Patent Publication Number: US-2018035101-A1

Title: See-through computer display systems

Description:
BACKGROUND 
     Field of the Invention 
     This invention relates to see-through computer display systems with stereo scene cameras. 
     Description of Related Art 
     Head mounted displays (HMDs) and particularly HMDs that provide a see-through view of the environment are valuable instruments. The presentation of content in the see-through display can be a complicated operation when attempting to ensure that the user experience is optimized. Improved systems and methods for presenting content in the see-through display are required to improve the user experience. 
     SUMMARY 
     Aspects of the present invention relate to methods and systems for the see-through computer display systems with stereo scene cameras. 
     These and other systems, methods, objects, features, and advantages of the present invention will be apparent to those skilled in the art from the following detailed description of the preferred embodiment and the drawings. All documents mentioned herein are hereby incorporated in their entirety by reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are described with reference to the following Figures. The same numbers may be used throughout to reference like features and components that are shown in the Figures: 
         FIG. 1  illustrates a head worn computing system in accordance with the principles of the present invention. 
         FIG. 2  illustrates a head worn computing system with optical system in accordance with the principles of the present invention. 
         FIG. 3 a    illustrates a large prior art optical arrangement. 
         FIG. 3 b    illustrates an upper optical module in accordance with the principles of the present invention. 
         FIG. 4  illustrates an upper optical module in accordance with the principles of the present invention. 
         FIG. 4 a    illustrates an upper optical module in accordance with the principles of the present invention. 
         FIG. 4 b    illustrates an upper optical module in accordance with the principles of the present invention. 
         FIG. 5  illustrates an upper optical module in accordance with the principles of the present invention. 
         FIG. 5 a    illustrates an upper optical module in accordance with the principles of the present invention. 
         FIG. 5 b    illustrates an upper optical module and dark light trap according to the principles of the present invention. 
         FIG. 5 c    illustrates an upper optical module and dark light trap according to the principles of the present invention. 
         FIG. 5 d    illustrates an upper optical module and dark light trap according to the principles of the present invention. 
         FIG. 5 e    illustrates an upper optical module and dark light trap according to the principles of the present invention. 
         FIG. 6  illustrates upper and lower optical modules in accordance with the principles of the present invention. 
         FIG. 7  illustrates angles of combiner elements in accordance with the principles of the present invention. 
         FIG. 8  illustrates upper and lower optical modules in accordance with the principles of the present invention. 
         FIG. 8 a    illustrates upper and lower optical modules in accordance with the principles of the present invention. 
         FIG. 8 b    illustrates upper and lower optical modules in accordance with the principles of the present invention. 
         FIG. 8 c    illustrates upper and lower optical modules in accordance with the principles of the present invention. 
         FIG. 9  illustrates an eye imaging system in accordance with the principles of the present invention. 
         FIG. 10  illustrates a light source in accordance with the principles of the present invention. 
         FIG. 10 a    illustrates a back lighting system in accordance with the principles of the present invention. 
         FIG. 10 b    illustrates a back lighting system in accordance with the principles of the present invention. 
         FIGS. 11 a  to 11 d    illustrate light source and filters in accordance with the principles of the present invention. 
         FIGS. 12 a  to 12 c    illustrate light source and quantum dot systems in accordance with the principles of the present invention. 
         FIGS. 13 a  to 13 c    illustrate peripheral lighting systems in accordance with the principles of the present invention. 
         FIGS. 14 a  to 14 h    illustrate a light suppression systems in accordance with the principles of the present invention. 
         FIG. 14 i    illustrates a head-worn computer with scene cameras in accordance with the principles of the present invention. 
         FIG. 15  illustrates an external user interface in accordance with the principles of the present invention. 
         FIGS. 16 a  to 16 c    illustrate distance control systems in accordance with the principles of the present invention. 
         FIGS. 17 a  to 17 c    illustrate force interpretation systems in accordance with the principles of the present invention. 
         FIGS. 18 a  to 18 c    illustrate user interface mode selection systems in accordance with the principles of the present invention. 
         FIG. 19  illustrates interaction systems in accordance with the principles of the present invention. 
         FIG. 20  illustrates external user interfaces in accordance with the principles of the present invention. 
         FIG. 21  illustrates mD trace representations presented in accordance with the principles of the present invention. 
         FIG. 22  illustrates mD trace representations presented in accordance with the principles of the present invention. 
         FIG. 23  illustrates an mD scanned environment in accordance with the principles of the present invention. 
         FIG. 23 a    illustrates mD trace representations presented in accordance with the principles of the present invention. 
         FIG. 24  illustrates a stray light suppression technology in accordance with the principles of the present invention. 
         FIG. 25  illustrates a stray light suppression technology in accordance with the principles of the present invention. 
         FIG. 26  illustrates a stray light suppression technology in accordance with the principles of the present invention. 
         FIG. 27  illustrates a stray light suppression technology in accordance with the principles of the present invention. 
         FIGS. 28 a  to 28 c    illustrate DLP mirror angles. 
         FIGS. 29 to 33  illustrate eye imaging systems according to the principles of the present invention. 
         FIGS. 34 and 34   a  illustrate structured eye lighting systems according to the principles of the present invention. 
         FIG. 35  illustrates eye glint in the prediction of eye direction analysis in accordance with the principles of the present invention. 
         FIG. 36 a    illustrates eye characteristics that may be used in personal identification through analysis of a system according to the principles of the present invention. 
         FIG. 36 b    illustrates a digital content presentation reflection off of the wearer&#39;s eye that may be analyzed in accordance with the principles of the present invention. 
         FIG. 37  illustrates eye imaging along various virtual target lines and various focal planes in accordance with the principles of the present invention. 
         FIG. 38  illustrates content control with respect to eye movement based on eye imaging in accordance with the principles of the present invention. 
         FIG. 39  illustrates eye imaging and eye convergence in accordance with the principles of the present invention. 
         FIG. 40  illustrates content position dependent on sensor feedback in accordance with the principles of the present invention. 
         FIG. 41  illustrates content position dependent on sensor feedback in accordance with the principles of the present invention. 
         FIG. 42  illustrates content position dependent on sensor feedback in accordance with the principles of the present invention. 
         FIG. 43  illustrates content position dependent on sensor feedback in accordance with the principles of the present invention. 
         FIG. 44  illustrates content position dependent on sensor feedback in accordance with the principles of the present invention. 
         FIG. 45  illustrates various headings over time in an example. 
         FIG. 46  illustrates content position dependent on sensor feedback in accordance with the principles of the present invention. 
         FIG. 47  illustrates content position dependent on sensor feedback in accordance with the principles of the present invention. 
         FIG. 48  illustrates content position dependent on sensor feedback in accordance with the principles of the present invention. 
         FIG. 49  illustrates content position dependent on sensor feedback in accordance with the principles of the present invention. 
         FIG. 50  illustrates light impinging an eye in accordance with the principles of the present invention. 
         FIG. 51  illustrates a view of an eye in accordance with the principles of the present invention. 
         FIGS. 52 a  and 52 b    illustrate views of an eye with a structured light pattern in accordance with the principles of the present invention. 
         FIG. 53  illustrates an optics module in accordance with the principles of the present invention. 
         FIG. 54  illustrates an optics module in accordance with the principles of the present invention. 
         FIG. 55  shows a series of example spectrum for a variety of controlled substances as measured using a form of infrared spectroscopy. 
         FIG. 56  shows an infrared absorbance spectrum for glucose. 
         FIG. 57  illustrates a scene where a person is walking with a HWC mounted on his head. 
         FIG. 58  illustrates a system for receiving, developing and using movement heading, sight heading, eye heading and/or persistence information from HWC(s). 
         FIG. 59  illustrates a presentation technology in accordance with the principles of the present invention. 
         FIG. 60  illustrates a presentation technology in accordance with the principles of the present invention. 
         FIG. 61  illustrates a presentation technology in accordance with the principles of the present invention. 
         FIG. 62  illustrates a presentation technology in accordance with the principles of the present invention. 
         FIG. 63  illustrates a presentation technology in accordance with the principles of the present invention. 
         FIG. 64  illustrates a presentation technology in accordance with the principles of the present invention. 
         FIG. 65  illustrates a presentation technology in accordance with the principles of the present invention. 
         FIG. 66  illustrates a presentation technology in accordance with the principles of the present invention. 
         FIG. 67  illustrates an optical configuration in accordance with the principles of the present invention. 
         FIG. 68  illustrates an optical configuration in accordance with the principles of the present invention. 
         FIG. 69  illustrates an optical configuration in accordance with the principles of the present invention. 
         FIG. 70  illustrates an optical configuration in accordance with the principles of the present invention. 
         FIG. 71  illustrates an optical configuration in accordance with the principles of the present invention. 
         FIG. 72  illustrates an optical element in accordance with the principles of the present invention. 
         FIG. 73  illustrates an optical element in accordance with the principles of the present invention. 
         FIG. 74  illustrates an optical element in accordance with the principles of the present invention. 
         FIG. 75  illustrates an optical element in accordance with the principles of the present invention. 
         FIG. 76  illustrates an optical element in a see-through computer display in accordance with the principles of the present invention. 
         FIG. 77  illustrates an optical element in accordance with the principles of the present invention. 
         FIG. 78  illustrates an optical element in accordance with the principles of the present invention. 
         FIG. 79 a    illustrates a schematic of an upper optic in accordance with the principles of the present invention. 
         FIG. 79  illustrates a schematic of an upper optic in accordance with the principles of the present invention. 
         FIG. 80  illustrates a stray light control technology in accordance with the principles of the present invention. 
         FIGS. 81 a  and 81 b    illustrate a display with a gap and masked technologies in accordance with the principles of the present invention. 
         FIG. 82  illustrates an upper module with a trim polarizer in accordance with the principles of the present invention. 
         FIG. 83  illustrates an optical system with a laminated multiple polarizer film in accordance with the principles of the present invention. 
         FIGS. 84 a  and 84 b    illustrate partially reflective layers in accordance with the principles of the present invention. 
         FIG. 84 c    illustrates a laminated multiple polarizer with a complex curve in accordance with the principles of the present invention. 
         FIG. 84 d    illustrates a laminated multiple polarizer with a curve in accordance with the principles of the present invention. 
         FIG. 85  illustrates an optical system adapted for a head-mounted display in accordance with the principles of the present invention. 
         FIG. 86  illustrates an optical system adapted for a head-mounted display in accordance with the principles of the present invention. 
         FIG. 87  illustrates an optical system adapted for a head-mounted display in accordance with the principles of the present invention. 
         FIG. 88  illustrates an optical system adapted for a head-mounted display in accordance with the principles of the present invention. 
         FIG. 89  illustrates an optical system adapted for a head-mounted display in accordance with the principles of the present invention. 
         FIG. 90  illustrates an optical system adapted for a head-mounted display in accordance with the principles of the present invention. 
         FIG. 91  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 92  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 93  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 94  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 95  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 96  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 97  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 98  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 99  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 100  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 101  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 102  illustrates an optical system in accordance with the principles of the present invention. 
         FIGS. 103, 103   a  and  103   b  illustrate optical systems in accordance with the principles of the present invention. 
         FIG. 104  illustrates an optical system in accordance with the principles of the present invention. 
         FIG. 105  illustrates a blocking optic in accordance with the principles of the present invention. 
         FIGS. 106 a , 106 b , and 106 c    illustrate a blocking optic system in accordance with the principles of the present invention. 
         FIG. 107  illustrates a full color image in accordance with the principles of the present invention. 
         FIGS. 108A and 108B  illustrate color breakup management in accordance with the principles of the present invention. 
         FIG. 109  illustrates timing sequences in accordance with the principles of the present invention. 
         FIG. 110  illustrates timing sequences in accordance with the principles of the present invention. 
         FIGS. 111 a  and 111 b    illustrate sequentially displayed images in accordance with the principles of the present invention. 
         FIG. 112  illustrates a see-through display with rotated components in accordance with the principles of the present invention. 
         FIG. 113  illustrates an optics module with twisted reflective surfaces in accordance with the principles of the present invention. 
         FIG. 114  illustrates PCB and see-through optics module positions within a glasses form factor in accordance with the principles of the present invention. 
         FIG. 115  illustrates PCB and see-through optics module positions within a glasses form factor in accordance with the principles of the present invention. 
         FIG. 116  illustrates PCB and see-through optics module positions within a glasses form factor in accordance with the principles of the present invention. 
         FIG. 117  illustrates a user interface in accordance with the principles of the present invention. 
         FIG. 118  illustrates a user interface in accordance with the principles of the present invention. 
         FIG. 119  illustrates a lens arrangement in accordance with the principles of the present invention. 
         FIGS. 120 and 121  illustrate eye imaging systems in accordance with the principles of the present invention. 
         FIG. 122  illustrates an identification process in accordance with the principles of the present invention. 
         FIGS. 123 and 124  illustrate combiner assemblies in accordance with the principles of the present invention. 
         FIG. 125  shows a chart of the sensitivity of the human eye versus brightness. 
         FIG. 126  is a chart that shows the brightness (L*) perceived by the human eye relative to a measured brightness (luminance) of a scene. 
         FIG. 127  is illustration of a see-through view of the surrounding environment with an outline showing the display field of view being smaller than the see-through field of view as is typical. 
         FIG. 128  is an illustration of a captured image of the surrounding environment which can be a substantially larger field of view than the displayed image so that a cropped version of the captured image of the environment can be used for the alignment process. 
         FIGS. 129 a  and 129 b    illustrate first and second target images with invisible markers. 
         FIGS. 130 and 131  illustrate targets overlaid onto a see-through view, wherein the target is moved using eye tracking control, in accordance with the principles of the present invention. 
         FIG. 132  shows an illustration of multiply folded optics for a head worn display that includes a solid prism in accordance with the principles of the present invention. 
         FIGS. 133 a , 133 b  and 133 c    show illustrations of steps associated with bonding the reflective plate to the solid prism in accordance with the principles of the present invention. 
         FIG. 134  shows an illustration of multiply folded optics for a reflective image source with a backlight assembly positioned behind the reflective plate in accordance with the principles of the present invention. 
         FIG. 135  shows an illustration of a prism film bonded to a reflective plate in accordance with the principles of the present invention. 
         FIG. 135 a    shows an illustration of multiply folded optics in which two cones of illumination light provided by the prism film are shown in accordance with the principles of the present invention. 
         FIGS. 136, 137 and 138  show illustrations of different embodiments of additional optical elements included in the solid prism for imaging the eye of the user in accordance with the principles of the present invention. 
         FIG. 139  shows an illustration of an eye imaging system for multiply folded optics in which the image source is a self-luminous display in accordance with the principles of the present invention. 
         FIGS. 140 a  and 140 b    are illustrations of an eye imaging system in accordance with the principles of the present invention. 
         FIGS. 141 a  and 141 b    are illustrations of folded optics that include a waveguide with an angled partially reflective surface and a powered reflective surface in accordance with the principles of the present invention. 
         FIGS. 142 a  and 142 b    are illustrations of folded optics for a head-worn display that include waveguides with at least one holographic optical element and image source in accordance with the principles of the present invention. 
         FIG. 143  is an illustration of folded optics for a head-worn display in which the illumination light is injected into the waveguide and redirected by the holographic optical element so that the user&#39;s eye is illuminated in accordance with the principles of the present invention. 
         FIG. 144  shows an illustration of folded optics for a head-worn display where a series of angled partial mirrors are included in the waveguide in accordance with the principles of the present invention. 
         FIG. 145  shows an illustration of a beam splitter based optical module for a head-worn display in accordance with the principles of the present invention. 
         FIG. 146  shows an illustration of an optical module for a head-worn display in accordance with the principles of the present invention. 
         FIG. 146 a    shows an illustration of a side view of an optics module that includes a corrective lens element. 
         FIG. 147  shows an illustration of left and right optics modules that are connected together in a chassis in accordance with the principles of the present invention. 
         FIG. 148  shows the left and right images provided at the nominal vergence distance within the left and right display fields of view in accordance with the principles of the present invention. 
         FIG. 149  shows how the left and right images are shifted laterally towards each other within the left and right display fields of view in accordance with the principles of the present invention. 
         FIGS. 150 a  and 150 b    show a mechanism for moving the image source in accordance with the principles of the present invention. 
         FIGS. 151 a  and 151 b    show illustrations of an upper wedge and lower wedge from the position of the image source in accordance with the principles of the present invention. 
         FIG. 152  shows an illustration of spring clips applying a force to an image source in accordance with the principles of the present invention. 
         FIGS. 153 a , 153 b    and  154  shows illustrations of example display optics that include eye imaging in accordance with the principles of the present invention. 
         FIGS. 155 a , 155 b , 156 a , 156 b , 157 a , 157 b , 158 a , 158 b , 159 a  and 159 b    show illustrations of focus adjustment modules in accordance with the principles of the present invention. 
         FIG. 160  shows an illustration of an example of multiply folded optics as viewed from the eye position in accordance with the principles of the present invention. 
         FIGS. 161 and 162  illustrate optical systems in accordance with the principles of the present invention. 
         FIG. 163A  illustrates an abrupt change in appearance of content in the field of view of a see-through display. 
         FIG. 163B  illustrates a managed appearance system where the content is reduced in appearance as it enters a transitional zone near the edge of the field of view. 
         FIG. 164  illustrates a hybrid field of view that includes a centered field of view and an extended field of view that is positioned at or near or overlapping with an edge of the centered field of view. 
         FIG. 165  illustrates a hybrid display system where the main, centered, field of view is generated with optics in an upper module and the extended field of view is generated with a display system mounted above the combiner. 
         FIGS. 166A-166D  illustrate examples of extended display, or extended image content optic, configurations. 
         FIG. 167  illustrates another optical system that uses a hybrid optical system that includes a main display optical system and an extended field of view optical system. 
         FIGS. 168A-168E  illustrate various embodiments where a see-through display panel is positioned directly in in front of the user&#39;s eye in the head-worn computer to provide the extended and/or overlapping field of view in a hybrid display system. 
         FIG. 169  shows a cross sectional illustration of an example optics assembly for a head worn display in accordance with the principles of the present invention. 
         FIG. 170  shows an illustration of the light trap operating to reduce stray light in accordance with the principles of the present invention. 
         FIG. 171  shows an illustration of a simple optical system that provides a 60 degree display field of view in accordance with the principles of the present invention. 
         FIG. 172  shows a chart of the acuity of a typical human eye relative to the angular position in the field of view. 
         FIG. 173  shows a chart of the typical acuity of the human eye vs the eccentricity in a simplified form that highlights the dropoff in acuity with eccentricity along with the difference between achromatic acuity and chromatic acuity. 
         FIGS. 174 a  and 174 b    show a typical chart of eye movements and head movements given in radians vs time. 
         FIG. 175  is a chart that shows the effective relative achromatic acuity, compared to the acuity of the fovea, provided by a typical human eye within the eye&#39;s field of view when the movement of the eye is included. 
         FIG. 176  is a chart that shows the minimum design MTF vs angular field position needed to provide a uniformly sharp looking image in a wide field of view displayed image. 
         FIG. 177  is a chart that shows the relative MTF needed to be provided by the display optics for a wide field of view display to provide a sharpness that matches the acuity of the human eye in the peripheral zone of the display field of view. 
         FIG. 178  shows a modeled MTF curve associated with the optical system of  FIG. 171  wherein MTF curves for a variety of different angular positions within the display field of view are shown. 
         FIG. 179  is an illustration of a resolution chart wherein the sharpness of the image has been reduced by blurring the peripheral portion of the image to simulate an image from optics that provide a central sharp zone of +/−15 degrees with a peripheral zone that is less sharp. 
         FIGS. 180 and 181  are illustrations that show how the image is shifted within the display field of view as the user moves their head in accordance with the principles of the present invention. 
         FIG. 182  illustrates the blank portion of the display field of view where the image has been shifted away from is displayed as a dark region to enable the user to see-through to the surrounding environment in the blank portion in accordance with the principles of the present invention. 
         FIG. 183  shows an illustration of a wide display field of view, wherein a user can choose to display a smaller field of view for a given image or application (e.g. a game) to improve the personal viewing experience in accordance with the principles of the present invention. 
         FIGS. 184 and 185  should physical arrangements of optical systems in accordance with the principles of the present invention. 
         FIG. 186  shows a 30:9 format field of view and a 22:9 format field of view, wherein the two fields of view have the same vertical field of view and different horizontal field of view in accordance with the principles of the present invention. 
     
    
    
     While the invention has been described in connection with certain preferred embodiments, other embodiments would be understood by one of ordinary skill in the art and are encompassed herein. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Aspects of the present invention relate to head-worn computing (“HWC”) systems. HWC involves, in some instances, a system that mimics the appearance of head-worn glasses or sunglasses. The glasses may be a fully developed computing platform, such as including computer displays presented in each of the lenses of the glasses to the eyes of the user. In embodiments, the lenses and displays may be configured to allow a person wearing the glasses to see the environment through the lenses while also seeing, simultaneously, digital imagery, which forms an overlaid image that is perceived by the person as a digitally augmented image of the environment, or augmented reality (“AR”). 
     HWC involves more than just placing a computing system on a person&#39;s head. The system may need to be designed as a lightweight, compact and fully functional computer display, such as wherein the computer display includes a high resolution digital display that provides a high level of emersion comprised of the displayed digital content and the see-through view of the environmental surroundings. User interfaces and control systems suited to the HWC device may be required that are unlike those used for a more conventional computer such as a laptop. For the HWC and associated systems to be most effective, the glasses may be equipped with sensors to determine environmental conditions, geographic location, relative positioning to other points of interest, objects identified by imaging and movement by the user or other users in a connected group, and the like. The HWC may then change the mode of operation to match the conditions, location, positioning, movements, and the like, in a method generally referred to as a contextually aware HWC. The glasses also may need to be connected, wirelessly or otherwise, to other systems either locally or through a network. Controlling the glasses may be achieved through the use of an external device, automatically through contextually gathered information, through user gestures captured by the glasses sensors, and the like. Each technique may be further refined depending on the software application being used in the glasses. The glasses may further be used to control or coordinate with external devices that are associated with the glasses. 
     Referring to  FIG. 1 , an overview of the HWC system  100  is presented. As shown, the HWC system  100  comprises a HWC  102 , which in this instance is configured as glasses to be worn on the head with sensors such that the HWC  102  is aware of the objects and conditions in the environment  114 . In this instance, the HWC  102  also receives and interprets control inputs such as gestures and movements  116 . The HWC  102  may communicate with external user interfaces  104 . The external user interfaces  104  may provide a physical user interface to take control instructions from a user of the HWC  102  and the external user interfaces  104  and the HWC  102  may communicate bi-directionally to affect the user&#39;s command and provide feedback to the external device  108 . The HWC  102  may also communicate bi-directionally with externally controlled or coordinated local devices  108 . For example, an external user interface  104  may be used in connection with the HWC  102  to control an externally controlled or coordinated local device  108 . The externally controlled or coordinated local device  108  may provide feedback to the HWC  102  and a customized GUI may be presented in the HWC  102  based on the type of device or specifically identified device  108 . The HWC  102  may also interact with remote devices and information sources  112  through a network connection  110 . Again, the external user interface  104  may be used in connection with the HWC  102  to control or otherwise interact with any of the remote devices  108  and information sources  112  in a similar way as when the external user interfaces  104  are used to control or otherwise interact with the externally controlled or coordinated local devices  108 . Similarly, HWC  102  may interpret gestures  116  (e.g captured from forward, downward, upward, rearward facing sensors such as camera(s), range finders, IR sensors, etc.) or environmental conditions sensed in the environment  114  to control either local or remote devices  108  or  112 . 
     We will now describe each of the main elements depicted on  FIG. 1  in more detail; however, these descriptions are intended to provide general guidance and should not be construed as limiting. Additional description of each element may also be further described herein. 
     The HWC  102  is a computing platform intended to be worn on a person&#39;s head. The HWC  102  may take many different forms to fit many different functional requirements. In some situations, the HWC  102  will be designed in the form of conventional glasses. The glasses may or may not have active computer graphics displays. In situations where the HWC  102  has integrated computer displays the displays may be configured as see-through displays such that the digital imagery can be overlaid with respect to the user&#39;s view of the environment  114 . There are a number of see-through optical designs that may be used, including ones that have a reflective display (e.g. LCoS, DLP), emissive displays (e.g. OLED, LED), hologram, TIR waveguides, and the like. In embodiments, lighting systems used in connection with the display optics may be solid state lighting systems, such as LED, OLED, quantum dot, quantum dot LED, etc. In addition, the optical configuration may be monocular or binocular. It may also include vision corrective optical components. In embodiments, the optics may be packaged as contact lenses. In other embodiments, the HWC  102  may be in the form of a helmet with a see-through shield, sunglasses, safety glasses, goggles, a mask, fire helmet with see-through shield, police helmet with see through shield, military helmet with see-through shield, utility form customized to a certain work task (e.g. inventory control, logistics, repair, maintenance, etc.), and the like. 
     The HWC  102  may also have a number of integrated computing facilities, such as an integrated processor, integrated power management, communication structures (e.g. cell net, WiFi, Bluetooth, local area connections, mesh connections, remote connections (e.g. client server, etc.)), and the like. The HWC  102  may also have a number of positional awareness sensors, such as GPS, electronic compass, altimeter, tilt sensor, IMU, and the like. It may also have other sensors such as a camera, rangefinder, hyper-spectral camera, Geiger counter, microphone, spectral illumination detector, temperature sensor, chemical sensor, biologic sensor, moisture sensor, ultrasonic sensor, and the like. 
     The HWC  102  may also have integrated control technologies. The integrated control technologies may be contextual based control, passive control, active control, user control, and the like. For example, the HWC  102  may have an integrated sensor (e.g. camera) that captures user hand or body gestures  116  such that the integrated processing system can interpret the gestures and generate control commands for the HWC  102 . In another example, the HWC  102  may have sensors that detect movement (e.g. a nod, head shake, and the like) including accelerometers, gyros and other inertial measurements, where the integrated processor may interpret the movement and generate a control command in response. The HWC  102  may also automatically control itself based on measured or perceived environmental conditions. For example, if it is bright in the environment the HWC  102  may increase the brightness or contrast of the displayed image. In embodiments, the integrated control technologies may be mounted on the HWC  102  such that a user can interact with it directly. For example, the HWC  102  may have a button(s), touch capacitive interface, and the like. 
     As described herein, the HWC  102  may be in communication with external user interfaces  104 . The external user interfaces may come in many different forms. For example, a cell phone screen may be adapted to take user input for control of an aspect of the HWC  102 . The external user interface may be a dedicated UI, such as a keyboard, touch surface, button(s), joy stick, and the like. In embodiments, the external controller may be integrated into another device such as a ring, watch, bike, car, and the like. In each case, the external user interface  104  may include sensors (e.g. IMU, accelerometers, compass, altimeter, and the like) to provide additional input for controlling the HWD  104 . 
     As described herein, the HWC  102  may control or coordinate with other local devices  108 . The external devices  108  may be an audio device, visual device, vehicle, cell phone, computer, and the like. For instance, the local external device  108  may be another HWC  102 , where information may then be exchanged between the separate HWCs  108 . 
     Similar to the way the HWC  102  may control or coordinate with local devices  106 , the HWC  102  may control or coordinate with remote devices  112 , such as the HWC  102  communicating with the remote devices  112  through a network  110 . Again, the form of the remote device  112  may have many forms. Included in these forms is another HWC  102 . For example, each HWC  102  may communicate its GPS position such that all the HWCs  102  know where all of HWC  102  are located. 
       FIG. 2  illustrates a HWC  102  with an optical system that includes an upper optical module  202  and a lower optical module  204 . While the upper and lower optical modules  202  and  204  will generally be described as separate modules, it should be understood that this is illustrative only and the present invention includes other physical configurations, such as that when the two modules are combined into a single module or where the elements making up the two modules are configured into more than two modules. In embodiments, the upper module  202  includes a computer controlled display (e.g. LCoS, DLP, OLED, etc.) and image light delivery optics. In embodiments, the lower module includes eye delivery optics that are configured to receive the upper module&#39;s image light and deliver the image light to the eye of a wearer of the HWC. In  FIG. 2 , it should be noted that while the upper and lower optical modules  202  and  204  are illustrated in one side of the HWC such that image light can be delivered to one eye of the wearer, that it is envisioned by the present invention that embodiments will contain two image light delivery systems, one for each eye. 
       FIG. 3 b    illustrates an upper optical module  202  in accordance with the principles of the present invention. In this embodiment, the upper optical module  202  includes a DLP (also known as DMD or digital micromirror device) computer operated display  304  which includes pixels comprised of rotatable mirrors (such as, for example, the DLP3000 available from Texas Instruments), polarized light source  302 , ¼ wave retarder film  308 , reflective polarizer  310  and a field lens  312 . The polarized light source  302  provides substantially uniform polarized light that is generally directed towards the reflective polarizer  310 . The reflective polarizer reflects light of one polarization state (e.g. S polarized light) and transmits light of the other polarization state (e.g. P polarized light). The polarized light source  302  and the reflective polarizer  310  are oriented so that the polarized light from the polarized light source  302  is reflected generally towards the DLP  304 . The light then passes through the ¼ wave film  308  once before illuminating the pixels of the DLP  304  and then again after being reflected by the pixels of the DLP  304 . In passing through the ¼ wave film  308  twice, the light is converted from one polarization state to the other polarization state (e.g. the light is converted from S to P polarized light). The light then passes through the reflective polarizer  310 . In the event that the DLP pixel(s) are in the “on” state (i.e. the mirrors are positioned to reflect light towards the field lens  312 , the “on” pixels reflect the light generally along the optical axis and into the field lens  312 . This light that is reflected by “on” pixels and which is directed generally along the optical axis of the field lens  312  will be referred to as image light  316 . The image light  316  then passes through the field lens to be used by a lower optical module  204 . 
     The light that is provided by the polarized light source  302 , which is subsequently reflected by the reflective polarizer  310  before it reflects from the DLP  304 , will generally be referred to as illumination light. The light that is reflected by the “off” pixels of the DLP  304  is reflected at a different angle than the light reflected by the ‘on” pixels, so that the light from the “off” pixels is generally directed away from the optical axis of the field lens  312  and toward the side of the upper optical module  202  as shown in  FIG. 3 . The light that is reflected by the “off” pixels of the DLP  304  will be referred to as dark state light  314 . 
     The DLP  304  operates as a computer controlled display and is generally thought of as a MEMs device. The DLP pixels are comprised of small mirrors that can be directed. The mirrors generally flip from one angle to another angle. The two angles are generally referred to as states. When light is used to illuminate the DLP the mirrors will reflect the light in a direction depending on the state. In embodiments herein, we generally refer to the two states as “on” and “off,” which is intended to depict the condition of a display pixel. “On” pixels will be seen by a viewer of the display as emitting light because the light is directed along the optical axis and into the field lens and the associated remainder of the display system. “Off” pixels will be seen by a viewer of the display as not emitting light because the light from these pixels is directed to the side of the optical housing and into a light trap or light dump where the light is absorbed. The pattern of “on” and “off” pixels produces image light that is perceived by a viewer of the display as a computer generated image. Full color images can be presented to a user by sequentially providing illumination light with complimentary colors such as red, green and blue. Where the sequence is presented in a recurring cycle that is faster than the user can perceive as separate images and as a result the user perceives a full color image comprised of the sum of the sequential images. Bright pixels in the image are provided by pixels that remain in the “on” state for the entire time of the cycle, while dimmer pixels in the image are provided by pixels that switch between the “on” state and “off” state within the time of the cycle, or frame time when in a video sequence of images. 
       FIG. 3 a    shows an illustration of a system for a DLP  304  in which the unpolarized light source  350  is pointed directly at the DLP  304 . In this case, the angle required for the illumination light is such that the field lens  352  must be positioned substantially distant from the DLP  304  to avoid the illumination light from being clipped by the field lens  352 . The large distance between the field lens  352  and the DLP  304  along with the straight path of the dark state light  354 , means that the light trap for the dark state light  354  is also located at a substantial distance from the DLP. For these reasons, this configuration is larger in size compared to the upper optics module  202  of the preferred embodiments. 
     The configuration illustrated in  FIG. 3 b    can be lightweight and compact such that it fits into a small portion of a HWC. For example, the upper modules  202  illustrated herein can be physically adapted to mount in an upper frame of a HWC such that the image light can be directed into a lower optical module  204  for presentation of digital content to a wearer&#39;s eye. The package of components that combine to generate the image light (i.e. the polarized light source  302 , DLP  304 , reflective polarizer  310  and ¼ wave film  308 ) is very light and is compact. The height of the system, excluding the field lens, may be less than 8 mm. The width (i.e. from front to back) may be less than 8 mm. The weight may be less than 2 grams. The compactness of this upper optical module  202  allows for a compact mechanical design of the HWC and the light weight nature of these embodiments help make the HWC lightweight to provide for a HWC that is comfortable for a wearer of the HWC. 
     The configuration illustrated in  FIG. 3 b    can produce sharp contrast, high brightness and deep blacks, especially when compared to LCD or LCoS displays used in HWC. The “on” and “off” states of the DLP provide for a strong differentiator in the light reflection path representing an “on” pixel and an “off” pixel. As will be discussed in more detail below, the dark state light from the “off” pixel reflections can be managed to reduce stray light in the display system to produce images with high contrast. 
       FIG. 4  illustrates another embodiment of an upper optical module  202  in accordance with the principles of the present invention. This embodiment includes a light source  404 , but in this case, the light source can provide unpolarized illumination light. The illumination light from the light source  404  is directed into a TIR wedge  418  such that the illumination light is incident on an internal surface of the TIR wedge  418  (shown as the angled lower surface of the TRI wedge  418  in  FIG. 4 ) at an angle that is beyond the critical angle as defined by Eqn 1. 
       Critical angle=arc−sin(1/ n )  Eqn 1
 
