Near eye display apparatus

Abstract

An apparatus includes a near eye display 1, an eye tracking system 36 and an array of shutter pixels 30. The eye tracking system determines a user pupil region 8 within an eye box 4 of the near eye display. The array of shutter pixels are across the optical path of the near eye display, or between the near eye display and the eye box. The apparatus controls the near eye display to output at least 24 frames-per-second. Each frame is output as a union of a sequence of two or more sub-frames. The apparatus controls the array of shutter pixels to provide one or more apertures 7. A location of each aperture is determined based on a combination of the user pupil location 33 and which sub-frame is being output, so that light passing that aperture reaches a primary display exit pupil 60 coinciding with the user pupil region. The one or more apertures are sized so that the primary display exit pupil 60 has a diameter of less than or equal to 5 mm at the user pupil region. The optic path of the near eye display may include an output optical element.

Description

Field of the invention The present invention relates to near eye display (NED) apparatus. In particular, the present invention relates to NEDs with a dynamic aperture. Background Mixed Reality (MR) displays, both see through Augmented Reality (AR) and occlusive Virtual Reality (VR) displays, suffer from Vergence Accommodation Conflict (VAC) and the need to wear prescription spectacles under the device. In addition, the image quality is often degraded by the complex refractive and reflective optics necessary to create a large eye box for the eye, i.e., the area where a user's eye can be to see the image. VR displays are likely to be the first mass-market platform for MR. However, optical hardware needs improvement before its wider adoption. A significant problem of MR displays is VAC. The conflict between accommodation and vergence is a significant reason for users' discomfort. This conflict occurs because our brains', after millennia of evolution, have linked "vergence" and "accommodation". However, in a Near Eye Display (NED), this link breaks, and our brains try to focus on one plane while the optical system displays the image on another plane. This invention reduces or eliminates the issue of VAC by creating an image that Is in focus for all depths. An equally challenging issue 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 eye relief and subsequently the size of the headset and causes discomfort) (b) purchase custom-made clip-on's (cumbersome and expensive) (c) correct only the spherical prescription by using the optics of the display (inferior correction as it ignores astigmatism, complex headset and expensive optomechanics). Existing NEDs attempt to optimise the entire eye box simultaneously, resulting in complex optics with poor performance. Existing solutions to the above problems include: Fixed plane A first example existing solution minimises the display's vergence variation about a single fixed accommodation plane. The single accommodation plane is somewhere in the middle of the working space. If it covers all the depth that a typical user can focus (30cm to Infinity), then the intermediate plane is between one meter and two meters. The experience is then designed to minimise the discrepancy between the accommodation plane (that is fixed) and the vergence plane (software controlled) and thus lessen the VAC-associated discomfort. With this approach, the users still need prescription glasses to correct their short- or long-slightness. Dynamic lenses 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 according to the experience content and the need to correct the user's prescription. Nevertheless, the technology for dynamic lenses Is In its embryonic state. Most dynamic lenses (e.g., Alvarez lenses, liquid crystal lenses, liquid lenses, etc.) correct for spherical aberration only over a very small field of view (FoV), introduce scattering, are bulky, and slow for fast-moving experiences (like video games). Retinal Scanners Retinal scanners eliminate the VAC problem and significantly reduce the need for prescription glasses by effectively delivering a very narrow beam of light into the user's eye pupil, forming a tiny eye box. Keeping the eye box small diminishes the aberrations of the optical system and the user's lens. However, existing retinal scanner solutions require careful headset adjustment to deliver the light into the user's eye pupil. As the field of view (FoV) increases, this adjustment becomes very complicated and impractical to Implement. Summary According to a first aspect of the invention there is provided an apparatus including a near eye display. The apparatus also includes an eye tracking system configured to determine a user pupil region within an eye box of the near eye display. The apparatus also includes an array of shutter pixels disposed across the optical path of the near eye display, or between the near eye display and the eye box. The apparatus is configured to control to the near eye display to output at least 24 frames-per-second. Each frame is output as a union of a sequence of two or more sub-frames. The apparatus is also configured to control the array of shutter pixels to provide one or more apertures. A location of each aperture is determined based on a combination of the user pupil location and which sub-frame is being output, such that light passing that aperture reaches a primary display exit pupil coinciding with the user pupil region. The one or more apertures are sized such that the primary display exit pupil has a diameter of less than or equal to 5 mm at the user pupil region. The user pupil region may correspond to the Eye Pupil of a user. The total time taken to output the sequence of two or more sub-frames may not exceed 1 / FPS seconds, where FPS is the overall rate of frames-per-second. In other words, if the number of sub-frames Is Nsub, then the time 6t per sub-frame may be 6t = l / (FPSxNSub) seconds. In this way, the near eye display uses a narrow display exit pupil, permitting an improved depth of field whilst reducing or even eliminating vergence accommodation conflict, without sacrificing the size of the eye box and / or the field of view. In a case where the apparatus controls the array of shutter pixels to provide two or more apertures, a number of display exit pupils will be formed. The primary display exit pupil (which is controlled to be collocated with the user pupil region) receives light from all of the apertures such that the primary display exit pupil includes the entire field-of-view output by the near eye display. Any other display exit pupils may be termed "secondary" display exit pupils. Secondary display exit pupils may not include the complete field-of-view, and will be offset relative to the primary display exit pupil. The apparatus may control the number and spacing of apertures to ensure that no secondary exit pupil intersects the user pupil region. In this way, the user will see the complete field of view via the primary display exit pupil, avoiding artefacts from the secondary display exit pupils. The diameter of the primary display exit pupil of the near eye display may be defined as the region within which the luminance is greater than or equal to 50% of maximum. The diameter may be evaluated on a plane perpendicular to a gaze direction of the users pupil and coincident with the user pupil region. The diameters of any secondary display exit pupils may be defined and evaluated in the same way as the diameter of the primary display exit pupil. Each sub-frame may correspond to a field-of view (FoV) segment as defined herein. During each sub-frame, the near eye display may output one image segment as defined herein. Each aperture may correspond to a shutter cell as defined herein. The primary display exit pupil of the near eye display may have a diameter of less than or equal to 4 mm at the user pupil region. The primary display exit pupil of the near eye display may have a diameter of less than or equal to 3 mm at the user pupil region. The primary display exit pupil of the near eye display may have a diameter of less than or equal to 2 mm at the user pupil region. Herein, when discussing optical paths, an element B is "before" an element A if light passes though the element B before passing through the element A on a path to the eye box (and the user pupil in use). Similarly, the element A would herein be described as "after" the element B. The optic path of the near eye display may include an output optical element. The array of shutter pixels is disposed before or after the output optical element. The output optical element may take the form of a waveguide and an outcoupling element. The array of shutter pixels may be disposed immediately after the outcoupling element. The output optical element may take the form of a lightguide. The output optical element may take the form of an output lens. The output lens may have a focal length f equal to a distance between the output lens and the eye box. In other words, the aperture formed by the shutter pixels may be at a distance at or around the focal length ffrom the user. The array of shutter pixels may be disposed immediately before or immediately after the output optical element. Immediately before / after may mean as physically close as is practical, and without any intervening further optical elements separating the output optical element and the array of shutter pixels. The near eye display may take the form of a laser beam scanner including a scanning mirror coupled to an in-coupling grating of a waveguide by a lens relay system. The waveguide may include an out-coupling element. The array of shutter pixels may be disposed after the out-coupling element, he apparatus may be configured to synchronise the position of a single aperture with the laser beam scanner such that light passing through the single aperture reaches the primary display exit pupil. The array of shutter pixels may be co-extensive with the out-coupling element. The waveguide may take