JP2009034994A - Image forming apparatus and control method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the degree of freedom in changing the shape, diameter, and light intensity distribution for the section of a laser beam. <P>SOLUTION: In an image forming apparatus, a laser controller 200 emission-controls a plurality of lasers arranged in a two-dimension on a surface-emitting laser 10 in an optional pattern based on image data of a pixel. The laser beams from the plurality of laser devices are bundled together by a collimator lens 11 to shape a laser beam to irradiate onto a photosensitive drum 101. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and a control method therefor, and more particularly to an image forming apparatus using an exposure technique when performing image formation by exposing and scanning a photosensitive member with a laser beam, and a control method therefor.

  Conventionally, an image forming apparatus using an electrophotographic process is known. In this type of image forming apparatus, generally, an electrostatic latent image is formed on the surface of the photosensitive member by exposing and scanning the photosensitive member with a laser beam whose emission is controlled based on image data. In this case, a semiconductor laser, a He—Ne laser, or the like is generally used as the laser light source. The cross-sectional shape (spot shape) of the laser beam emitted from these laser light sources and irradiated onto the photosensitive member is circular or elliptical, and the light intensity distribution of the beam cross-section has a peak value at the center. Yamagata Gaussian distribution (single transverse mode).

  Accordingly, the electrostatic latent image formed on the surface of the photoreceptor also has a mountain-shaped Gaussian distribution. As a result, the toner image obtained on the photosensitive member by developing the electrostatic latent image with toner also increases in the amount of toner loaded at the central portion, and rises into a mountain shape. In the case of using a transfer sheet having a stiff paper, the toner that faces the transfer sheet on the photosensitive member is crushed by the transfer roller, but finally, a transfer image in which the toner swells in the central portion is obtained. On the other hand, in order to ensure the necessary dot diameter and line width, since the electrostatic latent image has a mountain-shaped Gaussian distribution, it is necessary to increase the amount of toner loaded on the photoreceptor and the transfer paper. For this reason, this chevron-shaped toner image may be composed of a larger amount of toner than the maximum amount of toner that can ensure a predetermined dot diameter and line width, causing the output image to be crushed.

  Furthermore, the toner is scattered by the pressurization when the toner image on the photosensitive member is transferred and fixed on the recording paper, and the image quality is deteriorated. Further, the chevron-shaped toner image has also caused problems such as the toner does not melt uniformly and the fixability is deteriorated, and the electrostatic offset with respect to the fixing roller is generated. In order to deal with these problems, a technique described in Patent Document 1 below has been proposed.

  It is also known that the optimum spot diameter of the laser beam varies depending on the image to be formed, such as a binary image or a multi-tone image. In order to cope with this problem, a technique described in Patent Document 2 below has been proposed.

Furthermore, if the spot diameter of the laser beam is reduced in order to achieve high resolution, high definition of the image from highlight to halftone can be realized. However, since the spot size is small in the high density region, a gap is formed between the laser beams even if the entire surface is irradiated with light, and sufficient gradation cannot be obtained. In order to deal with this problem, a technique described in Patent Document 3 below has been proposed.
JP-A-8-276619 JP-A-8-164634 JP-A-2003-285466

  In the image forming apparatus according to the technique proposed in the above Patent Documents 1 to 3, it is possible to achieve a certain level of image quality. Recently, however, there has been a demand for higher image quality in image forming apparatuses such as printers as digital cameras have higher image quality. For this reason, in the exposure control of the image forming apparatus, it is desired to more freely control the shape, diameter, light intensity distribution and the like of the cross section of the laser beam.

  The present invention has been made under such a background, and an object of the present invention is to provide an image forming apparatus that improves the degree of freedom in changing the cross-sectional shape, diameter, and light intensity distribution of a laser beam, and a control method therefor. There is to do.