     Where the critical angle is the angle beyond which the illumination light is reflected from the internal surface when the internal surface comprises an interface from a solid with a higher refractive index (n) to air with a refractive index of 1 (e.g. for an interface of acrylic, with a refractive index of n=1.5, to air, the critical angle is 41.8 degrees; for an interface of polycarbonate, with a refractive index of n=1.59, to air the critical angle is 38.9 degrees). Consequently, the TIR wedge  418  is associated with a thin air gap  408  along the internal surface to create an interface between a solid with a higher refractive index and air. By choosing the angle of the light source  404  relative to the DLP  402  in correspondence to the angle of the internal surface of the TIR wedge  418 , illumination light is turned toward the DLP  402  at an angle suitable for providing image light  414  as reflected from “on” pixels. Wherein, the illumination light is provided to the DLP  402  at approximately twice the angle of the pixel mirrors in the DLP  402  that are in the “on” state, such that after reflecting from the pixel mirrors, the image light  414  is directed generally along the optical axis of the field lens. Depending on the state of the DLP pixels, the illumination light from “on” pixels may be reflected as image light  414  which is directed towards a field lens and a lower optical module  204 , while illumination light reflected from “off” pixels (generally referred to herein as “dark” state light, “off” pixel light or “off” state light)  410  is directed in a separate direction, which may be trapped and not used for the image that is ultimately presented to the wearer&#39;s eye. 
     The light trap for the dark state light  410  may be located along the optical axis defined by the direction of the dark state light  410  and in the side of the housing, with the function of absorbing the dark state light. To this end, the light trap may be comprised of an area outside of the cone of image light  414  from the “on” pixels. The light trap is typically made up of materials that absorb light including coatings of black paints or other light absorbing materials to prevent light scattering from the dark state light degrading the image perceived by the user. In addition, the light trap may be recessed into the wall of the housing or include masks or guards to block scattered light and prevent the light trap from being viewed adjacent to the displayed image. 
     The embodiment of  FIG. 4  also includes a corrective wedge  420  to correct the effect of refraction of the image light  414  as it exits the TIR wedge  418 . By including the corrective wedge  420  and providing a thin air gap  408  (e.g. 25 micron), the image light from the “on” pixels can be maintained generally in a direction along the optical axis of the field lens (i.e. the same direction as that defined by the image light  414 ) so it passes into the field lens and the lower optical module  204 . As shown in  FIG. 4 , the image light  414  from the “on” pixels exits the corrective wedge  420  generally perpendicular to the surface of the corrective wedge  420  while the dark state light exits at an oblique angle. As a result, the direction of the image light  414  from the “on” pixels is largely unaffected by refraction as it exits from the surface of the corrective wedge  420 . In contrast, the dark state light  410  is substantially changed in direction by refraction when the dark state light  410  exits the corrective wedge  420 . 
     The embodiment illustrated in  FIG. 4  has the similar advantages of those discussed in connection with the embodiment of  FIG. 3 b   . The dimensions and weight of the upper module  202  depicted in  FIG. 4  may be approximately 8×8 mm with a weight of less than 3 grams. A difference in overall performance between the configuration illustrated in  FIG. 3 b    and the configuration illustrated in  FIG. 4  is that the embodiment of  FIG. 4  doesn&#39;t require the use of polarized light as supplied by the light source  404 . This can be an advantage in some situations as will be discussed in more detail below (e.g. increased see-through transparency of the HWC optics from the user&#39;s perspective). Polarized light may be used in connection with the embodiment depicted in  FIG. 4 , in embodiments. An additional advantage of the embodiment of  FIG. 4  compared to the embodiment shown in  FIG. 3 b    is that the dark state light (shown as DLP off light  410 ) is directed at a steeper angle away from the optical axis of the image light  414  due to the added refraction encountered when the dark state light  410  exits the corrective wedge  420 . This steeper angle of the dark state light  410  allows for the light trap to be positioned closer to the DLP  402  so that the overall size of the upper module  202  can be reduced. The light trap can also be made larger since the light trap doesn&#39;t interfere with the field lens, thereby the efficiency of the light trap can be increased and as a result, stray light can be reduced and the contrast of the image perceived by the user can be increased.  FIG. 4 a    illustrates the embodiment described in connection with  FIG. 4  with an example set of corresponding angles at the various surfaces with the reflected angles of a ray of light passing through the upper optical module  202 . In this example, the DLP mirrors are provided at 17 degrees to the surface of the DLP device. The angles of the TIR wedge are selected in correspondence to one another to provide TIR reflected illumination light at the correct angle for the DLP mirrors while allowing the image light and dark state light to pass through the thin air gap, various combinations of angles are possible to achieve this. 
       FIG. 5  illustrates yet another embodiment of an upper optical module  202  in accordance with the principles of the present invention. As with the embodiment shown in  FIG. 4 , the embodiment shown in  FIG. 5  does not require the use of polarized light. Polarized light may be used in connection with this embodiment, but it is not required. The optical module  202  depicted in  FIG. 5  is similar to that presented in connection with  FIG. 4 ; however, the embodiment of  FIG. 5  includes an off light redirection wedge  502 . As can be seen from the illustration, the off light redirection wedge  502  allows the image light  414  to continue generally along the optical axis toward the field lens and into the lower optical module  204  (as illustrated). However, the off light  504  is redirected substantially toward the side of the corrective wedge  420  where it passes into the light trap. This configuration may allow further height compactness in the HWC because the light trap (not illustrated) that is intended to absorb the off light  504  can be positioned laterally adjacent the upper optical module  202  as opposed to below it. In the embodiment depicted in  FIG. 5  there is a thin air gap between the TIR wedge  418  and the corrective wedge  420  (similar to the embodiment of  FIG. 4 ). There is also a thin air gap between the corrective wedge  420  and the off light redirection wedge  502 . There may be HWC mechanical configurations that warrant the positioning of a light trap for the dark state light elsewhere and the illustration depicted in  FIG. 5  should be considered illustrative of the concept that the off light can be redirected to create compactness of the overall HWC.  FIG. 5 a    illustrates an example of the embodiment described in connection with  FIG. 5  with the addition of more details on the relative angles at the various surfaces and a light ray trace for image light and a light ray trace for dark light are shown as it passes through the upper optical module  202 . Again, various combinations of angles are possible. 
       FIG. 4 b    shows an illustration of a further embodiment in which a solid transparent matched set of wedges  456  is provided with a reflective polarizer  450  at the interface between the wedges. Wherein the interface between the wedges in the wedge set  456  is provided at an angle so that illumination light  452  from the polarized light source  458  is reflected at the proper angle (e.g. 34 degrees for a 17 degree DLP mirror) for the DLP mirror “on” state so that the reflected image light  414  is provided along the optical axis of the field lens. The general geometry of the wedges in the wedge set  456  is similar to that shown in  FIGS. 4 and 4   a . A quarter wave film  454  is provided on the DLP  402  surface so that the illumination light  452  is one polarization state (e.g. S polarization state) while in passing through the quarter wave film  454 , reflecting from the DLP mirror and passing back through the quarter wave film  454 , the image light  414  is converted to the other polarization state (e.g. P polarization state). The reflective polarizer is oriented such that the illumination light  452  with it&#39;s polarization state is reflected and the image light  414  with it&#39;s other polarization state is transmitted. Since the dark state light from the “off pixels  410  also passes through the quarter wave film  454  twice, it is also the other polarization state (e.g. P polarization state) so that it is transmitted by the reflective polarizer  450 . 
     The angles of the faces of the wedge set  450  correspond to the needed angles to provide illumination light  452  at the angle needed by the DLP mirrors when in the “on” state so that the reflected image light  414  is reflected from the DLP along the optical axis of the field lens. The wedge set  456  provides an interior interface where a reflective polarizer film can be located to redirect the illumination light  452  toward the mirrors of the DLP  402 . The wedge set also provides a matched wedge on the opposite side of the reflective polarizer  450  so that the image light  414  from the “on” pixels exits the wedge set  450  substantially perpendicular to the exit surface, while the dark state light from the ‘off’pixels  410  exits at an oblique angle to the exit surface. As a result, the image light  414  is substantially unrefracted upon exiting the wedge set  456 , while the dark state light from the “off” pixels  410  is substantially refracted upon exiting the wedge set  456  as shown in  FIG. 4   b.    
     By providing a solid transparent matched wedge set, the flatness of the interface is reduced, because variations in the flatness have a negligible effect as long as they are within the cone angle of the illuminating light  452 . Which can be f#2.2 with a 26 degree cone angle. In a preferred embodiment, the reflective polarizer is bonded between the matched internal surfaces of the wedge set  456  using an optical adhesive so that Fresnel reflections at the interfaces on either side of the reflective polarizer  450  are reduced. The optical adhesive can be matched in refractive index to the material of the wedge set  456  and the pieces of the wedge set  456  can be all made from the same material such as BK7 glass or cast acrylic. Wherein the wedge material can be selected to have low birefringence as well to reduce non-uniformities in brightness. The wedge set  456  and the quarter wave film  454  can also be bonded to the DLP  402  to further reduce Fresnel reflections at the DLP interface losses. In addition, since the image light  414  is substantially normal to the exit surface of the wedge set  456 , the flatness of the surface is not critical to maintain the wavefront of the image light  414  so that high image quality can be obtained in the displayed image without requiring very tightly toleranced flatness on the exit surface. 
     A yet further embodiment of the invention that is not illustrated, combines the embodiments illustrated in  FIG. 4 b    and  FIG. 5 . In this embodiment, the wedge set  456  is comprised of three wedges with the general geometry of the wedges in the wedge set corresponding to that shown in  FIGS. 5 and 5   a . A reflective polarizer is bonded between the first and second wedges similar to that shown in  FIG. 4 b   , however, a third wedge is provided similar to the embodiment of  FIG. 5 . Wherein there is an angled thin air gap between the second and third wedges so that the dark state light is reflected by TIR toward the side of the second wedge where it is absorbed in a light trap. This embodiment, like the embodiment shown in  FIG. 4 b   , uses a polarized light source as has been previously described. The difference in this embodiment is that the image light is transmitted through the reflective polarizer and is transmitted through the angled thin air gap so that it exits normal to the exit surface of the third wedge. 
       FIG. 5 b    illustrates an upper optical module  202  with a dark light trap  514   a . As described in connection with  FIGS. 4 and 4   a , image light can be generated from a DLP when using a TIR and corrective lens configuration. The upper module may be mounted in a HWC housing  510  and the housing  510  may include a dark light trap  514   a . The dark light trap  514   a  is generally positioned/constructed/formed in a position that is optically aligned with the dark light optical axis  512 . As illustrated, the dark light trap may have depth such that the trap internally reflects dark light in an attempt to further absorb the light and prevent the dark light from combining with the image light that passes through the field lens. The dark light trap may be of a shape and depth such that it absorbs the dark light. In addition, the dark light trap  514   b , in embodiments, may be made of light absorbing materials or coated with light absorbing materials. In embodiments, the recessed light trap  514   a  may include baffles to block a view of the dark state light. This may be combined with black surfaces and textured or fiberous surfaces to help absorb the light. The baffles can be part of the light trap, associated with the housing, or field lens, etc. 
       FIG. 5 c    illustrates another embodiment with a light trap  514   b . As can be seen in the illustration, the shape of the trap is configured to enhance internal reflections within the light trap  514   b  to increase the absorption of the dark light  512 .  FIG. 5 d    illustrates another embodiment with a light trap  514   c . As can be seen in the illustration, the shape of the trap  514   c  is configured to enhance internal reflections to increase the absorption of the dark light  512 . 
       FIG. 5 e    illustrates another embodiment of an upper optical module  202  with a dark light trap  514   d . This embodiment of upper module  202  includes an off light reflection wedge  502 , as illustrated and described in connection with the embodiment of  FIG. 5 and 5   a . As can be seen in  FIG. 5 e   , the light trap  514   d  is positioned along the optical path of the dark light  512 . The dark light trap  514   d  may be configured as described in other embodiments herein. The embodiment of the light trap  514   d  illustrated in  FIG. 5 e    includes a black area on the side wall of the wedge, wherein the side wall is located substantially away from the optical axis of the image light  414 . In addition, baffles  5252  may be added to one or more edges of the field lens  312  to block the view of the light trap  514   d  adjacent to the displayed image seen by the user. 
       FIG. 6  illustrates a combination of an upper optical module  202  with a lower optical module  204 . In this embodiment, the image light projected from the upper optical module  202  may or may not be polarized. The image light is reflected off a flat combiner element  602  such that it is directed towards the user&#39;s eye. Wherein, the combiner element  602  is a partial mirror that reflects image light while transmitting a substantial portion of light from the environment so the user can look through the combiner element and see the environment surrounding the HWC. 
     The combiner  602  may include a holographic pattern, to form a holographic mirror. If a monochrome image is desired, there may be a single wavelength reflection design for the holographic pattern on the surface of the combiner  602 . If the intention is to have multiple colors reflected from the surface of the combiner  602 , a multiple wavelength holographic mirror maybe included on the combiner surface. For example, in a three-color embodiment, where red, green and blue pixels are generated in the image light, the holographic mirror may be reflective to wavelengths substantially matching the wavelengths of the red, green and blue light provided by the light source. This configuration can be used as a wavelength specific mirror where pre-determined wavelengths of light from the image light are reflected to the user&#39;s eye. This configuration may also be made such that substantially all other wavelengths in the visible pass through the combiner element  602  so the user has a substantially clear view of the surroundings when looking through the combiner element  602 . The transparency between the user&#39;s eye and the surrounding may be approximately 80% when using a combiner that is a holographic mirror. Wherein holographic mirrors can be made using lasers to produce interference patterns in the holographic material of the combiner where the wavelengths of the lasers correspond to the wavelengths of light that are subsequently reflected by the holographic mirror. 
     In another embodiment, the combiner element  602  may include a notch mirror comprised of a multilayer coated substrate wherein the coating is designed to substantially reflect the wavelengths of light provided by the light source and substantially transmit the remaining wavelengths in the visible spectrum. For example, in the case where red, green and blue light is provided by the light source to enable full color images to be provided to the user, the notch mirror is a tristimulus notch mirror wherein the multilayer coating is designed to reflect narrow bands of red, green and blue light that are matched to the what is provided by the light source and the remaining visible wavelengths are transmitted through the coating to enable a view of the environment through the combiner. In another example where monochrome images are provided to the user, the notch mirror is designed to reflect a single narrow band of light that is matched to the wavelength range of the light provided by the light source while transmitting the remaining visible wavelengths to enable a see-thru view of the environment. The combiner  602  with the notch mirror would operate, from the user&#39;s perspective, in a manner similar to the combiner that includes a holographic pattern on the combiner element  602 . The combiner, with the tristimulus notch mirror, would reflect the “on” pixels to the eye because of the match between the reflective wavelengths of the notch mirror and the color of the image light, and the wearer would be able to see with high clarity the surroundings. The transparency between the user&#39;s eye and the surrounding may be approximately 80% when using the tristimulus notch mirror. In addition, the image provided by the upper optical module  202  with the notch mirror combiner can provide higher contrast images than the holographic mirror combiner due to less scattering of the imaging light by the combiner. 
     Light can escape through the combiner  602  and may produce face glow as the light is generally directed downward onto the cheek of the user. When using a holographic mirror combiner or a tristimulus notch mirror combiner, the escaping light can be trapped to avoid face glow. In embodiments, if the image light is polarized before the combiner, a linear polarizer can be laminated, or otherwise associated, to the combiner, with the transmission axis of the polarizer oriented relative to the polarized image light so that any escaping image light is absorbed by the polarizer. In embodiments, the image light would be polarized to provide S polarized light to the combiner for better reflection. As a result, the linear polarizer on the combiner would be oriented to absorb S polarized light and pass P polarized light. This provides the preferred orientation of polarized sunglasses as well. 
     If the image light is unpolarized, a microlouvered film such as a privacy filter can be used to absorb the escaping image light while providing the user with a see-thru view of the environment. In this case, the absorbance or transmittance of the microlouvered film is dependent on the angle of the light. Where steep angle light is absorbed and light at less of an angle is transmitted. For this reason, in an embodiment, the combiner with the microlouver film is angled at greater than 45 degrees to the optical axis of the image light (e.g. the combiner can be oriented at 50 degrees so the image light from the file lens is incident on the combiner at an oblique angle. 
       FIG. 7  illustrates an embodiment of a combiner element  602  at various angles when the combiner element  602  includes a holographic mirror. Normally, a mirrored surface reflects light at an angle equal to the angle that the light is incident to the mirrored surface. Typically, this necessitates that the combiner element be at 45 degrees,  602   a , if the light is presented vertically to the combiner so the light can be reflected horizontally towards the wearer&#39;s eye. In embodiments, the incident light can be presented at angles other than vertical to enable the mirror surface to be oriented at other than 45 degrees, but in all cases wherein a mirrored surface is employed (including the tristimulus notch mirror described previously), the incident angle equals the reflected angle. As a result, increasing the angle of the combiner  602   a  requires that the incident image light be presented to the combiner  602   a  at a different angle which positions the upper optical module  202  to the left of the combiner as shown in  FIG. 7 . In contrast, a holographic mirror combiner, included in embodiments, can be made such that light is reflected at a different angle from the angle that the light is incident onto the holographic mirrored surface. This allows freedom to select the angle of the combiner element  602   b  independent of the angle of the incident image light and the angle of the light reflected into the wearer&#39;s eye. In embodiments, the angle of the combiner element  602   b  is greater than 45 degrees (shown in  FIG. 7 ) as this allows a more laterally compact HWC design. The increased angle of the combiner element  602   b  decreases the front to back width of the lower optical module  204  and may allow for a thinner HWC display (i.e. the furthest element from the wearer&#39;s eye can be closer to the wearer&#39;s face). 
       FIG. 8  illustrates another embodiment of a lower optical module  204 . In this embodiment, polarized image light provided by the upper optical module  202 , is directed into the lower optical module  204 . The image light reflects off a polarized mirror  804  and is directed to a focusing partially reflective mirror  802 , which is adapted to reflect the polarized light. An optical element such as a ¼ wave film located between the polarized mirror  804  and the partially reflective mirror  802 , is used to change the polarization state of the image light such that the light reflected by the partially reflective mirror  802  is transmitted by the polarized mirror  804  to present image light to the eye of the wearer. The user can also see through the polarized mirror  804  and the partially reflective mirror  802  to see the surrounding environment. As a result, the user perceives a combined image comprised of the displayed image light overlaid onto the see-thru view of the environment. 
     While many of the embodiments of the present invention have been referred to as upper and lower modules containing certain optical components, it should be understood that the image light and dark light production and management functions described in connection with the upper module may be arranged to direct light in other directions (e.g. upward, sideward, etc.). In embodiments, it may be preferred to mount the upper module  202  above the wearer&#39;s eye, in which case the image light would be directed downward. In other embodiments it may be preferred to produce light from the side of the wearer&#39;s eye, or from below the wearer&#39;s eye. In addition, the lower optical module is generally configured to deliver the image light to the wearer&#39;s eye and allow the wearer to see through the lower optical module, which may be accomplished through a variety of optical components. 
       FIG. 8 a    illustrates an embodiment of the present invention where the upper optical module  202  is arranged to direct image light into a TIR waveguide  810 . In this embodiment, the upper optical module  202  is positioned above the wearer&#39;s eye  812  and the light is directed horizontally into the TIR waveguide  810 . The TIR waveguide is designed to internally reflect the image light in a series of downward TIR reflections until it reaches the portion in front of the wearer&#39;s eye, where the light passes out of the TIR waveguide  812  into the wearer&#39;s eye. In this embodiment, an outer shield  814  is positioned in front of the TIR waveguide  810 . 
       FIG. 8 b    illustrates an embodiment of the present invention where the upper optical module  202  is arranged to direct image light into a TIR waveguide  818 . In this embodiment, the upper optical module  202  is arranged on the side of the TIR waveguide  818 . For example, the upper optical module may be positioned in the arm or near the arm of the HWC when configured as a pair of head worn glasses. The TIR waveguide  818  is designed to internally reflect the image light in a series of TIR reflections until it reaches the portion in front of the wearer&#39;s eye, where the light passes out of the TIR waveguide  812  into the wearer&#39;s eye. 
       FIG. 8 c    illustrates yet further embodiments of the present invention where an upper optical module  202  is directing polarized image light into an optical guide  828  where the image light passes through a polarized reflector  824 , changes polarization state upon reflection of the optical element  822  which includes a ¼ wave film for example and then is reflected by the polarized reflector  824  towards the wearer&#39;s eye, due to the change in polarization of the image light. The upper optical module  202  may be positioned to direct light to a mirror  820 , to position the upper optical module  202  laterally, in other embodiments, the upper optical module  202  may direct the image light directly towards the polarized reflector  824 . It should be understood that the present invention comprises other optical arrangements intended to direct image light into the wearer&#39;s eye. 
     Another aspect of the present invention relates to eye imaging. In embodiments, a camera is used in connection with an upper optical module  202  such that the wearer&#39;s eye can be imaged using pixels in the “off” state on the DLP.  FIG. 9  illustrates a system where the eye imaging camera  802  is mounted and angled such that the field of view of the eye imaging camera  802  is redirected toward the wearer&#39;s eye by the mirror pixels of the DLP  402  that are in the “off” state. In this way, the eye imaging camera  802  can be used to image the wearer&#39;s eye along the same optical axis as the displayed image that is presented to the wearer. Wherein, image light that is presented to the wearer&#39;s eye illuminates the wearer&#39;s eye so that the eye can be imaged by the eye imaging camera  802 . In the process, the light reflected by the eye passes back though the optical train of the lower optical module  204  and a portion of the upper optical module to where the light is reflected by the “off” pixels of the DLP  402  toward the eye imaging camera  802 . 
     In embodiments, the eye imaging camera may image the wearer&#39;s eye at a moment in time where there are enough “off” pixels to achieve the required eye image resolution. In another embodiment, the eye imaging camera collects eye image information from “off” pixels over time and forms a time lapsed image. In another embodiment, a modified image is presented to the user wherein enough “off” state pixels are included that the camera can obtain the desired resolution and brightness for imaging the wearer&#39;s eye and the eye image capture is synchronized with the presentation of the modified image. 
     The eye imaging system may be used for security systems. The HWC may not allow access to the HWC or other system if the eye is not recognized (e.g. through eye characteristics including retina or iris characteristics, etc.). The HWC may be used to provide constant security access in some embodiments. For example, the eye security confirmation may be a continuous, near-continuous, real-time, quasi real-time, periodic, etc. process so the wearer is effectively constantly being verified as known. In embodiments, the HWC may be worn and eye security tracked for access to other computer systems. 
     The eye imaging system may be used for control of the HWC. For example, a blink, wink, or particular eye movement may be used as a control mechanism for a software application operating on the HWC or associated device. 
     The eye imaging system may be used in a process that determines how or when the HWC  102  delivers digitally displayed content to the wearer. For example, the eye imaging system may determine that the user is looking in a direction and then HWC may change the resolution in an area of the display or provide some content that is associated with something in the environment that the user may be looking at. Alternatively, the eye imaging system may identify different user&#39;s and change the displayed content or enabled features provided to the user. User&#39;s may be identified from a database of users eye characteristics either located on the HWC  102  or remotely located on the network  110  or on a server  112 . In addition, the HWC may identify a primary user or a group of primary users from eye characteristics wherein the primary user(s) are provided with an enhanced set of features and all other user&#39;s are provided with a different set of features. Thus in this use case, the HWC  102  uses identified eye characteristics to either enable features or not and eye characteristics need only be analyzed in comparison to a relatively small database of individual eye characteristics. 
       FIG. 10  illustrates a light source that may be used in association with the upper optics module  202  (e.g. polarized light source if the light from the solid state light source is polarized such as polarized light source  302  and  458 ), and light source  404 . In embodiments, to provide a uniform surface of light  1008  to be directed into the upper optical module  202  and towards the DLP of the upper optical module, either directly or indirectly, the solid state light source  1002  may be projected into a backlighting optical system  1004 . The solid state light source  1002  may be one or more LEDs, laser diodes, OLEDs. In embodiments, the backlighting optical system  1004  includes an extended section with a length/distance ratio of greater than 3, wherein the light undergoes multiple reflections from the sidewalls to mix of homogenize the light as supplied by the solid state light source  1002 . The backlighting optical system  1004  can also include structures on the surface opposite (on the left side as shown in  FIG. 10 ) to where the uniform light  1008  exits the backlight  1004  to change the direction of the light toward the DLP  302  and the reflective polarizer  310  or the DLP  402  and the TIR wedge  418 . The backlighting optical system  1004  may also include structures to collimate the uniform light  1008  to provide light to the DLP with a smaller angular distribution or narrower cone angle. Diffusers or polarizers can be used on the entrance or exit surface of the backlighting optical system. Diffusers can be used to spread or uniformize the exiting light from the backlight to improve the uniformity or increase the angular spread of the uniform light  1008 . Elliptical diffusers that diffuse the light more in some directions and less in others can be used to improve the uniformity or spread of the uniform light  1008  in directions orthogonal to the optical axis of the uniform light  1008 . Linear polarizers can be used to convert unpolarized light as supplied by the solid state light source  1002  to polarized light so the uniform light  1008  is polarized with a desired polarization state. A reflective polarizer can be used on the exit surface of the backlight  1004  to polarize the uniform light  1008  to the desired polarization state, while reflecting the other polarization state back into the backlight where it is recycled by multiple reflections within the backlight  1004  and at the solid state light source  1002 . Therefore by including a reflective polarizer at the exit surface of the backlight  1004 , the efficiency of the polarized light source is improved. 
       FIGS. 10 a  and 10 b    show illustrations of structures in backlight optical systems  1004  that can be used to change the direction of the light provided to the entrance face  1045  by the light source and then collimates the light in a direction lateral to the optical axis of the exiting uniform light  1008 . Structure  1060  includes an angled sawtooth pattern in a transparent waveguide wherein the left edge of each sawtooth clips the steep angle rays of light thereby limiting the angle of the light being redirected. The steep surface at the right (as shown) of each sawtooth then redirects the light so that it reflects off the left angled surface of each sawtooth and is directed toward the exit surface  1040 . The sawtooth surfaces shown on the lower surface in  FIGS. 10 a  and 10 b   , can be smooth and coated (e.g. with an aluminum coating or a dielectric mirror coating) to provide a high level of reflectivity without scattering. Structure  1050  includes a curved face on the left side (as shown) to focus the rays after they pass through the exit surface  1040 , thereby providing a mechanism for collimating the uniform light  1008 . In a further embodiment, a diffuser can be provided between the solid state light source  1002  and the entrance face  1045  to homogenize the light provided by the solid state light source  1002 . In yet a further embodiment, a polarizer can be used between the diffuser and the entrance face  1045  of the backlight  1004  to provide a polarized light source. Because the sawtooth pattern provides smooth reflective surfaces, the polarization state of the light can be preserved from the entrance face  1045  to the exit face  1040 . In this embodiment, the light entering the backlight from the solid state light source  1002  passes through the polarizer so that it is polarized with the desired polarization state. If the polarizer is an absorptive linear polarizer, the light of the desired polarization state is transmitted while the light of the other polarization state is absorbed. If the polarizer is a reflective polarizer, the light of the desired polarization state is transmitted into the backlight  1004  while the light of the other polarization state is reflected back into the solid state light source  1002  where it can be recycled as previously described, to increase the efficiency of the polarized light source. 
       FIG. 11 a    illustrates a light source  1100  that may be used in association with the upper optics module  202 . In embodiments, the light source  1100  may provide light to a backlighting optical system  1004  as described above in connection with  FIG. 10 . In embodiments, the light source  1100  includes a tristimulus notch filter  1102 . The tristimulus notch filter  1102  has narrow band pass filters for three wavelengths, as indicated in  FIG. 11 c    in a transmission graph  1108 . The graph shown in  FIG. 11 b   , as  1104  illustrates an output of three different colored LEDs. One can see that the bandwidths of emission are narrow, but they have long tails. The tristimulus notch filter  1102  can be used in connection with such LEDs to provide a light source  1100  that emits narrow filtered wavelengths of light as shown in  FIG. 11 d    as the transmission graph  1110 . Wherein the clipping effects of the tristimulus notch filter  1102  can be seen to have cut the tails from the LED emission graph  1104  to provide narrower wavelength bands of light to the upper optical module  202 . The light source  1100  can be used in connection with a combiner  602  with a holographic mirror or tristimulus notch mirror to provide narrow bands of light that are reflected toward the wearer&#39;s eye with less waste light that does not get reflected by the combiner, thereby improving efficiency and reducing escaping light that can cause faceglow. 
       FIG. 12 a    illustrates another light source  1200  that may be used in association with the upper optics module  202 . In embodiments, the light source  1200  may provide light to a backlighting optical system  1004  as described above in connection with  FIG. 10 . In embodiments, the light source  1200  includes a quantum dot cover glass  1202 . Where the quantum dots absorb light of a shorter wavelength and emit light of a longer wavelength ( FIG. 12 b    shows an example wherein a UV spectrum  1202  applied to a quantum dot results in the quantum dot emitting a narrow band shown as a PL spectrum  1204 ) that is dependent on the material makeup and size of the quantum dot. As a result, quantum dots in the quantum dot cover glass  1202  can be tailored to provide one or more bands of narrow bandwidth light (e.g. red, green and blue emissions dependent on the different quantum dots included as illustrated in the graph shown in  FIG. 12 c    where three different quantum dots are used. In embodiments, the LED driver light emits UV light, deep blue or blue light. For sequential illumination of different colors, multiple light sources  1200  would be used where each light source  1200  would include a quantum dot cover glass  1202  with a quantum dot selected to emit at one of the desired colors. The light source  1100  can be used in connection with a combiner  602  with a holographic mirror or tristimulus notch mirror to provide narrow transmission bands of light that are reflected toward the wearer&#39;s eye with less waste light that does not get reflected. 
     Another aspect of the present invention relates to the generation of peripheral image lighting effects for a person wearing a HWC. In embodiments, a solid state lighting system (e.g. LED, OLED, etc), or other lighting system, may be included inside the optical elements of an lower optical module  204 . The solid state lighting system may be arranged such that lighting effects outside of a field of view (FOV) of the presented digital content is presented to create an emersive effect for the person wearing the HWC. To this end, the lighting effects may be presented to any portion of the HWC that is visible to the wearer. The solid state lighting system may be digitally controlled by an integrated processor on the HWC. In embodiments, the integrated processor will control the lighting effects in coordination with digital content that is presented within the FOV of the HWC. For example, a movie, picture, game, or other content, may be displayed or playing within the FOV of the HWC. The content may show a bomb blast on the right side of the FOV and at the same moment, the solid state lighting system inside of the upper module optics may flash quickly in concert with the FOV image effect. The effect may not be fast, it may be more persistent to indicate, for example, a general glow or color on one side of the user. The solid state lighting system may be color controlled, with red, green and blue LEDs, for example, such that color control can be coordinated with the digitally presented content within the field of view. 
       FIG. 13 a    illustrates optical components of a lower optical module  204  together with an outer lens  1302 .  FIG. 13 a    also shows an embodiment including effects LED&#39;s  1308   a  and  1308   b .  FIG. 13 a    illustrates image light  1312 , as described herein elsewhere, directed into the upper optical module where it will reflect off of the combiner element  1304 , as described herein elsewhere. The combiner element  1304  in this embodiment is angled towards the wearer&#39;s eye at the top of the module and away from the wearer&#39;s eye at the bottom of the module, as also illustrated and described in connection with  FIG. 8  (e.g. at a 45 degree angle). The image light  1312  provided by an upper optical module  202  (not shown in  FIG. 13 a   ) reflects off of the combiner element  1304  towards the collimating mirror  1310 , away from the wearer&#39;s eye, as described herein elsewhere. The image light  1312  then reflects and focuses off of the collimating mirror  1304 , passes back through the combiner element  1304 , and is directed into the wearer&#39;s eye. The wearer can also view the surrounding environment through the transparency of the combiner element  1304 , collimating mirror  1310 , and outer lens  1302  (if it is included). As described herein elsewhere, various surfaces are polarized to create the optical path for the image light and to provide transparency of the elements such that the wearer can view the surrounding environment. The wearer will generally perceive that the image light forms an image in the FOV  1305 . In embodiments, the outer lens  1302  may be included. The outer lens  1302  is an outer lens that may or may not be corrective and it may be designed to conceal the lower optical module components in an effort to make the HWC appear to be in a form similar to standard glasses or sunglasses. 
     In the embodiment illustrated in  FIG. 13 a   , the effects LEDs  1308   a  and  1308   b  are positioned at the sides of the combiner element  1304  and the outer lens  1302  and/or the collimating mirror  1310 . In embodiments, the effects LEDs  1308   a  are positioned within the confines defined by the combiner element  1304  and the outer lens  1302  and/or the collimating mirror. The effects LEDs  1308   a  and  1308   b  are also positioned outside of the FOV  1305 . In this arrangement, the effects LEDs  1308   a  and  1308   b  can provide lighting effects within the lower optical module outside of the FOV  1305 . In embodiments the light emitted from the effects LEDs  1308   a  and  1308   b  may be polarized such that the light passes through the combiner element  1304  toward the wearer&#39;s eye and does not pass through the outer lens  1302  and/or the collimating mirror  1310 . This arrangement provides peripheral lighting effects to the wearer in a more private setting by not transmitting the lighting effects through the front of the HWC into the surrounding environment. However, in other embodiments, the effects LEDs  1308   a  and  1308   b  may be unpolarized so the lighting effects provided are made to be purposefully viewable by others in the environment for entertainment such as giving the effect of the wearer&#39;s eye glowing in correspondence to the image content being viewed by the wearer. 
       FIG. 13 b    illustrates a cross section of the embodiment described in connection with  FIG. 13 a   . As illustrated, the effects LED  1308   a  is located in the upper-front area inside of the optical components of the lower optical module. It should be understood that the effects LED  1308   a  position in the described embodiments is only illustrative and alternate placements are encompassed by the present invention. Additionallly, in embodiments, there may be one or more effects LEDs  1308   a  in each of the two sides of HWC to provide peripheral lighting effects near one or both eyes of the wearer. 
       FIG. 13 c    illustrates an embodiment where the combiner element  1304  is angled away from the eye at the top and towards the eye at the bottom (e.g. in accordance with the holographic or notch filter embodiments described herein). In this embodiment, the effects LED  1308   a  is located on the outer lens  1302  side of the combiner element  1304  to provide a concealed appearance of the lighting effects. As with other embodiments, the effects LED  1308   a  of  FIG. 13 c    may include a polarizer such that the emitted light can pass through a polarized element associated with the combiner element  1304  and be blocked by a polarized element associated with the outer lens  1302 . 
     Another aspect of the present invention relates to the mitigation of light escaping from the space between the wearer&#39;s face and the HWC itself. Another aspect of the present invention relates to maintaining a controlled lighting environment in proximity to the wearer&#39;s eyes. In embodiments, both the maintenance of the lighting environment and the mitigation of light escape are accomplished by including a removable and replaceable flexible shield for the HWC. Wherein the removable and replaceable shield can be provided for one eye or both eyes in correspondence to the use of the displays for each eye. For example, in a night vision application, the display to only one eye could be used for night vision while the display to the other eye is turned off to provide good see-thru when moving between areas where visible light is available and dark areas where night vision enhancement is needed. 
       FIG. 14 a    illustrates a removable and replaceable flexible eye cover  1402  with an opening  1408  that can be attached and removed quickly from the HWC  102  through the use of magnets  1404 . Other attachment methods may be used, but for illustration of the present invention we will focus on a magnet implementation. In embodiments, magnets may be included in the eye cover  1402  and magnets of an opposite polarity may be included (e.g. embedded) in the frame of the HWC  102 . The magnets of the two elements would attract quite strongly with the opposite polarity configuration. In another embodiment, one of the elements may have a magnet and the other side may have metal for the attraction. In embodiments, the eye cover  1402  is a flexible elastomeric shield. In embodiments, the eye cover  1402  may be an elastomeric bellows design to accommodate flexibility and more closely align with the wearer&#39;s face.  FIG. 14 b    illustrates a removable and replaceable flexible eye cover  1402  that is adapted as a single eye cover. In embodiments, a single eye cover may be used for each side of the HWC to cover both eyes of the wearer. In embodiments, the single eye cover may be used in connection with a HWC that includes only one computer display for one eye. These configurations prevent light that is generated and directed generally towards the wearer&#39;s face by covering the space between the wearer&#39;s face and the HWC. The opening  1408  allows the wearer to look through the opening  1408  to view the displayed content and the surrounding environment through the front of the HWC. The image light in the lower optical module  204  can be prevented from emitting from the front of the HWC through internal optics polarization schemes, as described herein, for example. 
       FIG. 14 c    illustrates another embodiment of a light suppression system. In this embodiment, the eye cover  1410  may be similar to the eye cover  1402 , but eye cover  1410  includes a front light shield  1412 . The front light shield  1412  may be opaque to prevent light from escaping the front lens of the HWC. In other embodiments, the front light shield  1412  is polarized to prevent light from escaping the front lens. In a polarized arrangement, in embodiments, the internal optical elements of the HWC (e.g. of the lower optical module  204 ) may polarize light transmitted towards the front of the HWC and the front light shield  1412  may be polarized to prevent the light from transmitting through the front light shield  1412 . 
     In embodiments, an opaque front light shield  1412  may be included and the digital content may include images of the surrounding environment such that the wearer can visualize the surrounding environment. One eye may be presented with night vision environmental imagery and this eye&#39;s surrounding environment optical path may be covered using an opaque front light shield  1412 . In other embodiments, this arrangement may be associated with both eyes. 
     Another aspect of the present invention relates to automatically configuring the lighting system(s) used in the HWC  102 . In embodiments, the display lighting and/or effects lighting, as described herein, may be controlled in a manner suitable for when an eye cover  1402  is attached or removed from the HWC  102 . For example, at night, when the light in the environment is low, the lighting system(s) in the HWC may go into a low light mode to further control any amounts of stray light escaping from the HWC and the areas around the HWC. Covert operations at night, while using night vision or standard vision, may require a solution which prevents as much escaping light as possible so a user may clip on the eye cover(s)  1402  and then the HWC may go into a low light mode. The low light mode may, in some embodiments, only go into a low light mode when the eye cover  1402  is attached if the HWC identifies that the environment is in low light conditions (e.g. through environment light level sensor detection). In embodiments, the low light level may be determined to be at an intermediate point between full and low light dependent on environmental conditions. 
     Another aspect of the present invention relates to automatically controlling the type of content displayed in the HWC when eye covers  1402  are attached or removed from the HWC. In embodiments, when the eye cover(s)  1402  is attached to the HWC, the displayed content may be restricted in amount or in color amounts. For example, the display(s) may go into a simple content delivery mode to restrict the amount of information displayed. This may be done to reduce the amount of light produced by the display(s). In an embodiment, the display(s) may change from color displays to monochrome displays to reduce the amount of light produced. In an embodiment, the monochrome lighting may be red to limit the impact on the wearer&#39;s eyes to maintain an ability to see better in the dark. 
     Another aspect of the present invention relates to a system adapted to quickly convert from a see-through system to a non-see-through or very low transmission see-through system for a more immersive user experience. The conversion system may include replaceable lenses, an eye cover, and optics adapted to provide user experiences in both modes. The lenses, for example, may be ‘blacked-out’ to provide an experience where all of the user&#39;s attention is dedicated to the digital content and then the lenses may be switched out for high see-through lenses so the digital content is augmenting the user&#39;s view of the surrounding environment. Another aspect of the invention relates to low transmission lenses that permit the user to see through the lenses but remain dark enough to maintain most of the user&#39;s attention on the digital content. The slight see-through can provide the user with a visual connection to the surrounding environment and this can reduce or eliminate nausea and other problems associated with total removal of the surrounding view when viewing digital content. 
       FIG. 14 d    illustrates a head-worn computer system  102  with a see-through digital content display  204  adapted to include a removable outer lens  1414  and a removable eye cover  1402 . The eye cover  1402  may be attached to the head-worn computer  102  with magnets  1404  or other attachment systems (e.g. mechanical attachments, a snug friction fit between the arms of the head-worn computer  102 , etc.). The eye cover  1402  may be attached when the user wants to cut stray light from escaping the confines of the head-worn computer, create a more immersive experience by removing the otherwise viewable peripheral view of the surrounding environment, etc. The removable outer lens may be of several varieties for various experiences. It may have no transmission or a very low transmission to create a dark background for the digital content, creating an immersive experience for the digital content. It may have a high transmission so the user can see through the see-through display and the lens to view the surrounding environment, creating a system for a heads-up display, augmented reality display, assisted reality display, etc. The lens  1414  may be dark in a middle portion to provide a dark background for the digital content (i.e. dark backdrop behind the see-through field of view from the user&#39;s perspective) and a higher transmission area elsewhere. The lenses  1414  may have a transmission in the range of 2 to 5%, 5 to 10%, 10 to 20% for the immersion effect and above 10% or 20% for the augmented reality effect, for example. The lenses  1414  may also have an adjustable transmission to facilitate the change in system effect. For example, the lenses  1414  may be electronically adjustable tint lenses (e.g. liquid crystal or have crossed polarizers with an adjustment for the level of cross). 
     In embodiments, the eye cover may have areas of transparency or partial transparency to provide some visual connection with the user&#39;s surrounding environment. This may also reduce or eliminate nausea or other feelings associated with the complete removal of the view of the surrounding environment. 
       FIG. 14 e    illustrates a head-worn computer  102  assembled with an eye cover  1402  without lenses in place. The lenses, in embodiments, may be held in place with magnets  1418  for ease of removal and replacement. In embodiments, the lenses may be held in place with other systems, such as mechanical systems. 
     Another aspect of the present invention relates to an effects system that generates effects outside of the field of view in the see-through display of the head-worn computer. The effects may be, for example, lighting effects, sound effects, tactile effects (e.g. through vibration), air movement effects, etc. In embodiments, the effect generation system is mounted on the eye cover  1402 . For example, a lighting system (e.g. LED(s), OLEDs, etc.) may be mounted on an inside surface  1420 , or exposed through the inside surface  1420 , as illustrated in  FIG. 14 f   , such that they can create a lighting effect (e.g. a bright light, colored light, subtle color effect) in coordination with content being displayed in the field of view of the see-through display. The content may be a movie or a game, for example, and an explosion may happen on the right side of the content, as scripted, and matching the content, a bright flash may be generated by the effects lighting system to create a stronger effect. As another example, the effects system may include a vibratory system mounted near the sides or temples, or otherwise, and when the same explosion occurs, the vibratory system may generate a vibration on the right side to increase the user experience indicating that the explosion had a real sound wave creating the vibration. As yet a further example, the effects system may have am air system where the effect is a puff of air blown onto the user&#39;s face. This may create a feeling of closeness with some fast moving object in the content. The effects system may also have speakers directed towards the user&#39;s ears or an attachment for ear buds, etc. 
     In embodiments, the effects generated by the effects system may be scripted by an author to coordinate with the content. In embodiments, sensors may be placed inside of the eye cover to monitor content effects (e.g. a light sensor to measure strong lighting effects or peripheral lighting effects) that would than cause an effect(s) to be generated. 
     The effects system in the eye cover may be powered by an internal battery and the battery, in embodiments, may also provide additional power to the head-worn computer  102  as a back-up system. In embodiments, the effects system is powered by the batteries in the head-worn computer. Power may be delivered through the attachment system (e.g. magnets, mechanical system) or a dedicated power system. 
     The effects system may receive data and/or commands from the head-worn computer through a data connection that is wired or wireless. The data may come through the attachment system, a separate line, or through Bluetooth or other short range communication protocol, for example. 
     In embodiments, the eye cover is made of reticulated foam, which is very light and can contour to the user&#39;s face. The reticulated foam also allows air to circulate because of the open-celled nature of the material, which can reduce user fatigue and increase user comfort. The eye cover may be made of other materials, soft, stiff, priable, etc. and may have another material on the periphery that contacts the face for comfort. In embodiments, the eye cover may include a fan to exchange air between an external environment and an internal space, where the internal space is defined in part by the face of the user. The fan may operate very slowly and at low power to exchange the air to keep the face of the user cool. In embodiments the fan may have a variable speed controller and/or a temperature sensor may be positioned to measure temperature in the internal space to control the temperature in the internal space to a specified range, temperature, etc. The internal space is generally characterized by the space confined space in front of the user&#39;s eyes and upper cheeks where the eye cover encloses the area. 
     Another aspect of the present invention relates to flexibly mounting an audio headset on the head-worn computer  102  and/or the eye cover  1402 . In embodiments, the audio headset is mounted with a relatively rigid system that has flexible joint(s) (e.g. a rotational joint at the connection with the eye cover, a rotational joint in the middle of a rigid arm, etc.) and extension(s) (e.g. a telescopic arm) to provide the user with adjustability to allow for a comfortable fit over, in or around the user&#39;s ear. In embodiments, the audio headset is mounted with a flexible system that is more flexible throughout, such as with a wire-based connection. 
       FIG. 14 g    illustrates a head-worn computer  102  with removable lenses  1414  along with a mounted eye cover  1402 . The head-worn computer, in embodiments, includes a see-through display (as disclosed herein). The eye cover  1402  also includes a mounted audio headset  1422 . The mounted audio headset  1422  in this embodiment is mounted to the eye cover  1402  and has audio wire connections (not shown). In embodiments, the audio wires&#39; connections may connect to an internal wireless communication system (e.g. Bluetooth, NFC, WiFi) to make connection to the processor in the head-worn computer. In embodiments, the audio wires may connect to a magnetic connector, mechanical connector or the like to make the connection. 
       FIG. 14 h    illustrates an unmounted eye cover  1402  with a mounted audio headset  1422 . As illustrated, the mechanical design of the eye cover is adapted to fit onto the head-worn computer to provide visual isolation or partial isolation and the audio headset. 
     In embodiments, the eye cover  1402  may be adapted to be removably mounted on a head-worn computer  102  with a see-through computer display. An audio headset  1422  with an adjustable mount may be connected to the eye cover, wherein the adjustable mount may provide extension and rotation to provide a user of the head-worn computer with a mechanism to align the audio headset with an ear of the user. In embodiments, the audio headset includes an audio wire connected to a connector on the eye cover and the eye cover connector may be adapted to removably mate with a connector on the head-worn computer. In embodiments, the audio headset may be adapted to receive audio signals from the head-worn computer through a wireless connection (e.g. Bluetooth, WiFi). As described elsewhere herein, the head-worn computer may have a removable and replaceable front lens. The eye cover may include a battery to power systems internal to the eye cover. The eye cover may have a battery to power systems internal to the head-worn computer. 
     In embodiments, the eye cover may include a fan adapted to exchange air between an internal space, defined in part by the user&#39;s face, and an external environment to cool the air in the internal space and the user&#39;s face. In embodiments, the audio headset may include a vibratory system (e.g. a vibration motor, piezo motor, etc. in the armature and/or in the section over the ear) adapted to provide the user with a haptic feedback coordinated with digital content presented in the see-through computer display. In embodiments, the head-worn computer includes a vibratory system adapted to provide the user with a haptic feedback coordinated with digital content presented in the see-through computer display. 
     In embodiments, the eye cover  1402  is adapted to be removably mounted on a head-worn computer with a see-through computer display. The eye cover may also include a flexible audio headset mounted to the eye cover, wherein the flexibility provides the user of the head-worn computer with a mechanism to align the audio headset with an ear of the user. In embodiments, the flexible audio headset is mounted to the eye cover with a magnetic connection. In embodiments, the flexible audio headset may be mounted to the eye cover with a mechanical connection. 
     In embodiments, the audio head set may be spring or otherwise loaded such that the head set presses inward towards the user&#39;s ears for a more secure fit. 
     Another aspect of the present invention involves a head-worn computer with stereo scene cameras. In embodiments, two or more cameras and/or sensors are mounted behind the outer lens(es) of the head-worn computer. In embodiments, the lens(es) is tinted to conceal the cameras. In embodiments, the lens(es) has a more transparent area in front of the camera(s) to allow for high transparency scene image capture. 
       FIG. 14 i    illustrates a head-worn computer  102  with see-through computer displays, where two cameras  1424   a  and  1424   b  are mounted on a frame front surface  1428  such that they are concealed by lenses (not shown in  FIG. 14 i   ). The front surface  1428  further conceals the image engine, or upper optics. The see-through displays, in this illustration, are mounted below the frame front surface  1428 . The user&#39;s view is through the portion of the optics that is mounted below the front surface  1428 . 
     In embodiments, a head-worn computer, comprising an optical system with at least two portions, a first optical portion that generates an image and a second optical portion that provides the image to a user&#39;s eye and through which the user views a surrounding environment; wherein the first portion is substantially concealed from view by a frame above the user&#39;s eye; the frame having a front surface concealing a portion of the first optical portion, the front surface further positioned to be at least partially concealed from view by a removable lens; and a scene camera positioned to view the surrounding environment through the frame front surface. In embodiments, the head-worn computer may have a removable lens, when mounted on the head-worn computer, that conceals the scene camera. In embodiments, the head-worn computer may further comprise a second scene camera, wherein the scene camera and the second scene camera are mounted with the scene camera over a first see-through display and the second scene camera over a second see-through display. 
     Referring to  FIG. 15 , we now turn to describe a particular external user interface  104 , referred to generally as a pen  1500 . The pen  1500  is a specially designed external user interface  104  and can operate as a user interface, such as to many different styles of HWC  102 . The pen  1500  generally follows the form of a conventional pen, which is a familiar user handled device and creates an intuitive physical interface for many of the operations to be carried out in the HWC system  100 . The pen  1500  may be one of several user interfaces  104  used in connection with controlling operations within the HWC system  100 . For example, the HWC  102  may watch for and interpret hand gestures  116  as control signals, where the pen  1500  may also be used as a user interface with the same HWC  102 . Similarly, a remote keyboard may be used as an external user interface  104  in concert with the pen  1500 . The combination of user interfaces or the use of just one control system generally depends on the operation(s) being executed in the HWC&#39;s system  100 . 
     While the pen  1500  may follow the general form of a conventional pen, it contains numerous technologies that enable it to function as an external user interface  104 .  FIG. 15  illustrates technologies comprised in the pen  1500 . As can be seen, the pen  1500  may include a camera  1508 , which is arranged to view through lens  1502 . The camera may then be focused, such as through lens  1502 , to image a surface upon which a user is writing or making other movements to interact with the HWC  102 . There are situations where the pen  1500  will also have an ink, graphite, or other system such that what is being written can be seen on the writing surface. There are other situations where the pen  1500  does not have such a physical writing system so there is no deposit on the writing surface, where the pen would only be communicating data or commands to the HWC  102 . The lens configuration is described in greater detail herein. The function of the camera is to capture information from an unstructured writing surface such that pen strokes can be interpreted as intended by the user. To assist in the predication of the intended stroke path, the pen  1500  may include a sensor, such as an IMU  1512 . Of course, the IMU could be included in the pen  1500  in its separate parts (e.g. gyro, accelerometer, etc.) or an IMU could be included as a single unit. In this instance, the IMU  1512  is used to measure and predict the motion of the pen  1500 . In turn, the integrated microprocessor  1510  would take the IMU information and camera information as inputs and process the information to form a prediction of the pen tip movement. 
     The pen  1500  may also include a pressure monitoring system  1504 , such as to measure the pressure exerted on the lens  1502 . As will be described in greater detail herein, the pressure measurement can be used to predict the user&#39;s intention for changing the weight of a line, type of a line, type of brush, click, double click, and the like. In embodiments, the pressure sensor may be constructed using any force or pressure measurement sensor located behind the lens  1502 , including for example, a resistive sensor, a current sensor, a capacitive sensor, a voltage sensor such as a piezoelectric sensor, and the like. 
     The pen  1500  may also include a communications module  1518 , such as for bi-directional communication with the HWC  102 . In embodiments, the communications module  1518  may be a short distance communication module (e.g. Bluetooth). The communications module  1518  may be security matched to the HWC  102 . The communications module  1518  may be arranged to communicate data and commands to and from the microprocessor  1510  of the pen  1500 . The microprocessor  1510  may be programmed to interpret data generated from the camera  1508 , IMU  1512 , and pressure sensor  1504 , and the like, and then pass a command onto the HWC  102  through the communications module  1518 , for example. In another embodiment, the data collected from any of the input sources (e.g. camera  1508 , IMU  1512 , pressure sensor  1504 ) by the microprocessor may be communicated by the communication module  1518  to the HWC  102 , and the HWC  102  may perform data processing and prediction of the user&#39;s intention when using the pen  1500 . In yet another embodiment, the data may be further passed on through a network  110  to a remote device  112 , such as a server, for the data processing and prediction. The commands may then be communicated back to the HWC  102  for execution (e.g. display writing in the glasses display, make a selection within the UI of the glasses display, control a remote external device  112 , control a local external device  108 ), and the like. The pen may also include memory  1514  for long or short term uses. 
     The pen  1500  may also include a number of physical user interfaces, such as quick launch buttons  1522 , a touch sensor  1520 , and the like. The quick launch buttons  1522  may be adapted to provide the user with a fast way of jumping to a software application in the HWC system  100 . For example, the user may be a frequent user of communication software packages (e.g. email, text, Twitter, Instagram, Facebook, Google+, and the like), and the user may program a quick launch button  1522  to command the HWC  102  to launch an application. The pen  1500  may be provided with several quick launch buttons  1522 , which may be user programmable or factory programmable. The quick launch button  1522  may be programmed to perform an operation. For example, one of the buttons may be programmed to clear the digital display of the HWC  102 . This would create a fast way for the user to clear the screens on the HWC  102  for any reason, such as for example to better view the environment. The quick launch button functionality will be discussed in further detail below. The touch sensor  1520  may be used to take gesture style input from the user. For example, the user may be able to take a single finger and run it across the touch sensor  1520  to affect a page scroll. 
     The pen  1500  may also include a laser pointer  1524 . The laser pointer  1524  may be coordinated with the IMU  1512  to coordinate gestures and laser pointing. For example, a user may use the laser  1524  in a presentation to help with guiding the audience with the interpretation of graphics and the IMU  1512  may, either simultaneously or when the laser  1524  is off, interpret the user&#39;s gestures as commands or data input. 
       FIGS. 16A-C  illustrate several embodiments of lens and camera arrangements  1600  for the pen  1500 . One aspect relates to maintaining a constant distance between the camera and the writing surface to enable the writing surface to be kept in focus for better tracking of movements of the pen  1500  over the writing surface. Another aspect relates to maintaining an angled surface following the circumference of the writing tip of the pen  1500  such that the pen  1500  can be rolled or partially rolled in the user&#39;s hand to create the feel and freedom of a conventional writing instrument. 
       FIG. 16A  illustrates an embodiment of the writing lens end of the pen  1500 . The configuration includes a ball lens  1604 , a camera or image capture surface  1602 , and a domed cover lens  1608 . In this arrangement, the camera views the writing surface through the ball lens  1604  and dome cover lens  1608 . The ball lens  1604  causes the camera to focus such that the camera views the writing surface when the pen  1500  is held in the hand in a natural writing position, such as with the pen  1500  in contact with a writing surface. In embodiments, the ball lens  1604  should be separated from the writing surface to obtain the highest resolution of the writing surface at the camera  1602 . In embodiments, the ball lens  1604  is separated by approximately 1 to 3 mm. In this configuration, the domed cover lens  1608  provides a surface that can keep the ball lens  1604  separated from the writing surface at a constant distance, such as substantially independent of the angle used to write on the writing surface. For instance, in embodiments the field of view of the camera in this arrangement would be approximately 60 degrees. 
     The domed cover lens, or other lens  1608  used to physically interact with the writing surface, will be transparent or transmissive within the active bandwidth of the camera  1602 . In embodiments, the domed cover lens  1608  may be spherical or other shape and comprised of glass, plastic, sapphire, diamond, and the like. In other embodiments where low resolution imaging of the surface is acceptable. The pen  1500  can omit the domed cover lens  1608  and the ball lens  1604  can be in direct contact with the surface. 
       FIG. 16B  illustrates another structure where the construction is somewhat similar to that described in connection with  FIG. 16A ; however this embodiment does not use a dome cover lens  1608 , but instead uses a spacer  1610  to maintain a predictable distance between the ball lens  1604  and the writing surface, wherein the spacer may be spherical, cylindrical, tubular or other shape that provides spacing while allowing for an image to be obtained by the camera  1602  through the lens  1604 . In a preferred embodiment, the spacer  1610  is transparent. In addition, while the spacer  1610  is shown as spherical, other shapes such as an oval, doughnut shape, half sphere, cone, cylinder or other form may be used. 
       FIG. 16C  illustrates yet another embodiment, where the structure includes a post  1614 , such as running through the center of the lensed end of the pen  1500 . The post  1614  may be an ink deposition system (e.g. ink cartridge), graphite deposition system (e.g. graphite holder), or a dummy post whose purpose is mainly only that of alignment. The selection of the post type is dependent on the pen&#39;s use. For instance, in the event the user wants to use the pen  1500  as a conventional ink depositing pen as well as a fully functional external user interface  104 , the ink system post would be the best selection. If there is no need for the ‘writing’ to be visible on the writing surface, the selection would be the dummy post. The embodiment of  FIG. 16C  includes camera(s)  1602  and an associated lens  1612 , where the camera  1602  and lens  1612  are positioned to capture the writing surface without substantial interference from the post  1614 . In embodiments, the pen  1500  may include multiple cameras  1602  and lenses  1612  such that more or all of the circumference of the tip  1614  can be used as an input system. In an embodiment, the pen  1500  includes a contoured grip that keeps the pen aligned in the user&#39;s hand so that the camera  1602  and lens  1612  remains pointed at the surface. 
     Another aspect of the pen  1500  relates to sensing the force applied by the user to the writing surface with the pen  1500 . The force measurement may be used in a number of ways. For example, the force measurement may be used as a discrete value, or discontinuous event tracking, and compared against a threshold in a process to determine a user&#39;s intent. The user may want the force interpreted as a ‘click’ in the selection of an object, for instance. The user may intend multiple force exertions interpreted as multiple clicks. There may be times when the user holds the pen  1500  in a certain position or holds a certain portion of the pen  1500  (e.g. a button or touch pad) while clicking to affect a certain operation (e.g. a ‘right click’). In embodiments, the force measurement may be used to track force and force trends. The force trends may be tracked and compared to threshold limits, for example. There may be one such threshold limit, multiple limits, groups of related limits, and the like. For example, when the force measurement indicates a fairly constant force that generally falls within a range of related threshold values, the microprocessor  1510  may interpret the force trend as an indication that the user desires to maintain the current writing style, writing tip type, line weight, brush type, and the like. In the event that the force trend appears to have gone outside of a set of threshold values intentionally, the microprocessor may interpret the action as an indication that the user wants to change the current writing style, writing tip type, line weight, brush type, and the like. Once the microprocessor has made a determination of the user&#39;s intent, a change in the current writing style, writing tip type, line weight, brush type, and the like may be executed. In embodiments, the change may be noted to the user (e.g. in a display of the HWC  102 ), and the user may be presented with an opportunity to accept the change. 
       FIG. 17A  illustrates an embodiment of a force sensing surface tip  1700  of a pen  1500 . The force sensing surface tip  1700  comprises a surface connection tip  1702  (e.g. a lens as described herein elsewhere) in connection with a force or pressure monitoring system  1504 . As a user uses the pen  1500  to write on a surface or simulate writing on a surface the force monitoring system  1504  measures the force or pressure the user applies to the writing surface and the force monitoring system communicates data to the microprocessor  1510  for processing. In this configuration, the microprocessor  1510  receives force data from the force monitoring system  1504  and processes the data to make predictions of the user&#39;s intent in applying the particular force that is currently being applied. In embodiments, the processing may be provided at a location other than on the pen (e.g. at a server in the HWC system  100 , on the HWC  102 ). For clarity, when reference is made herein to processing information on the microprocessor  1510 , the processing of information contemplates processing the information at a location other than on the pen. The microprocessor  1510  may be programmed with force threshold(s), force signature(s), force signature library and/or other characteristics intended to guide an inference program in determining the user&#39;s intentions based on the measured force or pressure. The microprocessor  1510  may be further programmed to make inferences from the force measurements as to whether the user has attempted to initiate a discrete action (e.g. a user interface selection ‘click’) or is performing a constant action (e.g. writing within a particular writing style). The inferencing process is important as it causes the pen  1500  to act as an intuitive external user interface  104 . 
       FIG. 17B  illustrates a force  1708  versus time  1710  trend chart with a single threshold  1718 . The threshold  1718  may be set at a level that indicates a discrete force exertion indicative of a user&#39;s desire to cause an action (e.g. select an object in a GUI). Event  1712 , for example, may be interpreted as a click or selection command because the force quickly increased from below the threshold  1718  to above the threshold  1718 . The event  1714  may be interpreted as a double click because the force quickly increased above the threshold  1718 , decreased below the threshold  1718  and then essentially repeated quickly. The user may also cause the force to go above the threshold  1718  and hold for a period indicating that the user is intending to select an object in the GUI (e.g. a GUI presented in the display of the HWC  102 ) and ‘hold’ for a further operation (e.g. moving the object). 
     While a threshold value may be used to assist in the interpretation of the user&#39;s intention, a signature force event trend may also be used. The threshold and signature may be used in combination or either method may be used alone. For example, a single-click signature may be represented by a certain force trend signature or set of signatures. The single-click signature(s) may require that the trend meet a criteria of a rise time between x any y values, a hold time of between a and b values and a fall time of between c and d values, for example. Signatures may be stored for a variety of functions such as click, double click, right click, hold, move, etc. The microprocessor  1510  may compare the real-time force or pressure tracking against the signatures from a signature library to make a decision and issue a command to the software application executing in the GUI. 
       FIG. 17C  illustrates a force  1708  versus time  1710  trend chart with multiple thresholds  1718 . By way of example, the force trend is plotted on the chart with several pen force or pressure events. As noted, there are both presumably intentional events  1720  and presumably non-intentional events  1722 . The two thresholds  1718  of  FIG. 4C  create three zones of force: a lower, middle and higher range. The beginning of the trend indicates that the user is placing a lower zone amount of force. This may mean that the user is writing with a given line weight and does not intend to change the weight, the user is writing. Then the trend shows a significant increase  1720  in force into the middle force range. This force change appears, from the trend to have been sudden and thereafter it is sustained. The microprocessor  1510  may interpret this as an intentional change and as a result change the operation in accordance with preset rules (e.g. change line width, increase line weight, etc.). The trend then continues with a second apparently intentional event  1720  into the higher-force range. During the performance in the higher-force range, the force dips below the upper threshold  1718 . This may indicate an unintentional force change and the microprocessor may detect the change in range however not affect a change in the operations being coordinated by the pen  1500 . As indicated above, the trend analysis may be done with thresholds and/or signatures. 
     Generally, in the present disclosure, instrument stroke parameter changes may be referred to as a change in line type, line weight, tip type, brush type, brush width, brush pressure, color, and other forms of writing, coloring, painting, and the like. 
     Another aspect of the pen  1500  relates to selecting an operating mode for the pen  1500  dependent on contextual information and/or selection interface(s). The pen  1500  may have several operating modes. For instance, the pen  1500  may have a writing mode where the user interface(s) of the pen  1500  (e.g. the writing surface end, quick launch buttons  1522 , touch sensor  1520 , motion based gesture, and the like) is optimized or selected for tasks associated with writing. As another example, the pen  1500  may have a wand mode where the user interface(s) of the pen is optimized or selected for tasks associated with software or device control (e.g. the HWC  102 , external local device, remote device  112 , and the like). The pen  1500 , by way of another example, may have a presentation mode where the user interface(s) is optimized or selected to assist a user with giving a presentation (e.g. pointing with the laser pointer  1524  while using the button(s)  1522  and/or gestures to control the presentation or applications relating to the presentation). The pen may, for example, have a mode that is optimized or selected for a particular device that a user is attempting to control. The pen  1500  may have a number of other modes and an aspect of the present invention relates to selecting such modes. 
       FIG. 18A  illustrates an automatic user interface(s) mode selection based on contextual information. The microprocessor  1510  may be programmed with IMU thresholds  1814  and  1812 . The thresholds  1814  and  1812  may be used as indications of upper and lower bounds of an angle  1804  and  1802  of the pen  1500  for certain expected positions during certain predicted modes. When the microprocessor  1510  determines that the pen  1500  is being held or otherwise positioned within angles  1802  corresponding to writing thresholds  1814 , for example, the microprocessor  1510  may then institute a writing mode for the pen&#39;s user interfaces. Similarly, if the microprocessor  1510  determines (e.g. through the IMU  1512 ) that the pen is being held at an angle  1804  that falls between the predetermined wand thresholds  1812 , the microprocessor may institute a wand mode for the pen&#39;s user interface. Both of these examples may be referred to as context based user interface mode selection as the mode selection is based on contextual information (e.g. position) collected automatically and then used through an automatic evaluation process to automatically select the pen&#39;s user interface(s) mode. 
     As with other examples presented herein, the microprocessor  1510  may monitor the contextual trend (e.g. the angle of the pen over time) in an effort to decide whether to stay in a mode or change modes. For example, through signatures, thresholds, trend analysis, and the like, the microprocessor may determine that a change is an unintentional change and therefore no user interface mode change is desired. 
       FIG. 18B  illustrates an automatic user interface(s) mode selection based on contextual information. In this example, the pen  1500  is monitoring (e.g. through its microprocessor) whether or not the camera at the writing surface end  1508  is imaging a writing surface in close proximity to the writing surface end of the pen  1500 . If the pen  1500  determines that a writing surface is within a predetermined relatively short distance, the pen  1500  may decide that a writing surface is present  1820  and the pen may go into a writing mode user interface(s) mode. In the event that the pen  1500  does not detect a relatively close writing surface  1822 , the pen may predict that the pen is not currently being used to as a writing instrument and the pen may go into a non-writing user interface(s) mode. 
       FIG. 18C  illustrates a manual user interface(s) mode selection. The user interface(s) mode may be selected based on a twist of a section  1824  of the pen  1500  housing, clicking an end button  1828 , pressing a quick launch button  1522 , interacting with touch sensor  1520 , detecting a predetermined action at the pressure monitoring system (e.g. a click), detecting a gesture (e.g. detected by the IMU), etc. The manual mode selection may involve selecting an item in a GUI associated with the pen  1500  (e.g. an image presented in the display of HWC  102 ). 
     In embodiments, a confirmation selection may be presented to the user in the event a mode is going to change. The presentation may be physical (e.g. a vibration in the pen  1500 ), through a GUI, through a light indicator, etc. 
       FIG. 19  illustrates a couple pen use-scenarios  1900  and  1901 . There are many use scenarios and we have presented a couple in connection with  FIG. 19  as a way of illustrating use scenarios to further the understanding of the reader. As such, the use-scenarios should be considered illustrative and non-limiting. 
     Use scenario  1900  is a writing scenario where the pen  1500  is used as a writing instrument. In this example, quick launch button  122 A is pressed to launch a note application  1910  in the GUI  1908  of the HWC  102  display  1904 . Once the quick launch button  122 A is pressed, the HWC  102  launches the note program  1910  and puts the pen into a writing mode. The user uses the pen  1500  to scribe symbols  1902  on a writing surface, the pen records the scribing and transmits the scribing to the HWC  102  where symbols representing the scribing are displayed  1912  within the note application  1910 . 
     Use scenario  1901  is a gesture scenario where the pen  1500  is used as a gesture capture and command device. In this example, the quick launch button  122 B is activated and the pen  1500  activates a wand mode such that an application launched on the HWC  102  can be controlled. Here, the user sees an application chooser  1918  in the display(s) of the HWC  102  where different software applications can be chosen by the user. The user gestures (e.g. swipes, spins, turns, etc.) with the pen to cause the application chooser  1918  to move from application to application. Once the correct application is identified (e.g. highlighted) in the chooser  1918 , the user may gesture or click or otherwise interact with the pen  1500  such that the identified application is selected and launched. Once an application is launched, the wand mode may be used to scroll, rotate, change applications, select items, initiate processes, and the like, for example. 
     In an embodiment, the quick launch button  122 A may be activated and the HWC  102  may launch an application chooser presenting to the user a set of applications. For example, the quick launch button may launch a chooser to show all communication programs (e.g. SMS, Twitter, Instagram, Facebook, email, etc.) available for selection such that the user can select the program the user wants and then go into a writing mode. By way of further example, the launcher may bring up selections for various other groups that are related or categorized as generally being selected at a given time (e.g. Microsoft Office products, communication products, productivity products, note products, organizational products, and the like) 
       FIG. 20  illustrates yet another embodiment of the present invention.  FIG. 2000  illustrates a watchband clip on controller  2000 . The watchband clip on controller may be a controller used to control the HWC  102  or devices in the HWC system  100 . The watchband clip on controller  2000  has a fastener  2018  (e.g. rotatable clip) that is mechanically adapted to attach to a watchband, as illustrated at  2004 . 
     The watchband controller  2000  may have quick launch interfaces  2008  (e.g. to launch applications and choosers as described herein), a touch pad  2014  (e.g. to be used as a touch style mouse for GUI control in a HWC  102  display) and a display  2012 . The clip  2018  may be adapted to fit a wide range of watchbands so it can be used in connection with a watch that is independently selected for its function. The clip, in embodiments, is rotatable such that a user can position it in a desirable manner. In embodiments the clip may be a flexible strap. In embodiments, the flexible strap may be adapted to be stretched to attach to a hand, wrist, finger, device, weapon, and the like. 
     In embodiments, the watchband controller may be configured as a removable and replacable watchband. For example, the controller may be incorporated into a band with a certain width, segment spacing&#39;s, etc. such that the watchband, with its incorporated controller, can be attached to a watch body. The attachment, in embodiments, may be mechanically adapted to attach with a pin upon which the watchband rotates. In embodiments, the watchband controller may be electrically connected to the watch and/or watch body such that the watch, watch body and/or the watchband controller can communicate data between them. 
     The watchband controller may have 3-axis motion monitoring (e.g. through an IMU, accelerometers, magnetometers, gyroscopes, etc.) to capture user motion. The user motion may then be interpreted for gesture control. 
     In embodiments, the watchband controller may comprise fitness sensors and a fitness computer. The sensors may track heart rate, calories burned, strides, distance covered, and the like. The data may then be compared against performance goals and/or standards for user feedback. 
     Another aspect of the present invention relates to visual display techniques relating to micro Doppler (“mD”) target tracking signatures (“mD signatures”). mD is a radar technique that uses a series of angle dependent electromagnetic pulses that are broadcast into an environment and return pulses are captured. Changes between the broadcast pulse and return pulse are indicative of changes in the shape, distance and angular location of objects or targets in the environment. These changes provide signals that can be used to track a target and identify the target through the mD signature. Each target or target type has a unique mD signature. Shifts in the radar pattern can be analyzed in the time domain and frequency domain based on mD techniques to derive information about the types of targets present (e.g. whether people are present), the motion of the targets and the relative angular location of the targets and the distance to the targets. By selecting a frequency used for the mD pulse relative to known objects in the environment, the pulse can penetrate the known objects to enable information about targets to be gathered even when the targets are visually blocked by the known objects. For example, pulse frequencies can be used that will penetrate concrete buildings to enable people to be identified inside the building. Multiple pulse frequencies can be used as well in the mD radar to enable different types of information to be gathered about the objects in the environment. In addition, the mD radar information can be combined with other information such as distance measurements or images captured of the environment that are analyzed jointly to provide improved object identification and improved target identification and tracking. In embodiments, the analysis can be performed on the HWC or the information can be transmitted to a remote network for analysis and results transmitted back to the HWC. Distance measurements can be provided by laser range finding, structured lighting, stereoscopic depth maps or sonar measurements. Images of the environment can be captured using one or more cameras capable of capturing images from visible, ultraviolet or infrared light. The mD radar can be attached to the HWC, located adjacently (e.g. in a vehicle) and associated wirelessly with the HWC or located remotely. Maps or other previously determined information about the environment can also be used in the analysis of the mD radar information. Embodiments of the present invention relate to visualizing the mD signatures in useful ways. 
       FIG. 21  illustrates a FOV  2102  of a HWC  102  from a wearer&#39;s perspective. The wearer, as described herein elsewhere, has a see-through FOV  2102  wherein the wearer views adjacent surroundings, such as the buildings illustrated in  FIG. 21 . The wearer, as described herein elsewhere, can also see displayed digital content presented within a portion of the FOV  2102 . The embodiment illustrated in  FIG. 21  is indicating that the wearer can see the buildings and other surrounding elements in the environment and digital content representing traces, or travel paths, of bullets being fired by different people in the area. The surroundings are viewed through the transparency of the FOV  2102 . The traces are presented via the digital computer display, as described herein elsewhere. In embodiments, the trace presented is based on a mD signature that is collected and communicated to the HWC in real time. The mD radar itself may be on or near the wearer of the HWC  102  or it may be located remote from the wearer. In embodiments, the mD radar scans the area, tracks and identifies targets, such as bullets, and communicates traces, based on locations, to the HWC  102 . 
     There are several traces  2108  and  2104  presented to the wearer in the embodiment illustrated in  FIG. 21 . The traces communicated from the mD radar may be associated with GPS locations and the GPS locations may be associated with objects in the environment, such as people, buildings, vehicles, etc, both in latitude and longitude perspective and an elevation perspective. The locations may be used as markers for the HWC such that the traces, as presented in the FOV, can be associated, or fixed in space relative to the markers. For example, if the friendly fire trace  2108  is determined, by the mD radar, to have originated from the upper right window of the building on the left, as illustrated in  FIG. 21 , then a virtual marker may be set on or near the window. When the HWC views, through it&#39;s camera or other sensor, for example, the building&#39;s window, the trace may then virtually anchor with the virtual marker on the window. Similarly, a marker may be set near the termination position or other flight position of the friendly fire trace  2108 , such as the upper left window of the center building on the right, as illustrated in  FIG. 21 . This technique fixes in space the trace such that the trace appears fixed to the environmental positions independent of where the wearer is looking. So, for example, as the wearer&#39;s head turns, the trace appears fixed to the marked locations. 
     In embodiments, certain user positions may be known and thus identified in the FOV. For example, the shooter of the friendly fire trace  2108  may be from a known friendly combatant and as such his location may be known. The position may be known based on his GPS location based on a mobile communication system on him, such as another HWC  102 . In other embodiments, the friendly combatant may be marked by another friendly. For example, if the friendly position in the environment is known through visual contact or communicated information, a wearer of the HWC  102  may use a gesture or external user interface  104  to mark the location. If a friendly combatant location is known the originating position of the friendly fire trace  2108  may be color coded or otherwise distinguished from unidentified traces on the displayed digital content. Similarly, enemy fire traces  2104  may be color coded or otherwise distinguished on the displayed digital content. In embodiments, there may be an additional distinguished appearance on the displayed digital content for unknown traces. 
     In addition to situationally associated trace appearance, the trace colors or appearance may be different from the originating position to the terminating position. This path appearance change may be based on the mD signature. The mD signature may indicate that the bullet, for example, is slowing as it propagates and this slowing pattern may be reflected in the FOV  2102  as a color or pattern change. This can create an intuitive understanding of wear the shooter is located. For example, the originating color may be red, indicative of high speed, and it may change over the course of the trace to yellow, indicative of a slowing trace. This pattern changing may also be different for a friendly, enemy and unknown combatant. The enemy may go blue to green for a friendly trace, for example. 
       FIG. 21  illustrates an embodiment where the user sees the environment through the FOV and may also see color coded traces, which are dependent on bullet speed and combatant type, where the traces are fixed in environmental positions independent on the wearer&#39;s perspective. Other information, such as distance, range, range rings, time of day, date, engagement type (e.g. hold, stop firing, back away, etc.) may also be displayed in the FOV. 
     Another aspect of the present invention relates to mD radar techniques that trace and identify targets through other objects, such as walls (referred to generally as through wall mD), and visualization techniques related therewith.  FIG. 22  illustrates a through wall mD visualization technique according to the principles of the present invention. As described herein elsewhere, the mD radar scanning the environment may be local or remote from the wearer of a HWC  102 . The mD radar may identify a target (e.g. a person) that is visible  2204  and then track the target as he goes behind a wall  2208 . The tracking may then be presented to the wearer of a HWC  102  such that digital content reflective of the target and the target&#39;s movement, even behind the wall, is presented in the FOV  2202  of the HWC  102 . In embodiments, the target, when out of visible sight, may be represented by an avatar in the FOV to provide the wearer with imagery representing the target. 
     mD target recognition methods can identify the identity of a target based on the vibrations and other small movements of the target. This can provide a personal signature for the target. In the case of humans, this may result in a personal identification of a target that has been previously characterized. The cardio, heart beat, lung expansion and other small movements within the body may be unique to a person and if those attributes are pre-identified they may be matched in real time to provide a personal identification of a person in the FOV  2202 . The person&#39;s mD signatures may be determined based on the position of the person. For example, the database of personal mD signature attributes may include mD signatures for a person standing, sitting, laying down, running, walking, jumping, etc. This may improve the accuracy of the personal data match when a target is tracked through mD signature techniques in the field. In the event a person is personally identified, a specific indication of the person&#39;s identity may be presented in the FOV  2202 . The indication may be a color, shape, shade, name, indication of the type of person (e.g. enemy, friendly, etc.), etc. to provide the wearer with intuitive real time information about the person being tracked. This may be very useful in a situation where there is more than one person in an area of the person being tracked. If just one person in the area is personally identified, that person or the avatar of that person can be presented differently than other people in the area. 
       FIG. 23  illustrates an mD scanned environment  2300 . An mD radar may scan an environment in an attempt to identify objects in the environment. In this embodiment, the mD scanned environment reveals two vehicles  2302   a  and  2302   b , en enemy combatant  2309 , two friendly combatants  2308   a  and  2308   b  and a shot trace  2318 . Each of these objects may be personally identified or type identified. For example, the vehicles  2302   a  and  2302   b  may be identified through the mD signatures as a tank and heavy truck. The enemy combatant  2309  may be identified as a type (e.g. enemy combatant) or more personally (e.g. by name). The friendly combatants may be identified as a type (e.g. friendly combatant) or more personally (e.g. by name). The shot trace  2318  may be characterized by type of projectile or weapon type for the projectile, for example. 
       FIG. 23 a    illustrates two separate HWC  102  FOV display techniques according to the principles of the present invention. FOV  2312  illustrates a map view  2310  where the mD scanned environment is presented. Here, the wearer has a perspective on the mapped area so he can understand all tracked targets in the area. This allows the wearer to traverse the area with knowledge of the targets. FOV  2312  illustrates a heads-up view to provide the wearer with an augmented reality style view of the environment that is in proximity of the wearer. 
     An aspect of the present invention relates to suppression of extraneous or stray light. As discussed herein elsewhere, eyeglow and faceglow are two such artifacts that develop from such light. Eyeglow and faceglow can be caused by image light escaping from the optics module. The escaping light is then visible, particularly in dark environments when the user is viewing bright displayed images with the HWC. Light that escapes through the front of the HWC is visible as eyeglow as it that light that is visible in the region of the user&#39;s eyes. Eyeglow can appear in the form of a small version of the displayed image that the user is viewing. Light that escapes from the bottom of the HWC shines onto the user&#39;s face, cheek or chest so that these portions of the user appear to glow. Eyeglow and faceglow can both increase the visibility of the user and highlight the use of the HWC, which may be viewed negatively by the user. As such, reducing eyeglow and faceglow is advantageous. In combat situations (e.g. the mD trace presentation scenerios described herein) and certain gaming situations, the suppression of extraneous or stray light is very important. 
     The disclosure relating to  FIG. 6  shows an example where a portion of the image light passes through the combiner  602  such that the light shines onto the user&#39;s face, thereby illuminating a portion of the user&#39;s face in what is generally referred to herein as faceglow. Faceglow be caused by any portion of light from the HWC that illuminates the user&#39;s face. 
     An example of the source for the faceglow light can come from wide cone angle light associated with the image light incident onto the combiner  602 . Where the combiner can include a holographic mirror or a notch mirror in which the narrow bands of high reflectivity are matched to wavelengths of light by the light source. The wide cone angle associated with the image light corresponds with the field of view provided by the HWC. Typically the reflectivity of holographic mirrors and notch mirrors is reduced as the cone angle of the incident light is increased above 8 degrees. As a result, for a a field of view of 30 degrees, substantial image light can pass through the combiner and cause faceglow. 
       FIG. 24  shows an illustration of a light trap  2410  for the faceglow light. In this embodiment, an extension of the outer shield len of the HWC is coated with a light absorbing material in the region where the converging light responsible for faceglow is absorbed in a light trap  2410 . The light absorbing material can be black or it can be a filter designed to absorb only the specific wavelengths of light provided by the light source(s) in the HWC. In addition, the surface of the light trap  2410  may be textured or fibrous to further improve the absorption. 
       FIG. 25  illustrates an optical system for a HWC that includes an outer absorptive polarizer  2520  to block the faceglow light. In this embodiment, the image light is polarized and as a result the light responsible for faceglow is similarly polarized. The absorptive polarizer is oriented with a transmission axis such that the faceglow light is absorbed and not transmitted. In this case, the rest of the imaging system in the HWC may not require polarized image light and the image light may be polarized at any point before the combiner. In embodiments, the transmission axis of the absorptive polarizer  2520  is oriented vertically so that external glare from water (S polarized light) is absorbed and correspondingly, the polarization of the image light is selected to be horizontal (S polarization). Consequently, image light that passes through the combiner  602  and is then incident onto the absorptive polarizer  2520 , is absorbed. In  FIG. 25  the absorptive polarizer  2520  is shown outside the shield lens, alternatively the absorptive polarizer  2520  can be located inside the shield lens. 
       FIG. 26  illustrates an optical system for a HWC that includes a film with an absorptive notch filter  2620 . In this case, the absorptive notch filter absorbs narrow bands of light that are selected to match the light provided by the optical system&#39;s light source. As a result, the absorptive notch filter is opaque with respect to the faceglow light and is transparent to the remainder of the wavelengths included in the visible spectrum so that the user has a clear view of the surrounding environment. A triple notch filter suitable for this approach is available from Iridian Spectral Technologies, Ottawa, ON: http://www.ilphotonics.com/cdv2/Iridian-Interference%20Filters/New%20filters/Triple%20Notch%20Filter.pdf 
     In embodiments, the combiner  602  may include a notch mirror coating to reflect the wavelengths of light in the image light and a notch filter  2620  can be selected in correspondence to the wavelengths of light provided by the light source and the narrow bands of high reflectivity provided by the notch mirror. In this way, image light that is not reflected by the notch mirror is absorbed by the notch filter  2620 . In embodiments of the invention the light source can provide one narrow band of light for a monochrome imaging or three narrow bands of light for full color imaging. The notch mirror and associated notch filter would then each provide one narrow band or three narrow bands of high reflectivity and absorption respectively. 
       FIG. 27  includes a microlouver film  2750  to block the faceglow light. Microlouver film is sold by 3M as ALCF-P, for example and is typically used as a privacy filter for computer. See http://multimedia.3m.com/mws/mediawebserver?mwsId=SSSSSuH8gc7nZxtUoY_xlY_eevUqe17zHVTSevTSeSSSSSS--&amp;fn=ALCF-P_ABR2_Control_Film_DS.pdf The microlouver film transmits light within a somewhat narrow angle (e.g. 30 degrees of normal and absorbs light beyond 30 degrees of normal). In  FIG. 27 , the microlouver film  2750  is positioned such that the faceglow light  2758  is incident beyond 30 degrees from normal while the see-through light  2755  is incident within 30 degrees of normal to the microlouver film  2750 . As such, the faceglow light  2758  is absorbed by the microlouver film and the see-through light  2755  is transmitted so that the user has a bright see-thru view of the surrounding environment. 
     We now turn back to a description of eye imaging technologies. Aspects of the present invention relate to various methods of imaging the eye of a person wearing the HWC  102 . In embodiments, technologies for imaging the eye using an optical path involving the “off” state and “no power” state, which is described in detail below, are described. In embodiments, technologies for imaging the eye with optical configurations that do not involve reflecting the eye image off of DLP mirrors is described. In embodiments, unstructured light, structured light, or controlled lighting conditions, are used to predict the eye&#39;s position based on the light reflected off of the front of the wearer&#39;s eye. In embodiments, a reflection of a presented digital content image is captured as it reflects off of the wearer&#39;s eye and the reflected image may be processed to determine the quality (e.g. sharpness) of the image presented. In embodiments, the image may then be adjusted (e.g. focused differently) to increase the quality of the image presented based on the image reflection. 
       FIGS. 28 a , 28 b  and 28 c    show illustrations of the various positions of the DLP mirrors.  FIG. 28 a    shows the DLP mirrors in the “on” state  2815 . With the mirror in the “on” state  2815 , illumination light  2810  is reflected along an optical axis  2820  that extends into the lower optical module  204 .  FIG. 28 b    shows the DLP mirrors in the “off” state  2825 . With the mirror in the “off” state  2825 , illumination light  2810  is reflected along an optical axis  2830  that is substantially to the side of optical axis  2820  so that the “off” state light is directed toward a dark light trap as has been described herein elsewhere.  FIG. 28 c    shows the DLP mirrors in a third position, which occurs when no power is applied to the DLP. This “no power” state differs from the “on” and “off” states in that the mirror edges are not in contact with the substrate and as such are less accurately positioned.  FIG. 28 c    shows all of the DLP mirrors in the “no power” state  2835 . The “no power” state is achieved by simultaneously setting the voltage to zero for the “on” contact and “off” contact for a DLP mirror, as a result, the mirror returns to a no stress position where the DLP mirror is in the plane of the DLP platform as shown in  FIG. 28 c   . Although not normally done, it is also possible to apply the “no power” state to individual DLP mirrors. When the DLP mirrors are in the “no power” state they do not contribute image content. Instead, as shown in  FIG. 28 c   , when the DLP mirrors are in the “no power” state, the illumination light  2810  is reflected along an optical axis  2840  that is between the optical axes  2820  and  2830  that are respectively associated with the “on” and “off” states and as such this light doesn&#39;t contribute to the displayed image as a bright or dark pixel. This light can however contribute scattered light into the lower optical module  204  and as a result the displayed image contrast can be reduced or artifacts can be created in the image that detract from the image content. Consequently, it is generally desirable, in embodiments, to limit the time associated with the “no power” state to times when images are not displayed or to reduce the time associated with having DLP mirrors in the “no power” state so that the affect of the scattered light is reduced. 
       FIG. 29  shows an embodiment of the invention that can be used for displaying digital content images to a wearer of the HWC  102  and capturing images of the wearer&#39;s eye. In this embodiment, light from the eye  2971  passes back through the optics in the lower module  204 , the solid corrective wedge  2966 , at least a portion of the light passes through the partially reflective layer  2960 , the solid illumination wedge  2964  and is reflected by a plurality of DLP mirrors on the DLP  2955  that are in the “no power” state. The reflected light then passes back through the illumination wedge  2964  and at least a portion of the light is reflected by the partially reflective layer  2960  and the light is captured by the camera  2980 . 
     For comparison, illuminating light rays  2973  from the light source  2958  are also shown being reflected by the partially reflective layer  2960 . Where the angle of the illuminating light  2973  is such that the DLP mirrors, when in the “on” state, reflect the illuminating light  2973  to form image light  2969  that substantially shares the same optical axis as the light from the wearer&#39;s eye  2971 . In this way, images of the wearer&#39;s eye are captured in a field of view that overlaps the field of view for the displayed image content. In contrast, light reflected by DLP mirrors in the “off” state form dark light  2975  which is directed substantially to the side of the image light  2969  and the light from eye  2971 . Dark light  2975  is directed toward a light trap  2962  that absorbs the dark light to improve the contrast of the displayed image as has been described above in this specification. 
     In an embodiment, partially reflective layer  2960  is a reflective polarizer. The light that is reflected from the eye  2971  can then be polarized prior to entering the corrective wedge  2966  (e.g with an absorptive polarizer between the upper module  202  and the lower module  204 ), with a polarization orientation relative to the reflective polarizer that enables the light reflected from the eye  2971  to substantially be transmitted by the reflective polarizer. A quarter wave retarder layer  2957  is then included adjacent to the DLP  2955  (as previously disclosed in  FIG. 3 b   ) so that the light reflected from the eye  2971  passes through the quarter wave retarder layer  2957  once before being reflected by the plurality of DLP mirrors in the “no power” state and then passes through a second time after being reflected. By passing through the quarter wave retarder layer  2957  twice, the polarization state of the light from the eye  2971  is reversed, such that when it is incident upon the reflective polarizer, the light from the eye  2971  is then substantially reflected toward the camera  2980 . By using a partially reflective layer  2960  that is a reflective polarizer and polarizing the light from the eye  2971  prior to entering the corrective wedge  2964 , losses attributed to the partially reflective layer  2960  are reduced. 
       FIG. 28 c    shows the case wherein the DLP mirrors are simultaneously in the “no power” state, this mode of operation can be particularly useful when the HWC  102  is first put onto the head of the wearer. When the HWC  102  is first put onto the head of the wearer, it is not necessary to display an image yet. As a result, the DLP can be in a “no power” state for all the DLP mirrors and an image of the wearer&#39;s eyes can be captured. The captured image of the wearer&#39;s eye can then be compared to a database, using iris identification techniques, or other eye pattern identification techniques to determine, for example, the identity of the wearer. 
