Abstract:
A light scanning apparatus comprising a photosynthesis device having a first light source unit and a second light source unit with a plurality of light sources spaced at equal intervals side by side in one direction, respectively for emitting rays in parallel from the plurality of light sources, a photosynthesis device for synthesizing transmitted rays from the first light source unit and reflected rays from the second light source unit on a single light path, and a light receiving device for focusing first synthesized rays synthesized by the photosynthesis device through an imaging lens, and an optical deflecting device for deflecting and scanning second synthesized rays synthesized by the photosynthesis device, and the second synthesized rays passed through the optical deflecting device is being externally emitted, wherein the photosynthesis device and the light receiving device are displaced relative to the optic axis of the first synthesized rays so as to position focused spots of the first synthesized rays at the center of a light receiving element provided in the light receiving device.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese application serial No. 2006-171140, filed on Jun. 21, 2006, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light scanning apparatus mounted in a digital copier, a laser printer, or another image forming apparatus as well as an image forming apparatus including the light scanning apparatus, and more particularly to a light scanning apparatus having a photosynthesis device for synthesizes rays from a plurality of light sources and an image forming apparatus equipped therewith. 
     2. Prior Art 
     A light scanning apparatus having a photosynthesis device for synthesizing rays from a plurality of light sources is already disclosed in, for example, Patent Document 1. 
     Patent Document 1: Japanese patent No. 3064347 
     SUMMARY OF THE INVENTION 
     The light scanning apparatus disclosed in the above Patent Document 1 uses a galvano mirror as a device for adjusting the positions of rays, so all rays from the plurality of light sources can be corrected simultaneously by use of the galvano mirror. 
     However, the rays from the plurality of light sources are superimposed, so individual rays cannot be adjusted separately. As a result, the angle of a sequence of focused spots formed by synthesizing rays emitted from the plurality light sources on a photosensitive body, which is a medium to be scanned, cannot be held at a fixed value, which makes it hard to switch a print dot density by changing scanning intervals equally. 
     An object of the present invention is to provide a light scanning apparatus that can hold at a fixed value the angle of a focused spot sequence formed by synthesizing rays from a plurality of light sources and enables a print dot switchover to be made easily, and to provide an image forming apparatus including the light scanning apparatus. 
     In a light scanning apparatus comprising a photosynthesis device having a first light source unit and a second light source unit with a plurality of light sources spaced at equal intervals side by side in one direction, respectively for emitting rays in parallel from the plurality of light sources, a photosynthesis device for synthesizing transmitted rays from the first light source unit and reflected rays from the second light source unit on a single light path, and a light receiving device for focusing first synthesized rays synthesized by the photosynthesis device through an imaging lens, and includes an optical deflecting device for deflecting and scanning second synthesized rays synthesized by the photosynthesis device, and the second synthesized rays passed through the optical deflecting device is being externally emitted, the light scanning apparatus implemented by the present invention to solve the above problem is structured in such a way that the photosynthesis device and the light receiving device are displaced relative to the optic axis of the first synthesized rays so as to position focused spots of the first synthesized rays at the center of a light receiving element provided in the light receiving device. 
     As described above, a light scanning apparatus can be obtained, which can hold at a fixed value the angle of a sequence of spots synthesized and focused on a medium to be scanned, by displacing the photosynthesis device and the light receiving device relative to the optic axis of the first synthesized rays, when a plurality of light sources of synthesized rays are arranged in one way, and enables a print dot switchover to be made easily; an image forming apparatus including the light scanning apparatus can also be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an approximate perspective view of a photosynthesis device according to an embodiment of the present invention. 
         FIG. 2  is an approximate side view of an image forming apparatus including a light scanning apparatus according to an embodiment of the present invention. 
         FIG. 3  is an approximate view of the light scanning apparatus in the present embodiment shown in  FIG. 2 . 
         FIG. 4  is a perspective view of a light source unit in the present embodiment. 
         FIG. 5  is a simulated view of a spot sequence formed by synthesized rays on a photosensitive body in the present embodiment. 
         FIG. 6  is a front view of an exemplary light receiving device in the present embodiment. 
         FIG. 7  is a front view of another exemplary light receiving device in the present embodiment. 
         FIG. 8  is a simulated view showing an aspect indicating a spot sequence formed by synthesized rays on the photosensitive body in the present embodiment. 
