Patent Publication Number: US-6219186-B1

Title: Compact biocular viewing system for an electronic display

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation In Part of U.S. application Ser. No. 09/241,828 filed Feb. 1, 1999, entitled “Color Superposition, Mixing And Correction For A Video Display System,” by Hebert, which issued Dec. 28, 1999 as U.S. Pat. No. 6,008,939, which is a Divisional Application of U.S. application Ser. No. 09/056,934, filed Apr. 6, 1998, entitled “Biocular Viewing System with Intermediate Image Planes for an Electronic Display Device,” by Hebert, which issued July 20, 1999 as U.S. Pat. No. 5,926,318. This application is also related to U.S. patent application Ser. No. 09/305,092 filed May 3, 1999, entitled “Infrared Audio/Video Interface For Head-Mounted Display,”, by Hebert and Hempson, which issued Aug. 8, 2000 as U.S. Pat. No. 6,101,038, commonly assigned, the specification of which are incorporated herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to image display systems and more particularly to dual off-axis stereographic systems with multiple illumination sources for biocular viewing of single microdisplays. 
     BACKGROUND 
     High quality, convenient, cost-effective medical remote imaging has gained increasing importance during recent years. This is particularly true of imaging during surgical procedures, most importantly minimally invasive procedures in which direct viewing of the surgical field by the surgeon is difficult. For example, a method for performing coronary artery bypass relies on viewing the cardiac region through a thoracoscope or other viewing scope (see for example Sterman et al. U.S. Pat. No. 5,452,733 and Gifford, III et al. U.S. Pat. No. 5,695,504). By way of further example, a surgeon may need to perform a delicate vascular- or neuro-microsurgical reconstruction through a minimal incision with the aid of remote viewing. Minimally invasive surgical procedures and their related need for remote imaging are now common in orthopedics, ophthalmology, urology, gynecology, anesthesiology, and other medical disciplines. 
     In a conventional surgical environment, remote imaging is accomplished by attaching a video camera to an endoscope, laparoscope, or other minimally invasive instrument and transmitting the video image via cable to a conventional CRT video monitor. This is typically cumbersome in a crowded, brightly lighted operating room, where surgical team members are frequently moving about and the surgeon&#39;s view of the image screen is often obstructed. Additionally, the CRT monitor is incapable of providing the surgeon with critical depth perception, since it is not stereographic. 
     Head-mounted displays (HMDs) potentially offer a method for convenient medical remote viewing without obstruction of the image typical in an operating room environment. While head-mounted displays have been designed, developed and deployed in military applications for many years, such displays are generally bulky, expensive, application-specific devices that are not well suited to commercial or surgical applications. 
     With the advent of inexpensive and increasingly complex commercial computing power and computer graphic devices and software, there has been increasing interest and activity in the field of commercial HMD devices. A number of such devices are presently available, but because of the high cost of appropriate display components and the generally cumbersome mechanical nature of the headgear, these devices are generally low in resolution and unattractive for professional computing applications. 
     High-resolution display device components are now emerging, that can significantly enhance commercial HMDs and related applications. However, they require integration into an ergonomic, well engineered and economical design. In the case of professional and consumer computing applications, visual quality and comfort are critical to long-term acceptance. A computing environment generally includes the use of a keyboard as well as peripheral devices and supporting paperwork. Therefore peripheral vision is also an important consideration. 
     A compact HMD system requires a very small display device, such as those found in modern camcorder viewfinders, but with significantly higher resolution. A number of such devices are now becoming available, including transmissive and reflective liquid-crystal microdisplay devices and micro-mirror devices having resolutions at or in excess of VGA quality (640 pixels by 480 pixels) with pixel sizes on the order of 15 microns or less. Most of these devices exhibit satisfactory image contrast only when illuminated and viewed at narrow angles of incidence, which compromises field of view, eye relief, and viewing comfort. 
     Due to the base costs of their materials, such devices are expensive for commercial applications, even in high volumes. In particular, for stereographic or other binocular applications, the use of dual display devices for two eye channels results in a high cost. A medical stereographic HMD system having dual display devices is described in Heacock et al. “Viewing Ocular Tissues with A Stereoscopic Endoscope Coupled to a Head Mounted Display (HMD),” http://www.hitl.washington.edu/publications/heacock/, Feb. 17, 1998. 
     Kaiser Electro-Optics (2752 Loker Avenue West, Carlsbad, Calif. 92008 manufactures the “CardioView,” “Series 8000,” and “StereoSite” HMD display systems for Vista Medical Technologies. These systems are bulky, heavy, and expensive, and include two LCD display devices. For peripheral vision correction they require the user to wear the HMD over conventional corrective eyeglasses, aggrevating user inconvenience and discomfort. 
     Attempts to use only a single display device for such applications have typically involved beamsplitters, and have not achieved true stereographic performance (see for example Meyerhofer et al. U.S. Pat. No. 5,619,373, issued Apr. 8, 1997). 
     Therefore, what is needed in the art is a compact, high resolution, high contrast, truly stereographic system for microdisplay viewing, particularly for surgical microdisplay viewing, that is suitable for head-mounted display use without requiring undue complexity or expense. The system should provide good color fidelity for color image viewing, and should incorporate ergonomic design for comfort and efficiency, including peripheral vision accommodation and minimal cabling. 
     SUMMARY 
     In accordance with the invention, true stereographic viewing is achieved using a single display device with appropriate less expensive optics and without a beamsplitter. Low-cost complex plastic optics allow biocular viewing of a single electro-optic display device, such as for use in a head-mounted display (HMD). A dual off-axis configuration provides two independent optical channels, intersecting only at the display surface of the display device. Each optical channel contains its own illumination source, eyepiece lens, and imaging optics. In some embodiments, nearly collimated illumination optics and intermediate field lenses are used to fill wide-aperture eyepieces. Various configurations have folded or unfolded optical paths. 
     Multiple illumination schemes are provided for either monochrome or color, and in either two-dimensional or time-sequential true stereographic presentation. An illumination source is provided in some embodiments by mixing the outputs from LEDs of differing color using dichroic mirrors. In compact versions, collimation of the illumination beam by a display field lens proximate to the display surface reduces optical complexity and minimizes collimated beam volume. In some embodiments, a shaping element radially redistributes the energy density of the illumination beam, thereby providing a substantially uniformly illuminated image. 
     Offsetting color overcorrection and undercorrection methods are applied to minimize optical element complexity. An appropriately sized aperture stop is positioned in the beam to set the system f/number, to reduce glare and scattered light, and to blend the display device&#39;s sub-pixel structure by blocking higher-order spatial frequency harmonics, thereby reducing “graininess” and improving image quality. Additional features include lightweight eyepiece construction and interchangeable lenslets for peripheral vision correction. 
     In some embodiments, true stereographic performance is achieved by sequential activation of the light sources in the two channels in synchronism with sequential video signals for the respective channels. Some embodiments include a video interface, which converts conventional RGB-VGA formatted video signals to sequential color data for storage in intermediate refresh frame memory. Some versions of the video interface incorporate wireless transmission, eliminating cumbersome cabling. 
     Therefore, in some embodiments of the invention, a compact, high resolution, high contrast, truly stereographic system is provided for microdisplay viewing, that is suitable for head-mounted display use without requiring undue complexity or expense. The system provides color correction for good color fidelity, and incorporates ergonomic features for comfort and efficiency, including peripheral vision accommodation. Wireless versions eliminate the need for cumbersome cabling. Particularly, high resolution, high color fidelity, truly stereographic viewing in a head-mounted display, including peripheral vision accommodation and wireless transmission, provides the depth perception and convenience required for remote viewing. Surgical applications will generally require a virtual image at the surgeon&#39;s nominal working distance of 22 inches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an unfolded optical path off-axis biocular system, in accordance with the invention; 
     FIGS. 2A-2G are schematic views of thin color-corrected lens elements, illustratively for use in a biocular display system, in accordance with the invention; 
     FIGS. 3A-3D are cross-sectional views of a folded optical path, reflective display biocular system; 
     FIGS. 4A and 4B are elevational views of a biocular display viewing system folded into a head-mounted display (HMD) housing, in accordance with the invention; 
     FIG. 4C is a top view illustrating schematically a biocular display viewing system installed into a head-mounted display (HMD) housing; 
     FIG. 4D is an elevational view illustrating a photodetector mounted with an input lens on a HMD housing; 
     FIG. 5 is a simplified schematic block diagram of a circuit interconnecting a photodetector with display light sources; 
     FIG. 6A is a block diagram illustrating the generation, transmission, reception, and processing of a video signal into a format suitable for a display system; 
     FIG. 6B is a block diagram illustrating the functions of video signal processing modules in connection with the use of intermediate frame memory with a display system; 
     FIG. 7 is a schematic view of a compact unfolded optical path off-axis biocular system, in accordance with the invention; 
     FIG. 8A is a schematic view of the left optical channel of a folded compact biocular display system; 
     FIGS. 8B and 8C are cross-sectional views of an embodiment of the compact folded biocular display system of FIG. 8A; and 
     FIG. 9 is an optical schematic diagram illustrating the performance of a shaping lens. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The invention relates generally to image display systems and more particularly to dual off-axis stereographic systems with multiple illumination sources for biocular viewing of single microdisplays. 
     In some embodiments of the invention, a compact biocular optical configuration provides a wide-angle image of a single display surface independently to each eye. 
