Method and system for calibrating a wearable head-up display with integrated prescription lenses to produce aligned and color-corrected images - Patents.com

Abstract

A system and method for calibrating a head mounted display (HMD) (100) by positioning an optical combiner (102) of the HMD in a holder (202) of a calibration station (200) so that the optical combiner is in a main optical path of light provided from a microdisplay (204) of the HMD and a camera (206) of the calibration station, adjusting a tunable correction unit (208) to correct defocus caused by a corrective prescription of the optical combiner, capturing an image of a reference target (224) seen through the tunable correction unit and the optical combiner by the camera, generating a distortion model from the captured image, and calibrating the HMD to apply the distortion model to an image projected by the HMD.

Description

[Background technology]

[0001] background In the field of optics, a combiner is an optical device that combines light transmitted from two light sources, e.g., a microdisplay and directed to the combiner via a light guide, and ambient light. Optical combiners are used in head-up displays (HUDs), examples of which include head-mounted displays (HMDs) or near-eye displays, that allow a user to see computer-generated content (e.g., text, images, or video content) superimposed on the user's environment seen through the HMD, generating what is known as augmented reality (AR) or mixed reality (MR). In some applications, the HMD is realized in an eyeglass frame form factor, where the optical combiner forms at least one of the lenses in the eyeglass frame. The HMD allows the user to see the displayed computer-generated content while still viewing the user's environment.

[0002] To ensure that users view computer-generated content in focus and with minimal distortion and chromatic aberration, the components of the HMD are generally calibrated using images captured by test setup cameras positioned where the user's eyes are expected to be when the user is wearing the HMD. These images simulate what a user is likely to see projected from the HMD and the real-world environment as seen through the optical combiner of an AR or MR HMD. Summary of the Invention

[0003] The present disclosure, its numerous features and advantages will become apparent to those skilled in the art by reference to the accompanying drawings, in which: The use of the same reference symbols in different drawings indicates similar or identical items. [Brief description of the drawings]

[0004] [Figure 1]1 illustrates an exemplary head mounted display (HMD) having an optical combiner with an integrated corrective prescription, according to some embodiments. [Diagram 2] 2 shows a block diagram of a calibration station where aspects of an HMD, such as the HMD of FIG. 1, are measured and calibrated, according to some embodiments. [Diagram 3] 3 shows a block diagram of a processing system associated with the microdisplay of the HMD of FIG. 1 and the calibration station of FIG. 2 in accordance with some embodiments. [Figure 4] 3 shows a block diagram of a calibration system including the calibration station and peripheral devices of FIG. 2 according to some embodiments. [Diagram 5] A method for calibrating an HMD, such as the HMD of FIG. 1, using the calibration system of FIG. 4 is shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0005] Detailed Description Conventional near-eye displays may require ophthalmic corrective lenses to accommodate users who require vision correction. A typical method for including a corrective optical prescription in an HMD involves configuring an optical combiner to accommodate both a light guide and a separate prescription lens, either as part of glasses worn by the user or as a lens inserted or attached to the optical combiner of the HMD. This often results in a bulky system that can be uncomfortable for the user to wear, thus compromising the user experience. Furthermore, the borders of the corrective prescription lens included in the combiner as an insert or attachment are often visible to the user, which also compromising the user experience. To avoid these issues, optical combiners with integrated corrective prescriptions have been developed. There are cumbersome issues with simultaneously correcting both the light from within the combiner and the ambient light so that the user does not experience undesirable optical aberrations or distortions when viewing the augmented reality scene, but these issues can be diagnosed and improved by calibrating the microdisplay so that the device provides the user with good vision.

