Patent Publication Number: US-11640064-B2

Title: Wearable pupil-forming display apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/496,102, filed Oct. 7, 2021, which is a continuation of U.S. patent application Ser. No. 17/389,484, filed Jul. 30, 2021 (now U.S. Pat. No. 11,181,747, issued Nov. 23, 2021), which is a continuation of U.S. patent application Ser. No. 17/139,167, filed Dec. 31, 2020 (now U.S. Pat. No. 11,112,611, issued Sep. 7, 2021), which claims the benefit of U.S. Provisional application Ser. No. 63/060,343, filed on Aug. 3, 2020, the disclosures of which are hereby incorporated by reference in their entirety and for all purposes. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to wearable display apparatus and more particularly to a wearable display device that provides augmented reality (AR) viewing with a catadioptric pupil-forming optical system that renders a binocular 3D virtual image from a pair of 2-dimensional (2D) displays. 
     BACKGROUND 
     Virtual image display has advantages for augmented reality (AR) presentation, including providing the capability for display of image content using a compact optical system that can be mounted on eyeglasses or goggles, generally positioned very close to the eye (Near-Eye Display) and allowing see-through vision, not obstructing the view of the outside world. Among virtual image display solutions for AR viewing are catadioptric optics that employ a partially transmissive curved mirror for directing image-bearing light to the viewer&#39;s eye and a partially reflective beam splitter for combining light generated at a 2D display with the real-world visible scene which forms a 3D image when viewed binocularly. 
     Vision correction applications have employed wearable display devices in order to enhance or compensate for loss of vision over portions of a subject&#39;s field of view (FOV). Support for these types of applications can require additional components and can introduce various factors related to wearability and usability that contribute to the overall complexity of the optical design and packaging. 
     Among challenges that must be addressed with wearable AR devices is obtaining sufficient brightness of the virtual image. Many types of AR systems, particularly those using pupil expansion, have reduced brightness and power efficiency. Measured in NITS or candelas per square meter (Cd/m2), brightness for the augmented imaging channel must be sufficient for visibility under some demanding conditions, such as visible when overlaid against a bright outdoor scene. Other optical shortcomings of typical AR display solutions include distortion, reduced see-through transmission, small eye box, and angular field of view (FOV) constraints. 
     Some types of AR solution employ pupil expansion as a technique for enlarging the viewer eye-box. However, pupil expansion techniques tend to overfill the viewer pupil which wastes light, providing reduced brightness, compromised resolution, and lower overall image quality. 
     Challenging physical and dimensional constraints with wearable AR apparatus include limits on component size and positioning and, with many types of optical systems, the practical requirement for folding the optical path in order that the imaging system components be ergonomically disposed, unobtrusive, and aesthetically acceptable in appearance. Among aesthetic aspects, compactness is desirable, with larger horizontal than vertical dimensions. 
     Other practical considerations relate to positioning of the display components themselves. Organic Light-Emitting Diode (OLED) displays have a number of advantages for brightness and overall image quality, but can generate perceptible amounts of heat. For this reason, it is advisable to provide some distance and air space between an OLED display and the skin, particularly since it may be necessary to position these devices near the viewer&#39;s temples. 
     Still other considerations relate to differences between users of the wearable display, such as with respect to inter-pupil distance (IPD) and other variables related to the viewer&#39;s vision. Further, problems related to conflict between vergence depth and accommodation have not been adequately understood or addressed in the art. 
     It has proved challenging to wearable display designers to provide the needed image quality, while at the same time allowing the wearable display device to be comfortable and aesthetically pleasing and to allow maximum see-through visibility. In addition, the design of system optics must allow wearer comfort in social situations, without awkward appearance that might discourage use in public. Providing suitable component housing for wearable eyeglass display devices has proved to be a challenge, making some compromises necessary. As noted previously, in order to meet ergonomic and other practical requirements, some folding of the optical path along one or both vertical and horizontal axes may be desirable. 
     SUMMARY OF INVENTION 
     The Applicants address the problem of advancing the art of AR display and addressing shortcomings of other proposed solutions, as outlined previously in the background section. 
     The Applicants&#39; solution uses pupil forming and can be distinguished from pupil expansion systems known to those skilled in the art. By comparison with pupil expansion approaches, the Applicants&#39; approach yields a more efficient optical system with improved image quality. Moreover, the eyes of the viewer can clearly see and be seen by others, with minimal impediment from the optics that provide the electronically generated virtual image. 
     Wearable display apparatus of the present disclosure are well-adapted for systems that complement viewer capabilities, such as where a viewer may have visual constraints due to macular degeneration or other condition of the eyes. 
     With these objects in mind, there is provided a wearable display apparatus comprising a wearable display apparatus comprising a headset that is configured for display from a left-eye optical system and a right-eye optical system, wherein each optical system defines a corresponding exit pupil for a viewer along a view axis and comprises: (a) an electroluminescent image generator that is energizable to direct image bearing light for a 2D image from an emissive surface; (b) a curved reflective surface disposed along the view axis and partially transmissive, wherein the curved reflective surface defines a curved intermediate focal surface; (c) a beam splitter disposed along the view axis and oriented to reflect light toward the curved reflective surface; (d) an optical image relay that is configured to optically conjugate the formed 2D image at the image generator with the intermediate focal surface, wherein the optical image relay comprises: (i) a prism having an input surface facing toward the emissive surface of the image generator, an output surface facing toward the intermediate focal plane, and a folding surface extending between the input and output surfaces and configured for folding an optical path for light generated by the image generator, wherein an aperture stop for the relay lies within the prism; (ii) at least a first plano-aspheric lens in optical contact against the prism input surface and configured to guide the image-bearing light from the image generator toward the folding surface; wherein the relay, curved mirror, and beam splitter are configured to form the exit pupil for viewing the generated 2D image superimposed on a portion of a visible object scene, wherein combined images from both left- and right-eye optical systems form a 3D image for the viewer; and (e) a plurality of sensors coupled to the headset and configured to acquire measured data relating to the viewer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a schematic front view showing placement of optical components of the system. 
