Patent Publication Number: US-9851565-B1

Title: Increasing effective eyebox size of an HMD

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/426,439, filed Mar. 21, 2012, the contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to the field of optics, and in particular but not exclusively, relates to head mounted displays. 
     BACKGROUND INFORMATION 
     A head mounted display (“HMD”) is a display device worn on or about the head. HMDs usually incorporate some sort of near-to-eye optical system to emit a light image within a few centimeters of the human eye. Single eye displays are referred to as monocular HMDs while dual eye displays are referred to as binocular HMDs. Some HMDs display only a computer generated image (“CGI”), while other types of HMDs are capable of superimposing CGI over a real-world view. This latter type of HMD can serve as the hardware platform for realizing augmented reality. With augmented reality the viewer&#39;s image of the world is augmented with an overlaying CGI, also referred to as a heads-up display (“HUD”), since the user can view the CGI without taking their eyes off their forward view of the world. 
     HMDs have numerous practical and leisure applications. Aerospace applications permit a pilot to see vital flight control information without taking their eye off the flight path. Public safety applications include tactical displays of maps and thermal imaging. Other application fields include video games, transportation, and telecommunications. There is certain to be new found practical and leisure applications as the technology evolves; however, many of these applications are limited due to the cost, size, weight, limited field of view, small eyebox, or poor efficiency of conventional optical systems used to implemented existing HMDs. In particular, HMDs that provide only a small eyebox can substantially detract from the user experience, since the CGI image can be impaired, or even disappear from the user&#39;s vision, with a slight bump of the HMD or from eye motions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles being described. 
         FIG. 1  is a functional block diagram illustrating components of a head mounted display (“HMD”) used to extend an effective size of an eyebox of the HMD using eye tracking data, in accordance with an embodiment of the disclosure. 
         FIG. 2  is a flow chart illustrating a process for extending the effective size of an eyebox of a HMD using eye tracking data, in accordance with an embodiment of the disclosure. 
         FIG. 3  is a block diagram illustrating an example HMD capable of extending an effective size of an eyebox using eye tracking data, in accordance with an embodiment of the disclosure. 
         FIGS. 4A-4C  illustrate how the effective size of an eyebox can be extended using eye tracking data, in accordance with an embodiment of the disclosure. 
         FIGS. 5A-C  are diagrams illustrating an example eyebox actuator for extending an effective size of an eyebox using a liquid lens, in accordance with an embodiment of the disclosure. 
         FIG. 6  is a diagram illustrating an example eyebox actuator for extending an effective size of an eyebox using a gradient refractive index (“GRIN”) lens, in accordance with an embodiment of the disclosure. 
         FIG. 7  is a diagram illustrating an example eyebox actuator for extending an effective size of an eyebox using a sliding end reflector, in accordance with an embodiment of the disclosure. 
         FIG. 8  is a diagram illustrating an example eyebox actuator for extending an effective size of an eyebox using a split sliding out-coupling beam splitter, in accordance with an embodiment of the disclosure. 
         FIG. 9  is a diagram illustrating an example eyebox actuator for extending an effective size of an eyebox using a pivoting out-coupling beam splitter, in accordance with an embodiment of the disclosure. 
         FIGS. 10A-C  illustrate an example eyebox actuator for extending an effective size of an eyebox using a sliding display panel, in accordance with an embodiment of the disclosure. 
         FIGS. 11A-B  illustrate an example eyebox actuator for extending an effective size of an eyebox using a display panel that translates a reduced size image, in accordance with an embodiment of the disclosure. 
         FIG. 12  illustrates an HMD with integrated near-to-eye display and eye tracking systems, in accordance with an embodiment of the disclosure. 
         FIG. 13  illustrates an example binocular HMD capable of extending effective sizes of eyeboxes through which CGI is delivered to the eyes using eye tracking data, in accordance with an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of a system, apparatus, and method for extending the effective size of an eyebox of a head mounted display (“HMD”) using eye tracking data are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
       FIG. 1  is a functional block diagram illustrating components of a head mounted display (“HMD”) system  100  used to extend an effective size of an eyebox of the HMD using eye tracking data, in accordance with an embodiment of the disclosure. The illustrated HMD system  100  includes an image source  105 , light bending optics  110 , an eyebox actuator  115 , an eye tracking camera system  120 , and an eyebox controller  125 . 
