Head-mounted display (HMD) with a spatially-varying retarder optical device

Abstract

The present invention discloses a head-mounted display or other near-eye display incorporating an optical device including a spatial-variant retarder (SVR). The SVR can be fabricated with correction factors applied thereto to compensate for one or more manufacturing errors exhibited in a molded lens and / or a polarization beam splitter included in the optical device of the system.

Description

[0001] Cross - Reference to Related Applications

[0002] This PCT application claims priority to U.S. Patent Application Serial No. 16 / 692,823, filed on November 22, 2019, entitled "HEAD - MOUNTED DISPLAY (HMD) WITH SPATIALLY - VARYING RETARDER OPTICS", which claims priority, under 35 U.S.C.§120, as a continuation - in - part, to U.S. Patent Application Serial No. 16 / 101,333 (now U.S. Patent No. 10,778,963), filed on August 10, 2018, entitled "HEAD - MOUNTED DISPLAY (HMD) WITH SPATIALLY - VARYING RETARDER OPTICS", the entire disclosures of all of these applications are incorporated herein by reference. Background Art

[0003] As part of a virtual reality ("VR") or augmented reality ("AR") system, near - eye display technology can be used to present information and images to a user. Such near - eye displays can be incorporated into a head - mounted display ("HMD") device or a headset. Although these near - eye information displays can be oriented for direct viewing, typically these information displays are coupled with one or more lenses in the HMD. The lens system can include lenses, various optical elements, an aperture stop, and a lens housing to contain the various components that are optically aligned with each other. Although such lenses can enhance the VR or AR experience, the performance of the lens system depends in part on the design of each element of the system and the overall design of the system, which elucidate the optical interactions between these elements.

[0004] Traditional information displays, such as liquid crystal displays, typically emit light over a wide angular cone or field of view. The angular divergence of these traditional displays can extend beyond 160 degrees and even approach 180 degrees. When viewed directly, this wide-angle light may not cause undesirable effects. However, when traditional information displays are employed in an HMD (such as in a VR or AR system), the wide-angle emission may lead to undesirable results. This is the case when light emitted by the information display strikes the lens of the HMD at an incident angle that exceeds the lens's imaging capabilities. Such light cannot be properly imaged onto the user's eye, at least in part due to the fact that off-axis light exhibits slightly elliptical polarization. That is, on-axis light is linearly polarized, while off-axis light exhibits a certain degree of elliptical polarization. Thus, light at angles beyond the focusing capabilities of the HMD lens may result in undesirable visual effects such as flood illumination, ghosting, glare, scattering, and other stray light effects. This stray non-imaging light may cause the user of the VR or AR headset to experience undesirable visual artifacts. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The detailed description is described with reference to the accompanying drawings. In these figures, the leftmost digit of a reference numeral identifies the figure in which the reference numeral first appears. The same reference numerals in different figures represent similar or identical items.

[0006] Figure 1 is a schematic diagram showing a user and a wearable device in which the techniques and configurations described herein may be implemented.

[0007] Figure 2 is a schematic cross-sectional view of a portion of an exemplary wearable device positioned relative to a user's eye.

[0008] Figure 3 is a schematic cross-sectional view of a system including a display and an optical subsystem that includes a spatially-varying retarder (SVR) according to some embodiments.

[0009] Figure 4 is a schematic cross-sectional view of a system including a display and an optical subsystem that includes a spatially-varying retarder (SVR) according to some embodiments.

[0010] Figure 5 is a schematic cross-sectional view of a system including a display and an optical subsystem that includes a spatially-varying retarder (SVR) according to some embodiments.

[0011] Figure 6 is a schematic diagram of an exemplary spatially-varying retarder (SVR) according to some embodiments.

[0012] Figure 7Schematic cross-sectional view of a system including a display and an optical subsystem, the optical subsystem including a spatially variable retarder (SVR), according to some embodiments.

[0013] Figure 8 Flowchart showing a process for providing a focused image to the eyes of a user of a display device, according to some exemplary embodiments.

[0014] Figure 9 Flowchart showing a process for applying a correction factor during the manufacturing process of a spatially variable retarder (SVR) to compensate for known or determined manufacturing errors in a molded lens and / or a polarization beam splitter. Detailed Description

[0015] In various examples described herein, techniques and architectures can be used to generate an image focused on the eye(s) of a user of a wearable device. Examples of wearable devices can include display devices worn on a user's head or as part of a helmet, such as a head-mounted display (“HMD”) device or a head-mounted headset, and can include position sensors and / or motion sensors to measure the inertial position or orientation of the wearable device. The display device can include a display in front of one eye, each eye, or both eyes. By way of example only, the display device can include a liquid crystal display (LCD), an organic light-emitting diode (OLED), a liquid crystal on silicon (LCOS), or a cathode ray tube (CRT). The wearable device can display computer-generated images, referred to as virtual images. For example, as described below, a processor of the wearable device can render and display a synthetic (virtual) scene such that an observer (the wearer of the wearable device) perceives the scene as reality (or augmented reality).

[0016] In some examples, an LCD display device includes a number of components. Two of these components are a display matrix and a light source or backlight, the display matrix blocking light in a granular or pixelated manner to create an image. The light source is typically located behind the display matrix and illuminates the image. For a color display, the backlight typically emits broad-spectrum light, such as white light.

[0017] As part of a virtual reality (“VR”) or augmented reality (“AR”) system, near-eye display technology can be used to present information and images to a user, and the near-eye display technology can be in the form of a wearable device such as those described above. A VR or AR HMD can use one or more lenses to direct light associated with an image displayed on one or more information displays (e.g., a pixelated LCD display device) to the user's eyes. Among other things, the lenses are also used to bend light from the information display so that the information display appears to the user to be farther away than it actually is. This provides a greater depth of field to the user in a virtual environment and allows the user to more easily focus on the displayed image. The lenses can also be used in a VR or AR headset to increase the field of view of the information display for the user. A greater field of view can enhance the immersive effect of the VR or AR system. The lenses can also be used in a VR or AR headset to shape the light from a single display such that the light received by the user is customized separately for the user's left and right eyes. For example, using separately customized images for each eye can enable the user to perceive a stereoscopic or three-dimensional image. The lenses are further designed for a near-eye environment with the constraint that the user's eyes are relatively close to the information display.