[0006] However, the introduction of a corrective prescription into the optical combiner of an HMD presents a challenge to the calibration of the HMD, as the corrective prescription, in some cases, introduces both defocusing and distortion into the image projected by the microdisplay and the light from the environment transmitted through the optical combiner. To avoid such aberrations, it is desirable that during the calibration process, the environment and the displayed content are viewed through the user's eyes to calibrate the microdisplay. Thus, using the techniques described herein, the components of a calibration system used to perform the calibration of the displayed content viewed through a prescription lens are configured to simulate the eye of a user in need of refractive correction. In contrast, existing calibration systems assume that the user has good vision during calibration and that the device to be calibrated uses an architecture that does not require the corrective prescription to be applied to the display, or that a modular corrective prescription can be added to the HMD after calibration. Because users' corrective prescriptions vary significantly, it is desirable for a single calibration system to be adjustable to support a wide range of user prescriptions.

[0007] 1-5 show a system and method for testing and calibrating an HMD having at least one optical combiner with an integrated user-specific corrective prescription. The output of the microdisplay of the HMD and a reference target, as seen through the optical combiner with the corrective prescription, are measured in a dedicated calibration station to calibrate the optical aberrations and artifacts caused by the corrective prescription. The calibration station includes a camera and at least one tunable correction unit that both mimic a user's eye that requires refractive correction. That is, the camera sensor of the camera mimics the retina of the user's eye to receive light to "see" an image, and the tunable correction unit mimics the lens of the user's eye to focus the light so that the image appears in focus by the camera sensor. In general, the tunable correction unit compensates for the focus shift caused by the corrective prescription of the optical combiner, allowing the camera of the calibration station to capture clear images required to measure and calibrate the HMD. The calibration of the HMD is relative such that the microdisplay of the HMD is calibrated to match the reference target, both of which are optically corrected by the corrective prescription and the tunable correction unit of the optical combiner. In other words, the optically corrected light from the microdisplay of the HMD is matched to the optically corrected light from the reference target, so that the user of the HMD sees the content displayed at the optical combiner and the environment behind the optical combiner with good visual acuity.

[0008] The tunable correction unit can be adjusted manually by an operator or by an automated process to produce a focused image at the camera during a calibration stage of the manufacturing process or as part of a recalibration of an HMD requiring repair. Because the components of the tunable correction unit can be adjusted, the calibration station can simulate a number of different corrective prescriptions, allowing for the simulation of how various users would see the real world and / or displayed content when wearing an HMD configured with a user's particular prescription.

[0009] 1 illustrates an exemplary HMD 100 that employs an optical combiner 102 with an integrated corrective prescription. The HMD 100 has a support structure 104 that includes a frame 106 that houses a microdisplay (shown in FIG. 2), such as a laser projector or light-emitting diode (LED) display, that generates visible light to project an image through the optical combiner 102 toward a user's eye such that the user perceives the projected image as displayed within a field of view (FOV) region 108 at the optical combiner 102. In some embodiments, the microdisplay also generates infrared light for eye-tracking purposes.

[0010] The support structure 104 also includes components that allow the support structure 104 to be worn in a position in front of the user's eyes. Examples of such components are the arms 110 and 112 that are supported by the user's ears. One or more straps (not shown) configured to be worn around and / or over the user's head may be used in place of the one or more arms in some embodiments to secure the support structure 104 in front of the user's eyes. In some embodiments, the HMD 100 is configured symmetrically such that the lens element 114 is also a combiner, and the microdisplay is housed in a portion of the frame 106 proximate the arms 112 to project an image to a FOV region within the lens element 114. Either or both of the combiner 102 and the lens element 114 can be configured with eye-side and world-side surfaces having curvatures that both provide a prescription correction for the light transmitted to the user's eyes.