     FIG. 1 is a schematic view of an unfolded optical path off-axis biocular system  100 , in accordance with the invention, incorporating a single transmissive display device  110  having a substantially planar display surface  108 , as described in detail below. Display device  110  is positioned at the intersection of two independent beams  112  and  114  of nearly collimated light, defining respective substantially rectilinear beam axes  113  and  115 . In some embodiments of the invention, independent beams  112  and  114  are individually produced from a single or multiple light sources, as described in greater detail below. Independent beams  112  and  114  preferably each have collimated beam width (e.g. diameter 116) approximately equal to the width (e.g. diameter) of display surface  108  projected onto planes perpendicular to respective beam axes  113 ,  115 . Beam axes  113  and  115  are tilted relative to one another and to a display axis  117  perpendicular to display surface  108 . 
     Independent beams  112  and  114  propagate through display surface  108  along respective beam axes  113  and  115  to respective intermediate image planes  122  and  124 . Single or multiple imaging elements  126  and  128  form intermediate images  132  and  134  of display surface  108  in space at intermediate image planes  122  and  124  respectively. In some embodiments, imaging elements  126  and  128  incorporate toroidal optical correction to correct for off-axis geometric distortion. 
     Intermediate image planes  122  and  124  are re-imaged by respective independent eyepiece lenses  136  and  138 , configured to magnify intermediate image planes  122  and  124  respectively and to provide virtual images (not shown) for independent viewing by each eye  146 ,  148  of an observer. Because of the collimated nature of independent beams  112  and  114  at display surface  108  and the specific paths of the resultant beams through intermediate image planes  122  and  124 , the optical energy of respective independent beams  112  and  114  does not fill the eyepiece apertures  142 ,  144 . It has long been recognized by those having ordinary skill in the art, that optical energy must fill eyepiece apertures  142 ,  144  sufficiently in order to provide complete and uniform virtual images to the eyes  146 ,  148  of an observer. 
     In some embodiments, this condition is met by inserting a field lens  152 ,  154  (see for example F. A. Jenkins and H. E. White: “Fundamentals of Optics,” McGraw-Hill New York, Toronto, London 3rd edition 1957, pp. 182-183) along beam axis  113 ,  115  at or near intermediate image plane  122 ,  124  to fill its respective eyepiece aperture  142 ,  144  for all possible fields of view. By definition, field lenses as described herein optionally include scattering screens and Fresnel lenses (see for example The Photonics Dictionary 1988, Laurin Publishing Co., Inc., Pittsfield, Mass.). In some embodiments, field lens  152 ,  154  is given on-axis or off-axis toroidal power to correct residual image distortions and/or to accommodate the field curvature of the eyepiece design. 
     Independent beam  112  propagating along beam axis  113 , together with intermediate imaging plane  122 , imaging element  126 , eyepiece lens  136 , eyepiece aperture  142 , and any other optical elements lying along independent beam  112  are defined collectively as left optical channel  140 . Likewise independent beam  114  propagating along beam axis  115 , together with intermediate imaging plane  124 , imaging element  128 , eyepiece lens  138 , eyepiece aperture  144 , and any other optical elements lying along independent beam  114  are defined collectively as right optical channel  150 . Left and right optical channels  140  and  150  intersect one another only at the display device  110 . Off-axis biocular system  100  avoids the loss of light, bulk and expense of conventional beamsplitters, but illumination off-axis relative to of display axis  117  induces geometric distortion of the image of display surface  108 , which must be accommodated or compensated. Accordingly the tilt angle between beam axes  113  and  115  is kept small enough to minimize geometric distortion but large enough so that optical components of the respective optical channels  140 ,  150  do not physically conflict with one another. 
     In some embodiments, left optical channel  140  uses single or multiple light-emitting diodes (LEDs)  162  as light sources, and their emissions are mixed and concentrated within a mixing light cone  166  to produce an approximate point light source  172 . Light cones are familiar in the art for collecting and concentrating light, and can be fabricated readily by methods including machining, molding, casting, and electroforming. 
     Light from point light source  172  is collected and collimated by a collimating lens  176  to produce substantially collimated independent beam  112 . Independent beam  112  is obliquely incident on substantially planar display device  110 , e.g. a miniature liquid-crystal display. Imaging element  126  in left optical channel  140  projects intermediate image  132  of display surface  108  at intermediate image plane  122 , where field lens  152  directs the optical energy in left optical channel  140  to fill left eyepiece aperture  142  uniformly. Left eyepiece lens  136  forms a virtual image (not shown) of intermediate image plane  122  for comfortable viewing. 
     In some embodiments, imaging element  126  incorporates toroidal correction to compensate for geometric distortion arising from the off-axis imaging of display surface  108 . 
     FIG. 1 also shows an aperture stop  182  inserted in at a plane of minimum convergence  186  of independent beam  112  between imaging element  126  and intermediate image plane  122 . As is familiar in the art, aperture stop  182  functions primarily to control system resolution by setting the f/number and to optimize contrast by blocking glare from stray and scattered light. If independent beam  112  is sufficiently collimated where it intersects display surface  108 , then plane of minimum convergence  186  becomes an approximate Fourier transform plane (hereinafter the transform plane). Since independent beam  112  is incoherent, the transform plane is a an approximate power Fourier transform plane and not an amplitude Fourier transform plane (which exists only in coherent optical systems). The detail in the display structure and image diffracts light at a small angle away from beam axis  113 , and consequently this diffracted light propagates through the transform plane displaced slightly off-axis relative to nondiffracted light, which propagates substantially along beam axis  113 . As is familiar in the art, the off-axis displacement of the diffracted light at the transform plane is proportional to the spatial frequency in the scattering display structure or image. Aperture stop  182  is appropriately sized to transmit spatial frequencies corresponding with the desired image resolution (for example VGA), while blending finer detail due to the display device&#39;s sub-pixel structure by blocking higher-order spatial frequency harmonics (e.g. light displaced farther off-axis at the transform plane), thereby reducing “graininess” and improving image quality. 
     Likewise in some embodiments, right optical channel  150  uses single or multiple light-emitting diodes (LEDs)  164  as light sources, and their emissions are mixed and concentrated within a mixing light cone  168  to produce an approximate point light source  174 . Light from point light source  174  is collected and collimated by a collimating lens  178  to produce substantially collimated independent beam  114 . Independent beam  114  is obliquely incident on substantially planar display device  110 . Imaging element  128  in right optical channel  150  projects intermediate image  134  of display surface  108  at intermediate image plane  124 , where a field lens  154  directs the optical energy in right optical channel  150  to fill right eyepiece aperture  144  uniformly. Right eyepiece lens  138  forms a virtual image (not shown) of intermediate image plane  124  for comfortable viewing. 
     In some embodiments, imaging element  128  incorporates toroidal correction to compensate for geometric distortion arising from the off-axis imaging of display surface  108 . 
     In some embodiments, FIG. 1 also shows an aperture stop  184  inserted at the plane of minimum convergence  188  of independent beam  114  propagating toward intermediate image plane  124 . As described above in relation to aperture stop  182 , aperture stop  184  functions primarily to control resolution by setting the f/number and to optimize contrast by blocking glare from stray and scattered light. If independent beam  114  is sufficiently collimated where it intersects display surface  108 , then plane of minimum convergence  188  is also a transform plane. As described above in relation to aperture stop  182 , aperture stop  184  is appropriately sized to transmit spatial frequencies corresponding with the desired image resolution, while blending finer detail due to the display device&#39;s structure by blocking higher-order spatial frequency harmonics, thereby reducing “graininess” and thus improving image quality. 
     Miniature display devices are available in both monochrome and color versions and in either transmissive or reflective configurations. Suppliers of transmissive microdisplays include Sony Semiconductor of San Jose, Calif.; and the Sharp, Canon, Seiko, Epson, and Kopin companies of Japan. Suppliers of reflective microdisplays include CRL of Dawley Road, Hayes, Middlesex, UK; CMD of Boulder, CO; Displaytech, Inc. of Longmont, CO; and Varitronix Limited of Hong Kong. A reflective micromirror display device is produced by Texas Instruments of Dallas, Tex. 
     Color display devices are generally supplied with microdot or microstrip color filters, requiring at least three pixels (red, green, and blue) to create a single color display element. As will be apparent to one skilled in the art, this compromises resolution and sacrifices optical efficiency. For display devices that are capable of sub-frame rate switching speeds, another option is to apply to each pixel sequential color techniques such as those used in early television with color wheels or, more recently, electro-optic switchable filters. 
     In some embodiments of the invention, light-emitting diodes  162  of left optical channel  140  comprise a group of red, green and blue color LEDs, for example  162   r ,  162   g ,  162   b . LEDs  162   r - g - b  illuminate the input aperture to mixing light cone  166 . Likewise light-emitting diodes  164  of right optical channel  150  comprise a group of red, green and blue color LEDs, for example  164   r ,  164   g ,  164   b , which illuminate the input aperture to mixing light cone  168 . LEDs  162   r - g - b ,  164   r - g - b  are sequentially switched for color in any manner that is compatible with an appropriately fast display device  110 . For example, red LEDs  162   r ,  164   r  are switched on and all other LEDs are switched off synchronously, while a red frame video signal is applied to display device  110 . For long-term stability of LED output powers and associated display color balance, electro-optic detectors  192 ,  194  are positioned to sample the optical outputs within appropriate color bands in respective left and right optical channels  140 ,  150  and appropriately adjust the drive currents with closed-loop electronic feedback. 
     In some embodiments the outputs from LEDs  162 ,  164  are captured by diffractive collectors  170 ,  171  and are mixed and homogenized in mixing light cones  166 ,  168 , as described in detail below. Diffractive collectors  170 ,  171  are configured to diffract and superimpose the respective color components from LEDs  162   r - g - b  within mixing light cones  166 ,  168 . 