  In order to achieve the above object, the present invention provides an image forming apparatus for forming an image by exposing a photosensitive member with laser light whose emission is controlled based on image data, and a light source means having a plurality of lasers and one pixel. Control means for individually controlling the light emission of the plurality of lasers based on the image data, and beam forming means for condensing the laser light emitted from the plurality of lasers by light emission control by the control means to form a laser beam And scanning means for exposing and scanning the photosensitive member with a laser beam from the beam forming means, and the control means changes the light emission patterns of the plurality of lasers in accordance with image data of each pixel. It is characterized by.

  According to the present invention, the degree of freedom in changing the cross-sectional shape, diameter, and light intensity distribution of the laser beam can be improved, and high image quality can be achieved.

  Hereinafter, the best mode for carrying out the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing a schematic configuration of an exposure scanning unit of an image forming apparatus according to an embodiment of the present invention.

  In FIG. 1, an image forming apparatus 1 according to the present embodiment includes an exposure scanning unit 100 and a photosensitive drum 101. The exposure scanning unit 100 generally includes a surface emitting laser (VCSEL) 10 as a multi-beam laser light source, a collimator lens 11, an aperture (optical aperture) 12, a cylindrical lens 13, and a polygon mirror 14. Further, the exposure scanning unit 100 includes an fθ lens group 15, a surface tilt correction lens 16, and a mirror 17. These are arranged in order, and are configured to scan the photosensitive drum 101. The exposure scanning unit 100 includes a synchronous mirror 19, a synchronous cylindrical lens 20, and a photodetector (BD sensor) 21.

  The surface emitting laser 10 has a plurality of laser light sources (L1 to L25) two-dimensionally arranged as will be described later with reference to FIG. 3, and the plurality of laser light sources are individually controlled to emit light based on image data. The collimator lens 11 (beam forming means) converts a plurality of laser beams emitted from the surface emitting laser 10 into a single parallel laser beam. The aperture 12 adjusts the cross-sectional shape (spot shape and spot diameter) of the laser beam from the collimator lens 11.

  The cylindrical lens 13 refracts the laser beam only in the sub-scanning direction in consideration of the width (corresponding to the thickness) of the polygon mirror 14 in the sub-scanning direction. The polygon mirror 14 is rotated at a constant speed (constant angular speed) in the direction of arrow A in FIG. 1 by a drive motor (not shown). The fθ lens group 15 corrects distortion so as to guarantee temporal linearity of scanning on the photosensitive drum 101. The surface tilt correction lens 16 corrects the surface tilt of the polygon mirror 14. The mirror 17 is a reflection mirror.

  Next, the optical path will be described in order. The laser beam emitted from the surface emitting laser 10 is converted into substantially parallel light by the collimator lens 11, the aperture 12 and the cylindrical lens 13, and then enters the polygon mirror 14 with a predetermined beam diameter. The polygon mirror 14 is rotated at a constant angular velocity in the rotation direction indicated by the arrow A in the figure, and with the rotation of the polygon mirror 14, the incident laser beam is continuously converted into a deflected beam whose angle is changed. Reflected. The deflected beam is focused by the fθ lens group 15. Further, the fθ lens group 15 corrects distortion aberration so as to guarantee temporal linearity of scanning on the photosensitive drum 101 at the same time with respect to the laser beam. The laser beam whose surface tilt of the polygon mirror 14 is corrected by the surface tilt correcting lens 16 is reflected by the mirror 17 and combined and scanned on the photosensitive drum 101 in the direction of arrow B at a constant speed.

  Further, the laser beam passing through the fθ lens group 15 from the polygon mirror 14 is reflected by the synchronous mirror 19, passed through the synchronous cylindrical lens 20, and received by the photodetector 21.

  The photodetector 21 generates a horizontal synchronization signal (BD signal) 201 (FIG. 4) that serves as a writing reference in the main scanning direction, which is the longitudinal direction (axial direction) of the photosensitive drum 101, at the timing when the laser beam is incident. The horizontal synchronization signal 201 is used as a signal for synchronizing the rotation of the polygon mirror 14 and the writing of image data to the photosensitive drum 101. The image signal 202 is output from the image processing unit 300 (FIG. 4) to the laser control unit 200 (FIG. 4) using the horizontal synchronization signal 201 as a reference.