         FIG.  1 B  is a schematic side view showing placement of optical components of the system. 
         FIG.  2 A  is a schematic front view showing the optical path through components of the system. 
         FIG.  2 B  is a schematic side view showing the optical path through components of the system. 
         FIG.  3 A  is a schematic that shows, in perspective view, components of an optical apparatus for AR viewing. 
         FIG.  3 B  is a simplified schematic of  FIG.  3 A . 
         FIG.  4    is a schematic that shows, from an alternate perspective view, components of an optical apparatus for AR viewing. 
         FIG.  5    is a side view schematic of an image relay. 
         FIGS.  6 A,  6 B  are schematic, showing the image relay and components for forming the exit pupil. 
         FIG.  6 C  is a schematic view that shows an alternate embodiment of the present disclosure that employs the imaging path for eye tracking. 
         FIGS.  7 A- 7 C  show various features of an embodiment useful for compensating for macular degeneration. 
         FIG.  8    is a block diagram illustration showing integrated components for an HMD system in an exemplary embodiment. 
         FIG.  9    is a perspective view that shows positions of various sensors, processor, and components of the head-mounted display according to an embodiment. 
         FIG.  10    is a schematic diagram that shows components of an interpupil distance adjustment system for the head-mounted display. 
         FIG.  11 A  is a schematic diagram showing optical relay components used for focal plane adjustment. 
         FIG.  11 B  is a perspective view that shows position of an actuator relative to the display and to corresponding optics. 
         FIG.  11 C  is a perspective view from behind the display component, showing a piezoelectric actuator mounted to a plate behind the display. 
         FIG.  11 D  is a side view showing components of a dynamic focus adjustment apparatus. 
         FIG.  12    is a schematic diagram showing visual accommodation and vergence-accommodation conflict. 
         FIG.  13    is a schematic diagram that shows aspects of dithering geometry for enhanced image resolution. 
     
    
    
     DETAILED DESCRIPTION 
     The following is a detailed description of the preferred embodiments of the disclosure, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the several figures. 
     While the devices and methods have been described with a certain degree of particularity, it is to be noted that many modifications may be made in the details of the construction and the arrangement of the devices and components without departing from the spirit and scope of this disclosure. It is understood that the devices and methods are not limited to the embodiments set forth herein for purposes of exemplification. It will be apparent to one having ordinary skill in the art that the specific detail need not be employed to practice according to the present disclosure. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present disclosure. 
     Reference throughout this specification to “one embodiment,” “an embodiment,” “one example,” or “an example” means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” “one example,” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable combinations and/or sub-combinations in one or more embodiments or examples. 
     In the context of the present disclosure, the term “eyebox” has its conventional meaning in the HMD arts, as functionally equivalent to “eye motion box” and similar phrases. The eyebox is that volume of space within which the viewable image is formed by an optical system or visual display. When the viewer&#39;s pupil is within this volume, the viewer can see all of the generated display content; with the pupil is outside of this volume, the user is typically not able to view at least some portion of the display content. A larger eyebox is generally desirable, as this allows for lateral and axial movement of the eye, while still maintaining a full field of view. The size of the eyebox relates directly to the size of the exit pupil for a display system. 
     Several (or different) elements discussed herein and/or claimed are described as being “coupled,” “in communication with,” “integrated,” or “configured to be in communication with” or a “system” or “subsystem” thereof. This terminology is intended to be non-limiting and, where appropriate, be interpreted to include, without limitation, wired and wireless communication using any one or a plurality of a suitable protocols, as well as communication methods that are constantly maintained, are made on a periodic basis, and/or made or initiated on an as-needed basis. 
     Where they are used, the terms “first”, “second”, and so on, do not necessarily denote any ordinal, sequential, or priority relation, but are simply used to more clearly distinguish one element or set of elements from another, unless specified otherwise. 
     In the context of the present disclosure, the term “coupled” is intended to indicate a mechanical association, connection, relation, or linking, between two or more components, such that the disposition of one component affects the spatial disposition of a component to which it is coupled. For mechanical coupling, two components need not be in direct contact, but can be linked through one or more intermediary components. 
     An embodiment of the present disclosure provides AR viewing and display having a large FOV with an optical system having an optical path that folds in the horizontal or x-direction, the direction substantially parallel (+/−15 degrees) to a line between left and right pupils of a viewer, for forming an intermediate image to the curved mirror. An embodiment of the AR system of the present disclosure has a component arrangement as shown schematically in the front view of  FIG.  1 A  and from the side in  FIG.  1 B . The corresponding light path is shown schematically in  FIGS.  2 A and  2 B , respectively. A flat-panel display is energized as an image generator  10  to form an image and to direct image-bearing light through beam-shaping optics and to a folding prism  20  that redirects the image-bearing light towards a beam splitter  24  and to a curved mirror  30  for forming the virtual image from electronically generated image content. Image generator  10  can be a display that emits light, such as an organic light-emitting device (OLED) array or a liquid crystal array or a micro-LED array with accompanying lenslets, or some other type of spatial light modulator useful for image generation. 
