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Facebook Patent | Multifocal system using adaptive lenses

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Patent: Multifocal system using adaptive lenses

Drawings: Click to check drawins

Publication Number: 20210003901

Publication Date: 20210107

Applicant: Facebook

Abstract

A device is provided. The device includes a first lens assembly controllable to switch between a first plurality of optical powers. The first lens assembly includes a plurality of directly optically coupled lenses. The device also includes a second lens assembly controllable to switch between a second plurality of optical powers that are the opposite of the first plurality of optical powers. The device further includes a half-wave plate disposed between the first adaptive lens assembly and the second adaptive lens assembly.

Claims

  1. A device, comprising: a first lens assembly controllable to switch between a first plurality of optical powers, the first lens assembly comprising a plurality of directly optically coupled lenses; a second lens assembly controllable to switch between a second plurality of optical powers that are the opposite of the first plurality of optical powers; and a half-wave plate disposed between the first adaptive lens assembly and the second adaptive lens assembly.

  2. The device of claim 1, wherein the half-wave plate is configured to convert a first light having a first polarization received from the first adaptive lens assembly to a second light having a second polarization orthogonal to the first polarization, and output the second light toward the second adaptive lens assembly.

  3. The device of claim 1, wherein at least one lens of the plurality of directly optically coupled lenses is switchable between a non-zero optical power state and a zero optical power state.

  4. The device of claim 3, wherein the at least one lens is a circular polarization dependent active liquid crystal (“LC”) lens.

  5. The device of claim 1, wherein the directly optically coupled lenses in the first lens assembly are first lenses, and the second lens assembly comprises a plurality of directly optically coupled second lenses.

  6. The device of claim 5, wherein: at least one first lens of the plurality of directly optically coupled first lenses is switchable between a non-zero optical power state and a zero optical power state, and at least one second lens of the plurality of directly optically coupled second lenses is switchable between a non-zero optical power state and a zero optical power state.

  7. The device of claim 6, wherein the at least one first lens and the at least one second lens are circular polarization dependent active LC lenses.

  8. The device of claim 7, wherein each of the at least one first lens and the at least one second lens is configured to operate as a converging lens for a circularly polarized incident light having a predetermined handedness, and as a diverging lens for a circularly polarized incident light having a handedness that is opposite to the predetermined handedness.

  9. The device of claim 6, wherein the at least one first lens is substantially identical to the at least one second lens.

  10. The device of claim 1, wherein the second adaptive lens assembly has the same number of lenses as the first adaptive lens assembly.

  11. The device of claim 1, wherein each of the first adaptive lens assembly and the second adaptive lens assembly includes a passive lens with a non-switchable optical power.

  12. A method, comprising: receiving a first light by a first adaptive lens assembly controllable to switch between a first plurality of optical powers, the first lens assembly comprising a plurality of directly optically coupled lenses; transmitting, by the first adaptive lens assembly, the first light to a half-wave plate disposed between the first adaptive lens assembly and a second adaptive lens assembly controllable to switch between a second plurality of optical powers that are the opposite of the first plurality of optical powers; and converting, by the half-wave plate, the first light into a second light propagating towards the second adaptive lens assembly, the first light and the second light having orthogonal polarizations.

  13. The method of claim 12, further comprising: determining an optical power of the second adaptive lens assembly; and determining, based on the determined optical power of the second adaptive lens assembly, an optical power for the first adaptive lens assembly, wherein the optical power of the first adaptive lens assembly is a first optical power, and the optical power of the second adaptive lens assembly is a second optical power opposite to the first optical power.

  14. The method of claim 13, further comprising: transmitting, by the first adaptive lens assembly having the first optical power, the first light to the half-wave plate; and transmitting, by the second adaptive lens assembly having the second optical power, the second light.

  15. The method of claim 13, wherein at least one lens of the plurality of directly optically coupled lenses in the first lens assembly is switchable between a non-zero optical power state and a zero optical power state, and the method further comprises: configuring an optical state of the at least one first lens to provide the first optical power.

  16. The method of claim 15, wherein the at least one lens is a circular polarization dependent active LC lens.

  17. A device, comprising: a first adaptive lens assembly controllable to switch between a first plurality of optical powers, the first lens assembly comprising at least two directly optically coupled first lenses including at least one first linear polarization dependent lens; and a second adaptive lens assembly controllable to switch between a second plurality of optical powers that are the opposite of the first plurality of optical powers, the second lens assembly comprising at least two directly optically coupled second lenses including at least one second linear polarization dependent lens.

  18. The device of claim 17, wherein: the at least one first linear polarization dependent lens is an active LC lens that is switchable between a non-zero optical power state and a zero optical power state, and the at least one second linear polarization dependent lens is an active LC lens that is switchable between a non-zero optical power state and a zero optical power state.

  19. The device of claim 17, wherein each of the first adaptive lens assembly and the second adaptive lens assembly includes a passive lens with a non-switchable optical power.