  Further, the laser control unit 200 drives (emits) the surface emitting laser 10 based on the image signal 202 input from the image processing unit 300 in an image section where a latent image is formed on the photosensitive drum 101 (FIG. 4). ) Current value and driving time are controlled. The laser beam emitted from the surface emitting laser 10 in this manner is converted into substantially parallel light by the collimator lens 11, the aperture (optical aperture) 12, and the cylindrical lens 13 and then enters the polygon mirror 14 with a predetermined beam diameter. Become.

  Further, a PD (not shown) is disposed as a light receiving element in a region at the peripheral end of the optical aperture 12. The PD detection signal is used for light emission amount control of the surface emitting laser 10, that is, auto power control (APC) for determining the current value of the drive (light emission) signal 204.

  FIG. 2 is a schematic configuration diagram of the image forming apparatus 1 including the exposure scanning unit 100, and is a diagram mainly showing devices related to the electrophotographic process provided around the photosensitive drum 101.

  In FIG. 2, the image forming apparatus 1 includes an exposure scanning unit 100, a photosensitive drum 101, a charging roller 102, a developing device 104, a transfer guide group 106, a transfer roller 105, a cleaner 107, a conveyance guide 108, and a fixing device 109. The image forming apparatus 1 also includes a bias power source 111, a developing bias power source 112, and a bias power source 113 as power sources.

  As shown in FIG. 2, the photosensitive drum 101 has a structure in which a photosensitive layer 101a is laminated on a cylindrical metal conductive substrate 101b. Around the photosensitive drum 101, a charging roller 102, a developing roller 104, a transfer guide group 106, a transfer roller 105, and a cleaner 107 are arranged in the rotation direction indicated by an arrow C.

  The charging roller 102 has a cored bar 102b and an elastic layer 102a as a surface layer thereof. The charging roller 102 is provided to uniformly charge the photosensitive drum 101 with a voltage applied to the cored bar 102b by the bias power source 111. For example, the bias power source 111 applies a DC bias voltage (DC = −800 V) and an AC bias voltage (AC) to the core metal 102 b of the charging roller 102. As a result, when the charging roller 102 comes into contact with the photosensitive drum 101 via the elastic layer 102a, the surface of the photosensitive drum 101 is uniformly charged with about −800 V (dark potential: Vd).

  The laser beam 103 from the exposure scanning unit 100 in FIG. 1 is applied to the photosensitive drum 101 charged with the dark potential. The potential (bright potential: VL) of the laser beam 103 is about −200V.

  By irradiating the laser beam 103 (about −200 V) whose absolute value is lower than the dark potential (about −800 V) in this way, the electrostatic latent image formed on the irradiated portion (exposed portion) is irradiated. The absolute value of the potential is lower than the dark potential (about −800 V).

  The developing device 104 develops the electrostatic latent image formed on the photosensitive drum 101 with toner. The developing device 104 has a developing sleeve 104a for charging the toner. A developing bias (for example, DC = −500 V and AC) is applied to the developing sleeve 104a by a developing bias power source 112.

  The transfer roller 105 transfers the toner image formed on the photosensitive drum 101 to a recording medium (not shown) such as recording paper. The transfer roller 105 includes a cored bar 105b to which a bias voltage is applied by a bias power source 113, and a medium resistance elastic layer 105a formed on the surface thereof. The recording medium is guided between the transfer roller 105 and the photosensitive drum 101 by the transfer guide 106. After transfer to the recording medium, toner, paper dust, and the like remaining on the photosensitive drum 101 are removed by the cleaning blade 107 a and accumulated in the cleaner 107.

  After the transfer, the recording medium is sent to the fixing device 109 through the conveyance guide 108. The fixing device 109 includes a fixing roller 109a and a pressure roller 109b, and fixes the toner image on the recording medium by pressing and heating.

