the field of view becomes small and the specific angular range that can be seen depends heavily on the eye position, at any given time. A pupil expander addresses the problem of how to increase the range of angles of light rays that are propagated from the display device that can successfully propagate through an eye’s pupil to form an image. The display device is generally (in relative terms) small and the projection distance is (in relative terms) large. In some embodiments, the projection distance is at least one - such as, at least two - orders of magnitude greater than the diameter, or width, of the entrance pupil and/or aperture of the display device (i.e., size of the array of pixels).

Use of a pupil expander increases the viewing area (i.e., user’s eye-box) laterally, thus enabling some movement of the eye/s to occur, whilst still enabling the user to see the image. As the skilled person will appreciate, in an imaging system, the viewing area (user’s eye box) is the area in which a viewer’s eyes can perceive the image. The present disclosure encompasses non-infinite virtual image distances - that is, near-field virtual images.

Conventionally, a two-dimensional pupil expander comprises one or more one-dimensional optical waveguides each formed using a pair of opposing reflective surfaces, in which the output light from a surface forms a viewing window or eye-box. Light received from the display device (e.g., spatially modulated light from a LCOS) is replicated by the or each waveguide so as to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide enlarges the viewing window due to the generation of extra rays or “replicas” by division of amplitude of the incident wavefront.

The display device may have an active or display area having a first dimension that may be less than 10 cms such as less than 5 cms or less than 2 cms. The propagation distance between the display device and viewing system may be greater than 1 m such as greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide may be up to 2 m such as up to 1 .5 m or up to 1 m. The method may be capable of receiving an image and determining a corresponding hologram of sufficient quality in less than 20 ms such as less than 15 ms or less than 10 ms.

In some embodiments - described only by way of example of a diffracted or holographic light field in accordance with this disclosure - a hologram is configured to route light into a plurality of channels, each channel corresponding to a different part (i.e. sub-area) of an image. The channels formed by the diffractive structure are referred to herein as “hologram channels” merely to reflect that they are channels of light encoded by the hologram with image information. It may be said that the light of each channel is in the hologram domain rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram and the hologram domain is therefore the Fourier or frequency domain. The hologram may equally be a Fresnel or Fresnel transform hologram. The hologram may also be a point cloud hologram. The hologram is described herein as routing light into a plurality of hologram channels to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-areas, wherein each hologram channel would correspond to each image sub-area. Importantly, the hologram of this example is characterised by how it distributes the image content when illuminated. Specifically and uniquely, the hologram divides the image content by angle. That is, each point on the image is associated with a unique light ray angle in the spatially modulated light formed by the hologram when illuminated - at least, a unique pair of angles because the hologram is two-dimensional. For the avoidance of doubt, this hologram behaviour is not conventional. The spatially modulated light formed by this special type of hologram, when illuminated, may be divided into a plurality of hologram channels, wherein each hologram channel is defined by a range of light ray angles (in two-dimensions). It will be understood from the foregoing that any hologram channel (i.e. sub-range of light ray angles) that may be considered in the spatially modulated light will be associated with a respective part or sub-area of the image. That is, all the information needed to reconstruct that part or sub-area of the image is contained within a sub-range of angles of the spatially modulated light formed from the hologram of the image. When the spatially modulated light is observed as a whole, there is not necessarily any evidence of a plurality of discrete light channels.

Nevertheless, the hologram may still be identified. For example, if only a continuous part or sub-area of the spatially modulated light formed by the hologram is reconstructed, only a sub-area of the image should be visible. If a different, continuous part or sub-area of the spatially modulated light is reconstructed, a different sub-area of the image should be visible. A further identifying feature of this type of hologram is that the shape of the cross-sectional area of any hologram channel substantially corresponds to (i.e. is substantially the same as) the shape of the entrance pupil although the size may be different - at least, at the correct plane for which the hologram was calculated. Each light I hologram channel propagates from the hologram at a different angle or range of angles. Whilst these are example ways of characterising or identifying this type of hologram, other ways may be used. In summary, the hologram disclosed herein is characterised and identifiable by how the image content is distributed within light encoded by the hologram. Again, for the avoidance of any doubt, reference herein to a hologram configured to direct light or angularly-divide an image into a plurality of hologram channels is made by way of example only and the present disclosure is equally applicable to pupil expansion of any type of holographic light field or even any type of diffractive or diffracted light field.

The system can be provided in a compact and streamlined physical form. This enables the system to be suitable for a broad range of real-world applications, including those for which space is limited and real-estate value is high. For example, it may be implemented in a head-up display (HUD) such as a vehicle or automotive HUD.

In accordance with the present disclosure, pupil expansion is provided for diffracted or diffractive light, which may comprise diverging ray bundles. The diffracted light field may be defined by a “light cone”. Thus, the size of the diffracted light field (as defined on a two- dimensional plane) increases with propagation distance from the corresponding diffractive structure (i.e. display device). It can be said that the pupil expander/s replicate the hologram or form at least one replica of the hologram, to convey that the light delivered to the viewer is spatially modulated in accordance with a hologram.

In some embodiments, two one-dimensional waveguide pupil expanders are provided, each one-dimensional waveguide pupil expander being arranged to effectively increase the size of the exit pupil of the system by forming a plurality of replicas or copies of the exit pupil (or light of the exit pupil) of the spatial light modulator. The exit pupil may be understood to be the physical area from which light is output by the system. It may also be said that each waveguide pupil expander is arranged to expand the size of the exit pupil of the system. It may also be said that each waveguide pupil expander is arranged to expand/increase the size of the eye box within which a viewer’s eye can be located, in order to see/receive light that is output by the system.

Light channelling

The hologram formed in accordance with some embodiments, angularly-divides the image content to provide a plurality of hologram channels which may have a cross-sectional shape defined by an aperture of the optical system. The hologram is calculated to provide this channelling of the diffracted light field. In some embodiments, this is achieved during hologram calculation by considering an aperture (virtual or real) of the optical system, as described above. Figures 2 and 3 show an example of this type of hologram that may be used in conjunction with a pupil expander as disclosed herein. However, this example should not be regarded as limiting with respect to the present disclosure.

