JP2005262620A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain density irregularities due to output irregularities of a recording head. <P>SOLUTION: The image forming device forms an image of a prescribed number of screen lines as a maximum number of screen lines. An LED print head (LPH) 14 is made by arranging a plurality of LEDs 64 in a row. When actually forming an image by using the LPH 14, the image forming device prepares a test pattern by the number of screen lines (300 lpi) greater than the number of screen lines selected (150 lpi, 200 lpi), and performs a luminous energy correction for the LEDs 64 forming the LPH 14 by using the test pattern prepared. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus that forms an image using a recording head configured by arranging a plurality of recording elements, and more particularly to output correction of each recording element.

  In image forming apparatuses such as printers, copiers, and facsimiles that employ an electrophotographic method, an electrostatic latent image is obtained by irradiating image information onto a uniformly charged photoreceptor by optical recording means. The electrostatic latent image is visualized by adding toner, and the image is formed by transferring and fixing on the recording paper. In addition to the optical scanning method in which a laser is used as the optical recording means and exposure is performed by scanning the laser beam in the main scanning direction, in recent years, an LED (Light Emitting Diode) has been received in response to a request for downsizing of the apparatus. A writing device using an LED print head (LPH) arranged in a large number in the main scanning direction is employed.

  LPH is generally an LED array in which a large number of LEDs are arranged in the main scanning direction, and a cell in which a large number of rod lenses are arranged to form an image of the light output from the LEDs on the surface of the photosensitive member (photosensitive drum). And a fock lens. In the image forming apparatus, each LED of the LPH is driven based on input image data, light is output toward the photosensitive member, and light is imaged on the surface of the photosensitive member by the SELFOC lens. An electrostatic latent image is formed in the sub-scanning direction by relatively moving the photoconductor and LPH.

  In this LPH, since a plurality of light emitting elements and lenses are arranged side by side in the main scanning direction, variation in each light emitting point has a great influence on image quality. In particular, when there is a variation in the amount of light emitted from the light emitting points or when the lens characteristics vary, streaks and density unevenness in the sub-scanning direction occur, and image quality defects are likely to occur. Therefore, as a conventional technique, there is a technique in which a test pattern formed and output on a sheet using LPH is read by a reading unit, and a light amount correction value of each LED in LPH is set based on the obtained reading result ( (See Patent Document 1).

In an electrophotographic image forming apparatus, for example, a square halftone cell is formed by a plurality of pixels, and the number of pixels to be recorded in each halftone cell, that is, the size of a halftone dot is changed. An area gradation method that expresses the density of an image with gradation (presence / absence of recording on a pixel) is widely used. In this area gradation method, the arrangement of halftone dots is regarded as a line, and the number of lines arranged per inch (number of halftone dots) is called the number of screen lines (unit: line per inch: lpi). The larger the number, the denser the halftone dots and the better the image reproducibility.
In such an image forming apparatus using LPH, a plurality of screen line numbers in the halftone processing mode can be set, and the light amount correction value of each LED is made different for each set screen line number. (See Patent Document 2).

Japanese Patent Laid-Open No. 11-240202 (page 3-7, FIG. 3) Japanese Patent Laying-Open No. 2003-112442 (page 5-6, FIG. 2)

However, in the technique described in Patent Document 1 described above, it is necessary to set a different light amount correction value for each LED in accordance with the selected halftone processing mode, that is, for each number of screen lines. There was a problem that became complicated.
Further, since it is necessary to store the light amount correction value of each LED for each intermediate processing mode, a large memory capacity is required, resulting in an increase in cost. In recent image forming apparatuses, colorization is progressing rapidly. For example, four image forming units of yellow (Y), magenta (M), cyan (C), and black (K) are arranged in parallel. In a so-called tandem type image forming apparatus that forms a full-color image, an LPH is provided for each image forming unit, so that more memory capacity is required and the cost increases.
Furthermore, according to the conventional method, depending on the configuration of the test pattern formed for correcting the light amount of each LED, the correction cannot be converged, and the density unevenness may not be completely eliminated.

  Such a problem is not limited to an image forming apparatus using a recording head using light such as LPH. For example, a plurality of nozzles are arranged in parallel and ink is ejected from the plurality of nozzles. This can also occur in an image forming apparatus using an inkjet head for image formation.

The present invention has been made to solve such a technical problem, and an object of the present invention is an image forming apparatus that forms an image with a predetermined number of screen lines as the maximum number of screen lines. This is to suppress density unevenness caused by output unevenness.
Another object is to reduce the capacity of output correction data for correcting the output unevenness of the recording head.