[0011] In the illustrated example, the HMD 100 is a near-eye display system in which the support structure 104 is configured to be worn on the head of a user and has the general shape and appearance (or "form factor") of an eyeglass frame. The support structure 104 houses or otherwise includes various components for facilitating the projection of such images toward the user's eye, such as a processing system, which will be described in more detail below with reference to FIG. 3. In some embodiments, the support structure 104 further includes various sensors, such as one or more forward-facing cameras, rear-facing cameras, other optical sensors, motion sensors, accelerometers, etc. The support structure 104 may further include one or more radio frequency (RF) interfaces or other wireless interfaces, such as a Bluetooth (trademark pending) interface, a WiFi interface, etc. Additionally, in some embodiments, the support structure 104 includes one or more batteries or other portable power sources for powering electrical components, such as one or more processors, and processing components of the processing system of the HMD 100. In some embodiments, some or all of these components of the HMD 100 are housed completely or partially within an interior volume of the support structure 104, such as within an area 116 within the arms 110 and portions of the frame 106 of the support structure 104. While an exemplary form factor is shown, it will be understood that in other embodiments, the HMD 100 may have a different shape and appearance than the eyeglass frame shown in FIG.

[0012] In the illustrated embodiment, the combiner 102 of the HMD 100 provides an AR display in which rendered graphical content can be superimposed on or otherwise provided in conjunction with a real-world view as viewed by a user through the combiner 102. For example, light used to form a perceptible image or series of images may be projected by a microdisplay of the HMD 100 to the user's eye through a series of optical elements, such as a light guide formed at least partially within the combiner 102 and one or more lenses and / or filters disposed between the microdisplay and the light guide. The optical combiner 102 includes at least a portion of a light guide that routes display light received by an in-coupler of the light guide to an out-coupler of the light guide that outputs the display light toward the eye of the user of the HMD 100. Additionally, the optical combiner 102 is sufficiently transparent to allow a user to see through the combiner 102 and provide a view of the user's real-world environment, with the image appearing to be superimposed on at least a portion of the user's real-world environment. To ensure that the user views their real-world environment and the images projected from the optical combiner 102 in focus, at the correct angular position, and with minimal undesirable distortion, the components of the HMD, including the microdisplay, are adjusted using a calibration station, an example of which is described in more detail below with reference to FIG. 2.

[0013] 2 shows a block diagram of a calibration station 200 in which aspects of a display system, such as HMD 100, are measured and calibrated. Calibration station 200 includes a holder 202 in which an optical combiner, such as optical combiner 102, is placed. The position of holder 202 is adjustable to allow optical combiner 102 to be positioned in a main optical path 220 of light traveling from a microdisplay 204 associated with optical combiner 102 to a camera 206 of calibration station 200. Although shown as a single component for ease of explanation, camera 206 is a combination of a camera sensor for sensing light and a camera lens or combination of lenses for focusing the light onto the camera sensor.

[0014] A tunable correction unit 208 is also disposed in the main optical path, located between the holder 202 and the camera 206. In some embodiments, the tunable correction unit 208 is a horopter, which includes a variety of lenses, including but not limited to spherical lenses, cylindrical lenses, filtered lenses, and prism lenses. The horopter also includes specialized measurement devices, such as a Maddox rod and a Jackson cross cylinder, in some embodiments. In some embodiments, the tunable correction unit 208 is a variable focus lens, such as a fluid-filled shape-changing lens, which can change the radius of the lens by deforming a membrane that contains the fluid or by pumping fluid in and out of the membrane. In some embodiments, the tunable correction unit 208 is a trial lens kit that includes a variety of spherical concave lenses, spherical convex lenses, cylindrical concave lenses, cylindrical convex lenses, prism lenses, and specialty lenses, such as tinted lenses, occluder lenses, pinhole lenses, and cross cylinder lenses, that can be placed in the optical path between the holder 202 and the camera 206. In some embodiments, a neutral density (ND) filter is positioned between the camera 206 and the tunable correction unit 208 to reduce or modify the intensity of light entering the camera 206 to avoid capturing images that are too bright or "overexposed."

[0015] To calibrate the HMD 100 so that the user sees in-focus images or text of uniform color and brightness across the FOV 108 of the optical combiner 102, the distortions caused by the optical combiner 102 are corrected by the tunable correction unit 208. In particular, if the optical combiner 102 includes an integrated corrective prescription based on the user's specific optical correction needs, the tunable correction unit 208 is used to reverse the blur caused by the corrective prescription, so that the camera 206 of the calibration station 200 can capture sharp in-focus images for performing display measurements and calibration.

