Patent Publication Number: US-6700552-B2

Title: Scanning display with expanded exit pupil

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. patent application Ser. No. 08/975,259 filed Nov. 20, 1997 for “Virtual Retinal Display with Expanded Exit Pupil,” now U.S. Pat. No. 6,157,352 which is a continuation of U.S. Pat. No. 5,701,132 issued Dec. 23, 1997 on application Ser. No. 08/624,950 filed Mar. 29, 1996 for “Virtual Retinal Display with Expanded Exit Pupil.” The content of such patent and patent application are incorporated herein by reference and made a part hereof. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to retinal display devices, and more particularly to optical configurations for retinal display devices and a method and apparatus for defining an exit pupil through which a user views an image. 
     A retinal display device is an optical device for generating an image upon the retina of an eye. Conventional retinal scanning displays use a coherent light source which is scanned in raster fashion onto the retina. Light is emitted from a light source, collimated through a lens, then passed through a scanning device. The scanning device defines a scanning pattern for the light. Following the scanning device, the scanned light passes through an objective lens which converges the light to focus an image. Conventionally such light is converged to a flat image plane. The light then diverges beyond the plane. An eyepiece is positioned along the light path beyond the objective lens at some desired focal length. An “exit pupil” (i.e., area of generated light pattern) occurs shortly beyond the eyepiece in an area where a viewer&#39;s eye pupil is to be positioned. 
     A viewer looks into the eye piece to view an image. The eye piece receives light that is being deflected along a raster pattern. Light thus impinges on the viewer&#39;s eye pupil at differing angles at different times during the scanning cycle. This range of angles determines the size of the image perceived by the viewer. Modulation of the light during the scanning cycle determines the content of the image. 
     Typically the exit pupil defined by the display device is less than 2 mm in diameter and often less than 1 mm in diameter. The viewer&#39;s eye pupil varies from approximately 2 mm in diameter under bright light to approximately 7 mm in a dark room. Because of the small exit pupil, a first step for a viewer is to adjust eye position to find the exit pupil. The viewer&#39;s pupil needs to achieve and maintain alignment with the display device&#39;s exit pupil. While in alignment, the light scans directly onto the viewer&#39;s retina without any intermediary screens, cathode ray tubes (CRT&#39;s) or liquid crystal display devices (LCD&#39;s). The result is an image perceived by the viewer. 
     A shortcoming of the conventional retinal display is the difficulty of maintaining alignment between the exit pupil and the viewer&#39;s pupil. If the viewer moves, alignment may be lost. Movement is problematic because a viewer has a tendency to move their eye when intending to view a peripheral portion of the image. Even blinking may cause movement of the eye. As a result, conventional exit pupils are inconvenient for the viewer. In particular a lay consumer using a virtual retinal display would find the alignment requirement difficult to maintain for entertainment or other long term viewing applications. Accordingly there is a need for a retinal display device having an exit pupil defined so as to enable easier viewing of the image. 
     Other shortcomings of conventional retinal display devices include the display&#39;s size and weight. As the retinal display device is to be positioned in the vicinity of a viewer&#39;s eye, there is a need to achieve a lightweight compact display device. 
     SUMMARY OF THE INVENTION 
     According to the invention, a lightweight, compact retinal display device is achieved using a simplified optical system which generates an expanded exit pupil without compromising magnification or resolution. 
     According to one aspect of the invention, a scanning device for deflecting light is located along the light path following an objective lens system. Significantly, the retinal display device of this invention avoids use of an objective lens system following the scanning device. The elimination of an objective lens system beyond the scanner shortens the light path through the retinal display device. One advantage of such a configuration is a lighter, more compact display device. 
     In a post-objective scanning system the scanning device receives converging light. Beyond the scanning device, the light continues to converge to an intermediate image plane. According to another aspect of this invention, the image plane is an intermediate curved image plane. The light then diverges beyond this plane in the direction of an eyepiece. 
     According to another aspect of the invention, an apparatus for expanding the exit pupil is positioned between the scanning device and the eyepiece at the curved image plane. To achieve a focused image with maximum resolution, the exit pupil expanding apparatus defines a curved surface which coincides with the intermediate curved image plane. The apparatus is positioned at the intermediate curved image plane so as to maintain maximum resolution and focus. 
