JP2015215576A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation device capable of reducing concentration unevenness caused by a surface state in a main scanning direction of each reflecting surface of a rotary polygon mirror.SOLUTION: An image formation device has a rotary polygon mirror mounted on a polarizer, and includes an exposure part operating in an underfilled method. In a memory part which the image formation device includes, an image height of a photoreceptor corresponding to a position in a main scanning direction of each reflecting surface that is acquired according to a surface state in the main scanning direction of each reflecting surface of the rotary polygon mirror and a light quantity correction value for correcting light quantity of an optical beam emitted to each reflecting surface are associated with each other and stored. The image formation device reads the light quantity correction value from the memory part, and the based on the light quantity correction value, adjusts light quantity of the optical beam that is emitted from a light source for each image height.

Description

  The present invention relates to an image forming apparatus equipped with a rotating polygon mirror in a polarizer and having an exposure unit that operates in an underfilled manner.

  Ideally, each reflecting surface of the rotary polygon mirror (polygon mirror) is a flat surface. However, fine unevenness (for example, a height difference from the reference surface of several nanometers to several tens of nanometers and a width of about mm) may exist locally along the main scanning direction due to an accuracy error in machining. The main scanning direction on the reflecting surface is a direction in which the light beam moves on the reflecting surface. When the light beam moves on the reflecting surface in the main scanning direction, the light beam reflected on the reflecting surface moves in the main scanning direction of the photosensitive member (corresponding to the width direction of the paper). The direction perpendicular to the main scanning direction is the sub-scanning direction. In general, in an exposure unit (print head) of an image forming apparatus, light is emitted from a light source of an exposure unit for the purpose of correcting a positional deviation in the sub-scanning direction due to surface tilt of a rotary polygon mirror used in a deflector in the sub-scanning direction. A light beam (light beam) is once formed as a line image at a predetermined position in the sub-scanning direction on the reflecting surface.

FIG. 9 is a diagram illustrating a state in which a light beam applied to a general rotary polygon mirror is reflected.
FIG. 10 is a diagram showing a line image by a light beam applied to one reflecting surface of a general rotary polygon mirror. In FIG. 10, the X-axis direction indicates a direction perpendicular to the reflecting surface.
As shown in FIG. 9, the rotating polygon mirror 230 reflects the light beam 231 of the light beam emitted from the light source on the reflection surface (scanning surface) and deflects it as a light beam 232 of the reflected light beam. The light beam is imaged as a line image 233 having a length of about several millimeters in the main scanning direction near the center of the reflecting surface of the rotating polygon mirror 230 (dotted line portion), and then the position of the line image 233 is the position of the rotating polygon mirror 230. Along with the rotation, the scanning surface moves in the main scanning direction. Thereby, the light beam is scanned in the main scanning direction of the photosensitive member.

  For example, in Patent Document 1, the reflection surface is specified from the scanning time of each reflection surface of the polygon mirror measured in advance, and the data indicating the degree of surface tilt of the polygon mirror in the sub-scanning direction is measured on the photoconductor. The variation in the position of the beam spot of the light beam formed in the sub-scanning direction is calculated. Then, by adjusting the light amount of the light beam emitted from the light source of the exposure unit based on the calculation result, density unevenness (pitch unevenness) due to the variation in the position of the beam spot in the sub-scanning direction is corrected.

JP 2011-148142 A

Incidentally, a polarizer using a rotating polygon mirror is divided into an underfilled optical system shown in FIG. 11 and an overfilled optical system shown in FIG. The rotating polygon mirror 230 illustrated in FIG. 9 is an underfilled optical system.
As shown in FIG. 11, in the underfilled optical system, the width of the incident light beam 201 is narrower than the width (length) of the reflecting surface of the rotary polygon mirror 200 in the main scanning direction (reflecting surface width> light beam width). Depending on the image height, the scanning position of the incident light beam 201 varies within the same reflecting surface. Here, the image height is a distance in the main scanning direction of the light beam applied to the photoconductor from a reference position (for example, exposure start position) of the photoconductor. As shown in FIG. 11, when the deflection angle is 0 °, the incident light beam 201 is irradiated near the center of the reflection surface, and the reflected light beam 202 is reflected in the opposite direction to the incident light beam 201. When the deflection angle is an arbitrary θ °, the rotary polygon mirror 200 rotates by θ °, and the position where the incident light beam 201 is reflected moves to a position away from the vicinity of the center of the reflecting surface. That is, the rotary polygon mirror 200 performs polarization of the light beam by using a part of the reflection surface according to the image height, so that the reflection condition of the reflection surface differs for each image height.

  On the other hand, in the overfilled optical system, the width of the incident light beam 211 is wider than the width (length) of the reflecting surface of the rotary polygon mirror 210 in the main scanning direction (reflecting surface width <light beam width), and has a predetermined value corresponding to the deflection angle. A part of the reflected light beam 212 reflected by the reflecting surface is irradiated onto the photosensitive member. That is, since the rotary polygon mirror 210 performs polarization of the light beam using the entire reflection surface at the entire image height, the reflection conditions of the reflection surface are the same for each image height.

  In the underfilled exposure unit described above, the line image moves along the main scanning direction with the rotation of the rotary polygon mirror. For this reason, when the line image overlaps and reflects the fine irregularities, the beam spot shape of the light beam changes at a specific position on the surface to be scanned, and a portion that is not originally exposed is exposed and developed. As a result, a reflective surface with unevenness has a problem that image deterioration such as density unevenness (pitch unevenness) occurs compared to a reflective surface without unevenness.

