In-coupling optimization

Abstract

To provide a display system comprising a waveguide pupil expander that optimizes light coupling into the waveguide pupil expander.SOLUTION: A display system comprises an optical sub-system, a coupling lens and a first one-dimensional replicator. The optical sub-system is arranged to form an optimized wavefront. The optimized wavefront comprises chief rays. Each chief ray comprises a first component and a second component. The first component is a first angle in a first direction and the second component is a second angle in a second direction. The second angle is a function of the second direction. The first angle may be zero or constant. The coupling lens is arranged to receive the optimized wavefront and form a first pupil corresponding to the first direction and a second pupil corresponding to the second direction. The second pupil is displaced from the first pupil owing to a difference between the first angle and second angle. That is, the first pupil and the second pupil are formed on different planes in the z-direction.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present disclosure relates to pupil dilation and wavefront replication. More specifically, the present disclosure relates to display systems including waveguide pupil expanders and methods for optimizing light coupling to waveguide pupil expanders. Some embodiments relate to two-dimensional pupil dilation or wavefront replication. Some embodiments relate to image generation units and head-up displays, such as automotive head-up displays (HUDs). [Background technology]

[0002] Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured, for example, by well-known interference techniques on a photosensitive plate to form a holographic recording containing interference fringes, or "hologram." The hologram can be reconstructed by illuminating it with appropriate light to form a two- or three-dimensional holographic reconstruction, or reconstructed image, that represents the original object.

[0003] Computer-generated holography can numerically simulate interference processes. Computer-generated holograms can be calculated using techniques based on mathematical transforms, such as the Fresnel transform or the Fourier transform. These types of holograms are sometimes called Fresnel / Fourier transform holograms, or simply Fresnel / Fourier holograms. Fourier holograms can be viewed as a Fourier domain / planar representation of an object, or a frequency domain / planar representation of an object. Computer-generated holograms can be calculated, for example, using coherent ray tracing or point cloud techniques.

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

[0005] Spatial light modulators are typically composed of multiple individually addressable pixels, also called cells or elements. The light modulation scheme may be binary, multilevel, or continuous. Alternatively, the device may be continuous (i.e., not pixelated), and thus the light modulation may be continuous across the device. Spatial light modulators may be reflective, meaning that modulated light is output through reflection. Spatial light modulators may also be transmissive, meaning that modulated light is output through transmission.

[0006] The systems described herein can be used to provide holographic projectors, which have applications in head-up displays (HUDs). Summary of the Invention

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

[0008] A first aspect of the present disclosure is a display system including an optical subsystem, a coupling lens, and a first one-dimensional replica. The optical subsystem is arranged to form an optimized wavefront. The optimized wavefront is composed of chief rays. Each chief ray is composed of a first component and a second component. The first component is at a first angle in a first (e.g., horizontal or x) direction, and the second component is at a second angle in a second (e.g., vertical or y) direction. More specifically, the first angle is an angle with the optical axis or Z axis in the xz plane, and the second angle is an angle with the optical axis or Z axis in the yz plane. The second angle is a function of the second (y) direction. The first angle is zero or constant. The coupling lens is arranged to receive the optimized wavefront and form a first (x) pupil corresponding to the first (x) direction and a second (y) pupil corresponding to the second (y) direction. The second (y) pupil is offset from the first (x) pupil by the difference between the first angle and the second angle. That is, the first and second pupils are formed on different planes in the Z direction. For example, the second pupil may be downstream of the first pupil. The first one-dimensional replica is positioned to replicate light in the first (x) direction. The entrance of the first one-dimensional replica is substantially aligned with the first (x) pupil.

[0009] The optical efficiency of a display system including a replicator can be challenged due to coupling losses at its entrance port. The inventors have found that optimal coupling is achieved by positioning the entrance port of the one-dimensional replica at the focal length of the coupling lens where the pupil is formed. Notably, the inventors have further recognized that by changing the direction of the chief ray of the wavefront received by the coupling lens, the position of the pupil formed by the lens can be changed and thus used to optimize coupling to the one-dimensional replica. For example, the chief ray angle of the wavefront incident on the coupling lens can be manipulated to match the pupil (e.g., in position, size, and / or shape) to the entrance port of the replica.

[0010] Optical efficiency is even more challenging for two-dimensional replicas where the entrance ports of the first and second replicas are at different distances from the coupling lens. A further improvement disclosed herein uses different x and y chief ray directions to create independent x and y pupil locations. The chief ray is optimized in the x direction, placing the x pupil at the entrance port of the first replica, and independently, the chief ray is optimized in the y direction, placing the y pupil at the entrance port of the second replica. The inventors have found that this disclosure significantly improves the efficiency of light coupling into the replicas.

[0011] The second angle of the optimized wavefront increases or decreases (in magnitude) with distance in the second (y) direction from the center of the coupling lens, and the increase or decrease with distance in the second (y) direction may be linear.

[0012] In this case, the chief ray is perpendicular to the plane of the coupling lens. The first angle may be constant (in the first direction), i.e., the first angle may not be a function of the distance from the center of the coupling lens in the first (x) direction.

[0013] In one embodiment, the display system further comprises a second one-dimensional replicator arranged to replicate in a second (y) direction, the entrance pupil of the second replicator being substantially aligned with the second (y) pupil.

[0014] The maximum difference between the first angle and the second angle may be less than 20 degrees, such as 5 to 12 degrees. The maximum difference between the first angle and the second angle may correspond to a chief ray at an edge of the wavefront or at a maximum (or minimum) of the wavefront in the second (y) direction.

[0015] The optical subsystem may include imaging optics positioned to receive a source wavefront. The source wavefront consists of chief rays characterized by a first angle in a first (x) direction and a second angle in a second (y) direction. The first angle may be constant with respect to the first (x) direction, and the second angle may be constant with respect to the first (y) direction. The imaging optics may be positioned to output an optimized wavefront.

[0016] The imaging optics comprises at least one toroidal lens surface or cylindrical surface. In some embodiments, the imaging optics comprises a complementary pair of toroidal lens or cylindrical surfaces. The imaging optics may be arranged to provide one-dimensional compression (of an image represented by or encoded in a wavefront). The imaging optics may be arranged to direct the wavefront substantially to one side of the optical axis. This is advantageous in embodiments where the image is formed by a hologram and therefore includes a zero-order spot or complex conjugate in the center of the reconstructed field. These features can be eliminated by utilizing half the reconstructed field or a sub-region of half the reconstructed field.

[0017] In some embodiments, the wavefront is an image or corresponds to an image. In some embodiments, the optical subsystem comprises a screen on which the image is formed (or displayed). The image may be a holographic reconstruction. The screen may be positioned to receive the (source) wavefront from a display device. The screen may be diffusing. In these embodiments, each received ray of the (source) wavefront received by the screen is effectively spread over the diffusion angle of the diffusing screen. However, the angular "spread" provided by the diffusing screen is the spread with respect to the ray angles received by the screen. For example, the screen may receive a wavefront characterized by chief rays. The general direction (i.e., the optical axis or propagation axis) of each chief ray is not altered by the diffusing screen; instead, the light is diffused or dispersed over a small range of angles relative to each chief ray. This small range of angles may be referred to as the "diffusion angle." In some embodiments, the diffusivity of the screen is isotropic. That is, the diffusion angle in the x-direction (i.e., the angle with the z-axis in the xz plane) is substantially equal to the diffusion angle in the y-direction (i.e., the angle with the z-axis in the yz plane).

[0018] The optical subsystem and coupling lens form an optimized wavefront for the (wavefront) replicator, which has an entrance or input port for receiving the wavefront for replication. In some embodiments, at least one of the size, shape, and position of the first pupil is substantially equal to the size of the entrance port of the first one-dimensional replica / s.

[0019] The optical subsystem may include a second one-dimensional replica. The second one-dimensional replicator may have an entrance port coupled to the output port of the first one-dimensional replicator. That is, the second one-dimensional replicator may be positioned to receive the output of the first one-dimensional replicator. The first one-dimensional replicator may replicate the optimized wavefront in a first (x) direction, and the second one-dimensional replicator may replicate a one-dimensional array of the optimized wavefronts (output by the first replicator) in a second (y) direction, where x and y are perpendicular. At least one of the size, shape, and position of the second pupil may be substantially equal to the size of the entrance port of the second one-dimensional replica. The first one-dimensional replica may include a first waveguide. Additionally or alternatively, the second one-dimensional replica may be comprised of a second waveguide. The first waveguide may be substantially elongated, such as rod-shaped. The second waveguide may be substantially planar, such as a slab.