    
    
     One advantage of this invention is that the shorter light path enabled by avoiding an objective after the scanning device allows for a more compact, lighter weight retinal display device. Another advantage is that a viewer has less difficulty aligning and maintaining alignment with an exit pupil formed at the eyepiece. In particular, the expanded exit pupil, the multiple exit pupils or the multiple expanded exit pupils make it easier for a viewer to align with an exit pupil. Another advantage with regard to the diffractive optical element embodiment is that image brightness is generally uniform among various groups of exit pupils which may form at the viewer&#39;s eye. These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a virtual retinal display according to an embodiment of this invention; 
     FIG. 2 is an optical schematic of the virtual retinal display of FIG. 1; 
     FIG. 3 is a block diagram of the image data interface, light source, and optics subsystem of FIG. 1 according to an embodiment of this invention; 
     FIG. 4 is a perspective drawing of the scanning subsystem of FIG. 1 according to an embodiment of this invention; 
     FIG. 5 is an exploded view of the scanning subsystem of FIG. 4; 
     FIG. 6 is an optical diagram of the exit pupil expanding apparatus according to one embodiment of this invention; 
     FIG. 7 is an optical diagram of the exit pupil expanding apparatus according to another embodiment of this invention; 
     FIG. 8 is an optical diagram of the exit pupil expanding apparatus according to another embodiment of this invention; and 
     FIG. 9 is an optical diagram of the exit pupil expanding apparatus according to another embodiment of this invention. 
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Overview 
     In one embodiment the exit pupil expanding apparatus is formed by a diffractive optical element. The diffractive optical element has a curved surface receiving light from the scanning device. Such curved surface coincides with the intermediate curved image plane scanned by the scanning device. The diffractive optical element replicates the incident light beams to produce multiple exit light beams. More specifically, the diffractive optical element passes one fraction of received light in the same direction as the incident light. Additional beams are passed at specific angles relative to the incident light. The percentage of light in each beam leaving the diffractive optical element is determined by the diffraction pattern. The angle spanned by each exiting light beam is defined by spacing among diffraction patterns and the light wavelengths. The cumulative angle of light beams exiting the diffractive optical element spans an angle greater than the cumulative angle of light beams incident to the diffractive optical element. Each exiting light beam output from the diffractive optical element passes through the eyepiece to define a separate exit pupil. In effect multiple closely spaced exit pupils are defined beyond the eyepiece in an area to coincide with a viewer&#39;s eye. One or more of such exit pupils of substantially the same size occur at the viewer&#39;s eye pupil. If a viewer moves their eye, the viewer&#39;s eye moves into alignment with other exit pupils. Also, by forming multiple exit pupils, the average brightness of the group of exit pupils formed within the viewer&#39;s eye at any given time stays approximately the same. When the viewer moves their eye to align with a newly defined group of exit pupils, the average brightness remains approximately the same as with the previous group of exit pupils. 
     In another embodiment the exit pupil expanding apparatus is formed by a bundle of aligned optical fibers, (e.g., a fiber optic face plate). One end of each fiber defines a portion of the curved plane which receives the light from the scanning device. Light enters a fiber over a given narrow angle, then exits over an enlarged angle. By creating an exit angle greater than the incident angle the exiting light impinges upon a larger surface of the ensuing eyepiece. The eye piece in turn passes light over an expanded exit pupil. According to variations, to best match the geometry of the eyepiece the fiber bundle defines at its exit surface either one of a flat planar surface or curved planar surface. 
     In another embodiment the exit pupil expanding apparatus is formed by a lens array. The lens array includes several small lenses in which each lens has a diameter on the order of 5-100 microns. Each lens is spaced as closely as possible to each adjacent lens in the array. The array defines a curved plane from sides of each lens facing the scanning device. Such curved plane receives the light from the scanning device. Light enters each lens over a given narrow angle, then exits over an enlarged angle. By creating an exit angle greater than the incident angle the exiting light impinges upon a larger surface of the ensuing eyepiece. As in the fiber bundle embodiment, the eyepiece in turn passes light over an expanded exit pupil. 
     In another embodiment the exit pupil expanding apparatus is formed by a diffuser. The diffuser defines a curved surface corresponding to the intermediate curved image plane. The diffuser evenly spreads the passing light. The light output from the diffuser spans a greater angle than the light incident to the diffuser. Thus, the light output from the diffuser is an expanded beam which passes through the eyepiece to define an expanded exit pupil. 
     In some embodiments the exit pupil expanding apparatus reflect light. The reflected light is used to form the expanded exit pupil(s). 
     FIG. 1 shows a block diagram of a virtual retinal display  10  according to one embodiment of this invention. The display  10  receives image data from a computer device, video device or other digital or analog image data source. Light generated by the display  10  is altered according to the image data to scan an image into the retina of a viewer&#39;s eye E. 
     The retinal display  10  generates and manipulates light to create color or monochrome images having narrow to panoramic fields of view and low to high resolutions. The display  10  does not generate a “real image” as done by CRTs, LCDs or an LED array. Instead, light modulated with video information is scanned directly onto the retina of a viewer&#39;s eye E to produce the perception of an erect virtual image. Because a real image is neither generated nor portrayed on a screen, the retinal display is small in size. In particular, the retinal display is suitable for hand-held operation or for mounting on the viewer&#39;s head. 
