Patent Publication Number: US-2013242053-A1

Title: Endoscopic apparatus and method for producing via a holographic optical element an autostereoscopic 3-d image

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application No. 12/408,447 filed Mar. 20, 2009, the disclosure of which is incorporated in its entirety by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to an apparatus and method for creating and displaying autostereoscopic three-dimensional images from an endoscope. 
     2. Background Art 
     Stereoscopic display devices separate left and right images corresponding to slightly different views or perspectives of a three-dimensional scene or object so that they can be directed to a viewer&#39;s left and right eye, respectively. The viewer&#39;s visual system then combines the left-eye and right-eye views to perceive a three-dimensional or stereo image. A variety of different strategies have been used over the years to capture or create the left and right views, and to deliver or display them to one or more viewers. Stereoscopic displays often rely on special glasses or headgear worn by the user to deliver the corresponding left and right images to the viewer&#39;s left and right eyes. These have various disadvantages. As such, a number of strategies have been, and continue to be, developed to provide autostereoscopic displays, which deliver the left and right images to corresponding eyes of one or more viewers without the use of special glasses or headgear. 
     Real-time medical imaging applications for diagnosis, treatment, and surgery have traditionally relied on equipment that generates two-dimensional images. For example, various types of endoscopy or minimally invasive surgery use an endoscope or similar device having a light source and camera to illuminate and provide a real-time image from within a body cavity. For some applications, special headgear or glasses have also been used to create a real-time three-dimensional view using stereo images. However, glasses or headgear may cause fatigue and/or vertigo in some individuals after extended viewing times due to visual cues from peripheral vision outside the field of view of the glasses or headgear. 
     SUMMARY OF THE INVENTION 
     This disclosure relates to systems and methods for generating a three-dimensionally perceived image by at least one viewer. Included in one embodiment is an autostereoscopic display having a left projector and a right projector that project corresponding left and right images received from corresponding left and right cameras of a stereo endoscope through a transmissive holographic optical element (“HOE”). The HOE functions as a Bragg diffraction grating to redirect light from the left projector to a left eye-box and to redirect light from the right projector to a right eye-box for viewing by left and right eyes of a viewer to create a three-dimensionally perceived image without glasses or optical headgear. 
     An endoscopic viewing apparatus according to one embodiment of the present disclosure includes a tube having a light delivery system for illuminating a body cavity for inspection and at least two cameras within the tube for capturing corresponding images of the body cavity. The at least two cameras provide corresponding video signals to at least two projectors that each project a corresponding real-time image from a different angle onto a common area of one side of a transmissive holographic diffraction grating. The diffraction grating redirects incident light passing therethrough to viewing zones for each one of a viewer&#39;s eyes to create a real-time stereo image for a viewer. In a two-projector embodiment that generates two eye-boxes for a single viewer, a left projector is positioned at a first azimuthal angle relative to the holographic diffraction grating to direct a projected image corresponding to a first camera to a left eye-box and a right projector is positioned at a second azimuthal angle to direct a projected image corresponding to a second camera to a right eye-box, such that a viewer perceives a stereo image in three-dimensions unaided by special glasses, optical headgear, or the like. 
     Various embodiments of an endoscopic viewing apparatus according to the present disclosure may include an eye/head tracking system to move the viewing system in response to viewer movement, such that the viewer&#39;s eyes remain within corresponding left and right eye-boxes. In one embodiment a tracking system includes an emitter/detector positioned above the holographic element and in communication with a tracking computer that generates signals for a computer-controlled actuator that repositions the display system in response to viewer movement. The actuator may be implemented by a servo-controlled rotary stage, for example. The system may also include a plurality of retro-reflectors worn by the viewer to facilitate detection of viewer movement. In one embodiment, a visor having three curved non-coplanar retro-reflectors facilitates detection of viewer head movements. 
