Image forming apparatus and image forming method

Abstract

To provide an image formation apparatus which can suppress the occurrence of a black stripe even when there is unevenness in a beam diameter of an exposure device.SOLUTION: A control unit (101) converts image data into binary raster image data on the basis of a dither matrix corresponding to the half-tone of image data. The dither matrix is configured such that a pattern is grown in a prescribed screen direction from an isolated pixel as the exposure area ratio of the image data becomes larger, and the pattern is grown in a direction intersecting the prescribed screen direction after the grown pattern is connected to an adjacent pattern. The control unit (101) exposes a photoreceptor drum (53) to light by an exposure device (ED) on the basis of the pattern.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus equipped with an LED (Light Emitting Diode) print head and an image forming method. [Background technology]

[0002] Conventionally, electrophotographic printers have been known to print halftone images using binary dither patterns. There are several types of dither patterns, but the most widely known is a line-type dither pattern that uses a pattern with a predetermined screen angle.

[0003] For example, Patent Document 1 relates to an image forming apparatus and method that can output an image with good color moiré by combining dither matrices, and describes a general line dither pattern. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-156394 Summary of the Invention [Problem to be solved by the invention]

[0005] When the beam diameter of an exposure device is uneven, white streaks may appear in areas with low print density, and black streaks may appear in areas with high print density. However, Patent Document 1 does not suppress the occurrence of white streaks or black streaks.

[0006] An aspect of the present invention aims to suppress the occurrence of black streaks even when there is unevenness in the beam diameter of an exposure device. [Means for solving the problem]

[0007] In order to solve the above problem, an image forming apparatus according to one aspect of the present invention comprises a photosensitive member, an exposure device that exposes the photosensitive member, a developer that develops an electrostatic latent image formed on the photosensitive member, and a control unit, wherein the control unit converts image data into binary raster image data based on a dither matrix corresponding to the halftones of the image data, and the dither matrix is ​​configured to grow a pattern from an isolated pixel in a predetermined screen direction as the exposure area ratio of the image data increases, and after the grown pattern connects with adjacent patterns, grow the pattern in a direction intersecting the predetermined screen direction, and the control unit exposes the photosensitive member using the exposure device based on the pattern.

[0008] According to the above configuration, the unexposed portions remain as concentrated areas rather than linear areas, so that even if there is unevenness in the beam diameter of the exposure device, the occurrence of black streaks can be suppressed.

[0009] The dither matrix may grow a pattern from an isolated pixel in the sub-scan direction before growing a pattern from the isolated pixel in a given screen direction.

[0010] According to the above configuration, even if there is unevenness in the beam diameter of the exposure device, the occurrence of white streaks can be suppressed.

[0011] The dither matrix may be configured such that after the grown pattern is connected to the adjacent pattern, the pattern is grown in the sub-scanning direction, and then the pattern is further grown in the screen direction.

[0012] According to the above configuration, the pattern can be grown so as to become thicker by the dither matrix.

[0013] The dither matrix may grow a pattern in a direction intersecting a predetermined screen direction, and connect to an adjacent pattern in the sub-scanning direction when the exposure area ratio reaches a predetermined value or more.

[0014] According to the above configuration, adjacent patterns in the sub-scanning direction are connected, so that the unexposed portions remain as concentrated areas rather than linear, and the occurrence of black streaks can be suppressed even if there is unevenness in the beam diameter of the exposure device.

[0015] The predetermined value may be 60%.

[0016] The direction intersecting the predetermined screen direction may be 90 degrees.

[0017] The exposure device may include a plurality of light-emitting elements arranged in the main scanning direction and a lens array that focuses light from the plurality of light-emitting elements onto a photosensitive member, and the lens array may be arranged in a row in the main scanning direction.

[0018] According to the above configuration, even in an exposure apparatus in which unevenness in the beam diameter is likely to occur, the occurrence of black or white streaks can be suppressed.

[0019] In order to solve the above problem, an image forming method according to one aspect of the present invention is configured such that a dither matrix grows a pattern from an isolated pixel in a predetermined screen direction as the exposure area ratio of the image data increases, and after the grown pattern connects with an adjacent pattern, the pattern grows in a direction intersecting the predetermined screen direction; the image data is converted into binary raster image data based on a dither matrix corresponding to the halftones of the image data, and a photosensitive member is exposed by an exposure device based on the pattern.

