Display system and light control film therefor

Abstract

A head-up display for a vehicle includes an optical component having a reflective surface arranged in a configuration to facilitate sunlight glare during operation of the head-up display. A light control layer is disposed on the optical component to receive sunlight in an optical path to the reflective surface. The light control layer includes a sunlight receiving surface and a core material separating an array of louvers. The sunlight receiving surface of the light control layer is serrated and cooperates with the array of louvers to deflect received sunlight away from an eyebox of the head-up display.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to display systems, in particular display systems using a diffractive light field comprising diverging light beams. More specifically, the present disclosure relates to display systems comprising a waveguide pupil expander and to methods of pupil expansion using a waveguide. The invention also relates to light control films for optical components of display systems, such as waveguide pupil expanders. Some embodiments relate to picture generation units and head-up displays, such as automotive head-up displays (HUD). 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 it 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 light modulation can be continuous across the device. Spatial light modulators can be reflective, meaning that modulated light is output in reflection. Spatial light modulators can equally be transmissive, meaning that modulated light is output in transmission.

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

[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 an observation 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 observation plane (e.g. any eye position within an observation 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 depends critically 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] Traditionally, 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 channels of light. 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, for the avoidance of 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 diffracted or diffractive light fields.

[0022] 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 heads-up display (HUD), such as a vehicle or car HUD.

[0023] According to the present disclosure, a pupil expansion is provided for diffracted or diffractive light, which can comprise diverging bundles of rays. The diffracted 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 diffracted light field can be defined by a "light cone". Thus, the size of the diffracted light field (as defined in a two-dimensional plane) increases with the propagation distance from the respective diffractive structure (i.e. the display device).

[0024] The spatial light modulator can be arranged to display a hologram. The diffracted or diverging light can comprise light encoded with / by the hologram, rather than light of a holographic reconstruction of an image. Thus, in such embodiments, it can be said that the pupil expander replicates the hologram or forms at least one copy of the hologram to convey that the light delivered to the observer is spatially modulated according to the hologram of the image rather than the image itself. That is, the diffracted light field propagates to the observer.

[0025] 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 copies or replicas 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 of the light output by the system. 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 in which an observer’s eye can be located in order to see / receive the light output by the system.

[0026] A head-up display for a vehicle is provided. The head-up display includes an optical component having a reflective surface. During head-up display operation, the optical component is arranged in a configuration that facilitates sunlight glare. It can be said that this configuration makes the optical component (e.g. its reflective surface) susceptible to causing glare to an observer of the head-up display as a result of sunlight incident thereon (e.g. by directing or propagating the sunlight to the eyebox). In particular, the reflective surface of the optical component can receive sunlight and direct the sunlight to the eyebox region (or viewing window) of the head-up display, thereby causing glare. A light control layer is disposed on the optical component to receive the sunlight on the optical path to the reflective surface. The light control layer includes a sunlight receiving surface and a core material of a louver array. The sunlight receiving surface of the light control layer is serrated, cooperating with the louver array.

[0027] Accordingly, the light control layer is configured to control the incident sunlight without significantly altering the functionality of the optical component. In particular, the light control layer can direct the received sunlight away from the eyebox of the head-up display. Accordingly, the risk of glare to an observer as a result of sunlight is reduced.

[0028] As used herein, a louver can be a slanted slat. Accordingly, the louver array can include a plurality of slanted slats. Each of the plurality of slanted slats can be parallel. In some embodiments, adjacent louvers of the louver array can be uniformly spaced (i.e. the louvers have a constant “louver spacing”). Accordingly, it can be said that the louver array has a periodicity. In embodiments, the periodicity of the serrations of the sunlight receiving surface is substantially equal to the periodicity of the louver array.

[0029] Generally, the louver array is a two-dimensional array having first and second dimensions. For example, the louvers are arranged in an array extending in the first dimension, i.e. the louvers are separated from one another by a spacing in the first dimension, and each louver has a length extending in the second dimension. The reflective surface of the optical component can be planar. The first and second dimensions can correspond to the dimensions of the planar reflective surface of the optical component. In embodiments, the serrated configuration of the sunlight receiving surface is one-dimensional and extends in the first dimension. For example, the serrated configuration can be a one-dimensional array of spaced serrations, protrusions or teeth separated by slanted or curved surfaces.

[0030] In examples, the first and second dimensions are perpendicular to each other. Sunlight can be incident on the planar reflective surface of the optical component in the first and second dimensions. Thus, it can be said that at least one component of the light ray angle of the incident sunlight is along the third dimension. However, due to the sawtooth configuration, the sunlight receiving surface of the light control layer is non-planar between the louvers along the first dimension.

[0031] In embodiments, the louvers are tilted. In particular, the louvers can be tilted from an orthogonal orientation relative to the plane of the light control layer and / or the planar reflective surface of the optical component. Additionally or alternatively, one or both sidewalls of the louvers can be tilted such that each louver has a trapezoidal cross-section.

[0032] In embodiments, the louvers are configured to attenuate sunlight, e.g., absorb sunlight. For example, the louvers can be formed by depositing a light attenuating or absorbing material in an array of louver-shaped grooves or recesses in a core material. In examples, all of the louvers in the array have substantially the same pitch, orientation, and geometry.