Figure 2 shows an image 252 for projection comprising eight image areas/components, V1 to V8. Figure 2 shows eight image components by way of example only and the image 252 may be divided into any number of components. Figure 2 also shows an encoded light pattern 254 (i.e., hologram) that can reconstruct the image 252 - e.g., when transformed by the lens of a suitable viewing system. The encoded light pattern 254 comprises first to eighth sub-holograms or components, H1 to H8, corresponding to the first to eighth image components/areas, V1 to V8. Figure 2 further shows how a hologram may decompose the image content by angle. The hologram may therefore be characterised by the channelling of light that it performs. This is illustrated in Figure 3. Specifically, the hologram in this example directs light into a plurality of discrete areas. The discrete areas are discs in the example shown but other shapes are envisaged. The size and shape of the optimum disc may, after propagation through the waveguide, be related to the size and shape of an aperture of the optical system such as the entrance pupil of the viewing system.

Figure 4 shows a system 400, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3.

The system 400 comprises a display device, which in this arrangement comprises an LCOS 402. The LCOS 402 is arranged to display a modulation pattern (or ‘diffractive pattern') comprising the hologram and to project light that has been holographically encoded towards an eye 405 that comprises a pupil that acts as an aperture 404, a lens 409, and a retina (not shown) that acts as a viewing plane. There is a light source (not shown) arranged to illuminate the LCOS 402. The lens 409 of the eye 405 performs a hologram-to-image transformation. The light source may be of any suitable type. For example, it may comprise a laser light source.

The viewing system 400 further comprises a waveguide 408 positioned between the LCOS 402 and the eye 405. The presence of the waveguide 408 enables all angular content from the LCOS 402 to be received by the eye, even at the relatively large projection distance shown. This is because the waveguide 508 acts as a pupil expander, in a manner that is well known and so is described only briefly herein. In brief, the waveguide 408 shown in Figure 4 comprises a substantially elongate formation. In this example, the waveguide 408 comprises an optical slab of refractive material, but other types of waveguide are also well known and may be used. The waveguide 408 is located so as to intersect the light cone (i.e., the diffracted light field) that is projected from the LCOS 402, for example at an oblique angle. In this example, the size, location, and position of the waveguide 408 are configured to ensure that light from each of the eight ray bundles, within the light cone, enters the waveguide 408. Light from the light cone enters the waveguide 408 via its first planar surface (located nearest the LCOS 402) and is guided at least partially along the length of the waveguide 408, before being emitted via its second planar surface, substantially opposite the first surface (located nearest the eye). As will be well understood, the second planar surface is partially reflective, partially transmissive. In other words, when each ray of light travels within the waveguide 408 from the first planar surface and hits the second planar surface, some of the light will be transmitted out of the waveguide 408 and some will be reflected by the second planar surface, back towards the first planar surface. The first planar surface is reflective, such that all light that hits it, from within the waveguide 408, will be reflected back towards the second planar surface. Therefore, some of the light may simply be refracted between the two planar surfaces of the waveguide 408 before being transmitted, whilst other light may be reflected, and thus may undergo one or more reflections, (or ‘bounces’) between the planar surfaces of the waveguide 408, before being transmitted.

Figure 4 shows a total of nine “bounce” points, B0 to B8, along the length of the waveguide 408. Although light relating to all points of the image (V1-V8) as shown in Figure 2 is transmitted out of the waveguide at each “bounce” from the second planar surface of the waveguide 408, only the light from one angular part of the image (e.g. light of one of 1 to V8) has a trajectory that enables it to reach the eye 405, from each respective “bounce” point, B0 to B8. Moreover, light from a different angular part of the image, V1 to V8, reaches the eye 405 from each respective “bounce” point. Therefore, each angular channel of encoded light reaches the eye only once, from the waveguide 408, in the example of Figure 4.

The waveguide 408 forms a plurality of replicas of the hologram, at the respective “bounce” points B1 to B8 along its length, corresponding to the direction of pupil expansion. As shown in Figure 4, the plurality of replicas may be extrapolated back, in a straight line, to a corresponding plurality of replica or virtual display devices 402’. This process corresponds to the step of “unfolding” an optical path within the waveguide, so that a light ray of a replica is extrapolated back to a “virtual surface” without internal reflection within the waveguide. Thus, the light of the expanded exit pupil may be considered to originate from a virtual surface (also called an “extended modulator” herein) comprising the display device 402 and the replica display devices 402’.

Although virtual images, which require the eye to transform received modulated light in order to form a perceived image, have generally been discussed herein, the methods and arrangements described herein can be applied to real images.

Two-Dimensional Pupil Expansion

Whilst the arrangement shown in Figure 4 includes a single waveguide that provides pupil expansion in one dimension, pupil expansion can be provided in more than one dimension, for example in two dimensions. Moreover, whilst the example in Figure 4 uses a hologram that has been calculated to create channels of light, each corresponding to a different portion of an image, the present disclosure and the systems that are described herebelow are not limited to such a hologram type.

Figure 5A shows a perspective view of a system 500 comprising two replicators, 504, 506 arranged for expanding a light beam 502 in two dimensions.

In the system 500 of Figure 5A, the first replicator 504 comprises a first pair of surfaces, stacked parallel to one another, and arranged to provide replication - or, pupil expansion - in a similar manner to the waveguide 408 of Figure 4. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially elongate in one direction. The collimated light beam 502 is directed towards an input on the first replicator 504. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), which will be familiar to the skilled reader, light of the light beam 502 is replicated in a first direction, along the length of the first replicator 504. Thus, a first plurality of replica light beams 508 is emitted from the first replicator 504, towards the second replicator 506.

The second replicator 506 comprises a second pair of surfaces stacked parallel to one another, arranged to receive each of the collimated light beams of the first plurality of light beams 508 and further arranged to provide replication - or, pupil expansion - by expanding each of those light beams in a second direction, substantially orthogonal to the first direction. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially rectangular. The rectangular shape is implemented for the second replicator in order for it to have length along the first direction, in order to receive the first plurality of light beams 508, and to have length along the second, orthogonal direction, in order to provide replication in that second direction. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), light of each light beam within the first plurality of light beams 508 is replicated in the second direction. Thus, a second plurality of light beams 510 is emitted from the second replicator 506, wherein the second plurality of light beams 510 comprises replicas of the input light beam 502 along each of the first direction and the second direction. Thus, the second plurality of light beams 510 may be regarded as comprising a two-dimensional grid, or array, of replica light beams.