     Some biocular vision systems and computer applications require true stereographic display viewing. A well established technique uses time-domain multiplexing of alternating right and left video signals with synchronous right-eye and left-eye electro-optic shutters (see for example Meyerhofer et al. U.S. Pat. No. 5,619,373; also Tektronix, Inc., Beaverton, OR, SGS 430 System bulletin). Similar products are offered by VRex (Elmsford, N.Y.), 3DTV Corporation (for example Model DMM StereoPlate), Kasan Model 3DMaX™, PCVR™, and Stereospace Model 1™. 
     In some embodiments of the invention, true stereographic viewing is provided by alternate sequencing of the right and left groups of LEDs  162 ,  164 , synchronously with time-domain multiplexing of alternate right and left video signals. Unlike previous systems that combine time-domain multiplexing of alternating right and left video signals with a common illumination source for both right-eye and left-eye optical channels (see Meyerhofer et al. U.S. Pat. No 5,619,373), the present embodiment provides for alternate sequencing of separate right and left groups of LEDs  162 ,  164  for separate optical channels  140 ,  150  respectively. Off-axis biocular system  100  provides unique true stereographic viewing capability with a single display device  110 , because left and right optical channels  140  and  150  remain optically independent of one another from illumination sources  162 ,  164  to the observer&#39;s eyes  146 ,  148 , except for sharing common display device  110 . In some embodiments stereographic video sequencing is performed concurrently with the above-described sequential color switching at a sequencing rate and in a combination that minimizes visual flicker. 
     In some embodiments, eyepiece lenses  136 ,  138  comprise reflective and/or refractive lens elements. For a compact optical geometry facilitating a wide angular field of view, refractive eyepiece lenses are preferable. In some embodiments, eyepiece lenses  136 ,  138  are color-corrected (achromatic) to enhance color and image fidelity. Eyepiece lenses  136 ,  138  have eye relief ranging from approximately 0.5 inch to 0.7 inch and angular aperture ranging from approximately 20 degrees to 40 degrees full angle. In some embodiments eyepiece lenses  136 ,  138  are further refined by using surfaces with higher order curvatures (aspheric surfaces) to minimize aberrations. Such lenses tend to be quite thick and heavy, generally require multiple elements for achromatic performance, and are difficult and expensive to fabricate. Conventional plastic injection-molded lenses can utilize the opposite color dispersions of refractive and diffractive surfaces to create simpler, lighter, and less expensive achromats, but the appropriate lenses are typically delicate and tend to scatter light. 
     FIGS. 2A-2G are schematic views of thin color-corrected lens elements, illustratively for use in biocular display system  100 , in some embodiments of the invention. FIG. 2A shows a conventional singlet refractive lens  210 , having significant color distortion. Illustratively red, green, and blue color images of point object  208  are focused in sequence at points labeled R, G, and B respectively (exaggerated for clarity). FIG. 2B shows a continuously-profiled Fresnelled refractive surface  220 , having reduced weight and thickness relative to conventional refractive lens  210 , but also having color distortion such that the red, green, and blue color images of point object  218  are focused at points labeled R, G, and B respectively, in the same sequence as for conventional refractive lens  210 . FIG. 2C shows a diffractive surface  230  used as a lens. Diffractive surface  230  is compact and light-weight, and also has color distortion, but in a reverse sequence relative to that for lens  210  and Fresnelled refractive surface  220 , as illustrated by red, green, and blue color image points of point object  228  labeled R, G, and B respectively. As is familiar in the art, two conventional refractive lenses  210  and  212  having different curvatures and materials can be combined for color correction, as illustrated by the superposition of color image points of point object  238  labeled R, G, B in FIG.  2 D. 
     In some embodiments, as is familiar in the art,  30  diffractive structures are superimposed on Fresnelled refractive surfaces for greater design versatility. Illustratively, FIG. 2E shows diffractive-refractive lens  240  combining diffractive surface  230  with Fresnelled refractive surface  220 . In addition to collapsing the thickness of the lens elements, thereby decreasing their weight and improving their moldability, diffractive-refractive lens  240  applies the offsetting color distortion sequences of diffractive surface  230  and Fresnelled refractive surface  220  to achieve color correction, as illustrated by the superposition of color image points of point object  248  labeled R, G, B in FIG.  2 E. 
     In some embodiments, multiple thin optical elements are used for each eyepiece with any combination of smooth, refractive, and diffractive surfaces for improved control of optical aberrations. In some embodiments (not shown), the outer surface (eye side) is preferably smooth for improved cleanability. The more delicate diffractive surfaces are thereby protected as intermediate surfaces. In some embodiments, Fresnelled refractive surfaces  220  and/or diffractive surfaces  230  are applied to complex substrate curvatures  216  for greater design versatility, as illustrated by a complex refractive-diffractive lens  250  in FIG.  2 F. Illustratively complex refractive-diffractive lens  250  corrects chromatic distortion (superposition of color image points labeled R, G, B) for both an on-axis point object  252  and an off-axis point object  254 . 
     FIG. 2G is a schematic view of a complex refractive-diffractive eyepiece lens  260 , incorporating chromatic and geometric correction. Eyepiece lens  260  is placed less than one focal length away from an object  262 , and creates a virtual image  264  of object  262  at a distance greater than one focal length from eyepiece lens  260 , as viewed through eyepiece lens  260  by an observer  266 . Virtual image  264  retains chromatic and geometric fidelity. Object  262  can be a physical object or a projected intermediate real image. 
     As described above, color correction of an eyepiece lens can require a superposition of multiple refractive and diffractive surfaces applied to complex substrate curvatures. This in turn increases manufacturing costs. In some embodiments, a simpler and less expensive system approach is employed, in which selected optical elements in the system are color overcorrected, whereas other optical elements are uncorrected or minimally corrected for color. Using such a system approach, overall system performance is made achromatic by balancing offsetting color distortions of individual optical elements. 
     Illustratively, a diffractive lens surface  230  (see for example FIG. 2C) is applied to imaging elements  126  and  128  (see FIG. 1) either alone or in combination with other optical structures (e.g. toroidal correction). This results in color overcorrection, i.e. red, green, and blue color image points labeled R, G, B of point object  228  occur in the sequence shown in FIG.  2 C. If imaging elements  126 ,  128  are appropriately color overcorrected, then field lenses  152 ,  154  and eyepiece lenses  136 ,  138  need not be individually achromatic for overall off-axis biocular system  100  to be achromatic. As needed, field lenses  152 ,  154  and/or eyepiece lenses  136 ,  138  are minimally color corrected. 
     It is also advantageous to have chromatic correction at planes of minimum convergence  186 ,  188 , which are images of point sources  172 ,  174  respectively. To filter the fine sub-image structure requires a small diameter aperture stop  182 ,  184  at planes of minimum convergence  186 ,  188 . Chromatic distortion separates the planes of minimum convergence for different colors, e.g. R, G, B, and thereby causes a loss of image resolution accompanying the sub-image structure filtering. Chromatic correction keeps the planes of minimum convergence  186 ,  188  in register for all colors. Therefore minimal sacrifice of image resolution accompanies sub-image structure filtering. 
     In some embodiments, the optical elements between point sources  172 ,  174  and their images in planes of minimum convergence  186 ,  188 , namely collimating lenses  176 ,  178  and imaging elements  126 ,  128 , are individually non-corrected but are overall corrected chromatically. In particular, collimating lenses  176 ,  178  are color undercorrected and imaging elements  126 ,  128  are color overcorrected so that the net result is chromatic correction at planes of minimum convergence  186 ,  188 . In some embodiments this is combined with color undercorrection of eyepiece lens  136 ,  138 . Since display surface  108  lies in a collimated beam, collimating lens  176 ,  178  has no effect on image color fidelity at eyepiece lens  136 ,  138 . Therefore the designer is free to use color overcorrection of imaging elements  126 ,  128  to compensate color undercorrection of eyepiece lens  136 ,  138 , as described above. 
     Plastic lenses are commonly injection molded from a variety of optical grade plastics, including acrylic, polycarbonate, styrene, and nylon, which are readily available from sources such as Seiko, Sola, and General Electric. As will be recognized by those skilled in the art, optical design and fabrication methods described in the above embodiments for particular optical elements are illustrative and are applicable to other optical elements in the system, as required for a particular configuration. Detailed selection of locations and properties of optical elements, e.g. focal lengths and aperture diameters, is performed in accordance with techniques familiar in the art. 
     FIGS. 3A-3D are cross-sectional views of a folded optical path, reflective display biocular system  300 , in accordance with the invention. This embodiment allows a more compact folded optical configuration, for example for use in a head-mounted display, than does the unfolded biocular off-axis system  100  shown in FIG.  1 . Except for the folded optical paths, reflective display biocular system  300  performs substantially the same functions as unfolded biocular off-axis system  100 , and each system contains elements that are essentially functional counterparts of elements of the other system. Some elements that are transmissive in biocular off-axis system  100  are reflective in reflective display biocular system  300 . Elements that are substantially similar in the various figures are designated by similar reference numbers. 
