Abstract:
A scanning system for a printer or scanner that has a beam source that produces a light beam at a point of origin and has a scanning mirror that sweeps the light beam from a position “A” to a position “B.” A lens receives the light beam from the scanning mirror and directs the light beam as a scanning beam to an image plane surface. A first synchronizing mirror intercepts and reflects a light ray to a photo detection system when the scanning beam is at a position “A′” that is in the vicinity of position “A” and between position “A” and “B”. A second synchronizing mirror intercepts a light ray when the scanning beam is at a position “B′” that is in the vicinity of position “B” and between position “A” and “B”, and reflects the light ray to a folding mirror that reflects the light ray to the photo detection system. In some configurations two photo detection systems are used in lieu of the folding mirror. In some instances, the lens is a focusing lens and when the scanning beam is at position “A′” and when the scanning beam is at position “B′”, equal optical path lengths are established between (1) the point of origin and the photo detection system and (2) the point of origin and virtually to the image plane surface such that the light ray arriving at the photo detection system is well focused.

Description:
FIELD 
   This invention relates to the field of light beam scanners. More particularly, this invention relates to mechanisms to control and synchronize bi-directional oscillating scanning devices. 
   BACKGROUND 
   Optical printers and scanners typically incorporate a mechanism to direct a light beam in a pattern that scans across the surface of an image plane surface. Often the light beam is a laser beam. For printers, the image plane surface is typically the outer cylindrical surface of a photo-sensitive drum. For scanners, the image plane surface is typically a piece of paper being scanned. Generally the light beam sweeps in one plane, called the horizontal plane, while the image plane moves in an orthogonal (vertical) plane. In cases where the image plane surface is a photo-sensitive drum, movement in the vertical plane is achieved by rotating the drum around its cylindrical axis. 
   In many optical systems the horizontal scanning motion of the light beam is achieved by reflecting the light beam off a moving mirror. Often the mirror is a series of planar surfaces on a polygonal wheel that rotates. In such systems, the light beam is focused at an oblique angle toward the axis of rotation of the mirror, and the light beam is deflected in a linear scanning mode by each planar mirror surface of the polygon as the mirror spins. In systems where the image plane surface is simultaneously moving in the vertical plane, the horizontal light scanning plane is tilted somewhat from the vertical recording medium plane such that the trace of the light scan is substantially orthogonal to the direction of movement of the of the image plane surface in the vertical plane. 
   In order to write information as in a printer, the ray of the light beam is typically modulated as it scans. In order to read information as in a scanner, the intensity of the reflected beam is typically monitored. To achieve an accurate recording of the image as successive scan lines are written on or read from the image plane surface, it is essential to synchronize the modulation of each scan of light beam with the modulation of the previous scans that are creating the image. Typically this synchronization is achieved by optically intercepting the light beam with one or more photo detectors at the start of each scan line. The detection of light beam by the photo detector creates a start of scan pulse that is used to synchronize the start of modulation of the light beam for each successive scan line. 
   Historically, in many optical scanning and printing systems, scanning occurs in only one direction. As a result, it has generally been sufficient to intercept the light beam on one side (the starting side) of each successive scan. However, optical printer and scanner users are continually demanding increases in processing speed. As speeds increase accurate synchronization of modulation (for printers) or detection (for scanners) of the light beam becomes more difficult using only a start-of-scan pulse. Also, in many applications it is desirable to provide bi-directional light beam scanning capability. Existing technology does not adequately address all the needs for synchronizing light beam modulation for high speed or bi-directional scanning or printing. What is needed is an improved means that has the ability to detect both the start and the end of each scan of the light beam. Also needed is an improved means of synchronizing bi-directional scanning systems. 
   SUMMARY 
   The above and other needs are met by a scanning system that has a beam source that produces a light beam and a scanning mirror that receives the light beam from the beam source and sweeps the light beam from a position “A” to a position “B”. A lens receives the light beam from the scanning mirror as the scanning mirror sweeps the light beam from position “A” to position “B” and directs the light beam as a scanning beam to an image plane surface. A photo detection system that has at least one photo detector and has a minimum light ray threshold detection level is provided. A first synchronizing mirror is used and the first synchronizing mirror is positioned to intercept and reflect at least a portion of the scanning beam as a synchronizing light ray when the light beam is at a position “A′” that is in the vicinity of position “A” and that is between position “A” and position “B” such that when the light beam is at position “A′” the first synchronizing mirror intercepts a first light ray of the scanning beam and reflects at least a portion of the first light ray as a second light ray of the scanning beam to the photo detection system, where the intensity of the second light ray of the scanning beam is at least equal to the minimum light ray threshold detection level of the photo detection system. A second synchronizing mirror is also used, and the second synchronizing mirror is positioned to intercept and reflect at least a portion of the scanning beam as a synchronizing light ray when the light beam is at a position “B′” that is in the vicinity of position “B” and that is between position “A” and position “B” such that when the light beam is at position “B′” the second synchronizing mirror intercepts a third light ray of the scanning beam and reflects at least a portion of the third light ray as a fourth light ray of the scanning beam to a folding mirror that is positioned to reflect at least a portion of the fourth light ray as a fifth light ray of the scanning beam to the photo detection system, where the intensity of the fifth light ray of the scanning beam is at least equal to the minimum light ray threshold detection level of the photo detection system. An electronic circuit is provided to generate an electronic signal when the photo detection system detects a light ray that is at least equal to the minimum light ray threshold detection level of the photo detection system. 