     In order to address the need for improved overall imaging performance, wider FOV, compactness, and other factors outlined in the background, embodiments of the present disclosure have a number of features shown particularly in  FIGS.  3 A,  3 B, and  4   . Specific features of interest include: (i) relay of the image generator  10  to form a curved intermediate image I as a conjugate image. As a type of “aerial” image, intermediate image I is formed in air, serving as the optical “object” for forming the virtual image. Intermediate image I is formed along the curved focal surface of curved mirror  30 , with the approximate position shown by a dashed line in  FIG.  5   . An optical relay  40 , with particular structure as described in more detail subsequently, conjugates the image formed from image generator  10  to the curved intermediate image I along the focal surface. Curved mirror  30  is partially transmissive, such as between about 30% to 70% transmissive, for example, allowing visibility of the real-world object scene to the viewer. A nominal transmission range of about 50% is useful in many applications. (ii) use of a cylindrically curved quarter-wave plate (QWP) between mirror  30  and beam splitter  24 . Curvature of this element helps to reduce variations of the retardation imparted to the image-bearing light by the QWP over the field of view. (iii) large exit pupil  44 . System optics can form a 10 mm exit pupil at the viewer&#39;s eye-box for eye E. Forming a suitably sized pupil for the viewer helps to provide an eyebox of reasonable dimensions to allow eye movement, without noticeable vignetting. Also, an enlarged eyebox permits the headset to move or slip without noticeable degradation of the viewed image(s). The apparatus does not need to provide pupil expansion, such as is used in existing wearable display apparatus, but uses pupil-forming optics for improved efficiency and brightness, as well as for improved image resolution. 
     Significantly, the eyes of the viewer can clearly see and be seen by others, with minimal impediment from the beam splitter and curved mirror optics that provide the electronically generated virtual image. 
     With the optical arrangement shown, the aperture stop AS lies within prism  20  of the image relay, along or very near the fold surface that is provided. This arrangement is advantageous for component packaging and spacing, allowing prism  20  to be reduced in size over other configurations using a folding prism. 
     The given design allows an FOV along the horizontal (x) axis, the axis parallel to a line between left and right pupils of the viewer&#39;s eyes, of greater than 50 degrees. The FOV aspect ratio (horizontal:vertical) equals or exceeds 1.5. Digital correction is not needed for distortion or lateral color. 
     According to an embodiment, curved mirror  30  has a conic surface shape. The conic shape is advantaged, in the embodiment shown herein, helping to control chief ray angles, thus correcting for distortion. 
     Depending on whether or not polarization is used for configuring light paths, beam splitter  24  can be either a polarization-neutral beam splitter or a polarization beam splitter. Beam splitter  24  can be, for example, a wire grid polarization beam splitter as shown in  FIG.  4   . 
     Image Relay  40   
       FIG.  5    shows an enlarged side view of relay  40  for relay of the display image from an electroluminescent display (such as an OLED in the examples shown) to the focal surface position of mirror  30  (shown in a perspective view in  FIG.  4   ) and for shaping the relayed image to suitable curvature to correct distortion. A concave-plano field lens LI, with sides truncated along the vertical direction as shown in  FIG.  4    in order to reduce weight and provide a more compact system, shapes the light from OLED display image generator  10 , providing a beam to a meniscus singlet lens L 2 . 
     From lens L 2 , the imaging light goes to a doublet L 3  having a concave/convex flint glass lens cemented to a crown glass lens. 
     An aspheric plano-convex lens L 4  is in optical contact with the input face of prism  20 , such as cemented to prism  20 . A second plano-aspheric lens L 5  can be cemented to the output face of prism  20 . This cemented arrangement facilitates alignment of these optical components. According to an alternate embodiment, only a single plano-aspheric lens L 4  is deployed at the prism  20  input surface. 
     The turning surface  26  of prism  20  is coated to enhance reflection. Hypotenuse or turning surface  26  of the prism is essentially the relay (and system) aperture stop. 
     Intermediate image I is formed in the shape and location of the focal surface of the curved mirror. Proceeding backward along the optical path from intermediate image I are the following components: Plano-asphere lens L 5 ; Folding prism  20  with turning surface  26 ; plano-asphere lens L 4 ; Doublet L 3 ; Meniscus singlet L 2 ; Field lens LI; Image source or generator, display  10   
       FIG.  6 A  and simplified  FIG.  6 B  show an alternate view of the display optics from exit pupil  44 . Chief rays are shown in  FIG.  6 B ; these chief rays converge at the position of exit pupil  44  at eye E.  FIG.  6 B  also shows the approximate position of an intermediate pupil P i  at the aperture stop, near the folding surface  26  of prism  20 . 
     As shown in  FIGS.  1 A and  4   , the image generator is disposed to direct image-bearing light beam in a horizontal direction and along a path that lies above eye-level, as the display optics are normally worn by a sitting or standing viewer. Prism  20  can be tilted slightly away from the forehead of the viewer, to direct light in front of the face at an oblique angle to vertical, as shown in the embodiment of  FIG.  2 B . 