[0016] To illustrate, the corrective prescription of the optical combiner 102 works in conjunction with the lens and cornea of ​​the user's eye to focus light onto the retina of the user's eye. Vision problems arise when the eye's focal accommodation is unable to focus on an object or when the eye is not symmetrical and suffers from astigmatism. The corrective prescription of the optical combiner 102 (measured in diopters) changes the focus of the light entering the user's eye so that the light is precisely focused onto the retina, allowing the user to see an in-focus environment. Thus, the corrective prescription is integrated into the optical combiner 102 by shaping the optical combiner 102 and shifting the perceived depth of the real world to a common plane in the case of astigmatism correction, or to a plane that falls within the patient's focal accommodation in the case of spherical correction. Because the degree to which the focus is shifted varies depending on the user's particular corrective prescription, an image viewed through the optical combiner 102 with the integrated prescription will appear blurry or defocused to another user, or in the case of the calibration station 200, to the camera 206. Thus, the lens or shape of the tunable correction unit 208 is adjusted to reverse the focus shift imposed by the optical combiner 102 with the corrective prescription so that light is properly focused at the camera 206 and the camera 206 can capture an in-focus image of the environment seen through the tunable correction unit 208 and the optical combiner 102.

[0017] In some embodiments, the calibration station 200 includes at least one optical relay. The exemplary calibration station 200 shown in FIG. 2 includes two optical relays 212, 214. The first optical relay 212 is disposed between the holder 202 and the tunable correction unit 208, and the second optical relay 214 is disposed between the tunable correction unit 208 and the camera 206. The first optical relay 212 has at least a first set of relay lenses 218 that relay light from the microdisplay 204 to the tunable correction unit 208, and the second optical relay 214 has at least a second set of relay lenses 218 that relay light from the tunable correction unit 208 to the camera 206. Because the camera 206 is used to simulate a user's field of view, without the at least one optical relay, the camera 206 would need to be positioned very close to the optical combiner 102 held in the holder 202 to emulate the position of the user's eye. As a result, there is not enough space between the holder 202 and the camera 206 to fit the tunable correction unit 208. The optical relays 212, 214 accommodate the positioning of the tunable correction unit 208 between the camera 206 and the holder 202 and serve to extend the main optical path to allow the camera 206 to capture an image of the projected digital content at the optical combiner 102 and / or the reference target 224 located beyond the optical combiner 102 when the user views the digital content and the reference target 224. In some embodiments, the reference target 224 is positioned at a set distance from the holder 202 such that the camera 206 views the reference target 224 through the optical combiner 102 positioned within the holder 202. The reference target 224 is generally a physical item or image, such as a checkerboard or other pattern with identifiable features, that is used to ensure that the camera 206 is capturing a focused image of the reference target 224. In some embodiments, the reference target 224 is a static or dynamic image projected onto a surface positioned a set distance from the holder.The reference target 224 is generally illuminated with white light, although colored lighting can also be used to illuminate the reference target 224.

[0018] A beam splitter 216 is positioned between the microdisplay 204 and the tunable correction unit 208 to sample a portion of the light 220 from the microdisplay 204 to calibrate color, intensity, and intensity uniformity. The beam splitter 216 redirects a portion of the light 220 from the microdisplay 204 away from the main optical path and toward a measurement device, such as a spectrometer, power meter, and / or integrating sphere, as described in more detail below with reference to FIG. 3. In some embodiments, the beam splitter 216 is used to inject light into the main optical path. In some embodiments, the beam splitter 216 is positioned between the tunable correction unit 208 and the camera to partially compensate for the effect of the tunable correction unit 208 on the light from the microdisplay 204. In some embodiments, the beam splitter 216 is positioned on either side of the tunable correction unit 208 to fully compensate for the effect of the tunable correction unit 208.