[0033] In embodiments, the core material is substantially transmissive to image light of the head-up display. For example, the core material can be optically transparent to light having a wavelength corresponding to the image light. In examples, the array of louvers is configured to transmit image light rays of the head-up display output by the optical component according to a range of transmission angles required for propagation to an observer eye pupil at an eyebox, such that the image can be perceived.

[0034] In some examples, the light control layer includes an optically transparent core material having an array of light-absorbing louvers disposed therein. The arrangement of louvers allows image light of the head-up display to propagate through the optical component without altering its functionality. Additionally, the arrangement of louvers can limit the range of angles of sunlight incident on the sunlight receiving surface that can enter the core material into the optical component.

[0035] In some embodiments, the core material can be disposed between adjacent pairs of louvers. The core material can be disposed between each adjacent pair of louvers. Each louver can be disposed within the core material.

[0036] In some embodiments, the optical component is a waveguide. The waveguide can be substantially planar. In embodiments, the waveguide is arranged in a substantially planar configuration (e.g., horizontal) relative to the ground during operation of the head-up display. The waveguide can form a waveguide pupil expander arranged to expand an exit pupil in the first and second dimensions.

[0037] In some embodiments, the sunlight-receiving surface is formed by an outer surface of a coating covering the array of louvers of the core material and the light control film. In an example, the coating is substantially transparent to the image light of the heads-up display. The coating can be formed of one or more materials for protecting the light control film, such as a cladding, fluid resistance, and / or scratch resistance.

[0038] In embodiments, the sunlight-receiving surface further comprises an anti-reflective coating. For example, the anti-reflective coating can minimize specular reflection.

[0039] In some embodiments, the serrations of the sunlight-receiving surface provide an array of tilted surfaces that direct sunlight away from the eyebox. In particular, each tilted surface forms an interface between the core material and air such that a substantial portion of sunlight incident thereon (e.g., 96% of the incident sunlight) is reflected (i.e., not coupled into the light control film). The tilted surfaces are oriented at an angle relative to the plane of the light control film / optical component in the first and second dimensions. Due to their angle of orientation, the tilted surfaces are arranged to direct the reflected sunlight away from a direction toward the eyebox.

[0040] In an example, the tilted surfaces of the sunlight-receiving surface change the angle of reflection of sunlight incident thereon. That is, the angle of sunlight rays received by the tilted surfaces is different from the angle of sunlight rays reflected by the tilted surfaces, where the angle of sunlight rays is measured relative to the plane of the light control film / optical component in the first and second dimensions (orthogonal to the plane). One can say that each tilted surface changes the course or path of the specular reflection of the incident sunlight compared to the specular reflection of a non-tilted surface parallel to the first and second dimensions.

[0041] In embodiments, the louvers in the array of louvers are substantially parallel. In some embodiments, the cross-section of each louver in the array of louvers is substantially rectangular or trapezoidal.

[0042] In embodiments, the heads-up display can be arranged such that the optical path of the image light of the heads-up display passes through the optical component before passing through the light control layer. In other words, the optical path of the image light of the heads-up display can not pass through the light control layer before passing through the optical component. In some embodiments, the heads-up display can be arranged such that the optical path of the image light of the heads-up display passes through the optical component before passing through the sunlight-receiving surface. In other words, the optical path of the image light of the heads-up display can not pass through the optical component before passing through the sunlight-receiving surface.

[0043] A component referred to herein as “contributing to sunlight glare” or “prone to cause glare” is a component that causes glare. More specifically, the physical form, position, and orientation of the component are such that received sunlight can be directed (e.g., reflected and / or propagated by the component) to the eyebox or viewing window of the head-up display. It can be said that, during normal operation or use, the component (e.g., a reflective surface thereof) is arranged in a configuration that supports glare. The reflective surface is disposed in the optical path of sunlight, such as direct sunlight. A configuration that “contributes to” or “supports” glare is a configuration in which the component can direct sunlight to the eyebox of the head-up display, e.g., in a way that reduces (perceived) image quality.

[0044] In this disclosure, the term “image light” is used to refer to light that propagates into the eyebox of a head-up display so that an observer can perceive an image. Thus, as described herein, image light can include light encoded with an image and spatially modulated light encoded with a hologram of an image.

[0045] In this disclosure, the term “replica” is used only 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 a replication event— such as a partial reflection-transmission of a pupil expander. Each replica propagates along a different optical path. Some embodiments of this disclosure relate to the propagation of light encoded with a hologram rather than an image— i.e., light spatially modulated with a hologram of an image rather than the image itself. Those skilled in the art of holography will appreciate that the composite light field associated with the propagation of light encoded with a hologram will vary with propagation distance. The term “replica” as used herein is independent of propagation distance, so two optical branches or paths associated with 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 this disclosure, two composite light fields are considered to be “replicas” even if they are associated with different propagation distances— provided they originate from the same replication event or series of replication events.

[0046] A “diffracted light field” according to this disclosure is a light field formed by diffraction. A diffracted light field can be formed by illuminating a corresponding diffraction pattern. According to this disclosure, an example of a diffraction pattern is a hologram and an example of a diffracted light field is a holographic light field or a light field forming a holographic reconstruction of an image. 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 diffracted light field is characterized by a diffraction angle determined by the smallest feature size of the diffractive structure and the wavelength of the light (of the diffracted light field). According to this disclosure, it can also be said that a “diffracted light field” is a light field forming a reconstruction on a plane spatially separated from the corresponding diffractive structure. An optical system for propagating a diffracted light field from a diffractive structure to an observer is disclosed herein. The diffracted light field can form an image.

