JP6519544B2 - Exposure apparatus and image forming apparatus provided with the same - Google Patents

Description

  The present invention relates to an exposure apparatus for scanning a laser beam on a photosensitive drum, and an image forming apparatus provided with the exposure apparatus.

  Some image forming apparatuses such as multifunction machines, copiers, printers, and facsimiles perform printing using toner. In an image forming apparatus using toner, an exposure device for forming an electrostatic latent image on a photosensitive drum is mounted. Some exposure apparatuses scan with a laser. Among exposure devices that perform scanning with a laser, there are devices that scan a single photosensitive drum with a plurality of lasers (multi-lasers). By scanning with a plurality of lasers, exposure of a plurality of lines can be performed in one scan, and high-speed scanning can be performed.

  Patent Document 1 describes an example of a circuit that performs scanning with such a multi-laser. Specifically, Patent Document 1 includes a plurality of light sources that respectively emit a plurality of beams, and a single light detector that collectively monitors a plurality of beam light amounts, and the monitor current detected by the light detector is A time average value of a plurality of beam light quantities is calculated based on the calculated time average value of the sum of image data of a plurality of lines respectively corresponding to a plurality of beams, and a time average value of a plurality of beam quantity A drive circuit of a multi-beam semiconductor laser array is described which performs negative feedback control so that a plurality of beam light amounts become constant based on a time average value of the sum of. With this configuration, it is attempted to reduce the number of light detectors (see Patent Document 1: Claim 1, paragraph [0042], etc.).

JP 2000-190563A

  There is an exposure apparatus which scans and exposes a photosensitive drum using a laser for exposure. In such an exposure apparatus, a mirror and a lens are provided in the light path from the exposure laser to the photosensitive drum. In addition, a polygon mirror is provided which moves the irradiation position of the laser beam along the main scanning direction. Reflective mirrors may be provided to redirect the laser beam. An fθ lens is also provided to make the moving speed of the irradiation position of the laser beam in the main scanning direction constant. Also, a lens may be provided to adjust the luminous flux of the laser beam. The reflectance of the mirror and the transmittance of the lens vary depending on the incident angle of the laser beam. That is, the attenuation factor of the laser beam changes according to the irradiation position of the laser beam in the main scanning direction.

  Depending on the position of the photosensitive drum in the main scanning direction, the degree of decrease in the amount of light varies. That is, when the light amount (output light amount) output by the exposure laser is maintained constant, the light amount (irradiated light amount, energy) of the laser beam irradiated to the photosensitive drum differs depending on the position in the main scanning direction. Which part of the attenuation rate increases and which part decreases in one main scanning direction is related to the installation position and installation angle of the exposure laser.

  In the technique described in Patent Document 1, the light quantity of a plurality of beams is made constant. However, in the technique described in Patent Document 1, since the scanning is performed with a constant light amount, the irradiation light amount is different for each pixel in one line. Therefore, density unevenness occurs in the image to be printed (toner image to be formed). For example, in the line in the main scanning direction, the amount of irradiated light tends to be insufficient at portions where the decrease in light amount is significant. The density of the image of the portion where the amount of irradiated light is insufficient becomes thinner than the portion where the amount of irradiated light is sufficient.

  Therefore, in order to maintain the actual irradiation light quantity of each pixel in one line at the target light quantity (constant light quantity), light quantity correction may be performed to increase or decrease the output light quantity of the exposure laser. In the light amount correction, the current supplied to the exposure laser is increased or decreased to increase or decrease the output light amount. For example, in the light amount correction, a correction current corresponding to the scanning position in the main scanning direction is added to the basic current. The correction current is increased as the amount of light attenuation decreases.

  Conventionally, a current at which the amount of light emitted from a certain reference pixel (a reference pixel, for example, a pixel having the smallest attenuation factor) becomes a target light amount may be used as a basic current. However, due to the accuracy of the device, there is a certain degree of error between the actual irradiation light quantity of the reference pixel and the target light quantity. Also, the current supplied to the exposure laser may be increased or decreased in proportion to the output light amount. For example, when the exposure of a certain pixel requires a light amount twice as high as the output light amount at the time of supplying the basic current, a current twice the basic current is supplied to the laser for exposure. Then, the error between the irradiation light quantity and the target light quantity is also doubled, and the difference between the actual light quantity and the ideal light quantity becomes large. As the amount of change (increase) in the amount of output light relative to the amount of output light at the time of basic current supply increases, the influence of the error increases. As a result, even if the light amount correction is performed, there is a problem that portions with large and small errors with the target light amount are formed in the main scanning direction, and unevenness in density may appear in the obtained image.

  The technique described in Patent Document 1 is to keep the output light quantity of a plurality of laser beams constant, and does not perform the light quantity correction during scanning in the main scanning direction. Even if the light amount correction is performed, it is not possible to cope with the problem that uneven density may appear in the image.

  In view of the above problems, the present invention reduces an error between an actual light amount (irradiated light amount) of a laser beam irradiated to each pixel and a target light amount, thereby reducing density unevenness.

  In order to achieve the above object, an exposure apparatus according to claim 1 includes an image signal supply unit, a light emitting unit, an optical unit, and a driver circuit. The image signal supply unit supplies an image signal. The plurality of light emitting units are provided and scan and expose a photosensitive drum by a laser beam. The optical unit includes a polygon mirror that reflects the laser beam while rotating and moves the irradiation position of the laser beam along the main scanning direction of the photosensitive drum, and an optical path from the light emitting unit to the photosensitive drum Provided in The driver circuit supplies a current to each of the light emitting units, turns on and off each of the light emitting units based on an image signal from the image signal supply unit, and the electrostatic latent image corresponding to the image signal is the photoconductor. In the case of forming on a drum and maintaining the irradiation light quantity, which is the light quantity of the laser beam irradiated to each pixel of one line in the main scanning direction of the photosensitive drum, at a predetermined target light quantity, The fluctuation range of the output light quantity which is the light quantity of the laser beam output from the light emitting part from the first pixel to the last pixel is divided into a plurality of light quantity bands by the number of the light emitting parts, and one light emitting part in the light quantity band Assigning, dividing one line in the main scanning direction into a plurality of sections based on pixels to be exposed with the light quantity of the light quantity band, and causing the light emitting unit allocated to the light quantity band corresponding to the section to scan the sections Said The current supplied to the light emitting portion of each so that the irradiation light amount of each pixel becomes the target amount of between varied within the interval, switching the light emitting unit for performing scanning for each of the sections.

  According to the present invention, an error between the irradiation light quantity of each pixel and the target light quantity is reduced, and density unevenness is reduced.