     One method for generating a three-dimensionally perceived image from an endoscope includes projecting substantially coextensive left and right images from corresponding left and right cameras disposed within the endoscope through a transmissive holographic diffraction grating from first and second azimuthal angles such that light projected at the first azimuthal angle is directed through the diffraction grating to a left eye of a viewer and light projected at the second azimuthal angle is directed through the diffraction grating to a right eye of the viewer. The method may also include video signal processing to combine video signals from the left and right cameras into a stereo video signal and transmitting the combined stereo video signal to an auxiliary display and/or recording the combined stereo video signal for subsequent playback. Three-dimensional viewing of the auxiliary display may include viewing aids, such as glasses, headgear, or the like, to separate or filter the left and right images for a viewer&#39;s left and right eyes. 
     In one embodiment, a method for generating an autostereoscopic three-dimensional image includes projecting first and second substantially overlapping images onto and through a transmissive viewing element having a holographically recorded diffraction pattern captured within a varying thickness photosensitive material, the diffraction pattern produced by an interference pattern being created by mutually coherent object and reference beams of a laser. In one embodiment, the interference pattern is captured in a master holographic plate having a photo-sensitive emulsion deposited on a substrate (such as glass or triacetate film), which is subsequently chemically processed to remove a portion of the emulsion. The remaining emulsion forms a desired master diffraction grating, sometimes referred to as a H 1  hologram. The master holographic plate is then copied using known holographic techniques to a second holographic plate, sometimes referred to as a H 2  hologram, which is chemically processed in a similar fashion to produce the holographic diffraction grating. 
     Embodiments according to the present disclosure have various associated advantages. For example, embodiments of the present disclosure provide real-time stereo images to corresponding eyes of at least one viewer to produce a three-dimensionally perceived image without viewing aids, such as glasses or headgear. The present disclosure provides real-time viewer position detection and image display synchronization to allow the viewer to move while staying within predetermined eye-boxes so that perception of the three-dimensional image is unaffected by viewer movement. Use of a transmissive holographic diffraction grating allows back illumination to facilitate packaging for endoscopic viewing applications. Transmissive holographic diffraction gratings according to the present disclosure may also provide better brightness and contrast for the viewer relative to reflection-type gratings or elements and exhibit reduced chromatic dispersion. 
     The above advantages and other advantages and features will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating operation of an apparatus and method for autostereoscopic display of an endoscopic image for three-dimensional perception by a viewer according to one embodiment of the present disclosure; 
         FIG. 2  illustrates a single-axis computer controlled actuator for positioning the display in response to viewer movement according to one embodiment of the present disclosure; 
         FIG. 3  illustrates a position tracking emitter and detector for use in synchronizing movement of the display with viewer movement according to one embodiment of the present disclosure; 
         FIG. 4  illustrates visor mountable retro-reflectors for use with the position tracking emitter and detector of  FIG. 3  according to one embodiment of the present disclosure; 
         FIG. 5  is a partial cross-sectional view of an endoscope having at least two cameras, a light source, and imaging optics for three-dimensional viewing of an image according to one embodiment of the present disclosure; 
         FIG. 6  is a back view of a display system according to one embodiment of the present disclosure; 
         FIG. 7  is a perspective view of a display system according to one embodiment of the present disclosure; 
         FIG. 8  is a front perspective view illustrating a projection sub-assembly of a display system according to one embodiment of the present disclosure; 
         FIG. 9  is an enlarged perspective view of imaging optics for the projection sub-assembly illustrated in  FIG. 8 ; 
         FIG. 10  is a back perspective view illustrating a projection sub-assembly of a display system according to one embodiment of the present disclosure; 
         FIG. 11  is a diagram illustrating electrical and video signal connections for a display system according to one embodiment of the present disclosure; 
         FIG. 12  is a flow diagram illustrating control logic for synchronizing the display system with viewer movement to provide a head tracking function of a system or method for three-dimensional image generation according to one embodiment of the present invention; and 
         FIG. 13  is a diagram illustrating operation of a system for making a holographic diffraction grating for a three-dimensional imaging system or method according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. The representative embodiments used in the illustrations relate generally to an autostereoscopic display system and method capable of displaying a stereo image in real-time using either live stereo video input from a stereo endoscope, or a standard video input processed to generate simulated stereo video that is perceived as a three-dimensional image by a properly positioned viewer. 
