Abstract:
An eyeglass-mount display (EMD) device includes a frame, a display pod, and a personalized module removably coupled to the frame. The frame has a cross-bracket and a pair of ear pieces each coupled to an end of the cross-bracket. The display pod is mounted to the cross-bracket and includes an electronic image generator for generating an image and optics for creating a virtual image. The personalized module includes preset fitting adjustments specific to a particular user. The module may also include corrective optical lenses. The removable personalized module enables multiple users to share the same EMD frame and display pod without making numerous fitting adjustments upon donning the EMD.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to imaging display systems. More particularly, the present invention relates to an eyeglass-mount display system with a removable personalized module. 
     2. Related art 
     Convenient, high-quality and cost-effective remote imaging has become increasingly popular in the medical field during recent years. This is particularly true for surgical procedures, such as minimally invasive surgery, in which direct viewing of the surgical field by the surgeon is difficult. In minimally invasive surgery, a minimally invasive instrument, such as an endoscope or a laparoscope, is inserted into a patient through a body orifice or small incision. The minimally invasive instrument includes a video camera which enables the surgeon to view the surgical field. In a conventional surgical environment, the video camera transmits the video image via a cable to a conventional CRT video monitor. This arrangement is cumbersome in an operating room environment because equipment or surgical team members can obstruct the surgeon&#39;s view of the video monitor. In addition, room ambient illumination or surgical lighting can reduce the CRT display contrast, and the surgeon&#39;s viewing angle and distance from the CRT may not be favorable to quality vision and eye-hand coordination. 
     Head-mounted displays (HMDs) provide a solution to this problem. The image from the video camera of the minimally invasive instrument is transmitted to the HMD that the surgeon wears on his or her head. Thus, the HMD provides the surgeon with a direct, unobstructed view of the surgical field. 
     HMDs have become increasingly popular, but they are relatively expensive. HMDs used in the medical field require small but high resolution displays. In addition, many stereoscopic or binocular HMDs use dual display devices for two eye channels. One such medical stereoscopic HMD system having dual display devices is described in Heacock et al., “Viewing Ocular Tissues with a Stereoscopic Endoscope Coupled to a Head Mounted Display (HMD)” (visited Feb. 17, 1998) &lt;http://www.hitl washington.edu/publications/heacock/&gt;. Because these HMDs include two LCD displays, they are typically heavy, bulky, and expensive. 
     Due to the high cost of HMDs, several users may choose to share a single HMD. Because different users have different head dimensions and vision requirements, sharing a HMD requires each user make numerous adjustments to the HMD in order for the HMD to fit on an individual user&#39;s head properly and to avoid eye fatigue. These adjustments include adjusting for the spacing between each user&#39;s eyes, known as the inter-pupillary distance (IPD), as well as for the position of each user&#39;s eyes relative to his or her nose and ears. Requiring users to make these adjustments every time they don the HMD is both time consuming and complex. In addition, if the user fails to adjust the HMD properly, not only will the HMD be uncomfortable to wear, but it can also result in eye strain or eye fatigue. Furthermore, HMDs can be especially awkward and uncomfortable for users wearing corrective eyeglasses because the HMD must be worn over the corrective eyeglasses. Allowing for eyeglass wearers adds size and weight to the HMD design with resulting discomfort. 
     Thus, there is a need for an easily adjustable eyeglass-mount display (EMD) that can be shared among multiple users. The EMD should minimize the number of adjustments that each user is required to make each time he or she dons the device. In addition, there is a need for an EMD with a small but high resolution display so as to preserve peripheral vision. 
     The following references are commonly assigned with the present application and are incorporated herein by reference: 
     a. U.S. Pat. No. 5,926,318 titled “Biocular Viewing System with Intermediate Image Planes for an Electronic Display Device” issued to Raymond T. Hebert; 
     b. U.S. patent application Ser. No. 09/241,828, filed Feb. 1, 1999, entitled “Color Superposition, Mixing, and Correction for a Video Display System,” by Raymond T. Hebert; 
     c. U.S. patent application Ser. No. 09/305,092, filed May 3, 1999, entitled “Infrared Audio/Video Interface for Head-Mounted Display,” by Raymond T. Hebert et al.; 
     d. U.S. patent application Ser. No. 09362,927, filed Jul. 27, 1999, entitled “Color Superposition and Mixing of Light Beams for a Visual Display” by Raymond T. Hebert; and 
     e. U.S. patent application Ser. No. 09373,807, filed Aug. 13, 1999, entitled “Compact Biocular Viewing System for an Electronic Display,” by Raymond T. Hebert. 
