Patent ID: 12222503

DETAILED DESCRIPTION

In general, the present invention relates to improvement in or relating to optical improvements for displays such as for example a head mounted display; a head worn display; or a heads up display (HMD, HWD, HUD respectively). In particular, the present invention relates to a folded optical arrangement for use in a display to transmit images from an image plane to a user's eye, and to a display incorporating the folded optical arrangement. Other aspects of the present invention relate to a display incorporating a waveguide.

An exemplary display such as for example a HMD or HWD according to the present invention comprises a folded optical arrangement, such as the optical arrangement ofFIG.5(which will be described later), in order to overcome the problems of existing arrangements in the art.

FIG.5shows a cross-sectional of a folded optical arrangement600according to an embodiment of the invention.FIG.5shows the path of light rays through the optical arrangement600from an image plane602to a user's eye604, and from the exterior environment606to the user's eye604.

The optical arrangement600comprises a collimating element608, also described as collimating optics, and a pupil expanding element610, also described as an exit pupil expander. Light forming an image from the image plane602is directed towards the collimating element608. The collimating element608receives the light forming the image. The light is collimated by the collimating element608and the collimated light is output from the collimating element608. The collimated light is incident on the input region of the pupil expanding element or waveguide optic610and the pupil expanding element610transmits the collimated light towards the location of the user's eye604. The pupil expanding element610receives the light over a first input area and effectively expands the exit pupil output from the collimating element608so that light leaves the pupil expanding element610towards the eye in use over a larger, second output area. As a result, the collimating element608can be reduced in size to be highly compact, by generating a small exit pupil, whilst the system still maintains a large exit pupil, via the exit pupil expanding element, which is directed towards the user's eye604for displaying the image. An optically absorbing element (not shown) may be placed behind the input region of the pupil expander to absorb any light not coupled into the pupil expander. Furthermore, since the user is not required to look through the collimating element608(as they can look through the waveguide optic610) there are fewer restrictions on the optical form, layout and makeup of those surfaces within the optical design, whereas the designs that require the user to look through the collimating element, such as with reference to the arrangement shown inFIGS.3and4would also have to be optimised to be see through whilst correcting the image light.

For the purposes of the figures, it will be assumed that the user's eye604is in the location shown, and references to the user's eye should be interpreted to mean that the typical use case is being described. However, it will be appreciated that the user's eye is not required for the invention to operate according to the principles set out herein. The optical arrangements described below ultimately generate exit pupils in the direction of an assumed position of the user's eye when the device is in use, regardless of where the user's eye actually is. Furthermore, the solid ray, dashed ray and dotted ray indicate the field of view of the optical arrangement. It should also be appreciated that the figures are illustrative, and do not show the exact ray paths through the optical arrangements.

Referring toFIGS.6A to6D, the HMD is wearable by means of an appropriate support shown generally as700, providing a look-through arrangement, such that the user may look through the display. The support may contain one or more optical elements702which can be viewed by one or both eyes704of the user. Optical elements702may comprise an outcoupling grating, incoupling grating and waveguide optics. Waveguide640is not shown inFIGS.6A to6D, and the support700is illustrative, it may take any shape or form. The HMD may further include a control system. The collimator element608ofFIG.5may be located relative to the arc or shape of the head706. InFIG.6Athe support is horizontal and inFIG.6Bthe frame is tilted for aesthetic reasons and to accommodate a different head shape and is viewed from above the top of the head. The tilt may also accommodate the waveguide tilt. In figures,6C and6D two possible positions: behind and in front of the frame, are shown for the collimator element608and are viewed from the side. It will be appreciated there are many alternatives to the arrangements shown. The eye gaze direction is shown by the arrow Z inFIGS.6A and6B.

For use with or as the invention, the HMD can be of any appropriate type including googles, glasses, a helmet or helmet visor suitable for use in multiple fields. Ideally, the device is portable or adapted to be portable by means of the support. Although not shown in detail the support may include a support adapted to support the optical elements in front of the eye. The support may include: frames; side arms and supports for goggles and glasses; a helmet or visor; a headband; a neck or shoulder worn support; a gaming headset; or any other support that could be worn to hold the optical elements in the desired position.

The control system is variable depending on the use of the display. The control unit may be in situ or remote from the display. The control device may include a communications module for communicating with the optical elements and with other modules either on the display or remote therefrom. The communications may be wireless and/or wired. The control module may include different modules for carrying out different functions. These functions are not limited in any way but may include imaging, tracking, scene generation, processing, storage, power supply, audio etc.