FIG. 1 is a diagram illustrating an example of a printer according to an embodiment. FIG. 1 is a diagram illustrating an example of a printer according to an embodiment. FIG. 1 is a view showing an example of an exposure apparatus according to an embodiment. It is a figure showing a part of exposure apparatus concerning an embodiment. It is a figure which shows an example of the output surface of the laser apparatus which concerns on embodiment. It is a figure which shows an example of the driver circuit mounted in the exposure apparatus which concerns on embodiment. It is a figure for demonstrating an example of density unevenness correction of the conventional exposure device. It is a figure which shows an example of a division | segmentation of the fluctuation range of the output light quantity in the exposure apparatus which concerns on embodiment. It is a flowchart which shows an example of the flow of the scan which concerns on embodiment, and exposure. It is a figure which shows an example of the scan in the exposure apparatus which concerns on embodiment.

  Hereinafter, an embodiment of the present invention will be described using FIGS. 1 to 10. The present invention can be applied to various image forming apparatuses such as multifunction machines, copying machines, and fax machines. In this description, a printer 100 (corresponding to an image forming apparatus) including the exposure apparatus 1 will be described as an example. However, the elements of the configuration and arrangement described in the present embodiment do not limit the scope of the invention, and are merely illustrative examples.

  In the following description, the light amount of the laser beam irradiated to each pixel of one line in the main scanning direction of the photosensitive drum 41 is referred to as “irradiated light amount”. In addition, the light amount of the laser beam output from each light emitting unit 6 (the first light emitting unit 61, the second light emitting unit 62, the third light emitting unit 63, and the fourth light emitting unit 64) is referred to as "output light amount". Further, a target value (ideal light amount) of the laser beam which is predetermined and is irradiated to each pixel of the photosensitive drum 41 is referred to as “target light amount”.

(Schematic Configuration of Image Forming Apparatus)
First, an outline of the printer 100 according to the embodiment will be described with reference to FIGS. 1 and 2. 1 and 2 are diagrams showing an example of the printer 100 according to the embodiment.

  As shown in FIG. 1, the printer 100 includes a control unit 2. The control unit 2 controls each unit of the printer 100. The control unit 2 includes a CPU 21 that performs various calculations, and an image processing unit 22 (corresponding to an image signal supply unit) that generates image data used for printing and supplies the image data to the exposure apparatus 1. The CPU 21 controls and calculates each unit of the printer 100 based on a control program and control data stored in the storage unit 3. The storage unit 3 is a combination of a volatile storage device such as a RAM and a non-volatile storage device such as a ROM and an HDD.

  The control unit 2 controls the printing unit 4 (the sheet feeding unit 4a, the conveyance unit 4b, the image forming unit 4c, and the fixing unit 4d) that performs sheet conveyance, toner image formation, transfer, and fixing and printing. The control unit 2 performs control, calculation, and processing relating to the part included in the printing unit 4 based on the storage content of the storage unit 3.

  At the time of printing, the control unit 2 supplies sheets one by one to the sheet feeding unit 4a. The control unit 2 causes the conveyance unit 4 b to convey the sheet supplied from the paper supply unit 4 a. The control unit 2 causes a toner image to be formed on the image forming unit 4c, and transfers the toner image formed on the transported sheet. The control unit 2 causes the fixing unit 4d to heat and press the sheet on which the toner image has been transferred, thereby fixing the toner image on the sheet. The sheet that has passed through the fixing unit 4d is discharged to the discharge tray 4e.

  As shown in FIG. 2, the image forming unit 4 c includes the exposure device 1, the photosensitive drum 41, the charger 42, the developing device 43, the transfer roller 44, and the cleaning device 45. The photosensitive drum 41 carries a toner image on its circumferential surface, and is rotationally driven at a predetermined process speed. The charger 42 charges the photosensitive drum 41 at a constant potential. The exposure apparatus 1 outputs a light signal (laser beam) obtained by converting the image signal transmitted from the image processing unit 22 and scans the charged photosensitive drum 41 to form an electrostatic latent image. The developing device 43 supplies toner to the electrostatic latent image on the photosensitive drum 41 and develops it. The transfer roller 44 transfers the toner image to the sheet. The cleaning device 45 cleans the photosensitive drum 41.

  The control unit 2 recognizes the setting contents made on the operation panel 23. Further, the communication unit 24 is connected to the control unit 2. The communication unit 24 is an interface for communicating with the computer 200 such as a PC or a server. The communication unit 24 receives, from the computer 200, print data including content data indicating print content such as image data and text data, and setting data indicating print setting content. Then, the control unit 2 (image processing unit 22) generates image data to be used for printing based on the received print data, and causes the printing unit 4 to perform printing based on the image data.

(Configuration of exposure apparatus 1)
Next, an example of the exposure apparatus 1 according to the embodiment will be described with reference to FIGS. FIG. 3 is a view showing an example of the exposure apparatus 1 according to the embodiment. FIG. 4 is a view showing a part of the exposure apparatus 1 according to the embodiment. FIG. 5 is a view showing an example of the emission surface of the laser device 6 according to the embodiment.

  The exposure apparatus 1 is included in a printer 100. Since the control unit 2 gives an operation instruction to the exposure apparatus 1, the control unit 2 also operates as the control unit 2 of the exposure apparatus 1. The storage unit 3 also stores data related to the exposure apparatus 1. The optical unit 5 (polygon motor 51, polygon mirror 52, fθ lens 53, lens 54) and the laser device 6 are provided in the housing of the exposure apparatus 1 (see FIG. 4).

  The laser device 6 has a plurality of light emitting units that emit a laser beam for scanning the photosensitive drum 41. Specifically, the laser device 6 includes each light emitting unit. Each light emitting unit is a laser diode. The specifications of each light emitting unit are the same. The exposure apparatus 1 scans the photosensitive drum 41 with four laser beams.

  The image processing unit 22 instructs the lighting of the laser beam in the pixels on which toner is to be put according to the contents of the image to be formed, and the image signals instructing the extinguishing of the laser beam on pixels on which the toner is not Turn off signal). The image processing unit 22 outputs an image signal used for the next scan before the start of one scan. Each light emitting unit turns on and off the laser beam based on the image signal supplied from the image processing unit 22.

  The polygon mirror 52 is a polygon mirror that reflects a laser beam. Each light emitting unit emits a laser beam toward the polygon mirror 52. The polygon mirror 52 is rotated at high speed by the polygon motor 51. Thus, the polygon mirror 52 reflects the laser beams from the light emitting units so as to move along the main scanning direction of the photosensitive drum 41.

  Fθ lens 53 for making the scanning speed (moving speed of the irradiation position of each laser beam) of the laser beam on the photosensitive drum 41 constant (constant speed) on the optical path between the polygon mirror 52 and the photosensitive drum 41 Is provided as the optical unit 5. The lens 54 uses refraction to adjust the laser beam to be parallel (collimator lens). Note that another member such as a mirror that reflects the laser beam to change the direction of the laser beam may be provided on the photosensitive drum 41.

