Abstract:
A multi-beam image forming apparatus includes a beam generating unit  101,  an optical scanner  102,  an image processing unit  103  for dividing image data into several areas in a primary scanning direction, and image writing units  106  and  107  for writing an image while switching multiple laser beams in accordance with each of the areas in the primary scanning direction in a printing region based on the divided image data, wherein the image processing unit  103  judges continuity of the image data when dividing the image data, so that the image processing unit  103  divides the image data at an image data portion having continuity and transmits the divided image data to the image writing units  106  and  107.  The multi-beam image forming apparatus is provided as an apparatus in which scan positions can be aligned accurately both in the primary scanning direction and the secondary scanning direction and which is suitable for wide printing.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a multi-beam image forming apparatus for forming an image based on image data transmitted from a host apparatus or the like. Particularly it relates to a multi-beam image forming apparatus suitable for wide printing of the image. 
         [0003]    2. Description of the Background Art 
         [0004]    An electrophotographic image forming apparatus such as a laser printer, a digital copying machine, etc. forms an image on a sheet of paper by a method including the steps of: forming an electrostatic latent image corresponding to recorded information on a photoconductor by a laser beam output from a beam generating unit after electrostatically charging a surface of the photoconductor evenly; developing the electrostatic latent image with toner to form a toner image; transferring the toner image to a sheet of paper by using a transfer portion; and fixing the toner image by using a fixing portion. 
         [0005]    As this type image forming apparatuses, there have been heretofore proposed various multi-beam image forming apparatuses using a polygon mirror for directing laser beams toward scan lines simultaneously to form an image. The multi-beam image forming apparatus has an advantage that an image can be formed at a high speed by use of a low-speed rotating polygon motor and a low-power semiconductor laser because an image corresponding to multiple scan lines can be formed by one surface of the polygon mirror. 
         [0006]    Use of an ASIC as an image writing means for the image forming apparatus has become the mainstream with the recent advance of semiconductor manufacturing technology. Provision of the ASIC as a general-purpose ASIC to be used in various image forming apparatuses makes mass production and drastic cost-cutting possible. 
         [0007]    On the other hand, there has been recently a demand for a laser beam high-definition image forming apparatus, for example, using a wide sheet of paper having a sheet size of more than 20 inches. In the case of 20 inches and 1200 dpi, 24000 dots are simply required as the number of image data in the primary scanning direction. If bit depth is taken into consideration, the number of image data increases to twice or three times. It has been necessary to design a product to use the background-art ASIC for forming such a wide image in accordance with the increase in the number of image data. 
         [0008]    As such a wide image forming method, there is a method using the background-art ASIC effectively by dividing an image forming area into parts on a photoconductor.  FIGS. 6A to 6C  are schematic configuration views of optical scanners applied to the background-art image forming apparatus. In  FIGS. 6A to 6C , the reference numeral  1  designates a beam generating unit;  2 , a polygon mirror as a scanning unit;  3 , an fθ lens; and  4 , a drum-shaped photoconductor. Each of the optical scanners has at least one beam generating unit  1 , at least one polygon mirror  2 , and at least one fθ lens  3 . 
         [0009]    Each of the optical scanners shown in  FIG. 6A and 6B  has two beam generating units  1 , and two polygon mirrors  2 . The optical scanner shown in  FIG. 6C  has two beam generating units  1 , and one polygon mirror  2 . 
         [0010]    The optical scanner shown in  FIG. 6A  is configured to rotate the two polygon mirrors  2  in the same direction so that laser beams output from the beam generating units  1  are scanned in the same direction. For this reason, scan positions of laser beams can hardly be aligned accurately in the primary scanning direction because the second scan start position needs to coincide with the first scan end position. There is another technical problem that scan positions of laser beams cannot be aligned in the secondary scanning direction unless rotations of the two polygon mirrors  2  are synchronized with each other. 
         [0011]    As shown in  FIG. 6B , the optical scanner described in JP-A-6-208066 is configured to rotate the two polygon mirrors  2  in opposite directions to scan laser beams from the center toward opposite ends. For this reason, scan positions of laser beams can be aligned easily in the primary scanning direction. There is however a technical problem that scan positions of laser beams cannot be aligned in the secondary scanning direction unless rotations of the two polygon mirrors  2  are synchronized with each other. 
         [0012]    An optical scanner described in JP-A-8-72308 is configured to rotate two polygon mirrors  2  by one drive source to synchronize rotations of the two polygon mirrors  2  with each other. It is however practically difficult that the two polygon mirrors  2  requiring high-speed rotations are rotated simultaneously by one drive source. 
