WO2024149589A1

WO2024149589A1 - Eyeglass lens with holographic optical element

Info

Publication number: WO2024149589A1
Application number: PCT/EP2023/087005
Authority: WO
Inventors: Helen Smith; Sophie VIAENE
Original assignee: Trulife Optics Ltd
Priority date: 2023-01-13
Filing date: 2023-12-20
Publication date: 2024-07-18
Also published as: GB2626181A; GB202300536D0

Abstract

The present disclosure relates to an eyeglass lens for a wearable head-up display, the eyeglass lens comprising: an optical centre, a primary gaze axis, and a holographic optical element having a vertex, wherein the holographic optical element is integrated with the eyeglass lens such that said vertex is coincident with an image viewing axis to allow digital content to be viewed, and wherein the optical centre of the lens is positioned with respect to the primary gaze axis and the vertex is positioned with respect to the optical centre. Also disclosed a wearable display, comprising: an eyeglass lens, a glasses frame; and an image source, the eyeglass lens is mounted in the frame, the image source is mounted on an arm of the frame adjacent to the eyeglass lens to project light on to the holographic optical element of the eyeglass lens to generate one or more image eyepoints.

Description

EYEGLASS LENS WITH HOLOGRAPHIC OPTICAL ELEMENT

Field

The present disclosure relates to an eyeglass lens integrated with a holographic optical element (HOE) for use with a wearable head-up display (HUD). More particularly, the present disclosure relates to such an eyeglass lens where the holographic optical element is formed of a thin-film integrated with the eyeglass lens. The present disclosure also relates to a wearable HUD AR system comprising such an eyeglass lens.

Background

Eyeglass lenses are typically manufactured via lens grinding or injection moulding techniques. The injection moulding process for an eyeglass lens consists of injecting the molten material into a mould under high pressure. The mould is shaped either with a blank shape, to create a lens ‘blank’ with the intention of further shaping the inner surface to create the final lens or with the exact contours of the desired eyeglass lens, including the inner and outer curvature profiles. After a short cooling time, the lens is complete. Due to the ease and speed of this process, injection moulding is the technique of choice for large batches and stock lenses such as sunglasses. For more bespoke applications, lens grinding techniques are used. The lens grinding process starts with a lens ‘blank’, which has a convex outer (world facing) surface and either a planar, or concave inner (eye-facing) surface. The curved surface or surfaces of these blanks are typically spherical or spherocylindrical in profile. The inner surface is then ground using specialist machinery to the desired curvature profile after which it is polished. The eyeglass lens is then edged to the desired edge profile to fit in a frame and the eyeglass is completed. With the advent of 3D printing (or additive manufacturing) technology, it is also possible to 3D print eyeglass lenses with entirely bespoke curvatures and edge profiles.

Integrating thin films with eyeglass lenses, or encapsulating them within eyeglass lenses, is desirable for many purposes. For example, some Augmented Reality (AR) systems (including associated eye tracking systems) require Holographic Optical Elements (HOEs) that can be produced on thin films which are affixed to, or encapsulated within, the eyeglass lens.

There are many existing approaches for integrating such thin films within eyeglasses or laminating thin films on eyeglasses. For example, US 2015/0131047 A1 details processes for the direct lamination of cellulose acetate laminated films onto eyeglasses of varying profile for use in safety glasses. US 2017/0068095 A1 describes a number of integration techniques. In a first of these, a lens is injection moulded, whereby a mould is made with a cavity to position a HOE photopolymer film prior to casting. The eyeglass material is then injected into the mould, encapsulating the HOE within. A second technique encapsulates the HOE between two half-lenses, comprising a back portion of the eyeglass lens and a front portion of the eyeglass lens. The HOE is sandwiched at the interface of the two eyeglass lens components or parts. In a third approach, a photopolymer film is directly laminated to the spherically concave inner surface of the eyeglass lens.

For AR applications such as wearable HU Ds, the integration and placement of the HOE with the eyeglass lens and alignment of the image light source with the HOE and the eyeglass lens is non-trivial. The HOE should be positioned carefully, to ensure that the image generated is incident on the user’s eye (taking into account diverse physiological differences from one user to the next) and has good resolution and colour uniformity.

Known approaches involve complex manufacturing techniques and risk misalignment of the HOE with respect to the eyeglass lens in which it is integrated, misalignment with respect to a user’s pupil and/or misalignment with respect to the image light source. Any misalignment could prevent or hinder a user of the wearable HUD from viewing the displayed content either completely or partially.

When HU Ds require a tight focus on the eye side, such as for virtual retinal display or eye tracking applications, typically for small form factor wearable HUDs, the eyebox (or eyepoint) needs to be small to support a sufficient field of view, typically in the order of 2mm² to 5mm², due to the smaller optics requirement of the light source. For AR HUD applications an eyebox typically describes the volume of space within which a viewable image is formed. The eyebox represents a combination of exit pupil area, field of view, and eye relief (or back vertex distance (BVD)). The virtual image or displayed content may only be seen when a user's pupil is located at specific location which corresponds to an eyebox (or eyepoint) location. Outside of an eyebox location, the virtual image or displayed content will be partially obscured or cannot be seen at all. Making the eyebox larger and/or sacrificing field of view can accommodate physiological differences such as IPD, wrap angle, pantoscopic tilt angle for different users. However, this can make the HUD larger and heavier to accommodate the larger projection system and image calibration needed to adjust the displayed image content from user to user.

Alternatively, one way to increase the eyebox size is to is to generate multiple replicated eyeboxes or eyepoints, for example in VRD based systems, such that for a range of typical users the range of IPDs for those users will fall within at least one of the eyepoints. Furthermore, gaze direction of the user and therefore positioning of the HOE is important to consider when designing HUDs to ensure that the displayed content is completely viewable by the user. In the context of the present disclosure, the virtual image, displayed content or other such similar term refers to the digital content or images displayed to the user of a wearable HUD.