[0020] In some embodiments, the distance from the image plane to the coupling lens is equal to the focal length of the coupling lens. Additionally or alternatively, the distance from the coupling lens to the first pupil may be equal to the focal length of the coupling lens.

[0021] A second aspect of the present disclosure is a method for increasing the optical efficiency of light coupling into a replicator or waveguide. The method includes optimizing the chief ray angle of a wavefront prior to incoupling. The method for optimizing the chief ray angle may include independently modifying the first or second component of at least one of the chief rays (e.g., received on an intermediate surface, e.g., a screen). The method for optimizing the chief ray angle may include forming a first pupil and a second pupil at different distances from the coupling lens. Thus, the second pupil can be said to be offset (in the z-direction) from the first pupil. For example, the second pupil is optically downstream of the first pupil.

[0022] In this disclosure, the term "replica" is used solely to reflect that the spatially modulated light is split and the composite light field is directed along multiple different optical paths. The word "replica" is used to refer to each occurrence or instance of the composite light field after a replication event, such as partial reflection / transmission by a pupil dilator. Each replica travels along a different optical path. Some embodiments of this disclosure relate to the propagation of light encoded in a hologram rather than an image, i.e., the light spatially modulated with the hologram of the image rather than the image itself. Thus, multiple replicas of a hologram are said to be formed. Those skilled in the art of holography will understand that the composite light field associated with the propagation of holographically encoded light varies with propagation distance. The use of the term "replica" here is independent of propagation distance; therefore, two light branches or paths associated with a replication event are still referred to as "replicas" of each other, even if the branches have different lengths. As a result, the composite light field evolves differently along each path. That is, two composite light fields are still considered "replicas" according to this disclosure, even if they are associated with different propagation distances. provided that they originate from the same replication event or series of replication events.

[0023] In some embodiments, the wavefront received by the display system is an image or a wavefront representing an image (e.g., spatially modulated with an image). In other embodiments, the wavefront received by the display system is an image or a wavefront representing a hologram of an image or a hologram of a hologram (e.g., spatially modulated with an image). In these embodiments, the wavefront is a diffracted wavefront or a holographic wavefront. The terms "wavefront" and "optical field" are used interchangeably herein. A "diffracted optical field" or "diffracted optical field" according to the present disclosure is an optical field formed by diffraction. A diffracted optical field may be formed by illuminating a corresponding diffraction pattern. According to the present disclosure, an example of a diffraction pattern is a hologram, and an example of a diffracted optical field is a holographic optical field or an optical field that forms a holographic reconstruction of an image. The holographic optical field forms a (holographic) reconstruction of the image on a reconstruction surface. The holographic optical field propagating from the hologram to the reconstruction surface can be said to consist of light encoded in the hologram or light in the hologram region. A diffracted optical field is characterized by a diffraction angle that depends on the minimum feature size of the diffracting structure and the wavelength of the light (of the diffracted optical field). In accordance with the present disclosure, a "diffracted light field" can also be referred to as a light field that forms a reconstruction on a spatially separated plane from a corresponding diffractive structure. Disclosed herein is an optical system for propagating the diffracted light field from the diffractive structure to a viewer. The diffracted light field can form an image.

[0024] The term "hologram" refers to a recording containing amplitude or phase information about an object, or a combination thereof. The term "holographic reconstruction" refers to an 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 separated from the hologram. The term "reconstruction field" refers to the 2D region in which the holographic reconstruction is formed and perfectly focused. When a hologram is displayed on a spatial light modulator containing pixels, the reconstructed field is repeated in multiple diffraction orders, each of which is a replica of the zeroth-order reconstructed field. The zeroth-order reconstructed field is the brightest reconstructed field and therefore generally corresponds to the dominant or primary reconstructed field. Unless explicitly stated otherwise, the term "reconstruction field" is interpreted to refer to the zeroth-order reconstructed field. The term "reconstruction plane" refers to the plane in space that contains all reconstructed fields. The terms "image," "reconstructed image," and "image region" refer to the region of the reconstructed field illuminated by the light of the holographic reconstruction. In some embodiments, the "image" is composed of individual spots, called "image spots" or, for convenience, "image pixels."

[0025] The terms "encoding," "writing," or "addressing" are used to describe the process of providing a plurality of pixels of an SLM with a plurality of control values ​​that respectively determine the modulation level of each pixel. The pixels of the SLM are said to be configured to "display" a light modulation distribution in response to receiving the plurality of control values. The SLM is therefore said to "display" a hologram, and a hologram can be thought of as an array of light modulation values ​​or levels.

[0026] It has been shown that holographic reconstructions of acceptable quality can be formed from "holograms" that contain only phase information related to the Fourier transform of the original object. Such holographic recordings are sometimes referred to as phase-only holograms. Although the embodiments relate to phase-only holograms, the present disclosure is equally applicable to amplitude-only holography.

[0027] The present disclosure is equally applicable to forming a holographic reconstruction using amplitude and phase information associated with the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called full complex hologram, which contains both amplitude and phase information associated with the original object. Such holograms are sometimes referred to as full complex holograms because the value (gray level) assigned to each pixel of the hologram has an amplitude and a phase component. The value (gray level) assigned to each pixel can be represented as a complex number having both an amplitude and a phase component. In some embodiments, a full complex computer-generated hologram is calculated.

[0028] The phase of a pixel of a computer-generated hologram or spatial light modulator, referred to as a phase value, phase component, phase information, or simply phase, is sometimes referred to as an abbreviation for "phase delay." That is, the described phase value is actually a number (e.g., ranging from 0 to 2π) representing the amount of phase delay provided by that pixel. For example, a spatial light modulator pixel described as having a phase value of π / 2 delays the phase of received light by π / 2 radians. In some embodiments, each pixel of a spatial light modulator is operable at one of multiple possible modulation values ​​(e.g., phase delay values). The term "gray level" is sometimes used to refer to multiple available modulation levels. For example, the term "gray level" is sometimes used for convenience to refer to multiple phase levels available in a phase-only modulator, even though the different phase levels do not provide different shades of gray. The term "gray level" is sometimes used for convenience to refer to multiple complex modulation levels available in a complex modulator.

[0029] A hologram therefore consists of an array of gray levels, i.e., an array of optical modulation values, such as an array of phase delay values ​​or complex modulation values. A hologram can also be considered a diffraction pattern, since it is a pattern displayed on a spatial light modulator and causes diffraction when illuminated with light of a wavelength comparable to (but usually shorter than) the pixel pitch of the spatial light modulator. Here, we refer to combining holograms with other diffraction patterns, such as diffraction patterns that act as lenses or gratings. For example, a diffraction pattern acting as a grating can be combined with a hologram to transform the reconstruction field on the reconstruction plane, or a diffraction pattern acting as a lens can be combined with a hologram to focus the holographic reconstruction on the reconstruction plane in the near field.

[0030] In the detailed description that follows, different embodiments and groups of embodiments may be disclosed separately, but 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, i.e., all possible combinations and permutations of features disclosed in this disclosure are contemplated. [Brief explanation of the drawings]

[0031] Specific embodiments will now be described, by way of example only, with reference to the following figures:

[0032] [Figure 1] FIG. 1 is a schematic diagram showing a reflective SLM generating a holographic reconstruction on a screen. [Figure 2] FIG. 2 shows an image for projection that includes cross sections of eight image regions / components V1-V8 and corresponding hologram channels H1-H8. [Figure 3] Figure 3 shows a hologram displayed on an LCOS that directs light to multiple discrete areas. [Figure 4]FIG. 4 shows a system including a display device that displays the holograms calculated as shown in FIGS. [Figure 5A] FIG. 5A is a perspective view of a first example of a two-dimensional pupil dilator made up of two replicas, each made up of a pair of laminated surfaces. [Figure 5B] FIG. 5B is a perspective view of a first example of a two-dimensional pupil expander, consisting of two replicas, each in the form of a solid waveguide. [Figure 6] FIG. 6 shows the principle of the chief ray angle that determines the pupil position. [Figure 7] Figure 7 shows how this principle can be applied to separate the x and y pupils. [Figure 8] FIG. 8 shows the horizontal plane of the embodiment. [Figure 9] FIG. 9 shows a vertical view of the embodiment. [Figure 10] FIG. 10 shows the distance from the coupling lens to the first and second waveguide input ports. [Figure 11A] FIG. 11A shows a cross section of a first waveguide according to a comparative example. [Figure 11B] FIG. 11B shows a cross section of a second waveguide according to an embodiment that provides improved optical coupling.

[0033] The same reference numbers are used throughout the drawings to refer to the same or similar parts. DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention is not limited to the embodiments described below, but rather encompasses the full scope of the appended claims, i.e., the present invention may be embodied in different forms and should not be construed as being limited to the embodiments set forth for illustrative purposes.