  20. The device of claim 17, wherein the first lens assembly comprises three directly optically coupled first lenses including the at least one first linear polarization dependent lens, and the second lens assembly comprises three directly optically coupled second lenses including the at least one second linear polarization dependent lens.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent application Ser. No. 16/265,435, entitled “MULTIFOCAL SYSTEM USING ADAPTIVE LENSES,” filed on Feb. 1, 2019, which claims the benefit of priority to U.S. Provisional Patent Application No. 62/754,414, filed on Nov. 1, 2018. Contents of the above-mentioned applications are incorporated herein by reference in their entirety.

BACKGROUND

[0002] Virtual reality (VR) headsets can be used to simulate virtual environments. For example, stereoscopic images can be displayed on an electronic display inside a headset to simulate the illusion of depth, and head tracking sensors can be used to estimate what portion of the virtual environment is being viewed by the user. However, because existing headsets are often unable to correctly render or otherwise compensate for vergence and accommodation conflicts, such simulation can cause visual fatigue and nausea of the users.

[0003] Augmented Reality (AR) headsets display a virtual image overlapping with real-world images. To create comfortable viewing experience, the virtual image generated by the AR headsets needs to be displayed at the right distance for the eye accommodations of the real-world images in real time during the viewing process.

SUMMARY OF THE DISCLOSURE

[0004] One aspect of the present disclosure provides a multifocal system. The multifocal system comprises a first adaptive lens assembly including a plurality of lenses arranged in optical series. The plurality of lenses includes at least one active liquid crystal (LC) lens having a plurality of optical states, such that the plurality of lenses provide a plurality of combinations of optical power, and the plurality of combinations of optical power provides a range of adjustment of optical power for the multifocal system.

[0005] Another aspect of the present disclosure provides a method for a multifocal system. The method comprises: stacking a plurality of lenses to form a first adaptive lens assembly, wherein the plurality of lenses includes at least one active liquid crystal (LC) lens having a plurality of optical states; determining a current optical state of the multifocal system; determining a next optical state required by the multifocal system in terms of the plurality of optical states of the at least one active LC lens; determining an optical state of the plurality of optical states of the at least one active LC lens to achieve the next optical state of the multifocal system; and switching the at least one active LC lens to the optical state to achieve the next optical state of the multifocal system, such that the plurality of lenses together provides an adjustment range from the current optical state to the next optical state for the multifocal system.

[0006] Other aspects of the present disclosure can be understood by those skilled in the art in light of the description, the claims, and the drawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] 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.

[0008] FIG. 1A illustrates the relationship between vergence and accommodation in the real word of the present disclosure;

[0009] FIG. 1B illustrates the conflict between vergence and accommodation in a three-dimensional (3D) display screen of the present disclosure;

[0010] FIG. 2A illustrates a wire diagram of an exemplary head-mounted display (HMD) consistent with the disclosed embodiments;

[0011] FIG. 2B illustrates a cross-section of a front rigid body of the head-mounted display in FIG. 2A consistent with the disclosed embodiments;

[0012] FIG. 2C illustrates a cross-section of an exemplary waveguide display of the head-mounted display in FIG. 2A consistent with the disclosed embodiments;

[0013] FIG. 2D illustrates a diagram of an exemplary multifocal block using adaptive lenses consistent with the disclosed embodiments;

[0014] FIGS. 3A-3B illustrate an exemplary linear polarization dependent liquid crystal (LC) lens consistent with the disclosed embodiments;

[0015] FIG. 4A illustrates an exemplary circular polarization dependent LC lens consistent with the disclosed embodiments;

[0016] FIG. 4B illustrates exemplary LC orientations in the LC lens of FIG. 4A consistent with the disclosed embodiments;

[0017] FIG. 4C illustrates a section of exemplary LC orientations taken along y-axis in the LC lens of FIG. 4A consistent with the disclosed embodiments;

[0018] FIG. 4D illustrates an exemplary neutral state of the LC lens of FIG. 4A consistent with the disclosed embodiments;

[0019] FIG. 4E illustrates another exemplary neutral state of the LC lens of FIG. 4A consistent with the disclosed embodiments;

[0020] FIG. 5 illustrates an exemplary polarization independent LC lens consistent with the disclosed embodiments;

[0021] FIG. 6A illustrates an exemplary multifocal block consistent with the disclosed embodiments;

[0022] FIGS. 6B-6Q illustrate exemplary optical power adjustments of the multifocal block in FIG. 6A consistent with the disclosed embodiments;

[0023] FIG. 7A illustrates another exemplary multifocal block consistent with the disclosed embodiments;

[0024] FIGS. 7B-7S illustrate exemplary optical power adjustments of the multifocal block in FIG. 7A consistent with the disclosed embodiments;

[0025] FIG. 8 illustrates an exemplary multifocal system in which an HMD operates consistent with the disclosed embodiments;

[0026] FIG. 9 illustrates an exemplary process for mitigating vergence-accommodation conflict by adjusting the focal length of an HMD consistent with the disclosed embodiments; and

[0027] FIG. 10 illustrates an exemplary process for mitigating vergence-accommodation conflict by adjusting a focal length of a multifocal block that includes multifocal structures consistent with the disclosed embodiments.