     HMD system  100  may be worn on the head of a user to provide a computer generated image (“CGI”)  130  (also referred to generically as display light since the light need not be computer generated) to the user&#39;s eye  135 . In one embodiment, HMD system  100  is a sort of wearable computing device capable of providing a near-to-eye display to the user. The CGI may be provided to eye  135  as a virtual reality (“VR”) display, or overlaid over the user&#39;s external scene light to augment the user&#39;s regular vision. In the later case, HMD system  100  may be referred to an augmented reality (“AR”) display. 
     CGI  130  is displayed to the user&#39;s eye  135  within an eyebox  140 . Eyebox  140  is a two-dimensional (“2D”) box in front of eye  135  from which CGI  130  can be viewed. If eye  140  moves outside of eyebox  140 , then CGI  140  misses eye  135  and cannot be seen by the user. If eyebox  140  is too small, then minor misalignments between eye  135  and an emission surface of light bending optics  110  from which CGI  130  is emitted can result in loss of the image. Thus, the user experience is substantially improved by increasing the size of eyebox  140 . In general, the lateral extent of eyebox  140  is more critical than the vertical extent of eyebox  140 . This is in part due to the significant variances in eye separation distance between humans, misalignments to eyewear tend to more frequently occur in the lateral dimension, and humans tend to more frequently adjust their gaze left and right, and with greater amplitude, than up and down. Thus, techniques that can increase the effective size of the lateral dimension of eyebox  140  can substantially improve a user&#39;s experience with a HMD. 
       FIG. 2  is a flow chart illustrating a process  200  for extending the effective size of eyebox  140  using eye tracking data, in accordance with an embodiment of the disclosure. HMD system  100  is capable of increasing or extending the effective lateral dimension of eyebox  140 . In a process block  205 , eye tracking camera system  120  obtains an eye image of eye  135  in real-time while eye  135  is viewing CGI  130 . The eye image is analyzed by eyebox controller  125  to determine a location or gazing direction of eye  135  (process block  210 ). The determined location is then used to adjust the position CGI  130  displayed to eye  130  (process block  215 ). In the illustrated embodiment, eyebox controller  125  is coupled to eyebox actuator  115 , which manipulates one or more components of light bending optics  110  and/or image source  105  to steer the CGI  130  to follow the motion of eye  135  thereby extending the effective size of eyebox  140 . For example, if eye  135  looks left, eyebox actuator  115  can be made to mechanically and/or electrically manipulate the position or emission angle of CGI  130  such that the displayed CGI  130  also moves left to track eye  135 . 
     Thus, eye tracking camera system  120  and eyebox controller  125  continuously monitor the position of eye  135  while it views CGI  130  (process block  220 ). If eye  135  moves (decision block  225 ), eyebox actuator  115  adjusts the image source  105  and/or light bending optics  110  to reposition CGI  130  (process block  230 ). Since eyebox  140  can move left or right to track eye movements, its lateral extent is increased, thereby extending the effective size of eyebox  140 . 
     Eyebox controller  125  may be implemented in hardware (e.g., application specific integrated circuit, field programmable gate array, etc.) or be implemented as software or firmware executing on a microcontroller. Eye tracking camera system  120  includes a camera that is positioned to obtain real-time images of eye  135 . This may be achieved by physically mounting an image sensor in directly line of sight of eye  135  (e.g., see  FIG. 3 ), such as mounted to eyeglass frames and pointed eye-ward to face the user&#39;s eye  135 . Alternatively, light bending optics may be used to deliver an eye image to a peripherally located camera sensor (e.g., see  FIG. 12 ). 
       FIG. 3  is a block diagram illustrating an example HMD system  300  capable of extending an effective size of eyebox  140  using eye tracking data, in accordance with an embodiment of the disclosure. HMD system  300  is one possible implementation of HMD system  100 . The illustrated embodiment of HMD system  300  includes an image source  305 , light bending optics  310 , an eye tracking camera system  315 , eyebox controller  125 , and one or more eyebox actuators (discussed in connection with  FIGS. 5-11 ). The illustrated embodiment of image source  305  includes a lamp source  320 , an in-coupling beam splitter  325 , a display panel  330 , and a half-wave plate polarization rotator  335 . The illustrated embodiment of light bending optics  310  includes a light relay  340 , an out-coupling beam splitter  345 , a quarter-wave plate polarization rotator  347 , and an end reflector  350 . The illustrated embodiment of eye tracking camera system  315  includes a camera  355  and a camera controller  360 . 