The one or more optical elements702may comprise waveguide optics, input and/or output coupling gratings. Although not shown in detail, the optical elements are a substantially transparent display medium. The user is able to view the exterior environment through the optical elements, as well as any image relayed to the eye of the user in use via the HMD.

The support also incorporates at least one collimator element608that may include or be separate to the optical elements702, for example on one of the arms of the support of an HMD. This is a good location due to the physical characteristics of the device. It will be appreciated that other locations and forms are equally applicable. In some embodiments, the support may incorporate two collimator elements as shown, one per optical element702.

To display images to the user via the optical arrangement, the display may also incorporates an image source corresponding to the optical arrangement. The image source may have a controlled numerical aperture or an uncontrolled numerical aperture and may comprise a flat panel display, emissive display, a reflective display, a projection optic, a relay lens or any other type of display source, image or light generation unit.

In an alternative, the user may use a heads up display as shown inFIG.7.

An enlarged representation of the collimating element608ofFIG.5is shown inFIG.8. The collimating element608comprises a first optically powered optical element612(hereinafter referred to as a prism element) and a second optically powered element614(hereinafter referred to as a wedge element). The prism element612and wedge element614operate as an optical lens system, to collimate the light for output into the pupil expanding element610. The prism element612and wedge element614may also be configured to optimise or counteract unwanted optical aberrations that are typically introduced by optical lens arrangements. For example, surface features of the prism element612and/or wedge element614may be configured to reduce aberrations and/or correct any other optical defects. The use of the two elements, whilst maintaining an air space between the elements allows the collimating element to operate as an air spaced optical doublet to improve chromatic correction. Furthermore a third optical element, not shown, may be added between the prism element612and image source to provide additional optical correction.

Prism element612comprises an input surface626that has optical power and is adjacent to a surface620of the wedge element614. Surfaces626and620may have shapes that are substantially dissimilar. For example, surface620may be linear, and surface626may have a non-linear shape, such as a shape defined by a multi-order polynomial. Surfaces620and626may have a non-complementary or non-sympathetic shape. A non-complementary shape or non-sympathetic may refer to a shape such that when placing the shapes together, there is always a gap between the surfaces.

A non-complementary shape or dissimilar shape of the surface allows an additional optical surface form for correction of aberrations.

The shape of surfaces that may be a polynomial or extended polynomial shape as mentioned above may be modelled by determining parameters of the lens. One parameter that is used is a determination of the surface sag. The surface sag for the surfaces that use this surface form could (for example) be described by the following equation, which perturbs a conic aspheric surface by adding additional polynomial terms.

z=cr21+1-(1+k)⁢c2⁢r2+∑i=1N⁢Ai⁢Ei⁡(x,y)Wherec=base surface curvaturer=base surface radial distancek=base surface conic constantN=number of polynomial coefficientsAiis the coefficient on the ithpolynomial term.

It will be appreciated that this is just one example of modelling the surfaces; other may equally well be used.

FIG.9shows a simple representation of the prism element612and the wedge element614to show example angular orientations. It should be noted that inFIG.9the surfaces of the elements are represented as having a linear shape for convenience, however the surfaces may have a curved shape as is described with reference toFIGS.5and8.FIG.9shows a normal900relative to a wedge surface628, from which the light exits the collimator. The normal is located at the centre of surface628and crosses surface628at point K. The normal extends to a point L where it intersects surface626; to a point M where it intersects surface620; and to a point N where it intersects surface622. In addition, two angles are indicated902for the prism element and904for the wedge element at point J. Angle902is defined by surfaces622and620whilst angle904is defined by surfaces628and626. The angle904can be determined based on the tangent of its angle. The tangent of angle904being:
Tan(angle 904)=KL/JK

Surfaces620and626are referred to herein as the adjacent surfaces of the respective wedge element and prism element as these are adjacent to one another in the normal orientation of the collimator elements. One of the angles (902,904) is defined at one end of the adjacent surfaces and the other angle is defined at the other end of the adjacent surfaces. In other words, the angles are at opposing end of the adjacent surfaces. The interface between the adjacent surfaces is referred to herein as the adjacent interface.

The two angles can be varied to obtain an optimal orientation of the two elements which in turn give rise to optimal optical properties for the collimator. The angular difference between the two angles is predetermined and in general, the angles are not equal and angle904is less than angle902. The specific angles are not essential but the angular difference is determined to ensure the correct path for the light coming from respective sources. By way of example angle904could be for example <30°.

