Facebook Patent | Multifocal System Using Pixel Level Polarization Controllers And Folded Optics

Patent: Multifocal System Using Pixel Level Polarization Controllers And Folded Optics

Publication Number: 10598945

Publication Date: 20200324

Applicants: Facebook

Abstract

A head-mounted display (HMD) that includes optical components that provide multiple focal distances for light emitted from an electronic display. The HMD includes a multifocal structure having a plurality of optical components positioned in series such that light from an electronic display is received and passes through the optical components at least once before being output from the multifocal structure. The plurality of optical components includes a pixel level polarizer positioned to receive light from the electronic display. The pixel level polarizer has a first configuration that causes the pixel level polarizer to linearly polarize light in a first direction and a second configuration that causes the pixel level polarizer to linearly polarize light in a second direction that is different than the first direction. The multifocal structure is configured to output image light different focal distances based in part on the configuration of the pixel level polarizer.

BACKGROUND

This invention relates generally to optical systems and specifically relates to a multifocal optical system with polarizing elements.

Head-mounted displays (HMDs) may be used to present augmented and/or virtual information to a user. For example, a virtual reality (VR) headset can be used to simulate virtual environments. Conventionally, a HMD presents stereoscopic images on an electronic display inside the HMD to simulate the illusion of depth. However, conventional HMDs are often unable to compensate for vergence and accommodation conflict when rendering content, which may cause double vision, visual fatigue, and nausea in users.

SUMMARY

A head-mounted display (HMD) presents images at multiple focal distances, such as at four or more different focal distances. The HMD includes a multifocal structure having a plurality of optical components positioned in series such that light from an electronic display is received and passes through the optical components at least once before being output from the multifocal structure. The plurality of optical components includes a pixel level polarizer positioned to receive light from the electronic display. The pixel level polarizer has a first configuration that causes the pixel level polarizer to linearly polarize light in a first direction and a second configuration that causes the pixel level polarizer to linearly polarize light in a second direction that is different than the first direction. The multifocal structure is configured to output image light different focal distances based in part on the configuration of the pixel level polarizer.

The multifocal structure may include a liquid crystal (LC) lens element that has a state that adjusts optical power of incident light and a neutral state that does not affect optical power of incident light. The multifocal structure may also include a folded optics assembly that directs light along different optical paths based in part on a polarization of light incident on the folded optics assembly, and wherein the different optical paths each have different optical path lengths. Different combinations of optical paths and optical power adjustments can be used to create multiple (e.g., four) image planes.

In some embodiments, the LC lens element is a Pancharatnam Berry Phase (PBP) LC lens having an additive state that adds optical power, subtractive state that subtracts optical power, and a neutral state. In some embodiments, the LC lens element is a liquid crystal lens having an additive state that adds optical power and a neutral state.

An eye tracking system (e.g., including processing circuitry) may be connected with the multifocal structure to provide focus instructions to the multifocal structure. Responsive to receipt of the focus instructions, the multifocal structure configures the pixel level polarizer and sets the state of the LC lens element to output light at a selected focal distance. In some embodiments, the focus instructions provide for cell level control of the pixel level polarizer to select focal distances for light emitted from the pixels of the electronic display.

Some embodiments may include an optical system for an electronic display. The optical system includes a multifocal structure having a plurality of possible focal distances. The multifocal structure includes: a plurality of optical components positioned in series such that light from an electronic display is received and passes through each of the plurality of optical components at least once before being output from the multifocal structure. The plurality of optical components include: a pixel level polarizer positioned to receive light from the electronic display, the pixel level polarizer having a first configuration that causes the pixel level polarizer to linearly polarize light in a first direction and a second configuration that causes the pixel level polarizer to linearly polarize light in a second direction that is different than the first direction, and a LC lens element having an active state that adjusts optical power of incident light and a neutral state that does not affect optical power of incident light, wherein the multifocal structure is configured to output image light at one or more different focal distances based in part on the configuration of the pixel level polarizer and the state of the LC lens element.

Some embodiments may include methods of operating the HMD to produce multiple focal distances.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an example of how a human eye experiences vergence and accommodation in a real world, according to one embodiment.

FIG. 1B shows an example conflict between vergence and accommodation that can occur with some three-dimensional displays, according to one embodiment.