     The layout and routing of the optical path are particularly suitable for providing augmented reality 2D and 3D viewing in a wearable device. Using relay  40  allows the positioning of image generator  10  to be out of the direct field of view; in addition, relay  40  allows image generator  10  to be positioned at sufficient distance away from the skin surface to avoid contact and consequent discomfort. The use of a first x-direction (horizontal) fold, followed by a y-direction (vertical) folding enables the imaging optics to be compactly packaged with reasonable optical path distance to allow a measure of light beam shaping and correction. Prism  20  can be rotated over at least a range of angles about the x axis, allowing a measure of alignment as well as adaptation to different mirror  30  curvatures. Employing a curved surface for an optional QWP component helps to reduce variations, over the FOV, of retardation imparted by the QWP; excessive variation over the field may otherwise cause some brightness fall-off. 
     Using a wire-grid polarizer reduces light loss, allowing high levels of visibility to the external, real-world object scene content, along with reduced light leakage over other polarization components. 
     Image source  10  may be unpolarized. In one embodiment, a polarizing beam splitter is used, such as a wire grid splitter made by Moxtek, Inc., Orem, Utah. This type of beam splitter reflects only one polarization, usually S polarization, towards the conic combiner. The orthogonal polarization, P polarization, is transmitted and is absorbed (absorber not shown). To prevent the small amount of P light from being reflected, an optional polarizer can be placed at the image source. 
     The mirror  30  provides a conic combiner in embodiments shown, with power only for the generated image and not for the visible field. The curved mirror  30  can be a double conic for improved image formation. Various types of coatings can be provided on the mirror  30  combiner, including, but not limited to dichroic coatings, metal coatings, such as to provide a half-silvered reflector, electrochromatic coatings, anti-reflection (AR) coatings. Mirror  30  can be fully or partially reflective or fully or partially transparent, with some amount of reflectivity. 
     Embodiments of the present disclosure provide a measure of distance between the image generator (OLED or other spatial light modulator device) and the face and temples of the viewer. This helps to prevent discomfort due to heat where the wearable display is worn for an extended period of time. 
     The particular arrangement of image-forming components provides suitable image quality and high resolution to allow reading and other visual activity involving fine detail. 
     According to an embodiment of the present disclosure, the optical system described herein is suitable for applications requiring sensitivity to the viewer, including not only viewer comfort, but some level of vision monitoring and adaptation. For example, the apparatus described herein can be used as part of a system for compensating for vision problems. By way of example,  FIGS.  6 C and  7 A- 7 C  show various features of an embodiment useful for compensating for macular degeneration. This type of application can require a measure of viewer monitoring and adaptation, possibly including adjustment of generated data content suitable for the viewer. 
     Gaze Tracking 
     According to an aspect of the present disclosure, gaze tracking can be provided as part of the wearable optics system and used to adjust system parameters according to perceived focus of attention for the viewer. Cameras and infrared (IR) light sources provided on a headset, as shown subsequently, can provide the gaze-tracking function and corresponding angular measurement data. Gaze tracking can be combined with the controller and with a camera image FOV intake. For instance, change of the image aspect ratio for generated image data content may be appropriate, allowing the system to adapt image content to the dimensional parameters available from the image generation system. Thus, for example, cameras associated with the HMD can oversample the real-world input from the object scene, acquiring a wider FOV than can be displayed by system optics. Gaze tracking identifies the actual FOV available to the viewer. The resulting buffered images are related to the reduced FOV video that can be generated, as controlled by using the sector of the FOV identified using eye gaze recognition. 
       FIG.  6 C  is a schematic view that shows an alternate embodiment of the present disclosure that employs the imaging path itself for eye tracking and provides 1:1 imaging of the viewer&#39;s iris. IR light, or other sensing light, is directed along the optical path, such as generated from, through, or at some other point along the output path of, image generator display  10 . Folding surface  26  can be formed as a dichroic surface, treated to direct the sensing light to beam splitter  24  and to curved mirror  30  and to the iris of eye E. Sensor light returning from the iris generally retraces the light path to prism  20 . A portion of this returned light from the viewer&#39;s eye, as shown by a dashed line in  FIG.  6 C , can be transmitted, rather than reflected, through surface  26  and be conveyed through a complementary facing prism  60  to a tracking sensor  62 , which can be a camera, imaging array, or other imaging sensor according to an embodiment. This returned light sensed at sensor  62  can be IR light, for example. The IR light source can be directed to the eye from any suitable position, and can be directed through or alongside components on the optical axis. Surface  26  of the prism is then configured to reflect most of the visible light, performing its turning function for image-bearing light, and to transmit IR light suitably for gaze tracking. 
     Other configurations are possible. Thus, for example, a dichroic coating can be employed for surface  26 , or some other coating can be employed that provides the needed redirection by reflection of the bulk of image-bearing light, while also allowing sufficient light leakage for sensing. 
     The embodiment of  FIG.  6 C  can be modified in a number of ways to allow eye gaze tracking through portions of the imaging path. For example, with corresponding changes to components, the use of IR light as a sensed light can be replaced by employing visible light. This would require surface  26  to be partially reflective in the visible range, such as 90% reflective and 10% transmissive, for example, wherein the transmitted light is the sensed portion. One or more additional lenses, not shown in  FIG.  6 C , could be provided in the path of light to sensor  62  for gaze tracking using the image path as described herein. 
