Compact head-up display

Abstract

A head-up display for a vehicle includes a first pupil replicator and a second pupil replicator. The first pupil replicator extends in a first direction. The first pupil replicator is arranged to receive a holographic light field from a spatial light modulator having an array of pixels defining a limiting aperture of the head-up display. The holographic light field is a composite light field spatially modulated according to a hologram displayed on the spatial light modulator. The second pupil replicator extends in the first direction and in a second direction perpendicular to the first direction. The second pupil replicator includes a first major surface forming an output and a second major surface parallel to the first major surface. The first pupil replicator is arranged within a planar layer substantially parallel and adjacent to the second major surface of the second pupil replicator.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to pupil expansion or replication, in particular for diffractive light fields comprising diverging bundles of light rays. More specifically, the present disclosure relates to display systems comprising waveguide pupil expanders and methods of pupil expansion using waveguides. Some embodiments relate to two-dimensional pupil expansion using first and second waveguide pupil expanders. Some embodiments relate to picture generation units and head-up displays, such as automotive head-up displays (HUDs). BACKGROUND

[0002] Light scattered from an object contains 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. A hologram can be reconstructed by illuminating with suitable light to form a two- or three-dimensional holographic reconstruction or replay image representative of the original object.

[0003] Computer-generated holography can numerically simulate the interference process. Computer-generated holograms can be computed by techniques based on mathematical transforms such as the Fresnel or Fourier transform. These types of holograms can be referred to as Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. Fourier holograms can be considered as a Fourier domain / planar representation of the object or a frequency domain / planar representation of the object. Computer-generated holograms can also be computed by, for example, coherent ray tracing or point cloud techniques.

[0004] Computer-generated holograms can be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of incident light. Light modulation can be achieved using, for example, electrically addressable liquid crystals, optically addressable liquid crystals or micro-mirrors.

[0005] Spatial light modulators typically comprise a plurality of individually addressable pixels, which can also be referred to as cells or elements. The light modulation scheme can be binary, multi-level or continuous. Alternatively, the device can be continuous (i.e. not comprising pixels) so that the light modulation can be continuous across the device. The spatial light modulator can be reflective, meaning that modulated light is output by reflection. The spatial light modulator can equally be transmissive, meaning that modulated light is output by transmission.

[0006] A holographic projector can be provided using the systems described herein. Such a projector has found application in head-up displays "HUDs". SUMMARY

[0007] Aspects of the present disclosure are defined in the accompanying independent claims.

[0008] The present disclosure relates generally to image projection. It relates to methods of image projection and image projectors comprising a display device. The present disclosure also relates to projection systems comprising an image projector and a viewing system, where the image projector projects or relays light from the display device to the viewing system. The present disclosure is equally applicable to monocular and binocular viewing systems. The viewing system can comprise one or more eyes of a viewer. The viewing system comprises an optical element with optical power (e.g. the lens of a human eye) and a viewing plane (e.g. the retina of a human eye). The projector can be referred to as a "light engine". The display device and the image formed (or perceived) using the display device are spatially separated from each other. The viewer forms or perceives the image on a display plane. In some embodiments, the image is a virtual image and the display plane can be referred to as a virtual image plane. In other embodiments, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. The image is formed by illuminating a diffractive pattern (e.g. a hologram) displayed on the display device.

[0009] The display device comprises pixels. The pixels of the display can display a diffractive pattern or structure of diffracted light. The diffracted light can form an image on a plane spatially separated from the display device. 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, according to well-known optical principles.

[0010] In embodiments, the display device is a spatial light modulator, such as a liquid crystal on silicon ("LCOS") spatial light modulator (SLM). Light propagates from the LCOS to a viewing entity / system such as a camera or an eye over a range of diffraction angles (e.g. from zero to the maximum diffraction angle). In some embodiments, magnification techniques can be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of the LCOS.

[0011] In some examples, the image (formed by the displayed hologram) propagates to the eye. For example, the spatially modulated light of an intermediate holographic reconstruction / image formed in free space between the display device and the viewer or on a screen or other light receiving surface can propagate to the viewer.

[0012] In some other examples, the hologram itself (of light) propagates to the eye. For example, the spatially modulated light of the hologram (which has not yet fully converted to a holographic reconstruction, i.e. an image) - which can be informally said to be "encoded" with the hologram - propagates directly to the viewer's eye. The viewer can perceive a real or virtual image. In these embodiments, an intermediate holographic reconstruction / image is not formed between the display device and the viewer. Sometimes it is said that, in these embodiments, the lens of the eye performs the conversion or transformation of the hologram to the image. The projection system or light engine can be configured so that the viewer effectively looks directly at the display device.

[0013] The "light field" referred to herein is a "complex light field". The term "light field" simply means a light pattern having a finite size in at least two orthogonal spatial directions, e.g. x and y. The term "light field" simply means a light pattern having a finite size in at least two orthogonal spatial directions (x and y). The word "complex" as used herein simply means that the light at each point in the light field can be defined by an amplitude value and a phase value, and thus can be represented by a complex number or a pair of values. For purposes of hologram computation, the complex light field can be a two-dimensional array of complex numbers, where the complex numbers define the light intensity and phase at a plurality of discrete locations within the light field. According to the methods disclosed herein, the complex light field propagates forward and backward between the hologram plane and the image plane - e.g. in the +z and -z directions. The light propagation can be simulated or modeled using any of a variety of different methods or mathematical transformations familiar to those skilled in the art of wave optics.

[0014] According to well-known optical principles, the range of angles of light propagating from a display device that can be observed by an eye or other observing entity / system varies with the distance between the display device and the observing entity. For example, at a viewing distance of 1 meter, only a small range of angles from an LCOS can propagate through the pupil of an eye to form an image on the retina at a given eye position. The range of angles of light rays propagating from a display device that determine the portion of the image that is "visible" to an observer, i.e. that can successfully propagate through the pupil of an eye to form an image on the retina at a given eye position. In other words, not all portions of the image are visible from any point on the observing plane (e.g. any eye position within an observing window such as an eyebox).

[0015] In some embodiments, the image perceived by the observer is a virtual image that appears upstream of the display device, that is, the observer perceives the image as being further away from them than the display device. Thus, conceptually, one can think of the observer as viewing a virtual image through a "display device size window" that can be very small, e.g. 1 cm in diameter, at a relatively large distance, e.g. 1 meter. And the user will be viewing the display device size window through the pupil of their eye, which can also be very small. Thus, at any given time, the field of view is small, and the specific range of angles that can be seen is heavily dependent on the eye position.

[0016] Pupil expanders solve how to increase the angular range of light rays propagating from a display device and that can successfully propagate through the pupil of an eye to form an image. The display device is typically (relatively) small and the projection distance (relatively) large. In some embodiments, the projection distance is at least one (such as at least two) order of magnitude larger than the diameter or width of the entrance pupil and / or aperture of the display device (i.e. the size of the pixel array). Embodiments of the present disclosure relate to a configuration in which a hologram of an image, rather than the image itself, propagates to the human eye. In other words, the light received by the observer is modulated according to a hologram of an image. However, other embodiments of the present disclosure can relate to a configuration in which an image, rather than a hologram, propagates to the human eye - for example by so-called indirect viewing in which light of a holographic reconstruction or "replayed image" formed on a screen (or even in free space) propagates to the human eye.

[0017] The use of a pupil expander increases the observation area (i.e. the eye box of the user) laterally, enabling some movement of the eye while still enabling the user to see an image. As the skilled person will appreciate, in an imaging system, the observation area (eye box of the user) is the area in which the eye of the observer can perceive an image. The present disclosure relates to a non-infinite virtual image distance - i.e. a near-field virtual image.

[0018] Conventionally, two-dimensional pupil expanders comprise one or more one-dimensional optical waveguides, each formed using a pair of opposing reflective surfaces, in which the output light from the surfaces forms an observation window - for example an eye box or eye movement box for an observer to view. Light received from a display device (for example spatially modulated light from an LCOS) is replicated by the or each waveguide in order to increase the field of view (or observation area) in at least one dimension. In particular, the waveguide expands the observation window as a result of creating additional light rays or "replicas" by dividing the amplitude of the incident wavefront.

[0019] The display device can have an active or pixel display area having a first dimension of less than 10 cm, for example less than 5 cm or less than 2 cm. The propagation distance between the display device and the viewing system can be greater than 1 m, for example greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide can be up to 2 m, for example up to 1.5 m or up to 1 m. The method is able to receive an image and determine a corresponding hologram of sufficient quality in less than 20 ms, such as less than 15 ms or less than 10 ms.

[0020] In some embodiments - described by way of example only in terms of diffractive or holographic light fields in accordance with the present disclosure - the hologram is configured to route light into a plurality of channels, each channel corresponding to a different portion (i.e. sub-region) of the image. The hologram can be exhibited, such as displayed, on a display device such as a spatial light modulator. When displayed on an appropriate display device, the hologram can spatially modulate light that can be converted into an image by an observation system. The channels formed by the diffractive structure are referred to herein as "hologram channels", simply to reflect that they are channels of light encoded by the hologram with image information. It can be said that the light of each channel is in the holographic domain, rather than in the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram, so the holographic domain is the Fourier or frequency domain. The hologram can equally be a Fresnel or Fresnel transform hologram. The hologram is described herein as routing light into a plurality of hologram channels, simply to reflect that the image that can be reconstructed by the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-regions, with each hologram channel corresponding to each image sub-region. Importantly, the hologram of the present example is characterised by how it distributes image content when illuminated. In particular, the hologram divides image content by angle. That is, each point on the image relates to a unique ray angle - at least a unique pair of angles, since the hologram is two-dimensional - in the spatially modulated light formed by the hologram when illuminated. For the avoidance of doubt, this behaviour of the hologram is not conventional. The spatially modulated light formed by this special type of hologram when illuminated can be arbitrarily divided into a plurality of hologram channels, with each hologram channel defined by a range of ray angles (two-dimensional). It will be appreciated from the foregoing that any hologram channel (i.e. a sub-range of ray angles) that can be considered in the spatially modulated light will relate to a corresponding portion or sub-region of the image. That is, all the information required to reconstruct that portion or sub-region of the image is contained within the sub-range of angles of the spatially modulated light formed by the hologram of the image. When the spatially modulated light is viewed as a whole, there is no evidence of the plurality of discrete light channels. However, in some arrangements, the plurality of spatially separated hologram channels are formed by deliberately leaving regions of the target image for the computational hologram blank or empty (i.e. without image content).

[0021] However, the hologram can still be identified. For example, if only a continuous portion or sub-region of the spatially modulated light formed by the hologram is reconstructed, only a sub-region of the image should be visible. If a different continuous portion or sub-region of the spatially modulated light is reconstructed, a different sub-region of the image should be visible. Another identifying feature of this type of hologram is that the shape of the cross-sectional area of any hologram channel substantially corresponds to the shape of the entrance pupil (i.e. is substantially the same), although the size can be different - at least at the correct plane of the hologram. Each light / hologram channel propagates from the hologram at a different angle or range of angles. While these are example ways of characterising or identifying this type of hologram, other ways can be used. In summary, the holograms disclosed herein are characterised and identified by how the image content is distributed within the light encoded by the hologram. Furthermore, to avoid any doubt, references herein to a hologram configured to direct light or to angularly divide an image into multiple hologram channels are by way of example only, and the disclosure is equally applicable to any type of holographic light field or even any type of pupil expansion of a diffractive or holographic light field.

[0022] In summary, a system is disclosed herein for providing pupil expansion for an input light field, wherein the input light field is a diffractive or holographic light field comprising diverging bundles of light rays. As described above, pupil expansion (which can also be referred to as “image replication” or “replication” or “pupil replication”) enables the size of the area from which / whereby a viewer can see an image (or can receive light of a hologram, the viewer’s eyes forming an image) to be increased by creating one or more copies of an input light ray (or bundle of light rays). Pupil expansion can be provided in one or more dimensions. For example, two-dimensional pupil expansion can be provided, wherein each dimension is substantially orthogonal to the respective other dimension.

[0023] The system can be provided in a compact and streamlined physical form. This makes the system suitable for a wide range of real-world applications, including those where space is limited and real-estate is valuable. For example, it can be implemented in a head-up display (HUD), such as a vehicle or car HUD.

[0024] In accordance with the present disclosure, pupil expansion is provided for a diffractive or diffractive light, which can comprise diverging bundles of light rays. The diffractive light can be output by a display device, such as a pixelated display device, such as a spatial light modulator (SLM) arranged to display a diffractive structure, such as a hologram. The diffractive light field can be defined by a “light cone”. Thus, the size of the diffractive light field (as defined in a two-dimensional plane) increases with the propagation distance from the respective diffractive structure (i.e. display device).

[0025] The spatial light modulator can be arranged to display a hologram. The diffracted or divergent light can comprise light encoded with / by the hologram, rather than light of a holographic reconstruction or image. Thus, in such embodiments, it can be said that the pupil expander replicates the hologram or forms at least one replica of the hologram to convey that the light passed to the observer is spatially modulated according to the hologram of the image rather than the image itself. That is, the diffracted light field is propagated to the observer.

[0026] In some embodiments, two one-dimensional waveguide pupil expanders are provided, each arranged to effectively increase the size of the exit pupil of the system by forming multiple replicas or copies of the exit pupil (or light of the exit pupil) of the spatial light modulator. The exit pupil can be understood as the physical area from which the system outputs light. It can also be said that each waveguide pupil expander is arranged to expand the size of the exit pupil of the system. It can also be said that each waveguide pupil expander is arranged to expand / increase the size of the eyebox within which an observer’s eye can be located in order to see / receive light output by the system.

[0027] In the present disclosure, the term “replica” is used simply to reflect that the spatially modulated light is split so that the composite light field is directed along multiple different optical paths. The term “replica” is used to refer to each occurrence or instance of the composite light field after the replication event— such as the partial reflection-transmission of the pupil expander. Each replica propagates along a different optical path. Some embodiments of the present disclosure relate to the propagation of light encoded with a hologram rather than an image— i.e. light spatially modulated with a hologram of the image rather than the image itself. Those skilled in the art of holography will understand that the composite light field relating to the propagation of light encoded with a hologram will evolve differently with propagation distance. The term “replica” used herein is independent of propagation distance, so two optical branches or paths relating to a replication event are still referred to as “replicas” of each other even if the branches have different lengths so that the composite light field evolves differently along each path. That is, according to the present disclosure, two composite light fields are considered to be “replicas” even if they relate to different propagation distances— provided they originate from the same replication event or series of replication events.

[0028] According to a first aspect, there is provided a display system comprising a first waveguide pupil expander. The first waveguide pupil expander comprises an input port, an output port, a first pair of parallel surfaces and a second pair of parallel surfaces. The first pair of parallel surfaces is orthogonal to the second pair of parallel surfaces. The first pair of parallel surfaces is arranged to light guide a diffractive or divergent (e.g. holographic) light field from the input port to the output port by internal reflections therebetween. A first surface of the first pair of parallel surfaces is partially transmissive-reflective such that the light field is split at each internal reflection and multiple copies of the light field are transmitted through a region of the first surface forming the output port. The second pair of parallel surfaces is also arranged to light guide the light field from the input port to the output port by at least one internal reflection. The input port can be formed on / by a first surface of the first pair of parallel surfaces or on / by a second surface.

[0029] A “diffractive light field” according to the present disclosure is a light field formed by diffraction. A diffractive light field can be formed by illuminating a corresponding diffraction pattern. According to the present disclosure, an example of a diffraction pattern is a hologram and an example of a diffractive light field is a holographic light field or a light field forming a (holographic) reconstruction of an image. A holographic light field forms a (holographic) reconstruction of an image on a replay plane. A holographic light field propagating from a hologram to a replay plane can be said to comprise light encoded with the hologram or light in the holographic domain. A diffractive light field is characterized by a diffraction angle determined by a minimum feature size of the diffractive structure and a wavelength of the light (of the diffractive light field). According to the present disclosure, a “diffractive light field” can also be said to be a light field forming a reconstruction on a plane spatially separated from the corresponding diffractive structure. An optical system for propagating a diffractive light field from a diffractive structure to an observer is disclosed herein. The diffractive light field can form an image. In some embodiments, the diffractive light field comprises divergent bundles of light rays. In some embodiments, the image formed by the diffractive light field is a virtual image.

[0030] In some embodiments, the first pair of parallel / complementary surfaces are elongate or extended surfaces, relatively longer along a first dimension and relatively shorter along a second dimension, e.g. relatively shorter along each of two other dimensions, each dimension being substantially orthogonal to each of the respective other dimensions. The arrangement of the light reflecting / transmitting between / from the first pair of parallel surfaces is such that the light propagates within the first waveguide pupil expander with a general direction of the light propagation being the direction in which the first waveguide pupil expander is relatively longer (i.e. in its “elongate” direction).

