Display system with steerable eye box

Abstract

A display includes an illumination engine which generates an input light beam with a first width. A light guide 50 is optically coupled to the illumination engine, receiving the input light beam and converting it into an illumination light beam with a second width larger than the first width. A display device is illuminated by the illumination light beam, converting it into an image light beam. The illumination engine controls the angle of the input light beam, rotating it relative to the orientation of the light guide. A light guide for use in a head mounted display includes a transparent substrate and an exit pupil expander. The thickness of the transparent substrate varies across the plane of the upper surface in a normal direction. A display includes a display device 60 with a pixel array, a converging lens 62, a shutter mechanism 64 defining an adjustable aperture, a dynamic lens 66 forming a display exit pupil, a telescopic optical system 68,72 imaging the display exit pupil in an eye box 74. A control device controls the position and size of the aperture and the power of the dynamic lens to locate the image of the display exit pupil in the eye box.

Description

FIELD OF THE INVENTION The invention relates to a display system producing a steerable eye box, referred to as a display exit pupil, for use in near eye displays (NEDs), such as those included in mixed reality (MR) and virtual reality (VR) head mounted displays (HMDs). BACKGROUND MR and VR devices allow a user to experience and / or interact with virtual information, without losing situational awareness of the real world in the case of MR. Such devices typically include a display device positioned close to the eye of a user, which displays images to the user but which may still allow the user to see past the displayed images to remain aware of the external environment. Often VR and MR devices take the form of a head mounted display (HMD), which is worn on the head of the user in the same way as a pair of glasses or a helmet. The display is positioned in front of one or both eyes of the user and in the case of MR devices typically takes the form of partially transparent optics through which an image is projected. The user can then see the image projected through the optics, overlayed onto the outside world. This type of device is known as an optical see-through device. MR headset devices can also operate by providing an opaque display set up and using an externally facing camera to incorporate images of the outside world into the image displayed to the user. This type of device is known as a digital pass-through device. There are many existing HMD products commercially available. HMD applications require the MR display to be compact, unintrusive, comfortable, and capable of producing high-quality images. Each HMD device has an eye box, which is defined as the area in which a user's eye can be physically located relative to the display and still see the complete projected image. It is desirable to have a large eyebox so that: (a) the user, as they move their eye in the eye box, is still able to see the image from different eye positions, (b) the eyebox size can compensate for any misalignments of the HMD, and (c) a greater proportion of users can view the projected image without readjusting the distance between the two displays of the HMD, taking into account the natural variation in Interpupil Distance (IPD) due to face shapes and eye separation distances between users. While it is defined as an area, the eye box must exist in many different planes, and thus it extends in three dimensions. A display's field of view (FOV) is defined as the angle subtended at the eye over which the user can see the displayed content. A large FOV increases the realism of the virtual content and the sense of immersion provided by a VR or MR device, as well as enabling larger images to be displayed. A small FOV is one of the most common complaints about existing VR and MR displays, with many currently available HMDs only achieving less than 56° diagonal FOV. The depth of field (DOF) of a display measures how fast the image becomes defocused, moving away from the focal plane along the optical axis of the display optics. When the F-number of the optics is larger, which corresponds to a smaller beam diameter in the optics, the depth of field is larger. A larger depth of field means that the image remains in focus for a larger distance away from the focal plane. One of the prevalent architectures for VR / MR displays uses catadioptric optics, i.e. optics using both reflective and refractive components. Catadioptric optics are used for their ability to form a large FOV while minimising the Total Track Length (TTL) of the display. The TTL is defined here as the length along the optical path from the light-emitting surface of the display device (i.e. the first surface of the optics) to the last optical surface closest to the user's eye. Well-known examples of this architecture include the pancake design by 3M (RTM) and the Apple (RTM) / Limbak (RTM) lens ThinEyes (RTM). Figure 1 shows a typical example of "pancake" optics for a Near Eye Display (NED) used in a VR or MR display. In this design, light is emitted by a display device 10, e.g. an LCD panel and passes through a circular polarizer 12. The light passes through a pancake lens having a halfmirror 14 and a quarter wave plate 16 formed on its front and back surfaces respectively and is then reflected by a surface, in this case a reflective polariser 18. The light is then reflected again by the half-mirror 14 and passes through the quarter wave plate before arriving into the eye box. Other configurations include surfaces having different reflectivities, different shapes for the surfaces and different polarization configurations. The pancake VR optics will be used as an exemplar in this document, but other optical architectures can use the principles described. A feature of the pancake optics design is that there is a one-to-one relationship between the angle of the light leaving the display and its arrival position in the eye box. Given that most of the time, the eye will not be in the central position of the eye box, the light finally arriving at the user's eye will usually leave the display at an oblique angle rather than a normal angle, especially at the edges of the FOV and eye box. Given that most displays emit light in a Lambertian-like power distribution across emission angles, only a small fraction of the display's optical power will arrive into the eye's pupil, reducing efficiency and contrast. In pancake optics, efficiency is especially important as a large amount of light (more than 75%) is lost in a partially reflective mirror of the optics. Light lost within the optical system reduces efficiency and contrast due to scattering or leaking through the reflective polariser. Another challenge with the pancake optics design realised by the inventors is the significant aberration introduced by the optics. The relatively steep reflection and refraction angles involved in directing light from the display to the user's eye using pancake optics will introduce aberrations even without any manufacturing errors. These aberrations arise due to the relatively small F-number of pancake optics and large eye box. Current approaches to reducing aberrations include using multiple high-quality glass elements. These optical elements increase cost and weight due to their material and number. In contrast, a single plastic optical element is more suitable for a consumer head-mounted VR display due to its lower cost and weight. Finally, the pancake optics design will cause users to suffer from Vergence Accommodation Conflict (VAC), which will be explained further below. The large eye box creates a narrow DOF, so the user's focus will be fixed on one plane while their vergence will change according to the displayed content, leading to VAC. VAC is a particularly significant problem for MR displays in general. The conflict between accommodation and vergence is a significant reason for users' discomfort. This conflict occurs because human brains link "vergence" and "accommodation", as defined below, and expect both of these parameters to match the distance of an object being viewed. In a NED, this link breaks, and the user's brain tries to focus on one plane matching the user's vergence while the optical system displays the image on another plane. Vergence is the movement of the eyes in opposite directions when an object moves closer to them. The effect of vergence is best described when the object is on the optical axis of the user. As the object moves closer to the eyes, the eyes look more towards the nose. On the other hand, accommodation is the ability of the eye to change focus from more distant to nearer objects by