Abstract:
linear or 2D bar code scanner, the results of a decode attempt may be used to determine the contents of projected information. When a “no decode” is returned, the user may be prompted to scan again. When a decoded symbol includes directly useful data, all or a portion of the data may be projected. When the data refers to a look-up table, information from the look-up table may be projected. The device may additionally project finder patterns to aid aiming.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to scanning devices and, more particularly, to scanned beam displays and imaging devices for viewing or collecting images.  
         BACKGROUND OF THE INVENTION  
         [0002]    A variety of techniques are available for providing visual displays of graphical or video images to a user. In many applications cathode ray tube type displays (CRTs), such as televisions and computer monitors produce images for viewing. Such devices suffer from several limitations. For example, CRTs are bulky and consume substantial amounts of power, making them undesirable for portable or head-mounted applications.  
           [0003]    Matrix addressable displays, such as liquid crystal displays and field emission displays, may be less bulky and consume less power. However, typical matrix addressable 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. Such displays have been reduced in size, at the cost of increasingly difficult processing and limited resolution or brightness. Also, improving resolution of such displays typically requires a significant increase in complexity.  
           [0004]    One approach to overcoming many limitations of conventional displays is a scanned beam display, such as that described in U.S. Pat. No. 5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, which is incorporated herein by reference. As shown diagrammatically in FIG. 1, in one embodiment of a scanned beam display  40 , a scanning source  42  outputs a scanned beam of light that is coupled to a viewer&#39;s eye  44  by a beam combiner  46 . In some scanned displays, the scanning source  42  includes a scanner, such as scanning mirror or acousto-optic scanner, that scans a modulated light beam onto a viewer&#39;s retina. In other embodiments, the scanning source may include one or more light emitters that are rotated through an angular sweep.  
           [0005]    The scanned light enters the eye  44  through the viewer&#39;s pupil  48  and is imaged onto the retina  59  by the cornea. In response to the scanned light the viewer perceives an image. In another embodiment, the scanned source  42  scans the modulated light beam onto a screen that the viewer observes. One example of such a scanner suitable for either type of display is described in 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.  
           [0006]    Sometimes such displays are used for partial or augmented view applications. In such applications, a portion of the display is positioned in the user&#39;s field of view and presents an image that occupies a region  43  of the user&#39;s field of view  45 , as shown in FIG. 2A. The user can thus see both a displayed virtual image  47  and background information  49 . If the background light is occluded, the viewer perceives only the virtual image  47 , as shown in FIG. 2B.  
           [0007]    One difficulty that may arise with such displays is raster pinch, as will now be explained with reference to FIGS.  3 - 5 . As shown diagrammatically in FIG. 3, the scanning source  42  includes an optical source  50  that emits a beam  52  of modulated light. In this embodiment, the optical source  50  is an optical fiber that is driven by one or more light emitters, such as laser diodes (not shown). A lens  53  gathers and focuses the beam  52  so that the beam  52  strikes a turning mirror  54  and is directed toward a horizontal scanner  56 . The horizontal scanner  56  is a mechanically resonant scanner that scans the beam  52  periodically in a sinusoidal fashion. The horizontally scanned beam then travels to a vertical scanner  58  that scans periodically to sweep the horizontally scanned beam vertically. For each angle of the beam  52  from the scanners  58 , an exit pupil expander  62  converts the beam  52  into a set of beams  63 . Eye coupling optics  60  collect the beams  63  and form a set of exit pupils  65 . The exit pupils  65  together act as an expanded exit pupil for viewing by a viewer&#39;s eye  64 . One such expander is described in U.S. Pat. No. 5,701,132 of Kollin et al., entitled VIRTUAL RETINAL DISPLAY WITH EXPANDED EXIT PUPIL, which is incorporated herein by reference. One skilled in the art will recognize that, for differing applications, the exit pupil expander  62  may be omitted, may be replaced or supplemented by an eye tracking system, or may have a variety of structures, including diffractive or refractive designs. For example, the exit pupil expander  62  may be a planar or curved structure and may create any number or pattern of output beams in a variety of patterns. Also, although only three exit pupils are shown in FIG. 3, the number of pupils may be almost any number. For example, in some applications a 15 by 15 array may be suitable.  
           [0008]    Returning to the description of scanning, as the beam scans through each successive location in the beam expander  62 , the beam color and intensity is modulated in a fashion to be described below to form a respective pixel of an image. By properly controlling the color and intensity of the beam for each pixel location, the display  40  can produce the desired image.  
           [0009]    Simplified versions of the respective waveforms of the vertical and horizontal scanners are shown in FIG. 4. In the plane  66  (FIG. 3), the beam traces the pattern  68  shown in FIG. 5. Though FIG. 5 shows only eleven lines of image, one skilled in the art will recognize that the number of lines in an actual display will typically be much larger than eleven. As can be seen by comparing the actual scan pattern  68  to a desired raster scan pattern  69 , the actual scanned beam  68  is “pinched” at the outer edges of the beam expander  62 . That is, in successive forward and reverse sweeps of the beam, the pixels near the edge of the scan pattern are unevenly spaced. This uneven spacing can cause the pixels to overlap or can leave a gap between adjacent rows of pixels. Moreover, because the image information is typically provided as an array of data, where each location in the array corresponds to a respective position in the ideal raster pattern  69 , the displaced pixel locations can cause image distortion.  
           [0010]    For a given refresh rate and a given wavelength, the number of pixels per line is determined in the structure of FIG. 3 by the mirror scan angle θ and mirror dimension D perpendicular to the axis of rotation. For high resolution, it is therefor desirable to have a large scan angle θ and a large mirror. However, larger mirrors and scan angles typically correspond to lower resonant frequencies. A lower resonant frequency provides fewer lines of display for a given period. Consequently, a large mirror and larger scan angle may produce unacceptable refresh rates.  
         SUMMARY OF THE INVENTION  
         [0011]    A display includes a primary scanning mechanism that simultaneously scans a beam of light both horizontally and vertically along substantially continuous scan paths. In the preferred embodiment, the scanning mechanism includes a mirror that pivots to sweep the beams horizontally. In other embodiments, the scanning mechanism may be electro-optic, acousto-optic, polymeric, refractive, or any other known scanning technique.  
           [0012]    Optical sources are aligned to provide the beam of light to the scanning mechanism from a respective input angle. The scanning mechanism sweeps the beam of light across a respective distinct region of an image field. As described above, the scan angle θ and the mirror dimensions determine the number of pixels drawn for the beam.  
           [0013]    In one embodiment, the scanning mechanism scans in a generally raster pattern with a horizontal component and a vertical component. A mechanically resonant scanner produces the horizontal component by scanning the beam sinusoidally. A non-resonant or semi-resonant scanner typically scans the beam vertically with a substantially constant angular speed.  
           [0014]    In one embodiment, the scanning mechanism includes a biaxial microelectromechanical (MEMs) scanner. The biaxial scanner uses a single mirror to provide both horizontal and vertical movement of each of the beams. In one embodiment, the display includes a buffer that stores data and outputs the stored data to the optical source. A correction multiplier provides correction data that adjusts the drive signals to the optical sources in response to the stored data. The adjusted drive signals compensate for variations in output intensity caused by pattern dependent heating or other effects.  
           [0015]    The correction data can be determined empirically by characterizing the light source, can be determined by monitoring the intensity vs. input signal, or can be predicted for given light source. In one embodiment, the intensity is monitored vs. an input signal by detecting the light intensity when the scanned beam is not in the field view. In another embodiment, an intensity is predicted based upon input signal and the predicted intensity is compared to an actual detected intensity. The difference between the predicted and detected intensity is produces an error signal that is used to determine the correction data.  
           [0016]    In another embodiment, an imager acquires images in tiles by a detector or an optical source and detector pair. One embodiment of the imager includes LEDs or lasers as the optical sources, where each of the optical sources is at a respective wavelength. The scanning assembly simultaneously directs light from each of the optical sources to respective regions of an image field. For each location in the image field, each of the detectors selectively detects light at the wavelength, polarization, or other characteristic of its corresponding source, according to the reflectivity of the respective location. The detectors output electrical signals to decoding electronics that store data representative of the image field.  
           [0017]    In one embodiment, the imager includes a plurality of detector/optical source pairs at each of red, green, and blue wavelength bands. Each pair operates at a respective wavelength within its band. For example, a first of the red pairs operates at a first red wavelength and a second of the red pairs operates at a second red wavelength different from the first.  
           [0018]    In one embodiment, a pair of optical sources alternately feed a single scanner from different angles. During forward sweeps of the scanner, a first of the sources emits light modulated according to one half of a line. During the return sweep, the second source emits light modulated according to the second half of the line. Because the second sweep is in the opposite direction from the first, data corresponding to the second half of the line is reversed before being applied to the second source so that light from the second source is modulated to write the second half of the line in reverse.  
           [0019]    In one embodiment of the alternate feeding approach, a single light emitter feeds an input fiber that is selectively coupled to one of two separate fibers by an optical switch. During forward sweeps, the optical switch couples the input fiber to a first of the separate fibers so that the first separate fiber forms the first optical source. During reverse sweep, the optical switch feeds the second separate fiber so that the second separate fiber forms the second source. This embodiment thus allows a single light emitter to provide light for both optical sources.  
           [0020]    The alternate feeding approach can be expanded to write more than just two tiles. In one approach, the input fiber is coupled to four fibers by a set of optical switches, where each fiber feeds the scanning assembly from a respective angle. The switches are activated according to the direction of the sweep and according to the tracked location of the user&#39;s vision. For example, when the user looks at the top half of the image, a first fiber, aligned to produce an image in the upper left tile feeds the scanner during the forward sweeps. A second fiber, aligned to produce an upper right tile feeds the scanner during reverse sweeps. When the user looks at the lower half of the image, a third fiber, aligned to produce the lower left tile, feeds scanner during forward sweeps. A fourth fiber, aligned to produce the lower right tile, feeds the scanner during reverse sweeps.  
           [0021]    To balance the intensity and color of the two tiles, the electronics can scale the data differently for each of the optical sources. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0022]    [0022]FIG. 1 is a diagrammatic representation of a display aligned to a viewer&#39;s eye.  
         [0023]    [0023]FIG. 2A is a combined image perceived by a user resulting from the combination of light from an image source and light from a background.  