     SUMMARY 
     In accordance with one embodiment of the invention, an eyeglass-mount display (EMD) apparatus includes a support frame, a display pod attached to the support frame, and a personalized module removably coupled to the frame. The frame has a cross bracket and a pair of spring-loaded ear pieces. Each ear piece is attached to an end of the cross bracket. The display pod is mounted on the cross bracket and includes an electronic image generator and optics for viewing a generated image. The display pod also includes an inter-pupillary distance adjustment and internal sighting mechanisms to aid proper image alignment for the user, thereby reducing long-term eyestrain. 
     The removable personalized module enables multiple users to share the same EMD frame and display pod without making numerous fitting adjustments upon swapping the display apparatus among each other. Some embodiments of the personalized module include a pair of corrective eye lenses that, if required, replace the user&#39;s normal corrective spectacles. The personalized module is fitted to a particular user by moving one or more integral adjustment mechanisms. The fitting adjustments accommodate the user&#39;s nose and ear heights in relation to his or her eyes. The adjustments also accommodate differences in eye level. An adjustable nose piece in the personalized module allows for horizontal and vertical adjustment of the display pod with respect to the user&#39;s eyes. Cams on the side of the personalized module adjust ear piece height, a movement that also moves the display pod with respect to the user&#39;s eyes. After donning the display apparatus, the user aligns the image generated in the display pod with his or her eyes by adjusting the nose piece and the ear pieces. 
     In accordance with the invention, each user first inserts his or her personalized module into the support frame and adjusts the nose piece and ear piece settings. The first user focuses the displayed image. Each user adjusts movable lenses in the display pod to accommodate his or her IPD and then makes fine fitting adjustments. 
     During use, when the EMD is swapped to a second user, the first user removes his or her personalized module, and the second user inserts their own preadjusted personalized module. The second user then adjusts the display pod for their IPD. But the second user need not adjust the fit or the focus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a support portion of a display apparatus in accordance with the invention. 
     FIG. 2 is another perspective view of the embodiment of the invention shown in FIG.  1 . 
     FIG. 3 is a side view of an embodiment of the invention showing a display pod attached to a support portion of a display apparatus. 
     FIG. 4 is an exploded perspective view showing a cross-bracket and latches. 
     FIGS. 5A and 5B illustrate how latches open and release a personalized module. 
     FIG. 6 is a perspective view of the interior of a spring housing. 
     FIG. 7 is an exploded view of an ear piece. 
     FIG. 8 is an exploded view showing several components of a personalized module. 
     FIG. 9 is a perspective view of a nose piece assembly. 
     FIG. 10 is an exploded view of the nose piece assembly shown in FIG.  9 . 
     FIGS. 11A and 11B are front and side views, respectively, of a nosepad support. 
     FIGS. 12A,  12 B, and  12 C are front, side, and top views, respectively, of a nosepad bracket. 
     FIGS. 13A,  13 B, and  13 C are front, side, and rear views, respectively, of a lock. 
     FIG. 14 is a front view of a nose pad. 
     FIG. 15 is a perspective view of a second embodiment of a nose piece assembly. 
     FIG. 16 is a cross-sectional perspective view showing the second embodiment of a nose piece assembly. 
     FIG. 17 is a top view diagram illustrating internal components of a display pod. 
     FIG. 18 is a split view diagram showing alternate positions of a carriage and lenses in a display pod. 
     FIG. 19 illustrates a sighting mechanism used in the right side of a display pod. 
     FIGS. 20A,  20 B, and  20 C illustrate reticle alignment patterns in a sighting mechanism. 
     FIG. 21 illustrates optical corrections within a user&#39;s field of view. 
    
    
     DETAILED DESCRIPTION 
     Some elements have been omitted from the drawings so as to more clearly show embodiments of the invention. In addition, some drawings are not to scale. 