Summary

According to a first aspect, there is provided an eyeglass lens for a wearable head-up display, the eyeglass lens comprising: an optical centre, a primary gaze axis, and a holographic optical element having a vertex, wherein the holographic optical element is integrated with the eyeglass lens such that said vertex is coincident with an image viewing axis to allow digital content to be viewed, and wherein the optical centre of the lens is positioned with respect to the primary gaze axis and the vertex is positioned with respect to the optical centre.

The image viewing axis may be offset from the primary gaze axis. The image viewing axis may be offset from the primary gaze axis in the horizontal plane by a horizontal image viewing angle and in the vertical plane by a vertical image viewing angle.

The horizontal viewing angle may be defined as any angle between zero, defined by the primary gaze direction, and the maximum possible leftward or rightward horizontal rotation of a user’s eye. The vertical viewing angle may be defined as any angle between zero, defined by the primary gaze direction and the maximum possible upward or downward vertical rotation of a user’s eye.

The horizontal image viewing angle may be between zero degrees and 40 degrees in adduction or abduction and preferably between 5 and 10 degrees in adduction or abduction; and the vertical image viewing angle is between zero and 28 degrees in elevation and 47 degrees in depression, and preferably between 5 and 10 degrees in elevation or depression.

The holographic optical element may comprise a first variation in hologram function configured and arranged to compensate for the horizontal and vertical image viewing angles being offset from the primary gaze axis. The first variation in hologram function may be a local variation in surface grating pitch. The image viewing axis may be substantially coincident with the primary gaze axis.

The optical centre of the eyeglass lens may be offset with respect to the primary gaze axis. The optical centre of the eyeglass lens may be offset with respect to the primary gaze axis and the vertex of the holographic optical element may be offset with respect to the optical centre of the eyeglass lens.

The optical centre of the lens may be offset with respect to the primary gaze axis by between 0 and 40 degrees and preferably between 5 and 10 degrees and the vertex of the holographic optical element (and thus the image viewing axis) may be offset with respect to the optical centre of the lens by between 0 and 40 degrees, and preferably 5 to 10 degrees.

The holographic optical element may comprise a second variation in hologram function to compensate for the optical centre of the lens being offset with respect to the primary gaze axis and to compensate for the vertex of the holographic optical element being offset with respect to the optical centre of the lens. The second variation in hologram function may be a local variation in surface grating pitch. The first and second variations in hologram function may be variations in phase gradient.

According to a second aspect there is provided a wearable head-up display, comprising: an eyeglass lens according to a first aspect; a glasses frame; and an image source, wherein the eyeglass lens is mounted in the frame, the image source is mounted on an arm portion of the frame adjacent to the eyeglass lens to project light on to the holographic optical element of the eyeglass lens to generate one or more image eyepoints.

The one or more eyepoints may be generated symmetrically about the gaze axis. Alternatively, the one or more image eyepoints may be generated asymmetrically about the gaze axis.

The angle of a chief ray from the image source with respect to the holographic optical element may be constant. The vertex of the holographic optical element may be aligned with respect to the chief ray of the image source such that the chief ray coincides with the vertex. The holographic optical element may be offset with respect to intersection of the chief ray in air with the holographic optical element plane to account for the refraction due to the eyeglass lens.

The image viewing axis may be offset from the primary gaze axis, in the horizontal plane, by an amount equal to a wrap angle of the eyeglass lens mounted in the glasses frame. The image viewing axis may be offset from the primary gaze axis, in the vertical plane, by an amount equal to a pantoscopic tilt angle of the eyeglass lens mounted in the glasses frame.

The first variation in hologram function is a variation of the phase of the hologram function to compensate for the wrap angle and pantoscopic tilt angle of the holographic optical element.

The eyeglass lens may comprise a first lens part formed on a second lens part, with an interface therebetween; and the holographic optical element interposed between the first and second lens parts at the interface.

The interface may be a curved interface, and the curved interface may be cylindrically shaped, and the eyeglass lens may have a major axis and a minor axis, and the cylindrically shaped interface may have a curved profile across the major axis and a straight profile across the minor axis. Alternatively, the curved interface may be spherically shaped. The curved interface surface may contribute to vertical and horizontal power and the holographic optical element may be configured and arranged to compensate for the vertical and horizontal power.

Arrangements according to the present disclosure address one or more of the above- mentioned issues, including but not limited to achieving an image viewing axis with god resolution and brightness for a range of users with physiologically differences without sacrificing field of view, compactness weight or prescription range. By correctly positioning the HOE with the eyeglass lens and the location of the HOE location in conjunction with the gaze direction, that is, for different proposed gaze positions, there will be a minimal change in digital image content directed to a user’s eye, which could otherwise be caused by the changes in distance of the HOE to the users pupil, IPD, wrap and tilt angles or other physiological factors related to the eyeglass and the frame design.

