Head-mounted display with reduced ghost images

Abstract

A head-mounted display can include a projector lens for coupling light from a display onto an input-coupler of a waveguide, which guides the light to an eye of a user. Reflections in the head-mounted display can create replicas (i.e., ghosts) of images displayed to the user. The head-mounted display can use a combination of aperture and non-uniformity to reduce the ghost-images. For example, an intensity profile at an output pupil of the projector lens can be adjusted to have a pupil hot-spot in which the light of the display is concentrated and a coupling efficiency of an input-coupler of the waveguide can be adjusted to have an input-coupler hot-spot in which the light from the output pupil is more efficiency coupled into the waveguide. The aperture and the hot-spots can be aligned to maximize display-light and minimize ghost-light. Additionally, the aperture may be adjustable based on lighting.

Description

HEAD-MOUNTED DISPLAY WITH REDUCED GHOST IMAGESCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 513,682, filed on July 14, 2023, which is hereby incorporated by reference in its entirety.FIELD OF THE DISCLOSURE

[0002] The present disclosure relates to a head-mounted display and more specifically to a head-mounted display that can be used in augmented-reality (AR), mixed-reality (MR), and / or extended-realty (XR) systems.BACKGROUND

[0003] A head-mounted display, such as used in AR glasses or AR goggles, may include optics to route images from a display to an eye of a user. The efficiency at which the light from the display can be routed through the optics to an eye of the user may correspond to its visibility. Increasing the effective aperture of the optics may increase this efficiency but may allow more stray light.SUMMARY

[0004] Stray light generated by a head-mounted display can create ghost images included with displayed images viewed by a user. The present disclosure describes a head-mounted display that can reduce the visibility of these ghost images without significantly reducing the visibility of the displayed images.

[0005] In some aspects, the techniques described herein relate to a head -mounted display including: a display configured to emit display-light from pixels of the display and reflect ghostlight from surfaces of the display; a projector lens configured output the display -light and the ghost-light at an output pupil of the projector lens; a waveguide including an input-coupler configured to collect the display -light and the ghost-light from the output pupil, the input-coupler having a hot-spot with a coupling efficiency that is at or above a threshold-efficiency; and an aperture covering an area of the output pupil to block light that is outside the hot-spot of theinput-coupler to increase a ratio of the display-light to the ghost-light coupled by the inputcoupler to the waveguide.

[0006] In some aspects, the techniques described herein relate to a method for reducing ghost images in a head-mounted display, the method including: projecting light to an input-coupler of a waveguide using a projector lens, the light including display -light emitted by pixels of a display and ghost-light reflected by surfaces of the display; covering an area of an output pupil of the projector lens using an aperture that is asymmetric with respect to the output pupil; and blocking the light that is outside a hot-spot of the input-coupler using the aperture, the hot-spot of the input-coupler being off-center with respect to the output pupil.

[0007] In some aspects, the techniques described herein relate to an AR system including: a micro-light-emitting-diode display configured to emit display-light from pixels of the microlight-emitting-diode display and reflect ghost-light from surfaces of the micro-light-emitting- diode display; a projector lens configured to transmit the display-light and the ghost-light from the micro-light-emitting-diode display to an output pupil; a waveguide configured to receive the display-light and the ghost-light from the output pupil at an input-coupler of the waveguide, the input-coupler having a hot-spot with a coupling efficiency that is at or above a thresholdefficiency; and an aperture covering an area of the output pupil to block light that is outside the hot-spot to increase a ratio of the display-light to the ghost-light coupled by the input-coupler to the waveguide.

[0008] The foregoing illustrative summary, as well as other exemplary objectives and / or advantages of the disclosure, and the manner in which the same are accomplished, are further explained within the following detailed description and its accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 illustrates an AR system according to a possible implementation of the present disclosure.

[0010] FIG. 2 illustrates aspects of the head-mounted display according to a possible implementation of the present disclosure.

[0011] FIG. 3 illustrates an input image and an output image of the head-mounted display according to a possible implementation of the present disclosure.

[0012] FIG. 4 illustrates a portion of a head-mounted display according to a possible implementation of the present disclosure.

[0013] FIGS. 5A-5C illustrate an input-coupler of a waveguide for a head-mounted display according to various implementations of the present disclosure.

[0014] FIGS. 6A-6C illustrate an output pupil of a projector lens for a head-mounted display according to various implementations of the present disclosure.

[0015] FIGS. 7A-7D illustrate output pupil and input-coupler combinations for headmounted displays according to various implementations of the present disclosure.

[0016] FIGS. 8A-8B illustrate aperture areas for higher and lower ambient light levels according to possible implementations of the present disclosure.

[0017] FIGS. 9A-9B illustrate aperture transmittances for higher and lower ambient light levels according to possible implementations of the present disclosure.

[0018] FIG. 10 is a flow chart of a method for reducing ghost images in a head-mounted display according to a possible implementation of the present disclosure.

[0019] The components in the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding parts throughout the several views.DETAILED DESCRIPTION

[0020] A head-mounted display may be used in devices, such as AR glasses or goggles, to transmit display-light from pixels of a display to an eye (or eyes) of a user as displayed content (i.e., displayed images). The optics in the head-mounted display may introduce reflections that create undesirable replicas (i.e., ghost images) of displayed images. Techniques to reduce these reflections may include painting, masking, baffling, tilting, and / or spacing elements in the optical system. Despite implementing these techniques, unwanted reflections (i.e., ghost-light) can still lead to noticeable ghost images (i.e., display ghosting).