FIG. 2A is a wire diagram of a head mounted display (HMD), according to one embodiment.

FIG. 2B is a cross section of a front rigid body of the HMD shown in FIG. 2A, according to one embodiment.

FIG. 3A illustrates an example of a pixel level polarizer, according to one embodiment.

FIG. 3B illustrates another example of a pixel level polarizer, according to one embodiment.

FIG. 4A illustrates a liquid crystal lens, according to one embodiment.

FIG. 4B illustrates a Pancharatnam Berry Phase (PBP) liquid crystal lens, according to one embodiment.

FIG. 5A illustrates a PBP lens, according to one embodiments.

FIG. 5B illustrates an example of liquid crystal orientations in the PBP lens of FIG. 4A, according to an embodiment.

FIG. 5C is a section of liquid crystal orientations of the PBP lens of FIG. 4A, according to an embodiment.

FIG. 6 illustrates optical components of an HMD including a multifocal structure,* according to one embodiment*

FIG. 7A illustrates an optical path within the multifocal structure of FIG. 6, according to one embodiment.

FIG. 7B illustrates a second optical path within the multifocal structure of FIG. 6, according to one embodiment.

FIG. 7C illustrates a third optical path within the multifocal structure of FIG. 6, according to one embodiment.

FIG. 7D illustrates a fourth optical within the multifocal structure of FIG. 6, according to one embodiment.

FIG. 8 illustrates an optical path in the multifocal structure shown in FIG. 6, according to one embodiment.

FIG. 9 illustrates an optical path in the multifocal structure shown in FIG. 6, according to one embodiment.

FIG. 10 illustrates optical components of an HMD including a multifocal structure, according to one embodiment.

FIG. 11 illustrates an optical path in the multifocal structure shown in FIG. 10, according to one embodiment.

FIG. 12 illustrates an optical path in the multifocal structure shown in FIG. 10, according to one embodiment.

FIG. 13 illustrates an optical path in the multifocal structure shown in FIG. 10, according to one embodiment.

FIG. 14 illustrates a multifocal system in which a HMD operates, according to an embodiment.

FIG. 15 illustrates a process for mitigating vergence-accommodation conflict by adjusting the focal length of a HMD, according to an embodiment.

FIG. 16 illustrates an example process for mitigating vergence-accommodation conflict by adjusting a focal length of a multifocal structure, according to an embodiment.

The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION

Vergence-accommodation conflict is a problem in many augmented reality or virtual reality systems. Vergence is the simultaneous movement or rotation of both eyes in opposite directions to obtain or maintain a 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 the shape of their lens) to provide accommodation at the new distance or vergence depth of the new object. FIG. 1A shows an example of how the human eyes experience vergence and accommodation in the real world. In the example of FIG. 1A, the user is looking at a real object 100A (i.e., the user’s eyes are verged on the real object 100A and gaze lines from the user’s eyes intersect at the real object 100A). As the real object 100A 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 100A. As the real object 100A gets closer, the eye 102 must “accommodate” for the closer distance by changing the shape of its lens to increase focusing power or reduce focal length. Thus, under normal conditions in the real world, the vergence depth (d.sub.v) equals the focal point (d.sub.f).

FIG. 1B shows an example conflict between vergence and accommodation that can occur with some three-dimensional (3D) electronic displays. In this example, 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 a 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 there is no increased focusing power or reduced focal length of each eye; hence, the user’s eyes do not accommodate as in FIG. 1A. Thus, instead of increasing focusing power or reducing focal length to accommodate for the closer vergence depth, each eye 102 maintains accommodation at a distance associated with the electronic display 104. Thus, the vergence depth (d.sub.v) often does not equal the focal point (d.sub.f) for the human eye for objects displayed on 3D electronic displays. This discrepancy between vergence depth and focal length is referred to as “vergence-accommodation conflict.” A user experiencing only vergence or accommodation and not both will eventually experience some degree of fatigue and nausea, which is undesirable for head mounted displays.

In addition to vergence-accommodation conflict, augmented reality systems can create real object-AR image mismatch. Here, a visual augmented image fails to be matched with a real-object. For example, an AR object that is displayed on a (e.g., transparent) electronic display as being on top of a real-world object (e.g., a desk) can result in real object-AR image mismatch if the vergence depth of the real-world object is at a different focal distance than the AR object in the electronic display. The techniques discussed herein reduce vergence-accommodation conflict and real object-AR image mismatch for HMDs.

