Patent Application
20240427151
The present disclosure relates to an optical system for a wearable heads-up display, and to a wearable heads-up display comprising such an optical system.
Off-axis retinal scanning displays (ORSDs) are a type of display that are used in virtual and augmented reality applications such as in wearable heads-up displays. They are designed to allow a user to see projected content in their field of view in a manner that allows the user to continue to see their external environment as well. ORSDs work by using a projector secured to a user's head to project an image onto the retina of the user which causes the user to see displayed content floating in space in front of them.
The projector is attached to the side of (i.e. off-axis to) a wearable frame, for example a headset or glasses frame with eye pieces. The eye pieces are provided with a holographic combiner which are illuminated by the projector. The illuminated holographic combiners cause the image to be projected through the user's pupil onto their retina.
As is known in the field, in near-eye optical devices such as ORSDs, the term “eyebox” refers to a volume of space relative to the ORSD in which the user has to position their eye to be able to correctly see the full, projected image. If an ORSD has a small eyebox, the range of eye positions at which the user can correctly see the full image is small. If an ORSD has a large eyebox, the range of eye positions at which the user can correctly see the full image is greater which thus provides a better user experience. If the user moves their eye position outside of the eyebox, they will see only part of the projected image or not see it at all. This is because it is only in the eyebox that the user's pupil and thus retina is correctly aligned with the optical path of the light projected by the ORSD. It is also known that the gaze direction of a user can effect whether or not the user's pupil lines up with the optical path of the light projected by the ORSD, particularly where the eyebox is small and only covers the eye position of a user gazing directly ahead.
A further problem is that users have different head shapes and sizes which means it is necessary to custom fit ORSDs individually to a user's head measurements to get the eyebox to line up with the eye of a given user—something which is inefficient to do at scale for mass production.
One way to overcome such issues is to actively steer the eyebox of the ORSD to overlap with the pupil of the eye of the user. For example by tracking the user's eye position and adapting the projection direction of the projector to match the user's eye position. However such systems are complex, may require motorised or actuated components and are therefore not practical.
An alternative approach is to use pupil replication. As is known in the field, the term “pupil” in pupil replication refers to copying the exit pupil of the optical system, that is, a copy of the full image being projected. Pupil replication works by replicating the full image a number of times and projecting each copy to a different position in front of the user's eye. As long as one of the replicated pupil's overlaps with the pupil of the user's eye, at least one copy of the image will be projected onto the user's retina and the user will be able to see the projected image correctly. This has the effect of increasing the size of the eyebox as the user's eye can be in any position where there is overlap with at least one replicated pupil. U.S. Pat. No. 10,031,338 B2 proposes a method for eyebox expansion by pupil replication in the projector. The projector then directs the replicated pupils onto a holographic combiner and then on to the user's eye.
However, known pupil replication approaches such as these only work when the replication typically occurs at an image plane (by diffusing the light and expanding etendue) or at the pupil (Fourier) plane where light is collimated (e.g. as is done in waveguide type approaches). In these cases, the replications can be collected and presented to the eye by a single optical function. For example, a single lens function if expanded at an image plane, or by a single grating function if replicated when the light is collimated (e.g. waveguide approaches).
This limitation has a major drawback in that it limits design flexibility of where the replication optical elements may be positioned. For example, in U.S. Pat. No. 10,031,338, pupil replication occurs in or at the projector using a bulky prism, which is impractical and limits the size of the replicated pupil that can be achieved, that is, it limits the pupil expansion that is possible.
There is a need for an optical system for a wearable heads-up display that is less bulky and has greater design flexibility in terms of positioning of the optical elements.
In general terms, in the present disclosure it is appreciated that the above described requirement that pupil replication must occur at an image or pupil (Fourier) plane can be overcome and that replication at a position in the optical path away from these two planes is possible provided that individual (i.e. selective) optical functions are applied to individual replicated beams. Various embodiments of the present disclosure describe achieving this by using a splitting element (for example a diffractive splitting element) positioned away from the image and Fourier planes to replicate the initially projected beam and subsequently using angle multiplexing in a volume phase holographic (VPH) element to collect and collimate the replicated beams with the correct angles to direct them towards a user's eye. This is possible because the plurality of multiplexed holograms of the VPH element apply a different optical function to each individual replicated beam according to its angle of incidence on the VPH element.
This allows for a much greater degree of design flexibility in terms of where the optical elements may be positioned. For example, this allows both the splitting element and the collimating element to be provided close together on/at the holographic combiner of a display for example on an eyepiece of the display and without the need for a splitting element to be provided in the projector or elsewhere on the display. Thus, the system of the present disclosure may be thinner, smaller and have a larger eyebox than known off-axis retinal displays.
Thus, according to a first aspect, there is provided an optical system for a wearable heads-up display, the optical system comprising: an image projector configured to project an image towards an eye of a user; and a combiner element positioned in a field of view of the user and in an optical path between the image projector and the eye of the user, the combiner element being configured to replicate the image onto a plurality of positions in a plane at the eye of the user, the combiner element comprising: a splitting element configured to split incident light into a plurality of directions, and a first volume phase holographic (VPH) element comprising a plurality of multiplexed holograms, each hologram configured to selectively collimate incident light according to a respective angle of incidence on the first VPH element.