[0020] According to the above configuration, the unexposed portions remain as concentrated areas rather than linear areas, so that even if there is unevenness in the beam diameter of the exposure device, the occurrence of black streaks can be suppressed. [Effects of the Invention]

[0021] According to one aspect of the present invention, even if there is unevenness in the beam diameter of an exposure device, the occurrence of black streaks can be suppressed. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram illustrating the configuration of the LED print head shown in FIG. [Figure 3] 3 is a diagram showing the LED print head shown in FIG. 2 as viewed from the Z direction. [Figure 4] FIG. 10 is a diagram illustrating the relationship between the screen angle and the interval between line segments. [Figure 5] 10A and 10B are diagrams illustrating a case where the beam diameter extends in the X direction or the Y direction in a dither pattern. [Figure 6] FIG. 10 is a diagram showing a dither matrix for suppressing the occurrence of black streaks. [Figure 7] 10A and 10B are diagrams showing a comparison between a general dither pattern and a dither pattern according to the present embodiment; [Figure 8] FIG. 10 is a diagram showing a dither matrix for suppressing the occurrence of white streaks and black streaks. [Figure 9] 10A and 10B are diagrams showing a comparison between a general dither pattern and a dither pattern according to the present embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0023] <Configuration of Image Forming Apparatus 1> 1 is a schematic diagram showing the configuration of an image forming apparatus 1 according to an embodiment of the present invention. In this embodiment, a color LED printer will be described as an example of the image forming apparatus 1. Note that the left side of the paper is the "front side," the right side of the paper is the "rear side," and the up-down direction of the paper is the "vertical direction."

[0024] 1, the image forming apparatus 1 includes, within a main body housing 10, a paper feed unit 20 that supplies paper P, an image forming unit 30 that forms an image on the fed paper P, a paper discharge unit 90 that discharges the paper P on which the image has been formed, and a main board 100 that controls each unit when forming an image. The main board 100 is equipped with a control unit 101 that controls each unit.

[0025] An upper cover 11, which opens and closes an opening provided in the main body housing 10, is provided on the top of the main body housing 10 and is rotatable up and down about a rotation shaft 12 provided on the rear side. The top surface of the upper cover 11 serves as a paper output tray 13 that accumulates paper P discharged from the main body housing 10, and the bottom surface is provided with multiple holding members 14 that hold LED units 40, which are exposure devices. Also provided within the upper cover 11 are an LED control board 110 and a shield plate 120 that faces the LED control board 110.

[0026] The paper feed unit 20 is provided in the lower part of the main body housing 10, and mainly includes a paper feed tray 21 that is detachably attached to the main body housing 10, and a paper feed mechanism 22 that transports paper P from the paper feed tray 21 to the image forming unit 30. The paper feed mechanism 22 is provided in front of the paper feed tray 21, and mainly includes a paper feed roller 23, a separation roller 24, and a separation pad 25.

[0027] In the paper feed section 20 configured in this manner, the paper P in the paper feed tray 21 is separated one by one and sent upward, and after paper dust is removed as it passes between the paper dust removal roller 26 and the pinch roller 27, it is turned backward through the conveying path 28 and supplied to the image forming section 30.

[0028] The image forming section 30 is mainly composed of four process cartridges 50, four LED units 40, a transfer unit 70, and a fixing unit 80.

[0029] The LED unit 40 is disposed above a photosensitive drum 53, which is an example of a photosensitive member, and mainly comprises an LED print head 41 and a support frame 42, with the LED print head 41 disposed opposite the photosensitive drum 53.

[0030] The support frame 42 is a member that supports the LED print head 41, and is swingably attached to the upper cover 11 via the holding member 14. As a result, the LED unit 40 (LED print head 41) moves from an exposure position facing the photosensitive drum 53 to an upper retracted position by rotating the upper cover 11 upward.