     FIG. 1 is an exploded perspective view of the support portion of a display apparatus in accordance with the invention. A display pod for generating and displaying an image to the user, as described below, is omitted for clarity. As shown, support frame  8  has a cross-bracket  10  and two spring housings  12  and  14  attached to bracket  10 . Housings  12  and  14  are spring-loaded forward and pivot around hinges  16  and  18 , respectively. Support frame  8  also has ear pieces  20  and  22  that are attached to housings  12  and  14  respectively and pivot around hinges described in detail below. Ear pieces  20  and  22  are spring-loaded so as to provide inward pressure against the user&#39;s head. Soft silicone rubber pads  24  and  26  are attached to ear pieces  20  and  22  respectively for user comfort. Some embodiments use pads somewhat smaller than shown. A hook  28  is shown mounted on ear piece  20  and, along with a similar hook (not shown) on ear piece  22 , holds a behind-the-head support strap (not shown) to snugly hold the support portion against the user&#39;s head. Portions of cross-bracket  10 , housings  12  and  14 , and ear pieces- 20  and  22  are made as light as possible using, for example, injection-molded magnesium. 
     FIG. 1 also shows optional speaker  30  that is mounted to socket  32  on ear piece  20 . Speaker  30  is held in socket  32  by tabs  34  (one of which is not shown) that fit into slits  36  in rim  38 . Slits  36  are positioned such that tabs  34  are behind rim  38  to hold speaker  30  in place. When inactive, speaker  30  is rotated upwards to be adjacent ear piece  20 . To use the speaker, the user rotates speaker  30  downwards to cover the ear. When rotated downwards, contacts electrically connect speaker  30  to wiring inside ear piece  20 . In some embodiments a second speaker may be similarly positioned on ear piece  22 . 
     FIG. 1 further shows personalized module  50  that is rigidly and removably attached to cross-bracket  10 . In the embodiment shown, personalized module  50  includes blade  51 , right lens  52 , left lens  54 , ratcheted cam  56 , and adjustable nose piece  60 . A second cam positioned on the left side of blade  51  opposite cam  56  is not shown. Two spring-loaded latches engage lips  62  and  64  on blade  51  to hold module  50  in cross-bracket  10 . Each of these components is described in detail below. In some embodiments blade  51  is transparent plastic. In some embodiments in which the user does not require a vision prescription lenses  52  and  54  are omitted and blade  51  is a single solid piece across the openings for these lenses. 
     Of importance is that a single personalized module  50  is adjusted to fit an individual user. Each individual user will have his or her own personalized module. Therefore the EMD is quickly exchanged between users by the first user removing a first personalized module and the second user inserting a second personalized module. No delay occurs when the second user dons the display because many personalized fittings are contained in the second user&#39;s personalized module. The second user only adjusts for his or her inter-pupillary distance (IPD) as described below. 
     FIG. 2 is another perspective view of the embodiment of the invention shown in FIG.  1 . Nose piece assembly  60  is more clearly shown in FIG. 2, although a silicone rubber pad normally covering a portion of nose piece assembly  60  has been omitted for clarity. Nose piece assembly  60  is adjustable left, right, up, and down with respect to frame  8 . Left head strap hook  29  is shown opposite hook  28 , as is ratcheted cam  57  opposite cam  56 . Speaker  30  is shown connected to ear piece  20  in the activated position. 
     FIG. 3 is a side view of an embodiment of the invention showing display pod  100  attached to front surface  11   a  of cross-bracket  10 . In other embodiments display pod  100  is attached to top surface  11   b  of cross-bracket  10 . In one embodiment display pod  100  is attached using three screws to cross-bracket  10 . Display pod  100  contains an electronic video image generator and optics that are described in more detail below, and in the references cited above. Display pod  100  is also made as light as possible and in one embodiment is made of injection-molded magnesium or magnesium alloy. FIG. 3 shows housing  12  spring-loaded against ratcheted cam  56 . The user adjusts the angle between housing  12  and blade  51  by turning cam  56 , thereby adjusting the angle of ear piece  20  in relation to the viewing angle into display pod  100 . The left ear piece  14  (FIG. 1) is similarly adjusted using cam  57  (FIG. 2) on the left side of blade  51 , opposite cam  56 . 
     The EMD uses a three-point mounting system to properly position display pod  100  with respect to the user&#39;s eyes. The EMD rests on the user&#39;s nose and on each of the user&#39;s ears. Small changes in the position of the EMD on the user&#39;s head significantly alter the user&#39;s line of sight into optics within pod  100 . The nose and ears provide natural reference points for accurate visual alignment every time the user dons the display. 
