Abstract:
According to an embodiment, an image capture apparatus comprises a light emitter, a beam scanner aligned to receive emitted light and operable to scan the light in a two-dimensional pattern, imaging optics aligned to receive the scanned two-dimensional pattern and image the pattern onto an object, and to collect light scattered from the object, a detector to receive scattered light from the imaging optics, an electronic controller c operable to receive an electrical signal from the detector corresponding to the received scattered light, and an actuator operable to modify the relative alignment between the beam scanner and the imaging optics to change an imaged location on the object. According to an embodiment, a method for capturing an image comprises scanning a beam of light through imaging optics onto a location on a surface, detecting light scattered by the surface, and steering the beam scanner relative to the imaging optics to change the trajectory of the scanned pattern. According to an embodiment, a system for scanning a field of view comprises a light source, an optical fiber, a MEMs scanner aligned to receive the light from the optical fiber and operable to scan the light through a partially-reflective imaging optic in a pattern, and a photo-detector aligned to receive light scattered from the field of view.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority benefit under 37 CFR §1.20 from U.S. patent application Ser. No. 10/601,921, copending at the time of the time of this application and filed Jun. 20, 2003 now U.S. Pat. No. 7,190,329; which depends from U.S. patent application Ser. No. 09/129,739, now U.S. Pat. No. 6,583,772, filed Aug. 5, 1998. 
     TECHNICAL FIELD 
     The present invention relates to optical imaging systems and, more particularly, to systems employing scanning inputs or outputs. 
     BACKGROUND OF THE INVENTION 
     A variety of techniques are available for providing visual displays of graphical or video images to a user. For example, cathode ray tube displays (“CRTs”), such as televisions and computer monitors, are very common. Such devices suffer from several limitations. Conventional CRTs are typically bulky and consume substantial amounts of power, making them undesirable for portable or head-mounted applications. 
     Flat panel displays, such as liquid crystal displays, plasma displays, and field emission displays, may be less bulky and consume less power. However, typical flat panel displays utilize screens that are several inches across. Such screens have limited use in head mounted applications or in applications where the display is intended to occupy only a small portion of a user&#39;s field of view. 
     More recently, very small displays have been developed for partial or augmented view applications and for various head-mounted applications. In augmented view applications, a portion of the display is positioned in the user&#39;s field of view and presents an image that occupies a small region  42  of the user&#39;s field of view  44 , as shown in  FIG. 1 . The user can thus see both a displayed image  46  and background information  48 . 
     One application of such small displays in found in dual-ended systems, i.e., systems in which images are acquired at one end and transmitted to a second end for output. For example, remote viewing systems typically utilize small detectors or cameras (such as CCD arrays) at a first end that convert images to electrical signals. Then, the electrical signals are either (a) transmitted along conductors; or (b) converted to optical data and transmitted along optical fibers to the second end. At the second end, the electrical signals or optical data are converted back to optical images by electronic or optoelectronic circuitry and a miniature display. Within the display, some form of the electronic or optoelectonic circuitry converts the electrical or optical signal to an electrical driving signal that is applied to the miniature display. The display then converts the signal to the viewable image. 
     Such approaches usually have several drawbacks. For example, conversion between electrical signals and optical signals typically induces image distortion and noise. Also, in typical systems, the image is reconstructed by combining light from red, green, and blue light sources (e.g., phosphors or laser diodes). Such systems can induce some form of color distortion. Moreover, electrical circuitry can be sensitive to temperature or other environmental variations and to electromagnetic fields. In many applications, temperature controllers and electrical shielding can protect the electrical circuitry. However, such controllers and shielding can impose significant weight and size limitations. In head-mounted applications, this additional weight can place stress on the wearer&#39;s neck and may also increase the difficulty of packaging. 
     SUMMARY OF THE INVENTION 
     In an optical imaging apparatus, light from an optical image is scanned by a first scanner at one location and transmitted by an optical transmission fiber to a second location without converting the optical information to electrical signals. In one embodiment, a second scanner receives light from the fiber and reconstructs the optical image by scanning substantially synchronously with the first scanner. 
     In one embodiment, a first light emitter is coupled to the transmission fiber through a fiber coupler. The first light emitter provides illuminating light to the transmission fiber and the transmission fiber transmits the illuminating light to the input scene. The first scanner scans the illuminating light over the input scene. The input scene reflects a portion of the scanned illuminating light and back to the input scanner which then couples the reflected light into the transmission fiber for transmission to the second scanner. 
     In one embodiment, the first emitter is a full spectrum illuminator, such as a mercury vapor lamp, white light laser or short arc lamp. If the full spectrum illuminator does not provide adequate luminance, the emitter can be formed from one or more monochrome sources, such as laser diodes. 