As described above, the use of a VPH element to collimate incident light split by a splitting element and to direct it towards the plurality of positions in the plane at the eye of the user expands the eyebox of the display while providing much greater design flexibility than known displays because the splitting element may be positioned anywhere in the optical path and not just in the image or pupil plane. This allows for a much more compact and simpler arrangement compared to known displays.
In some implementations, the splitting element comprises a second VPH element.
VPH elements have very high efficiencies and low losses for light propagating through them. Thus, synergistically, when the splitting element is also a VPH element, the optical system has a significantly greater optical efficiency than systems that use other, non-VPH optical components as splitting elements.
In some implementations, the splitting element is positioned in the optical path spaced apart from an image plane of the image projector.
As described above, this is made possible because of the use of the first VPH element to allow placement of the splitting element anywhere in the optical path, including for example at or on a surface of an eyepiece of the display, rather than on or at a projector or elsewhere on a heads up display.
In some implementations, the splitting element is a reflective optical element and wherein the first VPH element is a transmissive optical element. For example, in this arrangement, the image projector is arranged to (i) project light through the first VPH element on a first pass, (ii) reflect light from the splitting element, and (iii) project light through the first VPH element on a second pass after reflection from the splitting element, whereby the first VPH element is configured to selectively collimate said incident light on the second pass. An angle of incidence of light on the first VPH element on the first pass is greater than a critical angle of the first VPH element allowing said light to propagate through the first VPH element on the first pass without being collimated by said first VPH element.
This arrangement can allow the projector to be positioned off-axis on the heads up display relative to the combiner element as the initially projected image passes through the first VPH element on the first pass without interacting with it because it is outside a critical angle of the first VPH element. The light instead interacts first with the splitting element (e.g. a single-off-axis-point-to-many-plane-wave reflection VPH element) that functions as a pupil splitter not located at an image plane nor a pupil plane) which splits it into multiple directions in two dimensions (e.g. directions in a 2D rectangular or hexagonal arrangement of angles). The split light beams then pass through the first VPH element for the second pass where they now do interact with it as the angle is not outside the critical angle. The first VPH element may be angle-and-shift multiplexed such that each of the many plane waves generated by the splitting element are diffracted to a corresponding exit pupil point near or at the user's eye location. The first VPH element thus functions as a many-plane-to-many-points multiplexed transmission VPH element and as a selective collimator for each of the diffractions (i.e. split beams) of the splitting element. The term “selective” herein means that each multiplexed hologram acts on one of the diffraction angles produced by the splitting element.
Alternatively, in some implementations, the splitting element is a transmissive optical element and the first VPH element is a reflective optical element. For example, the image projector is arranged to (i) project light through the splitting element on a first pass, (ii) reflect light from the first VPH element, and (iii) project light through the splitting element on a second pass after reflection from the first VPH element, whereby the splitting element is configured to split said light into said plurality of directions on the first pass. An angle of incidence of light on the splitting element on the second pass is greater than a critical angle of the splitting element allowing said light to propagate through the splitting element on the second pass without being split into said plurality of directions.
This arrangement again allows the projector to be positioned off-axis on the heads up display relative to the combiner element as the initially projected image passes through the splitting element on the second pass without interacting with it.
In some implementations, the splitting element is configured to split said incident light into a plurality of directions having acute angles relative to the angle of incidence of said incident light onto the splitting element. For example, the holograms are configured to split the incident light when it is coming from the angle of the projector on the first pass but not the angle when reflected from the (reflective) VPH collimator on the second pass.
This helps to avoid cross-talk interactions on of the light through the splitting element on the second pass where it should not interact with the splitting element.
In some implementations, the splitting element comprises a polarising Bragg diffraction grating. Optionally, the image projector is configured to project the image using light of a first polarisation and wherein the polarising Bragg diffraction grating when diffracting said light of the first polarisation is configured to change the first polarisation to a second polarisation.
In this arrangement, crosstalk may be avoided by using polarisation control. That is, the light incident on the splitting element has one polarization whereas light exiting the splitting element has a different polarisation allowing filters to be used to filter out any light that does not have the desired polarisation, ensuring only the split light continues to propagate.
In some implementations, both the Bragg diffraction grating and the first VPH element are transmissive optical elements and wherein the system comprises a reflective element positioned in the optical path after the Bragg diffraction grating to allow said incident light to pass through the Bragg diffraction grating twice.
This double pass allows the splitting element (i.e. the Bragg diffraction grating) to be thinner than if the light passed through it only once because the light interacts in both directions with the Bragg diffraction grating, thus achieving the desired diffraction with half the material thickness.
In some implementations, the splitting element comprises a surface relief diffraction grating.
In some implementations, the image projector comprises a scanning laser projector.
