Patent Publication Number: US-6671100-B1

Title: Virtual imaging system

Description:
This application claims the benefit of provisional application Ser. No. 60/159,685, filed Oct. 14, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to a virtual imaging system, and more particularly, to a virtual imaging system suitable for use in a head-mounted imaging system. 
     BACKGROUND OF THE INVENTION 
     A virtual imaging system is a system in which a user views a virtual image of a display (or object) rather than the physical display itself. A typical virtual imaging system including a display, hereinafter referred to as “virtual display”, is shown in FIG. 1 in which a user&#39;s eye  2  looks through a lens  4  and sees a virtual image  6  of a physical display  8 . 
     In a virtual display, it is possible to create the appearance of a large display at a comfortable viewing distance from a user&#39;s eye. Recent developments have been made in microdisplays such that XGA (extended Graphics Array) computer screens can be made at lower cost on silicon chips having an area of approximately 1 cm 2 . It is highly desirable to provide a small virtual display system that can take advantage of such a microdisplay in various applications, such as a head-mounted, hand-held, body-worn, or other type of virtual display system. 
     In particular, a head-mounted display system is a virtual display system that is mounted on a user&#39;s head-and projects an image for one or both eyes. Because a head-mounted display does not restrict a user&#39;s movement, it offers a great potential for various practical uses, such as for viewing time and date, traffic and stock reports, or even e-mails. However, creating head-mounted displays typically involves tradeoffs between the following desirable factors: low weight, large field of view, large eye relief, large eyebox, and compact design. Ideally, one would prefer to have a head-mounted display that is no more intrusive than sunglasses and is capable of having a style desirable for consumers. The present invention provides a virtual imaging system suitable for forming, among other things, such an ideal head-mounted display. 
     SUMMARY OF THE INVENTION 
     The present invention offers a virtual imaging system that provides a user with an extended range of viewing (i.e., an enlarged “eyebox”). The system allows a user to view a virtual image of an object field, which may be of a physical display or other objects. The system includes an imaging subsystem including at least one lens. The imaging subsystem is arranged such that its object field is at or near its focal point, thereby positioning the virtual image of the object field at or near infinity. In one embodiment, the imaging subsystem also comprises an image generator that is separated from the lens by approximately the focal length of the lens. The system further includes an eyebox spreader that is arranged to receive the light transmitted from the imaging subsystem and to redirect the light to a user&#39;s eye. The eyebox spreader is adapted to effectively increase an eyebox of the imaging subsystem, i.e., the lateral range through which the user can see the complete virtual image. This eyebox spreading feature allows a user to more easily position himself to view the virtual image while at the same time allowing the virtual imaging system of the present invention to be compactly constructed and light in weight. The eyebox spreader requires that the virtual image of the imaging subsystem be positioned at or near infinity to project a clear image of the object field to the user&#39;s eye. 
     Various embodiments of an eyebox spreader for effectively increasing an eyebox of an imaging subsystem are disclosed in accordance with the present invention. In one embodiment, an eyebox spreader comprises a Fresnel surface. A Fresnel surface defines an array of parallel, optically flat facets thereon. Light transmitted from the imaging subsystem strikes the facets on the Fresnel surface and is either reflected therefrom or transmitted (e.g., refracted) therethrough to be redirected to a user&#39;s eye, while increasing its transverse “width”. A Fresnel surface may be provided on a thin substrate to form a Fresnel prism, or a Fresnel surface may be provided on a prism. Further, a plurality of Fresnel surfaces may be combined so that the facets of each Fresnel surface are offset from the facets of its adjacent Fresnel surface(s). 
     In a further embodiment, each of the facets on a Fresnel surface includes a first portion of a first reflectivity (50%, for example) and a second portion of a second reflectivity (100%, for example) to each form a beamsplitter. The first portions of the beamsplitters are adapted to partially transmit the light received from the imaging subsystem, while partially reflecting the received light toward a user&#39;s eye to form a first series of wavefronts. The second portions of the beamsplitters are adapted to receive the light transmitted through the first portions of the beamsplitters and to at least partially reflect the received light toward the user&#39;s eye to form a second series of wavefronts. The first and second series of wavefronts are alternately combined to form a contiguous wavefront. In other words, the second series of wavefronts fills in the gaps created by the first series of wavefronts, thereby eliminating dark gaps that the user&#39;s eye may otherwise see. 
     The present invention thus discloses a method of spreading an eyebox of a virtual imaging system used for a user to view a virtual image of an object field. According to the method, a virtual image of the object field is imaged by the imaging subsystem at or near infinity, so that the wavefront of each object field point is planar, with a transverse width defined by the aperture of the imaging subsystem. Next, the wavefront is sequentially sliced into a plurality of light ribbons from the transverse width of the wavefront. Finally, the plurality of light ribbons are redirected toward a user&#39;s eye so that the plurality of light ribbons will be separated along a collective transverse width of the plurality of light ribbons. The collective transverse width of the plurality of light ribbons is now greater than the transverse width of the original wavefront, thus the eyebox of the virtual imaging system is effectively increased. 
     In one aspect of the present invention, an eyebox spreader of a virtual imaging system may be configured to allow for light transmission therethrough. This “see-through” eyebox spreader may be suitable for use in constructing a head-mounted display system including a display, so that a user can see the real world through the eyebox spreader while also being able to view a virtual image of the display thereon. In this case, a virtual image of the display will be superimposed on the real-world image. 
     In another aspect of the present invention, a virtual imaging system including a display may further include an eye view switch adapted for activating the display only when the user&#39;s eye is viewing the display. In one embodiment, the eye view switch comprises an infrared light source, an infrared sensor, an infrared beamsplitter, and a dichroic beamsplitter. The light transmitted from the display is directed by the dichroic beamsplitter to the eyebox spreader, and then to the user&#39;s eye. The infrared light transmitted from the infrared source is directed by the infrared beamsplitter and by the dichroic beamsplitter to the eyebox spreader, and then to the user&#39;s retina. The infrared light reflected from the user&#39;s retina reflects from the eyebox spreader and is directed by the dichroic beamsplitter and by the infrared beamsplitter to the infrared sensor. The display includes a plurality of view field points, and the infrared sensor includes a plurality of sensor positions. There is a one-to-one correspondence between each view field point of the display and each sensor position of the infrared sensor. The display is adapted to be activated when any of the sensor positions of the infrared sensor detects infrared energy reflected from the user&#39;s eye, i.e., when the infrared sensor detects an eye view-angle directed to the display. 
     In a further aspect, a virtual imaging system of the present invention may be incorporated in a head-mounted virtual imaging system in the form of glasses to be worn by a user. The head-mounted virtual imaging system includes frames, and a virtual imaging system of the present invention mounted on the frames. As before, the virtual imaging system includes an imaging subsystem and an eyebox spreader. In one embodiment, the imaging subsystem includes a display located in the object field of the imaging subsystem, a display controller for supplying information to the display, and a battery for powering the display controller. For example, the display controller may provide information such as time, date, sensed data such as user&#39;s pulse, stored data such as addresses, and notification data such as “cell phone ringing”. 