     In one embodiment, both of the scanners act as transceivers. The first scanner thus scans images from the first scene and the transmission fiber transmits the light from the first scanner to the second scanner. The second scanner recreates the first scene from the scanned light. At the same time, the second scanner scans images from a second scene and the transmission fiber transmits the light from the second scanner to the first scanner. The first scanner recreates the second scene from the scanned light. To improve imaging, one or more of the scanners includes confocal optics that couple light to and from the respective scene. For viewing, one embodiment includes a beam splitter and imaging optics that display the image on a screen. 
     In another embodiment, one of the scanners couples light directly to the retina of a viewer. One embodiment of the retinal scanner includes a beam combiner that receives light from the fiber and light from a background. The combined light from the combiner is received through the user&#39;s pupil and strikes the retina. The light from the fiber forms a “virtual” image and the light from the background forms a “real” image. The user perceives an image that is a combination of the virtual image and the real image. 
     In one embodiment, the retinal scanner includes an eye tracking mechanism that monitors the position of the wearer&#39;s eye and adjusts the position of the scanned beam of light so that the wearer continues to see the virtual image as the wearer moves the eye to view the real image. 
     In another embodiment according to the invention, a separate fiber carries the illuminating light. To improve coupling of reflected light into the transmission fiber, the separate fiber and the transmission fiber are bonded together with their far or intermediate fields overlapped. Each of the transmission fiber and the separate fiber are formed as D-shaped fibers so that the cores of the fibers can be positioned substantially closely. 
     In one embodiment, the transmission fiber may include components that allow active or passive modification of the transmitted light. For example, in some applications it may be desirable to incorporate in-line fiber amplifiers to amplify the light being transmitted. In other amplifications, active switching can allow the transmitted light to be selectively directed along one or more alternative paths. In still other applications, the visible light may be directly down converted to typical communication system wavelengths for long distance transmissions and then up converted to visible wavelengths after transmission. Such wavelength shifting approaches may be adapted to wavelength division multiplex light from a plurality of input scanners along a common optical path. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a diagrammatic representation of a combined image perceived by a user resulting from the combination of light from an image source and light from a background. 
         FIG. 2A  shows a linked scanner system according to one embodiment of the invention including pair of scanners linked by a transmission fiber where a pair of light emitters are coupled to the transmission fiber to provide light for illumination. 
         FIG. 2B  shows an alternative approach to coupling light using a single fiber coupler. 
         FIG. 3  shows one application of a linked scanner system including three input scanners located at respective locations on an aircraft. 
         FIG. 4  shows another application of a linked scanner system including an input scanner located in a separated environment for remote viewing of the environment. 
         FIG. 5  is a diagrammatic representation of a scanning display suitable for use as one of the scanners of  FIG. 2A . 
         FIG. 6  is a diagrammatic representation of the display of  FIG. 4  showing displacement of the eye relative to the beam position and corresponding reflection of the positioning beam. 
         FIG. 7A  is a diagrammatic representation of reflected light striking the detector in the position of  FIG. 6 . 
         FIG. 7B  is a diagrammatic representation of reflected light striking the detector in the position of  FIG. 6 . 
         FIG. 8  is a diagrammatic representation of the display of  FIG. 2A  showing repositioning of the image source and positioning beam source responsive to detection of the displacement of  FIG. 6 . 
         FIG. 9  is a detail view of a portion of a display showing shape memory alloy-based positioners coupled to the substrate. 
         FIG. 10  is a diagrammatic representation of a scanning assembly in the scanning display of  FIG. 5 . 
         FIG. 11A  is a diagrammatic representation of a dual-ended imager where the output scanner includes an eye tracker coupled to positioners in the input scanner. 
         FIG. 11B  is a diagrammatic representation of the dual-ended imager of  FIG. 11A  showing realignment of the input scanner in response to detected movement of the viewer&#39;s eye 
         FIG. 12  is a diagrammatic representation of the dual-ended imager adapted for driving a scanner with an optical signal. 
         FIG. 13  is a diagrammatic representation of a dual-ended imager including separate fibers for illuminating a scene and transmitting light from the scene to a distal end. 
         FIG. 14  is a cross-sectional view of the fibers of  FIG. 13  showing D-shaped fibers. 
         FIG. 15  is a diagrammatic representation of alignment of fibers during fabrication by aligning images on a screen. 
         FIG. 16  is a diagrammatic representation of a dual-ended fiber imager showing effective magnification. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 2A , a dual-ended optical imager  8  is formed from a pair of scanners  10 ,  12  linked by a transmission fiber  14 , where each of the scanners  10 ,  12  acts as an optical transceiver. The structure and operation of the scanners  10 ,  12  are described in greater detail herein with respect to  FIGS. 5 and 6 . Also, an example of a fiber-coupled scanning display is found in U.S. Pat. No. 5,596,339 of Furness et. al., entitled VIRTUAL RETINAL DISPLAY WITH FIBER OPTIC POINT SOURCE which is incorporated herein by reference. As will also be explained below, the scanners  10 ,  12  are synchronized so that they scan in substantially the same pattern at substantially the same rate. 