     FIG.  4 . is an exploded perspective view showing cross-bracket  10  in more detail. Latch bracket  402  fits into housing  404  and is held in place by screws  406 . Two spring-loaded latches  408  are mounted on bracket  402 . Latches  408  are opened by pressing on buttons  410  that extend through holes  412  in housing  404 . As shown in FIGS. 5A and 5B, when buttons  410  are pushed down, latches  408  open and release lips  60  and  62  on blade  51  (FIG.  1 ). In some embodiments top surface  414  of housing  404  is flat so that buttons  410  may be simultaneously pressed by inverting the EMD and pressing it against a flat surface such as a table top. 
     FIG. 6 is a perspective view of the interior of spring housing  14 . As shown, housing  14  is connected to frame  10  using hinge pin  602 . Stop tab  604  is connected to frame  10  and stop post  606  is molded into housing  14 . A minimum angle ax between frame  10  and housing  14  is established when tab  604  and post  606  contact each other. The angle ax may be increased, and spring  608  provides tension against this increase. Spring  608  is mounted around support post  610 . One end  612  is anchored in housing  14  and the other end  614  rests against stop tab  604 . Referring again to FIG. 3, the angle between blade  51 , which is securely attached to cross-bracket  10 , and housing  12  is adjusted by turning cam  56  and a similar cam on the other side of blade  51  (not shown). A user adjusts his or her looking angle into display pod  100  by turning these cams. Each ear piece  20  and  22  may be independently adjusted because a user&#39;s ears are typically at different levels. In addition, a user&#39;s eyes are often different levels as well. Turning the cams compensates for any vertical displacement between the user&#39;s eyes and display pod  100 . Turning one cam individually, or both cams in opposite directions, rolls display pod  100  left or right with respect to the user&#39;s eyes. Simultaneously turning both cams in the same direction changes the pitch angle of pod  100  with respect to the vertical looking angle of the user&#39;s eyes. 
     FIG. 7 is an exploded view of left ear piece  22 . As shown, spring bracket  702  has upper hinge bracket  704  and lower hinge bracket  706  extending into ear piece  22 . Hinge pin  708  extends between brackets  704  and  706 , and spring  710  is positioned around pin  708 . One end  709  of spring  710  is connected to ear piece housing  712  and the other end  713  of spring  710  rests against lower hinge bracket  706 . Spring  710  is wound so as to produce a torsional force around pin  708 , thereby pulling ear piece  22  towards the user&#39;s head. Cover  714  protects the user from the interior of ear piece  22 , and rubber pad  26  is mounted on cover  714 . 
     FIG. 8 is an exploded view showing several components of personalized module  50 . As shown, lenses  52  and  54  are conventionally mounted in blade  51 . Lens  52  has a groove cut into the outer edge surface  802 . A tongue  804  is formed in the receptacle for lens  52  and fits into groove  802  when lens  52  is in place. Lens  52  is then held in place using wire or string  806  threaded along the bottom portion of groove  802  and through holes  808  in blade  51 . Lens  54  is similarly held in place. In some embodiments lenses  52  and  54  are ophthalmic plastic and are shaped as spectacle lenses to correct the user&#39;s vision and replace the user&#39;s spectacles during use. The prescription used is similar to one used for computer viewing, and is optimized for a 22-inch viewing distance. As described below, optics in pod  100  create an image at a 22-inch nominal distance from the user&#39;s eyes. This distance is close to the distance from the surgeon&#39;s eyes to the operating field. Thus a user does not change eye focus when looking between the image displayed in the optics, and the hands working in the surgical field. 
     Each lens  52  and  54  may contain two vision corrections. FIG. 21 represents a field of view  2100  that the user sees through left lens  54  when wearing the EMD. As shown, image  2102 , generated in display pod  100 , is positioned in the center of field  2100 . A portion of display pod  100  blocks the user&#39;s peripheral vision to the right of image  2102 . Lens  54  is given a prescription that allows the user to see at a 22-inch distance. Therefore, areas visible below, to the left, and above the image and display pod  100  are corrected to 22 inches. To enable the user to see at a far distance, however, lens  54  has a second vision correction. As shown, portion  2104  in field  2100  has a prescription for vision at infinity distance. In the embodiment shown, a clear, flexible plastic film  2106 , having approximately −1.0 diopter correction, is attached to lens  54  as depicted. Film  2106  allows the user to see at a distance. Other embodiments may use other custom-made configurations for dual vision correction. 