[0021] A ghost image can be a dimmed, shifted, and / or inverted version of a displayed image. In some cases, a user may recognize the ghost image based on its proximity to the displayed image and / or based on movements / changes of the ghost image that correspond to the movement / changes of the displayed image over time. While the ghost image may be dimmer than the virtual element, it may still be distracting in some environments.

[0022] One technical problem with using an aperture to block ghost-light leading to ghost images is that the aperture can reduce the visibility (e.g., brightness) of the ghost image at the expense of the visibility (e.g., brightness) of the displayed image when the aperture blocks the transmission of the ghost-light and display-light by the same percentage. In other words, a ratio of the display-light to the ghost-light may be unchanged despite a reduction in optical efficiency caused by the aperture.

[0023] The present disclosure describes a head-mounted display configured with at least one nonuniformity (i.e., hot-spot) and an aperture configured according to this nonuniformity to decrease display ghosting. This approach may have the technical effect of reducing the visibility (e.g., brightness) of a ghost image without significantly reducing the visibility (e.g., brightness) of the displayed image. In a possible implementation, the amount of reduction can be adjusted based on an ambient light condition because a ghost image may be easier to see (i.e., may appear brighter) in an ambient light condition that is relatively dim (e g., indoor environment).

[0024] FIG. 1 illustrates an AR system according to a possible implementation of the present disclosure. The AR system includes the head-mounted display included in AR glasses 110 worn by a user 100. While this implementation is useful for discussion, the techniques described herein can be applied to other AR systems (e.g., AR goggles).

[0025] As shown in FIG.l, a head-mounted display is configured to transmit displayed images in each of the lenses (i.e., L, R) of the AR glasses 110. In the inset view 120 the AR glasses 110 are shown from the user’s perspective. As shown, the head-mounted display for each lens is configured to display AR content (i.e., displayed images) on a transparent background.

[0026] The displayed images may correspond with applications running on the AR glasses 110 or in a network (i.e. cloud) coupled to the AR glasses 110. As shown, the displayed images may include graphics 111 corresponding to a navigation application. As shown, the displayed images may include text 112 corresponding to a text message. As shown, the displayed images may include icons 113 associated with time / weather / status. As shown, the displayed images may include images 114 associated with a map. The displayed images may be rendered in a monochromatic color (e.g., red) or in a full-color (e.g., white). A displayed image may be head- locked, meaning that its position on the head-mounted display (i.e., lens) is fixed as the user 100moves her head. Alternatively, a displayed image may be world-locked, meaning that its position on the head-mounted display (i.e., lens) changes as the user 100 moves her head.

[0027] It may be desirable for a size / shape (i.e., form factor) of the AR glasses 110 to substantially match the form factor of a conventional pair of glasses. This requirement may place constraints on the layout and devices used in the head-mounted display. For example, display-light of AR-content be emitted in from a display at a first position (e.g., temple arm) in the AR glasses 110 and then routed (i.e., guided) by a waveguide to a second position (e.g., lens) in front of the user’s eye.

[0028] In a possible implementation, the display is a micro-light-emitting-diode display (micro-LED display). The micro-LED display is well suited for the AR glasses 110 because it is small and relatively low complexity. The micro-LED display does not require an illumination subsystem to generate a flat field that is spatially modulated to create an image, as in digital light processing (DLP) or liquid crystal on silicon (LCDS) technologies. While DLP and LCOS technologies may not be well suited for AR glasses (or AR goggles) implementations, the ghostmitigation techniques described herein could be used with these technologies. For discussion purposes, however, AR glasses having a head-mounted display including a micro-LED display element will be described in what follows.

[0029] A micro-LED display may include an array of light emitting diodes arranged in a two-dimensional (2D) array. Each light emitting diode may represent a pixel in a monochromatic display. Alternatively, each light emitting diode may represent a subpixel (red, green, blue) in a color display. The display is referred to as the micro-LED display because each light emitting diode (LED) may each have dimension(s) that are on the order of a micrometer (e.g., < 10 microns). In a possible implementation, each LED is a Lambertian emitter so that the light (e g., viewed power per unit area) emitted by the LED is substantially independent of a viewing direction. Alternatively, each LED may be configured to radiate light in a particular direction.

[0030] In a possible implementation of the micro-LED, physical changes (e.g., density changes, pointing direction changes) to individual LEDs in the two-dimensional array of LEDs may cause the micro-LED display to emit display-light non-uniformly over the surface of the array.

[0031] In another possible implementation of the micro-LED display, an optical element corresponding to each pixel may alter the display-light emitted by the display. For example, one possible optical element is a two-dimensional array of micro-lenses (i.e. lenslet array). Each micro-lens of the lenslet array may correspond to a pixel in the pixel array and offsetting a lenslet can adjust how the light from a pixel is emitted spatially. Another possible optical element is a diffractive optical element (DOE), which can adjust how the light from one or more pixels is emitted spatially.

[0032] FIG. 2 illustrates aspects of the head-mounted display according to a possible implementation of the present disclosure. The head-mounted display 200 includes a display (e.g., micro-LED display 210) that is configured to generate images (e.g., text, video, photos, graphics, etc.) for display to an eye 220 of an observer (i.e., user). The head-mounted display 200 further includes a waveguide 230 that can be configured to collect and direct (i.e., couple) incident light 215 from an output pupil 217 of a projector lens 218 into the waveguide 230 where it can propagate by total internal reflection (TIR) based on an index of refraction (i.e., index) of the waveguide substrate (i.e., nsub).