  FIG. 3 is a diagram showing the arrangement relationship of the light sources of the surface emitting laser 10 in FIG. FIG. 4 is a diagram showing the relationship between the surface emitting laser 10 and the collimator lens 11 in FIG. As shown in FIG. 3, the surface emitting laser 10 has 25 lasers L1, L2, L3,..., L25, which are light sources in which laser light output portions are two-dimensionally arranged in a lattice shape. Yes. As shown in FIG. 4, the laser beams respectively emitted from the 25 lasers L1 to L25 are collected as one parallel laser beam by the collimator lens 11, and are incident on the aperture 12 in FIG. . In FIG. 4, for the sake of convenience, only the laser beams L12a and L14a corresponding to the lasers L12 and L14 are shown, and the laser beams corresponding to the other lasers are not shown. Further, the specifications and performance of the 25 lasers L1 to L25 are the same standard, and for example, the oscillation wavelength is unified to about 700 to 800 nm.

  The 25 lasers L1 to L25 individually have laser drive circuits LD1 to LD25 (FIG. 4), respectively. These laser drive circuits LD1 to LD25 turn on / off (select) the corresponding lasers L1 to L25 and switch their emission intensities under the control of the laser control unit 200. The laser control unit 200 controls laser light emission by the surface emitting laser 10 based on image data (rasterization signal) from the image processing unit 300. The image processing unit 300 performs various correction processes on the image data, and rasterizes the image data as vector information to convert it into bitmap information.

  The laser control unit 200 drives and controls the 25 lasers L1 to L25 of the surface emitting laser 10 based on the image data of one pixel rasterized by the image processing unit 300. In other words, the laser control unit 200 converts the light emission patterns of the 25 lasers L1 to L25 into image data attribute information (color, character / non-character, etc.), density data, and image processing data (halftone processing method). Etc., that is, change based on such information. Details thereof will be described later. That is, one pixel is expressed by the light emission patterns of the 25 lasers L1 to L25 of the surface emitting laser 10.

  That is, in the present embodiment, the 25 lasers L1 to L25 of the surface emitting laser 10 emit light in a pattern corresponding to the image data of each pixel, and the laser light is focused on one point on the photosensitive drum 101. As a result, the light emission pattern of the 25 lasers L1 to L25 can be changed to the shape, diameter, and light intensity distribution of the cross section of the laser beam when the electrostatic latent image relating to each pixel is written on the photosensitive drum 101. This makes it possible to improve the image quality.

  FIG. 5 is a relationship diagram of image attribute data (color, characters, graphics), density, gradation of each light source, and light emission pattern. FIG. 6 is a diagram used for explaining a dither matrix in the case where halftones are expressed by attributes of image data.

  In the present embodiment, the writing resolution of the electrostatic latent image is 600 DPI. Further, as shown in FIG. 5, the number of gradations of each of the lasers L1 to L25 is set to 3, and the level of each gradation is represented by an integer value of 0 to 2. Further, the number of gradations of one pixel is set to 20, and the levels of each gradation are represented by integer values of 0 to 19 using 25 lasers L1 to L25. In addition, the number of gradations is set to 256 by using a dither method for halftone expression.

  In addition to color and density information, the image processing unit 300 is input with information indicating whether it is a character image or a graphic image as attribute information. The image processing unit 300 supplies the attribute information (character image or graphic image) to the laser control unit 200 while adding the attribute information (character image or graphic image) after image processing, rasterization processing, and dither processing. Therefore, the laser control unit 200 determines the light emission patterns of the lasers L1 to L25 related to each pixel based on these pieces of information.

  As shown in FIG. 6, the image processing unit 300 switches the dither matrix in the case of expressing halftones according to the attribute of the image data.

  That is, when the attribute of the image data is a character image, the screen line number is 212 lpi (lines per inch), the screen angle (growth direction) is 45 °, and the growth method grows dots in a dot pattern for each color of CMYK. Use a spot growth screen.