         FIG. 9  is an estimated view indicating a sequence of light source positions in the present embodiment. 
         FIG. 10  is an approximate view of an inclined synthesized ray in the present embodiment when the print dot density is 1200 dpi. 
         FIG. 11  is an approximate view of an inclined synthesized ray in the present embodiment when the print dot density is 2400 dpi. 
         FIG. 12  is an approximate view of a positional relation of light receiving elements in the present embodiment when a print dot density is 1200 dpi and 2400 dpi. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of an image forming apparatus including a light scanning apparatus according to the present invention will be described below with reference to  FIGS. 1 to 4 . 
     As shown in  FIG. 2 , the image forming apparatus comprises a photosensitive body  1 , which is a recording medium for forming a toner image and is also used as a medium to be scanned, the photosensitive body being in a drum shape and being rotated in one direction, a charging apparatus  2  for uniformly charging the photosensitive body  1  to a particular polarity, an optic scanning apparatus  3  that is disposed downstream of the charging apparatus  2  in the rotational direction of the photosensitive body  1  and forms an electrostatic latent image by irradiating rays to the charged photosensitive body  1 , a developing apparatus  4  that opposes the photosensitive body  1  downstream of the optic scanning apparatus  3  in the rotational direction of the photosensitive body  1 , a carrying apparatus  5 , a print form  6  that is brought into contact with the photosensitive body  1  downstream of the developing apparatus  4  in the rotational direction of the photosensitive body  1  and carried by the carrying apparatus  5  in synchronization with the rotation of the photosensitive body  1 , a transfer apparatus  7  disposed at a position where the print form  6  touches the photosensitive body  1 , a cleaning apparatus  8  disposed upstream of the charging apparatus  2  in the rotational direction of the photosensitive body  1 , and a fixing apparatus  9  disposed downstream of the transfer apparatus  7  in a direction in which the print form  6  moves. 
     In the image forming apparatus structured as described above, the photosensitive body  1 , which rotates, is charged by the charging apparatus  2  so as to form a toner image. The photosensitive body  1  is then exposed to rays irradiated by the optic scanning apparatus  3  and forms an electrostatic latent image. When toner is then supplied from the developing apparatus  4  onto the photosensitive body  1 , a toner image corresponding to the electrostatic latent image is formed on the photosensitive body  1 . The print form  6  comes in contact with the photosensitive body  1  on which the toner image is formed. When the transfer apparatus  7  is operated to charge the back surface of the print form  6  with a polarity opposite to the polarity of the toner, the toner image on the photosensitive body  1  is transferred to the front surface of the print form  6 . After the transfer process is completed, toner not transferred to the print form  6  is removed from the photosensitive body  1  by the cleaning apparatus  8 , and the toner image transferred to the print form  6  is fixed by the fixing apparatus  9 . The fixing apparatus  9  has a heat role, the heating of which is controlled so that it is held at a fixed temperature, and a pressurizing roller pressed against the heat roller. When the print form  6  passes between these rollers, the toner image held on the print form  6  is pressed, melted, and fixed. After the fixing process is completed, the print form  6  is transferred to the outside of the image forming apparatus. 
     As shown in  FIG. 3 , the optic scanning apparatus  3  comprises a photosynthesis device  10  for synthesizing rays emitted from a plurality of light sources described later, a cylindrical lens  12  that passes synthesized rays (second synthesized rays)  11  synthesized by the photosynthesis device  10  and has a prescribed curvature only in a vertical scanning direction, a rotating polyhedral mirror  13 , which is an optical deflecting device for deflecting and scanning the second synthesized rays  11  that have passed through the cylindrical lens  12 , an Fθ lens  14  that passes the synthesized rays  11  deflected by the rotating polyhedral mirror  13 , a folding mirror  15  for reflecting the synthesized rays  11  that have passed through the Fθ lens  14  and forming an electrostatic latent image on the surface of the photosensitive body  1  shown in  FIG. 2 , and a sensor  17  for sensing part of the synthesized rays  11  that have passed through the Fθ lens  14  and then reflected by a mirror  16 . An output signal from the sensor  17  is used to start modulation for writing the synthesized rays  11  irradiated from the photosynthesis device  10 . 