[0031] In some embodiments, the second pair of parallel surfaces are elongate surfaces, relatively longer along a first dimension and relatively shorter along a second dimension, e.g. relatively shorter along each of two other dimensions, each dimension being substantially orthogonal to each of the respective other dimensions.

[0032] As the second pair of parallel surfaces is also arranged to direct the light field from the input port to the output port by at least one internal reflection, the first waveguide pupil expander ensures that light of the diffracted light field does not get lost through the second pair of parallel surfaces during propagation of the light within the first waveguide pupil expander. This can be particularly advantageous when the first waveguide pupil expander is relatively thin, especially when diffracted or divergent light propagates through the first waveguide pupil expander, if the dimensions of the first waveguide pupil expander along its one or two relatively shorter dimensions substantially equal or are smaller than the dimensions of the light field defined by the diffracted or divergent light.

[0033] Disclosed herein is a system that uses diffracted light to form an image and provides eyebox size and field of view suitable for real-world applications - for example through head-up displays in the automotive industry. Diffracted light is light that forms a holographic reconstruction of an image from a diffractive structure - for example a hologram such as a Fourier or Fresnel hologram. Using diffraction and diffractive structures requires a high density of display devices with very small pixels (e.g. 1 micron) - which in practice means small display devices (e.g. 1 cm). The inventors have solved the problem of how to provide a diffracted light field for 2D pupil expansion, for example including divergent (non-collimated) bundles of rays.

[0034] In some aspects, the display system includes a display device - such as a pixelated display device, for example a spatial light modulator (SLM) or liquid crystal on silicon (LCoS) SLM - arranged to provide or form diffracted or divergent light. In these aspects, the aperture of the spatial light modulator (SLM) is the limiting aperture of the system. That is, the aperture of the spatial light modulator - more specifically the dimensions of the area that bounds the array of light modulating pixels contained within the SLM - determines the size (e.g. spatial extent) of the bundle of rays that can exit the system. According to the present disclosure, it is stated that by using at least one pupil expander, the exit pupil of the system is expanded to reflect that the exit pupil of the system (which is limited by the small display device with pixel size for light diffraction) becomes larger in spatial extent.

[0035] The diffracted or diverging light field can be referred to as having a "light field size", which is defined in a direction substantially orthogonal to the propagation direction of the light field. Because the light is diffracted / divergent, the light field size increases with propagation distance. In some embodiments, the light field size within the first waveguide pupil expander exceeds the size of the first waveguide pupil expander - that is, the light field size is larger than the size of the first waveguide pupil expander in at least one dimension. In other words, the light field size can be substantially equal to or larger than at least one of: a "first separation dimension" defined between the first and second surfaces of the first pair of parallel surfaces; and a "second separation dimension" defined between the first and second surfaces of the second pair of parallel surfaces during internal reflections of the light field within the first waveguide pupil expander. In other words, in at least one dimension, the size of the light field can be equal to or larger than the thickness of the first waveguide pupil expander. According to the present disclosure, the second pair of parallel surfaces is configured to provide reflections of the light field to keep it within the first waveguide pupil expander and to ensure that it exits the first waveguide pupil expander only via the output port, due to internal reflections between the surfaces of the first pair of parallel surfaces.

[0036] In some embodiments, the diffracted light field is spatially modulated according to a hologram. In other words, in these aspects, the diffracted light field comprises a "holographic light field". The hologram can be displayed on a pixelated display device. The hologram can be a computer-generated hologram (CGH). It can be a Fourier hologram or a Fresnel hologram or a point-cloud hologram or any other suitable type of hologram. Optionally, the hologram can be computed so as to form channels of holographic light, each channel corresponding to a different portion of the image that the observer wants to observe (or perceive, if it is a virtual image). The pixelated display device can be configured to display a plurality of different holograms successively or in sequence. Each aspect and embodiment disclosed herein can be applied to the display of a plurality of holograms.

[0037] The output port of the first waveguide pupil expander can be coupled to an input port of a second waveguide pupil expander. The second waveguide pupil expander can be arranged to direct the diffracted light field - including some, preferably most, preferably all, copies of the light field output by the first waveguide pupil expander - from its input port to a respective output port by internal reflections between a third pair of parallel surfaces of the second waveguide pupil expander. The first surface of the third pair of parallel surfaces can be partially transmissive-reflective, such that the light field is split at each internal reflection, and multiple copies of the light field are transmitted through the area of the first surface of the second waveguide pupil expander forming its output port.

[0038] A first waveguide pupil expander can be arranged to provide pupil expansion or replication in a first direction, and a second waveguide pupil expander can be arranged to provide pupil expansion or replication in a different second direction. The second direction can be substantially orthogonal to the first direction. The second waveguide pupil expander can be arranged to maintain the pupil expansion already provided by the first waveguide pupil expander in the first direction, and expand (or replicate) some, preferably most, preferably all, copies it receives from the first waveguide pupil expander in the different second directions. The second waveguide pupil expander can be arranged to receive the optical field directly or indirectly from the first waveguide pupil expander. One or more other elements can be provided along the propagation path of the optical field between the first and second waveguide pupil expanders.

[0039] 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 its length along a first dimension. The planar or rectangular shape of the second waveguide pupil expander may be defined by its length along the first dimension and its width or width along a second dimension substantially orthogonal to the first dimension. The dimension or length of the first waveguide pupil expander along its first dimension corresponds respectively to the length or width of the second waveguide pupil expander along its first or second dimension. The shape, size, and / or position of the first surface of the third pair of parallel surfaces of the second waveguide pupil expander, including its input port, may be designed to correspond to the region defined by the output port on the first surface of the first pair of parallel surfaces on the first waveguide pupil expander, such that the second waveguide pupil expander is arranged to receive each copy output by the first waveguide pupil expander.

[0040] The first and second waveguide pupil expanders can jointly provide pupil expansion in a first direction and a second direction perpendicular to the first direction. Optionally, the planes containing the first and second directions are substantially parallel to the plane of the second waveguide pupil expander. In other words, the first and second dimensions defining the length and width of the second waveguide pupil expander, respectively, can be parallel to the first and second directions (or parallel to the second and first directions, respectively), wherein the waveguide pupil expander provides pupil expansion. The combination of the first and second waveguide pupil expanders is generally referred to as a "pupil expander".

[0041] In essence, the expansion / replication provided by the first and second waveguide expanders has the effect of expanding the exit pupil of the display system in each of two directions. The area defined by the expanded exit pupil can further define the expanded eyebox region from which the observer can receive light from the input diffracted or diverging light field. The eyebox region can be said to lie on or define the observation plane.

[0042] The two directions of the exit pupil expansion can be co-planar or parallel to the first and second directions of replication / expansion provided by the first and second waveguide pupil expanders. Alternatively, in an arrangement including other elements such as an optical combiner, e.g. a windshield of a vehicle, the exit pupil can be considered to be the exit pupil from that other element, such as from the windshield. In such an arrangement, the exit pupil can not be co-planar and not parallel to the first and second directions of replication / expansion provided by the first and second waveguide pupil expanders. For example, the exit pupil can be substantially perpendicular to the first and second directions of replication / expansion provided by the first and second waveguide pupil expanders.

[0043] The observation plane and / or eyebox region can not be co-planar or parallel to the first and second directions of replication / expansion provided by the first and second waveguide pupil expanders. For example, the observation plane can be substantially perpendicular to the first and second directions of replication / expansion provided by the first and second waveguide pupil expanders.

[0044] In some embodiments, a waveguide coupler can be provided between the first and second waveguide pupil expanders. The waveguide coupler can be configured to directly or indirectly receive some or all of the plurality of replicas of the light field output by the first waveguide pupil expander. The waveguide coupler can be further configured to output said some or all of the plurality of replicas and directly or indirectly transmit them to the second waveguide pupil expander.

[0045] The waveguide coupler can have a receiving face comprising input ports configured to receive some, preferably most, preferably all of the plurality of replicas of the light field output by the first waveguide pupil expander. The receiving face of the waveguide coupler can be arranged parallel to the first pair of parallel surfaces of the first waveguide pupil expander, but this is not essential. One or more other elements can be provided to ensure that the replicas of the light field enter the waveguide coupler at suitable angles of incidence. The waveguide coupler can further comprise a transmitting face comprising output ports configured to transmit some, preferably most, preferably all of the plurality of replicas of the light field to the second waveguide pupil expander.

[0046] The waveguide coupler can further comprise a fourth pair of parallel reflecting surfaces, wherein the fourth pair of parallel surfaces are arranged to direct the light field from the receiving face to the transmitting face of the waveguide coupler by at least one internal reflection. The fourth pair of parallel surfaces can substantially correspond to the second pair of parallel surfaces of the first waveguide pupil expander. For example, a first surface within the fourth pair of parallel surfaces can be co-planar with a first surface within the second pair of parallel surfaces, and / or a second surface within the fourth pair of parallel surfaces can be co-planar with a second surface within the second pair of parallel surfaces.

[0047] A second separation dimension, including a separation dimension between two surfaces of a second pair of parallel surfaces, can be substantially equal to a separation dimension between two surfaces of a fourth pair of parallel surfaces. Thus, a correspondence between the fourth pair of parallel surfaces and the second pair of parallel surfaces can determine that the first waveguide pupil expander and the waveguide coupler occupy a common plane or layer.

[0048] The first waveguide pupil expander and the optional waveguide coupler can be arranged to occupy a first plane or layer. The second waveguide pupil expander can be arranged to occupy a different second plane or layer. The second layer can be substantially parallel to the first layer. It can be said that the shape and size of the second waveguide pupil expander define a region or "footprint" on / in the second layer. Since the first layer and the second layer are substantially parallel, the "footprint" of the second pupil replicator can also be described as being defined on the first layer. The first waveguide pupil expander, and optionally also the waveguide coupler, can be arranged within a region of the first layer that is smaller than or equal to the size of the footprint of the second waveguide pupil expander on the second layer. One or more additional elements, such as mirrors, can also be arranged within this region of the first layer.

[0049] The first and second layers can be arranged relative to each other such that the footprint of the second waveguide pupil expander on the second layer covers the region of the first layer within which the first waveguide pupil expander and the optional waveguide coupler are arranged. In other words, in such an arrangement, a "plan" or "top-down" view with the second waveguide pupil expander on the second layer would prevent an observer from seeing the region of the first layer that is occupied by the first waveguide pupil expander and, optionally, also by the waveguide coupler and / or the one or more additional elements.

[0050] The waveguide coupler can have a shape that substantially fills a space or gap between the first and second waveguide pupil expanders. For example, the waveguide coupler can have a substantially triangular shape.

[0051] To provide suitable emission conditions for achieving internal reflections within the first and second waveguide pupil expanders, the elongate dimension of the first waveguide pupil expander can be tilted relative to the first and second dimensions of the second waveguide pupil expander. This tilt can result in an inherent triangular gap, which the waveguide coupler can be arranged to at least partially occupy.

[0052] The display system can further include an element arranged to fold the optical path of the light field replica between the first and second waveguide pupil expanders, e.g. between the waveguide coupler and the second waveguide pupil expander. The element can comprise a fold mirror. The fold mirror can be arranged to occupy a first layer occupied by the first waveguide pupil expander and optionally also by the waveguide coupler. The fold mirror can be arranged to direct light from the first layer towards a second layer. The fold mirror can be arranged to provide suitable launch conditions for internal reflection within the second waveguide pupil expander. The use of a fold mirror can enable the second layer to overlap the first layer, thereby enabling a compact and reduced volume system.

[0053] In embodiments in which the footprint of the second waveguide pupil expander on the second layer overlaps the area of the first layer within which the first waveguide pupil expander and optionally also the waveguide coupler are arranged, the second waveguide pupil expander can be considered to define respective first and second dimensions or axes of the footprint. Light output by the first waveguide pupil expander can be arranged parallel to one of said first and second dimensions of the footprint. The elongate dimension of the first waveguide pupil expander can be arranged oblique to the respective other one of said first and second dimensions of the footprint.

[0054] According to some embodiments, a control device can be provided within the display system. For example, it can be provided downstream of the first waveguide pupil expander, e.g. between the first waveguide pupil expander and the second waveguide pupil expander, wherein the control device comprises an array of apertures, each aperture being selectively operable between a transmissive state and a non-transmissive state.

[0055] The control device can thus be configured to selectively control which light is transmitted between the first waveguide pupil expander and the second waveguide pupil expander, and which is not, on a dynamic basis. In some aspects, the control device is controllable to allow some replicas of the output diffracted light field from the first waveguide pupil expander to be transmitted to the second waveguide pupil expander, and to prevent certain other replicas from doing so. The control device can be controlled to allow all replicas of a given diffracted light field to be transmitted to the second waveguide pupil expander, but not all at exactly the same time. For example, the transmission of the replicas can be staggered, or sequential, or time controlled in another way. This can for example enable the control device, and hence the display system, to adapt to motion of the observer’s head, and hence to motion of their eyebox. Alternatively or additionally, it can also be used to adapt to the fact that the observer almost certainly has multiple observation apertures (i.e. two eyes), and hence to ensure that these two eyes do not receive holographic light corresponding to the same image content at exactly the same time, since the human brain inherently expects each eye to have at least a slightly different view, because the eyes are physically displaced from each other and have different respective intrinsic fields of view.

[0056] The array of apertures of the control device can extend in the elongate direction of the first waveguide pupil expander. These apertures can be physically discrete apertures, or they can be software controlled portions of the control device. In this way, the size, number, and location of the apertures within the control device can be dynamically varied. The control device can comprise a plurality of liquid crystal cells or regions, which are independently switchable between transmissive and non-transmissive states.

[0057] The control device can be disposed between the first waveguide pupil expander and the waveguide coupler, or between the waveguide coupler and the second waveguide pupil expander, or between the fold mirror and the second waveguide pupil expander.

[0058] In some embodiments, the first waveguide pupil expander and the waveguide coupler can be joined together. In some embodiments, the first waveguide pupil expander, the waveguide coupler, and the control device can be joined together. In any such aspects, the joining can comprise any suitable type of attachment between the constituent parts. Joining the constituent parts of the display system together can enhance the overall mechanical and thermal stability of the display system. It can also enhance and ensure the optical alignment between the constituent parts, thereby ensuring that light propagates correctly through the system and reaches the observer.

[0059] Each "waveguide pupil expander" performs pupil expansion by replicating the light of the pupil.

[0060] According to a second aspect, the light engine is arranged in a stacked or layered configuration comprising a first layer and a second layer. The first layer comprises a first pupil replicator and a waveguide coupler. The first pupil replicator is arranged to receive a diffracted light field from a diffractive structure having a pupil. The first pupil replicator is substantially elongate. The second layer comprises a second pupil replicator. The second pupil replicator is substantially planar. The second pupil replicator comprises a first major surface arranged to form an input of the light engine and a second major surface arranged to form an output of the light engine. The waveguide coupler is arranged to couple the output of the first pupil replicator to the input of the second pupil replicator. The first layer and the second layer are substantially parallel and adjacent to each other.

[0061] The first pupil replicator can be arranged to replicate the pupil of the diffractive structure in a first direction, and the second pupil replicator can be arranged to replicate the pupil of the diffractive structure in a second direction. The first direction can be substantially perpendicular to the second direction.

[0062] The first pupil replicator and the waveguide coupler can be arranged within a footprint of the second pupil replicator, wherein the footprint comprises an area occupied by the layer when viewed in a direction substantially perpendicular to the respective layer.

[0063] The first pupil replicator can comprise a major pair of opposing surfaces arranged to provide a light guide and pupil replication therebetween.

[0064] The waveguide coupler can include a primary pair of opposing surfaces including input and output surfaces, respectively, where the input and output surfaces are at an angle to each other.

[0065] The first pupil replicator and the waveguide coupler can be substantially coplanar.

[0066] The first pupil replicator and the waveguide coupler of the first layer can be arranged to waveguide the diffracted light field in a plane substantially parallel to the second layer, where "waveguide" means to propagate light by internal reflection.

[0067] The second layer can be defined by first and second axes, where the elongate dimension of the first pupil replicator is at an angle with respect to at least one of the first and second axes of the second layer. The angle of the elongate dimension of the first pupil replicator with respect to the first or second axis of the second layer can be substantially equal to the angle of incidence of the diffracted light received by the first pupil replicator.

[0068] The second pupil replicator can have a substantially quadrilateral cross-sectional shape.

[0069] The input of the second pupil replicator can be elongate and correspond to the first axis of the second layer.

[0070] The first and second major surfaces of the second pupil replicator can form a primary pair of opposing surfaces arranged to provide light guiding and pupil replication therebetween.

[0071] The second pupil replicator of the second layer can be arranged to waveguide the diffracted light field in a plane substantially parallel to the first layer.

[0072] The first pupil replicator and the waveguide coupler can be fixed to the first major surface of the second pupil replicator.