[0035] Singular terms may include plurals unless otherwise specified.

[0036] A structure described as being formed on top / bottom of, or above / below, another structure is to be interpreted as including cases where the structures contact each other and even cases where a third structure is disposed between them.

[0037] When describing temporal relationships, for example, when the temporal order of events is described as "after," "succeeding," "next," "before," etc., the disclosure should be construed as including sequential and non-sequential events unless otherwise specified. For example, unless words such as "just," "immediately," "directly," etc. are used, the description should be construed as including non-sequential cases.

[0038] In this specification, terms such as "first" and "second" may be used to describe various elements, but these elements are not limited by these terms. These terms are used only to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of the appended claims.

[0039] Features of different embodiments may be partially or wholly combined or combined with one another and may interoperate with one another in various ways, and some embodiments may execute independently of one another or may execute together in an interdependent manner.

[0040] In the present disclosure, the term "substantially" when applied to a structural unit of an apparatus may be interpreted as meaning that the technical characteristics of the structural unit are produced within the technical limits of the method used to manufacture it.

[0041] Conventional optical configuration for holographic projection FIG. 1 illustrates an embodiment in which a computer-generated hologram is encoded onto a single spatial light modulator. The computer-generated hologram is the Fourier transform of the object for reconstruction. Therefore, the hologram can be said to be 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 onto the spatial light modulator, and a holographic reconstruction is formed at a replay field, e.g., a light-receiving surface such as a screen or diffuser.

[0042] A light source 110, e.g., a laser or laser diode, is positioned to illuminate the SLM 140 through a collimating lens 111. The collimating lens directs a substantially planar wavefront of light into the SLM. In FIG. 1, the wavefront direction is not perpendicular (e.g., 2 or 3 degrees away from true orthogonal to the plane of the transparent layer). However, in other embodiments, a substantially planar wavefront is provided at normal incidence, and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in FIG. 1, light from the light source is arranged to reflect off the back mirror of the SLM and interact with the light modulating layer to form an output wavefront 112. The output wavefront 112 is applied to an optical system including a Fourier transform lens 120 focused onto a screen 125. More specifically, the Fourier transform lens 120 receives the modulated beam of light from the SLM 140 and performs a frequency-space transformation to generate a holographic reconstruction on the screen 125.

[0043] Specifically, in this type of holography, each pixel of the hologram contributes to the overall reconstruction: there is no one-to-one correlation between a specific point on the replay field (or image pixel) and a specific light-modulating element (or hologram pixel). In other words, the modulated light leaving the light-modulating layer is distributed throughout the entire replay field.

[0044] In these embodiments, the position of the holographic reconstruction in space is determined by the refractive power (focusing power) of the Fourier transform lens. In the embodiment shown in FIG. 1 , the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens, and the Fourier transform is performed optically. Any lens can function as a Fourier transform lens, but the performance of the lens limits the accuracy of the performed Fourier transform. Those skilled in the art understand how to perform an optical Fourier transform using lenses. In some embodiments of the present disclosure, the lens in the observer's eye performs the conversion from hologram to image.

[0045] Hologram Calculation In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram, or a Fourier-based hologram, in which an image is reconstructed in the far field using the Fourier transform properties of a positive lens. A Fourier hologram is calculated by Fourier transforming the desired light field at the reconstruction plane back to the lens plane. A computer-generated Fourier hologram can be calculated using the Fourier transform. The embodiments relate, by way of example only, to Fourier holography and Gerchberg-Saxton-type algorithms. This disclosure is equally applicable to Fresnel holography and Fresnel holograms, which can be calculated in a similar manner. In some embodiments, the hologram is a phase or phase-only hologram. However, this disclosure is also applicable to holograms calculated by other techniques, such as those based on point cloud methods.

[0046] In some embodiments, the hologram engine is configured to exclude from the hologram calculation the contribution of light blocked by the display system's limiting aperture. UK Patent Application No. 2101666.2, filed February 5, 2021, and incorporated herein by reference, discloses a first hologram calculation method that uses eye tracking and ray tracing to identify subareas of a display device for the calculation of point cloud holograms that eliminate ghost images. The subareas of the display device correspond to the apertures of the present disclosure and are used to exclude light paths from the hologram calculation. UK Patent Application No. 2112213.0, filed August 26, 2021, and incorporated herein by reference, discloses a second method based on a modified Gerchberg-Saxton algorithm, including cropping the light field according to the pupil of the optical system during hologram calculation. Cropping the light field corresponds to determining the limiting aperture of the present disclosure. UK Patent Application 2118911.3, filed December 23, 2021, and incorporated herein by reference, discloses a third method for calculating a hologram, which includes determining an area of ​​a so-called extended modulator formed by a hologram replicator. According to this disclosure, the extended modulator area is also an aperture.

[0047] In some embodiments, a real-time engine is provided that is configured to receive image data and use an algorithm to calculate a hologram in real time. In some embodiments, the image data is a video that includes a series of image frames. In other embodiments, the hologram is pre-calculated, stored in computer memory, and recalled for display on the SLM as needed. That is, in some embodiments, a repository of pre-defined holograms is provided.

[0048] Wide field of view using a small display device Broadly, this disclosure relates to image projection. This disclosure relates to an image projector including a method of image projection and a display device. This disclosure also relates to a projection system including an image projector and a display system. In this projection system, the image projector projects or relays light from a display device to the display system. This disclosure is equally applicable to monocular and binocular display systems. The display system can include a viewer's eye or multiple eyes. The display system includes an optical element having optical power (e.g., a lens in a human eye) and a display surface (e.g., a retina in a human eye). The projector is sometimes 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 image is formed on the display surface or perceived by the viewer. In some embodiments, the image is a virtual image, and the display surface is sometimes referred to as a virtual image surface. In other examples, the image is a real image formed by holographic reconstruction, and the image is projected or relayed to the display surface. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed in free space or on a screen or other light-receiving surface between the display device and the viewer is propagated to the viewer. In both cases, the image is formed by illuminating a diffractive pattern (such as a hologram or kinoform) that is displayed on a display device.

[0049] A display device is made up of pixels. The pixels of a display can display a diffraction pattern or structure that diffracts light. The diffracted light forms an image at a plane spatially separated from the display device. According to well-known optics, the magnitude of the maximum diffraction angle depends on the size of the pixel and other factors such as the wavelength of the light.

[0050] In embodiments, the display device is a spatial light modulator, such as a liquid crystal on silicon ("LCOS") spatial light modulator (SLM). Light propagates from the LCOS toward a viewing entity / system, such as a camera or an eye, over a range of diffraction angles (e.g., from zero to a maximum diffraction angle). In some embodiments, magnification techniques can be used to expand the range of available diffraction angles beyond the conventional maximum diffraction angle of the LCOS.

[0051] In some embodiments, the hologram itself (light) is transmitted to the eye. For example, the hologram's spatially modulated light (not yet fully converted into a holographic reconstruction, i.e., an image), which may informally be referred to as "encoded" by the hologram, is transmitted directly to the viewer's eye. The viewer may perceive a real or virtual image. In these embodiments, no intermediate holographic reconstruction / image is formed between the display device and the viewer. In these embodiments, the lens of the eye is sometimes said to perform the hologram-to-image transformation or conversion. A projection system or light engine can be configured so that the viewer effectively views the display device directly.

[0052] Throughout the specification, references are made to a "light field," but this is a "complex light field." The term "light field" simply denotes a pattern of light that has finite size in at least two orthogonal spatial directions, e.g., x and y. The term "complex" is used herein simply to indicate that the light at each point in the light field is defined by an amplitude and a phase value and may therefore be represented by a complex number or pair of values. For purposes of hologram calculations, a complex light field is a two-dimensional array of complex numbers, where the complex numbers define the intensity and phase of light at multiple discrete locations within the light field.

[0053] According to well-known optical principles, the range of angles of light propagating from a display device that can be observed by the eye or other viewing object / system varies depending on the distance between the display device and the viewing object. For example, at a viewing distance of one meter, only a small portion of the angles from an LCOS can pass through the eye's pupil and form an image on the retina at a particular eye position. The range of angles of light rays propagating from the display device that can pass through the eye's pupil and form an image on the retina determines the portion of the image that is "visible" to the viewer. In other words, not all parts of the image are visible from any one point on the viewing surface (e.g., any one eye position within a viewing window such as the eyebox).

