Abstract:
A miniature optical scanner includes an electromagnetic drive having stationary magnets and stationary drive coils to minimize the rotational inertia of the scanner and increase the scanner&#39;s resonant frequency. The scanner is such that the resonant frequency is manually tunable as well as automatically adjustable to compensate for variables causing frequency drift. The optical scan angle is increased by employing a multiplying mirror with the optical scanner. For a two axis scanning system, the multiplying mirror may be formed of a second optical scanner to increase the optical scan angle relative to both of the axes.

Description:
This application is a divisional of U.S. patent application Ser. No. 08/664,103 filed on Jun. 13, 1996, U.S. Pat. No. 5,751,465, which is a divisional of U.S. patent application Ser. No. 08/329,508 filed Oct. 26, 1994, now U.S. Pat. No. 5,557,444 issued on Sep. 17, 1996. 
    
    
     TECHNICAL FIELD 
     The present invention is directed to a miniature optical scanner for a one or two axis scanning system and more particularly to a miniature optical scanner having stationary magnets and stationary drive coils to raise the resonant frequency of the system, the resonant frequency being manually tunable and automatically adjustable to compensate for variables causing frequency drift. The scanning system includes a multiplying mirror to substantially increase the optical scan angle of the optical scanner. Further, a second optical scanner may be used as the multiplying mirror to provide a two axis scan with substantially increased optical scan angles for both axes. 
     BACKGROUND OF THE INVENTION 
     Optical resonant scanners of various types are known but are in general not suitable for use in applications such as a head mounted display system that requires the scanners to be very small so that they may be comfortably supported on a user&#39;s head. Scanners used in such systems must also be operable at a high frequency of resonance such as 20 khz. One type of known optical resonant scanner includes moving magnets as part of an electro-magnetic circuit for oscillating a scanning mirror. However, because the magnets move, these scanners have a higher rotational inertia than desirable making it difficult to attain a sufficiently high resonant frequency for many applications. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the disadvantages of prior optical resonant scanners and multiple axis scanning systems have been overcome. The optical scanner of the present invention is a miniature optical resonant scanner capable of operating at very high frequencies. This miniature optical scanner is used with a multiplying mirror to provide a scanning system with an output optical scan angle that is substantially larger than the optical scan angle of the optical scanner itself. Further, a second optical scanner may be used as the multiplying mirror to provide a two axis scan with substantially increased optical scan angles for both axes while minimizing the size and weight of the overall system. The optical scanner and optical scanning system of the present invention have numerous uses such as in head mounted display systems, video displays in general, cameras, bar code readers, ophthalmological devices, etc. 
     More particularly, the miniature optical scanner of the present invention includes a ferromagnetic base with first and second stator poles formed thereon. The first and second stator poles are generally parallel to each other. Each stator pole includes a respective stator coil wound thereabout to produce magnetic fields in the stators of opposite polarity in response to a drive signal applied to each coil. First and second magnets are disposed on the base of the scanner on opposite sides of the first and second stators such that the magnets are equidistant to the first and second stators. The optical scanner also includes a thin metal spring plate having first and second support portions that rest respectively on the first and second magnets. The spring plate also includes a centrally located mirror mounting portion disposed above the stators, the mirror mounting portion having an axis of rotation that is equidistant to the stator poles. A mirror is formed on the mirror mounting portion of the spring plate so that when an alternating drive signal is applied to the first and second stator coils, magnetic fields are created that oscillate the mirror about the axis of rotation of the spring plate&#39;s mirror mounting portion. This oscillating mirror scans light incident thereto relative to a first axis. 
     This optical scanner may be made very small, having dimensions, for example, that are less than two centimeters in diameter or width and length, and less than one centimeter in height. Further, because the mass of the moving parts in the scanner is minimized and is as close to the axis of rotation as possible, the rotational inertia of the scanner is minimized. Thus, the optical scanner of the present invention has a high resonant frequency such as on the order of 24 khz. 
     The optical scanner of the present invention includes means for manually adjusting the resonant frequency of the scanner. Further, the scanner also includes means for automatically varying the amount of tension in the spring plate to maintain the resonant frequency constant. Thus, the tension in the spring plate can be automatically adjusted to accommodate for variations in temperature and other factors which if not compensated for would alter the resonant frequency of the scanner. 