[0073] The first pupil replicator and the waveguide coupler can each include a secondary pair of opposing surfaces arranged to trap the diffracted light field within their plane, where "trap" means to prevent the diffracted light from exiting therefrom. At least one surface of each secondary pair of opposing surfaces can include a reflective component (e.g., a mirror coating), and at least one surface of each secondary pair of opposing surfaces can be fixed to a common substrate by the reflective component. The common substrate can be the second pupil replicator or a vehicle component that houses the light engine.

[0074] The first pupil replicator and the waveguide coupler can be integrated together.

[0075] The light engine can further include a control device. The control device can include a plurality of independently controlled apertures arranged to determine which pupil replicas are relayed from the first pupil replicator to the second pupil replicator. The first pupil replicator, the waveguide coupler, and / or the control device can be integrated together.

[0076] According to a third aspect, a head-up display for a vehicle comprises a first pupil replicator, a second pupil replicator, and a waveguide coupler. The first pupil replicator extends in a first direction. The first pupil replicator is arranged to receive a holographic light field from a spatial light modulator having an array of pixels defining a limiting aperture of the head-up display. The holographic light field is a complex light field spatially modulated according to a hologram displayed on the spatial light modulator. The second pupil replicator extends in the first direction and in a second direction perpendicular to the first direction. The second pupil replicator comprises a first major surface forming an output and a second major surface parallel to the first major surface. The waveguide coupler is arranged to optically couple the output of the first pupil replicator to an input of the second pupil replicator. The first pupil replicator and the waveguide coupler are arranged within a planar layer substantially parallel and adjacent to the second major surface of the second pupil replicator. Optionally, the first pupil replicator and the waveguide coupler can be attached to the second major surface of the second pupil replicator or a vehicle structural frame housing the head-up display.

[0077] A light engine is also provided comprising a first layer comprising a first pupil replicator, wherein the first pupil replicator is substantially planar and comprises a first major surface arranged to form an input and a second major surface arranged to form an output. The light engine further comprises a second layer comprising a second pupil replicator and a waveguide coupler arranged to couple an output of the second pupil replicator to an input of the first pupil replicator, wherein the second pupil replicator is substantially elongate and the waveguide coupler is substantially planar, wherein the first layer is defined by first and second axes, and wherein an elongate dimension of the second pupil replicator is angled relative to at least one of the first and second axes so that a light footprint of the second layer falls within a light footprint of the first layer.

[0078] A surface area of the first layer can be substantially equal to or greater than, for example slightly greater than, a surface area of each major surface of the first pupil replicator. In other words, the first layer can comprise only the first pupil replicator or it can comprise other matter and / or one or more other components in addition to the first pupil replicator.

[0079] The first pupil replicator can be quadrilateral in shape. Lengths and widths of the quadrilateral can be substantially parallel to the first and second axes, respectively, defining the first layer of the light engine.

[0080] The first and second layers can be substantially parallel and adjacent. Thus, the first and second layers can be provided in a compact and space-saving form.

[0081] The first pupil replicator can be arranged to replicate in the first direction and the second pupil replicator can be arranged to replicate in the second direction, wherein the second pupil replicator is angled on the second layer.

[0082] The light footprint of each layer can be defined as the footprint or area it occupies when viewed in a direction perpendicular to the first and second layers.

[0083] The second pupil replicator can be angled on the second layer such that the light footprint of the second layer falls within the light footprint of the first layer. Thus, the two layers can together be provided in a form having a cross-sectional area no more than the cross-sectional area of the first layer.

[0084] There is also provided a light engine comprising a first pupil replicator extending in a first direction and a second direction perpendicular to the first direction, wherein the first pupil replicator comprises a first major surface forming an output and a second major surface parallel to the first major surface. The light engine further comprises a second pupil replicator extending in the first direction and arranged to receive a diffracted light field from a spatial light modulator; and a waveguide coupler located between the first pupil replicator and the second pupil replicator, wherein the second pupil replicator and the waveguide coupler are disposed on a plane substantially parallel to and adjacent to the second major surface of the first pupil replicator.

[0085] The second pupil replicator (which can be referred to as a“pupil expander”) can comprise a primary pair of opposing surfaces arranged to provide a light guide therebetween and pupil replication in the first direction. The waveguide coupler can comprise a primary pair of opposing surfaces forming an input and an output respectively, wherein the input surface and the output surface are angled to one another. The input surface and the output surface can meet or touch one another at a vertex or corner.

[0086] The second pupil replicator and the waveguide coupler can each comprise a respective secondary pair of opposing surfaces arranged to capture a diffracted light field within their plane. Thus, when diffracted or divergent light is input into the second pupil replicator or waveguide, it will be arranged to prevent the light from escaping from its secondary pair of opposing surfaces. For each of the second pupil replicator and the waveguide coupler, the respective secondary pair of opposing surfaces can be arranged substantially perpendicular to the respective primary pair of opposing surfaces.

[0087] The first pupil expander can comprise a primary pair of opposing surfaces arranged to provide a light guide therebetween and pupil replication in the second direction.

[0088] The second pupil replicator and the waveguide coupler can be fixed to the second major surface of the first pupil replicator.

[0089] At least one of the secondary pair of opposing surfaces (of the second pupil replicator and the waveguide coupler) can be fixed to a common substrate. The common substrate can be the second pupil replicator.

[0090] The light engine can form part of a head-up display (HUD), such as a vehicle HUD. Each of the pair of opposing surfaces can be incorporated into another component or part of the vehicle in which the HUD is to be provided. Thus, the common substrate can be a component of the vehicle that houses the light engine. For example, it can be a chassis of a motor vehicle.

[0091] Incorporating the second pupil replicator and waveguide coupler into the common substrate helps to reduce manufacturing costs and ensures robustness of the light engine, for example by ensuring that the light engine is provided in a compact form, for example in a relatively flat streamlined form.

[0092] The second pupil replicator and waveguide coupler can be arranged within the footprint of the first pupil replicator. The light engine can be arranged in a stacked / laminated configuration comprising a first layer and a second layer, wherein the first layer comprises the first pupil replicator and the waveguide coupler, and the second layer comprises the second pupil replicator, or vice versa.

[0093] The first and second layers can be substantially parallel and abut.

[0094] The second pupil replicator and waveguide coupler can be arranged to propagate light in a plane that is substantially parallel to the plane of the first pupil replicator.

[0095] The second pupil replicator can be arranged to replicate the diffracted light field in a first direction, and the first pupil replicator can be arranged to replicate the diffracted light field in a second direction.

[0096] The second pupil replicator can comprise a first pair of opposing surfaces arranged to waveguide in a plane that is parallel to the plane of the first pupil expander.

[0097] The second pupil expander can comprise a second pair of opposing surfaces arranged to prevent the diffracted light field from exiting the first pupil expander through said second pair of opposing surfaces.

[0098] The second pair of opposing surfaces can be perpendicular to the first pair of opposing surfaces. At least one of the second pair of opposing surfaces can comprise a reflective coating arranged to provide internal reflection within the second pupil replicator.

[0099] There is also provided a light engine comprising: a first pupil replicator extending in a first direction and arranged to receive a diffracted light field from a spatial light modulator (SLM); a second pupil replicator extending in the first direction and in a second direction perpendicular to the first direction, wherein the second pupil replicator comprises a first major surface forming an output and a second major surface parallel to the first major surface; and a waveguide coupler arranged to optically couple the output of the first pupil replicator to an input of the second pupil replicator, wherein the first pupil replicator and the waveguide coupler are fixed to the second major surface of the second pupil replicator.

[0100] By providing the first pupil replicator and the waveguide coupler fixed to the second major surface of the second pupil replicator, a compact, streamlined, and robust form of the light engine can be provided. For example, fixing the components to one another can protect the light engine from potential damage that might otherwise occur when the light engine is disposed in an unstable environment, such as in a moving or vibrating environment, for example, in a vehicle.

[0101] The first pupil replicator and the waveguide coupler can be disposed within a common plane or a common layer. The SLM, and optionally the light source, can also be disposed within the common layer. The second pupil replicator can be disposed in a different, second layer.

[0102] A head-up display (HUD) system is also provided that includes the display device of any of the above aspects. The HUD system can be implemented in a vehicle, including but not limited to an automotive vehicle. The HUD system can also include an optical combiner, such as a windshield. In some aspects, the display system can be configured to direct the output light toward the optical combiner, and the optical combiner can be arranged to direct (or redirect) the output light toward an eyebox of an intended observer. The eyebox can be substantially normal to a plane defined by the second waveguide pupil expander.

[0103] A method of providing pupil expansion for a diffractive light field is also provided, the method including directing the diffractive light field into a first waveguide pupil expander, where the first waveguide pupil expander includes an input port, an output port, a first pair of parallel surfaces, and a second pair of parallel surfaces, where the first pair of parallel surfaces is orthogonal to the second pair of parallel surfaces. The method further includes directing the diffractive light field from the input port to the output port through internal reflections between the first pair of parallel surfaces, where a first surface of the first pair of parallel surfaces is partially transmissive-reflective such that the light field is split at each internal reflection, and multiple copies of the light field are transmitted through an area of the first surface forming the output port; and where the second pair of parallel surfaces is further arranged to direct the light field from the input port to the output port through at least one internal reflection.

[0104] The term "hologram" is used to refer to a recording containing either amplitude information or phase information or some combination thereof about an object. The term "holographic reconstruction" is used to refer to the optical reconstruction of an object formed by illuminating a hologram. The systems disclosed herein are described as "holographic projectors" 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 is in focus. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of multiple diffraction orders, where each diffraction order is a copy of the zeroth order replay field. The zeroth order replay field typically 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 to mean the zeroth order replay field. The term "replay plane" is used to refer to the plane in space that contains all of the replay fields. The terms "image", "replay image", and "image area" refer to the area of the replay field that is illuminated by the holographic reconstruction. In some embodiments, an "image" can comprise discrete points, which can be referred to as "image points", or "image pixels" for convenience only.

[0105] The terms "encoding", "writing", and "addressing" are used to describe the process of providing a plurality of control values to a plurality of pixels of an SLM that respectively determine the modulation level of each pixel. It can 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, it can be said that the SLM "displays" a hologram, and the hologram can be considered to be an array of light modulation values or levels.

[0106] It has been found that a holographic reconstruction of acceptable quality can be formed from a "hologram" that contains only phase information related to the Fourier transform of the original object. Such a holographic recording can be referred to as a phase-only hologram. Embodiments relate to phase-only holograms, but the present disclosure is equally applicable to amplitude-only holography.

[0107] The present disclosure is also equally applicable to the use of amplitude and phase information related to the Fourier transform of the original object to form a holographic reconstruction. In some embodiments, this is achieved by using complex modulation of a so-called full-complex hologram containing amplitude and phase information related to the original object. Because the value (gray level) assigned to each pixel of the hologram has an amplitude and a phase component, such a hologram can be referred to as a full-complex hologram. The value (gray level) assigned to each pixel can be represented as a complex number having an amplitude and a phase component. In some embodiments, a full-complex computer-generated hologram is calculated.

[0108] Reference can be made to the phase value, phase component, phase information, or simply the phase of a pixel of a computer-generated hologram or spatial light modulator as shorthand for "phase delay." That is, any phase value described is actually a number (e.g., in the range of 0 to 2π) that represents the amount of phase delay that the pixel provides. For example, a pixel of a spatial light modulator described as having a phase value of π / 2 would delay the phase of received light by π / 2 radians. In some embodiments, each pixel of a spatial light modulator can operate in one of a plurality of possible modulation values (e.g., phase delay values). The term "gray level" can be used to refer to the plurality of available modulation levels. For example, the term "gray level" can be used to refer to the plurality of available phase levels in a phase-only modulator for convenience, even though different phase levels do not provide different shades of gray. The term "gray level" can also be used to refer to the plurality of available complex modulation levels in a complex modulator for convenience.

[0109] Accordingly, the hologram includes an array of gray levels, i.e., 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 relative to (typically 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 used as lenses or gratings. For example, a diffractive pattern used as a grating can be combined with the hologram to translate the replay field on a replay plane, or a diffractive pattern used as a lens can be combined with the hologram to focus the holographic reconstruction on a replay plane in the near field.

[0110] Although different embodiments and groups of embodiments can be disclosed separately in the following detailed description, any feature of any embodiment or group of embodiments can 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 the features disclosed in this disclosure are contemplated. BRIEF DESCRIPTION OF DRAWINGS

[0111] Particular embodiments are described by way of example only in reference to the following drawings:

[0112] Figure 1 is a schematic diagram showing a reflective SLM producing a holographic reconstruction on a screen;

[0113] Figure 2A shows a first iteration of an example Gerchberg-Saxton type algorithm;

[0114] Figure 2B shows a second and subsequent iteration of an example Gerchberg-Saxton type algorithm;

[0115] Figure 2CAlternative second and subsequent iterations of an example Gerchberg-Saxton type algorithm are shown;

[0116] Figure 3 is a schematic diagram of a reflective LCOS SLM;

[0117] Figure 4A An image comprising a plurality of image regions (bottom) and a corresponding hologram comprising a plurality of hologram components (top) are shown;

[0118] Figure 4B A hologram is shown, characterized by routing or directing the holographic encoded light into a plurality of discrete hologram channels;

[0119] Figure 5 A system is shown, arranged to route the optical content of each hologram channel of Figure 4B to the eye through different optical paths;

[0120] Figure 6 A perspective view of a pair of stacked image replicators is shown, arranged to expand the light beam in two dimensions;

[0121] Figure 7 An improved display system is shown, comprising a two-dimensional pupil expander;

[0122] Figure 8 A diffracted light cone output by a diffractive structure is shown;

[0123] Figure 9 A two-layer pupil expander is shown;

[0124] Figure 10 An enlarged view of the two-layer pupil expander of Figure 9 is shown;

[0125] Figure 11 A plan view of the two-layer pupil expander of Figure 10 is shown, further comprising a spatial light modulator (SLM); and

[0126] Figure 12 A heads-up display system is shown, comprising a pupil expander.

[0127] In all the drawings, like reference numerals will be used to designate like or similar parts. DETAILED DESCRIPTION

[0128] The present application is not limited to the embodiments described below, but extends to the full scope of the claims. That is, the present application can be embodied in different forms and should not be construed as limited to the embodiments described, which are set forth for illustrative purposes only.

[0129] The singular form of a term can include the plural unless otherwise specified.

[0130] Structures described as formed on top of / beneath or above / beneath another structure should be interpreted to include the case where the structures are in contact with each other, in addition to the case where a third structure is disposed therebetween.

[0131] In describing time relationships, for example when the time order of events is described as "after", "subsequently", "next", "before", etc., the present disclosure should be considered to include both consecutive and non-consecutive events, unless otherwise specified. For example, unless the wording such as "just", "immediately", or "directly" is used, the description should be considered to include non-consecutive cases.

[0132] Although the terms "first", "second", etc. can be used herein to describe various elements, these elements should not 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.

[0133] Features of different embodiments can be coupled or combined with each other, in part or whole, and can interoperate with each other differently. Some embodiments can be executed independently of each other, or can be executed together in interdependent relationships.

[0134] Optical configuration

[0135] Figure 1 An embodiment is shown in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is the Fourier transform of the object for reconstruction. Thus, it can 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.

[0136] A light source 110, for example a laser or laser diode, is arranged to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a substantially planar wavefront of light to be incident on the SLM. In Figure 1 In the embodiment, the direction of the wavefront is off-normal (for example two or three degrees from truly normal to the plane of the transparent layer). However, in other embodiments, a substantially planar wavefront is provided with normal incidence and a beamsplitter arrangement is used to separate the input and output optical paths. In Figure 1In the illustrated embodiment, the arrangement is such that light from the light source is reflected from the mirror back surface of the SLM and interacts with the light modulation layer to form an exit wavefront 112. The exit wavefront 112 is applied to optics including a Fourier transform lens 120, the focal point of which is located at a screen 125. More specifically, the Fourier transform lens 120 receives the modulated light beam from the SLM 140 and performs a frequency-space transform to produce a holographic reconstruction at the screen 125.

[0137] Notably, in this type of hologram, each pixel of the hologram contributes to the entire reconstruction. There is no one-to-one correlation between a particular point (or image pixel) on the replay field and a particular light modulating element (or hologram pixel). In other words, the modulated light that exits the light modulation layer is distributed across the entire replay field.

[0138] In these embodiments, the location of the holographic reconstruction in space is determined by the power (focusing) of the Fourier transform lens. In Figure 1 In the illustrated embodiment, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and performs a Fourier transform optically. Any lens can act as a Fourier transform lens, but the performance of the lens will limit the accuracy of the Fourier transform that it performs. The skilled person understands how to use a lens to perform an optical Fourier transform.

[0139] Hologram calculation

[0140] 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 exploiting the Fourier transform properties of a positive lens. A Fourier hologram is computed by Fourier transforming the desired light field in the replay plane back to the lens plane. The computer-generated Fourier hologram can be computed using a Fourier transform.