FIG. 2A is a wire diagram of a head mounted display (HMD) 200, according to one embodiment. The HMD 200 may be for use in a virtual reality (VR) system, an augmented reality (AR) system, a mixed reality (MR) system, or some combination thereof. The HMD 200 includes a front rigid body 205 and a band 210. In some embodiments, portions of the HMD 200 may be at least partially transparent (e.g., if the HMD 200 is part of an AR or MR system). For example, portions of the front side 206 and intervening components (e.g., electronic display) between the front side 206 and an eye of a user may be transparent. The front rigid body 205 includes other components of the HMD 200, such as the inertial measurement unit 215, position sensors 220, and locators 225. These and other components are discussed in greater detail below in connection with FIG. 14.

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 includes an electronic display 255 and a multifocal structure 260 that together provide image light to an exit pupil 263. The exit pupil 263 is 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 optics block, separate from the multifocal structure 260, provides altered image light to another eye of the user.

The electronic display 255 displays images to the user. In various embodiments, the electronic display 255 may include 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 QOLED, a QLED, some other display, or some combination thereof.

The multifocal structure 260 includes adjusts an orientation of light emitted from the electronic display 255 such that it appears at one or more particular focal distances from the user. The multifocal structure 260 includes a plurality of optical components such as, for example, a pixel level polarizer, a lens, a liquid crystal (LC) lens element (e.g., including a PBP (Pancharatnam Berry Phase) LC lens or other type of LC lens), a partially reflective surface, a polarizer, a passive waveplate (e.g., half waveplate, quarter waveplate), a switchable waveplate (e.g., SHWP), a beam splitter (e.g., 50:50 beam splitter), a beam splitting polarizer (e.g., reflective linear polarizer or wire grid polarizer), a polarization rotator (e.g., a Faraday rotator), or any other suitable optical component that affects the image light emitted from the electronic display 255. Moreover, a multifocal structure 260 may include combinations of different optical components. In some embodiments, one or more of the optical components in the multifocal structure 260 may have one or more coatings, such as anti-reflective coatings. A liquid crystal lens can provide different optical power adjustments for light emitted from the electronic display 255, and partially reflective surfaces created a folded optics system that provides a polarization dependent path (e.g. direct or folded) for the light emitted from the electronic display 255. Using different combinations of optical power and optical path configurations, the light emitted from pixels of the electronic display 255 can be placed at one or multiple different focal distances.

The multifocal structure 260 adjusts the focal distance by adjusting the length of the optical path of light (path length) emitted from the electronic display 255. Each focal distance corresponds to a respective focal plane, and each focal plane is associated with a respective path length of image light that propagates from the electronic display 255 to the exit pupil 263. The multifocal structure 260 varies the path length of the image light, and thus the focal planes, by varying a number of reflections the image light undergoes in the multifocal structure 260. A larger number of reflections increases the path length and, accordingly, presents a focal plane further away from the user. Likewise, a smaller number of reflections decrease the path length and, accordingly, presents a focal plane close to the user. The location of the focal planes also depends on focal lengths of the optical components within the optical path.

Additionally, in some embodiments, the multifocal structure 260 magnifies received light, corrects optical errors (e.g., field curvature, chromatic aberration, etc.) associated with the image light, and presents the corrected image light to a user of the HMD 200.

FIG. 3A illustrates a pixel level polarizer 300, in accordance with one embodiment. The pixel level polarizer 300 is an example of an optical component of the multifocal structure 260. The pixel level polarizer 300 is positioned to receive light emitted from the electronic display 255. The pixel level polarizer 300 polarizes light emitted from each pixel of the electronic display 255. Depending on the type of electronic display 255, the light emitted from the electronic display 255 may be polarized or unpolarized. The pixel level polarizer 300 receives light from each pixel of the electronic display 255, and is optically matched to the emitted light to transmit a portion of the light as linearly polarized light 302 aligned along 45 degrees and a portion of the light as linearly polarized light 304 aligned along 135 degrees.

