Optical display system

Abstract

The present disclosure relates to an off-axis optical display system for a head mounted display, the optical system comprising: an optical combiner defining a first optical axis for directing image rays to a pupil plane; and an aberration compensating element defining a second optical axis independent from the first optical axis; wherein the optical combiner is arranged to generate an intermediate focal plane on the second optical axis which is an optical conjugate of the pupil plane and wherein the aberration compensating element is located substantially at the intermediate focal plane.

Description

[0001] OPTICAL DISPLAY SYSTEM

[0002] FIELD OF THE DISCLOSURE

[0003] The present disclosure relates to an optical display system. In particular, the disclosure relates to an off-axis optical display system and more particularly a head mounted augmented reality display system.

[0004] BACKGROUND OF THE DISCLOSURE

[0005] In the field of optical display system design, so-called off-axis optical systems are systems in which at least two independent optical axes do not coincide. Off-axis optical systems relay an image to an image plane. Applications of off-axis optical systems include, but are not limited to, head mounted displays (HMDs), head up displays (HUDs), camera devices and more generally optical projection and display systems. Specific examples can include AR, VR or MR display systems where a user’s eye, and more specifically the retina of a user’s eye, is located at the image plane.

[0006] Figure 1 is a generalised view of an augmented reality display to superimpose an image from a projector or image source onto a real-world view at for example the eye of a user. The off- axis optical system 100 of Figure 1 generally comprises: a light source or image source 110 with associated beam shaping optics 120; imaging optics 130; and an optical combiner 140. Broadly speaking the combination of the light source 110, beam shaping optics 120 and the imaging optics 130 are termed a projector and when in use provide the image to be viewed at the eye of the user. The function of the optical combiner 140 is to direct light from the projector to a user’s eye while also allowing ambient external environmental light from a real-world view to pass through the optical combiner 140 to the user’s eye such that images from the projector are superimposed on the real-world view. An example of an optical combiner 140 is a holographic optical element (HOE). The HOE directs light from the light source 110 and to an image plane of the optical system, which would then be visible by a user’s eye. HOEs, or more generally, diffractive optical combiners are examples of free space optical combiners, so called because they do not rely on waveguides.

[0007] An optical combiner 140 has at least two independent optical axes 150, 160 as shown in Figure 1 , where the first optical axis 150 is that from the projector to the optical combiner 140 and the second optical axis 160 is that from the optical combiner 140 to the user’s eye / pupil plane. Such a system is therefore known as an off-axis optical system. When used in applications such as head-mounted display systems or augmented reality display systems, optical combiner 140 of the type described above are known to advantageously provide high transparency, good efficiency, that is, bright images at low light power and be compatible with ophthalmic glasses lens prescriptions and encapsulation into such lenses. However, such advantages need to be balanced against increased aberrations inherent with off-axis systems. The main difficulty associated with using free-space combiners in an optical system is that such combiners change the optical axis of the system, as shown in Figure 1 where the independent optical axis 150 of the projector is independent to the optical axis 160 of the combiner 140, the problems with which are discussed in more detail below. It should be noted that whilst light travels from the light source or image source 110 via the optical combiner 140 to a user’s eye, optical designers also consider systems in reverse, where notional light rays travel from the user’s eye via the optical combiner 140 to the light source or image source 110.

[0008] A challenge for the optical design of off-axis display devices comprising such optical combiners is that the key specifications of eye box, form factor, and field of view, are related and constrained by a quantity known as etendue. Another challenge is the issue of aberration control. Figure 2 shows the basic components of a head mounted display where the eyebox is the area on the surface at the desired eye position (or pupil plane) within which the pupil of the user can move while still seeing the full image from the display. The form factor of such a head mounted display is the three-dimensional volume that encloses the projector and rays in the head-mounted display device, and the field of view is the angular spread of rays at the centre of the eyebox carrying the image. Figure 3 depicts a simplified ray diagram showing that the eye relief is the distance from the combiner to the eye. Also shown is the field of view and eyebox for light rays from an optical combiner. The etendue of an optical system is related to the conservation of information bandwidth through the optical system which characterises how spread out the light is in area and angle and very generally is equal to the product of the eye box size and field of view. For head mounted displays, etendue restricts the optical design in two significant ways.