         [0024]    [0024]FIG. 2B is an image perceived by a user from the display of FIG. 1 where the background light is occluded.  
         [0025]    [0025]FIG. 3 is a diagrammatic representation of a scanner and a user&#39;s eye showing bi-directional scanning of a beam and coupling to the viewer&#39;s eye.  
         [0026]    [0026]FIG. 4 is a signal-timing diagram of a scan pattern scanner in the scanning assembly of FIG. 3.  
         [0027]    [0027]FIG. 5 is a signal position diagram showing the path followed by the scanned beam in response to the signals of FIG. 4, as compared to a desired raster scan path.  
         [0028]    [0028]FIG. 6 is a diagrammatic representation of a display according to the one embodiment invention including dual light beams.  
         [0029]    [0029]FIG. 7 is an isometric view of a head-mounted scanner including a tether.  
         [0030]    [0030]FIG. 8 is a diagrammatic representation of a scanning assembly within the scanning display of FIG. 6, including a correction mirror.  
         [0031]    [0031]FIG. 9 is an isometric view of a horizontal scanner and a vertical scanner suitable for use in the scanning assembly of FIG. 8.  
         [0032]    [0032]FIG. 10 is a diagrammatic representation of scanning with two input beams, showing slightly overlapped tiles.  
         [0033]    [0033]FIG. 11 is a top plan view of a biaxial scanner showing four feeds at spatially separated locations.  
         [0034]    [0034]FIG. 12 is a diagrammatic representation of four tiles produces by the four feed scanner of FIG. 11.  
         [0035]    [0035]FIG. 13 is a schematic of a system for driving the four separate feeds of FIG. 11, including four separate buffers.  
         [0036]    [0036]FIG. 14 is a signal-timing diagram comparing a ramp signal with a desired signal for driving the vertical scanner.  
         [0037]    [0037]FIG. 15 is a signal timing diagram showing positioning error and correction for the vertical scanning position.  
         [0038]    [0038]FIG. 16 is a side cross sectional view of a piezoelectric correction scanner.  
         [0039]    [0039]FIG. 17A is a top plan view of a microelectromechanical (MEMs) correction scanner.  
         [0040]    [0040]FIG. 17B is a side cross-sectional view of the MEMs correction scanner of FIG. 17A showing capacitive plates and their alignment to the scanning mirror.  
         [0041]    [0041]FIG. 18 shows corrected scan position using a sinusoidally driven scanner through 90% of the overall scan.  
         [0042]    [0042]FIG. 19 shows an alternative embodiment of a reduced error scanner where scan correction is realized by adding a vertical component to the horizontal mirror.  
         [0043]    [0043]FIG. 20 is a position diagram showing the scan path of a beam deflected by the scanner of FIG. 19.  
         [0044]    [0044]FIG. 21 is a diagrammatic view of a scanning system, including a biaxial microelectromechanical (MEMs) scanner and a MEMs correction scanner.  
         [0045]    [0045]FIG. 22 is a diagrammatic view of a correction scanner that shifts an input beam by shifting the position or angle of the input fiber.  
         [0046]    [0046]FIG. 23 is a diagrammatic view of a correction scanner that includes an electro-optic crystal that shifts the input beam in response to an electrical signal.  
         [0047]    [0047]FIG. 24 is a diagrammatic view of an imager that acquires external light from a target object.  
         [0048]    [0048]FIG. 25 is a diagrammatic view of an alternative embodiment of the imager of FIG. 24 that also projects a visible image.  
         [0049]    [0049]FIG. 26 is a signal timing diagram showing deviation of a sinusoidal scan position versus time from the position of a linear scan.  
         [0050]    [0050]FIG. 27 is a diagram showing diagrammatically how a linear set of counts can map to scan position for a sinusoidally scan.  
         [0051]    [0051]FIG. 28 is a system block diagram showing handling of data to store data in a memory matrix while compensating for nonlinear scan speed of the resonant mirror.  
         [0052]    [0052]FIG. 29 is a block diagram of a first system for generating an output clock to retrieve data from a memory matrix while compensating for nonlinear scan speed of the resonant mirror.  
         [0053]    [0053]FIG. 30 is a block diagram of an alternative embodiment of the apparatus of FIG. 29 including pre-distortion.  
         [0054]    [0054]FIG. 31 is a detail block diagram of a clock generation portion of the block diagram of FIG. 29.  
         [0055]    [0055]FIG. 32 is a representation of a data structure showing data predistorted to compensate for vertical optical distortion.  
         [0056]    [0056]FIG. 33 is a top plan view of a MEMs scanner including structures for electronically controlling the center of mass of each mirror half.  
         [0057]    [0057]FIG. 34 is a top plan view of the MEMs scanner of FIG. 32 showing flexing of protrusions in response to an applied voltage.  
         [0058]    [0058]FIG. 35 is a top plan view of a MEMs scanner including comb structures for laterally shifting the center of mass of each mirror half.  
         [0059]    [0059]FIG. 36 is a side cross sectional view of a packaged scanner including electrically controlled outgassing nodules.  
         [0060]    [0060]FIG. 37 is a top plan view of a MEMs mirror including selectively removable tabs for frequency tuning.  
         [0061]    [0061]FIG. 38 is a diagrammatic view of a four source display showing overlap of scanning fields with optical sources.  
         [0062]    [0062]FIG. 39 is a diagrammatic view of a four source display with small turning mirrors and offset optical sources.  
         [0063]    [0063]FIG. 40 is a diagrammatic view of the display of FIG. 39 showing beam paths with the small turning mirrors and a common curved mirror.  
         [0064]    [0064]FIG. 41 is a diagrammatic view of a single emitter display including switched optical fibers each feeding a separate tile.  
         [0065]    [0065]FIG. 42 is a diagrammatic view of a display including four separate fibers feeding a scanner through a set of optical switches in response to a detected gaze direction to produce four separate tiles. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0066]    As shown in FIG. 6, a scanned beam display  70  according to one embodiment of the invention is positioned for viewing by a viewer&#39;s eye  72 . While the display  70  is presented herein is scanning light into the eye  72 , the structures and concepts described herein can also be applied to other types of displays, such as projection displays that include viewing screens.  
         [0067]    The display  70  includes four principal portions, each of which will be described in greater detail below. First, control electronics  74  provide electrical signals that control operation of the display  70  in response to an image signal V IM  from an image source  76 , such as a computer, television receiver, videocassette player, DVD player, remote sensor, or similar device.  
         [0068]    The second portion of the display  70  is a light source  78  that outputs modulated light beams  80 , each having a modulation corresponding to information in the image signal V IM . The light source  78  may utilize coherent light emitters, such as laser diodes or microlasers, or may use non-coherent sources such as light emitting diodes. Also, the light source  78  may include directly modulated light emitters such as the light emitting diodes (LEDs) or may include continuous light emitters indirectly modulated by external modulators, such as acousto-optic modulators.  
         [0069]    The third portion of the display  70  is a scanning assembly  82  that scans the modulated beams  80  through two-dimensional scanning patterns, such as raster patterns. The scanning assembly  82  preferably includes a periodically scanning mirror or mirrors as will be described in greater detail below with reference to FIGS.  3 - 4 ,  8 ,  11 ,  19 - 22 .  
         [0070]    Lenses  84 ,  86  positioned on opposite sides of the scanning assembly  82  act as imaging optics that form the fourth portion of the display  70 . The lenses  86  are cylindrical graded index (GRIN) lenses that gather and shape light from the light source  78 . Where the light source  78  includes optical fibers that feed the lenses  86 , the lenses  86  may be bonded to or integral to the fibers. Alternatively, other types of lenses, such as doublets or triplets, may form the lenses  86 . Also, other types of optical elements such as diffractive elements may be used to shape and guide the light. Regardless of the type of element, the overall optical train may incorporate polarization sensitive materials, chromatic correction, or any other optical technique for controlling the shape, phase or other characteristics of the light.  
         [0071]    The lens  84  is formed from a curved, partially transmissive mirror that shapes and focuses the scanned beams  80  approximately for viewing by the eye  72 . After leaving the lens  84 , the scanned beams  80  enter the eye  72  through a pupil  90  and strike the retina  92 . As each beam of scanned modulated light strikes the retina  92 , the viewer perceives a respective portion of the image as will be described below.  
         [0072]    Because the lens  84  is partially transmissive, the lens  84  combines the light from the scanning assembly  82  with the light received from a background  89  to produce a combined input to the viewer&#39;s eye  72 . Although the background  89  is presented herein as a “real-world” background, the background light may be occluded or may be produced by another light source of the same or different type. One skilled in the art will recognize that a variety of other optical elements may replace or supplement the lenses  84 ,  86 . For example, diffractive elements such as Fresnel lenses may replace either or both of the lenses  84 ,  86 . Additionally, a beamsplitter and lens may replace the partially transmissive mirror structure of the lens  84 . Moreover, various other optical elements, such as polarizers, color filters, exit pupil expanders, chromatic correction elements, eye-tracking elements, and background masks may also be incorporated for certain applications.  
         [0073]    Although the elements of FIG. 6 are presented diagrammatically, one skilled in the art will recognize that the components are typically sized and configured for the desired application. For example, where the display  70  is intended as a mobile personal display the components are sized and configured for mounting to a helmet or similar frame as a head-mounted display  70 , as shown in FIG. 7. In this embodiment, a first portion  171  of the display  70  is mounted to a head-borne frame  174  and a second portion  176  is carried separately, for example in a hip belt. The portions  174 ,  176  are linked by a fiber optic and electronic tether  178  that carries optical and electronic signals from the second portion to the first portion. An example of a fiber-coupled scanner 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.  
         [0074]    An exemplary embodiment of the scanning assembly  82  will be described next with reference to FIG. 8. The scanning assembly.  82  includes several components that correspond to the scanning source  42  of FIG. 3, where components common to the scanning assembly  82  and scanning source  42  are numbered the same. Additionally, only central rays  55  are presented for the beams  52  for clarity of presentation.  