     FIG. 3A is a top cutaway projection showing a left eyepiece assembly  280  and a molded optical assembly  290 . A right eyepiece assembly, symmetric with left eyepiece assembly  280 , is not shown for clarity. Left eyepiece assembly comprises a field lens  352  located at or proximate to an intermediate image plane  122 , a deflecting mirror  282 , and an eyepiece lens  336 . In some embodiments eyepiece lens  336  consists of a single refractive or refractive-diffractive optical element. In other embodiments eyepiece lens  336  comprises multiple refractive, Fresnelled refractive, and/or refractive-diffractive surfaces applied to planar or curved substrates, as described above in connection with FIGS. 2A-2G. Molded optical assembly  290  and left eyepiece assembly  280  are separated by half an interocular distance  284  that is adjustable by an interocular adjustment mechanism (not shown), coupled to molded optical assembly  290  and left eyepiece assembly  280 . Similarly field lens  352  is separated from deflecting mirror  282  by a focus adjustment distance  286 , adjustable by a focus adjustment mechanism (not shown) coupled to field lens  352  and deflecting mirror  282 . 
     FIG. 3B is a cross-sectional view showing molded optical assembly  290  as viewed across section  3 B— 3 B of FIG.  3 A. FIG. 3C is a cross-sectional view of molded optical assembly  290  showing an illumination path as viewed across section  3 C— 3 C of FIG. 3A, and FIG. 3D is a cross-sectional view of molded optical assembly  290  showing an imaging path as viewed across section  3 D— 3 D of FIG.  3 A. Molded optical assembly  290  mounts on a substrate  292  by means of spacers  294 . In some embodiments substrate  292  is a printed circuit board. Substrate  292  also supports light sources, preferably LEDs  162 ,  164 , and a reflective display device  310  in a nominally planar relationship. In some embodiments LEDs  162  comprise red, green, and blue color LEDs  162   r - g - b  and  164   r - g - b  respectively (see FIG.  1 ). 
     Molded optical assembly  290  includes total internally reflective (TIR) surfaces  372  and  374 , collimating lens  376 , imaging elements  326  and  328 , and TIR stop  382 . Molded optical assembly  290  further includes mixing light cones  166 ,  168 , equipped with diffractive collectors  170 ,  171  respectively adjacent LEDs  162 ,  164 . 
     In some embodiments molded optical assembly  290  is fabricated in whole or in part as a monolithic, unitary structure with self-contained optical components, as described in Hebert U.S. Pat. No. 5,596,454, issued Jan. 21, 1997. Integrally molded mechanical features allow for precise registration of the optical elements of molded optical assembly  290  relative to one another and relative to substrate  292  and thereby relative to LEDs  162 ,  164  and reflective display device  310  mounted on substrate  292 . In other embodiments molded optical assembly  290  is fabricated as a frame onto which individual optical elements are assembled. 
     In operation, a video signal is applied to reflective display device  310 , thereby producing a reflective video image on reflective display surface  308 . In some embodiments the video signal comes from a remote video camera (not shown). In some embodiments a computer graphic generator synthesizes the video signal. In some embodiments the video signal is supplied from data previously stored on disk, tape, or other storage medium. Acquisition and processing of the video signal is described below in greater detail. 
     Reflective display surface  308  is illuminated by light beams defining independent left optical channel  340  and right optical channel  350  respectively. Arrows labeled  340  in FIGS. 3A-3D represent the beam propagation path of left optical channel  340 . The beam propagation path of right optical channel  350  is symmetric with that of left optical channel  340 , but is not labeled for clarity. Each optical element of right optical channel  350  is a symmetric counterpart and performs a function identical to that of a corresponding optical element of left optical channel  340 . Beam propagation paths are laid out so that left optical channel  340  and right optical channel  350  intersect only at reflective display surface  308 , ensuring that optical elements of respective left and right optical channels  340  and  350  do not interfere physically with one another. As described above in connection with FIG. 1, image distortion is produced in left and right optical channels  340 ,  350  respectively, because of off-axis illumination of reflective display surface  308 . 
     Referring to FIG. 3C, illumination for left optical channel  340  is generated by LEDs  162 . In some embodiments LEDs  162  comprise color LEDs  162   r - g - b , whereas in other embodiments the illumination is monochrome. The outputs from LEDs  162  are captured by diffractive collector  170  and are mixed and homogenized in mixing light cone  166 . 
     Diffractive collector  170  is configured to deflect and superimpose the respective color components from LEDs  162   r - g - b  within mixing light cone  166 . Illustratively each differing color component from LED  162   r ,  162   g ,  162   b  respectively impinges on diffractive collector  170  at a differing angle of incidence. Diffractive collector  170  is configured so that the respective color components are diffracted at differing angles, such that the diffracted color components are all substantially superimposed and propagate together within mixing light cone  166 , as if these color components were all emitted from a spatially common light source. This application of diffractive collector  170  is essentially the inverse of typical diffractive surfaces, wherein a single light beam comprising differing color components is diffracted into a plurality of separate light beams each having differing angles for differing color components. 
     Beam  340  emerges from mixing light cone  166  substantially as a point source of light, which is deflected by TIR reflective aperture  372 . After being collected and reflected by a TIR folding surface  374 , beam  340  passes through collimating lens  376 , producing a substantially parallel beam  340  that reflects obliquely from reflective display surface  308  (see diagonal arrow  340  in FIG.  3 A). At reflective display surface  308 , beam  340  is spatially modulated by a video image. In some embodiments color LEDs  162   r - g - b  are sequenced synchronously with a color video signal to reflective display device  310 , thereby producing a sequentially color modulated reflected beam, as described above in connection with FIG.  1 . 
     Referring to FIG. 3D, after modulation and reflection from reflective display surface  308 , beam  340  propagates to imaging elements  326  and  328 , which collectively perform functions analogous to those of imaging elements  126 ,  128  as described above in connection with FIG.  1 . In the present embodiment, imaging element  326  is a transmissive element that combines refractive, Fresnelled refractive, and/or diffractive properties. Imaging element  328  is a TIR reflective element disposed to fold beam  340  toward TIR stop  382 , which then deflects beam  340  toward intermediate image plane  122  (see FIGS.  3 A- 3 B). In some embodiments, imaging element  328  incorporates curvature, which combined with transmissive imaging element  326 , focuses beam  340  through a plane of minimum convergence at TIR stop  382  and then to form an intermediate image of reflective display surface  308  at intermediate image plane  122 . In some embodiments imaging element  326  and/or  328  incorporates toroidal correction to compensate for image distortion arising from off-axis imaging of display surface  308 . 
     TIR stop  382  combines the function of folding beam  340  toward left eyepiece assembly  280  with an aperture stop function, similar to aperture stops  182 ,  184  described above in connection with FIG.  1 . TIR stop  382  comprises a small TIR element that reflects the central portion of beam  340  at substantially a right angle toward left eyepiece assembly  280 , while letting undesired diffracted and scattered light pass harmlessly around it. 
     Molded optical assembly  290  advantageously uses TIR surfaces to reflect beam  340  efficiently without the need for expensive optical coatings. This design approach facilitates unitary fabrication of molded optical assembly  290 . 
     After reflecting from TIR stop  382 , beam  340  propagates to intermediate image plane  122 , where it forms an intermediate image  132  of reflective display surface  308 . The properties of intermediate image  132  of the present embodiment are substantially identical with those of intermediate image  132  described above in connection with FIG.  1 . As in the embodiment of FIG. 1, field lens  352  is placed at intermediate image plane  122 , in order to fill the aperture of eyepiece lens  336  with optical energy. For compactness a deflecting mirror  282  to fold beam  340  is inserted between intermediate image plane  122  and eyepiece lens  336 . 
     The full range of optical element configurations and, as described above, is applicable to reflective display biocular system  300 . These configurations include transmissive and reflective, off-axis, toroidal, refractive, diffractive, and Fresnelled refractive elements. In some embodiments, a system design approach to color correction, as described above in connection with FIGS.  1  and  2 A- 2 G, is applied to reflective display biocular system  300 . With such a system design approach, the overall system performance is made achromatic by balancing the offsetting color distortions of individual optical elements. 
     Illustratively, a diffractive lens surface  230  (see for example FIG. 2C) is applied to imaging element  326  (see FIG. 3D) either alone or in combination with other optical structures (e.g. toroidal correction). This results in color overcorrection, i.e. red, green, and blue color image points labeled R, G, B of point object  228  occur in the sequence shown in FIG.  2 C. If imaging element  326  is appropriately color overcorrected, then field lens  352  and eyepiece lens  336  need not be individually achromatic for overall reflective display biocular system  300  to be achromatic. As needed, field lens  352  and/or eyepiece lens  336  is minimally color corrected. 
     As described above, it is also advantageous to have chromatic correction at the plane of minimum convergence substantially coincident with TIR stop  382 , at which an image of TIR reflective aperture  372  is formed. Chromatic correction keeps the plane of minimum convergence in register for all colors, minimizing loss of image resolution with sub-image structure filtering. 
     In some embodiments, the optical elements between TIR reflective aperture  372  and its image at TIR stop  382 , namely collimating lens  376  and imaging elements  326 ,  328 , are individually non-corrected but are collectively corrected chromatically. For example, collimating lens  376  is color undercorrected, but imaging element  326  is color overcorrected, so that the net result is chromatic correction at TIR stop  382 . In some embodiments, this is combined with color undercorrection of eyepiece lens  336 . Since reflective display surface  308  lies in a collimated region of beam  340 , collimating lens  376  has no effect on image color fidelity at eyepiece lens  336 . Therefore a system designer is free to use color overcorrection of imaging element  326  to offset color undercorrection of eyepiece lens  336 , as described above. 
