Holographic waveguide

Abstract

A head-up display method for a vehicle. A first step includes displaying a hologram of an image on a display device and spatially modulating light according to the displayed hologram to form a holographic wavefront. A second step includes replicating the holographic wavefront in a first direction using a first pupil expander to form an array of 1D replicas of the holographic wavefront. A third step includes using a windshield of the vehicle as a second pupil expander to form an array of 2D replicas of the holographic wavefront from the array of 1D replicas.

Description

Technical Field

[0001] This disclosure relates to methods for optical engines and relaying diffracted light fields. More specifically, this disclosure relates to projection systems and methods for replicating holograms using waveguides. Some embodiments relate to holographic projectors, image generation units, or head-up displays. Background Technology

[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 using well-known interferometry techniques to form a holographic record, or "hologram," including interference fringes. The hologram can be reconstructed by illuminating it with appropriate light to form a two-dimensional or three-dimensional holographic reconstruction or replay image representing the original object.

[0003] Computer-generated holography can numerically simulate interference processes. Computer-generated holograms can be calculated using techniques based on mathematical transformations such as Fresnel or Fourier transforms. These types of holograms are called Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. A Fourier hologram can be considered a Fourier domain / plane representation of an object or a frequency domain / plane representation of an object. For example, computer-generated holograms can also be calculated using coherent ray tracing or point cloud techniques.

[0004] Computer-generated holograms can be encoded on spatial light modulators arranged to modulate the amplitude and / or phase of incident light. For example, optical modulation can be achieved using electrically addressable liquid crystals, optically addressable liquid crystals, or micromirrors.

[0005] Spatial light modulators typically comprise multiple individually addressable pixels, which may also be referred to as cells or elements. The light modulation scheme can be binary, multilevel, or sequential. Alternatively, the device can be sequential (i.e., excluding pixels), so the light modulation can be continuous on the device. Spatial light modulators can be reflective, meaning the modulated light is output as reflected light. Spatial light modulators can also be transmissive, meaning the modulated light is output as transmitted light.

[0006] The system described herein can be used to provide a holographic projector. This type of projector is already being used in head-up displays (HUDs). Summary of the Invention

[0007] The aspects of this disclosure are defined in the appended independent claims.

[0008] In summary, a display system is disclosed herein, including a display device, a pattern generator, and a waveguide. The display device is arranged to display a diffraction pattern corresponding to an image and spatially modulates light according to the diffraction pattern to form a diffracted light field. The pattern generator is arranged to determine the diffraction pattern. The diffraction pattern can be arranged to distribute light at an angle within the diffracted light field according to spatial positions within the image, such that consecutive angular ranges (i.e., angular channels) of the diffracted light field respectively correspond to consecutive regions of the image. The waveguide includes a pair of parallel reflective surfaces arranged therebetween to diffract the light field. The first surface of the pair of parallel reflective surfaces is partially reflective-transmissive to form an output (in the waveguide direction) including multiple emission or replication regions for the diffracted light field. The pattern generator can be arranged to modify the diffraction pattern to at least partially compensate for the reduction in emission intensity from each consecutive emission or replication region of the waveguide caused by partial reflection-transmission at the first surface during waveguide operation.

[0009] More specifically, a projection system is disclosed herein, including a display device, a hologram engine, and a waveguide. The display device is arranged to display a hologram of an image and spatially modulates light according to the hologram to form a holographic wavefront (or holographic light field). The hologram engine is arranged to compute the hologram. The hologram can be arranged to distribute light at angles within the holographic wavefront according to spatial locations within the image, such that consecutive angular ranges (i.e., angular channels) of the holographic wavefront correspond to consecutive regions of the image. The waveguide includes a pair of parallel reflective surfaces arranged to guide the holographic wavefront therebetween. The first surface of the pair of parallel reflective surfaces is partially reflective-transmissive to form an output (in the waveguide direction) including multiple emission regions for the holographic wavefront. The hologram engine is arranged to modify the hologram to at least partially compensate for the reduction in emission intensity from each consecutive emission region of the waveguide caused by partial reflection-transmission at the first surface during waveguide operation.

[0010] The inventors have recognized that a holographic engine can compensate for the reduction in waveguide emission intensity, thus eliminating the need for complex hierarchical coatings on the waveguide's output surface. This solution is possible because of the way holograms distribute image content in the spatial domain through angles within the holographic domain. This type of holographic behavior is not common, and the inventors have leveraged its characteristics to address fundamental problems with waveguide pupil expanders. In this respect, it can be said that the inventors have discovered a synergy between the so-called channel hologram of this disclosure and the transmission of holographically encoded light to the observer via a waveguide-based pupil expander. The holographically encoded light (i.e., the holographic wavefront or light field) can be transformed into a (meaningful) image by the observer (e.g., using the lens of the eye). This disclosure represents a significant difference from conventional techniques that rely on designing dielectric stacks to mitigate the intensity of waveguide emission. Fabrication is particularly complex when these stacks need to operate at broadband wavelengths, such as in the visible spectrum, because the thickness of at least some layers in the stack must be allowed to vary with the propagation distance, or a large number of different layers must be used.

[0011] In some embodiments, the angular distribution of light in the holographic domain and its correspondence with image content in the spatial domain are in a common direction, such as one-dimensionality in a projection system. The angular channels of the holographic wavefront formed by the hologram can extend in a plane. In some embodiments, the angular channels extend in the direction of waveguide and / or pupil replication / expansion. However, when non-uniform optical combiners (e.g., vehicle windshields) are used in optical systems, more complex correlations form between the angular channels and image content—falling within the scope of this disclosure. These interrelationships, however complex, can be identified or mapped using techniques such as ray tracing.

[0012] Projection systems can be arranged to form multiple observation positions within an observation area. The term "multiple observation positions" is used only to reflect the existence of more than one possible observation position, and that each observation position can receive the entire holographic light field. Therefore, an observer can see the entire field of view of the image from all observation positions. The term "multiple observation positions" is not used here to reflect the existence of a finite number of observation positions. There may be an infinite number of possible observation positions. Observation positions can be defined or fall within an observation window, region, or volume. In head-up display applications, the observation window is called an eyebox, and the observation position is called an eyebox position. The observer (or more broadly, the observation system) may include an entrance pupil or an observation pupil. The entire holographic wavefront can be received through the observation pupil at each observation position. However, it is worth noting that different successive angular ranges of the holographic wavefront can be received from different emission regions of the waveguide through the observation pupil, as explained in the detailed description below.

[0013] At least partial compensation for the reduction in emission intensity from each successive emission region may include altering (e.g., uniformly increasing or decreasing) the intensity of at least one angular channel of the holographic wavefront. The intensity of the angular channel may be altered by changing the intensity of the corresponding image portion before hologram computation, or by altering the intensity of image points (or waves propagating from them) while constructing the point cloud hologram, or by any other conceivable technique. Altering the intensity may include applying a gain or loss factor to image pixel values, image points, or propagating light waves before, during, or after hologram computation.

[0014] Compensating for at least partial reduction in emission intensity from each consecutive emission region may include altering the intensity of at least a consecutive region of the target image of the hologram before computing the hologram. Altering the intensity may include applying a gain or loss factor.

[0015] Hologram computation may include multiple iterations of phase retrieval algorithms and / or point cloud methods, but those skilled in the art of holography will understand that other techniques for hologram computation may be compatible with this disclosure.

[0016] The second surface in the pair of parallel reflecting surfaces can be substantially total reflective. However, this is not a necessary feature, and this disclosure can be readily adapted to compensate for defective reflections from the second surface during waveguide operation.