[0009] The first restriction imposed by the etendue of an optical system is that there is an inherent trade-off between the eyebox dimensions versus field of view. It is not possible to increase the eyebox size nor the field of view indefinitely It has generally been acknowledged by those skilled in the art that eyebox size is the more crucial design criteria for head mounted displays. Insufficient eyebox size, below approximately 10 mm2, provides a major hurdle for the supply chain of consumer-focused head mounted displays, because such displays would need to be custom-fitted to account for the spread in Inter-Pupillary Distance (IPD) in a population of such consumers.

[0010] The second restriction applies to the form factor of the components, as shown for example in Figure 2, used in head mounted display. However, this restriction is valid for any optical system where space to house the necessary components, including control electronics, is limited. As mentioned above, etendue is conserved as light rays propagate through the system. Therefore, each intermediate surface fully accepts light only if the product of the diameter of the surface and angular divergence is equal to or exceeds the etendue. In other words, the size of lenses must increase proportionally with the increase in eye box size. Clearly this is undesirable for head mounted display applications where lenses must be contained within a form factor suitable for mounting onto glasses frames which have a highly asymmetric form factor.

[0011] For head mounted display application several strategies have been attempted to overcome the limits of increasing etendue without negatively affecting form factor, namely the use of multiplexed free-space optical combiners, steerable imaging optics and / or beam splitting optics before the optical combiner. However, these strategies, or any combination of them result in reduced image resolution. Furthermore, waveguide based optical combiner solutions exist but come at the expense of increasing power consumption to maintain the displayed image brightness and cost.

[0012] It is difficult to achieve a high resolution and small form factor display if the etendue is large and techniques to circumvent etendue in a realistic imaging system for head-mounted display devices have proven to be challenging and many of their limitations have not, until now, been overcome.

[0013] In addition, for off-axis optical systems of the type described above management and reduction of aberrations is an important design consideration to ensure that the images from the light source have good quality at the pupil plane and therefore, at the user. It is known that optical aberrations depend on the image height h and the pupil coordinates. Aberrations are normally dependent on the field (that is image height h) namely, coma, astigmatism field curvature and distortion and pupil coordinates are referred to as field-dependent aberrations. The remaining aberrations, namely, spherical aberrations, defocus are known as field independent aberrations and depend on the pupil coordinates only. Field-dependent aberrations will be different for every image pixel, with a centre at height (hx, hy) each approaching the eye at a different angle. Pupil dependent aberrations are the same for all (virtual) pixels and change depending on where the eye is positioned in the pupil plane as shown in Figure 1. In the context of optical systems the term pixel can be used in it broadest sense, that is relating to either a physical pixel on the on a pixelated image source, such as for example Liquid Crystal on Silicon (LCoS), spatial light modulator (SLM), Liquid Crystal Display (LCD) or Digital Light Processing(DLP) or a virtual pixel related to the particular position of a scanning display (such as MEMS mirror display). SUMMARY OF THE DISCLOSURE

[0014] It is therefore and object of the embodiments disclosed herein to avoid or mitigate one or more of the disadvantages discussed above.

[0015] Against this background, there is provided off axis optical display system in accordance with the claims. Other preferred and optional features are defined in the other claims and discussed throughout this disclosure.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS

[0017] So that 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. It should be noted that the features as illustrated in the figures have been exaggerated for illustration purposes and no dimensions (unless stated in the text or drawings) should be inferred. Advantages of the embodiments 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:

[0018] Figure 1 shows a generalised arrangement of a known off-axis optical system comprising a projector and an optical combiner; Figure 2 shows a known off axis optical projection system of the type used in a head mounted display;

[0019] Figure 3 shows the concept of eye-relief in relation to optical combiners;

[0020] Figure 4a illustrates an off-axis optical system comprising imaging optics and an optical combiner according to embodiments;

[0021] Figure 4b illustrates the intermediate evaluation plane D-D of Figure 4a in more detail;

[0022] Figure 4c illustrates the off-axis optical system of Figure 4a from a side on view;

[0023] Figures 5a to 5f are transverse fan ray plots illustrating ray aberrations at the intermediate evaluation plane D-D between the combiner and projector;

[0024] Figure 6a illustrates the concept of asymmetric eyebox achieved by the combiner according to an embodiment; and

[0025] Figure 6b illustrates the concept of a symmetric eyebox for the purposes of comparison with the asymmetric eyebox of Figure 6a.