[0054] In some embodiments, the image perceived by the viewer is a virtual image displayed upstream of the display device. That is, the viewer perceives the image as being farther away than the display device. Conceptually, the viewer can be thought of as viewing the virtual image through a very small "display device-sized window," such as 1 cm in diameter, at a relatively large distance, e.g., 1 m. The user also sees the display device-sized window through a very small eye pupil. Thus, the field of view is narrowed, and the specific angular range that can be seen is highly dependent on the eye position at any given time.

[0055] Pupil expanders address the problem of how to expand the angular range of light rays propagating from a display device and successfully pass through the eye's pupil to form an image. Display devices are generally (relatively) small and have (relatively) large projection distances. In some embodiments, the projection distance is at least one order of magnitude, e.g., at least two orders of magnitude, larger than the diameter or width of the display device's entrance pupil and / or aperture (i.e., the size of the pixel array).

[0056] The use of a pupil expander expands the viewing zone (i.e., the user's eyebox) laterally, allowing for eye movement while still allowing the user to see the image. As a skilled artisan will appreciate, in an imaging system, the viewing area (the user's eyebox) is the area in which the observer's eyes can perceive an image. The present disclosure is directed to non-infinite virtual image distances, i.e., near-field virtual images.

[0057] Traditionally, a two-dimensional pupil expander consists of one or more one-dimensional optical waveguides, each formed using a pair of opposing reflective surfaces, with output light from the surfaces forming a viewing window or eyebox. Light received from a display device (e.g., spatially modulated light from an LCOS) is replicated by the or each waveguide so that the field of view (or viewing area) is expanded in at least one dimension. In particular, the waveguides generate additional light rays or "replicas" by amplitude division of the incident wavefront, thereby expanding the viewing window.

[0058] The display device may have an active or display area that is less than 10 cm, e.g., less than 5 cm or less than 2 cm. The propagation distance between the display device and the display system may be greater than 1 m, e.g., greater than 1.5 m or greater than 2 m. The light propagation distance within the waveguide may be up to 2 m, e.g., up to 1.5 m or up to 1 m. The method can receive an image and determine a corresponding hologram of sufficient quality in less than 20 ms, e.g., less than 15 ms or less than 10 ms.

[0059] In some embodiments, described solely as examples of diffractive or holographic light fields according to the present disclosure, a hologram is configured to route light into multiple channels, each corresponding to a different portion (i.e., subarea) of an image. The channels formed by the diffractive structures are referred to herein simply as "hologram channels" to reflect that they are channels of light holographically encoded with image information. The light in each channel is said to reside in the hologram domain, rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram, and thus the hologram domain is the Fourier or frequency domain. The hologram may similarly be a Fresnel or Fresnel transform hologram. The hologram may also be a point cloud hologram. Holograms are described herein as routing light into multiple hologram channels, each corresponding to a different image subregion, to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into multiple image subregions. Importantly, the hologram in this example is characterized by how it distributes image content when illuminated. Specifically, the hologram divides the image content by angle. That is, each point on the image is associated with a unique ray angle in the spatially modulated light formed by the hologram when illuminated—at least, a unique pair of angles, since the hologram is two-dimensional. For the avoidance of doubt, the operation of this hologram is unconventional. When illuminated, the spatially modulated light formed by this special type of hologram is split into multiple hologram channels, each defined by a range (in two dimensions) of ray angles. From the foregoing, it will be understood that each hologram channel (i.e., sub-range of ray angles) that may be considered in the spatially modulated light is associated with a respective portion or sub-region of the image. That is, all information necessary to reconstruct that portion or sub-region of the image is contained within the sub-range of angles of the spatially modulated light formed from the hologram of the image.When the spatially modulated light is viewed as a whole, there is not necessarily evidence of multiple individual light channels.

[0060] Nevertheless, the hologram is identifiable. For example, if only a continuous portion or subregion of the spatially modulated light formed by the hologram is reconstructed, only a subregion of the image should be visible. If a different continuous portion or subregion of the spatially modulated light is reconstructed, a different subregion of the image should be visible. A further distinguishing feature of this type of hologram is that the cross-sectional shape of the hologram channel substantially corresponds to (i.e., is substantially the same as) the shape of the entrance pupil, although the sizes may differ, at least in the correct plane from which the hologram was calculated. Each light / hologram channel propagates from the hologram at a different angle or range of angles. These are exemplary methods for characterizing or identifying this type of hologram, but other methods may also be used. In summary, the holograms disclosed herein are characterized and identifiable by how the image content is distributed within the light encoded by the hologram. Again, for the avoidance of doubt, references herein to holograms configured to direct light or angularly split an image into multiple holographic channels are made by way of example only, and the present disclosure is equally applicable to any type of holographic light field, as well as any type of diffracted light field or pupil dilation of a diffracted light field.

[0061] The system can be provided in a compact and streamlined physical form, making it suitable for a variety of practical applications where space is limited and real estate is at a premium, such as implementation in head-up displays (HUDs) in vehicles and automobiles.

[0062] According to the present disclosure, pupil dilation is provided for diffracted light or diffracted light comprising diverging ray bundles. The diffracted light field is defined by a "light cone." Thus, the size of the diffracted light field (defined on a two-dimensional plane) increases with the propagation distance from the corresponding diffractive structure (i.e., display device). The pupil dilator can be said to replicate a hologram or form at least one replica of a hologram, imparting that the light delivered to the viewer is spatially modulated according to the hologram.

[0063] In some embodiments, two one-dimensional waveguide pupil expanders are provided, each positioned to effectively expand the size of the system's exit pupil by forming multiple replicas or copies of the spatial light modulator's exit pupil (or the light at the exit pupil). An exit pupil can be understood to be the physical area from which light is output by the system. Each waveguide pupil expander can also be said to be positioned to expand the size of the system's exit pupil. Each waveguide pupil expander can also be said to be positioned to expand / increase the size of the eyebox where an observer's eye can be positioned to see / receive the light output by the system.

[0064] Light Modulation The display system, in an embodiment, comprises a display device that defines an exit pupil of the display system. The display device is a spatial light modulator. The spatial light modulator may be a phase modulator. The display device may be a liquid crystal on silicon, "LCOS," spatial light modulator.

[0065] Light Channeling Holograms formed according to some embodiments angularly split the image content to provide multiple hologram channels that can have cross-sectional shapes defined by the aperture of the optical system. The hologram is calculated to provide this channeling of the diffracted light field. In some embodiments, this is achieved during hologram calculation by taking into account the aperture (virtual or real) of the optical system, as described above.

[0066] 2 and 3 show examples of this type of hologram that can be used in conjunction with the pupil dilation devices disclosed herein, but this example does not limit the invention.

[0067] FIG. 2 illustrates a projection image 252 containing eight image regions / components V1 through V8. While FIG. 2 shows eight image components as an example, image 252 can be divided into any number of components. FIG. 2 also illustrates an encoded light pattern 254 (i.e., a hologram) that can reconstruct image 252, such as when transformed by lenses in an appropriate display system. Encoded light pattern 254 is composed of first through eighth sub-holograms or components H1 through H8, corresponding to the first through eighth image components / regions V1 through V8. FIG. 2 further illustrates how a hologram decomposes image content by angle. Thus, a hologram is characterized by the light channeling it performs. This is illustrated in FIG. 3. Specifically, the hologram in this example directs light into multiple discrete regions. In the illustrated example, the discrete regions are disks, but other shapes are also contemplated. The optimal disk size and shape may be related to the size and shape of the optical aperture, such as the entrance pupil of the observation system, after propagation through the waveguide.

[0068] FIG. 4 shows a system 400 including a display device for displaying the holograms calculated as shown in FIGS.

[0069] System 400 includes a display device, which in this arrangement includes an LCOS 402. LCOS 402 is positioned to display a modulation pattern (or "diffraction pattern") that includes a hologram and project the holographically encoded light toward eye 405, which includes a pupil that serves as aperture 404, a lens 409, and a retina (not shown) that serves as a viewing surface. There is a light source (not shown) positioned to illuminate LCOS 402. Lens 409 of eye 405 performs the conversion from the hologram to an image. The light source may be of any suitable type, for example, a laser light source.

[0070] The vision system 400 further comprises a waveguide 408 disposed between the LCOS 402 and the eye 405. The presence of the waveguide 408 ensures that all angular content from the LCOS 402 is received by the eye, even at the relatively long projection distances shown, because the waveguide 408 acts as a pupil expander. This method is well known and will only be briefly described here.

