Patent Description:
<CIT> relates to a near-eye-display (NED) including an eye tracking system and a waveguide display, the eye tracking system tracks locations based on a location of the user's eyes.

<CIT> relates to a display system comprising a display device operable to display an image, a first optical device positioned to receive light from the image displayed on the display device and reflect the light, and a second optical device positioned to receive light reflected by the first optical device and reflect the light back towards the first optical device.

<CIT> relates to a display assembly, comprising: a display device; a microlens array; and an eye tracker, for example a pupil tracker, and/or a head tracker.

One aspect of the present disclosure is set out in claim <NUM> appended hereto.

Another aspect of the present disclosure is set out in claim <NUM> appended hereto.

Other aspects of the present disclosure are set out in the dependent claims appended hereto.

The following drawings are provided for illustrative purposes according to various disclosed embodiments and are not intended to limit the scope of the present disclosure.

The present disclosure provides a waveguide display assembly capable of reducing the see-through artifacts, e.g., diffraction artifacts. The waveguide display assembly may be implemented into a near-eye display (NED). The waveguide display assembly may include a projector configured to generate an image light and a waveguide optically coupled with the projector and configured to guide the image light to an eye-box. The waveguide includes an in-coupling element configured to couple the image light into the waveguide, and an out-coupling element configured to decouple the image light out of the waveguide. The waveguide includes at least one switchable grating configured to: during a virtual-world subframe of a display frame, decouple the image light out of the waveguide via diffraction, and during a real-world subframe of the display frame, transmit a light from a real-world environment with a diffraction efficiency less than a predetermined threshold. In some embodiments, the predetermined threshold is about <NUM>%. In some embodiments, the predetermined threshold is about <NUM>%. In some embodiments, the predetermined threshold is about <NUM>%. In some embodiments, the at least one switchable grating is an out-coupling grating. The waveguide includes a plurality of switchable gratings. In some embodiments, each is configured to: during the virtual-world subframe of the display frame, perform at least one of directing, expanding or decoupling the image light out of the waveguide via diffraction, and during the real-world subframe of the display frame, transmit a light from a real-world environment with a diffraction efficiency less than a predetermined threshold.

During the real-world subframe, the display panel is switched off from generating the image light and the at least one switchable grating is switched to a non-diffracting state to transmit the light from a real-world environment with negligible diffraction, e.g., the diffraction efficiency is less than the predetermined threshold. During the virtual-world subframe, the display panel is switched on to generate the image light, and the at least one switchable grating is switched to a diffracting state to decouple the image light out of the waveguide via diffraction. A switching time of the virtual-world subframe and the real-world subframe may be sufficiently fast so that a user sees them combined without flicker, i.e., beyond a flicker fusion threshold. In other words, the real-world and virtual-world subframes are presented at a rate that exceeds the flicker fusion threshold of the user of the NED including the waveguide display assembly. In some embodiments, the flicker fusion threshold may be larger than or equal to about <NUM>.

The virtual-world subframe may have a shorter duration than the real-world subframe to minimize diffraction artifacts. To further reduce or eliminate the diffraction artifacts, the display may include an optical dimmer disposed at a side of the waveguide facing the real-world environment viewed through the NED to dim (including completely block) the light from the real-world environment during the virtual-world subframe. In some embodiments, the dimmer may also dim the light from the real-world environment during the real-world subframe according to the brightness of the real-world environment. The optical dimmer may be configured to have any number of light transmittance between <NUM> and <NUM>%, including <NUM> and <NUM>%. That is, the optical dimmer may completely transmit or completely block the incident light.

The present disclosure also provides a method for a waveguide display assembly. A display frame of a projector may be divided in two subframes for sequential transmission of light from real and virtual worlds, respectively. The method includes during a virtual-world subframe of the display frame, switching on the projector to generate an image light and switching at least one switchable grating to a diffracting state to decouple the image light out of a waveguide to an eye-box via diffraction. The method further includes during a real-world subframe of the display frame, switching off the projector from generating the image light and switching the at least one switchable grating to a non-diffracting state to transmit a light from a real-world environment to the eye-box with a diffraction efficiency less than a predetermined threshold. In some embodiments, the predetermined threshold is about <NUM>%. The duration or the time of the virtual-world subframe may be minimized in the display frame to minimize the diffraction artifacts. It may for example depend on brightness of the real-world environment. In some embodiments, the method may further include during the virtual-world subframe of the display frame, switching all switchable gratings to perform at least one of directing, expanding or decoupling the image light out of the waveguide to the eye-box via the diffraction; and during the real-world subframe of the display frame, switching all the switchable gratings to transmit the light from the real-world environment to the eye-box with the diffraction efficiency less than the predetermined threshold. In some embodiments, the method may further include switching an optical dimmer disposed at a side of the waveguide facing the real-world environment to block the light from a real-world environment during the virtual-world subframe. The method may further include switching the dimmer to dim (including completely block) the light from the real-world environment during the real-world subframe according to the brightness of the real-world environment.

<FIG> illustrates a schematic diagram of a near-eye display (NED) <NUM> according to an embodiment of the disclosure. In some embodiments, the NED <NUM> may be referred to as a head-mounted display (HMD). The NED <NUM> may present media to a user. Examples of media presented by the NED <NUM> include one or more images, video, audio, or some combination thereof. In some embodiments, audio is presented via an external device (e.g., speakers and/or headphones) that receives audio information from the NED <NUM>, a console (not shown), or both, and presents audio data based on the audio information. The NED <NUM> acts as a virtual reality (VR) device, an augmented reality (AR) device or a mixed reality (MR) device, or some combination thereof. In some embodiments, when the NED <NUM> acts as an augmented reality (AR) or a mixed reality (MR) device, portions of the NED <NUM> and its internal components may be at least partially transparent.

As shown in <FIG>, the NED <NUM> may include a frame <NUM> and a display <NUM>. Certain device(s) may be omitted, and other devices or components may also be included. The frame <NUM> may include any appropriate type of mounting structure to ensure the display assembly <NUM> to be viewed as a near-eye display (NED) by a user. The frame <NUM> may be coupled to one or more optical elements which together display media to users. In some embodiments, the frame <NUM> may represent a frame of eye-wear glasses. The display <NUM> is configured for users to see the content presented by the NED <NUM>. As discussed below in conjunction with <FIG>, the display <NUM> may include at least one display assembly (not shown) for directing image light to an eye of the user.

<FIG> is a cross-section <NUM> of the NED <NUM> shown in <FIG> according to an embodiment of the disclosure. The display <NUM> may include at least one waveguide display assembly <NUM>. An exit pupil <NUM> may be a location where the eye <NUM> is positioned in an eye-box region when the user wears the NED <NUM>. For purposes of illustration, <FIG> shows the cross section <NUM> associated with a single eye <NUM> and a single waveguide display assembly <NUM>, but in alternative embodiments not shown, another display assembly which is separate from the waveguide display assembly <NUM> shown in <FIG>, may provide image light to an eye-box located at an exit pupil of another eye of the user.

The waveguide display assembly <NUM>, as illustrated below in <FIG>, is configured to direct the image light to an eye-box located at the exit pupil <NUM> of the eye <NUM>. The waveguide display assembly <NUM> may be composed of one or more materials (e.g., plastic, glass, etc.) with one or more refractive indices that effectively minimize the weight and widen a field of view (FOV) of the NED <NUM>. In some embodiments, the waveguide display assembly <NUM> may be a component (e.g., the display <NUM>) of the NED <NUM>. In some embodiments, the waveguide display assembly <NUM> may be part of some other NED, or other system that directs display image light to a particular location. As shown in <FIG>, the waveguide display assembly <NUM> may be for one eye <NUM> of the user. The waveguide display assembly <NUM> for one eye may be separated or partially separated from the waveguide display assembly <NUM> for the other eye. In certain embodiments, a single waveguide display assembly <NUM> may be used for both eyes <NUM> of the user.

In some embodiments, the NED <NUM> may include one or more optical elements between the waveguide display assembly <NUM> and the eye <NUM>. The optical elements may act to, e.g., correct aberrations in image light emitted from the waveguide display assembly <NUM>, magnify image light emitted from the waveguide display assembly <NUM>, some other optical adjustment of image light emitted from the waveguide display assembly <NUM>, or some combination thereof. The example for optical elements may include an aperture, a Fresnel lens, a convex lens, a concave lens, a filter, or any other suitable optical element that affects image light. Exemplary waveguide display assembly <NUM> will be described in detail below in conjunction with <FIG> and <FIG>.