     In a further embodiment illustrated by  FIG. 29  all of the DLP mirrors are put into the “no power” state for a portion of a frame time (e.g. 50% of a frame time for the displayed digital content image) and the capture of the eye image is synchronized to occur at the same time and for the same duration. By reducing the time that the DLP mirrors are in the “no power” state, the time where light is scattered by the DLP mirrors being in the “no power” state is reduced such that the wearer doesn&#39;t perceive a change in the displayed image quality. This is possible because the DLP mirrors have a response time on the order of microseconds while typical frame times for a displayed image are on the order of 0.016 seconds. This method of capturing images of the wearer&#39;s eye can be used periodically to capture repetitive images of the wearer&#39;s eye. For example, eye images could be captured for 50% of the frame time of every 10th frame displayed to the wearer. In another example, eye images could be captured for 10% of the frame time of every frame displayed to the wearer. 
     Alternately, the “no power” state can be applied to a subset of the DLP mirrors (e.g. 10% of the DLP mirrors) within while another subset is in busy generating image light for content to be displayed. This enables the capture of an eye image(s) during the display of digital content to the wearer. The DLP mirrors used for eye imaging can, for example, be distributed randomly across the area of the DLP to minimize the impact on the quality of the digital content being displayed to the wearer. To improve the displayed image perceived by the wearer, the individual DLP mirrors put into the “no power” state for capturing each eye image, can be varied over time such as in a random pattern, for example. In yet a further embodiment, the DLP mirrors put into the “no power” state for eye imaging may be coordinated with the digital content in such a way that the “no power” mirrors are taken from a portion of the image that requires less resolution. 
     In the embodiments of the invention as illustrated in  FIGS. 9 and 29 , in both cases the reflective surfaces provided by the DLP mirrors do not preserve the wavefront of the light from the wearer&#39;s eye so that the image quality of captured image of the eye is somewhat limited. It may still be useful in certain embodiments, but it is somewhat limited. This is due to the DLP mirrors not being constrained to be on the same plane. In the embodiment illustrated in  FIG. 9 , the DLP mirrors are tilted so that they form rows of DLP mirrors that share common planes. In the embodiment illustrated in  FIG. 29 , the individual DLP mirrors are not accurately positioned to be in the same plane since they are not in contact with the substrate. Examples of advantages of the embodiments associated with  FIG. 29  are: first, the camera  2980  can be located between the DLP  2955  and the illumination light source  2958  to provide a more compact upper module  202 . Second, the polarization state of the light reflected from the eye  2971  can be the same as that of the image light  2969  so that the optical path of the light reflected from the eye and the image light can be the same in the lower module  204 . 
       FIG. 30  shows an illustration of an embodiment for displaying images to the wearer and simultaneously capturing images of the wearer&#39;s eye, wherein light from the eye  2971  is reflected towards a camera  3080  by the partially reflective layer  2960 . The partially reflective layer  2960  can be an optically flat layer such that the wavefront of the light from the eye  2971  is preserved and as a result, higher quality images of the wearer&#39;s eye can be captured. In addition, since the DLP  2955  is not included in the optical path for the light from the eye  2971 , and the eye imaging process shown in  FIG. 30  does not interfere with the displayed image, images of the wearer&#39;s eye can be captured independently (e.g. with independent of timing, impact on resolution, or pixel count used in the image light) from the displayed images. 
     In the embodiment illustrated in  FIG. 30 , the partially reflective layer  2960  is a reflective polarizer, the illuminating light  2973  is polarized, the light from the eye  2971  is polarized and the camera  3080  is located behind a polarizer  3085 . The polarization axis of the illuminating light  2973  and the polarization axis of the light from the eye are oriented perpendicular to the transmission axis of the reflective polarizer so that they are both substantially reflected by the reflective polarizer. The illumination light  2973  passes through a quarter wave layer  2957  before being reflected by the DLP mirrors in the DLP  2955 . The reflected light passes back through the quarter wave layer  2957  so that the polarization states of the image light  2969  and dark light  2975  are reversed in comparison to the illumination light  2973 . As such, the image light  2969  and dark light  2975  are substantially transmitted by the reflective polarizer. Where the DLP mirrors in the “on” state provide the image light  2969  along an optical axis that extends into the lower optical module  204  to display an image to the wearer. At the same time, DLP mirrors in the “off” state provide the dark light  2975  along an optical axis that extends to the side of the upper optics module  202 . In the region of the corrective wedge  2966  where the dark light  2975  is incident on the side of the upper optics module  202 , an absorptive polarizer  3085  is positioned with it&#39;s transmission axis perpendicular to the polarization axis of the dark light and parallel to the polarization axis of the light from the eye so that the dark light  2975  is absorbed and the light from the eye  2971  is transmitted to the camera  3080 . 
       FIG. 31  shows an illustration of another embodiment of a system for displaying images and simultaneously capturing image of the wearer&#39;s eye that is similar to the one shown in  FIG. 30 . The difference in the system shown in  FIG. 31  is that the light from the eye  2971  is subjected to multiple reflections before being captured by the camera  3180 . To enable the multiple reflections, a mirror  3187  is provided behind the absorptive polarizer  3185 . Therefore, the light from the eye  2971  is polarized prior to entering the corrective wedge  2966  with a polarization axis that is perpendicular to the transmission axis of the reflective polarizer that comprises the partially reflective layer  2960 . In this way, the light from the eye  2971  is reflected first by the reflective polarizer, reflected second by the mirror  3187  and reflected third by the reflective polarizer before being captured by the camera  3180 . While the light from the eye  2971  passes through the absorptive polarizer  3185  twice, since the polarization axis of the light from the eye  2971  is oriented parallel to the polarization axis of the light from the eye  2971 , it is substantially transmitted by the absorptive polarizer  3185 . As with the system described in connection with  FIG. 30 , the system shown in  FIG. 31  includes an optically flat partially reflective layer  2960  that preserves the wavefront of the light from the eye  2971  so that higher quality images of the wearer&#39;s eye can be captured. Also, since the DLP  2955  is not included in the optical path for the light reflected from the eye  2971  and the eye imaging process shown in  FIG. 31  does not interfere with the displayed image, images of the wearer&#39;s eye can be captured independently from the displayed images. 
       FIG. 32  shows an illustration of a system for displaying images and simultaneously capturing images of the wearer&#39;s eye that includes a beam splitter plate  3212  comprised of a reflective polarizer, which is held in air between the light source  2958 , the DLP  2955  and the camera  3280 . The illumination light  2973  and the light from the eye  2971  are both polarized with polarization axes that are perpendicular to the transmission axis of the reflective polarizer. As a result, both the illumination light  2973  and the light from the eye  2971  are substantially reflected by the reflective polarizer. The illumination light  2873  is reflected toward the DLP  2955  by the reflective polarizer and split into image light  2969  and dark light  3275  depending on whether the individual DLP mirrors are respectively in the “on” state or the “off” state. By passing through the quarter wave layer  2957  twice, the polarization state of the illumination light  2973  is reversed in comparison to the polarization state of the image light  2969  and the dark light  3275 . As a result, the image light  2969  and the dark light  3275  are then substantially transmitted by the reflective polarizer. The absorptive polarizer  3285  at the side of the beam splitter plate  3212  has a transmission axis that is perpendicular to the polarization axis of the dark light  3275  and parallel to the polarization axis of the light from the eye  2971  so that the dark light  3275  is absorbed and the light from the eye  2971  is transmitted to the camera  3280 . As in the system shown in  FIG. 30 , the system shown in  FIG. 31  includes an optically flat beam splitter plate  3212  that preserves the wavefront of the light from the eye  2971  so that higher quality images of the wearer&#39;s eye can be captured. Also, since the DLP  2955  is not included in the optical path for the light from the eye  2971  and the eye imaging process shown in  FIG. 31  does not interfere with the displayed image, images of the wearer&#39;s eye can be captured independently from the displayed images. 
     Eye imaging systems where the polarization state of the light from the eye  2971  needs to be opposite to that of the image light  2969  (as shown in  FIGS. 30, 31 and 32 ), need to be used with lower modules  204  that include combiners that will reflect both polarization states. As such, these upper modules  202  are best suited for use with the lower modules  204  that include combiners that are reflective regardless of polarization state, examples of these lower modules are shown in  FIGS. 6, 8   a ,  8   b ,  8   c  and  24 - 27 . 
     In a further embodiment shown in  FIG. 33 , the partially reflective layer  3360  is comprised of a reflective polarizer on the side facing the illumination light  2973  and a short pass dichroic mirror on the side facing the light from the eye  3371  and the camera  3080 . Where the short pass dichroic mirror is a dielectric mirror coating that transmits visible light and reflects infrared light. The partially reflective layer  3360  can be comprised of a reflective polarizer bonded to the inner surface of the illumination wedge  2964  and a short pass dielectric mirror coating on the opposing inner surface of the corrective wedge  2966 , wherein the illumination wedge  2964  and the corrective wedge  2966  are then optically bonded together. Alternatively, the partially reflective layer  3360  can be comprised of a thin substrate that has a reflective polarizer bonded to one side and a short pass dichroic mirror coating on the other side, where the partially reflective layer  3360  is then bonded between the illumination wedge  2964  and the corrective wedge  2966 . In this embodiment, an infrared light is included to illuminate the eye so that the light from the eye and the images captured of the eye are substantially comprised of infrared light. The wavelength of the infrared light is then matched to the reflecting wavelength of the shortpass dichroic mirror and the wavelength that the camera can capture images, for example an 800 nm wavelength can be used. In this way, the short pass dichroic mirror transmits the image light and reflects the light from the eye. The camera  3080  is then positioned at the side of the corrective wedge  2966  in the area of the absorbing light trap  3382 , which is provided to absorb the dark light  2975 . By positioning the camera  3080  in a depression in the absorbing light trap  3382 , scattering of the dark light  2975  by the camera  3080  can be reduced so that higher contrast images can be displayed to the wearer. An advantage of this embodiment is that the light from the eye need not be polarized, which can simplify the optical system and increase efficiency for the eye imaging system. 
     In yet another embodiment shown in  FIG. 32 a    a beam splitter plate  3222  is comprised of a reflective polarizer on the side facing the illumination light  2973  and a short pass dichroic mirror on the side facing the light from the eye  3271  and the camera  3280 . An absorbing surface  3295  is provided to trap the dark light  3275  and the camera  3280  is positioned in an opening in the absorbing surface  3295 . In this way the system of  FIG. 32  can be made to function with unpolarized light from the eye  3271 . 
     In embodiments directed to capturing images of the wearer&#39;s eye, light to illuminate the wearer&#39;s eye can be provided by several different sources including: light from the displayed image (i.e. image light); light from the environment that passes through the combiner or other optics; light provided by a dedicated eye light, etc.  FIGS. 34 and 34   a  show illustrations of dedicated eye illumination lights  3420 .  FIG. 34  shows an illustration from a side view in which the dedicated illumination eye light  3420  is positioned at a corner of the combiner  3410  so that it doesn&#39;t interfere with the image light  3415 . The dedicated eye illumination light  3420  is pointed so that the eye illumination light  3425  illuminates the eyebox  3427  where the eye  3430  is located when the wearer is viewing displayed images provided by the image light  3415 .  FIG. 34 a    shows an illustration from the perspective of the eye of the wearer to show how the dedicated eye illumination light  3420  is positioned at the corner of the combiner  3410 . While the dedicated eye illumination light  3420  is shown at the upper left corner of the combiner  3410 , other positions along one of the edges of the combiner  3410 , or other optical or mechanical components, are possible as well. In other embodiments, more than one dedicated eye light  3420  with different positions can be used. In an embodiment, the dedicated eye light  3420  is an infrared light that is not visible by the wearer (e.g. 800 nm) so that the eye illumination light  3425  doesn&#39;t interfere with the displayed image perceived by the wearer. 
       FIG. 35  shows a series of illustrations of captured eye images that show the eye glint (i.e. light that reflects off the front of the eye) produced by a dedicated eye light. In this embodiment of the invention, captured images of the wearer&#39;s eye are analyzed to determine the relative positions of the iris  3550 , pupil, or other portion of the eye, and the eye glint  3560 . The eye glint is a reflected image of the dedicated eye light  3420  when the dedicated light is used.  FIG. 35  illustrates the relative positions of the iris  3550  and the eye glint  3560  for a variety of eye positions. By providing a dedicated eye light  3420  in a fixed position, combined with the fact that the human eye is essentially spherical, or at least a reliably repeatable shape, the eye glint provides a fixed reference point against which the determined position of the iris can be compared to determine where the wearer is looking, either within the displayed image or within the see-through view of the surrounding environment. By positioning the dedicated eye light  3420  at a corner of the combiner  3410 , the eye glint  3560  is formed away from the iris  3550  in the captured images. As a result, the positions of the iris and the eye glint can be determined more easily and more accurately during the analysis of the captured images, since they do not interfere with one another. In a further embodiment, the combiner includes an associated cut filter that prevents infrared light from the environment from entering the HWC and the camera is an infrared camera, so that the eye glint is only provided by light from the dedicated eye light. For example, the combiner can include a low pass filter that passes visible light while absorbing infrared light and the camera can include a high pass filter that absorbs visible light while passing infrared light. 
     In an embodiment of the eye imaging system, the lens for the camera is designed to take into account the optics associated with the upper module  202  and the lower module  204 . This is accomplished by designing the camera to include the optics in the upper module  202  and optics in the lower module  204 , so that a high MTF image is produced, at the image sensor in the camera, of the wearer&#39;s eye. In yet a further embodiment, the camera lens is provided with a large depth of field to eliminate the need for focusing the camera to enable sharp image of the eye to be captured. Where a large depth of field is typically provided by a high f/# lens (e.g. f/#&gt;5). In this case, the reduced light gathering associated with high f/# lenses is compensated by the inclusion of a dedicated eye light to enable a bright image of the eye to be captured. Further, the brightness of the dedicated eye light can be modulated and synchronized with the capture of eye images so that the dedicated eye light has a reduced duty cycle and the brightness of infrared light on the wearer&#39;s eye is reduced. 
     In a further embodiment,  FIG. 36 a    shows an illustration of an eye image that is used to identify the wearer of the HWC. In this case, an image of the wearer&#39;s eye  3611  is captured and analyzed for patterns of identifiable features  3612 . The patterns are then compared to a database of eye images to determine the identity of the wearer. After the identity of the wearer has been verified, the operating mode of the HWC and the types of images, applications, and information to be displayed can be adjusted and controlled in correspondence to the determined identity of the wearer. Examples of adjustments to the operating mode depending on who the wearer is determined to be or not be include: making different operating modes or feature sets available, shutting down or sending a message to an external network, allowing guest features and applications to run, etc. 
     is an illustration of another embodiment using eye imaging, in which the sharpness of the displayed image is determined based on the eye glint produced by the reflection of the displayed image from the wearer&#39;s eye surface. By capturing images of the wearer&#39;s eye  3611 , an eye glint  3622 , which is a small version of the displayed image can be captured and analyzed for sharpness. If the displayed image is determined to not be sharp, then an automated adjustment to the focus of the HWC optics can be performed to improve the sharpness. This ability to perform a measurement of the sharpness of a displayed image at the surface of the wearer&#39;s eye can provide a very accurate measurement of image quality. Having the ability to measure and automatically adjust the focus of displayed images can be very useful in augmented reality imaging where the focus distance of the displayed image can be varied in response to changes in the environment or changes in the method of use by the wearer. 
     An aspect of the present invention relates to controlling the HWC  102  through interpretations of eye imagery. In embodiments, eye-imaging technologies, such as those described herein, are used to capture an eye image or series of eye images for processing. The image(s) may be process to determine a user intended action, an HWC predetermined reaction, or other action. For example, the imagery may be interpreted as an affirmative user control action for an application on the HWC  102 . Or, the imagery may cause, for example, the HWC  102  to react in a pre-determined way such that the HWC  102  is operating safely, intuitively, etc. 
       FIG. 37  illustrates a eye imagery process that involves imaging the HWC  102  wearer&#39;s eye(s) and processing the images (e.g. through eye imaging technologies described herein) to determine in what position  3702  the eye is relative to it&#39;s neutral or forward looking position and/or the FOV  3708 . The process may involve a calibration step where the user is instructed, through guidance provided in the FOV of the HWC  102 , to look in certain directions such that a more accurate prediction of the eye position relative to areas of the FOV can be made. In the event the wearer&#39;s eye is determined to be looking towards the right side of the FOV  3708  (as illustrated in  FIG. 37 , the eye is looking out of the page) a virtual target line may be established to project what in the environment the wearer may be looking towards or at. The virtual target line may be used in connection with an image captured by camera on the HWC  102  that images the surrounding environment in front of the wearer. In embodiments, the field of view of the camera capturing the surrounding environment matches, or can be matched (e.g. digitally), to the FOV  3708  such that making the comparison is made more clear. For example, with the camera capturing the image of the surroundings in an angle that matches the FOV  3708  the virtual line can be processed (e.g. in 2d or 3d, depending on the camera images capabilities and/or the processing of the images) by projecting what surrounding environment objects align with the virtual target line. In the event there are multiple objects along the virtual target line, focal planes may be established corresponding to each of the objects such that digital content may be placed in an area in the FOV  3708  that aligns with the virtual target line and falls at a focal plane of an intersecting object. The user then may see the digital content when he focuses on the object in the environment, which is at the same focal plane. In embodiments, objects in line with the virtual target line may be established by comparison to mapped information of the surroundings. 
     In embodiments, the digital content that is in line with the virtual target line may not be displayed in the FOV until the eye position is in the right position. This may be a predetermined process. For example, the system may be set up such that a particular piece of digital content (e.g. an advertisement, guidance information, object information, etc.) will appear in the event that the wearer looks at a certain object(s) in the environment. A virtual target line(s) may be developed that virtually connects the wearer&#39;s eye with an object(s) in the environment (e.g. a building, portion of a building, mark on a building, gps location, etc.) and the virtual target line may be continually updated depending on the position and viewing direction of the wearer (e.g. as determined through GPS, e-compass, IMU, etc.) and the position of the object. When the virtual target line suggests that the wearer&#39;s pupil is substantially aligned with the virtual target line or about to be aligned with the virtual target line, the digital content may be displayed in the FOV  3704 . 
     In embodiments, the time spent looking along the virtual target line and/or a particular portion of the FOV  3708  may indicate that the wearer is interested in an object in the environment and/or digital content being displayed. In the event there is no digital content being displayed at the time a predetermined period of time is spent looking at a direction, digital content may be presented in the area of the FOV  3708 . The time spent looking at an object may be interpreted as a command to display information about the object, for example. In other embodiments, the content may not relate to the object and may be presented because of the indication that the person is relatively inactive. In embodiments, the digital content may be positioned in proximity to the virtual target line, but not in-line with it such that the wearer&#39;s view of the surroundings are not obstructed but information can augment the wearer&#39;s view of the surroundings. In embodiments, the time spent looking along a target line in the direction of displayed digital content may be an indication of interest in the digital content. This may be used as a conversion event in advertising. For example, an advertiser may pay more for an add placement if the wearer of the HWC  102  looks ata displayed advertisement for a certain period of time. As such, in embodiments, the time spent looking at the advertisement, as assessed by comparing eye position with the content placement, target line or other appropriate position may be used to determine a rate of conversion or other compensation amount due for the presentation. 
     An aspect of the invention relates to removing content from the FOV of the HWC  102  when the wearer of the HWC  102  apparently wants to view the surrounding environments clearly.  FIG. 38  illustrates a situation where eye imagery suggests that the eye has or is moving quickly so the digital content  3804  in the FOV  3808  is removed from the FOV  3808 . In this example, the wearer may be looking quickly to the side indicating that there is something on the side in the environment that has grabbed the wearer&#39;s attention. This eye movement  3802  may be captured through eye imaging techniques (e.g. as described herein) and if the movement matches a predetermined movement (e.g. speed, rate, pattern, etc.) the content may be removed from view. In embodiments, the eye movement is used as one input and HWC movements indicated by other sensors (e.g. IMU in the HWC) may be used as another indication. These various sensor movements may be used together to project an event that should cause a change in the content being displayed in the FOV. 
     Another aspect of the present invention relates to determining a focal plane based on the wearer&#39;s eye convergence. Eyes are generally converged slightly and converge more when the person focuses on something very close. This is generally referred to as convergence. In embodiments, convergence is calibrated for the wearer. That is, the wearer may be guided through certain focal plane exercises to determine how much the wearer&#39;s eyes converge at various focal planes and at various viewing angles. The convergence information may then be stored in a database for later reference. In embodiments, a general table may be used in the event there is no calibration step or the person skips the calibration step. The two eyes may then be imaged periodically to determine the convergence in an attempt to understand what focal plane the wearer is focused on. In embodiments, the eyes may be imaged to determine a virtual target line and then the eye&#39;s convergence may be determined to establish the wearer&#39;s focus, and the digital content may be displayed or altered based thereon. 
       FIG. 39  illustrates a situation where digital content is moved  3902  within one or both of the FOVs  3908  and  3910  to align with the convergence of the eyes as determined by the pupil movement  3904 . By moving the digital content to maintain alignment, in embodiments, the overlapping nature of the content is maintained so the object appears properly to the wearer. This can be important in situations where 3D content is displayed. 
     An aspect of the present invention relates to controlling the HWC  102  based on events detected through eye imaging. A wearer winking, blinking, moving his eyes in a certain pattern, etc. may, for example, control an application of the HWC  102 . Eye imaging (e.g. as described herein) may be used to monitor the eye(s) of the wearer and once a pre-determined pattern is detected an application control command may be initiated. 
     An aspect of the invention relates to monitoring the health of a person wearing a HWC  102  by monitoring the wearer&#39;s eye(s). Calibrations may be made such that the normal performance, under various conditions (e.g. lighting conditions, image light conditions, etc.) of a wearer&#39;s eyes may be documented. The wearer&#39;s eyes may then be monitored through eye imaging (e.g. as described herein) for changes in their performance. Changes in performance may be indicative of a health concern (e.g. concussion, brain injury, stroke, loss of blood, etc.). If detected the data indicative of the change or event may be communicated from the HWC  102 . 
     Aspects of the present invention relate to security and access of computer assets (e.g. the HWC itself and related computer systems) as determined through eye image verification. As discussed herein elsewhere, eye imagery may be compared to known person eye imagery to confirm a person&#39;s identity. Eye imagery may also be used to confirm the identity of people wearing the HWCs  102  before allowing them to link together or share files, streams, information, etc. 
     A variety of use cases for eye imaging are possible based on technologies described herein. An aspect of the present invention relates to the timing of eye image capture. The timing of the capture of the eye image and the frequency of the capture of multiple images of the eye can vary dependent on the use case for the information gathered from the eye image. For example, capturing an eye image to identify the user of the HWC may be required only when the HWC has been turned ON or when the HWC determines that the HWC has been put onto a wearer&#39;s head, to control the security of the HWC and the associated information that is displayed to the user. Wherein, the orientation, movement pattern, stress or position of the earhorns (or other portions of the HWC) of the HWC can be used to determine that a person has put the HWC onto their head with the intention to use the HWC. Those same parameters may be monitored in an effort to understand when the HWC is dismounted from the user&#39;s head. This may enable a situation where the capture of an eye image for identifying the wearer may be completed only when a change in the wearing status is identified. In a contrasting example, capturing eye images to monitor the health of the wearer may require images to be captured periodically (e.g. every few seconds, minutes, hours, days, etc.). For example, the eye images may be taken in minute intervals when the images are being used to monitor the health of the wearer when detected movements indicate that the wearer is exercising. In a further contrasting example, capturing eye images to monitor the health of the wearer for long-term effects may only require that eye images be captured monthly. Embodiments of the invention relate to selection of the timing and rate of capture of eye images to be in correspondence with the selected use scenario associated with the eye images. These selections may be done automatically, as with the exercise example above where movements indicate exercise, or these selections may be set manually. In a further embodiment, the selection of the timing and rate of eye image capture is adjusted automatically depending on the mode of operation of the HWC. The selection of the timing and rate of eye image capture can further be selected in correspondence with input characteristics associated with the wearer including age and health status, or sensed physical conditions of the wearer including heart rate, chemical makeup of the blood and eye blink rate. 
       FIG. 40  illustrates an embodiment in which digital content presented in a see-through FOV is positioned based on the speed in which the wearer is moving. When the person is not moving, as measured by sensor(s) in the HWC  102  (e.g. IMU, GPS based tracking, etc.), digital content may be presented at the stationary person content position  4004 . The content position  4004  is indicated as being in the middle of the see-through FOV  4002 ; however, this is meant to illustrate that the digital content is positioned within the see-through FOV at a place that is generally desirable knowing that the wearer is not moving and as such the wearer&#39;s surrounding see through view can be somewhat obstructed. So, the stationary person content position, or neutral position, may not be centered in the see-through FOV; it may be positioned somewhere in the see-through FOV deemed desirable and the sensor feedback may shift the digital content from the neutral position. The movement of the digital content for a quickly moving person is also shown in  FIG. 40  wherein as the person turns their head to the side, the digital content moves out of the see-through FOV to content position  4008  and then moves back as the person turns their head back. For a slowly moving person, the head movement can be more complex and as such the movement of the digital content in an out of the see-through FOV can follow a path such as that shown by content position  4010 . 
     In embodiments, the sensor that assesses the wearer&#39;s movements may be a GPS sensor, IMU, accelerometer, etc. The content position may be shifted from a neutral position to a position towards a side edge of the field of view as the forward motion increases. The content position may be shifted from a neutral position to a position towards a top or bottom edge of the field of view as the forward motion increases. The content position may shift based on a threshold speed of the assessed motion. The content position may shift linearly based on the speed of the forward motion. The content position may shift non-linearly based on the speed of the forward motion. The content position may shift outside of the field of view. In embodiments, the content is no longer displayed if the speed of movement exceeds a predetermined threshold and will be displayed again once the forward motion slows. 
     In embodiments, the content position may generally be referred to as shifting; it should be understood that the term shifting encompasses a process where the movement from one position to another within the see-through FOV or out of the FOV is visible to the wearer (e.g. the content appears to slowly or quickly move and the user perceives the movement itself) or the movement from one position to another may not be visible to the wearer (e.g. the content appears to jump in a discontinuous fashion or the content disappears and then reappears in the new position). 
     Another aspect of the present invention relates to removing the content from the field of view or shifting it to a position within the field of view that increases the wearer&#39;s view of the surrounding environment when a sensor causes an alert command to be issued. In embodiments, the alert may be due to a sensor or combination of sensors that sense a condition above a threshold value. For example, if an audio sensor detects a loud sound of a certain pitch, content in the field of view may be removed or shifted to provide a clear view of the surrounding environment for the wearer. In addition to the shifting of the content, in embodiments, an indication of why the content was shifted may be presented in the field of view or provided through audio feedback to the wearer. For instance, if a carbon monoxide sensor detects a high concentration in the area, content in the field of view may be shifted to the side of the field of view or removed from the field of view and an indication may be provided to the wearer that there is a high concentration of carbon monoxide in the area. This new information, when presented in the field of view, may similarly be shifted within or outside of the field of view depending on the movement speed of the wearer. 
       FIG. 41  illustrates how content may be shifted from a neutral position  4104  to an alert position  4108 . In this embodiment, the content is shifted outside of the see-through FOV  4102 . In other embodiments, the content may be shifted as described herein. 
     Another aspect of the present invention relates to identification of various vectors or headings related to the HWC  102 , along with sensor inputs, to determine how to position content in the field of view. In embodiments, the speed of movement of the wearer is detected and used as an input for position of the content and, depending on the speed, the content may be positioned with respect to a movement vector or heading (i.e. the direction of the movement), or a sight vector or heading (i.e. the direction of the wearer&#39;s sight direction). For example, if the wearer is moving very fast the content may be positioned within the field of view with respect to the movement vector because the wearer is only going to be looking towards the sides of himself periodically and for short periods of time. As another example, if the wearer is moving slowly, the content may be positioned with respect to the sight heading because the user may more freely be shifting his view from side to side. 
       FIG. 42  illustrates two examples where the movement vector may effect content positioning. Movement vector A  4202  is shorter than movement vector B  4210  indicating that the forward speed and/or acceleration of movement of the person associated with movement vector A  4202  is lower than the person associated with movement vector B  4210 . Each person is also indicated as having a sight vector or heading  4208  and  4212 . The sight vectors A  4208  and B  4210  are the same from a relative perspective. The white area inside of the black triangle in front of each person is indicative of how much time each person likely spends looking at a direction that is not in line with the movement vector. The time spent looking off angle A  4204  is indicated as being more than that of the time spent looking off angle B  4214 . This may be because the movement vector speed A is lower than movement vector speed B. The faster the person moves forward the more the person tends to look in the forward direction, typically. The FOVs A  4218  and B  4222  illustrate how content may be aligned depending on the movement vectors  4202  and  4210  and sight vectors  4208  and  4212 . FOV A  4218  is illustrated as presenting content in-line with the sight vector  4220 . This may be due to the lower speed of the movement vector A  4202 . This may also be due to the prediction of a larger amount of time spent looking off angle A  4204 . FOV B  4222  is illustrated as presenting content in line with the movement vector  4224 . This may be due to the higher speed of movement vector B  4210 . This may also be due to the prediction of a shorter amount of time spent looking off angle B  4214 . 
     Another aspect of the present invention relates to damping a rate of content position change within the field of view. As illustrated in  FIG. 43 , the sight vector may undergo a rapid change  4304 . This rapid change may be an isolated event or it may be made at or near a time when other sight vector changes are occurring. The wearer&#39;s head may be turning back and forth for some reason. In embodiments, the rapid successive changes in sight vector may cause a damped rate of content position change  4308  within the FOV  4302 . For example, the content may be positioned with respect to the sight vector, as described herein, and the rapid change in sight vector may normally cause a rapid content position change; however, since the sight vector is successively changing, the rate of position change with respect to the sight vector may be damped, slowed, or stopped. The position rate change may be altered based on the rate of change of the sight vector, average of the sight vector changes, or otherwise altered. 
     Another aspect of the present invention relates to simultaneously presenting more than one content in the field of view of a see-through optical system of a HWC  102  and positioning one content with the sight heading and one content with the movement heading.  FIG. 44  illustrates two FOV&#39;s A  4414  and B  4420 , which correspond respectively to the two identified sight vectors A  4402  and B  4404 .  FIG. 44  also illustrates an object in the environment  4408  at a position relative to the sight vectors A  4402  and B  4404 . When the person is looking along sight vector A  4402 , the environment object  4408  can be seen through the field of view A  4414  at position  4412 . As illustrated, sight heading aligned content is presented as TEXT in proximity with the environment object  4412 . At the same time, other content  4418  is presented in the field of view A  4414  at a position aligned in correspondence with the movement vector. As the movement speed increases, the content  4418  may shift as described herein. When the sight vector of the person is sight vector B  4404  the environmental object  4408  is not seen in the field of view B  4420 . As a result, the sight aligned content  4410  is not presented in field of view B  4420 ; however, the movement aligned content  4418  is presented and is still dependent on the speed of the motion. 
       FIG. 45  shows an example set of data for a person moving through an environment over a path that starts with a movement heading of 0 degrees and ends with a movement heading of 114 degrees during which time the speed of movement varies from 0 m/sec to 20 m/sec. The sight heading can be seen to vary on either side of the movement heading while moving as the person looks from side to side. Large changes in sight heading occur when the movement speed is 0 m/sec when the person is standing still, followed by step changes in movement heading. 
     Embodiments provide a process for determining the display heading that takes into account the way a user moves through an environment and provides a display heading that makes it easy for the user to find the displayed information while also providing unencumbered see-through views of the environment in response to different movements, speed of movement or different types of information being displayed. 
       FIG. 46  illustrates a see-through view as may be seen when using a HWC wherein information is overlaid onto a see-through view of the environment. The tree and the building are actually in the environment and the text is displayed in the see-through display such that it appears overlaid on the environment. In addition to text information such as, for example, instructions and weather information, some augmented reality information is shown that relates to nearby objects in the environment. 
     In an embodiment, the display heading is determined based on speed of movement. At low speeds, the display heading may be substantially the same as the sight heading while at high speed the display heading may be substantially the same as the movement heading. In embodiments, as long as the user remains stationary, the displayed information is presented directly in front of the user and HWC. However, as the movement speed increases (e.g. above a threshold or continually, etc.) the display heading becomes substantially the same as the movement heading regardless of the direction the user is looking, so that when the user looks in the direction of movement, the displayed information is directly in front of the user and HMD and when the user looks to the side the displayed information is not visible. 
     Rapid changes in sight heading can be followed by a slower change in the display heading to provide a damped response to head rotation. Alternatively, the display heading can be substantially the time averaged sight heading so that the displayed information is presented at a heading that is in the middle of a series of sight headings over a period of time. In this embodiment, if the user stops moving their head, the display heading gradually becomes the same as the sight heading and the displayed information moves into the display field of view in front of the user and HMD. In embodiments, when there is a high rate of sight heading change, the process delays the effect of the time averaged sight heading on the display heading. In this way, the effect of rapid head movements on display heading is reduced and the positioning of the displayed information within the display field of view is stabilized laterally. 
     In another embodiment, display heading is determined based on speed of movement where at high-speed, the display heading is substantially the same as the movement heading. At mid-speed the display heading is substantially the same as a time averaged sight heading so that rapid head rotations are damped out and the display heading is in the middle of back and forth head movements. 
     In yet another embodiment, the type of information being displayed is included in determining how the information should be displayed. Augmented reality information that is connected to objects in the environment is given a display heading that substantially matches the sight heading. In this way, as the user rotates their head, augmented reality information comes into view that is related to objects that are in the see-through view of the environment. At the same time, information that is not connected to objects in the environment is given a display heading that is determined based on the type of movements and speed of movements as previously described in this specification. 
     In yet a further embodiment, when the speed of movement is determined to be above a threshold, the information displayed is moved downward in the display field of view so that the upper portion of the display field of view has less information or no information displayed to provide the user with an unencumbered see-through view of the environment. 
       FIGS. 47 and 48  show illustrations of a see-through view including overlaid displayed information.  FIG. 47  shows the see-through view immediately after a rapid change in sight heading from the sight heading associated with the see-through view shown in  FIG. 46  wherein the change in sight heading comes from a head rotation. In this case, the display heading is delayed.  FIG. 48  shows how at a later time, the display heading catches up to the sight heading. The augmented reality information remains in positions within the display field of view where the association with objects in the environment can be readily made by the user. 
       FIG. 49  shows an illustration of a see-through view example including overlaid displayed information that has been shifted downward in the display field of view to provide an unencumbered see-through view in the upper portion of the see-through view. At the same time, augmented reality labels have been maintained in locations within the display field of view so they can be readily associated with objects in the environment. 
     In a further embodiment, in an operating mode such as when the user is moving in an environment, digital content is presented at the side of the user&#39;s see-through FOV so that the user can only view the digital content by turning their head. In this case, when the user is looking straight ahead, such as when the movement heading matches the sight heading, the see-through view FOV does not include digital content. The user then accesses the digital content by turning their head to the side whereupon the digital content moves laterally into the user&#39;s see-through FOV. In another embodiment, the digital content is ready for presentation and will be presented if an indication for it&#39;s presentation is received. For example, the information may be ready for presentation and if the sight heading or predetermined position of the HWC  102  is achieved the content may then be presented. The wearer may look to the side and the content may be presented. In another embodiment, the user may cause the content to move into an area in the field of view by looking in a direction for a predetermined period of time, blinking, winking, or displaying some other pattern that can be captured through eye imaging technologies (e.g. as described herein elsewhere). 
     In yet another embodiment, an operating mode is provided wherein the user can define sight headings wherein the associated see-through FOV includes digital content or does not include digital content. In an example, this operating mode can be used in an office environment where when the user is looking at a wall digital content is provided within the FOV, whereas when the user is looking toward a hallway, the FOV is unencumbered by digital content. In another example, when the user is looking horizontally digital content is provided within the FOV, but when the user looks down (e.g. to look at a desktop or a cellphone) the digital content is removed from the FOV. 
     Another aspect of the present invention relates to collecting and using eye position and sight heading information. Head worn computing with motion heading, sight heading, and/or eye position prediction (sometimes referred to as “eye heading” herein) may be used to identify what a wearer of the HWC  102  is apparently interested in and the information may be captured and used. In embodiments, the information may be characterized as viewing information because the information apparently relates to what the wearer is looking at. The viewing information may be used to develop a personal profile for the wearer, which may indicate what the wearer tends to look at. The viewing information from several or many HWC&#39;s  102  may be captured such that group or crowd viewing trends may be established. For example, if the movement heading and sight heading are known, a prediction of what the wearer is looking at may be made and used to generate a personal profile or portion of a crowd profile. In another embodiment, if the eye heading and location, sight heading and/or movement heading are known, a prediction of what is being looked at may be predicted. The prediction may involve understanding what is in proximity of the wearer and this may be understood by establishing the position of the wearer (e.g. through GPS or other location technology) and establishing what mapped objects are known in the area. The prediction may involve interpreting images captured by the camera or other sensors associated with the HWC  102 . For example, if the camera captures an image of a sign and the camera is in-line with the sight heading, the prediction may involve assessing the likelihood that the wearer is viewing the sign. The prediction may involve capturing an image or other sensory information and then performing object recognition analysis to determine what is being viewed. For example, the wearer may be walking down a street and the camera that is in the HWC  102  may capture an image and a processor, either on-board or remote from the HWC  102 , may recognize a face, object, marker, image, etc. and it may be determined that the wearer may have been looking at it or towards it. 
       FIG. 50  illustrates a cross section of an eyeball of a wearer of an HWC with focus points that can be associated with the eye imaging system of the invention. The eyeball  5010  includes an iris  5012  and a retina  5014 . Because the eye imaging system of the invention provides coaxial eye imaging with a display system, images of the eye can be captured from a perspective directly in front of the eye and inline with where the wearer is looking. In embodiments of the invention, the eye imaging system can be focused at the iris  5012  and/or the retina  5014  of the wearer, to capture images of the external surface of the iris  5012  or the internal portions of the eye, which includes the retina  5014 .  FIG. 50  shows light rays  5020  and  5025  that are respectively associated with capturing images of the iris  5012  or the retina  5014  wherein the optics associated with the eye imaging system are respectively focused at the iris  5012  or the retina  5014 . Illuminating light can also be provided in the eye imaging system to illuminate the iris  5012  or the retina  5014 .  FIG. 51  shows an illustration of an eye including an iris  5130  and a sclera  5125 . In embodiments, the eye imaging system can be used to capture images that include the iris  5130  and portions the sclera  5125 . The images can then be analyzed to determine color, shapes and patterns that are associated with the user. In further embodiments, the focus of the eye imaging system is adjusted to enable images to be captured of the iris  5012  or the retina  5014 . Illuminating light can also be adjusted to illuminate the iris  5012  or to pass through the pupil of the eye to illuminate the retina  5014 . The illuminating light can be visible light to enable capture of colors of the iris  5012  or the retina  5014 , or the illuminating light can be ultraviolet (e.g. 340 nm), near infrared (e.g. 850 nm) or mid-wave infrared (e.g. 5000 nm) light to enable capture of hyperspectral characteristics of the eye. 
       FIG. 53  illustrates a display system that includes an eye imaging system. The display system includes a polarized light source  2958 , a DLP  2955 , a quarter wave film  2957  and a beam splitter plate  5345 . The eye imaging system includes a camera  3280 , illuminating lights  5355  and beam splitter plate  5345 . Where the beam splitter plate  5345  can be a reflective polarizer on the side facing the polarized light source  2958  and a hot mirror on the side facing the camera  3280 . Wherein the hot mirror reflects infrared light (e.g. wavelengths 700 to 2000 nm) and transmits visible light (e.g. wavelengths 400 to 670 nm). The beam splitter plate  5345  can be comprised of multiple laminated films, a substrate film with coatings or a rigid transparent substrate with films on either side. By providing a reflective polarizer on the one side, the light from the polarized light source  2958  is reflected toward the DLP  2955  where it passes through the quarter wave film  2957  once, is reflected by the DLP mirrors in correspondence with the image content being displayed by the DLP  2955  and then passes back through the quarter wave film  2957 . In so doing, the polarization state of the light from the polarized light source is changed, so that it is transmitted by the reflective polarizer on the beam splitter plate  5345  and the image light  2971  passes into the lower optics module  204  where the image is displayed to the user. At the same time, infrared light  5357  from the illuminating lights  5355  is reflected by the hot mirror so that it passes into the lower optics module  204  where it illuminates the user&#39;s eye. Portions of the infrared light  2969  are reflected by the user&#39;s eye and this light passes back through the lower optics module  204 , is reflected by the hot mirror on the beam splitter plate  5345  and is captured by the camera  3280 . In this embodiment, the image light  2971  is polarized while the infrared light  5357  and  2969  can be unpolarized. In an embodiment, the illuminating lights  5355  provide two different infrared wavelengths and eye images are captured in pairs, wherein the pairs of eye images are analyzed together to improve the accuracy of identification of the user based on iris analysis. 
       FIG. 54  shows an illustration of a further embodiment of a display system with an eye imaging system. In addition to the features of  FIG. 53 , this system includes a second camera  5460 . Wherein the second camera  5460  is provided to capture eye images in the visible wavelengths. Illumination of the eye can be provided by the displayed image or by see-through light from the environment. Portions of the displayed image can be modified to provide improved illumination of the user&#39;s eye when images of the eye are to be captured such as by increasing the brightness of the displayed image or increasing the white areas within the displayed image. Further, modified displayed images can be presented briefly for the purpose of capturing eye images and the display of the modified images can be synchronized with the capture of the eye images. As shown in  FIG. 54 , visible light  5467  is polarized when it is captured by the second camera  5460  since it passes through the beam splitter  5445  and the beam splitter  5445  is a reflective polarizer on the side facing the second camera  5460 . In this eye imaging system, visible eye images can be captured by the second camera  5460  at the same time that infrared eye images are captured by the camera  3280 . Wherein, the characteristics of the camera  3280  and the second camera  5460  and the associated respective images captured can be different in terms of resolution and capture rate. 
       FIGS. 52 a  and 52 b    illustrate captured images of eyes where the eyes are illuminated with structured light patterns. In  FIG. 52 a   , an eye  5220  is shown with a projected structured light pattern  5230 , where the light pattern is a grid of lines. A light pattern of such as  5230  can be provided by the light source  5355  show in  FIG. 53  by including a diffractive or a refractive device to modify the light  5357  as are known by those skilled in the art. A visible light source can also be included for the second camera  5460  shown in  FIG. 54  which can include a diffractive or refractive to modify the light  5467  to provide a light pattern.  FIG. 52 b    illustrates how the structured light pattern of  5230  becomes distorted to  5235  when the user&#39;s eye  5225  looks to the side. This distortion comes from the fact that the human eye is not spherical in shape, instead the iris sticks out slightly from the eyeball to form a bump in the area of the iris. As a result, the shape of the eye and the associated shape of the reflected structured light pattern is different depending on which direction the eye is pointed, when images of the eye are captured from a fixed position. Changes in the structured light pattern can subsequently be analyzed in captured eye images to determine the direction that the eye is looking. 
     The eye imaging system can also be used for the assessment of aspects of health of the user. In this case, information gained from analyzing captured images of the iris  5012  is different from information gained from analyzing captured images of the retina  5014 . Where images of the retina  5014  are captured using light  5357  that illuminates the inner portions of the eye including the retina  5014 . The light  5357  can be visible light, but in an embodiment, the light  5357  is infrared light (e.g. wavelength 1 to 5 microns) and the camera  3280  is an infrared light sensor (e.g. an InGaAs sensor) or a low resolution infrared image sensor that is used to determine the relative amount of light  5357  that is absorbed, reflected or scattered by the inner portions of the eye. Wherein the majority of the light that is absorbed, reflected or scattered can be attributed to materials in the inner portion of the eye including the retina where there are densely packed blood vessels with thin walls so that the absorption, reflection and scattering are caused by the material makeup of the blood. These measurements can be conducted automatically when the user is wearing the HWC, either at regular intervals, after identified events or when prompted by an external communication. In a preferred embodiment, the illuminating light is near infrared or mid infrared (e.g. 0.7 to 5 microns wavelength) to reduce the chance for thermal damage to the wearer&#39;s eye. In another embodiment, the polarizer  3285  is antireflection coated to reduce any reflections from this surface from the light  5357 , the light  2969  or the light  3275  and thereby increase the sensitivity of the camera  3280 . In a further embodiment, the light source  5355  and the camera  3280  together comprise a spectrometer wherein the relative intensity of the light reflected by the eye is analyzed over a series of narrow wavelengths within the range of wavelengths provided by the light source  5355  to determine a characteristic spectrum of the light that is absorbed, reflected or scattered by the eye. For example, the light source  5355  can provide a broad range of infrared light to illuminate the eye and the camera  3280  can include: a grating to laterally disperse the reflected light from the eye into a series of narrow wavelength bands that are captured by a linear photodetector so that the relative intensity by wavelength can be measured and a characteristic absorbance spectrum for the eye can be determined over the broad range of infrared. In a further example, the light source  5355  can provide a series of narrow wavelengths of light (ultraviolet, visible or infrared) to sequentially illuminate the eye and camera  3280  includes a photodetector that is selected to measure the relative intensity of the series of narrow wavelengths in a series of sequential measurements that together can be used to determine a characteristic spectrum of the eye. The determined characteristic spectrum is then compared to known characteristic spectra for different materials to determine the material makeup of the eye. In yet another embodiment, the illuminating light  5357  is focused on the retina  5014  and a characteristic spectrum of the retina  5014  is determined and the spectrum is compared to known spectra for materials that may be present in the user&#39;s blood. For example, in the visible wavelengths 540 nm is useful for detecting hemoglobin and 660 nm is useful for differentiating oxygenated hemoglobin. In a further example, in the infrared, a wide variety of materials can be identified as is known by those skilled in the art, including: glucose, urea, alcohol and controlled substances.  FIG. 55  shows a series of example spectrum for a variety of controlled substances as measured using a form of infrared spectroscopy (ThermoScientific Application Note 51242, by C. Petty, B. Garland and the Mesa Police Department Forensic Laboratory, which is hereby incorporated by reference herein).  FIG. 56  shows an infrared absorbance spectrum for glucose (Hewlett Packard Company 1999, G. Hopkins, G. Mauze; “In-vivo NIR Diffuse-reflectance Tissue Spectroscopy of Human Subjects,” which is hereby incorporated by reference herein). U.S. Pat. No. 6,675,030, which is hereby incorporated by reference herein, provides a near infrared blood glucose monitoring system that includes infrared scans of a body part such as a foot. United States Patent publication 2006/0183986, which is hereby incorporated by reference herein, provides a blood glucose monitoring system including a light measurement of the retina. Embodiments of the present invention provide methods for automatic measurements of specific materials in the user&#39;s blood by illuminating at one or more narrow wavelengths into the iris of the wearer&#39;s eye and measuring the relative intensity of the light reflected by the eye to identify the relative absorbance spectrum and comparing the measured absorbance spectrum with known absorbance spectra for the specific material, such as illuminating at 540 and 660 nm to determine the level of hemoglobin present in the user&#39;s blood. 
     Another aspect of the present invention relates to collecting and using eye position and sight heading information. Head worn computing with motion heading, sight heading, and/or eye position prediction (sometimes referred to as “eye heading” herein) may be used to identify what a wearer of the HWC  102  is apparently interested in and the information may be captured and used. In embodiments, the information may be characterized as viewing information because the information apparently relates to what the wearer is looking at. The viewing information may be used to develop a personal profile for the wearer, which may indicate what the wearer tends to look at. The viewing information from several or many HWC&#39;s  102  may be captured such that group or crowd viewing trends may be established. For example, if the movement heading and sight heading are known, a prediction of what the wearer is looking at may be made and used to generate a personal profile or portion of a crowd profile. In another embodiment, if the eye heading and location, sight heading and/or movement heading are known, a prediction of what is being looked at may be predicted. The prediction may involve understanding what is in proximity of the wearer and this may be understood by establishing the position of the wearer (e.g. through GPS or other location technology) and establishing what mapped objects are known in the area. The prediction may involve interpreting images captured by the camera or other sensors associated with the HWC  102 . For example, if the camera captures an image of a sign and the camera is in-line with the sight heading, the prediction may involve assessing the likelihood that the wearer is viewing the sign. The prediction may involve capturing an image or other sensory information and then performing object recognition analysis to determine what is being viewed. For example, the wearer may be walking down a street and the camera that is in the HWC  102  may capture an image and a processor, either on-board or remote from the HWC  102 , may recognize a face, object, marker, image, etc. and it may be determined that the wearer may have been looking at it or towards it. 
       FIG. 57  illustrates a scene where a person is walking with a HWC  102  mounted on his head. In this scene, the person&#39;s geo-spatial location  5704  is known through a GPS sensor, which could be another location system, and his movement heading, sight heading  5714  and eye heading  5702  are known and can be recorded (e.g. through systems described herein). There are objects and a person in the scene. Person  5712  may be recognized by the wearer&#39;s HWC  102  system, the person may be mapped (e.g. the person&#39;s GPS location may be known or recognized), or otherwise known. The person may be wearing a garment or device that is recognizable. For example, the garment may be of a certain style and the HWC may recognize the style and record it&#39;s viewing. The scene also includes a mapped object  5718  and a recognized object  5720 . As the wearer moves through the scene, the sight and/or eye headings may be recorded and communicated from the HWC  102 . In embodiments, the time that the sight and/or eye heading maintains a particular position may be recorded. For example, if a person appears to look at an object or person for a predetermined period of time (e.g. 2 seconds or longer), the information may be communicated as gaze persistence information as an indication that the person may have been interested in the object. 
     In embodiments, sight headings may be used in conjunction with eye headings or eye and/or sight headings may be used alone. Sight headings can do a good job of predicting what direction a wearer is looking because many times the eyes are looking forward, in the same general direction as the sight heading. In other situations, eye headings may be a more desirable metric because the eye and sight headings are not always aligned. In embodiments herein examples may be provided with the term “eye/sight” heading, which indicates that either or both eye heading and sight heading may be used in the example. 
       FIG. 58  illustrates a system for receiving, developing and using movement heading, sight heading, eye heading and/or persistence information from HWC(s)  102 . The server  5804  may receive heading or gaze persistence information, which is noted as persistence information  5802 , for processing and/or use. The heading and/or gaze persistence information may be used to generate a personal profile  5808  and/or a group profile  5810 . The personal profile  5718  may reflect the wearer&#39;s general viewing tendencies and interests. The group profile  5810  may be an assemblage of different wearer&#39;s heading and persistence information to create impressions of general group viewing tendencies and interests. The group profile  5810  may be broken into different groups based on other information such as gender, likes, dislikes, biographical information, etc. such that certain groups can be distinguished from other groups. This may be useful in advertising because an advertiser may be interested in what a male adult sports go&#39;er is generally looking at as oppose to a younger female. The profiles  5808  and  5810  and raw heading and persistence information may be used by retailers  5814 , advertisers  5818 , trainers, etc. For example, an advertiser may have an advertisement posted in an environment and may be interested in knowing how many people look at the advertisement, how long they look at it and where they go after looking at it. This information may be used as conversion information to assess the value of the advertisement and thus the payment to be received for the advertisement. 
     In embodiments, the process involves collecting eye and/or sight heading information from a plurality of head-worn computers that come into proximity with an object in an environment. For example, a number of people may be walking through an area and each of the people may be wearing a head worn computer with the ability to track the position of the wearer&#39;s eye(s) as well as possibly the wearer&#39;s sight and movement headings. The various HWC wearing individuals may then walk, ride, or otherwise come into proximity with some object in the environment (e.g. a store, sign, person, vehicle, box, bag, etc.). When each person passes by or otherwise comes near the object, the eye imaging system may determine if the person is looking towards the object. All of the eye/sight heading information may be collected and used to form impressions of how the crowd reacted to the object. A store may be running a sale and so the store may put out a sign indicating such. The storeowners and managers may be very interested to know if anyone is looking at their sign. The sign may be set as the object of interest in the area and as people navigate near the sign, possibly determined by their GPS locations, the eye/sight heading determination system may record information relative to the environment and the sign. Once, or as, the eye/sight heading information is collected and associations between the eye headings and the sign are determined, feedback may be sent back to the storeowner, managers, advertiser, etc. as an indication of how well their sign is attracting people. In embodiments, the sign&#39;s effectiveness at attracting people&#39;s attention, as indicated through the eye/sight headings, may be considered a conversion metric and impact the economic value of the sign and/or the signs placement. 
     In embodiments, a map of the environment with the object may be generated by mapping the locations and movement paths of the people in the crowd as they navigate by the object (e.g. the sign). Layered on this map may be an indication of the various eye/sight headings. This may be useful in indicating wear people were in relation to the object when then viewed they object. The map may also have an indication of how long people looked at the object from the various positions in the environment and where they went after seeing the object. 
     In embodiments, the process involves collecting a plurality of eye/sight headings from a head-worn computer, wherein each of the plurality of eye/sight headings is associated with a different pre-determined object in an environment. This technology may be used to determine which of the different objects attracts more of the person&#39;s attention. For example, if there are three objects placed in an environment and a person enters the environment navigating his way through it, he may look at one or more of the objects and his eye/sight heading may persist on one or more objects longer than others. This may be used in making or refining the person&#39;s personal attention profile and/or it may be used in connection with other such people&#39;s data on the same or similar objects to determine an impression of how the population or crowd reacts to the objects. Testing advertisements in this way may provide good feedback of its effectiveness. 
     In embodiments, the process may involve capturing eye/sight headings once there is substantial alignment between the eye/sight heading and an object of interest. For example, the person with the HWC may be navigating through an environment and once the HWC detects substantial alignment or the projected occurrence of an upcoming substantial alignment between the eye/sight heading and the object of interest, the occurrence and/or persistence may be recorded for use. 
     In embodiments, the process may involve collecting eye/sight heading information from a head-worn computer and collecting a captured image from the head-worn computer that was taken at substantially the same time as the eye/sight heading information was captured. These two pieces of information may be used in conjunction to gain an understanding of what the wearer was looking at and possibly interested in. The process may further involve associating the eye/sight heading information with an object, person, or other thing found in the captured image. This may involve processing the captured image looking for objects or patterns. In embodiments, gaze time or persistence may be measured and used in conjunction with the image processing. The process may still involve object and/or pattern recognition, but it may also involve attempting to identify what the person gazed at for the period of time by more particularly identifying a portion of the image in conjunction with image processing. 
     In embodiments, the process may involve setting a pre-determined eye/sight heading from a pre-determined geospatial location and using them as triggers. In the event that a head worn computer enters the geospatial location and an eye/sight heading associated with the head worn computer aligns with the pre-determined eye/sight heading, the system may collect the fact that there was an apparent alignment and/or the system may record information identifying how long the eye/sight heading remains substantially aligned with the pre-determined eye/sight heading to form a persistence statistic. This may eliminate or reduce the need for image processing as the triggers can be used without having to image the area. In other embodiments, image capture and processing is performed in conjunction with the triggers. In embodiments, the triggers may be a series a geospatial locations with corresponding eye/sight headings such that many spots can be used as triggers that indicate when a person entered an area in proximity to an object of interest and/or when that person actually appeared to look at the object. 
     In embodiments, eye imaging may be used to capture images of both eyes of the wearer in order to determine the amount of convergence of the eyes (e.g. through technologies described herein elsewhere) to get an understanding of what focal plane is being concentrated on by the wearer. For example, if the convergence measurement suggests that the focal plane is within 15 feet of the wearer, than, even though the eye/sight headings may align with an object that is more than 15 feet away it may be determined that the wearer was not looking at the object. If the object were within the 15 foot suggested focal plane, the determination may be that the wearer was looking at the object.  FIG. 59  illustrates an environmentally position locked digital content  5912  that is indicative of a person&#39;s location  5902 . In this disclosure the term “BlueForce” is generally used to indicate team members or members for which geo-spatial locations are known and can be used. In embodiments, “BlueForce” is a term to indicate members of a tactical arms team (e.g. a police force, secret service force, security force, military force, national security force, intelligence force, etc.). In many embodiments herein one member may be referred to as the primary or first BlueForce member and it is this member, in many described embodiments, that is wearing the HWC. It should be understood that this terminology is to help the reader and make for clear presentations of the various situations and that other members of the Blueforce, or other people, may have HWC&#39;s  102  and have similar capabilities. In this embodiment, a first person is wearing a head-worn computer  102  that has a see through field of view (“FOV”)  5914 . The first person can see through the FOV to view the surrounding environment through the FOV and digital content can also be presented in the FOV such that the first person can view the actual surroundings, through the FOV, in a digitally augmented view. The other BlueForce person&#39;s location is known and is indicated at a position inside of a building at point  5902 . This location is known in three dimensions, longitude, latitude and altitude, which may have been determined by GPS along with an altimeter associated with the other Blueforce person. Similarly, the location of the first person wearing the HWC  102  is also known, as indicated in  FIG. 59  as point  5908 . In this embodiment, the compass heading  5910  of the first person is also known. With the compass heading  5910  known, the angle in which the first person is viewing the surroundings can be estimated. A virtual target line between the location of the first person  5908  and the other person&#39;s location  5902  can be established in three dimensional space and emanating from the HWC  102  proximate the FOV  5914 . The three dimensionally oriented virtual target line can then be used to present environmentally position locked digital content in the FOV  5914 , which is indicative of the other person&#39;s location  5902 . The environmentally position locked digital content  5902  can be positioned within the FOV  5914  such that the first person, who is wearing the HWC  102 , perceives the content  5902  as locked in position within the environment and marking the location of the other person  5902 . 
     The three dimensionally positioned virtual target line can be recalculated periodically (e.g. every millisecond, second, minute, etc.) to reposition the environmentally position locked content  5912  to remain in-line with the virtual target line. This can create the illusion that the content  5912  is staying positioned within the environment at a point that is associated with the other person&#39;s location  5902  independent of the location of the first person  5908  wearing the HWC  102  and independent of the compass heading of the HWC  102 . 
     In embodiments, the environmentally locked digital content  5912  may be positioned with an object  5904  that is between the first person&#39;s location  5908  and the other person&#39;s location  5902 . The virtual target line may intersect the object  5904  before intersecting with the other person&#39;s location  5902 . In embodiments, the environmentally locked digital content  5912  may be associated with the object intersection point  5904 . In embodiments, the intersecting object  5904  may be identified by comparing the two person&#39;s locations  5902  and  5908  with obstructions identified on a map. In embodiments the intersecting object  5904  may be identified by processing images captured from a camera, or other sensor, associated with the HWC  102 . In embodiments, the digital content  5912  has an appearance that is indicative of being at the location of the other person  5902 , at the location of the intersecting object  5904  to provide a more clear indication of the position of the other person&#39;s position  5902  in the FOV  5914 . 
       FIG. 60  illustrates how and where digital content may be positioned within the FOV  6008  based on a virtual target line between the location of the first person  5908 , who&#39;s wearing the HWC  102 , and the other person  5902 . In addition to positioning the content in a position within the FOV  6008  that is in-line with the virtual target line, the digital content may be presented such that it comes into focus by the first person when the first person focuses at a certain plane or distance in the environment. Presented object A  6018  is digitally generated content that is presented as an image at content position A  6012 . The position  6012  is based on the virtual target line. The presented object A  6018  is presented not only along the virtual target line but also at a focal plane B  6014  such that the content at position A  6012  in the FOV  6008  comes into focus by the first person when the first person&#39;s eye  6002  focuses at something in the surrounding environment at the focal plane B  6014  distance. Setting the focal plane of the presented content provides content that does not come into focus until the eye  6002  focuses at the set focal plane. In embodiments, this allows the content at position A to be presented without when the HWC&#39;s compass is indicative of the first person looking in the direction of the other person  5902  but it will only come into focus when the first person focuses on in the direction of the other person  5902  and at the focal plane of the other person  5902 . 
     Presented object B  6020  is aligned with a different virtual target line then presented object A  6018 . Presented object B  6020  is also presented at content position B  6004  at a different focal plane than the content position A  6012 . Presented content B  6020  is presented at a further focal plane, which is indicative that the other person  5902  is physically located at a further distance. If the focal planes are sufficiently different, the content at position A will come into focus at a different time than the content at position B because the two focal planes require different focus from the eye  6002 . 
       FIG. 61  illustrates several BlueForce members at locations with various points of view from the first person&#39;s perspective. In embodiments, the relative positions, distances and obstacles may cause the digital content indicative of the other person&#39;s location to be altered. For example, if the other person can be seen by the first person through the first person&#39;s FOV, the digital content may be locked at the location of the other person and the digital content may be of a type that indicates the other person&#39;s position is being actively marked and tracked. If the other person is in relatively close proximity, but cannot be seen by the first person, the digital content may be locked to an intersecting object or area and the digital content may indicate that the actual location of the other person cannot be seen but the mark is generally tracking the other persons general position. If the other person is not within a pre-determined proximity or is otherwise more significantly obscured from the first person&#39;s view, the digital content may generally indicate a direction or area where the other person is located and the digital content may indicate that the other person&#39;s location is not closely identified or tracked by the digital content, but that the other person is in the general area. 
     Continuing to refer to  FIG. 61 , several BlueForce members are presented at various positions within an area where the first person is located. The primary BlueForce member  6102  (also referred to generally as the first person, or the person wherein the HWC with the FOV for example purposes) can directly see the BlueForce member in the open field  6104 . In embodiments, the digital content provided in the FOV of the primary BlueForce member may be based on a virtual target line and virtually locked in an environment position that is indicative of the open field position of the BlueForce member  6104 . The digital content may also indicate that the location of the open field BlueForce member is marked and is being tracked. The digital content may change forms if the BlueForce member becomes obscured from the vision of the primary BlueForce member or otherwise becomes unavailable for direct viewing. 
     BlueForce member  6108  is obscured from the primary BlueForce member&#39;s  6102  view by an obstacle that is in close proximity to the obscured member  6108 . As depicted, the obscured member  6108  is in a building but close to one of the front walls. In this situation, the digital content provided in the FOV of the primary member  6102  may be indicative of the general position of the obscured member  6108  and the digital content may indicate that, while the other person&#39;s location is fairly well marked, it is obscured so it is not as precise as if the person was in direct view. In addition, the digital content may be virtually positionally locked to some feature on the outside of the building that the obscured member is in. This may make the environmental locking more stable and also provide an indication that the location of the person is somewhat unknown. 
     BlueForce member  6110  is obscured by multiple obstacles. The member  6110  is in a building and there is another building  6112  in between the primary member  6102  and the obscured member  6110 . In this situation, the digital content in the FOV of the primary member will be spatially quite short of the actual obscured member and as such the digital content may need to be presented in a way that indicates that the obscured member  6110  is in a general direction but that the digital marker is not a reliable source of information for the particular location of obscured member  6110 . 
       FIG. 62  illustrates yet another method for positioning digital content within the FOV of a HWC where the digital content is intended to indicate a position of another person. This embodiment is similar to the embodiment described in connection with  FIG. 62  herein. The main additional element in this embodiment is the additional step of verifying the distance between the first person  5908 , the one wearing the HWC with the FOV digital content presentation of location, and the other person at location  5902 . Here, the range finder may be included in the HWC and measure a distance at an angle that is represented by the virtual target line. In the event that the range finder finds an object obstructing the path of the virtual target line, the digital content presentation in the FOV may indicate such (e.g. as described herein elsewhere). In the event that the range finder confirms that there is a person or object at the end of the presecribed distance and angle defined by the virtual target line, the digital content may represent that the proper location has been marked, as described herein elsewhere. 
     Another aspect of the present invention relates to predicting the movement of BlueForce members to maintain proper virtual marking of the BlueForce member locations.  FIG. 63  illustrates a situation where the primary BlueForce member  6302  is tracking the locations of the other BlueForce members through an augmented environment using a HWC  102 , as described herein elsewhere (e.g. as described in connection with the above figures). The primary BlueForce member  6302  may have knowledge of the tacticle movement plan  6308 . The tactical movement plan maybe maintained locally (e.g. on the HWCs  102  with sharing of the plan between the BlueForce members) or remotely (e.g. on a server and communicated to the HWC&#39;s  102 , or communicated to a subset of HWC&#39;s  102  for HWC  102  sharing). In this case, the tactical plan involves the BlueForce group generally moving in the direction of the arrow  6308 . The tactical plan may influence the presentations of digital content in the FOV of the HWC  102  of the primary BlueForce member. For example, the tactical plan may assist in the prediction of the location of the other BlueForce member and the virtual target line may be adjusted accordingly. In embodiments, the area in the tactical movement plan may be shaded or colored or otherwise marked with digital content in the FOV such that the primary BlueForce member can manage his activities with respect to the tactical plan. For example, he may be made aware that one or more BlueForce members are moving towards the tactical path  6308 . He may also be made aware of movements in the tactical path that do not appear associated with BlueForce members. 
       FIG. 63  also illustrates that internal IMU sensors in the HWCs worn by the BlueForce members may provide guidance on the movement of the members  6304 . This may be helpful in identifying when a GPS location should be updated and hence updating the position of the virtual marker in the FOV. This may also be helpful in assessing the validity of the GPS location. For example, if the GPS location has not updated but there is significant IMU sensor activity, the system may call into question the accuracy of the identified location. The IMU information may also be useful to help track the position of a member in the event the GPS information is unavailable. For example, dead reckoning may be used if the GPS signal is lost and the virtual marker in the FOV may indicate both indicated movements of the team member and indicate that the location identification is not ideal. The current tactical plan  6308  may be updated periodically and the updated plans may further refine what is presented in the FOV of the HWC  102 . 
       FIG. 64  illustrates a BlueForce tracking system in accordance with the principles of the present invention. In embodiments, the BlueForce HWC&#39;s  102  may have directional antenna&#39;s that emit relatively low power directional RF signals such that other BlueForce members within the range of the relatively low power signal can receive and assess it&#39;s direction and/or distance based on the strength and varying strength of the signals. In embodiments, the tracking of such RF signals can be used to alter the presentation of the virtual markers of persons locations within the FOV of HWC  102 . 
     Another aspect of the present invention relates to monitoring the health of BlueForce members. Each BlueForce member may be automatically monitored for health and stress events. For example, the members may have a watchband as described herein elsewhere or other wearable biometric monitoring device and the device may continually monitor the biometric information and predict health concerns or stress events. As another example, the eye imaging systems described herein elsewhere may be used to monitor pupil dilatations as compared to normal conditions to predict head trauma. Each eye may be imaged to check for differences in pupil dilation for indications of head trauma. As another example, an IMU in the HWC  102  may monitor a person&#39;s walking gate looking for changes in pattern, which may be an indication of head or other trauma. Biometric feedback from a member indicative of a health or stress concern may be uploaded to a server for sharing with other members or the information may be shared with local members, for example. Once shared, the digital content in the FOF that indicates the location of the person having the health or stress event may include an indication of the health event. 
       FIG. 65  illustrates a situation where the primary BlueForce member  6502  is monitoring the location of the BlueForce member  6504  that has had a heath event and caused a health alert to be transmitted from the HWC  102 . As described herein elsewhere, the FOV of the HWC  102  of the primary BlueForce member may include an indication of the location of the BlueForce member with the health concern  6504 . The digital content in the FOV may also include an indication of the health condition in association with the location indication. In embodiments, non-biometric sensors (e.g. IMU, camera, ranger finder, accelerometer, altimeter, etc.) may be used to provide health and/or situational conditions to the BlueForce team or other local or remote persons interested in the information. For example, if one of the BlueForce members is detected as quickly hitting the ground from a standing position an alter may be sent as an indication of a fall, the person is in trouble and had to drop down, was shot, etc. 
     Another aspect of the present invention relates to virtually marking various prior acts and events. For example, as depicted in  FIG. 66 , the techniques described herein elsewhere may be used to construct a virtual prior movement path  6604  of a BlueForce member. The virtual path may be displayed as digital content in the FOV of the primary BlueForce member  6602  using methods described herein elsewhere. As the BlueForce member moved along the path  6604  he may have virtually placed an event marker  6608  such that when another member views the location the mark can be displayed as digital content. For example, the BlueForce member may inspect and clear an area and then use an external user interface or gesture to indicate that the area has been cleared and then the location would be virtually marked and shared with BlueForce members. Then, when someone wants to understand if the location was inspected he can view the location&#39;s information. As indicated herein elsewhere, if the location is visible to the member, the digital content may be displayed in a way that indicates the specific location and if the location is not visible from the person&#39;s perspective, the digital content may be somewhat different in that it may not specifically mark the location. 
     Turning back to optical configurations, another aspect of the present invention relates to an optical configuration that provides digitally displayed content to an eye of a person wearing a head-worn display (e.g. as used in a HWC  102 ) and allows the person to see through the display such that the digital content is perceived by the person as augmenting the see through view of the surrounding environment. The optical configuration may have a variable transmission optical element that is in-line with the person&#39;s see-through view such that the transmission of the see-through view can be increased and decreased. This may be helpful in situations where a person wants or would be better served with a high transmission see-through view and when, in the same HWC  102 , the person wants or would be better served with less see-through transmission. The lower see-through transmission may be used in bright conditions and/or in conditions where higher contrast for the digitally presented content is desirable. The optical system may also have a camera that images the surrounding environment by receiving reflected light from the surrounding environment off of an optical element that is in-line with the person&#39;s see-through view of the surrounding. In embodiments, the camera may further be aligned in a dark light trap such that light reflected and/or transmitted in the direction of the camera that is not captured by the camera is trapped to reduce stray light. 
     In embodiments, a HWC  102  is provided that includes a camera that is coaxially aligned with the direction that the user is looking.  FIG. 67  shows an illustration of an optical system  6715  that includes an absorptive polarizer  6737  and a camera  6739 . The image source  6710  can include light sources, displays and reflective surfaces as well as one or more lenses  6720 . Image light  6750  is provided by the image source  6710  wherein, a portion of the image light  6750  is reflected toward the user&#39;s eye  6730  by a partially reflective combiner  6735 . At the same time, a portion of the image light  6750  may be transmitted by the combiner  6735  such that it is incident onto the absorptive polarizer  6737 . In this embodiment, the image light  6750  is polarized light with the polarization state of the image light  6750  oriented relative to the transmission axis of the absorptive polarizer  6737  such that the incident image light  6750  is absorbed by the absorptive polarizer  6737 . In this way, faceglow produced by escaping image light  6750  is reduced. In embodiments, the absorptive polarizer  6737  includes an antireflection coating to reduce reflections from the surface of the absorptive polarizer  6737 . 
       FIG. 67  further shows a camera  6739  for capturing images of the environment in the direction that the user is looking. The camera  6739  is positioned behind the absorptive polarizer  6737  and below the combiner  6735  so that a portion of light from the environment  6770  is reflected by the combiner  6735  toward the camera  6739 . Light from the environment  6770  can be unpolarized so that a portion of the light from the environment  6770  that is reflected by the combiner  6735  passes through the absorptive polarizer  6737  and it is this light that is captured by the camera  6739 . As a result, the light captured by the camera will have a polarization state that is opposite that of the image light  6750 . In addition, the camera  6739  is aligned relative to the combiner  6735  such that the field of view associated with the camera  6739  is coaxial to the display field of view provided by image light  6750 . At the same time, a portion of scene light  6760  from the environment is transmitted by the combiner  6735  to provide a see-through view of the environment to the user&#39;s eye  6730 . Where the display field of view associated with the image light  6750  is typically coincident to the see-through field of view associated with the scene light  6760  and thereby the see through field of view and the field of view of the camera  6739  are at least partially coaxial. By attaching the camera  6739  to the lower portion of the optical system  6715 , the field of view of the camera  6739  as shown by the light from the environment  6770  moves as the user moves their head so that images captured by the camera  6739  correspond to the area of the environment that the user is looking at. By coaxially aligning the camera field of view with the displayed image and the user&#39;s view of the scene, augmented reality images with improved alignment to objects in the scene can be provided. This is because the captured images from the camera  6739  provide an accurate representation of the user&#39;s perspective view of the scene. As an example, when the user sees an object in the scene as being located in the middle of the see-through view of the HWC, the object will be located in the middle of the image captured by the camera and any augmented reality imagery that is to be associated with the object can be located in the middle of the displayed image. As the user moves their head, the relative position of the object as seen in the see-through view of the scene will change and the position of the augmented reality imagery can be changed within the displayed image in a corresponding manner. When a camera  6739  is provided for each of the user&#39;s eyes, an accurate representation of the 3D view of the scene can be provided as well. This is an important advantage provided by the invention because images captured by a camera located in the frame of the HWC (e.g. between the eyes or at the corners) capture images that are laterally offset from the user&#39;s perspective of the scene and as a result it is difficult to align augmented reality images with objects in the scene as seen from the user&#39;s perspective. 
     In the optical system  6715  shown in  FIG. 67 , the absorptive polarizer  6737  simultaneously functions as a light trap for escaping image light  6750 , a light blocker of the image light  6750  for the camera  6739  and a window for light from the environment  6770  to the camera  6739 . This is possible because the polarization state of the image light  6750  is perpendicular to the transmission axis of the absorptive polarizer  6737  while the light from the environment  6770  is unpolarized so that a portion of the light from the environment  6770  that is the opposite polarization state to the image light is transmitted by the absorptive polarizer  6737 . The combiner  6735  can be any partially reflective surface including a simple partial mirror, a notch mirror and a holographic mirror. The reflectivity of the combiner  6735  can be selected to be greater than 50% (e.g. 55% reflectivity and 45% transmission over the visible wavelength spectral band) whereby a majority of the image light  6750  will be reflected toward the user&#39;s eye  6730  and a majority of light from the environment  6770  will be reflected toward the camera  6739 , this system will provide a brighter displayed image, a brighter captured image with a dimmer see-through view of the environment. Alternatively, the reflectivity of the combiner  6735  can be selected to be less than 50% (e.g. 20% reflectivity and 80% transmission over the visible wavelength spectral band) whereby the majority of the image light  6750  will be transmitted by the combiner  6735  and a majority of light from the environment  6770  will be transmitted to the user&#39;s eye  6730 , this system will provide a brighter see-through view of the environment, while providing a dimmer displayed image and a dimmer captured image. As such, the system can be designed to favor the anticipated use by the user. 
     In embodiments, the combiner  6735  is planar with an optical flatness that is sufficient to enable a sharp displayed image and a sharp captured image, such as a flatness of less than 20 waves of light within the visible wavelengths. However, in embodiments, the combiner  6735  may be curved in which case the displayed image and the captured image will both be distorted and this distortion will have to be digitally corrected by the associated image processing system. In the case of the displayed image, the image is digitally distorted by the image processing system in a direction that is opposite to the distortion that is caused by the curved combiner so the two distortions cancel one another and as a result the user sees an undistorted displayed image. In the case of the captured image, the captured image is digitally distorted after capture to cancel out the distortion caused by the curved combiner so that the image appears to be undistorted after image processing. 
     In embodiments, the combiner  6735  is an adjustable partial mirror in which the reflectivity can be changed by the user or automatically to better function within different environmental conditions or different use cases. The adjustable partial mirror can be an electrically controllable mirror such as for example, the e-Transflector that can be obtained from Kent Optronics (http://www.kentoptronics.com/mirror.html) where the reflectivity can be adjusted based on an applied voltage. The adjustable partial mirror can also be a fast switchable mirror (e.g. a switching time of less than 0.03 seconds) wherein the perceived transparency is derived from the duty cycle of the mirror rapidly switching between a reflecting state and a transmitting state. In embodiments, the images captured by the camera  6739  can be synchronized to occur when the fast switchable mirror is in the reflecting state to provide an increased amount of light to the camera  6739  during image capture. As such, an adjustable partial mirror allows for the transmissivity of the partial mirror to be changed corresponding to the environmental conditions, e.g. the transmissivity can be low when the environment is bright and the transmissivity can be high when the environment is dim. 
     In a further embodiment, the combiner  6735  includes a hot mirror coating on the side facing the camera  6739  wherein visible wavelength light is substantially transmitted while a spectral wavelength band of infrared light is substantially reflected and the camera  6739  captures images that include at least a portion of the infrared wavelength light. In these embodiments, the image light  6750  includes visible wavelength light and a portion of the visible wavelength light is transmitted by the combiner  6735 , where it is then absorbed by the absorptive polarizer  6737 . A portion of the scene light  6760  is comprised of visible wavelength light and this is also transmitted by the combiner  6735 , to provide the user with a see-through view of the environment. The light from the environment  6770  is comprised of visible wavelength light and infrared wavelength light. A portion of the visible wavelength light along with substantially all of the infrared wavelength light within the spectral wavelength band associated with the hot mirror, is reflected by the combiner  6735  toward the camera  6739  thereby passing through the absorptive polarizer  6737 . In embodiments, the camera  6739  is selected to include an image sensor that is sensitive to infrared wavelengths of light and the absorptive polarizer  6737  is selected to substantially transmit infrared wavelengths of light of both polarization states (e.g. ITOS XP44 polarizer which transmits both polarization states of light with wavelengths above 750 nm: see http://www.itos.de/english/polarisatoren/linear/linear.php) so that an increased % of infrared light is captured by the camera  6739 . In these embodiments, the absorptive polarizer  6737  functions as a light trap for the escaping image light  6750  and thereby blocking the image light  6750  that is in the visible wavelengths from the camera  6739  while simultaneously acting as a window for infrared wavelength light from the environment  6770  for the camera  6739 . 
     By coaxially aligning the camera field of view with the displayed image and the user&#39;s view of the scene, augmented reality images with improved alignment to objects in the scene can be provided. This is because the captured images from the camera provide an accurate representation of the user&#39;s perspective view of the scene. In embodiments, the camera that is coaxially aligned with the user&#39;s view captures an image of the scene, the processor then identifies an object in the captured image and identifies a field of view position for the object, which can be compared to the displayed field of view correlated position so digital content is then displayed relative to the position of the object. 
     Another aspect of the present invention relates to an optical assembly that uses a reflective display where the reflective display is illuminated with a front light arranged to direct the illumination at angles around 90 degrees from the active reflective surface of the reflective display. In embodiments, the optical configuration is light weight, small and produces a high quality image in a head-worn see-through display. 
       FIG. 68  provides a cross sectional illustration of the compact optical display assembly for a HWC  102  according to principles of the present invention along with illustrative light rays to show how the light passes through the assembly. The display assembly is comprised of upper optics and lower optics. The upper optics include a reflective image source  6810 , a quarter wave film  6815 , a field lens  6820 , a reflective polarizer  6830  and a polarized light source  6850 . The upper optics convert illumination light  6837  into image light  6835 . The lower optics comprise a beam splitter plate  6870  and a rotationally curved partial mirror  6860 . The lower optics deliver the image light to a user who is wearing the HWC  102 . The compact optical display assembly provides the user with image light  6835  that conveys a displayed image along with scene light  6865  that provides a see-through view of the environment so that user sees the displayed image overlaid onto the view of the environment. 
     In the upper optics, linearly polarized light is provided by the polarized light source  6850 . Where the polarized light source  6850  can include one or more lights such as LEDs, QLEDs, laser diodes, fluorescent lights, etc. The polarized light source  6850  can also include a backlight assembly with light scattering surfaces or diffusers to spread the light uniformly across the output area of the polarized light source. Light control films or light control structures can be included as well to control the distribution of the light (also known as the cone angle) that is provided by the polarized light source  6850 . The light control films can include, for example, diffusers, elliptical diffusers, prism films and lenticular lens arrays. The light control structures can include prism arrays, lenticular lenses, cylindrical lenses, Fresnel lenses, refractive lenses, diffractive lenses or other structures that control the angular distribution of the illumination light  6837 . The output surface of the polarized light source  6850  is a polarizer film to ensure that the illumination light  6837  provided to the upper optics is linearly polarized. 
     The illumination light  6837  provided by the polarized light source  6850  is reflected by a reflective polarizer  6830 . Where the polarizer on the output surface of the polarized light source  6850  and the reflective polarizer  6830  are oriented so that their respective transmission axes are perpendicular to one another. As a result, the majority of the illumination light  6837  provided by the polarized light source  6850  is reflected by the reflective polarizer  6830 . In addition, the reflective polarizer  6830  is angled so that the illumination light  6837  is reflected toward the reflective image source  6810  thereby illuminating the reflective image source  6810  as shown in  FIG. 68 . 
     The illumination light  6837  passes through a field lens  6820  and is then incident onto the reflective image source  6810 . The illumination light  6837  is then reflected by the reflective image source (otherwise referred to as a reflective display herein elsewhere)  6810 . Wherein the reflective image source  6810  can comprise a liquid crystal on silicon (LCOS) display, a ferroelectric liquid crystal on silicon (FLCSO) display, a reflective liquid crystal display, a cholesteric liquid crystal display, a bistable nematic liquid crystal display, or other such reflective display. The display can be a monochrome reflective display that is used with sequential red/green/blue illumination light  6837  or a full color display that is used with white illumination light  6837 . The reflective image source  6810  locally changes the polarization state of the illumination light  6837  in correspondence to the pixel by pixel image content that is displayed by the reflective image source  6810  thereby forming image light  6835 . Wherein if the reflective image source  6810  is a normally white display, the areas of the image light  6835  that correspond to bright areas of the image content end up with a polarization state that is opposite to the polarization state of the illumination light and dark areas of the image light  6835  end up with a polarization state that is the same as the illumination light  6837  (it should be noted that the invention can be used with normally black displays which provide an opposite effect on polarization in the image light). As such, the image light  6835  as initially reflected by the reflective image source  6810  has a mixed polarization state pixel by pixel. The image light  6835  then passes through the field lens  6820  which modifies the distribution of the image light  6835  while preserving the wavefront to match the requirements (such as for example, magnification and focus) of the lower optics. As the image light  6835  passes through the reflective polarizer  6830 , the bright areas of the image light  6835  that have a polarization state that is opposite to the illumination light  6837  are transmitted through the reflective polarizer  6830  and the dark areas of the image light  6835  that have the same polarization state as the illumination light  6837  are reflected back toward the polarized light source  6850 , as a result, the image light  6835  after passing through the reflective polarizer  6830  is linearly polarized with a single polarization state in all the pixels of the image but now with different intensities pixel by pixel. Thus the reflective polarizer  6830  acts first as a reflector for the illumination light  6837  and then second as an analyzer polarizer for the image light  6835 . 
     As such, the optical axis of the illumination light  6837  is coincident with the optical axis of the image light  6835  between the reflective polarizer  6830  and the reflective image source  6810 . The illumination light  6837  and the image light 
       6835  both pass through the field lens  6820 , but in opposite directions. Wherein the field lens acts to expand the illumination light  6837  so it illuminates the entire active area of the reflective image source  6810  and also to expand the image light  6835  so it fills the eyebox  6882  after passing through the rest of the compact optical display system. By overlapping the portion of the compact optical display assembly associated with the illumination light  6837  with the portion of the compact optical display assembly associated with the image light  6835 , the overall size of the compact optical display assembly is reduced. Given that the focal length associated with the field lens  6820  requires some space in the compact optical display assembly, the reflective polarizer  6830  and the polarized light source  6850  are located in space that would otherwise be unused so the overall size of the display assembly is more compact. 
     The reflective polarizer  6830  can be a relatively thin film (e.g. 80 microns) or thin plate (e.g. 0.2 mm) as shown in  FIG. 68 . The reflective polarizer  6830  can be a wiregrid polarizer such as is available from Asahi Kasei under the name WGF, or a multilayer dielectric film polarizer such as is available from 3M under the name DBEF. As previously described, the reflective polarizer  6830  has two functions. First, the reflective polarizer  6830  reflects the illumination light  6837  provided by the polarized light source  6850  and redirects the illumination light  6837  toward the reflective image source  6810 . Second, the reflective polarizer  6830  acts as an analyzer polarizer to the image light  6835  thereby converting the mixed polarization state of the image light  6835  above the reflective polarizer  6830  to linearly polarized light with a single polarization state below the reflective polarizer  6830 . While the illumination light  6837  incident on the reflective polarizer  6830  is incident on a relatively small portion of the reflective polarizer  6830 , the image light  6835  is incident on the majority of the area of the reflective polarizer  6830 . Consequently, the reflective polarizer  6830  extends at least across the entire area of the field lens  6820  and may extend across the entire area between the field lens  6820  and the beam splitter  6870  as shown in  FIG. 68 . In addition, the reflective polarizer  6830  is angled at least in the portion where the illumination light  6837  is incident to redirect the illumination light  6837  toward the reflective image source  6810 . However, since reflective polarizers (such as a wiregrid polarizer) can be relatively insensitive to the incident angle, in a preferred embodiment, the reflective polarizer  6830  is a flat surface angled to redirect the illumination light  6837  toward the reflective image source  6810  wherein the flat surface extends substantially across the entire area between the field lens  6820  and the beam splitter  6870  in one continuously flat surface to make manufacturing easier. The thin film or thin plate of the reflective polarizer  6870  can be retained at the edges to position it at the desired angle and to make the surface flat. 
     The systems and methods described herein with respect to  FIGS. 68 through 71  have a number of advantages. By avoiding grazing angles of the illumination light  6837  and the image light  6835  at all the surfaces in the compact optical display assembly, scattering of light in the assembly is reduced and as a result the contrast of the image presented to the user&#39;s eye  6880  is higher with blacker blacks. In addition, the reflective image source  6810  can include a compensating retarder film  6815  as is known to those skilled in the art, to enable the reflective image source  6810  to provide a higher contrast image with more uniform contrast over the area of the displayed image. Further, by providing an optical display assembly that is largely comprised of air, the weight of the compact optical display assembly is substantially reduced. By using coincident optical axes for the illumination light  6837  and the image light  6835  and overlapping the illumination light  6837  and image light  6835  for a substantial portion of the optical display assembly, the overall size of the compact optical display assembly is reduced. Where the coincident optical axes are provided by passing the illumination light  6837  and the image light  6835  in opposite directions through the field lens  6820 . To maintain a uniform polarization state for the illumination light  6837 , the field lens  6820  is made from a low birefringence material such as glass or a plastic such as OKP4 as available from Osaka Gas Chemicals. By positioning the polarized light source  6850  and the associated illumination light  6837  below the field lens  6820 , and by folding the optical path of both the illumination light  6837  at the reflective polarizer  6830  and the image light  6835  at the beam splitter  6870 , the overall height of the compact optical display assembly is greatly reduced. For example the overall height of the compact optical display assembly can be less than 24 mm as measured from the reflective image source  6810  to the bottom edge of the rotationally curved partial mirror  6860  for a display that provides a 30 degree diagonal field of view with a 6×10 mm eyebox. 