[0018] In various examples, the optical system of a wearable device incorporates a spatially varying retarder (SVR). Among other things, a system that also includes an information display allows for near-eye applications that involve focusing an image onto the eyes of a user that are relatively close to the information display. The SVR is used to modify the phase of the light of an image generated by the information display by different amounts for different portions of the SVR. Thus, the SVR provides an appropriate level of retardation to varying angles of incidence, which mitigates undesired visual effects such as flood illumination, ghosting, glare, scattering, and other stray light effects present in conventional optical system designs. As described below, such phase modification by the SVR works in concert with various optical elements in the optical system.

[0019] The present disclosure also describes a spatially variable retarder (SVR) that can be fabricated with correction factors to correct for spatial variations in manufacturing errors that may occur in components of the systems described herein. An exemplary manufacturing error that can be corrected during the fabrication of the SVRs described herein is stress birefringence in a molded lens, which can be a byproduct of molding the lens during the manufacturing process. Another exemplary manufacturing error that can be corrected during the fabrication of the SVRs described herein is polarization variations on a polarization beam splitter (sometimes referred to herein as a "reflective polarizer" or a "reflective film"). In an exemplary process, the extent of the manufacturing error in at least one of the molded lens or the polarization beam splitter can be determined. At least in part based on the extent of the manufacturing error, a correction factor for the spatially variable retarder (SVR) can be determined. The SVR can then be formed by applying the correction factor during the fabrication of the SVR to compensate for (or correct) the manufacturing error. The present disclosure also describes a system that includes an SVR fabricated using the correction factor to compensate for manufacturing errors in at least one of the molded lens or the polarization beam splitter as described herein. When the correction factor is applied during the fabrication of the SVR, the overall performance of the optical system can be improved. For example, an optical subsystem that includes the SVR can further mitigate undesired visual effects that may be caused by manufacturing defects in other constituent components of the optical subsystem.

[0020] Those of ordinary skill in the art will recognize that the following description is merely illustrative and is not limiting in any way. Benefiting from this disclosure, these skilled artisans will readily conceive of other embodiments. Reference will now be made in detail to specific embodiments as illustrated in the accompanying drawings. The same reference numerals will be used throughout the drawings and the following description to refer to the same or like parts.

[0021] Figure 1 FIG. 1 is a schematic diagram showing a user 102 and a wearable device 104 in accordance with some embodiments. A computing device associated with the wearable device 104 can provide rendering data associated with respective virtual content items to the wearable device 104 and cause the respective virtual content items to be presented on a display associated with the wearable device 104. The rendering data can include instructions for rendering graphical representations of the virtual content items via the device's display. For example, the rendering data can include instructions describing the geometry, viewpoints, textures, lighting, shadows, etc. associated with the virtual content items. In one illustrative example, the virtual content items can be presented on the display of the wearable device 104 as part of a game that the user 102 can play using the wearable device 104.

[0022] In some examples, the computing device may be located in a network (such as the Internet) at a location remote from the wearable device 104. In other embodiments, the computing device may be co-located with the wearable device 104 (e.g., embedded within the wearable device 104). Additionally, the wearable device 104 may be communicatively coupled to the network in any manner, such as via a global or local wired or wireless connection (e.g., local area network (LAN), intranet, Bluetooth, etc.). The network may facilitate communication between the computing device and the wearable device 104 associated with one or more users (such as user 102).

[0023] Figure 2 FIG. 204 is a schematic cross-sectional view of a portion 202 of a wearable device 204 positioned relative to a user's eye 206. For example, the wearable device 204 may be the same as or similar to the wearable device 104. In certain embodiments, the wearable device 204 may be configured to display images that will be seen by both the user's left and right eyes. This may be achieved using separate left and right LCD displays, or may be achieved using a single LCD display. Similarly, the wearable device 204 (e.g., in the form of a VR or AR headset) may include a single lens assembly, or the wearable device may use separate left and right lens assemblies.

[0024] Exemplary light rays 208 and 210 illustrate possible paths of light from the wearable device 204 to the cornea 212 of the eye 206. The cornea 212 may be considered to have a substantially spherical shape. The wearable device 204 may include a near-eye display such that the paths of the light rays 208 and 210 are relatively short, such as providing an eye relief of approximately 20 millimeters. In such a case, the optics of the wearable device 204 are configured to focus the light onto a surface relatively close to the wearable device (e.g., the cornea 212). Such a configuration may involve a pancake optical system having a relatively thin profile that enables the pancake optical system to be assembled into the wearable device 204 while allowing a physical gap with the user's eye 206.

[0025] Figure 3 FIG. 300 is a schematic cross-sectional view of a system 300 including a display and an optical subsystem including a spatially-varying retarder (SVR) according to some embodiments. The system 300 may be incorporated in a head-mounted device, such as the wearable device 104 / 204. However, it should be understood that the system 300 may be incorporated in other types of devices, including but not limited to cameras, binoculars, office equipment, scientific instruments, etc. The system 300 may include a pixelated display device 302 (sometimes referred to as an information display 302), a backlight assembly 304, and an optical subsystem 306. A schematic of a user's eye 308 is also shown. Such elements are aligned along an optical axis 310.

[0026] The light-emitting backlight assembly 304 may include a light source, such as one or more light-emitting diodes (LEDs), one or more OLEDs, one or more cold cathode fluorescent lamps (CCFLs), one or more lasers, one or more quantum dots, or any combination of these exemplary light sources. The light source in the backlight assembly 304 may emit light across a broad spectrum (e.g., white light) such that the information display 302 may produce a color image across the visible spectrum. The backlight assembly 304 may emit light uniformly across its entire front face in a range of, for example, from about 160 degrees to 180 degrees.

[0027] The information display 302 works in concert with the backlight assembly 304 and may emit light within an angular range of up to about 180 degrees (except for light parallel to the face of the backlight assembly 304). This range of emission angles is sometimes referred to as the field of view of the backlight assembly 304 or the light cone of the backlight assembly 304. In some embodiments, the information display 302 may be an LCD matrix that includes one or more polarizing layers, a liquid crystal layer, and a thin film transistor layer. The LCD matrix creates an image by obscuring portions of the backlight in a pixelated manner. When light 312 is emitted from the backlight assembly 304 and passes through the information display 302 (e.g., the LCD matrix), an image is displayed. For clarity, Figure 3 the spacing between the backlight assembly 304 and the information display 302 is shown. However, the two components may be sandwiched together with little (if any) space between them.