  In addition, a beam detection unit 55 is provided in the irradiation range (scanning range) of the laser beam by the polygon mirror 52 and out of the irradiation range on the photosensitive drum 41. The beam detection unit 55 changes the output current (output voltage) when the laser beam is irradiated. The beam detection unit 55 includes a light receiving element such as a photodiode 65 or a phototransistor. Then, based on the output of the beam detection unit 55, the control unit 2 sets the start timing of exposure (writing) on the photosensitive drum 41. The control unit 2 causes the driver circuit 7 to start scanning of one line when a predetermined time has elapsed since the detection of light reception of the laser beam by the beam detection unit 55.

  Next, an example of the laser device 6 will be described with reference to FIG. One laser device 6 includes four light emitting units (a first light emitting unit 61, a second light emitting unit 62, a third light emitting unit 63, and a fourth light emitting unit 64). The laser device 6 can emit four laser beams simultaneously. It is also possible to use a laser device 6 including a plurality of light emitting units other than four (for example, eight beams). In the present description, a four-beam laser device 6 will be described as an example.

  As shown on the left side of FIG. 5, in the laser device 6, the emission parts (the respective light emitting parts) of the laser beam are arranged in a line. The intervals between the light emitting units are equal. The pitch L0 of each laser beam in the aligned state differs depending on the laser device 6, but does not match the width of one pixel (one dot). The width (size) of one dot is determined by the specifications (resolution of printing) of the printer 100. In the case of 600 dpi, the width of one dot is 25.4 mm (one inch) ÷ 600 ≒ 42.3 μm.

  Therefore, as shown on the right side of FIG. 5, the laser device 6 is attached to the exposure device 1 by rotating so that the beam pitch L1 in the sub-scanning direction matches the interval of one dot. In other words, the laser device 6 is installed such that the irradiation position of the laser beam of each light emitting unit is shifted by one dot in the sub scanning direction. In the case of 600 dpi, the laser device 6 is attached to the exposure device 1 using a jig or a measuring device such that the beam pitch L1 in the sub-scanning direction is 42.3 μm. Thereby, the width of one line and the beam pitch L1 in the sub scanning direction in the exposure apparatus 1 coincide with each other, and the reproducibility of one dot in the sub scanning direction is secured.

  On the other hand, since the laser device 6 is rotated and attached, positional deviation occurs in each light emitting portion in the main scanning direction (beam pitch L2 in FIG. 5). The beam pitch L2 is an image of the photosensitive drum 41 in the main scanning direction when the first light emitting unit 61, the second light emitting unit 62, the third light emitting unit 63, and the fourth light emitting unit 64 start scanning at the same timing. It appears as a gap of

  When image signals (image data for instructing drawing) are input at the same timing to start drawing, the exposure start position and the exposure end position of the first light emitting unit 61 and the second light emitting unit 62 are the beam pitch Shift by L2 minutes. Therefore, the image processing unit 22 (control unit 2) makes the writing start timing of the second light emitting unit 62 earlier than the writing start timing of the first light emitting unit 61 by the time required for the movement of the beam pitch L2. The control unit 2 makes the writing start timing of the third light emitting unit 63 earlier than the writing start timing of the first light emitting unit 61 by the time required to move the distance twice the beam pitch L2. In addition, the control unit 2 causes the writing start timing of the fourth light emitting unit 64 to be earlier than the writing start timing of the first light emitting unit 61 by the time required to move the distance three times the beam pitch L2. Thereby, it is possible to prevent the displacement of the position of the pixel in the main scanning direction.

(Driver circuit 7)
Next, the driver circuit 7 according to the embodiment will be described with reference to FIG. FIG. 6 is a view showing an example of the driver circuit 7 mounted on the exposure apparatus 1 according to the embodiment.

  The driver circuit 7 is mounted on the exposure apparatus 1. The driver circuit 7 includes a controller 70, four voltage conversion circuits (a first voltage conversion circuit 71a, a second voltage conversion circuit 71b, a third voltage conversion circuit 71c, and a fourth voltage conversion circuit 71d), and four comparison circuits (first Comparison circuit 72a, second comparison circuit 72b, third comparison circuit 72c, fourth comparison circuit 72d), four current output circuits (first current output circuit 73a, second current output circuit 73b, third current output circuit 73c, And a fourth current output circuit 73d).

  The first current output circuit 73 a supplies a current to the first light emitting unit 61. The second current output circuit 73 b supplies a current to the second light emitting unit 62. The third current output circuit 73 c supplies a current to the third light emitting unit 63. The fourth current output circuit 73 d supplies a current to the fourth light emitting unit 64. That is, each current output circuit (the first current output circuit 73a, the second current output circuit 73b, the third current output circuit 73c, and the fourth current output circuit 73d) of the driver circuit 7 supplies current to the corresponding light emitting unit. . Each current output circuit changes the current supplied to the corresponding light emitting unit according to the instruction of the controller 70.

  The controller 70 of the driver circuit 7 instructs each current output circuit to turn on / off each light emitting unit based on the image signal supplied from the image processing unit 22 (image signal supply unit), and an electrostatic latent image according to the image signal Is formed on the photosensitive drum 41. The controller 70 also includes a reference voltage generation unit 70a.

  The output of one current output circuit and one light emitting unit are connected. Each current output circuit controls an output current. By using each voltage conversion circuit, each comparison circuit, and each current output circuit, it is possible to perform automatic power control (APC) control that holds the light amount (output light amount) of the laser beam output from the light emitting unit constant. For the APC control, each light emitting unit is provided with a photodiode 65 for monitoring the light amount of the laser beam.

  As the output light quantity is larger, a larger current flows in the photodiode 65. The output current of the photodiode 65 of the first light emitting unit 61 is input to the first voltage conversion circuit 71a. The output current of the photodiode 65 of the second light emitting unit 62 is input to the second voltage conversion circuit 71 b. The output current of the photodiode 65 of the third light emitting unit 63 is input to the third voltage conversion circuit 71 c. The output current of the photodiode 65 of the fourth light emitting unit 64 is input to the fourth voltage conversion circuit 71 d.

  Each voltage conversion circuit converts the output current of the corresponding photodiode 65 into a monitor voltage. Specifically, the first voltage conversion circuit 71a converts the output current of the photodiode 65 of the first light emitting unit 61 into a first monitor voltage Vm1. The second voltage conversion circuit 71 b converts the output current of the photodiode 65 of the second light emitting unit 62 into a second monitor voltage Vm2. The third voltage conversion circuit 71c converts the output current of the photodiode 65 of the third light emitting unit 63 into a third monitor voltage Vm3. The fourth voltage conversion circuit 71d converts the output current of the photodiode 65 of the fourth light emitting unit 64 into a fourth monitor voltage Vm4.