     In a preferred case, the light control structure in the polarized light source  6850  includes a positive lens, such as for example a positive Fresnel lens, a positive diffractive lens or a positive refractive lens. Wherein a positive Fresnel lens or a positive diffractive lens is preferred because they can be very thin. The illumination light  6837  is thereby focused to form a smaller area or pupil at the reflective polarizer  6830  that has a direct relationship to the area of an eyebox  6882  at the other end of the optics wherein image light  6835  is provided to the user&#39;s eye  6880  as shown in  FIG. 68 . Where the positive lens concentrates the illumination light  6837  from the polarized light source  6850  both in terms of intensity and angular distribution to match the etendue of the optical system and thereby fills the eyebox with image light  6835 . By using the positive lens to converge the light from the polarized light source  6850  as provided to the reflective polarizer  6830  and then using the field lens  6820  to expand the illumination light  6837  to illuminate the active area of the reflective image source  6810 , efficiency is improved since illumination light  6837  is substantially delivered only where needed to form image light  6835 . Further, illumination light  6837  outside the pupil can be controlled by the positive lens and clipped by masked edges of the positive lens. By focusing the illumination light  6837  and clipping light outside the pupil, illumination light  6837  is prevented from impinging adjacent surfaces at grazing angles in the compact optical display assembly to reduce scattering of light and thereby increase contrast in the image provided to the user&#39;s eye  6880  by providing blacker blacks. 
     It should be noted that while  FIGS. 68, 69 and 70  show optical layouts wherein the illumination light  6837  is provided from behind the rotationally curved partial mirror  6860 , other optical layouts are possible within the invention. The location of the polarized light source  6850  can be changed for example to be at the side of the rotationally curved partial mirror  6860  wherein the reflective polarizer  6830  is oriented to receive the illumination light  6837  from the side. And reflect it toward the reflective image source  6810  (not shown). 
     In a further embodiment, the portion of the image light  6835  that is reflected back toward the polarized light source  6850  is recycled in the polarized light source  6850  to increase the efficiency of the polarized light source  6850 . In this case, a diffuser and a reflective surface is provided behind the polarized light source  6850  so the polarization of the light is scrambled and reflected back toward the reflective polarizer  6830 . 
     In yet another embodiment, another reflective polarizer is provided in the polarized light source  6850  and behind the linear polarizer previously disclosed. Wherein the respective transmission axes of the reflective polarizer and the linear polarizer are parallel to one another. The other reflective polarizer then reflects the light back into the backlight that has the polarization state that would not be transmitted by the linear polarizer. The light that is reflected back into the backlight passes through diffusers associated with the polarized light source  6850  where the polarization state is scrambled and reemitted thereby recycling the light and increasing efficiency. 
     In another embodiment, the system according to the principles of the present invention includes an eye imaging system.  FIG. 69  is an illustration of a compact optical display assembly, which includes an eye imaging camera  6992  that captures an image of the user&#39;s eye  6880  that is coaxial with the displayed image provided to the user so that a full image of the user&#39;s iris can be reliably captured. The eye imaging camera  6992  is reflected into the lower optics by a reflective polarizer  6930  that includes a notch mirror coating, facing the eye imaging camera  6992 , that reflects the wavelengths of light that are captured by the eye imaging camera  6992  (e.g. near infrared wavelengths) while transmitting wavelengths associated with the image light  6835  (e.g. visible wavelengths). Eye light rays  6995  shown in  FIG. 69  illustrate how the field of view associated with the eye imaging camera  6992  is a relatively narrow field of view because it is multiply reflected through the lower optics to capture an image of the user&#39;s eye  6880 . However, to enable the eye imaging camera  6992  to focus onto the user&#39;s eye  6880 , the eye imaging camera  6992  needs to have a very near focus distance (e.g. 35 mm). In addition, the field of view and focus distance of the eye imaging camera must take into account the reducing effect of the optical power provided by the rotationally curved partial mirror  6860 . To increase the efficiency of capturing the light reflected from the user&#39;s eye  6880  and thereby enable a brighter image of the eye, the rotationally curved partial mirror  6860  can be coated with a partial mirror coating that acts as a full mirror in the wavelengths being captured by the eye imaging camera  6992 , for example the coating can reflect 50% of visible light associated with the image light and 90% of near infrared light associated with the eye light  6995 . Where the reflections and associated changes in polarization state are similar to those associated with the image light  6835  but in the opposite order since the eye light rays  6995  are coming from the user&#39;s eye  6880 . LEDs or other miniature lights are provided adjacent to the user&#39;s eye  6880  to illuminate the user&#39;s eye  6880  wherein the wavelengths associated with the LED&#39;s or other miniature lights are different than the wavelengths associated with the image light  6835  such as for example near infrared wavelengths (e.g. 850 nm, 940 nm or 1050 nm). Alternatively, the image light  6835  is used to illuminate the user&#39;s eye  6880  and a reflective polarizer  6930  with a low extinction ratio in reflection (e.g. reflective extinction ratio &lt;15) is used so that some of the eye light rays are reflected toward the eye imaging camera  6992 . 
     In an alternative embodiment, the reflective and partially reflective surfaces can extend laterally to the sides of the areas used for displaying an image to the user. In this case, the eye imaging camera can be located adjacent to the field lens and pointed in a direction to image the user&#39;s eye after reflecting from the beam splitter and the rotationally curved partial mirror as shown in  FIG. 70 . Where  FIG. 70  is an illustration that shows an eye imaging camera  7092  positioned to the side of the field lens  6820  and reflective polarizer  6830 . The eye imaging camera  7092  is pointed such that the field of view captured by the eye imaging camera  7092  includes the user&#39;s eye  6880  as illustrated by the eye light rays  7095 . The quarter wave film  6890  is also extended laterally to change the polarization state of the eye light  7095  in the same way that the polarization state of the image light is changed so that the eye light passes through the beam splitter  6870  and quarter wave  6890 , is partially reflected by the rotationally curved partial mirror  6860  and is then reflected by the beam splitter  6870  and is then captured by the eye imaging camera  7092 . By positioning the eye imaging camera  7092  to the side of the field lens  6820  and reflective polarizer  6830 , the complexity of the optics associated with displaying an image to the user is reduced. In addition, the space available for the eye imaging camera  7092  is increased since interferences with the display optics are reduced. By positioning the eye imaging camera  7092  adjacent to the display optics, the eye image is captured nearly coaxially with the displayed image. 
     In a yet another embodiment, the systems according to the principles of the present invention include a field lens with an internal reflective polarizer and one or more surfaces with optical power.  FIG. 71  is an illustration of the upper optics including a field lens  7121  comprised of upper prism  7122  and lower prism  7123 . The upper prism  7122  and the lower prism  7123  can be molded to shape or grind and polished. A reflective polarizer  7124  is interposed on the flat surface between the upper prism  7122  and the lower prism  7123 . The reflective polarizer  7124  can be a wiregrid polarizer film or a multilayer dielectric polarizer as previously mentioned. The reflective polarizer  7124  can be bonded into place with a transparent UV curable adhesive that has the same refractive index as the upper prism  7122  or the lower prism  7123 . Typically the upper prism  7122  and the lower prism  7123  would have the same refractive index. Wherein upper prism  7122  includes an angled surface for illumination light  6837  to be provided to illuminate the reflective image source  6810 . The illumination light is provided by a light source that includes lights such as LEDs, a backlight  7151 , a diffuser  7152  and a polarizer  7153  as has been previously described. The lower prism  7123  includes a curved surface on the exit surface for controlling the wavefront of the image light  6835  as supplied to the lower optics. The upper prism may also include a curved surface on the upper surface next to the reflective image source  6810  as shown in  FIG. 71  for manipulating the chief ray angles of the light at the surface of the reflective image source  6810 . Illumination light  6837  is polarized by the polarizer  7153  prior to entering the upper prism  7122 . The transmission axes of the polarizer  7153  and the reflective polarizer  7124  are perpendicular to one another so that the illumination light  6837  is reflected by the reflective polarizer  7124  so that the illumination light is redirected toward the reflective image source  6810 . The polarization state of the illumination light  6837  is then changed by the reflective image source  6810  in correspondence with the image content to be displayed as previously described and the resulting image light  6835  then passes through the reflective polarizer  7124  to form the bright and dark areas associated with the image that is displayed to the user&#39;s eye  6880 . 
     In another embodiment, the field lens  7121  of  FIG. 71  comprises a polarizing beam splitter cube including two prisms, upper prism  7122  and lower prism  7123 . In this case, the reflective polarizer  7124  is replaced by a coating that is polarization sensitive so that light of one polarization state (typically S polarized light for example) is reflected and light of the other polarization state is transmitted. The illumination light  6837  is then provided with the polarization state that is reflected by the coating and the image light is provided with the polarization state that is transmitted by the coating. As shown in  FIG. 71 , the beam splitter cube includes one or more curved surfaces in the upper prism  7122  or the lower prism  7123 . The beam splitter cube can also include one or more angled surfaces where the illumination light is supplied. The angled surface can include light control structures such as a microlens array to improve the uniformity of the illumination light  6837 , or a lenticular array to collimate the illumination light  6837 . 
     In yet another embodiment, the curved surface(s) or the angled surface(s) illustrated in  FIG. 71  can be molded onto a rectangularly shaped beam splitter cube by casting a UV curable material (e.g. UV curable acrylic) onto a flat surface of a beam splitter cube, placing a transparent mold with a cavity that has the desired curve onto the flat surface to force the UV curable material into the desired curve and applying UV light to cure the UV curable material. The beam splitter cube can be made of a material that has the same or different refractive index than the UV curable material. 
     In a further embodiment, polarization sensitive reflective coatings such as dielectric partial mirror coatings, can be used in place of reflective polarizers or beam splitters as shown in  FIG. 68 . In this case, the reflective films and plates that comprise the reflective polarizers  6830  and beam splitters  6870  include polarization sensitive coatings that substantially reflect light with one polarization state (e.g. S polarization) while substantially transmitting light with the other polarization state (e.g. P polarization). Since the illumination light source includes a polarizer  7153 , the illumination light  6837  is one polarization state and it is not important that the reflective polarizer  7124  be sensitive to the polarization state in reflection, the polarization state just needs to be maintained and presented uniformly over the surface of the reflective image source  6810 . However, it is important that the reflective polarizer  7124  be highly sensitive to polarization state in transmission (e.g. extinction ratio &gt;200) to be an effective polarizer analyzer and to provide a high contrast image (e.g. contrast ratio &gt;200) to the user&#39;s eye  6880 . 
     In a further embodiment, the field lens  7121  shown in  FIG. 71  can comprise a reflective polarizer  7124  with a curved surface (not shown) instead of a flat surface and wherein the reflective polarizer  7124  is not a film and instead is a polarization sensitive coating, a printed wiregrid polarizer or a molded wiregrid pattern that is then metallized. In this case, the upper prism  7122  and the lower prism  7123  are made as a matched pair with mating curved surfaces that together form the surface of the reflective polarizer. Wherein the polarization sensitive coating, the printed wiregrid or the molded wiregrid pattern are applied to the mating curved surface associated either the upper prism  7122  or the lower prism  7123  and a transparent adhesive is applied to the other mating surface to bond the upper prism  7122  and lower prism  7123  together to form the field lens  7121  with an internal curved reflective polarizer  7121 . 
     Another aspect of the present invention relates to manufacturing and providing an optical element for use in a see-through computer display system. In embodiments, a lightweight low-cost and high optical quality optical element. 
     In a head mounted display, a beam splitter can be used to direct illuminating light from a light source toward a reflective image source such as an LCOS or a DLP. Where it is desirable to have a low weight beam splitter with a flat partially reflective surface to provide good image quality. The flat partially reflective surface is particularly important when an eye camera is provided for eye imaging that utilizes the flat partially reflective surface for directing the field of view of the eye camera toward the user&#39;s eye. 
     Systems and methods provide for a lightweight beam splitter comprised of molded plastic elements and an internal plate element to provide a flat partially reflective surface. Together the pieces form a triplet beam splitter optic including two molded elements and a plate element. By providing the plate element internal to the beam splitter, the matching surfaces of the molded elements do not have to be optically flat, instead the plate element provides the flat surface and an index matching material is used to join the plate element to the molded elements. All three elements can be plastic elements to reduce the weight and cost of the lightweight beam splitter. To provide a more uniform refractive effect, the molded elements and the plate element are preferentially made from plastic materials with similar refractive index and have low birefringence. 
       FIG. 72  shows an illustration of the two molded elements  7210  and  7220 . These molded elements are molded with a relatively uniform thickness to provide uniform flow of the plastic material during molding (either injection molding, compression molding or casting) and thereby enable a low birefringence in the elements as molded. To further reduce birefringence in the molded elements as molded, materials with low viscosity and low stress optic coefficients are preferred including: OKP4 from Osaka Gas Company; Zeonex F52R, K26R or 350R from Zeon Chemical; PanLite SP3810 from Teijin. 
     The molded elements  7210  and  7220  can include flat surfaces and surfaces with optical power, where the surfaces with optical power can include spherical or aspheric curved surfaces, diffractive surfaces or Fresnel surfaces. Flat surfaces, diffractive surfaces or Fresnel surfaces are preferred on the surfaces associated with light that illuminates the image source and flat surfaces, spherical surfaces or aspheric surfaces are preferred on the surfaces associated with image light. Molded element  7210  is shown with a spherical or aspheric surface  7215  and molded element  7220  is shown with a flat surface  7225 , however, any of the surfaces shown can be molded as flat surfaces or surfaces with optical power. 
     After molding the molded elements  7210  and  7220  are machined to provide matching angled surfaces. Molded element  7210  is shown in  FIG. 73  where a milling cutter  7328  is shown machining angled surface  7329 .  FIG. 74  shows an illustration of molded elements  7210  and  7220  after they have been machined to respectively provide beam splitter elements  7430  and  7440  that are prisms. The angled surface of beam splitter elements  7430  and  7440  are machined to have matching angles. Alternatively, beam splitter elements  7430  and  7440  can be machined from sheet material or molded pucks. In either case of using machined angled surfaces or molded angled surface in the beam splitter elements, the surfaces will not be optically flat. 
       FIG. 75  shows an illustration of the assembled triplet beam splitter optic, wherein the beam splitter elements  7430  and  7440  have been assembled with a partially reflecting plate element  7560  to form a beam splitter cube. Wherein the beam splitter elements  7430  and  7440  are made from either the same material or different materials that have a very similar refractive index (e.g. within 0.05 of each other). An index matching material is used at the interfaces between the beam splitter elements and the plate element. The index matching material can be a fluid, a light curing adhesive, a moisture curing adhesive or a thermally curing adhesive. The index matching material should have a refractive index that is very similar to that of the beam splitter elements (e.g. within 0.1). 
     The partially reflective plate element  7560  can be a transparent plate with a partially reflective layer that is either a partially reflective coating or a laminated partially reflective film. The transparent plate is preferably a cast sheet such as cell cast acrylic that has low birefringence, or a molded plaque of a low birefringence material such as OKP4, Zeonex F52R, Zeonex K26R, Zeonex 350R or PanLite SP3810. In addition, the transparent plate should be optically flat (e.g. within 20 microns over the surface and with a surface finish of less than 15 nanometers), however optically flat surfaces are easily obtained in sheet stock. By using an index matching material at the interfaces between the beam splitter elements  7430  and  7440  and the partially reflective plate element  7560 , the lack of optical flatness of the surface of the beam splitter elements  7430  and  7440  can be filled by the index matching material so that the flatness of the reflective surface is determined by the flatness of the more easily obtained partially reflective plate element  7560  thereby providing a manufacturing advantage. The partially reflective layer can be a partial mirror, a reflective polarizer or a wiregrid polarizer where the reflective polarizer can be a coating or a film and the wiregrid polarizer can be a film or a molded structure that is partially coated with a conductive layer. Where a suitable reflective polarizer film can be obtained from 3M available under the trade name of DBEFQ and a wiregird polarizer film can be obtained from Asahi-Kasei available under the trade name of WGF. In a preferred embodiment, the transparent plate of the partially reflective plate element  7560  has a refractive index that is very similar (e.g. within 0.1) to the refractive indices of the beam splitter elements  7430  and  7440   
       FIG. 76  shows an illustration of an optical system for a head mounted display system. The system includes a reflective display as an image source  7667 , a light source  7665  that can be a white light source or a sequential color light source as appropriate for the image source  7665 . Wherein the light source  7665  provides illumination light  7674  that can be polarized light provided that a quarter wave layer is associated with the image source  7667  or the partially reflecting plate element  7560  so that the polarization of the illumination light  7674  is changed before becoming image light  7672 . The illumination light  7674  is reflected by a surface of the partially reflecting plate element  7560 , and then reflected by the image source  7667 , whereupon it passes through the partially reflective plate element  7560  thereby becoming image light  7672 . The image light  7672  is then reflected by a partially reflective combiner  7682  so that the image light is directed toward the user&#39;s eye  7680  to display an image to the user while simultaneously providing a see-through view of the environment. In the optical system, an index matching material can be used at the interface between the image source  7665  and the beam splitter element  7440  sop that the surface of the beam splitter element  7440  does not have to be flat. It is contemplated by the current inventions that the optical system may include additional lenses and other optical structures that are not shown to improve the image quality or change the form factor of the optical system. 
     In another embodiment, beam splitter elements  7430  and  7440  are molded directly to shape using injection molding or casting. The molded beam splitter elements are then assembled as shown in  FIG. 75  as described previously herein. 
     In further embodiments, surfaces of the beam splitter elements are molded or machined to have additional structures to provide further features.  FIG. 77  shows an illustration of lightweight beam splitter  7750  that includes an extended partially reflective plate element  7760  and an extended beam splitter element  7740 , wherein the partially reflective surface is extended to provide additional area for the illumination light  7674  to be reflected toward the image source  7665 . Where having an extended partially reflective surface is particularly useful when the image source  7665  is a DLP and the illumination light  7665  must be delivered at an oblique angle.  FIG. 78  shows a lightweight beam splitter  7850  that includes an entrance surface  7840  for the illumination light  7674  that is angled so the illumination light  7674  passes substantially perpendicularly through the entrance surface  7840 . 
     In yet further embodiments, beam splitter element elements  7430  and  7440  are machined from a single molded element. Where the single molded element is designed to provide the desired optical surfaces. For example, the molded element  7210  as shown in  FIG. 72  has surfaces that could be used for both surface  7215  and  7225 . A series of molded elements  7210  could then be molded and some would be used to make machined beam splitter elements  7430  and some for beam splitter elements  7440 . A partially reflective plate element  7560  would then be bonded with the beam splitter element  7430  and  7440  using index-matched adhesive as previously described herein. Alternatively, the single molded element  7210  could be designed with extra thickness across the dimension where the partially reflective plate element  7560  will be added, so that a single molded element  7210  could be sawn, machined or laser cut into beam splitter elements  7430  and  7440 . 
     In another embodiment, a first molded optical element is molded in a geometry that enables improved optical characteristics including: low birefringence; more accurate replication of the optical surfaces of the mold (reduced power and irregularity deviation). The first molded optical element is then cut to a different shape wherein the cutting process leaves an optically rough surface finish. A second optical element with an optically smooth surface is then bonded to the optically rough surface of the first molded optical element using an index matched adhesive to provide a combined optical element. The index matched adhesive fills in the optically rough surface on the first molded optical element so that the optically rough surface is no longer visible and an optically smooth surface is provided in the combined optical element by the second optical element. The optical characteristics of the combined optical element are improved as compared to a directly molded optical element that has the geometry of the combined optical element. The cut surface can be flat or curved, as long as the cut surface of the first molded optical element is substantially similar to the bonding surface of the second optical element. In addition, both the first molded optical element and the second optical element can provide optical surface with independent optical features such as optical power, wedge, diffraction, grating, dispersion, filtering and reflection. For example, optically flat surfaces can be difficult to mold on plastic lenses. A lens can be molded to provide a spherically curved surface and another surface that is subsequently milled off to provide a flat surface with a rough surface finish. An optically flat sheet can then be bonded to the milled surface using an index matched adhesive to provide a combined optical element with an optically flat surface. 
     In yet further embodiments, surfaces of the beam splitter elements include molded or machined structures to collimate, converge, diverge, diffuse, partially absorb, redirect or polarize the illumination light  7674  or the image light  7672 . In this way, the number of parts in the lightweight beam splitter is reduced and the cost and manufacturing complexity is reduced. 
     The multi-piece lightweight solid optic has been described in connection with certain embodiments; it should be understood that the multi-piece lightweight solid optic may be used in connection with other optical arrangements (e.g. other see-through head-worn display optical configuration described herein elsewhere). 
     In embodiments, the invention provides methods for aligning images, along with methods and apparatus for controlling light within the optics of the display assembly associated with a HMD to prevent scattering and also to trap excess light to thereby improve the image quality provided to the user. 
       FIG. 79 a    is a schematic illustration of a cross section of a display assembly for a HMD. Wherein, the display assembly includes upper optics  795  and lower optics  797  that operate together to display an image to a user while simultaneously providing a see-through view of the environment. Aspects of the upper optics  795  will be discussed in more detail herein. The lower optics  797  can comprise optical elements to manipulate image light  7940  from the upper optics  795  and thereby present the image light  7940  to the user&#39;s eye  799 . Lower optics  797  can comprise one or more lenses  7950  and a combiner  793 . The combiner  793  presents the image light  7940  to the user&#39;s eye  799  while simultaneously allowing light from the environment  791  to pass through to the user&#39;s eye  799  so that the user sees the displayed image overlaid onto a view of the environment. 
       FIG. 79  is a schematic drawing of a cross section of the upper optics  795  for a HMD. Included are a light source  7910 , a partially reflective layer  7930 , a reflective image source  7935  and a lens  7950 . The light source  7910  provides illumination light  7920  to the HMD. The illumination light  7920  is redirected by the partially reflective layer  7930  to illuminate the reflective image source  7935 . The illumination light  7920  is then reflected by the reflective image source  7935  in correspondence with the image content in the displayed image so that it passes through the partially reflective layer  7930  and thereby forms image light  7940 . The image light  7940  is optically manipulated by the lens  7950  and other optical elements (not shown) in the lower optics  797  so that a displayed image is provided to a user&#39;s eye  799 . Together, the light source  7910 , the partially reflective layer  7930  and the reflective image source  7935  form a frontlighted image source. Where, the reflective image source  7935  can comprise a LCOS, a FLCOS, DLP or other reflective display.  FIGS. 79, 80, 82 and 83  are shown with the illumination light  7920  provided so that it is incident on the reflective image source  7935  at an oblique angle as is required for a DLP.  FIGS. 84 c , 84 d   ,  85 ,  86 ,  87 ,  88  and  89  are shown with the illumination light  7920  provided perpendicular to the reflective image source  8535  as is required for an LCOS or FLCOS. The principles of the invention described herein apply to any type of reflective image source where stray reduction is needed. The light source  7910  can include light sources such as LEDs, laser diodes or other light sources (e.g. as described herein) along with various light control elements including: diffusers, prismatic films, lenticular films, Fresnel lenses, refractive lenses and polarizers. Polarizers included in the light source  7910  polarize the light exiting the light source  7910  so that the illumination light  7920  is polarized. The partially reflective layer  7930  can be a partial mirror coating on a substrate or it can be a reflective polarizer film such as a wire grid film supplied by Asahi-Kasei under the name WGF or a multilayer polarizer film supplied by 3M under the name DBEF. When the partially reflective layer  7930  is a reflective polarizer, the illumination light  7920  is supplied as polarized light wherein the polarization axis of the illumination light  7920  is oriented relative to the polarization axis of the reflective polarizer so that the illumination light  7920  is substantially reflected. The reflective image source  7935  then includes a quarter wave retarder (e.g. a quarter wave film) so that the polarization state of the illumination light  7920  is reversed in the process of being reflected by the reflective image source  79345 . This enables the reflected illumination light  7920  to then be substantially transmitted by the reflective polarizer. After passing through the partially reflective layer  7930 , the light becomes image light  7940 . The image light  7940  then passes into a lens  7950  which is part of the lower optics  797  or display optics which manipulates the light to provide a displayed image to the user&#39;s eye. While the partially reflective layer  7930  is illustrated as a flat surface, the inventors have contemplated that the surface may be curved, shaped, have simple or complex angles, etc. and such surface shapes are encompassed by the principles of the present invention. 
     In HMDs that provide images to both eyes of the user, it is desirable to provide the images so that they are aligned to one another. This is particularly important when the images are viewed as stereo images where the perceived alignment of the images seen with each eye is critical to achieving the perception of depth. To provide an accurate alignment of the images, an active alignment of the optics can be performed after the optics have been assembled into a rigid frame of the HMD. Where active alignment includes aligning the images for each eye to one another by moving portions of the display assembly and affixing the portions into position relative to one another. To this end,  FIG. 79  shows space  7952  that extends around the reflective image source  7935  so that the reflective image source  7935  can be moved laterally and rotationally. The light source  7910  and partially reflective layer  7930  are arranged to illuminate the area that includes the reflective image source  7935  and a portion of the adjacent space  7952 . As a result, the reflective image source  7935  can be moved within the space  7952  during the active alignment process without losing illumination or degrading the brightness of the displayed image. Where movements of the reflective image source  7935  during the active alignment can include movements that correspond to horizontal, vertical and rotational movements of the image provided to one eye relative to the image provided to the other eye of the user. The movements can be 0.5 mm in size for example when the reflective image source  7935  is approximately 5×8.5 mm in size (this equates to approximately 10% of the reflective image source dimension) and as such the space  7952  can be 0.5 mm wide or wider. 
     However, by including the space  7952 , in the illuminated area, visible artifacts can occur due to light scattering or reflecting from the edges of the reflective image source  7935  or from structures adjacent to the space  7952 . Consequently, a mask  8055  is provided that extends from the edge of the active area of the reflective image source  7935  across the space  7952  to cover the edges of the reflective image source  7935  and structures adjacent to the space  7952  as shown in  FIG. 80 . The mask  8055  is black and non-reflecting so that incident illumination light  7920  is absorbed. In addition the mask  8055  is designed to not interfere with the movements of the reflective image source  7935  that occur during active alignment. To this end, the mask  8055  can be stiff (e.g. a black plastic or a black coated metal) and designed to slide under the adjacent structures such as the light source  7910 , the edge of the partially reflective layer  7930  and the sides of the housing that contain the frontlight. Alternatively, the mask  8055  can be flexible (e.g. a black plastic film or a black rubber film or tape) so that it deforms when it contacts the adjacent structures.  FIG. 81 a    shows an illustration of the reflective image source  7935 , the light source  7910  and the space  7952  as viewed from above. As is typically found with image source of all kinds, there is a mask  8168  applied to the surface of the image source that surrounds the active area  8165 , however this mask  8168  does not cover the space  7952 .  FIG. 81 b    shows a further illustration of the system shown in  FIG. 81 a    wherein the mask  8055  is applied to the reflective image source  7935  so that it attaches within the mask  8168  in a way that covers the space  7952  and does not block the active area  8165 . 
     In another embodiment, the image produced by the image source does not use all of the active display area of the image source so there is room to shift the image in an x and/or y perspective within the active display area for alignment of the content. For example, if a misalignment is observed (as indicated above) rather than physically moving the image source, or in addition to moving the image source, the image is digitally shifted in the x and/or y direction to create better combined content alignment. The originally inactive display area around the content may be referred to as a content shift buffer zone. 
     In a further embodiment for aligning images in a HMD with see-through, a first image containing features is provided to one eye of the user using a display assembly similar to that shown in  FIG. 79 a    or  FIG. 85 . A second image containing features in the same locations is provided to the other eye of the user. The position of at least one of the image sources is then moved within the space provided for adjustment to align the first image to the second image as seen by the user&#39;s eyes. This image alignment can also be done using cameras in place of the user&#39;s eyes. 
     In the case where the first and second images are smaller in size than the active area of the reflective image source, thereby leaving a digital space adjacent to the images that can be used for digital shifting of the images for further alignment adjustment. This adjustment can be used in combination with physical movements of the reflective image sources to align the first image to the second image. 
       FIG. 82  is an illustration of upper optics  825  that includes the elements of upper optics  795  with the addition of a trim polarizer  8260 . Where the polarization axis of the trim polarizer  8260  is oriented so the image light  7940  is transmitted to the lower optics (not shown). Light that has the opposite polarization state compared to the image light  7940  is absorbed by the trim polarizer  8260 . As such, light that is scattered from surfaces such as the walls of the housing  8262  that typically has a mixed polarization state will be partially absorbed by the trim polarizer  8260 . The trim polarizer  8260  can also absorb a portion of colored light caused by birefringence in the lens  7950  provided the trim polarizer  8260  is located after the lens  7950 . In this case, the trim polarizer  8260  absorbs the light that has the opposite polarization state caused by the birefringence and transmits the light that has the polarization state of the image light  7940  prior to the lens  7950 . In some cases, it is advantageous to change the polarization state of the image light  7940  to improve the reflection of the image light  7940  from the combiner  793  so that a half wave retarder is needed in addition to the trim polarizer  8260 . For proper operation, the half wave retarder is positioned with it&#39;s fast axis oriented at 45 degrees to the transmission axis of the trim polarizer  8260 . In this case, it is advantageous to position the half wave retarder (not shown) below the trim polarizer  8260  so that the trim polarizer can absorb any elliptical polarization that may be present due to birefringence in the lens  7950  before the image light is acted upon by the half wave retarder. In this way, any variation in retardation with wavelength that may be present in the half wave retarder will not act to increase the elliptical polarization or act to increase color artifacts in the image light  7940  caused by birefringence in the lens  7950 . In an example, the trim polarizer can be a polarizer film that is laminated to a half wave retarder film and antireflection coatings can be applied to the outer surfaces. 
     In  FIG. 83 , the partially reflective layer  8330  is a laminated multiple polarizer film comprised of a reflective polarizer film  8332  laminated to an absorptive polarizer film  8331 . Where, the reflective polarizer film  8332  is only big enough to reflect the illumination light  7920  that illuminates the active area  8165  of the reflective image source  7935 . The absorptive polarizer film  8331  is larger than the reflective polarizer film  8332  and extends across the entire aperture between the reflective image source  7935  and the lens  7950 , so that no edges of the absorptive polarizer film  8331  are visible and all the light reflected from the reflective image source  7935  passes through the absorptive polarizer  8331 . For the case when the reflective image source  7935  is an LCOS, the absorptive polarizer  8331  acts as an analyzer polarizer to only allow the polarization state of the image light to be transmitted. As such, the reflective polarizer film  8332  only covers a portion of the absorptive polarizer film  8331 . The polarization axes of the reflective polarizer film  8332  and the absorptive polarizer film  8331  are aligned so that polarized light that is transmitted by the reflective polarizer film  8332  is also transmitted by the absorptive polarizer film  8331 . In contrast, polarized light that is reflected by the reflective polarizer film  8332  is absorbed by the absorptive polarizer film  8331 . Thereby, illumination light  7920  that is incident onto the reflective polarizer film  8332  is reflected toward the reflective image source  7935  where the polarization state is reversed so that it is transmitted by the reflective polarizer film  8332  and the absorptive polarizer film  8331  as it becomes image light  7940 . At the same time, illumination light  7920  that is incident onto the absorptive polarizer film  8331  in the area surrounding the reflective polarizer film  8332  is absorbed by the absorptive polarizer film  8331 . By absorbing this excess illumination light  7920 , that would not illuminate the active area  8165  of the reflective image source  7935 , stray light is reduced within the display assembly and the contrast in the image presented to the user&#39;s eye is increased as a result. By aligning the polarization axes of the reflective polarizer film  8332  and the absorptive polarizer film  8331 , the transmission is only reduced by approximately 12%, in the regions that include both reflective polarizer film  8332  and absorptive polarizer film  8331  compared to the regions that include just absorptive polarizer film  8331 . Given the location of the partially reflective layer  8330  in the optical system and the fact that it is remote from the reflective image source  7935 , having local differences in transmission on the partially reflective layer  8330  comprised of a laminated multiple polarizer will have a very small effect on the brightness uniformity in the image provided to the user&#39;s eye. In addition, the fact that the partially reflective layer  8330  is remote from the reflective image source  8330  makes the edges of the reflective polarizer film  8332  indistinct as seen by the user. 
       FIGS. 84 a  and 84 b    show illustrations of examples of partially reflective layers  8330 , comprised of a reflective polarizer film  8430  and  8431  laminated to an absorptive polarizer film  8432 . The reflective polarizer films  8430  and  8431  are cut to a shape that covers only the area where illumination light  7920  will be reflected to illuminate the active area  8165  of the reflective image source  7935 . The shape required for the reflective polarizer film will vary depending on the type of frontlight. For the frontlight shown in  FIG. 83  where the partially reflective layer  8330  is located adjacent to the reflective image source  7935 , the shape of the reflective polarizer film  8431  will be rectangular or oval such as shown in  FIG. 84 b   . For the frontlight included in the display assembly shown in  FIG. 85  where the lens  8550  is located between the partially reflective layer  8530  and the reflective image source  8535 , the influence of the illumination light  8520  passing through the lens  8550  changes the distribution of illumination light  8520  needed from the light source  8510 . As a result, the illumination light  8520  can cover only a portion of the partially reflective layer  8530  and the use of a laminated multiple polarizer is advantageous. In embodiments, the reflective polarizer film can cover less than 80% of the area of the absorptive polarizer film in the laminated partially reflective layer. In further embodiments, the reflective polarizer film can cover less than 50% of the area of the absorptive polarizer film in the laminated partially reflective layer. In this case, the partially reflective layer  8530  can include a reflective polarizer film  8430  with a shape similar to that shown in  FIG. 84 a   . In any case, the shape of the reflective polarizer film is selected in concert with the optical elements in the frontlight and display optics associated with the display assembly of the HMD. 
       FIG. 84 c    shows an example illustration of a frontlight for a display assembly similar to that shown in  FIG. 85  wherein a laminated multiple polarizer film  8436  is shown with a complex curved shape that resembles an S with a central flat portion and curved ends. The laminated multiple polarizer  8436  includes a reflective polarizer film  8438  and an absorptive polarizer film  8437 . Illumination light  8520  includes rays  8522  that are incident on the reflective polarizer film  8438  and rays  8521  that are incident on the absorptive polarizer film  8437 . Due to the alignment of the polarization of the illumination light  8520  to the polarization axes of the reflective polarizer film  8438  and the absorptive polarizer film  8437  as previously described herein, rays  8522  are reflected by the reflective polarizer film  8438  and rays  8521  are absorbed by the absorptive polarizer film  8437 . In this way, rays  8521  are prevented from contributing to stray light. It is beneficial to absorb rays  8521  since they cannot contribute to image light  8540  because if they were reflected by the laminated multiple polarizer  8436  they would be incident on the reflective image source  8535  outside of the active area  8165 , and if they were transmitted by the laminated multiple polarizer  8436 , they would be incident on the housing sidewalls  8262 . Consequently, by absorbing rays  8521 , the laminated multiple polarizer  8436  reduces stray light and thereby increases the contrast in the image displayed to the user. 
       FIG. 84 d    shows a further example illustration of a frontlight for a display assembly similar to that shown in  FIG. 79  wherein the partially reflective layer  7930  comprises a laminated multiple polarizer film with a curved surface. The laminated polarizer includes an absorptive polarizer film  8442  with a laminated reflective polarizer film  8441 . The reflective polarizer film  8441  is positioned in the central portion of the absorptive polarizer film  8442  where the illumination light  7920  is reflected toward the reflective image source  7935 . The polarization axes of the reflective polarizer film  8441  and the absorptive polarizer film  8442  are aligned in parallel to each other and perpendicular to the polarization axis of the illumination light  7920  as provided by the polarized light source  7910 . The rays  8421  of the illumination light  7920  that are incident on the partially reflective layer  7930  outside of the reflective polarizer film  8441  are absorbed by the absorptive polarizer film  8442 . The reflective light source  8535  includes a quarter wave layer  8443  so that the polarization axis of the illuminating light  7920  is changed during the process of being reflected from the reflective image source  8535 . As a result, the reflected illumination light  7920  is transmitted by the reflective polarizer film  8441  and the absorptive polarizer film  8442 , thereby becoming image light  7940 . By absorbing the rays  8421 , before they are incident on external surfaces such housing walls or other optical surfaces, stray light is reduced and as a result the contrast in the image provided to the user&#39;s eye is increased. It should be noted that while  FIGS. 84 c  and 84 d    show the reflective polarizer film being positioned to reduce stray light from the left and right sides as shown in the figure, the reflective polarizer can similarly be positioned to reduce stray light in the direction in and out of the paper as shown in the figure.  FIGS. 84 a  and 84 b    show reflective polarizer films  8430  and  8431  positioned in a center portion of the absorptive polarizer  8432  so that stray light can be reduced in all directions. An important aspect of the invention is that this stray light reduction is obtained without a reduction in the brightness of the image provided to the user&#39;s eye since the reflective polarizer films  8430  and  8431  reflect illumination light over the entire area that is needed to fully illuminate the reflective image source. 
       FIG. 85  shows a schematic illustration of a display assembly for a HMD wherein the optical elements of the frontlight are overlapped with the display optics, as the lens  8550  is located between the partially reflective layer  8530  and the reflective image source  8535 . The display assembly is then comprised of upper optics and lower optics. The upper optics include a reflective image source  8535 , a lens  8550 , a partially reflective layer  8530  and a light source  8510 . The upper optics convert illumination light  8520  into image light  8540 . As shown, the lower optics comprise a beam splitter plate  8580 , a quarter wave film  8575  and a rotationally curved partial mirror  8570  (lower optics similar to those shown in  FIG. 79 a    are also possible). The lower optics deliver the image light  8540  to a user&#39;s eye  8582 . As previously stated herein, the display assembly provides the user with image light  8540  that conveys a displayed image along with scene light  8583  that provides a see-through view of the environment so that the user sees the displayed image overlaid onto a view of the environment. 
       FIG. 85  shows a display assembly wherein the partially reflective layer  8530  is a single flat film. However, it can be advantageous to use a segmented partially reflective layer  8630  such as is shown in  FIG. 86 . In this way, the angle of the central portion  8631  of the partially reflective layer  8630  can be selected to position the light source  8610  differently to reduce the clipping of illumination light  8620  by the lens  8550  or other portions of the supporting structure associated with the display assembly and thereby improve brightness uniformity in the displayed image seen by the user&#39;s eye  8582 . To this end, a comparison of  FIG. 85  to  FIG. 86  shows that by changing the angle of the central portion of the partially reflective film, the position of the light source  8610  is moved downward and the clearance of the illumination light  8620  is increased relative to the lens  8550 . 
     Segmented partially reflective layers can be used which a variety of geometries and makeups.  FIG. 86  shows a segmented partially reflective layer  8630  that includes a folded Z shape with three flat sections.  FIG. 87  shows a segmented partially reflective layer that includes an S shape with a central flat section  8731  and ends that are curved similar to that shown in  FIG. 84 c   . The segmented partially reflective layer can comprise a single partially reflective layer such as a reflective polarizer film or a partial mirror film. In addition, illumination light  8620  can be reflected from just the central flat section or it can be reflected from the central flat section plus one or more of the other segments of the segmented partially reflective layer. Alternatively, the partially reflective layer  8630  can comprise a multiple polarizer film to selectively provide a partially reflective layer over just the portions of the partially reflective layer that are actually needed to reflect illumination light to uniformly illuminate the reflective image source  7935  as previously described herein.  FIG. 88  shows a display assembly wherein the partially reflective layer  8830  is comprised of a laminated multiple polarizer film with a central portion  8831  that includes a reflective polarizer film and the remainder of which is an absorptive polarizer as previously described herein. Where the segmented shape of the partially reflective layer  8830  is similar to that shown in  FIG. 86 .  FIG. 89  shows a display assembly wherein the partially reflective layer  8930  is comprised of a laminated multiple polarizer film with a central portion  8931  that includes a reflective polarizer film and the remainder of which is an absorptive polarizer as previously described herein. Where the segmented shape of the partially reflective layer  8930  is similar to that shown in  FIG. 87 . While  FIGS. 88 and 89  show the reflective polarizer film as just occupying the flat central segment of the segmented partially reflective layers  8830  and  8930  respectively, the reflective polarizer can extend into the adjacent segments as needed to reflect the illumination light  8620  in the pattern needed to uniformly illuminate the reflective image source  8535 . Alternatively the segments associated with the segmented partially reflective layers  8830  and  8930  can have three dimensional shapes when the reflective polarizer portion is shaped like that shown in  FIG. 84 a    to keep the reflective polarizer  8430  portion flat. 
     In a further embodiment, the reflective polarizer film is laminated to a flexible transparent carrier film to increase the flexibility and the absorptive polarizer film is a separate layer.  FIG. 90  shows a partially reflective layer  9030  comprised of a reflective polarizer film  8441  laminated to a flexible transparent carrier film  9043 . Where the flexible transparent carrier film  9043  does not reflect the illumination light  7920  or change polarization state of the illumination light  7920  and as a result rays  8421  pass through the flexible transparent carrier film  9043 . The purpose of the flexible transparent carrier film is to support the reflective polarizer film  8441  while allowing the partially reflective layer  9030  to be substantially as flexible as the reflective polarizer film  8441  alone. Absorptive polarizer film  9042  is then provided as a separate layer positioned adjacent to the partially reflective layer  9030 . While the absorptive polarizer film  9042  can be flat or curved as needed to fit within the available space, in a preferred embodiment, the absorptive polarizer film  9042  is curved to be better positioned to absorb rays  8421  that are incident on the partially reflective layer  9030  outside of the reflective polarizer film  8441  as shown in  FIG. 90 . 
     In yet another embodiment, the reflective polarizer film is modified to make the portions transparent and non-reflective where illumination light is incident that is not needed to illuminate the active area of the reflective image source and a separate absorptive polarizer is provided to absorb light that is transmitted through the non-reflective portions.  FIG. 91  is an illustration of a partially reflective layer  9130  comprised of a reflective polarizer film wherein portions  9143  are modified to be transparent and non-reflective while the portion  9141  is a reflective polarizer. As such, polarized illumination light  7920  is reflected by the reflective polarizer portion  9141  and is transmitted by the modified portions  9143 . An absorptive polarizer  9042  is provided as a separate layer adjacent to the partially reflective layer  9130  so that rays  8421  of the illumination light  7920  are transmitted by the modified portions  9143  and absorbed by absorptive polarizer. Wherein the transmission axis of the reflective polarizer portion  9141  is parallel aligned to the transmission axis of the absorptive polarizer  9042 . The modification of the reflective polarizer film can be accomplished by etching the reflective polarizer film, when the reflective polarizer film is a wiregrid polarizer, and thereby removing the metal wires of the wiregrid in the modified portions. Alternatively the wiregrid polarizer can be masked during the metal deposition step to provide shaped portions of wire grid polarizer during manufacturing. An advantage provided by modifying the reflective polarizer film is that the flexibility of the partially reflective layer  9130  is substantially unchanged by the modification and as a result the partially reflective layer  9130  remains uniformly flexible in both the modified portions  9143  and the reflective polarizer portion  9141 . Another advantage provided by using a modified reflective polarizer film is that the transition from the modified portion  9143  to the reflective polarizer portion  9141  does not include a sharp edge that can cause visible artifacts in the image provided to the user&#39;s eye due to scattering by the edge or a change in optical density from a thickness change. This embodiment can also be applied to other types of display assemblies such as for example that shown in  FIG. 85 . 
     In a yet further embodiment, the partially reflective layer comprises a reflective polarizer film laminated to an absorptive polarizer and the partially reflective layer includes a flat portion and a curved portion.  FIG. 92  is an illustration of a frontlight for a display assembly similar to that shown in  FIG. 79 a    with the addition of a laminated partially reflective layer  9230  that has a portion that is a reflective polarizer laminated to an absorptive polarizer  9230 . Where the partially reflective layer  9230  is segmented with a flat segment and a curved segment. By including a flat segment in the portion of the partially reflective layer  9230  that is a reflective polarizer  9241 , the uniformity of illumination light  7920  that is reflected onto the reflective image source  7935  is improved because a larger portion of the light source  7910  is mapped to the image as can be seen in  FIG. 92 . Wherein when using a small scale light source and associated light control films such as diffusers, it is important to map a large portion of the light source area to avoid darker or brighter lines across the image produced by a dark or bright spot on the light source. Including a flat segment in the partially reflective layer  9230  also reduces local distortions in the image provided to the user&#39;s eye that are caused by local changes in optical path length or localized refraction due to changes in the surface angles that the light is exposed to. This embodiment can also be applied to other types of display assemblies such as for example that shown in  FIG. 85 . 
     In head mounted displays that provide a displayed image overlaid onto a see-through view of the environment, it is advantageous to have high see-through transmission both so the user can better interact with the environment and so that people in the environment can see the user&#39;s eyes so they feel more engaged with the user. It is also advantageous to have a thin optics module with low height to make the head mounted display more compact and thereby more attractive. 
       FIG. 93  shows an illustration of an optics module that provides the user with a displayed image while simultaneously providing high see-thru transmission. In this way, the user is provided with a displayed image overlaid onto a clear view of the environment. The optics module includes a combiner  9320  that can have a partial mirror coating that transmits a majority (greater than 50% transmission of visible light) of the available light from the environment, with transmission higher than 70% preferred. For example, the combiner  9320  can have a broadband partial mirror that reflects less than 30% and transmits over 70% of the entire visible wavelength band. Alternatively, the combiner  9320  can have a notch mirror coating where the reflectivity band of the notch mirror coating is matched to the wavelength bands provided by the light source  9340 , where the light source  9340  can include one or more LEDs, QLEDs, diode lasers or other light source, each with narrow wavelength bands (e.g. 50 nm wide bands or less, full width half max). The notch mirror coating can provide for example, greater than 20% reflectivity (e.g. 50% reflectivity) in the wavelengths bands provided by the light source  9340  while providing greater than 80% transmission in the remaining wavelength bands in the visible. For full color images to be provided by the optics module, at least three LEDs with complimentary colors are required such as red, green and blue light or, cyan, magenta and yellow light. In a preferred embodiment, the combiner  9320  has a tristimulus notch mirror that reflects over 50% of the light within the wavelength bands provided by the light source  9340  and transmits an average of over 80% across the entire visible wavelength band. In this way, the tristimulus notch mirror coating provides improved efficiency compared to the partial mirror coating previously described. In an example, if the combiner is to provide 75% transmission of visible light from the environment  9362 , the partial mirror coating will reflect only 25% of image light  9360  so that 75% of the image light will be transmitted through the combiner and will not contribute to the brightness of the image provided to the user&#39;s eye  9310 . In contrast, a tristimulus notch mirror coating can be used to reflect over 50% of the image light  9360  over the wavelengths of light provided by the LEDs in the light source  9340  while transmitting over 90% of the remaining wavelengths of visible light that are not provided by the LEDs so that the average transmission over the entire range of visible light is over 75%. Consequently, the tristimulus notch mirror is twice as efficient as the partial mirror in terms of the ability to reflect image light  9360  toward the user&#39;s eye  9310 . 
     To enable the optics module to operate with a combiner  9320  as shown in  FIG. 93 , image light  9360  is provided to a lens  9330  which focuses the image light  9360  at the user&#39;s eye  9310 . Wherein lens  9330  is shown as a single lens element for simplicity, but multiple lens elements are also possible. The image light  9360  is provided from illumination light  9364  that comes from the light source  9340 . Where, the illumination light  9364  is reflected by a beam splitter  9352  toward a reflective image source  9350 . The image source  9350  can be a liquid crystal on silicon display (LCOS), a ferroelectric liquid crystal display (FLCOS) or other such reflective display. A polarizer  9342  can be associated with the light source  9340  to provide polarized illumination light  9364 . The beam splitter  9352  can then be a reflective polarizer that is oriented to substantially reflect the polarized illumination light  9364 . The image source  9350  changes the polarization state of the illumination light  9364  when the light is reflected by the image source  9350  to form image light  9360  that has a polarization state that is opposite to that of the illumination light  9364 . By changing the polarization state of the illumination light  9364  to the polarization state of the image light  9360 , the image light  9360  can then be transmitted by the reflective polarizer of the beam splitter  9352 . It is important to note that the image light  9360  is polarized to enable a folded illumination system and not because polarized light is required by the combiner  9320 . In fact, to provide a transmission of light from the environment  9362  that is greater than 50%, the combiner  9320  cannot include a polarizer. 
       FIG. 94  is an illustration of an optics module than includes multiply folded optics to reduce the overall height of the optics module. In this case, illumination light  9464  is transmitted by the beam splitter  9452  so that it passes directly toward the image source  9450  wherein the beam splitter  9452  is a reflective polarizer and the light source  9340  includes a polarizer  9342  that is oriented so the transmission axis of the polarizer  9342  is parallel to the transmission axis of the beam splitter  9452 . The illumination light  9464  is then reflected and changed in polarization state by the image source  9450  so that the image light  9360  with it&#39;s changed polarization state is reflected by beam splitter  9452  toward the lens  9330 . As can be seen by comparing  FIG. 93  to  FIG. 94 , the overall height of the optics module shown in  FIG. 94  is substantially reduced. 
     However, the orientation of the additional fold in the optical path of the image light  9360  in the optics module of  FIG. 94  increases the thickness of the optics module, where thickness is defined as the distance from the closest back surface of the optics module that is nearest to the user&#39;s eye to the farthest front surface of the optics module that is farthest from the user&#39;s eye.  FIGS. 95 and 96  show illustrations of an optical module where the added fold in the optical path of the image light  9360  is oriented perpendicular to the fold shown in  FIG. 94 . In this case, the optics module in  FIGS. 95 and 96  is wider but thinner than that shown in  FIG. 94 .  FIG. 95  shows the optics module from the side and  FIG. 96  shows the optics module from the position of the user&#39;s eye  9310 . As such, in the multiply folded optics shown in  FIGS. 95 and 96 , optical axis  935  associated with the illumination light  9464  is perpendicular to both the optical axis  934  associated with the image light  9360  as it passes through the lens  9330  and the optical axis  933  associated with the image light  9360  as it proceeds toward the user&#39;s eye  9310  in the eyebox. In the case of a head mounted display, it can be very important to have a thin optics module because a thick optics module can cause the head mounted display to stick outward from the user&#39;s forehead, which can be uncomfortable and unattractive. Thus, the multiply folded optics module shown in  FIGS. 95 and 96  are shorter and thinner than the optic module shown in  FIG. 93 . The optics module shown in  FIGS. 95 and 96  is wider than the optics module shown in  FIG. 93 , but in a glasses configuration of the head mounted display, wider optics modules can be better fit into the glasses frames than taller or thicker optics modules. 
     A further advantage that is provided by an optics module that includes multiply folded optics is that twists can be introduced at the fold surfaces to modify the orientation of different portions of the optics module relative to each other. This can be important when the optics module needs to fit into a thin curved glasses frame, a visor or a helmet where the increased width associated with the upper portion of the multiply folded optics module can make it more difficult to fit into structures that are not parallel to the combiner. In this case, the upper portion including for example (based on  FIG. 96 ), the light source  9340 , the polarizer  9342 , the beam splitter  9452  and the image source  9450 , can be twisted relative to the lower portion including the lens  9330  and the combiner  9320 . Where to avoid distortion of the image due to the compound angles between the fold surfaces, a twist of the upper portion about the axis  934  must be combined with a corresponding twist of the lower portion about the axis  933 . In this way, the effects of the increased width of the upper portion of the multiply folded optics can be reduced when fitting the optics module into a curved structure such as glasses frames, a visor frame or a helmet structure. 
       FIG. 99  shows a further embodiment wherein the lens  9930  includes a diffractive surface  9931  to enable a more compact and shorter optical design with reduced chromatic aberration. Where the diffractive surface  9931  can be comprised of a series of small annular sections of a refractive lens curve such as for example in a Fresnel lens. The diffractive surface  9931  can be flat as shown in  FIG. 99  or it can have a base curve to provide additional optical power. The diffractive surface  9931  can be a single order diffractive or a multiple order diffractive. To reduce scattering of wide angle illumination light  9964  that could be incident on the diffractive surface  9931 , an absorptive polarizer  9932  is provided and is oriented with it&#39;s transmission axis perpendicular to the transmission axis of the reflective polarizer of the beam splitter  9452 . In this way, illumination light  9964  that is transmitted by the beam splitter  9452  in the direction that would cause it to be incident on the diffractive surface  9931  is absorbed by the absorptive polarizer  9932  before it can be scattered by the diffractive surface  9931 . At the same time, image light  9360  has a polarization state that is opposite to that of the illumination light  9964  so that it is reflected by the beam splitter  9452  and transmitted by the absorptive polarizer  9932  as it passes into the lens  9930 . 
       FIG. 100  shows an illustration of an optics module that includes a reduced angle between the beam splitter  9452  and the lens  9930  to reduce the overall height of the optics module. The fold angle of the image light  9360  (the deflection angle between 934 and 1005) is then more than 90 degrees and as a result, the upper edge of the beam splitter is closer to the lens  9330  thereby providing a reduced overall height of the optics module. 
       FIG. 100  also shows a compact planar light source  10040  comprised of a thin edge-lit backlight similar to what is provided in displays used in displays for mobile devices like cellphones. The compact planar light source  10040  is positioned directly behind the beam splitter  9452  to reduce the overall size of the optics module. The compact planar light source can include a light guide film or light guide plate with an edge lit light such as one or more LEDs and a reflector on the side opposite the beam splitter  9452 . The compact planar light source can include a polarizer so the illumination light  10064  is polarized as previously described herein. To direct the illumination light  10064  toward the image source  9450  for improved efficiency, a turning film  10043  is positioned between the compact planar light source  10040  and the beam splitter  9452 . A 20 degree prismatic turning film can be obtained for example, from Luminit 103C (Torrance, Calif.) under the name DTF. To obtain greater degrees of turning, such as 40 degrees, multiple layers of turning film  10043  can be stacked together provided they are oriented such that the turning effect is additive. A diffuser layer (not shown) can be used in addition to the turning film  10043  to reduce artifacts such as linear shadows that can be associated with prismatic structures that are typically associated with turning films  10043 .  FIG. 101  shows an illustration of an optics module as seen from the position of the user&#39;s eye, which is similar to that shown in  FIG. 100  but with a perpendicular orientation of the added fold in the image light  10164  to reduce the thickness of the optics module as previously described herein. As in the optics module shown in  FIGS. 95 and 96 , the multiply folded optics shown in  FIG. 101  have an optical axis  1005  associated with the illumination light  10164  that is perpendicular to both the optical axis  934  associated with the image light  9360  as it passes through the lens  9330  and the optical axis  933  associated with the image light  9360  as it proceeds toward the user&#39;s eye  9310  in the eyebox. As a result, the optics module of  FIG. 101  is thinner and shorter than the optics module of  FIG. 93 .  FIG. 101  also includes a field lens  10130  to improve the optical performance of the optics module. The addition of this second lens element is possible because of the change in fold orientation so that the field lens  10130  does not increase the thickness of the optics module, instead the added length of the optical path from the field lens  10130  occurs in the width of the optics module where space is more readily available in the head mounted display. 
       FIG. 102  shows an illustration of an optics module similar to that shown in  FIG. 99  but with a different orientation of the upper portion of the optics module relative to the combiner so that the combiner  10220  can be more vertical. This rearrangement of the elements within the optics module can be important to achieve a good fit of the head mounted display onto the user&#39;s face. By making the combiner  10220  more vertical, the optics module can be made to have less interference with the user&#39;s cheekbones. 
       FIGS. 103, 103   a  and  103   b  show illustrations of optics modules as seen from the position of the user&#39;s eye, that include multiply folded optics and digital light projector (DLP) image sources  10350 . In this case, the illumination light  10364  is provided at an oblique angle to the image source  10350  as required by the micromirrors in the DLP, to reflect image light  9360  along the optical axis  934  of the lens  9930 . Where, in the case of a DLP image source  10350 , image light  9360  is comprised of on-state light reflected by on-state micromirrors in the DLP image source  10350  along optical axis  934 , in correspondence to the brightness of pixels in the image to be displayed to the user&#39;s eye  9310  in the eyebox. The micromirrors in the DLP image source  10350  also reflect off-state light  10371  to the side of the optics module in correspondence to the dark image content and as a result, a light trap  10372  is provided in the optics module to absorb light  10371 . The light trap  10372  can be a black absorptive surface or a textured black surface. The purpose of the light trap  10372  is to absorb incident light  10371  and thereby reduce stray light and subsequently improve the contrast of the image displayed to the user&#39;s eye  9310 . As previously described in other embodiments herein, the light source  10340  is provided to the side of the optics module with a multiply folded optical path to reduce the overall thickness and height of the optics module.  FIG. 103  provides the DLP image source  10350  at the top of the optics module so that the image light  9360  proceeds straight along the optical axis  934 , through the lens  9930  and down to the combiner  9320  where the image light is reflected toward the user&#39;s eye  9310  located in the eyebox. A polarizer  10341  is provided with the light source  10340  so that polarized illumination light  10364  is reflected by the beam splitter  9452  to illuminate the DLP image source  10350 . Where, the beam splitter  9452  in this case, is a reflective polarizer that is aligned with the polarizer  10341  so that the polarized illumination light  10364  is reflected by the beam splitter  9452  and image light  9360  is transmitted by the beam splitter  9452 . A quarter wave film  10351  is located adjacent to the surface of the DLP image source  10350  so that the polarization state of the image light  9360  is opposite to that of the illumination light  10364  after being reflected by the DLP image source  10350 . The light source  10340  and the reflective polarizer  9452  are angularly arranged so that the illumination light  10364  is incident onto the DLP image source  10350  at the oblique angle required so that the image light  9360  when reflected by the on-state pixels in the DLP image source  10350  proceeds along the optical axis  934  of the lens  9930 . A field lens (similar to  10130  as shown in  FIG. 101 ) or other lens elements may be included in the optics of  FIG. 103  but is not shown, in which case, the illumination light  10364  and the image light  9360  may pass thru the field lens or other lens elements in opposite directions. 
       FIG. 103 a    is an illustration of another optics module with a multiply folded optical path that includes a DLP image source  10350  and is shown from the position of the user&#39;s eye. The light source  10340  is again provided to the side of the optics module to reduce the thickness of the optics module. In this case, the light source  10340  is provided on the same side of the lens  9930  and combiner  9320 , as the DLP image source  10350 . Lens  9930  can optionally include one or more diffractive surfaces  9931 . The light source  10340  directly illuminates the DLP image source  10350  where the illumination light  10364  is incident on the DLP image source  10350  at an oblique angle so that the image light  9360 , after being reflected by the on-state micromirrors in the DLP image source  10350 , proceeds along the folded optical axis  934 . At least one light trap  10372  is also provided to absorb light  10371  that is reflected from off-state micromirrors in the DLP and thereby improve the contrast of the displayed image as seen by the user. A field lens  10332  is provided between the DLP image source  10350  and the fold mirror  10352 . The illumination light L 64  in this case can be unpolarized light whereupon the fold mirror  10352  can be comprised of a full mirror coating (e.g. a coating that reflects the entire visible light spectrum) on a substrate. The field lens  10332  can be a single lens element as shown in  FIG. 103 a    or it can include multiple lens elements as needed. The field lens  10332  is designed to provide a large air gap between the field lens  10352  and the DLP image source  10350 , so that the illumination light  10364  can be introduced to the optics module to directly illuminate the active area associated with the DLP image source  10350 . By using unpolarized illumination light  10364 , the optics module shown in  FIG. 103 a    has improved efficiency over the optics module with DLP image sources  10350  shown in  FIGS. 103 and 103   b.    
       FIG. 103 b    is an illustration of another optics module with multiply folded optical path that includes a DLP image source  10350  and is shown from the position of the user&#39;s eye  9310  in the eyebox. As in the optics modules shown in  FIGS. 103 and 103   a , the optics module of  FIG. 103 b    has the light source  10340  positioned at the side of the optics module to reduce the height and thickness of the optics module. The DLP image source  10350  is positioned opposite the light source  10340  however in this embodiment they do not share an optical axis. The illumination light  10364  passes through the beam splitter  10352 , which in this case can be a first reflective polarizer. A second reflective polarizer  10332  is positioned adjacent to the lens  9930  so that the illumination light  10364  is reflected toward the DLP image source  10350 . To reflect the illumination light  10364 , the first reflective polarizer (beam splitter  10352 ) and the second reflective polarizer  10332  are oriented with perpendicular transmission axes. A quarter wave film  10351  (or quarter wave coating on the DLP cover glass) is provided adjacent to the DLP image source  10350  so that the polarization state of the illumination light  10364  is changed upon reflection from the DLP image source  10350  as it becomes image light  9360 . As a result, the polarization of the illumination light  10364  is opposite to that of the image light  9360 . Consequently, the illumination light  10364  is transmitted by the beam splitter  10352  and reflected by the second reflective polarizer  10332 , while the image light  9360  is reflected by the beam splitter  10352  and transmitted by the second reflective polarizer  10332 . The light source  10340  is oriented relative to the second reflective polarizer  10332  so that it is reflected at an oblique angle relative to the DLP image source  10350  as required to provide image light  9360  reflected from on-state micromirrors in the DLP image source  10350  along the folded optical axis  934 . The second reflective polarizer  10332  can be extended beyond the lens  9930  to provide the required oblique angle to fully illuminate the DLP image source  10350  as shown in  FIG. 103 b   . Because the light source  10340  is located behind the beam splitter  10352 , which is a reflective polarizer, the light source  10340  does not affect the image light  9360  and as a result, the light source  10340  can be a different size and orientation than the beam splitter  10352 . One or more light traps  10372  are provided to absorb light  10371  that is reflected from off-state micromirrors in the DLP image source  10350  and thereby improve the contrast of the displayed image. In this case, the light trap  10372  can be positioned under the second reflective polarizer  10332  because the polarization state of the light  10371  is such that it is reflected by the beam splitter  10352  and transmitted by the second reflective polarizer  10332 . The combined orientation of the light source  10340 , the beam splitter  10352  and the DLP image source  10350  provides an optics module that is relatively thin and relatively short compared to optics modules where the image source or the light source are positioned above the fold mirror or beam splitter (e.g. such as the optics module shown in  FIG. 103 ). 
       FIGS. 97 and 98  show illustrations of optics modules similar to those shown in  FIG. 94  but with the addition of an eye imaging camera  979  for capturing images of the user&#39;s eye  9310  during use. In these cases, the light source  9340  and image source  9450  are positioned opposite one another so that the eye imaging camera  979  can be positioned directly above the lens  9340  so that the optical axis  934  is shared between the optics module and the eye imaging camera  979 . By sharing a common optical axis, the eye imaging camera  979  can capture an image of the user&#39;s eye  9310  that has a perspective from directly in front of the user&#39;s eye  9310 . Image light  9360  can then be used to illuminate the user&#39;s eye  9310  during image capture. A portion of the light reflected from the user&#39;s eye  9310 , which can be unpolarized, passes through the beam splitter  9452  before being captured by the eye imaging camera  979 . Because the eye imaging camera  979  is located above the beam splitter  9452 , if the beam splitter  9452  is a reflective polarizer, the polarization state of the image light  9360  will be opposite to that of the light  978  captured by the eye imaging camera  979 . The eye imaging camera  979  can be used to capture still images or video. Where video images can be used to track movements of the user&#39;s eye when looking at displayed images or when looking at a see-through view of the environment. Still images can be used to capture images of the user&#39;s eye  9310  for the purpose of identifying the user based on patterns on the iris. Given the small size of available camera modules, an eye imaging camera  979  can be added to the optics module with little impact on the overall size of the optics module. Additional lighting can be provided adjacent to the combiner  9320  to illuminate the user&#39;s eye. The additional lighting can be infrared, so the user can simultaneously view images displayed with visible light. If the additional lighting is infrared, the eye camera  979  must be capable of capturing images at matching infrared wavelengths. By capturing images of the user&#39;s eye from the perspective of directly in front of the user&#39;s eye, undistorted images of the user&#39;s eye can be obtained over a wide range of eye movement. 
       FIG. 120  shows an illustration of another embodiment of an eye imaging camera associated with the optics module shown in  FIG. 101 , however the eye imaging camera can be similarly included in optics modules such as those shown in  FIGS. 99, 100, 103, 103   b . These optics modules include absorptive polarizers  9932  to reduce stray light as previously disclosed herein. These optics modules can also include a diffractive surface  9931 , but the diffractive surface  9931  is not required for the operation of the eye imaging camera  979 . In this embodiment, the polarization state of the image light  9360  is the same as that of the light that is reflected by the user&#39;s eye and captured by the eye imaging camera  979  since they both pass through the absorptive polarizer  9932 . In this embodiment, the eye imaging camera  979  is positioned adjacent to the beam splitter  9452  and the compact planar light source  10040  and between the beam splitter and the field lens  10130 . The optical axis  12034  of the light reflected by the eye is then angled somewhat relative to the optical axis  934  of the image light  9360 , so that the center of the user&#39;s eye  9310  and the associated eyebox are within the field of view of the eye imaging camera  979 . In this way, the eye imaging camera  979  captures images of the user&#39;s eye from nearly directly in front and only slightly to the side of the user&#39;s eye  9310  as shown in  FIG. 120 . While  FIG. 120  shows the eye imaging camera  979  positioned adjacent to an end of the beam splitter  9452 , it is also possible to position the eye imaging camera  979  adjacent to a side of the beam splitter  9452 . The advantage of this embodiment is that the eye imaging camera  979  is provided with a simple optical path so that high image quality is possible in the captured images of the user&#39;s eye  9310 . It should be noted that the optics associated with the eye imaging camera must take into account the effect of the lens  9930  since the light reflected by the user&#39;s eye  9310  that is captured by the eye imaging camera passes through the lens  9930 . Also, the addition of the eye imaging camera  979  does not substantially increase the volume of the optics module as can be seen by comparing  FIG. 120  to  FIG. 101 . 
       FIG. 121  shows an illustration of a further embodiment of an optics module that includes an eye imaging camera  979 . Similar to the embodiment shown in  FIG. 120 , this optics module also includes an absorptive polarizer  9932  to reduce stray light and a diffractive surface  9931  may be included, but is not required. In this embodiment, the eye imaging camera  979  is positioned between the beam splitter  9452  and the field lens  10130  and pointed towards the beam splitter  9452 . In this way, light reflected by the user&#39;s eye  9310  is reflected upwards by the combiner  9320 , passes through the lens  9930  and the absorptive polarizer  9932  and then is reflected laterally toward the eye imaging camera  979  by the beam splitter  9452 . The light captured by the eye imaging camera  979  is thereby the same polarization state as the image light  9360 , so that it is reflected by the beam splitter  9452  and transmitted by the absorptive polarizer  9932 . The light reflected by the user&#39;s eye  9310  can be unpolarized as initially reflected by the user&#39;s eye  9310 , however, after passing through the absorptive polarizer  9932 , the light becomes polarized with the same polarization state as the image light  9360 . An advantage of this embodiment is that it is even more compact than the embodiment shown in  FIG. 120 . This arrangement of the eye imaging camera  979  is also possible in the optics modules shown in  FIGS. 99, 100, 103, 103   a  and  103   b.    
     In the embodiments shown in  FIGS. 120 and 121 , the user&#39;s eye  9310  and the associated eyebox can be illuminated by image light  9360  or an additional light source can be provided for example, by an LED positioned adjacent to the combiner  9320 . Where the LED can provide visible light or infrared light, provided the eye imaging camera can capture at least a portion of the wavelengths of light provided by the LED. 
     In an alternative embodiment for the optics module shown in  FIG. 103 a   , the light source  10340  provides polarized illumination light  10364  and the fold mirror  10352  is a reflective polarizer plate so that an eye camera (not shown) can be positioned above the fold mirror  10352  and along the optical axis  934  for capturing images of the user&#39;s eye  9310  similar to that shown in  FIGS. 97 and 98 . The eye camera and the optics module then share a common optical axis  934  so that images of the user&#39;s eye  9310  are captured from directly in front of the eye. In this arrangement, the polarization state of the image light  9360  is opposite to that of the light captured by the eye camera because the image light  9360  is reflected by the fold mirror  10352  and the light captured by the eye camera is transmitted by the fold mirror  10352 . 
       FIG. 104  shows an illustration of the optics module of  FIG. 95  with the additional element of a controllable light blocking element to improve contrast in portions of the displayed image and also to improve the appearance of opacity in displayed objects such as augmented reality objects. Where the controllable light blocking element can operate by absorbing the incident light or scattering the incident light as provided, for example, by an electrochromic element, a polymer stabilized liquid crystal or a ferroelectric liquid crystal. Examples of suitable light blocking elements includes: 3G Switchable Film from Scienstry (Richardson, Tex.); Switchable Mirror or Switchable Glass from Kent Optronics (Hopewell Junction, N.Y.). The controllable light blocking element  10420  is shown in  FIG. 104  as being attached to the lower surface of the combiner  9320  so that it doesn&#39;t interfere with the displayed image while blocking see-thru light from the environment  9362 . Provided the combiner  9320  is flat, the addition of controllable light blocking elements  10420  adjacent to the combiner  9320  is easily done either by attaching directly to the combiner or attaching to the sidewalls of the optics module housing. The controllable light blocking element  10420  can have a single area that can be used to block a selectable portion of the see-through light from the environment over the entire combiner  9320  area thereby enabling a selectable optical density. Alternatively the controllable light blocking element  10420  can provide an array of areas  10520 , as shown in  FIG. 105 , that can be separately selectably controlled to block portions of the combiner  9320  area that correspond to areas in the displayed image where high contrast areas of the image are located.  FIG. 105  shows an illustration of an array of separately controllable light blocking elements  10520 .  FIGS. 106 a , 106 b  and 106 c    are illustrations of how the array of separately controllable light blocking elements  10520  can be used.  FIG. 106 a    shows how the array of separately controllable light blocking elements  10520  can be put into blocking modes in areas  10622  and non-blocking modes in areas  10623 . Where the blocking mode areas  10622  correspond to areas where information or objects are to be displayed such as is shown in the corresponding areas in the illustration of  FIG. 106 b   .  FIG. 106 c    shows what the user sees when the image of  FIG. 106 b    is displayed with the array of controllable light blocking elements  10520  used in light blocking modes  10622  and non-blocking modes  10623 . The user then sees the displayed information or objects overlaid onto a see-through view of the environment, but in the areas where information of objects are displayed, the see-through view is blocked to improve the contrast of the displayed information or object and provide a sense of solidness to the displayed information or objects. 
     In addition,  FIG. 104  shows a rear optical element  10490  that can be a protective plate or a corrective optic. The protective plate can be connected to sidewalls and other structural elements to stiffen the positioning of the combiner  9320  and to prevent dust and dirt from getting onto the inner surface of the combiner  9320 . The corrective optics can include a prescriptive optic, which includes the ophthalmic prescription (optical power and astigmatism for example) of the user to improve the viewing experience. 
     Head mounted displays provide the user with freedom to move their head while watching displayed information. See-through head mounted displays also provide the user with a see-through view of the environment whereupon the displayed information is overlaid. While head mounted displays can include various types of image sources, image sources that provide sequential color display typically provide higher perceived resolution relative to the number of pixels in the displayed images because each pixel provides image content for each of the colors and the image perceived by the user as a displayed full color image frame is actually the sum of a series of rapidly displayed sequential color subframes. For example, the image source can sequentially provide subframe images comprised of a red image, a green image and then a blue image that are all derived from a single full color frame image. In this case, full color images are displayed at an image frame rate that includes a series of at least three sequentially colored subframes that are displayed at a subframe rate which is at least 3× the image frame rate. Sequential color images sources include reflective image sources such as LCOS and DLP. 
     The color breakup that occurs with a sequential color display occurs because the different color subframe images that together provide the user with a full color frame image are displayed at different times. The inventors realized that with sequential color display in a head mounted display, when there is movement of the head-mounted display or movement of the user&#39;s eyes, such that the user&#39;s eyes do not move in synch with the displayed image that under such movement conditions the perceived locations of each of the sequential color image subframes are different within the user&#39;s field of view. This can happen when the user moves his head and the user&#39;s eyes do not follow the same trajectory as the head mounted display, which can be due to the user&#39;s eyes moving in a jerky trajectory as the eyes pause to look at an object in the see-through view of the environment. Another way this can happen is if an object passes through the see-through view of the environment and the user&#39;s eyes follow the movement of the object. Due to this difference in perceived locations within the user&#39;s field of view, the user sees the sequential color images slightly separated at the edges of objects. This separation of colors at the edge of objects is referred to as color breakup. Color breakup may be easily perceived during certain movements because the sequential colors are vividly colored in areas where they do not overlap one another. The faster the user moves their head or the faster the user&#39;s eyes move across the display field of view, the more noticeable the color breakup becomes, because the different color subframe images are separated by a greater distance within the field of view. Color breakup is particularly noticeable with see-through head mounted displays, because the user can see the environment and the user&#39;s eyes tend to linger on objects seen in the environment as the user turns his head. So even though the user may turn his head at a steady rotational rate, the user&#39;s eye movement tends to be jerky and this creates the conditions where color breakup is observed. As such there are two different conditions that tend to be associated with color breakup: rapid head movement and rapid eye movement. 
     It is important to note that when the user is not moving his head and the head mounted display is not moving on the user&#39;s head, color breakup will not be observed because the subframe images are provided at the same positions within the field of view of the user&#39;s eyes. Also, if the user were to move his head and the user moves his eyes in synch with the head movement, color breakup will not be observed. So movement of the head mounted display is indicative of conditions that can lead to color breakup and is also indicative of the degree of color breakup that can occur if the user moves his eyes relative to the movement of the head mounted display. Color breakup is less of an issue with head mounted displays that do not have see-through to the environment, because only the displayed image content is visible to the user and it moves in synch with the movement of the head mounted display. Color breakup is also not an issue if a monochrome image is displayed with a monochrome light source (i.e. there are no sequential color subframes, instead there are only single color frames) since all the displayed images are comprised of the same color. Thus, color breakup is an issue that is most noticeable with head mounted displays that provide a see-through view of the environment. 
     Systems and methods according to the principles of the present invention reduce color breakup and thereby improve the viewing experience provided by a head-mounted display with see-through when the user is moving through the environment. 
     In embodiments, systems and methods are provided where the head-mounted display detects the speed of movement of the head-mounted display and in response, the resolution of the image is reduced or the bit depth of the image is reduced, while the image frame rate at which the image is displayed and the associated subframe rate are correspondingly increased. In this way, the bandwidth associated with the display of the image can be maintained constant, in spite of the frame rate being increased. Where, by increasing the frame rate associated with the display of images, the time between the display of each sequential color subframe image is reduced and as a result the visually perceived separation between the sequential color images is reduced. Similarly the image frame rate can be reduced while the subframe rate is increased by increasing the number of subframes displayed for each image frame. 
     In further embodiments, systems and methods are provided where the sequential color subframe images are shifted laterally or vertically relative to one another by a number of pixels that corresponds to the detected movement of the head mounted display. In this way, the color sequential subframe images are displayed to the user such that they are visually overlaid on top of each other within the displayed field of view. This compensates for separation between subframes and thereby reduces color breakup. 
     In yet another embodiment, systems and methods are provided where an eye-imaging camera in the head-mounted display is used to track the movement of the user&#39;s eyes. The movement of the head-mounted display may be simultaneously measured. An accommodation in the presentation may then be made to reduce color breakup. For example, the resolution of the images and the frame rate may be changed or the image frame rate can be reduced while increasing the subframe rate, in correspondence to the difference in movement of the user&#39;s eyes and the movement of the head mounted display. As another example, the subframes may be shifted to align the subframes in correspondence to the determined difference in movement between the user′ eyes and the head mounted display. As a further example, the color saturation of the content may be reduced to reduce the perception of color breakup due to the fact that the colors, while positionally separated as perceived by the user, are not as separated in color space. In yet a further example, the content could be converted to monochrome imagery which is displayed as a single color image (e.g. white) during the detected movement so that color breakup is not visible. 
       FIG. 107  shows an example of a full color image  10700  that includes an array of pixels, including portions of red, green and blue pixels. For sequential color display, three subframe images are created that are each comprised of only one color, such as only red or only green or only blue. Those skilled in the art will recognize that sequential color images that together provide a perceived full color image can also be comprised of subframes of cyan, magenta and yellow. These subframe images are rapidly displayed in sequence to the user on the head-mounted display so that the user perceives a full color image that combines all three colors. With a reflective display such as an LCOS or a DLP, the subframe images are displayed by changing the reflective display to provide the respective image content associated with the particular subframe image and then illuminating the reflective display with the associated color light, so the light is reflected to provide the subframe image to the optics of the head-mounted display and from there to the user&#39;s eye. 
     If the subframe images are accurately aligned with each other, then the full color image perceived by the user will be full color out to the edges of the image and there will be no color breakup. This is what is typically seen by the user of a head-mounted display when the head-mounted display is stationary on the user&#39;s head and the user is not moving his eyes. However, if the user moves his head or the head-mounted display moves on the user&#39;s head (such as due to vibration) and the user&#39;s eyes are not moved in unison with the displayed image, the user will perceive the subframe images to be laterally (or vertically) offset relative to one another as shown by illustrations  10802  and  10804  in  FIGS. 108A and 108B . The perceived amount of lateral offset between the displayed subframe images is related to the speed of movement of the head-mounted display and the time between the display of the sequential subframe images, which is also known as subframe time or 1/subframe rate. The lateral shifting between subframe images, that is perceived by the user, is the color breakup and color breakup is perceived as fringes of color at the edges of objects. When the user moves his head (or eyes) quickly and the subframe rate is slow, color breakup can be substantial as illustrated in  FIG. 108A . If the user moves his head slowly or the subframe rate is higher, the color breakup is less as illustrated in  FIG. 108B . If the color breakup is less than one pixel, in digital imaging, in lateral shifting, the user will perceive there to be no color breakup. 
     Display frame rate in a head-mounted display is typically limited by either the bandwidth of the processor and associated electronics or by the power required to drive the processor and associated electronics, which translates into battery life. The bandwidth required to display images at a given frame rate is related to the number of frames displayed in a period of time and the number of pixels in each frame image. As such, simply increasing the frame rate to reduce color breakup is not always a good solution as it requires a higher bandwidth which the processor or associated electronics may not be able to support and power usage will be increased thereby reducing battery life. Instead, systems and methods in accordance with the principles of the present invention provide a method of display wherein the number of pixels in each subframe image is reduced thereby reducing the bandwidth required to display each subframe image while simultaneously increasing the subframe rate by a corresponding amount to maintain bandwidth while reducing color breakup. This embodiment is suitable for situations wherein subframe images can be provided with different numbers of pixels and different frame rates. For example, it would be suitable in camera and display systems where the capture conditions can be changed to provide images with a lower resolution that can then be displayed with a faster subframe rate. Static images such as text or illustrations can be displayed with a lower frame rate and a faster subframe rate to reduce color breakup since the image content doesn&#39;t change quickly. Alternatively, images can be modified to be displayed at lower resolution (fewer pixels) with a faster frame rate or subframe rate to reduce color breakup 
       FIG. 109  shows an illustration of the timing of a sequential color image comprised of sequential display of a red subframe image  10902  followed by a green subframe image  10904  followed by a blue subframe image  10908  in a repeating process. As long as the subframes together are displayed at a full color image frame rate that is greater than approximately 24 frames/sec, such that the sequential color subframes are displayed at a subframe rate of greater than 72 subframes/sec, the human eye will perceive full color moving images without flicker. This condition is suitable for displaying a video image without color breakup when the head-mounted display is stationary or moving relatively slowly. However, if the user moves his head such that the head-mounted display moves rapidly, color breakup will occur. This color breakup occurs because rapid head movements are typically a reaction of the user to something occurring in the environment (e.g. a loud noise) so that the user&#39;s eyes are searching the environment during the rapid head movement, which leads to jerky eye movements and substantial color breakup. 
     Movement of the head-mounted display can be detected by an inertial measurement unit, which can include accelerometers, gyro sensors, magnetometers, tilt sensors, vibration sensors, etc. Where only the movements within the plane of the display field of view (e.g. x and y movements and not z movement) are important for detecting conditions where color breakup may occur. If the head-mounted display is detected to be moving above a predetermined threshold where color breakup is predicted to occur (e.g. greater than 9 degrees/sec), in embodiments, the resolution of the images may be reduced (thereby reducing the number of pixels in the images and effectively making each pixel larger within the display field of view) and the subframe rate may be correspondingly increased. Note that the subframe rate can be increased without changing the image frame rate by increasing the number of subframes that are displayed sequentially, for example six subframes could be displayed for each image frame wherein the sequential color subframe images are each displayed twice. By increasing the number of subframes displayed for each image frame, the subframe rate can be increased without having to increase the image frame rate, which can be more difficult to change because the image frame rate is typically provided by the source of the image content such as in a movie.  FIG. 110  shows an illustration of a faster subframe rate, wherein the display time for each subframe,  11002 ,  11004 , and  11008  is reduced and the time between display of each sequential subframe is also reduced.  FIG. 110  shows a subframe rate that is approximately twice as fast as that shown in  FIG. 109 . The associated image frame rate can be twice as fast in  FIG. 110  as compared to  FIG. 109 , where both the image frame rate and the subframe rate are doubled. Alternatively, as previously described, the image frame rate can be unchanged between  FIGS. 109 and 110 , where only the subframe rate is doubled to reduce color breakup. To enable the bandwidth associated with the display of the images shown in  FIG. 110  to be approximately the same as the bandwidth associated with the display of subframe images shown in  FIG. 109 , the resolution (number of pixels in each subframe image) is reduced by approximately a factor of two. 
     While reducing the resolution of the displayed subframe images in correspondence to an increase in the subframe rate may seem to degrade the image quality perceived by the user, the human eye is not capable of perceiving high resolution when there is substantial movement. As such, color breakup is more visible than a reduction in the resolution of the image when the eye is moving. Consequently, the systems and methods of the present invention trade reduced image resolution for increased image frame rate to reduce color breakup without a perceptible loss in resolution, and bandwidth is thereby maintained. This technique can be used, for example, to reduce color breakup by a factor of up to 16, where the resolution of the displayed image is reduced to 1/16th the original resolution and the frame rate of the displayed image is increased by 16×. 
     In another embodiment of the invention, when movement of the head-mounted display is detected, the subframe images associated with a full color frame image are digitally shifted relative to one another in a direction counter to the detected direction of movement and with an amount that corresponds to the detected speed of movement. This effectively compensates for the perceived offset between the displayed subframe images that causes color breakup. The digital shifting is applied only to the subframes that together comprise a full color frame image. This is different from typical digital image stabilization wherein full color frame images are digitally shifted relative to one another to compensate for movement as described, for example, in United States patent publication 2008/0165280. By applying the digital shifting to the subframes that constitute a single full color frame image, the amount of digital shifting required to reduce color breakup is typically only a few pixels even when the detected movement speed is high, this is in contrast to typical digital image stabilization where fast movements result in accumulating shifts of the frame image so that the image effectively moves outside of the display field of view or the amount of digital stabilization that can be applied is limited.  FIGS. 111 a  and 111 b    illustrate this embodiment.  FIG. 111 a    shows how sequentially displayed subframe images,  11102 ,  11104 , and  11108  would be perceived by the user when there is substantial movement, wherein the different colors associated with the subframes are separately visible along the edges of objects, evenly spaced across the field of view in the direction of movement. In contrast,  FIG. 111 b    shows how the visibility of the subframes is changed when the subframes are digitally shifted to compensate for the detected movement and thereby reduce the separation between the subframes across the field of view, and as a result the user perceives a series of full color frame images  11120  with reduced color breakup. As shown in  FIG. 111 b   , the full color frame images are not image stabilized or digitally shifted in response to the detected movement. 
     In embodiments, movement direction and speed of the head-mounted display is detected by the IMU sensor immediately prior to the display of each full color frame image. If the movement speed is above a predetermined threshold, the sequentially displayed color subframes associated with each full color frame are digitally shifted relative to one another so that they are displayed in an aligned position within the display field of view. The magnitude of the shift corresponds to the speed of the detected movement and the direction of the shift is counter to the detected direction of movement. 
     In an example, the movement of the head-mounted display is detected immediately prior to display of a first subframe associated with a full color frame image. The first subframe associated with the full color frame image can then be displayed without a shift. The second subframe can be shifted by an amount and direction that compensates for the movement that occurs between the display of the first and second subframes and then is displayed. The third subframe can be shifted by an amount and direction that compensates for the movement that occurs between the display of the first subframe and the third subframe and is then displayed. The movement of the head-mounted display is then detected again to determine the shifts to be applied to the subframes associated with the next full color frame image. Alternatively, the subframes can be shifted by an amount that compensates for a portion of the movement that occurs between the subframes. 
     In a further example, the direction and speed of movement of the head-mounted display is detected immediately prior to the display of a reference subframe. Subsequent subframes are then shifted to compensate for movement that occurs between the time the reference subframe is displayed and the time that the subsequent subframe is displayed. Wherein the time that the reference subframe is displayed and the time that the subsequent subframe is displayed may be up to 5 frame times. 
     An advantage of this embodiment is illustrated by examining the effective frame rates associated with the color breakup and the blur of the image. If the full color image is displayed with an image frame rate of 60 frames/sec, the subframes would typically be displayed at a subframe rate of 180 frames/sec to provide three subframes for each image frame. The described system and method effectively shifts the subframes so that they are positioned on top of one another, so the color breakup is reduced to an amount that corresponds to 180 frames/sec. At the same time, the blur perceived by the user between image frames corresponds to 60 frames/sec since each of the subframes is derived from the same full color frame image. 
     In further embodiments, the digital shifting of the subframes that is based on detected movement immediately prior to the display of each full color frame image can be combined with digital image stabilization that is applied between the full color frame images. 
     In yet further embodiments, the method of digital shifting of subframes is combined with the method of increasing frame rate with a simultaneous reduction in image resolution. These two methods of reducing color breakup operate on different aspects of the image processing associated with displaying an image in a head mounted display, as such they can be independently applied in either order in the image processing system associated with the processor. 
     In yet another embodiment, the head mounted display includes a camera for detecting the eye movements of the user (e.g. as described herein) relative to the movement of the head mounted display. The eye camera can be used to measure the speed of eye movement and the direction of eye movement. In embodiments, the resolution of eye cameras can be relatively low (e.g. QVGA or VGA) so that the frame rate can be relatively high (e.g. 120 frames/sec) without introducing bandwidth limitations. The detected eye movements relative to the head-mounted display can be used to determine when to apply methods to reduce color breakup including, for example, increasing the frame rate and digitally shifting the subframes as has been previously described herein. For example, if the detected eye movement is above a predetermined angular speed, the resolution of the displayed images can be reduced and the subframe rate can be increased. In another example, the detected eye movement can be used to determine the amount and direction of digital shifting applied to subframes within an image frame prior to display of the subframes. In yet another example, measured eye movements can be used in combination with detected movements of the head-mounted display to determine the amount and direction of digital shifting applied to subframes within an image frame prior to display of the subframes. The amount and direction of digital shifting applied to the subframes can be in correspondence to the difference between the detected movements of the head mounted display and the detected eye movements of the user. Where the detection of a condition where the user&#39;s eye is moving one direction and the head mounted display is moving in an opposing direction represents a situation where particularly bad color breakup can occur. In this case, combined methods for reducing color breakup are advantageous. 
     In another yet further embodiment, when movement of the head-mounted display or eye movement is detected above a predetermined threshold, the images are changed from color sequentially displayed full color images to monochrome images. The monochrome images can be comprised of combined image content from each of the color sequential subframes associated with each full color image frame. Where the monochrome images can be grey scale or luma images wherein the luma code values (Y) for each pixel can be calculated for example as given in Equation 1 below as taken from http://en.wikipedia.org/wiki/Grayscale and as referenced to the CIE 1931 standard for digital photography: 
         Y= 0.2126 R+ 0.7152 G+ 0.0722 B   Equation 1
 