         [0013]    As shown in  FIG. 6C , another optical scanner described in JP-A-8-72308 is configured so that laser beams from two beam generating units  1  are made incident on different planes of polarization of one polygon mirror  2  and joined together in the primary scanning direction on the photoconductor  4 . In this configuration, scan positions of laser beams can be aligned easily in the secondary scanning direction because only one polygon mirror  2  is provided. It is however difficult to accurately align scan positions of laser beams in the primary scanning direction because laser beams are scanned in the same direction so that the second scan start position need to coincide with the first scan end position. 
         [0014]    In the aforementioned background-art configurations, it is technically difficult to align scan positions accurately though rotations of two polygon mirrors need to be synchronized with each other. Even when only one polygon mirror is provided, there is a technical problem that it is difficult to align scan positions accurately in the primary scanning direction because laser beams from two beam generating units are scanned in the same direction. 
       SUMMARY OF THE INVENTION 
       [0015]    In order to solve the problems in the background art, an object of the present invention is to provide a multi-beam image forming apparatus in which scan positions can be aligned accurately both in the primary scanning direction and the secondary scanning direction and which is suitable for wide printing. 
         [0016]    To achieve the foregoing object, in accordance with a first aspect of the present invention, there is provided a multi-beam image forming apparatus including: an optical scanner which has a beam generating unit for generating multiple laser beams, and a scanning unit for scanning the multiple laser beams simultaneously; an image processing unit which divides image data into areas in a primary scanning direction; and image writing units which write an image while switching the multiple laser beams in accordance with each of the areas in the primary scanning direction in a printing region based on the image data divided by the image processing unit. 
         [0017]    In the first aspect of the invention, the image processing unit judges continuity of the image data when dividing the image data, so that the image processing unit divides the image data at an image data portion having continuity and transmits the divided image data to the respective image writing units. 
         [0018]    According to a second aspect of the invention, there is provided a multi-beam image forming apparatus according to the first aspect, wherein the image processing unit has: line memories which store the image data before dividing; a latch circuit which temporarily holds the image data stored in the line memories; a recording medium which stores an image pattern used for judging whether the image data have continuity or not; and a comparison circuit which compares an image pattern of the image data held in the latch circuit with the image pattern stored in the recording medium and outputs a dividing signal when the comparison circuit makes a decision that the image data have continuity. 
         [0019]    According to a third aspect of the invention, there is provided a multi-beam image forming apparatus according to the first aspect, wherein the judgment of continuity of the image data results in a decision that the image data have continuity when several white dots are lined continuously. 
         [0020]    According to the invention, displacement in scan position in the secondary scanning direction can be eliminated because the invention is made up of one beam generating unit and one scanning unit. Moreover, even when displacement occurs in scan position in the primary scanning direction, the displacement can be made inconspicuous because the timing of dividing image data can be changed. In addition, there is a large advantage in terms of cost because the image writing units are made up of ASICs to eliminate the necessity of producing wide-range ASICs newly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic configuration view of a multi-beam image forming apparatus having an optical scanning system as a main body according to a basic embodiment of the present invention; 
           [0022]      FIG. 2  is a block diagram showing the configuration of an image processing unit used in the embodiment of the invention; 
           [0023]      FIGS. 3A and 3B  are views showing examples of configuration of a beam generating unit used in the embodiment of the invention; 
           [0024]      FIG. 4  is an image view showing image data dividing position in the embodiment of the invention; 
           [0025]      FIG. 5  is a schematic configuration view of the multi-beam image forming unit according to the embodiment of the invention; and 
           [0026]      FIGS. 6A to 6C  are views showing examples of an optical scanning system according to the background art. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    An embodiment of the present invention will be described below with reference to  FIGS. 1 to 5 . The schematic configuration of a multi-beam image forming apparatus having an image forming system as a main body will be described first with reference to  FIG. 5 . 
         [0028]    After a drum-shaped photoconductor  500  for forming a toner image is electrostatically charged evenly by an electrostatic charging device  501 , the photoconductor  500  is exposed to laser beams output from an optical scanner  502  in accordance with image data which are to be recorded and which are transmitted from a host apparatus  507  such as a host computer. In this manner, an electrostatic latent image is formed on the photoconductor  500 . Then, a developing agent is fed onto the photoconductor  500  by a developing device  503 , so that the electrostatic latent image is developed to a toner image. 
