Patent Publication Number: US-2009237628-A1

Title: Optical reflection device and image projector includng the same

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optical reflection device and an image projector including the device. 
     BACKGROUND OF THE INVENTION 
       FIG. 16  is a perspective view of conventional optical reflection device  501 . Optical reflection device  501  includes mirror  201 , two first meander vibration beams  202  joined to respective ones of both ends of mirror  201 , movable frame  203  joined to first meander vibration beams  202 , second meander vibration beams  204  joined to respective ones of both ends of movable frame  203 , and supporter  205  supporting second meander vibration beam  204 . Movable frame  203  encloses first meander vibration beam  202  and mirror  201 . First meander vibration beams  202  swing about rotation axis  206 . Second meander vibration beams  204  swing about rotation axis  207 . Rotation axes  206  and  207  are perpendicular to each other. 
     First meander vibration beams  202  extend meanderingly along rotation axis  206 , and cause mirror  201  to rotate about rotation axis  206 . Second meander vibration beams  204  extend meanderingly along rotation axis  207 , and cause mirror  201  to rotate about rotation axis  207 . 
     Mirror  201  is supported at both ends thereof with first meander vibration beams  202  and has a both-end-supported structure. Movable frame  203  having the both-end-supported structure is supported at both ends thereof with second meander vibration beams  204 . 
     In optical reflection device  501 , swinging vibration beams  202  and  204  swing to cause mirror  201  rotates about rotation axes  206  and  207 . Light enters onto mirror  201  which rotates, and is reflected on mirror  201 , thereby moving and scanning a screen along an X-axis and a Y-axis so as to project an image, such as characters, on the screen. That is, when mirror  201  rotates about rotation axis  206 , the reflected light moves and scans along the X-axis on the screen. When mirror  201  rotates about rotation axis  207 , the reflected light moves and scans along the Y-axis on the screen. 
     In order to project the image on the screen, the reflected light generally scans along the X-axis plural times while scanning along the Y-axis once. That is, the scanning frequency along the X-axis is higher than that along the Y-axis. 
     In order to make the scanning frequency along the X-axis higher than that along the Y-axis, the meander length of first meander vibration beam  202  is determined to be adequately shorter than that of second meander vibration beam  204 . However, a smaller meander length of first meander vibration beam  202  decreases the angle by which mirror  201  rotates about rotation axis  206 , and decreases the scanning length along the X-axis. 
     In order to project a precise image having a high resolution, the ratio of the vibration frequency of first meander vibration beam  202  to that of second meander vibration beam  204  is required to be large. In order to increase this ratio, second meander vibration beam  204  is required to be long, accordingly increasing the size of optical reflection device  501 . 
     SUMMARY OF THE INVENTION 
     An optical reflection device includes a mirror adapted to reflect light thereon, a first meander vibration beam supporting the mirror rotatably about the first rotation axis, a movable frame connected to the first meander vibration beam, a second meander vibration beam supporting the movable frame rotatably about a second rotation axis, and a supporter connected to the second meander vibration beam. The first meander vibration beam meanderingly extends along a first rotation axis, and has a first end and a second end opposite to the first end. The movable frame is connected to the second end of the first meander vibration beam. The second meander vibration beam extends meanderingly along the second rotation axis perpendicular to the first rotation axis, and has a third end and a fourth end opposite to the third end. The supporter is connected to the fourth end of the second meander vibration beam. The mirror is coupled to the movable frame only via the first meander vibration beam. 
     This optical reflection device has a feature, a large angle by which the mirror rotates about the first rotation axis. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a top view of an optical reflection device according to Exemplary Embodiment 1 of the present invention. 
         FIG. 1B  is an enlarged top view of the optical reflection device according to Embodiment 1. 
         FIG. 2  is a sectional view of the optical reflection device according to Embodiment 1. 
         FIG. 3  is a schematic diagram of an image projector according to Embodiment 1. 
         FIG. 4  is a top view of an optical reflection device according to Exemplary Embodiment 2 of the invention. 
         FIG. 5  is a top view of a mirror of the optical reflection device according to Embodiment 2. 
         FIGS. 6A to 6E  are sectional views of mirrors of the optical reflection device according to Embodiment 1. 
         FIG. 7A  shows evaluation results of the optical reflection device according to Embodiment 2. 
         FIG. 7B  is a top view of the mirror of the optical reflection device according to Embodiment 2. 
         FIG. 8  is a top view of an optical reflection device according to Exemplary Embodiment 3 of the invention. 
         FIG. 9A  is a sectional view of the optical reflection device according to Embodiment 3. 
         FIG. 9B  is a sectional view of the optical reflection device according to Embodiment 3. 
         FIG. 10  is a schematic diagram of an image projector according to Embodiment 3. 
         FIG. 11  is a top view of another optical reflection device according to Embodiment 3. 
         FIG. 12  shows evaluation results of the optical reflection device according to Embodiment 3. 
         FIG. 13  is a schematic diagram of a movable frame of the optical reflection device according to Embodiment 3. 
         FIG. 14  is a top view of a comparative example of an optical reflection device. 
         FIG. 15  is a top view of an optical reflection device according to Exemplary Embodiment 4 of the invention. 
         FIG. 16  is a perspective view of a conventional optical reflection device. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Exemplary Embodiment 1  