         [0075]    In this embodiment, a pair of fibers  50  emit light from the light sources  78  (not shown) and the lens  84  is represented as a common refractive lens rather than as a partially transmissive mirror. Unlike the scanning source  42  of FIG. 3, the scanning assembly  82  includes an active correction mirror  100  that can pivot to scan the light beam  80  along the vertical axis. As will be explained below, the correction mirror  100  produces a varying corrective shift along the vertical axis during each sweep (forward or reverse) of the horizontal scanner  56 . The corrective shift offsets vertical movement of the beams  80  caused by the vertical scanner  58  to reduce the overall deviation of the scanning pattern from the desired pattern shown in broken lines in FIG. 5.  
         [0076]    Before describing the effects of the correction mirror  100  and the relative timing of the various signals, exemplary embodiments of mechanically resonant scanner  200 ,  220  suitable for use as the horizontal scanner  56  and vertical scanner  58  will be described with reference to FIG. 9.  
         [0077]    The principal scanning component of the horizontal scanner  200  is 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  have a high Q with a natural oscillatory (“resonant”) frequency on the order of 1-100 kHz, where the selected resonant frequency depends upon the application. For VGA quality output with a 60 Hz refresh rate and no interlacing, the resonant frequency is preferably about 15-20 kHz. As will be described below, the selected resonant frequency or the achievable resolution may be changed through the use of a plurality of feeds.  
         [0078]    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. The ferromagnetic material is preferably integral to the spring plate  204  and body of the mirror  202 . Drive electronics  218  provide electrical signals to activate the coils  206 ,  208 , as described above. Responsive to the electrical signals, the coils  206 ,  208  produce periodic electromagnetic fields that apply force to the ferromagnetic material, thereby causing oscillation of the mirror  202 . If the frequency and phase of the electric signals are properly synchronized with the movement of the mirror  202 , the mirror  202  oscillates at its resonant frequency with little power consumption.  
         [0079]    The vertical scanner  220  is structured very similarly to the resonant scanner  200 . Like the resonant 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  220  is typically not resonant. The mirror  222  receives light from the horizontal scanner  201  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 . The details of virtual retinal displays 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 incorporated herein by reference.  
         [0080]    One skilled in the art will recognize a variety of other structures that may scan a light beam through a generally raster pattern. For example, spinning polygons or galvanometric scanners may form either or both of the scanners  56 ,  58  in some applications.  
         [0081]    In another embodiment, a bi-axial microelectromechanical (MEMs) scanner may provide the primary scanning. Such scanners are described in U.S. Pat. No. 5,629,790 to Neukermanns et al., entitled MICROMACHINED TORSIONAL SCANNER, which is incorporated herein by reference. Like the scanning system described above, the horizontal components of the MEMs scanners are typically defined by mechanical resonances of their respective structures, as will be described in greater detail below with reference to FIGS.  17 A-B and  21 . Like the two scanner system described above with reference to FIGS. 3 and 8, such biaxial scanners may suffer similar raster pinch problems due to movement along the slower scan axis during sweeps along the faster scan axis. Other scanning approaches may also apply. For example, acousto-optic scanners, electrooptic scanners, spinning polygons, or some combination of scanning approaches can provide the scanning function. Some of these approaches may not require pinch correction.  
         [0082]    Returning to FIGS. 6, 8 and  9 , the fibers  50  output light beams  80  that are modulated according to the image signal from the drive electronics  218 . At the same time, the drive electronics  218  activate the coils  206 ,  208 ,  224 ,  226  to oscillate the mirrors  202 ,  222 . The modulated beams of light strike the oscillating horizontal mirror  202  (of the horizontal scanner  56 ), and are deflected horizontally by an angle corresponding to the instantaneous angle of the mirror  202 . The deflected beams then strike the vertical mirror  222  (of the vertical scanner  58 ) and are deflected at a vertical angle corresponding to the instantaneous angle of the vertical mirror  222 . After expansion by the beam expander  62 , the beams  52  pass through the lens  84  to the eye. As will also be described below, the modulation of the optical beams is synchronized with the horizontal and vertical scans so that, at each position of the mirrors, the beam color and intensity correspond to a desired virtual image. Each beam therefore “draws” a portion of the virtual image directly upon the user&#39;s retina.  
         [0083]    One skilled in the art will recognize that several components of the scanning assembly  82  have been omitted from the FIG. 9 for clarity of presentation. For example, the horizontal and vertical scanners  200 ,  220  are typically mounted to a frame. Additionally, lenses and other optical components for gathering, shaping, turning, focusing, or collimating the beams  80  have been omitted. Also, no relay optics are shown between the scanners  200 ,  220 , although these may be desirable in some embodiments. Moreover, the scanner  200  typically includes one or more turning mirrors that direct the beam such that the beam strikes each of the mirrors a plurality of times to increase the angular range of scanning. Further, in some embodiments, the scanners  200 ,  220  are oriented such that the beam can strike the scanning mirrors a plurality of times without a turning mirror.  
         [0084]    Turning to FIGS. 10 and 11, the effect of the plurality of beams  80  will now be described. As is visible in FIG. 10, two fibers  50  emit respective light beams  80 . The GRIN lenses  86  gather and focus the beams  80  such that the beams  80  become converging beams  80 A,  80 B that strike a common scanning mirror  1090 .  
         [0085]    For clarity of presentation, the embodiment of FIG. 10 eliminates the mirror  84 , as is desirable in some applications. Also, the embodiment of FIG. 10 includes a single mirror  1090  that scans biaxially instead of the dual mirror structure of FIG. 9. Such a biaxial structure is described in greater detail below with reference to FIGS.  11 ,  17 A-B and  21 . One skilled in the art will recognize that a dual mirror system may also be used, though such a system would typically involve a more complex set of ray traces and more complex compensation for differing optical path lengths.  
         [0086]    Also, although the fibers  50  and lenses  84  of FIG. 10 appear positioned in a common plane with the scanning mirror  1090 , in many applications, it may be desirable to position the fibers  50  and lenses  84  off-axis, as is visible in FIG. 11. Moreover, where four fiber/lens pairs are used, as in FIG. 11, a beam splitter or other optical elements can allow the fiber/lens pairs to be positioned where they do not block beams  80 A-D from other fiber/lens pairs. Alternatively, other approaches, such as small turning mirrors can permit repositioning of the fiber/lens pairs in non-blocking positions with little effect on the image quality. Such approaches are described in greater detail below with reference to FIGS. 11 and 38- 40 .  
         [0087]    After exiting the lens  86 , the first beam  80 A strikes the scanning mirror  1090  and is reflected toward an image field  1094 . The second beam  80 B is also reflected by the scanning mirror  1090  toward the image field  1094 . As shown by the ray tracing of FIG. 10, the horizontal position of the beams  80 A-B in the image field  1094  will be functions of the angular deflection from the horizontal scanner  56  and the position and orientation of the lens  86  and fiber  50 .  
         [0088]    At the image field  1092 , the first beam  80 A illuminates a first region  1092  of the image field  1094  and the second beam  80 B illuminates a second region  1096  that is substantially non-overlapping with respect to the first region  1092 . To allow a smooth transition between the two regions  1092 ,  1096 , the two regions  1092 ,  1096  overlap slightly in a small overlap region  1098 . Thus, although the two regions are substantially distinct, the corresponding image portions may be slightly “blended” at the edges, as will be described below with reference to FIGS. 12 and 13.  
         [0089]    While only two beams  80 A-B are visible in FIG. 10, more than two fiber/lens pairs can be used and the fiber/lens pairs need not be coplanar. For example, as can be seen in FIG. 11, four separate lenses  86  transmit four separate beams  80 A-D from four spatially separated locations toward the mirror  1090 . As shown in FIG. 12, the mirror  1090  reflects each of the four beams  80 A-D to a respective spatially distinct region  1202 A-D of the image field  1094 .  
         [0090]    Thus, the four beams  80 A-D each illuminate four separate “tiles”  1202 A-D that together form an entire image. One skilled in the art will recognize that more than four files may form the image. For example, adding a third set of fiber/lens pairs could produce a 2-by-3 tile image or a 3-by-2 tile image.  
         [0091]    To produce an actual image, the intensity and color content of each of the beams  80 A-D is modulated with image information as the mirror  1090  sweeps through a periodic pattern, such as a raster pattern. FIG. 13 shows diagrammatically one embodiment where the beams  8 OA-D can be modulated in response to an image signal V IM  to produce the four tiles  1202 A-D.  
         [0092]    The image signal V IM  drives an A/D converter  1302  that produces corresponding data to drive a demultiplexer  1304 . In response to the data and a clock signal CK from the controller  74  (FIG. 8), the demultiplexer  1304  produces four output data streams, where each data stream includes data corresponding to a respective image tile  1202 A-D. For example, the demultiplexer  1304  outputs data corresponding to the first half of the first line of the image to a first buffer  1306 A and the data corresponding to the second half of the first line to a second buffer  1306 B. The demultiplexer  1304  then outputs data corresponding to the second line of the image to the second lines of the first two buffers  1306 A, B. After the first two buffers  1306 A, B contain data representing the upper half of the image, the demultiplexer  1304  then begins filling third and fourth buffers  1306 C, D. Once all of the buffers  1306 A-D are full, an output clock CKOUT clocks data simultaneously from all of the buffers  1306 A-D to respective D/A converters  1308 A-D. The D/A converters  1308 A-D then drive respective light sources  78  to produce light that is scanned into the respective regions  2102 A-D, as described above. The actual timing of the pixel output is controlled by the output clock CKOUT, as described below with reference to FIGS.  28 - 31 .  
         [0093]    One skilled in the art will recognize that, although the system of FIG. 13 is described for four separate regions  1201 A-D, a larger or smaller number of regions may be used. Also, where some overlap of the regions  1202 A-D is desired, common data can be stored in more than one buffer  1202 A-D. Because the sets of common data will duplicate some pixels in the overlapping region, the data may be scaled to limit the intensity to the desired level.  
         [0094]    One approach to improving image quality that is helpful in “matching” the image portions  1202 A-D to each other will now be described with reference to FIGS. 14 and 15. Because the angle of the beams  80 A-D is determined by the angles of the vertical and horizontal scanner (for the uniaxial, two scanner system) or the horizontal and vertical angles of the single mirror (for the biaxial scanner), the actual vector angle of the beams  80 A-D at any point in time can then be determined by vector addition. In most cases, the desired vertical portions of the scan patterns will be a “stair step” scan pattern, as shown by the broken line in FIG. 14.  