     A pair of light emitters  16 ,  18  are coupled to the transmission fiber  14  by respective fiber couplers  20 ,  22  to provide input light for illumination. As shown in  FIG. 2B , a single fiber coupler  22 A can couple light form both of the light emitters  16 ,  18  in the transmission fiber. Returning to  FIG. 2A , the light emitters  16 ,  18  are preferably full spectrum light sources, although monochrome sources may be desirable for some applications. Where full spectrum light is desired, the light emitters  16 ,  18  may be small mercury vapor lamps, white light lasers or short arc lamps. For monochrome applications, laser diodes or other light emitting diodes may be used. In some color applications, each of the light emitters may include a plurality of light sources. For example, each of the light emitters  16 ,  18  may include red, green and blue lasers. Alternatively, the light emitters may use non-visible illuminating light in some applications. For example, in scanning ophthalmascope applications ultraviolet or infrared light may be useful for detection of certain types of body tissue. In such applications, a wavelength converting viewer, such as an infrared viewer, may be added the output scanner. 
     As described in the following discussion and as indicated by the arrows in  FIG. 2A , the left scanner  10  is operating as the input scanner and the right scanner  12  is operating as the output scanner. However, in many applications, each of the scanners  10 ,  12  may act as both an input scanner and an output scanner to achieve bi-directional communication. Communication in a single direction will be described first. 
     To image a scene  24 , the right emitter  18  emits light that is coupled to the transmission fiber  14  through the fiber coupler  22 . The transmission fiber  14  transmits the light to the input scanner  10  where the light is scanned onto the scene  24  in a two dimensional pattern, such as a raster pattern, to illuminate the scene  24 . At each position of the input scanner  10 , a portion of the illuminating light is reflected back to the input scanner  10  by the scene  24 . Because light travels to and from the scene  24  very quickly, the position of the input scanner  10  does not change significantly before the reflected light reaches the input scanner  10 . Therefore, the input scanner  10  couples the reflected light back into the transmission fiber  14 , which then transmits the reflected light to the output scanner  12 . 
     The output scanner  12  scans the light from the transmission fiber  14  in the same pattern and at the same frequency as the input scanner  10  to reconstruct the scene  24  as a virtual scene  26 . Where the input scanner  10  is sufficiently distant from the output scanner  12 , propagation delay through the transmission fiber may affect the displayed images. In such applications, it may be desirable to adjust the timing of the output scanner  12  to compensate for the delay. As shown in  FIG. 2A , light output from the scanner  12  strikes a viewing screen  27 , thereby producing an image for viewing by the user. One skilled in the art will recognize that the output scanner  12  may be incorporated into a retinal scanning display in some applications, such as where the imager  8  is operated as a unidirectional imager. 
     To improve coupling of the illuminating and reflected light into the scanners  10 ,  12 , the scanners  10 ,  12  include confocal optics similar to those of a confocal microscope. Although the confocal optics  28 ,  30  are represented as simple lenses, one skilled in the art will recognize that a variety of confocal optics  28 ,  30  may be used. Also, though the confocal optics  28 ,  30  are shown as separate from the scanners  10 ,  12  the optics  28 ,  30  are typically integrated into the scanners  10 ,  12 , as will be described below with reference to  FIG. 5 . 
     One application of the imager  8  is shown in  FIG. 3 , where an aircraft  50  includes three scanners  10 A-C that are located along each wing  51  and at the rear of the aircraft  50 . In these locations, the input scanners  10 A-C can image the wings and flaps to show icing, vibration, deformation, or flap position. The transmission fibers  14  for each of the scanners  10 A-C extend from the scanners  10 A-C to the cockpit, where they are coupled to a single output scanner  12  through an optical switch  54 . The switch may be one of many known optical switches, such as an in-line fiber optic switch. By controlling the switch  54 , a pilot can selectively view images from the input scanners  10 A-C. 
     As shown in  FIG. 4 , the imager  8  may also be used to remotely view a hazardous or otherwise inaccessible environment  56 , such as a toxic gas or high-temperature environment or an internal cavity of a human body. In this embodiment, the input scanner  10  is inserted into the environment  56  and oriented to view an appropriate portion of the environment  56 . A single emitter  18  is coupled to the input scanner  10  by the transmission fiber  14  to provide light that illuminates the environment  56 . Light reflected from the environment  56  is received by the input scanner  10  and coupled back into the transmission fiber  14 . The transmission fiber  14  transmits the received light to the output scanner  12  for viewing by a viewer&#39;s eye  52 . Because the light is not converted to an electrical signal and re-converted to an optical signal, the viewer perceives light originating from the environment  56  without conversion artifacts. 