The pixel level polarizer may include a plurality of cells 330. Each cell 330 receives light from one or more pixels of the electronic display 255, and transmit the light as linearly polarized light along the first or second direction. The light transmitted from adjacent cells 330 of the pixel level polarizer 300 may have polarizations aligned tangentially from each other. The linearly polarized light 302 and linearly polarized light 304 provide two image planes that can take different optical paths through the multifocal structure 260.

FIG. 3B illustrates pixel level polarizer 350, in accordance with one embodiment. The pixel level polarizer 350 is another embodiment of the pixel level polarizer 300 that can be positioned to receive light emitted from the electronic display 255. The pixel level polarizer 350 polarizes light emitted from each pixel of the electronic display in orientations different from the orientations of the pixel level polarizer 300. The pixel level polarizer 350 transmit linearly polarized light 352 aligned along 90 degrees and linearly polarized light 354 aligned along 0 degrees. The other optical components may be aligned accordingly to accommodate the 0 and 90 degree alignments output from the pixel level polarizer 350, or some other (e.g., tangential) alignment along other axes.

In some embodiments, the pixel level polarizer 300/350 has a first configuration that causes the pixel level polarizer 300/350 to linearly polarize light in a first direction and a second configuration that causes the pixel level polarizer 300/350 to linearly polarize light in a second direction that is different than the first direction. The multifocal structure 260 is configured to output image light at different focal distances based in part on the configuration of the pixel level polarizer.

In some embodiments, the pixel level polarizer 300/350 includes at least a first set and a second set of cells 330, and the pixel level polarizer has a third configuration that causes the pixel level polarizer to linearly polarize light passing through the first set of cells 330 in the first direction and to linearly polarize light passing through the second set of cells 330 in the second direction. For example, the first set of cells 330 of the pixel level polarizer 300 may include cells that output the linearly polarized light 302 aligned along 45 degrees and the second set of cells 330 may include cells that output the linearly polarized light 304 aligned along 135 degrees. The first and second set of cells 330 may be arranged in alternating patterns across the pixel level polarizer. In another example, the pixel level polarizer selectively causes each cell to linearly polarize light in either the first direction or second direction. In some embodiments, the first set of cells of the pixel level polarizer 350 may include cells that output the linearly polarized light 352 aligned along 90 degrees and the second set of cells may include cells that output the linearly polarized light 354 aligned along 0 degrees.

In some embodiments, the configuration of each cell of a pixel level polarizer may be set by a focus signal. The focus signal may provide for a cell-by-cell addressing and selection for orientations of light emitted from each cell 330 of the pixel level polarizer. The pixel level polarizer may include a layer of liquid crystals that form the cells 330. The liquid crystals of a cell 330 change their orientation based on the focus signal to control the orientation of linear light emitted from the cell 330, which results in different orientations of linear light being transmitted through the liquid crystals.

In some embodiments, the cells 330 are defined by a mask having a fixed arrangement of liquid crystals to transmit light at the first and second directions. Here, adjacent cells 330 may transmit light in tangential directions. The mask may be coupled to or formed on a display surface of the electronic display 255.

FIG. 4A illustrates a liquid crystal lens 400, according to an embodiment. The LC lens 400 is an example of a LC lens element of the multifocal structure 260. The LC lens 400 has two states: an additive state (with positive focus of “f”) and a neutral state. In either state, the LC lens 400 can receive incident right circularly polarized light 402 or incident left circularly polarized light 404, and outputs light without changing the polarization of the incident light.

FIG. 4B illustrates a Pancharatnam Berry Phase (PBP) liquid crystal lens 450, according to an embodiment. The PBP liquid crystal lens 450 (or “PBP lens 450”) is another example of the LC lens element of the multifocal structure 260. The PBP lens 450 has three states: an additive state, a subtractive state, and a neutral state. In an additive state, the PBP lens 450 adds optical power R and has a positive focus of “f,” where R (step resolution) is a positive number (e.g., 0.1, 0.25, 0.5 etc.). Conversely, in a subtractive state, the PBP lens 450 subtracts–R of optical power and has a negative focus of `-f.” In the neutral state, the PBP lens 450 does not add or subtract optical power. The additive and subtractive states are also referred to as active states because the PBP lens 450 changes the optical power of incident light.