[0028] The optical subsystem 306 may include a lens assembly to direct light from the information display 302 toward the user's eye 308. For example, the optical subsystem 306 may have a pancake configuration. In this case, as described below, the optical subsystem 306 may include an assembly of optical elements that are configured to direct light from the information display 302 toward the user's eye 308 using coaxial optical folding that is at least partially based on the polarization of the light. In some embodiments, the lens assembly of the optical subsystem 306 includes various optical elements in addition to lenses. For example, the optical subsystem 306 may include at least one polarization beam splitter 338 and a spatially varying retarder (SVR) 316. The polarization beam splitter 338 may be located between the SVR 316 and the exit surface (or side) 314 of the optical subsystem 306. The SVR 316 is configured to modify the phase of the light passing through the SVR 316 by different amounts for different portions of the SVR 316. For example, light emitted from the perimeter of the display toward the user's eye may enter the optical subsystem 306 at a relatively large angle of incidence. If the SVR 316 is configured for different degrees of phase modification across the SVR 316, it may provide an appropriate level of delay to light from any and all angles in order to mitigate undesired visual effects (e.g., removing unwanted stray light from reaching the user's eye 308).

[0029] The polarization beam splitter 338 may represent a beam splitter that allows only linearly polarized light to pass through it, thereby reflecting all other non-linearly polarized light. The polarization beam splitter 338 may be considered a linear polarizer reflector or a reflective linear polarizer. That is, the polarization beam splitter 338 may combine the functions of a linear polarizer and a beam splitter into a single element.

[0030] In some embodiments, the polarization beam splitter 338 may be replaced with a partial mirror (e.g., a 50 / 50 mirror). That is, according to some embodiments, the optical subsystem 306 may include at least one partial mirror located between the SVR 316 and the exit surface (or side) 314 of the optical subsystem 306 (e.g., the partial mirror may be at the Figure 3 position of reference numeral 338 in).

[0031] Figure 3 An exemplary optical path 318 of light of an image generated by the information display 302 is shown. For simplicity, the optical subsystem 306 is schematically shown as including at least one lens-shaped element. However, the optical subsystem 306 may include any one of a variety of types of optical elements that need not be lenses. A specific example of the optical subsystem 306 is now described.

[0032] The optical subsystem 306 may include a first quarter-wave plate 326 having a front surface 322 and a back surface 324. The front surface 322 may be considered the incident side of the optical subsystem 306 (e.g., a lens assembly) through which light enters the optical subsystem 306. The front surface 322 of the first quarter-wave plate 326 may be disposed on the information display 302. As used herein, "disposed on" may mean "in contact with" or "adjacent to", such that there may be a space between the layers disposed on another layer. Thus, the first quarter-wave plate 326 may be in contact with the information display 302, or spaced apart from the information display 302, but interposed between the information display and the lens 330. It should also be understood that "disposed on" may mean directly disposed on, or indirectly disposed on (e.g., having one or more intermediate layers). The lens 330 may be interposed between the first quarter-wave plate 326 and the SVR 316. The SVR 316 may be disposed on the polarization beam splitter 338 (alternatively, in an alternative embodiment, the SVR 316 may be disposed on a partial mirror that is at the Figure 3 position of reference numeral 338 in).

[0033] In one example, SVR 316 can be part of a second quarter-wave plate. In this way, the second quarter-wave plate (interposed between lens 330 and polarization beam splitter 338 (or partial mirror)) can include materials, features, or another suitable mechanism to modify the phase of the light passing through SVR 316 by different amounts for different portions of SVR 316. Although Figure 3 SVR 316 is shown and described as being interposed between lens 330 and polarization beam splitter 338 (or partial mirror), it should be understood that alternatively, SVR 316 can instead be part of the first quarter-wave plate 326. In this alternative embodiment, Figure 3 the SVR 316 shown in can be replaced with a standard quarter-wave plate (i.e., a quarter-wave plate without SVR). That is, the optical subsystem 306 can include two quarter-wave plates (one at the position of 326 and one at the position of 316), where the SVR can be part of either of the two quarter-wave plates. In some embodiments, when the optical subsystem 306 includes two quarter-wave plates (one at the position of 326 and one at the position of 316), the SVR can be part of both quarter-wave plates.

[0034] To illustrate the operation principle of the optical subsystem 306, the light path 318 enters the first quarter-wave plate 326 and is polarized (e.g., linearly polarized to one orientation, p) at the front face 322 of the first quarter-wave plate 326. The light exiting from the back face 324 of the first quarter-wave plate 326 can be circularly polarized. This light passes through lens 330. Then, the light exiting from lens 330 passes through SVR 316 and is reflected from the polarization beam splitter 338 (alternatively, in an alternative scenario, the light passing through SVR 316 is reflected from the partial mirror at the position of reference numeral 338 in Figure 3 . The reflected light passes through SVR 316 in the opposite direction, such that the light changes its handedness while remaining circularly polarized and having a 45-degree orientation. The light passing through SVR 316 in the opposite direction is reflected again by lens 330, causing the light to pass through SVR 316 a third time in the forward direction (i.e., towards the user's eye 308). The reflected light passes through the polarization beam splitter 338 (alternatively, in an alternative scenario, the light passes through the partial mirror at the position of reference numeral 338 in Figure 3 to exit from the exit side 314 (or surface) of the optical subsystem 306.

[0035] The SVR 316 modifies the phase of the light passing through the SVR 316 with an appropriate level of delay based on the position at which the incident light impinges on the SVR 316. In this way, the light exiting the SVR 316 is horizontally polarized. Since the polarization beam splitter 338 allows horizontally polarized light to pass through to reach the user's eye 308 and otherwise blocks vertically polarized light, the SVR 316 performs post-phase modification to allow this light to pass through the polarization beam splitter 338. In Figure 3 the example, since the exemplary optical path 318 is off-axis, the optical path 318 entering the SVR 316 is elliptically polarized, and the position on the SVR 316 where the optical path 318 impinges on the SVR 316 includes materials (e.g., birefringent materials) and / or features that modify the phase of the light by an appropriate amount so that the incident elliptically polarized light exits as horizontally polarized light. This can be contrasted with a coaxial optical path (not shown) that impinges on the optical subsystem 306 at an angle of incidence equal to zero. Such a coaxial optical path does not have its polarization state changed by the SVR 316 because the amount of this phase modification varies across the region of the SVR 316 and there is little phase modification at the center of the SVR 316. However, for more off-axis incident light, the SVR 316 is configured to modify the phase of the off-axis incident light by an appropriate amount based on the angle of incidence and thus based on the position on the SVR 316. Although this example describes a polarization beam splitter 338 that allows horizontally polarized light to pass through to reach the user's eye 308 and otherwise reflects all other non-horizontally polarized light, it should be understood that the polarization beam splitter 338 may allow vertically polarized light to pass through to reach the user's eye 308 and otherwise reflect all other non-vertically polarized light.

