Abstract:
An image forming apparatus includes a semiconductor laser array to emit a plurality of laser beams through a plurality of independently controllable laser elements according to an image signal, an imaging optical system to magnify the laser beams emitted from the laser array, and a photosensitive drum having an imaging surface to form an electrostatic latent image by being photo-exposed through the imaging optical system. The imaging optical system includes a first mirror group having a plurality of reflection mirrors to reflect the laser beams emitted from the laser array, disposed at a given distance from the laser array, a second mirror group having a plurality of reflection mirrors to reflect the laser beams reflected from the first group, and a third mirror group having a plurality of reflection mirrors to reflect the laser beams reflected from the second group onto the imaging surfaces

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims the benefit of Korean Application No. 2002-1199, filed Jan. 9, 2002, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an imaging optical system, image forming apparatus having the same, and a method therefor, which are adapted for use in a printer, a copier, a facsimile machine, etc. More particularly, the present invention relates to an imaging optical system and image forming apparatus having the same, in which image light fluxes, such as a laser beam, are emitted on an imaging surface of a photosensitive material by using a plurality of combined reflection mirrors.  
           [0004]    2. Description of the Related Art  
           [0005]    Generally, an image forming apparatus which is used in a printer, a copier, a facsimile machine, etc., has a laser scanning unit (LSU) to form an electrostatic latent image on a photosensitive material, such as a photosensitive drum. The image is formed by irradiating light fluxes according to an image signal . The apparatus further has an image forming part to develop the electrostatic latent image formed on the photosensitive material into a toner image through a developer, such as colored toner, and transfer the image onto a sheet of printing paper to form a desired image.  
           [0006]    To form the electrostatic latent image on the photosensitive material, the LSU has a light source having a plurality of luminous elements such as light emitting diodes (LED), laser diodes (LD), or vertical cavity surface emitting lasers (VCSEL), which are one or two-dimensionally arranged.  
           [0007]    As an example of an LSU has a polygon mirror. The LSU changes light fluxes emitted from the light source into parallel light beams through a collimating lens, leads the fluxes to the polygon mirror to deflect a reflective direction of the light beams, and then scans the beams as light spots conforming to a printing pattern on an imaging surface of the photosensitive material through an fθ lens to form the electrostatic latent image thereon.  
           [0008]    However, the LSU employing the polygon mirror has a problem in that mechanical vibration and noise occur because of the mechanical rotation of a motor to rotate the polygon mirror during scanning. Specifically, in order to achieve a high scanning speed, the speed of revolution of the polygon mirror is increased, and the increase in noise is ever greater, and rotation accuracy and reliability of the motor are lowered because of an increase in a rotation load.  
           [0009]    In order to overcome these problems, an LSU using an imaging optical system as a magnification optical system to image light fluxes by using a plurality of combined lenses has been proposed. This LSU does not require a polygon mirror, and is shown in FIG. 1.  
           [0010]    [0010]FIG. 1 illustrates an image forming apparatus  10  having a semiconductor laser array  1  in which a plurality of laser elements la are one or two-dimensionally arranged, and an imaging optical system  4  as a magnification optical system to magnify a plurality of laser beams emitted from the laser array  1  at a given angle and position . The apparatus  10  also includes a photosensitive drum  3  having an imaging surface to image the laser beams outputted from the imaging optical system  4  thereon, a drum driving circuit  5  to drive the photosensitive drum  3  at a given rotation speed and to output a timing signal synchronously with the rotation of the photosensitive drum  3 , a signal processing circuit  7  to read an image signal from an image memory  6  and process the image signal to output a printing signal conforming to a printing pattern to a laser driving circuit  8 , and a control circuit  9  to output a control signal to the laser driving circuit  8  synchronously with the timing signal from the drum driving circuit  5 .  
           [0011]    The imaging optical system  4  includes a first lens group  4   a  disposed at a photosensitive drum side of the imaging optical system  4  to image incident laser beams  2  traveling vertically from the laser elements  1   a  on the imaging surface of the photosensitive drum  3 . The imaging optical system  4  further includes a second lens group  4   b  having a stop  4   e  and a composite focus  4   d  adjacent to the first lens group  4   a  toward the laser array  1 , and a third lens group  4   c  disposed between the second lens group  4   b  and the laser array  1 .  