     where R is the red code value for the pixel, G is the green code value for the pixel and B is the blue code value for the pixel. Alternatively, monochrome images can be comprised of single color images such as the green subframe image, and this image can be displayed either with a single color or preferably with simultaneous application of all the sequential colors (e.g. red, green and blue) so that the applied illumination onto the reflective image source is white light and as a result, the displayed image appears as a grey scale image. 
     Several more specific examples are provided below. 
     Example 1 
     For a 26 deg display field of view and a 1280 pixel horizontally wide image, a pixel occupies 0.020 deg within the display field of view. If the frame rate of the full color images is 60 Hz, with three color sequential subframes images, the subframe time is 0.006 sec. The rotational speed of the head mounted display needed to produce one pixel of color breakup is then 3.6 deg/sec. If the number of horizontal pixels in the display field of view is reduced to 640 pixels and simultaneously the frame rate of the full color images is increased to 120 Hz, with three color sequential subframes images, the subframe time is reduced to 0.003, the size of a pixel is increased to 0.041 deg and the rotational speed to produce one pixel of color breakup is 14.6 deg/sec. 
     Example 2 
     For a 26 deg display field of view and a 1280 pixel horizontally wide image, a pixel is 0.020 deg within the display field of view. If the smallest size that the user can detect for color breakup is one pixel wide, then a rotational speed of over 3.6 deg/sec is required if the subframe rate is 180 Hz, before color breakup is detected by the user. Even though the color breakup is an analog effect, the user&#39;s eye does not have the resolution to detect the color fringes that are present during movement below this speed. So below this rotational speed, color breakup management is not required. 
     Example 3 
     For a 26 deg display field of view and a 1280 pixel horizontally wide image, a pixel is 0.020 deg within the display field of view. If the user can detect color breakup as small as one pixel wide, then a rotational speed of 3.6 deg/sec will require a shift of the subframes relative to each other of one pixel if the subframe rate is 180 Hz, to align the subframes so that color breakup is not visible to the user. If the user rotates their head at 15 deg/sec, then the subframes will require a shift of 4 pixels relative to one another to align the subframes so that color breakup is not visible. If the image frame begins with the display of the red subframe image, then no digital shifting is required for the red subframe image. A 4 pixel shift is required for the green subframe image. And, an 8 pixel shift is required for the blue subframe image. The next red subframe associated with the next image frame would then be effectively shifted 12 pixels relative to the previous red subframe within the field of view. 
     Each of the color breakup reduction technologies described herein may be used in combination with each of the other color breakup reduction technologies. 
     The inventors appreciated that fitting see-through computer displays into certain head-worn form factors is a challenge, even when reduced in size as described herein. A further advantage that is provided by an optics module that includes multiply folded optics is that twists can be introduced at the fold surfaces to modify the orientation of different portions of the optics module relative to each other. This can be important when the optics module needs to fit into a thin curved glasses frame, a visor or a helmet where the increased width associated with the upper portion of the multiply folded optics module can make it more difficult to fit into structures that are not parallel to the combiner. As such, another aspect of the present invention relates to twisting certain optical components within the see-through computer display such that the optical components better fit certain form factors (e.g. glasses) yet continue to perform as high quality image displays. In embodiments, optics systems with dual mirror systems to fold the optical path (e.g. optical systems described herein with respect to  FIGS. 6, and 93 through 106 ) are provided such that the image production module (e.g. upper module), which includes a first image light reflective surface, is turned about a first optical axis leading from the upper module to the lower module and in a direction to fit the upper module more compactly into a frame of a head-worn computer. At the same time, to avoid distorting the image provided to the eye of the user, the image delivery optics (e.g. lower module), which includes a second image light reflective surface, is turned about a second optical axis that leads to the user&#39;s eye and in the opposite direction relative to the image, thereby introducing a compound angle between the first image light reflective surface and the second image light reflective surface. Provided that the first and second optical axes are perpendicular to one another in the non-twisted state, the distortion in the image associated with the twist about the first axis is compensated by a twist of the same angular magnitude about the second axis so that the image presented to the eye of the user is undistorted by the twisting. 
       FIG. 112  illustrates a head-worn computer with see-through displays in accordance with the principles of the present invention. The head-worn computer has a frame  11202  that houses/holds the optics modules in position in front of the users eyes. As illustrated in  FIG. 112 , the frame  11202  holds two sets of optical modules  11204  and  11208  each of which have upper and lower optics modules. Optics module  11204  is non-twisted and is presented to illustrate the difficulty in fitting the non-twisted version into the frame. One will note that the dotted box, which represents the outer bounds of the optics module  11204  doesn&#39;t fit within the bounds of the frame  11202 . Fitting optics module  11204  into the frame  11202  would normally require that the frame  11202  become thicker, from front to back, which would lead to more offset of the glasses form factor from the face of the user, which is less desirable and is less compact. In contrast, optics module  11208  is a twisted optics module, where the upper module is twisted (or rotated) to better fit into the confines of the frame  11202  as shown in  FIG. 112 .  FIG. 113  shows a more detailed illustration of the twists imparted within multiply folded optics in optics module  11208 . Upper module  11214  is twisted relative to the lower module  11218  along optical axis  934  to better fit into the frame  11202 , It is this twist which enables optics module  11208  to better fit within the frame  11202  as shown in  FIG. 112  and as a result frame  11202  can be thinner and more compact than if non-twisted optics modules were used. To avoid distorting the image provided to the user, a second twist is required to introduce a compound angle between the first reflecting surface  11225  in the upper module  11214  and second reflecting surface  11226  in the lower optics module  11218 . The second twist is imparted to the second reflecting surface about the optical axis  933  and in an opposite direction relative to the image from the twist in the upper module  11214 . In this way, the effects of the increased width of the upper portion of the multiply folded optics can be reduced when fitting the optics module into a curved structure such as glasses frames, a visor frame or a helmet structure. Where it is preferred, but not required that the optical axis  934  be perpendicular to the optical axis  933  so that the magnitude of the angular twist imparted to the first reflecting surface  11225  can be the same as the twist imparted to the second reflecting surface  11226  to provide an image to the user&#39;s eye that is not distorted due to the twisting. 
     Another aspect of the present invention relates to the configuration of the optics and electronics in a head-worn frame such that the frame maintains a minimal form factor to resemble standard glasses. In embodiments, a see through optical display with multiply folded optics to provide a reduced thickness (e.g. as described herein) may be mounted in the frame. In embodiments, the multiply folded optical configuration may be twisted at the fold surfaces (e.g. as described herein) to better fit the optics into the frame. In embodiments, the electronics that operate the displays, processor, memory, sensors, etc. are positioned between, above, below, on a side, etc. of the optical modules and oriented to provide a reduced thickness in the frame to match the thickness of the optics. Orienting the board can be particularly important when the board includes large components that limit the width of the board, such as for example the processor chip. For example, an electronics board or components on the electronics board may be mounted in a vertical orientation between and/or above the optical modules to reduce the thickness of the electronics board as mounted into the frame. In another configuration the board may be mounted between the optical modules at a height near the top of the optical modules to minimize the height of the glasses frame. In yet another configuration the board may be mounted such that it extends over the optical modules to minimize the thickness of the frame. In further embodiments, the board may be mounted in an angled configuration to enable the thickness and height of the frame to be reduced simultaneously. In embodiments, the electronics may be divided between multiple boards. For example, a longer board over a shorter board where the space between the optical modules is used for the lower board. This configuration uses some of the space between the eyes for some of the electronics. 
       FIG. 114  illustrates a top view and front view of a configuration including optical modules  11208 , electronics board  11402  and a heat sink  11404 . The board  11402  is mounted in a vertical orientation to maintain a thin frame portion that sits across the user&#39;s brow. As illustrated, the optical modules  11208  include upper modules  11214  and a second reflecting surface  11226  in front of the user&#39;s eye. The upper module may have a flat reflecting surface and the upper  11214  may be turned or twisted with respect to the second reflecting surface  11226  as described herein. The second reflecting surface  11226  may be a partial mirror, notch filter, holographic filter, etc. to reflect at least a portion of the image light to the eye of the user while allowing scene light to transmit through to the eye. 
       FIG. 115  illustrates a front view of a configuration that includes optics illustrated in  FIG. 114 ; however, the electronics board  11402  is mounted in the space between the optical modules at a height that is similar to the height of the optical modules. This configuration reduces the overall height of the frame. 
       FIG. 116  illustrates a front view of a configuration that includes optics illustrated in  FIGS. 114 and 115 . The electronics layout in this configuration is done with multiple boards,  11402 ,  11602  and  11604 . The multiple board configuration allows the boards to be thinner from front to back thereby enabling the brow section of the frame to be thinner. A heat sink  11404  (not shown in  FIG. 116 ) may be mounted on the front face between the optical modules. This configuration also causes the heat to be drawn in a direction away from the user&#39;s head. In embodiments, the processor, which is a main heat generator in the electronics, is mounted vertically (e.g. on board  11604 ) and the heat sink  11404  may be mounted in front such that it contacts the processor. In this configuration, the heat sink  11404  causes heat to spread to the front of the device, away from the user&#39;s head. In other embodiments, the processor is mounted horizontally (e.g. on board  11602  or  11402 ). In embodiments, the board(s) may be tilted (e.g. 20 degrees) from front to back to create an even thinner brow section. 
     Another aspect of the present invention relates to concealing the optical modules such that a person viewing the user does not clearly see the optical modules, electronics or boards. For example, in configurations described herein, the optical modules include lenses that hang below the top of the brow section of the head-worn device frame and the electronics board(s) hang down as well so that the see-through view is partially blocked. To conceal these features and thereby provide the head worn computer with the appearance of conventional glasses, an outer lens may be included in the glasses frame so that it covers a portion of the frame that contain the optical modules or electronics, and the outer lens may include a progressive tint from top to bottom. In embodiments, the tint may have less transmission at the top for concealment of a portion of the frame that includes the optical modules or electronics board while having higher transmission below the concealment point such that a high see-through transmission is maintained. 
     Aspects of the present invention provide multiply folded optics to reduce the thickness of the optics modules along with vertically oriented or angled electronics to reduce the mounted thickness of the electronics and progressively tinted outer lenses to conceal a portion of the optics or electronics. In this way, a head worn computer is provided with a thinner form factor and an appearance of conventional glasses. 
     Another aspect of the present invention relates to an intuitive user interface mounted on the HWC  102  where the user interface includes tactile feedback to the user to provide the user an indication of engagement and change. In embodiments, the user interface is a rotating element on a temple section of a glasses form factor of the HWC  102 . The rotating element may include segments such that it positively engages at certain predetermined angles. This facilitates a tactile feedback to the user. As the user turns the rotating element it ‘clicks’ through it&#39;s predetermined steps or angles and each step causes a displayed user interface content to be changed. For example, the user may cycle through a set of menu items or selectable applications. In embodiments, the rotating element also includes a selection element, such as a pressure-induced section where the user can push to make a selection. 
       FIG. 117  illustrates a human head wearing a head-worn computer in a glasses form factor. The glasses have a temple section  11702  and a rotating user interface element  11704 . The user can rotate the rotating element  11704  to cycle through options presented as content in the see-through display of the glasses.  FIG. 118  illustrates several examples of different rotating user interface elements  11704   a ,  11704   b  and  11704   c . Rotating element  11704   a  is mounted at the front end of the temple and has significant side and top exposure for user interaction. Rotating element  11704   b  is mounted further back and also has significant exposure (e.g. 270 degrees of touch). Rotating element  11704   c  has less exposure and is exposed for interaction on the top of the temple. Other embodiments may have a side or bottom exposure. 
     As discussed above, a specially designed lens may be used to conceal portions of the optics modules and/or electronics modules.  FIG. 119  illustrates an embodiment of one such lens  11902 . Two lenses,  11902  are illustrated with Base 6 and 1.3 mm thickness but other geometries with, for example, different curvatures and thicknesses can be used. The lenses  11902  are shaped to look like conventional glasses lenses with features including magnetic mounting attachment and special tinting in portions of the lenses  11902  where opaque structures such as electronics are located behind the lenses. 
     The lenses  11902  includes blind holes  11904  for the mounting of a magnetic attachment system (not shown). The magnetic attachment system may include magnets, magnetic material, dual magnets, opposite polarization magnets, etc. such that the lenses  11902  can be removed and remounted to the head-worn computer (e.g. HWC  102 ). In the magnetic attachment system, the lenses  11902  are held by magnetic force into the frame of the HWC. The magnets can be inserted into the blind holes  11904  or inserted into the frame of the HWC in corresponding matching positions. As long as either the lens  11902  or the matching position on the frame of the HWC includes a magnet and the other position has a similar sized piece of magnetic material or another magnet oriented to attract the lens  11902  and hold it in the frame of the HWC. To this end, the frame of the HWC can provide guidance features to position the lens  11902  in front of the optics modules in the HWC. Where the guidance features can be a ridge or flange that the lens is seated in so the lens  11902  cannot move laterally when held in place by the magnetic attachment system. In this way, the function of the magnetic attachment system is simply to hold the lenses  11902  in place, while the guidance features position the lenses  11902 . The guidance features can be robustly made to hold the lenses  11902  in place when dropped or subjected to impact even when the force provided by the magnetic attachment system is relatively low, so that the lenses  11902  can be easily removed by the user for cleaning or replacement. Where easy replacement enables a variety of lenses with different optical features (e.g. polarized, photochromic, different optical density) or different appearance (e.g. colors, level of tinting, mirror coating) to be changed out by the user as desired. 
       FIG. 119  also illustrates an example of how the lens  11902  may be tinted to conceal or at least partially conceal certain optical components (e.g. the non-see-through components or opaque components) such as, electronics, electronics boards, auxiliary sensors such as an infrared camera and/or other components. As illustrated, the blind holes  11904  may also be concealed or at least partially concealed by the tinting. As illustrated in  FIG. 119 , a top portion  11908 , approximately 15 mm as illustrated, may be more heavily tinted (e.g. 0 to 30% transmission) or mirrored to better conceal the non-see through portions of the optics and other components. Below the top portion  11908 , the lens  11902  may have a gradient zone  11909  where the tinting level gradually changes from top to bottom and leads into the lower zone  11910 . The lower zone  11910  includes the area where the user primarily views the see-through surrounding and this zone may be tinted to suit the viewing application. For example, if the application requires a high see through, the lower zone  11910  may be tinted, between 90% and 100% transmissive. If the application requires some see-through tint, than the lower area may be more heavily tinted or mirrored (e.g. 20% to 90%). In embodiments, the lower area  11910  may be a photochromic layer, an electrochromic layer, a controllable mirror or other variable transmission layer. In embodiments, the entire lens or portions thereof may have a variable transmission layer such as a photochromic layer, electrochromic layer, controllable mirror, etc. In embodiments, any of the areas or whole lens  11902  may include polarization. 
     Another aspect of the present invention relates to cooling the internal component through the use of micro-holes sized such they are large enough to allow gas to escape but small enough to not allow water to pass through (e.g. 25 μm, 0.2 mm, 0.3 mm, etc.). The micro-holes may be included in a heat sink, for example. The heat sink, or other area, may be populated with hundreds or thousands of such micro-holes. The micro-holes may be laser cut or CNC holes, for example, that are small enough to keep large droplets of water out of the device but allow air to exchange through the heat sink. Besides increasing surface area of the heat sink, they also have matching holes on the underside of the frame to enable convective cooling where cool air is pulled in from the bottom as the heat raises from the top, like a chimney and as such, the heat sink with the micro-holes is preferably located on the top or side of the frame of the HWC. In embodiments, the micro-holes are aligned in the troughs formed by the fins on the top of the heat sink. This causes the exiting air to flow through the troughs thereby increasing the heat transfer from the fins. In embodiments, the micro-holes may be angled such that the length of the hole in the heat sink material is increased and the air flow can be directed away from the head of the user. In addition, the micro-holes may be of a size to cause turbulence in the air flow as it passes through the micro-holes. Where, turbulence substantially increases the heat transfer rate associated with the air flow through the heat sink. In embodiments, the heat management system of the HWC  102  is passive, including no active cooling systems such as fans or other energized mechanical cooling systems to force air flow through the micro-holes. In other embodiments, the heat management system includes energized mechanical cooling, such as a fan or multiple fans or other systems to force air movement through the HWC and the micro-holes. 
     Another aspect of the present invention relates to finding items in the surrounding environment based on similarity to items identified. Augmented reality is often rigidly defined in terms of what is included and how it is used, it would be advantageous to provide a more flexible interface so people can use augmented reality to do whatever they want it to do. An example is to use the HWC camera, image analysis and display to designate items to be found.  FIG. 122  shows an illustration of an image  12210  of a scene containing an object that the user would like the HWC to assist in looking for the object as the user moves through the environment. In this example, the user has circled the object  12220  that is being looked for, where in this case the object is a cat. The HWC then analyses the circled region of the image for shapes, patterns and colors to identify the target to be searched for. The HWC then uses the camera to capture images of the scene as the user moves about. The HWC analyses the images and compares the shapes, patterns and colors in the captured images of the scene and compares them to the shapes, patterns and colors of the target. When there is a match, the HWC alerts the user to a potential find. The alert can be a vibration, a sound or a visual cue in an image displayed in the HWC such as a pointer, a flash or a circle that corresponds to the location of the potential find in the scene. This method provides a versatile and flexible augmented reality system wherein an item is described visually and a command of “find something like this” is given to the HWC. Examples of ways to identify an object to be searched for include: circle an item in a previously captured image that is stored on the HWC (as shown in  FIG. 122 ); point to an item in a physical image held in front of the camera in the HWC; point to an item in the live image provided by the camera in the HWC and viewed in the see-through display of the HWC, etc. Alternately, text can be input to the HWC with a command of “find wording like this”, e.g. a street sign or an item in a store and the HWC can then search for the text as the user moves through the environment. In another example, the user can indicate a color with a command of “find a color like this”. The camera used to search for the item can even be a hyperspectral camera in HWC to search for the item using infrared or ultraviolet light to thereby augment the visual search that the user is conducting. This method can be extended to any pattern that the user can identify for the HWC such as sounds, vibrations, movements, etc. and the HWC can then use any of the sensors included in the HWC to search for the identified pattern as the target. As such the finding system provided by the invention is very flexible and can react to any pattern that can be identified by the sensors in the HWC, all the user has to do is provide an example of the pattern to look for as a target. In this way the finding system assists the user and the user can do other things while the HWC looks for the target. The finding system can be provided as an operating mode in the HWC where the user selects the mode and then inputs the pattern to be used as the search target by the HWC. Examples of items that can be searched for include: household objects, animals, plants, street signs, weather activity (e.g. cloud formations), people, voices, songs, bird calls, specific sounds, spoken words, temperatures, wind direction shifts as identified by wind sound relative to the compass heading, vibrations, objects to be purchased, brand names in stores, labels on items in a warehouse, bar codes or numbers on objects and colors of objects to be matched. In a further embodiment, the rate of searching (e.g. how often an analysis is conducted) can be selected by the user or the rate can be automatically selected by the HWC in response to the rate of change of the conditions related to the target. In a yet further embodiment, the sensors in the HWC include a rangefinder or a camera capable of generating a depth map to measure the distance to an object in an image captured by the camera. The HWC can then analyze the image along with the distance to determine the size of the object. The user can then input the size of the object to the finding system as a characteristic of the target pattern to enable the HWC to more accurately identify potential finds. 
     Another aspect of the present invention relates to assisting a person in reading text that is presented in a physical form, such as a book, magazine, on a computer screen or phone screen, etc. In embodiments, the camera on the HWC can image the page and the processor in the HWC can recognize the words on the page. Lines, boxes, or other indicators may be presented in the HWC to indicate which words are being captured and recognized. The user would then be viewing the page of words through the see-through display with an indication of which words have been recognized. The recognized words can then be translated or converted from text that is then presented to the user in the see-through display. Alternately, the recognized words can be converted from text to speech, which is then presented to the user through the head worn speakers, headphones, visual displays, etc. This gives the user a better understanding of the accuracy associated with the text recognition relative to the translated text or converted speech. 
     In a further aspect of the invention, a magnetic attachment structure is provided for the combiner to enable the combiner to be removable. In the optics associated with a HWC  102  such as for example the optics shown in  FIG. 6 , it is important that the combiner  602  be accurately positioned and rigidly held below the frame of the HWC and the upper optical module  202  located inside the frame. At the same time, the combiner  602  can become damaged so that it needs to be replaced, or it may need to be cleaned periodically so that it is advantageous for the combiner to be removeable.  FIG. 123  shows an illustration of a cross section of a single combiner  12360  with the magnetic attachment structure as shown from the side to show the angle of the combiner  12360 .  FIG. 124  shows an illustration of two combiners  12360  with magnetic attachment structures attaching the combiners  12360  to the frame of the HWC  12350  as shown from the front of the HWC. The combiner  12360  has two or more pins  12365  that are attached to the combiner  12360  such that the pins have parallel axes. The pins  12365  are shown as being inserted into holes drilled through the combiner  12365  and attached in place with adhesive such as UV cured adhesive. The pins  12365  are made of a magnetic material such as for example 420 stainless steel. The pins  12365  extend into parallel bores in the frame of the HWC  12350  so that the combiner  12360  is fixedly held in place relative to the frame of the HWC  12350 . The attachment and bend of the pins  12365  establish the angle between the combiner  12360  and the optics in the frame of the HWC  12350 . A magnet  12370  is bonded into the frame of the HWC  12350  such that the pin  12365  attracted by the magnet  12370  and thereby the pin  12365  and the attached combiner  12360  are held in place relative to the frame of the HWC  12350 . The magnet  12370  is selected so that the force exerted by the magnet  12370  onto the pin  12365  is strong enough to hold the combiner  12360  in place during normal use, but weak enough that removal of the combiner  12350  is possible by the user. By having the pins  12365  and associated bores parallel, the combiner  12350  can be easily removed for cleaning, or replaced if damaged. To provide a more rigid and repeatable connection between the combiner  12360  and the frame of the HWC  12350 , the pins can fit into an extended tight bore in the frame of the HWC  12350 . In addition, the pins  12365  can include a flange as shown that seats onto an associated flat surface of the frame of the frame  12350  or a flat surface of the magnet  12370  to further establish the angle of the combiner  12360  and the vertical position of the combiner  12360 . In a preferred embodiment, the magnet  12370  is a ring magnet and the pin  12365  extends through the center of the ring magnet. The magnet  12370  can also be included in an insert (not shown) that further includes a precision bore to precisely align and guide the pin  12365 . The insert can be made of a hardened material such as a ceramic to provide a bore for the pin  12365  that is resistant to wear during repeated removal and reinstallation of the combiner  12360 . The pins can be accurately positioned within the combiner through the use of a jig that holds the pins and the combiner. The holes for the pins in the combiner are then made larger than the pins so there is a clearance to allow the combiner and pins to be fully positioned by the jig. An adhesive such as a UV curing adhesive is then introduced to the holes and cured in place to fasten the pins to the combiner in a position that is established by the jig. In a further embodiment, the combined structure of the pins  12365  and the combiner  12350  are designed to break if subjected to a high impact force, to thereby protect the user from injury. Where the pin  12365  or the combiner are designed to break at a previously selected impact force that is less than the impact force required to break the frame of the HWC  12350  so that the combiner  12350  with the attached pins  12365  can be simply replaced when damaged. In yet a further embodiment, by providing a method for easily replacing the combiners  12360 , different types of combiners can also be provided to the user such as: polarized combiners, combiners with different tints, combiners with different spectral properties, combiners with different levels of physical properties, combiners with different shapes or sizes, combiners that are partial mirrors or combiners that are notch mirrors, combiners with features to block faceglow as previously described herein. 
     In typical computer display systems, automatic brightness control is a one dimensional control parameter; when the ambient brightness is high, the display brightness or light source is increased, when the ambient brightness is low, the display brightness or light source is decreased. The inventors have discovered that this one-dimensional paradigm has significant limitations when using see-through computer displays. Aspects of the present invention relate to improving the performance of the head-worn computer by causing it to understand the relative brightness of the content to be presented in addition to understanding the brightness of the surrounding environment and to then adjust the brightness of the content, based on both factors, to create a viewing experience that has the appropriate viewability. 
     An aspect of the present invention relates to improving the viewability of content displayed in a see-through head-worn display. Viewability involves a number of factors. The inventor&#39;s have discovered that, in addition to image resolution, contrast, sharpness, etc., the viewability of an image presented in a see-through display is effected by (1) the surrounding scene that forms the backdrop for the image, and (2) the relative or apparent brightness of the image displayed. If the user, for example, is looking towards a bright scene, the viewability of the presented content may be washed out our or hard to see if the display settings are not altered and, in the event that the content itself is relatively low in brightness (e.g. the content has a lot of dark colors or black areas in it), it may continue to be washed out unless the content is also altered. In this situation, the brightness of the display may be increased even higher than what would normally be required in a dark environment in order to compensate for the dark content of the image. As an additional example, if the user is looking towards a dark scene, the presented content may be perceived by the user as overly bright and washing out the scene, or making it hard to interact with the scene if the display settings are not altered. In addition, if the content itself is relatively bright (e.g. mainly light colors or areas of white content), the content may require further alteration to obtain the proper viewability. In this situation, the display brightness may be decreased further than if it were only dependent on the environmental lighting conditions to make the viewablilty of the content appropriate. In embodiments, the head-worn computer is adapted to measure the scene that forms the backdrop for the presented content, understand the relative brightness of the content itself (i.e. the innate content brightness) to be presented and then adjust the presentation of the content based on the scene brightness and the innate content brightness to achieve a desired content viewability. 
     While embodiments herein use the terms “content brightness” and “display brightness” in the context of altering the viewability of the content, it should be understood that the step of making the alteration in content and/or display in response to meeting a viewability need may include causing the system to leave the image content alone and increase the light source brightness of the display, use the available light and increase the digital brightness of the image content by adjusting the parameters of the entire display using the display driver, adjust the actual content that is being displayed, etc. The viewability adjustment may be made by adjusting a lighting system used to illuminate a reflective display (e.g. changing the pulse width modulation duty cycle of the LEDs, changing the power delivered to the lighting system, etc.), changing the brightness settings of an emissive display, changing an aspect of how the display presents all content by adjusting settings in the display driver or changing an aspect of the content its self through image processing (e.g. changing brightness, hue, saturation, color value (e.g. red, green, blue, cyan, yellow, magenta, etc.) exposure, contrast, saturation, tint, etc.), of the all the content, select regions of the content, types of content which may be shown at the same time but have innate differences in visibility regardless of location, etc. 
     To improve the viewing experience for a user when viewing content in a see-through head-worn display, the visual interaction between the displayed image and the see-through view of the environment must be considered. The viewability of a given displayed image is highly dependent on a variety of attributes such as its size, color, contrast and brightness as well as the perceived brightness as seen by the user. Where the color and brightness of the displayed image can be determined by the pixel code values within the digital image (e.g. average pixel code). Alternatively, the brightness of the displayed image can be determined from the luma of the displayed image (see “Brightness Calculation in Digital Image Processing”, Sergey Bezryadin et. al., Technologies for Digital Fulfillment 2007, Las Vegas, Nev.). Other attributes of the displayed image can be calculated based the code value distributions in the image similar to the brightness. Depending on the mode of operation, the type of activity the user is engaged in and a perceived brightness of the image being displayed, it may be important for the displayed image to match the see-through view of the environment, contrast with the see-through view of the environment, or blend into the see-through view of the environment. The content adjustment may be based on the perceived user need in addition to the scene that will form the backdrop for the content. Embodiments provide methods and systems to automatically adjust viewability of the image depending on, for example: 
     1. the percent of the display field of view that is covered by displayed content, (where in a see-through head worn display the portions of the displayed image that are black are seen as portions with no displayed content and instead the user is provided with a see-through view of the environment in that portion); 
     2. a brightness metric of image being displayed (e.g. hue, saturation, color, individual color contribution (e.g. red content, blue content, green content) average brightness, highest brightness, lowest brightness, statistically calculated brightness (e.g. mean, median, mode, range, distribution concentration), etc.); 
     3. sensor feed back indicative of a user use scenario (e.g. the amount of motion measured by sensors in the IMU in the head-worn display used to determine that the user is stationary, walking, running, in a car, etc.); 
     4. the operating mode of the head-worn display (which can be selected by the user or automatically selected by the head-worn display based on for example: the environmental conditions, the GPS location, the time or date, indicated or determined user scenerion). 
     5. the type of content (e.g. still pictures (e.g. either high or low contrast, monochrome or color such as icons or markers), moving pictures (e.g. either high or low contrast, monochrome or color such as scrolling icons on our launcher or a bouncing marker), video content (e.g. where location and intensity of pixels are varying such as a bouncing and blinking marker, other normal types of video content like hollywood movies, step by step tutorials or your last run down the ski slope recorded on your glasses), text (e.g. small, large, monochrome, outlined, blinking, etc.), etc.; and/or 
     6. a user use scenario (e.g. a predicted scenario based on sensor feedback, based on an operating application, based on a user setting) such as sitting still in a safe location such as your living room and viewing a movie (e.g. where it might not need to defeat ambient), walking around and getting notifications or viewing turn by turn directions (e.g. where it might depend on the amount of display covered but probably best to match ambient), driving in a car and erasing the blind spots such as vertical pillars (e.g. where it may need to match ambient), driving in a car and trying to display HUD data over the external illumination (e.g. where it may need to defeat ambient), getting instructions on repairing and engine (e.g. where some areas need to defeat ambient such as pages in the service manual and some need to match such as augmented overlays where you still need to see what you&#39;re working on), etc. 
     For example, in a night vision mode using the camera with a live feed to the head-worn display, sensors associated with the head-worn display indicate that the user is moving at a speed and with an up and down movement that indicates jogging. As a result, the head-worn display can automatically determine that the displayed images should be provided with a brightness that provides good viewing without regard to the see-through view of the surrounding environment since it is too dark for the user to see a see-through view of the environment. In addition, the head-worn display may switch the displayed image from full color to a monochrome image such as green where the human eye is more sensitive and the human eye responds faster. 
     In another example of a mode, the brightness of the displayed image is increased relative to the see-through view of the surrounding environment when eye tracking is being used in a user interface. In this embodiment, the type of user interface being used determines the brightness of the displayed image relative to the brightness of the see-through view of the surrounding environment. In this way, the see-through view is made to be dimmer than the displayed image so that the see-through view is made less noticeable to the user. By making the see-through view less noticeable to the user, the user can more easily move his eyes to control the user interface without being distracted by the see-through view of the surrounding environment. This approach reduces the jittery eye movement that is typically encountered when using eye tracking in a head mounted display that also provides the user a see-through view of the environment.  FIG. 126  is a chart that shows the brightness (L*) perceived by the human eye relative to a measured brightness (luminance) of a scene. In this chart, it can be readily seen that the human eye has a non-linear response to luminance wherein the eye is more sensitive to differences at lower levels and less sensitive to differences at higher levels. In embodiments, the displayed image can be provided with an average brightness that is perceived as being 2× or more brighter than the average brightness of the see-thru view of the environment (i.e. L* of the displayed image is 2× the L* of the see-thru view) when using a mode that includes eye tracking control of a user interface. 
     Further, the displayed image can be changed in response to the average color, hue or spatial frequency of the environment surrounding the user. In this case, a camera in the head-worn display can be used to capture an image of the environment that includes a portion of the see-through field of view as seen by the user. Attributes of the captured image of the environment can then be digitally analyzed as previously described herein to calculate attributes for the displayed image. In this case, the attributes of the captured image of the environment can include an average brightness, a color distribution or spatial frequency of the see-through view of the environment. The calculated attribute of the environment can then be compared relative to attributes of the image being displayed to determine how distracting the see-through view will be versus the type of displayed image being displayed. The attributes of the displayed image can then be modified in terms of color, hue or spatial frequency to improve the viewability in the head-worn display with see-through. This comparison of image content versus see-through view and the associated modification of the displayed image can be applied within large blocks of the field of view or within small localized blocks of the field of view comprised of only a few pixels each such as may be required for some types of augmented reality objects. Wherein the captured image of the environment that is used to calculate the attributes of at least a portion of the see-through view of the environment provided to the user does not have to be the same resolution as the displayed image. In a further embodiment, a brightness sensor or a color sensor included in the head-worn display can be used to measure the average brightness or average color within a portion of the see-through field of view of the environment. By using a dedicated sensor for measuring brightness or color, the calculation of the attribute in the see-through view of the environment can be provided with little processing power thereby reducing the power required and increasing the speed of the calculation. 
     It has often been said that color is very subjective and there are several reasons for this including things like dependencies on ambient lighting of the environment, the proximity of other colors and whether you are using one eye or two. To compensate for these effects, the head-worn display may measure the color balance and intensity of the ambient light either with a light sensor or with a camera to infer how colors of objects in the environment will appear, then the color of the displayed image can be modified to improve viewability in the head-worn display with see-through. In the case of augmented reality objects, viewability can be improved by rendering the augmented reality object so that it better contrasts with the environment for example for a marker, or the so that it blends into the environment for example when viewing architectural models. To this end, light sensors can be provided to determine the brightness and color balance of the ambient lighting in front of the user or from other directions in the environment such as above the user. In addition, objects in the environment can be identified that typically have standard colors (e.g. stop signs are red) and these colors can be measured in a captured image to determine the ambient lighting color balance. 
     Color perception by the human eye gets even more complicated at the extremes of very bright and very dark, because the human eye responds non-linearly. For example in direct sunlight, colors begin to wash out as nerves in the brain begin to saturate and lose the ability to detect subtle differences in color. On the other hand, when the environment is dim, the contrast perceived by the human eye decreases. As such, when bright conditions are detected, colors can be enhanced in the displayed image. When dim conditions are detected, the contrast in the displayed image can be enhanced to provide a better viewing experience for the user. Where contrast can be enhanced by digitally sharpening the image, increasing the code value differences between adjacent areas in the digital image or by adding a narrow line comprised of a complimentary color around the edge of displayed objects. 
     In dim conditions, color sensitivity of the human eye varies by color as well, so that blue colors look brighter than red colors. As a result, in dim viewing conditions, the color of objects changes toward the blue. Consequently, when the displayed image is provided as a dim image such as for example when using the head-worn display in dim lighting where viewbility of both the displayed image and the see-through view are important, the color balance of the image can be shifted to be more red to provide a more accurate color rendition of the displayed image as perceived by the user. If the image is displayed as a very dim image, the image can be further changed to a monochrome red to better preserve the user&#39;s night vision. 
     In embodiments, the head-worn display uses sensors or a camera to determine the brightness of the surrounding environment. The type of image to be displayed is then determined and the brightness of the image is adjusted in correspondence with the type of image and the operating mode of the head-worn display. The combined brightness, comprised of the brightness of the see-through view in combination with the brightness of the displayed image, is determined. The operating zone of the human eye is then determined based on the combined brightness and the known sensitivity of the human eye as shown in  FIG. 125 . Attributes of the image (e.g. color balance, contrast, color of objects, size of text) are then adjusted to improve viewability in correspondence to the determined operating zone, the type of image and the operating mode. 
       FIG. 125  shows a chart of the sensitivity of the human eye versus brightness as provided in Chapter 2.1 page 38 in the book by Gonzalez, R. C. and Woods, R. E., “Digital Image Processing Second Edition”, copyright 2002, Prentice Hall Inc ISBN 0-201-18075-8 and also available at http://users.dcc.uchile.cl/˜jsaavedr/libros/dip_gw.pdf. As can be seen, the sensitivity is quite non-linear. To make this non-linearity easier to understand, the chart has been broken up into four zones. 
     Zone 1: Top end of Photopic vision (glare limit) where relative differences in brightness are less noticeable and colors shift to red. Sharpness of focus is good with contracted pupil but glare inside the eye starts to obscure details. 
     To improve viewability, the displayed image is modified to increase contrast and increase green and/or blue. 
     Zone 2: Standard range of color vision where cones dominate in the human eye. Color perception is basically uniform and brightness perception follows a standard Gamma curve. Maximum sharpness possible due to small pupil and manageable levels of brightness. Viewability is good with standard brightness and color. 
     Zone 3: Transition zone from cones to rods for primary sensitivity. Color perception becomes non-linear as the red cones lose sensitivity faster than blue and green. Contrast perception is reduced due to flattening response to changes in brightness. Focus sharpness also begins to reduce with larger pupils, especially in older eyes that aren&#39;t as capable of adapting freely. Viewability is improved by increasing font and object sizes for legibility and reducing blue and green colors while increasing red and increasing contrast. 
     Zone 4: Bottom end of scotopic vision where rods dominate for sensitivity and motion is more apparent than content. Viewability is improved by changing the displayed images to eliminate high spatial frequency such as small text and instead provide iconography and use motion or blinking to increase visibility of critical items. 
     In a further embodiment, changes in operating mode are considered. So that if the user changes operating mode, the displayed image is modified in correspondence to the mode change and the environmental conditions to improve viewability. This can be a temporary state as the user&#39;s eyes adapt to the new operating mode and the associated change in viewing conditions. For example, if the display settings were based on darker ambient conditions than are detected when the head-worn display wakes up, the brightness of the displayed image is modified to match the environmental conditions to avoid hurting the user&#39;s eyes. In another example, an entertainment mode is used and the brightness of the displayed image is slowly increased from the environmental conditions up to level for best viewability of a video with saturated color and high sharpness (Zone 2). In yet another example, if the displayed image includes a limited area of icons or white on black text for nighttime viewing, the brightness is reduced before showing a photo or white background page to account for the increased perception of brightness. 
     In a yet further embodiment, an eye camera is used to determine which portion of the displayed image that the user is directly looking at and attributes of the displayed image are adjusted in correspondence to the brightness of that portion of the displayed image. In this way, the attributes of the image are adjusted in correspondence to the portion of the image that the user&#39;s eye is reacting to. This approach recognizes that the human eye adapts very quickly to local changes in brightness within the area that the eye is looking. When the brightness increases rapidly such as when a light is turned ON in a dark room, the pupil diameter can decrease by 30% in 0.4 sec as shown in studies by Pamplona (Pamplona, V. F., Oliveira, M. M., and Baranoski, G. V. G. 2009, Photorealistic models for pupil light reflex and iridal pattern deformation, ACM Trans. Graph. 28, 4, Articles 106 (August 2009), 12 pages). As a result, the user&#39;s eye can rapidly adapt to local changes in brightness as the user moves his eye to look at different portions of the displayed image or different portions of the see-through view of the surrounding environment. In order to provide a more consistent perceived brightness for different portions of the displayed image, systems or methods in accordance with the principles of the present invention adjust the overall brightness of the displayed image in correspondence to the local brightness of the portion of the displayed image or the local brightness of the portion of the see-through view that the user&#39;s eye is looking at. In this way, changes in the size of the pupil of the user&#39;s eye are reduced and the user is then provided with a more consistent brightness distribution within a displayed image. Wherein the portion of the displayed image or the portion of the see-through view that the user&#39;s eye is looking at is determined by analyzing images of the user&#39;s eye that have been captured by the eye camera. The eye camera can be used in a video mode to capture images of the user&#39;s eye continuously and the captured images are then analyzed continuously to track the position of the user&#39;s eye over time. The position of the user&#39;s eye within the captured images of the eye is correlated to the portion of the displayed image or the portion of the see-through view that the user is looking at. The overall brightness of the displayed image can then be adjusted in correspondence to the local brightness of the portion of the displayed image or the portion of the see-through view that the user&#39;s eye is looking at. The rate of adjustment of the overall brightness of the displayed image can be further correlated to the measured diameter of the pupil of the user or to the measured change in diameter of the pupil of the user as determined from analysis of the captured images of the user&#39;s eye. 
     In a yet further embodiment, adjustments to attributes of the overall image can be made based on the local attributes of the portion of the displayed image or the portion of the see-through view that the user&#39;s eye is looking at. The adjusted attributes of the displayed image can include: color, color balance, contrast, sharpness, spatial frequency and resolution. Where the eye camera is used to capture images of the user&#39;s eye, which are then analyzed to determine the portion of the displayed image or the portion of the see-through view that the user&#39;s eye is looking at. The portion of the displayed image or the portion of the see-through view that the user&#39;s eye is looking at is then analyzed to determine the relative intensity of the attribute. Adjustments are then made to the overall displayed image in correspondence to the local intensity of the attribute in the area that the user&#39;s eye is looking at to improve viewability. Where a camera in the head-worn display can be used to capture images of the surrounding environment that at least partly correspond to the see-through view provided to the user&#39;s eye. 
     In embodiments, the head-worn computer has an outward facing camera to capture a scene in front of the person wearing the head-worn computer. The camera and image processing used to determine the area in the surrounding scene that will be used for brightness and/or color consideration in the process of adjusting the displayed content may take a number of forms. For example:
         Camera positioned to capture forward facing scene—the brightness measure would consider the captured scene and determine a relevant brightness and/or color. For example, the entire scene average color/brightness may be considered, a bright or color saturated portion may be considered, a dark area may be considered, etc.;   The forward facing camera may have a field of view larger than that of the see-through display&#39;s field of view and image processing may be used to assess the overlapping areas such that a captured image brightness and/or color may be representative of the see-through display&#39;s field of view brightness and/or color;   The forward facing camera may have a field of view similar to that of the see-through display&#39;s field of view such that a captured image brightness and/or color may be representative of the see-through display&#39;s field of view brightness and/or color;   The forward facing camera may have a narrow field of view to better target a scene directly in front of the user;   The forward facing camera may be a mechanically movable camera that follows the eye-position (e.g. as determined through eye-imaging as described herein) to capture a scene that follows the user&#39;s eyes;   The forward facing camera may have a wide field of view to capture the scene. Once the image is captured, a segment of the image may be identified as being the segment that the user is looking towards (e.g. in accordance with eye imaging information) then the brightness and/or color in that segment may be considered;   An object in the captured scene image may be identified (e.g. as determined based on eye-imaging and position determination) and the object may be considered; and   An object in the captured scene image may be identified as an object for which the displayed content is going to relate (e.g. an advertisement to be associated with a store) and the object&#39;s brightness and/or color may be considered.       

