Image formation device, control method therefor, and program

Abstract

To suppress occurrence of density unevenness due to variations in light volume on a photoreceptor.SOLUTION: Image data, in which image data of a printing object has been halftone-processed, is acquired, and in order to correct variations in light volume on a photoreceptor due to light radiated from a plurality of light emitters, and a plurality of pieces of correction information is generated corresponding to types of factors contributing to variations in light volume. Although the halftone-processed image data is corrected by use of the correction information, the image data of the printing object is not corrected.SELECTED DRAWING: Figure 11

Description

[Technical Field]

[0001] The present invention relates to a technique for suppressing the occurrence of density unevenness in an electrophotographic image forming apparatus that uses a line head as a light source. [Background technology]

[0002] In electrophotographic image forming apparatuses that use a line head with multiple light-emitting elements as an exposure means, the amount of light may not be uniform across the main scanning direction on the photosensitive member. Such non-uniform light amount can result in density variations in the main scanning direction in the image formed on the recording medium. To prevent this density variation, a circuit is provided that corrects the amount of light on the photosensitive member for each main scanning direction, and this correction circuit corrects the image data used by the exposure means. Specifically, when forming an image on the recording medium, the image data is corrected to turn off light-emitting elements corresponding to areas with high reproduced density based on a correction profile that is stored in advance in memory and indicates the light intensity differences corresponding to each light-emitting element.

[0003] In this regard, Patent Document 1 discloses a technique in which a correction process for correcting variations in the amount of light caused by elements other than light-emitting elements is performed on image data before halftone processing. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-151566 Summary of the Invention [Problem to be solved by the invention]

[0005] In the above-mentioned Patent Document 1, the size of the portion where the correction is performed is affected by the halftone processing, and therefore the above-mentioned correction is not properly reflected in the image data after the halftone processing. [Means for solving the problem]

[0006] An image forming apparatus according to one aspect of the present disclosure is an electrophotographic image forming apparatus that uses a line head having a plurality of light-emitting elements arranged thereon as a light source, and includes: an acquisition means for acquiring halftone-processed image data of image data to be printed; a generation means for generating a plurality of correction information corresponding to types of factors causing variations in the amount of light on a photosensitive body due to light irradiated from the plurality of light-emitting elements; and a correction means for correcting the halftone-processed image data using the correction information, wherein the correction means does not correct the image data to be printed. [Effects of the Invention]

[0007] According to the present disclosure, it is possible to suppress the occurrence of density unevenness due to variations in the amount of light on the photosensitive member. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating a system configuration. [Figure 2] FIG. 1 is a block diagram illustrating a configuration of an image forming apparatus. [Figure 3] FIG. 2 is a cross-sectional view of a printing unit of the image forming apparatus. [Figure 4] FIG. 2 is a diagram illustrating a configuration example of an LED line head. [Figure 5] FIG. 10 is a diagram showing an example of the arrangement of LED chips. [Figure 6] 10 is a diagram showing the correspondence relationship between an LED light emitting element, a rod lens, and a current source. FIG. [Figure 7] FIG. 2 is a functional block diagram illustrating the configuration of an image processing unit. [Figure 8] 10A and 10B are diagrams showing an example of an HT image and an example of a printout thereof to which light intensity correction has not been performed. [Figure 9] 10A and 10B are diagrams showing an example of an HT image and an example of a printout thereof after light intensity correction. [Figure 10] 10 is a flowchart showing the flow of processing by an image processing unit. [Figure 11] FIG. 2 is a functional block diagram illustrating the configuration of an HT density correction processing unit. [Figure 12] FIG. 10 is a diagram illustrating an example of a correction profile. [Figure 13] 1A and 1B are diagrams for explaining examples of matrices and examples of how to use the matrices; [Figure 14] FIG. 10 is a diagram for explaining generation of a correction signal using correction data and a matrix. [Figure 15] 10 is a flowchart showing the flow of processing by an HT density correction processing unit. [Figure 16] 10 is a flowchart showing a detailed flow of processing by a composite profile acquisition unit. [Figure 17] 10 is a flowchart showing a detailed flow of processing for updating a third correction profile. [Figure 18] FIG. 2 is a schematic diagram of a calibration chart. [Figure 19] FIG. 2 is a functional block diagram illustrating the configuration of an HT density correction processing unit. [Figure 20] 10 is a flowchart showing the flow of processing by an HT density correction processing unit. [Figure 21] 10 is a flowchart showing a detailed flow of processing by a composite profile acquisition unit. [Figure 22] 10A and 10B are diagrams for explaining the correspondence between matrix data, HT image data, and threshold values. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments for carrying out the present disclosure will be described with reference to the drawings. Note that the following embodiments do not limit the present disclosure according to the claims, and not all of the combinations of features described in the present embodiments are necessarily essential to the solutions of the present disclosure. Note that the same components will be described with the same reference numerals.

[0010] [Embodiment 1] FIG. 1 illustrates the configuration of a system including an image forming apparatus 101 according to this embodiment. The system includes the image forming apparatus 101, a host computer 102, a mobile terminal 103, and a server 104. These devices are connected to a network 105 so that they can send and receive data to and from each other. The image forming apparatus 101 forms (prints) images using an electrophotographic method. The image forming apparatus 101 receives image data from the host computer 102, the mobile terminal 103, the server 104, or another image processing apparatus (not shown) via the network 105 and performs printing (image formation). The image forming apparatus 101 can also transmit image data obtained by scanning an original document using its own image reading unit 202 to the host computer 102, the mobile terminal 103, or the server 104 via the network 105. The image forming apparatus 101 can also perform a copy operation by printing the scanned image data using its own printing unit 206.

[0011] In this embodiment, an example will be described in which the image forming apparatus 101 is an image processing apparatus that performs halftone processing on image data to be printed, but this is not limiting. Image processing such as halftone processing may be executed by the host computer 102 or the like that is the sender of the image data to be printed. Furthermore, the image forming apparatus 101 may cooperate with the host computer 102, mobile terminal 103, server 104, or the like that is the sender of the image data to distribute the processing described below.

[0012] <Configuration of image forming device> 2 is a functional block diagram illustrating the configuration of the image forming apparatus 101. The image forming apparatus 101 includes a data input unit (receiving unit) 201, an image reading unit 202, a control unit 203, a storage unit 204, a UI (user interface) unit 205, a printing unit 206, and an image processing unit 207.

[0013] The data input unit 201 receives and inputs print data transmitted from the server 104, for example, via the network 105. The image reading unit 202 has a scanner and reads an image of a document to acquire image data of the document. The control unit 203 is configured to control the operation of the image forming apparatus 101 and includes a CPU 208, a ROM 209, and a RAM 210. The CPU 208 executes a program stored in the ROM 209 to perform the processing described below. The storage unit 204 is a storage device that stores large amounts of data, such as a hard disk drive (HDD). The CPU 208 may be configured to load the program stored in the storage unit 204 into the RAM 210 and perform the processing described below. The UI unit 205 includes an operation panel and a display unit, and displays messages to the user and accepts operation instructions from the user. The UI unit 205 may also have a touch panel function.

[0014] The printing unit 206 is a printer engine, and in this embodiment, it uses an electrophotographic method and a tandem method to form an image by superimposing toner images of multiple colors (for example, cyan / magenta / yellow / black, hereinafter referred to as C / M / Y / K) on a recording medium. Note that the color material configuration is not limited to this. It may be a black-only image forming device, or may have colors other than the above four colors. In this embodiment, the printing unit 206 will be described as having a printing resolution of 2400 dpi in the main scanning direction and sub-scanning direction. The above printing resolution is an example and is not limited to this.

[0015] The printing unit 206 also has a ROM 211 for each line head of each color used to control exposure to the photosensitive member. The ROM 211 stores manufacturing variation information for each line head, such as information about the assembly position and tilt of the LED chip, measured with a jig during production processes such as the line head manufacturing process. The ROM 211 also stores a correction profile that summarizes correction values ​​for correcting variations in light intensity. Details of the correction profile will be described later.