     Referring now to  FIG. 1 , a schematic diagram illustrating an endoscopic apparatus and method for producing a three-dimensional image via a holographic optical element of an autostereoscopic display according to embodiments of the present disclosure is shown. System  100  includes a display system  110  for projecting an autostereoscopic image captured from a stereo endoscope  112  so that user  114  perceives a three-dimensional image of the interior of a cavity  116  of a body  118  or other object unaided by special glasses or optical headgear. Stereo endoscope  112  may provide left video  132  and right video  134  to a video processor  130 , or directly to display system  110 , depending on the particular application and implementation. Video signal processor  130  may combine or encode the stereo video signals into a multiplexed signal for display on a local or remote auxiliary screen  190  and/or for recording on a recording device  196 , such as a VCR or DVD recorder, for example. Three-dimensional viewing of auxiliary display  190  by another viewer  192  may require viewing glasses  194 , such as polarized or active shutter glasses depending upon the particular implementation. 
     In one embodiment, video processor  130  is implemented by a stereo encoder/decoder commercially available from 3-D ImageTek Corp. of Laguna Niguel, Calif. and combines the two stereo input signals into a single field-multiplexed output video signal, or vice versa. Video signal processor  130  may also include a pass-through mode where video feeds  132 ,  134  pass through to output feeds  136 ,  138  without any signal multiplexing, but may provide noise filtering, amplification, or other functions, for example, between the stereo inputs and corresponding stereo outputs. 
     As also shown in  FIG. 1 , stereo video output signal lines  136 ,  138  are provided to at least two associated projectors  140 ,  142  within enclosure  110  via a cable panel ( FIG. 11 ). Projectors  140 ,  142  project corresponding images in real-time through various optical elements including lenses  144 ,  146  and (optionally) mirrors  148 ,  150 ,  160 ,  170 , to focus substantially co-extensive overlapping images on, and through, transmissive holographic element  180 . Holographic element  180  (sometimes referred to as a transmissive “screen” even though the resulting three-dimensional image perceived by the viewer may appear in front of and/or behind the element) may be implemented by a holographic optical element (HOE) that functions as a Bragg diffraction grating, and may therefore also be referred to as a diffractive optical element (DOE). Holographic element  180  diffracts light passing therethrough from projector  140  to a first viewing zone or eye-box  182  and light passing therethrough from projector  142  to a second viewing zone or eye-box  184 . When viewer  114  is properly positioned, each eye will see only one of the images of a corresponding eye-box. The slightly different perspective provided by each image is combined by the visual processing of the viewer&#39;s brain and the viewer perceives a three-dimensional image of the interior of cavity  116  as captured by a stereo imaging system within tube  106  of stereo endoscope  112  as illustrated and described with reference to  FIG. 5 . 
     System  100  may also include a head tracking subsystem  120  that synchronizes or aligns a viewer&#39;s eyes with a stereoscopic viewing zone corresponding to the left eye-box  182  and right eye-box  184 . Head tracking subsystem  120  may include means for moving eye-boxes  182 ,  184  in response to movement of viewer  114 . In the embodiment illustrated in  FIG. 1 , the means for moving eye-boxes  182 ,  184  includes means for moving enclosure  110 , which includes projectors  140 ,  142 , lenses  144 ,  146 , mirrors  148 ,  150 ,  160 ,  170 , and holographic element  180 , and means for detecting movement of viewer  114 . The means for moving enclosure  110  may be implemented by a single or multi-axis microprocessor controlled actuator  188 . In one embodiment, the means for moving enclosure  110  corresponds to actuator  188 , which includes a base  192 , stepper motor  194 , and rotary stage  196  with stepper motor and controller  194  commanded by control logic or software executed by a computer  178 . The means for detecting movement of viewer  114  may include computer  178 , which communicates with motor /controller  194  and tracking emitter/detector  172  with computer  178  generating commands to rotate stage  196  in response to changes in position of viewer  114 . 
     Tracking emitter/detector  172  may be mounted on enclosure  110  above holographic element  180  and emit an electromagnetic signal  174  in the direction of viewer  114 . In the illustrated embodiment, viewer  114  is wearing a visor  122  having three non-coplanar retro-reflectors  124 ,  126 , and  128  that generate a one or more reflected signals  176  indicative of the position of the head of viewer  114 . The detected signal is processed by software running on head-tracking computer  178  to synchronize movement of eye-boxes  182 ,  184  with eyes of viewer  114 . One embodiment of a head tracking synchronization function is illustrated and described in greater detail with respect to  FIG. 12 . In one embodiment, tracking emitter/detector  172  is implemented by the TRACKIR™ sensor commercially available from NaturalPoint, Inc. of Corvallis, Oreg. 