[0036] Figure 4 is a schematic cross-sectional view of a system 400 including a display and an optical subsystem that includes a spatially-varying retarder (SVR) according to some embodiments. The system 400 may be incorporated in a head-mounted device, such as the wearable device 104 / 204. However, it should be understood that the system 400 may be incorporated in other types of devices, including but not limited to cameras, binoculars, office equipment, scientific instruments, etc. The system 400 may include a pixelated display device 402 (sometimes referred to as an information display 402), a backlight assembly 404, and an optical subsystem 406. A schematic of the user's eye 408 is also shown. Such elements are aligned along the optical axis 410.

[0037] The light-emitting backlight assembly 404 may include a light source, such as one or more light-emitting diodes (LEDs), one or more OLEDs, one or more cold cathode fluorescent lamps (CCFLs), one or more lasers, one or more quantum dots, or any combination of these exemplary light sources. The light source in the backlight assembly 404 may emit light across a broad spectrum (e.g., white light) such that the information display 402 can produce a color image across the visible spectrum. The backlight assembly 404 may emit light uniformly across its entire front face in a range of, for example, from about 160 degrees to 180 degrees.

[0038] The information display 402 works in conjunction with the backlight assembly 404 and can emit light in an angular range of up to about 180 degrees (except for light parallel to the face of the backlight assembly 404). This range of emission angles is sometimes referred to as the field of view of the backlight assembly 404 or the light cone of the backlight assembly 404. In some embodiments, the information display 402 may be an LCD matrix that includes one or more polarization layers, a liquid crystal layer, and a thin film transistor layer. The LCD matrix creates an image by obscuring portions of the backlight in a pixelated manner. When light 412 is emitted from the backlight assembly 404 and passes through the information display 402 (e.g., the LCD matrix), an image is displayed. For clarity, Figure 4 the spacing between the backlight assembly 404 and the information display 402 is shown. However, these two components may be sandwiched together with little or no space between them.

[0039] The optical subsystem 406 may include a lens assembly to direct light from the information display 402 toward the user's eye 408. For example, the optical subsystem 406 may have a pancake configuration. In this case, as described below, the optical subsystem 406 may include an assembly of optical elements configured to use coaxial optical folding based at least in part on the polarization of light to direct light from the information display 402 toward the user's eye 408. In some embodiments, the lens assembly of the optical subsystem 406 includes various optical elements in addition to lenses. For example, the optical subsystem 406 may include at least one linear polarizer 438 and a spatially variable retarder (SVR) 416. The linear polarizer 438 may be located between the SVR 416 and the exit surface 414 of the optical subsystem 406. The SVR 416 is configured to modify the phase of the light passing through the SVR 416 by different amounts for different portions of the SVR 416. For example, light emitted from the perimeter of the display toward the user's eye may enter the optical subsystem 406 at a relatively large angle of incidence. If the SVR 416 is configured for different degrees of phase modification across the SVR 416, it can provide an appropriate level of delay to light from any and all angles in order to mitigate undesired visual effects (e.g., removing unwanted stray light so that it does not reach the user's eye 408).

[0040] Figure 4 Shows an exemplary optical path 418 of light of an image generated by the information display 402. For simplicity, the optical subsystem 406 is schematically shown as including at least one lens-shaped element. However, the optical subsystem 406 can include any of a variety of types of optical elements that need not be lenses. A specific example of the optical subsystem 406 is now described.

[0041] The optical subsystem 406 can include a reflection and refraction element 430 (sometimes referred to as "element 430"). The reflection and refraction element 430 can include a 50 / 50 mirror that allows approximately 50% of the incident light to pass through the element 430 while approximately 50% of the incident light is reflected away by the element 430. The optical subsystem 406 can also include a first quarter-wave plate 426 having a front surface 422 and a back surface 424. The first quarter-wave plate 426 can be disposed on a beam splitter 436 that has a front surface adjacent to the back surface 424 of the first quarter-wave plate 426 and a back surface 428. The beam splitter 436 can be disposed on the SVR 416. The SVR 416 can be disposed on a linear polarizer 438. It should be understood that "disposed on" can mean directly disposed on or indirectly disposed on (e.g., having one or more intermediate layers). In addition, it should be understood that "disposed on" can mean "in contact with" or "adjacent to" such that there can be a space between the layers disposed on another layer.

[0042] In one example, the SVR 416 can be part of a second quarter-wave plate. Thus, the second quarter-wave plate (interposed between the beam splitter 436 and the linear polarizer 438) can include a material, feature, or another suitable mechanism to modify the phase of the light passing through the SVR 416 by different amounts for different portions of the SVR 416. Although Figure 4 the SVR 416 is shown and described as being interposed between the beam splitter 436 and the linear polarizer 438, it should be understood that alternatively, the SVR 416 can be part of the first quarter-wave plate 426. In this alternative embodiment, Figure 4 the SVR 416 shown can be replaced with a standard quarter-wave plate. That is, the optical subsystem 406 can include two quarter-wave plates (one in the position of 426 and one in the position of 416), where the SVR can be part of either of the two quarter-wave plates. In some embodiments, when the optical subsystem 406 includes two quarter-wave plates (one in the position of 426 and one in the position of 416), the SVR can be part of both quarter-wave plates.

[0043] To illustrate the operation of the optical subsystem 406, an optical path 418 enters the optical subsystem 406 and is polarized (e.g., linearly polarized to one orientation, p) at the incident side (or surface) of the optical subsystem 406 (e.g., at the incident side of the element 430). A portion of the light passes through the reflection and refraction element 430 while the remainder of the light is reflected away by the element 430. The optical path 418 exiting the exit side of the element 430 represents the portion of the light that passes through the element 430. Then, the light exiting the element 430 passes through the first quarter-wave plate 426 and becomes circularly polarized. This circularly polarized light is reflected from the beam splitter 436 and passes through the first quarter-wave plate 426 in the opposite direction, such that the light changes its handedness while remaining circularly polarized and at a 45-degree orientation. In some embodiments, the beam splitter 436 includes another 50 / 50 mirror. The light passing through the first quarter-wave plate 426 in the opposite direction is reflected again so that the light passes through the first quarter-wave plate 426 a third time in the forward direction (i.e., toward the user's eye 408). The reflected light passes through the beam splitter 436, then through the SVR 416 (e.g., a second quarter-wave plate including the SVR 416), and subsequently through the linear polarizer 438 to exit from the exit side 414 (or surface) of the optical subsystem 406.