[0016] The image processing unit 207 performs various image processing operations on the image data included in the input print data. Note that the image processing unit 207 may be a processing unit such as hardware specialized for each type of image processing, or may be configured to realize its functions by the CPU 208 executing the above-mentioned program.

[0017] <Printing department configuration> Fig. 3 is a cross-sectional view of the printing unit 206 in the image forming apparatus 101 that uses a tandem electrophotographic method and employs the intermediate transfer body 28. The operation of the printing unit 206 will be described using Fig. 3. Note that in the drawing, components provided for each color are indicated by adding a letter (C / M / Y / K) indicating the respective color at the end of the reference numeral, but when the explanation does not require any particular distinction between colors, the letter at the end of the reference numeral will be omitted.

[0018] First, an overview will be given. The printing unit 206 exposes the photoconductor 22 in accordance with image data processed by the image processing unit 207, forms an electrostatic latent image, and develops the electrostatic latent image to form a monochromatic toner image. The printing unit 206 then forms a multicolor toner image by superimposing the monochromatic toner images of each color on the intermediate transfer body 28. The printing unit 206 transfers the multicolor toner image to the recording medium 11, and the fixing device 31 fixes the multicolor toner image on the recording medium.

[0019] Next, the configuration of the printing unit 206 will be described in detail with reference to Fig. 3. The injection charger 23 is used to uniformly charge the surface of the photoconductor 22 to a predetermined potential, and has a sleeve 23S.

[0020] The photoconductor 22 is rotated by the driving force of a drive motor (not shown). The drive motor rotates the photoconductor 22 counterclockwise in FIG. 3 in accordance with the image forming operation. The exposure unit is configured to irradiate the photoconductor 22 with exposure light from LEDs from an LED line head 24 arranged parallel to the photoconductor 22, selectively exposing the surface of the photoconductor 22 to light, thereby forming an electrostatic latent image. In other words, the LED line head 24 is used as a light source for the exposure unit in an electrophotographic image forming apparatus.

[0021] The developing unit 26 is for visualizing the electrostatic latent image on the photosensitive member 22 with a single color toner, and has a sleeve 26S. The developing unit 26 is detachable from the photosensitive member 22.

[0022] 3 to receive the single-color toner image from the photosensitive member 22. As the photosensitive member 22 and the primary transfer roller 27 positioned opposite it rotate, the single-color toner image is transferred to the intermediate transfer member 28. By applying an appropriate bias voltage to the primary transfer roller 27 and by creating a difference between the rotation speeds of the photosensitive member 22 and the intermediate transfer member 28, the single-color toner image is efficiently transferred onto the intermediate transfer member 28. This is called primary transfer.

[0023] Furthermore, the single-color toner images for each CMYK station are superimposed on the intermediate transfer body 28. The superimposed multi-color toner image is transported to the secondary transfer roller 29 as the intermediate transfer body 28 rotates. The recording medium 11 is then sandwiched and transported from the paper feed tray 21 between the secondary transfer roller 29, and the multi-color toner image on the intermediate transfer body 28 is transferred to the recording medium 11. At this time, the toner image is electrostatically transferred by applying an appropriate bias voltage to the secondary transfer roller 29. This is called secondary transfer. The secondary transfer roller 29 contacts the recording medium 11 at position 29a while the multi-color toner image is being transferred onto the recording medium 11, and then moves away to position 29b after the printing process.

[0024] The fixing device 31 has a fixing roller 32 that heats the recording medium 11 and a pressure roller 33 that presses the recording medium 11 against the fixing roller 32 in order to melt and fix the multicolor toner image transferred onto the recording medium 11 to the recording medium 11. The fixing roller 32 and the pressure roller 33 are formed in a hollow shape, and heaters 34 and 35 are built therein respectively. The fixing device 31 conveys the recording medium 11 holding the multicolor toner image by the fixing roller 32 and the pressure roller 33, applies heat and pressure, and fixes the toner to the recording medium 11.

[0025] After the toner is fixed, the recording medium 11 is then discharged to a paper discharge tray (not shown) by a discharge roller (not shown), and the image forming operation ends. The cleaning means 30 cleans the toner remaining on the intermediate transfer member 28, and the waste toner remaining after transferring the four-color multicolor toner image formed on the intermediate transfer member 28 to the recording medium 11 is stored in a cleaner container.

[0026] <Configuration of LED Line Head> FIG. 4 is a diagram showing a configuration example of an LED line head 24 arranged in parallel with the photoreceptor 22. In the present embodiment, the LED line head 24 includes a printed circuit board 40 on which a circuit for supplying various signals for controlling the driving of the LED line head 24 is formed, and a lens array 41. Further, a plurality of LED chips 42 are arranged staggeredly on the printed circuit board 40. The size of the LED line head 24 may be, for example, substantially the same size as the photoreceptor 22 in the main scanning direction, or may be smaller than the photoreceptor 22.

[0027] The configuration example of the LED chip 42 will be described with reference to the drawings. FIG. 5 is a diagram schematically showing the configuration example of the LED chip 42. As shown in FIG. 5, 512 LED light-emitting elements 43 are arranged at equal intervals in the LED chip 42. As the LED light-emitting element, for example, an LED (Light Emitting Diode) or an organic LED (Organic Light Emitting Diode) can be used. At the end portion in the LED chip 42, the main scanning positions of the LED light-emitting elements 43 included in different LED chips 42 overlap. In FIG. 5, the light-emitting elements are arranged in a state where four of them overlap between different chips. Note that the arrangement of the LED chip 42 and the number of light-emitting elements are not limited to this. For example, the LED light-emitting elements 43 may be two-dimensionally arranged in the main scanning direction and the sub-scanning direction. In addition to the description of the configuration of the LED line head 24 below, the factors causing variations in the light amount on the photoreceptor corrected in this embodiment will be described.

[0028] First, the correspondence relationship between the LED light-emitting element 43 in the LED chip 42 and the rod lens current source described later will be described with reference to the drawings. FIG. 6 is a diagram showing the correspondence relationship between the LED light-emitting element 43 in the LED chip 42 and the rod lens current source described later. FIG. 6(a) shows the case where there is no temperature change in the LED line head, and FIG. 6(b) shows the case where there is a temperature change in the LED line head. The variations in the light amount on the photoreceptor caused by the LED line head

[0029] <Factors caused by the LED line head 24> The lens array 41 is a device disposed between the LED chips 42 and the photoconductor 22 and functions as an imaging lens. The lens array 41 is configured with gradient index rod lenses arranged at a pitch such that one rod lens corresponds to, for example, four LED light-emitting elements 43, as shown in FIG. 6( a). The lens array 41 focuses light emitted from each LED light-emitting element 43 onto the photoconductor 22. Because the rod lenses have different transmittances between their centers and edges, the amount of light on the photoconductor 22 varies. In this embodiment, a first correction profile 901 contains correction values ​​for correcting this variation in the amount of light due to lens position. In other words, the first correction profile 901 can be considered correction information for correcting the variation in the amount of light on the photoconductor due to lens position. Furthermore, the printed circuit board 40 expands and contracts depending on the temperature inside the LED line head 24. This expansion and contraction of the printed circuit board 40 also changes the position of the attached LED light-emitting elements 43. However, the position of the lens array 41 does not change. As a result, the LED light-emitting element 43 and the rod lens move in parallel while maintaining the distance between them, as shown in FIGS. 6(a) and 6(b). To account for this misalignment, the lens array 41 is prepared with a width wider than that of the LED chip 42. In FIG. 6(a), the LED line head 24 further includes a rod lens 41001 disposed outside the LED chip 42 at its left end. The LED line head 24 also includes a temperature sensor 212 that detects the temperature inside the LED line head 24. A change in the detected temperature beyond a certain level triggers the update of the reference position within the first correction profile 901. The first correction profile 901 includes correction profiles corresponding to all rod lenses, which are stored in the ROM 211 of the LED line head 24. The correspondence between the LED light-emitting element 43 and the rod lens is adjusted in response to temperature changes within the LED line head 24. The updated reference position is stored in the RAM 210 of the control unit 203 and used to generate a correction signal. The format of the correction profile and the method for generating the correction signal will be described in detail later.