   In an alternate embodiment, the scanning system has a beam source producing a light beam and a scanning mirror that receives the light beam from the beam source and sweeps the light beam from a position “A” to a position “B”. A lens is provided to receive the light beam from the scanning mirror as the scanning mirror sweeps the light beam from position “A” to position “B” and directs the beam as a scanning beam to an image plane surface. A single photo detector having a minimum light ray threshold detection level is used. A first synchronizing mirror is positioned to intercept and reflect at least a portion of the scanning beam when the light beam is at a position “A′” that is in the vicinity of position “A” and that is between position “A” and position “B” such that when the light beam is in position “A′” the first synchronizing mirror intercepts a first light ray of the scanning beam and reflects at least a portion of the first light ray as a second light ray of the scanning beam to the photo detector without passing through the lens, where the intensity of the second light ray of the scanning beam is at least equal to the minimum light ray threshold detection level of the photo detector. A second synchronizing mirror is positioned to intercept and reflect at least a portion of the scanning beam when the light beam is at a position “B′” that is in the vicinity of position “B” and that is between position “A” and position “B” such that when the light beam is at position “B′” the second synchronizing mirror intercepts a third light ray of the scanning beam and reflects at least a portion of the third light ray as a fourth light ray of the scanning beam to the photo detector, where the intensity of the fourth light ray of the scanning beam is at least equal to the minimum light ray threshold detection level of the photo detector. An electronic circuit generates an electronic signal when the photo detector detects a light ray that is at least equal to the minimum light ray threshold detection level of the photo detector. 
   Another embodiment incorporates a beam source producing a light beam. A scanning mirror receives the light beam from the beam source and sweeps the light beam from a position “A” to a position “B”. A lens receives the light beam from the scanning mirror as the scanning mirror sweeps the light beam from position “A” to position “B” and directs the light beam as a scanning beam to an image plane surface. A plurality of photo detectors are provided, each having its own minimum light ray threshold detection level. A first synchronizing mirror is positioned to intercept and reflect at least a portion of the scanning beam as a synchronizing light ray when the light beam is at a position “A′” that is in the vicinity of position “A” and that is between position “A” and position “B”, such that when the light beam is in position “A′” the first synchronizing mirror intercepts a first light ray of the scanning beam and reflects at least a portion of the first light ray as a second light ray of the scanning beam to a first photo detector, where the intensity of the second light ray of the scanning beam is at least equal to the minimum light ray threshold detection level of the first photo detector. A second synchronizing mirror is positioned to intercept and reflect at least a portion of the scanning beam as a synchronizing light ray when the light beam is at a position “B′” that is in the vicinity of position “B” and that is between position “A” and position “B” such that when the light beam is at position “B′” the second synchronizing mirror intercepts a third light ray of the scanning beam and reflects at least a portion of the third light ray as a fourth light ray of the scanning beam to a second photo detector, where the intensity of the fourth light ray of the scanning beam is at least equal to the minimum light ray threshold detection level of the second photo detector. At least one synch focusing lens is provided to intercept at least one synchronizing light ray and focus at least one synchronizing light ray onto at least one photo detector. An electronic circuit is included to generate an electronic signal when a photo detector detects a light ray that is at least equal to the minimum light ray threshold detection level of the photo detector. 
   In a different configuration, the scanning system has a beam source that produces a light beam at a light beam point of origin, and a scanning mirror receives the light beam from the beam source and sweeps the light beam from a position “A” to a position “B”. A lens receives the light beam from the scanning mirror as the scanning mirror sweeps the light beam from position “A” to position “B” and directs the light beam as a scanning beam to an image plane surface. A plurality of photo detectors are provided, each having its own minimum light ray threshold detection level. A first synchronizing mirror is positioned to intercept and reflect at least a portion of the scanning beam when the light beam is at a position “A′” that is in the vicinity of position “A” and that is between position “A” and position “B” such that when the light beam is in position “A′” the first synchronizing mirror intercepts a first light ray of the scanning beam and reflects at least a portion of the first light ray as a second light ray of the scanning beam to a first photo detector, where the intensity of the second light ray of the scanning beam is at least equal to the minimum light ray threshold detection level of the first photo detector. A second synchronizing mirror is positioned to intercept and reflect at least a portion of the scanning beam when the light beam is at a position “B′” that is in the vicinity of position “B” and that is between position “A” and position “B” such that when the light beam is at position “B′” the second synchronizing mirror intercepts a third light ray of the scanning beam and reflects at least a portion of the third light ray as a fourth light ray of the scanning beam to a second photo detector, where the intensity of the fourth light ray of the scanning beam is at least equal to the minimum light ray threshold detection level of the second photo detector. A first recording optical path length is established as measured from the light beam point of origin to the scanning mirror and then through the lens and then virtually to the image plane surface when the light beam is at position “A′”, and a second recording optical path length is established as measured from the light beam point of origin to the scanning mirror and then through the lens and then virtually to the image plane surface when the light beam is at position “B′”. A first synchronizing optical path length is established as measured from the light beam point of origin to the scanning mirror and then to the first synchronizing mirror and then to the first photo detector when the light beam is at position “A′”, and a second synchronizing optical path length is established as measured from the light beam point of origin to the scanning mirror and then to the second synchronizing mirror and then to the second photo detector when the light beam is at position “B′”. The first recording optical path length is substantially equal to the first synchronizing optical path length and the second recording optical path length is substantially equal to the second synchronizing optical path length such that the focused scanning beam is both focused on the photo detector and virtually focused on the image plane surface. The scanning system further comprises an electronic circuit that generates an electronic signal when a photo detector detects a light ray that is at least equal to the minimum light ray threshold detection level of the photo detector. 