changing the shape of the eye's lens. Muscles in the eye change the shape of the lens and hence its optical power to keep an image of an object on the retina sharp as the distance of the object from the eye varies. The movement of the muscles controlling vergence is highly linked to the eye's accommodation and the ciliary muscles bringing an object to focus. The human brain controls the ocular muscles in such a way that vergence and accommodation are linked because they normally occur in unison, i.e. as the eye muscles are controlled to increase vergence, they are also controlled to perform accommodation to focus closer to the eye. When vergence and accommodation are inconsistent or in conflict, this normal connection between the two in the brain is broken, and there is a vergence accommodation conflict (VAC). This conflict is believed to be a cause of fatigue and sickness in NEDs. An equally challenging issue for VR and MR displays is the necessity for users to use prescription glasses. Users with prescription glasses have the option to: (a) wear the display on top of their spectacles (this increases the size of the headset, pushes the centre of gravity away from the nose, and causes discomfort) (b) purchase custom-made clip-ons (which tend to be expensive), or (c) partially correct for the user's optical prescription, usually only spherical correction, by using the optics of the display (this leads to inferior correction and a more complex headset containing expensive optomechanics). The currently proposed solutions for minimising both the VAC problem and the prescription glasses problem in NEDs are inadequate, as described below. (i) Fixed accommodation plane display: minimising the display's vergence variation about a single fixed accommodation plane. The single accommodation plane is positioned in the working space to minimise the user's overall discomfort. If the working space covers all the depth a user can focus on (e.g. 30cm to infinity), then the intermediate plane is between one meter and two meters away. The display is then designed to minimise the discrepancy between the accommodation plane (fixed) and the vergence plane (software-controlled) and thus lessen the VAC-associated discomfort. However, with this approach, users still need prescription glasses to correct their short- or long-sightedness. (ii) Dynamic lenses: using dynamic lenses that change the accommodation plane in real-time. The principle is to have a dynamic lens between the user and the display and change the power of the dynamic lens according to the content being displayed and the need to correct the user's optical prescription. However, current dynamic lenses (e.g. Alvarez lenses, liquid crystal lenses and liquid lenses) correct for spherical aberration only over a very small FOV, introduce scattering, are bulky, and are too slow for fast-moving experiences such as video games. SUMMARY According to an aspect of the invention, there is provided a display system comprising: an illumination engine configured to generate an input light beam having a first diameter; a light guide optically coupled to the illumination engine and configured to receive the input light beam, the light guide being configured to convert the input light beam into an illumination light beam having a second diameter larger than the first diameter; and a display device configured to be illuminated by the illumination light beam and to convert the illumination light beam into an image light beam, wherein the illumination engine is configured to rotate the angle of the input light beam relative to the orientation of the light guide. Preferably, the light guide is a planar waveguide. Preferably, the light guide comprises an in-coupling element and an out-coupling element. Preferably, each of the in-coupling element and the out-coupling element is one of: a diffractive optical element, a metasurface, a prism and a prism array. Preferably, the illumination engine comprises: a light source configured to output light; a collimation module configured to collimate light from the light source to generate a collimated beam; a rotatable scanning mirror configured to reflect the collimated beam; an actuator configured to rotate the scanning mirror; and a control device configured to control operation of the actuator to rotate the scanning mirror. Preferably, the display system further comprises: a camera configured to capture images of a user's eye and output the images; and a processing unit configured to receive the images output by the camera, calculate a position of the user's eye based on the images, and calculate a rotation angle of the input light beam required to direct the image light beam onto an eye pupil of the user's eye based on the calculated position, wherein the processing unit is configured to output the calculated rotation angle to the illumination engine, and the illumination engine is configured to rotate the angle of the input light beam by the calculated rotation angle so as to direct the image light beam onto the eye pupil of the user's eye. Preferably, the illumination engine further comprises relay optics configured to receive light reflected by the scanning mirror and to convert the reflected light into the input light beam. Preferably, the display system further comprises an imaging optical system configured to project the image light beam generated by the display device onto a user's eye. Preferably, the imaging optical system comprises catadioptric optics. Preferably, the imaging optical system has an effective f-number greater than 4. Preferably, the display system has a display exit pupil diameter in the range of 0.5mm to 3mm. Preferably, the illumination input light beam has a divergence or convergence angle in the range of 0° to 15°. Preferably, the light guide comprises an exit pupil expander including the out-coupling element, wherein the out-coupling element is a diffractive optical element configured to expand the beam diameter of the input light beam along at least a first axis. Preferably, the exit pupil expander further comprises another diffractive optical element configured to expand the beam diameter of the input light beam along a second axis perpendicular to the first axis. According to another aspect of the invention, there is provided a light guide for use in a head mounted display, the light guide comprising: a transparent substrate having upper and lower surfaces; an in-coupling element on the upper surface of the substrate configured to couple light incident on the in-coupling element into the light guide; and an exit pupil expander comprising an out-coupling element on the upper surface of the substrate configured to couple light incident on the out-coupling grating out of the light guide; wherein the upper surface is planar, and the thickness of the transparent substrate in a direction normal to the upper surface varies across the plane of the upper surface. Preferably, the lower surface has at least one of a curved profile and a profile that is inclined relative to the upper surface when viewed in a direction parallel to the upper surface. Preferably, the transparent substrate has a thickness variation of more than 1%. Suitably, the transparent substrate has a thickness variation of more than 5%. Suitably, the transparent substrate has a thickness variation of more than 10%. Suitably, the transparent substrate has a thickness variation of more than 25%. Suitably, the transparent substrate has a thickness variation of more than 50%. Preferably, the transparent substrate comprises an upper layer including the upper surface and a lower layer including the lower surface, the upper layer and the lower layer being stacked in a vertical direction, wherein the upper layer and lower layers are formed from different optical materials. Preferably, the upper surface has a partially reflective coating. Preferably, each of the in-coupling element and the out-coupling element is one of: a diffractive optical element, a metasurface, a prism and a prism array. According to another aspect of the invention, there is provided a light guide for use in a head mounted display, the light guide comprising: a transparent substrate having upper and lower surfaces; an in-coupling element on one of the upper and lower surfaces of the substrate configured to couple light incident on the in-coupling element into the light guide; and an out-coupling element on the other of the upper and lower surfaces of the substrate configured to couple light incident on the out-coupling element out of the light guide; wherein the upper surface is planar, and the lower surface is reflective and has a curved profile when viewed in a direction parallel to the upper surface. According to another aspect of the invention, there is provided a display system comprising: a display device comprising an array of