So that the way the features of the present disclosure can be understood in detail, a more particular description is made with reference to embodiments, some of which are illustrated in the appended figures. It is to be noted, however, that the appended figures illustrate only typical embodiments and are therefore not to be considered limiting of its scope. The figures are for facilitating an understanding of the disclosure and thus are not necessarily drawn to scale. Advantages of the subject matter claimed will become apparent to those skilled in the art upon reading this description in conjunction with the accompanying figures, in which like reference numerals have been used to designate like elements, and in which:

Figure 1a illustrates a front on view of an eyeglass lens for use with a wearable head- up display (HUD) according to embodiments;

Figure 1b illustrates a side view of the eyeglass lens of Figure 1a;

Figure 2a illustrates an eyeglass lens comprising a holographic optical element with a plurality eyepoints of a holographic optical element arranged symmetrically about a gaze direction;

Figure 2b illustrates the wrap angle of the eyeglass lens of Figure 2a relative to a user’s eye direction;

Figure 3 illustrates an eyeglass lens comprising a holographic optical element with a plurality eyepoints of a holographic optical element arranged asymmetrically about a gaze direction; Figure 4a illustrates the pantoscopic tilt of the eyeglass lens of Figures 1 a and 1 b with respect to a user’s pupil;

Figure 4b illustrates a close up of the eyeglass lens as shown in Figure 4a;

Figure 4c illustrates a side on view of the eyeglass lens of Figures 1a and 1 b, with respect to a user’s pupil;

Figure 5a illustrates a cross-section of the eyeglass lens of Figures 1a and 1b;

Figure 5b is an exploded view of area A of Figure 5a;

Figure 6a illustrates a perspective view of the eyeglass lens according to an embodiment;

Figure 6b illustrates a cross section through the eyeglass lens of Figure 6a; and

Figure 7a illustrates an exploded perspective view of the eyeglass lens components according to an embodiment;

Figure 7b illustrates a perspective view of the eyeglass lens components according to an embodiment;

Figure 8 illustrates a layer stack of front and back lens parts with a holographic optical element stack made up plurality of hologram layers therebetween; and

Figure 9 illustrates an AR system in the form of a pair of AR glasses comprising an eyeglass lens according to embodiments.

Before describing embodiments of the present disclosure it is necessary to define some common optometry terms used throughout this specification.

The term primary gaze axis/direction is defined as the user looking straight ahead at a point on the horizon at infinity. The position of a lens optical centre relative to the primary gaze direction can affect a user visual acuity and eye alignment when wearing glasses.

The term pantoscopic tilt (or simply “tilt”) defines an angle in the vertical plane with respect to the primary gaze axis/direction. More specifically it is the intersection of the primary gaze axis/direction with the plane of the lenses. It is determined by measuring the angle between the vertical axis of the eyeglass frames and the person's corneal plane, which is the imaginary plane that passes through the centre of a user’s eye. A pantoscopic tilt that is too steep can cause a user eyestrain and discomfort, while a pantoscopic tilt that is too shallow can cause the frames to slide down the nose and interfere with vision. In AR applications pantoscopic tilt may be between 0 and 20 degrees depending on the design of the frame.

The term frame wrap (or simply “wrap”) is the angle and describes the horizontal angle of the lens plane with respect to the primary gaze axis/direction. It is determined by measuring the angle between the vertical axis of the eyeglass frames and the imaginary plane that passes through the centre of the lenses. The wrap angle can affect the amount of peripheral vision that the lenses provide. A higher wrap angle can provide more coverage of the peripheral visual field, which can be beneficial for certain activities, such as sports or driving. However, a higher wrap angle can also cause the lenses to be larger and more obtrusive, which may not be desirable for all users. In AR applications the wrap angle may be between 0 and 20 degrees depending on the design of the frame.

The term interpupillary distance (IPD) is the distance measured in millimetres between the centres of the pupils in each eye. The IPD varies across the human population in the range of 42mm to 75mm.

The term eyeglass plane is defined by both the wrap and tilt of the frame design. The optical axis is normal to the eyeglass plane and passes through the optical centre of the eyeglass lens. In this context the optical centre of a lens is the point at which light rays pass through the lens without deviation. In a concave corrective lens this will typically be the thinnest portion of the lens.

The term eye relief (also known as back vertex distance or BVD) refers to the distance along the primary gaze axis between the pupil and the eyeglass plane. In the following discussion of embodiments, the eyeglass plane is the same as the plane of the holographic optical element integrated with an eyeglass lens.

The term gaze offset refers to the angular offset from the primary gaze axis in which the displayed content may be viewed. The skilled person will understand that the gaze offset may be zero, in which case it would be substantially coincident with the primary gaze direction. More generally, where an angle subtended between two axes is given as zero, the skilled person will understand that to mean that the axes coincide or points along the axes are coincident.

Figs. 1a and 1b illustrate respectively front and a top- cross-section view of an eyeglass lens 100 according to embodiments integrated with a holographic optical element (HOE) 102 suitable for use with a wearable head-up display (HUD). The eyeglass lens 100 may be any appropriate shape where the shape is largely dictated by the design and outline of the glasses frame into which the eyeglass lens 100 will be mounted (see Figure 9 for example). The appropriate shape of the eyeglass lens 100 integrated with the holographic optical element (HOE) 102 is manufactured from a preformed lens blank or “puck” as discussed in more detail below.

The eyeglass lens 100 comprises an optical centre 104 and a geometrical centre 106. The optical centre of a lens is the point where the lens materials - air interfaces are parallel so that light rays pass thought that lens without deviation this is the thinnest part of the lens for a concave lens. As illustrated the geometrical centre 106 of the lens may be coincident with the optical centre 104 of the lens. However, the geometrical centre 106 of the lens may not coincide with the optical centre 104 of the lens. Whether the geometrical centre 106 coincides with the optical centre 104 of the lens or not depends on the shape of the lens in the frame, that is how the lens is formed from the lens blank to fit within the frame (known as glazing). This is typically a design choice based on wrap and tilt angles, content direction (imaging viewing direction).

When mounted in glasses frames the optical centre 104 of the eyeglass lens may be generally coincident with the pupil of a user’s eye based on the IPD of the user. This is particularly important where the eyeglass lens has corrective optical power to correct for a user’s prescription. Therefore, depending on a specific user’s IPD and how the eyeglass lens 100 is cut from the from lens blank, the optical centre 104 may not coincide with the geometric centre 104 of the eyeglass lens 100. In the case of zero prescription lenses of AR applications a user’s IPD will determine the optimum location of the HOE.