     By incorporating an eyebox spreader to effectively increase the eyebox, the present invention permits lowering the cost and also the size of a virtual imaging system. Further, the eyebox spreader redirects the virtual image in a way that conforms to the desired shape and form of eyeglasses, thereby improving design of head-mounted displays. Indeed, a compact, lightweight, and high-performance virtual imaging system of the present invention may ideally be used in a head-mounted virtual imaging system that “wraps” around a user&#39;s head, such as a head-mounted display system in the form of sunglasses. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram illustrating the concept of a virtual imaging system; 
     FIG. 2A is a schematic diagram illustrating an embodiment of a virtual imaging system of the present invention, including an imaging subsystem and an eyebox spreader in the form of a Fresnel surface defining an array of parallel, optically flat facets thereon; 
     FIG. 2B illustrates a display that may be located in the object field of the imaging subsystem of FIG. 2A; 
     FIG. 2C illustrates a scanner-based display that may be provided to form animage in the object field of the imaging subsystem of FIG. 2A; 
     FIG. 2D is a schematic illustration of a wavefront of a light beam exiting the lens of the imaging subsystem of FIG. 2A; 
     FIG. 2E is a schematic illustration of wavefronts of beamlets exiting the eyebox spreader of the virtual imaging system of FIG. 2A; 
     FIG. 3 is a diagram illustrating the concept of eyebox, eye relief, and field of view (FOV); 
     FIG. 4 is a diagram illustrating the lens formula relationship for a virtual imaging system; 
     FIG. 5 is a diagram illustrating the concept of a planar wavefront; 
     FIG. 6 is a schematic diagram illustrating the operation of a Fresnel surface-based eyebox spreader for use in a virtual imaging system of the present invention; 
     FIG. 7 illustrates an alternative embodiment of an eyebox spreader for use in a virtual imaging system of the present invention, wherein the eyebox spreader is formed with a Fresnel prism&#39;s back surface facing toward incident light; 
     FIG. 8 illustrates yet another embodiment of an eyebox spreader for use in a virtual imaging system of the present invention, wherein the eyebox spreader comprises a Fresnel faceted surface provided on a right-angle prism; 
     FIG. 9 illustrates a variation of the eyebox spreader of FIG. 8, wherein a Fresnel prism and a right-angle prism are combined together with the faceted surface of the Fresnel prism facing the right-angle prism; 
     FIG. 10A schematically illustrates a further embodiment of an eyebox spreader suitable for use in a virtual imaging system of the present invention, wherein each of an array of facets on a Fresnel surface includes plural portions with different reflectivities to each for a beanisplitter; 
     FIG. 10B is an enlarged schematic view of a portion of FIG. 10A; 
     FIG. 10C is s schematic illustration of a wavefront of a light beam exiting the lens of the virtual imaging system incorporating the eyebox spreader of FIG. 10A; 
     FIG. 10D is a schematic illustration of wavefronts of light beamlets exiting the eyebox spreader of FIG. 10A; 
     FIG. 10E is a schematic illustration of a modification of the eyebox spreader of FIGS. 10A and 10B, wherein each of the beamsplitters includes four portions with different reflectivities; 
     FIGS. 11A-11D illustrate a method of forming beamsplitters having plural portions with different reflectivities; 
     FIG. 12 illustrates an alternative method of forming a beamsplitter having plural portions with different reflectivities, using an oblique evaporative coating technique; 
     FIG. 13 illustrates a still further alternative embodiment of an eyebox spreader suitable for use in a virtual imaging system of the present invention, wherein the eyebox spreader includes a plurality of Fresnel surfaces arranged in an offset manner to produce random reflections; 
     FIG. 14 is yet another alternative embodiment of an eyebox spreader suitable for use in a virtual imaging system of the present invention, wherein the eyebox spreader includes one beam splitter and one mirror; 
     FIG. 15 illustrates use of an adjustment lens to adjust the depth of focus of a virtual image, which may be incorporated in a virtual imaging system of the present invention; 
     FIGS. 16A and 16B both illustrate “see-through” eyebox spreaders, which are configured to allow for light transmission therethrough; 
     FIG. 17 illustrates use of a light pipe to guide light transmission, which may be incorporated in a virtual imaging system of the present invention; 
     FIG. 18 illustrates another use of a light pipe to guide light transmission, which may be incorporated in a virtual imaging system of the present invention; 
     FIG. 19 illustrates use of an infrared (IR) sensor and other optical elements to activate a display only when a user is looking at the display, which may be incorporated in a virtual imaging system of the present invention; 
     FIG. 20A is a head-mounted display design in the form of eyeglasses, incorporating a virtual imaging system of the present invention; 
     FIG. 20B is an enlarged view of a portion of FIG. 20A; and 
     FIG. 21 is an alternative embodiment of a virtual imaging system of the present invention suitable for incorporation in a head-mounted display design in the form of eyeglasses. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2A, the invention provides a virtual imaging system  10  for a user to view a virtual image of an object field. The system  10  includes an imaging subsystem  11  comprising at least one lens  14 . The imaging subsystem  11  is positioned such that its object field  13  is at or near its focal point, thereby positioning the virtual image of the object field at or near infinity. The virtual imaging system  10  further includes an eyebox spreader  16  for receiving a light beam  18  from the imaging subsystem  11  and redirecting the received light  18 ′ to a user&#39;s eye  2 . The eyebox spreader  16  is adapted to increase the “eyebox” of the imaging subsystem  11 . Specifically, the eyebox spreader  16  increases the width of a planar wavefront, from “A” to “B” as illustrated in FIG. 2A, thus increasing the lateral range (i.e., the “eyebox”) throughout which the user&#39;s eye  2  can move and still see the virtual image. This allows for the overall virtual imaging system  10  to be of compact construction and lightweight, while still permitting a user to view the virtual image at a comfortable focal distance. It should be understood that the lens  14  may be a single lens or may be formed of a plurality of lenses in combination, as will be apparent to those skilled in the art. 
     To aid in the description of the present invention, the following terms are defined and are depicted in FIG.  3 : eyebox width  20 , eye relief  22 , and field of view (FOV)  24 . First, an eyebox width  20  is a transverse range through which an eye  2  can move with respect to a virtual imaging system and still see the entire image of a display  8  (or an object field) for a given eye relief. An eye relief  22  is the distance between the eye  2  and a lens  4  of the virtual imaging system. The eyebox width  20  and eye relief  22  are related to one another, as shown in equation (3) below, in that the larger the eye relief is, the smaller the eyebox width is. Finally, the FOV  24  is the angle that the virtual image of the display  8  subtends at the eye  2 . 