DETAILED DESCRIPTION

[0028] A multifocal system includes a head-mounted display (HMD). The HMD includes a multifocal block. The HMD presents content via an electronic display to a wearing user at a focal distance. The multifocal block adjusts the focal distance in accordance with instructions from the HMD to, e.g., mitigate vergence accommodation conflict of eyes of the wearing user. The focal distance is adjusted by adjusting an optical power associated with the multifocal block, and specifically by adjusting the optical power associated with one or more multifocal structures within the multifocal block.

[0029] In some embodiments, a virtual object is presented on the electronic display of the HMD that is part of the multifocal system. The light emitted by the HMD is configured to have a particular focal distance, such that the virtual scene appears to a user at a particular image plane. As the content to be rendered moves closer/farther from the user, the HMD correspondingly instructs the multifocal block to adjust the focal distance to mitigate a possibility of a user experiencing a conflict with eye vergence and eye accommodation. Additionally, in some embodiments, the HMD may track a user’s eyes such that the multifocal system is able to approximate gaze lines and determine a gaze point including a vergence distance (an estimated point of intersection of the gaze lines) to determine an appropriate amount of accommodation to provide the user. The gaze point identifies an object or plane of focus for a particular frame of the virtual scene and the HMD adjusts the distance of the multifocal block to keep the user’s eye in a zone of comfort as vergence and accommodation change.

[0030] Vergence-accommodation conflict is a problem in many virtual reality systems. Vergence is the simultaneous movement or rotation of both eyes in opposite directions to obtain or maintain single binocular vision and is connected to accommodation of the eye. Under normal conditions, when human eyes look at a new object at a distance different from an object they had been looking at, the eyes automatically change focus (by changing their shape) to provide accommodation at the new distance or vergence distance of the new object.

[0031] FIG. 1A shows an exemplary of how the human eye experiences vergence and accommodation in the real world. As shown in FIG. 1A, the user is looking at a real object 100 (i.e., the user’s eyes are verged on the real object 100 and gaze lines from the user’s eyes intersect at real object 100.). As the real object 100 is moved closer to the user, as indicated by the arrow in FIG. 1A, each eye 102 rotates inward (i.e., convergence) to stay verged on the real object 100. As the real object 100 gets closer, the eye 102 must “accommodate” for the closer distance by changing its shape to reduce the power or focal length. The distance to which the eye must be focused to create a sharp retinal image is the accommodative distance. Thus, under normal conditions in the real world, the vergence distance (dv) is equal to the accommodative distance (da).

[0032] FIG. 1B shows an exemplary conflict between vergence and accommodation that can occur with some three-dimensional displays. As shown in FIG. 1B, a user is looking at a virtual object 100B displayed on an electronic screen 104. However, the user’s eyes are verged on and gaze lines from the user’s eyes intersect at virtual object 100B, which is at greater distance from the user’s eyes than the electronic screen 104. As the virtual object 100B is rendered on the electronic display 104 to appear closer to the user, each eye 102 again rotates inward to stay verged on the virtual object 100B, but the power or focal length of each eye is not reduced; hence, the user’s eyes do not accommodate as in FIG. 1A. Thus, instead of reducing power or increasing focal length to accommodate for the more distant vergence distance, each eye 102 maintains accommodation at a distance associated with the electronic display 104. Thus, the vergence distance (dv) often is not equal to the accommodative distance (da) for the human eye for objects displayed on 3D electronic displays. This discrepancy between vergence distance and accommodative distance is referred to as a “vergence-accommodation conflict.” A user who is experiencing only vergence or accommodation but not both will eventually experience some degree of fatigue and nausea, which is undesirable for virtual reality system creators.

[0033] FIG. 2A illustrates a wire diagram of an exemplary head-mounted display (HMD) 200 consistent with the disclosed embodiments. As shown in FIG. 2A, the HMD 200 may include a front rigid body 205 and a band 210. The front rigid body 205 may include one or more electronic display elements of an electronic display (not shown), an inertial measurement unit (IMU) 215, one or more position sensors 220, and locators 225. In the embodiment shown by FIG. 2A, the position sensors 220 may be located within the IMU 215, and neither the IMU 215 nor the position sensors 220 may be visible to the user. The IMU 215, the position sensors 220, and the locators 225 may be discussed in detail below with regard to FIG. 7. The HMD 200 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 HMD 200 acts as an augmented reality (AR) or a mixed reality (MR) device, portions of the HMD 200 and its internal components may be at least partially transparent.