For this purpose, the present invention is an image forming apparatus for forming an image with a predetermined number of screen lines as the maximum number of screen lines, and is larger than the maximum number of screen lines and a recording head in which a plurality of recording elements are arranged. Input means for inputting test image data created with other screen lines to the recording head, output means for outputting test image data input to the recording head by the input means using the recording head, and output means Storage means for storing output correction data of each recording element obtained based on the result of reading the output image by the reading means.
Here, the input means can input a test image pattern created with a screen line number of 200 to 300 as another screen line number to the recording head.

From another viewpoint, the image forming apparatus to which the present invention is applied includes a recording head in which a plurality of recording elements are arranged, and a first image forming mode in which an image is formed with a first screen line number. And an image forming unit that operates in a second image forming mode for forming an image with a second screen line number larger than the first screen line number, and a second screen line number output using the recording head. A storage unit for storing output correction data of each recording element obtained based on the test image data having a large third screen line number, and each recording element in the recording head using the output correction data stored in the storage unit And a correction unit for correcting the output.
Here, the first image forming mode is a speed priority mode that prioritizes production speed, and the second image forming mode is an image quality priority mode that prioritizes image quality.

Further, from another viewpoint, an image forming apparatus to which the present invention is applied includes a recording head in which a plurality of recording elements are arranged, and a first image forming mode in which an image is formed with a first screen line number. An image forming unit that operates in a second image forming mode for forming an image with a second screen line number that is larger than the first screen line number, and an output correcting unit that corrects the output of each recording element in the recording head; And the output correction unit performs correction using the same output correction data in the first image forming mode and the second image forming mode.
Here, the output correction unit uses output correction data created based on test image data having a third screen line number larger than the second screen line number output using the recording head. be able to.

  Furthermore, from another viewpoint, an image forming apparatus to which the present invention is applied includes a recording head in which a plurality of recording elements are arranged, and test image data formed with a screen line number of 200 to 300. Obtained based on the result of reading the image output by the output means, the output means outputting the test image data input to the recording head by the input means, the output means using the recording head. Storing means for storing output correction data of each recording element.

  According to the present invention, in the image forming apparatus that forms an image with a predetermined number of screen lines as the maximum number of screen lines, uneven density due to output unevenness of the recording head can be suppressed.

The best mode for carrying out the present invention (hereinafter referred to as an embodiment) will be described below in detail with reference to the accompanying drawings.
FIG. 1 is a diagram showing an overall configuration of an image forming apparatus using an LED print head to which this embodiment is applied, and shows a so-called tandem type digital color printer. An image forming apparatus shown in FIG. 1 includes an image processing system 10 that forms an image corresponding to gradation data of each color, an image output control unit 30 that controls the image processing system 10, such as a personal computer (PC). 2 and an image reading device (IIT) 3 and an image processing system (IPS) 40 that performs predetermined image processing on image data received from these.

  The image processing system 10 as an output unit includes an image forming unit 11 as an image forming unit including a plurality of engines arranged in parallel at a constant interval in the horizontal direction. The image forming unit 11 is composed of four image forming units 11Y, 11M, 11C, and 11K of yellow (Y), magenta (M), cyan (C), and black (K). A photosensitive drum 12 that is an image carrier (photosensitive member) that forms an image and carries a toner image, a charger 13 that uniformly charges the surface of the photosensitive drum 12, and a photosensitive drum that is charged by the charger 13. 12 is an LED print head (LPH) 14 that is a recording head that exposes 12 and a developing unit 15 that develops a latent image obtained by the LPH 14. Further, the image process system 10 conveys the recording paper in order to multiplex-transfer the toner images of the respective colors formed on the photosensitive drums 12 of the image forming units 11Y, 11M, 11C, and 11K onto the recording paper. The sheet conveying belt 21, the driving roll 22 which is a roll for driving the sheet conveying belt 21, the transfer roll 23 for transferring the toner image on the photosensitive drum 12 to the recording sheet, and the toner image on the recording sheet after the transfer are fixed. A fixing device 24 is provided.

  Each of the image forming units 11Y, 11M, 11C, and 11K has substantially the same configuration except for the toner accommodated in the developing device 15. Image signals input from the PC 2 or IIT 3 are subjected to image processing by the image processing unit 40 and supplied to the image forming units 11Y, 11M, 11C, and 11K through the interface. The image process system 10 operates based on a control signal such as a synchronization signal supplied from the image output control unit 30.