Here, with reference to FIG. 13 and FIG. 14, the influence on the image quality due to the difference in the reflecting surface of the rotary polygon mirror will be described.
FIG. 13 is an explanatory diagram when the reflecting surface of the rotary polygon mirror is an ideal plane. FIG. 13A shows a position [mm] with the center of the reflecting surface in the main scanning direction as the origin and the reference plane position of the reflecting surface. FIG. 13B shows a beam spot shape formed on the surface to be scanned of the photoconductor, and FIG. 13C shows the output. An example of an image is shown.
FIG. 14 is an explanatory diagram when the reflecting surface of the rotary polygon mirror is uneven. FIG. 14A shows the position [mm] of the reflecting surface in the main scanning direction and the error [nm] in the height direction (X-axis direction). 14B shows a beam spot shape on the surface to be scanned, and FIG. 14C shows an example of an output image.

  As shown by the error characteristic 240 in FIG. 13A, when the reflecting surface is an ideal plane, the error in the height direction of the reflecting surface is almost zero in an arbitrary line image region 241. The beam spot 242 on the reflection surface (scanned surface) at this time has a shape that is close to a substantially uniform circle vertically and horizontally (FIG. 13B). Here, the beam spot 242 is a beam spot generated by a light beam having an intensity equal to or higher than a predetermined development threshold value. As a result, density unevenness does not occur in the output image 243 formed on the paper (FIG. 13C).

  On the other hand, as shown by the error characteristic 250 in FIG. 14A, when fine irregularities exist on the reflection surface, an error exists in the height direction of the reflection surface in an arbitrary line image region 251. At this time, the beam spot 252 on the reflection surface (scanned surface) changes in shape and does not have a uniform shape vertically and horizontally (FIG. 14B). In FIG. 14B, a small and slightly dark beam spot 252a (second beam spot) exists on the right side of the main beam spot 252 (first beam spot) having the highest density. As a result, there is a problem that density unevenness 254 (pitch unevenness) occurs in the output image 253 formed on the paper (FIG. 14C).

  The technique described in Patent Document 1 relates to correction of density unevenness caused by position fluctuation (surface tilt) of the beam spot in the sub-scanning direction of the photosensitive member. Patent Document 1 does not mention density unevenness caused by the surface shape (fine irregularities) of the reflecting surface of the rotary polygon mirror in the main scanning direction, and such density unevenness cannot be sufficiently corrected.

  From the above situation, there has been a demand for a method of reducing density unevenness due to the surface state (fine uneven shape) of each reflecting surface of the rotary polygon mirror in the main scanning direction.

An image forming apparatus according to one embodiment of the present invention is formed by a light source having a light source that emits a light beam for exposing a photosensitive member and a length in a main scanning direction in which the light beam emitted from the light source moves. A rotating polygon mirror that has a plurality of reflecting surfaces longer than the luminous flux width to be scanned and reflects the light beam emitted from the light source by the reflecting surfaces while scanning the photosensitive member.
In addition, a reflecting surface specifying unit that specifies a reflecting surface that reflects the light beam of the rotating polygon mirror, and the surface of each reflecting surface of the rotating polygon mirror that is obtained in accordance with the surface state in the main scanning direction. A storage unit that stores light amount correction data in which the image height of the photoconductor corresponding to the position in the main scanning direction and the light amount correction value for correcting the light amount of the light beam emitted to each reflecting surface are associated with each other.
Furthermore, a control unit is provided that reads light amount correction data from the storage unit and adjusts the light amount of the light beam emitted from the light source for each image height based on the read light amount correction data.

  In the above configuration, the light amount correction in which the image height of the photosensitive member corresponding to the position of each reflecting surface of the rotary polygon mirror in the main scanning direction and the light amount correction value for correcting the light amount of the light beam emitted to each reflecting surface are associated with each other. Using the data, the amount of light beam at the time of exposure is adjusted.

  According to the present invention, density unevenness caused by the surface shape (fine irregularities) of the reflecting surface in the main scanning direction of the rotary polygon mirror is reduced, and the image quality is improved.

1 is an overall configuration diagram illustrating an image forming apparatus according to an embodiment of the present invention. It is a block diagram which shows the optical system of the exposure part in an image forming apparatus. FIG. 3A is an explanatory diagram of a method for specifying a reflecting surface of a rotating polygon mirror, and FIG. 3B is a perspective view of the rotating polygon mirror. 2 is a block diagram illustrating a hardware configuration of each unit of the image forming apparatus. FIG. It is a flowchart which shows the process which stores the light quantity correction value for every reflective surface and image height of a rotary polygon mirror in memory. FIG. 6A is a graph for explaining the light beam intensity on the reflecting surface, and FIG. 6A is a graph showing the relationship between the position of the reflecting surface of the light beam in the main scanning direction and the light beam intensity when the reflecting surface of the rotary polygon mirror is uneven. FIG. 6B is a graph showing the relationship between the position of the light beam reflecting surface in the main scanning direction and the light beam intensity when the reflecting surface of the rotary polygon mirror is an ideal flat surface. 5 is a flowchart illustrating image forming processing by the image forming apparatus. It is a figure for explanation of the light beam intensity on the reflection surface after the light amount correction, and is a graph showing the relationship between the position of the light beam reflection surface in the main scanning direction and the light beam intensity before and after correcting the light amount of the light beam. Indicates. It is a figure which shows a mode that the light beam irradiated to the general rotary polygon mirror reflects. It is a figure which shows the line image by the light beam irradiated to the reflective surface of one common rotary polygonal mirror. It is the schematic which shows an underfill optical system. It is the schematic which shows an overfilled optical system. FIG. 13A is an explanatory diagram when the reflecting surface of the rotary polygon mirror is an ideal plane. FIG. 13A is a height direction with the center of the reflecting surface as the origin in the main scanning direction and the reference plane position of the reflecting surface as the origin. FIG. 13B shows a beam spot shape formed on the surface to be scanned of the photoconductor, and FIG. 13C shows an example of an output image. FIG. 14A is an explanatory diagram in the case where there are irregularities on the reflecting surface of the rotary polygon mirror, FIG. 14A is a graph showing an example of the relationship between the position of the reflecting surface in the main scanning direction and the error in the height direction, and FIG. FIG. 14C shows an example of the output image.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the following description and each drawing, the same elements or elements having the same functions are denoted by the same reference numerals, and redundant descriptions are omitted.