Primary gaze direction (or axis) is defined as the axis P-P from the user’s eye through the eyeglass lens 100 where a user would look straight ahead with the head in an un-tilted direction and looking at the horizon in the distance. The HOE 102 also comprises a geometrical centre and a vertex, where the geometrical centre and the vertex may or may not be coincident. The vertex of a HOE is defined by a reference pixel corresponding the centre pixel where the virtual or digital image content can be viewed and is the point where the chief ray from an image source is incident on the HOE. The vertex may also be defined with respect to a reference pixel of the projected image about which all other pixels are calibrated. Based on the known position of he holographic optical element, the vertex is positioned so that the image viewing axis is achieved.

The vertex of the HOE 102 is substantially aligned or coincident with the gaze offset direction also known as the image viewing axis l-l. For augmented reality applications it may be desirable that the digital image content is offset from the primary gaze direction P-P so that the display of the digital content does not interrupt a user’s world view along the primary gaze direction. In this case, the vertex of the HOE 102 is offset from the primary gaze axis of the eyeglass lens 100 so that digital content is not viewed as the user looks through the eyeglass lens in the primary gaze direction. The display of digital image content, viewable along the image viewing axis l-l is offset from primary gaze direction P-P by a horizontal and/or vertical angular offset and the digital image content is thus offset from the primary gaze axis P-P by an angular offset referred to as the gaze offset 0GO as illustrated in Fig. 1b. The skilled person will appreciate that Fig. 1b illustrates the gaze offset in the horizontal direction and that the gaze offset may also be angled in the vertical direction. The gaze offset axis may be defined by angles in the horizontal and/or vertical direction with respect primary gaze direction. The gaze offset may be defined by polar and azimuthal coordinate relative to the primary gaze axis and the plane tangent to the surface of the lens normal to the primary gaze axis. The gaze offset may be defined at the maximum horizontal rotation of a user’s eye (maximum condition) with respect to the primary gaze axis (minimum condition). The gaze offset is therefore the angular offset from the primary gaze axis in which the digital content will be visible and may intersect the HOE 102 at a different point different to the primary gaze axis. Similarly, it may be desirable in certain augmented reality applications that the gaze offset direction coincides with the primary gaze direction. In this case digital content can be viewed at the primary gaze direction as a user looks through the eyeglass lens 100. The maximum limit on the gaze offset direction is thus defined by the maximum rotation of the user’s eye from the primary gaze position and based on a human eyeball of approximately 25mm can be approximately ±40 degrees horizontally (adduction or abduction) and 28 degrees in elevation and 47 degrees in depression. The skilled person will appreciate that the rotating the eye to it maximum limit for an extended period of time may be uncomfortable for some users. Therefore, the horizontal and vertical image viewing angles are preferably no greater than about 12 degrees and more preferably between about 4 degrees and 11 degrees, where the lower limit of 4 degrees ensures that the digital image content is not placed on the primary gaze direction which could act as a distraction to a user.

A plurality of eyepoints (or eyeboxes) may be arranged as in a spaced apart pattern, such as a grid-like pattern, linear array, n x m array (where n and m are positive integers). The plurality of eyepoints may be arranged symmetrically about the gaze axis l-l as illustrated in Figures 2a and 2b and discussed in more detail below (the so-called symmetric condition). Alternatively, the plurality of eyepoints may be arranged asymmetrically about the gaze axis as illustrated in Figures 3 and discussed in more detail below (the so-called asymmetric condition). Alternatively, a single eyepoint defining an entire eyebox may be arranged to coincide with or be offset from the gaze axis l-l.

As mentioned above, Figures 2a and 2b illustrate, the symmetric arrangement of the plurality of eyepoints 108 according to an embodiment. As illustrated in Figure 2a, each of the eyepoints 108 are symmetrically arranged about the vertex of the HOE 102 and the geometric centre of the HOE is substantially coincident with the optical centre 104 of the eyeglass lens 100. The geometric centre 106 of the lens is also illustrated. The plurality of eyepoints 108 may be arranged symmetrically about the optical centre 104 of the eyeglass lens 100 and because the optical centre of the eyeglass lens coincides with the gaze offset direction the plurality of eyepoints will be arranged symmetrically about this axis l-l (reference 112 of Figure 2b). Whilst Figure 2a illustrates four eyepoints 108, the skilled person will appreciate that any number of eyepoints 108 may be used, provided that they are symmetrically arranged. The skilled person will also appreciate that the eyepoints 108 may be spaced apart defining an effective replicated eyebox area in which the digital image content can be viewed by a user.

Figure 2b is a top view showing the wrap of the lens and illustrates the eyeglass lens 100 and HOE 102 of Figures 1a to 2b positioned with respect to a user’s eye. The primary gaze axis P-P indicated by axis 110 from the pupil of the eye to the vertex of the HOE 102. The gaze offset direction 112 is indicated by axis l-l. As an example, the gaze offset direction 112 can be any angle as determined by the maximum comfortable rotation of the user’s eye. A light beam 114 from an image source 116 is incident on the holographic optical element 102 at the vertex. The chief ray from image source 116 is incident on the HOE at an angle of which is dictated by the position of the image source with respect to the HOE and the maximum achievable numerical aperture of the HOE. For example, the angle of the chief ray from the image source 116 may be 56 degrees at the vertex of the HOE relative to the surface normal of the HOE. A typical wrap angle for the eyeglass lens may be between 0 and 20 degrees. Beamlets 118, 118’ are diverted by the holographic optical element 102 from the projector beam 114 through the pupil of the user’s eye on to the retina of the eye forming the eyepoints 108 which are collimated at the pupil of the user’s eye.