[0031] The process cartridges 50 are arranged side by side in the front-to-rear direction between the upper cover 11 and the paper feed section 20, and include a drum unit 51 and a developing device 61 that is detachably attached to the drum unit 51. The process cartridge 50 can be replaced through an opening in the main body casing 10 after the upper cover 11 is rotated upward. The process cartridges 50 are identical in configuration, with the only difference being the color of the toner (developer) contained in the toner containing chamber 66 of the developing device 61.

[0032] The drum unit 51 mainly includes a drum frame 52, a photosensitive drum 53 rotatably supported by the drum case 52, and a charger .

[0033] The developing device 61 is a device that develops the electrostatic latent image formed on the photosensitive drum 53, and is equipped with a developing frame 62, a developing roller 63 and a supply roller 64 that are rotatably supported in the developing case 62, a layer thickness regulating blade 65, and a toner storage chamber 66 that stores toner.

[0034] The transfer unit 70 is provided between the paper feed section 20 and each process cartridge 50 , and mainly includes a drive roller 71 , a driven roller 72 , a conveyor belt 73 , a transfer roller 74 , and a cleaning section 75 .

[0035] The drive roller 71 and the driven roller 72 are arranged parallel to each other and spaced apart in the front-to-rear direction, and an endless conveyor belt 73 is stretched between them. The outer surface of the conveyor belt 73 is in contact with each of the photosensitive drums 53. Four transfer rollers 74 are arranged inside the conveyor belt 73, facing each of the photosensitive drums 53, and sandwich the conveyor belt 73 between themselves and the photosensitive drums 53. A transfer bias is applied to these transfer rollers 74 by constant current control during transfer.

[0036] The cleaning unit 75 is disposed below the conveyor belt 73, and is configured to remove toner adhering to the conveyor belt 73 and drop the removed toner into a toner reservoir 76 disposed below.

[0037] The fixing unit 80 is disposed behind the process cartridges 50 and the transfer unit 70, and includes a heating roller 81 and a pressure roller 82 disposed opposite the heating roller 81 and pressing the heating roller 81.

[0038] In the image forming unit 30 configured as above, first, the surface of each photosensitive drum 53 is uniformly charged by the charger 54, and then exposed to LED light emitted from each LED print head 41. As a result, an electrostatic latent image based on image data is formed on each photosensitive drum 53.

[0039] In addition, the toner in the toner storage chamber 66 is supplied to the developing roller 63 by the rotation of the supply roller 64, and as the developing roller 63 rotates, it enters between the developing roller 63 and the blade assembly 65 and is carried on the developing roller 63 as a thin layer of a certain thickness.

[0040] When the developing roller 63 comes into contact with the photosensitive drum 53, the toner carried on the developing roller 63 is supplied to the electrostatic latent image formed on the photosensitive drum 53. As a result, the toner is selectively carried on the photosensitive drum 53, making the electrostatic latent image visible, and a toner image is formed by reversal development.

[0041] Then, as the sheet P supplied onto the conveying belt 73 passes between each photosensitive drum 53 and each transfer roller 74 disposed inside the conveying belt 73, the toner images formed on each photosensitive drum 53 are sequentially transferred onto the sheet P. When the sheet P passes between the heating roller 81 and the pressure roller 82, the toner image transferred onto the sheet P is thermally fixed.

[0042] The paper discharge unit 90 mainly includes a paper discharge side conveyance path 91 that extends upward from the outlet of the fixing unit 80 and is formed to reverse toward the front side, and a plurality of pairs of conveyance rollers 92 that convey the sheet P. The sheet P onto which the toner image is transferred and thermally fixed is conveyed along the paper discharge side conveyance path 91 by the conveyance rollers 92, discharged to the outside of the main body housing 10, and accumulated in the paper discharge tray 13.

[0043] <Configuration of the LED print head 41> FIG. 2 is a schematic diagram showing the configuration of the LED print head 41 shown in FIG. 1. As shown in FIG. 2, a plurality of LED arrays 43 are arranged in the LED print head 41. Further, a lens array 44 is arranged at a position facing the plurality of LED arrays 43. The LED array is a semiconductor having a plurality of light-emitting elements arranged in the X direction. The lens array 44 is formed by arranging refractive index distribution type lenses 44A in the X direction, and forms an upright and equal magnification image of the light from each light-emitting element of the LED array 43 on the photosensitive drum 53 which is the image plane.