           [0012]    The second and third lens groups  4   b ,  4   c  form a telecentric optical system, so that the curvature of the laser beams  2  incident on the third lens group  4   c  and an aperture of the third lens group  4   c  can be reduced. Also, principal light beams  2   a  of the laser beams  2  from the laser array  1  are bent at two stages by the second lens group  4   b  and the third lens group  4   c.    
           [0013]    During operation of the apparatus  10 , according to a printing command from a computer or central processing unit (CPU) (not shown), the signal processing circuit  7  reads an image signal from the image memory  6  and processes the signal to output a printing signal conforming to a printing pattern to the laser driving circuit  8 .  
           [0014]    At this time, the drum driving circuit  5  controls the photosensitive drum  3  to rotate at a given speed of rotation and simultaneously outputs a timing signal synchronous with the rotation of the photosensitive drum  3  to the control circuit  9 . As the drum driving circuit  5  outputs the timing signal, the control circuit  9  outputs a control signal to the laser driving circuit  8  synchronously with the timing signal.  
           [0015]    In response to the control signal from the control circuit  9  and the printing signal from the signal processing circuit  7 , the laser driving circuit  8  drives the laser array  1 . At this time, the laser array  1  emits the plurality of laser beams  2  in parallel with the optical axis (direction Z). The laser beams  2  first enter the imaging optical system  4 , and the laser beams  2  incident upon the imaging optical system  4  are bent by light gathering action of the third and second lens groups  4   c ,  4   b . Then, the laser beams  2  are gathered at the composite focus  4   d  positioned at an imaging surface side of the second lens group  4   b , and are scanned in a main scanning direction as a plurality of light spots on the imaging surface of the photosensitive drum  3 . At this time, since the photosensitive drum  3  is driven to be rotated in a sub-scanning direction by a driving motor (not shown), an electrostatic latent image conforming to the image signal is formed on the photosensitive drum  3 . This is the result of the movement of the light spots in the main scanning direction and the movement of the photosensitive drum  3  in the sub-scanning direction.  
           [0016]    However, in the above-described image forming apparatus  10 , to reduce an optical aberration, distances D 1 ′, D 2 ′ between the laser array  1  and the second and third lens groups  4   b ,  4   c  are set beyond certain values, thereby resulting in the total length of the optical system being increased. Also, as the distances D 1 ′, D 2 ′ between the laser array  1  and the second and third lens groups  4   b ,  4   c  are increased, an area of the laser beams  2  incident on the imaging optical system  4  is enlarged and as a result, an effective aperture of the lenses forming the first and second lens groups  4   a ,  4   b  is increased.  
           [0017]    Thus, in a case in which the total length of the optical system and size of the lenses are increased, the total size of the image forming apparatus is enlarged, and at the same time, a weight thereof is increased. Also, when the lenses have large apertures, designing surface shapes of the lenses into aspherical surfaces becomes impossible, and manufacturing processes such as lens design, mold production, injection molding, etc. become difficult, thereby resulting in increased manufacturing time and cost.  
           [0018]    Also, in the conventional image forming apparatus  10 , at least three lenses are required in the telecentric optical system in order to realize a resolution of more than 600 dpi, This complicates the design and also increases manufacturing costs.  
           [0019]    Also, since the conventional image forming apparatus  10  has refracting surfaces of the lenses which are disposed with rotational symmetry with respect to the optical axis to thereby reduce the asymmetrical aberration, magnification powers in the main and sub-scanning directions are equal. Thus, if the magnification powers in the main and sub-scanning directions should differ, additional structures are required.  
           [0020]    Another problem of the conventional imaging optical system  10  is that temperature changes cause the wavelength of the laser beams emitted from the laser array  1  to change, and thus, the refractive index of the lens material is changed, and image quality deteriorates.  
         SUMMARY OF THE INVENTION  
         [0021]    Accordingly, it is an object of the present invention to provide an improved imaging optical system and image forming apparatus having the same, in which image light fluxes are emitted from a light source on an imaging surface of photosensitive material by using a plurality of combined reflection mirrors having aspherical surfaces. Thus, design tolerances are increased and polygon mirrors and lenses having temperature-dependent refractive indices are not used.  
           [0022]    Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.  
           [0023]    At this time, the toner image transferred onto the printing paper is fixed by the fixing part and then the printing paper on which the toner image is fixed is discharged to the outside by a paper-discharging part. In this way, a high quality image is formed on the printing paper.  
