Pupil expander integrity

Abstract

A system and method includes a display device including a spatial light modulator arranged to output spatially modulated light to form an image. The system also includes a waveguide pupil expander configured to receive the spatially modulated light from the display device at an input port thereof and to expand a viewing window of the system. The system also includes a controller. In an example, the controller is configured to control the spatially modulated light output by the display device in response to a signal indicative of a detection of a glass break, such as controlling (e.g., turning off) a light source of the display device. The signal indicative of the detection of the glass break can be generated in response to a detection of stray laser light from the holographic system by an eye tracking system.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to pupil expanders, such as planar waveguide pupil expanders. The present disclosure also relates to projectors and projection methods, such as holographic projectors and holographic projection methods. Some embodiments relate to head-up displays. Some embodiments relate to waveguide pupil expanders that maintain functional and / or structural integrity in case of breakage. BACKGROUND

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

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

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

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

[0006] Holographic display devices, such as holographic projectors, can be provided using the systems described herein. For example, such projectors have found application in head-up displays "HUDs" and light detection and ranging "LIDARs". SUMMARY

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

[0008] Disclosed herein is a system comprising a display device and a waveguide pupil expander. The display device is arranged to output spatially modulated light to form an image. The waveguide pupil expander comprises a pair of parallel planar reflective surfaces. The waveguide pupil expander defines an input port and an output port or observation surface. The input port is arranged to receive spatially modulated light from the display device. The output port is arranged to output the spatially modulated light to an observation window of the system. The observation window is typically an area or volume where an observer can observe or perceive an image. The pair of parallel reflective surfaces are arranged to guide the spatially modulated light from the input port to the output port through a series of internal reflections. A first reflective surface of the pair of parallel reflective surfaces is partially transmissive and partially reflective, and a second reflective surface of the pair of parallel reflective surfaces is substantially completely reflective (i.e., a near-perfect mirror).

[0009] According to the present disclosure, a waveguide pupil expander includes a glass structure that may be subject to mechanical, structural, and / or optical failure due to, for example, glass breakage. In an embodiment, the glass structure includes a layered glass structure that is arranged to maintain the integrity of the waveguide pupil expander in the event of glass breakage, for example. In particular, the layered glass structure includes at least one layer of glass material and at least one layer of another material having properties that maintain the integrity of the waveguide pupil expander. In the following description, the term "glass" refers to any form of optically transparent (solid) material that is brittle (i.e., breakable upon impact), including silica-based glass or crystalline optically transparent material.

[0010] In this disclosure, reference to the "integrity" of a waveguide pupil expander may refer to its structural integrity. The structural integrity of a waveguide pupil expander may be considered maintained when the (external) reflective surfaces remain parallel in the event of a glass breakage therein. Furthermore, the integrity of a waveguide pupil expander may refer to its functional integrity. The functional integrity of a waveguide pupil expander may be considered maintained when at least a portion of light received at its input port is directed to an observation area in the event of a glass breakage therein.

[0011] In an embodiment, a layered glass structure of a waveguide pupil expander includes a glass layer and an optically transparent non-glass layer. In some examples, the layered glass structure includes a glass layer laminated together with or laminated onto a polymer-based layer. The polymer-based layer may include polymer-based louvers, polymer-based polarizers, etc. In other examples, the layered glass structure includes a glass layer and a resin layer. In these examples, the layered glass structure may form an optically transparent material between the first and second reflective surfaces. The layered glass structure may have low birefringence and / or the resin may include a resin material having a refractive index matched to that of the glass. The layered glass structure may include a resin interlayer between the inner major surfaces of the first and second glass layers. The layered glass structure may have a fully reflective coating on the outer major surface of the first glass layer and a partially reflective coating on the outer major surface of the second glass layer. The resin material may be arranged to maintain parallelism between the outer major surfaces of the first and second glass layers. For example, the resin material may include polyvinyl butyral.

[0012] In other embodiments, the layered glass structure includes two or more layers of tempered glass. In these examples, the layered glass structure can form an optically transparent material between the first and second reflective surfaces. In some examples, the layered glass structure includes a first layer of glass under compressive strain, a second layer of glass under tensile strain, and optionally, a third layer of glass under compressive or tensile strain.

[0013] The display device and the waveguide pupil expander may be referred to as a projection system or an image projection system. In some embodiments, the projection system is a holographic system. The holographic system includes a display device including a spatial light modulator configured to display a diffraction pattern of / corresponding to an image (e.g., a hologram) and output spatially modulated light according to the diffraction pattern. The projection system also includes a waveguide pupil expander configured to receive the spatially modulated light from the display device at its input port and expand the viewing window of the projection system.

[0014] In some embodiments, the display device further includes a controller (e.g., a system controller or a holographic controller) configured to control the spatially modulated light output by the display device. In an example, the controller is configured to control (e.g., turn off) a light source of the display device in response to a signal indicating detection of glass breakage.

[0015] In an embodiment, the system further comprises an observer tracking system (or eye tracking system) in communication with a system controller (e.g. a holographic controller). The observer tracking system may be arranged to determine an observation position (e.g. an eye position) within an observation window. The observer tracking system is further arranged to monitor the observer's face to detect stray light incident thereon, and may be arranged to provide a signal to the controller when stray light is detected. In an example, the observer tracking system comprises a light detector directed towards the observer's face to detect stray visible light, such as a visible laser, on the observer's face. In an example, the observer tracking system comprises an infrared light source configured to periodically illuminate the observer's face. The light detector may be configured to detect stray infrared light. The light detector may be configured to detect light of both infrared and visible wavelengths.

[0016] A signal indicative of one or more of a mechanical, structural, or optical failure of the waveguide pupil expander can be generated in response to detection of stray laser light from the projection system by the eye tracking system. In some embodiments, the eye tracking system is configured to use infrared light for eye tracking. In embodiments, the light detector of the eye tracking system is also configured to respond to visible light, optionally between pulses or gates of infrared light emission for eye tracking purposes. In some embodiments, a wavelength conversion element (e.g., quantum dots) is incorporated to allow the infrared camera of the eye tracking system to detect non-infrared light (e.g., visible laser light from the projection system). In other embodiments, a broadband (i.e., infrared and visible light) camera serves dual purposes for the eye tracking system and stray laser detection—e.g., in a gating / timing scheme as described above.

[0017] In an embodiment, the waveguide pupil expander is further configured so that its first totally reflective surface can be mounted to or integrated with a relatively large and stable component that is arranged to absorb any impact, thereby reducing the risk of glass shattering. In an example, the component comprises metal to improve thermal stability.

[0018] In an embodiment, the system further comprises a light detection system for monitoring scattered light. In an example, the light detection system is configured to detect scattered light from one or more of the following: the waveguide pupil expander, optical components of the system, and external reflective components around the system, such as a windshield or mirrors.

[0019] In an embodiment, the system further comprises a container of expandable foam arranged to release foam to surround one or more optical components of the system in response to detection of an event, such as an impact or rupture event.In some examples, the expander foam comprises a light absorbing filler material.

[0020] In an embodiment, fragile optical components of the system such as a pupil expander waveguide are immersed in a (transparent) fluid; a high refractive index (transparent) liquid is provided between the waveguide and the cover glass, and / or an optically clear adhesive at least partially surrounds the waveguide pupil expander.

[0021] A method of operating a system including a display device and a waveguide pupil expander is also provided. The method includes displaying a diffraction pattern corresponding to an image (e.g., a hologram of the image) on a display device (e.g., a spatial light modulator). The method also includes outputting spatially modulated light by the display device to an input port of a waveguide pupil expander having an output port or an observation surface. The method also includes guiding the spatially modulated light to the output port by the waveguide pupil expander through a series of internal reflections, and outputting the spatially modulated light from the output port to form an image at an observation window of the system. The method also includes monitoring stray light incident on an observer's face in the image by an observer tracking system. The method also includes outputting a signal by the observer tracking system when stray light is detected. The signal indicates a system failure event, such as one or more of a mechanical, structural, or optical failure of the waveguide pupil expander.