[0044] FIG. 3 shows a view of the LED print head 41 shown in FIG. 2 as viewed from the Z direction. The refractive index distribution type lenses 44A constituting the lens array are arranged in a single row in the X direction. The number of light-emitting elements of the LED array 43 is provided to be larger than the number of refractive index distribution type lenses 44A of the lens array 44.

[0045] In this way, when the gradient index lenses 44A that make up the lens array 44 are arranged in a row in the X direction, variations in the individual gradient index lenses 44A cause the exposure beam diameter to deform in the vertical direction (Y direction) or horizontal direction (X direction) from the ideal state. Therefore, the deformation of the exposure beam diameter may have an effect and cause degradation of image quality.

[0046] <Overview of Linear Dither> Line dithering is constructed by periodically arranging line segments in a specific angular direction (hereafter referred to as the screen direction, and the angle at that time is called the screen angle) at arbitrary intervals. In the case of color printers, it is common to use dither patterns with different screen angles for each of C (cyan), M (magenta), Y (yellow), and K (black) to avoid inter-color moiré, which occurs when the dither cycles of each color interfere with each other.

[0047] As an example, the screen angle for M (magenta) can be set to 75 degrees, the screen angle for K (black) to 45 degrees, the screen angle for C (cyan) to 15 degrees, and the screen angle for Y (yellow) to 0 degrees. For example, when growing a dither pattern for M (magenta), the pattern is grown in the direction where the screen angle is 75 degrees.

[0048] Any combination of colors and screen angles can be used. For example, the screen angle of the M (magenta) dither pattern may be 45 degrees, the K (black) screen angle may be 15 degrees, the C (cyan) screen angle may be 0 degrees, and the Y (yellow) screen angle may be 75 degrees. Although the screen angles are 75 degrees, 45 degrees, 15 degrees, and 0 degrees, other screen angles may also be used.

[0049] Figure 4 shows the relationship between the screen angle and the spacing between line segments. Figure 4(a) shows the relationship between the screen angle and the spacing between line segments when the screen angle is high (75 degrees). If the spacing between line segments is R (inches), the number of line segments that appear per inch can be expressed as the screen frequency 1 / R (lpi (lines per inch)). The higher the screen frequency, the less noticeable the halftone dots are to the naked eye, and higher quality images can be obtained, but there are also disadvantages such as a deterioration in gradation (collapse) and a tendency for banding to occur.

[0050] Furthermore, when the screen angle is θ, the interval ΔX between the line segments in the X direction and the interval ΔY between the line segments in the Y direction are given by the following equations (Equation 1) and (Equation 2).

[0051] ΔX=R / sinθ (inch) (Formula 1) ΔY=R / cosθ (inch) (Formula 2) Therefore, if abs(θ)≧45deg, then ΔY≧ΔX. The angle of the normal to the line segment is -15deg. Note that abs(θ) is the absolute value of θ.

[0052] FIG. 4(b) shows the relationship between the screen angle and the spacing between line segments when the screen angle is low (15 degrees). If the screen angle is θ, the spacing ΔX between line segments in the X direction and the spacing ΔY between line segments in the Y direction are as shown in (Equation 1) and (Equation 2) above. Therefore, if abs(θ)<45 degrees, then ΔX>ΔY. The angle of the normal direction of the line segments is -75 degrees. Here, the range of the screen angle θ is -90 degrees≦θ≦90 degrees.

[0053] Thus, when we look at the spacing between line segments in the X and Y directions, even for the same line spacing, the spacing in the X and Y directions will differ depending on the dither screen angle. Below, we will explain the mechanism by which dark streaks (black streaks) occur depending on the screen angle.

[0054] <Mechanism of black streaks> FIG. 5 illustrates a dither pattern in which the beam diameter extends in the X or Y direction. FIG. 5(a) illustrates a dither pattern in which the beam diameter extends in the X direction with a high screen angle (75°). In FIG. 5(a), because the exposure beam diameter extends in the X direction, the exposure of left and right pixels interferes to form an electrostatic latent image, and toner adheres to areas not exposed as pixels. This results in streaky areas with a large amount of toner adhesion extending in the Y direction, known as black streaks. As described above, when the screen angle is high, the spacing between the lines in the X direction becomes narrower, and therefore, when the beam diameter extends in the X direction, the interference between the exposure of left and right pixels becomes more pronounced. Thus, when the screen angle is high and the exposure beam diameter extends in the X direction, black streaks are more likely to occur.