     In accordance with another feature of the present invention the optical scanner may be disposed in a housing forming a vacuum chamber so as to minimize problems with loss of energy, air turbulence and noise when operating at high speeds. The housing may also be formed with a scanning window to allow light to enter the housing and be reflected out from the housing by the scanning mirror. The scanning window is disposed in the housing at an angle with respect to the mirror when the mirror is in its rest position to minimize the effect of reflections from the housing window. 
     These and other objects, advantages and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and from the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view of a first embodiment of a miniature optical resonant scanner in accordance with the present invention; 
     FIG. 2 is an exploded perspective view of the optical scanner of FIG. 1; 
     FIG. 3 is a crosssectional view of a miniature optical resonant scanner of a second embodiment of the present invention; 
     FIG. 4 is a partial exploded perspective view of the optical scanner of FIG. 3; 
     FIG. 5 is a diagrammatic illustration of the angular relation between the scanning mirror and housing window of the scanner of FIG. 3; 
     FIG. 6 is a top view of the spring plate of the scanner of FIG. 3; 
     FIG. 6A is a diagram illustrating a spring plate tension adjuster. 
     FIG. 7 is a diagram illustrating a single axis scanning system with a multiplying mirror; 
     FIG. 8 is a second embodiment of the scanning system in accordance with the present invention for a single axis; 
     FIG. 9 is a diagram illustrating series scanners; 
     FIG. 10 is a diagram illustrating a second optical scanner used as the multiplying mirror to provide a two axis scanning system; 
     FIG. 11 is a perspective view of a stator-base insert mounted on the base of the optical scanner of FIG. 1; and 
     FIG. 12 is a perspective view of the stator-base insert of FIG. 11 with the stator coils mounted thereon. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The miniature optical resonant scanner  10  of the present invention as shown in FIGS. 1-2 includes a mirror  12 . The mirror  12  is driven by a magnetic circuit to oscillate at a high frequency about an axis of rotation  14  where the only moving parts are the mirror  12  and a spring plate  16  which may be integrally formed. 
     The optical scanner  10  includes a base plate  17  with a pair of stator posts  18 ,  20  centrally located thereon. The base plate  17  and stator posts  18 ,  20  are integrally formed in one piece of a soft iron. Stator coils  22  and  24  are wound in opposite directions about the respective stator posts  18  and  20 . The electrical coil windings  22  and  24  may be connected in series or in parallel to a drive circuit as discussed below. Mounted on opposite ends of the base plate  17  are first and second magnets  26  and  28 , the magnets  26 ,  28  being equidistant from the stators  18  and  20 . In order to locate the magnet  26 , the base  17  is formed with a seat  30  having a back stop  32  extending up from one end of the seat  30  and having a front stop  34  extending up from the seat at the opposite end thereof. Similarly, to locate the magnet  28 , the base  17  is formed with a seat  36  at the end of the base opposite the seat  30 . The seat  36  includes a back stop  38  and a front stop  40  that extend upwardly from the seat  36  at the back and front thereof. 
     The spring plate  16  is formed of spring steel and is a torsional type of spring having a spring constant determined by its length and width. The spring plate  16  has enlarged opposite ends  42  and  44  that rest directly on a pole of the respective magnets  26  and  28 . The magnets  26  and  28  are oriented such that they have like poles adjacent the spring plate. For example, the North poles of the magnet  26  and  28  could be adjacent the spring plate  16  with the South poles of the magnets  26  and  28  adjacent the base  17 . Alternatively the South poles of both magnets  26 ,  28  could be adjacent the spring plate with the North pole of the magnets  26 ,  28  adjacent the base  17 . A narrower arm portion  46 ,  48  of the spring plate  16  extends from each of the enlarged ends  42 ,  44  to an enlarged central mirror mounting portion  50  of the spring plate  16 . The mirror mounting portion  50  forms the armature of the optical resonant scanner  10  and is mounted directly over the stator posts  18  and  20  such that the axis of rotation  14  of the mirror mounting portion  50  is equidistant from the stator posts  18  and  20 . The mirror  12  is mounted on or coated on the mirror mounting portion  50  of the spring plate. 