[0044] The SVR 416 modifies the phase of the light passing through the SVR 416 with an appropriate level of delay based on the position at which the incident light impinges on the SVR 416. In this way, the light exiting the SVR 416 is horizontally polarized. Since the linear polarizer 438 allows horizontally polarized light to pass through to reach the user's eye 408 and blocks vertically polarized light otherwise, the SVR 416 performs a late-phase modification to allow the light to pass through the linear polarizer 438. In Figure 4 the example of, since the exemplary optical path 418 is off-axis, the optical path 418 entering the SVR 416 is elliptically polarized, and the position on the SVR 416 at which the optical path 418 impinges on the SVR 416 includes materials (e.g., birefringent materials) and / or features that modify the phase of the light by an appropriate amount to cause the incident elliptically polarized light to exit as horizontally polarized light. This can be contrasted with a coaxial optical path (not shown) that impinges on the optical subsystem 406 at an angle of incidence equal to zero. Such a coaxial optical path does not have its polarization state changed by the SVR 416 because the amount of this phase modification varies across the region of the SVR 416 and there is little phase modification at the center of the SVR 416. However, for more off-axis incident light, the SVR 416 is configured to modify the phase of the off-axis incident light by an appropriate amount based on the angle of incidence and thus based on the position on the SVR 416.

[0045] Figure 5FIG. 500 is a schematic cross-sectional view of a system 500 including a display and an optical subsystem, according to some embodiments. System 500 may be incorporated in a head-mounted device, such as wearable device 104 / 204. However, it should be understood that system 500 may be incorporated in other types of devices, including but not limited to cameras, binoculars, office equipment, scientific instruments, etc. System 500 may include a pixelated display device 502 (sometimes referred to as information display 502), a backlight assembly 504, and an optical subsystem 506. A schematic of a user's eye 508 is also shown. Such elements are aligned along an optical axis 510.

[0046] The backlight assembly 504 that emits light may include a light source, such as one or more light-emitting diodes (LEDs), one or more OLEDs, one or more cold cathode fluorescent lamps (CCFLs), one or more lasers, one or more quantum dots, or any combination of these exemplary light sources. The light source in the backlight assembly 504 may emit light across a broad spectrum (e.g., white light) such that the information display 502 can produce a color image across the visible spectrum. The backlight assembly 504 may emit light uniformly across its entire front surface in a range, for example, of about 160 degrees to 180 degrees.

[0047] The information display 502 works in conjunction with the backlight assembly 504 and may emit light within an angular range of up to about 180 degrees (merely avoiding light parallel to the surface of the backlight assembly 504). This range of emission angles is sometimes referred to as the field of view of the backlight assembly 504 or the light cone of the backlight assembly 504. In some embodiments, the information display 502 may be an LCD matrix including one or more polarization layers, a liquid crystal layer, and a thin film transistor layer. The LCD matrix creates an image by obscuring portions of the backlight in a pixelated manner. When light 512 is emitted from the backlight assembly 504 and passes through the information display 502 (e.g., the LCD matrix), an image is displayed. For clarity, Figure 5 the spacing between the backlight assembly 504 and the information display 502 is shown. However, the two components may be sandwiched together with little or no space between them.

[0048] The optical subsystem 506 may include a lens assembly to direct light from the information display 502 towards the user's eye 508. For example, the optical subsystem 506 may have a pancake configuration. In such a case, as described below, the optical subsystem 506 may include an assembly of optical elements configured to direct light from the information display 502 towards the user's eye 508 using coaxial optical folding that is at least partially based on the polarization of light. In some embodiments, the lens assembly of the optical subsystem 506 includes various optical elements in addition to lenses. For example, the optical subsystem 506 may include at least one linear polarizer 538 and a spatially varying retarder (SVR) 516. The linear polarizer 538 may be located between the SVR 516 and the exit surface 514 of the optical subsystem 506. The SVR 516 is configured to modify the phase of the light passing through the SVR 516 by different amounts for different portions of the SVR 516. For example, if the backlight / display emits light at varying angles, the light emitted from the perimeter of the display towards the user's eye may enter the optical subsystem 506 at a relatively large angle of incidence. If configured for different degrees of phase modification across the SVR 516, the SVR 516 may provide an appropriate level of delay to light from any and all angles in order to mitigate undesired visual effects (e.g., removing unwanted stray light from reaching the user's eye 508).

[0049] Figure 5 An exemplary light path 518 of light of an image generated by the information display 502 is shown. For simplicity, the optical subsystem 506 is schematically shown as a box including at least one lens-shaped element. However, the optical subsystem 506 may include any of a variety of types of optical elements that need not be lenses. A specific example of the optical subsystem 506 is now described.

[0050] The optical subsystem 506 may include a linear polarizer 520 having a front face 522 and a back face 524. The linear polarizer 520 may be disposed on a quarter-wave plate 526, which has a front face adjacent to the back face 524 of the linear polarizer 520 and a back face 528. The quarter-wave plate 526 may be disposed on a doublet lens 530. In one example, the doublet lens 530 may include a plano-concave singlet lens having a flat face 532 and a concave surface 534. The curvature of the concave surface 534 may establish the focal length of the optical subsystem 506. A first reflective polarizing beam splitter, which may include an optical coating (e.g., a metal thin film or a multilayer dielectric film), may be located (e.g., disposed on) the concave surface 534 of the doublet lens 530. The doublet lens 530 may be disposed on the SVR 516. The SVR 516 may be disposed on a second reflective polarizing beam splitter 536. The second reflective polarizing beam splitter 536 may be disposed on an optional second linear polarizer 538. It should be understood that "disposed on" may mean directly disposed on or indirectly disposed on (e.g., having one or more intermediate layers). In addition, it should be understood that "disposed on" may mean "in contact with" or "adjacent to", such that there may be a space between the layers disposed on another layer.