     During operation, lamp source  320  generates lamp light used to illuminate display panel  330 , which modulates image data onto the lamp light to create CGI  130 . The lamp light generated by lamp source  320  is output into in-coupling beam splitter  325 . In one embodiment, in-coupling beam splitter  325  is a polarizing beam splitter (“PBS”) cube that substantially passes light of a first polarization (e.g., P polarization), while substantially reflecting light of a second polarization (e.g., S polarization). These two polarization components are typically orthogonal linear polarizations. The emitted light may be pre-polarized (e.g., P polarized) or unpolarized light. In either event, the P polarization component passes through the PBS cube to illuminate display panel  330  while most of any remaining portions of S polarization are back reflected. Thus, in the illustrated embodiment, display panel  330  is mounted in opposition to lamp source  305  with in-coupling beam splitter  325  disposed in between. 
     Display panel  330  (e.g., liquid crystal on silicon panel, digital micro-mirror display, etc.) imparts image data onto the lamp light via selective reflection by an array of reflective pixels. In an embodiment using an LCOS panel, reflection by display panel  330  rotates the polarization of the incident lamp light by 90 degrees. Upon reflection of the incident lamp light, the CGI light (which has been rotated in polarization by 90 degrees to be, for example, S polarized) is re-directed by in-coupling beam splitter  325  and propagates down light relay  340  along a forward propagation path towards end reflector  350 . In an embodiment using a digital micro-mirror display, in-coupling beam splitter  325  may be implemented as a standard 50/50 non-polarizing beam splitter and the CGI light may be non-polarized light. 
     In one embodiment, CGI  130  is directed along the forward propagation path within light relay  340  without need of total internal reflection (“TIR”). In other words, the cross sectional shape and divergence of the light cone formed by CGI  130  is confined such that the light rays reach end reflector  350  without need of TIR off the sides of light relay  340 . 
     In-coupling beam splitter  325 , light relay  340 , and out-coupling beam splitter  345  may be fabricated of a number of materials including glass, optical grade plastic, fused silica, PMMA, Zeonex-E48R, or otherwise. The length of light relay  340  may be selected based upon the temple-eye separation of the average adult and such that the focal plane of end reflector  350  substantially coincides with an emission aperture of display panel  330 . To achieve focal plane alignment with the emission aperture of display panel  330 , both the length of light relay  340  and the radius of curvature of end reflector  350  may be selected in connection with each other. 
     In the illustrated embodiment, light relay  340  includes half-wave plate polarization rotator  335  disposed within the forward propagation path of CGI  130 . Half-wave plate polarization rotator  335  servers to rotate the polarization of CGI  130  by 90 degrees (e.g., convert the S polarized light back to P polarized light again). 
     The illustrated embodiment includes a partially reflective surface formed within out-coupling beam splitter  345 . In one embodiment, out-coupling beam splitter  345  is partially transparent, which permits external (ambient) scene light  346  to pass through an external scene side and eye-ward side of light bending optics  310  to reach eye  135 . A partially transparent embodiment facilitates an augmented reality (“AR”) where CGI  130  is superimposed over external scene light  346  to the user eye  135 . In another embodiment, light bending optics  310  is substantially opaque (or even selectively opaque) to external scene light  346 , which facilitates a virtual reality (“VR”) that immerses the user in the virtual environment displayed by CGI  130 . 
     In one embodiment, out-coupling beam splitter  345  is an out-coupling PBS cube configured to pass one linear polarization (e.g., P polarization), while reflecting the other linear polarization (e.g., S polarization). Thus, the out-coupling PBS passes CGI  130  propagating along the forward propagation path to quarter wave-plate polarization rotator  347 . After passing through quarter-wave plate polarization rotator  347 , CGI  130  is reflected back along a reverse propagation path back towards out-coupling beam splitter  345 . Thus, CGI  130  is rotated a total of 90 degree in polarization during its double pass through quarter-wave plate polarization rotator  347  and is S polarized by the time it strikes the out-coupling beam splitter  345  on the reverse propagation path. 
     In one embodiment, end reflector  350 , both reflects and collimates CGI  130  such that CGI  130  traveling along the reverse propagation path is substantially collimated. Collimating (or nearly collimating) CGI  130  helps eye  135  to focus on CGI  130  emitted out the emission surface on the eye-ward side of light bending optics  310  in a near-to-eye configuration (e.g., emission surface placed within 10 cm of eye  135  and typically less than 5 cm of eye  130 ). CGI  130  is directed towards eye  135  due to the oblique orientation of the out-coupling beam splitter  345 . In other embodiments, end reflector  350  merely reduces the divergence of CGI  130  without fully collimating CGI  130 . In embodiments where one or both of in-coupling beam splitter  325  and out-coupling beam-splitter  345  are regular non-polarizing beam splitters, half-wave plate polarization rotator  335  and/or quarter-wave plate polarization rotator  347  may be omitted. 