     The retinal display  10  includes an image data interface  11 , a light source  12 , an optics subsystem  14 , a scanning subsystem  16 , an exit pupil expanding apparatus  18 , and an eyepiece  20 . The image data interface  11  receives a video or other image signal, such as an RGB signal, NTSC signal, VGA signal or other formatted color or monochrome video or image data signal. The light source  12  includes one or more sources of light. In one embodiment red, green and blue light sources are included. The light sources or their output beams are modulated according to the input image data signal content to produce light which is input to an optics subsystem  14 . In one embodiment the emitted light is coherent. In another embodiment the emitted light is noncoherent. 
     Referring to FIG. 2, the optics subsystem  14  serves as an objective to focus the light. For some embodiments in which noncoherent light is received, the optics subsystem  14  also collects the light. The light exiting the optics subsystem  14  converges toward a focal point at image plane  15 . Prior to the image plane  15  is the scanning subsystem  16 . The scanning subsystem  16  deflects the light and the ensuing focal point to define an image plane of focal points. Typically the light is deflected along a raster pattern, although other display formats such as vector imaging also can be used. In one embodiment the scanning subsystem  16  receives a horizontal deflection signal and a vertical deflection signal derived from the image data interface  11 . In another embodiment, the scanning subsystem includes a mechanical resonator for deflecting passing light. FIG. 2 shows deflection of light  19  along one axis. As the light  19  is deflected, the focal point moves. Over the course of scanning a raster pattern the focal points define the intermediate curved image plane  15 . 
     The exit pupil expanding apparatus  18  is positioned at the location where the intermediate curved image plane  15  is to occur. Specifically the exit pupil expanding apparatus  18  has a curved surface upon which light  19  impinges. Such curved surface coincides with the image plane  15 . The light exiting the apparatus  18  exits over an angle qo which is larger than an angle qi of incident light. As a result the light exiting the apparatus  18  spreads over a larger surface area of the eyepiece  20 . This, in turn, causes a larger exit pupil  21  to occur. 
     The exit pupil expanding apparatus  18  in various embodiments expands the exit pupil to define an enlarged exit pupil  21  by generating multiple closely spaced (or overlapping) exit pupils and/or by enlarging the exit pupil(s). A diffractive optical element embodiment generates multiple exit pupils. A fiber-optic face plate embodiment, lens array embodiment or diffuser embodiment enlarges a single exit pupil. The light output from the exit pupil expanding apparatus  18  travels to the eyepiece  20 . The expanded exit pupil(s) occur slightly beyond the eyepiece  20  at a location where a viewer positions the pupil of their eye E. 
     FIG. 2 shows light paths for three pixels of an image to be formed on the eye E retina  23 . Light impinging on the apparatus  18  results in formation of an exit pupil  21 ,  21 ′,  21 ″ at a distance d from the eyepiece  20 . The part numbers  21 ,  21 ′,  21 ″ depict the exit pupil at respective points in time receiving light at differing respective angles. The light rays forming the exit pupil  21  for a given pixel impinge upon the eye&#39;s pupil  31  at a common angle. As a result, the light is focused to a point on the retina  23 . Such point  25  corresponds in effect to a pixel of an image. 
     A short time after the imaging of point  25 , the scanning subsystem  16  deflects light  19  to another focal point on the intermediate curved image plane  15 . As a result, the exit pupil  21 ′ occurs. Exit pupil  21 ′ corresponds to exit pupil  21  and occurs at approximately the same 3-dimensional position relative to the eyepiece  20 . This is evident from the common boundary points  33 ,  35  where the exit pupil  21 ,  21 ′forms at the eye pupil  31 . Exit pupil  21 ′ is formed by light rays impinging the eye pupil at a common angle. Such angle, however, differs from the angle of the light rays forming exit pupil  21 . Due to the differing angle, the light rays forming exit pupil  21 ′ focus at a different point  27  on the eye retina  23 . Thus, by deflecting the light  19 , the image point formed in the retina moves from point  25  to point  27 . 
     FIG. 2 further depicts formation of another image point  29  at the retina  23 . Such point  29  is formed as the scanning subsystem  16  deflects light  19  so as to change the current focal point location within the image plane  15 . In turn the altered focal point causes light rays of a different angle to define an exit pupil  21 ″. The light rays defining the exit pupil  21 ″ impinge the eye pupil  31  at a common angle. Such common angle differs than that for exit pupils  21 ,  21 ′. The result is a different image point  29  formed on the eye retina  23 . Thus, as the scanning subsystem deflects the light  19 , the light rays forming the exit pupil  21  ( 21 ′,  21 ″)at different moments in time impinge upon the eye pupil  31  at differing angles. For each variation in angle the focal point on the retina varies. As the scanning subsystem  16  deflects light  19  along a raster pattern, a raster of focal points occurs on the retina. The raster of focal points defines an image scanned directly on the retina. 