   In a further alternate implementation, the scanning system has a beam source producing a light beam having a point of origin and a scanning mirror that receives the light beam from the beam source and oscillates the light beam between a position “A” and a position “B”. A focusing lens is provided to receive the light beam from the scanning mirror as the scanning mirror oscillates the light beam between position “A” and position “B” and to focus the beam as a focused scanning beam to an image plane surface. A photo detection system is provided having at least one photo detector with each photo detector having a minimum light ray threshold detection level. A first synchronizing mirror is positioned to intercept and reflect at least a portion of the focused scanning beam when the light beam is at a position “A′” that is in the vicinity of position “A” and is between position “A” and position “B”. The position is such that as the light beam approaches position “A” from position “B”, when the light beam is at position “A′” the first synchronizing mirror intercepts a first light ray of the focused scanning beam and reflects at least a portion of the first light ray as a second light ray of the focused scanning beam to a photo detector, the second light ray having an intensity at least equal to the minimum light ray threshold detection level of the second light ray&#39;s photo detector. When the light beam is at position “A” the first synchronizing mirror is positioned to not reflect a portion of the focused scanning beam to the second light ray&#39;s photo detector that has intensity equal to or greater than the minimum light ray threshold detection level of the second light ray&#39;s photo detector. Then when the light beam is again at position “A′” moving toward position “B” the first synchronizing mirror is positioned to intercept a third light ray of the focused scanning beam and reflect at least a portion of the third light ray as a fourth light ray of the focused scanning beam to the second light ray&#39;s photo detector, the fourth light ray having an intensity at least equal to the minimum light ray threshold detection level of the second light ray&#39;s photo detector. A first recording optical path length is established as measured from the light beam point of origin to the scanning mirror and through the lens and then virtually to the image plane surface when the light beam is at position “A′”. A first synchronizing optical path length is established as measured from the light beam point of origin to the scanning mirror and then through the lens and then to the first synchronizing mirror and then to the second light ray&#39;s photo detector when the light beam is at position “A′”. A second synchronizing mirror is positioned to intercept and reflect at least a portion of the focused scanning beam when the light beam is at a position “B′” that is in the vicinity of position “B” and is between position “A” and position “B”. The position is such that as the focused scanning beam approaches position “B” from position “A”, when the light beam is at position “B′” the second synchronizing mirror intercepts a fifth light ray of the focused scanning beam and reflects at least a portion of the fifth light ray as a sixth light ray of the focused scanning beam to a photo detector, the sixth light ray having intensity at least equal to the minimum light ray threshold detection level of the sixth light ray&#39;s photo detector. When the light beam is at position “B” the second synchronizing mirror is positioned to not reflect a portion of the focused scanning beam to the sixth light ray&#39;s photo detector that is equal to or greater than the minimum light ray threshold detection level of the sixth light ray&#39;s photo detector. Then when the light beam is again at the position “B′” moving toward position “A” the second synchronizing mirror is positioned to intercept a seventh light ray of the focused scanning beam and reflect at least a portion of the seventh light ray as an eighth light ray of the focused scanning beam to the sixth light ray&#39;s photo detector, the eighth light ray having an intensity at least equal to the minimum light ray threshold detection level of the sixth light ray&#39;s photo detector. A second recording optical path length is established as measured from the light beam point of origin to the scanning mirror and then through the lens and then virtually to the image plane surface when the light beam is at position “B′”. A second synchronizing optical path length is established as measured from the light beam point of origin to the scanning mirror and then through the lens and then to the second synchronizing mirror and then to the photo detector when the light beam is at position “B′”. The first recording optical path length and the first synchronizing optical path length are substantially equal and the second recording optical path length and the second synchronizing optical path length are substantially equal such that the focused scanning beam is focused on both the photo detector and the image plane surface. The scanning system further incorporates an electronic circuit that generates an electronic signal when the photo detection system detects a light ray that is at least equal to the minimum light ray threshold detection level of the photo detection system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
       FIG. 1  is a graph that illustrates the scan angle versus time for a scanning system according to the invention. 
       FIG. 2  is perspective and somewhat schematic illustration of a scanning system according to the invention. 
       FIG. 3  is a perspective and somewhat schematic illustration of details of a portion of  FIG. 2  according to the invention. 
       FIG. 4  is a schematic view of a scanning system using a single photo detector according to the invention. 
       FIG. 5  is a partial cutaway schematic view of a photo detection system according to the invention. 
       FIG. 6  is a schematic view of a scanning system using two photo detectors according to the invention. 
   

   DETAILED DESCRIPTION 
   Various embodiments are described herein that provide electronic signals to synchronize the start and the end of a horizontal scan line in a device, such as an optical scanner or a printer, that use light beams to sweep a field of view, at least in a horizontal direction, and either read or write information along the horizontal scan line.  FIG. 1  illustrates a graph of the sinusoidal path over time of an light beam in a printer or scanner as it sinusoidally sweeps horizontally across scan angles in one embodiment of the invention. The scan angle is depicted on the ordinate  10  and time is depicted on the abscissa  12  of the graph. The light beam scan is centered about an axis of symmetry S  14 . The light beam scan angle starts at S  14  at time=0 and increases to a first intercept point  16  at scan angle position A′  18  at time tA′ 1    20 . In preferred embodiments, the light beam is detected by a sensor at scan angle position A′  18  at time  20 . The scan angle continues to a first maximum amplitude  21  at position A  22 , and then decreases to a second intercept point  24  at time tA′ 2    26 , where second intercept point  24  is substantially at scan angle position A′  18 . The elapsed time between the first intercept point  16  and the second intercept point  24  is t 0   28 . In preferred embodiments, the light beam is also detected by a sensor at second intercept point  24  (scan angle position A′  18 ) which occurs at time tA′ 2    26 . The scan angle continues to decrease passing through the axis of symmetry S  14  to a third intercept point  30  at scan angle position B′  32  at time tB′ 1    36 . In preferred embodiments, the light beam is also detected by a sensor at intercept point  30  (scan angle position B′  32 ) occurring at time tB′ 1    36 . The elapsed time between the second intercept point  24  and the third intercept point  30  is t 1   38 . The scan angle continues to a minimum amplitude  51  at position B  34  and then begins to increase where it reaches a fourth intercept point  40  at time tB′ 2    42 , where the fourth intercept point is substantially at scan angle position B′  32 . The elapsed time between the third intercept point  30  and the fourth intercept point  40  is t 2   44 . In preferred embodiments, the light beam is also detected by a sensor at fourth intercept point  40  (scan angle position B′  32 ) at time tB′ 2    42 . The scan angle continues to increase until it reaches a fifth intercept point  46  which occurs at time tA′ 3    48 . In preferred embodiments, the light beam is also detected by a sensor at intercept point  46  (scan angle position A′  18 ) at time tA′ 3    48 . The elapsed time between the fourth intercept point  40  and the fifth intercept point  46  is t 3   50 . The scan angle then continues to a sixth intercept point  52 , and seventh intercept point  54 , an eighth intercept point  56 , and so forth, with the preferred instance continuing of the light beam being detected by a sensor at each intercept point. Time t 1  represents a horizontal scan in the direction A to B, and time t 3  represents a horizontal scan in the direction B to A. 