[0141] A Fourier transform hologram can be computed using an algorithm such as the Gerchberg-Saxton algorithm. Furthermore, the Gerchberg-Saxton algorithm can be used to compute a hologram (i.e. a Fourier transform hologram) in the Fourier domain from only amplitude information in the spatial domain (such as a photograph). The phase information relating to the object is effectively “retrieved” from the only amplitude information in the spatial domain. In some embodiments, the computer-generated hologram is computed from only amplitude information using the Gerchberg-Saxton algorithm or a variant thereof.

[0142] The Gerchberg-Saxton algorithm considers that the intensity cross-sections I A (x,y) and I B(x, y) and I A (x, y) and I B (x, y) and I A (x, y) and I B (x, y). The Gerchberg-Saxton algorithm solves this problem by following an iterative procedure. More specifically, the Gerchberg-Saxton algorithm iteratively applies spatial and spectrum constraints while repeatedly transferring data sets (amplitude and phase) representing I A (x, y) and I B (x, y). A corresponding computer-generated hologram in the spectral domain is obtained by at least one iteration of the algorithm. The algorithm is convergent and arranged to produce a hologram representing the input image. The hologram can be an amplitude-only hologram, a phase-only hologram or a full-complex hologram.

[0143] In some embodiments, a phase-only hologram is computed using an algorithm based on the Gerchberg-Saxton algorithm, such as the algorithm described in UK patent 2498170 or 2501112, the entire contents of which are incorporated herein by reference. However, the embodiments disclosed herein describe the computation of a phase-only hologram by way of example only. In these embodiments, the Gerchberg-Saxton algorithm retrieves the phase information of the Fourier transform of the data set, Ψ[u, v], which produces known amplitude information, T[x, y], where the amplitude information T[x, y] represents a target image (e.g. a photograph). Since the amplitude and phase are intrinsically bound in the Fourier transform, the transformed amplitude and phase contain useful information about the accuracy of the computed data set. Therefore, the algorithm can be used iteratively with feedback of the amplitude and phase information. However, in these embodiments, only the phase information, Ψ[u, v], is used as a hologram to form a holographic representation of the target image at the image plane. The hologram is a data set (e.g. 2D array) of phase values.

[0144] In other embodiments, an algorithm based on the Gerchberg-Saxton algorithm is used to compute a full-complex hologram. A full-complex hologram is a hologram having an amplitude component and a phase component. The hologram is a data set (e.g. 2D array) comprising an array of complex data values, where each complex data value comprises an amplitude component and a phase component.

[0145] In some embodiments, the algorithm processes complex data, and the Fourier transform is a complex Fourier transform. The complex data can be considered to comprise either (i) real and imaginary components, or (ii) amplitude and phase components. In some embodiments, the two components of the complex data are treated differently at various stages of the algorithm.

[0146] Figure 2A A first iteration of the algorithm for computing a phase-only hologram is shown, according to some embodiments. The input to the algorithm is an input image 210 comprising a 2D array of pixels or data values, where each pixel or data value is an amplitude or magnitude value. That is, each pixel or data value of the input image 210 does not have a phase component. Thus, the input image 210 can be considered to be an amplitude-only or magnitude-only or intensity-only distribution. An example of such an input image 210 is a photograph or a frame of a video comprising a time sequence of frames. The first iteration of the algorithm begins with a data formation step 202A, which comprises assigning a random phase value to each pixel of the input image using a random phase distribution (or random phase seed) 230 to form a starting complex data set, where each data element of the data set comprises an amplitude and a phase. It can be said that the starting complex data set represents the input image in the spatial domain.

[0147] The first processing block 250 receives the starting complex data set and performs a complex Fourier transform to form a Fourier-transformed complex data set. The second processing block 253 receives the Fourier-transformed complex data set and outputs a hologram 280A. In some embodiments, the hologram 280A is a phase-only hologram. In these embodiments, the second processing block 253 quantizes each phase value and sets each amplitude value to 1 in order to form the hologram 280A. Each phase value is quantized according to the phase level that can be represented on the pixels of a spatial light modulator that will be used to "display" the phase-only hologram. For example, if each pixel of the spatial light modulator provides 256 different phase levels, then each phase value of the hologram is quantized to one of the 256 possible phase levels. The hologram 280A is a phase-only Fourier hologram that represents the input image. In other embodiments, the hologram 280A is a full complex hologram comprising an array of complex data values (each comprising an amplitude component and a phase component) derived from the received Fourier-transformed complex data set. In some embodiments, the second processing block 253 constrains each complex data value to one of a plurality of allowable complex modulation levels to form the hologram 280A. The constraining step can comprise setting each complex data value to the closest allowable complex modulation level in the complex plane. It can be said that the hologram 280A represents the input image in the frequency or Fourier or spectral domain. In some embodiments, the algorithm stops at this point.

[0148] However, in other embodiments, the algorithm continues, as Figure 2Athe dashed arrow in FIG. 2B. In other words, the steps following Figure 2A The steps following the dashed arrow in FIG. 2B are optional (i.e. not essential to all embodiments).

[0149] The third processing block 256 receives the modified complex dataset from the second processing block 253 and performs an inverse Fourier transform to form an inverse Fourier transformed complex dataset. It can be said that the inverse Fourier transformed complex dataset represents the input image in the spatial domain.

[0150] The fourth processing block 259 receives the inverse Fourier transformed complex dataset and extracts a distribution of magnitude values 211 A and a distribution of phase values 213A. Optionally, the fourth processing block 259 evaluates the distribution of magnitude values 211 A. In particular, the fourth processing block 259 can compare the distribution of magnitude values 211 A of the inverse Fourier transformed complex dataset with the input image 510, which is of course itself a distribution of magnitude values. If the difference between the distribution of magnitude values 211 A and the input image 210 is sufficiently small, then the fourth processing block 259 can determine that the hologram 280A is acceptable. That is, if the difference between the distribution of magnitude values 211 A and the input image 210 is sufficiently small, then the fourth processing block 259 can determine that the hologram 280A is a sufficiently accurate representation of the input image 210. In some embodiments, the distribution of phase values 213A of the inverse Fourier transformed complex dataset is ignored for the purposes of comparison. It will be understood that any number of different methods can be employed to compare the distribution of magnitude values 211 A and the input image 210, and the present disclosure is not limited to any particular method. In some embodiments, a mean squared difference is calculated, and if the mean squared difference is less than a threshold value, then the hologram 280A is deemed to be acceptable. If the fourth processing block 259 determines that the hologram 280A is not acceptable, then a further iteration of the algorithm can be performed. However, this comparison step is not essential, and in other embodiments the number of iterations of the algorithm performed is predetermined or pre-set or user-defined.

[0151] Figure 2B The second iteration of the algorithm, and any further iterations of the algorithm, are represented in FIG. 2C. The distribution of phase values 213A of the previous iteration is fed back through the processing blocks of the algorithm. The distribution of magnitude values 211 A is rejected in favour of the distribution of magnitude values of the input image 210. In the first iteration, the data formation step 202A forms a first complex dataset by combining the distribution of magnitude values of the input image 210 with a random phase distribution 230. However, in the second and subsequent iterations, the data formation step 202B comprises forming a complex dataset by combining (i) the distribution of phase values 213A from the previous iteration of the algorithm with (ii) the distribution of magnitude values of the input image 210.

[0152] Then, with reference to FIG. 2D, the second iteration of the algorithm is performed. The distribution of phase values 213A of the previous iteration is fed back through the processing blocks of the algorithm. The distribution of magnitude values 211 A is rejected in favour of the distribution of magnitude values of the input image 210. In the first iteration, the data formation step 202A forms a first complex dataset by combining the distribution of magnitude values of the input image 210 with a random phase distribution 230. However, in the second and subsequent iterations, the data formation step 202B comprises forming a complex dataset by combining (i) the distribution of phase values 213A from the previous iteration of the algorithm with (ii) the distribution of magnitude values of the input image 210. Figure 2AThe complex data set formed by the data of Figure 2B The complex data set output by the third processing block 256 is processed by the data formation step 202B to form a second iteration hologram 280B. Thus, the description of the process is not repeated here. When the second iteration hologram 280B has been calculated, the algorithm can stop. However, any number of further iterations of the algorithm can be performed. It will be appreciated that the third processing block 256 is only required if a fourth processing block 259 is required or if further iterations are required. The output hologram 280B generally gets better with each iteration. However, in practice, there will generally be a point at which no measurable improvement can be observed, or the positive benefits of performing further iterations are offset by the negative impact of the additional processing time. Thus, the algorithm is described as iterative and convergent.

[0153] Figure 2C An alternative embodiment is represented by the second and subsequent iterations. The distribution of phase values 213A of the previous iteration is fed back through the processing blocks of the algorithm. The distribution of amplitude values 21 1A is rejected in favour of an alternative distribution of amplitude values. In this alternative embodiment, the alternative distribution of amplitude values is derived from the distribution of amplitude values 21 1 of the previous iteration. Specifically, the processing block 258 subtracts the distribution of amplitude values of the input image 210 from the distribution of amplitude values 21 1 of the previous iteration, scales the difference by a gain factor a, and subtracts the scaled difference from the input image 210. This is expressed mathematically by the following equations, where the subscript text and number represents the iteration number:

[0154] R n+1 [x,y] = F'{exp(iψ n [u,v])}

[0155] ψ n [u,v] = ∠F{η·exp(i∠R n [x,y])}

[0156] η = T[x,y] - a(|R n [x,y]| - T[x,y])

[0157] Where:

[0158] F' is the inverse Fourier transform;

[0159] F is the forward Fourier transform;

[0160] R[x,y] is the complex data set output by the third processing block 256;

[0161] T[x,y] is the input or target image;

[0162] ∟ is the phase component;

[0163] Ψ is the phase-only hologram 280B;

[0164] η is a new distribution of the magnitude values 211B; and

[0165] α is a gain factor.

[0166] The gain factor α can be fixed or variable. In some embodiments, the gain factor α is determined based on the size and rate of the input target image data. In some embodiments, the gain factor α depends on the number of iterations. In some embodiments, the gain factor α is only a function of the number of iterations.

[0167] In all other respects, Figure 2C the embodiments of Figure 2A and Figure 2B are identical. It can be said that only the phase hologram Ψ(u,v) comprises a phase distribution in the frequency or Fourier domain.

[0168] In some embodiments, the Fourier transform is performed using a spatial light modulator. Specifically, the hologram data is combined with a second data that provides optical power. That is, the data written to the spatial light modulation comprises hologram data representing the object and lens data representing a lens. When displayed on the spatial light modulator and illuminated with light, the lens data emulates a physical lens - that is, it focuses light in the same way as the corresponding physical optical element. Thus, the lens data provides optical power or focusing power. In these embodiments, the Figure 1The data representing the lens can be referred to as software lens. For example, a phase-only lens can be formed by calculating the phase delay caused by each point of the lens due to its refractive index and spatially varying optical path length. For example, the optical path length at the centre of a convex lens is greater than at the edge of the lens. An amplitude-only lens can be formed from a Fresnel zone plate. It is also known in the art of computer generated holography how to combine data representing a lens with a hologram so that the Fourier transform of the hologram can be performed without the need for a physical Fourier lens. In some embodiments, the lensing data is combined with the hologram by simple addition such as simple vector addition. In some embodiments, a physical lens is used in combination with a software lens to perform the Fourier transform. Alternatively, in other embodiments, the Fourier transform lens is omitted entirely so that the holographic reconstruction occurs in the far field. In further embodiments, the hologram can be combined with grating data - i.e. data arranged to perform a grating function such as image steering - in the same way. Again, it is known in the art how to calculate such data. For example, a phase-only grating can be formed by modelling the phase delay caused by each point on the surface of a blazed grating. An amplitude-only grating can simply be superimposed with an amplitude-only hologram to provide angular steering of the holographic reconstruction. The second data providing the lensing and / or steering can be referred to as an optical processing function or optical processing pattern to distinguish from the hologram data which can be referred to as an image forming function or image forming pattern.

[0169] In some embodiments, the Fourier transform is performed jointly by a physical Fourier transform lens and a software lens. That is, some of the optical power to assist the Fourier transform is provided by the software lens and the remainder of the optical power to assist the Fourier transform is provided by one or more physical optical devices.

[0170] In some embodiments, a real-time engine is provided which is arranged to receive image data and calculate holograms in real-time using an 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 required for display on the SLM. That is, in some embodiments, a repository of predetermined holograms is provided.

[0171] The embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The disclosure is equally applicable to Fresnel holography and Fresnel holograms which can be calculated by similar methods. The disclosure can also be applicable to holograms calculated by other techniques such as point cloud based methods.

[0172] Light modulation

[0173] A spatial light modulator can be used to display a diffraction pattern comprising a computer-generated hologram. If the hologram is a phase-only hologram, a spatial light modulator that modulates phase is required. If the hologram is a full complex hologram, a spatial light modulator that modulates both phase and amplitude can be used, or a first spatial light modulator that modulates phase and a second spatial light modulator that modulates amplitude can be used.

[0174] In some embodiments, the light modulating elements (i.e. pixels) of the spatial light modulator are cells containing liquid crystals. That is, in some embodiments, the spatial light modulator is a liquid crystal device in which the optically active component is a liquid crystal. Each liquid crystal cell is configured to selectively provide a plurality of light modulation levels. That is, each liquid crystal cell is configured to operate at one light modulation level selected from a plurality of possible light modulation levels at any one time. Each liquid crystal cell can be dynamically reconfigured to a different light modulation level from the plurality of light modulation levels. In some embodiments, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) spatial light modulator, although the present disclosure is not limited to this type of spatial light modulator.

[0175] LCOS devices provide a dense array of light modulating elements or pixels within a small aperture (e.g. a few centimetres wide). The pixels are typically around 10 microns or less, which results in diffraction angles of a few degrees, meaning that the optical system can be compact. It is much easier to sufficiently illuminate the small aperture of an LCOS SLM than the large aperture of other liquid crystal devices. LCOS devices are typically reflective, meaning that the circuitry driving the pixels of the LCOS SLM can be buried under the reflective surface. This results in a higher aperture ratio. In other words, the pixels are densely packed, meaning that there is little dead space between the pixels. This is advantageous as it reduces optical noise in the replay field. LCOS SLMs use a silicon substrate, which has the advantage that the pixels are optically flat. This is particularly important for phase modulating devices.

[0176] A suitable LCOS SLM is described below by way of example only with reference to Figure 3 An LCOS device is formed using a single-crystal silicon substrate 302. It has a 2D array of square planar aluminium electrodes 301, spaced apart by gaps 301a, arranged on the upper surface of the substrate. Each electrode 301 can be addressed by circuitry 302a buried in the substrate 302. Each electrode forms a respective planar mirror. An orientation layer 303 is provided over the array of electrodes, and a liquid crystal layer 304 is provided over the orientation layer 303. A second orientation layer 305 is provided over a planar transparent layer 306, for example made of glass. A single transparent electrode 307, for example made of ITO, is provided between the transparent layer 306 and the second orientation layer 305.

[0177] Each square electrode 301, together with the overlying area of the transparent electrode 307 and the intervening liquid crystal material, defines a controllable phase-modulating element 308, commonly referred to as a pixel. Taking into account the spaces between the pixels 301a, the effective pixel area or fill factor is the percentage of the total pixels that are optically active. By controlling the voltage applied to each electrode 301 relative to the transparent electrode 307, the properties of the liquid crystal material of the individual phase-modulating elements can be varied, providing a variable retardation to light incident thereon. The effect is to provide a phase-only modulation to the wavefront, i.e. no amplitude effects occur.

[0178] The described LCOS SLM outputs spatially modulated light in a reflective manner. Reflective LCOS SLMs have the advantage that the signal lines, the grating lines and the transistors are located below the mirror surface, which leads to a high fill factor (typically more than 90%) and a high resolution. Another advantage of using a reflective LCOS spatial light modulator is that the thickness of the liquid crystal layer can be half of the thickness required when using a transmissive device. This greatly improves the switching speed of the liquid crystal (a key advantage for projecting moving video images). However, the teachings of the present disclosure can equally be implemented using a transmissive LCOS SLM.

[0179] Light channel

[0180] The optical systems disclosed herein are suitable for pupil expansion with any diffractive light field. In some embodiments, the diffractive light field is a holographic light field - i.e. a complex light field that is spatially modulated according to a hologram of an image rather than the image itself. In some embodiments, the hologram is a special type of hologram that angularly divides / directs image content. This type of hologram is further described herein, merely as an example of a diffractive light field that is compatible with the present disclosure. Other types of holograms can be used in conjunction with the display systems and light engines disclosed herein.

