JP2014028458A - Image forming apparatus, and method for setting dither pattern - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the consumption of a memory while making stripes of an image inconspicuous.SOLUTION: In a method for setting a dither pattern Z formed by using a light-emitting array 41 obtained by arranging light-emitting chips CH having a plurality of light emitting elements P in a main scanning direction, and representing gradation of an image, angles of two dither patterns to be used in the same color are set so as to make a difference between the angle being parameters representing easiness to cause a density difference in peripheral pixels of a pixel corresponding to an edge part of the light-emitting chips CH.

Description

  The present invention relates to a technique for making an image stripe inconspicuous.

  Japanese Patent Application Laid-Open No. 2004-151620 discloses a technique for making white stripes inconspicuous by correcting the light amount of the light emitting element at the boundary according to the distance between the LED chips. Patent Document 2 below proposes a technique in which the gradation of an image is represented by a dither pattern composed of a repetitive pattern of diagonal lines.

JP 2001-080111 A JP 2004-364084 A

In order to improve the image quality, it is preferable to make the white stripes and color stripes as inconspicuous as possible. When forming an image using a dither pattern, pay attention to the angle of the dither pattern, or pay attention to the area fluctuation ratio of the blank portion existing between the dither patterns, specifically, the area fluctuation ratio with respect to the change in the distance between elements. By correcting the light intensity of the light emitting element located at the chip edge, the density of the pixel at the location corresponding to the chip edge can be made closer to the density of the peripheral pixel, and the image streak may be less noticeable There is. An image forming apparatus mainly has a plurality of image forming modes such as a high-speed image forming mode for forming images at high speed and a high-quality image forming mode for forming high-quality images. A dither pattern suitable for each image forming mode is often prepared. As described above, when correcting the light amount of the light emitting element located at the chip edge by paying attention to the dither pattern angle and area variation ratio, the correction value is determined for each prepared dither pattern. Even if the difference in values does not significantly affect the image quality, if each has a correction value, the memory of the image forming apparatus may be wasted.
The present invention has been completed based on the above-described circumstances, and an object of the present invention is to suppress the consumption of memory while making the streak of an image inconspicuous.

An image forming apparatus disclosed in the present specification includes, as an exposure apparatus, a light emitting array in which light emitting chips having a plurality of light emitting elements are arranged in the main scanning direction, and is formed on a photosensitive member exposed by the light emitting array. An image forming unit that forms an image on a recording medium using an electrostatic latent image, and a dither pattern that represents the gradation of the image, and a plurality of dither patterns that are used when the image is formed by the image forming unit A memory to be stored, and a control unit, wherein the control unit has a first value of a parameter indicating the ease of occurrence of a density difference with respect to a peripheral pixel of a pixel corresponding to an end of the light emitting chip. Among the pixels constituting a certain first dither pattern, the pixel located at the end according to the first value so that the density of the pixel corresponding to the end of the light emitting chip approaches the density of a peripheral pixel. A first calculation process for calculating a first correction value for correcting the amount of light of the optical element, and a parameter value representing the ease of occurrence of a density difference with respect to peripheral pixels of the pixel corresponding to the end of the light emitting chip are Among the pixels constituting the second dither pattern that is binary and used for the same color as the first dither pattern, the density of the pixel corresponding to the edge of the light emitting chip is made closer to the density of the surrounding pixels. In accordance with the second value, a second calculation process for calculating a second correction value for correcting the light amount of the light emitting element located at the end, and the first calculation process calculated by the first calculation process. Based on the correction value and the second correction value calculated in the second calculation process, a common correction value calculation process for calculating a common correction value, and the common correction calculated in the common correction value calculation process A storage process for storing a value in the memory;
In the storage process, the image forming mode includes both a first image forming mode for forming an image using the first dither pattern and a second image forming mode for forming an image using the second dither pattern. Correction processing for correcting the light amount of the light emitting element located at the end using the common correction value stored in the memory is performed. In this way, the correction value stored in the memory can be reduced as compared with the case where a different correction value is used for each prepared dither pattern. Therefore, memory consumption can be suppressed. In addition, since it is not necessary to change the correction value every time a different dither pattern is used, the operation speed can be increased.

  The dither pattern setting method disclosed in this specification is a dither pattern setting method that is formed by using a light emitting array in which light emitting chips having a plurality of light emitting elements are arranged in the main scanning direction and represents the gradation of an image. The parameters of the two dither patterns used for the same color so that the difference between the values of the parameters corresponding to the edges of the light emitting chip and the peripheral pixels is reduced. Set the value of. In this way, the difference in pixel density at the edge of the light emitting chip is reduced by the two dither patterns.

In the image forming apparatus or the dither pattern setting method disclosed in this specification, the following is preferable.
The parameter is an angle with respect to the main scanning direction of the line segment constituting the dither pattern. If it is an angle, it is suitable for easily grasping the ease of occurrence of the density difference.

The parameter is an area variation ratio of a blank portion existing between the dither patterns with respect to a change in the distance between two light emitting elements located at the end of the light emitting chip. The area variation ratio is suitable for accurately grasping the ease of occurrence of the density difference.

  According to the present invention, it is possible to suppress memory consumption while making an image stripe inconspicuous.