     FIG. 2  illustrates an embodiment that uses a laser as the light beam in a scanning system. Laser scanning unit  70  incorporates a housing  72  that contains a laser beam source  74 . The laser beam source  74  includes a collimation unit  76  that is bolted onto housing  72 . The laser beam  78  emanates from a beam point of origin in the laser beam source  74  and exits the collimation unit  76  and passes through a pre-scan lens  80  that is mounted on a pre-scan lens sled  82  that is adjusted to focus the laser beam  78  in the process direction on the image plane  116 . The laser beam  78  strikes a pre-scan mirror  84  that reflects the beam onto the torsion oscillator mirrored surface  86 . The oscillation of the torsion oscillator sweeps the laser beam into a scan path having an axis of symmetry  88 . Thus the laser beam becomes a series of light rays which are segments of the laser beam. A representative light ray  90  is shown leaving torsion oscillator mirrored surface  86  along the axis of symmetry  88  and passing through first f-theta lens F 1   92 . Light ray  90  then passes through the second f-theta lens F 2   94 . First f-theta lens F 1   92  and second f-theta lens F 2   94  are optical elements that may be transmissive or reflective. In this embodiment, one of the functions of first f-theta lens F 1   92  and second f-theta lens F 2   94  is to focus the light rays (e.g. light ray  90 ) as a scanning beam on image plane  116 . Also, light rays typically will bend as they pass through first f-theta lens F 1   92  and second f-theta lens F 2   94 , but for clarity of illustration in  FIGS. 2 and 3 , light rays are shown as passing straight through first f-theta lens F 1   92  and second f-theta lens F 2   94 . The depiction of light ray  90  is terminated at the housing  72  in order to not obscure the axis of symmetry  88  with which light ray  90  is co-linear. 
   As further illustrated in  FIG. 2 , after exiting the F 2  lens  90 , light rays are directed toward image plane  116  across a span ranging from a maximum position A  120  to a maximum position B  122 , through a maximum scan angle  121 . Light rays  96 ,  98 , and  100  are shown as the scan approaches maximum position A  120 . However, when the scan angle reaches position A′  126 , as represented by light ray  102  (labeled on its virtual extension for clarity), the light ray ( 102 ) is intercepted by the A HSYNC mirror  124  and reflected as light ray  130  to the HSYNC photo detection system  140 . Light ray  102  is an example of a synchronizing light ray. The HSYNC photo detection system  140  can be seen more clearly in  FIG. 3 . In the most preferred embodiments the HSYNC photo detection system  140  incorporates a collection lens and a circuit board ( 142 ) that incorporates a single PIN photodiode sensor as the photo detector. 
   To capture a robust signal at the photo detector in the photo detection system  140 , it is highly desired to have a focused laser beam illuminate the photo detector. In the most preferred embodiments the focusing of the laser beam onto the photo detector is accomplished by positioning the Sensor A HSYNC mirror  124  and the photo detection system  140  such that two specific optical path lengths related to light ray  102  are substantially equal. The first optical path length is from the light beam point of origin in the laser beam source  74  to the pre-scan mirror  84 , and then on to the torsion oscillator mirrored surface  86 , and then on through first f-theta lens F 1   92  and second f-theta lens F 2   94 , and then to Sensor A HSYNC mirror  124 , and then finally on to the HSYNC photo detection system  140 . The second (and equal) optical path length is from the beam point of origin in the laser beam source  74  to the pre-scan mirror  84 , and then on to the torsion oscillator mirrored surface  86 , and then on through first f-theta lens F 1   92  and second f-theta lens F 2   94 , virtually past Sensor A HSYNC mirror  124  to position A′  126  at the image plane  116 . The second optical path length is a “virtual” optical path length because light ray  102  is intercepted by the A HSYNC mirror  124  before it reaches image plane  116 . Having these two optical path lengths substantially equal takes advantage of using the focusing function of first f-theta lens F 1   92  and second f-theta lens F 2   94  (that focuses light rays onto image plane  116 ) to also cause light rays that are intercepted by the A HSYNC mirror  124  to focus onto the photo detector in the HSYNC photo detection system  140 . 
   As the torsion oscillator mirrored surface  86  continues to oscillate light rays are directed beyond position A′  126  toward maximum scan position A  120 , until a light ray, shown as light ray  104  (labeled on its virtual extension) reaches virtual position A  120 . 
   Torsion oscillator mirrored surface  86  then reverses direction and begins to direct light rays toward maximum position B  122 . In bi-directional embodiments such as this the information encoded by the printer or scanner has to be constructed in forwards and backwards formats as the bi-directional scanning occurs. Light rays  106  and  108  are shown as scanning approaches maximum position B  122 . When the scan angle reaches position B′  128 , as represented by light ray  110  (labeled on its virtual extension for clarity), the light ray ( 110 ) is intercepted by the B HSYNC mirror  132 . Light ray  110  is an example of a synchronizing light ray. Thus it is seen that although torsion oscillator mirrored surface  86  sweeps light rays through a maximum scan angle  121 , only light rays of the scanning beam that are within recording scan angle  127  actually reach image plane  116 . 
   Referring to  FIG. 3 , light ray  110  (again labeled on its virtual extension for clarity) is reflected off of the B HSYNC mirror  132  as light ray  134  onto a B HSYNC folding mirror  136 . Note that folding mirror  136  is located in a depression  154  in the laser scanning system housing  72  such that folding mirror  136  is lower than the main laser beam sweep path  158  so that folding mirror  136  does not interfere with the main laser beam sweep path  158  exiting the F 2  lens  94  and traveling toward the image plane  116  (as seen in  FIG. 2 ). Achieving proper alignment requires that the B HSYNC mirror  132  be tilted at such an angle that the light ray  134  is reflected off of folding mirror  136  at an angle that is out of the plane of the main laser sweep path  158 . 