the form of a diffractive waveguide. The outcoupling element may include or take the form of, a grating. The outcoupling element may include or take the form of, a partially reflective mirror. The outcoupling element may include or take the form of, an array of microprisms. The single aperture need not be the same size at every position. The single aperture may correspond to a single shutter pixel. The single aperture may correspond to a group of shutter pixels forming a square, rectangle or hexagon. Alternatively the single aperture may correspond to a group of shutter pixels approximating or bounded by a circle, ellipse, or any other desired shape. Each sub-frame may correspond to a single pixel output by the laser beam scanner. The single aperture formed by the shutter pixels may have a size permitting (the light from) a single pixel at a time to pass through to the primary display exit pupil. In other words, the shutter pixels may be controlled to form an aperture which passes a single pixel generated the laser beam scanner, and the single aperture position may be scanned across the array of shutter pixels at the same rate as the laser beam of the laser beam scanner. In this way, the single aperture position may always be in the right position to allow the beam to pass to form the frame at the primary display exit pupil. Each sub-frame may correspond to a subset of pixels output by the laser beam scanner. The single aperture formed by the shutter pixels may have a size permitting (the light from) any pixel belonging to the subset of pixels to pass through to the primary display exit pupil. In other words, the shutter pixels may be controlled to form an aperture which passes multiple pixels as generated by the laser beam scanner. Typically this may correspond to a section of a line output by the laser beam scanner. In this case, the position of the single aperture may be scanned across the array of shutter pixels at a reduced rate compared to the laser beam of the laser beam scanner, since the position of the single aperture need only be moved as the pixel formed by the laser beam reaches the edge of the single aperture. The near eye display may include a pixelated display. The apparatus may be configured such that, during each sub-frame, the pixelated display outputs one or more image segments of an overall Image corresponding to a frame which that subframe forms part of. The apparatus may be configured such that, during each subframe, the positions of one or more apertures are synchronised to permit a field-of-view segment corresponding to each of the one or more image segments to pass to the primary display exit pupil via a respective aperture. Concurrently open apertures during a single sub-frame may be the same size and / or shape, but need not be. Apertures may be the same size and / or shape across the array of shutter pixels. Alternatively, apertures formed at different positions (centre position of the aperture) within the array may vary in one or more of shape, size, and / or refresh rate, in dependence on position within the array of shutter pixels. The single aperture may correspond to a single shutter pixel. The single aperture may correspond to a group of shutter pixels forming a square, rectangle or hexagon. Alternatively the single aperture may correspond to a group of shutter pixels approximating or bounded by a circle, ellipse, or any other desired shape. Each sub-frame may include, or take the form of, two or more image segments, which are separated from one another by pixels of the pixelated display which are inactive for that sub-frame. The spacing of the two or more image segments on the pixelated display, and the positions and spacing of the corresponding apertures on the array of shutter pixels may be controlled by the apparatus to ensure that the spacing of any secondary display exit pupil(s) from the primary display exit pupil is sufficient that no secondary display exit pupil(s) intersect the user pupil region. Each image segment may correspond to a single pixel of the overall image. Alternatively, each image segment may correspond to two or more adjacent pixels of the overall image. The apparatus may also be configured to output two or more image segments during each sub-frame via an equal number of apertures to form the primary display exit pupil and one or more secondary display exit pupils. The apparatus may also be configured to control the positions and spacings of the two or more apertures within the array of shutter pixels such that a minimum separation between the primary display exit pupil and any one of the one or more secondary display exit pupils is greater than or equal to 1 mm. In this way, the secondary display exit pupil(s) may be prevented from intersecting the user eye pupil region. The minimum separation between the primary display exit pupil and any one of the one or more secondary display exit pupils may be controlled to be greater than or equal to 2 mm. The minimum separation between the primary display exit pupil and any one of the one or more secondary display exit pupils may be controlled to be greater than or equal to 3 mm. The minimum separation between the primary display exit pupil and any one of the one or more secondary display exit pupils may be controlled to be greater than or equal to 4 mm. The minimum separation between the primary display exit pupil and any one of the one or more secondary display exit pupils may be controlled to be greater than or equal to 5 mm. The minimum separation between the primary display exit pupil and any one of the one or more secondary display exit pupils may be controlled to be greater than or equal to 6 mm. The image segments of each sub-frame may form an array. Arrays corresponding to each sub-frame may have the same lattice type and spacing as each other sub-frame. Arrays corresponding to each sub-frame may be offset from, and may not overlap with, one another. The lattice type may be any one of the five 2D Bravais lattices. The lattice type may be square. The lattice type may be rectangular. The lattice type may be hexagonal. In this way, an overall image of resolution N by M pixels may be broken down Into clusters of, for example, 2 by 2 pixels, 3 by 3 pixels, 10 by 10 pixels, 20 by 20 pixels, 50 by 50 pixels and so forth. During each subframe, a number of pixels from each cluster may be output to the primary display exit pupil via a respective aperture, for the sake of explanation say that during each subframe, an image segment of 10 by 10 pixels is output per cluster of 50 by 50 pixels (though more or fewer than 10 by 10 pixels may be output as each image segment). In such an example, there will be a number of sub-frames equal to the number of such image segments in each cluster, in this example 2500 / 100 = 25 subframes. The larger the number of pixels per image segment, the faster the shutter pixels must be switched for a fixed number of clusters (i.e. they must have higher bandwidth). Using a larger number of smaller clusters will reduce bandwidth requirements, but will have more closely spaced active output image segments. This in turn will require more closely spaced apertures, making it harder to achieve sufficient separation of the primary and secondary display pupils. A balance must be found for a given apparatus between providing sufficient separation of the primary and secondary display pupils and a required bandwidth of the shutter pixels. The shutter pixels may have shapes corresponding to the lattice type. For example the shutter pixels may be shaped to permit tessellation. The array of shutter pixels may include, or take the form of, a transmissive liquid crystal display. The apparatus may be configured to control the transmissive liquid crystal display providing the array of shutter pixels to form apertures in the form of binary masks. In other words, within each aperture the shutter pixels may be switched to maximum transmission and outside the one or more apertures the shutter pixels may be switched to minimum transmission (in the ideal case, complete extinction). The shutter pixels may be square. Alternatively, the shutter pixels may be rectangular. The shutter pixels may be hexagonal. The pixelated display may take the form of a transmissive or reflective display having a segmented backlight. Each image segment may be output by illumination of a respective segment of the segmented backlight. The segmented backlight may be controllable at a resolution equal to (and preferably aligned with) the pixelated display. In other words, each segment of the segmented backlight may be a pixel of the same size and shape as (and preferably collocated with) a corresponding pixel of the pixelated display. Alternatively, the segmented backlight may be controllable at a resolution lower than the pixelated display. For example, each image segment may correspond to a number of pixels of the pixelated display which are illuminated by a corresponding segment of the segmented backlight. The pixellated display may take the form of a transmissive liquid crystal display (LCD). The pixellated display may take the form of a reflective liquid crystal on silicon (LCOS) display. The pixellated display may take the form of an e-ink display. The pixellated display may take the form of an electrochromic display. In this way, the liquid crystal pixels may be outputting the overall image for the entire frame, but only those pixels being illuminated by the segmented backlight will be output to the primary display exit pupil during each sub-frame. The pixelated display may take the form of an emissive display. During each subframe only the pixels corresponding to the one or more image segments output during that sub-frame may be emitting light. Pixels which are not used to form the one or more image segments output during that sub-frame may not be activated / on (i.e. do not emit light). The emissive display may take the form of an inorganic light emitting diode (LED) display. The emissive display may take the form of an organic light-emitting diode (OLED) display. The apparatus may control one or