FIG. 3 is a side sectional view of a main part of the color printer according to the first embodiment. Enlarged view of LED unit and process cartridge LED unit viewed from the exposure side Block diagram of light quantity adjustment unit and control device Diagram showing dither pattern (showing white streaks and white streaks disappeared by light intensity correction) Enlarged view of white streaks (when dither pattern angle is large) Enlarged view of white streaks (when dither pattern angle is small) Illustration explaining how to measure the angle difference between dither patterns Illustration explaining how to measure the angle difference between dither patterns Diagram showing dither pattern combinations The flowchart figure which shows the flow of the process which correct | amends light emission time. Enlarged view of dither pattern according to actual form 2 (shows the case where the distance between lines is small) Enlarged view of dither pattern (shows the case where the distance between lines is large) Enlarged view of dither pattern (shows small angle) Enlarged view of dither pattern (shows the case of large angle)

<Embodiment 1>
The first embodiment will be described with reference to FIGS.
1. Overall Configuration of Color Printer As shown in FIG. 1, an electrophotographic color printer 1 includes a paper feed unit 20 that supplies paper (an example of a “recording medium” of the present invention) S into a main body housing 10. The image forming unit 30 that forms an image on the fed paper S, the paper discharge unit 90 that discharges the paper S on which the image is formed, and the control device 100 that controls the operation of each of these units are provided. . In the following description, the direction will be described with reference to the user when using the color printer. That is, in FIG. 1, the left side toward the paper surface is “front side” and the right side toward the paper surface is “rear side”. The direction perpendicular to the transport direction of the paper S is defined as the main scanning direction (the direction perpendicular to the paper surface in FIG. 1, the left-right direction of the printer), and the direction orthogonal to the main scanning direction, that is, the transport direction of the paper S 1 in the left-right direction and the front-rear direction of the printer).

  An upper cover 12 that can be opened and closed relative to the main body housing 10 is provided on the upper portion of the main body housing 10 so as to be rotatable up and down around a hinge 12A provided on the rear side. An upper surface of the upper cover 12 serves as a paper discharge tray 13 for accumulating the paper S discharged from the main body housing 10, and an LED unit 40 serving as an exposure device is provided below the upper cover 12.

  Further, a cartridge drawer 15 that detachably accommodates each process cartridge 50 is provided in the main body housing 10. The cartridge drawer 15 is provided with a pair of metal side plates 15A (only one side is shown) provided on the left and right and a cross member 15B connecting the pair of side plates 15A on the front and rear. The side plates 15A are members that are disposed on both sides of the LED array 41 as the exposure head of the LED unit 40 in the left-right direction, and directly or indirectly support and position the photosensitive drum 53. Light emission of the LED array 41 is controlled by the control device 100 and the light emission control device 110. The LED array 41 is an example of the light emitting array of the present invention.

  The paper feeding unit 20 is provided in the lower part of the main body housing 10, and is a paper feeding tray 21 that is detachably attached to the main body housing 10, and a paper that conveys the paper S from the paper feeding tray 21 to the image forming unit 30. A supply mechanism 22 is mainly provided. The paper supply mechanism 22 is provided on the front side of the paper feed tray 21 and mainly includes a paper feed roller 23 and a separation roller 24.

  In the paper feed unit 20 configured as described above, the paper S in the paper feed tray 21 is separated one by one and sent upward, and is turned backward through the transport path 28 to the image forming unit 30. Supplied.

  The image forming unit 30 includes four LED units 40, four process cartridges 50, a transfer unit 70, and a fixing unit 80. The four LED units 40 and the four process cartridges 50 correspond to four colors of black, yellow, magenta, and cyan.

  The process cartridge 50 is arranged side by side in the front-rear direction between the upper cover 12 and the paper feeding unit 20, and as shown in FIG. 2, the drum unit 51 and the developing unit 61 that is detachably attached to the drum unit 51. And. The side plate 15 </ b> A supports the process cartridge 50, and the process cartridge 50 supports the photosensitive drum 53. Each process cartridge 50 has the same configuration except that the color of the toner stored in the toner storage chamber 66 of the developing unit 61 is different.

  The drum unit 51 mainly includes a drum frame 52, a photosensitive drum 53 as an example of a photosensitive member rotatably supported by the drum frame 52, and a scorotron charger 54.

  The developing unit 61 includes a developing frame 62, a developing roller 63 and a supply roller 64 that are rotatably supported by the developing frame 62, and includes a toner storage chamber 66 that stores toner. In the process cartridge 50, the developing unit 61 is mounted on the drum unit 51, whereby an exposure hole 55 is formed between the developing frame 62 and the drum frame 52 so as to face the photosensitive drum 53 from above. The LED unit 40 holding the LED array 41 at the lower end is inserted into the exposure hole 55. Details of the LED array 41 will be described later.

  As shown in FIG. 1, the transfer unit 70 is provided between the paper feeding unit 20 and each process cartridge 50, and mainly includes a drive roller 71, a driven roller 72, a conveyance belt 73, and a transfer roller 74.

  The driving roller 71 and the driven roller 72 are spaced apart and arranged in parallel in the front-rear direction, and a conveyor belt 73 is stretched therebetween. The outer surface of the conveyor belt 73 is in contact with each photosensitive drum 53. In addition, four transfer rollers 74 that sandwich the conveyor belt 73 between the photosensitive drums 53 are arranged inside the conveyor belt 73 so as to face the photosensitive drums 53. A transfer bias is applied to the transfer roller 74 by constant current control during transfer.

  The fixing unit 80 is disposed on the back side of each process cartridge 50 and the transfer unit 70, and includes a heating roller 81 and a pressure roller 82 that is disposed to face the heating roller 81 and presses the heating roller 81.