     In some embodiments, true stereographic viewing is provided by alternate sequencing of the right and left groups of LEDs  162 ,  164 , synchronously with time-domain multiplexing of alternating right and left video signals, as described above in connection with FIG.  1 . Reflective display biocular system  300  provides unique true stereographic viewing capability with a single reflective display device  310 , because left and right optical channels  340  and  350  remain optically independent of one another, except for sharing common reflective display device  310 . In some embodiments, stereographic video sequencing is performed concurrently with the above-described sequential color switching at a sequencing rate and in a combination that minimizes visual flicker. 
     FIG. 4A is a front elevational view of a biocular display viewing system  400  incorporating a head-mounted display (HMD) housing  410  containing, for example, an off-axis biocular system  100  or a reflective display biocular system  300  and incorporating interchangeable corrective peripheral vision lens elements  422 ,  424 , in accordance with the invention. 
     FIG. 4B is a side elevational view of the biocular display viewing system of FIG.  4 A. For users requiring corrected vision, an appropriate HMD device must either provide eye relief space to accommodate eyeglasses, or provide eyepiece-focusing adjustments. Accommodating eyeglasses compromises the ergonomic mechanical design of the device in that the eye relief must be extended beyond the surface of the eyeglasses. Thus, the device becomes bulkier, heavier and generally less comfortable. 
     In some embodiments of the invention, biocular display viewing system  400  provides eyepiece-focusing adjustments (not shown). However, true peripheral vision correction is required for a typical user environment. In some embodiments, interchangeable corrective peripheral vision lens elements  422 ,  424  are integrated into a HMD housing, as illustrated in FIGS. 4A-4B. As with over-the-counter conventional corrective eyewear, a few general optical prescriptions potentially satisfy a range of peripheral vision requirements; generally in the range of −0.5 to +1.5 diopter. This is especially true for a limited-distance peripheral vision requirement, as in a computer workplace environment. Illustratively, interchangeable corrective peripheral vision lenses  422 ,  424  are made of injection-molded optical grade plastic with integrally molded snap features (not shown), which permit easy, interchangeable installation onto a compatibly designed HMD housing  410 . Alternatively, commercially available eyeglass lens blanks may be milled and shaped to fit the HMD. 
     While many configurations are possible for corrective peripheral vision lens elements  422 ,  424 , a wrap-around configuration as illustrated in FIG. 4A is generally preferred for optimum peripheral vision, wherein corrective peripheral vision lens elements  422 ,  424  include cutout segments to surround respective left and right eyepiece lenses  336 ,  337 . 
     FIG. 4C is a top view illustrating schematically how a biocular display viewing system  430  similar to that described in connection with FIGS. 3A-3D fits into a head-mounted display (HMD) housing. HMD housing  432  (shown in shaded outline in FIG. 4C) with earpieces  434 ,  436  and corrective peripheral vision lens elements  422 ,  424  contains a reflective display biocular system  300 , incorporating molded optical assembly  290 , as viewed in FIG.  3 A. Molded optical assembly  290  incorporates a substrate  292  (preferably a printed circuit board) to which a reflective display device and LED light sources (hidden beneath molded optical assembly  290  are attached. Left and right imaging beams emerge from molded optical assembly  290  and are directed to left and right eyes respectively by left field lens  352 , left deflecting mirror  282 , and left eyepiece lens  336 , and by right field lens  353 , right deflecting mirror  283 , and right eyepiece lens  337 . 
     Some embodiments include a photodetector with a lens to measure ambient light intensity and to enable automatic adjustment of the display intensity for improved visualization. FIG. 4D is an elevational view illustrating a photodetector  452  mounted with an input lens  454  on HMD housing  410 . Photodetector  452  is disposed to have an ambient field of view  456  through input lens  454  that approximates the peripheral field of view  458  of observer  460 . In some embodiments equipped with corrective peripheral vision lens elements  424  and  422  (see FIG.  4 A), peripheral field of view  458  of observer  460  is taken through corrective peripheral vision lens elements  424  and  422 . 
     FIG. 5 is a simplified schematic block diagram of a circuit  500  interconnecting photodetector  452  with display light sources (for example LEDs  162   r - g - b ,  164   r - g - b , see FIG.  1 ). Photodetector  452  is positioned behind input lens  454  and has an output terminal connected to an input terminal of an amplifier  472 . Amplifier  472  in turn has an output terminal connected to a reference input terminal of a summing junction  474 . A signal input terminal of summing junction  474  is connected to the output terminal of a nominal contrast and intensity control module  476 . The output terminal of summing junction  474  is connected to the input terminal of a light source driver  478 , which has an output terminal connected to the light source. 
     In some embodiments, light source driver  478  represents a plurality of light source drivers, each driving an individual light source, for example LED  162   r . In some embodiments, circuit  500  represents a plurality of individual circuits comprising individual photodetectors, amplifiers, and light source drivers. In some embodiments, circuit  500  includes input terminals (not shown) interconnected with light source sequencing apparatus. In some embodiments, all of the elements of circuit  500  are contained in or on HMD housing  410 . However, other embodiments apparent to those skilled in the art, having any and all of the elements of circuit  500 , including photodector  452  and input lens  454 , located separately from HMD housing  410 , are also within the scope of the invention. 
     In operation, illustratively input lens  454  collects ambient light over ambient field of view  456  and concentrates the collected ambient light on the sensing element of photodetector  452 , which generates an output signal proportional to the intensity of the collected ambient light. The output signal from photodetector  452  is amplified by amplifier  472 , which generates a reference signal and applies it to the reference terminal of summing junction  474 . Summing junction  474  combines the reference signal with control signals provided by nominal contrast and intensity control module  476 , and generates adjusted contrast and intensity control signals, which are applied to the input terminals of light source drivers  478 . The adjusted contrast and intensity control signals automatically adjust the intensity of light sources, e.g. LEDs  162 ,  164 , for a near-constant ratio to the ambient light intensity, without the need for continual manual intensity adjustment in a continually changing ambient light environment. 
     In operation, the display system requires a video input signal. The video signal is typically generated by a source device (e.g. video camera or VCR) remote from the display system, and is then transmitted, received, and processed to meet the particular requirements of the display device. FIG. 6A is a block diagram illustrating the generation, transmission, reception, and processing of a video signal into a format suitable for a display system, in an embodiment of the invention. 
     A source device  602  generates video signals in a conventional RGB-VGA format in the analog domain. These signals are transmitted either by cable (not shown) or by an appropriate wireless data link  606  comprising a transmitter  608  and a receiving module  610  connected with a processing module  612 . Processing module  612  is physically located at or adjacent to a display device  620  as described above in connection with FIGS.  1  and  3 A- 3 D, or located independently. Optionally a preprocessing module  604  (shown in dashed outline) is located at or adjacent to source device  602  and is interconnected between source device  602  and transmitter  608 . An independently located processing module  612  is typically connected to display device  620  by a short cable  614 . For example, a processing module  612  can be incorporated into an optional head-mounted display housing  618  (shown in dashed outline). Alternatively, to reduce bulk and weight of head-mounted display housing  618 , processing module  612  is attached conveniently for example to a user&#39;s clothing (not shown) and connected into head-mounted display housing  618  by a short, minimally encumbering cable  614 . 
     Processing modules  604 ,  612  convert video signals into a format required by display device  620 . In practice, visual quality of a display requires a frame rate that minimizes the eye&#39;s perception of flicker. But the flicker frame rate requirement is about four times faster than that required to create a sense of continuous motion of an object within an image. 
     Unlike conventional analog CRT-based VGA monitors, many miniature display devices appropriate to the invention are not directly scan-compatible with the standard RGB interface and require an intermediate memory (frame grabber) for scan format conversion. Where intermediate memory is a requirement, the display device refreshes from the intermediate memory at a frame rate to minimize flicker, but the intermediate memory is updated only at a slower frame rate consistent with image motion requirements. 
     FIG. 6B is a block diagram illustrating the functions of processing modules  604 ,  612  in connection with the use of intermediate frame memory with a display system, in an embodiment of the invention. Inputs from a conventional RGB video source (not shown) are received by a serial multiplexer  640  and a shift register  644  incorporated into processing module  604 . The vertical sync pulse from the video source causes shift register  644  to control multiplexer  640  to generate large sequential color frames of video data. These are combined with vertical and horizontal sync pulses in a combiner module  642 . optionally, audio signals are embedded into the horizontal sync pulses of the video waveform in combiner module  642 . 
     A combined video signal from combiner module  642  is transmitted over wireless date link  606  from transmitter  608  to receiving module  610  and is applied to processor module  612 . In processor module  612  the video signal is optionally amplified in conventional automatic gain control module  630  and is then separated into its respective video, sync, and audio components by sync separator  632 . The various signal components are appropriately applied to the intermediate memory (not shown), which interfaces with the input electronics of display device  620 . 
     In this case, it is sufficient to serial-multiplex large strings of analog video data of each color (frame-by-frame or interleaved line-by-line, for example) prior to transmission to reduce the required transmission data rate, eliminate tracking phase-locked loop clocks, and simplify the analog-to-digital conversion and memory requirements of processing module  612 . This technique results in a slower but visually equivalent replication of a standard RGB interface. 
     FIG. 7 is a schematic view of a compact unfolded optical path off-axis biocular system  700 , in accordance with the invention, incorporating a single transmissive display device  710  having a substantially planar display surface  708 , similar to that described above in connection with FIG.  1 . Unless otherwise described below, respective components of compact unfolded biocular system  700  are substantially identical to counterpart components of biocular system  100 . Display device  710  is positioned at the intersection of two independent beams  712  and  714  of nearly collimated light, defining respective substantially rectilinear beam axes  713  and  715 . In some embodiments, independent beams  712  and  714  are individually produced from a single or multiple light sources, as described in greater detail below. Independent beams  712  and  714  preferably each have collimated beam width (e.g. diameter  716 ) approximately equal to the width (e.g. diameter) of display surface  708  projected onto planes substantially perpendicular to respective beam axes  713 ,  715 . Beam axes  713  and  715  are tilted relative to one another and relative to a display axis  717  substantially perpendicular to display surface  708 . 