     For the pixels described above, at a given time the light rays defining an exit pupil are said to impinge on the eye at a common angle. For such an embodiment the eyepiece  20  preferably is positioned at one focal distance from the intermediate curved image plane  15 . In an alternative embodiment the relative distance between the image plane  15  and eyepiece is variable. In the case where the relative distance is slightly less than one focal length, the size and apparent depth of the image formed in the viewer&#39;s eye changes. 
     Light Source 
     The light source  12  includes a single or multiple light sources. For generating a monochrome image a single monochrome source typically is used. For color imaging, multiple light sources are used. Exemplary light sources are colored lasers, laser diodes or light emitting diodes (LEDs). Referring to FIG. 3, an embodiment having a respective red photon source  22 , green photon source  24  and blue photon source  26  are shown, although other colors may be used. 
     Although LEDs do not output coherent light, lenses are used in one embodiment to shrink the apparent size of the LED light source and achieve flatter wave fronts. In a preferred LED embodiment a single mode monofilament optical fiber receives the LED output to define a point source which outputs light approximating coherent light. 
     Additional detail on these and other light source  12  embodiments are found in U.S. patent application Ser. No., 08/437,818 for “Virtual Retinal Display with Fiber Optic Point Source” filed May 9, 1995, and incorporated herein by reference. 
     According to alternative embodiments, the light sources or the light generated by the light sources are modulated to include red, green and or blue components at a given point (e.g., pixel) of a resulting image. Referring to FIG. 3 respective beams of light sources  22 ,  24 ,  26  are modulated to introduce color components at a given pixel. Red light from source  22  is output to a modulator  28  then to a beam combining apparatus  34 . Green light from source  24  is output to a modulator  29  then to the beam combining apparatus  34 . Lastly, blue light from source  26  is output to a modulator  30  then to the beam combining apparatus  34 . The modulators  28 ,  29 ,  30  modulate the respective beams of light according to R, G and B component signals derived from the image data signal received into the display  10 . 
     In one embodiment the beam combining apparatus  34  is formed by an arrangement of dichroic mirrors or dichroic beam splitters which direct a substantial portion of each beam into a common beam. The light output along such common path is light  36  which subsequently enters the optical subsystem  14 . 
     Image Data Interface 
     The retinal display device  10  is an output device which receives image data to be displayed. Such image data is received as an image data signal at the image data interface  11 . In various embodiments, the image data signal is a video or other image signal, such as an RGB signal, NTSC signal, VGA signal or other formatted color or monochrome video or graphics signal. Referring to FIG. 3, an exemplary embodiment of the image data interface  11  extracts color component signals and synchronization signals from the received image data signal. In an embodiment in which an image data signal has embedded red, green and blue components, the red signal is extracted and routed to the modulator  28  for modulating the red light source  22  output. Similarly, the green signal is extracted and routed to the modulator  29  for modulating the green light source  24  output. Also, the blue signal is extracted and routed to the modulator  30  for modulating the blue light source  26  output. 
     The image data signal interface  11  also extracts a horizontal synchronization component and vertical synchronization component from the image data signal. In one embodiment, such signals define respective frequencies for horizontal scanner and vertical scanner drive signals routed to the scanning subsystem  16 . 
     Pre-Scanning Optics Subsystem 
     The optics subsystem  14  receives the output beam  36  from the beam combining apparatus  34  and converges the beam. Left undisturbed the beam converges to a focal point then diverges beyond such point. As the converging light is deflected, however, the focal point is deflected the pattern of deflection defines a pattern of focal points. Such pattern is referred to as an intermediate image plane. Referring to FIG. 2, such intermediate image plane  15  occurs as a curved image plane. 
     In an exemplary embodiment the optics subsystem  14  includes an objective lens  44  which converges the light  36  received from the light source  12 . In an embodiment for a laser diode light source, the optic subsystem  14  includes a cylindrical lens  40  and an objective lens  44  (see FIG.  3 ). The cylindrical lens  40  equalizes the divergence angle of the light output from the laser diode sources. The objective lens  44  then converges the light toward the intermediate curved image plane  15 —(see FIG.  2 ). 