pixels and configured to emit image light representing an image; a converging lens configured to collect image light from the display device; a shutter mechanism configured to partially block light that has passed through the converging lens from the display device, wherein the shutter mechanism defines an adjustable aperture through which light can pass; a dynamic lens configured to collect light that has passed through the aperture to form a display exit pupil of the display system; a telescopic optical system configured to image the display exit pupil in an eye box of the display system; and a control device configured to control the shutter mechanism and the dynamic lens, wherein the control device is configured to control the position and size of the aperture and the power of the dynamic lens so as to locate the image of the display exit pupil in the eye box. Preferably, the display system further comprises an eye-tracking system including: an eye tracking camera configured to image an eye of a user of the display system and output image data; and an eye position calculating unit configured to calculate a position of the user's eye based on the image data from the eye tracking camera; wherein the eye position calculating unit is configured to output the calculated position of the user's eye to the control device and the control device is configured to control the position and size of the aperture and the power of the dynamic lens so as to locate the image of the display exit pupil at the calculated position of the user's eye. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described by way of further example only and with reference to the accompanying drawings, in which: Fig. 1 shows an example of the pancake-type optics used in a conventional VR or MR display; Fig. 2 shows a display system according to an embodiment of the invention; Fig. 3 is a diagram showing how the focus of a light beam from an object varies for two different beam diameters as the object moves from a distance of 0.3m from the eye to infinity through optics with a focal plane at a distance of 2m. The diagram shows that the depth of field of the optics is larger when using a beam diameter of 1mm than when using a beam diameter of 5mm; Fig. 4 is a spot diagram showing the performance of pancake lens optics with an eye box of 12mm diameter; Fig. 5 is a spot diagram showing the performance of pancake lens optics with an eye box of 5mm diameter; Fig. 6 is a spot diagram showing the performance of pancake lens optics with an eye box of 1mm diameter; Fig. 7 shows an illumination light guide having an exit pupil expander used in an embodiment of the invention; Fig. 8 shows the path of light rays from the display device through a pancake-type lens to the eye in the optical system of the invention for three different eye positions within the eye box; Fig. 9A shows the path light takes from the display device to the display exit pupil. The relationships between the pixel position, the angle of light leaving the display device 0y, and the position in the eye box y are illustrated for three different pixels. Fig. 9B shows how the angle of light leaving the display device 0y, and the position in the eye box y are related for rays leaving the display device to the display exit pupil for the three different pixels of Fig. 9A. Figs. 10A and 10B show variable thickness illumination light guides according to embodiments of the invention; Fig. 11 shows a variable thickness illumination light guide according to another embodiment of the invention; Fig. 12 shows a 6f (six focal length) optical system that can be used to generate a steerable exit display pupil in a display system according to an alternative embodiment of the invention; Fig. 13 shows a display system according to the alternative embodiment of the invention incorporating the 6f optical system. DETAILED DESCRIPTION Introduction The invention can be applied to any Near Eye Display (NED) that uses a backlit pixelated display device. Possible architectures for the NED optical system include pancake lenses, segmented lens architectures (segmented lens architectures are lenses having discontinuity at the first derivative of the shape of the lens), bird bath optics, beam splitter architectures, curved mirrors, holographic reflectors and TIR prisms. This list is not exhaustive, and other NED optical arrangements are possible and compatible with the invention. For simplicity, this description will focus on embodiments using a pancake lens optical architecture. The skilled person will appreciate how the invention can be applied to other optical architectures. The invention provides a NED display with a larger FOV, greatly reduced or eliminated VAC, and reduced need for prescription glasses. In some embodiments, these enhancements are achieved by combining an illumination light guide with a reflective / refractive NED to create a smaller display exit pupil within the eye box and centered on the eye pupil of the user. This reduction in the display exit pupil diameter leads to a display with a larger DOF and improved contrast. The eye pupil is the opening of the user's eye through which light enters. The iris determines the diameter of the eye pupil, which usually varies between 3mm and 5mm. The display exit pupil is the pupil formed by the NED in the eye box. The display exit pupil of the optical system moves within the eye box and follows the eye pupil in the use of the NED, as will be described below. A display system as described below uses a small steerable display exit pupil that only illuminates the user's pupil. Eye tracking can be used to relay the exact position of the eye pupil within the eye box. Constant steering or constant discrete switching is achieved by using a light guide that illuminates the pixelated display device with a steerable light beam. Light leaving the pixelated display device forms only a narrow display exit pupil within the eye box that coincides with the user's eye pupil. While the eye box size remains large, the display exit pupil is small. The display system keeps the diameter of the display exit pupil narrow (for example approximately 1 mm in diameter). This reduction in the beam diameter and hence the optics' diameter reduces the effect on the displayed image of aberrations in both the eye and the optical system. The term "diameter" is used here to mean a width or more generally a dimension of the beam in a direction perpendicular to the propagation direction of the beam. It does not imply that the beam is necessarily circular in cross-section. For example, the beam diameter could be the width dimension of a beam having a rectangular cross-section. By reducing the effect of aberrations in the optical system, the display system relaxes manufacturing tolerances for the optics, which allows the cost of the display system to be reduced. The use of a narrow-angle beam allows the optics to be simplified and consequently also has the effect of reducing the overall size of the optical system. Using the display system of the embodiment, the user does not need prescription glasses and will not experience VAC, the system's optics are simplified, and contrast improves. The display system reduces or eliminates VAC by creating an image that is in focus for all depths. The F-number of an optical system is the ratio of the focal distance to the diameter. Estimates for the F-number of the eye vary from 2~8, depending on the light conditions (the darker the conditions, the larger the pupil diameter and hence the lower the F-number). A pancake lens can have an F-number below 1. The inventors have found that the problems described above can be reduced or eliminated using optics with a large F-number. Increasing the F-number of the optics of the display system increases the depth of field and reduces the effect of optical aberrations on the displayed image in a NED. By increasing the optical system's F-number above the human eye's F-number, aberrations introduced by either the optical system or the eye are reduced, and the depth of field increases and resolution increases. The F-number of the optical system can be increased by dynamically reducing the illumination cone emitted by the display device onto the optics at any one time. Components of the display system An embodiment of the invention comprises the following components: (1) A NED display device. (2) An illumination light guide (also referred to as a waveguide). The light guide can be flat (as shown in Fig. 2) or have one of its surfaces be curved (see Fig. 10 and 11). Its purpose is to act as a backlight and illuminate the display of the NED. (3) An illumination engine. The illumination engine includes a light source and a beam steering unit that steers light emitted from the light source to illuminate an in-coupling element of the light guide or waveguide. The