[0017] The projection system may also include an audience tracking system arranged to determine the observer's position within an observation window downstream of the waveguide. The hologram engine may be arranged to identify the correlation between the angular channels and emission regions of the holographic wavefront based on the determined observer position. In some embodiments, prior to the core method of this disclosure, a lookup table of data (similar to Table 1 in the detailed description below) is used. The correlation may depend on the observer's observation position and other attributes, or the observation system, such as interpupillary distance. A one-to-one or one-to-many association may exist between the emission region and the image portion. The size of the image portion may be constant or variable. The size of the image portion may also depend on characteristics of the observation system, such as interpupillary distance.

[0018] A head-up display method for a vehicle is also disclosed herein. The method includes first, second, and third steps that can be performed sequentially. The first step includes displaying a hologram of an image on a display device and spatially modulating light according to the displayed hologram to form a holographic wavefront. The second step includes replicating the holographic wavefront in a first direction using a first pupil expander to form a 1D replica array of the holographic wavefront. The third step includes using the vehicle's windshield as a second pupil expander to form a 2D replica array of the holographic wavefront from the 1D replica array.

[0019] The inventors have recognized that a second waveguide and an optical combiner can be combined. More specifically, the inventors were surprised to find that the two surfaces of a vehicle windshield (more broadly, a transparent optical combiner) are sufficient to provide the function of a second waveguide. That is, the two surfaces of the vehicle windshield can waveguide a holographic wavefront between them. This reduces the size, complexity, and cost of head-up displays.

[0020] Vehicle windshields typically have high transmittance, making it generally undesirable to apply optical coatings that improve waveguide visibility. However, this disclosure relates to waveguides on holographic wavefronts, in some embodiments of which use at least one narrowband light source (e.g., for highly coherent lasers). Therefore, correspondingly, narrowband filters can be applied to the windshield without significantly reducing visibility in the visible spectrum. Furthermore, vehicle windshields typically have complex curvature, making them undesirable for pupil expansion through the waveguide, but the effects of this complex curvature are not catastrophic for image quality when the hologram is characterized by image content distributed angularly in the spatial domain within the holographic domain. In fact, curvature merely causes a change in the correlation between the position on the waveguide and the position of the image content (or the light angle within the holographic light field). More specifically, the angular distribution of the image content is stretched or compressed in the holographic domain. These effects are tolerable.

[0021] A 2D duplicate array can be formed by arranging a waveguide 1D duplicate array between the inner and outer surfaces of the windshield.

[0022] Waveguides can include multiple portions of transmission and reflection at multiple different locations on the inner surface of a windshield.

[0023] The method may also include arranging holograms and display devices to guide holographic wavefronts of different continuous angular ranges from different portions of the inner surface of the windshield to an observer inside the vehicle.

[0024] The method may also include converting the holographic wavefront into a (meaningful) image. The image content can appear inside the vehicle, outside the vehicle, or both. The image can include virtual images, real images, and / or both.

[0025] The method may also include computing a hologram such that, when illuminated, the hologram divides the image content in the spatial domain by angles within the holographic domain. The image content is then transmitted to the observer in the holographic domain. That is, the holographic wavefront is transmitted to the observer. The observer performs a hologram-to-image conversion.

[0026] A system comprising a light engine and a windshield for a vehicle is also disclosed. The light engine includes a display device and a first pupil expander. The display device is arranged to display a hologram of an image and spatially modulates light according to the displayed hologram to form a holographic wavefront. The first pupil expander is arranged to receive the holographic wavefront and replicate the holographic wavefront in a first direction to form a 1D replica array of the holographic wavefront. The windshield of the vehicle is arranged to receive a 1D replica array of the holographic wavefront from the first pupil expander and expand the 1D replica array in a second direction to form a 2D replica array of the holographic wavefront. The second direction may be substantially perpendicular to the first direction.

[0027] This system can be configured to distribute light at angles within the holographic wavefront based on spatial location within the image, such that successive angular ranges or "channels" of the holographic wavefront correspond to successive regions of the image. An observer can receive different successive angular ranges of the holographic wavefront from different parts of the windshield. Different parts of the windshield may correspond to different numbers of internal reflections within the windshield.

[0028] The windshield and / or hologram can be arranged such that the replica has substantially uniform intensity. The windshield may include optical elements (e.g., coatings, such as dielectric stacks) configured to increase the reflectivity of the windshield surface within at least one narrow band of the visible spectrum. At least one narrow band may correspond to the wavelength of spatially modulated light. The term "narrow band" may refer to less than 20 nm or less than 10 nm, such as 7 ± 2 nm full-width half-peak (or full-width half-peak).

[0029] In this disclosure, the term "replica" is used only to reflect that spatially modulated light is segmented such that a 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 copying event—such as partial reflection-transmission of a pupil dilator. Each replica propagates along a different optical path. Some embodiments of this disclosure relate to the propagation of light encoded with a hologram rather than an image—that is, light spatially modulated with a hologram of an image rather than the image itself. Thus, it can be said that multiple replicas of a hologram are formed. Those skilled in the art of holography will understand that the composite light field associated with the propagation of hologram-encoded light will vary with the propagation distance. The term "replica" as used herein is independent of propagation distance, so two optical branches or paths associated with a copying event are still referred to as "replicas" of each other, even if the branches have different lengths, causing the composite light field to evolve differently along each path. That is, according to this disclosure, even if two composite light fields are associated with different propagation distances, they are still considered "replicas"—assuming they originate from the same copying event or a series of copying events.

[0030] According to this disclosure, a "diffracted light field" is a light field formed by diffraction. A diffracted light field can be formed by illuminating a corresponding diffraction pattern. According to this disclosure, an example of a diffraction pattern is a hologram, and an example of a diffracted light field is a holographic light field or a holographically reconstructed light field that forms an image. The holographic light field forms a (holographic) reconstruction of an image on the playback plane. A holographic light field can also be referred to as a holographic wavefront to avoid confusion with a light field display. A holographic light field or wavefront propagating from a hologram to the playback plane can be said to include light encoded by the hologram or light in the holographic domain. A diffracted light field is characterized by a diffraction angle determined by the minimum characteristic size of the diffracting structure and the wavelength of the light (of the diffracted light field). According to this disclosure, a "diffracted light field" can also be said to be a reconstructed light field formed on a plane spatially separated from the corresponding diffracting structure. This document discloses an optical system for propagating a diffracted light field from a diffracting structure to an observer. The diffracted light field can form an image.

[0031] The term "hologram" is used to refer to a record containing amplitude or phase information about an object, or some combination thereof. The term "holographic reconstruction" is used to refer to the optical reconstruction of an object formed by illuminating a hologram. The system disclosed herein is described as a "holographic projector" because the holographic reconstruction is a real image and spatially separate from the hologram. The term "reproduced field" is used to refer to the 2D region within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator including pixels, the reproduced field will be repeated in the form of multiple diffraction orders, where each diffraction order is a copy of the zero-order reproduced field. The zero-order reproduced field typically corresponds to the preferred or master reproduced field because it is the brightest reproduced field. Unless otherwise explicitly stated, the term "reproduced field" should be considered to refer to the zero-order reproduced field. The term "reproduced plane" is used to refer to a plane in space containing all reproduced fields. The terms "image," "reproduced image," and "image region" refer to the region of the reproduced field illuminated by the light reconstructed by the hologram. In some embodiments, an “image” may include discrete points, which may be referred to as “image points” or simply as “image pixels” for convenience.

[0032] The terms “encoding,” “writing,” and “addressing” are used to describe the process of providing multiple corresponding control values ​​to multiple pixels of an SLM, each determining the modulation level of the pixel. In other words, the pixels of an SLM are configured to “display” an optical modulation distribution in response to receiving multiple control values. Therefore, it can be said that an SLM “displays” a hologram, and a hologram can be considered an array of optical modulation values ​​or levels.

[0033] It has been found that acceptable-quality holographic reconstructions can be formed from "holograms" containing only phase information related to the Fourier transform of the original object. Such holographic records can be referred to as phase-only holograms. While the embodiments relate to phase-only holograms, this disclosure is equally applicable to amplitude-only holography.