         [0029]    The toner image formed on the photoconductor  500  is transferred onto a printing sheet  508  by a transfer device  504 . The printing sheet  508  having the transferred toner image thereon is conveyed to a fixing device  505 , so that the toner image on the printing sheet is pressure-melted and fixed onto the printing sheet  508 . A part of toner which remains on the photoconductor  500  because it has not been transferred by the transfer device  504  is removed by a cleaning device  506  in preparation for next image formation. 
         [0030]      FIG. 1  is a schematic configuration view of a multi-beam image forming apparatus having an optical scanning system as a main body according to a basic embodiment of the invention. As shown in  FIG. 1 , the multi-beam image forming apparatus includes an optical scanner  100  (equivalent to the optical scanner  502  in  FIG. 5 ), an image processing unit  103 , and an image writing unit  104 . These constituent parts  100 ,  103  and  104  are connected as shown in  FIG. 1 . The image processing unit  103  is connected to the host apparatus  507 . 
         [0031]    The optical scanner  100  has a beam generating unit  101 , a polygon mirror  102  as a scanning unit, and an fθ lens  105 . The fθ lens  105  faces the photoconductor  500 . The image writing unit  104  has two writing devices, that is, a write ASIC device ( 1 )  106  and a write ASIC device ( 2 )  107 . 
         [0032]    For example, the beam generating unit  101  is configured as shown in  FIGS. 3A and 3B .  FIG. 3A  shows an example in which two semiconductor laser arrays are used.  FIG. 3B  shows an example in which one semiconductor laser array is used. In this embodiment, a beam generating unit which generates 20 beams is used as the beam generating unit  101 . 
         [0033]    In  FIG. 3A , the reference numerals  300  and  301  designate semiconductor laser arrays (hereinafter referred to as LDAs) each of which generates 10 beams. The laser beams generated by the LDAs  300  and  301  enter a beam splitter  302 , so that the laser beams are combined and output as 20 laser beams from the beam splitter  302 . In  FIG. 3B , the reference numeral  303  designates a semiconductor laser array which is composed of 20 laser components and which outputs 20 laser beams. 
         [0034]    Referring back to  FIG. 1 , the 20 laser beams output from the beam generating unit  101  are irradiated on deflective and reflective surfaces of the polygon mirror  102  which is a scanning unit for scanning the surface of the photoconductor  500 . The laser beams reflected by the polygon mirror  102  pass through an imaging unit such as the fθ lens  105 , so that an image is formed on the photoconductor  500 . 
         [0035]    In  FIG. 1 , the reference symbol X designates the number of image data in the primary scanning direction, that is, an image forming region. In this embodiment, the image forming region X has 24000 dots. If the image forming region X is simply allocated equally to the write ASIC device ( 1 )  106  and the write ASIC device ( 2 )  107 , the write ASIC device ( 1 )  106  performs image writing for a region of from the first dot to the 12000th dot while the write ASIC device ( 2 )  107  performs image writing for a region of from the 12001st dot to the 24000th dot. There is however a possibility that positional displacement will occur in a printing image at the time of switching of writing in accordance with characteristic difference between the write ASIC devices  106  and  107 , image data of the 12000th dot and the 12001st dot on the turn and image data of several lines in the secondary scanning direction. 
         [0036]    Therefore, the image processing unit  103  has a function of properly changing the dividing position of image data based on image data of ambient dots so that positional displacement caused by switching of image data writing becomes inconspicuous in a printing image corresponding to image data to be recorded. 
         [0037]      FIG. 2  is a block diagram showing the configuration of the image processing unit  103 . As shown in  FIG. 2 , the image processing unit  103  has a speed change line memory  200 , a first line memory  201 , a second line memory  202 , a third line memory  203 , a latch circuit  204 , a comparison circuit  205 , an image data counter  206 , a recording medium  207 , a dividing circuit  208 , and a delay circuit  209 . These constituent parts  200  to  209  are connected as shown in  FIG. 2 . As shown in  FIG. 2 , the latch circuit  204  is composed of a large number of shift registers  213  provided in accordance with the line memories  201  to  203 . 
         [0038]    The host apparatus  507  (see  FIG. 1 ) transmits image data  210  of 20 lines per scan and a transfer clock signal  211 , for example, of 20 MHz to the image processing unit  103 . The image processing unit  103  having the plurality of line memories  200  to  203  writes the image data into the line memories  200  to  203  in synchronization with the transfer clock signal. 