[0181] A display system and method are described below that include a waveguide pupil expander, which will be described from Figure 7and the description that follows. As will be familiar to the skilled reader, a waveguide can be thought of as a "pupil expander" in that it can be used to increase the area over which light emitted by a relatively small light emitter (such as a relatively small SLM or other pixelated display device used in the apparatus described herein) can be observed by a human observer or other observation system located a distance (such as a relatively large distance) away from the light emitter. The waveguide achieves this by increasing the number of transmissive points at which light is output to the observer. As a result, the light can be seen from a plurality of different observer positions, e.g. an observer can move their head, and thus their line of sight, whilst still being able to see light from the light emitter. Thus, it can be said that the "eye box" or "eye movement box" of the observer is magnified by the use of the waveguide pupil expander. This has a number of useful applications, such as but not limited to head-up displays, such as but not limited to automotive head-up displays.

[0182] The display systems described herein can be configured to direct light, such as a diffracted light field, through a waveguide pupil expander in order to provide pupil expansion in at least one dimension, e.g. in two dimensions. The diffracted light field can comprise light output by a spatial light modulator (SLM), such as an LCOS SLM. For example, the diffracted light field can comprise light encoded by a hologram displayed by the SLM. For example, the diffracted light field can comprise light of a holographic reconstructed image, corresponding to a hologram displayed by the SLM. The hologram can comprise a computer generated hologram (CGH), such as but not limited to a point cloud hologram, a Fresnel hologram or a Fourier hologram. The hologram can be referred to as a "diffractive structure" or a "modulation pattern". The SLM or other display device can be arranged to display the diffractive pattern (or modulation pattern) in a manner familiar to the skilled reader, which comprises the hologram and one or more other elements, such as a software lens or a diffractive grating.

[0183] The hologram can be calculated to provide the guidance of the diffracted light field. This is described in detail in GB2101666.2, GB2101667.0 and GB2112213.0, all of which are incorporated herein by reference. In general, the hologram can be calculated to correspond to an image that is to be holographically reconstructed. The image to which the hologram corresponds can be referred to as an "input image" or a "target image". The hologram can be calculated so that when it is displayed on the SLM and suitably illuminated, it forms a light field (output by the SLM) comprising a spatially modulated light cone. In some embodiments, the light cone comprises a plurality of continuous channels of spatially modulated light, corresponding to respective continuous regions of the image. However, the present disclosure is not limited to this type of hologram.

[0184] Although we refer to them here as "holograms" or "computer-generated holograms (CGHs)", it will be appreciated that the SLM can be configured to display a plurality of different holograms continuously or dynamically according to a sequence. The systems and methods described herein are applicable to the dynamic display of a plurality of different holograms.

[0185] Figure 4A to 5 An example of the type of hologram that can be displayed on a display device such as an SLM, which can be used in conjunction with the pupil expanders disclosed herein, is shown. This example should not, however, be seen as limiting the disclosure.

[0186] Figure 4A An image 452 for projection is shown, comprising eight image regions / components V1 to V8. By way of example only, Figure 4A Eight image components are shown, and the image 452 can be divided into any number of components. Figure 4A An encoded light pattern 454 (i.e. a hologram) is also shown, which can reconstruct the image 452 - for example, when converted by a lens of a suitable viewing system. The encoded light pattern 454 comprises first to eighth sub-holograms or components H1 to H8, corresponding to the first to eighth image components / regions V1 to V8. Figure 4A It is further shown how the hologram decomposes the image content by angle. Thus, the hologram is characterised by its guidance of light. This is shown in Figure 4B Specifically, the hologram in this example guides light into a plurality of discrete regions. In the example shown, the discrete regions are discs, but other shapes are also envisaged. The size and shape of the optimal discs can be related to the size and shape of the entrance pupil of the viewing system, after propagation through the waveguide.

[0187] Figure 5 A viewing system 500 is shown, comprising a display device displaying a hologram calculated as shown in Figure 4A and 4B

[0188] The viewing system 500 comprises a display device, which in this arrangement comprises an LCOS 502. The LCOS 502 is arranged to display a modulated pattern (or "diffraction pattern") comprising a hologram, and to project light that has been holographically encoded to an eye 505, which comprises a pupil acting as an aperture 504, a lens 509 and a retina (not shown) acting as a viewing plane. There is a light source (not shown) arranged to illuminate the LCOS 502. The lens 509 of the eye 505 performs the conversion of the hologram to an image. The light source can be of any suitable type. For example, it can comprise a laser source.

[0189] ​The viewing system 500 also includes a waveguide 508 positioned between the LCOS 502 and the eye 505. The presence of the waveguide 508 enables all angular content from the LCOS 502 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 well-known manner, and so is only briefly described here.

[0190] In brief summary, Figure 5 The waveguide 508 shown comprises a substantially elongate structure. In this example, the waveguide 508 comprises an optical slab of refractive material, although other types of waveguide are also well known and can be used. The waveguide 508 is positioned to intersect the cone of light (i.e. the diffracted light field) projected from the LCOS 502, for example at an oblique angle. In this example, the size, position and positioning of the waveguide 508 is configured to ensure that light from each of the eight beams within the cone of light enters the waveguide 508. Light from the cone of light enters the waveguide 508 via a first planar surface of the waveguide 508 (positioned closest to the LCOS 502) and is guided at least partially along the length of the waveguide 508 before being emitted via a second planar surface of the waveguide 508 (positioned closest to the eye) which is substantially opposite the first surface. It will be readily appreciated that the second planar surface is partially reflective, partially transmissive. In other words, when each light ray propagates within the waveguide 508 from the first planar surface and hits the second planar surface, some of the light will be transmitted out of the waveguide 508 and some of the light will be reflected back to the first planar surface by the second planar surface. The first planar surface is reflective such that all light hitting it from within the waveguide 508 will be reflected back to the second planar surface. Thus, some light can simply refract between the two planar surfaces of the waveguide 508 before being transmitted, while other light can be reflected and thus can experience one or more reflections (or “bounces”) between the planar surfaces of the waveguide 508 before being transmitted.

[0191] Figure 5 Nine “bounce” points B0to B8are shown along the length of the waveguide 508. Although as Figure 4A shown, light associated with all points of the image (V1-V8) is transmitted out of the waveguide at each “bounce” from the second planar surface of the waveguide 508, only light from one angular portion of the image (e.g. light from one of V1to V8) has a trajectory which enables it to reach the eye 505 from each respective “bounce” point B0to B8. Furthermore, light from different angular portions of the image (V1to V8) reaches the eye 505 from each respective “bounce” point. Thus, in the example shown, each angular channel of encoded light reaches the eye from the waveguide 508 only once. Figure 5

[0192] ​The methods and apparatus described above can be implemented in a variety of different applications and observation systems. For example, they can be implemented in head-up displays (HUDs) or in head-mounted or helmet-mounted devices (HMDs) such as augmented reality (AR) HMDs.

[0193] Although virtual images have been discussed in general terms here, which require the eye to convert received modulated light to form a perceived image, the methods and apparatus described here can be applied to real images.

[0194] Two-dimensional pupil expansion

[0195] Although Figure 5 The arrangement shown includes a single waveguide providing pupil extension in one dimension, but pupil extension can be provided in more than one dimension, such as in two dimensions. Furthermore, although Figure 5 The examples in the document use holograms that have been calculated to create optical channels, each corresponding to a different part of the image, but this disclosure and the system described below are not limited to this type of hologram.

[0196] Figure 6 A perspective view of system 600 is shown, which includes two replicators 604 and 606 arranged to extend beam 602 in two dimensions.

[0197] exist Figure 6 In system 600, the first replicator 604 includes a first pair of surfaces stacked parallel to each other, arranged in a manner similar to Figure 5 The waveguide 508 provides replication or pupil expansion. The first pair of surfaces are similar in size and shape to each other (in some cases identical) and are substantially elongated in one direction. The collimated beam 602 is guided to the input on the first replicator 604. Due to the internal reflection process between the two surfaces, and from one of the surfaces (the upper surface, such as...) Figure 6 As shown, the light from each of the plurality of output points on the beam 602 is partially transmitted, which will be familiar to a skilled reader, and the light from beam 602 is replicated along the length of the first replicator 604 in a first direction. Thus, the first plurality of replica beams 608 are emitted from the first replicator 604 toward the second replicator 606.

[0198] The second replicator 606 comprises a second pair of surfaces stacked parallel to each other, arranged to receive each collimated light beam of the first plurality of light beams 608, and further arranged to provide replication, or pupil expansion, by expanding each of these light beams in a second direction substantially orthogonal to the first direction. The first pair of surfaces are similar in size and shape to each other (in some cases identical), and are substantially rectangular in shape. The rectangular shape is implemented for the second replicator to have a length along the first direction in order to receive the first plurality of light beams 608, and to have a length along the second orthogonal direction in order to provide replication in this second direction. Due to the internal reflection process between the two surfaces, and the partial transmission of light from each of a plurality of output points on one of the surfaces (such as the upper surface as shown), the light of each light beam within the first plurality of light beams 608 is replicated in the second direction. Thus, a second plurality of light beams 610 is emitted from the second replicator 606, wherein the second plurality of light beams 610 comprises replicas of the input light beam 602 along each of the first and second directions. Thus, the second plurality of light beams 610 can be viewed as comprising a two-dimensional grid or array of replica light beams. Figure 6

[0199] Thus, it can be said that Figure 6 The first and second replicators 604, 605 in combination provide a two-dimensional replicator (or "two-dimensional pupil expander").

[0200] Improved two-dimensional pupil expander

[0201] The inventors have recognised limitations in the usefulness and efficiency of pupil expanders in practical applications. For example, the skilled reader will appreciate that many practical applications requiring the use of a pupil expansion have physical space limitations. For example, in automotive head-up displays, it can be desirable for the pupil expander to be provided in a limited space, for example under the dashboard of a vehicle. Furthermore, such spaces are often unstable environments that are mobile or vibrating, which is problematic for many conventional pupil expanders. Furthermore, pupil expanders are often provided as part of a larger display or viewing system that includes other optical elements, all of which can have to be physically constrained within a limited space. However, known pupil expanders often make non-optimal trade-offs between the extent (or range) of pupil expansion provided, and the physical volume of space occupied by the two-dimensional pupil expander and its adaptability to its surrounding environment.

[0202] ​Even more so, when the light field is a diffractive light field, such as a holographic light field from a small display device, the inventors have discovered technical problems associated with using waveguides to replicate / expand the pupil. Unlike conventional imaging, holography involves diffraction, and it is advantageous to propagate diffracted / divergent light within the waveguide, rather than collimated light, when the image is formed by holographic reconstruction at the eye. The important problem is that the display device required is small due to the small pixel size required for diffraction, the projection distance needs to be large (relatively speaking), and thus the size of the light field (in cross-sectional dimension) becomes large. Notably, the inventors have discovered that by using simple internal reflections from two complementary surfaces of the elongated waveguide (i.e., the other two elongated surfaces that do not contribute to pupil replication / expansion), the holographic light field can be effectively "folded" in this dimension and all image content is preserved (albeit in the holographic domain). Furthermore, the inventors have devised a compact stacked configuration that leverages waveguide couplers to further fold the holographic light field, facilitating optimal (i.e., small volume) packaging.

[0203] The inventors have recognized that it is possible to provide a two-dimensional pupil expander for holography that enhances the balance between efficiency, robustness, and compactness. The two-dimensional pupil expander disclosed herein enables a user to receive all (or at least a desired portion) of the light output by a light emitter (such as an SLM or another pixelated device) used with the two-dimensional pupil expander, and also provides the user with a larger eyebox than is conventionally achievable, enabling them to move their head and still see the desired light, enabling the user to have a wide field of view. All of this is provided in a compact, robust, and space-saving manner. This can be further understood from the figures that follow. Figure 7

[0204] Figure 7 An improved system 700 is shown, including a first waveguide pupil expander 702, a waveguide coupler 704 (which is optional in some embodiments), and a second waveguide pupil expander 706.

[0205] The first waveguide pupil expander 702 includes three-dimensional elements. The first waveguide pupil expander 702 is substantially cuboid in shape, having three pairs of mutually orthogonal faces, the first waveguide pupil expander 702 being relatively long along one dimension, and relatively short along each of the other two dimensions. However, the present disclosure is not limited to Figure 7 the particular dimensions or shape of the first waveguide pupil expander 702 shown.

[0206] The first waveguide pupil expander 702 includes a first pair of parallel elongated faces 708a, 708b, which are shown in the particular arrangement shown as the upper and lower faces, respectively. The first waveguide pupil expander 702 also includes a second pair of parallel elongated faces 710a, 710b, which are shown in the particular arrangement shown as the left and right faces, respectively. Figure 7 The first waveguide pupil expander 702 also includes a third pair of parallel elongated faces 712a, 712b, which are shown in the particular arrangement shown as the front and back faces, respectively.​Figure 7 In the particular arrangement shown, they are shown as side faces. The first waveguide pupil expander 702 also includes a pair of opposing smaller end faces 712a, 712b, which are also parallel to each other. For ease of understanding and convenience, the relevant terms “upper”, “lower”, “side” and “end” will be used in the description of Figure 7 However, it should be understood that the present disclosure is not limited to these relevant terms, and the system 700 can be moved, rotated or translated in any suitable manner while still functioning as described herein.

[0207] The input port on the upper elongated face 708a is arranged to receive input light 714. The upper elongated face 708a also includes an output port 716 at an end of the upper elongated face 708a substantially opposite the input port (not specifically shown in the Figure 7 The output port is the last of a plurality of transmissive points defined along the upper elongated face 708a.

[0208] Although the input light 714 is depicted in Figure 7 with a single line, the inventors have recognized that it is desirable / advantageous for the input light 714 to be non-collimated and / or to include a diverging beam of light. Thus, in embodiments, the input light 714 includes diffracted or diverging light. The input light 714 can be received directly or indirectly from an SLM, e.g., there can be one or more other optical elements between the SLM and the first waveguide pupil expander 702.

[0209] The first pair of parallel elongated faces 708a, 708b are arranged to function as a waveguide pupil expander (or replicator) in a manner analogous to that described above with respect to Figure 5 and 6 The inner surface of the lower face 708b is reflective, and the upper face 708a is partially transmissive and reflective. Thus, the first pair of parallel elongated faces 708a, 708b are arranged to internally reflect or “bounce” light between them along the elongated direction of the first waveguide pupil expander 702, and to transmit some light from each of the plurality of transmissive points on the upper face 708a between the input port and the output port. Thus, the light of the hologram is replicated or expanded in the first direction. In short, it can also be said that the hologram is replicated.

[0210] However, the inventors have recognized that if the input light 714 is diffracted or diverging light (i.e., light that includes a diverging beam of light), it will include an expanding cone of light (as opposed to one or more parallel beams of light included within collimated light), such that the size of the cone of light— i.e., the size ‘L’ of the diffracted light field defined by the end or mouth of the cone of light, as shown in Figure 8 — increases as the light propagates along its optical path. This is a recognized principle of diffracted / diverging light, and can be found in, e.g., Figure 8This is understood from the light cone 802 emitted from the SLM804 shown. As illustrated, the light cone 802 is emitted with a diffraction angle θ, which is the distance from the optical axis 'A' (extended from the center point of the emitted light on the plane substantially perpendicular to the SLM804) and the outermost part or limit of the light cone 802 in the positive or negative direction relative to that axis 'A' (i.e., in the direction of the light cone 802). Figure 8 In the example shown, the angle is defined between (above or below) the optical axis 'A'. According to the accepted principle of trigonometry, it can be seen that the size 'L' of the diffracted light field depends on the diffraction angle θ and the distance 'd' from the SLM804 measuring this size 'L'.

[0211] Therefore, the inventors further recognize that if diffracted / divergent light is input into the elongated facet of the first image replicator in a conventional arrangement, there is a risk that the size of the light cone (i.e., the size 'L' of the diffracted light field defined by the end or aperture of the light cone) will exceed the size of the pupil replicator along one of the shorter dimensions of the pupil replicator. In other words, if Figure 7 The first waveguide pupil expander 702 shown is a conventional arrangement, which carries the risk that at some point along the propagation path between its upper surface 708a and lower surface 708b, the size 'L' of the diffracted light field will exceed the left and right thicknesses of the first waveguide pupil expander 702. If this occurs, in a conventional pupil expander, at least a portion of the light from the input light cone 714 will therefore escape through one or both sides and thus will not reach the intended observer correctly.

[0212] Therefore, the inventors provided an improved first waveguide pupil expander 702, wherein the second pair of parallel surfaces—in Figure 7 In the example, a pair of elongated sides 710a, 710b are also arranged to guide the light field from the input port to the output port via at least one internal reflection. This internal reflection allows the light from the spatially modulated light cone to remain trapped within the first waveguide pupil expander, even when the size of the diffracted light field extends beyond the size of the first waveguide pupil expander 702 in one or more dimensions. Figure 7 In the example shown, light is transmitted only through multiple transmission points defined on an elongated surface (upper elongated surface 708a) specifically designed for pupil expansion. Therefore, light containing desired information, such as image-related information (e.g., coded light corresponding to a hologram of the image), is not lost through the subsurface of the first waveguide pupil expander 702. In short, the inventors have found it possible to effectively “fold” the diffraction / holographic light field in this direction (using additional reflections from other opposing surfaces) to preserve all necessary diffraction / holographic light content for reconstructing a high-quality image at the eye.