[0051] To illustrate the operating principle of the optical subsystem 506, the light path 518 enters the optical subsystem 506 unpolarized at the incident side (or surface) of the optical subsystem 506, and the light path 518 is polarized to one orientation, e.g., p, by the linear polarizer 520. After passing through the quarter-wave plate 526, the light is right-circularly polarized. After passing through the doublet lens 530 and then through the SVR 516, the resulting s-polarized light is reflected from the second reflective polarizing beam splitter 536, passes through the SVR 516 in the opposite direction, is reflected again from the first reflective polarizing beam splitter at the concave surface 534 of the doublet lens 530, and passes through the SVR 516 for the third time to exit the optical subsystem 506 from the exit side 514 (or surface) as p-polarized light. Any stray s-polarized light may be removed by the optional second linear polarizer 538 such that pure p-polarized light reaches the eye 508.

[0052] Figure 6 is a schematic diagram of an exemplary spatially-varying retarder (SVR) 600 according to some embodiments. For example, the SVR 600 may be the same as or similar to the SVR 316 / 416 / 516 shown in Figure 3 / Figure 4 / Figure 5 the SVR 316 / 416 / 516 shown therein.

[0053] In some examples, the SVR 600 can be a monolithic structure having a substrate 602, an alignment layer 604, and M twisted birefringence layers 606A, 606B, ..., 606M of liquid crystal elements, where M is an integer of 2 or greater. Such layers can include polymerizable liquid crystals. Figure 6 The cylinder in Figure 6 corresponds to the optical axis (and the nematic director field). Subsequent twisted birefringence layers 606 are aligned with the previous twisted birefringence layers 606. Ellipses 608 and 610 schematically represent the polarization of incident light 612 and outgoing light 614, respectively.

[0054] The SVR 600 provides many benefits, such as effective control of broadband polarization conversion. Since the SVR 600 includes subsequent liquid crystal layers that are directly aligned with the previous layers, the fabrication of the SVR 600 is relatively simple, enabling automatic layer alignment and resulting in a monolithic film with a continuously varying optical axis. The SVR 600 is generally well-suited for patterning achromatic retarders and can achieve large bandwidths and / or low retardation variations in the range from visible to infrared wavelengths. For example, the SVR 600 can operate with relatively high achromaticity across bandwidths of 450 nanometers (nm) - 650 nm and 400 nm - 800 nm.

[0055] It should be understood that Figure 6 the exemplary SVR 600 shown in Figure 6 is just one exemplary type of SVR 416 / 516 suitable for implementation in the techniques and systems described herein. In some embodiments, the exemplary SVR 416 / 516 can include one or more birefringent material layers (such as polymers) that act as means for changing the polarization state of light passing through the SVR by different amounts for different parts of the SVR. In some embodiments, the exemplary SVR 416 / 516 can include thin-film dichroic materials (e.g., stacks) that act as means for changing the polarization state of light passing through the SVR by different amounts for different parts of the SVR. In some embodiments, the exemplary SVR 416 / 516 can include a substrate having nano-features that act as means for changing the polarization state of light passing through the SVR by different amounts for different parts of the SVR. In any of these exemplary configurations, the SVR 416 / 516 changes the polarization by different degrees / amounts depending on the position on the SVR 416 / 516 (e.g., changing the polarization state by a larger amount towards the periphery of the SVR 416 / 516 and by a smaller amount towards the center of the SVR 416 / 516).

[0056] Figure 7FIG. 0 is a schematic cross-sectional view of a system 700 including a display and an optical subsystem, according to some embodiments. The system 700 may be incorporated into a head-mounted device, such as the wearable device 104 / 204. The system 700 may include an information display 702, a backlight assembly 704, and an optical subsystem 706. A schematic of a user's eye 708 is also shown. Such elements are aligned along an optical axis 710.

[0057] The system 700 may be similar to Figure 5 the system 500 shown in, with the addition of a Fresnel lens 712 disposed on an incident side 714 (or surface) of the optical subsystem 706 or a Fresnel lens 716 disposed on an exit side 718 (or surface) of the optical subsystem 706. For example, as referenced in Figure 4 it may be disposed on a front face 422 of the first quarter-wave plate 426, or as referenced in Figure 5 it may be disposed on a front face 522 of the linear polarizer 520. Alternatively, a polarization beam splitter 338, a second reflective polarization beam splitter 536, or a linear polarizer 438 / 538 (as referenced in Figure 3 / Figure 4 / Figure 5 it may be disposed on the Fresnel lens 716. Although Figure 7 both Fresnel lenses 716 and 718 are shown as part of the optical subsystem 706, it should be understood that the optical subsystem 706 may include the Fresnel lens 712 without the Fresnel lens 716, or the optical subsystem 706 may include the Fresnel lens 716 without the Fresnel lens 712. Such addition of the Fresnel lens (712 or 716) to the optical subsystem 706 may be used to modify the focal length of the light exiting the optical subsystem 706. Other types of lenses may be used in addition to or alternatively on the side 714 and / or the side 718 of the optical subsystem 706

[0058] Figure 8is a flowchart showing a process 800 for providing a focused image to a user's eye of a display device, according to some exemplary embodiments. For example, such a display device may be included in an HMD that is the same as or similar to the wearable device 104. At block 802, the display device may generate an image using a pixelated display device, such as an LCD display. For example, such generation may work in conjunction with an illuminating backlight. At block 804, the display device may direct light of the image to a lens assembly, such as the optical subsystem 506, to modify the focal length of the image to a modified focal length. For example, the lens assembly may include a spatially variant retarder (SVR) 316 / 416 / 516 / 600 that is configured to modify the phase of light passing through the SVR 316 / 416 / 516 / 600 by different amounts for different portions of the SVR 316 / 416 / 516 / 600. At block 806, the display device may project the image with the modified focal length onto the user's eye.