     In a further embodiment, the present invention provides a method for improving the alignment of a displayed image to the see-through view of the surrounding environment. The method can also be used for correlating eye tracking to where the user is looking in the see-through view of the surrounding environment. This is an important feature for making adjustments to attributes in the displayed image when the adjustments are based on local attributes in the portion of the see-through view that the user is looking at. The adjustment process can be used for each user using the head-worn display to improve the viewing experience for different individuals and compensate for variations in eye position or head shape between individuals. Alternatively, the adjustment process can be used to fine-tune the viewing experience for a single individual to compensate for different positioning of the head-worn display on the user head each time the user uses the head-worn display. The method can also be important for improving the accuracy of positioning of augmented reality objects. The method includes using an externally facing camera in the head-worn display to capture an image of the surrounding environment that includes at least a portion of then user&#39;s field of view of the see-through view of the surrounding environment. A visible marker such as for example, a cross, is provided in a corner of the captured image to provide a first target image. The first target image is then displayed to the user so the user simultaneously sees the displayed image of the surrounding environment from the first target image overlaid onto the see-through view of the surrounding environment. The user looks at the visible marker and then uses eye tracking control to move the displayed image to the position where the portion of the displayed image adjacent to the visible marker is aligned with objects in the see-through view of the environment. Where eye tracking controls include an eye camera to determine the movements of the user&#39;s eye and blinks of one or both eyes (head movements can be used in conjunction with eye controls in the user interface) which are used to in a user interface to input control inputs. A second image of the surrounding environment is then captured and a visible marker is provided in a corner to provide a second target image wherein the visible marker in the second target image is positioned in a corner that is opposite to the visible marker in the first target image. The second target image is then displayed to the user. The user then looks at the visible marker in the second target and uses eye control to move the displayed image to align objects in the second target image that are adjacent to the visible marker with objects in the see-through view of the environment. During the period when the user is viewing the first and second target images, it is important that the user not move their head relative to the environment. The displayed image is then adjusted in correspondence with the relative amounts that the first and second target images had to be moved to align portions of the displayed image with corresponding portions of the see-through view of the surrounding environment. 
       FIG. 127  shows an example of an illustration of a see-through view of the surrounding environment with an outline showing the display field of view  12723  being smaller than the see-through field of view  12722  as is typical. 
       FIG. 128  shows an illustration of a captured image of the surrounding environment which can be a substantially larger field of view than the displayed image so that a cropped version of the captured image of the environment can be used for the alignment process. 
       FIG. 129 a    shows an illustration of a first target image  12928  and  FIG. 129 b    shows an illustration of a second target image  12929 , wherein the target images  12928  and  12929  each include visible markers  12926  and  12927  in opposite respective corners. 
       FIG. 130  shows an illustration of a first target image  12928  overlaid onto a see-through view wherein the first target image  12928  has been moved using eye tracking control to align the portion of the first target image that is adjacent to the visible marker  12926  in relation to corresponding objects in the see-through view. Note that objects in the displayed image are shown in  FIG. 130  to be smaller in overall size compared to the see-through view before being adjusted to improve alignment, but it is also possible that the overall size could be larger before adjustment. 
       FIG. 131  shows an illustration of a second target image  12929  overlaid onto a see-through view wherein the second target image  12929  has been moved using eye tracking control to align the portion of the second target image that is adjacent to the visible marker  12927  in relation to corresponding objects in the see-through view. The movements needed to align the first target image  12928  and the second target image  12929  are then used to determine adjustments to the displayed image so that the accuracy of the alignment of the displayed image field of view  12723  with the see-through field of view  12722  is improved. Where the determined adjustments to the displayed image can include adjustments in overall size, cropping of the image and vertical and horizontal position of the displayed image within the displayed image field of view  12723 . By adding at least one more visible marker to the target images and using at least one more step to position the target images relative to the see-through view of the environment, rotational adjustments can be determined to further improve the alignment of the displayed image to the see-through view of the environment. A separate figure showing an illustration of the displayed image sized and aligned to match the see-through view of the surrounding environment is not shown because it would look like  FIG. 127 . The determined adjustments can then be used to improve the alignment of other displayed images to the see-through view of the surrounding environment so that areas in the displayed image can be mapped to the corresponding areas in the see-through view that would be located behind the displayed image when viewed in the head-worn display. The determined adjustments can also be used to map the movements of the user&#39;s eye to areas in the see-through view of the environment as captured in images of the surrounding environment from the externally facing camera, so that it can be determined where the user is looking in the surrounding environment. Further, by analyzing a captured image of the environment, it can be determined what the user is looking at in the surrounding environment. 
     In a yet further embodiment, eye tracking controls are used by the user to adjust the size of the displayed image and adjust the position of the displayed image to match the see-through view of the surrounding environment. In this method, an image of the surrounding environment is captured by the externally facing camera in the head-worn display. The image of the surrounding environment is then displayed to the user within the displayed image field of view  12723  so the user simultaneously sees the displayed image of the surrounding environment overlaid onto the see-through view of the surrounding environment. The user then uses eye tracking controls to perform two adjustments to the displayed image to improve the alignment of the displayed image of the surrounding environment with the see-through view of the surrounding environment. The first adjustment is to adjust the size of the displayed image of the surrounding environment in relation to the size of the see-through view of the surrounding environment. This adjustment can be performed by the user, for example by a long blink of the eye to begin the adjustment, followed by a sliding movement of the eye to increase or decrease the size of the displayed image. Another long blink ends the resizing process. The second adjustment is to position the displayed image to improve the alignment of the displayed image of the surrounding environment with the see-through view of the surrounding environment. This adjustment can be performed by the user for example, by a long blink of the eye to begin the adjustment followed by a sliding directional movement of the eye to indicate the movement to align the displayed image to the see-through view of the environment. This adjustment process can be performed for one eye at a time so that the displayed images for the left and right eyes can be positioned independently for improved viewing of stereo images. The determined adjustments are then used with other displayed images to improve the alignment of the other displayed images to the see-through view of the environment and to determine the mapping of the see-through view as seen behind the displayed image in the head-worn display. The determined adjustments can also be used to map the movements of the user&#39;s eye to areas in the see-through view of the environment as captured in images of the surrounding environment from the externally facing camera, so that it can be determined where the user is looking in the surrounding environment. Further, by analyzing a captured image of the environment, it can be determined what the user is looking at in the surrounding environment. 
     While some of the embodiments above have been described in connection with the use of eye tracking input for display content control and adjustment, it should be understood that an external user interface may be used in conjunction with or instead of eye-tracking control. For example, when the displayed content is presented in the field of view of the head-worn display, a touch pad, joy stick, button arrangement, etc. may be used to align the content with the surrounding environment. 
     In embodiments, the displayed content may be color adjusted depending on the scene background that will be behind the displayed content in the see-through display to compensate for the color of the scene background such that the displayed content appears to be properly color balanced. For example, if the scene background over which the displayed content will be overlaid is red (e.g. a red brick wall), the displayed content may be adjusted to reduce its red content because some of the scene&#39;s red content will be seen through the displayed content and hence contribute to the red content in the displayed content. 
     In embodiments, the displayed content may be adjusted as described herein (e.g. to blend or be distinguished from the scene as viewed through the see-through display) by adjusting a color and/or intensity of light produced by a lighting system adapted to light a reflective display, adjusting the image content through software image processing, adjusting an intensity of one or more colors of an emissive display, etc. 
     In embodiments, the see-through scene brightness and/or color may be based on an average see-through brightness and/or color of the scene as viewed through the display or otherwise proximate the head-worn display, a brightness and/or color of an object apparently in view through the see-through display, an eye heading (e.g. eye position based on eye imaging as described herein), compass heading, etc. 
     The inventors have discovered that, in head-worn displays that include multiply folded optics, it can be advantageous to use a solid prism with an included fold surface to improve image quality and enable a more compact form factor. They have also discovered that manufacturing of the solid prism by molding can be challenging due to sink marks, which often appear on planar surfaces. In addition, providing the illumination light into the solid prism at the required angle requires special considerations. Imaging of the user&#39;s eye can be an important feature in head-worn displays for user identification and as a user interface. Eye imaging apparatus are provided herein for a variety of head-worn displays. 
     An aspect of the present invention relates to a solid prism with improved manufacturability along with design modifications that enable illumination light to be effectively supplied into the solid prism at the required angle to illuminate the image source. 
     An aspect of the present invention relates to a solid prism with a fold surface platform, wherein an optically flat fold surface is mounted on the prism&#39;s fold surface platform such that the fold surface maintains a high optical flatness that minimizes aberrations in the prism&#39;s fold surface platform. 
     An aspect of the present invention relates to providing additional optical features in the solid prism that are used for capturing images of the user&#39;s eye with an eye imaging camera. 
     An aspect of the present invention relates to providing a solid prism with a fold surface, wherein the solid prism includes shaped input and/or output surfaces that act as optical power producing optical systems. 
     An aspect of the present invention relates to a solid prism with optical power producing surfaces with an additional power lens above the combiner such that the physical size of the power lens above the combiner is reduced thereby reducing the overall size of the optical system. 
     An aspect of the present invention relates to a solid prism with an optically powered surface at the image light-receiving end of the optical path from the display, wherein an additional optically powered field lens is positioned between the display and the optically powered surface to further increase the optical power of the optical system. 
     An aspect of the present invention relates to a solid prism with a fold surface that includes optically powered input and/or output surfaces and material selection amongst related optical materials that are adapted to reduce lateral color aberrations and thereby improve image quality provided to the user. 
     An aspect of the present invention relates to an angled backlight assembly that redirects illumination light toward an image source through the inclusion of a prism film, wherein the prism is positioned on the side of the backlight so that it acts like a Fresnel wedge. 
     An aspect of the present invention relates to a stray light management system adapted to manage stray light produced by a prism film used in a backlighting system, wherein the prism film causes significant stray light and an analyzer polarizer film is positioned in an image light optical path to absorb such stray light. 
     An aspect of the invention relates to an emissive display system that projects image light into a solid prism with a fold surface for delivery of the image light to the user&#39;s eye. 
     An aspect of the present invention relates to projecting illuminating light through a portion of the display optics and towards a combiner surface, wherein the illuminating light reflects off the combiner surface and directly towards an eye of the user to thereby illuminate the eye for eye imaging. In embodiments, the display optics includes a solid prism and a light source is mounted above the fold surface of the solid prism. 
     An aspect of the present invention relates to capturing eye images directly from the combiner, wherein the eye-imaging camera is mounted above the combiner. In embodiments, an eye light is positioned at the top edge of the combiner so the eye is illuminated directly. 
     An aspect of the present invention relates to a surface applied to the combiner, wherein the surface is applied outside of the field of view of the see-through display and adapted to reduce stray light reflections from reflecting off the combiner and towards an eye of the user. 
     An aspect of the present invention relates to a surface applied to the combiner, wherein the surface is adapted to reflect infrared light and pass visible light such that visible stray light reflections towards the user&#39;s eye are minimized and such that infrared light from an infrared light source is reflected towards the user&#39;s eye. The infrared reflections may then be used for eye imaging. 
     An aspect of the present invention relates to eye imaging through a waveguide optic adapted to transmit image light and to be see-through for a user&#39;s view of the surroundings, wherein the eye imaging camera is positioned to receive eye images through the waveguide optic such that the image is captured from a position in front of the user&#39;s eye. 
     An aspect of the present invention relates to eye imaging by capturing reflected light off of an outer surface of a waveguide optic adapted to transmit image light and to be see-through for a user&#39;s view of the surroundings. 
       FIG. 132  shows an illustration of multiply folded optics for a head worn display that includes a solid prism  13250 . Where the solid prism  13250  includes a planar surface  13254  (i.e. a first fold surface) that is reflective to redirect the image light  13230  and thereby provide a first fold to the optical axis  13235  to enable the multiply folded optics to be more compact than optics which do not include this fold. As shown in  FIG. 132 , a second fold of the optical axis  13235  is provided in the lower portion of the multiply folded optics where the image light  13230  is reflected by the combiner  13210  (i.e. a second fold surface) so the image light  13230  is directed into the eyebox  13220  where the user&#39;s eye is located as has been previously described herein. The planar surface  13254  can be a full mirror so that all of the image light  13230  is reflected, wherein the image source  13260  must be a self-luminous image source such as an OLED or a backlit image source such as an LCD so that the image light  13230  is provided directly by the image source  13260 . However if the image source  13260  is a reflective image source such as a LCOS, FLCOS or DLP illumination light must be supplied which is then reflected by the image source  13260  to provide image light  13230 . In the case where the reflective image source is an LCOS or FLCOS, where illumination light is needed at a high incidence angle, the planar surface  13254  can be a partial mirror so that illumination light can be provided from a light source located behind the planar surface  13254  and pointed at the image source  13260 . In the case where the reflective image source is a DLP, where illumination light is needed at an angle commensurate with the mirror angles, the planar surface  13254  may be extended, or an additional surface may be provided, such that light can be provided from a light source located behind the planar surface  13254  or the additional surface. In embodiments, a first advantage provided by the solid prism  13250  is that the cone angle of the image light  13230  is reduced inside the solid prism  13250  thereby extending the optical path length so that a fold can be provided to the optical axis  13235  thereby enabling a more compact size of the multiply folded optics. A second advantage of the solid prism  13250  is that the planar surface  13254  provides an internal reflection so that dust cannot collect on the reflective surface. A third advantage of the solid prism  13250  is that stray light is easier to control by blackening the external surfaces that do not need to transmit light. 
     In addition to folding the optical axis  13235  by reflecting off the planar surface  13254 , the solid prism  13250  can also provide optical power since the input and output surfaces  13252  can be curved.  FIG. 132  shows two surfaces  13252  that have optical power. By providing some of the optical power needed in the multiply folded optics, the power lens  13240  doesn&#39;t need to provide as much optical power and as a result, the power lens  13240  is thinner and the overall size of the multiply folded optics is thereby reduced. A field lens  13270  can also be provided to act in conjunction with the solid prism  13250  and the power lens  13240 . By selecting the materials of the field lens  13270 , the solid prism  13250  and the power lens  13240  to be different in terms of refractive index and Abbe number (combining flint and crown glass properties as is known by those skilled in the art), the lateral color aberration in the image light  13230  provided to the eyebox  13220  can be substantially reduced thereby improving the sharpness of the image as perceived by the user particularly in the corners of the image. 
     In the multiply folded optics, the surfaces ( 13254  and  13210 ) that fold the optical axis  13235  are preferentially optically flat (e.g. flatness better than 10 microns) to maintain the wavefront of the image light  13230  and thereby provide a high quality image to the user. These surfaces can be tilted relative to the optical axis  13235  to compensate for twists of the upper portion of the optics (extending from the image source  13260  to the bottom surface of the solid prism) relative to the lower portion of the optics (extending from the power lens to the eyebox) as has been described previously herein. 
     Manufacturing of a plastic solid prism  13250  by molding can be difficult, because the solid prism  13250  has non-uniform thickness and it can include curved surfaces and flat surfaces. Injection molding of curved surfaces requires a different process setup than that required for injection molding flat surfaces. In particular, optically flat surfaces can be very difficult to injection mold without sink marks when the thickness of plastic under the flat surface is not uniform as is the case for the solid prism  13250 . To overcome this difficulty, the present disclosure provides a separate reflective plate  13275  that is used to establish an improved flat surface  13254 . The reflective plate  13275  can be manufactured using a sheet manufacturing process so that a high degree of optical flatness is provided. In a preferred embodiment, the reflective plate  13275  is a glass plate that has been coated to provide reflectivity. Where the coating can be a full mirror if the image source  13260  is a self-luminous display or it can be a partial mirror if the image source  13260  is a reflective display. In a further preferred embodiment, the reflective plate  13275  includes a glass plate with a reflective polarizer such as a Proflux wire grid polarizer by Moxtek (Orem, Utah) so that light of one polarization state is reflected and light of the opposite polarization state is transmitted. 
     The reflective plate  13275  can be bonded to the planar surface  13254  of the solid prism  13250  using a transparent adhesive that has a refractive index that is very similar (within for example +/−0.05) to that of the solid prism material (also known as index matched). By matching the refractive index of the adhesive to the refractive index of the solid prism  13250 , the interface between the solid prism material and the adhesive becomes optically invisible. In this way, the adhesive can fill in any spaces between the reflective plate  13275  and the planar surface  13254  of the solid prism  13250  that are caused by sink marks, scratches, grooves or other non-flatness of the planar surface of the solid prism. The flatness of the planar surface as molded on the solid prism  13250  is then not important to the optical performance of the multiply folded optic, and instead the flatness of the reflective plate  13275  determines the a new flat surface  13254  with improved flatness. In this way, the manufacturing of the solid prism  13250  becomes easier and less expensive because the planar surface  13254  does not have to be an optically flat surface as molded (or otherwise manufactured) and the manufacturing process used to make the solid prism  13250  can be optimized for the powered surfaces  13252 . In addition, by bonding the reflective surface of the reflective plate  13275  to the planar surface  13254 , the optically flat reflective surface is protected from being damaged during the further assembly process of the multiply folded optics. 
       FIGS. 133 a , 133 b  and 133 c    show illustrations of steps associated with bonding the reflective plate  13275  to the solid prism  13250 . As shown in  FIG. 133 a   , the solid prism  13250  is mounted for bonding so that the planar surface  13254  is approximately horizontal. A drop  13377  of relatively low viscosity (e.g. 200 centipoise) transparent adhesive is then applied to the flat surface  13254 . Where the adhesive is selected to have a refractive index that is very similar to the material of the solid prism  13250  so that the adhesive and the solid prism are index matched. The reflective plate  13275  is then brought into contact with the drop  13377  as shown in  FIG. 133 b   . The adhesive is then allowed to wick across the interface between the reflective plate  13275  and the planar surface  13254  until the entire interface is covered by the adhesive as shown in  FIG. 133 c   . Importantly, in embodiments, no pressure is applied to the reflective plate  13275  during the bonding process so that the reflective plate  13275  is not distorted and the optical flatness of the reflective plate  13275  is maintained. The drop  13377  used is relatively small so the interface is covered without adhesive oozing out at the edges. The adhesive is then cured, by waiting the appropriate length of time, applying heat or applying UV light as appropriate for the adhesive. In a preferred embodiment, a UV curing adhesive is used to provide a rapid cure. The advantage of bonding the reflective plate  13275  to the solid prism  13250  is that the adhesive can fill any sink marks that may be present on the planar surface of the prism so that the surface of the reflective plate  13275  establishes a planar surface  13254  with improved flatness and a desired level of reflectivity to reflect the image light  13230 . Since the adhesive is index matched to the material of the solid prism  13250  the image light  13230  passes from the solid prism  13250  through the layer of adhesive to the surface of the reflective plate  13275  without disturbing the wavefront of the image light  13230  so that high image quality is maintained. 
       FIG. 134  shows an illustration of multiply folded optics for a reflective image source with a backlight assembly positioned behind the reflective plate  13275 . Where, as shown in  FIG. 134 , the reflective plate  13275  is a partial mirror that transmits at least a portion of the light from the backlight to illuminate the image source  13260  and then reflects at least a portion of the image light  13230 . In a preferred embodiment, the reflective plate  13275  is a reflective polarizer that transmits one polarization state while reflecting the opposite polarization state. In this case, the illumination light  13432  is provided with a first polarization state (for example P polarization) and the image light  13230  is a second polarization state (for example S polarization). This change in polarization state occurs in the bright areas of the displayed image when the illumination light  13432  is reflected by the image source  13260  if the image source  13260  is for example a normally white LCOS. As a result, image light  13230  in the bright areas of the displayed image are reflected by the reflective polarizer of the reflective plate  13275  and image light in the dark areas of the displayed image is transmitted by the reflective polarizer, so that image light of only the second polarization state passes into the lens  13240 . The backlight assembly includes a prism film  13477  to deflect, at least a portion of the illumination light  13432  provided by the light guide  13480 , toward the image source  13260 . Where the prism film  13477  can be a turning film such as DTF provided by Luminit Corporation (Torrance, Calif.) or alternatively the prism film can be a brightness enhancement film such as Vikuiti BEF4-GT-90 provided by 3M (St. Paul, Minn.). A diffuser film  13478  is also included in the backlight assembly to provide the desired cone angle of light within the illumination light  13432 . A light source  13479  is also included in the backlight assembly to provide light to the light guide  13480 , where the light source  13479  can be one or more LEDs. The light source  13479  can provide white light or sequential color illumination (e.g. a repeating sequence of red then blue then green illumination, or cyan then magenta then yellow illumination) depending on whether the reflective image source includes a color filter array or not. 
     In a solid prism  13250 , the angle that the illumination light  13432  can be provided at is limited by refraction effects at the interface where the light enters the solid prism  13250 . As an example, following Snell&#39;s law for refraction across an interface 
         n 1 sin θ1= n 2 sin θ2
 