Thus, it can be said that the first and second replicators 504, 505 of Figure 5A combine to provide a two-dimensional replicator (or, “two-dimensional pupil expander”). Thus, the replica light beams 510 may be emitted along an optical path to an expanded eye-box of a display system, such as a head-up display.

In the system of Figure 5A, the first replicator 504 is a waveguide comprising a pair of elongate rectilinear reflective surfaces, stacked parallel to one another, and, similarly, the second replicator 504 is a waveguide comprising a pair of rectangular reflective surfaces, stacked parallel to one another. In other systems, the first replicator may be a solid elongate rectilinear waveguide and the second replicator may be a solid planar rectangular shaped waveguide, wherein each waveguide comprises an optically transparent solid material such as glass. In this case, the pair of parallel reflective surfaces are formed by a pair of opposed major sidewalls optionally comprising respective reflective and reflective- transmissive surface coatings, familiar to the skilled reader.

Figure 5B shows a perspective view of a system 500 comprising two replicators, 520, 540 arranged for replicating a light beam 522 in two dimensions, in which the first replicator is a solid elongated waveguide 520 and the second replicator is a solid planar waveguide 540.

In the system of Figure 5B, the first replicator/waveguide 520 is arranged so that its pair of elongate parallel reflective surfaces 524a, 524b are perpendicular to the plane of the second replicator/ waveguide 540. Accordingly, the system comprises an optical coupler arranged to couple light from an output port of first replicator 520 into an input port of the second replicator 540. In the illustrated arrangement, the optical coupler is a planar/fold mirror 530 arranged to fold or turn the optical path of light to achieve the required optical coupling from the first replicator to the second replicator. As shown in Figure 5B, the mirror 530 is arranged to receive light - comprising a one-dimensional array of replicas extending in the first dimension - from the output port I reflective-transmissive surface 524a of the first replicator/waveguide 520. The mirror 530 is tilted so as to redirect the received light onto an optical path to an input port in the (fully) reflective surface of second replicator 540 at an angle to provide waveguiding and replica formation, along its length in the second dimension. It will be appreciated that the mirror 530 is one example of an optical element that can redirect the light in the manner shown, and that one or more other elements may be used instead, to perform this task.

In the illustrated arrangement, the (partially) reflective-transmissive surface 524a of the first replicator 520 is adjacent the input port of the first replicator/waveguide 520 that receives input beam 522 at an angle to provide waveguiding and replica formation, along its length in the first dimension. Thus, the input port of first replicator/waveguide 520 is positioned at an input end thereof at the same surface as the reflective-transmissive surface 524a. The skilled reader will understand that the input port of the first replicator/waveguide 520 may be at any other suitable position.

Accordingly, the arrangement of Figure 5B enables the first replicator 520 and the mirror 530 to be provided as part of a first relatively thin layer in a plane in the first and third dimensions (illustrated as an x-z plane). In particular, the size or “height” of a first planar layer - in which the first replicator 520 is located - in the second dimension (illustrated as the y dimension) is reduced. The mirror 530 is configured to direct the light away from a first layer/plane, in which the first replicator 520 is located (i.e. the “first planar layer"), and direct it towards a second layer/plane, located above and substantially parallel to the first layer/plane, in which the second replicator 540 is located (i.e. a “second planar layer”). Thus, the overall size or “height” of the system - comprising the first and second replicators 520, 540 and the mirror 530 located in the stacked first and second planar layers in the first and third dimensions (illustrated as an x-z plane) - in the second dimension (illustrated as the y dimension) is compact. The skilled reader will understand that many variations of the arrangement of Figure 5B for implementing the present disclosure are possible and contemplated.

The image projector may be arranged to project a diverging or diffracted light field. In some embodiments, the light field is encoded with a hologram. In some embodiments, the diffracted light field comprises diverging ray bundles. In some embodiments, the image formed by the diffracted light field is a virtual image. In some embodiments, the first pair of parallel / complementary surfaces are elongate or elongated surfaces, being relatively long along a first dimension and relatively short along a second dimension, for example being relatively short along each of two other dimensions, with each dimension being substantially orthogonal to each of the respective others. The process of reflection/transmission of the light between/from the first pair of parallel surfaces is arranged to cause the light to propagate within the first waveguide pupil expander, with the general direction of light propagation being in the direction along which the first waveguide pupil expander is relatively long (i.e., in its “elongate” direction).

There is disclosed herein a system that forms an image using diffracted light and provides an eye-box size and field of view suitable for real-world application - e.g. in the automotive industry by way of a head-up display. The diffracted light is light forming a holographic reconstruction of the image from a diffractive structure - e.g. hologram such as a Fourier or Fresnel hologram. The use diffraction and a diffractive structure necessitates a display device with a high density of very small pixels (e.g. 1 micrometer) - which, in practice, means a small display device (e.g. 1 cm). The inventors have addressed a problem of how to provide 2D pupil expansion with a diffracted light field e.g. diffracted light comprising diverging (not collimated) ray bundles.

In some embodiments, the display system comprises a display device - such as a pixelated display device, for example a spatial light modulator (SLM) or Liquid Crystal on Silicon (LCoS) SLM - which is arranged to provide or form the diffracted or diverging light. In such aspects, the aperture of the spatial light modulator (SLM) is a limiting aperture of the system. That is, the aperture of the spatial light modulator - more specifically, the size of the area delimiting the array of light modulating pixels comprised within the SLM - determines the size (e.g. spatial extent) of the light ray bundle that can exit the system. In accordance with this disclosure, it is stated that the exit pupil of the system is expanded to reflect that the exit pupil of the system (that is limited by the small display device having a pixel size for light diffraction) is made larger or bigger or greater in spatial extend by the use of at least one pupil expander.