[0022] In some embodiments, monitoring further comprises providing pulsed illumination of the observer's face by the observer tracking system. In some examples, the pulsed illumination is infrared (IR) light. In embodiments, the method further comprises detecting (reflections of) stray visible light incident on the observer's face by the observation tracking system. In examples using pulsed IR illumination, stray visible light can be detected between the IR pulses, and optionally, IR light can be detected during the IR pulses for observer eye tracking.

[0023] In an embodiment, the method further comprises responding, by a controller of the system, to a signal output by the viewer tracking system. In an example, the response may include reducing a drive signal for a light source of the display device or turning off the light source.

[0024] The term "hologram" is used to refer to a recording containing amplitude information or phase information, or some combination thereof, about an object. The recording of the hologram can be stored in a data storage device (i.e., a memory) or embodied in light (e.g., as an optical signal) that forms a carrier wave of the amplitude and / or phase information. In other words, the light can be described as being "encoded with the hologram" or "modulated according to the hologram" so as to transmit a hologram rather than an image.

[0025] The term "holographic reconstruction" is used to refer to the optical reconstruction of an object formed by illuminating a hologram. The system disclosed herein is described as a "holographic projector" because the holographic reconstruction is a real image and is spatially separated from the hologram. The term "replay field" is used to refer to the 2D region within which the holographic reconstruction is formed and is perfectly focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in multiple diffraction orders, where each diffraction order is a copy of the zeroth-order replay field. The zeroth-order replay field generally corresponds to the preferred or primary replay field because it is the brightest replay field. Unless explicitly stated otherwise, the term "replay field" should be considered to refer to the zeroth-order replay field. The term "replay plane" is used to refer to the plane in space that contains all replay fields. The terms "image," "replay image," and "image region" refer to the region of the replay field illuminated by the light of the holographic reconstruction. In some embodiments, an "image" may include discrete points, which may be referred to as "image points" or, for convenience only, as "image pixels."

[0026] The terms "writing" and "addressing" can be used to describe the process of providing a plurality of control values, respectively determining the modulation level of each pixel, to a plurality of pixels of the SLM. It can be said that the pixels of the SLM are configured to "display" a light modulation profile in response to receiving the plurality of control values. Thus, the SLM can be said to "display" a hologram, and a hologram can be considered an array of light modulation values ​​or levels.

[0027] It has been found 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 may be referred to as phase-only holograms. The embodiments relate to phase-only holograms, but the present disclosure is equally applicable to amplitude-only holography.

[0028] The present disclosure is also 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 using complex modulation of a so-called full complex hologram that contains amplitude and phase information related to the original object. Because the value (gray level) assigned to each pixel of the hologram has both amplitude and phase components, such a hologram can be referred to as a full complex hologram. The value (gray level) assigned to each pixel can be represented as a complex number having both amplitude and phase components. In some embodiments, a full complex computer-generated hologram is calculated.

[0029] Reference may be made to the phase value, phase component, phase information, or simply phase of a pixel of a computer-generated hologram or spatial light modulator as shorthand for "phase delay". That is, any phase value described is actually a number (e.g., in the range of 0 to 2π) that represents the amount of phase delay provided by that pixel. For example, a pixel of a spatial light modulator described as having a phase value of π / 2 will delay the phase of received light by π / 2 radians. In some embodiments, each pixel of the spatial light modulator can operate at one of a plurality of possible modulation values ​​(e.g., phase delay values). The term "grayscale" may be used to refer to a plurality of available modulation levels. For example, the term "grayscale" may be used for convenience to refer to a plurality of available phase levels in a phase modulator alone, even though the different phase levels do not provide different shades of gray. For convenience, the term "grayscale" may also be used to refer to a plurality of available complex modulation levels in a complex modulator.

[0030] Thus, a hologram comprises an array of gray levels, i.e. an array of light modulation values, such as phase delay values ​​or an array of complex modulation values. A hologram is also considered a diffraction pattern, since it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light of a wavelength relative to (usually less than) the pixel pitch of the spatial light modulator. Reference is made herein to combining a hologram with other diffraction patterns, such as diffraction patterns used as lenses or gratings. For example, a diffraction pattern used as a grating can be combined with a hologram to shift the replay field on the replay plane, or a diffraction pattern used as a lens can be combined with a hologram to focus the holographic reconstruction on the replay plane in the near field.

[0031] Although different embodiments and groups of embodiments may be disclosed separately in the detailed description below, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of the features disclosed in this disclosure are contemplated. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Certain embodiments are described, by way of example only, with reference to the following drawings:

[0033] Figure 1 is a schematic diagram showing a reflective SLM producing a holographic reconstruction on a screen;

[0034] Figure 2A A first iteration of an example Gerchberg-Saxton type algorithm is shown;

[0035] Figure 2B The second and subsequent iterations of an example Gerchberg-Saxton type algorithm are shown;

[0036] Figure 2CAn alternative second and subsequent iteration of an example Gerchberg-Saxton type algorithm is shown;

[0037] Figure 3 is a schematic diagram of a reflective LCOS SLM;

[0038] Figure 4 An example pupil expander including a waveguide is shown;

[0039] Figure 5 A holographic display device including a pupil expander according to an embodiment is shown;

[0040] Figure 6 shows a holographic display device including a pupil expander according to a further embodiment;

[0041] Figure 7 shows a layered structure of a pupil expander according to an embodiment;

[0042] Figure 8 shows a layered structure of a pupil expander according to a further embodiment;

[0043] Figure 9 shows a layered structure of a pupil expander according to other embodiments; and

[0044] Figure 10 A layered structure of a pupil expander according to other embodiments is shown.

[0045] The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION

[0046] The present invention is not limited to the embodiments described below, but extends to the full scope of the appended claims. That is, the present invention can be embodied in different forms and should not be construed as limited to the described embodiments, which are set forth for illustrative purposes.

[0047] Unless otherwise stated, terms in the singular may include plural forms.

[0048] A structure described as being formed on / under or above / below another structure should be construed to include a case where the structures are in contact with each other and, further, a case where a third structure is provided therebetween.

[0049] When describing a temporal relationship, for example, when the temporal order of events is described as "after," "followed," "next," "before," etc., the present disclosure should be considered to include both consecutive and non-consecutive events unless otherwise specified. For example, unless terms such as "just," "immediately," or "directly" are used, the description should be considered to include non-consecutive situations.

[0050] Although the terms "first," "second," etc. may be used herein to describe various elements, these elements are not limited by these terms. These terms are only used to distinguish between the various elements. For example, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element without departing from the scope of the appended claims.

[0051] The features of different embodiments may be coupled or combined with each other in part or in whole, and may interoperate with each other in different ways. Some embodiments may be performed independently of each other, or may be performed together in a mutually dependent relationship.

[0052] Optical configuration

[0053] Figure 1 An embodiment is shown in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object being reconstructed. Thus, 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 on the spatial light modulator, and a holographic reconstruction is formed at a replay field, such as a light-receiving surface (e.g., a screen or diffuser).

[0054] A light source 110, such as a laser or laser diode, is arranged to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a substantially planar wavefront of light to be incident on the SLM. Figure 1 In embodiments, the direction of the wavefront is off-normal (e.g., two or three degrees from a plane that is truly normal to 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. Figure 1 In the embodiment shown, the arrangement is such that light from the light source reflects from the mirrored back surface of the SLM and interacts with the light modulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to an optical device including a Fourier transform lens 120, the focus of which is located at a screen 125. More specifically, the Fourier transform lens 120 receives the modulated light beam from the SLM 140 and performs a frequency-to-space transform to produce a holographic reconstruction at the screen 125.

[0055] It's important to note that in this type of hologram, every pixel of the hologram contributes to the entire reconstruction. There is no one-to-one correlation between a specific point on the replay field (or image pixel) and a specific light modulation element (or hologram pixel). In other words, the modulated light leaving the light modulation layer is distributed across the entire replay field.