     Further terms and concepts used in the present description are described in reference to FIG. 4, which illustrates a virtual display system based upon a paraxial thin lens  26 . Characteristics of this imaging system are modeled according to the following equation:                1     S   obj       =       1   f     +     1     S   img                 (   1   )                         
     where f is the focal length; S img  is the position of the virtual image  6  with respect to the lens plane; and S obj  (=D) is the position of the display (object)  8  with respect to the lens plane. In accordance with the above equation (1), if the virtual image  6  of the display is at 2 m from the lens  26  and the focal length of the lens is 20 mm, then D=19.8 mm. For most applications of interest in accordance with the present invention, D is approximately equal to f so that S img &gt;&gt;f, i.e., S img  is “near infinity”. 
     Still referring to FIG. 4, for a display size L, the FOV is given by the following equation:              FOV   =     2                 a                   tan        (     L     2      D       )                 (   2   )                         
     Under thin lens assumption, for a given eye relief ER, the eyebox width EB is given by the following equation:                    EB   =                A   -     2      ER                   tan        (     FOV   2     )                       =                A   -     ER        (     L   D     )                       (   3   )                         
     where A is the lens aperture. 
     Referring back to FIG. 2A, the virtual imaging system  10 , in particular the eyebox spreader  16  for increasing the eyebox of the system  10 , is now described in detail. A planer wavefront  18  is projected by the lens  14  from a point of the object field  13 . The eyebox spreader  16  increases the width “A” of the planar wavefront exiting the lens aperture of the imaging subsystem  11  to a larger width “B”, thereby increasing the eyebox width of the virtual imaging system (see Equation (3) above). To work effectively, the eyebox spreader  16  must be arranged so that the virtual image of the object field  13  appears to be at infinity or near infinity. In the illustrated embodiment, this is accomplished by arranging the object field  13  at or near the focal length “F” from the objective lens  14 . When a virtual image is thus positioned at or near infinity, the wavefronts from object field points are nearly planar and thus can be manipulated by the eyebox spreader  16  to effectively expand or spread the eyebox width without adding serious astigmatism, defocus, or double-image artifacts to the virtual image. FIG. 5 illustrates wavefronts A′ and B′, which are produced from object points A and B, respectively, and are flat (or planar). 
     The object field  13  may simply capture a real landscape image. For example, an imaging subsystem  11  may be binoculars that create a virtual image of a real scene with some magnification. Alternatively, the imaging subsystem  11  may optionally include an image generator  15  for creating an image in the object field  13 . Nonlimiting examples of an image generator are shown in FIGS. 2B and 2C. FIG. 2B illustrates a display  12 , such as a microdisplay, that may be located in the object field  13  of the imaging subsystem  11  of FIG.  2 A. FIG. 2C illustrates another type of scanner-based display  40 , which forms an image in the object field  13  of the imaging subsystem  11 . As known in the art, in a scanner-based display  40 , collimated light from a point source  41  is directed via a lens  43  toward a scanner  45 , which is rapidly moving to redirect the virtual image of the source  41  at different angles to build a composite virtual image in the object field  13 . 
     A. Eyebox Spreader 
     The eyebox spreader  16  may be constructed in various ways, as long as it effectively increases the eyebox width of a virtual imaging system of the present invention. The following describes some nonlimiting examples of an eyebox spreader suitable for use in the present invention. 
     (1) Front Surface Reflective Fresnel Eyebox Spreader. 
     FIG. 2A illustrates the eyebox spreader  16  constructed in the form of a reflective Fresnel prism  28 . Referring additionally to FIG. 6, the reflective Fresnel prism  28  is a generally flat optical element including a Fresnel surface  29 . The Fresnel surface  29  defines a regular array of parallel, optically flat facets  30 . As used herein, a facet is “optically flat” when the behavior of light incident on the facet is reflected from or transmitted through the facet in a controllable manner to achieve the goal of the present invention, i.e., effectively spreading an eyebox. In FIG. 2A, the Fresnel surface  29  is used to reflect the light transmitted from the imaging subsystem  11  and each facet  30  is a flat mirror. Each facet  30  is tilted out of the nominal surface plane (or “base plane”  32 ) of the prism  28  by the facet base angle φ fb . The facet pitch P is the width of the facet  30  projected onto the base plane  32 . In FIG. 6, the facet base angle is illustrated as φ fb =30°. The wavefront angle of incidence relative to base plane normal  31  is illustrated as φ i =60°, thus the angle of reflection relative to the base plane normal  31  is φ r =φ i −2φ fb =0°. Each facet  30  of the Fresnel surface  29  reflects a light ribbon  34  of the incident wavefront at φ r . As illustrated, the reflected wavefront is not continuous but is a fabric of these light ribbons  34  spaced out by dark gaps. The width of each light ribbon  34  of reflected light W is given by: 
     
       
           W =cos(φ i )cos(φ r )  P   (4) 
       
     
     Similarly, the width of the dark gap G is given by: 
     
       
           G =(1−cos(φ i ))cos(φ r )  P   (5) 
       
     
     In the example of FIG. 6, W=G=½P. 
     It is important to design W to be in the correct size range. If W is too small, diffractive effects reduce the image resolution and blurring occurs. Continuing with the example of FIG. 6, the diffraction from each light ribbon  34  can be approximated by the diffraction from a thin slit aperture of width ½P, for which the angular spread of the center lobe is 4λ/P, where λ is the wavelength of the light. To maintain diffraction below the human vision resolution limit of 0.6×10 −3  radians, the angular spread is constrained by the relation “0.6×10 −3 &gt;4λ/P”, which means that the pitch P should be larger than 3.4 mm where λ=500 nm light. On the other hand, if W is too large, the pupil sees the dark gaps and may lose the image. Even when the image is visible, the dark gaps may still be visible under many conditions. It is noted, however, that under conditions of limited information, where much of the background is black, the dark gaps may not be very visible. 
     If the dark gap “G” between the light ribbons  34  is larger than the user&#39;s pupil diameter, the image becomes shuttered and image detail is lost. Therefore, G should be small enough for the user to see the complete image. At the same time, it should be noted that when G is small an artifact can occur where a light/dark shutter is superimposed on the view. 
     Considering all of the above, one preferred design space in accordance with the present invention dictates W&gt;0.5 mm and G&lt;2 mm, to balance eyebox spreading performance against diffraction and shuttering artifacts. Those skilled in the art will appreciate that other designing of W and G is also possible and might be preferable in particular applications. 
     Referring back to FIG. 2A, the ratio of the reflected wavefront width B to the incident wavefront width A, or B/A, is the eyebox spreading ratio R ES , which is given by: 
       R   ES =cos(φ r )/cos(φ j )  (6) 
     An embodiment of FIG. 6 has R ES =2. The larger this number is, the larger the eyebox width is (at the expense of possible shutter and diffraction artifacts). 
     Eyebox spreading has been described thus far in the context of a single planar wavefront with a single angle of incidence. A practical virtual imaging system, however, has a nonzero field of view (FOV). Therefore, in practice, an eyebox spreader must be configured to perform for a range of incident angles that covers the FOV of the virtual image, as will be appreciated by those skilled in the art. 