  The reference voltage generation unit 70a of the controller 70 generates a reference voltage corresponding to the amount of light to be emitted by the APC for each light emission unit. For example, the reference voltage generation unit 70a is a D / A conversion circuit. Specifically, the reference voltage generation unit 70a generates a first reference voltage Vref1 as a reference voltage of the first light emitting unit 61, generates a second reference voltage Vref2 as a reference voltage of the second light emitting unit 62, and generates a third light emission. The third reference voltage Vref3 is generated as the reference voltage of the unit 63, and the fourth reference voltage Vref4 is generated as the reference voltage of the fourth light emitting unit 64.

  Each comparison circuit compares the monitor voltage Vm of the APC control with the reference voltage Vref to confirm whether or not both voltages are equal. Specifically, the first comparison circuit 72a compares the first monitor voltage Vm1 with the first reference voltage Vref1. The second comparison circuit 72b compares the second monitor voltage Vm2 with the second reference voltage Vref2. The third comparison circuit 72c compares the third monitor voltage Vm3 with the third reference voltage Vref3. The fourth comparison circuit 72d compares the fourth monitor voltage Vm4 with the fourth reference voltage Vref4.

  Each comparison circuit increases or decreases the current supplied to the light emitting unit connected to the corresponding current output circuit based on the comparison result. Specifically, when the first reference voltage Vref1> the first monitor voltage Vm1, the first comparison circuit 72a causes the first current output circuit 73a to increase the current flowing through the first light emitting portion 61, and the first reference voltage Vref1 < At the first monitor voltage Vm1, the current flowing through the first light emitting unit 61 is decreased to the first current output circuit 73a. Also, when the first reference voltage Vref1 = the first monitor voltage Vm1, or when the voltage difference between them is within a predetermined allowable range, the first current output circuit 73a supplies the first light emitting portion 61 at that time. The stored current value is stored as a first reference current value.

  When the second reference voltage Vref2> the second monitor voltage Vm2, the second comparison circuit 72b causes the current flowing to the second light emitting unit 62 to increase to the second current output circuit 73b, and the second reference voltage Vref2 <the second monitor voltage When it is Vm2, the current flowing through the second light emitting unit 62 is decreased to the second current output circuit 73b. Further, when the second reference voltage Vref2 = the second monitor voltage Vm2, or when the voltage difference between them is within a predetermined allowable range, the second current output circuit 73b supplies the second light emitting unit 62 at that time. The stored current value is stored as a second reference current value.

  When the third reference voltage Vref3> the third monitor voltage Vm3, the third comparison circuit 72c causes the current flowing to the third light emitting unit 63 to increase to the third current output circuit 73c, and the third reference voltage Vref3 <the third monitor voltage When it is Vm3, the current flowing to the third light emitting unit 63 is decreased to the third current output circuit 73c. Also, when the third reference voltage Vref3 = the third monitor voltage Vm3, or when the voltage difference between them is within a predetermined allowable range, the third current output circuit 73c supplies the third light emitting portion 63 at that time. The stored current value is stored as a third reference current value.

  When the fourth reference voltage Vref4> the fourth monitor voltage Vm4, the fourth comparison circuit 72d causes the current flowing through the fourth light emitting unit 64 to increase to the fourth current output circuit 73d, and the fourth reference voltage Vref4 <the fourth monitor voltage When it is Vm4, the current flowing through the fourth light emitting unit 64 is decreased to the fourth current output circuit 73d. Further, when the fourth reference voltage Vref4 = the fourth monitor voltage Vm4, or when the voltage difference between them is within a predetermined allowable range, the fourth current output circuit 73d supplies the fourth light emitting portion 64 at that time. The stored current value is stored as a fourth reference current value.

(Uneven density in the main scanning direction)
Next, an example of density unevenness correction in the main scanning direction in the conventional exposure apparatus 1 will be described with reference to FIG. FIG. 7 is a view for explaining an example of density unevenness correction in the main scanning direction in the conventional exposure apparatus 1.

  An exposure apparatus using a semiconductor laser device is provided with various lenses and various mirrors for scanning and exposing the photosensitive drum. The transmittance of the lens and the reflectance of the mirror depend on the incident angle of the laser beam. On the other hand, the irradiation position of the laser beam moves in the main scanning direction. Therefore, the incident angle of the laser beam to various mirrors and various lenses changes during scanning.

  Therefore, when the photosensitive drum is scanned along the main scanning direction with the output light amount fixed, the irradiated light amount (the light amount incident on the photosensitive drum, light energy) does not become constant. As a result, even when the output light quantity is constant, density unevenness may be noticeable in an image obtained by printing.

  Therefore, the current supplied to the semiconductor laser device may be corrected so that the irradiation light quantity of each pixel in the main scanning direction is maintained at the target light quantity. For example, a current of a certain magnitude is used as a basic current, and a current obtained by adding a correction current to the basic current is supplied to the semiconductor laser device. The light amount correction is performed by increasing or decreasing the correction current.

  For example, depending on the incident angle of the laser beam, among the pixels in the main scanning direction, the pixels with small lens transmittance and mirror reflectance have a semiconductor laser device more than pixels with large lens transmittance and mirror reflectance. Increase the supplied current (correction current). Thereby, the irradiation light quantity can be maintained at the target light quantity at any pixel in the main scanning direction. As a result, unevenness in the main scanning direction is eliminated.

  FIG. 7 shows an example of increase / decrease of the output light quantity of the laser beam in one line in the main scanning direction. FIG. 7 shows an example of light amount correction in an exposure apparatus in which the light amount attenuation is the largest at the center of the photosensitive drum. In the example of FIG. 7, the amount of output light (correction current) is gradually increased from the start of scanning toward the center of the photosensitive drum, and the amount of output light (correction current) is gradually reduced from the center to the last pixel. Note that which part of the photosensitive drum 41 has the highest attenuation factor is related to the installation position and the installation angle of the semiconductor laser device. Depending on the exposure apparatus, the attenuation of the light amount may be the largest at the start position and the end position of the scan. In that case, the graph of light intensity correction is convex downward.

  Here, due to various error factors such as the accuracy (accuracy) of the device, the error between the target light amount and the irradiation light amount may increase. In addition, in the exposure apparatus of the type that supplies the same correction current to all the light emitting parts (exposure apparatus in which the current source of the correction current is common), the error in the light amount is Sometimes it gets bigger. In particular, it is known that the error between the target light amount and the irradiation light amount tends to be large in the vicinity of the maximum light amount (near the maximum value of the correction current). In the part where the light amount error is large, the density becomes thin or thick.