[0055] Figure 5(b) shows a dither pattern with a high screen angle (75 degrees) in which the beam diameter extends in the Y direction. In Figure 5(b), the exposure beam diameter extends in the Y direction, but interference does not occur because the spacing between lines is large. As mentioned above, when the screen angle is high, the spacing between lines in the X direction narrows, but because the beam diameter extends in the Y direction, exposure interference does not occur between left and right pixels. In this way, when the screen angle is high and the exposure beam diameter extends in the Y direction, black streaks do not occur.

[0056] Figure 5(c) is a diagram showing a case where the beam diameter extends in the X direction in a dither pattern with a low screen angle (15 degrees). In Figure 5(c), the exposure beam diameter extends in the X direction, but interference does not occur because the spacing between lines is large. As mentioned above, when the screen angle is low, the spacing between lines in the X direction is wide, so even if the beam diameter extends in the X direction, exposure interference does not occur between upper and lower pixels. In this way, when the screen angle is low and the exposure beam diameter extends in the X direction, no black streaks occur.

[0057] FIG. 5(d) is a diagram showing a case where the beam diameter extends in the Y direction in a dither pattern with a low screen angle (15 degrees). In FIG. 5(d), because the exposure beam diameter extends in the Y direction, exposure of upper and lower pixels interferes, resulting in the occurrence of black streaks. As described above, when the screen angle is low, the spacing between lines in the Y direction becomes narrower, so when the beam diameter extends in the Y direction, the exposure interference of upper and lower pixels becomes more pronounced. In this way, when the screen angle is low and the exposure beam diameter extends in the Y direction, black streaks are more likely to occur.

[0058] <Countermeasures against black streaks> Fig. 6 shows a dither matrix for suppressing the occurrence of black streaks. Fig. 6(a) shows the growth order of the minimum matrix, and "1" to "13" in Fig. 6(b) show the entire image in the dither repeat unit when the dither pattern is grown in the dither growth order of the minimum matrix. For example, "3" in Fig. 6(b) shows the entire image in the dither repeat unit when the pattern is grown to "3" in the dither growth order of the minimum matrix.

[0059] The control unit 101 converts the image data into binary raster image data based on a dither matrix corresponding to the halftones of the image data. The dither matrix is ​​configured to grow a pattern from an isolated pixel in a predetermined screen direction as the exposure area ratio of the image data increases. The control unit 101 grows the pattern according to the growth order "1" to "13" of the minimum matrix, and then exposes the photosensitive drum 53 with the LED unit 40 based on the pattern.

[0060] As shown in the growth order of the minimum matrix in Figure 6(a), the dither matrix is ​​configured to form isolated pixels with growth order "1", and to grow patterns in a predetermined screen direction with growth orders "2" and "3". Growth order "3" connects to adjacent patterns.

[0061] The dither matrix then grows the pattern from the isolated pixel in the sub-scanning direction in growth order "4," and grows the pattern in the specified screen direction to make the pattern thicker in "5" and "6." Then, in growth order "7," the pattern is grown in a direction that intersects with the specified screen direction.

[0062] Therefore, the dither matrix is ​​grown in a direction intersecting the screen direction (90°) with a growth order of "8". This connects the adjacent patterns above and below. In this state, the exposure area ratio is equal to or greater than the specified value (60%).

[0063] In this way, the unexposed portion remains as a concentrated area rather than a linear area, so that even if there is unevenness in the beam diameter of the LED unit 40, differences in the toner adhesion state are less likely to occur, and the occurrence of black streaks can be suppressed.

[0064] Fig. 7 is a diagram showing a comparison between a general dither pattern and the dither pattern of this embodiment. Fig. 7 corresponds to the pattern of growth order "8" in the dither pattern of this embodiment shown in Fig. 6(b).