details of the relay optics between the beam steering unit and the in-coupling element (such as a 4f, i.e. four focal length, system for example) are not shown here for simplicity and clarity. Suitable 4f relay optics are known. (4) An eye-tracking (ET) system. The ET system may utilise an image sensor and a plurality of light sources emitting light towards the user's eye. Images captured by the image sensor can be used to determine the position of the user's eye. Such an ET system may detect reflections from the eye to determine the 3D position and direction of the eye. (5) A controller that receives the position of the eye from the ET system and sets the illumination engine to the appropriate illumination angle by controlling the beam steering unit, such that light from the illumination engine enters the eye via the display exit pupil and the eye pupil. For example, if the beam steering unit uses a rotatable scanning mirror, this illumination angle will correspond to the angle of the mirror. The illumination engine injects collimated light or light with a low beam divergence into the illumination light guide at a controlled angle by controlling the orientation of a scanning mirror operating as the beam steering unit. The divergence or convergence of the collimated beam is preferred to be less than + / - 2 degrees. This control over the angle of the illumination light also controls the position of the display exit pupil produced by the display system after the light from the illumination engine has passed through the display device and the subsequent NED imaging optics. The technique of moving the display exit pupil to different positions to illuminate the user's eye pupil is known as pupil steering. The illumination engine may compromise any suitable number or combination of components that emit light in visible wavelengths, components for controlling the emission angle and optical elements to relay the collimated beam into the lightguide. Examples of light sources for the illumination engine include Light Emitting Diodes (LEDs), lasers, VCSELS, and arrays of such sources, like inorganic LED displays. Examples of components controlling the emission angle include scanning mirrors and arrays of emitters at different angles that are selectively switched. When a scanning mirror is used as the beam steering unit, its rotation needs to be controlled at a frequency on the order of 100 Hz to ensure that the beam angle remains aligned with the user's eye. This frequency is lower than the frequency at which a scanning laser is used to form an image by scanning a beam across the FoV in known display devices, thus making driving the embodiment's scanning mirror easier. First embodiment Figure 2 shows one embodiment of the invention. In this example, the illumination engine 32 of the display system comprises a light source, which is a LED, a laser or a superluminescent diode, and a scanning mirror 30 with relay optics 28 such as a relay lens as a beam steering unit. The illumination engine illuminates an in-coupling diffractive optical element (DOE) 22 of a planar light guide 20, forming the illumination light guide, which also acts as an exit pupil expander (EPE). "In-coupling" means that the DOE provides optical coupling into the light guide 20, in this case, from the illumination engine 32. The function of the illumination light guide 20 is to receive a narrower beam of light and output a larger-area backlight illumination for the display device 26. Note that the term illumination light guide includes both planar waveguides, i.e. waveguides having a substantially constant thickness, and freeform light guides, i.e. light guides having a variable thickness across their length and / or width. The thickness direction of the light guide 20 is defined as the direction substantially along the optical path in the optical system. The in-coupling DOE 22 diffracts the beam sufficiently that the beam is totally internally reflected within the illumination light guide 20. Light trapped in the light guide 20 by total internal reflection (TIR) will eventually hit the out-coupling element 24 provided on the light guide 20 and diffract towards the display 26. The out-coupling element 24 is also a grating, for example another DOE or a metasurface. The out-coupling element 24 has the function of coupling light out of the light guide 20 and into the next stage of the display optics, in this case the display device 26 itself, which is a transmissive liquid crystal display panel in the embodiment. The display device 26 can, in other embodiments, be any image-forming device that utilises a back light unit (BLU). Examples of suitable display devices include liquid crystal on silicon (LCOS) and liquid crystal display (LCD) devices. The light guide's output angle determines the position of the display exit pupil 38 in the eye box 40, and the position of the display exit pupil 38 will move as the angle of the scanning mirror 30 changes. An eye-tracking system, including an eye-tracking image sensor 36, is also provided in the display system. The image sensor or camera 36 is used to determine the eye's exact position and thus correctly set the angle of the light from the illumination engine 32 so that the display exit pupil 38 is formed where the user's pupil is. In this embodiment, the eye-tracking system includes an array of infrared (IR) light emitting diodes (LEDs) positioned on a part of the display system facing the eye. For example, if the display system is incorporated into a HMD resembling a pair of glasses, the LEDs are located around the frame of the spectacles facing the user's eye. In a larger headset, such as a typical VR helmet-type display, the IR LEDs are located inside the headset. For example, the LEDs can be placed around the edge of the aperture through which image light from the display device passes to reach the user's eye. Infrared light from the LEDs is reflected from the eye and sensed by the eye-tracking camera 36, which can be a standard digital camera of the kind commonly used in smartphones, provided that the camera sensor is sensitive to infrared light. The information is then analyzed to extract eye rotation from changes in the reflections of the infrared light from the eye, which lead to corresponding changes in the images captured by the camera. The 2D image from the camera is fitted into a 3D model by an eye position calculating unit in the display system. Within a few frames of image data from the camera, the eye position calculating unit creates a 3D model of the eye representing the eye's position and orientation in 3D space. The eye position calculating unit is provided to perform the analysis of image data from the camera and calculate the position of the eye. The eye position calculating unit can be implemented as software running on a general purpose processor or as a dedicated hardware component. The eye position calculating unit can calculate the eye position by tracking various features of the eye using the image of the reflected light, including the corneal reflection (the first Purkinje image) and the center of the eye pupil in this example. The eye-tracking system of the invention may also track both the reflection from the front of the cornea (the first Purkinje image) and the back of the lens (fourth Purkinje image). This type of eye-tracking system is known as a dual-Purkinje eye tracker. Another alternative or additional feature of the eye-tracking system is to image features from inside the eye, such as the retinal blood vessels, and follow these features as the eye rotates. This leads to a more accurate determination of the eye orientation. Illumination light guide The display system includes an illumination light guide. The light guide acts as an exit pupil expander (EPE) in one or two dimensions, thus creating a large illumination area with compact optics. This light guide (unlike more conventional waveguides used for AR devices) is used for illumination purposes and is not used as an image-conveying device, i.e. the illumination light guide is positioned before the display device in the optical path of the display system. An example of the illumination light guide is shown in Figure 7. This illumination light guide 1 benefits from a greater degree of design freedom compared to more conventional light guides, resulting from its use for illumination purposes and the properties of the display system of the invention. The illumination light guide 1 includes an in-coupling element 2 having an exit pupil expander (EPE) 3 and an out-coupling element 4 such as a DOE grating. The following features can be included in the illumination light guide 1 as appropriate for the particular application of the display system: (1) Out-coupling optical power. The out-coupling grating (item 4 in Figure 7) may be configured to have variable orientation