     Headset Configurations 
       FIGS.  7 A- 7 C  show a stereoscopic display headset  100  that has separate optical paths for the left and right eyes of a viewer. As particularly shown in  FIGS.  7 A and  7 C , the optical relay  40  can be compactly packaged as part of a headset  100  within an optical module  42  that is disposed above eye level, as the system is normally worn. Beam splitter  24  and curved mirror  30  are arranged to lie along the visual axis of the viewer. An adjustable strap  102  can be provided to allow adaptation to viewer anatomy. A control logic processor  34  can include the needed electronics for controlling operation of the optical apparatus; processor  34  components can be mounted above the visual axis and disposed away from the viewer&#39;s forehead. Control logic processor  34  can alternately be separated from, but in wired or wireless signal communication with, headset  100 . 
     Eye-tracking can be provided from the headset using the arrangement previously described with respect to  FIG.  6 C  or using one or more eye-tracking cameras  46 , working in conjunction with illumination LEDs  48 , typically infra-red (IR) sources, as shown in  FIG.  7 B . 
       FIG.  8    shows a headset  100  that incorporates the optical system described herein as part of a wearable system for providing user information and guidance in performing a task or assignment. Optical relay  40  and its associated components are packaged within an optical module  42 . A variety of sensors  80 , described in detail subsequently, can include one or more SLAM sensors  82 , as well as sensors for various environmental or ambient conditions such as temperature, humidity, and the like, viewer-related, contextual data, cameras, and other information-gathering devices can be integrated with or in wireless or wired signal communication with control logic and related processing components of the HMD headset. Sensors can be provided to support AR imaging, VR imaging, or mixed AR/VR imaging modes of operation. 
     Connection to power and to signal sources for the headset can be obtained by connection of the headband of headset  100  with external power and signal sources, such as other computing and processing equipment worn or carried by the viewer. 
     System Block Diagram 
       FIG.  8    is a diagrammatic block diagram illustration showing, for an exemplary embodiment, interrelation of integrated components for an HMD system including input sensors  80 , output components and systems  90 , logic and control components including processors, graphic processing units (GPU), MVC and other devices. Sensors  80  can include high-resolution cameras, multiple displays per eye, 6 to 9 degrees of freedom sensor or other sensors necessary for detection of hand-gesturing, head-gesturing, voice control, positional location, and estimation or navigation, as well as optical character recognition (OCR), tracking, marker-based or markerless-based AR, location, SLAM sensors, concurrent odometry and mapping sensors, microphones and noise-cancelling microphones, and any other sensors which could be coupled to and used on an AR/VR headset. Previous figures, for example, gave a diagrammatic illustration of a placement of an IR light for the eye-tracking subsystem. The perspective view of  FIG.  9    shows positions of various sensors, processor, and components coupled to headset  100  according to an embodiment of the present disclosure. 
     Among other sensor technologies which may be housed on the HMD are manual control inputs. These can include digital buttons, which may include power buttons, and a D-Pad or control-pad for accessing and controlling functions by the user, which may or may not be in a dongle; and if not in a dongle then it may exist on the headset or in a wired or wireless remote control. The sensors listed above may include their operating systems and output. The control mechanism may also respond to other types of input, including voice command, SLAM, eye tracking, head or hand gesturing, or any other method which can be employed with the sensors and systems mentioned above. 
     HMD  100  may also house connectors such as power connection for recharging a battery or for direct connection to an AC source, for the HMD as well as for related input and output devices. There can also be additional external connectors for HDMI, sound, and other input/outputs, such as additional image overlay display, or for a diagnostics protocol for upgrading the system. The HMD may also house the microprocessor(s) control circuits. HMD  100  may also contain one or more display per eye, allowing the use of any number of additional projectors, like Pico projectors, or micro-displays. The displays may be used to project though either catoptric system, a dioptric system, or catadioptric system, or combinations thereof, such as to generate an ultra-short-throw image onto reflective lenses or to project to some other surface, which can be clear plastic, like a polycarbonate resin thermoplastic (Lexan). 
     The HMD  100  may also house a rechargeable battery which is not typically removed, thus, providing spare energy to continue to power the HMD when the removable battery is exhausted or removed. While this battery may be smaller and only have a run-time of several minutes, it can provide the HMD with a “hot-swap” battery system that permits a user to keep viewing from the HMD for a time after the removeable battery has died or been removed. 
     HMD  100  may also include a strap and counterweight or other headgear to balance the HMD and maintain its position on the head. The HMD may contain a “pinch adjustor” to adjust strap  102 . In addition, the HMD may or may not include a “dongle” whereby one or more of the systems or subsystems may be connected via wired or wireless to another device, such as could be worn on a belt or carried in a pocket to reduce the overall weight of the HMD  100 . In one embodiment, the HMD may be connected to another device which is providing power, while in an alternative embodiment, the HMD may have its own power from the mains or from wireless power transmission or from a battery. 
     Further, in another embodiment, the HMD may house other subsystems such as the cameras, the microcontrollers, the connectors, central processing unit, graphics processing unit, software, firmware, microphones, speakers, display, and collector lens; the displays, and other subsystems. 
     In another embodiment, the HMD may contain a front facing sensor array along with other sensors mentioned above and optical character recognition (OCR) and/or cameras to read and/or measure information from the real world object scene. Additionally, the HMD may contain one or more connectors to connect via wire to the outside world for power and data (i.e. USB, HDMI, MiniUSB). 