[0034] This disclosure is equally applicable to forming holographic reconstructions using amplitude and phase information associated with the Fourier transform of the original object. In some embodiments, this is achieved by using complex modulation of a so-called fully complex hologram containing amplitude and phase information associated with the original object. Because the value (gray level) assigned to each pixel of the hologram has amplitude and phase components, such a hologram may be called a fully complex hologram. The value (gray level) assigned to each pixel can be represented as a complex number with amplitude and phase components. In some embodiments, a fully complex computer-generated hologram is computed.

[0035] The term "phase delay" can be a shorthand for the phase value, phase component, phase information, or simply phase of a pixel in a computer-generated hologram or spatial light modulator. That is, any phase value described is actually a number representing the amount of phase delay provided by that pixel (e.g., in the range of 0 to 2π). For example, a spatial light modulator is described as a pixel having a π / 2 phase value causing a π / 2 radian phase delay in the received light. In some embodiments, each pixel of a spatial light modulator can operate on one of a plurality of possible modulation values ​​(e.g., phase delay values). The term "gray level" can be used to refer to a plurality of available modulation levels. For example, the term "gray level" can be used for convenience to refer to a plurality of available phase levels in a phase modulator, even if different phase levels do not provide different shades of gray. For convenience, the term "gray level" can also be used to refer to a plurality of available complex modulation levels in a complex modulator.

[0036] Therefore, a hologram comprises an array of gray levels, i.e., an array of optical modulation values, such as phase delay values ​​or complex modulation values. A hologram is also considered a diffraction pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light whose wavelength is relative to (typically less than) the pixel spacing of the spatial light modulator. Reference is made herein to combining holograms with other diffraction patterns, such as those used as lenses or gratings. For example, a diffraction pattern used as a grating can be combined with a hologram to translate the playback field on the playback plane, or a diffraction pattern used as a lens can be combined with a hologram to focus the holographic reconstruction onto the playback plane in the near field.

[0037] Although different embodiments and groups of embodiments may be disclosed separately in the detailed description below, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and substitutions of the features disclosed in this disclosure are contemplated. Attached Figure Description

[0038] Referring to the following figures, specific embodiments are described by way of example only:

[0039] Figure 1This is a schematic diagram illustrating a reflective SLM that generates holographic reconstruction on a screen;

[0040] Figure 2 The image used for projection is shown, including eight image regions / components V1 to V8, and cross-sections of the corresponding holographic channels H1-H8;

[0041] Figure 3 A hologram is shown on an LCOS that directs light to multiple discrete regions;

[0042] Figure 4 The diagram shows, including, as shown Figure 2 and 3 The system of the display device for the calculated hologram;

[0043] Figure 5A A perspective view of a first example two-dimensional pupil expander including two replicators is shown;

[0044] Figure 5B A perspective view of a second example two-dimensional pupil expander including two replicators is shown;

[0045] Figure 6 An example of using a waveguide pupil extension in a vehicle windshield according to this disclosure is shown;

[0046] Figure 7 An example waveguide pupil expander including eight emission zones is shown;

[0047] Figure 8 This disclosure illustrates the problem it addresses; and

[0048] Figure 9 An embodiment is shown that includes an initial step of preprocessing the target image before hologram computation.

[0049] In all the accompanying drawings, the same reference numerals will be used to refer to the same or similar parts. Detailed Implementation

[0050] This invention is not limited to the embodiments described below, but extends to the full scope of the appended claims. That is, the invention may be implemented in different forms and should not be construed as limited to the described embodiments, which are illustrated for illustrative purposes.

[0051] Unless otherwise stated, singular terms may include plural forms.

[0052] A structure described as being formed above or below another structure should be interpreted as including situations where the structures are in contact with each other, and also including situations where a third structure is placed between them.

[0053] When describing temporal relationships, such as when the chronological order of events is described as “after,” “following,” “next,” “before,” etc., this disclosure should be considered to include both consecutive and discontinuous events, unless otherwise stated. For example, unless terms such as “exactly,” “immediately,” or “directly” are used, the description should be considered to include discontinuous cases.

[0054] Although the terms “first,” “second,” etc., may be used herein to describe various elements, these elements are not limited by these terms. These terms are used only to distinguish individual elements. For example, without departing from the scope of the appended claims, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element.

[0055] Features of different embodiments may be coupled or combined with each other in part or in whole, and may interoperate differently with each other. Some embodiments may be implemented independently of each other, or may be implemented together in a mutually dependent relationship.

[0056] In this disclosure, the term "basic" when applied to a structural unit of an apparatus can be interpreted as the technical feature of the structural unit produced within the technical tolerances of the method used to manufacture it.

[0057] Traditional optical configuration of holographic projection

[0058] Figure 1 An embodiment is shown in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object used for reconstruction. Therefore, the hologram can be described as a Fourier domain, frequency domain, or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) device. The hologram is encoded on the spatial light modulator, and a holographic reconstruction is formed at the reproduction field, such as a light-receiving surface like a screen or diffuser.

[0059] A light source 110, such as a laser or laser diode, is configured to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes the approximately plane wavefront of the light to be incident on the SLM. Figure 1 In this embodiment, the wavefront is oriented off-normal (e.g., two or three degrees away from a plane that is truly orthogonal to the transparent layer). However, in other embodiments, a generally planar wavefront is provided with normal incidence, and beam splitters are arranged to separate the input and output optical paths. Figure 1In the illustrated embodiment, the arrangement is such that light from the light source is reflected from the mirrored rear surface of the SLM and interacts with the light modulation layer to form an outgoing wavefront 112. The outgoing wavefront 112 is applied to an optics device including a Fourier transform lens 120, the focal point of which is located at screen 125. More specifically, the Fourier transform lens 120 receives the modulated beam from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at screen 125.

[0060] It is worth noting that in this type of hologram, each pixel of the hologram contributes to the overall reconstruction. There is no one-to-one correlation between a specific point (or image pixel) on the playback field and a specific optical modulation element (or hologram pixel). In other words, the modulated light leaving the optical modulation layer is distributed across the entire playback field.

[0061] In these embodiments, the spatial position of the holographic reconstruction is determined by the diopter (focusing) of the Fourier transform lens. Figure 1 In the illustrated embodiments, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and performs a Fourier transform optically. Any lens can act as a Fourier transform lens, but the performance of the lens will limit the accuracy of the Fourier transform it performs. Those skilled in the art understand how lenses can be used to perform optical Fourier transforms. In some embodiments of this disclosure, the lens of an observer's eye performs a hologram-to-image conversion.

[0062] Holographic computation

[0063] In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or a Fourier-based hologram, wherein the image is reconstructed in the far field by utilizing the Fourier transform properties of a positive lens. The Fourier hologram is computed by Fourier transforming the desired light field in the reproduction plane back to the lens plane. The Fourier transform can be used to compute computer-generated Fourier holograms. By way of example only, the embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms. This disclosure is equally applicable to Fresnel holography and Fresnel holograms that can be computed using similar methods. In some embodiments, the hologram is a phase or pure phase hologram. However, this disclosure is also applicable to holograms computed using other techniques, such as point cloud-based methods.

[0064] In some embodiments, the hologram engine is arranged to exclude the contribution of light blocked by the limiting aperture of the display system from the hologram calculation. UK Patent Application 2101666.2, filed February 5, 2021 and incorporated herein by reference, discloses a first hologram calculation method in which eye tracking and ray tracing are used to identify sub-regions of the display device for calculating a point cloud hologram of a ghost-free image. The sub-regions of the display device correspond to the aperture of this disclosure and are used to exclude light paths from the hologram calculation. UK Patent Application 2112213.0, filed August 26, 2021 and incorporated herein by reference, discloses a second method based on an improved Gerchberg-Saxton type algorithm, which includes a step of clipping the light field according to the pupil of the optical system during hologram calculation. The clipping of the light field corresponds to the determination of the limiting aperture of this disclosure. UK Patent Application 2118911.3, filed on December 23, 2021, and also incorporated herein by reference, discloses a third method for calculating holograms, which includes the step of determining a region of a so-called extended modulator formed by a hologram replicator. According to this disclosure, the region of the extended modulator is also the aperture.