[0019] FIG. 3 illustrates a block diagram of a processing system 300 associated with the microdisplay 204 and calibration station 200 of the HMD. In the illustrated example, the processing system 300 includes an application processor (AP) 302, which is an integrated circuit (e.g., a microprocessor) that executes one or more software programs for controlling the microdisplay 204 and other components of the HMD 100. In the example illustrated in FIG. 3, the AP 302 includes a processor 304, a GPU 306, and a memory 308. The processor 304 and the GPU 306 are communicatively coupled to the memory 308. In some embodiments, the memory 308 is configured as a temporary storage device that holds data and instructions that can be quickly accessed by the processor 304 and the GPU 306. In some embodiments, the storage 310 is a more permanent storage device for holding the data and instructions. Each of the memory 308 and storage 310 is a non-transitory processor-readable storage medium that stores data and instructions and may include one or more of a random access memory (RAM), a read-only memory (ROM), a flash memory, a solid-state drive, or other processor-readable storage medium. In some embodiments, the processor 304 is a programmed computer that performs computational operations. For example, the processor 304 is implemented as a central processing unit (CPU), a microprocessor, a controller, an application specific integrated circuit (ASIC), a system on a chip (SOC), or a field programmable gate array (FPGA).

[0020] To form a virtual image in the field of view (FOV) region 108 seen by a user of the HMD 100, the GPU 306 receives source images from the processor 304 and writes or renders the source images into a projector frame buffer, which is sent to a display controller 322 of the microdisplay 204. To project the source images onto the optical combiner 102, the microdisplay 204 uses the frame buffer data to generate drive controls for laser diodes or other light sources within the microdisplay 204. During normal operation of the HMD 100, any corrections to the source images to be projected onto the optical combiner 102 are applied when rendering the source images into the projector frame buffer. In some embodiments, the display controller 322 applies the corrections before providing the frame buffer data to the microdisplay 204. Some corrections, such as geometric distortions (determined by a calibration process described herein), color corrections, and / or other corrections due to physical changes in the optical system (e.g., thermal changes), are applied to the source images to achieve a corrected image displayed to the user.

[0021] For example, an HMD employing microdisplays 204 with laser diodes that each project different wavelengths of light generates a virtual image in the FOV 108 by projecting multiple source images from different regions of the projector frame buffer. The HMD is designed so that the virtual images overlap in the FOV 108 and appear as one image. However, manufactured HMDs may not automatically generate aligned virtual images in the FOV 108, resulting in the user viewing them as defocused or "ghost" images (i.e., slightly offset, overlapping images). Even after the optical elements in the system are precisely aligned, there may still be misalignment in the projected virtual images in the FOV 108 due to the unique path and unique nonlinear distortions that generated each virtual image. Thus, a distortion model may be applied to the source images such that the source images are distorted ("corrected") and projected into the FOV 108, forming a virtual image that is aligned within the target region of the FOV 108. Similarly, color correction and / or intensity models may be applied to the source images. These models vary the distribution of color and brightness within certain regions of the source image, so that when projected in the FOV 108, the resulting virtual image has uniform color and brightness as seen by the user.

[0022] The processing system 300 further includes a calibration processor 312 communicatively coupled to the camera 206 to receive image and / or light intensity data captured by the camera 206 during a calibration process, as further described with reference to FIG. 4. In some embodiments, the calibration processor can be disconnected from the HMD 100 once the calibration is complete. In general, a processor that performs the calibration process described herein may be referred to as a calibration processor. In some embodiments, the calibration processor 312 is a programmed computer that performs computational operations. For example, the calibration processor 312 can be a central processing unit (CPU), a microprocessor, a controller, an application specific integrated circuit (ASIC), a system on a chip (SOC), or a field programmable gate array (FPGA). Although not shown, a display screen may be communicatively coupled to the calibration processor 312 to enable interaction with a calibration program 314 running on the calibration processor 312 and / or to enable the calibration processor 312 to display calibration results from the calibration program 314.