[0056] In these embodiments, the position of the holographic reconstruction in space is determined by the power (focus) of the Fourier transform lens. Figure 1 In the illustrated embodiment, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and optically performs a Fourier transform. Any lens can function as a Fourier transform lens, but the accuracy of the Fourier transform it performs will be limited by the lens's performance. Those skilled in the art understand how to use lenses to perform an optical Fourier transform.

[0057] Figure 1 Embodiments of the invention can be used as part of a holographic system in which a holographic reconstruction or replay image is relayed to an observation area. As will be appreciated by those skilled in the art, in other embodiments, the holographic system can be used in a holographic system in which the outgoing wavefront 112 is relayed to an observation area without forming an intermediate holographic reconstruction. It is sometimes said that in these embodiments, the lens of the eye performs the conversion or transformation of the hologram into an image.

[0058] Hologram computing

[0059] In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or a Fourier-based hologram, in which the image is reconstructed in the far field by utilizing the Fourier transform properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back to the lens plane. The computer-generated Fourier hologram can be calculated using the Fourier transform.

[0060] Algorithms such as the Gerchberg-Saxton algorithm can be used to calculate Fourier transform holograms. Furthermore, the Gerchberg-Saxton algorithm can be used to calculate a hologram in the Fourier domain (i.e., a Fourier transform hologram) from amplitude-only information in the spatial domain (e.g., a photograph). Phase information about the object is effectively "retrieved" from amplitude-only information in the spatial domain. In some embodiments, computer-generated holograms are calculated from amplitude-only information using the Gerchberg-Saxton algorithm or a variation thereof.

[0061] The Gerchberg-Saxton algorithm considers that when the intensity cross section I of the beam in planes A and B is known, A (x,y) and I B (x,y) and I A (x,y) and I B (x,y) is related by a single Fourier transform. For a given intensity cross section, the phase distribution approximation Ψ in planes A and B is obtained separately A (x,y) and Ψ B(x,y). The Gerchberg-Saxton algorithm finds the solution to this problem by following an iterative process. More specifically, the Gerchberg-Saxton algorithm iteratively applies spatial and spectral constraints while repeatedly transferring representations of I between the spatial domain and the Fourier (spectral or frequency) domain. A (x,y) and I B A data set (amplitude and phase) of (x, y) is obtained. A corresponding computer-generated hologram in the spectral domain is obtained by at least one iteration of the algorithm. The algorithm is convergent and is arranged to produce a hologram representing the input image. The hologram can be an amplitude-only hologram, a phase-only hologram, or a full complex hologram.

[0062] In some embodiments, a phase-only hologram is calculated using an algorithm based on the Gerchberg-Saxton algorithm, such as the algorithm described in UK Patent 2498170 or 2501112, the entire contents of which are incorporated herein by reference. However, the embodiments disclosed herein describe calculating a phase-only hologram by way of example only. In these embodiments, the Gerchberg-Saxton algorithm retrieves the phase information Ψ[u,v] of the Fourier transform of a data set, which produces known amplitude information T[x,y], where the amplitude information T[x,y] represents a target image (e.g., a photograph). Since amplitude and phase are inherently combined in the Fourier transform, the transformed amplitude and phase contain useful information about the accuracy of the calculated data set. Therefore, the algorithm can be used iteratively with feedback of the amplitude and phase information. However, in these embodiments, only the phase information Ψ[u,v] is used as a hologram to form a holographic representation of the target image at the image plane. A hologram is a data set (e.g., a 2D array) of phase values.

[0063] In other embodiments, an algorithm based on the Gerchberg-Saxton algorithm is used to calculate a full complex hologram. A full complex hologram is a hologram having an amplitude component and a phase component. A hologram is a data set (e.g., a 2D array) comprising an array of complex data values, where each complex data value comprises an amplitude component and a phase component.

[0064] In some embodiments, the algorithm processes complex data and the Fourier transform is a complex Fourier transform. The complex data can be viewed as comprising (i) a real component and an imaginary component, or (ii) an amplitude component and a phase component. In some embodiments, the two components of the complex data are processed differently at various stages of the algorithm.

[0065] Figure 2AA first iteration of an algorithm for calculating a phase-only hologram according to some embodiments is shown. The input to the algorithm is an input image 210 comprising a 2D array of pixels or data values, wherein each pixel or data value is an amplitude or magnitude value. That is, each pixel or data value of the input image 210 does not have a phase component. Therefore, the input image 210 can be regarded as an amplitude-only or amplitude-only or intensity-only distribution. An example of such an input image 210 is a frame of a photograph or a video comprising a time sequence of frames. The first iteration of the algorithm begins with a data formation step 202A, which comprises assigning a random phase value to each pixel of the input image using a random phase distribution (or random phase seed) 230 to form a starting complex data set, wherein each data element of the data set comprises an amplitude and a phase. It can be said that the starting complex data set represents the input image in the spatial domain.

[0066] First processing block 250 receives an initial complex data set and performs a complex Fourier transform to form a Fourier-transformed complex data set. Second processing block 253 receives the Fourier-transformed complex data set and outputs hologram 280A. In some embodiments, hologram 280A is a phase-only hologram. In these embodiments, second processing block 253 quantizes each phase value and sets each amplitude value to 1 to form hologram 280A. Each phase value is quantized according to a phase level that can be represented at a pixel of a spatial light modulator that will be used to "display" the phase-only hologram. For example, if each pixel of the spatial light modulator provides 256 different phase levels, each phase value of the hologram is quantized to one of the 256 possible phase levels. Hologram 280A is a phase-only Fourier hologram representing the input image. In other embodiments, hologram 280A is a full complex hologram, comprising an array of complex data values ​​(each including an amplitude component and a phase component) derived from the received Fourier-transformed complex data set. In some embodiments, second processing block 253 constrains each complex data value to one of a plurality of permissible complex modulation levels to form hologram 280A. The constraining step may include setting each complex data value to the closest permissible complex modulation level in the complex plane. Hologram 280A can be said to represent the input image in the spectral, Fourier, or frequency domain. In some embodiments, the algorithm stops at this point.

[0067] However, in other embodiments, the algorithm continues as Figure 2A In other words, following Figure 2A The steps indicated by dashed arrows are optional (ie, not essential for all embodiments).

[0068] The third processing block 256 receives the modified complex data set from the second processing block 253 and performs an inverse Fourier transform to form an inverse Fourier transformed complex data set. The inverse Fourier transformed complex data set can be said to represent the input image in the spatial domain.

[0069] Fourth processing block 259 receives the inverse Fourier transformed complex data set and extracts the distribution of amplitude values ​​211A and the distribution of phase values ​​213A. Optionally, fourth processing block 259 evaluates the distribution of amplitude values ​​211A. Specifically, fourth processing block 259 may compare the distribution of amplitude values ​​211A of the inverse Fourier transformed complex data set with input image 510, which itself is a distribution of amplitude values. If the difference between the distribution of amplitude values ​​211A and input image 210 is sufficiently small, fourth processing block 259 may determine that hologram 280A is acceptable. In other words, if the difference between the distribution of amplitude values ​​211A and input image 210 is sufficiently small, fourth processing block 259 may determine that hologram 280A is a sufficiently accurate representation of input image 210. In some embodiments, the distribution of phase values ​​213A of the inverse Fourier transformed complex data set is ignored for comparison purposes. It will be appreciated that any number of different methods may be employed to compare the distribution of amplitude values ​​211A to the input image 210, and the present disclosure is not limited to any particular method. In some embodiments, a mean square error is calculated, and if the mean square error is less than a threshold, the hologram 280A is deemed acceptable. If the fourth processing block 259 determines that the hologram 280A is unacceptable, further iterations of the algorithm may be performed. However, this comparison step is not required, and in other embodiments, the number of iterations of the algorithm performed is predetermined, preset, or user-defined.

[0070] Figure 2B 2 represents the second iteration of the algorithm and any further iterations of the algorithm. The distribution of phase values ​​213A from the previous iteration is fed back through the processing blocks of the algorithm. The distribution of amplitude values ​​211A is rejected in favor of the distribution of amplitude values ​​of the input image 210. In the first iteration, the data forming step 202A forms a first complex data set by combining the distribution of amplitude values ​​of the input image 210 with the random phase distribution 230. However, in the second and subsequent iterations, the data forming step 202B includes forming a complex data set by combining (i) the distribution of phase values ​​213A from the previous iteration of the algorithm with (ii) the distribution of amplitude values ​​of the input image 210.