and spatial frequency. The spatial frequency of the diffractive grating (the equivalent of optical power in refractive optics) can change the diffraction angle for each position and add an angular offset required for the angles on the pixelated device backlight. (2) Variable thickness. Variable thickness adds another degree of freedom in matching the target illumination angle for each position in the eye box and each image pixel. The light guide thickness can be varied to achieve the desired central and range of illumination angles. (3) Including both refractive and reflective components. (4) Low-refractive index (RI) materials. The range of angles within the illumination light guide will be reduced compared to the range required in the waveguide of a conventional AR device. The reduced range of angles allows for the use of low-RI materials like plastic in the light guide of the invention. Light from the illumination light guide 1 passes through the transmissive LCD panel forming the display device. The display device causes the beam from the illumination light guide to diverge in this embodiment. The light from the display device enters the pancake lens, which is located at one focal length from the display device and one focal length from the eye box. The angular spread of the pixel output beam from the display device will determine the size of the display exit pupil. The smaller the angular spread of the pixel output beam, the smaller the display exit pupil in the eyebox and hence the larger the depth of field of the image. On the other hand, the display exit pupil needs to be large enough to reliably form an image in the user's eye. If the angular spread of the pixel output beam and hence the display exit pupil would otherwise be too small, a diffusing function can be added on one of the surfaces in the optical path of the beam. For example, a diffusing optical surface can be added prior to the display so that the angular spread of the pixel output beam increases and the size of the display exit pupil also increases. The pancake lens is formed of a plurality of lenses that are used in both reflection and refraction. A polarizer and a quarter waveplate are used to selectively reflect the first bounce while transmitting the second. A half-silvered mirror is used to reflect the backwards propagating ray towards the user's eye. Alternative types of lens can be used in place of the pancake lens in other embodiments. Suitable alternatives include achromatic converging lenses, Fresnel lenses, freeform reflectors and birdbath lenses. Eliminating VAC The invention reduces or eliminates VAC by reducing the beam diameter in the optics of the display system. By keeping the beam diameter narrower, the image is more in focus and the user can experience focused images without requiring the significant eye accommodation that can lead to VAC. There are two opposing factors in deciding the beam diameter within the display system. On the one hand, the larger the beam is, the more "defocus" the eye will experience due to the narrower DOF that results. This means that more accommodation will be required in the eye to produce a sharp image. On the other hand, making the beam diameter too narrow will introduce aperture diffraction and thus reduce the resolution of the display system. The optimal value of the beam diameter depends on the specific optics used in a given embodiment, but it is usually in the range 0.5 mm to 1 mm. Fig. 3 shows how the spot diameter of an image changes on the retina of an idealised eye model. The eye was modelled as a paraxial lens at a perfect focus at 2 meters. The object is placed at five different positions, from infinity to 30cm. The spot radius on the retina is then calculated for an eye with a 1mm eye pupil (top row) and a 5mm eye pupil (bottom row). The five columns represent an object at different distances, with the eye's focus fixed at 2 meters. When the pupil diameter is only 1mm, the geometric spot diameter is small for the full depth range (shown by black dots in the diagram). On the other hand, when the pupil diameter is 5mm the DOF is much narrower. This leads to much larger geometric spot diameters at depths (i.e. distances) further away from the focal distance of 2m. This is shown by the large and diffuse discs represented by "+" symbols shown at infinity and 0.3m in the diagram for the 5mm pupil diameter. This means that there is significant defocus at distances away from the focal plane when the pupil diameter is 5mm, and the user will likely experience VAC. By reducing the beam diameter of the display system, the invention creates the same larger DOF that is normally associated with a small pupil diameter as shown in Fig. 3. Even if the user's pupil diameter is in fact 5mm, using the display system of the invention the spot diameters at different distances will be the same as if the user's pupil diameter was the same as the smaller beam diameter output by the display system (e.g. around 1mm). While the diffraction spot diameter (shown by a circular black outline in the diagram) is larger when the pupil diameter is 1mm, the large reduction in geometric aberrations relative to a pupil diameter of 5mm keeps the small eye box in focus for all depths. The increase in the spot diameter due to diffraction is significantly less compared to the spot diameter reduction due to the lower geometrical aberrations in the optical system of the embodiment. The inventors have found that a projection optical system with a small display exit pupil also decreases the aberrations associated with the optical system. The display exit pupil is the exit pupil formed by the display optics. The display exit pupil can be replicated at multiple places in the eye box. For example, this replication can be achieved by using fan-out gratings. As the beam diameter output by the illumination engine and hence the display exit pupil diameter becomes smaller, the beam samples a smaller area of the display optics and thus collects fewer aberrations as it passes through the optics. The display exit pupil diameter in the optical system is preferred to be in the range 0.5 - 3 mm, more preferably in the range 2-3 mm. Fig. 4 shows the optical diagram of a simple pancake optics-based VR display with a horizontal FoV (shown vertically on the diagram) of 110 degrees. The user's eye is on the left, and the display device emitting light is on the right. Each line type corresponds to a different angle in the eye box and a different image pixel, which is often referred to as a different "field". In this case, each field corresponds to a single image pixel. In the example shown in this diagram, the eye box formed is fixed and has a length of 12mm, which in many cases will not be sufficiently large. The rays are traced from the eye to the display in the diagram. The system was simulated using OpticStudio (RTM) in this example, which is an example of optical design software. This reversal of rays allows us to compare the resolution of the display system optics to the device pixel size. The right-hand image shows the spot size on the display. The total size of the display is 50.6mm. In the design of Fig. 4, the spot diameter for the central 22 degrees of the FoV is approximately 80pm. Dividing the display size by the spot size for the example of Fig. 4 leads to a maximum central display resolution of approximately 632 pixels, which is very low for a 110-degree FoV VR display. Fig. 5 shows the same optical system as Fig. 4 except that the eye box was cropped to 5mm diameter, which corresponds to the usual maximum diameter of a human eye pupil. This simulation enables us to determine the image produced by the optical system as perceived by the user. This means that in theory all the rays from the display system shown in Fig. 5 could enter the user's eye. With a 5mm eye box, the spot diameter for the range of 0-22 degrees away from the central position of the display is approximately 45pm. This leads to a total display resolution of around 1224 pixels, which is still low for a 110 degrees FoV VR display. Fig. 6 shows the same optical system as Figs. 4 and 5, except that the eye box is now cropped down to 1 mm. This 1mm diameter disk now corresponds to the display exit pupil that moves within the eye box in use of the optical system. The consequent improvement in spot size (and thus resolution) on the display device is significant. Using a 1 mm display exit pupil, the spot diameter for the range of 0-22 degrees away from the central position of the display is approximately 10 pm. For a 50.6mm display, this represents a resolution of approximately 5,000 pixels. This is more than enough for a 110 degree FoV VR display, meaning that with this optical performance the limiting factor for display resolution is likely to be the pixel size of the display device rather than the maximum