both of an aperture size and an aperture shape as a function of an aperture centroid position within the array of shutter pixels. The apparatus may take the form of a head-mounted display apparatus. The apparatus may take the form of an augmented reality apparatus. The apparatus may also comprises a combiner between the near eye display and the eye box. The combiner may merge light from the real world with light from the near eye display. The array of shutter pixels may be disposed between the near eye display and the combiner. The array of shutter pixels may be disposed between the combiner and the eye-box. In other words, the light from the real world may pass through the array of shutter pixels. However, even when the closed / inactive / opaque / off shutter pixels block light from the real word, the rapid switching of the aperture positions means that a user will still be able to perceive the real world. In other examples, the shutter pixels may be configured to only block light from the near eye display. The array of shutter pixels may include, or take the form of, a transmissive liquid crystal display which only includes a single polarizer layer. The single polarizer layer may be arranged to extinguish light from the near eye display. If the near eye display outputs polarised light, the single polarizer layer of the array of shutter pixels may be arranged to extinguish this output polarised light except where one or more shutter pixels are actuated to allow transmission. Alternatively, if the near eye display outputs unpolarised light or a superposition of two or more polarisations, then a pre-polarisation layer may be positioned between the near eye display and the combiner. In this way, the shutter pixels may be used to apply one or more apertures to light output from the near eye display, whilst at most polarising light arriving from the real world. The apparatus may be configured to distort and / or shift frames output by the near eye display to compensate for a user prescription. The apparatus may be configured such that each aperture has an opening less than or equal to 2 mm. According to a second aspect of the invention there is provided a method for an apparatus which includes a near eye display; an eye tracking system configured to determine a user pupil region within an eye box of the near eye display; and an array of shutter pixels disposed across the optical path of the near eye display or between the near eye display and the eye box. The method includes controlling to the near eye display to output at least 24 frames-per-second. Each frame Is output as a union of a sequence of two or more sub-frames. The method also Includes controlling the array of shutter pixels to provide one or more apertures. A location of each aperture Is determined based on a combination of the user pupil location and which sub-frame is being output, such that light passing that aperture reaches a primary display exit pupil coinciding with the user pupil region. The one or more apertures are sized such that the primary display exit pupil has a diameter of less than or equal to 5 mm at the user pupil region. The method of the second aspect may include features corresponding to any features 5 of the apparatus of the first aspect. Definitions applicable to the apparatus of the first aspect (and / or features thereof) may be equally applicable to the method of the second aspect (and / or features thereof). Brief description of the drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram showing the rays emitted by a near eye display (NED) and how they are received by a user; Figures 2A and 2B are schematic diagrams illustrating that the effect of reducing a lens's diameter (or increasing the ratio of the focal length fof a lens divided by the diameter of the lens) is that the depth of field (DoF) increases; Figure 3 is a schematic diagram illustrating different ways in which a small aperture at a user's eye can be achieved: (a) using a 4f system and imaging a small moving pupil (like a scanning mirror) at Plane C into the user's eye, (b) by controlling the cone of the angle at Plane A or E or (c) by having a moving pupil on Plane F which would also require a time sequentially formed image as done in a mirror scanner; Figure 4A is a schematic diagram which illustrates how a small Display Exit Pupil can be achieved by emitting light from selected areas on the output of the NED for a first pixel; Figure 4B is a schematic diagram which illustrates how a small Display Exit Pupil can be achieved by emitting light from selected areas on the output of the NED for a second pixel; Figure 4C is a schematic diagram which illustrates how a small Display Exit Pupil can be achieved by emitting light from selected areas on the output of the NED for a third pixel; Figures 5A and 5B are schematic diagrams that illustrate a shutter after the exit pupil of the NED blocking most of the light, only leaving a small aperture for the light to pass through. The position of the aperture on the shutter moves from a first position (Figure 5A) to a second position (Figure 5B) according to which part of the image is displayed; Figure 6 is a schematic diagram showing three components working in synchronisation: the eye tracker, display engine and shutter; Figure 7 is a schematic diagram of a 4f relay system used to image the mirror into the user's eye or the in-coupling grating of the Exit Pupil Expansion (EPE) waveguide; Figures 8A to 8C are example plots for an NED of (8A) the angle from the scanning mirror and the waveguide, 6(t), at time t, (8B) the aperture position on the shutter h(t) as a function of time, and (8C) where p(t) is the position of the display exit pupil, and pe is the position of the eye pupil. This figure shows a case when the error between p(t) and pe is always zero. The duration shown here is less than a frame; Figures 9A to 9C are example plots for an NED of, in Figure 9A, the angle 0(t) from the scanning mirror and the shutter position on the waveguide at time t. Figure 9B illustrates the aperture position on the shutter h(t) as a function of time t when a pixelated shutter is used (solid line) and an ideal position when a laser scanner is used (dotted line). Figure 9C illustrates the position error on the Display Exit Pupil (solid line), which is introduced relative to the Eye Pupil position (dotted line) by the pixelated nature of the shutter; Figures 10A and 10B are schematic diagrams of how a transmissive liquid crystal display (LCD), or a liquid crystal on silicon (LCOS) in a reflective arrangement, can form the field of view (FoV) time sequentially as different parts of the backlight switch on, different parts of the FoV are formed, and the position of the aperture, h(t), on the shutter can change so that the position of the Display Exit Pupil, p(t) remains the same; Figures 11A to 11C are plots showing an LCD illuminated at different positions during the duration of the video frame. Figure 11A shows the ideal angle (formed by the scanning mirror) leaving the waveguide 9(t) at time t (dashed line), and the angles formed by a pixelated LCD leaving the waveguide, at time t. Figure 11B shows the aperture position on the shutter h(t) as a function of time for the idealised position (dashed line) and the aperture position on the shutter h(t) as a function of time for a pixelated LCD. Figure 11C shows where p(t) is the position of the display exit pupil, and pe is the position of the eye pupil for eight sub-frames of 5t duration each; Figures 12A and 12B are diagrams of how an emissive display such as an inorganic micro-LED) or an organic light-emitting diode (OLED) can form the field of view (FoV) time sequentially as different parts of the emissive display switch on, different parts of the FoV are formed, and the position of the aperture, h(t), on the shutter can change so that the position of the Display Exit Pupil, p(t) remains the same; Figures 13A to 13C show a shutter for three different time points within a single frame as the shutter moves horizontally for a single aperture; Figure 14 illustrates formation of a primary display exit pupil and several secondary display exit pupils when multiple apertures are ON simultaneously. Figures 15A to 15C show a shutter for three different time points within a single frame as the shutter moves horizontally for multiple apertures where a single frame is split into 3x3=9 different time slots; Figure 16 is a schematic diagram of the eye box light distribution when multiple apertures are open and a single display image is ON; Figure 17 is a schematic diagram of hexagonal packing of apertures; Figures 18A to 18C show a hexagonal pattern for a unit cell of seven hexagons, the dashed outline indicates the shape of the aperture cell where a single frame is split into 3x3=9 different time slots; Figures 19A to 19C are schematic diagrams of three example packings (cells) of apertures; Figures 20A and 20B are grids of pixels (dark grey) with shutter pixels highlights (light grey); Figures 21A to 21D are schematic diagrams of light rays passing through an eye lens and reaching the retina for a user that requires prescription correction for hypermetropia; and Figure 22 is a schematic diagram of the paths of light when multiple pixels are displayed in the same timeslot. Detailed description Terminology Vergence Accommodation Conflict (VAC): 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 (alternatively "viewer"). As the object moves closer to the eyes, the eyes look more towards the nose. The movement of the muscles controlling vergence is highly linked to the eye's accommodation and the muscles bringing an object to focus. After many years of evolution, human ocular muscles and visual systems have linked these two factors. Thus, when they are in conflict, it is said that there is a Vergence Accommodation Conflict, and without wishing to be bound by theory it is presently believed to be a cause of fatigue and sickness in NEDs. Field of View (FoV): FOV defines the perceived image size. For a NED, the range of angles represents the image size on the retina