[0059] In some embodiments, the spatially variant retarder (SVR) 316 / 416 / 516 / 600 described herein may be fabricated with a correction factor to correct for manufacturing errors that may occur in components of the systems described herein. An exemplary manufacturing error that may be corrected during the fabrication process of the SVR 316 / 416 / 516 / 600 described herein is stress birefringence in a molded lens, which may be a byproduct of molding the lens during the manufacturing process. By way of example, stress birefringence is created when an isotropic material is stressed or deformed (i.e., stretched or bent), resulting in a loss of physical isotropy and, thus, a loss of isotropy in the dielectric constant tensor of the material. Thus, when the lens is molded, this may create stress birefringence in the molded lens. For example, referring to Figure 3, the optical subsystem 306 may include a lens 330 interposed between the first quarter-wave plate 326 and the SVR 316. The lens 330 may be a molded lens manufactured using a molding technique, where the material of the lens 330 is stressed or deformed to produce the desired characteristics (e.g., shape) of the finished component. As a result of this manufacturing process, as explained herein, the manufactured lens 330 may exhibit stress birefringence. This stress birefringence in the molded lens 330 may be compensated for during the manufacturing process of the SVR 316 described herein, which may effectively restore the polarization of the light exiting the lens 330 to the state it would be in if there were no stress birefringence in the molded lens 330. In other words, the molded lens 330 including stress birefringence (as a result of manufacturing the lens 330) may polarize the light passing through it in an undesired manner, and these undesired polarization effects may be effectively "eliminated" by manufacturing the SVR 316 with a correction factor that compensates for these undesired polarization effects. For example, during the manufacturing process of the SVR 316 (e.g., the manufacturing process of arranging the material sheet for the SVR 316), a correction factor may be applied to the SVR 316 to account for the known or determined amount of stress birefringence in the manufactured or to-be-manufactured molded lens 330. The correction factor may include, but is not limited to, an adjustment to the type of birefringent material used to manufacture the SVR 316, an adjustment to the number M of birefringent layers 606 (see Figure 6 ) to be included in the SVR 316, an adjustment to the features (e.g., nano-features) to be included in the SVR 316 (e.g., by changing the type, number, and / or density of the features), etc. In some embodiments, if the stress birefringence is localized, the correction factor may be applied to the SVR 316 at a location corresponding to the local stress birefringence in the molded lens 330 on the SVR 316. In some embodiments, the SVR 316 may be applied as a film with a correction factor to correct or compensate for manufacturing errors in the molded lens 330. Thus, the manufactured SVR 316 may provide an appropriate level of retardation based on the degree and / or location of manufacturing defects in the molded lens 330.

[0060] Another exemplary manufacturing error that may be corrected during the manufacturing process of the SVR 316 / 416 / 516 / 600 described herein is polarization variation on a polarization beam splitter (sometimes referred to herein as a "reflection polarizer" or "reflection film"). By way of example, a roll-to-roll process may be used to manufacture a polarization beam splitter. During such a manufacturing process, a roll of material for the polarization beam splitter may be flattened (e.g., into a sheet), the material may be stretched while it is being flattened, and then a thin film may be applied to the flattened material to produce the finished component. Referring again to Figure 3, by way of example, the optical subsystem 306 may include a polarization beam splitter 338 located between the SVR 316 and the exit surface (or side surface) 314 of the optical subsystem 306. The polarization beam splitter 338 may be fabricated using the roll-to-roll techniques described herein, where the material of the polarization beam splitter 338 is stretched or deformed to produce the finished component. As a result of this manufacturing process, the fabricated polarization beam splitter 338 may have manufacturing errors, which may include undesired polarization variations on the polarization beam splitter 338. These manufacturing errors in the polarization beam splitter 338 may also be compensated for when forming the SVR 316 described herein. For example, during the manufacturing process of the SVR 316 (e.g., the manufacturing process of arranging the material sheet for the SVR 316), a correction factor may be applied to the SVR 316 to account for the known or determined amount of polarization variation in the fabricated or to-be-fabricated polarization beam splitter 338. The correction factor may include, but is not limited to, adjustments to the type of birefringent material used to fabricate the SVR 316, adjustments to the number M of birefringent layers 606 (see Figure 6 ) to be included in the SVR 316, adjustments to the features (e.g., nano-features) to be included in the SVR 316 (e.g., by changing the type, number, and / or density of the features), etc. In some embodiments, if the undesired polarization in the polarization beam splitter 338 is localized, the correction factor may be applied to the SVR 316 at a location on the SVR 316 corresponding to the local undesired polarization in the polarization beam splitter 338. In some embodiments, the SVR 316 may be applied to the polarization beam splitter 338 as a film with a correction factor in order to correct or compensate for the known or determined manufacturing errors in the polarization beam splitter 338. In some embodiments, the correction factor (applied when forming the SVR 316) may be directly applied to the polarization beam splitter 338 during the manufacturing process, which may contribute to the overall performance of the polarization beam splitter 338 and thus to the overall performance of the system including these components as described herein.

[0061] Although the first manufacturing error in the molded lens 330 and the second manufacturing error in the polarization beam splitter 338 have been discussed separately above, it should be understood that the SVR 316 / 416 / 516 / 600 may be fabricated with a correction factor to correct or compensate for the manufacturing errors in both the molded lens 330 and the polarization beam splitter 338 as described herein. That is, during the manufacturing process of the SVR 316 / 416 / 516 / 600, the correction factor applied to the SVR 316 / 416 / 516 / 600 may be based on both the first manufacturing error in the molded lens 330 and the second manufacturing error in the polarization beam splitter 338. In this way, the SVR 316 / 416 / 516 / 600 may be fabricated in a manner that compensates for both manufacturing errors.

[0062] Figure 9FIG. 0 is a flowchart showing a process 900 for applying correction factors during the manufacturing process of a spatial variable retarder (SVR) 316 / 416 / 516 / 600 to compensate for known or determined manufacturing errors in a molded lens and / or a polarization beam splitter. Process 900 may be implemented during the manufacturing process of an optical subsystem of a display system to be included in an HMD, such as an HMD that is the same as or similar to wearable device 104.

[0063] In block 9**02**, the extent of manufacturing errors in the manufacturing components of the optical subsystem may be determined. For example, at sub-block 9**04**, the amount of stress birefringence in the molded lens may be determined. This may be measured using polarization techniques, such as by measuring the polarization change of light passing through the molded lens material. In some embodiments, the difference in the refractive indices of components within the molded lens material may be measured to determine the amount of stress birefringence. In some embodiments, if the stress birefringence in the molded lens is non-uniform, the locations on the molded lens where stress birefringence is exhibited may be determined at sub-block 9**04**. As another example, at sub-block 9**06**, the polarization change in the polarization beam splitter may be determined. This may be measured based on the extinction ratio, the laser-induced damage threshold (LIDT), the wavelength range, and / or the thickness of the entire polarization beam splitter. In some embodiments, if the unwanted polarization in the polarization beam splitter is localized, the locations on the polarization beam splitter where the unwanted polarization is exhibited may be determined at sub-block 9**06**.