resolution of the optics. In the same way that the display system reduces the effect of aberrations in the optical system, it also reduces the effect of eye aberrations. Fig. 3 shows how the effect of defocus is reduced by reducing the beam diameter. In the same way, a narrow beam diameter will sample fewer aberrations in the eye and affect the image quality less when users suffer from ocular aberrations. In addition, creating a focus-free image can help people with presbyopia (the progressive loss of near focusing ability of the eye due to ageing). Contrast is often reduced in an optical system by scattering on the surfaces or in the bulk of the optics, producing scattered light that subsequently arrives in the eye to wash out the image produced by the intended image light. In pancake optics, this is particularly challenging as a large portion of the light is reflected by the half-silvered mirror. Contrast can be improved by reducing the amount of scattered light. The current conventional approach to achieving this is to make better optics with more complex coatings to eliminate scattering. In the display system of the invention on the other hand, scattering is reduced by reducing the amount of unutilised light produced, i.e. light that will never arrive at the user's eye. This is achieved by using only a narrow beam of light that is directed accurately through the optics to the user's eye. The invention reduces unutilised light by one or two orders of magnitude in this way. Therefore, there is less light to cause scattering and contrast improves without requiring more expensive coatings and lenses. Rays from different pixels on the display device should arrive in the same position as the eye moves across the eye box. Effectively for any specific pixel (i,j) on the pixelated display device with coordinates (Hi, Hj), there should be a target emission angle (0i, 0j) that will make the rays from the specific pixel arrive at the target eye box position (Pm, Pn). For a fixed eye position, the light input into the illumination light guide by the illumination engine is a beam directed substantially at a single angle 4>m, 4>n (with a relatively small beam divergence around that angle). On the other hand, light emerging from the illumination light guide is a two-dimensional matrix that must satisfy specific angular conditions for all i's and j's. The requirement to control a two-dimensional matrix using a single value is an overconstrained problem, which becomes an optimisation problem. Effectively, with a single value input to the illumination light guide (the angle of incident illumination light), all the conditions for emission angles (0i, 0j) must be satisfied for all i's and j’s (i.e. all pixels). There will inevitably be some degree of error in satisfying the conditions for the illumination angles (0i, 0j) for all pixels (i,j) in this way. The first way to reduce the error is by choosing an input angle 4>m, <t>n that approximates the curve of emission angles (0i, 0j) into a single plane of fixed value; effectively the emission angle across the display device is fixed for all pixels. The exact shape of the curve relating target emission angle (0i, 0j) to a specific display exit position (Pm,Pn) depends on the optics of the projection optical system. One way of approximating the 2D relationship between input angle and display exit position in the eye box is to minimise the root mean square error between the optimal curve for (0i, 6j) and the plane approximation corresponding to a fixed emission angle. However, the error will remain significant for some angles resulting in the beam not arriving at the user's eye. The second approach is to have different out-coupling gratings (different in direction and / or spatial frequency) at different areas on the illumination light guide. The resulting non-uniform out-coupling grating will effectively enable the central angle of the waveguide to be shifted to any desired angle. Therefore, there will be at least one position of the display exit pupil in the eye box where the conditions on the angle (0i, 0j) of light from each pixel are perfectly satisfied. At this position in the eye box, the light is emitted from all the pixels at the target angles and the conditions are met for the display exit pupil to be formed at the same target position in the eye box. In the example shown in Fig. 8, the out-coupling grating would be varied across the illumination light guide so that for each position on the display device the light is emitted at the target angle (0i, 0j) and therefore satisfies the conditions for forming the display exit pupil at the target position. The third approach is to create a light guide with a variable thickness, i.e. an illumination light guide 5 in which one of its surfaces is free-form (see Figs. 10A and 10B). A free-form light guide adds more degrees of freedom to the optics at each pixel position. The thickness variation allows the beam to be magnified or demagnified to different extents across the surface of the illumination light guide, effectively increasing or decreasing the angle of light output by the light guide at each position relative to the input angle of light into the illumination light guide. The variable thickness illumination light guides 5 of Figs. 10A and 10B can be made from plastic, glass, or a combination of both. In some embodiments, the light guide can be moulded in a shape that allows for additional functions to be carried out in it, like focusing and angle magnification. Such light guides may be wedge-shaped, have a concave shape with a constant radius of curvature or have a freeform shape. In addition, a stack of different materials can be used to form the light guide. For example, the light guide nanostructures (i.e. the in-coupling 2 and out-coupling 4 gratings on the input side of the light guide) can be made in a substrate suitable for nanofabrication such as glass or quartz. Fig. 10B shows a light guide having a layer 6 of glass or quartz on which the in-coupling 2 and out-coupling 4 gratings are formed. The part of the light guide 5 on the output side forming the inclined, curved and / or freeform output surface can be formed from plastics, for example using injection moulding. This allows for greater freedom in the shape of the output surface and reduces cost compared to a light guide formed entirely of glass or quartz. Fig. 11 shows another illumination light guide 50 of variable thickness including in-coupling 54 and out-coupling 52 gratings. Like the waveguide shown in Fig. 7, the light guide 50 of Fig. 11 also includes another grating (not shown) to perform the EPE in the second dimension. The light guide 50 of Fig. 11 has a concave curved reflective lower surface, while the upper surface is flat where the in-coupling and out-coupling gratings 54 and 52 are formed. The lower surface may have a reflective coating or may utilise the RI difference between the light guide material (which can be made of materials such as optical glass or optical plastic) and the air to reflect light by TIR. Light enters the light guide 50 from an illumination engine via the in-coupling grating 54 and the upper surface and is then reflected from the lower surface. The reflected light is incident on the out-coupling grating 52 on the upper surface and then exits the light guide at different angles across the light guide as shown. The exit angles of the light across the light guide 50 are controlled by the variation in the angle of the lower surface across the light guide. The in-coupling and out-coupling gratings 54 and 52 may each independently have any shape in plan view (i.e. viewed in a direction perpendicular to the upper surface of the light guide), including for example a circle, ellipse, rectangle or square. Compared to a uniform-thickness planar waveguide, a variable-thickness illumination light guide provides additional design freedom, allowing the out-coupled rays output by the light guide to change in angle across the light guide for a constant angle of input light. Starting from the angle range of illumination light required by the optical system of a NED (for example the pancake lens NED described above), an optical engineer designing the optical system can select the shape of the lower surface of a wedge-shaped, curved and / or freeform light guide to produce the desired output angles across the light guide. In particular, the curved light guide 50 shown in Fig. 11 allows an in-coupled ray at a single angle to exit at different angles at different positions across the light guide, breaking the one-to-one relationship between the input and output angles of the waveguide that exists in planar waveguides. A single input angle is entered into the waveguide, while the output angle is varied across the out-coupling element. The rays of interest are shown as bold dashed