     Referring again to FIG. 8, blade  51  is shaped to accommodate adjustable nose piece assembly  60  (FIG.  1 ). Groove  810  is molded into blade  51  and a plurality of detents  812  are molded into the sides of groove  810 . Cross piece  814  closes across groove  810  and forms opening  816  into which nose piece assembly  60  is mounted. 
     FIG. 9 shows in more detail nose piece assembly  60  mounted in blade  51 , and FIG. 10 is an exploded view of nose piece assembly  60 . Nosepad bracket  902  rests in and slides vertically in groove  810  on the front side of blade  51 . Spring tabs  904  push into detents  812  to hold bracket  902  in a desired vertical position. 
     Nosepad support  906  is placed behind blade  51 . Spring latch tabs  908  on nosepad support  906  extend through opening  816  (FIG. 8) and clip to bracket  902 . Thus blade  51  is sandwiched between bracket  902  and nosepad support  906 . Nosepad support  906  is also spaced apart from nosepad bracket  902  using an alignment tab on bracket  902 , shown in detail below. 
     Nosepad support  906  slides horizontally on bracket  902 . A series of horizontal index detents  910  are molded into bracket  902  as shown. Horizontal spring index tab  912  pushes into horizontal index detents  910  to hold nosepad support  906  in a desired horizontal position. 
     Support bracket  902  and nosepad support  906  are locked into position using lock  914 . Front posts  915  of lock  914  fit in front of cross piece  814  (FIG. 8) to lock bracket  902  in place. When the user slides lock  914  upwards, vertical lock tabs  916  on posts  915  prevent spring tabs  904  on bracket  902  from moving inward and support bracket  902  is locked in its vertical position. Back posts  917  of lock  914  fit behind cross piece  814  to lock nosepad support  906  in place. Horizontal lock tabs  918  on posts  917  engage detents  920  on nosepad support  906 , and support  906  is locked in its horizontal position. Lock  914  is held in the lock position by spring catch  922  that engages a groove ( 1102  in FIG. 11A) in support  906 . When the user unlocks the nosepiece assembly, catches ( 1304 , FIGS. 13A and 13B) on the tops of posts  915  engage cross piece  814  to prevent lock piece  914  from falling out. 
     FIGS. 11A and 11B are front and side views, respectively, of nosepad support  906 . Shown are spring latch tabs  908 , spring index tab  912 , and horizontal lock detents  920 , as described above. Also shown is lock groove  1102 , into which spring catch  922  on lock  914  engages. Alignment groove  1104  is also shown. Nose pad holders  1106  hold the nosepad described below. In one embodiment nosepad support  906  is made of DUPONT® DELRIN® type 500 AF (20% Teflon PTFE fiber in acetal). In other embodiments support  906  may be made of other material such as plastic over molded spring steel. 
     FIGS. 12A,  12 B, and  12 C are front, side, and top views, respectively, of nosepad bracket  902 . As shown in FIG. 12A, spring latch tabs  908  of nosepad support  906  (FIGS. 11A and 11B) engage and slide along lips  1202 . Spring index tab  912  of nosepad support  906  (FIGS. 11A and 11B) engages horizontal index detents  910 . Spring tabs  904  push into vertical detents  812  of blade  51  (FIG.  9 ). Spaces  1204  exist between spring tabs  904  and bracket body  1206 . FIG. 12B shows alignment tab  1208  that fits into alignment groove  1104  in nosepad support  906  (FIGS.  11 A and  11 B). FIG. 12C shows the width of alignment tab  1208 . In one embodiment bracket  902  is made of DUPONT ® DELRIN ® type 500 AF. 
     FIGS. 13A,  13 B, and  13 C are front, side, and rear views, respectively, of lock  914 . When the user slides lock  914  upwards, vertical lock tabs slide into spaces  1204  on bracket  902  (FIG.  12 A), thereby holding bracket  902  in position with respect to blade  51 . Simultaneously, horizontal lock tabs  918  engage horizontal lock detents  920  (FIG.  11 A), thereby holding nose piece support  906  in position with respect to bracket  902 . FIG. 13B shows ridge  1302  on spring catch  922  that engages lock groove  1102  on nosepad support  906  when lock  914  is in the locked position. When the user wants to make an adjustment, he or she slides lock  914  downwards. Spring catches  1304  engage cross piece  814  (FIG. 8) to keep lock  914  from falling out of nose piece assembly  60 . Lock  914  has a cutaway portion  1306  to accommodate the user&#39;s nose. In one embodiment lock  914  is made of grade HF 1110  LEXAN®. 