     As will be appreciated by those of ordinary skill in the art, light projected from projectors  140 ,  142  exits the projectors at substantially the same altitudinal angle but a different azimuthal angle, i.e. into/out of the plane of the paper. In the illustrated embodiment, commercially available projectors (Model NP-40 from NEC Corporation) are used with projector  140  mounted upside-down to provide a desired lens-to-lens distance between projector  140  and  142 . These projectors are single-chip, DLP-based projectors with various embedded color correction, focusing, and keystone correction functions. Mounting one projector upside-down results in the projector housings being at different altitudinal angles, but the output lenses are positioned at substantially the same altitudinal angle as described in greater detail herein. The embedded projector processor functions are used to flip the image of projector  140 , and to provide various color and keystone adjustments for both projectors  140 ,  142  so that the images projected on holographic element  180  are substantially rectangular and co-extensive or completely overlapping with right-angle corners. Appropriate keystone correction provides accurate depth perception for viewer  114  based on the projected stereo images. 
     Referring now to  FIG. 2 , a perspective view of a representative computer-controlled actuator for use in a head tracking system of an autostereoscopic display for viewing three-dimensional endoscopic images according to the present disclosure is shown. While a single-axis actuator is illustrated, those of ordinary skill in the art will recognize that multi-axis actuators could be used to synchronize movement of eye-boxes  182 ,  184  with movement of viewer  114 . In this embodiment, actuator  188  includes a stationary base  192  with a rotatable stage or platform  196  that may be directly-driven or belt-driven by a stepper motor/controller  194 . In one representative embodiment, system  100  includes a precision rotary stage, which is commercially available from Newmark Systems, Inc of Mission Viejo, Calif. (Model RM-8). 
       FIG. 3  is a perspective view of a representative sensor  172  that may be used in a head tracking system  120  ( FIG. 1 ) to detect the position of a viewer  114  according to embodiments of the present disclosure. As previously described, sensor  172  may include one or more infrared emitters and one or more infrared detectors within a curved housing  202  with an infrared filter cover  204 . A standard  206  or custom mount may be used to secure sensor  172  to enclosure  110  ( FIG. 1 ) such that sensor  172  is positioned approximately in the center of holographic element  180 , and either above or below holographic element  180  such that it does not obstruct the view of viewer  114 . Of course various other types of sensor(s) and sensor positioning may be used to provide a head tracking function according to the teachings of the present disclosure. 
       FIG. 4  is a perspective view illustrating a representative embodiment of a retro-reflector unit including three curved and non-coplanar retro-reflectors  124 ,  126 , and  128 . The retro-reflectors may be worn by, or positioned on, a viewer  114  ( FIG. 1 ) to facilitate motion tracking as previously described. In the illustrated embodiment retro reflectors  126 ,  128  are spaced to correspond to an approximate average inter-pupillary distance for viewers. Of course, various other types of reflectors may be used and positioned to suit a particular application or implementation in accordance with the teachings of the present disclosure. 
     Referring now to  FIG. 5 , a partial cross-section of a representative stereo endoscope for use in embodiments of an apparatus or method according to the present disclosure is shown. Stereo endoscope  112  ( FIG. 1 ) may include a tube  106  and an annular light delivery system optionally having one or more optic fibers  210 ,  212  to illuminate a distal end of tube  106  as generally represented by areas  230  and  240  for viewing of an object  222  being inspected. Light reflected from object  222  is collected and imaged by one or more cameras  214 ,  216  that may be optically coupled by a lens or lens system  220 , which is at least partially disposed within tube  106 . Lens system  220  may include a single lens or multiple optical components, such as lenses, mirrors, and the like. First camera  214  and second camera  216  may also include associated optic elements to provide corresponding focused images that are converted to video signals delivered through tube  216  via wired or wireless connections for display on display system  108  as previously described. 