       FIG. 1A  is a top view of optical reflection device  1001  according to Exemplary Embodiment 1 of the present invention. Optical reflection device  1001  includes mirror  107  adapted to reflect light thereon, single first meander vibration beam  108 A connected to mirror  107 , movable frame  109  connected to first meander vibration beam  108 A, second meander vibration beam  110 A connected to movable frame  109 , third meander vibration beam  110 B connected to movable frame  109 , supporter  111  connected to second meander vibration beam  110 A and third meander vibration beam  110 B, and further meander vibration beam  108 B connected to mirror  107 . Movable frame  109  surrounds meander vibration beams  108 A and  108 B and mirror  107 . Supporter  111  has a frame shape surrounding meander vibration beams  110 A and  110 B and movable frame  109 . Mirror  107  is arranged substantially at the center of movable frame  109 . First meander vibration beam  108 A has end  1108 A connected to end  107 C of mirror  107 , and causes mirror  107  to rotate about first rotation axis  1001 A. Meander vibration beam  108 B has end  1108 B connected to end  107 D of mirror  107  opposite to end  107 C in a direction along first rotation axis  1001 A. Meander vibration beams  108 A and  108 B extend meanderingly along first rotation axis  1001 A. Second meander vibration beam  110 A has end  1110 A connected to end  109 C of movable frame  109 , and causes movable frame  109  to rotate about second rotation axis  1001 B. Third meander vibration beam  110 B has end  1110 B connected to end  109 D of movable frame  109  opposite to end  109 C in a direction along second rotation axis  1001 B, and causes movable frame  109  to rotate about second rotation axis  1001 B. Meander vibration beams  110 A and  110 B extend meanderingly along second rotation axis  1001 B. The periphery of mirror  107  except end  107 C is not connected to movable frame  109 . 
       FIG. 1B  is an enlarged top view of optical reflection device  1001  for showing details of mirror  107 . Rotation axes  1001 A and  1001 B are substantially perpendicular to each other. Rotation axes  1001 A and  1001 B cross each other preferably at center  107 E of mirror  107 . 
     End  114  of first meander vibration beam  108 A opposite to end  1108 A is connected to movable frame  109 , thus being a fixed end. End  112  of meander vibration beam  108 B opposite to end  1108 B is not connected to movable frame  109 , thus being an open end. Meander vibration beams  108 A and  108 B and mirror  107  provides combination structure  113 , and has a cantilever structure in which end  114  of first meander vibration beam  108 A is the fixed end. 
     First meander vibration beam  108 A faces meander vibration beam  108 B across mirror  107  along first rotation axis  1001 A. 
     End  2110 A of second meander vibration beam  110 A opposite to end  1110 A is connected to supporter  111 . End  2110 B of third meander vibration beam  110 B opposite to end  1110 B is connected to supporter  111 . Second meander vibration beam  110 A faces third meander vibration beam  110 B across movable frame  109  along second rotation axis  1001 B. 
     In optical reflection device  1001  according to Embodiment 1, ends  1110 A and  1110 B of meander vibration beams  110 A and  110 B are positioned at ends  109 G and  109 H of sides  109 E and  109 F of movable frame  109  opposite to each other, respectively, however, may be positioned at ends  109 J and  109 K of sides  109 E and  109 F opposite to ends  109 G and  109 H, respectively. 
     First meander vibration beam  108 A extending meanderingly along first rotation axis  1001 A has plural portions  3108 A extending in a direction of second rotation axis  1001 B perpendicular to first rotation axis  1001 A. Meander vibration beam  108 B extending meanderingly along second rotation axis  1001 B has plural portions  3108 B extending in a direction of second rotation axis  1001 B perpendicular to first rotation axis  1001 A. Second meander vibration beam  110 A extending meanderingly along second rotation axis  1001 B has plural portions  3110 A extending in the direction of first rotation axis  1001 A. Third meander vibration beam  110 B extending meanderingly along second rotation axis  1001 B has plural portions  3110 B extending in the direction of first rotation axis  1001 A. 
       FIG. 2  is a sectional view of portions  3108 A,  3108 B,  3110 A, and  3110 B of meander vibration beams  108 A,  108 B,  110 A, and  110 B. Each of meander vibration beams  108 A,  108 B,  110 A, and  110 B includes silicon substrate  115 , silicon oxide film  116  provided on silicon substrate  115 , and piezoelectric actuator  151  provided on silicon oxide film  116 . Piezoelectric actuator  151  includes lower electrode layer  117  provided on silicon oxide film  116 , piezoelectric layer  118  provided on lower electrode layer  117 , and upper electrode layer  119  provided on piezoelectric layer  118 . Lower electrode layer  117  may be grounded. Piezoelectric layer  118  is made of piezoelectric material. 
     Optical reflection device  1001  is formed by etching silicon substrate  115 . Particularly, portions of silicon substrate  115  constituting meander vibration beams  108 A,  108 B,  110 A, and  110 B and mirror  107  are etched and are thinner than the other portions of silicon substrate  115 . The thickness of meander vibration beams  108 A,  108 B,  110 A, and  110 B and mirror  107  is 120 μm. The thickness of movable frame  109  and supporter  111  is 525 μm. Meander vibration beams  108 A,  108 B,  110 A, and  110 B and mirror  107  are thus thinner than movable frame  109  and supporter  111 . 
     Meander vibration beams  108 A,  108 B,  110 A, and  110 B are thinner than movable frame  109  and supporter  111 , and elastically deform more easily than movable frame  109  and supporter  111 , thereby vibrating at a large amplitude. Movable frame  109  is thicker than meander vibration beams  108 A,  108 B,  110 A, and  110 B and functions as a weight for meander vibration beams  110 A and  110 B, which increases the amplitude of the vibration of meander vibration beams  110 A and  110 B about second rotation axis  1001 B. Supporter  111  is thick and allows optical reflection device  1001  to be easily handled, thus increasing the mechanical strength of optical reflection device  1001 . 