[0213] The inventors have recognized that, at least in some embodiments, the light emission conditions should be actively controlled so that the light within the first waveguide pupil expander 702 is reflected not only by the first pair of opposing surfaces but also by the interior of the second pair of opposing surfaces. For example, from Figure 7 As can be seen from the example, the input light 714 should enter the first waveguide pupil expander 702 at an angle of incidence (AOI), which is defined relative to the normal of the surface 708a into which it enters. Such an angle is required in order to establish propagation at least between the first pair of opposing surfaces.

[0214] The inventors discovered that the combination of the relatively small thickness and angle of incidence (AOI) of the first waveguide pupil expander 702 enables the first waveguide pupil expander 702 to provide high-quality pupil expansion in the first direction (e.g., without any vertical black / white bands).

[0215] The first waveguide coupler 702 can be formed of any suitable material to function as a waveguide as described herein. In an embodiment, the inherent difference between the refractive index 'n' of the first waveguide coupler 702 and air will enable the second pair of parallel surfaces to provide internal reflection, such as total internal reflection, thereby keeping the diffracted light inside the first waveguide coupler 702, except when it is transmitted in a controlled manner from the output port on the reflective / transmitting surface of the first pair of parallel surfaces. In other embodiments, at least one surface of the second pair of parallel surfaces may be coated or added with another material to achieve the desired internal reflection and light trapping. This will be discussed further below in conjunction with the accompanying drawings.

[0216] The light output from the first waveguide pupil expander 702 includes a plurality of corresponding transmission points on the upper elongated surface 708a. Figure 7 Multiple duplicate beams (not specifically shown) are output. The output light is at an angle ( Figure 7 (Not specifically shown) emitted from the transmission point, where each output beam is substantially parallel to each of the corresponding other beams. This can be seen from... Figure 10 As can be seen from the optical path shown, Figure 10 An embodiment of the improved system disclosed herein is shown, which will be discussed further below. Return Figure 7 In some embodiments, pupil expansion is provided only in one direction, thus eliminating the need for... Figure 7 The waveguide coupler 704 and the second waveguide pupil expander 706 are shown.

[0217] Accordingly, there is provided a display system comprising a first waveguide pupil expander comprising an input port, an output port, a first pair of parallel faces and a second pair of parallel faces, wherein the first pair of parallel faces is orthogonal to the second pair of parallel faces, wherein the first pair of parallel faces is arranged to light guide a diffracted light field from the input port to the output port by internal reflections therebetween, and wherein a first face of the first pair of parallel faces is partially transmissive-reflective such that the light field is split at each internal reflection and multiple replicas of the light field are transmitted through a region of the first face forming the output port, and wherein the second pair of parallel faces is also arranged to light guide the light field from the input port to the output port by at least one internal reflection. The second pair of parallel faces is not arranged to provide pupil replication by allowing partial transmission, and optionally, both faces thereof can be arranged for perfect light guiding to the output port, i.e. 100% (or close to 100%) reflection.

[0218] The improved first waveguide pupil expander 702 disclosed herein enables a diffracted light field to propagate therethrough and thus be expanded in a first dimension. Accordingly, for example, light of a hologram (i.e. light of a hologram that has been spatially modulated on an SLM or other display device but has not been converted to form a holographic reconstructed image) can be propagated and expanded by the improved first waveguide pupil expander 702. Each "replica" or output beam formed by the first waveguide pupil expander is in fact a replica of the hologram, as the light is spatially modulated according to the hologram. In colloquial terms, it can be said that the light is "encoded" with the hologram.

[0219] In embodiments in which the first waveguide pupil expander is provided in a compact form - for example, in the relatively thin, elongate form shown in Figures 7 to 11 herein, the first waveguide pupil expander is advantageous in that it reduces the overall size and weight of the display system in which it is contained.

[0220] In embodiments in which two-dimensional pupil expansion is required, the first waveguide pupil expander 702 is oriented such that the multiple output beams are directed (directly or indirectly, e.g. via one or more other components, as discussed further below) to a receiving face 716b of a second waveguide pupil expander 706, which is arranged to provide pupil expansion in a second direction substantially perpendicular to the first direction.

[0221] In the example shown in Figure 7 , the second waveguide pupil expander 706 comprises three-dimensional elements. The second waveguide pupil expander 706 is substantially planar in shape, having three pairs of mutually orthogonal faces, the second waveguide pupil expander 706 being relatively long along two of its dimensions and relatively short along its third dimension. However, the present disclosure is not limited to the particular size or shape of the second waveguide pupil expander 706 shown in Figure 7 .

[0222] The second waveguide pupil expander 706 includes a first pair of parallel rectangular (or quadrilateral or planar) faces 716a, 716b, in this particular arrangement, which are respectively shown as an upper face and a lower face. These can be referred to as the "major faces" or "major surfaces" of the second waveguide pupil expander. Each of the parallel rectangular faces 716a, 716b has a relatively large surface area, the length and width of the rectangle being defined along the first and second elongate dimensions of the second waveguide pupil expander 706. The second waveguide pupil expander 706 also includes a pair of parallel elongate side faces 718 and a pair of parallel elongate end faces 720, all of which have a relatively small surface area. Figure 7 In the particular arrangement shown, they are respectively shown as an upper face and a lower face. These can be referred to as the "major faces" or "major surfaces" of the second waveguide pupil expander. Each of the parallel rectangular faces 716a, 716b has a relatively large surface area, the length and width of the rectangle being defined along the first and second elongate dimensions of the second waveguide pupil expander 706. The second waveguide pupil expander 706 also includes a pair of parallel elongate side faces 718 and a pair of parallel elongate end faces 720, all of which have a relatively small surface area.

[0223] The first waveguide pupil expander 702 and the second waveguide pupil expander 706 are oriented relative to each other so that the plurality of output beams from the first waveguide pupil expander 706 are directed toward the receiving face 716b of the second waveguide pupil expander 706— in this non-limiting example, the receiving face is the lower face 716b. Preferably, they are directed toward one end of the lower face 716b so that along / near this end of the lower face 716b define a plurality of input ports, where these input ports receive the plurality of output beams from the first waveguide pupil expander 706. The system 700 is configured so that the beams enter the second waveguide pupil expander 706 via the input ports at an oblique angle relative to the surface normal of the lower face 716b. Furthermore, the second waveguide pupil expander 706 is preferably dimensioned and oriented so as to receive each of the output beams from the first waveguide pupil expander so as to preserve the pupil expansion that the first waveguide pupil expander 706 has already provided in one direction, and then expand each of these beams in a second substantially orthogonal direction defined by the second elongate dimension of the second waveguide pupil expander 706.

[0224] The inner surface of the lower face 716b is reflective, and the upper (transmissive) face 716a is partially transmissive-reflective. Thus, due to the internal reflection process between the two faces, and the partial transmission of light from each of the plurality of output points on the upper face 716a, the light of each of the beams received via the input ports of the second waveguide pupil expander 706 is expanded (or replicated) in the second direction. Thus, a second plurality of beams is emitted from the waveguide pupil expander 706, where the second plurality of beams includes a plurality of replicas of the input beam 714 along each of the first and second directions. Thus, the second plurality of beams can be viewed as a two-dimensional grid or array of replicated beams. In embodiments where the input diffracted / divergent light is that of a hologram— i.e., light that has been spatially modulated according to a hologram and has not yet been converted to form a holographic reconstructed image—each replica is in effect a replica of the hologram.

[0225] Those skilled in the art will understand, for example, how to determine the thickness and angle of incidence AOI of the second waveguide pupil expander for optimal two-dimensional pupil expansion, in which the replicates are perfectly stitched together, i.e. abut.

[0226] In a further technical advance, a waveguide coupler 704 is disposed between the first waveguide pupil expander 702 and the second waveguide pupil expander 706. The waveguide coupler 704 is arranged to couple or direct light between the first waveguide pupil expander 702 and the second waveguide pupil expander 706. The shape of the waveguide coupler 704 can vary depending on the physical configuration and / or constraints of any given display system, but in the example arrangement shown in Figures 7 to 11 In the example arrangement, the waveguide coupler 704 is substantially triangular in cross-section. The waveguide coupler 704 comprises two parallel triangular faces 722 which have a relatively large surface area and can be said to form the “major faces” of the waveguide coupler 704. In this example they are right-angled triangles, but this should not be considered limiting. It also has three substantially rectangular faces 724 which form connecting walls or sides between the two triangular faces 722 and have a relatively small surface area, so they can be said to form the “minor faces” of the waveguide coupler 704.

[0227] The waveguide coupler 704 is arranged to receive the plurality of replicate light rays output from the first waveguide pupil expander 702 and output them towards the second waveguide pupil expander 706. As Figure 7 shown in the example arrangement, the plurality of replicate light rays are received by a first one of the minor faces 724 and output by a different second one of the minor faces 724 which directs the plurality of replicate light rays towards the lower surface 716b of the second waveguide pupil expander 706. The waveguide coupler 704 can have any suitable shape depending on the desired or required relative positioning of the first and second waveguide pupil expanders 702, 706. In Figure 7 In the particular arrangement shown, factors such as the angle requirements for inputting light into each waveguide pupil expander 702, 706 to achieve the desired internal reflections and pupil expansion inherently result in a triangular gap between the two waveguide pupil expanders 702, 706. The waveguide coupler 704 is configured to occupy this gap and couple or direct light between the two waveguide pupil expanders 702, 706 so that there is no loss of light during propagation between the two waveguide pupil expanders 702, 706.

[0228] In embodiments where the input light 714 includes diffracted or divergent light, each replica output from the first waveguide pupil expander 702 will also include diffracted or divergent light. Therefore, each replica includes an uncollimated light cone, where the light field defined at the cone's opening increases with increasing light propagation distance. Consequently, if light propagates uncontrollably between the first waveguide pupil expander 702 and the second waveguide pupil expander 706, there is a risk that some light will deviate from the area defined by the input port on the second waveguide pupil expander 706 and will therefore be lost, or at least will not reach the end observer correctly. In many applications, increasing the surface area of ​​the second waveguide pupil expander 706 is undesirable because physical compactness is often required, such as in head-up display (HUD) systems, particularly in vehicle HUD systems.

[0229] Therefore, the inventors have recognized that providing a waveguide coupler 704 between the two waveguide pupil expanders 702, 706 is an effective and advantageous solution because the waveguide coupler 704 can be suitably formed, shaped, and sized to fit into the gap that is necessary between the two waveguide pupil expanders anyway, and within this gap, it can provide very useful and important optical control functions. The inventors also recognize that although including additional components within the pupil expander system may be counterintuitive, especially if the intention is to integrate the system into an environment where compactness and / or weight reduction are advantageous, the presence of the waveguide coupler and the benefits it brings, as described herein, can outweigh the potential disadvantages of introducing additional elements. Furthermore, they have realized that, at least in some embodiments, the waveguide coupler can be formed to fit into the gap that is inherently necessary between the first and second waveguide pupil expanders, in any case, so that the waveguide coupler does not significantly increase the overall size of the system, if any.

[0230] For example, the size and shape of the facet of waveguide coupler 704 may correspond to the output facet of the first waveguide pupil expander, or at least to multiple output rays from the first waveguide pupil expander 702, in order to receive some or all of these copies. This can be seen from here. Figure 10 and 11 This is most clearly seen in the arrangement. Among these arrangements, it is also... Figure 7 In this case, the cross-sectional shape of waveguide coupler 704 is essentially a right-angled triangle. The first face 724 of waveguide coupler 704 (in this example, the second face on the 'hypotenuse' side in trigonometric terms) is arranged to receive light from the first waveguide pupil expander 702. The light passes through waveguide coupler 704 and exits from the second face 724 to the second waveguide pupil expander 706. Figure 10 and 11In the example arrangement of FIG. 10, light output from waveguide coupler 704 is indirectly directed to second waveguide pupil expander 706 via mirror 1002, as will be discussed further below. However, direct propagation of light from waveguide coupler 704 to second waveguide pupil expander 706, as well as indirect propagation of light therebetween via any suitable one or more other elements, is also contemplated in the present disclosure.

[0231] Waveguide coupler 704 is formed of any suitable material or materials that enable it to retain light therein and direct it to second waveguide pupil expander 706. In embodiments in which input light 714, and thus the multiple copies output by first waveguide pupil expander 702, include diffracted or divergent light, one or more faces of waveguide coupler 704 can be arranged to provide internal reflections so as to prevent the divergent light from escaping waveguide coupler 704, except through designated regions intended to direct light to second waveguide coupler 706, such as output ports. Thus, in such embodiments, waveguide coupler 704 can be configured to provide internal reflections so as to prevent the divergent light from escaping waveguide coupler 704, except through the output ports. Figure 10 and 11 In the example arrangement of FIG. 10, triangular primary face 722 and / or one or more secondary faces 724 that do not include output ports can be configured to provide internal reflections so that light is not allowed to escape through another secondary face (which can be referred to as an “output face”) through which light is intended to propagate to second waveguide pupil expander 706. The output face can include a transmissive surface through which light is intended to propagate to second waveguide pupil expander 706.

[0232] The position, size, and / or shape of the output face of waveguide coupler 704 can correspond to an input port on second waveguide coupler 706. One or more other elements can also be used between waveguide coupler 704 and second waveguide pupil expander 706 to appropriately direct the multiple copies of light rays to second waveguide pupil expander 706 for pupil expansion in the second direction. Again, this can be seen most clearly in FIGS. 10A and 10B, in which light rays output from waveguide coupler 704 are directed to mirror 1002, which reflects the light rays to a region near one end of the receiving side (in these examples, the lower side) of second waveguide pupil expander 706, from which region the light rays undergo the reflections and transmissions described above to achieve two-dimensional pupil expansion. Figure 10 and 11 In addition to ensuring that the copies of light rays reach second waveguide pupil expander 706, waveguide coupler 704 can also ensure that they reach it at the desired angles. This can ensure that internal reflections are achievable in second waveguide pupil expander 706, and that the output of second waveguide pupil expander 706 is correspondingly properly directed. This helps to ensure the overall collimation function of the display system, and for example, to ensure proper positioning of the desired observer eyebox. The inventors have found that waveguide couplers significantly help to minimize horizontal black / white bands in 2D pupil expansion.

[0233] As noted above, in many practical applications it is desirable and in some cases necessary to provide pupil expansion in a compact and space-efficient arrangement, in order to implement the pupil expansion system into a larger system, such as into a vehicle. Compactness is also beneficial if the environment in which the pupil expansion system is to be provided is a mobile, vibrating, or otherwise unstable environment, such as a vehicle. The inventors have recognized that one or more of the waveguide pupil expanders described above, and optionally the waveguide coupler, can be arranged in a highly compact and efficient manner. For example, they have recognized that the first waveguide pupil expander and the waveguide coupler, optionally along with another suitable optical element such as a turning mirror (or "fold mirror"), can be arranged within the physical area defined by the "footprint" of the second waveguide pupil expander (which must be larger than the first waveguide pupil expander, as it is arranged to preserve the pupil expansion in the first direction and increase the pupil expansion in the second, substantially orthogonal direction), and provide the efficiency and effectiveness of the two-dimensional pupil expansion described herein even for diffracted / divergent input light rays.

[0234] Accordingly, in at least some cases, and in accordance with the recognition of the inventors, it is possible to provide a display system (or light engine) in which the first waveguide pupil expander, the waveguide coupler, and optionally one or more other elements can be disposed within a first layer (e.g., a lower layer) of a compact two-dimensional pupil expansion system, while the second waveguide pupil expander can be disposed within a second layer (e.g., an upper layer). The compact two-dimensional pupil expansion system can have a cross-sectional area that is equal to or substantially equal to the cross-sectional area of the major face of the second waveguide pupil expander. In some cases, the compact two-dimensional pupil expansion system can have a cross-sectional area that is slightly larger than the cross-sectional area of the major face of the second waveguide pupil expander, such as within a predetermined threshold or tolerance level.