The state of the PBP lens 450 is determined by whether the incident light has left circular polarization (LCP) or right circular polarization (RCP), and applied voltage. The PBP lens 450 operates in the additive state (f) responsive to incident light 452 with a right handed circular polarization and an applied voltage of zero (or more generally below some minimal value), operates in the subtractive state (-f) responsive to incident light 404 with a left handed circular polarization and the applied voltage of zero (or more generally below some minimal value), and operates in a neutral state (0) (regardless of polarization) responsive to an applied voltage larger than a threshold voltage which aligns liquid crystal with positive dielectric anisotropy along with the electric field direction. Note that if the PBP lens 450 is in the additive or subtractive state, light output from the PBP lens 450 has a handedness opposite that of the light input into the PBP lens 450. In contrast, if the PBP lens 450 is in the neutral state, light output from the PBP lens 450 has the same handedness as the light input into the PBP lens 400.

In some embodiments, the PBP lens 450 is a passive PBP liquid crystal lens having two optical states, specifically, the additive state and the subtractive state. The state of a passive PBP lens 450 is determined by the handedness of polarization of light incident on the passive PBP lens 450. A passive PBP liquid crystal lens operates in a subtractive state responsive to incident light with a left handed polarization and operates in an additive state responsive to incident light with a right handed polarization. Note that the passive PBP lens 450 outputs light that has a handedness opposite that of the light input into the passive PBP lens 450 lens.

In some embodiments, the LC lens element of the multifocal structure 260 may include a series of LC lenses and/or PBP lenses. Each lens can be placed in various states, with combinations of states producing different combined optical powers.

FIG. 5A is an example PBP lens 500, according to an embodiment. The PBP lens 500 is an example of a PBP lens 450 shown in FIG. 4B. The PBP lens 500 creates a respective lens profile via an in-plane orientation (.THETA., azimuth angle) of a liquid crystal molecule, in which the phase difference T=2.THETA.. In contrast, a (non-PBP) liquid crystal lens creates a lens profile via a birefringence (.DELTA.n) and layer thickness (d) of liquid crystals, and a number (#) of Fresnel zones (if it is Fresnel lens design), in which the phase difference T=.DELTA.nd*#*2.pi./.lamda.. Accordingly, in some embodiments, a PBP lens 500 may have a large aperture size and can be made with a very thin liquid crystal layer, which allows fast switching speed to turn the lens power on/off.

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., .+-.2 Diopters or more), fast switching speeds (e.g., .about.300 ms), and a good quality image. The PBP lens may be desirable over other types of liquid crystal lenses having relatively high index of refraction or be relatively larger thick (which reduces switching speeds). The PBP liquid includes a liquid crystal having a relatively low index of refraction, is thin (e.g., a single liquid crystal layer can be .about.2 .mu.m), and has high switching speeds (e.g., 300 ms).

FIG. 5B is an example of liquid crystal orientations 510 in the PBP lens 500 of FIG. 5A, according to an embodiment. In the PBP lens 500, an azimuth angle (.THETA.) of a liquid crystal molecule is continuously changed from a center 520 of the PBP lens 500 to an edge 530 of the PBP lens 500, 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. 5C is a section of liquid crystal orientations 540 taken along a y axis in the PBP lens 500 of FIG. 5A, according to an embodiment. The liquid crystal orientation 540 has a rate of pitch variation as a function of distance from the lens center 520. The rate of pitch variation increases with distance from the lens center. For example, pitch at the lens center (.LAMBDA..sub.0), is the slowest and pitch at the edge 520 (.LAMBDA..sub.r) is the highest, i.e., .LAMBDA..sub.0>.LAMBDA..sub.1> … >.LAMBDA..sub.r. In the x-y plane, to make a PBP lens with lens radius (r) and lens power (+/-f), the azimuth angle needs to meet: 2.THETA.=r.sup.2/f*(.pi./.lamda.), where .lamda. is the wavelength of light. Along with the z-axis, a dual twist or multiple twisted structure layers offers achromatic performance on efficiency in the PBP lens 500. Along with the z-axis, the non-twisted structure is simpler to fabricate then a twisted structure, but is optimized for a monochromatic light.

Note that a PBP lens may have a twisted or non-twisted structure. In some embodiments, a LC lens element may include one or more PBP lenses having a twisted structure, one or more PBP lenses having a non-twisted structure, or some combination thereof.