[0030] On the printed circuit board 40, a ROM 211 storing manufacturing variation information of the LED line head 24 measured in the manufacturing process and a current source for supplying current to the LED light-emitting elements 43 are arranged on the back side of the board or the like. Similar to the rod lenses, there are a plurality of current sources, and a plurality of LED light-emitting elements 43 correspond to one current source. The correspondence between the current source and the LED light-emitting elements 43 is shown in Fig. 6(a). As shown in Fig. 6(a), for example, they are arranged at a pitch corresponding to 8 LED light-emitting elements and one current source. In the LED chip 42 of Chip1, the current source 4001 at the left end corresponds to the 8 LED light-emitting elements 43 at the left end, and the current source 4020 at the right end corresponds to the 8 LED light-emitting elements 43 at the right end. In the LED chip 42 of Chip1, the 4 LED light-emitting elements 43 at the right end correspond to the rod lens 41129, and the 4 LED light-emitting elements 43 adjacent to the left of the 4 LED light-emitting elements 43 at the right end correspond to the rod lens 41128 adjacent to the left of the rod lens 41129. Also, the 4 LED light-emitting elements 43 at the left end in the LED chip 42 of Chip2 are arranged so as to overlap with the 4 LED light-emitting elements 43 at the right end in the LED chip 42 of Chip1 in the main scanning direction. In the LED chip 42 of Chip2, the current source 4021 at the left end corresponds to the 8 LED light-emitting elements 43 at the left end. However, due to the individual differences of the above-mentioned current sources, the light emission amounts of the LED light-emitting elements 43 vary, and as a result, the light amounts on the photoreceptor 22 vary. A correction value for suppressing the variation in the light amount is provided as the second correction profile 902. The second correction profile 902 can be said to be correction information for correcting the variation in the light amount on the photoreceptor due to the individual differences of the current sources. Different from the correspondence between the above-mentioned rod lens and the LED light-emitting elements 43, the correspondence between the LED light-emitting elements 43 and the current source does not change over time. Therefore, when assembling the LED line head 24, the difference in the light emission amount due to the difference in the current source is measured, and the second correction profile 902 created based on the measurement result is held in advance in the ROM 211 for each line head and used for generating the correction signal. The form of the correction profile and the details of the method for generating the correction signal will be described later.

[0031] <Factors other than those caused by the LED line head 24> The first and second correction profiles 901 and 902 represent variations in the light intensity of the LED light-emitting elements 43 due to the LED line head 24. Next, we will explain variations in the light intensity on the photoconductor 22 due to the positional relationship between the LED line head 24 and the photoconductor 22. For example, eccentricity of the photoconductor 22 can be cited. If the rotation axis is tilted relative to the axis of the photoconductor 22, the distance from the LED line head 24 to the photoconductor 22 changes in the main scanning position, causing variations in the light intensity on the photoconductor. The variations in the light intensity on the photoconductor occur at a lower frequency than those due to lens position or current source. In this embodiment, the third correction profile 903 has correction values ​​that correct variations in the light intensity due to the positional relationship between the LED line head 24 and the photoconductor 22. The third correction profile 903 can also be considered correction information for correcting variations in the light intensity on the photoconductor due to the positional relationship between the LED line head 24 and the photoconductor 22. For example, a correction value is provided for each of 16 light-emitting elements. Furthermore, the positional relationship between the LED line head 24 and the photoconductor 22 changes over time depending on the replacement of parts including the photoconductor 22 and the number of pages printed since the replacement. Therefore, the third correction profile 903 is updated in response to a calibration instruction when a part is replaced, or an instruction from the user who has confirmed that density unevenness in the main scanning direction has become apparent. The third correction profile 903 is stored in the storage unit 204. The form of the correction profile and the method of generating the correction signal will be described in detail later.

[0032] As described above, in the LED line head 24, the many LED light-emitting elements 43 arranged in the main scanning direction correspond to different units and other components within the LED line head 24. This causes variations in the amount of light with a mixture of different periods. In addition to being caused by the LED line head 24, variations in the amount of light on the photoconductor 22 also occur due to the positional relationship between the LED line head 24 and the photoconductor 22.

[0033] On the other hand, whether or not the correction profile needs to be updated and the trigger for updating differ depending on the factor that causes variations in the amount of light on the photoconductor. Therefore, if one correction profile that integrates variations in the amount of light on the photoconductor due to multiple factors is stored, all data must be updated at any one update timing, which increases the processing cost for updating. Therefore, in this embodiment, a correction profile is stored for each factor so that only the correction profile that is necessary at the update timing can be updated. In other words, multiple types of correction profiles are stored so that multiple types of correction profiles (multiple types of correction information) can be updated at different timings.

[0034] <Configuration of image processing unit> Next, a description will be given of the configuration of the image processing unit 207 that performs image processing on image data included in input print data when the image forming apparatus 101 according to this embodiment forms (prints) an image using the printing unit 206. Fig. 7 is a functional block diagram illustrating the configuration of the image processing unit 207. As mentioned above, the functions of the image processing unit 207 may be realized by hardware, or may be realized by the CPU 208 executing a program.

[0035] The image processing unit 207 includes an input unit 301, a color conversion processing unit 302, a rendering processing unit 303, a gradation correction processing unit 304, a halftone processing unit 305, an output unit 307, and an HT density correction processing unit 306. Note that the HT at the beginning of the HT density correction processing unit 306 is an abbreviation for halftone, indicating that it accepts and processes image data that has already been halftone processed.

[0036] The input unit 301 receives image data written in a PDL (page description language), for example, included in print data received by the data input unit 201. The color conversion processing unit 302 converts image data, for example, from an RGB color space to image data in a CMYK color space. The rendering processing unit 303 renders the received PDL data and converts it into image data. The rendering processing unit can switch between rendering processes in response to an instruction of fine or super fine. When the instruction is fine, image data is generated at a resolution of 600 dpi in the main scanning direction and the sub-scanning direction. When the instruction is super fine, image data is generated at a resolution of 1200 dpi in the main scanning direction and the sub-scanning direction. These resolution settings can be specified by the user via the UI unit 205, and are also selected according to the resolution instruction included in the print data received by the data input unit 201.

[0037] The gradation correction processing unit 304 performs gradation correction so that the printer density characteristics according to the dither matrix of the halftone processing applied to the intermediate tones of the image data will achieve the target output density of the image data for each CMYK color plate. Note that the printer density characteristics described here are obtained by measuring the printed matter shown below. In other words, the HT density correction processing unit 306 corrects for density unevenness and streaks caused by variations in the light intensity of the LED line head 24, prints halftone dot patches that have been halftoned for the signal values ​​of each color plate, and then measures the printed matter.

[0038] The halftone processing unit 305 performs halftone processing on the image data of each CMYK color plate after gradation correction, and converts it into an N-value screened image (halftone image data) in which the intermediate tones of the image data are expressed in area gradation. Hereinafter, the halftone image data after halftone processing is abbreviated as HT and is referred to as HT image data. In this embodiment, resolution conversion to a printing resolution of 2400 dpi is simultaneously performed. That is, for input image data with a resolution of 600 dpi, in order to perform halftone processing according to a printing resolution of 2400 dpi, the input image data with a resolution of 600 dpi is quadrupled in the main scanning direction and quadrupled in the sub-scanning direction, and halftone processing is performed while repeatedly referring to it. As a feature of this embodiment, at the stage of halftone processing, it is not necessarily required to be at the printing resolution, and in the HT density correction processing unit 306 described later, it is sufficient if it is at the resolution of the printing resolution. For example, a processing unit having a resolution conversion function for converting to the printing resolution may be provided after the halftone processing unit 305.