[0033] For the coupling, the waveguide 230 can include an input-coupler (IC) grating (i.e., input-coupler 240). The input-coupler 240 can be a grating (e.g., reflection grating) that includes a periodic (e.g., nanometer) changes in reflectivity and / or index in order to preferentially reflect the light in a direction for propagation in the waveguide 230. An efficiency that the grating can perform this preferential reflection is referred to herein as the coupling efficiency of the inputcoupler 240. In a possible implementation, the coupling efficiency of the input-coupler 240 is non-uniform over an area of the input-coupler 240. For example, the input-coupler 240 may have a coupler hot-spot (i.e., hot-spot 270) in which the coupling efficiency is at or above a threshold-efficiency (e.g., 50%). In other words, the coupling efficiency within the hot-spot 270 may be higher (e.g., multiple times higher) than in areas of the input-coupler 240 outside the hotspot 270.

[0034] In some examples, the IC is characterized by a first average coupling efficiency (el), which is averaged over the IC area. The IC hot-spot is characterized by a second average coupling efficiency (e2), which is averaged over the area of the hot-spot. The ratio of the first average coupling efficiency to the second average coupling efficiency (i.e., e2 / el) is larger than 1.5 (e.g., 2 < e2 / el< 10).

[0035] The waveguide 230 can further include an output-coupler (OC) grating (i.e., output coupler 250). The output coupler 250 can be a grating (e.g., transmission grating) that includes periodic (e.g., having a period on a nanometer scale) changes in reflectivity and / or index in order to preferentially transmit projected light 225 towards the eye 220 of the user. The waveguide can further include other gratings that are not shown. For example, the waveguide 230 may include a fold grating to help guide the light to the eye of a user. For example, the outputcoupler grating may be configured to expand an eye-box of the light in a first direction (e.g., in a plane of the waveguide), while the folded grating may be configured to expand the eye-box of the light in a second direction, which is perpendicular to the first direction.

[0036] The input-coupler 240, the output coupler 250, and the substrate of the waveguide 230 may be transparent at the visible wavelengths so that the user can see a display image 211 from the micro-LED display 210 superimposed on the user’s view through the waveguide 230.

[0037] In practice, the grating (e.g., due to imperfections) may reflect or rebound (i.e., decouple after being coupled) a portion of the incident light 215 back through the output pupil 217 and the projector lens 218 so that the rebounded light reaches the micro-LED display 210. The micro-LED display 210 may reflect a portion of the rebound light off of surfaces, including (but not limited to) a cover plate and metal (e.g., copper) electrodes / traces. These reflections may then pass back through the projector lens 218, the output pupil 217, the input-coupler 240 and the output coupler 250 to form ghost images in the eye 220 of the user. In other words, the display may emit display-light from pixels as well as reflect ghost-light from reflections off surfaces (e.g., electrodes) of the display.

[0038] FIG. 3 illustrates an input image and an output image of the head-mounted display according to a possible implementation of the present disclosure. As shown, a display 310 can be configured to emit display-light from pixels to form a display image 311 at the input of the head-mounted display. Based on the display image 311, the head-mounted display may be configured to output an output image 320 to a user at an output of the head-mounted display. The output image 320 includes the display image 311 and a ghost image 322. The ghost image 322 may be due to the rebound and reflections described above. In other words, the display image 311 is based on display -light emitted from the display 310 and the ghost image 322 is based on ghost- light reflected from the display 310. The ghost image 322 may appear dimmerthan the display image 31 1 because the intensity of the display-light in the output image 320 may be higher than the intensity of the ghost-light in the output image 320.

[0039] The relative intensities of the display image 311 and the ghost image 322 in the output image 320 may be represented as a ratio of the display-light to the ghost-light coupled by the input-coupler 240 to the waveguide 230. It may be desirable to maximize this ratio so that the ghost image 322 is reduced or eliminated from the view of the user. This ratio may be due to changes in the head-mounted display made in response to an ambient light condition.

[0040] The brightness of the display image 311 may be increased (e.g., by driving pixels of a micro-LED display with a relatively high current) when the ambient light condition is relatively bright (e.g., daytime, outdoor lighting) so that the brightness of the display image 311 in the output image 320 may be seen superimposed on the bright environment (e.g., through a lens of the AR glasses 110). In a possible implementation, the ghost image 322 is not viewable during the bright (i.e., high) ambient light condition. As a result, reducing ghost-light may not be as important in a relatively bright ambient light condition as it is in a relatively dim ambient light condition.

[0041] The brightness of the display image 311 may be decreased (e.g., by driving pixels of a micro-LED display with a relatively low current) when the ambient light condition is relatively dim (e.g., nighttime and / or outdoor lighting) to save power because the brightness of the display image 311 in the output image 320 may be seen superimposed on the dim environment (e g., through a lens of AR glasses 110) at a lower intensity. In a possible implementation, the ghost image 322 is viewable during the dim (i.e., low) ambient light condition. As a result, reducing ghost-light may be more important in a relatively dim ambient light condition than it is in a relatively bright ambient light condition.

[0042] FIG. 4 illustrates a portion of a head-mounted display according to a possible implementation of the present disclosure. As shown, a head-mounted display 400 includes a display 410 (e.g., micro-LED display) configured to emit display-light 425 from pixels. The head-mounted display 400 further includes a projector lens configured to output the display-light at an output pupil 419 of the projector lens 418, which can include multiple lenses. The headmounted display further includes a waveguide 430 that includes an input-coupler 440 (e.g., IC grating). The input-coupler 440 is configured to generate guided light that can propagate in the waveguide 430 through total internal reflection (TIR). The input-coupler 440 is furtherconfigured to generate reflected light 441 or rebound light 442. The reflected light 441 may be due to reflections from a grating interface and the rebound light 442 may be due to leakage (i.e., decoupling) of light out of the waveguide 430. The head-mounted display 400 further includes an aperture 450 covering an area of the output pupil 419. In a possible implementation, the aperture 450 is movable to adjust an amount of blockage of the output pupil 419. The aperture 450 can block ghost-light 435 from reaching the input-coupler 440.