  Therefore, even if the light amount correction for avoiding density unevenness in the main scanning direction is performed, density unevenness may still occur. Therefore, in the exposure apparatus 1 of the present embodiment, the error between the irradiation light quantity and the target light quantity is reduced in all the pixels in one line in the main scanning direction, and the printed image has high image quality without noticeable density unevenness. Turn

(Division of fluctuation range of light quantity)
Next, an example of division of the fluctuation range of the output light quantity in the exposure apparatus 1 according to the embodiment will be described with reference to FIG. FIG. 8 is a view showing an example of division of the fluctuation range of the output light quantity in the exposure apparatus 1 according to the embodiment.

  In the exposure apparatus 1, the fluctuation range of the output light amount (light emission amount of the light emitting portion) in light amount correction is divided into a plurality of light amount bands 8 in order to reduce the error between the irradiation light amount and the target light amount.

  First, one of the light emitting units is used as a reference light emitting unit. The fluctuation range of the output light amount for maintaining the irradiation light amount between the start and the end of the scanning of one line in the main scanning direction of the photosensitive drum 41 at the predetermined target light amount in the reference light emission part is It can be grasped by experiment etc. For example, a light intensity measuring device is used to measure the output light intensity necessary to maintain the irradiation light intensity of each pixel at a predetermined target light intensity. The range between the minimum value and the maximum value of the required output light quantity is the fluctuation range of the output light quantity.

  Also, the monitor by each photodiode 65 when current is supplied to the reference light emitting portion so as to maintain the irradiation light amount at the target light amount from the start to the end of scanning of one line in the main scanning direction of the photosensitive drum 41 The variation range of the output light quantity may be grasped based on the variation range of the voltage.

  FIG. 8 shows the fluctuation range of the output light quantity of the reference light emitting part when the attenuation of the light quantity is maximum at the center of the photosensitive drum 41 in the main scanning direction and the attenuation of the light quantity is minimum at both ends in the main scanning direction. ing. The controller 70 of the driver circuit 7 divides the fluctuation range into a plurality of light intensity bands 8 by the number of light emitting units. FIG. 8 corresponds to the laser apparatus 6 according to the embodiment including four light emitting parts, and four light quantity bands 8 (a first light quantity band 81, a second light quantity band 82, a third light quantity band 83, a fourth light quantity An example of division into bands 84) is shown. In FIG. 8, when the controller 70 of the driver circuit 7 maintains the irradiation light amount at the target light amount during the period from the start to the end of the scanning of the pixels of one line in the main scanning direction of the photosensitive drum 41. An example is shown in which the maximum value and the minimum value of the output light quantity are set as the fluctuation range, and the fluctuation range is divided so as to be equal or almost even with the number of light emitting portions.

  Hereinafter, the light amount band 8 with the largest light amount is referred to as a first light amount band 81. The light amount band 8 having the second largest light amount is referred to as a second light amount band 82. The light amount band 8 having the third largest light amount is referred to as a third light amount band 83. The fourth largest (smallest) light intensity band 8 is referred to as a fourth light intensity band 84.

  The reference voltage (the first reference voltage Vref1, the second reference voltage Vref2, the third reference voltage Vref3, and the fourth reference voltage Vref4) of each light emitting unit which has any light amount in the range of the first light amount band 81, and the second light amount Any of the reference voltage of each light emitting portion which becomes any light amount of the range of the band 82, the reference voltage of each light emitting portion which becomes light amount of any of the range of the third light amount band 83, and the range of the fourth light amount band 84 The reference voltage of each light emitting unit which becomes the light amount is stored in the storage unit 3 as the light amount data D1.

  Specifically, the reference voltage of each light emitting portion corresponding to the central light amount in the range of the first light amount band 81, the reference voltage of each light emitting portion corresponding to the central light amount in the range of the second light amount band 82, and the third light amount band The reference voltage of each light emitting portion corresponding to the central light amount in the range of 83 and the reference voltage of each light emitting portion corresponding to the central light amount in the range of the fourth light amount band 84 are stored in the storage portion 3 as light amount data D1.

  Then, the controller 70 of the driver circuit 7 divides one line (scanning range in the main scanning direction) into a plurality of sections 9 based on the pixels to be exposed with the light quantity within the range of each light quantity band 8 for each light quantity band 8 Do. In other words, the controller 70 causes the light emitting unit to emit light with a light amount in the range of the light amount band 8 and divides one line into a plurality of sections 9 in units of pixels to be exposed. Specifically, when the current corresponding to the boundary value of each light amount band 8 is supplied to the reference light emitting portion, the driver circuit 7 sets the position of the pixel whose irradiation light amount becomes the target light amount as the boundary of the section 9.

  In the example of FIG. 8, the controller 70 performs exposure with the light intensity in the range of the second light intensity band 82 and the section 9 of the pixel to be exposed with the light intensity of the first light intensity band 81 (hereinafter “first interval 91”). A section 9 of the pixel to be exposed (hereinafter "the second section 92"), a section 9 of the pixel to be exposed with the light intensity in the range of the third light intensity band 83 (hereinafter "the third interval 93") A line in the main scanning direction is divided into sections 9 of pixels to be exposed with light quantity (hereinafter, "fourth sections 94").

  In the example of FIG. 8, since the correction current is larger toward the center of the photosensitive drum 41 (because it is convex upward), the middle portion of one line in the main scanning direction is the first section 91. The fourth section 94, the third section 93, and the second section 92 are sequentially from the leading pixel of one line. From the rear end pixel of the first section 91 to the final pixel, a second section 92, a third section 93, and a fourth section 94 are formed. If one line in the main scanning direction is viewed from the leading side (tail side), the order of section 9 is fourth section 94 → third section 93 → second section 92 → first section 91 → second section 92 → third A section 93 → a fourth section 94 is established.

  The driver circuit 7 assigns one light emitting unit to the section 9 corresponding to one light amount band 8. Then, the driver circuit 7 changes the light emission amount of the light emitting unit within the range of the allocated light amount band 8 so that the irradiation light amount of each pixel becomes the target light amount. As described above, the driver circuit 7 switches the light emitting unit which performs scanning in each section 9 to scan and expose one line.

  Conventionally, one laser beam is used for scanning of one line in the main scanning direction of the photosensitive drum 41. For example, in the four-beam exposure apparatus 1, four main scan lines are scanned in one scan. Thus, a plurality of lines in the main scanning direction can be scanned by one scan, and the photosensitive drum 41 can be scanned at high speed. However, as described above, even when the light amount correction is performed, the uneven density may be noticeable.

  In the exposure apparatus 1 of the present embodiment, one light emitting unit is assigned to the section 9 in the main scanning direction. Then, the driver circuit 7 changes the light amount of the light emitting unit within the range of the light amount band 8 corresponding to the allocated section 9. As a result, the amplitude (increase / decrease range) of the current in light amount correction can be suppressed, and the error between the target light amount and the irradiation light amount can be prevented from increasing.