[0065] In some embodiments, a real-time engine is provided, arranged to receive image data using an algorithm and compute holograms in real time. In some embodiments, the image data is video comprising a sequence of image frames. In other embodiments, the holograms are pre-computed, stored in computer memory, and retrieved as needed for display on an SLM. That is, in some embodiments, a library of predetermined holograms is provided.

[0066] Large viewing window and long range using a small display device

[0067] In summary, this disclosure relates to image projection. It relates to methods of image projection and image projectors including display devices. This disclosure also relates to projection systems including an image projector and an observation system, wherein the image projector projects or relays light from the display device to the observation system. This disclosure is equally applicable to monocular and binocular observation systems. An observation system may include one or more eyes of an observer. An observation system includes optical elements with optical power (e.g., the lens of the human eye) and an observation plane (e.g., the retina of the human eye). The projector may be referred to as a “light engine.” The display device and the image formed (or perceived) using the display device are spatially separated from each other. The observer forms or perceives the image on the display plane. In some embodiments, the image is a virtual image, and the display plane may be referred to as a virtual image plane. In other examples, the image is a real image formed by holographic reconstruction, and this image is projected or relayed to the observation plane. In these other examples, spatially modulated light from an intermediate holographic reconstruction formed in free space or on a screen or other light-receiving surface between the display device and the observer is propagated to the observer. In both cases, an image is formed by illuminating a diffraction pattern (such as a hologram or phase hologram) displayed on a display device.

[0068] Display devices consist of pixels. The pixels of a display can show diffraction patterns or structures of diffracted light. Diffracted light can form an image on a plane spatially separate from the display device. According to well-known optical principles, the magnitude of the maximum diffraction angle is determined by the pixel size and other factors such as the wavelength of light.

[0069] In this embodiment, the display device is a spatial light modulator, such as a liquid crystal on silicon (“LCOS”) spatial light modulator (SLM). Light travels from the LCOS to the observing entity / system, such as a camera or eye, within a diffraction angle range (e.g., from zero to the maximum diffraction angle). In some embodiments, amplification techniques may be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of the LCOS.

[0070] In some embodiments, the hologram itself (light) propagates to the eye. For example, the spatially modulated light of the hologram (not yet fully converted into a holographic reconstruction, i.e., an image)—which can be informally referred to as being “encoded” by / with the hologram—propagates directly to the observer's eye. The observer can perceive a real or virtual image. In these embodiments, no intermediate holographic reconstruction / image is formed between the display device and the observer. Sometimes it is said that in these embodiments, the lens of the eye performs the holographic-to-image conversion or transformation. The projection system or light engine can be configured to allow the observer to effectively look directly at the display device.

[0071] The "light field" mentioned here refers to a "composite light field." The term "light field" simply means a light pattern of finite size in at least two orthogonal spatial directions, such as x and y. The term "complex" as used here simply indicates that the light at each point in the light field can be defined by amplitude and phase values, and therefore can be represented by a complex number or a pair of values. For holographic computation purposes, a composite light field can be a complex two-dimensional array, where complex numbers define the light intensity and phase at multiple discrete locations within the light field.

[0072] According to well-known optical principles, the range of angles at which light propagating from a display device can be observed by an eye or other observing entity / system varies with the distance between the display device and the observing entity. For example, at an observation distance of 1 meter, only a small range of angles from the LCOS can propagate through the pupil of the eye to form an image on the retina at a given eye location. The range of angles at which light propagates from the display device determines the portion of the image that the observer can successfully propagate through the pupil of the eye to form an image on the retina at a given eye location. In other words, not all parts of the image are visible from any point on the observation plane (e.g., any eye position within the observation window of an eyebox).

[0073] In some embodiments, the image perceived by the observer is a virtual image appearing upstream of the display device; that is, the observer perceives the image as being farther away than the display device. Therefore, conceptually, it can be thought of as the observer viewing the virtual image through a "window the size of the display device," which can be very small, for example, 1 centimeter in diameter, at a relatively large distance, such as 1 meter. And the user will view the window the size of the display device through the pupil of their eye, which can also be very small. Therefore, at any given time, the field of view is small, and the range of specific angles that can be seen is heavily dependent on eye position.

[0074] A pupil expander addresses how to increase the angular range of light propagating from a display device so that the light can successfully pass through the pupil of the eye to form an image. Display devices are typically (relatively) small, with (relatively) large projection distances. In some embodiments, the projection distance is at least an 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), for example, at least two orders of magnitude larger.

[0075] The use of a pupil expander laterally increases the viewing area (i.e., the user's eyebox), allowing for some eye movement while still enabling the user to see the image. As those skilled in the art will understand, in an imaging system, the viewing area (the user's eyebox) is the region where the observer's eye can perceive the image. This disclosure includes non-infinite virtual image distances—i.e., near-field virtual images.

[0076] Traditionally, two-dimensional pupil expanders comprise one or more one-dimensional optical waveguides, each formed using a pair of opposing reflective surfaces, where output light from the surfaces forms an observation window or eyebox. Light received from a display device (e.g., spatially modulated light from LCOS) is replicated by this or each waveguide to increase the field of view (or observation area) in at least one dimension. In particular, the waveguides enlarge the observation window by generating additional light rays or "copies" through the division of the amplitude of the incident wavefront.

[0077] The display device can have an active or pixel display area, which has 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 observation 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. This method is capable of receiving images and determining corresponding holograms of sufficient quality in less than 20 ms, such as less than 15 ms or less than 10 ms.

[0078] In some embodiments described herein by way of examples of diffraction or holographic light fields 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 diffraction structure are referred to herein as “holographic channels” simply to reflect that they are light channels encoded by a hologram containing image information. It can be said that the light in 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, and therefore the holographic domain is Fourier or the frequency domain. The hologram can also 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 holographic channels to reflect that the image reconstructable from the hologram has a finite size and can be arbitrarily divided into multiple image sub-regions, where each holographic channel will correspond to each image sub-region. Importantly, the hologram of this example is characterized by how it distributes the image content when illuminated. Specifically and uniquely, the hologram divides the image content by angle. In other words, each point on the image is associated with a unique ray angle in the spatially modulated light formed by the hologram during illumination—at least a unique pair of angles, since the hologram is two-dimensional. To avoid confusion, this holographic behavior is not conventional. When illuminated, the spatially modulated light formed by this particular type of hologram can be divided into multiple holographic channels, each defined by a range of ray angles (in two dimensions). As understood above, any holographic channel that can be considered in the spatially modulated light (i.e., a subrange of ray angles) will be associated with a corresponding part or subregion of the image. That is, all the information needed to reconstruct that part or subregion of the image is contained within the subrange of angles of the spatially modulated light formed by the hologram of the image. When the spatially modulated light is viewed as a whole, there is not necessarily any evidence of multiple discrete light channels.

[0079] Nevertheless, holograms 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, then only that sub-region of the image should be visible. If different continuous portions or sub-regions of the spatially modulated light are reconstructed, then different sub-regions of the image should be visible. Another identifying feature of this type of hologram is that the shape of the cross-section of any hologram channel substantially corresponds to (i.e., substantially the same as) the shape of the incident pupil, although the size may differ—at least in the correct plane in which the hologram is computed. Each light / hologram channel propagates from the hologram at a different angle or angular range. While these are example ways of characterizing or identifying this type of hologram, other methods may also be used. In summary, the holograms disclosed herein are characterized and identified by how the image content is distributed within the hologram-encoded light. Furthermore, to avoid any doubt, references herein to holograms configured to guide light or to divide an image at an angle into multiple hologram channels are by way of example only, and this disclosure is equally applicable to pupil expansion of any type of holographic light field or even any type of diffractive or diffracted light field.