[0026] DETAILED DESCRIPTION

[0027] With reference to Figure 4a the off-axis optical system 400, according to embodiments, generally comprises projection optics 401 and an optical combiner 402. The off-axis optical system 400 as illustrated in Figure 4a is viewed in the plane perpendicular to the plane containing the optical axes A-A and B-B of the system. Optical axis A-A is the optical axis of the projection optics 401 up to the optical combiner 402, and optical axis B-B is the optical axis from the optical combiner 402 to a pupil plane of the optical system. Axes A-A and BB are not coincident and thus the optical system 400 is off-axis. The projector 401 also includes relay optics 404 for generating an intermediate pupil plane C-C and relaying the intermediate pupil plane to C-C to the exit pupil plane (at the user’s eye). The compensating element 406, which provides aberration control, is located at or around the intermediate pupil plane C-C to correct for pupil-dependent aberrations. Focusing optics 408 and tilted diffuser 410 are included for clarity and are not a fundamental part of the optical system. The focusing optics 408 may be used to couple light from a light source (not illustrated) to the intermediate pupil plane C-C when the light source illuminates the tilted diffuser. This arrangement may be used where the light source is used to illuminate a spatial light modulator (SLM), such as an LCoS. The focusing optics 408 may also be used to correct for image aberrations.

[0028] In the example of Figure 4a, the projection optics 401 comprises a compensating element 406, which can be any suitable optical element such as a mirror or lens with a cubic phase function. In the present example the compensating element 406 is a lens. The cubic phase function of the compensating element 406 may be implemented in practice as an off centred cut away section of an odd or even asphere. The compensating element 406 is located at the plane C- C, which is the conjugate of the pupil plane (or eyebox) generated by the optical combiner 402. At the plane C-C in the optical system 400, field independent aberrations merge or overlap due to a parabolic phase function of the optical combiner 402. In other words, plane C-C is the location in the optical system 400 where rays from different fields but from the same pupil coordinates intersect. Due to the intersection of the field independent aberrations at the plane C-C the compensating element 406 can be used to compensate for all field dependent aberrations. The optical combiner 402 is optically coupled to the projection optics 401 along optical axis A-A and is configured and arranged to produce only field independent aberrations, no aberrations at the pupil plane (or eyebox) and only filed independent aberrations at the pupil plane conjugate plane C-C. The projector 401 may comprise an image source 410 for providing images to be displayed at the pupil plane of the optical system 400. In the case a head mounted augmented reality display, the pupil plane may coincide with a user’s eye. For convenience and ease of explanation the image source 410 is illustrated in Figure 4a as a tilted diffuser. However, the skilled person will appreciate that the image source 410 may be any suitable source of images such as a laser (or LED) beam scanner (such as a MEMS based scanner) or a flat panel or curved display (such an LCoS, MicroLED or OLED display). The image source 410 may also be a diffuser or a holographic diffuser illuminated by a laser MEMS scanner. The skilled person will also appreciate that the image source 410 may be provided outside the projector 401. Focusing optics 408 may be included as part of the image source 410. Alternatively, the focusing optics 408 may be located externally from the image source 410 or the projector 401 , or the focusing optics 408 may be included in the projector 401. Regardless of the location of the focusing optics 408, it may be arranged on the optical axis A-A to couple light from the image source 410 to the optical combiner 402 and provide an intermediate image of the image source 410 at the plane D-D before the optical combiner 402. Plane D-D is an intermediate plane for evaluating aberrations in the optical system 400 according to embodiments. In other words, image source 410 needs to be relayed to plane D-D. The skilled person will use focusing optics between image source 410 and plane D-D. Plane C-C will result before, after or between the elements making up the focusing optics 408. Figure 4b illustrates the evaluation plane D-D in closer detail. Rays along axis B-B have been omitted for clarity. It can be seen that field independent aberrations for rays of a pixel (Pixel 1 , Pixel 2, Pixel 3) along axis A-A are substantial, in this case coma, but that for each pixel the aberrations are translationally symmetric along the evaluation plane D-D. In other words, whilst the aberrations are substantial, they are the same for each pixel. This is can also be seen from a comparison of Figures 5a to 5f.