  On the other hand, when the attribute of the image data is a graphics image, the growth method uses a line growth screen that grows the thickness of a line of lines for each color of CMYK. Then, the number of screen lines and the screen angle are changed as follows according to the colors of CMYK. That is, in the case of C (Cyan), the number of screen lines is 166 lpi and the screen angle is 124 °. In the case of M (Magenta), the number of screen lines is 166 lpi and the screen angle is 56 °. In the case of Y (Yellow), the screen line number is 145 lpi and the screen angle is 166 °. In the case of K (Black), the screen line number is 145 lpi and the screen angle is 14 °.

  As shown in FIG. 5, when the image data is a character image, the uppermost light emission pattern is adopted regardless of the color. That is, when the density (gradation of one pixel) is 0 gradation, each of the lasers L1 to L25 is expressed as 0 gradation (non-light emission). When the density is 4 gradations, each of the lasers L8, L12, L14, and L18 is expressed as 1 gradation, and the other lasers are expressed as 0 gradations. When the density is 8 gradations, each of the lasers L8, L12, L14, and L18 is expressed as 1 gradation, each of the lasers L7, L9, L17, and L19 is expressed as 2 gradations, and the other lasers are expressed as 0 gradations. . When the density is 12 gradations, each of the lasers L8, L12, L14, and L18 is 1 gradation, each of the lasers L3, L7, L9, L11, L15, L17, L19, and L23 is 2 gradations, and the others The laser is expressed as 0 gradation. When the density is 15 gradations, each of the lasers L8, L12, L14, and L18 is 1 gradation, and the lasers L2 to L4, L6, L7, L9, L10, L11, L15, L16, L17, L19, L20, and L22. Each of .about.L24 is expressed as 2 gradations, and the other lasers are expressed as 0 gradations. In other words, in the case of pixel data related to the character image portion, the spot shape of the laser beam is close to a circle, and the light intensity distribution is such that a higher light intensity is obtained at the peripheral portion than the light intensity at the central portion of the spot diameter. ing. This is because the edge of the character image portion is emphasized at a high resolution so that the toner image after transfer becomes close to a rectangle so that image deterioration due to toner scattering or the like does not occur.

  By controlling the spot shape and light intensity distribution of the laser beam, the electrostatic latent image formed on the surface of the photosensitive drum 101 has a potential distribution as shown in FIG. 7A, and the toner height on the recording paper. Is as shown in FIG.

  As a result, the edge of the character image portion is emphasized, and the reproducibility of the character portion can be improved without causing image degradation due to toner scattering or the like.

  In the case of the image data related to the graphics portion, the light emission pattern that gives a spot shape along the growth direction of the screen is emphasized with respect to gradation reproducibility. That is, as shown in FIG. 6, a spot shape inclined at 135 ° in the case of Cyan and 45 ° in the case of Magenta is obtained, and in the case of Yellow and Black, a light emission that produces a horizontally long spot shape in the main scanning direction is obtained. Pattern was used.

  That is, when the image data is graphics, the light emission pattern shown in FIG. 5 is adopted for C, M, Y, and K in consideration of the screen angle shown in FIG. That is, in the case of 0 gradation, each of C, M, Y, and K and each of the lasers L1 to L25 are expressed as 0 gradation (non-light emission).

  Specifically, considering that the screen angle of C is 124 ° as shown in FIG. 6, when the density of Cyan is 4 gradations, each of lasers L7, L13, and L19 is 1 gradation, and the others This laser is expressed as 0 gradation. When the Cyan density is 8 gradations, each of the lasers L1 and L25 is expressed as 1 gradation, each of the lasers L7, L13, and L19 is expressed as 2 gradations, and the other lasers are expressed as 0 gradations. When the Cyan density is 12 gradations, each of the lasers L2, L6, L20, and L24 is 1 gradation, each of the lasers L1, L7, L8, L12 to L14, L18, L19, and L25 is 2 gradations, Other lasers are expressed as 0 gradation. When the Cyan density is 15 gradations, each of the lasers L1 to L3, L6 to L9, L11 to L15, L17 to L20, and L23 to L25 is expressed as 2 gradations, and the other lasers are expressed as 0 gradations. Similarly, in consideration of the screen angles of Magenta, Yellow, and Black being 56 °, 166 °, and 14 °, respectively, the gradation of each density of Magenta, Yellow, and Black is expressed by the pattern shown in FIG. .