     A scanning device  60  suitable for use as one of the scanners  10 ,  12 , shown in  FIG. 5 , is positioned for viewing by a viewer&#39;s eye  52  or for viewing features of the eye  52 . One skilled in the art will recognize that, the embodiment of the device  60  is described herein for viewing the eye  52  for convenience of presentation, the device  60  may also be used to view many other objects. For example, the device  60  may be used to view an inaccessible or hazardous environment, as described above with reference to  FIGS. 3 and 4 . 
     The device  60  includes two principal portions, each of which will be described in greater detail below. The first portion of the device  60  is a scanning assembly  76  that scans an input beam  53  through a two-dimensional scanning pattern, such as a raster pattern. One example of such a scanning assembly is a mechanically resonant scanner, such as that described U.S. Pat. No. 5,557,444 to Melville, et. al. entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which is incorporated herein by reference. However, other scanning assemblies, such as acousto-optic scanners may be used in such displays. 
     Imaging optics  61  form the second portion of the device  60 . The imaging optics  61  in the embodiment of  FIG. 5  include a pair of mirrors  62  and  64  that shape and focus the beam  53  appropriately for imaging or for viewing by a viewer&#39;s eye  52 . One skilled in the art will recognize that the mirror  62  may be partially transmissive so that a portion of the light will be reflected and a portion of the light will be transmitted. 
     To image the eye  52 , the fiber  14  provides illuminating light that exits the fiber  14  and enters a scanning assembly  76 . The scanning assembly  76  scans the illuminating light through a substantially raster pattern onto the imaging optics  61  so that the mirrors  62 ,  64  direct light toward the eye  52 . The scanned light strikes the eye  52  (in this case the retina  59 ) and a portion of the light is reflected back to the mirrors  62 ,  64 . The mirrors  62 ,  64  direct the reflected light to the scanning assembly  76 . Because the time for light to travel from the scanning assembly  76 , to the mirrors  62 ,  64  and retina  59  is very small, the scanning assembly  76  is in substantially the same condition as when the light first arrived from the fiber  14 . Accordingly, the scanning assembly  76  couples the light from the mirrors  62 ,  64  back into the fiber  14 . While the preferred embodiment described herein uses light reflected from the retina  59 , other embodiments may se different optical structures or position the components differently to image other portions of the eye  52 , such as the iris. 
     Viewing will now be described with reference to the same figure ( FIG. 5 ), because the device  60  can operate bi-directionally. During viewing, the fiber  14  outputs the transmitted scanned light to the scanning assembly  76 . The scanning assembly  76  scans the light both horizontally and vertically in a repetitive pattern, such as a raster pattern. The imaging optics  61  redirects and magnifies scanned light from the scanning assembly  76  toward the user&#39;s eye  52 , where the light passes through the pupil  65  and strikes the retina  59  to produce a perceived virtual image. To ease the user&#39;s acquisition of light from mirrors  62 ,  64 , the imaging optics  78  may also include an exit pupil expander that increases the effective numerical aperture of the beam of scanned light. The exit pupil expander is omitted from  FIG. 5  for clarity of presentation of the beam  53 . 
     As can be seen from  FIG. 5 , the user&#39;s eye  52  is typically in a substantially fixed location relative to the imaging optics  61  because the viewer&#39;s head is typically in a fixed location relative to the scanning assembly  76 . For example, the scanner  10  may be rigidly mounted and include an eyecup against which the user&#39;s eye socket is pressed. For clarity, this description therefore does not discuss head movement in describing operation of the device  60 . One skilled in the art will recognize that the user&#39;s head may be free for relative movement in some applications. In such applications, a known head tracking system may track the user&#39;s head position for coarse positioning. 
     When the user&#39;s eye  52  moves, the pupil  65  may move out of alignment with light from the fiber  14 . In the embodiment of  FIG. 6 , eye tracking reduces such misalignment by monitoring the position of the user&#39;s eye  52  and adjusting the beam alignment, as will now be described with reference to  FIGS. 6-9 . 
     In addition to light from the fiber  14 , the imaging optics  78  also receive a locator beam  90  from an infrared light source  92  carried in a common housing with the output scanner  12  ( FIG. 2A ). The output scanner  12  also includes combining optics that combine the infrared light with the scanned light so that the infrared and visible light are substantially collinear. Thus, the output of the imaging optics  78  includes light from the infrared light source  92 . One skilled in the art will recognize that, although the infrared light source  92  is shown as being positioned orthogonally relative to the fiber  14 , other implementations are easily realizable 
     When the user&#39;s eye  52  moves, all or a portion of the light from the light source  74  and infrared source  92  may no longer enter the pupil  65  or may enter the pupil  65  at an orientation where the pupil  65  does not direct the light to the center of the retina  59 . Instead, some of the light from the fiber  14  and source  92  strikes a non-pupil portion  96  of the eye. As is known, the non-pupil portion  96  of the eye has a reflectance different and typically higher than that of the pupil  65 . Consequently, the non-pupil portion  96  reflects light from the sources  74 ,  92  back toward the imaging optics  78 . The imaging optics  78  redirect the reflected light toward an optical detector  88  positioned on the substrate  85  adjacent to the source  92 . In this embodiment, the detector  88  is a commercially available CCD array that is sensitive to infrared light. As will be described below, in some applications, other types of detectors may be desirable. 
