Light control film

Abstract

A method of manufacturing a light control film. The method includes providing a plurality of alternating layers of a transparent material and a light absorbing material. Each layer of the light absorbing material is interspersed between a respective layer of the transparent material. The plurality of alternating layers of the transparent material and the light absorbing material are stacked along a stacking direction. The stacking direction is perpendicular to a surface plane of at least one of the plurality of alternating layers. The method also includes cutting through the plurality of alternating layers of the transparent material and the light absorbing material in a first cutting plane and a second cutting plane with a straight blade, thereby producing an intermediate film including a first cut surface parallel to the first cutting plane and a second cut surface parallel to the second cutting plane. Each of the first and second cutting planes is oriented at an angle less than or equal to 90° from the stacking direction. The method further includes hot embossing a repeating prism structure on the first cut surface of the intermediate film using a tool having the repeating prism structure, thereby producing a sawtooth / undulating first surface.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method of manufacturing a light control film. Some embodiments relate to using the light control film in a holographic projector, image generation unit or head-up display. 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 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] In one aspect, a method of manufacturing a light control film is provided. The method includes providing a plurality of alternating layers of a transparent material and a light absorbing material. Each layer of the light absorbing material is interspersed between a respective layer of the transparent material. The plurality of alternating layers of the transparent material and the light absorbing material are stacked along a stacking direction. The stacking direction is perpendicular to a surface plane of at least one of the plurality of alternating layers. The method further includes cutting through the plurality of alternating layers of the transparent material and the light absorbing material in a first cutting plane and a second cutting plane with a straight blade, thereby producing an intermediate film comprising a first cut surface parallel to the first cutting plane and a second cut surface parallel to the second cutting plane. Each of the first and second cutting planes is oriented at an angle of less than or equal to 90° to the stacking direction. The method further includes hot embossing a repeating prism structure on the first cut surface of the intermediate film using a tool having the repeating prism structure, thereby producing a sawtooth / undulating first surface. Advantageously, the sawtooth / undulating surface of the light control film prevents direct reflection of sunlight into the eyebox. Furthermore, the light control film having a sawtooth surface can eliminate sunlight glare on a transparent optical surface. Moreover, the hot embossing process tends to provide a more reliable and cost effective method of manufacturing a light control film.

[0009] The method can further include hot embossing a repeating prism structure on the second cut surface of the intermediate film using the tool, thereby producing a sawtooth / undulating second surface.

[0010] The method can further include aligning ridges of the prism structure with edges of the layers of the light absorbing material prior to hot embossing the repeating prism structure. The first and second cutting planes can be parallel to each other.

[0011] The sawtooth first surface can include a first front facet at an angle greater than 30°, for example greater than 35°, to the first cut surface. The sawtooth second surface can include a second front facet at an angle greater than 30°, for example greater than 35°, to the second cut surface. Advantageously, the inventors have found that this means that reflected sunlight propagating through the sawtooth first and / or second surface cannot reach the eyebox. At the same time, due to the orientation of the strips of light absorbing material, any internal reflections are absorbed by the strips of light absorbing material. In some embodiments, the louver angle (measured from the surface normal) required for effective sunlight / reflection suppression is less than 60 degrees, for example less than 55 degrees.

[0012] The step of providing a plurality of alternating layers can include providing a first layer of the transparent material, providing a first layer of the light absorbing material on the first layer of the transparent material, providing a second layer of the transparent material on the first layer of the light absorbing material such that the first layer of the light absorbing material is between the first and second layers of the transparent material, and providing a second layer of the light absorbing material on the second layer of the transparent material such that the second layer of the transparent material is between the first and second layers of the light absorbing material.

[0013] The step of providing the first and / or second layer of the light absorbing material can include spray coating or solution coating.

[0014] Each transparent material layer includes an opposing rough surface configured to scatter light. Each rough surface can be a boundary between the respective layer of transparent material and the respective layer of light absorbing material. Advantageously, the rough surface of the transparent layer (e.g. PMMA sheet) tends to scatter light, thereby further preventing reflected sunlight from reaching the eyebox. Furthermore, the layer of light absorbing material deposited on the rough surface also results in a rough surface on the layer of light absorbing material. The rough surface on the layer of light absorbing material that increases light scattering means that the light absorbing layer can more effectively absorb reflected sunlight.

[0015] The layer of light absorbing material is a black adhesive or a black paint or a black silicone rubber or any combination thereof. The transparent material layer is made of PMMA or transparent silicone rubber or any combination thereof. Each layer of light absorbing material has a thickness of 10 pm - 100 pm. Each layer of transparent material has a thickness of 0.1 mm to 10 mm. The method of each of the above aspects can further comprise applying an anti-reflective coating on the jagged first and / or second surface. Each layer of transparent material is wedge shaped. The layers in the plurality of alternating layers are parallel to each other.

[0016] In another aspect, there is provided a method of manufacturing a vehicle head-up display. The method comprises: providing an optical component having a reflective surface arranged in a configuration that facilitates sunlight glare during operation of the head-up display; and disposing a light control film manufactured using the method of any preceding aspect on the reflective surface of the optical component.

[0017] In yet another aspect, there is provided a head-up display for a vehicle. The head-up display comprises: an optical component having a reflective surface arranged in a configuration that facilitates or is susceptible to sunlight glare during operation of the head-up display; and a light control layer manufactured using any preceding aspect disposed on the reflective surface of the optical component. The light control layer can be a reflection suppression component.

[0018] In the present disclosure, the term“replica” is used only to reflect that the spatially modulated light is split so that the composite light field is guided 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 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 an image rather than the image itself. Thus, it can be said that multiple replicas of the hologram are formed. 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” used herein is independent of propagation distance, so two light 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 the present 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.

[0019] 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 reproduction 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 the smallest feature size of the diffractive structure and the wavelength of the light (of the diffractive light field). According to the present disclosure, it can also be said that a“diffractive 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 diffractive light field from a diffractive structure to an observer is disclosed herein. The diffractive light field can form an image.