     Independent beams  712  and  714  propagate through display surface  708  along respective beam axes  713  and  715  to respective intermediate image planes  722  and  724 . Single or multiple imaging elements  726  and  728  form intermediate images  732  and  734  of display surface  708  in space at respective intermediate image planes  722  and  724 . In some embodiments, imaging elements  726  and  728  incorporate off-axis aspheric correction to correct for off-axis geometric image distortion. 
     Intermediate image planes  722  and  724  are re-imaged by respective independent eyepiece lenses  736  and  738 , configured to magnify respective intermediate image planes  722  and  724  and to provide virtual images (not shown) for independent viewing by each eye  746 ,  748  of an observer. Because of the collimated nature of independent beams  712  and  714  at display surface  708  and the specific paths of the resultant beams through intermediate image planes  722  and  724 , the optical energy of respective independent beams  712  and  714  typically does not fill the eyepiece apertures  742 ,  744 . It has long been recognized by those having ordinary skill in the art, that optical energy must fill eyepiece apertures  742 ,  744  sufficiently in order to provide substantially complete and uniform virtual images to the eyes  746 ,  748  of an observer. 
     In some embodiments, this condition is met by inserting intermediate field lenses  752 ,  754 , as described in connection with FIG. 1, along beam axes  713 ,  715  at or near respective intermediate image plane  722 ,  724 , to fill respective eyepiece aperture  742 ,  744  for all probable fields of view. In some embodiments, field lenses  752 ,  754  include on-axis or off-axis optical power to correct residual image distortions and/or to accommodate the field curvature of the eyepiece design, as described above in connection with FIG.  1 . 
     Independent beam  712  propagating along beam axis  713 , together with intermediate imaging plane  722 , imaging element  726 , eyepiece lens  736 , eyepiece aperture  742 , and any other optical elements lying along independent beam  712  are defined collectively as a left optical channel  740 . For convenience, the portion of independent beam  712  illuminating display surface  708  is defined as an illumination beam  718 , whereas the portion of independent beam  712  propagating from display surface  708  is defined as an imaging beam  720 . Likewise independent beam  714  propagating along beam axis  715 , together with intermediate imaging plane  724 , imaging element  728 , eyepiece lens  738 , eyepiece aperture  744 , and any other optical elements lying along independent beam  714  are defined collectively as right optical channel  750 . For convenience, the portion of independent beam  714  illuminating display surface  708  is defined as an illumination beam  719 , whereas the portion of independent beam  714  propagating from display surface  708  is defined as an imaging beam  721 . Left and right optical channels  740  and  750  intersect one another only at display device  710 . Compact unfolded biocular system  700  avoids the bulk, expense, and loss of light of conventional beamsplitters. However, off-axis illumination relative to display axis  717  produces geometric distortion of the image of display surface  708 , as described above in connection with FIG.  1 . 
     In some embodiments, left optical channel  740  uses single or multiple light-emitting diodes (LEDs)  762  as light sources, and their emissions are collimated with collimating lenses  785 , deflected by TIR surface  701 , and mixed at wavelength graded dichroic mirrors  703   a  and  703   b  to produce an approximate collimated mixed beam  780 . Dichroic mirrors are familiar in the art for efficiently mixing light of differing wavelengths, and can be readily purchased from optical suppliers such as Omega Optical Inc., 3 Grove Street, Brattleboro, Vt. 05302. Collimated mixed beam  780  is decollimated by a decollimating lens  766  and folded by planar folding elements  771 . The non-uniform optical energy distribution from LEDs  762  is shaped into a uniform distribution by a shaping element  772 , described in more detail below. In some embodiments, decollimating lens  766 , shaping element  772 , and folding elements  771  are fabricated from a single unitary block of optical grade plastic. 
     Light from shaping element  772  is collected and collimated by a first display field lens  776  to produce substantially collimated independent beam  712 . Independent beam  712  is obliquely incident on substantially planar display device  710 , e.g. a miniature liquid-crystal display. Light propagated through display device  710  is collected and decollimated by a second display field lens  778  to produce an approximate point of minimum convergence  786 . Imaging element  726  in left optical channel  740  projects intermediate image  732  of display surface  708  at intermediate image plane  722 , where intermediate field lens  752  directs the optical energy in left optical channel  740  to fill left eyepiece aperture  742  substantially uniformly. Left eyepiece lens  736  forms a virtual image (not shown) of intermediate image plane  722  for comfortable viewing. 
     In some embodiments, imaging element  726  incorporates off-axis aspheric correction to compensate for geometric image distortion arising from the off-axis illumination of display surface  708 . 
     FIG. 7 also shows an aperture stop  782  inserted near point of minimum convergence  786  of independent beam  712  proximate to imaging element  726 , as described in connection with FIG.  1 . 
     Likewise, in some embodiments right optical channel  750  uses single or multiple light-emitting diodes (LEDs)  764  as light sources, and their emissions are collected and collimated with collimating lenses  787 , deflected by TIR surface  705 , and mixed at wavelength graded dichroic mirrors  707   a  and  707   b  to produce an approximate collimated mixed beam  781 . Collimated mixed beam  781  is decollimated by a decollimating lens  768  and folded by folding elements  773 . The non-uniform optical energy distribution from LEDs  764  is shaped into a substantially uniform distribution by shaping element  774 . Light from shaping element  774  is collected and collimated by first display field lens  776  to produce substantially collimated independent beam  714 . Independent beam  714  is obliquely incident on substantially planar display device  710 . Light propagated through display device  710  is collected and decollimated by second display field lens  778  to produce an approximate point of minimum convergence  788 . Imaging element  728  in right optical channel  750  projects intermediate image  734  of display surface  708  at intermediate image plane  724 , where an intermediate field lens  754  directs the optical energy in right optical channel  750  to fill right eyepiece aperture  744  uniformly. Right-eyepiece lens  738  forms a virtual image (not shown) of intermediate image plane  724  for comfortable viewing. 
     In some embodiments, imaging element  728  incorporates off-axis aspheric correction to compensate for geometric image distortion arising from the off-axis illumination of display surface  708 . In some embodiments, FIG. 7 also shows an aperture stop  784  inserted near point of minimum convergence  788  of independent beam  714 , as described above in connection with aperture stop  782 . 
     In some embodiments of the invention, light-emitting diodes  762  of left optical channel  740  comprise a group of red, green and blue color LEDs, for example  762   r ,  762   g ,  762   b . LEDs  762   r - g - b  illuminate the input aperture to a dichroic mirror combiner  702 . Likewise light-emitting diodes  764  of right optical channel  750  comprise a group of red, green and blue color LEDs, for example  764   r ,  764   g ,  764   b , which illuminate the input aperture to dichroic mirror combiner  706 . LEDs  762   r - g - b ,  764   r - g - b  are sequentially switched for color in any manner that is compatible with an appropriately fast display device  710 . For example, red LEDs  762   r ,  764   r  are switched on and all other LEDs are switched off synchronously, while a red frame video signal is applied to display device  710 . For long-term stability of LED output powers and associated display color balance, electro-optic detectors  792 ,  794  are disposed to sample the optical outputs within appropriate color bands in respective left and right optical channels  740 ,  750  and to appropriately adjust the drive currents with closed-loop electronic feedback. 
     In some embodiments, true stereographic viewing is provided by alternately sequencing the right and left groups of LEDs  762 ,  764 , synchronously with time-domain multiplexing of alternate right and left video signals, as described above in connection with FIG.  1 . 
     In some embodiments, eyepiece lenses  736 ,  738  comprise reflective and/or refractive lens elements. For a compact optical geometry facilitating a wide angular field of view, refractive eyepiece lenses are preferred. In some embodiments, eyepiece lenses  736 ,  738  are color-corrected (achromatic) to enhance color and image fidelity. Eyepiece lenses  736 ,  738  have eye relief ranging from approximately 0.5 inch to 0.7 inch and angular aperture ranging from approximately 20 degrees to 40 degrees full angle. In some embodiments eyepiece lenses  736 ,  738  are further refined by using surfaces with higher order curvatures (aspheric surfaces) to minimize aberrations. Such lenses tend to be quite thick and heavy, generally require multiple elements for achromatic performance, and are difficult and expensive to fabricate. Conventional plastic injection-molded lenses can utilize the opposite color dispersions of refractive and diffractive surfaces to create simpler, lighter, and less expensive achromats, but the appropriate lenses are typically delicate and tend to scatter light. Lens considerations, discussed above in connection with FIG.  1  and FIGS. 2A-2G, are equally applicable to FIGS.  7  and  8 A- 8 C below. 
     In FIG. 7, polarizers  796 ,  797  and analyzers  798 ,  799  are shown in respective left and right optical channels  740 ,  750 . As is well known to those skilled in the art, these are typically required when using liquid crystal displays. The location of these elements in the beam path is somewhat arbitrary, and in some embodiments is integral with the display device. The configuration of FIG. 7 shows separate polarizing and analyzing elements positioned as close as possible to display device  710  without overlap between left and right optical channels  740  and  750 . Some form of polarizer/analyzer configuration is typically required in all embodiments of the invention. 