  In the image forming unit 30 configured as described above, first, the surface (photosensitive surface 53A) of each photosensitive drum 53 is uniformly charged by the scorotron charger 54 and then irradiated from each LED array 41. It is exposed by LED light. As a result, the potential of the exposed portion is lowered, and an electrostatic latent image based on the image data is formed on each photosensitive drum 53 (charging process, exposure process).

  Further, the toner in the toner storage chamber 66 is supplied and carried on the developing roller 63 by the rotation of the supply roller 64. The toner carried on the developing roller 63 is supplied to the electrostatic latent image formed on the photosensitive drum 53 when the developing roller 63 comes into contact with the photosensitive drum 53. As a result, the toner is selectively carried on the photosensitive drum 53 to visualize the electrostatic latent image, and a toner image is formed by reversal development (development process).

  Next, the sheet S supplied onto the conveyance belt 73 passes between each photosensitive drum 53 and each transfer roller 74 disposed inside the conveyance belt 73, thereby forming on each photosensitive drum 53. The toner image thus transferred is transferred onto the paper S (transfer process). Then, as the sheet S passes between the heating roller 81 and the pressure roller 82, the toner image transferred onto the sheet S is thermally fixed (fixing process).

  The paper discharge unit 90 mainly includes a paper discharge side transport path 91 formed so as to extend upward from the exit of the fixing unit 80 and to be reversed to the front side, and a plurality of pairs of transport rollers 92 that transport the paper S. I have. The sheet S on which the toner image has been transferred and heat-fixed is transported along a paper discharge side transport path 91 by a transport roller 92, discharged outside the main body housing 10, and accumulated in the paper discharge tray 13.

2. Configuration of LED Array The LED array 41 functions to expose the photosensitive drum 53 based on print data, and is in the main scanning direction (left and right in FIG. 3) orthogonal to the paper feed direction (up and down direction in FIG. 3). The light emitting element P is arranged in the direction).

  The LED array 41 is divided from a plurality of LED array chips (an example of the “light emitting chip” of the present invention) CH. Each LED array chip CH is formed by forming a plurality of light emitting diodes as light emitting elements P in a line on a semiconductor substrate by a semiconductor process (256 pieces are formed as an example). In this embodiment, 20 pieces are formed on a circuit board CB. The LED array chips CH are arranged in a staggered manner with their positions shifted in the sub-scanning direction. Thus, the LED array chips CH are arranged in a staggered manner, and the chip ends are overlapped in the main scanning direction, whereby the distance Dx between the light emitting elements at the joint between the chips is matched with the reference pitch Dp. FIG. 3 is a schematic diagram of the LED array 41, showing the number of light emitting elements P formed in each LED array chip CH smaller than the actual number.

  Each LED array 41 is provided with an EEPROM 43 as a nonvolatile storage means. In the EEPROM 43, data necessary for light quantity control of the LED array 41, for example, data on the inter-element distance Dx between the light emitting elements Pa and Pb at the joint of each LED array chip CH, and the reference pitch Dp of the inter-element distance Dx are stored. Data is stored.

3. Description of the control device 100 and the light emission control device 110 The control device 100 controls the entire color printer 1, and includes a calculation control unit 100A composed of a CPU and the like, and a memory M composed of a ROM 100B and a RAM 100C. It has become. The light emission control device 110 includes an ASIC 120 as shown in FIG. 4, and performs light emission control (light amount adjustment) on each light emitting element P of the LED array 41 according to a command from the control device 100. The arithmetic control unit 100A is an example of the “control unit” in the present invention.

  Four sets of LED arrays 41 are connected in common to the light emission control device 110, and the ASIC 120 of the light emission control device 110 is configured to collectively control the light emission of the four sets of LED arrays 41. The ROM 100C of the control device 100 is configured to store data such as a dither pattern that represents the gradation of an image. The dither pattern Z is a repetitive pattern (dot pattern) of diagonal lines inclined with respect to the main scanning direction, and is used when a gradation is added to an image.

4). Light emission control of the LED array 41 The light emitting elements P on the LED array are arranged at a constant reference pitch Dp in the main scanning direction, specifically, at a pitch of 42 μm when the image resolution is 600 dpi. And also about the joint of each LED array chip CH, LED array chip CH is arrange | positioned so that the element distance Dx between light emitting elements may become the reference | standard pitch Dp.

  However, when each LED array chip CH is mounted on the circuit board CB, the mounting position may deviate from the normal position. At the joint between the LED array chips, the inter-element distance Dx between the light emitting elements is smaller than the reference pitch Dp. May increase or decrease. As described above, when the inter-element distance Dx between the light emitting elements increases or decreases with respect to the reference pitch Dp, as shown in FIG. 5, an image is obtained using a dither pattern Z that is a repetition of oblique lines inclined with respect to the main scanning direction. When the is printed, streaks are likely to occur in the image (in the frame in FIG. 5). That is, when the inter-element distance Dx between the light emitting elements Pa and Pb is larger than the reference pitch Dp, the light density decreases at the joint portion. For this reason, among the pixels constituting the dither pattern Z, in the pixels corresponding to the joints of the LED array chip CH, the exposure becomes thin and it is difficult to place toner. Therefore, in the pixel corresponding to the joint of the LED array chip CH (for example, the pixel G1 shown in FIG. 5), the density is lower than that of the peripheral pixel (for example, the pixel G2 shown in FIG. 5), and white stripes are generated. It becomes easy to do.

  As described above, when the inter-element distance Dx between the light emitting elements Pa and Pb is larger than the reference pitch Dp, the amount of light is reduced only in that portion, and white lines are likely to occur. Therefore, in the present embodiment, when the inter-element distance Dx between the light emitting elements Pa and Pb at the joint of each LED array chip CH is larger than the reference pitch Dp, the light emission time T of the light emitting element P located at the joint is controlled to emit light. The occurrence of white streak is suppressed by adjusting the device 110 to compensate for the shortage of light quantity.