[0047] 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 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 replica 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, for convenience only, as "image pixels".

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] In this disclosure, the term "substantially" when applied to a structural unit of a device can be interpreted as a technical feature of the structural unit that is produced within the technical tolerances of the method used to manufacture it. BRIEF DESCRIPTION OF DRAWINGS

[0055] Particular embodiments are described by way of example only in reference to the following drawings:

[0056] Figure 1 is a schematic diagram showing a reflective SLM producing a holographic reconstruction on a screen;

[0057] Figure 2 shows an image for projection, including eight image regions / components, i.e., V1 to V8;

[0058] Figure 3A hologram is shown displayed on an LCOS that directs light to a plurality of discrete areas;

[0059] Figure 4 A system is shown including a display device displaying a hologram calculated as Figure 2 and 3 A system of a display device displaying a hologram calculated as shown;

[0060] Figure 5 A perspective view is shown of a system including two replicators arranged to expand a light beam in two dimensions;

[0061] Figure 6A is a schematic side view showing the optical path of image light from a head-up display in a vehicle to an eyebox having a replicator with a light control film, Figure 6B is a schematic top view showing the optical path of image light from the light control film to the eyebox;

[0062] Figure 7 is a cross-sectional view of an example light control film including a louver array;

[0063] Figure 8 is a cross-sectional view of a light control film including a louver array according to a first embodiment of the invention;

[0064] Figure 9 is a cross-sectional view of a light control film according to a second embodiment of the invention, and

[0065] Figure 10 is a cross-sectional view of a light control film according to a third embodiment of the invention.

[0066] Throughout the drawings, like reference numerals will be used to refer to like or similar portions. DETAILED DESCRIPTION

[0067] 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 set forth, which are set forth for illustrative purposes only.

[0068] The singular form "a", "an", and "the" can include plural references unless otherwise specified.

[0069] A structure described as formed on the upper portion / lower portion of another structure or above / below another structure should be construed to include the case where the structures contact each other, in addition to the case where a third structure is disposed therebetween.

[0070] In describing a relationship of time, for example, when an order of timing of events is described as "after," "subsequently," "next," "before," or the like, the present disclosure should be considered to include both consecutive and non-consecutive events, unless otherwise indicated. For example, unless the word such as "just," "immediately," or "directly" is used, the description should be considered to include non-consecutive cases.

[0071] 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 accompanying claims.

[0072] 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.

[0073] Optical configuration

[0074] 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 a 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.

[0075] 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 shown, the wavefront is directed off-normal (for example, two or three degrees off a plane that is truly normal to 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 1 In the embodiment shown, the arrangement is such that light from the light source is reflected from a mirror back surface of the SLM and interacts with the light modulating 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 particularly, the Fourier transform lens 120 receives the modulated light beam from the SLM 140 and performs a frequency-to-space transform to produce a holographic reconstruction at the screen 125.

[0076] Notably, in this type of hologram, each pixel of the hologram contributes to the overall 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 modulating layer is distributed across the entire replay field.

[0077] In these embodiments, the location in space of the holographic reconstruction is determined by the power (focusing) of the Fourier transform lens. In Figure 1 In the illustrated embodiments, 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.

[0078] Hologram calculation

[0079] In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier or Fourier-based hologram, in which the 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. By way of example only, embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms which can be computed by similar methods. In some embodiments, the hologram is a phase or pure phase hologram. However, the present disclosure is also applicable to holograms computed by other techniques, for example techniques based on point cloud methods. An example hologram computation method that can be combined with the present disclosure is disclosed in UK patent application GB2112213.0, filed on 26 August 2021, which is incorporated by reference.

[0080] In some embodiments, a real-time engine is provided which is arranged to receive image data and compute 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-computed, 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.

[0081] Light modulation

[0082] The display system comprises a display device defining an exit pupil of the display system. The display device is a spatial light modulator. The spatial light modulation can be phase modulation. The display device can be a liquid crystal on silicon, LCOS, spatial light modulator.

[0083] Light channel

[0084] 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 - that is, 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.

[0085] A display system and method is described below that includes a waveguide pupil expander. As will be familiar to the skilled reader, a waveguide can be configured 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 does this by increasing the number of transmissive points at which light 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, while still being able to see light from the light emitter. Thus, it can be said that the observer's "eye box" or "eye movement box" is magnified by the use of a waveguide pupil expander. This has a number of useful applications, such as but not limited to heads-up displays, such as but not limited to automotive heads-up displays.

[0086] The display systems described herein can be configured to direct light, such as a diffractive light field, through a waveguide pupil expander in order to provide pupil expansion in at least one dimension, e.g., in two dimensions. The diffractive light field can include light output by a spatial light modulator (SLM), such as an LCOS SLM. For example, the diffractive light field can include light encoded by a hologram displayed by the SLM. For example, the diffractive light field can include light of a holographically reconstructed image, corresponding to a hologram displayed by the SLM. The hologram can include 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 includes the hologram and one or more other elements, such as a software lens or a diffractive grating.