<FIG> illustrates a schematic diagram of a waveguide display assembly <NUM> of the NED in <FIG> according to an embodiment of the disclosure. The waveguide display assembly <NUM> may be the waveguide display assembly <NUM> in <FIG>. As shown in <FIG>, the waveguide display assembly <NUM> may include a source assembly <NUM>, a waveguide <NUM>, and a controller <NUM>. The source assembly <NUM> may be a projector <NUM> that includes a source <NUM> and an optics system <NUM>. The source <NUM> may be a light source that generates coherent or partially coherent light. The source <NUM> may include, e.g., a laser diode, a vertical cavity surface emitting laser, and/or a light emitting diode. In some embodiments, the source <NUM> may be a display panel, such as a liquid crystal display (LCD) panel, an liquid-crystal-on-silicon (LCoS) display panel, an organic light-emitting diode (OLED) display panel, a micro-LED (micro light-emitting diode) display panel, a digital light processing (DLP) display panel, or some combination thereof. In some embodiments, the source <NUM> may be a self-emissive panel, such as an OLED display panel or a micro-LED display panel. In some embodiments, the source <NUM> may be a display panel that is illuminated by an external source, such as an LCD panel, an LCoS display panel, or a DLP display panel. Examples of an external sources may include a laser, an LED, an OLED, or some combination thereof. The optics system <NUM> may include one or more optical components that condition the light from the source <NUM>. Conditioning light from the source <NUM> may include, e.g., transmitting, attenuating, expanding, collimating, and/or adjusting orientation in accordance with instructions from the controller <NUM>.

The projector <NUM> may generate image light <NUM> and output the image light <NUM> to an in-coupling element <NUM> located at the waveguide <NUM>. The waveguide <NUM> may receive the image light <NUM> at one or more in-coupling elements <NUM>, and guide received image light <NUM> to an out-coupling element <NUM> located at the waveguide <NUM>, such that the received input image light <NUM> is decoupled out of the waveguide <NUM> via the out-coupling element <NUM> towards the eye <NUM> of the user.

The waveguide <NUM> may include a first surface <NUM>-<NUM> facing the real-world and an opposing second surface <NUM>-<NUM> facing the eye <NUM>. The in-coupling element <NUM> may be part of, or affixed to, the first surface <NUM>-<NUM> or the second surface <NUM>-<NUM> of the waveguide <NUM>. In some embodiments, as shown in <FIG>, the in-coupling element <NUM> may be part of, or affixed to, the first surface <NUM>-<NUM> of the waveguide <NUM>. In some embodiments, the in-coupling element <NUM> may be part of, or affixed to, the second surface <NUM>-<NUM> of the waveguide <NUM>. In some embodiments, the in-coupling element <NUM> may include a surface relief grating, a volume hologram, a polarization grating, a polarization volume hologram, a metasurface, or other types of diffractive elements, or some combination thereof. In some embodiments, the in-coupling element <NUM> may include a diffraction grating, and a pitch of the diffraction grating may be chosen such that the total internal reflection occurs in the waveguide <NUM>, and the image light <NUM> may propagate internally in the waveguide <NUM> (e.g., by total internal reflection). The in-coupling element <NUM> is also referred to as an in-coupling grating.

The out-coupling element <NUM> may be part of, or affixed to, the first surface <NUM>-<NUM> or the second surface <NUM>-<NUM> of the waveguide <NUM>. In some embodiments, as shown in <FIG>, the out-coupling element <NUM> may be part of, or affixed to, the second surface <NUM>-<NUM> of the waveguide <NUM>. In some embodiments, the out-coupling element <NUM> may be part of, or affixed to, the first surface <NUM>-<NUM> of the waveguide <NUM>. In some embodiments, the out-coupling element <NUM> may include a surface relief grating, a volume hologram, a polarization grating, a polarization volume hologram, a metasurface, or other types of diffractive elements, or some combination thereof. In some embodiments, the out-coupling element <NUM> may include a diffraction grating, and the pitch of the diffraction grating may be configured to cause the incident image light <NUM> to exit the waveguide <NUM>, i.e. redirecting image light <NUM> so that total internal reflection no longer occurs. Such a grating is also referred to as an out-coupling grating.

In some embodiments, the waveguide display assembly <NUM> may include additional gratings that redirect/fold and/or expand the pupil of the projector <NUM>, and an exemplary structure of the waveguide display assembly will be described in conjunction with <FIG>. In some embodiments, multiple functions, e.g., redirecting/folding and/or expanding the pupil of the projector <NUM> may be combined into a single grating, e.g. an out-coupling grating. In some embodiments, some above-mentioned gratings may be divided in several sections (subgratings), for example for tiling field of view (FOV).

The waveguide <NUM> may be composed of one or more materials that facilitate total internal reflection of the image light <NUM>. The waveguide <NUM> may be composed of, for example, plastic, glass, and/or polymers, or some combination thereof. The waveguide <NUM> may have a relatively small form factor. For example, the waveguide <NUM> may be approximately <NUM> wide along the x-dimension, <NUM> long along the y-dimension and <NUM>-<NUM> thick along the z-dimension. In some embodiments, the waveguide display assembly <NUM> may include a stack of waveguides, where each waveguide is designed to handle, e.g., some portion of the FOV and color spectrum of the virtual image.

The controller <NUM> may control the operations of the source assembly <NUM>, and determine scanning instructions for the source assembly <NUM>. In some embodiments, the waveguide <NUM> may output the expanded image light <NUM> to the eye <NUM> with a large FOV. For example, the expanded image light <NUM> may be provided to the eye <NUM> with a diagonal FOV (in x and y) of <NUM> degrees and or greater and/or <NUM> degrees and/or less. The waveguide <NUM> may be configured to provide an eye-box with a width of <NUM> or greater and/or equal to or less than <NUM>, and/or a height of <NUM> or greater and/or equal to or less than <NUM>.

In some embodiments, the waveguide display assembly <NUM> may include a plurality of source assemblies <NUM> and a plurality of waveguides <NUM>. Each of the source assemblies <NUM> may emit a monochromatic image light of a specific wavelength band corresponding to a primary color (e.g., red, green, or blue). Each of the waveguides <NUM> may be stacked together with a distance of separation to output an expanded image light <NUM> that is multi-colored. Using the waveguide display assembly <NUM>, the physical display and electronics may be moved to the side of the front rigid body and a fully unobstructed view of the real world may be achieved, therefore opening up the possibilities to true AR experiences.

To reduce the artifacts caused by the diffractive structures at the waveguide <NUM>, the projector <NUM> may have a high turning on and off speed and, desirably, increased brightness. Such a projector is referred to as a low-persistence projector or a high-speed projector. During an operation of the waveguide display assembly <NUM>, each frame may include two subframes: a real-world subframe and a virtual-world subframe. The projector <NUM> may be switched on to display a virtual image during the virtual-world subframe, and switched off to stop displaying a virtual image during the real-world subframe, in accordance with instructions from the controller <NUM>. The switching on and off of the projector <NUM> may be sufficiently fast such that the real-world and virtual-world subframes are presented at a rate that exceeds a flicker fusion threshold of the user, i.e., beyond a flicker fusion threshold.

In some embodiments, the projector <NUM> may be switched on to display a virtual image at the beginning of the virtual-world subframe, and switched off to stop displaying the virtual image at the beginning of the real-world subframe. The projector <NUM> may include any appropriate projectors of fast switching, such as a DLP (digital light processing) projector, an LCoS (liquid crystal on silicon) projector (for example, ferroelectric LCoS (FLCoS) projector), a LCD (liquid crystal display) projector, an OLED (organic light-emitting diode) projector, or a micro-LED (light-emitting diode) projector, or some combination thereof. In some embodiments, the fast on/off switching of the projector <NUM> may be realized by using a fast-switchable light source, such as a laser, an LED, an OLED or some combination thereof.

In some embodiments, the fast on/off switching of the projector <NUM> may be realized by using a high-speed optical shutter to reduce the switching time of the projector <NUM>, at an expense of light efficiency. The high-speed optical shutter may be capable of being selectively switched between an opaque state (off-state) for blocking an incident light and a transparent state (on-state) for transmitting an incident light. The switchable optical shutter may be configured to be switched between, ideally, <NUM> and <NUM>% light transmittance. In some embodiments, during the real-world subframe, the high-speed optical shutter may be switched to the opaque state blocking the image light <NUM>, and during the virtual-world subframe, the high-speed optical shutter may be switched to the transparent state transmitting the image light <NUM>. The switching on and off of the high-speed optical shutter may be sufficiently fast such that the real-world and virtual-world subframes are presented at a rate and switch from one to another with a speed that exceeds a flicker fusion threshold of the user.