<1. Embodiment>
[Configuration example of image forming apparatus]
First, an outline of an image forming apparatus according to an embodiment of the present invention will be described with reference to FIG.
FIG. 1 is an overall configuration diagram showing an image forming apparatus according to an embodiment of the present invention.

  As shown in FIG. 1, the image forming apparatus 1 forms an image on a sheet by an electrophotographic method, and has four colors of yellow (Y), magenta (M), cyan (C), and black (Bk). This is a tandem color image forming apparatus that superimposes toner. The image forming apparatus 1 includes a document transport unit 10, a paper storage unit 20, an image reading unit 30, an image forming unit 40, an intermediate transfer belt 50, a secondary transfer unit 70, and a fixing unit 80. .

  The document transport unit 10 includes a document feed table 11 on which a document G is set, a plurality of rollers 12, a transport drum 13, a transport guide 14, a document discharge roller 15, and a document discharge tray 16. Yes. The documents G set on the document feeder 11 are conveyed one by one to the reading position of the image reading unit 30 by the plurality of rollers 12 and the conveying drum 13. The conveyance guide 14 and the document discharge roller 15 discharge the document G conveyed by the plurality of rollers 12 and the conveyance drum 13 to the document discharge tray 16.

  The image reading unit 30 reads an image of the document G transported by the document transport unit 10 or the document placed on the document table 31 and generates image data. Specifically, the image of the original G is irradiated by the lamp L. The reflected light from the document G is guided in the order of the first mirror unit 32, the second mirror unit 33, and the lens unit 34 and forms an image on the light receiving surface of the image sensor 35. The image sensor 35 photoelectrically converts incident light and outputs a predetermined image signal. The output image signal is created as image data by A / D conversion.

  The image reading unit 30 has an image reading control unit 36. The image reading control unit 36 performs processing such as shading correction, dither processing, and compression on the image data created by the A / D conversion, and stores it in the RAM 103 (see FIG. 2). The image data is not limited to data output from the image reading unit 30 and may be data received from an external device such as a personal computer connected to the image forming apparatus 1 or another image forming apparatus.

  The paper storage unit 20 is disposed at the lower part of the apparatus main body, and a plurality of paper storage units 20 are provided according to the size and type of paper. The sheet is fed by the sheet feeding unit 21 and sent to the transport unit 23, and is transported by the transport unit 23 to the secondary transfer unit 70 having a transfer position. That is, the transport unit 23 functions to transport the paper fed from the paper feed unit 21 to the secondary transfer unit 70 and forms a transport path for transporting the paper. Further, a manual feed portion 22 is provided in the vicinity of the paper storage portion 20. From the manual feed section 22, paper of a size not stored in the paper storage section 20, tag paper having a tag, special paper such as an OHP sheet is sent to the transfer position. In FIG. 1, a sheet S fed by the sheet feeding unit 21 is denoted by a symbol “S”.

  An image forming unit 40 and an intermediate transfer belt 50 are disposed between the image reading unit 30 and the paper storage unit 20. The image forming unit 40 includes four image forming units 40Y, 40M, 40C, and 40K in order to form toner images of respective colors of yellow (Y), magenta (M), cyan (C), and black (Bk). .

  The first image forming unit 40Y forms a yellow toner image, and the second image forming unit 40M forms a magenta toner image. The third image forming unit 40C forms a cyan toner image, and the fourth image forming unit 40K forms a black toner image. Since these four image forming units 40Y, 40M, 40C, and 40K have the same configuration, only the first image forming unit 40Y will be described here.

  The first image forming unit 40Y includes a drum-shaped photoconductor 41, a charging unit 42 arranged around the photoconductor 41, an exposure unit 43, a developing unit 44, and a cleaning unit 45. The photoreceptor 41 is rotated by a drive motor (not shown). The charging unit 42 applies a charge to the photoconductor 41 and uniformly charges the surface of the photoconductor 41. The exposure unit 43 performs spot exposure on the surface of the photoconductor 41 by performing exposure scanning on the surface of the photoconductor 41 based on image data generated by the image reading unit 30 or image data transmitted from an external device. An electrostatic latent image having a shape is formed.

Here, the exposure unit 43 will be described with reference to FIGS. 2 and 3.
FIG. 2 is a configuration diagram showing an optical system of the exposure unit 43.
FIG. 3 is a diagram for explaining the rotary polygon mirror included in the exposure unit 43, FIG. 3A shows an explanatory diagram of a method for specifying the reflecting surface of the rotary polygon mirror, and FIG. 3B shows a perspective view of the rotary polygon mirror. .