[0087] Holograms can be computed to provide guidance for a 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, a hologram can be computed 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 "target image". A hologram can be computed so that when it is displayed on an 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 successive light channels of spatially modulated light corresponding to respective successive regions of the image. However, the present disclosure is not limited to this type of hologram.

[0088] Although we refer to it here as a "hologram" or "computer generated hologram (CGH)", it will be appreciated that the SLM can be configured to display a plurality of different holograms successively or dynamically according to a sequence. The systems and methods described herein are applicable to the dynamic display of a plurality of different holograms.

[0089] Figure 2 and 3 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. However, this example should not be taken as limiting the present disclosure.

[0090] Figure 2 An image 252 for projection is shown, comprising eight image regions / components V1 to V8. By way of example only, Figure 2 Eight image components are shown, and the image 252 can be divided into any number of components. Figure 2 An encoded light pattern 254 (i.e. a hologram) is also shown, which can reconstruct the image 252 - 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 2 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 3 In particular, 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 can also be 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.

[0091] Figure 4 A system 400 is shown, comprising a display device displaying a hologram computed as Figure 2 and 3 shown. The hologram is computed to correspond to an image 252 to be holographically reconstructed.

[0092] The system 400 comprises a display device which in this arrangement comprises an LCOS 402. The LCOS 402 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 405 comprising a pupil acting as an aperture 404, a lens 409 and a retina (not shown) acting as a viewing plane. There is a light source (not shown) arranged to illuminate the LCOS 402. The lens 409 of the eye 405 performs 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.

[0093] The viewing system 400 also comprises a waveguide 408 located between the LCOS 402 and the eye 405. The presence of the waveguide 408 enables all angular content from the LCOS 402 to be received by the eye, even at the relatively large projection distance shown. This is because the waveguide 408 acts as a pupil expander in a manner well known and so is only briefly described here.

[0094] In brief summary, Figure 4 The waveguide 408 shown comprises a generally elongate structure. In this example, the waveguide 408 comprises an optical slab of refractive material, although other types of waveguide are also well known and can be used. The waveguide 408 is positioned to intersect the cone of light (i.e. the diffracted light field) projected from the LCOS 402, for example at an oblique angle. In this example, the size, position and orientation of the waveguide 408 are configured to ensure that light from each of the eight beams within the cone of light enters the waveguide 408. Light from the cone of light enters the waveguide 408 via a first planar surface of the waveguide 408 (positioned closest to the LCOS 402) and is guided at least partially along the length of the waveguide 408 before being ejected via a second planar surface of the waveguide 408 (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 408 from the first planar surface and hits the second planar surface, some of the light will be transmitted out of the waveguide 408 and some of the light will be reflected by the second planar surface back to the first planar surface. The first planar surface is reflective such that all light hitting it from within the waveguide 408 will be reflected back to the second planar surface. Thus, some light can simply be refracted between the two planar surfaces of the waveguide 408 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 408 before being transmitted.

[0095] Figure 4 Nine "bounce" points B0 to B8 along the length of the waveguide 408 are shown. Although as Figure 2As shown, light associated with all points of the image (V1-V8) is transmitted out of the waveguide on each "bounce" from the second planar surface of the waveguide 408, but only light from one angular portion of the image (e.g. light from one of V1 to V8) has a trajectory that enables it to reach the eye 405 from each respective "bounce" point B0 to B8. Furthermore, light from different angular portions of the image (V1 to V8) reaches the eye 405 from each respective "bounce" point. Thus, in the example of Figure 4 each angular channel of encoded light reaches the eye only once from the waveguide 408.

[0096] The above described methods and apparatus can be implemented in a variety of different applications and viewing systems. For example, they can be implemented in a head-up display (HUD) or in a head or helmet mounted device (HMD) such as an augmented reality (AR) HMD.

[0097] Although virtual images, which require the eye to transform received modulated light to form a perceived image, have been discussed generally herein, the methods and apparatus described herein can be applied to real images.

[0098] Two-dimensional pupil expansion

[0099] Although Figure 4 The illustrated arrangement comprises a single waveguide providing pupil expansion in one dimension, but pupil expansion can be provided in more than one dimension, for example in two dimensions. Furthermore, although Figure 4 The examples in

[0100] Figure 5 A perspective view of a system 500 is shown, the system comprising two replicators 504, 506, arranged to expand a light beam 502 in two dimensions.

[0101] In the system 500 of Figure 5 The first replicator 504 comprises a first pair of surfaces stacked parallel to each other, arranged to provide replication or pupil expansion in a similar way to the waveguide 408 of Figure 4 The first pair of surfaces are similar in size and shape to each other (in some cases identical), and are substantially elongate in one direction. The collimated light beam 502 is directed onto an input on the first replicator 504. Due to the internal reflection processes between the two surfaces, and from one of the surfaces (the upper surface, as shown in Figure 5The skilled reader will be familiar with the partial transmission of light from each of a plurality of output points on the upper surface (as shown) and the replication of light from each of the first plurality of light beams 502 in the first direction along the length of the first replicator 504. Thus, a first plurality of replicated light beams 508 is emitted from the first replicator 504 towards the second replicator 506.