The diffracted or diverging light field may be said to have “a light field size”, defined in a direction substantially orthogonal to a propagation direction of the light field. Because the light is diffracted I diverging, the light field size increases with propagation distance.

In some embodiments, the diffracted light field is spatially-modulated in accordance with a hologram. In other words, in such aspects, the diffractive light field comprises a “holographic light field”. The hologram may be displayed on a pixelated display device. The hologram may be a computer-generated hologram (CGH). It may be a Fourier hologram or a Fresnel hologram or a point-cloud hologram or any other suitable type of hologram. The hologram may, optionally, be calculated so as to form channels of hologram light, with each channel corresponding to a different respective portion of an image that is intended to be viewed (or perceived, if it is a virtual image) by the viewer. The pixelated display device may be configured to display a plurality of different holograms, in succession or in sequence. Each of the aspects and embodiments disclosed herein may be applied to the display of multiple holograms.

The output port of the first waveguide pupil expander may be coupled to an input port of a second waveguide pupil expander. The second waveguide pupil expander may be arranged to guide the diffracted light field - including some of, preferably most of, preferably all of, the replicas of the light field that are output by the first waveguide pupil expander - from its input port to a respective output port by internal reflection between a third pair of parallel surfaces of the second waveguide pupil expander.

The first waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a first direction and the second waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a second, different direction. The second direction may be substantially orthogonal to the first direction. The second waveguide pupil expander may be arranged to preserve the pupil expansion that the first waveguide pupil expander has provided in the first direction and to expand (or, replicate) some of, preferably most of, preferably all of, the replicas that it receives from the first waveguide pupil expander in the second, different direction. The second waveguide pupil expander may be arranged to receive the light field directly or indirectly from the first waveguide pupil expander. One or more other elements may be provided along the propagation path of the light field between the first and second waveguide pupil expanders.

The first waveguide pupil expander may be substantially elongated and the second waveguide pupil expander may be substantially planar. The elongated shape of the first waveguide pupil expander may be defined by a length along a first dimension. The planar, or rectangular, shape of the second waveguide pupil expander may be defined by a length along a first dimension and a width, or breadth, along a second dimension substantially orthogonal to the first dimension. A size, or length, of the first waveguide pupil expander along its first dimension make correspond to the length or width of the second waveguide pupil expander along its first or second dimension, respectively. A first surface of the pair of parallel surfaces of the second waveguide pupil expander, which comprises its input port, may be shaped, sized, and/or located so as to correspond to an area defined by the output port on the first surface of the pair of parallel surfaces on the first waveguide pupil expander, such that the second waveguide pupil expander is arranged to receive each of the replicas output by the first waveguide pupil expander.

The first and second waveguide pupil expander may collectively provide pupil expansion in a first direction and in a second direction perpendicular to the first direction, optionally, wherein a plane containing the first and second directions is substantially parallel to a plane of the second waveguide pupil expander. In other words, the first and second dimensions that respectively define the length and breadth of the second waveguide pupil expander may be parallel to the first and second directions, respectively, (or to the second and first directions, respectively) in which the waveguide pupil expanders provide pupil expansion. The combination of the first waveguide pupil expander and the second waveguide pupil expander may be generally referred to as being a “pupil expander”.

It may be said that the expansion/replication provided by the first and second waveguide expanders has the effect of expanding an exit pupil of the display system in each of two directions. An area defined by the expanded exit pupil may, in turn define an expanded eyebox area, from which the viewer can receive light of the input diffracted or diverging light field. The eye-box area may be said to be located on, or to define, a viewing plane.

The two directions in which the exit pupil is expanded may be coplanar with, or parallel to, the first and second directions in which the first and second waveguide pupil expanders provide replication/expansion. Alternatively, in arrangements that comprise other elements such as an optical combiner, for example the windscreen (or, windshield) of a vehicle, the exit pupil may be regarded as being an exit pupil from that other element, such as from the windscreen. In such arrangements, the exit pupil may be non-coplanar and non-parallel with the first and second directions in which the first and second waveguide pupil expanders provide replication/expansion. For example, the exit pupil may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication/expansion.

The viewing plane, and/or the eye-box area, may be non-coplanar or non-parallel to the first and second directions in which the first and second waveguide pupil expanders provide replication/expansion. For example, a viewing plane may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication/expansion.

In order to provide suitable launch conditions to achieve internal reflection within the first and second waveguide pupil expanders, an elongate dimension of the first waveguide pupil expander may be tilted relative to the first and second dimensions of the second waveguide pupil expander.

Combiner shape compensation

An advantage of projecting a hologram to the eye-box is that optical compensation can be encoded in the hologram (see, for example, European patent 2936252 incorporated herein by herein). The present disclosure is compatible with holograms that compensate for the complex curvature of an optical combiner used as part of the projection system. In some embodiments, the optical combiner is the windscreen of a vehicle. Full details of this approach are provided in European patent 2936252 and are not repeated here because the detailed features of those systems and methods are not essential to the new teaching of this disclosure herein and are merely exemplary of configurations that benefit from the teachings of the present disclosure.

Control device

The present disclosure is also compatible with optical configurations that include a control device (e.g. light shuttering device) to control the delivery of light from a light channelling hologram to the viewer. The holographic projector may further comprise a control device arranged to control the delivery of angular channels to the eye-box position. British patent application 2108456.1 , filed 14 June 2021 and incorporated herein by reference, discloses the at least one waveguide pupil expander and control device. The reader will understand from at least this prior disclosure that the optical configuration of the control device is fundamentally based upon the eye-box position of the user and is compatible with any hologram calculation method that achieves the light channeling described herein. It may be said that the control device is a light shuttering or aperturing device. The light shuttering device may comprise a 1 D array of apertures or windows, wherein each aperture or window independently switchable between a light transmissive and a light non-transmissive state in order to control the delivery of hologram light channels, and their replicas, to the eye-box. Each aperture or window may comprise a plurality of liquid crystal cells or pixels. Eye-tracking

Some holographic display devices include user tracking such as eye-tracking, using an eyetracking device. Figure 6 shows an example of such a holographic display device comprising a waveguide forming a waveguide pupil expander and further comprising an eye-tracking device. Such holographic display devices may be arranged to receive an input from the user I eye tracking device. The holographic display device may be arranged to determine a current position of the user (or the eye or eyes of a user) based on the input, for example. The holographic display device may be arranged to calculate, recalculate or modify a hologram to be displayed by the holographic display device based on this determined position. An example of this is described in relation to Figure 6.