[0020] 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 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".

[0021] 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.

[0022] 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 pure phase hologram. Embodiments relate to pure phase holograms, but the present disclosure is equally applicable to pure amplitude holography.

[0023] 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.

[0024] 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 a plurality of available phase levels in a pure phase 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 a plurality of available complex modulation levels in a complex modulator for convenience.

[0025] 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.

[0026] 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

[0027] With reference to the following drawings, particular embodiments are described by way of example only:

[0028] FIG. 1 is a schematic diagram showing a reflective SLM producing a holographic reconstruction on a screen;

[0029] FIG. 2 shows a cross-section of an image for projection, including eight image regions / components V1-V8, and corresponding hologram channels H1-H8;

[0030] FIG. 3 shows a hologram displayed on an LCOS that directs light to a plurality of discrete areas;

[0031] FIG. 4 shows a hologram including a display asFIG. 2 and 3 a system of display devices for holograms computed as shown;

[0032] FIG. 5A a perspective view (not to scale) showing a system including two replicators arranged to expand a light beam in two dimensions;

[0033] FIG. 5B a perspective view (not to scale) showing another system including two replicators arranged to replicate a light beam in two dimensions;

[0034] FIG. 6A is a schematic side view (not to scale) 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, FIG. 6B is a schematic top view showing the optical path of image light from the light control film to the eyebox;

[0035] FIG. 7 is a cross-sectional view (not to scale) of an example light control film including a louver array;

[0036] FIG. 8 is a cross-sectional view (not to scale) of a light control film including a louver array;

[0037] FIG. 9A is a cross-sectional view (not to scale) of a layer of transparent material;

[0038] FIG. 9B is a cross-sectional view (not to scale) depicting the deposition of a layer of light-absorbing material on a layer of transparent material;

[0039] FIG. 9C is a cross-sectional view (not to scale) of a plurality of alternating layers of transparent material and light-absorbing material;

[0040] FIG. 9D is a cross-sectional view (not to scale) of an intermediate layer cut from the plurality of alternating layers; and

[0041] FIG. 9E is a cross-sectional view (not to scale) of a light control film manufactured from the intermediate layer.

[0042] In all of the drawings, like reference numerals will be used to designate like or similar portions. DETAILED DESCRIPTION

[0043] The application is not limited to the embodiments described below, but extends to the full scope of the claims. That is, the 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.

[0044] The singular form of a term can include the plural form unless otherwise specified.

[0045] A structure described as formed on top of / under or above / below 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 provided therebetween.

[0046] In describing a temporal relationship, for example when a temporal 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.

[0047] Although the terms "first", "second", and the like 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.

[0048] Features of different embodiments can be coupled or combined with each other, in part or in whole, and can interoperate with each other in different ways. Some embodiments can be performed independently of each other, or can be performed together in interdependent relationships.

[0049] In the present disclosure, the term "substantial" when applied to a structural unit of a device can be interpreted as a technical feature of the structural unit produced within the technical tolerance of the method used to manufacture it.

[0050] Traditional optical configuration for holographic projection

[0051] FIG. 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 silicon-based liquid crystal "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.

[0052] 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 FIG. 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 FIG. 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.

[0053] 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.

[0054] In these embodiments, the location in space of the holographic reconstruction is determined by the power (focusing) of the Fourier transform lens. In FIG. 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. In some embodiments of the disclosure, the lens of the observer’s eye performs the conversion of the hologram to the image.

[0055] Hologram computation

[0056] 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 property 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, such as techniques based on point cloud methods.

[0057] In some embodiments, the hologram engine is arranged to exclude from the hologram calculation the contribution of light blocked by the limiting aperture of the display system. UK patent application 2101666.2, filed on 5 February 2021 and incorporated herein by reference, discloses a first method of hologram calculation in which eye tracking and ray tracing are used to identify sub-regions of the display device for calculating a point cloud hologram that cancels ghost images. The sub-regions of the display device correspond to the apertures of the present disclosure and are used to exclude light paths from the hologram calculation. UK patent application 2112213.0, filed on 26 August 2021 and incorporated herein by reference, discloses a second method based on an improved Gerchberg-Saxton type algorithm that includes a step of light field clipping according to the pupil of the optical system during the hologram calculation. The clipping of the light field corresponds to the determination of the limiting apertures of the present disclosure. UK patent application 2118911.3, filed on 23 December 2021 and also incorporated herein by reference, discloses a third method of calculating a hologram that includes a step of determining the area of a so-called extended modulator formed by the hologram replicator. According to the present disclosure, the area of the extended modulator is also an aperture.

[0058] In some embodiments, a real-time engine is provided that 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 needed for display on the SLM. That is, in some embodiments, a repository of predetermined holograms is provided.

[0059] Eyeglass case using small display devices

[0060] In general terms, the present disclosure relates 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 examples, the image is a real image formed by a holographic reconstruction and the image is projected or relayed to the viewing plane. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed in free space or on a screen or other light receiving surface between the display device and the viewer is propagated to the viewer. In both cases, the image is formed by illuminating a diffractive pattern (e.g. a hologram or a phasegram) displayed on the display device.

[0061] 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 extent 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.

[0062] 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.

[0063] In some embodiments, 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 referred to as being "encoded" with / by the hologram - propagates directly to the eye of the viewer. The viewer can perceive a real or virtual image. In these embodiments, there is no intermediate holographic reconstruction / image 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.

[0064] 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 the 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.

[0065] 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 the LCOS can propagate through the pupil of the eye to form an image on the retina at a given eye position. The range of angles of light rays propagating from the display device that determine the portion of the image that is "visible" to the observer, which light rays can successfully propagate through the pupil of the 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 the observing window such as an eyebox).

[0066] 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.

[0067] A pupil expander addresses how to increase the range of angles of light rays propagating from the display device that can successfully propagate through the pupil of the eye to form an image. The display device is typically (relatively) small and the projection distance is (relatively) large. In some embodiments, the projection distance is at least one 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), e.g. at least two orders of magnitude larger.