         [0095]    If the turning mirror  100  (FIG. 8) is disabled, the pattern traced by the ray will be the same as that described above with respect to FIGS.  3 - 5 . As represented by the solid line in FIG. 14, the actual vertical scan portion of the pattern, shown in solid line, will be an approximate ramp, rather than the desired stair step pattern.  
         [0096]    On approach to providing the stair step pattern would be to drive the vertical scanner  58  with the stair step voltage. However, because the vertical mirror is a physical system and the stair step involves discontinuous motion, the vertical mirror will not follow the drive signal exactly. Instead, as the vertical mirror attempts to follow the stair step pattern, the vertical mirror will move at a maximum rate indicated largely by the size and weight of the vertical mirror, the material properties of the mirror support structure, the peak voltage or current of the driving signal, and electrical properties of the driving circuitry. For typical vertical scan mirror size, configuration, scan angle and driving voltage, the vertical scanner  58  is limited to frequencies on the order of 100 to 3000 Hz. The desired scan pattern has frequency components far exceeding this range. Consequently, driving the vertical scanner  58  with a stair step driving signal can produce a vertical scan pattern that deviates significantly from the desired pattern.  
         [0097]    To reduce this problem, the scanning assembly  82  of FIG. 8 separates the vertical scan function into two parts. The overall vertical scan is then a combination of a large amplitude ramp function at about 60 Hz and a small amplitude correction function at twice the horizontal rate (e.g., about 30 kHz). The vertical scanner  58  can produce the large amplitude ramp function, because the 60 Hz frequency is well below the upper frequency limit of typical scanning mirrors. Correction mirrors  100  replace the turning mirrors  100  and provide the small amplitude corrections. The correction mirrors  100  operate at a much higher frequency than the vertical scanner, however, the overall angular swings of the correction mirrors  100  are very small.  
         [0098]    As can be seen from the signal timing diagram of FIG. 15, the correction mirror  100  travels from approximately its maximum negative angle to its maximum positive angle during the time that the horizontal scanner scans from the one edge of the field of view to the opposite edge (i.e. from time t 1  to t 2  in FIG. 15). The overall correction angle, as shown in FIGS. 14 and 15, is defined by the amount of downward travel of the vertical scan mirror during a single horizontal scan. The correction angle will vary for various configurations of the display; however, the correction angle can be calculated easily.  
         [0099]    For example, for a display where each image region  1202 A-D has  1280  vertical lines and a total mechanical vertical scan angle of 10 degrees, the angular scan range for each line is about 0.008 degrees (10/1280=0.0078125). Assuming the vertical scanner  58  travels this entire distance during the horizontal scan, an error correction to be supplied by the correction mirror  100  is about plus or minus 0.0039 degrees. The angular correction is thus approximately θ/N, where θ is the vertical scan angle and N is the number of horizontal lines. This number may be modified in some embodiments. For example, where the horizontal scanner  56  is a resonant scanner, the correction angle may be slightly different, because the horizontal scanner  56  will use some portion of the scan time to halt and begin travel in the reverse direction, as the scan reaches the edge of the field of view. The correction angle may also be modified to correct for aberrations in optical elements or optical path length differences. Moreover, the frequency of the correction scanner  100  may be reduced by half if data is provided only during one half of the horizontal scanner period (“unidirectional scanning”), although raster pinch is typically not problematic in unidirectional scanning approaches.  
         [0100]    As can be seen from the timing diagrams of FIGS. 14 and 15, the correction mirror  100  will translate the beam vertically by about one half of one line width at a frequency of twice that of the horizontal scanner  56 . For a typical display at SVGA image quality with bi-directional scanning (i.e., data output on both the forward and reverse sweeps of the horizontal scanner  56 ), the horizontal scanner  56  will resonate at about 15 kHz. Thus, for a typical display, the correction scanner  100  will pivot by about one-tenth of one degree at about 30 kHz. One skilled in the art will recognize that, as the resolution of the display increases, the scan rate of the horizontal scanner  56  increases. The scan rate of the correction mirror  100  will increase accordingly; but, the pivot angle will decrease. For example, for a display having 2560 lines and an overall scan of 10 degrees, the scan rate of the correction mirror  100  will be about 60 kHz with a pivot angle of about 0.002 degrees. One skilled in the art will recognize that, for higher resolution, the minimum correction mirror size will typically increase where the spot size is diffraction limited.  
         [0101]    [0101]FIG. 16 shows a piezoelectric scanner  110  suitable for the correction mirror  100  in some embodiments. The scanner  110  is formed from a platform  112  that carries a pair of spaced-apart piezoelectric actuators  114 ,  116 . The correction mirror  100  is a metallized, substantially planar silicon substrate that extends between the actuators  114 ,  116 . The opposite sides of the piezoelectric actuators  114 ,  116  are conductively coated and coupled to a drive amplifier  120  such that the voltage across the actuators  114 ,  116  are opposite. As is known, piezoelectric materials deform in the presence of electric fields. Consequently, when the drive amplifier  120  outputs a voltage, the actuators  114 ,  116  apply forces in opposite directions to the correction mirror  100 , thereby causing the correction mirror  100  to pivot. One skilled in the art will recognize that, although the piezoelectric actuators  114 ,  116  are presented as having a single set of electrodes and a single layer of piezoelectric material, the actuators  114 ,  116  would typically be formed from several layers. Such structures are used in commercially available piezoelectric devices to produce relatively large deformations.  
         [0102]    A simple signal generator circuit  122 , such as a conventional ramp generator circuit, provides the driving signal for the drive amplifier  120  in response to the detected position of the horizontal scanner  56 . The principal input to the circuit  122  is a sense signal from a sensor coupled to the horizontal scanner  56 . The sense signal can be obtained in a variety of approaches. For example, as described in U.S. Pat. No. 5,648,618 to Neukermanns et al., entitled MICROMACHINED HINGE HAVING AN INTEGRAL TORSIONAL SENSOR, which is incorporated herein by reference, torsional movement of a MEMs scanner can produce electrical outputs corresponding to the position of the scanning mirror. Alternatively, the position of the mirror may be obtained by mounting piezoelectric sensors to the scanner, as described in U.S. Pat. No. 5,694,237 to Melville, entitled POSITION DETECTION OF MECHANICAL RESONANT SCANNER MIRROR, which is incorporated herein by reference. In other alternatives, the position of the beam can be determined by optically or electrically monitoring the position of the horizontal or vertical scanning mirrors or by monitoring current induced in the mirror drive coils.  
         [0103]    When the sense signal indicates that the horizontal scanner  56  is at the edge of the field of view, the circuit  122  generates a ramp signal that begins at its negative maximum and reaches its zero crossing point when the horizontal scanner reaches the middle of the field of view. The ramp signal then reaches its maximum value when the horizontal scan reaches the opposite edge of the field of view. The ramp signal returns to its negative maximum during the interval when the horizontal scan slows to a halt and begins to return sweep. Because the circuit  122  can use the sense signal as the basic clock signal for the ramp signal, timing of the ramp signal is inherently synchronized to the horizontal position of the scan. However, one skilled in the art will recognize that, for some embodiments, a controlled phase shift of the ramp signal relative to the sense signal may optimize performance. Where the correction mirror  100  is scanned resonantly, as described below with reference to FIG. 18, the ramp signal can be replaced by a sinusoidal signal, that can be obtained simply be frequency doubling, amplifying and phase shifting the sense signal.  
         [0104]    The vertical movements of the beams  80 A-D induced by the correction mirrors  100  offset the movement of the beams  80 A-D caused by the vertical scanner  58 , so that the beams  80 A-D remain stationary along the vertical axis during the horizontal scan. During the time the horizontal scan is out of the field of view, the beams  80 A-D travel vertically in response to the correction mirrors  100  to the nominal positions of the next horizontal scan.  
         [0105]    As can be seen from the above discussion, the addition of the piezoelectrically driven correction mirrors  100  can reduce the raster pinching significantly with a ramp-type of motion. However, in some applications, it may be undesirable to utilize ramp-type motion. One alternative embodiment of a scanner  130  that can be used for the correction mirror  100  is shown in FIGS. 17A and 17B.  
         [0106]    The scanner  130  is a resonant micorelectromechanical (MEMs) scanner, fabricated similarly to the uniaxial embodiment described in the Neukermanns &#39;790 patent. Alternatively, the scanner  130  can be a mechanically resonant scanner very similar to the horizontal scanner  54  of FIG. 9; however, in such a scanner it is preferred that the dimensions and material properties of the plate and mirror be selected to produce resonance at about 30 kHz, which is twice the resonant frequency of the horizontal scanner  200 . Further, the materials and mounting are preferably selected so that the scanner  130  has a lower Q than the Q of the horizontal scanner  56 . The lower Q allows the scanner  130  to operate over a broader range of frequencies, so that the scanner  130  can be tuned to an integral multiple of the horizontal scan frequency.  
         [0107]    The use of the resonant scanner  130  can reduce the complexity of the electrical components for driving the scanner  130  and can improve the scanning efficiency relative to previously described approaches. Resonant scanners tend to have a sinusoidal motion, rather than the desired ramp-type motion described above. However, if the frequency, phase, and amplitude of the sinusoidal motion are selected appropriately, the correction mirror  100  can reduce the pinch error significantly. For example, FIG. 18 shows correction of the raster signal with a sinusoidal motion of the correction mirror where the horizontal field of view encompasses  90  percent of the overall horizontal scan angle. One skilled in the art will recognize that the error in position of the beam can be reduced further if the field of view is a smaller percentage of the overall horizontal scan angle. Moreover, even further reductions in the scan error can be realized by adding a second correction mirror in the beam path, although this is generally undesirable due to the limited improvement versus cost. Another approach to reducing the error is to add one or more higher order harmonics to the scanner drive signal so that the scanning pattern of the resonant correction scanner  130  shifts from a sinusoidal scan closer to a sawtooth wave.  