     to provide illumination light  13432  inside the solid prism with the approximately 30 degree angle from the interface normal that is shown in  FIG. 134 , the light from the backlight assembly would have to be provided to the interface at approximately 50 degrees if the prism material has a refractive index of 1.5. Where n1 is the refractive index of the first medium where the light is coming from, θ1 is the angle of the light relative to the surface normal in the first medium, n2 is the refractive index of the second medium where the light is going and θ2 is the angle of the light relative to the surface normal in the second medium. Providing illumination light  13432  with a 50 degree angle from the backlight assembly can be difficult as turning films are not available that deflect light by such a large angle. To reduce refraction effects, a prism film  13477  is used as a Fresnel wedge with the smooth side bonded to the reflective plate  13275  and the prism structure pointed toward the backlight assembly.  FIG. 135  shows an illustration of a prism film  13477  bonded to a reflective plate  13275 , where the prism film  13477  shown is a brightness enhancement film with linear prismatic surfaces oriented at approximately 45 degrees to the interface (thereby forming linear prisms with a 90 degree included angle) and an optically clear adhesive  13578  such as 8142KCL available from 3M (St. Paul, Minn.) used to bond the prism film  13477  to the reflective plate  13275 . It should be noted that this orientation with the prismatic structure pointed toward the light source is opposite to the orientation typically used for a brightness enhancement film, which is typically used to collimate light in a backlight. Instead, with the orientation shown in  FIG. 135 , following Snell&#39;s Law as previously described herein, the 45 degree surfaces of the brightness enhancement film split the incoming light into two cones of light (illustrated in  FIG. 135 a    as light  13532  and  13533 ) with respective deflection angles of approximately +/−17 degrees inside the prism film  13477  relative to the incident illumination light from the diffuser which is approximately perpendicular to the plane of the light guide  13480  and the plane of the reflective plate  13275 . Importantly, the prism film provides a substantially reduced amount of light between the two cones of light. Where the cone angle of the light within each of the cones of light is determined by the cone angle of the diffuser  13478 . The deflection angle of the illumination light  13432  can be modified by adding a turning film (not shown) on top of the prism film, where the turning film changes the angle of the illumination light provided to the prism film  13477 . Typical turning film such as the DTF film available from Luminit (Torrance, Calif.) provides a 20 degree deflection of light. The illumination light is then incident onto one surface of the prism film at 65 degrees and 25 degrees on the other surface of the prism film. The two cones of illumination light inside the prism film have deflection angles of +28 and −8 degrees relative to the incident illumination light from the diffuser which is approximately perpendicular to the plane of the light guide  13480  and the plane of the reflective plate  13275 . Since the prism film  13477  is bonded to the reflective plate  13275  and the reflective plate is bonded to the solid prism  13250 , the angle of light inside the prism film  13477  is essentially maintained into the solid prism  13250 , provided the refractive indices of the prism film  13477 , the reflective plate  13275  and the solid prism  13250  are reasonably similar. In this way, the system deflects the illumination light  13432  provided by the backlight assembly in a direction that directs the illumination light  13432  toward the image source  13260 . The image source  13260  is thereby illuminated by the light guide  13480  in a way that allows the multiply folded optics to have a more compact form factor as provided by the multiple folds of the optical axis  13235 . In manufacturing, the prism film  13477  can be bonded to the reflective plate  13275  either before or after the reflective plate  13275  is bonded to the solid prism  13250 . 
       FIG. 135 a    shows an illustration of multiply folded optics in which the two cones of illumination light  13532  and  13533  provided by the prism film  13477  are shown. While illumination light D 32  illuminates the image source  13260 , illumination light  13533  is a form of stray light in the multiply folded optics that must be controlled to provide high contrast image light  13230  to the eyebox  13220  so that the user experiences a high contrast image. The advantage provided by the prism film  13477  is that approximately half of the illumination light ( 13532 ) is deflected toward the image source  13260  while the other half of the illumination light ( 13533 ) is deflected in a direction where stray light can be controlled and little light is provided between  13532  and  13533  where control of stray light is more difficult.  FIG. 135 a    includes an analyzer polarizer  13582  to absorb the portion of light  13533  from the backlight that is not used to illuminate the image source  13260 . Analyzer polarizer  13582  is shown positioned between the power lens  13240  and the combiner  13210 , however, the analyzer polarizer  13582  could also be positioned in the gap between the solid prism  13250  and the power lens  13240 . The analyzer polarizer  13582  is oriented with it&#39;s transmission axis so that light with the polarization state of the bright areas of the image light is transmitted and light with the polarization state of the dark areas of the image light and the illumination light  13533  is absorbed. As such, the analyzer polarizer provides a dual purpose by reducing stray light associated with the illumination light  13533  and associated with image light in the dark areas of the image. 
     In multiply folded optics with a solid prism, additional optical elements can be added for imaging the eye of the user for the purpose of eye tracking in a user interface or eye identification for security purposes.  FIGS. 136, 137 and 138  show illustrations of different embodiments of additional optical elements included in the solid prism for imaging the eye of the user.  FIGS. 136 and 137  show illustrations of various views of an optical element  13612  attached to the side of the solid prism  13250  such that eye camera  13610  can image the user&#39;s eye in the eyebox  13220 . The optical element  13612  is shown as a single lens surface angled relative to optical axis  13235  to provide a field of view that includes light  13613  reflected from the user&#39;s eye. In this way, the light  13613  reflected from the user&#39;s eye is multiply folded in a way that is similar to the image light  13230 . However, the optical element  13612  can include more than one lens surface and more than one lens element to improve the resolution of the eye imaging.  FIG. 137  shows how the optical element  13612  can be positioned adjacent to surfaces  13252  on the solid prism  13250 . With the optical element  13612  positioned as shown in  FIGS. 136 and 137 , the eye camera  13610  is provided with a field of view that includes light  13613  reflected by the eye and the field of view associated with the optical element  13612  tends to extend to the upper portion of the user&#39;s eye. Where the user&#39;s eye can be passively illuminated by image light  13230  or actively illuminated by additional lights (not shown) adjacent to the eyebox  13220  or adjacent to the optical element  13612 . The additional lights can be infrared lights provided the eye camera  13610  can capture infrared images of the user&#39;s eye.  FIG. 138  shows an illustration of another solid prism  13250  with an optical element  13812  positioned adjacent to the top of the solid prism  13250  to enable the eye camera  13814  to image the eyebox  13220 . In this case, the optical element  13812  is attached to the solid prism  132450  and designed to provide a field of view that includes light  13813  that is reflected from the user&#39;s eye. The light  13613  is reflected by the user&#39;s eye and captured by the eye camera  13814  following a singly folded path. Where the field of view associated with the optical element  13812  being positioned as shown in  FIG. 138  tends to extend to the side of the user&#39;s eye. In both of the embodiments shown for eye imaging in  FIGS. 136, 137 and 138 , the optical elements  13612  and  13812  are designed to take into account the fact that the light reflected by the user&#39;s eye passes through the power lens  13240  and at least a portion of the solid prism  13250 . From a manufacturing perspective, the optical elements  13612  and  13812  can be made as attachments to the solid prism  13250  or made as an integral part of the solid prism  13250  that is molded along with the other surfaces of the solid prism  13250 . 
     In a further embodiment, eye imaging is included for the multiply folded optics shown in  FIG. 132 .  FIG. 139  shows an illustration of an eye imaging system for multiply folded optics in which the image source is a self-luminous display such as for example an OLED or a backlit LCD. In this case, the reflective plate  13275  is a partial mirror that is bonded to the planar surface  13254  of the solid prism  13250  as previously described herein. Alternatively a partial mirror coating can be applied directly to the planar surface  13254 , provided the planar surface  13254  is optically flat. The partial mirror then reflects a portion of the image light  13230  thereby redirecting it toward the lens  13240  and the combiner  13210  where the image light  13230  is reflected a second time and thereby redirected toward the user&#39;s eye to provide an image to the eyebox  13220 . Simultaneously, a portion of the light  13923  reflected by the user&#39;s eye is transmitted by the partial mirror and captured by an eye camera  13922 . Where, the user&#39;s eye can be passively illuminated by the image light  13230  and additional active illuminating light  13913  can be provided by an eye light  13912  to illuminate the user&#39;s eye. In a preferred embodiment, the eye light  13912  provides infrared illuminating light  13913  and the eye camera  13922  is sensitive to infrared light, in this way the illuminating light  13913  doesn&#39;t interfere with the images displayed to the user by the image light  13230 . In a further preferred embodiment, the partial mirror is a cold mirror that reflects a majority of visible light (e.g. greater than 80% of visible light, 400-700 nm) and transmits a majority of infrared light (e.g. greater than 80% of infrared light 800-1000 nm). In a yet further preferred embodiment, the combiner is at least partially coated with a hot mirror coating that reflects infrared light and transmits visible light. Wherein for example, the hot mirror coating can reflect greater than 80% of the infrared light provided by the eye light and transmit greater than 50% of the visible light associated the see-through view of the surrounding environment. By including a cold mirror on the planar surface or the reflective plate  13275  along with a hot mirror on the combiner  13210 , losses of the light  13923  reflected by the user&#39;s eye can be reduced thereby enabling bright images to be captured of the user&#39;s eye and reducing power needed for active illumination of the user&#39;s eye by the eye light  13912 . 
       FIGS. 140 a  and 140 b    show illustrations of folded optics with a combiner  14010  that redirects image light  13230  that has been provided by upper optics  1406  that includes an image source and associated optics. A camera  14022  is provided for imaging the user&#39;s eye  1408  when positioned adjacent to the eyebox  13220 . An eye light  14012  is provided to provide illuminating light  14013  that is reflected by the combiner  14010  and thereby directed toward the user&#39;s eye  1408 . Where, the camera  14022  is positioned to one side of the upper optics  1406  so that light  14023  reflected by the user&#39;s eye is reflected by the combiner  14010  and captured by the camera  14022 . As previously described herein, the eye light  14012  can provide infrared illuminating light  14013  (e.g. 850 nm) and the combiner  13220  can include a hot mirror coating to reflect the majority of the infrared illuminating light  14013 , while providing a see-through view of the surrounding environment. The eye light  14012  can be positioned to one side of the upper optics  1406  and preferably the eye light  14012  is positioned adjacent to the camera  14022  so that the illuminating light  14013  causes light  14023  to be reflected from the user&#39;s eye with a distribution that can be efficiently captured by the camera  14022 . For example, the eye light  14012  can be positioned on an adjacent side of the upper optics  1406 , as in  FIGS. 140 a  and 140 b    where the eye light  14012  is shown positioned on the back side of the upper optics  1406  so the illuminating light is reflected by the combiner back toward the user&#39;s eye  140   b  and the camera  14022  is shown on the left side of the upper optics  1406 , but other arrangements are also possible. In a preferred embodiment, the eye light  14012  is a small LED that is mounted on the lower front edge of the upper optics  1406  and pointed directly back toward the user&#39;s eye  1408 . 
     In embodiments, the combiner  14010  includes a surface that prevents visible light reflections outside of the field of view. The surface may include an anti-reflective coating and it may only be applied outside of the field of view. This arrangement can be useful in preventing environmental stray light from reflecting into the user&#39;s eyes. Without such a surface, light from the environment may reflect off of the combiner surface and into the user&#39;s eye. 
       FIGS. 141 a  and 141 b    show illustrations of folded optics that include a waveguide  14132  with an angled partially reflective surface  14135  and a powered reflective surface  14136 . Where an image source  14153  provides image light  14130  that is reflected by reflective plate  14175  so that the image light  14130  is conveyed by the waveguide  14132  to the partially reflective surface  14135  where it is transmitted to the powered reflective surface  14136  where it is condensed and reflected back toward the partially reflective surface  14135 . The partially reflective surface then reflects and redirects the image light so that the image light  14130  is provided to the user&#39;s eye  1408 . In the embodiment shown in  FIG. 141 a   , an eye light  14112  is positioned adjacent to one end of the waveguide  14132  so that illuminating light  14113  is directed at the user&#39;s eye  1408 . A camera  14122  is positioned behind the reflective plate  14175  wherein the reflective plate reflects at least a portion of the image light  14130  and transmits at least a portion of the light  14123  that is reflected by the user&#39;s eye  1408 . Wherein the reflective plate  14175  can be a partially reflecting mirror, a reflective polarizer or in a preferred embodiment the reflective plate  14175  is a cold mirror that reflects visible light and transmits infrared light (e.g. the cold mirror reflects greater than 80% of visible light, 400 to 700 nm, and transmits greater than 80% of the infrared light provided by the eye light, 800 to 1000 nm). It will be noted that in some cases the reflective plate can be replaced by a coating applied directly to the underlying planar surface of the waveguide  14132  provided the planar surface is optically flat. As previously described herein, eye light  14112  can provide infrared illuminating light  14113  provided the camera  14122  is sensitive to infrared. By positioning the camera  14122  behind the angled reflective plate  14175 , the image light  14130  and the light  14123  reflected by the user&#39;s eye  1408  can be coaxial so that images captured of the user&#39;s eye  1408  are from a perspective directly in front of the user.  FIG. 141 b    shows another embodiment in which the eye light  14112  is positioned adjacent to the camera  14122  so that the illuminating light  14113  is transmitted by the reflective plate  14175  and conveyed by the waveguide  14132  in a manner similar to the image light  14130  so that it is redirected toward the user&#39;s eye  1408 . 
       FIGS. 142 a  and 142 b    show illustrations of folded optics for a head-worn display that include waveguides  14232  with at least one holographic optical element  14242  and image source  14253 . In this embodiment, the image source  14253  provides image light  14230  to the waveguide  14232  (not shown) so that the holographic optical element  14242  can redirect the image light  14230  at approximately 90 degrees towards the user&#39;s eye  1408 . A camera  14222  is provided to capture images of the user&#39;s eye  1408 . An eye light  14212  provides illumination light  14213  to the user&#39;s eye  1408 . Light  14223  is reflected by the user&#39;s eye  1408  and is captured by the camera  14222 . As shown in  FIG. 142 a   , the eye light  14212  is positioned to one side of the waveguide  14232  and adjacent to the camera  14222 . A hot mirror coating with it&#39;s reflection spectrum matched to the infrared spectrum provided by the eye light  14212 , is applied to at least a portion  14224  of the waveguide  14232  so that the majority of light  14223  is reflected toward the camera  14222  and a bright see-through view of the surrounding environment is provided simultaneously.  FIG. 142 b    shows an illustration of similar folded optics for a head-worn display in which the waveguide  14232  is positioned at an angle to the user&#39;s eye  1408  to provide a closer fit of the folded optics to the user&#39;s head. In this case the holographic optical element  14242  is designed to redirect the image light  14230  at approximately 110 degrees to the waveguide and towards the user&#39;s eye  1408 . The camera  14222  is then positioned at the end of the waveguide  14232  that is opposite to the image source  14253  to enable the angle between the light  14223  reflected from the user&#39;s eye  1408  and the illumination light  14213  to be reduced. In this way an image of the user&#39;s eye  1408  with more uniform brightness can be captured by the camera  14222 . As previously described herein, at least a portion  14224  of the waveguide  14232  can be a hot mirror to reflect a majority of the light  14223  reflected by the user&#39;s eye  1408  while a bright see-through view of the surrounding environment is provided simultaneously. 
       FIG. 143  shows an illustration of folded optics for a head-worn display in which the illumination light is injected into the waveguide and redirected by the holographic optical element so that the user&#39;s eye is illuminated. Eye light  14312  is positioned at one end of the waveguide  14232  so that the illumination light  14313  can be injected into the waveguide  14232  and conveyed along with the image light  14230  to the holographic optical element  14242 . The holographic optical element  14242  then redirects the image light  14230  and the illumination light  14313  towards the user&#39;s eye  1408 . The holographic optical element  14242  must then be capable of redirecting both the image light  14230  and the illumination light  14313 , where the image light  14230  is visible light and the illumination light  14313  can be infrared light. Light  14223  reflected by the user&#39;s eye is then reflected by the waveguide surface and captured by the camera  14222 . A hot mirror coating with it&#39;s reflection spectrum matched to the infrared spectrum provided by the eye light  14212 , is applied to at least a portion  14224  of the waveguide  14232  so that the majority of light  14223  is reflected toward the camera  14222  and a bright see-through view of the surrounding environment is provided simultaneously. The advantage of this design is that the illumination lighting system including eye light  14312  can be made more compact.  FIG. 144  shows an illustration of folded optics for a head-worn display that is similar to the system shown in  FIG. 143  where a series of angled partial mirrors  14442  are included in the waveguide instead of a holographic optical element. In this case, illumination light  14413  is injected into the waveguide  14432  along with image light  14230  provided by the image source  14253 . The illumination light  14413  and the image light  14230  are conveyed by the waveguide  14432  to the series of angled partial mirrors  14442  which redirect the illumination light  144134  and image light  14230  towards the user&#39;s eye  1408 . Light  14223  reflected by the user&#39;s eye  1408  is reflected by a hot mirror coating applied at least to a portion  14224  of the waveguide  14432  wherein the reflection spectrum of the hot mirror is matched to the infrared spectrum of the illumination light  144134  provided by the eye light  14212 . The advantage of this design is that the illumination lighting system is compact and the series of angled partial mirrors can be easily made to operate on both the visible image light  14230  and the infrared illumination light  14413 . 
     When using a head-worn display for augmented reality applications, particularly when the head-worn display provides a see-through view of the surrounding environment, it can be important to be able to change the focus depth that the displayed image is presented at. It is also important to present stereo images at the proper vergence distance to provide the intended perception of depth to the user. Where focus distance is the distance the user&#39;s eye must be focused at to view a sharp image and vergence distance is the distance the user&#39;s two eyes come together to view the same spot in an image or on a real object. Within a stereo image, objects intended to be perceived to be at different depths are presented with a rendered lateral shift between the relative locations of the object within the left and right images, which is know as disparity. The rendering of typical stereo imagery as viewed in theaters or on televisions is mostly directed at disparity mapping of objects to create the 3D effect because the focus distance is limited to the theater screen or television (see the paper “Nonlinear disparity mapping for stereoscopic 3D”, M. Lang, A. Hornung, O. Wang, S. Poulakos, A. Smolic, M. Gross, ACM Transactions on Graphics (Impact Factor: 3.73). July 2010; 29. DOI: 10.1145/1833349.1778812). To make the stereo viewing experience more comfortable for the user, the vergence distance associated with viewing an augmented reality object should closely match the focus distance associated with the same augmented reality object thereby enabling the augmented reality object to more closely resemble a real object as seen by the user of the head-worn display. The systems and methods in accordance with the principles of the present invention provide methods of changing the focus distance and vergence distance associated with augmented reality objects and imagery viewed in a head-worn display in ways that more closely match real objects in a see-through view of the surrounding environment. 
     The focus distance of an image displayed in any head-worn display is determined by the elements in the optics of the head-worn display. The focus distance of the image can be changed by changing the elements in the optics, or by changing the relative positioning of some of the elements in the optics. The vergence distance associated with stereo images is determined by the lateral positioning of the images within the field of view of the user&#39;s left and right eyes. The vergence distance can be changed by laterally shifting the left and right images relative to one another within the user&#39;s fields of view either by repointing the left and right optics thereby establishing a different point of convergence between the left and right optics or by digitally shifting the displayed images within the display fields of view. To provide a stereo viewing experience of augmented reality objects that more closely resemble the viewing experience associated with a real object, it is important that the focus distance match the vergence distance for augmented reality objects in displayed stereo images in a head-worn display within the limitations of the user&#39;s eyes. Given that augmented reality objects are often positioned at different distances within stereo images and as different augmented reality activities are conducted at different distances, the inventors have discovered that methods are needed to change focus distance with a corresponding change in vergence distance within all types of head-worn displays. 
       FIG. 145  shows an illustration of a beam splitter based optical module for a head-worn display (shown from the side and from the eye position) that includes upper optics  14510  and a combiner  14520 . Wherein the upper optics  14510  include an image source, a light source and one or more lens elements. The combiner  14520  is a beam splitter that reflects a portion of the image light associated with the displayed image toward the user&#39;s eye while also allowing light from the surrounding environment to be transmitted so that the user sees the displayed image overlaid onto a see-through view of the surrounding environment.  FIG. 146  shows an illustration of an optical module for a head-worn display (also shown from the side and from the eye position) that has been modified to change the focus distance by adding a focus shift element  14625 . Where the focus shift element  14625  is a thin lens with optical power. For example, the focus shift element  14625  required to change the focus distance from infinity to 1 meter needs to provide −1 diopter of optical power. As such the focus shift element  14625  can be a refractive lens such as a portion of an ophthalmic lens, which is 1 to 1.5 mm thick. Alternatively, the focus shift element  14625  can be a Fresnel lens, which can be thinner than a refractive lens. By positioning the focus shift element  14625  above the combiner  14520 , the optical power of the focus shift element  14625  only acts on the displayed image and does not change the see-through view of the surrounding environment. This method can be used in any type of optics for a head-worn display (e.g. projected optics with a see-through combiner, holographic image projection with a see-through combiner, see-through optics with a see-through waveguide, TIR wave guide, etc.) wherein space is available to insert a focus shift element with optical power into the optical path such that the focus distance is changed without changing the see-through view. In the event that the upper optics  14510  utilize polarized image light, a polarization control element  14515  can be included to modify the polarization state of the image light. Where the polarization control element can include one or more of the following: a polarizer to cut unwanted polarization states, a retarder such as a quarter wave film to change the image light to circularly polarized or a half wave film to change the polarization state. 
     For the case where the user&#39;s eyes are not capable of focusing at the focus distance associated with the displayed image, a corrective lens element can be provided behind the optics module to improve the sharpness of the displayed image as perceived by the user. In this case, the corrective lens element is based on the user&#39;s ophthalmic prescription and the corrective lens element improves the view for the user of both the displayed image and the see-through view of the surrounding environment.  FIG. 146 a    shows an illustration of a side view of an optics module that includes a corrective lens element  14624 . The corrective lens element  14624  can have a positive optical power or a negative optical power as required by the user for viewing the displayed image at the focus distance. In addition, the corrective lens element can also include astigmatism and wedge as included in the user&#39;s ophthalmic prescription. Corrective lens elements  14624  for the left and right eyes can be connected to each other to provide a corrective unit that is attached and aligned to the optics module or frame of the head-worn display with either a built-in interpupillary spacing or a flexible interpupillary spacing. Alternatively, the left and right corrective lens elements  14624  can be separate and be attached and aligned individually to the optics module or frame of the head-worn display. For example, for applications where the displayed image is presented with a focus distance and vergence distance of 0.6 meters so that augmented reality objects or information can be provided for a task performed at arm&#39;s length, the focus shift element  14625  could have an optical power of −1.6 diopter and could provide only optical power and the corrective lens element  14624  could have an optical power of +2 diopters and also provide correction for astigmatism and wedge per the user&#39;s ophthalmic prescription. Where a +2 diopter corrective lens element  14624  would be a fairly typical prescription for reading glasses for a person of approximately 55 years old and as such would enable the person to view objects and images clearly that are positioned at arm&#39;s length. The corrective lens element  14624  shown in  FIG. 146 a    is a refractive lens, but other types of lenses are also possible, such as Fresnel lenses. 
     While lenses with fixed optical power are shown for the focus shift elements  14625  and the corrective lens element  14624 , lenses with adjustable optical power can also be used. Adjustable lenses using sliding lens elements (see U.S. Pat. No. 3,305,294) or liquid injection can be obtained for example from Adlens located in Oxford, United Kingdom: https://www.adlens.com/. Electrically adjustable lenses can also be used as corrective lenses such as: liquid crystal lenses available from LensVector (Sunnyvale, Calif.) or liquid lenses available from Varioptic (Lyon, France). 
     In addition, the optical modules can be mounted in the frame of the head-worn display such that they are slightly pointed toward one another (also known as toe-in) to provide a convergence distance. Thus, the convergence distance is established by the structural setup of the optics in the head-worn display and vergence distance can be adjusted by lateral digital shifting of similar portions of the left and right images that are displayed to create disparity for a portion of an image. The convergence distance then establishes the baseline vergence distance perceived by the user for stereo images that are rendered without disparity. To provide an improved stereo viewing experience, the convergence distance associated with the structural setup of the optics must be taken into account when rendering the disparity associated with displayed objects in stereo images. This is particularly important in a head-worn display system wherein the focus distance and vergence distance are matched for augmented reality objects in stereo images. As such the rendering of stereo images that were originally rendered for viewing in a theater, may need to be adjusted for improved viewing in a head-mounted display. The convergence distance can also be used to establish the perceived distance to the entire image if the stereo image is rendered without disparity, this can be useful for applications such as a head-worn computer wherein the desktop screen associated with the computer is perceived to be at a distance that is established by the convergence distance. However, the convergence distance cannot be too close to the user since the left and right images will experience opposing versions of keystone distortion. For example, a convergence distance of 2.4 meters can be provided by pointing the optics modules towards&#39; each other by 0.75 degrees if the user&#39;s eyes are separated by approximately 63.5 mm. The inventors have discovered that 0.75 degrees of toe-in results in a negligible level of keystone distortion. Closer convergence distances require larger angles of toe-in and as such the keystone distortion between the left and right images degrades the perceived sharpness in the corners of a stereo image. This keystone distortion can be compensated for by rendering the left and right images with matching and opposite levels of keystone predistortion. 
       FIG. 147  shows an illustration of left and right optics modules that are connected together in a chassis  14727  where the illustration is shown from behind the chassis  14727  where the user&#39;s eyes would be. The chassis  14727  allows the optics modules to be built as a separate unit that is assembled into a head-worn display. By making the chassis  14727  structurally stiff, the optics modules can be physically aligned relative to one another and the focus distance and convergence distance can be checked and adjusted as necessary prior to being assembled into the head-worn display thereby providing additional manufacturing flexibility. 
       FIG. 147  also shows the focus shift elements  14625  for the left and right optics modules connected in a focus shift element pair  14731 . By connecting the focus shift elements  14625  together, it is easier to add a pair of focus shift elements when needed for augmented reality imaging at different distances. The connection between the focus shift elements  14625  in a focus shift element pair  14731  can be rigid as shown in  FIG. 147  or flexible to enable the focus shift element pair  14731  to adjust to different spacing between the left and right optics modules with chassis&#39;s that have different widths for user&#39;s with different spacing between their eyes. Where focus shift elements  14625  with various optical powers are used to provide displayed images with different focus distances for augmented reality activities that require the image to be displayed at different working distances. The focus shift elements  14625  can also be different for the left and right eye to provide different focus distances for the left and right eyes. Focus shift elements  14625  can also be provided without optical power so that they function as a protective window for the upper optics  14510 . 
     In the simplest form, a mode change associated with changing the focus distance and vergence distance, can be accomplished by the user inputting information and selecting options through a user interface such as buttons or a graphical user interface. Confirmation of the mode change can then be provided to the user on the displayed image such as for example a colored box around the edge of the display field of view or a message stating “Mode change initiated for arm&#39;s length display”. In a more automatic mode change, a sensor  14730  can be provided that senses the focus shift element pair  14731  so that the images can be automatically presented with a lateral shift that provides a different vergence distance that matches the focus distance provided by the focus shift elements  14625 . The sensor  14730  can simply sense whether a focus shift element pair  14731  is present or not. Alternatively, the sensor  14730  can detect a code (e.g. a barcode) on the focus shift element pair  14731  that corresponds to the optical power or focus distance provided by the focus shift elements  14625  so that the displayed images can be automatically digitally shifted laterally to provide a matching vergence distance. The sensor can be located in the center as shown in  FIG. 147 , but other locations are also possible such as to one side. The code can be on one of the optical surfaces or on the edge of the focus shift element  14625  and the sensor  14730  can be oriented in a corresponding fashion to read the code. If the focus shift elements  14625  are not connected in a focus shift element pair  14731 , then two sensors  14730  can be provided with one sensor  14731  on each side. When a focus shift element  14625  is detected, the displayed image can be automatically changed in response to the change in operating mode that is implied by the detected presence of a focus shift element  14625 . In addition to the lateral shift to change the vergence distance as previously discussed herein, other changes can be made to the presentation of the displayed image when a focus shift element is present including: the size, the magnification, the format (e.g. 4:3 instead of 16:9), the color, the contrast, the dynamic range or the resolution. Where these changes to the image, are done to improve the viewing experience for the user when operating at different display distances such as in augmented reality activities. Changes in magnification and format are particularly important with this mode change as the lateral shift of the image to change the vergence distance results in some clipping of the available display field of view and the optical power associated with the focus shift element  14625  changes the overall optical power of the display optics. 
       FIGS. 148 and 149  show how displayed images can be digitally shifted laterally within the display field of view to change the vergence distance seen by the user.  FIG. 148  shows the left and right images,  14841  and  14843  respectively, as provided at the nominal vergence distance within the left and right display fields of view,  14840  and  14842  respectively. Where the nominal vergence is established by the alignment of the optics modules relative to one another in the head-worn display. The nominal vergence distance can be for example, infinity wherein the optical axes of the left and right display fields of view would be parallel to each other. In a preferred embodiment, the optical axes of the left and right display fields of view ( 14840  and  14842 ) are toed-in by approximately 0.75 degrees each, so that the nominal vergence distance is established at approximately 2.4 meters for a typical user with an interpupillary spacing between their eyes of 63.5 mm.  FIG. 149  shows how the left and right images  14941  and  14943  are shifted laterally towards each other within the left and right display fields of view  14940  and  14942  respectively, to provide a shorter vergence distance. By shifting the left and right images  14941  and  14943  towards each other, the user&#39;s eyes must be pointed towards each other somewhat to view the left and right images  14941  and  14943  as a stereo pair with a shorter vergence distance. For improved comfort when viewing the stereo pair, the focus distance should be matched to the vergence distance. In shifting the left and right images  14941  and  14943  laterally, portions of the left and right display fields of view (shown as  14945  and  14946 ) become unusable for stereo imaging since those areas do not overlap in the user&#39;s field of view. As such, the usable size of the left and right display fields of view  40  and  14942  is reduced when the head-worn display is used with a vergence distance other than the nominal vergence distance. The advantage of doing a digital shift of the left and right images  14941  and  14943  to provide a different vergence distance is that switching from the nominal vergence distance to a different vergence distance can be done without having to change the physical setup of the optics modules in the head-worn display. To reduce the clipping of the display field of view, extra pixels can be used on the image source that are not normally used to display images when operating in a mode where lateral shifting of the image is required. For example an image source with 1310×768 pixels can normally be used to display images that have 1280×720 pixels so that additional pixels around the edge are only used when the displayed image is digitally shifted to change the vergence distance. Due to vignetting, the brightness of the portion of the displayed image that is displayed with the pixels around the edge may need to be increased. 
     As previously mentioned herein, changes in focus distance can also be provided by changing the relative positioning of some of the elements in the optics.  FIGS. 150 a  and 150 b    show a mechanism for moving the image source  15040  relative to one or more lens elements  15012  in the upper optics  14510  to provide a change in the focus distance of the displayed image. Where typically moving the image source  15040  upward as shown in  FIG. 150 b    moves the focus distance further away and vice versa. The mechanism shown includes an upper wedge  15042  and a lower wedge  15043  along with solenoids  15035  and  15036  that respectively act on cores  15037  and  15038 . Where cores  15037  and  15038  are made of ferromagnetic materials and are attached to the lower wedge  15043 . Solenoids  15035  and  15036  include cylindrical windings of conductive wiring so that when an electrical current is applied to the wiring, the respective core  15037  or  15038  is drawn into the solenoid and the attached lower wedge thereby is moved to one side or the other. the solenoids are fixed in position relative to the housing of the upper optics  14510 . As the lower wedge  15043  is moved laterally, the upper wedge  15042  is moved up and down along with the image source  15040  which is attached to the upper wedge  15042 . Consequently, when a current is applied to the solenoid  15035 , the lower wedge  15043  is moved to the left as shown in  FIG. 150 b   , and as a result, the upper wedge  15042  is moved upward along with the image source  15040  and the focus distance is increased. Similarly, when a current is applied to the solenoid  15036 , the lower wedge  15043  is moved to the right as shown in  FIG. 150 a   , the upper wedge  15042  is moved downward along with the image source  15040  and the focus distance is decreased. By using upper and lower wedges  15042  and  15043  with a relatively shallow wedge angle (e.g. 5 to 15 degrees), the wedges tend to stay in place when the current to the solenoid is turned 0150. Opposing permanent magnets (not shown) can be added to the wedges  15042  and  15043  to increase the friction between the wedges and thereby assist in holding the wedges in place when the current to the solenoids is turned 0150. In this way, the power required to operate the solenoids ( 15035  and  15036 ) can be very small even if a relatively large current is required to generate enough force to move the lower wedge  15043 . By alternating the application of current to solenoids  15035  and  15036 , the focus distance can be alternately switched between two focus distances such between a 2.4 meter focus distance and a 0.6 meter focus distance. This method of changing the focus distance can be used with any optics that use a microdisplay at a focus plane of optics such as waveguide based optics or beam splitter cube based optics. This arrangement may also be used with a pulsed application of current to the solenoids  15035  and  15038  to cause a stepped change in wedge position and an associated stepped change in focus distance that spans over a continuous range, multiple stepped range, etc. In addition, guidance to the movement of the image source  15040  can be provided by sliding pins that pass through the upper wedge  15042  or an associated structure (not shown), where the pins allow vertical movement and prevent lateral movement. 
       FIGS. 151 a  and 151 b    show illustrations of upper wedge  15042  and lower wedge  15043  from the position of the image source  15040 . As shown, the wedges  15042  and  15043  comprise rectangular structures with their centers removed like a window frame so that illumination light and image light can pass through the wedges ( 15042  and  15043 ) to enable an image to be displayed. This is important when the mechanism to move the image source  15040  is positioned below the image source  15040  (along the optical path of the image light).  FIG. 151 a    corresponds to the wedge positioning shown in  FIG. 150 a    and  FIG. 151 b    corresponds to the wedge positioning shown in  FIG. 150 b   . The advantage of the layout shown in  FIGS. 150 a  and 150 b    is that the wedges  15042  and F43 and other pieces in the mechanism do not increase the overall height of the upper optics  14510 . 
     In an alternate embodiment (not shown) the mechanism for moving the image source  15040  is positioned above the image source  15040  and then the wedges ( 15042  and  15043 ) can be solid wedges or have portions of the center removed to enable wires to connect to the image source  15040 . The advantage of positioning the wedges and other pieces of the mechanism above the image source  15040  is that the image source can be positioned closer to the lens elements  15012  which can be important in some optical designs. 
     In another embodiment, the wedges ( 15042  and  15043 ) can be transparent and can cover the entire aperture of the image source  15040 . The transparent wedges (f 15042  and  15043 ) can operate as previously described to move the image source  15040 . In addition, as the wedges move laterally, the combined optical thickness of the two wedges is a function of the relative wedge position in the area that covers the active area of the image source  15040 . This is due to the fact that the transparent wedges have a higher index of refraction than the air that they are replacing. Because the wedges are matched in slope, the combined optical thickness of the area where the wedges are overlapped is uniform. As such, changes in the combined optical thickness of the overlapped wedges contributes to changes in the focus distance. 
     To further improve the repeatability of the movement of the image source  15040  and the upper wedge  15042  when the lower wedge  15043  moves, spring clips can be used to apply a force to the image source  15040  or the upper wedge  15042  to insure contact is maintained between the surfaces.  FIG. 152  shows an illustration of spring clips  15250  and  15252  applying a force to an image source  15040  where the image source  15040  is attached to the upper wedge  15042 . The spring clips  15250  and  15252  are attached to the housing of the upper optics  14510  using screws  15253 , ultrasonic welding, adhesive or other connecting systems. To reduce lateral movement of the image source  15040  as the lower wedge  15043  is moved, one or both of the spring clips  15250  and  15252  can be connected to the image source  15040  or the upper wedge  15042 . In this way, vertical movement (as shown) is allowed for changing the focus distance by flexing the spring clips  15250  and H 52 , while lateral movement is not allowed due to the higher stiffness of the spring clips in the lateral direction particularly if both spring clips  15250  and  15252  are connected to the image source  15040  or upper wedge  15042 . 
     In another embodiment, the movement of the lower wedge  15043  is controlled by an electric motor and a lead screw instead of solenoids. Where the electric motor is connected to the housing of the optics module and a lead screw or core is connected to the lower wedge  15043 . The electric motor can be a conventional rotating motor, a linear motor, a vibrating piezoelectric motor, an induction motor, etc. The electric motor can also be controlled to move the lower wedge  15043  different distances to provide various focus distances. The electric motor can be a stepper motor in which the number of steps determines the distance of movement. Sensors can also be provided to detect the movement of the lower wedge, lead screw or core to improve the accuracy of the movement and associated accuracy of the focus distance change. 
     In yet another embodiment, the movement of the lower wedge  15043  is provided by a manually operated knob (not shown). The knob is connected to a lead screw that is threaded into the lower wedge  15043 . The user turns the knob to move the lower wedge and thereby affect a change in the focus distance. This can be used for fine tuning of the sharpness of the displayed image as well for changing the focus distance to match a given vergence distance or to match the focus distance to the distance to a real object in the see-through view of the surrounding environment. 
     In a further embodiment, the corrective lens element  14624  can include a mechanism (not shown) to enable the corrective lens element  14624  to slide upward or swing to the side, to thereby move out of the display field of view while still being attached to the head-worn display. In this way, the corrective lens element  14624  can be readily available for use with the head-worn display. This can be useful as the corrective acts simultaneously on both the displayed image and the see-through view of the surrounding environment. There can be times when the user would want to be able to change the focus distance of the displayed image or change the focus of the see-through view of the surrounding environment depending on the activity that he is engaged in and having a readily available corrective lens element  14624  would enable that. In particular, a corrective lens may be needed by the user when operating at extreme focus distance such as arm&#39;s length or nearer, or at infinity. In embodiments, the corrective lens  14624  may be manually or automatically shifted into position. 
     In a yet further embodiment, eye cameras are included in the left and right optics modules to determine where the relative direction the user&#39;s eyes are looking. This information can then be used to determine the portion of the displayed image the user is looking at. The focus distance can then be adjusted to match the vergence distance associated with augmented reality objects in that portion of the displayed image. The focus distance is then automatically adjusted as the user moves his eye to different augmented reality objects or different portions of augmented reality objects within the displayed image. Alternatively the eye cameras can be used to determine the vergence of the user&#39;s eyes and thereby determine the distance that the user is looking at in the see-through view of the surrounding environment. The focus distance or vergence distance can then be adjusted in correspondence to the distance the user is looking at. Where the focus distance or vergence distance can be automatically adjusted to either match the distance the user is looking at in the see-through view of the surrounding environment or to be at a different distance so the displayed image doesn&#39;t interfere with the user&#39;s view of the surrounding environment. 
       FIGS. 153 a , 153 b    and  154  shows illustrations of example display optics that include eye imaging.  FIGS. 153 a  and 153 b    show display optics that include upper optics  14510  and a combiner  14520  to provide image light  15370  to an eyebox  15366  where the user&#39;s eye would be positioned when viewing a displayed image overlaid onto a see-through view of the surrounding environment. An eye camera  15364  is provided on the side of the upper optics  14510  and angled towards the combiner  14520  to capture light  15368  from the user&#39;s eye in the eyebox  15366  as reflected by the combiner  14520 . One or more LEDs  15362  are provided adjacent to the upper optics  14510  and pointed to provide illuminating light  15367  to the eyebox  15366  and the user&#39;s eye either directly or as reflected from an optical surface such as the combiner  14520 , when the head-worn display is being used by a user. Where the LED&#39;s  15362  can provide infrared light  15367 , provided the eye camera  15364  is sensitive to infrared light. 
       FIG. 154  shows an illustration of display optics viewed from above, that include projection optics  15410 , a waveguide  15415  and holographic optical elements  15417  and  15413 . The projection optics  15410  can include one or more optical elements  15412  to modify the image light  15470  as required to couple the image light  15470  into the holographic optical element  15413  and into the waveguide  15415 . The optical elements  15412  can change the wavelengths of the image light  15470 , change the format of the image light  15470 , change the size of the image light  15470  or predistort the image light  15470  as needed to enable the image light  15470  to be presented to the user&#39;s eye  15466  in the desired format with reduced distortion. The optical elements  15412  can include: refractive lenses, diffractive lenses, toroidal lenses, freeform lenses, gratings or filters. Where the holographic optical element  15413  deflects image light  15470  that has been provided by the projection optics  15410  into the waveguide  15415  where it is transported to the holographic optical element  15417 . The holographic optical element  15417  then deflects the image light  15470  toward the user&#39;s eye  15466  where the displayed image is viewed as an image overlaid onto a see-through view of the surrounding environment. An eye camera  15464  is provided for capturing images of the user&#39;s eye, as reflected by a surface of the waveguide, when the head-worn display is being used by a user. One or more LEDs  15462  are provided adjacent to the waveguide  15415  to illuminate the user&#39;s eye  15466  either directly or reflected from a surface of the waveguide and thereby increase the brightness of the captured images of the user&#39;s eye  15466 . Where the LED&#39;s  15362  can provide infrared light, provided the eye camera  15364  is sensitive to infrared light. 
     To improve the efficiency of the eye imaging systems shown in  FIGS. 153 a , 153 b    and  154 , coatings can be applied to the surface that reflects light from the eye toward the eye camera. The coating can be a hot mirror coating that reflects infrared light and transmits visible light. In this way, the eye camera can capture bright images of the user&#39;s eye while simultaneously providing the user with a bright see-through view of the surrounding environment. 
     The eye camera ( 15364  or  15464 ) can include autofocus to automatically adjust a focus setting of the eye camera when the user&#39;s eye is in a different positions such as when the head-worn display is positioned differently on the user&#39;s head or when a different user is using the head-worn display. Where the autofocus adjusts the relative position of lens elements or adjusts the optical power associated with adjustable lens elements in the optics associated with the eye camera to provide a higher contrast in the images of the user&#39;s eye. In addition, the autofocus can automatically adjust focus when corrective lenses  14624  are present and thereby compensate for the corrective lenses  14624 . In this case, metadata saved with the images of the user&#39;s eye records the relative focus setting of the eye camera ( 15364  or  15464 ) and changes in the metadata can be used to determine whether a corrective lens  14624  is present or not. If a corrective lens  14624  is present, adjustments to the focus distance of the display optics can be made that take into account the presence of the corrective lens  14624 . 
     Images of the user&#39;s eyes can be used to determine the viewing direction the user is looking by determining the relative position of the user&#39;s pupil within the eyebox or within the field of view of the eye camera  15364 . From this information the relative direction that the left and right eyes are looking can be determined. This relative direction information can be used to identify which portion of the displayed image the user is looking at. By comparing the relative direction of the user&#39;s left and right eyes within simultaneously captured images, the difference in relative direction between the left and right eyes and the interpupillary distance between the user&#39;s eye scan be used to determine the vergence viewing distance that the user is looking at. The vergence viewing distance can be used to determine the focus distance and vergence distance needed in the displayed image to provide the user with a sharply focused augmented reality object in the displayed image. The determined vergence viewing distance can also be compared to the vergence distance associated with the portion of the displayed image that the user is looking at, to determine whether the user is looking at the displayed image or the see-through view of the surrounding environment. Adjustments can be made to the focus distance and vergence distance for different portions of the displayed image to present the user a sharply focused image in the portion of the image that the user is looking at or present the user with a blurry image in the portion of the image that the user is looking at as needed for the mode of operation or use case. Where digital blurring of portions of the image can be used to make portions of the image appear to have a focus distance that is closer or farther away than the portions of the image that left with sharp imagery. In addition, the vergence viewing distance can be compared with the disparity associated with the portion of a stereo image that the user is looking at. The disparity of the stereo image can then be adjusted locally at the portion of the image the user is looking at or scaled over the entire stereo image to present the user with adjusted stereo depth over the entire image. 
     The head-worn display can include an inertial measurement unit to determine the location, movement and gaze direction of the head-worn display. Where the inertial measurement unit can include: a location determining system such as GPS, an electronic compass to determine gaze direction in the compass directions, accelerometers and gyroscopes to determine movements and a tilt sensor to determine a vertical gaze direction. Comparing the viewing direction determined from the images of the user&#39;s eyes to the gaze direction determined by the inertial measurement unit can allow a compass heading to be determined for the direction the user is looking. Combining the determined location with the compass heading of the direction the user is looking can allow objects in the surrounding environment to be identified that the user is looking at. This identification can be further improved by comparing the vergence viewing distance and the compass heading for the direction the user is looking with objects in the surrounding environment known to be that distance and direction from the user. This type of determination can be important for augmented reality and the display of augmented reality objects relative to real objects. 
     To enable the focus distance to be adjusted as the user moves his eyes around the field of view, the focus viewing distance must be determined rapidly and a fast focus adjustment system is required. Vergence and disparity within the stereo images must be adjusted in correspondence to the determined changes in focus viewing distance. A response time of 0.033 sec or less is typically required for imaging modifications within head-worn display systems to prevent the user&#39;s viewing experience from being adversely affected by latency such as the user experiencing nausea (see the paper “Tolerance of Temporal Delay in Virtual Environments” R. Allison, L. Harris, M. Jenkin, U, Jasiobedzka, J. Zacher, I149E Virtual Reality 2001, March 2001, p 247-254, ISBN 0-7695-0948-7). When a person&#39;s gaze changes from a far object to a near object, the human eye can change vergence viewing distance quickly while the focus adjusts more slowly. To enable this, a fast frame rate (e.g. 60 frames/sec or greater) is needed for capture of images of the user&#39;s eyes and the images need to have high contrast to enable fast image analysis to determine the relative positions of the user&#39;s eyes. The user&#39;s viewing direction and the focus viewing distance can then be determined to further determine where and what the user is looking at. A fast focus distance adjustment system is then needed to adjust the focus distance in 0.5 sec or less as the user moves his eyes. 
       FIGS. 153 a , 153 b   ,  154  show display optics that include a focus distance adjustment module  15360  in the upper optics  14510  and projection optics  15410  respectively. Where the focus distance adjustment modules  15360  can provide fast mechanisms for moving the position of the image source relative to the remaining lens elements thereby changing the focus distance of the displayed image. It is important to realize that focus adjustment modules can be used in any type of display optics for head-worn displays (e.g. wedge waveguides, waveguides with multiple reflective strips, holographic projection systems, (with the exception of laser scanning projection systems because they are not focused) because the movement of the image source relative to the other display optics to adjust the focus distance is fundamental to display optics and as such the focus adjustment modules are broadly useable in head-worn displays. 
       FIGS. 155 a , 155 b , 156 a , 156 b , 157 a , 157 b , 158 a , 158 b , 159 a  and 159 b    show illustrations of focus adjustment modules  15360  with mechanisms that can provide fast focus distance adjustment. To be effective for fast focus distance adjustment in a head-worn display, the focus adjustment modules  15360  need to be fast, quiet, provide approximately 0.5 mm travel, compact, provide guidance to maintain alignment between the image source  15040  and the remaining optics without tilt, controllable over the focus distance range, low cost and low weight. 
     In a preferred embodiment, to provide a change in focus distance without changing the size of the displayed image, display optics are provided that are telecentric at the image source. Where telecentric display optics provide parallel light ray bundles so that the area of the image source that is imaged by the display optics remains constant regardless of changes in the distance between the image source and the remaining optics as required to change the focus distance for the displayed image. In certain embodiments the image source is reflective and the illumination light provided by the illumination source may be telecentric as well. Where, telecentric illumination light can be provided by an illumination source that is at least the same size as the image source and provides a wider cone of light where only the telecentric portion of the cone is reflected by the image source. Thus, telecentric display optics at the image source provide an improved viewing experience for augmented reality, particularly when rapid changes to focus distance are being provided as the user moves their eyes around the field of view. Under this use case scenario, using non-telecentric display optics at the image source would result in displayed augmented reality objects that changed slightly in size each time the user moved their eyes and nausea would likely result. In contrast, by using telecentric display optics, focus distance can be comfortably changed continuously as the user moves their eyes around the field of view.  FIG. 161  provides an illustration of an example of non-telecentric display optics where the rays bundles of the image light  16150  are converging as the image light  16150  proceeds from the image source  15040  toward the display optics including the powered prism  16140 . As a result, if the image source  15040  is moved closer to the powered prism  16140 , the lens  16145  and combiner  16150  in the display optics, the image appears to get smaller when viewed by the user from the position of the eyebox  16155  and vice versa. In contrast,  FIG. 162  shows an illustration of example telecentric optics including for example powered prism  16240  and lens  16245  wherein the ray bundles of the image light  16250  are parallel to each other. Consequently as the image source  15040  is moved closer or farther from the powered prism  16140 , the image remains the same size in the displayed image as viewed by the user from the eyebox  16155 . 
       FIGS. 155 a , 155 b , 156 a , 156 b , 157 a , 157 b , 158 a  and 158 b    show actuators and guidance mechanisms positioned between the image source and the remaining optics. Each of the  FIGS. 155, 156, 157, and 158  illustrate different mechanisms in two states. In contrast,  FIGS. 159 a  and 159 b    show actuators and guide mechanisms positioned between the image source  15040  and the top of the housing for the focus adjustment module  15360 . Any of the actuators and guidance mechanisms shown can be used in either position with some modifications (not shown). The choice of where to position the actuators and guidance mechanisms depends on where space is available in the display optics and the housing for the head-worn display. If the space for the actuators and guidance mechanisms is limited in the display optics, the actuators and guidance mechanisms are positioned above the image source as shown in  FIGS. 159 a  and 159 b   . However, by positioning the actuators and guidance mechanisms above the image source, the height of the display optics can be substantially increased. Therefore in a preferred embodiment, multiply folded (also known as compound folded) display optics are included so the actuators and guidance mechanisms can be positioned adjacent to the image source, and as a result, the height of the display optics is reduced.  FIG. 160  shows an illustration of an example of multiply folded optics as viewed from the eye position, wherein the optical axis is folded to the side in the upper optics  16010  to reduce the height of the upper optics  16010 . The image source  15040  is then positioned to the side of the upper optics  16010  and the image source  15040  is approximately vertical instead of horizontal. Where in the example folded optics shown in  FIG. 160  are included, one or more lenses  16012 , a fold mirror  16013  that redirects image light  15370  from the upper optics  16010  toward a combiner  14520 , that redirects the image light toward the eyebox  15366  and the user eye. In the folded optics shown in  FIG. 160 , the fold mirror  16013  is a reflective polarizer so that a backlight  16014  can be positioned behind the fold mirror  16013  to provide P polarized illumination light  16071  that illuminates a reflective image source in the focus adjustment module  15360  such as an LCOS. In reflecting the illumination light  16071 , the image source  15040  changes the polarization state from P to S, thereby providing S polarized image light  15370 , which is reflected by the fold mirror  16013 . By using multiply folded optics, the focus adjustment module  15360  including actuators and guidance mechanisms can be positioned to one side of the upper optics  16010  where more space can be available in the frame of the head-worn display. Alternatively, the fold mirror can be included in a prism as shown in  FIGS. 161 and 162 , that can also include surfaces with optical power to further reduce the size of the display optics. As a result, multiply folded display optics provide the advantage of enabling a more compact head-worn display when the display optics include focus adjustment modules  15360 . 
       FIGS. 155 a  and 155 b    show an illustration of a focus adjustment module that includes a set of wedges  15042  and  15043  as actuators, wherein the lower wedge  15043  moves laterally to move the image source  15040  vertically (as shown) to change the position of the image source  15040  relative to the remaining optics comprising lens elements  15012  or lens elements  15412 . Solenoids  15035  and  15036  are provided to act on ferromagnetic cores  15037  and  15038  respectively, where the cores  15037  and  15038  are attached to the lower wedge  15043 . Because the wedges  15042  and  15043  are positioned between the image source  15040  and the remaining optics of display optics, the wedges  15042  and  15043  are made with a center window as shown in  FIGS. 151 a  and 151 b    so that light can pass from the remaining optics to the image source  15040 . Applying an electrical current to solenoid  15035  will attract core  15037  and cause the lower wedge  15043  to move to the left, thereby causing the upper wedge  15042  and the attached image source  15040  to move downward which decreases the focus distance as shown in  FIG. 155 a   . Similarly, applying an electrical current to solenoid  15036  will attract core  15038  and cause the lower wedge  15043  to move to the right, thereby causing the upper wedge  15042  and the attached image source  15040  to move upwards which increases the focus distance as shown in  FIG. 155 b   . A leaf spring  15570  has been provided to apply a force against the upper wedge  15042  or image source  15040  so that the wedges are help in alignment during the movement of the wedges. The leaf spring can also be attached to the housing of the focus adjustment module  15360  and to the image source  15040  or the upper wedge  15042  to prevent lateral movement of the image source during movement of the wedges, thereby providing guidance to the image source during focus adjustments. 
       FIGS. 156 a  and 156 b    show illustrations of a focus adjustment module  15360  that includes a pair of bimorph piezoelectric actuators  15675  and M 76  to move the image source  15040  for focus adjustments. Where a bimorph piezoelectric actuator is comprised of two laminated strips of piezoelectric material arranged so that when a voltage is applied to the two strips, one side of the bimorph contracts while the other side of the bimorph expands, thereby causing the actuator to go from flat to curved. Bimorph piezoelectric actuators are advantageous for use in a focus adjustment module  15360  because they are fast acting, compact and they can provide much more displacement than piezoelectric stack actuators. With the bimorph piezoelectric actuators  15675  and  15676  shown in  FIGS. 156 a  and 156 b   , one end is attached to the housing of the focus adjustment module  15360  and the other end pushes on a carrier  15677  that is attached to the image source  15040 .  FIG. 156 a    shows a flat state for the bimorph piezoelectric actuators  15675  and  15676 , while  FIG. 156 b    shows a curved state for the bimorph piezoelectric actuators  15675  and  15676 . Where the carrier  15677  supports the image source  15040  around the edge and the center portion of the carrier is removed to form a window so that light including illumination light and image light, can pass from the image source  15040  to the remaining optics as previously described herein for wedges  15042  and  15043 . When a voltage is applied to the two bimorph piezoelectric actuators  15675  and  15676 , both of the actuators  15675  and  15676  curl upwards thereby causing the carrier  15677  and attached image source  15040  to move upwards as shown in  FIG. 156 b    and the focus distance then increases. If more voltage is applied the bimorph piezoelectric actuators  15675  and  15676  will curl more. When the voltage is removed, the bimorph piezoelectric actuators  15675  and  15676  to return to a flat state, as shown in  FIG. 156 a    and the focus distance decreases. The actuators are shown arranged to lift opposite corners of the carrier to provide a vertical lifting force. If a faster response is desired in the movement from the curved state shown in  FIG. 156 b    to the flat state shown in  FIG. 156 a   , the voltage applied to the bimorph piezoelectric actuators can be reversed in sign for a short period of time. However, if the reversed voltage is applied for a long enough time for the actuators  15675  and  15676  to reach steady state, the actuators will curve in the reverse direction which will cause the carrier  15677  and the attached image source  15040  to be lifted somewhat. In addition, as shown in  FIGS. 156 a  and 156 b   , a four bar linkage  15679  has been provided. Wherein the four bar linkage  15679  is attached to the sidewall of the housing of the focus adjustment module  15360  and to four points on the carrier  15677 . The function of the four bar linkage  15679  is to provide guidance of the carrier  15677  and attached image source  15040  so that the image source  15040  doesn&#39;t move laterally or tilt relative to the remaining optics so that alignment is maintained during movements associated with focus adjustments. The four bar linkage  15679  shown in  FIGS. 156 a  and 156 b    is a thin metal or plastic structure with flexible fingers that extend from the sidewall attachment to the attachment points on the carrier  15679 . The flexibility of the fingers allows for unimpeded vertical movement while preventing lateral movement. The carrier  15677  is designed to provide attachment points that are spaced apart vertically as shown thereby enabling the fingers of the four bar linkage  15679  to prevent tilt of the carrier and attached image source during vertical movement. The four bar linkage  15679  can be further designed to be a leaf spring so that a slight downward force is applied to the carrier  15677  to ensure that the carrier  15677  remains in contact with the bimorph piezoelectric actuators  15675  and  15676  during focus adjustments. The advantage of this arrangement of the bimorph piezoelectric actuators is that a large displacement can be provided for a larger focus adjustment. In embodiments, the linkage  15679  may have a stop at an upper position to more accurately stop the translation of the carrier  15677  in an upper position. In embodiments, a stop may be otherwise positioned to create an upper boundary for the carrier. In further embodiments, the voltage applied to the bimorph piezoelectric actuators can be reversed to cause the bimorph piezoelectric actuators to bend in the opposite direction (not shown) and thereby extend the useable displacement range for focus adjustment. 
       FIGS. 157 a  and 157 b    show illustrations of another version of a focus adjustment module  15360  that includes bimorph piezoelectric actuators  15781  and  15782 . In this case, the lower bimorph piezoelectric actuator  15781  is attached in the middle to the lower surface of the housing of the focus adjustment module  15360  and the upper bimorph piezoelectric actuator  15782  is attached in the middle to the lower surface of the carrier  15677 . The ends of the upper bimorph piezoelectric actuator  15782  and the lower bimorph piezoelectric actuator  15781  are attached together.  FIG. 157 a    shows the flat state wherein no voltage is applied to the bimorph piezoelectric actuators  15781  and  15782 . When a voltage is applied to the bimorph piezoelectric actuators  15781  and  15782 , they both change to a curved state, which causes the carrier  15677  and the image source to move vertically thereby increasing the focus distance. As more voltage is applied, the curve of the actuators  15781  and  15782  becomes more pronounced and the movement of the carrier  15677  and the change in focus distance is increased. The advantage this arrangement of the bimorph piezoelectric actuators  15781  and  15782  is that a larger lifting force and faster movement can be provided, but the displacement is less. Consequently, the bimorph piezoelectric actuators  15781  and  15782  are arranged back-to-back so they curl in opposite directions when a voltage is applied thereby doubling the displacement of the carrier for a given voltage. The use of more than two bimorph piezoelectric actuators (e.g. four bimorph piezoelectric actuators) in a stack is possible. As previously described herein, a four bar linkage is provide to guide the movement of the carrier  15677  and attached image source  15040  to prevent lateral movement or tilt during focus adjustments. 
       FIGS. 158 a  and 158 b    show illustrations of a focus adjustment module  15360  that includes one or more scissors jack actuator actuators  15883 . Where the scissors jack actuator includes a frame that flexes so that the upper point moves further upward as a center shaft  15885  shortens. In this way, the frame of the scissors jack actuator  15883  acts as a displacement amplifier so that the movement of the carrier  15677  is greater than the change in length of the center shaft  15885 .  FIG. 158 a    shows the state when the center shaft  15885  is long, thereby causing the upper point to be lower and the carrier  15677  that sits on the scissors jack actuator  15883  to be lower and as a result the focus distance is nearer to the user.  FIG. 158 b    shows the state when the center shaft  15885  is short, thereby causing the upper point to be higher and the carrier  15677  that sits on the scissors jack actuator  15883  to also be higher and as a result the focus distance is farther from the user. The center shaft  15885  can be a variety of devices that effectively change the distance between the ends of the scissors jack actuator  15883  for example, the center shaft  15885  can be a piezoelectric stack actuator that is actuated with an applied voltage or a screw that is actuated manually by turning by hand or actuated electrically by an electric motor. In any case, the scissors jack actuator  15883  pushes on the carrier  15677  to lift the image source  15040  thereby increasing the focus distance. As previously described herein, a four bar linkage  15679  can be provided to guide the carrier during focus adjustments to preserve the alignment of the image source  15040  relative to the remaining optics in the upper optics  14510 . Piezoelectric stack actuators can provide very fast and precise movements so that if a piezoelectric stack is used as the center shaft  15885 , very fast and precise focus adjustments can be provided by a focus adjustment module  15360  if it includes a piezoelectric stack actuator with a scissors jack actuator  15883 . 
       FIGS. 159 a  and 159 b    show illustrations of focus adjustment modules  15360  with voice coil motor actuators  15987 . As previously described herein, in this case the image source  15040  is shown positioned below the actuator and the guidance mechanisms. A carrier  15977  is attached to the image source  15040  to support the image source  15040  and provide attachment points for the four bar linkage  15679 . Where the four bar linkage  15679  provides guidance to the carrier  15977  and attached image source  15040  during movement associated with focus adjustments. The outer portion of the voice coil motor  15987  is attached to the upper surface (as shown) of the housing of the focus adjustment module  15360  and the inner portion is attached to the carrier  15977 .  FIG. 159 a    shows the relative positions of the components when no voltage is applied to the voice coil motor  15987 . As shown, in  FIG. 159 a   , the inner portion of the voice coil motor  15987  is extended so that the carrier is in a lower position thereby providing a decreased focus distance.  FIG. 159 b    shows the relative positions of the components when a voltage is applied to the voice coil motor  15987 . Under these conditions as shown in  FIG. 159 b   , the inner portion of the voice coil motor  15987  is retracted so that the carrier is in a raised position thereby providing an increased focus distance. As more voltage is applied to the voice coil motor  15987 , the inner portion of the voice coil motor  15987  is retracted further thereby providing a greater change in focus distance. A spring (not shown) can be included in the focus adjustment module  15360  to apply a force to the carrier to decrease the time for the carrier to move back to the position shown in  FIG. 159 a    when the voltage is removed from the voice coil motor  15987 . The spring can also assist in holding the carrier  15977  in the position shown in  FIG. 159 a    to provide a default focus setting when no power is applied to the voice coil motor  15987  to thereby provide a low power operating mode. 
     A position measurement device (not shown) can be added to any of the focus adjustment modules  15360  shown in  FIGS. 155 a , 155 b , 156 a , 156 b , 157 a , 157 b , 158 a , 158 b , 159 a  and 159 b    to measure the relative position of the image source. The position measurement device can then provide a measurement that can be used in a control system for focus distance that can be a closed loop control system to improve the accuracy and repeatability of focus distance adjustments. 
     In a yet further embodiment, the position of the image sources  15040  in the left and right optics modules can be adjusted in alignment step to provide a reliable convergence distance. Where the alignment step includes positioning the chassis C 27  in a jig that is aligned with a target located in front of the jig and at the desired convergence distance. A matched image is then displayed on the image source  15040  and the image source  15040  is moved to align the displayed to the target as viewed through the optics module. The advantage of adjusting the position of the image source  15040  in an alignment step is that the effects of variations in the dimensions of the chassis  14727 , upper optics  14510  and combiner  14520  can be compensated for to provide a reliable convergence distance in a manufacturing environment. 
     In another embodiment, one or more of the following elements can be connected to provide a removeable assembly, including: the focus shift element, the combiner and the corrective lens element. This can provide a more easily replaceable assembly which can be changed when damage occurs, when the use case changes or the user changes. In particular, it is useful to change the focus shift element and the corrective lens element at the same time when changing from a use case where the vergence viewing distance changes from a longer distance to a shorter distance and vice versa. As in this use case, one or the other of the vergence viewing distances may be beyond what the user&#39;s eyes can comfortable focus at. For example, if the user is near sighted then a corrective is needed when the vergence viewing distance is longer and not needed when the vergence viewing distance is shorter. 
     The inventors have discovered that when world-locked digital content shifts out of the field of view of a user&#39;s head-worn see-through computer display it can create a less than optimal experience. When the user&#39;s turns his head away from the point in the world where the digital content is locked, for instance, the digital content shifts towards the side of the field of view. As the user turns his head even further, the content shifts out of the field of view and abruptly cuts off at the edge of the field of view. The abruptness of the change in appearance and the ultimate complete loss of the content once the head turns far enough does not create a natural impression of the content being fixed in the real world. Normally, when viewing an actual object in our environment, the object stays visually present, even if slightly present, until we shift our vision completely away from the object. An object that is shifted to the side of our direct line of sight vision may be slightly blurry do to the nature of our vision (i.e. foviated vision), but it remains present to some extent. In a typical see-through head-worn display the field of view has a limited area (e.g. width and height). Typically, one can see through to the environment outside of the field of view so it seems odd when the content begins and ultimately disappears from the user&#39;s vision when the user can still see into the environment where the content was once present and locked. 
     An aspect of the present invention relates to generating a smooth transition of world-locked augmented reality content that is shifting out of a see-through field of view. In embodiments, the world-locked content is modified to appear less apparent to the user as the content shifts towards the edge of the field of view. This may take the form of de-focusing, blurring, reducing the resolution, reducing the brightness, reducing the sharpness, reducing the contrast, etc. of the content as it is shifted towards the edge. The content may decrease in appearance gradually as it approaches the edge such that as it shifts past the edge it&#39;s appearance is minimal or non-existent such that it appears to have gradually disappears from the user&#39;s sight. This may work particularly well in a system that has a field of view that is large enough to accommodate sharp content in the middle of the field of view but large enough such that the user does not use the edges very much. For example, in a system with a horizontal field of view of 60 degrees, the outer 10 degrees on both sides may be used as a transitional area where world-locked content is managed to reduce its appearance in preparation for its disappearance from the field of view. 
     In one embodiment of a system for generating a smooth transition of world-locked augmented reality content that is shifting out of a see-through field of view, a head-worn see-through display that includes a see-through optical element mounted such that it is positioned in front of a user&#39;s eye when the head-worn see-through display is worn by the user also includes a processor that is adapted to present digital content in a field of view on the see-through optical element. The digital content may have a position within the field of view that is dependent upon a position in the surrounding environment. The processor may be further adapted to modify an appearance of the content as the content approaches an edge of the field of view such that the content appears to disappear as the content approaches the edge of the field of view. The appearance modification may be a change in the content&#39;s brightness, a change in the content&#39;s contrast, a change in the content&#39;s sharpness, or a change in the content&#39;s resolution. The processor may include a display driver or an application processor. The processor may be further adapted to generate a secondary field of view (e.g. through an additional optical system as described herein) in which the user views presented digital content and through which the user sees the surrounding environment, the processor further adapted to transition the content from the field of view to the secondary field of view. In this further adaptation, the appearance of the content in the secondary field of view may be diminished as compared to the appearance of the content in the field of view. In this further adaptation, the secondary field of view may have a lower resolution than a resolution of the field of view, and may be generated by one of reflecting image light onto a combiner that directs the image light directly to an eye of the user or towards a culminating partial mirror that reflects the image light to an eye of the user, an OLED that projects light onto a combiner, an LED array that projects light onto a combiner, or an edge lit LCD that projects light onto a combiner. In this further adaptation, the secondary field of view may be presented by a see-through panel positioned directly in front of an eye of the user, wherein the see-through panel is mounted on a combiner and/or vertically. The see-through panel may be an OLED or an edge lit LCD. The processor may be further adapted to predict when the content is going to approach the edge of the field of view and to base the appearance transition at least in part on the prediction. The prediction may be based at least in part on an eye-image. 
     In embodiments, the prediction that the content is going to approach and/or go past the edge of the field of view may be determined based on a compass in the head-worn computer (e.g. monitoring the compass heading as compared to the world-locked position for the content), movement of the content within the field of view (e.g. monitoring where the content is within the field of view and monitoring a direction and speed of it&#39;s movement towards an edge), eye position (e.g. monitoring eye position and movement as an indication of how the head-worn computer may move. There are times when the eyes shift prior to the head turning and the eye shift may provide the indication that the content appearance should be managed), and/or a combination of these techniques. 
     In one embodiment of a system for prediction based transition of world-locked content, a head-worn see-through display may include a see-through optical element mounted such that it is positioned in front of a user&#39;s eye when the head-worn see-through display is worn by the user and a processor adapted to present digital content in a field of view on the see-through optical element, wherein the digital content has a position within the field of view that is dependent upon a position in the surrounding environment. The processor may be further adapted to predict when the digital content is going to shift out of the field of view due to a positional change of the head-worn see-through display and to modify the appearance of the content as the content approaches an edge of the field of view such that the content appears to disappear as the content approaches the edge of the field of view. The prediction may be based on a compass heading indicative of a forward facing direction of the head-worn see-through display or a tracked eye movement of the user, wherein the tracked eye movement is indicative that the user is going to turn the user&#39;s head. The appearance modification may be a change in the content&#39;s brightness, a change in the content&#39;s contrast, a change in the content&#39;s sharpness, or a change in the content&#39;s resolution. The processor may include a display driver or an application processor. The processor may be further adapted to generate a secondary field of view in which the user views presented digital content and through which the user sees the surrounding environment, the processor further adapted to transition the content from the field of view to the secondary field of view. In this further adaptation, the appearance of the content in the secondary field of view may be diminished as compared to the appearance of the content in the field of view. In this further adaptation, the secondary field of view may have a lower resolution than a resolution of the field of view, and may be generated by one of reflecting image light onto a combiner that directs the image light directly to an eye of the user or towards a culminating partial mirror that reflects the image light to an eye of the user, an OLED that projects light onto a combiner, an LED array that projects light onto a combiner, or an edge lit LCD that projects light onto a combiner. In this further adaptation, the secondary field of view may be presented by a see-through panel positioned directly in front of an eye of the user, wherein the see-through panel is mounted on a combiner and/or vertically. The see-through panel may be an OLED or an edge lit LCD. The processor may be further adapted to predict when the content is going to approach the edge of the field of view and to base the appearance transition at least in part on the prediction. The prediction may be based at least in part on an eye-image. 
       FIG. 163A  illustrates an abrupt change in appearance of content  16302  in the field of view of a see-through display.  FIG. 163B  illustrates a managed appearance system where the content is reduced in appearance as it enters a transitional zone  16304  near the edge of the field of view. 
     An aspect of the present invention relates to a hybrid see-through display system where a high quality display system presents content to a field of view that is centered on the user&#39;s straight forward line of sight and another lower quality system is used to present content outside of the straight forward line of sight. The content appearance transition may then be managed in part in the center field of view and in the extended field of view. The extended field of view may have more than one section as well, such that imagery may be presented in a near edge portion and lighting effects are presented further out. 
     To illustrate, a front lit reflective display, emissive display, holographic display (e.g. as described herein) may be used to present high quality content in a 40 degree field of view and another display system may be used to present content or visually perceptive effects from the edge of the 40 degree point (or overlapping or with a gap) out to some other point (e.g. 70 degrees). In embodiments, the outer field of view coverage (generally referred to as the “outer display”) may operate through an optical system in an upper module, proximate the main field of view display system, and the optical path may include folds (e.g. as generally described herein). In other embodiments, the outer display may be a direct system where, for example, the image light or effects light is generated and directed to the combiner. For example, a display may be mounted above the combiner and arranged to direct lighting effects directly to the combiner. 
     In embodiments, the outer display may be included within the main display. For example, the lensing system in the upper module may be adapted to generate high quality content in the middle but then lower quality toward the edges of a larger field of view. In this system, there may be only one display (e.g. LCoS, OLED, DLP, etc.) and the content towards the edge of the display may be managed to effect the appearance transition. 
       FIG. 164  illustrates a hybrid field of view that includes a centered field of view  16402  for the presentation of sharp and transitional content and an extended field of view  16404  that is positioned at or near or overlapping with an edge of the centered field of view  16402  and adapted to provide lower appearance content and/or lighting effects that assist in the transition of the world locked content as it shifts out of the center field of view  16402 . 
       FIG. 165  illustrates a hybrid display system where the main, centered, field of view is generated with optics in an upper module  16502  (e.g. as described herein elsewhere) and the extended field of view is generated with a display system mounted  16504  above the combiner and providing image content and/or lighting effects in the extended area. In embodiments, the extended field of view display  16504  may include an OLED, edge lit LCD, LED, or other display and the display may include micro-lenses, macro-lens, or other optics to properly align and focus the light. In embodiments, the extended field of view may include a single lighting element, such as an LED, line or elements, array of elements, etc. 
     In yet other embodiments, the extended field of view area may be created by mounting a see through display on the combiner. For example, a see-through OLED display, edge lit LCD, etc. may be mounted in the extended field of view area and controlled to produce the transitional images and/or lighting effects. 
     In embodiments, a head-worn see-through display may be adapted to transition content to an extended FOV with reduced display resolution. The head-worn see-through display may include a see-through optical element mounted such that it is positioned in front of a user&#39;s eye when the head-worn see-through display is worn by the user and a processor adapted to present digital content in a main field of view on the see-through optical element in which a user views presented digital content and through which the user sees a surrounding environment, the processor further adapted to present digital content in an extended field of view in which the user views presented digital content and through which the user sees the surrounding environment. The main field of view may have a higher resolution than the extended field of view; and the processor further adapted to present a world-locked positioned digital content in the main field of view and transition the presentation of the world-locked positioned digital content to the extended field of view as the head-worn display changes position causing the world-locked positioned digital content to transition out of the main field of view. The processor may include display driver or an application processor. The extended field of view has a resolution that generates a substantial blur to content as compared with the content as presented in the main field of view. The extended field of view may be generated by reflecting image light onto a combiner that directs the image light directly to an eye of the user, by reflecting image light onto a combiner that directs the image light towards a culminating partial mirror that reflects the image light to an eye of the user, by an OLED that projects light onto a combiner, by an LED array that projects light onto a combiner, by an edge lit LCD that projects light onto a combiner, or by a see-through panel positioned directly in front of the eye of the user. The panel may be mounted on a combiner or vertically and may be an OLED or edge lit LCD. The processor may be further adapted to predict when the content is going to approach the edge of the field of view and to base the appearance transition at least in part on the prediction. The prediction may be at least in part based on an eye-image. 
       FIGS. 166A-166D  illustrate examples of extended display, or extended image content optic, configurations. As illustrated, the extended display configuration may be adapted to produce extended content and/or lighting effects around each side of the center display, on multiple sides of the center display or on one side of the center display. 
       FIG. 167  illustrates another optical system that uses a hybrid optical system that includes a main display optical system  16502  and an extended field of view optical system  16504 . In this embodiment, both optical systems project image light, extended image light, and/or lighting effects to a combiner that reflects the light to a forward culminating partial mirror, which in turn reflects the light towards the wearer&#39;s eye. 
     In yet further embodiments, the extended field of view display may be provided by a see-through display positioned in front of the user&#39;s eye such that the user looks directly through the see-through display. For example, a see-through OLED display or edge lit transparent LCD display may be positioned on either side of the combiner as illustrated in figures C and E or on either side of a waveguide or other display system (e.g. as illustrated in  FIGS. 8 a , 8 b , 8 c , 141 a , 141 b , 142 a , 142 b   ,  143 , and  144 ). 
     In embodiments, a head-worn see-through display may be adapted to provide an extended FOV for large content. The head-worn see-through display may include a see-through optical element mounted such that it is positioned in front of a user&#39;s eye when the head-worn see-through display is worn by the user, and a processor adapted to present digital content in a main field of view on the see-through optical element in which a user views presented digital content and through which the user sees a surrounding environment, the processor adapted to present digital content in an extended field of view in which the user views presented digital content and through which the user sees the surrounding environment. The main field of view may have a higher resolution than the extended field of view. The processor may be further adapted to present a first portion of the digital content in the main field of view and a second portion of the digital content in the extended field of view. For example, when the digital content is too large to fit in the main field of view, the processor may create a soft transition between the first portion of the digital content in the main field of view and the second portion of the digital content in the extended field of view such that it does not appear to be abruptly cut off at the edge of the main field of view. The processor may be adapted to generate a soft appearance towards the edges of the main field of view. The processor may modify how pixels towards an edge of the display render content. The head-worn display of may further include a display driver that modifies how pixels towards an edge of the head-worn display render content. The head-worn display may have pixels towards an edge of the head-worn display that render content differently than pixels towards a center portion of the head-worn display. The pixels towards the edge may have less gain than the pixels towards the center portion of the head-worn display. The pixels towards the edges of the main field of view may be altered digitally through a content transition algorithm. The extended field of view may be generated by reflecting image light onto a combiner that directs the image light directly to an eye of the user, by reflecting image light onto a combiner that directs the image light towards a culminating partial mirror that reflects the image light to an eye of the user, by an OLED that projects light onto a combiner, by an LED array that projects light onto a combiner, by an edge lit LCD that projects light onto a combiner, or by a see-through panel positioned directly in front of the eye of the user. The panel may be mounted on a combiner or vertically. The see-through panel may be an OLED or an edge lit LCD. The processor may be further adapted to predict when the content is going to approach an edge of the field of view and to base the appearance transition at least in part on the prediction. The prediction may be at least in part based on an eye-image. 
     In embodiments, a head-worn see-through display may be adapted to adjust content for transition to an extended FOV. The head-worn see-through display may include a see-through optical element mounted such that it is positioned in front of a user&#39;s eye when the head-worn see-through display is worn by the user and a processor adapted to present digital content in a main field of view in which a user views presented digital content and through which the user sees a surrounding environment. The processor may be further adapted to present digital content in an extended field of view in which a user views presented digital content and through which the user sees the surrounding environment. The main field of view may have a higher resolution than the extended field of view. The processor may be further adapted to present digital content in the main field of view and reduce an appearance of the content as the content approaches an edge of the main field of view. The processor may yet be further adapted to further reduce the appearance of the content when the content is presented in the extended field of view. The processor may gradually reduce the appearance of the content in the extended field of view the closer the content gets to an edge of the extended field of view. The content may be substantially not apparent when the content is at the edge of the extended field of view. The appearance reduction may be a reduction in the content&#39;s brightness, a reduction in the content&#39;s contrast, a reduction in the content&#39;s sharpness, or a reduction in the content&#39;s resolution. The extended field of view may be generated by reflecting image light onto a combiner that directs the image light directly to an eye of the user, by reflecting image light onto a combiner that directs the image light towards a culminating partial mirror that reflects the image light to an eye of the user, by an OLED that projects light onto a combiner, by an LED array that projects light onto a combiner, by an edge lit LCD that projects light onto a combiner, or by a see-through panel positioned directly in front of the eye of the user. The panel may be mounted on a combiner or vertically. The see-through panel may be an OLED or an edge lit LCD. The processor may be further adapted to predict when the content is going to approach an edge of the field of view and to base the appearance transition at least in part on the prediction. The prediction may be at least in part based on an eye-image. 
       FIGS. 168A-168E  illustrate various embodiments where a see-through display panel  16802  (e.g. OLED, edge lit transparent LCD display) is positioned directly in in front of the user&#39;s eye in the head-worn computer to provide the extended and/or overlapping field of view in a hybrid display system.  FIG. 168A  illustrates a system where the extended field of view is provided by the transparent display panel  16802  mounted on or near the combiner optic. In this embodiment, the see-through display panel  16802  is mounted on or near the back of the combiner such that it does not interfere with the center display system that reflects image light off the combiner directly to the user&#39;s eye.  FIG. 168B  illustrates a hybrid display system where the see-through extended field of view display panel  16802  is positioned vertically proximate the combiner.  FIG. 168C  illustrates a hybrid display system where the see-through extended field of view display panel  16802  is mounted vertically in front of a curved partial mirror of the main field of view display. 
       FIGS. 168D and 168E  illustrate hybrid display systems from the rear (i.e. user&#39;s view).  FIG. 168D  illustrates a system where the see-through extended field of view display panel  16802  surrounds the main field of view see-through display.  FIG. 168E  illustrates a system where the extended field of view see-through display panel  16802  is on the sides of the main field of view display system. It should be understood that the inventors envision that the extended field of view display panel may be configured in a number of different ways to provide the extension on one or more sides of the main field of view and in a balanced (i.e. similar extension on more than one side) or unbalanced (i.e. more or less extension on one or more sides) configuration. It should also be understood that the inventors envision that the extended field of view may overlap the main field of view, appear adjacent to the main field of view, have a gap between the main field of view and the extended field of view, etc., depending on the specific needs of the situation. 
     While the configurations described herein with respect to the extended field of view have been illustrative of creating a system where smooth transitioning of world-locked content, these configurations may further be used to create additional lighting effects and or shadowing effects for content displayed in the main field of view. For example, in a configuration where the extended field of view see-through display overlaps the main field of view, the extended field of view system may provide a backdrop for content displayed in the main field of view. The backdrop may be a lighting effect, for example, that is behind the content or near the content to provide context to the content. The backdrop may be a non-lighting effect where the pixels of the see-through display (e.g. the pixels of a see-through LCD) are changed to be opaque or less transparent to provide a dark back drop behind the content or adjacent the content (e.g. to form the appearance of a shadow). In such embodiments, the extended field of view system may overlap the main field of view and the extended field of view system may or may not extend past the edges of the main field of view. 
     In embodiments, a head-worn see-through display may be adapted to provide a hybrid multi-FOV display. In an aspect, an optical system of a head-worn see-through display may include a main image content optic for the production of center-eye image content, an extended image content optic for the production of off-center-eye image content, and a combiner positioned to present content to a user and through which the user views a surrounding environment, wherein each of the main image content optic and extended image content optic are positioned to project their respective image light to the combiner, which reflects the respective image light to a user&#39;s eye. The combiner may directly reflect the respective image light to the user&#39;s eye. The combiner may indirectly reflect the respective image light to the user&#39;s eye, wherein the combiner may reflect the respective image light towards a collimating partial mirror. The center-eye image content and the off-center-eye image content may pass through at least one fold in the optical system before reflecting off of the combiner. The extended image content optic may be mounted directly above the combiner such that the off-center-eye image content is directly projected to the combiner. The optical system may further include a processor adapted to coordinate a smooth disappearing transition of world-locked content as the content moves from a field of view of the main image content optic to a field of view of the extended image content optic and to an edge of the field of view of the extended image content optic. The extended image content optic may be an OLED, an LCD display, an array of LEDs, linear, two-dimensional, or curved. The extended image content optic may generate lighting effects corresponding to image content. The extended image content optic may include a lens system to modify the projection. The lens system may include an array of micro lenses. 
     In embodiments, a head-worn see-through display may be a hybrid display with a see-through panel. In an aspect, a head-worn see-through display may include a main image content display adapted to produce image light and project the image light in a direction to be reflected by a see-through combiner such that it reaches an eye of a user, and a secondary image content display, wherein the secondary image content display is a see-through panel positioned directly in front of the eye of the user and used to augment the visual experience delivered by the main image content display. The secondary display may provide content or effects in an area outside of a main field of view that is produced by the main image display. The area outside may be adjacent to the main field of view, surrounding the main field of view, or overlapping with the main field of view. The secondary display may provide content or effects in an area overlapping a main field of view produced by the main image display. The secondary display may be mounted on a combiner adapted to reflect image light to an eye of the user or may be mounted vertically outside of an image light optical path established by the main image display. The head-worn display may further include a processor that is adapted to track an eye position of the user, the processor further adapted to alter a position of content as presented in the secondary display. The altered position may substantially maintain an alignment of the main image display and the secondary image display from the user&#39;s perspective as the user&#39;s eye moves. The see-through panel may be an OLED or an edge lit LCD. 
     In embodiments, a head-worn see-through display may be adapted to blend types of content. In an aspect, a head-worn see-through display may include a field of view generated by an image display, wherein a user views digital content in the field of view and sees through the field of view to view a surrounding environment, and a processor adapted to generate two types of content, wherein the two types of content are presented in the field of view. The first type of content may be world-locked content with a field of view position that is dependent on a place in the surrounding environment, wherein an appearance of the first type of content is diminished as it approaches an edge of the field of view. The second type of content may not be world-locked, wherein the second type of content maintains a substantially constant appearance as it approaches the edge of the field of view. The diminished appearance may include a reduction in resolution, a reduction in brightness, a reduction in contrast, regulated by a display driver, regulated by an application processor, or regulated by altered pixels of a display that generates the field of view. The head-worn display may further include a secondary field of view generated by the image display in which the user views presented digital content and through which the user sees the surrounding environment, the processor further adapted to transition the content from the field of view to the secondary field of view. The appearance of the content in the secondary field of view is diminished as compared to the appearance of the content in the field of view. The secondary field of view may have a lower resolution than a resolution of the field of view. The secondary field of view may be generated by reflecting image light onto a combiner that directs the image light directly to an eye of the user, reflecting image light onto a combiner that directs the image light towards a culminating partial mirror that reflects the image light to an eye of the user, an OLED that projects light onto a combiner, an LED array that projects light onto a combiner, an edge lit LCD that projects light onto a combiner, or a see-through panel positioned directly in front of the eye of the user. The panel is mounted on a combiner or vertically. The see-through panel is an OLED or an edge lit LCD. The processor may be further adapted to predict when the content is going to approach the edge of the field of view and to base the appearance transition at least in part on the prediction. The prediction may be at least in part based on an eye-image. 
     In embodiments, a head-worn see-through display may be adapted to adjust an FOV alignment. The head-worn see-through display may include a hybrid optical system adapted to produce a main see-through field of view for the presentation of content with high resolution and a secondary see-through field of view for the presentation of content with lower resolution, wherein the main and secondary fields of view are presented proximate one another, a processor adapted to adjust the relative proximity of the main and the secondary fields of view, and an eye position detection system adapted to detect a position of an eye of a user, wherein the processor adjusts the relative proximity of the main and secondary fields of view based on the position of the eye of the user. The secondary field of view may be produced on a see-through OLED panel positioned directly in front of the eye of the user, on a see-through edge lit LCD panel positioned directly in front of the eye of the user, or on a see-through combiner positioned directly in front of the eye of the user. The relative proximity may be a horizontal proximity or a vertical proximity. The relative proximity may define a measure of overlap between the main and secondary fields of view or a measure of separation between the main and secondary fields of view. The eye position detection system may image the eye from a perspective substantially in front of the eye, as a reflection off a see-through optic in a region including the main field of view, or as a reflection off a see-through optic in a region including the secondary field of view. 
     When using head mounted displays (HMDs) (e.g. as part of an HWC  102 ) for purposes such as augmented reality imaging, it is desirable to provide a wide field of view (e.g. 60 degrees). However, in viewing a wide field of view with a head mounted display it should be recognized that viewing an image with a head mounted display is different than viewing an image on a rigidly mounted screen in the environment (e.g. a television mounted on the wall or a movie theater screen). With a head mounted display, as the user moves their head, the head mounted display and it&#39;s associated display field of view moves as well in relation to the surrounding environment. This makes it difficult for the user of an HMD to view the edge or corner of an image that is displayed with a wide field of view because head movements do not assist the user, eye movements alone must be used to view the corner of the image. To improve the viewing experience when using an HMD to view images displayed with a wide field of view, the relationship between eye movement and head movement that a person uses when viewing the surrounding environment should be substantially replicated. For example, a viewer would normally turn his head, at least somewhat, when viewing an image with a wide field of view on a rigidly mounted screen such as in a movie theater when looking towards an edge of the movie screen, as opposed to only moving his eyes towards the edge. The inventors have discovered that certain accommodations have to be made to provide comfortable and intuitive viewing of the areas towards the outer edges of a wide field of view in an HMD system. In embodiments, the content being displayed in the wide field of view may not necessarily be world-locked (i.e. where the position of the content in the field of view is dependent on an object&#39;s position in the environment such that the content appears to the user as positionally connected to the environment) but may still include a process that shifts a position of the presented content based on a position or motion of the user&#39;s eye or head. 
     Because a head mounted display is worn on the head of the user, compactness is important to provide a comfortable viewing experience. Compact optical system typically include short focal length optics with low f# to reduce the physical size. Optics with these characteristics generally require a wide cone angle of light from the image source. Where wide cone angles are associated with image sources that emit image light from their front surfaces as, for example, in small displays or microdisplays such as: OLED, backlit LCD, etc. These displays can emit unpolarized or polarized image light. The optical system receives the image light from the image source and then manipulates the image light to form a converging cone of image light that forms an image at the eye of the user with an associated wide field of view. To enable the user to simultaneously interact with the displayed image and the surrounding environment, it is advantageous to provide an undistorted and bright see-thru view of the surrounding environment along with a bright and sharp displayed image. However, providing an undistorted and bright see-thru view and a bright and sharp displayed image can be competing requirements, especially when a wide field of view image is being provided. 
     For the purpose of viewing augmented reality imagery, it can be desirable to provide a wide field of view of 50 degrees or greater. However, the design of compact optics with a wide field of view that is suitable for use in a compact head mounted display can be challenging. This is further complicated by the fact that the human eye is only capable of high resolution in a very narrow portion of the field of view know as the fovea and a much lower resolution at the periphery of the field of view. To observe the whole area of a high resolution image, a person must move their eyes over a wider field of view. 
     The inventor&#39;s have discovered that optical systems are needed that provide high transparency to the surrounding environment to provide an undistorted and bright view of the surrounding environment while also displaying bright and sharp images over a wide display field of view. To provide a comfortable viewing experience, the optical system should take into account how the user moves their eyes and their head to view the environment. This is particularly important when the user is viewing augmented reality imagery. 
     Systems and methods in accordance with the principles of the present invention provide an HMD which displays images with wide fields of view overlaid onto a see-through view of the surrounding environment, with an improved see-through view and a high contrast displayed image. An optical system is provided that includes upper optics comprised of an emissive image source (e.g. OLED, backlit LCD, etc.), one or more lenses and a stray light trap, and non-polarized lower optics comprised of a planar angled beam splitter and a curved partial mirror. The emissive image source provides image light comprised of one or more narrow spectral bands of image light. Wherein, one or more of the reflective surfaces on the beam splitter and the curved partial mirror is treated to reflect a majority of incident light within the narrow spectral bands and transmit a majority of incident light within the visible band thereby providing a bright displayed image and a bright see-through view of the surrounding environment (e.g. using a tri-stimulus mirror on the beam splitter). 
     A stray light trap is also provided to enable higher contrast images to be displayed in concert with a high transmission view of the surrounding environment. Where the stray light can come from various sources including: see-through light from the surrounding environment; image light that has been reflected back into the optics by the curved partial mirror; or light from below that has passed through the beam splitter. By trapping this stray light, the contrast of the displayed image as seen by the user is greatly improved. 
     A display operating mode is also provided for improved viewing of wide field of view images wherein the displayed image is laterally shifted within the display field of view in correspondence to movements of the user&#39;s head. Wherein the lateral shifting of the displayed image is triggered by detecting an eye movement followed by a head movement in the same direction. The displayed image is then laterally shifted in correspondence to and in an opposite direction to ensuing head movements. The purpose of this mode is to enable the user to view peripheral portions of the image without having to move their eyes to the full extent of the wide displayed field of view. Thereby the user views the wide field of view of the displayed image through a combination of eye movement and head movement to obtain a more comfortable viewing experience. 
     Systems and methods in accordance with the principles of the present invention provide a head worn display with a high transmission see-through view of the surrounding environment and a high contrast displayed image that is overlaid onto the see-through view of the surrounding environment. In this way, the systems and methods provide a head worn display that is well suited for use with augmented reality imagery because the user is provided with a bright and sharp displayed image while still being able to easily view the surrounding environment. The systems and methods also provide a wide field of view with a sharpness that corresponds to the acuity distribution of the human eye when typical eye movement and head movement is taken into consideration. Where the wide field of view head mounted display can provide a displayed field of view for example at least +/−25 degrees (50 degree included angle). In addition, compact optics are provided with reduced thickness to improve a compact form factor of the head worn display. Operating modes are provided that take into account the viewing conditions of the head worn display where the display is attached to the user&#39;s head. 
       FIG. 169  shows a cross sectional illustration of an example optics assembly  16900  for a head worn display. The optics assembly  16900  include upper optics  16903  comprised of an emissive image source  16910 , one or more lenses  16920  and a light trap  16930 , and lower optics  16907  comprised of an angled beam splitter  16950  and a curved partial mirror  16960 . The emissive image source  16910  provides image light  16940 , with image content, that is optically manipulated by the lenses  16920  and the curved partial mirror  16960  to form a wide field view that is presented to a user&#39;s eye in the eyebox  16970 . Where the eyebox is defined as the region wherein the user&#39;s eye can see the displayed image. The optics are folded to make the optics assembly  16900  more compact, so that the optics have a first optical axis  16946  that extends perpendicularly from the emissive image source  16910 . The angled beam splitter  16950  redirects a portion of the image light  16940  by reflection so that the image light  16940  passes out along a second optical axis  16943 . The curved partial mirror  16960  reflects a portion of the image light  16940  so that it passes back along the second optical axis  16943  and towards the eyebox  16970 . Simultaneously, scene light  16973  from the surrounding environment is transmitted by the curved partial mirror  16960  and the angled beam splitter  16950  to provide a see-through view of the surrounding environment to the eyebox  16970 . As such the curved partial mirror  16960  acts as a combiner wherein the user sees the displayed image provided by the image light  16940  overlaid onto the see-through view of the surrounding environment provided by the scene light  16973 . 
     The emissive image source  16910  can be any type of luminous display that doesn&#39;t require supplemental light to be applied (e.g. a transmissive front light as described herein elsewhere) within the upper optics  16903  including: an OLED, a backlit LCD, a micro-sized LED array, a laser diode array, edgelit LCD or a plasma display. Typically an emissive display provides image light with narrow wavelength bands of light within the visible range. For example, for a full color display the bands can include a red, green and blue band with full width halfmax (FWHM) wavelengths of 615-635, 510-540 and 450-470 nm respectively. In addition, the emissive image source  16910  provides a wide cone of image light (e.g. 100 or more degrees). There are a number of advantages associated with using an emissive image source  16910  that has a wide cone angle in that, the optical system can be designed with a shorter focal length and a faster f# (e.g.  2 . 5  or faster) which enables the optics to be much more compact. In addition, by eliminating the need for an illumination system to apply light to the front surface of the image source such as is typically required for a reflective image source like an LCOS or a DLP, the overall size of the upper optics can be reduced substantially. 
     In embodiments, to provide a high transmission (e.g. greater than 50% transmission of scene light to the eye) see-through view of the surrounding environment, the lower optics are a non-polarized design, wherein the optical surfaces allow some portion of unpolarized visible light to be transmitted. This is to avoid the greater than 50% losses of light that occur when an absorptive polarizer or reflective polarizer is used in transmission along the optical path of scene light  16973 . Instead, the reflective surfaces on the angled beam splitter  16950  and the curved partial mirror  16960  are treated to be partially reflective. Where the partially reflective treatment can be a base partial mirror that has a relatively uniform level of reflectivity across the entire visible range, or the partially reflective treatment can be a notch mirror that provides higher levels of reflectivity in one or more narrow wavelength bands within the visible range that have been selected to match the output bands of the emissive image source and higher levels of transmission in the wavelengths between the narrow wavelength bands (e.g. as described herein elsewhere). The partially reflective treatment can be a coating such as a multilayer coating, a phase matched nanostructure or a film such as a multilayer film or a coated film that has partial mirror properties or notch mirror properties. 
     By using non-polarized lower optics  16907  in the portion of the optics where a see-through view of the surrounding environment is provided, there is an added benefit in that chromatic abberations are avoided when viewing a polarized image source in the environment such as a liquid crystal television or computer monitor or natural sources like clouds and reflections that could be very distracting to the user. These chromatic abberations typically take the form of rainbow patterns with bright colors that can be very distracting to the head worn experience. The chromatic abberations are caused by interference between the polarized light of the polarized image source and any polarizers or circular polarizers that are present in the see-through portion of the optics. As a result, the systems and methods described provide non-polarized optics in the see-through portion of the optics to enable the user to view polarized image sources such as liquid crystal computer monitors without being exposed to rainbow patterns while wearing a head worn display. 
     With a high transmission see-through view of the surrounding environment, a high level of scene light  16973  passes through the lower optics on the way to the eyebox  16970 . This opens up the possibility for a loss of contrast in the displayed image due to stray light from a portion of the scene light  16973  being reflected by the angled beam splitter  16950  back to the emissive image source  16910 , and also from a portion of the image light  16940  being reflected by the angled beam splitter  16950  back toward the emissive image source  16910 . The combined stray light from the portions of the scene light  16973  and the image light  16940  being reflected back to the emissive image source  16910  is then scattered off of the sidewalls in the upper optics  16903  and reflected by the surface of the emissive image source  16910  so that it joins the image light  16940  that is presented to the eyebox  16970  for viewing by the user. Since this stray light does not have image content, the net effect is that the contrast in the displayed image is reduced. To reduce the stray light from these two sources, a light trap  16930  is provided. 
       FIG. 170  shows an illustration of the light trap  16930  operating to reduce stray light. The light trap  16930  is comprised of a sandwich structure including quarter wave films  17032  and  17034  on either side of a linear polarizer film  17033 . The sandwich structure can be loosely connected or laminated together with adhesive layers. The light trap  16930  functions by allowing unpolarized image light  17025  from the emissive image source  16910  to pass through quarterwave film  17032 , which doesn&#39;t affect the image light  17025  because it is unpolarized. The image light  17025  then passes through the polarizer  17033 , which causes the image light to become linearly polarized. The linearly polarized image light then passes through quarterwave film  17034 , which causes the image light to become circularly polarized image light  17026 . A portion of the circularly polarized image light  17026  is reflected toward the curved partial mirror  16960  by the angled beam splitter  16950 , while another portion of the circularly polarized image light is transmitted by the angled beam splitter  16950  to become faceglow. The curved partial mirror  16960  reflects a portion of the circularly polarized image light  17026  back toward the angled beam splitter  16950  while transmitting apportion that becomes eyeglow. From the circularly polarized image light  17026  that passes back toward the angled beam splitter  16950 , a portion is transmitted to the eyebox  16970  and a further portion is reflected by the angled beam splitter  16950  so that it passes toward the emissive image source  16910 . However when the returning circularly polarized image light  17026  passes through the quarterwave film  17034 , it is transformed into linearly polarized light with the opposite polarization orientation compared to the image light  17025  so that the polarizer  17033  absorbs the returning light. As such the portion of the image light that is reflected back toward the emissive image source  16910  can be essentially eliminated by the light trap  16930  considering that typical absorptive polarizers absorb approximately 99.99% of light with the opposite polarization state. 
     Scene light  17045  is unpolarized and is transmitted by the curved beam splitter  16960 . When the unpolarized scene light  17045  encounters the angled beam splitter  16950 , a portion is transmitted toward the eyebox  16970  to provide a see-through view of the environment and a portion is reflected toward the emissive image source  16910 . The unpolarized scene light  17045  passes through the quarterwave film  17034  unchanged. As the scene light passes through the polarizer  17033  it becomes polarized light. The scene light then becomes circularly polarized scene light  17046  as it passes through quarterwave film  17032 . The circularly polarized scene light  17046  is reflected by the surface of the emissive image source  16910 . This returning circularly polarized scene light  17046  is transformed into polarized scene light with an opposite polarization state when it passes back through quarterwave film  17032 , which is then absorbed by the polarizer  17033 . 
     The net effect of the light trap  16930  is that stray light from returning image light and scene light is essentially eliminated and as a result, the contrast in the displayed image is greatly increased. This is particularly important when using the head worn display in a bright environment where the incoming scene light  17045  can be substantial. By using a light trap  16930  with a sandwich structure comprised of quarterwave films  17032  and  17034  on either side of a linear polarizer film  17033 , stray light from unpolarized light  17025  and  17045  coming in opposing directions can be effectively trapped. The effect on the portion of the image light  17025  that is reflected by the angled beam splitter  16950 , is reflected by the curved partial mirror  16960  and is transmitted by the angled beam splitter  16950  so that it becomes the displayed image that is viewed by the user, is that this image light  16940  is circularly polarized light. In addition, since the image light  17025  passes through a polarizer film  17033 , there is a reduction in brightness of approximately 50%. However the increase in contrast is much higher, so that the perceived image quality of the displayed image is greatly improved especially in a bright environment. The inventors have performed measurements of the effectiveness of such a light trap positioned above an OLED display surrounded by a black textured plastic frame. Wherein the quarter wave film was selected to have a retardation level that provides excellent extinction of the stray light after it passes through the quarterwave film twice without imparting a color bias to the remaining stray light. The result was that light reflected from the OLED display surface was reduced by 117× and light reflected from the black textured plastic was reduced by 6×. 
     The light trap  16930  can also be simplified to be a circular polarizer by eliminating one of the quarter wave films. In this case, the light trap  16930  works on only one of the unpolarized stray light sources. If quarterwave film  17032  is eliminated, the light trap  16930  traps only stray light from the image light  17025  and the scene light  17046  reflected back toward the image source  16910  is then polarized. Alternately, if quarterwave film  17034  is eliminated, the light trap  16930  traps only stray light from the scene light  17045  and the image light  17026  is then polarized. 
     In an alternative embodiment, the light trap  16930  can be positioned on the surface of the image source  16910 . The light trap can be a polarizer  17033  sandwiched between quarter wave films  17032  and  17034  to trap stray light from both scene light  17045  and image light  17025  that is reflected back toward the image source  16910 . By positioning the light trap  16930  directly on the surface of the image source  16910 , stray light from scene light  17045  is trapped very efficiently because birefringence in the lenses  16920  don&#39;t affect the polarization state of the circularly polarized scene light  17046 . As such, the light trap  16930  can be a circular polarizer that is positioned on the image source  16910  with the quarter wave film of the circular polarizer against the surface of the image source  16910  to trap just the stray light associated with the scene light  17045  as previously described herein. The light trap  16930  can be sized to cover the surface of the image source  16910  in addition to covering adjacent reflective portions of the image source package or the adjacent housing to trap stray light associated with reflected light from these surfaces. 
     To trap stray light from image light  17025  that is reflected back toward the image source  16910 , a second circular polarizer (e.g. comprised of polarizer  17033  and quarter wave film  17034 ) can be positioned between the lenses  16920  and the lower optics, wherein the quarter wave film  17034  of the second circular polarizer is positioned to face the lower optics. The polarization axis of the first circular polarizer should be aligned with the polarization axis of the second circular polarizer to transmit the most image light  17025 . This second circular polarizer provides an efficient light trap for stray light from image light  17025  that is reflected by the partial mirror  16960  and the angled beam splitter  16950  back toward the image source  16910 . However, if a first and second circular polarizer are included, birefringence in the lenses  16920  in the upper optics will affect the brightness uniformity and contrast uniformity of the image seen by the user. This is because the image light  17025  will be polarized by the first circular polarizer, the image light will then pass through the lenses  16920  where any birefringence present will cause portions of the image light to become elliptically polarized. The elliptically polarized image light will then pass through the second circular polarizer where the elliptically polarized portions of the image light will be filtered in correspondence to the degree of elliptical polarization present. If the lenses  16920  have low birefringence (e.g. &lt;50 nm retardation), using two circular polarizers will provide an image with barely noticeable degradation of brightness uniformity and contrast uniformity, however if the birefringence is high then the brightness uniformity and contrast uniformity will be noticeably degraded. 
     Table 1, below, shows a comparative analysis of a variety of non-polarized partially reflective treatments for the angled beam splitter  16950  and the curved partial mirror  16960  where all the numbers are presented in terms of % of the image light  17025  emitted by the image source  16910 . This analysis shows the effects of using notch mirror treatments compared to base partial mirror (i.e. a partial mirror that reflects all visible wavelengths substantially equally) treatments on the angled beam splitter  16950  and the curved partial mirror  16960  along with the effects of the light trap  16930 . Phase matched nano-structures that reflect narrow wavelength bands of light can be provided as an embossed film or as a molded in structure on an optical surface, to provide a notch mirror treatment, but they are not shown in Table 1. In this analysis, the reflectivities of the angled beam splitter  16950  and the curved partial mirror  16960  have been chosen to deliver at least 50% “See-through light to the eye” (this is scene light  16973  that reaches the eyebox  16970 ) with at least 20% “See-through light at the wavelengths of the image light”, which takes into account the narrow band of reflectivity provided by any notch mirror treatments on the reflective surfaces. Case 1 includes triple notch mirror treatments (also known as a tristimulus notch mirror for reflecting narrow bands of red, green and blue light) to the angled beam splitter  16950  and the curved partial mirror  16960  and it does not include a light trap  16930 . In this analysis, the notch mirror was assumed to reflect at a selected reflectivity % within a 20 nm wide band for each color (for example the triple notch mirror can provide high reflectivity in the following bands: 450-470 nm for blue, 515-535 nm for green, 615-635 nm for red) and transmit the remaining visible light at 95%. Case 2 includes triple notch mirror treatments to the angled beam splitter  16950  and the curved partial mirror  16960  along with a light trap  16930 . Case 3 includes a base partial mirror treatment on the curved partial mirror  16960  and a triple notch mirror treatment on the angled beam splitter  16950  along with a light trap  16930 . Case 4 includes a base partial mirror treatment on the angled beam splitter  16950  and a triple notch mirror treatment on the curved partial mirror  16960  along with a light trap  16930 . Case 5 includes base partial mirror treatments on both the angled beam splitter  16950  and the curved partial mirror  16960  along with a light trap  16930 . 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Case Number 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Coating on beam splitter 
                 Tristim 
                 Tristim 
                 Tristim 
                 Simple 
                 Simple 
               