[0068] The use of a pupil expander laterally increases the viewing area (i.e. the user's eyebox) so that some movement of the eye is possible while still enabling the user to see the image. As will be appreciated by the skilled person, in an imaging system, the viewing area (the user's eyebox) is the area in which the observer's eye can perceive the image. The present disclosure encompasses non-infinite virtual image distances - i.e. near-field virtual images.

[0069] Traditionally, two-dimensional pupil expanders comprise one or more one-dimensional optical waveguides, each formed using a pair of opposing reflective surfaces, where the output light from the surfaces forms a viewing window or eyebox. Light received from a display device (e.g. spatially modulated light from an LCOS) is replicated by the or each waveguide in order to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide expands the viewing window as a result of creating additional light rays or “replicas” by dividing the amplitude of the incoming wavefront.

[0070] The display device can have an active or pixel display area, the active or 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.

[0071] In some embodiments described by way of example according to the present disclosure by way of diffractive or holographic light field only, the hologram is configured to route light into multiple channels, each corresponding to a different portion (i.e. sub-region) of the image. The channels formed by the diffractive structure are referred to herein as “hologram channels”, simply to reflect that they are light channels 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 can also be a point cloud hologram. The hologram is described herein as routing light into multiple hologram channels to reflect that the image reconstructable from the hologram has a finite size, and can be arbitrarily divided into multiple image sub-regions, with each hologram channel to correspond to each image sub-region. Importantly, the hologram of this example is characterised by how it distributes image content when illuminated. Specifically and uniquely, the hologram divides image content by angle. That is, each point on the image relates to a unique light ray angle (at least a unique pair of angles, as the hologram is two-dimensional) in the spatially modulated light formed by the hologram when illuminated. For the avoidance of doubt, this hologram behaviour is not conventional. The spatially modulated light formed by this special type of hologram when illuminated can be divided into multiple hologram channels, with each hologram channel defined by a range of light ray angles (in two dimensions). It can be understood from the foregoing that any hologram channel (i.e. sub-range of light 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 angular sub-range 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 necessarily any evidence of the multiple discrete light channels.

[0072] Nonetheless, 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 different continuous portions or sub-regions of the spatially modulated light are reconstructed, different sub-regions of the image should be visible. Another identifying feature of this type of hologram is that the shape of a cross-section of any hologram channel substantially corresponds to (i.e. is substantially the same as) the shape of the entrance pupil, 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 a plurality of 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 diffracted light field.

[0073] The system can be provided in a compact and streamlined physical form. This makes the system suitable for a wide range of practical applications, including those where space is limited and asset value is high. For example, it can be implemented in a head-up display (HUD), such as a vehicle or car HUD.

[0074] According to the present disclosure, a pupil expansion is provided for the diffracted light, which can comprise diverging bundles of light rays. 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). 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 delivered to the observer is spatially modulated according to the hologram.

[0075] 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 a plurality of replicas or copies of the exit pupil (or of the 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 the eye of the observer can be located in order to see / receive the light output by the system.

[0076] Light tunnel

[0077] Holograms formed according to some embodiments angularly divide the image content to provide multiple hologram channels, which can have a cross-sectional shape defined by the aperture of the optical system. The holograms are computed to provide such guidance of the diffracted light field. In some embodiments, as mentioned above, this is achieved during the hologram computation by taking into account the aperture of the optical system (virtual or real).

[0078] FIG. 2 and 3 An example of this type of hologram, which can be used in conjunction with the pupil expanders disclosed herein, is shown. However, this example should not be seen as limiting the disclosure.

[0079] FIG. 2 An image 252 for projection is shown, comprising eight image regions / components VI to V8. Merely by way of example, FIG. 2 Eight image components are shown, and the image 252 can be divided into any number of components. FIG. 2 An encoded light pattern 254 (i.e. a hologram) is also shown, which can reconstruct the image 252 - e.g. when converted by a lens of a suitable viewing system. The encoded light pattern 454 comprises first to eighth sub-holograms or components HI to H8, corresponding to the first to eighth image components / regions VI to V8. FIG. 2 It is further shown how the hologram angularly resolves the image content. Thus, the hologram is characterized by its guidance of light. This is illustrated in FIG. 3 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 can also be envisaged. After propagation through the waveguide, the size and shape of the optimal discs can be related to the size and shape of the aperture of the optical system (e.g. the entrance pupil of the viewing system).

[0080] FIG. 4 A system 400 is shown, comprising a display device displaying a hologram computed as FIG. 2 and 3 shown. The display device is shown in more detail in

[0081] 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, which comprises 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.

[0082] The viewing system 400 also includes a waveguide 408 positioned between the LCOS 402 and the eye 405. The presence of the waveguide 408 enables all angular content from the LCOS 402 to be received by the eye, even at the relatively large projection distance shown. This is because the waveguide 408 acts as a pupil expander in a well-known manner, and so is only briefly described here.

[0083] In brief summary, FIG. 4 The waveguide 408 shown comprises a substantially 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 positioning of the waveguide 408 is 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 emitted 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 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 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.

[0084] FIG. 4 A total of nine "bounce" points B0to B8are shown along the length of the waveguide 408. Although as FIG. 2 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 408, 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 405 from each respective "bounce" point B0to B8. Furthermore, light from different angular portions of the image (V1to V8) reaches the eye 405 from each respective "bounce" point. Thus, in the example shown, each angular channel of encoded light reaches the eye from the waveguide 408 only once. FIG. 4

[0085] ​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.

[0086] Two-dimensional pupil expansion

[0087] Although FIG. 4 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 FIG. 4 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.

[0088] FIG. 5A A perspective view of system 500 is shown, which includes two replicators 504 and 506 arranged to extend beam 502 in two dimensions.