[0029] The relay optics 404 may be a pair of conic imaging lenses for generating the intermediate pupil image in the horizontal plane C-C. The compensating element 406 may be a displaced element in the horizontal plane arranged at the intermediate pupil plane C-C. The relay optics 404 and the compensating element 406 may be formed of any appropriate material such as lightweight PMMA or high-index materials such as S-LAH.

[0030] Importantly, because the optical combiner 402 achieves field-independent aberrations (that is field dependent aberrations are removed by the parabolic phase function of the combiner as mentioned above), it remains therefore to correct for the pupil-dependent aberrations (field independent aberrations). This correction is achieved by putting an optical element, in this case the compensating element 406 at the plane C-C, that is the intermediate image pupil plane where chief rays of the optical system 400 are focus. This design choice goes against known conventional optical design practices, where optical elements should not be positioned near any intermediate pupils, because doing so would re-introduce pupil independent aberrations that have previously been corrected for. However, for the optical system 400 according to embodiments, the pupil dependent aberrations have been removed by the optical combiner 402. The intermediate image of the pupil plane C-C is the conjugate of the pupil plane (or eyebox) generated by the optical combiner 402 because at this location in the optical system due to the parabolic phase function of the optical combiner 402 field independent aberrations merge or overlap. In other words, plane C-C is the location in the optical system 400 where rays from different fields but from the same pupil coordinates intersect. Therefore, because the field independent aberrations merge or overlap at the plane C-C one or more elements, in this example the compensating element 406, can be used to compensate for field independent aberrations.

[0031] As an alternative to the odd or even asphere, the compensating element 406 may be a cylindrically symmetric freeform lens with a substantial fourth-order contribution to sag. For example, coma (which is a field dependent aberration) is compensated by displacing the compensating element 406 relative to the local optical axis of the system (in this case axis A- A), so the effective sag profile has a substantial third-order contribution. Note that any alternative optical equivalent, for example, a freeform lens or lenses, a freeform mirror or mirrors, a diffractive element diffractive or elements and so on, could have been used to perform the aberration correction.

[0032] The optical combiner 402 according to embodiments is arranged to allow external or environmental light to pass through the to the user’s eye, while also redirecting image light rays to the user’s eye such that the external light and light ray are both are visible to a user. The optical combiner 402 may include at least one diffractive element such as holographic optical element, volume diffraction grating, surface relief diffraction grating or a reflection grating. The optical combiner 402 may be provided on or in a transparent substrate such as an ophthalmic lens. The optical combiner 402 may have a known physical shape, that is a flat profile or with a certain sag (curve) profile. In this context the sag profile may be either the convex or concave curvature of a surface and represents the physical distance between the vertex (highest or lowest point) point along the curve and the centre point of a line drawn perpendicular to the curve from one edge of the optical combiner 402 to the other. The physical shape of the optical combiner 402 may change the effective path length of light rays that cross it or are reflected by an amount ( / >(X, Y) at each point (X, Y) on the optical combiner 402. The change in effective path length may be achieved in various ways, namely through reflection, refraction, or diffraction. The optical combiner may be applied to a surface of a lens or encapsulated in a lens. Alternatively, the optical combiner may be applied to the surface of a waveguide, such as a curved or planar waveguide.

[0033] The horizontal and vertical coordinates (X, Y) of the optical combiner 402 are defined by those of the plane tangential to the surface at the vertex (0,0) of the optical combiner 402. Any reference to a point (X, Y) on the combiner will lay on the physical surface of the combiner. Embodiments presented herein relate to the situation where the optical combiner 402 is flat and diffractive, and the change in effective path length has contributions from the phase function (X, Y) on the surface. However, based on the following discussion the skilled person will understand that the surface of the optical combiner 402 may be adapted to have a curved surface as required to integrate the optical combiner 402 into for example a curve helmet visor or a curved prescription glasses lens.