[0065] Figure 7(a) shows a pattern corresponding to a typical dither pattern growth order of "8," and Figure 7(b) shows the case where the third exposure beam diameter from the left extends in the Y direction. As shown in Figure 7(b), the upper and lower exposure beams come into close proximity in areas where the exposure beam diameter extends in the Y direction, causing toner adhesion in those areas. On the other hand, in areas where the exposure beam diameter does not extend in the Y direction, the upper and lower exposure beams do not come into close proximity, so toner adhesion does not occur in those areas. In this way, black streaks occur in areas where the exposure beam diameter extends in the Y direction.

[0066] FIG. 7(c) is a pattern corresponding to the dither pattern growth order "8" in this embodiment, and FIG. 7(d) shows the case where the third exposure beam diameter from the left extends in the Y direction. As shown in FIG. 7(d), the upper and lower exposure beams overlap in areas where the exposure beam diameter extends in the Y direction, causing toner adhesion in those areas. On the other hand, the upper and lower exposure beams also overlap in areas where the exposure beam diameter does not extend in the Y direction, causing toner adhesion in those areas. In this way, black streaks do not occur even if there are areas where the exposure beam diameter extends in the Y direction.

[0067] <Countermeasures against white and black streaks> Fig. 8 shows dither matrices for suppressing the occurrence of white and black streaks. Fig. 8(a) shows the growth order of the minimum matrix, and "1" to "13" in Fig. 8(b) show the entire image in the dither repeat unit when the dither pattern is grown in the dither growth order of the minimum matrix. For example, "3" in Fig. 8(b) shows the entire image in the dither repeat unit when the pattern is grown to "3" in the dither growth order of the minimum matrix.

[0068] As shown in the growth order of the minimum matrix in Figure 8(a), the dither matrix is ​​configured to form an isolated pixel with growth order "1". Next, the dither matrix grows a pattern in the sub-scanning direction from the isolated pixel with growth order "2". This makes it possible to suppress the occurrence of white streaks, as will be described later.

[0069] The dither matrix grows a pattern to the left of the isolated pixel in growth order "3", and grows the pattern in the specified screen direction in growth order "4". The growth order "4" connects to adjacent patterns.

[0070] The dither matrix then grows the pattern in the sub-scanning direction in growth order "5" to "6", grows the pattern below the pixel formed in growth order "3" in growth order "7", and grows the pattern in the specified screen direction to make the pattern thicker.

[0071] Therefore, the dither matrix grows a pattern above the pixel formed with growth order "3" in a direction intersecting the screen direction with growth order "8." This connects the adjacent patterns above and below. In this state, the exposure area ratio is equal to or greater than the specified value (60%).

[0072] In this way, the unexposed portion remains as a concentrated area rather than a linear area, so that even if there is unevenness in the beam diameter of the LED unit 40, differences in the toner adhesion state are less likely to occur, and the occurrence of black streaks can be suppressed.

[0073] 9 is a diagram showing a comparison between a general dither pattern and the dither pattern of this embodiment. Fig. 9 corresponds to the pattern of growth order "2" in the dither pattern of this embodiment shown in Fig. 8.

[0074] Figure 9(a) shows a pattern corresponding to growth order "2" of a typical dither pattern, in which the pattern grows to the right of an isolated pixel. Because the exposure beam diameter near the center extends in the Y direction, adjacent exposure beams do not overlap, making it difficult for toner to adhere. As a result, the amount of toner adhesion is low in this area. This makes it easy for streaky areas with low toner adhesion extending in the Y direction, known as white streaks, to occur. On the other hand, as shown in Figure 9(b), when the exposure beam diameter near the center extends in the X direction, adjacent exposure beams overlap, stabilizing toner adhesion. As a result, white streaks do not occur in this area.

[0075] FIG. 9(c) shows a pattern corresponding to the dither pattern growth order "2" of this embodiment, in which the pattern is grown below the isolated pixel. Because the exposure beam diameter near the center extends in the Y direction, the overlap between adjacent exposure beams is large, stabilizing toner adhesion. Therefore, no white streaks appear in this area. On the other hand, as shown in FIG. 9(d), even if the exposure beam diameter near the center extends in the X direction, the exposure time Δt is sufficient, resulting in overlap between the exposures of both pixels, stabilizing toner adhesion. Therefore, no white streaks appear in this area.