[0023] In some embodiments, the calibration processor 312 is communicatively coupled to the AP processor 302 for calibration purposes. FIG. 3 shows the calibration processor 312 executing instructions of a calibration program 314. The calibration program 314 may be stored in a memory 316 and accessed by the calibration processor 312 during execution. The calibration program 314 includes decision logic 318, which, when executed by the calibration processor 312, in some embodiments provides the AP 302 with a test pattern in a prescribed sequence. The prescribed sequence may be adjusted during the calibration process. The AP 302 renders the prescribed sequence of the test pattern into a projector frame buffer. The camera 206 captures images of the test pattern as it is projected by the microdisplay 204 through the optical combiner 102 and generates display data 320. The camera 206 also captures images of the reference target 224 as seen through the optical combiner 102 and the tunable correction unit 208 and generates reference target data 324. The calibration processor 312 receives view data 320 and reference target data 324 from the camera 206, and the calibration program 314 uses the view data 320 and the reference target data 324 to determine an eye space to projector space mapping that is subsequently used to generate a warped or distorted source image. In some embodiments, the spectral and intensity measurements of the test pattern projected by the microdisplay 204 are obtained by peripheral components such as those described in more detail with reference to FIG.

[0024] 4 shows a block diagram of a calibration system 400 including a calibration station 200 and peripheral devices. In some embodiments, the peripheral devices include a spectrometer 404 and a power meter 406 positioned in the optical path of the light 222 redirected by the beam splitter 216. In some embodiments, the peripheral devices include an integrating sphere 408 for measuring the total power (flux) of the light 222 redirected by the beam splitter 216. The color uniformity and intensity measurements obtained by the spectrometer 404, the integrating sphere 408, and the power meter 406 are communicated to a calibration processor 312 associated with the processing system 300 and configured to determine calibration parameters for the particular HMD 100 being analyzed at the calibration station 200. Additionally, images captured by the camera 206 are sent to the calibration processor 312 for measurement and analysis. Thus, the calibration processor 312 is communicatively coupled to the camera 206, the measurement devices such as the spectrometer 404 and the power meter 406, and the AP processor 130.

[0025] FIG. 5 illustrates a method 500 for calibrating an HMD, such as HMD 100, using calibration system 400. In block 502, optical combiner 102 is positioned in holder 202 of calibration station 200. Typically, optical combiner 102 will be mounted in a frame, such as frame 106, that houses microdisplays 204 and other HMD components, such as display controller 322 and AP 302. Tunable correction unit 208 is adjusted in block 504 to correct for defocus caused by any corrective prescription included in optical combiner 102. For example, if the optical combiner is configured to provide a +4 spherical correction, tunable correction unit 208 is adjusted to compensate for the defocus that would be seen by camera 206 by providing a -4 spherical correction. Thus, camera 206 and tunable correction unit 208 act together to simulate a user's eye requiring a +4 spherical correction. The tunable correction unit 208 can be adjusted manually by an operator or automatically during a calibration process.

[0026] In block 506, the camera 206 captures an image of a reference target, such as the reference target 224, as seen through the tunable correction unit 208 and the optical combiner 102. Although the tunable correction unit 208 acts to correct the defocus of the reference target 224 introduced by the correction prescription of the optical combiner 102, as described above with reference to FIG. 2, in some cases, some distortion of the reference target 224 will still be present in the captured image. In some embodiments, this distortion is desirable in the final calibrated HMD because a user who requires refractive correction will adapt to viewing the world with this distortion. Thus, the final display calibration should match the user's expectations when viewing their environment. In block 508, the captured image of the reference target 224 is sent to the calibration processor 312, where the captured image is measured and analyzed for distortion. In block 510, a distortion model is generated. The distortion model is provided to the HMD's AP processor 302 in block 512 to calibrate the microdisplay 204 to project an image with the same geometric distortion as the captured image of the reference target 224.