[0071] Briefly, the waveguide 408 shown in FIG. 4 comprises a substantially elongated structure. In this example, the waveguide 408 comprises an optical slab of refractive material, although other types of waveguides are well known and may be used. The waveguide 408 is positioned, for example, at an oblique angle, to intersect with the light cone (i.e., the diffracted light field) projected from the LCOS 402. In this example, the size, location, and position of the waveguide 408 are configured so that light from each of eight ray bundles within the light cone enters the waveguide 408 through a first planar surface of the waveguide 408 (closest to the LCOS 402) and is guided at least partially along the length of the waveguide 408 before exiting through a second planar surface (closest to the eye) substantially opposite the first surface. As will be appreciated, the second planar surface may be partially reflective and partially transmissive. In other words, as each ray of light travels within the waveguide 408 from a first plane and strikes a second plane, some of the light is transmitted out of the waveguide 408, and some is reflected off the second plane back to the first plane. The first plane is reflective, so all of the light that strikes the first plane from within the waveguide 408 is reflected off the second plane. Thus, some of the light is refracted between the two planes of the waveguide 408 before being transmitted, while other light is reflected and undergoes one or more reflections (or "bounces") between the planes of the waveguide 408 before being transmitted.

[0072] FIG. 4 shows a total of nine "reflection" points B0 through B8 along the length of waveguide 408. As shown in FIG. 2, light associated with all points (V1-V8) of the image is transmitted from the waveguide at each "reflection" from the second plane of waveguide 408, but only light from one angular portion of the image (e.g., any light from V1 through V8) has a trajectory that allows it to reach eye 405 from each "reflection" point B0 through B8. Furthermore, light from different angular portions of the image V1 through V8 reaches eye 405 from each "reflection" point. Thus, in the example of FIG. 4, each angular channel of encoded light reaches the eye only once from waveguide 408.

[0073] The waveguide 408 forms multiple replicas of the hologram along its length at respective "bounce" points B1-B8 corresponding to the pupil dilation direction. As shown in FIG. 5, the multiple replicas are extrapolated in straight lines to corresponding multiple replica or virtual display devices 402'. This process corresponds to "unfolding" the light path within the waveguide so that the replica light rays are extrapolated to a "virtual surface" without internal reflection within the waveguide. Thus, the light at the expanded exit pupil can be considered to originate from the virtual surfaces (also referred to herein as "expansion modulators") that make up the display device 402 and replica display device 402'.

[0074] Although this specification has generally discussed virtual images, in which the eye must transform received modulated light to form a perceived image, the methods and arrangements described herein can also be applied to real images.

[0075] 2D pupil dilation While the arrangement shown in Figure 4 includes a single waveguide that provides pupil dilation in one dimension, pupil dilation can be provided in multiple dimensions, such as two. Additionally, while the example of Figure 4 uses a calculated hologram to create channels of light that correspond to different portions of the image, this disclosure and the systems described below are not limited to such types of holograms.

[0076] FIG. 5A shows a perspective view of a system 500 that includes two replicators 504, 506 arranged to expand a light beam 502 in two dimensions.

[0077] In system 500 of FIG. 5A, first replicator 504 comprises a first pair of surfaces stacked parallel to one another and arranged to provide replication (or pupil dilation) similar to waveguide 408 of FIG. 4. The first pair of surfaces have similar (possibly identical) sizes and shapes and are substantially elongated in one direction. A collimated light beam 502 is directed to the input of first replicator 504. As is well known to the skilled reader, due to the process of internal reflection between the two surfaces and partial transmission of light from each of multiple output points on one of the surfaces (the top surface as shown in FIG. 5A), the light of light beam 502 is replicated in a first direction along the length of first replicator 504. Thus, a first plurality of replica light beams 508 are emitted from first replicator 504 toward second replicator 506.

[0078] The second replicator 506 comprises a second pair of surfaces stacked parallel to one another and positioned to receive each of the collimated rays of the first plurality of light beams 508 and to provide replication, i.e., pupil dilation, by expanding each of those rays in a second direction substantially perpendicular to the first direction. The first pair of surfaces are similar (possibly identical) in size and shape to one another and are substantially rectangular. The second replicator is implemented in a rectangular shape so that it has a length along the first direction to receive the first plurality of light beams 508 and a length along a second, orthogonal direction to provide replication in the second direction. Through a process of internal reflection between the two surfaces and partial transmission of light from each of a plurality of output points on one of the surfaces (the top surface as shown in FIG. 5A ), the light of each ray in the first plurality of light beams 508 is replicated in the second direction. Thus, a second plurality of light beams 510 is emitted from the second replica device 506, the second plurality of light beams 510 comprising replicas of the input light beam 502 along each of the first and second directions. Thus, the second plurality of light beams 510 can be considered to comprise a two-dimensional grid or array of replica light beams.

[0079] 5A , the combination of the first and second replicators 504, 505 may be said to provide a two-dimensional replicator (or a “two-dimensional pupil dilator”). Thus, a replica light beam 510 may be emitted along a light path to an extended eyebox of a display system, such as a head-up display.

[0080] In the system of Figure 5A, the first replicator 504 is a waveguide including a pair of elongated, linear reflective surfaces stacked parallel to one another, and similarly, the second replicator 504 is a waveguide including a pair of rectangular reflective surfaces stacked parallel to one another. In other systems, the first replicator is a solid, elongated, linear waveguide and the second replicator is a solid, planar, rectangular waveguide, each including a solid, optically transparent material such as glass. In this case, the pair of parallel reflective surfaces is formed by a pair of opposing major sidewalls, each optionally including a reflective and a reflective-transmissive surface coating, familiar to the skilled reader.

[0081] FIG. 5B shows a perspective view of a system 500 including two replicators 520, 540 arranged to replicate a light beam 522 in two dimensions, the first replicator being a solid, elongated waveguide 520 and the second replicator being a solid, planar waveguide 540.

[0082] In the system of FIG. 5B, the first replicator / waveguide 520 is positioned so that its pair of elongated, parallel reflective surfaces 524a, 524b are perpendicular to the plane of the second replicator / waveguide 540. Thus, the system includes an optical coupler positioned 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 flat / folding mirror 530 positioned to fold and rotate the optical path of the light to achieve the required optical coupling from the first replicator to the second replicator. As shown in FIG. 5B, the mirror 530 is positioned to receive light from the output port / reflective-transmissive surface 524a of the first replicator / waveguide 520, which contains a one-dimensional array of replicas extending in the first dimension. Mirror 530 is tilted at an angle that provides waveguiding and replication along its length in the second dimension to redirect the received light onto a path to an input port on the (fully) reflective surface of second replicator 540. Mirror 530 is one example of an optical element capable of redirecting light in the illustrated manner, and it will be appreciated that one or more other elements may be used instead to perform this task.

[0083] In the illustrated arrangement, the (partially) reflective-transmissive surface 524a of the first replicator 520 is adjacent to the input port of the first replicator / waveguide 520 and receives the input beam 522 at an angle for guiding and replicating along its length in the first dimension. The input port of the first replicator / waveguide 520 is therefore located at the input end of the same surface as the reflective-transmissive surface 524a. The skilled reader will appreciate that the input port of the first replicator / waveguide 520 may be located in other suitable locations.

[0084] 5B allows first replicator 520 and mirror 530 to be provided as part of a relatively thin first layer in a plane in the first and third dimensions (illustrated as the x-z plane). Notably, the size or "height" of the first planar layer in which first replicator 520 is disposed is reduced in the second dimension (illustrated as the y-dimension). Mirror 530 is configured to direct light away from the first layer / plane in which first replicator 520 is disposed (i.e., the "first planar layer") and toward a second layer / plane above and substantially parallel to the first layer / plane in which second replicator 540 is disposed (i.e., the "second planar layer"). Thus, the overall size or "height" of the system, including first and second replicators 520, 540 and mirrors 530 arranged in stacked first and second planar layers in the first and third dimensions (illustrated as the xz planes), is compact in the second dimension (illustrated as the y dimension). The skilled reader will appreciate that many variations of the arrangement of Figure 5B are possible and contemplated for implementing the present disclosure.

[0085] The image projector may be arranged to project a diverging light field or a diffracted light field. In some embodiments, the diffracted light field is encoded with a hologram. In some embodiments, the diffracted light field consists of diverging light ray bundles. In some embodiments, the image formed by the diffracted light field is a virtual image.

[0086] In some embodiments, the first pair of parallel / complementary surfaces are elongated or narrow surfaces that are relatively long along a first dimension and relatively short along a second dimension, e.g., relatively short along each of two other dimensions, each dimension being substantially orthogonal to each other dimension. The process of reflection / transmission of light between / from the first pair of parallel surfaces is arranged such that light propagates within the first waveguide pupil expander, and the general direction of propagation of light is the direction in which the first waveguide pupil expander is relatively long (i.e., its "elongated" direction).