In some embodiments, the high-speed optical shutter may be disposed in front of the projector <NUM>. In some embodiments, the high-speed optical shutter may be disposed in front of the light source <NUM> of the projector <NUM>. In one embodiment, as shown in <FIG>, the optical system <NUM> may include a high-speed optical shutter <NUM> that is disposed in front of the light source <NUM> and controlled by the controller <NUM>. In some embodiments, the high-speed optical shutter <NUM> may be a switchable shutter having fast switching speed, for example, in the order of milliseconds (ms) or microseconds (µs). In some embodiments, the high-speed optical shutter <NUM> may include liquid crystal (LC) materials, which is referred to as an LC shutter. Examples of LC shutters will be described in <FIG>.

Further, the out-coupling element <NUM> may include a high-speed switchable out-coupling grating capable of being selectively switched between an on-state (or a diffracting state) having a grating effect of diffracting light and an off-state (or a non-diffracting state) that transmits light with negligible diffraction, for example, with a diffraction efficiency less than a predetermined threshold, in accordance with instructions from the controller <NUM>. In some embodiments, the predetermined threshold may be about <NUM>%. In some embodiments, the predetermined threshold may be about <NUM>%. In some embodiments, the predetermined threshold may be about <NUM>%. For illustrative purposes, <FIG> shows the waveguide <NUM> includes one out-coupling grating <NUM>. The waveguide <NUM> includes a plurality of out-coupling gratings each is capable of being selectively switched between the on-state (or the diffracting state) having a grating effect of diffracting light and the off-state (or the non-diffracting state) that transmits light with negligible diffraction, for example, with the diffraction efficiency less than the predetermined threshold.

In some embodiments, the high-speed switchable out-coupling grating may include an LC layer where the grating structures are formed. In some embodiments, the out-coupling element <NUM> may be switched to the on-state to diffract an incident light during the virtual-world subframe, and switched to the off-state to transmit an incident light with negligible diffraction during the real-world subframe. The switching of the out-coupling element <NUM> may be sufficiently fast such that the real-world and virtual-world subframes are presented at a rate that exceeds a flicker fusion threshold of the user. In some embodiments, the out-coupling element <NUM> may be switched to the on-state at the beginning of the virtual-world subframe, and switched to the off-state at the beginning of the real-world subframe. It should also be understood that any additional grating arranged at the waveguide <NUM> that are visible to the user (e.g. fold gratings, pupil expansion gratings), may also be switchable in addition to the out-coupling element <NUM>.

During the virtual-world subframe, the out-coupling element <NUM> may decouple the image light <NUM> out of the waveguide <NUM> via the diffraction and, thus, the eye <NUM> may observe a virtual image. Meanwhile, light <NUM> from a real-world environment (i.e., real-world light <NUM>) may also be diffracted by the out-coupling element <NUM> when the real-world light <NUM> could be transmitted to be incident onto the out-coupling element <NUM>. During the real-world subframe, the real-world light <NUM> may be directly transmitted through the out-coupling element <NUM> with the negligible diffraction. When the virtual-world subframe is controlled to only last for a short period of the total frame period, the out-coupling element <NUM> may have a negligible impact on the see-through view, such that the see-through artifacts caused by the diffractive structures of the out-coupling element <NUM> at the waveguide <NUM> may be minimized. In some embodiments, the out-coupling element <NUM> may include a polarization sensitive grating, which has a grating effect of diffracting light having a first polarization direction, and no grating effect but transmitting light having a second polarization direction with the negligible diffraction.

The second polarization direction may be different from the first polarization direction. In some embodiments, the out-coupling element <NUM> may be configured to diffract the image light <NUM> and transmit the real-world light <NUM> with negligible diffraction, where the image light <NUM> and the real-world light <NUM> may have different polarization directions, e.g., orthogonal polarization direction. The polarization of the real-world light may, for example, be controlled with a polarizer placed at the first surface <NUM>-<NUM> of the waveguide <NUM> facing the real-world environment viewed through the NED. The real-world light <NUM> may become a polarized light after transmitted through the polarizer, then incident onto the waveguide <NUM>. In some embodiments, the polarizer may be a linear polarizer. In some embodiments, the polarizer may be a circular polarizer. Further, polarization of the polarized light may change by traveling in the waveguide <NUM> so that a polarization correction element (e.g., an anisotropic plate disposed adjacent to the waveguide <NUM>) may be desired. The polarization sensitive grating may be switchable or non-switchable.

When the out-coupling element <NUM> includes a polarization sensitive grating, during the virtual-world subframe, the out-coupling element <NUM> may diffract the image light <NUM> but transmit the real-world light <NUM> with negligible diffraction. During the real-world subframe, the out-coupling element <NUM> may not receive the image light <NUM>, but transmit the real-world light <NUM> with negligible diffraction. When the display time of the virtual-world subframe is much shorter than the display time of the real-world subframe, for example, when the display time of the virtual-world subframe is <NUM>% of the total frame period, the see-through artifacts caused by the diffractive structures of the out-coupling element <NUM> at the waveguide <NUM> may be significantly reduced. In some embodiments, each display frame may be no longer divided into the virtual-world subframe and the real-world subframe because these two operation may happen simultaneously. That is, during the total display frame, the out-coupling element <NUM> may diffract the image light <NUM> but transmit the real-world light <NUM> with negligible diffraction. The advantage of dividing the display frame into the virtual-world subframe and the real-world subframe is that, even when a polarizer is arranged at the first surface <NUM>-<NUM> of the waveguide <NUM> facing the real-world environment, the real-world light incident onto the polarizer at a high incident angle (e.g., larger than <NUM>°) may be not completely polarized. A polarization component that is not perfectly absorbed by the polarizer may be incident onto and diffracted by the out-coupling element <NUM>, resulting a rainbow effect. Exemplary high-speed switchable LC gratings and polarization sensitive gratings will be described in <FIG>.

<FIG> illustrates an operation of the waveguide display shown in <FIG> according to an embodiment of the disclosure. As shown in <FIG>, during the operation of the waveguide display, each frame may include two subframes: a real-world subframe (stage <NUM>) and a virtual-world subframe (stage <NUM>). The virtual-world subframe may have a shorter duration than the real-world subframe. For example, the real-world subframe may last for <NUM>% to <NUM>%, <NUM>% to <NUM>%, or <NUM>% to <NUM>% of time of the total frame period, and the real-world subframe may last for <NUM>% to <NUM>%, <NUM>% to <NUM>%, or <NUM>% to <NUM>% time of the total frame period.

Referring to <FIG> and <FIG>, during the real-world subframe (stage <NUM>), the projector <NUM> may be switched to the off-state that does not display a virtual image and/or the high-speed optical shutter <NUM> may be switched to the opaque state to block the image light <NUM> and, accordingly, the out-coupling element <NUM> may not receive the image light <NUM>. Meanwhile, the out-coupling element <NUM> may be switched to the off-state to transmit the real-world light <NUM> with negligible diffraction. During the virtual-world subframe (stage <NUM>), the projector <NUM> may be switched to the on-state that displays a virtual image and/or the high-speed optical shutter <NUM> may be switched to the transparent state to transmit the image light <NUM> and, accordingly, the out-coupling element <NUM> may receive the image light <NUM> via the in-coupling element <NUM>. Meanwhile, the out-coupling element <NUM> may be switched to the on-state to diffract both the image light <NUM> and the real-world light <NUM> incident on the out-coupling element <NUM>, such that the image light <NUM> may be decoupled out of the waveguide <NUM> to be incident onto the eye <NUM>, and the user may observe the virtual image.

That is, in one frame, the projector <NUM> may be switched on and/or the high-speed optical shutter <NUM> may be in the transparent state, and the out-coupling element <NUM> may exhibit the grating effect for a short period of the total frame period (e.g. <NUM>% to <NUM>%, <NUM>% to <NUM>%, or <NUM>% to <NUM>% of the total frame period) for the user to observe the virtual image, and the projector <NUM> may be switched off and/or the high-speed optical shutter <NUM> may be in the opaque state, and the out-coupling element <NUM> may transmit light with negligible diffraction for the remainder of the total frame period (e.g. <NUM>% to <NUM>%, <NUM>% to <NUM>%, or <NUM>% to <NUM>% of the total frame period) for the see-through view. Such a projector is referred to as a low persistence projector. Pulse-like operation of the low persistence projector may decrease power consumption of the NED.