[0043] As shown in FIG. 4, at a first time (1), the display 410 may emit display-light 425 that is focused by the projector lens 418 to generate display -light 425 at the output pupil 419. At a second time (2), a first portion of the display-light 425 is converted by the input-coupler 440 into guided light. A second portion of the display -light 425 is converted by the input-coupler 440 into reflected light 441 and / or rebound light 442, which is decoupled after being coupled so it can be shifted laterally with respect to the reflected light 441. The intensity of the first portion may be greater than the intensity of the second portion. The first portion and the second portion may be related to a coupling efficiency of the input-coupler 440. For example, a higher coupling efficiency may correspond to a higher first portion and a lower second portion, while a lower coupling efficiency may correspond to a lower first portion and a high second portion. The display-light 425 is directed in a first direction away from the output pupil 419 and the reflected light 441 and / or rebound light 442 is directed in a second direction towards the output pupil 419. As shown, the waveguide 430 may be tilted relative to an optical axis 405 of the head-mounted display 400.

[0044] At a third time (3), the reflected light 441 and / or rebound light 442 is focused by the projector lens 418 onto the display 410. Reflections off surfaces of the display 410 can generate ghost-light 435 that is focused by the projector lens 418 to generate ghost-light 435 at the output pupil 419. The aperture 450 is configured to cover an area of the output pupil to block the ghostlight 435 at a fourth time (4). The aperture may increase a ratio of the intensity of the displaylight to the ghost-light coupled by the input-coupler 440 to the waveguide 430.

[0045] The projector lens may output light at an infinite focus. For example, divergent light rays originating from a center of the display 410 may be focused by the projector lens 418 to parallel light rays distributed over the output pupil 419 and directed in a direction aligned with the optical axis 405. Divergent light rays originating from a point of the display 410 that is off- center may be focused by the projector lens 418 to parallel light rays distributed over the outputpupil 419 and directed in a direction at an angle with the optical axis 405. Accordingly, blocking a portion of the output pupil 419 with an aperture 450 does not change the content (size, shape, color, etc.) of display-image and a ghost-image, but does change the intensity of the displayimage and the ghost-image.

[0046] In a possible implementation, the aperture 450 may be symmetric around the optical axis 405 (e.g., annular ring aperture centered on the optical axis). Alternatively, the aperture 450 may be asymmetric around the optical axis (e.g., semi-circular aperture having a centroid offset form the optical axis). An aperture 450 that is asymmetric (with respect to the output pupil) may offer some advantage in blocking ghost light when the ghost light is concentrated in a portion of the output pupil 419. This concentration may be based on the angle of the waveguide 430 relative to the optical axis 405. Accordingly, the aperture 450, which is asymmetric, may block proportionally more ghost-light than display -light from the display 410. The amount of blockage may be based on the area of the output pupil 419 covered by the aperture 450 and / or may be based on a transmittance of the aperture covering the output pupil 419.

[0047] In some examples, the aperture blocks a first fraction (fl) of a first power (e.g., average intensity) of the di splay ed-image and a second fraction (f2) of a second power (e.g., average intensity) of the ghost-image. In this case the ratio of the second fraction to the first fraction (i.e., f2 / fl) is greater than 1.5 (e.g., 2 < f2 / fl < 100).

[0048] The amount of ghost-light coupled to the waveguide (i.e., converted to guided light) may be further reduced based on a spatial non-uniformity corresponding to the input-coupler 440 and / or based on a spatial non-uniformity corresponding to the output pupil 419. The spatial nonuniformity corresponding to the output pupil 419 may be generated by adjusting the light emitted by the display. The light emitted by the display may be adjusted by micro-lenses (e.g., a two- dimensional array of micro-lenses), a diffractive optical element (or elements), or by a pixel configuration and / or arrangement.

[0049] FIGS. 5A-5C illustrate an input-coupler grating (i.e., input-coupler, IC grating) of a waveguide for a head-mounted display according to various implementations of the present disclosure. The shape of IC grating 540 shown in each figure is square for discussion purposes, but any shape (e.g., circular, oval, etc.) for the IC grating 540 may be implemented. FIG. 5A illustrates an IC grating 540 with a uniform (e.g., < 20% variation) coupling efficiency over the surface area of the IC grating. FIGS. 5B and 5C illustrate IC gratings with non-uniform couplingefficiency of the surface area of the IC grating. In particular, the IC gratings shown in these figures have an IC hot-spot (i.e., hot-spot 510) that divides the surface area of each IC grating into two portions: a first portion inside (i.e., within) the hot-spot 510 and a second portion outside the hot-spot 510. The hot-spot 510 shown in FIGS. 5B-5C are circular but can be selected to be another shape (e.g., square, oval, etc.) to help reduce the amount of ghost light coupled by the IC grating 540. Likewise, the hot-spot 510 shown in FIGS. 5B-5C can have a size (e.g., diameter) selected to help reduce the amount of ghost light coupled by the IC grating 540. In effect, the hot-spot 510 may function as an aperture integrated in the IC grating 540, where a lower percentage of the light received in the second portion outside the hot-spot 510 is coupled to the waveguide and a higher percentage of the light received in the first portion inside the hotspot 510 is coupled to the waveguide. The border of the hot-spot 510 may be based on a threshold coupling efficiency (i.e., threshold-efficiency). A threshold-efficiency (e.g., 50%) may be selected so that the first portion of the surface area of the IC grating 540 within the hot-spot 510 has a coupling efficiency that is at or above the threshold-efficiency and the second portion of the surface area of the IC grating 540 outside the hot-spot 510 has a coupling efficiency that is below the threshold-efficiency.