[0034] FIG. 2B is a cross section 250 of the front rigid body 205 of the embodiment of the HMD 200 shown in FIG. 2A. As shown in FIG. 2B, the front rigid body 205 may include an electronic display 255 and a multifocal block 260 that together provide image light to an exit pupil 263. The exit pupil 263 may be the location of the front rigid body 205 where a user’s eye 265 is positioned. For purposes of illustration, FIG. 2B shows a cross section 250 associated with a single eye 265, but another multifocal block 260, which is separated from the multifocal block 260, may provide altered image light to another eye of the user.

[0035] Additionally, the HMD 200 may include an eye tracking system 270. The eye tracking system 270 may include, e.g., one or more sources that illuminate one or both eyes of the user, and one or more cameras that captures images of one or both eyes of the user. The HMD 200 may include an adaptive dimming system 290, which includes a dimming element. The dimming element may dynamically adjust the transmittance of the real-world objects viewed through the HMD 280, thereby switching the HMD 200 between a VR device and an AR device or between a VR device and a MR device. In some embodiments, along with switching between the AR/MR device and the VR device, the dimming element may be used in the AR device to mitigate difference in brightness of real and virtual objects.

[0036] The electronic display 255 may display images to the user. In various embodiments, the electronic display 255 may comprise a single electronic display or multiple electronic displays (e.g., a display for each eye of a user). Examples of the electronic display 255 include: a liquid crystal display (LCD), an organic light-emitting diode (OLED) display, an active-matrix organic light-emitting diode display (AMOLED), a quantum dot organic light-emitting diode (QOLED), a quantum dot light-emitting diode (QLED), some other display, or some combination thereof.

[0037] In some embodiments, the electronic display 255 may include a stack of one or more waveguide displays 275 including, but not limited to, a stacked waveguide display. In some embodiments, the stacked waveguide display may be a polychromatic display (e.g., a red-green-blue (RGB) display) created by stacking waveguide displays whose image light is from respective monochromatic sources of different colors. In some embodiments, the stacked waveguide display may be a monochromatic display.

[0038] FIG. 2C is a cross section of a waveguide display 275 of the head-mounted display in FIG. 2A consistent with the disclosed embodiments. As shown in FIG. 2C, the waveguide display 275 may include a source assembly 271, an output waveguide 272, and a source controller 273. The source assembly 271 includes a source 279 and an optics system 281. The source 279 may be a light source that generates coherent or partially coherent light. The source 279 may include, e.g., a laser diode, a vertical cavity surface emitting laser, and/or a light emitting diode. The optics system 281 may include one or more optical components that condition the light from the source 279. Conditioning light from the source 279 may include, e.g., expanding, collimating, and/or adjusting orientation in accordance with instructions from the source controller 273.

[0039] The source assembly 271 may generate image light 274 and output the image light 274 to a coupling element 276 located on a first side 272-1 of the output waveguide 272. The output waveguide 272 may include an optical waveguide that outputs expanded image light 274 to the eye 265 of the user. The output waveguide 272 may receive the image light 274 at one or more coupling elements 276 located on the first side 272-1, and guide received image light 274 to a directing element 277. In some embodiments, the coupling element 276 may couple the image light 274 from the source assembly 271 into the output waveguide 272.

[0040] The coupling element 276 may include, for example, a diffraction grating, a holographic grating, one or more cascaded reflectors, one or more prismatic surface elements, and/or an array of holographic reflectors. In some embodiments, the coupling element 276 may be a diffraction grating, and a pitch of the diffraction grating may be chosen such that total internal reflection occurs in the output waveguide 282, and the image light 274 may propagate internally in the output waveguide 272 (e.g., by total internal reflection), toward a decoupling element 278.

[0041] The directing element 277 may redirect the received input image light 274 to the decoupling element 278, such that the received input image light 274 is decoupled out of the output waveguide 272 via the decoupling element 278. The directing element 277 may be part of, or affixed to, the first side 272-1 of the output waveguide 272. The decoupling element 278 may be part of, or affixed to, the second side 272-2 of the output waveguide 272, such that the directing element 277 is opposed to the decoupling element 278.

[0042] In some embodiments, the directing element 277 and/or the decoupling element 278 may be structurally similar. The directing element 277 and/or the decoupling element 278 may be, e.g., a diffraction grating, a holographic grating, one or more cascaded reflectors, one or more prismatic surface elements, and/or an array of holographic reflectors. In some embodiments, the directing element 277 may be a diffraction grating, the pitch of the diffraction grating is chosen to cause incident image light 274 to exit the output waveguide 272 at angle(s) of inclination relative to a surface of the decoupling element 278.

[0043] The output waveguide 272 may be composed of one or more materials that facilitate total internal reflection of the image light 274. The output waveguide 272 may be composed of, for example, silicon, plastic, glass, and/or polymers. The output waveguide 272 may have a relatively small form factor. For example, the output waveguide 272 may be approximately 50 mm wide along the x-dimension, 30 mm long along the y-dimension and 0.5-1 mm thick along the z-dimension.