     The spring plate  16 , magnets  26  and  28  and the base  17  are tightly clamped together by respective spring plate caps  52  and  58 . Each cap  52 ,  58  is formed as a block with openings  60  and  62 . The openings  60 ,  62  are formed so that the caps  52 ,  58  can accommodate the ends  42 ,  44  of the spring plate, the magnets  26 ,  28  and the seats  30 ,  36  as well as the arms  46  and  48  of the spring plate  16  when the caps  52 ,  58  are resting on the base  17 . The cap  52  is held securely to the base  17  by a pair of screws  54  and  56  so as to clamp the spring plate  16  and magnet  26  to the base. The screws  54  and  56  extend up through apertures  58  in the base  17  on opposite sides of the seat  30  and into threaded apertures formed in the cap  52  on opposite sides of the opening  60 . The cap  58  is similarly clamped to the base  17  by respective screws  61  and  63  that extend up through respective apertures  64  formed on opposite sides of the cap opening  62 . 
     Magnetic circuits are formed in the optical scanner  10  so as to oscillate the mirror  12  about the axis of rotation  14  in response to an alternating drive signal. One magnetic circuit extends from the top pole of the magnet  26  to the spring plate end  42 , through the arm  46  and mirror mounting portion  50  of the spring plate  16 , across a gap to the stator  18  and through the base  17  back to the magnet  26  through its bottom pole. Another magnetic circuit extends from the top pole of the magnet  28  to the spring plate end  44  through the arm  48  and mirror mounting portion  50  of the spring plate  16 , across a gap to the stator  18  and through the base  17  back to the magnet  28  through its bottom pole. Similarly, magnet circuits are set up through the stator  20  as follows. One magnetic circuit extends from the top pole of the magnet  26  to the spring plate end  42 , through the arm  46  and mirror mounting portion of the spring plate  16 , across the gap to the stator  20  and through the base  17  back to the magnet  26  through its bottom pole. Another magnetic circuit extends from the top pole of the magnet  28  to the spring plate end  44 , through the arm  48  and mirror mounting portion  50  of the spring plate  16 , across the gap to the stator  20  and then through the base  17  back to the magnet  28  through its bottom pole. 
     When a periodic drive signal such as a square wave is applied to the oppositely wound coils  22  and  24 , magnetic fields are created which cause the mirror  12  to oscillate back and forth about the axis of rotation  14 . More particularly, when the square wave is high for example, the magnetic field set up by the magnetic circuits through the stator  18  and magnets  26  and  28  cause an end  66  of the mirror mounting portion  50  to be attracted to the stator  18 . At the same time, the magnetic field created by the magnetic circuits extending through the stator  20  and the magnets  26  and  28  cause the opposite end  68  of the mirror mounting portion  50  to be repulsed by the stator  20 . Thus, the mirror is caused to rotate about the axis of rotation in one direction. When the square wave goes low, the magnetic field created by the stator  18  repulses the end  66  of the spring plate  50  whereas the stator  20  attracts the end  68  of the spring plate portion  50  so as to cause the mirror  12  to rotate about the axis  14  in the opposite direction. 
     It is noted that for high frequency operations the impedance of each coil  22 ,  24  must not become so large to effectively decrease the current flow therethrough. Therefore, as the frequency of operation increases the number of turns in each coil  22 ,  24  should be decreased. 
     Further, at high frequency operations although losses are not seen in the magnetic circuits through the magnets  26  and  28  described above, eddy current losses do exist in another magnetic circuit of the scanner. More particularly, eddy current losses exist in the magnetic circuit from the stator  18  to the base  17 ; through the base  17  to the stator  20 ; from the stator  20  across the gap to the spring plate  16 ; through the mirror mounting portion  50  of the spring plate  16 ; and across the gap back to the stator  18  with the magnetic flux induced from the coils also circulating through this circuit in the opposite direction as well. 
     The eddy current losses affecting this circuit increase with the square of the frequency. However, these losses are inversely proportional to the volume resistivity of the materials used to form the circuit. Therefore, by lowering the volume resistivity of at least the stators and base, the eddy current losses at high frequencies of operation can be reduced. The volume resistivity can be lowered, for example, by utilizing laminations of the material, such as soft iron, forming the base and stators  18 ,  20 ; by utilizing powdered iron pressed into the desired shape of the base and stators; or by utilizing ferrite to form the base and stators. These are just a few of the possible methods that can be used to reduce the volume resistivity. The entire base  17  and stators  18  and  20  can be manufactured in accordance with one of these methods. Alternatively, however, only that portion of the base  17  that forms part of the magnetic circuit between the stators as well as the stators themselves could be manufactured in accordance with one of these methods. This latter method can be achieved with a stator-base insert  65  shown in FIGS. 11 and 12. The stator-base insert  65  includes stators  67  and  69  that are integrally formed with a base portion  71  so as to have low volume resistivity. The base  71  of the insert  65  may rest directly on the base  17  of the scanner as shown in FIGS. 11 and 12. Alternatively, the base  17  of the scanner may be formed with an aperture therein to accept the base  71  of the insert  65  so that the base  71  of the insert is mounted flush with the base  17  of the scanner. 