               
                   
                 notch 
                 notch 
                 notch 
                 partial 
                 partial 
               
               
                   
                 mirror 
                 mirror 
                 mirror 
                 mirror 
                 mirror 
               
               
                 Coating on curved partial 
                 Tristim 
                 Tristim 
                 Simple 
                 Tristim 
                 Simple 
               
               
                 mirror 
                 notch 
                 notch 
                 partial 
                 notch 
                 partial 
               
               
                   
                 mirror 
                 mirror 
                 mirror 
                 mirror 
                 mirror 
               
               
                 Quarterwave/polarizer 
                 No 
                 Yes 
                 Yes 
                 Yes 
                 20 
               
               
                 sandwich trap for light 
               
               
                 reflected back to display 
               
               
                 Beam splitter reflectivity image 
                 50 
                 50 
                 60 
                 30 
                 75 
               
               
                 light (%) 
               
               
                 Beam splitter transmission 
                 83 
                 83 
                 80.6 
                 65 
                 28 
               
               
                 overall (%) 
               
               
                 Curved partial mirror image 
                 80 
                 80 
                 33 
                 75 
                 67 
               
               
                 light reflectivity (%) 
               
               
                 Curved partial mirror overall 
                 75.8 
                 75.8 
                 62 
                 77 
                 15 
               
               
                 transmission (%) 
               
               
                 Reflectivity of display panel (%) 
                 15 
                 15 
                 15 
                 15 
                 15 
               
               
                 Image light to the eye 
                 20.0 
                 8.4 
                 3.3 
                 6.1 
                 1.8 
               
               
                 See-thru light to the eye 
                 62.9 
                 62.9 
                 50.0 
                 50.1 
                 50.3 
               
               
                 Eyeglow 
                 10.0 
                 4.2 
                 16.9 
                 3.2 
                 5.6 
               
               
                 Faceglow 
                 50.0 
                 21.0 
                 16.8 
                 27.3 
                 31.5 
               
               
                 Light from below reflected 
                 12.0 
                 12.0 
                 14.4 
                 30.0 
                 20.0 
               
               
                 toward eye 
               
               
                 Image light back to panel 
                 20.0 
                 0.00084 
                 0.000499 
                 0.000284 
                 0.00005 
               
               
                 See-thru light with image light 
                 10.0 
                 4.2 
                 15.6 
                 3.2 
                 5.6 
               
               
                 wavelengths back to panel 
               
               
                 See-thru light with image light 
                 1.5 
                 0.000063 
                 0.000234 
                 4.73E−05 
                 0.00008 
               
               
                 wavelengths back to panel and 
               
               
                 reflected back toward eye 
               
               
                 See-thru light with image light 
                 20.1 
                 20.1 
                 31.0 
                 25.2 
                 50.3 
               
               
                 wavelengths to eye 
               
               
                 Ratio image light to eye/image 
                 7 
                 10000 
                 6667 
                 21667 
                 37500 
               
               
                 light back to panel 
               
               
                 Ratio image light to eye/See- 
                 13 
                 133333 
                 14194 
                 130000 
                 20896 
               
               
                 thru light with image light 
               
               
                 wavelengths back to panel and 
               
               
                 reflected back into system 
               
               
                   
               
            
           
         
       
     
     The effects of the light trap  16930  on image contrast can be seen in the two rows at the bottom of Table 1 that relate to image contrast as shown by ratios of the “Image light to the eye”, which represents the displayed image brightness, divided by the “light back to the image source” where the light back to the image source comes from either the image light being reflected back to the image source or from scene light being reflected back to the image source. In both sets of numbers, the ratio is dramatically higher (1000× or more) in Cases 2-5 where there is a light trap  16930  compared to Case 1 where there is not a light trap. The light loss produced by having a light trap can also be seen in the numbers for the “Image light to the eye” wherein Case 1 shows approximately 2× higher numbers indicating a brighter displayed image. 
     The effects of the notch mirror treatments on the numbers for the “Image light to the eye” (image light  16940 ) and “See-through light to the eye” (scene light  16973 ) can be seen by comparing Cases 2-4 which have various combinations of tristimulus notch mirror treatments to Case 5 which has base partial mirror treatments on the angled beam splitter  16950  and the curved partial mirror  16960 . The tristimulus notch mirror treatment on one or both reflective surfaces increases the portion of image light  16940  that is delivered to the eyebox  16970  while also increasing the portion of scene light  16973  that is provided to the eye. Using base partial mirror treatments on both the angled beam splitter  16950  and the curved partial mirror  16960  reduces the efficiency of the optics to deliver image light to the user&#39;s eye by a factor of approximately 2× to 4.5×. It should be noted that if either the angled beam splitter  16950  or the curved partial mirror  16960  included a polarizer (absorptive or reflective), only about 42% of the scene light would be transmitted to the user&#39;s eye based on typical transmission % of unpolarized light by polarizers. And if one of the surfaces is a polarizer and the other is a 50% partial mirror, only about 21% of the scene light would be transmitted to the user&#39;s eye. 
     Other light losses are also shown by the numbers in Table 1. “Eyeglow” is the portion of image light  16940  that is transmitted by the curved partial mirror  16960 . “Faceglow” is the portion of image light that is transmitted downward by the angled beam splitter  16950 . The determination of which Case is better in terms of eyeglow and faceglow for a given head worn display will depend on whether there are other controls present to mitigate eyeglow or faceglow. If there are eyeglow controls present, then Case 3 may be the best choice because the faceglow is lower. If there are faceglow controls present than Case 4 may be the best choice because it has lower eyeglow. 
     In general, Case 2 with tristimulus notch mirror treatments on both the angled beam splitter  16950  and the curved partial mirror  16960  has a good combination of characteristics for providing a bright and high contrast image to the user&#39;s eye along with a high see-through transmission. This is because Case 2 has relatively good numbers for efficiency for delivering image light to the eye, high transmission see-through, low eyeglow, low faceglow, acceptable see-through at the wavelengths of the image light and excellent contrast. 
     Tristimulus notch mirror treatments can be obtained that reflect S polarized light more than P polarized light. However, given the narrow bands of reflection provided by the tristimulus notch mirror treatment, the transmitted portion of the light can be substantially non-polarized and as such still provide transmission of scene light that is over 50% and provide a view of polarized light sources that do not contain chromatic aberrations such rainbows. Under this scenario, Case 4 can be more efficient for delivering image light to the eye and providing high see-through transmission. 
     In many uses cases, such as for example augmented reality imaging, it is desirable to use a head mounted display that provides a wide field of view, e.g. greater than 40 degrees. However it can be difficult to design any type of optics that provide uniformly high MTF for a uniformly sharp image over the entire wide field of view. As a result, the optics can be very complicated and the physical size of the optics can become unsuitably large for use in a head mounted display. To avoid this problem, it is important to understand the acuity of the human eye in the peripheral portions of the field of view and to understand the angular range of eye movement typically used before a person moves their head. 
       FIG. 172  shows a chart of the acuity of a typical human eye relative to the angular position in the field of view (S. Anderson, K Mullen, R Hess; “Human peripheral spatial resolution for achromatic and chromatic stimuli: limits imposed by optical and retinal factors”, Journal of Physiology (1991), 442, pp 47-64). The fovea at the center of the human eye provides very high acuity over an angular range of approximately 2 degrees. The acuity then drops off rapidly as the angular position in the field of view (also known as eccentricity) increases. In addition, the chromatic acuity is substantially lower than the achromatic acuity. As shown in  FIG. 172 , the achromatic acuity goes from approximately 50 cycles/degree at the fovea to 5 cycles/degree at 15 degrees and the chromatic acuity goes from approximately 30 cycles/degree at the fovea to 3 cycles/degree at 15 degrees.  FIG. 173  shows a chart of the typical acuity of the human eye vs the eccentricity in a simplified form that highlights the dropoff in acuity with eccentricity along with the difference between achromatic acuity and chromatic acuity. 
     However, the acuity of the eye that is experienced by the user has to take into account the rapid movements of the eye within the field of view. These rapid movements of the eye effectively expand the high acuity portion of the field of view seen by the user. In an augmented reality application, movement of the head by the user must also be taken into account. When the user perceives an object near the edge of the eye&#39;s field of view, the user first moves their eyes toward the object and then moves their head. These combined movements enable the user to view a wider field of view while also making it more comfortable to view an object at the edge of the field of view by reducing the angular movement of the eyes. Human&#39;s tend to only move their eyes a limited amount before they move their head. 
       FIGS. 174A and 174B  show typical examples of eye movements and head movements given in charts showing angular movements in radians vs time for a variety of situations (A Doshi, M Trivedi; “Head and eye gaze dynamics in visual attention and Context Learning”, 2009 IEEE, 978-1-4244-3993-5/09, pp 77-84). As seen in the data given in the lower panel of  FIG. 174A , the user&#39;s head tends to move quickly after an eye movement to recenter the eye within the field of view so that the head and the eye have the same angle.  FIG. 174B  shows the converse situation in which the head moves first followed by an eye movement. Angular disparities between the eye and the head tend to be limited to less than approximately 0.25 radians (which is equal to approximately 15 degrees) except for very brief excursions. This is different from a head movement that occurs when a person reacts to a sound wherein the eyes and the head move together with minimal disparity, as in the top panel of  FIG. 174A . If the user wants to look at an object that is more than approximately 15 degrees from the direction the head is pointed, the user will first move their eyes and then move their head, as seen in the lower panel of  FIG. 174A , to reduce the angular disparity between the eyes and the head to less than 15 degrees to look at the object. This relationship between the movement of the eyes and the movement of the head is important to take into account when designing and operating a head worn display with a wide display field of view. Based on the acuity of the human eye and the movement of the eye relative to the movement of the head, sharp images with high resolution and high contrast are needed within the central +/−15 degree to +/−20 degree portion of the display field of view to provide the user with an image that is perceived as sharp and high contrast. This is the central region of the display field of view wherein the user will move their eyes to look at the image with the fovea. Outside of this region of the display field of view, the displayed image does not have to be as sharp because the user will not typically look directly at that region of the display field of view. Instead for example, to view an augmented reality object that is located 30 degrees from the center of the displayed field of view, the user will move their eyes approximately 15 degrees toward the object and then turn their head the remaining 15 degrees toward the object. If the augmented reality object is world locked (i.e. where the object is displayed in a constant position relative to real objects in the surrounding environment), as the user moves their head, the augmented reality object will move toward the center of the displayed field of view and as such it will move into the central sharp region of the display field of view. 
       FIG. 175  is a chart that shows the effective relative achromatic acuity, compared to the acuity of the fovea, provided by a typical human eye within the eye&#39;s field of view when the movement of the eye is included. Within the +/−15 degree portion of the field of view that is viewed with the fovea by moving the eyes, the relative acuity is equal to that provided by the fovea. Beyond the portion of the field of view that is viewed with the fovea, the acuity decreases at the rate associated with eccentricity in the eye as shown in  FIG. 173 . This acuity chart corresponds to the sharpness distribution that needs to be provided by a head worn display with a wide field of view. As long as the displayed image is provided with a relative sharpness that is above the acuity distribution shown in  FIG. 175 , the human eye will perceive the displayed image to be uniformly sharp. This is because when an image is presented with a field of view that is wider than the portion of the field of view that can be comfortably viewed by the fovea, the acuity of the eye is substantially decreased. For example, based on the acuity chart in  FIG. 175 , an image can be presented with a central sharp zone that is +/−15 degree to a +/−20 degree in size and as long as the image sharpness decreases to no less than 20% of the sharpness of the sharp zone by approximately +/−25 degrees, the image will be perceived by the user as being uniformly sharp.  FIG. 176  is a chart that shows the minimum design MTF vs angular field position needed to provide a uniformly sharp looking image in a wide field of view displayed image. In this figure the design MTF is given as a spatial modulation at 20% MTF relative to Nyquist, where Nyquist MTF is 100% and reduced MTF is less. The chart shows a uniform design MTF of 100% Nyquist across the central sharp zone (+/−15 degrees) and a rapidly decreasing design MTF in the peripheral zone (greater than 15 degrees). By providing a reduced design MTF in the outer portions of the angular field, the optics can be greatly simplified, thereby reducing cost and reducing the overall size of the optics. 
       FIG. 177  is a chart that shows the relative MTF needed to be provided by the display optics for a wide field of view display to provide a sharpness that matches the acuity of the human eye in the peripheral zone of the display field of view, wherein the resolvable sharpness for optics is determined to be the spatial frequency at which the MTF is 20%. In the figure, simple two point MTF curves (100% MTF and 20% MTF) are shown for a variety of angular field positions in the display field of view: 0 to 15 degrees (this is the top right curve), 20 degrees, 25 degrees, 30 degrees, 35 degrees, 40 degrees, 45 degrees and 50 degrees (this curve is the bottom left curve). These curves show the minimum MTF (from  FIG. 176 ) that needs to be provided across the display field of view to match the acuity of the human eye. As can be seen, this result shows that the MTF for wide field of view optics can drop off substantially in the outer portions of the display field of view. For example, the MTF of the wide field of view optics can be above 20% at the Nyquist frequency of the image source in the central sharp zone while the MTF can be much lower in the peripheral zone, such as 2% or 20% a t½ the Nyquist frequency. It should also be noted that since the chromatic acuity of the human eye is lower than the achromatic acuity, substantial lateral color (e.g. 5 pixels or more at 25 degrees) can be present in the peripheral regions of the wide field of view displayed image and the lateral color will not be noticeable. Thus, lateral color in the peripheral regions of the displayed wide field of view image contribute more to reducing the perceived sharpness of the image but the low acuity of the eye in the peripheral regions makes the loss in sharpness imperceptible. Similarly, the low acuity of the human eye in the peripheral regions makes distortion less perceptible in the peripheral regions. The combination of the loss of acuity and reduction in chromatic acuity that makes distortion less noticeable all add together to a reduce need for image quality in the peripheral regions of the display field of view. 
     As an example,  FIG. 171  shows an illustration of a simple optical system that provides a 60 degree display field of view (i.e. +/−30 degrees from center). This includes an emissive image source  16910 , a single lens element  16920 , an angled beam splitter  16950  and a curved partial mirror  16960  as previously described herein. The optical system provides a displayed image to eyebox  16970  with a displayed field of view of approximately 60 degrees included angle. Simultaneously, the user is provided with a see-through view of the surrounding environment through the angled beam splitter  16950  and the curved beam splitter  16960 , wherein the see-through field of view can be larger than the display field of view by enabling a view of the surrounding environment through areas adjacent to or extensions of the angled beam splitter  16950  and the curved partial mirror  16960 .  FIG. 178  shows a modeled MTF curve associated with the optical system of  FIG. 171  wherein MTF curves for a variety of different angular positions within the display field of view are shown. The MTF curve for the 15, 6 degree position (expressed in horizontal, vertical degrees within the field of view) in the display field of view is indicated with an arrow in  FIG. 178 , where it can be seen that the 15 degree MTF curve ends at 20% MTF at the Nyquist point for the image source which in this case corresponds to the right hand end of the spatial frequency axis or 75 cycles/mm. The MTF curves below the indicated 15.6 degree MTF curve are 30 degree MTF curves. For the 30 degree points in the display field to have the same perceived sharpness as the 15 degree point in the display field, according to  FIG. 176  the 30 degree MTF curve needs to be have at least 20% MTF at 7.5 cycles/mm (10% of Nyquist). It can be seen that all of the 30 degree MTF curves shown in  FIG. 178  are easily above 20% MTF at 7.5 cycles/mm point, so as such the image will be perceived as sharp in the peripheral regions by the human eye, when limited movements of the human eye are considered. Thus, even though the MTF curves shown in  FIG. 178  corresponding to the peripheral angular positions in the display field of view do not meet the Nyquist performance conditions for this display of 20% MTF at 75 cycles/mm, the peripheral points in the field of view will still be perceived by the user as providing the same level of sharpness as that provided by the central angular points in the field of view. 
       FIG. 179  is an illustration of a resolution chart wherein the sharpness of the image has been reduced by blurring the peripheral portion of the image to simulate an image from optics that provide a central sharp zone of +/−15 degrees with a peripheral zone that is less sharp. Looking directly at different portions of the image, it is can be seen that the outer portions  179100  are much less sharp than the central zone 17920. However, if the image is viewed at a distance where the central zone 17920 between the vertical bars occupies approximately +/−15 degrees in the viewer&#39;s field of view, the image will appear to be uniformly sharp to the outer edge as long as the viewer keeps their gaze inside the inner edge of the vertical bars. 
     As a result, the systems and methods described herein in accordance with the principles of the present invention can be used to design any type of optics for head mounted displays with a wide field of view including optics with a beam splitter, optics with a waveguide or projected optics with a holographic optical element, wherein a central sharp zone is provided that delivers a level of MTF that corresponds to the acuity of the fovea and a peripheral zone adjacent to the central sharp zone that provides a reduced level of sharpness in correspondence to the acuity of the human eye when limited movement of the eye is considered. In embodiments, the central sharp zone comprises a +/−15 degrees about the optical axis (30 degree included angle) and the peripheral zone extends beyond the central sharp zone to the edge of the field of view of the displayed image. The MTF in the central sharp zone should be above 20% at the Nyquist level of the display to provide a sharp image. The MTF in the peripheral zone can reduce with increasing angle at a rate that is less than the decrease in acuity of the human eye as the eccentricity increases. For example, if the peripheral zone extends from +/−15 degrees to +/−30 degrees (60 degree included angle), the MTF can be as low as 10% of the Nyquist spatial modulation at 20% MTF. By limiting the angular zone where high MTF is required and reducing the design MTF in the peripheral zone, the optics can include fewer elements and simpler elements with lower cost materials, thereby reducing the overall cost of the optics, in addition, the optics can be made more compact to enable the wide field of view optics to better fit into the head mounted display. This effect is shown by the compact optics shown in  FIG. 171  which as previously stated herein provide a 60 degree field of view while including a single plastic field lens, a beam splitter and a curved partial mirror. Wherein the treatments for the beam splitter and the curved partial mirror have been discussed previously herein to provide high see-through with a non-polarized lower to eliminate rainbows when looking at a polarized light source. And in addition, a light trap can be added to the compact optics to increase contrast as also discussed previously herein. 
     The systems and methods described herein in accordance with the principles of the present invention can be used for making compact optics for a head mounted display with a wide display field of view that has improved contrast and has a high transparency for the see-through view of the surrounding environment. By using an emissive display, the need for a frontlight is eliminated thereby reducing the space between the emissive image source and the lower optics. By limiting the high MTF zone to a central sharp zone surrounded by a lower MTF peripheral zone, the number of lens elements required to display a wide field of view is reduced, thereby also reducing the size of the optics. As shown in  FIG. 171 , a 60 degree field of view is possible with only one or two lens elements in the upper. As a result, the height of the optics can be reduced. 
     In embodiments, the emissive image source  16910  and the angular size of the display field of view are selected so that a single pixel in the emissive image source  16910  subtends an angle in the displayed image that is smaller than the achromatic acuity of the fovea of the human eye, so that black and white portions of displayed images don&#39;t have a pixelated look when viewed by the user. This provides the user with an image that has smooth lines and curves without the jagged look produced when individual black and white pixels can be resolved. For example, based on the data shown in  FIGS. 172 and 173 , the human eye has an achromatic acuity of approximately 50 cycles/degrees, for adjacent black and white pixels to not be separately resolvable in the sharp zone of a displayed image that includes 1920×1080 pixels (1080p), the displayed field of view should be less than 38×22 degrees or 43 degrees diagonal. 
     In embodiments, the emissive image source  16910  and the angular size of the display field of view are selected so that a single pixel in the emissive image source  16910  subtends an angle in the displayed image that is smaller than the chromatic acuity of the human eye, so that colored portions of displayed images don&#39;t have a pixelated look when viewed by the user. This provides the user with an image that has smooth lines and curves on colored areas without the jagged look produced when individual colored pixels can be resolved. For example, based on the data shown in  FIGS. 172 and 173 , the human eye has a chromatic acuity of approximately 30 cycles/degrees, for adjacent colored pixels to not be separately resolvable in the sharp zone of a displayed image that includes 1920×1080 pixels, the displayed field of view should be less than 64×36 degrees or 73 degrees diagonal. 
     In embodiments, the emissive image source  16910  and the angular size of the display field of view are selected so that the subpixels (typically each full color pixel includes adjacent red, green and blue subpixels, and the relative brightness of the subpixels together determine the perceived color of the pixel) that makeup each pixel in the emissive image source subtend an angle that is smaller than can be resolved by the human eye so that each pixel appears to be comprised of a single color and the subpixels are not visible to the user. This provides the user with an image is comprised of consistent blocks of colors without the speckled look that can be perceived when individual subpixels can be resolved. For example, based on the data shown in  FIGS. 172 and 173 , the human eye has an achromatic acuity of approximately 50 cycles/degrees, for the subpixels to not be resolvable in an image that includes 1920×1080 pixels, the displayed field of view is less than 115×64 degrees or 131 degrees diagonal. 
     In embodiments, the optics include a telecentric zone in the image light optical path wherein lens elements can be moved relative to one another to affect a change in focus distance without changing the magnification of the displayed image. Changes in focus distance can be accomplished in a variety of ways in a head mounted display by changing the spacing between optical elements. For example, focus adjustments can be accomplished by moving the image source in relation to the remainder of the optical system. However, in a display system with a wide field of view, the image light  16940  emitted by the emissive image source  16910  must be expanded in area to fill the area of the curved partial mirror  16960  which establishes the angular size of the display field of view as seen from the eyebox  16970  as shown in  FIG. 171 . To this end, the ray bundles between the emissive image source  16910  and the lens element  16920  are rapidly diverging (e.g. a 100 degree or more included angle). Because of the diverging ray bundles emitted by the emissive image source  16910 , any change in spacing between the emissive image source  16910  and the lens element  16920  done to change the focus distance or focus quality is accompanied by a change in the visual size of the displayed image seen by the user. In a head mounted display that is presenting augmented reality imagery, particularly when focus adjustments are done automatically as the user moves or as augmented reality objects move, it is important that the visual size of the augmented reality objects be consistent with the movements to provide comfortable viewing conditions for the user. Changes in the visual size of displayed image can also cause the image to be clipped by portions of the housing that are adjacent to the optics so that the edges of the displayed image are not viewable from the eyebox or the effective size of the eyebox is reduced. As such, the ability to makes changes in the focus distance for the displayed image or portions of the displayed image without changing the visual size of the image is an important feature for a head mounted display that is used to display augmented reality imagery. The telecentric zone can be provided in a number of locations within the optics such as between lenses in the upper optics or between the upper and lower optics.  FIG. 171  shows a telecentric zone  17140  between the upper and lower optics where the central rays in each ray bundle are parallel. Within this telecentric zone  17140 , focus adjustments can be made by moving the lens element  16920  and emissive image source  16910  as a first unit relative to a second unit comprised of the angled beam splitter  16950  and curved partial mirror  16960  to change the focus. As an example, for the optics shown in  FIG. 171 , a reduction in spacing between the upper optics  16903  and lower optics  16907  of 0.5 mm can provide a change in focus distance from infinity to 1 meter (this is the same as adding 1 diopter corrective lens behind the optics). This ability to adjust focus distance can be used to fine tune the sharpness of the displayed image for the user or to change the apparent distance that the displayed image is presented to the user. Where changes in the apparent distance of the displayed image can be used for augmented reality use cases where the displayed image is presented at a distance that matches an object in the environment or at a specific distance such as at arm&#39;s length. 
     Manual mechanisms such as screws or cams can be positioned to change the space in the telecentric zone by moving the relevant optical elements. Where manual adjustments are useful for adjusting focus during manufacturing or to enable user&#39;s to fine tune focus for their ophthalmic power prescription. Electronic actuators can be mounted to automatically adjust the spacing in the telecentric zone for augmented reality applications or for mode changes that include a change in focus distance. 
     In embodiments, a telecentric zone may not be provided or it may be only nearly telecentric and focal plane adjustments may be made by moving optical elements and also adjusting, digitally, the content to compensate for a magnification effect caused by the shifting elements in the non-telecentric zone. 
     In embodiments, a mode for viewing a wide angle displayed image (e.g. greater than 50 degrees included angle) with a head mounted display of any type is provided wherein the image is moved laterally within the display field of view in correspondence to a detected eye movement followed by a head movement by the user. This mode mimics the experience of sitting in the front row of a movie theater where to view the wide angle movie image, the viewer cannot comfortably view the whole movie screen with eye movement alone and instead must move their eyes along with their head to see the peripheral areas of the movie screen. To enable this mode, the head worn display requires apparatus for detecting eye movements that are associated with the optics assembly  16900 , along with an inertial measurement unit to detect head movement. As such, the mode detects the desire of the user to view a peripheral portion of the displayed image with the portion of the eye&#39;s field of view that has higher acuity, by detecting a movement of the eye followed by a movement of the head in the same direction. 
     The displayed image is then moved laterally across the display field of view in a direction that is opposite to the detected movements of the eye and head, wherein the magnitude and speed of the lateral movement correspond to the magnitude and speed of the detected movements of the eye and head. This lateral movement of the displayed image within the display field of view provides the user with an improved view of the peripheral portion of the displayed image by moving the peripheral portion of the displayed image into the central sharp zone of the display field of view and moving the peripheral portion of the displayed image into a position where the user&#39;s eye is relatively centered. In addition, the lateral movement of the displayed image within the display field of view can be limited to that needed to center the edge of the displayed image within the display field of view. This mode addresses the fact that it is uncomfortable for a user to move their eyes beyond an angle of approximately 15 to 20 degrees relative to their head for more than a short period of time and since head mounted displays are attached to the user&#39;s head, eye movement is the only way to visually look at different portions of the display field of view. This makes it difficult for a user of a head worn display to comfortably view an image that has a visual size of larger than a 30 to 40 degrees included angle. The disclosed mode overcomes this limitation, by detecting when the user would like to view a peripheral portion of a displayed image and then laterally moving the displayed image within the display field of view to a position where the peripheral portion of the displayed image can be more comfortably viewed and where the peripheral portion of the displayed image is displayed with improved sharpness and higher contrast. 
     By triggering the lateral movement of the displayed image within the display field of view based on the detection of a combined eye movement in a direction followed by a head movement in the same direction, the mode is different from a world locked or body locked presentation of the displayed image in which lateral movement of the image occurs in correspondence to head movement regardless of eye movement. A description of body locking of virtual objects in a head worn display is provided for example in US Patent Publication 2014204759. In embodiments, the lateral movement of the displayed image is limited within the display field of view to that required to position the edge of the displayed image in the center of the display field of view or some other comfortable point within the field of view. Another example wherein lateral movement of the image would not be wanted is when the user only momentarily looks towards an edge or corner (e.g. a warning light is blinking in the corner of the image and the user simply moves their eye momentarily to verify the blinking light). In this case, the user does not move their head and as a result lateral movement of the image is not triggered and the displayed image remains stationary within the display field of view. 
     After an eye movement above a predetermined threshold has been detected followed by a head movement in the same direction, the displayed image is laterally moved (note that the method can also be used in a corresponding way for transverse or radial movements of the displayed image within the display field of view) across the display field of view in correspondence to and in an opposite direction to the detected angular movement of the user&#39;s head. Eye movements can be detected for example with an eye camera (e.g. as disclosed herein elsewhere) that captures images of the user&#39;s eye while viewing the displayed image or by detecting changes in electric fields associated with the eye. Angular movements of the user&#39;s head can be detected relative to the world, relative to the user&#39;s body through a motion sensor (e.g. IMU), etc. Fixing the displayed image in relation to the environment is good for viewing a wide angle image when the user is sitting or standing still. Fixing the displayed image in relation to the user&#39;s body is good for viewing a wide angle image when the user is walking, running or riding in a vehicle. Angular movements of the user&#39;s head relative to the environment can be measured by, for example, either an inertial measurement unit in the head worn display or by image tracking of objects in the environment with a camera in the head worn display. Angular movement of the user&#39;s head relative to the user&#39;s body can be measured by a downward facing camera that can for example, capture images of a portion of the user&#39;s body. The images of the portion of the user&#39;s body are then analyzed to detect relative changes that can be used to detect movements of the user&#39;s head relative to the user&#39;s body. Alternatively, two inertial measurement units can be used to detect movements of the user&#39;s head relative to the user&#39;s body, wherein one is attached to the head worn display and one is attached to the user&#39;s body and differential measurements are used to determine movements of the user&#39;s head relative to the user&#39;s body. After an eye movement above the threshold has been detected and a movement of the user head above a threshold has been detected as following the eye movement, lateral movement of the displayed image across the display field of view is begun. The speed of the lateral movement of the displayed image is in correspondence to and in an opposite direction to the ensuing detected head movement. The lateral movement of the displayed image continues until either the edge of the displayed image reaches the center of the display field of view or the eye is detected to be looking at the center of the display field of view (or within a predetermined threshold of the center of the display field of view) thereby indicating that the peripheral portion of the image that the user wanted to look at has been reached. 
       FIGS. 180 and 181  are illustrations that show how the image is shifted within the display field of view as the user moves their head. Note that the user&#39;s head is shown to the side of the image, because the image is actually presented to the user inside the head worn display.  FIG. 180  shows an image  18055  centered within the display field of view and the user&#39;s head pointed straight ahead  18050 .  FIG. 181  shows the user&#39;s head pointed to the side  18150  and as a result, the image  18155  is shifted within the display field of view in a direction that is opposed to the movement of the user&#39;s head, thereby leaving a blank portion  18130  where there is now no image content to display. In  FIG. 182 , the blank portion of the display field of view  18230  where the image has been shifted away from is displayed as a dark region to enable the user to see-through to the surrounding environment in the blank portion. However, in different use cases it may be advantageous to display the blank portion as a neutral gray or a color. 
     In embodiments, the user of a wide field of view head mounted display is provided with an option to select the size (e.g. angular size) of displayed images associated with different images or applications. The displayed image is then resized to provide the selected angular image size for display to the user. For instance in a movie viewing mode, the user may choose the displayed image to be approximately 30 degrees in size which mimics the experience of sitting in the back row of a movie theater where it is comfortable for the user to view the entire displayed image with eye movements alone. Alternately, the user may choose the displayed image to be 50 degrees in size which mimics the experience of sitting in the front row of a movie theater where the displayed image needs to be viewed with a combination of eye movements and head movements with image shifting as previously described herein to comfortably view the entire displayed image.  FIG. 183  shows an illustration of a wide display field of view  18360 , wherein a user can choose to display a smaller field of view  18365  for a given image or application (e.g. a game) to improve the personal viewing experience. Where the smaller field of view  18365  enables the user to view the image or application without having to move their eyes as much to see the entire image. 
     In embodiments, the display format is selected to have a narrow vertical field of view relative to the horizontal field of view to enable the thickness of the optics to be reduced as measured across the lower optics. Due to the angled orientation of the angled beam splitter  16950  in the lower optics, the vertical field of view in the displayed image is directly proportional to the thickness of the optics assembly. For a given display field of view as measured along the diagonal of the display field of view, reducing the vertical field of view and thereby increasing the format ratio of the displayed image enables the thickness of the optics assembly to be reduced. For example, for a 16:9 format image with a 50 degree diagonal field of view the thickness  18410  of the optical assembly  18415  can be approximately 17 mm as shown illustratively in  FIG. 184 . If the format of the displayed image is increased to 30:9 with a 50 degree diagonal field of view, the thickness  18510  of the optical assembly  18515  can be approximately 10 mm as shown illustratively in  FIG. 185 . This represents approximately a 40% reduction in thickness of the optical assembly provided by changing to a higher format ratio.  FIG. 186  shows a 30:9 format field of view  18620  and a 22:9 format field of view  18625 , wherein the two fields of view have the same vertical field of view and different horizontal field of view. By using a higher format ratio, a wide field of view can be displayed for use with augmented reality imagery in a relatively thin head mounted display to improve the form factor of the head mounted display. The high format ratio can be obtained by using a high format ratio emissive display or by using a normal format ratio emissive display (e.g. 4:3, 16:9 or 22:9) and then using portions of the upper and lower regions of the emissive display. For example, the head mounted display can include a 1080p emissive display which has 1920×1080 pixels and a 30:9 image can be displayed by using 1920×576 pixels on the emissive display. A thin optics assembly would then be provided which was only capable of displaying an image comprised of the 576 pixels in the vertical direction, but the optics can display an image comprised of up to 1920 pixels horizontally. In the event that an image with a different format is to e displayed, it would be resized to fit the available display space (e.g. a 16:9 format image could be displayed as a 1024×576 pixel image). In a preferred embodiment, the display field of view has a format ratio that is greater than 22:9. By having a format ratio such as 30:9, the center portion can be used for displaying 22:9 image such as a movie, while the areas  18627  outside the 22:9 display field of view can be used for displaying auxiliary information that doesn&#39;t need to be as easily viewable or be presented with high resolution such as battery life, time, temperature, directional heading, whether new emails or texts are available. 
     In another embodiment, the central sharp zone of the display can be used to display different types of images than the outer peripheral zone. For example, the central sharp zone can be used to display  22 : 9  or  16 : 9  movie images that are resized to fit the number of pixels contained in the central sharp zone. The outer peripheral zone can then be used like a second display where other types of information are displayed that can be viewed at a lower resolution for a short period of time so that the uncomfortable eye position required is acceptable. 
     In yet another embodiment, the information displayed in the outer peripheral zone is rendered differently compared to the central sharp zone. This can include using larger font letters, higher contrast settings or different colors to make the information presented in the outer peripheral zone more easily viewable. 
     In a further embodiment, the displayed image is adjusted in correspondence to changes in the focus distance. To enable a measurement of the focus distance, a sensor may be provided to measure the distance between optical elements that are used to change the focus distance such as between the image source  16910  and the lens elements  16920  or between the lens elements  16920  and the lower optics. Wherein the displayed image can be digitally adjusted to be larger or smaller to compensate for magnification that may occur if the light rays between optical elements is not telecentric. The displayed image can also be digitally adjusted for distortion that may occur as the optical elements are moved to change the distance between the optical elements in accomplishing a change in focus distance. Where the change in focus distance may be associated with an augmented reality operating mode such as a mode where the focus distance needs to be at a specific distance such as for example at arm&#39;s length to allow the user to interact with displayed augmented reality objects. 
     In a yet further embodiment, the optical assembly is designed to provide telecentric light to an optical surface that includes a triple notch mirror treatment to reduce the angular extent of the incident light and thereby improve the performance of the triple notch mirror. Where the telecentric light can be incident onto the angled beam splitter or onto the curved partial mirror. This embodiment can be particularly important when the head worn display provides a wide field of view because triple notch mirror are designed to be used at a specific angle with a limited angular distribution around the specific angle. By providing telecentric light to the triple notch mirror, the color uniformity and brightness uniformity can be improved. In a further improvement, the wide angle displayed image can be rendered to compensate for radially based color and brightness rolloff by radially increasing the digital brightness (e.g. radially increase the code values and associated luma in the image) and radially changing the color balance (e.g. color rendering) in the image. In this way, the user is provided with an image that is perceived to have uniform brightness and uniform color in spite of angular limitations of the triple notch mirror treatment affecting the displayed image over the wide display field of view. 
     Although embodiments of HWC have been described in language specific to features, systems, computer processes and/or methods, the appended claims are not necessarily limited to the specific features, systems, computer processes and/or methods described. Rather, the specific features, systems, computer processes and/or and methods are disclosed as non-limited example implementations of HWC. All documents referenced herein are hereby incorporated by reference.