  In addition to the inter-element distance Dx, there is an angle θ with respect to the main scanning direction of the dither pattern Z as a parameter indicating the ease of occurrence of a density difference with respect to the peripheral pixel G2 of the pixel G1 corresponding to the joint of the LED array chip CH. . The reason why the angle θ of the dither pattern Z is related to the ease of occurrence of the density difference is that the angle θ of the dither pattern Z is large (see FIG. 6) or the angle θ is small (see FIG. 7). In comparison, the low density region expands in the sub-scanning direction (see A part in FIGS. 6 and 7), so that a density difference is likely to occur. When the angle θ is large, it is necessary to increase the light amount correction value or increase the number of elements for correcting the light amount in order to compensate for the spread of the density difference compared to the case where the angle θ is small.

  Therefore, if you try to correct the amount of light using a common correction value for multiple dither patterns with a large angle difference, only one of them can be corrected satisfactorily. Don't be. Therefore, in the present embodiment, as described below, the combination of dither patterns Z used for the same color is determined so that the angle difference becomes small.

  The angle difference Δ of the dither pattern Z is obtained by the following method. That is, as shown in FIG. 8A, when both the dither patterns Za and Zb are 90 ° or less with respect to the X axis, the angle difference Δ is the difference between the angle θa of the dither pattern Za and the angle θb of the dither pattern Zb. To do. On the other hand, as shown in FIG. 8B, when one angle exceeds 90 °, the difference between the angle θc of the line Zc and the angle θa of the dither pattern Za when the dither pattern Zb is folded back with respect to the Y axis. Is the angle difference Δ.

5. Example of Combination of Image Quality Mode and Dither Pattern In this printer 1, three types of image quality modes, normal, fine, and super fine (the image quality is higher in the order of normal → fine → super fine) are set. The dither pattern Z needs to have a shorter distance between lines as the image quality becomes higher. The dither pattern needs to be determined so as to satisfy the conditions of the number of dots determined by the density and the distance between lines, and the angle of the dither pattern is determined so as to satisfy these conditions. The image quality mode of the present embodiment corresponds to the “image formation mode” of the present invention.

  For example, FIG. 9 shows a list of dither patterns Z that satisfy the conditions for each image quality mode when “color is black” and “density is 50%”. As shown in FIG. 9, in the fine mode, a dither pattern Z1 having an angle of 45 ° satisfies the condition. In the normal mode, two dither patterns Z2 and Z3 having an angle of 23 ° and 67 ° satisfy the condition, and in the super fine mode, two dither patterns Z4 and Z5 having an angle of 12 ° and 78 ° are satisfied. Meet.

In this case, as combinations of dither patterns, the following four combinations can be considered as use candidates.
(A) First dither pattern Z1 (fine), third dither pattern Z3 (normal), combination of fifth dither pattern Z5 (super fine) (b) first dither pattern Z1 (fine), third (C) first dither pattern Z1 (fine), second dither pattern Z2 (normal), and fifth dither pattern Z5 ( (D) First dither pattern Z1 (fine), second dither pattern Z2 (normal), fourth dither pattern Z4 (super fine)

  When the combination (a) is selected, the angle difference between the first dither pattern Z1 and the third dither pattern Z3 is 22 °, the angle difference between the third dither pattern Z3 and the fifth dither pattern Z5 is 11 °, The angle difference between the fifth dither pattern Z5 and the first dither pattern Z1 is 33 °. In this case, the maximum angle difference between dither patterns is 33 °.

  On the other hand, when the combination (b) is selected, the angle difference between the first dither pattern Z1 and the third dither pattern Z3 is 22 °, the angle difference between the fourth dither pattern Z4 and the first dither pattern Z1 is 33 °, The angle difference between the third dither pattern Z3 and the fourth dither pattern Z4 is 55 °. In this case, the maximum angle difference between dither patterns is 55 °.

  When the combination (c) is selected, the angle difference between the first dither pattern Z1 and the second dither pattern Z2 is 22 °, the angle difference between the fifth dither pattern Z5 and the first dither pattern Z1 is 33 °, The angle difference between the second dither pattern Z2 and the fifth dither pattern Z5 is 55 °. In this case, the maximum angle difference between dither patterns is 55 °.

  When the combination (d) is selected, the angle difference between the first dither pattern Z1 and the second dither pattern Z2 is 22 °, the angle difference between the fourth dither pattern Z4 and the first dither pattern Z1 is 33 °, The angle difference between the second 23 ° dither pattern Z2 and the fourth 12 ° dither pattern Z4 is 11 °. In this case, the maximum angle difference between dither patterns is 33 °.

  Therefore, when the combination (a) or the combination (d) is selected from the combinations (a) to (d) described above, the combination (b) or the combination (c) is selected. The maximum angle difference between the dither patterns used is reduced.

  If the maximum angle difference is small, the density difference of the pixel G1 corresponding to the joint of the LED array chip CH with respect to the peripheral pixel G2 is generated in substantially the same manner in each dither pattern, and there is no difference in the magnitude of the density difference. Therefore, when the dither pattern Z of the three patterns used in the fine mode, normal mode, and super fine mode is subjected to light amount correction using a common correction value, the density difference is reduced in common with each dither pattern. For all dither patterns Z, it is possible to eliminate the uneven density difference between the pixel G1 corresponding to the joint of the LED array chip CH and the peripheral pixel G2.