In some implementations, the optical system comprises an actuator configured to move one of the splitting element or first VPH element relative to the other. For example, the actuator may comprise a mechanical motor that displaces or rotates one or both of the splitting element or first VPH element, or an electrically addressable grating configured to change the angle at which light is emerges from the respective elements. This has the effect of changing the optical path between the splitting element and the first VPH element depending on the user's vergence which may be tracked for example using known eye or gaze-tracking software and hardware methods.
This implementation allows for vergence-accommodation conflict and the discomfort it causes to some users to be addressed using hardware instead of using UI design or other presentational or software methods (for example using long viewing distances, matching simulated distance in the display and focal distance as best as possible, moving objects in and out of depth at a slow pace, maximising other depth cues for the user, avoiding the stacking of smaller objects at widely varying depths in the scene, and others). Thus, user discomfort caused by vergence-accommodation conflict is significantly reduced and the user experience is greatly improved in a reliable and efficient manner regardless of UI design. In turn, a much greater freedom and flexibility in app or UI design is provided as the above described approaches to minimise the discomfort caused by vergence accommodation conflict no longer need to be deployed.
According to a second aspect of the disclosure, there is provided a wearable heads-up display comprising the optical system described above, and a support frame for mounting the optical system thereon. The support frame may be for example, an eyeglass frame.
Optionally, the wearable heads-up display may comprise at least one eye piece, wherein the splitting element is positioned on a first surface of the at least one eye piece, and wherein first VPH element is positioned on a second surface of the at least eye piece opposite the first surface. In some implementations, the wearable heads-up display comprises an off-axis retinal scanning display.
As described above, this arrangement means the splitting element can be positioned no further apart from the VPH element than the thickness of the eyepiece and can thus be provided on the eyepiece to provide a significantly more compact arrangement than known systems.
These and other aspects will now be further described, by way of example only, with reference to the accompanying figures in which:
The optical system 200 further comprises a combiner element 204 positioned in a field of view of the user and in an optical path between the image projector 201 and the eye 202 of the user. The combiner element 204 is configured to replicate the image projected by the image projector 201 onto a plurality of positions to create replicated images (i.e. exit pupils) at each of those positions in a plane 205 at the eye 202 of the user. As described above, this has the effect of expanding the size of the eyebox of the display as the user may now position his eye anywhere where his pupil overlaps with one of the replicated images (i.e. exit pupils) of the display.
The combiner element 204 comprises a splitting element 206 and a first volume phase holographic (VPH) element 207. The splitting element 206 in the example of
The path the light takes through the optical system will now be described. The light 203a, 203b, 203c from the projector 201 propagates through the first VPH element 207 first without interacting with it to reach the splitting element 206. Interaction is avoided by ensuring that the angle of incidence 207a onto the first VPH element 207 is greater than a critical angle of the first VPH element 207 i.e. an angle under which the VPH element 207 would begin to have an effect. The light 203a, 203b, 203c upon reaching the splitting element 206 is reflected and split into a plurality of directions, back in the general direction of the eye 202 of the user. The split light, now incident on the first VPH element 207 from an angle at which interaction will occur, is selectively collimated by the first VPH element 207 (i.e. a different optical function is applied to each ray depending on the angle of incidence) and thereby redirected towards said plurality of positions in the plane 205 to form the replicated images in the plane 205 at the user's eye to expand the eyebox of the display. As the collimation of each ray depends on its angle of incidence on the first VPH element 207 neither the splitter element 206 nor the first VPH element 207 that functions as a collimator need to be placed at in image or pupil (Fourier) plane of the projector 201, thereby providing the ability to position both elements significantly closer together than in known systems. For example, they may be placed as close to each other as the thickness of an eyepiece of the wearable heads-up display in which they are used, which means neither components takes up space in the projector or elsewhere on or in the wearable heads-up display.
Whilst
The light 213 from the projector 201 is split by passing twice through the PVG material (reflecting off the reflective element after the first pass) which means the PVG may be half the thickness it would have needed to be had it only passed through the PVG once. This results in a splitting element that is thinner than in the examples described above in
Further, the projector 201 may be configured to project light in one polarisation which is rotated upon reflection from the reflective element 215 so that the light that the user ultimately sees has a different (for example orthogonal) polarisation to the light that the projector initially projects. This reduces crosstalk and image artefacts as polarising filters may be used to filter out unwanted polarisations so that the user sees only the light with the correct polarisation from the combiner element 204.
Other effective alternatives will occur to the skilled person. It will be understood that the present disclosure is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.
For example, whilst
| Number | Date | Country | Kind |
|---|---|---|---|
| 2116770.5 | Nov 2021 | GB | national |
This application is a US National Stage Application of International Application PCT/EP2022/078809, filed on 17 Oct. 2022, and claims priority under 35 U.S.C. § 119 (a) and 35 U.S.C. § 365 (b) from United Kingdom Patent Application GB 2116770.5, filed on 22 Nov. 2021, the contents of which are incorporated herein by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/078809 | 10/17/2022 | WO |