In the present invention the prism element as referred to throughout is used for ease and is not intended to be a limitation as to the form and shape of the element612. The prism element is thus intended to include any optical element comprising a plurality of optically powered surfaces. This could be a three surface prism or may have two or more surfaces. The preferred form is a three surface prism, but other shapes and forms are equally applicable. At least one of the optically powered surfaces on the first optical element may be described by a multiple order polynomial.

Similarly the wedge element is used of ease of reference but could be different shapes and forms. The wedge element is thus intended to include any optical element comprising one or more optically powered surfaces.

The combination of the prism element and the wedge element define a plurality of interfaces between the optical powered surfaces of each element. As light passes through the combination (also referred to as the collimating element) and as a result of the interfaces there is a change in refractive index, which when combined with the surface shape, leads to a change in direction of the light passing through the interface. This enables light beams to be directed by the combination. Due to the relative positioning of the prism and wedge there are different interfaces and different shapes of interface this helps define a “folded path” through the collimator.

The prism and wedge elements may be in direct contact or have a gap between their surfaces. The gap can be an air gap or may comprise other material such as glue etc. The addition of the air gaps adds a further interface (for example, from the prism to air; from air to the wedge and so on). The further interfaces have effect of further directing the light. The overall light direction will be described in greater detail below. The result of the combination is that due to the multiple interfaces light can be guided in a very controlled manner and in a component that is compact and light as is the optimum for head mounted optics. The juxtaposition of the first and second optically powered elements (with or without a gap) define a compact folder path through which light can be directed.

The collimating element608produces a collimated exit pupil that is well corrected whilst maintaining a low volume and size. As used herein, ‘well-corrected’ is intended to mean that defects of the light, such as aberrations, artefacts, chromatic distortion, are minimised in order to provide a predefined standard of optical performance which may be different for different applications.

The prism612, a 3D representation of which is also shown inFIG.5or8, is a single-piece three-sided element having by way of example an elongate, substantially triangular cross-section. The prism612therefore has two three-edged bases (only one of which is shown inFIG.9)616and three surfaces618,620,622joining corresponding edges of the two bases616. Any number of the surfaces618,620,622of the prism612are optically powered and the surface powers may be different from one surface to the next. In the embodiment ofFIG.8, the prism element612and wedge element are separated by a gap624such as for example an air gap. The gap624, wedge614and prism612may be of any combination of materials and as each may be different the interfaces between the three can provide a difference in refractive index from one element to the next. This can be exploited in the present invention to control the direction of light passing through the combination of elements. The gap may be formed from any type of spacing material having a different refractive index than the other elements. In theFIG.8example, the prism612is surrounded by air, which has a lower refractive index than the other elements. Many other different combinations of material can be used.

The operation of the prism element612alone and in combination with the wedge element614and the gap624will now be described. Light from the image plane602enters the prism element612at a first, receiving surface618. The receiving surface618is optically powered, and may be described by a spherical, aspherical, cylindrical, toroidal or multiple order polynomial surface shape.

The light travels through the prism element612and undergoes Total Internal reflection (TIR) at a second surface620. The TIR occurs because the surrounding material, air, has a lower refractive index than the material of the prism612and because the angle of incidence of the light is greater than the critical angle for the interface at the surface620between the prism element612and the air. The surface620is also optically powered and may be described by a spherical, aspherical, cylindrical, toroidal or multiple order polynomial surface shape. It should be noted that partial internal reflections or a reflection due to a reflective coating may be used instead of TIR, however these may be less efficient.

The surface620is tilted relative to a central axis A of the prism612. Tilting the surface620relative to the normal axis can help to reduce TIR breakdown that would occur if the transmission surface were aligned closer to the normal axis. Tilting the surface620in this way beneficially enables the image plane to be oriented at a shallower angle relative to the normal axis, permitting a more compact arrangement.

By virtue of having undergone TIR at the surface620, the light is reflected towards the reflective surface622. The reflective surface622is also optically powered and may be described by a spherical, aspherical, cylindrical, toroidal or multiple order polynomial surface shape. The reflective surface622is optically coated to reflect light. Light reflected within the prism612towards the reflective surface622therefore experiences the reflective surface622as a second surface mirror. The reflective surface622may be untilted or minimally tilted relative to the normal axis A to reduce off axis aberration. The light reflected by the coating applied to the reflective surface622returns toward the surface620.