  As shown in FIG. 2, the exposure unit 43 mainly includes a light source 61, an incident optical system 62, a rotary polygon mirror 63 (an example of a polarizer), a scanning optical system 64, a reflection mirror 65, and a writing start synchronization. A sensor 66 and a surface detection sensor 67 are provided.

The light source 61 emits a light beam in accordance with a pulse current input from a pulse generator (not shown) based on the image data. As the light source 61, for example, a laser light generator is used.
The incident optical system 62 includes, for example, a collimator lens that converts a light beam emitted from the light source 61 into parallel light, a cylinder lens that condenses the light beam in a linear shape, and the like.
The rotary polygon mirror 63 is a so-called polygon mirror that has a regular polygonal cross-sectional shape perpendicular to the rotation center axis, is perpendicular to the side of the regular polygon, and has a plurality of reflecting surfaces of the same shape. In the example of FIG. 2, a polygon mirror having six reflecting surfaces is illustrated as an example, but the number of reflecting surfaces is not limited to this example. The light beam is reflected by the reflecting surface of the rotating polygon mirror 63. The rotary polygon mirror 63 rotates at a constant speed, and the light beam is deflected and scanned. The exposure unit 43 operates by an underfill method. That is, the width of the reflecting surface of the rotary polygon mirror 63 in the main scanning direction is wider than the width of the light beam incident on the reflecting surface.

The scanning optical system 64 includes, for example, an fθ lens that corrects the scanning speed of the light beam reflected by the rotary polygon mirror 63, a flat lens that corrects the angle of the light beam incident on the photoconductor 41 in the sub-scanning direction, and the like. . The light beam deflected by the rotary polygon mirror 63 is incident on the surface of the photoconductor 41 via the scanning optical system 64.
The reflection mirror 65 is disposed between the scanning optical system 64 and the photoconductor 41, on a straight line connecting the side of the photoconductor 41 close to the exposure scanning start position in the main scanning direction and the rotary polygon mirror 63 or in the vicinity thereof. Has been. The reflection mirror 65 reflects the light beam deflected by the rotary polygon mirror 63 via the scanning optical system 64 toward the synchronization sensor 66 at the start of writing.

  The writing start synchronization sensor 66 is an example of a main scanning direction synchronization detection unit, and outputs a detection signal when a reflected light beam from the reflection mirror 65 is detected. This detection signal is used to synchronize the rotation of the rotary polygon mirror 63 and the exposure scanning (writing) to the photosensitive member 41 based on the image data. After a predetermined time has elapsed after the writing start synchronization sensor 66 outputs the detection signal, exposure scanning on the photoconductor 41 is started.

  The exposure unit 43 deflects the light beam emitted from the light source 61 by the rotary polygon mirror 63 in accordance with an instruction from the CPU 101 described later, and periodically scans the deflected light beam by moving it in the main scanning direction of the photoconductor 41. To do. When the scanning in the main scanning direction is completed at a certain position in the sub-scanning direction of the photoconductor 41, the exposure unit 43 moves the position in the sub-scanning direction at the end point by a predetermined pitch, and at the position in the sub-scanning direction after the movement. Scanning in the main scanning direction is performed, and this process is repeated.

  As shown in FIGS. 2 and 3A, the surface detection sensor 67 is disposed in the vicinity of the rotating polygon mirror 63 so as to face the reflecting surface of the rotating polygon mirror 63. The surface detection sensor 67 detects a specific reflecting surface of the rotating polygon mirror 63 and outputs a detection signal when the rotating polygon mirror 63 enters a stable constant speed rotation state and processing for specifying the reflecting surface is started. As shown in FIGS. 3A and 3B, six reflecting surfaces 63 a to 63 f are formed around the rotating body 68. Further, a surface detection member 63M, which is a surface detection mark, is provided at an arbitrary position of the rotating body 68 that does not interfere with the reflection surfaces 63a to 63f. When the surface detection sensor 67 detects the surface detection member 63M, the surface detection sensor 67 outputs a detection signal. In the example of FIG. 3, a detection signal is output from the surface detection sensor 67 when the surface detection sensor 67 and the reflection surface 63 d face each other. As an example, a light detector is used for the surface detection sensor 67, and the surface detection sensor 67 emits light to a reflection plate used for the surface detection member 63M and receives the reflected light of the surface detection member 63M. . A magnetic detector may be used for the surface detection sensor 67, and a magnetic material may be used for the surface detection member 63M.

  The surface detection sensor 67 outputs a detection signal every time the reflecting surface of the rotary polygon mirror 63 passes through the vicinity of the surface detection sensor 67 six times, that is, every time the rotary polygon mirror 63 makes one rotation. At present, the reflection surface (hereinafter referred to as “current reflection surface”) on which deflection scanning is performed by the rotating polygon mirror 63 is the time elapsed from the time when the surface detection sensor 67 outputs the detection signal, and the rotation. It can be calculated from the rotation period of the polygon mirror 63.

  Returning to the description of the image forming apparatus 1 in FIG. The developing unit 44 attaches yellow toner to the electrostatic latent image formed on the photoconductor 41 using, for example, a two-component developer composed of toner and carrier. As a result, a yellow toner image is formed on the surface of the photoreceptor 41.

  The developing unit 44 of the second image forming unit 40M attaches magenta toner to the photoconductor 41, and the developing unit 44 of the third image forming unit 40C attaches cyan toner to the photoconductor 41. Then, the developing unit 44 of the fourth image forming unit 40K adheres black toner to the photoconductor 41.