           [0024]    As is apparent from the foregoing description, it can be appreciated that in the image forming apparatus  100 , since laser beams are imaged on the imaging surface of the photosensitive drum  103  by the imaging optical system  104  having combined reflection mirrors noise, size, weight and manufacturing costs are reduced.  
           [0025]    Also, in the image forming apparatus  100  having the imaging optical system  104 , since reflection mirrors are used, surface shapes of the reflection mirrors can be formed of aspherical surfaces and main and sub-scanning magnification powers can be designed to be the same or different from one another, and thereby design tolerances can be increased to obtain a high quality image.  
           [0026]    Also, in the image forming apparatus  100  having the imaging optical system  104 , even though the wavelength of the laser beams  102  emitted from the laser array  101  changes according to temperature, deterioration in image quality as a result of a corresponding change in refractive index of lens materials can be prevented.  
           [0027]    Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.  
           [0028]    The foregoing and other objects of the invention are achieved by providing an imaging optical system for imaging a plurality of light fluxes emitted from a light source on an imaging surface, comprising a first group having at least more than one reflection mirror for reflecting the light fluxes emitted from the light source, disposed in a spaced-apart relation with a given distance from the light source, a second group having at least more than one reflection mirror for reflecting the light fluxes reflected from the first group, and a third group having at least more than one reflection mirror for reflecting the light fluxes reflected from the second group onto the imaging surface, disposed in a spaced-apart relation with a given distance from the imaging surface an imaging optical system to image a plurality of light fluxes emitted from a light source onto an imaging surface, including a first reflection mirror to reflect the light fluxes emitted from the light source, spaced at a first given distance D 1  from the light source; a second reflection mirror to reflect the light fluxes reflected from the first mirror; and a third reflection mirror to reflect the light fluxes reflected from the second mirror onto the imaging surface, spaced at a second distance D 2  from the imaging surface.  
           [0029]    According to an aspect of the present invention, the first, second and third groups have a positive refractive power to converge the light fluxes, a negative refractive power to diverge the light fluxes, and a positive refractive power to converge the light fluxes, respectively.  
           [0030]    According to another aspect of the invention, the light source includes luminous elements which are one or two-dimensionally arranged. The luminous elements are light emitting diodes, laser diodes, or vertical cavity surface emitting lasers.  
           [0031]    According to another aspect of the invention, the distances D 1  and D 2  satisfy the following mathematical formula:  
       1   &lt;          D2   D1          &lt;   10                         
 
           [0032]    According to another aspect of the invention, surface shapes of the reflection mirrors are spherical. Alternatively, the surface shapes of the reflection mirrors are aspherical and satisfy the following mathematical formula:  
       Z   =           C   1          y   2         1   +       1   -       (     K   +   1     )          C   1   2          y   2               +       ∑     m   =   3     ∞            A   m          Y   m         +         x   2            C   2          (     1   +       ∑     n   =   3     ∞            B   n          Y   n           )           1   +       1   -       (     x                     C   2          (     1   +       ∑     n   =   3     ∞            B   n          Y   n           )         )     2                                     
 
           [0033]    in which:  
           [0034]    m, n are positive numbers greater than 3,  
           [0035]    K is a conical coefficient,  
           [0036]    A, B are aspherical coefficients,  
           [0037]    C is a curvature coefficient,  
           [0038]    x, y are positions or distances to a point on the aspherical surface, and  
           [0039]    Z is a distance from an optical axis.  
           [0040]    The foregoing and other objects of the present invention may also be achieved by providing an image forming apparatus including a light source, having a plurality of luminous elements, to emit a plurality of light fluxes according to an image signal, the luminous elements being independently controlled; an imaging optical system to magnify the light fluxes emitted from the light source; and a photosensitive material having an imaging surface to form an electrostatic latent image by being photo-exposed by the light received from the imaging optical system, wherein the imaging optical system includes a first mirror group having a plurality of first reflection mirrors to reflect the light fluxes emitted from the light source spaced from the light source; a second mirror group having a plurality of second reflection mirrors to reflect the light fluxes reflected from the first mirror group; and a third mirror group having a plurality of third reflection mirrors to reflect the light fluxes reflected from the second mirror group onto the imaging surface, spaced from the imaging surface of the photosensitive material. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]    These and other objects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:  
         [0042]    [0042]FIG. 1 is a schematic diagram of a known image forming apparatus;  
         [0043]    [0043]FIG. 2 is a schematic perspective view of an image forming apparatus having an imaging optical system in accordance with an embodiment of the present invention; and  
         [0044]    [0044]FIG. 3 is a side elevation view of the image forming apparatus shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]    Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.  