  In other words, if the maximum angle difference is greater than or equal to the predetermined value, the three dither patterns cannot be corrected with the common correction value. In this case, some dither patterns are set with dedicated correction values. There is a need to. For example, when the allowable value of the angle difference is less than 45 °, the combination of (b) and (c) can be selected to prevent the angle difference from exceeding 45 ° out of three dither patterns. It is necessary to set a correction value exclusively for one dither pattern. However, having many correction values consumes the RAM 100B accordingly.

  Accordingly, at the stage of designing the printer 10, a common correction value can be used by determining in advance the combination of dither patterns Z used for the same color so that the angle difference between the dither patterns Z is small. Thus, the number of data stored in the RAM 100B can be reduced. Based on the above, the value of the parameter (here, the angle) representing the ease of occurrence of a density difference with respect to the peripheral pixel G2 of the pixel G1 corresponding to the end of the light emitting chip (here, the LED array chip CH) of the present invention. “Set the values of the parameters of the two dither patterns used for the same color so that the difference is reduced (here, the combination of angles is determined)” is realized.

  Since the dither pattern Z needs to be set according to the density, it is necessary to set the combination of the dither patterns Z as described above for each density. In addition to black, it is necessary to prepare different patterns for each color of yellow, magenta, and cyan in addition to black, so the dither pattern Z for each color other than black also has a small angular difference between the dither patterns. Thus, the combination of dither patterns Z must be determined at the design stage. The data of each dither pattern Z determined at the design stage is stored in advance in the ROM 100C of the control device 100.

  Next, a procedure for correcting the light emission time T executed by the printer 1 during printing will be described. In the following description, it is assumed that the dither pattern Z1 is used for normal, the dither pattern Z3 is used for fine, and the dither pattern Z5 is used for super fine.

  First, when print data is received from an information terminal such as a PC, the arithmetic control unit 100A of the control device 100 executes a charging process in which the photosensitive drums 53 are charged by the image forming unit 30. The arithmetic control unit 100A of the control device 100 determines whether the received print data is normal printing (printing that does not use the dither pattern Z) or gradation printing (printing that uses the dither pattern Z).

  In the case of normal printing (printing that does not use the dither pattern Z), the arithmetic control unit 100A executes an exposure process via the light emission control device 110. That is, the initial value “To” is applied to the light emission time T, and each light emitting element P on each LED array 41 is turned on via the light emission control device 110 in accordance with the print data to irradiate each photosensitive drum 53 with light. To do. Thus, each photosensitive drum 53 is exposed. Thereafter, under the control of the control device 100, the image forming unit 30 sequentially executes a development process, a transfer process, and a fixing process, and print data is printed on the paper.

  On the other hand, in the case of gradation printing (printing using the dither pattern Z), the control device 100 stores data on the inter-element distance Dx between the light emitting elements Pa and Pb at the joint of each LED array chip CH from the EEPROM 43 of each LED array 41. Is read. Further, the angle data of the dither pattern Z is read from the ROM 100C (S10).

  Then, the calculation control unit 100A of the control device 100 performs a calculation process for calculating the correction value X of the light emission time T for each dither pattern Z based on the following equation (1), and for each LED array 41, each LED array chip. It carries out for every joint of CH (S20-S40). Specifically, in the case of the dither pattern Z1, the correction value X1 of the light emission time T is calculated by substituting the data of “ΔD” and the data of “θ1” into the following equation (1) (first calculation process: S20). ). In the case of the dither pattern Z3, the “ΔD” data and the “θ3” data are substituted into the following equation (1) to calculate the light emission time correction value X3 (second calculation process: S30). In the case of the dither pattern Z5, “ΔD” data and “θ5” data are substituted into the following equation (1) to calculate the light emission time correction value X5 (third calculation process: S40). “Θ1” is an example of the “first value” of the present invention, “correction value X1” is an example of the “first correction value” of the present invention, and “θ3” is the “second value” of the present invention. An example of “correction value X3” is an example of the “second correction value” of the present invention.

X = F (ΔD, θ) (1) Formula ΔD = | Dx−Dp | (2) Formula “X” is the light emission time. Is the correction value. “F” is a function for determining the correction value X.
“Θ” is the angle θ of the dither pattern Z to be used.
The function F is set to increase the value as “ΔD” increases, and to increase the value as “θ” increases.

  Thereafter, the arithmetic control unit 100A of the control device 100 calculates the common correction value Xv from the correction values X1, X3, and X5 of the light emission times of the dither patterns Z1, Z3, and Z5 (common correction value calculation process: S50). The common correction value Xv is a correction value commonly used in each dither pattern Z1, Z3, Z5 when correcting the light emission time of the light emitting elements Pa, Pb at the joint of each LED array chip CH. 41 is calculated for each joint of each LED array chip CH. In this embodiment, as shown in the following equation (3), the common correction value Xv is an average value of X1, X3, and X5.

  Xv = (X1 + X3 + X5) / 3 (3)

  The common correction value Xv may be determined within the range from the minimum value to the maximum value of the three correction values X1 to X3. In addition to the average value, the common correction value Xv is the second of the three correction values X1 to X3. Correction values (intermediate correction values) can be used.

  Then, when the arithmetic control unit 100A of the control device 100 calculates the common correction value Xv for each LED array chip CH for each LED array 41, the arithmetic control unit 100A performs storage processing for storing the value in the RAM 100B (S60). ). Thereafter, the arithmetic control unit 100A of the control device 110 executes the exposure process at a predetermined timing. Specifically, the stored common correction value Xv is read from the RAM 100B. Then, the read common correction value Xv is substituted into the equation (4) to determine the light emission times T of the light emitting elements Pa and Pb.