  The potential distribution of the electrostatic latent image formed by these spot-shaped laser beams is as shown in FIG. That is, electrostatic latent images related to adjacent pixels are connected at an early stage, and a more stable output image can be obtained. Thereby, the reproducibility of the graphics part can be improved.

  Note that the present invention is not limited to the above embodiment, and the number of lasers whose emission is individually controlled based on image data of one pixel may be other than “25”. In addition, laser light emitted from a plurality of lasers can be condensed by a device other than the collimator lens 11 into a single laser beam.

1 is a diagram showing a schematic configuration of an exposure scanning unit of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic configuration diagram of an image forming apparatus including an exposure scanning unit, and is a diagram mainly showing devices related to an electrophotographic process provided around a photosensitive drum. It is a figure which shows the arrangement | positioning relationship of the light source on the surface emitting laser in FIG. It is a figure which shows the relationship between the surface emitting laser and collimating lens in FIG. FIG. 6 is a relationship diagram of image data attributes (color, characters, graphics), density, gradation of each light source, and light emission pattern. It is a figure used in order to demonstrate the dither matrix in the case of expressing a halftone with the attribute of image data. 4A and 4B are conceptual diagrams illustrating a potential distribution and a toner height according to a character portion, where FIG. 5A illustrates a potential distribution of an electrostatic latent image, and FIG. It is a conceptual diagram which shows the electric potential distribution of the electrostatic latent image of the pixel data which concerns on a graphics part.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Surface emitting laser 11 Collimator lens 12 Aperture 101 Photosensitive drum 200 Laser control part 300 Image processing part L1-L25 Laser LD1-LD25 Laser drive circuit

Claims (7)

In an image forming apparatus that forms an image by exposing a photoreceptor with laser light whose emission is controlled based on image data.
Light source means having a plurality of lasers;
Control means for individually controlling emission of the plurality of lasers based on image data of one pixel;
Beam forming means for condensing laser light emitted from the plurality of lasers by light emission control by the control means to form a laser beam;
Scanning means for exposing and scanning the photosensitive member with a laser beam from the beam forming means;
Have
The image forming apparatus according to claim 1, wherein the control unit changes a light emission pattern of the plurality of lasers according to image data of each pixel.   2. The image forming apparatus according to claim 1, wherein the plurality of lasers of the light source means have laser light output units arranged two-dimensionally.   The control unit selects a laser to emit light from the plurality of lasers according to image data of each pixel, and changes a light emission pattern of the plurality of lasers by switching the emission intensity. The image forming apparatus according to claim 1.   The image data of each pixel has at least one of a group consisting of color, density, and halftone processing method information, and the control means changes the light emission patterns of the plurality of lasers according to the information. The image forming apparatus according to claim 1, wherein: In a control method of an image forming apparatus for forming an image by exposing a photosensitive member with laser light whose emission is controlled based on image data,
A light emission control step of individually controlling light emission of a plurality of lasers provided in the light source means based on image data of one pixel;
A beam forming step of condensing laser beams emitted from the plurality of lasers to form a laser beam;
A scanning step of exposing and scanning the photosensitive member with the laser beam,
In the light emission control step, a light emission pattern of the plurality of lasers is changed in accordance with image data of each pixel.   In the light emission control step, according to the image data of each pixel, the laser to be emitted is selected from the plurality of lasers, and the light emission pattern of the plurality of lasers is changed by switching the light emission intensity. 6. A method for controlling an image forming apparatus according to claim 5, wherein:   The image data of each pixel includes at least one of a group consisting of information on a color, density, and halftone processing method, and the light emission control step includes the light emission patterns of the plurality of lasers according to the information. The method of controlling an image forming apparatus according to claim 6, wherein the image forming apparatus is changed.