     It should also be appreciated by those skilled in the art that a Fresnel surface can be used in a transmission form as well as in a reflection form. In the transmission form, the light striking the facets of the Fresnel surface is refracted rather than reflected. In either transmissive or reflective form, the eyebox spreader serves to widen the eyebox width. 
     FIG. 2D schematically illustrates a wavefront of a light beam exiting the lens  14  of the imaging subsystem  11  of FIG. 2A, and FIG. 2E schematically illustrates an effective wavefront of a light beam exiting the eyebox spreader  16  of FIG.  2 A. As illustrated in FIG. 2E; use of a Fresnel faceted surface  29  as an eyebox spreader serves to break the wavefront into smaller “beamlets”  38 , with dark gap areas  39  inserted therebetween, thereby stretching out (or flattening out) the beam wavefront. The phase lags due to path differences of the light ribbons are not finctionally significant and are not illustrated. The net result is that the wavefront made up of the beamlets  38  becomes a good approximation of the original wavefront form as shown in FIG.  2 D. Thus, it should be appreciated that use of such a faceted surface as an eyebox spreader is advantageous in reducing the curvature that may be included in the wavefront exiting the eyebox spreader. It is noted that the limit of curvature that is acceptable depends upon the acceptable resolution. Since the eye resolves about 30 arc seconds, the deviation of a wavefront from a best-fit spherical wavefront that enters into the eye&#39;s pupil should be less than 30 arc seconds. In lower resolution applications, this constraint can be relaxed. 
     The Fresnel prism  28  described above, and other optical elements to be described in the present description, may be formed of suitable glass, or of plastic by injection molding, or cast or compression molding. 
     (2) Back Surface Reflective Fresnel Eyebox Spreader. 
     FIG. 7 illustrates another embodiment of an eyebox spreader in accordance with the present invention. It should be understood that the eyebox spreader of FIG. 7 is suitable for incorporation in a virtual imaging system of the invention, such as the virtual imaging system  10  shown in FIG.  2 A. The eyebox spreader of FIG. 7 also comprises a Fresnel prism  28 , as with the embodiment of FIG. 2A, but in this embodiment the Fresnel prism  28  is flipped over so that the light  18  from the imaging subsystem  11  enters the optically flat base plane  32  of the Fresnel prism  28 . The light passing through the base plane  32  and the Fresnel prism  28  is then reflected internally from the facets  30  of the prism  28  to reemerge from the base plane  32  again. It is noted that the effectiveness of this embodiment may be limited by the fact that the incident and reflected beams refract through the base plane  32  of the prism  28  at different angles, causing a lateral chromatic aberration. This limitation, however, would not be an issue if narrowband illumination were used. 
     As before, the Fresnel prism  28  described above may be formed of suitable glass, or of plastic by injection molding, or cast or compression molding. A limitation to using injection-molded (or cast or compression-molded) devices is that the stresses near the comers and edges of the facets  30  are often significant and may lead to chromatic distortion. Some applications may use monochromatic displays where this is not an issue. Low-stress molding processes may also be possible. 
     (3) Reflective Fresnel Surface on Prism Surface Eyebox Spreader. 
     FIG. 8 illustrates a variation of the back surface reflective Fresnel eyebox spreader described above in reference to FIG.  7 . By adding and index matching (i.e., matching the index of refraction of) a prism  42 , such as a right-angle prism, to the front (incident side) of a Fresnel prism  28  having the Fresnel faceted surface  29  with appropriate face angles, lateral chromatic aberrations can be eliminated. Alternatively, the eyebox spreader shown in FIG. 8 may be integrally formed in a unitary piece. This embodiment has the advantage of reducing the overall size of the virtual imaging system. Specifically, when a lens  14  (see FIG. 2A) is used, the lens width (or diameter) should be large enough to maximally fill the Fresnel prism  28  with light from the object field (e.g., display). In other words, the light rays that define the left and right extremes of the object field image must reflect off the left and right extremes of the Fresnel prism  28 , respectively, or the user&#39;s eyebox will be reduced. In FIG. 8, for example, dashed lines and solid lines represent propagation paths of the rays through the prism  42 , wherein the index of refraction of the prism  42  are 1.0 and 1.5, respectively. As illustrated, increase in the index of refraction reduces the angle between the left and right extreme rays, thereby rendering the rays to converge less as they propagate from a lens through the Fresnel prism  28 . Consequently, increase in the index of refraction reduces the required lens width, from “B” to “A”. This in turn serves to reduce the overall size of the virtual imaging system. 
     Further alternatively, referring to FIG. 9, another method of implementing this embodiment is to index match the faceted surface  29  of the Fresnel prism  28  to the flat surface on the back of the right-angle prism  42 . Preferably, a liquid, gel, or other transparent material  44  that will not exhibit index nonuniformities as the material flows and sets around the facet  30  edges will be applied between the two components. 
     (4) Gap-Filling Faceted Reflector Eyebox Spreader. 
     FIGS. 10A-10E illustrate yet another embodiment of an eyebox spreader. This embodiment is a variation of an eyebox spreader based on a Fresnel surface as shown in FIGS. 2A,  7 ,  8 , and  9 , in that this embodiment also employs a regular pattern of parallel facets; however the dark gap areas between plural wavefronts (see  39  in FIG. 2E) are filled with a second image to reduce the light/dark shutter effect. This second image may be created in several ways. 
     Specifically, referring to FIGS. 10A and 10B, an eyebox spreader  46  of this embodiment includes a Fresnel surface defining an array of parallel facets, wherein each of the facets forms a beamsplitter  48 . In the illustrated embodiment, the facets (beamsplitters)  48  are embedded in a transparent substrate having two optically flat surfaces, and are provided on a surface of a prism  42 . Each of the beamsplitters  48  includes plural portions having different reflectivities, for example, a first portion  52  of 50% reflectivity and a second portion  54  of 100% reflectivity. Thus, the light incident on the 100% reflective portion  54  of a first beamsplitter  48   a  is reflected therefrom, while the light incident on its 50% reflective portion  52  is partially reflected therefrom ( 18   a ) and partially transmitted therethrough. The light transmitted through the 50% reflective portion  52  of the first beamsplitter  48   a  is then received by the 100% reflective portion  54  of a second beamsplitter  48   b , which then reflects the received light toward the user&#39;s eye ( 18   b ). Referring additionally to FIG. 10D, the series of beamsplitters  48  thus arranged will produce a first series of wavefronts  50   a  that are reflected from the 50% reflective portions  52 , respectively, and a second series of wavefronts  50   b  that are reflected from the 100% reflective portions  54 , respectively. As illustrated, the first and second series of wavefronts  50   a  and  50   b  are alternately combined so as to advantageously fill in the dark gaps of each other to form a contiguous wavefront, effectively reducing the light/dark shutter effect. In depicting the wavefronts in FIG. 10D, phase lags due to path differences in the light ribbons are not functionally significant and are not illustrated. 
     Each of the beamsplitters  48  must include material of uniform refractive index applied to both sides of the beamsplitter. This arrangement keeps any wavefronts transmitted through the beamsplitters  48  on an undeviated path. 