     Lower electrode layer  117  may be made of platinum. Upper electrode layer  119  may be made of gold. Piezoelectric layer  118  may be made of lead zirconate titanate, Pb(Zr x ,Ti 1−x )O 3 (x=0.525). These layers may be formed by a film-forming method, such as deposition, sol-gel method, chemical vapor deposition (CVD), or sputtering. 
     An operation of optical reflection device  1001  will be described below. 
     Alternating-current (AC) voltages having respective resonance frequency of first meander vibration beam  108 A, second meander vibration beam  110 A, third meander vibration beam  110 B, and meander vibration beam  108  are applied to upper electrode layer  119  of first meander vibration beam  108 A, second meander vibration beam  110 A, third meander vibration beam  110 B, and meander vibration beam  108  so as to drive piezoelectric actuator  151  of each of first meander vibration beam  108 A, second meander vibration beam  110 A, third meander vibration beam  110 B, and meander vibration beam  108 B. First meander vibration beam  108 A, second meander vibration beam  110 A, third meander vibration beam  110 B, and meander vibration beam  108 B vibrate at large amplitude due to resonance, thereby rotating and swinging mirror  107  by a large angle about rotation axes  1001 A and  1001 B. 
     The polarity of the AC voltage applied to upper electrode layer  119  changes, and causes meander vibration beams  110 A and  110 B to vibrate so that the warping directions of portions  3110 A and  3110 B of meander vibration beams  110 A and  110 B parallel to first rotation axis  1001 A change. This vibration causes movable frame  109  to vibrate so that ends  109 G and  109 H of movable frame  109  are displaced in a direction opposite to a direction in which ends  109 J and  109 K are displaced, thereby rotating and swinging mirror  107  about second rotation axis  1001 B while center  107 E of mirror  107  is not displaced. 
     The polarity of the AC voltage supplied to upper electrode layer  119  changes, and causes meander vibration beams  108 A and  108 B to vibrate so that the warping direction of portions  3108 A and  3108 B of meander vibration beams  108 A and  108 B parallel to second rotation axis  1001 B change. This vibration causes movable frame  109  to vibrate so that ends  109 G and  109 J of movable frame  109  are displaced in a direction opposite to a direction in which ends  109 H and  109 K are displaced, thereby rotating and swinging mirror  107  about first rotation axis  1001 A while center  107 E of mirror  107  is not displaced. 
       FIG. 3  is a schematic diagram of image projector  1100  including optical reflection device  1001 . Light  121  is emitted onto mirror  107  from light source  122 , such as a laser light source. Mirror  107  reflects light  121  so that reflected light  122  reaches screen  123  while rotating and swinging about rotation axes  1001 A and  1001 B. While mirror  107  rotates about first rotation axis  1001 A, reflected light  122  scans screen  123  in a direction of an X-axis perpendicular to first rotation axis  1001 A. Similarly, while mirror  107  rotates about second rotation axis  1001 B, reflected light  122  scans screen  123  in a direction of a Y-axis perpendicular to second rotation axis  1001 B. Thus, mirror  107  of optical reflection device  1001  allows reflected light  122  to scan screen  123  in the directions of the X-axis and Y-axis, thereby projecting image  124  on screen  123 . Rotation axis  1001 A is perpendicular to rotation axis  1001 B, however may not be exactly perpendicular to rotation axis  1001 B by about 1 degree due to manufacturing error or measuring error. Thus, rotation axis  1001 A substantially perpendicular to rotation axis  1001 B provides the same effects. 
     In optical reflection device  1001  according to Embodiment 1, combination structure  113  including mirror  107  and meander vibration beams  108 A and  108 B has a cantilever structure. End  112  of meander vibration beam  108 B is a free end which opens, and is connected to nothing, mirror  107  vibrates and rotates by a large angle about first rotation axis  1001 A, thereby allowing reflected light  122  to scan widely in the direction of the Y-axis. 
     In conventional optical reflection device  501  shown in  FIG. 16 , both ends of mirror  201  are fixed to movable frame  203  via meander vibration beams  202 . The fixed ends of meander vibration beams  202  restrain the rotation of mirror  201  about rotation axis  206 , thereby preventing mirror  208  from rotating by a large angle. 
     In image projector  1100  shown in  FIG. 3 , reflected light  122  scans in the direction of the X-axis at a higher frequency than in the direction of the Y-axis. In other words, mirrors  107  and  201  vibrate and rotate about rotation axes  206  and  1001 A at a higher frequency than about rotation axes  207  and  1001 B. Hence, conventional optical reflection device  501  rotates about rotation axis  206  by a further small angle. 
     In optical reflection device  1001  according to Embodiment 1, mirror  107  has the free end, namely, end  112  of the meander vibration beam opens, and coupled to movable frame  109  only via single first meander vibration beam  108 A, thus preventing meander vibration beam  108 B from receiving a reactive force from movable frame  109 . Consequently, mirror  107  and meander vibration beams  108 A and  108 B deform and are displaced more freely than mirror  201  and meander vibration beam  202  of conventional optical reflection device  501 . Hence, mirror  107  rotates about first rotation axis  1001 A by a large angle, thereby allowing reflected light  122  to scan widely in the direction of the X-axis direction. In optical reflection device  1001  according to Embodiment 1, mirror  107  rotates about first rotation axis  1001 A by an angle approximately 4.8 times that about rotation axis  206  in conventional optical reflection device  501  shown in  FIG. 16 . 
     In optical reflection device  1001  according to Embodiment 1, mirror  107  is located substantially at the center of movable frame  109 . Center  107 E of mirror  107  does not move while mirror  107  vibrates, thus reducing the variation at center  107 E of mirror  107 . This prevents image  124  projected by optical reflection device  1001  from distorting. 
     Although optical reflection device  1001  can have a small size, meander vibration beams  108 A,  108 B,  110 A, and  110 B extend meanderingly and can be long, thereby increasing the rotation angle of mirror  107 . 
     Exemplary Embodiment 2  