[0050] The IC grating 540 may be spatially aligned with the output pupil 419 of the projector lens so that a center 520 of the IC grating 540 may be spatially offset (e.g., in a z-direction (see FIG. 4)) but otherwise spatially aligned (e g., in x-direction and y-direction (see FIG. 4)) with a center of the output pupil 419. FIG. 5B illustrates a hot-spot 510 that is symmetric and centered with the center 520 of the IC grating 540 (i.e., symmetric and centered with respect to the output pupil). FIG. 5C illustrates a hot-spot 510 that is asymmetric (i.e., off-center) with the center 520 of the IC grating (i.e., off-center with respect to the output pupil).

[0051] In some examples, the IC grating has a first center (e g., a geometric center-of-mass) and a characteristic lateral dimension (L). The hot-spot of the IC grating has a second center, which is located at a distance from the first center that is at lest L / 10 (or at least L / 5). In some examples, the aperture does not obstruct any light impinging on the IC grating within a distance less than L / 5 (or L / 10) of the center.

[0052] Just as non-uniformity (i.e., IC hot-spot) in the input-coupler may function as an aperture based on coupling efficiency, a non-uniformity in the illumination of the IC grating 540by the output pupil 419 of the projector lens 418 may function as an aperture based on illumination (i.e., brightness).

[0053] FIGS. 6A-6C illustrate an output pupil of a projector lens for a head-mounted display according to various implementations of the present disclosure. FIGS. 6A-5C illustrate an output pupil (i.e., exit pupil) of a projector lens for a head-mounted display. The output pupil may be the diameter of the light leaving the projector. A larger output pupil may correspond to a bright image at the output of the head-mounted display, while a smaller output pupil may correspond to a dimmer image at the output of the head-mounted display. The output pupil can be aligned with the input-coupler of the waveguide of the head-mounted display to maximize the light coupled into the waveguide.

[0054] FIG. 6A illustrates an output pupil 619 with a uniform (e.g., < 20% variation) brightness (i.e. light intensity) over an area of the output pupil 619. FIGS. 5B and 5C illustrate output pupil 619 with non-uniform brightness over the area of the output pupil 619. In other words, the output pupil 619 has an intensity profile that is non-uniform. In particular, the output pupil 619 shown in these figures each have a pupil hot-spot 610 that divides the area of each pupil into two portions: a first portion inside (i.e., within) the pupil hot-spot 610 and a second portion outside the pupil hot-spot 610. The pupil hot-spot 610 shown in FIGS. 6B-6C are circular but can be selected to be another shape (e.g., square, oval, etc.) to help reduce the amount of ghost light projected onto the IC grating 540. Likewise, the pupil hot-spot 610 shown in FIGS. 6B-6C can have a size (e g., diameter) selected to help reduce the amount of ghost light transmitted to the IC grating 540. In effect, the pupil hot-spot 610 may function as an aperture integrated in the output pupil 619, where light in the second portion outside the pupil hot-spot 610 is transmitted at a lower intensity to the IC grating 540 while light in the first portion inside the pupil hot-spot 610 is transmitted at a higher intensity to the IC grating 540. The border of the pupil hot-spot 610 may be based on a brightness (i.e., threshold-intensity). A threshold-intensity may be selected so that light at an intensity above the intensity threshold (i.e., more light) is transmitted from the first portion of output pupil 619 to a corresponding first portion of the IC grating 540. Light at an intensity below the threshold-intensity (i.e. less light) is transmitted from the second portion of the output pupil 619 to a corresponding second portion of the IC grating.

[0055] The pupil hot-spot 610 can be positioned relative to a center 620 of the output pupil 619. The pupil hot-spot 610 may be spatially offset (e.g., in a z-direction (along the optical axis)) but otherwise spatially aligned (e.g., in x-direction and y-direction) with the IC hot-spot. FIG. 6B illustrates a pupil hot-spot 610 that is symmetric and centered with the center 620 of the output pupil 619. FIG. 6C illustrates a pupil hot-spot 610 that is asymmetric (i.e., off-center) with the center 620 of the output pupil 619.

[0056] As discussed, the ghost light can be reduced in relation to the display light by spatially reducing light from a portion of the output pupil that is coupled into the waveguide of the head-mounted display. This reduction can be carried out using a physical aperture, a pupil hot-spot, and / or an IC hot-spot. Table 1 illustrates various combinations that can be implemented to reduce a relative intensity of a ghost image.TABLE 1: Possible head-mounted display configurations to reduce ghost images

[0057] FIGS. 7A-7D illustrate output pupil and input-coupler combinations for headmounted displays according to various implementations of the present disclosure. A first possible configuration, shown in FIG. 7A, includes an aperture 750 that blocks a portion of the output pupil 719. The illumination (i.e., brightness) of the remaining portion of the output pupil not covered by the aperture 750 is uniform. The coupling efficiency of the input-coupler 740 is uniform. A projection of the output pupil 719 and the aperture 750 is shown on the input-coupler 740. The input-coupler 740 will couple more light in a first area 725 corresponding to the portion of the output pupil 719 not covered by the aperture 750. The input-coupler 740 will couple less light (e.g., no light) in a second area 726 corresponding to the portion of the output pupil 719 covered by the aperture 750. More ghost-light may be in the second area 726 than the first area 725. The aperture 750 may increase a ratio of the display-light to the ghost light coupled by the input-coupler 740 to the waveguide. Some display-light will be blocked by the aperture 750.