     As before, the eyebox spreader  46  of the present embodiment is suitable for use in a virtual imaging system, such as the one shown in FIG.  2 A. FIG. 10C illustrates a wavefront of a light beam exiting the lens  14  of the imaging subsystem  11  of FIG. 2A incorporating the eyebox spreader  46  of the present embodiment. As will be appreciated by comparing FIG. 10D against FIG. 10C, the eyebox spreader  46  of the illustrated embodiment is designed to double the wavefront width from “A” to “ 2 A”, and hence increase the eyebox width. 
     The coating along each facet (i.e., a beamsplitter)  48  is preferably optimized for brightness uniformity. For this purpose, in the illustrated embodiment utilizing first and second facet reflections from the portions  52  and  54 , respectively, the coating is selected so that the portions  52  and  54  will have 50% and 100% reflectivities, respectively. As a result, at the nominal angle of incidence, 50% of the light is reflected at the first facet reflection from the portion  52  ( 18   a ) and 100% of the remaining light (50%) is reflected at the second facet reflection from the portion  54  ( 18   b ). This produces a uniform brightness from each reflection. 
     By careful choice of facet angles and reflective coatings applied on the facets, the present embodiment effectively fills in any dark gaps that may otherwise be present. Further, depending upon the geometry, this design may employ a third facet reflection or more to effectively fill in any dark gaps. The goal here is to produce a good approximation (FIG. 10D) of the original wavefront (FIG. 10C) for a beam exiting an eyebox spreader  46 . 
     For example, FIG. 10E shows an embodiment that employs first through fourth facet reflections, which are optimized for brightness uniformity. In this case, each beamsplitter  48  is optimally coated so that its reflectivity varies along its surface in four stages: a first portion  56  where a coating reflectivity is 25% (R=¼); a second portion  58  where a coating reflectivity is 33% (R=⅓); a third portion  60  where a coating reflectivity is 50% (R=½); and a fourth portion where a coating reflectivity is 100% (R=1). In this way, the first, second, third, and fourth facet reflections achieve brightness uniformity. 
     To generalize the brightness uniformity coating aspect, if an eyebox spreader has a maximum N th  facet reflection, the reflectivity along each facet (or beamsplitter) is stepped to obtain equal brightness from each of the N facet reflections. The coating reflectivity “R” increases in steps along the facet as: 
     1/N for the 1 st  facet reflection; 
     1/(N−1) for the 2 nd  facet reflection; 
     1/(N−m+1) for the m th  facet reflection; and 
     Unitary reflectivity for the N th  facet reflection. 
     Next referring to FIGS. 11A thorough  11 D, a method of forming a gap-filling faceted reflector eyebox spreader  46  is described. This method uses laminated and coated sheets of glass (or plastic) that are then obliquely sliced and polished. 
     First, referring to FIG. 11A, a sheet of glass  64  is provided. Alternatively, a sheet of plastic may be used. The glass size is width (W) by length (L) by thickness (T, see FIG.  11 B). The glass  64  is vacuum coated with a sliding mask that produces a repeating series of precision mirrored stripes  66 ,  67 , and  68  having different reflectivities. For example, three reflective stripes  66 ,  67 ,  68  may be 100%, 50%, and 0% reflective, respectively, each of width 1 mm and being repeated in every 3 mm across the width of the glass  64 . Preferably, some stripe width is allocated for the kerf of the subsequent wafer sawing (see FIG. 11C) and for polishing (see FIG.  11 D), as will be fully appreciated later in reference to FIGS. 11C and 11D. The same coating process is repeated to produce plural sheets  64  of coated glass (or plastic). 
     Referring to FIG. 11B, plural sheets of the coated glass  64  are aligned by shifting each successive sheet by Δx along the width of the sheet. The aligned sheets are then laminated using a suitable optical adhesive to produce a laminated block  70 . Choice of lamination shift “Δx” and the glass thickness “T” determines the facet base angle φ fb  of the eyebox spreader  46  of this embodiment. In the illustrated embodiment, nine sheets of glass  64  are laminated with Δx=1.73 mm. If the glass thickness T is 1 mm, the facet base angle X is arc tangent (1.73)=60 to glass plane normal  69  (FIG.  11 C). 
     In FIG. 11C, the coated and laminated block  70  is sawed into a plurality of wafers  72  at the determined facet base angle φ fb . As illustrated, each wafer  72  is cut along and parallel to the coating stripes  66 ,  67 ,  68 , i.e., intersecting each glass sheet  64  at the same coating stripe. 
     Finally, referring to FIG. 11D, the cut sides of each wafer  72  are polished using suitable polishers  74 . Thus, polished glass wafer  72  is then diced to form the eyebox spreader  46  (FIGS. 10A-10E) or other optical elements. Eyebox spreaders or other optical elements produced using the present method do not suffer from index nonuniformity and thus can advantageously reduce any chromatic distortion. 
     FIG. 12 illustrates an alternative method of forming a gap-filling faceted reflector eyebox spreader  46  of FIGS. 10A-10E. This method applies a special oblique coating process to a Fresnel prism.  28 . In this method, first portions  76  of the facets  30  have a first reflectivity (50%, for example, which may be the reflectivity of the original coating on the facets  30 ). When evaporative coating of metal (or dielectric) mirror is applied from an oblique angle, as indicated by arrows  80 , the facets  30  shadow each other so that the coating (having 100% reflectively, for example) will be applied only on second portions  78  of the facets  30 . Though this method requires index matching of the first and second portions  76  and  78  to the Fresnel prism  28 , it is less costly than the method described above in reference to FIGS. 11A-11D. 
     (5) Randomized Reflection Eyebox Spreader. 
     Referring to FIG. 13, an eyebox spreader  81  of this embodiment includes a plurality of Fresnel prisms  16   a - 16   d , each with a Fresnel surface including an array of parallel, optically flat facets  30 . The plurality of Fresnel prisms  16   a - 16   d  are combined such that the facets  30  of each Fresnel surface are offset from the facets  30  of the adjacent surface(s). Thus constructed, the eyebox spreader  81  receives incident light beams  18   a - 18   d  and reflects them in a random, or checkerboard, pattern. Such random reflection does not fill dark gaps in the wavefront but, rather, makes it difficult for a user to see any pattern of lines caused by the gaps. Those skilled in the art will appreciate that there are various other embodiments that could produce such randomized reflections. 
     (6) Beamsplitter Eyebox Spreader. 
     FIG. 14 illustrates yet another embodiment of an eyebox spreader  82  including a beamsplitter  83  and a mirror  84 . The beamsplitter  83  splits the incident wavefront  18  into two by partially transmitting it and partially reflecting it to a user&#39;s eye ( 18   a ). The mirror  84  then receives the light transmitted through the beamsplitter  84  and reflects it to the user&#39;s eye ( 18   b ). The reflected light beams  18   a  and  18   b  thus form two wavefronts  85   a ,  85   b  along an edge. As illustrated, the eyebox spreader  82  of the present embodiment effectively doubles the wavefront width from “A” to “ 2 A”, and thus substantially increases the eyebox also. 