       FIG. 4  is a top view of optical reflection device  1002  according to Exemplary Embodiment 2 of the present invention. In  FIG. 4 , components identical to those of optical reflection device  1001  according to Embodiment 1 shown in  FIG. 1A  are denoted by the same reference numerals, and their description will be omitted. Optical reflection device  1002  according to Embodiment 2 does not include meander vibration beam  108 B of optical reflection device  1001  shown in  FIG. 1A . 
       FIG. 5  is a top view of mirror  107 . Mirror  107  located substantially at the center of movable frame  109  has a rectangular shape having sides  107 G and  107 H in the direction of first rotation axis  1001 A and sides  107 E and  107 F in the direction of second rotation axis  1001 B perpendicular to first rotation axis  1001 A. According to Embodiment 2, width W 1  of sides  107 G and  107 H is 1100 μm, and width W 2  of sides  107 E and  107 F is 1800 μm. 
       FIGS. 6A to 6E  are sectional views of mirror  107  at line  66  shown in  FIG. 5 . Mirror  107  shown in  FIGS. 6A to 6E  has a thickness in direction  1001 C perpendicular to rotation axes  1001 A and  1001 B. First meander vibration beam  108 A has thickness TA. Thickness TA of first meander vibration beam  108 A is 120 μm. 
     Thickness T 1  of mirror  107  shown in  FIG. 6A  in direction  1001 C is 120 μm, which is the same as thickness TA of first meander vibration beam  108 A. Thickness T 2  of mirror  107  shown in  FIG. 6C  in direction  1001 C is 525 μm, and is larger than thickness TA of first meander vibration beam  108 A. Thickness T 3  of mirror  107  in direction  1001 C shown in  FIG. 6E  is 930 μm, which is larger than thickness TA of first meander vibration beam  108 A. 
     Mirror  107  shown in  FIGS. 6B and 6D  has recess  125  formed in lower surface  107 B of mirror  107 . Recess  125  is surrounded by projection  126  projecting from an outer edge of mirror  107 . Projection  126  has a frame shape. Height T 6  of projection  126  in direction  1001 C from bottom  125 A of recess  125  is 405 μm. Thickness T 7  of mirror  107  from bottom  125 A of recess  125  to upper surface  107 A of mirror  107  is 120 μm, which is substantially identical to thickness TA of first meander vibration beam  108 A. The sum of height T 6  of projection  126  and thickness T 7  of mirror  107  is larger than thickness TA of first meander vibration beam  108 A. 
     In the fabricating of optical reflection device  1002 , meander vibration beams  108 A,  110 A, and  110 B are thinned by etching as well as the fabricating of optical reflection device  1001  shown in  FIG. 1A . Mirror  107  shown in  FIG. 6A  is formed in the same etching process in which meander vibration beams  108 A,  110 A, and  110 B are formed. Recess  125  shown in  FIGS. 6B and 6D  is formed in the same etching process in which meander vibration beams  108 A,  110 A, and  110 B are formed. Hence, thickness TA of meander vibration beams  108 A,  110 A, and  110 B is identical to thickness T 1  of mirror  107  shown in  FIG. 6A  and to thickness T 7  of mirror  107  at recess  125 , that is, between bottom upper surface  107 A of mirror  107  and bottom  125 A of recess  125 , shown in  FIGS. 6B and 6D . 
     Weight layer  127  is provided on upper surface  107 A of mirror  107  shown in  FIGS. 6D and 6E . Weight layer  127  can be formed by, e.g. depositing silicon identical to the material of silicon substrate  115 , or can be formed by depositing other material having a high density and can be strongly bonded with silicon substrate  115 . 
     In optical reflection device  1002 , mirror  107  is coupled to movable frame  109  only via single first meander vibration beam  108 A, and optical reflection device  1002  does not include meander vibration beam  108 B shown in  FIG. 1A . This structure allows first meander vibration beam  108 A to have a small length in the direction of first rotation axis  1001 A, and the small length of first meander vibration beam  108 A realizes high frequency fH of mirror  107  about first rotation axis  1001 A to allow reflected light  122  to scan at high speed in the direction of the X-axis direction shown in  FIG. 3 . Higher frequency fH generally results in smaller rotation angle θH by which mirror  107  rotates about first rotation axis  1001 A. In optical reflection device  1002 , mirror  107  has a cantilever structure coupled to movable frame  109  only via first meander vibration beam  108 A, allowing mirror  107  and first meander vibration beam  108 A to be displaced and deform flexibly, thereby providing relatively large rotation angle θH of mirror  107 . 
     Optical reflection device  1002  shown in  FIG. 4  does not include meander vibration beam  108 B of optical reflection device  1001  shown in  FIG. 1A . Hence, the area in which meander vibration beam  108 B is located in device  1001  shown in  FIG. 1A  is a portion of movable frame  109  in device  1002  in  FIG. 4 . This structure increases frequency fV at which mirror  107  vibrates and rotation angle θV by which mirror  107  rotates about second rotation axis  1001 B, caused by the larger mass of movable frame  109  in device  1002  in  FIG. 4  than in device  1001  in  FIG. 1 . 
     Thus, frequency fV at which mirror  107  vibrates and rotates about second rotation axis  1001 B is decreased to increase ratio fH/fV. This increases scanning lines of image  124  parallel to the X-axis, allowing optical reflection device  1002  to project image  124  at a high resolution on screen  123 . 
     Larger rotation angles θH and θV provide larger image  124 . 
     As shown in  FIGS. 6B to 6E , the thickness of at least a portion of mirror  107  in direction  1001 C is larger than thickness TA of first meander vibration beam  108 A so as to increase the mass of mirror  107 , thereby increasing rotation angle θH about first rotation axis  1001 A. 
       FIG. 7A  shows evaluation results of samples  1  to  5  of optical reflection device  1002  including mirrors  107  shown in  FIGS. 6A to 6E , respectively. In samples  2  and  4  for optical reflection device  1002  including mirrors  107  shown in  FIGS. 6B and 6D , respectively, the mass of mirror  107  is adjusted by adjusting width W 4  of projection  126  on lower surface  107 B of mirror  107  in the direction of second rotation axis  1001 B. Hence, the mass of mirror  107  is adjusted to a predetermined mass to increase rotation angle θH. 