[0089] exist FIG. 5A In system 500, the first replicator 504 includes a first pair of surfaces stacked parallel to each other, arranged in a manner similar to FIG. 4 The waveguide 408 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 502 is guided to the input on the first replicator 504. Due to the internal reflection process between the two surfaces, and from one of the surfaces (the upper surface, such as...) FIG. 5A As shown, the light from each of the plurality of output points on the beam 502 is partially transmitted, which will be familiar to a skilled reader, and the light from beam 502 is replicated along the length of the first replicator 504 in a first direction. Thus, the first plurality of replica beams 508 are emitted from the first replicator 504 toward the second replicator 506.

[0090] The second replicator 506 includes a second pair of surfaces stacked parallel to each other, arranged to receive each collimated beam of the first plurality of beams 508, and further arranged to provide replication or pupil expansion by extending each of these 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. The second replicator is made rectangular so that it has a length along the first direction to receive the first plurality of beams 508, and a length along the second orthogonal direction to provide replication in that second direction. Due to the internal reflection process between the two surfaces, and from one of the surfaces (such as... FIG. 5APartial transmission of light from each of the plurality of output points on the upper surface shown, and the light from each beam within the first plurality of beams 508 being replicated in the second direction. Therefore, a second plurality of beams 510 is emitted from the second replicator 506, wherein the second plurality of beams 510 comprises a copy of the input beam 502 along each of the first and second directions. Thus, the second plurality of beams 510 can be considered as a two-dimensional grid or array comprising the replicated beams.

[0091] Therefore, it can be said that FIG. 5A The first and second replicators 504, 505 are combined to provide a two-dimensional replicator (or "two-dimensional pupil expander"). Thus, the replica beam 510 can be emitted along the optical path to the expanded eye box of a display system, such as a head-up display.

[0092] exist FIG. 5A In this system, the first replicator 504 is a waveguide comprising a pair of elongated, straight, linear reflective surfaces stacked parallel to each other; similarly, the second replicator 504 is a waveguide comprising a pair of rectangular reflective surfaces stacked parallel to each other. In other systems, the first replicator may be a solid, elongated, straight waveguide, and the second replicator may be a solid planar rectangular waveguide, wherein each waveguide comprises an optically transparent solid material, such as glass. In this case, the pair of parallel reflective surfaces are formed by a pair of opposing main sidewalls, optionally including corresponding reflective and reflective-transmitting surface coatings, which will be familiar to the skilled reader.

[0093] FIG. 5B A perspective view of system 500 is shown, which includes two replicators 520 and 540 arranged to replicate beam 522 in two-dimensional space, wherein the first replicator is a solid elongated waveguide 520 and the second replicator is a solid planar waveguide 540.

[0094] exist FIG. 5B In the system, the first replicator / waveguide 520 is arranged such that its pair of elongated parallel reflective surfaces 524a, 524b are perpendicular to the plane of the second replicator / waveguide 540. Therefore, the system includes an optical coupler arranged to couple light from the output port of the first replicator 520 to the input port of the second replicator 540. In the illustrated arrangement, the optical coupler is a plane / folding mirror 530, which is arranged to fold or redirect the optical path of light to achieve the desired optical coupling from the first replicator to the second replicator. FIG. 5BAs shown, the mirror 530 is arranged to receive light from the output port / reflective-transmissive surface 524a of the first replicator / waveguide 520, including a one-dimensional array of replicates extending in the first dimension. The mirror 530 is tilted so as to redirect the received light at an angle onto the light path of the input port in the (fully) reflective surface of the second replicator 540 to provide waveguide and replicate formation along its length in the second dimension. It will be appreciated that the mirror 530 is one example of an optical element capable of redirecting light in the manner shown, and that one or more other elements can alternatively be used to perform this task.

[0095] In the arrangement shown, the (partially) reflective-transmissive surface 524a of the first replicator 520 is adjacent to the input port of the first replicator / waveguide 520 that receives the input beam 522 at an angle to provide waveguide and replicate formation along its length in the first dimension. Thus, the input port of the first replicator / waveguide 520 is co-planar with the reflective-transmissive surface 524a at its input end. The skilled reader will appreciate that the input port of the first replicator / waveguide 520 can be at any other suitable location.

[0096] Thus, FIG. 5B The arrangement of the first replicator 520 and mirror 530 enables them to be provided as part of a first relatively thin layer in the first and third dimension planes (shown as the x-z plane). In particular, the first planar layer in which the first replicator 520 lies has a reduced dimension or "height" in the second dimension (shown as the y dimension). The mirror 530 is configured to direct light out of the first layer / planar layer in which the first replicator 520 lies (i.e. the "first planar layer") and towards the second layer / planar layer in which the second replicator 540 lies (i.e. the "second planar layer"), which lies above and is substantially parallel to the first layer / planar layer. Thus, the overall size or "height" of the system - including in the first and third dimensions (illustrated as the x-z plane) - in the second dimension (illustrated as the y dimension) - the first and second replicators 520, 540 and mirror 530 in the first and second planar layers of the stack - is compact. The skilled reader will appreciate that many variations of the arrangement of the first and second replicators 520, 540 and mirror 530 for implementing the present disclosure are possible and contemplated. FIG. 5B Many variations of the arrangement of the first and second replicators 520, 540 and mirror 530 are possible and contemplated.

[0097] The image projector can be arranged to project a diverging or diffractive light field. In some embodiments, the light field is encoded with a hologram. In some embodiments, the diffractive light field comprises diverging bundles of light rays. In some embodiments, the image formed by the diffractive light field is a virtual image.

[0098] In some embodiments, the first pair of parallel / complementary surfaces are elongate or elongated 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 between / from the first pair of parallel surfaces is such that the light propagates within the first waveguide pupil expander, the general direction of the light propagation being in the direction in which the first waveguide pupil expander is relatively longer (i.e. in its "elongate" direction).

[0099] Disclosed herein is a system that uses diffracted light to form an image and provides eye-box 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. The use of 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 diffracted light fields for 2D pupil expansion, for example, including beams of diverging (non-collimated) light rays.

[0100] In some embodiments, a display system includes a display device - such as a pixelated display device, for example a spatial light modulator (SLM) or a liquid crystal on silicon (LCoS) SLM - arranged to provide or form diffracted or diverging 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 beams of light 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 become larger in spatial extent than the exit pupil of the system (which is limited by the small display device having pixel sizes for light diffraction).