In the example of Figure 6, the holographic display device comprises a picture generating unit arranged to form a first picture (also called “first image”) and a second picture (also called “second image”). In this example, a first single colour channel (also called “first display channel”) is arranged to form the first picture and comprises a first light source 610, a first collimating lens 612 and a first dichroic mirror 614. First dichroic mirror 614 is arranged to reflect light of a first wavelength along a common optical path so as to illuminate a spatial light modulator (SLM) 640. The first wavelength of light corresponds to the first display channel of a first colour (e.g. red). A second single colour channel (also called “second display channel”) is arranged to form the second picture and comprises a second light source 620, a second collimating lens 622 and a second mirror 624. Second mirror 624 is arranged to reflect light of a second wavelength along the common optical path so as to illuminate the SLM 640. The second wavelength of light corresponds to the second single colour channel of a second colour (e.g. green). In other embodiments, the picture generating unit may comprises a third single colour/display channel (equivalent to the first and second channels) arranged to form a third picture, wherein the third colour channel corresponds to a wavelength of light of a third colour (e.g. blue). In the illustrated embodiment, SLM 640 comprises a single array of light modulating pixels (e.g. LCOS) that is illuminated by light of both the first and second wavelengths. In other embodiments, SLM 640 may comprise separate arrays of light modulating pixels that are illuminated by light of the respective first and second wavelengths.

Holographic display device further comprises a holographic controller 602 arranged to control the picture generating unit, specifically the light output by picture generating unit as described herein. First spatially modulated light of the first colour corresponding to the first picture is output by SLM 640 to form a first single colour image (e.g. red image). A first single colour computer-generated hologram is calculated by a holographic controller 602 and encoded on SLM 640, for example by a display driver 642. The SLM 640 displays the first hologram and is illuminated by light of the first colour from the first colour/display channel to form a first holographic reconstruction at an intermediate plane 670 which may also be referred to as a replay plane. Similarly, second spatially modulated light of the second colour corresponding to the second picture is output by SLM 640 to form a second single colour image (e.g. green image) at the intermediate 670. A second single colour computergenerated hologram is encoded on SLM 640 by holographic controller 602. The SLM 640 displays the second hologram and is illuminated by light of the second colour from the second colour/display channel to form a second holographic reconstruction at the replay plane. In the illustrated arrangement, a beam splitter cube 630 is arranged to separate input light to SLM 640 and spatially modulated light output by SLM 640. A Fourier lens 650 and mirror 660 are provided in the optical path of the output spatially modulated light to the intermediate plane 670. Thus, a composite colour reconstruction may be formed at the intermediate plane 670. A second lens 680 is arranged to project the first and second pictures formed on the light receiving surface 672 to an input port of a pupil expander in the form of a waveguide 690. A viewer 608 may receive spatially modulated light from the expanded eye box - the “viewing window” - formed by waveguide 690. Waveguide 690 comprises an optically transparent medium separated by first and second reflective surfaces as described above with reference to Figure 4. Thus, holographic display device has a “direct view” configuration - that is the viewer directly receives spatially modulated light that has been modulated in accordance with a picture, rather than image light.

The holographic display device further comprises a viewer-tracking system comprising an eye tracking camera 606 and an eye tracking controller 604. As known in the art, the eye tracking camera is arranged to capture images of the eye(s) of the viewer for tracking the eye position, and thus the viewing position within the viewing window. Eye tracking controller 604 provides feedback to holographic controller 602 indicating the current viewing position. In example implementations, holographic controller 602 is arranged to dynamically adjust a brightness of the first and second images according to the current viewing position. In particular, a brightness of the first and second images may be adjusted to compensate for a difference in the reflectivity of light of the first and second wavelengths of the first (partially) reflective surface of the slab waveguide at the propagation distance corresponding to the current viewing position. In some examples, in a given viewing position, different content may be received from different replicas formed by the waveguide. Given the differences in reflectivity, and the difference in viewing distance, the brightness of the content from different replicas may vary. Without correction, this may result in the brightness of the holographic reconstruction being unintentionally non-uniform and the non-uniformity of brightness may vary as a user moves around a viewing window of the system. It may be said that the holographic controller 602 is arranged to adjust a brightness of the first and I or second images (or one or more portions of the first and I or second images) as seen at the current viewing position to compensate for the difference in reflectivity response of the second reflective surface to light of the respective first and second wavelengths. This maintains the perceived colour balance at different viewing positions within the viewing window. Calibration data may be used to fine-tune the brightness of one or more of the single colour images in real-time in order to maintain colour balance. The calibration data may be obtained by a calibration process comprising measuring the relative brightness of each single colour image at a plurality of different viewing positions within the viewing window.

In some implementations, the holographic controller 602 may be arranged to adjust the relative brightness of the first and second pictures according to the current viewing position by adjusting one or more drive signals (e.g. provided by a light source controller) to the first light source 610 and second light source 620. A drive signal to a light source controls the power to the light source and thus the optical power of the output light. In other implementations, the holographic controller 602 may be arranged to adjust the relative brightness of the first and second pictures by adjusting one or more of the first and second computer-generated holograms. For example, the quantisation scheme used for calculation of the first and/or second hologram may be changed in accordance with the current viewing position. The quantisation scheme may be changed to reduce the light modulation range within which allowable light modulation levels are distributed, which may change the intensity of pixels of the calculated hologram.