     Alternatively, some parts of the system mentioned herein may be in a dongle attached to the HMD via wire or wireless connection. Alternatively, some portions of the system mentioned herein may be contained in a connected device, like a laptop, smart phone, or Wi-Fi router. Alternatively, some parts of the system mentioned herein may be contained in a smartphone or may be transferred back and forth from a smartphone to the HMD, when synced, such as the HMD displaying the smartphone apps and other features of the smartphone that would otherwise be displayed on the smartphone display. Alternatively, the HMD may contain and display all the features that a smartphone can. 
     In another aspect of the present disclosure, HMD  100  may contain all the features of a typical smartphone and no connection may be needed with a smartphone to have all the smartphone features, like web or cell calling, app use, SMS, MMS, or similar texting, emailing, logging on to the internet, and the like. 
     According to an aspect of the present disclosure, the HMD headset may provide a computer mediated video shown on the reflective lens layer such that the wearer may see both the real world and the augmented video at the same time. In this aspect of the disclosure, such features as voice/speech recognition, gesture recognition, obstacle avoidance, an accelerometer, a magnetometer, gyroscope, GPS, spatial mapping (as used in simultaneous localization and mapping (SLAM)), cellular radio frequencies, Wi-Fi frequencies, Bluetooth and Bluetooth Light connections, infrared cameras, and other light, sound, movement, and temperature sensors may be employed, as well as infrared lighting, and eye-tracking. 
     Batteries and other power connections may be needed for various devices, but are omitted from schematic figures for clarity of other features. 
     SLAM Sensors 
     Embodiments of the disclosure can further include mechanisms and logic that provide SLAM (simultaneous localization and mapping) capabilities to support the viewer. SLAM uses statistical techniques to map the viewer&#39;s environment and to maintain information on the viewer&#39;s relative position within that environment. For example, an image from a Simultaneous Localization and Mapping (SLAM) camera configured for the wearable unit can detect a location of the HMD wearer within the given environment. SLAM capabilities can also be useful where some portion of the viewer&#39;s FOV is blocked or otherwise obscured. Using SLAM allows the system to present portions of the real-world object scene in the electronically generated image. SLAM capability allows generation and display of image content related to the real-world viewer environment. 
     To support SLAM capabilities, a headset as shown in the example of  FIG.  9    can include one or more SLAM sensors  82 , such as cameras. In some cases, the SLAM camera may include a visual spectrum camera, an infrared (IR) camera, or a near-IR (NIR) camera. Additionally, or alternatively, the SLAM camera may be include or exclude the viewer from its field of view. Techniques for providing SLAM capabilities are known to those skilled in the mapping arts and can include processing of local image content as well as use of tracking data. 
     SLAM logic can be provided by control logic processor  34  or by an external processor that is in signal communication with processor  34 , including processors that are connected to the wearable display device by a wired connection or, alternately, processors that are in wireless communication with control processor  34 . 
     IPD Adjustment System 
     Embodiments of the present disclosure address the need for HMD system adjustment to viewer anatomy, with benefits for system efficiency and usability. One aspect of variable viewer anatomy relates to inter-pupil distance (IPD). This well-known characteristic relates to overall head dimensions and position of the eye sockets. Mismatch of IPD by the device can make it difficult to provide image content and needed functions and can make the HMD difficult to wear and use for some viewers. Some systems provide manual methods for IPD adjustment. As shown in the perspective view of  FIG.  10   , the Applicant&#39;s HMD can provide an automated IPD adjustment system  110  that measures the IPD for the viewer and responds by adjusting the inter-pupil spacing, changing the distance between the left and right optical modules  421  and  42   r , respectively. 
     Referring to the  FIG.  10    depiction, IPD adjustment system  110  can have an actuator  112  that urges one or both left and right optical modules  421 ,  42   r  either farther apart, as shown in distance IPD 2 , or closer together as in distance IPD 1 . This adjustment can be performed at power-up or other system activation, at operator command, or at predetermined periods, or at some other time or in response to other event. According to an embodiment of the present disclosure, IPD adjustment settings can be stored for each viewer who has previously used the HMD. Then, upon entry of information identifying the viewer, the stored IPD adjustment settings can be restored as presets. 
     There may even be an external device, motorized or mechanical, which effects the activation of the preset IPD upon instruction prior to wearing the HMD. 
     In one embodiment, to execute the IPD adjustment function, eye tracking cameras  46  on both left and right optical modules  421 ,  42   r  obtain image content that allows pupil center detection by processor  34  logic. This logic determines the relative location of actual pupil centers and IP distance and determines whether or not the IPD between pupil centers is compatible with the positioning of left and right optical modules  421 ,  42   r . If positioning is appropriate, no IPD adjustment is necessary. Otherwise, an actuator  112  can be energized to translate one or both left and right optical modules  421 ,  42   r  in the horizontal or x-direction as shown in  FIG.  10   . At various incremental positions, feedback logic can again measure and calculate any needed adjustment until a suitable IPD is achieved. For IPD adjustment, at least one of the left and right optical modules  421  and  42   r  can be movable; the other module  421  or  42   r  can also be movable or may be stationary. 
     Adjustable Focus 
     As a useful default for most virtual image viewability, HMD optics are typically designed to form the virtual image so that it appears to be at optical infinity, that is, with rays substantially in parallel to the optical axis. In an embodiment, the HMD optics are designed to provide the augmented virtual image at about 1 m distance from the viewer eyebox, equivalent to approximately 1 diopter prescription glasses. 