[0101] A 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 direction of propagation of the light field. Because the light is diffracted / divergent, the light field size increases with propagation distance.

[0102] In some embodiments, the diffracted light field is spatially modulated in accordance with 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 of the aspects and embodiments disclosed herein can be applied to the display of a plurality of holograms.

[0103] 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 (comprising 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 reflection between a third pair of parallel surfaces of the second waveguide pupil expander.

[0104] The first waveguide pupil expander can be arranged to provide pupil expansion or replication in a first direction, while the 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 preserve the pupil expansion already provided by the first waveguide pupil expander in the first direction, and to expand (or replicate) some, preferably most, preferably all, copies it receives from the first waveguide pupil expander in the different second direction. The second waveguide pupil expander can be arranged to receive the light field directly or indirectly from the first waveguide pupil expander. One or more other elements can be provided along the propagation path of the light field between the first and second waveguide pupil expanders.

[0105] The first waveguide pupil expander can be substantially elongated, and the second waveguide pupil expander can be substantially planar. The elongated shape of the first waveguide pupil expander can be defined by a length along a first dimension. The planar or rectangular shape of the second waveguide pupil expander can be defined by a length along a first dimension and a width or breadth along a second dimension substantially orthogonal to the first dimension. The dimension or length of the first waveguide pupil expander along its first dimension can correspond to the length or width of the second waveguide pupil expander along its first or second dimension, respectively. A first surface of the pair of parallel surfaces of the second waveguide pupil expander including its input port can be shaped, dimensioned and / or positioned to correspond to the area defined by the output port on the first surface of the pair of parallel surfaces on the first waveguide pupil expander, such that the second waveguide pupil expander is arranged to receive each copy output by the first waveguide pupil expander.

[0106] The first and second waveguide pupil expanders can collectively provide pupil expansion in a first direction and a second direction perpendicular to the first direction, optionally wherein a plane containing the first and second directions is substantially parallel to a plane of the second waveguide pupil expander. In other words, first and second dimensions defining a length and a width, respectively, of the second waveguide pupil expander can be parallel to the first and second directions, respectively (or parallel to the second and first directions, respectively), in which the waveguide pupil expanders provide pupil expansion. The combination of the first waveguide pupil expander and the second waveguide pupil expander can be generally referred to as a “pupil expander”.

[0107] It can be said that the expansion / copying provided by the first and second waveguide expanders has the effect of expanding an exit pupil of the display system in each of two directions. The area defined by the expanded exit pupil can in turn define an expanded eyebox area from which an observer can receive light of the input diffractive or divergent light field. It can be said that the eyebox area lies in or defines an observation plane.

[0108] The two directions of exit pupil expansion can be coplanar or parallel to the first and second directions in which the first and second waveguide pupil expanders provide copying / expansion. Alternatively, in an arrangement including other elements such as an optical combiner, for example 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 be non-coplanar and non-parallel to the first and second directions in which the first and second waveguide pupil expanders provide copying / expansion. For example, the exit pupil can be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide copying / expansion.

[0109] The observation plane and / or eyebox area can be non-coplanar or non-parallel to the first and second directions in which the first and second waveguide pupil expanders provide copying / expansion. For example, the observation plane can be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide copying / expansion.

[0110] In order to provide suitable launch conditions to enable internal reflections within the first and second waveguide pupil expanders, the elongate dimension of the first waveguide pupil expander can be tilted with respect to the first and second dimensions of the second waveguide pupil expander.

[0111] Combiner shape compensation

[0112] An advantage of projecting a hologram to an eyebox is that optical compensation can be encoded in the hologram (see, for example, European patent 2936252, incorporated herein by reference). The present disclosure is compatible with holograms that compensate for the complex curvature of an optical combiner that serves as part of the projection system. In some embodiments, the optical combiner is the windshield of a vehicle. Full details of this approach are provided in European patent 2936252 and are not repeated here as the detailed features of these systems and methods are not essential to the new teachings of this disclosure herein and are merely examples of configurations that benefit from the teachings of the present disclosure.

[0113] Control device

[0114] The present disclosure is also compatible with optical configurations that include control devices (e.g., light shutter devices) to control the delivery of light from the light passage hologram to the observer. The holographic projector can also include control devices arranged to control the delivery of the angular passage to the eyebox location. UK patent application 2108456.1, filed on June 14, 2021 and incorporated herein by reference, discloses at least one waveguide pupil expander and control devices. The reader will understand from at least this prior disclosure that the optical configuration of the control devices is based essentially on the eyebox location of the user and is compatible with any hologram computation method that implements the light passages described herein. It can be said that the control devices are light shutter or aperture devices. The light shutter devices can include a 1D array of apertures or windows, where each aperture or window can be independently switched between light transmitting and non-light transmitting states in order to control the delivery of the hologram light passages and their replicas to the eyebox. Each aperture or window can include a plurality of liquid crystal cells or pixels.

[0115] FIG. 6A and 6B The use of such light control films in head-up displays operating in vehicles is shown. In the arrangement shown, the head-up display 608 is an optical component at an interface with air (e.g., the second replicator 506 in Figure 5) 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 a vehicle dashboard. The transmissive surface of the optical component is covered by a light control film 606 that includes an array of one-dimensional louvers. The louvers can be in a substantially vertical plane, shown as the x-z plane. In the arrangement shown, the louvers are tilted with respect to the vertical plane (i.e., tilted from the orthogonal direction 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.

[0116] As FIG. 6AAs shown, image light from the heads-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 in the y-direction, toward the eyebox 612, where a virtual image (combined with the external scene viewed through the windshield) can be perceived by an observer (e.g., a vehicle driver).

[0117] FIG. 7 An example light control film including an array of louvers is shown in more detail.