In other examples, the user / eye-tracking can alternatively or additionally be used for calculating the hologram so as to reduce or eliminate the risk of ghost images being formed. As noted above, such “ghost images” are different from the secondary or ghost reflections formed by the optical combiner, as described herein. In particular, so-called ghost images may be formed because a user, in a particular viewing position, may receive the same content from more than one replica formed by the waveguide. Because the propagation path differs for the different replicas, ghosts (i.e. secondary copies of the content, generally having a lower intensity) can be formed. This can adversely affect the viewing experience. User or eye-tracking can be used to determine a current viewing position and based on that, modify the hologram to reduce ghosts and I or control a control device (as described previously) to prevent light associated with ghosts from reaching the viewing window. Suppression of Secondary or Ghost Reflections of an Image

Figure 7 shows a conventional optical combiner 700 arranged to receive display light 710 corresponding to a target image for display from a head-up display system (not shown) and to reflect the display light 710 towards a viewing plane (not shown). The optical combiner 700 comprises an optically transparent substrate, which, in the illustrated example, is a windscreen of a vehicle. Other forms of optical combiner are possible and contemplated. A viewer, such as a driver, positioned at the viewing plane therefore perceives a virtual image, corresponding to the target image, in the optical combiner 700 formed by reflection of the display light 710.

The optical combiner 700 is formed of safety glass having a multi-layered structure. In particular, the optical combiner comprises an inner glass layer 702, an outer glass layer 706 and an intermediate polymer layer 704 positioned therebetween. For convenience of illustration, Figure 7 shows a portion or area of the optical combiner 700 that receives display light 710, in which the layers 702, 704 706 are substantially planar. As the skilled person will appreciate, in practice, a windscreen may have a planar or curved configuration. The layers 702, 704, 706 of the optical combiner 700 each have a substantially uniform thickness are arranged substantially parallel to each other. Thus, the optical combiner 700 has a substantially uniform thickness. Accordingly, the optical combiner 700 has substantially parallel major outer surfaces 702A, 706A, corresponding to the outer surfaces of inner glass layer 702 and outer glass layer 706, respectively. The optical combiner 700 is tilted towards the viewing plane. As the skilled person will appreciate, the tilt angle is dependent upon the windscreen design. In an example, the area or portion of the optical combiner 700 that receives display light 710 is tilted at an angle relative to the vertical substantially in the range of 40 to 65 degrees, such as 45 to 60 degrees, typical of different vehicle designs.

Figure 7 shows the optical paths of head-up display light 710, corresponding to a target image for display, received from a head-up display (not shown). In particular, display light 710 is incident on a primary surface 702A of the optical combiner 700, corresponding to the outer surface of inner glass layer 702. Thus, the primary surface 702A may be the inner surface of the windscreen facing a viewer at the viewing plane. A first portion 722 of the display light 710 is reflected by the primary surface 702A towards the viewing plane to form a primary image 730. As the skilled person will appreciate, the primary image is a virtual image and corresponds to the target image. As shown in Figure 7, display light 710 is incident on the primary surface 702A at an angle of incidence greater than zero (e.g. in an example about 55 degrees), and, in accordance with the law of reflection, the angle of reflection of the first portion 722 of display light 710 corresponds to the angle of incidence. In addition, the remaining portion of display light 710 is transmitted through, and refracted by, the primary surface 702A into the optical combiner 700. The transmitted display light 710 passes through the optical combiner 700 and is incident on a secondary surface 706A of the optical combiner 700, which corresponds to the outer surface of outer glass layer 706. The secondary surface 706A may be the outer surface of the windscreen facing away from the viewer at the viewing plane. The angle of incidence on the secondary surface 706A is less than the angle of incidence on the primary surface 702A due to refraction of the display light 710 at the air-glass interface. It is noted that, in the illustrated example, the layers 702, 704, 706 of the optical combiner 700 are refractive index matched, so that refraction of the display light 710 only takes place at the primary surface 702A. The display light 710 transmitted through the optical combiner 700 is at least partially reflected by the secondary surface 706A, and so directed back towards the primary surface 702A, such that a second portion 722' of the display light 710 is transmitted through the primary surface 702A towards the viewing plane to form a secondary or ghost reflection 730' of the target or primary image.

As shown in Figure 7, the display light 710 reflected from the secondary surface 706A and transmitted by the primary surface 702A out of the optical combiner 700 undergoes refraction for a second time at the air-glass interface thereof. In consequence, the optical path of the second portion 722' of display light 710 is substantially parallel to, but spatially offset, from the optical path of the first portion 722 of display light 710. Thus, the secondary or ghost reflection 730' of the target image is spatially offset from the primary image 730 at the viewing plane. Accordingly, the viewer perceives an undesirable ghost reflection 730' of the target image, which is typically dimmer and blurred in comparison to the primary image 730, and is additionally offset from the primary image 730. This may cause a distraction and detrimentally impacts the quality of the image perceived by the viewer.

Figure 8 shows an optical combiner 800 of a head-up display system in accordance with the present disclosure. Similar to the conventional optical combiner 700 of Figure 7, the optical combiner 800 comprises an optically transparent substrate, in particular, a windscreen of a vehicle arranged to receive display light 810 corresponding to a target image for display from a head-up display system (not shown) and to reflect the display light 810 towards a viewing plane. Optical combiner 800 has an similar multi-layered structure to the optical combiner 700 of Figure 7 as described above. In particular, the optical combiner 800 comprises an inner glass layer 802, an outer glass layer 806 and an intermediate polymer layer 804 positioned therebetween. Thus, optical combiner 800 comprises a primary surface 802A and a secondary surface 806A corresponding to the outer surfaces of inner glass layer 802 and outer glass layer 806, respectively. However, optical combiner 800 has an additional layer optical layer 808, which, in the illustrated example, is positioned between inner glass layer 802 and intermediate polymer layer 804. Optical layer 808 comprises an optical polarisation layer arranged to change a polarisation state of light propagating therethrough. The optical layer 808 is referred to herein as a polarisation layer or polarisation converter.

In some embodiments, polarisation converter 808 comprises a half-wave component, such as a half-waveplate or half-wave film. As the skilled person will appreciate, a half-wave component is configured to rotate the polarisation direction of linear polarised light propagating therethrough. In other embodiments, polarisation layer 808 comprises a quarterwave component, such as a quarter-waveplate or quarter-wave film. As the skilled person will appreciate, a quarter-wave component is configured to convert the polarisation direction of linear polarised light propagating therethrough to circularly polarised light.