     Another feature of the optical scanner  10  in accordance with the present invention is that the resonant frequency of the scanner  10  is tuneable after the scanner is manufactured. This is accomplished by a pair of frequency adjustment screws  70  and  72  that cooperate with a pair of rods  74  and  76 . The screws  70  and  72  can be adjusted so as to increase the tension in the spring plate  16  thereby increasing the resonant frequency of the optical scanner  10  or to decrease the tension in the spring plate  16  to thereby decrease the resonant frequency of the optical scanner  10 . 
     More particularly, the frequency adjustment screws  70  and  72  are screwed into threaded apertures  78  and  79  that extend through the body of the cap  52  on opposite sides of the opening  60 . The screws  70  and  72  also extend into apertures  80  and  82  in respective, enlarged ends  84  and  86  of the temperature compensation rods  74  and  76 , discussed in detail below and held in place by locknuts  73  and  75 . The opposite ends  88  and  89  of the rods  74  and  76  are inserted into non-threaded apertures  90  and  92  formed in the body of the cap  58  on opposite sides of the opening  62 . Enlarged sections  94  and  96  adjacent to the ends  88  and  89  of the rods  74  and  76  form stops to limit the length of the rods  74 ,  76  that extend into the apertures  90 ,  92  of the cap  58 . Tension in the spring plate  16  is lowered to decrease the resonant frequency of the optical scanner by unscrewing the frequency adjustment screws  70  and  72  farther out of the apertures  78  and  79 . By increasing the amount of the screws  70 ,  72  that is screwed into the apertures  78  and  79  so that the effective length of the rods between the caps  52  and  58  is increased, the screws and rods can exert a force on the caps  52  and  58  to push the caps  52  and  58  apart from each other. The caps  52  and  58  act on the ends  42  and  44  of the spring plate  16  so as to pull the ends  42  and  44  of the spring plate  16  apart. This force increases the tension in the spring plate to increase the resonant frequency thereof and thus increase the resonant frequency of the optical scanner  10  as a whole. 
     It is desirable to maintain the resonant frequency of the optical scanner  10  constant over an operating range of temperatures. As the temperature increases, the spring plate  16  expands resulting in an increase in the rotational inertia of the scanner  10 . This increase in the rotational inertia causes the resonant frequency of the scanner  10  to decrease. The rods  74  and  76  compensate for increases in temperature and the resulting decrease in the resonant frequency by increasing the tension in the spring plate  16  as the temperature increases. When the tension in the spring plate  16  is increased, the resonant frequency of the spring plate also increases. Thus the rods  74  and  76  compensate for temperature increases by increasing the tension in the spring plate  16  to maintain the resonant frequency of the scanner  10  relatively constant. 
     More particularly, in one embodiment of the rods  74  and  76 , a passive form of temperature compensation is employed. In the passive embodiment, a portion or all of the rods  74  and  76  are made of a material having a greater thermal expansion coefficient than that of the material out of which the spring plate  16  is formed. Thus, as the temperature increases, the rods  74  and  76  expand more than the spring plate  16  expands. The expansion of the rods  74  and  76  causes a force to be applied to the caps  52  and  58  to push the caps  52 ,  58  apart from each other. As the caps  52  and  58  are pushed apart, the caps act on the ends  42  and  44  of the spring plate  16  causing the ends  42  and  44  to move apart thereby increasing the tension in the arms  46  and  48  of the spring plate. This increase in tension in the spring plate  16  causes the resonant frequency to increase. Thus, as the temperature causes the spring plate to expand, lowering the resonant frequency, the temperature compensation rods  74  and  76  compensate for this decrease in frequency by applying a force to the spring plate  16  via the caps  52  and  58  so as to increase the tension in the spring plate and thus increase the resonant frequency so as to maintain it relatively constant over the operating range of temperatures of the optical scanner  10 . It is noted that the rods  74  and  76  may be made of one material or of a combination of a number of materials having different thermal expansion coefficients arranged in a series along the length of the rods  74  and  76  so as to obtain the desired expansion characteristics. 