     In one embodiment of a method according to the present disclosure, a first endoscope image is captured by first camera  214  disposed within tube  106  of endoscope  112  ( FIG. 1 ) and transmitted to a first projector  140  ( FIG. 1 ) for projection onto and through holographic diffraction grating  180  ( FIG. 1 ) from a first angle to a first eye-box  182  ( FIG. 1 ). The method also includes capturing a second endoscope image at substantially the same time as the first image with second camera  216  disposed within tube  106  of endoscope  112  ( FIG. 1 ), and transmitting the second image to a second projector  142  ( FIG. 1 ) for projection onto and through holographic diffraction grating  180  ( FIG. 1 ) from a second angle to a second eye-box  184  ( FIG. 1 ). 
     As illustrated in  FIGS. 1-5 , holographic optical element  180  is a diffractive optical element (DOE), which is a kind/class of holographic optical element (HOE) created using holographic techniques as known in the art and modified as described herein. The illustrated embodiment of system  100  incorporates a transmissive element  180  with light from at least two projectors  140 ,  142  shining from behind element  180  (relative to viewer  114 ) and passing through element  180  to corresponding left/right eye-boxes  182 ,  184  or viewing zones to create the image perceived as a three-dimensional image by viewer  114 . Element  180  functions to diffract incident light from first projector  140  positioned at a first azimuthal angle of incidence relative to element  180  to a first eye-box  182  or viewing zone. Likewise, light from second projector  142  positioned at a second azimuthal angle of incidence relative to element  180  passes through element  180  and is diffracted toward a second eye-box  184  or viewing zone. A viewer  114  properly positioned in front of display device  108  at the viewing “sweet spot” sees only the left image  182  with the left eye and the right image  184  with the right eye. If the left image and right images are appropriately shifted one relative to the other, i.e. contain an appropriate amount of horizontal parallax, the viewer&#39;s brain combines the left and right images and the viewer  114  perceives a three-dimensional image. The horizontal parallax provides the third dimension or depth to the image, which appears in front of, within, or spanning the plane of element  180 . The position of the perceived image relative to the viewing element can be controlled by appropriate positioning of the holographic plate used to create the DOE during the holographic recording process as illustrated and described with reference to  FIG. 13 . If viewer  14  moves out of the “sweet spot”, the three-dimensional effect is at least partially lost and viewer  14  no longer perceives a three-dimensional image. 
     To reduce or eliminate loss of the three-dimensional image, head tracking system  120  attempts to synchronize movement of eye-boxes  182 ,  184  with movement of viewer  114  to maintain alignment of a viewer&#39;s eyes with the “sweet spot” or stereoscopic viewing zone of the display. Although numerous other head/eye tracking strategies are possible, the strategy illustrated and described for above for a prototype display rotates the entire display enclosure  110  in response to viewer movement. 
     As previously described, the left and right video signals provided to the left and right projectors may be captured in real-time by corresponding left and right cameras positioned within an endoscope to provide appropriate parallax. Alternatively, the left and right video signals may be generated by a video signal processor, such as processor  130  ( FIG. 1 ) or the like, that processes a standard format video input signal captured by a single camera (two-dimensional) to create a stereo left/right output signal provided to the left/right projectors by adding horizontal parallax to the left/right video output signals. As another alternative, either or both of the left/right video input signals could be based on images generated entirely by computer, i.e. CG images. 