[0235] A compact two-dimensional pupil expansion system can form part of a viewing system, light engine or display system, such as a head-up display (HUD) system, which includes an SLM or other pixelated display device on which an image or hologram can be displayed. At least in some cases, the SLM or other display device can also be located within the footprint of the second waveguide pupil expander. For example, it can be disposed within an underlying layer with the first waveguide pupil expander and waveguide coupler and optionally one or more other optical elements, the second waveguide pupil expander forming at least part of a second overlying layer. The optical path of light travelling from the display device to the second waveguide pupil expander via the first waveguide pupil expander, waveguide coupler and any other optical elements provided can also be positioned to fall within the physical footprint defined by the cross-sectional area of the second waveguide pupil expander, or at least within the physical footprint defined by the layer containing the second waveguide pupil expander. The first layer, which includes the first waveguide pupil expander and waveguide coupler, can be contiguous with the second layer, which includes the second waveguide pupil expander. For example, the first and second layers can be contiguous with one another. For example, the first and second layers can be attached to one another. For example, the first and second layers can be bonded to one another. To provide a compact and efficient two-dimensional pupil expansion, the bonding can be provided by any suitable material that enables the propagation of light in the two-dimensional pupil expansion system to occur as disclosed herein. This can be further understood from the detailed description of the Figures 9 to 12 above.

[0236] Figure 10 An exemplary configuration is shown that includes a first waveguide pupil expander 702, a waveguide coupler 704 and a second waveguide pupil expander 706, which operate in a similar manner to that described in detail above with respect to the Figure 7 above. Thus, Figure 10 the arrangement embodies the inventor’s realisation as described above. In Figure 10 (and in Figure 7In the first waveguide pupil expander 702 (as described above), the principal surface 722 of the waveguide coupler 704 is coplanar with the second pair of elongated parallel surfaces 710a, 710b within the first waveguide pupil expander 702. The first waveguide pupil expander 702 is arranged to provide internal reflection to trap light within the first waveguide pupil expander 702, thereby ensuring that light escapes only through an output port defined on one of the other first pair of parallel elongated surfaces 708a, 708b of the first waveguide pupil expander 702. Light enters and exits through the first pair of parallel elongated surfaces 708a, 708b of the first waveguide pupil expander 702, which are arranged substantially parallel to the secondary “input” surface 724 of the waveguide coupler 704. The waveguide coupler 704 is configured to receive multiple copies of light rays from the first waveguide pupil expander 702. In this example, in trigonometric terms, the secondary surface 724 is the “hypotenuse” of a substantially right-angled triangle defined by the principal surface of the waveguide coupler. This arrangement allows the first waveguide pupil expander 702 and waveguide coupler 704 to be provided as part of a first relatively thin layer. Figure 10 A mirror 1002, which may be referred to as a "redirecting mirror," is also provided within the first relatively thin layer. The mirror 1002 is arranged to change the direction of light output from the waveguide coupler 702. It should be understood that the mirror 1002 is one example of an optical element capable of redirecting light in the manner shown, and one or more other elements may be used alternatively to perform this task.

[0237] exist Figure 10 In the arrangement shown, mirror 1002 is configured to guide light away from the layer / plane where the first waveguide pupil expander 702 and waveguide coupler 704 are located, and instead guide light toward a second layer substantially parallel to the first layer, wherein the second waveguide pupil expander 706 is located within this second layer. Mirror 1002 is suitably positioned within the first layer and is tilted to guide light toward the second layer, which, in the example shown, is above the first layer; however, this relative positioning should not be considered a limitation of this disclosure. Figure 11 As can be seen more clearly in the plan view shown, in this example arrangement, the first waveguide pupil expander 702, waveguide coupler 704 and mirror 1002 included in the first layer all fall within the physical coverage area of ​​the second layer, which is defined by the cross-sectional area of ​​the main surface of the second waveguide pupil expander 706. Figure 11 The SLM1102, provided within the first layer, is also shown, within the same coverage area. Figure 11 In this process, the light output from the SLM1102 is directed by one or more suitable optical elements to the input port of the first waveguide pupil expander 702, which are not shown but preferably fall within the same coverage area.

[0238] Despite Figure 10 and 11A mirror 1002 is shown in FIG. 10B, but alternative arrangements are contemplated in which no mirror or other optical element is needed to direct light replicas from the first layer in which the first waveguide pupil expander 702 and waveguide coupler 704 reside to the preferably parallel second layer in which the second waveguide pupil expander 706 resides. For example, one or more surfaces of the waveguide coupler 704 can be formed or coated or otherwise arranged so that light is output directly by the waveguide coupler 704 in a direction that is not coplanar with the first layer, and directed toward the appropriate input port region of the second waveguide pupil expander 706.

[0239] From Figure 11 It can also be seen in FIG. 10B that in order to ensure the correct input and output angles of the waveguide pupil expanders 702, 706, e.g., to implement internal reflections therein, the elongate dimension of the first waveguide pupil expander 702 is tilted with respect to each of the dimensions that define the major face of the second waveguide pupil expander 706. The inventors have recognized that when the first waveguide pupil expander 702 is tilted in this way, it can be positioned within the physical footprint of the major face of the second waveguide pupil expander 706. In contrast, if the first waveguide pupil expander 702 were not tilted on this plane, then the input and output angles would be incorrect. Figure 10 And 11 It can be seen in FIGS. 10B and 10C that instead, the second waveguide pupil 706 needs to be tilted on its plane, otherwise the optical performance would be compromised. Thus, the tilt of the first waveguide pupil expander 702 on the lower layer provides a technical advance in optimizing the packaging, i.e., minimizing the volume, by allowing the bottom layer components to fall within the footprint of the top layer components. This tilt naturally creates a substantially triangular gap between the perimeters of the footprints of the first and second waveguide pupil expanders. Thus, this inherent triangular gap shape facilitates the formation of a substantially triangular waveguide coupler 704, as shown in FIG. 10B and described in detail above. Figure 11

[0240] As noted herein Figure 9 ​As shown, the first layer, including the first waveguide pupil expander 702 and waveguide coupler 704, and optional mirror 1002, can be directly adjacent or abutting the second layer, including the second waveguide pupil expander 706. Optionally, any surface within the first layer that abuts the second layer can be provided with a reflective coating. In some embodiments, the second pair of parallel faces 710a, 710b of the first waveguide pupil expander 702 and the triangular major face 722 of the waveguide coupler 704 provide light guiding due to total internal reflection based on the difference in refractive index and appropriate angles of incidence (as will be familiar to those skilled in the art). However, in other embodiments, a suitable mirror coating can be provided on the second pair of parallel faces 710a, 710b of the first waveguide pupil expander 702 and / or the triangular major face 722 of the waveguide coupler 704 to provide internal reflection within the first waveguide pupil expander 702 and waveguide coupler 704, respectively, which compensates for the diffraction angle of the diffracted light.

[0241] As used herein Figure 12 , the inventors' recognition enables the provision of systems including or incorporating the two-dimensional pupil expansion systems described herein in a highly compact and stable form. For example, Figure 12 A heads-up display (HUD) system is shown, provided as a HUD package 1202, which includes a plurality of substantially quadrilateral layers that are adjacent to one another, e.g., they can be bonded to one another in any suitable manner. The HUD package 1202 includes the elements shown in Figure 11 and described above with respect to Figure 11 . It can also include additional elements within these layers and / or within a third (or subsequent) substantially parallel and relatively thin (planar) layer that is adjacent to one or more respective other layers within the HUD package 1202. As a result, the HUD package 1202 is formed in a compact and regular shape that can be incorporated into a variety of different environments. For example, in the example of Figure 12 , it is shown positioned beneath the dashboard 1203 of an automotive vehicle, but this should not be considered limiting of the present disclosure. Due to the regular shape and layered configuration of the HUD package 1202, it is relatively straightforward for a user (e.g., a manufacturer) to incorporate it into a surrounding environment in a desired orientation and position. For example, in Figure 12 , the HUD package 1202 is oriented such that the major surfaces of its layers are substantially horizontal, as a result, light is directed at a suitable angle toward the windshield 1204 of the vehicle to ensure that the light is reflected or otherwise redirected from the windshield 1204 to the eyebox 1206 of an observer, which is defined in a substantially vertical plane. However, in other examples, the HUD package 1202 is tilted with respect to the horizontal in order to provide an optimal package - e.g., a minimum volume.

[0242] It is further beneficial in terms of simplification and reduced manufacturing costs to provide the two-dimensional pupil expansion (or pupil replication) systems described herein in a compact form, e.g., in a regular shaped compact form, with components as close to each other as possible while still ensuring the correct angles of light propagation. In other words, it is simpler and more cost effective to manufacture the first and second waveguide pupil expanders and, if applicable, the waveguide coupler together, e.g., as a single layered component, than to manufacture them separately and then arrange them together afterwards. Moreover, it is beneficial in terms of manufacturing efficiency and in terms of financial costs to reduce the physical size of the waveguide pupil expanders, e.g., to make the first waveguide pupil expander in a thin elongated shape. This in turn can enable the use of higher quality optical surfaces or optical materials than can be possible with traditional, less compact, and / or more irregular system arrangements. Moreover, the regular and compact shape of the two-dimensional pupil expansion systems described herein makes it physically more stable and robust, e.g., when located in challenging environments, such as in a vehicle, than can typically be achieved with traditional two-dimensional pupil expansion systems.

[0243] The improved systems disclosed herein enable diffracted or divergent light to be replicated in at least one dimension via one or more waveguide pupil expanders, which is not possible with traditional viewing systems. As a result, light output by a diffractive structure, including but not limited to light encoded by a hologram, can be replicated or expanded in one or more dimensions before being transmitted to a viewer. This expansion enables the viewer to have a larger eyebox, within which their eyes can be located while still capturing the required light, thereby enabling the viewer to see or perceive an image (e.g., an image corresponding to a hologram) from a large number of different eye positions.

[0244] Waveguide aperture

[0245] The apparatus and methods described herein are applicable to viewing systems having a single viewing aperture or entrance pupil, as well as to viewing systems having multiple entrance pupils, e.g., most commonly but not limited to, a human viewer having two eyes.

[0246] The inventors have recognised that, at least in some cases, it can be appropriate to take account of the potential impact of the observation system having multiple entrance pupils. In other words, they have recognised that it can be appropriate to provide control over how and when multiple copies of a common diffracted light field reach an observer or other observation system. For example, it can be appropriate, at least in some cases, to prevent two copies of the same diffracted light field reaching an observer’s left and right eyes at the same time, since the human brain does not expect both eyes to receive the same content at the same time, given that the two eyes are physically displaced from one another. The inventors have recognised that control can be provided so that different respective positions of the observer’s two eyes (and correspondingly, of two or more entrance pupils in any multi-entrance pupil observation system) can be taken into account to ensure that the two eyes do not receive the same image or part of an image (or, holographic light belonging to the same image or part of an image) at substantially the same time. This will be described in more detail in particular in relation to Figure 4A to 5 The type of hologram described is described in more detail in co-pending UK patent application GB2108456.1, the entire contents of which are hereby incorporated by reference. However, the present disclosure, including the use of waveguide apertures as described further below, is applicable to pupil expansion of diffracted or divergent light fields of any type, including but not limited to diffracted light fields modulated by any type of hologram, including but not limited to a Fourier hologram, a point cloud hologram or a Fresnel hologram.

[0247] Accordingly, in some embodiments, a control device is provided within the display system and arranged to control the transmission of at least some copies of the diffracted light within the system. The control device can be referred to as an “aperture” or “waveguide aperture” since it can be configured to provide the aperture through which one or more components of the system are configured to output, or selective blocking and transmission of a particular light path. It is configured to do this by selectively having one or more “open regions” which transmit light and one or more “closed regions” (i.e. non-transmitting regions) which block light. The “regions” can be referred to as “apertures”, but it will be appreciated that they can not be physically distinct or discrete structures, but rather they can be controlled by software and so can be dynamic in position and form. For example, the ratio of open regions to closed regions can be dynamic, as can the size and position of any given region. The period of time during which a particular configuration of open and closed regions of the control device is employed can be referred to as a “phase”. The control device can be controlled to cycle or change between phases on a dynamic, typically very rapid basis.

[0248] In some embodiments, a control device is provided downstream of the first waveguide pupil expander, e.g. immediately downstream. For example, the control device can be provided between the first waveguide pupil expander and (if present) the waveguide coupler, and more generally it can be provided between the first and second waveguide pupil expanders. In some embodiments, instead of or in addition to providing a control device between the first and second waveguide pupil expanders, a control device can be provided downstream of the second waveguide pupil expander.

[0249] Returning to the embodiment shown here Figure 10 and 11 The control device can be provided substantially within the same layer as the first waveguide pupil expander 702 and (if present) the waveguide coupler 704. For example, the control device can be fixed to (e.g. bonded to) the first waveguide pupil expander 702 and / or the waveguide coupler 702.

[0250] For example, the control device can comprise an elongate structure which can be located between the first waveguide pupil expander 702 and the waveguide coupler 704. It can be arranged substantially parallel to the first waveguide pupil expander 702, and / or it can have an elongate dimension such that it is able to intercept some, preferably most, preferably all of the replicas of the diffracted light field that the first waveguide pupil expander 704 is configured to output. The control device can be controlled so as to selectively transmit or block the onward transmission of at least some of these replicas, so as to dynamically control the hologram content that reaches the observer, e.g. to control which hologram content reaches each eye of the observer. The control device can be switchable between not allowing, or allowing all, or allowing a selected number of the replicas to be transmitted therethrough.

[0251] In some embodiments, the control device comprises a substantially planar liquid crystal display panel arranged to provide a customised light shutter which selectively lets through particular replicas based on the observer eye position. The eye position can be known to a controller of the control device by any suitable sensor and / or feedback means. The control device can be substantially co-planar with the waveguide pupil expander, the control device being configured to operate with the waveguide pupil expander.

[0252] In embodiments, the control device can also be configured to selectively control which parts of the diffracted light field within a single replica of the input diffracted light field will reach the observer at any given time. For example, it can be configured to dynamically control which range(s) of diffraction angles within the cone of light defined by the diffracted light are transmitted and which are blocked.

[0253] The control device is referred to above as a "waveguide aperture", but any suitable control device can be used to provide the functionality described herein. The control device, such as a waveguide aperture, can be formed of any suitable material. For example, it can comprise one or more (such as an array) liquid crystal devices, each of which can be switched between opaque and transmissive. For example, the control device can comprise "smart glass" or "switchable glass", the light transmission properties of which can be changed when a voltage, light or heat is applied. The control device can be controlled by any suitable processor or controller. To coordinate or synchronize with the dynamic display of a plurality of different holograms on the display device, for example to transmit holographic light corresponding to each different target image and / or to accommodate movement of the observer or observation system, its configuration can be changed rapidly.

[0254] The control device disclosed herein can take many different forms. In some embodiments, the control device comprises a plurality (such as a 2D array) of individually controllable light receiving / processing elements, such as pixels. In some embodiments, the control device comprises a pixelated liquid crystal device or display. In some embodiments, the elements or pixels are operable in continuous groups to form transmissive and non-transmissive "shutter zones". Each group of pixels can be switched between a first mode, for example transmissive, and a second mode, for example reflective. Those skilled in the art will be familiar with how to control a pixelated display device so as to change the size and position of the pixel groups or zones in operation, for example in real time, each zone having a different response to light. In embodiments, each zone is larger than the pixel size of the control device. Thus, each zone can comprise a plurality of pixels. Those skilled in the art will also be familiar with how to implement optical components such as polarizers and waveplates in conjunction with a pixelated liquid crystal device to provide a reconfigurable light gate. Other schemes based on other characterization properties of light are equally applicable, by way of example only, to polarization selection. In some embodiments, the control device comprises a pixelated liquid crystal display, and optionally other optical elements are collectively configured to transmit light having a first polarization and to absorb or reflect light having a second polarization, optionally wherein the first and second polarizations are opposite or complementary. For the avoidance of doubt, any number of different optical systems can be used to form the control device depending on the properties of the light forming the image, for example polarization and wavelength, and thus the present disclosure is not limited by the configuration of the control device. It will therefore be understood that the control device disclosed herein is defined by its functionality rather than its structure.

[0255] The control devices are dynamically reconfigurable. The reader should appreciate that the total area of the control devices that is blocked / non-transmissive or not blocked / transmissive is not generally constant during its operation. In some embodiments, the control devices are pixelated. That is, the control devices include an array of individually controllable pixels. Each pixel can include, for example, liquid crystals that can be configured between a transmissive state and a non-transmissive state. Any imperfections in the alignment between the pixel edges and the edges of the ideal aperture region can be handled by letting too much or too little light through.

[0256] The examples described above should not be considered limiting. For example, the viewing system can have more than two viewing apertures or entrance pupils. For example, the operation of the control devices can be controlled according to any suitable “phase sequence” or timing scheme. For example, the control devices can be selectively put to sleep.

[0257] In embodiments, two or more holograms or two or more other diffractive patterns can be interleaved with one another. In other words, two holograms can be alternately displayed in rapid succession such that the observer perceives that the two respective images are formed substantially simultaneously.

[0258] Some phases of the control devices can pass more light content than some respective other phases. Similarly, when multiple phases of the control devices are interleaved with one another, the two eyes need not receive the same amount of light content from each phase or collectively. For example, depending on their relative positions and / or other factors, one eye can see more light content than the respective other eye.