         [0108]    Another alternative embodiment of a reduced error scanner  140  is shown in FIG. 19 where the scan correction is realized by adding a vertical component to a horizontal mirror  141 . In this embodiment, the horizontal scanner  140  is a MEMs scanner having an electrostatic drive to pivot the scan mirror. The horizontal scanner  140  includes an array of locations  143  at which small masses  145  may be formed. The masses  145  may be deposited metal or other material that is formed in a conventional manner, such as photolithography. Selected ones of the masses  145  are removed to form an asymmetric distribution about a centerline  147  of the mirror  141 . The masses  145  provide a component to scan the correction along the vertical axis by pivoting about an axis orthogonal to its primary axis. As can be seen in FIG. 20, the vertical scan frequency is double the horizontal scan frequency, thereby producing the Lissajous or “bow-tie” overall scan pattern of FIG. 20. The masses  145  may be actively varied (e.g. by laser ablation) to tune the resonant frequency of the vertical component. This embodiment allows correction without an additional mirror, but typically requires matching the resonant frequencies of the vibration and the horizontal scanner.  
         [0109]    To maintain matching of the relative resonant frequencies of the horizontal scanner  56  and the correction scanner  100 , the resonant frequency of either or both scanners  56 ,  100  may be tuned actively. Various frequency control techniques are described below with reference to FIGS.  33 - 36 . Where the Q of the scanners  56 ,  100  are sufficiently low or where the scanners  56 ,  100  are not resonant, simply varying the driving frequency may shift the scanning frequency sufficiently to maintain synchronization.  
         [0110]    As shown in FIG. 21, another embodiment of a scanner  150  according to the invention employs a biaxial scanner  152  as the principal scan component, along with a correction scanner  154 . The biaxial scanner  152  is&#39; a single mirror device that oscillates about two orthogonal axes. Design, fabrication and operation of such scanners are described for example in the Neukermanns &#39;790 patent, in Asada, et al, Silicon Micromachined Two-Dimensional Galvano Optical Scanner, IEEE Transactions on Magnetics, Vol.30, No. 6, 4647-4649, November 1994, and in Kiang et al, Micromachined Microscanners for Optical Scanning, SPIE proceedings on Miniaturized Systems with Micro-Optics and Micromachines II, Vol. 3008, Feb. 1997, pp. 82-90 each of which is incorporated herein by reference. The bi-axial scanner  152  includes integral sensors  156  that provide electrical feedback of the mirror position to terminals  158 , as is described in the Neukermanns &#39;618 patent.  
         [0111]    The correction scanner  154  is preferably a MEMs scanner such as that described above with reference to FIGS.  17 A-B, although other types of scanners, such as piezoelectric scanners may also be within the scope of the invention. As described above, the correction scanner  154  can scan sinusoidally to remove a significant portion of the scan error; or, the correction mirror can scan in a ramp pattern for more precise error correction.  
         [0112]    Light from the light source  78  strikes the correction mirror  154  and is deflected by a correction angle as described above. The light then strikes the biaxial scanner  152  and is scanned horizontally and vertically to approximate a raster pattern, as described above with reference to FIGS.  3 - 5 .  
         [0113]    Another embodiment of a display according to the invention, shown in FIG. 23, eliminates the correction mirror  100  by physically shifting the input beam laterally relative to the input of an optical system  500 . In the embodiment of FIG. 23, a piezoelectric driver  502  positioned between a frame  504  and an input fiber  506  receives a drive voltage at a frequency twice that of the horizontal scan frequency. Responsive to the drive voltage, the piezoelectric driver  502  deforms. Because the fiber  506  is bonded to the piezoelectric driver  502 , deformation of the piezoelectric driver  502  produces corresponding shifting of the fiber  506  as indicated by the arrow  508  and shadowed fiber  510 . One skilled in the art will recognize that, depending upon the characteristics of the optical system  500 , the piezoelectric driver  502  may produce lateral translation of the fiber  506  or angular shifting of the fiber  506  output. The optical system  500  then translates movement of the fiber output into movement of the perceived pixel location as in the previously described embodiments. While the embodiment of FIG. 23 translates a fiber, the invention is not so limited. For example some applications may incorporate translation of other sources, such as LEDs or laser diodes, may translate the position of the lens  50 , or may translate or rotate an entire scanner, such as a biaxial MEMs scanner.  
         [0114]    Although the embodiment of FIG. 23 shifts the input beam by shifting the position of the input fiber, other methods of shifting the input beam may be within the scope of the invention. For example, as shown in FIG. 24, an electro-optic crystal  300  shifts the input beam  83  in response to an electrical signal. In this embodiment, the beam  83  enters a first face  302  of a trapezoidally shaped electro-optic crystal  300 , where refraction causes a shift in the direction of propagation. When the beam  83  exits through a second face  304 , refraction produces a second shift in the direction of propagation. At each face, the amount of changes in the direction or propagation will depend upon difference in index of refraction between the air and the crystal  300 .  
         [0115]    As is known, the index of refraction of electro-optical crystals is dependent upon the electric field through the crystal. A voltage applied across the crystal  300  through a pair of electrodes  306  can control the index of refraction of the crystal  300 . Thus, the applied voltage can control the index of refraction of the crystal  300 . Thus the applied voltage can control the angular shift of the beam  83  as it enters and exits the crystal  300  as indicated by the broken line  83   a . The amount of shift will correspond to the applied voltage. Accordingly, the amount of shift can be controlled by controlling the voltage applied to the electrodes  306 . The crystal  300  thus provides a voltage controlled beam shifter that can offset raster pinch.  
         [0116]    Although the embodiments described herein have been displays, other devices or methods may be within the scope of the invention. For example, as shown in FIG. 24, an imager  600  includes a biaxial scanner  602  and correction scanner  604  that are very similar to the scanners  152 ,  154  of FIG. 21. The imager  600  is an image collecting device that may be the input element of a digital camera, bar code reader, two dimensional symbol reader, document scanner, or other image acquisition device. To allow the imager  600  to gather light efficiently, the imager  600  includes gathering optics  606  that collect and transmit light from a target object  608  outside of the imager  600  onto the correction scanner  604 . The gathering optics  606  are configured to have a depth of field, focal length, field of view and other optical characteristics appropriate for the particular application. For example, where the imager  600  is a two dimensional symbology reader, the gathering optics  606  may be optimized for red or infrared light and the focal length may be on the order of  10 - 50   cm . For reading symbols at a greater distance, the focusing optics may have longer focusing distance or may have a variable focus. The optics may be positioned at other locations along the optical path to allow smaller, cheaper components to be used.  
         [0117]    The correction scanner  604  redirects light received from the gathering optics  606  as described above for the display embodiments, so that the gathered light has a correction component before it reaches the biaxial scanner  602 . The biaxial scanner  602  scans through a generally raster pattern to collect light arriving at the gathering optics  606  from a range of angles and to redirect the light onto a group of stationary photodetectors  610 , each positioned at a respective location and orientation, such that it images a respective “tile” of the image field.  
         [0118]    Movement of the biaxial scanner  602  thus translates to imaging successive points of the target object  608  onto the photodetectors  610 . The photodetectors  610  convert light energy from the scanner  602  into electrical signals that are received by decoding electronics  612 . Where the imager  600  is a symbology reader, the decoding electronics  612  may include symbol decoding and storing circuitry and further electronics for assembling the image form the stored files. Where the imager is a portion of a camera, the decoding electronics  612  may include digital-to-analog converters, memory devices and associated electronics for storing a digital representation of the scanned tile and further electronics for assembling the image from the stored files. One skilled in the art will recognize that, although the correction scanner  604  is positioned before the bi-axial scanner  602 , it may be desirable to position the correction scanner  604  following the bi-axial scanner  602  in some applications.  
         [0119]    Another feature of the imager  600  shown in FIG. 24 is a set of illumination sources  614  that provide light for illuminating respective locations on a target object. The illumination sources  614  are preferably of different wavelengths to ease differentiation of beams, although in some applications common wavelength devices may be used. In one example of a multiwavelength structure where imager  600  is a symbol reader, the illumination sources  614  may include infrared or red light emitters that emit beams of light into a beam splitter  616 . The beam splitter  616  directs the illuminating light beams into the biaxial scanner  602  where the illuminating light is redirected to the correction scanner.  604 . Because the illuminating light beams are collinear with the paths of light from the target object  608 , the illuminating light beams strike the target object  608  at the same locations that are imaged by the photodetectors  610 . The illuminating light beams are reflected by the target object  608  in pattern corresponding to the reflectivity of the respective regions of the target object  608 . The reflected illuminating light travels to the photodetectors  610  to image the respective regions light that can be used only by the photodetectors  610  to image the respective regions of the target object  608 . For high resolution, the area illuminated by the sources  614  or imaged by the photodetectors  610  may be made small through a variety of known optical techniques. One skilled in the art will recognize that, although FIG. 24 shows the correction scanner  604  positioned after the horizontal scanner  602 , it will often be preferable to position the correction scanner  604  between the beam splitter  616  and the horizontal scanner  602 . This allows for the mirror of the correction scanner  604  to be made small.  
         [0120]    Alternatively, the photodetectors  610  may be mounted externally to the scanners  602 ,  604  and oriented to capture light directly from their respective tiles. Because each photodetector  610  is wavelength matched to its respective source and because the photodetectors  610  are aligned to spatially distinct regions, crosstalk between signals from the respective tiles may be adequately suppressed.  
         [0121]    In one application of the imager  600  of FIG. 24, one or more of the illumination sources  614  includes a visible, directly modulated light source, such as a red laser diode or a visible wavelength light emitted diode (LED). As shown in FIG. 25, the visible illumination source  614  can thus produce a visible image for the user. In the exemplary embodiment of FIG. 25, the imager can operate as a symbology scanner to identify information contained in a symbol on the target object  608 . Once the decoding electronics  612  identifies a desired image to be viewed, such as an item price and identity, the decoding electronics  612  modulates the drive current of the illumination sources  614  to modulate the intensity of the emitted light according to the desired image. When the user directs the imager  600  toward a screen  619  (or the target object), the illuminating light is scanned onto the screen  619  as described above. Because the illuminating light is modulated according to the desired image, the visible light reflected from the screen  619  is spatially modulated according to the desired image. The imager  600  thus acts as an image projector in addition to acquiring image data. In addition to, or as an alternative to, modulating the diode to produce an image, the diodes corresponding to each of the regions of the target object  608  may also output continuous or pulsed beams of light that fill the entire field of view of the imager  600 . The imager  600  thus provides a spotter frame  618  that indicates the field of view to the user. Similarly, the illumination sources  614  can be modified to outline the field of view or to produce other indicia of the field of view, such as cross hatching or fiducials, to aid the user in aligning the imager  600  to the target object  608 .  