  First, in the yellow image forming unit 11Y, an electrostatic latent image is formed on the surface of the photosensitive drum 12 charged by the charger 13 by the LPH 14 based on the image signal obtained from the image processing unit 40. A yellow toner image is formed on the electrostatic latent image by the developing unit 15, and the formed yellow toner image is transferred to the recording paper on the paper transport belt 21 that rotates in the direction of the arrow in the figure. It is transcribed using. Similarly, magenta, cyan, and black toner images are formed on the respective photosensitive drums 12, and sequentially transferred onto the recording paper on the paper transport belt 21 using the respective transfer rolls 23. The multiple transferred toner images on the recording paper are conveyed to the fixing device 24 and fixed on the recording paper by heat and pressure.

  FIG. 2 is a diagram showing the configuration of the LED print head (LPH) 14 described above. The LPH 14 includes an LED array 51 formed by arranging a large number of LED chips 63 (described later), a printed circuit board 52 on which a circuit for supporting the LED array 51 and controlling the drive of the LED array 51 is formed, and each LED. A SELFOC lens array (SLA: registered trademark) 53 for forming an image of the light beam emitted from the photosensitive drum 12 on the surface of the photosensitive drum 12 is provided, and the printed circuit board 52 and the SLA 53 are held in a housing 54.

  The LED array 51 is formed by arranging a plurality of LED chips in the main scanning direction (the axial direction of the photosensitive drum 12) and attaching the LED chip to a printed circuit board 52. These LED chips may be arranged in a line or a staggered pattern. In the LED array 51, LEDs 64 (see FIG. 3 described later) for the number of pixels corresponding to the resolution are arranged in the main scanning direction. For example, when the A3 size short (297 mm) is used as the main scanning direction, 14040 LEDs are arranged every about 21.2 μm at a resolution of 1200 dpi.

  In the SLA 53, a gradient index lens as an imaging lens is formed corresponding to the number of pixels corresponding to the resolution (for example, 1070 in the present embodiment). The SLA 53 condenses the light beams emitted from the LED arrays 51 of the image forming units 11Y, 11M, 11C, and 11K and forms an image on the corresponding photosensitive drum 12 to thereby form a main image on the photosensitive drum 12. An electrostatic latent image for one line is formed in the scanning direction. Further, the printed circuit board 52 is mounted on the housing 54 so that the mounting surface of the LED array 51 faces the surface of the photosensitive drum 12.

  FIG. 3 is a circuit diagram illustrating a relationship between the image output control unit 30 serving as a correction unit (output correction unit) and the LPH 14. As shown in FIG. 1, the LPH 14 is connected to the image output control unit 30, and the operation of each circuit built in the LPH 14 is controlled by the image output control unit 30. The printed circuit board 52 shown in FIG. 2 has a shift register 61 that stores image data for one line, a latch circuit 62 that latches image data, and a current value corresponding to the image data latched in the latch circuit 62. A driver 65 for driving each LED 64 of the LED chip 63 by being supplied to the LED chip 63 is provided. The driver 65 is connected to an EEPROM (Electrically Erasable and Programmable ROM) 66 for storing correction data to be described later.

  When image data is input from the image processing unit 40, the image output control unit 30 outputs image data for one line to the shift register 61 in synchronization with the transfer clock. Further, the image output control unit 30 outputs the SET signal to the latch circuit 62 after outputting the image data for one line to the shift register 61. As a result, the image data for one line stored in the shift register 61 is latched in the latch circuit 62. When the image output control unit 30 outputs the STROB signal to the driver 65, each driver 65 supplies a current having a value corresponding to the corresponding pixel data to the corresponding LED 64 as the recording element. As a result, each LED 64 emits light with an intensity corresponding to the current value supplied from the driver 65.

  Each driver 65 is provided with a register (not shown) for correcting a current value output (supplied) from the driver 65 to the LED 64. This register holds correction data for correcting the light emission amount of the LED 64. The driver 65 corrects the current value based on the image data latched by the latch circuit 62 with the correction data held in the register, and then supplies the current value to the LED 64. The correction data is calculated in advance and stored in the EEPROM 66, and is written from the EEPROM 66 to the register of each driver 65 and held at a predetermined timing such as when the image forming apparatus is turned on.