The toner image formed on the photoreceptor 41 is transferred to the intermediate transfer belt 50. The intermediate transfer belt 50 is formed in an endless shape and is stretched around a plurality of rollers. The intermediate transfer belt 50 is rotationally driven in a direction opposite to the rotation (movement) direction of the photoconductor 41 by a drive motor (not shown).
The cleaning unit 45 removes the toner remaining on the surface of the photoreceptor 41 after the toner image is transferred to the intermediate transfer belt 50.

  A primary transfer portion 51 is provided at a position on the intermediate transfer belt 50 that faces the photoreceptor 41 of each of the image forming units 40Y, 40M, 40C, and 40K. The primary transfer unit 51 primarily transfers the toner image formed on the photoreceptor 41 to the intermediate transfer belt 50 by applying a voltage having a polarity opposite to that of the toner to the intermediate transfer belt 50.

  When the intermediate transfer belt 50 is driven to rotate, the toner images formed by the four image forming units 40Y, 40M, 40C, and 40K are sequentially transferred onto the surface of the intermediate transfer belt 50. As a result, yellow, magenta, cyan, and black toner images are superimposed on the intermediate transfer belt 50 to form a color toner image.

  A belt cleaning device 53 faces the intermediate transfer belt 50. The belt cleaning device 53 cleans the surface of the intermediate transfer belt 50 that has finished transferring the toner image onto the paper.

  A secondary transfer unit 70 is disposed near the intermediate transfer belt 50 and downstream of the transport unit 23 in the sheet transport direction. The secondary transfer unit 70 secondarily transfers the toner image formed on the outer peripheral surface of the intermediate transfer belt 50 onto a sheet.

  The secondary transfer unit 70 has a secondary transfer roller 71. The secondary transfer roller 71 is in pressure contact with the counter roller 52 with the intermediate transfer belt 50 interposed therebetween. A portion where the secondary transfer roller 71 and the intermediate transfer belt 50 come into contact becomes a secondary transfer nip portion 72. The secondary transfer nip portion 72 is a transfer position where the toner image formed on the outer peripheral surface of the intermediate transfer belt 50 is transferred to the paper S.

  A fixing unit 80 is provided on the paper discharge side of the secondary transfer unit 70. The fixing unit 80 pressurizes and heats the paper to fix the transferred toner image on the paper. The fixing unit 80 includes, for example, a fixing upper roller 81 and a fixing lower roller 82 which are a pair of fixing members. The upper fixing roller 81 and the lower fixing roller 82 are arranged in pressure contact with each other, and a fixing nip portion is formed as a pressing portion at a position where the upper fixing roller 81 and the lower fixing roller 82 are in contact with each other.

  A heating unit is provided inside the fixing upper roller 81. The outer peripheral portion of the fixing upper roller 81 is warmed by the radiant heat from the heating portion. Then, the heat of the fixing upper roller 81 is transmitted to the sheet, whereby the toner image on the sheet is thermally fixed.

  The sheet is conveyed so that the surface on which the toner image is transferred by the secondary transfer unit 70 (the surface to be fixed) faces the fixing upper roller 81 and passes through the fixing nip portion. Accordingly, the sheet passing through the fixing nip portion is pressed by the upper fixing roller 81 and the lower fixing roller 82 and heated by the heat of the upper fixing roller 81.

  A switching gate 24 is disposed downstream of the fixing unit 80 in the sheet conveyance direction. The switching gate 24 switches the conveyance path of the sheet that has passed through the fixing unit 80. That is, the switching gate 24 moves the paper straight when performing face-up paper discharge in which the image forming surface is discharged upward in single-sided image formation. As a result, the paper is discharged by the pair of paper discharge rollers 25. Further, the switching gate 24 guides the sheet downward when performing face-down paper discharge and double-sided image formation in which the image forming surface in single-sided image formation is discharged downward.

When face-down paper discharge is performed, after the sheet is guided downward by the switching gate 24, the sheet reverse transport unit 26 reverses the front and back and transports the sheet upward. As a result, the sheet whose front and back sides are reversed and whose image forming surface faces downward is discharged by the pair of discharge rollers 25.
When performing double-sided image formation, after the sheet is guided downward by the switching gate 24, the front and back are reversed by the sheet reversing conveyance unit 26, and sent again to the transfer position of the secondary transfer unit 70 by the refeed path 27.

  A post-processing device that folds the paper or performs stapling processing or the like on the paper may be disposed on the downstream side of the pair of paper discharge rollers 25.

[Configuration of control system of image forming apparatus]
Next, a control system of the image forming apparatus 1 will be described with reference to FIG.
FIG. 4 is a block diagram illustrating a control system of the image forming apparatus 1.

  As shown in FIG. 4, the image forming apparatus 1 is used as, for example, a CPU (Central Processing Unit) 101, a ROM (Read Only Memory) 102 for storing a program executed by the CPU 101, and a work area of the CPU 101. RAM (Random Access Memory) 103. Further, it has a hard disk drive (HDD) 104 as a mass storage device and an operation display unit 105. As the ROM 102 which is an example of the storage unit, for example, an electrically erasable programmable ROM is used.

  The CPU 101 is an example of a control unit, and is connected to the ROM 102, the RAM 103, the HDD 104, and the operation display unit 105 via the system bus 107, and controls the entire apparatus. The CPU 101 is connected to the image reading unit 30, the image processing unit 106, the image forming unit 40, the paper feeding unit 21, and the transport unit 23 via the system bus 107.

  The CPU 101 includes a clock generation unit 101a that generates a clock signal therein. The CPU 101 can calculate the elapsed time from an arbitrary reference time by counting the clock signals generated by the clock generator 101a.