[0034] As mentioned the optical combiner 402 is configured and arranged to have substantially only field-independent aberrations immediately after rays in the off-axis optical system 400 are redirected by the combiner the conjugate plane C-C. Due to optical symmetry and because plane C-C is a conjugate of the pupil plane, there will be no field dependent aberrations at the either the plane C-C or the pupil plane and because the field dependent aberrations have been removed only field-independent aberrations will remain. As a result, this means that field dependent aberrations have been removed by the optical combiner 402 and so the aberration compensating element 406 is only required to manage field-independent aberrations. The optical combiner 402 results in field independent aberrations only due to an optimised phase function <t>(X, Y)tot. The substantially field independent nature of the aberrations due to the optimised phase function of the optical combiner 402 is demonstrated by the ray aberration diagrams of Figures 5a to 5f. Figures 5a to 5f show that the optical combiner 402 achieves identical aberrations for different fields. Whilst the aberrations are substantial and dominated by coma (field-independent aberrations), the aberrations are identical and can thus be corrected for by the compensating element 406.

[0035] The optimised phase function comprises a tilt component (X, y)ti|t(applying a tilt to all fields and in this context fields are rays or pixels from the projector) applying the same tilt to all fields and a parabolic or substantially parabolic phase function (X, y)paraboiic used to focus the fields in the same way, independent of their incidence angle. The total optimised phase function of the combiner is thus given by:

[0036] <J (X, y)tot= O (X, y)tiit + O (X, r)paraboiic (Eqn. 1 ) This phase function may be implemented a binary polynomial surface: O(X,y) = Eijiy)(Eqn. 2) with X and Y appropriately normalized coordinates.

[0037] The tilt (linear) and parabolic (quadratic) contribution respectively correspond to the coefficients01, X20, and X02. It is convenient to optimise the phase function so that chief rays are approximately parallel or telecentric to one another. This ensures the form factor of the imaging optics is bounded. When the chief rays are approximately parallel or telecentric, the parabolic phase function is: where is the wave vector k and parameter Zo is the eye relief distance.

[0038] The tilt component is given by:

[0039] <P(X,Y)tilt= aX + bY (Eqn. 4)

[0040] The eye relief distance is dictated by the parabolic (quadratic) component and in this way the skilled person will understand that the eye relief may be controlled by appropriate selection of the parabolic component. Similarly, the angle of incidence of image light from the projector 401 is dictated by the tilt component and in this way the skilled person will understand that projector incidence angle may be chosen by appropriate selection of the tilt component.

[0041] It is a realization of the optical system 400 according to embodiments that pupil-dependent aberrations can be corrected for by placing an optical element in the form compensating element 406, or a combination of such optical elements, substantially close to the intermediate image of the pupil (plane C-C) of Figure 4a. Whilst the foregoing examples illustrate compensating element as a de-centred even asphere or freeform lens, the skilled person will appreciate that diffractive elements, and / or a combination of lenses may be used provided that they meet the requirement to correct for pupil dependent aberrations. The skilled person will also appreciate that the required corrections are different dependent on the pupil coordinates Px, and Pybecause of symmetry, that is the optical axes of the system coincide with the yz-plane so that aberrations are very mild in terms of Px, i.e., mainly defocus and spherical aberrations, but considerable in terms of Py, dominated by coma. The embodiment identifies the following corrections, analyzed through optimized binary phase functions at the intermediate plane C-C: where contributions are largest forPyand lowest-order terms in <PxpY(X, Y). Correcting initially for coma by implementing the correctionsPypushes the order of magnitude of off- axis induced aberrations down to similar levels as those forPX. A possible example implementation to make such corrections is a cylindrically symmetric even asphere, displaced along the Y-axis to obtain odd contributions in Y.

[0042] Whilst the above design describes the use appropriate focusing power to position the intermediate pupil plane C-C, the skilled person will appreciate that the position is not limited to the illustrated location and may, depending on the focusing power of the focusing be positioned at any convenient location. Furthermore, the skilled person will appreciate that there are many equivalent options to create the intermediate pupil plane C-C without departing from the inventive concepts presented here. These options may include for example, powerdependent contribution to the phase function of the optical combiner, using combinations of lenses that are less prone to introduction aberrations, using either concave or convex lenses, and so on. The use of an asymmetric eyebox relaxes the requirements on optical elements of the system, such as the focusing optics 408, compensating element 406, additional focusing optics 408. The optical combiner 402 implements field dependent aberrations by inducing a tilt <Y>(Y)tiitand substantial parabolic phase contribution O( , Y)paraboiic. It is important to note that the parabolic phase contribution may be replaced by any contribution that achieves field independent aberrations. In addition to the tilt and phase functions the combiner may also impose an astigmatic defocus coefficient C’(X)astig= AastgX2(Eqn. 8) solely dependent on X to implement the asymmetric eyebox. The phase function required to implement the asymmetric eye box can therefore be given by:

[0043] The skilled person will appreciate that the astigmatic defocus coefficient is required to implement the asymmetric eyebox and the tilt and parabolic functions are optional and included here for completeness, where aberration control is required in off-axis optical systems such as for example head mounted displays using optical combiners.