     FIG. 14 is a front view of nose pad  1402 . Nose pad holders  1106  on nose pad support  906  (FIG. 11A) slide into holes  1404 . In the embodiment shown, nose pad  1402  is molded of conventional silicone rubber in a U-shape. When pad  1402  is spread apart and mounted on nose pad holders  1106 , the molded U-shape provides tension that holds the nose pad in place. Embodiments of the invention use nose pads of different shapes and thicknesses to accommodate the nose shapes and sizes of various users. When fitting the personalized module, each user chooses the nose pad that is most comfortable. 
     FIG. 15 is a perspective view of another embodiment of a nose piece assembly  1502 . As shown, nosepad support  1504  has two support pieces- 1506  extending downward to rest against the nose (soft pads have been omitted for clarity). Nosepad support  1504  rests against blade  51  and is held in position by lock piece  1508  that fits over both blade  51  and nosepad support  1504 . 
     FIG. 16 is a cross-sectional view showing nose piece assembly  1502  in more detail. A vertical row of bumps, such as bump  1602 , is formed on the back side of blade  51 . A horizontal row of detents (e.g., holes), such as detent  1604 , are formed in support  1504 . A raised portion, such as annular boss  1606 , surrounds the detents in support  1504 . Support  1504  is horizontally adjusted by moving it so that one bump in the vertical row of bumps on blade  51  is in one of the horizontal detents. Similarly, support  1504  is vertically adjusted by sliding it so that a particular bump in the vertical line of bumps is engaged in one of the horizontal detents. When support  1504  is in the desired position, the user slides lock piece  1508  down to hold support  1504  firmly against blade  51 . During adjustment, lock tab  1610  on lock piece  1508  slides in groove  1612  until reaching stop surface  1614  in support  1504 . Lock tab  1610  keeps nose piece assembly  1502  together during adjustment. 
     Display pod  100  houses the optics and a miniature electronic display device for producing virtual rectangular video images with a nominal diagonal of  12  inches at a nominal distance of  22  inches from the user&#39;s eyes. Persons skilled in the art will understand that images include text and graphics, as well as video pictures. Display pod  100  has a receiver (not shown) for receiving infrared signals from a remote transmitting system. These infrared signals contain the information to be displayed to the user. Details of the infrared system can be found in U.S. patent application Ser. No. 09305,092, referred to above. The present invention deals primarily with an adjustable and interchangeable EMD. Therefore only those elements of display pod  100  necessary for properly aligning the video image for the users is described. Further details of the internal optics of display pod  100  are described in the references cited above (e.g., U.S. Pat. No. 5,926,318) and incorporated herein by reference. Pod  100  is approximately 3.5 inches wide, 1.3 inches high and 1.5 inches deep. Its small size minimally impacts the user&#39;s peripheral vision. Thus, when a user wearing the EMD looks straight ahead, the user looks directly into display pod  100  (see FIG.  21 ). However, by looking up, down, or to either side, the user will be able to see around display pod  100 . 
     FIG. 17 illustrates the internal components of display pod  100 . An image is created by electronic image generator  1706 , shown in outline in order to more clearly illustrate the mechanisms, and optics  1708  direct the image along left and right folded optical centerlines, creating virtual images in intermediate image planes  1710  and  1712 , respectively. Left mirror  1714  and right mirror  1716  are nominally angled at 43.3 degrees to fold the optical centerlines of intermediate image planes  1710  and  1712  to coincide with the slightly convergent optical centerlines of eyepiece lenses  1718  and  1720 . To avoid eyestrain, this convergence is nominally set at 3.4 degrees for each eyepiece so that the visual centerlines nominally converge at the virtual image distance of 22 inches. 
     Display pod  100  includes a biocular viewing system having a movable left eyepiece lens  1718  and a movable right eyepiece lens  1720 . Lenses  1718  and  1720  are located within the housing of display pod  100  and slide along tracks and grooves (omitted for clarity) in the housing. In some embodiments each lens is surrounded by a plastic cup and is sealed to the housing to prevent foreign matter, e.g. dust, from entering display pod  100 . FIG. 3 shows the position of lens  1720  in relation to blade  51 . Lenses  1718  and  1720  may be moved closer together or farther apart to accommodate the IPD of the user&#39;s eyes. To maintain focus, however, it is important that the total optical path length between the respective intermediate image planes and the eyepiece lenses be kept constant as the eyepiece lenses move. Thus, optics assembly  1708  must move in fixed relationship to eyepiece lenses  1718  and  1720 . 