[0118] The light control film 712 includes first (bottom / inner) and second (top / outer) surfaces defining a thickness of the light control film 712. In the arrangement shown, an optically transparent coating (also referred to as a “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 layers 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 needed on the first (bottom / inner) surface. The light control film 712 includes 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 pitch between louvers is small enough to optimize light absorption without causing diffraction or ghosting. In examples, the pitch between louvers can be on the order of 10-1000 pm, such as 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.

[0119] In the arrangement shown, each louver has a trapezoidal cross-section (i.e., with non-parallel, sloped sidewalls) that 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 sloped 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 slope angle 714. In the arrangement shown, the geometry of the louvers is identical, and the opposing sidewalls of each louver are sloped 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-sloped 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 oriented at any desired slope angle relative thereto.

[0120] Accordingly, image light 704 incident from the transmissive surface of the optical component of the head-up display passes through the relevant coating / planarization layer between the light control film 712 and 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, the angular range of which is required for the image to be visible at all locations within the eyebox. Accordingly, in examples where a special hologram is implemented, as described herein with reference to FIG. 2 to 4 the 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 light path angles (e.g. as a beam of light from a plurality of transmission points of the optical component, as described herein), which pass between the louvers to the eyebox, for example as shown by the solid arrows in FIG. 7 Due to the small spacing between the louvers, each replica formed at one of the plurality of transmission points along the transmissive surface of the optical component can be incident on a plurality of louvers of the light control film 712.

[0121] However, as described above, sunlight can be incident on the second (top / outer) surface of the light control film 712, as shown by 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 angle of the light ray shown by 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 the “first pass”, as shown by arrow 700. Alternatively, sunlight coupled into the light control film 712 at the planar top surface 720 can enter at an angle such that the light passes between the louvers into the optical component (not shown). In this case, sunlight reflected from the optical component is absorbed by one of the louvers in the “second pass” and is therefore 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 light path. FIG. 7Example light rays 702 of reflected sunlight are shown in dashed lines at an angle parallel to the light rays from the image light 704 of the head-up display that are transmitted between the louvers and out of the light control film 712. It will be appreciated that the illustrated image light rays 704 are merely one example of a plurality of light rays of a diverging bundle of replicas that are transmitted from a point of transmission of the optical component at an angle through between the louvers. Further, it will be appreciated that the same replicas can pass between other pairs of adjacent louvers at the same or different light ray angles. Thus, the reflected sunlight light rays 702 will follow the same optical path as the illustrated image light rays 704 to the eyebox and cause glare to the observer. As shown in 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 examples of FIG. 7

[0122] FIG. 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 FIG. 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 FIG. 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 sloped or ramped sidewalls). As described above, the light control film 812 can be formed on a planar reflective surface of an optical component of a head-up display, such as the second replicator 506 of FIG. 5.

[0123] According to the present application, the sunlight receiving surface 820 of the light control film 812 is serrated. In particular, the sunlight receiving surface 820 is serrated in coordination with the array of louvers. For example, the arrangement of serrations of the sunlight receiving surface 820 is coordinated, synchronized or aligned with the arrangement of the underlying array of louvers. Thus, the serrated configuration of the sunlight receiving surface 820 is one-dimensional and extends in a first dimension. In FIG. 8 embodiments of the present application, the serrated configuration includes a one-dimensional array of uniformly spaced serrations (also referred to as “bumps” or “teeth”) separated by surfaces that are angled relative to the plane of the light control film 812 / optical component (i.e., angled relative to the horizontal or x-y plane). Thus, in FIG. 8 embodiments of the present application, the cross-section of the sunlight receiving surface 820 has a generally serrated configuration. Further, in FIG. 8 embodiments of the present application, the uniformly spaced serrations are aligned with the louvers. Thus, it can be said that the periodicity of the serrations of the sunlight receiving surface 820 is substantially equal to the periodicity of the array of louvers.

[0124] As described above with reference to​FIG. 7 As noted, light transmitted by the light control film 812 is limited to an angular range 806 defined by the orientation, spacing, and geometry (e.g., sidewall angle) of the louvers. Thus, image light 804 from the optical components of the head-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 712 between the louvers and along the optical path to the eyebox. Moreover, as noted above with reference to FIG. 7 As noted, sunlight 800 coupled into the light control film 812 is either absorbed by one of the louvers and / or prevented from being transmitted to the eyebox due to the limited range of transmission angles 806.

[0125] Moreover, according to the present disclosure, 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 eyebox (e.g., in the direction away from the windshield that reflects image light to the eyebox in automotive applications, as described herein), as shown by the dashed line. In particular, due to the angled second (top / external) surface 820 of the light control film 812 over the optically transparent core region between the louvers, the surface normal is similarly angled relative to the normal of the plane (i.e., horizontal or x-y plane) of the light control film 812 / optical components. Thus, due to the law of reflection, the angle of the sunlight 800 changes (e.g., increases) compared to the angle of reflection from the planar surface 720 of the light control film 712. In examples, the angled surface is angled or tilted relative to the plane (normal) of the light control film 812 / optical components by an angle in the range of 15° to 75°, such as 30° to 60° in either direction. Importantly, the slope or degree of tilt of the tilted surface of the light receiving surface 820 is such that the light ray angle of the reflected sunlight 802 is not within the strict range of angles 806 permitted / required for the image light 804 from the head-up display, and thus the light rays of the sunlight do not reach the eyebox along the optical path and cause glare. FIG. 7

[0126] Method of manufacturing light control film

[0127] ​In some holographic HUDs, glass waveguides are used to achieve 2D pupil expansion to meet the required eyebox size and field of view. The coated waveguides, together with the optical turning components, form flat and highly reflective surfaces on the dashboard of the car, any of which can be a source of sun glare (i.e. sun reflection into the observer’s eye). The reflected sun light will enter the eyebox at an angle along the light path emitted by the HUD. This means that the driver can be blinded and / or distracted. Special light control films, also known as louver films, are commonly used to reduce unwanted reflections of light emitted by displays (e.g. LCDs) on the car windows. The light control films, typically made of plastic materials, have multiple transparent regions separated by light-absorbing louvers, which limit the light transmission to a fixed angular range defined by the size and orientation of the louvers. However, these conventional light control films cannot eliminate reflections from external light sources (e.g. the sun).