[0044] The source controller 273 may control scanning operations of the source assembly 271, and determine scanning instructions for the source assembly 271. In some embodiments, the output waveguide 272 may output expanded image light 274 to the user’s eye 265 with a large field of view (FOV). For example, the expanded image light 274 may be provided to the user’s eye 265 with a diagonal FOV (in x and y) of 60 degrees and or greater and/or 150 degrees and/or less. The output waveguide 272 may be configured to provide an eye-box with a length of 20 mm or greater and/or equal to or less than 50 mm, and/or a width of 10 mm or greater and/or equal to or less than 50 mm.

[0045] In some embodiments, the waveguide display 275 may include a plurality of source assemblies 271 and a plurality of output waveguides 272. Each of the source assemblies 271 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 output waveguides 272 may be stacked together with a distance of separation to output an expanded image light 274 that is multi-colored. Using the waveguide display 275, the physical display and electronics may be moved to the side (near the user’s temples) and a fully unobstructed view of the real world may be achieved, therefore opening up the possibilities to true AR experiences.

[0046] FIG. 2D illustrates an exemplary multifocal block 260 using adaptive lenses consistent with the disclosed embodiments. As shown in FIG. 2B and FIG. 2D, the multifocal block 260 may include one or more multifocal structures in optical series. A multifocal structure is an optical device that is configured to dynamically adjust its focus in accordance with instructions from a multifocal system. Optical series refers to relative positioning of a plurality of optical elements, such that light, for each optical element of the plurality of optical elements, is transmitted by that optical element before being transmitted by another optical element of the plurality of optical elements. Moreover, ordering of the optical elements does not matter. For example, optical element A placed before optical element B, or optical element B placed before optical element A, are both in optical series. Similar to electric circuitry design, optical series represents optical elements with their optical properties compounded when placed in series.

[0047] The multifocal block 260 may comprise a first adaptive lens assembly 280 which adjusts a focal distance/an image plane at which images from the electronic display 255 are presented to a user of the HMD. The first adaptive lens assembly 280 may include a plurality of lenses 282 arranged in optical series, at least one of which is an active lens having a plurality of optical states (i.e., focal states), such that the plurality of lenses 282 provides a plurality of combinations of optical power, and the plurality of combinations of optical power provides a range of adjustment of optical power for the multifocal block 260. The range of adjustment of optical power for the multifocal block 260 may be a set of discrete values of optical power, and a minimum number of the discrete values of optical power is two. That is, the multifocal block 260 may generate multiple (at least two) image planes.

[0048] In some embodiments, the active lens may be an active or switchable liquid crystal (LC) lens that is switchable between a lens switched-on state with non-zero optical power of d and a lens switched-off state with zero optical power. Herein the unit of the optical power is Diopter. It should be noted that, depending on different structures of the active LC lenses, the active LC lens may provide optical power of d with an applied voltage and optical power of zero without an applied voltage, or vice versa. The active LC lens may be polarization dependent (e.g., linear or circular polarization dependent) or polarization independent. For example, the active LC lens may be one of a linear polarization dependent active LC lens, a circular polarization dependent active LC lens, and a polarization independent active LC lens.

[0049] In some embodiments, the first adaptive lens assembly 280 may also include at least one passive lens having non-switchable optical power, i.e., fixed optical power. In some embodiments, the passive lens may be conventional lens made of, for example, glass, plastic or polymer. In some embodiments, the passive lens may be a polarization dependent or a polarization independent LC lens.

[0050] The minimum number of the discrete values of optical power or the optical power combinations provided by the first adaptive lens assembly 280 may be two, which may be achieved by using one active lens and one passive lens. The maximum number of the discrete values of optical power or optical power combinations provided by the first adaptive lens assembly 280 may be unlimited, without considering size or other optical property factors. When the number of the optical power provided by the first adaptive lens assembly 280 increases, the performance of the first adaptive lens assembly 280 may gradually approach that of a varifocal lens. That is, the size of the range of adjustment for the multifocal block 260 may scale with the number of active lenses included in the first adaptive lens assembly 280.

[0051] In some embodiments, to provide a series of optical power, the linear polarization dependent active LC lens may be arranged in optical series with other linear polarization dependent active LC lenses and/or passive lenses (such as conventional lenses, linear polarization dependent passive LC lenses, or polarization independent passive LC lenses), and the incident light may be linearly polarized. The circular polarization dependent active LC lens may be arranged in optical series with other circular polarization dependent active LC lenses and/or passive lenses (such as conventional lenses, circular polarization dependent passive LC lenses, or polarization independent passive LC lenses), and the incident light may be circularly polarized. The polarization independent active LC lenses may be arranged in optical series other active and/or passive polarization independent lenses (such as conventional lenses or polarization independent LC passive lenses).