     In another embodiment of the rods  74  and  76 , active temperature compensation is employed. For active temperature compensation, a piezoelectric material is utilized that is responsive to an electrical signal so as to vary its length. For example, each of the temperature compensation rods  74 ,  76  may be formed in two separate sections as opposed to being integrally formed as described above. In such an embodiment, the rod  74 , for example, may be split into two distinct parts so that the end  88  and enlarged portion  94  form one section and the mid-portion of the rod  98  and the enlarged end  84  form a second section of the rod. In this embodiment, the enlarged section  94  would be formed with an aperture therein to allow the end of the mid-section  98  to be inserted therein. Prior to insertion of the end of the mid-section  98  into the aperture of the enlarged section  94 , a piezoelectric element may be positioned in the aperture of the enlarged section  94 . A temperature sensor, not shown, may be used to sense the temperature of the scanner  10  and in particular the spring plate  16  so as to provide an electrical signal representative thereof. The piezoelectric element in the rod  74  would be responsive to an electrical signal indicating an increase in temperature to expand causing the mid-portion  98  of the rod to be pushed out so as to increase the effective length of the rod  74 . Thus, as the temperature is increased, the piezoelectric element causes the rod to increase in length to push apart caps  52  and  58 , increasing the tension in the spring plate as well as the resonant frequency to compensate for the temperature change. The above is just one example of an active temperature compensation rod. Many modifications can be made to the active compensation rod embodiment of the present invention. For example, a portion of the rod  74 ,  76  may be made of a piezoelectric material, obviating the need for a separate piezoelectric element. Non-electrical expansion elements may also be utilized. For example, mechanical means including a lead screw and motor may be used to adjust the frequency adjustment screws  70  and  72  to change the tension in the spring plate as discussed above wherein a controller that is responsive to increases in temperature would be employed to control the motor and the positioning of the frequency adjustment screws  70  and  72 . These are just a few examples of active and passive temperature compensation techniques for the optical scanner  10  in accordance with the present invention. 
     A second embodiment of  10 ′ the optical resonant scanner in accordance with the present invention is shown in FIGS. 3-6. The optical scanner  10 ′ includes a base  17 ′ that supports a pairs of stators  18 ′ and  20 ′ having coils  22 ′ and  24 ′ respectively wound thereabout. The base  17 ′ also supports a pair of magnets  26 ′ and  28 ′ positioned on opposite sides of the base and equidistant from stator posts  18 ′ and  20 ′. A spring plate  100  rests on the magnets  26 ′ and  28 ′. Four screws, only two of which  102  and  104  being shown, extend through apertures in respective clamp plates  101  and  103  and through respective apertures  106 - 109  in the spring plate  100  to screw into threaded apertures  110 - 113  formed in the base  17 ′. These screws clamp the spring plate  100  and magnets  26 ′ and  28 ′ to the base  17 ′ of the scanner  10 ′. 
     As shown in FIG. 6, the spring plate  100  has a generally circular outer periphery  120 . A pair of arms  122  and  124  extend between the spring plate supporting portions  126  and  128  of the outer periphery  120  and a mirror mounting portion  130 . As discussed above, the mirror  12 ′ may be coated onto the mirror mounting portion  130  of the spring plate  100  or a separately formed mirror may be bonded onto the mirror mounting portion  130 . The magnetic circuits set up in the optical scanner  10 ′ are essentially the same as described above for the optical scanner  10  such that the mirror  12 ′ is caused to oscillate about an axis of rotation  132  that extends through the arms  122  and  124  of the spring plate  100 . 