     In some embodiments, end reflector  350  is an adjustable end reflector with eyebox actuator  115  incorporated into the adjustable end reflector to adjust a position or other optical properties of the adjustable end reflector to steer emitted CGI  130  (e.g.,  FIG. 5, 6 , or  7 ). In these embodiments, eyebox controller  125  outputs a control signal CTRL 1  for manipulating eyebox actuator  115  within the adjustable end reflector. In some embodiments, out-coupling beam splitter  345  is adjustable with eyebox actuator  115  to adjust a slidable position or angle of out-coupling beam splitter to steer emitted CGI  130  (e.g.,  FIG. 8 or 9 ). In these embodiments, eyebox controller  125  outputs a control signal CTRL 2  for manipulating out-coupling beam splitter  345 . In some embodiments, display panel  330  is adjustable to translate CGI output from display panel  330  (e.g., see  FIGS. 10 and 11 ). In these embodiments, eyebox controller  125  outputs a control signal CTRL 3  for manipulating the CGI output from display panel  330 . 
       FIGS. 4A-4C  illustrate how the effective size of eyebox  140  can be extended using eye tracking data, in accordance with an embodiment of the disclosure. Referring to  FIG. 4A , eyebox  140  is typically approximately determined by the projection of out-coupling beam splitter  345  onto emission surface  405 . However, the effective size of eyebox  140  may be somewhat smaller than this projection, since the light is not uniformly bright across its lateral extents (x-dimension). Rather, the light emitted in the middle (illustrated by a solid line on eyebox  140 ) may in fact represent the effective size of the eyebox  140 . When eye  135  is looking straight forward, the effective size of eyebox  140  may be sufficient. However, when eye  135  changes its gaze either left  FIG. 4B ) or right ( FIG. 4C ), then the pupil may enter the reduced brightness peripheral region of the emission surface  405 . Thus, embodiments disclosed herein are capable of steering or diverting the CGI  130  towards the peripheral regions in response to a determination of the current position of eye  135 . If eye  135  is gazing left ( FIG. 4B ), then eyebox actuator  115  can bias CGI  135  exiting emission surface  405  towards the left to increase the brightness and image contrast on the left side of eyebox  140 . If eye  135  is gazing right ( FIG. 4C ), then eyebox actuator  115  can bias CGI  135  exiting emission surface  405  towards the right to increase the brightness and image contrast on the right side of eyebox  140 . This dynamic adjustment has the effect of extending the effective or useful size of eyebox  140 . 
       FIGS. 5A-C  are diagrams illustrating an adjustable end reflector  500  for extending an effective size of an eyebox using a liquid lens, in accordance with an embodiment of the disclosure. Adjustable end reflector  500  is one possible implementation of end reflector  350 . The illustrated embodiment of adjustable end reflector  500  includes liquid lens  505  and end reflector  510 . Liquid lens  505  is an enclosure of two non-mixing liquids (e.g., oil and water). The surface tension at the interface between the two non-mixing liquids forms a lens shape. In this embodiment, eyebox actuator  115  may be implemented as a voltage controller coupled to apply one or more control voltages CTRL 1  across liquid lens  505 . The applied control voltages have the effect of adjusting the shape of the liquid lens using electrostatic forces. As illustrated in  FIGS. 5B and 5C , the control voltages CTRL 1  can control the shape such that the CGI traveling along reverse propagation path  515  after reflecting off end reflector  510  is selectively angled relative to the CGI traveling along the forward propagation path  520 . 
       FIG. 6  is a diagram illustrating an adjustable end reflector  600  for extending an effective size of an eyebox using a gradient refractive index (“GRIN”) lens, in accordance with an embodiment of the disclosure. Adjustable end reflector  600  is one possible implementation of end reflector  350 . The illustrated embodiment of adjustable end reflector  600  includes GRIN lens  605  and end reflector  510 . GRIN lens  605  is an enclosure of liquid crystal material. When a gradient voltage potential is applied across GRIN lens  605 , an index of refraction gradient is formed within the liquid crystal material. This refraction gradient can be controlled to form different effective lens shapes. In this embodiment, eyebox actuator  115  may be implemented as a voltage controller coupled to apply one or more control voltages CTRL 1  across GRIN lens  605 . The applied control voltages have the effective of adjusting the lensing shape of GRIN lens  605  using electrostatic forces. In this manner, the control voltages CTRL 1  can selectively control the angle of the CGI traveling along reverse propagation path  615  after reflecting off end reflector  610  relative to the CGI traveling along the forward propagation path  620 . 