For example, when a total frame period is <NUM>, the projector <NUM> may be switched on and/or the high-speed optical shutter <NUM> may be in the transparent state, and the out-coupling element <NUM> may exhibit the grating effect for <NUM> to <NUM> for the virtual image. Thus, the out-coupling element <NUM> may have a negligible impact on the see-through view, and the rainbowing effects caused by the diffractive structures of the out-coupling element <NUM> may not be visible in the see-through view. For example, when the out-coupling element <NUM> exhibits the grating effect only for <NUM>% of the total frame period (i.e., the stage <NUM> lasts for <NUM>% of the total frame period), it is expected that see-through artifacts may be reduced to about <NUM>% as compared to the situation where the out-coupling element <NUM> exhibits the grating effect throughout the total frame period.

In some embodiments, when the out-coupling element <NUM> includes a high-speed switchable grating, during the real-world subframe (stage <NUM>), the out-coupling element <NUM> may be switched to the off-state (or the non-diffracting state) to transmit the real-world light <NUM> with negligible diffraction. During the virtual-world subframe (stage <NUM>), the out-coupling element <NUM> may be switched to the on-state (or the diffracting state) to diffract both the image light <NUM> and the real-world light <NUM>. In some embodiments, when the out-coupling element <NUM> includes a polarization sensitive grating, during the real-world subframe (stage <NUM>), the out-coupling element <NUM> may be configured to receive the real-world light <NUM> having the second polarization direction and transmit the real-world light <NUM> with negligible diffraction. During the virtual-world subframe (stage <NUM>), the out-coupling element <NUM> may be configured to receive both the image light <NUM> having the first polarization direction and the real-world light <NUM> having the second polarization direction, and diffract the image light <NUM> but transmit the real-world light <NUM> with negligible diffraction. The polarization of the real-world light <NUM> may be controlled by disposing a linear polarizer at the first surface <NUM>-<NUM> of the waveguide <NUM>. In some embodiments, each display frame may be no longer divided into the virtual-world subframe and the real-world subframe because these two operation may happen simultaneously. That is, during the total frame period of the display frame, the out-coupling element <NUM> may diffract the image light <NUM> but transmit the real-world light <NUM> with negligible diffraction. Thus, the see-through artifacts caused by the diffractive structures of the out-coupling element <NUM> at the waveguide <NUM> may be significantly reduced.

Referring to <FIG>, in some embodiments, in addition to the out-coupling element <NUM>, the in-coupling element <NUM> may also include a grating similar to the out-coupling element <NUM>. The grating may be a high-speed switchable grating that exhibits a grating effect for a short period of the total frame. The in-coupling element <NUM> may be periodically switched off and on together with the out-coupling element <NUM>, in accordance with instructions from the controller <NUM>. The details may be referred to the out-coupling element <NUM> and are not repeated herein.

In some embodiments, the waveguide display assembly <NUM> may further include a high-speed optical dimmer <NUM> disposed at the first side <NUM>-<NUM> of the waveguide <NUM>, i.e., a side facing the real-world environment. In some embodiments, the dimmer <NUM> may be activated to block the see-through view when the out-coupling element <NUM> exhibits the grating effect, through which the see-through artifacts may be reduced to nearly <NUM>% at the expense of a see-through attenuation. For example, in the real-world subframe that lasts for <NUM>% of the total frame period, the dimmer <NUM> may be switched to the transparent state to transmit the real-world light <NUM>, and in the virtual-world subframe that lasts for <NUM>% of the total frame period, the dimmer <NUM> may be switched to the opaque state to block the real-world light <NUM>, such that the see-through artifacts may be almost eliminated at the expense of a <NUM>% reduction of the see-through brightness. However, the dark background in case of see-though attenuation may increase contrast of virtual images demonstrated in this subframe. In addition, the use of the dimmer <NUM> may also allow optimizing the grating structures for a virtual image, rather than finding a trade-off between the diffraction of the virtual image and the see-through view quality (including minimization of rainbow) for real world light. In some embodiments, the dimmer <NUM> may adaptively dim an incident light, i.e., the dimmer <NUM> may function as a controllable dimming element rather than a shutting element with only two transmittance states. The attenuation provided by the dimmer <NUM> may be controlled by, for example, an external electric field, a magnetic field, or light or some combination thereof. Exemplary high-speed optical shutters or dimmers will be described in <FIG>.

<FIG> illustrates a cross-section of another waveguide display assembly <NUM> according to an embodiment of the disclosure. The similarities between <FIG> and <FIG> are not repeated, while certain differences may be explained. In some embodiments, as shown in <FIG>, the waveguide display assembly <NUM> may further include a directing element <NUM> that redirects the received input image light <NUM> to the out-coupling element <NUM>, such that the received input image light <NUM> is decoupled out of the waveguide <NUM> via the out-coupling element <NUM>. The directing element <NUM> may be part of, or affixed to, the first side <NUM>-<NUM> of the waveguide <NUM>. The out-coupling element <NUM> may be part of, or affixed to, the second side <NUM>-<NUM> of the waveguide <NUM>, such that the directing element <NUM> is arranged opposed to the out-coupling element <NUM>.

In some embodiments, the directing element <NUM> and the out-coupling element <NUM> may be structurally similar. The directing element <NUM> may include a surface relief grating, a volume hologram, a polarization grating, a polarization volume hologram, a metasurface, or other types of diffractive elements or some combination thereof. In some embodiments, the directing element <NUM> may be a diffraction grating, and in this case the directing element <NUM> is also referred to as a folding grating. In some embodiments, the waveguide display assembly <NUM> may include the directing element <NUM> but not include the out-coupling element <NUM>, and the directing element <NUM> may be disposed at the first surface <NUM>-<NUM> or the second surface <NUM>-<NUM> of the waveguide <NUM>. In this case, the directing element <NUM> may function similarly to the out-coupling element <NUM> in <FIG>, i.e., the directing element <NUM> may cause light to exit the waveguide <NUM>.

In some embodiments, similar to the out-coupling element <NUM>, the directing element <NUM> may also include a high-speed switchable grating capable of being selectively switched between an on-state (or a diffracting state) having a grating effect of diffracting incident light and an off-state (or a non-diffracting state) transmitting incident light with negligible diffraction, in accordance with instructions from the controller <NUM>. In some embodiments, similar to the out-coupling element <NUM>, the directing element <NUM> may also include a polarization sensitive grating, which has a grating effect of diffracting incident light having a first polarization direction, and no grating effect but transmitting incident light having a second polarization direction different from the first polarization direction. When the directing element <NUM> includes a high-speed switchable grating c, the directing element <NUM> may be periodically switched off and on together with the out-coupling element <NUM>. Similar to the out-coupling element <NUM>, the grating effect of the directing element <NUM> may be configured to last for a short period of the total frame period during the operation of the display assembly <NUM>. Thus, the directing element <NUM> may also have a negligible impact on the see-through view, such that the see-through artifacts caused by the diffractive structures of the directing element <NUM> at the waveguide <NUM> may be suppressed. The details may be referred to the out-coupling element <NUM> and are not repeated herein.

For illustrative purposes, <FIG> and <FIG> shows the projector <NUM>, the out-coupling element <NUM>, the dimmer <NUM>, the directing element <NUM>, and the in-coupling element <NUM> may be all controlled by the controller <NUM>. In some embodiments, the projector <NUM>, the out-coupling element <NUM>, the dimmer <NUM>, the directing element <NUM>, and the in-coupling element <NUM> may be controlled by individual controllers, or some of them may share a controller, which is not limited by the present disclosure.

Further, in addition to the diffractive components at the waveguide display assembly <NUM> (e.g., the out-coupling element <NUM>, the directing element <NUM>, the in-coupling element <NUM>, which are referred to as waveguide gratings), the NED may also include other diffractive components such as eye tracking combiner, accommodation lenses, etc., which may also cause rainbow artifacts that is not so strong as the waveguide gratings. In some embodiments, to maximally suppress the rainbow artifacts caused by the diffractive components, all these elements (all diffractive components, projector and optical shutters) may be configured to be switchable, for example, switched to the on-state during the virtual-world subframe and switched to the off-state during the real-world subframe. In some embodiments, a switching of all diffractive elements, projector and shutters may be desired be sufficiently fast such that the real-world and virtual-world subframes are presented at a rate that exceeds a flicker fusion threshold of the user, i.e., beyond a flicker fusion threshold.