     In order to tune the optical scanner  10 ′, a gap  134  is formed in the outer periphery  120  of the spring plate  100 . As shown in FIG. 6A, a tuning screw  136  extends through the aperture of a bent washer  138  and through an enlarged aperture  140  of the spring plate  100 . The screw  136  also extends through a flat washer  142  and is held in place by a nut  144 . Opposite ends  146  and  148  of the bent washer extend into arcuate slots  152  formed in the outer periphery  120  of the spring plate  100  adjacent to opposite sides of the aperture  140 . As the screw  136  is screwed farther and farther into the nut  140 , the screw  136  exerts a force on the bent washer  138  tending to flatten it out. This causes the ends  146  and  148  of the washer to exert a force on the ends  154  and  156  of the spring plate  100  adjacent the gap  134 , pushing the ends  154  and  156  apart. As the ends  154  and  156  are pushed apart, the tension in the spring plate arms  122  and  124  increases so as to increase the resonant frequency of the spring plate  100  and thus the scanner  10 ′. Thus, the spring plate  100  has a resonant frequency that is tunable by adjusting the frequency adjustment screw  136  to increase or decrease the flatness of the bent washer  138 . It is noted that the spring plate  100  may also include an identification tag area  158  if desired. 
     The optical scanner of the present invention may be disposed in a housing that forms a vacuum chamber such as the housing  160  for the scanner  10 ′ shown in FIGS. 3 and 4. The use of a vacuum chamber housing minimizes the problems due to loss of energy, air turbulence and noise when operating at high frequencies. The housing  160  is generally cylindrical in shape having a sidewall  162  extending generally perpendicular to a housing base  164  which in turn is generally parallel to the base  17 ′ of the scanner  10 ′. The base  17 ′ of the optical scanner  10 ′ is supported on the housing base  164  by posts, two electrical posts  166  and  168  being shown, that extend through apertures  170  formed in the base  17 ′. The posts  166  and  168  extend through apertures  172  in the housing base  164  wherein an epoxy is filled into the apertures  172 . Silicone rubber is filled into the apertures  170  of the scanner base  17 ′ to secure the posts  166 ,  168  therein but to accommodate shearing forces which may be exerted thereon. An O-ring  176  is disposed in a channel  178  extending about the upper periphery of the housing base  164  wherein the O-ring seals the housing sidewall  162  to the base  164 . Further a vacuum port  180  is formed in the base  164  of the housing  160 . 
     The top wall  182  of the housing  160  is not parallel to the housing base  164  or to the scanner base  17 ′ but is disposed at an angle thereto. The top wall  182  is angled to support a window  184  formed therein at an angle with respect to the mirror  12 ′ when the mirror is in its rest or stationary position such that the mirror  12 ′ is parallel for example to the base  17 ′ as shown in FIG.  3 . The window  184  allows light to enter the housing  160  so that it can be reflected out of the housing  160  and scanned by the mirror  12 ′. By angling the window  184  with respect to the scanning mirror  12 ′ in its rest position, multiple reflections caused by the window  184  may be minimized. Preferably, the angle formed between the window  184  and the mirror  12 ′ in its rest position is greater than the optical scan angle of the mirror  12 ′, where the optical scan angle is twice the mechanical scan angle through which the mirror  12 ′ is rotated by the magnetic circuits described above. 
     The optical resonant scanners  10  and  10 ′ as described above can be made extremely small for those applications in which it is desirable to minimize the size of the scanner. For example, the housing  160  may have a diameter that is less than two centimeters and a height that is less than one centimeter. Further, because only the mirror and a portion of the spring plate move, the mass of the moving parts in the optical scanner  10 ,  10 ′ is minimized. Because the mass of the moving parts in the scanner is minimized and is as close to the axis of rotation as possible, the rotational inertia of the scanner is minimized. Thus the optical scanner of the present invention may be made with a high resonant frequency such as on the order of 24 khz or higher. 
     In accordance with another feature of the present invention, the optical scanner  10 ,  10 ′ may be used with a multiplying mirror so as to substantially increase the optical scan angle of the optical scanner. For example, as shown for the mirror  12  of the scanner  10  in FIG. 7, by employing a multiplying mirror  200  placed at a location with respect to the scanning mirror  12  such that incoming light  202  from a source first reflects off of the scanning mirror  12  onto the multiplying mirror  200  from which the light is reflected back onto the scanning mirror  12 , a doubling of the optical scan angle ψ off of the scanning mirror  12  is achieved. The positioning of the scanning mirror  12  and the multiplying mirror  200  as shown in FIG. 7 forms a retro-reflective scanning system such that the exit light beam  204  is reflected almost 360° back towards the source beam. In FIG. 7, a mechanical scan angle a is shown between the mirror  12  in a first position  206  shown by the solid line and the mirror  12  after it has been rotated to a second position  208  shown by the dotted line. By simple geometry it can be shown that the optical scan angle ψ is equal to twice the mechanical scan angle α and that the optical scan angle ψ is doubled by the retro-reflective scanning system of FIG.  7 . More particularly, the output optical scan angle β is equal to 2ψ, where β is the angle between the exit beam  204  which is the input beam  202  after it has been reflected twice off of the scanning mirror  12  in the first position  206  and a beam  210  which is the input beam  212 , from the same source as the input beam  202 , after the beam  212  has been reflected twice off of the scanning mirror  12  in the second position  208 . If the angles θ and φ between the mirrors  12  and  200  and the source beams  202  and  212  are changed such that the light beams reflect off of the scanning mirror  12  multiple times n, the optical scan angle is multiplied by the number of times n that the beam reflects off of the scanning mirror  12 . For example, if the position of the mirrors  200  and  12  and the angles θ and φ are such that the source beam is reflected off of the scanning mirror  12  four times, then the output optical scanning angle β will be four times the optical scan angle ψ. 