[0071] Then, with reference to Figure 2A The same method as described is handled by Figure 2BThe complex data set formed in step 202B is used to form the second iterative hologram 280B. Therefore, the description of the process is not repeated here. When the second iterative hologram 280B has been calculated, the algorithm can stop. However, any number of further iterations of the algorithm can be performed. It will be understood that the third processing block 256 is only required when the fourth processing block 259 is required or further iterations are required. The output hologram 280B generally gets better with each iteration. However, in practice, a point is often reached where no measurable improvement can be observed, or the positive benefits of performing further iterations are offset by the negative effects of the additional processing time. Therefore, the algorithm is described as iterative and convergent.

[0072] Figure 2C 213A for the previous iteration is fed back through a processing block of the algorithm. The distribution of amplitude values ​​211A is rejected in favor of an alternative distribution of amplitude values. In this alternative embodiment, the alternative distribution of amplitude values ​​is derived from the distribution of amplitude values ​​211 for the previous iteration. Specifically, processing block 258 subtracts the distribution of amplitude values ​​of input image 210 from the distribution of amplitude values ​​211 for the previous iteration, scales the difference by a gain factor α, and subtracts the scaled difference from input image 210. This is mathematically expressed by the following equation, where the subscript text and number represent the number of iterations:

[0073] R n+1 [x,y]=F'{exp(iψ n [u,v])}

[0074] ψ n [u,v]=∠F{η·exp(i∠R n [x,y])}

[0075] η=T[x,y]-α(|R n [x,y]|-T[x,y])

[0076] in:

[0077] F' is the inverse Fourier transform;

[0078] F is the forward Fourier transform;

[0079] R[x,y] is the complex data set output by the third processing block 256;

[0080] T[x,y] is the input or target image;

[0081] ∠ is the phase component;

[0082] Ψ is the phase-only hologram 280B;

[0083] η is the new distribution of amplitude values ​​211B; and

[0084] α is the gain factor.

[0085] The gain factor α may be fixed or variable. In some embodiments, the gain factor α is determined based on the size and rate of the input target image data. In some embodiments, the gain factor α depends on the number of iterations. In some embodiments, the gain factor α is only a function of the number of iterations.

[0086] In all other respects, Figure 2C Examples and Figure 2A and Figure 2B It can be said that only the phase hologram Ψ(u,v) comprises the phase distribution in the frequency or Fourier domain.

[0087] In some embodiments, a spatial light modulator is used to perform a Fourier transform. Specifically, the hologram data is combined with second data that provides optical power. That is, the data written to the spatial light modulation includes hologram data representing the object and lens data representing the lens. When displayed on the spatial light modulator and illuminated with light, the lens data simulates a physical lens - that is, it focuses light in the same way as a corresponding physical optical element. Therefore, the lens data provides optical power or focusing power. In these embodiments, the lens data can be omitted. Figure 1A physical Fourier transform lens 120 is shown. It is known how to calculate data representing a lens. This data representing a lens can be referred to as a software lens. For example, a phase-only lens can be formed by calculating the phase delay caused by each point of the lens due to its refractive index and spatially varying optical path length. For example, the optical path length at the center of a convex lens is greater than the optical path length at the edge of the lens. An amplitude-only lens can be formed using a Fresnel zone plate. In the field of computer-generated holography, it is also known how to combine data representing a lens with a hologram, thereby performing a Fourier transform of the hologram without requiring a physical Fourier lens. In some embodiments, the lensing data is combined with the hologram via a simple addition, such as simple vector addition. In some embodiments, a physical lens is used in conjunction with a software lens to perform the Fourier transform. Alternatively, in other embodiments, the Fourier transform lens is omitted entirely, allowing holographic reconstruction to occur in the far field. In further embodiments, the hologram can be combined in the same manner with grating data—data arranged to perform a grating function, such as image steering. Again, it is known in the art how to calculate such data. For example, a phase-only grating can be formed by modeling the phase delay caused by each point on the surface of the blazed grating. An amplitude-only grating can simply be superimposed with an amplitude-only hologram to provide angular steering for the holographic reconstruction. The second data providing lensing and / or steering can be referred to as a light processing function or light processing pattern to distinguish it from the hologram data, which can be referred to as an image forming function or image forming pattern.

[0088] In some embodiments, the Fourier transform is performed jointly by a physical Fourier transform lens and a software lens. That is, some of the optical power that contributes to the Fourier transform is provided by the software lens, while the remaining optical power that contributes to the Fourier transform is provided by one or more physical optical devices.

[0089] In some embodiments, a real-time engine is provided that uses an algorithm to receive image data and calculate holograms in real time. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are pre-calculated, stored in computer memory, and recalled as needed for display on the SLM. That is, in some embodiments, a repository of pre-determined holograms is provided.

[0090] The embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms that can be calculated using similar methods. The present disclosure is also applicable to holograms calculated using other techniques, such as point cloud-based techniques.

[0091] Optical Modulation

[0092] A spatial light modulator can be used to display diffraction patterns, including computer-generated holograms. If the hologram is a phase-only hologram, a spatial light modulator that modulates the phase is required. If the hologram is a fully complex hologram, a spatial light modulator that modulates both phase and amplitude can be used, or a first spatial light modulator that modulates the phase and a second spatial light modulator that modulates the amplitude can be used.

[0093] Aspects of the present disclosure are not limited to holographic projection systems. Thus, a display device such as a spatial light modulator can be used to display an image. In this case, a single spatial light modulator that (only) modulates amplitude can be used.

[0094] In some embodiments, the light modulation elements (i.e., pixels) of the spatial light modulator are cells comprising liquid crystals. That is, in some embodiments, the spatial light modulator is a liquid crystal device in which the optically active component is a liquid crystal. Each liquid crystal cell is configured to selectively provide multiple light modulation levels. That is, each liquid crystal cell is configured to operate at a light modulation level selected from a plurality of possible light modulation levels at any time. Each liquid crystal cell can be dynamically reconfigured to a light modulation level that is different from the plurality of light modulation levels. In some embodiments, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) spatial light modulator, but the present disclosure is not limited to this type of spatial light modulator.

[0095] LCOS devices provide a dense array of light modulating elements or pixels within a small aperture (e.g., a few centimeters wide). Pixels are typically around 10 microns or smaller, which results in a diffraction angle of a few degrees, meaning the optical system can be compact. It is much easier to fully illuminate the small aperture of an LCOS SLM than the large apertures of other liquid crystal devices. LCOS devices are typically reflective, which means that the circuitry that drives the LCOS SLM pixels can be buried under the reflective surface. This results in a higher aperture ratio. In other words, the pixels are densely packed, meaning there is almost no dead space between pixels. This is advantageous because it reduces optical noise in the playback field. LCOS SLMs use a silicon backplane, which has the advantage that the pixels are optically flat. This is particularly important for phase modulation devices.

[0096] The following is just an example, Figure 3A suitable LCOS SLM is described below. An LCOS device is formed using a single crystal silicon substrate 302. It has a 2D array of square planar aluminum electrodes 301, separated by gaps 301a, arranged on the upper surface of the substrate. Each electrode 301 can be addressed by circuitry 302a embedded in the substrate 302. Each electrode forms its own plane mirror. An orientation layer 303 is disposed on the electrode array, and a liquid crystal layer 304 is disposed on the orientation layer 303. A second orientation layer 305 is disposed on a planar transparent layer 306, for example, made of glass. A single transparent electrode 307, for example, made of ITO, is disposed between the transparent layer 306 and the second orientation layer 305.