[0039] The HT density correction processing unit 306 acquires the first and second correction profiles from the ROM 211 of the line head of each color plate in the printing unit 206 and the third correction profile from the storage unit 204, and generates a correction signal for each main scanning position based on these correction profiles. Density correction based on the correction signal for each main scanning position is performed on the image data after halftone processing.

[0040] <Method for Light Quantity Correction by Adjusting HT Image Data> A method for adjusting variations in light intensity by correcting HT image data will be described using diagrams. FIG. 8 illustrates an example of HT image data without light intensity correction, and a resulting printout. FIG. 8(a) shows the lit and unlit pixels in the HT image data, and FIG. 8(b) shows a printout based on the HT image data of FIG. 8(a). For example, even if an attempt is made to generate a printout with uniform density based on HT image data in which all LED light-emitting elements 43 are illuminated, as shown in FIG. 8(a), variations in the amount of light on the photoconductor 22 can result in the following: That is, variations in the amount of light at each main scanning position on the photoconductor result in a printout with varying density at each main scanning position, as shown in FIG. 8(b). Therefore, LED light-emitting elements 43 corresponding to areas with relatively higher light emission levels than other areas are stochastically turned off.

[0041] FIG. 9 illustrates an example of processed HT image data and its printout after light intensity correction. FIG. 9(a) shows lit and unlit pixels, and FIG. 9(b) shows an example of the HT image based on FIG. 9(a). As shown in FIG. 9(a), pixels in the HT image corresponding to the central region in the main scanning direction, which is an area with relatively high light intensity, are stochastically turned off, thereby minimizing the difference in light intensity per unit area consisting of multiple pixels. As a result, although the HT image data is nonuniform, the difference in density reproduced as a printout is reduced by adding the variation in light intensity on the photoconductor, as shown in FIG. 9(b). In this embodiment, the probability of the above-mentioned extinction is determined according to a correction profile, minimizing the variation in light intensity at each main scanning position. Details of the HT density correction processor 306, a feature of this embodiment, will be described later.

[0042] The output unit 307 passes the density correction signal data generated by the HT density correction processing unit 306 to the printing unit 206 .

[0043] <Processing flow of the image processing unit 207> The processing flow by each processing unit constituting the image processing unit 207 will be explained using the drawings. Fig. 10 is a flowchart showing the flow of image processing by the image processing unit 207. The processing shown in Fig. 10 is achieved by the CPU 208 reading out a program stored in the ROM 209, expanding it into the RAM 210, and executing it.

[0044] In S1001, the CPU 208 passes the document data received by the data input unit 201 to the rendering processing unit 303 via the input unit 301 of the image processing unit 207. The rendering processing unit 303 then converts the input document data into RGB raster image data with a resolution of 600 dpi in the main scanning direction and the sub-scanning direction, and supplies the image data to the color conversion processing unit 302.

[0045] In S1002, the CPU 208 controls the color conversion processing unit 302 to execute a process of color converting the generated RGB data into CMYK data. The image data after the color conversion process is passed to the tone correction processing unit 304.

[0046] In S1003, the CPU 208 controls the gradation correction processing unit 304 to perform gradation correction processing on the image data of each color plane, taking into account the gradation characteristics of the printing unit 206 of the image forming apparatus 101 for the dither matrix of the halftone processing applied to the halftones of the image data. The image data that has undergone gradation correction processing is passed to the halftone processing unit 305. There are multiple types of dither matrices that can be applied to halftones, and the user can specify the type of dither matrix in the UI unit 205. Because the gradation characteristics of the printing unit 206 vary depending on the dither matrix of the halftone processing, it is necessary to switch the gradation correction processing depending on the dither matrix. Therefore, the gradation correction processing is performed depending on the type of dither matrix specified in the UI unit 205.

[0047] In S1004, the CPU 208 controls the halftone processing unit 305 to perform resolution conversion on the CMYK data after gradation correction, and generates HT image data, which is 1-bit data indicating on or off of the light emission of the LED light-emitting element 43. In the resolution conversion, conversion from 600 dpi to the printing resolution of 2400 dpi is executed. The generated HT image data is sent to the HT density correction processing unit 306.

[0048] In S1005, the CPU 208 controls the HT density correction processing unit 306 to acquire a correction profile from the ROM 211 held in the LED line head 24 of each CMYK color and the storage unit 204, and generates a correction signal for each main scanning position. Based on the correction signal for the HT image data, the correction processing described using FIGS. 8 and 9 is sequentially executed. The data after the correction processing is passed to the output unit 307. Details of the correction profile and correction values will be described later.

[0049] <Detailed Configuration of HT Density Correction Processing Unit 306> A configuration example of the HT density correction processing unit 306 in the present embodiment will be described with reference to the drawings. FIG. 11 is a functional block diagram for explaining the configuration of the HT density correction processing unit 306 in the present embodiment. The HT density correction processing unit 306 includes a first correction profile (also referred to as the first correction PF) 901, a second correction profile (also referred to as the second correction PF) 902, and a third correction profile (also referred to as the third correction PF) 903. The HT density correction processing unit 306 includes a composite profile acquisition unit (also referred to as the composite PF acquisition unit) 904, a first generation unit 905, a matrix 906 of the first correction signal, a third acquisition unit 907, and a first correction processing unit 910. With these functional units, density correction is performed based on the profile acquired by the composite profile acquisition unit 904. Further, the HT density correction processing unit 306 includes a second generation unit 908, a matrix 909 of the second correction signal, and a second correction processing unit 911. With these functional units, density correction is performed based on the profile acquired by the third acquisition unit 907. Furthermore, the HT density correction processing unit 306 includes an update unit (912). With this functional unit, the third correction profile 903 is updated according to changes over time.

[0050] <First, second, and third correction profiles> The first, second, and third correction profiles will be described with reference to Fig. 12. Fig. 12 is a diagram showing an example of data of the first, second, and third correction profiles.

[0051] The first correction profile 901 is data that stores correction values ​​for correcting variations in the light intensity of light-emitting elements due to differences in transmittance at each rod lens position. In this embodiment, as shown in FIG. 12, the first correction profile 1201 corresponding to the first correction profile 901 stores data corresponding to each of the 512 LED light-emitting elements 43 per LED chip 42. In other words, the first correction profile 901 can be said to store data for each light-emitting element. As described above, the range of deviation between the LED light-emitting elements 43 and the rod lenses from their initial position depending on the temperature inside the LED line head is determined in advance, and data corresponding to the number of rod lenses and the amount of deviation in the correspondence relationship depending on the temperature are stored. In this embodiment, an example is shown in which a maximum deviation of one rod lens (four light-emitting elements) occurs due to expansion and contraction of the printed circuit board 40. Therefore, the first correction profile 1201 stores 512 pieces of data plus eight pieces, for a total of 520 pieces of data. The data is stored in 4 bits, with 15 set for main scanning positions where the transmittance is high and the light emission frequency is low to reduce the light intensity. Conversely, if the transmittance is low and the light emission frequency of the LED light emitting element 43 is maintained, the value is set to 0. These numbers indicate the extinguishing probability.

[0052] The second correction profile 902 is data that holds correction values ​​for correcting variations in the light intensity of the LED light-emitting elements 43 due to differences in the current source. In this embodiment, as shown in FIG. 12 , the second correction profile 1202 corresponding to the second correction profile 902 holds one piece of data for every eight light-emitting elements. In this embodiment, since each LED chip 42 has 512 LED light-emitting elements 43, the second correction profile 902 holds 64 pieces of data for each LED chip 42. Like the first correction profile 901, the data is held in 4 bits, and 15 is set when the current supplied from the current source is strong and the light emission frequency of the LED light-emitting elements 43 is reduced. Conversely, 0 is set when the current is weak and the light emission frequency of the LED light-emitting elements 43 is maintained.