[0044] The astigmatic defocus coefficient is chosen so that the intermediate pupil plane image separates into two locations. The first location corresponds to the intermediate image of the horizontal pupil plane in Figure 4a, at intermediate pupil plane C-C corresponding to the location of the compensating element 406. The second location is located at the intermediate image of the vertical pupil plane and is moved further away or closer to the combiner depending on the value of the astigmatic coefficient. Optionally, additional optics may be placed at the intermediate image of the vertical pupil plane where there is a design requirement to flatten rays in that direction thus maintaining an asymmetric footprint throughout the imaging optics. Moreover, optionally, the intermediate image of the vertical pupil plane can be positioned at a nodal plane of the relay optics 404 so that the lenses of the system can be appropriately shaped or cut down to fit for example within the form factor of the asymmetrically profiled smart glasses frame arm (see Figure 7). In addition, by controlling the tilt component it is possible to mount the projector on the smart glasses frame arm at any appropriate angle with respect to the optical combiner. For example, the optical axis A-A may be an angle of between 10 and 80 degrees with respect to a surface normal of the combiner. This allows the optical designer more design freedom to incorporate the within the form factor of the asymmetrically profiled smart glasses frame arm without any loss of image quality. Figure 4c illustrates the optical system of Figure 4a from a perspective view and shows that the optical elements: relay optics 404; compensating element 406; and the focusing optics 408 can be cut or truncated in order to fit within the form factor of the arm of a pair of glasses. Specifically, one or more edges of the optical elements can be advantageously trimmed (when compared to standard optical elements) to accommodate the footprint of the passing light rays. In addition, with the geometry of the optical elements being smaller in one dimension, it becomes advantageously possible to fold the optical path using fold mirrors as known in optical design (such as periscopes). The fold mirrors may be located in the optical path between the optical elements. This allows the projector 401 to be folded so that it can conform to the profiled smart glasses frame arm and thus human head temples.

[0045] As a result of the asymmetric eyebox, pupil dependent aberrations are reduced in the vertical direction. As a result of the reduced pupil dependent aberrations along the vertical pupil plane the skilled person will therefore appreciate that aberration need only be corrected along the horizontal pupil plane. This has the advantage that the optical design for the optical system as a whole can be simplified because the control aberrations along the horizontal pupil plane can be achieved by the compensating element 406, example a lens with a cylindrically symmetric freeform lens or a de-centred asphere 406 in Figures 4a. The cylindrically symmetric freeform lens or de-centred asphere should have a fourth order contribution to sag. In particular, coma is corrected for by displacing this lens relative to the optical axis A-A of the system so that the effective sag profile has a third order contribution. The skilled person will appreciate that any optical alternative to the cylindrically symmetric freeform lens de-centred even asphere may be used, for example a freeform lens or lenses, a freeform mirror or mirrors, diffractive element or elements or any combination thereof.

[0046] With reference to Figures 6a and 6b the concept of asymmetric and symmetric eyeboxes will be described. Figure 6a illustrates a symmetric eyebox 600 with a representation of a pupil plane 602 of a user’s eye. The eyebox 600 is symmetric in that the vertical dimension 604 of the eyebox is equal in length to the horizontal dimension 606 of the eyebox. The horizontal 606 and vertical 604 dimensions of the eyebox are greater than those of the pupil plane 602. Figure 6b illustrates an asymmetric eyebox 610 with a representation of a pupil plane 602 of a user’s eye. For comparison purposes the pupil plane is identical to that of Figure 6a. The eyebox 610 is asymmetric in that the vertical dimension 604 of the eyebox is shorter in length to the horizontal dimension 606 of the eyebox. In embodiments the vertical extent of the asymmetric eyebox 610 may be greater than or equal to the vertical extent of the pupil plane 602. Alternatively, the vertical extent of the pupil plane 602 may be less than the height of the pupil plane as illustrated in Figure 6b. If the eyebox is very asymmetric and the vertical dimension 604 of the eyebox is less than the vertical dimension of the pupil plane, then only a part of the eye pupil is needed to capture the full image. Therefore, because the asymmetric eyebox is smaller than the eye pupil in the vertical dimension, aberrations are reduced in the vertical direction. The remaining aberrations in the horizontal direction can be targeted fully by degrees of freedom in the optical design as discussed above. In other words, because of the highly asymmetric eye box, pupil-dependent aberrations are highly reduced along the vertical pupil coordinate and only need to be corrected in the horizontal plane, i.e. ‘J’p -