[0097] Each square electrode 301, together with the footprint of the transparent electrode 307 and the intervening liquid crystal material, defines a controllable phase-modulating element 308, often referred to as a pixel. The effective pixel area, or fill factor, is the percentage of the total pixel that is optically active, taking into account the spaces between pixels 301a. By controlling the voltage applied to each electrode 301 relative to the transparent electrode 307, the properties of the liquid crystal material of each phase-modulating element can be varied, thereby providing variable retardation to light incident thereon. The effect is to provide only phase modulation of the wavefront, i.e., no amplitude effects occur.

[0098] The described LCOS SLM outputs spatially modulated light in a reflective manner. A reflective LCOS SLM has the advantage that the signal lines, grating lines and transistors are located below the mirror surface, which results in a high fill factor (typically greater than 90%) and high resolution. Another advantage of using a reflective LCOS spatial light modulator is that the thickness of the liquid crystal layer can be half that required when using a transmissive device. This greatly increases the switching speed of the liquid crystal (a key advantage for projecting moving video images). However, the teachings of the present disclosure can also be implemented using a transmissive LCOS SLM.

[0099] Waveguide Pupil / Observation Window Expander

[0100] In projection systems such as heads-up displays (HUDs), it is desirable to expand the system's exit pupil corresponding to the eyebox area or observation window. In particular, the aperture of a display device such as a spatial light modulator is the limiting aperture of the system. That is, the aperture of the spatial light modulator (more specifically, the size of the area that defines the array of light modulating pixels) determines the size (e.g., spatial extent) of the light beam that can leave the system. This is typically quite small. However, the observer needs to be able to move their head around so that the full image can be seen from anywhere within the area / volume at the eyebox / observation distance. This is known as the eye movement box (EMB) or observation window. Therefore, a pupil expander can be used to expand the size of the EMB or observation window. According to the present disclosure, it is therefore stated that the system's exit pupil is expanded by a waveguide pupil expander. It can also be said that the pupil expander expands / increases the size of the received pupil.

[0101] Figure 4 An example pupil expander comprising a planar waveguide is shown. The general principles of waveguides are known in the art and will not be described in detail here. The waveguide guides light within a layer between a pair of parallel reflective surfaces by internal reflection. The pupil expander is formed by a waveguide comprising a first partially reflective surface 420 (e.g., a mirror with partial reflectivity / transmittance) and a second fully reflective surface 410 (e.g., a mirror with substantially 100% reflectivity). The first reflective surface 420 can have a reflectivity that varies with distance, so that the transmitted light has a desired intensity along the length of the waveguide. In particular, the first reflective surface 420 can include a reflective coating whose reflectivity decreases along the length of the waveguide. The layer can be glass or organic glass. Thus, the waveguide can be a block or plate of glass or organic glass. This can be referred to as a "block optic" or "slab waveguide." The first reflective surface can be a first surface of a glass block, and the second reflective surface can be a second surface of the glass block, where the first surface is opposite and parallel to the second surface. Alternatively, the layer may be air and the first and second reflecting surfaces may be separate components, such as first and second mirrors that are spatially separated to form an air gap within which the light is propagated by internal reflection.

[0102] Therefore, if Figure 4An input light beam 402, which can include spatially modulated light encoded with a picture (i.e. light of a picture / image or just a picture) or spatially modulated light encoded with a hologram, as described below, is shown entering the waveguide through an input port of the waveguide. The waveguide is arranged to direct light received at the input port to an output port or viewing surface for output to a viewing window. In the arrangement shown, the input port comprises a gap in a first partially reflective surface 420 near one end of the waveguide, although other locations for the input port are possible. The viewing window is the area or volume within which a viewer can view the image, as described herein. The input light beam 402 has an angle of incidence such that the light rays propagate along the length of the waveguide due to internal reflections off the first partially reflective surface 420 and the second total internal reflection surface 410. Exemplary light rays are shown in Figure 4 Due to the graded reflectivity of the first reflective surface 420, a portion of the light is transmitted by the first reflective surface 420 to provide a plurality of output light rays 404a-f along the length of the waveguide. The first reflective surface 420 thus forms an output port or viewing surface. It can be said that the pupil (or viewing window) is expanded by the plurality of output light rays 404a-f along the length of the waveguide, such that the size of the viewing window is increased. Each light ray 404a-f corresponds to a portion of the amplitude (intensity or brightness) of the input light beam 402. Ideally, the graded coating provides a decrease in the reflectivity (or conversely, an increase in the transmissivity) of the first reflective surface 420 along the length of the waveguide, such that each output light ray 404a-f has substantially the same amplitude. A viewer 430 at an eyebox at a viewing distance from the first reflective surface 420 is thus able to see the image at any position within the expanded viewing window, as indicated by the arrow 440.

[0103] Light rays 404a-f may generally be referred to as "replicas". The term "replica" may generally be understood to refer to the light rays output by the pupil expander at the expanded exit pupil as a result of the propagation (and amplitude splitting) of the input light, and the term "replica" has a corresponding meaning. In more detail, the term "replica" is used herein simply to reflect that the light is split and directed along different optical paths as it propagates within the waveguide. In the case of spatially modulated light encoded with a hologram, the spatially modulated light is split so that the composite light field is directed along multiple different optical paths. The term "replica" is used to refer to each occurrence or instance of the composite light field following a replication event, such as a partial reflection-transmission from a pupil expander. Each replica propagates along a different optical path. Some embodiments of the present disclosure relate to light propagation encoded with a hologram rather than an image, i.e., light that is spatially modulated with a hologram of an image rather than the image itself. Those skilled in the art of holography will understand that the composite light field associated with light propagation encoded with a hologram will vary with propagation distance. The term "replica" as used herein is independent of propagation distance, so two light branches or paths associated with a replication event are still referred to as "replicas" of each other, even if the branches have different lengths such that the composite light field evolves differently along each path. That is, according to the present disclosure, even if two composite light fields are associated with different propagation distances, they are still considered "replicas"—assuming they originate from the same replication event or series of replication events.

[0104] like Figure 4 The waveguide shown extends the observation window in one dimension, which corresponds to the longitudinal direction along which the light beam propagates within the waveguide, as indicated by arrow 440. As the skilled person will appreciate, the observation window can be extended in two dimensions if desired by using two orthogonal waveguides.

[0105] Example embodiments disclosed herein include holographic display devices and methods using waveguides as pupil expanders as described above. However, the present disclosure is not limited to holographic displays. Therefore, other example embodiments include image display devices and methods using waveguide pupil expanders as described above.

[0106] First embodiment

[0107] Figure 5 A holographic display system including a waveguide pupil expander according to a first example embodiment of the present disclosure is shown.

[0108] The holographic system comprises a display device arranged to form an image. In the arrangement shown, the display device is arranged to form two monochromatic images. Figure 5, a first monochrome / display channel (e.g., a red channel) includes a first light source 510, a first collimating lens 512, and a first dichroic mirror 514, which are arranged to illuminate a spatial light modulator (SLM) 540 with light of a first wavelength. A second monochrome / display channel (e.g., a green channel) includes a second light source 520, a second collimating lens 522, and a second mirror 524, which are arranged to illuminate the SLM 540 with light of a second wavelength.

[0109] The holographic system further comprises a holographic controller 502, which is arranged to control the system described herein. A first monochrome computer-generated hologram of a first monochrome image (e.g., a red image) is calculated by the holographic controller 502 and encoded on the SLM 540, for example, by the display driver 542. The SLM 540 displays the first hologram and is illuminated by light of a first color from a first color / display channel so as to output a first spatially modulated light of the first color encoded with the first hologram. Similarly, a second monochrome computer-generated hologram of a second monochrome image (e.g., a green image) is calculated by the holographic controller 502 and encoded on the SLM 540. The SLM 540 displays the second hologram and is illuminated by light of a second color from a second color / display channel so as to output a second spatially modulated light of a second color encoded with the second hologram.