     Post-Objective Scanning Subsystem 
     The scanning subsystem  16  receives the converging light output from the optics subsystem at a position prior to the curved image plane  15 . In one embodiment the scanning subsystem  16  includes two resonant scanners  200 . One scanner is for deflecting light along a horizontal axis. The other scanner is for deflecting light along a vertical axis. FIGS. 4 and 5 show a miniature optical resonant scanner  200 . The function of the scanner  200  is to deflect light along a horizontal axis or a vertical axis. In one embodiment a pair of scanners  200  deflect light along a raster pattern so as to define the intermediate image plane  15 . A scanner  200  serving as the horizontal scanner receives a drive signal having a frequency defined by the horizontal synchronization signal HSYNC extracted at the image data interface  11 . Similarly, a scanner  200  serving as the vertical scanner receives a drive signal having a frequency defined by the vertical synchronization signal VSYNC extracted at the image data interface. Such drive signals tune a resonance circuit of the scanner  200 . 
     Referring to FIG. 4 the scanner  200  includes a mirror  212  driven by a magnetic circuit so as to oscillate at a high frequency about an axis of rotation  214 . In one embodiment the only moving parts are the mirror  212  and a spring plate  216 . The optical scanner  200  also includes a base plate  217  with a pair of stator posts  218 ,  220  centrally located thereon. The base plate  217  and stator posts  218 ,  220  are integrally formed in one piece of a soft iron. Stator coils  222  and  224  are wound in opposite directions about the respective stator posts  218  and  220 . The electrical coil windings  222  and  224  may be connected in series or in parallel to a drive circuit as discussed below. Mounted on opposite ends of the base plate  217  are first and second magnets  226  and  228 , the magnets  226 ,  228  being equidistant from the stators  218  and  220 . In order to locate the magnet  226 , the base  217  is formed with a seat  230  having a back stop  232  extending up from one end of the seat  230  and having a front stop  234  extending up from the seat at the opposite end thereof. Similarly, to locate the magnet  228 , the base  217  is formed with a seat  236  at the end of the base opposite the seat  230 . The seat  236  includes a back stop  238  and a front stop  240  that extend upwardly from the seat  236  at the back and front thereof. 
     The spring plate  216  is formed of spring steel and is a torsional type of spring having a spring constant determined by its length and width. The spring plate  216  has enlarged opposite ends  242  and  244  that rest directly on a pole of the respective magnets  226  and  228 . The magnets  226  and  228  are oriented such that they have like poles adjacent the spring plate. For example, the North poles of the magnet  226  and  228  could be adjacent the spring plate  216  with the South poles of the magnets  226  and  228  adjacent the base  217 . Alternatively the South poles of both magnets  226 ,  228  could be adjacent the spring plate with the North pole of the magnets  226 ,  228  adjacent the base  217 . A narrower arm portion  246 ,  248  of the spring plate  216  extends from each of the enlarged ends  242 ,  244  to an enlarged central mirror mounting portion  250  of the spring plate  216 . The mirror mounting portion  250  forms the armature of the optical resonant scanner  200  and is mounted directly over the stator posts  218  and  220  such that the axis of rotation  214  of the mirror mounting portion  250  is equidistant from the stator posts  218  and  220 . The mirror  212  is mounted on or coated on the mirror mounting portion  250  of the spring plate. 
     The spring plate  216 , magnets  226  and  228  and the base  217  are tightly clamped together by respective spring plate caps  252  and  258 , each cap  252 ,  258  is formed as a block with openings  260  and  262 . The openings  260 ,  262  are formed so that the caps  252 ,  258  can accommodate the ends  242 ,  244  of the spring plate, the magnets  226 ,  228  and the seats  230 ,  236  as well as the arms  246  and  248  of the spring plate  216  when the caps  252 ,  258  are resting on the base  217 . The cap  252  is held securely to the base  217  by a pair of screws  254  and  256  so as to clamp the spring plate  216  and magnet  226  to the base. The screws  254  and  256  extend up through apertures  258  in the base  217  on opposite sides of the seat  230  and into threaded apertures formed in the cap  252  on opposite sides of the opening  260 . The cap  258  is similarly clamped to the base  217  by respective screws  261  and  263  that extend up through respective apertures  264  formed on opposite sides of the cap opening  262 . 
     Magnetic circuits are formed in the optical scanner  200  so as to oscillate the mirror  212  about the axis of rotation  214  in response to an alternating drive signal. One magnetic circuit extends from the top pole of the magnets  226  to the spring plate end  242 , through the arms  246  and mirror mounting portion  250  of the spring plate  216 , across a gap to the stator  218  and through the base  217  back to the magnet  226  through its bottom pole. Another magnetic circuit extends from the top pole of the magnet  228  to the spring plate end  244  through the arm  248  and mirror mounting portion  250  of the spring plate  216 , across a gap to the stator  218  and through the base  217  back to the magnet  228  through its bottom pole. Similarly, magnet circuits are set up through the stator  220  as follows. One magnetic circuit extends from the top pole of the magnet  226  to the spring plate end  242 , through the arm  246  and mirror mounting portion of the spring plate  216 , across the gap to the stator  220  and through the base  217  back to the magnet  226  through its bottom pole. Another magnetic circuit extends from the top pole of the magnet  228  to the spring plate end  244 , through the arm  248  and mirror mounting portion  250  of the spring plate  216 , across the gap to the stator  220  and then through the base  217  back to the magnet  228  through its bottom pole. 