[0128] Reference is made below to FIG. 9A to 9E A method of manufacturing a light control film is described. The method comprises providing a layer of transparent material 902. FIG. 9A A cross-sectional view (not to scale) of the layer of transparent material 902 is shown. The layer of transparent material 902 has a first surface 904 and a second surface 906 opposite the first surface 904. The first and second surfaces 904, 906 are rough / uneven. Any light propagating through the first or second surfaces 904, 906 will be scattered. In this embodiment, the thickness d of the layer of transparent material 902 is 1 mm. In this embodiment, the layer of transparent material 902 is a PMMA layer.

[0129] The method further comprises depositing a layer of light-absorbing material 908 on the layer of transparent material 902. FIG. 9B A cross-sectional view (not to scale) of the layer of light-absorbing material 908 being sprayed onto the layer of transparent material 902 is depicted. In this embodiment, the light-absorbing layer 908 is deposited on the first surface 904. The light-absorbing layer 908 is a black adhesive or black paint. The light-absorbing layer 908 can be deposited using spray or solution coating techniques. The thickness of the layer of light-absorbing material 908 is 10 pm - 100 pm.

[0130] The method further comprises laminating another layer of transparent material onto the light-absorbing layer 908. The other layer of transparent material can be the same as the layer of transparent material 902, e.g. another PMMA sheet having opposing rough surfaces. The two layers of transparent material are parallel to each other. The light-absorbing layer 908 between the two layers of transparent material forms a cushion for the rough surfaces of the two layers of transparent material.

[0131] The method further comprises depositing another layer of light-absorbing material onto the layer of transparent material, similar to depositing the layer of light-absorbing material 908.

[0132] The stack of layers of transparent material and light absorbing material is repeated until a plurality of alternating layers 910 of transparent material and light absorbing material having a predetermined total thickness is obtained. The layers in the plurality of alternating layers 910 are stacked in a stacking direction 911. The thickness is defined in the stacking direction 911, i.e. the direction in which the alternating layers 910 are stacked. FIG. 9C A cross-sectional view (not to scale) of the plurality of alternating layers 910 of transparent material and light absorbing material is shown.

[0133] The method further comprises making a first cut 912 through the plurality of alternating layers 910 of transparent material and light absorbing material at a first location and at a cutting angle θ c with a straight blade. The cutting angle θ c may be less than or equal to 90°. The first cut 912 results in a top cut surface. The method further comprises making a second cut 914 through the plurality of alternating layers 910 of transparent material and light absorbing material at a second location and at a cutting angle θ c with a straight blade. The second cut 914 results in a bottom cut surface. The first and second cuts 912, 914 are substantially parallel to each other. The first and second cuts 912, 914 are substantially planar.

[0134] Each of the first and second cuts 912, 914 can further comprise orienting a cutting tool comprising the straight blade such that the straight blade faces the top 916 of the plurality of alternating layers 910. Once oriented in this way, the cutting tool is moved along a cutting path at the cutting angle θ c of the first or second cut plane is moved, thereby cutting through the plurality of alternating layers 910.

[0135] The first and second cuts will result in an intermediate layer comprising strips of light absorbing material located at periodic positions. The intermediate layer also has a top and a bottom cut surface. The top and bottom cut surfaces of the intermediate film can be polished. The angle between each strip of light absorbing material and the top cut surface is θ a, θ a = 90° - θ c . The intermediate film has the structure of a louver film. FIG. 9D A cross-sectional view (not to scale) of the intermediate layer cut from the plurality of alternating layers 910 is shown. The angle θ c is also the louver angle (measured from the surface normal) in use.

[0136] The method also includes a tool 919 for hot-pressing a 1D prism structure. Tool 919 may be a hot-pressing mold. Tool 919 includes a substrate 920 and a repeating prism structure 922 extending from the substrate. The repeating prism structure 922 includes a front facet 924 and a rear facet 926. The front facet 924 and the rear facet 926 at least partially define the prism. Tool 919 also includes a ridge defined by the portion where the front facet 924 and the rear facet 926 intersect. An angle θ exists between the front facet 924 and the top cut surface. f The back facet 926 is oriented in the same manner as the light-absorbing material strip. In other words, the angle θ between the back facet and the substrate... b The angle θ between the light-absorbing material strip and the cut surface of the intermediate film a same.

[0137] FIG. 9E A cross-sectional view (not to scale) of an intermediate layer undergoing thermo-imprinting to form a light-controlling film is shown. The method also includes aligning a tool 919 with the top cut surface of the intermediate layer such that the ridge of the tool 919 covers the edge of a periodically positioned strip of light-absorbing material, and the front facet 924 is covered with a transparent material.

[0138] The method also includes using an aligned tool 919 to thermally press the top cut surface of the intermediate film to form a serrated first surface. Each serration is formed between each pair of adjacent light-absorbing stripes. Each serration of the serrated first surface is at an angle of 180-θ. f Therefore, angle θ f The angle of the serrations on the serrated first surface is determined. In some embodiments, the angle θ is used to effectively suppress sunlight reflection. f Less than 150°, for example, less than 145°.

[0139] The method also includes aligning the tool 919 with the bottom cutting surface of the intermediate layer, such that the ridge of the tool 919 covers the edge of the periodically positioned light-absorbing material strips, and the front facet 924 is covered with a transparent material.

[0140] The method also includes using an aligned tool 919 to thermally emboss the bottom cut surface of the intermediate film to form a serrated second surface, thereby manufacturing a light-controlling film. Each serration is formed between pairs of adjacent light-absorbing strips. Each serration of the serrated second surface is at an angle of 180°-θ. f Therefore, angle θ f The angle of the serrations on the serrated second surface is determined. The serrations on the first and second serrated surfaces are parallel to each other.

[0141] The method may also include applying an anti-reflective coating to the serrated first and / or second surfaces of the light control film.