  Next, a description will be given of the correction data generation device 70 that functions as a correction device and is used as a jig for generating the correction data written in the EEPROM 66 described above. FIG. 4 is a block diagram illustrating a functional configuration of the correction data generation device 70. The correction data generation device 70 includes a reading unit 71 that reads an image formed on a recording sheet as image data, and an image data calculation that performs a predetermined calculation based on the image data read by the reading unit 71. 72, density unevenness calculation unit 73 for obtaining density unevenness data in the image data calculated by image data calculation unit 72, and density unevenness suppression based on the density unevenness data calculated by density unevenness calculation unit 73. A correction data calculation unit 74 that calculates correction data to be corrected, a correction data generation unit 75 that generates correction data to be stored in the image forming apparatus based on a calculation result in the correction data calculation unit 74, and a correction data generation unit And a driver 76 for writing the correction data generated in 75 to an EEPROM 66 provided in the image forming apparatus. . The driver 76 is connected to the EEPROM 66 provided in the image forming apparatus via, for example, a predetermined cable and an interface (both not shown). Then, the correction data formed by the correction data generation device 70 based on the density information of the test pattern formed on the recording paper is written into the EEPROM 66 via the cable and interface described above. The image data calculation unit 72, the density unevenness calculation unit 73, the correction data calculation unit 74, and the correction data generation unit 75 constitute a scanner correction unit.

The reading unit 71 reads a predetermined output image (hereinafter referred to as a test pattern) as test image data output from the image forming apparatus as image data, and outputs the image data to the image data calculation unit 72. As the reading unit 71, for example, IIT3 provided in the image forming apparatus can be used.
The image data calculation unit 72 detects the inclination of the image data obtained by reading the test pattern formed on the recording paper by the reading unit 71 and corrects the inclination. Then, the image data whose inclination is corrected is output to the density unevenness calculation unit 73.
The density unevenness calculation unit 73 obtains density unevenness data from the image data input from the image data calculation unit 72 by averaging the density data for each LED 64 in the main scanning direction and the sub-scanning direction. In addition, the magnification correction is performed by performing a scaling process (enlargement or reduction) in a one-dimensional manner in the main scanning direction (main scanning direction at the time of test pattern formation), and the LED 64 in which the number of pixels of density unevenness data and the test pattern are formed. Are matched with the position of each LED 64. Thus, the density distribution in the main scanning direction generated on the image formed by the image forming apparatus is obtained, and the result is output to the correction data calculation unit 74 as density unevenness data.

The correction data calculation unit 74 calculates correction data (referred to as correction data B) for making the density distribution in the main scanning direction substantially flat based on the density unevenness data input from the density unevenness calculation unit 73. That is, the correction data B is a correction value for correcting the light emission intensity of each LED 64 so that the density of each pixel of the image formed by each LED 64 of the image forming apparatus is uniform.
The correction data generation unit 75 can access an EEPROM 66 as a storage means or a storage unit via a driver 76, and the correction data (referred to as correction data A) stored in advance in the EEPROM 66 from the correction data calculation unit 74. By combining with the input correction data B, new correction data (referred to as correction data C) is generated, and this correction data C is written to the EEPROM 66 via the driver 76. Thereby, the correction data C as the output correction data generated by the correction data generation device 70 is stored in the EEPROM 66 of the image forming apparatus. Thereafter, in each of the image forming units 11Y, 11M, 11C, and 11K of the image forming apparatus, An image forming process based on the correction data C is performed.

Next, creation and reading of a test pattern used for generating the correction data described above will be described in detail. This test pattern is formed on a recording sheet by the image forming apparatus and is read by the reading unit 71 of the correction data generating apparatus 70.
FIG. 5 schematically shows the relationship between the LED array 51 and the test pattern formed on the recording paper P. The LED array 51 has 14040 LEDs 64 as a plurality of recording elements arranged in parallel in the main scanning direction, specifically, L1, L2, L3,. When the test pattern is formed, A4 size recording paper P is set so that the longitudinal direction thereof is the main scanning direction, and these LEDs 64 are used to form the same test pattern in the sub-scanning direction in different colors. Specifically, the test image Y (Cin50) of 50% density of yellow, the test image C (Cin50) of 50% density of cyan, the test image M (Cin50) of 50% density of magenta, black An electrostatic latent image is formed so as to be a test image K (Cin 50) having a density of 50%. The test pattern creation method will be described later.