         [0046]    Referring now to FIG. 2, there is illustrated an image forming apparatus  100  having an imaging optical system in which reflection mirrors are used in accordance with an embodiment of the present invention.  
         [0047]    The image forming apparatus  100  includes a semiconductor laser array  101  in which a plurality of laser elements are one or two-dimensionally arranged, an imaging optical system  104  to magnify a plurality of laser beams  102  emitted from the laser array  101  at a given angle and position by using reflection mirrors, and a photosensitive drum  103  having an imaging surface to image the laser beams  102  emitted from the imaging optical system  104  thereon . The apparatus  100  further includes a drum driving circuit  105  to drive the photosensitive drum  103  at a given speed of rotation and to output a timing signal synchronous with the rotation of the photosensitive drum  103 , a control circuit  109  to output a printing signal and a driving signal to a laser driving circuit  108  and the drum driving circuit  105 , respectively, by processing an image signal  107  outputted from a computer or CPU (not shown), and outputting a control signal to the laser driving circuit  108  synchronously with the timing signal of the drum driving circuit  105 .  
         [0048]    The laser elements of the laser array  101  may include LEDs, LDs or VCSELs.  
         [0049]    The imaging optical system  104  includes a first reflection mirror  104   a  to reflect the laser beams  102  emitted from the laser array  101 , and is spaced at a given distance from the laser array  101 ; a second reflection mirror  104   b  to reflect the laser beams  102  reflected from the first reflection mirror  104   a ; and a third reflection mirror  104   c  to reflect the laser beams  102  reflected from the second reflection mirror  104   b  onto the imaging surface of the photosensitive drum  103 .  
         [0050]    The first, second and third reflection mirrors  104   a ,  104   b ,  104   c  have a positive refractive power to converge the laser beams  102 , a negative refractive power to diverge the laser beams  102 , and a positive refractive power to converge the laser beams  102 , respectively.  
         [0051]    Accordingly, the laser beams  102  incident on the first reflection mirror  104   a  from the laser array  101  are reflected from a surface of the first reflection mirror  104   a , and are changed into converging or condensing laser beams and are then reflected toward the second reflection mirror  104   b . The converging laser beams incident on the second reflection mirror  104   b  from the first reflection mirror  104   a  are reflected from a surface of the second reflection mirror  104   b , and are changed into diverging laser beams and are then reflected toward the third reflection mirror  104   c . Also, at the third reflection mirror  104   c , the diverging laser beams are again changed into converging laser beams and are then imaged on the imaging surface of the photosensitive drum  103 .  
         [0052]    This design of the imaging optical system  104  ensures magnification, i.e., focus of the imaging optical system  104 , to be easily adjusted by changing a distance D 1  (see FIG. 3) between the laser array  101  and the first reflection mirror  104   a , a distance between the first reflection mirror  104   a  and the second reflection mirror  104   b , and a distance D 2  (see FIG. 3) between the third reflection mirror  104   c  and the imaging surface of the photosensitive drum  103 .  
         [0053]    Particularly, the distance D 1  between the laser array  101  and the first reflection mirror  104   a  and the distance D 2  between the third reflection mirror  104   c  and the imaging surface of the photosensitive drum  103  satisfy the following mathematical formula (1):  
       1   &lt;          D2   D1          &lt;   10                         
 
         [0054]    Also, in the imaging optical system  104 , since the path of the laser beams  102  can be freely adjusted according to respective angles and positions of the reflection mirrors  104   a ,  104   b ,  104   c , the imaging optical system  104  can be designed so that magnifications in main and sub-scanning directions are different from one another, or that surface shapes of the reflection mirrors  104   a ,  104   b ,  104   c  are formed of spherical or aspherical surfaces.  