     To meet this requirement, optics  1708  are mounted on carriage  1722  that is movable toward and away from the user&#39;s eyes in display pod  100 . Flexible metal bands  1724  and  1726  couple carriage  1722  to eyepieces  1718  and  1720 , respectively. In the embodiment shown, coupling is done by punching holes in the metal bands and inserting molded plastic index tabs on the carriage and lenses into the holes. Metal band  1724  is routed through channel  1728  and metal band  1726  is routed through channel  1730 . Channels  1726  and  1728  are formed using TEFLON®/acetal bearing surfaces in molded plastic parts. Therefore, as carriage  1722  moves forwards and backwards along its track, lenses  1718  and  1720  move inward and outward in a direction approximately orthogonal to the movement of carriage  1722 . This coupled movement effectively eliminates the need for refocus when the IPD changes, such as when a new user dons, adjusts, and uses the EMD. 
     Lead screw  1732  is coupled using mating threads molded into carriage  1722 . Knob  1734  is attached to the outer end of screw  1732  so that the user can easily turn screw  1732  while wearing the EMD to accommodate his or her IPD. FIG. 3 provides another view of knob  1734 . In other embodiments a drive pin or other arrangement may be provided to move carriage  1722 . 
     FIG. 18 is a split view diagram showing alternate positions of carriage  1722  and lenses  1718  and  1720 . FIG. 18 illustrates the full range of IPD settings for display pod  100 . The left side of the drawing depicts an optical center of eyepiece lens  1718  at a farthest distance D MAX  to centerline  1802  of display pod  100 . The right side of the drawing depicts an optical center of eyepiece lens  1720  at a closest distance D MIN  to centerline  1802 . Both D MAX  and D MIN  values are one half of the maximum or minimum IPD setting, respectively. When lenses  1718  and  1720  are at the maximum IPD setting, as indicated by the left half of FIG. 18, carriage  1722  is closest to the user&#39;s eyes. When lenses  1718  and  1720  are at the minimum IPD setting, as indicated by the right half of FIG. 18, carriage  1722  is farthest from the user&#39;s eyes. Eyepiece lenses  1718  and  1720 , along with carriage  1722 , may be positioned anywhere between the maximum and minimum IPD settings by turning screw  1732  using knob  1734 . 
     It can be seen that the optical path length remains essentially constant between intermediate image planes  1710  and  1712  and the lenses  1718  and  1720 , respectively, as the IPD is adjusted. When the IPD is at maximum setting, as illustrated in the left half of FIG. 18, the optical path along optical centerline  1740  is the sum of the distance from image plane  1710  to mirror  1714  and from mirror  1714  to lens  1718 . When the IPD is at minimum setting, as illustrated in the right half of FIG. 18, the optical path along optical centerline  1742  is the sum of the distance from image plane  1712  to mirror  1716 , and from mirror  1716  to lens  1720 . The length of optical paths  1740  and  1742  is essentially equal. 
     In other display devices, any available adjustments are often made intuitively, without the benefit of any visual target reference. Since the human vision system can briefly accommodate some vertical misalignment and a fair amount of horizontal misalignment, a user can easily misalign such display devices and endure these errors for a short period of time before noticing eye fatigue and other related discomforts. Accordingly, the present invention uses sighting mechanisms to provide visual references to avoid optical misadjustment. 
     FIG. 19 illustrates the sighting mechanism  1900  as used in the right side of display pod  100 . A similar mechanism is used in the left side. Sighting mechanism  1900  allows the user to properly adjust his or her lines of sight to an intermediate image plane, e.g. image plane  1712 . As shown, light source  1902 , e.g., a LED, is located behind sighting block  1904 , which is preferably molded plastic. Light source  1902  is activated by a conventional push-button switch (not shown) so that sighting alignment will not be distracting during normal use. Sighting block  1904  is mounted above intermediate image plane  1712  so as to have an optical axis different from path  1742  (FIG.  18 ). Sighting block  1904  has a front surface  1906  and a back surface  1908 . Reticle image  1910  is placed on front surface  1906  and reticle image  1912  is placed on back surface  1908 . Reticle images  1910  and  1912  are contrasting patterns. Sighting block  1904  is placed such that when the user&#39;s line of sight is aligned with the center of image plane  1712 , reticle images  1910  and  1912  will be aligned and coincident. Therefore, sighting block  1904  acts as a proxy for aligning the user&#39;s actual line of sight with the center of intermediate image plane  1712 . 