[0110] The holographic system further comprises a beam splitter cube 530 arranged to separate input light to and output light from the SLM 540. Figure 5 In some embodiments, the holographic display system is arranged in a direct view configuration. In the arrangement shown, lens 550 is located in the optical path of the spatially modulated light output by SLM540. Lens 550 is optional. Observer 508 can directly observe the spatially modulated light based on the hologram from SLM540. In some embodiments, as described above, the lens of the observer's eye forms a holographic reconstruction on the retina of the eye. Thus, the display device can be said to form an image at an image plane corresponding to the retina of the eye. In these embodiments, the observer can be said to receive spatially modulated light encoded with the hologram. In other words, the observer receives light encoded with the hologram of the image, rather than the image itself. Waveguide 590 includes an optically transparent medium separated by a first and second reflective surface as described herein. Thus, the holographic display device has a "direct view" configuration - that is, the observer looks directly at the display device / SLM.

[0111] Second embodiment

[0112] Figure 6 A holographic display system according to a second example embodiment of the present disclosure is shown, which includes a waveguide forming a waveguide pupil expander.

[0113] Figure 6 The holographic system shown is similar to Figure 5holographic system, and is also arranged to form two monochromatic images. Thus, a first monochromatic channel (also referred to as the "first display channel") includes a first light source 610, a first collimating lens 612, and a first dichroic mirror 614. The first dichroic mirror 614 is arranged to reflect light of a first wavelength along a common optical path to illuminate a spatial light modulator (SLM) 640. The first wavelength of light corresponds to the first display channel of a first color (e.g., red). A second monochromatic channel (also referred to as the "second display channel") includes a second light source 620, a second collimating lens 622, and a second mirror 624. The second mirror 624 is arranged to reflect light of a second wavelength along a common optical path to illuminate the SLM 640. The second wavelength of light corresponds to the second monochromatic channel of a second color (e.g., green). In the illustrated embodiment, the SLM 640 includes a single array of light-modulating pixels (e.g., LCOS) illuminated by light of the first and second wavelengths. In other embodiments, the SLM 640 may include separate arrays of light-modulating pixels illuminated by light of the respective first and second wavelengths.

[0114] The holographic system also includes a holographic controller 602, which is arranged to control the system described herein. A first monochrome computer-generated hologram is calculated by the holographic controller 602 and encoded on the SLM 640, for example, by the display driver 642. The SLM 640 displays the first hologram and is illuminated by light of a first color from a first color / display channel so as to output spatially modulated light and form a first holographic reconstruction on a light receiving surface 670 located in the playback plane. The SLM 640 outputs first spatially modulated light of a first color to form a first monochrome image (e.g., a red image) on the light receiving surface 670 (e.g., a screen or diffuser). Similarly, a second monochrome computer-generated hologram is encoded on the SLM 640 by the holographic controller 602. The SLM 640 displays the second hologram and is illuminated by light of a second color from a second color / display channel so as to output spatially modulated light and form a second holographic reconstruction on the light receiving surface in the playback plane. The SLM 640 outputs second spatially modulated light of a second color to form a second monochrome image (e.g., a green image) on the light receiving surface 670.

[0115] In the arrangement shown, the beam splitter cube 630 is arranged to separate the input light to the SLM 640 and the spatially modulated light output by the SLM 640. A Fourier lens 650 and a mirror 660 are provided in the optical path of the output spatially modulated light to the light receiving surface 670. It can be said that a first / second image is formed on the light receiving surface 670. The first / second image is a first / second holographic reconstruction of the corresponding first / second hologram. Therefore, a composite color picture can be formed on the light receiving surface 670 combining the first and second monochromatic images. A projection lens 680 is arranged to project the first and second images formed on the light receiving surface 672 to the input port of a pupil expander in the form of a waveguide 690. The observer 608 can observe a magnified image of the picture from an expanded eye box ("observation window") formed by the waveguide 690 due to the optical power of the projection lens 680. The waveguide 690 comprises an optically transparent medium which is separated by a first and a second reflecting surface, as described above with reference to Figure 4 Thus, the holographic display apparatus has an “indirect viewing” configuration—that is, the observer does not directly view the display device / SLM, but rather views the image formed on the light receiving surface 670 .

[0116] The holographic system also includes an observer tracking system (eye tracking system), which includes an eye tracking camera 606 and an eye tracking controller 604. As is known in the art, the eye tracking camera is arranged to capture images of the observer's eyes to track the eye position and thus the viewing position within the viewing window. The eye tracking controller 604 provides feedback to the holographic controller 602 indicating the current viewing position.

[0117] In addition to feedback input from the observer tracking system, the holographic controller 602 may also receive other external and internal inputs 600 for generating computer-generated holograms as known in the art. Such inputs may determine the image content to be displayed by the holographic display device.

[0118] For example only, Figure 5 and 6 In each of the holographic systems shown in the embodiments, the display device includes a first color (e.g., red) display channel arranged to display a first monochrome hologram and a second color (e.g., green) display channel arranged to display a second monochrome hologram. In other example embodiments, three or more display channels may be provided, each configured to display a corresponding monochrome hologram. For example, a full-color composite image / picture may be formed by displaying corresponding red, green, and blue monochrome holograms. In other example embodiments, only a single display channel is provided.

[0119] Waveguide pupil expander with improved integrity

[0120] As described above, a system (e.g., a projection system) including a display device such as a spatial light modulator includes one or more pupil expanders to expand the viewing window in one or more dimensions. The pupil expander is typically implemented as a bulk optic waveguide including a plane-parallel (primary) reflective surface, as described above with reference to FIG. Figure 4 As described above. However, in practice, this type of waveguide pupil expander is a relatively large component (e.g., the two largest dimensions / major surface dimensions are each several hundred millimeters) and is formed from an optically transparent solid material such as glass. Therefore, when the system is used in certain applications, such as for a head-up display in a vehicle, the waveguide pupil expander is at risk of breaking in the event of an impact (such as a vehicle collision). Breaking of the glass within the waveguide may compromise the structural integrity of the pupil expander. For example, such broken glass may undesirably result in the generation of large, uncontained glass fragments, which may cause injury. In addition, even if the glass of the waveguide breaks and the fragments are not expelled, the broken glass may compromise the functional integrity of the pupil expander. For example, cracks in the glass may change the propagation direction of light through the waveguide, resulting in undesirable reflections and scattering. This may not only reduce image quality, but may also cause laser light to scatter towards the observer, which may be dangerous.

[0121] Thus, embodiments of the present disclosure provide improved integrity of bulk light waveguides comprising fragile optically transparent (solid) materials such as glass.

[0122] Layered glass structure

[0123] The structure of the waveguide pupil expander according to the first aspect of the present disclosure can take various different layered forms, including fragile optically transparent (solid) materials such as glass, referred to herein as "layered glass structures". Figures 7 to 10 Describe the example.

[0124] Figure 7 A schematic cross section of a waveguide pupil expander 700 comprising a layered glass structure of a first example is shown. The waveguide 700 comprises first and second reflective major surfaces 720, 710, generally as described above with reference to Figure 4The arrangement is described. In particular, the waveguide includes a first substantially planar partially reflective surface 720 (e.g., having partial reflectivity / transmittance) and a second substantially planar fully reflective surface 710 (e.g., having substantially 100% reflectivity), which are arranged parallel to the first partially reflective surface 720 and spatially separated therefrom. According to a first example, the waveguide 700 includes a layered glass structure comprising a glass layer 730 laminated together with or laminated to a polymer-based layer 740. In one example, the polymer-based layer 740 includes a plurality of parallel polymer-based wires or slats, for example formed as polymer-based louvers or a grid. In another example, the polymer-based layer may include a polymer-based polarizer, such as a circular polarizer. In the example shown, the polymer-based layer is laminated to the second partially reflective / partially transmissive surface 720.

[0125] Laminating the polymer-based layer 740 with or to the glass (e.g. float glass) layer 730 serves two functions. Firstly, the polymer helps to maintain the (structural) integrity of the waveguide 700 in the event that the glass layer 730 breaks. Secondly, the louver / polarizer arrangement of the polymer-based layer 740 serves to mitigate unwanted glare. In particular, as described in UK patent application GB2016616.1 (which is incorporated herein by reference), a circular polarizer can be arranged near a partially reflective surface of the waveguide to reduce glare at a viewing window of a waveguide pupil expander in a holographic projection system, for example when used in a head-up display (HUD). Thus, by using a polymer-based circular polarizer, glare can be reduced while maintaining the integrity of the waveguide 700, as described herein.