[0080] The system can be offered in a compact and streamlined physical form. This makes it suitable for a wide range of practical applications, including those with limited space and high asset value. For example, it can be implemented in a head-up display (HUD), such as a vehicle or automotive HUD.

[0081] According to this disclosure, a pupil expander is provided for diffracted light, which may include a diverging beam of light. The diffracted light field can be defined by a "light cone". Therefore, the magnitude of the diffracted light field (as defined in a two-dimensional plane) increases with the propagation distance from the corresponding diffractive structure (i.e., the display device). It can be said that the pupil expander replicates a hologram or forms at least one copy of a hologram to convey light transmitted to the observer that is spatially modulated according to the hologram.

[0082] In some embodiments, two one-dimensional waveguide pupil expanders are provided, each arranged to effectively increase the size of the system's exit pupil by forming multiple copies or replicas of the exit pupil (or the light from the exit pupil) of the spatial light modulator. The exit pupil can be understood as the physical region where the system outputs light. Alternatively, each waveguide pupil expander can be arranged to expand the size of the system's exit pupil. Also, each waveguide pupil expander can be arranged to expand / increase the size of the eyebox in which an observer's eye can reside, in order to see / receive the light output by the system.

[0083] Optical Channel

[0084] Holograms formed according to some embodiments angularly divide image content to provide multiple holographic channels, which may have a cross-sectional shape defined by the aperture of an optical system. The hologram is calculated to provide this guidance of the diffracted light field. In some embodiments, as described above, this is achieved during hologram calculation by taking into account the aperture (virtual or real) of the optical system.

[0085] Figure 2 and 3 An example of this type of hologram is shown, which can be used in conjunction with the pupil expander disclosed herein. However, this example should not be considered as a limitation of this disclosure.

[0086] Figure 2 An image 252 for projection is shown, comprising eight image regions / components V1 to V8. This is for illustrative purposes only. Figure 2 Eight image components are shown, and image 252 can be divided into any number of components. Figure 2 Also shown is an encoded light pattern 254 (i.e., a hologram) that can reconstruct image 252—for example, when transformed by a lens of a suitable observation system. The encoded light pattern 454 includes first to eighth sub-holograms or components H1 to H8, corresponding to first to eighth image components / regions V1 to V8. Figure 2 This further demonstrates how holograms decompose image content through angles. Therefore, a characteristic of holograms lies in their ability to guide light. This is in Figure 3 As shown in the example. Specifically, the hologram in this example guides light into multiple discrete regions. In the example shown, the discrete regions are disks, but other shapes can also be envisioned. After propagation through the waveguide, the optimal size and shape of the disk can be related to the size and shape of the aperture of the optical system (e.g., the entrance pupil of an observation system).

[0087] Figure 4 System 400 is shown, including displays such as Figure 2 and 3 The display device for the calculated hologram.

[0088] System 400 includes a display device comprising an LCOS 402 in this arrangement. The LCOS 402 is arranged to display a modulation pattern (or “diffraction pattern”) including a hologram and to project holographically encoded light onto an eye 405, which includes a pupil acting as an aperture 404, a lens 409, and a retina (not shown) acting as a viewing plane. A light source (not shown) is arranged to illuminate the LCOS 402. The lens 409 of the eye 405 performs the hologram-to-image conversion. The light source can be of any suitable type. For example, it can include a laser source.

[0089] The observation system 400 also includes a waveguide 408 located between the LCOS 402 and the eye 405. The presence of the waveguide 408 allows 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 therefore will only be briefly described here.

[0090] in short, Figure 4 The waveguide 408 shown comprises a generally elongated structure. In this example, waveguide 408 comprises an optical plate of refractive material, but other types of waveguides are also well known and can be used. Waveguide 408 is positioned to intersect, for example, at an oblique angle, a light cone (i.e., a diffracted light field) projected from LCOS 402. In this example, the dimensions, position, and orientation of waveguide 408 are configured to ensure that light from each of the eight beams within the light cone enters waveguide 408. Light from the light cone enters waveguide 408 via a first planar surface (closest to LCOS 402) and is guided at least partially along the length of waveguide 408 before being emitted via a second planar surface (closest to the eye) substantially opposite the first surface. It is readily understood that the second planar surface is partially reflective and partially transmissive. In other words, as each ray of light propagates within waveguide 408 from the first planar surface and strikes the second planar surface, some light will be transmitted out of waveguide 408, and some 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 striking it from within waveguide 408 will be reflected back to the second planar surface. Thus, some light can be simply refracted between the two planar surfaces of waveguide 408 before being transmitted, while other light can be reflected and thus undergo one or more reflections (or "bouncing") between the planar surfaces of waveguide 408 before being transmitted.

[0091] Figure 4 A total of nine "bounce" points B0 to B8 are shown along the length of waveguide 408. Although... Figure 2 As shown, light associated with all points in the image (V1-V8) is transmitted out of the waveguide at each "bounce" from the second planar surface of waveguide 408, but only light from one angular portion of the image (e.g., light from one of V1 to V8) has a trajectory that allows it to reach the eye 405 from each corresponding "bounce" point B0 to B8. Furthermore, light from different angular portions (V1 to V8) of the image reaches the eye 405 from each corresponding "bounce" point. Therefore, in Figure 4 In the example, each angular channel of the encoded light reaches the eye only once from waveguide 408.

[0092] 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.

[0093] Two-dimensional pupil expansion—Example 1

[0094] Although Figure 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 Figure 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.

[0095] Figure 5A A perspective view of system 500 is shown, which includes two replicators 504 and 506 arranged to extend beam 502 in two dimensions.

[0096] exist Figure 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 Figure 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...) Figure 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.

[0097] 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... Figure 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.

[0098] Therefore, it can be said that Figure 5A The first and second replicators 504 and 505 are combined to provide a two-dimensional replicator (or "two-dimensional pupil expander").

[0099] Two-dimensional pupil expansion—Example 2

[0100] Figure 5B A perspective view of a second example system 500 is shown, which includes two replicators 520, 540 arranged to extend the beam 522 in two dimensions.

[0101] In the second example system 500, the first replicator is a solid elongated waveguide 520, and the second replicator is a solid planar waveguide 540. Specifically, 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.

[0102] The second example system 500 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 planar / folding mirror 530, which is arranged to fold or redirect the optical path of the light to achieve the desired optical coupling from the first replicator to the second replicator. Figure 5B As shown, mirror 530 is arranged to receive light from the output port / reflective-transmitting surface 524a of the first replicator / waveguide 520, comprising a one-dimensional array of replicas extending in the first dimension. Mirror 530 is tilted to redirect the received light at an angle onto the optical path of the input port in the (total) reflective surface of the second replicator 540, to provide waveguide and replica formation along its length in the second dimension.

[0103] In the illustrated arrangement, the reflective-transmitting surface 524a of the first replicator 520 is adjacent to the input port of the first replicator / waveguide 520, which receives the input beam 522 at an angle to provide a waveguide and copy formation along its length in the first dimension. Therefore, the input port of the first replicator / waveguide 520 is located at its input end, on the same surface as the reflective-transmitting surface 524a. The skilled reader will understand that the input port of the first replicator / waveguide 520 can be located in any other suitable position.