In some embodiments, to suppress the rainbow artifacts caused by the diffractive components, among all diffractive components, projector and optical shutters, at least the projector <NUM> and the out-coupling element <NUM> may be switched to the on-state during the virtual-world subframe and switched to the off-state during the real-world subframe. In some embodiments, a switching of the projector <NUM> and the out-coupling element <NUM> may be sufficiently fast such that the real-world and virtual-world subframes are presented at a rate that exceeds a flicker fusion threshold of the user, i.e., beyond a flicker fusion threshold. In some embodiments, the out-coupling element <NUM> and an optical shutter capable of reducing the switching time of the projector <NUM> may be configured to be switched on and switched off in a sufficiently fast way such that the real-world and virtual-world subframes are presented at a rate that exceeds a flicker fusion threshold of the user, i.e., beyond a flicker fusion threshold. Such a simplification is on expense of some weak rainbow artifacts caused by diffractive components other than the waveguide gratings.

In some embodiments, to suppress the rainbow artifacts caused by the diffractive components, among all diffractive components, projector and optical shutters, the projector <NUM>, the out-coupling element <NUM> and the directing element <NUM> may be switched to the on-state during the virtual-world subframe and switched the off-state during the real-world subframe. In some embodiments, a switching of the projector <NUM>, the out-coupling element <NUM> and the directing element <NUM> may be sufficiently fast such that the real-world and virtual-world subframes are presented at a rate that exceeds a flicker fusion threshold of the user, i.e., beyond a flicker fusion threshold. In some embodiments, the out-coupling element <NUM>, the directing element <NUM>, and an optical shutter capable of reducing the switching time of the projector <NUM> may be configured to be switched on and switched off in a sufficiently fast way such that the real-world and virtual-world subframes are presented at a rate that exceeds a flicker fusion threshold of the user, i.e., beyond a flicker fusion threshold. Such a simplification is on expense of some weak rainbow artifacts caused by diffractive components other than the waveguide gratings.

In the following, exemplary high-speed switchable LC shutters will be explained. Switchable LC shutters may be divided into different categories based on operation principle, such as an LC shutter based on polarization, an LC shutter based on absorption, an LC shutter based on scattering, and an LC shutter based on scattering and absorption, etc..

An LC shutter based on polarization may be simply a liquid crystal display (LCD) that has a single large cell or "pixel" that covers the entire display area. The shutter may be simply "open" (in a transparent state) or "closed" (in an opaque state). The shutter may be switched between its open and closed state by applying a simple square wave drive voltage. Examples of LC shutters based on polarizations may include a nematic LC shutter, a ferroelectric LC (FLC) shutter, a guest-host LC shutter, a polymer stabilized blue phase LC (BPLC) shutter, etc. A nematic LC shutter having a normal twist nematic LC (TNLC) cell may be only switched at a rate of about <NUM> to <NUM>. For faster switching rates up to about <NUM>, a special type of LC cell called a Pi-cell may be used in the LC shutter. The name of Pi-cell comes from the twist of LC molecules, which is <NUM>° formed by the parallel alignment directions on two opposite substrates sandwiching an LC layer. The fast speed is the result of fluid dynamics. When the voltage is removed from a Pi-cell operated at a relative high voltage state (e.g., a homeotropic state), the LC molecules may feel very little torque to return to their low voltage state (e.g., a bend state). In the operation of the Pi-cell, a "holding" voltage at the desired low voltage state may be desired to maintain pure polarization switching, and the Pi-cell is operated between a high voltage state (e.g., about 10V) and a low holding voltage state (e.g., about 2V) with fast switching speed in the order of milliseconds (ms). In addition, the Pi-cell may have naturally high viewing angles due to the symmetrical LC molecules alignment at surface boundaries. High contrast is easily achieved with a compensation film to subtract residual birefringence at the surfaces.

<FIG> illustrates a schematic diagram of a Pi-cell LC shutter <NUM> according to an embodiment of the disclosure. As shown in <FIG>, the Pi-cell LC shutter <NUM> may include a first linear polarizer <NUM> that is referred to as a polarizer, a second linear polarizer <NUM> that is referred to as an analyzer, and an LC cell <NUM> sandwiched between the polarizer <NUM> and the analyzer <NUM>. The polarizer <NUM> and the analyzer <NUM> may be crossed linear polarizers, i.e., a transmission axis of the polarizer <NUM> may be arranged orthogonal to a transmission axis of the analyzer <NUM>. The transmission axes of the polarizer <NUM> and the analyzer <NUM> may be disposed at angles of plus or minus <NUM>° to <NUM>°with respect to an alignment direction of the LC cell <NUM>. In some embodiments, the transmission axes of the polarizer <NUM> and the analyzer <NUM> may be disposed at angles of plus or minus <NUM>°with respect to an alignment direction of the LC cell <NUM>.

The LC cell <NUM> may include upper and lower substrates <NUM> arranged opposite to each other. The substrates <NUM> may be substantially transparent in the visible band (about <NUM> to about <NUM>). In some embodiments, the substrates <NUM> may also be transparent in some or all of the infrared (IR) band (about <NUM> to about <NUM>). The substrates <NUM> may include a suitable material that is substantially transparent to the light of above-listed wavelengths range, e.g., glass, plastic, sapphire, etc. A conductive electrode <NUM> such as an indium tin oxide (ITO) electrode may be disposed on opposing surfaces of the substrates <NUM> to apply an electric field to the LC cell <NUM>.

An alignment layer <NUM> may be disposed on opposing surfaces of the electrodes <NUM>, and an LC layer including nematic LC molecules <NUM> may be sandwiched between the two alignment layers <NUM>. The two alignment layers <NUM> may be configured with a homogeneous parallel alignment direction, for example, in an x-direction indicated by an arrow <NUM>, through which the nematic LC molecules <NUM> near the upper and lower substrates <NUM> may be oriented in a parallel direction. The directors of the LC molecules <NUM> may be twisted by <NUM>° one substrate to the other, formed by the parallel alignment direction (e.g., parallel rubbing direction) on each substrate <NUM>. As a comparison, in a TNLC cell, the alignment directions on two substrates are perpendicular to each other and, thus, a <NUM>° twist of LC directors from one substrate to the other is formed inside the TNLC cell.

When a relatively low electric field is applied (e.g., when a relatively low voltage Vi (e.g., Vi=2V) is applied), as shown in <FIG>, the LC cell <NUM> may be operated at a bend state in which the LC molecules <NUM> at the middle of the LC layer are reoriented by the electric field E to be perpendicular to the substrates <NUM>, while other LC molecules <NUM> are still oriented parallel to the alignment direction <NUM> because of the surface constraints. In some embodiments, a polarization direction of an incident light <NUM> may be parallel to the transmission axis of the polarizer <NUM>. After propagating through the LC cell <NUM>, the polarization direction of the incident light <NUM> may be rotated by <NUM>° to be parallel to the transmission axis of the analyzer <NUM>, such that the incident light <NUM> may be transmitted through the analyzer <NUM>, realizing a transparent state of the LC shutter <NUM>.

When a relatively high electric field is applied (e.g., when a relatively high voltage V<NUM> (e.g., V<NUM>=10V) is applied), as shown in <FIG>, the LC cell <NUM> may be switched to a homeotropic state, in which the majority of the LC molecules <NUM> are reoriented by the electric field E to be perpendicular to the substrates <NUM>. After propagating through the LC cell <NUM>, the polarization direction of the incident light <NUM> may substantially remain the same, which is perpendicular to the transmission axis of the analyzer <NUM>. Thus, the incident light <NUM> may be blocked by the analyzer <NUM>, realizing an opaque state of the LC shutter <NUM>. Through switching the LC cell <NUM> between a relative low voltage state (e.g., a bend state) and a relative high voltage state (e.g., a homeotropic state), the Pi-cell LC shutter <NUM> may be switched between a transparent state (i.e., incident light is transmitted) and an opaque state (i.e., incident light is block), and the switching time may be substantially fast, e.g., in the order of millisecond (ms).