  On the other hand, FIG. 6 is a diagram schematically showing the relationship between the line sensor 77 provided in the reading unit 71 of the correction data generation device 70 and the test pattern formed on the recording paper P. The line sensor 77 has 7020 sensors S1, S2, S3,... Arranged in the main scanning direction and has a resolution of 600 dpi. When the test pattern is read, the A4 size recording is performed so that the short direction is the main scanning direction, that is, the main scanning direction and the sub-scanning direction are switched from the time when the test pattern is created. The paper P is set, and the yellow 50% density test image Y (Cin50), cyan 50% density test image C (Cin50), and magenta 50% density test image M ( Cin 50), black test image K (Cin 50) of 50% density is read by the line sensor 77. By using this method, it becomes possible to read a test image of the same color (for example, test image K (Cin50) of 50% density of black) with the same sensor (for example, sensors S1, S2, and S3), and different sensors are read. It is possible to reduce the influence of errors caused by this. Further, there is an advantage that image data substantially corresponding to each of the LEDs L1, L2, L3,... Can be obtained every time one line is read by the line sensor 77.

  Next, a procedure for setting correction data performed before shipment of the image forming apparatus according to the present embodiment will be described with reference to FIG. FIG. 7 is a flowchart showing a procedure for setting correction data. First, prior to generating the correction data, the main body 1 for forming an image such as a test pattern on the recording paper P is assembled by mounting the LPH 14 or the like on the image forming apparatus, and attaching each part. Correction is performed. The EEPROM 66 incorporated in the image forming apparatus holds an initial value (correction data A) of correction data. When the power of the image forming apparatus is turned on, the correction data A is read out and each of the LPHs 14 is read. It is stored in a register of each driver 65 for driving the LED 64.

  Then, an instruction to form a test pattern is given together with the contents of the test pattern from the PC 2 as input means. By instructing this test pattern formation, each LED 64 of the LPH 14 emits light whose light amount is corrected based on the correction data A, and an electrostatic latent image is formed on the photosensitive drum 12 by this light emission. On the recording paper P, as shown in FIG. 5, a test image Y (Cin50) of 50% density of yellow and a test image C (Cin50) of 50% density of cyan, each having a predetermined length in the sub-scanning direction. ), A test pattern comprising a magenta 50% density test image M (Cin50) and a black 50% density test image K (Cin50) is output (step 101). Details of the test pattern will be described later. Based on the image formed on the recording paper P, the correction data generation device 70 generates correction data for suppressing density unevenness. For this purpose, the main body 1 of the image forming apparatus and the correction data generating apparatus 70 are connected.

  As shown in FIG. 6, the image formed on the recording paper P in step 101 is read by the reading unit 71 of the correction data generating device 70 in a state where the main scanning direction and the sub-scanning direction at the time of image formation are switched. (Step 102), the test pattern included in this image is acquired as image data. Then, the image data calculation unit 72 performs skew correction of the image data based on the read image data (step 103). Here, as shown in FIG. 6, the skew correction is performed by, for example, the outer edge a on the rear end side of the recording paper P in the 50% density test image Cin50 and the rear end side of the recording paper P in the 20% density test image Cin20. This is realized by detecting the position information of the outer end b of the image, calculating the inclination of the entire image data from the position information of the outer end a and the outer end b, and correcting the inclination. In addition, since the outer end part a and the outer end part b are formed by the same LED 64 in Step 101, it is possible to improve the accuracy of the detected position. Thereby, an inclination error component between the test pattern actually formed on the recording paper P and the image data read by the reading unit 71 is removed.

  The density unevenness calculation unit 73 averages the density of the image data subjected to the skew correction in step 103 in the main scanning direction and the sub-scanning direction, and noise is removed (step 104). In addition, the density unevenness calculation unit 73 performs magnification correction in the main scanning direction (main scanning direction during test pattern formation), thereby correcting the number of pixels in the main scanning direction so as to match the number of LEDs 64 (14040). (Step 105).

  In the image data that has been subjected to the pixel number correction in step 105, the average density for each color is calculated in the correction data calculation unit 74 (step 106), and the average density for each color obtained in step 106 and the input density Cin for each color are calculated. The deviation from the output density of each LED 64 is calculated (step 107). Then, correction data (correction data B) is calculated by dividing the deviation obtained for each input density and for each LED 64 by the correction resolution (step 108). Further, the correction data B calculated by the correction data calculation unit 74 and the correction data A acquired from the EEPROM 66 of the image forming apparatus via the driver 76 are combined in the correction data generation unit 75, thereby density unevenness. Correction data (correction data C) for forming an image having no image is generated.