     As shown in  FIG. 7A , when the user&#39;s eye is positioned so that light from the fiber  14  and source  92  enters the pupil (i.e., when the eye is positioned as shown in  FIG. 4 ), a central region  100  of the detector  88  receives a low level of light from the imaging optics  78 . The area of low light resulting from the user&#39;s pupil will be referred to herein as the pupil shadow  106 . When the eye  52  shifts to the position shown in  FIG. 6 , the pupil shadow  106  shifts relative to the detector  88  as shown in  FIG. 7B . In response the detector  88  outputs data, which are indicative of the position of the pupil shadow  106 . The data are input to control electronics  108 , such as a microprocessor or application specific integrated circuit (ASIC). Responsive to the data, the control electronics  108  accesses a look up table in the memory device  110  to retrieve positioning data indicating an appropriate positioning correction for the light source  74 . The positioning data may be determined empirically or may be calculated based upon known geometry of the eye  52  and the scanner  12 . 
     In response to the retrieved positioning data, the control electronics  108  activates X, Y and Z drivers  112 ,  114 ,  116  to provide voltages to respective piezoelectric positioners  118 ,  120 ,  122  coupled to the substrate  85 . As is known, piezoelectric materials deform in the presence of electrical fields, thereby converting voltages to physical movement. Therefore, the applied voltages from the respective drivers  112 ,  114 ,  116  cause the piezoelectric positioners  118 ,  120 ,  122  to move the fiber  14  and source  92 , as indicated by the arrows  124 ,  126 ,  128  in  FIG. 8 . 
     As shown in  FIG. 8 , shifting the positions of the fiber  14  and source  92  shifts the locations at which light from the fiber  14  and source  92  strikes the user&#39;s eye, so that the light once again enters the pupil  65 . The pupil shadow  106  once again returns to the position shown in  FIG. 7A . One skilled in the art will recognize that the deformation of the piezoelectric positioner  116  is exaggerated in  FIG. 8  for demonstrative purposes. However, because the mirrors  62 ,  64  may have a magnification greater than one, small shifts in the position of the substrate  85  can produce larger shifts in the location at which the light from the light source  74  arrives at the eye. Thus, the piezoelectric positioners  118 ,  120 ,  122  can produce sufficient beam translation for many positions of the eye. Where even larger beam translations are desirable, a variety of other types of positioners, such as electronic servomechanisms may be used in place of the piezoelectric positioners  118 ,  120 ,  122 . 
     Alternatively, shape memory alloy-based positioners  113  can be used to reposition the substrate as shown in  FIG. 9 . Shape memory alloys are known materials, such as equiatomic nickel-titanium alloys, that change shape in response to energy inputs, such as heat induced by electrical currents. The positioners  113  may be spirally located, as shown in  FIG. 9  or may be in any other appropriate configuration. One skilled in the art will also recognize that the imaging optics  78  does not always require magnification, particularly where the positioners  118 ,  120 ,  122  are formed from a mechanism that provides relatively large translation of the scanner  70 . 
       FIG. 10  shows one embodiment of a mechanically resonant scanner  200  suitable for use in some applications of the scanners  10 ,  12 . The resonant scanner  200  includes as the principal horizontal scanning element, a horizontal scanner  201  that includes a moving mirror  202  mounted to a spring plate  204 . The dimensions of the mirror  202  and spring plate  204  and the material properties of the spring plate  204  are selected so that the mirror  202  and spring plate  204  have a natural oscillatory frequency on the order of 10-100 kHz. A ferromagnetic material mounted with the mirror  202  is driven by a pair of electromagnetic coils  206 ,  208  to provide motive force to mirror  202 , thereby initiating and sustaining oscillation. Drive electronics  218  provide electrical signal to activate the coils  206 ,  208 . 
     Vertical scanning is provided by a vertical scanner  220  structured very similarly to the horizontal scanner  201 . Like the horizontal scanner  201 , the vertical scanner  220  includes a mirror  222  driven by a pair of coils  224 ,  226  in response to electrical signals from the drive electronics  218 . However, because the rate of oscillation is much lower for vertical scanning, the vertical scanner is typically not resonant at high frequencies. The mirror  222  receives light from the horizontal scanner  200  and produces vertical deflection at about 30-100 Hz. Advantageously, the lower frequency allows the mirror  222  to be significantly larger than the mirror  202 , thereby reducing constraints on the positioning of the vertical scanner  220 . 