[0102] The second replicator 506 comprises a second pair of surfaces stacked parallel to each other, arranged to receive each of the collimated light beams of the first plurality of light beams 508, and further arranged to provide replication or pupil expansion by expanding each of 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 second replicator is implemented with a rectangular shape so as to have a length along the first direction in order to receive the first plurality of light beams 508, and 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 the upper surface (as shown), the light of each of the first plurality of light beams 508 is replicated in the second direction. Thus, a second plurality of light beams 510 is emitted from the second replicator 506, wherein the second plurality of light beams 510 comprises replicas of the input light beam 502 along each of the first and second directions. The second plurality of light beams 510 can thus be viewed as comprising a two-dimensional grid or array of replicated light beams. Figure 5

[0103] Thus, the first and second replicators 504, 505 of the two-dimensional replicator (or “two-dimensional pupil expander”) can be said to provide a two-dimensional replication of the input light beam 502. Figure 5

[0104] Light control film

[0105] In operation, Figure 5 The transmissive surface of the second replicator 506 of the two-dimensional pupil expander (i.e. the expanded exit pupil) forms an external surface from which image light is transmitted through air to the eyebox region for viewing. Thus, the transmissive surface can be exposed to sunlight from the environment in which the head-up display is used. The received sunlight can cause glare to the observer. For example, glare can occur if sunlight is directly reflected from the external transmissive surface at an angle such that the sunlight rays reach the eyebox along the optical path. In another example, glare can occur if sunlight is coupled into the second replicator at an angle such that the sunlight reaches the eyebox within the replicator along the same optical path as the image rays.

[0106] ​​Accordingly, the inventors propose using a light control film on the transmissive surface of the second replicator 506 to control the direction of the received sunlight to reduce the risk of glare for the observer. An example light control film for controlling the direction of transmitted light includes an optically transparent film having a plurality of parallel louvers formed of a light absorbing material. Such light control films are commonly used to control the direction of transmitted light from a vehicle display system to prevent light emitted by the display system from being received by and reflected from the vehicle window, thereby causing glare to the driver or passenger (e.g., at night). However, the inventors have recognized that due to the upwardly facing orientation of the second replicator 506 adjacent to the vehicle windshield, the same type of light control film can be used to control the direction of direct sunlight that can be incident on the second replicator 506 (i.e., during the day). The orientation, pitch, and geometry (e.g., sidewall angle) of the louvers of the light control film can be selected to allow image light to be transmitted from the transmissive surface of the second replicator 506 only over a limited range of angles required to reach the eyebox.

[0107] Figure 6A and 6B An example light control film is shown for use with a head-up display operating in a vehicle. In the illustrated arrangement, the optical component (e.g., second replicator 506 in Figure 5 ) at the interface with air is oriented substantially horizontally, as shown by the x-y plane. For example, the optical component can be located in an opening in the upward facing surface of the vehicle dashboard. The transmissive surface of the optical component is covered by a light control film 606 including an array of one-dimensional louvers. The louvers can be in a substantially vertical plane, illustrated as the x-z plane. In the illustrated arrangement, the louvers are tilted with respect to the vertical plane (i.e., tilted from the orthogonal orientation with respect to the plane of the light control layer). In particular, the louvers have tilted sidewalls. The louvers can be light absorbing or light attenuating.

[0108] As shown in Figure 6A , image light from the head-up display 608 passes between the louvers of the light control film 606 and is transmitted to the optical combiner 602 (e.g., the windshield). The optical combiner 602 redirects the image light substantially horizontally, as shown by the y-direction, toward the eyebox 612, where an observer (e.g., the driver of the vehicle) can perceive a virtual image (combined with the external scene viewed through the windshield).

[0109] Figure 7An example light control film comprising an array of louvers is shown in more detail. The light control film 712 comprises first (bottom / inner) and second (top / outer) surfaces defining the thickness of the light control film 712. In the arrangement shown, an optically transparent coating (which can also be referred to as an “cladding layer”) is provided on each of the first and second surfaces for planarization, as the surfaces of the light control film 712 can not be flat. Thus, the light control film 712 is sandwiched between a pair of optically transparent coatings or cladding or films having respective outer surfaces 720, 722. Those skilled in the art will appreciate that in arrangements where the light control film 712 is formed directly on a planar transmissive surface of an optical component (e.g. the second replicator 506), no coating / planner layer is required on the first (bottom / inner) surface. The light control film 712 comprises a core of optically transparent material in which a plurality of light-absorbing louvers are disposed. The louvers are arranged in a one-dimensional array, shown extending in the y-direction, with uniform spacing or pitch 708 between adjacent louvers. The spacing between the louvers is small enough to optimize light absorption without causing diffraction or ghosting. In examples, the spacing between the louvers can be in the order of 10-1000 pm, for example 50-250 pm. The louvers extend through the entire thickness of the light control film 712 and are configured to have an orientation, pitch and geometry arranged to limit the range of transmission angles 706 from the second (top / outer) surface, as shown by the dashed lines.