     FIG. 8 illustrates the positioning of the scanning mirror  12  and a multiplying mirror  220  so as to form a trans-reflective scanning system such that the exit beam is almost parallel and travels in generally the same direction as the incoming source beam  224  as the beam  224  first hits the scanning mirror  12 . Again in this embodiment, it can be seen that the output optical scan angle β in FIG. 8 is two times the optical scan angle ψ. Again by altering the angles θ and φ between the mirrors  12 ,  220  and the source beam  224  to increase the number of times that the source beam reflects off of the scanning mirror  12 , the output optical scan angle can be further increased over the optical scan angle ψ. 
     The use of a multiplying mirror  200 ,  222  that is positioned relative to a scanning mirror  12  such that a source beam reflects off of the same scanning mirror  12  a number of times has several advantages over using two scanners in series in order to increase the optical scan angle, this later method being depicted for the scanners  240 - 243  in FIG. 9 wherein the source beam is reflected off of each of the scanners only once. In particular, the embodiment of the present invention depicted in FIGS. 7 and 8 does not have problems caused by phase differences between the series scanning mirrors of the configuration depicted in FIG.  9 . Further, the embodiments of the invention depicted in FIGS. 7 and 8 require less power than the embodiment shown in FIG. 9 since only one of the mirrors is scanned. In addition, the size and weight of each of the scanning systems shown in FIGS. 7 and 8 is less than that of the series scanning system shown in FIG.  9 . 
     FIG. 10 illustrates another embodiment of a multiplying mirror scanning system for applications requiring the light beam to be scanned in two orthogonal directions. In this embodiment the multiplying mirror  250  is an optical scanner that scans the light in a direction orthogonal to the direction that the first scanning mirror  12  scans the light. For example, as shown in FIG. 10, the scanning mirror  12  oscillates about the x axis; whereas the multiplying scanning mirror  250  oscillates about the z axis. The output optical scan angle of the mirror  12  is doubled because the light beam reflects off the scanning mirror  12  twice using the multiplying scanning mirror  250  as discussed above for the retro-reflective type of system. However, the output optical scan angle of the multiplying scanning mirror  250  is not doubled since the light reflects only once off of the multiplying scanning mirror  250 . If the multiplying scanning mirror  250  and first scanning mirror  12  are set up as discussed above for optical scan angle multiplying that is greater than two times in a retro-reflective type system as depicted in FIG. 10 then the optical scan angle of the multiplying scanning mirror  250  can also be increased. For example, if the system is set up so that the light beam reflects off of the scanning mirror  12  four times and reflects off of the multiplying scanning mirror  250  three times in a retro-reflective type system, the output optical scan angle for the scanning mirror  12  will be four times the optical scan angle ψ and the output optical scan angle of the multiplying scanning mirror  250  will be three times the optical scan angle ψ′ where the ψ′ is equal to twice the mechanical scan angle through which the mirror  250  rotates. It is noted that a multiplying scanning mirror may also be utilized instead of the stationery mirror  220  in the trans-reflective type of system. In this embodiment the optical scan angle multiplier is the same as the number of reflections for both of the scanning mirrors. 
     Positioning a second axis optical scanner to form a multiplying mirror has several advantages. In particular, fewer mirrors are required to increase the optical scan angle for the multiplying scanning mirror for a second axis scan than in prior systems. Further, the overall size and weight of this two axis scanning system is minimized while providing large scan angles. 
     Many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as described hereinabove.