and the perceived image size. FOV is usually described as an angle in degrees though there are many different ways to define it explicitly. Eye box: The eye box is the area where the eye can be and see the image. While it is defined as an area, the eye box can exist in many different planes and thus extends in three dimensions. Eye Pupil: Eye Pupil Is the opening of the eye where the light enters. The iris determines the diameter of the eye pupil. Usually, the eye pupil varies between 3 mm and 5 mm. Within the context of the examples of the present invention, the term "user pupil region" Is used herein to refer to the location of the Eye Pupil measured by an eye tracking system. Display Exit Pupil: The display exit pupil is the small pupil formed by the NED in the eye box. The Display Exit Pupil can move within the eye box and follow the Eye Pupil. Display Engine Exit Pupil: Display Engine Exit Pupil is the exit pupil formed by the display engine. The display engine exit pupil can be relayed at multiple places, but It is usually relayed on the incoupling grating, or other in-coupling element of the waveguide. Shutter: The shutter is an optical element that blocks the light leaving the NED and stops it from entering the eye box. For the purposes of specific examples described herein, the shutter can be considered to take the form of a dynamic and pixelated device like a liquid crystal shutter. However, other forms of shutter may be used provided that they are switchable at the rates required to implement the methods described herein. Shutter Pixel: The shutter consists of multiple openings called shutter pixels. These pixels can be switched ON or OFF. When a shutter pixel is ON, light can go through, and the pixel shutter is OPEN; when OFF, light Is blocked, and the shutter pixel is CLOSED. Aperture: Here, an aperture is a single or a group of shutter pixels that are ON (or "OPEN") and form a continuous opening where the light can pass through. FoV Segment: The FoV segment is the image portion the user views within a single sub-frame, and it is formed when the light destined for the eye box passes through a single aperture. The FoV segment can be as little as a single pixel or 10's or 100's pixels square. As the FoV segment increases in angular size, the Display Exit Pupil increases in diameter. Image Segment: The image Segment is the portion of the image the user views when a single aperture is ON. The Image Segment is matched with a specific FoV segment. The Image Segment refers to the image on the display, while the FoV Segment refers to the image in the Eye Box. Sub-frame: A video frame consists of multiple sub-frames. A sub-frame is the time duration, St, of which a single image segment is formed. When a backlight is used, it is the time a single backlight segment is ON. For emissive displays, it is the duration for which a segment of the emissive display (for example inorganic micro-LED, OLED and so forth) is ON (i.e. emitting) within a single frame. Shutter Cell: k Shutter Cell is a group of Shutter Pixels where only one can be ON (or "OPEN") at any time. The Shutter Cell may have a shape similar to the Eye Pupil, i.e. circular, to minimise the number of Shutter Pixels within each. The Shutter Cell reconfigures for each sub-frame and repeats itself across the shutter. Pupil Steering: Pupil steering refers to moving the Display Exit Pupil to different positions so it illuminates the Eye Pupil of the user as the Eye Pupil moves. IPD: Interpuplllary distance. The distance between the two eyes. DoF: Depth of Field. How fast the image becomes defocused as the optics move away from the focal plane. When the F# is large (i.e. small beam diameter), the DoF is large, and the image remains in focus for much longer away from the focal plane. The present specification concerns methods and apparatuses for artificially reducing the effective eye pupil diameter using a dynamic aperture. It uses a combination of components not previously combined in a NED system. Namely, (a) a shutter that reduces the pupil diameter of the optics, (b) an eye-tracking system that detects the position of the user's eye (c) an NED. The methods and apparatuses described herein may employ specific display driving (emissive displays like inorganic micro-LED & OLED) or Illumination (for LCOS &LCD) schemes on the display, in which only part of the display Is on at any time. Operation of a Near-Eye Display In its simplest form, a NED has an aperture where it emits parallel rays and forms a virtual image at infinity. Referring to Figure 1, a simplified diagram is shown of a NED 1 in which parallel rays 2 are emitted from the device. The user's eye 3 can see (via Eye Pupil 8) the entire image where all rays exist, in this case, where dashed-dot line, solid line, and dashed line rays 2i, 22, 23 mutually intersect. This area in the x-y plane is called the eye box 4. In typical NEDs 1, the entire field of view (FoV) is created simultaneously within a single frame. The NED 1 of the present invention forms the FoV time-sequentially within a single video frame. If the object is not at infinity, the rays will slightly diverge. The eye 3 cannot focus on distances closer to ~30cm; therefore, the purpose of the NED 1 is to collimate (i.e., make parallel) or nearly collimate the rays 2. All NEDs 1 can partially create the perception of 3D by shifting the two images in the two eyes 3 and forcing the eyes to verge at a different point. However, most NEDs cannot move the virtual image in the z-direction (depth); thus, the VAC arises. F#, DoF and Aberrations The "F-number" (also F# of f / #) Is the ratio of the focal length f of a lens divided by the diameter of the lens. Increasing the F# increases the depth of focus (DoF) and reduces the effect of aberrations on the image quality Referring also to Figures 2A and 2B, optical systems with small F#, i.e., large diameter compared to their focal distance f, have a limited DoF 5. This phenomenon is shown In Figure 2A, where a lens 6 with a relatively large aperture 7 (formed through opaque screen 11) creates a more blurred image of the virtual world 9 for the exact depth change. Referring in particular to Figure 2A, lens 6 with a large F# (large diameter) focuses on an object at three different planes. The defocus spots 101, IO2, IO3, on the retina 15 also known as Circles of Confusion (CoC), increase fast; thus, the DoF 5 is small. When the effective lens 6 diameter decreases, for example by using an smaller aperture 7 that reduces the effective lens 6 size as shown in Figure 2B, the optical system's DoF 5 increases. As the aperture 7 decreases, any aberrations (including defocus) caused by the lens 6 also have reduced effect on the image. Referring in particular to Figure 2B, the lens 6 diameter is decreased with respect to that in Figure 2A with an aperture resulting in a smaller CoC 10i, 10z, IO3, for the same depth discrepancy; thus, the image will remain sharper for a larger range of depths. Thus, a defocused and highly aberrated optical system can be improved by reducing the size of the aperture 7 used in the system. Reducing a lens's 6 effective diameter (or increasing the F#) increases the DoF 5. In conventional NEDs 1, the eye pupil 8 of the eye 3 is the smallest diameter and sets the DoF 5. In the present NED 1, the optics create a beam diameter smaller than the eye pupil 8 of the eye 3. In this way the optical hardware controls the DoF 5 instead of the eye pupil 8 of the eye 2. Beam diameter As the eye pupil 8 diameter decreases, aberrations associated with the display and the human eye lens reduce, and the DoF 5 increases. While the eye 3 will automatically reduce its iris diameter (and hence eye pupil 8 diameter) in a bright environment, this is not controlled and cannot be utilised in a practical system. In contrast, conventional NEDs 1 create a large eye box 4 that often covers an area of a few square centimetres. Therefore, the eye pupil 8 diameter is not controlled by the optical system but by the individual's physiology. Near Eye Display The present specification describes methods and apparatuses which can control and reduce the beam diameter within the NED's 1 optics, creating a display exit pupil smaller than the eye pupil 8. This can allow obtaining the same benefits as reducing the eye pupil 8 diameter. The display exit pupil can be moved anywhere in the eye box (using pupil steering) to coincide with where the eye pupil 8 is located. The eye pupil 8 position can be determined by eye tracking. By creating a reduced light beam diameter, the effective F# of the eye 3 is increased. Thus, the DoF increases, and consequently, the effect of VAC is reduced or eliminated. In addition, the optical aberrations of both the eye and the NED 1 to the user are reduced. Pupil Steering A NED 1 has a sufficiently large eye box so it can accommodate the movement of the eye 3, the movement of the headset, and different interpupillary distances (IPDs) across the population. Nearly all existing NEDs 1 create a large eye box 4 fixed in space. An alternative approach described herein is to have a small Display Exit Pupil that can move within a much larger eye box 4. Display Exit Pupil steering (sometimes referred to as "pupil steering") is more complex but the inventors have realised that this may also enable some advantages. For example, the optical hardware (and not the eye pupil diameter) may be used to control the DoF 5, and can be changed by the diameter of the Display Exit Pupil. The present invention describes a range of methods and apparatuses for creating a NED 1 with pupil steering. Controlling Pupil position from different planes Referring also to Figure 3, different combinations of how a NED 1 can operate are all superimposed on the same diagram. A pixelated display (e.g. an LCD) can be placed at Plane E or even at Plane A, and a virtual image will be created in the eye box (not shown). Similarly, a Laser Beam Scanner (LBS) can be used on Plane C to form the same virtual image. A small aperture 7 at the user's eye 3 can be achieved, for example, by either using a 4f system and