   Light ray  134  is reflected off of the B HSYNC folding mirror  136  as light ray  138  back up to the photo detection system  140  which incorporates a photo detector (not shown) mounted on the HSYNC board  142 . The B HSYNC folding mirror  136  is mounted at such an angle as to reflect the laser beam  138  back up to the HSYNC photo detection system  140 . Note that clearance features  154  and  156  are molded into the housing  72  to insure that the laser beams  134  and  138  are not clipped in any way as they arrive at and leave from folding mirror  136 . 
   To focus light ray  138  as it illuminates the photo detector in photo detection system  140 , Sensor B HSYNC mirror  132 , B HSYNC folding mirror  136 , and the photo detection system  140  are located such that two additional specific optical path lengths related to light ray  110  are substantially equal. The first optical path length is from the light beam point of origin in the laser beam source  74  to the pre-scan mirror  84 , and then on to the torsion oscillator mirrored surface  86 , and then on through first f-theta lens F 1   92  and second f-theta lens F 2   94  to B HSYNC mirror  132  and on to B HSYNC folding mirror  136  to the HSYNC photo detection system  140 . The second (and equal) optical path length is from the beam point of origin in the laser beam source  74  to the pre-scan mirror  84 , and then on to the torsion oscillator mirrored surface  86 , and then on through first f-theta lens F 1   92  and second f-theta lens F 2   94 , virtually past Sensor B HSYNC mirror  132  to position B′  128  at the image plane  116 . The second optical path length is also a “virtual” optical path length because light ray  110  is intercepted by the B HSYNC mirror  132  before it reaches image plane  116 . Having these two optical path lengths substantially equal again takes advantage of using the focusing function of first f-theta lens F 1   92  and second f-theta lens F 2   94  (that focuses light rays onto image plane  116 ) to also cause light rays that are intercepted by the B HSYNC mirror  132  to focus onto the photo detector in the HSYNC photo detection system  140 . 
   In alternate embodiments the focusing of the laser beam onto the photo detector of photo detection system  140  may be accomplished at least in part by methods not related to maintaining substantially equal optical path lengths. Instead, for example, again referring to  FIGS. 2 and 3 , the focusing of the laser beam onto the photo detector of photo detection system  140  may be accomplished by establishing an optical step function in the first f-theta lens  92  or the second f-theta lens  94 , or an optical step function in both the first f-theta lens  92  and the second f-theta lens  94 . The optical step function operates such that as the scan approaches position A′  126  from the direction of position B′  128 , such as depicted by light rays  96 ,  98 , and  100 , the first f-theta lens  92  or the second f-theta lens  94  or both first f-theta lens  92  and second f-theta lens  94  operate as a focusing lens system to help focus the light rays onto the image plane  116 . However, when the scan reaches position A′  126  an optical step function built into the f-theta lens systems operates such that the light ray  102  is focused on the photo detector in photo detection system  140  without necessarily having the synchronizing path length equal to the virtual path length to the image plane  116 . Similarly in such embodiments, as the scan approaches position B′  128  from the direction of position A′  126 , such as depicted by light rays  106  and  108 , the first f-theta lens  92  or the second f-theta lens  94  or both first f-theta lens  92  and second f-theta lens  94  operate as a focusing lens system to focus the light rays on the image plane  116 . However, when the scan reaches position B′  128  an optical step function built into the f-theta lens systems operate such that the light ray  110  is focused on the photo detector in photo detection system  140  without necessarily having the synchronizing path length equal to the virtual path length to the image plane  116 . In such embodiments the focusing lens system is referred to as a synch focusing lens. 
   In other embodiments involving a synch focusing lens, the focusing of the laser beam onto the photo detector is accomplished by optically configuring the A HSYNC mirror  124 , the B HSYNC mirror  132 , the B HSYNC folding mirror  136 , or combinations thereof, to converge or diverge light ray  102  or light ray  110 , or both light ray  102  and light ray  110  of the scanning beam such that light ray  102  and light ray  110  are focused on the photo detector in photo detection system  140  without necessarily having both synchronizing path lengths equal to their comparable virtual path lengths to the image plane  116 . In such embodiments A HSYNC mirror  124 , the B HSYNC mirror  132 , or the B HSYNC folding mirror  136  is referred to as a synch focusing lens. 
   In yet other embodiments, the focusing of the laser beam onto the photo detector is accomplished by adding a focusing lens somewhere in the path of a synchronizing light ray directed to a photo detector. Such a focusing lens would preferably take into account the length of the beam path from the beam point of origin to the focusing lens. For example, a focusing lens may be added in front of the photo detection system  140  to focus either light ray  102  or light ray  110  onto the photo detector in the photo detection system  140 . This type of synch focusing lens is discussed in more detail later and illustrated in  FIG. 5 . 
   Thus it is seen that these embodiments provide a method of implementing a single HSYNC sensor to detect light rays at both the A′  126  position and the B′  128  position. This embodiment allows for a very compact, yet robust, laser scanning system with the added benefit of the cost reduction that accompanies a reduction in the number of HSYNC mirror and photo detection system components. 
   The depiction in  FIGS. 2 and 3  of maximum position A  120  as being on the right side of laser scanning unit  70  (when looking from torsion oscillator mirrored surface  86  toward image plane  116 ), and the depiction of maximum position B  122  as being on the left side right side of laser scanning unit  70  (when looking from torsion oscillator mirrored surface  86  toward image plane  116 ), is arbitrary. That is, maximum position A  120  and associated position A′  126  could be on the left, and maximum position B  122  and associated position B′  128  could be on the right, and the same principles of operation would apply. This feature of alternate left and right orientations is similarly applicable to alternate embodiments also described elsewhere herein. In addition, in some embodiments position A′  126  and maximum position A  120  may be at the same locus. Position B′  128  may be at the same locus as maximum position B  122 . In such embodiments, scanning would stop at position A′  126  and position B′  128 . 