[0126] Figure 8 A schematic cross-section of a waveguide pupil expander 800 including a second example layered glass structure is shown. Similar to the first example, the waveguide 800 includes parallel first and second reflective major surfaces 820, 810 with an optically transparent material therebetween for propagating light. According to the second example, the optically transparent material comprises a layered glass structure comprising one or more glass layers 830 and a resin layer 840. In the illustrated arrangement, the layered glass structure comprises a resin layer 840 sandwiched between first and second glass layers 830A, 830B. The first and second glass layers 830A, 830B may have low birefringence, and the resin layer 840 may comprise a refractive index matching resin material.

[0127] The resin layer 840 may include a polyvinyl butyral (PVB) resin interlayer. PVB is particularly suitable because it can easily match the refractive index of the glass material and flows relatively freely. In addition, the layer structure can be formed in a press.

[0128] Laminating the resin layer 740 with or to the glass layer 730 helps maintain the structural integrity of the waveguide 700 in the event of a breakage of the glass layer 730. In particular, the resin is selected to provide the necessary parallelism (between the outer surfaces) in the glass layer 730. Additionally, index matching of the resin layer can help maintain the functional integrity of the waveguide 770 so that any breakage of the glass does not adversely affect the propagation of light therein.

[0129] Those skilled in the art will understand that Figure 8 A second example adds one or more interlayers of glass within an optically transparent material between the reflective surfaces of the waveguide. However, other forms of layered glass structures are possible.

[0130] Figure 9 A cross-section of a waveguide pupil expander 900 comprising a layered glass structure according to a third example is shown. Similar to the first and second examples, waveguide 900 comprises parallel, first and second reflective major surfaces 920, 910 with an optically transparent material therebetween for propagating light. According to the third example, the optically transparent material between the first and second reflective surfaces 920, 910 comprises a layered glass structure comprising tempered glass. Specifically, the optically transparent material comprises glass that has been tempered to form an inner glass region or layer 940 under tensile strain, which is disposed between a pair of outer glass regions or layers 930, 950 under compressive strain.

[0131] Tempering the glass forming the optically transparent material acts to strengthen the glass, thereby preventing or mitigating significant breakage and reducing shattering upon impact. As will be appreciated by those skilled in the art, the use of tempered glass according to the third example may be implemented in conjunction with one or more other examples.

[0132] Figure 10 A cross-section of a waveguide pupil expander 1000 comprising a layered glass structure of a fourth example is shown. Similar to the previous examples, the waveguide 1000 comprises first and second reflective main surfaces 1020, 1010 arranged in parallel, with an optically transparent material therebetween for propagating light. However, unlike the previous examples, the optically transparent material (primarily) comprises an air gap 1030. In this example, the first and second reflective surfaces 1020, 1010 comprise mirrors, each in the form of a relatively thin, optically transparent substrate (e.g., glass) having one or more reflective coatings thereon. In this example, one or more additional layers 1040A, 1040B of another material, such as a polymer-based layer, a resin layer, and / or a tempered glass layer, are provided between the air gap 1030 and one or both of the first and second reflective surfaces 1020, 1010.

[0133] It has been discovered that the use of bulk optical waveguides as described herein can be effectively used as pupil expanders. Based on the above examples of the first aspect, the present disclosure provides a layered glass structure for a waveguide pupil expander that is adapted, configured, or arranged to maintain the integrity of the waveguide in the event of glass breakage. This improves safety and, depending on the extent of damage to the glass, maintains functionality in certain circumstances.

[0134] Implementation in projection systems

[0135] As will be appreciated by those skilled in the art, compared to conventional glass or air cavities, the above reference Figures 8 to 10 The layers of the described example layered glass structures can alter the birefringence of the optical path of light that is guided through the layered structure of the waveguide between the input port and the exit pupil / observation window. Therefore, the system according to the present disclosure (i.e., in which a waveguide pupil expander is implemented) needs to be adapted to account for any such birefringence and its effects at different locations along the length of the waveguide. Thus, for example, Figure 5 or Figure 6 The holographic projection system shown can use an observer tracking system to determine the observer's eye position and adjust the diffraction pattern displayed on the SLM to compensate for wavefront errors caused by light propagation (e.g., through different propagation distances / multiple layers of a layered structure).

[0136] In some embodiments, the eye tracking system of the projection system can be used to detect fault events, such as collisions and / or dangerous impacts of collisions, and provide feedback to the system controller to take safety measures. Figure 5 and Figure 6 The described eye tracking system can use an infrared (IR) light source to provide pulsed illumination of the face of an observer (e.g., a driver). A camera that can detect both IR and visible light can be used not only for eye tracking, but also for detecting visible light. In the periods between the IR pulses, visible light scattered by the waveguide or directed toward the observer's face due to broken glass or due to a collision can be detected by the camera. During the pulsed IR, the camera can be used to detect laser light (IR light), which is directed toward the driver's face due to a collision and presents a safety hazard. In the event that such a safety hazard is detected, the eye tracking system can provide a feedback signal to the holographic controller. The system controller can respond to the feedback signal, for example by reducing the drive signal to the relevant laser or turning off the relevant laser.

[0137] Other safety features

[0138] Other security features may be used in the systems described herein.

[0139] For example, the techniques described above for adapting the system to the specific birefringence associated with different transparent optical materials within the waveguide cavity can also be used in conjunction with transparent plastic waveguides. For ease of manufacturing, the transparent plastic waveguides can be injection molded. The plastic material can be selected to have a low risk of shattering or other damage upon impact. For example, the transparent optical material can include transparent polycarbonate or silicon-grade materials (e.g., soft polymers) suitable for TIR optics.

[0140] Other example safety features may include the use of expanding foam, optionally with black filler, which releases in the same manner as an airbag when a collision is detected. The foam may prevent the release of shattered parts and further help maintain the parallelism and structural integrity of the waveguide. Additionally, the use of black filler may absorb stray / scattered light, thereby preventing it from reaching the observer's eyes. According to further examples, the waveguide (and optionally other fragile optical components) may be immersed in a (transparent) fluid, a high refractive index (transparent) liquid may be used between the waveguide and the cover glass (e.g., a curved cover), or an optically clear adhesive (OCA) may be pumped around the waveguide at the end of manufacturing to form a protective coating. In each of these further examples, the glass of the waveguide may be protected and / or dampened from shock from the impact of a collision in order to prevent the release of shattered parts and further help maintain the parallelism and structural integrity of the waveguide.

[0141] The system may also include one or more light detectors arranged to monitor and detect the location of scattered light (e.g., laser light) generated by glass breakage. This scattered light may pose a safety hazard to the eyes of an observer. Due to the broken glass, the light detectors may be associated with the waveguide pupil expander to detect scattered light directly from the waveguide pupil expander, or may be arranged to detect other scattered light from other reflective surfaces (e.g., a vehicle windshield). This light detection system may provide a light detection signal to the system controller as part of a closed-loop feedback system to maintain safety (e.g., by shutting down the laser source).

[0142] The waveguide pupil expander can be adapted or configured so that the outer surface can be mounted to a relatively large and stable object, such as a large metal component within a vehicle. In particular, the outer surface corresponding to the second totally reflective layer can be mounted to a stable component, which can absorb the impact of a collision and reduce the risk of breakage. Mounting the waveguide on a metal component has the additional advantage of improved thermal stability. Furthermore, the totally reflective layer can be integrated into the planar metal component to which it is mounted, for example by providing a totally reflective coating on the planar surface of the metal component, which thereby acts as part of the waveguide. Other optical components (of the projection system) can be attached (to the metal component) using foam or low-refractive-index materials.

[0143] As the skilled artisan will appreciate, the teachings of the present disclosure may be combined by implementing the other security features described above in a system including a waveguide pupil expander having a layered glass structure, as described above.