An FLC shutter could offer a superfast switching typically less than <NUM> microsecond (µs), excellent viewing angle and low residual retardance due to intrinsic in-plane switching behavior of the FLC molecules. The FLC shutter may include an FLC cell sandwiched between a crossed polarizer and analyzer. The FLC cell may include an FLC layer sandwiched between two opposite substrates, and each substrate may be disposed with an electrode and an alignment layer. Within the ferroelectric smectic C* phase (the symbol "*" refers to the chiral nature), FLC molecules in the FLC layer may be arranged in a layered geometry where the smectic layers are perpendicular to the substrates, and the directors of the FLC molecules may move along the surface of a cone whose axis is normal to the smectic layers and parallel to the substrates. The FLC molecules may have a helical structure with spontaneous polarization perpendicular to the FLC molecules which are tilted with respect to the normal of the smectic layers, and a tilt angle is θ. In a thin cell of surface-stabilized configuration where the thickness of the LC cell is much smaller than the helix pitch, the helixes of the FLC may be unwound and, thus, resulting in a net spontaneous polarization. Such a FLC mode is known as surface stabilized (SSFLC) mode. In some embodiments, the thickness of LC layer is comparable with helix pitch, the complete suppression of helical structure may be achieved only when the electric field is applied. Such a FLC mode is known as electrically suppressed helix mode (ESHFLC mode). Other operation modes of FLC can be referred to the review paper [<NPL>)].

Upon applying a voltage, the FLC molecules may be rotated along the cone and quickly align themselves in a state where the spontaneous polarization is parallel to the direction of the electric field. In particular, the directors of the FLC molecules may be reoriented from one final to the other when an external electric field changes its polarity. The total angle of switching equals the double tilt angle θ. In each final state, the directors of the FLC molecules may remain parallel to the substrates, thus transforming the FLC cell into a uniaxial phase plate. The maximum variation of the transmitted light intensity may be achieved when the FLC cell is placed between crossed polarizer and analyzer, and a transmission axis of the polarizer is configured to coincide with the director of FLC molecules at one of the final states so that the optical axis of the FLC layer forms angle 2θ with polarizer, which is usually designed to be close to <NUM>°. A high speed and a high contrast light shutter may be obtained under applied electric fields of opposite polarity.

An example of LC shutters based on absorption is a guest-host LC shutter. <FIG> illustrates a schematic diagram of a guest-host LC shutter <NUM> according to an embodiment of the disclosure. As shown in <FIG>, the guest-host LC shutter <NUM> may include two opposite substrates <NUM> and an LC layer sandwiched between the two opposite substrates <NUM>. Each substrate <NUM> may be provided with a transparent electrode <NUM> and an alignment layer <NUM>. The LC layer may include a mixture of host LCs <NUM> and guest dichroic dyes <NUM> doped into the LCs <NUM>. The dye molecules <NUM> may be aligned together with the LC molecules <NUM> in a voltage-off state, and reoriented with the LC molecules <NUM> under an applied electric field E in a voltage-on state. The absorption properties of a dye molecule <NUM> may depend on its orientation relative to the incident light <NUM>. In some embodiments, when the absorption axes of the dye molecule <NUM> and the polarization direction of the incident light <NUM> are parallel, the dye molecule <NUM> may strongly absorb the incident light <NUM>. On the contrary, the dye molecule <NUM> may weakly absorb the incident light <NUM> when the axes of the dye molecule <NUM> and the polarization direction of the incident light <NUM> are crossed with each other. Thus, through switching the orientation of the dye molecules <NUM> by an electric field, the incident light <NUM> may be absorbed or transmitted. Accordingly, the guest-host LC shutter <NUM> may be switched between an opaque state and a transparent state.

The LCs <NUM> in the LC layer may have positive or negative dielectric anisotropy. For illustrative purposes, <FIG> show the LCs <NUM> have positive dielectric anisotropy (Δε><NUM>). As shown in <FIG>, when the directors of the LCs <NUM> change from a planar orientation to a perpendicular orientation along with an applied voltage V, the long molecular axis of the dyes <NUM> may also change orientation along with the LCs <NUM>, i.e., the dyes <NUM> may change from a planar orientation (a strong absorption state) at V=<NUM> to a perpendicular orientation (a weak absorption state) at V≠<NUM>. Accordingly, the guest-host LC shutter <NUM> may be changed from an opaque state at V=<NUM> to a transparent state at V≠<NUM>. In some embodiments, the LCs <NUM> may have negative dielectric anisotropy (Δε<<NUM>), and the opaque state and the transparent state of the guest-host LC shutter <NUM> may be reversed, i.e., the guest-host LC shutter <NUM> may have a transparent state at V=<NUM> and an opaque state at V≠<NUM>. The guest-host LC shutter <NUM> may be free of polarizers that absorb more than <NUM>% of incident light. Also, the production cost of the guest-host LC shutter <NUM> may be significantly reduced. In addition, flexible substrates may be allowed to be used in the guest-host LC shutter, which may enable great design flexibility of the overall NED design.

Examples of LC shutters based on scattering may include a polymer dispersed liquid crystal (PDLC) shutter, a polymer network liquid crystal (PNLC) shutter, a filled LC shutter, etc. LC/polymer composites may appear in the form of network or droplet depending on the polymer and its concentration. In the low concentration regime of polymer (about <NUM>-<NUM> wt%), the response time may be significantly improved, however, the resulted light scattering may be very strong in the visible range. When the polymer concentration increases to about <NUM>-<NUM> wt%, the polymer network liquid crystal (PNLC, also known as gel) may be formed in a homogeneous or homeotropic cell which exhibits an anisotropic light scattering behavior. When the polymer concentration increases to <NUM>-<NUM> wt%, the polymer dispersed liquid crystal (PDLC) may be formed. Alignment layers may be no longer required in a PDLC cell. Micron-sized LC droplets are dispersed in the polymer matrix, in which visible light may be strongly scattered independently of polarization. In the high polymer concentration regime (about <NUM>-<NUM> wt%), nanoscale PDLC (nano-PDLC) may be formed. Because the LC droplet size is much smaller than the visible wavelength, nano-PDLC may be free of light scattering and its response time is fast (<NUM>-<NUM>). However, to reorient LC molecules in such small droplets, a relatively high switching electric field may be desired. Alignment layers may be no longer required in a nano-PDLC cell.

<FIG> illustrates a schematic diagram of a PDLC shutter <NUM> according to an embodiment of the disclosure. As shown in <FIG>, the PDLC shutter <NUM> may include two opposite substrates <NUM> and a PDLC layer sandwiched between the two substrates <NUM>. Each substrate <NUM> may be provided with a transparent electrode <NUM>, such as an ITO electrode, for applying a voltage to the PDLC layer. The PDLC layer may be a composite material layer that includes micro-sized nematic LC droplets <NUM> randomly dispersed in an optically isotropic polymer matrix <NUM>. The LC droplets <NUM> each may have a bipolar configuration that exhibits a dielectric anisotropy. The PDLC may be obtained either by an encapsulation from the emulsion of LCs in a liquid or by a polymerization-induced phase separation process, for example, using photopolymerization or thermal polymerization. The ordinary refractive index no of the LCs within the LC droplets <NUM> may be chosen to be sufficiently close to (including match) refractive index np of the polymer matrix material. The PDLC shutter <NUM> may work without polarizers.

In a voltage-off state, as shown in <FIG>, the symmetry axis of each LC droplet <NUM> may be in general randomly oriented. The difference between the refractive index of the polymer matrix material and the effective refractive index of the LCs may result in the scattering of a substantially normally incident light <NUM>, giving the PDLC shutter <NUM> a milky appearance. An opaque state of the PDLC shutter <NUM> may be realized. Note that the LC droplets <NUM> with diameter of the same order of wavelength of visible light may more efficiently scatter an incident visible light. In a voltage-on state, as shown in <FIG>, an electric field may be applied along the normal direction of the PDLC layer, the symmetry axis of the LC droplets <NUM> having LCs of positive dielectric anisotropy may be reoriented by the electric field, and trend to be parallel to the electric field direction. Because the ordinary refractive index no of the LCs is sufficiently close to (or match) the refractive index np of the polymer matrix material, the substantially normally incident light <NUM> may encounter negligible variation of refractive index. Thus, the incident light <NUM> may be transmitted through the PDLC layer with negligible scattering. A transparent state of the PDLC shutter <NUM> may be realized. Varying the strength of the applied electric field may allow the PDLC shutter <NUM> to be continuously tuned from opaque to almost transparent. After removal of the electric field, the anchoring force of the polymer on the LCs may restore the LC droplets <NUM> to the original orientation and again the PDLC shutter <NUM> may appear milky. Hence, the PDLC shutter <NUM> may be opaque in the voltage-off state and clear in the voltage-on state. In some embodiments, the PDLC shutter <NUM> may be opaque in the voltage-on state and clear in the voltage-off state.