[0052] In some embodiments, when the HMD acts as an AR or an MR device, the multifocal block 260 may further include a second adaptive lens assembly 285 configured to compensate the distortion of the real-world images caused by the first adaptive lens assembly 280, such that the real-world objects viewed through the HMD may stay unaltered. The second adaptive lens assembly 285 may provide a plurality of combinations of optical power, which is opposite to the plurality of combinations of optical power but having a same absolute value as the plurality of combinations of optical power provided by the first adaptive lens assembly 280.

[0053] In some embodiments, the first adaptive lens assembly 280 may be configured to provide positive optical power and, thus, the second adaptive lens assembly 285 may be configured to negative optical power of the same magnitude to compensate the first adaptive lens assembly 280. When the HMD acts as a VR device, the second adaptive lens assembly 285 may be omitted.

[0054] Similar to the first adaptive lens assembly 280, the second adaptive lens assembly 285 may also include a plurality of lenses 284 arranged in optical series, and at least one of the plurality of lenses 284 may be an active lens having a plurality of optical states. The second adaptive lens assembly 285 may provide a series of optical power which are formed by various combinations of optical power of the plurality of lenses, which in turn are achieved by the plurality of optical states of the active lens. The second adaptive lens assembly 285 may include the same number of lenses as the first adaptive lens assembly.

[0055] In some embodiments, the active lens included in the second adaptive lens assembly 285 may be an active LC lens having switchable optical power, whose features may be similar to that of the active LC lens in the first adaptive lens assembly 280. Further, the plurality of lenses 284 in the second adaptive lens assembly 285 may have the same polarization dependency as the plurality of lenses 282 in the first adaptive lens assembly 280. For example, the active LC lenses in the first adaptive lens assembly 280 and the second adaptive lens assembly 285 may be both linear polarization dependent active LC lenses, circular polarization dependent active LC lenses, or polarization independent active LC lenses. In some embodiments, the second adaptive lens assembly 285 may also include at least one passive lens with unswitchable optical power, whose features may be similar to that of the passive lens in the first adaptive lens assembly 280. The passive lens in the second adaptive lens assembly 285 may have the same lens type or polarization dependency as the passive lens in the first adaptive lens assembly 280. The details of the active LC lens and the passive lens included in the second adaptive lens assembly 285 are not repeated here.

[0056] Further, the multifocal block 260 may include one or more substrate layers, a linear polarizer, a quarter-wave plate, a half-wave plate, a circular polarizer or some combination thereof. For example, the linear polarizer may be optically coupled to the first adaptive lens assembly, to ensure the light incident onto the first adaptive lens assembly is linearly polarized light. The linear polarizer and the quarter-wave plate may be optically coupled to the first adaptive lens assembly, to ensure the light incident onto the first adaptive lens assembly is circularly polarized light. The half-wave plate may switch the polarization direction of incident polarized light to the orthogonal one. For example, the half-wave plate may reverse the handedness of the incident circularly polarized light. In some embodiments, the half-wave plate may include a switchable half-wave plate (SHWP), which may reverse the handedness of the incident circularly polarized light in accordance with a switching state (i.e., active or non-active). The circular polarizer and the quarter-wave plate may be optically coupled to the first adaptive lens assembly, to ensure the light incident onto the first adaptive lens assembly is linear polarized light. In some embodiments, the circular polarizer may include a cholesteric circular polarizer.

[0057] The substrate layers are layers which other elements (e.g., switchable half-wave plate, liquid crystal, etc.) may be formed upon, coupled to, etc. The substrate layers are substantially transparent in the visible band (.about.380 nm to 750 nm). In some embodiments, the substrate may also be transparent in some or all of the infrared (IR) band (.about.750 nm to 1 mm). The substrate layers may be composed of, e.g., SiO2, plastic, sapphire, etc.

[0058] Additionally, in some embodiments, the multifocal block 260 may magnify received light, corrects optical errors associated with the image light, and presents the corrected image light is presented to a user of the HMD 200. The multifocal block 260 may additionally include one or more optical elements in optical series. An optical element may be an aperture, a Fresnel lens, a convex lens, a concave lens, a filter, or any other suitable optical element that affects the blurred image light. Moreover, the multifocal block 260 may include combinations of different optical elements. In some embodiments, one or more of the optical elements in the multifocal block 260 may have one or more coatings, such as anti-reflective coatings.

[0059] FIGS. 3A-3B illustrate an exemplary linear polarization dependent LC lens 300 consistent with the disclosed embodiments. As shown in FIGS. 3A-3B, the linear polarization dependent LC lens 300 is often named as a GRIN (Gradient of Refractive Index) lens, which is a lens with gradient of refractive index. The gradient of refractive index may be obtained by gradient of LC alignment. In some embodiments, the gradient of LC alignment may be obtained by gradient of electric field generated in the LC lens 300, which may be realized by adopting, for example, a set of discrete ring-patterned electrodes addressed individually with different voltages, a hole-patterned electrode plate, or a spherical shape electrode. In some embodiments, the gradient of LC alignment may be obtained by gradient of anchoring which determines gradient of LC pretilt angles.