[0027] Once the distortions are provided to the AP processor 302, in block 514, the microdisplay 204 provides light representing the image or text to the optical combiner 102, which outputs light 220 along a main optical path between the optical combiner 102 and the camera 206 of the calibration station 200. In block 516, a portion of the output light is redirected by the beam splitter 216 away from the main optical path and toward measurement devices of the calibration station 200, such as the spectrometer 404 and the power meter 406, where the light is analyzed for color and luminance uniformity. A uniformity correction model is generated in block 518 and provided to the AP processor 302 in block 520. The uniformity correction model is applied to the light 220 projected by the microdisplay 204 to compensate for non-uniformity in the light output by the optical combiner 102 and match a predetermined target white point. In some embodiments, in the case of a binocular HMD, the method of calibration described in blocks 502-520 is repeated for the second lens of the HMD 100. The result is an HMD 100 that is optimally calibrated for the user's eye requiring refractive correction.

[0028] In some embodiments, some aspects of the techniques described above may be realized by one or more processors of a processing system executing software. The software includes a set of one or more executable instructions stored or otherwise tangibly embodied on a non-transitory computer-readable storage medium. The software may include instructions and some data that, when executed by one or more processors, operate the one or more processors to perform one or more aspects of the techniques described above. The non-transitory computer-readable storage medium may include, for example, magnetic or optical disk storage devices, solid-state storage devices such as flash memory, caches, random access memory (RAM) or other non-volatile memory devices, etc. The executable instructions stored on the non-transitory computer-readable storage medium may be source code, assembly language code, object code, or other instruction formats that are interpreted or otherwise executable by one or more processors.

[0029] A computer-readable storage medium may include any storage medium or combination of storage media that can be accessed by a computer system during use to provide instructions and / or data to the computer system. Such storage media may include, but are not limited to, optical media (e.g., compact discs (CDs), digital versatile discs (DVDs), Blu-ray discs), magnetic media (e.g., floppy disks, magnetic tapes, or magnetic hard drives), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or flash memory), or microelectromechanical system (MEMS)-based storage media. A computer-readable storage medium may be embedded in a computing system (e.g., system RAM or ROM), fixedly attached to a computing system (e.g., magnetic hard drives), removably attached to a computing system (e.g., optical disks or universal serial bus (USB)-based flash memory), or coupled to a computer system via a wired or wireless network (e.g., network-accessible storage (NAS)).

[0030] It should be noted that not all of the activities or elements described above in the general description are required, some of the specific activities or devices may not be required, and one or more additional activities may be performed in addition to those described, or one or more additional elements may be included. Furthermore, the order in which the operations are listed is not necessarily the order in which they are performed. Also, the concepts have been described with reference to specific embodiments. However, those skilled in the art will appreciate that various modifications and changes may be made without departing from the scope of the present disclosure as set forth in the claims. Thus, the specification and drawings should be viewed in an illustrative rather than restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure.

[0031] Benefits, other advantages, and solutions to problems have been described above with respect to specific embodiments. However, the benefits, advantages, solutions to problems, and any features that may give rise to or make more prominent any benefits, advantages, or solutions should not be construed as critical, necessary, or essential features of any or all claims. Moreover, the specific embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners that will be apparent to those skilled in the art having the benefit of the teachings herein. No limitations are intended to the details of construction or design shown herein, other than as set forth in the claims. It is therefore apparent that the specific embodiments disclosed above may be altered or modified, and all such variations are considered to be within the scope of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims.

Claims

1. 1. An apparatus comprising: A camera and a holder for fixing an optical combiner of a head-mounted display (HMD) in an optical path between a microdisplay of the HMD and the camera; a tunable correction unit disposed in the optical path and configured to adjust a focus of light transmitted by the optical combiner; the camera is configured to capture at least one image of the adjusted focus light output from the tunable correction unit; The apparatus further comprising means for calibrating the microdisplay based on the at least one captured image.