         [0122]    In addition to compensating for raster pinch, one embodiment of the scanning system, shown in FIG. 28, also addresses effects of the nonlinearity of resonant and other nonlinear scanning systems. One skilled in the art will recognize that, although this correction is described for a single light source or single detector system, the approaches described herein are applicable to systems using more than one light source, as presented in FIG. 10 above. For example, in one application, the corrected output clock signal described below with reference to FIG. 28, drives all of the buffers  1306 A-D (FIG. 13) to output data in parallel from buffers  1306 A-D.  
         [0123]    As shown by broken line in FIG. 26, the timing of incoming data is premised upon a linear scan rate. That is, for equally spaced subsequent locations in a line, the data arrive at constant intervals. A resonant scanner, however, has a scan rate that varies sinusoidally, as indicated by the solid line in FIG. 26. For a start of line beginning at time to (note that the actual start of scan for a sinusoidal scan would likely be delayed slightly as described above with respect to FIG. 26), the sinusoidal scan initially lags the linear scan. Thus, if the image data for position P 1  arrive at time t 1A , the sinusoidal scan will place the pixel at position P 2 .  
         [0124]    To place the pixel correctly, the system of FIG. 28 delays the image data until time t 1B , as will now be described. As shown in FIG. 28, arriving image data V IM  are clocked into a line or frame buffer  2200  by a counter circuit  2202  in response to a horizontal synchronization component of the image data signal. The counter circuit  2202  is a conventional type circuit, and provides an input clock signal having equally spaced pulses to clock the data into the buffer  2200 . In the multisource system of FIG. 13, the four buffers  1306 A-D, and demultiplexer  1304  replace the frame buffer and the image data are clocked sequentially through the demultiplexer  1304  into the four buffers  1306 A-D, rather than being clocked into a single frame buffer or line buffer.  
         [0125]    A feedback circuit  2204  controls timing of output from the buffer  2200  (or buffers  1306 A-D of FIG. 13). The feedback circuit  2204  receives a sinusoidal or other sense signal from the scanning assembly  82  and divides the period of the sense signal with a high-speed second counter  2206 . A logic circuit  2208  produces an output clock signal in response to the counter output.  
         [0126]    Unlike the input clock signal, however, pulses of the output clock signal are not equally spaced. Instead, the pulse timing is determined analytically by comparing the timing of the linear signal of FIG. 26 to the sinusoidal signal. For example, for a pixel to be located at position P 1 , the logic circuit  2208  provides an output pulse at time t 1B , rather than time t 1A , as would be the case for a linear scan rate.  
         [0127]    The logic circuit  2208  identifies the count corresponding to a pixel location by accessing a look-up table in a memory  2210 . Data in the look-up table  2210  are defined by dividing the scanning system period into many counts and identifying the count corresponding to the proper pixel location. FIG. 27 shows this evaluation graphically for a 35-pixel line. One skilled in the art will recognize that this example is simplified for clarity of presentation. A typical line may include hundreds or even thousands of pixels. As can be seen, the pixels will be spaced undesirably close together at the edges of the field of view and undesirably far apart at the center of the field of view. Consequently, the image will be compressed near the edges of the field of view and expanded near the middle, thereby forming a distorted image.  
         [0128]    As shown by the upper line, pixel location varies nonlinearly for pixel counts equally spaced in time. Accordingly, the desired locations of each of the pixels, shown by the upper line, actually correspond to nonlinearly spaced counts. For example, the first pixel in the upper and lower lines arrives at the zero count and should be located in the zero count location. The second pixel arrives at the 100 count, but, should be positioned at the 540 count location. Similarly, the third pixel arrives at count  200  and is output at count  720 . One skilled in the art will recognize that the figure is merely representative of the actual calculation and timing. For example, some output counts will be higher than their corresponding input counts and some counts will be lower. Of course, a pixel will not actually be output before its corresponding data arrives. To address this condition, the system of FIG. 28 actually imposes a latency on the output of data, in a similar fashion to synchronous memory devices. For the example of FIG. 27, a single line latency ( 3400  count latency) would be ample. With such a latency, the first output pixel would occur at count  3400  and the second would occur at count  3940 .  
         [0129]    [0129]FIG. 29 shows an alternative approach to placing the pixels in the desired locations. This embodiment produces a corrected clock from a pattern generator rather than a counter to control clocking of output data. A synch signal stripper  2500  strips the horizontal synchronization signal form an arriving image signal V IM . Responsive to the synch signal, a phase locked loop  2502  produces a series of clock pulses that are locked to the synch signal. An A/D converter  2504 , driven by the clock pulses, samples the video portion of the image signal to produce sampled input data. The sampling rate will depend upon the required resolution of the system. In the preferred embodiment, the sampling rate is approximately 40 Mhz. A programmable gate array  2506  conditions the data from the A/D converter  2504  to produce a set of image data that are stored in a buffer  2508 . One skilled in the art will recognize that, for each horizontal synch signal, the buffer will receive one line of image data. For a 1480×1024 pixel display, the system will sample and store  1480  sets of image data during a single period of the video signal.  
         [0130]    Once each line of data is stored in the buffer  2508 , the buffer is clocked to output the data to a RAMDAC  2509  that includes a gamma correction memory  2510  containing corrected data. Instead of using the buffer data as a data input to the gamma correction memory  2510 , the buffer data is used to produce addressing data to retrieve the corrected data from the gamma correction memory  2510 . For example, a set of image data corresponding to a selected image intensity I 1  identifies a corresponding location in the gamma correction memory  2510 . Rather than output the actual image data, the gamma correction memory  2510  outputs a set of corrected data that will produce the proper light intensity at the user&#39;s eye. The corrected data is determined analytically and empirically by characterizing the overall scanning system, including the transmissivity of various components, the intensity versus current response of the light source, diffractive and aperture effects of the components and a variety of other system characteristics.  
         [0131]    In one embodiment shown in FIG. 30 according to the invention, the data may be corrected further for temperature-versus-intensity or age-versus-intensity variations of the light source. Reference data drives the light source while the vertical and horizontal position is out of the user&#39;s field of view. For example, at the edge of the horizontal scan, the reference data is set to a predetermined light intensity. A detector  2519  monitors the power out of the light source  2516  and a temperature compensation circuit  2521 . If the intensity is higher than the predetermined light intensity, a gain circuit  2523  scales the signal from the RAMDAC  2506  by a correction factor that is less than one. If the intensity is higher than the predetermined light intensity, the correction factor is greater than one. While the embodiments described herein pick off a portion of the unmodulated beam or sample the beam during non-display portions of the scanning period, the invention is not so limited. For example, a portion of the modulated beam can be picked off during the display portion of the scanning period or continuously. The intensity of the picked off portion of the modulated beam is then scaled and compared to the input video signal to determine shifts in the relative intensity of the displayed light versus the desired level of the displayed light to monitor variations.  
         [0132]    In addition to monitoring the intensity, the system can also compensate for pattern dependent heating through the same correction data or by multiplying by a second correction factor. For example, where the displayed pattern includes a large area of high light intensity, the light source temperature will increase due to the extended period of high level activation. Because data corresponding to the image signal is stored in a buffer, the data is available prior to the actual activation of the light source  2516 . Accordingly, the system can “look-ahead” to predict the amount of heating produced by the pattern. For example, if the light source will be highly activated for the 50 pixels preceding the target pixel, the system can predict an approximate pattern dependent heat effect. The correction factor can then be calculated based upon the predicted pattern dependent heating. Although the correction has been described herein for the intensity generally, the correction in many embodiments can be applied independently for red, green and blue wavelengths to compensate for different responses of the emitters and for variations in pattern colors. Compensating for each wavelength independently can help limit color imbalance due to differing variations in the signal to intensity responses of the light emitters.  
         [0133]    Returning to FIG. 29, the corrected data output from the gamma correction memory  2510  (as it may be modified for intensity variations) drives a signal shaping circuit  2514  that amplifies and processes the corrected analog signal to produce an input signal to a light source  2516 . In response, the light source  2516  outputs light modulated according to the corrected data from the gamma correction memory  2510 . The modulated light enters a scanner  2518  to produce scanned, modulated light for viewing.  
         [0134]    The clock signal that drives the buffer  2508 , correction memory  2510 , and D/A converter  2512  comes from a corrected clock circuit  2520  that includes a clock generator  2522 , pattern memory  2524  and rising edge detector  2526 . The clock generator  2522  includes a phase locked loop (PLL) that is locked to a sense signal from the scanner  2518 . The PLL generates a high frequency clock signal at about 80 MHz that is locked to the sense signal. The high frequency clock signal clocks data sequentially from addresses in the pattern memory  2524 .  
         [0135]    The rising edge detector  2526  outputs a pulse in response to each 0-to-1 transition of the data retrieved from the pattern memory  2524 . The pulses then form the clock signal CKOUT that drives the buffer output, gamma correction memory  2510 , and D/A converter  2512 .  
         [0136]    One skilled in the art will recognize that the timing of pulses output from the edge detector  2526  will depend upon the data stored in the pattern memory  2524  and upon the scanning frequency f SCAN  of the scanner  2518 . FIG. 31 shows a simplified example of the concept One skilled in the art will recognize that, in FIG. 31, the data structure is simplified and addressing and other circuitry have also been omitted for clarity of presentation.  
         [0137]    In the example of FIG. 31, if the scanning frequency f SCAN  is 20 kHz and clock generator  2522  outputs a clock signal at 4000 times the scanning frequency f SCAN , the pattern memory  2524  is clocked at 80 MHz. If all bits in an addressed memory location  2524 A are 0, no transitions of the output clock occur for 16 transitions of the generator clock. For the data structure of location  2524 B, a single transition of the output clock occurs for 16 transitions of the generator clock. Similarly, location  2524 C provides two pulses of the generator clock in one period of the scan signal and location  2524 E provides eight pulses of the generator clock in one period.  
         [0138]    The number and relative timing of the pulses is thus controlled by the data stored in the pattern memory  2524 . The frequency of the generator clock on the other hand depends upon the scanner frequency. As the scanner frequency varies, the timing of the pulses thus will vary, yet will depend upon the stored data in the pattern memory.  