       FIG. 7  is a diagram illustrating an adjustable end reflector  700  for extending an effective size of an eyebox using a sliding end reflector, in accordance with an embodiment of the disclosure. Adjustable end reflector  700  is one possible implementation of end reflector  350 . The illustrated embodiment of adjustable end reflector  700  includes one or more eyebox actuator(s)  705  and end reflector  710 . Eyebox actuator(s)  705  operate to slide end reflector  710  back and forth in response to control signal(s) CTRL 1 . End reflector  710  may be implemented as a concave lensing reflector that is slidably mounted to the distal end of light relay  340 . Eyebox actuator  705  may be implemented as piezoelectric crystals that expand and contract in response to control signal(s) CTRL 1 , a micro-electro-mechanical-systems (“MEMS”) actuator responsive to control signal(s) CTRL 1 , an electrostatic actuator, or otherwise. By adjusting the position of end reflector  710 , the forward propagating CGI  715  strikes a different part of end reflector  710 , causing the backward propagating CGI  720  to be selectively reflected at different angles relative to the forward propagating CGI  715 . In this manner, the control signal(s) CTRL 1  can selectively control the angle of the CGI traveling along reverse propagation path  720  after reflecting off end reflector  710 . 
       FIG. 8  is a diagram illustrating an example eyebox actuator for extending an effective size of an eyebox using a split sliding out-coupling beam splitter  800 , in accordance with an embodiment of the disclosure. Out-coupling beam splitter  800  is one possible implementation of out-coupling beam splitter  345 . Out-coupling beam splitter  800  includes two prism portions  805  and  810  capable of sliding along the oblique partially reflective surface. In one embodiment, portion  810  is held fixed while portion  805 , upon which the partially reflective surface is disposed, slides. Portion  805  is moved by eyebox actuator  815 . Eyebox actuator  815  may be implemented with a piezo-electric crystal or a MEMS actuator coupled to be responsive to control signal CTRL 2 . Eyebox actuator  815  can be made to expand or contract, thereby pushing or pulling the movable portion  805  along the oblique axis of the out-coupling beam splitter  800 . By moving the partially reflective surface, CGI  130  can be dynamically extended at the peripheries. 
       FIG. 9  is a diagram illustrating an example eyebox actuator for extending an effective size of an eyebox using a rotating out-coupling beam splitter  900 , in accordance with an embodiment of the disclosure. Out-coupling beam splitter  900  is one possible implementation of out-coupling beam splitter  345 . Out-coupling beam splitter  900  is mounted to light relay  340  at pivot joints  905  (only one is illustrated). Eyebox actuator  910  is coupled to out-coupling beam splitter  900  to rotate it, thereby selectively changing the emission angle of CGI  130  in response to control signal CTRL 2 . Eyebox actuator  910  may be implemented with a piezo-electric crystal or a MEMS actuator coupled to be responsive to control signal CTRL 2 . Eyebox actuator  815  can be made to expand or contract, thereby pushing or pulling on out-coupling beam splitter  900  causing a moment about pivot joints  905 . 
       FIGS. 10A-C  illustrate an example eyebox actuator for extending an effective size of an eyebox using a sliding display panel  1000 , in accordance with an embodiment of the disclosure. Display panel  1000  is mounted to be illuminated by lamp source  320  via in-coupling beam splitter  325 . Display panel  1000  may be secured in place using a sliding mount and coupled to eyebox actuator  1005 . Display panel  1000  is moved by eyebox actuator  1005  in response to control signal CTRL 3 . Eyebox actuator  1005  may be implemented with with a piezo-electric crystal or a MEMS actuator coupled to be responsive to control signal CTRL 2 . Eyebox actuator  1005  can be made to expand or contract, thereby pushing or pulling the display panel  1000  along its sliding axis. By sliding display panel  1000 , CGI  130  can be dynamically extended at the peripheries. For example, display panel  1000  may include a pixelated reflective center portion  1010  with blackout non-reflective portions  1015  on either side (e.g., see  FIG. 10B ). By moving display panel  1000  to the right, the peripheral region on the right side of CGI  130  is extended. 