In accordance with the present disclosure, a head-up display comprises a head-up display system (not shown), which is configured to provide polarized head-up display light corresponding to a target image to the optical combiner 800. In particular, the head-up display light is S-polarised light, i.e., the display light is linearly polarised in a direction perpendicular to the plane of incidence. In consequence, a secondary or ghost reflection of the target image at the viewing plane, due to reflection from a surface of the optical combiner 800 other than a primary surface 802A thereof, may be minimized or even eliminated, as described below.

Figure 8 shows the optical path of the head-up display light 810, corresponding to a target image for display, received from a head-up display system (not shown). In particular, S- polarised display light 810 is incident on the primary surface 802A of the optical combiner 800, corresponding to the outer surface of inner glass layer 802. Thus, the primary surface 802A may be the inner surface of the windscreen facing a viewer at the viewing plane. A first portion 822 of the display light 810 is reflected by the primary surface 802A towards a viewing plane to form a primary image 830 corresponding to the target image. As shown in Figure 8, display light 810 is incident on the primary surface 802A at an angle of incidence greater than zero (e.g. in an example about 55 degrees) and, in accordance with the law of reflection, the angle of reflection of the first portion 822 of display light 810 corresponds to the angle of incidence. In addition, a remaining portion 821 of the S-polarized display light 810 is transmitted through the primary surface 802A into the optical combiner 800. The transmitted portion 821 of the display light 821 , which is refracted at the air-glass interface, passes through the inner glass layer 808 and through the polarization layer 808, where its polarization state is changed from S-ploarised light. In the illustrated example, the polarisation layer 808 is half-wave converter so that the polarisation layer 808 converts the S-polarised display light to P-polarised display light 823. In other examples, the polarisation layer 808 may convert the S-polarised display light to circularly polarised display light 823. The optical path of the P-polarised display light 823 continues through polymer layer 802 and outer glass layer 806 to secondary surface 806A, which forms the outer surface of outer glass layer 806. The secondary surface 806A may be the outer surface of the windscreen facing away from the viewer at the viewing plane. For the reasons discussed below, since the display light 823 at the secondary surface 806A is P-polarised rather than S-polarised, it is transmitted out of the optical combiner 800 through the secondary surface 806A as shown by arrow 824. Thus, unlike the optical combiner 700 of Figure 7, the display light 821 that is transmitted into optical combiner 800 at primary surface 802A is not reflected by the secondary surface 806A back through the optical combiner 800 towards the viewing plane to form a secondary or ghost reflection of the target image.

The outer or external major surfaces of an optical combiner, which form primary and secondary surfaces for reflection of display light described above, are found to preferentially reflect incident display light that is S-polarised, i.e., light with an electric field that is polarised perpendicular to the plane of incidence, at typical angles of incidence. The dominant or greater reflectivity of light that is S-polarised is due to the geometry of the system. For example, when the optical combiner is a windscreen, the inner and outer major surfaces of the windscreen forming the above primary and secondary surfaces are tilted, typically at an angle from the vertical in the range of 40 to 65 degrees, and may be curved, such that the top of the windscreen is closer to the viewing plane that the bottom of the windscreen.

Furthermore, the angle of incidence of display light on the primary surface is relatively high, typically greater than 40°, for example in the range of 45 to 70 degrees such as 50 to 65 degrees. This geometry leads to the preferential reflection of S-polarised light, such that the reflectivity of S-polarised light by the primary surface of the optical combiner is high, whilst P- polarised light is preferentially transmitted by the primary surface. The secondary surface of the optical combiner is substantially parallel to the primary surface. Although the angle of incidence of display light is reduced as a result of refraction, for example the angle of incidence is in the range of 25 to 45 degrees such as 30 to 37 degrees, the reflectivity of S polarised light by the secondary surface remains relatively high whilst P polarised light is preferentially transmitted by the secondary surface. As described herein, the head-up display light may be received from a holographic display system comprising a spatial light modulator that outputs spatially modulated light in accordance with a hologram. Thus, the head-up display light may comprise a spatially modulated wavefront or a holographic wavefront. The spatially modulated light may be provided to a waveguide pupil expander, which output a plurality of replicas of the holographic wavefront. As described herein, the spatially modulated light field may comprise a diverging light field. Accordingly, each replica may comprise a diverging light ray bundle. The head-up display light may be passed through a polariser, to convert the head-up display light is polarised in the direction perpendicular to the plane of incidence (i.e., S-polarised), before output by the display system.

Accordingly, the head-up display comprises a head-up display system configured to output head-up display light corresponding to a target image that is S-polarised. The display light may be entirely linearly polarised in a direction perpendicular to the plane of incidence (i.e. S-polarised). Thus, as shown in Figure 8, S-polarised head-up display light 810 is incident on the primary surface 802A of the optical combiner 800 so as to maximise the first portion 822 of the display light 810, which is (preferentially or dominantly) reflected by the primary surface 802A towards the viewing plane to form the primary image 830. A remaining portion 821 of the S-polarised head-up display light 810 is transmitted by primary surface 802A into the optical combiner 800 and is converted by polarisation converter 802 to form display light having a different polarisation state 823 before reaching the secondary surface 806A. For example, the head-up display light 821 is (partly or fully) converted to P-polarised light 823. The (partly or fully) P-polarised head-up display light 823 incident on the secondary surface 806A is therefore preferentially transmitted out of the optical combiner as shown by arrow 824. In this way, reflection of the head-up display light by the secondary surface 806A, and consequential formation of secondary or ghost reflections of the target image at the viewing plane, is reduced or suppressed or in come cases eliminated entirely.

In particular, as the skilled person will appreciate, the head-up display system may be optimised so that reflection of head-up display light by the secondary surface 806A is reduced to substantially zero, thereby eliminating ghost reflections of the target image at the viewing plane. In particular, based on the geometry of the system, the head-up display system may be arranged to provide head-up display light that has an angle of incidence on the primary surface 802A of the optical combiner that lead to an angle of incidence on the secondary surface 806A such that reflection of P-polarised light by the secondary surface 806A is substantially zero. For example, an angle of incidence on the secondary surface in the range of 30 to 37 degrees (e.g. 33.5 degrees) may reduce reflection of P-polarised light to substantially zero. This may correspond to an angle of incidence of the head-up display light on the primary surface 802A of 53 to 58 degrees (e.g. 55.5 degrees). Thus, in the case that the head-up display light is fully converted from S polarised light to P-polarised light, such as by a half-wave retarder, ghost reflections at the viewing plane are eliminated entirely.