[0104] Therefore, the arrangement of the second example system allows the first replicator 520 and mirror 530 to be provided as part of a first relatively thin layer in the first and third dimensional planes (shown as the xz plane). Specifically, the size or "height" of the first planar layer (in which the first replicator 520 is located) is reduced in the second dimension (shown as the y dimension). Mirror 530 is configured to guide light from the first layer / plane (i.e., the "first planar layer") where the first replicator 520 is located and direct it to the second layer / plane (i.e., the "second planar layer") where the second replicator 540 is located, which is located above and substantially parallel to the first layer / plane. Thus, it can be said that the first planar layer containing the first replicator 520 and mirror 530 is within the coverage area of ​​the second planar layer containing the second replicator 540. Therefore, the overall size or "height" of the system—including in the first and third dimensions (shown as the xz plane) and in the second dimension (shown as the y dimension)—of the first and second replicators 520, 540 and mirror 530 in the stacked first and second layers / planes, is compact. Technical readers will understand that the methods used to implement this disclosure Figure 5B Many variations in the arrangement are possible and anticipated.

[0105] UK patent application GB2113454.9, filed on 21 September 2021 and incorporated herein by reference, discloses an image projector that includes a more advanced two-dimensional pupil expander arranged according to the present disclosure.

[0106] An image projector can be arranged to project a diverging or diffracted light field. In some embodiments, the light field is encoded with a hologram. In some embodiments, the diffracted light field comprises a diverging beam of light. In some embodiments, the image formed by the diffracted light field is a virtual image.

[0107] In some embodiments, the first pair of parallel / complementary surfaces are elongated or slender surfaces, relatively long along a first dimension and relatively short along a second dimension, for example, relatively short along each of two other dimensions, each dimension being substantially orthogonal to each of the corresponding other dimensions. The process of light reflection / transmission between / from the first pair of parallel surfaces is arranged such that light propagates within the first waveguide pupil expander, with the general direction of light propagation being the direction of the relatively longer portion of the first waveguide pupil expander (i.e., its "elongated" direction).

[0108] This paper discloses a system that uses diffracted light to form an image and provides eyebox sizes and fields of view suitable for real-world applications—such as head-up displays in the automotive industry. Diffracted light is light used to holographically reconstruct an image from a diffracted structure—such as a hologram like a Fourier or Fresnel hologram. The use of diffraction and diffracted structures requires high-density display devices with very small pixels (e.g., 1 micrometer)—which in practice means small display devices (e.g., 1 cm). The inventors have solved the problem of how to provide a diffracted light field for 2D pupil expansion, for example, by including diffracted light comprising divergent (non-collimated) beams.

[0109] In some embodiments, the display system includes display devices—such as pixelated display devices, such as spatial light modulators (SLMs) or liquid crystal on silicon (LCoS) SLMs—arranged to provide or form diffracted or diverging light. In these respects, 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 region defining the light-modulating pixel array contained within the SLM—determines the size (e.g., spatial extent) of the light beam that can exit the system. According to this disclosure, it is stated that by using at least one pupil expander, the exit pupil of the system is expanded to reflect that the exit pupil of the system (which is limited by the small display device having pixel sizes for light diffraction) becomes larger in spatial extension.

[0110] A diffracted or diverging light field can be described as having a "light field size," which is defined in a direction substantially orthogonal to the direction of light field propagation. Because light is diffracted / divered, the light field size increases with the propagation distance.

[0111] In some embodiments, the diffracted light field is spatially modulated according to the hologram. In other words, in these aspects, the diffracted light field includes 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, a Fresnel hologram, a point cloud hologram, or any other suitable type of hologram. Optionally, the hologram can be computed to form channels of holographic light, each channel corresponding to a different portion of the image that an observer wants to observe (or perceive, if it is a virtual image). The pixelated display device can be configured to display multiple different holograms consecutively or sequentially. Each aspect and embodiment disclosed herein can be applied to the display of multiple holograms.

[0112] The output port of the first waveguide pupil expander can be coupled to the input port of the second waveguide pupil expander. The second waveguide pupil expander can be arranged to guide the diffracted light field (including some, preferably most, preferably all copies of the light field output by the first waveguide pupil expander) from its input port to the corresponding output port through internal reflection between the third pair of parallel surfaces of the second waveguide pupil expander.

[0113] A first waveguide pupil expander can be arranged to provide pupil expansion or replication in a first direction, while a second waveguide pupil expander can be arranged to provide pupil expansion or replication in a different second direction. The second direction can be substantially orthogonal to the first direction. The second waveguide pupil expander can be arranged to maintain the pupil expansion already provided by the first waveguide pupil expander in the first direction, and expand (or replicate) some, preferably most, preferably all, copies it receives from the first waveguide pupil expander in the different second directions. The second waveguide pupil expander can be arranged to receive the optical field directly or indirectly from the first waveguide pupil expander. One or more other elements can be provided along the propagation path of the optical field between the first and second waveguide pupil expanders.

[0114] The first waveguide pupil expander may be substantially elongated, and the second waveguide pupil expander may be substantially planar. The elongated shape of the first waveguide pupil expander may be defined by its length along a first dimension. The planar or rectangular shape of the second waveguide pupil expander may be defined by its length along the first dimension and its width or span along a second dimension substantially orthogonal to the first dimension. The dimension or length of the first waveguide pupil expander along its first dimension may correspond respectively to the length or width of the second waveguide pupil expander along its first or second dimension. The first surface of a pair of parallel surfaces of the second waveguide pupil expander, including its input port, may be shaped, sized, and / or positioned to correspond to the region 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.

[0115] The first and second waveguide pupil expanders can jointly provide pupil expansion in a first direction and a second direction perpendicular to the first direction. Optionally, the planes containing the first and second directions are substantially parallel to the plane of the second waveguide pupil expander. In other words, the first and second dimensions defining the length and width of the second waveguide pupil expander, respectively, can be parallel to the first and second directions (or parallel to the second and first directions, respectively), wherein the waveguide pupil expander provides pupil expansion. The combination of the first and second waveguide pupil expanders is generally referred to as a "pupil expander".

[0116] In essence, the expansion / replication provided by the first and second waveguide expanders has the effect of expanding the exit pupil of the display system in each of two directions. The area defined by the expanded exit pupil can further define the expanded eyebox region from which the observer can receive light from the input diffracted or diverging light field. The eyebox region can be said to lie on or define the observation plane.

[0117] The two directions of pupil expansion can be coplanar or parallel to the first and second directions of replication / expansion provided by the first and second waveguide pupil expanders. Alternatively, in arrangements that include other elements such as optical combiners, e.g., a vehicle's windshield, the pupil can be considered as the pupil of that other element, such as the windshield. In such an arrangement, the pupil can be non-coplanar and non-parallel to the first and second directions of replication / expansion provided by the first and second waveguide pupil expanders. For example, the pupil can be substantially perpendicular to the first and second directions of replication / expansion provided by the first and second waveguide pupil expanders.

[0118] The observation plane and / or eyebox region may not be coplanar or parallel to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. For example, the observation plane may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion.

[0119] In order to provide suitable emission conditions to achieve internal reflection within the first and second waveguide pupil expanders, the elongated dimension of the first waveguide pupil expander may be tilted relative to the first and second dimensions of the second waveguide pupil expander.

[0120] To avoid any doubt, the teachings of this disclosure can be applied to one or both replicators of a two-dimensional pupil expander that includes two replicators.

[0121] Combiner shape compensation

[0122] The advantage of projecting a hologram onto an eyebox is that optical compensation can be encoded in the hologram (see, for example, European Patent 2936252, incorporated herein by reference). This disclosure is compatible with holograms that compensate for the complex curvature of an optical combiner used as part of a projection system. In some embodiments, the optical combiner is the windshield of a vehicle. The full details of this method 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 novel teachings of this disclosure and are merely examples of configurations that benefit from the teachings of this disclosure.