[0053] The third correction profile 903 is data that stores correction values ​​for correcting variations in the amount of light on the photoconductor 22 due to the inclination between the LED line head 24 and the photoconductor 22. In this embodiment, as shown in FIG. 12, the third correction profile 1203 corresponding to the third correction profile 903 stores one data item for every 16 light-emitting elements. In this embodiment, since each LED chip 42 has 512 LED light-emitting elements 43, 32 data items are stored per chip. The data is stored in 8 bits, and a print chart is output with all LED light-emitting elements 43 turned on (illuminated), and correction values ​​are determined based on the density difference at each main scanning position. The correction value corresponding to an area where the distance between the LED line head 24 and the photoconductor 22 is long and the amount of light is low is set to 0, and the correction value corresponding to an area where the distance between the LED line head 24 and the photoconductor 22 is short and the amount of light is high is set to a value greater than 0. The process of generating correction values ​​from the print chart is similar to the process performed by the update unit 912 and will be described in detail later in the process performed by the update unit 912. The pitch and data amount for holding each correction profile are not limited to the above examples. For example, the number of corresponding LED light-emitting elements 43 may be uneven depending on the position of the data. Also, the data may be multi-bit. By using multi-bit data, correction can be performed with finer precision.

[0054] The composite profile acquisition unit 904 has a first acquisition unit 913, a second acquisition unit 914, and a first synthesis processing unit 915. The first acquisition unit 913 selects a data area corresponding to the LED light emitting element 43 from the acquired first correction profile based on the first correction profile acquired from the ROM 211 in the LED line head 24 and temperature information acquired by the temperature sensor 212. The second acquisition unit 914 acquires the second correction profile from the ROM 211 in the LED line head 24. The first synthesis processing unit 915 generates data by synthesizing the data selected by the first acquisition unit 913 and the data acquired by the second acquisition unit 914 on a light emitting element basis. Details of the composite profile acquisition unit 904 will be described later.

[0055] The first generation unit 905 acquires a threshold value corresponding to a light-emitting element by referencing the data for each light-emitting element acquired by the composite profile acquisition unit 904 and a first correction signal matrix (also referred to as the first matrix) 906. The first generation unit 905 then generates a first correction signal for correcting the HT image data based on the first and second correction profiles 901 and 902. The correction signal in this embodiment is a signal value representing the extinction rate for determining the pixels to be extinguished probabilistically, as described above in the method for correcting light intensity by adjusting HT image data. In this embodiment, the first correction signal is generated using the data for each light-emitting element and the first correction signal matrix 906, which will be described later.

[0056] <Matrix of the first correction signal> The details of the matrix 906 of the first correction signal will be described with reference to the drawings. FIG. 13 is a diagram for explaining the matrix of the first correction signal and an example of its use, with FIG. 13(a) showing an example of the matrix of the first correction signal and FIG. 13(b) showing an example of its use. The matrix 906 of the first correction signal is matrix data for 128 × 128 pixels. FIG. 13(a) shows a schematic diagram of the matrix 906 of the first correction signal. In this embodiment, matrix data for 128 × 128 pixels is stored. When this matrix data is used, it is repeatedly arranged and referenced in the vertical and horizontal directions based on the matrix data and the shift amount stored together, as shown in FIG. 13(b).

[0057] <Generation of First Correction Signal> The generation of the first correction signal by the first generation unit 905 will be described with reference to the drawings. FIG. 14 is a diagram illustrating the generation of the first correction signal. Correction data 1401 for the main scanning position is compared with a threshold value 1402 obtained by referencing the matrix of the first correction signal. Pixels for which the correction data exceeds the matrix data are designated as unlit pixels (0), and other pixels are designated as lit pixels (1), generating a first correction signal 1403. A threshold matrix with known blue noise characteristics is used as the matrix. By using blue noise characteristics, the halftone dot period is finer than that of an AM screen-type dither matrix, dispersing the unlit pixels. Since the positions of the unlit pixels are determined using a fine halftone dot period, it is possible to suppress the occurrence of broken thin lines and jerky characters due to light intensity correction. Furthermore, it is possible to suppress stripes caused by interference between the HT image data and the correction signal. The frequency characteristics of the matrix data are not limited to blue noise. Any dot-dispersed matrix that minimizes adjacent dots and does not affect jerky thin lines or jerky characters may be used. A threshold matrix with known green noise characteristics or pink noise characteristics may also be used.

[0058] The third acquisition unit 907 acquires the third correction profile from the storage unit 204 in the control unit 203 and converts it into data for each light-emitting element. The conversion process is performed by acquiring the profile value of the main scanning position corresponding to the LED light-emitting element 43.

[0059] The second generation unit 908 refers to the data for each light-emitting element acquired and converted by the third acquisition unit 907, and a matrix of second correction signals (also referred to as a second matrix) 909. Then, the second generation unit 908 generates a second correction signal for correcting the HT image data based on the third correction profile 903.

[0060] The format of the matrix 909 of the second correction signal is the same as that of the matrix 906 of the first correction signal, except for the threshold value of the matrix data that is stored. The matrix 909 of the second correction signal has known blue noise characteristics, similar to the matrix 906 of the first correction signal, and uses matrix data with a different phase from that of the matrix 906 of the first correction signal. The matrix 909 of the second correction signal stores matrix data with different phases so that the correction positions resulting from the two correction processes do not overlap. The method of generating correction data by the second generation unit 908 is the same as the method of generating the first correction signal by the first generation unit 905, and therefore a description thereof will be omitted.

[0061] The first correction processing unit 910 acquires the HT image data from the HT processing unit 305, and acquires the first correction signal from the first generation unit 905. Then, the first correction processing unit 910 performs an AND process on the acquired HT image data and the acquired first correction signal for each pixel. That is, even if the HT image data is a lit state (1), if the first correction signal is an off pixel (0), the first correction processing unit 910 corrects it to an off state (0).

[0062] The second correction processing unit 911 acquires the first light quantity corrected HT image data from the first correction processing unit 910 and acquires the second correction signal from the second generation unit 908. Then, the second correction processing unit 911 performs an AND process on the acquired first light quantity corrected HT image data and the acquired second correction signal for each pixel. That is, similar to the first correction process, even if the first light quantity corrected HT image data is in the lit state (1), when the second correction signal is for a pixel that is off (0), the second correction processing unit 911 corrects it to the off state (0).

[0063] The HT image data on which the first and second light quantity corrections are sequentially performed is sent to the printing unit 206. Then, the printing unit 206 performs image formation on the recording medium based on the processed HT image data.

[0064] Furthermore, the update unit 912 updates the third correction profile 903 based on a user instruction from the UI unit 205. That is, first, the update unit 912 reads, with the image reading unit 202, the result of printing the calibration chart previously held in the ROM 209 based on a user instruction from the UI unit 205, and acquires the density distribution for each main scanning position. Then, the update unit 912 updates the third correction profile 903 based on the acquired density distribution for each main scanning position. With the position having the lowest density among the main scanning positions as a reference, a correction value is calculated such that the emission frequency of the region corresponding to the region darker than the reference becomes lower, and the third correction profile 903 is updated. Details of the update process of the third correction profile will be described later.

[0065] <Processing flow of the HT density correction processing unit 306> The processing flow of the HT density correction processing unit 306 of this embodiment will be described using the drawings. Fig. 15 is a flowchart showing the flow of image processing by the HT density correction processing unit 306 of this embodiment. The processing shown in Fig. 15 is achieved by the CPU 208 reading out a program stored in the ROM 209, expanding it in the RAM 210, and executing it. In the following steps S1501 to S1509, target toner colors and pixel positions are selected sequentially from the HT image data of the four colors of CMYK, and processing is performed. The selected pixel is changed and the processing is repeated until processing is completed for all pixels.

[0066] In S1501, the CPU 208 acquires the signal value In of the pixel to be processed from the HT image data after halftone processing stored in the RAM 210.