imaging a small moving pupil at Plane C into the user's eye 3, like a scanning mirror or by controlling the cone of the angle at Plane A or E, or by having a moving pupil on Plane F which combined with a time sequentially formed image as done in a mirror scanner. While Figure 3 is shown as a transmissive 6f system, a practical system will only use one or two lenses 6 (2f or 4f system) depending on whether it uses a pixelated device on Plane E or a scanning mirror on Plane C. In addition, other components can be used in the optical path, like an eye-pupil expanding (EPE) waveguide. As shall be described with reference to Figure 3, there are three possible ways to reduce the beam diameter at the eye 3. Controlling position at 4f First, a small LBS can be used on Plane C. As the LBS mirror is imaged by the 4f system in the eye box 4 (see Figure 1), it is possible to set the mirror size (of the LBS) at the target size. However, this approach creates a small, fixed eye box 4 that cannot move. Thus, as the eye 3 moves, the image disappears. Instead, it is possible to have a dynamic pixelated display (e.g., LCD, OLED) at 6f from the eye 3, and then place a dynamic shutter to form an aperture 7 at the 4f plane. The shutter could be an LCD with large pixels. While this would work, a 6f system is not preferred because it will tend to be larger. Dynamic control of position at f Referring also to Figures 4A to 4C, in terms of size, placing the aperture 7 at a distance of f from the user's eye 3 is the most compact system. Not only this, it also may allow for any architecture that may not use a relay of lenses 6. However, because the lens closest to the eye pupil (for example lens 6f on plane F in Figure 3), the aperture's 7 position may differ for each pixel displayed. A small Display Exit Pupil can be achieved by emitting light from selected areas on the output of the NED 1. The examples shown in Figures 4A to 4C illustrate how three different pixels may be formed in the user's eye 3 within three different time slots (sub-frames) within the duration of a single frame. Eye tracking using camera 36 is used to locate the position of the eye pupil 8. Referring also to Figures 5A and 5B, a moving aperture 7 at Plane F (immediately following lens 6f) can control the Display Exit Pupil in the eye box (not shown). The aperture 7 is formed using a dynamic array of shutter pixels 12 For this approach to work, the display outputs different parts (or "segments") of the image at different times. The display does not display the entire image at once. The time-multiplexing of the image frame allows the aperture 7 to be moved to the correct position when a specific part of the image is projected, allowing that specific part of the image to be steered to the eye pupil 8. A laser beam scanner (LBS) (Figure 6) can be used in this application as it only displays a single pixel at any time. Hereinafter, use of a LBS is described in the example shown in Figure 6, though other technologies can be used and examples of transmissive / reflective and emissive displays are also described hereinafter. The array of shutter pixels 12 positioned after the exit pupil of the NED 1 blocks most of the light and only leaves a small aperture 7 for the light to pass through. The position of the aperture 1 within the array of shutter pixels 12 is controlled according to which part of the image is displayed. Example 1: LBS with non-pixelated shutter Referring also to Figures 6 and 7 an example NED 1 includes: (1) An eye tracker that accurately detects the eye's 3 position and its eye pupil 8 using camera 36. (2) An LBS which includes a scanning mirror 20 and a lens relay system 34 (shown in detail in Figure 7). (3) An Exit Pupil Expanding waveguide 23 with at least an in-coupling grating 26 and out-coupling grating 27. The LBS and lens relay system form an input pupil 39 into the waveguide 23. Non-waveguide-based NEDs 1 can also be used. (4) An electrically controlled shutter 30 in front of the out-coupling 27 grating that occludes or transmits light. (5) A single aperture 7 on the shutter 30, the position of which is controllable according to the electrical signals on the shutter 30. (6) An area called the eye box 4 which the user's eye 3 can be positioned within (the user pupil region 33). (7) The user's eye 3. In Figure 6, the system is shown in a single plane. However, the position of the aperture 7, the rays leaving the LBS and the eye 3 can move in two dimensions. In addition, the exit pupil expansion (EPE) may include another grating to expand the eye box in the direction perpendicular to the page. The operation of the system is as follows. A laser illuminates a scanning mirror, and then the mirror is imaged onto the in-coupling grating 26 using the relay optics. The in-coupling grating 26 bends the light and then is totally internally reflected into the waveguide 23. Light propagates into the waveguide 23 until it is incident onto the out-coupling grating 27. After each bounce towards the eye box 4, the out-coupling grating 27 "leaks" some of the light. Collimated (parallel) rays arriving at the eye box 4 appear as an object to infinity for the user. The angle of these rays, 0(t), translates to a pixel position. Therefore, the angle of the mirror maps to the pixel position while the laser intensity, modulated by the system's electronics, controls the pixel intensity. The LBS also links the angle emitted by the display, 9(t), (same out-coupled by the waveguide 23), to time, t. The rays arriving into the pupil at the eye can be backwards propagated and will meet the waveguide 23 at a specific position, h(t), forming a circle with diameter d. Therefore, the pixel position is linked to a time, t, within each frame and related to a specific waveguide p(t) area. Referring also to Figure 8 the position of the aperture, h(t) and the angle of the rays 0(t) are related to time, t. Referring also to Figures 8A to 8C, parameters corresponding to a single line scan of the LBS are illustrated for small angles so that tan(0)=0. Figure 8A is a plot showing the angle 9(t) from the scanning mirror 20 and the waveguide 23 at time t. The duration shown here spans less than a frame. Referring in particular to Figure 8B, the aperture 7 position h(t) on the shutter 30 is shown as a function of time t. Referring to Figure 8C, p(t) plots the position of the display exit pupil, and pe is the position of the eye pupil 8. Figure 8C illustrates the ideal case when the error between p(t) and pe is always zero. The Display Exit Pupil size formed in the eye box 4 can be controlled by controlling the diameter, d, of the aperture 7 on the shutter 30. Note that the diameter d of the aperture 7 is not limited to be the same size across the surface of the shutter 30 (although it may be). The shutter 30 may have theoretically infinite spatial resolution (in practice limited by the precision of the scanning mirror 20). Figure 8B illustrates how the aperture position, h(t), may take any value and match precisely the target user pupil region within the Eye Box 4. Thus an apparatus or system including a NED 1 as described above may help reduce the size, eliminate prescription spectacles (in case of VR or passthrough AR), reduce the device's size (and thus improve comfort), and relax manufacturing tolerances, which in turn may reduce cost. The NED 1 may split a single video frame into subframes, and within each subframe, only a part of the image / FoV may be formed. The NED can therefore steer the Display Exit Pupil to create a large eye box but with a small Display Exit Pupil (created by the display) that follows the eye 3 at any moment. Eye tracking can be used to determine the exact or approximate position of the eye pupil 8 relative to the NED 1. By reducing the beam diameter to around 1 mm, the DoF of the virtual world decreases, and thus VAC may be reduced or eliminated. In addition, since the diameter of the beam Is reduced (F# increases), the image quality may be less susceptible to the aberrations of the display and the human eye, which may provide improved image quality, decreased tolerance for the optical components, and potentially reduced need for prescription correction spectacles. Existing NEDs attempt to optimise the entire eye box 4 simultaneously, resulting in complex optics with poor performance. In contrast, the present invention keeps the diameter of the beam narrow (~lmm). Using a narrow beam can reduce the effects of aberrations caused by the optics, and it can minimise high tolerance requirements, resulting in simpler and lower-cost optics and better image performance. Example 2: LBS with pixelated aperture Referring also to Figures 9A to 9C, the requirement of "Infinite" resolution on the positioning of the aperture 7 on the shutter 30 may be relaxed if the size and position of the target Display Exit Pupil position, p(t), is also relaxed. In this second example, the same architecture may be used as was described in relation to Figures 6 and 7. However, in this example, the shutter 30 is pixelated. This means that the aperture 7 is controllable to discrete positions, as illustrated in Figures 9A to 9C. The restriction to pixel positions of an array of shutter pixels 12 introduces an error on the target Display Exit Pupil position p^t). The error in the position of p(t) should not affect the user experience if it remains small. Typically, the human eye has a diameter of 3 mm to 5 mm; therefore, some error (fraction of a mm) is acceptable (and will typically be compensated for by the users eye and brain). Referring in particular to Figure 9A, the angle of the rays emerging from the waveguide for a fraction of the frame period are shown. Referring in particular to Figure 9B, when a pixelated shutter 30 is used, the aperture takes discrete positions (solid line). This discretisation results in an error relative to the ideal position (dotted line). Referring in particular to Figure 9C, the pixelated nature of the shutter 30 also introduces position error on the Display Exit Pupil (shown as a solid line) relative to the Eye Pupil 8 position (shown