[0123] Control device

[0124] This disclosure is also compatible with optical configurations that include control devices (e.g., optical shutter devices) to control the transmission of light from the optical channel hologram to the observer. The holographic projector may also include control devices arranged to control the transmission of the angular channel to the eyebox position. UK Patent Application 2108456.1, filed June 14, 2021 and incorporated herein by reference, discloses at least one waveguide pupil expander and control device. The reader will understand from at least this prior disclosure that the optical configuration of the control device is substantially based on the user's eyebox position and is compatible with any hologram calculation method for implementing the optical channel described herein. The control device can be described as an optical shutter or aperture device. The optical shutter device may include a 1D array of apertures or windows, wherein each aperture or window can be independently switched between transparent and opaque states to control the transmission of the hologram optical channel and its replica to the eyebox. Each aperture or window may include multiple liquid crystal cells or pixels.

[0125] Windshield waveguide

[0126] Figure 5A and 5B A pair of orthogonal waveguides, including a first waveguide and a second waveguide, are shown. Figure 5A and 5B The output from a pair of waveguides is not shown being directed to an observation system using an optical combiner. In head-up display applications, the optical combiner could be a vehicle's windshield.

[0127] Figure 6 The windshield waveguide is shown. Figure 6 A projector 602 arranged to output a holographic wavefront according to the present disclosure is shown. The projector 602 may include a hologram engine arranged to determine a hologram for projecting an image, and a display device arranged to display the hologram and spatially modulate light according to the displayed hologram. Figure 6 The first waveguide 604 is also shown, which corresponds to Figure 5A 504 waveguide or Figure 5B Waveguide 520. First waveguide 604 replicates the holographic wavefront in the first dimension to form a 1D replica array of the holographic wavefront. Figure 6 A windshield 620 having an inner surface 622 and an outer surface 624 is also shown. The terms “inner” and “outer” are not restrictive and are used merely for the convenience of referring to the main vehicle. Figure 6 An observer 640, possibly the vehicle driver, is also shown. Projection axis 606 shows the approximate orientation of the 1D replica array of the holographic wavefront waveguided between the inner surface 622 and the outer surface 624 of the windshield 620. As the reader will see from... Figure 4As understood from the description, different angular ranges or channels of the holographic wavefront are transmitted, in their function, from different portions of the windshield 620, which acts as a second waveguide, to the observer 640 (through their pupil). This is merely an example. Figure 6 A first axis 642 for a first angular range, a second axis 644 for a second angular range, and a third axis 646 for a third angular range are shown. As the reader will understand, according to the channel hologram of this disclosure, each different angular range corresponds to a different consecutive region of the image. Figure 6 The image shows only three angular channels as examples. Holograms and waveguides can be configured to deliver the complete image content to the observer (in the holographic domain) using any number of holographic channels and any number of replicas of the holographic wavefront. In summary, Figure 6 Thus, a vehicle windshield is shown as a second pupil expander to form a 2D replica array of a holographic wavefront from a 1D replica array.

[0128] In one embodiment, the outer surface 624 of the windshield is coated (e.g., with a dielectric laminate) to increase the reflectivity of the output surface to the holographic wavefront. In one embodiment, red, green, and blue holographic wavefronts are waveguided by the windshield to provide full-color display, and the outer surface includes a coating that provides narrowband increases in reflectivity at discrete wavelengths of the projector. For example, each narrowband may have a bandwidth of only 7 nm, such that the effect on the perceived color balance of a real-world scene within the 200 nm visible range is not significantly distorted. This is possible because the embodiments use narrowband light sources, such as laser diodes, to generate the holographic wavefront, since holography requires coherence. In other embodiments, the head-up display is arranged such that internal reflections within the windshield meet the criteria for total internal reflection. In a further improvement explained in the next section, the hologram is modified to compensate for varying replication intensities.

[0129] Compensation for replication intensity

[0130] The output surface of each waveguide is partially reflective-transmissive so that a portion of the holographic light field (i.e., a percentage of its intensity) is output, and during waveguide operation, a portion is internally reflected with each interaction with the output surface. The result of this method of replicating the light field is that the intensity of the replica decreases with each reflection. Figure 7 and Figure 8 This illustrates the point.

[0131] Figure 7An example waveguide is shown, arranged with an input 700 and multiple light emitters 701 to 708. Each emission is a copy of the holographic light field. Alternatively, each emission can be described as a copy of the hologram. Each emission is also a copy of the pupil of a display device showing the hologram; therefore, the waveguide can be referred to as a pupil expander. That is, multiple emission regions provide pupil expansion. Each emission corresponds to a different number of internal reflections within the waveguide. For example, light emission 701 corresponds to zero internal reflection (i.e., direct transmission of the input light 700 through the waveguide), and the light output from emission 702 corresponds to one internal reflection from the waveguide output surface, and so on. The reader can understand from the illustration that each emission occurs in a corresponding emission region of the waveguide. Figure 7 The approximate orientation of waveguide 710 is shown, which is also the direction of pupil expansion.

[0132] Figure 8 A target image 800 comprising multiple image regions 801 to 808 is shown, these image regions being illustrated only as examples of vertical cross-sections of the target image 800. The target image is the input image that the projector will project to the observer. Figure 8 Hologram 810 of target image 800 is also shown, which is formed by the first step of computing hologram 851, including any suitable method, such as iterative phase retrieval or point cloud method. Figure 8 A second step of holographic illumination 852 is also shown to form a holographic reconstruction 820 of the target image 800 from the hologram 810. The holographic reconstruction 820 includes the same vertical cross-section as the image 800. In a perfect system, the holographic reconstruction 820 is a perfect reproduction of the target image 800. In this example, the shadow density used in the illustrated holographic reconstruction 820 represents the relative intensity of different vertical cross-sections of the holographic reconstruction 820. In this illustration, high-density shadows represent low brightness levels (i.e., relatively dark), and low-density shadows represent high brightness levels (i.e., relatively bright). The hologram of the image 800 is angularly distributed according to this disclosure, as described in detail elsewhere herein. Further according to this disclosure, there is a correlation between portions of the image and the emission region. Figure 8 In China, for example only, the correlations are as follows:

[0133] 801 706 802 707 803 708 804 701 805 702 806 703 807 704 808 705

[0134] Table 1—Example correlations between image cross-sections and emission areas of example projection systems

[0135] Those skilled in the art of optics will understand, for example, how ray tracing can be used to determine the correlation between different parts of an image and emission areas. They will also understand that this correlation depends on the observer's location, such as the observer's so-called eyebox position within the eyebox region or volume.

[0136] Figure 8The diagram illustrates how the intensity of each image section in the holographic reconstruction 820 depends on the location of the emission region from which the image section is emitted, due to the decrease in intensity with each successive internal reflection from the output surface. The fourth image section 814 has the highest intensity because, as shown in Table 1, it corresponds to the first emission 701 with zero internal reflection. The third image section 813 has the lowest intensity (and is therefore shown as the darkest / most shadowed) because it is associated with the eighth emission 708 with seven internal reflections.

[0137] In some embodiments, the transmittance T(n) of the first surface in each emission region along the waveguide direction satisfies the following equation:

[0138]

[0139] Where L is the optical loss factor of the waveguide material, and n is the emission number. Equation 1 is merely an example of T(n), and this disclosure is not limited to Equation 1. Those skilled in the art will understand that the principles disclosed herein can be applied to any relationship between T and n.

[0140] In some examples, a graded coating (with variable transmittance-reflectance) is applied to the output surface to compensate for the decrease in waveguide light intensity with each internal reflection. For example, the coating could have increasing transmittance and decreasing reflectance with each internal reflection so that the waveguide emission has a uniform intensity. This may require complex dielectric stacks comprising more than 20 separate layers of different materials, most with varying thicknesses, especially when color projection requires three monochromatic (e.g., red, green, and blue) holographic light fields.