[0064] In block 9**08**, the correction factors to be applied during the manufacturing process of the spatial variable retarder (SVR) 316 / 416 / 516 / 600 (e.g., during formation or fabrication) may be determined. The correction factors determined at block 9**08** are configured to compensate for the manufacturing errors determined at block 9**02**. For example, correction factors may be determined to correct a first manufacturing error of the molded lens, a second manufacturing error of the polarization beam splitter, or manufacturing errors of both the molded lens and the polarization beam splitter. In some embodiments, the correction factor is a correction factor based on an adjustment of the type of birefringent material used to manufacture the SVR 316 / 416 / 516 / 600, an adjustment of the number M of birefringent layers 606 to be included in the SVR 316 / 416 / 516 / 600 (see Figure 6 ), an adjustment of the features (e.g., nano-features) to be included in the SVR 316 / 416 / 516 / 600 (e.g., by changing the type, number, and / or density of the features), etc. In some embodiments, if the manufacturing defects to be corrected in the defective component are localized, the correction factor may be applied to the SVR 316 / 416 / 516 / 600 at the location on the SVR 316 / 416 / 516 / 600 corresponding to the localized manufacturing defect in the defective component.

[0065] In block 910, the SVRs 316 / 416 / 516 / 600 can be fabricated (e.g., formed) at block 908 by applying the determined correction factors. Many variations and modifications can be made to the above examples, and the elements in these examples will be understood to be in other acceptable examples. All such modifications and variations are intended to be included within the scope of this disclosure.

Claims

1. A method for manufacturing a spatially variant retarder, the method comprising: Determining the degree of manufacturing error in at least one of a molded lens or a polarization beam splitter to be included in a lens assembly, wherein the degree of manufacturing error includes at least one of the following: The amount of stress birefringence in the molded lens; or The polarization variation in the polarization beam splitter; Determining a correction factor for a spatially variant retarder to be included in the lens assembly, wherein the correction factor is determined based on at least one of the following: The amount of stress birefringence in the molded lens; or The polarization variation in the polarization beam splitter; and Forming the spatially variant retarder by applying the correction factor.

2. The method according to claim 1, wherein determining the degree of manufacturing error includes determining the amount of stress birefringence in the molded lens and determining the polarization variation in the polarization beam splitter, and wherein the correction factor is determined based on the amount of stress birefringence in the molded lens and the polarization variation in the polarization beam splitter.

3. The method according to claim 1, the method further comprising: Determining the location of the manufacturing error in at least one of the molded lens or the polarization beam splitter, wherein forming the spatially variant retarder by applying the correction factor includes applying the correction factor at a location on the spatially variant retarder corresponding to the location of the manufacturing error in at least one of the molded lens or the polarization beam splitter.

4. The method according to claim 1, wherein the correction factor is a correction factor according to at least one of the following: Adjustment of the type of birefringent material used to manufacture the spatially variant retarder; Adjustment of the number of birefringent layers to be included in the spatially variant retarder; or Adjustment of one or more nano-features to be included in the spatially variant retarder.

5. The method according to claim 1, wherein the lens assembly is to be incorporated into a head-mounted display.

6. A method for manufacturing a spatially variant retarder, the method comprising: Determining the degree of manufacturing error in at least one of a lens or a reflective polarizer to be included in an optical subsystem of a head-mounted display, wherein the degree of manufacturing error includes at least one of the following: The amount of stress birefringence in the lens; or The polarization variation in the reflective polarizer; Determining a correction factor to be applied in manufacturing a spatially variant retarder to be included in the optical subsystem, wherein the correction factor is determined based on at least one of the following: The amount of stress birefringence in the lens; or The polarization variation in the reflective polarizer; and Manufacturing the spatially variant retarder by applying the correction factor.

7. The method according to claim 6, wherein determining the degree of the manufacturing error includes determining the amount of stress birefringence in the lens and determining the polarization change in the reflective polarizer, and wherein the correction factor is determined based on the amount of stress birefringence in the lens and the polarization change in the reflective polarizer.

8. The method according to claim 6, wherein the correction factor is a correction factor based on at least one of the following: Adjustment of the type of birefringent material used to fabricate the spatially varying retarder; Adjustment of the number of birefringent layers of polymerizable liquid crystal to be included in the spatially varying retarder; or Adjustment of at least one of the type, number, or density of nano-features to be included in the spatially varying retarder.

9. The method according to claim 6, wherein the spatially varying retarder is fabricated as part of a quarter-wave plate.

10. The method according to claim 6, wherein the correction factor is applied to a position on the spatially varying retarder corresponding to the position of the manufacturing error.

11. A system having a spatially varying retarder, the system comprising: An information display including pixels configured to jointly form an image; A backlight assembly for illuminating the pixels of the information display; And An optical subsystem for directing light of the image from the information display toward a user's eye, wherein the optical subsystem includes: A molded lens; A polarization beam splitter located between the molded lens and the exit surface of the optical subsystem; and A spatially varying retarder interposed between the molded lens and the polarization beam splitter, wherein the spatially varying retarder is configured to compensate for manufacturing errors in at least one of the molded lens or the polarization beam splitter.

12. The system according to claim 11, wherein the spatially varying retarder is configured to compensate for the amount of stress birefringence in the molded lens.

13. The system according to claim 11, wherein the spatially varying retarder is configured to compensate for the polarization change in the polarization beam splitter.

14. The system according to claim 11, wherein the spatially varying retarder is configured to compensate for the amount of stress birefringence in the molded lens and the polarization change in the polarization beam splitter.

15. The system according to claim 11, wherein the spatially varying retarder includes characteristics for compensating for the manufacturing errors in at least one of the molded lens or the polarization beam splitter, the characteristics including at least one of the following: A specific type of birefringent material used to fabricate the spatially varying retarder; A specific number of birefringent layers included in the spatially varying retarder; or At least one of a specific type, a specific number, or a specific density of nano-features included in the spatially varying retarder.

16. The system according to claim 11, wherein the spatially varying retarder is part of a quarter-wave plate.

17. The system according to claim 11, wherein the optical subsystem is part of a head-mounted display.

18. The method according to claim 1, wherein the spatially variable retarder is formed as part of a quarter-wave plate.

19. The method according to claim 4, wherein the birefringent layer comprises a polymerizable liquid crystal birefringent layer.

20. The method according to claim 4, wherein the adjustment of the one or more nano-features comprises an adjustment of at least one of a type, a quantity, or a density of the one or more nano-features.

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