     The depth of recess  125  of mirror  107 , i.e., height T 6  of the projection, is identical to a depth by which silicon substrate  115  is etched to form meander vibration beams  108 A,  110 A, and  110 B. This structure allows recess  125  to be formed simultaneously to meander vibration beam  108 A,  110 A, and  110 B, thereby allowing optical reflection devices  1002  to be manufactured at high productivity. 
     In samples  4  and  5  of optical reflection device  1002  including mirrors  107  shown in  FIGS. 6D and 6E , respectively, weight layer  127  is provided entirely on upper surface  7 A of mirror  107 . Mirror  107  is joined to weight layer  127  on upper surface  107 A of mirror  107  to provide mirror body  157 . Center  157 P of mirror body  157  is positioned at a position roughly the same as center  108 P of first meander vibration beam  108 A. In sample  1  including mirror  107  shown in  FIG. 6A , center  107 P of mirror  107  is placed at a position roughly the same as center  108 P of first meander vibration beam  108 A in direction  1001 C. 
     In samples  2  and  3  including mirrors  107  shown in  FIGS. 6B and 6C , respectively, gravity center  107 P of mirror  107  deviates from gravity center  108 P of first meander vibration beam  108 A in direction  1001 C. Therefore, as shown in  FIG. 7A , in samples  2  and  3 , the position of center  107 E of mirror  107  deviates in the direction of first rotation axis  1001 A while first meander vibration beam  108 A vibrates.  FIG. 7B  is a top view of mirror  107  having gravity center  109 P deviate. As shown in  FIG. 7B , the deviation of gravity center  107 P of mirror  107  in the direction  1001 C adds unnecessary swing vibration mode to mirror  107 A in the plane including rotation axes  1001 A and  1001 B shown in  FIG. 4  while mirror  107  swings about rotation axis  1001 A. The addition of the swing vibration mode to mirror  107  causes the deviation of mirror center  107 E in the direction of rotation axis  1001 A as shown in  FIG. 7B . Therefore, as shown in  FIG. 7A , in samples  2  and  3 , the position of center  107 E deviates in the direction of rotation axis  1001 A while first meander vibration beam  108 A vibrates. 
     In samples  4  and  5  including mirrors  107  shown in  FIGS. 6D and 6E , respectively, gravity center  157 P of mirror body  157  is located at a position substantially identical to gravity center  108 P of first meander vibration beam  108 A in direction  1001 C. This arrangement prevents the position of gravity center  107 E of mirror  107  from deviating in the direction of first rotation axis  1001 A while first meander vibration beam  108 A vibrates, thereby projecting image  124  with small distortion. In mirror  107  according to Embodiment 2 shown in  FIG. 6D , height T 6  is 405 μm, thickness T 7  is 120 μm, which is identical to thickness TA, thickness T 8  is 585 μm, and thickness T 9  is 110 μm. Width W 3  is 1600 μm, and width W 4  is 100 μm. In the case that mirror  107  is made of the same material, i.e., material having the same density, as material of weight layer  127 , the above dimensions locate gravity center  157 P of mirror body  157  at a position substantially identical to gravity center  108 P of first meander vibration beam  108 A in direction  1001 C. In mirror  107  according to Embodiment 2 shown in  FIG. 6E , thickness T 4  of mirror  107  in direction  100 C is 525 μm, and thickness T 5  of weight layer  127  is 405 μm. In the case that mirror  107  is made of the same material, i.e., material having the same density, as material of weight layer  127 , the above dimension locate the meander vibration beam  108 A at the center of mirror body  127  in direction  1001 C, and locate gravity center  157 P of mirror body  157  substantially at gravity center  108 P of first meander vibration beam  108 A. 
     In sample  5  including mirror  107  shown in  FIG. 6E , mirror  107  has an excessively large weight, accordingly lowering frequency fH while rotation angle θH is large, as shown in  FIG. 7A . In sample  1  including mirror  107  shown in  FIG. 6A , mirror  107  has a small weight, accordingly decreasing rotation angle θH, as shown in  FIG. 7A   
     In sample  4  including mirror  107  shown in  FIG. 6D , the volume of projection  126 , namely, widths W 3  and W 4  and height T 9  of weight layer  127  are effectively adjusted to adjust the weight of mirror body  157 . Even if mirror body  157  vibrates, the position of center  107 E of mirror  107  is prevented from deviation, and the weight of mirror body  157  is easily adjusted so that mirror body  157  of sample  4  vibrates at predetermined frequency fH by predetermined rotation θH. 
     In mirror  107  shown in  FIG. 6D , projection  126  is provided along the outer periphery of lower surface  107 B of mirror  107 . Projection  126  may be provided at the center of lower surface  107 B of mirror  107 , or at both the outer periphery and the center. These structures allow the weight of mirror body  157  to be adjusted appropriately. 
     Projection  126  of mirror  107  functions as a weight provided on lower surface  107 B of mirror  107 . Projection  126  can be formed by etching lower surface  107 B of mirror  107 , and may be formed by stacking a weight layer having a film shape on lower surface  107 B of mirror  107 . 
     Exemplary Embodiment 3  
       FIG. 8  is a top view of optical reflection device  1003  according to exemplary embodiment 3. Optical reflection device  1003  includes mirror  208  adapted to reflect light thereon, first meander vibration beam  209  connected to mirror  208 , movable frame  210  connected to first meander vibration beam  209 , second meander vibration beam  211  connected to movable frame  210 , and supporter  212  connected to second meander vibration beam  211 . Movable frame  210  has a frame shape surrounding first meander vibration beam  209  and mirror  208 . Supporter  212  supports second meander vibration beam  211  and has a frame shape surrounding second meander vibration beam  211  and movable frame  210 . 
     First meander vibration beam  209  extends meanderingly along first rotation axis  213 , and has end  1209  and end  2209  opposite to end  1209 . Meander vibration beam  209  includes plural portions  3209  extending in parallel with rotation axis  214  perpendicular to rotation axis  213 . End  1209  of first meander vibration beam  209  is connected to mirror  208 , and end  2209  is connected to movable frame  210 . Mirror  208  has a cantilever structure coupled to movable frame  210  only via single first meander vibration beam  209 . First rotation axis  213  is perpendicular to second rotation axis  214 . 