     As shown in  FIG. 1 , the photosynthesis device  10  of the optic scanning apparatus  3  structured as described above comprises a first light source unit  18 , a second light source  19  for irradiating a ray, the optic axis of which differs 90 degrees from the optic axis of a ray from the first light source  18 , a photosynthesis device  20  including a prism for synthesizing the two rays irradiated from the first light source  18  and second light source  19  by reflecting either of the rays and transmitting the other, an imaging lens  22  for transmitting the first synthesized ray  21  synthesized by the photosynthesis device  20  in the optic axis direction of the second light source  19 , and a supporting box  23  for supporting these elements; a light receiving device  24  for receiving the first synthesized ray  21  that has passed through the imaging lens  22  is disposed at a position separated by the focal length of the imaging lens  22 . The photosynthesis device  10  has a pair of oscillating mechanisms  25  on both sides in a direction in which the second synthesized ray  11  synthesized by the photosynthesis device  20  in the supporting box  23  propagates, as well as a driving arm  26  extending from the supporting box  23  in the same direction as the direction in which the first synthesized ray  21  propagates. A threaded rod  27  is threaded into an end of the driving arm  26 . A motor  28  is connected to the threaded rod  27  for rotating the threaded rod  27  in the normal or reverse direction. The light receiving device  24  determines whether the first synthesized ray  21  is received, the result is output to a control device  29 , and the motor  28  is driven according to the result to control the inclination of the supporting box  23 . The driving arm  26 , threaded rod  27 , motor  28 , and control device  29  constitute an inclining device for inclining the photosynthesis device  10  of the embodiment in the present invention. 
     In the photosynthesis device  10  structured as described above, the light receiving device  24  detects a spot of the first synthesized ray  21  that has passed through the imaging lens  22 . The spots are arranged in a plane including the optic axis of the first synthesized rays  21  from the first light source  18  and second light source  19  so that the spots are in parallel to the optic axis of the second synthesized ray  11 . When the supporting box  23  is inclined, the position of a spot near the light receiving device  24  changes in the vertical scanning direction. 
     A displacement of spots from the light receiving device  24  indicates that the angle of the spot sequence formed by the second synthesized rays  11  on the surface of the photosensitive body  1  has changed. In this case, the control device  29  drives the motor  28  to rotate it in the normal or reverse direction. The threaded rod  27  is then rotated to swing the driving arm  26  upward or downward with the oscillating mechanisms  25  being a fulcrum so as to adjust the inclination of the supporting box  23 . The adjustment of the inclination of the supporting box  23  enables the light receiving device  24  to receive the spot of the first synthesized ray  21 . When control is performed so that the amount of light received is maximized, the angle of the spot sequence formed by the second synthesized rays  11  on the photosensitive body  1  can be held at a fixed value. This spot position change is represented as a change turned around the second synthesized ray  11  with a radius of the sum of the focal length of the imaging lens  22  and the distance between the imaging lens  22  and the photosynthesis device  20 , so a small change can be magnified at the time of detection. 
     The image forming apparatus may cause pitch flecks on a printout due to assembly error or parts error even if the angle of the spot sequence is set as designed. When this happens, it is necessary to adjust the angle of the spot sequence so that pitch flecks are not generated while the print state is being checked. Specifically, horizontal lines are actually printed; if there are variations in intervals of the printed horizontal lines, the printing is repeated with different angles of the spot sequence on the photosensitive body  1 . The angle of the spot sequence is adjusted and the spot sequence is fixed at an angle at which the variations in the intervals are minimized. 
     When a worker performs this adjustment by manually inclining the supporting box  23  little by little, a slight inclination causes a large inclination of the optic axis of the first synthesized ray  21 , making fine adjustment of the angle extremely difficult. 
     In this embodiment of the present invention, however, this fine adjustment can be performed by changing the position of the light receiving device  24 . 
     Specifically, when the position of the light receiving device  24  is changed, the control device  29  drives the motor  28  to adjust the inclination of the supporting box  23  so that the spot of the first synthesized ray  21  from the imaging lens  22  is positioned at the center of the light receiving device  24 . When the position of the light receiving device  24 , which is a target position of a spot position near the light receiving device  24 , is adjusted, the same effect as when the target angle is adjusted is provided. Since a magnified spot sequence angle can be adjusted instead of manually adjusting the angle of the supporting box  23 , adjustment can be performed with ease. If the distance between the rotational center of the supporting box  23  and the light receiving device  24  is five times as long as the distance between the center of the supporting box  23  and the end of the supporting box  23 , adjustment by changing the position of the light receiving device  24  is five times as fine as adjustment by manually inclining the supporting box  23 . 