         [0039]    An operation sequence of the line memories  200  to  203  will be described below. First, image data of first 20 lines (called 20 lines a) are written into the speed change line memory  200  successively in synchronization with the transfer clock signal  211 . The written 20 lines a are read in synchronization with a printing pixel sync clock signal  212  (for example, of 60 MHz) in a next scan. At the same time, next 20 lines (called 20 lines b) are written in synchronization with the transfer clock signal  211 . The aforementioned operation is performed in accordance with each scan in order to change the speed of image data. 
         [0040]    On the other hand, the read 20 lines a are written into the line memory  201  in synchronization with the printing pixel sync clock signal  212 . In a further next scan, the 20 lines a are read from the line memory  201  and 20 lines b are written into the line memory  201 . In a further next scan, 20 lines c are written into the line memory  201  while the 20 lines b are read from the line memory  201 , the 20 lines b are written into the line memory.  202  while the 20 lines a are read from the line memory  202 , and the 20 lines a are written into the line memory  203 . In this manner, repetition of data reading and data writing permits the image processing unit  103  to hold image data of 60 lines at all times. 
         [0041]    The data of 60 lines read from the group of line memories  201  to  203  are transferred to the latch circuit  204 . The latch circuit  204  shifts the image data to the right in  FIG. 2  in synchronization with the printing pixel sync clock signal  212  and holds the data up to next synchronization timing of the printing pixel sync clock signal  212 . The number of image data held in the primary scanning direction is decided based on the number of latches in the latch circuit  204 . For example, assuming now that the number of image data held is 60, then image data of 60 dots in the primary scanning direction and 60 dots in the secondary scanning direction are transmitted, as information for making a decision for dividing image data, to the comparison circuit  205 . 
         [0042]    The image processing unit  103  has the image data counter  206  for counting the number of image data in synchronization with the printing pixel sync clock signal  212 . In this embodiment, the image processing unit  103  validates the comparison circuit  205  when the number of counts reaches  11970 , and the image processing unit  103  invalidates the comparison circuit  205  when the number of counts reaches  12030 . 
         [0043]    When the comparison circuit  205  is validated, the comparison circuit  205  compares the image pattern with an image pattern stored in advance in the recording medium  207  while shifting the image data dot by dot in the primary scanning direction to judge whether the image data have continuity or not. When the comparison circuit  205  makes a decision that the image data have continuity, the comparison circuit  205  outputs a dividing signal for dividing image data to the dividing circuit  208  so that image data transfer is changed from the write ASIC device ( 1 )  106  to the write ASIC device ( 2 )  107 . When the comparison circuit  205  makes a decision that the image data have no continuity, the comparison circuit  205  outputs a dividing signal for dividing image data at the time of the 12000th dot as the count number so that image data are allocated equally to the write ASIC device ( 1 )  106  and the write ASIC device ( 2 )  107 . 
         [0044]    The image processing unit  103 , which has the delay circuit  209  for delaying image data to absorb the time required for judgment of continuity, eliminates the time lag between the image data and the dividing signal. In this embodiment, the delay circuit  209  is provided as a circuit for delaying image data for 60 dots. 
         [0045]      FIG. 4  shows an image example in which image data dividing is performed based on image data to be printed in the condition that an ordinary boundary of the dividing position of image data is set between the 12000th dot and the 12001st dot as described above. 
         [0046]    A character “R” and a Japanese character           are shown in this example. Whether or not the image data have continuity is judged in the condition that each character is divided by 60 dots in the primary scanning direction and by 60 dots in the secondary scanning direction ( FIG. 4  shows a state in which the number of divisions is small for the sake of simplification). When several white or black dots (e.g. about 5 to 10 dots) are continuous, a decision is made that the image data have continuity. Since there is a possibility that white stripes will occur when the dividing position is decided based on black dots, and since decision of the dividing position based on white dots has an advantage that displacement becomes inconspicuous, the dividing position in this embodiment is decided based on white dots. The line  400  in  FIG. 4  is a dividing line by which the image data are divided. 
         [0047]    In the character “R” shown in  FIG. 4 , image data dividing in the ordinary boundary is used because the character “R” is high in both continuity of black dots and continuity of write dots. On the other hand, in the Japanese character          , the dividing position of image data is set at a high-continuity position as shown in  FIG. 4  so that image displacement caused by switching in the image writing unit can be made inconspicuous, because the Japanese character           is low in continuity in the ordinary boundary. Incidentally, the position of the vertical line attached to each of the characters “R” and           expresses the dividing position of image data in  FIG. 4 .