     In some applications, closer focal length is advantageous. To achieve this, the Applicant&#39;s solution provides measurement and adjustment for diopter adjustment of the optical relay  40  optics. Referring to  FIG.  11 A , there is shown a schematic for relay  40  components that includes an actuator  122  and associated components as part of a focal plane adjustment system  120 . By changing image generator  10  position along an axis A, a change in focal position is effected, such as with image generator  10  shifted to the dashed outline position denoted for image generator  10 ′ in  FIG.  11 A . This movement causes a corresponding shift of intermediate image I to the position shown as image I′ in relay  40 . The splitter  24  and combiner, curved mirror  30 , then condition the image-bearing light to provide a virtual image at a shifted spatial location. Actuator  122  can be a linear piezoelectric actuator, for example, capable of high-speed change between positions. One or more actuators  122  can be used for moving any of the components described hereinabove with relation to optical relay  40  in order to adjust the position of the focal plane. 
       FIGS.  11 B,  11 C, and  11 D  show different views of relay optics with focal plane adjustment system  120 .  FIG.  11 B  is a perspective view that shows position of actuator  122  relative to image generator  10  and corresponding optics.  FIG.  11 C  is a perspective view from behind image generator  10 , showing a piezoelectric actuator  122  mounted to a stationary plate  124  behind image generator  10  (not visible in the view of  FIG.  11 C ). Actuator  122 , such as a piezoelectric actuator, translates image generator  10  along axis A, which extends orthogonally to the electroluminescent display component surface of image generator  10 .  FIG.  11 D  shows a side view of adjustment system  120  components. One or more stabilizing posts  126  has a screw and a compression spring  162  for maintaining image generator  10  in position along the optical path, so that movement of image generator  10  is constrained to the axial direction (axis A) during actuation. 
     One difficulty with the change in focal length relates to vergence-accommodation conflict, as shown in the schematic diagram of  FIG.  12   . Vergence-accommodation conflict (VAC) is a vision phenomenon that is familiar to developers of three-dimensional (3D) displays and virtual-reality (VR) display devices. Under real-world normal viewing conditions, where only the object scene is in view as shown in part (a) of  FIG.  12   , the viewer&#39;s eyes converge, rotating toward one another to focus on closer objects and, correspondingly, away from one another to focus on objects at further distance. Accommodation, as the process where the lenses of the eyes focus on a close or far away object, is consistent with convergence for normal viewing in the part (a) depiction and involves epipolar geometry of the HMD cameras and the wearer&#39;s eyes. Thus, accommodation and convergence can be considered to be coupled. 
     Focal plane adjustment system  120  ( FIGS.  11 A-D ) also employs eye tracking sensors and related data for determining when there is a discrepancy between focal planes for the object world FOV and the generated image content. The focal plane position for the generated image can be computed according to system optical geometry for the components defined hereinabove. Later adjustments to the geometry, executed by system logic for shifting focal plane position, can be recorded and used to recalculate focal plane position. 
     In  FIG.  12    part (a), the viewer sees a real object, i.e., the viewer&#39;s eyes are verged on the real object and gaze lines from the viewer&#39;s eyes intersect at the real object. As the real object moves nearer the user, as indicated by the arrow, each eye rotates inward (i.e., converges) to stay verged on the real object. As the real object gets closer, the eye “accommodates” to the closer distance, through reflexive, automatic muscle movements, by adjusting the eye&#39;s lens to reduce the focal length. In this way, accommodation adjustment is achieved. Thus, under normal viewing conditions in the real world, the vergence distance (d v ) equals the accommodation distance (d f ). 
     In 3D displays and VR systems, however, these two processes can be decoupled, as shown in part (b) of  FIG.  12   .  FIG.  12    shows an example conflict between vergence and accommodation that can occur with stereoscopic three-dimensional displays, in accordance with one or more embodiments. In this example, an observer is looking at the virtual object displayed on a 3D electronic display; however, the observer&#39;s eyes are verged on and gaze lines mapped from the observer&#39;s eyes intersect at the virtual object, which is at a greater distance from the observer&#39;s eyes than the image formed by the 3D electronic display. As the virtual object from the 3D electronic display is rendered to appear closer to the viewer, each eye again rotates inward to remain verged on the virtual object, but the focus distance of the image is not reduced; hence, the observer&#39;s eyes do not accommodate, as in part (a) Thus, instead of increasing the optical power to accommodate for the closer vergence depth, the eye maintains accommodation at a display distance associated with the 3D electronic display. Thus, the vergence depth (dv) often does not equal the focal length (df) for the human eye for objects displayed from 3D electronic displays. This discrepancy between vergence depth and focal length is referred to as “vergence-accommodation conflict” or VAC. A user experiencing only vergence or only accommodation, and not both vergence and accommodation, can eventually experience some degree of fatigue, dizziness, discomfort, disorientation, and even nausea in some cases. 
     In order to compensate and correct VAC, the  FIGS.  11 A-D  relay optics can adjust the relative position of intermediate image I to I′, moving the virtual image focal plane more closely toward the real-world focal plane. 