  Then, the correction data C generated as described above is written into the EEPROM 66 of the image forming apparatus via the driver 76 (step 109) and stored in the EEPROM 66. When the correction data C is stored in the EEPROM 66, the correction data setting is completed. The main body 1 of the image forming apparatus is shipped after the correction data generating apparatus 70 attached to the main body 1 is removed.

  Therefore, the image forming apparatus is shipped in a state where correction data (correction data C) changed so as not to cause density unevenness is stored in the EEPROM 66. Therefore, the LPH 14 performs this correction when used by the user after shipment. An image can be formed while performing light amount correction using data. Thereby, it is possible to form a good image in which uneven density is suppressed.

  By the way, in the image forming apparatus according to the present embodiment, different image processing modes are selected depending on whether high speed or high image quality is required when forming an image, and the image processing unit 40 responds to the request. Image processing is performed. In the high-speed mode (speed priority mode) as the first image formation mode, image formation is performed with a screen line number of 150 lpi (first screen line number), and the high-quality mode (image quality) as the second image formation mode. In the priority mode), image formation is performed with a screen line number of 200 lpi (second screen line number).

  In this image forming apparatus, as described above, it is possible to form an image with a plurality of screen lines (150 lpi, 200 lpi). In the present embodiment, a test pattern to be formed when correcting the light quantity of each LED 64 in the LPH 14 of each of the image forming units 11Y, 11M, 11C, and 11K of the image forming apparatus using the correction data generation device 70 described above. Is larger than the maximum screen line number in the actual image formation (predetermined screen line number, 200 lpi in the high image quality mode in this embodiment). It is set to 300 lpi.

Next, the reason why the number of screen lines of the test pattern is made larger than the maximum number of screen lines in actual image formation when correcting the light quantity of each LED 64 in each LPH 14 will be described.
FIGS. 8A to 8D schematically show images of 50% density for each screen line number (150 lpi, 200 lpi, 300 lpi, 600 lpi). 8A to 8D show an example in which the screen angle of halftone dots constituting an image is set to 90 °. As is apparent from the figure, the area of the halftone cell HC for forming one halftone dot decreases as the number of screen lines increases. For example, when the number of screen lines doubles from 150 lpi to 300 lpi, The area of the tone cell HC becomes a quarter.

  Further, the number of dots constituting one side of the halftone cell HC can be obtained by dividing the resolution of the LED array 51 (1200 dpi in the present embodiment) by the number of screen lines. For example, in the case of 150 lpi shown in FIG. 8A, one halftone cell HC is composed of pixels of 1200/150 = 8, that is, 8 × 8 = 64 dots. Further, in the case of 200 lpi shown in FIG. 8B, a halftone cell HC is composed of pixels of 6 × 6 = 36 dots. Further, in the case of 300 lpi shown in FIG. 8C, a halftone cell HC is configured with 4 × 4 = 16 dots. In the case of 600 lpi shown in FIG. 8D, the halftone cell HC is configured with 2 × 2 = 4 dots. When image formation is performed with a larger number of screen lines, the number of gradations that can be expressed in this way is reduced. However, in forming a test pattern, for example, a predetermined density such as an input density of 50% is used. Since only an image is formed, increasing the number of screen lines (reducing the number of gradations) is not particularly problematic.

  Further, FIGS. 9A, 9B, and 10A, 10B are obtained by reading the test pattern created with each screen line number shown in FIG. 8 using the correction data generating apparatus 70 described above. FIG. 6 is a graph showing the relationship between the position in the main scanning direction (scanner pixel number) and the scanner output value obtained by dividing the average of the output values at each pixel number by the average of the output values of all pixel numbers. is there. Here, 700 to 900 dots are illustrated as examples of all pixel numbers. In this embodiment, the resolution of the LPH 14 is 1200 dpi, whereas the resolution of the line sensor 77 is 600 dpi. For example, the scanner output value of the scanner pixel number 700 shown in FIG. It will be used to create correction data for the LED 64.

  From the results shown in FIGS. 9A and 9B and FIGS. 10A and 10B, the smaller the number of screen lines (the larger the halftone cell HC constituting the halftone dot), the greater the variation in the scanner output value. It is understood. For example, in the case of the screen line number 150 lpi shown in FIG. 9A, the dot formation pitch (hereinafter referred to as the screen pitch) appears in the output pattern as it is, whereas for example in FIG. In the case of the screen line number 600 lpi shown, it can be seen that the screen pitch is crushed.