[0058] In a possible implementation a head-mounted display includes a display configured to emit display-light from pixels of the display and reflect ghost-light from surfaces (e.g., electrodes, cover plate, etc.) of the display. The head-mounted display further includes a projector lens configured to output the display-light and the ghost-light at an output pupil of the projector lens. The head -mounted display further includes an input-coupler (i.e. IC grating) configured to collect the display -light and the ghost-light from the output pupil, where the input coupler has a relatively uniform coupling efficiency (i.e., over the surface of the input coupler). The head-mounted display further includes an aperture covering an area of the output pupil to block light in a portion of the input-coupler to increase a ratio of the display-light to the ghostlight coupled by the input-coupler to the waveguide. In a possible implementation, the portion is rotationally symmetric with respect to a center of the IC coupler. In an alternative implementation, the portion is asymmetric (e g., off-center) with respect to the center of the IC coupler.

[0059] A second possible configuration, shown in FIG. 7B, includes an aperture 750 that blocks a portion of the output pupil 719. The illumination (i.e., brightness) of the remaining portion of the output pupil not covered by the aperture 750 is uniform. The coupling efficiency of the input-coupler 740 is not uniform. The input-coupler 740 includes an IC hot-spot 710 in the first area 725 corresponding to the portion of the output pupil 719 not covered by the aperture. The IC hot-spot 710 and the aperture 750 will increase the ratio of the display light to the ghost-light coupled by the input-coupler 740 to the waveguide. The IC hot-spot 710 may increase the coupling efficiency of light in the first area 725 to compensate for the light lost due to the aperture 750.

[0060] A third possible configuration, shown in FIG. 7C, includes an aperture 750 that blocks a portion of the output pupil 719. The illumination (i.e., brightness) of the remaining portion of the output pupil not covered by the aperture 750 is not uniform. The output pupil 719 includes a pupil hot-spot 720 so that the first area 725 of the input-coupler 740 is illuminated with more display light. The pupil hot-spot 720 and the aperture 750 will increase the ratio of the display light to the ghost-light coupled by the input-coupler 740 to the waveguide. The pupil hot-spot 720 may increase the amount of display-light in the first area 725 to reduce its loss to the aperture 750.

[0061] A fourth possible configuration, shown in FIG. 7D, includes an aperture 750 that blocks a portion of the output pupil 719. The illumination (i.e., brightness) of the remaining portion of the output pupil not covered by the aperture 750 is not uniform. The output pupil 719 includes a pupil hot-spot 720 so that the first area 725 of the input-coupler 740 is illuminated with more display light. The input-coupler 740 includes an IC hot-spot 710 in the first area 725 corresponding to the (projected) position of the pupil hot-spot 720. The pupil hot-spot 720, the IC hot-spot 710, and the aperture 750 will increase the ratio of the display light to the ghost-light coupled by the input-coupler 740 to the waveguide.

[0062] The aperture 750 shown in FIGS. 7A-7D is D-shaped. The area covered (i.e., blocked) by the aperture is asymmetric with respect to a center of the output pupil 719. The location of the area covered is horizontally offset from the center of the output pupil. Variations to these parameters may be implemented. For example, the aperture may be crescent shaped. In another example, the aperture may be an annular ring that is rotationally symmetric with the output pupil 719. The location of the area covered may be vertically offset from the center of the pupil. The exact choice of these parameters may be based on details of the optical design (e.g., projection lens f-number) and based on acceptable levels of ghost light in the system. As discussed, the acceptable levels of the ghost light in the head-mounted display may change based on ambient light levels. Accordingly, in a possible implementation, the ghost light blocked by the aperture may be adjustable.

[0063] In some examples, the projector lens has a cylindrical symmetry, and the aperture lacks cylindrical symmetry. In some examples, the aperture does not obstruct light passing through an axis of symmetry of the projector lens.

[0064] FIGS. 8A-8B illustrate aperture areas for higher and lower ambient light levels according to possible implementations of the present disclosure. As shown, the area covered by the aperture is adjustable according to an ambient light level. In practice, the aperture may be mechanically adjusted (moved) in directions (e.g., see movement 451 in FIG. 4) to open and close the output pupil 819. As shown in FIG. 8 A, the area of the output pupil 819 blocked by an aperture 810, which is crescent shaped in this example, is relatively small (i.e., reduced) when the ambient light level is at a relatively high level (e.g., daytime). As shown in FIG. 8B, the area of the output pupil 819 blocked by the aperture 810 is relatively large (i.e., increased) when the ambient light level is at a relatively high low (e.g., nighttime).

[0065] The aperture 810 may be an opaque, low-reflectivity element that is movable so that the light blocked by the aperture is based on a position of the aperture relative to the output pupil 819. The ambient light level may be measured with a light sensor of the device (e.g. AR glasses) that utilizes the head-mounted display. In a possible implementation, a signal (i.e., measurement) from the light sensor may be compared to a light-level threshold 820. Actuators may be controlled to move the aperture 810 to one of two positions in or out of the output pupil 819 based on the comparison (i.e., discrete adjustment). More positions may be implemented based on more complex thresholding and logic. In another possible implementation, the aperture may be adjusted over a range of positions corresponding to a range of ambient light levels (i.e., continuous adjustment). Additionally, a brightness of the display (e.g., see FIG. 4) or a dimmer level of a dimmable (i.e., adjustable transmittance) lens (e.g., see L or R in FIG. 1) may be received and used to help determine the aperture adjustment.