as a dotted line). Example 3: LCD and LCOS with segmented backlight In the present invention, an LCOS or an LCD can be used to form the image (i.e. to provide the NED 1). However, an LCD or LCOS device that is constantly ON cannot create a small NED display exit pupil in the eye box. If the entire LCD / LCOS is on, as the aperture 7 moves along the plane of the array of shutter pixels 12, the entire eye box 4 will be formed for each pixel (but in a time-sequential manner). By forming the entire eye box 4, all the artefacts the narrow beam tries to eliminate will appear. Referring also to Figure 10, this may be avoided by using a transmissive LCD or a reflective LCOS combined with a segmented backlight 42. The different segments of the backlight illuminate different segments of the pixelated device so that the temporal formation of the FoV is introduced. In other words, different parts of the FoV are formed at different times in the video frame. In the extreme case, if the segmented backlight 42 illuminated one pixel of the LCD / LCOS at a time, the LCD / LCOS would behave as an LBS regarding the temporal FoV formation. The third example illustrated in Figures 10A and 10B is similar to the first example illustrated In Figure 6. This architecture can be used with or without an EPE waveguide. In Figures 10A and 10B, only the Display Engine 45 and the shutter 30 (for example a dynamic array of shutter pixels 12) are shown. The NED 1 of the third example includes the following: (1) A backlight 42 where different areas can be switched ON or OFF (the area which is ON is illustrated by the hatched LED symbol). Such an arrangement is already used in LCD screens to improve contrast ratio. (2) A transmissive LCD 45 (a reflective LCD could be substituted). (3) A lens 6 that collimates rays and forms an image at infinity (the same principle applies even if the image is not at infinity). (4) A shutter 30 (for example a dynamic array of shutter pixels 12) that only allows light to pass through an aperture 7 of diameter d at position p{t) As different parts (or "segments") of the backlight 42 switch on, different parts of the FoV are formed, and the position of the aperture 7, h(t), on the shutter 30 changes to allow the position of the Display Exit Pupil, p(t) to remain the same (i.e. coincident with the user eye pupil 8. Referring also to Figures 11A to 11C, this arrangement of the third example works as follows. At any moment, only a small part of the LCD 45 is illuminated. The illumination area is represented in Figure 11A as the height of the rectangles 46. The height of each rectangle 46 represents the size of the FoV segment in degrees and the width the duration that the aperture 7 of the shutter 30 will remain open. Only a small section of the image is formed for a specific time by keeping all the backlight's LEDs off except one segment's LEDs. The segment LEDs are then synchronised with the pixelated aperture 7 so that the aperture 7 is at the correct position, h(t) and restricts the beam to the target diameter, d. As the backlight 42 will illuminate many pixels during each subframe, rays from different pixels will propagate at different positions in the eye box 4, resulting in a Display Exit Pupil position that is different for each part of the FoV segment and spread over a small area. This variation in the Display Exit Pupil position is represented by the height of the rectangles 47 in Figure 11C. This approach has the advantage that it can use an LCD or LCOS instead of a LBS. LCD and LCOS can have advantages in reliability, cost, and resolution. Referring in particular to Figure 11A, an LCD 45 can be Illuminated at different positions during the duration of the video frame. Here eight sub-frames are shown, each of duration St. During each sub-frame, the image segment contains a range of angles, 0(t), represented by the height of the rectangles 46. Referring in particular to Figure 11B, the aperture position, h(t), is synchronised with the illumination from the segmented backlight 42, so the pixelated shutter 30 changes in synchronisation with the display engine of the LCD. Referring in particular to Figure 11C, the variation of angles, 0(t), creates an error in the position of the Display Exit Pupil depending on the angle, represented here as the heights of the rectangle 47. The dotted line shows the eye pupil 8 position, corresponding to the target user pupil region . Example 4: Emissive displays forming image temporally sequential Referring also to Figures 12A and 12B, in a fourth example emissive displays 48 can be used by switching on their different segments at different timings within a frame. Emissive displays 48 such as inorganic micro-LEDs and OLEDs have advantages compared to LBS and transmissive / reflective displays like LCDs and LCOS. The same approach used for LCOS and LCDs and described in relation to Figures 10A to 11C can be adapted for inorganic micro-LEDs and OLEDs. Currently, Inorganic micro-LEDs and OLEDs 51 are already switched on and off at high frequencies to increase efficiency. Within the context of the methods of the present specification, this switching can be adjusted so different display segments are switched on to output different sub-frames. The timing of each segment can be synchronised with the pixelated shutter 30 as described hereinbefore, so the aperture position, p{t), is controlled to direct the beam to the user eye pupil 8. To allow the use of emissive displays 45 with the present invention, it is possible to use a significantly faster display in order to sacrifice speed for achieving the desired pattern on the display pixels. Alternatively, where a display 51 supports a "Region of Interest", for example, in smartphones where power consumption is critical, the display may be configured so that it only updates the area where the ON pixels are present. Example 5: Multiple apertures ON at the same time In the preceding examples, it has been assumed that at any moment, there is a single aperture 7 only. Having a single aperture 7 per shutter 12, 30 per frame requires the aperture 7 to scan the entire area of the NED 1 waveguide 23, lens 6 or other optics within a single frame, usually ~10 ms, placing high requirements on the shutter's 30 speed (alternatively "bandwidth"). In addition, the display backlight 42 or emissive display 48 may be required to be very bright as it will only be on for a tiny fraction of the frame (which may for example accelerate OLED aging). However, the methods of the present specification are not limited to the single-aperture 7 approach. Referring also to Figures 13A to 13B, a pixelated shutter 30 with sixty shutter pixels 54 is illustrated. The aperture 7 can be in any of these positions and traverses them all during a frame. For example, in a 90fps display, the shutter 30 would be only open for ~370ps; thus, the switching speed would have to be significantly higher. When a single aperture 7 is used, the shutter 30 can configured to be fast enough to cover the entire area within a single frame. In Figures 13A, 13B and 13C the shutter 30 is shown for three consecutive time points within a single frame as the aperture 7 moves horizontally across the shutter 30. The aperture 7 being able to more across the 10x6 grid of the shutter 30 within a single frame imposed a high-speed requirement. A single frame is split into 10x6=60 different time slots for the example shown in Figures 13A to 13C. An alternative approach used in the fifth example is to open multiple apertures 7 concurrently during each sub-frame to create multiple Display Exit Pupils in the Eye Box 4. However, the spacing of the multiple Display Exit Pupils should be controlled to be larger than the Eye Pupil 8 Diameter. This ensures that the user does not experience any repercussions / artefacts from the use of larger and multiple Display Exit Pupils. The spacing of the apertures 7 also determines the spacing between the multiple Display Exit Pupils. Referring also to Figure 14, it is illustrated how multiple apertures 7 can be ON simultaneously, and the eye 3 only receives light from one aperture 7 for each part of the FoV. Three apertures 7a, 7b, 7c are open, and the eye pupil 8 of the eye 3 receives light from three segments of the display. All three apertures 7a, 7b, 7c allow light to go in all three different directions (FoVla to FoV3c), creating nine bundles of rays. However, the eye pupil 8 only receives light from three FoVs (here, FoV3a, FoV2b and FoVlc from apertures 7a, 7b and 7c respectively), forming a primary display exit pupil. The exclusion of the other six FoVs (FoV3b, FoV2a and FoVla, FoV3c, FoV2c and FoVlb) is achieved by placing the apertures 7a, 7b, 7c far enough apart so that at any moment, the pupil of the eye only receives the desired parts of the FoV. At the same time, the unwanted copies of the rays, forming secondary display exit pupils, do not intersect the eye pupil 8 diameter dep. Referring also to Figures 15A to 15C, an example is shown of how multiple apertures 7 can move across the pixelated shutter 30. Here, there Is a repetition of a 3x3 cluster (or "cell") of shutter pixels (delineated by the dashed square) that have the same pattern across the entire area of the shutter 30. Within a single sub-frame, multiple image segments are output via the multiple apertures 7. Each image segment will be output from all the ON apertures 7, but only the aperture 7 corresponding to each image segment will pass light to the primary display exit pupil collocated with the user eye pupil 8 (the other beams forming secondary display exit pupils offset from the eye pupil 8 as illustrated in Figure 14). In a case as illustrated in Figures 15A to 15B, when multiple apertures are used, the speed requirement of the shutter 30 goes down. In Figures 15A, 15B and 15C, the shutter 30 is shown for three different time points within a single frame as the apertures 7 move along the horizontal. A single frame here is split into 3x3=9 different time slots. As this illustrates, the shutter 30 switching speed requirements reduce rapidly with the number of concurrently