         [0139]    The approaches of FIGS. 29 and 30 are not limited to sinusoidal rate variation correction. The clock pattern memory  2524  can be programmed to address many other kinds of nonlinear effects, such as optical distortion, secondary harmonics, and response time idiosyncrasies of the electronic and optical source.  
         [0140]    Moreover, the basic structure of FIG. 29 can be modified easily to adapt for vertical scanning errors or optical distortion, by inserting a bit counter  2530 , look up table  2532 , and vertical incrementing circuit  2534  before the buffer  2508 , as shown in FIG. 30. The counter  2530  addresses the look up table  2532  in response to each pulse of the input clock to retrieve two bits of stored data. The retrieved data indicate whether the vertical address should be incremented, decremented or left unaffected. The data in the look up table  2532  is determined empirically by measuring optical distortion of the scanning system and optics or is determined analytically through modeling. If the address is to be incremented or decremented, the incrementing circuit increments or decrements the address in the buffer  2508 , so that data that was to be stored in a nominal memory location are actually stored in an alternate location that is one row higher or lower than the nominal location.  
         [0141]    A graphical representation of one such data structure is shown in the simplified example FIG. 32. In this example, the first three sets of data bits  3202  for the first line of data (line  0 ) are stored in the first memory row, the next three sets of data bits  3204  for the first line are stored in the second memory row, and the last three sets of data bits are stored in the third memory row. One skilled in the art will recognize that this example has been greatly simplified for clarity of presentation. An actual implementation would include many more sets of data and may utilize decrementing of the row number as well as incrementing.  
         [0142]    The result is that some portion of the data for one line is moved to a new line. The resulting data map in the buffer  2508  is thus distorted as can be seen from FIG. 32. However, distortion of the data map can be selected to offset vertical distortion of the image caused by scanning and optical distortion. The result is that the overall system distortion is reduced. Although the embodiment of FIG. 30 shows correction of vertical distortion by adjusting the position of data stored in the buffer  2508 , other approaches to this correction may be implemented. For example, rather than adjusting the addresses of the storage locations, the addresses used for retrieving data from the buffer  2508  to the RAMDAC  2509  can be modified.  
         [0143]    As noted above, in many applications, it is desirable to control the scanning frequencies of one or more scanners. In non-resonant or low Q applications, simply varying the frequency of the driving signal can vary the scanning frequency. However, in high Q resonant applications, the amplitude response of the scanners may drop off dramatically if the driving signal differs from the resonant frequency of the scanner. Varying the amplitude of the driving signal can compensate somewhat, but the magnitude of the driving signal may become unacceptably high in many cases. Consequently, it is undesirable in many applications to try to control the scanner frequency f SCAN  simply by controlling the driving signal frequency and/or amplitude.  
         [0144]    One approach to controlling the frequency f SCAN  is shown in FIGS. 33 and 34 for a MEMs scanner  3300 . The scanner  3300  includes four tuning tabs  3302 A-D positioned at corners of a mirror body  3304 . The tuning tabs  3302 A-D are flexible projections that are integral to the mirror body  3304 . Fixed rigid projections  3305  project from the mirror body  3304  adjacent to the tuning tabs  3302 A-D, leaving a small gap therebetween.  
         [0145]    Each of the tuning tabs  3302 A-D carries a ground electrode  3306  coupled by a conductor  3310  to an external electrode  3312  to form an electrical reference plane adjacent to the respective tab  3302 A-D. Each of the rigid projections  3306 A-D carries a respective hot electrode  3308  controlled by a respective external electrode  3316 A-D, that allows control of the voltage difference between each tuning tab  3302 A-D and its corresponding rigid projection  3306 A-D.  
         [0146]    Each flexible tab  3302 A-D is dimensioned so that it bends in response to an applied voltage difference between the tab  3302 A-D and the adjacent rigid projection  3306 , as shown in FIG. 34. The amount of bending will depend upon the applied voltage, thereby allowing electrical control of tuning tab bending.  
         [0147]    One skilled in the art will recognize that the resonant frequency of the scanner  3300  will be a function of the mass of the mirror  3304 , the dimensions and mechanical properties of torsion arms  3317  supporting the mirror  3304 , and the locations  3318  of the centers of mass of each half of the mirror  3304  (including its tabs  3302 A-D and rigid projections  3306 ) relative to the axis of rotation of the mirror  3304 . Bending the flexible tabs shifts the centers or mass slightly inwardly from the original locations  3318  to new locations  3320 . Because the centers of mass are located closer to the axis of rotation, the scanning frequency increases slightly. Increasing the voltage on the fixed projections  3306  thus can increase the resonant frequency of the scanner  3300 .  
         [0148]    The use of electronically controlled elements to control resonance in a scanner is not limited to controlling the horizontal scanning frequency. For example, in the embodiment of FIG. 35, a mirror body  3500  has interdigitated comb drives  3502  that extend from the body&#39;s edges. Comb driven actuators are known structures, being described for example in Tang, et al., ELECTROSTATIC-COMB DRIVE OF LATERAL POLYSILICON RESONATORS, Transducers &#39;89, Proceedings of the 5 th  International Conference on Solid State Sensors and Actuators and Eurosensors III, Vol. 2, pp.328-331, June 1990, which is incorporated herein by reference. Respective conductors  3504  extend from each of the comb drives  3502  to allow tuning voltages Vtune 1 , Vtune 2  to control the comb drives  3502 . As is known, applied voltages produces lateral forces F 1 , F 2  in the comb drives  3502 . Flexible arms  3506  at the distal ends of the comb drives  3502  bend in response to the forces F 1 , F 2 , thereby shifting the mass of the flexible arms  3506  relative to the center of mass  3508  of the respective half of the mirror body. Because the position shift is parallel to the axis of rotation of the mirror body  3500 , the horizontal resonant frequency does not shift significantly. However, if the voltages are set such that the flexible arms experience different position shifts, the mirror body  3500  can be made slightly unbalanced. The mirror body  3500  will then begin to approximate the Lissajous pattern of FIG. 20. Adjusting the tuning voltages Vtune 1 , Vtune 2  produces a corresponding adjustment in the scan pattern. Where the masses of the flexible portions  3506  and the voltages Vtune 1 , Vtune 2  are chosen appropriately, the resonant frequency of vibrations from the unbalanced mirror body will be an integral multiple of the horizontal scanning frequency and the Lissajous pattern will be stable. By monitoring the scan pattern and adjusting the tuning voltages Vtune 1 , Vtune 2  accordingly, the Lissajous pattern can be kept stable. Thus, the electronically controlled structures can assist in pinch correction.  
         [0149]    [0149]FIG. 36 shows an alternative approach to controlling the resonant frequency of a scanner  3600 . In this embodiment, the scanner  3600  is housed on a platformi  3602  in a sealed package  3604  having a transparent lid  3606 . The package  3604  also contains a gas, such as a helium or argon mix, at a low pressure. The resonant frequency of the scanner  3600  will depend, in part, upon the pressure of within the package  3602  and the properties of the gas, as is described in Baltes et. al., THE ELECIRONIC NOSE IN LILLIPUT, IEEE Spectrum, Sept. 1998, pp. 35-39, which is incorporated here by reference. Unlike conventional sealed packages, the package  3602  includes a pair of outgassing nodules  3610  concealed beneath the platform  3602 .  
         [0150]    The nodules  3610  are formed from an outgassing material, such as isopropanol in a polymer, atop a resistive heater  3611 . Electrical current causes resistive heating of the heater  3611 , which, in turn causes the nodule  3610  to outgas. An electronic frequency controller  3614 , controls the amount of outgassing by applying a controlled current through pairs of electrodes  3612  positioned on opposite sides of each of the nodules  3610 . The increased gas concentration reduces the resonant frequency of the scanner  3600 . For greater frequency variation, absorptive polymer segments  3618  coat the scanners torsion arms  3620  to “amplify” the absorptive effect on resonant frequency.  
         [0151]    Typically, the above-described variable or “active” tuning approaches are most desirable for producing small frequency variations. For example, such small frequency adjustments can compensate for resonant frequency drift due to environmental effects, aging, or internal heat buildup. To reduce the difficulty of active tuning approaches or to eliminate active tuning entirely, it is desirable in many applications to “tune” the resonant frequency of a scanner to minimize the difference between the scanner&#39;s uncompensated resonant frequency and the desired scan frequency. Such frequency differences may be caused by processing variations, material property variations, or several other effects.  
         [0152]    [0152]FIG. 37 shows one approach to tuning the scanner&#39;s uncompensated resonant frequency, in which a scanner  3700  is fabricated with integral tuning tabs  3702 A-B,  3704 A-B,  3706 A-B,  3708 A-B,  3710 , and  3712 . Initially, the scanner&#39;s mirror body  3714  and torsional arms  3716  are dimensioned to produce a resonant frequency (with all of the tuning tabs  3702 A-B,  3704 A-B,  3706 A-B,  3708 A-B,  3710 , and  3712  attached) that is slightly below the desired resonant frequency. Once the scanner  3700  is assembled, the resonant frequency can be measured in a variety of fashions. For example, the scanner  3700  can be driven in one of the techniques described previously and the mirror response can be monitored optically. Alternatively, impedance versus frequency measurements may also provide the resonant frequency relatively quickly.  
         [0153]    The determined resonant frequency is then compared to the desired resonant frequency to identify a desired frequency compensation. Based upon the identified frequency compensation some of the tuning tabs  3702 A-B,  3704 A-B,  3706 A-B,  3708 A-B,  3710 , and  3712  can be removed, for example by laser trimming or mechanical force to reduce the mass of the mirror body  3714 . As is known, lowering the mass of the mirror body  3714  (in the absence of other variations) will increase the resonant frequency. The number and position of the tabs to be removed for the identified frequency compensation can be determined through modeling or empirical data. Preferably, the removed tuning tabs are positioned symmetrically relative to the center of mass of the respective half of the mirror body and with respect to the axis of rotation of the mirror body  3714 . To make this symmetricity easier, the tuning tabs  3702 A-B,  3704 A-B,  3706 A-B,  3708 A-B,  3710 , and  3712  are positioned in the symmetric locations about the mirror body  3714 . For example, tuning tabs  3702 A-B and  3704 A-B form a quartet of tabs that would typically be removed as a group. Similarly, tuning tabs  3710  and  3712  form a pair of tabs that would typically be removed as a pair.  