     FIG. 4  illustrates an alternate embodiment also utilizing a single photo detection system. A beam source  160  having a light beam point of origin  161  transmits a light beam  162  to a pre-scan mirror  164 . In various alternate implementations, light beam  162  may be a laser, or a collimated light beam, or a focused light beam. In the embodiment illustrated, pre-scan mirror  164  reflects substantially all of light beam  162  to scanning mirror  168  as light beam  166 . Scanning mirror  168  may, for example, be a Micro Electro-Mechanical System (MEMS) device, or a rotating polygonal mirror. Scanning mirror  168  directs the light beam as a series of light rays (segments of the light beam, as illustrated by light rays  170 ,  172 ,  182 ,  190 ,  192 , and  202 ), through lens  169  toward image plane  116  as a scanning beam between a maximum position A  184  and a maximum position B  204 . While graphically depicted as a single transmissive device in  FIG. 4 , in alternate embodiments the lens  169  may be a compound lens, or a reflective optical device, or other refraction-type device. 
   Light ray  170  is shown as scanning approaches maximum position A  184 . When the scan angle reaches position A′  176 , as represented by light ray  172 , the light ray ( 172 ) is intercepted by the A HSYNC mirror  174  and is reflected as light ray  178  to the HSYNC photo detection system  180  having a photo detector. This signals the end of transmission of light rays onto image plane  116  for this horizontal scan. When the scan angle reaches maximum position A  184  as represented by light ray  182 , the A HSYNC mirror  174  reflects light ray  182  as light ray  186  which, in preferred embodiments, at least partially misses the photo detector in HSYNC photo detection system  180  (in the case depicted here, misses the entire photo detection system) such that the intensity of light that reaches the photo detector in the HSYNC photo detection system  180  is less than the minimum light ray threshold detection level of the photo detector. In preferred embodiments, lens  169  is a focusing lens that focuses the light rays on image plane  116 . In the most preferred embodiments, the optical path length from the beam point of origin  161  to pre-scan mirror  164  to scanning mirror  168  along the path of light ray  172  to A HSYCH mirror  174  and on to photo detection system  180  along the path of light ray  178  is equal to the virtual optical path length from the beam point of origin  161  to pre-scan mirror  164  to scanning mirror  168  along the path of light ray  172  to A HSYCH mirror  174  and virtually on to image plane  116  along the path of virtual light ray  210 . When lens  169  is a focusing lens, and these two optical path lengths are substantially equal, focusing lens  169  causes light ray  172  that has been intercepted by the A HSYNC mirror  174  to focus onto the photo detector in the HSYNC photo detection system  180 . 
   Continuing with  FIG. 4 , as the scanning mirror  168  continues to sweep light rays toward image plane  116 , light rays are directed beyond position A′  176  toward maximum scan position A  184 . Then, if scanning mirror  168  is a uni-directional scanner scanning in the direction B to A, transmission of light rays in the general direction of maximum position A  184  will then cease, and scanning mirror  168  will start to direct the light rays in the direction of maximum position B  204 , as represented by light ray  202 . Light ray  202  strikes B HSYNC mirror  194  and is reflected toward HSYNC photo detection system  180 , but in preferred embodiments, at least partially misses the photo detector in photo detection system (as illustrated by light ray  206  which illustrated here misses the entire photo detection system  180 ) such that the intensity of light that reaches the photo detector in the HSYNC photo detection system  180  is less than the minimum light ray threshold detection level of the photo detector. Scanning mirror  168  then continues to direct light rays in a scanning direction that is moving toward position B′  196 . When a light ray, such as light ray  192  reaches position B′  196 , it is reflected by B HSYNC mirror  194  as light ray  198  to HSYNC photo detection system  180 . This signals the start of scanning onto image plane  116  for the next scan line. Scanning proceeds as scanning mirror  168  sweeps light rays to position A′  176  which signals the end of the scan line, and the repeat of the process starting at A′  176  which was previously described. 
   If scanning mirror  168  is a bi-directional scanner, after light rays are directed toward maximum scan position A  184 , scanning mirror  168  then reverses direction and begins to sweep the light rays toward maximum position B  204 . In bi-directional embodiments such as this the information encoded by the printer or scanner has to be constructed in forwards and backwards formats as the bi-directional scanning occurs. Light ray  190  is shown as scanning approaches maximum position B  204 . When the scan angle reaches position B′  196 , as represented by light ray  192 , the light ray ( 192 ) is intercepted by the B HSYNC mirror  194  and reflected as light ray  198  to the HSYNC photo detection system  180  having a photo detector. This signals the end of transmission of light rays onto image plane  116  for this horizontal scan. When the scan angle reaches maximum position B  204  as represented by light ray  202 , the B HSYNC mirror  194  reflects light ray  202  as light ray  206  which, in preferred embodiments, at least partially misses the HSYNC photo detection system  180  such that the intensity of light that reaches the photo detector in the HSYNC photo detection system  180  is less than the minimum light ray threshold detection level of the photo detector. In preferred embodiments, lens  169  is a focusing lens that focuses the light rays as a scanning beam on image plane  116 . In the most preferred embodiments, the optical path length from the beam point of origin  161  to pre-scan mirror  164  to scanning mirror  168  along the path of light ray  192  to B HSYCH mirror  194  and on to photo detection system  180  along the path of light ray  198  is equal to the virtual optical path length from the beam point of origin  161  to pre-scan mirror  164  to scanning mirror  168  along the path of light ray  192  to B HSYCH mirror  194  and virtually on to image plane  116  along the path of virtual light ray  212 . When lens  169  is a focusing lens, and these two optical path lengths are substantially equal, focusing lens  169  causes light ray  192  that has been intercepted by the B HSYNC mirror  194  to focus onto the photo detector in the HSYNC photo detection system  180 . 
   In alternate embodiments the focusing of the laser beam onto the photo detector of photo detection system  180  may be accomplished at least in part by methods not related to maintaining substantially equal optical path lengths. Instead, for example, again referring to  FIG. 4 , the focusing of the laser beam onto the photo detector of photo detection system  180  may be accomplished by establishing an optical step function in lens  169 . The optical step function operates such that as the scan approaches position A′  176  from the direction of position B′  196 , such as depicted by light ray  170 , lens  169  operates as a focusing lens to focus the light rays on the image plane  116 . However, when the scan reaches position A′  176  an optical step function built into lens  169  operates such that the light ray  172  (when reflected by A HSYNC mirror  174  as light ray  178 ) is focused on the photo detector in photo detection system  180  without necessarily having the synchronizing path length equal to the virtual path length to the image plane  116 . Similarly, as the scan approaches position B′  196  from the direction of position A′  176 , such as depicted by light ray  190 , the lens  169  operates as a focusing lens to focus the light rays on the image plane  116 . However, when the scan reaches position B′  196  an optical step function built into lens  169  operates such that the light ray  192  (when reflected by B HSYNC mirror  194  as light ray  198 ) is focused on the photo detector in photo detection system  180  without necessarily having the synchronizing path length equal to the virtual path length to the image plane  116 . In such embodiments lens  169  is referred to a synch focusing lens. 