T = To ± Xv (4) Equation (4) The symbol “+” indicates that the inter-element distance Dx is greater than the reference pitch Dp, and the symbol “−” indicates the inter-element distance. This is a case where Dx is smaller than the reference pitch Dp.

  Then, the arithmetic control unit 100A of the control device 100 executes an exposure process via the light emission control device 110. That is, according to the print data, each light emitting element P on each LED array 41 is turned on via the light emission control device 110 to irradiate each photosensitive drum 53 with light.

  At this time, the arithmetic control unit 100A of the control device 100 sets the light emission times of the light emitting elements Pa and Pb at the joints of the LED array chips CH to the normal, fine, and super fine dither patterns Z1, Z3, and Z5 for each LED array 41. In common, correction is performed using the common correction value Xv (correction processing: S70). That is, regardless of which of normal, fine, and super fine modes is selected as the image quality mode, the light emitting elements Pa and Pb at the joints of the LED array chips CH are lit at the light emission time T determined based on the equation (4). Let

  Thereby, among the pixels constituting the dither pattern Z, the density of the pixels corresponding to the joint portion of each LED array chip CH can be made close to the density of the peripheral pixels in common for the three modes. Thereafter, under the control of the control device 100, the image forming unit 30 sequentially executes a development process, a transfer process, and a fixing process, and print data is printed on a sheet.

  As described above, in the present embodiment, since the light emission time T is corrected in consideration of the dither pattern angle θ in addition to the inter-element distance Dx, the dither pattern Z is configured as compared with the case where the angle θ is not considered. Among the pixels to be processed, the density of the pixel G1 corresponding to the joint portion of each LED array chip CH can be made close to the density of the peripheral pixel G2. For this reason, white stripes and color stripes can be made inconspicuous, and the image quality is improved.

  In the present embodiment, the light emission times of the light emitting elements Pa and Pb at the joint of the LED array chip CH are set to the common correction value Xv for the three types of dither pattern Z1, dither pattern Z3, and dither pattern Z5 used for the same color. Correct by In this way, the correction value stored in the RAM 100B can be reduced as compared with the case where the light amount correction is performed using a different correction value for each dither pattern prepared as the same color. Therefore, consumption of the RAM 100B can be suppressed. Further, since it is not necessary to change the correction value X for each dither pattern prepared as the same color, the operation speed can be increased.

  In particular, in this case, at the stage of designing the printer 10, the combination of dither dither patterns Z used for the same color is determined so that the angle difference between the dither patterns Z is small. When the light amount correction is performed by using it, the unevenness of the density difference can be eliminated in the same manner for each dither pattern.

<Embodiment 2>
Next, a second embodiment of the present invention will be described with reference to FIGS.
In the first embodiment, the angle “θ” of the dither pattern Z is exemplified as an example of the parameter indicating the ease of occurrence of the density difference with respect to the peripheral pixel G2 of the pixel G1 corresponding to the joint of the LED array chip CH. In the above description, the combination of the dither dither patterns Z used for the same color is determined so that the angle difference between the dither patterns Z is reduced.

  In the second embodiment, as an example of a parameter indicating the ease of occurrence of a density difference with respect to the peripheral pixel G2 of the pixel G1 corresponding to the joint of the LED array chip CH, the inter-element distance Dx of the blank portion N existing between the dither patterns The area variation ratio “Sr” with respect to the change is illustrated.

  The blank portion “N” is a non-printing area that is not printed with a space between dither patterns. In other words, it is a space between the dots U constituting the dither pattern Z. Further, the area variation ratio Sr with respect to the change in the inter-element distance Dx of the blank portion N is expressed by the following equation (5).

Sr = Sx / Sp (5) Equation “Sx” is the area of the blank portion N when the element distance is Dx, and “Sp” is Dx = Dp. This is the area (reference value) of the blank portion N in some cases.

  The relationship between the area variation ratio Sr and the ease of occurrence of the density difference will be described in detail with a specific example. For example, when a dither pattern Z11 having a short line-to-line distance L and a dither pattern Z12 having a long line-to-line distance L are compared under a condition where the angle is constant, the area (reference value) Sp of the blank portion N is The dither pattern Z11 of 11 is smaller than the dither pattern Z12 of FIG. Here, as an example, the area (reference value) Sp of the blank portion N of the dither pattern Z11 having a short line-to-line distance L is set to “2”, and the area (reference value) of the blank portion N of the dither pattern Z12 having a long line-to-line distance L is set. ) Let Sp be "3".

  As shown in FIGS. 11 and 12, when the dots U forming the dither pattern Z are arranged with 2 dots continuous in the main scanning direction as one unit, the blank portion “N” has 2 dots. 11 to 12 are repeated, the horizontal width (width in the main scanning direction) of the blank portion N is divided in units of 2 dots in FIGS.

  If the error amount “ΔD” is the same, the non-printing area (indicated by hatching in FIGS. 11 and 12) is enlarged by the same number of dots, so that a blank portion is formed between the dither pattern Z11 side and the Z12 side. The increase in the area of N is the same. Here, the increase in the area of the blank portion N is assumed to be “0.2 (one dot)”.

  In this case, the area Sx of the blank portion N at the joint portion of the dither pattern Z11 having a short line-to-line distance L is “2” + “0.2” and becomes “2.2”. Therefore, the area variation ratio “Sr” on the dither pattern Z11 side is “2.2” / “2.0” and becomes “110”%.