[0047] The concept of the asymmetric eyebox as described above applies to eyeboxes with modest asymmetry, that is where both dimensions are larger than the dimensions of the eye pupil, and eyeboxes with high asymmetry, that is where one of the eyebox dimensions is smaller than the pupil of the eye. Following the embodiment of Figure 4a, the following discussion details how the optical design benefits from an asymmetric eyebox with high aspect ratio, so that the eyebox is smaller than the eye pupil (as discussed with reference to Figures 6a and 6b. The plane containing both optical axes of the system will be referred to as the horizontal plane (H), that is the YZ plane in Figure 4a, and the plane perpendicular to that plane will be referred to as the vertical plane (V). The eyebox has the dimensions of H= 10mm by V= 2.5 mm, the field of view is H= 20 deg by V=15 deg, and the eye relief is 37.5 mm. The resulting etendue is 200 deg mm which is equal to the product of the eyebox H dimension and field of view in the horizontal plane, implemented with lenses with a diameter below 23 mm in the YZ plane and with a variation in diameter between 5mm and 16mm in the XZ plane. The foregoing dimensions are given as examples and are not intended to limit the scope in any way.

[0048] During operation, the image source emits light (illustrated in Figures 4a, 4b, 4c as the ray patterns) and the components of the optical system 400 direct emitted light along axis A-A and the optical combiner 402 directs the light to an eyebox of the system, at the pupil plane, which is spaced from the optical combiner 402 by a distance corresponding to the eye-relief of the optical system 400. The optical combiner 402 is arranged to direct the light from along a second optical axis B-B of the system where the second optical axis B-B is oblique to the first optical axis A-A. The emitted light from the image source is collimated at the eyebox by the optical combiner 402. The optical combiner 402 is transmissive so that light from the external world passes therethrough to the pupil plane.

[0049] Whilst the foregoing generally refers to diffractive or holographic optical combiners, the skilled person would recognise, on the basis of the above disclosure that other possibilities exist. The optical combiner can be implemented in various ways to impose the accumulated phase difference and path length difference. Options include but are not limited to curved partially reflective optical combiners or equivalent micro-mirror systems, diffractive optical combiners such as surface-relief or holographic optical combiners liquid-crystal based optical combiners. Moreover, the combiner can be used either in a transmissive or reflective configuration, whichever one is more appropriate as understood by the skilled person. The optical combiner may be planar or curved.

[0050] Whilst the foregoing describes the relay optics 404 as a discrete lens forming part of the projection optics, the skilled person will understand that this lens may be implemented as a lens function on the optical combiner 402. In other words, optical combiner may include appropriate optical power to implement the functionality of the focusing optics as discussed above. The relay optics 404 and the focusing optics 408 may be multifocal length lenses such as adaptive or liquid lenses. This allows the focal length to be varied from infinity to several centimetres thereby allowing images to appear at infinity or closer to the users eye.

[0051] The prior art approach is rooted in the conventional optical design strategy for on-axis optical systems, where symmetry means that pupil-dependent aberrations are first eliminated before adding or tuning optics to manage field-dependent aberrations. This strategy does not work well for off-axis systems, where field-dependent aberrations such as coma and distortion dominate and increase after rays are deflected by the combiner. It is a realisation of display system according to embodiments that field-dependent aberrations should and can, by appropriate selection of the phase function for the optical combiner, be eliminated before going off axis.

[0052] Figure 7 illustrates a wearable augmented reality display 700 comprising at least one of the of axis optical system 400 described above. This wearable augmented reality display takes the form of a wearable heads-up display, such as for example, a pair of glasses. As with known types of glasses, wearable augmented reality display 700 includes a frame 702. The frame 702 includes arms 704, and lens mounting portions 706 connected by a bridge portion 708. One of the arms 704 includes a mounting portion 710 in the projector 401 (discussed above) is fixedly mounted such that the light from projector will be incident on an eyeglass lens 712 including an optical combiner 402 as described above.