lines for clarity. Figs 9A and 9B illustrate further how the display exit pupil 108 is moved across the eye box 107 of an optical system according to an embodiment of the invention. Fig. 9A shows the optical system in cross-section, including an illumination engine 100, a light guide 102, a transmissive display device 104, a projection optical system 106, and the eye box 107. An input illumination light beam 101 is output by the illumination engine 100 into the light guide, which outputs an illumination light beam 103 into the display device 104 via an outcoupling element 105. The display device modifies the light passing through it at pixels including pixels A, B and C labelled 119,120 and 121 respectively. Image light beams from pixels A, B and C are labelled 122,123 and 124 respectively, and the chief image light rays within those beams are labelled 125,126 and 127 respectively. The image light beams from the display device are incident on the projection optical system 106, which converts these beams into pixel A projection output light beam 116, pixel B projection output light beam 117, and pixel C projection output light beam 118. The projection output light beams from the projection optical system 106 are all incident on the display exit pupil 108 within the eye box 107. There is a requirement that the Pixel A projection output light 116, Pixel B projection output light 117, Pixel C projection output light 118, and projection output light from all other pixels on the display device 104 arrive at the display exit pupil 108 position. This requirement ensures that the user's eye lens 109 will collect light from all the pixels of the display device 104 so that the entire displayed image is viewable. The display exit pupil 108 can be at any position in the eye box 107. The position of the display exit pupil 108 is defined by the local vertical coordinate axis y 111 shown in Fig. 9A. In this document the local vertical coordinate axis y 111 is assumed to be in the range between 0 and 1 where 0 is the lowest point in the eye box 107 and 1 is the highest point. It will be understood that the user's eye 112 can move also in the horizontal dimension x within the eye box 107 but for simplicity this is not shown in Fig. 9A. The Pixel A projection output light 116, Pixel B projection output light 117, Pixel C projection output light 118, and projection output light from all other pixels on the display device 104 will coincide on the display exit pupil 108 if the Pixel A image light chief ray 125, Pixel B image light chief ray 126 and Pixel C image light chief ray 127 each have a respective correct angle 0y(i=O), 0y(i=O.5), and 0y(i=l). The correct angle for each chief ray is defined by the optics of the system and particularly the projection optical system 106. The correct angle for each chief ray is a function of the eye 112 position in the eye box 107 on the eye box local vertical coordinate axis y 111 and the pixel position on the display device 104, given by the display device local vertical coordinate axis i 110. Fig. 9B shows the possible variation of the angles of the Pixel A image light chief ray 125, Pixel B image light chief ray 126 and Pixel C image light chief ray 127. As a first approximation, the variation of the chief ray angle as a function of the display exit pupil 108 position in the eye box 107, i.e. the local vertical coordinate axis y 111, can be modelled as a straight line with a given offset 0O(i) and a slope of d0y / dy. Therefore, these model lines each need two independent variables to be independently defined for a fixed pixel position. This means that two degrees of freedom are required to control 0y sufficiently for each chief ray. The first degree of freedom can be the grating period of the out-coupling element 105. However, the local grating frequency of the out-coupling element 105 will have the slope and the offset of the 0y(y) function linked. Therefore, another variable is required to create fully independent control of the slope and offset of the 0y(y) function for each chief ray. A second degree of freedom can be to use a light guide 102 with variable thickness, as described above in relation to Figs. 10A, 10B and 11. The variable thickness of the light guide 102 can shift the angle along its thickness to the desired offset so that the Oy angle offset 128 for Pixel A and the Oy angle offset 129 for Pixel C shown in Fig. 9B have the required target values. Combining adjustment of the grating period from the out-coupling element 105 and the thickness of the light guide 102 can provide sufficient control of the 0y(y) function for each chief ray. In the embodiments above, a method and system have been described for steering the display exit pupil of a display system, using a directional illumination engine producing a low-divergence light beam and an illumination light guide located behind a pixelated display device. In such a display system, the display device is typically 2f (two focal distances) from the user's eye. However, it is also possible to steer the display exit pupil by using a 6f display system in which the display device is six focal distances from the user's eye. Fig. 12 schematically illustrates different options for controlling the position of the display exit pupil of the invention. In the case of the directional backlight created by the illumination engine and the illumination light guide of the embodiment discussed above, the position of the display exit pupil is controlled by changing the angle of light on Plane E of Fig. 12. In an alternative embodiment, the position of the display exit pupil can be controlled instead by a shutter at Plane C and a pixelated display device at Plane A of Fig. 12, in a 6f optical system. Fig. 13 shows this alternative embodiment using a 6f optical system. A pixelated display device 60 at Plane A (position 6f from the eye) emits light that is then collimated by a lens 62 at Plane B (position 5f). The collimated light is then selectively blocked by a shutter mechanism 64 having an adjustable aperture at Plane C (4f) to create an exit pupil having the required size at a desired position on plane C. A converging dynamic lens 66 is also provided after the shutter 64 in the optical path in this embodiment, to adjust the path of light passing out of the aperture of the shutter 64. This lens 66 can be omitted in other embodiments. In this embodiment, the shutter 64 can be a liquid crystal shutter. For example, a liquid crystal shutter modulating polarizations between two crossed polarizers can be used. As a voltage is selectively applied to each pixel of the shutter 64, the light passing through the first of the two polarizers is either transmitted or blocked through the second polarizer according to the voltage applied to that pixel, forming an aperture and a light blocking part. In this way, the liquid crystal shutter provides an aperture whose shape and size can be freely controlled. The shape and size of the aperture can also be modified quickly by changing the voltages applied to pixels across the shutter 64. Using a telescope comprising two lenses 68 and 72 at Plane D (3f) and Plane F (If), the aperture created by the shutter 64 as the display exit pupil is re-imaged from the shutter plane C into the eye box 74 at Of. The lenses 68 and 72 at Plane D and Plane F can be formed in the same solid piece of plastic and / or formed from diffractive surfaces as an alternative to using conventional separate converging lenses. In this example, a reflective surface 70 is provided in the optical path between the lens 68 on plane D and a diffractive element or metasurface 72 as the lens at plane F. The reflective surface 70 can be used to produce an optical system with the desired form factor. The reflective surface 70 may also be curved to assist with imaging the display exit pupil in the eye box 74. With this arrangement, the position of the eye box 74 can be adjusted by changing the position of the aperture in the shutter plane C by controlling the shutter mechanism 64, thus moving the display exit pupil. To produce a larger eye box using the optical system of Fig. 13 without increasing the burden on the display engine, display exit pupil replication can be used. Display exit pupil replication can be implemented by providing a DOE on Plane F that splits each incident ray into multiple output rays. By using display exit pupil replication, an array of display exit pupils is formed in the eye box 74. In this arrangement, the DOE on plane F is configured to produce an array of display exit pupils with a separation distance larger than the maximum usual eye pupil diameter, i.e. larger than 5mm. This ensures that the user only sees a single display exit pupil at any point in the eye box. The foregoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention as defined by the claims.