   In other embodiments, the focusing of the laser beam onto the photo detector is accomplished by optically configuring the A HSYNC mirror  174  or the B HSYNC mirror  194 , or both, to converge or diverge light ray  172  or light ray  192  respectively, or both light ray  172  and light ray  192  respectively, which are considered examples of synchronizing light rays, such that light ray  172  and light ray  192  are focused on the photo detector in photo detection system  180  without necessarily having both synchronizing path lengths equal to their comparable virtual path lengths to the image plane  116 . In such embodiments (where optically configured to converge or diverge a light ray)A HSYNC mirror  174  or the B HSYNC mirror  194  is (are) referred to a synch focusing lens. 
   In yet other embodiments, the focusing of the laser beam onto the photo detector is accomplished by adding an additional synch focusing lens somewhere in the path of a synchronizing light ray directed to the photo detector. Such a synch focusing lens would preferably take into account the length of the beam path from the beam point of origin to the focusing lens. For example, a synch focusing lens could be added just in front of the photo detection system  180  to focus the synchronizing light rays onto the photo detector in the photo detection system  180 . 
   Finally, with reference to  FIG. 4 , in some embodiments position A′  176  and maximum position A  174  may be at the same locus. Position B′  196  may be at the same locus as maximum position B  194 . In such embodiments, scanning would stop at position A′  176  and position B′  196 . 
     FIG. 5  illustrates details of some embodiments of a photo detection system  220 . A housing  222  supports a circuit board  224  upon which is mounted photo detector  226 . Also mounted on circuit board  224  are electronic components  228  and  230  which form an electronic circuit that generates an electronic signal when the photo detection system detects a light ray that is at least equal to the minimum light ray threshold detection level of the photo detector. A first synchronizing light ray  232 , shown in expanded scale to illustrate its focus, is focused on photo detector  226 . A second synchronizing light ray  234 , also shown in expanded scale to illustrate its focus, would not be focused on photo detector  226  except that synch focusing lens  236  intercepts synchronizing light ray  234  and focuses it on photo detector  226 . In some embodiments, multiple photo detectors may be applied to form a photo detection system, and each photo detectors may have its own electronic circuit or several photo detectors may utilize at least in part a common electronic circuit to generate an electronic signal when a photo detector detects a light ray that is at least equal to the minimum light ray threshold detection level of the photo detector. When multiple photo detectors are used as a photo detection system, the lowest minimum light ray threshold detection level among the multiple photo detectors is referred to as the minimum light ray threshold detection level of the photo detection system. 
     FIG. 6  illustrates an alternate embodiment utilizing two photo detection systems. A beam source  160  having a light beam point of origin  161  transmits a light beam  162  to a pre-scan mirror  164 . In various alternate implementations, light beam  162  may be a laser, or a collimated light beam, or a focused light beam. Pre-scan mirror  164  reflects substantially all of light beam  162  to scanning mirror  168  as light beam  166 . Scanning mirror  168  may, for example, be a Micro Electro-Mechanical System (MEMS) device, or a rotating polygonal mirror. Scanning mirror  168  directs light rays (segments of the light beam, represented by light rays  171 ,  173 ,  183 ,  191 ,  193 , and  203 ) through lens  169  toward image plane  116  as a scanning beam from a maximum position A  185  to a maximum position B  205 . As in  FIG. 4 , for clarity of illustration light rays are shown following a straight path through lens  169  in  FIG. 6 , but in practice light rays would typically bend as they pass through lens  169 . 
   Light ray  171  is shown as scanning approaches maximum position A  185 . When the scan angle reaches position A′  177 , as represented by light ray  173 , the light ray ( 173 ) is intercepted by the A HSYNC mirror  175  and is reflected as light ray  179  to the A HSYNC photo detection system  181  having a photo detector. This signals the end of transmission of light rays onto image plane  116  for this horizontal scan. When the scan angle reaches position A  185  as represented by light ray  183 , the A HSYNC mirror  175  reflects light ray  183  as light ray  187  which, in preferred embodiments, at least partially misses the photo detector in A HSYNC photo detection system  181  (as illustrated here misses the entire photo detection system  181 ) such that the intensity of light that reaches the photo detector in the A HSYNC photo detection system  181  is less than the minimum light ray threshold detection level of the photo detector. In preferred embodiments, lens  169  is a focusing lens that focuses the light rays as a scanning beam on image plane  116 . In the most preferred embodiments, the optical path length from the beam point of origin  161  to pre-scan mirror  164  to scanning mirror  168  along the path of light ray  173  to A HSYCH mirror  175  and on to photo detection system  181  along the path of light ray  179  is equal to the virtual optical path length from the beam point of origin  161  to pre-scan mirror  164  to scanning mirror  168  along the path of light ray  173  to A HSYCH mirror  175  and virtually on to image plane  116  along the path of virtual light ray  211 . When lens  169  is a focusing lens, and these two optical path lengths are substantially equal, focusing lens  169  causes light ray  173  that has been intercepted by the A HSYNC mirror  175  to focus onto the photo detector in the HSYNC photo detection system  181 . 