[0066] Alternatively, the aperture 810 may be implemented using an array of shutters (i.e., pixels) which can be open / closed to change the size and shape of the aperture to increase or decrease an amount of blocking provided by the aperture. The shutters may be implemented using liquid crystal display technology or electronic shutter technology. In other words, the aperture may include pixels that can be electronically controlled to adjust the amount of blocking provided by the aperture.

[0067] In some examples, the aperture is configured such that, at the output coupler (OC) of the waveguide (e.g., see FIG. 2), the displayed-image has a first brightness (bl) and the ghostimage has a second brightness (b2). A brightness ratio (b2 / b 1 ) is less than 10% (e.g., 0.01% < b2 / b 1 < 10%) for a given background light level.

[0068] In a possible implementation, an aperture 910 may be a semi-transparent element that is fixed (not movable) so that the light block by the aperture is based on the transmittance of the aperture 910.

[0069] FIGS. 9A-9B illustrate aperture transmittances for higher and lower ambient light levels according to possible implementations of the present disclosure. As shown, the aperture covers a fixed area of the output pupil 919 but the transmittance is adjusted according to an ambient light level. As shown in FIG. 9A, the transmittance of aperture 910, which is crescent shaped in this example, is relatively large (i.e., increased) (e.g., greater than zero) when the ambient light level is at a relatively high level (e.g., daytime). As shown in FIG. 9B, thetransmittance of the aperture 910 is relatively small (i.e., decreased) (e.g., zero) when the ambient light level is at a relatively high low (e.g., nighttime).

[0070] The aperture 910 may be an element with an electrically adjustable transmittance so that the light blocked by the aperture is based on the transmittance of aperture and its area covering the output pupil 919. For example, the aperture may be a liquid crystal element or an electrochromic element. The ambient light level may be measured with a light sensor of the device (e.g. AR glasses) that utilizes the head-mounted display. In a possible implementation, a signal (i.e., measurement) from the light sensor may be compared to a light-level threshold 920. Signals may adjust the aperture 910 to have one of two transmittances based on the comparison (i.e., discrete adjustment). More transmittance levels may be implemented based on more complex thresholding and logic. In another possible implementation, the aperture may be adjusted over a range of transmittances corresponding to a range of ambient light levels (i.e., continuous adjustment).

[0071] FIG. 10 is a flow chart of a method for reducing ghost images in a head-mounted display according to a possible implementation of the present disclosure. The method 1000 includes projection 1010 light (e.g., display-light, ghost-light) in an input-coupler of a waveguide using a projector lens. The output pupil of the projector lens may be aligned with the inputcoupler of the waveguide so that the light coupled by the input-coupler to the waveguide is based on the light at the output pupil of the projector lens. The method 1000 further includes covering 1020 an area of the output pupil using an aperture. The aperture may be configured so that light in the output pupil is blocked from reaching an area outside a hot-spot (i.e., area of enhanced coupling efficiency) of the input-coupler of the waveguide. In other words, the method 1000 includes blocking 1030 the light outside a hot-spot (i.e., IC hot-spot) of the input-coupler and / or blocking light outside a pupil hot-spot of the output pupil. The method further includes transmitting 1040 the light coupled into the waveguide to an eye of a user. The light transmitted has a ratio of display-light to ghost-light that is increased by the blocking and the hot-spot. The increased ratio corresponds to a ghost image being less visible (e.g., invisible) to the user.

[0072] In the specification and / or figures, typical embodiments have been disclosed. The present disclosure is not limited to such exemplary embodiments. The use of the term “and / or” includes any and all combinations of one or more of the associated listed items. The figures are schematic representations and so are not necessarily drawn to scale. Unless otherwise noted,specific terms have been used in a generic and descriptive sense and not for purposes of limitation.

[0073] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure. As used in the specification, and in the appended claims, the singular forms “a,” “an,” “the” include plural referents unless the context clearly dictates otherwise. The term “comprising” and variations thereof as used herein is used synonymously with the term “including” and variations thereof and are open, non-limiting terms. The terms “optional” or “optionally” used herein mean that the subsequently described feature, event or circumstance may or may not occur, and that the description includes instances where said feature, event or circumstance occurs and instances where it does not. Ranges may be expressed herein as from "about" one particular value, and / or to "about" another particular value. When such a range is expressed, an aspect includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0074] Some implementations may be implemented using various semiconductor processing and / or packaging techniques. Some implementations may be implemented using various types of semiconductor processing techniques associated with semiconductor substrates including, but not limited to, for example, Silicon (Si), Gallium Arsenide (GaAs), Gallium Nitride (GaN), Silicon Carbide (SiC) and / or so forth.

[0075] While certain features of the described implementations have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the scope of the implementations. It should be understood that they have been presented by way of example only, not limitation, and various changes in form and details may be made. Any portion of the apparatus and / or methods described herein may be combined in any combination, except mutually exclusive combinations. The implementations described herein can include various combinations and / or sub-combinations of the functions, components and / or features of the different implementations described.

[0076] It will be understood that, in the foregoing description, when an element is referred to as being on, connected to, electrically connected to, coupled to, or electrically coupled to another element, it may be directly on, connected or coupled to the other element, or one or more intervening elements may be present. In contrast, when an element is referred to as being directly on, directly connected to or directly coupled to another element, there are no intervening elements present. Although the terms directly on, directly connected to, or directly coupled to may not be used throughout the detailed description, elements that are shown as being directly on, directly connected or directly coupled can be referred to as such. The claims of the application, if any, may be amended to recite exemplary relationships described in the specification or shown in the figures.