Accordingly, a method of suppressing a ghost reflection head-up display light from a secondary surface of an optical combiner, is achieved. The method comprises providing S- polarised head-up display light to the optical combiner. The method further comprises converting the polarisation of the head-up display light within the optical combiner, after partial transmission through a primary surface thereof. In this way, the intensity of reflection of the head-up display light from the secondary surface is reduced or minimised.

Additional features

The methods and processes described herein may be embodied on a computer-readable medium. The term “computer-readable medium” includes a medium arranged to store data temporarily or permanently such as random-access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" shall also be taken to include any medium, or combination of multiple media, that is capable of storing instructions for execution by a machine such that the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methodologies described herein, in whole or in part.

The term "computer-readable medium" also encompasses cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example form of a solid-state memory chip, an optical disc, a magnetic disc, or any suitable combination thereof. In some example embodiments, the instructions for execution may be communicated by a carrier medium. Examples of such a carrier medium include a transient medium (e.g., a propagating signal that communicates instructions).

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. The present disclosure covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

1 . A head-up display system comprising: an optical combiner, and a display system arranged to output head-up display light to the optical combiner, wherein the head-up display light is linearly polarised light that is S-polarised, wherein the optical combiner comprises a primary surface and a secondary surface, wherein the primary surface is arranged to direct a first portion of the display light towards a viewing plane to form a primary image, and the secondary surface is arranged to direct a second portion of the display light towards the viewing plane to form a ghost reflection of the primary image; wherein the optical combiner further comprises a polarisation converter between the primary surface and the secondary surface arranged to reduce the intensity of the ghost reflection.

2. A head-up display system as claimed in claim 1 wherein the optical combiner is arranged to receive the head-up display light at the primary surface before the secondary surface.

3. A head-up display system as claimed in claim 1 or 2 wherein the polarisation converter is arranged to change the polarisation of the S-polarised head-up display light.

4. A head-up display system as claimed in claim 1 , 2 or 3 wherein the polarisation converter comprises a half-wave component arranged to convert the S-polarised head-up display light to P-polarised light.

5. A head-up display system as claimed in claim 1 , 2 or 3 wherein the polarisation converter comprises a quarter-wave component arranged to convert the S-polarised head- up display light to circularly polarised light.

6. A head-up display system as claimed in any preceding claim wherein the optical combiner and display system are arranged such that polarisation converter reduces the intensity of the ghost reflection to substantially zero.

7. A head-up display system as claimed in any preceding claim wherein the head-up display light comprises a plurality of replicas of a spatially-modulated wavefront.

8. A head-up display system as claimed in claim 7 further comprising a substantially planar waveguide having a reflective surface arranged, during head-up display operation, in a configuration that is conducive to sunlight glare, the substantially planar waveguide arranged to output the plurality of replicas of the spatially-modulated wavefront and further arranged such that it directs at least a portion of the sunlight towards the viewing plane via the first and second surfaces of the optical combiner, wherein polarisation converter is further arranged to reduce the intensity of the sunlight directed towards the viewing plane by the substantially planar waveguide.

9. A head-up display system as claimed in claim 7 or claim 8 wherein the spatially- modulated wavefront comprises a holographic wavefront, optionally wherein each replica of the plurality of replicas comprises a diverging light ray bundle.

10. A head-up display system as claimed in any preceding claim wherein a refractive index of the optical combiner is greater than that of air, optionally, wherein the primary surface comprises an air-glass interface and the secondary surface comprises a glass-air interface.

11. A head-up display system as claimed in any preceding claim wherein the optical combiner comprises a windscreen of a vehicle, wherein the primary surface is an inner surface of the windscreen facing a viewer disposed at the viewing plane and the secondary surface may be an outer surface of the windscreen facing away from the viewer.

12. A head-up display system as claimed in any preceding claim wherein the optical combiner comprises a polymer layer disposed between the primary surface and the secondary surface, optionally wherein the polymer layer comprises a polyvinyl butyral layer or the like.

13. A head-up display system as claimed in claim 12 wherein the polarisation converter is arranged adjacent the polymer layer, such as disposed on the polymer layer, and/or the polymer layer is refractive index matched to an optically transparent material of the optical combiner.

14. A head-up display system as claimed in any preceding claim wherein the polarisation converter comprises a polarisation film, such as half-wave film or quarter-wave film.

15. A head-up display system as claimed in any preceding claim wherein the optical combiner is tilted from the vertical towards the viewing plane, optionally wherein the tilt angle is in the range of 40 to 65 degrees such as 45 to 60 degrees.

16. A head-up display system as claimed in any preceding claim wherein the primary surface and the secondary surface of the optical combiner are substantially parallel.

17. A head-up display system as claimed in any preceding claim arranged such that the angle of incidence of the head-up display light on the primary surface is in the range of 45 to 70 degrees, such as 50 to 65 degrees, and/or the angle of incidence of the head-up display light on the secondary surface is in the range of 25 to 45 degrees such as 30 to 37 degrees.

18. A method of suppressing a ghost reflection of head-up display light from a secondary surface of an optical combiner of a head-up display, the method comprising providing S- polarised head-up display light to the optical combiner, and converting the polarisation of the head-up display light within the optical combiner after partial transmission through a primary surface thereof such that the intensity of the ghost reflection of the head-up display light from the secondary surface is reduced.

19. A method as claimed in claim 18 comprising providing the S-polarised head-up display light at an angle of incidence on the primary surface of 45 to 70 degrees and an angle of incidence on the secondary surface of 25 to 45 degrees.

20. A windscreen comprising a primary surface, a secondary surface and a polarisation converter therebetween, wherein the polarisation converter is arranged to reduce the intensity of a ghost reflection from the secondary surface when the windscreen is arranged as an optical combiner for head-up display using S-polarised light.