[0087] Disclosed herein is a system that uses diffracted light to form an image, providing an eyebox size and field of view suitable for real-world applications, such as head-up displays in the automotive industry. Diffracted light is light that forms a holographic reconstruction of an image from a diffractive structure, such as a Fourier hologram or a Fresnel hologram. The use of diffraction and diffractive structures requires a display device with a high density of very small pixels (e.g., 1 micrometer), which in practice means a small display device (e.g., 1 centimeter). The inventors addressed the problem of how to provide 2D pupil expansion with a diffracted light field, such as diffracted light consisting of diverging (uncollimated) ray bundles.

[0088] In some embodiments, the display system includes a display device, e.g., a pixelated display device such as a spatial light modulator (SLM) or a liquid crystal on silicon (LCoS) SLM, arranged to provide or shape diffracted or divergent light. In such embodiments, the aperture of the spatial light modulator (SLM) is the limiting aperture of the system. That is, the aperture of the spatial light modulator, more specifically, the size of the area bounding the array of light-modulating pixels configured within the SLM, determines the size (e.g., spatial extent) of the bundle of light rays that can exit the system. In accordance with the present disclosure, the system's exit pupil (limited by the small display device pixel size for light diffraction) is described as being expanded to reflect the increased or increased spatial extent achieved by using at least one pupil expander.

[0089] A diffracted or diverged optical field can be said to have an "optical field magnitude" defined in a direction substantially perpendicular to the direction of propagation of the optical field. Because light is diffracted / diverged, the optical field magnitude increases with propagation distance.

[0090] In some embodiments, the diffracted light field is spatially modulated according to a hologram. In other words, in such aspects, the diffracted light field constitutes a "holographic light field." The hologram may be displayed on a pixelated display device. The hologram may be a computer-generated hologram (CGH). The hologram may be a Fourier hologram, a Fresnel hologram, a point cloud hologram, or any other suitable type of hologram. The hologram may optionally be calculated to form channels of holographic light, each channel corresponding to a different respective portion of the image intended to be seen (or perceived, if virtual) by the viewer. The pixelated display device may be configured to display multiple different holograms, either sequentially or sequentially. Each aspect and embodiment disclosed herein may be applicable to the display of multiple holograms.

[0091] The output port of the first waveguide pupil expander may be coupled to the input port of a second waveguide pupil expander, which may be arranged to direct the diffracted optical field—including some, preferably most, preferably all, of the replica of the optical field output by the first waveguide pupil expander—from its input port to a respective output port by internal reflection between a third pair of parallel faces of the second waveguide pupil expander.

[0092] The first waveguide pupil expander may be positioned to provide pupil expansion or replication in a first direction, and the second waveguide pupil expander may be positioned to provide pupil expansion or replication in a second, different direction. The second direction may be substantially orthogonal to the first direction. The second waveguide pupil expander may be positioned to maintain the pupil expansion provided by the first waveguide pupil expander in the first direction and expand (or replicate) a portion, preferably a majority, or preferably all, of the replica received from the first waveguide pupil expander in a second, different direction. The second waveguide pupil expander may be positioned to receive the optical field directly or indirectly from the first waveguide pupil expander. One or more other elements may be provided along the propagation path of the optical field between the first and second waveguide pupil expanders.

[0093] The first waveguide pupil expander may have a substantially elongated shape, and the second waveguide pupil expander may be substantially planar. The elongated shape of the first waveguide pupil expander may be defined by a length along a first dimension. The planar, i.e., rectangular, shape of the second waveguide pupil expander may be defined by a length along the first dimension and a width along a second dimension, i.e., a width, that is substantially orthogonal to the first dimension. The length along the first dimension of the first waveguide pupil expander corresponds to the length or width along the first or second dimension, respectively, of the second waveguide pupil expander. A first of the pair of parallel sides of the second waveguide pupil expander, which constitutes its input port, may be shaped, sized, and / or positioned to correspond to an area defined by an output port on the first of the pair of parallel sides of the first waveguide pupil expander, such that the second waveguide pupil expander is positioned to receive each of the replicas output by the first waveguide pupil expander.

[0094] The first and second waveguide pupil dilators may collectively provide pupil dilation in a first direction and a second direction perpendicular to the first direction, and optionally, a plane containing the first and second directions is substantially parallel to the plane of the second waveguide pupil dilator. In other words, the first and second dimensions defining the length and width, respectively, of the second waveguide pupil dilator may be parallel to the first and second directions, respectively, in which the waveguide pupil dilator provides pupil dilation (or may be parallel to the second and first directions, respectively). The combination of the first waveguide pupil dilator and the second waveguide pupil dilator may be generally referred to as a "pupil dilator."

[0095] The magnification / duplication provided by the first and second waveguide expanders can be said to have 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 define an expanded eyebox region, from which the viewer can receive light from the input diffracted or divergent light field. The eyebox region can be said to be located on or define the viewing plane.

[0096] The two directions in which the exit pupil is expanded may be coplanar with or parallel to the first and second directions in which the first and second waveguide pupil expanders provide duplication / magnification. Alternatively, in an arrangement that includes another element, such as an optical combiner, e.g., a vehicle windshield (or windshield), the exit pupil can be considered the exit pupil from the other element, such as the windshield. In such an arrangement, the exit pupil may be non-parallel to the first and second directions in which the first and second waveguide pupil expanders provide duplication / magnification. For example, the exit pupil may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide duplication / magnification.

[0097] The field of view plane and / or eyebox region may be non-planar or non-parallel to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion. For example, the field of view plane may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication / expansion.

[0098] The elongated dimension of the first waveguide pupil expander may be angled relative to the first and second dimensions of the second waveguide pupil expander to provide suitable launch conditions for achieving internal reflection within the first and second waveguide pupil expanders.

[0099] Combiner shape correction An advantage of projecting a hologram into the eyebox is that optical compensation can be encoded into the hologram (see, for example, EP 2936252, incorporated herein). The present 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 a vehicle windshield. Details of this approach are described in EP 2936252, and the detailed features of those systems and methods are not repeated here as they are not essential to the novel teachings of the present disclosure and are merely exemplary of configurations that would benefit from the teachings of the present disclosure.

[0100] Control device The present disclosure is also compatible with optical configurations including a control device (e.g., an optical shutter device) for controlling the delivery of light from a light-channeling hologram to a viewer. The holographic projector may further include a control device arranged to control the delivery of the angular channels to an eyebox location. UK Patent Application No. 2108456.1, filed June 14, 2021, and incorporated herein by reference, discloses at least one waveguide pupil expander and control device. From at least this prior disclosure, the reader will understand that the optical configuration of the control device is essentially based on the user's eyebox location and is compatible with any hologram computation method that achieves light channeling as described herein. The control device can be described as an optical shutter or iris device. The optical shutter device includes a 1D array of apertures or windows, each of which can be independently switched between a light-transmitting state and a light-non-transmitting state to control the delivery of the hologram light channel and its replica to the eyebox. Each aperture or window may include multiple liquid crystal cells or pixels.

[0101] Chief ray optimization FIG. 6 shows a rough outline of the principle of the concept devised by the inventors. The inventors addressed the problem of how to achieve efficient light coupling into at least one-dimensional waveguides or pupil expanders. To improve light coupling within the waveguide, a coupling lens is used. The coupling lens receives a wavefront corresponding to an image. The coupling lens can be used (i.e., positioned) to form a pupil that receives all ray angles of the wavefront. The focal plane of the coupling lens may be aligned (e.g., coincident or coplanar) with the (intermediate) image plane (of the image). The angle of the chief ray forming the image on the (intermediate) image plane determines the distance from the coupling lens to the pupil. First, the inventors recognized that the angle of the chief ray can therefore be used to align the pupil formed by the coupling lens with the entrance port or window of the waveguide. Notably, the inventors further recognized that when the chief ray angle has different components in a first (e.g., horizontal) direction and a second (e.g., vertical) direction, a first (e.g., horizontal) and a second (e.g., vertical) pupil are formed. Furthermore, the inventors recognized that this is highly synergistic with display systems using one-dimensional replicators or pupil dilation, since one pupil (e.g., a horizontal pupil) can be aligned with a one-dimensional replicator (e.g., to provide a horizontal replica) and, optionally, another pupil (e.g., a vertical pupil) can be aligned with a second one-dimensional replicator (e.g., to provide a vertical replica). The pupil direction and the replica direction are coincident, i.e., the same. Therefore, optimized light coupling (replica direction) can be provided. Most notably, if the display system consists of two one-dimensional replicas (e.g., a first replica in the x direction and a second replica in the y direction), the first (x) pupil of the coupling lens is aligned with the input to the first replica, and the second (y) pupil of the coupling lens is aligned with the input to the second replica. The process of adjusting the chief ray angle to align a pupil (e.g., the pupil of the coupling lens) with the entrance port or input window of a waveguide or pupil expander is referred to herein as chief ray optimization.