[0077] As used in this specification, a singular form may, unless definitely indicating a particular case in terms of the context, include a plural form. Spatially relative terms (e.g., over, above, upper, under, beneath, below, lower, and so forth) are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. In some implementations, the relative terms above and below can, respectively, include vertically above and vertically below. In some implementations, the term adjacent can include laterally adjacent to or horizontally adjacent to.

Claims

CLAIMS1. A head-mounted di spl ay compri sing : a display configured to emit display-light from pixels of the display and reflect ghost-light from surfaces of the display; a projector lens configured to output the display-light and the ghost-light at an output pupil of the projector lens; a waveguide including an input-coupler configured to collect the display-light and the ghost-light from the output pupil, the input-coupler having a hot-spot with a coupling efficiency that is at or above a threshold-efficiency; and an aperture covering an area of the output pupil to block light that is outside the hot-spot of the input-coupler to increase a ratio of the display-light to the ghost-light coupled by the input-coupler to the waveguide.

2. The head-mounted display according to claim 1, wherein the display is a micro-light- emitting-diode display.

3. The head-mounted display according to claim 2, wherein the micro-light-emitting-diode display includes micro-lenses to generate a pupil hot-spot at the output pupil that is outside the area covered by the aperture to further increase the ratio of the display-light to the ghost-light coupled by the input-coupler to the waveguide.

4. The head-mounted display according to claim 2, wherein the pixels of the micro-light- emitting-diode display are arranged to generate a pupil hot-spot at the output pupil that is outside the area covered by the aperture to further increase the ratio of the display-light to the ghost-light coupled by the input-coupler to the waveguide.

5. The head-mounted display according to claim 1, wherein: the hot-spot of the input-coupler is off-center with respect to the output pupil; and the aperture is asymmetric with respect to the output pupil so that the hot-spot of the inputcoupler is not blocked by the aperture.

6. The head-mounted display according to claim 5, wherein the aperture is D-shaped.

7. The head-mounted display according to claim 5, wherein the aperture is crescent shaped.

8. The head-mounted display according to claim 1, wherein the area covered by the aperture is adjustable according to an ambient light level.

9. The head-mounted display according to claim 8, wherein: the area is adjusted to be smaller to block less light when the ambient light level is at a relatively high level; and the area is adjusted to be larger to block more light when the ambient light level is at a relatively low level.

10. The head-mounted display according to claim 1, wherein a transmittance of the aperture is adjustable according to an ambient light level.

11. The head-mounted display according to claim 10, wherein: the transmittance is adjusted to be higher to block less light when the ambient light level is at a relatively high level; and the transmittance is adjusted to be lower to block more light when the ambient light level is at a relatively low level.

12. The head-mounted display according to claim 11, wherein the aperture is liquid crystal.

13. The head-mounted display according to claim 11, wherein the aperture is electrochromic.

14. A method for reducing ghost images in a head-mounted display, the method comprising: projecting light to an input-coupler of a waveguide using a projector lens, the light including display-light emitted by pixels of a display and ghost-light reflected by surfaces of the display; covering an area of an output pupil of the projector lens using an aperture that is off-center with respect to the output pupil; and blocking the light that is outside a hot-spot of the input-coupler using the aperture, the hotspot having a coupling efficiency that is at or above a threshold-efficiency and the hotspot of the input-coupler being off-center with respect to the output pupil.

15. The method for reducing the ghost images in the head-mounted display according to claim14, wherein the blocking increases a ratio of the display -light to the ghost-light transmitted by the waveguide.

16. The method for reducing the ghost images in the head-mounted display according to claim15, further comprising: generating a pupil hot-spot at the output pupil that is outside the area covered by the aperture to further increase the ratio of the display-light to the ghost-light coupled by the input-coupler to the waveguide.

17. The method for reducing the ghost images in the head-mounted display according to claim 14, further comprising: adjusting an amount of the blocking using the aperture based on an ambient light condition.

18. The method for reducing the ghost images in the head-mounted display according to claim 17, wherein adjusting the amount of the blocking includes: adjusting a transmittance of the aperture based on the ambient light condition, the aperture being mechanically fixed.

19. The method for reducing the ghost images in the head-mounted display according to claim 17, wherein adjusting the amount of the blocking includes:mechanically moving the aperture to adjust the amount of blocking.

20. The method for reducing the ghost images in the head-mounted display according to claim17, wherein adjusting the amount of blocking includes: electronically controlling pixels of the aperture to adjust the amount of blocking.

21. An AR sy stem compri si n : a micro-light-emitting-diode display configured to emit display-light from pixels of the micro-light-emitting-diode display and reflect ghost-light from surfaces of the microlight-emitting-diode display; a projector lens configured to transmit the display-light and the ghost-light from the microlight-emitting-diode display to an output pupil; a waveguide configured to receive the display-light and the ghost-light from the output pupil at an input-coupler of the waveguide, the input-coupler having a hot-spot with a coupling efficiency that is at or above a threshold-efficiency; and an aperture covering an area of the output pupil to block light that is outside the hot-spot to increase a ratio of the display -light to the ghost-light coupled by the input-coupler to the waveguide.

22. The AR system according to claim 21, wherein the display-light at the output pupil has an intensity profile that includes a pupil hot-spot that is aligned by the micro-light-emitting- diode display and the projector lens to the hot-spot to further increase the ratio of the display-light to the ghost-light could by the input-coupler to the waveguide.

23. The AR system according to claim 21, wherein the aperture is adjustable to change an amount of the ghost-light blocked according to an ambient light condition.

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