(Flow of exposure and scanning of exposure apparatus 1)
Next, an example of the flow of scanning and exposure in the exposure apparatus 1 according to the embodiment will be described using FIGS. 9 and 10. FIG. 9 is a flowchart showing an example of the scanning and exposure flows of the exposure apparatus 1 according to the embodiment. FIG. 10 is a view showing an example of scanning by the exposure apparatus 1 according to the embodiment.

  In the present description, the flow of scanning on the photosensitive drum 41 in printing of one page will be described. When printing on a plurality of sheets in succession, the flowchart of FIG. 9 is repeated. The start of the flowchart of FIG. 9 is the time to start scanning the photosensitive drum 41 for printing.

  First, the driver circuit 7 assigns one light emitting unit to the light amount band 8 (section 9) (step # 1). At this time, the driver circuit 7 may switch the light emitting units allocated to the light amount band 8 every printing (every one sheet). For example, in one light emitting portion, a first light amount band 81 (first section 91), a second light amount band 82 (second section 92), a third light amount band 83 (third section 93), and a fourth light amount band 84 The fourth section 94) is assigned in order. As a result, even if the number of pixels in each section 9 varies, the lighting time of each light emitting unit becomes substantially uniform in the long run. Therefore, the life of the laser device 6 is extended. The light emitting unit allocated to the section 9 may be switched for each of a plurality of pages or for each print job.

  Next, before the start of scanning of each pixel, the controller 70 of the driver circuit 7 grasps a reference current value corresponding to the central light amount of the allocated light amount band 8 for each light emitting unit (step # 2). The controller 70 of the driver circuit 7 inputs a reference voltage corresponding to the central light amount in the light amount band 8 corresponding to the section 9 to each comparison circuit, and monitor voltage = reference voltage or difference between monitor voltage and reference voltage is within tolerance The reference current value which is the current value which becomes is obtained for each light emitting unit and stored in each current output circuit.

  Specifically, when obtaining the reference current value, the controller 70 performs APC control. The reference voltage generation unit 70a generates a reference voltage (first reference voltage Vref1) corresponding to the central light amount of the light amount band 8 of the section allocated to the first light emitting unit 61, and inputs the reference voltage to the first comparison circuit 72a. The controller 70 (first current output circuit 73a) recognizes a current value where monitor voltage = reference voltage as a first reference current value. The reference voltage generation unit 70a generates a reference voltage (second reference voltage Vref2) corresponding to the central light amount of the light amount band 8 of the section allocated to the second light emitting unit 62, and inputs the reference voltage to the second comparison circuit 72b. The controller 70 (second current output circuit 73b) recognizes a current value at which monitor voltage = reference voltage as a second reference current value. The reference voltage generation unit 70a generates a reference voltage (third reference voltage Vref3) corresponding to the central light amount of the light amount band 8 of the section allocated to the third light emitting unit 62, and inputs the reference voltage to the third comparison circuit 72c. The controller 70 (third current output circuit 73c) recognizes a current value where monitor voltage = reference voltage as a third reference current value. The reference voltage generation unit 70a generates a reference voltage (fourth reference voltage Vref4) corresponding to the central light amount of the light amount band 8 of the section allocated to the fourth light emitting unit 62, and inputs the reference voltage to the fourth comparison circuit 72d. The controller 70 (fourth current output circuit 73d) recognizes a current value where monitor voltage = reference voltage as a fourth reference current value.

  The image processing unit 22 transmits an image signal for four lines to the driver circuit 7 while shifting the line of the image signal supplied in units of sections 9 so as to become an electrostatic latent image of the image to be formed (step # 3).

  The driver circuit 7 scans and exposes the photosensitive drum 41 based on the image signal supplied from the image processing unit 22 (step # 4). At this time, the driver circuit 7 (each current output circuit) supplies a current to each light emitting unit so that the irradiation light amount of each pixel becomes the target light amount when the light emitting unit is lit (step # 4). In addition, the driver circuit 7 switches and scans the light emitting unit which performs scanning in each section 9 (step # 4).

  In each section, the difference between the reference current value and the current value to be supplied to each light emitting unit is determined in advance in each section so that the irradiation light amount becomes the target light amount, and stored in the storage unit 3 as difference data in a nonvolatile manner. Ru. In other words, in the section, the change pattern of the current value supplied to the light emitting unit is predetermined. The driver circuit 7 (each current output circuit) changes the current supplied to the light emitting unit centering on the reference current value based on the difference data.

  Specifically, based on the difference data, the driver circuit 7 (current output circuit) increases or decreases the current of the first reference current value with a predetermined change pattern so that the irradiation light amount becomes the target light amount, thereby the first light emitting unit The current supplied to 61 is changed. In addition, the driver circuit 7 (current output circuit) causes the second light emitting unit 62 to increase or decrease the current of the second reference current value with a predetermined change pattern so that the irradiation light quantity becomes the target light quantity based on the difference data. Vary the current supplied. The driver circuit 7 (current output circuit) supplies the third light emitting unit 63 by increasing or decreasing the current of the third reference current value in a predetermined change pattern so that the irradiation light quantity becomes the target light quantity based on the difference data. Change the current. The driver circuit 7 (current output circuit) supplies the fourth light emitting unit 64 by increasing or decreasing the current of the fourth reference current value with a predetermined change pattern so that the irradiation light quantity becomes the target light quantity based on the difference data. Change the current.

The points at which the lines of the image signal are shifted and the actual scanning will be described with reference to FIG. There are a plurality of patterns of allocation of the light amount bands 8 and the light emitting portions. The allocation of the light amount zone 8, the section 9 and the light emitting portions in FIG. 10 is as follows.
(1) The first light emitting unit 61 is the light emitting unit whose scanning line is at the top of the sub scanning direction. The assignment of the first light emitting unit 61 is a first light amount band 81 (first section 91).
(2) The second light emitting unit 62 is the light emitting unit whose scanning line is second from the top side in the sub scanning direction. The assignment of the second light emitting unit 62 is a second light amount band 82 (second section 92).
(3) The third light emitting unit 63 is the light emitting unit whose scanning line is the third from the top side in the sub scanning direction. The assignment of the third light emitting unit 63 is a third light amount band 83 (third section 93).
(4) The light emitting unit having the scanning line closest to the rear end in the sub scanning direction is the fourth light emitting unit 64. The assignment of the fourth light emitting unit 64 is a fourth light intensity band 84 (fourth section 94).