FIG. 6 illustrates optical components of an HMD 200 including a multifocal structure 604, according to one embodiment. The multifocal structure 604 is an embodiment of the multifocal structure 260. The multifocal structure 604 includes a pixel level polarizer 602, a quarter wave plate (QWP) 612, a LC lens element 606, and a folded optics assembly 608. Image light emitted from pixels of the electronic display 255 propagate along a z-axis in a Cartesian coordinate system toward the exit pupil 263. The multifocal structure 604 receives light emitted from the pixels of the electronic display 255, and outputs light associated with each pixel at one of multiple focal distances at the exit pupil 263. In some embodiments, multifocal structure 604 provides for four different selectable focal distances.

The pixel level polarizer 602 is positioned in front of the electronic display 255 (e.g., along the Z axis, from the perspective of the eye 265) to polarize light emitted from each pixel of the electronic display 255 in two polarizations defined as P1 and P2, respectively. For example, P1 may be linear light aligned along a first axis, and P2 may be linear light aligned along a second axis. The first and second axes may be orthogonal to each other. For example, the first axis may be aligned along 45 degrees and the second axis may be aligned along 135 degrees. In another example, the first axis is aligned along 0 degrees and the second axis is aligned along 90 degrees.

The LC lens element 606 may be a PBP lens 450 having an additive state that adds optical power and changes the polarization of incident circularly light, a subtractive state that subtracts optical power and changes the polarization of incident circularly light, and a neutral state that does not change the optical power or polarization of incident light. In other embodiments, the LC lens element 606 may be the LC lens 400 having an additive state that adds optical power and does not change the polarization of incident circularly polarized light, and a neutral state that does not change the optical power or polarization of incident light. The QWP 612 receives linearly polarized light emitted from the pixel level polarizer 602, and converts the linearly polarized light into a circularly polarized light for the LC lens element 606.

The folded optics assembly 608 cause the light output from the LC lens element 606 to take either a direct path or a folded path through the multifocal structure 604 based on a polarization of the light. The folded optics assembly 608 includes a 50:50 reflective element 614, a QWP 616, and a polarizer 618.

The LC lens element 606 can control whether light takes the direct or folded path based on outputting the light at different (e.g., circular left or right) polarizations. Light propagating along the direct path through the multifocal structure 604 propagates along the z-axis in a direction from the electronic display 255 toward the exit pupil 263. Light propagating along the folded path also propagates from the electronic display 255 toward the exit pupil 263, but takes a bi-directional path within the folded optics assembly 608 that includes propagation along the z-axis in a first direction from the electronic display 255 toward the exit pupil 263, and a second direction from the exit pupil 263 toward the electronic display 255. As such, the folded path provides a larger focal distance for light than the direct path. Multiple optical paths for the multifocal structure 604 are discussed in detail below with regard to FIGS. 7A through 9. By controlling the operation of the pixel level polarizer 602 and the LC lens element 606, the multifocal structure 604 selectively outputs light from the electronic display 255 at 4 different focal distances.

The optical components of the multifocal structure 604 are not necessarily arranged in the order shown in FIG. 6. For example, multiple focal distances can also be achieved by placing the LC lens element 606 between the 50:50 reflective element 614 and the QWP 616.

The QWPs 612 and 616 convert incident linearly polarized light into circularly polarized light, and vice versa. The orientation of an optic axis relative to the axis orientation incident linearly polarized light controls the handedness of the emitted circularly polarized light. Similarly, the handedness of incident circularly polarized light controls orientation of linearly polarized light output by the QWPs 612 and 616.

The 50:50 reflective element 614 transmits a portion of incident light and reflect a portion of incident light (e.g., without changing polarization of the light). The 50:50 reflective element 614 may reflect 50% of incident light and transmit the remaining 50% of incident light. A partially reflective mirror transmits some percentage of incident light and reflects the remaining percentage of incident light regardless of polarization.

The polarizer 618 linearly polarizes light in accordance with the orientation of its polarization axis. The polarizer 618 may be, for example, a reflective polarizer, a linear polarizing beam splitter, or some other optical element that linearly polarizes light. A reflective polarizer polarizes light in accordance with its associated axis orientation. For example, a polarizer passes incident light whose electric field component is aligned with an axis orientation of the reflective polarizer, and reflects light whose electric field component is orthogonal to the axis orientation of the reflective polarizer. A linear polarizing beam splitter splits light into two beams whose respective electric field components are linearly polarized. In some embodiments, one of the beams output from the linear polarizing beam splitter is blocked (e.g., via baffles, some type of light absorptive mechanism, etc.).