open apertures 7, which facilitates finding the right balance between the number of apertures 7 whilst keeping the secondary display exit pupils well separated from the primary display exit pupil (and away from the user eye pupil 8). Many factors including (but not limited to) maximum eye pupil 8 diameter dep, target beam diameter, eye relief 57 (see Figure 14), image segment size determine the number of shutter pixels per cell. However, the maximum eye pupil 8 diameter deP and target beam diameter give a lower estimate of the number of shutter pixels per cell. In one dimension, considering a single pixel (i.e., a single angle 0( t)), the spacing between Display Exit Pupils will be, at most, the spacing between two apertures 7. Thus the spacing between two apertures 7 should preferably be around the maximum diameter dep of the Eye Pupil 8 (for example, based on population averages). Referring also to Figure 16, the appearance of the Display Exit Pupils 60a, 60b within the Eye Box 4 is illustrated for the case that a single image pixel is on. The Display Exit Pupils 60a, 60b include the primary display exit pupil 60a which is steered to coincide with the user eye pupil 8, and also a number of secondary display exit pupils 60b. The repeated pattern on the Pixelated Shutter 30 create an array of Display Exit Pupils 60a, 60b in the eye box 4. As long as the eye pupil 8 cannot see more than the primary Display Exit Pupil 60a, the spacing between the open apertures 7 on the shutter 30 is sufficient. Example 6: Shape &Arrangement of the apertures Figure 16 shows how the display exit pupils would appear In the eye box 4 if the apertures 7 were arranged in a square grid, as shown in Figures 15A to 15C. However, rectangular packing of the apertures is not essential. Referring also to Figure 17, the eye pupil 8 has a circular shape, and thus a packing of the apertures 7 that most resemble circular packing may be preferable. Hexagonal packing is an approach to maximise the area of each aperture 7 and thus minimise the time they need to be ON and the number of the Shutter Pixels. In the example shown in Figure 17, a "cell" or "cluster" of apertures 67 has seven apertures 7. Other packings are possible with more square, rectangular, or hexagonal apertures. All the apertures within a cluster may open within a single frame. In the Figure 17 example, an example opening order of the seven apertures 7 of a hexagonal packed cluster is shown (tl to t7), each corresponding to a sub-frame. Referring also to Figures 18A to 18C, a larger area of a shutter 30 made up using the hexagonal cells of Figure 17 is shown. Figures 18A, 18B and 18C respectively show the first three sub-frames (tl to t3). The dashed outline indicates the shape of the cell 67 as shown in Figure 17. Referring also to Figures 19A to 19C, three possible packings (cells) of hexagonal shutter pixels for forming apertures 7 are shown having three, seven, and nine hexagons respectively. By keeping the packing shape as close to a circle as possible, the display exit pupil 60 may better matche the circular shape of the eye's pupil 63. The number of apertures 7 is minimised; thus, the speed requirements are reduced. The hexagonal shutter pixels shown in Figures 17 to 19C may be used with conventionally square pixel arrays of a NED 1. In practice, each hexagonal shutter pixels may allow pixels from a region of a pixellated NED 1 to pass to the primary display exit pupil. Of course, a pixellated NED 1 using hexagonal pixels could be used if available. Example 7: Shutter with adjacent apertures ON In the pixel shutter cluster arrangements described hereinbefore, only a single aperture 7 per cluster / cell is ON per sub-frame. This abrupt change from one part of the image segment to another is not ideal for all types of NED 1. First, if a segmented backlight 42 Is used, a uniform illumination profile over a specific area and then transition to zero may be preferred. Second, if an emissive display 48 is used, the addressing (or at least the inorganic micro-LED / OLED switching) may happen In blocks and can complicate the addressing circuit. In such cases, a rolling addressing of shutter pixels may be preferable. In such examples, the shutter 30 is still quantised, but at any moment, many adjacent Shutter Pixels are ON. By creating each aperture 7 using multiple Shutter Pixels 54, the movement of the aperture 7 can be made smoother. For example, referring also to Figures 20A and 20B, an illustration is provided of how a single aperture 7 may be made of 2x2 Shutter Pixels 54. Comparing this cluster arrangement with the one illustrated in Figures 15A to 15C shows how the aperture may move more smoothly. In other words, an aperture 7 may consist of many smaller Shutter Pixels 54 to create a smoother shift. The smoother transition relaxes the speed requirements on the backlight 42 (for LCDs and LCOS) and the display backplane 51 (for ILEDS). In the example shown in Figures 20A and 20B, the opening of the aperture 7 is twice as large in each dimension as the shutter pixels 54. The smooth movement may also allow the backlight 42 or the display 48 to work analogously to a "rolling shutter camera". Example 8: Variable Shutter Pixel size In the example described hereinbefore the apertures 7 and Shutter Pixels 54 used to form them were presumed to have the same size and shape in all sub-frames and at any position within the shutter 30. However, human vision varies across its FoV, and as a consequence any of the preceding examples may be modified to have different aperture 7 and / or shutter pixel 54 sizes (and / or shapes) across the area of the shutter 30. The shutter pixel size may reflect requirements for depth perception and spatial or temporal response of the eye 3. 9: Pre-compensation for prescription and aberration correction A small primary Display Exit Pupil 60a can help reduce defocus caused by the eye 3 and the optical system. Referring also to Figure 21A, an eye that requires positive ophthalmic correction (hypermetropic eye) is illustrated. The eye lens 70 focuses behind the retina 15 when this user looks at an object at infinity. On the retina 15, the image is defocused, and they perceive a blurred image (illustrated by the dashed ellipse). Reducing the effect pupil diameter by controlling the width of an incident light beam (as described in this specification) helps to minimises or diminishes the effect of the blur, as shown referring also to Figure 21B. However, while blur is reduced, another undesirable effect is introduced. If the primary Display Exit Pupil 60a of the optical system p(t) is not always at the exact position of the eye pupil 8, the user will experience image shifting. Referring again to Figure 21B and also to Figure 21C, it is illustrated how the position on the retina shifts as the position of the primary Display Exit Pupil 60a changes. This can creates an undesirable effect of the image moving which, depending on the prescription of a user, may need to be corrected (smaller shifts in users who do not require corrective lenses may remain below a threshold of perceptibility). In this example, accurate eye tracking can be used to determine the exact position of the eye pupil 8. By doing so, it is also possible to determine the position of the primary Display Exit Pupil 60a relative to the user's eye lens 70. For users of corrective lenses, correction can then be applied in the software to distort or shift the image being output so as to reduce or eliminate the shift on the user's retina 15. For example, referring also to Figure 21D the situation is shown wherein an additional tilt in the rays forming the image is introduced to correct for errors due to the user's prescription and keep the image they perceive at a stable location. In this way, a users prescription, if severe enough to need correction, may be accounted for in software, instead of requiring special / customised optics. NED 1 pupil spread Referring also to Figure 22, rays 2 from multiple pixels displayed in the same timeslot (sub-frame) passing through the aperture 7 will spread as they propagate from the NED 1 towards the eye 3. This spread will increase as the FoV segment and the eye 3 relief increase, creating a larger Display Exit Pupil 60 (as each pixel is at a different angle). If the Display Exit Pupil 60 expands significantly, the eye pupil 8 may crop some of It, and the image will be affected. The Display Exit Pupil 60 preferably remains small relative to the Eye Pupil Diameter dep in order to avoid the iris cropping the beam in this way. For the case when the eye relief 57 is 15 mm, the FoV segment is 5° (or ±2.5°), pixel resolution is 45 ppd, and the beam diameter is 1 mm, the error is ±0.52 mm. Therefore, the Display Exit Pupil 60 expands from the target value of 1 mm to 2.04 mm. This value is still smaller than the typical smallest eye pupil 8 diameter dep '3mm Smaller FoV / image segment size may be required if the Eye Tracking is slow or inaccurate. Modifications It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of display apparatuses, devices, systems and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. An apparatus comprising:a near eye display;an eye tracking system configured to determine a user pupil region within an eye box of the near eye display; andan array of shutter pixels disposed across the optical path of the near eye display or between the near eye display and the eye box;the apparatus configured:to control to the near eye display to output at least 24 frames-per-second, wherein each frame is output as a union of a sequence of two or more sub-frames;to control the array of shutter pixels to provide one or more apertures, wherein a location of each aperture is determined based on a combination of the user pupil location and which sub-frame is being output, such that light passing that aperture reaches a primary display exit pupil coinciding with the user pupil region;wherein the one or more apertures are sized such that the primary display exit pupil has a diameter of less than or equal to 5 mm at the user pupil region.