  One light emitting unit is assigned to each light amount band 8 (section 9). Then, the light emitting unit which performs scanning in each section 9 is switched and scanned. Therefore, in the case of the assignment of FIG. 10, the fourth light emitting unit 64 is supplied with an image signal instructing turning on / off of the laser beam only to the pixels of the fourth section 94 in the image to be generated. The image processing unit 22 sets dummy data (data instructing light-out, data not indicating exposure) as an image signal while the fourth light emitting unit 64 scans the third section 93, the second section 92, and the first section 91. give. Therefore, the fourth light emitting unit 64 lights only in the fourth section 94.

  Further, in the case of the assignment of FIG. 10, the third light emitting unit 63 is given an image signal instructing turning on / off of the laser beam only to the pixels of the third section 93 in the image to be generated. The image processing unit 22 supplies dummy data as an image signal while the third light emitting unit 63 scans the fourth section 94, the second section 92, and the first section 91. Therefore, the third light emitting unit 63 is turned on only in the third section 93.

  Further, in the case of the assignment of FIG. 10, the second light emitting unit 62 is provided with an image signal instructing turning on / off of the laser beam only to the pixels of the second section 92 in the image to be generated. The image processing unit 22 supplies dummy data as an image signal while the second light emitting unit 62 scans the fourth section 94, the third section 93, and the first section 91. Therefore, the second light emitting unit 62 is turned on only in the second section 92.

  Further, in the case of the assignment of FIG. 10, the first light emitting unit 61 is supplied with an image signal instructing turning on / off of the laser beam only to the pixels of the first section 91 among the images to be generated. The image processing unit 22 supplies dummy data as an image signal while the first light emitting unit 61 scans the fourth section 94, the third section 93, and the second section 92. Therefore, the first light emitting unit 61 lights only in the first section 91. As described above, the image processing unit 22 transmits a signal for scanning a pixel (a signal instructing turning on / off of the laser beam) to the section 9 to which the light emitting section is allocated. Dummy data is supplied to the driver circuit 7 as an image signal.

  Here, in FIG. 10, the head line of printing of one page is illustrated by the arrow of a two-dot chain line. Further, inside the rectangle indicating the section 9, a number indicating the number of times of the scanning is described. As described above, a plurality of light emitting units are provided side by side in one laser device 6. Then, the laser device 6 is installed so that the irradiation position of the laser beam of each light emitting unit is shifted by one dot in the sub scanning direction (see FIG. 5). Therefore, when the fourth light emitting unit 64 scans the leading line, the scanning position of the third light emitting unit 63 is the position of the line one before the leading line, and the scanning position of the second light emitting unit 62 is The scanning position of the first light emitting unit 61 is the position of the line three lines before the leading line.

  That is, the scanning position of the fourth light emitting unit 64 and the scanning position of the third light emitting unit 63 are shifted by one dot (one line) in the sub scanning direction. The scanning position of the fourth light emitting unit 64 and the scanning position of the second light emitting unit 62 are shifted by two dots in the sub-scanning direction. The scanning position of the fourth light emitting unit 64 and the scanning position of the first light emitting unit 61 are shifted by three dots in the sub-scanning direction.

  In consideration of this deviation, the image processing unit 22 shifts the line of the image signal to be transmitted to the driver circuit 7 in units of sections 9. Specifically, the image processing unit 22 does not shift the image signal supplied to the light emitting unit (the fourth light emitting unit 64 in the present description) in which the scanning position reaches the top line earliest. The image processing unit 22 shifts the line of the image signal by the dot that is shifted from the light emitting unit that reaches the top line at the earliest scanning position. The data of the section 9 allocated to the light emitting unit (third light emitting unit 63) shifted by one dot is delayed by one line (one dot). The data of the section 9 allocated to the light emitting unit (the second light emitting unit 62) shifted by 2 dots is delayed by two lines (for two dots). The data of the section 9 allocated to the light emitting unit (first light emitting unit 61) shifted by 3 dots is delayed by 3 lines (3 dots). That is, the image processing unit 22 shifts the data of the section 9 assigned to the light emitting unit in a direction to delay the data by the number of dots shifted from the light emitting unit that reaches the head line the scanning position most quickly.

  Because of this deviation, in the example of FIG. 10, the first line scans and exposes the fourth section 94 based on the image signal in the first scan, and the second scan makes the third section 93 based on the image signal. Scanning and exposure are performed, and scanning and exposure of the second section 92 based on the image signal are performed in the third scan, and scanning and exposure of the first section 91 based on the image signal are performed in the fourth scan. Thereafter, the scanning after the first line is sequentially performed. As described above, the scanning of each light emitting unit is combined to perform scanning of one line in the main scanning direction.

  Note that, even if the scanning position of the fourth light emitting unit 64 reaches the leading line, the third light emitting unit 63, the second light emitting unit 62, and the first light emitting unit 61 are not positions for scanning the leading line. For the third light emitting unit 63, the second light emitting unit 62, and the first light emitting unit 61, the image processing unit 22 drives the dummy data (a signal instructing light-off) to the driver circuit until the head line is scanned. Give to seven. In the example of FIG. 10, the dummy data portion is illustrated by a dashed rectangle. As described above, the image processing unit 22 supplies dummy data to the driver circuit 7 as an image signal to the light emitting unit whose scanning position is outside the drawing area.

  After completion of one scan, the driver circuit 7 confirms whether or not scanning of all lines in one page is completed (step # 5). When scanning of all the lines is completed (Yes in step # 5), this flow ends (end). When scanning of all the lines is not finished (No in step # 5), the flow returns to step # 3.

In light intensity correction where exposure is performed while maintaining one line in the main scan at the target light intensity, in the portion of one line where the current for correction is increased, the target light intensity and the pixel are actually irradiated Errors in light intensity may increase. Therefore, the exposure apparatus 1 according to the embodiment includes an image signal supply unit (image processing unit 22), a light emitting unit (first light emitting unit 61, second light emitting unit 62, third light emitting unit 63, fourth light emitting unit 64), The optical unit 5 and the driver circuit 7 are included. The image signal supply unit supplies an image signal. A plurality of light emitting units are provided, which scan and expose the photosensitive drum 41 with a laser beam. The optical unit 5 also includes a polygon mirror 52 that reflects the laser beam while rotating and moves the irradiation position of the laser beam along the main scanning direction of the photosensitive drum 41, and the light from the light emitting unit to the photosensitive drum 41 It is provided on the street. The driver circuit 7 is
A current is supplied to each light emitting unit, and each light emitting unit is turned on and off based on the image signal from the image signal supply unit to form an electrostatic latent image according to the image signal on the photosensitive drum 41. Light emission from the first pixel to the last pixel of the scanning of one line when the irradiation light quantity, which is the light quantity of the laser beam irradiated to each pixel of one line in the main scanning direction, is maintained at a predetermined target light quantity The variation range of the output light quantity which is the light quantity of the laser beam which the part outputs is divided into a plurality of light quantity bands 8 (the first light quantity band 81, the second light quantity band 82, the third light quantity band 83, the fourth light quantity band 84 Divided into two, and one light emitting portion is allocated to the light amount band 8, and one line in the main scanning direction is divided into a plurality of sections 9 (first section 91, second section 92, Divided into a third section 93 and a fourth section 94), section 9 The section 9 is scanned by the light emitting unit allocated to the corresponding light quantity band 8, and the current supplied to each light emitting section is changed in the section 9 so that the irradiation light quantity of each pixel included in the section 9 becomes the target light quantity. , Switch the light emitting unit to scan every interval 9