Although illustrated as being flat and separate elements, some or all of the optical elements (e.g., the polarizer 618, the QWPs 612 and 616, and the 50:50 reflective element 614) may be coatings on optical surfaces that are curved to add optical power to a system (e.g., a system including the front rigid body 105 including the electronic display 255 and the multifocal structure 604).

With reference to an orientation coordinate circle illustrated at a left hand side of FIG. 6, the QWP 612 has an optic axis Q1 which is .+-.45 degrees relative to the polarization axis P1 (or alternatively P2) of the pixel level polarizer 602. The QWP 616 has an optic axis Q2 which is .+-.90 degrees relative to the optic axis Q1 of the QWP 612. The polarizer 618 has a polarization axis P3 which is .+-.45 degrees relative to the optic axis Q2 of the QWP 616.

In some embodiments, the axes P1 and P2 of the pixel level polarizer 602 are aligned along 45 degrees and 135 degrees, respectively, and the optic axis Q1 of the QWP 612 is aligned along 0 degrees, the optic axis Q2 of the QWP 616 is aligned along 90 degrees, and the polarization axis P3 is aligned along 135 degrees.

In other embodiments, the axes P1 and P2 of the pixel level polarizer 602 are aligned, respectively, along 45 degrees and 135 degrees, the optic axis Q1 of the QWP 612 is aligned along 0 degrees, the optic axis Q2 of the QWP 616 is aligned along 90 degrees, and the polarization axis P3 is aligned along 45 degrees.

An eye tracking system controls the operation of the multifocal structure 604 by generating and sending focus instructions to the pixel level polarizer 602 and/or LC lens element 606. The focus instructions may cause the pixel level polarizer 602 to selectively output light at different orientations for each cell of the pixel level polarizer 602. The focus instructions may also cause the LC lens element 606 to switch between states (e.g., additive, subtractive, or neutral). The eye tracking system may be integrated within the HMD 200, or separate from the HMD 200.

FIGS. 7A-7D illustrates various optical paths within a multifocal structure (e.g., the multifocal structure 604), in accordance to one embodiment. The optical paths are shown for a configuration of the HMD 200 where the LC lens element 606 is a PBP lens 450, the axis P1 of the pixel level polarizer 602 are aligned along 45 degrees, the axis P2 of the pixel level polarizer 602 is aligned along 135 degrees, the optic axis Q1 of the QWP 612 is aligned along 0 degrees, the optic axis Q2 of the QWP 616 is aligned along 90 degrees, and the polarization axis P3 of the polarizer 618 is aligned along 135 degrees. As discussed above, the PBP lens 450 can be placed in different states for different polarizations of light output from the pixel level polarizer 602, resulting in multiple optical paths. The optical paths include direct paths and folded paths through the folded optics assembly 608, with different levels of optical power addition or subtraction by the PBP lens 450. Table 1 shown below illustrates four states of the multifocal structure that result in four different focal distances of the multifocal structure–and each focal distance is associated with a respective image plane. The associated optical path for image planes 1 through 4 are respectively shown in FIGS. 7A through 7D.

TABLE-US-00001 TABLE 1 Four states of the multifocal structure 604 including PBP lens 450 that result in four different focal distances. Image Pixel Level PBP Lens Folded Plane Polarizer Optical Power Path 1 Linear 45.degree. 0 Y 2 Linear 45.degree. f N 3 Linear 135.degree. -f Y 4 Linear 135.degree. 0** N**

FIG. 7A illustrates a first optical path in a multifocal structure (e.g., the multifocal structure 604), according to one embodiment. The first optical path corresponds with image plane 1 shown in Table 1. Orientation blocks are illustrated between each of the optical components. The orientation blocks illustrate an orientation of light between the optical components. The optical path traverses along the z-axis, with polarization orientation defined along the x-y axes (with reference to an orientation coordinate circle illustrated at a left hand side of the figure).

The electronic display 255 generates an image light 702, and provides the image light 702 to the pixel level polarizer 602. The image light 702 may be unpolarized light. The pixel level polarizer 602 has a configuration that transmits a portion of the image light 702 as linearly polarized light 704 oriented along 45 degrees. The linearly polarized light 704 propagates to the QWP 612. The QWP 612 converts the linearly polarized light 704 into left circularly polarized light (LCP) 706.