     Second meander vibration beam  211  extends meanderingly along second rotation axis  214 , and has end  1211  and end  2211  opposite to end  1211 . End  1211  of second meander vibration beam  211  is connected to movable frame  210 , and end  2211  is connected to supporter  212 . Movable frame  210  has a cantilever structure coupled to supporter  212  only via single second meander vibration beam  211 . 
     First meander vibration beam  209  rotates, for example, about first rotation axis  213  and swings mirror  208  at frequency fH while rotating mirror  208  by rotation angle θH about first rotation axis  213 . 
     Second meander vibration beam  211  rotates, for example, about second rotation axis  214  and swings movable frame  210  at frequency fV while rotating movable frame  210  by rotation angle θV about second rotation axis  214 . Second meander vibration beam  211  rotates movable frame  210  to swing mirror  208  at frequency fV while rotating movable frame  210  by rotation angle θV about second rotation axis  214 . 
     Mirror  208  is arranged substantially at the center of the frame shape of movable frame  210 . Rotation axes  213  and  214  cross each other at crossing point  208 E preferably inside mirror  208 . While mirror  208  rotates and swings about rotation axes  213  and  214 , crossing point  208 E does not move. Mirror  208  receives light at crossing point  208 E to reflect the light and projects the light on the screen. The light enters to crossing point  208 E and reflected by mirror  208  reaches the screen along a fixed optical path even while first meander vibration beam  209  and second meander vibration beam  211  vibrate, thereby projecting an image on the screen precisely. In optical reflection device  1003  according to Embodiment 3, crossing point  208 E is positioned at the center of mirror  208 . This arrangement positions crossing point  208 E inside mirror  208  even if the positions of rotation axes  213  and  214  deviates due to a manufacturing error or other problems. 
     Movable frame  210  has ends  210 A and  210 B opposite to each other along first rotation axis  213 . That is, ends  210 A and  210 B are positioned opposite to each other across second rotation axis  214  in between, and second rotation axis  214  is positioned between ends  210 A and  210 B. End  2209  of first meander vibration beam  209  having end  1209  connected to mirror  208  is connected to end  210 B of movable frame  210 . End  1211  of second meander vibration beam  211  is connected to end  210 A of movable frame  210 . 
     Mirror  208  has ends  208 A and  208 B opposite to each other along second rotation axis  214 . That is, ends  208 A and  208 B are positioned opposite to each other across first rotation axis  213  in between, and first rotation axis  213  is positioned between ends  208 A and  208 B. End  1209  of first meander vibration beam  209  is connected to end  208 A of mirror  208 . First meander vibration beam  209  of optical reflection device  1003  can rotate and swing mirror  208  by larger amplitude due to leverage effects than an optical reflection device in which end  1209  of first meander vibration beam  209  is connected to the center of a side of mirror  208  Meanwhile, as compared to an optical reflection device in which end  1211  of second meander vibration beam  211  is connected to the center of the side of movable frame  210 , second meander vibration beam  211  of optical reflection device  1003  can rotate and swing movable frame  210  (i.e. mirror  208 ) with large amplitude due to leverage. 
     End  2209  connected to movable frame  210  of first meander vibration beam  209  is positioned on first rotation axis  213 . End  2211  connected to supporter  212  of second meander vibration beam  211  is positioned on second rotation axis  214 . This arrangement stabilizes the positions of rotation axes  213  and  214 , thereby preventing unnecessary vibration. 
       FIGS. 9A and 9B  are sectional views of optical reflection device  1003  shown in  FIG. 8  at first rotation axis  213  and second rotation axis  214 , respectively. Piezoelectric actuators  215  and  255  are provided on surfaces of meander vibration beams  209  and  211  directed in direction  1003 C perpendicular to rotation axes  213  and  214 , respectively. Mirror  208 , meander vibration beams  209  and  211 , movable frame  210 , and supporter  212  have common silicon substrate  216 . Silicon oxide film  217  is provided on silicon substrate  216 . Piezoelectric actuator  215  provided on first meander vibration beam  209  includes lower electrode layer  218  provided on silicon oxide film  217 , piezoelectric layer  219  provided on lower electrode layer  218 , and upper electrode layer  220  provided on piezoelectric layer  219 . Piezoelectric actuator  255  provided on second meander vibration beam  211  includes lower electrode layer  258  provided on silicon oxide film  217 , piezoelectric layer  259  provided on lower electrode layer  258 , and upper electrode layer  220  provided on piezoelectric layer  259 . Upper electrode layers  220  and  221  are patterned to have predetermined patterns by etching. Lower electrode layers  218  and  258  may be grounded. 
     In optical reflection device  1003 , portions of a lower surface of silicon substrate  216  corresponding to meander vibration beams  209  and  211  and mirror  208  are etched to make meander vibration beams  209  and  211  and mirror  208  thinner than movable frame  210  and supporter  212 . Meander vibration beams  209  and  211  are thin to elastically deform, hence increasing rotation angles θH and θV. A large thickness of movable frame  210  allows movable frame  210  to function as a weight connected to second meander vibration beam  211 , increasing rotation angle θV about second rotation axis  214 . A large thickness of supporter  212  allows optical reflection device  1003  to be handled easily and increases the mechanical strength of optical reflection device  1003 . 
     Lower electrode layers  218  and  258  may be made of platinum. Upper electrode layers  220  and  221  may be made of gold. Piezoelectric layer  219  and  259  may be made of lead zirconate titanate, Pb(Zr x , Ti 1−x )O 3 , (x=0.525). These layers can be formed by a film-forming method, such as deposition, sol-gel method, chemical vapor deposition (CVD), or sputtering. 