[0102] More specifically, FIG. 6 illustrates a diffusion screen or diffuser 650 positioned to display an image. The image may be a holographic reconstruction formed by illuminating a hologram of the image. The image is formed by a display device. Light received by the diffuser 650 is characterized by a chief ray. At least conceptually, the image may be composed of multiple image points, such as a first image point 652 and a second image point 654. As will be understood by those skilled in the art of optics, each image point is formed by a bundle of rays that reach the diffuser 650, characterized by a chief ray or chief ray angle. The chief ray angle is an angle in three-dimensional space that may be defined by a first component / angle with the optical axis in a first plane (e.g., a horizontal plane) and a second component / angle with the optical axis in a second plane (e.g., a vertical plane).

[0103] FIG. 6 shows that the first image point 652 is located on the optical axis 640, i.e., at the point where the diffuser 650 intersects with the optical axis 640. Regarding the second image point 654, FIG. 6 shows two different examples to explain the concepts of the present disclosure. In the first example, the second image point 654 corresponds to a first chief ray 601 that is perpendicular to the plane of the diffuser 650. More specifically, the first chief ray 601 is parallel to the optical axis 640 in the yz plane. The first chief ray 601 is perpendicular to the plane of the diffuser 650. The diffuser 650 diffuses the incoming light characterized by the first chief ray 601. Therefore, a first diffused cone of light 613 corresponding to the first chief ray 601 is emitted from the diffuser. The first diffused cone of light 613 is characterized by a first axis 621, which is a continuation of the first chief ray 601. The first axis 621 corresponds to the chief ray of the light exiting the diffuser 650. The angle of the first chief ray 601 that forms the first image point 601 is not changed by the diffuser 650. However, the diffusive nature of the diffuser 650 effectively spreads the light over a cone. Thus, the first chief ray 601 reaching the diffuser 650 corresponds to a first image point 654 on the diffuser 650 and a first ray cone 613 about the first axis 621 downstream of the diffuser 650. In the second example, the second image point 654 corresponds to a second chief ray 603 that is not perpendicular to the plane of the diffuser 650. More specifically, there is an acute angle between the second chief ray 603 and the optical axis 640 in the yz plane. The diffuser 650 also diffuses the second chief ray 603 to form a second ray cone 611 centered on a second axis 621. The second axis 621 is a continuation of the second chief ray 603. Therefore, the first example corresponds to a first chief ray angle (with the z-axis in the yz plane), and the second example corresponds to a second chief ray angle (with the z-axis in the yz plane). The light emitted from the diffuser 650 is received by a coupling lens 660. The distance from the diffuser 650 to the coupling lens 660 is equal to the focal length of the coupling lens 660. In other words, the diffuser 650, or the image formed thereon, is located on the focal plane of the coupling lens 660. Therefore, the coupling lens 660 collimates the received light.Importantly, it has been found that the coupling lens 660 forms a pupil, and the angle of the chief rays forming the image determines the position of the pupil, more specifically, the (perpendicular) distance from the coupling lens to the plane of the pupil (z-direction). Thus, FIG. 6 shows how a first example, corresponding to a first chief ray 601, forms a first pupil 670 at a first distance from the coupling lens 660, and a second example, corresponding to a second chief ray 603, forms a second pupil 680 at a second distance from the coupling lens 660. The first distance is smaller than the second distance. In other words, the second pupil 680 is downstream of the first pupil 670. In summary, FIG. 6 shows how the angle of the chief rays forming an image on the diffuser 650 determines the position of the pupil formed by the coupling lens 660.

[0104] It is worth noting that the inventors used a coupling lens to form two pupils because the two orthogonal components of the chief ray angle are different. That is, the first component of the chief ray angle is different from the second component of the chief ray angle. This concept is illustrated in FIG. 7. FIG. 7 shows a diffuser 750 positioned at the focal plane of a coupling lens 760 to display an image thereon. FIG. 7 shows how the first component of the chief ray angle forms a first pupil 770, and the second component of the chief ray angle forms a second pupil 780 downstream of the first pupil. The first component is a first angle the chief ray makes with the z-axis in the x-z plane, and the second component is a second angle the chief ray makes with the z-axis in the y-z plane. The first pupil may be formed in the x-z plane, and the second pupil may be formed in the x-y plane. The first pupil may be parallel to the x-axis, and the second pupil may be parallel to the y-axis. For the avoidance of doubt, an image is composed of a plurality of image points, each characterized (e.g., formed) by a chief ray having a first (angular) component and a second (angular) component. The first component corresponds to a first pupil, and the second component corresponds to a second pupil formed by the coupling lens 760. The first pupil extends in a first direction, and the second pupil extends in a second direction. In some embodiments, all image points have the same first component but different second components. The first component of all image points may be zero or constant. The second component may be a function of the second direction, i.e., may vary, for example, linearly, in the second direction. The maximum or minimum value of the function corresponds to the center of the image. Figure 7 illustrates the distance or displacement between a first pupil 770 and a second pupil 780 achieved by forming an image using chief rays with different components in two perpendicular directions.

[0105] 8 and 9 further illustrate the effect of the chief ray (or chief ray angle) on pupil position.

[0106] FIG. 8 depicts a horizontal plane and shows three exemplary chief rays 841-843 forming an image on a diffuser 850. On the illustrated horizontal plane, the angles of incidence of the chief rays (including the three example chief rays 841-843) on the diffuser 850 are 90 degrees (i.e., normal incidence). The three exemplary chief rays 841-843 form respective angles 801h-803h with the diffuser 850 on the illustrated horizontal plane. In this embodiment, 801h = 802h = 803h = 90 degrees (normal incidence). The diffuser 850 effectively diffuses the light of each chief ray over a range of angles (e.g., ±5 degrees). Thus, each image point and chief ray produces a diffuse cone of light. The three exemplary chief rays 841-843 shown in FIG. 8 produce respective diffuse cones of light 851-853. Therefore, the axis of each diffused light cone 851-853 is also perpendicular to the plane of the diffuser plate.

[0107] The divergent cones of light 851-853 are received by a coupling lens 860, which forms a first (horizontal) pupil 820 corresponding to the horizontal direction. At least one of the size, shape, and position of the first pupil 820 may correspond to the size, shape, and position of a first input port 830 of a first (horizontal) waveguide or pupil expander (not shown in FIG. 8 ). The first pupil 820 may be substantially aligned with the first input port 830 of the first (horizontal) waveguide or pupil expander. The distance from the coupling lens 860 to the first (horizontal) pupil 820 is equal to the focal length of the coupling lens 860. The position of the first input port 830 ensures that the full range of (ray) angles from the image is coupled into the first (horizontal) waveguide or pupil expander.

[0108] FIG. 9 depicts a vertical plane of the same embodiment, showing three example chief rays 841-843 forming an image on a diffuser 850. In the illustrated vertical plane, the chief rays are not all perpendicular to the diffuser 850. Furthermore, the angles of incidence of the chief rays on the diffuser are not uniform or constant. In the illustrated vertical plane, the angles of incidence are a function of the vertical direction y. Three example chief rays 841-843 make angles 901v-903v with the diffuser 850 in the illustrated vertical plane. In this embodiment, 902v = 90 degrees (normal incidence), but 901v > 90 degrees and 903v < 90 degrees. Because diffuser 850 is isotropic in this embodiment—although this disclosure encompasses anisotropic diffusers characterized by different diffusion angles in the x and y directions as well—diffuser 850 effectively diffuses the light of each chief ray over the same diffusion angle (e.g., ±5 degrees), as per FIG. 8 . Thus, each image point and chief ray gives rise to a diffused cone of light not only in the horizontal plane ( FIG. 8 ) but also in the vertical plane ( FIG. 9 ). The three example chief rays 841-843 shown in FIG. 9 give rise to respective diffused cones of light 951-953. Thus, the axis of each diffused light cone 951-953 is also a function of y.