     Model Controller 
     Embodiments in accordance with the present disclosure may be provided as an apparatus, method, computer program, hardware/software, state machine, firmware, and/or product. All of the systems and subsystems may exist, or portions of the systems and subsystems may exist to form the apparatus described in the present disclosure. Accordingly, one or more portions of the Applicant&#39;s solution may take the form of an entirely or partial hardware embodiment, a predominantly software embodiment (including firmware, resident software, micro-code, etc), or an embodiment combining software and hardware aspects in some combination that may all generally be referred to herein, without limitation, as a “unit,” “module,” or “system.” Furthermore, one or more portions or functions for the present disclosure may take the form of a computer program product or products embodied in any tangible media of expression or storage having computer-usable program code embodied in or otherwise represented using the media. Any combination of one or more computer-usable or computer-readable media (or medium) may be utilized, including networked combinations that utilize remote processing components. For example, a random-access memory (RAM) device, a read-only memory (ROM) device, an erasable programmable read-only memory (EPROM or Flash memory) device, a portable compact disc read-only memory (CDROM), an optical storage device, and a magnetic storage device. Computer program code for carrying out operations of the present disclosure may be written in any combination of one or more programming languages. Further, the intelligence in the main circuitry may be software, firmware, or hardware, and can be microcontroller based or included in a state machine. The disclosure may be a combination of the above intelligence and memory and this can exist in a central processing unit or a multiple of chips including a central graphics chip. The computer portion of the disclosure may also include a model view controller (MVC) as shown in  FIG.  8   , which is also called herein a “model controller.” 
     Dithering 
     According to an embodiment of the present disclosure, dithering can be employed to modify and improve the visual experience of the viewer. Dithering can be effected, for example, by rapid in-plane vibration of a camera or image generator  10  using a piezoelectric actuator  122 , as was described previously with respect to  FIGS.  11 A-D . Dithering, imparted to the displayed image content using synchronous, timed spatial displacement, can be a desirable solution for helping to mask or eliminate display-related artifacts, for example. 
     Dithering can also be used to enhance image resolution using Timed Spatial Displacement. Improved image resolution is goal, and holds promise for future use of AR/VR glasses in various applications, such as in critical use cases such as surgery visualization. In these applications, for example, detail visualization of fine layers and blood vessels can be critical to successful outcomes. Micro displays continue to mature, with pixel counts of 2 million in a single small device, with further improvements likely. These higher resolution displays impose steeply increased demands on system resources, including higher power and computation speed and complexity, for example. 
     The Applicant addresses this problem using dithering of display components. This solution can make higher resolution images available to users at a discounted power cost, which in turn can provide lighter, cooler running systems. 
     Increased pixel resolution is obtained by using the capability to shift image generator  10  in-plane, that is, in one or more (x-y plane) directions parallel to the emissive display surface of image generator  10 , synchronously with corresponding changes in image data content. With respect to  FIG.  11 D , image generator  10  translation is in the x-y plane, orthogonal to axis A (conventionally considered the z-axis) for dithering to increase pixel resolution using synchronous, timed spatial displacement. Actuator  122  is configured to provide dithering using in-plane translation of image generator  10 . 
     As shown schematically in  FIG.  13   , multiplication of the image generator  10  resolution is accomplished by physically shifting an array  13  of pixels that form image generator display  10  in the x-y plane. The shift distance is a proper fraction of the pixel pitch. At right in  FIG.  13    is represented a pixel  130  of array  13 , shifted in the x-direction to pixel positions  134  and  138  and shifted in the y-direction to positions  132  and  136 . Synchronous with the shift action is modulation of image data for each pixel  130  at its original position and at each shifted position  132 ,  134 ,  136 , and  138 . Thus, for example, the state of a pixel at its position  134  (e.g., brightness or color) can differ from its state at position  138 , according to the image data content that is provided with the shift. With a half-pixel shift in each x- and y-direction, for example, the effective pixel count can be increased by at least 4 times. With a half-pixel shift only along one axis, such as only along the x or y axis as shown, the effective resolution along the axis parallel to the shift can be doubled. Overall, the power cost of nano scale piezo shifting is much lower than the cost to design and implement  4   x  the pixel count using a higher resolution image generator  10  element. An array  14  represents increased pixel resolution. 
     For the embodiment of  FIG.  13   , an image generator  10  was provided, having a 240 frames per second (fps) refresh rate. In terms of the piezoelectric actuation provided, each pixel element can be relocated at ½ the delta of the pixel element center-to-center distance in the array  13 . This arrangement can provide 60 fps display at 4× the resolution of the original image generator  10 . 
     By way of example, an embodiment of the present disclosure employs QNP-XY Series Two-Axis, CY Piezo Nanopositioners for image generator  10  dithering actuation. 
     A number of features allow piezoelectric dithering to provide enhanced resolution, including the following: (1) Adjustable travel range, such as from 100 um to 600 um, for example; (2) Long device lifetimes; (3) Superior positioning resolution; and (4) High stiffness and other factors. 
     A number of piezoelectric actuators provide the option of closed-loop feedback that allows sub-nanometer resolution and high linearity. 
     Dithering for increased resolution can utilize any of a number of movement patterns for in-plane displacement of pixels. Patterns for dithering the pixels in the display (or the displays) for increased resolution include transposing with rectilinear motion, curvilinear motion, or in a translational, rotational, periodic, or non-periodic motion, or any combination of the above. Also, the pattern could include a rectangle pattern which may increase the resolution by 4 times. Another alternative way to address the pixel movement is to have pixels that are approximately the same size as the non-emissive dark or “black” space between the pixels where the dithering translation of each pixel in a display is dithered to the next adjacent unused space existing between the pixels in the display. 
     For viewer comfort, a strap adjustment can be provided, allowing both a one-time fastener positioning adjustment and a flexible stretch band. 
     The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the disclosure. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by any appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.