[0060] In one embodiment, as shown in FIG. 3A, when applying a zero voltage (or more generally below some minimal value which is too small to reorient LC molecules 230) to the LC lens 300, the LC molecules 320 may be uniformly aligned along an x-direction in an LC layer. For incident light 330 having a polarization direction along the alignment directions of the LC molecules 320 or the LC layer, i.e., x-direction, the optical power of the LC lens 300 may be zero. That is, no lens effect is provided.

[0061] When applying a voltage of certain amplitude (or more generally above some threshold value which is large enough to reorient the LC molecules 320) to the LC lens 300, the LC molecules 320 may be reoriented to generate the gradient of LC alignment from the center to the edge of the LC layer. In one embodiment, as shown in FIG. 3A, from the center to the edge of the LC layer, the LC alignment may change from being parallel to a surface of the LC lens 300 to being closer to perpendicular to the surface of the LC lens 300. For the incident light 330 having the polarization direction along the alignment directions of the LC molecules 320 or the LC layer, i.e., x-direction, the effective refractive index of the LC molecules 320 may gradually decrease from the center to the edge of the LC layer. Then a positive lens profile 310 may be obtained, and the incident light having the polarization direction 330 along the x-direction may be focused. That is, the LC lens 330 may be a positive LC lens providing positive optical power d.

[0062] In particular, the optical power d of the switched-on LC lens 300 may be calculated by d=8.delta.n*L/D2, where L is the thickness of the LC layer, D is the aperture size of the LC lens 300, .delta.n is the refractive indices difference between the center and the edge of the LC lens 300. As long as the aperture size (D) and the thickness (L) are fixed, the optical power d of the LC lens 300 may be determined by the refractive indices difference (.delta.n) between the center and the edge. .delta.n is always smaller than or equal to .DELTA.n, where .DELTA.n is the birefringence of the LC materials of the LC layer.

[0063] As shown in FIG. 3B, for the incident light 340 having the polarization direction perpendicular to the alignment directions of the LC molecules 320 or the LC layer, i.e., y-direction, the LC lens 300 may appear to be a transparent plate without gradient of refractive index regardless of the applied voltage. That is, the LC lens 300 may not provide a lens effect to the incident light 340 polarized in y-direction.

[0064] It should be noted that, the LC molecules orientations shown in FIGS. 3A-3B are merely for illustrative purposes and are not intended to limit the scope of the present disclosure. The LC lens 300 may have any appropriate structure which utilize the change in polar angle (or tilt angle) to create the lens profile, and the optical power of the LC lens 300 could be switched between some value d (e.g., at the lens switched-on state) and 0 (e.g., at the lens switched-off state), such as a Fresnel LC lens based on light diffraction. In some embodiments, the LC lens 300 may be a negative lens providing negative optical power. In some embodiments, the LC lens 300 may be a hybrid lens, whose optical power could be changed from a certain negative value to zero to a certain positive value.

[0065] FIG. 4A illustrates an exemplary circular polarization dependent LC lens 400 consistent with the disclosed embodiments. In some embodiments, the circular polarization dependent LC lens 400 may be a Pancharatnam Berry Phase (PBP) LC lens, which creates a lens profile via an in-plane orientation (.theta., azimuth angle) of LC molecules. The phase difference of the PBP LC lens may be calculated as T=20.

[0066] FIG. 4B illustrates exemplary LC orientations 410 in the LC lens of FIG. 4A. As shown in FIG. 4B, in the PBP LC lens 400, an azimuth angle (.theta.) of an LC molecule 406 may be continuously changed from a center 402 to an edge 404 of the PBP LC lens 400, with a varied pitch A. Pitch is defined in a way that the azimuth angle of LC is rotated 180.degree. from the initial state. FIG. 4C illustrates a section of exemplary LC orientations 420 taken along y-axis in the LC lens of FIG. 4A. As shown in FIG. 4C, a rate of pitch variation may be a function of distance from the lens center 402. The rate of pitch variation may increase with distance from the lens center. For example, the pitch at the lens center 402 (.LAMBDA.0) may be the highest or the pitch variation is the slowest, and the pitch at the edge 404 (.LAMBDA.r) may be the smallest or the pitch variation is the fastest, i.e., .LAMBDA.0>.LAMBDA.1> … >.LAMBDA.r.

[0067] Referring to FIGS. 4A-4C, in the x-y plane, to obtain a PBP LC lens with lens radius (r) and lens power (+/-f), the azimuth angle .theta. may satisfy: 2.theta.=.pi.r2/(f.lamda.), where .lamda. is the wavelength of incident light. In addition, along the light propagation direction z-direction, a dual twist or multiple twisted structure layers offers achromatic performance on efficiency in the PBP LC lens 400. Along the z-direction, the non-twisted structure is simpler to fabricate then a twisted structure, but is optimized for a monochromatic light.