The light reflects from the reflective surface622and is re-incident on the surface620at an angle that is less than the critical angle for the interface at the surface620between the prism element612and the air, so the light is transmitted through the surface620and exits the prism element612. The light exits the prism element612and travels into the gap624between the prism element612and the wedge element614.

The light travels through the gap624and enters the wedge element614. The refractive index of the air in the gap624is lower than the refractive index of the material of the wedge element614. The wedge element614is for example, an elongate element having quadrilateral cross-section, and so has two surfaces626,628connected by upper and lower ends630,632.

The surfaces626,628and ends630,632extend between bases (not shown inFIG.5). The wedge element614may also take a substantially triangular cross section, without an upper end630.

The wedge element614receives light from the gap624at the first, input surface626. The light leaves the wedge614at the second, output surface628.

In the embodiment ofFIG.5or8, the input surface626of the wedge614may also be described by a spherical, aspherical, cylindrical, toroidal or multiple order polynomial surface shape. The light is transmitted through the wedge614from the input surface626to the output surface628.

The output surface628is typically planar but may also be described by a spherical, aspherical, cylindrical, toroidal or multiple order polynomial surface shape. At the output surface628, the light exits the wedge element614because the light is incident on the output surface628at an angle that is less than the critical angle for the surface. The light that exits the wedge element614is now collimated and forms a well corrected exit pupil.

In the embodiment ofFIG.5or8, the prism element612, gap624, and wedge element614provide a number of degrees of freedom that are used to manipulate the light in order to result in collimated, well-corrected exit pupil. The collimated light exits the collimating element608at the wedge element614. In some embodiments, further components may be incorporated to increase the number of degrees of freedom of the arrangement and collimated light may enter or exit the collimating element608via a different component. For example, a third optical element, a field lens, may be incorporated into the collimating element608to further reduce residual aberrations such as distortion and/or field curvature. In some embodiments, a further corrective element may be incorporated in the collimating element608to chromatically correct the light.

Moreover, in the embodiment ofFIG.5or8, any of the three surfaces618,620,622of the prism element612and either of the two surfaces626,628of the wedge614may have varied optical power and surface forms. In some embodiments, the optical properties of the surface620of the prism element612and the input surface626of the wedge element614may be matched or designed to be complementary for specific applications.

The materials of the prism element612and wedge element614may be the same or may be different to take advantage of the optical characteristics such as refractive indices that different materials have. Similarly, the surrounding material and/or spacing material of the gap624are variable to optimise the optical characteristics of the collimating element608. Materials for the optical elements can be of any appropriate nature. For example, the materials may comprise one or more of optical glasses, polymers and plastics of varying refractive index and abbe number may be used, such as: N-BK7 (low index glass), N-SF6 (high index glass), 7980_0F (low index fused silica), PMMA (low index polymer) and E48R (low index polymer). It may be advantageous to have both optical elements made from different materials so as to combine materials with different indices and different dispersion characteristics. For example, the optical wedge may be manufactured from a material with low dispersion to mitigate chromatic splitting of light during the interaction with the optically powered surface.

Returning toFIG.5, collimated light from the collimating element608enters the pupil expanding element610. The pupil expanding element610comprises a waveguide640, an incoupling region642, and an outcoupling region644. The pupil expanding element610is arranged so that the collimated light is incident on the waveguide640. A first end646of the waveguide640is aligned with the wedge element614so that the collimated light is incident upon a first surface648of the waveguide640, in use.

The waveguide640is a planar slab waveguide. The waveguide640comprises an optically transmissive substrate. The waveguide640is arranged adjacent to the collimating element608. The light exiting the collimating element608passes into the waveguide640via the first surface648. In some embodiments, the waveguide may also be curved in one or two dimensions.

The in coupling region642inFIG.5may comprise a mirror, a diffraction grating, a hologram or other suitable optical coupling device. The incoupling region642abuts the second surface650of the waveguide640at the first end646of the waveguide640. The incoupling region642couples the light into the waveguide640under TIR, the light is then able to travel along the waveguide640from the first end646to a second end652of the waveguide640.

The incoupling region642is dimensioned to couple light that is incident on the waveguide640into the waveguide640over a coupling area or aperture. The dimensions, typically the height and width, of the incoupling region642dictate the size of the coupling area. Light from the collimating element608incident on both the waveguide640and the incoupling region642is coupled into the waveguide640. Light from the collimating element608incident on the waveguide640but not on the incoupling region642is not coupled to the waveguide640and passes through the waveguide640.