Claims

1. A display system comprising:An illumination engine configured to generate an input light beam having a first width;a light guide optically coupled to the illumination engine and configured to receive the input light beam, the light guide being configured to convert the input light beam into an illumination light beam having a second width larger than the first width; and a display device configured to be illuminated by the illumination light beam and to convert the illumination light beam into an image light beam, wherein the illumination engine is configured to rotate the angle of the input light beam controllably relative to the orientation of the light guide.

2. The display system of claim 1, wherein the light guide is a planar waveguide.

3. The display system of claim 1 or claim 2, wherein the light guide comprises an incoupling element and an out-coupling element.

4. The display system of claim 3, wherein each of the in-coupling element and the out-coupling element is one of: a diffractive optical element, a metasurface, a prism and a prism array.

5. The display system of any of claims 1-4, wherein the illumination engine comprises: a light source configured to output light;a collimation module configured to collimate light from the light source to generate a collimated beam;a rotatable scanning mirror configured to reflect the collimated beam;an actuator configured to rotate the scanning mirror; anda control device configured to control operation of the actuator to rotate the scanning mirror.

6. The display system of any of claims 1-5, further comprising:a camera configured to capture images of a user's eye and output the images; and a processing unit configured to receive the images output by the camera, calculate a position of the user's eye based on the images, and calculate a rotation angle of the input light beam required to direct the image light beam onto an eye pupil of the user's eye based on the calculated position, wherein the processing unit is configured to output the calculated rotation angle to the illumination engine, and the illumination engine is configured to rotate the angle of the input light beam by the calculated rotation angle so as to direct the image light beam onto the eye pupil of the user's eye.

7. The display system of claim 5, wherein the illumination engine further comprises relay optics configured to receive light reflected by the scanning mirror and to convert the reflected light into the input light beam.

8. The display system of any preceding claim, further comprising an imaging optical system configured to project the image light beam generated by the display device onto a user's eye.

9. The display system of claim 8, wherein the imaging optical system comprises catadioptric optics.

10. The display system of claim 8 or claim 9, wherein the imaging optical system has an effective F-numbergreater than 4.

11. The display system of any preceding claim having a display exit pupil diameter in the range of 0.5mm to 3mm.

12. The display system of any preceding claim, wherein the illumination input light beam has a divergence or convergence angle in the range of 0° to 15°.

13. The display system of claim 3 or claim 4, wherein the light guide comprises an exit pupil expander including the out-coupling element, wherein the out-coupling element is a diffractive optical element configured to expand the beam width of the input light beam along at least a first axis.

14. The display system of claim 13, wherein the exit pupil expander further comprises another diffractive optical element configured to expand the beam width of the input light beam along a second axis perpendicular to the first axis.

15. A light guide for use in a head mounted display, the light guide comprising: a transparent substrate having upper and lower surfaces;an in-coupling element on the upper surface of the substrate configured to couple light incident on the in-coupling element into the light guide; andan exit pupil expander comprising an out-coupling element on the upper surface of the substrate configured to couple light incident on the out-coupling grating out of the lightguide; whereinthe upper surface is planar, and the thickness of the transparent substrate in a direction normal to the upper surface varies across the plane of the upper surface.

16. The light guide according to claim 15, wherein the lower surface has at least one of a curved profile and a profile that is inclined relative to the upper surface when viewed in a direction parallel to the upper surface.

17. The light guide according to claim 15 or claim 16, wherein the transparent substrate has a thickness variation of more than 1%.

18. The light guide according to any of claims 15-17, wherein the transparent substrate comprises an upper layer including the upper surface and a lower layer including the lower surface, the upper layer and the lower layer being stacked in a vertical direction, wherein the upper layer and lower layers are formed from different optical materials.

19. The light guide according to any of claims 15-18, wherein the upper surface has a partially reflective coating.

20. The light guide according to any of claims 15-19, wherein each of the in-coupling element and the out-coupling element is one of: a diffractive optical element, a metasurface, a prism and a prism array.

21. A light guide for use in a head mounted display, the light guide comprising: a transparent substrate having upper and lower surfaces;an in-coupling element on one of the upper and lower surfaces of the substrate configured to couple light incident on the in-coupling element into the light guide; andan out-coupling element on the other of the upper and lower surfaces of the substrate configured to couple light incident on the out-coupling element out of the light guide; whereinthe upper surface is planar, andthe lower surface is reflective and has a curved profile when viewed in a direction parallel to the upper surface.

22. A display system comprising:a display device comprising an array of pixels and configured to emit image light representing an image;a converging lens configured to collect image light from the display device;a shutter mechanism configured to partially block light that has passed through the converging lens from the display device, wherein the shutter mechanism defines an adjustable aperture through which light can pass;a dynamic lens configured to collect light that has passed through the aperture to form a display exit pupil of the display system;a telescopic optical system configured to image the display exit pupil in an eye box of the display system; anda control device configured to control the shutter mechanism and the dynamic lens, wherein the control device is configured to control the position and size of the aperture and the power of the dynamic lens so as to locate the image of the display exit pupil in the eye box.

23. The display system of claim 22, further comprising an eye-tracking system including: an eye tracking camera configured to image an eye of a user of the display system and output image data; andan eye position calculating unit configured to calculate a position of the user's eye based on the image data from the eye tracking camera;wherein the eye position calculating unit is configured to output the calculated position of the user's eye to the control device and the control device is configured to control the position and size of the aperture and the power of the dynamic lens so as to locate the image of the display exit pupil at the calculated position of the user's eye.

Images