  On the other hand, the area Sx of the blank portion N at the joint portion of the blank portion N of the dither pattern Z12 having a long line-to-line distance L is “3.2” as “3” + “0.2”. Therefore, the area variation ratio “Sr” on the dither pattern Z12 side is “3.2” / “3.0” and becomes “107”%.

  As described above, the dither pattern Z11 having the short line-to-line distance L has a larger area variation ratio “Sr” of the blank portion N at the joint portion with respect to the error amount “ΔD” than the dither pattern Z12 having the long line-to-line distance L. . The large area variation ratio “Sr” means that the expansion ratio of the blank portion N with respect to the error amount “ΔD” is large, that is, the exposure becomes thin and the density of the pixel tends to decrease.

  Next, the case where the dither pattern angle θ is different will be described with reference to FIGS. If the line-to-line distance L is the same, the area (reference value) Sp of the blank portion N substantially matches regardless of the angle θ of the dither pattern Z. Here, as an example, the area (reference value) Sp of the blank portion N is “2” for both the dither pattern Z13 having a small angle θ and the dither pattern Z14 having a large angle θ.

  If the error amount “ΔD” is the same, in the non-printing area, the dither pattern with a larger angle θ is enlarged than the dither pattern with a smaller angle θ. This is because the number of dots that form a non-printing area increases as the angle θ increases. Here, the increase in the area of the blank portion N of the dither pattern Z3 with a small angle θ is “0.2 (one dot)”, and the increase in the area of the blank portion N of the dither pattern Z4 with a large angle θ is “ 0.4 (2 dots) ".

  In this case, the area Sx of the blank portion N in the joint portion of the dither pattern Z13 having a small angle θ shown in FIG. 13 is “2” + “0.2” and becomes “2.2”. Therefore, the area variation ratio “Sr” on the dither pattern Z3 side with a small angle becomes “110”% at “2.2” / “2.0”.

  On the other hand, the area Sx of the blank portion N in the joint portion of the blank portion N of the dither pattern Z14 having a large angle θ shown in FIG. 14 is “2.4” as “2” + “0.4”. Therefore, the area variation ratio “Sr” of the dither pattern Z4 is “2.4” / “2.0” and becomes “120”%.

  Thus, the dither pattern Z14 having a large angle θ has a larger area variation ratio “Sr” of the blank portion N at the joint portion with respect to the error amount “ΔD” than the dither pattern Z13 having a small angle θ. The large area variation ratio “Sr” means that the expansion ratio of the blank portion N with respect to the error amount “ΔD” is large, that is, the exposure becomes thin and the density of the pixel tends to decrease.

  As described above, the area variation ratio “Sr” indicates the ease of occurrence of a density difference with respect to the peripheral pixel G2 of the pixel G1 corresponding to the joint of the LED array chip CH. In the second embodiment, as described with reference to FIG. 9 in the first embodiment, the combination of the three dither patterns Z used in the fine mode, the normal mode, and the super fine mode is represented by an area variation ratio “Sr”. The difference is set to be small. That is, for each use dither pattern, the area variation ratio “Sr” is calculated based on data such as the pattern angle θ, the line distance L, the dot arrangement, and the size, and the calculated area variation ratio is calculated. A combination of three dither patterns Z used in the fine mode, normal mode, and super fine mode is set so that the difference in “Sr” is reduced.

  By doing in this way, similarly to the first embodiment, when the light amount correction is performed on the three dither patterns Z used in the fine mode, the normal mode, and the super fine mode using a common correction value, Similarly to each dither pattern, it is possible to eliminate the uneven density difference.

  As described above, the area variation ratio “Sr” tends to increase as the angle θ of the dither pattern Z increases, and decreases as the interline distance “L” increases. Therefore, determining the combination of dither patterns so that the difference in the area variation ratio Sr is small means that, in the normal mode in which the inter-line distance L is relatively wide, a dither pattern having a large angle is selected and the inter-line distance L The medium fine mode is substantially equivalent to selecting a dither pattern having a medium angle, and the super fine mode having a relatively small distance L between lines is substantially equivalent to selecting a dither pattern having a small angle θ. .

  Then, the area variation ratio “Sr” of the three types of dither patterns Z determined in the design stage is stored in advance in the RAM 100B, and the light quantity correction of the light emitting elements Pa and Pb at the joint of the LED array chip CH is performed with printing. It is configured to be utilized when performing.

  Specifically, in the second embodiment, the area variation ratio Sr is applied to a parameter indicating the ease of occurrence of a density difference with respect to the peripheral pixel G2 of the pixel G1 corresponding to the joint of the LED array chip CH. Therefore, for the fine mode dither pattern Z, the normal mode dither pattern Z, and the super fine mode dither pattern Z, the correction values X of the light amounts of the light emitting elements Pa and Pb are based on the following equation (6). Calculate each.

X = F (ΔD, Sr) (6)

  Then, the common correction value Xv is calculated from the obtained correction value X of each dither pattern Z in the same manner as in the first embodiment, and the LED array chip CH of each dither pattern is calculated for each dither pattern using the calculated common correction value Xv. The light emission times of the light emitting elements Pa and Pb at the joint are corrected.

  In this way, the correction value stored in the RAM 100B can be reduced as compared to the case where the light amount correction is performed using a different correction value for each prepared dither pattern, as in the first embodiment. Therefore, consumption of the RAM 100B can be suppressed. In addition, since it is not necessary to change the correction value every time a different dither pattern is used, the operation speed can be increased.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention.