     An operation of optical reflection device  1003  will be described below. 
     An alternating-current (AC) voltage having a resonance frequency intrinsic to first meander vibration beam  209  is applied between upper electrode layer  220  and lower electrode layer  218  of piezoelectric actuator  215  provided on first meander vibration beam  209  to drive piezoelectric actuator  215 . Similarly, an AC voltage having a resonance frequency intrinsic to second meander vibration beam  211  is applied between upper electrode layer  221  and lower electrode layer  258  of piezoelectric actuator  255  provided on second meander vibration beam  211  to drive piezoelectric actuator  255 . 
     The polarity of the AC voltage supplied to upper electrode layer  220  changes, and accordingly, first meander vibration beam  209  vibrates about first rotation axis  213 . Mirror  208  rotates and swings about first rotation axis  213  while crossing point  208 E inside mirror  208  is not displaced due to this vibration. 
     Similarly, with the polarity of the AC voltage supplied to upper electrode layer  221  changes, and accordingly, second meander vibration beam  211  vibrates about second rotation axis  214 . This vibration causes movable frame  109  to vibrate about second rotation axis  214 , and rotates and swings mirror  208  about second rotation axis  214  while crossing point  208 E inside mirror  208  is not displaced. 
     Meander vibration beams  209  and  211  are driven at their respective resonance frequencies to increase rotation angles θH and θV by which mirror  208  and movable frame  210  rotate. 
       FIG. 10  is a schematic diagram of image projector  2100  including optical reflection device  1003 . Light  225  is emitted onto mirror  208  from light source  222 , such as a laser light source. Mirror  208  reflects light  225  to reflect light  225  to screen  223  while rotating and swinging about rotation axes  213  and  214 . While mirror  208  rotates about first rotation axis  213 , reflected light  266  scans screen  223  in a direction of an X-axis perpendicular to first rotation axis  213 . Similarly, while mirror  208  rotates about second rotation axis  214 , reflected light  266  scans screen  223  in a direction of a Y-axis perpendicular to second rotation axis  214 . Thus, mirror  208  of optical reflection device  1003  allows reflected light  266  to scan screen  223  in the directions of the X-axis and the Y-axis, thereby projecting image  264  on screen  223 . 
     Movable frame  210  is coupled to supporter  212  only via single second meander vibration beam  211 , thereby allowing optical reflection device  1003  to have a small size. In conventional optical reflection device  501  shown in  FIG. 16 , two meander vibration beams  204  located opposite to each other across movable frame  203  are connected to movable frame  203 . Meander vibration beams  204  occupy a certain area. Hence, optical reflection device  1003  according to Embodiment 3 has a smaller size than conventional optical reflection device  501 . 
     In conventional optical reflection device  501  shown in  FIG. 16 , meander vibration beams  204  apply restraint forces on movable frame  210  from both sides, thereby increasing a vibration frequency of meander vibration beams  204 . In optical reflection device  1003  according to Embodiment 3, movable frame  210  is coupled to supporter  212  only via single second meander vibration beam  211 , and have a restraint force restraining vibration of movable frame  210  reduced, thereby decreasing frequency fV at which second meander vibration beam  211  vibrates. Thus, ratio fH/fV of frequency fH of vibration of mirror  208  about first rotation axis  213  to frequency fV about second rotation axis  214  can increase, and accordingly increases the resolution of image  264  projected, allowing image projector  2100  to project high-resolution image  264  on screen  223 . 
     Movable frame  210  is preferably rotates and vibrates in parallel with second rotation axis  214  about second rotation axis  214 . However, in the case that movable frame  210  is supported by being coupled to supporter  212  only via single second meander vibration beam  211 , the gravity center of movable frame  210  may be displaced more largely than movable frame  203  supported by two meander vibration beams  204  shown in  FIG. 16 , and thus, movable frame  210  may incline with respect to second rotation axis  214 . 
     In optical reflection device  1003  according to Embodiment 3, first meander vibration beam  209  is connected to end  210 B of movable frame  210 , and second meander vibration beam  211  is connected to movable frame  210  at end  210 A opposite to end  210 B across second rotation axis  214 . This structure prevents movable frame  210  from inclining with respect to second rotation axis  214  while movable frame  210  vibrates, thereby preventing unnecessary vibration. 
     In order to evaluate examine unnecessary vibration of movable frame  210 , samples of optical reflection device  1003  according to Embodiment 1 shown in  FIG. 8  as example 1 were produced.  FIG. 11  is a top view of comparative example 1 of optical reflection device  502 . In  FIG. 11 , components identical to those of optical reflection device  1003  shown in  FIG. 8  are denoted by the same reference numerals, and their description will be omitted. In example  2  of optical reflection device  502  shown in  FIG. 11 , end  2209  of first meander vibration beam  209  is connected not to end  210 B of movable frame  210 , but to end  210 A to which second meander vibration beam  211  is connected. In order to evaluate examine unnecessary vibration of movable frame  210 , samples of example 2 of optical reflection device  502  shown in  FIG. 11  were produced as well. 
       FIG. 13  is a schematic diagram of movable frame  210 . Movable frame  210  is a rectangular shape having four vertices P 1 , P 2 , P 3 , and P 4 , sides P 1 P 2  and P 3 P 4  parallel to second rotation axis  214 , and sides P 2 P 3  and P 4 P 1  parallel to first rotation axis  213 . Angle θE of the rotation axis of movable frame  210  with respect to second rotation axis  214  was determined. As shown in  FIG. 13 , four vertices P 1 , P 2 , P 3 , and P 4  of movable frame  210  vibrated at amplitudes Z 1 , Z 2 , Z 3 , and Z 4 , respectively, in direction  1003 C perpendicular to rotation axes  213  and  214 . Angle θE is expressed by formula 1 with length Wt of sides P 1 P 2  and P 3 P 4 . 
     