     Among the differences between unfolded off-axis biocular system  100  and compact unfolded biocular system  700  are the following. Mixing light cones  166 ,  168  are replaced by optional dichroic combiner assemblies  702 ,  706 . Shaping elements  772 ,  774  are added in illumination beams  718 ,  719  to provide improved uniformity of beam energy density. Collimating lenses  176 ,  178  are replaced by a single display field lens  776  proximate to display device  710  collimating both left and right independent beams  712  and  714 . This reduces the number of optical elements and substantially reduces the collimated volume occupied by collimated portions of independent beams  712 ,  714 , permitting reduction of both physical size of the system and of the off-axis angle between left and right optical channels  740  and  750  and thereby reducing the need to compensate for off-axis image distortion. Imaging elements  126 ,  128  are replaced by a single decollimating display field lens  778  proximate to display device  710  in both left and right independent beams  712  and  714 , further reducing the collimated volume of the system. Imaging elements  726 ,  728  are positioned in respective optical channels  740 ,  750  behind aperture stops  782 ,  784  near points of minimum convergence  786 ,  788  such that their sizes are minimized. 
     FIG. 8A is a schematic view of a left optical channel  840  of a folded compact biocular display system  800 , functionally similar to left optical channel  740  of compact unfolded biocular system  700  shown in FIG.  7 . The embodiment of FIG. 8A allows a more compact folded optical configuration, for example for head-mounted display use, than does compact unfolded biocular off-axis system  700  shown in FIG.  7 . Folded compact biocular display system  800  and unfolded compact biocular off-axis system  700  each contain elements that are essentially functional counterparts of elements of the other system. Some elements that are transmissive in compact unfolded biocular off-axis system  700  are reflective in folded compact biocular display system  800 . Elements that are substantially similar in the various figures are designated by similar reference numbers. 
     A left beam  813  is tilted relative to a display axis  817  perpendicular to a reflective display surface  808 . A reflective display device  810  folds left optical channel  840  within a region of nearly collimated light  812 , defining incident collimated illumination beam region  812   a  and reflected collimate imaging beam region  812   b . Left optical channel  840  intersects a similarly configured right optical channel (not shown for clarity) only at reflective display device  810 . As in compact unfolded biocular display system  700 , left beam  813  is divided into two portions, namely a left illumination beam  818 , illuminating reflective display surface  808 , and a left imaging be am  820 , reflected and propagating from reflective display surface  808 . In some embodiments, illumination beam  818  originates from a single or multiple light sources, as described in more detail below. Illumination beam  818  preferably has a width (e.g. diameter 816) at reflective display surface  808  approximately equal to the projected width (e.g. diameter) of reflective display surface  808  onto a plane substantially perpendicular to illumination beam  818 . 
     Collimated beam  812  propagates from reflective display surface  808  along left beam axis  813  to an intermediate image plane  822 . 
     In some embodiments, left optical channel  840  uses single or multiple light-emitting diodes (LEDs)  862  a light sources, and their emissions are collimated wit collimating lenses  885  and mixed with wavelength grad d dichroic mirrors  803   a  and  803   b  to produce an approximate collimated mixed beam  880 . Beam folding elements  873 , e.g. planar mirrors or plano-TIRs (total internal reflectors) redirect collimated mixed beam  880 , and the beam is decollimated by a decollimating lens  866  and folded by folding elements  871 . The nonuniform optical energy distribution in collinated mixed beam  880  from LEDs  862  is then shaped by a shaping element  872  into an illumination beam  818  having an optical energy distribution described in more detail below. 
     Illumination beam  818  from shaping element  872  is collected and collimated by a display field lens  876  to produce substantially collimated beam region  812 . Illumination beam  818  is obliquely incident on substantially planar reflective display device  810 , e.g. a miniature liquid-crystal display. Imaging beam  820  reflected from reflective display surface  808  is collected and decollimated by display field lens  876 , producing an approximate point of minimum convergence  886 . Single or multiple imaging elements  826  in imaging beam  820  project an intermediate image  832  of reflective display surface  808  at intermediate image plane  822 , where an optional intermediate field lens  852  directs the optical energy in imaging beam  820  to fill left eyepiece aperture  842  substantially uniformly. Left eyepiece lens  836  forms a virtual image (not shown) of intermediate image plane  822  for comfortable viewing by the left eye  846  of an observer. 
     A beam folding element  805 , e.g., a front surface mirror or prism between reflective display surface  808  and intermediate image plane  822 , redirects imaging beam  820 , reducing the lateral dimension of left optical channel  840 . 
     A deflecting element  806 , e.g., a front surface mirror, redirects imaging beam  813  and simplifies lateral placement of eyepiece lens  836 . Because of the collimated property of beam  813  at reflective display surface  808  and the path of resultant imaging beam  820  through intermediate image plane  822 , the optical energy of imaging beam  820  typically does not fill the eyepiece aperture  842 . In some embodiments, an intermediate field lens  852  is inserted in imaging beam  820  at or near intermediate image plane  822 , for reasons discussed above in connection with FIG.  1 . 
     In some embodiments, imaging element  826  incorporates off-axis aspheric correction to compensate for geometric image distortion arising from the off-axis illumination of reflective display surface  808 . 
     FIG. 8A also shows an aperture stop  882  inserted near point of minimum convergence  886  of imaging beam  820  proximate to imaging element  826 , as described above in relation to aperture stop  782 . 
     FIGS. 8B and 8C are cross-sectional views of an embodiment of compact folded biocular display system  800 . FIG. 8B is a top cutaway projection showing a left eyepiece assembly  888  and a molded optical assembly  890  as viewed across section  8 B— 8 B of FIG. 8C. A right eyepiece assembly, substantially symmetric with left eyepiece assembly  888 , is omitted for clarity. Left eyepiece assembly  888  comprises deflecting element  806  and eyepiece lens  836 . In some embodiments eyepiece lens  836  consists of a single refractive or refractive-diffractive optical element. In other embodiments eyepiece lens  836  comprises multiple refractive, Fresnelled refractive, and/or refractive-diffractive surfaces applied to planar or curved substrates, as described above in connection with FIGS. 2A-2G. Respective axes  893  and  895  of molded optical assembly  890  and left eyepiece assembly  888  are separated by nominally half an interocular distance  896  that is adjustable by an interocular adjustment mechanism (not shown), coupled to both molded optical assembly  890  and left eyepiece assembly  888 . 
     In some embodiments, a focus adjustment is effected by changing the separation  894  of eyepiece lens  836  from deflecting element  806  using a focus adjustment mechanism (not shown). Optionally, intermediate field lens  852  is separated from deflecting mirror  806  by a focus adjustment distance  898 , adjustable by a focus adjustment mechanism (not shown) coupling intermediate field lens  852  and deflecting mirror  806 . 
     FIG. 8C is a cross-sectional view showing molded optical assembly  890  as viewed across section  8 C— 8 C of FIG.  8 B. Molded optical assembly  890  supports light sources, preferably LEDs  862 , and a reflective display device  810  in a nominally orthogonal relationship. In some embodiments LEDs  862  include red, green, and blue color LEDs  862   r - g - b.    
     Molded optical assembly  890  includes total internally reflective (TIR) surfaces  875 , display field lens  876 , molded aperture stop  882 , and imaging element  826 . Molded optical assembly  890  further includes dichroic mirror combiner  802 , equipped with LEDs  862 , collimating lenses  885 , and dichroic mirrors  803   a-b . Further included are decollimating lens  866 , and shaping element  872 . 
     In some embodiments, molded optical assembly  890  is fabricated in whole or in part as a monolithic, unitary structure with self-contained optical components, as described above in connection with FIGS. 3A-3D (see Hebert U.S. Pat. No. 5,596,454, cited above). Integrally molded mechanical features allow for precise registration of the optical elements of molded optical assembly  890  relative to one another, such as the relationship between LEDs  862  and reflective display device  810 , as well as other elements in respective optical channels  840  and  850 . In other embodiments, molded optical assembly  890  is fabricated as a frame onto which individual optical elements are assembled. In some embodiments, molded optical assembly  890  includes a base plate  892 , which serves as a carriage for moving molded optical assembly  890  relative to other assemblies of folded compact biocular display system  800 . 
     Referring to FIGS. 8A-8C, in operation a video signal is applied to reflective display device  810 , thereby producing a reflective video image at reflective display surface  808 , as described above in connection with FIGS. 6A and 6B. 
     Reflective display surface  808  is illuminated by illumination beam  818  of independent left optical channel  840  (substantially symmetric right optical channel  850  not shown). Arrows labeled  840  in FIGS. 8B-8C show the beam propagation path of left optical channel  840 . Points marked with encircled letters X and Y in FIGS. 8B-8C indicate substantially identical respective points in left optical channel  840 . The beam propagation path in right optical channel  850  is symmetric with that of left optical channel  840 , but is not shown for clarity. Each optical element of right optical channel  850  is a symmetric counterpart and performs a function substantially identical to that of a corresponding optical element of left optical channel  840 . Beam propagation paths are configured so that left optical channel  840  intersects right optical channel  850  only at reflective display surface  808 , a discussed above in connection with FIGS.  1  and  3 A- 3 D. 
     Referring to FIG. 8C, illumination beam  818  for left optical channel  840  is generated by LEDs  862 . In some embodiments LEDs  862  include color LEDs  862   r - g - b , whereas in other embodiments the illumination can be monochrome. The outputs from LEDs  862  are captured by collimating lenses  885  and are mixed in dichroic combiner  802 . 