[0067] In S1502, the CPU 208 acquires the first correction profile 901 and the second correction profile 902 stored in the ROM 211 of the LED line head 24, and acquires temperature information within the LED line head 24 from the temperature sensor 212. The acquired temperature information is compared with past temperature information stored in the storage unit 204 to determine whether a temperature change greater than a pre-stored threshold has occurred. Examples of past temperature information include temperature information stored in the storage unit 204 in the previous process. If the determination result indicates that a temperature change greater than the pre-stored threshold has occurred, the CPU 208 determines data for each light-emitting element based on the temperature information from the first correction profile 901, and updates the data stored in the RAM 210. Furthermore, the CPU 208 calculates a composite correction value by combining data corresponding to the second correction profile 902 with data corresponding to the first correction profile 901 stored in the RAM 210, and stores the result in the RAM 210. Details of the method for calculating the composite profile will be described later.

[0068] In S1503, the CPU 208 acquires the matrix 906 of the first correction signal stored in the RAM 210.

[0069] In S1504, the CPU 208 selects a value Coeff_1st corresponding to the main scanning position of the pixel selected for processing from the composite profile stored in the RAM 210. Furthermore, it selects a threshold Mat_1st corresponding to the pixel position selected for processing from the matrix 906 of the acquired first correction signal. Then, it calculates a correction signal Corr_1st from these pieces of data. Pseudocode for calculating the correction signal Corr_1st is shown below. The calculation results are stored in the RAM 210. Both Coeff_1st and Mat_1st are stored as 8-bit values ​​and compared. Details of the method for calculating the 8-bit Coeff_1st from the 5-bit first correction profile 901 and second correction profile 902 will be described later.

[0070] Pseudocode: if(Coeff_1st > Mat_1st){ Corr_1st = 0; }else{ Corr_1st = 1; } In S1505, the CPU 208 performs an AND operation on the In and Corr_1st acquired in S1501 to calculate the HT image data In_trim corrected using the first correction signal. That is, the first light intensity correction process is performed. The calculation result is stored in the RAM 210.

[0071] In S1506, the CPU 208 acquires the third correction profile 903 stored in the storage unit 204. Furthermore, the CPU 208 references the data corresponding to each light-emitting element and converts it into data for each light-emitting element. Furthermore, the CPU 208 references the correction data ProcCoeff corresponding to each light-emitting element and converts it into correction data ProcCoeff_full for each light-emitting element. The converted correction data ProcCoeff_full for each light-emitting element is stored in the RAM 210. For example, the conversion is performed using the following equation (1), where x represents the main scanning position. ProcCoeff_full[x] = ProcCoeff[x / 16] ···(1)

[0072] In S1507, the CPU 208 acquires the matrix 906 of the second correction signal stored in the RAM 210.

[0073] In S1508, the CPU 208 selects a value Coeff_2nd corresponding to the pixel position selected as the processing target from the data corresponding to the third correction profile 903 stored in the RAM 210. Furthermore, the CPU 208 selects a threshold Mat_2nd corresponding to the pixel position selected as the processing target from the matrix 909 of the acquired second correction signal. Then, the correction signal Corr_2nd is calculated from these data. The calculation method is the same as the pseudo code in S1504, and a description thereof will be omitted.

[0074] In S1509, the CPU 208 performs an AND operation on In_trim and Corr_2nd calculated in S1505 to calculate HT image data In_trim_2nd corrected using the second correction signal. That is, the second light intensity correction process is performed. The calculated HT image data In_trim_2nd corrected using the second correction signal is sent to the printing unit 206.

[0075] In the above, S1501 and S1502 to S1504 are parallel processes, but this is not limiting. For example, S1501 to S1504 may be sequential processes. In the above, S1501 to S1505 and S1506 to S1508 are parallel processes, but this is not limiting. For example, S1501 to S1508 may be sequential processes.

[0076] <Detailed processing flow of composite profile acquisition processing S1502> The details of the process (S1502) of the composite profile acquisition unit 904 will be described with reference to Fig. 16. Fig. 16 is a flowchart showing the detailed flow of the process by the composite profile acquisition unit 904.

[0077] In S1601, the CPU 208 acquires, from the temperature sensor 212, temperature information obtained by the temperature sensor 212 measuring the temperature inside the LED line head 24.

[0078] In S1602, CPU 208 compares the temperature information acquired in S1601 with past temperature information previously stored in storage unit 204, and determines whether a temperature difference has occurred that would cause printed circuit board 40 to expand or contract. Examples of past temperature information include temperature information stored in storage unit 204 in the previous process. If the determination result shows that a temperature difference has occurred that would cause printed circuit board 40 to expand or contract and that there has been a temperature change (YES in S1602), the process proceeds to S1603. On the other hand, if a temperature difference has not occurred that would cause printed circuit board 40 to expand or contract and there has been no temperature change (NO in S1602), S1603 is skipped and the process proceeds to S1604.

[0079] In S1603, the CPU 208 acquires the temperature information acquired in S1601 and the amount of misalignment between the rod lens and the LED light-emitting element 43 corresponding to the temperature information, which is stored in the ROM 211 of the LED line head 24. The CPU 208 then selects a correction value corresponding to the current LED light-emitting element 43 from the first correction profile 901 and updates the correction data stored in the RAM 210. The amount of misalignment corresponding to the temperature information is stored in the same units as the main scanning resolution. The correction value is selected by setting the main scanning position obtained by shifting the initial correspondence start position by the amount of misalignment as the start position. The temperature information stored in the storage unit 204 is also updated to the latest information.

[0080] In S1604, the CPU 208 acquires the first correction profile 901 for each light-emitting element that has already been selected and stored in the RAM 210.

[0081] In S1605, the CPU 208 acquires the second correction profile 902 stored in the ROM 211 of the LED line head 24. Furthermore, the CPU 208 refers to the correction data PowCoeff corresponding to each light-emitting element, converts it into correction data PowCoeff_full for each light-emitting element, and stores it in the RAM 210. For example, the conversion is performed using the following equation (2), where x represents the main scanning position. PowCoeff_full[x] = PowCoeff[x / 8] ···(2)

[0082] In S1606, the CPU 208 calculates the composite correction value TotalCoeff[x], which is composite information, based on the data shown below and using the following pseudo code. The calculation process of the composite correction value TotalCoeff[x] is performed based on the data shown below. That is, the calculation is performed based on the data LensCoeff[x] corresponding to the first correction profile 901 acquired in S1604 and the data PowCoeff_full[x] corresponding to the second correction profile 902 acquired in S1605. Here, x represents the main scanning position. The calculation is performed for each main scanning position.

[0083] First, the lens transmittance correction value LensCoeff[x], which is data corresponding to the first correction profile 901 acquired in S1604, is converted into a light intensity equivalent value Lumi that takes the lens transmittance into consideration. The lower the amount of light emitted from the LED light-emitting element 43 and transmitted through the rod lens, the smaller the correction value. In other words, the correction value and the light intensity are inversely proportional to each other. Therefore, the light intensity equivalent value can be found by inverting the correction value.

[0084] Next, the correction value PowCoeff_full[x] due to variations in the current source is converted into a light intensity correction ratio Rate that takes into account differences in current. The smaller the current to the LED light emitting element 43, the smaller the correction value. In other words, as with LensCoeff, the correction value and the light intensity correction ratio are inversely proportional. Therefore, a value equivalent to the light intensity correction ratio can be obtained by inverting the correction value.

[0085] Next, the composite correction value TotalCoeff[x] is determined based on the corrected light intensity (LumiTrim) obtained by multiplying the light intensity by the light intensity adjustment ratio. In addition, when determining TotalCoeff[x], it is multiplied by the adjustment value Gain. The adjustment value Gain is used to expand the correction resolution due to the small amount of data held in the correction profile. If the correction resolution is sufficient, 1 is set. If the correction resolution is insufficient, a value greater than 1 is set.

[0086] Finally, clipping is performed so that TotalCoeff[x] falls within the 8-bit threshold range held in the matrix 906 of the first correction signal.