The dimensions of the incoupling region642are chosen to correspond to a portion of the exit pupil output from the collimating element608where the light is well corrected. Therefore, if a well corrected image is output from the collimating element608, the incoupling region642can be matched dimensionally to the dimensions of the well corrected exit pupil and remove any surrounding area of exit pupil that is not well corrected by permitting them to pass directly through the waveguide640and remain uncoupled to the waveguide640.

Light is coupled to the waveguide640by the incoupling region642by causing it to reflect or diffract towards the second end652of the waveguide640. The angle at which the light is reflected or diffracted within the waveguide640by the incoupling region642is greater than critical angle for the interface of the waveguide640with the external environment606, i.e. the surrounding air. Therefore, the coupled light undergoes total internal reflection at each interface along the waveguide640until it reaches the outcoupling region644.

The outcoupling region644ofFIG.5may comprise a mirror array, a diffraction grating, a hologram or other suitable optical decoupling device. InFIG.5, the outcoupling region644abuts the first surface648of the waveguide640and the user's eye is spaced from the second surface650of the waveguide640. In other embodiments, the outcoupling region644may abut the second surface, and the user may view the waveguide's first surface. The outcoupling region644is positioned at the second end of the waveguide.

The outcoupling region644effectively performs the opposite function to the incoupling region642and decouples the light travelling along the waveguide640from the waveguide640.

The decoupling is achieved by reflection or diffraction, dependent upon the type of decoupling element used. The light is therefore transmitted out of the waveguide640at the outcoupling region644.

The outcoupling region644is dimensioned to decouple light from the waveguide640over a decoupling area or aperture. In other words, the exit pupil of the light exiting the waveguide640has a particular area or aperture because the outcoupling region644decouples light over that area of the waveguide. The dimensions of the exit pupil generated out of the waveguide are dictated by the dimensions of the outcoupling region644. To achieve an expanded exit pupil, the dimensions of the outcoupling region are greater than those of the incoupling region. Accordingly, the input and output regions ofFIG.5are dimensioned so that the input region is smaller than the output region.

Once the light has been decoupled from the waveguide640and exits the waveguide640, it is directed to the user's eye. The decoupled light remains collimated and exits over a larger area than the area of the exit pupil of the collimated light exiting the collimating element608and of the collimated light entering the waveguide640that is coupled to the waveguide640.

In use, the user's eye is positioned to view the second surface650of the waveguide640. The user's eye is also aligned with the outcoupling region644. The outcoupling region644and waveguide640permit light from the external environment to pass through them towards the user's eye so that the user can view the external environment. The outcoupling region644and waveguide are transparent or translucent to permit the user to view the external environment.

By viewing the outside world through the waveguide640the user does not have to look through a thick and powered optical element. This can help to mitigate issues encountered with the external view from the outside world being distorted after the light from the outside world has been transmitted through a thick, powered optical element, which can induce refractive errors.

In some embodiments, the incoupling region and/or outcoupling region may be fully or partially optically coated, optionally using a dielectric, to vary the optical qualities of each. If the outcoupling region is fully coated so as not to transmit light from an external environment, the user will be able to see the image alone when viewing the waveguide.

The folded optical arrangement600described above is particularly beneficial for use in head-mounted displays. The use of a thin waveguide optic removes the requirement for a user to view the generated image and outside world through a thick or powered optical element. For the user to view the external environment, light from the external environment needs to only travel through the outcoupling region and the waveguide to reach the user's eye. In addition, the collimating element608can be specifically designed to improve the quality of the image displayed to the user because the user does not have to view the external environment through the collimating element608. Furthermore, the expansion of the exit pupil performed by the pupil expanding element610permits a smaller, more compact collimating element608with improved exit pupil performance, thereby reducing the volume occupied by the arrangement and the mass of the arrangement. It is envisaged that a significant volume reduction can be achieved as compared to a conventional eyepiece optic. Furthermore, the use of a waveguide permits the folding of the optical system to move the bulk of the optical elements away from the user's eye.

FIGS.10A and110Bshow respectively a 2D and a 3D representation of the prism and the wedge combination.FIG.10Ashows a 2D representation side on with an exemplary ray path shown and10B shows a 3D view of the combination of the prism and wedge with optically powered surfaces shown.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term ‘comprising’ does not exclude the presence of other elements or steps.

Furthermore, the order of features in the claims does not imply any specific order in which the features must be performed and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus, references to ‘a’, ‘an’, ‘first’, ‘second’, etc. do not preclude a plurality. In the claims, the term ‘comprising’ or “including” does not exclude the presence of other elements.