     The parallax effect of the distance between reticles  1910  and  1912  alerts the user that his or her line of sight is improper and that an adjustment is required. Thus FIG. 20A illustrates a condition when the user&#39;s line of sight is both horizontally and vertically misaligned. FIG. 20B illustrates a horizontal misalignment only. FIG. 20C illustrates a condition when the user&#39;s line of sight is properly aligned. Although the sighting mechanism is shown using an optical axis different from the normal viewing axis, some embodiments may-use a sighting mechanism coincident with the viewer&#39;s normal viewing axis. 
     Referring again to FIG. 17, sighting mechanism  1900  is shown for the right side of display pod  100 . The left side has a similar, mirror-image configuration. As shown, sighting block  1904  is placed such that front surface  1906  is coplanar with image plane  1712 . Back surface  1908  is shown opposite front surface  1906 . Reticles (not shown) are placed on surfaces  1906  and  1908 . The user aligns the reticles by making nose and ear adjustments on personal module  50  (FIG.  1 ). 
     The following acts illustrate a user&#39;s first-time adjustments made in conjunction with sighting mechanisms  1900  while wearing the EMD: 
     1. Insert personalized module  50  into frame  10 . Adjust the cams to their mid-range point. 
     2. Turn on the light sources for the left and right sighting mechanisms. 
     3. Adjust the IPD setting by turning knob  1734  until both left and right sighting mechanisms are at least marginally visible. 
     4. Release the lock on the adjustable nose piece and adjust the nose piece vertically until the left and right sighting mechanism reticles are, on average, vertically balanced. For example, adjust until the left reticles are misaligned low by an equal distance as the right reticles are misaligned high. 
     5. Adjust the nose piece horizontally until the left and right sighting mechanism reticles are, on average, horizontally balanced. For example, adjust until the left reticles are too far left and the right reticles are too far right by an equal distance. 
     6. Lock the nose piece into position. 
     7. Adjust one cam, for example the right cam, until the reticles in the sighting mechanisms show that no roll exists. That is, both left and right reticles have the same vertical position. If additional roll adjustment is required, adjust the other cam, for example the left cam, as well. If the setting in step  4  above was correct, the vertical alignment will be proper. 
     8. Adjust the IPD setting by turning knob  1734  until both the left and right reticles are horizontally aligned. 
     9. Turn off the light sources for the sighting mechanisms. Once the initial adjustments are made, the user may remove his or her personalized module and a second user will follow the above steps. 
     The following acts illustrate actions taken when the EMD is to be exchanged from the first user to the second user during operation: 
     1. First user doffs the EMD and removes his or her personalized module. 
     2. Second user inserts his or her personalized module and dons the EMD. The second user&#39;s personalized fittings for nose and ears, and corrective eyeglass lenses if required, are contained in the second personalized module. 
     3. Second user turns on the light sources for the sighting mechanisms and turns knob  1734  to adjust for his or her IPD. 
     4. Second user turns of the light sources. 
     The interconnection of carriage  1722  and lenses  1718  and  1720  eliminates the need for the second user to refocus the image in display pod  100  when adjusting for his or her IPD. The internal sighting mechanisms guarantee that the user&#39;s line of sight will extend to the center of intermediate image planes  1710  and  1712 , thereby maximizing the user&#39;s light box and reducing eye fatigue. 
     Some embodiments of the EMD may include additional features. The EMD may include a battery pack (not shown) to provide power to display pod  100 . The battery pack may be mounted to the head strap (not shown) connecting the ear pieces so that the battery pack&#39;s weight counterbalances the weight of display pod  100 . The EMD may also include a microphone attached, for example, to frame  10  or display pod  100 . And FIG. 1 shows speaker  30  mounted on right ear piece  20 , but a second speaker may be mounted on left ear piece  22  as well. 
     The present invention has been described with reference to specific embodiments. These embodiments are illustrative of the invention and are not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the following claims.