[0110] In the arrangement shown, each louver has a trapezoidal cross-section (i.e. with non-parallel sloping sidewalls) which tapers (narrows) to a thickness 710 at the second (top / outer) surface of the light control film 712. Thus, the sidewalls of each louver are inclined relative to a plane normal to the first and second surfaces of the light control film 712 (shown as the vertical or x-z plane), as shown by the inclination angle 714. In the arrangement shown, the geometry of the louvers is identical and the opposing sidewalls of each louver are inclined at different angles. As those skilled in the art will appreciate, in other arrangements the louvers can have a rectangular cross-section (i.e. with parallel non-inclined sidewalls) and / or can be oriented in a plane normal to the first and second surfaces of the light control film 700, as well as at any desired inclination angle relative thereto.

[0111] Thus, image light 704 incident from the transmissive surface of the optical component of the head-up display passes through the light control film 712 and the associated coating / planner layer between the louvers. In embodiments, the louvers are geometrically configured to allow the image light 704 to pass through the optically transparent core between the louvers at the required range of angles for the image to be visible at all locations within the eyebox. Thus, in examples where a special hologram is implemented, as referred to herein with reference to Figure 2, the louvers are configured to have a pitch and orientation which is the same as the pitch and orientation of the special hologram. In this way, the special hologram is replicated by the array of louvers. Figures 2 to 4The range of transmission angles can allow all angular channels of the hologram to reach the eyebox. For example, the image light 704 can be transmitted at one or more defined optical path angle (e.g., as a light beam from a plurality of transmission points of the optical component, as described herein) that passes between the louvers to the eyebox, e.g., as shown by the solid arrows in FIG. 7A. Due to the small spacing between the louvers, each replica formed at one of the plurality of transmission points along the transmission surface of the optical component can be incident on a plurality of louvers of the light control film 712. Figure 7

[0112] However, as described above, sunlight can be incident on the second (top / outer) surface of the light control film 712, as shown by the arrow 700. Accordingly, the second (top / outer) surface of the light control film 712 is described herein as the “light receiving surface” of the light control film 712. It will be appreciated that sunlight can be incident on the surface of the light control film 712 at any angle, the light ray angle shown by the arrow 700 being merely an example. Sunlight coupled into the light control film 712 at the planar top surface 720 (i.e., the outer interface with air) can enter at an angle such that the light is directly absorbed by one of the louvers in a “first pass”, as shown by the arrow 700. Alternatively, sunlight coupled into the light control film 712 at the planar top surface 720 can enter through the louvers into the optical component (not shown). In this case, the sunlight reflected out of the optical component is absorbed by one of the louvers in a “second pass” and thus is not transmitted by the light control film 712. However, a portion of the sunlight can be reflected at the planar top surface 720 (i.e., the outer interface with air). In this case, there is a risk that the sunlight is reflected at an angle to reach the eyebox along the optical path. Figure 7 An example light ray of the reflected sunlight 702 is shown with a dashed line at an angle parallel to the light rays of the image light 704 from the head-up display that is transmitted between the louvers and out of the light control film 712. It will be appreciated that the illustrated light rays of the image light 704 are merely an example of a plurality of light rays of a diverging light ray bundle of a replica that is transmitted at an angle through between the louvers from a transmission point of the optical component. Further, it will be appreciated that the light rays of the same replica can pass between other pairs of adjacent louvers at the same or different light ray angles. Accordingly, the light rays of the reflected sunlight 702 will reach the eyebox along the same optical path as the illustrated light rays of the image light 704 and cause glare to the observer. As shown by the embodiments described below, the present disclosure addresses this problem. In the description of the embodiments, like reference numerals are used to represent similar features as in the example of FIG. 7A. Figure 7

[0113] Figure 8 ​​A light control film 812 according to an embodiment of the present application is shown. The light control film 812 is generally the same as the examples described above Figure 7 , except that the coating on the second (top / outer) surface of the light control film 812 is omitted. Thus, the second (top / outer) surface of the light control film 812 forms a sunlight receiving surface 820. Thus, as shown in the examples described above Figure 7 , the light control film 812 includes a core of optically transparent material in which a one-dimensional array of uniformly spaced louvers is disposed. The louvers extend through the entire thickness of the light control film 812 and are configured in a trapezoidal configuration (i.e., with angled or sloped sidewalls). As described above, the light control film 812 can be formed on a planar reflective surface of an optical component of a heads-up display, such as the second replicator 506 of the example described above Figure 5

[0114] According to the present application, the light receiving surface 820 of the light control film 812 is sawtoothed. In particular, the light receiving surface 820 is sawtoothed in coordination with the array of louvers. For example, the arrangement of sawteeth of the sunlight receiving surface 820 is coordinated, synchronized or aligned with the arrangement of the underlying array of louvers. Thus, the sawtoothed configuration of the sunlight receiving surface 820 is one-dimensional and extends in a first dimension. In the embodiment of the example described above Figure 8 , the sawtoothed configuration includes a one-dimensional array of uniformly spaced sawteeth (also referred to as "protrusions" or "teeth") separated by a surface that is angled with respect to the plane of the light control film 812 / optical component (i.e., angled with respect to the horizontal or x-y plane). Thus, in the embodiment of the example described above Figure 8 , the cross-section of the sunlight receiving surface 820 has a generally sawtoothed configuration. Further, in the embodiment of the example described above Figure 8 , the uniformly spaced sawteeth are aligned with the louvers. Thus, it can be said that the periodicity of the sawteeth of the sunlight receiving surface 820 is substantially equal to the periodicity of the array of louvers.