2. The apparatus of claim 1, wherein the optic path of the near eye display comprises an output optical element, and wherein the array of shutter pixels is disposed before or after the output optical element.

3. The apparatus of claims 1 or 2, wherein the near eye display takes the form of a laser beam scanner comprising a scanning mirror coupled to an in-coupling grating of a waveguide by a lens relay system, wherein the waveguide comprises an out-coupling element;wherein the array of shutter pixels are disposed after the out-coupling element and the apparatus is configured to synchronise the position of a single aperture with the laser beam scanner such that light passing through the single aperture reaches the primary display exit pupil.

4. The apparatus of claim 3, wherein each sub-frame corresponds to a single pixel output by the laser beam scanner, and wherein the single aperture formed by the shutter pixels has a size permitting a single pixel at a time to pass through to the primary display exit pupil.

5. The apparatus of claim 3, wherein each sub-frame corresponds to a subset of pixels output by the laser beam scanner, and wherein the single aperture formed by the shutter pixels has a size permitting any pixel belonging to the subset of pixels to pass through to the primary display exit pupil.

6. The apparatus of claims 1 or 2, wherein the near eye display comprises a pixelated display, and the apparatus is configured such that, during each sub-frame: the pixelated display outputs one or more image segments of an overall image corresponding to a frame which that sub-frame forms part of;the positions of one or more apertures are synchronised to permit a field-of-view segment corresponding to each of the one or more image segments to pass to the primary display exit pupil via a respective aperture.

7. The apparatus of claim 6, wherein each image segment corresponds to a single pixel of the overall image.

8. The apparatus of claim 6, wherein each image segment corresponds to two or more adjacent pixels of the overall image.

9. The apparatus of any one of claims 6 to 8, wherein the apparatus is configured: to output two or more image segments during each sub-frame via an equal number of apertures to form the primary display exit pupil and one or more secondary display exit pupils;to control the positions and spacings of the two or more apertures within the array of shutter pixels such that a minimum separation between the primary display exit pupil and any one of the one or more secondary display exit pupils is greater than or equal to 1 mm.

10. The apparatus of claim 9, wherein the image segments of each sub-frame form an array, wherein arrays corresponding to each sub-frame:have the same lattice type and spacing as each other sub-frame; and are offset from, and do not overlap with, one another.

11. The apparatus of any one of claims 1 to 10, wherein the array of shutter pixels comprises a transmissive liquid crystal display.

12. The apparatus of any one of claims 1 to 11, wherein the shutter pixels are square.

13. The apparatus of any one of claims 6 to 12, wherein the pixelated display takes the form of a transmissive or reflective display having a segmented backlight, wherein each image segment is output by illumination of a respective segment of the backlight.

14. The apparatus of any one of claims 6 to 12, wherein the pixelated display takes the form of an emissive display, wherein during each sub-frame only the pixels corresponding to the one or more image segments output during that sub-frame are emitting light.

15. The apparatus of any one of claims 1 to 14, wherein the apparatus controls one or both of an aperture size and an aperture shape as a function of an aperture centroid position within the array of shutter pixels.

16. The apparatus of any one of claims 1 to 15, wherein the apparatus takes the form of a head-mounted display apparatus.

17. The apparatus of any one of claims 1 to 16, wherein the apparatus takes the form of an augmented reality apparatus, and further comprises a combiner between the near eye display and the eye box.

18. The apparatus of claim 17, wherein the array of shutter pixels is disposed between the near eye display and the combiner.

19. The apparatus of claim 17, wherein the array of shutter pixels is disposed between the combiner and the eye-box.

20. The apparatus of claim 19, wherein the array of shutter pixels comprises a transmissive liquid crystal display which only includes a single polarizer layer;wherein the single polarizer layer is arranged to extinguish light from the near eye display.

21. The apparatus of any one of claims 1 to 20, wherein the apparatus is configured to distort and / or shift frames output by the near eye display to compensate for a user prescription.

22. The apparatus of any one of claims 1 to 21, wherein the apparatus is configured such that each aperture has an opening less than or equal to 2 mm.

23. A method for an apparatus comprising:a near eye display;an eye tracking system configured to determine a user pupil region within an eye box of the near eye display; andan array of shutter pixels disposed across the optical path of the near eye display or between the near eye display and the eye box;the method comprising:controlling to the near eye display to output at least 24 frames-per-second, wherein each frame is output as a union of a sequence of two or more sub-frames;controlling the array of shutter pixels to provide one or more apertures, wherein a location of each aperture is determined based on a combination of the user pupil location and which sub-frame is being output, such that light passing that aperture reaches a primary display exit pupil coinciding with the user pupil region;wherein the one or more apertures are sized such that the primary display exit pupil has a diameter of less than or equal to 5 mm at the user pupil region.

Images