   As the scanning mirror  168  continues to sweep light rays toward image plane  116 , light rays are directed beyond position A′  177  toward maximum scan position A  185 . Then, if scanning mirror  168  is a uni-directional scanner, transmission of light rays in the general direction of maximum position A  185  will then cease, and scanning mirror  168  will start to direct the light rays in the general direction of maximum position B  205 , as represented by light ray  203 . Light ray  203  strikes B HSYNC mirror  195  and is reflected toward HSYNC photo detection system  180 , but in preferred embodiments, at least partially misses the photo detector in the HSYNC photo detection system  180  (as illustrated by light ray  207  which misses the entire photo detection system  180 ) such that the intensity of light that reaches the photo detector in the HSYNC photo detection system  180  is less than the minimum light ray threshold detection level of the photo detector. Scanning mirror  168  then continues to direct light rays in a scanning direction that that is moving toward position B′  197 . When a light ray, such as light ray  193  shown in  FIG. 6  reaches position B′  197 , it is reflected by B HSYNC mirror  195  to B HSYNC photo detection system  201 . This signals the start of scanning onto image plane  116  for the next scan line. Scanning proceeds as scanning mirror  168  sweeps light rays to position A′  177  which signals the end of the scan line, and the repeat of the process starting at A′  177  which was previously described. 
   If scanning mirror  168  is a bi-directional scanner, after light rays are directed toward maximum scan position A  185 , scanning mirror  168  then reverses direction and begins to sweep the light rays to maximum position B  205 . In bi-directional embodiments such as this the information encoded in the printer or scanner has to be constructed in forwards and backwards formats as the bi-directional scanning occurs. Light ray  191  is shown as scanning approaches maximum position B  205 . However, when the scan angle reaches position B′  197 , as represented by light ray  193 , the light ray ( 193 ) is intercepted by the B HSYNC mirror  195  and reflected as light ray  199  to the B HSYNC photo detection system  201  having a photo detector. This signals the end of transmission of light rays onto image plane  116  for this horizontal scan. When the scan angle reaches position B  205  as represented by light ray  203 , the B HSYNC mirror  195  reflects light ray  203  as light ray  207  which, in preferred embodiments, at least partially misses the B HSYNC photo detection system  201  such that the intensity of light that reaches the photo detector in the B HSYNC photo detection system  201  is less than the minimum light ray threshold detection level of the photo detector. In preferred embodiments, lens  169  is a focusing lens that focuses the light rays as a scanning beam on image plane  116 . In the most preferred embodiments, the optical path length from the beam point of origin  161  to pre-scan mirror  164  to scanning mirror  168  along the path of light ray  193  to B HSYCH mirror  195  and on to photo detection system  201  along the path of light ray  199  is equal to the virtual optical path length from the beam point of origin  161  to pre-scan mirror  164  to scanning mirror  168  along the path of light ray  193  to B HSYCH mirror  195  and virtually on to image plane  116  along the path of virtual light ray  213 . When lens  169  is a focusing lens, and these two optical path lengths are substantially equal, focusing lens  169  causes light ray  193  that has been intercepted by the B HSYNC mirror  195  to focus onto the photo detector in the B HSYNC photo detection system  201 . 
   In alternate embodiments the focusing of the laser beam onto the photo detector of photo detection system  181  or  201  may be accomplished at least in part by methods not related to maintaining substantially equal optical path lengths. Instead, for example, again referring to  FIG. 6 , the focusing of the laser beam onto the photo detector of photo detection system  181  may be accomplished by establishing an optical step function in lens  169 . The optical step function operates such that as the scan approaches position A′  177  from the direction of position B′  197 , such as depicted by light ray  171 , lens  169  operates as a focusing lens to focus the light rays on the image plane  116 . However, when the scan reaches position A′  177  an optical step function built into lens  169  operates such that the light ray  173  reflected a light ray  179  is focused on the photo detector in photo detection system  181  without necessarily having the synchronizing path length equal to the virtual path length to the image plane  116 . Similarly, as the scan approaches position B′  197  from the direction of position A′  177 , such as depicted by light ray  191 , the lens  169  operates as a focusing lens to focus the light rays on the image plane  116 . However, when the scan reaches position B′  197  an optical step function built into lens  169  operates such that the light ray  193  reflected as light ray  199  is focused on the photo detector in photo detection system  201  without necessarily having the synchronizing path length equal to the virtual path length to the image plane  116 . In such embodiments, lens  169  is referred to a synch focusing lens. 
   In other embodiments, the focusing of the laser beam onto the photo detector is accomplished by optically configuring the A HSYNC mirror  175  or the B HSYNC mirror  195 , or both, to converge or diverge light ray  173  or light ray  193  respectively, or both light ray  173  and light ray  193  respectively, which are considered examples of synchronizing light rays, such that light ray  172  reflected as light ray  173  and light ray  192  reflected as light ray  193  are focused on the photo detectors in photo detection systems  181  and  201  respectively without necessarily having both synchronizing path lengths equal to their comparable virtual path lengths to the image plane  116 . In such embodiments A HSYNC mirror  175  or the B HSYNC mirror  195  is referred to a synch focusing lens. 
   In yet other embodiments, the focusing of the laser beam onto the photo detector is accomplished by adding a focusing lenses somewhere in the path of a synchronizing light ray directed to a photo detector. Such a focusing lens would preferably take into account the length of the beam path from the beam point of origin to the focusing lens. For example, a focusing lens could be added just in front of the photo detection system  181  to focus the synchronizing light rays onto the photo detector in the photo detection system  181  and a second focusing lens could be added just in front of the photo detection system  201  to focus the synchronizing light rays onto the photo detector in the photo detection system  201 . 
   Finally, with reference to  FIG. 6 , in some embodiments position A′  177  and maximum position A  185  may be at the same locus. Position B′  197  may be at the same locus as maximum position B  205 . In such embodiments, scanning would stop at position A′  177  and position B′  197 . 
   In designing an effective scanning unit it is constructive to ensure that the minimum light sensitivity of the photo detector system, the illumination power of the light source, and the reflectivity coefficient of each mirror surface are compatible such that under worst-case conditions a ray of light that reaches the photo detection system for the purpose of detection is of sufficient power to trigger the photo detector sensor in the photo detection system. Scanning synchronization also requires the photo detector in a photo detection system be connected to an electronic circuit that generates an electronic signal when the photo detection system detects a light ray that is at least equal to the minimum light ray threshold detection level of the photo detection system. 
   The foregoing description of preferred embodiments for this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.