     When a periodic drive signal such as a square wave is applied to the oppositely wound coils  222  and  224 , magnetic fields are created which cause the mirror  212  to oscillate back and forth about the axis of rotation  214 . More particularly, when the square wave is high for example, the magnetic field set up by the magnetic circuits through the stator  218  and magnets  226  and  228  cause an end  266  of the mirror mounting portion  250  to be attracted to the stator  218 . At the same time, the magnetic field created by the magnetic circuits extending through the stator  220  and the magnets  226  and  228  cause the opposite end  268  of the mirror mounting portion  250  to be repulsed by the stator  220 . Thus, the mirror is caused to rotate about the axis of rotation in one direction. When the square wave goes low, the magnetic field created by the stator  218  repulses the end  266  of the spring plate  250  whereas the stator  220  attracts the end  268  of the spring plate portion  250  so as to cause the mirror  212  to rotate about the axis  214  in the opposite direction. 
     In alternative embodiments, the scanning subsystem  14  instead includes acousto-optical deflectors, electro-optical deflectors, rotating polygons or galvanometers to perform the horizontal and vertical light deflection. In some embodiments, two of the same type of scanning device are used. In other embodiments different types of scanning devices are used for the horizontal scanner and the vertical scanner. 
     Exit Pupil Expanding Apparatus Embodiments 
     Following are descriptions of four preferred embodiments of the exit pupil expanding apparatus, various alternative embodiments and corresponding methods for forming the exit pupil(s). The preferred embodiments are a diffractive optical element, fiber optic face plate, lens array and a diffuser. In each preferred embodiment the exit pupil expanding apparatus defines a curved entry surface coinciding with the intermediate curved image plane. In various alternative embodiments, light is partially or fully reflected with the resulting reflected light used in defining the exit pupil(s). 
     Diffractive Optical Element 
     FIGS. 6 a  and  6   b  show the light path through a diffractive optical element  300  embodiment of the exit pupil expanding apparatus  18  of this invention. A diffractive optical element is a device which uses diffraction to control an optical wavefront. Exemplary diffractive optical elements include diffraction gratings, surface-relief diffractive lenses such as Fresnel lenses, holographic optical elements and computer generated holograms. Fabrication techniques include diamond machining, interference of coherent beams (holography), injection molding and microlithographic techniques. In a preferred embodiment a diffraction grating is used. 
     FIG. 6 a  shows the light path through a diffraction grating embodiment  300  for scanning a pixel onto the retina of an eye. FIG. 6 b  shows the light path through the diffraction grating embodiment  300  for scanning another pixel onto the retina of an eye. The diffraction grating  300  has a curved surface  301  which is coincident with the intermediate curved image plane  15 . In some embodiments the diffraction grating surface  301  extends beyond the image plane  15 . 
     The diffraction grating  300  receives light  302  from the scanning subsystem  16 , then passes a fraction  304  of the incident light  302 . One portion  304  is passed in an unchanged direction. Additional portions  306 ,  308  are deflected at specific angles relative to the incident light  302 . The result is multiple beams of light  304 - 308 . The percentage of light in each beam  304 - 308  is determined by the diffraction pattern. The angles of the exiting light beams  304 - 308  are defined by grating  300  spacing and the light wavelengths. Each of the multiple beams  304 - 308  is an expanding beam which passes through the eyepiece  20  to define a separate exit pupil  310 ,  312 ,  314  at a known distance d from the eyepiece  20 . The viewer positions their eye pupil at the distance d from the eyepiece  20 . In effect multiple closely spaced exit pupils  310 - 314  are defined beyond the eyepiece  20 . When the viewer&#39;s eye E is in position, one or more exit pupils  310 - 314  are formed at the viewer&#39;s eye pupil. 
     In an exemplary embodiment a 2-dimensional array of 7×7 exit pupils are defined at the distance d. Each exit pupil is approximately 1.5 mm in diameter. A spacing of approximately 1 mm occurs between each exit pupil. Depending on the ambient lighting, the eye pupil spans a diameter of approximately 2 to 7 mm. Thus, between 1 and approximately 9 exit pupils occur at the pupil of the viewer&#39;s eye E. 