  (1) In Embodiments 1 and 2, an LED array using a light emitting diode as the light emitting element P is illustrated as an example of the light emitting array. However, an organic EL array using an organic EL (electroluminescence) element as the light emitting element is used. It is also possible to use it.

  (2) In the first and second embodiments, the example in which the light amounts of the light emitting elements Pa and Pb located at the chip end of the LED array chip CH are corrected has been described. The correction target of the light amount is not limited to the light emitting elements Pa and Pb, and a plurality of light emitting elements located at the chip end may be targeted. For example, a total of four light emitting elements including the light emitting elements Pa and Pb and two light emitting elements adjacent thereto may be used.

  (3) In the above embodiment, the calculation control unit 100A of the control device 100 is configured by a CPU. However, the calculation control unit 100 may be configured by a hardware circuit such as an ASIC, or the CPU and the ASIC. The hardware circuit may be combined.

  (4) In the above embodiment, an example in which three types of modes of “fine mode”, “normal mode”, and “super fine mode” are set as the image mode has been described. For example, there may be only two image modes, such as “fine mode” and “normal mode”. In this case, it is only necessary that the distance between the two modes is different, and the dither patterns of the respective colors can be combined as follows. That is, in the case of “normal mode”, “black” is a dither pattern corresponding to “normal mode” at the time of setting the three types of modes of the first embodiment, “yellow” is a dither pattern corresponding to “fine mode”, “Magenta” is a dither pattern corresponding to “normal mode”, and “cyan” is a dither pattern corresponding to “normal mode”. On the other hand, in the case of the “fine mode”, “black” is a dither pattern corresponding to the “super fine mode” in the three types of mode setting of the first embodiment, and “yellow” is a dither pattern corresponding to the “super fine mode”. , “Magenta” is a dither pattern corresponding to “fine mode”, and “cyan” is a dither pattern corresponding to “fine mode”.

1. Printer (an example of the “image forming apparatus” of the present invention)
30 ... Image forming unit 40 ... LED unit 41 ... LED array (an example of the “light emitting array” of the present invention)
53. Photosensitive drum (an example of the “photosensitive member” of the present invention)
DESCRIPTION OF SYMBOLS 100 ... Control apparatus 100A ... Calculation control part (an example of the "control part" of this invention)
110 ... light emission control device CH ... LED array chip (an example of the "light emitting chip" of the present invention)
M ... Memory P ... Light emitting element T ... Light emission time X ... Correction value θ ... Dither pattern angle Sr ... Area variation ratio Z ... Dither pattern

Claims (6)

A light emitting array in which light emitting chips having a plurality of light emitting elements are arranged in the main scanning direction is provided as an exposure device, and an image is recorded on a recording medium using an electrostatic latent image formed on a photosensitive member exposed by the light emitting array. An image forming unit to be formed;
A dither pattern representing the gradation of the image, a memory storing a plurality of dither patterns used when an image is formed by the image forming unit;
A control unit,
The controller is
Of the pixels constituting the first dither pattern in which the value of the parameter representing the ease of occurrence of the density difference with respect to the peripheral pixels of the pixel corresponding to the edge of the light emitting chip is the first value, the edge of the light emitting chip A first calculation process for calculating a first correction value for correcting the light amount of the light emitting element located at the end in accordance with the first value so that the density of the pixel corresponding to the pixel approaches the density of the surrounding pixels. When,
A second dither value used for the same color as the first dither pattern is a parameter value representing the ease of occurrence of a density difference with respect to the peripheral pixels of the pixel corresponding to the edge of the light emitting chip. Among the pixels constituting the pattern, the light amount of the light emitting element located at the end is set according to the second value so that the density of the pixel corresponding to the end of the light emitting chip approaches the density of the peripheral pixel. A second calculation process for calculating a second correction value for correction;
A common correction value calculation process for calculating a common correction value based on the first correction value calculated in the first calculation process and the second correction value calculated in the second calculation process;
A storage process for storing the common correction value calculated in the common correction value calculation process in the memory;
In the storage process, the image forming mode includes both a first image forming mode for forming an image using the first dither pattern and a second image forming mode for forming an image using the second dither pattern. An image forming apparatus that performs a correction process for correcting the light amount of the light emitting element located at the end using the common correction value stored in the memory.   The image forming apparatus according to claim 1, wherein the parameter is an angle with respect to a main scanning direction of a line segment constituting the dither pattern.   The image forming apparatus according to claim 1, wherein the parameter is an area variation ratio of a blank portion existing between the dither patterns with respect to a change in a distance between two light emitting elements located at an end portion of the light emitting chip. A method of setting a dither pattern formed using a light emitting array in which light emitting chips having a plurality of light emitting elements are arranged in the main scanning direction and representing the gradation of an image,
The parameter values of the two dither patterns used for the same color are reduced so that the difference in the parameter value indicating the ease of occurrence of the density difference with respect to the peripheral pixels of the pixel corresponding to the edge of the light emitting chip is reduced. How to set the dither pattern to be set. The parameter is an angle with respect to a main scanning direction of a line segment constituting the dither pattern,
The dither pattern setting method according to claim 4, wherein an angle of a line segment of two dither patterns used for the same color is set so that an angle difference with respect to the main scanning direction is small. The parameter is an area variation ratio of a blank portion existing between the dither patterns with respect to a change in a distance between two light emitting elements located at an end portion of the light emitting chip.
The dither pattern setting method according to claim 4, wherein an angle and a distance between the line segments of two dither patterns used for the same color are set so that the area variation ratio becomes small.

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