     Next, the specific structure of the first light source  18  will be described with reference to  FIG. 4 . The description for the structure of the first light source  18  is also applied to that of the second light source  19 , so its description will be omitted. 
     A light source  30  is secured to a light source holder  31  with screws  32 . A collimator lens holder  33  is secured to the light source holder  31  with screws  34 , and a collimator lens  35  is secured thereto with a screw  36 . When the screws  34  and the screw  36  are loosened, the optic axis direction of the collimator lens  35  can be adjusted and the direction perpendicular to the optic axis between the light source  30  and the collimator lens  35  can be adjusted Upon the completion of the adjustment, the position of the collimator lens  35  is fixed by tightening the screws  34  and the screw  36 . 
     The focal length of the imaging lens  22  of the photosynthesis device  10  may be determined according to an allowable value of the inclination angle of the supporting box  23  and the position detection precision of the light receiving device  24 . Specifically when the position detection precision of the light receiving device  24  is ±1 mm and the allowable value of the inclination angle of the supporting box  23  is ±0.5 degree, the focal length may be set to 114.6 mm. The equation to calculate it is 1/tan (0.5 degree). 
     Next, how to switch the print dot density by using the photosynthesis device  10  will be described with reference to  FIGS. 5 to 9 . 
       FIG. 5  shows an exemplary spot sequence on the photosensitive body  1 . In this example, a spot sequence  40  from five first light sources  18  aligned at equal intervals and a spot sequence  41  from five second light sources  19  aligned at equal intervals are synthesized. These spot sequences  40  and  41  are also formed on the light receiving device  24  in a similar manner, and used to detect the inclination of the supporting box  23 . 
     As shown in  FIG. 6 , the light receiving device  24  has a light receiving element  42  in such a way that it can move vertically. Specifically, the light receiving element  42  is mounted in an element holder  43 , and the element holder  43  is included in such a way that it can move only vertically within a frame  44 . The element holder  43  is always pressed downward by a spring  45 . The element holder  43  is also pressed by a pressing rod  47  from below against the force of the spring  45 , the pressing rod  47  being extruded or retracted by the rotation of a motor  46 . When the pressing rod  47  is a threaded rod and a threaded hole is formed at the bottom of the element holder  43  to accept the threaded rod, the spring  45  can be eliminated. 
     When the vertical position of the light receiving element  42  is changed in this way, the rotational radius for adjustment of the spot sequences  40  and  41  on the photosensitive body  1  can be enlarged, as compared with a case in which the inclination of the supporting box  23  is adjusted. As a result, fine angle adjustment can be performed with high precision, enabling the angle to be adjusted precisely. 
     When, in the above structure, the element holder  43  is raised or lowered by rotating the motor  46  and thereby the position of the light receiving element  42  is changed, the angle of the spot sequence on the photosensitive body  1  can be changed responsive to the changed position of the light receiving element  42 . Accordingly, when the position of the light receiving element  42  is changed so as to lessen the angle, a switchover to a high print dot density can be made. 
     A light receiving element  42   a  for a reference print dot density and a light receiving element  42   b  for another print dot density may be provided in the fixed element holder  43  as shown in  FIG. 7  so that a switchover between the print dot densities can be made by inclining the supporting box  23  to select the light receiving element  42   a  or light receiving element  42   b.  When a switchover is made between the print dot densities, it is necessary to change the revolutions of the rotating polyhedral mirror  13  and the modulated frequencies of the light sources besides the changing of the scanning interval involved in the change of the angle of the above spot sequence. 
     Each of the light receiving elements  42 ,  42   a,  and  42   b  may be a single optic sensor, a two-part sensor, or a CCD camera if it can easily detect the positions, in the vertical scanning direction, of the light spot sequences. 
     An aspect of the photosynthesis device  10  will be described below, assuming that the reference print dot density is 1200 dpi. Five light sources are arranged at intervals of 150 μm, and the focal length of the collimator lens  35  is 15 mm. The magnification, in the scanning direction, of the scanning lens system including the Fθ lens  14  is 13 times, and that in the vertical scanning direction is 4 times. 