     An example of one such resonant scanner suitable for use in the device  60  is found in U.S. Pat. No. 5,557,444 of Melville, et. al. entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM which is incorporated herein by reference. One skilled in the art will recognize that other types of vertical scanners, such as acousto-optically driven scanners or commercially available magnetically driven scanners may also be within the scope of the invention. 
     For output scanning, the fiber  14  outputs light obtained by scanning the input scene. At the same time, the drive electronics  218  activate the coils  206 ,  208 ,  224 ,  226  to oscillate the mirrors  202 ,  222 . The light strikes the oscillating horizontal mirror  202 , and is deflected horizontally by an angle corresponding to the instantaneous angle of the mirror  202 . The deflected light then strikes the vertical mirror  222  and is deflected at a vertical angle corresponding to the instantaneous angle of the vertical mirror  222 . The scanning of the horizontal and vertical mirrors  202 ,  222  is synchronized with the horizontal and vertical scans of corresponding mirrors in the input scanner so that at each position of the mirrors, the output light comes from the corresponding position in the input scene. The scanner therefore “draws” the virtual image using the same light received from the input scene. As noted above, in some applications it may be desirable to phase shift the output mirrors  202 ,  222  to compensate for delays through the fiber  14 . The output light may be scanned directly upon the user&#39;s retina in some applications to produce a perceived image. Displays employing scanned beams imaged upon a retina and mechanical resonant scanning are described in greater detail in U.S. Pat. No. 5,467,104, of Furness III, et. al. entitled VIRTUAL RETINAL DISPLAY, which is also incorporated herein by reference. 
     In certain applications, environmental factors may shift the natural resonant scanning of one of the scanners  10 ,  12 . For example, where the environment of  FIG. 4  is a high temperature environment, material properties in the scanner  10  may vary, thereby causing variations in the scanning frequency. 
     Still referring to  FIG. 4 , to ensure that the scanners  10 ,  12  remain substantially synchronized, the resonant frequencies of the scanners  10 ,  12  are tunable. The frequency of the input scanner  10  is allowed to vary in response to environmental factors. A detector  270  coupled to the input scanner  10  provides an output signal indicating the frequency of oscillation of the input scanner  10 . The detector  270  is an optical fiber  271  aligned to an edge of the scanning range of the scanner  10 , so that during each scan of the scanner  10 , the fiber  14  receives a brief pulse of light. The optical fiber  271  transmits the light to the scanner  12  where a photodiode  272  converts the pulse of light to an electrical pulse. A second detector  276  in the scanner  12  provides a second set of pulses indicating the scanning frequency of the scanner  12 . The pulses from the detectors  270 ,  276  are input to a comparing circuit  278  that outputs an error signal Ve having a voltage level corresponding to the error between the frequencies. Alternatively, it may be desirable in some applications to determine the synchronization information directly from the transmitted light. For example, a small light source or reflector can be positioned at the edge of the field of view of the input scanner  10  to provide a light pulse at the edge of each scan. Alternatively, signal-processing techniques can produce the synchronization signal directly from the light received by the scanner. 
     Returning to  FIG. 6 , the error signal drives a piezoelectric transducer  274  (visible in  FIG. 10 ) in the scanner  12  that applies a force to the spring plate  204  ( FIG. 10 ) in response. The applied force places stress in the spring plate  204  thereby shifting the spring constant. The adjusted spring constant shifts the resonant frequency of the scanner  12  to minimize the error signal and thus the error. The input scanner  10  thus acts as a master and the output scanner  12  acts as a slave. Although the frequency adjustment is described herein as being controlled by mechanical pressure on the spring plate, a variety of other approaches to frequency control may be within the scope of the invention. For example, the error signal Ve can drive a thermoelectric temperature controller to control the resonant frequency of output scanner  12 . Alternatively, the Q of the slave scanner  12  can be damped so that the scanner  12  will scan adequately at frequencies varying slightly from the resonant frequency. 
     Although the detectors  270 ,  276  are described herein as being fully optical, one skilled in the art will recognize that several other types of detectors may be used. For example, where all-optical communication between the scene and the scanner  12  is not desired, the detectors may be photoelectric devices such as photodiodes. In such a configuration, electrical conductors would couple electrical signals from the photodiodes to the comparing circuit  278 . Alternatively, the detectors  270 ,  276  may be electrical structures, such as piezoelectric detectors. In another alternative, where the scanners  10 ,  12  are MEMs devices, the detectors may be piezoresistive. 