     Referring now to  FIGS. 6 and 7 , a back view ( FIG. 6 ) and back perspective view ( FIG. 7 ) of a representative embodiment of a display system  108  for use in a medical imaging system  100  according to the present disclosure are shown. A back panel normally in place during operation has been removed for illustration purposes. Enclosure  110  includes a common (shared) upper mirror mount  310  for securing mirror  160  within enclosure  110 . Similarly, a common (shared) lower mirror mount  312  is provided to secure lower mirror  170  within enclosure  110 . In one embodiment, upper mirror mount  310  and lower mirror mount  312  are fixed mounts with angles and distances determined so that the projected images from projectors  140 ,  142  substantially overlap (are co-extensive) with common boundaries and completely fill holographic element  180 . Of course, manually or electromechanically adjustable mounts may be used for one or more mirrors or other optical elements depending on the particular application and implementation. For example, single-axis or multiple-axis gimbal mounts may be used for one or more optical elements to adjust the angle(s) of projected light from one or both projectors  140 ,  142 . Upper and lower mirrors  160 ,  170  may be positioned to match the optical path length or beam length of projectors  140 ,  142  to the corresponding beam length and angle selected during recording of holographic element  180  as described herein while folding the beam path to meet desired packaging constraints. As such, the number and position of optical elements used may vary by application and implementation. Upper mirror  160  and lower mirror  170  are preferably front (first) surface enhanced aluminum mirrors having a reflectivity of about 95%. In one embodiment, upper mirror  160  is about 356 mm×130 mm×3.17 mm while lower mirror  170  is about 356 mm×180 mm×3.17 mm. 
     In the illustrated embodiment, projectors  140 ,  142  are arranged to project the image through the holographic element  180  to the viewer  114  using various front-surface mirrors to fold the optical path and provide a more compact display unit. However, the optical path of the projected images may be modified for particular applications to improve aesthetics, hide the projectors from direct view, or for implementation of a display using a different HOE while maintaining a desired beam path. 
     Enclosure  110  may include one or more passive ventilation ports  330  that may be aligned with vents on projectors  140 , and  142  to provide proper heat dissipation from enclosure  110  and manage internal operating temperatures. Enclosure  110  may also include one or more powered ventilation fans  320 ,  322  that may be manually or automatically controlled to manage operating temperatures of projectors  140 ,  142 . 
     As also shown in  FIGS. 6-7  and in the perspective view of  FIG. 8 , enclosure  110  may include a projection sub-assembly  314  to position projectors  140 ,  142  in a desired orientation and to secure projectors  140 ,  142  within enclosure  110 . In the illustrated embodiment, commercially available projectors are used as previously described. As such, to achieve a desired lens-to-lens distance between first projector  140  and second projector  142 , projector  140  is mounted upside-down as previously described, which requires a different orientation (angle) of the projector housing relative to the housing of projector  140  to align the corresponding projected images on the holographic element  180 . In addition, the embedded projector controls are used to flip the image projected by projector  140  so it has the same (right-side-up) orientation as the image projected by projector  142 . As illustrated in the front perspective of  FIG. 8  and the rear perspective of  FIG. 10 , projector  140  is mounted upside-down with the projector housing angled generally upward relative to enclosure  110 , while projector  142  is mounted right-side-up with its associated projector housing angled generally downward relative to the bottom of enclosure  110 . 
     Projection sub-assembly  314  may optionally include projector optics  350  depending on the particular optical characteristics of the projectors and desired beam path length to achieve the desired packaging for enclosure  110 . In one embodiment, projector optics  350  include a lens  144  upstream of a first mirror  148  associated with projector  142  and a lens  146  upstream of a second mirror  150  associated with projector  140 . In this embodiment, lenses  144 ,  146  are achromatic lenses having a diameter of about 51 mm×750 mm focal length and are commercially available from ThorLabs (Model AC508-750-A1). Lenses  144 ,  146  are fixed in corresponding mounts and secured to adjustable mirror mounts  352 ,  354 , which provide for independent adjustment of mirrors  148 ,  150 , respectively. 
     Referring now to  FIGS. 10 and 11 , representative connector interfaces for projectors  142  and  144  are shown. Projector  142  includes a connector panel or interface  400  for various standardized power and video signal connectors. For example, as illustrated in  FIG. 11 , projectors  400 ,  402  may include connections for composite video signal input, high-definition (HDMI) input, component (RGB) input, and S-video input. The connector interfaces  400 ,  402  are connected by corresponding signal lines or cabling, generally represented by lines  406 , to a back panel  420  of enclosure  110 . 