[0141] Figure 9 An embodiment of this disclosure is shown. Figure 9 To a large extent correspond to Figure 8However, it includes an initial step 950 of modifying the target image. Initial step 950 includes individually changing the intensity of each image section 801 to 808 based on the intensity loss associated with the corresponding number of internal reflections. The length of the vertical arrow in image 800' represents the intensity increase associated with each image section. The process of modifying the target image 800 may include multiplying each image section 801 to 808 by its respective gain or loss factor. This process may include changing the gray level associated with each pixel of the image section. For example, modifying the third image section 803 of the target image may include multiplying each pixel value of the third image section 803 by a gain factor, such as gain factor 2. This has the effect of enhancing the intensity of the last emission 708 of the waveguide (as shown in Table 1) to compensate for the intensity loss caused by the seven previous internal reflections from the output surface. The opposing surfaces of the waveguide may be substantially totally reflective. The reader will understand how different gain or loss factors are used to process each image section based on the corresponding number of internal reflections. To avoid confusion, according to this example, each pixel value of the image section is multiplied by the same gain or loss factor. Figure 8 Hologram 810' of the modified image 800' formed by the step of computing hologram 951 is also shown, which includes any suitable method, such as iterative phase retrieval or point cloud methods. Figure 9 The third step of holographic illumination 952 is also shown to form a holographic reconstruction 820' of the target. (See diagram below.) Figure 9 As shown, according to this disclosure, the intensity ratio of the holographic reconstruction 820' is... Figure 8 The intensity of the holographic reconstruction 820 is more uniform, and it is an improved reproduction of the target image 800.

[0142] In some embodiments, the amplification or gain applied to each corresponding image section (pixel value) is a function of the amount of reflection (from / on the output surface) associated with the corresponding emitting region. In some embodiments, the amplification or gain applied to each corresponding image section (pixel value) increases with the amount of reflection (from / on the output surface) associated with the corresponding emitting region, and / or with the propagation or waveguide distance. In some embodiments, an example of the gain factor basic compensation T(n) is given in Equation 1.

[0143] In other embodiments, it is not necessary to calculate the modified image 800'; instead, the method for determining the hologram includes the step of modulating the contribution of data points or light waves based on the respective emission regions. For example, in point cloud methods, the intensity of the simulated light waves used to form a point cloud hologram can be adjusted based on the respective emission regions. Those skilled in the art will understand how the concept of compensating for intensity loss during waveguide movement in a hologram can be implemented in various different ways. While the invention can be implemented in various different ways, the basic principle can be expressed as changing (e.g., uniformly increasing or decreasing) the intensity of at least one angular channel of the holographic light field based on the transmittance of the respective emission regions.

[0144] As described above, in some embodiments, an improved Gerchberg-Saxton type algorithm, such as that described in UK Patent Application 2112213.0 filed August 26, 2021, which is incorporated herein by reference, is used to compute the hologram. This algorithm is particularly synergistic with this disclosure. The method computes a sub-hologram corresponding to each replica, thus the full dynamic range for computing the Fourier transform can be used independently for each replica. That is, since the intensity decreases with each successive replica, any difficulties that might typically exist during hologram computation do not arise, as each sub-hologram is computed independently and therefore the full dynamic range available in the computation can be benefited from.

[0145] In these embodiments, when the windshield is used as a waveguide, the holographic algorithm handles well the large losses that may occur with each reflection. This is because the method involves using computational Fourier transforms that provide high dynamic range and floating-point numbers to compute sub-holograms for each holographic replica.

[0146] To avoid any doubt, although the method of modifying a hologram to compensate for the intensity reduction of a replica with a waveguide is highly compatible with the method of using a vehicle's windshield as a waveguide pupil expander, the two methods work independently and their combination is not important.

[0147] Additional features

[0148] The methods and processes described herein can be embodied on a computer-readable medium. The term "computer-readable medium" includes media arranged for temporary or permanent storage of data, such as random access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" should also be considered to include any medium or combination of media capable of storing instructions for machine execution, such that when the instructions are executed by one or more processors, the machine performs, wholly or partially, any or all of the methods described herein.

[0149] The term "computer-readable medium" also covers cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example forms of solid-state storage chips, optical discs, disks, or any suitable combinations thereof. In some example embodiments, instructions for execution may be transmitted by a carrier medium. Examples of such carrier media include transient media (e.g., propagation signals for transmitting instructions).

[0150] It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. This disclosure covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

1. A head-up display method for a vehicle, the method comprising: A hologram of an image is displayed on a display device, and light is spatially modulated according to the displayed hologram to form a holographic wavefront; The holographic wavefront is replicated in the first direction using a first pupil expander to form a 1D replica array of the holographic wavefront; Using the windshield of a vehicle as a second pupil expander, a 2D replica array of a holographic wavefront is formed from a 1D replica array, wherein the 2D replica array is formed by waveguides between the inner and outer surfaces of the windshield of the 1D replica array. as well as A coating with different graded transmittance and reflectance is formed on the inner surface of the windshield to compensate for the decrease in intensity of the waveguide light with each internal reflection. Different parts of the windshield correspond to different numbers of internal reflections within the windshield, thereby giving the 2D replica array uniform intensity.

2. The method as described in claim 1, wherein, The waveguide includes multiple portions of transmission and reflection at multiple different locations on the inner surface of the windshield.

3. The method of claim 1 or 2, further comprising arranging the hologram and display device to guide holographic wavefronts of different continuous angular ranges from different portions of the inner surface of the windshield to an observer inside the vehicle.

4. The method of claim 1 or 2, further comprising converting the holographic wavefront into an image.

5. The method of claim 4, wherein, The image content appears inside the vehicle, outside the vehicle, or both.

6. The head-up display method for a vehicle as described in claim 1 or 2, further comprising calculating the hologram such that, when illuminated, the hologram divides the image content in the spatial domain by angles in the hologram domain.

7. A display system, comprising: A light engine, the light engine comprising: a display device arranged to display a hologram of an image and spatially modulate light according to the displayed hologram to form a holographic wavefront; and a first pupil expander arranged to receive the holographic wavefront and replicate the holographic wavefront in a first direction to form a 1D replica array of the holographic wavefront. The vehicle's windshield, having an inner surface and an outer surface, is arranged to receive a 1D replica array of holographic wavefronts from a first pupil expander, and the 1D replica array is extended in a second direction orthogonal to a first direction using the inner and outer surfaces to form a 2D replica array of holographic wavefronts. The 2D replica array is formed by forming a waveguide between the inner and outer surfaces of the windshield of the 1D replica array. In this process, a coating with different graded transmittance and reflectance is formed on the inner surface of the windshield to compensate for the decrease in intensity of the waveguide light with each internal reflection. Different parts of the windshield correspond to different numbers of internal reflections within the windshield, thereby giving the 2D replica array a uniform intensity.

8. The display system as claimed in claim 7, wherein, The hologram is configured to distribute light at an angle within the holographic wavefront according to the spatial position within the image, such that continuous angular ranges of the holographic wavefront correspond to continuous regions of the image.

9. The display system as claimed in claim 8, wherein, An observer receives different consecutive angular ranges of the holographic wavefront from different portions of the windshield.

10. The display system of claim 9, wherein, The holograms are arranged such that the 2D replica array has uniform intensity.

11. The display system according to any one of claims 7 to 10, wherein, The windshield includes optical elements configured to increase the reflectivity of the windshield surface within at least one narrow band of the visible spectrum.

12. The display system of claim 11, wherein, The optical element is a coating.

13. The display system of claim 11, wherein, The at least one narrow band corresponds to the wavelength of spatially modulated light.

14. The display system of claim 11, wherein, Narrowband means a band with a full-wave half-peak of less than 20 nm.

15. The display system of claim 11, wherein, Narrowband means a band with a full-wave half-peak of less than 10 nm.

16. The display system of claim 11, wherein, Narrowband means a band with a full-wave half-peak of 7+ / -2nm.

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