2. The device of claim 1 , wherein the optical combiner includes an integrated corrective prescription.

3. The apparatus of claim 1 or 2, wherein the tunable correction unit is a horopter including at least one of a spherical lens, a cylindrical lens, a filtered lens, and a prismatic lens.

4. The apparatus of claim 1 or 2, further comprising at least one optical relay disposed in the optical path between the microdisplay and the camera.

5. 3. The apparatus of claim 1, further comprising a beam splitter disposed between the microdisplay and the tunable correction unit and configured to redirect a portion of light from the microdisplay towards at least one measurement device.

6. The apparatus of claim 5 , wherein the at least one measurement device includes at least one of a spectrometer and a power meter.

7. 1. A method for calibrating a head mounted display (HMD), comprising: Positioning an optical combiner of the HMD in a holder of a calibration station so that the optical combiner is in a main optical path of light traveling from a microdisplay of the HMD to a camera of the calibration station; adjusting a tunable correction unit to correct defocus caused by the corrective prescription of the optical combiner; capturing an image of a reference target located beyond the optical combiner with the camera through the tunable correction unit and the optical combiner; generating a distortion model from the captured images, calibrating the microdisplay of the HMD, and applying the distortion model to images projected by the microdisplay of the HMD onto the optical combiner.

8. The method of claim 7 , wherein the tunable correction unit is at least one of a horopter or a tunable lens.

9. The method of claim 7 or 8, further comprising diverting a portion of the light provided by the microdisplay of the HMD from the main optical path to at least one measurement device.

10. The method of claim 9 , wherein the at least one measurement device includes at least one of a spectrometer and a power meter.

11. The method of claim 9 , wherein the portion of the light is redirected by a beam splitter positioned between the microdisplay and the tunable correction unit.

12. 10. The method of claim 9, further comprising generating a uniformity correction model by analyzing the portion of the light at the at least one measurement device, and calibrating the microdisplay based on the uniformity correction model to compensate for non-uniformity of the light output by the optical combiner.

13. A camera; means for fixing an optical combiner of a head mounted display (HMD) in an optical path between a microdisplay of the HMD and the camera; a tunable correction unit disposed in the optical path and configured to be adjusted to correct defocusing of light caused by the optical combiner; a calibration station, wherein the camera is configured to capture at least one image of the corrected light output from the tunable correction unit, and to transmit the at least one captured image to means for calibrating the microdisplay based on the at least one captured image.

14. 1. A system comprising: a calibration station including a camera and means for fixing an optical combiner of a head mounted display (HMD) in an optical path between a microdisplay of the HMD and the camera, the calibration station further including a tunable correction unit disposed in the optical path and configured to be adjusted to correct defocusing of light caused by the optical combiner, the camera configured to capture at least one image of the corrected light output from the tunable correction unit for use in calibrating the microdisplay, the system further including: A system comprising: a calibration processor configured to receive the at least one captured image and generate a distortion model based on the at least one captured image, the calibration processor further configured to provide the distortion model to a display controller of the HMD.

15. The system of claim 14 , wherein the optical combiner includes an integrated corrective prescription.

16. 16. The system of claim 14 or 15, wherein the tunable correction unit is a horopter including at least one of a spherical lens, a cylindrical lens, a filtered lens, and a prismatic lens.

17. The system of claim 14 or 15, wherein the tunable correction unit is a tunable lens.

18. 16. The system of claim 14 or 15, further comprising at least one optical relay disposed in the optical path between the microdisplay and the camera.

19. 16. The system of claim 14 or 15, further comprising a beam splitter disposed between the microdisplay and the tunable correction unit and configured to redirect a portion of light from the microdisplay towards at least one measurement device.

20. Fixing an optical combiner of a head mounted display (HMD) in an optical path between a microdisplay and a camera of the HMD; adjusting a focus of the light transmitted by the optical combiner at a tunable correction unit disposed in the optical path; capturing at least one image of the adjusted focus light output from the tunable correction unit; and calibrating the microdisplay based on the at least one captured image.

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