     The spot sequences  40  and  41  are synthesized into a single sequence on the photosensitive body  1 , as shown in  FIG. 8 . Since the reference print dot density is 1200 dpi, the intervals in the vertical scanning direction are uniformly fixed to 21.17 μm. The light sources are arranged so that element sequences  50  and  51  are aligned as shown in  FIG. 9 . Since the magnitude in the vertical scanning direction is 4 times, one interval, in the vertical scanning direction, in each element sequence before it is synthesized is 10.58 μm (=42.33/4). Since one interval in the element sequence before it is synthesized is 150 μm, an inclination angle α of 4.0446 is obtained from sin (α)=10.58/150. One interval, in the scanning direction, in each element sequence before it is synthesized is determined to be 149.6264 μm from 150×cos (α). One interval in the scanning direction on the photosensitive body  1  shown in  FIG. 8  is then 1945.14 μm because the magnification in the scanning direction is 13 times. The inclination angle θ of the spot sequence shown in  FIG. 8  is determined to be 1.2468 degrees from tan (θ)=42.33/1945.14. It is known that a deviation of about ±10% is allowed for the inclination angle θ from the viewpoint of print quality, variations need to be about ±0.4 degree or less. 
     Next, the focal length of the imaging lens  22  in the photosynthesis device  10  will be described. The use of a single optic sensor as the light receiving element of the light receiving device  24  poses a problem; if its dimension in the vertical scanning direction is large, the range of variations becomes large; if the dimension in the vertical scanning direction is small, light does not enter the optic sensor in a stable manner. When the dimension in the vertical scanning direction is 1 mm, if the focal length of the imaging lens  22  is set to 143.2 mm or more, variations in the spot sequence angle can be suppressed to ±0.4 degree or less. According to this, when the focal length of the imaging lens  22  is set to 150 mm, the interval in the spot sequence on the light receiving device  24  is 0.75 mm and thereby a dimension of 7.5 mm or more is needed. In this aspect, the dimension is set to 10 mm to provide a margin. Accordingly, it can be known that the dimensions of the light receiving device  24  should be set to 1 mm in the scanning direction and 10 mm in the vertical scanning direction. 
     When a switchover from the basic print dot density 1200 dpi to 2400 dpi is explained, it suffices to use the same process speed, double the revolutions of the rotating polyhedral mirror  13 , quadruple the modulated frequency of the light source, and change the interval, in the vertical scanning direction on the photosensitive body  1 , of the synthesized rays  11  from the light source from 21.17 μm to 10.58 μm. In this method, the light source is rotated around the optic axis. 
     In  FIGS. 10 and 11 , the height of one light receiving element  42  held in the element holder  43  is changed and the supporting box  23  is inclined to match the change. In  FIG. 10 , the print dot density is 1200 dpi, so the inclination angle of the supporting box  23  needs to be 4.0446 degrees. To achieve this, the motor  28  in  FIG. 1  is run to rotate the threaded rod  27  so that a driving arm  26  inclines downward so as to incline the first synthesized rays  21  from the imaging lens  22  with a focal length of 150 mm downward 10.6 mm with respect to the optic axis when the supporting box  23  is horizontal.  FIG. 11  shows a case in which the print dot density is 2400 dpi. The same operation as in  FIG. 10  is performed so that the first synthesized rays  21  from the imaging lens  22  incline downward 5.3 mm, which is half the downward displacement in  FIG. 10 , with respect to the optic axis when the supporting box  23  is horizontal. 
     In  FIG. 12 , two light receiving elements  42   a  and  42   b  are provided in the element holder  43  in the vertical direction with an interval, as in  FIG. 7 . When the print dot density is 1200 dpi, the light receiving element  42   a  is positioned 10.6 mm downward from the optic axis when the supporting box  23  is horizontal; when the print dot density is 2400 dpi, the light receiving element  42   b  is positioned 5.3 mm downward from the horizontal optic axis. 
     In an exemplary method of doubling the revolutions of the rotating polyhedral mirror  13  and quadrupling the modulated frequency of the light source, a plurality of revolutions settings and a plurality of modulated frequency settings are given and ones of them are selectively selected. In another method, a single reference revolutions setting and a single reference frequency setting are given, and a plurality of device for switching their ratios are provided to make a switchover.