As illustrated in Figure 3, each of the eyepoints 108 are asymmetrically arranged about the vertex of the HOE 102 (and the axis l-l). The plurality of eyepoints 108 are therefore arranged asymmetrically about the about the gaze direction. Whilst Figure 3 illustrates four eyepoints 108, the skilled person will appreciate that any number of eyepoints 108 may be used, provided that they are asymmetrically arranged. As with the symmetric arrangement discussed above, the skilled person will also appreciate that the eyepoints 108 may be spaced apart defining an effective replicated eyebox area in which the digital image content can be viewed by a user. As an example, the gaze offset direction can be any angle dependent on the maximum comfortable rotation of the users eye. A projector beam 114 from the image source 116 is incident on the holographic optical element 102 as discussed above. In Figures 4a and 4b, which illustrate the concept of pantoscopic tilt of the eyeglass lens 100 according to embodiments, the primary gaze direction is indicated by axis 402, the gaze offset direction is indicated by axis 404 and the pantoscopic tilt is the angle Qtiit, between the primary gaze axis 402 and a normal at the centre of the pupil to a back surface of the eyeglass lens. Variation in this distance, also known as the back vertex distance (BVD) can change the field of view (FOV) or the size of the viewable digital image content. Large variations in BVD may result in image clipping, image shifting or a reduction in image brightness. Here the BVD is taken from the pupil centre to the holographic optical element centre 104. The gaze offset axis is arranged to coincide with the pantoscopic tilt. By positioning the location of the vertex of the holographic optical element 102 with respect to the gaze offset direction 112 there will be minimal change in beamlet location on the retina which is caused by the slight change in distance from the holographic optical element 102 to the pupil of the user’s eye. The BVD is inversely proportional to the FOV for a given HOE area.

In Figure 4c, which illustrates the wrap angle of the eyeglass lens 100 according to an embodiment, the primary gaze direction is indicated by axis 402, the gaze offset direction is indicated by axis 404 and the wrap is the angle Qwrap, between the primary gaze axis 402 and the wrap. As with the pantoscopic tilt angle Qtiit, the gaze offset angle in the horizontal direction may correspond to wrap angle. In this way, the optical entre of the eyeglass lens 100 is aligned with the vertex of the holographic optical element 102. The arrangement of pantoscopic tilt and wrap angle is further illustrated in Figure 4d. The optical centre of the eyeglass lens 100 coincides with wrap and tilt offset point 406. A typical tilt angle for the eyeglass lens may be between 0 and 20 degrees.

The image viewing axis is offset from the primary gaze axis in the horizontal plane by a horizontal image viewing angle and in the vertical plane by a vertical image viewing angle. The horizontal image viewing angle and the vertical image viewing angle is limited by the maximum rotation of the eye. In this case the holographic optical element may comprise a first variation in hologram function configured and arranged compensate for the horizontal and vertical image viewing angles being offset from the primary gaze axis. The first variation in hologram function is formed by a local variation in surface grating pitch when recording the hologram function. The variation in surface grating pitch may be a variation in local phase gradient. Similarly, the variation in holographic functions may be achieved by variation in fringe spacings in the bulk or volume of the holographic material of the HOE.

The optical centre 104 of the eyeglass lens 100 may be offset with respect to the primary gaze axis P-P and the vertex of the holographic optical element 102 and thus the image viewing axis is offset with respect to the optical centre 104 of the eyeglass lens 100. The optical centre of the eyeglass lens 100 is offset with respect to the primary gaze axis by between 0 and 20 degrees, and preferably between 4 and 11 degrees and the vertex of the holographic optical element is offset with respect to the optical centre of the lens by between 0 and 20 degrees and preferably between 4 and 11 degrees.

The holographic optical element 102 may comprise a second variation in hologram function to compensate for the optical centre 104 of the eyeglass lens 100 being offset with respect to the primary gaze axis and to compensate for the vertex of the holographic optical element 102 being offset with respect to the optical centre of the eyeglass lens 100. This second further variation in hologram function is formed by a local variation in surface grating pitch when recording the hologram function. The further variation is surface grating pitch may be a variation in phase by varying the local grating gradient. The further variation accounts for the shape of the world side and eye side surfaces of the eyeglass lens.

The image viewing axis is offset from the primary gaze axis, in the horizontal plane, by an amount equal to a wrap angle of the eyeglass lens mounted in a glasses frame (discussed below with respect to Figure 9). Similarly, the image viewing axis is offset from the primary gaze axis, in the vertical plane, by an amount equal to a pantoscopic tilt angle of the eyeglass lens 100 mounted in the glasses frame. The skilled person will therefore understand that the first variation in hologram function is a variation of the phase of the hologram function to compensate for the wrap angle and pantoscopic tilt angle of the holographic optical element integrated with the eyeglass lens and where the eyeglass lens 100 is mounted into the glasses frame.

Where the holographic optical element 102 is embedded within the eyeglass lens 100, as illustrated for example in Figure 5a, it is necessary to align the angle of the beam light 114 from the projector 116 in order to compensate for the refractive index and thickness of the eyeglass lens 100 material. Based on a projector beam angle of 0p_rOj, which is the angle between the projector beam and a local surface normal 120 to the point on the eyeglass lens 100 where the light beam is incident the on eyeglass lens at an eye side surface 122, and a refractive index of the eyeglass lens material of 1.5 and a lens material thickness of 1.6mm an adjustment in the position X between the notional head on position and the point where the chief ray (projector beam 114) from the projector 116 is incident on the eye side surface of the eyeglass lens. More broadly, and following the above, the skilled person will appreciate that it is not possible to align the vertex holographic optical element 102 at the centre of rotation such that the beam 114 from the projector 116 will hit the vertex. Figure 5b is an exploded view of area A from Figure 5a. The holographic optical element comprises an optical function to compensate for the curvature of the lens surface where the beam light 114 from the projector 116 is incident on the lens and also to compensate for the refractive index of the lens material. This optical function takes account of the deviation in incidence angle Qinc due to the refractive index of and thickness of the lens material. If this compensation were not included the light beam would not be incident on the vertex of the holographic optical element.