Next, the relationship between the number of screen lines of the test pattern and the created correction data will be examined.
FIG. 11 illustrates the relationship between the position in the main scanning direction of the LED 64 (LED number: referred to as LPH pixel number) and the density percentage when a 50% density image is created with a screen line number of 200 lpi. In this case, since one halftone cell HC is composed of 36 dots, the LEDs 64 have a period of every six. Here, focusing on the 7000, 7006, ... 7030, 7036, 7042, ... 7054, 7060th LED 64 where the concentration percentage is the smallest, if there is no problem, it is indicated by ● in the figure Since correction data for the 7000, 7006,..., 7030, 7036, 7042,..., 7054, 7060th LED 64 is generated based on the concentration percentage, there is no particular problem. On the other hand, for example, when the reading pattern is read by the reading unit 71, for example, if the reading speed fluctuates for some reason, even if the number of pixels in the main scanning direction is corrected in step 105 shown in FIG. The LPH pixel number is shifted from the actual density percentage value after the site where the fluctuation occurs. Now, for example, if speed fluctuation occurs due to slip of the recording paper P between the LPH pixel numbers 7030 and 7036, for example, correction data is created for the 7036th LED 64 based on the concentration percentage originally indicated by ● in the figure. However, the position is shifted due to the speed fluctuation. Then, correction data for the LED 64 of the LPH pixel numbers 7036, 7042, 7048, 7054, and 7060 is generated at the density percentage indicated by ○ in the figure, and the error in the correction increases, and the output image This will cause uneven density and streaks.

On the other hand, for example, when a 50% density image is created with a screen line number of 300 lpi, the density percentage (corresponding to the scanner output value) is clear from the comparison between FIG. 9B and FIG. Therefore, even if the LPH pixel number and the density percentage are slightly shifted back and forth due to the speed fluctuation, the error caused by the shift is reduced. For this reason, errors in correction can be reduced, and density unevenness and streaks can be suppressed.
On the other hand, for example, when a 50% density image is created with a screen line number of 600 lpi, as is apparent from FIG. 10B, the screen pitch is crushed, making it difficult to generate accurate correction data. Become. According to the experiment results of the present inventors, it has been confirmed that 200 to 300 lpi is favorable for the number of screen lines of the test pattern.

The number of screen lines described above is a numerical value when the screen angle is 90 °. If the screen angle is different, the number of screen lines at that time is also different. Here, FIG. 12 shows 50% density images with screen line numbers of 150 lpi, 200 lpi, and 300 lpi when the screen angle is 90 °. When the screen angle is 90 °, the screen lines actually obtained are 150 lpi, 200 lpi, and 300 lpi for the target screen lines 150 lpi, 200 lpi, and 300 lpi. On the other hand, for example, FIG. 13 shows 50% density images with screen line numbers of 150 lpi, 200 lpi, and 300 lpi when the screen angle is 45 °. When the screen angle is 45 °, the screen line numbers actually obtained are 141 lpi, 212 lpi, and 283 lpi with respect to the target screen lines 150 lpi, 200 lpi, and 300 lpi, which are slightly different. Further, for example, FIG. 14 shows 50% density images with screen line numbers of 150 lpi, 200 lpi, and 400 lpi when the screen angle is 72 °. When the screen angle is 72 °, the screen lines actually obtained are 127 lpi, 190 lpi, and 380 lpi with respect to the target screen line numbers 150 lpi, 200 lpi, and 400 lpi. .
Therefore, the above-mentioned screen line number of 200 to 300 lpi is a numerical value when the screen angle is 90 °. For example, when a screen angle other than 90 ° is used to create a full-color image. The number of screen lines may be less than 200 lpi or more than 300 lpi.

As described above, in the present embodiment, a test pattern is created with a screen line number (300 lpi) larger than the screen line number (150 lpi, 200 lpi) at the time of actual image formation. Thus, the light intensity correction of each LED 64 constituting the LPH 14 is performed, so that even if the number of screen lines in image formation is changed, uneven density due to uneven light intensity of the LPH 14 can be suppressed.
In this embodiment, the high-speed mode for forming an image with a screen line number of 150 lpi and the high-quality mode for forming an image with a screen line number of 200 lpi are both set based on a test pattern created with a screen line number of 300 lpi. Since the light amount correction is performed using the output correction data, it is not necessary to create and hold the output correction data for each screen line number, and the capacity of the EEPROM 66 can be reduced.
Furthermore, in the present embodiment, the number of screen lines at the time of test pattern creation is made larger than the number of screen lines used in actual image formation, so that the light quantity correction of each LED 64 constituting the LPH 14 can be performed more accurately. As a result, the correction data setting operation can be converged at an early stage.