[0087] Pseudocode: Lumi = 16―LensCoeff[x] / / Correction value → Light intensity Rate = (16―PowCoeff_full[x]) / 16 / / Correction value → ratio LumiTrim = Lumi × Rate / / Corrected light intensity = Light intensity × Ratio TotalCoeff[x] = (16-LumiTrim) × Gain / / Corrected light intensity → composite correction value if(TotalCoeff[x]>255){TotalCoeff[x]=255} The first correction profile 901 and the second correction profile are factors that cause variations in the amount of light caused by the same LED line head 24. In other words, when one of the conditions changes, the period of the variation caused by the remaining factor remains the same, but only the amplitude changes. Therefore, the two conditions are combined to form a composite.

[0088] In the above, steps S1601 to S1604 and step S1605 are performed in parallel, but this is not limiting. For example, steps S1601 to S1605 may be performed sequentially.

[0089] <Processing flow of the update unit 912> The details of the processing by the update unit 912 will be explained using the drawings. FIG. 17 is a flowchart showing a detailed flow of the processing by the update unit 912 to update the third correction profile. The processing shown in FIG. 17 is achieved by the CPU 208 reading out a program stored in the ROM 209, expanding it into the RAM 210, and executing it. The following processing is executed based on a calibration instruction from the user input via the UI unit 205. For example, calibration needs to be performed when parts including the photoconductor 22 are replaced, or when a certain number of pages or more have been printed and there has been a change over time in the surface of the photoconductor. Therefore, it is desirable to display a notification on the UI unit 205 prompting the user to perform calibration at the above timing.

[0090] In S1701, the CPU 208 prints a calibration chart 1801 stored in the ROM 209 using the printing unit 206. The calibration chart will be described using diagrams. FIG. 18 is a schematic diagram of the calibration chart 1801. As shown in FIG. 18, in the calibration chart 1801, patches 1802, 1803, 1804, and 1805 represent different color materials to be printed. In this embodiment, print data for printing using C (cyan) for patch 1802, M (magenta) for patch 1803, Y (yellow) for patch 1804, and K (black) for patch 1805 is stored as the calibration chart 1801. As shown in FIG. 8A, all pixels in each patch are lit pixels for each color material. Note that, although only an example in which the signal values ​​of each patch are all lit pixels has been shown in this embodiment, this is not limiting. A calibration chart including halftone patches with lit pixels of less than 100% may also be used.

[0091] In S1702, the CPU 208 reads (reads) the printout of the calibration chart (print chart) 1801 output in S1701 using the image reading unit 202, and stores the signal values ​​resulting from the reading in the RAM 210.

[0092] In S1703, the CPU 208 converts the read signal values held in the RAM 210 into density values for each main scanning position using the conversion formula held in the ROM 209. In this embodiment, the resolution of the image reading unit 202 is 600 dpi, and density values for each 600 dpi unit of main scanning are calculated and held in the RAM 210. That is, in this embodiment, one pixel of 600 dpi in main scanning is set as a predetermined region, and correction data is generated so as to reduce density variations. Note that the resolution of the predetermined region is an example and is not limited to the above example. In order to correct finer streaks, higher-resolution read signal values may be used.

[0093] In S1704, the CPU 208 calculates a correction profile. That is, the CPU 208 calculates an updated value ProcCoeff[subx] of a third correction profile for each 150 dpi unit of main scanning based on the density values Dens[scanx][comp] for each 600 dpi unit of main scanning held in the RAM 210. The pseudo-code for calculating the correction profile is shown below. Here, scanx represents the main scanning position in units of 600 dpi, subx represents the main scanning position in units of 150 dpi, and comp represents the ID of the colorant. The calculated updated value ProcCoeff[subx] is held in the RAM 210.

[0094] Pseudo-code: for(int c=0; c<4; c++){ / / Repeat for 4 colors of CMYK min = Dens[0][c]; for(int scanx=1; scanx<width_600; scanx++){ / / width_600: Maximum value of main scanning position in units of 600 dpi if(min > Dens[x][c]) min = Dens[x][c];<​​​​​​​​​for(int i=0; i <4;i++){ sum = Dens[subx*4+i][c] - min;} ProcCoeff[subx]=sum / 4; } } Finally, in S1705, the CPU 208 updates the value of the third correction profile stored in the storage unit 204 with the updated value ProcCoeff[subx] stored in the RAM 210.

[0095] As described above, in this embodiment, the following correction process is performed on HT image data after halftoning to suppress variations in the amount of light on the photoconductor. Specifically, the correction process suppresses variations in the amount of light on the photoconductor based on a correction signal created using a dot-dispersed matrix that allows for higher-frequency extinction than the AM screen dither matrix used in halftoning. This allows the correction process to be performed without being affected by the low-frequency dither matrix used in halftoning, thereby suppressing the occurrence of broken thin lines and jerky characters. Furthermore, because a correction profile is maintained for each cause of variations in the amount of light, the processing cost associated with updating the correction profile can be reduced. In other words, it is possible to suppress density unevenness caused by variations in the amount of light on the photoconductor.

[0096] In this embodiment, an example in which two light intensity corrections are performed after halftone processing has been described, but the number of light intensity corrections is not limited to this. Another correction process may be added and performed after the second correction process.

[0097] [Embodiment 2] The image forming apparatus according to this embodiment will be described with reference to the drawings. However, in this embodiment, the differences from embodiment 1 will be mainly described. Fig. 19 is a functional block diagram illustrating the configuration of the HT density correction processing unit 306 included in the image forming apparatus according to this embodiment.

[0098] In this embodiment, a mode will be described in which correction profiles corresponding to different factors are synthesized in advance and aggregated into one correction process using the synthesized profile. In this embodiment, it is different from Embodiment 1 in that the synthesis profile acquisition unit 904 that calculates the synthesis correction value for each light-emitting element is replaced with the synthesis profile acquisition unit 916 that includes the third acquisition unit 907. Also, in this embodiment, the generation unit of the correction signal value is one of the first generation units 905, and the light amount correction unit is one of the first correction processing units 910, which is also different from Embodiment 1.

[0099] The HT concentration correction processing unit 306 of this embodiment includes the first correction profile 901, the second correction profile 902, the third correction profile 903, the first generation unit 905, the matrix 906 of the first correction signal, and the first correction processing unit 910. The HT concentration correction processing unit 306 of this embodiment further includes a synthesis profile acquisition unit 916. The synthesis profile acquisition unit 916 includes a third acquisition unit 907, a first acquisition unit 913, a second acquisition unit 914, a first synthesis processing unit 915, and a second synthesis processing unit 917.

[0100] After calculating the first synthesis profile obtained by synthesizing the first and second correction profiles 901 and 902 in the same manner as in the first embodiment, the synthesis profile acquisition unit 916 executes the following processing. That is, the synthesis profile acquisition unit 916 (second synthesis processing unit 917) generates a second synthesis profile obtained by synthesizing the first synthesis profile and the third correction profile 903. The processing after generating the second synthesis profile is the same as that in Embodiment 1, so the description is omitted. <so000413> <Processing Flow of HT Concentration Correction Processing Unit> The processing flow of the HT density correction processing unit 306 of this embodiment will be described using the drawings. Fig. 20 is a flowchart showing the flow of image processing by the HT density correction processing unit 306 of this embodiment. The processing shown in Fig. 20 is achieved by the CPU 208 reading out a program stored in the ROM 209, expanding it into the RAM 210, and executing it. In this embodiment, after executing the processing of S1502, the processing of S1510, which is different from that of the first embodiment, is executed. Then, after executing the processing of S1510, the processing of S1503, S1504, and S1505 is executed, as in the first embodiment.

[0102] In S1510, the CPU 208 calculates a second composite profile by combining the composite profile calculated in S1502 with the third correction profile 903 acquired in S1607, which will be described in detail later. The calculated second composite profile is stored in the RAM 210. Details of the processing in S1510 will be described with reference to the drawings.

[0103] <Detailed processing flow of composite profile acquisition processing S1502 and S1510> The details of the processing (S1502, S1510) of the composite profile acquisition unit 916 will be described with reference to the drawings. Fig. 21 is a flowchart showing the detailed flow of the processing by the composite profile acquisition unit 916. Steps S1607 and S1608, which are different from the first embodiment, will be described.