[0144] Additional Features

[0145] The embodiments relate to electrically activated LCOS spatial light modulators by way of example only. The teachings of the present disclosure may equally be implemented on any spatial light modulator capable of displaying computer-generated holograms according to the present disclosure, such as any electrically activated SLM, optically activated SLM, digital micromirror device, or microelectromechanical device.

[0146] In some embodiments, the light source is a laser such as a laser diode. In some embodiments, the detector is a photodetector such as a photodiode. In some embodiments, the light receiving surface is a diffuser surface or screen, such as a diffuser. The holographic projection system of the present disclosure can be used to provide an improved head-up display (HUD) or head-mounted display. In some embodiments, a vehicle is provided, which includes a holographic projection system installed in the vehicle to provide a HUD. The vehicle can be a motor vehicle, such as a car, truck, van, delivery truck, motorcycle, train, airplane, ship, or boat.

[0147] The quality of the holographic reconstruction can be affected by the so-called zero-order problem, which is a consequence of the diffraction properties of the pixelated spatial light modulators used. This zero-order light can be considered "noise" and includes, for example, specularly reflected light and other unwanted light from the SLM.

[0148] In the example of Fourier holography, this "noise" is concentrated at the focal point of the Fourier lens, resulting in a bright spot at the center of the holographic reconstruction. The zero-order light can simply be blocked out, however this means replacing the bright spot with a dark spot. Some embodiments include an angle-selective filter to remove only the collimated light of the zero order. Embodiments also include methods for managing the zero order as described in European Patent 2030072, which is incorporated herein by reference in its entirety.

[0149] In an embodiment, only primary playback fields are utilized, and the system includes physical blocks, such as baffles, arranged to limit the propagation of higher-level playback fields through the system.

[0150] In the above embodiments, the holographic reconstruction is a composite color image. In some embodiments, a method called spatially separated colors "SSC" is used to provide color holographic reconstruction. In other embodiments, a method called frame sequential color "FSC" is used.

[0151] The SSC method uses three spatially separated light modulating pixel arrays for the three monochrome holograms. The advantage of the SSC method is that the images can be very bright because all three holographic reconstructions can be formed simultaneously. However, if the three spatially separated light modulating pixel arrays are provided on a common SLM due to spatial constraints, the quality of each monochrome image will be sub-optimal because only a subset of the available light modulating pixels is used for each color. Thus, a relatively low resolution color image is provided.

[0152] The FSC method can use all of the pixels of a common spatial light modulator to display the three monochrome holograms sequentially. The monochrome reconstruction cycle (e.g., red, green, blue, red, green, blue, etc.) is fast enough that a human observer perceives a multicolor image from the combination of the three monochrome images. The advantage of FSC is that the entire SLM can be used for each color. This means that the quality of the three color images produced is optimal because all of the pixels of the SLM are used for each color image. However, the disadvantage of the FSC method is that the brightness of the composite color image is lower than the SSC method - about 3 times - because each monochrome illumination event can only occur for one third of the frame time. This deficiency can be addressed by overdriving the laser or using a more powerful laser, but this requires more power, resulting in higher cost and an increase in system size.

[0153] The methods and processes described herein can be embodied in a computer-readable medium. The term "computer-readable medium" includes media that are arranged to temporarily or permanently store data, such as random access memory (RAM), read only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" should also be taken to include any medium or combination of media capable of storing instructions for execution by a machine such that, when the instructions are executed by one or more processors, the machine becomes configured to perform any one or more of the methods described herein in whole or in part.

[0154] The term "computer-readable medium" also encompasses cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data storage repositories (e.g., data volumes) in the example form of a solid-state storage chip, an optical disc, a magnetic disk, or any suitable combination thereof. In some example embodiments, instructions for execution can be delivered by a carrier medium. Examples of such carrier media include transitory media (e.g., a propagating signal that delivers the instructions).

[0155] It will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope of the appended claims. The disclosure encompasses all such modifications and variations as within the scope of the appended claims and their equivalents.

Claims

1. A system comprising: a display device comprising a spatial light modulator arranged to output spatially modulated light to form an image; a waveguide pupil expander configured to receive spatially modulated light from a display device at its input port and expand an observation window of the system; as well as controller, wherein the system further comprises an observer tracking system arranged to monitor an observer's face to detect stray light incident thereon and, upon detection of stray light, provide a signal to the controller indicative of a mechanical, structural or optical failure of the waveguide pupil expander; Wherein the controller is arranged to respond to said signal indicative of a mechanical, structural or optical failure of the waveguide pupil expander.

2. The system of claim 1, wherein: The controller is configured to control the spatially modulated light output by the display device in response to the signal to prevent further emission of the spatially modulated light.

3. The system of claim 2, wherein: The controller is configured to control the light source of the display device in response to the signal to reduce a driving signal of the light source or turn off the light source.

4. The system of claim 1, wherein: The observer tracking system includes a light detector directed toward the observer's face to detect stray visible light.

5. The system of claim 4, wherein: The visible light is visible laser light of a projection system including the display device and the waveguide pupil expander.

6. The system of claim 4, wherein: The observer tracking system includes an infrared light source configured to periodically illuminate an observer's face, and the light detector is configured to detect stray infrared light.

7. The system of claim 6, wherein: The light detector is configured to detect infrared light and visible wavelengths of light.

8. The system of claim 1 or 2, wherein: The waveguide pupil expander is further configured such that its first totally reflective surface can be mounted to or integrated with a relatively large and stable component arranged to absorb any impact, thereby reducing the risk of glass shattering.

9. The system of claim 8, wherein: The components arranged to absorb any impact comprise metal in order to improve thermal stability.

10. The system of claim 1 or 2, further comprising a light detection system for monitoring scattered light.

11. The system of claim 10, wherein: The light detection system is configured to detect scattered light from one or more of: the waveguide pupil expander, optical components of the system, and external reflective components surrounding the system.

12. The system of claim 1 or 2, further comprising a container of expandable foam arranged to release foam to surround one or more optical components of the system in response to detection of an event.

13. The system of claim 12, wherein: The expander foam includes a light absorbing filler material.

14. The system of claim 1 or 2, wherein: The pupil expander waveguide of the system is immersed in a fluid; Providing a high refractive index liquid between the waveguide and the cover glass, and / or An optically clear adhesive at least partially surrounds the waveguide pupil expander.

15. The system of claim 1 or 2, wherein: The waveguide pupil expander comprises: first and second substantially planar reflective surfaces arranged in parallel with an optically transparent material therebetween, and an input port for receiving input light; wherein the first reflective surface is totally reflective and the second reflective surface is partially reflective such that input light is directed from the input port to the output port at the second partially reflective surface by a series of internal reflections; Wherein the waveguide is formed from a layered glass structure arranged to maintain the integrity of the waveguide in the event of glass breakage.

16. The system of claim 1 or 2, comprising a holographic projection system, wherein: The display device includes a spatial light modulator arranged to display a diffraction pattern of an image and to output spatially modulated light according to the diffraction pattern to form a holographic reconstruction corresponding to the image.

17. A method for operating a system comprising a display device and a waveguide pupil expander, comprising: displaying the diffraction pattern of the image on a display device; outputting the spatially modulated light by the display device to an input port of a waveguide pupil expander having an output port; The spatially modulated light is guided to the output port by a waveguide pupil expander through a series of internal reflections; outputting spatially modulated light from an output port by the waveguide pupil expander to form an image at an observation window of the system; as well as The stray light incident on the observer's face in the image is monitored by the observer tracking system; In response to detecting stray light, the method further includes: Tracking the output of a signal from the system by an observer indicating a system failure event; A controller of the system responds to a fault event signal output by the observer tracking system.

18. The method of claim 17, wherein: The monitoring includes: providing illumination of the observer's face by the observer tracking system, and Stray light incident on the observer's face is detected by the observer tracking system.

19. The method of claim 18, wherein: The illumination is pulsed illumination, and detecting stray light incident on the viewer's face includes detecting reflections of the stray light between pulses of the pulsed illumination.

20. The method according to claim 17 or 18, wherein The response includes reducing a driving signal of a light source of the display device or turning off the light source.

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