1 is a diagram illustrating an overall configuration of an image forming apparatus. It is the figure which showed the structure of the LED print head (LPH). It is a circuit diagram which shows the relationship between an image output control part and LPH. It is a block diagram which shows the function structure of a correction data generation apparatus. It is a figure which shows typically the relationship between a LED array and the test pattern formed on a recording paper. It is a figure which shows typically the relationship between the line sensor provided in the reading part, and the test pattern formed on a recording paper. It is a flowchart which shows the setting procedure of correction data. It is a figure which shows typically the image of 50% density in each screen line number. It is a figure which shows an example of the reading result of a test pattern, (a) is the case of the test pattern produced with the screen line number of 150 lpi, (b) is the case of the test pattern produced with the screen line number of 200 lpi. It is a figure which shows an example of the reading result of a test pattern, (a) is the case of the test pattern produced with the screen line number 300 lpi, (b) is the case of the test pattern produced with the screen line number 600 lpi. It is a figure for demonstrating the shift | offset | difference of correction data. It is a figure which shows a 50% density image of the screen line numbers 150 lpi, 200 lpi, and 300 lpi in case a screen angle is 90 degrees. It is a figure which shows a 50% density image of the screen line numbers 150 lpi, 200 lpi, and 300 lpi in case a screen angle is 45 degrees. It is a figure which shows a 50% density image of the screen line number 150 lpi, 200 lpi, and 400 lpi in case a screen angle is 72 degrees.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Main body, 2 ... Personal computer (PC), 3 ... Image reader (IIT), 10 ... Image process system, 11Y, 11M, 11C, 11K ... Image forming unit, 12 ... Photosensitive drum, 13 ... Charger, DESCRIPTION OF SYMBOLS 14 ... LED print head (LPH), 15 ... Developing device, 21 ... Paper conveyance belt, 30 ... Image output control part, 40 ... Image processing part (IPS), 51 ... LED array, 52 ... Printed circuit board, 53 ... Selfock Lens array (SLA) 54 ... Housing 63 ... LED chip 64 ... LED 65 ... Driver 66 ... EEPROM 70 ... Correction data generator 71 ... Reading unit 72 ... Image data calculation unit 73 ... Uneven density Calculation unit, 74 ... correction data calculation unit, 75 ... correction data generation unit, 76 ... driver, 77 ... line sensor

Claims (7)

An image forming apparatus for forming an image with a predetermined screen line number as a maximum screen line number,
A recording head in which a plurality of recording elements are arranged;
Input means for inputting test image data created with another screen line number larger than the maximum screen line number to the recording head;
Output means for outputting the test image data input to the recording head by the input means using the recording head;
An image forming apparatus comprising: a storage unit that stores output correction data of each recording element obtained based on a result of reading an image output by the output unit by a reading unit.   The image forming apparatus according to claim 1, wherein the input unit inputs a test image pattern created with a screen line number of 200 to 300 as the other screen line number to the recording head. A recording head having a plurality of recording elements arranged therein, and a first image forming mode for forming an image with a first screen line number and an image with a second screen line number larger than the first screen line number. An image forming unit that operates in a second image forming mode to be formed;
A storage unit for storing output correction data of each recording element obtained based on the test image data having a third screen line number larger than the second screen line number output using the recording head;
An image forming apparatus including: a correction unit that corrects an output of each recording element in the recording head using output correction data stored in the storage unit.   4. The image forming apparatus according to claim 3, wherein the first image forming mode is a speed priority mode that prioritizes production speed, and the second image forming mode is an image quality priority mode that prioritizes image quality. A recording head having a plurality of recording elements arranged therein, and a first image forming mode for forming an image with a first screen line number and an image with a second screen line number larger than the first screen line number. An image forming unit that operates in a second image forming mode to be formed;
An output correction unit for correcting the output of each recording element in the recording head,
The output correction unit performs correction using the same output correction data in the first image forming mode and the second image forming mode.   The output correction unit uses the output correction data created based on test image data having a third screen line number larger than the second screen line number output using the recording head. The image forming apparatus according to claim 5. A recording head in which a plurality of recording elements are arranged;
Input means for inputting test image data formed with a screen line number of 200 to 300 to the recording head;
Output means for outputting the test image data input to the recording head by the input means using the recording head;
An image forming apparatus comprising: a storage unit that stores output correction data of each recording element obtained based on a result of reading an image output by the output unit by a reading unit.

Images