[0115] As described above with reference to the example described above Figure 7 , the light transmitted by the light control film 812 is limited to an angular range 806 defined by the orientation, pitch and geometry (e.g., sidewall angle) of the louvers. Thus, the image light 804 from the optical component of the heads-up display (not shown) is transmitted only at light ray angles that fall within the limited range of angles 806 in order to pass through the light control film 812 between the louvers and along the optical path to the eyebox. Further, as described above with reference to the example described above Figure 7 , the sunlight 800 that is coupled into the light control film 812 is either absorbed by one of the louvers and / or is prevented from being transmitted to the eyebox due to the limited range of transmission angles 806.

[0116] ​Furthermore, according to the invention, due to the sawtooth configuration, sunlight 800 reflected by the top surface 820 (i.e., the external interface with air) is reflected in a direction away from the eyepiece (e.g., in automotive applications, as described herein, in a direction away from the windshield that reflects the image light to the eyepiece), as shown by the dashed lines. Specifically, due to the inclined second (top / outer) surface 820 of the light control film 812 above the optically transparent core region between the louvers, the surface normal is similarly angled relative to the normal of the plane (i.e., the horizontal or xy plane) of the light control film 812 / optical component. Therefore, due to the law of reflection, with Figure 7 The angle of sunlight 800 changes (e.g., increases) compared to the reflection angle of the planar surface 720 of the light control film 712. In the example, the tilted surface is angled or tilted relative to the plane (normal) of the light control film 812 / optical component, with the angle in either direction ranging from 15° to 75°, for example, from 30° to 60°. Importantly, the slope or inclination of the tilted surface of the light receiving surface 820 causes the angle of the reflected sunlight 802 to be outside the strictly permissible / required angle 806 of the image light 804 from the head-up display, thus preventing the sunlight from reaching the eyepiece along the optical path and causing glare.

[0117] Figure 9 A light control film 912 according to another embodiment is shown. Except for an optically transparent coating of uniform thickness disposed on the second (top / outer) surface of the light control film 912 forming the light receiving surface 920, the light control film 912 of this embodiment is generally consistent with… Figure 8 The embodiments are the same. Therefore, similar to Figure 7 For example, the light control film 912 is sandwiched between a pair of optically transparent coatings or claddings or films having corresponding outer surfaces 920, 922.

[0118] Because the coating formed on the second (top / outer) surface of the light control film 912 has a uniform thickness, the light receiving surface 920 has the same serrated configuration as the second (top / outer) surface of the underlying light film, such as... Figure 8 As illustrated in the embodiments. However, using a coating or cladding on the second (top / outer) surface of the light-receiving film 912 can provide surface planarization, thereby reducing any surface roughness (e.g., related to the formation of the light control film 912) that could otherwise degrade image quality at the eye box.

[0119] The coating can include any suitable optically transparent layer or film, including a multilayer structure, which can be provided with a desired thickness uniformity over the jagged light-receiving surface of the light control film 912. In some examples, the coating can include an anti-reflective coating for minimizing specular reflection. Further, as the coating is exposed to the environment, for example on a dashboard of a vehicle cabin, the material of the coating can also be selected to protect the light control film 912 from external causes of damage, for example to provide heat resistance, scratch / impact resistance, fluid resistance, and other similar properties.

[0120] Figure 10 A light control film 1012 according to yet another embodiment is shown. The light control film 1012 of this embodiment is substantially the same as the embodiment of Figure 9 except that the optically transparent coating provided over the second (top / outer) surface forming the light-receiving surface 1020 does not have a uniform thickness. It should be noted that the inclined surfaces formed by the second (top / outer) surface of the light control film 1012 between louvers are shown as being inclined in the opposite direction (relative to the horizontal or x-y plane) than the corresponding inclined surfaces of the light control film 912 of Figure 9

[0121] As shown in the embodiment of Figure 10 the coating thickness over the optically transparent core region (between louvers) on the second (top / outer) surface of the light control film 1012 is substantially uniform. Thus, as in the embodiment of Figure 9 the coating does not affect the transmission of image light from the head-up display through the core material by the light control film 1012 over the range of transmission angles 1006. However, the thickness of the coating varies with distance over the louvers of the light control film 1012. In the illustrated configuration, the coating thickness over the louvers increases from one side of each louver to the other in the direction of the louver array (illustrated as the y-direction). Thus, as described herein, the jagged configuration of the light-receiving surface 1020 has larger and sharper serrations, protrusions, or teeth. It can be said that the angle at the corner of the serrations, protrusions, or teeth varies with distance along the serration dimension. In the illustrated example, the angle at the corner of each serration decreases from approximately 90° in Figure 9 to an acute angle of less than 90° in Figure 10 This alternative configuration of the coating allows the coating and sunlight reflection properties to be tailored according to application requirements, for example the geometry of the head-up display when in use.