A PNLC shutter may include a PNLC layer sandwiched between two opposite substrates. Each substrate may be provided with a transparent electrode, such as an ITO electrode, for applying a voltage to the PNLC layer. In some embodiments, the PNLC layer may include nematic LCs, and the operation principle of the PNLC shutter may be similar to the PDLC shutter, but have a reduced operating voltage due to the lower polymer concentration. The switching speed of the PNLC shutter may be further improved by using sheared PNLC, where the PNLC layer is subjected to a shearing force parallel to the substrates. The shearing force may orientate the polymer chains within the PNLC in the direction of the shearing movement. The resulting sheared PNLC shutter may have a switching speed of couple tens of microseconds. However, the PNLC shutter including nematic LCs may be still polarization sensitive, and alignment layers may be disposed at the internal surfaces of the substrates. The polarization independency of the PNLC shutter may be achieved by stacking the above two PNLC layers with orthogonal alignment directions or by forming PNLC structure in a <NUM>° twisted nematic (TN) cell.

In some embodiments, the PNLC layer may include cholesteric LCs, where the pitch of the cholesteric LC may be a few micrometers (ms), and alignment layers may be no longer required. Polymerization of monomer in the cholesteric LC may occur when a high voltage is applied on the LC cell, which unwinds the cholesteric structure and reorients the cholesteric LC molecules to the homeotropic state (i.e., perpendicular to the substrate). The polymer network may be formed perpendicular to the substrate in the direction of homeotropically aligned cholesteric LCs under the high field. After polymerization, in a voltage-off state, the cholesteric LCs may tend to have a helical structure, while the polymer network may tend to keep the LC director parallel to the polymer network. The competition between the two factors may result in the focal conic texture. The PNLC layer may have a polydomain structure and may be optically scattering, realizing an opaque state of the PNLC shutter. In a voltage-on state, the LCs with positive dielectric anisotropy (Δε><NUM>) may be switched to the homeotropic texture, and the incident light may experience the ordinary reflective index no of LCs which is matched with the refractive index np of the polymer and, thus, may be not scattered but transmitted through the PNLC layer, realizing a transparent state of the PNLC shutter.

A filled LC shutter may include liquid crystal (LC)-colloidal nanoparticle (NP)- polymer (P) composites, which are formed by, for example, photoinduced phase separation. The nanoparticles may be used as filling materials for LCs. For PDLC filled with NPs, in the process of photoinduced phase separation of the LC-NP-prepolymer mixture, the nanoparticles may be mainly involved with the polymer, serving as building blocks for the polymer matrix. For an LC shutter including PDLC filled with NPs, when the aggregation rate of the nanoparticles is high or the size of the nanoparticles is large, the PDLC filled with NPs may have an enhanced light scattering as compared to pure PDLC. When the aggregation rate of nanoparticles is low, the nanoparticles may not influence the optical uniformity, but modify the refractive index of the polymer, which may modify the ratio of the refractive indices of LC drops and polymer matrix and, as a result, the scattering characteristics. Accordingly, the electro-optic contrast of the PDLC may be modified and, in particular, the off-axis haze of the PDLC, which is caused by the refractive index mismatch of the LC drops and the polymer matrix if they are matched for normal incident light (np=no), may be efficiently suppressed.

An LC shutter based on scattering and absorption may be a combination of an LC shutter based on scattering shown in <FIG> and an LC shutter based on absorption shown in <FIG>. <FIG> illustrate a schematic diagram of an LC shutter <NUM> based on scattering and absorption according to an embodiment of the disclosure. As shown in <FIG>, the LC shutter <NUM> may include two opposite substrates <NUM>, and a composite layer <NUM> of PDLC (or PNLC) doped with dyes (e.g., dichroic dyes) sandwiched between the two substrates <NUM>. Each substrate may be provided with a planar transparent electrode <NUM>, e.g., an ITO electrode. Each electrode <NUM> may be provided with an alignment layer (not drawn), through which LC molecules <NUM> with negative dielectric anisotropy (Δε<<NUM>) may be homeotropically (or vertically) aligned, and dye molecules <NUM> may align together with the LC molecules <NUM>. The dye molecules <NUM> may have an absorption axis in the long axis.

In a voltage-off state, as shown in <FIG>, the LC molecules <NUM> and the dye molecules <NUM> may be vertically aligned on each substrate <NUM>, and a substantially normally incident light <NUM> that is unpolarized may be weakly absorbed. On the other hand, because the ordinary refractive index no of the LCs is sufficiently close to (or match) the refractive index np of the polymer matrix material, the substantially normally incident light <NUM> may encounter negligible variation of refractive index. Thus, the incident light <NUM> may be transmitted through with negligible scattering. A transparent state of the LC shutter <NUM> may be realized.

In a voltage-on state, as shown in <FIG>, an electric field may be applied along the normal direction of the composite layer <NUM>, the LC molecules <NUM> having negative dielectric anisotropy (Δε<<NUM>) may be reoriented by the electric field to be parallel to the substrate <NUM>. The orientation of the dye molecules <NUM> may also change along with the LC molecules <NUM>, consequently, the absorption axis orientation of the dye molecules <NUM> may be changed, for example, from the weakly-absorbing/homeotropic orientation to the strongly-absorbing/homogeneous orientation. Accordingly, the incident light <NUM> may be strongly absorbed. On the other hand, the difference between the refractive index of the polymer matrix material and the effective refractive index of the LCs may result in the scattering of the incident light <NUM>. Thus, an opaque state of the LC shutter <NUM> may be realized.

In the following, exemplary high-speed switchable LC gratings and polarization sensitive gratings will be presented. Examples of high-speed switchable LC gratings may include an H-PDLC grating, a surface relief grating filled with LCs, and an LC grating based on modulation of LC alignment, etc..

An H-PDLC grating may be fabricated by polymerizing an isotropic photosensitive liquid mixture of monomers and LCs under a laser interference irradiation. H-PDLC gratings include gratings with droplet structure of LC and layer structure of LC. <FIG> illustrate a schematic diagram of a droplet type H-PDLC grating <NUM> according to an embodiment of the disclosure. As shown in <FIG>, the H-PDLC grating <NUM> may include layers of randomly oriented or partially oriented nematic LC droplets <NUM> embedded in a polymer matrix <NUM> that is sandwiched between two substrates <NUM>. Each substrate <NUM> may be provided with a transparent conductive electrode <NUM>, such as an ITO electrode. The ordinary refractive index no of the LCs within the LC droplets <NUM> may be chosen to be sufficiently close to the refractive index np of the material of the polymer matrix <NUM>. Because of the refractive index difference between the LC droplets <NUM> and material of the polymeric matrix <NUM>, the spatial modulation of the LC content may produce a modulation in the average refractive index, resulting in an optical phase grating which diffracts a normally incident light <NUM>. The LC droplets <NUM> in H-PDLC are usually very small (subwavelength size) so that scattering due to refractive index mismatch of the LC and polymer may be minimized, and only phase modulation may play a role. In other words, H-PDLC often belongs to class of nano-PDLC.

As shown in <FIG>, the resulting optical phase grating may be switched off by applying an external voltage, such that the droplet directors may be aligned along with the electric field direction. Because the ordinary refractive index no of the LCs within the LC droplets <NUM> is chosen to be sufficiently close to the refractive index np of the material of the polymer matrix <NUM>, the normally incident light <NUM> may not experience a refractive index modulation and, thus, may be transmitted through with negligible diffraction.

Due to a high refractive index modulation, e.g., the refractive index modulation amplitude may be close to birefringence of the LCs, the H-PDLC grating <NUM> may have a substantially thin thickness of a few micrometers (ms) even in Bragg regime. The diffraction efficiency of the H-PDLC grating <NUM> may be close to about <NUM>%. In some embodiments, the H-PDLC grating <NUM> may be polarization sensitive. The diffraction efficiency of the H-PDLC grating <NUM> may have polarization dependence because of partial alignment of the LCs in the H-PDLC grating <NUM>. Depending on the materials used, either p-polarized light or s-polarized light may be strongly diffracted while the light with the other polarization may be negligibly diffracted.

For illustrative purposes, <FIG> shows the H-PDLC grating <NUM> is switchable by an external voltage, when the voltage is switched off, the H-PDLC grating <NUM> may exhibit the grating effect, and when the voltage is switched on, the grating effect may vanish. In some embodiments, the H-PDLC grating may be non-switchable. In this case, the substrates may not contain conductive electrodes.