     Referring now to  FIG. 12 , a block diagram illustrating operation of a viewer tracking function for use with a medical imaging system  100  according to one embodiment of the present disclosure is shown. The diagram of  FIG. 12  provides a representative strategy or means for synchronizing or moving eye-boxes of an autostereoscopic display in response to viewer movement, which is sometimes referred to as head/eye tracking The illustrated blocks represent a control strategy and/or logic generally stored as code or software executed by a microprocessor of a general purpose computer. However, code or software functions may also be implemented in dedicated hardware, such as FPGA&#39;s, ASIC&#39;s, or dedicated micro-controllers in communication with sensor  172  and motor/controller  194 . In general, various functions are implemented by software in combination with hardware as known by those of ordinary skill in the art. Code may be processed using any of a number of known strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like depending upon the particular implementation. As such, various steps or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Although not explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending upon the particular processing strategy being used. Similarly, the order of processing is not necessarily required to achieve the features and advantages described herein, but is provided for ease of illustration and description. 
     Block  500  of  FIG. 12  represents a zeroing or homing function for actuator  188 , typically performed on a system reset or during a power-on self-test (POST) procedure so that the starting position of the actuator is known. The tracking sensor/detector  172  is then initialized as represented by block  502 . The user or viewer may initiate a tracking mode via keyboard input from computer  178 , for example, which results in the current position of viewer  114  being stored in memory as represented by block  506 . In the embodiment illustrated, sensor/detector  172  provides a position vector having six degrees of freedom (DOF) and a vector containing x-axis, y-axis, z-axis information as well as pitch, roll, and yaw axis information (rx, ry, rz) corresponding to the detected central position between retro-reflectors  124 ,  126 , and  128  to provide an indication of the position of the viewer&#39;s head and eyes. For the representative embodiment illustrated in  FIG. 12 , a reference angle is determined using only the x-axis and z-axis information by calculating the arc-tan(x/z) as represented by block  508 . In block  510  keyboard input is monitored to determine whether to continue in tracking mode. The current tracking state (on or off) is toggled when appropriate keyboard input is received. Block  512  then determines whether tracking is in progress, i.e. retro-reflectors  124 ,  126 , and  128  are detected, then tracking is in progress and control continues with block  514 . If viewer  114  moves out of the field of view of sensor  172 , then tracking is no longer in progress and must be re-initiated by the user as represented by block  504 . 
     The current tracked position is obtained at block  514  with a corresponding current angle offset determined at block  516  in a similar manner as described above with reference to block  508 . A delta or change in angle from the previously stored reference angle is determined as represented by block  518 . If the change in angle exceeds a corresponding threshold associated with the eye-box tolerance, such as 0.5 degrees, for example, then block  524  determines the direction of rotation and generates an actuator command to rotate the stage to correct for the change of angle as represented by block  526 . Control then returns to block  510 . 
     If the change in angle is less than the corresponding threshold as determined by block  520 , then the actuator is stopped as represented by block  522  and control continues with block  510 . 
     As previously described, the viewing element in one embodiment is implemented by a transmissive HOE screen (also referred to as a transmissive DOE screen). The method or process for recording this element is generally known to those of ordinary skill in the art of holography and is described in greater detail in U.S. Pat. No. 4,799,739 to Newswanger, the disclosure of which is hereby incorporated by reference in its entirety. The process can be summarized with respect to making a transmissive holographic screen as shown in  FIG. 13 , which generally corresponds to  FIG. 2  of the &#39;739 patent. However, advances in various photosensitive materials developed since the &#39;739 patent have resulted in the ability to produce more efficient transmissive HOEs with less chromatic dispersion and better contrast than previously available. As such, the process described in the &#39;739 patent has been modified according to the teachings of the present disclosure to provide an autostereoscopic display  108  particularly suited for use in medical imagining applications, such as endoscopy, for example. 
     In general, as described with reference to  FIG. 13 , the process includes a single exposure of a master holographic plate or film  618  to create a Bragg diffraction grating for use as holographic element  180  ( FIG. 1 ). The master holographic plate  618  captures an interference pattern created by a generally monochromatic laser  600  having a beam split by beam splitter  602  into a mutually coherent object beam  604  and reference beam  606 . Reference beam  606  is steered by mirrors  608 ,  610 , through a spatial filter  612 , which expands or spreads reference beam to illuminate concave mirror  614 . The reflected reference beam illuminates holographic plate or film  618  and interferes with object beam  604 , which passes through a spatial filter  622  and diffuser  624  (the object) implemented by a ground glass plate before illuminating the opposite side of plate or film  618 . The relative angle between the object and reference beams determines the size and position/depth of the resulting viewing zone. The entire holographic plate  618  is exposed at one time using a continuous wave (cw) laser  600  after the laser stabilizes and is operating in a single longitudinal mode (TEM 0,0 ) during the exposure. In one embodiment, a Nd:YAG laser having a frequency doubled primary line (wavelength) of 532 nm was used to create the master holographic plate. The plate was then chemically processed/developed as known in the art. A contact copy of the master holographic plate was made using known holographic techniques using the same laser operating as previously described with a frequency doubled primary wavelength of 532 nm to produce the transmissive viewing element  180  for the autostereoscopic display  108 . 