         [0154]    While the tuning tabs  3702 A-B,  3704 A-B,  3706 A-B,  3708 A-B,  3710 , and  3712  in FIG. 37 are shown as equally sized for ease of presentation, it is not always necessary or even desirable to make them the same size. In some applications, such tabs may be variably sized to allow greater flexibility in tuning.  
         [0155]    As described above with respect to FIG. 12, tiling in two dimensions can allow a large, high resolution display with less demand upon a scanner. FIG. 38 shows one difficulty that may arise when four separate sources  3800 ,  3802 ,  3804 ,  3806  feed a common scanner  3808 . As can be seen from the ray tracing for the lower left scanner  3800 , the upper right source  3804  is positioned within an expected scanning field  3810  of the lower left source  3800 . With no further adjustment, the upper right source  3804  would be expected to occlude a portion of the image from the lower left source  3800 , producing an unilluminated region in the corresponding tile.  
         [0156]    [0156]FIG. 39 shows one approach in which the effects of overlapping of sources and beams can be reduced. In this embodiment, light arrives through separate fibers  3900 ,  3902 ,  3904 ,  3906  and is gathered and focused by respective GRIN lenses  3908 ,  3910 ,  3912 ,  3914  onto respective turning mirrors  3916 ,  3918 ,  3920 ,  3922 . As is visible for two of the mirrors  3916 ,  3922  in FIG. 40, the turning mirrors  3916 ,  3922  are very small mirrors that redirect light from their respective GRIN lenses  3908 ,  3914  toward a curved, partially reflective mirror  3924 . The mirror  3924  returns the incident light toward a centrally positioned scanner  3926  that scans periodically, as described previously. The scanned light passes through the partially transmissive mirror  3924  toward an image field  3928  where an image can be viewed.  
         [0157]    As can be seen in FIG. 40, the GRIN lenses  3908 ,  3914  gather diverging light from the respective fibers  3900 ,  3906  and reduce the beam width to substantially its minimum diameter at the respective turning mirror  3916 ,  3922 . The beam  3930  then expands as it travels to the curved mirror  3924 . The curved mirror  3924  converts the expanding beam  3930  into a substantially collimated or slightly converging beam  3932  having a diameter slightly smaller than the mirror width W of the scanner  3926 .  
         [0158]    It can be seen in FIG. 40 that the turning mirrors  3916 ,  3918 ,  3920 ,  3922  will block light from other turning mirrors during a portion of their scans. However, because the turning mirrors block only small section of the beams and because the beams converge at the image field  3924 , the effect will be a slight dimming of the corresponding pixel. Uncompensated, this might produce a slight variation from the desired pixel intensity. However, the programmable gate array  2506  described above with respect to FIG. 29 can pre-weight the intensity to offset the dimming effects of the turning mirrors  3916 ,  3918 ,  3920 ,  3922 .  
         [0159]    To further improve efficiency the display of FIGS. 39 and 40 can also take advantage of properties of polarized light. In some applications, the fibers  3900 ,  3902 ,  3904 ,  3906  (or other light sources such as laser diodes) emit polarized light. A polarization dependent reflector  3934 , such as 3M&#39;s Dual Brightness Enhancement Film coats the inner surface of the mirror and reflects the polarized incident beam  3930 . As the reflected beam  3932  travels to the scanner  3926 , the beam  3932  passes through a quarter wave plate that rotates the polarization by 45 degrees. The beam  3932  is then reflected by the scanner  3926  and passes through the quarter wave plate once again, so that the polarization rotates by a total of 90 degrees and is orthogonal to the original beam  3930 . The orthogonally polarized beam passes efficiently through the polarization dependent reflector  3934  and travels to the image field  3928 .  
         [0160]    [0160]FIG. 41 shows how the use of a tiling approach can reduce raster pinch without a correction scanner. In this embodiment, modulated light from an input fiber  4102  enters one or the other of a pair of transmission fibers  4104 ,  4106  as dictated by an optical switch  4108 . Light exits the transmission fibers  4104 ,  4106  and strikes a common scanner  4110  that scans light from the first fiber  4104  onto a first region  4112  of an image field  4114  and scans light from the second fiber  4106  onto a second region  4116  of the image field  4114 . The fibers  4104 ,  4106  are oriented so that the first and second regions  4112 ,  4116  overlap very slightly in an overlap area  4118 .  
         [0161]    During forward sweeps of the scanner  4110 , an electronic controller  4120  activates the switch  4108  so that light passes through the second fiber  4106 . The scanner  4110  thus redirects the light along a first scan line  4122  in the second region  4116 . At the end of the forward sweep, the controller  4120  activates the switch  4108  so that light now passes through the first fiber  4104  and is scanned along a first scan line  4124  in the first region  4112 . For each subsequent sweep of the scanner  4110 , the controller  4120  activates the switch to produce sets of lines in each of the regions  4112 ,  4116 . Because the vertical scan continues during the forward sweeps, the lines may be slightly tilted, as shown in FIG. 41. While such tilt is typically not observable by a viewer, if desired, custom optics can produce a “counter”-tilt that offsets the scanning tilt. Alternatively, the image data may be predistorted by the programmable gate array  2506  described above with respect to FIG. 29 to compensate.  
         [0162]    This structure is not limited to two horizontal tiles or to a single light emitter. For example, as shown in FIG. 42, light from two fibers can be switched into four fibers to produce a 2-by-2 tiled image.  
         [0163]    In this approach, an input fiber  4200  is coupled to four fibers  4202 ,  4204 ,  4206 ,  4208  by a set of optical switches  4210 ,  4212 ,  4214 , where each fiber feeds a scanning assembly  4216  from a respective angle. A switch controller  4220  activates the switches  4210 ,  4212 ,  4214  according to the direction of the sweep and according to the tracked location of the user&#39;s gaze, as provided by a gaze tracker (not shown). The gaze tracker may be any known apparatus for determining gaze direction.  
         [0164]    For example, when the user looks at the top half of the image, a first fiber  4206 , aligned to produce an image in the upper left tile  4222  feeds the scanning assembly  4216  during the forward sweeps. A second fiber  4208 , aligned to produce an upper right tile  4224  feeds the scanning assembly  4216  during reverse sweeps. When the user looks at the lower half of the image, a third fiber  4204 , aligned to produce the lower left tile  4226 , feeds scanning assembly  4216  during forward sweeps. A fourth fiber  4202 , aligned to produce the lower right tile  4228 , feeds the scanning assembly  4216  during reverse sweeps. While each of the fibers  4200 ,  4206 ,  4208 ,  4204  is represented as a single fiber, in some applications each fiber  4200 ,  4206 ,  4208 ,  4204  may actually include a plurality of fibers  4200 ,  4206 ,  4208 ,  4204 . In such applications each fiber  4200 ,  4206 ,  4208 ,  4204  is fed by a plurality of input fibers  4200  and a corresponding plurality of switch sets. Such an embodiment advantageously allows a plurality of lines to be written simultaneously. Writing a plurality of lines simultaneously reduces the frequency of the horizontal scanner relative to the single line writing approaches described above, thereby reducing the difficulty of scanning. Also, providing light simultaneously from a plurality of light emitters reduces the amount of light energy required from each source for a given display brightness and reduces the modulation frequency of the beam. This reduces the performance requirements of the light sources, thereby decreasing the cost and complexity of the overall display.  
         [0165]    While the embodiments of FIGS. 41 and 42 have been described herein using fibers and optical switches, in some applications, discrete light sources, such as laser diodes, LEDs, microlasers, or gas lasers may replace each fiber. In such applications, electrical switches (e.g., transistors) selectively control drive currents to the respective sources or control external modulators aligned with the respective sources to control feeding of light during forward an reverse sweeps of the mirror.  
         [0166]    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. In one example of repositioning, the correction scanners can be positioned in the optical path either before or after the other scanners. Also, an exit pupil expander may be added or omitted in many applications. In such embodiments, conventional eye tracking may be added to ease coupling of the scanned beam to the eye. Moreover, the scanning system can be used for projection displays, optical storage and a variety of other scanned light beam applications, in addition to scanned retinal displays. Further, a variety of other timing control mechanisms, such as programmable delays, may be used to compensate for the variable speed of the scanner in place of the approaches described with reference to FIGS.  24 - 31 . Additionally, in some applications it may be desirable for ease of positioning or for other reasons to use a plurality of scanners, each of which may be fed by one or more beams. In such a structure, each scanner and its corresponding light sources produce respective sets of tiles. The overall image is than formed by combining the sets of tiles from each of the scanners, either by adjacent positioning or by overlapping. Although overlapping is generally preferred only where each scanner is used for a respective wavelength, in some applications overlapping may be used for interlacing or other approaches to image combination.  
         [0167]    In another alternative approach to timing and distortion correction, the memory map may be undistorted and addressed at a constant rate. To compensate for nonlinearity of the scanner, the data for each location is derived from the retrieved image data and output at fixed increments. Referring to FIG. 27, for example, data would be output at a time  1500 , even though this time did not correspond directly to a pixel time. To compensate, the buffer  2508  is addressed at the 10 th  and 11 th  locations for this line. Then, the output data is a weighted average of the data from the 10 th  and 11 th  locations. Thus, the buffer  2508  is clocked at a constant rate and pixels are output at a constant rate. Yet, by controlling the addressing circuitry carefully and performing a weighted averaging, the output data is sinusoidally corrected. Also, although the light emitters and light sources described herein utilize laser diodes or LEDs, with or without fibers, a variety of other light emitters such as microlasers, gas lasers, or other light emitting devices may desirable in some applications. Moreover, although the exemplary scanning assemblies described herein utilize torsionally mounted mirrors, other scanning assembly structures, such as spinning polygons, comb drive mirrors, acousto-optic scanners, and other scanning structures may be within the scope of the invention. Also, while the beams are shown as converging upon a single scanner, in some applications it may be desirable to use separate scanners for each beam of light or to use a plurality of scanners that each reflect a plurality of beams. Accordingly, the invention is not limited except as by the appended claims.