[0142] The light control film manufactured in the above manner can be positioned to cover the exit surface of a holographic HUD and be in direct contact with sunlight (transmitted through the windshield). The holographic HUD can output an emission requiring all light rays in a bundle to maintain their relative angle, phase, and intensity through any optical surface. In such a holographic HUD, a rough surface is not a suitable solution to mitigate glare. The inventors have found that a periodic sawtooth flat surface can solve the direct reflections from the outer surface. In particular, the inventors have found that an angle > 30° or even > 35° (defined as the angle between the sawtooth surface and the horizontal axis) of the sawtooth surface means that any reflected sunlight cannot reach the eyebox. At the same time, any reflection from the inner surface (e.g. glass or coating) is absorbed by the louvers (i.e. strips of light absorbing material) that are aligned with the HUD light rays.

[0143] Examples

[0144] The manufacturing process can comprise the following steps:

[0145] 1. Prepare a PMMA (about 1 mm thick) plate with rough (scattering) surfaces on both sides. Any light passing through the surface is scattered to different angles.

[0146] 2. Apply a thin (about 10-100 pm thick) black viscous paint / adhesive on one side of the PMMA plate, e.g. by spray or solution coating techniques.

[0147] 3. Laminate another rough PMMA plate to the black viscous surface. The two PMMA plates remain overall parallel, while the black viscous material forms a backing for the rough surface.

[0148] 4. Repeat 2 and 3 to make the total thickness of the stack close to the size required for the display application.

[0149] 5. Cut the stack at an angle using a knife with a straight blade. After cutting, sand the surface if necessary. The resulting film has periodic black viscous strips aligned at an angle. This angle can be parallel to the light rays of the display lamp in use.

[0150] 6. Prepare the tool for hot embossing of a 1D prism structure. The front facet of the prism determines the angle of the sawtooth surface, while the side facet of the prism is aligned with the louver angle in 5. The prism structure must have the same pitch as the black viscous strips in 5.

[0151] 7. In 5, hot emboss both surfaces of the louver film using the tool, ensuring that the prism angle falls exactly on the edge of the black viscous strips so as to form a flat sawtooth PMMA surface between each pair of adjacent black viscous strips. The sawtooth PMMA surfaces on both sides of the film are always parallel.

[0152] 8. Optionally, the anti-reflective coating is applied over the sawtooth surface.

[0153] In a variant, if the structure requires a louver angle variation through the part, the PMMA sheets used in steps 1 and 3 can be wedge-shaped, so that there is an angle difference between adjacent louver baffles.

[0154] Additional features

[0155] The methods and processes described herein can be embodied on a computer-readable medium. The term "computer-readable medium" includes media arranged to temporarily or permanently store data, such as random access memory (RAM), read only memory (ROM), a buffer, 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 the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methods described herein, in whole or in part.

[0156] 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 media include transitory media (e.g., a propagating signal that conveys the instructions).

[0157] 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 method of manufacturing a light control film, the method comprising: providing a plurality of alternating layers of transparent material and light absorbing material, wherein the alternating layers of transparent material and light absorbing material are stacked in a stacking direction; cutting through the plurality of alternating layers of transparent material and light absorbing material in a first cutting plane and a second cutting plane with a straight blade, thereby producing an intermediate film comprising a first cut surface parallel to the first cutting plane and a second cut surface parallel to the second cutting plane, wherein each of the first and second cutting planes is oriented at an angle of less than 90° to the stacking direction; and thermally embossing a repeating prism structure on the first cut surface of the intermediate film using a tool having the repeating prism structure, thereby producing a serrated first surface.

2. The method of claim 1, further comprising thermally embossing the repeating prism structure on the second cut surface of the intermediate film using the tool, thereby producing a serrated second surface.

3. The method of claim 1 or 2, further comprising aligning ridges of the prism structure with edges of the layers of light absorbing material prior to thermally embossing the repeating prism structure. The repeating prism structure comprises:

4. The method of claim 3, wherein, a substrate; a front facet extending from the substrate; and a back facet extending from the substrate; wherein the front facet and the back facet at least partially define a prism, and wherein a ridge of the prism structure is defined by an intersection of the front facet and the back facet. An angle between the back facet and the substrate is the same as an angle between the layers of light absorbing material and the first and / or second cut surface.

5. The method of claim 4, wherein, The first and second cutting planes are parallel to each other.

6. The method of claim 1, wherein, The serrated first surface comprises first front facets having an angle of greater than 30°.

7. The method of claim 1, wherein, The serrated second surface comprises second front facets having an angle of greater than 30°.

8. The method of claim 2, wherein, Providing the plurality of alternating layers comprises:

9. The method of claim 1, wherein, providing a first layer of transparent material; providing a first layer of light absorbing material on the first layer of transparent material; providing a second layer of transparent material on the first layer of light absorbing material, such that the first layer of light absorbing material is between the first and second layers of transparent material; and providing a second layer of light absorbing material on the second layer of transparent material, such that the second layer of transparent material is between the first and second layers of light absorbing material.

10. The method of claim 1, wherein, Providing the plurality of alternating layers of transparent material and light absorbing material comprises spray coating or solution coating at least one layer of light absorbing material on at least one layer of transparent material.

11. The method of claim 1, wherein, Each layer of transparent material comprises opposing rough surfaces configured to scatter light.

12. The method of claim 3, wherein, The layers of light absorbing material are black adhesive or black paint or black silicone rubber or any combination thereof.

13. The method of claim 1, wherein, The transparent material comprises PMMA or transparent silicone rubber or any combination thereof.

14. The method of claim 1, wherein, Each layer of light absorbing material has a thickness of 10 pm to 100 pm.

15. The method of claim 1, wherein, Each layer of transparent material has a thickness of 0.1 mm to 10 mm.

16. The method of claim 2, applying an anti-reflective coating on the serrated first and / or second surface.

17. The method of claim 1, wherein, Each layer of transparent material is wedge-shaped.

18. The method of claim 1, wherein, The layers of the plurality of alternating layers are parallel to each other.

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