     For the wavefronts  85   a ,  85   b  to successfully focus to the same spot on a user&#39;s retina, the two wavefronts must be substantially planar. This means that the wavefront surface normal (i.e., a line normal to the wavefront surface) must vary less than 0.6×10 −3  radians across the pupil of the eye to maintain the human vision resolution limit. If the wavefront curvature is too great, two distinctive wavefronts will be created for the user&#39;s eye, and the user&#39;s pupil will see a blurred or double image at a position  86  where the two wavefronts  85   a  and  85   b  interface. Therefore, the eyebox spreader  82  of the present embodiment requires excellent optics that can produce very planar wavefronts. 
     While the eyebox spreader  82  of the present embodiment is illustrated to include a single beamsplitter, which, in combination with a mirror, doubles the eyebox, this embodiment can be extended to achieve even larger eyebox spreading by using two or more beamsplitters in series and adjusting their reflective coatings appropriately. Specifically, N−1 beamsplitters with one mirror will spread the eyebox N times. For getting equal brightness (an equal amount of light) from each beamsplitter to provide a uniform intensity output, the reflectivity of the coating applied to each beamsplitter should be as follows: 
     1/N on the 1 st  beamsplitter; 
     1/(N−1) on the 2 nd  beamsplitter; and 
     1/(N+1−m) on the m th  beamsplitter. 
     (7) Diffraction Grating Eyebox Spreader. 
     As will be appreciated by those skilled in the art, an eyebox spreader may be formed of a linear diffraction grating with a constant grating vector. Like the faceted surface eyebox spreader described above in reference to FIGS.  2 A and  7 - 9 , the diffraction grating eyebox spreader can be used in a transmission form as well as in a reflection form. Diffraction gratings typically have large chromatic dispersion, but the diffraction grating eyebox spreader would be effective with narrowband illumination, such as a laser source. 
     As apparent from the description of various eyebox spreaders above, the present invention provides a method of spreading an eyebox of a virtual imaging system used for a user to view a virtual image of an object field. According to the method, a virtual image of the object field is positioned at or near infinity. As described above, this results in a wavefront of each object field point being planar. Next, the wavefront is sequentially sliced from the transverse width of the wavefront into a plurality of light ribbons (see  34  of FIG.  6 ). Finally, the plurality of light ribbons are redirected toward a user&#39;s eye, so that the plurality of light ribbons will be separated along a collective transverse width of the plurality of light ribbons. At this point, the collective transverse width of the plurality of light ribbons “W 2 ” (FIG. 6) is greater than the transverse width of the original wavefront “W 1 ” (FIG.  6 ), thus effectively increasing the eyebox of a virtual imaging system. 
     B. Related Techniques 
     The following describes several related techniques that may be used with an eyebox spreader to improve the performance, characteristics, or implementation of a virtual imaging system of the present invention. In the following description, an “eyebox spreader” is understood to be any of the embodiments hereinabove described. 
     (1) Noninfinite Fixed Focus. 
     Referring to FIG. 15, a virtual imaging system  10  includes an imaging subsystem  11  including a display  8  and a lens  14 ; and an eyebox spreader  16  (illustrated to be in the form of a Fresnel prism). The display  8  and the lens  14  are separated by the focal length F of the lens  14  so as to place the virtual image of the display  8  at or near infinity. In some applications, however, it may be preferable to adjust the depth of focus of the virtual image. In this regard, to set the focus of the virtual imaging system  10  with eyebox spreading at a distance other than infinity, a low-profile (low diopter) lens  88  can be added across the entire effective eyebox width between the eyebox spreader  16  and the user&#39;s eye (not shown). The lens  88  can move the focus to an appropriate focal distance dictated by the particular application. The lens  88  may comprise the user&#39;s prescription lens also. 
     (2) See-through Eyebox Spreader. 
     Referring to FIGS. 16A and 16B, see-through eyebox spreaders  90   a ,  90   b  are illustrated. The see-through eyebox spreaders  90   a ,  90   b  allow a user to see through the eyebox spreaders without serious optical distortion. Essentially, see-through eyebox spreaders  90   a ,  90   b  are provided with means through which light can pass. Such a see-thorough eyebox may be formed by embedding a faceted Fresnel surface within a transparent substrate having two optically flat surfaces. Specifically, in FIG. 16A, the see-through eyebox spreader  90   a  of one embodiment is formed of two elements  92   a ,  92   b . The two elements  92   a  and  92   b  have a generally triangular cross section and both include faceted surfaces, which are mated along a faceted interface  93 . A semitransmissive (i.e., semireflective) coating is applied along one of the faceted surfaces, prior to combining the two elements  92   a ,  92   b  along the faceted interface  93 . Thus constructed, the faceted interface  93  partially passes light as indicated by dotted arrows  94 . Alternatively or additionally, a gap (no coating area) could be left on the faceted interface  93 , through which light could pass. In the present description, the faceted interface  93  including gaps for passing light therethrough is also characterized as “semitransmissive”. A flat incident side  96  of the first element  92   a  and a flat exit side  98  of the second element  92   b  are maintained parallel with each other so that light can pass without distortion. Further, the two elements  92   a  and  92   b  are preferably index matched to minimize thre optical distortion that may otherwise be caused around the faced interface  93 . 
     FIG. 16B illustrates the see-through eyebox spreader  90   b  of another embodiment, which also includes two elements  100   a  and  100   b  having faceted surfaces mated along a faceted interface  93 . In this embodiment, the two elements  100   a  and  100   b  are substantially flat, as illustrated. As before, the faceted interface  93  may include a semitransmissive coating and/or a gap so as to allow for light transmission therethrough. Further as before, to minimize optical distortion, a flat incident side  97  and a flat exit side  98  are maintained parallel with each other. Preferably, the two elements  100 A and  100 B are index matched. 
     See-through eyebox spreaders may be particularly useful in head-mounted display applications, to allow a user to see the real world through an eyebox spreader. 
     (3) Light Pipe with Internal Bounces. 
     FIG. 17 illustrates a light pipe  100 , which may be incorporated into a virtual imaging system of the present invention. As illustrated, the light pipe  100  is arranged to direct the light  102  from a lens  14  toward an eyebox spreader  116  and then to a user&#39;s eye (not shown). The thickness T of the pipe  100  can be reduced to as little as A/(2 sin (φ i )), where A is the lens aperture and φ i  is the angle of incidence of the optic axis in the pipe  100 . It should be appreciated that, as the FOV of the virtual image increases, T also increases to provide an allowance for the extreme ray angles in the image to diverge along the pipe. 
     (4) Polarization Folded Path with Reflective Lens. 