[0109] The divergent cones of light 951-953 are received by a coupling lens 860, which forms a second (vertical) pupil 920 corresponding to the vertical direction. At least one of the size, shape, and position of the second pupil 920 may correspond to the size, shape, and position of a second input port 930 of a second (vertical) waveguide or pupil expander (not shown in FIG. 9 ). The second pupil 920 may be substantially aligned with the second input port 930 of the second (vertical) waveguide or pupil expander. The distance from the coupling lens 860 to the second (vertical) pupil 920 is not equal to the focal length of the coupling lens 860. In this embodiment, the distance from the coupling lens 860 to the second (vertical) pupil 920 is greater than the focal length of the coupling lens 860. The shortest propagation distance from the coupling lens 860 to the second pupil 920 is represented by a first distance d1 in FIG. 9 , and the corresponding longest propagation distance is represented by a second distance d2. The third distance d3 (= d2 - d1) represents the range of propagation distances to the second input port 930. In an embodiment, the second pupil 920 is positioned at the center of the third distance d3. The position of the second input port 930 of the second waveguide or pupil expander ensures that the full range of (ray) angles from the image is coupled into the second (vertical) waveguide or pupil expander. The fourth distance d4 (extending evenly on both sides of the second (vertical) pupil 920) represents the range in which all (ray) angles in the image exist. The fourth distance d4 may be greater than the third distance d3.

[0110] FIG. 10 is a further representation of an embodiment showing a diffuser 850 positioned to display an image thereon, a coupling lens 860 positioned to receive diffused light from the diffuser, a first waveguide 1010 positioned to replicate in a first (x) direction, and a second waveguide 1020 positioned to replicate in a second (y) direction. The first waveguide 1010 has a first input port 1012, and the second waveguide 1020 has a second input port 1022. FIG. 10 illustrates the distances d1 and d2 of FIG. 9, i.e., the shortest and longest propagation distances d1 and d2 from the coupling lens 860 to the second input port 1022 of the second waveguide 1020. A 2D array of replicas 1050 output by the display system is also shown in FIG. 10.

[0111] Those skilled in the art of optical design will know how to manipulate the chief ray angle, as described herein and shown by way of example only in FIGS. 8 and 9 . Those skilled in the art will also know how to position the x-pupil and y-pupil formed by the coupling lens so that they are optically aligned or coupled with the corresponding input port / s of the waveguide / s. Those skilled in the art will understand that the optical system for this purpose—referred to herein as the “imaging optics”—may actually be comprised of multiple optical systems, such as lenses, lens pairs, curved surfaces, mirrors, or prisms. In advantageous embodiments, the image-forming optics comprises at least one toroidal or cylindrical optical surface. The image-forming optics may additionally be positioned to provide one-dimensional compression and / or to substantially direct the wavefront to one side of its optical axis. The image-forming optics may be positioned to be centered (or nearly centered) with an offset image for better performance. In some embodiments, the image-forming optics is substantially offset to one side of the optical axis and nearly centered with the image content. In some embodiments, the image forming optics have different properties in the x and y directions and compress the image in the y direction (e.g., x2 or x3).

[0112] In one embodiment—disclosed herein by way of example only—an image-forming optical system (or optical system) is comprised of first, second, and third lens groups. The first group is comprised of (i) a cylindrical lens and a standard spherical lens, or (ii) a cylindrical lens. The second group is comprised of (i) a standard spherical lens and an aspherical lens, or (ii) a single standard lens. The third group consists of only a single cylindrical lens. For the avoidance of doubt, the present disclosure is not limited to any one design for achieving pupil separation as disclosed herein. Those skilled in the art of optical design will be able to provide the functionality of aspects of the present disclosure using any number of different designs.

[0113] 11A and 11B illustrate the benefits of the chief ray optimization of the present disclosure in terms of light coupling efficiency into a waveguide or waveguide, with Fig. 11A representing a non-optimized configuration and Fig. 11B representing an optimized configuration according to an embodiment.

[0114] FIG. 11A shows an arrangement without chief ray optimization. FIG. 11A shows a first waveguide 1110 with a first input port 1120. The entire height of the first waveguide is filled with a pupil 1180. In the horizontal (x) direction, light couples onto a first edge 1160 in the plane of the page. In the vertical (y) direction, light couples onto a second edge 1170 beyond the plane of the page. A projection of a second input port 1130 of the second waveguide is shown running along the longitudinal axis of the first waveguide 1110. A "negative" ray bundle 1140, corresponding to one side of the image (the "negative" angle of the ray bundle in FIG. 8), and a "positive" ray bundle 1150, corresponding to the other side of the image (the "positive" angle of the ray bundle in FIG. 8), are shown.

[0115] Figure 11B shows an arrangement with Chief Ray optimization. Figure 11B also shows the first waveguide 1110 and the first input port 1120. In this embodiment, the angle content input is matched to the propagation direction and location where the light is coupled into the second waveguide. Negative angles are represented by negative ray bundles 1240, and "positive" angles are represented by positive ray bundles 1250. In this embodiment, more efficient light coupling is achieved, as represented by the increased overlap between the (second) input port 1130 and the two ray bundles 1240, 1250.

[0116] Additional Features The methods and processes described herein can be embodied in a computer-readable medium. The term "computer-readable medium" includes a medium configured to store data temporarily or permanently, such as random access memory (RAM), read-only memory (ROM), buffer memory, flash memory, cache memory, etc. The term "computer-readable medium" is also intended to include any medium, or combination of media, capable of storing instructions for execution by a machine. The instructions, when executed by one or more processors, cause the machine to perform, in whole or in part, one or more of the methods described herein.

[0117] The term "computer-readable medium" also includes cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible, non-transitory data repositories (e.g., data volumes), such as solid-state memory chips, optical disks, magnetic disks, or any suitable combination thereof. In some embodiments, executable instructions may be carried by a carrier medium. Examples of such carrier media include transitory media, such as a propagated signal carrying the instructions.

[0118] 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 that come within the scope of the appended claims and their equivalents.

Claims

1. 1. A display system comprising: an optical subsystem configured to form an optimized wavefront including chief rays each characterized by a first angle in a first direction and a second angle in a second direction, the second angle being a function of the second direction; a coupling lens configured to receive the optimized wavefront and form a first pupil corresponding to the first direction and a second pupil corresponding to the second direction, the second pupil being displaced from the first pupil by a difference between the first angle and the second angle; a first one-dimensional replicator configured to replicate light in the first direction, an entrance port of the first one-dimensional replicator substantially aligned with the first pupil; A display system comprising:

2. 2. The display system of claim 1, wherein the second angle of the optimized wavefront increases or decreases with distance from a center of the coupling lens in the second direction.

3. 3. The display system of claim 2, wherein the increase is linear.

4. 4. The display system of claim 1, wherein the first angle is zero.

5. The display system of any one of claims 1 to 4, wherein the first angle is constant.

6. 6. A display system as claimed in any preceding claim, further comprising a second one-dimensional replicator configured to replicate in the second direction, an entrance port of the second one-dimensional replicator being substantially aligned with the second pupil.

7. 7. The display system of claim 1, wherein the maximum difference between the first angle and the second angle is less than 20 degrees.

8. 8. The display system of claim 1, wherein the optical subsystem comprises an image-forming optics configured to receive a source wavefront including chief rays characterized by a first angle in a first direction and a second angle in a second direction, where the first angle is constant with respect to the first direction and the second angle is constant with respect to the first direction, and to output the optimized wavefront.

9. 10. The display system of claim 8, wherein the image-forming optics comprises at least one toroidal or cylindrical optical surface.

10. 10. A display system according to claim 8 or 9, wherein the image-forming optics are configured to provide one-dimensional compression.

11. 11. A display system according to any one of claims 8 to 10, wherein the imaging optics is configured to direct the source wavefront substantially to one side of an optical axis of the imaging optics.

12. The display system of any preceding claim, wherein the optical subsystem further comprises a screen configured to receive the source wavefront and form an image thereon.

13. 13. The display system of claim 12, wherein the screen is diffusing such that the image is diffuse and is characterized by a diffusion angle.

14. 14. The display system of claim 13, wherein the diffusion angle in the x-direction is substantially equal to the diffusion angle in the y-direction.

15. A display system according to any preceding claim, wherein at least one of the size, shape and position of the first pupil is substantially equal to at least one of the size, shape and position of an entrance port of one or more of the first one-dimensional replicators.

16. 16. A display system as claimed in any preceding claim, wherein at least one of the size, shape and position of the second pupil is substantially equal to at least one of the size, shape and position of an entrance port of a second one-dimensional replicator, the second one-dimensional replicator having an entrance port coupled to an output port of the first one-dimensional replicator.

17. 17. A display system according to any one of claims 1 to 16, wherein a distance from an image plane to the coupling lens is equal to a focal length of the coupling lens, and / or a distance from the coupling lens to the first pupil is equal to the focal length of the coupling lens.

18. A display system according to any preceding claim, wherein the first one-dimensional replicator comprises a waveguide and / or the second one-dimensional replicator comprises a waveguide.

19. 20. The display system of claim 18, wherein the first one-dimensional replicator is substantially elongated and the second one-dimensional replicator is substantially planar.

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