[0076] The occurrence of white streaks is more pronounced when the screen angle of the dither pattern is low. Therefore, the dither pattern for preventing white streaks in this embodiment may be used only when the screen angle is smaller than a predetermined angle.

[0077] [Software implementation example] The functional blocks of the image forming apparatus 1 (particularly the control unit 101) may be realized by a logic circuit (hardware) formed on an integrated circuit (IC chip) or the like, or may be realized by software.

[0078] In the latter case, the image forming apparatus 1 includes a computer that executes instructions of a program, which is software that realizes each function. This computer includes, for example, one or more processors and a computer-readable recording medium that stores the program. The object of the present invention is achieved by having the processor in the computer read and execute the program from the recording medium.

[0079] The processor may be, for example, a CPU. The recording medium may be a "non-transitory tangible medium," such as a ROM, tape, disk, card, semiconductor memory, or programmable logic circuit. The computer may further include a RAM for expanding the program. The program may be supplied to the computer via any transmission medium capable of transmitting the program (such as a communication network or broadcast waves). One aspect of the present invention may also be realized in the form of a data signal embedded in a carrier wave, in which the program is embodied by electronic transmission.

[0080] The present invention is not limited to the above-described embodiment, and various modifications are possible within the scope of the claims. The technical scope of the present invention also includes configurations obtained by appropriately combining the technical means disclosed in the embodiments. [Explanation of symbols]

[0081] 1. Image forming device 10 Main unit housing 20 Paper feed section 30 Image forming unit 40 LED units 43 LED array 44 SELFOC (registered trademark) lens array 50 Process cartridge 53 Photosensitive drum 61 Developer 70 Transcription Unit 80 Fuser unit 90 Paper output section 100 Main board 101 Control section

Claims

1. A photoreceptor; an exposure device that exposes the photosensitive member; a developing device that develops the electrostatic latent image formed on the photosensitive member; a control unit, the control unit converts the image data into binary raster image data based on a dither matrix corresponding to the halftones of the image data; the dither matrix is ​​configured to grow a pattern from an isolated pixel in a predetermined screen direction as the exposure area ratio of the image data increases, and after the grown pattern connects with an adjacent pattern, grow the pattern so that line segments extend in a direction intersecting the predetermined screen direction at predetermined intervals in the screen direction; The control unit causes the exposure device to expose the photosensitive member based on the pattern.

2. 2. The image forming apparatus according to claim 1, wherein the dither matrix grows the pattern from the isolated pixel in a sub-scanning direction before growing the pattern from the isolated pixel in the predetermined screen direction.

3. 3. The image forming apparatus according to claim 1, wherein the dither matrix grows the pattern in the sub-scanning direction after the grown pattern is connected to an adjacent pattern, and then further grows the pattern in the screen direction.

4. 4. The image forming apparatus according to claim 1, wherein the dither matrix grows the pattern in a direction intersecting the predetermined screen direction, and connects the pattern to the adjacent pattern in the sub-scanning direction when the exposure area ratio reaches a predetermined value or more.

5. 5. The image forming apparatus according to claim 4, wherein the predetermined value is 60%.

6. 6. The image forming apparatus according to claim 1, wherein the direction intersecting the predetermined screen direction is 90 degrees.

7. the exposure device includes a plurality of light-emitting elements arranged in a main scanning direction, and a lens array that focuses light from the plurality of light-emitting elements onto the photosensitive member; The image forming apparatus according to claim 1 , wherein the lens array is arranged in a line in the main scanning direction.

8. the dither matrix is ​​configured to grow a pattern from an isolated pixel in a predetermined screen direction as the exposure area ratio of the image data increases, and after the grown pattern connects with an adjacent pattern, grow the pattern so that line segments extend in a direction intersecting the predetermined screen direction at predetermined intervals in the screen direction; converting the image data into binary raster image data based on the dither matrix corresponding to the halftones of the image data; An image forming method in which a photosensitive member is exposed to light based on the pattern by an exposure device.

Images