       FIGS. 11A-B  illustrate an example eyebox actuator for extending an effective size of an eyebox using a display panel  1100  that translates a reduced size image  1105 , in accordance with an embodiment of the disclosure. Display panel  1100  is mounted to be illuminated by lamp source  320  via in-coupling beam splitter  325 . However, display panel  1100  is mounted in a fixed location and does not slide. Rather, display panel  1100  displays a reduced size image  1105  relative to the actual size of display panel  1100 . In this embodiment, the eyebox actuator is the video controller, which translates reduced size image  1105  electronically within display panel  1100 . The lamp light emitted from lamp source  320  illuminates the entire display panel  1100 ; however, only a subset of pixel corresponding to reduced side image  1105  are activated to modulate the CGI onto the reflected light. By way of example, by moving reduced side image  1105  to the left (see  FIG. 11B ), the peripheral region on the left side of CGI  130  is extended. 
       FIG. 12  illustrates an HMD system  1200  with integrated near-to-eye display and eye tracking systems, in accordance with an embodiment of the disclosure. HMD system  1200  is similar to HMD system  300 , except that the eye tracking system is integrated to use the same internal optics as the image display system. In particular, HMD system  1200  includes tracking camera  1205  and infrared (“IR”) emitters  1210 . IR emitters  1210  may be configured to emit non-polarized or S-polarized light  1215  to illuminate eye  335 . IR light  1215  is reflected off of eye  335  as eye image  1220  back into out-coupling beam splitter  345 . From there, eye image  1220  reverse traces the path taken by CGI  130  back to in-coupling beam splitter  325 . When eye image  1220  reaches in-coupling beam splitter  325  it is p-polarized due to polarization rotators  335  and  347 . Thus eye image  1220  passes through in-coupling beam splitter  325  and is impingent upon tracking camera  1205 . Tracking camera  1205  captures eye image  1220  and generates eye tracking data, which is provided to eyebox controller  125  for analysis, as discussed above. The eye tracking data may be the eye image itself or preprocessed data. In one embodiment, an IR cut filter  1225  is disposed on the external scene side over the view region of light relay  340 . IR cut filter  1225  block external IR light from interfering with the operation of the eye tracking system. The IR emitters  1210  may be positioned in various other locations than illustrated and may include only a single emitter or multiple emitters that emit non-polarized IR light, polarized IR light, or circularly polarized light. 
       FIG. 13  is a top view of a binocular HMD system  1300  using a pair of HMDs  1301 , in accordance with an embodiment of the disclosure. Each HMD  1301  may be implemented with embodiments of HMD systems  100 ,  300 , or  1200 , or combination thereof. Furthermore, each HMD  1301  may be implemented with an eyebox actuator and adjustable optics as disclosed in connection with any of  FIGS. 5-11 . The HMDs  1301  are mounted to a frame assembly, which includes a nose bridge  1305 , left ear arm  1310 , and right ear arm  1315 . Although  FIG. 13  illustrates a binocular embodiment; however, a single HMD  1301  may also be mounted to a frame for use as a monocular HMD with only a single eyepiece. 
     The two HMDs  1301  are secured into an eyeglass arrangement that can be worn on the head of a user. The left and right ear arms  1310  and  1315  rest over the user&#39;s ears while nose assembly  1305  rests over the user&#39;s nose. The frame assembly is shaped and sized to position an emission surface of the eyepiece in front of a corresponding eye  135  of the user. Of course, other frame assemblies having other shapes may be used (e.g., a visor with ear arms and a nose bridge support, a single contiguous headset member, a headband, goggles type eyewear, etc.). 
     The illustrated embodiment of binocular HMD system  1300  is capable of displaying an augmented reality to the user. The viewing region of each HMD  1301  is partially transparent and permits the user to see a real world image via external scene light. Left and right (binocular embodiment) CGI light may be generated by one or two CGI engines (not illustrated) coupled to a respective image source of each HMD  1301 . The CGI light is seen by the user as virtual images superimposed over the real world as an augmented reality. In some embodiments, external scene light may be blocked or selectively blocked to provide a head mounted virtual reality display or heads up display. 
     The processes explained above are described in terms of computer software and hardware. The techniques described may constitute machine-executable instructions embodied within a tangible or non-transitory machine (e.g., computer) readable storage medium, that when executed by a machine will cause the machine to perform the operations described. Additionally, the processes may be embodied within hardware, such as an application specific integrated circuit (“ASIC”) or otherwise. 
     A tangible machine-readable storage medium includes any mechanism that provides (i.e., stores) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-readable storage medium includes recordable/non-recordable media (e.g., read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc.). 
     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.