[0104] In S1607, the CPU 208 acquires the third correction profile 903 stored in the storage unit 204. In S1608, the CPU 208 performs a second synthesis process using the synthesis correction value TotalCoeff[x] calculated in S1502 and ProcCoeff_full[x] obtained by converting the third correction profile 903 acquired in S1607 into 8-bit data for each light-emitting element. That is, the CPU 208 adds the synthesis correction value TotalCoeff[x] and the converted correction data ProcCoeff_full[x] for each light-emitting element as shown in the following equation (3) to calculate a second synthesis value TotalCoeff_2nd[x]. That is, the second synthesis value TotalCoeff_2nd[x] can also be considered synthesis information. Here, x represents the main scanning position. Calculation is performed for each main scanning position. TotalCoeff[x] and ProcCoeff_full[x] are converted in advance into 8-bit data for each light-emitting element. Clipping is performed so that TotalCoeff_2nd[x] falls within the 8-bit threshold range held in the matrix 906 of the first correction signal. TotalCoeff_2nd[x]=TotalCoeff[x]+ProcCoeff_full[x] ····(3)

[0105] The composite profile calculated in S1502 and the third correction profile 903 are factors that cause variations in light quantity due to different phenomena. In other words, even if one of the conditions changes, the period and amplitude of the variation in the remaining factor are not affected. Therefore, the two conditions are combined.

[0106] As described above, according to this embodiment, in addition to the influence of the dither matrix used in halftone processing, it is also possible to suppress interference caused by correction processing between different correction profiles, making it possible to further suppress image quality degradation caused by correction processing for variations in light quantity.

[0107] [Embodiment 3] An image processing device according to this embodiment will be described with reference to the drawings. However, in this embodiment, differences from the first and second embodiments will be mainly described. In the first and second embodiments, an example was described in which a matrix for generating a correction signal is shared for each of the CMYK colors. However, if a correction signal is generated using a matrix shared by CMYK, correction-induced light-off occurs at the same position, exposing the paper white. When the paper white is exposed, correction-induced light-off becomes more visible.

[0108] In this embodiment, an aspect of shifting the position of the off pixels for each color will be described. As a method of shifting the position of the off pixels for each color, for example, a method of providing a plurality of types of matrices with different phases corresponding to the respective color plates in the printing unit 206 and switching the matrix to be referenced for each color may be used. Alternatively, as a method of shifting the position of the off pixels for each color, a method of providing a processing unit that shifts the reference start position (reference reference position) of the matrix so that each color has a different phase, and shifting the reference start position of the matrix by this processing unit so that each color has a different phase may be used.

[0109] An example of changing the phase by shifting the reference start position will be described using the drawings. FIG. 22 is a diagram illustrating the correspondence between matrix data, HT image data, and thresholds. FIG. 22(a) shows a case where the reference start position 2201 is at a threshold of 16, and FIG. 22(b) shows a case where the correspondence between the HT image data and thresholds is determined for the matrix data in FIG. 22(a) based on the shift amount. FIG. 22(c) shows a case where the reference start position 2201 is at a threshold of 32, and FIG. 22(d) shows a case where the correspondence between the HT image data and thresholds is determined for the matrix data in FIG. 22(c) based on the shift amount. For simplicity's sake, the matrix size will be described as 8×8. The reference start position 2201 is set as the origin of the matrix (main scanning position = 0, sub-scanning position = 0), and the data is repeatedly aligned and referenced in the vertical and horizontal directions based on the shift amount stored together with the matrix data.

[0110] By changing the reference start position 2201, it becomes possible to refer to matrices of different phases from the same matrix data, as shown in FIG. 22(b) and FIG. 22(d).

[0111] As described above, according to this embodiment, in addition to the influence of the dither matrix used in halftone processing, it is possible to prevent overlapping of pixels that are turned off for each CMYK color, making it possible to further suppress deterioration in image quality due to correction processing for variations in light intensity.

[0112] Furthermore, in the first and second embodiments, the reference start position 2201 of the matrix for generating the correction signal is always the same. However, this is not limiting. It is known that the LED element 43 deteriorates as its illumination time increases, resulting in a decrease in the amount of light emitted. Differences in illumination time among the multiple light-emitting elements in the LED line head 24 can cause localized deterioration and streaking. If the reference start position 2201 of the matrix for the correction signal is always fixed regardless of the number of printed sheets, certain LED element 43 are likely to be turned off, resulting in differences in illumination time. Therefore, a processing unit may be provided that shifts the reference start position 2201 of the matrix in the main scanning direction when either the page or job changes, such as between pages, at startup, or between jobs.

[0113] Also, a processing unit may be provided that has a plurality of matrices with different phases and changes the matrix to be referenced when either the page or the job changes.

[0114] [Other embodiments] The present disclosure can also be realized by providing a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) that realizes one or more functions. [Explanation of symbols]

[0115] 905 First Generator 908 Second Generator 910 First correction processing unit 911 Second correction processing unit

Claims

1. An electrophotographic image forming apparatus using a line head having a plurality of light emitting elements arranged thereon as a light source, An acquisition unit that acquires print data to be printed by the electrophotographic image forming apparatus; an execution unit that executes halftone processing on the print data acquired by the acquisition unit; a generating unit for generating a plurality of correction information for correcting a variation in the amount of light on a photoconductor due to the light irradiated from the plurality of light emitting elements; a correction unit that corrects the print data that has been halftoned using the correction information; having The plurality of pieces of correction information include a first profile for correcting the variation in the amount of light caused by the light emitting element, a second profile for correcting the variation in the amount of light caused by individual differences in current sources of the light emitting element, and a third profile for correcting the variation in the amount of light caused by a positional relationship between the line head and the photoconductor.

1. An image forming apparatus comprising:

2. The correction information has a dot period that is finer than the dot period generated in the print data that has been halftone processed.

2. The image forming apparatus according to claim 1,

3. The plurality of pieces of correction information hold correction values ​​for determining the probability of turning off the plurality of light-emitting elements in different units.

3. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction.

4. The correction information storing unit further includes an update unit that updates at least one of the plurality of pieces of correction information.

4. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction.

5. The update means executes the update at different timings for each of the plurality of pieces of correction information.

5. The image forming apparatus according to claim 4.

6. The timing of the update is either a calibration instruction from a user or a change in the state of the line head.

6. The image forming apparatus according to claim 5,

7. The generating means generates the correction information based on a matrix and a profile stored in advance.

7. The image forming apparatus according to claim 1, wherein the image forming apparatus is a multi-color image forming apparatus.

8. 8. The image forming apparatus according to claim 7, wherein the reference position for referring to the matrix is ​​changed at a timing when any one of a toner color, a page, and a job is changed.

9. As the matrix, a plurality of matrices with different phases are held, and the matrix to be referenced is changed at the timing when any one of the toner color, page, and job is changed.

8. The image forming apparatus according to claim 7,

10. A method for controlling an electrophotographic image forming apparatus using a line head having a plurality of light emitting elements arranged thereon as a light source, comprising the steps of: acquiring print data to be printed by the electrophotographic image forming apparatus; an execution step of executing halftone processing on the print data acquired in the acquisition step; a generating step of generating a plurality of correction information for correcting a variation in the amount of light on a photoconductor due to the light irradiated from the plurality of light emitting elements; a correction step of correcting the print data that has been halftoned using the correction information; Including, The plurality of pieces of correction information include a first profile for correcting the variation in the amount of light caused by the light emitting element, a second profile for correcting the variation in the amount of light caused by individual differences in current sources of the light emitting element, and a third profile for correcting the variation in the amount of light caused by a positional relationship between the line head and the photoconductor. 2 is a control method for an image forming apparatus according to claim 1 .

11. A program for causing a computer to function as each of the units of the image forming apparatus according to any one of claims 1 to 9.

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