[0068] Design specifications for HMDs used for VR, AR, or MR applications typically requires a large range of optical power to adapt for human eye vergence-accommodation (e.g., .about..+-.2 Diopters or more), fast switching speeds (e.g., <20 ms), and a good quality image. The PBP LC lens 400 may be able to meet design specs using LC materials having a relatively low index of refraction and, moreover, the PBP LC lens 400 may have a large aperture size, a thin thickness (e.g., a single LC layer can be .about.2 .mu.m) and high switching speeds (e.g., <20 ms) to turn the lens power on/off.

[0069] Returning to FIG. 4A, in some embodiments, the PBP LC lens 400 may be an active PBP LC lens, which has three discrete focal states (also referred to as optical states). The three optical states are an additive state, a neutral state, and a subtractive state. In particular, the additive state may add optical power to the system (i.e., have a positive focus of f), and the subtractive state may subtract optical power from the system (i.e., have a negative focus of -f). When not in the neutral state, the PBP LC lens 400 may reverse the handedness of circularly polarized light passing through the PBP LC lens 400 in addition to focusing/defocusing the incident light. The neutral state may not affect the optical power of the system, however, the handedness of circularly polarized light passing through the PBP LC lens 400 may be unchanged or changed.

[0070] The optical state of the PBP LC lens 400 may be determined by the handedness of circularly polarized light incident on the PBP LC lens and an applied voltage. In some embodiments, as shown in FIG. 4A, the PBP LC lens 400 may operate in an additive state that adds optical power to the system in response to incident light with a right-handed circular polarization and an applied voltage of zero (or more generally below some minimal value), operate in a subtractive state that removes optical power from the system in response to incident light with a left-handed circular polarization and the applied voltage of zero (or more generally below some minimal value), and operate in a neutral state (regardless of polarization) that does not affect the optical power of the system in response to an applied voltage larger than a threshold voltage which aligns LCs along with the electric field.

[0071] Through flipping the PBP LC lens 400, the additive state and the subtractive state of the PBP LC lens 400 may be reversed for the circularly polarized incident light with the same handedness. For example, after flipping the PBP LC lens 400 in FIG. 4A, the flipped PBP LC lens (at the right side of FIG. 4A) may operate in an additive that adds optical power to the system in response to incident light with a left-handed circular polarization and an applied voltage of zero (or more generally below some minimal value), operate in a subtractive state that removes optical power from the system in response to incident light with a right-handed circular polarization and the applied voltage of zero (or more generally below some minimal value), and operate in a neutral state (regardless of polarization) that does not affect the optical power of the system in response to an applied voltage larger than a threshold voltage which aligns LCs along with the electric field.

[0072] Although the PBP LC lens 400 does not provide any lens power in the neutral state, the polarization handedness of the light transmitted through the PBP LC lens 400 may be changed or unchanged. FIG. 4D illustrates an exemplary neutral state 430 of the BPB LC lens 400 which does not change the polarization handedness of the transmitted light. As shown in FIG. 4D, when the generated electric field is perpendicular to the LC layer (e.g., a vertical electrical field which is applied across the LC layer), the LC molecules 406 having positive dielectric anisotropy may be reoriented along the direction of the vertical electrical field to be perpendicular to the LC layer, i.e., out-of-plane switched. That is, the orientation of the LC molecules 406 having positive dielectric anisotropy may be in a homeotropic state and, thus, the LC layer may act as isotropic medium for normally incident light. When the optical power of the BPB LC lens 400 is out-of-plane switched off, the polarization handedness of the light transmitted through the PBP LC lens 400 may not be affect.

[0073] FIG. 4E illustrates an exemplary neutral state 440 of the BPB LC lens 400 which reverses the polarization handedness of the transmitted light transmitted. As shown in FIG. 4E, when the generated electric field is parallel to the LC layer (e.g., a horizontal electric field which is applied in plane of the LC layer), the LC molecules 406 having positive dielectric anisotropy may be reoriented along the direction of the horizontal electrical field to be parallel to the LC layer, i.e., in-plane switched, and the orientation of the LC molecules 406 having positive dielectric anisotropy may be in a homogeneous state. That is, the patterned LC alignment structure giving optical power of the BPB LC lens 400 may be transformed to the uniform uniaxial planar structure functioning as a half-wave plate. When the optical power of the BPB LC lens 400 is in-plane switched off, for the circularly polarized incident light, the polarization handedness of the light transmitted through the PBP LC lens 400 may be reversed.

[0074] For illustrative purposes, FIGS. 4D-4E merely illustrate the orientation of the LC molecules 406 having positive dielectric anisotropy under a generated electric field. In some embodiments, the LC molecules may have negative dielectric anisotropy, then the generated vertical electric field may enable the LC molecules 406 to be reoriented towards a direction parallel to the LC layer, which leads to a reversed polarization handedness of the light transmitted through the PBP LC lens 400. The horizontal electric field may enable the LC molecules 406 to be reoriented towards a direction perpendicular to the LC layer, and the polarization handedness of the light transmitted through the PBP LC lens 400 may not be affected.

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