[0122] ​The light control film of the present disclosure prevents an observer from experiencing glare due to sunlight at the eyebox of a head-up display, such as when the head-up display is used in a vehicle, where a pupil expander (or other optical components thereof) can be arranged to receive sunlight. The light control film has an angular-dependent transmittance due to the geometry of the light-absorbing louver array, allowing image light to be transmitted from the head-up display to the eyebox at a desired range of angles. For example, the range of transmission angles can allow all angular channels of a special hologram (as described herein with reference to Figures 2 to 4 The spacing, orientation, and geometry of the louvers can be configured to avoid ghosting due to diffraction. According to the present invention, the jagged configuration of the surface at the interface between the light control film and air prevents specularly reflected sunlight from reaching the eyebox and causing glare. In particular, the jagged configuration of the light-receiving surface formed by the coating in some embodiments is arranged to deflect sunlight away from the eyebox while ensuring that image light still propagates to the eyebox in order to provide a complete image. In an example, the jagged configuration includes a slanted surface— which can be at any angle along the dimension of the louver array— in order to deflect sunlight off the trajectory to the eyebox. The angle of the slanted surface can be selected in situ based on the geometry of the head-up display, such as the location within a vehicle, to prevent specularly reflected sunlight from reaching the eyebox. In embodiments where the jagged light-receiving surface is formed by a coating, the coating can be made of a flexible plastic or other optically transparent material, made of a material that provides sufficient protective properties, such as thermal, mechanical, and fluid resistance properties for automotive applications, as described herein.

[0123] As will be appreciated by those skilled in the art, many variations can be made to the embodiments described herein. For example, the louver array of the light control film can be arranged to have a variable louver spacing, orientation, and geometry, depending on application requirements. Moreover, while the embodiments have a jagged light-receiving surface with slanted surfaces between the jags, protrusions, or teeth, in other arrangements, curved surfaces or surfaces with two or more slanted portions can be provided between the jags in order to redirect reflected sunlight away from the eyebox. Thus, while the described embodiments include jags that are aligned with one or more sidewalls of the louvers, the jags can be aligned with another periodic location along the dimension of the louver array.

[0124] Disclosed herein is a system that uses diffracted light to form an image and provides an eyebox size and field of view suitable for real-world applications— such as through a head-up display in the automotive industry. Diffracted light is light that forms a holographic reconstruction of an image from a diffractive structure— such as a hologram like 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 a small display device (e.g., 1 cm). The inventors have solved the problem of how to provide diffracted light for a 2D pupil expansion, such as diffracted light that includes diverging (non-collimated) light beams.

[0125] In some aspects, the display system includes a display device— such as a pixelated display device, e.g., a spatial light modulator (SLM) or a 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 size of the area bounding the array of light-modulating pixels contained within the SLM— determines the size (e.g., spatial extent) of the bundle of light 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 having pixel sizes for light diffraction) becomes larger in spatial extent.

[0126] Additional features

[0127] The methods and processes described herein can be embodied in a computer- readable medium. The term "computer-readable medium" includes media that are arranged 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, the machine becomes configured to perform any one or more of the methods described herein in whole or in part.

[0128] 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 solid-state storage chips, optical discs, magnetic discs, or any suitable combination thereof. In some example embodiments, instructions for execution can be delivered by a carrier medium. Examples of such carrier media include transitory media (e.g., propagating signals that deliver the instructions).

[0129] 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.

Claims

1. A head-up display for a vehicle, wherein, The head-up display comprises: an optical component having a reflective surface arranged during head-up display operation in a configuration conducive to sunlight glare; and a light control layer disposed on the optical component for receiving sunlight in an optical path to the reflective surface, wherein the light control layer comprises a sunlight receiving surface and a core material separating an array of louvers, all louvers in the array of louvers having substantially the same pitch and geometry, wherein the sunlight receiving surface of the light control layer is serrated in coordination with the array of louvers such that the serrations are aligned with the louvers; wherein the optical component is a waveguide, wherein a periodicity of the serrations of the sunlight receiving surface is substantially equal to a periodicity of the array of louvers, wherein the serrations provide an array of tilted surfaces having any directional angle to direct sunlight away from an eyebox of the head-up display, wherein the tilted surfaces change a reflection angle of sunlight received thereby.

2. The head-up display of claim 1, wherein, The array of louvers is a two-dimensional array having first and second dimensions, and the serrations are one-dimensional and extend in the first dimension.

3. The head-up display of claim 1 or 2, wherein, The louvers are tilted relative to a plane of the light control layer.

4. The head-up display of claim 1 or 2, wherein, The louvers are configured to attenuate sunlight, such as to absorb sunlight.

5. The head-up display of claim 1 or 2, wherein, The array of louvers is configured to transmit image light of the head-up display output by the optical component.

6. The head-up display of claim 1 or 2, wherein, The waveguide is substantially planar.

7. The head-up display of claim 6, wherein, The waveguide is arranged during head-up display operation in a substantially planar configuration relative to the ground.

8. The head-up display of claim 1 or 2, wherein, The core material is substantially transmissive to image light of the head-up display.

9. The head-up display of claim 1 or 2, wherein, The sunlight receiving surface is a coating covering the core material and array of louvers.

10. The head-up display of claim 9, wherein, The coating is substantially transparent to image light of the head-up display.

11. The head-up display of claim 10, wherein, The coating is a cladding layer.

12. The head-up display of claim 1 or 2, wherein, The sunlight receiving surface further comprises an anti-reflective coating.

13. The head-up display of claim 1 or 2, wherein, The louvers in the array of louvers are substantially parallel.

14. The head-up display of claim 1 or 2, wherein, A cross-section of each louver in the array of louvers is substantially rectangular or trapezoidal.

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