     In general, a wide variety of materials have been used to capture/record a holographic interference pattern for subsequent use, such as photo-sensitive emulsions, photo-polymers, dichromated gelatins, and the like. The selection of a particular material/medium and corresponding recording process may vary depending upon a number of considerations. In one prototype display, the recording process described above was performed with a holographic plate including two optical quality glass (float glass) pieces each having a thickness of about 3 mm (0.125 in.) and approximately 30 cm by 40 cm in size. A silver halide emulsion having an initial thickness of about 10-12 micrometers was applied to a triacetate substrate, followed by drying and cooling, and cutting to a final size, with the coated film placed between the glass plates. 
     According to embodiments of the present disclosure, the photosensitive material on plate or film  618  is a nano-structured silver halide emulsion having an average grain size of 10 nm, such as the commercially available PFG-03C holographic plates, for example. Such film/emulsions/plates are commercially available from Sphere-s Co, Ltd. company located in Pereslazl-Zalessky, Russia. 
     Another suitable emulsion has been developed by the European SilverCross Consortium, although not yet commercially available. Similar to the PFG-03C material, the emulsion developed by the European SilverCross Consortium is a nano-structured silver halide material with an average grain size of 10 nm in a photographic gelatin having sensitizing materials for a particular laser wavelength. In general, the finer the particles, the higher efficiency and better resolution in the finished screen, but the less sensitive the material is to the laser frequency, which results in higher power density and generally longer exposure times. The photo-sensitive emulsion is sensitized using dyes during manufacturing to improve the sensitivity to the frequency doubled wavelength of the laser used during the recording process. 
     After the holographic plate  618  has been exposed, it is developed using generally known techniques that include using a suitable developer for fine-grain material, using a bleaching compound to convert the developed silver halide grains into a silver halide compound of a different refractive index than the surrounding gelatin matrix, and washing and drying. The emulsion and processing/developing process should be selected so that there is minimal or no shrinkage of the emulsion during processing. Depending on the particular application, a panchromatic photopolymer could be used rather than a silver halide emulsion. 
     After the master holographic plate has been completed, one or more copies may be made by illuminating the master plate to be copied with the same wavelength used for recording the master plate, scanning or full-beam exposure of the copy plate through master plate, and applying a developing process similar to the master plate as previously described. 
     The copy may also be made using a photopolymer having desired characteristics as previously described with respect to the master. The resulting master and/or copy may be coated or processed to enhance stability and durability, and/or with anti-reflective coatings to improve visibility, and the like. 
     As such, the present disclosure includes embodiments having various associated advantages. For example, embodiments of the present disclosure provide real-time stereo images to corresponding eyes of at least one viewer to produce a three-dimensionally perceived image without viewing aids, such as glasses or headgear. The present disclosure provides real-time viewer position detection and image display synchronization to allow the viewer to move while staying within predetermined eye-boxes so that perception of the three-dimensional image is unaffected by viewer movement. Use of a transmissive holographic diffraction grating according to the present disclosure allows back illumination to facilitate packaging for endoscopic viewing applications. Transmissive holographic diffraction gratings according to the present disclosure may also provide better brightness and contrast for the viewer relative to reflection-type gratings or elements while also exhibiting reduced chromatic dispersion. 
     While the best mode has been described in detail, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. While various embodiments may have been described as providing advantages or being preferred over other embodiments with respect to one or more desired characteristics, as one skilled in the art is aware, one or more characteristics may be compromised to achieve desired system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments discussed herein that are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.