         [0055]    When the surface shapes of the reflecting mirrors are formed of the aspherical surfaces, the following mathematical formula 2 is satisfied:  
             Z   =           C   1          y   2         1   +       1   -       (     K   +   1     )          C   1   2          y   2               +       ∑     m   =   3     ∞            A   m          Y   m         +       (       x   2            C   2          (     1   +       ∑     n   =   3     ∞            B   n          Y   n           )               (       1   +         1   -     (     xC   2                (     1   +       ∑     n   =   3     ∞            B   n          Y   n           )         )     )     2                 (   2   )                               
 
         [0056]    in which:  
         [0057]    m, n are positive numbers greater than 3,  
         [0058]    K is a conical coefficient,  
         [0059]    A, B are aspherical coefficients,  
         [0060]    C is a curvature coefficient,  
         [0061]    x, y are positions or distances to a point on the aspherical surface, and  
         [0062]    Z is a distance from an optical axis.  
         [0063]    Thus, the imaging optical system  104  of the present invention has a compact structure and superior image quality by using a mirror optical system having combined reflection mirrors to bend the path of the laser beams  2 , instead of the conventional telecentric optical system having lenses. Although three reflection mirrors are described above, the number of the reflection mirrors may vary to change the path of the laser beams, or to increase or decrease the number of times the laser beams are reflected.  
         [0064]    The image forming apparatus  100  of the present invention further includes an image forming part (not shown) having a charger, a cleaner, a development roller, a developer, and a transferrer, to carry out a series of image forming process to form a toner image on a printing material such a sheet of printing paper; a paper-feeding part (not shown) to feed the printing paper to the transferrer; a fixing part (not shown) to fix the toner image transferred onto the printing paper, and a paper-discharging part (not shown) to discharge the printing paper on which the toner image is fixed. Since the structure of these components is the same as that of the conventional image forming apparatus, a detailed explanation will be omitted.  
         [0065]    Operation of the image forming apparatus  100  having the imaging optical system  104  in accordance with the present invention will now be explained with reference to FIG. 2 and FIG. 3.  
         [0066]    First, according to an image signal inputted from the computer or CPU of the image forming apparatus  100 , the control circuit  109  processes the image signal  107  and outputs printing and driving signals to the laser driving circuit  108  and the drum driving circuit  105 , respectively.  
         [0067]    At this time, the drum driving circuit  105  provides control to rotate the photosensitive drum  103  at a given rotation speed and simultaneously outputs a timing signal synchronous with the rotation of the photosensitive drum  103  to the control circuit  109 . As the drum driving circuit  105  outputs the timing signal, the control circuit  109  outputs a control signal to the laser driving circuit  108  synchronously with the timing signal.  
         [0068]    In response to the control signal and the printing signal from the control circuit  109 , the laser driving circuit  108  drives the laser array  101 . At this time, the laser array  101  emits the plurality of laser beams  102  at predetermined angles from respective laser elements. The laser beams  102  emitted from the laser array  101  enter the imaging optical system  104 .  
         [0069]    The laser beams  102  incident on the imaging optical system  104  are first incident on the first reflection mirror  104   a  having the positive refractive index. Then, the laser beams  102  are reflected from a surface of the first reflection mirror  104   a  and are simultaneously changed into converging laser beams. The converging laser beams  102  are then incident on the second reflection mirror  104   b  having the negative refractive index. At the second reflection mirror  104   b , the laser beams  102  are reflected from the surface thereof and are changed into diverging laser beams, and then are incident on the third reflection mirror  104   c  having the positive refractive index. At the third reflection mirror  104   c , the diverging laser beams  102  are again changed into converging laser beams and are then scanned in a main scanning direction as a plurality of light spots on the imaging surface of the photosensitive drum  103 . At this time, since the photosensitive drum  103  is driven to be rotated in the sub-scanning direction (indicated by the arrow) by a driving motor (not shown), an electrostatic latent image conforming to the image signal is formed on the photosensitive drum  103  as a result of the movement of the light spots in the main scanning direction, and the movement of the photosensitive drum  103  in the sub-scanning direction.  
         [0070]    Thereafter, the electrostatic latent image on the photosensitive drum  103  is developed into a toner image by the development roller and a developer such as colored toner. As the photosensitive drum  103  on which the toner image is formed is further rotated, the toner image is transferred onto the printing paper fed from the paper-feeding part by the transfer roller of the transferrer. Next, the photosensitive drum  103  continues to rotate, and a cleaning blade (not shown) of the cleaner removes developer remaining on the imaging surface of the photosensitive drum  103 , so that another electrostatic latent image can be formed thereon.