     FIG. 18 illustrates another use of a light pipe  104  in a virtual imaging system of the present invention. In this embodiment, the light  102   a  from a display  12  is plane-polarized and propagates to the reflective lens  14  after one or more total internal reflections from the walls of the light pipe  104 . On one reflection near the lens  14 , the light  102   a  strikes a polarization beamsplitter  106 , which may be a coating applied on the wall of the light pipe  104  or on a thin element bonded between the light pipe  104  and the eyebox spreader  16 . The polarization of the light  102   a  is chosen so that the light  102   a  will reflect from the polarization beamsplitter  106  to propagate through the light pipe  104  to. the lens  14 . Between the reflection from the polarization beamsplitter  106  and the lens  14 , the light passes through a quarter wave (¼-wave) plate  108  to be changed to circular-polarized light. The light is then reflected from the lens  14  and again passes through the ¼-wave plate  108  to proceed back toward the polarization beamsplitter  106 . After twice passing the ¼-wave plate  108 , the polarization of the light  102   b  is now rotated 90 degrees from its original polarization (in the light  102   a ) so that the light  102   b  can now transmit through the polarization beamsplitter  106  to the eyebox spreader  16 . The transmitted light  102   b  is then redirected by the eyebox spreader  16  and propagates through and out of the light pipe  104  to the user&#39;s eye  2 . 
     (5) Eye View Switch for Powering Display. 
     FIG. 19 illustrates a virtual imaging system  109  including an imaging subsystem comprising a display  12  and a lens  14 ; and an eyebox spreader  16 , which is preferably in the form of a head-mounted display (HMD) to be worn on a user&#39;s head. To minimize power requirements of the HMD, the HMD further includes an eye view switch  110  that activates the display  12  to an on-state from an off-state (or standby-state) only when the user&#39;s eye  2  is viewing the display  12 . 
     For example, when the eye  2  looks at the display  12 , the eye&#39;s viewing direction may be sensed by a sensor in the eye view-angle switch  110 , which triggers a switch to activate the display  12  to an on-state. When the eye  2  looks away, the display  12  is switched to an off-state. 
     In one embodiment, the eye view switch  110  includes an infrared (IR) source  111 , an IR sensor  112 , a dichroic beamsplitter  114 , and an IR beamsplitter  115 . The IR sensor  112  is arranged in the HMD  109  such that there is a one-to-one correspondence between each view field point of the display  12  and a position on the IR sensor  112  when the user&#39;s eye  2  is viewing that field point. This is accomplished without user calibration by arranging the eye view switch  110  and the virtual imaging system of the HMD  109  to share the optical path along the eyebox spreader  16  and the lens  14 . In FIG. 19, the optical path of the eye view switch  110  is shown in a solid arrow  116   a  and the optical path of the virtual imaging system  109  is shown in a broken-line arrow  116   b . At the same time, as illustrated, the dichroic beamsplitter  114  is arranged so that the optical path is split to place both the display  12  of the virtual imaging system  109  and the IR sensor  112  of the eye view-angle switch  110  at the focal point of the lens  14  along the split paths. Further, the IR source  111  is isolated from the IR sensor  112  by the IR beamsplitter  115 . 
     Preferably, an IR filter  119  is provided between the display.  12  and the dichroic beamsplitter  114  to block any IR illumination from the display  12  from entering the eye view switch system  110 . At least part of the design may be conveniently incorporated in a light pipe  117  including a total-intemal-reflection mirror wall  118 , which is generally contoured to match the shape of the HMD that wraps around the user&#39;s head. Preferably, the eyebox spreader  16  is coupled to a prism  120 , such as a right-angle prism (see FIGS.  8  and  9 ). In the illustrated embodiment, a light-exit surface  121  of the light pipe  117  and a light-incident surface  122  of the prism form the first and second lenses, respectively, of the objective lens  14 . 
     In operation, IR energy from the IR source  111  is directed by the IR beamsplitter  115  to the dichroic beamsplitter  114 , which directs the IR illumination through the lens  14  and via the eyebox spreader  16  to the eye  2  to project an IR image on the retina  123 . The IR retinal image then follows the same optical path backward, via the dichroic beamsplitter  114  and the IR beamsplitter  115  to the IR sensor  112 . At the same time, an image on the display  12  is directed by the dichroic beamsplitter  114  via the lens  14  and the eyebox spreader  16  to the eye  2 . 
     Functionally, a user looks at a field point in the display  12  and that field point is imaged onto a unique focal point of the eye&#39;s lens on the retina  123  (located in the fovea). The reflection from this focal point on the retina is very strong, much like a coniscopic microscope reflection. This focal point on the retina is simultaneously imaged onto the IR sensor  112  at a specific sensor position that has a one-to-one correspondence with the field point on the display  12 . If the foveal image point is on the IR sensor  112 , a signal threshold is exceeded and the signal is processed in a conventional manner to trigger activation of the display  12 . If the signal-to-noise ratio is not sufficient for simple thresholding, a multielement two-dimensional (2D) sensor can be used with suitable signal processing to determine the presence of the foveal point. When the foveal image point is off the IR sensor, the display  12  is switched to an off-state. 
     The HMD with an eye view switch, as described herein, can be modified to achieve other applications. Specifically, the retinal imaging system using IR energy as illustrated in FIG. 19 may be used in eyetracking and cursor control on the HMD, security clearance by matching retinal maps, or pulse sensing of the retinal blood vessels. The pulse sensing application will be more fully described later. 
     (6) Adjustment of the FOV Direction. 
     Still referring to FIG. 19, the direction of the HMD&#39;s FOV (and the position of the eyebox) can be adjusted by the user by a small rotation or tilt of the eyebox spreader  16  about a preferred axis, for example, about an axis  124  in the direction of an arrow  125 , relative to the rest of the HMD imaging system. This is a very sensitive adjustment because the change in the HMD&#39;s FOV direction is double the adjustment angle. The advantage of making the eyebox spreader tiltable to adjust the HMD&#39;s FOV direction (and the position of the eyebox) is that a smaller adjustable virtual imaging system in an HMD can accommodate all users, instead of having to use an oversized, nonadjustable virtual imaging system. 
     C. Head-Mounted Display (HMD) Designs 
     Various embodiments of a virtual imaging system of the present invention, which are compact and lightweight in design, are well suited for forming head-mounted display (HMD) systems in a stylish manner. For example, in accordance with the present invention, an HMD in the form of a pair of glasses (sunglasses, safety glasses, etc.) is provided. 
     Referring to FIGS. 20A and 20B, an HMD  126  in the form of glasses to be worn by a user includes frames  127  and a virtual imaging system  10  mounted on the frames  127 . In the illustrated embodiment, the virtual imaging system  10  includes an imaging subsystem  11  comprising a display  12  and a lens  14 ; and an eyebox spreader  16 . The display  12  is placed at approximately the focal length of the lens  14  to position a virtual image at or near infinity. The imaging subsystem  11  may further include a display controller  128 , preferably mounted on the frames  127 , for supplying information to the display  12  via a line  130 , and a battery  129 , also preferably mounted on the frames  127 , for powering the display controller  128 . As illustrated, the display controller  128  and the battery  129  may be housed in a single module. 