  The HDD 104 (an example of a storage unit) stores image data of an image of a document obtained by reading by the image reading unit 30, and stores output image data and the like. The operation display unit 105 is a touch panel including a display such as a liquid crystal display (LCD) or an organic ELD (Electro Luminescence Display). The operation display unit 105 displays an instruction menu for the user, information about the acquired image data, and the like. Furthermore, the operation display unit 105 includes a plurality of keys, receives input of various instructions, data such as characters and numbers, and outputs an input signal to the CPU 101.

  Image data generated by the image reading unit 30 and image data transmitted from a PC (personal computer) 120 which is an example of an external device connected to the image forming apparatus 1 are sent to the image processing unit 106 for image processing. Is done. The image processing unit 106 performs image processing such as shading correction, image density adjustment, and image compression on the received image data as necessary.

  The image forming unit 40 receives the image data processed by the image processing unit 106, performs exposure on the photosensitive member 41 by the exposure unit 43 and development by the developing unit 44 based on the image data, and performs an image on the paper S. Form. The exposure unit 43 in the image forming unit 40 has a nonvolatile memory 43m (an example of a storage unit). The memory 43m stores light amount correction value data (hereinafter referred to as “light amount correction data”) corresponding to the surface shapes of the reflecting surfaces 63a to 63f of the rotary polygon mirror 63. The light amount correction data is calculated on the basis of the intensity distribution of the beam spot of the light beam obtained from the surface shape in the main scanning direction of each of the reflecting surfaces 63a to 63f of the rotary polygonal mirror 63 measured in advance as will be described later. Note that light quantity correction value data may be stored in the ROM 102 or the HDD 104.

The CPU 101 is an example of a reflection surface specifying unit that specifies the current reflection surface. The CPU 101 receives the detection signal from the surface detection sensor 67 based on the time when the detection signal is received from the surface detection sensor 67 and the count value of the clock signal generated by the clock generation unit 101a after receiving the detection signal. The elapsed time from is calculated. Then, based on the elapsed time after receiving the detection signal from the surface detection sensor 67 and the rotation period of the rotary polygon mirror 63, the reflection surface (current reflection surface) on which the current rotary polygon mirror 63 is deflected and scanned is determined. Identify.
Further, the CPU 101 detects the detection signal from the write start synchronization sensor 66 based on the time when the detection signal is received from the write start synchronization sensor 66 and the count value of the clock signal generated by the clock generation unit 101a after receiving the detection signal. Elapsed time after receiving Then, based on the elapsed time after receiving the detection signal from the writing start synchronization sensor 66 and the scanning speed of the exposure unit 43 in the main scanning direction, the position (image) of the light beam on the current reflecting surface in the main scanning direction. High).

  The CPU 101 reads out the light amount correction data corresponding to the current reflecting surface of the identified rotary polygon mirror 63 from the memory 43m, and adjusts the light amount of the light beam emitted from the light source 61 for each image height based on the read light amount correction data. To do. For example, the light amount of the light beam can be adjusted by changing the intensity of the light beam, the emission time, or both.

  For example, the communication unit 108 receives job information transmitted from the PC 120 which is an external information processing apparatus via a communication line. The received job information is sent to the CPU 101 via the system bus 107.

  In this embodiment, an example in which a personal computer is applied as an external device has been described. However, the present invention is not limited to this, and various other devices such as a facsimile device can be applied as the external device. .

[Process for storing light amount correction value in storage]
FIG. 5 is a flowchart showing a process of storing the light quantity correction value for each reflecting surface and image height of the rotary polygon mirror 63 in the memory 43m. The following processing is performed at the manufacturing stage of the image forming apparatus 1 as an example.

  First, the surface shape of one of the reflecting surfaces 63a to 63f of the rotating polygon mirror 63 is measured with an interferometer (not shown) (step S1). That is, the CPU 101 acquires, from the interferometer, data associating the position in the main scanning direction of the reflecting surface with the error (unevenness) in the X-axis direction (height direction) as shown in FIG. Save to etc. An interferometer is a device that measures the wavelength, optical path length, refractive index, and the like using light interference, and is widely used to measure the fine structure of an object.

  Next, the CPU 101 calculates the intensity distribution of the beam spot for each image height of the photoreceptor 41 based on the surface shape of the reflecting surface of the rotary polygon mirror 63 measured in step S1 (step S2). For example, the intensity distribution of the beam spot corresponding to the surface shape of the reflecting surface of the rotary polygon mirror 63 is calculated with reference to the intensity of the beam spot when a light beam having a predetermined amount of light is irradiated onto an ideal reflecting surface. .

  Next, the CPU 101 calculates a light amount correction value according to the image height for the current reflecting surface (step S3).

Here, the light beam intensity on the reflecting surface will be described.
FIG. 6 is a diagram for explaining the light beam intensity on the reflecting surface. FIG. 6A is a graph showing the relationship between the position of the light beam reflecting surface in the main scanning direction and the light beam intensity when the reflecting surface of the rotating polygon mirror has irregularities (level difference), and FIG. The graph which shows the relationship between the position of the reflective surface of a light beam in the main scanning direction and light beam intensity when a reflective surface is an ideal plane is shown. 6A and 6B, the horizontal axis indicates the position in the main scanning direction, and the vertical axis indicates the intensity of the light beam. 6A and 6B, the numerical values on the horizontal axis and the vertical axis are normalized.