[0053] The skilled person will appreciate that the projector 401 will be mounted on the arm 704 adjacent the lens mounting portion 706 holding the eyeglass lens 712 and optical combiner 402. The other lens mounting portion may have a standard ophthalmic lens inserted therein. Alternatively, an eyeglass lens including the optical combiner may be mounted in the other lens mounting portion and there may be an optics projector 401 mounted on a corresponding mounting portion on second arm 704.

[0054] One or both of the arms 704 may also be adapted to house a battery (not illustrated) to power the projector 401. In addition, one or both of the arms 704 may also include control electronics (not illustrated) for controlling the operation of the illumination optics projector 401.

[0055] 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.

[0056] 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.

[0057] The term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality. Reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

CLAIMS1. An off-axis optical display system for a head mounted display, the optical system comprising: an optical combiner defining a first optical axis for directing image rays to a pupil plane; and an aberration compensating element defining a second optical axis independent from the first optical axis; wherein the optical combiner is arranged to generate an intermediate focal plane on the second optical axis which is an optical conjugate of the pupil plane and wherein the aberration compensating element is located substantially at the intermediate focal plane.

2. The off axis optical display system of claim 1 , wherein the optical combiner comprises a diffractive optical element having a phase function and wherein the phase function comprises a tilt component and a quadratic component.

3. The off axis optical display system of claim 2, wherein the quadratic component is a parabolic phase function.

4. The off axis optical display system of claims 2 to 3, wherein the tilt component and a quadratic component are configured to achieve substantially field-independent aberrations at the optical conjugate of the pupil plane.

5. The off axis optical display system of claims 2 to 4, wherein the tilt component and the quadratic component are configured to achieve substantially telecentric rays along at least a portion of the second optical axis from the optical combiner to the aberration compensating element.

6. The off axis optical display of claims 2 to 5, wherein the tilt component is configured and arranged to apply substantially the same tilt to wavefronts incident on the optical combiner.

7. The off axis optical display system of claims 2 to 6, wherein the quadratic component is arranged to focus all wavefronts incident on the optical combiner independent of the angle of incidence of the wavefronts on the optical combiner.

8. The off axis optical display system of any preceding claim, wherein the tilt component is a diffractive tilt component and the quadratic component is a focusing component.

9. The off axis optical display system of any preceding claim, wherein the diffractive optical element is one of a holographic diffractive pattern, volume diffraction grating, a surface relief diffraction grating, or a reflection grating and is configured and arranged to introduce a phase retardation to wavefronts incident thereon.

10. The off axis optical display system of claims 2 to 9, wherein the total phase function is given a sum of the tilt component and the quadratic (parabolic) component:<J ( , y)tot= O ( , y)tiit + O ( , y)paraboiic11. The off axis optical display system of claim 10, wherein the quadratic phase function is given by:where X and Y are coordinates on the optical combiner, k is a wave vector, and Zo is the eye relief from the pupil plane to the optical combiner.

12. The off axis optical display system of claim 10, wherein the tilt component is given by:*(X, Yilt= aX + bY13. The off axis optical display system of any preceding claim, further comprising focusing optics arranged between an image source and the compensating element, wherein the focusing optics are configured and arranged to image source to the optical conjugate of the pupil plane.

14. The off axis optical display system of any preceding claim, further comprising additional focusing optics arranged between the compensating element and the optical combiner, wherein the additional focusing optics are arranged to relay intermediate pupil to exit the pupil.

15. The off axis optical display system of any preceding claim, wherein additional focusing optics may be implemented on the optical combiner arranged to relay intermediate pupil to exit the pupil.

16. The off axis optical display system of claim 15, wherein chief rays are substantially parallel to the second optical axis between the additional focusing optics and the optical combiner.

17. The off axis optical display system of any preceding claim, wherein the optical combiner is a free-space optical combiner.

18. The off axis optical display system of claim any preceding claim wherein an eyebox of the system is asymmetric.

19. A wearable head mounted display comprising the off axis optical display system of claims 1 to 18.

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