     Collimating lenses  885  collect and collimate the light from LEDs  862   r - g - b  into collimated mixed beam  880  within dichroic combiner  802 . Each color component from LEDs  862   r ,  862   g ,  862   b  respectively reflects on either TIR surface  801  or dichroic mirrors  803   a  and  803   b . TIR surface  801  reflects the short wavelength blue light from LED  862   b . Dichroic mirror  803   a  reflects the medium wavelength green light from LED  862   g  while transmitting the shorter wavelength blue light. Dichroic mirror  803   b  reflects the long wavelength red light from LED  862   r  while transmitting both the shorter blue and green wavelength components. Thus, within dichroic combiner  802 , LED  862  color components are all substantially superimposed into one collimated mixed beam  880  and propagate thereafter as if these color components were all emitted from a spatially common multicolor light source. The functions of elements  801 ,  803   a , and  803   b  of dichroic combiner  802  are similarly performed by respective counterpart element  701 ,  703   a , and  703   b  of dichroic mirror combiner  702  and by respective elements  705 ,  707   a , and  707   b  of dichroic mirror combiner  706 , shown in FIG.  7 . 
     Collimated mixed beam  880  emerges from dichroic combiner  802  and decollimated by decollimating lens  866  to produce illumination beam  818 , which is deflected by TIR reflective surfaces  875  through shaping element  872 . TIR reflective surfaces  875 , typically comprising three or more planar surfaces, combine the functions of folding elements  871  and beam folding elements  873 . In some embodiments, as shown in FIG. 8C, TIR reflective surfaces  875 , decollimating lens  866 , and shaping element  872  are fabricated from a single unitary block of optical grade plastic. 
     FIG. 9 is a diagram illustrating the configuration and performance of shaping element  872 . In some embodiments, shaping element  872  transforms an input beam having an energy distribution concentrated along the beam axis into an illumination beam  818  having a substantially peripherally concentrated optical energy distribution. LEDs, such as LED  862   b , are typically Lambertian light sources generating a cosine beam energy distribution having higher optical energy density on axis than at the edges. Thus, collimating lenses  885  of dichroic combiner module  802  collect more energy near the axis than near the edges of the beam, and the core of collimated mixed beam  880  within dichroic combiner  802  contains a higher energy density than do the edges. This is illustrated in FIG. 9 by substantially parallel lines closer together near the core and farther apart near the edges of collimated mixed beam  880  between collinator lenses  885  and decollinating lens  866 . This nonuniform energy distribution is substantially preserved as the beam passes through decollimating lens  866  and makes progressive reflections at three TIR surfaces  875 . Other LED sources, reflective surfaces, and dichroic mirrors are omitted in FIG. 9 for clarity. 
     The beam then enters shaping element  872 , which spreads beam energy from near the axis more than it spreads energy from near the edges of the beam, thereby offsetting the energy density nonuniformity of the input beam delivering illumination beam  818 . 
     Referring to FIG. 8C, illumination beam  818  is collimated by display field lens  876 , propagates through collimated beam region  812 , and illuminates reflective display surface  808 . At reflective display surface  808 , illumination beam  818  is spatially modulated by a video image and is reflected obliquely (see folded arrow  840  in FIGS. 8B-8C) as imaging beam  820 , which produces a final image, as described below in more detail. 
     If the energy distribution of illumination beam  818  is not shaped by shaping element  872 , then the illumination of reflective display surface  808  is weaker at the edges than at the center. Likewise, this optical energy nonuniformity propagates through optical channel  840  on imaging beam  820 , thereby creating a nonuniformly illuminated final image. Typically, the illumination nonuniformity is aggravated by the transmission properties of subsequent optical elements within optical channel  840 . Shaping of illumination beam  818  by shaping element  872  can overcome this image illumination nonuniformity. Advantageously, shaping element  872  is configured to overcorrect the radial distribution of optical energy in illumination beam  818 , as shown by the spacings of parallel lines in collimated beam region  812  of FIG.  9 . This method provides compensation for edge degradation of optical energy density in imaging beam  820  subsequent to reflection from reflective display surface  808 , thereby resulting in substantially uniform illumination of the final image. 
     A variety of basically aspheric lens designs, including diffractive and refractive surfaces, can provide the function of shaping element  872 . In the present embodiment, shaping element  872  has a substantially conical aspheric surface  910 . In some embodiments, decollimating lens  866 , shaping element  872 , and TIR surfaces  875  are all fabricated from a single unitary block of optical grade plastic. 
     In some embodiments, color LEDs  862   r - g - b  are sequenced synchronously with a color video signal to reflective display device  810 , thereby producing a sequentially color modulated reflected beam, as described above in connection with FIG.  1 . After modulation and reflection from reflective display surface  808 , imaging beam  820  propagates back through display field lens  876 , which de-collimates the beam. Thus, display field lens  876  of folded compact biocular display system  800  performs the dual collimating and decollimating functions of display field lenses  776  and  778  of unfolded compact biocular display system  700 . Further, single display field lens  876  performs these functions for both left and right optical channels in folded compact biocular display system  800 . Display field lens  876  thereby functionally replaces four optical elements of unfolded off-axis biocular system  100 , namely left and right collimating lenses  176 ,  178 , and left and right imaging elements  126 ,  128 . 
     After decollimation, imaging beam  820  passes through aperture stop  882  proximate to imaging element  826 . In some embodiments, imaging element  826  is a transmissive element that combines refractive, Fresnelled refractive, and/or diffractive properties. Folding element  805  is a reflective element disposed to fold imaging beam  820  toward intermediate field lens  852 , which then images reflective display surface  808  onto intermediate image plane  822  (see FIGS.  8 A- 8 B). In some embodiments, imaging element  826  incorporates off-axis aspheric correction to compensate for geometric image distortion arising from off-axis imaging of reflective display surface  808 . 
     Referring to FIG. 8B, after reflecting from folding element  805 , imaging beam  820  propagates to intermediate image plane  822 , where it forms an intermediate image  832  of reflective display surface  808 . The properties of intermediate image  832  of the present embodiment are substantially identical with those of intermediate image  732  described above in connection with FIG.  7 . Eyepiece lens  836  images intermediate image  832  to form a final virtual image (not shown). As in the embodiment of FIG. 7, intermediate field lens  852  is placed at intermediate image plane  822 , thereby filling the aperture  842  of eyepiece lens  836  with optical energy. For compactness, a deflecting element  806  folding imaging beam  820  is inserted between intermediate image plane  822  and eyepiece lens  836 . 
     The full range of optical element configuration and corrections, as described above in connection with FIGS.  1  and  2 A- 2 G, is applicable to folded compact biocular display system  800 . In some embodiments, a system design approach to color correction, is applied to folded compact biocular display system  800 . With such a system design approach, the overall system performance is made achromatic by balancing the offsetting color distortions of individual optical elements. 
     Illustratively, a diffractive lens surface  830  (see for example FIG. 8A) is applied to imaging element  826  (see FIGS. 8B-8C) either alone or in combination with other optical structures (e.g. off-axis correction). This results in color overcorrection, as described in connection with FIG. 2C, and intermediate field lens  852  and eyepiece lens  836  need not be individually achromatic for overall folded compact biocular display system  800  to be achromatic. As needed, intermediate field lens  852  and/or eyepiece lens  836  includes a minimally offsetting color correction. A similar function is provided by diffractive lens surface  730  in unfolded compact off-axis biocular system  700 . 
     In some embodiments, true stereographic viewing is provided by alternate sequencing of respective left and right groups of LEDs  862 ,  864 , synchronously with time-domain multiplexing of alternating right and left video signals, as described above in connection with FIG.  7 . Folded compact biocular display system  800  provides unique true stereographic viewing capability with a single reflective display device  810 , because respective left and right optical channels  840  and  850  remain optically independent of one another, except for sharing common reflective display device  810 . In some embodiments, stereographic video sequencing is performed concurrently with the above-described sequential color switching at a sequencing rate and in a combination that minimizes visual flicker. 
     In accordance with the invention, true stereographic viewing is achieved using a single display device with appropriate inexpensive optics and without a beamsplitter. Low-cost, complex optical grade plastic optics allow biocular viewing of a single electro-optic display device, such as for use in a head-mounted display (HMD). A dual off-axis configuration provides two independent optical channels, intersecting only at the image surface of the display device. Each optical channel provides its own illumination source, eyepiece lens, and imaging optics. In some embodiments, color illumination beams are optically combined in a dichroic mixer. Image uniformity is enhanced using an aspheric shaping element. In some embodiments, nearly collimating illumination optics and intermediate field lenses are used to fill wide-aperture eyepieces without the need for a beamsplitter. In some embodiments, compact design is facilitated by restricting collimated beam space to a small volume proximate to the display surface. 
     Multiple illumination schemes are described for either monochrome or color, and in either two-dimensional or time-sequential true stereographic presentation. In some embodiments, true stereographic performance is achieved by sequential activation of the light sources in the two channels in synchronism with sequential video signals for the respective channels. Offsetting color overcorrection and undercorrection methods are applied to minimize optical element complexity. Additional features include lightweight achromatic eyepiece construction. A video interface converts conventionally formatted video signals to serially multiplexed color data. Versions of the video interface include wireless transmission, eliminating cumbersome cabling. 
     Particularly, embodiments of the invention provide the depth perception, high resolution, high color fidelity, peripheral vision correction, and convenience of use in a head-mounted configuration to satisfy the demanding requirements of medical and surgical remote viewing. 
     Although the invention has been described in terms of a certain preferred embodiment, other embodiments apparent to those skilled in the art are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined only by the claims which follow.