  The example of FIG. 6A corresponds to the case where the unintended small beam spot 252a shown in FIG. 14B is generated. If there are fine irregularities on the reflecting surface, for example, as shown in FIG. 6A, the beam spot shape collapses, and the two peak portions 90a, 90b exceeding the development threshold 92 in the intensity distribution 90 of the light beam. Will occur. The development threshold 92 is a value set so that development is performed when the intensity of the light beam exceeds that value. The first peak portion 90a that appears in the vicinity of “0” in the main scanning direction is the target, but the second peak portion 90b that appears in the vicinity of “20” in the main scanning direction is not intended. Such a beam spot where the intensity of the light beam unintentionally exceeds the development threshold value 92 is developed and appears as uneven density.

  On the other hand, the example of FIG. 6B corresponds to the beam spot 242 having a shape similar to a circle that is substantially uniform vertically and horizontally as shown in FIG. 13B. When the reflecting surface is an ideal plane, for example, as shown in FIG. 6B, the peak distribution exceeding the development threshold value 92 in the light beam intensity distribution 91 is one place (near “0” in the main scanning direction). Only. Therefore, the light amount correction value in step S3 is determined so that the intensity of the unintended second peak portion 90b in the intensity distribution of the light beam approaches the development threshold 92 or less than the development threshold 92. As an example, the light amount correction value is determined so that the intensity of the intensity distribution 90 of the light beam including the unintended second peak portion 90b in FIG. Alternatively, for example, referring to the intensity distribution 91 of the light beam when the reflecting surface is an ideal plane, the area of the region where the intensity exceeds the development threshold 92 in the intensity distribution of the light beam irradiated on the reflecting surface is Then, a light amount correction value that is equivalent to that in the case of the light beam intensity distribution 91 is determined. The light amount correction value is a value indicating how much light amount is corrected with respect to the light amount before correction. For example, the amount (percentage) when compared with the light amount before correction or the light amount before correction is increased or decreased. Defined as a percentage.

  Next, the CPU 101 determines whether or not the calculation of the light amount correction values for all the reflecting surfaces of the rotary polygon mirror 63 has been completed (step S4). If the calculation of the light amount correction values for all the reflecting surfaces is completed, the process proceeds to step S5, and if not completed, the process proceeds to step S1, and the surface shape of the next reflecting surface is measured.

  When the calculation of the light amount correction values for all the reflecting surfaces is completed in step S4 (YES in step S4), the CPU 101 stores the reflecting surfaces 63a to 63f of the rotary polygon mirror 63 in the memory 43m in the exposure unit 43. The light quantity correction value data (light quantity correction data) is stored in association with each other (step S5). After the process of step S5 is completed, the series of processes in FIG.

[Operation of image forming apparatus]
Hereinafter, an operation when the image forming apparatus 1 forms an image will be described.
FIG. 7 is a flowchart showing image forming processing by the image forming apparatus 1.
The CPU 101 implements the processing shown in FIG. 7 by executing the program recorded in the ROM 102.

  First, the CPU 101 of the image forming apparatus 1 detects the start of a job related to image formation based on an operation signal input from the operation display unit 105 or job information transmitted from the PC 120 via the communication unit 108. When the CPU 101 detects the start of a job related to image formation, the CPU 101 sets image data corresponding to the job in the RAM 103. Then, the CPU 101 controls the exposure unit 43 of the image forming unit 40 based on the image data. The CPU 101 identifies the current reflecting surface of the rotating polygon mirror 63 based on the elapsed time from the time when the detection signal is received from the surface detection sensor 67 and the rotation period of the rotating polygon mirror 63 (step S11).

  Next, the CPU 101 reads the light amount correction value corresponding to the current reflecting surface from the memory 43m of the exposure unit 43 (step S12).

  Next, the CPU 101 performs the main scanning direction of the light beam on the current reflecting surface based on the elapsed time since receiving the detection signal from the writing start synchronization sensor 66 and the scanning speed of the exposure unit 43 in the main scanning direction. Specify the position of. Then, the CPU 101 applies the light amount correction value for each position (image height of the photoconductor 41) of the current reflecting surface in the main scanning direction (step S13). That is, the CPU 101 stores a light amount correction value in the RAM 103 for each position in the main scanning direction of the current reflecting surface (image height of the photoconductor 41).

  Next, the CPU 101 determines whether or not the application of the light amount correction value to all the reflecting surfaces of the rotary polygon mirror 63 has been completed (step S14). If the application of the light amount correction value to all the reflecting surfaces is completed, the process proceeds to step S15. If not, the process proceeds to step S11, the next reflecting surface is specified, and the processes of steps S11 to S14 are repeated.

  Next, the CPU 101 forms an image based on the image data corresponding to the job and the light amount after the light amount correction value is applied to each of the reflecting surfaces 63 a to 63 f of the rotary polygon mirror 63 stored in the RAM 103. This is performed (step S15). That is, the CPU 101 identifies the current reflecting surface while rotating the rotating polygonal mirror 63 at a constant speed, and based on the image data and the corrected light amounts for the reflecting surfaces 63a to 63f of the rotating polygonal mirror 63, the reflecting surfaces 63a to 63f. The amount of light beam irradiated to the scanning surface of the photoconductor 41 is adjusted for each position in the main scanning direction 63f (image height of the photoconductor 41).

  After such exposure by the exposure unit 43, development by the development unit 44 is performed, and a toner image is formed on the surface of the photoreceptor 41. The toner image formed on the photoconductor 41 is transferred to the intermediate transfer belt 50, transferred to the paper S in the secondary transfer unit 70, fixed in the fixing unit 80, and discharged.