To produce the image plane 1, the PBP lens 450 is set to the neutral state. The PBP lens 450 can be set to the neutral state by applying a voltage to the PBC lens 450 that is larger than a threshold voltage. The neutral state results in the PBP lens 450 neither adding nor subtracting optical power (“0”), and the PBP lens 450 also does not change the polarization of the incident LCP light 706. As such, the PBP lens 450 transmits the incident light as LCP light 708.

The LCP light 708 propagates to the folded optics assembly 608, and takes a folded path through the folded optics assembly 608 because of the left circular polarization. The folded assembly 608 includes the 50:50 reflective element 614, the QWP 616, and the polarizer 618. The polarizer 618 has a polarization axis aligned along 135 degrees, and thus transmits linearly polarized light oriented along 135 degrees and reflects linearly polarized light oriented along 45 degrees.

The 50:50 reflective element 614 reflects 50% of the incident LCP light 708, and transmits 50% of the LCP light 708 as LCP light 710 propagating in the +z direction. The QWP 616 converts the incident LCP light 710 into linearly polarized light 712 oriented along 45 degrees propagating in the +z direction. The polarizer 618 reflects the linearly polarized light 712 as linearly polarized light 714 oriented along 135 degrees propagating in the -z direction. The QWP 616 converts the incident linearly polarized light 714 into LCP light 716 propagating in the -z direction. The 50:50 reflective element 614 transmits 50% of the incident LCP light 716, and reflects 50% of the incident LCP light 716 as right circular polarized (RCP) light 718 propagating in the +z direction. The QWP 616 converts the incident RCP light 718 into linearly polarized light 720 oriented along 135 degrees and propagating in the +z direction. The polarizer 618 transmits the linearly polarized light 720 as linearly polarized light 722 oriented along 135 degrees and propagating out of the folded optics assembly 608 in the +z direction.

FIG. 7B illustrates a second optical path in the multifocal structure (e.g., the multifocal structure 604), according to one embodiment. The second optical path corresponds with image plane 2 shown in Table 1. The electronic display 255 generates an image light 724, and provides the image light 724 to the pixel level polarizer 602. The pixel level polarizer 602 has a configuration that transmits a portion of the image light 724 as linearly polarized light 726 oriented along 45 degrees. The linearly polarized light 726 propagates to the QWP 312. The QWP 312 converts the linearly polarized light 704 into LCP light 728.

To produce the image plane 2, the PBP lens 450 is set to an active state. The PBP lens 400 is set to the active state by applying a voltage to the PBP lens 450 that is below a threshold voltage (or not applying a voltage). In the active state, the PBP lens 450 converts the LCP light 728 into RCP light 730. The orientation of the incident LCP light 728, and the PBP lens 450 being in the active state, results in the PBP lens 450 having increased optical power (“f”), and changes the polarization of the incident LCP light 728.

The RCP light 730 propagates to the folded optics assembly 608, and takes a direct path through the folded optics assembly 608 because of the right circular polarization. The 50:50 reflective element 614 reflects 50% of the incident RCP light 730, and transmits 50% of the RCP light 730 as RCP light 732 propagating in the +z direction. The QWP 616 converts the incident RCP light 732 into linearly polarized light 734 oriented along 135 degrees propagating in the +z direction. The polarizer 618 transmits the linearly polarized light 734 as linearly polarized light 736 oriented along 135 degrees and propagating out of the folded optics assembly 608 in the +z direction. The differences in the optical paths shown in FIGS. 7A and 7B result in different focal distances and image planes. In some embodiments, one or more optical components of the multifocal structure 260 may be curved to further add or subtract optical power.

FIG. 7C illustrates a third optical path in the multifocal structure (e.g., multifocal structure 260), according to one embodiment. The third optical path corresponds with image plane 3 shown in Table 1. The electronic display 255 generates an image light 738, and provides the image light 738 to the pixel level polarizer 602. The pixel level polarizer 602 transmits a portion of image light 738 as linearly polarized light 740 oriented along 135 degrees. The linearly polarized light 740 propagates to the QWP 312. The QWP 312 converts the linearly polarized light 740 into RCP light 742.