For the above-mentioned refractive index and thickness of 1.5 and 1 ,6mm respectively, by not accounting for refraction of the beam 114 from the projector 116 a displacement X, with respect to a non-refracted beam 114, of up to 1.07mm may occur.

The eyeglass lens according to embodiments may have the holographic optical element embedded within the eyeglass lens material by any appropriate process. For example, Figure 6a illustrates that the holographic optical element (not illustrated for clarity) is encapsulated in the eyeglass lens 100 by forming the eyeglass lens from two components, where the interface between those two parts is a cylindrical profile. The holographic optical element may be formed from a thin film material, such as photopolymer or silver halide. The parameters of a thin film are generally well understood, but in this context, the thin film (for example photopolymer thin films or silver halide thin films) may have a thickness of the order of 100 microns or less. It is typically a low absorption, optically clear (or transparent) low haze film, suitable for incorporation within a lens without significantly affecting see through. There is highlighted a reference plane 202 central to the eyeglass lens in the xz plane. Reference is also made to Figure 6b, in which there is illustrated a cross section through the eyeglass lens of Figure 6a at the reference plane 202. Here, a cylindrical interface profile 203 between the two component parts is shown as a dotted line.

The two component parts are typically fabricated separately but may optionally be formed from a divided eyeglass lens. A cylindrical surface on a lens or lens component is not typical. The cylindrical surface between the two lens components may be made by grinding or bespoke moulds (injection moulding). Optionally, 3D printing may be used to make the lens components. Manufacturing lens components with a cylindrical interface is not a standard technique. Nevertheless, it is possible using injection moulding (for example of plastic), grinding or 3D printing. Standard lens grinding and mould manufacture may use diamond turning, which would normally imply a spherical surface, but this technique and other techniques may be used to achieve a cylindrical surface instead.

Referring next to Figure 7a, there is depicted component parts of the eyeglass lens of Figure 6a. These comprise: a first component part (incorporating the eye-facing surface) 701 ; and a second component part (incorporating the world-facing surface) 702. The interface between the first component part 701 and the second component part 702 has a cylindrical shape (as shown in Figure 6b), which is curved (in this case, a circular segment) in the xz plane and flat in the yz plane (and also flat in the xy plane). The holographic optical element (not shown in Figure 7a) of the same size in x-dimension and y-dimension and profile of the eyeglass is laminated on the cylindrical interface between the first component part 701 and the second component part 702. When the first component part 701 and the second component part 702 are then attached to each other, the holographic optical element is entirely encapsulated within the eyeglass lens. The lamination on a cylindrical surface avoids the deformations associated with laminating on a spherical surface. The interface is preferably a pure cylinder, but can be partially spherocylindrical or toric, provided that any deviation from a pure cylindrical shape is negligible at the HOE position. The closer the interface is to a perfect cylinder, the more stresses in the thin film are minimised.

As noted above, the cylindrical interface between the first component part 701 and the second component part 702 is flat in one axis. Therefore, it exhibits in this axis the same geometric restrictions as using a flat thin film. However, since eyeglass lenses typically have a significant aspect ratio, this restriction becomes less important if it only applies across the shorter (minor) axis of the eyeglass lens, as will now be further discussed below. Attachment of the thin film to the cylindrically shaped surface of one of the lens components may be achieved in a number of ways. Lamination of the thin film may not require adhesion of the thin film to one or both surfaces of the cylindrical interface (the first component part 701 and the second component part 702 may simply be attached to each other with the thin film therebetween), but additional adhesion may be provided. The HOE may be a photopolymer, for example Bayfol (RTM) HX (as marketed by Covestro AG), which typically has a substrate of about 60 microns thickness and a polymer layer of around 20 microns thickness. Then, one side of the photopolymer is typically a tacky film that will adhere to glass or plastic once laminated onto the surface using a roller (or similar). It is alternatively or additionally possible to use glue, double sided tapes, vacuum treatment, thermal treatment or pressure treatment to adhere the HOE to the substrate. The HOE could instead be a silver halide film and the above-mentioned techniques may also be used to adhere the thin film to the substrate surface. The choice of lens material can help with adhesion, for example, polycarbonate may give better adhesion. The thin film typically does not extend quite to the edge of the cylindrical interface. This may allow for better encapsulation (no moisture ingress), if there is a good glue seal all around.

Referring to Figure 7b, there is schematically illustrated an assembled eyeglass lens 100 from the component parts of Figure 7a. Also shown is thin film 104 encapsulated between the first component part 701 and the second component part 702. Hence, the finished eyeglass lens 703 is created by the two components part mated together by the cylindrical interface with the thin film 704 between them. The eye-facing component (first component part 701) may be moulded with the desired shape for the eyeglass lens. More typically, it is moulded as a lens blank, in which the component will have the cylindrical profile on a front surface (to be bonded to the world-facing component 702), but the back surface may be planar or have some arbitrary surface curvature. The first component part 701 , second component part 702 and thin film 104 are then bonded to form the eyeglass lens 100 as a lens ‘blank’ with embedded HOE.