[0259] A display system including control devices such as waveguide apertures as described herein can be configured to display multiple different diffractive patterns and to output multiple respective different diffracted or divergent light fields in succession and / or at different respective times. Accordingly, the display devices in such a system can be configured to display different respective holograms, sometimes in rapid succession. The control devices can be configured to be dynamically adaptable to accommodate changes in the diffracted light fields and / or changes in viewing requirements.

[0260] The system can be configured to display a diffraction pattern corresponding to a sequence of images, e.g., a video rate sequence of images. Each image can correspond to a frame in a sequence of frames having a frame rate such as 50 or 60 Hz. Each frame can include a plurality of sub-frames. For example, the sub-frame rate can be 4 or 8 times the frame rate. For each successive sub-frame, the hologram displayed can change. Each sub-frame can be considered a separate display event. Each sub-frame can correspond to an image or at least a portion of an image. While embodiments have been shown delivering light to both eyes in each display event, the present disclosure is not so limited. For example, the light engine can be configured to deliver light to only one eye / entrance pupil in each display event. The configuration of the waveguide apertures, i.e., the size and / or distribution of open and closed apertures / openings, can change in each display event or every n display events, where n is an integer. In some embodiments, only a range of angles of light is delivered to one eye for each display event / aperture configuration. In some embodiments, the control system is configured to deliver light to each eye / entrance pupil in turn.

[0261] In some embodiments, the hologram for a target image can be computed for a particular size and position of the viewing aperture, e.g., for a particular size and position of the entrance pupil of the observer’s eye. If a constraint such as the entrance pupil diameter or position changes, the hologram can be recomputed, even if the target image to be reconstructed (and thus the image content that the observer will see or perceive) remains the same. Each hologram need not have the same number or size of areas, even if two holograms represent the same target image.

[0262] Additional features

[0263] Embodiments refer to electrically activated LCOS spatial light modulators by way of example only. The teachings of the present disclosure can be equally implemented on any spatial light modulator capable of displaying computer-generated holograms according to the present disclosure, e.g., any electrically activated SLM, optically activated SLM, digital micromirror device, or microelectromechanical device.

[0264] In some embodiments, the light source is a laser, such as a laser diode. In some embodiments, the detector is a photodetector, such as a photodiode. In some embodiments, the light receiving surface is a diffuse surface or screen, such as a diffuser. The holographic projection system of the present disclosure can be used to provide an improved heads-up display (HUD). In some embodiments, a vehicle is provided that includes a display system mounted in the vehicle to provide a HUD. The vehicle can be a motor vehicle, such as an automobile, truck, van, delivery truck, motorcycle, train, airplane, boat, or ship.

[0265] The quality of the holographic reconstruction can be affected by so-called zero order problems, which are a result of the diffractive nature of using a pixelated spatial light modulator. This zero order light can be considered as "noise" and includes, for example, specularly reflected light and other unwanted light from the SLM.

[0266] In the example of Fourier holography, this "noise" is concentrated in the focal point of the Fourier lens, resulting in a bright spot in the centre of the holographic reconstruction. The zero order light can simply be blocked off, however this means replacing the bright spot with a dark spot. Some embodiments include an angle selective filter to remove only the collimated light of the zero order. Embodiments also include the method of managing the zero order described in European patent 2030072, which is incorporated herein in its entirety by reference.

[0267] In some embodiments, the size of the hologram (the number of pixels in each direction) is equal to the size of the spatial light modulator, such that the hologram fills the spatial light modulator. That is, the hologram uses all of the pixels of the spatial light modulator. In other embodiments, the hologram is smaller than the spatial light modulator. More specifically, the number of hologram pixels is less than the number of light modulating pixels available on the spatial light modulator. In some of these other embodiments, a portion of the hologram (i.e. a contiguous subset of the hologram pixels) is repeated in the unused pixels. This technique can be referred to as "tiling", in which the surface area of the spatial light modulator is divided into a plurality of "tiles", each of which represents at least a subset of the hologram. Thus, the size of each tile is smaller than the size of the spatial light modulator. In some embodiments, the "tiling" technique is implemented to improve the image quality. In particular, some embodiments implement the tiling technique to minimize the size of the image pixels, while maximizing the amount of signal content that enters the holographic reconstruction. In some embodiments, the hologram pattern written to the spatial light modulator includes at least one complete tile (i.e. a complete hologram) and at least a portion of a tile (i.e. an adjacent subset of the hologram pixels).

[0268] In embodiments, only the primary replay field is utilized, and the system includes a physical block, such as a baffle, arranged to limit the propagation of higher order replay fields through the system.

[0269] In embodiments, the holographic reconstruction is in colour. In some embodiments, a method known as spatially separated colour "SSC" is used to provide a colour holographic reconstruction. In other embodiments, a method known as frame sequential colour "FSC" is used.

[0270] The SSC method uses three spatially separated light modulating pixel arrays for three monochrome holograms. The advantage of the SSC method is that the image can be very bright because all three holographic reconstructions can be formed simultaneously. However, if the three spatially separated light modulating pixel arrays are provided on a common SLM due to spatial constraints, the quality of each monochrome image is suboptimal because only a subset of the available light modulating pixels is used for each color. Thus, a relatively low resolution color image is provided.

[0271] The FSC method can use all the pixels of a common spatial light modulator to display three monochrome holograms sequentially. The cyclic monochrome reconstruction (e.g., red, green, blue, red, green, blue, etc.) is fast enough so that a human observer perceives a multicolor image from the synthesis of the three monochrome images. The advantage of FSC is that the entire SLM is used for each color. This means that the quality of the three color images produced is optimal because all the pixels of the SLM are used for each color image. However, the disadvantage of the FSC method is that the brightness of the synthesized color image is about 3 times lower than the SSC method because each monochrome illumination event can only occur for one third of the frame time. This disadvantage can be addressed by overdriving the laser or by using a larger power laser, but this requires more power, resulting in higher cost and an increase in system size.

[0272] Examples describe illuminating the SLM with visible light, but those skilled in the art will appreciate that, for example, the light source and SLM can equally be used to direct infrared or ultraviolet light, as disclosed herein. For example, to provide information to a user, those skilled in the art will be aware of techniques for converting infrared and ultraviolet light into visible light. For example, the disclosure extends to the use of phosphor and / or quantum dot technology for this purpose.

[0273] Some arrangements describe 2D holographic reconstruction by way of example only. In other arrangements, the holographic reconstruction is a 3D holographic reconstruction. That is, in some arrangements, each computer-generated hologram forms a 3D holographic reconstruction.

[0274] The methods and processes described herein can be embodied on a computer-readable medium. The term "computer-readable medium" includes media laid out to temporarily or permanently store data, such as random access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" should also be taken to include any medium or combination of media capable of storing instructions for execution by a machine, such that, when the instructions are executed by one or more processors, cause the machine as a whole or in part to perform any one or more of the methods described herein.

[0275] 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 storage repositories (e.g., data volumes) in the example form of a solid-state storage chip, an optical disk, a magnetic disk, or any suitable combination thereof. In some example embodiments, instructions for execution can be delivered by a carrier medium. Examples of such carrier mediums include transitory media (e.g., a propagating signal that conveys the instructions).

[0276] It will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope of the appended claims. The present disclosure encompasses all such modifications and variations as within the scope of the appended claims and their equivalents.

[0277] The following numbered items are also disclosed.

[0278] Item 1. A light engine comprising:

[0279] a first layer comprising: a first pupil replicator arranged to receive a diffracted light field from a diffractive structure defining a pupil; and a waveguide coupler, wherein the first pupil replicator is substantially elongate;

[0280] a second layer comprising a second pupil replicator, wherein the second pupil replicator is substantially planar and comprises a first major surface arranged to form an input of the light engine and a second major surface arranged to form an output of the light engine, wherein the waveguide coupler is arranged to couple an output of the first pupil replicator to an input of the second pupil replicator,

[0281] wherein the first layer and the second layer are substantially parallel and adjacent to one another.

[0282] Item 2. The light engine of any preceding item, wherein the first pupil replicator and the waveguide coupler are arranged within a footprint of the second pupil replicator.

[0283] Item 3. The light engine of any preceding item, wherein the first pupil replicator comprises a major pair of opposing surfaces arranged to provide light guiding and pupil replication therebetween.

[0284] Item 4. The light engine of any preceding item, wherein the waveguide coupler comprises a major pair of opposing surfaces comprising an input surface and an output surface respectively, wherein the input surface and the output surface are at an angle to one another.

[0285] Item 5. The light engine of any preceding item, wherein the first pupil replicator and the waveguide coupler are substantially co-planar.

[0286] Item 6. The light engine of any preceding item, wherein the first pupil replicator and the waveguide coupler of the first layer are arranged to waveguide diffracted light fields in a plane substantially parallel to the second layer.

[0287] Project 7. The light engine of any of the preceding projects, wherein the second layer is defined by first and second axes, wherein the elongated dimension of the first pupil replicator is angled relative to at least one of the first and second axes of the second layer.

[0288] Project 8. As in Project 7, the light engine, wherein the elongated dimension of the first pupil replicator is at an angle relative to the first or second axis of the second layer that is approximately equal to the incident angle of the diffracted light received by the first pupil replicator.

[0289] Project 9. A light engine as described in any of the preceding projects, wherein the second pupil replicator has a basic quadrilateral cross-sectional shape.

[0290] Project 10. A light engine as described in any of the preceding projects, wherein the input to the second pupil replicator is elongated and corresponds to the first axis of the second layer.

[0291] Project 11. The light engine of any of the preceding projects, wherein the first and second main surfaces of the second pupil replicator form a main-to-opposite surface, the surfaces being arranged to provide a light guide and pupil replication therebetween.

[0292] Project 12. The light engine of any of the preceding projects, wherein the second pupil replicator of the second layer is arranged to waveguide diffract the light field in a plane substantially parallel to the first layer.

[0293] Project 13. The optical engine of any of the preceding projects, wherein the first pupil replicator and the waveguide coupler are fixed to the second main surface of the second pupil replicator.

[0294] Project 14. The optical engine of any of the preceding projects, wherein each of the first pupil replicator and the waveguide coupler includes a corresponding pair of opposing surfaces arranged to capture a diffracted light field in its plane.

[0295] Item 15. The light engine of Item 14, wherein at least one surface of each pair of opposing surfaces includes a reflective element, and at least one surface of each pair of opposing surfaces is fixed to a common substrate by the reflective element.

[0296] Project 16. A light engine as described in Project 15, wherein the common base is a second pupil replicator or a component of a vehicle housing the light engine.

[0297] Project 17. An optical engine as described in any of the preceding projects, wherein a first pupil replicator and a waveguide coupler are combined together.

[0298] Item 18. The light engine of any preceding item, wherein the light engine further comprises a control device, wherein the control device comprises a plurality of independently controlled apertures arranged to determine which pupil replicas are relayed from the first pupil replicator to the second pupil replicator, optionally wherein the first pupil replicator, the waveguide coupler, and the control device are combined together.

[0299] Item 19. A head-up display for a vehicle, wherein the head-up display comprises:

[0300] a first pupil replicator extending in a first direction and arranged to receive a holographic light field from a spatial light modulator having an array of pixels defining a limiting aperture of the head-up display, wherein the holographic light field is a complex light field spatially modulated according to a hologram displayed on the spatial light modulator;

[0301] a second pupil replicator extending in the first direction and a second direction perpendicular to the first direction, wherein the second pupil replicator comprises a first major surface forming an output and a second major surface parallel to the first major surface;

[0302] a waveguide coupler arranged to optically couple an output of the first pupil replicator to an input of the second pupil replicator,

[0303] wherein the first pupil replicator and the waveguide coupler are arranged within a planar layer substantially parallel and adjacent to the second major surface of the second pupil replicator.

[0304] Item 20. The head-up display of item 19, wherein the first pupil replicator and the waveguide coupler are attached to the second major surface of the second pupil replicator or to a structural frame of a vehicle housing the head-up display.

Claims

1. A light engine comprising: a first layer comprising a first pupil replicator extending in a first direction and arranged to receive a diffracted light field from a diffractive structure defining a pupil; wherein the first pupil replicator is substantially elongate; a second layer comprising a second pupil replicator, wherein the second pupil replicator is substantially planar and comprises a first major surface arranged to form an input of the light engine and a second major surface arranged to form an output of the light engine, wherein the first layer and the second layer are substantially parallel and adjacent to each other; wherein the second pupil replicator defines an footprint on the first layer extending in the first direction and a second direction; and wherein the first pupil replicator is arranged such that light output by the first pupil replicator is parallel to the second direction of the footprint.

2. The light engine of claim 1, wherein, the first pupil replicator is arranged within the footprint of the second pupil replicator.

3. The light engine of claim 1 or 2, wherein, the first layer further comprises a waveguide coupler arranged to couple the output of the first pupil replicator to the input of the second pupil replicator.

4. The light engine of claim 3, wherein, the waveguide coupler comprises a primary pair of opposing surfaces comprising an input surface and an output surface respectively, wherein the input surface and the output surface are at an angle to each other.

5. The light engine of claim 3, wherein, the first pupil replicator and the waveguide coupler are substantially co-planar.

6. The light engine of claim 3, wherein, the first pupil replicator and the waveguide coupler of the first layer are arranged to waveguide the diffracted light field in a plane substantially parallel to the second layer.

7. The light engine of claim 3, wherein, the second pupil replicator of the second layer is arranged to waveguide the diffracted light field in a plane substantially parallel to the first layer.

8. The light engine of claim 3, wherein, the first pupil replicator and the waveguide coupler are fixed to the first major surface of the second pupil replicator.

9. The light engine of claim 3, wherein, the first pupil replicator and the waveguide coupler each comprise a respective secondary pair of opposing surfaces arranged to trap the diffracted light field within their plane.

10. The light engine of claim 9, wherein, at least one of each secondary pair of opposing surfaces comprises a reflective component, and at least one of each secondary pair of opposing surfaces is fixed to a common substrate by the reflective component.

11. The light engine of claim 10, wherein, the common substrate is the second pupil replicator or a component of a vehicle housing the light engine.

12. The light engine of claim 3, wherein, the first pupil replicator and the waveguide coupler are bonded together.

13. The light engine of claim 3, wherein, the light engine further comprises a control device, wherein the control device comprises a plurality of independently controlled apertures arranged to determine which pupil replicas are relayed from the first pupil replicator to the second pupil replicator.

14. The light engine of claim 13, wherein, the first pupil replicator, the waveguide coupler and the control device are bonded together.

15. The light engine of claim 1, wherein, the second layer is defined by first and second axes, wherein the elongate dimension of the first pupil replicator is angled relative to at least one of the first and second axes of the second layer.

16. The light engine of claim 1, wherein, the elongate dimension of the first pupil replicator is arranged to be inclined relative to a respective other of the first and second directions of the footprint.

17. The light engine of claim 15 or 16, wherein, the angle of the elongate dimension of the first pupil replicator relative to the first or second axis of the second layer is substantially equal to an angle of incidence of the diffracted light received by the first pupil replicator.

18. The light engine of claim 1 or 2, wherein, the second pupil replicator has a substantially quadrilateral cross-sectional shape.

19. The light engine of claim 1 or 2, wherein, the input of the second pupil replicator is elongate and corresponds to the first axis of the second layer.

20. The light engine of claim 1 or 2, wherein, The first and second major surfaces of the second pupil replicator form a major pair of opposing surfaces arranged to provide light guidance and pupil replication therebetween.

21. The light engine of claim 1 or 2, wherein, The first pupil replicator comprises a major pair of opposing surfaces arranged to provide light guidance and pupil replication therebetween.

22. A head-up display for a vehicle, wherein the head-up display comprises: a first pupil replicator extending in a first direction and arranged to receive a holographic light field from a spatial light modulator having an array of pixels defining a limiting aperture of the head-up display, wherein the holographic light field is a complex light field spatially modulated according to a hologram displayed on the spatial light modulator; a second pupil replicator extending in the first direction and in a second direction perpendicular to the first direction, wherein the second pupil replicator comprises a first major surface forming an output and a second major surface parallel to the first major surface; wherein the first pupil replicator is arranged within a planar layer substantially parallel and adjacent to the second major surface of the second pupil replicator; wherein the first pupil replicator is arranged such that light output by the first pupil replicator is parallel to the second direction.

23. The head-up display of claim 22, wherein, The first pupil replicator is attached to the second major surface of the second pupil replicator, or to a structural frame of a vehicle housing the head-up display.

24. The head-up display of claim 22 or 23, wherein, The head-up display further comprises a waveguide coupler arranged to optically couple an output of the first pupil replicator to an input of the second pupil replicator, the waveguide coupler being arranged within the planar layer of the first pupil replicator.

Images