       
         
           
             
               
                 
                   
                     θ 
                      
                     
                         
                     
                      
                     E 
                   
                   = 
                   
                     
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                          
                         
                           ( 
                           
                             
                                
                               
                                 
                                   Z 
                                   1 
                                 
                                 - 
                                 
                                   Z 
                                   2 
                                 
                               
                                
                             
                             Wt 
                           
                           ) 
                         
                       
                       + 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                          
                         
                           ( 
                           
                             
                                
                               
                                 
                                   Z 
                                   3 
                                 
                                 - 
                                 
                                   Z 
                                   4 
                                 
                               
                                
                             
                             Wt 
                           
                           ) 
                         
                       
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
       FIG. 12  shows angle θE of movable frame  210  inclining with respect to second rotation axis  214  of optical reflection device  1003  according to Embodiment 1 and example 2 of optical reflection device  502 . As shown in  FIG. 12 , example 2 of optical reflection device  502  exhibited angle θE of 0.274 degrees of movable frame  210  inclining with respect to second rotation axis  214 , while optical reflection device  1003  according to Embodiment 1 exhibited angle θE of 0.075 degrees. Thus, optical reflection device  1003  according to Embodiment 1 provides smaller angle θE of movable frame  210  inclining with respect to second rotation axis  214  than example 2 of optical reflection device  502 , accordingly preventing unnecessary vibration. 
     Movable frame  210  receives a force due to the rotation and vibration of second meander vibration beam  211  at a portion (end  210 A) where movable frame  210  is connected to second meander vibration beam  211 , and thus, is displaced with end  210 A as a point for receiving the force. Since second meander vibration beam  211  is connected to end  210 A of movable frame  210 , end  210 A of movable frame  210  is displaced largely, and end  210 B opposite to end  210 A is less displacement. Hence, the rotation axis of movable frame  210  moves to a position deviating from second rotation axis  214  toward end  210 B. 
     Movable frame  210  connected to second meander vibration beam  211  is influenced by second meander vibration beam  211  rotating about second rotation axis  214 . The rotation axis of movable frame  210  deviating from the rotation axis of second meander vibration beam  211  causes the rotation axis of movable frame  210  to incline with respect to second rotation axis  214 . 
     In optical reflection devices  501  and  1003  shown in  FIGS. 8 and 11 , first meander vibration beam  209  is formed by providing slit  210 E in movable frame  210  to arrange first meander vibration beam  209  inside movable frame  203 . Hence, the weight of a portion of movable frame  210  wherein first meander vibration beam  209  is formed therein is smaller than the weight of a portion of movable frame  210  where first meander vibration beam  209  is not formed therein. 
     In example 2 of optical reflection device  502  shown in  FIG. 11 , the weight of a portion of movable frame  210  between second rotation axis  214  and end  210 A is smaller than that of a portion of movable frame  210  between second rotation axis  214  and end  210 B. Hence, in example 2 of optical reflection device  502 , the portion of movable frame  210  between second rotation axis  214  and end  210 A may be displaced more than that of the portion of movable frame  210  between second rotation axis  214  and end  210 B. This increases a deviation of the rotation axis of movable frame  210  from second rotation axis  214 , accordingly increasing angle θE of movable frame  210  inclining toward second rotation axis  214 . 
     In optical reflection device  1003  according to Embodiment 1 shown in  FIG. 8 , the weight of the portion of movable frame  210  between second rotation axis  214  and end  210 B is smaller than that of the portion of movable frame  210  between second rotation axis  214  and end  210 A. Hence, the portion of movable frame  210  between second rotation axis  214  and end  210 B may be displaced more than that of the portion of movable frame  210  between second rotation axis  214  and end  210 A. Hence, optical reflection device  1003  has the rotation axis of movable frame  210  closer to second rotation axis  214  than example 2 of optical reflection device  502  is, thus decreasing angle θE. 
       FIG. 14  is a top view of comparative example 1 of optical reflection device  503 . In  FIG. 14 , components identical to those of optical reflection device  1003  shown in  FIG. 8  are denoted by the same reference numerals, and their description will be omitted. Optical reflection device  503  shown in  FIG. 14  includes movable frame  225  instead of movable frame  210  of optical reflection device  1003  shown in  FIG. 8 , and further includes meander vibration beam  224  connected to mirror  208 . Meander vibration beam  224  extends meanderingly along first rotation axis  213  and is connected to end  225 A of movable frame  210 . More specifically, in optical reflection device  503 , mirror  208  is support by being connected to ends  210 A and  210 B of movable frame opposite to each other about movable frame  210  via first meander vibration beam  209  and meander vibration beam  224 . That is, movable frame  225  has a shape symmetrical about second rotation axis  214 , and the weight of the portion of movable frame  225  between second rotation axis  214  and end  210 A is the same as the portion of movable frame  225  between second rotation axis  214  and end  210 B. A sample of comparative example 1 of optical reflection device  502  was produced. In optical reflection device  503 , the angle θE by which movable frame  225  inclines with respect to second rotation axis  214  was 0.330 degrees. Thus, optical reflection device  1003  shown in  FIG. 8  has smaller angle θE by which the rotation axis of movable frame  210  inclines with respect to second rotation axis  214  than optical reflection device  503  shown in  FIG. 14 , thus reducing unnecessary vibration. 
     In optical reflection device  1003  according to Embodiment 3 shown in  FIGS. 9A and 9B , recess  226  is provided in the lower surface of mirror  208 , and a portion of silicon substrate  216  inside mirror  208  is thinner than a portion of silicon substrate  216  at the outer periphery of mirror  208 . Optical reflection device  1003  may further include weight layer  227 A provided on the upper surface of mirror  208  and reflection layer  227 B provided on weight layer  227 A. Reflection layer  227 B is made of material, such as silicon, having high optical reflectance. Weight layer  227 A is made of material, such as copper, having a high specific gravity, thereby functioning as a weight even if weight layer  227 A is thin. Thin weight layer  227 A can be formed in a short time. If weight layer  227 A is made of material, such as silicon, having high optical reflectance, optical reflection device  1003  does not necessarily include reflection layer  227 B. 
     Recess  226  is provided in the lower surface of mirror  208 , and weight layer  227 A is provided on the upper surface of mirror  208 . This structure locates the gravity center of mirror  208  on first rotation axis  213  of first meander vibration beam  209 . This arrangement prevents the axis about which mirror  208  rotates and vibrates from inclining due to deviation of the center of mirror  208  from first rotation axis  213 , thereby reducing unnecessary vibration of mirror  208  while rotating and vibrating. The depth of recess  226  may be identical to the depth to which silicon substrate  216  is etched in order to thin meander vibration beams  209  and  211 . This arrangement allows recess  226  to be formed by the same process as meander vibration beams  209  and  211 , thereby allowing optical reflection devices  1003  to be manufactured efficiently. 
     Optical reflection device  1003  shown in  FIG. 8  does not include meander vibration beam  224  of optical reflection device  502  shown in  FIG. 14 . Hence, movable frame  210  can be larger by expanding movable frame  210  to a portion of movable frame  210  corresponding to meander vibration beam  224 . This structure increases frequency fV of the vibration and rotation angle θV of the rotation of the mirror  208  (movable frame  210 ) about second rotation axis  214  due to deformation of second meander vibration beam  211 . 
     Thus, frequency fV of the vibration during the rotation about second rotation axis  214  is decreased, and ratio fH/fV of frequency fH to frequency fV is increased. This increases the number of scanning lines of image  264  in the X-axis direction, accordingly allowing optical reflection device  1003  to project high resolution image  264  on screen  223 . 
     Exemplary Embodiment 4  
       FIG. 15  is a top view of optical reflection device  1004  according to Embodiment 4. In  FIG. 15 , components identical to those of optical reflection device  1003  according to Embodiment 3 shown in  FIG. 8  are denoted by the same reference numerals, and their description will be omitted. Optical reflection device  1004  shown in  FIG. 15  includes optical reflection device  1003  according to Embodiment 3 shown in  FIG. 8  and further includes gimbal shaft  228  connecting movable frame  210  to supporter  212 . Gimbal shaft  228  is connected to end  210 C opposite to end  210 A, across first rotation axis  213 , to which second meander vibration beam  211  of movable frame  210  is connected. 
     Gimbal shaft  228  is rotatably supported by supporter  212 , fir example, by groove  228 A formed in supporter  212 ). Gimbal shaft  228  is connected to movable frame  210  on second rotation axis  214 . Gimbal shaft  228  is not fixed to supporter  212 , but is supported on supporter  212  rotatably about second rotation axis  214 , and supports movable frame  210  so that movable frame  210  rotates about second rotation axis  214 . This structure prevents the gravity center of movable frame  210  from deviating while vibrating, thereby preventing unnecessary vibration of movable frame  210  and mirror  208 . Second meander vibration beam  211  rotates and vibrates movable frame  210 , but restrains the rotation of movable frame  210 . Gimbal shaft  228  does not substantively restrain the rotation of movable frame  210  about second rotation axis  214  except for inevasible physical actions, such as friction. Gimbal shaft  228  does not decrease rotation angle θV of movable frame  210 , i.e., mirror  208 , about second rotation axis  214 . 
     Optical reflection devices  1003  and  1004  according to Embodiments 3 and 4 can have small sizes, and are applicable to small image projectors included in, e.g. portable phones.