No-droplet H-PDLC gratings may include Policryps (polymer-liquid-crystal-polymer-slices) and Poliphem (polymer-liquid crystal-polymer holograms electrically manageable) gratings. In both types of gratings, the LC droplet nucleation may be inhibited, so that their morphology may simply consist of alternate homogeneous films of polymer and LCs. Compared to most H-PDLC gratings, a higher refractive index modulation may be achieved in Policryps and Poliphem gratings and, thus, a higher diffraction efficiency may be obtained given the same grating thickness as the H-PDLC grating. In addition, the scattering loss of Policryps and Poliphem gratings may be strongly reduced, and the switching voltage may be much lower because the dimension of the LC domains is not fixed by the droplet size but by the grating spacing. However, Policryps and Poliphem gratings may have slower response time.

<FIG> illustrate a schematic diagram of a grating <NUM> which is a surface relief grating filled with LCs. As shown in <FIG>, the grating1000 may be a surface relief grating (SRG) <NUM> filled with LCs <NUM>, which is referred to as an LC filled SRG in the following description. The surface relief grating <NUM> may be optically recorded in a photopolymer, for example, an acrylamide-based photopolymer. The LC filled SRG <NUM> may have two opposite transparent substrates <NUM> that sandwiches the surface relief grating <NUM> and the filled LCs <NUM>. Each substrate <NUM> may be provided with a transparent electrode (e.g., ITO electrode) <NUM>. The ordinary refractive index no of the LCs <NUM> may be configured to be sufficiently close to the refractive index np of the polymer material of the SRG <NUM>. The surface relief grating <NUM> may align the LCs <NUM> in a certain direction. In one embodiment, as shown in <FIG>, the LCs <NUM> may have a homogeneous alignment in the direction of grating grooves, e.g., in the direction of y-axis.

The LC grating <NUM> may be sensitive to linear polarization and may be switchable. At a voltage-off state, as shown in <FIG>, due to the refractive index difference between the homogeneously aligned LCs <NUM> and the photopolymer of the surface relief grating <NUM> (i.e., the refractive index difference between ne and np), a periodic modulation of the refractive index may be realized in the LC filled SRG <NUM>. In some embodiments, an s-polarized incident light <NUM> incident on the LC filled SRG <NUM> may be diffracted, realizing an on-state of the LC filled SRG <NUM>. At a voltage-on state, as shown in <FIG>, with the application of an electric field, the LCs <NUM> may be aligned along with the electric field direction (e.g., in the z-axis), such that the refractive index of LCs <NUM> matches the refractive index of photopolymer of the surface relief grating <NUM>. In this state, the periodic modulation of the refractive index in the LC filled SRG <NUM> may vanish, and the s-polarized incident light <NUM> may be directly transmitted, realizing an off-state of the LC filled SRG <NUM>. In particular, the diffraction efficiency of the LC filled SRG <NUM> may be controlled by varying the applied electric field, and only the reorientation of LCs <NUM> within the LC filled SRG grating <NUM> may affect the diffraction efficiency. A p-polarized incident light may not be diffracted when the LC filled SRG <NUM> in the on-state or off-state, because the p-polarized incident light always sees the ordinary refractive index no of the LCs <NUM> that is matched with the photopolymer. In other words, the LC filled SRG <NUM> may appear as a uniform transparent plate to the p-polarized incident light.

For illustrative purposes, <FIG> shows the LC filled SRG <NUM> is switchable by an external voltage. When the voltage is switched off, the LC filled SRG <NUM> may exhibit the grating effect, and when the voltage is switched on, the grating effect may vanish. In some embodiments, the LC filled SRG <NUM> may be non-switchable, for example, when the substrates are not provided with the electrodes.

The grating effect in LC layers may also be caused by the modulation of the LC alignment. To switch the LC grating between an on-state and an off-state in a fast speed, fast LC modes, such as FLC mode, Pi-cell mode, dual-frequency nematic mode, etc., may be highly desired. In some embodiments, the modulation of the LC alignment may be realized by an electric field, such as using striped electrodes, pixelated electrodes. In some embodiments, the modulation of the LC alignment may be realized by modulation of anchoring conditions of the LCs, for example, in-plane modulation of easy axis of LCs, modulation of pretilt angle of LCs.

The present discourse also provides a method for a waveguide display assembly. <FIG> illustrates a flow chart of a method for a waveguide display assembly according to an embodiment of the disclosure. As shown in <FIG>, the method includes during a virtual-world subframe of a display frame of a projector, switching on the projector to generate an image light and switching at least one switchable grating to a diffracting state to decouple the image light out of a waveguide to an eye-box via diffraction (S1110). The method further includes during a real-world subframe of the display frame, switching off the projector from generating the image light and switching the at least one switchable grating to a non-diffracting state having a diffraction efficiency lower than a predetermined threshold (S1120). The at least one switchable grating is an out-coupling grating.

In some embodiments, the method may further include during the virtual-world subframe of the display frame, switching all switchable gratings to perform at least one of directing, expanding or decoupling the image light out of the waveguide to the eye-box via the diffraction; and during the real-world subframe of the display frame, switching all the switchable gratings to transmit the light from the real-world environment to the eye-box with the diffraction efficiency less than the predetermined threshold. In some embodiments, all switchable gratings may include one or more out-coupling gratings, one or more fold gratings, one or more pupil expansion gratings, and any other gratings disposed in the optical path of the light from the real-world environment towards the eye-box.

In some embodiments, the method may further include during the real-world subframe, switching an optical shutter disposed in front of the projector to an opaque state to block the image light from being incident onto the waveguide; and during the virtual-world subframe, switching the switchable optical shutter to a transparent state to transmit the image light to be incident onto the waveguide. A switching time of the switchable optical shutter is sufficiently fast such that the real-world and virtual-world subframes are switched for the time and presented at a rate that exceeds a flicker fusion threshold of the user, i.e., beyond a flicker fusion threshold.

In some embodiments, the method may further include during the real-world subframe, switching a high speed optical dimmer disposed opposite to the outcoupling grating to a transparent state to transmit the light from the real-world environment to be incident onto the waveguide; and during the virtual-world subframe, switching the dimmer to an opaque state that dims the light from the real-world environment from being incident onto the waveguide. In some embodiments, the dimmer may be switched to the transparent state to transmit the real-world light during the real-world subframe, and switched to the opaque state to dim (including completely block) the real-world light during the virtual-world subframe. The dark background in case of see-though attenuation may increase contrast of virtual images demonstrated in this subframe. When the virtual-world subframe is configured to last for <NUM>% of the total frame period, the see-through artifacts may be almost eliminated at the expense of a <NUM>% reduction of the see-through brightness. In some embodiments, the dimmer may adaptively dim an incident light, i.e., the dimmer may function as a controllable dimming element rather than a shutting element with only two transmittance states. The attenuation provided by the dimmer may be controlled by, for example, an external electric field, a magnetic field, or light or some combination thereof.

The foregoing description of the embodiments of the disclosure have been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure.

Some portions of this description describe the embodiments of the disclosure in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations are commonly used by those skilled in the data processing arts to convey the substance of their work effectively to others skilled in the art. These operations, while described functionally, computationally, or logically, are understood to be implemented by computer programs or equivalent electrical circuits, microcode, or the like. Furthermore, it has also proven convenient at times, to refer to these arrangements of operations as modules, without loss of generality. The described operations and their associated modules may be embodied in software, firmware, hardware, or any combinations thereof.

Embodiments of the disclosure may also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, and/or it may comprise a general-purpose computing device selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a non-transitory, tangible computer readable storage medium, or any type of media suitable for storing electronic instructions, which may be coupled to a computer system bus. Furthermore, any computing systems referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability.

Embodiments of the disclosure may also relate to a product that is produced by a computing process described herein. Such a product may comprise information resulting from a computing process, where the information is stored on a non-transitory, tangible computer readable storage medium and may include any embodiment of a computer program product or other data combination described herein.

Claim 1:
An optical device, comprising:
a projector (<NUM>) configured to generate an image light;
a waveguide (<NUM>) optically coupled with the projector (<NUM>) and configured to guide the image light to an eye-box, the waveguide comprising:
an in-coupling element (<NUM>) configured to couple the image light into the waveguide (<NUM>), and
an out-coupling element (<NUM>) configured to decouple the image light out of the waveguide;
wherein the waveguide includes a plurality of switchable gratings; and
a controller (<NUM>) configured to switch each of the plurality of switchable gratings to:
during a virtual-world subframe of a display frame, perform at least one of directing, expanding or decoupling the image light out of the waveguide via diffraction, and the controller (<NUM>) is configured to switch each of the plurality of switchable gratings to:
during a real-world subframe of the display frame, transmit a light from a real-world environment to the eye-box with a diffraction efficiency less than a predetermined threshold.