Once the assembled eyeglass lens 100 is formed in this way, it will then appear as a circular profile lens blank and the back surface may be milled as a standard lens blank to produce the final lens. To fit eyeglasses, the front and back curvatures are typically edged (ground down around the edges) to fit in frames of eyeglasses. This processing can be performed to the first component part 701 and the second component part 702, before encapsulation of the thin film and assembly, or after assembly. A grinding process, when the front and/or back surfaces are ground to a curvature is a more violent process than edging and if required, it is preferred that this is performed to the first component part 701 and/or the second component part 702, before encapsulation of the thin film and assembly.

Hence, the world-facing component is typically formed to comprise a front surface which is typically spherical with the desired base curvature and a back surface which has the cylindrical profile. The eye-facing component may be formed to comprise a front surface with cylindrical profile and a back surface. This back surface may be formed with the desired curvature for the final prescription. Alternatively, the back surface may be left planar (or with arbitrary curvature) to create a lens blank, whereby further milling of this surface will be used to set the prescription.

The holographic optical element may be configured to act as a plane mirror. In this case, the holographic optical element may add no optical power itself. The total optical power of the holographic optical element may then be determined by the cylindrical curvature of the inner surface onto which the HOE is laminated. The eyeglass lens exhibits optical power on the real world as determined by the inner and outer surface curvatures of the eyeglass lens only. Alternatively, the eyeglass lens may be configured such that the total optical power of the holographic optical element is determined by a sum of an optical power of the hologram of the holographic optical element and an optical power due to a curvature of the cylindrically shaped interface. Then, the HOE may act as a reflective powered optical element (as is typical for AR applications). The holographic optical element may be a photosensitive material and a hologram may be recorded on the photosensitive material to form the holographic optical element. In certain cases, recording a hologram on the photosensitive material may comprise: providing the photosensitive material on a planar substrate; and recording the hologram on the photosensitive material when on the planar substrate, the hologram being recorded so as to compensate for an optical power of the cylindrically-shaped interface. This may thereby provide the holographic optical element, which may then be applied to the cylindrically shaped interface. In other cases, recording a hologram on the photosensitive material may comprise: applying the photosensitive material to the cylindrically-shaped interface; and recording the hologram on the photosensitive material when on the cylindrically-shaped interface. In other cases the projector or image source may be compensated rather than the hologram.

In terms of compensating for the optical power, the back surface part, or in other words the eye facing surface, of the eyeglass lens is the most critical to the holographic optical element function. The effective curvature of the back surface part of the eyeglass lens is made up of optical power components in both the horizontal and vertical planes where the power is different in these planes. The total optical power of the back lens part will be equal to the sum of the optical power due to the spherical curvature of the eye facing side, the optical power of the cylindrical curvature of the eye facing part of the eyeglass lens and the back centre thickness. To avoid astigmatisms due to the eyepoints being too far or too close to the holographic optical element it is necessary to record the holographic functions on the holographic optical element to compensate the total optical power. For example, the optical power due to the spherical curvature of the eye facing side and the cylindrical power of the world-facing side, may contribute -4D in the vertical and +8.9D in the horizontal.

The eyeglass lens according to embodiments may include a holographic optical element 102 comprising a stack of hologram layers HOL1 , HOL2 between two lens parts as illustrated in Figure 8. The stack of hologram layers HOL1 , HOL2 includes a plurality of hologram layers. Whilst Figure 8 illustrates two hologram layers HOL1 , HOL2 the skilled person will appreciate that any number of holographic layers HOL1 , HOL2 may be included depending on the specific applications. For example, one or more of the hologram layers may be a visible wavelength hologram. One or more of the hologram layers may be an Infrared (IR) wavelength hologram. IR holograms may be used where it is necessary to implement eye-tracking in the AR system.

Where there are plurality of hologram layers making up the holographic optical element it will be necessary to align the holograms with respect to each other, and also with respect to the lens. This process is known as registration. To achieve this the skilled person will understand that a system of registration marks or fiducials may be used to align adjacent hologram layers with respect to each other and also align the stack of hologram layers with the eyeglass lens parts in accordance with embodiments described above.

As discussed above, the eyeglass lens 100 may be any appropriate shape, where the shape is largely dictated by the design and outline of the glasses frame into which the eyeglass lens 100 will be mounted. The required shape of the eyeglass lens 100 integrated with the HOE 102 may be cut or milled from a lens blank or “puck”. Alternatively, the eyeglass lens or parts of the eyeglass lens may be injection moulded and/or overcast.

Figure 9 illustrates an augmented reality system comprising at least one of the eyeglass lenses described above. This augmented reality system takes the form of a wearable heads-up display, such as for example, a pair of glasses 900. As with known types of glasses, the glasses 900 according to an embodiment include a frame 902. The frame includes arms 904, and lens mounting portions 906 connected by a bridge portion 908. One of the arms 904 includes a mounting portion 910 in which the projector (as mentioned above) is fixedly mounted to such that the beam 114 from the projector 116 (not illustrated in Figure 9) will hit the eyeglass lens 100 according to embodiments as discussed above. The skilled person will appreciate that the projector 116 will be mounted on the arm 904 adjacent the lens mounting portion 906 holding the eyeglass lens 100 according to embodiments. The other lens mounting portion may have a standard ophthalmic lens inserted therein. Alternatively, an eyeglass lens according to embodiments may be mounted in the other lens mounting portion and there may be an additional projector system 116 mounted on a corresponding mounting portion on arm 904.

One or both of the arms 904 may also be adapted to house a battery (not illustrated) to power the projector as discussed above. In addition, one or both of the arms 904 may also include control electronics (not illustrated) for controlling the operation of the projector 116. The projector may be a MEMS based projector system or a pixelated projection system such as LOCOS, microLED or OLED based system. In addition, the operation of the projector may be controlled via the control electronics, by an eye tracking system (not illustrated). The eye tracking system may include a non-visible light source, such as an infra-red LED, directed to the user’s eye and a photo sensor arranged to capture the non-visible light reflected by the user eye. Embodiments as described herein apply equally to waveguide or free-space based AR systems.