     In a given dimension, between 1 and 3 exit pupils occur at the eye pupil. More significantly, there are between 4 and 6 more exit pupils occurring along a given direction outside the eye pupil. Thus, if a viewer moves their eye, they will move into alignment with one or more of such other exit pupils. From a centered position, a viewer would need to move by 2 or 3 exit pupils (e.g., 5 to 7.5 mm) before losing alignment with all retinal display exit pupils. This corresponds a substantially increased freedom of movement for the viewer. More specifically, given the conventional single exit pupil of 1.5 mm centered at an eye pupil of 2 mm in diameter, a viewer loses alignment by moving approximately 1 mm. Given a centered 7×7 array of exit pupils according to this invention, the viewer would need to move 5 to 7.5 mm to lose alignment. Further, by forming multiple exit pupils  308 , the average brightness of the exit pupils  308  formed at the viewer&#39;s eye stays approximately the same when the viewer movers their eye to align with other exit pupils  308 . 
     Although a 7×7 array of exit pupils is described in which each exit pupil is 1.5 mm in diameter and spaced 1 mm apart, arrays of different size and exit pupils of different diameter and spacing are used in alternative embodiments. In one embodiment the exit pupil has a diameter within a range of 2 mm to less than 1 mm. In other embodiments larger exit pupils are formed having a diameter larger than 2 mm (e.g., 4 mm diameter or larger). 
     Referring to FIG. 6 a  light from each exit pupil  310 - 314  aligned with the eye E enters the eye at a common angle qA. As a result the light from each exit pupil  310 - 312  focuses at the same point  316  on the retina  315 . This point corresponds to one pixel of the image being scanned. In comparison, in FIG. 6 b  light from each exit pupil  310 ′,  312 ′ also enters at a common angle qβ. Such angle qβ, however, differs from the angle qA in FIG. 6 a . The different incidence angle causes light to focus at a different point  318  on the retina  315 . This point  318  corresponds to a different pixel of the image being scanned. As the scanning subsystem  16  deflects the light  302  the focal point on the image plane  15  moves. As the light  302  scans the image plane  15 , the exit pupils  310 - 314  define light impinging the eye pupil at changing angles. Thus, the point where the light strikes the retina changes. Over the scanning cycle the light scans many points or pixels directly onto the retina  315 . 
     Fiber Optic Face Plate 
     FIG. 7 shows the light path through a fiber optic face plate  320  embodiment of the exit pupil expanding apparatus  18 . The fiber optic face plate  320  is formed by a bundle of aligned optical fibers  322   a - 322   n . One end  324  of each fiber  322   i  defines a portion of a curved plane  326  receiving incident light  302  from the scanning subsystem  16 . The curved plane  326  is coincident with the intermediate curved image plane  15 . Light  302  impinges upon the face plate  320  at surface  326 . As the scanning subsystem  16  deflects the light in a raster pattern an image plane  15  forms at the surface  326 . For each discrete deflection of light a pixel is defined. FIG. 7 shows the light  302  focussing at one point (i.e., pixel) of the image plane  15 . 
     Light  302  enters a fiber  322 i over a given narrow angle q 1 , then exits over an enlarged angle q 2 . In some embodiments the fiber approximates a Lambertian source. The enlarged angle causes a larger surface of the eyepiece to receive light  328 . In turn the eyepiece creates a larger exit pupil  330  at the eye pupil. For each pixel imaged on the retina there is the same enlarged exit pupil. Note that for each pixel scanned, the light defining the exit pupil impinges on the eye pupil at a different angle. Thus, the light entering the eye is focused at differing points as the subsystem deflects the light  302  along a raster pattern. The differing points are, in effect, pixels of an image being scanned onto the retina. 
     In one embodiment the fiber bundle  320  defines a diameter of 2-5 cm. Each fiber  322   i  defines a diameter of approximately 5 microns and a length of approximately 3 mm. The specific dimensions of each fiber  322   i , however may vary. According to variations, to best match the geometry of the eyepiece  20  the fiber bundle  320  defines at its exit surface  332 , either one of a flat planar surface or curved planar surface. 
     Lens Array 
     FIG. 8 shows the light path through a lens array  340  embodiment of the exit pupil expanding apparatus  18 . The array  340  includes several small lenses  342   a - 342   n . Each lens  342   i  is on the order of 5-100 microns in diameter. In a preferred embodiment lenses  342   i  having a diameter of approximately 10 microns are used. Each lens  342   i  is spaced as closely as possible to each adjacent lens  342 (i+1),  342 (i−1) in the array  340 . In one embodiment the lens array  340  defines a diameter of 2-5 cm. The side  344  of each lens  342   i  facing the scanning subsystem  16  defines a curved plane  346 . The cumulative plane  346  of the lenses coincides with the image plane  15  scanned by the scanning subsystem  15 . In alternative embodiments the lens array  340  is defined as a holographic optical element. 