     Examples of the types of information that could be supplied by the display controller  128  to be presented on the display  12  are time data (date, time, timer function, etc.), sensor data (the user&#39;s pulse, speed, altitude, pitch, roll, yaw, temperature, etc.), stored (received) data (PDA functions, addresses, calculator functions, E-mail, etc.), and notification data (E-mail arrived, cell phone ringing, appointment notification, etc.). Most of the data described above require use of a clock  131 , which may also be mounted on the frames  127  in the same module as the display controller  128  and the battery  129 . In one embodiment, the display  12  is advantageously formed of a passive LC transmissive display that displays data with natural illumination. This makes the design low power, thereby reducing battery weight requirements. 
     The outer dimensions of the display  12  often significantly exceed the actual active area of the display  12  because of the area devoted to connectors, edge tolerances, etc. Thus, keeping the plane of the display  12  parallel to the surface of the frames  127  minimizes the thickness of the virtual imaging system  10  and preserves the wrap-around-the-head style of the HMD  126 . The data in the display  12  are preferably about 3 mm by 5 mm in size and are back-illuminated by the ambient light through a diffuser  132  located near the display  12  on the outside of the frames  127 . The diffuser  132  provides spatial averaging of the ambient lighting and can be a styling feature for the HMD  126 . The diffuser  132  can also provide some limited temporal averaging of light by using phosphorescent dyes therein. By using ambient lighting, the display  12  achieves good contrast because the light level of the display  12  adapts to the environment and becomes brighter relative to the transmission through the sunglasses. 
     The display  12  is wired via the line  130  to the display controller/battery module  128 ,  129 , which may be shifted back along the temple of the frames  127 , as illustrated in FIG. 20A. A suitable imaging subsystem  11 , including the lens  14 , is provided for focusing a virtual image of the display  12  at or near infinity. For example, in the illustrated embodiment, the image is relayed from the display  12  to the objective lens  14  through two mirror reflections, specifically, via a first mirror  133   a  and a second mirror  133   b . The first mirror  133   a  may be on the hypotenuse of a prism  134 , which reflects the received image to the second mirror  133   b . Bending the light path with the mirrors  133   a ,  133   b  in this manner wraps the light path very closely about the frames  127 , thereby allowing for compact styling of the HMD  126 . 
     In the illustrated embodiment, the lens  14  has a 25 mm focal length and is placed at one focal length from the display  12 . The eyebox spreader  16  comprises a front surface reflective Fresnel eyebox spreader ( 28  in FIG. 2A) located aft of the lens  14 . The optic axis of the lens  14  is oriented at 60 degrees to the surface normal 135 of the eyebox spreader  16  (φ 1 =60) and the facet base angle 4 fb  of the eyebox spreader (see FIG. 6) is 20 degrees. This means that the optic axis emerging from the eyebox spreader  16  is 80 degrees rotated from the incident optic axis and −20 degrees from the surface normal  135  of the eyebox spreader  16 . Referring back to equation (6) above, the eyebox spreading ratio R ES  is the ratio of the cosines of the reflected and incident optic axis angles relative to the eyebox spreader surface normal, or cos(φ r )/cos(4)). The eyebox is increased by this ratio in the dimension that is normal to the optic axis emerging from the eyebox spreader  16  and in the plane of reflection. In this illustrated case, the eyebox spreading ratio R ES  is cos(−20)/cos(60)=1.88. Thus, for a lens with 6 mm horizontal aperture, the user sees a horizontal aperture of 11.3 mm. 
     FIG. 21 illustrates a virtual imaging system that is also suitable for forming an HMD in the form of a pair of glasses, as illustrated in FIGS. 20A and 20B, but with the following differences. The embodiment of FIG. 21 includes first and second prisms  136  and  137 , which may be plastic molded parts as before. The optical path from the display  12  to the emergence from an eyebox spreader  46  is substantially contained in these prisms  136  and  137 . Specifically, the first prism  136  includes a light entry surface  138   a  positioned adjacent the display  12 , first and second total internal reflective surfaces (or mirrors)  138   b  and  138   c  that function, substantially in the same manner as the first and second mirrors  133   a  and  133   b  of FIGS. 20A and 20B, and an exit face  138   d  that forms the first lens of the objective lens  14 . The second prism  137  is a right-angle prism including an entry face  139   a  that forms the second lens of the objective lens  14 , a hypotenuse  139   b  that includes thereon a gap-filling faceted reflector eyebox spreader  46  (FIGS.  10 A- 10 E), and an exit face  139   c . The exit face  139   c  of the second prism  137  may have a low magnification lens for moving the virtual image focus closer in from infinity, as described above in reference to FIG.  15 . 
     As noted above, a head-mounted display (HMD) system of the present invention has various applications and may be adapted to present various data on the display. As a nonlimiting example, an HMD system, as described in reference to FIGS. 20A-21, may include a sensor, which captures and relays sensed data to the display. For example, an HMD system may include a pulse sensor. The pulse detected by the pulse sensor is relayed to the display controller  128 , which then processes and forwards the pulse information to the display  12  to allow the user to monitor his pulse. Three specific embodiments of a pulse sensor suitable for use in the present invention are described below. 
     First, a pulse monitor may be an infrared sensor. Specifically, a combination of an IR source and an IR sensor may be provided to “look at” blood vessels of the user and measure the modulation of the IR light signal to determine the user&#39;s pulse. Four places at which the user&#39;s blood vessels may be monitored are the soft tissue on the ear (examples: earlobe, connective tissue joining ear to head), the temple, the nose, and the eye (either the retina or the cornea). In particular, the retinal pulse can be monitored using the retinal imaging system described above in reference to FIG.  19 . 
     Second, a pulse sensor may be formed of a pressure sensor, which is used on a blood vessel to detect the pulse signal. Two places at which a pressure sensor may be applied are the temple and behind the ear. 
     Third, a two-point electrical potential sensor may be used as a pulse sensor. In this sensor, two contacts are made with the user&#39;s head at separated locations and the differential electrical potential of skin between these contacts is measured. The pulse is then extracted from the electrical potential signal. Several pairs of contact locations are: behind both ears, behind one ear and the nose bridge, between both temples, one temple and the nose bridge, and one temple and behind one ear. 
     Optionally, an HMD system as described above may further include a wireless transceiver  140  (FIG. 20A) to exchange data, such as sensed data, with a remote data transceiver (not shown). 
     A virtual imaging system of the present invention, by effectively increasing the width of an eyebox, makes it possible to construct a virtual imaging system that is compact in construction and light in weight. Further, the arrangement of optical elements in the virtual imaging system is such that a virtual image is placed at or near infinity and thus is presented clearly to the user&#39;s eye. A compact, lightweight, and high-performance virtual imaging system of the present invention may ideally be used in a head-mounted virtual imaging system that “wraps” around a user&#39;s head, as in the form of sunglasses, without compromising its style or function. 
     While the preferred embodiments of the invention have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.