  Thus, the fluctuation range is divided into a plurality of light intensity bands 8. One light emitting unit is assigned to the light amount band 8. Further, one line is divided into a plurality of lines according to the light amount band 8. The section 9 is scanned only with the corresponding light emitting unit. As a result, the plurality of light emitting units jointly scan one line. Furthermore, each light emitting unit is turned on so that the irradiation light amount becomes constant. Thereby, the absolute value of the correction amount of the current can be reduced, and the error between the irradiation light amount and the target light amount can be suppressed to a small value. The error does not become so large that uneven density is noticeable. Accordingly, it is possible to provide the exposure apparatus 1 capable of forming an image of high quality with less unevenness in density.

  Further, a plurality of light emitting units are provided side by side in one laser device 6. The laser device 6 is installed so that the irradiation position of the laser beam of each light emitting unit is shifted by one dot in the sub scanning direction. The image signal supply unit shifts the line of the image signal supplied to the driver circuit 7 in units of sections 9 so as to form an electrostatic latent image to be formed. As a result, even if one line is divided into sections 9, it is possible to form an image with little density unevenness and no deviation.

  In addition, a photodiode 65 is provided to monitor the amount of output light for each light emitting unit. The driver circuit 7 changes the current supplied to each light emitting unit centering on the reference current value corresponding to the center value of the light amount band 8 corresponding to the section 9. Thereby, the error between the irradiation light quantity and the target light quantity becomes smaller on average. Therefore, no noticeable density unevenness occurs. It is possible to provide an exposure apparatus 1 capable of forming a high quality image with less density unevenness.

  When the range of the specific light intensity band 8 is wide, there is a possibility that a pixel may appear in which an error between the actual light intensity of the laser beam irradiated to the pixel and the target light intensity is large. Therefore, in the reference light emitting unit which is one light emitting unit among the light emitting units, the driver circuit 7 applies the irradiation light amount between the start and the end of scanning of the pixels of one line in the main scanning direction of the photosensitive drum 41. The fluctuation range is determined based on the maximum value and the minimum value of the output light quantity when maintaining the target light quantity, and the fluctuation range is divided so as to be equal to the number of light emitting parts. Thereby, the widths of the light amount bands 8 can be made uniform. Then, the error between the irradiation light quantity and the target light quantity can be reduced on average. Therefore, it is possible to prevent noticeable density unevenness.

  In addition, the driver circuit 7 switches the light amount band 8 to be allocated to the light emitting unit every printing. Thereby, it is possible to avoid that the light amount band 8 having a large current is allocated to only a specific light emitting unit. Therefore, only the specific light emitting portion does not deteriorate, and the life of the laser device 6 can be extended.

  Further, the image forming apparatus (printer 100) includes the exposure apparatus 1 described above. Since the exposure apparatus 1 with less density unevenness is included, it is possible to provide an image forming apparatus with high image quality.

  As mentioned above, although embodiment of this invention was described, the range of this invention is not limited to this, A various change can be added and implemented in the range which does not deviate from the main point of invention.

  The present invention is applicable to an exposure apparatus and an image forming apparatus which perform exposure with a laser beam.

100 printer (image forming apparatus) 1 exposure device 22 image processing unit (image signal supply unit) 41 photosensitive drum 5 optical unit 52 polygon mirror 6 laser device 61 first light emitting unit 62 second light emitting unit 63 third light emitting unit 64 4 light emitting portion 7 driver circuit 8 light amount band 81 first light amount band 82 second light amount band 83 third light amount band 84 fourth light amount band 9 section 91 first section 92 second section 93 third section 94 fourth section

Claims (6)

An image signal supply unit that supplies an image signal;
A plurality of light emitting units for scanning and exposing a photosensitive drum by a laser beam;
An optical unit provided on an optical path from the light emitting unit to the photosensitive drum, including a polygon mirror that reflects the laser beam while rotating and moves the irradiation position of the laser beam along the main scanning direction of the photosensitive drum When,
A current is supplied to each of the light emitting units, and each of the light emitting units is turned on and off based on an image signal from the image signal supply unit to form an electrostatic latent image according to the image signal on the photosensitive drum. In the case where the irradiation light quantity which is the light quantity of the laser beam irradiated to each pixel of one line in the main scanning direction of the photosensitive drum is maintained at a predetermined target light quantity, the first pixel to the last of the scanning of one line The fluctuation range of the output light quantity, which is the light quantity of the laser beam output by the light emitting part, is divided into a plurality of light quantity bands by the number of the light emitting parts, and one light emitting part is allocated to the light quantity band One line in the main scanning direction is divided into a plurality of sections based on the pixel to be exposed by the light quantity, and the section is scanned by the light emitting unit allocated to the light quantity band corresponding to the Of each pixel And changing the current supplied to each of the light emitting units so that the irradiation light amount becomes the target light amount in the section, and switching the light emitting section to perform scanning in each section. Exposure device. A plurality of the light emitting units are provided side by side in one laser device,
The laser device is installed such that the irradiation position of the laser beam of each of the light emitting units is shifted by one dot in the sub scanning direction,
2. The exposure apparatus according to claim 1, wherein the image signal supply unit shifts the line of the image signal supplied to the driver circuit in units of sections so as to form an electrostatic latent image to be formed. A photodiode is provided for monitoring the output light quantity for each of the light emitting units,
The exposure according to claim 1 or 2, wherein the driver circuit changes the current supplied to each of the light emitting units centering on a reference current value corresponding to a central value of the light amount band corresponding to the section. apparatus.   The driver circuit is configured such that, in the reference light emitting unit, which is one of the light emitting units, of the light emitting units, the irradiation light amount during a period from the start to the end of scanning of pixels of one line in the main scanning direction of the photosensitive drum. The fluctuation range is determined based on the maximum value and the minimum value of the output light quantity when maintaining the target light quantity at the target light quantity, and the fluctuation range is divided so as to be equal by the number of the light emitting parts. An exposure apparatus according to any one of 1 to 3.   The exposure apparatus according to any one of claims 1 to 4, wherein the driver circuit switches the light amount band assigned to the light emitting unit every printing.   An image forming apparatus comprising the exposure device according to any one of claims 1 to 5.

Images