     Also, although piezoelectric transducers control the scanning frequency in the embodiment of  FIG. 4 , other methods for controlling the scanning frequency may be within the scope of the invention. For example, the scanning frequency of the output scanner  12  may be controlled by adjusting the temperature of the output scanner  12  with a thermoelectric controller. Similarly, electronic servomechanisms can adjust the position of or stress on various components of the scanner  10 ,  12  to vary the frequency. For example, an Acme gear driven by servomechanism can convert rotational force from the servomechanism to longitudinal force on the spring plate  204 . 
     Alternately, the scanners  10 ,  12  may be realized with acousto-optic scanners in some applications. One skilled in the art will recognize that such scanners  10 ,  12  can be synchronized by synchronizing the electrical signals used to drive the modulators. In such applications, an amplitude control loop will adjust the drive signal to compensate for changes in the deflection angle-versus-voltage that are caused by Q variations. 
     As shown in  FIGS. 11A and 11B , control electronics  108  can use the eye position information from the detector  88  to control the remote scanner  12 . In this embodiment, the control electronics  108  activates positioners  150 ,  152  coupled in the scanner  12  in addition to the positioners  118 ,  120 ,  122  in the scanner  10 . As the user&#39;s eye  52  moves ( FIG. 11B ), the control electronics  108  determines the angle at which the user is attempting to look. The control electronics  108  retrieves additional positioning information from a memory  110  and activates the positioners  150 ,  152  accordingly, through an electrical cable  154 . Such information need not be transmitted electrically. For example, this information can be transmitted along the transmission fiber using wavelength division multiplexing or similar techniques for transmitting a plurality of signals along a common transmission path. In response, the positioners  150 ,  152  reposition and reorient the scanner  10  to change its effective field of view. Thus, as the user attempts to look left, right, up or down, the input scanner field of view can shift in response. At the same time, the output scanner  12  can track the viewer&#39;s eye position to help insure the viewer&#39;s cornea receives the scanned light. 
     In addition to following the viewer&#39;s field of view, the input scanner  10  also includes a z-axis positioner  156  that allows the user to effectively “focus” the input scanner  10 . The user controls the z-axis positioner  156  by manually adjusting an electronic driver  158  that provides an input signal to the z-axis positioner  156 . In response, the z-axis positioner  156  shifts the position of a low power lens  157  to establish the distance from which the scanner  12  optimally receives light reflected from the scene. One skilled in the art will recognize that, although the lens  157  and positioner  156  are represented diagrammatically for clarity of presentation, a variety of z-axis variability structures can be used. For example, variable lenses, such as those found in auto focus cameras can provide effective shifts along the z-axis. 
       FIG. 12  shows another alternative embodiment where input signals for driving the input scanner  10  are optical, thereby eliminating the transmission of electrical driving signals to the scanner  10 . In this embodiment, the scanner  10  is a magnetically driven mechanical resonant scanner that has an optoelectric device  302 , such as a photodiode or phototransistor, that receives control signals from a secondary fiber  304 . The optoelectric device  302  draws power from a battery  306  in response to optical signals from the secondary fiber  304  and drives a coil  308  to propel mirrors  310 . The mirrors  310  scan light from the fiber  14  to scan an image as described above with reference to  FIG. 4 . Although the exemplary embodiment of  FIG. 12  includes magnetic coils to drive the mirrors  310 , other driving approaches, such as electrostatic (i.e., capacitive) or piezoelectric drivers, may be appropriate depending upon the particular application. 
     The optical signals for activating the optoelectric device  302  are provided by an infrared laser diode  312  driven by the control electronics  108  in response to the sensed scanning position of the output scanner  12 . To synchronize the scanners  10 ,  12 , the input scanner can be made with a relatively low Q or the master-slave approach described with reference to  FIG. 10  can be applied. 
     As is also visible in  FIG. 12 , the output scanner  12  need not be a scanned retinal display. Instead, where sufficient light is available, the scanner  12  can scan light onto a screen or other target to produce a visible projected image  318 . 
       FIG. 13  shows another embodiment of an imaging apparatus in which separate fibers are used for illuminating and imaging. In this embodiment, an illuminating fiber  170  extends from the light emitter  16  to the scanner  10 . The illuminating fiber is etched to reduce its diameter and the transmission fiber  14  and illuminating fiber  170  are D-shaped, as shown in  FIG. 14 . The D-shapes and reduced diameters allow the fiber cores  172  to be positioned very close to each other. As a consequence, the light emitted by the illuminating fiber is substantially aligned with the field sensitivity of the transmission fiber  14 . Thus, when light from the illuminating fiber  170  is reflected from the scene, a portion of the reflected light couples into the transmission fiber  14 . 
     During fabrication, alignment is aided by using overlapped images, as shown in  FIG. 15 . Each of the fibers  14 ,  170  receives input light from a respective source  174 ,  176  and both fibers  14 ,  170  output light onto a screen  178 . In response, each fiber  14 ,  170  outputs light that is imaged onto a to form respective images  180 A-B. The fibers  14 ,  170  are then adjusted until the images  180 A-B overlap. Then, an optically cured epoxy is activated to fix the relative positions of the fibers  14 ,  170 . Additionally or alternatively, other lenses, prisms, beam splitters or other optical elements can be used to properly align the optical paths defined by the fibers  14 ,  170 . 