Particular and preferred aspects of the disclosure are set out in the accompanying independent claims. Combinations of features from the dependent and/or independent claims may be combined as appropriate and not merely as set out in the claims.

The scope of the present disclosure includes any novel feature or combination of features disclosed therein either explicitly or implicitly or any generalisation thereof irrespective of whether or not it relates to the claimed disclosure or mitigate against any or all of the problems addressed by the present disclosure. The applicant hereby gives notice that new claims may be formulated to such features during prosecution of this application or of any such further application derived therefrom. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in specific combinations enumerated in the claims.

Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination.

Claims

1. An eyeglass lens for a wearable head-up display, the eyeglass lens comprising: an optical centre, a primary gaze axis, and a holographic optical element having a vertex, wherein the holographic optical element is integrated with the eyeglass lens such that said vertex is coincident with an image viewing axis to allow digital content to be viewed, and wherein the optical centre of the lens is positioned with respect to the primary gaze axis and the vertex is positioned with respect to the optical centre.

2. The eyeglass lens of claim 1 , wherein the image viewing axis is offset from the primary gaze axis.

3. The eyeglass lens of claim 2, wherein the image viewing axis is offset from the primary gaze axis in the horizontal plane by a horizontal image viewing angle and in the vertical plane by a vertical image viewing angle.

4. The eyeglass lens of claim 3 wherein the horizontal image viewing angle is between zero degrees and 40 degrees in adduction or abduction and preferably 4 and 12 degrees; and the vertical image viewing angle is between zero and 28 degrees in elevation and 47 degrees in depression and preferably between 4 and 12 degrees.

5. The eyeglass lens of claim 1 , wherein the holographic optical element comprises a first variation in hologram function configured and arranged to compensate for the horizontal and vertical image viewing angles being offset from the primary gaze axis.

6. The eyeglass lens of claim 5, wherein the first variation in hologram function is a local variation in surface grating pitch and/or fringe spacing within a holographic material of the holographic optical element.

7. The eyeglass lens of claim 1 , wherein the image viewing axis substantially coincident with the primary gaze axis.

8. The eyeglass lens of claim 1 wherein the optical centre of the lens is offset with respect to the primary gaze axis.

9. The eyeglass lens of claim 1 , wherein the optical centre of the eyeglass lens is offset with respect to the primary gaze axis and the vertex of the holographic optical element is offset with respect to the optical centre of the eyeglass lens.

10. The eyeglass lens of any preceding claim, wherein the optical centre of the lens is offset with respect to the primary gaze axis by between 0 and 20 degrees, and preferably between 5 to 10 degrees and the vertex of the holographic optical element is offset with respect to the optical centre of the lens by between 0 and 20 degrees, and preferably 5 to 10 degrees.

11. The eyeglass lens of any preceding claim, wherein the holographic optical element comprises a second variation in hologram function to compensate for the optical centre of the lens being offset with respect to the primary gaze axis and to compensate for the vertex of the holographic optical element being offset with respect to the optical centre of the lens.

12. The eyeglass lens of claim 11 , wherein the second variation in hologram function is a local variation in surface grating pitch.

13. The eyeglass lens of claims 6 and 11 wherein the first and second variations in hologram function are variations in phase gradient.

14. A wearable head-up display, comprising: an eyeglass lens according to claims 1 to 13; a glasses frame; and an image source, wherein the eyeglass lens is mounted in the frame, the image source is mounted on an arm portion of the frame adjacent to the eyeglass lens to project light on to the holographic optical element of the eyeglass lens to generate one or more image eyepoints.

15. The wearable head up display of claim 14, wherein the one or more eyepoints are generated symmetrically about the gaze axis.

16. The wearable head up display of claim 14, wherein the one or more image eyepoints are generated asymmetrically about the gaze axis.

17. The wearable head up display of claims 14 to 16, wherein the angle of a chief ray from the image source with respect to the holographic optical element is constant.

18. The wearable head up display of claims 14 to 17, wherein the vertex of the holographic optical element may be aligned with respect to the chief ray of the image source such that the chief ray coincides with the vertex.

19. The wearable head up display of claims 14 to 18, wherein the holographic optical element may be offset with respect to intersection of the chief ray in air with the holographic optical element plane to account for the refraction due to the eyeglass lens.

20. The wearable head up display of claims 14 to 19, wherein the image viewing axis is offset from the primary gaze axis, in the horizontal plane, by an amount equal to a wrap angle of the eyeglass lens mounted in the glasses frame.

21. The wearable head up display of claims 14 to 20, wherein the image viewing axis is offset from the primary gaze axis, in the vertical plane, by an amount equal to a pantoscopic tilt angle of the eyeglass lens mounted in the glasses frame.

22. The wearable head up display of claims 14 to 21 , wherein the first variation in hologram function is a variation of the phase of the hologram function to compensate for the wrap angle and pantoscopic tilt angle of the holographic optical element.

23. The eyeglass lens of claims 1 to 13 wherein the eyeglass lens comprises a first lens part formed on a second lens part, with an interface therebetween; and the holographic optical element interposed between the first and second lens parts at the interface.

24. The eyeglass lens of claim 23, wherein the interface is a curved interface, and the curved interface is cylindrically shaped, and the eyeglass lens has a major axis and a minor axis, the cylindrically shaped interface having a curved profile across the major axis and a straight profile across the minor axis.

25. The eyeglass lens of claim 24, wherein the curved interface surface contributes vertical and horizontal power to the eyeglass lens and wherein the holographic optical element is configured and arranged to compensate for the vertical and horizontal power of the lens.