     Light  302  enters a lens  342   i  over a given narrow angle q 3 , then exits over an enlarged angle q 4 . The enlarged angle q 4  causes a larger surface of the eyepiece  20  to receive light  350 . In turn the eyepiece creates a larger exit pupil  352  at the eye pupil. For each pixel imaged on the retina there is the same enlarged exit pupil. Note that for each pixel scanned, the light defining the exit pupil impinges on the eye pupil at a different angle. Thus, the light entering the eye is focused at differing points as the subsystem deflects the light  302  along a raster pattern. The differing points are, in effect, pixels of an image being scanned onto the retina. 
     Diffuser 
     FIG. 9 shows the light path through a diffuser  360  embodiment of the exit pupil expanding apparatus  18 . The diffuser  360  receives the light  302  from the scanning subsystem  16 . The intermediate curved image plane  15  scanned by the scanning subsystem  16  coincides with a curved surface  362  of the diffuser. In one embodiment the diffuser  360  is engineered to evenly spread the passing light. In alternative embodiments the diffuser  360  is a reflective diffuser or is defined as a holographic optical element or a holographic random phase element. 
     The light  364  output from the diffuser  360  is an expanding beam which passes through the eyepiece  20  to define an expanded exit pupil  366  at a position where the viewer is to place their eye E. Note that for a given pixel of the image plane  15 , the angle of light  364  exiting the diffuser is larger than the angle at which light  302  enters the diffuser. As a result, a larger portion of the eyepiece  20  receives light  364 . In turn a larger exit pupil (i.e., larger diameter) occurs at a distance d beyond the eyepiece  20 . For each pixel scanned, the light defining the exit pupil impinges on the eye pupil at a different angle. Thus, the light entering the eye is focused at differing points as the subsystem deflects the light  302  along a raster pattern. The differing points are, in effect, pixels of an image being scanned onto the retina. 
     Eyepiece 
     The eyepiece  20  typically is a multi-element lens or lens system receiving the light beam(s) exiting from the exit pupil enlarging apparatus  18 . In an alternative embodiment the eyepiece  20  is a single lens. The eyepiece  20  serves to relay the rays from the light beam(s) toward a viewer&#39;s eye. In particular the eyepiece  20  contributes to the location where an exit pupil of the retinal display  10  forms. The eyepiece  20  defines one or more exit pupils at a known distance d from the eyepiece  20  as shown in FIGS.  2  and  6 - 9 . Such location is the expected location for a viewer&#39;s eye E. As a result, one or more exit pupils are formed coincident with a viewer&#39;s eye, the eye being positioned adjacent to the eyepiece. 
     In one embodiment the eyepiece  20  is an occluding element which does not transmit light from outside the display device  10 . In an alternative embodiment, an eyepiece lens system  20  is transmissive so as to allow a viewer to view the real world in addition to the virtual image. In yet another embodiment the eyepiece is variably transmissive to maintain contrast between the real world ambient lighting and the virtual image lighting. For example a photosensor detects ambient lighting. A bias voltage is generated which applies a voltage across a photochromatic material to change the transmissiveness of the eyepiece  20 . 
     Alternative Embodiments 
     The components of the retinal display  10  can be made small, compact and lightweight so as to embody a hand-held display or to be mounted on a viewer&#39;s head without requiring a helmet or an elaborate head mounting support. Also the light source  12  and image data interface  11  can be separated from the rest of the display  10  to further reduce the volume and weight of the display portion adjacent to the viewer&#39; eye. For example the modulating light emitted from the light source  12  is coupled to the optical subsystem  14  in an alternative embodiment via one or a bundle of monofilament optical fibers. 
     For a display device providing stereoscopic viewing two retinal display devices  10  are used. If combining two monocular systems to define binocular viewing, however, there is a potential conflict between distance cues and focus. 
     Meritorious and Advantageous Effects 
     One advantage of this invention is that the shorter light path allows for a more compact, lighter weight retinal display device. Another advantage is that a viewer has less difficulty aligning and maintaining alignment with an exit pupil formed at the eyepiece. In particular, the expanded exit pupil, the multiple exit pupils or the multiple, expanded exit pupils make it easier for a viewer to find an exit pupil. Another advantage with regard to the diffractive optical element embodiment is that image brightness is generally uniform among various groups of exit pupils which form at the viewer&#39;s eye. 
     Although a preferred embodiment of the invention has been illustrated and described, various alternatives, modifications and equivalents may be used. Therefore, the foregoing description should not be taken as limiting the scope of the inventions which are defined by the appended claims.