     As will now be explained with reference to  FIG. 16 , the imager  8  can easily provide apparent magnification of a scanned object  240 . As can be seen by comparing the angular range of the input scanner  10  to that of the output scanner  12 , the output scanner  12  has a larger angular swing than the input scanner  10 . However, the scanners  10 ,  12  are synchronized so that each sweeps through its respective field of view in the same amount of time. Thus, the output scanner  12  sweeps at a larger angular rate than the input scanner  10 . Consequently, the output scanner  12  outputs light from the scanned object  240  over the same amount of time, but over a larger angle than the input scanner  10 . The viewer perceives a larger angular swing as a larger field of view and a reproduced object  240 A appears enlarged to the viewer. Thus, with no data manipulation, the output image is an enlarged image of the input image, i.e., the image is effectively magnified. One skilled in the art will recognize that magnification can also be obtained by placing the input scanner close to the scanned object and adjusting coupling to the fiber with fiber coupling lens for sharp focus. 
     By varying the field of view of either the input or output scanner  10 ,  12 , the imager  8  can vary its effective magnification. For example, increasing the amplitude of the driving signal to the output scanner  12  increases the angular swing of the output scanner  12 , increasing the effective magnification. One skilled in the art will recognize a variety of approaches to increasing the drive signal amplitude either continuously, for a “zoom” effect, or incrementally. 
     Although the invention has been described herein by way of exemplary embodiments, variations in the structures and methods described herein may be made without departing from the spirit and scope of the invention. For example, the positioning of the various components may also be varied. Where imaging in a single direction is predominant, the optics of each of the scanners  10 ,  12  may differ. For example, the first scanner  10  may have a focal length selected for viewing as the input element of a microscope or a camera and the second scanner  12  may have a focal length selected for displaying an image to a viewer. 
     Also, although the system described herein has been describes as including one input scanner and one output scanner, a plurality of input or output scanners may be used. For example, light from the transmission fiber  14  can be split among a plurality of output scanners to provide an imaging system with a plurality of outputs. Such a system could allow a user to view the scene while a film or electronic detector records the viewed scene. Moreover, the light form the transmission fiber  14  can be directed to a photodetector, such as a photodiode, or may be scanned onto a two dimensional detector, such as a commercially available CCD. The photodetector produces an electrical signal indicative of the light form the transmission fiber that can be processed according to known techniques to drive an electronically driven display or to identify information about the remote environment. For example, if the remote environment includes one or two-dimensional symbols, such as barcodes or similar symbologies, the electrical signal can be decoded to identify information represented by the symbols. 
     Further, although the exemplary embodiment of the scanner described herein is a magnetically driven resonant scanner, other types of scanners may also be used. For example, a microelectomechanical (MEMs) scanner may be used in some applications. Examples of MEMs scanners are described in U.S. Pat. No. 5,629,790 entitled MICROMACHINED TORSIONAL SCANNER to Neukermans, et. al., U.S. Pat. No. 5,648,618 entitled MICROMACHINED HINGE HAVING AN INTEGRAL TORSIONAL SENSOR to Neukermans, et. al., and in U.S. Pat. No. 5,673,139 entitled MICROELECTROMECHANICAL TELEVISION SCANNING DEVICE AND METHOD FOR MAKING THE SAME to Johnson, each of which is incorporated herein by reference. 
     Additionally, although the light described herein is generally visible light, non-visible radiation may be used in some applications. For example, where the remote viewer is used to view body tissue, an ultraviolet or infrared wavelength may be desirable. In such applications, the user can view the image at the output scanner using a wavelength converter, such as an infrared viewer. Alternatively, the user can direct the light from the output scanner onto a screen containing a wavelength converting phosphor. The phosphor absorbs the non-visible radiation and, in response emits visible light for viewing. 
     Also, in some high ambient light applications or in applications where a photographic film or sensitive electronic detector detect the light from the output scanner  12 , it may be desirable to eliminate the emitters  16 ,  18 . In such an embodiment, the scanners  10 ,  12  can monitor visible light originating from their respective environments. In addition, the components and configurations described herein can be combined in a variety of ways and remain within the scope of the invention. For example, the structure for viewing a remote location using projection of an image, as described with reference to  FIG. 2A  can be combined with the structure of  FIGS. 11A-B . Such a combination allows a user to view a remote environment with light scanned onto the user&#39;s retina. With appropriate beam splitting and filtering the combination allows bi-directional communication between two locations. Such a structure could be particularly useful if combined with audio for closed loop video telephony. Accordingly, the invention is not limited except as by the appended claims.