FIG. 8 is a diagram for explaining the light beam intensity on the reflection surface after the light amount correction, and the position of the light beam in the main scanning direction and the light beam intensity before and after correcting the light amount of the light beam. The graph which shows the relationship of is shown.
The intensity distribution 93 of the light beam shown in FIG. 8 is an intensity distribution of the light beam obtained by correcting the light amount of the light beam applied to the reflective surface having the unevenness illustrated in FIG. 6A. The intensity distribution 93 of the light beam after light amount correction is such that the area of the region where the intensity exceeds the development threshold 92 is equivalent to the intensity distribution 91 (FIG. 6B) of the light beam on an ideal flat reflecting surface. As a result of the correction, the intensities of the first peak portion 93a and the second peak portion 93b are reduced. Further, the second peak portion 93b corresponding to the second peak portion 90b of the light beam intensity distribution 90 (FIG. 6A), which is the cause of the density unevenness, has a strength close to the development threshold value 92. The toner density after development of the portion becomes light. In the beam spot corresponding to the intensity distribution 91 of the light beam, the area of the region where the intensity exceeds the development threshold 92 is equivalent to the intensity distribution 91 of the light beam on the ideal flat reflecting surface. Density unevenness is reduced.

  In the present embodiment having the above-described configuration, the photoconductors corresponding to the positions of the reflecting surfaces 63a to 63f of the rotating polygon mirror 63 in the main scanning direction according to the surface shapes of the reflecting surfaces 63a to 63f of the rotating polygon mirror 63. The light amount correction data in which the image height 41 is associated with the light amount correction value for correcting the light amount of the light beam emitted to each of the reflection surfaces 63a to 63f is generated. And the light quantity of the light beam at the time of exposure is adjusted for every reflective surface 63a-63f based on the light quantity correction data according to the surface shape of the reflective surfaces 63a-63f. Therefore, it is possible to reduce unevenness in density caused by the surface state (fine irregularities) of the reflecting surface of the underfilled rotary polygon mirror 63 in which the scanning position of the incident light beam is different within the same reflecting surface. Therefore, the image quality is improved.

  The embodiment to which the invention made by the present inventor is applied has been described above. However, the present invention is not limited by the description and drawings which form part of the disclosure of the invention according to the above-described embodiments, and various modifications may be made without departing from the spirit of the invention described in the claims. Is possible.

  For example, in the embodiment described above, the configuration in which the CPU 101 executes the processing in FIG. 6 has been described. However, the image forming unit 40 includes a microcomputer as a control unit, and the microcomputer specifies the reflection surface in FIG. Correction value reading and application processing may be executed. Although the example in which the light amount correction data is stored in the memory 43m in the exposure unit 43 has been described, the light amount correction data may be stored in the ROM 102.

  In addition, the exposure unit 43 of the image forming apparatus 1 includes a plurality of light sources that are separated from each other by a certain distance in the main scanning direction and the sub-scanning direction, and the light beams emitted from the plurality of light sources are reflected on the reflecting surface of the rotary polygon mirror 63. It is good also as a structure reflected and deflected by. Then, the plurality of light beams deflected by the rotary polygon mirror 63 are scanned in the main scanning direction while providing a predetermined pitch in the sub scanning direction with respect to the surface of the photoconductor 41.

  In the above-described embodiment, the electrophotographic image forming apparatus 1 has been described. However, the present invention can be applied to image forming apparatuses other than the electrophotographic system.

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 40 ... Image forming part, 41 ... Photoconductor, 43 ... Exposure part, 43m ... Memory, 63 ... Rotating polygon mirror, 63M ... Surface detection member, 66 ... Write synchronous sensor, 67 ... Surface detection sensor 91 ... (ideal) light beam intensity distribution 101 ... CPU 101a ... clock generator 102 ... ROM 103 ... RAM

Claims (4)

A light source that emits a light beam for exposing the photoreceptor;
The light beam emitted from the light source while rotating has a plurality of reflecting surfaces whose length in the main scanning direction in which the light beam emitted from the light source moves is longer than the width of the light beam formed by the light beam. A rotating polygon mirror that scans the photoreceptor by reflecting the light by the reflecting surface;
A reflecting surface specifying unit for specifying a reflecting surface reflecting the light beam of the rotary polygon mirror;
The image height of each photosensitive surface corresponding to the position of each reflecting surface in the main scanning direction, which is obtained according to the surface state of each reflecting surface of the rotary polygon mirror in the main scanning direction, is emitted to each reflecting surface. A storage unit for storing light amount correction data in association with a light amount correction value for correcting the light amount of the light beam;
A controller that reads out the light amount correction data from the storage unit and adjusts the light amount of the light beam emitted from the light source for each image height based on the read out light amount correction data;
An image forming apparatus comprising: The light amount correction value is an intensity of a beam spot of a light beam formed in the main scanning direction on the photosensitive member, which is obtained from a surface state in the main scanning direction of each reflecting surface of the rotary polygon mirror measured in advance. The image forming apparatus according to claim 1, wherein the image forming apparatus is calculated based on the distribution. The light amount correction value is such that, for each reflection surface, the area of the region that is equal to or greater than the development threshold in the intensity distribution of the beam spot of the light beam formed in the main scanning direction of the photoconductor is ideal for the reflection surface. 3. The calculation is performed so that the area of the light spot formed in the main scanning direction of the photosensitive member is equal to the area of the region that is equal to or greater than the development threshold in the intensity distribution of the light spot formed in the main scanning direction of the photoconductor. The image forming apparatus described. The image forming apparatus according to any one of claims 1 to 3, wherein the control unit adjusts a light amount for each image height by changing an intensity or an emission time of the light beam emitted from the light source.